HK1049551B

HK1049551B - Electrode for use in lithium battery and rechargeable lithium battery

Info

Publication number: HK1049551B
Application number: HK03101624.9A
Authority: HK
Inventors: 池田博昭; 藤本正久; 藤谷伸; 岛正树; 八木弘雅; 樽井久树; 黑河宏史; 浅冈贤司; 松田茂树; 堂本洋一; 大下竜司; 加藤善雄; 中岛宏
Original assignee: 三洋电机株式会社
Priority date: 1999-10-22
Filing date: 2000-10-20
Publication date: 2006-04-07

Description

Electrode for lithium battery and rechargeable lithium battery

Technical Field

The present invention relates to a novel electrode for use in a lithium battery, a lithium battery using the same, and a rechargeable lithium battery.

Background

Recently, intensive research and development into battery performance of rechargeable lithium batteries, such as charge-discharge voltage, cycle life characteristics or storage characteristics, are mostly related to electrodes used. This has led to attempts to obtain better battery performance by improving electrode active materials.

Although it is possible to construct a battery having a high energy density per unit weight and volume using metallic lithium as a negative active material, there is a problem in that lithium deposited upon charging generates dendrites, which may cause internal short circuits.

Rechargeable lithium batteries (Solid State Ionics, 113-115, p57(1998)) have been reported that use electrodes composed of aluminum, silicon tin, or the like, which electrochemically mix with lithium during charging. In particular, silicon electrodes offer particularly high theoretical capacities and are being allowed as high capacity negative electrodes. For this reason, various rechargeable lithium batteries using silicon as a negative electrode are recommended (Japanese patent laid-open No. Hei 10-255768). However, such a mixture negative electrode reduces the current collecting ability due to the mixture itself as an electrode active material becoming powder upon charging and discharging, and cannot provide sufficient cycle characteristics.

Disclosure of the invention

The object of the present invention is to provide a novel electrode which, when used as an electrode for a rechargeable lithium battery, can provide a high charge-discharge capacity and excellent charge-discharge cycle characteristics; and also to provide a lithium battery and a rechargeable lithium battery using the novel electrode.

The present invention is an electrode for use in a lithium battery, characterized by comprising a thin film provided on a current collector, wherein the thin film comprises an active material capable of storing and releasing lithium, and is divided into a plurality of columns by gaps formed thereon and extending in the thickness direction thereof, and the bottoms of the column portions are bonded to the current collector.

In the present invention, the thin film composed of the active material is divided into columns by gaps formed thereon and extending in the thickness direction thereof so as to provide spaces in such a manner as to surround the columnar portions. These spaces serve to relieve stress caused by expansion and contraction of the film during charge and discharge, thereby preventing the stress from increasing to such an extent that the film comes off the current collector. Accordingly, the bottom of the columnar portion can be kept adhered to the current collector.

In the present invention, it is preferable that at least half of the film thickness portion is divided into pillars by the gaps. That is, it is preferable that the film thickness portion occupying at least half of the film thickness be divided into the pillars by the gaps.

Also, in the case where the irregularities are formed on the surface of the film and the gaps formed on the film are formed starting from the depressions of the irregularities, the gaps may be formed in such a manner that each of the pillar portions surrounds at least one of the convex portions of the irregularities on the surface of the film. The gap may be formed in such a manner that each pillar portion surrounds a plurality of the convex portions.

In the present invention, a gap may be formed in the thin film during the first or subsequent charge-discharge cycle. To explain this, a film having irregularities on the film surface may be provided before charge-discharge. In the first or subsequent charge-discharge cycle, gaps are formed starting from the irregularities on the surface of the film to divide the film into pillars.

Asperities may be formed on the surface of the film such that its shape substantially conforms to the shape on the underlying current collector surface. That is, when the film is placed on the uneven surface, the current collector may have such unevenness thereon.

The surface roughness Ra (roughness average) of the collector is preferably 0.01 μm or more, more preferably in the range of 0.01 to 1 μm, and most preferably in the range of 0.05 to 0.5. mu.m. For example, the surface roughness Ra specified in Japanese Industrial Standard (JIS B0601-.

In the present invention, it is preferable that the surface roughness Ra of the current collector satisfy the relationship Ra ≦ t, where t is the thickness of the active material thin film. Preferably, the surface roughness Ra of the collector and the average interval S of the local peaks of the distribution satisfy the relation 100Ra ≧ S. For example, the average interval S of local peaks of a distribution that can be measured by a surface roughness meter is specified in Japanese Industrial Standard (JIS B0601-1994).

For example, the shape of the convex portion on the surface of the current collector is not particularly specified, but is preferably substantially tapered.

Preferably, the convex portion has a rounded top, making it suitable to avoid local current concentration during charge-discharge.

In the present invention, a gap is formed in a thin film made of an active material so as to extend in its thickness direction. Such gaps may be formed in the first or subsequent charge-discharge cycle, or alternatively, pre-formed prior to charge-discharge. One method of forming such a gap in the film before the film is subjected to a charge-discharge process is shown, in which the film of the electrode is allowed to store lithium and then release lithium or the like before it is mounted in the battery, so that the volume of the film expands and then contracts, thereby forming a gap. In an exemplary case where a lithium-free active material is used as the positive electrode, mounting with storage of lithium in a thin film may be performed. Also, the division of the thin film into pillars by gaps can be accomplished lithographically using a protective film patterned by photolithography.

According to the present invention, the active material thin film may be formed from one or more materials capable of producing a compound or solid solution with lithium, for example, at least one element selected from elements of groups IIB, IIIB, IVB, and VB of the periodic table, and oxides and sulfides of transition metal elements of period 4, period 5, and period 6 of the periodic table.

In the present invention, examples of the elements of groups IIB, IIIB, IVB and VB of the periodic table that can produce compounds or solid solutions with lithium include carbon, aluminum, silicon, phosphorus, zinc, gallium, germanium, arsenic, cadmium, indium, tin, antimony, mercury, thallium, lead and bismuth. Specific examples of transition metal elements of periods 4, 5 and, 6 of the periodic Table include scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, lanthanide, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold and mercury.

Preferred among the above listed elements is at least one selected from carbon, silicon, germanium, tin, lead, aluminum, indium, zinc, cadmium, bismuth and mercury. Silicon and/or germanium are more preferred.

Generally, silicon is roughly classified into amorphous (amorphous) silicon, microcrystalline silicon, polycrystalline silicon, and single crystal silicon by its crystallinity. The term "amorphous silicon" as used herein is meant to include amorphous silicon and microcrystalline silicon, with the exclusion of polycrystalline silicon and single crystal silicon. About 520cm when Raman (Raman) spectrometry detects features defining the crystalline region^-1When there is substantially no peak, silicon is identified as amorphous silicon, and about 520cm corresponding to the crystalline region is detected by Raman spectrometry^-1And about 480cm representing an amorphous region^-1When a peak is substantially present, silicon is identified as microcrystalline silicon. Therefore, microcrystalline silicon basically includes crystalline regions and amorphous regions. When Raman spectrometry detects about 480cm corresponding to the amorphous region^-1And substantially no peak, silicon is identified as single crystal silicon or polycrystalline silicon.

In the present invention, a microcrystalline or amorphous form of silicon thin film is preferably used for the active material thin film.

In addition to the above-mentioned silicon thin film, a germanium thin film or a silicon-germanium mixed thin film can be preferably used as the thin film in the present invention. Germanium films in amorphous or microcrystalline form are preferably used. It is preferable to use a silicon-germanium mixed film in a microcrystalline or amorphous form. The above-described process applied to silicon thin films can be similarly used to determine the microcrystalline or amorphous characteristics of germanium and silicon-germanium mixed films. In the present invention, it was found that silicon and germanium which can be mixed with each other to form a homogeneous solid solution can provide excellent results. Accordingly, it is believed that similar results may be obtained using mixtures thereof, i.e., silicon-germanium mixtures.

In the present invention, the thin film may be formed on the current collector by various methods including, for example, CVD, spray coating, vapor evaporation, sputtering, and plating processes. Particularly recommended among such thin film forming methods are CVD, spray coating, and vapor evaporation treatment.

The type of current collector used in the present invention is not particularly specified as long as it ensures good adhesion to the active material-coated film. In particular, for example, the current collector may include at least one selected from copper, nickel, stainless steel, molybdenum, tungsten, and tantalum.

The current collector is preferably thin and is therefore preferably in the form of a metal foil. Preferably, the current collector comprises a material that is not capable of forming a mixture with lithium, and particularly, preferably copper. The current collector is preferably in the form of a copper foil, preferably the surface of said copper foil is roughened. A typical example of such a copper foil is an electrolytic copper foil. For example, electrolytic copper foil is obtained from electrolysis including copper ions: the metal drum is immersed in an electrolytic solvent and rotated. An electric current is introduced, causing copper to deposit on the surface of the drum. Then, the deposited copper was removed from the drum to obtain an electrolytic copper foil. The electrolytic copper foil may be roughened on one or both surfaces thereof or subjected to other surface treatment.

A rolled copper foil, which has its surface roughened by depositing copper on its surface through an electrolytic process, may be used as the current collector.

Also, a separator layer may be provided between the current collector and the active material film. In this case, the barrier layer preferably comprises a material that contains a component that readily diffuses into the film. Preferably, the spacer layer is a copper layer. Such a copper layer may be superimposed on a roughened nickel foil (e.g., an electrolytic nickel foil) to provide a current collector. On the other hand, copper may be deposited on the nickel foil by an electrolytic process, during which the surface of the nickel foil is roughened.

In the present invention, the low-density region may be previously formed in the active film to extend in its thickness direction, so that a gap will be formed later along the low-density region. For example, such a low-density region may be formed so as to extend upward from an uneven depression on the surface of the current collector.

In the present invention, it is preferred that the current collector components be diffused into the active material film. Diffusion of the current collector components into the film improves adhesion between the current collector and the film. When the collector composition is an element such as copper that does not form a mixture with lithium, the film in the diffusion region is less mixed with lithium and less expands and contracts during charge-discharge action, so that the generation of stress can be prevented to the extent that it may cause the film to fall off the collector.

Preferably, the concentration of the collector component diffused into the film is higher near the collector and lower near the surface of the film. Due to the presence of such a concentration gradient of the collector components, the active material film undergoes less expansion and contraction in the vicinity of the collector during charge-discharge action, so that the generation of stress in the vicinity of the collector can be prevented to such an extent that the film may be peeled off from the collector. Also, the concentration of the collector component is decreased toward the surface of the film, making it possible to maintain a high charge-discharge capacity.

Preferably, the current collector component forms a solid solution with the components of the film when diffused into the film, replacing the intermetallic compound. As used herein, an intermetallic compound refers to a compound having a specific crystalline structure formed by a mixture of metals in a specific ratio. The formation of a solid solution of the film component and the current collector component instead of the intermetallic compound improves the adhesion between the film and the current collector, resulting in an increase in charge-discharge capacity.

In the present invention, the active material film may be doped with impurities. Examples of such impurities include elements of groups IIIB, IVB and VB of the periodic table, such as phosphorus, aluminum, arsenic, antimony, boron, gallium, indium, oxygen and nitrogen.

