CA2636471C

CA2636471C - A method of fabricating fibres composed of silicon or a silicon-based material and their use in lithium rechargeable batteries

Info

Publication number: CA2636471C
Application number: CA2636471A
Authority: CA
Inventors: Mino Green; Feng-Ming Liu
Original assignee: Nexeon Ltd
Current assignee: Nexeon Ltd
Priority date: 2006-01-23
Filing date: 2007-01-23
Publication date: 2016-12-13
Anticipated expiration: 2027-01-23
Also published as: CN101371381B; HK1118959A1; KR101024900B1; RU2444092C2; US20120100427A1; TWI513083B; CN103560226A; IL192940A0; US9583762B2; GB0601319D0; EP1982370B1; US8101298B2; US20100151324A1; BRPI0706946A2; JP5752509B2; EP2472653A3; JP2011216496A; TWI398980B; CN103560226B; KR20080111440A

Abstract

A method of fabricating fibres of silicon or silicon-based material comprises the steps of etching pillars on a substrate and detaching them. A battery anode can then be created by using the fibres as the active material in a composite anode electrode.

Description

A METHOD OF FABRICATING FIBRES COMPOSED OF SILICON OR A
SILICON-BASED MATERIAL AND THEIR USE IN LITHIUM
RECHARGEABLE BATTERIES.
The invention relates to a method of fabricating fibres composed of silicon or a silicon-based material and their use the active anode material in rechargeable lithium battery cells.
It is well known that silicon can be used 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. 0.
Besenhard, M. E. Spahr, and P.Novak in Adv. Mater. 1998, 10, No. 10). The basic composition of a conventional lithium-ion rechargeable battery cell is shown in Fig. 1 including a graphite-based anode electrode, the component to be replaced by the silicon-based anode. The battery cell includes a single cell but may also include more than one cell.
The battery cell generally comprises a copper current collector for the anode 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 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

2 such cases the polymer electrolyte material is present within both the composite anode layer 14 and the composite cathode layer 16.
When the battery cell is fully charged, lithium has been transported from the lithium containing metal oxide via the electrolyte into the graphite-based layer where it reacts with the graphite to create the compound, LiC6. The graphite, being the electrochemically active material in the composite anode layer, has a maximum capacity of 372 mAh/g. It will be noted that the terms "anode" and "cathode" are used in the sense that the battery is placed across a load.
It is generally believed that silicon, when used as an active anode material in a lithium-ion rechargeable cell, provides a significantly higher capacity than the currently used graphite. Silicon when converted to the compound Lin Si5 by reaction with lithium in an electrochemical cell, has a capacity of 4,200 mAh/g.
Existing approaches of using a silicon or silicon-based active anode material in a lithium-ion electrochemical cell have failed to show sustained capacity over the required number of charge/discharge cycles and are thus not commercially viable.
One approach uses silicon in the form of a powder, in some instances made into a composite with optionally an electronic additive and containing an appropriate binder such as polyvinylidene difluoride coated onto a copper current collector. However, this electrode fails to show sustained capacity when subjected to charge/discharge cycles. It is believed that this capacity loss is due to partial mechanical isolation of the silicon powder mass arising from the volumetric expansion/contraction associated with lithium insertion/extraction into and from the host silicon. In turn this gives rise to agglomeration of the powder mass in electrically isolated "islands".

