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

抄録

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 α-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 β-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 β-Ni(OH)₂ system, γ-NiOOH, which was formed during overcharging, re-transited into β-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 valence number of Fe³⁺ in LDH did not change during the overcharge–discharge process. Conversely, the Fe K-edge extended XAFS spectrum indicated that only the Fe³⁺ sites were significantly distorted in the overcharged state. These results indicate that the redox-inactive Fe³⁺ sites absorbed the strain caused by Ni²⁺ oxidation up to 3.5 valences and maintained the structural stability of the LDH system.