TFSI Anions Research Articles

Controlling Long-Term Plasticity in Neuromorphic Computing Through Modulation of Ferroelectric Polarization.

Electrolyte-gated transistors (EGTs) have significant potential for neuromorphic computing because they can control the number of ions by mimicking neurotransmitters. However, fast depolarization of the electric double layer (EDL) makes it difficult to achieve long-term plasticity (LTP). Additionally, most research utilizing organic ferroelectric materials has been focused on basic biological functions, and the impact on nonvolatile memory properties is still lacking. Herein, we present a polyvinylidene fluoride (PVDF)-based ion-gel synaptic device using PVDF and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) to implement LTP through the introduction of ferroelectric materials. The PVDF-based polymer slows the escape rate of TFSI anions from the electrolyte/channel layer through residual polarization. The fabricated synaptic devices successfully demonstrate LTP by controlling ion adsorption under the influence of PVDF-based polymers. Furthermore, it implements synaptic functions including paired pulse facilitation (PPF), high-pass filtering, and neurotransmitter control. To validate the potential of neuromorphic computing, we successfully achieved high recognition rates for artificial/convolutional neural network (A/CNN) simulations via sequential adsorption and desorption under ferroelectric polarization with long-term potentiation/depression (LTP/D). This study provides a rational ion adsorption strategy utilizing the ferroelectric polarization caused by the introduction of a PVDF-based polymer in the dielectric layer.

Electrolytes from Alkoxyalkyl-substituted Imidazolium ionic liquids. Synthesis, characterization, properties and cell performance in LiFePO4 half-cells

Electrolytes based on lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt and alkoxyalkyl-substituted imidazolium ionic liquids (ILs) have been studied. We have synthesized three different 1-alkoxyalkyl-3-methylimidazolium TFSI salts with different chain lengths (from 4 to 10 atoms) and with one to three oxygen atoms at position 1- of the imidazole ring. Both the neat ILs as well as the cognate lithium doped electrolytes have been structurally and physicochemically characterized and electrochemically evaluated.Thermal analyses indicate that the glass transition temperature of the electrolytes is displaced to higher temperatures than those of the corresponding neat ILs, being also higher (more positive) upon increasing the number of oxyethylene units. On the other hand, all 1 M LiTFSI doped synthesized electrolytes are thermally stable up to 300 °C.Raman spectroscopy results indicate that the fraction of TFSI anions coordinated to lithium ion decreases on increasing the number of alkoxy units. This finding implies that there is more “free” lithium ion available to link to the ethereal oxygen of the side chain of the imidazolium ring system. This fact influences the ionic conductivity of the neat ILs and their cognate electrolytes.The electrochemical properties —namely high room temperature conductivities as well as wide electrochemical stability windows— indicate that these materials are promising electrolytes for lithium-ion batteries (LIBs). Lithium half-cells using LiFePO4 (LFP) olivine have been assembled in order to assess the electrochemical performance by means of both cyclic voltammetry and galvanostatic charge/discharge tests. Overall, we have found that the electrochemical performance of these cells is better than that reported 1 M LiTFSI [Pyr14][TFSI], a common electrolyte in LIBs. The generated half-cell from the 1-alkoxyalkyl-3-methylimidazolium TFSI salts possess a high capacity up to 145 mAh∙g−1 at a current of 2C.The results reported herein show a striking relationship between structure and properties.

Understanding Electrode-Electrolyte Interfaces in Gel Polymer Electrolytes Using in-Operando Raman Spectroscopy

