Silicon (Si) is considered as a promising anode material due to its exceptionally high specific capacity. However, Si anodes undergo rapid capacity fade upon lithiation/delithiation cycles, even when nanostructured Si was used and fracture is greatly reduced. [1] The structural evolution of the amorphous Si-Si network during cycling and its impact on trapping of Li atoms during delithiation critically affect the capacity retention. To reveal the fundamental reasons for this degradation mechanism, reactive molecular dynamics simulations along with a newly developed continuous delithiation algorithm were used to investigate the response of fully lithiated amorphous Si to different delithiation rates.

In order to create a chemical potential gradient mimicking delithiation from the electrode surface, a starting structure of amorphous Li_3.75Si slab sandwiched by amorphous Li₈Al₂O₃ coating was designed. The a-Li₈Al₂O₃ coating mainly served as a Li reservoir, from which a small amount of Li atoms can be randomly removed. The removal of Li atoms generated a chemical potential gradient between the a-Li_xSi slab and a-Li_yAl₂O₃ coating, which provided the driving force for Li to diffuse out of the lithiated Si region. The chemical potential difference is calibrated with the computed open circuit voltage of the a-Li_xSi and a-Li_yAl₂O₃, shown in Figure 1a. This diffusion process occurs during MD relaxation of the structure. A series of delithiation steps followed by MD simulations were repeated until there were not enough Li left in the coating or in the a-Li_xSi slab. The corresponding rate of delithiation is defined by the number of Li removed in each step divided by the MD simulation time. The Li concentration in the Si and coating regions were tracked and more detailed bonding, angle and excess volume analysis were performed to study the structural evolution.

Although the same numbers of Li have been removed from the coatings in each step, the final structures after the delithiation simulation with different rates are quite different (shown Figure 1b and 1c). Delithiation with slow rate (200Li/10ps) allowed the a-Li_3.75Si to delithiate to a-Li_0.15Si and contract to its original Si volume before lithiation, indicating the delithiated a-Li_0.15Si has negligible amount of pores or voids. In contrast, if the delithiation rate is 100 times faster, the delithiated Si remained at a higher state of charge of a-Li_0.75Si, with more residual Li trapped in the Si slab. The volume of the a-Li_0.75Si is 40% larger than its theoretical volume, indicating some voids and pores are formed. Furthermore, delithiation with fast rate eventually resulted in delamination of the Li_xAl₂O₃ coating.

The structural changes represented by formation of porous amorphous Si and corresponding Li trapping clearly demonstrate the effect of delithiation rates on capacity loss. The loss of active Li due to trapping in the porous amorphous Si upon delithiation with fast rate contributes to the capacity loss.

Reference

[1]        W. J. Zhang, “A review of the electrochemical performance of alloy anodes for lithium-ion batteries,” J. Power Sources, vol. 196, no. 1, pp. 13–24, 2011.