Sacrificial Anion Reduction Mechanism for Electrochemical Stability Improvement in Highly Concentrated Li-Salt Electrolyte

Abstract

Li-salt concentration has been recently proposed as an important control parameter of reduction stability of electrolytes in lithium-ion battery (LIB). Here we theoretically investigated low (LC) and high (HC) concentration systems of LiN(SO2CF3)2 (Li-TFSA) salt in acetonitrile (AN) solution, to elucidate the mechanism of improving the low reduction stability of AN at the HC condition, by density functional theory based molecular dynamics (DFT-MD) sampling of the solvation character with extra electron(s). We demonstrated that TFSA anions sacrificially accept the reductive electron at the HC condition, which is ascribed to formation of specific network structure and the resulting shift of electron affinity of the anions. We also found that, even in the LC condition, TFSA eventually decomposes with one electron reduction. This sacrificial anion reduction hinders two electron reductive decomposition of AN, leading to improved electrochemical stability. The mechanism may give a guiding principle for the design of better LIB electrolytes.

Developing New Functionalities of Superconcentrated Electrolytes for Lithium-ion Batteries

“…This suggests that a TFSI anion, rather than AN solvent molecules, should receive an electron under a reductive atmosphere. The DFT-MD simulation further demonstrates that the C-S Q* orbital of a TFSI anion receives an excess electron, leading to the cleavage of the C-S bond, 18 which might be the first step of the SEI formation reaction. All those theoretical simulations reasonably account for the formation of the anion-derived SEI in concentrated electrolytes.…”

“…Electrochemistry, 85(9), 559-565 (2017) solution obtained via density functional theory-based molecular dynamics (DFT-MD) simulations. 16,18 Notably, the lowest unoccupied molecular orbital (LUMO), which contributes to reduction reactions, is located at a TFSI anion in concentrated electrolytes owing to the downward shift of the anion's orbital level. This suggests that a TFSI anion, rather than AN solvent molecules, should receive an electron under a reductive atmosphere.…”

“…4). 16,18,25 First, i) the extensive formation of ion pairs should be the origin of anionderived SEIs. The DFT-MD simulation shows that the TFSI anion that receives an excess electron is in an aggregate (AGG) state, 18 in which the anion is coordinated with two or more Li + cations.…”

“…16,18,25 First, i) the extensive formation of ion pairs should be the origin of anionderived SEIs. The DFT-MD simulation shows that the TFSI anion that receives an excess electron is in an aggregate (AGG) state, 18 in which the anion is coordinated with two or more Li + cations. In such an extensive ion pair, the TFSI anion partially donates its electron to Li + cations (a strong Lewis acid) and thus has less negative charge; this situation makes the TFSI anion more susceptible for reduction, as is evident from the downward shift of the anion's orbital level.…”

“…1), and hence, the highly concentrated region has long been outside the major research focus of battery electrolytes. However, various new functionalities have recently been discovered in the highly concentrated region, [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] which bring about vigorous research worldwide on superconcentrated (highly concentrated) electrolytes. Notably, highly concentrated electrolytes show high stabilities toward both negative and positive electrodes.…”