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Ion Transport Properties in Novel and Traditional Liquid Electrolytes for Lithium-Based Batteries


Enabling next-generation lithium-based batteries is of paramount importance for the transportation sector. The energy density, cycle life, and safety are all prime considerations for electric vehicles. In addition, these batteries must be able to operate in a wide variety of temperatures and a range of power requirements. The electrolyte is fundamental to the ion transport within a battery, is a constraint on the operational voltage of a cell, and is integral to the safety of a device. Limitations on power delivered by a battery are specifically due to ion transport within the electrolyte and these properties are often not well understood within conventional and novel electrolytes. In this work, we measure ion transport properties in a variety of nonaqueous liquid electrolytes utilizing Newman’s concentrated solution theory. We then use these properties to predict lithium-symmetric cell behavior and compare theoretical predictions to experimental results without the use of any adjustable parameters.

Mixtures of perfluoropolyethers (PFPE) and lithium salts with fluorinated anions are a new class of electrolytes for lithium batteries. Unlike conventional electrolytes wherein electron-donating oxygen groups interact primarily with the lithium cations, the properties of PFPE-based electrolytes appear to be dependent on interactions between the fluorinated anions and the fluorinated backbones. We study these interactions by examining a family of lithium salts wherein the size of the fluorinated anion is systematically increased: lithium bis(fluorosulfonyl)imide (LiFSI), bis(trifluoromethanesulfonyl)imide (LiTFSI) salts and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI). Two short chain perfluoroethers (PFE), one with three repeat units, C6-DMC, and another with four repeat units, C8-DMC were studied; both systems have dimethyl carbonate end groups. We find that LiFSI provides the highest conductivity in both C6-DMC and C8-DMC. These systems also present the lowest interfacial resistance against lithium metal electrodes. The ideal transference number (t_(+,id)) was above 0.6 for all of the electrolytes and was an increasing function of anion size. The product of conductivity and the steady-state transference number is a convenient measure of the efficacy of the electrolytes for lithium battery applications. Amongst the systems studied, LiFSI/PFE mixtures were the most efficacious electrolytes.

The performance of binary electrolytes is governed by three transport properties: conductivity, salt diffusion coefficient, and transference number. Rigorous methods for measuring conductivity and the salt diffusion coefficient are well established and used routinely in the literature. The commonly used methods for measuring transference number are the steady-state current method, t_(+,id), and pulsed field gradient NMR, t_(+,NMR). These methods yield the transference number only if the electrolyte is ideal, i.e., the salt dissociates completely into non-interacting anions and cations. In this work, we present a complete set of ion transport properties for mixtures of a functionalized perfluoroether, dimethyl carbonate terminated perfluorinated tetraethylene ether, and lithium bis(fluorosulfonyl)imide (LiFSI). The equations used to determine these properties from experimental data are based on Newman’s concentrated solution theory. The concentrated-solution-theory-based transference number, t_+^0, is negative across all salt concentrations, and it increases with increasing salt concentration. In contrast, the ideal transference number, t_(+,id), is positive across all salt concentrations and it decreases with salt concentration. The NMR-based transference number, t_(+,NMR), is approximately 0.5, independent of salt concentration. The disparity between the three transference numbers, which indicates the dominance of ion clustering, is resolved by the use of Newman’s concentrated solution theory.

Imposing a steady ionic current through an electrolyte results in the formation of salt concentration gradients that compromise battery performance. The limiting current is usually defined as the current at which the salt concentration at the cathode approaches zero. Higher currents cannot be imposed on the cell as larger concentration gradients are unsustainable. We study the limiting current in electrolytes comprising a perfluorinated oligomer, C8-DMC, and lithium bis(fluorosulfonyl)imide salt in symmetric lithium cells. The time-dependence of the potential, which increases as salt concentration gradients develop, was also measured. Both steady-state and transient behaviors are modeled using Newman’s concentrated solution theory; transport and thermodynamic parameters needed to perform the calculations were measured independently. The limiting current is a non-monotonic function of salt concentration in both theory and experiment. The model shows that at low salt concentrations (below 0.88 mol/kg solvent), the concentration at the cathode approaches zero at limiting current. In contrast, at high salt concentrations (above 0.88 mol/kg solvent), the concentration at the anode approaches the solubility limit (2.03 mol/kg solvent). The experimentally determined salt concentration at which the limiting current is maximized is in excellent agreement with theoretical predictions made without resorting to any adjustable parameters.

We apply concentrated solution theory to measure a complete set of ion transport properties to mixtures of low molecular weight poly(ethylene oxide), tetraglyme, and LiTFSI at T = 30 °C and 90 °C. We found that while conductivity and the salt diffusion coefficient are lower at 30 °C compared to 90 °C, the ideal transference number is greater at all salt concentrations. The rigorously defined transference number is lower at 90 °C than at 30 °C for most concentrations, except for in the limit of large concentrations. We use the measured properties at 90 °C to predict the limiting current of mixtures of LiTFSI/tetraglyme and compare those results to experimental behavior in lithium symmetric cells. We posit that the lack of qualitative agreement at high starting salt concentrations is due to solubility constraints at the anode surface.

Carbonate electrolytes are often used in commercial lithium-ion batteries, however their transport properties are not well understood. We apply concentrated solution theory to study mixtures of LiFSI/PC and lithium hexafluorophosphate (LiPF6)/PC over a wide range of salt concentrations at room temperature. We find that while the conductivity and salt diffusion coefficient for both mixtures are similar for all salt concentration, the ideal transference number is greater for mixtures of LiFSI/PC compared to mixtures of LiPF6/PC at all concentrations. The rigorously defined transference number for mixtures of LiFSI/PC is also found to be greater than that of mixtures of LiPF6/PC at all salt concentrations. We use the measured transport properties for mixtures of LiFSI/PC and use them to predict the electrolyte potential drop within a lithium symmetric cell. We find that there is good qualitative agreement between the measured experimental drop and theoretical predictions as a function of applied current density and salt concentration. Both the model and experimental results predict that the largest potential drop within mixtures of LiFSI/PC occurs at a concentration of 1 M, which is when conductivity is also maximized.

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