(601d) Tailored Solvation Structure Enabled By an Advanced Fluorinated Ether Solvent with a Sulfonimide Salt for Silicon-Based Li-Ion Battery Anodes

Despite the great effort for adapting lithium metal (LM) anodes to secondary batteries after their promising results with primary batteries back in the 1970s, the commercialization of lithium ion batteries (LIBs) realized after graphite anode discovery.¹ Passivation layers form because of the parasitic reactions between the electrodes and the electrolyte, which referred as solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) on the anode and cathode surfaces, respectively. ¹ Graphite is relatively more stable than LM anode and can be passivated by conventional ethylene carbonate (EC) based electrolytes¹. However, the energy density that can be attained with graphite as an intercalation-type anode is limited as its theoretical specific capacity (372 mAh/g for LiC₆) is only one-tenth of that of LM anode (3860 mAh/g).¹

In this respect, silicon emerges as a strong candidate for high energy density LIB anode with a theoretical specific capacity of 3580 mAh/g (Li₁₅Si₄) and low reduction potential (~0.2V) in addition to being an abundant and environmentally friendly material.² Since carbonate esters form a soft, organic SEI and react rigorously with the LM anode due to their instability at lower voltages, the same phenomenon applies to silicon anodes decreasing the Coulombic efficiency (CE) and cycle lifetime as well as impeding commercialization of high silicon content anodes.¹ Silicon undergoes an alloying reaction during lithiation, which is a different reaction mechanism than graphite and results in a huge volumetric expansion (~300%).³ Several strategies have been developed to minimize the mechanical stress in the electrode and the extent of pulverization including engineering of the SEI layer. The chemical composition and morphology of the SEI strongly depends on the solvation structure of Li⁺, in other words, preferential decomposition of certain species can be promoted by tuning the solvating species near Li⁺. ⁴

F-including sulfonimide salts facilitate Li⁺ transport in the bulk electrolyte, which has a high impact on the reaction rate as it controls Li⁺ concentration at the electrode surface.⁵ Also, they stabilize the anode by forming inorganic rich SEI layer with decomposition of the fluorinated anion.⁴ Several studies have shown that the SEI formed by LiPF₆ is not as stable as the fluorinated sulfonimide salts, although PF^-₅anion is a F donor too.⁶ Therefore, sulfonimide functional group is believed to be one of the key contributors for a stable, inorganic-rich SEI. However, its exact role in fast reaction kinetics and passivation of silicon anode SEI hasn’t been investigated thoroughly. Due to the high viscosity and compromised ionic conductivity of HCEs, localized highly concentrated electrolytes (LCHEs) have been developed by including nonsolvating, highly fluorinated ethers as diluents.

Having accounted for possible parasitic reactions between diluents and solvated species as well as very poor oxidative stability of dilute ether-based electrolytes above 4V, a novel, fluorinated dilute ether electrolyte design is proposed in this work. When coupled with a sulfonimide salt, a tailored, F-functionalized dilute ether-based electrolyte succeeded in giving reasonable ionic conductivity and oxidative stability, high CE, capacity, and long cycle life for silicon-based anodes. The improvements in the electrochemical performance of silicon anode are attributed to a unique, anion-rich solvation structure at low salt concentrations and an inorganic-rich, robust SEI layer on the silicon anode.⁷

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(7) Aydemir, E.; Li, Z.; Pol, V. G. Interplay between Si Anodes and Fluorinated Electrolyte for Advanced Li-ion Batteries, 2024. (In Progress)