Unraveling the Anionic Redox Chemistry in Aqueous Zinc-MnO₂ Batteries

Activating anionic redox reaction (ARR) has attracted a great interest in Li/Na-ion batteries owing to the fascinating extra-capacity at high operating voltages. However, ARR has rarely been reported in aqueous zinc-ion batteries (AZIBs) and its possibility in the popular MnO₂-based cathodes has not been explored. Herein, the novel manganese deficient MnO₂ micro-nano spheres with interlayer "Ca²⁺-pillars" (CaMnO-140) are prepared via a low-temperature (140 °C) hydrothermal method, where the Mn vacancies can trigger ARR by creating non-bonding O 2p states, the pre-intercalated Ca²⁺ can reinforce the layered structure and suppress the lattice oxygen release by forming Ca-O configurations. The tailored CaMnO-140 cathode demonstrates an unprecedentedly high rate capability (485.4 mAh g^-1 at 0.1 A g^-1 with 154.5 mAh g^-1 at 10 A g^-1) and a marvelous long-term cycling durability (90.6 % capacity retention over 5000 cycles) in AZIBs. The reversible oxygen redox chemistry accompanied by CF₃SO₃ ^- (from the electrolyte) uptake/release, and the manganese redox accompanied by H⁺/Zn²⁺ co-insertion/extraction, are elucidated by advanced synchrotron characterizations and theoretical computations. Finally, pouch-type CaMnO-140//Zn batteries manifest bright application prospects with high energy, long life, wide-temperature adaptability, and high operating safety. This study provides new perspectives for developing high-energy cathodes for AZIBs by initiating anionic redox chemistry.