Overcharge-Discharge Mechanism of Nickel–Iron Layered Double Hydroxide as Cathode Materials for Nickel Batteries

High-capacity cathode materials must be developed to improve the energy densities of Ni-Zn batteries.Ni-containing layered double hydroxide (LDH), with a similar structure to -Ni(OH)2, represents a promising candidate for developing such cathode materials.However, the detailed reaction mechanisms of this LDH system during overcharging until the Ni 3.5+ valence state and during discharging remain unknown.Here, the structural changes of -Ni(OH)2 and Ni-Fe LDH in their overcharged as well as subsequent discharged states were investigated by ex-situ XRD and XAFS analyses.In the -Ni(OH)2 system, -NiOOH, which was formed during overcharging, re-transited into -Ni(OH)2 during discharging, with a drastic loss of its crystallinity.On the other hand, these drastic structural changes were not observed in the Ni-Fe LDH system even after repeated overcharge-discharge cycles, and its crystal structure was maintained during cycling.Moreover, the ex-situ XAFS measurements revealed that the ACCEPTED MANUSCRIPT 2 valence number of Fe 3+ in LDH did not change during the overcharge-discharge process.Conversely, the Fe K-edge extended XAFS spectrum indicated that only the Fe 3+ sites were significantly distorted in the overcharged state.These results indicate that the redox-inactive Fe 3+ sites absorbed the strain caused by Ni 2+ oxidation up to 3.5 valences and maintained the structural stability of the LDH system.