Anode active materials that are low cost and have excellent capacity retention are suitable for large secondary battery systems such as energy saving systems (ESSs). In this study, inexpensive low-purity natural graphite was used as a matrix. This matrix was coated with amorphous carbon to improve the cycle characteristics and inhibit dissolution of impurities. The natural graphite coated with amorphous carbon was prepared by the pyrolysis of sucrose during heat treatment. The shape and crystal structure of the natural graphite powder were maintained even after a coating process that included heat treatment. The thickness of the amorphous carbon layer measured by TEM was less 10 nm. After amorphous carbon coating, the coulombic efficiency during the first cycle decreased from 91.3 to 87.9%. On the other hand, the capacity retention rate during 50 cycles increased from 90.9 to 94.7%. Interestingly, impurities in the natural graphite such as Al, Fe, and Zn did not affect the stability of the electrolyte.

Keywords: Lithium-ion batteries, Natural graphite, Hard carbon, Impurity, Dissolution