Gel polymer electrolytes hold enormous potential for advancing battery performance and technology, promising high energy densities and safe, rechargeable quasi solid-state batteries. GPEs offer higher ionic conductivities than solid electrolytes and enhanced safety in comparison to liquid electrolytes. A particular advantage of these electrolytes has been their ability to create robust electrode electrolyte interfaces that guide uniform deposition and stripping of lithium via their mechanical strength from the polymer and interface species that form during initial cycling.Herein we report that 1:1 dimethoxyethane (DME) and dioxolane (DOL) with lithium bistrifluoromethanosulfonyl imide (LiTFSI) and lithium nitrate entrapped in Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) gel polymer electrolytes have shown stability for up to 500 hours in lithium metal symmetric cells at 1 mAh/cm2 and performs at 5mAh/cm2 in dynamic symmetric cell cycling. To understand the fundamental interfacial phenomenon governing lithium deposition and growth in this system in-operando Raman was conducted on an anodeless Cu|PVDF-HFP|Li cell. Copper was sputtered onto the polymer electrolyte before assembly with a thickness of 20nm, providing an optically transparent electrode through which the Raman laser could pass through to detect interfacial species. We observe that PVDF-HFP acted as a chemically stable polymer electrolyte while Raman shifts associated with TFSI anions disappear near the electrode surface. The spectral and electrochemical data allow us to build a model for the growth and evolution of interfaces in an anodeless lithium metal battery cell with a GPE.Acknowledgement: This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the DOE under contract number DE‐SC0014664. Figure 1

Universal Copolymerization of Crosslinked Polyether Electrolytes for All-Solid-State Lithium-Metal Batteries.

Solid polymer electrolytes (SPEs) are pivotal in advancing the practical implementation of all-solid-state batteries. Poly(1,3-dioxane) (PDOL)-based electrolytes have attracted significant attention due to the pseudo-high conductivity achieved through sophisticated in situ polymerization methods; however, such PDOL-based electrolytes present challenges of crystallization over time and monomers residual during processing. In this study, integrating LiTFSI and LiDFOB as a universal copolymerization strategy for developing high-performance PDOL electrolytes with a wide range of epoxy crosslinkers is proposed. It is discovered that this approach leverages the protective effects of TFSI anions on the boron active center and catalyzes polymer chain growth via crosslinking. The homogenously crosslinked (benzene-centered) PDOL electrolyte exhibits remarkable thermo-mechanical stability (up to 100°C), high ion migration number (tLi+ = 0.42), a wide electrochemical window (≈5.0V vs Li+/Li), and high ionic conductivity (4.5×10-4 S cm-1). Notably, the crosslinked PDOL electrolyte is in the all-solid-state with minimal monomer/oligomer residual, exhibiting no crystallization during relaxation, delivering a robust performance in all-solid-state lithium metal batteries.

Are NaTFSI and NaFSI Salt-Based Water-in-Salt Electrolytes Structurally Similar or Different?

Water-in-salt electrolytes (WiSEs) are a promising class of electrolytes due to their wide electrochemical stability window and nonflammability. In this study, we explore the structural organization of sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) and sodium bis(fluorosulfonyl)imide (NaFSI) salt-based aqueous electrolytes, covering dilute to highly concentrated regions, by employing an all-atom molecular dynamics simulation. For the NaTFSI-based electrolyte, we observe that Na+ ions are mostly surrounded by water molecules at all the salt concentrations due to the very strong interaction between them. While TFSI anions weakly coordinate with Na+ ions and other TFSI anions, they also mostly prefer to be surrounded by water molecules. These interactions were found to have moderate dependence on the concentration of the NaTFSI salt. For the NaFSI-based electrolyte, while the Na+-water interaction is stronger at lower salt concentrations, the number of nearest neighbor FSI anions is found to be more than that of water at higher concentrations (≥20 m). This is because the increase in the salt concentration leads to expulsion of water molecules from the solvation shell of Na+ ions and enhances the interaction between Na+ ions and oxygen atoms of FSI. At the highest salt concentration (solubility limit), the bulk-like water structure is completely disrupted and dominated by an anionic network in the FSI-based electrolyte. In contrast, water-water hydrogen bonding network is still present even in the highly concentrated TFSI-based electrolyte. The simulated X-ray scattering pattern displays a low-q peak, revealing the presence of an intermediate range ordering due to alternating anion-rich and water/Na+-rich regions in both the electrolytes. However, the characteristic length scale corresponding to the low-q peak decreases with increasing the salt content in both the electrolytes.

Low Solvating Power of Acetonitrile Facilitates Ion Conduction: A Solvation-Conductivity Riddle.

