JP2011071113A - Negative electrode for lithium secondary battery, method of manufacturing negative electrode for lithium secondary battery, and lithium secondary battery - Google Patents

Abstract

An object of the present invention is to improve the charge / discharge cycle characteristics of a lithium secondary battery.
[Solution]
A negative electrode 100 for a lithium secondary battery comprising a current collector 1 and a negative electrode active material layer 5 formed on the surface of the current collector 1, wherein the negative electrode active material layer 5 is mainly composed of silicon and carbon. The ratio x of the number of carbon atoms to silicon in the negative electrode active material layer 5 is 0.03 or more and 1.0 or less, and the number of bonds of silicon-carbon bonds to the number of carbon-carbon bonds in the negative electrode active material layer 5 is The ratio R is greater than 0 and less than 2.5.
[Selection] Figure 1

Description

  The present invention relates to a negative electrode for a lithium secondary battery, a method for producing a negative electrode for a lithium secondary battery, and a lithium secondary battery using the negative electrode for a lithium secondary battery.

  The demand for small electronic / electrical devices such as portable communication devices has been increasing in recent years, and the production of secondary batteries used for them has also increased. Especially, the increase in the production amount of a lithium secondary battery with a high energy density is remarkable.

  As the applications of small electronic / electrical equipment are diversified and further miniaturization is attempted, it is desired to improve the performance of lithium secondary batteries. Specifically, there is an increasing demand for increased discharge capacity and extended life.

Currently available lithium secondary batteries use a lithium-containing composite oxide such as LiCoO ₂ for the positive electrode and graphite for the negative electrode. However, the negative electrode material made of graphite can only absorb lithium ions up to the composition of LiC ₆ , and the maximum values of lithium ion storage and release are 372 mAh / g and 835 mAh / cm ³ . This value is smaller than the theoretical capacity of metallic lithium (3878 mAh / g, 1939 mAh / cm ³ ).

On the other hand, metal elements such as Al, Ga, In, Si, Ge, Sn, Pb, As, Sb, and Bi or alloys thereof are known as elements capable of reversibly inserting and extracting lithium ions. The theoretical capacity of these elements is
Si: 4192 mAh / g, 9767 mAh / cm ³ (Si—Li _4.4 Si),
Ge: 1621 mAh / g, 8625 mAh / cm ³ (Ge-Li _4.4 Ge),
Sn: 958 mAh / g, 6973 mAh / cm ³ (Sn—Li _4.25 Sn),
Al: 2231 mAh / g, 6024 mAh / cm ³ (Al—Li _2.25 Al),
Sb: 659 mAh / g, 4410 mAh / cm ³ (Sb—Li ₃ Sb),
Bi: 384 mAh / g, 3745 mAh / cm ³ (Bi-Li ₃ Bi),
Pb: 426 mAh / g, 4837 mAh / cm ³ (Pb—Li _3.3 Pb)
It is much larger than the theoretical capacity of carbonaceous materials such as graphite.

  However, the negative electrode active material using the metal element as described above greatly expands and contracts by inserting and extracting lithium ions during charge and discharge. Therefore, in a negative electrode having a structure in which a negative electrode active material layer containing the negative electrode active material as described above is formed on a sheet-like current collector, when charge and discharge are repeated, the negative electrode active material layer and the current collector are in the vicinity of the interface. Large stress is generated and distortion occurs. For this reason, the negative electrode may be wrinkled or cut, or the negative electrode active material layer may be peeled off. As a result, the electrical connection between the active material layer and the current collector cannot be maintained, and the capacity is reduced. Therefore, there is a problem that a lithium secondary battery excellent in charge / discharge cycle characteristics cannot be obtained.

  For this problem, Patent Document 1 proposes to use a silicon material containing impurities such as carbon, oxygen, nitrogen, argon, and fluorine as a negative electrode active material. In Patent Document 1, monosilane gas, which is a silicon material, and methane gas, which is a carbon material, are decomposed in Ar plasma generated with high-frequency power and deposited on a copper foil used as a current collector. A negative electrode active material layer made of a mixture of carbon is formed. Hereinafter, a method of forming a film by decomposing a material gas in plasma is referred to as “plasma CVD method”. Further, in Patent Document 1, in an example in which the atomic ratio x of carbon to silicon in the negative electrode active material layer is 2% or less, the ratio of the first cycle discharge capacity to the first cycle charge capacity of the lithium secondary battery (capacity) (Retention rate) shows a high value of 87% or more, but it is also described that the capacity maintenance rate is reduced to 40% in the example where the atomic ratio x is 50%.

  On the other hand, although not applied to a lithium secondary battery, Patent Documents 2 and 3 disclose a method for forming a thin film containing silicon and carbon. In Patent Document 2, it is proposed to form a silicon-carbon mixed film by plasma CVD as in Patent Document 1, using a gas having a silicon-carbon bond typified by tetramethylsilane.

  Patent Document 3 discloses a method of forming a SiC film by plasma CVD using monosilane and methane as material gases.

  Patent Document 4 discloses a method for vapor-phase growth of an active material film using a raw material containing silicon and carbon. For example, an active material film containing silicon and carbon is formed on a current collector by heating and evaporating a raw material obtained by adding carbon to silicon at 1650 ° C. or more and 2500 ° C. or less. In addition, it has also been proposed that carbon is mixed in vapor phase grown silicon by containing silicon in a crucible made of a carbon material and heating it.

  Patent document 5 is disclosing providing the negative electrode active material layer containing a silicon, carbon, and oxygen. In this negative electrode active material layer, the carbon content is 0.2 to 10 atomic%, the oxygen content is 0.5 to 40 atomic%, and 0.1 to 17.29% of silicon is bonded to carbon. (Si-C bond). In patent document 5, said negative electrode active material layer is formed by performing vapor deposition, introducing oxygen gas into a vapor deposition treatment tank, using the evaporation source containing silicon and carbon.

JP-A-2005-235397 JP 2007-308774 A JP 7-315822 A JP 2007-184252 A JP 2009-283366 A

  As disclosed in Patent Documents 1 to 3, the present inventor has found that when a negative electrode active material layer containing silicon and carbon is formed by a plasma CVD method, there are the following problems.

In a lithium secondary battery using a negative electrode active material layer formed by plasma CVD, the ratio of the discharge capacity measured immediately thereafter to the initial charge capacity (hereinafter referred to as “initial charge / discharge efficiency”) is about 40%. And lower. Thereafter, the charge / discharge capacity remains at about 40%, and the capacity density per volume also remains at about 2300 mAh / cm ³ .

  The present inventor examined this cause. First, a large amount of hydrogen remained in the negative electrode active material layer formed by the plasma CVD method, and in the first charge / discharge, the charge was not only occluded by lithium but also hydrogen gas. Also used to generate As a result, it is considered that the charge capacity (initial charge capacity) is increased by the amount of hydrogen gas generated. Secondly, the negative electrode active material layer produced by the plasma CVD method contains a large amount of SiC that does not occlude lithium or does not release even if occluded. For this reason, subsequent discharge capacity becomes small, and as a result, it is thought that initial stage charge-and-discharge efficiency and capacity density fall rather than before.

  On the other hand, in the methods disclosed in Patent Documents 4 and 5, vapor deposition is performed using a mixture containing silicon and carbon as a raw material, so that the above-described problems hardly occur.

  However, as a result of investigation by the present inventors, when the negative electrode active material layer is formed by vapor deposition using the above mixture as a raw material, the capacity retention rate of the secondary battery is greatly reduced, and it is difficult to sufficiently improve the charge / discharge cycle characteristics. I found out that

  According to the experimental results to be described later, when a negative electrode active material layer obtained by vapor-depositing a mixed powder of silicon and carbon (carbon: silicon = 1: 1 by atomic ratio) is used, the secondary battery is charged and discharged. The capacity was remarkably lowered, and the capacity retention rate after only 5 charging / discharging stayed at 79%. Here, the “capacity maintenance ratio” refers to the ratio of the discharge capacity in each cycle to the reference capacity, with the maximum charge capacity observed in the charge / discharge cycle test of the secondary battery as the reference capacity.

