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

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.