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

Abstract

Fluorine based additives have a tremendously beneficial effect on the performance of lithium ion batteries, yet the origin of this phenomenon is unclear. In this paper we show that the formation of a solid electrolyte interphase (SEI) on the anode surface in the first 5 charge / discharge cycles is affected by the stereochemistry of the electrolyte molecules on the anode surface starting at open circuit potential. We have studied an anode specific model system, the reduction of 1,2-diethoxy ethane with LiTFSI, lithium Bis(trifluoromethane)sulfonimide, as salt on amorphous silicon anode and compared the electrochemical response and SEI formation to its fluorinated version BTFEOE, Bis(2,2,2-trifluoroethoxy) ethane by sum frequency generation (SFG) vibrational spectroscopy under reaction conditions. Our SFG results suggest that the -CF3 end groups of the linear ether BTFEOE change its adsorption orientation on the a-Si surface at open circuit potential, leading to a better protective layer. Supporting evidence from ex situ scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) depth profiling measurements show that the fluorinated ether, BTFEOE, yields a smooth SEI on the a-Si surface and enables lithium ion to intercalate deeper into the a-Si bulk.

Fluorine and Lithium: Ideal Partners for High‐Performance Rechargeable Battery Electrolytes

“…This leads to an ordered interface, which allows better Li + diffusion. In contrast, 1,2‐DEE molecules are oriented parallel to the electrode surface, and methyl groups are free to rotate and bend, thus leading to a decrease in the Li + diffusion, as illustrated in Figure …”

Effect of Building Block Connectivity and Ion Solvation on Electrochemical Stability and Ionic Conductivity in Novel Fluoroether Electrolytes

“…Within both works mentioned above, the same fluoroether design strategy was employed, where the fluorinated moieties are sandwiched by ether moieties ( Figure 1 ). Although prior work by Horowitz et al 29 illustrated a case, bis(2,2,2-trifluoroethoxy) ethane, where the building block connectivity was flipped, their study was limited to the anode/electrolyte interface whereas other physicochemical or electrochemical properties were not investigated. Furthermore, while the improvements in electrochemical stability were usually attributed to the new molecular designs of fluoroethers, 15 , 26 studies on HCEs have already emphasized the important role of ionic solvation structure.…”

The Electrolyte Frontier: A Manifesto

“…Tools such as sum frequency generation (SFG) would find great utility with this approach. 6 Other important properties to decouple are mechanical stability from conductivity. As solid-state electrolytes have garnered fervent research and commercial interest, their benefits beyond nonflammability/ safety need to be clearly elucidated.…”