  In addition, such a problem is not only in the case of using a mixture containing silicon and carbon as a raw material, but as described in Patent Document 4, silicon is contained in a container made of a carbon material and carbon and silicon are contained. It is considered that this also occurs when evaporating water.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a lithium secondary battery having high capacity and excellent charge / discharge cycle characteristics while ensuring initial charge / discharge efficiency and capacity density. is there.

  The negative electrode for a lithium secondary battery of the present invention is a negative electrode for a lithium secondary battery comprising a current collector and a negative electrode active material layer formed on a surface of the current collector, wherein the negative electrode active material layer is The ratio x of the number of carbon atoms to silicon in the negative electrode active material layer is 0.03 or more and 1.0 or less, and the number of carbon-carbon bond bonds in the negative electrode active material layer. The ratio R of the number of silicon-carbon bonds to R is greater than 0 and less than 2.5.

  In a preferred embodiment, a ratio R of the number of silicon-carbon bonds to the number of carbon-carbon bonds is 1.0 or less.

  The ratio R of the number of silicon-carbon bonds to the number of carbon-carbon bonds may be a value determined by photoelectron spectroscopy.

  In a preferred embodiment, the ratio x of the number of carbon atoms to silicon in the negative electrode active material layer is greater than 0.18.

  Another negative electrode for a lithium secondary battery of the present invention is a negative electrode for a lithium secondary battery comprising a current collector and a negative electrode active material layer formed on a surface of the current collector, wherein the negative electrode active material The layer is a vapor deposition film containing silicon and carbon, and the vapor deposition film is provided separately from the first evaporation source for evaporating the negative electrode active material mainly composed of silicon and the first evaporation source, A second evaporation source that evaporates a carbon material containing carbon as a main component, and silicon and carbon are evaporated from the first and second evaporation sources, respectively, and incident on the surface of the current collector. Yes.

  A method for producing a negative electrode for a lithium secondary battery according to the present invention is a method for producing a negative electrode for a lithium secondary battery comprising a current collector and a negative electrode active material layer formed on a surface of the current collector, A negative electrode containing silicon and carbon on the surface of the current collector by evaporating a negative electrode active material comprising silicon as a main component and a carbon material containing carbon as a component and allowing the material to enter the surface of the current collector A step of depositing the active material layer, wherein the step of depositing the negative electrode active material layer is provided separately from the first evaporation source for evaporating the negative electrode active material and the first evaporation source, and the carbon material And a second evaporation source for evaporating the water.

  In a preferred embodiment, in the step of depositing the negative electrode active material layer, the negative electrode active material of the first evaporation source is heated to a temperature of 1600 ° C. or more and 2200 ° C. or less, and the carbon material of the second evaporation source is used. Is heated to a temperature of 2600 ° C. or higher and 3200 ° C. or lower.

  In a preferred embodiment, in the step of depositing the negative electrode active material layer, the silicon and carbon are evaporated simultaneously.

  The carbon material may be graphite.

  In a preferred embodiment, the negative electrode active material and the carbon material are evaporated by irradiating the negative electrode active material and the carbon material with an electron beam.

  In a preferred embodiment, a ratio R of the number of silicon-carbon bonds to the number of carbon-carbon bonds in the negative electrode active material layer is greater than 0 and less than 2.5.

  In a preferred embodiment, a ratio R of the number of silicon-carbon bonds to the number of carbon-carbon bonds is 1.0 or less.

  The number R of silicon-carbon bonds relative to the number of carbon-carbon bonds may be a value determined by photoelectron spectroscopy.

  In a preferred embodiment, the ratio x of the number of carbon atoms to silicon in the negative electrode active material layer is 0.03 or more and 1.0 or less.

  In a preferred embodiment, the ratio x of the number of carbon atoms to silicon in the negative electrode active material layer is greater than 0.18.

  The other negative electrode for lithium secondary batteries of this invention is a negative electrode for lithium secondary batteries manufactured by one of the methods as described above.

  A lithium secondary battery of the present invention is disposed between a positive electrode capable of inserting and extracting lithium ions, a negative electrode for a lithium secondary battery as described above, and the positive electrode and the negative electrode for a lithium secondary battery. And an electrolyte having lithium ion conductivity.

  According to the present invention, in a negative electrode active material layer containing silicon and carbon, carbon is made to function as an aggregate, thereby suppressing expansion and contraction of the negative electrode active material layer due to repeated charge and discharge, and peeling of the negative electrode active material layer And deformation of the negative electrode can be prevented. Therefore, it is possible to realize a lithium secondary battery having high capacity and excellent charge / discharge cycle characteristics while ensuring initial charge / discharge efficiency and capacity density.

  Moreover, if the manufacturing method of the negative electrode for lithium secondary batteries of this invention is used, said negative electrode active material layer can be formed simply.

It is typical sectional drawing of the negative electrode for lithium secondary batteries of Embodiment 1 by this invention. It is typical sectional drawing of the negative electrode for lithium secondary batteries of Embodiment 1 by this invention. It is a schematic diagram which illustrates the structure of the vapor deposition apparatus used for formation of the negative electrode active material body in Embodiment 1 of this invention. It is a phase phase diagram of silicon (Si) and carbon (C), the horizontal axis is the ratio of the number of silicon atoms to the number of atoms constituting the entire system, and the vertical axis is the temperature of the system. It is typical sectional drawing which illustrates the coin-type lithium ion secondary battery using the negative electrode by this invention. In a comparative example, it is a schematic diagram which illustrates the structure of the plasma CVD apparatus used for formation of a negative electrode active material body. It is a graph which shows the evaluation result of the capacity maintenance rate in the charging / discharging cycle test of the sample cell of an Example and a comparative example, a horizontal axis represents the number of charging / discharging cycles, and the vertical axis | shaft represents the capacity maintenance rate, respectively. (A) And (b) is a figure which shows the result of having analyzed the carbon bond state in the negative electrode active material layer of Example 1, (a) is the signal derived from the carbon 1s orbital electron of a photoelectron spectroscopy. It is a graph which shows a waveform, (b) is the graph which carried out waveform separation of the signal waveform derived from the carbon 1s orbital electron of photoelectron spectroscopy analysis by the Voigt function. (A) And (b) is a figure which shows the result of having analyzed the carbon bond state in the negative electrode active material layer of Example 3 and Comparative Example 4, respectively, and originates in the carbon 1s orbital electron of a photoelectron spectroscopy. It is the graph which separated the signal waveform with the Voigt function.

(First embodiment)
Hereinafter, a first embodiment of a negative electrode for a lithium secondary battery (hereinafter simply referred to as “negative electrode”) according to the present invention will be described with reference to the drawings.

  First, refer to FIG. FIG. 1 is a schematic cross-sectional view of a negative electrode for a lithium secondary battery according to this embodiment.

  The negative electrode 100 includes a current collector 1 and a negative electrode active material layer 5 made of silicon and carbon formed on the current collector 1. The negative electrode active material layer 5 in the present embodiment has a plurality of columnar negative electrode active material bodies 2. A plurality of convex portions 3 are arranged on the surface of the current collector 1, and each negative electrode active material body 2 is formed on each convex portion 3.

  According to the present embodiment, the negative electrode active material layer 5 has a function of occluding and releasing lithium during charge and discharge, and in addition to silicon having a high theoretical capacity, carbon having a small volume change due to insertion and extraction of lithium is used. Contains. Although the function of occluding and releasing lithium of carbon is smaller than that of silicon, carbon functions as an aggregate that prevents silicon from dropping from the current collector when silicon expands and contracts. The negative electrode active material layer 5 in the present embodiment contains silicon and carbon as main components, but may further contain other components (for example, oxygen or the like).

  In this way, since the negative electrode active material body 2 contains carbon that retains its shape without changing its volume in the charge / discharge process, the negative electrode active material layer 5 as a whole reduces the volume change rate due to charge / discharge. be able to. As a result, it is possible to suppress a decrease in charge / discharge cycle characteristics due to the volume change of the negative electrode active material layer.

