Non-aqueous electrolytes comprising the fluoride ion are characterized using a variety of experimental and computational methods.

Current and future demands for increasing the energy density of batteries without sacrificing safety has led to intensive worldwide research on all solid state Li-based batteries. Given the physical limitations on inorganic ceramic or glassy solid electrolytes, development of polymer electrolytes continues to be a high priority. This brief review covers several recent alternative approaches to polymer electrolytes based solely on poly(ethylene oxide) (PEO) and the use of nuclear magnetic resonance (NMR) to elucidate structure and ion transport properties in these materials.

The lithium oxygen battery has a theoretical energy density potentially meeting the challenging requirements of electric vehicles. However, safety concerns and short lifespan hinder its application in practical systems. In this work, we show a cell configuration, including a multiwalled carbon nanotube electrode and a low flammability glyme electrolyte, capable of hundreds of cycles without signs of decay. Nuclear magnetic resonance and electrochemical tests confirm the suitability of the electrolyte in a practical battery, whereas morphological and structural aspects revealed by electron microscopy and X-ray diffraction demonstrate the reversible formation and dissolution of lithium peroxide during the electrochemical process. The enhanced cycle life of the cell and the high safety of the electrolyte suggest the lithium oxygen battery herein reported as a viable system for the next generation of high-energy applications.

The Li-ion battery has served as the workhorse of many industries for a generation. The technology has advanced incrementally over the years, but the underlying chemistry has undergone only minimal changes since its commercialization in the early 90’s. A novel solvent-free polymer electrolyte developed by Ionic Materials, Inc. threatens to shift this paradigm. The polymer electrolyte is based on a crystalline thermoplastic polymer which is treated with an oxidizing agent and then reacted with a lithium salt. This electrolyte displays transport properties suitable for commercial battery use, while retaining the mechanical and safety advantages associated with a solid, and sufficient electrochemical stability to allow its use with Li-metal and high voltage intercalation electrodes. We report here Nuclear Magnetic Resonance (NMR) investigations of this polymer electrolyte. Pulsed-field gradient self-diffusion studies, in tandem with relaxometry and magic angle spinning (MAS), allow characterization of the structure and dynamics of this revolutionary material. Our measurements show room-temperature Li + self-diffusion coefficients on the order of 10 -9 m 2 /s (an order of magnitude higher than previously reported solid electrolyte candidates), as well as cation transference numbers approaching 0.5. We have also established further evidence that the ionic motion is decoupled from polymer segmental or chain motion, in contrast to the mechanism governing ionic transport in the widely studied polyether-based electrolytes. We discuss some of the challenges of studying this material, as well as the implications of the results for its suitability as an electrolyte for secondary Li metal batteries.

We report an electrolyte with low flammability, based on diethylene glycol dimethyl ether (DEGDME) dissolving lithium bis-trifluoromethane sulfonimidate (LiTFSI), and lithium nitrate (LiNO3) for high-performances lithium/sulfur battery. Self-diffusion coefficients, conductivity, and lithium transport number of the electrolyte are obtained by nuclear magnetic resonance and electrochemical impedance spectroscopy. Interface stability, lithium stripping/deposition ability, and the electrochemical stability window of the electrolyte are determined by voltammetry and impedance spectroscopy. The tests suggest conductivity higher than 10(-2) S cm(-1), lithium transport number of about 0.5, electrochemical stability extending from 0 V to 4.6 V, and excellent compatibility with lithium metal. A composite cathode using sulfur and multi walled carbon nanotubes (MWCNTs) is characterized in terms of structure and morphology by X-ray diffraction and scanning electron microscopy. The study shows spherical flakes in which the carbon nanotubes protect the crystalline sulfur from excessive dissolution, and create the optimal host for allowing the proper cell operation. The Li/S cell reveals highly reversible process during charge/discharge cycles, fast kinetic, and lithium diffusion coefficient in the sulfur electrode ranging from 10(-12) to 10(-10) cm(2) s(-1). The cell evidences a coulombic efficiency approaching 100%, capacity from 1300 mAh g(-1) to 900 mAh g(-1) and practical energy density higher than 400 Wh kg(-1).

Triethylene glycol dimethyl ether (TREGDME) dissolving lithium trifluoromethanesulfonate (LiCF3SO3) is studied as a suitable electrolyte medium for lithium battery. Thermal and rheological characteristics, transport properties of the dissolved species, and the electrochemical behavior in lithium cell represent the most relevant investigated properties of the new electrolyte. The self-diffusion coefficients, the lithium transference numbers, the ionic conductivity, and the ion association degree of the solution are determined by pulse field gradient nuclear magnetic resonance and electrochemical impedance spectroscopy. The study sheds light on the determinant role of the lithium nitrate (LiNO3) addition for allowing cell operation by improving the electrode/electrolyte interfaces and widening the voltage stability window. Accordingly, an electrochemical activation procedure of the Li/LiFePO4 cell using the upgraded electrolyte leads to the formation of stable interfaces at the electrodes surface as clearly evidenced by cyclic voltammetry, impedance spectroscopy, and ex situ scanning electron microscopy. Therefore, the lithium battery employing the TREGDME-LiCF3SO3-LiNO3 solution shows a stable galvanostatic cycling, a high efficiency, and a notable rate capability upon the electrochemical conditions adopted herein.

