Nonpolarizing oxygen-redox capacity without O-O dimerization in Na₂Mn₃O₇

Reversibility of an electrode reaction is important for energy-efficient rechargeable batteries with a long battery life. Additional oxygen-redox reactions have become an intensive area of research to achieve a larger specific capacity of the positive electrode materials. However, most oxygen-redox electrodes exhibit a large voltage hysteresis >0.5 V upon charge/discharge, and hence possess unacceptably poor energy efficiency. The hysteresis is thought to originate from the formation of peroxide-like O₂^2- dimers during the oxygen-redox reaction. Therefore, avoiding O-O dimer formation is an essential challenge to overcome. Here, we focus on Na_2-xMn₃O₇, which we recently identified to exhibit a large reversible oxygen-redox capacity with an extremely small polarization of 0.04 V. Using spectroscopic and magnetic measurements, the existence of stable O^-• was identified in Na_2-xMn₃O₇. Computations reveal that O^-• is thermodynamically favorable over the peroxide-like O₂^2- dimer as a result of hole stabilization through a (σ + π) multiorbital Mn-O bond.