Silicon-Carbon Composite Materials and Methods

Abstract

The invention provides methods for providing composite particles with a carbon coating and the resulting core-shell particulate material. The process comprises subjecting a plurality of precursor composite particles to a heat treatment in contact with a pyrolytic carbon precursor such that an outer shell of a pyrolytic conductive carbon material is formed on the precursor composite particles, wherein the heat treatment is carried out at a temperature of no more than 700° C.

Claims

1-45. (canceled)

46. A process for preparing core-shell composite particles, the process comprising: (a) providing a plurality of precursor composite particles comprising: i. a porous carbon framework comprising micropores and/or mesopores, wherein the total pore volume of micropores and mesopores as measured by gas adsorption is at least 0.4 cm.sup.3/g, and wherein the PD.sub.50 pore diameter of the porous carbon framework is no more than 10 nm; and ii. a plurality of nanoscale electroactive material domains disposed with the porous carbon framework; (b) subjecting the plurality of precursor composite particles to a heat treatment in contact with a pyrolytic carbon precursor such that an outer shell of a pyrolytic conductive carbon material is formed on the precursor composite particles, wherein the heat treatment is carried out at a temperature of no more than 700° C.

47. A process according to claim 46, wherein the heat treatment is carried out at a temperature of no more than 650° C.

48. A process according to claim 46, wherein the heat treatment is carried out at a temperature of at least 500° C.

49. A process according to claim 46, wherein the pyrolytic carbon precursor is contacted with the composite particles as a vapour.

50. A process according to claim 49, wherein the pyrolytic carbon precursor is selected from polyaromatic hydrocarbons comprising from 10 to 25 carbon atoms and optionally from 1 to 3 heteroatoms; bicyclic monoterpenoids; and C.sub.2-C.sub.10 hydrocarbons.

51. A process according to claim 50, wherein the pyrolytic carbon precursor is selected from (i) naphthalene, anthracene, tetracene, pentacene, fluorene, acenapthene, phenanthrene, fluoranthrene, pyrene, chrysene, perylene, coronene, fluorenone, anthraquinone, anthrone and alkyl-substituted derivatives thereof; (ii) camphor, borneol, eucalyptol, camphene, careen, sabinene, thujene and pinene; and (iii) methane, ethylene, propylene and acetylene.

52. A process according to claim 46, wherein the pyrolytic carbon precursor vapour is contacted with a transition metal catalyst at a temperature of at least 500° C. prior to contact with the composite particles.

53. A process according to claim 52, wherein the transition metal catalyst comprises nickel, iron, cobalt, copper and mixtures thereof.

54. A process according to claim 52, wherein the transition metal catalyst is disposed on the surface of the composite particles.

55. A process according to claim 52, wherein gaseous nickel carbonyl is thermally decomposed to deposit nickel onto the surface of the composite particles prior to step (b).

56. A process according to claim 55, wherein the carbon-coated particulate material is contacted with carbon monoxide gas after step (b) to form gaseous nickel carbonyl, thereby removing nickel from the carbon-coated composite particles.

57. A process according to claim 46, wherein step (b) comprises contacting the composite particles with a solution or dispersion of a pyrolytic carbon precursor in a solvent, and removing the solvent to provide composite particles coated with the pyrolytic carbon precursor, prior to the heat treatment.

58. A process according to claim 57, wherein the pyrolytic carbon precursor is a polymer or oligomer comprising a carbon-containing backbone.

59. A process according to claim 46, wherein the outer shell of pyrolytic conductive carbon material has a thickness of no more than 10 nm.

60. A process according to claim 46, wherein the total pore volume of micropores and mesopores of the porous carbon framework as measured by gas adsorption is at least 0.45 cm.sup.3/g and no more than 2.2 cm.sup.3/g; the PD.sub.50 pore diameter of the porous carbon framework is no more than 8 nm; the precursor composite particles have a D.sub.50 particle diameter of at least 1 μm and no more than 50 μm.

61. A process according to claim 46, wherein the electroactive material is silicon.

62. A process according to claim 46, wherein the duration of the heat treatment in step (b) is from 1 to 3 hours.

63. A particulate material consisting of a plurality of core-shell composite particles obtainable by a process according to claim 46.

