An aluminum‐ion battery was assembled with potassium nickel hexacyanoferrate (KNHCF) as a cathode and Al foil as an anode in aqueous electrolyte for the first time, based on Al3+ intercalation and deintercalation. A combination of ex situ XRD, X‐ray photoelectron spectroscopy (XPS), galvanostatic intermittent titration technique (GITT), and differential capacity analysis was used to unveil the crystal structure changes and the insertion/extraction mechanism of Al3+. Al3+ could reversibly insert/extract into/from KNHCF nanoparticles through a single‐phase reaction with reduction/oxidation of Fe and Ni. Over long‐term cycling, it was Fe rather than Ni that contributed to more capacity owing to the dissolution of Ni from the KNHCF structure, which could be expressed as a compensation effect of mixed redox centers in KNHCF. KNHCF delivered an initial discharge capacity of 46.5 mAh g−1. The capacity decay could be attributed to the unstable interface between Al foil and the aqueous electrolyte owing to the catalytic activity of the Ni transferring from Ni dissolution of KNHCF to the Al foil anode, rather than KNHCF structure collapse; KNHCF maintained its 3 D framework structure for 500 cycles. This work is expected to inspire more exhaustive investigations of the mechanisms that occur in aluminum‐ion batteries.

LIBs) as the most popular energy storage system have been widely adopted in portable electronic devices and electric vehicles. [1] Besides, sodium-ion batteries (SIBs, NIBs) that have potential beyond LIBs have also received much interest recently due to the naturally abundant sodium source. [2] Both of these two energy storage systems require significant improvement in cycling life and energy density to meet the increasing need for smart power grids, which depends on the development of electrode materials. For the anode materials, carbon-based materials are deeply studied by researchers because of their good cycling stability and low cost. [3] However, the tapping density and specific capacity of carbon are relatively low, which lead to the poor volumetric capacity and limit the commercial application. As typical functional materials, 2D transitionmetal chalcogenides (TMCs) based on conversion reaction are able to give much higher theoretical capacity, which originates from multiple electron transfer per metal anion. [4] The reversible electrochemical conversion reaction of TMCs can be described as follows [5] A M X e A X M 0 nm mn m n n m nin which A = Li + , Na + , etc.; M = Co 2+ , Ni 2+ , Cu 2+ , etc.; X = S 2− , Se 2− , etc. Using the high-profile transition-metal sulfides (MS x ) for LIBs as an example, they usually exhibit poor cycle performance, which can be ascribed to the severe irreversibility happening in the initial delithiation. [6] Under the voltage range of 2.0-2.4 V, a portion of Li 2 S should be oxidized to polysulfides that prefer to dissolve into the electrolyte, which is similar to that of the reason for shuttle effect in Li-S batteries. [7] Therefore, improving the reversibility of the abovementioned reaction and further suppressing the generation of polysulfides is a challenge. As we all know, the electrochemical reversibility depends on the diffusion kinetics of Li + as well as the conductivity of activity materials. [8] Morphology control of electrode material is demonstrated to be an effective technology to elevate its specific capacity and cycling life through providing preferential diffusion pathways for ions, conduction pathways for electrons, and increasing contact areas between active materials and electrolyte. [9] For example, Zhao and co-workers [10] used the two-step synthetic approach to prepare a layer-by-layer 2D cobalt selenide based on conversion reaction has attracted much attention due to its open layered structure and high specific capacity. However, effectively suppressing the fast capacity fade caused by the irreversible Se dissolution and serious volume change during the cycling process is still a challenge. Herein, the concentration of dispersion liquid under supercritical conditions is tuned to induce the CoSe crystal to grow along the graphene oxide (GO), and finally obtain the Tremella-like CoSe-reduced GO (rGO) hybrid. The nature of epitaxial growth leads to the formation of stable CSe bonds, which maintain a favorable conductive connection between CoSe ...

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Aqueous rechargeable metal ion batteries (ARMBs), featuring safety, facile manufacturing and environmental benignity, have recently attracted extensive attention as promising energy storage systems. Particularly, pursuit of electrode materials with abundance,...