Effect of Fluorinated Carboxylic Acid Ester on Lithium Solvation as an Additive in Electrolyte and Low-Temperature Insight on Battery Performance

Abstract

The fluorinated carboxylic acid esters are effective additives to improve the performance of lithium-ion batteries (LIBs). Among them, 2,2,2-trifluoroethylbutyrate (TFEB) has been reported as a good additive to enhance battery performance, especially at low temperatures. It becomes a consensus that the solvation structure around lithium ion plays a crucial role in LIBs. However, it remains a mystery how such additives impact lithium solvation and how the impact changes at low temperatures. In this work, classical molecular dynamics (MD) simulations were conducted to investigate microcosmic mechanisms of the influence of TFEB on a typical commercial electrolyte composed of lithium hexafluorophosphate (LiPF6)/ethylene carbonate (EC)/ethyl methyl carbonate (EMC) in LIBs. Although TFEB is a minority species, it can significantly modify the lithium solvation structure. The introduction of TFEB facilitates the participation of anion in the solvation shell, due to its weaker coordination ability compared to EC/EMC, which was elucidated by the detailed analyses including radial distribution functions, cluster analysis, cage residence time, etc. We further investigated a series of TFEB analogs (TFEA, ETFA, and MPFP) and found that a moderately weak coordination ability is essential in the design of electrolytes. Furthermore, it was uncovered not only that the coordination number of the anions will decrease at low temperatures but also that their orientations are significantly changed. We demonstrated that the introduction of TFEB will alleviate this tendency, which may be advantageous to low-temperature performance. In summary, the critical role of fluorinated additives is uncovered by our MD simulations in modulating the Li+ solvation structure and improving the low-temperature performance of LIB, which provides references to the design strategy of novel low-temperature fluorine-containing additive-based electrolytes.