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 degrees C. The symmetric, bulky ClO4- anion effectively disrupts hydrogen bonds between water molecules and provides intrinsic ion diffusion even while frozen, and enables approximate to 260 mAh g(-1) on MnO2 cathodes at -30 degrees 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 MnOx that are largely disordered and appeared as crumpled paper sheets. The Zn2+ insertion at -30 degrees C is faster and much more stable than at 20 degrees C, and delivers approximate to 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 degrees C with approximate to 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.