HK1138942B - Method - Google Patents

Description

The invention relates to a method of fabricating a composite electrode comprising an active component directly bonded to a current collector and its use as the active anode material in rechargeable lithium battery cells.

Lithium battery cells, as illustrated in figure 1, are well known in the art. A battery cell generally comprises a copper current collector for the anode 10 and an aluminium current collector for the cathode 12 which are externally connectable to a load or to a recharging source as appropriate. A graphite-based composite anode layer 14 overlays the current collector 10 and a lithium containing metal oxide-based composite cathode layer 16 overlays the current collector 12. A porous plastic spacer or separator 20 is provided between the graphite-based composite anode layer 14 and the lithium containing metal oxide-based composite cathode layer 16 and a liquid electrolyte material is dispersed within the porous plastic spacer or separator 20, the composite anode layer 14 and the composite cathode layer 16. In some cases, the porous plastic spacer or separator 20 may be replaced by a polymer electrolyte material and in such cases the polymer electrolyte material is present within both the composite anode layer 14 and the composite cathode layer 16.

A number of investigations have been carried out using silicon as the active anode material of a rechargeable lithium-ion electrochemical cell (see, for example, Insertion Electrode Materials for Rechargeable Lithium Batteries, M. Winter, J. O. Besenhard, M. E. Spahr, and P.Novak in Adv. Mater. 1998, 10, No. 10). The silicon anode would replace the graphite-based anode electrode in the conventional lithium-ion rechargeable battery cell illustrated in Fig. 1 The battery cell includes a single cell but may also include more than one cell. The silicon electrode structure generally utilises powdered silicon, evaporated silicon or fibres of silicon which are attached onto a current collector with a polymer binder and an electronic additive by a solvent casting process to form a composite electrode structure.

It will be appreciated that the formation of a conventional composite electrode requires multiple components hence increasing the cost and complexity of the production of the electrode. Furthermore, the use of a solvent casting process results in the formation of waste solvent, which requires costly disposal. It will be further appreciated that, in order to allow the battery to deliver high levels of power, it is very important to create a low resistance contact between the active material and the current collector and this can be hard to achieve in a composite film electrode structure. In addition, the low resistance contacts will be preserved during the volume changes induced by charging and discharging the electrode in an electrochemical cell.

JP 2003 168426 A discloses a nanaqueous electrolyte secondary battery which uses silicon for the negative electrode active material.

JP 2003 017040 A discloses a manufacturing method for the electrode of lithium secondary batteries comprising etching a void in the thickness direction of an active material thin film.

US 2004/224231 discloses an electrode for a rechargeable lithium battery which includes a thin film composed of native material characterised in that the current collector has irregularities on its surface.

The present invention provides an improved method of composite electrode production. In particular, the first aspect of the invention provides an electrode comprising an active component directly bonded to a current collector. The direct bonding provides a low resistance contact between the current collector and the active material.

It will be appreciated that the electrode of the first aspect does not require the use of a polymer binder and an electronic additive. Instead the active component is directly bonded to the current collector by the formation of a compound between the active component and the current collector. Furthermore, the production of the composite electrode is not carried out using a solvent casting process thus avoiding the use of solvent and the need to dispose of waste solvent.

For the purposes of the present invention, the active component and the current collector are directly bonded such that a physical and/or chemical interaction occurs between the active component and the current collector. In particular, the physical and/or chemical interaction occurs at the interface between the active component and the current collector (i.e. at the contacting surfaces of the active component and the current collector). The direct bonding of the active component and the current collector results in a permanent or temporary attachment. The bonding can therefore be irreversible or reversible. The bonding can result in an interaction at an atomic or molecular level between the active component and the current collector. In particular, the interaction may result in chemical bonding between the active component and the current collector such as covalent, ionic, van der Waals bonding or the formation of an alloy, such as a metal alloy. The direct bonding causes the active component and the current collector to adhere to each other. It will be appreciated that the direct bonding causes a physical and/or chemical change in the contacting surfaces of the active component and/or the current collector.

The direct bonding of the active component and the current collector can result in the formation of a compound between the active component and the current collector. In this case, the direct bonding of the active component and the current collector is via the compound formed between the active component and the current collector. The compound therefore has two functions. It causes the adherence of the active component to the current collector and allows electrons to be conducted therethrough. The compound therefore provides a high conductivity and low resistance binding between the current collector and the active component.