64. A particulate material consisting of a plurality of core-shell composite particles, wherein the core-shell composite particles comprise: (a) a core comprising: i. a porous carbon framework comprising micropores and/or mesopores, wherein the micropores and/or mesopores have a total pore volume as measured by gas adsorption of at least 0.4 cm.sup.3/g, and wherein the PD.sub.50 pore diameter of the porous carbon framework is no more than 10 nm, preferably no more than 5 nm; and ii. a plurality of electroactive material domains disposed within the micropores and/or mesopores of the porous carbon framework; and (b) an outer shell of a pyrolytic conductive carbon material at least partially surrounding the core.

65. A particulate material according to claim 64, wherein the outer shell of a pyrolytic conductive carbon material penetrates into the pores of the porous carbon framework.

66. A particulate material according to claim 64, wherein the electroactive material is silicon.

67. A composition comprising a particulate material as defined in claim 64 and at least one other component selected from one or more of: (i) a binder; (ii) a conductive additive; and (iii) an additional particulate electroactive material.

68. An electrode comprising a particulate material as defined in claim 64 in electrical contact with a current collector.

69. A rechargeable metal-ion battery comprising: (i) an anode, wherein the anode comprises an electrode as described in claim 68; (ii) a cathode comprising a cathode active material capable of releasing and reabsorbing metal ions; and (iii) an electrolyte between the anode and the cathode.

Description

[0120] The invention will now be described with reference to the following examples and the accompanying figures in which:

[0121] FIG. 1 shows the characteristic TGA trace for a particulate material according to the invention, comprising a low level of coarse silicon.

[0122] FIG. 2 shows thermogravimetric analysis (TGA) data for Example 4;

[0123] FIG. 3 shows X-Ray Diffraction (XRD) data for Example 4;

[0124] FIG. 4 shows cell data for Example 5;

[0125] FIG. 5 shows the effect of temperature on coarse silicon content in Example 4;

[0126] FIG. 6 shows XRD data for Example 6;

[0127] FIG. 7 shows the effect of temperature on coarse silicon content in Example 7.

[0128] FIG. 8 shows the effect of temperature on capacity retention at 100 cycles. The numbers against each data point indicate the level of coarse silicon.

[0129] FIG. 9 shows the effect of temperature on capacity retention at 200 cycles. The numbers against each data point indicate the level of coarse silicon.

EXAMPLES

[0130] Porous carbon frameworks C1 to C3 used in the following examples have the characteristics set out in Table 1.

TABLE-US-00001 TABLE 1 Carbon No. C1 C2 C3 P.sub.1 (cm.sup.3/g) 0.82 0.76 0.88 BET (m.sup.2/g) 1757 1637 1860 D.sub.50 (μm) 98.8 2.9 3.1 PD.sub.50 (nm) 1.18 1.26 1.41 Micropore volume (cm.sup.3/g) 0.50 0.44 0.48

Example 1: Preparation of Particulate Materials in a Fluidized Bed Reactor

[0131] Silicon-carbon composite particles were prepared in a vertical bubble-fluidized bed reactor comprising an 83 mm internal diameter stainless steel cylindrical vessel. A quantity of a powder of carbon framework particles with the properties listed in Table 1 is placed in the reactor. An inert gas (nitrogen) at a low flow rate is injected into the reactor to remove any oxygen. The reactor is then heated to a reaction temperature between 430 and 500° C. and 4% v/v monosilane gas diluted in nitrogen is supplied to the bottom of the reactor at a flow rate sufficient to fluidize the carbon framework particles, for a length of time sufficient to deposit the target mass of silicon. The reactor is purged for 30 minutes under nitrogen before being cooled down to room temperature over several hours. The atmosphere is then switched over to air gradually over a period of two hours by switching the gas flow from nitrogen to air from a compressed air supply.

Example 2: Preparation of the Particulate Material in a Static Furnace

[0132] Silicon-carbon composite particles were prepared by placing 1.8 g of a particulate porous framework with the properties listed in Table 1 on a stainless-steel plate at a constant thickness of 1 mm along its length. The plate was then placed inside a stainless-steel tube of outer diameter 60 mm with gas inlet and outlet lines located in the hot zone of a retort furnace. The furnace tube was purged with nitrogen gas for 30 minutes at room temperature, then the sample temperature was increased to between 450 and 475° C. The nitrogen gas flow-rate is adjusted to ensure a gas residence time of at least 90 seconds in the furnace tube and maintained at that rate for 30 minutes. Then, the gas supply is switched from nitrogen to a mixture of monosilane in nitrogen at 1.25 vol % concentration. Dosing of monosilane is performed over a period of up to 5-hours with a reactor pressure maintained at 101.3 kPa (1 atm). After dosing has finished the gas flow rate is kept constant whilst the silane is purged from the furnace using nitrogen. The furnace is purged for 30 minutes under nitrogen before being cooled down to room temperature over several hours. The atmosphere is then switched over to air gradually over a period of two hours by switching the gas flow from nitrogen to air from a compressed air supply.