Acetonitrile (AN) electrolyte solutions display uniquely high ionic conductivities, of which the rationale remains a long-standing puzzle. This research delves into the solution species and ion conduction behavior of 0.1 and 3.0 M LiTFSI AN and propylene carbonate (PC) solutions via Raman and dielectric relaxation spectroscopies. Notably, LiTFSI-AN contains a higher fraction of free solvent uncoordinated to Li ions than LiTFSI-PC, resulting in a lower viscosity of LiTFSI-AN and facilitating a higher level of ion conduction. The abundant free solvent in LiTFSI-AN is attributed to the lower Li-solvation power of AN, but despite this lower Li-solvation power, LiTFSI-AN exhibits a level of salt dissociation comparable to that of LiTFSI-PC, which is found to be enabled by TFSI anions loosely bound to Li ions. This work challenges the conventional notion that high solvating power is a prerequisite for high-conductivity solvents, suggesting an avenue to explore optimal solvents for high-power energy storage devices.

Ab Initio Molecular Dynamics Simulations of Aqueous LiTFSI Solutions─Structure, Hydrogen Bonding, and IR Spectra.

Simulations of Density Functional Theory-based ab initio molecular dynamics (AIMD) have been performed for a series of aqueous lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) solutions with concentrations ranging from salt-in-water to water-in-salt systems. Analysis of the structure of electrolytes has revealed a preference of Li+ cations to interact with water molecules. In concentrated LiTFSI solutions, water molecules form small associates. The total number of hydrogen bonds (HBs) in the system decreases with salt concentrations, with bonds to water acceptors being only partially replaced by interactions with TFSI anions. Infrared (IR) spectra in the region of the O-H stretching frequency calculated from AIMD trajectories are in good agreement with experimental data. Statistics of oscillations of individual O-H bonds have shown correlations between vibrational frequencies and the structure of HBs formed by water. The changes in the IR spectrum have been related to the varying contributions of different local environments of the water molecules. The abundances of the three spectral components calculated from the simulations agree well with the decomposition of the experimental IR spectra reported in the literature.

High-performance pure red perovskite light-emitting diodes utilizing conformational transformation of ionic liquid additive

The performance of perovskite light-emitting diodes (PeLEDs) depends highly on the defect density and carrier transport properties of the perovskite layer. Some long-chain ligands are good at passivating the defects, but they hinder the charge injection into the perovskite because of their insulating nature. Notably, red PeLEDs suffer from spectral shift under bias caused by ion migration inside the mixed cation perovskite layer. This study proposes the use of silver bis(trifluoromethanesulfonyl)imide (AgTFSI) a conducting ionic liquid as a multifunctional additive to the perovskite to overcome the aforementioned problems. The TFSI anion comprises strong electronegative -CF3 and Lewis base -SO2 groups. These functional groups interact with undercoordinated Pb2+, efficiently suppressing defects and reducing nonradiative recombination in the perovskite film. AgTFSI displays two distinct conformations: cisoid and transoid. The opposing orientations of the two functional groups enable the cisoid structure to exclusively engage with the perovskite through the -SO2 groups. In contrast, the semi-linear arrangement of the transoid structure provides accessibility to both -SO2 and -CF3 groups for interaction. Owing to their conformational flexibility, the TFSI anions in AgTFSI undergo a transition from the zigzag cisoid to a more linear transoid structure when added to the perovskite, enabling efficient packing and tight binding with the Pb defects. As a result, the perovskite film shows a twofold increase in photoluminescence quantum yield and decay lifetime. Consequently, the red PeLEDs achieve a maximum external quantum efficiency of 13.52%, nearly double that of the control device. This study confirms AgTFSI's potential as an effective additive for high-performance pure red PeLEDs.