Using molecular dynamics simulations, small-angle neutron scattering, and a variety of spectroscopic techniques, we evaluated the ion solvation and transport behaviors in aqueous electrolytes containing bis(trifluoromethanesulfonyl)imide. We discovered that, at high salt concentrations (from 10 to 21 mol/kg), a disproportion of cation solvation occurs, leading to a liquid structure of heterogeneous domains with a characteristic length scale of 1 to 2 nm. This unusual nano-heterogeneity effectively decouples cations from the Coulombic traps of anions and provides a 3D percolating lithium-water network, via which 40% of the lithium cations are liberated for fast ion transport even in concentration ranges traditionally considered too viscous. Due to such percolation networks, superconcentrated aqueous electrolytes are characterized by a high lithium-transference number (0.73), which is key to supporting an assortment of battery chemistries at high rate. The in-depth understanding of this transport mechanism establishes guiding principles to the tailored design of future superconcentrated electrolyte systems.

Nuclear magnetic resonance (NMR) has been productively employed to investigate ion transport and solvation in standard carbonate-based Li ion battery electrolytes and in solid polymer electrolytes based on poly(ethylene oxide) (PEO). Future electrochemical power sources require new electrolytes to adapt to disruptive changes in the basic working chemistry, such as moving from Li ion to Na ion, or to Li metal electrodes. We highlight two recent collaborative activities on electrolytes in our group (i) Na ion; (ii) Li metal polymer. Using a combination of 23Na, 19F and natural abundance 17O NMR, and pulsed field gradient diffusion in a collaboration with the U.S. Army Research Lab, we have examined NaPF6 solutions in binary solvent mixtures ethylene carbonate (EC) and ethylmethyl carbonate (EMC) for possible cation solvation preference as a function of EC/EMC ratio. Spectroscopic evidence demonstrate that the Li+ and Na+ cations share a number of similar ion–solvent and ion-ion interaction trends, such as a preference for a solvation shell rich in cyclic carbonates over linear carbonates and fluorinated carbonates. However some differences are noted, in particular a somewhat weaker ion-solvent interaction for Na+. Ionic Materials, Inc. has invented a novel polymer with extremely high ionic conductivity over a range of temperatures, even surpassing that of a commercial porous separator containing the standard liquid carbonate-based electrolyte at room temperature. This solid polymer can be reliably extruded into very thin films, is non-flammable, has attractive mechanical properties for lithium dendrite suppression, is electrochemically stable against Li, and is compatible with a variety of different anodes and cathodes. The polymer electrolyte is based on an inexpensive semicrystalline commercially available polymer such as polyether ether ketone or polyphenylene sulfide and Li salts familiar to the battery and polymer electrolyte communities. The ionic transport mechanism is unlike that which characterizes the PEO salt complexes – that is ion mobility is completely decoupled from polymer host motion, as verified by differential scanning calorimetry and NMR. In fact NMR pulsed gradient measurements reveal Li self-diffusion coefficients at room temperature that are an order of magnitude higher than in the ceramic ion conductors of the Li10GeP2S12 class and thus the highest in any known solid. Furthermore, the Li+ transference number determined electrochemically and by NMR is > 0.6.

Glyme electrolytes are prepared by dissolving sodium trifluoromethane sulfonate (NaCF3SO3) either in dimetoxyethane (DME) or in diethyleneglycoledimethylether (DEGDME). The solutions, designed for sodium battery applications, are thermally characterized by TGA and studied in terms of transport properties by combining pulse field gradient nuclear magnetic resonance (PFG NMR) and electrochemical techniques. Both electrolytes reveal suitable characteristics for sodium batteries, such as ion conductivity of about 10−3Scm−1, sodium transport number of 0.5, a stable stripping-deposition trend, and electrochemical stability windows extending from 0 to 4V. However, the more volatile DME leads to a higher ion association degree. The suitability of both electrolytes is then verified in sodium-sulfur cells by cyclic voltammetry and galvanostatic test. The measurements confirm the reversibility of the sodium-sulfur process, and reveal the expected trend of the sulfur electrode in sodium cell with average working voltage of about 1.8V, with a higher polarization and lower capacity for the cell using the DME-based electrolyte. Accordingly, the DEGDME-based solution appears to be more suitable for sodium battery applications.

