Topological Considerations in Electrolyte Additives for Passivating Silicon Anodes with Hybrid Solid–Electrolyte Interphases

Unlike most anodes used in high energy density batteries, lithiated Si does not form long-lasting passivating solid-electrolyte interphases (SEI) during formation or on charge due to SEI delamination, reconstruction, or dissolution. As a result, electrolyte degradation is continuous and results in a permanent loss of the Li inventory, shortening the useful life of the battery. Here, we show that perfluoroether electrolyte additives featuring either sulfonyl fluorides or trifluorovinyl ethers, when introduced in prescribed amounts to locally superconcentrated electrolytes, exhibit preferential reactivity at Si during formation due to their higher reduction potential than salts and solvents, creating a hybrid SEI that is simultaneously enriched with LiF and organics tethered to the reactive functionality. While both reactive motifs are effective in creating a hybrid SEI, perfluoroether additives bearing sulfonyl fluorides show more substantial integration. More important, however, is the combined influence of additive topology on anchoring efficacy and tether flexibility between anchoring sites on SEI resiliency. Top-performing Si|LFP cells featuring ditopic additive-enriched SEI improve capacity retention by as much as 45% over 100 cycles when compared to additive-free cells.