  In the negative electrode active material layer 5 (that is, the negative electrode active material body 2) of the present embodiment, the ratio of carbon atoms to silicon (composition ratio) x is 0.03 or more and 1.0 or less (0.03 ≦ x ≦ 1). .0) is preferably controlled. When x is larger than 1.0 (x> 1.0), the mobility of lithium in the negative electrode active material body 2 is remarkably lowered, making it difficult to use as a negative electrode. If x is less than 0.03 (x <0.03), it becomes difficult to suppress silicon expansion / contraction due to carbon, and the shape of the negative electrode active material body 2 may not be sufficiently retained.

  As a result of further detailed analysis by the present inventor, the function of carbon as an aggregate that hardly absorbs and releases lithium is larger than the function as an aggregate of silicon that increases and decreases the volume by inserting and extracting lithium. . Moreover, the knowledge that the carbon which comprises the carbon-carbon bond among the carbon contained in the negative electrode active material body 2 shows the aggregate function which was superior to the carbon which comprises the carbon-silicon bond is shown. Obtained.

Therefore, based on the above findings, the present inventor has determined that the number of silicon-carbon bonds N _Si—C with respect to the number of carbon-carbon bonds N _CC present in the negative electrode active material layer 5 (that is, the negative electrode active material body 2). The relationship between the ratio R (= N _Si-C / N _CC ) and the charge / discharge cycle characteristics was examined. As a result of intensive studies, in order to obtain a secondary battery having a high capacity and excellent charge / discharge cycle characteristics, the ratio R of the negative electrode active material layer 5 is greater than 0 and less than 2.5 (0 <R < 2.5) was found to be important.

As will be described later, the ratio R of the number of silicon-carbon bonds N _{Si-C to} the number of carbon-carbon bonds N _CC in the negative electrode active material layer 5 can be measured by, for example, photoelectron spectroscopy.

  When the ratio R of the number of bonds is 0 (R = 0), since silicon is not bonded to carbon, even if carbon has a function as an aggregate, the function is exerted on silicon. I can't. On the other hand, when the ratio R is 2.5 or more (R ≧ 2.5), the ratio of carbon constituting the carbon-carbon bond is decreased, so that the aggregate function is lowered and the silicon is sufficiently dropped by expansion / contraction. May not be able to be suppressed.

  The ratio R is preferably 1 or less (0 <R ≦ 1). Thereby, since the ratio of carbon constituting the carbon-carbon bond is increased, a stronger aggregate function can be ensured.

The negative electrode active material body 2 (negative electrode active material layer 5) satisfying 0 <R <2.5 can be formed, for example, by performing deposition by separately providing a silicon evaporation source and a carbon evaporation source. At the time of vapor deposition, the material contained in each evaporation source is heated to a temperature at which the material evaporates (evaporation temperature). The heating temperature (evaporation temperature) T _C of the carbon material contained in the carbon evaporation source is set to be higher than the heating temperature T _Si of the silicon material contained in the silicon evaporation source.

As a specific example, the melting point of silicon is 1414 ° C., whereas the melting point of carbon is 3000 to 3500 ° C. Accordingly, the temperature T _Si of the silicon evaporation source is set to a temperature equal to or higher than the melting point of silicon, for example, 1400 ° C. to 2200 ° C. Preferably, it is set to 1600 ° C. or higher and 2200 ° C. or lower. The temperature T _Si of the carbon evaporation source containing carbon as a main component is set to, for example, 2600 ° C. or more and 3200 ° C. or less. Thereby, silicon and carbon evaporate from each evaporation source, and the vapor deposition film (negative electrode active material layer 5) containing silicon and carbon can be formed on the current collector.

  According to this method, it becomes possible to evaporate carbon having a carbon-carbon bond in the carbon evaporation source while maintaining the bonded state, so that the ratio of carbon-carbon bonds in the negative electrode active material layer 5 is increased. Can do.

  In the above vapor deposition method, each evaporation source is heated at the same time. That is, each evaporation source is heated so that carbon and silicon are simultaneously evaporated from the respective evaporation sources, and the evaporated carbon and silicon are simultaneously present in the vicinity of the surface of the current collector.

  It is preferable to use graphite as the carbon evaporation source. Graphite has a layered structure and has the property of sublimating at about 3000 ° C. Graphite evaporates while maintaining the carbon-carbon bond. The carbon that sublimates and disperses from graphite becomes a lump (carbon cluster) composed of 50 or more carbon atoms. Accordingly, carbon clusters composed of a large number of carbon atoms and silicon evaporated from a silicon evaporation source continuously fly to the current collector 1 to form the negative electrode active material body 2. In the negative electrode active material body 2 formed in this way, the carbon-carbon bond derived from the carbon cluster is taken over, so that the ratio of the carbon-carbon bond can be further increased.

  When silicon powder and carbon powder are brought into contact with each other and evaporated from one place as in the methods disclosed in Patent Documents 4 and 5 described above, only carbon constituting silicon-carbon bonds can be deposited. The reason for this will be described below.

  As described above, since the melting point of carbon is much higher than the temperature range suitable for silicon deposition (for example, 1600 to 2200 ° C.), even if the raw material containing carbon and silicon is heated to the above temperature range, It is thought that it does not gasify itself. At this time, two processes of carbon scattering are considered. One is a process of evaporating from a silicon melt in which carbon is dissolved, and the other is a process of forming silicon carbide and then sublimating as silicon carbide. However, by either process, the form of carbon that is scattered becomes carbon that is not bonded to other atoms (one atomic unit) or carbon that holds a silicon-carbon bond. The proportion of carbon that forms a carbon-carbon bond is extremely low.

  For example, when a container (evaporation source) containing a mixture of carbon and silicon is heated, a silicon melt in which carbon is dissolved is obtained. When this silicon melt is maintained at a temperature in the above temperature range (1600 to 2200 ° C.), silicon and carbon bonded to silicon sublimate, but evaporation of carbon constituting the carbon-carbon bond hardly occurs.

  Therefore, as disclosed in Patent Documents 4 and 5, when silicon and carbon are vapor-deposited by heating a material containing silicon and carbon, the obtained vapor-deposited film has silicon rather than the number of carbon-carbon bonds. -The number of carbon bonds is remarkably increased, and the ratio R of the number of bonds is 2.5 or more (R ≧ 2.5).

  In order to evaporate carbon constituting the carbon-carbon bond from an evaporation source having a mixture of silicon and carbon, when the above mixture is heated to a temperature higher than the melting point of carbon (3,000 to 3500 ° C.), silicon explosively evaporates. This is not practical.

  As described above, in the negative electrode active material layer 5 (that is, the negative electrode active material body 2) of this embodiment, the ratio of carbon atoms to silicon (composition ratio) x is 0.03 or more and 1.0 or less (0. 03 ≦ x ≦ 1.0). The composition ratio x of carbon to silicon can be freely controlled by adjusting the evaporation rate of each evaporation source when the negative electrode active material body 2 is formed using a silicon evaporation source and a carbon evaporation source.

  Further, when the negative electrode active material body 2 is formed by separately heating the silicon evaporation source and the carbon evaporation source, the ratio of silicon carbide contained in the negative electrode active material body 2 is the silicon and carbon formed using the CVD method. This is significantly lower than the proportion of silicon carbide in a conventional negative electrode active material layer (eg, Patent Document 1). Therefore, according to the present embodiment, a reduction in discharge capacity due to the generation of silicon carbide can be suppressed. The negative electrode active material body 2 preferably does not substantially contain silicon carbide, but may contain silicon carbide to such an extent that the discharge capacity is not significantly reduced.

  The surface of the current collector 1 in the present embodiment is preferably provided with irregularities. This makes it possible to form a plurality of negative electrode active material bodies 2 at intervals on the surface of the current collector 1 using oblique vapor deposition. Therefore, a space for expansion of the negative electrode active material body 2 can be formed between the adjacent negative electrode active material bodies 2. More preferably, the current collector 1 has convex portions 3 regularly arranged on the surface. Thereby, the magnitude | size of the negative electrode active material body 2 and the space | interval between the negative electrode active material bodies 2 which adjoin can be controlled by adjusting the shape, magnitude | size, arrangement pitch, etc. of the convex part 3 suitably. As a result, a space for expansion can be more reliably secured between the adjacent negative electrode active material bodies 2, and the expansion stress applied to the interface between the negative electrode active material body 2 and the current collector 1 can be effectively reduced. A method for forming such a convex portion 3 will be described later.

