Structural and electrochemical properties of Mn-based DRX cathodes via metal composition control

Cation-disordered rocksalt (DRX) cathodes consisting of redox-active and redox-inactive transition metals (TMs) have received much attention due to their high capacity, low cost and wide selection of TMs. Presently, few systematic studies have identified the effect of the ratio of the metal composition on the electrochemical performance of lithium-ion batteries. Here, the metal compositions were controlled following the formula of Li_1.2Mn _x Ti_0.8-x O₂ (0.2 ≤ x ≤ 0.7). Li_1.2Mn_0.7Ti_0.1O₂ showed the highest capacity of 213 mAh g^-1 at 10 mA g^-1 without capacity fading after 40 cycles, which is attributed to a multi-stage manganese redox reaction, with low contributions from the oxygen redox reaction. DRXs with manganese content below the stoichiometric ratio of 0.5 exhibited a relatively lower capacity retention of about 80% after 40 cycles at 20 mA g^-1, influenced by the irreversible oxygen redox reaction. To explore the extractible lithium within the DRX structure, the short range ordering (SRO) was investigated via Raman spectroscopy. As the amount of manganese increases, a lower binding energy and a higher portion of local ordering of the Mn-O axial A_1g stretching-like vibration were observed. The structural evolution of a new secondary phase was observed for high-Mn DRXs, as evidenced by ex situ XRD analysis and electrochemical characterization. This study gives guidelines for designing the SRO and structural stability of DRXs depending on the TM composition.