Improving Electrochemical Performance of High-Voltage Spinel LiNi_0.5Mn_1.5O₄ Cathode by Cobalt Surface Modification

High-voltage spinel LiNi_0.5Mn_1.5O₄ is considered as a promising cathode material for the next generation of lithium ion battery, but it is plagued by poor long cycling performances especially at high temperature. The as-prepared LiNi_0.5Mn_1.5O₄ material is surface modified by cobalt at 500 and 700 °C in this work. The obtained samples are studied by scanning transmission X-ray microscopy and electrochemical tests in detail. After surface modification at 500 °C, cobalt oxide coating is formed on the surface of LiNi_0.5Mn_1.5O₄ and electrochemical performance is not improved. Whereas after surface modification at 700 °C, the cobalt enters into the LiNi_0.5Mn_1.5O₄ surface layer, leading to cobalt surface doping. Cobalt surface doping decreases the Ni concentration and increases the oxidation state of Mn on the surface, which enhances the long cycling and high-temperature cycling performances. After 200 cycles at 55 °C, the capacity retention increases from 82% to 93% because of cobalt surface doping. After 2000 cycles at 5 C, the capacity of cobalt surface doped LiNi_0.5Mn_1.5O₄ remains 93 mAh g^-1 with capacity retention of 81%. Also, surface doping increases the disordered phase and Mn³⁺ content in LiNi_0.5Mn_1.5O₄. This helps to improve the rate performance.