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 >0.5V 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 Na2-xMn3O7, which we recently identified to exhibit a large reversible oxygen-redox capacity with an extremely small polarization of 0.04V. Using spectroscopic and magnetic measurements, the existence of stable O-center dot was identified in Na2-xMn3O7. Computations reveal that O-center dot is thermodynamically favorable over the peroxide-like O-2(2-) dimer as a result of hole stabilization through a (sigma+) multiorbital Mn-O bond. Majority of oxygen-redox electrodes exhibit a large voltage hysteresis but its origin is not fully understood. Here, the authors use combined RIXS and magnetic measurements to provide insights into the origin of the typically large voltage hysteresis observed upon oxygen redox.

Automated summary

This study focuses on the origin of voltage hysteresis in oxygen-redox reactions, particularly exploring the stable O· oxygen atoms identified in the Na2-xMn3O7 material. The results indicate that the presence of O· oxygen atoms and the hole stabilization through a (sigma+) multiorbital Mn-O bond are key factors for its small polarization and large capacity.