Reversible Transformation of a Zinc Salt-Boosted High Areal Capacity Manganese Dioxide Cathode for Energy-Dense and Stable Aqueous Zinc Batteries

Recently, rechargeable aqueous zinc–manganese dioxide batteries have attracted extensive attention due to their intrinsic safety and high energy density. However, due to the poor reversibility and substrate-dependent feature of the MnO2 dissolution/deposition reaction, MnO2 is barely cycled at a low areal capacity and short period, which considerably limits its practical applications. Herein, a reversible transformation reaction of an inorganic salt Zn4(OH)6SO4·5H2O (ZHS) was coupled with the dissolution/deposition reaction of layered K- and Na-inserted MnO2 (KNMO) to realize a high areal capacity KNMO–ZHS composite cathode for energy-dense and stable aqueous zinc batteries. During the dissolution (discharging process) and deposition (charging process) of KNMO, ZHS transforms in the opposite direction and plays the role of a pH stabilizer during discharging and an in situ pore former during charging, thus considerably enhancing the reaction kinetics and reversibility of KNMO. The KNMO–ZHS cathode could deliver a high specific capacity of 335 mAh g–1 at 0.1 A g–1 and show stable cycling for 5000 cycles at 6 A g–1. In particular, for the first time, a zinc-ion battery with a record high mass loading of more than 50 mg cm–2 cathode was demonstrated and showed decent capacity retention, which paves the way for practical applications of zinc–manganese dioxide batteries.