Zinc ion batteries (ZIBs) have attracted extensive attention in recent years, benefiting from their high safety, eco-friendliness, low cost, and high energy density. Although many cathode materials for ZIBs have been developed, the poor stability of zinc anodes caused by uneven deposition/stripping of zinc has inevitably limited the practical application of ZIBs. Herein, we report a highly stable 3D Zn anode prepared by electrodepositing Zn on a chemically etched porous copper skeleton. The inherent excellent electrical conductivity and open structure of the 3D porous copper skeleton ensure the uniform deposition/stripping of Zn. The 3D Zn anode exhibits reduced polarization, stable cycling performance, and almost 100% Coulombic efficiency as well as fast electrochemical kinetics during repeated Zn deposition/stripping processes for 350 h. Furthermore, full cells with a 3D Zn anode, ultrathin MnO 2 nanosheet cathode, and Zn 2+ -containing aqueous electrolyte delivered a record-high capacity of 364 mAh g −1 at a current density of 0.1 A g −1 and good cycling stability with a retained capacity of 173 mAh g −1 after 300 charge/discharge cycles at 0.4 A g −1 . This work provides a pathway for developing high-performance ZIBs.

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HIGHLIGHTS• Pourbaix diagram of Mn-Zn-H 2 O system was used to analyze the charge-discharge processes of Zn/MnO 2 batteries.• Electrochemical reactions with the participation of various ions inside Zn/MnO 2 batteries were revealed.• A detailed explanation of phase evolution inside Zn/MnO 2 batteries was provided.ABSTRACT Aqueous rechargeable Zn/MnO 2 zinc-ion batteries (ZIBs) are reviving recently due to their low cost, non-toxicity, and natural abundance.However, their energy storage mechanism remains controversial due to their complicated electrochemical reactions. Meanwhile, to achieve satisfactory cyclic stability and rate performance of the Zn/MnO 2 ZIBs, Mn 2+ is introduced in the electrolyte (e.g., ZnSO 4 solution), which leads to more complicated reactions inside the ZIBs systems. Herein, based on comprehensive analysis methods including electrochemical analysis and Pourbaix diagram, we provide novel insights into the energy storage mechanism of Zn/MnO 2 batteries in the presence of Mn 2+ . A complex series of electrochemical reactions with the coparticipation of Zn 2+ , H + , Mn 2+ , SO 4 2− , and OH − were revealed. During the first discharge process, co-insertion of Zn 2+ and H + promotes the transformation of MnO 2 into Zn x MnO 4 , MnOOH, and Mn 2 O 3 , accompanying with increased electrolyte pH and the formation of ZnSO 4 ·3Zn(OH) 2 ·5H 2 O. During the subsequent charge process, Zn x MnO 4 , MnOOH, and Mn 2 O 3 revert to α-MnO 2 with the extraction of Zn 2+ and H + , while ZnSO 4 ·3Zn(OH) 2 ·5H 2 O reacts with Mn 2+ to form ZnMn 3 O 7 ·3H 2 O. In the following charge/discharge processes, besides aforementioned electrochemical reactions, Zn 2+ reversibly insert into/extract from α-MnO 2 , Zn x MnO 4 , and ZnMn 3 O 7 ·3H 2 O hosts; ZnSO 4 ·3Zn(OH) 2 ·5H 2 O, Zn 2 Mn 3 O 8 , and ZnMn 2 O 4 convert mutually with the participation of Mn 2+ . This work is believed to provide theoretical guidance for further research on high-performance ZIBs.

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