1.College of Mechanical and Vehicle Engineering, Taiyuan University of Technology, Taiyuan 030024, China 2.Shanxi Key Laboratory of Material Strength and Structural Impact, Taiyuan University of Technology, Taiyuan 030024, China 3.Institute of Applied Mechanics, College of Mechanical and Vehicle Engineering, Taiyuan University of Technology, Taiyuan 030024, China 4.Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education Affiliation, Dalian University of Technology, Dalian 116024, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11872265) and the Natural Science Foundation of Shanxi Province of China (Grant Nos. 201901D111087, 201801D121281)
Received Date:10 March 2021
Accepted Date:23 April 2021
Available Online:07 June 2021
Published Online:05 September 2021
Abstract:Silicon is considered as a first candidate for ideal anode material of the next-generation lithium-ion battery due to its high theoretical capacity to meet the demand for higher energy density. On the other hand, high theoretical capacity is accompanied by massive volume expansion, which gives arise to high stress and crack and pulverization of anode particles. Finally, the capacity of the battery fades gradually. While some kinds of factors contribute to the failure of silicon-based electrodes, the most important one is the diffusion-induced stress generated in silicon-based electrode particles. The cyclic processes of lithiation and delithiation are accomplished by the intercalation into and deintercalation from the silicon particles of lithium ions. During the cycle, physical processes and chemical processes, such as diffusion of lithium ions, phase transition, and volume expansion, take place simultaneously, making the cyclic process a strong-coupling problem to be addressed. For example, the intercalation of lithium ions into the electrode results in volume expansion and phase transition of anodes, thereby inducing stress; in turn, stress affects the diffusion process of lithium ions. Aiming to probe this problem, with the finite deformation hypothesis, an electrochemical-mechanical coupling model is used to study the variation and distribution of concentration and stress of core-shell structure during lithiation. And more importantly, great emphasis is put on the optimal design of core-shell structure. The numerical results show that the shell is useful in prohibiting the volume expansion of silicon core, but large compressive radial stress in silicon core may cause the core and shell to be detached, while the tangential tensile stress at the core-shell interface leads the shell to fracture. To improve the electrochemical and mechanical performance and hence lengthen the cycle life of lithium-ion batteries, two kinds of optimal designs are considered: 1) single-layered core-shell structure and 2) double-layered core-shell structure. The numerical results suggest that the softer shell material is suitable for a single-layered core-shell structure and the inner-soft & outer-hard design is optimal for the double-layered core-shell structure. Furthermore, the effects of Young's modulus of the inner and outer carbon layer materials on the chemical and mechanical performance of anode are explored. The simulation shows that the optimal Young's modulus of the inner shell is less than 10 GPa, and that of the outer shell is not higher than 70 GPa. This research is helpful in designing and optimizing the silicon-based anode electrodes of lithium-ion batteries. Keywords:lithium-ion battery/ electrochemical-mechanical coupling/ core-shell structure/ finite deformation
表1核壳结构材料参数[16,32,33] Table1.Material parameters of core-shell structure[16,32,33]
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3.1.有限变形效应
针对高能量密度所带来的体积膨胀, 计算了小变形理论的径向应力与切向应力, 与有限变形理论的数值结果进行了对比, 如图4所示. 随后模拟计算得到浓度场和应力场, 结果如图5所示. 图 4 不同假设条件下充电过程中沿着半径方向的径向应力和切向应力 (a)小变形下的径向应力; (b)有限变形下的径向应力; (c)小变形下的切向应力; (d)有限变形下的切向应力 Figure4. Radial stress and tangential stress along the radial direction during charging under different hypothesis: (a) Radial stress under small deformation; (b) radial stress under finite deformation; (c) tangential stress under small deformation; (d) tangential stress under finite deformation.
图 5 嵌锂过程中沿着半径方向的(a)浓度和(b) von Mises 应力分布示意图 Figure5. Schematic diagram of (a) concentration; (b) von Mises stress along the radius during lithium insertion.
表2包覆层碳的材料参数 Table2.Material parameters of the outer shell.
23.2.单层核壳结构 -->
3.2.单层核壳结构
为了降低核壳结构有限变形模型下的应力, 本文数值模拟了不同的包覆层碳, 以比较包覆层材料对活性颗粒结构的电化学和力学性能的影响. 图6所示为嵌锂过程中浓度、von Mises应力、径向应力和切向应力随着时间的分布示意. 图 6 嵌锂过程中(a)浓度, (b) von Mises应力, (c)径向应力, (d)切向应力随着时间的分布示意 Figure6. Schematic diagram of the distribution of (a) concentration, (b) von Mises stress, (c) radial stress, and (d) tangential stress over time during lithium insertion.