Structural transformations and mechanics of amorphous silicon anodes during initial lithian-delithiation cycle

Structural transformations and mechanics of amorphous silicon anodes during initial lithian-delithiation cycle

Silicon (Si) is widely regarded as one of the most promising anode materials for Li ion battery (LIB) due to its highest known theoretical specific capacity (4200mAhg-1). Particularly, amorphous silicon has recently attracted great interest due to its robust lithiation behavior. Silicon electrodes are subjected to charge-discharge via an alloying-dealloying (lithiation-delithiation) mechanism. It enables the host silicon to store up to 4.4 lithium atoms per silicon atom, which gives more than ten times greater capacity compared with conventional graphite based electrodes. However, this enormous capacity comes with the expense of significant structural changes to the host silicon, which results in poor mechanical integrity. To address this issue, it is important to understand the transitional structural and mechanical properties of silicon during charge-discharge process. In this study, molecular dynamic simulation is employed to study the structure and mechanics of an amorphous silicon thin film (2.7nm) during a complete lithiation-delithiation cycle. The microstructure evolution during lithiation process is associated with the break of covalent amorphous structure into small clusters, when Li concentration exceeds ~Li0.25Si. Also, it is evident that Li induced stress is largely dependent on lithium concentration in silicon. Steep stress gradients were observed when the lithium content in the anode is very low (<Li0.25Si). Plastic flow and stress relaxation are dominant in higher Li concentrations. Furthermore, we show that the rate of si-si bond breaking is lithium concentration dependent. Also, the Radial Distribution Function (RDF) analysis revealed a defective amorphous structure in silicon film after a complete lithiation/delithiation cycle, which is attributed to breaking down of covalent silicon network and subsequent plastic flow. These results are useful in understanding lithiation-delithiation mechanisms of silicon and the favourable charge-discharge depths to avoid extreme stresses. Furthermore, these results are believed to be able to help improve the materials selection and design of next generation Li ion batteries.

Poster. c. 2016 QUT.