Improving the Electrochemical Performance of High Power and Energy Aqueous Rechargeable Zn/MnO2 Batteries

Abstract

Rechargeable aqueous zinc ion batteries (ZIBs) have many advantages such as high abundance and low cost of Zn and safer battery chemistry in aqueous ZnSO4 electrolytes. Tunnel and layer structured manganese dioxides (MnO2) are promising cathodes for ZIBs due to their high theoretical capacity (~300mAh/g), voltage (~1.4 V vs. Zn/Zn2+) and large tunnel openings facilitating fast Zn2+ insertion/removal. However, MnO2 suffers from severe capacity fading as the battery is put through extended charge-discharge cycles due to the cathode dissolution and formation of electrochemically inactive phases such as ZnMn2O4 and ZnMn3O7·xH2O.In order to address the shortcomings of MnO2 cathodes and improve their electrochemical performances in ZIB, we have investigated the effects of (1) electrolyte concentration and type, (2) sol-gel surface coating of the cathodes, and (3) the tunnel sizes of MnO2. Here, we report that electrochemical performance of 0.46 nm x 0.46 nm tunnel sized α-MnO2 can significantly be enhanced at low concentration (≤0.5 M) ZnSO4 and ZnTFSI electrolytes. Low concentration ZnSO4 and ZnTFSI electrolytes demonstrated higher discharge capacities and capacity retentions at long battery cycling tests. At 0.1 M ZnSO4, α-MnO2 delivered 62 and 103 mAh/g capacities with 0.3 A/g rate at the 1st and 100th cycles, respectively (166% retention). At a higher charge/discharge rate of 0.9 A/g, α-MnO2 delivered 85mAh/g capacity at the 500th cycle, where the retention was 167% with respect to the 1st cycle. On the other hand, the capacity retentions were below 30% and discharge capacities were below 60 mAh/g after 100 cycles in high concentration electrolytes. Detailed characterization of spent electrodes showed that low concentration electrolytes mitigate the formation of the impurity phases and suppresses cathode dissolution. Additionally, further improvements in the ZIB performance of MnO2 have been achieved in 0.5 M ZnTFSI electrolytes and MnO2 materials with larger tunnel and layer openings.