How the PEG terminals affect the electrochemical properties of polymer electrolytes in lithium metal batteries

Poly(ethylene oxide) (PEO) or poly(ethylene glycol) (PEG) based electrolytes for lithium batteries have arouse great interest, however their application is still hindered by low ion conductivity (σ) at room temperature, narrow electrochemical stability window (ESW) and low lithium ion transference number (LTN). Herein, we report the synthesis and comparative investigation of four kinds of methoxy poly(ethylene glycol) (MPEG) based electrolytes with different terminals, including MPEG with a hydroxyl terminal (MPEG-OH), a propyl carbonate terminal (MPEG-PC), a methoxy terminal (MPEG–OCH3) and a trifluoroethoxy terminal (MPEG-3F). It is found that the terminals in MPEG electrolytes exert great influences on the physical and electrochemical properties, e.g., crystallization, viscosity, dissociation capacity of Li salts, chemical environment of fluorine, σ, ESW, LTN and the cycling performance of lithium metal batteries. The -OH terminal is reactive with less stability, while -PC and –OCH3 result in moderate ionic transport. In comparison, MPEG-3F containing a trifluoroethoxy terminal affords high σ because of the good dissociation of LiTFSI, high LTN because of encapsulating the TFSI anions, and wide ESW due to high-voltage-resistant fluorocarbon.

Effects of Me-Solvent Interactions on the Structure and Infrared Spectra of MeTFSI (Me = Li, Na) Solutions in Carbonate Solvents-A Test of the GFN2-xTB Approach in Molecular Dynamics Simulations.

We investigated the performance of the computationally effective GFN2-xTB approach in molecular dynamics (MD) simulations of liquid electrolytes for lithium/sodium batteries. The studied systems were LiTFSI and NaTFSI solutions in ethylene carbonate or fluoroethylene carbonate and the neat solvents. We focused on the structure of the electrolytes and on the manifestations of ion-solvent interactions in the vibrational spectra. The IR spectra were calculated from MD trajectories as Fourier transforms of the dipole moment. The results were compared to the data obtained from ab initio MD. The spectral shifts of the carbonyl stretching mode calculated from the GFN2-xTB simulations were in satisfactory agreement with the ab initio MD data and the experimental results for similar systems. The performance in the region of molecular ring vibrations was significantly worse. We also found some differences in structural data, suggesting that the GFN2-xTB overestimates interactions of Me ions with TFSI anions and Na+ binding to solvent molecules. We conclude that the GFN2-xTB method is an alternative worth considering for MD simulations of liquids, but it requires testing of its applicability for new systems.

Thin Film Transistors Incorporating Ultrapure Semiconducting Single-Walled Carbon Nanotubes and Green Dielectrics

