Structured silicon particles

Abstract

A composite particle is provided. The particle comprises a first particle component and a second particle component in which: (a) the first particle component comprises a body portion and a surface portion, the surface portion comprising one or more structural features and one or more voids, whereby the surface portion and body portion define together a structured particle; and (b) the second component comprises a removable filler; characterized in that (i) one or both of the body portion and the surface portion comprise an active material; and (ii) the filler is contained within one or more voids comprised within the surface portion of the first component. The use of the particle in applications such as electrochemical cells, metal-ion batteries such as secondary battery applications, lithium air batteries, flow cell batteries, fuel cells, solar cells, filters, sensors, electrical and thermal capacitors, micro-fluidic devices, gas or vapor sensors, thermal or dielectric insulating devices, devices for controlling or modifying the transmission, absorption or reflectance of light or other forms of electromagnetic radiation, chromatography or wound dressings is disclosed.

Claims

1. A composite particle comprising a first particle component and a second particle component in which: (a) the first particle component comprises a body portion and a surface portion, the surface portion comprising one or more structural features and one or more voids, wherein the one or more structural features and the one or more voids extend between the body portion and a particle boundary of the composite particle, whereby the surface portion and body portion define together a structured porous or pillared particle; and (b) the second particle component consists of a removable filler that is removable from the composite particle during processing; characterised in that (i) the one or more structural features of the surface portion comprise an electroactive material comprising a member selected from the group consisting of silicon, tin, germanium, gallium, lead, zinc and aluminium; (ii) the filler is contained around the one or more structural features and within the one or more voids comprised within the surface portion of the first particle component; and (iii) the filler has a sublimation or disintegration temperature of at least 50? C. and/or is soluble in an ionic liquid or an electrolyte solution.

2. The composite particle according to claim 1, wherein the filler is soluble in an electrolyte solution having a salt concentration of at least 0.7 M.

3. The composite particle according to claim 1, wherein the filler is soluble in a solution comprising a member selected from the group consisting of N-methylpyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma butyro lactone, 1,2-dimethoxy ethane, 2-methyl tetrahydrofuran, dimethyl sulphoxide, 1,3-dioxolane, formamide, dimethylformamide, acetonitrile, nitromethane, methylformate, methyl acetate, phosphoric acid trimester, trimethoxy methane, sulpholane, methyl sulpholane and 1,3-dimethyl-2-imidazolidione.

4. The composite particle according to claim 1, wherein the filler sublimes or degrades at a temperature in the range 70 to 200? C.

5. The composite particle according to claim 1, wherein the removable filler comprises a member selected from the group consisting of o- and p-cresol, 3-nonanol, 1-methyl cyclohexanol, p-toluenenitrile, 2-methoxyphenol, 2-phenol-2-propanol, 2,3-dimethylanisole, phenol, 2,4-diemthylphenol, 3,4,4-trimethylpentanol, butanediol, oxalic acid, a surfactant and a natural wax or a synthetic wax selected from the group comprising 12-hydroxystearic acid, low molecular weight polyethylene, petroleum waxes such as paraffin wax, and microcrystalline waxes.

6. The composite particle according to claim 1, wherein the first particle component is selected from a pillared particle, a porous particle having voids distributed there through, a porous particle fragment or a fractal, or a scaffold structure.

7. The composite particle according to claim 6, wherein the first particle component is the pillared particle and wherein the surface portion comprises one or more pillars distributed over a body portion.

8. The composite particle according to claim 6, wherein the first particle component is the pillared particle and wherein the voids of the surface portion comprise channels extending from the body portion to the particle boundary.

9. The composite particle according to claim 6, wherein the first particle component is the porous particle having voids distributed there through and wherein the porous particle is a fibre, wire, thread, tube, flake, or ribbon with a minor dimension of at least 10 nm.

10. The composite particle according to claim 6, wherein the first particle component is the porous particle having voids distributed there through and further comprising voids distributed at the surface of the particle.

11. The composite particle according to claim 1, wherein the particle has a principal diameter in the range 0.5 to 10 ?m.

12. The composite particle according to claim 1, wherein the body portion and the surface portion are formed integrally and the active material comprising the body portion is the same as or is similar to the active material of the surface portion.

13. The composite particle according to claim 1, wherein the active material of the body portion is different to the active material of the surface portion.

