Stress Analysis And Optimization Of Silicon Anode For Porous Lithium-Ion Batteries

Negative electrode material is one of the important factors affecting the overall performance of lithium ion battery.The theoretical specific capacity of silicon material is very high and the lithium intercalation potential is low.However,the main problem of silicon electrode materials is that the huge volume expansion seriously affects the cycle performance of silicon materials,and the improper battery internal parameter design and lithium mechanism problems aggravate the silicon anode materials rupture and pulverization,resulting in the damage of electrode structure,resulting in battery capacity attenuation.Based on the electrochemical-mechanical coupling model,the effects of electrode material design parameters and lithium mechanism on the stress of silicon anode are studied.The effect of porous structure on the stress of porous silicon electrode is studied theoretically and experimentally.Finally,the lithium process of porous silicon electrode is optimized on the basis of reducing the stress,which provides a theoretical basis for restraining the irreversible deformation of materials.The main research content of this paper includes the following aspects:（1）Under the premise of finite deformation,based on the principle of electrochemical and mechanical coupling,the electrochemical-mechanical coupling model is established,and the main factors affecting diffusion stress are analyzed by single factor analysis method.The internal factors affecting the stress include particle radius,solid diffusion coefficient,Poisson’s ratio and Young’s elastic modulus,etc,while the external factors are mainly lithium ratio.The results show that when the silicon electrode is lithium ion embedded in the active particles,the center of the particles is stretched and the surface is compressed.The radial tangential stress at the particle center is equal,and the radial stress on the particle surface is 0.Von Mises stress is maximum at the particle surface and decreases from the particle surface to the center,so the surface is most prone to rupture.The model analysis shows that the structure of active materials and the lithium ratio have great influence on the stress,so it should be taken into account in the design of battery.（2）Three different porous silicon electrode materials are prepared by chemical etching aluminum-silicon alloys with different silicon ratios,and the effectiveness of the porous structure design is further studied based on electrochemical experiments.The results show that the porous silicon anode has better cycling and multiplier properties than the solid silicon anode.Moreover,the porous silicon material with higher porosity has better performance.The stress evolution of porous silicon electrode is studied theoretically,and the effect of pore size on electrode stress is analyzed.It is found that compared with solid structure,the silicon stress of porous structure decreases obviously,and the evolution process of porous stress still follows the distribution trend of lithium concentration.With the increase of pore size,the stress of porous electrode decreases gradually.（3）In view of the influence of the lithium mechanism on the stress,the multi-stage constant current strategy is selected to optimize the de-intercalation process of porous silicon electrode battery on the basis of reducing the stress.F@1 and F@3 strategies are adopted to reduce the peak stress during the process of lithium intercalation,and F@2 and F@4 strategies are adopted to reduce the stress during the whole charging and discharging cycle.The results show that the battery cycle performance under the four optimization strategies is excellent,and the capacity retention rate is high at high temperature.At different temperatures,the cyclic stress can be alleviated to a great extent,the electrode damage can be reduced.The cyclic induced stress on the electrode surface and cross section is characterized by scanning electron microscopy（SEM）,and the results are in good agreement with the numerical simulation results.The selected electron diffraction（SAED）test reflects the actual volume expansion size of silicon particles from the microscopic point of view.The minimum volume expansion of silicon particles at 5,25 and 55℃is 45.88%,18.75%and 11.99%,which is much lower than the volume expansion of Li₁₅Si₄ alloy phase at room temperature（280%）.