Conflicting roles of F doping on electrochemical performance of layered-spinel Li1.2Mn0.75Ni0.25O2-zFz as cathode materials for Li-ion batteries

Abstract

Herein, the conflicting roles of F doping on the electrochemical performance of layered-spinel Li_1.2Mn_0.75Ni_0.25O_2-zF_z (z = 0, 0.05, 0.1) cathode materials are studied. The compound consists of Li_1.2Mn_0.6Ni_0.2O₂ and LiMn_1.5Ni_0.5O₄ with their phase ratio depending on F doping content. When z = 0.05, it favors the spinel phase formation, leading to increased first Coulombic efficiency and improved cycle stability. However, it reduces the cathode capacity to 250 mAh g⁻¹ at C/10 (the capacity of the pristine is 280 mAhg⁻¹). With F doping content increasing to z = 0.1, the cycle stability of the cathode is the best, with 96 % capacity retention after 200 cycles at C/2. The high level of F stabilizes the structure, prevents phase transition, and retards voltage decay. The result demonstrates a promising strategy for the design of composite cathode materials by F doping.