3 In another approach described by Ohara et al. in Journal of Power Sources 136 (2004) 303-306 silicon is evaporated onto a nickel foil current collector as a thin film and this structure is then used to form the anode of a lithium-ion cell.
However, although this approach gives good capacity retention, this is only the case for very thin films and thus these structures do not give usable amounts of capacity per unit area and increasing the film thickness to give usable amounts of capacity per unit area causes the good capacity retention to be eliminated.
In another approach described in US2004/0126659, silicon is evaporated onto nickel fibres which are then used to form the anode of a lithium battery.
However this is found to provide an uneven distribution of silicon on the nickel fibres hence significantly affecting operation.
In another approach described in US 6,887,511, silicon is evaporated onto a roughened copper substrate to create medium-thickness films of up to 10 um.
During the initial lithium ion insertion process, the silicon film breaks up to form pillars of silicon. These pillars can then reversibly react with lithium ions and good capacity retention is achieved. However, the process does not function well with thicker film and the creation of the medium-thickness film is an expensive process. Furthermore the pillared structure caused by the break up of the film has no inherent porosity such that issues may arise with long term capacity retention.
In the present invention, due to the fact that the anode electrode structure uses fibres of silicon or silicon-based material, the problems of reversibly reacting these silicon or silicon-based fibres with lithium are overcome. In particular by arranging the fibres in a composite structure, that is a mixture of fibres a polymer binder and an electronic additive, the charge/discharge process

4 becomes reversible and repeatable and good capacity retention is achieved. In addition the manner in which the fibres are laid can provide advantages. By providing a dis-ordered non-woven mat of fibres, a fully reversible and repeatable charging capability is introduced without risk of significant mechanical isolation.
For example the fibres may be deposited as a felt or felt-like structure. In the case of a composite structure this can be with the additional components, or the felt can be with a simple binder or, where structurally appropriate, loose.
Furthermore, a simplified method of fabricating fibres is provided comprising etching a substrate to produce pillars and detaching the pillars providing a robust and high-yield approach.
In accordance with an aspect of the present invention, there is provided a method of fabricating fibres of silicon or a silicon-based material comprising the steps of etching a silicon substrate or a silicon-based substrate to make pillars and detaching the pillars from the substrate.
In accordance with another aspect of the present invention, there is provided a method of creating a cell electrode comprising depositing a slurry containing silicon-based fibres to form a layer of silicon-based fibres.
In accordance with another aspect of the present invention, there is provided a lithium rechargeable cell anode comprising a layer of silicon-based single crystal fibres in which the fibres form a felt.
Embodiments of the invention will now be described, by way of example, with reference to the figures, of which:
Fig. 1 is a schematic diagram showing the components of a battery cell;
Fig. 2 is a magnified photograph of an electrode according to the present invention;
Fig.3 shows a first cycle voltage plot for a silicon fibre/PVDF/Super P
composite electrode.
In overview the invention allows creation of fibres or hairs of silicon or silicon-based material and the use of these fibres to create both a composite anode 4a structure with a polymer binder, an electronic additive (if required) and a metal foil current collector and a felt-like electrode structure. In particular it is believed that the structure of the silicon elements that make up the composite overcomes the problem of charge/discharge capacity loss.
In accordance with a further aspect of the present invention, there is provided a method of fabricating a rechargeable lithium cell electrode comprising fibres of silicon or a silicon-based material, the method comprising the steps of:
etching a silicon substrate or a silicon-based substrate to make pillars; detaching the pillars from the substrate to obtain fibres; and forming an electrode, wherein the fibres are an active material of the electrode.
In accordance with a further aspect of the present invention, there is provided an electrode structure for a lithium rechargeable cell, the electrode structure comprising a plurality of elongate silicon-comprising fibres, in which said silicon-comprising fibres crossover to provide multiple intersections, wherein the intersections comprise a disrupted crystalline structure, optionally, wherein the disrupted crystalline structure is an amorphous structure.
In accordance with a further aspect of the present invention, there is provided an electrode for a lithium rechargeable cell, the electrode comprising an electrically interconnected mass comprising: elongated silicon comprising fibres, wherein the elongated silicon comprising fibres are for being reversibly charged and discharged, and at least some of the elongated silicon comprising fibres cross over each other to provide intersections and a porous structure; and at least one of a binder and an electronic additive; wherein the elongated silicon comprising fibres and the at least one of the binder and the electronic additive cooperate to define a porous composite electrode layer.
In accordance with a further aspect of the present invention, there is provided an electrode for a lithium rechargeable, the electrode comprising an electrically interconnected mass comprising: elongated silicon comprising fibres, wherein the elongated fibres are electrochemically active such that they are for lithium 4b insertion and removal, and at least some of the elongated fibres cross over each other to provide intersections and a porous structure; and wherein the electrically interconnected mass has a percentage pore volume of about 10 to 30 per cent.
In accordance with a further aspect of the present invention, there is provided an electrode for a lithium rechargeable cell, the electrode comprising:elongated silicon comprising fibres, wherein at least some of the elongated fibres cross over each other to provide intersections and a porous structure; and wherein the intersections are disrupted crystalline forming an amorphous structure which welds the elongated fibres to one another.

