Redox-active manganese dioxide@polypropylene hybrid separators for advanced lithium ion batteries

Polypropylene (PP) separators, popular in lithium ion batteries, suffer from poor wettability towards liquid electrolytes because of their hydrophobic surfaces. Herein, thin manganese dioxide (MnO2) layers with profuse nanopores are in situ grown on a commercial PP separator via facile reduction of KMnO4 in an acidic aqueous solution. High specific capacity is achieved by the hydrophilic MnO2@PP separator, irrespective of C-rate. Importantly, MnO2 layers have an extra capacity contribution in the potential range between 3.0 and 3.4 V. It thus results in an extremely high specific capacity of 202.5 mAh/g at 0.2 C, far above the theoretical value of the cathode LiFePO4 (170 mAh/g). The electrochemical reaction of MnO2 is involved with reversible insertion/extraction of Li+ ions, similar to that of the cathode.