Investigation of Transport and Kinetic Nonideality in Solid Li-Ion Electrodes through Deconvolution of Electrochemical Impedance Spectra

The development of Li-ion battery management systems (BMSs) is highly dependent on high fidelity computer simulations. In traditional physics-based models (PBMs), Fick’s law coupled with the Butler–Volmer equation has been employed to describe both Li-ion diffusion and solid/liquid interfacial Li-ion intercalation/deintercalation, but this methodology makes the primary assumption that there is no nonideality caused by Li–Li interactions. Such nonideality phenomena are usually described by the activity coefficient (γ) of Li-ions in a solid solution. With the nonideality, the activity of Li-ion species is not equivalent to the concentration of Li-ions. This research has demonstrated, through the deconvolution of electrochemical impedance spectra, that significant nonideality exists in the solid active materials during charge/discharge cycles, and it leads to nonlinear variation of both transport and kinetic parameters of the electrodes. We also show that PBMs with new pre-factors derived from nonequilibrium thermodynamics of concentrated solution theory can make battery-level predictions that are consistent with EIS data. The pre-factors developed in this paper are functions of the activity coefficient of the solid phase. They show a three order-of-magnitude variation for diffusivity in the solid active material and a one-to-two order of magnitude change in the reaction rate constant at the solid/liquid interface. The results presented here could provide baseline parameters for PBMs and improve their accuracy for high-fidelity BMSs.