Printed electronics is a burgeoning field that has received intense research interest and is beginning to experience commercial successes. Single-walled carbon nanotubes (SWNTs) are a unique and promising building block for incorporation into next generation superfast electronic devices. SWNTs have very high carrier mobilites, with band gaps compatible for integration into logic circuits. Their excellent mechanical flexibility allows for potential incorporation into flexible printed electronics, enabling fully-printed transistors and circuits with performances that support low cost, large area fabrication. Progress in incorporating SWNTs into commercial devices has been hindered by the presence of metallic SWNTs, which are produced alongside semiconducting SWNTs during synthesis, and negatively impact device performance. Fabrication of ambipolar SWNT organic thin film transistors (OTFTs) with high carrier mobilities and high on/off ratios remains particularly challenging; many examples in the literature required high temperature, expensive and energy-demanding processes.Since the initial discovery of conjugated polymer-assisted dispersion and purification of SWNTs in 2008, several polymer families have been successfully shown to selectively disperse semiconducting SWNTs. However, only a relatively small number of these supramolecular complexes have been incorporated into OTFTs. We used a novel conjugated polymer to exclusively disperse semiconducting SWNTs. The dispersal procedure requires a simple sonication and centrifugation, during which the metallic SWNTs sediment out. Solution purity was evaluated using UVVis- NIR and Raman spectroscopies. The resulting dispersions are amenable to solution processing techniques such drop casting and spin coating, allowing for the potential for large area device fabrication at room temperature. Ambipolar OTFTs were fabricated under ambient conditions using this solution and tested in both air and under inert atmosphere. The presence of excess conjugated polymer, solution deposition techniques, SWNT density, surface treatment, and post-fabrication treatment were all investigated to determine which parameters facilitated the production of OTFTs with high mobilities (>20 cm2V-1s-1), high on/off ratios (10^6-10^8), negligeble hysteresis, controlled threshold voltages, and high bias stability.[1] Protocols for sorting and dispersing ultrapure sc-SWNTs with conjugated polymers for thin-film transistor (TFT) applications have been well refined. Conventional wisdom dictates that removal of excess unbound polymer through filtration or centrifugation is necessary to produce high- erformance TFTs. However, this is time-consuming, wasteful, and resource-intensive. We challenge this paradigm and demonstrate that excess unbound polymer during semiconductor film fabrication is not necessarily detrimental to device performance.[2] With focus on further improving device performance, we look to green, compostable dielectrics to pair with SWCNTs. We report a tri-layer dielectric using poly (lactic acid) (PLA), poly(vinyl alcohol)/cellulose nanocrystals (PVAc) and toluene diisocyanate terminated poly(caprolactone) (TPCL) which we integrated into SWCNT based TFTs in a top gate bottom contact architecture. The PVA provides a high dielectric constant due to the hydroxy groups, the cellulose is used to optimize the viscosity, the TPCL layer provides a robust hydrophobic surface[3], and the PLA eliminates the interfacial charge traps present in the PVAc. This leads to a decrease in leakage currents and reduced the polarity at the dielectric/semiconductor interface. The TFTs fabricated using tri-layer dielectrics led to air stable and balanced hole and electron mobilities which was not observed for the PVAc/TPCL bilayer systems with supressed hole mobility. These TFTs were then used to study the impact of electrochemical doping on the performance of sc-SWCNT TFTs when switching from n-type, where an electrical double layer is formed, to p-type, where the TFSI anions are free to interact with the sc-SWCNTs.[4]The following presentation will focus on the engineering of sc-SWNTs based electronic devices. The choice of conjugated wrapping polymer, dielectric and processing conditions on film formation and the TFT device performance.

Direct imaging of micrometer-thick interfaces in salt–salt aqueous biphasic systems

Unlike the interface between two immiscible electrolyte solutions (ITIES) formed between water and polar solvents, molecular understanding of the liquid-liquid interface formed for aqueous biphasic systems (ABSs) is relatively limited and mostly relies on surface tension measurements and thermodynamic models. Here, high-resolution Raman imaging is used to provide spatial and chemical resolution of the interface of lithium chloride - lithium bis(trifluoromethanesulfonyl)imide - water (LiCl-LiTFSI-water) and HCl-LiTFSI-water, prototypical salt-salt ABSs found in a range of electrochemical applications. The concentration profiles of both TFSI anions and water are found to be sigmoidal thus not showing any signs of a positive adsorption for both salts and solvent. More striking, however, is the length at which the concentration profiles extend, ranging from 11 to 2µm with increasing concentrations, compared to a few nanometers for ITIES. We thus reveal that unlike ITIES, salt-salt ABSs do not have a molecularly sharp interface but rather form an interphase with a gradual change of environment from one phase to the other. This knowledge represents a major stepping-stone in the understanding of aqueous interfaces, key for mastering ion or electron transfer dynamics in a wide range of biological and technological settings including novel battery technologies such as membraneless redox flow and dual-ion batteries.

Ambipolar Electrochemistry of Pre‐Intercalated Ti3C2Tx MXene in Ionic Liquid Electrolyte

