Nonpolarizing oxygen-redox capacity without O-O dimerization in Na2Mn3O7
Akihisa Tsuchimoto, Xiang‐Mei Shi, Kosuke Kawai, Benoît Mortemard de Boisse, Jun Kikkawa, Daisuke Asakura, Masashi Okubo, Atsuo Yamada
Abstract
Abstract 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 O 2 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- x Mn 3 O 7 , 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- x Mn 3 O 7 . Computations reveal that O −• is thermodynamically favorable over the peroxide-like O 2 2− dimer as a result of hole stabilization through a (σ + π) multiorbital Mn-O bond.