LFP/Graphite 18650 Cells Designed to Operate from 20 to 100oC for Studies of Degradation Mechanisms: Part 2 – Arrhenius Parameter Extraction and Lifetime Modelling

Five sets of LFP/artificial graphite 18650 cells were assembled with the aim of limiting degradation to only lithium inventory loss. When this is the case, the well known “square root time model” for cell capacity, Q, as a function of initial capacity, Q_0, and time, t: Q=Q_0 (1-At^(1/2)) can be used to model capacity loss fairly well. Data collected at the same charge and discharge rate at various temperatures can be fitted to extract the parameter, A, as a function of temperature, T. In this work, plots of log⁡(A) versus 1/T were used to extract the activation energy, E_a, and frequency factor, γ, for A = γ exp-Ea/kT. At extreme temperatures, significant Fe dissolution from LFP and deposition on the graphite electrode occurs, which accelerates capacity loss. In these cases, a modified square root time model is used to extract A. Model parameters for capacity loss were extracted using three different methods, compared, and used to make lifetime predictions as a function of temperature.