Effects of stress-assisted migration on potentiostatic intermittent titration technique (PITT) in spherical particles

Abstract

Determining the chemical diffusion in active materials is crucial to the modeling and simulation of the chemo-mechanical interaction during electrochemical cycling of lithium-ion batteries, in which the diffusion coefficient of lithium plays an important role in describing the migration of lithium. Among the existing electrochemical measurement techniques, potentiostatic intermittent titration technique (PITT) is relatively reliable in determining diffusion coefficient in an electrochemical system since this technique relies only on the electrode dimension, i.e. the electrode thickness or the particle radius. In this work, a two-way coupling chemo-mechanical model is established to investigate the electrode transient current under PITT operation by incorporating the contribution of mechanical energy (hydrostatic stress) in chemical potential. The electrode transient current with the contribution of hydrostatic stress is compared to the counterpart with classical Fick diffusion law. Two key factors controlling the stress-diffusion coupling in the analysis of the electrode transient current under PITT operation are investigated, i.e. partial molar volume and elastic modulus of spherical particle. A ratio of the “intrinsic” diffusion coefficient to the nominal diffusion coefficient is defined to quantitively analyze the effects of the stress-diffusion coupling on the diffusion coefficient determined from PITT operation. The numerical results reveal that a large partial molar volume and/or a large elastic modulus will result in significant deviation of the diffusion coefficient by using PITT technique without considering the stress-diffusion coupling effects.