14. The composite particle according to claim 1, wherein the active material is an electroactive material.

15. The composite particle according to claim 1, wherein the electroactive material is silicon, a silicon alloy or an electroactive silicon compound.

16. A method of forming a composite particle according to claim 1, the method comprising steps of mixing the first particle component with the filler in liquid form.

17. The method according to claim 16, wherein the filler is provided in a solution comprising a solvent and wherein the solvent is removed by evaporation.

18. A composite material comprising: a composite particle, the composite particle comprising a first particle component and a second particle component in which: (a) the first particle component comprises a body portion and a surface portion, the surface portion comprising one or more structural features and one or more voids, wherein the one or more structural features and the one or more voids extend between the body portion and a particle boundary of the composite particle, whereby the surface portion and body portion define together a structured particle; and (b) the second particle component comprises a removable filler; characterised in that (i) the one or more structural features of the surface portion comprise an electroactive material comprising a member selected from the group consisting of silicon, tin, germanium, gallium, lead, zinc and aluminium; (ii) the filler is contained around the one or more structural features within one or more voids comprised within the surface portion of the first particle component; and (iii) the filler has a sublimation or disintegration temperature of at least 50? C. and/or is soluble in an ionic liquid or an electrolyte solution; and a binder; and optionally further comprising one or more components selected from an electroactive carbon, a conductive carbon and an electroactive component.

19. A composition comprising: a stable suspension of a composite particle, the composite particle comprising a first particle component and a second particle component in which: (a) the first particle component comprises a body portion and a surface portion, the surface portion comprising one or more structural features and one or more voids, wherein the one or more structural features and the one or more voids extend between the body portion and a particle boundary of the composite particle, whereby the surface portion and body portion define together a structured particle; and (b) the second particle component comprises a removable filler; characterised in that (i) the one or more structural features of the surface portion comprise an electroactive material comprising a member selected from the group consisting of silicon, tin, germanium, gallium, lead, zinc, and aluminium; (ii) the filler is contained around the one or more structural features within one or more voids comprised within the surface portion of the first particle component; and (iii) the filler has a sublimation or disintegration temperature of at least 50? C. and/or is soluble in an ionic liquid or an electrolyte solution; and wherein the stable suspension is in a liquid carrier; and optionally wherein the removable filler includes a surfactant to maintain the composite particles in a stable suspension.

20. The composite particle according to claim 1, wherein the filler is soluble in an electrolyte solution having a salt concentration of between about 0.7 and about 2 M.

21. The composite particle according to claim 1, wherein the filler sublimes or degrades at a temperature in a range between about 70 and about 110? C.

22. The composite particle according to claim 1, wherein the body portion comprises an electroactive material.

23. The composite particle according to claim 1, wherein the electroactive material comprises an electroactive alloy of a member selected from the group consisting of silicon, tin, germanium, gallium, lead, zinc, and aluminium.

24. The composite particle according to claim 1, wherein the electroactive material comprises a compound comprising a member selected from the group consisting of silicon, tin, germanium, gallium, lead, zinc, and aluminium.

Description

(1) The invention will now be described with reference to the accompanying drawings and examples in which:

(2) FIG. 1 is a composite particle comprising as a first particle component, a pillared particle (1), having a particle core (2) and a plurality of pillars (3) extending there from. A filler (4) occupies the void space (5) between adjacent pillars. Although not illustrated, partial filling of voids may involve the formation of a thin coat of filler on the void walls.

(3) FIG. 2 is a composite particle comprising as a first particle component a porous particle (6) or a porous particle fragment (6a) having a plurality of voids or pores (7, 7a). A filler (8, 8a) occupies some or all of the void spaces within the particle (6) or particle fragment (6a). The depth (from the surface of the particle) to which the filler penetrates is defined as the surface region.

(4) FIG. 3 is a composite particle comprising as a first particle component a fractal (9), being a porous particle fragment, the fractal comprising a body portion (10) and a plurality of spikes (11) extending there from. Void spaces (12) separate the spikes (11) and are fully or partially occupied by filler (13).

(5) FIG. 4 is a composite particle comprising as a first particle component a fibre core (14) having pores (15) formed on the surface thereof, each pore defining a void in the surface structure. The voids are either fully or partially occupied by a filler (16).

