METHOD OF FABRICATING STRUCTURED PARTICLES COMPOSED OF SILICON OR A SILICON-BASED MATERIAL AND THEIR USE IN LITHIUM RECHARGEABLE BATTERIES

Abstract

Pillared particles of silicon or silicon-comprising material and a method of fabricating the same are disclosed. These particles may be used to create both a composite anode structure with a polymer binder, a conductive additive and a metal foil current collector, and an electrode structure. The structure of the particles overcomes the problems of charge/discharge capacity loss.

Claims

1. A particle comprising a particle core and a plurality of elongate structures coating one or more surfaces of the particle, each extending outwardly from the particle core from a first end to a second end, wherein each of the plurality of elongate structures is attached to the core at the first end of the elongate structure and the second end of the elongate structure is an unattached free end, and wherein the elongate structures are formed from silicon.

2. A particle as claimed in claim 1, wherein the fraction of the surface area of the particle core occupied by the elongate structures is in the range of 0.10 to 0.50.

3. A particle as claimed in claim 2, wherein the particle has a diameter of at least 0.5 m and the elongate structures have an aspect ratio of greater than 20:1.

4. A particle as claimed in claim 1 wherein the particles have a first dimension in the range of 10 m to 1 mm.

5. A particle as claimed in claim 1, wherein the elongate structures have a diameter in the range of 0.08 to 0.70 microns.

6. A particle as claimed in claim 1 in which the elongate structures have a length from the first end to the second end in the range of 4 to 100 microns.

7. A particle as claimed in claim 1, wherein the elongate structures are formed from n-type silicon, p-type silicon, or metallurgical grade silicon.

8. A particle as claimed in claim 1 wherein the elongate structures have a silicon purity of 90.00 to 99.95% by mass.

9. A particle as claimed in claim 1 in which the plurality of elongate structures coat one or more surfaces of each particle.

10. A particle as claimed in claim 1, wherein in each of the plurality of discrete particles, the plurality of elongate structures extend over all the surfaces of the particle core.

11. A particle as claimed in claim 1, wherein the elongate structures and the particle core are formed from the same material.

12. A particle as claimed in claim 1, wherein the fraction of the surface area of the particle core occupied by the elongate structures is in the range of 0.10 to 0.50; the elongate structures have an aspect ratio of greater than 20:1; the elongate structures have a diameter in the range of 0.08 to 0.70 microns; the elongate structures have a length from the first end to the second end in the range of 4 to 100 microns; the elongate structures have a silicon purity of 90.00 to 99.95% by mass; and the plurality of elongate structures coat one or more surfaces of each particle.

13. A plurality of discrete particles as claimed in claim 1.

14. A solvent-based slurry comprising a solvent and a plurality of discrete particles as claimed in claim 1.

15. A porous composite structure for an electrode comprising a plurality of discrete particles, each discrete particle comprising a particle core and a plurality of elongate structures coating one or more surfaces of the particle, each extending outwardly from the particle core from a first end to a second end, wherein each of the plurality of elongate structures is attached to the core at the first end of the elongate structure, and wherein the elongate structures are formed from silicon; and a binder binding the particles into the composite structure.

16. A porous composite structure as claimed in claim 15, further comprising an electronic additive bound into the composite structure by the binder.

17. A porous composite structure according to claim 16, having a pore volume of 10-30%.

18. An electrochemical cell comprising an anode and a cathode, the anode comprising a porous composite structure according to claim 15 disposed against a current collector.

19. A method of fabricating a porous composite structure according to claim 15, the method comprising providing a solvent-based slurry of the plurality of discrete particles and a binder; coating the slurry onto a surface; and evaporating the solvent to create the porous composite structure.

20. A method of fabricating a plurality of discrete particles, each particle comprising a particle core and a plurality of elongate structures coating one or more surfaces of the particle, each extending outwardly from the particle core from a first end to a second end, wherein each of the plurality of elongate structures is attached to the core at the first end of the elongate structure and the second end of the elongate structure is an unattached free end, and wherein the elongate structures are formed from silicon, the method comprising providing a plurality of silicon particles, and etching the silicon particle to form the elongate structures.