Example 3: Determination of Coarse Silicon Wt %

[0133] The procedure used to calculate the coarse silicon for the composite materials of the examples was as follows. 10-20 mg of the sample under test was loaded into a 70 μL crucible. The sample was loaded into a Mettler Toledo TGA/DSC 3+ instrument with an Ar purge gas, N.sub.2 padding gas and air reaction gas at 100 mL/min. The TGA furnace chamber was ramped from 25 to 1400° C. at a rate of 10° C./min. Data was collected at 1 s intervals.

[0134] The amount of coarse silicon was determined by finding the final mass of ash at the end of the TGA experiment and the mass at 800° C. The formula outlined above is used to calculate the Coarse Silicon (Z) value.

Example 4—Heat Treatment of Uncoated Particles

[0135] Silicon-carbon composite particles were prepared using 125 g of carbon C1 and the method of Example 1. The silicon-carbon composite particles were divided into five samples. Each sample was then individually jet milled for 25 minutes using a Hosokawa Alpine 50 AS spiral jet mill with feed gas pressure of 8 bar and a feeding rate of 8 rpm. The resulting volumetric particle size distribution after jet milling and before any heat treatment step was measured using laser diffraction as described herein above and is provided in Table 2.

[0136] The first sample was used as a reference sample and was not subjected to any further processing. The other four samples were flushed for 30 minutes in argon flow at 1 L/min flow rate and then the temperature was ramped at 5° C./min to a heat treatment temperature of 600° C., 700° C., 800° C., or 900° C. correspondingly, then held at the heat treatment temperature for 1 hour. The properties of the silicon-carbon composite precursor particles were measured after the heat treatment and compared to the reference sample.

Results

[0137] The reference sample and the heat-treated samples were subjected to various analyses to compare the effects of each temperature on the material properties.

[0138] Thermogravimetric analysis (TGA) in air (FIG. 2) demonstrated that the samples subjected to heat treatments of 800° C. and 900° C. suffered from an annealing of the fine micropore structure and amalgamation of nanoscale silicon domains within the pore structure to form larger domains of silicon. The sample subjected to a heat treatment at 600° C. exhibited a TGA profile largely identical to the reference sample, demonstrating that a carbon coating step on these silicon-carbon composite precursor particles is highly unlikely to damage the fine micropore structure that is so desirable for use of the particulate material in an anode.

[0139] The amount of coarse silicon, determined according to the method of Example 3, is shown for each sample in FIG. 5, with the reference sample denoted as 430 and other samples denoted according to the temperature of the heat treatment step to which they were subjected. In the final product, fine silicon is more desirable than coarse silicon. FIG. 5 demonstrates that any processing step carried out at a temperature above 700° C. is likely to give the finished particulate product a higher proportion of coarse silicon compared to products that were processed (such as coated with carbon) at temperatures of 700° C. or lower.

[0140] The samples were tested for the total pore volume and BET surface area using the methods described earlier.

TABLE-US-00002 TABLE 2 Heat Heat Heat Heat treated treated treated treated Sample Reference 600° C. 700° C. 800° C. 900° C. BET, m.sup.2/g.sup.1 141.09 165.41 142.16 112.56 104.37 Total pore 0.0908 0.0993 0.0917 0.0779 0.069 volume, cm.sup.3/g D.sub.10, μm 0.94 2.79 0.97 0.97 0.86 D.sub.50, μm 2.52 5.63 2.71 2.76 2.52 D.sub.90, μm 5.0 7.50 5.5 5.5 4.99 D.sub.98, μm 6.6 11.0 7.4 7.5 6.66 Density, g/cm.sup.3 2.14 2.16 2.24 2.36

[0141] These data show that the total pore volume decreases as the processing temperature is increased. The resulting total pore volume for the 800° C. and 900° C. samples is undesirably low and can be explained by the pores annealing out at these higher temperatures.

