Electrochemical performance of thermally-grown SiO2 as diffusion barrier layer for integrated lithium-ion batteries

Abstract

Direct integration of lithium-ion battery (LIB) with electronic devices on the same Si substrate can significantly miniaturize autonomous micro systems. For achieving direct integration, a barrier layer is essential to be inserted between LIB and the substrate for blocking Li+ diffusion. In this paper, the feasibility of thermal SiO2 film as the barrier layer is investigated by electrochemical characterization and X-ray photoelectron spectroscopy (XPS). Due to the negligible side reactions of thermal SiO2 with electrolyte, the solid electrolyte interphase (SEI) layer formed on the surface of the barrier layer is thin and the SEI content mainly consists of hydrocarbon together with slight polyethylene oxide (PEO), Li x PO y F z , and Li2CO3. Although 8-nm thermal SiO2 effectively prevents the substrate from alloying with Li+, the whole film changes to Li silicate after electrochemical cycling due to the irreversible chemical reactions of SiO2 with electrolyte. This degrades the performance of the barrier layer against the electrolyte penetration, thus leading to the existence of Li+ (in the form of F-Si-Li) and solvent decompositions (with the products of hydrocarbon and PEO) near the barrier layer/substrate interface. Moreover, it is found that the reaction kinetics of thermal SiO2 with electrolyte decrease significantly with increasing the SiO2 thickness and no reactions are found in the bulk of the 30-nm SiO2 film. Therefore, thermal SiO2 with an appropriate thickness is a promising barrier layer for direct integration.