Complete Electrochemical Characterization and Limiting Current of Polyacetal Electrolytes

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.

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.

Phase transitions in block copolymer electrolytes induced by ionic current

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.

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 deployment of safe and high‐energy density lithium metal polymer batteries (LMPBs) still requires further advances in the quest for new solid polymer electrolytes (SPEs). In this regard, salt anions have a decisive role in the overall SPE performance. While lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was chosen earlier to have a highly flexible sulfonimide center and an extensively delocalized negative charge, it still suffers from several drawbacks ascribed to its poor interfacial compatibility with the lithium metal (LiM) anode and the fact that it is a PFAS. In this work, a novel lithium salt is cunningly designed, aiming to combine the advantages of previously reported lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(difluoromethanesulfonyl)imide (LiDFSI) to overcome the limitations of the state‐of‐the‐art SPE based on LiTFSI/poly(ethylene oxide) (PEO). The SPE containing the developed (difluoromethanesulfonyl)(fluorosulfonyl)imide (LiDFFSI) salt presented reduced interfacial resistance and improved compatibility with the lithium metal (LiM) anode compared with LiTFSI/PEO, enabled by the formation of a stable, uniform, and ionically conductive solid–electrolyte interphase (SEI). In addition, LiDFFSI‐based SPEs demonstrated a prolonged cycling stability, achieving over 125 cycles at C/10 with minimal capacity fading in LiM||LiFePO4 cell configuration. These findings evidence how a rational design of the lithium salt chemistry allows tuning the formed SEI, directly impacting the overall SPE performance. Thus, LiDFFSI is presented as a promising alternative lithium salt to improve electrochemical performance and interfacial stability in next‐generation LiM batteries.

Room temperature sphere-to-rod growth and gelation of PEO–PPO–PEO triblock copolymers in aqueous salt solutions

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.

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.

The effect of salt and temperature on the infinite dilution activity coefficients of volatile organic chemicals in water

Solid polymer electrolytes (SPEs) are regarded as promising candidates for next-generation solid-state lithium metal batteries (SSLMBs) due to their high flexibility and excellent processability, whereas their practical application remains a challenge to simultaneously achieve high ionic conductivity, superior inherent stability, and desirable interfacial compatibility with the Li metal anode. Here, we report a composite solid electrolyte (CSE) incorporating a three-dimensional zeolite network (3D Zeo) into a poly(ethylene oxide) (PEO) matrix containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt (3D Zeo/PEO). The interconnected 3D Zeo provides continuous Li+-conduction pathways and promotes the amorphization of the polymer and the dissociation of the Li salt via Lewis acid–base interactions. The resulting 3D Zeo/PEO CSE exhibits an ionic conductivity of 1.62 × 10−4 S cm−1 at room temperature and an electrochemical stability window up to 5.7 V vs. Li+/Li, significantly surpassing those of LiTFSI–PEO SPE (3.23 × 10−6 S cm−1, 4.9 V). Leveraging the regulatory effect of the 3D Zeo/PEO CSE on Li+ deposition and solid electrolyte interphase formation, the symmetric cell demonstrates stable cycling over 2300 h with an overpotential of 0.13 V, and the full cells also deliver superior high-rate capability and long-term stability with a capacity retention of 92% after 500 cycles at room temperature, far exceeding the performance of batteries using LiTFSI–PEO SPE and CSE with zeolite-based particle fillers. This work provides a promising approach for the design of CSEs to achieve high interfacial compatibility and uniform Li+ deposition for next-generation SSLMBs.

Realizing compatibility of high voltage cathode and poly (ethylene oxide) electrolyte in all-solid-state lithium batteries by bilayer electrolyte design

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

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

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.