  The current collector 1 only needs to have a plurality of convex portions 3 on the surface. For example, as the current collector 1, a metal foil in which convex portions of various sizes and shapes are randomly provided is used. You can also. Even in this case, if the oblique deposition is used, the negative electrode active material bodies 2 can be formed on the convex portions 3 with a space therebetween, so that a gap can be secured between the adjacent negative electrode active material bodies 2.

  The thickness H of the negative electrode active material layer 5 is preferably, for example, from 5 μm to 100 μm, and more preferably from 5 μm to 50 μm. If the thickness H of the negative electrode active material layer 5 is 5 μm or more, a sufficient energy density can be secured. Further, when the thickness H of the negative electrode active material layer 5 exceeds 100 μm, not only the formation of the negative electrode active material body 2 becomes difficult, but also the aspect ratio of the negative electrode active material body 2 becomes large. Breakage such as 2 breakage is likely to occur, causing a reduction in capacity and rate characteristics. In the present embodiment, the thickness H corresponds to the height of the negative electrode active material body 2. The “height of the negative electrode active material body 2” refers to the height of the negative electrode active material body 2 along the normal direction of the current collector 1 from the upper surface or the apex of the convex portion 3 of the current collector 1.

  The structures of the current collector 1 and the negative electrode active material layer 5 are not particularly limited to the structure shown in FIG. For example, like the negative electrode 200 shown in FIG. 2, the current collector 1 may have a substantially smooth surface, and the negative electrode active material layer 5 may be formed on the surface. In this case, the negative electrode active material layer 5 may be a continuous film formed by depositing silicon and carbon from the normal direction of the surface of the current collector 1.

<Carbon - Measurement method of carbon bonds N _Si-C bond number ratio _{R (= N Si-C /} N CC) - silicon to bond number N _CC carbon bonds>
An example of a method for evaluating the bonding state between silicon and carbon in the negative electrode active material layer 5 by photoelectron spectroscopy will be described. In this method, for example, an X-ray photoelectron spectrometer such as PHI Quantera SXM manufactured by ULVAC PHI can be used.

  First, a negative electrode active material layer is formed by vapor-depositing silicon and carbon on a current collector using a silicon evaporation source and a carbon evaporation source.

  Since adhesion of components in the air and dust is considered on the outermost surface of the negative electrode active material layer, the outermost surface is removed by etching. Here, argon (Ar) is used as an etchant to etch from the outermost surface to a depth of 20 nm. Thereafter, measurement is started on the exposed surface (measurement surface).

Next, the energy of photoelectrons emitted by irradiating the measurement surface with X-rays is measured by photoelectron spectroscopy to obtain energy derived from carbon 1s orbital electrons. The obtained energy waveform is a waveform having a peak coordinate of 284.7 eV (a waveform derived from a carbon-carbon bond) W _CC and a waveform having a peak coordinate of 283.4 eV (a waveform derived from a silicon-carbon bond) W. Separated into _Si-C . The area ratio of these waveforms (area of W _Si-C ) / (area of W _CC ) is calculated and is defined as the ratio R of the number of bonds.

<Method for Manufacturing Negative Electrode 100>
Next, an example of the manufacturing method of the negative electrode 100 of this embodiment is demonstrated.

  First, a concavo-convex pattern is formed on the surface of the metal foil to produce a sheet-like current collector having a plurality of convex portions 3 on the surface.

  As the metal foil, for example, a copper foil whose surface is roughened can be used. The copper foil may contain elements that do not react with lithium such as zirconium and titanium, and inevitable elements such as oxygen, selenium, and tellurium in addition to copper as a main component. Here, for example, a copper foil (manufactured by Furukawa Circuit Foil Co., Ltd.) having a thickness of 35 μm and a surface roughness Ra of 2.0 μm is used. “Surface roughness Ra” refers to “arithmetic mean roughness Ra” defined in Japanese Industrial Standard (JISB 0601-1994), and is measured using, for example, a surface roughness meter or a confocal laser microscope. it can.

  The current collector may be produced by providing grooves with a predetermined pattern on the surface of the metal foil using a cutting method, or a plurality of convex portions are formed on the surface of the metal foil by a plating method or a transfer method. You may produce by doing. Suitable ranges such as the shape, height, and arrangement pitch of the protrusions 3 will be described later. Note that a commercially available metal foil (uneven foil) having a large surface roughness can also be used as the current collector.

  Next, a negative electrode active material body made of silicon and carbon is formed on the surface of the current collector by vapor deposition.

  FIG. 3 is a schematic cross-sectional view illustrating the configuration of a vapor deposition apparatus used when forming the negative electrode active material body.

  The vapor deposition apparatus 300 includes a chamber 30 and an exhaust pump (not shown) for exhausting the chamber 30.

  Inside the chamber 30 are a fixing base 40 for fixing the current collector 1, a silicon evaporation source 31 for supplying silicon to the surface of the current collector 1 fixed to the fixing base 40, and a fixing base 40. A carbon evaporation source 32 for supplying carbon to the surface of the current collector 1 fixed to the current collector, and a current collector heating heater 35 for heating the current collector 1 installed on the fixed base 40 are installed. ing.

  The silicon evaporation source 31 and the carbon evaporation source 32 are, for example, electron beam gun heating type copper crucibles. The electron beam gun only needs to have an acceleration voltage of 5 to 10 kV and an output of an irradiation current of about 0.3 to 1 A. For example, a JEBG-203UB type electron gun or JEBG-303UA type electron gun manufactured by JEOL Ltd. can be used. . In particular, the JEBG-303UA type electron gun is preferable because it can adjust the power of the electron beam applied to the silicon evaporation source 31 and the carbon evaporation source 32, respectively.

  Note that in this specification, the evaporation source includes a container such as a crucible for storing a deposition material and a heating device for evaporating the deposition material. The vapor deposition material and the container may be configured to be detachable as appropriate. When vapor deposition is performed, the vapor deposition raw materials (here, silicon and carbon) accommodated in the crucible are heated by the heating device, evaporate from the upper surface (evaporation surface), and are supplied to the surface of the current collector 1. The

  A shutter 38 is disposed between the fixed base 40 and the evaporation surfaces of the silicon evaporation source 31 and the carbon evaporation source 32. Further, rate monitors 36 and 37 for controlling the evaporation speed are disposed between the evaporation source to be used and the shutter 38. Here, the rate monitor 36 is used when controlling the evaporation rate of silicon, and the rate monitor 37 is used when controlling the evaporation rate of carbon.

  Although not shown, an oxygen introduction pipe for introducing oxygen and an argon introduction pipe for introducing argon are provided in the chamber 30 as necessary, and the gas in the chamber 30 is supplied by supplying argon to the chamber 30 using this. The pressure may be adjusted.

For example, since the amount of silicon and the amount of carbon evaporated from the evaporation sources 31 and 32 vary depending on the gas pressure in the chamber 30, a predetermined amount of argon is introduced into the chamber 30 and the gas pressure in the chamber 30 is set to 1 × 10 ⁻⁴ Pa. It may be kept constant in a range of ˜1 × 10 ⁻² Pa.

  A method of forming the negative electrode active material body 2 using the vapor deposition apparatus 300 will be specifically described.

  First, as shown in FIG. 3, the silicon evaporation source 31 and the carbon evaporation source 32 are arranged below the fixed base 40. The fixed base 40 is inclined with respect to the horizontal plane 45 by a predetermined angle θ (0 ° <θ <90 °, preferably 20 ° or more and 85 ° or less). The incident direction E of silicon and carbon (that is, the vapor deposition direction) with respect to the normal direction N of the current collector 1 can be adjusted by the inclination direction of the fixed base 40 from the horizontal plane 45. The absolute value of the inclination angle θ is an angle between the incident direction E of silicon and carbon and the normal direction N of the current collector 1 with respect to the current collector 1 installed on the fixed base 40 (incident angle of silicon and carbon) α. Is equal to Therefore, the growth direction S of the negative electrode active material body 2 grown on the surface of the current collector 1 can be controlled by adjusting the inclination angle θ of the fixed base 40.

