Reactivity with Water and Bulk Ruthenium Redox of Lithium Ruthenate in Basic Solutions

Abstract The reactivity of water with Li‐rich layered Li 2 RuO 3 and partial exchange of Li 2 O with H 2 O within the structure is studied under aqueous (electro)chemical conditions. Upon slow delithiation in water over long time periods, micron‐sized Li 2 RuO 3 particles structurally transform from an O3 structure to an O1 structure with a corresponding loss of 1.25 Li ions per formula unit. The O1 stacking of the honeycomb Ru layers is imaged using high‐resolution high‐angle annular dark‐field scanning transmission electron microscopy, and the resulting structure is solved by X‐ray powder diffraction and electron diffraction. In situ X‐ray absorption spectroscopy suggests that reversible oxidation/reduction of bulk Ru sites is realized on potential cycling between 0.4 and 1.25 V RHE in basic solutions. In addition to surface redox pseudocapacitance, the partially delithiated phase of Li 2 RuO 3 shows high capacity, which can be attributed to bulk Ru redox in the structure. This work demonstrates that the interaction of aqueous electrolytes with Li‐rich layered oxides can result in the formation of new phases with (electro)chemical properties that are distinct from the parent material. This understanding is important for the design of aqueous batteries, electrochemical capacitors, and chemically stable cathode materials for Li‐ion batteries.