[0142] The total pore volume and the BET surface area are dependent on the particle size of the sample. The increase in the BET surface area and the total pore volume for the sample that was heat treated at 600° C. compared to the reference sample is accounted for by the fact that the 600° C. heat-treated sample had a larger particle size than all the other samples, because the samples were jet-milled individually.

[0143] The five samples were further analysed for their chemical composition using X-Ray Diffraction (XRD). The results can be seen in FIG. 3.

[0144] Amorphous silicon characteristics, i.e. a broad peak, can be seen up to 700° C. and especially broad up to 600° C. Crystalline silicon is apparent in varying degrees in the 700° C., 800° C. and 900° C. samples and is particularly pronounced for the 800° C. and 900° C. samples.

[0145] Evidence of silicon carbide can also be clearly seen for the 900° C. sample. The formation of silicon carbide is undesirable for the silicon-carbon composite particles, because silicon carbide does not exhibit electrochemical activity and is a poor conductor and so formation of this compound will reduce the overall effectiveness of the product in the context of an anode.

[0146] Further tests were made on electrodes each incorporating one of the test samples of particulate material.

Example 5—Electrochemical Testing

[0147] Test coin cells were made with negative electrodes comprising the composite material prepared as described in Example 4. A dispersion of Carbon Super P (conductive carbon) in CMC binder was mixed in a Thinky™ mixer. The silicon-based material was added to the mixture and mixed for 30 min in the Thinky™ mixer. SBR binder was then added to give a CMC:SBR ratio of 1:1, yielding a slurry with a weight ratio of silicon-based material:CMC/SBR:conductive carbon of 70%:16%:14%. The slurry was further mixed for 30 minutes in the Thinky™ mixer, then was coated onto a 10 μm thick copper substrate (current collector) and dried at 50° C. for 10 minutes, followed by further drying at 110° C. for 12 hours to thereby form an electrode comprising an active layer on the copper substrate.

[0148] Coin half cells were made using circular electrodes of 0.8 cm radius cut from the electrode with a porous polyethylene separator, a lithium foil as the counter electrode and an electrolyte comprising 1M LiPF.sub.6 in a 7:3 solution of EC/FEC (ethylene carbonate/fluoroethylene carbonate) containing 3 wt % vinylene carbonate.

[0149] These half cells were used to measure the initial volumetric energy density (VED2, mAh/cm.sup.3), first cycle loss (FCL) and first delithiation capacity (DC1) of the active layer. The half cells were tested by applying a constant current of C/25, (wherein “C” represents the specific capacity of the electrode in mAh, and “25” refers to 25 hours), to lithiate the electrode comprising the porous particles, with a cut off voltage of 10 mV. When the cut off is reached, a constant voltage of 10 mV is applied with a cut off current of C/100. The cell is then rested for 10 minutes in the lithiated state. The electrode is then delithiated at a constant current of C/25 with a cut off voltage of 1V and the cell is then rested for 10 minutes. A constant current of C/25 is then applied to lithiate the cell a second time with a 10 mV cut off voltage, followed by a 10 mV constant voltage with a cut off current of C/100 and rest for 5 minutes.

[0150] The impact of the formation of silicon carbide on cell performance can be seen in FIG. 4. The delithiation capacity, measured as described above, markedly drops for the 900° C. heat-treated sample, demonstrating the deleterious effect of formation of silicon carbide on anode performance.

[0151] FIG. 4 also shows the thickness change of the electrode coating (excluding the current collector) for electrodes prepared and tested using half cells as described above. The thickness change in percentage is measured ex situ in the charged state, after removal of the anode following lithiation, delithiation and a second lithiation cycle in the lithium-ion half-cell. In other words, the thickness change is measured for the anode in its lithiated state.

[0152] The anodes that incorporated the particulate material that was subjected to 900° C. heat treatment suffered the largest increase of thickness, demonstrating that a carbon coating step at the same temperature would have an undesirable impact on the usefulness of the particulate material in an anode. Ordinarily the degree of electrode expansion would be expected to have a positive correlation with the amount of active silicon. However, the opposite was observed with the electrode containing the 900° C. sample. Without wishing to be bound by theory, it is believed that the fact that this electrode had the highest degree of expansion of all samples indicates that the structural and chemical relationship within the particulate material has been so badly degraded that particle expansion is no longer controlled. This excessive expansion is expected to quickly lead to isolated particles and poor cycle retention.

[0153] Further electrode performance data for anodes each incorporating one of the samples was conducted. The results of these tests are shown in Table 3 and in FIG. 4.