  Next, with the shutter 38 closed, silicon is evaporated from the silicon evaporation source 31 and carbon is evaporated from the carbon evaporation source 32. The rate of film formation of silicon incident on the current collector 1 is monitored by the rate monitor 36, and the rate of film formation of carbon incident on the current collector 1 is monitored by the rate monitor 37, and the current of the electron beam is adjusted. Then, the shutter 38 is opened, and silicon and carbon vapors are incident on the surface of the current collector 1.

  Silicon and carbon vapors are incident on the surface of the current collector 1 from the direction inclined by the angle α described above with respect to the normal direction N of the surface of the current collector 1.

  At this time, the evaporated silicon atoms and carbon atoms are incident on the surface of the current collector 1 from the direction E inclined from the normal direction N of the current collector 1. It is easy to deposit on the top, so that the silicon oxide grows in a columnar shape on the protrusion. For this reason, the surface of the current collector 1 becomes a shadow of the silicon oxide that grows in a convex portion or a columnar shape, and a region in which the silicon oxide is not deposited without the incidence of silicon atoms is formed (shadowing effect). ). Therefore, a plurality of negative electrode active material bodies can be formed on the surface of the current collector 1 at intervals. Thus, the negative electrode 100 of this embodiment is obtained.

  Note that, although the method for forming the negative electrode 100 illustrated in FIG. 1 has been described above, the negative electrode 200 illustrated in FIG. 2 can be formed using the vapor deposition apparatus 300. In this case, the current collector 1 having a substantially flat surface is installed on the fixed base 40, and the fixed base 40 is fixed so that the inclination angle θ is 0 °. Next, silicon and carbon vapor may be incident from the normal direction N of the current collector 1 to form a negative electrode active material layer on the surface of the current collector 1.

  The silicon evaporation source 31 is not particularly limited as long as it is a silicon material in which the proportion of silicon is 50% or more. This is because if the ratio is lower than 50%, the silicon ratio in the negative electrode active material layer is low, so that the mobility of lithium is lowered and the negative electrode may not operate. However, inclusions other than silicon contained in the silicon evaporation source 31 exhibit a sufficiently low vapor pressure at the temperature (1400 to 2200 ° C.) at which the silicon evaporation source 31 is evaporated, and are substantially contained in the negative electrode active material layer. If not, that is not the case.

  The silicon evaporation source 31 may contain some carbon, but preferably does not contain carbon. As described above, the carbon contained in the silicon evaporation source 31 evaporates from the silicon melt or reacts with silicon to form silicon carbide to evaporate and retains one atomic unit or a silicon-carbon bond. Spatter in the state. For this reason, if carbon is contained in the silicon evaporation source 31, it becomes a factor for increasing the ratio of silicon-carbon bonds having a low aggregate function. When a negative electrode having a negative electrode active material layer with a high ratio of silicon-carbon bonds (ie, R ≧ 2.5) is used, the capacity retention rate in the charge / discharge cycle test of the lithium secondary battery is rapidly reduced, as will be described later. .

  The heating temperature of the silicon material of the silicon evaporation source 31 is 2200 ° C., for example. If it exceeds 2200 ° C, silicon may bump. According to the phase phase diagram of silicon and carbon shown in FIG. 4, when the heating temperature of the silicon evaporation source 31 is 2200 ° C., a molten metal in which 15% carbon and 85% silicon are mixed at an atomic level in terms of atomic ratio can be formed. It is therefore possible to evaporate both silicon and carbon by irradiating it with an electron beam.

  However, according to the technique of evaporating both silicon and carbon, there is an upper limit for the composition ratio x of carbon to silicon contained in the negative electrode active material body 2. The upper limit value of the carbon / silicon composition ratio x is a value that substantially reflects the composition of the molten metal of silicon and carbon (carbon 15%, silicon 85%, x = 0.18). That is, if the composition ratio x of carbon to silicon is in the range of 0 <x ≦ 0.18, only a single evaporation source for evaporating silicon and carbon is obtained by dissolving carbon in the silicon evaporation source 31. It can be seen that it can be formed using.

  Conversely, when a film having a higher carbon concentration (x> 0.18) than the above is formed, a silicon evaporation source containing a silicon material containing silicon as a main component and a carbon material containing carbon as a main component are included. It is necessary to use a carbon evaporation source. This is because it is impossible to form a deposited film having a composition such that x is 0.18 or more using only a single evaporation source containing silicon and carbon. Even if x ≦ 0.18, a carbon evaporation source can be used separately from the silicon evaporation source. By separately providing the carbon evaporation source and the silicon evaporation source, the evaporation rate from each evaporation source can be controlled independently, so that the composition of the negative electrode active material layer can be controlled with higher accuracy.

  The material contained in the carbon evaporation source 32 is not particularly limited, but hexagonal carbon such as graphite is preferably used. When cubic carbon (melting point: 3550 ° C., boiling point: 4800 ° C.) is used, the vapor pressure is extremely low. Therefore, when the carbon evaporation source 32 is heated, it is necessary to raise the temperature to about 3800 ° C. In contrast, the sublimation temperature of hexagonal carbon is 3370 ° C., and carbon can be evaporated at a relatively low temperature, which is practical.

  As a method for heating the silicon evaporation source 31 and the carbon evaporation source 32, resistance heating by a tungsten filament, heating by electron beam irradiation, or the like can be suitably applied. It is particularly preferable to heat the evaporation sources 31 and 32 by electron beam irradiation because tungsten can be kept from being mixed into the negative electrode active material layer.

  Next, the shape and size of the convex portion 3 formed on the current collector 1 will be described. When the plurality of convex portions 3 are formed on the surface of the current collector 1, each convex portion 3 may be, for example, a quadrangular prism with a rhombus on the upper surface. Or the orthographic projection image of the convex part 3 seen from the normal line direction N of the electrical power collector 1 may be polygons, such as a rhombus, a square, a rectangle, a trapezoid, and a parallelogram, a circle, an ellipse. The shape of the cross section parallel to the normal direction N of the current collector 1 may be a square, a rectangle, a polygon, a semicircle, or a combination thereof. Moreover, the shape of the convex part 3 in a cross section perpendicular | vertical with respect to the surface of the electrical power collector 1 may be a polygon, a semicircle, an arcuate shape etc., for example. In addition, when the boundary of the convex part 3 and parts other than a convex part (it is also called a groove | channel, a recessed part, etc.) is not clear, such as when the cross section of the uneven | corrugated pattern formed in the electrical power collector 1 has the shape comprised by the curve. A portion having an uneven pattern having a height equal to or higher than the average height of the entire surface is defined as “convex portion 3”, and a portion less than the average height is defined as “groove” or “recess”.

  The height of the convex part 3 is preferably 3 μm or more in order to ensure a void by a shadowing effect during oblique vapor deposition. On the other hand, in order to ensure the strength of the convex portion 3, it is preferably 20 μm or less, and more preferably 15 μm or less. The width (maximum width) of the upper surface of the convex portion 3 is not particularly limited, but is preferably 50 μm or less, whereby the deformation of the negative electrode 100 due to the expansion stress of the negative electrode active material body 2 can be more effectively suppressed. More preferably, it is 20 μm or less. On the other hand, if the width of the upper surface of the convex portion 3 is too small, the contact area between the negative electrode active material body 2 and the current collector 1 may not be sufficiently secured, so the width of the upper surface of the convex portion 3 is 1 μm or more. Preferably there is.

