Assessing the impact of transport and kinetic mechanisms during the analysis of a LiFePO₄ cathode: A different perspective during the operation and modeling of a battery cell

Although (de)lithiation in the anisotropic and nonconductive LiFePO₄ solid phase has been broadly studied, yet there are some uncertainties about transport and kinetic mechanisms related to this active material, that have not been fully understood during the operation of a cell battery due to today's limitations on the development of in-depth experiments, and first principle calculations. Particularly, this work combines experimentation and theoretical foundations to elucidate the impact of these mechanisms on the performance of a LiFePO₄/Csp:PVDF cathode. As part of the experimental strategy, the composition of the active material and additives in the solid phase, and the initial concentration of Li⁺ in the active material were intentionally manipulated to analyze their influences on transport and kinetic mechanisms during the cathode operation. The experimental evidence led to the proposal of a heterogeneous model that overcomeslimitations identified in conventional pseudo-continuous models by including in its conceptualization the effect of the solid-electrolyte interphase transport, the diffusion of Li⁺ over the surface of the solid phase and intra-solid phase transport on electrochemical kinetics. To minimize uncertainties related to the reaction model, a new kinetic approach, based on the mass action law, the transition state theory, the mean-field approximation and thermodynamics was also proposed and coupled to the heterogeneous model. The modelling approach showed its adequacy by describing observation trends properly. Based on the non-conventional experimental strategy and the modelling approach, transport resistances associated with the solid-electrolyte interphase and intra-solid phase are identified as the main processes limiting kinetics and, hence, the macroscopic performance of the cell. Although the experimental and modeling framework was only applied for a LiFePO₄ cathode, it is still valid for any other active material involved in a battery cell.