As an anode active material, Si experiences large deformations during battery cycling, which has been cited as a primary cause for the electrode fracture and thus capacity fading. The problems caused by the electrode deformations can be remedied through electrode design, and the numerical models will greatly facilitate this design process. However, the lack of key material properties for Si/LixSi, especially the Li diffusivity in Si, has prevented the efficient use of such numerical tools in the design of lithium-ion batteries with Si anodes. In this work, finite element (FE) analysis was used to study the Li diffusivity in amorphous Si (a-Si) anodes. To simulate the large deformation, a large strain based formulation for the concentration induced volume expansion was proposed. The Li diffusion and volume expansion were coupled and solved in COMSOL Multiphysics®. By comparing with the lithiation experimental observations for the a-Si nanospheres, the Li diffusivity was estimated to be on the order of 2.0 × 10 − 15 m 2 /s for the Li-rich phase and 2.0 × 10 − 17 m 2 /s for the Li-poor phase. These values were further validated by comparing the thickness growth of the Li-rich phase and the lithiation time.