Also, in the present invention, the active material film may be constructed with a series of overlapping layers. These layers may be different from each other depending on the composition, crystallinity, impurity concentration, and the like. These layers may provide a thin film structure that is layered in the thickness direction. For example, these layers may provide a thin film structure in which the composition, crystallinity, impurity concentration, and the like are changed in the thickness direction thereof.

Preferably, the active material thin film in the present invention stores lithium by forming a mixture with lithium.

In the present invention, lithium may be incorporated in the active thin film in advance. Lithium may be added during the formation of the active material thin film. That is, lithium can be introduced by forming a lithium-containing thin film. On the other hand, lithium may be added or stored after the thin film is formed. One approach is to use an electrochemical mechanism that adds or stores lithium in the film.

In the present invention, the thickness of the active material thin film is not particularly specified, but may be 20 μm or less. For the purpose of obtaining a high charge-discharge capacity, it is preferably 1 μm or more.

In the present invention, a separator may be provided between the current collector and the film to improve adhesion therebetween. Preferably, such a separator layer may comprise a material that is a mixture, preferably a solid solution, with the current collector material and the active material.

The lithium battery of the present invention is characterized by comprising a negative electrode positive electrode constituted of the above-described electrode of the present invention and an electrolyte.

The term "lithium battery" used herein includes a lithium primary battery and a lithium secondary battery. Accordingly, the electrode of the present invention can be applied to a lithium primary battery as well as a rechargeable lithium secondary battery.

A rechargeable lithium battery (lithium secondary battery) of the present invention is characterized by comprising a negative electrode positive electrode constituted of the above-described electrode of the present invention and an electrolyte containing no water.

The type of electrolytic solvent used in the rechargeable battery of the present invention is not particularly limited, but may be exemplified by mixed solvents including cyclic carbides such as carbonized ethylene, carbonized propylene, or carbonized butylene, and also chain carbides such as carbonized ethane, carbonized butanone, or carbonized diethyl ether. Also employable are mixed solvents of the above-listed cyclic carbides and ether solvents, such as 1, 2-dimethoxyethane or 1, 2-diethoxyethane, or chain esters such as γ -butyrolactone, sulfolane or methyl acetate. An exemplary electrolytic solute is LiPF₆、LiBF₄、LiCF₃SO₃、LiN(CF₃SO₂)₂、LiN(C₂F₅SO₂)₂、LiN(CF₃SO₂)(C₄F₉SO₂)、LiC(CF₃SO₂)₃、LiC(C₂F₅SO₂)₃、LiAsF₆、LiClO₄、Li₂B₁₀Cl₁₀、Li₂B₁₂Cl₁₂And mixtures thereof. Other applicable electrolytes include gelled polymer electrolytes, for example, by impregnating electrolytic solvents such as polyethylene oxide, polyacrylonitrile or polyvinylidene fluoride and also electrolytes such as LiI and Li₃A polymer electrolyte such as an inorganic solid electrolyte like N to form the polymer electrolyte. The electrolyte of the rechargeable lithium battery of the present invention can be used without limitation as long as the lithium compound (as a solute of the electrolyte imparting ionic conductivity), and the solvent of the electrolyte (dissolving and retaining the lithium compound) remain undecomposed at the voltage during charge, discharge, and battery storage.

Examples of positive active materials for use in the present invention include: transition metal oxides containing lithium, such as LiCoO₂、LiNiO₂、LiMn₂O₄、LiMnO₂、LiCo_0.5Ni_0.5O₂And LiNi_0.7Co_0.2Mn_0.1O₂(ii) a Such as MnO₂Such as lithium-free metal oxides, and the like. Other substances can also be used without limitation if they can electrochemically add and release lithium.

It is believed that the electrode of the present invention is also useful for nonaqueous electrolyte batteries and nonaqueous electrolyte rechargeable batteries that contain electrode active materials capable of storing and releasing alkali metals other than lithium, such as sodium and potassium, and alkaline earth metals such as manganese and calcium.

According to another aspect of the present invention, an electrode for use in a rechargeable battery is characterized by comprising an electrode material layer in the form of a film and a current collector bonded to the electrode material layer, wherein the film has low-density regions which are connected to each other in a planar direction as a mesh and extend in a thickness direction toward the current collector. Such an electrode for a rechargeable lithium battery in a state before a gap is formed along the low density region to extend in a thickness direction thereof can be explained by the electrode for a rechargeable lithium battery according to the present invention.

Accordingly, it is desirable to diffuse the current collector components into the film. Such diffused current collector components preferably form solid solutions with the film components in the film instead of the intermetallic compound.

Preferably, the thin film is formed on the current collector by a thin film forming method. Examples of the thin film forming method include CVD, spray coating, vapor evaporation, sputtering, and plating processes.

The formation of the low density region in the thin film is similar to the method of forming the low density region in the above-described electrode of the present invention for a lithium battery.

Also, the thin film and the current collector used in the above-described electrode of the present invention for a lithium battery can be similarly used.

Brief Description of Drawings

Fig. 1 is a schematic sectional view of a rechargeable lithium battery manufactured in an example according to the present invention;

FIG. 2 is a photomicrograph (at a magnification of 2,000X) taken using a scanning electron microscope, showing an electrode according to one embodiment of the invention, the electrode being in a state prior to charging and discharging;

FIG. 3 is a photomicrograph (at a magnification of 5,000X) taken using a scanning electron microscope, showing an electrode according to one embodiment of the invention, the electrode being in a state prior to charging and discharging;

FIG. 4 is a photomicrograph (at 500X magnification) taken using a scanning electron microscope showing an electrode according to one embodiment of the invention, after charging and discharging;

FIG. 5 is a photomicrograph (at a magnification of 2,500) taken using a scanning electron microscope, showing an electrode according to one embodiment of the invention, after charging and discharging;

FIG. 6 is a photomicrograph (at a magnification of 1,000X) taken using a scanning electron microscope, showing the silicon thin film of an electrode according to one embodiment of the invention when viewed from above;

FIG. 7 is a photomicrograph (at a magnification of 5,000X) taken using a scanning electron microscope, showing the silicon thin film of an electrode according to one embodiment of the invention when viewed from above;

FIG. 8 is a photomicrograph (at a magnification of 2,000X) taken using a scanning electron microscope, showing the silicon thin film of an electrode according to one embodiment of the invention when viewed from a small angle;

FIG. 9 is a photomicrograph (at a magnification of 5,000) taken using a scanning electron microscope, showing a silicon thin film when viewed from a small angle with an electrode according to one embodiment of the present invention;

FIG. 10 is a schematic cross-sectional view showing a silicon thin film in the course of being divided into pillars by gaps;

FIG. 11 is a photomicrograph (at a magnification of 12,500X) taken using a transmission electron microscope, showing a cross-section of the silicon thin film of electrode a3 according to the invention;

FIG. 12 is a photomicrograph (at a magnification of 25,000X) taken using a transmission electron microscope, showing a cross-section of the silicon thin film of electrode a6 according to the invention;

FIG. 13 is a diagrammatic representation of the photomicrograph of FIG. 11;

FIG. 14 is a diagrammatic representation of the photomicrograph of FIG. 12;

fig. 15 is a photomicrograph (at a magnification of 1,000X) taken using a scanning electron microscope, showing the silicon thin film surface of electrode a3 according to the present invention when viewed from above;

fig. 16 is a photomicrograph (at a magnification of 1,000X) taken using a scanning electron microscope, showing the silicon thin film surface of electrode a6 according to the present invention when viewed from above;

FIG. 17 is a graphical representation showing the concentration profile of the constituent elements in the silicon thin film of electrode a6 along the depth of the film in accordance with the present invention;

fig. 18 is a schematic view showing the structure of an apparatus used when a thin film is formed using a vacuum vapor evaporation technique in an example of the present invention;

fig. 19 is a photomicrograph (at a magnification of 2,000X) taken using a scanning electron microscope, showing electrode a7 according to the present invention, in a state prior to charging and discharging;

FIG. 20 is a photomicrograph (at a magnification of 10,000X) taken using a scanning electron microscope, showing electrode a7 according to the invention, in a state prior to charging and discharging;

fig. 21 is a photomicrograph (at a magnification of 2,000X) taken using a scanning electron microscope, showing electrode a8 according to the invention, in a state prior to charging and discharging;

FIG. 22 is a photomicrograph (at a magnification of 10,000X) taken using a scanning electron microscope, showing electrode a8 according to the invention, in a state prior to charging and discharging;

fig. 23 is a photomicrograph (at 500X magnification) taken using a scanning electron microscope, showing electrode a7 according to the invention, after charging and discharging;

FIG. 24 is a photomicrograph (at a magnification of 2,500X) taken using a scanning electron microscope, showing electrode a7 according to the invention, after charging and discharging;

FIG. 25 is a photomicrograph (at 500X magnification) taken using a scanning electron microscope, showing electrode a8 according to the invention, after charging and discharging;

FIG. 26 is a photomicrograph (at a magnification of 2,500X) taken using a scanning electron microscope, showing electrode a8 according to the invention, after charging and discharging;

fig. 27 is a photomicrograph (at a magnification of 1,000X) taken using a scanning electron microscope, showing the germanium film of electrode a7 according to the invention when viewed from above, the germanium film of electrode a7 being in a state after charging and discharging;

fig. 28 is a photomicrograph (at a magnification of 5,000X) taken using a scanning electron microscope, showing the germanium film of electrode a7 according to the invention when viewed from above, the germanium film of electrode a7 being in a state after charging and discharging;

FIG. 29 is a photomicrograph (at a magnification of 1,000X) taken using a scanning electron microscope, showing the germanium film of electrode a7 according to the invention when viewed from a small angle, the germanium film of electrode a7 being after charge and discharge;

FIG. 30 is a photomicrograph (at a magnification of 5,000X) taken using a scanning electron microscope, showing the germanium film of electrode a7 according to the invention when viewed from a small angle, the germanium film of electrode a7 being after charge and discharge;

fig. 31 is a photomicrograph (at a magnification of 1,000X) taken using a scanning electron microscope, showing the germanium film of electrode a8 according to the invention when viewed from above, the germanium film of electrode a8 being in a state after charging and discharging;

fig. 32 is a photomicrograph (at a magnification of 5,000X) taken using a scanning electron microscope, showing the germanium film of electrode a8 according to the invention when viewed from above, the germanium film of electrode a8 being in a state after charging and discharging;

FIG. 33 is a photomicrograph (at a magnification of 1,000X) taken using a scanning electron microscope, showing the germanium film of electrode a8 according to the invention when viewed from a small angle, the germanium film of electrode a8 being after charge and discharge;

FIG. 34 is a photomicrograph (at 5,000X magnification) of a germanium film of a8 using a scanning electron microscope, showing an electrode according to the invention when viewed from a small angle, the germanium film of electrode a8 being after charge and discharge;

fig. 35 is a photomicrograph (at a magnification of 1,000X) taken using a scanning electron microscope, showing the germanium film of electrode a7 according to the invention when viewed from above, the germanium film of electrode a7 being in a state prior to charging and discharging;

fig. 36 is a photomicrograph (at a magnification of 1,000X) taken using a scanning electron microscope, showing the germanium film of electrode a8 according to the invention when viewed from above, the germanium film of electrode a8 being in a state prior to charging and discharging;

FIG. 37 is a graphical representation showing the concentration profile of a constituent element in a germanium film of electrode a7 along the depth of the film in accordance with the present invention;

FIG. 38 is a graphical representation showing the concentration profile of a constituent element in a germanium film of electrode a8 along the depth of the film in accordance with the present invention;

FIG. 39 is a photomicrograph (at a magnification of 2,000X) taken using a scanning electron microscope, showing a cross-section of an electrode a-11 according to the invention before charging and discharging;

FIG. 40 is a photomicrograph (at a magnification of 10,000X) taken using a scanning electron microscope, showing a cross-section of an electrode a-11 according to the invention before charging and discharging;

FIG. 41 is a photomicrograph (at a magnification of 1,000X) taken using a scanning electron microscope, showing the silicon thin film of electrode a-11 according to the present invention before charging and discharging when viewed from above;

FIG. 42 is a photomicrograph (at a magnification of 1,000X) taken using a scanning electron microscope, showing the silicon thin film of electrode a-11 according to the present invention after charging and discharging when viewed from above;

FIG. 43 is a photomicrograph (at 500,000X magnification) taken using a transmission electron microscope showing the connection between the copper foil and the silicon film and its neighbors;

FIG. 44 is a photomicrograph (at a magnification of 1,000,000X) taken using a transmission electron microscope showing the interface between the copper foil and the silicon thin film and its neighbors;

FIG. 45 is a graphical representation showing the concentration profile of copper and hydrogen in the mixed layer along the depth of the layer in electrode c 1;

fig. 46 is a graphical representation showing the concentration distribution of copper and hydrogen in the mixed layer in electrode c3 along the depth of the layer.

