The presence of lithium hexafluorophosphate (LiPF6) ion pairs in carbonate-based electrolyte solutions is widely accepted in the field of battery electrolyte research and is expected to affect solution transport properties. No existing techniques are capable of directly quantifying salt dissociation in these solutions. Previous publications by others have provided estimates of dissociation degrees using dilute solution theory and pulsed field gradient nuclear magnetic resonance spectroscopy (PFG-NMR) measurements of self-diffusivity. However, the behavior of a concentrated electrolyte solution can deviate significantly from dilute solution theory predictions. This work, for the first time, instead uses Onsager–Stefan–Maxwell concentrated solution theory and the generalized Darken relation with PFG-NMR measurements to quantify the degrees of dissociation in electrolyte solutions (LiPF6 in ethylene carbonate/diethyl carbonate, 1:1 by weight). At LiPF6 concentrations ranging from 0.1 M to 1.5 M, the salt dissociation degree is found to range from 61% to 37%. Transport properties are then calculated through concentrated solution theory with corrections for these significant levels of ion pairing.

Nanostructured block copolymers are of particular interest as electrolytes in batteries with lithium metal anodes. The performance of electrolytes in batteries can be predicted only if three transport coefficients (ionic conductivity, κ, salt diffusion coefficient, D, and cation transference number, t+0) are known. We present complete electrochemical transport characterization of a microphase-separated SEO block copolymer electrolyte by reporting κ, D, and t+0 as functions of salt concentration. We compare the properties of the block copolymer electrolyte with those of PEO homopolymer electrolytes. Negative values of t+0 are observed in many cases. Recasting the transport parameters in terms of Stefan-Maxwell coefficients provides insight into the nature of ion transport in these electrolytes.

We develop a model based on concentrated solution theory for predicting the cycling characteristics of a lithium-polymer-lithium symmetric cell containing an electrolyte with known transport properties. The electrolytes used in this study are mixtures of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt, prepared over a wide range of salt concentrations. The transport properties of PEO/LiTFSI previously reported in the literature are used as inputs for our model. We calculate salt concentration and potential profiles, which develop in these electrolytes under a constant dc polarization, as a function of current density, electrolyte thickness, and salt concentration. These profiles are nonlinear at steady-state due to the strong concentration dependence of the transport properties of this electrolyte. The effect of this nonlinearity on limiting current is demonstrated. Cycling characteristics of a series of lithium symmetric cells were measured to test the validity of our model, without resorting to any adjustable parameters. The time-dependence and steady-state value of the potential measured during cycling experiments were in excellent agreement with model predictions.