Reactive diffusion of lithium in silicon in anode materials for Li-ion batteries

In this work, we perform atomistic simulations on the insertion of lithium (Li) in crystalline silicon (c-Si) by using a modified embedded atom method potential. Novel structural analyzes definitively reveal the mechanism of reactive diffusion Li in c-Si. The results show that Li atoms diffuse preferably along the 112 directions on the {111} planes, and the energy barrier is close to 0.6 eV/atom, which is very high for thermally activated diffusion around room temperature. This diffusion path leads to the formation of an interface between the a-LixSi and the c-Si, i.e., ACI. In the ACI region, the Li atoms appear to be aligned along the (111) planes. The ACI is a supersaturated Si-Li solid solution with large lattice distortion. Insertion of more Li atoms causes the ACI region, which is still crystalline, to collapse, but the amorphization is incomplete. A new ACI is then formed and migrates into the c-Si. Our results also demonstrate that the diffusion rate of Li in the a-LixSi is about 50-100 times faster than in the c-Si. This leads to an interesting growth mechanism for the a-LixSi. The earlier inserted Li atoms are pushed outward by the later inserted Li atoms. The value of x is measured from our simulation results: x approximate to 0.17 in the ACI and x approximate to 0.5 right next to the ACI, respectively. Inside the a-LixSi, x varies between 0.9 and 2.3 from the ACI toward the surface; close to the surface, x approximate to 4.0.