Improving the cycle life of cryptomelane type manganese dioxides in aqueous rechargeable zinc ion batteries: The effect of electrolyte concentration
Chemistry and Biochemistry
Rechargeable aqueous zinc ion batteries (ZIBs) have many advantages such as high abundance and low cost of Zn, as well as safer battery chemistry in aqueous electrolytes. Cryptomelane-type manganese dioxides (α-MnO2s) are promising cathodes for ZIBs due to their high theoretical capacity, voltage, and abundance. However, α-MnO2 suffers from severe capacity fading as the battery is put through extended charge-discharge cycles. Here, we show that galvanostatic cycling performance of α-MnO2 can significantly be improved at ZnSO4 concentrations below 0.2 M. Low-electrolyte concentrations 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 85 mAh/g capacity at the 500th cycle, where the retention was 167% with respect to the 1st cycle in 0.1 M ZnSO4. On the other hand, at high electrolyte concentrations (>1.0 M), the capacity retentions were below 30% and discharge capacities were below 60 mAh/g after 100 cycles. Better electrochemical performance of α-MnO2 at low electrolyte concentrations is attributed to the suppression of cathode dissolution and the mitigation of the formation of impurity phases such as ZnMn2O4 and ZnMn3O7·xH2O.
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