21. A method as claimed in claim 20 in which the elongate structures are created by galvanic exchange etching.

Description

[0029] Embodiments of the invention will now be described, by way of example, with reference to the figures, of which:

[0030] FIG. 1 is a schematic diagram showing the components of a battery cell;

[0031] FIG. 2 is a electron micrograph of a pillared particle according to embodiments of the present invention;

[0032] FIG. 3 shows the overall galvanic exchange etching mechanism; and

[0033] FIG. 4 shows hypothetical kinetic curves in the form of the partial currents in the galvanic exchange etching process.

[0034] In overview the invention allows creation of pillared particles of silicon or silicon-comprising material and the use of these particles to create both a composite anode structure with a polymer binder, an conductive additive (if required) and a metal foil current collector and an electrode structure. In particular it is believed that the structure of the particles that make up the composite overcomes the problem of charge/discharge capacity loss. By providing a particle with a plurality of elongate or long thin pillars the problem of charge/discharge capacity loss is reduced.

[0035] Typically the pillars will have a length to diameter ratio of approximately 20:1. The insertion and removal of lithium into the pillars, although causing volume expansion and volume contraction, does not cause the pillars to be destroyed and hence the intra-fibre electronic conductivity is preserved.

[0036] The pillars can be made on the particles 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, incorporated herewith by reference. 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), 387-394; K. Peng, Z. Huang and J. Zhu, Adv. Mater., 16 (2004), 127-132; and T. Qui, L. Wu, X. Yang, G. S. Huang and Z. Y. Zhang, Appl. Phys. Lett., 84 (2004), 3867-3869. The above-mentioned methods are used to produce pillars from a high purity silicon wafer.

[0037] In a preferred embodiment of the present invention, pillars are produced on crystalline particles of relatively low purity silicon such as metallurgical grade silicon. The method comprises the five steps of: grinding and sieving; washing; nucleation; etching; and silver removal, as explained, by way of example only, later. An electron micrograph of pillared particle made in accordance with present invention is shown in FIG. 2.

[0038] Any appropriate grinding process is suitable such as power grinding or ball milling. The skilled person will appreciate that a minimum particle size will exist below which pillars cannot be etched onto the surface and instead the particle will be homogeneously etched away. Particles having a diameter less than 0.5 m may be too small.

[0039] A more uniform pillar array, in terms of density and height, is produced by nucleating before etching. This step produces a uniform distribution of silver nuclei/islands (nuclei combine and form silver islands that are the site for pillar growth).

[0040] Silver islands delineate the formation of pillars and galvanic fluoride etching of the {100} planes: see FIG. 3. Referring to FIG. 3 there is shown a silicon surface 301 having a pillar 307. An electron 305 is transferred from a fluoride ion 303 to the silicon surface 301. The reaction of fluorine with silicon 301 and fluoride ions 303 gives rise to fluorosilicate ions 305. This is the anodic etching process. The cathode process is the discharge of silver ions 309 to produce metallic silver 311.

[0041] The structure is explained by supposing that silicon-fluoride bonds are formed as an essential step in the etch process. And furthermore that structures that are SiF (mono-fluoride) are stable and those that are FSiF (di-fluoride) and Si[F]3 (tri-fluoride) are not stable. This is because of steric interference on the Si surface of nearest neighbour groups. The case of the {111} plane is that, a mono-fluoride surface, stable except at the edges, inevitably proceeds to a tri-fluoride surface and consequent instability. The {110} surface is the only stable major crystal plane of Si that will have exclusively mono-fluoride bonds-hence its stability and the etch rate ratio [etch rate <100>]:[etch rate <110>] of about three orders of magnitude. So the sides of the pillars will be terminated on {110} planes.

[0042] A pillar surface density may be used to define the density of the pillars on the surface of the particle. Herein, this is defined as F=P/[R+P] wherein: F is the pillar surface density; P is the total surface area of the particle occupied by pillars; and R is the total surface area of the particle unoccupied by pillars.

[0043] The larger the pillar surface density, the larger the lithium capacity per unit area of a silicon particle electrode and the larger the amount of harvestable pillars available to create fibres.

[0044] For example, using the above-mentioned silicon powder from Elken of Norway having a pre-etching size of 400300200 m, pillars are produced all over the surface having a pillar height of approximately 25 to 30 m, a diameter of approximately 200 to 500 nm and a pillar surface density, F, of 10-50%, more typically, 30%.

