Enhanced Electrochemical Performance of LiNi0.5Mn1.5O4 Cathode Material by YPO4 Surface Modification

Cathode material LiNi0.5Mn1.5O4 (LNMO) for lithium-ion batteries is successfully synthesized by a sol–gel method and is further modified by a thin layer of YPO4 (1, 3, and 5 wt %) through a simple wet chemical strategy. Physical characterizations indicate that the YPO4 nanolayer has a little impact on the cathode structure. Electrochemical optimization reveals that the 3 wt % YPO4-coated LNMO could still deliver a high specific capacity of 107 mAh g–1 after 240 cycles, with a capacity retention of 77.5%, much higher than that of the pristine electrode. Electrochemical impedance spectroscopy (EIS) analysis proves that the rapid increase of surface impedance could be suppressed by the YPO4 coating layer and thus facilitates the surface kinetics behavior in repeated cycling. Through further material aging experiments, the improvement of electrochemical performances could be attributed to the formation of Lewis acid YF3, converted from the YPO4 coating layer in the LiPF6-based electrolyte, which not only scavenges the surface insulating alkaline species with a high acidity but also accelerates ion exchange on the material surface and thus helps to generate the solid solution Li–Ni–Mn–Y–O on the surface of YPO4-coated LNMO.