Methods for fabricating improved graphite granules

Abstract

This invention relates to a method for the fabrication of improved graphite granules from unimproved graphite granules, where a polymer solution is uniformly sprayed onto a fluidized bed of unimproved graphite granules that are being stirred three dimensionally in high speed. The granules are then heat to dry to obtain coated graphite granules. To further improve the electrochemical properties of the final improved graphite granules, the coated graphite granules are immersed in a surface modifier, filtered, heat to dry and sifted. Then, they are solidified and carbonized in an inert environment to obtain final improved graphite granules. This invention is simple, easy to implement, and easy to implement for industrial production. The improved graphite granules made using embodiments of this invention, when used as the material for the negative electrode of a rechargeable battery produces a battery that has high initial charge/discharge efficiency, high reversible specific capacity and good cycle ability.

We claim:  
     
         1 . A method for fabricating improved graphite granules from unimproved graphite granules, comprising the steps of: 
 dissolving a first polymer in a first solvent to form a polymer solution;    fluidizing said unimproved graphite granules;    spraying said polymer solution to evenly coat said fluidized unimproved graphite granules;    drying said sprayed graphite granules of excessive polymer solution;    sifting said dried graphite granules;    solidifying said sifted graphite granules; and    carbonizing said solidified graphite granules to form said improved graphite granules.    
     
     
         2 . The method of  claim 1  wherein said first polymer is one or more polymers selected from the group consisting of: epoxy resin, phenolic aldehyde resin, polyacrylonitrile, polyvinyl alcohol, polystyrene, coal pitch, petroleum pitch, and coal tar.  
     
     
         3 . The method of  claim 1  wherein said first solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, N,N-dimethyl formamide, benzene, toluene, chloroform, and cyclohexane.  
     
     
         4 . The method of  claim 1  wherein the concentration of said first polymer in said polymer solution is between 1% and 30%.  
     
     
         5 . The method of  claim 1  wherein said unimproved graphite granules have an average diameter between 5 μm and 40 μm and the weight ratio between said polymer solution and said unimproved graphite granules is between 1:5 and 1:1.  
     
     
         6 . The method of  claim 1  wherein in said solidification step, said sifted graphite granules are held in an inert environment at a first holding temperature of between 200° C. and 500° C. for 0.5 hours to 3 hours.  
     
     
         7 . The method of  claim 6  wherein in said solidification step, the rate for increasing temperature to the first holding temperature is between 5° C./min and 20° C./min.  
     
     
         8 . The method of  claim 1  wherein in said carbonizing step, said solidified graphite granules are held in an inert environment at a second holding temperature of between 800° C. and 1100° C. for 2 hours to 10 hours.  
     
     
         9 . The method of  claim 8  wherein in said carbonizing step, the rate for increasing temperature to said second holding temperature is 3° C./min to 20° C./min.  
     
     
         10 . The method of  claim 8  wherein in said carbonizing step, after the solidified graphite granules have been held at said second holding temperature for said predetermined period of time, the rate for decreasing the temperature is 5° C./min to 15° C./min.  
     
     
         11 . The method of  claim 8  wherein in said spraying step, said fluidized graphite granules are being stirred at between 500 rpm and 4000 rpm in three dimensions and in directions that are parallel and perpendicular to the flow of said sprayed polymer solution.  
     
     
         12 . The method of  claim 9  wherein in said carbonizing step, after the solidified graphite granules have been held at said second holding temperature for said predetermined period of time, the rate for decreasing the temperature is 5° C./min to 15° C./min.  
     
     
         13 . The method of  claim 1  wherein after said sifting step and before said solidification step, the following steps are added: 
 immersing said sifted graphite granules in a polymer surface modifier comprising of a second polymer and a second solvent;  
 filtering said immersed graphite granules from said polymer surface modifier;  
 drying said filtered graphite granules with heat; and  
 sifting said dried graphite granules.  
 
     
     
         14 . The method of  claim 13  wherein said second polymer is one or more polymers selected from the group consisting of: coal pitch, coal tar, petroleum pitch, petroleum tar, benzene, naphthalene, copolymers of benzene and naphthalene, petroleum wax, and petroleum resin.  
     
     
         15 . The method of  claim 13  wherein said second solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, chloroform, tetrahydrofuran, carbon tetrachloride, and cyclohexane.  
     
     
         16 . The method of  claim 14  wherein said second solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, chloroform, tetrahydrofuran, carbon tetrachloride, and cyclohexane.  
     
     
         17 . The method of  claim 4  wherein said first polymer is one or more polymers selected from the group consisting of: epoxy resin, phenolic aldehyde resin, polyacrylonitrile, polyvinyl alcohol, polystyrene, coal pitch, petroleum pitch, and coal tar and said first solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, N,N-dimethyl formamide, benzene, toluene, chloroform, and cyclohexane.  
     
     
         18 . The method of  claim 17  wherein said unimproved graphite granules have an average diameter between 5 μm and 40 μm and the weight ratio between said polymer solution and said unimproved graphite granules is between 1:5 and 1:1.  
     
