Realization of an anion insertion mechanism for high-rate electrochemical energy storage in highly crystalline few-layered potassium manganese dioxide nanosheets

Abstract

Aqueous anionic energy storage with a non-flammable electrolyte has the advantage of high power density but suffers from limitations in terms of cycling performance. Herein, we report few-layered potassium manganese dioxide (K_0.5Mn₂O_4.3(H₂O)_0.5) with high crystallinity that exhibits high-capacity anion storage and rapid insertion in aqueous K₂SO₄ electrolyte. In contrast to the storage of cations such as H⁺, Li⁺, Na⁺, and K⁺, the insertion of SO₄²⁻ and OH⁻ anions inhibits the migration of cations in the birnessite anode. The nanosheets with rapid anion insertion have excellent electrochemical properties, including a specific volumetric capacitance of 350 F cm⁻³ at a current density of 1 mA cm⁻² at a mass loading of 10 mg cm⁻² and even 315 F cm⁻³ at 20 mA cm⁻², showing their extremely high-rate performance and stable cycling performance of up to 10 000 cycles with 85% capacity retention. This impressive electrode material uses anion storage technology to complement existing cation storage for the preparation of a material that exhibits long-lifespan and high-power energy storage.