High‐Energy and Stable Subfreezing Aqueous Zn–MnO2 Batteries with Selective and Pseudocapacitive Zn‐Ion Insertion in MnO2

One major challenge of aqueous Zn-MnO₂ batteries for practical applications is their unacceptable performance below freezing temperatures. Here the use of simple Zn(ClO₄ )₂ aqueous electrolytes is described for all-weather Zn-MnO₂ batteries even down to -60 °C. The symmetric, bulky ClO₄ ^- anion effectively disrupts hydrogen bonds between water molecules and provides intrinsic ion diffusion even while frozen, and enables ≈260 mAh g^-1 on MnO₂ cathodes at -30 °C . It is identified that subfreezing cycling shifts the reaction mechanism on the MnO₂ cathode from unstable H⁺ insertion to predominantly pseudocapacitive Zn²⁺ insertion, which converts MnO₂ nanofibers into complicated zincated MnO_x that are largely disordered and appeared as crumpled paper sheets. The Zn²⁺ insertion at -30 °C is faster and much more stable than at 20 °C, and delivers ≈80% capacity retention for 1000 cycles without Mn²⁺ additives. In addition, simple Zn(ClO₄ )₂ electrolyte also enables a nearly fully reversible and dendrite-free Zn anode at -30 °C with ≈98% Coulombic efficiency. Zn-MnO₂ prototypes with an experimentally verified unit energy density of 148 Wh kg^-1 at a negative-to-positive ratio of 1.5 and an electrolyte-to-capacity ratio of 2.0 are further demonstrated.