Direct carbon coating at high temperature on LiNi_0.5Mn_1.5O₄ cathode: Unexpected influence on crystal structure and electrochemical performances

Carbon coated LiNi_0.5Mn_1.5O₄/C composite has been directly synthesized at 800 °C without using reducing gas or introducing additional low temperature treatments. Scanning/transmission electron microscope (S/TEM) observation shows clear coating layer on the particle's surface, which has been verified by Energy Dispersive X-Ray Spectroscopy (EDX), Raman spectra and Brunauer-Emmett-Teller (BET); and the results indicate the coating layer is amorphous carbon which significantly increases the BET surface area, and the carbon content is around 0.6 wt%. Unlike conventional low temperature coating process, the high temperature carbon coating process puts significant influence on the crystal structure in terms of degree of cation ordering and Mn³⁺ concentration. The LiNi_0.5Mn_1.5O₄/C exhibits improved rate capability, smaller polarization and better cyclic performances at high rates. These improvements can be attributed to the conductive carbon layer with better chemical stability, as well as the smaller concentration of Mn³⁺ resulted from the high temperature coating process.