AbstractCation intercalation with or without redox remains the dominant charge storage mechanism for two‐dimensional (2D) Ti3C2Tx MXene. Anion‐based charge storage remains unexplored due to intrinsic negative surface charge of MXenes preventing spontaneous intercalation of anions and irreversible oxidation of Ti at anodic potentials in aqueous electrolytes. In this work, we report on the ambipolar electrochemical behavior of the Ti3C2Tx in ionic liquid electrolyte over a 2.5 V electrochemically stable window. The experiments are conducted on a thin Ti3C2Tx film current collector coated with an electroactive layer of small flakes (∼150 nm) of Ti3C2Tx pre‐intercalated with 1‐ethyl‐3‐methylimidazolium bis‐(trifluoromethylsulfonyl)‐imide (EMIM‐TFSI) ionic liquid. Couples of redox peaks with a very small potential separation during the voltage sweep are observed at high negative (−0.75 V vs. Ag wire) and high positive (+0.75 V vs. Ag wire) potentials. Our experimental electrochemical data combined with density functional theory (DFT) calculations suggest feasibility of pseudo‐intercalation of TFSI anions between Ti3C2Tx flakes. This study provides a pathway for elucidating anion intercalation for different MXene chemistries in solvent‐free electrolytes, which can lead to development of MXene based energy storage devices with improved performance.

Hydrogen Bonding and Infrared Spectra of Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide/Water Mixtures: A View from Molecular Dynamics Simulations

Simulations of ab initio molecular dynamics have been performed for mixtures of ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) ionic liquid and water. Statistics of donors and acceptors of hydrogen bonds has revealed that with increasing water content, hydrogen bonds between EMIM cations and TFSI anions are replaced by bonds to water molecules. In the mixture of liquids, the total number of bonds (from EMIM cations or water molecules) formed by TFSI acceptors increases. IR spectra obtained from ab initio molecular dynamics trajectories are in good agreement with literature data for ionic liquid/water systems. Analysis of oscillations of individual C-H and O-H bonds has shown correlations between vibrational frequencies and hydrogen bonds formed by an EMIM cation or water molecule and has indicated that the changes in the IR spectrum result from the decreased number of water-water hydrogen bonds in the mixture. The tests of DFTB methodology with tailored parameterizations have yielded reasonably good description of the IR spectrum of bulk water, whereas available parameterizations have failed in satisfactory reproduction of the IR spectrum of EMIM-TFSI/water mixtures in the region above 3000 cm-1.

Triazine crosslinked polyionic liquid: An electrolyte-stable, lithiophilic and electrostatic shielding layer for uniform lithium plating/stripping

Rational design of functional layer on Cu current collector with polymer material is an effective strategy to improve electrochemical performance of batteries with Li anode. Here, a multifunctional solvent-stable layer, which contains triazine crosslinked polyionic liquid (cPIL) as the polymer matrix and in-situ formed LiF nanoparticle as filler (cPIL-LiF), is constructed on Cu foil to regulate the plating/stripping behavior of Li. The cPIL with excellent lithiophilicity and electrostatic shielding performance ensures homogeneous Li+ flux and promotes uniform Li deposition. The LiF nanoparticles and TFSI anions in cPIL-LiF contribute to the formation of the stable LiF- and Li3N-rich solid electrolyte interphase (SEI). Due to these superiorities, excellent interfacial stability is achieved and, consequently, dense and dendrite-free Li deposition is well maintained during cycling. The cPIL-LiF coated Cu enables an improved cycling stability with coulombic efficiency of ∼98% for 200 cycles, much better than the bare Cu and the Cu modified with the commonly used polymers. Moreover, the full cells with Li4Ti5O12/LiFePO4 cathodes and cPIL-LiF-Cu@Li (limited amount of deposited Li) demonstrate apparently enhanced electrochemical performance. This work provides new perspectives to construct electrolyte-stable artificial protective layer on Cu current collector for stable Li plating/stripping and enriches the understanding on the development of high-performance Li metal-based batteries.

How Temperature, Pressure, and Salt Concentration Affect Correlations in LiTFSI/EMIM-TFSI Electrolytes: A Molecular Dynamics Study.

Classical polarizable molecular dynamics simulations have been performed for LiTFSI solutions in the EMIM-TFSI ionic liquid. Different temperature or pressure values and salt concentrations have been examined. The structure and dynamics of the solvation shell of Li+ cations, diffusion coefficients of ions, conductivities of the electrolytes, and correlations between motions of ions have been analyzed. The results indicated that regardless of the conditions, significant correlations are present in all systems. The degree of correlations depends mainly on the salt fraction in the electrolyte and is much less affected by temperature and pressure changes. A positive correlation between motions of Li+ cations and TFSI anions, leading to the occurrence of negative Li+ transference numbers, exists for all conditions, although temperature and pressure changes affect the speed of anion exchange in Li+ solvation shells.