Best Mode for Carrying Out The Invention

The invention is described in more detail below by way of example. It will be understood that the following examples are merely illustrative of the practice of the invention and are not intended to limit the invention, and that suitable variations and modifications may be made without departing from the scope of the invention.

(experiment 1)

(production of negative electrode)

A microcrystalline silicon thin film was formed on a rolled copper foil (18 μm thick) by a CVD method, and Silane (SiH) was applied using the rolled copper foil as a substrate₄) As the source gas and hydrogen as the carrier gas. Specifically, a copper foil as a substrate was placed on a heater in a reaction chamber. The vacuum extractor vacuums the interior of the reaction chamber to a pressure of 1Pa or less. Silane gas as a source gas and hydrogen (H) as a carrier gas are introduced through a source gas inlet port₂) And (4) qi. The substrate was heated to 180 ℃ on a heater. The vacuum pump apparatus adjusted the vacuum to the reaction pressure. An RF (radio frequency) power supply is operated to excite radio frequency waves, which are introduced through electrodes to induce a glow discharge. The detailed conditions for film formation are listed in table 1. In Table 1, the volume unit sccm indicates the volume flow rate (cm) of the liquid at 0 ℃ under 1 atmosphere (101.33kPa) per minute³Per minute), is an abbreviation for standard cubic centimeters per minute.

TABLE 1

Condition	During film formation
Condition	During film formation	Source gas (SiH)₄) Flow rate of	10sccm
Carrier gas (H)₂) Flow rate of	200sccm	Source gas (SiH)₄) Flow rate of	10sccm
Carrier gas (H)₂) Flow rate of	200sccm	Substrate temperature	180℃
Reaction pressure	40Pa	Substrate temperature	180℃
Reaction pressure	40Pa	RF power	555W

The microcrystalline silicon thin film was deposited to a thickness of about 10 μm under the above-specified conditions. The non-crystallinity of the thin film was determined by a method of arranging the non-crystalline region to surround the crystalline region including the fine crystal particles by observation with an electron microscope (at a magnification of 2,000,000X). A piece of sample having a diameter of 17mm was punched out from the produced sample to provide an electrode a 1. A piece of sample equivalent to the electrode a1 was subjected to a heat treatment at 400 ℃ for 3 hours to provide an electrode a 2.

For comparison purposes, 90 parts by weight of commercially available single-crystal silicon powder (particle diameter of 10 μm) and 10 parts by weight of polytetrafluoroethylene as a binder were mixed. This mixture was pressed in a 17mm diameter die to obtain a disk-shaped electrode b 1.

(production of Positive electrode)

Distribution of the starting materials Li₂CO₃And CoCO₃So that the atomic ratio of Li to Co is 1: 1, and then mixed in a mortar. This mixture was pressed in a 17mm diameter die and calcined in air at 800 ℃ for 24 hours to obtain a catalyst containing LiCoO₂The calcined product of (a). This was then ground into particles having an average particle diameter of 20 μm.

80 parts by weight of the produced Li₂CO₃10 parts by weightAcetylene black as a conductive material in an amount and 10 parts by weight of polytetrafluoroethylene as a binder were mixed. This mixture was pressed in a 17mm diameter die to obtain a positive electrode in the form of a disc.

(preparation of electrolytic solvent)

1 mol/l LiPF₆Dissolved in a mixed solvent comprising equal amounts of carbonized ethylene and carbonized diethyl ether to prepare an electrolytic solvent used in the following battery constitution.

(construction of Battery)

A coin-type rechargeable lithium battery was constructed using the electrode a1, a2, or b1 produced above as the negative electrode, as well as the positive electrode produced above and the electrolytic solvent prepared above.

Fig. 1 is a schematic sectional view showing a rechargeable lithium battery of such a constitution, which includes a positive electrode 1, a negative electrode 2, a separator 3, a positive case 4, a negative case 5, a positive current collector 6, a negative current collector 7, and an insulating gasket 8 made of polypropylene.

The positive electrode 1 and the negative electrode 2 are deposited on opposite sides of the separator 3. These are sealed in a battery case which is composed of a positive case 4 and a negative case 5. The positive electrode 1 is connected to the positive housing 4 by a positive current collector 6. The negative electrode 2 is connected to the negative casing 5 through a negative current collector 7. This structure makes it possible to charge and discharge into a secondary battery.

As a result, batteries a1, a2, and B1 were constructed using the electrodes a1, a2, and B1 as negative electrodes, respectively.

(measurement of Charge-discharge cycle Life characteristics)

Except for battery B1, each battery was charged at 25 ℃ with a current of 100 μ a until the negative electrode capacity reached 2,000mAh/g, and then discharged. This charge-discharge cycle was recorded as a unit. Cycles were performed to measure the capacity retention at the 50 th cycle for each cell. The battery B1, which was not charged to 2,000mAh/g, was subjected to a cycle test in which it was charged to 4.2V and then discharged. The results are given in table 2.

In Table 2, the hydrogen concentration obtained from SIMS measurement, determined by Raman spectroscopy, is also given at about 480cm^-1And 520cm^-1The peak intensity ratio of (a) and the crystal particle size calculated from the X-ray diffraction spectrum and the Scherrer formula were all found for the negative active material of each cell. Also, the crystal particle size of the negative active material of battery B1 was given by the particle diameter of the powder, because both were considered to have almost equal values to each other.

TABLE 2

Battery with a battery cell	Capacity retention ratio at 50 th cycle	Hydrogen content	Peak intensity ratio (480 cm)^-1/520cm^-1)	Crystal particle size
Battery with a battery cell	Capacity retention ratio at 50 th cycle	Hydrogen content	Peak intensity ratio (480 cm)^-1/520cm^-1)	Crystal particle size	A1	85％	4％	0.1	1nm
A2	78％	0.01％	0.1	1nm	A1	85％	4％	0.1	1nm
A2	78％	0.01％	0.1	1nm	B1	5％	0％	0	10μm

As is clear from the results shown in table 2, both the batteries a1 and a2 according to the present invention exhibited significantly higher capacity retention rates as compared to the comparative battery B1.

Therefore, the use of the microcrystalline silicon thin film as a negative active material results in a significant improvement in charge-discharge cycle characteristics of a rechargeable lithium battery. The reasons for this are believed to be: in the microcrystalline silicon thin film, expansion and contraction occurring at the time of storing and releasing lithium are alleviated, and the negative active material is prevented from being pulverized, thereby suppressing the possibility of lowering the current collecting ability.

(experiment 2)

The procedure used in experiment 1 to construct cell a1 was followed except that an electrolytic copper foil (18 μm thick) was used as the substrate for the current collector. That is, a microcrystalline silicon thin film (about 10 μm thick) was deposited on the electrolytic copper foil to manufacture an electrode a 3. Using this electrode, a battery a3 was constructed.

Also, the rolled copper foil used in experiment 1 was subjected to a grinding treatment for 1-minute with #400 or #120 diamond paper to provide a ground copper foil. Except that this ground copper foil was used for the current collector as a substrate. That is, a microcrystalline silicon thin film (about 10 μm thick) was deposited on a copper foil to fabricate an electrode, following the procedure used in experiment 1 to construct battery a 1. An electrode made using copper foil polished with #400 diamond paper was designated electrode a4, and an electrode made using copper foil polished with #120 diamond paper was designated electrode a 5. The same method as in experiment 1 was used to construct cells a4 and a 5.

These batteries A3-a5 and batteries a1 and B1 constructed in experiment 1 were subjected to charge-discharge cycle tests under the same conditions as used in experiment 1 to obtain the 10 th cycle capacity retention rate for each battery. The results are given in table 3. The surface roughness Ra of the copper foil as a current collector and the average interval of local peaks S are also given in table 3 for each of the cells a1, B1, and A3-a 5.

The surface roughness Ra of each copper foil and the average interval of local peaks S were measured by using a needle profiler Dektak ST (available from ULVAC) with a scanning distance of 2.0 mm. The surface roughness Ra was calculated after correction of the defective portion. The defective portion is corrected using correction values having a low pass of 200 μm and a high pass of 20 μm. The surface roughness Ra was automatically calculated and read from the graph. Average interval of local peaks S.

TABLE 3

Battery with a battery cell

Capacity retention ratio at 10 th cycle

Current collector (copper foil)

		Surface roughness Ra (mum)	Mean spacing S (μm)
		Surface roughness Ra (mum)	Mean spacing S (μm)	A1	97％	0.037	14
A3	99％	0.188	11	A1	97％	0.037	14
A3	99％	0.188	11	A4	98％	0.184	9
A5	99％	0.223	8	A4	98％	0.184	9
A5	99％	0.223	8	B1	20％	0.037	14

As is clear from the results given in table 3, the batteries A3-a5 using a copper foil having a higher surface roughness Ra value as a current collector exhibited improved capacity retention at cycle 10, as compared to the battery a1 using a copper foil having a lower surface roughness Ra value. This is believed to be due to the following reasons: when a copper foil having a higher surface roughness Ra value is used as the current collector, adhesion between the current collector and the active material is improved. This adhesion improvement reduces the effects of structural changes, such as the shedding of the active material that occurs when it expands or contracts during storage or release of lithium.

(experiment 3)

The batteries a1 and A3 respectively constructed in experiments 1 and 2 were further subjected to a charge-discharge cycle test under the same test conditions as used in experiment 1 to measure the capacity retention rate at the 30 th cycle. The results are shown in table 4.

