Li₄SiO₄-Based Artificial Passivation Thin Film for Improving Interfacial Stability of Li Metal Anodes

An amorphous SiO₂ (a-SiO₂) thin film was developed as an artificial passivation layer to stabilize Li metal anodes during electrochemical reactions. The thin film was prepared using an electron cyclotron resonance-chemical vapor deposition apparatus. The obtained passivation layer has a hierarchical structure, which is composed of lithium silicide, lithiated silicon oxide, and a-SiO₂. The thickness of the a-SiO₂ passivation layer could be varied by changing the processing time, whereas that of the lithium silicide and lithiated silicon oxide layers was almost constant. During cycling, the surface of the a-SiO₂ passivation layer is converted into lithium silicate (Li₄SiO₄), and the portion of Li₄SiO₄ depends on the thickness of a-SiO₂. A minimum overpotential of 21.7 mV was observed at the Li metal electrode at a current density of 3 mA cm^-2 with flat voltage profiles, when an a-SiO₂ passivation layer of 92.5 nm was used. The Li metal with this optimized thin passivation layer also showed the lowest charge-transfer resistance (3.948 Ω cm) and the highest Li ion diffusivity (7.06 × 10^-14 cm² s^-1) after cycling in a Li-S battery. The existence of the Li₄SiO₄ artificial passivation layer prevents the corrosion of Li metal by suppressing Li dendritic growth and improving the ionic conductivity, which contribute to the low charge-transfer resistance and high Li ion diffusivity of the electrode.