Effect of the charge rate on the mechanical response of composite graphite electrodes: in situ experiment and mathematical analysis.

Abstract

The cycling lifespan and coulombic efficiency of lithium-ion batteries are crucial to high C-rate applications. The Li-ion concentration is crucial in determining the mechanical integrity and structural stability of electrodes. In this work, graphite is selected as the working electrode due to its widespread use in the electric vehicle industry. The experimental data have shown that the electrodes with a mass loading of 6.54 mg cm-2 exhibited poor cycling performance and high charge transfer resistance at high charge rates. To explain this phenomenon, an in situ stress measurement system and a C-rate-dependent stress model are established to study the mechanical properties of the composite graphite electrode during the electrochemical process at various C-rates. Moreover, the effect of the Li-ion concentration-dependent modulus and C-rate-dependent partial molar volume is taken into account in the mathematical model. The computational curvature data fit well with the corresponding experimental data, highlighting the importance of considering lithium-ion concentration in mechanical stress. It has been found that stresses along the thickness of the active layer switch between compressive and tensile stresses due to the competition between bending stress and diffusion-induced stress. The stress at the outer surface of the composite graphite electrodes reaches a maximum magnitude of 27.5 MPa at a 1.5C-rate. In contrast, the stress at the interface of the active layer is maximum at a 0.5C-rate due to the existence of more lithium ions. Our study provides a direct insight into the quantitative analysis of electrode stresses at different C-rates.