Negative Transference Numbers in Poly(ethylene oxide)-Based Electrolytes

New experimental techniques such as electrophoretic NMR (eNMR) are emerging as powerful methods for directly measuring ion velocities in electrolytes under applied electric fields. The aim of this theoretical study is to predict the spatial- and temporal-dependence of these velocities of ions as a function of the magnitude of the applied field and salt concentration. It has recently been shown that mixtures of poly(ethylene oxide)-based (PEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) electrolytes exhibit negative cation transference numbers in a certain salt concentration range. In this range, the cation motion at early times is directed to the positive electrode at all locations in the cell; ion migration dominates in this regime. As time progresses, the cation velocity in finite zones near both the electrodes changes sign. These zones grow rapidly with time, reflecting the increasing importance of diffusion, and a point in time is reached beyond which the cation velocity in the entire cell is directed toward the negative electrode. Our work reveals the limited time window over which the results of eNMR can be used to determine the transference number. More importantly, it shows how to account for the effect of diffusional flux in such experiments.

The effect of salts on the micellization temperature of aqueous poly(ethylene oxide)‐b‐poly(propylene oxide)‐b‐poly(ethylene oxide) solutions and the dissolution rate and water diffusion coefficient in their corresponding gels

We investigate a polyacetal-based electrolyte, poly(1,3,6-trioxocane) (P(2EO-MO)) mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, and report full electrochemical characterization of the transport parameters and a thermodynamic property in comparison to the previously reported poly(ethylene oxide) (PEO) electrolyte data [D. Gribble et al., J. Electrochem. Soc., 166, A3228 (2019)]. While the steady-state current fraction (ρ +) of P(2EO-MO) electrolyte is greater than that of PEO electrolyte in the entire salt concentration window we explored, the rigorously defined transference number using Newman’s concentrated solution theory ( t+0 ) appears to be similar to that of PEO electrolyte. On the basis of full electrochemical characterization, we calculate the salt concentration profile as a function of position in the cell and predict limiting current density (i L L) as a function of salt concentration. Experimental data were compared to the predicted values. The non-monotonic behaviors were observed both in prediction and experimental results with offset peak positions. We find that the limiting current density of P(2EO-MO) electrolyte is systematically lower than that of PEO electrolyte in most of the salt concentrations with the exception of r av = 0.05. It is noteworthy that even though one measure of electrolyte efficacy (κρ +) is superior in P(2EO-MO) electrolyte, the limiting current density, which is another metric of electrolyte efficacy at high currents, is not greater in P(2EO-MO).

Even though batteries operate at different temperatures depending on their use and state of charge, little work has been done to understand the effects of temperature on the ionic transport properties of the electrolyte. The temperature dependence of these properties is important for predicting how the performance of the battery will change as a function of temperature, along with gaining fundamental insights into the underpinnings of ion transport in these electrolytes. In this study we provide the first investigation of the effect of temperature on ionic conductivity, salt diffusion coefficient, transference number, and the thermodynamic factor of a model polymer electrolyte: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt dissolved in poly(ethylene oxide) (PEO). These properties were measured at 70, 90, and 110 °C. As expected, we see monotonic increases in conductivity and diffusion with increasing temperature. Additionally, monotonic dependencies on temperature were obtained for the transference number and the thermodynamic factor. One presumes that concentration polarization decreases with increasing temperature due to more rapid ion transport. We use concentrated solution theory to predict concentration polarization in lithium-PEO/LiTFSI‑lithium symmetric cells and thereby quantify the effect of temperature on concentration polarization.

We examine the phase behavior of a hybrid organic–inorganic diblock copolymer/salt mixtures. The experimental system comprises poly(ethylene oxide)-block-polyhedral oligomeric silsesquioxane (PEO–POSS) mixed with a lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) salt. Although the diblock copolymers without salt exhibit a classical order-to-disorder transition behavior with increasing temperature, the PEO–POSS/salt mixtures exhibit disorder-to-order transitions with increasing temperature. The analysis of a small-angle X-ray scattering data from the disordered state using Leibler’s random phase approximation enables the determination of an effective Flory–Huggins interaction parameter, χeff, for the electrolytes. Unlike conventional systems, χeff increases with increasing temperature. A simple expression is proposed to describe the dependence of χeff on temperature and salt concentration. This enables the calculation of the segregation strength, χeffN, for both ordered and disordered electrolytes. The composition of the electrolytes is quantified by fEO/LiTFSI, the volume fraction of the salt-containing poly(ethylene oxide)-rich phase. The morphology of electrolytes is presented on a χeffN versus fEO/LiTFSI phase diagram. Over the values of fEO/LiTFSI studied (0.61–0.91), only two ordered phases were found: lamellae and co-existing lamellae/hexagonally packed cylinders.

