Rechargeable aqueous zinc-ion batteries have been intensively studied as novel promising large-scale energy storage systems recently, owing to their advantages of high abundance, cost effectiveness, and high safety. However, the development of suitable cathode materials with superior performance is severely hampered by the sluggish kinetics of Zn 2+ with divalent charge in the host structure. In the present work, a highly reversible aqueous Zn 2+ battery is demonstrated in aqueous electrolyte using V 6 O 13 •nH 2 O hollow microflowers composed of ultrathin nanosheets. Benefiting from the synthetic merits of its favorable architecture and expanded interlamellar spacing that results from its structural water, the V 6 O 13 •nH 2 O cathode exhibits outstanding electrochemical performances with a high reversible capacity of 395 mAh g −1 at 0.1 A g −1 , superior rate capability, and durable cycling stability with a capacity retention of 87% up to 1000 cycles. In addition, the reaction mechanism is significantly investigated in detail. This study demonstrates that the V 6 O 13 •nH 2 O nanostructure is emerging as a promising cathode material for the high-potential rechargeable aqueous zinc-ion battery, and it may shed light on the water-initiated effective interlayer engineering strategy for the construction of high-performance cathode materials for grid-scale energy storage devices.

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Salinity gradient (SG) is a natural and renewable energy source existing in estuaries, and can also be produced during various desalination and industrial processes. Here, a new method is proposed to efficiently recover SG energy based on chloride-ion (Cl − ) extraction and insertion with metal chloride electrodes and the Donnan potential over a cation-exchange membrane in a concentration flow cell. Three different metal chloride electrodes (BiCl 3 , CoCl 2 , and VCl 3 ) were investigated in the cell, and their properties after discharging in 30 g L −1 (seawater) and 1 g L −1 (river water) NaCl solutions were studied by cyclic voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy. The cell with BiCl 3 electrodes yielded the largest power density (max. = 3.17 W m −2 ) compared to that of CoCl 2 and VCl 3 electrodes, which was higher than those of most previous technologies for SG energy recovery. Fast Cl − extraction and insertion processes were observed on BiCl 3 electrodes due to small charge transfer resistance and Cl − diffusion resistance. BiCl 3 was reduced to metal Bi as Cl − released from the electrode to river water, while metal Bi was oxidized to BiCl 3 as Cl − inserted into the electrode from seawater.