Processes to fabricate porous silicon and its use as feedstock for secondary battery electrodes

Abstract

Provided are processes to form microporous silicon useful as mi active material in an electrode of an electrochemical cell the processes including subjecting a mixture of silicon oxide and a metal reducing agent, optionally aluminum, to mechanical milling to form mechanically activated silicon oxide/aluminium, thermally treating the silicon oxide/aluminium to reduce the silicon oxide and form Si/Al 2 O 3 and removing at least a portion of the alumina from the Si to form a microporous silicon. The resulting electrochemically active microporous silicon is also provided with residual alumina present at 15% by weight or less that demonstrates excellent cycle life and safety.

1 . A process for the formation of microporous silicon comprising:
 subjecting a combination of powdered silicon oxide and aluminum to mechanical milling to form a mechanically activated silicon oxide/aluminum;   thermally treating said silicon oxide/aluminum by exposure to heat at a temperature of 500 degrees Celsius to 700 degrees Celsius under an inert or reducing atmosphere to form Si/Al 2 O 3 ; and   removing at least a portion of alumina from said Si/Al 2 O 3  by exposing said Si/Al 2 O 3  to an etchant to form microporous silicon.   
     
     
         2 . The process of  claim 1  wherein said silicon oxide has a linear dimension of 500 micrometers or less. 
     
     
         3 . The process of  claim 1  wherein said heat is at a temperature of 500 degrees Celsius to 600 degrees Celsius. 
     
     
         4 . The process of  claim 1  wherein said step of subjecting is by ball milling at a ball to powder mass ratio of 4:1 to 16:1. 
     
     
         5 . The process of any one of  claims 1 - 4  wherein said etchant excludes fluorine. 
     
     
         6 . The process of any one of  claims 1 - 4  wherein said etchant excludes hydrogen fluoride. 
     
     
         7 . The process of any one of  claims 1 - 4  wherein 85 alumina weight percent to 100 alumina weight percent of said alumina is removed by said step of removing. 
     
     
         8 . The process of any one of  claims 1 - 4  wherein 85 alumina weight percent to 99 alumina weight percent alumina is removed by said step of removing. 
     
     
         9 . The process of any one of  claims 1 - 4  wherein said acid comprises HCl, H 2 SO 4 , H 3 PO 4 , HNO 3  or combinations thereof. 
     
     
         10 . The process of any one of  claims 1 - 4  wherein said step of subjecting is in a high energy ball mill. 
     
     
         11 . The process of any one of  claims 1 - 4  wherein said step of subjecting is for a milling time of 0.5 to 24 hours. 
     
     
         12 . The process of any one of  claims 1 - 4  wherein said step of thermally treating is for a thermal treatment time of 10 minutes to 12 hours. 
     
     
         13 . A process of forming an electrode comprising:
 providing an anode active material comprising microporous silicon produced as in any one of  claims 1 - 4 ;   combining said microporous silicon with a binder to form a slurry; and   coating said slurry on an electrically conductive substrate to form an electrode.   
     
     
         14 . The process of  claim 13  wherein said anode active material further comprises graphite. 
     
     
         15 . The process of  claim 13  wherein said step of providing comprises:
 subjecting a combination of powdered silicon oxide and aluminum to mechanical milling to form a mechanically activated silicon oxide/aluminum, optionally SiO 2 /Al, powder; 
 thermally treating said silicon oxide/aluminum powder by exposure to heat at a temperature of 500 degrees Celsius to 700 degrees Celsius under an inert or reducing atmosphere to form Si/Al 2 O 3 ; 
 removing at least a portion of alumina from said Si/Al 2 O 3  by exposing said Si/Al 2 O 3  to an acid to form microporous silicon. 
 
     
     
         16 . The process of  claim 13  wherein said microporous silicon further comprises alumina at a weight percent of 0.1 to 1.5. 
     
     
         17 . An electrochemically active material comprising:
 a microporous silicon; and   alumina, said alumina present at 15% or less by weight of said material;   said microporous silicon and said alumina intermixed and mechanically activated.   
     
     
         18 . The electrochemically active material of  claim 17 , said alumina present at 15% by weight or less. 
     
     
         19 . The electrochemically active material of  claim 17 , said alumina present at 5% by weight or less. 
     
     
         20 . The electrochemically active material of  claim 17  further comprising carbon. 
     
     
         21 . The electrochemically active material of any one of  claims 17 - 20  characterized by a cycle life of 80 percent capacity or greater at cycle 40. 
     
     
         22 . The electrochemically active material of any one of  claims 17 - 20  characterized by a cycle life of 80 percent capacity or greater at cycle 80. 
     
     
         23 . The electrochemically active material of any one of  claims 17 - 20  characterized by an initial capacity loss of less than 20%. 
     
     
         24 . The electrochemically active material of any one of  claims 17 - 20 , said alumina comprising alpha-alumina. 
     
     
         25 . The electrochemically active material of any one of  claims 17 - 20 , said alumina comprising gamma-alumina. 
     
     
         26 . The electrochemically active material of any one of  claims 17 - 20 , said alumina comprising free of alpha-alumina or gamma-alumina. 
     
     
         27 . The electrochemically active material of any one of  claims 17 - 20  having a porosity of 10% to 90%. 
     
     
         28 . The electrochemically active material of any one of  claims 17 - 20  having a porosity of 30% to 60%. 
     
     
         29 . The electrochemically active material of any one of  claims 17 - 20  having a BET surface area of 10 m 2 /g to 500 m 2 /g. 
     
     
         30 . The electrochemically active material of any one of  claims 17 - 20  having a BET surface area of 20 m 2 /g to 100 m 2 /g. 
     
     
         31 . The electrochemically active material of any one of  claims 17 - 20  formed by the process of  claim 1 . 
     
     
         32 . An electrochemically active material produced by the process of any one of  claims 1 - 4 . 
     
     
         33 . A process of forming as electrochemically active microporous silicon substantially as described herein. 
     
     
         34 . An electrochemically active microporous silicon substantially as described herein.