Effect of Fluorine Substitution on the Electrochemical Property and Structural Stability of a Lithium-Excess Cation Disordered Rock-Salt Cathode

Lithium-excess cation disordered rock-salt materials have received much attention because of their high-capacity as a candidate for cathodes for lithium-ion batteries. The ultra-high specific capacity comes from the coordinated charge compensation of both transition metal and lattice oxygen. However, the oxygen redox at high voltage usually leads to irreversible oxygen release, thereby degrading the structure stability and electrochemical performance. Lithium-excess Li_1.14Ni_0.57+0.5 xTi_0.19-0.5 xMo_0.10O_2-xF_x (x=0, 0.05, 0.10, 0.15, and 0.20) with different amounts of fluorine substitution were synthesized. Among them, Li_1.14Ni_0.620Ti_0.140Mo_0.10O_1.85F_0.15 exhibits a lower capacity decline, better rate performance, and lower structure damage. The effects of fluorine substitution on the electrochemical property and structural stability were systematic studied by x-ray photoelectron spectroscopy and in situ XRD etc. Results show that fluorine substitution reduces the average valence of the anion, allowing a larger proportion of low-valent redox active transition metals, increasing the transition metal redox capacity, inhibiting irreversible oxygen release and side reaction. Fluorine substitution further improves the structural stability and suppresses lattice deformation of the material.