[0045] For example, particles having a pre-etching size of approximately 63-805035 m are found to produce pillars with a height of approximately 10 to 15 m, with a coverage of approximately 30% and a diameter of approximately 200 to 500 nm

[0046] In a preferred embodiment, pillars of for example 100 microns in length and 0.2 microns in diameter are fabricated on and from a silicon-comprising particle. More generally pillars of length in the range of 4 to 100 microns and diameter or transverse dimension in the range of 0.08 to 0.70 microns are fabricated from a particle having an initial size of 10 to 1000 m.

[0047] According to the process, the silicon particles may be predominantly n- or p-type and, according to the chemical approach, and may be etched on any exposed (100), (111) or (110) crystal face. Since the etching proceeds along crystal planes, the resulting pillars are single crystals. Because of this structural feature, the pillars will be substantially straight facilitating length to diameter ratio of greater than 20:1.

[0048] The pillared-particles may then be used to form a composite electrode as described later. Alternatively, the pillars may be detached from the particle and used to form a fibre-based electrode. The detached pillars may also be described as fibres.

[0049] The invention encompasses the detachment of the pillars from the particle. The particle, with pillars attached, can be placed in a beaker or any appropriate container, covered in an inert liquid such as ethanol or water 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 the particle.

[0050] In an embodiment, the pillars are removed from the particle in a two stage process. In the first stage, the particles are washed several times in water and, if necessary, dried in a low vacuum system to remove the water. In the second stage, the particles are agitated in an ultrasonic bath to detach the pillars. These are suspended in water and then filtered using different various filter paper sizes to collect the silicon fibres.

[0051] It will be appreciated that alternative methods for harvesting the pillars include scraping the particle surface to detach them or detaching them chemically. One chemical approach appropriate to n-type silicon material comprises etching the particle in an HF solution in the presence of backside illumination.

[0052] Once the pillared particles have been fabricated they can be used as the active material in a composite anode for lithium-ion electrochemical cells. To fabricate a composite anode, the pillared particles are 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 composite film has a porous structure in which the mass of silicon-based pillared particles is typically between 70 percent and 95 percent. The composite film will have a percentage pore volume of 10-30 percent, preferably about 20 percent.

[0053] 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-comprising active anode material rather than a graphite active anode material. For example the silicon particle-based composite anode layer is covered by the porous spacer 20, 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.

[0054] Some embodiments provide an electrode containing as its active material a plurality of pillared particles of silicon. Capacity retention is improved as the pillared structure of the silicon allows for accommodation of the volume expansion associated with insertion/extraction (charging and discharging) of lithium. Advantageously, the pillared particles may be created by etching lumps of low purity, silicon (termed metallurgical grade silicon) such that a core of silicon remains covered by pillars that are between 0.08 m and 0.5 m in diameter and between 4 m and 150 m in length.

[0055] A particular advantage of the approach described herein is that large sheets of silicon-based anode can be fabricated and then rolled 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.

[0056] The invention will now be illustrated by reference to one or more of the following non-limiting examples:

Grinding and Seiving

[0057] In the first stage, widely-available metallurgical grade silicon, such as Silgrain from Elkem of Norway, was ground and sieved to produce particles in the range 10 to 1000 m, preferably 30 to 300 m and more preferably 50 to 100 m.

Washing

[0058] The second stage comprised washing the ground and sieved particles in water to remove any fine particles stuck to the big particles. The washed particles were then treated in diluted HNO.sub.3 (1 mol.Math.L) or H.sub.2SO.sub.4/H.sub.2O.sub.2 (1:2 in volume) or H.sub.2O.sub.2/NH.sub.3H.sub.2O/H.sub.2O.sub.2 (1:1:1 in volume) in 10 minutes to get rid of the possible organic or metal impurities.

Nucleation

[0059] In the third stage, a nucleation reaction was carried out in a solution of 17.5 ml HF (40%)+20 ml AgNO.sub.3 (0.06 mol/l)+2.5 ml EtOH (97.5%)+10 ml H.sub.2O for 710 minutes at room temperature (23 C.) using 0.1 g of silicon particles with the dimension of about 400300200 m. For the same weight of silicon, smaller silicon particles required a larger solution volume due to the increased surface area to volume ratio.

[0060] The effect of ethanol at room temperature was to slow the chemical processes which gives a more uniform distribution of silver islands. The time (especially at the upper limit) was sufficient to consume a significant amount of the solution silver.