TABLE 4

Battery with a battery cell	Capacity retention at 30 th cycle
Battery with a battery cell	Capacity retention at 30 th cycle	A1	91％
A3	97％	A1	91％

As is clear from the results given in table 4, the batteries a1 and A3 exhibited excellent capacity retention rates even at the 30 th cycle. In particular, battery a3 using a copper foil having a higher surface roughness Ra value as a current collector exhibited an excellent capacity retention rate.

The electrode a3 incorporated in the cell a3 was observed under an electron microscope to observe the state of its silicon thin film. First, electrode a3 was observed using a scanning electron microscope, the electrode a3 being in a state before being incorporated into a battery. Fig. 2 and 3 are micrographs (secondary electron images) obtained with a scanning electron microscope, both showing the state of the electrode a3 before charging and discharging. Fig. 2 and 3 were obtained at 2,000X and 5,000X magnification, respectively.

The electrodes were embedded in epoxy and then sectioned to provide samples. In the upper and lower parts of fig. 2, and in the upper part of fig. 3, a layer of embedded epoxy is found.

In fig. 2 and 3, the portion that appears slightly brighter represents the copper foil. The deposited silicon thin film (about 10 μm thick) was found in the dark portion on the copper foil. As shown in fig. 2 and 3, irregularities are formed on the surface of the copper foil. In particular, the protruding portion generally has a tapered shape. Similar asperities are formed on the surface of the silicon film deposited on the copper foil. Accordingly, the surface irregularities of the silicon thin film appear to generally conform to the shape of the irregularities formed on the surface of the copper foil.

Next, after 30 cycles, electrode a3 was taken out of cell a3, embedded in epoxy resin, and then observed under a scanning electron microscope in the same manner as described above. Here, the electrode a3 is taken out after the discharge. Thus, the observed electrode a3 is in a state after it is discharged.

Fig. 4 and 5 are micrographs (secondary electron images) obtained with a scanning electron microscope, each showing electrode a3 after discharge. FIGS. 4 and 5 were obtained at 500X and 2,500X magnification, respectively.

As shown in fig. 4 and 5, the silicon thin film has a gap extending in its thickness direction and is divided into pillars. It is difficult to find a gap extending in the planar direction. The bottom of each post section was found to adhere well to the copper foil used as the current collector. Also, each post section has a dome. It can therefore be understood that these gaps are formed so as to start from the depressions of the irregularities that can be found on the surface of the silicon thin film when the silicon thin film is in its state before charging and discharging.

Further, the surface of the silicon thin film of the electrode a3 after charging and discharging was observed with a scanning electron microscope. Fig. 6 and 7 are micrographs (secondary electron images) obtained with a scanning electron microscope, each showing the surface of the silicon thin film when observed from above. FIGS. 6 and 7 were obtained at 1,000 and 5,000 magnification, respectively. Fig. 8 and 9 are micrographs (secondary electron images) obtained with a scanning electron microscope, each showing the surface of the silicon thin film when observed from a small angle. FIGS. 8 and 9 were obtained at 1,000 and 5,000 magnification, respectively.

As shown in fig. 6 to 9, the gap is formed in a manner of surrounding the columnar portion of the silicon thin film so as to define a space between the adjacent columnar portions. When the silicon thin film stores lithium during charging, the columnar portion will expand and increase in volume. However, it is believed that the space provided around the cylindrical portion accommodates this increase in volume. Upon discharge, the columnar portion of the silicon thin film releases lithium and shrinks to reduce the volume. It is believed that the reduction in volume restores the space surrounding the cylindrical portion. Such a columnar structure of the silicon thin film is effective for releasing stress caused by expansion and contraction of the active material at the time of charge and discharge, so that the active silicon thin film can be prevented from falling off from the current collector.

The formation of gaps, which divide the silicon thin film into pillars, causes the contact area of the silicon thin film with the electrolytic solvent to increase significantly. Also, the sizes of the cylindrical portions are almost comparable to each other. It is believed that this allows the charge-discharge action accompanying lithium storage and release to occur efficiently in a thin film of the active material.

As shown in fig. 4 and 5, since the respective cylindrical portions of the silicon thin film are adhered to the current collector, excellent electrical contact is provided between the active material and the current collector. It is believed that this allows the charge-discharge action to occur efficiently.

As also shown in fig. 6-9, each cylindrical portion has a dome. This provides an electrode structure that prevents local current concentration and reduces the occurrence of deposition reactions such as lithium metal.

Fig. 10 is a schematic sectional view showing a process by which a silicon thin film deposited on a copper foil is divided into pillars through gaps formed on the silicon thin film.

As shown in fig. 10(a), the copper foil 10 has irregularities on its surface 10 a. The copper foil having an increased surface roughness Ra value has large irregularities.

Fig. 10(b) shows the amorphous silicon thin film layer 11 deposited on the roughened surface 10a of the copper foil 10. The surface 11a of the silicon thin film 11 is affected by the irregularities on the surface 10a of the copper foil 10 to have similar irregularities. The silicon thin film 11 remains undivided before charging and discharging, as shown in fig. 10 (b). When charging is performed, the silicon thin film 11 stores lithium therein and expands in volume. During charging, the silicon membrane 11 appears to expand in both the thickness and planar directions of the membrane, although the details are not clear. During the subsequent discharge action, the silicon thin film 11 releases lithium therefrom and shrinks in volume. At this time, tensile stress is generated in the silicon thin film 11. Presumably, such stress concentrates at the rugged depressions 11b on the surface 11a of the silicon thin film 11, resulting in the formation of gaps 12 which start from the depressions 11b and extend in the thickness direction, as shown in fig. 10 (c). It is conceivable that the gap 12 thus formed relieves stress, allowing the silicon thin film 11 to shrink without coming off the copper foil 10.

In the silicon thin film divided into pillars in the above-described manner, the space provided around the pillar-shaped portion functions to relieve stress generated by expansion and contraction of the active material during successive charge-discharge cycles. This appears to ensure repeated charge-discharge cycles while preventing the active material from falling off the current collector.

In addition, electrode a3, which was observed under a transmission electron microscope to investigate a mechanism of forming a gap on a silicon thin film, was combined with a microcrystalline silicon thin film deposited on an electrolytic copper foil, which was about 10 μm thick, by electrode a 3. Fig. 11 is a photomicrograph (at 12,500X magnification) taken with a transmission electron microscope, showing a portion of electrode a3 prior to charging and discharging. The observation sample was prepared by slicing the epoxy embedded electrode.

Fig. 13 is a graphical representation of the photomicrograph of fig. 11. In the photomicrograph of fig. 11 obtained with a transmission electron microscope, a silicon thin film 11 is deposited on the surface 10a of the electrolytic copper foil 10 as shown in the graph in fig. 13. The silicon thin film 11 appeared brighter than the copper foil 10 in the photomicrograph obtained by a transmission electron microscope. In the silicon thin film shown in fig. 11, bright portions are observed in regions extending between the silicon thin film 11 and the respective depressions 11b and 10b of the irregularities on the surfaces 11a and 10a of the copper foil 10. In fig. 13, these bright portions are indicated by single-dot chain lines A, B and C. In particular, a bright portion is more clearly observed in the region indicated by a. It is considered that the density of these regions is low, i.e., the low density region of the silicon thin film 11. For the purpose of observing such a low density region more closely, the electrode a6 was fabricated by depositing a microcrystalline silicon thin film of about 2 μm thickness on an electrolytic copper foil under the same conditions as those used for fabricating the electrode a 3.

Fig. 12 is a photomicrograph taken with a transmission electron microscope, showing electrode a6 when observed in the same manner as described above. Fig. 12 was obtained at 25,000X magnification. Fig. 14 is a graphical representation of the photomicrograph of fig. 12. As is clear from fig. 12, a low-density region is also observed in the region D of the electrode a6, which extends between the silicon thin film 11 and the respective recesses 11b, 10b of the irregularities on the surfaces 11a, 10a of the copper foil 10. Careful observation of the photomicrograph of fig. 12 shows many thin lines extending in the direction shown by the arrows in fig. 14. It appears that such lines are most likely to be formed when a silicon thin film is grown. Accordingly, it is shown that the silicon thin film 11 is generated generally perpendicular to the surface 10a of the copper foil 10. It is also shown that the silicon thin film layer grown in this direction collides with the adjacent silicon thin film layer deposited and grown on the adjacent inclined surface of the copper foil at the region D. The formation of the low-density region D may be extremely likely to be caused by such collision at the region D. It has also been shown that the collision of the silicon thin film layers with each other continues until the end of the film formation, while the formation of the low density regions continues until the surface of the silicon thin film is reached.

Fig. 15 is a photomicrograph (secondary electron image) obtained with a scanning electron microscope, showing the surface of the silicon thin film of the electrode a3 when viewed from above. The electrode a3 is shown in fig. 15 in a state before it is charged and discharged. Figure 15 was observed at 1,000X magnification. In fig. 15, the bright portions are shown to represent convex portions on the surface of the silicon thin film, and the dark surrounding portions are shown to represent concave portions on the surface of the silicon thin film. As shown in fig. 15, the depressions on the surface of the silicon thin film are connected to each other to form a mesh. Accordingly, it was found that the low density regions define a continuous network in the plane of the silicon thin film. As shown in fig. 11 and 13, such a reticulated low-density region also extends in the thickness direction of the current collector. The shaded portion in fig. 15 does not indicate a gap (space). It is apparent that, in fact, in the micrographs of fig. 2 and 3 obtained with a scanning electron microscope, no gap was observed to extend in the thickness direction of the film.

Fig. 16 is a photomicrograph (secondary electron image) obtained with a scanning electron microscope at a magnification of 1,000, showing the silicon thin film surface of electrode a6 when viewed from above, the electrode a6 being in its state before charging and discharging. As is apparent from fig. 16, the depressions on the electrode a6 are also interconnected in a net. Accordingly, it was found that the low density areas were arranged as a continuous web in the planar direction.

Fig. 17 is a graph showing the concentration distribution of the constituent elements in the thickness direction of the silicon thin film in the electrode a 6. Using O₂ ⁺As a spray source, by SIMS via copper (A), (B), (C), (D⁶³Cu⁺) And silicon (Si)²⁺) The concentration of the constituent element is measured to obtain the concentration distribution of the constituent element. In fig. 17, the abscissa represents the depth from the surface of the silicon thin film, and the ordinate represents the intensity (count) of each constituent element.

As is apparent from fig. 17, it was found that the constituent element of the current collector, copper (Cu), diffused into the silicon thin film at a position adjacent to the current collector. At a position closer to the surface of the silicon thin film, the copper (Cu) concentration decreases. It was also found that the copper (Cu) concentration was varied in a continuous manner. This means that a solid solution of silicon and copper is formed in the copper (Cu) diffusion region, replacing the intermetallic compound thereon.

From the above discussion, the obtained results are most likely to explain a mechanism by which a gap extending in its thickness direction is formed on the silicon thin film when the silicon thin film expands and contracts during charge and discharge. That is, the stress caused by the volume expansion or contraction of the silicon thin film is concentrated at the rugged depressions on the surface of the silicon thin film, as explained above with reference to fig. 10. Also, in the silicon thin film, there are low-density regions extending from the recesses to the current collector at first, and the mechanical strength of these regions is rather poor. As a result of the above, it is possible to form gaps (spaces) along these low-density regions.