  Furthermore, when the convex part 3 is a columnar body having a side surface perpendicular to the surface of the current collector 1, the distance between adjacent convex parts 3, that is, the width of the groove is preferably the width of the convex part 3. 30% or more, more preferably 50% or more. Thereby, a sufficient space | gap can be ensured between the negative electrode active material bodies 2, and an expansion stress can be relieve | moderated significantly. On the other hand, if the distance between the adjacent convex portions 3 is too large, the height of the negative electrode active material body 2 increases in order to ensure capacity, and therefore the distance is 250% or less of the width of the convex portions 3. Preferably, it is 200% or less. Note that the width of the upper surface of the convex portion 3 and the distance between the adjacent convex portions 3 indicate the width and distance in a cross section perpendicular to the surface of the current collector 1 and including the growth direction of the negative electrode active material body 2. Shall.

  Although the upper surface of each convex part 3 may be flat, it is preferable to have an unevenness | corrugation, and it is preferable that the surface roughness Ra is 0.3 micrometer or more and 5.0 micrometers or less. If the upper surface of the convex portion 3 has irregularities with a surface roughness Ra of 0.3 μm or more, the negative electrode active material body 2 is likely to grow on the convex portion 3, and as a result, the negative electrode active material body 2 A sufficient gap can be reliably formed between them. On the other hand, if the surface roughness Ra of the convex portion 3 is too large, the current collector 1 becomes thick. Therefore, the surface roughness Ra is preferably 5.0 μm or less. Furthermore, if the surface roughness Ra of the current collector 1 is within the above range (0.3 μm or more and 5.0 μm or less), the adhesive force between the current collector 1 and the negative electrode active material body 2 can be sufficiently secured. Peeling of the negative electrode active material body 2 can be prevented.

<Configuration of lithium secondary battery>
Next, an example of the configuration of a sample cell obtained by applying the negative electrode 100 of the present embodiment will be described with reference to the drawings.

  FIG. 5 is a schematic cross-sectional view illustrating a coin-type lithium ion secondary battery using the negative electrode 100. The lithium ion secondary battery 600 has a positive electrode 62, a negative electrode 64, and an electrode group having a separator 63 provided between the negative electrode 64 and the positive electrode 62. The electrode group has lithium ion conductivity. An electrolyte (not shown) is impregnated. The positive electrode 62 is electrically connected to a positive electrode case 61 that also serves as a positive electrode terminal, and the negative electrode 64 is electrically connected to a sealing plate 66 that also serves as a negative electrode terminal. Further, the open end portion of the positive electrode case 61 is caulked by a gasket 65 provided on the peripheral edge portion of the sealing plate 66, thereby sealing the entire battery.

Note that FIG. 5 shows an example of a coin-type battery, but the shape of the lithium secondary battery of the present invention is not limited to the coin type, and may be a button type, a sheet type, a cylinder type, a flat type, a square type, or the like. May be. Moreover, the lithium secondary battery of this invention should just be equipped with the negative electrode 100 or the negative electrode 200 which was mentioned above with reference to FIG. 1 and FIG. 2, and components other than a negative electrode are not specifically limited. As the material for the current collector of the positive electrode, Al, Al alloy, Ti, or the like can be used. Moreover, lithium-containing transition metal oxides such as lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), and lithium manganate (LiMn ₂ O ₄ ) are used for the negative electrode active material layer (positive electrode negative electrode active material layer) of the positive electrode. Can be used. The positive electrode negative electrode active material layer may be composed of only the positive electrode negative electrode active material, or may include a mixture containing a positive electrode negative electrode active material, a binder, and a conductive agent. Furthermore, a positive electrode negative electrode active material layer can also be comprised from a some columnar negative electrode active material body. Various lithium ion conductive solid electrolytes and non-aqueous electrolytes are used as the lithium ion conductive electrolyte. As the non-aqueous electrolyte, a solution obtained by dissolving a lithium salt in a non-aqueous solvent is preferably used. The composition of the nonaqueous electrolytic solution is not particularly limited. Further, the material of the separator 63 is not particularly limited, and materials used in various forms of lithium secondary batteries can be applied.

(Examples and Comparative Examples)
Examples of the negative electrode according to the present invention and comparative examples will be described. In the first to third embodiments, the silicon and carbon evaporation sources are heated and vapor-deposited to an appropriate temperature, so that the silicon-carbon composition ratio x is 0.4, 0.03, 1.0. A negative electrode active material body made of silicon and carbon was formed on a current collector to prepare a sample negative electrode.

  On the other hand, in Comparative Example 1, a negative electrode active material layer made of silicon and carbon was formed by the plasma CVD method on the same current collector as that used in the example, thereby preparing a sample negative electrode.

  In Comparative Examples 2 and 3, a mixture of silicon and carbon is heated and deposited as a single evaporation source, thereby forming silicon and carbon having a composition ratio x of silicon and carbon of 0.03 and 0.05. A negative electrode active material body was formed on the current collector to produce a sample negative electrode.

  In Comparative Examples 4 and 5, a mixture of silicon and silicon carbide is heated and deposited as a single evaporation source, thereby forming silicon and carbon having a silicon to carbon composition ratio x of 0.4 and 0.7. A negative electrode active material body was formed on the current collector to produce a sample negative electrode.

  In Comparative Example 6, a negative electrode active material body having a composition ratio of silicon and carbon of 1: 1 was formed on a current collector by heating and depositing silicon carbide as an evaporation source, and a sample negative electrode was produced.

  Using these obtained sample negative electrodes, sample cells having the structure shown in FIG. 5 were prepared, and their characteristics were evaluated.

  Below, the preparation methods of the sample negative electrode of an Example and a comparative example, the preparation method of the sample cell for evaluation, and the evaluation method and evaluation result of a sample cell are demonstrated.

<Method for producing sample negative electrode>
(I) Example 1
In Example 1, as the current collector, an electrolytic copper foil having a core material thickness of 35 μm (manufactured by Furukawa Circuit Foil Co., Ltd., _Ra : 2.0 μm) was used. On this electrolytic copper foil, the negative electrode active material body 2 was formed using the vapor deposition apparatus 300 shown in FIG.

  Refer to FIG. 3 again. First, the silicon evaporation source 31 of the vapor deposition apparatus 300 contained 50 g of silicon having a purity of 99.9999%, and the carbon evaporation source 32 contained 40 g of graphite having a purity of 99.9%. Further, the current collector 1 is installed on the fixed base 40, and the tilt of the fixed base is set so that silicon is incident on the surface of the current collector 1 in parallel to the normal direction N of the current collector 1 (α = 90 °). The angle θ was adjusted (θ = 0 °). Thereafter, the lid of the chamber 30 was closed.

After reducing the pressure inside the chamber 30 to 7 × 10 ⁻⁵ Pa, oxygen was introduced through a mass flow controller, and the pressure in the chamber 30 was adjusted to 4.5 × 10 ⁻³ Pa. Further, the current collector 1 was heated to 300 ° C. using the current collector heating heater 35.

  Next, the silicon evaporation source 31 was irradiated with electrons to heat and evaporate the silicon, and the carbon evaporation source 32 was irradiated with electrons to heat and sublimate the graphite. The heating temperature of the silicon evaporation source 31 was 1600 ° C., and the heating temperature of the carbon evaporation source 32 was 2800 ° C. Moreover, the sample negative electrode of Example 1 whose atomic ratio x of carbon to silicon is 0.4 was obtained by adjusting the evaporation rate and the sublimation rate with the rate monitors 36 and 37, respectively.

(Ii) Example 2
The sample of Example 2 in the same manner as in Example 1 except that the amount of electron irradiation to the silicon evaporation source 31 and the carbon evaporation source 32 was adjusted so that the atomic ratio x of carbon to silicon was 0.03. A negative electrode was obtained.

(Iii) Example 3
Sample of Example 2 in the same manner as in Example 1 except that the amount of electron irradiation to the silicon evaporation source 31 and the carbon evaporation source 32 was adjusted so that the atomic ratio x of carbon to silicon was 1.0. A negative electrode was obtained.

(Iv) Comparative Example 1
In Comparative Example 1, an electrolytic copper foil similar to that used in the examples was used as the current collector. A negative electrode active material body is formed on the electrolytic copper foil by a method described below using a Penning discharge plasma gun type plasma CVD apparatus (plasma CVD apparatus AAPIG-16110DS manufactured by Shinko Seiki Co., Ltd.) 500 shown in FIG. 2 was formed.

