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

Abstract One major challenge of aqueous Zn–MnO 2 batteries for practical applications is their unacceptable performance below freezing temperatures. Here the use of simple Zn(ClO 4 ) 2 aqueous electrolytes is described for all‐weather Zn–MnO 2 batteries even down to −60 °C. The symmetric, bulky ClO 4 − anion effectively disrupts hydrogen bonds between water molecules and provides intrinsic ion diffusion even while frozen, and enables ≈260 mAh g −1 on MnO 2 cathodes at −30 °C . It is identified that subfreezing cycling shifts the reaction mechanism on the MnO 2 cathode from unstable H + insertion to predominantly pseudocapacitive Zn 2+ insertion, which converts MnO 2 nanofibers into complicated zincated MnO x that are largely disordered and appeared as crumpled paper sheets. The Zn 2+ insertion at −30 °C is faster and much more stable than at 20 °C, and delivers ≈80% capacity retention for 1000 cycles without Mn 2+ additives. In addition, simple Zn(ClO 4 ) 2 electrolyte also enables a nearly fully reversible and dendrite‐free Zn anode at −30 °C with ≈98% Coulombic efficiency. Zn–MnO 2 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.