Investigation of carbon-coated SiO phase changes during charge/discharge by X-ray absorption fine structure

Abstract

To improve the cycling performance of carbon-coated SiO (SiO–C) as a high-capacity negative electrode material, we have used X-ray absorption fine structure measurements to assess the electronic state of Si and investigate phase changes during charge/discharge cycles. The results suggest that Si4+ continuously absorbs Li during the charging process, mainly forming Li4SiO4. Li silicates with a lower valence are also formed. Neutral Si and Si2+ mainly contribute to Li insertion during the initial charging stage, whereas low-valence oxides like Si1+ and Si2+ form during the middle charging stage. The fully charged state contains all Si phase states. These results suggest that during the discharge process, Li is mainly released from composites with Si2+-oxidation state, and Li is partially released from Li4SiO4 at a higher voltage forming Si0. SiO composites with Si1+–Si3+ reversibly shift to a higher valence state during cycling. We confirmed that Li4SiO4 readily releases Li after long-term cycling, and SiO composites at the end of the cycle change to a higher valence state. The results indicate that disproportionation is a primary factor decreasing cell cycle performance and confirm that it is possible to design LiB cells with improved cycling performance by controlling the disproportionation linked to the structural deterioration of SiO–C.