| [Multiscale Computational Mechanics ] Study of Stress Evolution in Spherical Electrode Particles Study of Stress Evolution in Spherical Electrode Particles Bo Wang1; Katerina Aifantis2; 1UNIV. OF FLORID, Gainesville, United States; 2UNIVERSITY OF FLORIDA, Gainesville, United States; PAPER: 151/Multiscale/Regular (Oral) SCHEDULED: 18:15/Tue. 29 Nov. 2022/Similan 1 ABSTRACT: Silicon electrode is the most promising candidate for the next generation anodes for Li-ion batteries due to its highest theoretical capacity and abundance on earth. However, lithium ion insertion and de-insertion can lead to significant volume changes. As a result, diffusion-induced stress (DIS) can occur. Especially for these active materials with high theoretical capacity, phase transformation is often involved. The high stresses arising from mismatch between the swelling part and non-swelling part can lead to capacity decay, failure and fracture of the active particles and strongly affects the cycle life. In addition, silicon would experience decrease in elastic properties due to lithium insertion and plastic deformation can occur due to large volume expansions and contractions. In this study, phase field models for DISs in spherical phase-transformation electrode materials are developed. For electrodes with relatively small volume variations, elastic models can be employed while for electrodes with large volume changes, plastic models are preferred. The models account for the effects of phase change, chemo-mechanical coupling and concentration-dependent material properties. The sharp phase boundary is naturally captured by the phase field model. Concentration field is obtained by a mixed formulation of the fourth-order Cahn-Hilliard equation. DISs are obtained by solving the variational form of the mechanical equilibrium equations. It is found that the DISs arise from the inhomogeneous volume expansions resulting from Li concentration gradients and the hydrostatic stress facilitates the diffusion of Li-ions under elastic deformation while hinders diffusion in plastic case. Material softening shows decreases in DISs but increases in strains under elastic deformation. It’s the opposite for plastic case. Under elastic deformation, radial stress is always positive and, hoop stress is positive in core region and is negative in the shell. In plastic case, radial stress shows a transition from tension in initial stage to compression at late stage. Hoop stress in the core region also shows similar trend while hoop stress in the shell shows transition from compression to tension. Furthermore, if strain softening due to plastic deformation is assumed, smaller stresses and higher plastic strains are predicted than strain hardening case. To sum up, the models highlight the importance of chemo-mechanical coupling effects, concentration-dependent material properties and plastic deformation on diffusion-induced stresses. To sum up, concentration-dependent material properties due to Li insertion and hardening behavior of the material due to plastic deformation plays a significant role on DISs in spherical phase transformation electrodes. By taking these factors into consideration, more accurate predictions of the DISs can be achieved, thus providing an improved theoretical basis and insight for designing next-generation mechanically stable phase transforming electrode materials. |
