Towards Rest Period based Li-Ion Battery Diagnostics: Part 2: Estimating Solid Phase Diffusivity

Abstract Accurate estimation of solid phase diffusivity, a rate-limiting step in Li-Ion batteries, is extremely important for battery management system (BMS) algorithms for precise state estimations and safe, stable battery operation. The present state-of-the-art diffusivity estimation methods give erroneous results, due to inherent limitations and invalid assumptions. Here, a new method is proposed, diffusivity estimation from long-term voltage relaxation technique (DELVRT), to estimate solid diffusivity using battery rest period voltage relaxation. As solid phase diffusion dominates rest period voltage relaxation, the derivation of the spatio-temporal evolution of the concentration profile presented in Part-I of this work is further built, to demonstrate accurate estimation of diffusivity. For times greater than the diffusion time scale, the temporal voltage variation is directly proportional to the diffusivity, leading to an analytical expression for estimating diffusivity. The validation for this model is performed for various different conditions, with data generated using the Pseudo-2D electrochemical-thermal model. It is demonstrated that the diffusivity estimation using DELVRT is significantly more accurate compared to the widely used galvanostatic intermittent titration technique (GITT), especially where GITT performed poorly owing to high non-linearity in the open circuit potential-state of charge relationship of the active material.