AC impedance, FT-Raman and pulsed field gradient (pfg) NMR measurements have been conducted on solutions of poly(ethylene oxide) dimethyl ether (MW 400) complexed with LiCF3SO3 as a function of temperature and salt concentration. From an analysis of the νS(SO3) and δS(CF3) vibrational band envelopes of the CF3SO3 anion, respectively, the relative concentrations of anions in various chemical environments have been calculated. We find spectroscopic evidence for a redissociation of associated ionic species into spectroscopically “free” anions with increasing salt concentration in dilute solutions. The relative abundance of associated ionic species increases with increasing temperature. Pfg-NMR measurements show that D−(19F) and D+(7Li) are very similar for all concentrations (i.e., O:Li⩾53:1) and temperatures (25–80 °C) investigated. Most notably, the diffusivity of the oligomer solvent, D(1H), is significantly faster than the self-diffusion coefficients of the dissolved ions in all cases. Predicted values for the ionic conductivity were obtained from the NMR diffusivities, using the Nernst–Einstein relation, and compared with those from direct measurement. We find that the calculated values are higher for all concentrations; the discrepancy increases with decreasing salt concentration and increasing temperature. A good correlation is found between the concentration dependence of the ionic redissociation pattern, as determined from the νS(SO3) Raman band envelope, and an increase in equivalent ionic conductivity with increasing salt concentration in dilute solutions (i.e., O:Li⩾110:1). We suggest that fluctuating, salt-rich heterogeneities of dissolved ions and polymer segments form at low salt concentrations, and that this may be a general behavior of dilute polymer–salt complexes.

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) incorporating lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) offer excellent potential for solid-state lithium metal batteries (SSLMBs) due to their wide electrochemical stability window and high lithium salt solubility. LiTFSI supplies Li+ ions and also acts as a plasticizer, promoting the formation of the amorphous phase, which is vital for ionic conduction. Reduced ionic conductivity can result from very high crystallinity, spherulitic morphology, and secondary crystallization disrupting the ion pathways. The [EO:Li] molar ratio critically governs crystallization behavior and ionic transport; a ratio of 10:1, close to the stable amorphous eutectic composition (9:1), promotes amorphous phase formation and high ionic conductivity while limiting excessive crystallization. However, high salt content at this ratio leads to membrane stickiness, low mechanical strength, and handling difficulties. Here, we present a parafilm-assisted peeling technique that enables the fabrication of freestanding PEO/LiTFSI membranes while preserving their structural integrity. SEM analysis shows that conventional Teflon-mold peeling yields cracked, stretched surfaces, whereas parafilm-assisted films are smooth and defect-free. FTIR confirms Li+–PEO coordination with no parafilm–electrolyte interaction. Both methods deliver similar electrochemical performance: ∼4.30 V stability window, ionic conductivity 3.96 × 10−3 S cm−1 at 50 ° C (E a ∼ 0.898 eV), and Li+ transference number ∼0.14–0.15. Symmetric Li|SPE|Li cells cycle stably for 300 h at 50 ° C , while LiFePO4 full cells retain ∼161 mAh g−1 and >98% Coulombic efficiency over 30 cycles, confirming parafilm-assisted peeling as an effective fabrication method. This approach presents an effective means to address processing difficulties, thereby extending the viability of PEO based SPEs in next-generation energy storage devices.