First, the current collector 1 is attached to the substrate holder 53, the door of the vacuum vessel 50 is closed, and the inside of the vessel is depressurized to 10 ⁻⁴ Pa. Next, the current collector heater 55 is energized to heat the substrate so that the substrate is maintained at 150 ° C., and Ar gas is introduced into the plasma emission portion 51a of the plasma gun to maintain the internal pressure of the vacuum vessel 50 at 0.1 Pa. In this state, the filament is energized and a voltage is applied to the anode to excite plasma inside the vacuum vessel 50. Subsequently, tetramethylsilane (TMS) is introduced into the vacuum vessel 50 while applying a voltage of −550 V to the substrate holder 53 at a repetition frequency of 1000 pulses / second and a pulse width of 20 μs by the pulse power source 54. The internal pressure of the vacuum vessel 50 was set to 0.14 Pa, and this state was maintained for 30 minutes, whereby a negative electrode active material layer made of silicon and carbon having a thickness of 0.5 μm was formed on the surface of the current collector 1. During the formation of the negative electrode active material body, the output of the plasma gun was about 1 kW (anode voltage 60 V, thermionic current 17 A). In this way, a sample negative electrode of Comparative Example 1 was obtained.

(V) Comparative Example 2
As the current collector, the same electrolytic copper foil as that used in Example 1 was used. Moreover, the same apparatus as Example 1 was used as a vapor deposition apparatus. In Comparative Example 2, the silicon evaporation source 31 contained a material obtained by adding 8 g of carbon powder to Si powder 28 g as an evaporation material, and nothing was put in the carbon evaporation source 32. In vapor deposition, the silicon evaporation source 31 was irradiated with an electron beam and heated to a temperature of 1800 ° C., and the carbon evaporation source 32 was not irradiated with an electron beam. Except for these points, a negative electrode active material body was formed on a current collector in the same manner as in Example 1 to obtain a sample negative electrode of Comparative Example 2.

(Vi) Comparative Example 3
A sample negative electrode of Comparative Example 3 was obtained in the same manner as Comparative Example 2 except that the silicon evaporation source 31 contained 20 g of silicon powder plus 8 g of carbon powder as the evaporation material.

(Vii) Comparative Example 4
A sample negative electrode of Comparative Example 4 was obtained in the same manner as Comparative Example 2 except that the silicon evaporation source 31 contained a material obtained by adding 20 g of silicon carbide powder to 20 g of silicon powder as the evaporation material.

(Viii) Comparative Example 5
A sample negative electrode of Comparative Example 5 was obtained in the same manner as Comparative Example 2, except that the silicon evaporation source 31 contained 12 g of silicon powder plus 60 g of silicon carbide powder as the evaporation material.

(Ix) Comparative Example 6
A sample negative electrode of Comparative Example 6 was obtained in the same manner as Comparative Example 2 except that silicon carbide powder was housed in the silicon evaporation source 31 as an evaporation material and the silicon carbide powder was heated to a temperature of 1400 ° C.

<Composition of negative electrode active material>
The amount of silicon contained in the negative electrode active material layers in Examples 1 to 3 and Comparative Examples 1 to 6 was measured by inductively coupled plasma spectroscopy, and the amount of carbon was measured by a combustion-infrared absorption method. From this result, the ratio (number of carbon atoms / number of silicon atoms) x of silicon and carbon constituting the negative electrode active material layer was calculated. The results are shown in Tables 1-3.

<Analysis of bonding state of carbon contained in negative electrode active material>
Subsequently, the carbon bonding state of the negative electrode active material layers of Examples 1 to 3 and Comparative Examples 2 to 6 was examined.

  For each negative electrode active material layer, the binding energy is measured by photoelectron spectroscopy, and the ratio R of the number of silicon-carbon bonds to the number of carbon-carbon bonds present in the negative electrode active material body R based on the measurement result R Was calculated. Hereinafter, the analysis method will be described more specifically.

  First, Ar was etched from the outermost surface of each negative electrode active material layer to a depth of 20 nm with Ar to expose the measurement surface. This is because components in the air or dust may adhere to the outermost surface of the negative electrode active material layer, which makes accurate measurement difficult.

  Next, using a photoelectron spectrometer (for example, PHI Quantera SXM manufactured by ULVAC PHI), the energy of photoelectrons emitted by irradiating the measurement surface with X-rays is measured, and the energy derived from carbon 1s orbital electrons is measured. Asked.

  FIG. 8A is a graph showing measurement results of energy derived from carbon 1s orbital electrons of the negative electrode active material layer of Example 1. FIG. The absolute value (horizontal axis) of the binding energy is a value obtained by calibrating the signal waveform peak derived from Ar2p orbital electrons as 243.3 eV.

  From the graph shown in FIG. 8, a peak near 284.7 eV derived from a carbon-carbon bond and a peak near 283.4 eV derived from a silicon-carbon bond showing energy smaller than this can be confirmed.

When the waveform shown in FIG. 8A is separated using two Voigt functions having peak coordinates of 284.7 eV and 283.4 eV, respectively, as shown in FIG. 8B, the waveform is derived from a carbon-carbon bond. Waveform W _CC and a waveform W _Si-C derived from a silicon-carbon bond were obtained. The area ratio of these waveforms, that is, the ratio of the area of the waveform W _{Si-C to} the area of the waveform W _CC was calculated and used as the ratio R of the number of bonds.

FIGS. 9A and 9B are graphs showing measurement results of energy derived from carbon 1s orbital electrons of the negative electrode active material layers of Example 3 and Comparative Example 4, respectively. Also in these examples, waveform separation is performed in the same manner as described above, and the area ratio of the two separated waveforms (the area of the waveform W _Si-C / the area of the waveform W _CC ) is calculated, and the ratio of the number of couplings R.

  Although not shown, for other examples and comparative examples, the ratio R of the number of bonds was determined by photoelectron spectroscopic analysis in the same manner as described above.

  Tables 1 to 3 show the values of the obtained ratio R of the number of bonds. As can be seen from these results, 0 <R <2.5 in Examples 1 to 3, but R was 2.5 or more in Comparative Examples 2 to 6. Therefore, when silicon and carbon are contained in separate containers and heated at a temperature equal to or higher than their melting points, the carbon and silicon carbide powders mixed with silicon and carbon are heated more than when heated at the deposition temperature of silicon. -It was confirmed that an active material layer having a high carbon bond ratio was formed. In Comparative Example 1, R was 0.04. This is considered to be because the ratio of carbon to silicon (composition ratio x) is high.

<Method for producing sample cell for evaluation>
The sample negative electrodes of Examples and Comparative Examples prepared by the above methods were cut into circles each having a diameter of approximately 11.2 mm to obtain cell negative electrodes. A sample cell for evaluation having the configuration described above with reference to FIG. 5 was produced using the negative electrode for each cell. In each sample cell, a metal lithium plate having a diameter of 13 mm was used as the positive electrode. Further, as an electrolytic solution, 1 mol of LiPF6 was dissolved in a mixed solvent of 30% by volume of ethylene carbonate, 50% by volume of methyl ethyl carbonate, and 20% by volume of diethyl carbonate to adjust to 1 liter. Was used.

<Evaluation method and result of sample cell>
(I) Evaluation of sample cells of Examples 1 to 3 and Comparative Examples 1 to 6 The sample cells of Examples and Comparative Examples obtained by the above methods were subjected to charge / discharge cycle tests under the following conditions, and the number of cycles. And the capacity retention rate were measured.

  In the charge / discharge cycle test, the charge / discharge modes 1 and 2 shown in Table 4 were carried out in this order to obtain one cycle, and this was repeated. In each cycle, the amount of electricity until the end voltage is reached by charging or discharging in mode 1 and mode 2 is measured, and the sum of these amounts of electricity is taken as the measured charge capacity or measured discharge capacity in that cycle. . In addition, the capacity retention rate was calculated based on the measured values of these capacities. Here, the “capacity maintenance ratio” refers to the ratio of the discharge capacity in each cycle to the reference capacity, with the maximum value of the actually measured charge capacity observed in the charge / discharge cycle test as the reference capacity.