By laying down the fibres 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 problem of charge/discharge capacity loss is

5 reduced.
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. Also, 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 fibres may be manufactured by detaching pillars from a substrate. 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. 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 (CsC1) 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 CsC1 develops into an island array of hemispheres whose dimensional characteristics depend on initial thickness,

6 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 CsC1 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. 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 at, Angew. Chem. Int. Ed., 44 2737-2742; and K. Peng et at, Adv. Funct. Mater., 16 (2006), 387-394.
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 20 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 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

7 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 the silicon pillars have been detached they can be used as the active material in a composite anode for lithium-ion electrochemical cells. To fabricate a composite anode, the harvested silicon is filtered from solution and can be mixed with polyvinylidene difluoride and made into a slurry with a casting solvent such as n-methyl pyrrolidinone. This slurry can then be applied or coated onto a metal plate or metal foil or other conducting substrate for example physically with a blade or in any other appropriate manner to yield a coated film of the required thickness and the casting solvent is then evaporated from this film using an appropriate drying system which may employ elevated temperatures in the range of 50 degrees C to 140 degrees C to leave the composite film free or substantially from casting solvent. The resulting mat or composite film has a porous and/or felt-like structure in which the mass of silicon fibres is typically between 70 percent and 95 percent. The composite

8 film will have a percentage pore volume of 10-30 percent, preferably about 20 percent.
An SEM of a composite electrode structure obtained by the method set out above is shown in Fig. 2. Alternatively a felt or felt-like structure may be produced as a sheet material (not necessarily on a current collector) and act as its own current collector.
Fabrication of the lithium-ion battery cell thereafter can be carried out in any appropriate manner for example following the general structure shown in Fig. 1 but with 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 18, 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.
Please see the following examples:
0.0140 g of silicon fibres were weighed out into a 2 cm2 Eppendorf centrifuge tube, and 0.0167 g of Super P conductive carbon was added. N-methyl pyrrolidinone (NMP) was then pipetted into the tube, until all the materials were dispersed (0.92 g). Previously, PVDF had been dissolved in NMP, at 7.8 wt % PVDF. A quantity of this solution was added to the tube, containing 0.0074 g of PVDF. The mix composition was thus Si : PVDF: Super P = 85.3:
4.5: 10.1 wt%.

9 The tube was placed in an ultrasonic bath for one hour, to homogenise the mixture, and then stirred for a further hour. The slurry was then coated onto pm copper foil, using a doctor blade with a blade gap of 0.8 mm. The coating was then dried in an oven at 100 C for one hour, to evaporate the NMP solvent.
After drying, the thickness of the coated layer was 30 ¨ 40 pm. Figure 2 shows an SEM of a similar mix and coating, with no Super P carbon.
The coating was lightly rolled, and then electrode disks were cut out with a diameter of 12 mm. These were assembled into electrochemical cells in an argon filled glove box. The counter electrode and reference electrode were both lithium metal. The electrolyte was LiPF6 in a mixture of organic carbonates.
The cell was tested on a VMP3 device. After a thirty minute soak, the cell was held at - 0.1 mA for one hour, and then at ¨ 0.2 mA until the required lithiation capacity was achieved. The electrode was then delithiated at + 0.2 mA, up to a voltage of 1.0 V vs. Li/Li+. Figure 3 shows the cell voltage during this first cycle.
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.
A further advantage of the arrangement described herein is that the structural strength in fact increases with each recharging operation. This is because the fibres are found to "weld" to one another as a result of the disrupted crystalline structure at the fibre junctions creating an amorphous structure. This in turn reduces the risk of capacity loss over multiple cycles as there is less risk of mechanical isolation of the fibres once the fibres become connected in the manner described above.
It will be appreciated, of course, that any appropriate approach can be adopted 5 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

10 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 SixGei,. 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 LiCo02, LiMnõNixCo1.2x02 or LiFePO4. The features of different embodiments can be interchanged or juxtaposed as appropriate and the method steps performed in any appropriate order.

