Multifunctional Iron Selenate Sheath of Fe‐Based Anode Achieving High‐Rate Capacity‐Durability Combination of Aqueous Hybrid Energy Storage Devices

Abstract The introduction of battery‐type cathode has been commonly considered a preferred approach to boost the energy density of aqueous hybrid energy storage devices (AHESDs) in alkalic systems, but AHESDs with both high energy density and power density are rare due to the great challenge in designing battery‐type anode materials with high rate and durability comparable to capacitive‐type carbon anodes. In this paper, a well‐hydrated iron selenate (FeSeO) sheath is constructed around FeOOH nanorods by a facile electrochemical activation, demonstrating the unique multifunction in fasting charge diffusion, promoting the dissociation of H 2 O, and inhibiting the irreversible phase transition of FeOOH to inert γ‐Fe 2 O 3 , which endow the hydrated sheath coated Fe‐based anodes with an impressive rate capability and superior durability. Thanks to the comprehensive performance of this Fe‐based anode, the assembled AHESD delivered a high energy density of 117 Wh kg −1 with the extraordinary durability of almost 100% capacity retention after 40 000 cycles. Even at an ultrahigh power density of 27 000 W kg −1 , an impressive energy density of 65 Wh kg −1 can be achieved, which rivals previously reported energy‐storage devices.