2 to 5 microns. The surface coating of the negative electrode material adopts the "dry method" coating. The coating agent asphalt ground to 2 to 5 microns is evenly mixed with the graphite powder that has been modified, shaped and graded by the honeycomb mill system, and then the air is isolated to promote the asphalt to dissolve in the graphite. A layer of asphalt film is formed on the surface of the micropowder, and finally graphitization or carbonization is performed to obtain a composite negative electrode material with a core structure. Asphalt is a dark-brown complex mixture composed of hydrocarbons of different molecular weights and their non-metallic derivatives.