Study On The Evolution Of Structure And Mechanical Properties Of Silicon During The Lithium Intercalation Process

With the promotion of new energy vehicles and green energy,lithium-ion batteries have great application valueon above fields.At this stage,the anode material of lithium-ion battery is mainly graphite material,but its actual specific capacity is relatively low,which is close to its theoretical capacity.Therefore,one of the research hotspots of large-capacity lithium-ion batteries currently is to find new anode materials.Silicon has a high mass specific capacity（about ten times that of graphite）,low reactivity with the electrolyte,abundant reserves of silicon materials,easy mining,and low preparation cost,so silicon is the ideal anode material for lithium ion batteries.Although it has the above advantages,silicon material undergoes a large volume expansion during the lithium intercalation and deintercalation process;after multiple charge-discharge cycles,silicon electrode material is prone to fragmentation or even powderization;At the end,the electrical contact between the active particles and the current collector fails,and the battery cycle performance deteriorates.In this paper,the above problems are studied.The first-principles method is used to study the intercalation process of a single lithium ion in the crystalline silicon material.Binding energy,lattice structure changes of bulk silicon materials,were studied.The results show that the doping of lithium ions in bulk silicon is interstitial doping rather than substitutional doping,and the tetrahedral central site（Td site）is the best site for lithium intercalation in crystalline silicon.At the same time,the diffusion form of lithium ions in the bulk silicon material was studied,and the results showed that the diffusion of lithium ions into the bulk silicon material was along the Td site-Hex site-Td site mode,and the diffusion barrier was about 0.62 eV.When lithium is inserted in a large concentration,lithium and silicon will form a silicon-lithium alloy,and the structure of the lithium-silicon alloy generated is different according to the lithium-insertion concentration.In order to better understand the changes in the structure and mechanical properties of the silicon-lithium alloy during the intercalation process,the mechanical properties and structural changes of the silicon-lithium alloy system after lithium intercalation were studied in this paper.Through the analysis of the MT calculation module,it is known that with the increase of the lithium intercalation concentration,the mechanical properties of the silicon-lithium alloy gradually decrease.When the lithium intercalation concentration reaches 81.5%,a saturated silicon-lithium alloy is formed,and the mechanical properties of the alloy are reduced to 50%of that of silicon.The reduction of the overall mechanical properties of the silicon anode material further leads to the fragmentation of the anode.And with the incomplete transformation of lithium intercalation,a part of silicon cannot be completely delithiated after lithium intercalation,which makes the cycle performance of silicon intercalation worse.Many studies have shown that at high temperature,silicon crystals can form Li Si,Li₁₂Si₇,Li₁₃Si₄,Li₁₅Si₄,Li₂₂Si₅and other crystalline silicon-lithium alloys during the lithium intercalation process.At room temperature,the lithium intercalation process of the silicon anode material is more inclined to form an amorphous silicon-lithium alloy.In this paper,the mechanical properties and structural changes of the amorphous silicon-lithium alloy system after lithium intercalation were studied,and the structure,charge state and mechanical properties of the crystalline silicon-lithium alloy were compared.Amorphous silicon has a similar Td site to crystalline silicon,which has a larger binding energy than the Td site in crystalline silicon,so the system is more stable.Using the NEB method,the diffusion process of lithium in amorphous silicon was studied,and the diffusion barrier of lithium in amorphous silicon was calculated.It was found that the diffusion barrier of lithium in amorphous silicon was0.53 eV,which was 0.62 eV in crystalline silicon lower,indicating that lithium diffuses more easily in amorphous silicon.Amorphous silicon will also generate amorphous silicon lithium alloys of different structures according to the concentration of lithium intercalation when large concentrations of lithium are inserted.The resulting amorphous silicon lithium alloys have lower mechanical properties than lithium silicon alloys as a whole due to the softening of the amorphous phase,and With the increase of the lithium intercalation concentration,the mechanical properties of the amorphous silicon lithium alloy decrease significantly when the a-Li₂₂Si₅generated,and it is very prone to damage such as deformation and cracking.The research results of this paper provide a reference for the development and design of large-capacity,long-life lithium-ion batteries.