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The development of Mg‐ion batteries is hindered by the lack of cathode materials that allow facile and reversible Mg2+ intercalation at high potential. Herein, the authors present a polyanion cathode material of K2(VO)2(HPO4)2(C2O4)⋅4.5H2O (KVPCH) for Mg‐ion batteries. The inductive effect of polyanions ensures the high redox potential of the vanadium centers. In addition, the material contains structural water located between the layers. It helps with Mg2+ desolvation at the electrode–electrolyte interface and facilitates its diffusion in the structure, as confirmed by experimental analysis and theoretical calculations. Thanks to those factors, the KVPCH electrode presents excellent Mg storage capability at room temperature. It delivers 121 mAh g–1 capacity at 1C with a high average discharge potential of 0.16 V versus Ag/Ag+ (3.2 V vs Mg/Mg2+). A capacity retention of 87% is realized after 1500 cycles at 5C. A rocking‐chair Mg‐ion full cell is also demonstrated with the KVPCH cathode and a MoOx anode, which achieves 46 mAh g–1 capacity (based on the total active material mass of two electrodes). This work would bring effective paths for the design of cathode materials for Mg‐ion batteries. [ABSTRACT FROM AUTHOR]

The Mn2+ dissolution of MnO2 cathode materials causes rapid capacity decay in aqueous zinc batteries. We herein show that the dissolved Mn2+ can be deposited back to the cathode with the aid of a suitable conductive agent. The active material is thus retained for energy storage, and this MnO2/Mn2+ redox process also provides capacity. In the Mn2+ free ZnSO4 electrolyte, MnO2 delivers 325 mA h g−1 capacity at 0.1 A g−1, and 90.4% capacity retention is achieved after 3000 cycles at 5 A g−1. Our work demonstrates an effective strategy to realize stable cycling of MnO2 cathodes in aqueous zinc batteries without Mn2+ additives. [ABSTRACT FROM AUTHOR]

The energy storage behavior of the Li3V2(PO4)3 cathode in zinc batteries is evaluated. The dissolution or decomposition into vanadium oxide in aqueous electrolytes is revealed. Using the optimal combination of water and acetonitrile solvents in electrolyte, those processes are effectively prevented without sacrificing the Zn2+ de/insertion kinetics. Further investigation demonstrates a water induced phase transformation into a VOPO4 type structure, which is still a polyanion material and preserves the high voltage. It delivers 128 mA h g−1 capacity at 1C with 1.45 V discharge voltage, and 87 mA h g−1 capacity is retained at 10C. A stable cycling is obtained for 1000 cycles. [ABSTRACT FROM AUTHOR]

Rechargeable Mg-ion batteries offer high energy density and good safety. However, the lack of cathode materials with high operating potential and considerable capacity is a great challenge for their practical applications. This is limited by the sluggish diffusion of multivalent Mg2+ cations in solid lattices. Here, we propose a co-insertion strategy to facilitate cation diffusions and present a polyanion cathode of Li 3 V 2 (PO 4) 3 to provide high redox potential. The initial charge of the material is associated with the extraction of two Li+ cations, which are co-inserted with Mg2+ from the 0.5 m Mg(ClO 4) 2 /PC electrolyte during the following electrochemical processes. The cation diffusion coefficients are largely enhanced in comparison to the electrode without Li+ co-insertion, and higher capacity is obtained. At room temperature, the Li 3 V 2 (PO 4) 3 electrode delivers 124 mAh g−1 capacity at 100 mA g−1 with discharge plateaus at 0.46 V and −0.44 V vs. Ag/Ag+. A capacity retention of 80% is realized after 300 cycles at 500 mA g−1. A rocking-chair Mg-ion full cell coupling the Li 3 V 2 (PO 4) 3 cathode with an anthraquinone anode achieves 41 mAh g−1 capacity (based on the mass of both electrodes). The work demonstrates a promising pathway for the design of high performance cathode materials for Mg-ion batteries. [Display omitted] • A high potential polyanion cathode of Li 3 V 2 (PO 4) 3 is proposed for Mg-ion batteries. • A co-insertion strategy is revealed to enhance cation diffusion in solid lattices. • The Li 3 V 2 (PO 4) 3 electrode delivers 124 mAh g−1 capacity at 100 mA g−1. • High discharge potentials of 0.46 V and −0.44 V vs. Ag/Ag+ are achieved. • A capacity retention of 80% is realized over 300 cycles with 99.3% CE. [ABSTRACT FROM AUTHOR]

[Display omitted] • Doping Zn into Fe lattice enhances the reversibility and stability of Fe electrode. • It is realized by introducing [ZnI]+ to adsorb on negatively charged Fe surface. • Stable Fe plating/stripping for over 2500 h is achieved in symmetric cells. • The Fe plating/stripping coulombic efficiency stabilizes at 98% for 1200 cycles. • The iron-iodine battery delivers 96.7% coulombic efficiency for 1200 cycles. The iron metal electrode based on the Fe2+/Fe redox reaction is a promising anode candidate for aqueous batteries. It possesses high capacity, large abundance, low price, as well as a little higher redox potential than Zn2+/Zn which corresponds to suppressed hydrogen evolution reaction. However, the reversibility and stability of Fe electrodes require improvement. Herein, we regulate the Fe deposition behavior with Zn doping. The positively charged [ZnI]+ complex is introduced into FeCl 2 electrolytes by adding ZnI 2 , which is adsorbed on the negatively charged surface of Fe metal and realizes the doping of Zn into Fe lattice during deposition. In comparison to the large chunks with sole Fe deposition, the Zn doped Fe forms homogeneous small particles with the (110) plane preferred to grow parallel to the substrate. Enhanced coulombic efficiencies of 98% are achieved for over 1200 cycles. Stable Fe plating/stripping is accomplished for over 2500 h at the current density of 1 mA cm−2 and capacity of 1 mAh cm−2, as well as for over 300 h under ultra-high current density and capacity of 10 mA cm−2 and 10 mAh cm−2. Benefiting from the iodine species in electrolyte, an iron-iodine full cell is demonstrated. The battery exhibits excellent capacity retention of 99.8% with average coulombic efficiency of 96.7% for over 1200 cycles. The work provides a potential path for high-performance aqueous Fe-metal batteries. [ABSTRACT FROM AUTHOR]