The active component of the electrode preferably comprises one or more of silicon, aluminium, tin, lead, bismuth, antimony or silver. The current collector of the electrode preferably comprises one or more of copper, nickel, aluminium, iron, iridium, gold, silver or titanium. The electrode of the first aspect can therefore comprise for example, an active component comprising silicon directly bonded via a silicon-copper compound to a copper current collector.

The active component comprises silicon-comprising fibres. In a particular feature of the invention, the silicon-comprising fibres are single crystal fibres.

The silicon comprising fibres may have transverse dimensions in the range 0.08 to 0.5 microns, preferably 0.2 microns and a length in the range 12 to 300 microns preferably 100 microns. The fibres may have an aspect ratio of about 250:1. The fibres may have a substantially circular cross-section or a substantially non-circular cross-section.

The electrode of the first aspect is preferably provided as an anode.

There is also provided a method of fabricating a composite electrode according to the first aspect of the invention comprising contacting an active component with a current collector and heating to the reaction temperature of the active component and current collector. Alternatively, the active component and the current collector are heated to above their reaction temperature.

For the purposes of this invention, the reaction temperature is the temperature required to form a direct bond between the active component and the current collector, in particular to form to a compound between the active component and the current collector. It will be appreciated that the required temperature will vary depending on the identity (i.e. the chemical composition) of the active component and the current collector.

The method may require that the active component and the current collector are brought into contact. Preferably the active component and the current collector are contacted under an excess pressure of 34.5 kPa (5 pounds per square inch) or above, more preferably 506.6 kPa (5 atmospheres) or above.

It will be appreciated that it is advisable to avoid the formation of excess metal oxides at the contacting surfaces of the active component and/or the current collector. The method may therefore preferably carried out in an inert atmosphere, such as nitrogen, argon etc or in vacuo.

In a preferred feature there is provided a method of fabricating an electrode comprising contacting a silicon comprising active component with a copper current collector and heating to or above the silicon-copper alloy temperature.

The fibres may be free or attached to a support. The fibres of the present invention can be created from a substrate (e.g. a chip or a wafer) comprising silicon by reactive ion etching, chemical reaction etching or galvanic exchange etching and, where applicable, detached by one or more of scraping, agitating or chemical etching.

The silicon-comprising active component can comprise undoped silicon, doped silicon or a silicon germanium mixture. The fibre can be single crystal silicon or single crystal silicon germanium. The fibres may extend from silicon-comprising particles.

The fibres of silicon can be contacted on the current collector in a composite or felt or a felt-like structure, that is a plurality of elongate or long thin fibres which crossover to provide multiple intersections, for example by being laid down in a random or disordered or indeed ordered manner. The arrangement of the active component in this fashion, reduces the problem of charge/discharge capacity loss. Typically the fibres will have a length to diameter ratio of approximately 100:1 and hence in an anode layer such as a composite anode layer, each fibre will contact other fibres many times along their length giving rise to a configuration where the chance of mechanical isolation arising from broken silicon contacts is negligible. The insertion and removal of lithium into the fibres, although causing volume expansion and volume contraction, does not cause the fibres to be destroyed and hence the intra-fibre electronic conductivity is preserved.

The active component and the current collector are brought into contact and form an interaction at the contacting surfaces. The interaction results in a change in the structure and/or composition of the active component and/or the current collector at the contacting surfaces, such that the active component and the current collector are bonded together. This bonding can be irreversible or reversible. Preferably, the active component and the current collector form a metal alloy at the contacting surfaces. The invention does not encompass merely contacting the active component and the current collector (i.e. by plating or depositing the active component on the current collector) and hence creates a very low electrical resistance pathway between the current collector and the active material.

There is also provided a method of fabricating a lithium rechargeable cell comprising the steps of creating an anode according to the second aspect of the invention and adding a cathode and an electrolyte. The method of the third aspect may further comprise adding a separator between cathode and the anode, and additionally further providing a casing around the cell.

There is further provided an electrochemical cell containing an anode according to the present invention. The cathode may comprise a lithium-containing compound capable of releasing and reabsorbing lithium ions as its active material. More particularly, there is provided an electrochemical cell in which the cathode comprises lithium-based metal oxide or phosphate as its active material preferably LiCoO₂ or LiMn_xNi_xCo_1-2xO₂ or LiFePO₄

There is also provided a lithium rechargeable cell anode comprising an active component directly bonded to a current collector.