Also, as shown in fig. 17, the constituent element of the collector, copper, diffuses into the silicon thin film, creating a concentration gradient of copper therein, so that the concentration of copper is higher nearer to the collector and lower nearer to the silicon thin film. Accordingly, there is a higher concentration of copper that does not react with lithium and a lower concentration of silicon that does react with lithium at locations closer to the current collector. In the vicinity of the current collector, it is believed that the silicon thin film stores and releases less lithium, undergoes less expansion and contraction, and thus generates less stress, which results in reduced formation of gaps (spaces) that may incidentally cause the silicon thin film to peel off or flake off from the current collector.

The silicon thin film divided into pillars by such gaps maintains strong adhesion to the current collector even during charge-discharge cycles. Also, the space provided around the columnar portion functions to relieve stress caused by expansion and contraction of the film that occurs with charge-discharge cycles. It is believed that these contribute to excellent charge-discharge cycle characteristics.

(experiment 4)

(production of electrode a 7)

An electrolytic copper foil similar to that used in the manufacture of the electrode a3 was used as a substrate for the current collector. An amorphous germanium film (about 2 μm thick) was formed on the copper foil by an RF spray technique to manufacture an electrode a 7.

The thin film was formed using germanium as a target at a flow rate of 100sccm of the spray gas (Ar), an ambient substrate temperature (no heating), a reaction pressure of 0.1Pa, and an RF power of 200W.

The resulting germanium film was analyzed by Raman spectroscopy and detected at about 274cm^-1A peak appears at, and is about 300cm^-1No peak appears. This exhibits the amorphous nature of germanium films.

(production of electrode a 8)

Electrode a8 was fabricated by forming an amorphous germanium film (about 2 μm thick) on an electrolytic copper foil having a shape similar to that of the current collector of electrode a7 using a vapor evaporation technique.

Specifically, a germanium film was deposited on the substrate using the apparatus of the structure shown in fig. 18. Referring to fig. 18, the ECR plasma source 21 includes a plasma generation chamber 22 to which microwave power 25 and Ar gas 26 are supplied. When the microwave power 25 is supplied to the plasma generation box 22, Ar plasma is generated. This Ar plasma 23 is directed out of the plasma box 22 and bombards the substrate 20. A germanium film may be deposited on the substrate 20 by using an electron beam from an Electron Beam (EB) gun placed under the substrate 20.

The electrolytic copper foil substrate is previously treated by Ar plasma irradiation before depositing a germanium film on the electrolytic copper foil substrate. The degree of vacuum in the reaction chamber was adjusted to about 0.05Pa (about 5X 10)^-4Torr). The substrate was exposed to the Ar plasma with an Ar gas flow rate of 40sccm and a supplied microwave power of 200W. During the Ar plasma irradiation, a bias voltage of-100V was applied to the substrate. The pre-treatment was completed by exposing the substrate to Ar plasma for 15 minutes.

Next, a germanium film was deposited at a deposition rate of 1nm/sec (10 /sec) using an electron beam gun. The substrate temperature is ambient (no heating).

Analysis of the resulting germanium film by Raman spectroscopy revealed amorphous characteristics of the germanium film, similar to electrode a 7.

(production of electrode b 2)

80 parts by weight of germanium powder having an average particle diameter of 10 μm, 10 parts by weight of acetylene black as a conductive material, and 10 parts by weight of polytetrafluoroethylene as a binder were mixed. This mixture was pressed in a 17mm diameter die to obtain a disk-shaped electrode b 2.

(construction of Battery)

The procedure of the experiment was repeated except for the above-described electrodes a7, a8, and B2 as negative electrodes to constitute batteries a7, a8, and B2.

(estimation of Charge-discharge cycle characteristics)

Each cell was charged to 4.2V at 25 ℃ with a current of 0.1mA and then discharged to 2.75V. This standard charge-discharge cycle was repeated to measure the capacity retention on the 10 th cycle. The measurement results are given in table 5.

TABLE 5

Battery with a battery cell	Capacity retention ratio at 10 th cycle
Battery with a battery cell	Capacity retention ratio at 10 th cycle	A7	96％
A8	93％	A7	96％
A8	93％	B2	39％

As is apparent from table 5, the batteries a7 and A8 as negative electrodes using the electrode according to the present invention, i.e., the electrode incorporating the germanium thin film formed on the current collector, exhibited significantly improved capacity retention rates as compared to the battery B2 using germanium powder as the negative electrode.

(Observation with an electron microscope)

Fig. 19 and 20 are micrographs (reflection electron images) obtained with a scanning electron microscope, each showing a part of the electrode a7, the electrode a7 being in its state before charging and discharging. FIGS. 19 and 20 were obtained at 2,000 and 10,000 Xmagnifications, respectively.

Each electrode was embedded in epoxy and then sectioned to provide a sample. The embedded epoxy resin was observed as a layer at the upper and lower end portions in fig. 19 and a layer at the upper end portion in fig. 20.

In fig. 19 and 20, the copper foil and the germanium film appear lighter than the other portions. The thin layer of the overlapping copper foil is a germanium film. Irregularities are defined on the surface of the copper foil. Similar asperities are also found on the surface of the germanium film. This suggests that the irregularities formed on the germanium film conform to the shape of the irregularities defined on the surface of the copper foil.

In fig. 20, a dark portion is observed, which is located at the leftmost depression of the overlapping copper foil and in the germanium film region extending to the thickness direction of the film. This portion is most likely to represent a low density region, i.e., a low density region of the germanium film.

Fig. 21 and 22 are micrographs (reflection electron images) obtained with a scanning electron microscope, each showing a part of the electrode a8, the electrode a8 being in its state before charging and discharging. FIGS. 21 and 22 were obtained at 2,000 and 10,000 Xmagnifications, respectively. Like electrode a7 shown in fig. 19 and 20, a sample of this electrode was embedded in epoxy.

In fig. 21 and 22, the lighter portion represents the copper foil, and the portion slightly darker thereon is the germanium film (about 2 μm thick). Irregularities were determined on both surfaces of the germanium film and the copper foil of the electrode a8, similar to the electrode a 7.

Fig. 23 and 24 are micrographs (reflection electron imaging) obtained with a scanning electron microscope, each showing a portion of electrode a7 taken out of cell a7 after 10 cycles. Fig. 25 and 26 are micrographs (reflection electron imaging) obtained with a scanning electron microscope, each showing a portion of electrode A8 taken out of cell A8 after 10 cycles. In each case, the electrodes were embedded in epoxy and then sectioned to provide samples. Both fig. 23 and 25 were obtained at 500X magnification. Both fig. 24 and 26 were obtained at a magnification of 2,500X.

In fig. 23-26, the portion of the germanium film that exhibits a white color on its surface is gold plated thereon before it is embedded in the epoxy. A gold plating is provided to prevent any reaction between the germanium film and the epoxy and also to define a clear boundary between them.

As is clear from fig. 23 to 26, the charge-discharge cycle results in the formation of gaps which extend in the thickness direction of the germanium film and divide the film into pillars, similarly to the case of the silicon film. Although there is a small difference between copper foils as current collectors compared with and the germanium film makes it difficult to distinguish the boundary between them, careful observation shows that columnar portions of the germanium film exist on the convex portion of the current collector, and therefore, the germanium film is preferably adhered to the current collector.

Laterally extending gaps are also observed in germanium films, unlike in the case of silicon films. However, it is highly likely that such a gap will be formed when the germanium film is polished while a partial observation is being made.

Also, it was found that the width of the gap (space) between adjacent columnar portions in the germanium film was larger than that in the silicon film. After the charge-discharge cycle, the height of the columnar portion was measured to be about 6 μm, which is about 3 times the initial film thickness, 2 μm, of the germanium film before the charge-discharge cycle. It is considered that this means that after the film has expanded due to lithium storage during charging, the film shrinks upon discharge, and shrinkage mainly occurs in the transverse direction, that is, in the planar direction. Accordingly, it is believed that the smaller percentage of contraction of the germanium film in its thickness direction results in a wide gap (spacing) between the columnar portions.

Fig. 27 and 28 are micrographs (secondary electron images) obtained with a scanning electron microscope, each showing the germanium film of electrode a7 when viewed from above, said electrode a7 being in its state after charging and discharging. FIGS. 27 and 28 were obtained at 1,000 and 5,000 Xmagnifications, respectively. Fig. 29 and 30 are micrographs (secondary electron images) obtained with a scanning electron microscope, each showing the germanium film of electrode a7 when viewed from a small angle, said electrode a7 being in its state after charging and discharging. FIGS. 29 and 30 were obtained at 1,000 and 5,000 Xmagnifications, respectively.

Fig. 31 and 32 are micrographs (secondary electron images) obtained with a scanning electron microscope, each showing a germanium film of the electrode a8 when viewed from above, the electrode a8 being in its state after charging and discharging. FIGS. 31 and 32 were obtained at 1,000 and 5,000 Xmagnifications, respectively. Fig. 33 and 34 are micrographs (secondary electron images) obtained with a scanning electron microscope, each showing the germanium film of electrode a8 when viewed from a small angle, said electrode a8 being in its state after charging and discharging. FIGS. 33 and 34 were obtained at 1,000 and 5,000 Xmagnifications, respectively.

As shown in fig. 27-34, gaps (spaces) are formed to surround the columnar portions of the germanium film in such a manner as to define spaces between adjacent columnar portions. It is believed that the role of these spacers is to relieve the stress caused by the expansion and contraction of the active material during charging and discharging, as also in the case of the previous silicon thin film.

Fig. 35 is a photomicrograph (secondary electron image) obtained with a scanning electron microscope, showing the surface of the germanium film of electrode a7 when viewed from above, said electrode a7 being in its state before charging and discharging. Fig. 36 is a photomicrograph (secondary electron image) obtained with a scanning electron microscope, showing the surface of the germanium film of electrode a8 when viewed from above, said electrode a8 being in its state before charging and discharging. Both fig. 35 and 36 were obtained at a magnification of 1,000X.

As shown in fig. 35 and 36, the germanium film has irregularities on its surface, which are modeled on those defined on the underlying electrolytic copper foil. The depressions of the germanium film are interconnected to form a web. It will be appreciated that the gap extends along the depth of such a recess to define a pillar portion in the germanium film.

(SIMS analysis of concentration distribution along depth)

Fig. 37 is a graphical representation showing the concentration distribution of the constituent elements in electrode a7 in the depth direction along electrode a7 prior to incorporation of electrode a7 into the cell, i.e., prior to charging and discharging. Drawing (A)38 is a graphical representation showing the concentration profile of the constituent elements in electrode a8 in the depth direction along electrode a8 prior to charging and discharging. Concentration distribution of component elements obtained by Secondary Ion Mass Spectrometry (SIMS), wherein O is used₂ ⁺As a spray source, measuring copper along a depth from the surface of the film: (⁶³Cu^-) And silicon (a)⁷³Ge^-) The concentration of (c). The abscissa represents the depth (μm) from the surface of the germanium film, and the ordinate represents the intensity (count) of each constituent element.

As is clear from fig. 37 and 38, copper (Cu) as a component of the current collector diffuses into the germanium film at a position adjacent to the current collector, and shows that the concentration is lower at a position closer to the surface of the germanium film.

