Alloying with Ge and Hollowing Reduces Lithiation-Induced Stresses in Si Nanopillar Anodes

Abstract

High energy-density batteries are crucial to energy storage solutions. In lithium-on batteries (LIBs), Si nanopillars are promising anodes due to their highest theoretical specific capacity. However, volume expansion and fracture during cycling inhibit its widespread adaptation. Ge, which is isomorphic with Si, shows better fracture resistance and higher cycle life but has higher molecular weight and cost. Alloying Si with Ge offers a trade-off in optimizing stresses, weight and cost. Here, we computationally evaluate the effect of alloying Si with Ge in reducing stresses generated during lithiation. Hollowing, which creates additional free surface for expansion is also considered. First, we model the stress evolution in nanopillars of Si, Ge, Si–Ge core-shell and Si0.5Ge0.5 alloy. Alloying Si with Ge uniformly, reduces the maximum circumferential stress by around 17%, however, the Si core-Ge shell structure shows stress reduction only when lithiation is confined only to the Ge. Stresses in Si/Ge alloyed nanotubes considering lithiation from the outer boundary as well as from both boundaries are considered. We find a non-monotonous change in lithiation stress with varying radius ratio (R in/R out) and R in/R out = 0.4 leads to the least maximum Hoop stress. The stress reduction in Si-nanotubes in such configuration is found to be 16%.