Kinetic square scheme in oxygen-redox battery electrodes

Abstract

Integrating an anionic-redox (or oxygen-redox) capacity with a conventional cationic-redox capacity is a promising strategy for large-capacity battery cathodes exceeding present technical limits. However, most oxygen-redox cathodes exhibit a large charge/discharge voltage hysteresis (>0.5 V), resulting in poor energy efficiency and impractical implementation. Here, we show that nonpolarizing O⁻ ↔ O²⁻ (4.4 V vs. Li/Li⁺) and polarizing O₂²⁻ → O²⁻ (3.3 V vs. Li/Li⁺) coexist and kinetically compete in O2-type Li_1.12–yNi_0.17Mn_0.71O₂. The oxygen-redox reaction is described as a square scheme, involving bond-forming 2O⁻ → O₂²⁻ and bond-cleaving O₂⁴⁻ → 2O²⁻ processes, where preventing the formation of O₂²⁻ is essential to realize non-polarizing and energy-efficient oxygen-redox reactions.