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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]

VOPO4⋅x H2O has been proposed as a cathode for rechargeable aqueous zinc batteries. However, it undergoes significant voltage decay in conventional Zn(OTf)2 electrolyte. Investigations show the decomposition of VOPO4⋅x H2O into VOx in the electrolyte and voltage drops after losing the inductive effect from polyanions.PO43− was thus added to shift the decomposition equilibrium. A high concentration of cheap, highly soluble ZnCl2 salt in the electrolyte further prevents VOPO4⋅x H2O dissolution. The cathode shows stable capacity and voltage retentions in 13 m ZnCl2/0.8 m H3PO4 aqueous electrolyte, in direct contrast to that in Zn(OTf)2 where the decomposition product VOx provides most electrochemical activity over cycling. Sequential H+ and Zn2+ intercalations into the structure are revealed, delivering a high capacity (170 mAh g−1). This work shows the potential issue with polyanion cathodes in zinc batteries and proposes an effective solution using fundamental chemical principles. [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]