Improved Mn4+/Mn2+ Contribution in High-Voltage Zn–MnO2 Batteries Enabled by an Al3+-Ion Electrolyte

Abstract

Rechargeable aqueous Zn–MnO2 batteries are attracting attention as a cost-effective and safe energy storage solution, but their commercialization faces challenges due to limited stability, output voltage, and energy density. Herein, a hybrid-ion Zn–MnO2 system with enhanced Mn4+/Mn2+ electrochemical contribution is introduced using an Al3+-based electrolyte. Compared with conventional Zn2+ electrolytes, the hybrid Al3+/Zn2+ cell offers higher output voltage of 1.75 V, capacities up to 469 mAh g−1, and outstanding energy densities up to ≈730 Wh kg−1 at 0.3 A g−1. Besides, the Al3+-enabled Zn–MnO2 battery shows 100% capacity and energy density retention after 10,000 cycles at 2 A g−1. Even at a high mass–loading of 6.2 mg cm−2, a capacity of ≈200 mAh g−1 is maintained for over 100 cycles. This outstanding performance is related to the contribution of different intercalation and reaction mechanisms, as proved by the combination of electrochemical analysis and ex-situ x-ray diffraction characterization of the cells at different discharge stages. Al3+ ions, as Lewis strong acid, contribute to capacity in two significant ways: through a highly reversible intercalation/de-intercalation that substantially boosts capacitance at low current rates, and promoting the Mn4+/Mn2+ reaction aided by H+ that dominates the capacitance at higher current rates. Overall, this work demonstrates a practical Zn–MnO2 battery with a high potential for low-cost stationary energy storage habilitated by multiple ion co-intercalation.