Lithium-Ion Transport and Cycling Characteristics of Fluorinated Electrolytes

Abstract

Electrical energy is ubiquitous in the modern world, and in order to continue pushing boundaries on consumer wearables, electric vehicles, and grid storage, secondary lithium-based rechargeable batteries need to be enhanced. The performance of binary electrolytes is governed by the electrolyte thermodynamic factor and three transport properties: conductivity, salt diffusion coefficient, and transference number. Rigorous methods for measuring conductivity and the salt diffusion coefficient are well established in literature. NMR and the steady-state current method are often used to measure the transference number, but are predicated on the dilute solution assumption and do not fully consider the non-ideality of the electrolyte. Within this work, we elucidate a complete set of lithium-ion transport properties for mixtures of dimethyl carbonate terminated perfluorinated tetraethylene ether and lithium bis(fluorosulfonyl)imide (LiFSI). The steady-state current transference number is positive across all concentrations and decreases with salt concentration, while the NMR transference number is approximately 0.5 everywhere. In contrast, we find that the transference number using Newman’s concentrated solution theory is negative across all concentrations. We show that Newman’s concentrated solution theory allows for accurate potential and limiting current modeling of lithium half cells whereas the steady-state current transference number severely over-estimates the electrolyte’s limiting current. The disparity between the dilute solution theory measurements and that of Newman’s concentrated solution theory approach indicates the dominance of ion clustering. Figure 1