Claims

1. A method of fabricating a rechargeable lithium cell electrode comprising fibres of silicon or a silicon-based material, the method comprising the steps of:
etching a silicon substrate or a silicon-based substrate to make pillars;
detaching the pillars from the substrate to obtain fibres; and forming the electrode, wherein the fibres are an active material of the electrode.

2. The method as claimed in claim 1 wherein the step of forming the electrode comprises laying down a plurality of the fibres in a composite felt structure.

3. The method as claimed in claim 2 wherein the composite structure comprises a polymer binder and, optionally, an electronic additive.

4. The method as claimed in any one of claims 1 to 3 in which the fibres have transverse dimensions in the range 0.08 to 0.5 microns and a length in the range 20 to 300 microns.

5. The method as claimed in any one of claims 1 to 4 in which the fibres have an aspect ratio of 100:1.

6. The method as claimed in any one of claims 1 to 5 in which the fibres have a substantially circular cross-section.

7. The method as claimed in any one of claims 1 to 5 in which the fibres have a substantially non-circular cross-section.

8. The method as claimed in any one of claims 1 to 7 in which the pillars are created by reactive ion etching.

9. The method as claimed in any one of claims 1 to 7 in which the pillars are created by chemical reaction etching.

10. The method as claimed in claim 9 in which the pillars are created by galvanic exchange etching.

11. The method as claimed in any one of claims 1 to 10 in which the pillars are detached by one or more methods selected from the group comprising scraping, agitating and chemical etching.

12. The method as claimed in any one of claims 1 to 11 in which the silicon or silicon-based material comprises material selected from the group consisting of undoped silicon, doped silicon and a silicon germanium mixture.

13. The method as claimed in any one of claims 1 to 12 in which the fibre is a single crystal silicon, a silicon-based material or silicon germanium.

14. The method as claimed in claim 12 wherein the fibres are provided in a slurry and deposited to form a layer of silicon based fibres.

15 The method as claimed in claim 14 wherein the fibres are single crystal fibres.

16. The method according to claim 15 wherein the fibres are deposited on a current collector.

17. The method according to any one of claims 14 to 16 in which the electrode is an anode.

18. The method according to claim 2, in which the felt has a percentage pore volume of 10-30 per cent.

19. A method of fabricating a rechargeable lithium cell comprising the steps of creating the cell electrode according to any one of claims 1 to 18, and adding a cathode and an electrolyte.

20. The method according to claim 19 further comprising adding a separator between the cathode and the cell electrode.

21. The method according to claim 19 or 20 further comprising providing a casing around the cell.

22. An electrode structure for a lithium rechargeable cell, the electrode structure comprising a plurality of elongate silicon-comprising fibres, in which said silicon-comprising fibres crossover to provide multiple intersections, wherein the intersections comprise a disrupted crystalline structure, optionally, wherein the disrupted crystalline structure is an amorphous structure.

23. The electrode structure according to claim 22 wherein the fibres are arranged in a random or disordered manner.

24. The electrode structure according to claim 22 or 23 wherein the structure is arranged to preserve intra-fibre electronic conductivity during charge and discharge.

25. The electrode structure according to any one of claims 22 to 24 wherein the structure is such that the chance of mechanical isolation arising from broken fibre contacts is negligible.

26. The electrode structure according to any one of claims 22 to 25 wherein the fibres are crystalline.

27. The electrode structure according to any one of claims 22 to 26 wherein the fibres are substantially straight.

28. The electrode structure according to any one of claims 22 to 27 wherein the structure is a felt structure.

29. The electrode structure according to claim 28 wherein the felt structure is a sheet material.

30. The electrode structure according to any one of claims 22 to 27 wherein the structure is a composite structure comprising at least one of a binder and electronic additive.

