(399d) Mechanistic Elucidation of Inorganic-Rich Solid-Electrolyte-Interphase Enabled By Advanced Fluorinated Ether Electrolyte Design for Silicon-Based Anodes

Today, the specific energy density attainable by lithium-ion batteries (LIBs) is around 250 Wh/kg, which mainly due to the limited theoretical specific capacity of graphite anode (372 mAh/g for LiC₆).^1,2 Therefore, a great amount of research effort has been devoted for alternative anode materials such as silicon that is of high theoretical specific capacity (3580 mAh/g for Li₁₅Si₄) and low reduction potential (~0.2V).³ As an earth-abundant material with low environmental impact, silicon-based LIBs have a projected energy density of 500 Wh/kg.⁴ However, silicon’s breakthrough as a promising anode material has been impeded by exacerbated active material loss by pulverization and cracking after continuous cycling due to huge volumetric change reaching 300% after full lithiation.¹ As a result, strategies to mitigate the adverse effects of volume expansion of silicon have been developed, and engineering of the electrode-electrolyte interface has been of the particular interest to researchers.

When exposed to the electrolyte, parasitic reactions occur and passivation layers form on both of the electrode surfaces known as the solid-electrolyte-interphase (SEI) and the cathode-electrolyte-interphase (CEI) for the anode and the cathode, respectively. The degradation products of conventional carbonate esters usually form an organic-rich, soft, and porous SEI, which is prone to rupturing and cracking under mechanical stress.⁵ Therefore, conventional electrolyte solutions often fail to deliver a high Coulombic efficiency (CE) and suffers from fast capacity decay due to poor passivation of the silicon-based anode. Since the stability of the electrolyte near the electrode surface can be tuned by changing the solvation structure, the formation of an inorganic-rich SEI layer can be promoted by designing an anion-rich solvation sheath.⁶

In contrast to conventional carbonate esters, ethers have a lower viscosity and higher wettability of the electrodes boosting active material utilization and ion transport.⁷ Also, ethers possess a higher reductive stability than carbonate esters, which makes them less prone to decomposition at low voltages and facilitates formation of inorganic-rich SEI. An anion-rich solvation shell promotes the formation of an inorganic-rich SEI by preferential decomposition of anions, which has a higher mechanical strength and low electronic conductivity, preventing reduction of solvent molecules. LiF stands out among the other inorganic SEI constituents and integral to a robust SEI. The wide band gap of LiF prevents electron tunneling, which prevents continuous solvent reduction and limits growth of the SEI. Low solubility and high elastic modulus of LiF (~65GPa) makes it a suitable SEI component for silicon anodes.⁸ Additionally, the high surface energy and therefore less adhesion of LiF to lithiated silicon surface prevents rupture of SEI after continuous volume changes.

In this work, we demonstrate the impact of anion-abundant solvation structure on the cycling performance for silicon-based anodes. A fluorinated ether-based system is compared with conventional and non-fluorinated ether-based electrolytes to stress the importance of fluorinated SEI layer for longer cycling life of silicon anodes. This study provides a mechanistic understanding for the influence of the design of the solvation structure and SEI on faster interfacial reaction kinetics and improved cycling life attained with the fluorinated-ether based electrolyte.

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