Mechanical and electrochemical properties of Sn anode for lithium ion batteries

Abstract

Sn is a promising anode material for lithium ion battery with high theoretical capacity, but it can easily pulverize due to repeated application of extreme volumetric strain of 260 % during cycling, which leads to the fracture of the Sn anode material. Moreover, Sn is compliant in nature and has a low melting point which mitigates the failure based on Li diffusion-induced stresses (DIS) via creep-based stress relaxations. In this thesis, intrinsic mechanical properties of lithiated Sn at various stages in the lithiation were explored, and by incorporating these results, Sn as an anode material for lithium ion battery was evaluated. First, in chapter 2, in order to avoid oxidation of the highly reactive lithiated Sn, nanoindentation was performed on specimens submerged in a mineral oil. After careful calibration, hardness and modulus of different phases of lithiated Sn were evaluated. With an increase in the lithium content, both the modulus and hardness of the lithiated Sn decreases as expected, where the modulus and hardness are 28.6 GPa and 0.37 GPa respectively, for fully lithiated Sn ($Li_{22}Sn_5$) and 58.0 GPa and 0.77 GPa, respectively, for unlithiated Sn. Second, in chapter 3, by using the material properties obtained in chapter 2, the effect of Li concentration-dependent material properties and concentration-independent material properties on the performance of Sn micropillar is investigated and found that the maximum tensile stress is formed in the center of the micropillar. Therefore, by incorporating these maximum tensile DIS results, critical size for failure of Sn micropillar was determined to be 5.3 μm for C/10 charging rate. This was then correlated to experimental observations, where fracture occurred in Sn micropillars with sizes larger than 6 μm while 4.4 μm sized Sn micropillar survived the lithiation cycle. Last, in chapter 4, 3D – structured porous Sn anode is considered. To overcome the volume expansion problems in high capacity anodes, nanostructures are considered due to many advantages such as increased surface area, reduced ion diffusion distance and stress relief generated by volume expansion under electrochemical conditions. However, in manufacturing such a nanostructure is very costly, and therefore, by using one-step freeze-drying method, we were able to produce a 3D porous Sn anode in a single process that shows excellent electrochemical performance.