[0154] These data show that the first lithiation capacity and the first delithiation capacity are both decreased when the heat treatment temperature of the sample incorporated into the electrode increased. The percentage of active silicon as a percentage of the total particle mass was also decreased as the heat treatment temperature increased. The active silicon content of the composite particles is calculated by dividing the first de-lithiation capacity of the helf-cell (in mAh per of the composite particles) by the theoretical capacity of silicon (3579 mAh/g) and representing the result as a percentage. The first cycle loss increases as the heat treatment temperature is increased and is particularly increased for the 900° C. treated sample. The anode thickness increased for all samples after 1.5 cycles; however the amount of thickness increase was roughly constant for the reference sample, the 600° C. treated sample and the 700° C. sample, whereas the 800° C. treated sample and the 900° C. treated samples exhibited a greater increase of electrode thickness. In the context of a battery, it is desirable to keep the electrode thickness as constant as possible.

[0155] Overall, the volumetric capacity of the anode was roughly the same for the 600° C. treated sample and the reference sample. It was slightly decreased for the 700° C. treated sample and was significantly decreased for the 800° C. and 900° C. treated samples.

[0156] These tests were conducted in a half cell with silicon-carbon composite precursor particles without a carbon coating. However, the results are expected to be replicated with the carbon coating and demonstrate the negative impact that a high-temperature carbon coating process would have on the properties of a carbon-coated particulate silicon-carbon composite product.

TABLE-US-00003 TABLE 3 Heat treatment temperature, ° C. Reference sample 600 700 800 900 total wt % silicon, measured 48 47 44 47 46 active silicon wt %, estimated 42 39 37 32 19 1st lithiation capacity, mAh/g 2077 ± 22 2003 ± 20 1941 ± 16 1754 ± 18 1237 ± 31 1st de-lithiation capacity, mAh/g 1513 ± 18 1408 ± 11 1317 ± 9  1162 ± 13  687 ± 12 % first cycle loss  27.2 ± 0.5  29.7 ± 0.3  32.1 ± 0.4  33.7 ± 0.3  44.4 ± 0.5 initial density, g/cm.sup.3 0.28 0.3 0.28 0.28 0.27 final density, g/cm.sup.3 0.25 0.27 0.26 0.24 0.21 % change in coating thickness  11 ± 8  11 ± 3  8 ± 1  13 ± 4  25 ± 7 volumetric capacity, mAh/cm.sup.3 263 266 241 197 104

Example 6—Uncoated Samples

Test Protocol

[0157] Silicon-carbon composite particles were prepared using 125 g of carbon C1 and the method of Example 2. The heat treatment test protocol of Example 4 was repeated except that there was no jet milling step before the heat treatment step, and the heat treatment step was carried out in a nitrogen atmosphere instead of an argon atmosphere.

Results

[0158] The heat-treated samples and the reference sample were analysed using XRD. The results are shown in FIG. 6; the heat treatment temperature for each sample can be seen on the right-hand side of the XRD plot with the 600° C. sample XRD trace shown at the front and the 900° C. sample XRD trace shown at the back.

[0159] An important effect that was observed to depend on the heat treatment temperature was formation of compounds. In particular, some non-electroactive compounds formed in the 800° C. and 900° C. samples, such as Si.sub.3N.sub.4 and SiC. This is an undesirable outcome because it reduces the overall capacity of the material, thereby reducing its value as an anode material. Furthermore, SiC is a poor electrical conductor; presence of SiC in the final product is therefore further undesirable because it could frustrate lithiation of silicon within the particulate material.

[0160] The absence of silicon nitride formation after heat treatment at temperatures of 700° C. and below is an important benefit because it shows that the method of the invention can be carried out in a nitrogen environment rather than an argon environment.

[0161] Likewise, crystallisation of silicon was observed in the 800° C. and 900° C. samples and this is undesirable: amorphous silicon is the preferred form of silicon for these materials.

[0162] With the 700° C. sample, a mixture of amorphous and crystalline silicon was observed from the XRD data. A small peak for crystalline silicon is superimposed on the broad amorphous peak at this temperature.

[0163] Where crystallisation of silicon occurred, it is believed that this may additionally be symptomatic of an increase in the average length scale of the silicon domains within the particulate material, caused by the heat treatment.

