The direct connection between the microscopic structure of lithium-ion electrolytes and the macroscopic properties of the system remains elusive. Lately, high concentration electrolytes have appeared as a possible solution to the problems of current electrolytes, since they show low electrochemical degradation and low flammability among other properties. However, high concentration electrolytes use large quantities of lithium salts, which makes then unsuitable for large scale production of batteries. Pseudo-concentrated electrolytes, wherein a typical lithium electrolyte is diluted with a co-solvent, have been proposed as substitutes for high concentration electrolytes because they maintain the desirable properties of the electrolyte while utilizing lower amounts of the salt. However, previous work has focused on co-solvents which were not truly a non-interacting material. We here report a study of the electrochemical properties and structure of a new pseudo-concentrated electrolyte composed of two organic solvents, one of which is a fluorinated alkane (pentafluorobutane, PFB). The new electrolyte is studied as a function of the non-interactiving co-solvent concentration using an assortment of spectroscopies (FTIR, ATR-FTIR, and NMR) and experimental methods (cyclic voltammetry, conductivity, and viscosity), as well as computational methods (density functional theory calculations). In particular, we focus our attention on two lithium salts (LiPF6 and LiTFSI) dissolved in a mixture of dimethyl carbonate (DMC) and different concentrations of PFB. Our results indicate that even though the highly-fluorinated solvent is supposed to act as a filler (non-interacting solvent), it alters the speciation of ionic species in the electrolyte, which leads to a higher conductivity for the LiPF6/DMC/PFB system. Moreover, the addition of PFB results in a loss of electrochemical stability in the LiTFSI/DMC/PFB system, which is not observed in the other electrolyte. These results highlight the critical roles that the anion and secondary solvent play in shaping the macroscopic properties of new electrolytes, such that the structure-property relations cannot be accurately assessed by concentration alone and/or chemical identity of the components.

Effect of secondary solvent on macroscopic properties of pseudo-concentrated organic electrolyte

ACS Fall 2021

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