Optimal Design Of Core-shell Structures Of Silicon Electrode

In order to meet the development needs of a new generation of high specific energy lithium-ion battery,the research and application of new high-capacity anode materials has always been a research hotspot.Because of its extremely high theoretical specific capacity and low lithium intercalation potential,silicon materials are considered to be one of the ideal negative electrode materials and have attracted researcher’s attention.However,a major problem in the application of silicon-based electrode materials is the huge volume expansion,and the resulting cracking and pulverization of the electrode materials.However,a major problem in the application of silicon-based electrode materials is the huge volume expansion,and the resulting cracking and powdering of the electrode materials.This non-negligible problem seriously affects the cycle performance of silicon materials and limits its further commercial application.Therefore,the stress evolution of the electrode material during charging is analyzed,and based on which optimal structural design is discussed to mitigate the structural damage and to improve the cycle stability of the battery.This is useful for the development of electric energy with high energy density and high cycle life.The cycling process of lithium-ion battery is the result of the interaction of the intercalation and de intercalation of lithium ions,the change of the lattice of electrode particles and the evolution of internal stress field.It is a multi-physical field strong coupling problem of physical diffusion,electrochemistry and mechanics.The intercalation and deintercalation of lithium ions in the electrode will cause changes in the material lattice,thereby inducing stress;on the contrary,stress will also affect the diffusion process of lithium ions.Therefore,studying the distribution and evolution of diffusion-induced stress in electrode particles is helpful to understand the failure mechanism of lithium-ion battery.The main content of this article is as follows:（1）Under the assumption of finite deformation,this paper studied the evolution of the concentration and stress field of the spherical Si/C core-shell structure during the lithium insertion process based on the electrochemical-mechanical coupling theory.The calculation results show that compared with the small deformation assumption The finite deformation can more truly reflect the distribution and evolution of the diffusion-induced stress in the core-shell structure,and the shell layer can well limit the expansion of silicon particles;however,the large radial compressive stress generated in the core and the core-shell interface Tangential tensile stress may cause the failure of the core-shell structure.（2）Aiming at the possible failures of the core-shell structure under limited deformation conditions,considering the influence of material and structural effects,the optimization of two structures is simulated:single-layer core-shell structure and double-layer core-shell structure.The results show that selecting a softer coating material can improve the ratio performance of lithium ion battery,and significantly reduce the Von Mises stress,radial stress and tangential stress at the core-shell interface,thus reducing the mechanical damage of active particles;Two different materials are used to coat the silicon anode material to form a double-layer core-shell structure,which can not only accelerate the smooth insertion of lithium ions into the double-layer core-shell structure,but also maintain the stability of the internal single-layer core-shell structure,which is conducive to avoiding the failure of the carbon structure of the coated silicon material.（3）Because the double-layer core-shell structure still has the case of shell collapse failure,in order to effectively improve the electrochemical and mechanical properties of the double-layer core-shell structure,so as to achieve a longer cycle life of lithium-ion battery,the optimization of the two-layer shell structure is considered,including the optimization of the two-layer shell material and the influence of the thickness of the outer shell.The internal and external hard double-layer core-shell structure was formed by exchanging the materials of inner and outer shell.It was found that the difference of inner and outer shell materials affected the reaction kinetics of lithium ion diffusion.The inner soft and outer hard double-layer core-shell structure was lower than that of the inner hard outer soft double core shell structure along the radial Von Mises stress and radial stress.The shear tensile stress is still lower than that of the core shell structure although the shear stress increases after the internal and external materials exchange.Three sets of parametric scanning results(5×10^-9m、1×10^-8 m、1.5×10 ^8-m)are established for the thickness of outer shell.The results show that the double-layer core-shell structure has better ratio performance when the thickness of the outer layer is5×10^-9 m,and von Mises stress and radial stress are relieved,which is beneficial to the stability and reliability of lithium-ion battery.