[0164] The deposition of a carbon coating in the method of the invention has a broadly similar time period to the length of time at which the samples in these tests were held at their respective temperatures. By testing the uncoated particles, the effect of the temperature on the fine interior structure of the composite particles can be more easily observed than when they are coated. It is expected that the same effects would be observed when carrying out carbon deposition at the same temperatures.

[0165] Therefore the method of the invention, in which the carbon deposition is carried out a temperature of no more than 700° C., is expected to avoid or lessen the deleterious effects that were observed at 800° C. and 900° C. in this experiment and therefore retain the beneficial fine microstructure that the inventors developed previously, whilst also obtaining the benefits of an electrically conductive coating and pore capping. At temperatures 600° C. and lower, the material properties of the particles are even better for use in an anode.

Example 7: Carbon Coating

[0166] A series of composite particle samples were prepared according to the method of Example 1 using carbon frameworks C2 and C3.

[0167] The precursor particles had the properties set out in Table 3. The amount of coarse silicon was determined using the TGA method described in Example 3. All of the composite particle samples contained less than 4 wt % of coarse silicon.

TABLE-US-00004 TABLE 3 BET Composite Carbon Surface Area Coarse No. Framework (m.sup.2/g) Si wt % C wt % Si wt % N1 C2 215 48.8 45.8 1.7 N2 C2 134 51.6 43.1 1.71 N3 C2 156 49.8 43.8 2.56 N4 C2 263 45.8 49.5 2.58 N5 C3 212 48.6 44.0 2.64 N6 C3 259 48.9 46.64 3.78 N7 C2 179 52.1 45.5 3.79

[0168] The composite particle samples described in Table 3 were carbon coated according to the following method. The composite particles (60 g) were placed into a stainless-steel tube (diameter 57 mm, length 500 mm) loaded into the heated zone of a rotary furnace (and sealed. The reactor space was purged with nitrogen at 0.2 L/min for 30 min. The furnace temperature was ramped up to the temperature specified in Table 4 under nitrogen flow. An excess amount of styrene was placed in a Dreschel bottle and heated in a water bath, up to 75° C. After 10 minutes of furnace temperature stabilisation, styrene was allowed to flow into the reactor tube for up to 90 minutes (as indicated in Table 4) by bubbling nitrogen of 2 L/min into the Dreschel bottle. The reactor was then purged with nitrogen and cooled down to ambient temperature under nitrogen, resulting in a carbon coated material.

[0169] Carbon-coated particles from Example 7 were analysed for their elemental composition and their coarse silicon content, again using the TGA method described in Example 3. The results are shown in Table 4 and in FIG. 7. All of the carbon-coated particles had a BET surface area in the range of 6-12 m.sup.2/g except for S1 (58 m.sup.2/g) and S10 (22 m.sup.2/g).

TABLE-US-00005 TABLE 4 Coarse Com- Si C Coarse Si Sample posite T Time wt wt Si increase No. No. (° C.) (min) % % wt % (%) S1 N1 608 90 44.9 47.8 2.51 147 S2 N2 640 90 48.4 45.0 2.67 156 S3 N6 640 90 43.2 50.0 5.23 138 S4 N3 665 90 45.6 45.8 4.42 172 S5 N3 665 90 48.0 45.0 4.32 169 S6 N3 665 90 46.7 44.9 4.47 175 S7 N7 665 90 42.9 45.5 6.38 168 S8 N7 665 90 43.0 47.3 6.19 163 S9 N6 665 90 42.1 50.7 6.07 160  S10 N5 675 60 43.3 48.1 4.90 186  S11 N6 690 67 41.6 50.8 7.04 186  S12* N1 710 60 47.3 49.0 5.73 337  S13* N4 735 60 30.8 56.3 9.03 350  S14* N5 750 60 40.6 50.4 10.30 390  S15* N1 758 60 40.8 51.4 8.73 513 *Comparative examples

[0170] The data in Table 4 shows that the relative increase in coarse silicon is below 190% in all cases when carbon coating is carried out at a temperature below 700° C. However, when the temperature is increased above 700° C., there is a significant increase in the amount of coarse silicon. This indicates that the particles coated at temperatures above 700° C. suffered from annealing of the fine micropore structure and amalgamation of nanoscale silicon domains within the pore structure to form larger domains of silicon. This therefore supports the hypothesis in Example 4 that the changes to the silicon domains observed when heating uncoated particles are also observed during the carbon coating process.

