The viability of next generation lithium and beyond-lithium battery technologies hinges on the development of electrolytes with improved performance. Comparing electrolytes is not straightforward, as multiple electrochemical parameters affect the performance of an electrolyte.Additional complications arise due to the formation of concentration gradients in response to dc potentials. We propose a modified version of Ohm's law to analyze current through binary electrolytes driven by a small dc potential. We show that the proportionality constant in Ohm's law is given by the product of the ionic conductivity, κ, and the ratio of currents in the presence (i ss ) and absence (i Ω ) of concentration gradients, ρ +¿ ¿ . The importance of ρ +¿ ¿ was recognized by J. Evans, C.A. Vincent, and P.G. Bruce [Polymer 28, 2324[Polymer 28, (1987]. The product κ ρ +¿ ¿ is used to rank order a collection of electrolytes. Ideally, both κ and ρ +¿ ¿ should be maximized, but we observe a trade-off between these two parameters, resulting in an upper bound. This trade-off is analogous to the famous Robeson upper bound for permeability and selectivity in gas separation membranes. Designing polymer electrolytes that overcome this trade-off is a worthwhile but ambitious goal.

Lithium metal is a promising anode material for next-generation rechargeable batteries, but non-uniform electrodeposition of lithium is a significant barrier. These non-uniform deposits are often referred to as lithium "dendrites," although their morphologies can vary. We have surveyed the literature on lithium electrodeposition through three classes of electrolytes: liquids, polymers and inorganic solids. We find that the non-uniform deposits can be grouped into six classes: whiskers, moss, dendrites, globules, trees, and cracks. These deposits were obtained in a variety of cell geometries using both unidirectional deposition and cell cycling. The main result of the study is a figure where the morphology of electrodeposited lithium is plotted as a function of two variables: shear modulus of the electrolyte and current density normalized by the limiting current density. We show that specific morphologies are confined to contiguous regions on this two-dimensional plot.

We present experimental results on the phase behavior of block copolymer/salt mixtures over a wide range of copolymer compositions, molecular weights, and salt concentrations. The experimental system comprises polystyrene- block-poly(ethylene oxide) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. It is well established that LiTFSI interacts favorably with poly(ethylene oxide) relative to polystyrene. The relationship between chain length and copolymer composition at fixed temperature is U-shaped, as seen in experiments on conventional block copolymers and as anticipated from the standard self-consistent field theory (SCFT) of block copolymer melts. The phase behavior can be explained in terms of an effective Flory-Huggins interaction parameter between the polystyrene monomers and poly(ethylene oxide) monomers complexed with the salt, χ, which increases linearly with salt concentration. The phase behavior of salt-containing block copolymers, plotted on a segregation strength versus copolymer composition plot, is similar to that of conventional (uncharged) block copolymer melts, when the parameter χ replaces χ in segregation strength.

The uncontrollable non-planar electrodeposition of lithium is a significant barrier to the widespread adoption of high energy density rechargeable batteries with a lithium metal anode. A promising approach for preventing the growth of lithium dendrites is the use of solid polymer electrolytes with a high shear modulus. Current density is the key variable in the electrodeposition of lithium. The present study is the first attempt at quantifying the effect of current density on the geometry and density of dendrites and other protrusions during electrodeposition through a solid polymer electrolyte. The geometry and density of defects formed on the lithium electrode were determined by X-ray microtomography. The tomograms revealed protrusions on the electrodeposited lithium electrodes that were either globular or dendritic, or void defects. The range of current densities over which stable, planar deposition was observed is quantified. At higher current densities, globular protrusions were observed. At the highest current density, both globular and dendritic protrusions were observed. The areal density of protrusion defects increased sharply with current density, while the overall defect density is a weak function of current density. Our work enables comparisons between the experimentally determined onset of non-planar electrodeposition and prevailing theoretical predictions with no adjustable parameters.

The limiting current is an important transport property of an electrolyte as it provides an upper bound on how fast a cell can be charged or discharged. We have measured the limiting current in lithium-lithium symmetric cells with a standard polymer electrolyte, a mixture of poly(ethylene oxide) and lithium bis(trifluoromethane) sulfonamide salt at 90°C. The cells were polarized with increasing current density. The steady-state cell potential was a smooth function of current density until the limiting current was exceeded. An abrupt increase in cell potential was taken as an experimental signature of the limiting current. The electrolyte mixture was fully characterized using electrochemical methods to determine the conductivity, salt diffusion coefficient, cation transference number, and thermodynamic factor as a function of salt concentration. We used Newman's concentrated solution theory to predict both cell potential and salt concentration profiles as functions of position in the cell at the experimentally applied current density. The theoretical limiting current was taken to be the current at which the calculated salt concentration at the cathode was zero. We see quantitative agreement between experimental measurements and theoretical predictions for the limiting current. This agreement is obtained without resorting to any adjustable parameters.

