Synthesis and Redox Mechanism of Cation-Disordered, Rock-Salt Cathode-Material Li–Ni–Ti–Nb–O Compounds for a Li-Ion Battery

Abstract

Cation-disordered oxide materials working as cathodes for Li-ion batteries have been at a standstill because of their structurally limited specific capacities (below 175 mAh g^-1 in most cases). In this work, we have introduced 4d⁰ Nb⁵⁺ into host material LiNi_0.5Ti_0.5O₂ to synthesize Ni-based cation-disordered <i>Fm</i>3̅<i>m</i> Li-Ni-Ti-Nb-O compounds of Li_{1+<i>x</i>/100}Ni_{1/2-<i>x</i>/100}Ti_{1/2-<i>x</i>/100}Nb_<i>x</i>/100O₂ (<i>x</i> = 0, 5, 10, 15, 20) through a sol-gel method, showing particle sizes of less than 200 nm. Taking Li_1.2Ni_0.3Ti_0.3Nb_0.2O₂ with the best performance (an average voltage of ∼2.7 V and high discharge capacity of 221.5 mAh g^-1) among oxides as a model, we study the relationship between the structure, morphology, redox mechanism, and electrochemical performance of cation-disordered oxides through a combination of X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption near-edge spectroscopy tests and in situ XRD with electrochemistry. The obtained results indicate that the improved capacity is mainly ascribed to Nb⁵⁺, which optimizes the Ni²⁺/Ni⁴⁺ practical capacity and effectively stabilizes the O^2-/O^- redox reaction. The results emphasize that Li-Ni-Ti-Nb-O compounds are promising members in the family of cation-disordered transition-metal oxide materials.