Advancing solid-state battery technology is essential for achieving sustainable energy storage and accelerating the transition to electric transportation. Lithium metal batteries are particularly promising due to the high theoretical specific capacity of lithium metal (3860 mA h g⁻¹). Composite polymer electrolytes (CPEs) are a compelling candidate for solid-state lithium-metal batteries (SSBs) due to their favorable ionic conductivity, mechanical properties, and ease of processing. In this work, poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were used as the polymer-salt matrix due to their low cost, mechanical flexibility, and favorable interfacial contact with lithium. The addition of 20 wt% tetraethylene glycol dimethyl ether (TEGDME) as a plasticizer significantly enhanced room-temperature ionic conductivity and improved mechanical flexibility and processability. To further improve electrolyte performance, 500 nm nanoparticles were incorporated at 20 wt% into the PEO–LiTFSI–TEGDME matrix. Neat aluminum oxide (Al₂O₃) nanoparticles showed no significant effect on conductivity, transference number, limiting current, or interfacial resistance. Surface modification of Al₂O₃ with γ-methacryloxypropyltrimethoxysilane (A-174@Al₂O₃) similarly resulted in no observable improvement. In contrast, incorporating lithium-conducting lithium lanthanum zirconium oxide (LLZO) nanoparticles modified with the same silane (A-174@LLZO) largely impacted the cell performance. While the ionic conductivity remained unchanged, a substantial reduction in interfacial resistance and an increase in transference number were observed. As a result, the limiting current was tripled compared to the system with neat Al₂O₃. Modification of LLZO with an alternative silane, (3-aminopropyl)triethoxysilane (APTES@LLZO), resulted in poorer electrochemical performance compared to A-174@LLZO. Solid-state NMR measurements revealed that both silane-modified LLZO samples had reduced lithium content compared to unmodified LLZO and provided insights into changes in the lithium environment in the modified samples. These findings help explain the observed electrochemical performance, suggesting that changes in the lithium concentration and the local environment may influence overall cell behavior. These results demonstrate that lithium-active, silane-modified LLZO nanoparticles can enhance the electrochemical performance of CPEs. Further mechanistic studies and optimization are ongoing to fully understand and exploit these enhancements for high-rate solid-state batteries.

Electrolyte interfaces with freshly plated lithium metal are crucial for the development of reservoir‐free all‐solid‐state batteries (ASSBs). This study provides an in‐depth characterization of the polyethylene oxide (PEO) lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) interphase with lithium metal resembling reservoir‐free cell conditions. Using an in situ setup in both secondary ion mass spectrometry (SIMS) and X‐ray photoelectron spectroscopy (XPS), the dynamic evolution of the solid electrolyte interphase (SEI) layer is investigated at 60 °C. Lithium metal plating is conducted with an electron flood gun, ensuring controlled lithium deposition and enabling direct identification of intermediate reaction products. Time‐of‐flight‐ (ToF‐) and Orbitrap‐SIMS are employed to provide high‐resolution insights into the formation of inorganic and organic SEI components coming from PEO and LiTFSI degradation after plating. XPS further reveals chemical transformations occurring at the interface. Additionally, coulometric titration time analysis (CTTA) captures the kinetic aspect of PEO: LiTFSI reaction with lithium metal, highlighting the influence of molecular weight and salt concentration on initial reaction speed and long‐term stability. By integrating high‐resolution SIMS analysis with surface‐sensitive XPS and electrochemical methods, a comprehensive approach is offered to better understand the dynamic behavior of the polymer electrolyte | lithium metal interphase under conditions relevant to reservoir‐free battery concepts.

Solvation structure plays a crucial role in determining ion transport in electrolytes. We combine wide-angle X-ray scattering (WAXS) and molecular dynamics (MD) simulation to identify the solvation cage structure in two polymer electrolytes, poly(pentyl malonate) (PPM) and poly(ethylene oxide) (PEO) mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. As the salt concentration increases, the amorphous halo in the pure polymers is augmented by an additional peak at low scattering angles. The location of this peak and its height are, however, different in the two electrolytes. By decoupling the total intensity into species contributions and mapping scattering peaks to position-space molecular correlations, we elucidate distinct origins of the additional peak. In PPM, it arises from long-range charge-ordering between solvation cages and anions, while in PEO it is dominated by correlations between anions surrounding the same cage. TFSI- ions are present in the PPM solvation cage, but expelled from the PEO solvation cage.

