Strong Magnetic Exchange Interactions and Delocalized Mn–O States Enable High-Voltage Capacity in the Na-Ion Cathode P2–Na_0.67[Mg_0.28Mn_0.72]O₂

High Resolution Image Download MS PowerPoint Slide The increased capacity offered by oxygen-redox active cathode materials for rechargeable lithium- and sodium-ion batteries (LIBs and NIBs, respectively) offers a pathway to the next generation of high-gravimetric-capacity cathodes for use in devices, transportation and on the grid. Many of these materials, however, are plagued with voltage fade, voltage hysteresis and O 2 loss, the origins of which can be traced back to changes in their electronic and chemical structures on cycling. Developing a detailed understanding of these changes is critical to mitigating these cathodes’ poor performance. In this work, we present an analysis of the redox mechanism of P2–Na 0.67 [Mg 0.28 Mn 0.72 ]O 2, a layered NIB cathode whose high capacity has previously been attributed to trapped O 2 molecules. We examine a variety of charge compensation scenarios, calculate their corresponding densities of states and spectroscopic properties, and systematically compare the results to experimental data: 25 Mg and 17 O nuclear magnetic resonance (NMR) spectroscopy, operando X-band and ex situ high-frequency electron paramagnetic resonance (EPR), ex situ magnetometry, and O and Mn K -edge X-ray Absorption Spectroscopy (XAS) and X-ray Absorption Near Edge Spectroscopy (XANES). Via a process of elimination, we suggest that the mechanism for O redox in this material is dominated by a process that involves the formation of strongly antiferromagnetic, delocalized Mn–O states which form after Mg 2+ migration at high voltages. Our results primarily rely on noninvasive techniques that are vital to understanding the electronic structure of metastable cycled cathode samples.