Microstructure dependent chemo-mechanical behavior of amorphous Si anodes for Li-ion batteries upon delithiation

Alloying-type anodes exhibit the solid-state amorphization during charging/discharging cycles. The mechanical and electrochemical properties of amorphous reaction phases have been widely explored recently. However, there is still lack of understanding of the underlying mircostructure-property relation in the delithiation behavior of alloying anodes. Here we perform molecular dynamics simulations to investigate the microstructure effect on the chemo-mechanical properties of amorphous Si (a-Si) anodes upon delithiation. It is indicated that stress-free delithiation without sufficient structural relaxation leads to the gradual accumulation of structural disorder (the increase of excess energy) in amorphous Li-Si systems (a-LixSi). The creation of structural disorder during delithiation not only facilitates the plastic deformation of a-LixSi at lower stress, but also thermodynamically destabilizes a-LixSi associated with the drop of open-cell potentials. While upon constrained delithiation, the initial value of excess energy and reaction stress both contribute to the increase of structural disorder during delithiation process. Based on the stress-dependent chemical-potential model, the tensile stress increases open-cell potentials, and reduces the Li chemical potential which weakens the driving force for delithiation. As a result, the structural disorder and tensile reaction stress may cause the undesirable capacity fading of a-Si anodes, and is detrimental to the battery performance.