Time resolved luminescence and vibrational spectroscopic studies on complexes of poly(ethylene oxide) oligomers and EuTFSI 3 salt

Ionic mobility in ternary polymer electrolytes for lithium-ion batteries

Battery safety is increasingly at the forefront of consumer concerns, especially regarding electric vehicles. Current lithium ion battery technology utilizes liquid electrolytes, which have very high ionic conductivities but are extremely flammable. To improve safety, researchers are investigating replacing liquid electrolytes in lithium ion batteries with solid electrolytes. A promising approach is the use of solid polymer composite electrolytes where lithium conductive ceramic powders are dispersed into polymer electrolyte matrix. These materials are intended to improve lithium conductivities and mechanical properties relative to pure polymer electrolyte, lower manufacturing and battery cell construction costs relative to using pure ceramic electrolytes, and eliminate volatile and flammable electrolyte solvents. Currently, the archetypal polymer matrix used in composite electrolytes is polyethylene oxide (PEO). When mixed with lithium salts, such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and ceramics, such as Li7La3Zr2O12 (LLZO), PEO based composites have conductivities (~10-7 S/cm at room temperature) that are still insufficient for today’s applications. In addition, PEO tends to crystallize and has poor oxidative stability (decomposition ca. 4V). We are exploring poly(polyethylene glycol methyl ether methacrylate) (PEGMA) and polyethylene carbonate (PEC) as alternatives to PEO to address issues with crystallization and oxidative stability, respectively. Composite electrolytes are fabricated by blending bulk polymer with LiTFSI and LLZO powders. We have formed rubbery PEGMA-based composites with comparable conductivities (~10-7 S/cm at room temperature, 70 vol% polymer, 10:1 ethylene oxide:lithium). Lithium symmetric cycling was observed to be stable over 250 hours at 1C. We have also recently formed stand-alone PEC based composites with high conductivities (~10-6 S/cm at room temperature, 60 vol% polymer, 10:1 ethylene carbonate:lithium). Investigations into the origin of this high conductivity and measurements of electrochemical stability are currently underway.

Synthesis and characterization of amorphous poly(ethylene oxide)/poly(trimethylene carbonate) polymer blend electrolytes

Liquid electrolytes (LEs) commonly show severe side reactions at the electrode-electrolyte interface, especially with alkali metal anodes, leading to rapid capacity fade of metal-ion batteries. Solid polymer electrolytes (SPEs), however, contribute to the suppression of side reactions due to their inherent inertness and high mechanical strength, providing long-term stable battery operation. Herein, we investigated physical and electrochemical properties of SPEs based on our previously reported microphase-separated poly(vinyl benzyl methoxy poly(ethylene oxide) ether)-block-polystyrene block copolymer (PVBmPEO-b-PS) with different alkali metal ions (A+ = Li+, Na+ or K+) and their use in the respective metal batteries, showing the potential for the transition from lithium to post-lithium batteries. Rheological and thermal properties as well as ion transport in the SPEs with different bis(trifluoromethanesulfonyl)imide (TFSI) salts concentrations revealed similar shear storage moduli (G’) for the investigated SPEs, while the lowest glass transition temperatures (Tg) were found for KTFSI-based films. By contrast, the highest total ionic conductivity was found for the LiTFSI-based SPEs. To quantify the A+ transference numbers (TA+), the Bruce-Vincent method and pulsed-field gradient (PFG) NMR were conducted, revealing significant challenges for TA+ determination of post-Li systems. Further, the examination of the interfacial stability of SPE/A interfaces by conducting plating/stripping experiments revealed significantly higher resistances for sodium- and potassium-based systems in comparison to their lithium-based counterpart. Nonetheless, A-metal/SPE/cathode cells with PVBmPEO-b-PS-based Na- and K-SPEs with the Prussian Blue analogues (PBAs, Na2-xFe[Fe(CN)6] and K2-xFe[Fe(CN)6]) positive electrodes and the respective alkali metal negative electrodes enabled cycling at elevated temperature of 55 °C. Herein, both sodium and potassium metal batteries exhibited stable cycling with capacity retentions of 73% over 100 cycles for the Na-cell, and 94% over the same cycle number for the K-cell, (and a high coulombic efficiency (CE) of 98% at the 100th cycle).