Methods for forming silicon-silicon oxide-carbon composites for lithium ion cell electrodes
Composite silicon based materials are described that are effective active materials for lithium ion batteries. The composite materials comprise processed, e.g., high energy mechanically milled, silicon suboxide and graphitic carbon in which at least a portion of the graphitic carbon is exfoliated into graphene sheets. The composite materials have a relatively large surface area, a high specific capacity against lithium, and good cycling with lithium metal oxide cathode materials. The composite materials can be effectively formed with a two-step high energy mechanical milling process. In the first milling process, silicon suboxide can be milled to form processed silicon suboxide, which may or may not exhibit crystalline silicon x-ray diffraction. In the second milling step, the processed silicon suboxide is milled with graphitic carbon. Composite materials with a high specific capacity and good cycling can be obtained in particular with balancing of the processing conditions.
1. A method for forming a composite material comprising processed silicon suboxide and graphitic carbon at least a portion of which is in graphene sheets, the method comprising:
performing high energy ball milling or jar milling of graphite powder with processed silicon suboxide material having a BET surface area from about 2.5 m 2 /g to about 20 m 2 /g and a D50 volume average secondary particle size of no more than about 10 microns, to form the composite material, wherein the processed silicon suboxide is prepared by high energy mechanical milling of silicon suboxide prior to blending with graphite powder to form a blend and performing the high energy ball milling or jar milling with the blend.
2. The method of claim 1 wherein the silicon suboxide is represented by the formula SiO x where 0.1≤x≤1.9.
3. The method of claim 1 wherein the silicon suboxide material is processed silicon suboxide formed by high energy mechanical milling of silicon suboxide.
4. The method of claim 3 wherein the processed silicon suboxide is substantially free of milling media.
5. The method of claim 1 wherein the high energy ball milling is performed with zirconium oxide milling beads.
6. The method of claim 1 wherein the product composite material has a BET surface area from about 12 m 2 /g to about 35 m 2 /g and a D50 volume average secondary particle size of no more than about 5 microns.
7. The method of claim 1 wherein the product composite material comprises graphene sheets visible in micrographs and residual graphite as determined by x-ray diffraction.
8. The method of claim 1 wherein the product composite material has a discharge capacity of at least about 1200 mAh/g at a rate of C/20 discharged from 1.5V to 0.005V against lithium metal, a specific discharge capacity of at least about 1250 mAh/g at a rate of C/3 discharged from 1.5V to 0.005V against lithium metal, and a 50th cycle discharge capacity that is at least about 87% of the 5th cycle discharge capacity when cycled against lithium from 1.5V to 0.005V at a discharge rate of C/3.
9. The method of claim 1 wherein the product composite material has from about 2 weight percent to about 95 weight percent graphitic carbon.