Critical lithiation for C-rate dependent mechanical stresses in LiFePO4

Abstract

The prevention of capacity loss after electrochemical cycling is of paramount importance to the development of lithium-ion batteries, especially for applications in the electric vehicle industry. The objective of this research is to investigate C-rate dependent diffusion-induced stresses in electrode materials. LiFePO4 is selected as the model system in this study since it is one of the most promising cathode materials used in electric vehicle applications. Finite element models incorporating several factors with concentration dependency are developed in this study including concentration-dependent anisotropic material properties, concentration-dependent and C-rate-dependent volume expansion coefficients, and concentration-dependent lithium ion diffusivity. Our simulation results show that the effect of concentration dependency on mechanical properties and lithium diffusivities cannot be neglected in mechanical stress predictions. We also observe that C-rate has a great effect on how fast the surface concentration is saturated, suggesting that C-rate dependency of the diffusion-induced stresses occurs at a critical lithiation stage: 47.5, 26.5, 10.1, and 6.8 % lithiation for 1, 2, 6, and 10 C, respectively. Mechanical stresses in perfect and cracked particles are also studied. It is observed that the crack surface orientation plays an important role in the diffusion-induced stress. The existence of the crack surface increases mechanical stresses, suggesting that particles inside the material may undergo fractures faster and may accelerate the material deterioration, leading to capacity loss at higher C-rate (dis)charging.