The design and engineering of composite materials is one strategy to satisfy the materials needs of systems with multiple orthogonal property requirements. In the case of rechargeable batteries with lithium metal anodes, the system requires a separator with fast lithium ion transport and good mechanical strength. In this work, we focus on the system polystyrene-blockpoly(ethylene oxide) (SEO) with bis(trifluoromethane)sulfonimide lithium salt (LiTFSI). Ion transport occurs in the salt-containing poly(ethylene oxide)-rich domains. Mechanical rigidity arises due to the glassy nature of polystyrene (PS). If we assume that the salt does not interact with the PS-rich domains, we can describe ion transport in the electrolyte by three transport parameters (ionic conductivity, , salt diffusion coefficient, , and cation transference number, + 0) and a thermodynamic factor, f. By systematically varying the volume fraction of the conducting phase, c between 0.29 and 1.0, and chain length, between 80 and 8000, we elucidate the role of morphology on ion transport. We find that is the strongest function of morphology, varying by three full orders of magnitude, while is a weaker function of morphology. To calculate + 0 and f , we measure the current fraction, + , and the open circuit potential, , of concentration cells. We find that + and follow universal trends as a function of salt concentration, regardless of chain length, morphology, or c , allowing us to calculate + 0 for any SEO/LiTFSI or PEO/LiTFSI mixture when and are known. The framework developed in this paper enables predicting the performance of any block copolymer electrolyte in a rechargeable battery. MAIN TEXT

It is known that the addition of salts to symmetric block copolymers leads to stabilization of ordered phases and an increase in domain spacing; both trends are consistent with an increase in the effective Flory-Huggins interaction parameter between the blocks, χ. In this work, we show that the addition of salt to a disordered asymmetric block copolymer first leads to the formation of coexisting ordered phases which give way to a reentrant disordered phase at a higher salt concentration. The coexisting phases are both body centered cubic (BCC) with different domain spacings, stabilized by partitioning of the salt. Further increase in salt concentration results in yet another disorder-to-order transition; hexagonally packed cylinders are obtained in the high salt concentration limit. The coexisting phases formed at intermediate salt concentration, elucidated by electron tomography, showed the absence of macroscopic regions with distinct BCC lattices. A different asymmetric block copolymer with composition in the vicinity of the sample described above only showed only a single disorder-to-order transition. However, the dependence of domain spacing on salt concentration was distinctly non-monotonic, and similar to that of the sample with the reentrant phase behavior. This dependence appears to be an announcement of reentrant phase transitions in asymmetric block copolymer electrolytes. These results cannot be mapped on to the traditional theory of block copolymer electrolyte self-assembly based on an effective χ.

Successful prevention of lithium dendrite growth would enable the use of lithium metal as an anode material in next-generation rechargeable batteries. Mechanically stiff block copolymer electrolytes have been shown to prolong the life of lithium metal cells by suppressing lithium dendrite growth. However, impurity particles that are invariably present in the lithium metal nucleate electrodeposition defects that eventually lead to short-circuits. In this study, we use X-ray tomography to study the morphology of electrodeposited lithium in symmetric cells containing a block copolymer electrolyte. An "electrochemical filtering" treatment was performed on these cells in order to reduce the concentration of impurity particles near the electrode-electrolyte interface, and cells were cycled to determine the effects of the treatment on lifetime. Depending on the treatment details, average charge passed before failure was improved by 30-400%. For a cell in which the treatment was most effective, cycle life was increased by more than an order of magnitude and the measured defect density was negligible. Other treated cells, however, in which the treated lithium was imperfect, had a higher areal density of defects compared to control cells. A majority of the defects in treated cells were confined within the electrodes. In contrast, most of the defects seen in the control cells were protrusions that invaded the electrolyte phase. The increased lifetime in these imperfectly treated cells was not due to a reduction in defect density, but rather due to the differences in defect morphology. These results motivate the development of deposition defect-and impurity-free lithium metal electrodes.