Blocked-Diffusion with Frequency Dispersion for Study of EIS in NiMH Batteries

Based on fundamental electrochemical theory, an impedance model of a Nickel-Metal Hydride (NiMH) battery considering blocked-diffusion with frequency dispersion has been developed in this study. The impedance model accounts for electrochemical mechanisms during the decrease in the state of charge of a NiMH battery. The a.c. diffusion mechanisms in the NiMH battery during electrochemical impedance spectroscopy (EIS) tests can be modelled through a Warburg element considering blocked-diffusion with frequency dispersion. The Warburg element is analogous to a transmission line circuit comprised of distributed constant phase elements connected in parallel with resistors attributed to the resistance of diffusion processes. The NiMH impedance model is applied to EIS measurements carried out after discharging a NiMH battery pack. The impedance model can reproduce the straight-line EIS measurements of the NiMH battery represented in the low frequency range of the Nyquist plot. The change in slope of the straight-line EIS measurements at low frequencies can be related to nonhomogeneous electrode ion concentration and can be attributed to roughness of the NiMH electrode. This study has demonstrated that it is possible to gain an insight into the electrochemical processes of NiMH by combining fundamental theory of battery electrode and EIS measurements in a complementary manner. (C) 2019 The Electrochemical Society.