Molecular insights into vacancy defect formation in silicon anodes induced by femtosecond laser

Silicon anodes in lithium-ion batteries have garnered attention due to their high capacity. However, the volume changes during charging and discharging lead to fracturing and reduced lifespan. To address this, femtosecond laser processing technology has been introduced to improve silicon anodes. This involves structural adjustments and the introduction of vacancy defects to enhance the diffusion rate of lithium ions and mitigate volume expansion. This study employs molecular dynamics simulation methods and, based on the Lambert-Beer law, constructs a complex three-dimensional model of a femtosecond laser-ablated silicon anode. The model enables atomic-level observation and tracking of the formation and evolution of vacancy defects in silicon anodes during the heating process induced by femtosecond laser pulses. The results reveal the dynamics of femtosecond laserinduced silicon vacancies, which can be divided into ballistic, combination, and stabilization phases. It was found that the concentration of vacancy defects significantly depends on the laser energy density and pulse width. Statistical analysis of the microstructural evolution of femtosecond laser-processed silicon anodes indicates that the locally induced high temperature and pressure by the femtosecond laser are the primary drivers of changes in vacancy defect concentration. Additionally, a quantitative analysis of laser-induced high-coordination structures was conducted, and an appropriate balance between vacancy concentration and high-coordination structures was established. The study shows that increasing the laser energy density or reducing the pulse width, while inducing more vacancy defects, also results in a higher number of high-coordination structures, which may affect the diffusivity of lithium ions. These findings not only provide in-depth molecular-level insights into the variations of femtosecond laser-induced vacancy defects but also offer significant theoretical support for the application of femtosecond laser-ablated silicon anodes.