Revealing and suppressing surface Mn(II) formation of Na_0.44MnO₂ electrodes for Na-ion batteries

Understanding and controlling the surface activities of electrode materials is critical for optimizing the battery performance, especially for nanoparticles with high surface area. Na_0.44MnO₂ is a promising positive electrode material for large-scale sodium-ion batteries. However, its application in grid-scale energy storage requires improvements in cycling stability at high rate. Here, we performed comprehensive surface-sensitive soft x-ray spectroscopic studies of the Na_0.44MnO₂ electrode. We are able to quantitatively determine the Mn evolution upon the potentials and cycle numbers. We reveal the Mn²⁺ formation on the top 10nm of Na_0.44MnO₂ particles when the electrochemical potential is below 2.6V, which does not occur in the bulk. A portion of the surface Mn²⁺ compounds become electrochemically inactive after extended cycles, contributing to the capacity fading. Based on the spectroscopic discoveries, we demonstrate that cycling Na_0.44MnO₂ above 3V could efficiently suppress the Mn²⁺ formation.