31. The electrode structure according to any one of claims 22 to 27 wherein the fibres are etched fibres.

32. The electrode structure according to claim 28 or 29 wherein the structure functions as its own current collector.

33. The electrode structure according to any one of claims 22 to 32 in which the fibres have transverse dimensions in the range 0.08 to 0.5 microns.

34 The electrode structure according to any one of claims 22 to 33 in which the fibres have a length in the range of 20 to 300 microns

35. The electrode structure according to any one of claims 22 to 34 wherein the fibres are n-type or p-type doped silicon.

36. An electrode for a lithium rechargeable cell, the electrode comprising an electrically interconnected mass comprising:
elongated silicon comprising fibres, wherein the elongated silicon comprising fibres are for being reversibly charged and discharged, and at least some of the elongated silicon comprising fibres cross over each other to provide intersections and a porous structure; and at least one of a binder and an electronic additive;
wherein the elongated silicon comprising fibres and the at least one of the binder and the electronic additive cooperate to define a porous composite electrode layer.

37. The electrode as claimed in claim 36, wherein the elongated fibres are n-type or p-type silicon.

38. The electrode as claimed in claim 36, wherein the elongated fibres have a resistivity of 100 to 0.001 Ohm cm.

39. The electrode as claimed in claim 36, wherein the elongated fibres have an aspect ratio of 100:1.

40. The electrode as claimed in claim 36, wherein the elongated fibres are straight.

41. A lithium-ion battery comprising the electrode of any one of claims 36 to 40.

42. The electrode as claimed in claim 36, wherein the elongated fibres are arranged in a random or disordered manner.

43. The electrode as claimed in claim 36, wherein the fibres are doped silicon.

44. The electrode as claimed in claim 36, wherein the composite layer has a percentage pore volume of about 10 to 30 per cent.

45. The electrode as claimed in claim 36, wherein the elongated fibres have a transverse dimension in the range 0.08 to 0.5 microns and a length in the range of 20 to 300 microns.

46. The electrode as claimed in claim 36, wherein the mass of silicon fibres in the electrically interconnected mass is 70 to 95 per cent.

47. The electrode as claimed in claim 36 wherein the intersections comprise a disrupted crystalline or amorphous structure.

48. An electrode for a lithium rechargeable cell, the electrode comprising an electrically interconnected mass comprising:
elongated silicon comprising fibres, wherein the elongated fibres are electrochemically active such that they are for lithium insertion and removal, and at least some of the elongated fibres cross over each other to provide intersections and a porous structure; and wherein the electrically interconnected mass has a percentage pore volume of about 10 to 30 per cent.

49. The electrode as claimed in claim 48, wherein the elongated structures comprise n-type or p-type silicon.

50. The electrode as claimed in claim 48, wherein the elongated structures have a resistivity of 100 to 0.001 Ohm cm.

51. The electrode as claimed in claim 48, wherein the elongated structures have an aspect ratio of 100:1.

52. The electrode as claimed in claim 48, wherein the elongated structures are substantially straight.

53. A lithium-ion battery comprising the electrode of any one of claims 48 to 52.

54. An electrode for a lithium rechargeable cell, the electrode comprising:
elongated silicon comprising fibres, wherein at least some of the elongated fibres cross over each other to provide intersections and a porous structure; and wherein the intersections are disrupted crystalline forming an amorphous structure which welds the elongated fibres to one another.

55. The electrode as claimed in claim 54, wherein the elongated structures are n-type or p-type silicon.

56. The electrode as claimed in claim 54, wherein the elongated structures have a resistivity of 100 to 0.001 Ohm cm.

57. The electrode as claimed in claim 54, wherein the silicon comprising fibers have an aspect ratio of 100:1.

58. The electrode as claimed in claim 54, wherein the elongated structures are substantially straight.

59. A lithium-ion battery comprising the electrode of any one of claims 54 to 58.