Carbon powder suitable as a negative electrode material for nonaqueous secondary batteries
Carbon powder having low temperature calcined carbon derived from pitch adhered to a portion of the surface of natural graphite powder is obtained by solids mixing of natural graphite powder and pitch powder as a carbon precursor followed by heat treatment at 900-1500° C. to carbonize the pitch. The amount of pitch powder is such that the ratio V 2 /V 1 of the pore volume V 2 of pores having a diameter of 50-200 nm to the pore volume V 1 of pores having a diameter of 2-50 nm in a pore size distribution curve obtained by analysis of the nitrogen desorption isotherm of the resulting carbon powder by the BJH method is at least 1. This carbon powder can be used as a negative electrode material for a nonaqueous secondary battery able to operate at low temperatures.
1. A carbon powder comprising natural graphite powder having turbostratic carbon deposited on a portion of the surface thereof, wherein the carbon powder has a ratio of V 2 /V 1 of at least 1 wherein V 1 is the pore volume of pores having a diameter of 2-50 nm and V 2 is the pore volume of pores having a diameter of 50-200 nm both determined in a pore size distribution curve obtained by analyzing a nitrogen desorption isotherm by the BJH method.
2. A process for preparing a carbon powder as claimed in claim 1 comprising performing solids mixing of pitch powder having an average particle diameter of at most 500 μm with graphite powder and then performing heat treatment of the resulting mixture in a nonoxidizing atmosphere at a temperature of 900-1500° C.
3. A carbon powder as claimed in claim 1 wherein the ratio V 2 /V 1 is at least 1.3 and at most 2.0.