Structural water enhanced intercalation of magnesium ions in copper hexacyanoferrate nonaqueous batteries

In this article we investigate copper hexacyanoferrate (CuHCF) as a nonaqueous magnesium ion battery (MIB) cathode, and the positive influence of structural water on its Mg2+ storage properties. The hydrated and dehydrated forms of CuHCF (H- and D-CuHCF) each displayed excellent rate capabilities and extended lifetimes with zero capacity loss. However, H-CuHCF delivers a specific capacity 25% higher than that of D-CuHCF. Cyclic voltammetry and galvanostatic charge discharge reveal a redox reaction present only in the hydrated CuHCF, which accounts for 25% of its total reversible capacity (63 mAh g(-1) at 25 mA g(-1)). Furthermore, ex-situ FTIR reveals that the structural water of H-CuHCF remains in the lattice during charging/discharging. We propose that water provides partial electrostatic screening between the CuHCF lattice and Mg ions, while allowing an additional redox transition to become thermodynamically accessible. (C) 2020 Elsevier Ltd. All rights reserved.