Bivalence Mn₅O₈ with hydroxylated interphase for high-voltage aqueous sodium-ion storage

Aqueous electrochemical energy storage devices have attracted significant attention owing to their high safety, low cost and environmental friendliness. However, their applications have been limited by a narrow potential window (∼1.23 V), beyond which the hydrogen and oxygen evolution reactions occur. Here we report the formation of layered Mn₅O₈ pseudocapacitor electrode material with a well-ordered hydroxylated interphase. A symmetric full cell using such electrodes demonstrates a stable potential window of 3.0 V in an aqueous electrolyte, as well as high energy and power performance, nearly 100% coulombic efficiency and 85% energy efficiency after 25,000 charge-discharge cycles. The interplay between hydroxylated interphase on the surface and the unique bivalence structure of Mn₅O₈ suppresses the gas evolution reactions, offers a two-electron charge transfer via Mn²⁺/Mn⁴⁺ redox couple, and provides facile pathway for Na-ion transport via intra-/inter-layer defects of Mn₅O₈.