Atomistics of the lithiation of oxidized silicon (SiOx) nanowires in reactive molecular dynamics simulations

Abstract

Although silicon oxide (SiO_x) nanowires (NWs) are recognized as a promising anode material for lithium-ion batteries (LIBs), a clear understanding of their lithiation mechanism has not been reported yet. We elucidate the lithiation mechanism of SiO_x NWs at the atomic scale based on molecular dynamics (MD) simulations employing the ReaxFF reactive force field developed through first-principles calculations. SiO_x NWs with crystalline Si (c-Si) core and amorphous SiO₂ (a-SiO₂) shell structures of ∼1 nm in thickness show smaller volume expansion than pristine Si NWs, as found in previous experiments. Lithiation into SiO_x NWs creates two interfaces: c-Si/a-Li_xSi and a-Li_xSi/a-Li_ySiO₂. The mobility of the latter, which is located farther toward the outside of the NW, is slower than that of the former, which is one of the reasons why the thin SiO₂ layer can suppress the volume expansion of SiO_x NWs during lithiation. Another reason can be found from the stress distribution, as the SiO_x NWs show stress distribution different from the pristine case. Moreover, the lithiation of SiO_x NWs leads to the formation of Li₂O and Li₄SiO₄ compounds in the oxide layer, where several Li atoms (not a majority) in Li₄SiO₄ can escape from the compound and diffuse into the c-Si, in contrast to the Li₂O case. However, Li atoms that pass through the SiO₂ layer penetrate into the c-Si preferentially along the 〈110〉 or 〈112〉 direction, similar to the mechanism observed in pristine Si NWs. We expect that our comprehensive understanding of the lithiation mechanism of SiO_x NWs will provide helpful guidance for the design of SiO_x anodes to obtain better performing LIBs.