Structural factors for activating anionic redox in Li-rich Ti-based cathodes

Abstract

A systematic investigation of the anionic redox (AR) activities of four types of Li-rich Ti-based cathodes based on their crystal structures (monoclinic and rocksalt) and anions (oxygen and sulphur) was performed to exploit the full potential of the anionic capacity in lithium-ion batteries (LIBs). The phase (in)stabilities of Ti-based cathode models were investigated, demonstrating that a rocksalt-type structure consisting of sulphur ions destabilises its phase. Considering the prediction of the activation of (de)lithiation, the phase transition barrier under the narrowed spinodal decomposition region was observed. The ionic interactions, (i) cation–anion and (ii) anion–anion, revealed that the decrease in chemical hardness induced by S is a universal design factor for facilitating reversible and non-hysteretic AR reactions, leading to the shortening of the S–S bond distance, which was proposed to trigger sulphur redox. Based on the crystal structure and anionic species, our understanding provides an intriguing pathway for enabling active AR of inert Ti-oxide cathodes for advanced LIBs.