Mechanistic Insight on the Solid Electrolyte Interphase (SEI) Formed By a Superconcentrated [Li][TFSI] in Acetonitrile Electrolyte Near Lithium Metal

Superconcentrated salt solutions are promising alternatives to conventional electrolytes in Li-ion batteries because of their enhanced redox stability and tendency to form a stable and rigid solid electrolyte interphase (SEI). Herein, we performed density functional theory-based molecular dynamics (DFT-MD) simulations to understand the initial electrode reactions between the Li metal and superconcentrated electrolyte solution of Li bis(trifluoromethanesulfonyl)imide ([Li][TFSI]) in acetonitrile (AN). We thoroughly analyzed the structure and composition of the SEI formed near the Li metal. We show that reductive decomposition of the TFSI anions contributes majorly in the initial SEI formation and the AN molecules are relatively stable against the redox processes occurring near the Li metal. The atomized C, O, F, and S atoms contributed by the TFSI anions forms the stable inorganic layer leading to highly structured and inhomogeneous SEI. The AN molecules and undissociated N–SO2–CF3 fragments are mostly aligned away from the Li layers. Figure 1

Free Energy Sampling to Explore Ion Solvation Environments and Understand Transport and Glass Transition in Solid State Electrolytes for Battery Materials

Molecular dynamics simulations using newly developed, accurate interaction potentials have helped elucidate possible mechanisms for various transport properties in systems of lithium salts dissolved in poly(ether-acetals) ( P(nEO-mMO) where EO is ethoxyl and MO is methoxyl ) that are prime candidates for solid state polymer electrolytes. The presence of a second coordination shell around the Li ion, observed in polymers that have MO groups, seems to distort the first coordination shell, creating a more open cage structure that may help facilitate transport in these polymers. Furthermore, this distorted coordination may also assist in the formation of a lower coordination transition state while the Li ions jump from one cage to another during diffusion. Free energy calculations further elucidate the relative stability of these different Li-ether coordination environments and the possible pathways to move between them (See figure 1 for example).Studies on poly(ether-acetals) also tell us that polymers with low glass transition temperature (Tg) are not necessarily the best foundation for high transport electrolytes as addition of salt may disproportionately increase Tg and reduce transport at comparable absolute temperatures. At battery working temperatures, PEO still exhibits superior ionic conductivity as compared to other polymers in this series due a smaller increase in glass transition with salt loading. Hence it is very important for practical applications to understand how salt loading affects the glass transition temperatures in these polymers and how we can tune it to our advantage. It was observed that the softness of the polymer chain controlled by the strength of the dihedral angles is an important parameter that has a significant effect on the glass transition and hence provide insight and tune further improved potentials. We will show the associated correlations of glass transition temperature in these systems with microscopic chemical details like the polymer backbone chemistry, ion clustering and coordination by extensively studying the trends in these different systems with extensive statistics from long time molecular dynamics simulations at different temperatures and compositions and understand transport at different timescales and help design new polymers electrolytes with improved transport at battery working temperatures. Figure 1: Pathways:2D Free energy analysis of Solvation of Li cation and TFSI anion in PEO and PEOMO. Coordination w/r to oxygens of ether/acetyl and oxygens of TFSI anions are used as collective variables a)Serial Process: PEO shows distinct stepwise pathways to move from one coordination state to another (arrows for guide showing TFSI forming a bond in the first step and then the polymer bond breaking in the second step b)Concerted Process: PEOMO shows a more direct path that implies bond breaking and forming happens in a more concerted way that allows for easier cation transport from one cage to another Figure 1

