Effect of stress on electrochemical performance of hollow carbon-coated silicon snode in lithium ion batteries

Electrochemical-mechanical coupling mechanism plays an important role in stress relaxation and cycle stability during charging and discharging of lithium ion batteries. The hollow core-shell structure has become a research hotspot in recent years due to the dual effects of its carbon layer and internal voids on volume expansion. However, the theory of diffusion induced stress has not been used to determine how the elastoplastic deformation of amorphous silicon affects the electrochemical performance of silicon anodes with more complex geometries. Based on the Cahn-Hilliard type of material diffusion and finite deformation, a fully coupled diffusion-deformation theory is developed to describe the electrochemical-mechanical coupling mechanism of silicon-polar particles. According to the interface reaction kinetics, the voltage response curve is obtained. The overall trend of the calculated results accords well with the experimental results, and the predicted stress response is also consistent with the experimental result, and thus verifying the effectiveness of the method. Taking the hollow carbon-coated silicon structure that has received much attention in recent years as an example, we study the electrochemical and mechanical behavior during lithiation of hollow carbon-coated silicon anodes and the capacity decay and stress evolution after charge and discharge cycles. The numerical simulation results show that the stress level of the hollow carbon-coated silicon electrode is significantly lower than that of the solid silicon electrode during the whole lithiation. With the lithiation, the stress difference becomes larger and the stress value at the end of lithiation is reduced by about 27%. It fully shows the dual effects of carbon layer and internal pores on stress relaxation and release. In addition, the concentration gradient in the solid silicon negative electrode is too large, which will result in greater stress. In contrast, the lithium ion concentration inside the hollow carbon-coated silicon particles during lithiation is significantly higher than that of the solid silicon particles, and tends to be evenly distributed, which conduces to alleviating the mechanical degradation of the electrode. At the same time, the hollow carbon coated silicon electrode reaches the fully lithiated state earlier, which fully shows the excellent electrochemical performance of the hollow core-shell structure. Finally, the numerical calculation shows that the capacity attenuation is quite consistent with the experimental measurements. Mitigation of stress levels under structural control delays the attenuation of the capacity of hollow carbon-coated silicon anodes. The excellent cycle stability can be attributed to the dual effect of carbon coating and internal pores on volume expansion and stress relief.