Voltage induced lattice contraction enabling superior cycling stability of MnO2 cathode in aqueous zinc batteries

The cycling stability of MnO2 cathodes in aqueous zinc batteries is highly relied on Mn2+ additives in electrolytes. Herein, we demonstrate a voltage induced lattice contraction mechanism for a-MnO2 to realize stable cycling in Mn2+ free electrolytes. Analysis reveals the accumulation of electrochemically inactive parts is mainly responsible for capacity decay when cycled with 1.8 V top voltage cut-off, whereas the contribution from irreversible dissolution of active material is smaller. With the extension of voltage limit to 2.2 V and a voltage hold process during the initial charge, the inactive parts are activated to undergo reversible two electron transfer reaction. Further studies suggest that high voltage induces the contraction of original lattice. It stabilizes entered structural water and enhances Mn release. Meanwhile, more oxygen vanacies are generated, which also leads to the weakening of Mn binding in the lattice. Therefore, MnO2 achieves 323 mAh g 1 capacity with 94.6% retention over 300 cycles (>1800 h) at 0.1 A g 1 in the Mn2+ free electrolyte of 3 M ZnSO4. This work provides an effective path to realize stable cycling of MnO2 cathodes in zinc batteries.

Automated summary

The cycling stability of MnO2 cathodes in aqueous zinc batteries is highly dependent on the presence of Mn2+ additives in the electrolyte. However, this study demonstrates a voltage-induced lattice contraction mechanism that enables stable cycling of a-MnO2 in Mn2+-free electrolytes. The accumulation of electrochemically inactive parts is found to be the main cause of capacity decay during cycling at a top voltage cut-off of 1.8 V. By extending the voltage limit to 2.2 V and implementing a voltage hold process during the initial charge, the inactive parts are activated to undergo reversible two-electron transfer reactions. The high voltage induces lattice contraction, stabilizes structural water, enhances Mn release, and generates more oxygen vacancies, weakening the Mn binding in the lattice. As a result, MnO2 achieves a capacity of 323 mAh g 1 with 94.6% retention over 300 cycles (>1800 h) at 0.1 A g 1 in a Mn2+-free electrolyte of 3 M ZnSO4. This work provides an effective approach to achieve stable cycling of MnO2 cathodes in zinc batteries.