Example 8—Electrochemical Testing of Coated Particles

[0171] Negative electrode coatings (anodes) were prepared using the Si—C composite materials of Table 4 and were tested in full coin cells. To make the electrodes, a dispersion of carbon black in CMC binder was mixed in a Thinky™ mixer. The Si—C composite material was added to the mixture and mixed for 30 min in the Thinky™ mixer. SBR binder was then added to give a CMC:SBR ratio of 1:1, yielding a slurry with a weight ratio of Si—C composite material:CMC/SBR:carbon black of 70%:16%:14%. The slurry was further mixed for 30 min in the Thinky™ mixer, then was coated onto a 10 μm thick copper substrate (current collector) and dried at 50° C. for 10 minutes, followed by further drying at 110° C. for 12 hours to form a negative electrode with a coating density of 0.7±0.5 g/cm.sup.3.

[0172] Full coin cells were made using circular negative electrodes of 0.8 cm radius cut from the negative electrodes with a porous polyethylene separator and a nickel manganese cobalt (NMC532) positive electrode. The positive and negative electrodes were designed to form a balanced pair, such that the capacity ratio of the positive to negative electrodes was 0.9. An electrolyte comprising 1M LiPF.sub.6 in a solution of fluoroethylene carbonate, ethylene carbonate and ethyl methyl carbonate containing 3 wt % vinylene carbonate was then added to the cell before sealing.

[0173] The coin cells were cycled as follows: A constant current was applied at a rate of C/25, to lithiate the anode, with a cut off voltage of 4.3 V. When the cut off was reached, a constant voltage of 4.3 V is applied until a cut off current of C/100 is reached. The cell was then rested for 10 minutes in the lithiated state. The anode is then delithiated at a constant current of C/25 with a cut off voltage of 2.75 V. The cell was then rested for 10 minutes. After this initial cycle, a constant current of C/2 was applied to lithiate the anode with a 4.3 V cut off voltage, followed by a 4.3 V constant voltage with a cut off current of C/40 with rest time of 5 minutes. The anode was then delithiated at a constant current of C/2 with a 2.75V cut off. This was then repeated for the desired number of cycles. The capacity retention at 100 cycles (CR100) and 200 cycles (CR200) was calculated and is given in Table 5 along with the 1st lithiation capacity, the 1st delithiation capacity and the first cycle loss (FCL).

[0174] The charge (lithiation) and discharge (delithiation) capacities for each cycle are calculated per unit mass of the silicon-carbon composite material and the capacity retention value is calculated for each discharge capacity as a percentage of the discharge capacity on the second cycle. The first cycle loss (FCL) is (1−(1.sup.st delithiation capacity/1.sup.st lithiation capacity))×100%. The values in Table 5 are averaged over 3 coin cells for each material.

[0175] The active silicon level is determined from half cell experiments as described in Example 5.

[0176] The data in Table 5 shows that the increase in the coarse Si wt % as the carbon coating temperature increases is reflected in inferior electrochemical performance. As the level of coarse silicon increases, the amount of active silicon in the material (as determined from half cells) decreases, the initial lithiation and delithiation capacity of the material is reduced, and normalised capacity retention over multiple charge discharge cycles is reduced. The normalised capacity retention of these materials is shown in FIGS. 8 and 9.

TABLE-US-00006 TABLE 5 Sample No. Coating Coarse Active 1st 1st (composite T Si Si lith. delith. FCL CR100** CR200** No.) ° C. wt % % mAh/g mAh/g % % % S3 (N6)  640 5.23 40 1669 ± 1054 ± 36.8 ± 77 67 14 5 0.2 S9 (N6)  665 6.07 37 1586 ±  992 ± 37.4 ± 73 65 3 14 1 S11 (N6) 690 7.04 36 1523 ±  984 ± 35.4 ± 71 64 2 6 0.3 S10 (N5) 675 4.9 40 1773 ± 1242 ± 29.9 ± 76 67 5 13 0.5 S14 (N5) 750 10.3 35 1534 ± 1068 ± 30.4 ± 65 63 7 10 0.9 S12 (N1) 710 5.73 39 1674 ± 1212 ± 27.6 ± 78 70 12 23 0.9 S15 (N1) 758 8.73 34 1458 ± 1067 ± 26.8 ± 67 61 8 10 0.4 *Comparative samples **To facilitate comparison between samples having different initial capacities, the capacity retention values in Table 5 are normalised to 45 wt % active silicon by multiplying the percent capacity retention by the Active Si value and dividing by 45.