     
         19 . The method of  claim 12  wherein in said solidification step, said sifted graphite granules are held in an inert environment at a first holding temperature of between 200° C. and 500° C. for 0.5 hours to 3 hours; and the rate for increasing temperature to the first holding temperature is between 5° C./min and 20° C./min.  
     
     
         20 . The method of  claim 18  wherein after said sifting step and before said solidification step, the following steps are added: 
 immersing said sifted graphite granules in a polymer surface modifier comprising of a second polymer and a second solvent;  
 filtering said immersed graphite granules from said polymer surface modifier;  
 drying said filtered graphite granules with heat; and  
 sifting said dried graphite granules;  
 wherein said second polymer is one or more polymers selected from the group consisting of: coal pitch, coal tar, petroleum pitch, petroleum tar, benzene, naphthalene, copolymers of benzene and naphthalene, petroleum wax, and petroleum resin; and said second solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, chloroform, tetrahydrofuran, carbon tetrachloride, and cyclohexane.  
 
     
     
         21 . The method of  claim 18  wherein in said solidification step, said sifted graphite granules are held in an inert environment at a first holding temperature of between 200° C. and 500° C. for 0.5 hours to 3 hours and the rate for increasing temperature to said first holding temperature is between 5° C./min and 20° C./min; and in said carbonizing step, said solidified graphite granules are held in an inert environment at a second holding temperature of between 800° C. and 1100° C. for 2 hours to 10 hours and the rate for increasing temperature to said second holding temperature is 3° C./min to 20° C./min; and, after the solidified graphite granules have been held at said second holding temperature for said predetermined period of time, the rate for decreasing the temperature is 5° C./min to 15° C./min.  
     
     
         22 . The method of  claim 18  wherein in said spraying step, said fluidized graphite granules are being stirred at between 500 rpm and 4000 rpm in three dimensions and in directions that are parallel and perpendicular to the flow of said sprayed polymer solution.  
     
     
         23 . The method of  claim 22  wherein in said solidification step, said sifted graphite granules are held in an inert environment at a first holding temperature of between 200° C. and 500° C. for 0.5 hours to 3 hours and the rate for increasing temperature to said first holding temperature is between 5° C./min and 20° C./min; and in said carbonizing step, said solidified graphite granules are held in an inert environment at a second holding temperature of between 800° C. and 1100° C. for 2 hours to 10 hours and the rate for increasing temperature to said second holding temperature is 3° C./min to 20° C./min; and, after the solidified graphite granules have been held at said second holding temperature for said predetermined period of time, the rate for decreasing the temperature is 5° C./min to 15° C./min.  
     
     
         24 . A method for fabricating improved graphite granules from unimproved graphite granules, comprising the steps of: 
 dissolving a first polymer in a first solvent to form a polymer solution wherein said first polymer is one or more polymers selected from the group consisting of: epoxy resin, phenolic aldehyde resin, polyacrylonitrile, polyvinyl alcohol, polystyrene, coal pitch, petroleum pitch, and coal tar, said first solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, N,N-dimethyl formamide, benzene, toluene, chloroform, and cyclohexane, and, the concentration of said first polymer in said polymer solution is between 1% and 30%;    fluidizing said unimproved graphite granules wherein said unimproved graphite granules have an average diameter between 5 μm and 40 μm and the weight ratio between said polymer solution and said unimproved graphite granules is between 1:5 and 1:1;    spraying said polymer solution to evenly coat said fluidized unimproved graphite granules wherein said fluidized graphite granules are being stirred at between 500 rpm and 4000 rpm in three dimensions and in directions that are parallel and perpendicular to the flow of said sprayed polymer solution;    drying said sprayed graphite granules of excessive polymer solution;    sifting said dried graphite granules;    immersing said sifted graphite granules in a polymer surface modifier comprising of a second polymer and a second solvent wherein said second polymer is one or more polymers selected from the group consisting of: coal pitch, coal tar, petroleum pitch, petroleum tar, benzene, naphthalene, copolymers of benzene and naphthalene, petroleum wax, and petroleum resin, and said second solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, chloroform, tetrahydrofuran, carbon tetrachloride, and cyclohexane;    filtering said immersed graphite granules from said polymer surface modifier;    drying said filtered graphite granules with heat;    sifting said dried graphite granules;    solidifying said sifted graphite granules wherein said sifted graphite granules are held in an inert environment at a first holding temperature of between 200° C. and 500° C. for 0.5 hours to 3 hours and the rate for increasing temperature to the first holding temperature is between 5° C./min and 20° C./min; and    carbonizing said solidified graphite granules to form said improved graphite granules wherein, said solidified graphite granules are held in an inert environment at a second holding temperature of between 800° C. and 1100° C. for 2 hours to 10 hours; the rate for increasing temperature to said second holding temperature is 3° C./min to 20° C./min; and, after the solidified graphite granules have been held at said second holding temperature for said predetermined period of time, the rate for decreasing the temperature is 5° C./min to 15° C./min.