Regulating d0 transition metals and facilitating high-performance Li-excess cation-disordered rock salt cathode materials

Abstract

Cation-disordered rock salt (DRX) cathode materials have attracted significant attention owing to their high specific capacity, low cost, and utilization of earth-abundant elements. The electrochemical properties of DRX materials are influenced by multiple factors. Among these, doping with d⁰ elements, which have no valence d electrons in transition metals, is a widely accepted approach within the research community. However, it is still dependent on the d⁰-TM concentration applied to DRX materials, structure stability, and electrochemical properties. Herein, a systematic study was conducted on a series of Li_1.2Mn_0.4³⁺Ti_xMn_0.4−x⁴⁺O₂ DRX materials with varying contents of Ti⁴⁺. Li_1.2Mn_0.4³⁺Ti_0.1Mn_0.3⁴⁺O₂ (x = 0.1, denoted as MT_0.1) exhibited excellent electrochemical performance, with an initial discharge capacity of 283.9 mA h g⁻¹ at a current density of 20 mA g⁻¹ and a capacity retention of 86% after 50 cycles. The charge compensation process, including both Mn³⁺/Mn⁴⁺ cationic redox (Mn³⁺/Mn⁴⁺) and oxygen redox, and structural stability within MT_0.1 during charging–discharging are elucidated from X-ray photoelectron spectroscopy (XPS) and in situ X-ray diffraction (XRD) characterization results, respectively. A fundamental understanding was also achieved using density functional theory (DFT) calculations, which revealed that MT_0.1 has a lower Li⁺ diffusion energy barrier and thermodynamic transition state stability among these compounds. These thorough studies offer crucial information for comprehending DRX cathode materials for Li-ion batteries (LIBs) that have excellent structural stability. Our research unveiled the concentrations of d⁰ elements that play a crucial role in DRX materials, providing a strategy to rationally design Li-excess DRX materials for advanced energy storage systems.