A medium-entropy transition metal oxide cathode for high-capacity lithium metal batteries

The limited capacity of the positive electrode active material in non-aqueous rechargeable lithium-based batteries acts as a stumbling block for developing high-energy storage devices. Although lithium transition metal oxides are high-capacity electrochemical active materials, the structural instability at high cell voltages (e.g., >4.3 V) detrimentally affects the battery performance. Here, to circumvent this issue, we propose a Li_1.46Ni_0.32Mn_1.2O_4-x (0 < x < 4) material capable of forming a medium-entropy state spinel phase with partial cation disordering after initial delithiation. Via physicochemical measurements and theoretical calculations, we demonstrate the structural disorder in delithiated Li_1.46Ni_0.32Mn_1.2O_4-x, the direct shuttling of Li ions from octahedral sites to the spinel structure and the charge-compensation Mn³⁺/Mn⁴⁺ cationic redox mechanism after the initial delithiation. When tested in a coin cell configuration in combination with a Li metal anode and a LiPF₆-based non-aqueous electrolyte, the Li_1.46Ni_0.32Mn_1.2O_4-x-based positive electrode enables a discharge capacity of 314.1 mA h g^-1 at 100 mA g^-1 with an average cell discharge voltage of about 3.2 V at 25 ± 5 °C, which results in a calculated initial specific energy of 999.3 Wh kg^-1 (based on mass of positive electrode's active material).