As described above, the germanium film includes the collector component diffused therein, copper, has a higher copper concentration in the vicinity of the collector, and has a concentration gradient such that the copper concentration becomes lower at a position near the surface of the germanium film. Thus, the germanium film adjacent to the current collector includes a higher concentration of copper that is non-reactive with lithium, and a lower concentration of germanium that is reactive with lithium. It is believed that the germanium film stores and releases less lithium, undergoes less expansion and contraction, and produces lower strength stresses in the vicinity of the current collector. This may result in the formation of a reduced gap (space) that may cause the germanium film to peel off or peel off from the current collector, so that the bottom of the columnar portion of the germanium film may be kept adhered to the current collector.

As described above, in the case of division into columnar shapes, the germanium film maintains strong adhesion to the current collector even during charge-discharge cycles. Also, the gap formed in such a manner as to surround the columnar portion functions to relieve stress caused by expansion and contraction during charge-discharge. Thus, excellent charge-discharge cycle characteristics are obtained.

(experiment 5)

(production of electrode a 9)

Electrolytic copperFoil (18 μm thick) was used for the current collector as substrate. A silicon thin film was formed on this electrolytic copper foil by an RF spray technique. At a flow rate of 100sccm of the spray gas (Ar), ambient substrate temperature (unheated), 0.1Pa (1.0X 10)^-3Torr) and an RF power of 200W. The silicon thin film was deposited to a thickness of about 2 μm.

The resulting silicon thin film was analyzed by Raman spectroscopy and detected at about 480cm^-1A peak appears at, and is at about 520cm^-1No peak appears. This exhibits the amorphous nature of the silicon thin film.

After the silicon thin film was deposited on the electrolytic copper foil, it was cut into a size of 2cm × 2cm to prepare an electrode a 9.

Using a needle profiler Dektat³ST (available from ULVAC corporation), the surface roughness Ra and the average spacing S of the electrolytic copper foil were measured with a scanning distance of 2.0 mm. The surface roughness Ra and the average spacing S were determined to be 0.188 μm and 11 μm, respectively.

(production of electrode a 10)

An electrolytic copper foil similar to that used for the production of the electrode a9 was used for the current collector as a substrate. Under the same conditions as used for manufacturing the electrode a1 of experiment 1, a silicon thin film was formed to a thickness of about 2 μm on the electrolytic copper foil. Electrode a10 is prepared following the procedure used to prepare electrode a 9.

The resulting silicon thin film was analyzed by Raman spectroscopy and detected at about 480cm^-1At and at about 520cm^-1A peak appears. This demonstrates the microcrystalline nature of the silicon thin film.

(production of comparative electrode b 3)

The rolled copper foil used in experiment 1 above was used for the current collector as a substrate. A thin film of amorphous silicon (about 2 μm thick) was formed on the rolled copper foil by RF spraying technique following the procedure used for manufacturing electrode a 9.

The resulting amorphous silicon thin film was subjected to calcination at 650 ℃ for 1-hour. Then pass throughAnalysis of the calcined silicon film by Raman spectroscopy revealed about 480cm^-1And at about 520cm^-1A unique peak was detected. This indicates that calcination results in the formation of a polycrystalline silicon thin film.

The electrode b3 was prepared from a polysilicon film formed on a rolled copper foil according to the procedure for preparing the electrode a 9.

The surface roughness Ra and the average spacing S of the rolled copper foil were measured using the above procedure. The rolled copper foil exhibited a surface roughness Ra of 0.037 μm and an average spacing S of 14 μm.

(measurement of Charge-discharge characteristics)

Each of the electrodes a9, a10, and b3 manufactured as above was used as a working electrode. Metallic lithium was used for both the counter electrode and the reference electrode. These electrodes were used to construct a battery for experiments. The electrolytic solvent was the same as that used in experiment 1 described above. In a single electrode cell, the reduction of the working electrode is a charging reaction, while its oxidation is a discharging reaction.

Each experimental cell was charged at 25 ℃ with a constant current of 0.5mA until the potential relative to the reference electrode reached 0V, and then discharged to 2V. This is recorded as a unit charge-discharge cycle. Cycles were performed to measure the 1 st and 5 th cycle discharge capacities and charge-discharge efficiencies. The results are given in table 6.

TABLE 6

			Electrode a9	Electrode a10	Electrode b3
			Electrode a9	Electrode a10	Electrode b3	Substrate			Electrolytic copper foil	Electrolytic copper foil	Rolled copper foil
Thickness of silicon film			2μm	2μm	2μm	Substrate			Electrolytic copper foil	Electrolytic copper foil	Rolled copper foil
Thickness of silicon film			2μm	2μm	2μm	Calcination of			Is free of	Is free of	650 ℃ at 1 hour
Crystallinity of silicon thin film			Amorphous form	Microcrystals	Polycrystalline	Calcination of			Is free of	Is free of	650 ℃ at 1 hour
Crystallinity of silicon thin film			Amorphous form	Microcrystals	Polycrystalline	Charge-discharge characteristics	Cycle 1	Discharge capacity (mAh/g)	3980	4020	1978
Charge-discharge efficiency (%)	100	96	83					Discharge capacity (mAh/g)	3980	4020	1978
Charge-discharge efficiency (%)	100	96	83	Cycle 5	Discharge capacity (mAh/g)			3990	4020	731
Charge-discharge efficiency (%)	100	100	75		Discharge capacity (mAh/g)			3990	4020	731

As is apparent from the results shown in table 6, even on cycle 5, the electrode a9 using an amorphous silicon thin film as an electrode active material and the electrode a10 using a microcrystalline silicon thin film as an electrode active material according to the present invention exhibited a higher discharge capacity and a superior charge-discharge efficiency relative to the comparative electrode b 3.

(experiment 6)

(examples 1 to 7 and comparative examples 1 to 2)

(production of Current collector)

The samples specified in table 7 were used as a current collector as a substrate. Sample 1 used a rolled copper foil similar to the current collector of electrode b 3. Samples 2-4 were prepared according to the following procedure: the rolled copper foil was ground with #100, #400 or #1000 diamond paper so that its surface was roughened, washed with purified water, and then dried.

Watch (A)7

Sample numbering	1	2	3	4
Sample numbering	1	2	3	4	Copper foil thickness (mum)	18	18	18	18
Surface roughness Ra (mum)	0.037	0.1	0.18	1	Copper foil thickness (mum)	18	18	18	18

Each of the above copper foils was used as a substrate. With the aid of an RF argon spray apparatus, a silicon thin film was deposited on the substrate under the conditions specified in tables 8-10. In comparative example 2, the deposited film was then subjected to heat treatment (calcination). In examples 1 to 7 and comparative example 1, each substrate was previously treated before thin film deposition. The preprocessing is performed as follows: an ECR plasma was generated in a separately mounted plasma generator and the plasma was directed to bombard the substrate with a microwave power of 200W and an argon partial pressure of 0.06Pa for 10 minutes.

The characteristics of each silicon thin film were identified by Raman spectroscopy analysis. The results are shown in tables 8 to 10.

(measurement of Charge-discharge characteristics)

The silicon-deposited copper foils obtained in examples 1 to 7 and comparative examples 1 to 2 were cut into pieces of 2cm × 2cm, and then used to construct experimental cells in the same manner as in experiment 5 described above. For each battery, charge-discharge tests were performed in the same manner as in experiment 5 above to measure the discharge capacity and charge-discharge efficiency at the 1 st, 5 th and 20 th cycles. The results are shown in tables 8 to 10.

TABLE 8

		Example 1	Example 2	Example 3	Example 4
		Example 1	Example 2	Example 3	Example 4	Substrate	Type of substrate	Sample 2	Sample 3	Sample No. 4	Sample 3
Surface roughnessRa	0.1	0.18	1	0.18			Type of substrate	Sample 2	Sample 3	Sample No. 4	Sample 3
Surface roughnessRa	0.1	0.18	1	0.18	Thickness of substrate		18μm	18μm	18μm	18μm
Film formation conditions	Thickness of silicon film	2μm	2μm	2μm	Thickness of substrate		18μm	18μm	18μm	18μm	2μm
	Thickness of silicon film	2μm	2μm	2μm	Film forming process	Spraying of paint	Spraying of paint	Spraying of paint	Spraying of paint		2μm
	Spraying gas	Argon gas	Argon gas	Argon gas	Film forming process	Spraying of paint	Spraying of paint	Spraying of paint	Spraying of paint	Argon gas
	Spraying gas	Argon gas	Argon gas	Argon gas	Rate of Ar flow	100sccm	100sccm	100sccm	100sccm	Argon gas
	Target	99.999% silicon single crystal	99.999% silicon single crystal	99.999% silicon single crystal	Rate of Ar flow	100sccm	100sccm	100sccm	100sccm	99.999% silicon single crystal
	Target	99.999% silicon single crystal	99.999% silicon single crystal	99.999% silicon single crystal	Atmospheric pressure of spraying	0.10Pa	0.10Pa	0.10Pa	0.10Pa	99.999% silicon single crystal
	Spraying power	200W	200W	200W	Atmospheric pressure of spraying	0.10Pa	0.10Pa	0.10Pa	0.10Pa	200W
	Spraying power	200W	200W	200W	Substrate temperature	20℃	20℃	20℃	200℃	200W
	Pretreatment of	Is provided with	Is provided with	Is provided with	Substrate temperature	20℃	20℃	20℃	200℃	Is provided with
	Pretreatment of	Is provided with	Is provided with	Is provided with	Time of spraying	2 hours	2 hours	2 hours	2 hours	Is provided with
	Conditions of heat treatment	Thermal treatment	Is free of	Is free of	Time of spraying	2 hours	2 hours	2 hours	2 hours	Is free of	Is free of
Time of heat treatment		Thermal treatment	Is free of	Is free of	-	-	-	-		Is free of	Is free of
Time of heat treatment		Identification of crystallinity	480cm^-1Raman peak of (A)	Is provided with	-	-	-	-	Is provided with	Is provided with	Is provided with
520cm^-1Raman peak of (A)	Is free of		480cm^-1Raman peak of (A)	Is provided with	Is free of	Is free of	Is free of		Is provided with	Is provided with	Is provided with
520cm^-1Raman peak of (A)	Is free of		Crystallinity of the compound	Amorphous form	Is free of	Is free of	Is free of	Amorphous form	Amorphous form	Amorphous form
Cycle 1	Discharge capacity (mAh/g)		Crystallinity of the compound	Amorphous form	3980	3978	3975	Amorphous form	Amorphous form	Amorphous form	3980
	Discharge capacity (mAh/g)	Charge-discharge efficiency (%)	100	100	3980	3978	3975	100	100		3980
	Cycle 5	Charge-discharge efficiency (%)	100	100	Discharge capacity (mAh/g)	3990	3981	100	100	3980	3990
Charge-discharge efficiency (%)		100	100	100	Discharge capacity (mAh/g)	3990	3981	100		3980	3990

Cycle No. 20	Discharge capacity (mAh/g)	3990	3980	3981	3990
	Discharge capacity (mAh/g)	3990	3980	3981	3990	Charge-discharge efficiency (%)	100	100	100	100