The Hydrotropic Effect of Ionic Liquids in Water-in-Salt Electrolytes

The water-in-salt approach has played a pivotal role in extending the electrochemical stability of aqueous electrolytes far beyond the thermodynamic stability limit of water.[1–4] At the molality of the original water-in-salt electrolyte of 21 moles of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) per kilogram of water, the water-to-lithium ratio equals 2.65. Strong cation-water and cation-anion interactions, the formation of a solid-electrolyte interphase on the anode side, and the alignment of TFSI anions in a water-blocking electrochemical double-layer on the cathode surface result in a particularly high oxidative stability.[1,5] However, even at this concentration, a small fraction of free water molecules, which are prone to hydrolysis reactions, remains. Hence, focus was put on further decreasing the water-to-lithium ratio to eliminate the remaining fraction of free water and to improve the quality of the solid-electrolyte interphase, either by partially substituting water with organic solvents[6] or ionic liquids.[7] Interestingly, the presence of ionic liquid boosts the LiTFSI solubility, resulting in a very low water-to-lithium ratio of ≤1.3.[7] Here we report on this enhancement of the LiTFSI solubility by examining ion-ion and ion-water interactions using Raman and NMR spectroscopy.[8] We identify a hydrotropic effect of ionic liquids as origin of the enhanced LiTFSI solubility. The increased Li concentration leads to an improved reductive stability and enables cycling of Li4Ti5O12 (LTO) anodes when coated with a layer of niobium oxide. Furthermore, we exploit the reduced water content of the water-in-salt/ionic liquid hybrid electrolytes and demonstrate compatibility with LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes. For full cells based on the LTO/NMC811 electrode couple, we obtain relatively high Coulombic efficiencies of 99.4% and 99.2% at rates of 1C and C/2, respectively, for such aqueous high-voltage batteries. This cell also displays a very high initial energy density of up to 150 Wh/kg on the active material level owing to the high capacity of NMC811.[1] L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, K. Xu, Science 2015, 350, 938–943.[2] Y. Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama, A. Yamada, Nat. Energy 2016, 1, 16129.[3] R. S. Kühnel, D. Reber, C. Battaglia, ACS Energy Lett. 2017, 2, 2005–2006.[4] M. Becker, R.-S. Kühnel, C. Battaglia, Chem. Commun. 2019, 55, 12032–12035.[5] J. Vatamanu, O. Borodin, J. Phys. Chem. Lett. 2017, 8, 4362–4367.[6] J. Chen, J. Vatamanu, L. Xing, O. Borodin, H. Chen, X. Guan, X. Liu, K. Xu, W. Li, Adv. Energy Mater. 2020, 10, 1902654.[7] L. Chen, J. Zhang, Q. Li, J. Vatamanu, X. Ji, T. P. Pollard, C. Cui, S. Hou, J. Chen, C. Yang, L. Ma, M. S. Ding, M. Garaga, S. Greenbaum, H.-S. Lee, O. Borodin, K. Xu, C. Wang, ACS Energy Lett. 2020, 5, 968–974.[8] M. Becker, D. Rentsch, D. Reber, A. Aribia, C. Battaglia, R.-S. Kühnel, Angew. Chemie Int. Ed. 2021, accepted manuscript, 10.1002/ange.202103375.

Aqueous interphase formed by CO2 brings electrolytes back to salt-in-water regime.

Super-concentrated water-in-salt electrolytes make high-voltage aqueous batteries possible, but at the expense of high cost and several adverse effects, including high viscosity, low conductivity and slow kinetics. Here, we observe a concentration-dependent association between CO2 and TFSI anions in water that reaches maximum strength at 5 mol kg-1 LiTFSI. This TFSI-CO2 complex and its reduction chemistry allow us to decouple the interphasial responsibility of an aqueous electrolyte from its bulk properties, hence making high-voltage aqueous Li-ion batteries practical in dilute salt-in-water electrolytes. The CO2/salt-in-water electrolyte not only inherits the wide electrochemical stability window and non-flammability from water-in-salt electrolytes but also successfully circumvents the numerous disadvantages induced by excessive salt. This work represents a deviation from the water-in-salt pathway that not only benefits the development of practical aqueous batteries, but also highlights how the complex interactions between electrolyte components can be used to manipulate interphasial chemistry.