Formation and conversion of hydrate zinc sulfate hydroxide in aqueous Zn/MnO2 batteries

Aqueous Zn/MnO 2 batteries (AZMBs) are suitable for large-scale energy storage because of the advantages of safety, low cost and environmentally friendliness. However, the energy storage mechanism in AZMBs is still inconclusive, especially in sulfate-based electrolytes. In this work, we investigate the formation and conversion mechanisms of hydrate zinc sulfate hydroxide, Zn 4 SO 4 ·(OH) 6 ·xH 2 O (ZSH), aiming to elucidate the reaction mechanism and capacity origin of AZMBs with ZnSO 4 and MnSO 4 as the electrolyte. Results show that ZSH is an important intermediate in the electrolyte. It is observed that the electrochemical reaction occurs at the cathode/electrolyte interface involving the generation of ZSH-induced ZnMn 3 O 7 ‧2H 2 O during a long-term cycling. In addition, transformation of ZSH to Zn at the anode/electrolyte interface is observed, implying that ZSH not only induces the uneven deposition of Zn 2+ from electrolyte, but also in-situ converts into metallic Zn during the repeated cycling. This research provides an in-depth understanding of the formation and conversion of ZSH in AZMBs with potential impacts on Zn anode protection. Zn 4 SO 4 ‧(OH) 6 ‧xH 2 O (ZSH) is believed to be an important intermediate in sulfate electrolytes. This research provides an in-depth understanding of the formation and conversion of ZSH in AZMBs with potential impacts on Zn anode protection. • The normalization mechanism of MnO 2 with different crystal types is proposed. • The generation of ZSH-induced ZnMn 3 O 7 ‧2H 2 O occurs during a long-term cycling. • ZSH induces the uneven deposition of Zn 2+ from electrolyte. • ZSH also in-situ converts into metallic Zn during the repeated cycling.