Silicon composite, making method, and non-aqueous electrolyte secondary cell negative electrode material

Abstract

A silicon composite comprises silicon particles whose surface is at least partially coated with a silicon carbide layer. It is prepared by subjecting a silicon powder to thermal CVD with an organic hydrocarbon gas and/or vapor at 900-1,400° C., and heating the powder for removing an excess free carbon layer from the surface through oxidative decomposition.

1 - 7 . (canceled) 
     
     
         8 . A negative electrode material for a non-aqueous electrolyte secondary cell, wherein said negative electrode comprises:
 a silicon composite comprising silicon particles having an average particle size of from 50 nm to 50 μm whose surfaces are coated with a fused layer of silicon carbide and comprise free carbon in an amount of from 0 wt. % to 5 wt. % based on the total weight of said silicon composite, wherein said silicon carbide is present in an amount of from 10 wt. % to 36 wt. % based on the total weight of said silicon composite,   wherein said silicon composite is in the form of a powder having an average particle size of from 0.08 μm to 52 μm, and   wherein said silicon composite further comprises zero-valent silicon in an amount of from 62.7 wt. % to 90 wt. % based on the total weight of said silicon composite, wherein said zero-valent silicon is capable of generating hydrogen gas when reacted with an alkali hydroxide solution.   
     
     
         9 . The negative electrode material according to  claim 8 , wherein said silicon carbide is present in an amount of from 20 wt. % to 36 wt. % based on the total weight of said silicon composite. 
     
     
         10 . The negative electrode material according to  claim 8 , wherein said silicon particles have an average particle size of from 100 nm to 20 μm. 
     
     
         11 . The negative electrode material according to  claim 8 , wherein said silicon composite is in the form of a powder having an average particle size of from 0.5 μm to 40 μm. 
     
     
         12 . The negative electrode material according to  claim 8 , wherein said zero-valent silicon is present in an amount of from 62.7 wt. % to 80 wt. % based on the total weight of said silicon composite. 
     
     
         13 . The negative electrode material according to  claim 8 , wherein a diffraction peak attributable to silicon is observed when said silicon composite is analyzed by diffractometry. 
     
     
         14 . The negative electrode material according to  claim 13 , wherein said diffraction peak centers at approximately 2θ=28.4° and is attributable to Si(111) when said silicon composite is analyzed by x-ray diffractometry. 
     
     
         15 . The negative electrode material according to  claim 8 , wherein said silicon particles comprise free carbon in an amount of from 0 wt. % to 3 wt. % based on the total weight of said silicon composite. 
     
     
         16 . The negative electrode material according to  claim 8 , wherein said silicon particles comprise free carbon in an amount of from 0 wt. % to 2 wt. % based on the total weight of said silicon composite. 
     
     
         17 . The negative electrode material according to  claim 8 , wherein said silicon particles comprise free carbon in an amount of from 1.3 wt. % to 5 wt. % based on the total weight of said silicon composite. 
     
     
         18 . A method for preparing the silicon composite according to  claim 8 , wherein said method comprises:
 subjecting silicon particles to a thermal chemical vapor deposition treatment at a temperature of from 900° C. to 1400° C. in the presence of a fluidizing gas comprising an organic material in gaseous and/or vaporous form; and   heating at a temperature of from 600° C. to 1400° C. under an oxidizing atmosphere thereby removing excess free carbon from said surfaces of the silicon particles to produce the silicon particles having an average particle size of from 50 nm to 50 μm whose surfaces are coated with a fused layer of silicon carbide of said silicon composite.   
     
     
         19 . The method for preparing the silicon composite according to  claim 18 , wherein said organic material is one or more selected from the group consisting of an aliphatic hydrocarbon, an aromatic hydrocarbon, a heterocyclic hydrocarbon, and mixtures thereof. 
     
     
         20 . The method for preparing the silicon composite according to  claim 18 , wherein said thermal chemical vapor deposition treatment is carried out under a non-oxidizing atmosphere and said fluidizing gas further comprises an inert gas. 
     
     
         21 . The method for preparing the silicon composite according to  claim 18 , wherein said thermal chemical vapor deposition treatment is carried out at a temperature of from 900° C. to 1300° C. 
     
     
         22 . The method for preparing the silicon composite according to  claim 18 , wherein said heating is carried out at a temperature of from 600° C. to 900° C. 
     
     
         23 . A method of confirming the preparation of the silicon composite according to  claim 8 , wherein said method comprises:
 dissolving said silicon composite in a solution comprising hydrofluoric acid and an oxidizing agent; and   evaporating said solution to dryness,   whereby the presence of a precipitate of silicon carbide after said dissolving, and the presence of a grayish greenish evaporation residue of silicon carbide after said evaporating, confirms that said silicon composite comprises silicon particles whose surfaces are at least partially coated with a layer of silicon carbide.   
     
     
         24 . The negative electrode material according to  claim 8 , wherein said silicon carbide is present in an amount of from 10 wt. % to 36.0 wt. % based on the total weight of said silicon composite. 
     
     
         25 . The method for preparing the silicon composite according to  claim 18 , wherein said silicon carbide is present in an amount of from 10 wt. % to 36.0 wt. % based on the total weight of said silicon composite.