| Summary: | A novel in-situ FTIR method is developed to probe the Li anode/liquid electrolyte interface. Three different conventional electrolyte systems were tested: 1.2 M LiPF₆ in EC, 1.0 M LiPF₆ in EMC, and LP57 (1.0 M LiPF₆ in EC:EMC (3/7 vol %)). Using the spectroelectrochemical cell, FTIR measurements for first plating step and cycled cells (up to 50 cycles) were collected to look for new species formation. In the case of 1.2 M LiPF₆ in EC, LEMC formation was observed when the potential was brought below 1.50 VLi. LEMC growth accelerated when the potential was reduced below 0.0 VLi, upon contact with freshly plated Li metal. When 1.0 M LiPF₆ in EMC was used for the same study, either lithium methyl carbonate or lithium ethyl carbonate were formed. Upon switching to LP57, Li₂CO₃ became the dominant SEI component. When the three electrolytes were cycled in the spectroelectrochemical cell, the SEI peaks continued to grow for the first 10 cycles. After the first 10 cycles, LEMC and Li₂CO₃ growth plateaued, indicating SEI stabilization. On the other hand, LRC signal diminished, indicating an unstable SEI formed by EMC. Additionally, anion decomposition was observed to be more pronounced under high concentrations of EC. Since anion decomposition can be used as a proxy for LiF formation, high concentration electrolytes perform better possibly due to larger amounts of LiF formation.
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