Understanding the Low-Voltage Hysteresis of Anionic Redox in Na₂Mn₃O₇

The large-voltage hysteresis remains one of the biggest barriers to optimizing Li/Na-ion cathodes using lattice anionic redox reaction, despite their very high energy density and relative low cost. Very recently, a layered sodium cathode Na₂Mn₃O₇ (or Na4/7Mn6/71/7O₂, is vacancy) was reported to have reversible lattice oxygen redox with much suppressed voltage hysteresis. Yet, the structural and electronic structural origin of this small-voltage hysteresis has not been well understood. In this article, through systematic studies using ex situ/in situ electron paramagnetic resonance and X-ray diffraction, we demonstrate that the exceptional small-voltage hysteresis (<50 mV) between charge and discharge curves is rooted in the well-maintained oxygen stacking sequence in the absence of irreversible gliding of oxygen layers and cation migration from the transition-metal layers. In addition, we further identify that the 4.2 V charge/discharge plateau is associated with a zero-strain (de)intercalation process of Na⁺ ions from distorted octahedral sites, while the 4.5 V plateau is linked to a reversible shrink/expansion process of the manganese-site vacancy during (de)intercalation of Na⁺ ions at distorted prismatic sites. It is expected that these findings will inspire further exploration of new cathode materials that can achieve both high energy density and efficiency by using lattice anionic redox.