Etching

[0061] The fourth stage comprised etching. The etching reaction used a solution of 17.5 ml HF (40%)+12.5 ml Fe(NO.sub.3).sub.3 (0.06 mol.Math.l)+2 ml AgNO.sub.3 (0.06 mol.Math.l)+18 ml H.sub.2O for 11.5 hours at room temperature (23 C.) using 0.1 g of silicon particles with the dimension of about 400300200 m. For the same weight of silicon, smaller silicon particles required a larger solution volume due to the increased surface area to volume ratio. In addition, as the particle size decreases, a shorter time is needed for smaller silicon particles, for example, 30 min for 100120 m (sieve size) sample and 20 min for 6380 m sample.

[0062] In further modifications, stirring increased the etch rate possibly owing to the discharge of hydrogen. Here, the out diffusion of fluorosilicate ion was rate limiting.

[0063] The skilled person will understand that oxidizing agents other than Ag.sup.+ may be equally suitable. For example: K.sub.2PtC.sub.16; Cu(NO.sub.3).sub.2; Ni(NO.sub.3).sub.2; Mn(NO.sub.3).sub.2; Fe(NO.sub.3).sub.3; Co(NO.sub.3).sub.2; Cr(NO.sub.3).sub.2; Mg(NO.sub.3).sub.2. Compounds involving Cu and Pt, having potentials higher than hydrogen, give metal deposition (Cu and Pt) but the others, except for Ni, do not.

[0064] The overall galvanic exchange etching mechanism can be illustrated using FIGS. 3 and 4. In FIG. 3 the anodic process,

Si+6F.sup.=SiF.sub.6.sup.2+4e.sup.(1.24 Volts)

is the local etching of silicon. While the removal of the electrons accompanied by the discharge of silver ions is the cathodic process

Ag.sup.++e.sup.=Ag(+0.8 Volts)

[0065] For standard conditions the overall cell voltage is 2.04 volts. The other cathodic couples of interest are Cu/Cu.sup.2+ (+0.35V); PtCl.sub.6.sup.2/PtCl.sub.4.sup.2 (+0.74V); Fe.sup.3+/Fe.sup.2+ (+0.77V), since they are all positive with respect to hydrogen. Couples that are more negative than H+/H.sub.2 will be in competition with hydrogen and will be largely ineffective. FIG. 4 shows a schematic version of the partial electrode reactions.

Silver Removal

[0066] The final stage of the process involved removing the silver which was left on the etched silicon particles from the third and fourth stages. The silver was removed (and saved) using a solution of 15% HNO.sub.3 for 510 min.

[0067] 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 particles.

[0068] The particles can have any appropriate dimension and can for example be pure silicon or doped silicon other silicon-comprising material such as a silicon-germanium mixture or any other appropriate mixture. The particles 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.sub.xGe.sub.1-x. The particles may be metallurgical grade silicon.

[0069] The particles and/or the detached pillars 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 LiCoO.sub.2, LiMn.sub.xNi.sub.xCo.sub.1-2xO.sub.2 or LiFePO.sub.4. The features of different embodiments can be interchanged or juxtaposed as appropriate and the method steps performed in any appropriate order.

[0070] Although relatively high purity single crystal wafers of silicon can be etched to produce pillars of the desired parameters, the wafers themselves are very expensive owing to their high purity. Furthermore, it is difficult to arrange a pillared-wafer into an electrode-geometry. Embodiments of the present invention are advantageous because metallurgical grade silicon is relatively cheap and pillared particles may themselves be incorporated into a composite electrode without further processing. Also, pillared particles are a good source of silicon fibres and can be used themselves as the active ingredient in a battery electrode.

[0071] The particles used for etching may be crystalline for example mono- or poly-crystalline with a crystallite size equal to or greater than the required pillar height. The polycrystalline particle may comprise any number of crystals from example two or more.

[0072] Advantageously, metallurgical grade silicon is particularly suitable as a battery electrode because of the relatively high density of defects (compared to silicon wafers used in the semiconductor industry). This leads to a low resistance and hence high conductivity.

[0073] As the skilled person will understand, both n-type and p-type silicon can be etched and any density of charge carriers is appropriate provided the material does not become significantly degenerate.