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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]

Mn‐based materials for aqueous energy storage are reaching the capacity ceiling due to the limited Mn4+/Mn3+ redox. The disproportionation of Mn3+ often occurs, forming soluble Mn2+ and thus leading to severer capacity decays. Here, an amorphous manganese phosphate material [AMP, Na1.8Mn4O1.4(PO4)3] is fabricated using an electrochemical method for the first time. Benefitting from the open framework and the insoluble nature of Mn2+ in AMP, the Mn3+/Mn2+ and Mn4+/Mn3+ redox couples can participate in the charge storage processes. The AMP electrode shows a high capacity of 253.4 mAh g−1 (912.4 F g−1 or 912.4 C g−1) at the current density 1 A g−1 and good rate capability. Experimental results indicate AMP experiences a mixed charge storage mechanism (i.e., cation intercalation and conversion reactions) in Na2SO4 electrolyte. Besides, electrolyte engineering can effectively prevent the decomposition of AMP during cycling test, achieving capacity retention of 97% in 5000 cycles. Importantly, AMP can accommodate different cations (e.g., Mg2+, Ca2+, etc.), exhibiting great potential for aqueous energy storage. [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]

[Display omitted] • A zinc-iodine battery with high coulombic efficiency and stability is proposed. • The polyiodide is confined at cathode by doping on polyaniline chains. • The complex in electrolyte is regulated to eliminate free iodide anions. • It delivers 99.2% coulombic efficiency with 2 mAh cm−2 capacity at 6 mA cm−2. • A stable capacity retention of 99.9% is achieved after 1000 cycles. Aqueous zinc-iodine batteries are promising electrochemical energy storage systems due to the high safety and low cost. The application of zinc halide solution as the electrolyte allows the dual-plating mechanism on both electrodes, i.e. the redox reactions of Zn2+/Zn and I 2 /I- at the anode and cathode, respectively. These solid–liquid conversion processes guarantee excellent reaction kinetics. However, soluble polyiodide (I 3 -, I 5 -, etc.) are formed at the cathode either during the oxidation of I- or from the reaction between I- and I 2. The dissolution of polyiodide in electrolytes causes rapid loss of charged products, leading to poor coulombic efficiency and fast self-discharge. Herein, we apply the synergistic regulation of electrode and electrolyte to confine the charged products. The conducting polymer polyaniline (PANI) is used as the polyiodide binder. It contains positively charged nitrogen sites, allowing the doping and effective binding of polyiodide anions through electrostatic attraction. At the same time, the complex in zinc halide electrolytes is regulated to eliminate free iodide anions and prevent the reaction with I 2 to form more polyiodide. The optimized zinc-iodine aqueous battery delivers excellent rate capability thanks to the facile solid–liquid reactions as well as the high electrical conductivity of PANI. More importantly, it achieves a high coulombic efficiency of 99.2% with the capacity of 2 mAh cm−2 at 6 mA cm−2, and an excellent capacity retention of 99.9% after 1000 cycles is realized upon long-term cycling. The work proposes a potential pathway to realize stable energy storage in aqueous zinc-halogen batteries. [ABSTRACT FROM AUTHOR]

[Display omitted] • Quinone sites are introduced to PANI by a facile electrochemical method. • Polymer chains transform into expanded coil conformation at the same time. • Quinone provides extra redox sites, and expanded polymer enhances conductivity. • The electrode delivers 186 mAh g−1 capacity with quinone at 1.4 V and PANI at 1.1 V. • A stable capacity retention of 88.0% is achieved after 1500 cycles. Rechargeable aqueous batteries with zinc metal anodes are promising energy storage systems owing to their high safety and low lost. A suitable cathode material is in urgent demand. Organic materials offer adjustable electrochemical activity, and conducting polymers provide good electrical conductivity. Herein, we combine those advantages in one cathode material by the controlled introduction of quinone-type active sites onto polyaniline (PANI) nanorod-arrays through a facile electrochemical treatment. The process simultaneously transforms the polymer with a compact coil conformation into an expanded coil structure, resulting in facilitated electron transportations and charge transfer processes. In comparison to the original PANI, the electrochemically treated Q-PANI electrode exhibits extra electrochemical activity from quinone sites as well as enhanced capacity from PANI itself. Q-PANI delivers a high capacity of 186 mAh g−1 at 0.2 A g−1. Upon long-term cycling, 88.0% capacity retention is achieved after 1500 cycles at 2 A g−1. Detailed mechanism studies demonstrate the redox reaction on quinone sites at 1.4 V and nitrogen centers in PANI at 1.1 V. The work proposes a potential pathway towards high performance electrode materials for zinc batteries. [ABSTRACT FROM AUTHOR]

• Fe 3 O 4 /FeOOH heterostructure is constructed during electrochemical activation. • A built-in electric field is formed near the Fe 3 O 4 /FeOOH interface. • The activated electrode shows an enhanced capacity of 634 mAh g−1. • The assembled Ni-Fe cell shows a high energy density of 161.3 Wh kg−1 at 5.7 kW kg−1. Aqueous Ni-Fe batteries show promise for grid level energy storage due to their high safety and low cost. However, high capacities of Fe-based anodes can only be achieved under slow discharging rates. Moreover, an activation process is often required, the mechanism of which has not been fully understood. Herein, we present a facile and controllable method to uniformly deposit Fe 3 O 4 nanoparticles on a 3D graphite substrate. Post-mortem analysis demonstrates the partial conversion of Fe 3 O 4 to FeOOH during the subsequent in-situ electrochemical activation process, forming a Fe 3 O 4 /FeOOH heterostructure. Density functional theory calculations suggest that a built-in electric field is formed near the Fe 3 O 4 /FeOOH interface, which facilitates the charge transfers and lowers the adsorption energy of OH−. The above modification on the active material significantly improves its electrochemical activity. The activated electrode delivers a high capacity of 509 mAh g−1 at the ultra-high current density of 100 A g−1. A Ni-Fe cell assembled with activated Fe 3 O 4 /FeOOH anode and Ni-Co double hydroxide cathode provides a high energy density of 161.3 Wh kg−1 and a maximum power density of 43 kW kg−1, making it a good candidate for high safety, low cost, and environmental friendliness energy storage systems. [ABSTRACT FROM AUTHOR]