TABLE 9

		Example 5	Example 6	Example 7
		Example 5	Example 6	Example 7	Substrate	Type of substrate	Sample 3	Sample 3	Sample 3
Surface roughness Ra	0.18	0.18	0.18			Type of substrate	Sample 3	Sample 3	Sample 3
Surface roughness Ra	0.18	0.18	0.18	Thickness of substrate		18μm	18μm	18μm
Film formation conditions	Thickness of silicon film	2μm	2μm	Thickness of substrate		18μm	18μm	18μm	2μm
	Thickness of silicon film	2μm	2μm	Film forming process	Spraying of paint	Spraying of paint	Spraying of paint		2μm
	Spraying gas	Argon gas	Argon gas	Film forming process	Spraying of paint	Spraying of paint	Spraying of paint	Argon gas
	Spraying gas	Argon gas	Argon gas	Rate of Ar flow	100sccm	100sccm	100sccm	Argon gas
	Target	99.999% silicon single crystal	99.999% silicon single crystal	Rate of Ar flow	100sccm	100sccm	100sccm	99.999% silicon single crystal
	Target	99.999% silicon single crystal	99.999% silicon single crystal	Atmospheric pressure of spraying	0.10Pa	1.0Pa	10Pa	99.999% silicon single crystal
	Spraying power	200W	200W	Atmospheric pressure of spraying	0.10Pa	1.0Pa	10Pa	200W
	Spraying power	200W	200W	Substrate temperature	50℃	20℃	20℃	200W
	Pretreatment of	Is provided with	Is provided with	Substrate temperature	50℃	20℃	20℃	Is provided with
	Pretreatment of	Is provided with	Is provided with	Time of spraying	2 hours	1.5 hours	2.5 hours	Is provided with
	Conditions of heat treatment	Thermal treatment	Is free of	Time of spraying	2 hours	1.5 hours	2.5 hours	Is free of	Is free of
Time of heat treatment		Thermal treatment	Is free of	-	-	-		Is free of	Is free of
Time of heat treatment		Identification of crystallinity	480cm^-1Raman peak of (A)	-	-	-	Is provided with	Is provided with	Is provided with
520cm^-1Raman peak of (A)	Is free of		480cm^-1Raman peak of (A)	Is free of	Is free of		Is provided with	Is provided with	Is provided with
520cm^-1Raman peak of (A)	Is free of		Crystallinity of the compound	Is free of	Is free of	Amorphous form	Amorphous form	Amorphous form
Cycle 1	Discharge capacity (mAh/g)		Crystallinity of the compound	4060	3585	Amorphous form	Amorphous form	Amorphous form	2500
	Discharge capacity (mAh/g)	Charge-discharge efficiency (%)	100	4060	3585	100	100		2500
	Cycle 5	Charge-discharge efficiency (%)	100	Discharge capacity (mAh/g)	4060	100	100	3592	2505
Charge-discharge efficiency (%)		100	100	Discharge capacity (mAh/g)	4060	100		3592	2505

Cycle No. 20	Discharge capacity (mAh/g)	4060	3590	2505
	Discharge capacity (mAh/g)	4060	3590	2505	Charge-discharge efficiency (%)	100	100	100

Watch 10

		Comparative example 1	Comparative example 2
		Comparative example 1	Comparative example 2	Substrate	Type of substrate	Sample 3	Sample 1
Surface roughness Ra	0.18	0.037			Type of substrate	Sample 3	Sample 1
Surface roughness Ra	0.18	0.037	Thickness of substrate		18μm	18μm
Film formation conditions	Thickness of silicon film	2μm	Thickness of substrate		18μm	18μm	2μm
	Thickness of silicon film	2μm	Film forming process	Spraying of paint	Spraying of paint		2μm
	Spraying gas	Argon gas	Film forming process	Spraying of paint	Spraying of paint	Argon gas
	Spraying gas	Argon gas	Rate of Ar flow	100sccm	100sccm	Argon gas
	Target	99.999% silicon single crystal	Rate of Ar flow	100sccm	100sccm	99.999% silicon single crystal
	Target	99.999% silicon single crystal	Atmospheric pressure of spraying	0.10Pa	0.10Pa	99.999% silicon single crystal
	Spraying power	200W	Atmospheric pressure of spraying	0.10Pa	0.10Pa	200W
	Spraying power	200W	Substrate temperature	450℃	20℃	200W
	Pretreatment of	Is provided with	Substrate temperature	450℃	20℃	Is free of
	Pretreatment of	Is provided with	Time of spraying	2 hours	2 hours	Is free of
	Conditions of heat treatment	Thermal treatment	Time of spraying	2 hours	2 hours	Is free of	650℃
Time of heat treatment		Thermal treatment	-	1 hour		Is free of	650℃
Time of heat treatment		Identification of crystallinity	-	1 hour	480cm^-1Raman peak of (A)	Is free of	Is free of
520cm^-1Raman peak of (A)	Is provided with		Is provided with		480cm^-1Raman peak of (A)	Is free of	Is free of
520cm^-1Raman peak of (A)	Is provided with		Is provided with	Crystallinity of the compound	Polycrystalline	Polycrystalline
Cycle 1	Discharge capacity (mAh/g)		1250	Crystallinity of the compound	Polycrystalline	Polycrystalline	1978
	Discharge capacity (mAh/g)	Charge-discharge efficiency (%)	1250	81	83		1978
	Cycle 5	Charge-discharge efficiency (%)	Discharge capacity (mAh/g)	81	83	900	731
Charge-discharge efficiency (%)		75	Discharge capacity (mAh/g)	75		900	731

Cycle No. 20	Discharge capacity (mAh/g)	700	350
	Discharge capacity (mAh/g)	700	350	Charge-discharge efficiency (%)	69	59

As is clear from the results in tables 8 to 10, an increase in discharge capacity and an improvement in charge-discharge cycle characteristics were obtained with the electrodes obtained with examples 1 to 7, relative to the electrodes obtained with comparative examples 1 to 2, which used a polycrystalline silicon thin film as an electrode active material, with examples 1 to 7, which used an amorphous silicon thin film according to the present invention as an electrode active material.

(experiment 7)

An amorphous silicon thin film (about 3 μm thick) was formed on an electrolytic copper foil (18 μm thick, surface roughness Ra 0.188 μm, average spacing S6 μm) by an RF spray technique to manufacture an electrode a-11. The thin film was deposited using single crystal silicon as a target at a flow rate of 100sccm of the spray gas (Ar), an ambient substrate temperature (no heating), a reaction pressure of 0.1Pa, and a RF power of 200W.

The resulting silicon thin film was analyzed by Raman spectroscopy to detect about 480cm^-1Peaks were present at about 520cm^-1There was no peak. This shows the amorphous nature of the silicon thin film.

The thus-obtained electrode a-11 was used to construct a battery A11 in the same manner as in experiment 1 described above. The battery was subjected to a charge-discharge cycle test under the same conditions as in the above experiment 1 to measure the capacity retention rate at the 30 th cycle. In table 11, the results of batteries a1 and A3 are also shown.

TABLE 11

Battery with a battery cell	First cycle capacity retention ratio of 30%
Battery with a battery cell	First cycle capacity retention ratio of 30%	A1	91％
A3	97％	A1	91％
A3	97％	A11	97％

As can be understood from the results shown in table 11, the battery a11 using a spray-deposited amorphous silicon thin film as an active material also exhibited excellent capacity retention rate, compared with those of the batteries a1 and A3 using a microcrystalline silicon thin film as an active material.

The state of the silicon thin film in the electrode a-11 was observed using an electron microscope. First, a part of the electrode a-11 before its charge and discharge was observed with a scanning electron microscope. Fig. 39 and 40 are micrographs (secondary electron images) obtained with a scanning electron microscope, each showing a part of the electrode a-11 before charging and discharging. FIGS. 39 and 40 were obtained at 2,000 and 10,000 Xmagnifications, respectively. The samples were prepared according to the procedure for preparing samples shown in fig. 2 and 3, i.e., the electrodes were embedded in epoxy resin, and then the epoxy-embedded electrodes were cut into pieces.

In fig. 39 and 40, a portion which appears quite bright shows the electrolytic copper foil. The deposited silicon film (about 3 μm thick) was found to be a dark portion on the copper foil. As shown in fig. 39 and 40, unevenness was determined on the surface of the electrolytic copper foil. In particular, the projecting portion has a generally conical shape. Similar irregularities having such tapered convex portions are also formed on the surface of the silicon thin film deposited on the copper foil. Accordingly, the surface irregularities of the silicon thin film show shapes in accordance with those defined on the surface of the copper foil.

FIG. 41 is a photomicrograph (secondary electron image) taken with a scanning electron microscope, showing the silicon thin film surface in electrode a-11 when viewed at a magnification of 1,000X. As shown in fig. 41, a plurality of projections are formed on the surface of the silicon thin film. These projections are formed in a manner that emulates those projections defined on the surface of the copper foil, as shown in fig. 39 and 40.

Fig. 42 is a photomicrograph (reflection electron image) obtained with a scanning electron microscope, showing the surface of electrode a-11 taken out of cell a11 after 30 cycles in the charge-discharge test. Fig. 42 was obtained at a magnification of 1,000X.

As shown in fig. 42, gaps (spaces) extending in its thickness direction are formed in the silicon thin film, and these gaps (spaces) divide the silicon thin film into pillars. In the silicon thin film shown in fig. 6 to 9, the gaps are formed such that a plurality of columnar portions each surrounding a single convex portion are defined on the thin film. On the other hand, in the silicon thin film shown in fig. 42, the gap is formed such that a plurality of columnar portions respectively surrounding the plurality of convex portions are defined in the thin film. It was also found that the gap (spacing) in the silicon thin film shown in fig. 42 is wider than the gap (spacing) in the silicon thin films shown in fig. 6 to 9.

Battery a11 exhibited excellent capacity retention similar to battery A3. It is believed that the effect of the spacers provided to surround the columnar portions in one way is to relieve the stress caused by the expansion and contraction of the active material, so that even in the case where it is determined that each columnar portion surrounds a plurality of convex portions on the surface of the thin film, the charge-discharge cycle can be repeated without occurrence of the detachment of the active material from the current collector.

(experiment 8)

A microcrystalline silicon thin film having a thickness of about 2 μm was formed on both the rolled copper foil and the electrolytic copper foil (18 μm thick) under the same film forming conditions used for manufacturing the electrode a1 in experiment 1. Then, a 17mm diameter piece was punched out from each sample to provide an electrode c1 bonded to the silicon film formed on the rolled copper foil and an electrode c3 bonded to the silicon film formed on the electrolytic copper foil. The blocks equivalent to the electrodes c1 and c3 were subjected to heat treatment at 400 ℃ for 3 hours to provide electrodes c2 and c4, respectively.

The procedure of experiment 1 was followed except that the electrode C1-C4 was used as a negative electrode to constitute a rechargeable lithium battery C1-C4. The charge-discharge cycle life characteristics of these batteries were measured in the same manner as in experiment 1. Further, the hydrogen content and the Raman peak intensity ratio (480 cm) were measured for the silicon thin film of each electrode in the same manner as in experiment 1^-1/520cm^-1) And crystal particle size. The results are shown in table 12.