Furthermore, there is provided a cell comprising an anode as described in the fifth aspect of the invention and a cathode. In particular, the cathode comprises lithium-based material, preferably the cathode comprises lithium cobalt dioxide.

A device may be powered by a cell as described above.

The present invention is illustrated with reference to one or more of the following non-limiting examples:

• The silicon fibres may be manufactured by detaching pillars from a substrate such as a chip or a wafer. In addition the manner of fabrication of the pillars may be
• provided by a simple repeatable chemical process.

One manner in which the pillars can be made is by dry etching, for example deep reactive ion etching of the type, for example, described in US application number 10/049736 which is commonly assigned herewith. The skilled person will be familiar with the process such that detailed description is not required here. Briefly, however, a silicon substrate coated in native oxide is etched and washed so as to give a hydrophilic surface. Caesium chloride (CsCl) is evaporated on the surface and the coated substrate is transferred under dry conditions to a chamber of fixed water vapour pressure. A thin film of CsCl develops into an island array of hemispheres whose dimensional characteristics depend on initial thickness, water vapour pressure and time of development. The island array provides an effective mask after which etching is carried out for example by reactive ion etching leaving an array of pillars corresponding to the hemispherical islands. The CsCl resist layer is highly soluble in water and can be readily washed away.

Alternatively the pillars can be made by wet etching/using a chemical galvanic exchange method for example as described in our co-pending application GB 0601318.9 with common assignees and entitled "Method of etching a silicon-based material,". A related method which may also be used has been disclosed in Peng K-Q, Yan, Y-J Gao, S-P, Zhu J., Adv. Materials, 14 (2004), 1164-1167 ("Peng"); K. Peng et al, Angew. Chem. Int. Ed., 44 2737-2742; and K. Peng et al., Adv. Funct. Mater., 16 (2006), 387.

In the preferred embodiment pillars of for example 100 microns in length and 0.2 microns in diameter are fabricated on and from a silicon substrate. More generally pillars of length in the range of 12 to 300 microns and diameter or largest transverse dimension in the range of 0.08 to 0.5 microns may be used to provide the fibres. According to the process the silicon substrate may be n- or p- type and, according to the chemical approach, and may be etched on any exposed (100) or (110) crystal face. Since the etching proceeds along crystal planes, the resulting fibres are single crystals. Because of this structural feature, the fibres will be substantially straight facilitating length to diameter ratio of approximately 100:1 1 and, when in a composite anode layer, allowing each fibre to contact other fibres many times along their length. The etching process can also be carried out either on very large scale integration (VLSI) electronic grade wafers or rejected samples of the same (single crystal wafers). As a cheaper alternative, photovoltaic grade polycrystalline material, as used for solar panels, may also be used.

In order to detach the pillars to obtain the fibres, the substrate, with pillars attached, is placed in a beaker or any appropriate container, covered in an inert liquid such as ethanol and subjected to ultra-sonic agitation. It is found that within several minutes the liquid is seen to be turbid and it can be seen by electron microscope examination that at this stage the pillars have been removed from their silicon base.

It will be appreciated that alternative methods for "harvesting" the pillars include scraping the substrate surface to detach them or detaching them chemically. One chemical approach appropriate to n-type silicon material comprises etching the substrate in an HF solution in the presence of backside illumination of the silicon wafer.

Once harvested, silicon fibres were washed in a dilute HF (1%) solution for several minutes. Water was removed by filtration, but sufficient water was retained to keep the silicon fibres wet. The fibres were dispersed on the copper foil (current collector). The silicon fibres coated copper substrate was then transferred into a Silicon Bonder chamber. At the vacuum of ca 2x 10^-6 mbar, a temperature of 400°C and a pressure of 5 atmospheres, the silicon fibres annealed with copper for about 1 hour.

Alternatively dry and clean silicon fibres were placed on a copper substrate. The sample was transferred into a Rapid Thermal Annealing chamber and annealed in air or low flowing Argon ambient for 10 s at 800°C. After cooling, the sample was annealed in H₂/N₂(10%H₂) for 10 s at 800°C to reduce the oxide.

Alternatively silicon fibres were coated onto a copper current collector to create a felt or felt-like structure and the sample passed through heated rollers to provide the required temperature and pressure to create the silicon-copper compound.

