Theoretical And Experimental Study On Mechanical-electrochemical Coupling Behavior Of Silicon Electrode In Lithium Ion Batteries

Secondary lithium-ion battery is a rechargeable battery that can be reused many times.Due to its high energy density,high open circuit voltage,low self-discharge rate and environmental protection,it is widely used in the fields of aerospace,transportation,civil electronics and the like.The anode material of lithium ion batteries is a key factor affecting the overall performance of lithium ion batteries.Therefore,the development of a new generation of high specific capacity anode materials is one of the important directions for the future development of the lithium ion battery industry,and is of great significance for improving the performance of lithium ion batteries.Silicon electrode materials are considered to be the most potential new generation of lithium ion battery anode energy storage materials due to their high volumetric specific capacity,high specific capacity,safe performance,abundant reserves,mature technology and low cost.However,when the silicon interacts with lithium ions,lithium ions can destroy the chemical bonds between the silicon atoms,causing a large change in the internal microstructure and volume.The volume change can be up to 400%.Such large volume changes will directly lead to mechanical failure and failure of the electrode structure,which in turn will cause its electrical performance to be greatly attenuated or even failed.In view of this problem,the existing research mainly improves from two aspects:(1)Silicon-carbon composite electrode material is used to coat silicon nano-particles in the hard shell of amorphous carbon to form a silicon-core-shell spherical structure electrode;(2)A nano-silicon film structure is used as an electrode.Both of the above methods can significantly improve the mechanical and electrical properties of the silicon electrode compared to the bulk morphology.However,both the spherical shell silicon-carbon composite electrode and the thin film electrode have serious mechanical damage and failure problems in the process of charging and discharging,and the reaction process involves the coupling of mechanics and electrochemistry.The physical boundary conditions are also complex and its destruction.The mechanism is still unclear.In addition,most of the existing researches are analyzed from the perspective of theory and calculation,and there are deficiencies in the experiment.In summary,this paper studies the silicon-core-shell spherical structure electrode and the nano-silicon film electrode.The main contents include:(1)The diffusion stress of silicon-based multi-layer spherical core-shell structure electrode is deduced theoretically.Considering the influence of interface/surface modulus of three-phase material,the distribution of radial stress and tangential stress of silicon-based multi-layer spherical core-shell structure electrode during charging and discharging is numerically analyzed.(2)Based on laser scanning confocal microscope,we developed an in-situ observation system for silicon film electrode material-electrode of lithium-ion battery.This in-situ observation system contains electrode-visualization button battery system,electrochemical test system and laser scanning confocal microscope system.A method for in-situ three-dimensional measurement of full-field deformation of a silicon thin film electrode material of lithium-ion battery is proposed..(3)Using in-situ observation system,the morphology and deformation damage evolution process of silicon film electrode structure was real-time detected during the charge and discharge.In this part,a 50μm copper foil was used as the substrate.A silicon thin film electrode with a thickness of 800 nm was prepared by RF magnetron sputtering.The surface of the silicon thin film electrode was observed in situ by in-situ observation system to obtain two-dimensional image and three-dimensional coordinates of the thin film electrode.The three-dimensional digital image correlation method was used to process and analyze the image.The in-plane strain and out-of-plane displacement of the silicon film electrode during charge and discharge were obtained.The relationship between mechanical damage and electrochemical performance degradation of silicon thin film electrodes was analyzed by introducing semiquantitative analysis method of mean strain gradient parameters.The research shows that the mean strain gradient parameter based on the full field strain information can more accurately describe the mechanical/electrochemical relationship between the deformation damage and the electrochemical failure of the silicon film electrode.