由于具有高的电能密度、良好的协调变形能力，纳米Si材料已经成为最具潜力的锂离子电池负极材料。如何理解锂化过程中纳米Si材料的力学行为和力学性能对于推动锂离子电池的发展具有非常重要的意义。本文采用分子动力学模拟、理论分析和实验相结合的方法，研究了锂化过程中纳米Si材料的变形、断裂及应力效应。针对锂化后生成的非晶锂硅合金（a-LixSi），研究了其裂纹扩展过程中的微结构演化以及相关的断裂机制。大规模分子动力学模拟表明，随着锂化浓度的增加，a-LixSi由裂尖孔洞形核主导的脆性断裂转变为裂尖塑性变形主导的韧性断裂。基于理论分析，建立孔洞形核准则，该准则预测a-LixSi的韧脆转变可以由材料孔洞形核的临界强度与材料剪切屈服强度的比值来确定，且预测结果与分子动力学模拟结果相符合。鉴于孔洞形核对a-LixSi断裂的重要性，进一步采用分子动力学模拟了a-LixSi中孔洞的三维生长。模拟结果从原子尺度上揭示了，随锂化浓度增加，孔洞由非均质生长转变为各向同性生长。利用锂离子电池Si负极材料的锂化变形，构建了一种基于锂离子电池（其中Si做负极、LiFePO4做正极）的致动器。采用分子动力学方法模拟了密排结构c-Si颗粒电极的初次锂化过程，从微结构的相界迁移与Si-Si键伸长率的角度，解释了实验测量的载荷-时间曲线中拐点产生的原因。通过分子动力学模拟首先验证了预弯曲c-Si样品中拉压两侧不对称锂化的现象。同时，模拟了定常外加双向应力状态下c-Si锂化过程中的相界迁移，并采用电荷平衡的方法识别了相界。模拟结果表明，在界面反应主导的初始锂化阶段，随着外加双向应力的增加，相界迁移速度增加。另外，也计算了不同的外加静水应力状态下，晶态Li0.5Si和非晶LiSi中Li原子的扩散系数。计算结果表明，拉应力促进扩散，压应力抑制扩散。为了解释c-Li13Si4化学脱锂后形成层状a-Si纳米片的实验现象，建立了自发分层的理论模型。采用分子动力学方法计算了a-Si纳米片与c-Li13Si4基底之间的错配应力，结合原子尺度的内聚力模型方法计算了界面的断裂能。理论模型预测纳米片形成的临界厚度为5.57 nm，与实验结果（3-5 nm）相一致。

Nano-structured Si has been considered as a promising next-generation anode material for lithium-ion batteries due to its high theoretical energy density and excellent accommodation for large volume change. It is of great significance for development of lithium-ion battery to understand the mechanical behaviors and properties of nano-structured Si anode during lithiation. In this dissertation, we therefore investigated the deformation, fracture and stress effect of nano-structured Si during lithiation using a combination of large-scale molecular dynamics simulations, theoretical modelling and experiments.A series of large-scale molecular dynamic simulations were performed to investigate the crack propagation behaviors and associated fracture mechanisms of various a-LixSi. The simulation results reveal that as the lithium concentration increases, there exists a transition in fracture mechanism from intrinsic nanoscale cavitation to extensive shear banding ahead of the crack tip. Such fracture-mechanism transition can be understood from the changing ratio between critical stresses for cavitation and plastic yield under increasing lithium content. Atomistic J-integral method was used to calculate the fracture energies of various a-LixSi. The obtained results are consistent with to recent experimental measurements. Furthermore, we have investigated the mechanistic details of cavitation in a-LixSi using fully three-dimensional molecular dynamics simulations. It was revealed that in a low lithium concentration environment, an initial void grows heterogeneously by merging with neighboring nucleated voids. However, at high lithium concentrations, the initial void continues to grow in a homogeneous mode.Taking advantage of lithiation induced deformation, we constructed an actuator based on a Li-ion battery device with a Si anode and a LiFePO4 cathode. Such actuator can generate a stress of about 10 MPa stress and has a response time of about 1 s. To reveal the underlying mechanisms behind the turning point of loading-time curves from experiments, we carried out molecular dynamics simulations for first lithiation of close-packed single-crystalline Si nano-spheres. The simulation results indicate that the turning point is associated with lattice transformation of c-Si during lithiation. Such transformation involves the elongation and breakage of Si-Si bonds, as well as crystalline-to-amorphous transition caused by phase boundary migration.A series of molecular dynamics simulations were first used to verify the asymmetric lithiation in the pre-bent c-Si. Then we further simulated the lithiation of c-Si under the constant external bi-axial stress, which is normal to the direction of Li transport. The simulation results show that as the external bi-axial stress increases, the velocity of phase boundary at initial stage controlled by interfacial reaction significantly increases. Furthermore, we calculated the diffusivity of Li atoms in crystalline Li0.5Si and amorphous LiSi under different applied hydrostatic stress. The dependence of diffusion coefficient on the external hydrostatic stress follows the Arrhenius equation, where the activation energy is linearly proportional to the external stress. These results indicate that the external stress has significant effect on both interfacial reaction and diffusion during lithiation, and also demonstrate that the tensile stress accelerates the lithiation rate, while the compressive stress retards it. Our calculations of diffusivity of amorphous LiSi indicate that the sampling amorphous structures and duration time have significant influence on calculations of diffusivity of amorphous materials.To explain the formation of Si nanosheets by simply chemical leaching of Li from c-Li13Si4 (actually chemical delithiation), we proposed a theoretical model based on spontaneous delamination. The model shows that when the thickness of delithiated layer reaches up to a critical value hcr, the mismatch strain energy stored in delithiated layer drives a spontaneous delamination, leading to formation of Si nanosheets. We used molecular dynamics simulations and atomistic cohesive-zone-volume-element methodology to estimate the mismatch stress and interfacial fracture energy. Substituting these calculated key parameters to theoretical model, we obtained the critical film thickness hcr as 5.57 nm, which is in agreement with the experimental results of 3~5 nm.