  Tables 1 to 3 show the capacity retention ratio of each sample for each cycle. In Tables 1 to 3, the numerical value in the “Charge” column indicates the charge capacity density per volume, and the numerical value in the “Discharge” column indicates the discharge capacity density per volume.

  Further, FIG. 6 shows the relationship between the number of cycles and the capacity retention rate in the examples and comparative examples.

  As can be seen from the results of Tables 1 to 3 and FIG. 6, in Examples 1 to 3, the capacity retention rate at the fifth cycle is 80% or more, which is higher than any comparative example. This is because the aggregate function by the carbon-carbon bond is sufficiently functioning by setting the ratio R of the number of silicon-carbon bonds to the number of carbon-carbon bonds in the range of 0 <R <2.5. I think that the.

  In particular, in Example 3, the decrease in the capacity maintenance rate is greatly suppressed. This is presumably because the ratio of carbon-carbon bonds is higher than in the other examples.

  In Comparative Examples 2 to 5, the capacity retention rate is relatively high, but the rate of decrease for each cycle is large. This is probably because the carbon-carbon bond ratio is low (R ≧ 2.5), and the aggregate function due to the carbon-carbon bond is not exhibited.

  In Comparative Example 1, the carbon atom content was about twice as high as that of silicon atoms, so the charge / discharge mechanism was not clear, but the subsequent discharge capacity density was about 40% of the charge capacity density. Considering this, there is a possibility that the charge capacity density in the first cycle reflects a value different from the storage amount of lithium.

  In Comparative Example 6, since the negative electrode active material body contains more SiC that does not release even when lithium is occluded than in other Examples and Comparative Examples, the capacity maintenance rate at the first cycle is considerably low. . In the subsequent charge / discharge cycle, the capacity retention rate is further greatly reduced. This is probably because the carbon constituting the carbon-carbon bond is small and the aggregate function of carbon is not fully exhibited.

  The negative electrode of the present invention can be applied to various forms of lithium secondary batteries, but is particularly advantageous when applied to lithium secondary batteries that require high charge / discharge cycle characteristics. It is also useful as an electrode plate of a lithium ion migration type electrochemical capacitor.

  The shape of the lithium secondary battery to which the present invention is applicable is not particularly limited, and may be any shape such as a coin shape, a button shape, a sheet shape, a cylindrical shape, a flat shape, and a square shape. Further, the form of the electrode plate group including the positive electrode, the negative electrode, and the separator may be a wound type or a laminated type. Further, the size of the battery may be a small size used for a small portable device or the like, or a large size used for an electric vehicle or the like. The lithium secondary battery according to the present invention can be used for a power source of, for example, a portable information terminal, a portable electronic device, a small electric power storage device for home use, a motorcycle, an electric vehicle, a hybrid electric vehicle, etc., but the application is not particularly limited.

DESCRIPTION OF SYMBOLS 1,11 Current collector 2,12 Negative electrode active material body 3 Protrusion part 100,200 Negative electrode 300 Vapor deposition apparatus 30 Chamber 31 Silicon evaporation source 32 Carbon evaporation source 35 Heater for collector heating 36 Rate monitor for silicon evaporation rate measurement 37 Carbon Evaporation rate measurement rate monitor 38 Shutter 40 Fixing table 45 Horizontal surface 500 Plasma CVD apparatus 51a Plasma emission part of Penning discharge type plasma gun 51b Plasma reflection part of Penning discharge type plasma gun 52 Plasma reflection plate 53 Fixing base (substrate holder)
54 Pulse power supply 55 Heater for current collector heating 60 Coin type battery 61 Positive electrode case 62 Positive electrode 63 Separator 64 Negative electrode 65 Gasket 66 Sealing plate 600 Coin type lithium ion secondary battery

Claims (17)

A negative electrode for a lithium secondary battery comprising a current collector and a negative electrode active material layer formed on a surface of the current collector,
The negative electrode active material layer is mainly composed of silicon and carbon,
The ratio x of the number of carbon atoms to silicon in the negative electrode active material layer is 0.03 or more and 1.0 or less,
The negative electrode for a lithium secondary battery, wherein a ratio R of the number of silicon-carbon bonds to the number of carbon-carbon bonds in the negative electrode active material layer is greater than 0 and less than 2.5.   2. The negative electrode for a lithium secondary battery according to claim 1, wherein a ratio R of the number of silicon-carbon bonds to the number of carbon-carbon bonds is 1.0 or less.   3. The negative electrode for a lithium secondary battery according to claim 1, wherein the ratio R of the number of silicon-carbon bonds to the number of carbon-carbon bonds is a value determined by photoelectron spectroscopy.   4. The negative electrode for a lithium secondary battery according to claim 1, wherein a ratio x of the number of carbon atoms to silicon in the negative electrode active material layer is larger than 0.18. 5. A negative electrode for a lithium secondary battery comprising a current collector and a negative electrode active material layer formed on a surface of the current collector,
The negative electrode active material layer is a deposited film containing silicon and carbon,
The vapor deposition film is provided separately from the first evaporation source for evaporating the negative electrode active material mainly composed of silicon, and the second evaporation for evaporating the carbon material mainly composed of carbon. A negative electrode for a lithium secondary battery, which is formed by evaporating silicon and carbon from the first and second evaporation sources and making them enter the surface of the current collector. A method for producing a negative electrode for a lithium secondary battery comprising a current collector and a negative electrode active material layer formed on a surface of the current collector,
A negative electrode containing silicon and carbon on the surface of the current collector by evaporating a negative electrode active material comprising silicon as a main component and a carbon material containing carbon as a component and allowing the material to enter the surface of the current collector Including a step of depositing an active material layer,
The step of depositing the negative electrode active material layer uses a first evaporation source that evaporates the negative electrode active material and a second evaporation source that is provided separately from the first evaporation source and evaporates the carbon material. The manufacturing method of the negative electrode for lithium secondary batteries performed.   In the step of depositing the negative electrode active material layer, the negative electrode active material of the first evaporation source is heated to a temperature of 1600 ° C. or higher and 2200 ° C. or lower, and the carbon material of the second evaporation source is heated to 2600 ° C. or higher and 3200 ° C. The manufacturing method of the negative electrode for lithium secondary batteries of Claim 6 heated to the following temperature.   The method for producing a negative electrode for a lithium secondary battery according to claim 6 or 7, wherein, in the step of depositing the negative electrode active material layer, the silicon and carbon are simultaneously evaporated.   The method for producing a negative electrode for a lithium secondary battery according to claim 6, wherein the carbon material is graphite.   The method for producing a negative electrode for a lithium secondary battery according to claim 6, wherein the negative electrode active material and the carbon material are evaporated by irradiating the negative electrode active material and the carbon material with an electron beam. .   11. The lithium secondary battery according to claim 6, wherein a ratio R of the number of silicon-carbon bonds to the number of carbon-carbon bonds in the negative electrode active material layer is greater than 0 and less than 2.5. Manufacturing method for negative electrode.   The method for producing a negative electrode for a lithium secondary battery according to claim 11, wherein a ratio R of the number of silicon-carbon bonds to the number of carbon-carbon bonds is 1.0 or less.   The method for producing a negative electrode for a lithium secondary battery according to claim 11 or 12, wherein the number R of silicon-carbon bonds relative to the number of carbon-carbon bonds is a value determined by photoelectron spectroscopy.   14. The method for producing a negative electrode for a lithium secondary battery according to claim 6, wherein the ratio x of the number of carbon atoms to silicon in the negative electrode active material layer is 0.03 or more and 1.0 or less.   The method for producing a negative electrode for a lithium secondary battery according to claim 14, wherein the ratio x of the number of carbon atoms to silicon in the negative electrode active material layer is greater than 0.18.   The negative electrode for lithium secondary batteries manufactured by the method in any one of Claim 6 to 15. A positive electrode capable of inserting and extracting lithium ions;
A negative electrode for a lithium secondary battery according to any one of claims 1 to 5 and 16,
A separator disposed between the positive electrode and the negative electrode for a lithium secondary battery;
A lithium secondary battery comprising an electrolyte having lithium ion conductivity.

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