Electrochemical performance and kinetics on lithium ion insertion/deinsertion of sulfuric acid surface-modified natural graphite

The natural graphite was modified by impregnation method with sulfuric acid as surface modification agent. FTIR results show that -OH and C=C bonds on the surface of natural graphite disappear while the number of -COO- increases after sulfuric acid surface-modification. The electrochemical performance reveals that the cycle performance and ratio performance are improved. The lithium-deinsertion capacities of natural graphite impregnated by 3 mol/L H2SO4 for 12 h (NGS3) and unmodified natural graphite (NG) are respectively 320.5 and 299.9 mA·h/g after 20 cycles at 0.5C. AC impedance was used to investigate kinetics on lithium ion insertion/ deinsertion. It is found that the membrane resistance (RSEI) and charge transfer resistance (Rct) decrease, while the membrane capacitance (CSEI) and extracted capacitance (CCPE) increase. Moreover, RSEI keeps constant after several cycles. The activation energies of NG and NGS3 are 87.7 and 77.2 kJ/mol, respectively, which indicates that sulfuric acid treatment is beneficial to the improvement of the desolvation of lithium ion from solvent molecules and the formation of a stable solid electrolyte interface (SEI).