SOC-dependent high-rate dischargeability of AB_5-type metal hydride anode: Mechanism linking phase transition to electrochemical H-desorption kinetics

The current application of nickel-metal hydride (Ni-MH) batteries places a particular emphasis on the high-rate dischargeability (HRD) at varying state-of-charges (SOCs). However, most research on the HRD of AB5-type MH anodes only considers the fully charged case but overlooks the significant impact of SOC. In this work, at first, the great SOC effect on the HRD or pulse power of AB_5-type MH anode is presented. Then, by crosschecking the SOC dependence of both 'in situ' polarization and 'ex situ' kinetic parameters, a definite SOC-dependent H-desorption kinetics for AB_5-type MH anode is acquired. Finally, a novel mechanism linking phase transition to H-desorption kinetics for AB_5-type MH anode is proposed. The HRD or pulse power of AB5-type MH anode significantly improves when SOC decreases from 100% to an appropriate range (90-60%) and suddenly deteriorates when SOC drops below ∼20%. The former improvement relates to the formation of saturated solid solution that simultaneously facilitates both charge-transfer reaction and hydrogen diffusion. The latter deterioration is due to the complete depletion of hydride causing an insufficient supply of hydrogen atoms.