Fluorinated End‐Groups in Electrolytes Induce Ordered Electrolyte/Anode Interface Even at Open‐Circuit Potential as Revealed by Sum Frequency Generation Vibrational Spectroscopy

• Published in: Advanced energy materials 2017-09, Vol.7 (17), p.n/a
• Main Authors: Horowitz, Yonatan, Han, Hui‐Ling, Ralston, Walter T., de Araujo, Joyce Rodrigues, Kreidler, Eric, Brooks, Chris, Somorjai, Gabor A.
• Format: Article
• Language: English
• Subjects: Additives; Amorphous silicon; Depth profiling; Electrolytes; Ethane; fluorinated electrolytes; Fluorination; Fluorine; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Lithium; Lithium-ion batteries; MATERIALS SCIENCE; Photoelectron spectroscopy; Rechargeable batteries; solid-electrolyte interface; Spectrum analysis; Stereochemistry; sum frequency vibrational spectroscopy; Trifluoromethane
• ISSN: 1614-6832; 1614-6840
• DOI: 10.1002/aenm.201602060

Fluorine‐based additives have a tremendously beneficial effect on the performance of lithium‐ion batteries, yet the origin of this phenomenon is unclear. This paper shows that the formation of a solid‐electrolyte interphase (SEI) on the anode surface in the first five charge/discharge cycles is affected by the stereochemistry of the electrolyte molecules on the anode surface starting at open‐circuit potential (OCP). This study shows an anode‐specific model system, the reduction of 1,2‐diethoxy ethane with lithium bis(trifluoromethane)sulfonimide, as a salt on an amorphous silicon anode, and compares the electrochemical response and SEI formation to its fluorinated version, bis(2,2,2‐trifluoroethoxy) ethane (BTFEOE), by sum frequency generation (SFG) vibrational spectroscopy under reaction conditions. The SFG results suggest that the CF3 end‐groups of the linear ether BTFEOE change their adsorption orientation on the a‐Si surface at OCP, leading to a better protective layer. Supporting evidence from ex situ scanning electron microscopy and X‐ray photoelectron spectroscopy depth profiling measurements shows that the fluorinated ether, BTFEOE, yields a smooth SEI on the a‐Si surface and enables lithium ions to intercalate deeper into the a‐Si bulk. In this sum frequency generation vibrational spectroscopy study, it is revealed that a fluorinated ether in contact with an amorphous silicon anode has a different adsorption orientation than its hydrocarbon counterpart even at open‐circuit potential, leading to a better protective layer while enabling lithium ions to intercalate deeper into the amorphous silicon bulk.