Investigating the electrochemical performance of MnO₂ polymorphs as cathode materials for aqueous proton batteries

Aqueous proton batteries (APBs) have attracted considerable attention due to the small size and fast kinetics of protons. However, the study of the cathode materials for APBs is unsatisfactory due to either low specific capacity or poor stability in acidic electrolytes, which greatly hinders the development of APBs. In this work, for the first time, six kinds of MnO₂ polymorphs (α-, β-, δ-, γ-, λ-, and R-MnO₂) are synthesized and systematically investigated to evaluate their potential as cathodes for APBs, among which the λ-MnO₂ stands out by virtue of the 235 mAh g⁻¹ specific capacity and 85% capacity retention rate over 500 cycles. To explain the reason for the stability of the λ-MnO₂, DFT calculations are conducted, and the results indicate that the λ-MnO₂ sustains minute volume expansion after proton insertion due to the slight Jahn-Teller distortion and robust all edge-sharing structure. The full battery performance shows the potential of the λ-MnO₂ as a promising cathode for APBs with an excellent capacity of 202 mAh g⁻¹ at 0.1 A g⁻¹ and an outstanding retention rate of 80% after 1000 cycles. These results broaden the range of cathode materials and shed new light on the performance improvement of APBs.