TABLE 12

Battery with a battery cell	50 th cycle capacity retention	Hydrogen content	Peak intensity ratio (480 cm)^-1/520cm^-1)	Crystal particle size
Battery with a battery cell	50 th cycle capacity retention	Hydrogen content	Peak intensity ratio (480 cm)^-1/520cm^-1)	Crystal particle size	C1	90％	4％	0.1	1nm
C2	85％	0.01％	0.1	1nm	C1	90％	4％	0.1	1nm
C2	85％	0.01％	0.1	1nm	C3	91％	4％	0.1	1nm
C4	87％	0.01％	0.1	1nm	C3	91％	4％	0.1	1nm

The results shown in Table 12 show that the batteries C1-C4, which have a microcrystalline silicon thin film of about 2 μm thickness, also gave significantly high capacity retention.

The electrode c1 bonded to the microcrystalline silicon thin film formed on the rolled copper foil was cut into pieces in its thickness direction to provide a sample, which was then observed with a transmission electron microscope.

Fig. 43 and 44 are micrographs obtained by a transmission electron microscope, showing the interface between the copper foil and the silicon thin film in the electrode c1 and its vicinity. FIGS. 43 and 44 were obtained at 500,000X and 1,000,000X magnification, respectively. The copper foil was found on the lower side of each photomicrograph, while the silicon film was on the upper side.

In fig. 43 and 44, the bright lower portion appears like a copper foil portion. A portion located near the boundary between the copper foil and the silicon thin film appears to be darkened upward. This portion (about 30nm to about 100nm) appears to be part of the mixed layer where the diffusion of copper from the copper foil to the silicon is particularly important. In the mixed layer, copper (Cu) may be mixed with silicon (Si). Also, in FIGS. 43 and 44, a specific portion is observed in the vicinity of the interface between the like mixed layer and the copper foil. This particular portion was found to determine the distribution of asperities along the interface as a result of diffusion of copper (Cu) into silicon (Si).

Next, the concentration distribution of the component elements along the depth of the mixed layer was observed. For this purpose, O is used₂ ⁺Copper as a source for spraying by SIMS measurement: (⁶³Cu⁺) And hydrogen (¹H⁺) The concentration of (c). Fig. 45 shows the concentration distribution of each constituent element. The abscissa represents depth (. mu.m), and the ordinate represents atomic density (atomic number/cm)³)。

As shown in fig. 45, the concentration of copper (Cu) in the mixed layer increases at a deeper position, that is, at a position close to the copper foil. If the mixed layer is defined as one of the layers in a silicon thin film, the silicon thin film comprises at least 1% (10)²⁰Atom/cm³If expressed in atomic density), the mixed layer was found to be present in a thickness region extending from a depth of about 1.9 μm to a depth of about 2.7 μm.

Similarly, for the electrode c3 incorporating a microcrystalline silicon thin film of about 2 μm thickness formed on the electrolytic copper foil, the concentration distribution of each component element along the depth of the mixed layer was observed using SIMS. The results are shown in fig. 46. As shown in FIG. 46, at the surface of the silicon thin film in the electrode c3, the atomic density of copper has exceeded 10²⁰Atom/cm³. This clearly indicates that the copper diffusing across the silicon film to its surface causes the silicon film to become in the form of a mixed layer in its bulk. Also, the battery C3 using this electrode C3 exhibited excellent charge-discharge cycle characteristics. This means that the silicon thin film functions as an electrode active material even if it is changed into a form of a mixed layer in its entirety.

As is clear from fig. 45 and 46, the copper concentration varies continuously across the silicon film. This correspondingly means that the copper present in the silicon thin film is not in the form of an intermetallic compound but in the form of a solid solution with silicon.

As described above, it was confirmed that a mixed layer in which copper in the copper foil is mixed with silicon in the silicon thin film is formed at the interface between the copper foil and the silicon thin film. It is believed that the presence of this mixed layer improves adhesion of the silicon film and the copper foil, prevents the silicon film from peeling off from the copper foil as a substrate even if the silicon film undergoes expansion and contraction upon charge and discharge, and provides excellent charge-discharge cycle characteristics.

Utilization in industry

A rechargeable lithium battery exhibiting high charge-discharge capacity and excellent charge-discharge cycle characteristics can be obtained using the electrode according to the present invention.

Claims

1. An electrode for a lithium battery having a thin film composed of an active material having lithium storage and release capacity and provided on a current collector;

the electrode is characterized in that the film surface is uneven in conformity with the shape of unevenness on the underlying collector surface, the film is divided into columns by gaps formed on the film in such a manner as to extend in the thickness direction of the film from the depressions of the unevenness on the film surface, and the bottoms of the columnar portions are bonded to the collector.

2. An electrode for a lithium battery having a thin film composed of an active material having lithium storage and release capacity and provided on a current collector;

the electrode is characterized in that a gap formed in the thickness direction thereof divides the thin film into pillars, the bottoms of the columnar portions are bonded to the current collector, and the gap is formed in the thin film along a low-density region of the thin film, which extends in the film thickness direction.

3. The electrode for a lithium battery as claimed in claim 1 or 2, wherein at least half of the thickness portion of the thin film is divided into pillars by the gaps.

4. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the gap is formed such that each of the columnar portions surrounds at least one of the rugged projections on the surface of the thin film.

5. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the gap is formed during a first or subsequent charge-discharge cycle.

6. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the rugged projections on the surface of the current collector have a tapered shape.

7. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the columnar portion has a dome shape.

8. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the gap is formed on the thin film before being subjected to charge and discharge.

9. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the thin film comprises at least one material selected from elements of groups IIB, IIIB, IVB and VB of the periodic table, and oxides and sulfides of transition metal elements of periods 4, 5 and 6 of the periodic table, which can produce compounds or solid solutions with lithium.

10. The electrode for a lithium battery as claimed in claim 9, wherein the element is at least one element selected from carbon, silicon, germanium, tin, lead, aluminum, indium, zinc, cadmium, bismuth and mercury.

11. The electrode for a lithium battery as claimed in claim 9, wherein the element is silicon or germanium.

12. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the thin film is formed by one of chemical vapor deposition, spray coating, vapor evaporation, sputtering and plating.

13. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the thin film is an amorphous thin film.

14. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the thin film is an amorphous thin film.

15. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the thin film is an amorphous silicon thin film.

16. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the thin film is a microcrystalline or amorphous silicon thin film.

17. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the thin film is an amorphous germanium thin film.

18. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the thin film is a microcrystalline or amorphous germanium thin film.

19. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the thin film is a microcrystalline or amorphous silicon-germanium mixed thin film.

20. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the current collector includes at least one selected from copper, nickel, stainless steel, molybdenum, tungsten and tantalum.

21. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the current collector has a surface roughness Ra in the range of 0.01 to 1 μm.

22. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the current collector is a copper foil.

23. The electrode for a lithium battery as claimed in claim 22, wherein the copper foil is an electrolytic copper foil.

24. The electrode for a lithium battery as claimed in claim 2, wherein the low-density region extends upward from the rugged depressions on the surface of the current collector.

25. The electrode for a lithium battery as claimed in claim 1 or 2, wherein a component of the current collector is diffused to the thin film.

26. The electrode for a lithium battery as claimed in claim 25, wherein the diffused current collector component forms a solid solution in the thin film in place of the intermetallic compound having the thin film component.

27. The electrode for a lithium battery as claimed in claim 1 or 2, wherein the thin film is an active material thin film that stores lithium by forming a mixture with lithium.

28. An electrode for a lithium battery as claimed in claim 1 or 2, wherein lithium is pre-stored or incorporated in a thin film.

29. A lithium battery comprising a negative electrode, a positive electrode and an electrolyte, wherein the negative electrode comprises an electrode as claimed in any one of claims 1 to 28.

30. A rechargeable lithium battery comprising a negative electrode, a positive electrode and a non-aqueous electrolyte, wherein the negative electrode comprises an electrode as claimed in any one of claims 1 to 28.

31. A rechargeable lithium battery as claimed in claim 30, characterized in that the positive electrode comprises an oxide capable of storing and releasing lithium as an active material.

32. A rechargeable lithium battery as claimed in claim 30, characterized in that the positive electrode comprises a lithium-containing oxide as active material.

33. An electrode for a rechargeable battery, comprising an electrode material layer in the form of a film and a current collector bonded to the electrode material layer,

the electrode is characterized in that the film has low-density regions connected to each other in a net shape in a planar direction and extending toward the current collector in a thickness direction of the film, and the low-density regions of the film are formed in such a manner as to extend from uneven depressions on the surface of the current collector.

34. An electrode for a rechargeable battery as claimed in claim 33 wherein the composition of the current collector is diffused into the film.

35. The electrode for a rechargeable battery according to claim 34 wherein the diffused current collector component forms a solid solution in the thin film in place of the intermetallic compound having the thin film component.

36. An electrode for a rechargeable battery as claimed in any one of claims 33 to 35 wherein the thin film is formed on the current collector by a thin film forming method.

37. The electrode for a rechargeable battery as claimed in claim 36, wherein the thin film forming method is one of chemical vapor deposition, spray coating, vapor evaporation, sputtering or plating process.

38. An electrode for a rechargeable battery as claimed in any of claims 33 to 35 wherein the rugged projections on the surface of the current collector have the shape of said cone.

39. An electrode for a rechargeable battery as claimed in any one of claims 33 to 35 wherein the film has irregularities on its surface conforming to the shape of the irregularities on the surface of the current collector.

40. An electrode for a rechargeable battery as claimed in any of claims 33-35 wherein gaps formed in the thickness direction along the low density regions as a result of expansion and contraction of the membrane divide the membrane into pillars.

41. The electrode for a rechargeable battery as claimed in claim 40, wherein charging and discharging causes the film to expand and contract.

42. An electrode for a rechargeable battery as claimed in any of claims 33 to 35 wherein the membrane is a silicon membrane.

43. An electrode for a rechargeable battery according to claims 33-35 characterised in that the silicon thin film is an amorphous or microcrystalline silicon thin film.

44. An electrode for a rechargeable battery as claimed in any of claims 33 to 35 wherein the film is a germanium film.

45. The electrode for a rechargeable battery as claimed in claim 44 wherein the germanium film is an amorphous or microcrystalline germanium film.

46. An electrode for a rechargeable battery as claimed in any of claims 33 to 35 wherein the film is a silicon germanium hybrid film.

47. The electrode for a rechargeable battery as claimed in claim 46 wherein the silicon germanium mixed thin film is an amorphous or microcrystalline silicon germanium mixed thin film.

48. An electrode for a rechargeable battery as claimed in any one of claims 33 to 35 wherein the current collector has a surface roughness Ra in the range 0.01 to 1 μm.

49. An electrode for a rechargeable battery as claimed in any one of claims 33 to 35 wherein the current collector is copper foil.

50. The electrode for a rechargeable battery as claimed in claim 49, wherein the copper foil is an electrolytic copper foil.

51. A rechargeable battery, characterised in that an electrode according to any of claims 33-50 is used.

52. The rechargeable battery of claim 51, wherein the electrode is used as a positive and/or negative electrode of a rechargeable battery.

53. The rechargeable battery of claim 50 or 52, wherein the rechargeable battery is a non-aqueous rechargeable battery.

54. The rechargeable battery of claim 53, wherein the non-aqueous rechargeable battery is a rechargeable lithium battery.