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Chemomechanical Modeling of Silicon Electrode in Lithium-Ion Battery by Considering the Effects of Large Plastic Deformation and Strain Rate

Bagheri, Afsar | 2022

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 55505 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Naghdabadi, Reza; Argavani Hadi, Jamal
  7. Abstract:
  8. In Li-ion batteries, large volume changes of electrodes with high capacity, such as silicon electrodes during charging and discharging processes (insertion/extraction of lithium into/from the electrode), lead to restrictions on the use of this type of electrode. Given the very high capacity of silicon, predicting and modeling the behavior of silicon electrodes during electrochemical cycles is important and is also addressed in this thesis. First, using the theory of linear elasticity, relations are proposed that couple the diffusion of particles with small elastic deformation. The effect of hydrostatic stress on diffusion and stresses is investigated in lithiation processes. Then, the simultaneous theory of swelling and small strain plasticity (mass transfer coupling and small inelastic deformation) is formulated. The proposed model is applied to investigate two different rate-dependent and rate-independent behaviors for silicon spherical particles under lithiation at different charging rates. In addition, the the large plastic deformation behavior of electrode is modeled as elastic-perfectly plastic and elastic-viscoplastic. The distribution of concentration, stress, strain and chemical potential for both types of behavior are analyzed during electrochemical cycles. In addition, the effects of charging rate and material properties on the behavior of the electrode are investigated. Finally, another structural theory is proposed that couples the large viscoplastic deformation by the insertion and extraction of lithium in the amorphous structure of electrode materials. The theory relies on the concept of shear transformation zone (STZ) as a plastic flow carrier in amorphous materials and is able to estimate microstructural changes using the internal free volume variable. The model is able to predict several plasticity properties of electrodes with amorphous structure during electrochemical processes, including dependence on lithium rate (strain rate) and pressure dependence, as well as structural changes. The predictions made by the model are also compared and evaluated with experimental results for silicon electrodes with amorphous structure. The results of various modelings and the study of the effects of charging rate indicate that hydrostatic stress leads to acceleration and increase in diffusion of lithium, resulting in a larger charge state ( ). It also reduces the concentration gradient of lithium ( reduction of concentration gradient at the end of lithiation process) and the magnitude of the stresses ( reduction of radial stress at the center). Larger gradient of lithium concentration and larger plastic deformation are experienced by the silicon electrode with perfectly plastic behavior compared to viscoplastic behavior at higher charging rates. Simulation of charge / discharge cycles at different charging rates shows three cycle modes for deformation: (visco)plastic cycles, elastic shakedown cycles, and elastic cycles. Charging capacity for charging rates and is reported and , respectively. The results show that increasing the charge rate, which leads to plastic deformation, reduces the charging capacity and battery performance efficiency. The results of this thesis can be a basis for the correct selection of electrode materials and the appropriate charging and discharging strategy with maximum efficiency and minimum stresses in electrodes of Li-ion batteries
  9. Keywords:
  10. Lithium Ion Batteries ; Free Volume ; Elastic-Viscoplastic Behavior ; Hydrostatic Stress Effect ; Silicon Electrode ; Elastic-Plastic Behavior ; Severe Plastic Deformation ; High Strain Rate

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