Sn as a fracture-resistant electrode for high performance lithium ion batteries

Abstract

Sn anode has maximum theoretical capacity of 994 mAhg-1, which is three times higher than that of the commercialized graphite anode (372 mAhg-1). Sn, being a metal, is compliant and ductile in nature and thus is expected to readily relax the Li diffusion-induced stresses. Most importantly, low melting point of Sn permits time-dependent or creep deformations even at room temperature and therefore allows for further relaxations of diffusion-induced stresses. In the first part of this dissertation, numerical modeling is used to analyze first the elastic stresses followed by the effect of plasticity and creep inelastic deformations for Sn micropillars that revealed the significance of plasticity and creep based stress relaxations. For instance, the maximum elastic tensile hoop stresses for 1 μm Sn micropillar with 1C charging rate condition decreases from ~1 GPa to ~200 MPa when Sn is allowed to plastically deform at yield strength of ~150 MPa. Nanoindentation testing is performed to find out the creep response of Sn micropillar and are incorporated in numerical modeling to show that the maximum tensile hoop stress is further reduced from ~200 MPa to ~0.45 MPa under the same conditions. The critical size to prevent fracture is also calculated which is calculated to be ~5.3 μm for C/10 charging rate, that is notably bigger than as compared to Si at same conditions. In the second part of the dissertation, Li concentration-dependent material properties are incorporated in numerical modeling and its influence on diffusion induced stresses is revealed. The stresses in Sn anode are found to be lowered due to lowered modulus i.e. elastic compliance effect. The diffusivity of Li is enhanced and thus stresses are reduced when diffusivity is incorporated in numerical modeling. Interestingly, the hoop stresses at surface of Sn micropillars transform from compressive to tensile in nature due to surface stress relaxation based on plasticity. The creep deformations steadily relax these transformed-tensile hoop stresses at surface. The results suggest that as the composition-dependent material properties are not insignificant, therefore, considering these properties in numerical calculation is more realistic. In the last part of the dissertation, the merits of mechanical stability with Sn/Ni core-shell morphology is re-vealed. Relatively stiffer metal Ni was used to reduce the outward volume expansion of Sn core. With increasing lithiation, tensile stresses at the center of Sn core are transformed to compressive and thus making it more frac-ture-resistant. The maximum tensile hoop stress at the center of Sn pillar is reduced from ~130 MPa to ~30 MPa when Ni shell thickness increases from ~5 nm to ~100 nm and even entirely vanished for Ni shell thickness of ~120 nm. For Sn micropillar of 1 μm size, the optimum Ni shell thickness is found to be below ~ 100 nm in order to minimize the self-delithiation effect due to huge compressive stresses.