The performance of battery electrolytes depends on three independent transport properties: ionic conductivity, diffusion coefficient, and transference number. While rigorous experimental techniques for measuring conductivity and diffusion coefficients are well-established, popular techniques for measuring the transference number rely on the assumption of ideal solutions. We employ three independent techniques for measuring transference number, t + , in mixtures of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt. Transference numbers obtained using the steady-state current method pioneered by Bruce and Vincent, t +,SS , and those obtained by pulsed-field gradient NMR, t +,NMR , are compared against a new approach detailed by Newman and coworkers, t +,Ne , for a range of salt concentrations. The latter approach is rigorous and based on concentrated solution theory, while the other two approaches only yield the true transference number in ideal solutions. Not surprisingly, we find that t +,SS and t +,NMR are positive throughout the entire salt concentration range, and decrease monotonically with increasing salt concentration. In contrast, t +,Ne has a non-monotonic dependence on salt concentration and is negative in the highly-concentrated regime. Our work implies that ion transport in PEO/LiTFSI electrolytes at high salt concentrations is dominated by the transport of ionic clusters. Energy density and safety of conventional lithium-ion batteries is limited by the use of liquid electrolytes comprising mixtures of flammable organic solvents and lithium salts. Polymer electrolytes have the potential to address both limitations. However, the power and lifetime of batteries containing solvent-free polymer electrolytes remain inadequate for most applications. The performance of electrolytes in batteries depends on three independent transport properties: ionic conductivity, σ, salt diffusion coefficient, D, and cation transference number, t + .1 The poor performance of batteries with polymer electrolytes is generally attributed to low conductivity, which is on the order of 10 −3 S/cm at 90• C for mixtures of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt, 2,3 compared to that of liquid electrolytes which is 10 −2 S/cm at ambient temperatures. 4 Much of the literature in this field has been devoted to increasing the ionic conductivity of these materials.5-32 The purpose of our work is to shed light on another transport property of polymer electrolytes, the transference number.In a pioneering study, Ma and coworkers showed that the transference number of a mixture of PEO and a sodium salt is negative. 33Following this approach, others have obtained t + <0 in polymers containing lithium or sodium salts. [34][35][36] Nevertheless, the majority of reports for t + in polymer electrolytes fall between zero and one. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] In contrast, all reports of t + in non-aqueous liquid electrolytes containi...

Nanostructured block copolymer electrolytes have the potential to enable solid-state batteries with lithium metal anodes. We present complete continuum characterization of ion transport in a lamellar polystyrene-b-poly(ethylene oxide) copolymer/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte as a function of salt concentration. Electrochemical measurements are used to determine the Stefan-Maxwell salt diffusion coefficients [Formula: see text], [Formula: see text], and [Formula: see text]. Individual self-diffusion coefficients of the lithium- and TFSI-containing species were measured by pulsed-field gradient NMR (PFG-NMR). The NMR data indicate that salt diffusion is locally anisotropic, and this enables determination of a diffusion coefficient parallel to the lamellae, D, and a diffusion coefficient through defects in the lamellae, D. We quantify anisotropic diffusion by defining an NMR morphology factor and demonstrate that it is correlated to defect density seen by transmission electron microscopy. We find agreement between the electrochemically determined Stefan-Maxwell diffusion coefficients and the diffusion coefficient D determined by PFG-NMR. Our work indicates that the performance of nanostructured block copolymer electrolytes in batteries is strongly influenced by ion transport through defects.

Nanostructured block copolymer electrolytes composed of organic and inorganic moieties have the potential to enable solid-state batteries.