Superlattice Formation of Crystal Water in Layered Double Hydroxides for Long-Term and Fast Operation of Aqueous Rechargeable Batteries

Aqueous rechargeable batteries (ARBs) are gaining increasing attention as alternatives to conventional nonaqueous lithium ion batteries. However, finding electrode materials with competitive electrochemical properties in various aspects is challenging. Moreover, the operation mechanism of some of high performance electrode materials is not fully understood. Here, an α-phase layered double hydroxide (α-LDH) working in alkaline electrolytes as an ARB cathode is reported. On charge, OH⁻ carrier ions intercalate into the interlayer space and react with protons detached from the host structure to yield crystal water. This crystal water is then arranged in a superlattice during charging to accommodate carrier ions and stabilize the structure. The solid solution mixing of cobalt and nickel also stabilizes the structure during the wide range of redox swing of Ni from 2+ to 4+. In pairing with Fe₃O₄/Fe(OH)₂ mixture, the α-LDH exhibits 198.0 mA h g⁻¹ at 3 A g⁻¹, 68.3% capacity retention after 10 000 cycles, and 172.5 mA h g⁻¹ at 1 min charge, demonstrating the promise of hydrated compounds for ARB electrodes. The present study elucidates that the arrangement of crystal water within the host framework plays a critical role in determining the electrochemical performance of the corresponding hydrated active compound in aqueous media.