(6) FIG. 5 is a composite particle comprising as a first particle component a scaffold structure (17) including a plurality of elongate structures (18) bounding void spaces (19). The void spaces (19) are occupied by a filler (20).

EXAMPLES

Example 1Formation of a Polyacrylic Acid Polyethylene Glycol Ester

Example 1a

(7) An ester of polyacrylic acid and polyethylene glycol (MW=4000) was dissolved in deionised water. The final solution contained 0.05 wt % polyacrylic acid/polyethylene glycol ester.

Example 2Formation of a Perfluoro-Octane Sulfonic Acid Ester Solution

(8) Perfluoro-octane sulfonic acid ester was dissolved in an aqueous solution to give a final solution containing 0.05 wt % of perfluoro-octane sulfonic acid ester.

(9) Suitable surfactants are sold by 3M as FC4330, which contains fluoroaliphatic polymeric esters.

Example 3aFormation of Coated Particles

(10) Silicon pillared particles having an average D50 value in the range 3 to 10 ?m were added to a solution as described in Examples 1 or 2 and stirred for between 2 and 3 hours. The wet particles were separated from the bulk mixture by filtration. The coated particles were then dried in an airstream until dry. The resulting particles were characterised by the presence of either an oxalic acid or a perfluoro-octane sulfonic acid ester.

Example 3bElectrode Mix

(11) A slurry was formed by shear mixing 85 parts by weight of spherical synthetic graphite (d50=27 ?m), 3 parts by weight of VGCF, 9.2 parts by weight of a composite silicon particle (9 parts silicon particle as specified in Example 1 and 0.005 parts oxalic acid), and 2.8 parts by weight of a PVdF(9200) binder in NMP as the carrier liquid using a T25 IKA High Shear Mixer? 15. The final solids content of the slurry is in the range 30 to 50%. The viscosity of the slurry is in the range 1000 to 4500 mPa.Math.s. The resulting slurry was cast onto a copper foil to a thickness of 60 g/cm2.

Example 3c

(12) The procedure was repeated using a perfluoro-octane sulfonic acid ester to give a composite having perfluoro-octane sulphonic acid coated silicon particles.

Example 4Preparation of Cells

(13) Electrode and Cell Fabrication

(14) Anode Preparation

(15) The desired amount of composite particle was added to a carbon mixture that had been bead milled in deionised water as specified above. The resulting mixture was then processed using a T25 IKA High Shear? overhead mixer at 1200 rpm for around 3 hours. To this mixture, the desired amount of binder in solvent or water was added. The overall mix was finally processed using a Thinky? mixer for around 15 minutes to give the composite materials described in Examples 3a and 3b above.

(16) The anode mixture (either 3a or 3b) was applied to a 10 ?m thick copper foil (current collector) using a doctor-blade technique to give a 20-35 ?m thick coating layer. The resulting electrodes were then allowed to dry. Composite materials comprising oxalic acid released CO.sub.2 gas during the drying process.

(17) Cathode Preparation

(18) The cathode material used in the test cells was a commercially available lithium MMO electrode material (e.g. Li1+xNi0.8Co0.15Al0.05O2) on a stainless steel current collector.

(19) Electrolyte

(20) The electrolyte used in all cells was a 1.2M solution of lithium hexafluorophosphate dissolved in solvent comprising a mixture of ethylene carbonate and ethyl methyl carbonate (in the ratio 3:7 by volume) (82%), FEC (15 wt %) and VC (3 wt %). The electrolyte was also saturated with dissolved CO2 gas before being placed in the cell.

(21) Cell Construction

(22) Swagelok test cells were made as follows:

(23) Anode and cathode discs of 12 mm diameter were prepared and dried over night under vacuum. The anode disc was placed in a 2-electrode cell fabricated from Swagelok? fittings. Two pieces of Tonen separator of diameter 12.8 mm and 16 um thick were placed over the anode disc. 40 ?l of electrolyte was added to the cell. The cathode disc was placed over the wetted separator to complete the cell. A plunger of 12 mm diameter containing a spring was then placed over the cathode and finally the cell was hermetically sealed. The spring pressure maintained an intimate interface between the electrodes and the electrolyte. The electrolyte was allowed to soak into the electrodes for 30 minutes. Structured silicon particles including a coating of pefluorobenzene sulphonic acid ester were observed to have lost their coating material on formation of a cell.