Figure 2 illustrates a cycle number plot for an electrode produced using a Silicon Bonder chamber as described above. The plot shows the capacity of the electrode with repeated cycles of lithiation and delithiation.

A particular advantage of the approach described herein is that large sheets of silicon-based anode can be fabricated, rolled if necessary, and then slit or stamped out subsequently as is currently the case in graphite-based anodes for lithium-ion battery cells meaning that the approach described herein can be retrofitted with the existing manufacturing capability.

Fabrication of the lithium-ion battery cell can be carried out in any appropriate manner, for example following the general structure shown in Figure 1 but with for example, a silicon or silicon-based active anode material rather than a graphite active anode material. For example, the silicon fibres-based composite anode layer is covered by the porous spacer, the electrolyte added to the final structure saturating all the available pore volume. The electrolyte addition is done after placing the electrodes in an appropriate casing and may include vacuum filling of the anode to ensure the pore volume is filled with the liquid electrolyte.

It will be appreciated, of course, that any appropriate approach can be adopted in order to arrive at the approaches and apparatus described above. For example the pillar detaching operation can comprise any of a shaking, scraping, chemical or other operation as long as pillars are removed from the substrate to create fibres. Reference to silicon-based material includes silicon where appropriate. The fibres can have any appropriate dimension and can for example be pure silicon or doped silicon or other silicon-based material such as a silicon-germanium mixture or any other appropriate mixture. The substrate from which pillars are created may be n- or p- type, ranging from 100 to 0.001 Ohm cm, or it may be a suitable alloy of silicon, for example Si_xGe_1-x. The fibres may be grown on particles of silicon such that they do not have to be detached. The fibres can be used for any appropriate purpose such as fabrication of electrodes generally including cathodes. The cathode material can be of any appropriate material, typically a lithium-based metal oxide or phosphate material such as LiCoO₂, LiMn_xNi_xCo_1-2xO₂ or LiFePO₄. The features of different embodiments can be interchanged or juxtaposed as appropriate and the method steps performed in any appropriate order.

Claims (16)

1. An electrode comprising an active component directly bonded to a current collector, wherein the active component comprises silicon-comprising fibres, arranged in a felt-like structure, and the direct bonding is via a compound formed from the active component and current collector at the contacting surfaces of the active component and current collector.
2. The electrode as claimed in claim 1 wherein the active component comprises one or more of silicon, aluminium, tin, lead, bismuth, antimony and silver, and/or wherein the current collector comprises one or more of copper, nickel, aluminium, iron, iridium, gold, silver or titanium.
3. The electrode as claimed in any preceding claim in which the fibres are single crystal fibres.
4. The electrode as claimed in any one of claims 1 to 3 wherein at least some of the fibres extend from silicon-comprising particles.
5. The electrode as claimed in any preceding claim wherein the fibres comprise n-type or p-type doped silicon.
6. The electrode as claimed in any preceding claim wherein the electrode is a composite anode.
7. An electrochemical cell comprising the electrode as claimed in any preceding claim.
8. A lithium-ion battery comprising the electrode as claimed in any of claims 1 to 6 and further comprising a cathode and electrolyte.
9. A device powered by the electrochemical cell as claimed in claim 7 or the battery as claimed in claim 8.
10. A method of fabricating an electrode comprising the steps of: depositing silicon-comprising fibres as an active component on a current collector; and heating to at least the reaction temperature of the active component and current collector until the silicon-comprising fibres directly bond to the current collector via a compound formed from the silicon-comprising fibres and current collector at the contacting surfaces of the active component and current collector.
11. The method of claim 10 wherein the silicon-comprising fibres are deposited on the current collector as a felt.
12. The method as claimed in claim 10 or 11 wherein the active component and the current collector are contacted under an excess pressure of 34.5 kPa (5 pounds per square inch) or above.
13. The method as claimed in any one of claims 10 to 12 wherein the formation of the compound is carried out in an inert atmosphere.
14. The method as claimed in any one of claims 10 to 13 wherein the active component comprises silicon-comprising fibres and the current collector comprises copper and the method comprises heating the active component and current collector to at least the silicon-copper alloy temperature.
15. The method of any of claims 10 to 14 further comprising the steps of etching a silicon-comprising substrate to form pillars and detaching the pillars from the silicon-comprising substrate to form the silicon-comprising fibres.
16. The method of any of claims 10 to 15 further comprising the step of arranging the electrode with a cathode and electrolyte to form a lithium-ion battery.