Abstract
<h2>Summary</h2> The increasing need for sustainable energy storage has rekindled interest for Na-ion batteries. Their energy density can be enhanced using anionic redox (AR), as reported in Na-deficient P2 phases. Contrary to their Li-rich counterparts with O3 stacking, these Na-deficient P2 phases show surprisingly good structural stability during AR. Understanding the fundamental relationship between O and P stacking and AR reversibility thus becomes critical. Herein, using density functional theory (DFT) analysis and modeling of O2- and P2-Na<sub>2∕3</sub>Mg<sub>1∕3</sub>Mn<sub>2∕3</sub>O<sub>2</sub>, we show that during AR, the oxygen network is stabilized through either (1) a highly reversible collective distortion, in P stacking, or (2) a disproportionation of oxygen pairs leading to voltage hysteresis, in O stacking. Using this 2-distortions model, we describe a magnetic-constrained DFT methodology to predict the critical state of charge for reversible cycling that we successfully extend to other Mn-based cathodes. This article provides fundamental understanding, powerful computational methods, and practical guidelines to design next-generation cathode materials.
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