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.