Currently used lithium ion batteries for portable electronics utilize flammable and often toxic non-aqueous electrolytes in order to achieve high energy densities. They also require a low humidity manufacturing environment resulting in an increased cost. Aqueous electrolytes have recently emerged as potential intrinsically nonflammable alternatives after their electrochemical stability window was expanded beyond 3.0 V by employing a new class of “Water-in-Salt” electrolytes. In such super-concentrated electrolyte, the decomposition of salt anion occurs preferentially on the anode before hydrogen evolution takes place, creating a kinetic protection against electrochemical decomposition via a dense solid electrolyte interphase (SEI). In this presentation, results from classical molecular dynamics (MD) simulations using a polarizable APPLE&P force field are analyzed in order to examine in detail the ion transport mechanism in bis(trifluoromethane sulfonyl)imide (LiTFSI-water) “Water-in-Salt” electrolytes (WiSE) for safe, green and low cost aqueous lithium ion batteries. They are complemented by Born Oppenheimer MD simulations of smaller systems that yield similar structural features. Simulations revealed an unusually low activation energy and fast ion transport for highly concentrated solutions even at low temperatures that is quite different from the dramatic increase of the activation energy for conductivity found in traditional battery electrolytes. A high conductivity and lithium transference number in WiSE is attributed to the formation of fast ion transporting pathways that are connected to the unexpected structure of WiSE electrolytes, which was confirmed by small angle neutron scattering experiments (SANS). The ability of MD simulations to describe dynamics of ion and solvent in WiSE electrolytes was further validated via pfg-NMR and conductivity measurements, while IR spectroscopy measurements provide a comprehensive picture of the salt electrolyte aggregation that is coupled with ion transport. The connection between the double layer structure of WiSE electrolytes and its electrochemical stability will be briefly discussed.

The physical and transport properties (density, viscosity, ionic conductivity, and ion diffusion coefficients) are reported for solutions containing 1-ethyl-3-methylimidazolium acetate (EMIAc) and cellulose, either with no molecular solvent, with water, or with acetonitrile (AN). Ionicity values, or the fraction of ions present in the ionic liquid (IL) that are "free" were calculated for each solution. The addition of molecular solvents to IL-cellulose solutions significantly changes the association behavior of the ions, with the presence of acetonitrile increasing aggregation while water decreases aggregation. Adding cellulose to all solutions results in more ideal behavior from the IL, with the increase in ideality being a function of the type of solvent present. The observed behavior suggests that water and AN have fundamentally different interactions with EMIAc and these differences in interaction may explain the varying cellulose solvation behavior of solutions containing these solvents.

Some ionic liquids (ILs) are notable for their ability to solubilize traditionally difficult to dissolve natural materials such as cellulose, silk, and wool keratin. NMR studies have shown that this ability is due predominantly to the interaction of chaotropic anions present in the IL, such as Cl - or acetate, with the hydroxyl groups present on polymeric materials. When molecular solvents are added to such mixtures, differing behaviors occur depending on the nature of the added solvent. Polar protic solvents generally serve as antisolvents, inducing the precipitation of dissolved materials. In contrast, polar aprotic solvents behave as cosolvents and are able to be added in significant fractions without causing the reconstitution of dissolved materials. It is generally understood that this differing behavior is the result of whether or not the molecular solvent preferentially interacts with the IL anion, thus preventing the anion from interacting with the polymer hydroxyl groups. However, little experimental work has been published to verify this hypothesis. In this study, the physical and transport properties (density, viscosity, ionic conductivity, and ion diffusion coefficients) are reported for 1-ethyl-3-methylimidazolium acetate (EMIAc) containing dissolved cellulose. The properties of these EMIAc-cellulose solutions were studied with no added molecular solvent, with added acetonitrile, or with added water. The fraction of ions present in the IL that are “free” (the Ionicity values) were calculated for each solution. The addition of molecular solvents to an EMIAc-cellulose solution results in significant changes in the mobility of all species present, and offers a means to better understand the molecular interactions that govern biopolymer solvation in ILs.

The electrolytes currently used in lithium ion batteries are a source of many cost and safety issues, which can be mitigated by a switch to aqueous electrolyte. This was, until recently, precluded by the narrow electrochemical stability window of the aqueous electrolyte, a problem which severely limited the practical voltage and energy density of the battery. However, recent developments have shown that it is possible to open up this stability window by increasing concentration of the salt (LiTFSI), enabling the formation of an interphase. This opens the door for a fresh focus on the properties of aqueous electrolytes. (1) Nuclear Magnetic Resonance (NMR) is a powerful tool for exploring ionic and molecular transport in electrolytes. The nuclei in question (7Li , 19F, and1H) are particularly well-suited to measurement by NMR, due to their gyromagnetic ratios and abundances. We performed NMR measurements on a range of concentrations of LiTFSI aqueous electrolyte, varying from 1 m to 21 m. In particular, pulsed gradient spin-echo self-diffusion experiments were performed on a 300 MHz spectrometer. Self-diffusion coefficients for both ions and water molecules were obtained over a range of temperatures from 20°C to -60°C. The diffusion results are displayed in Figs. 1 and 2. As expected, we note a general trend of smaller diffusion coefficients with lower temperature, as well as smaller diffusion coefficients with higher salt concentration. Cation transference numbers were determined directly from the diffusion measurements and those results are listed in Table 1. The cation transference number increases as the temperature falls and the salt concentration rises, as well. Although this investigation focuses on the transport properties of the liquid phase, ongoing work will address SEI formation on a variety of anodes in contact with this unusual aqueous system. References 1. Liumin Suo et al. "Water-in-salt" electrolyte enables high-voltage aqueous lithium-ion chemistries. Science. 350, 938 (2015). Figure 1