Property evolution of Al2O3 coated and uncoated Si electrodes: A first principles investigation

In order to compare the difference between coated and uncoated Si electrode, the basic material properties of atomic layer deposition ALD-Al₂O₃ coating and native SiO₂ layer on Si are investigated by using density functional theory calculations. These properties include: the open circuit voltage curve with respect to lithium metal, the elastic properties, and Li ion diffusion characteristics in SiO₂ and Al₂O₃ and their lithiated compounds (such as Li4SiO4 and LiAlO₂). Results show that lithiation occurs at the voltage above EC (Ethylene Carbonate) decomposition(~0.8V) causing initial irreversible capacity loss. Also, lithiation softens Al₂O₃ mechanically, while slightly stiffens SiO₂. For the Li ion diffusion, SiO₂ and Al₂O₃ have totally different behavior upon lithiation. For SiO₂, Li diffuses very fast, but the energy barrier becomes higher as lithiation goes on. When SiO₂ is lithiated to Li4SiO4, the diffusion energy barrier becomes lower again. For Al₂O₃, its intrinsic ionic conductivity is very low, but the Li diffusion energy barrier becomes lower (as low as Li₂Si₂O₅) as lithiation proceeds and LiAlO₂ is formed. Therefore, designing ALD coating should target the coating properties after lithiation.