Ion transport in polycarbonate based solid polymer electrolytes: experimental and computational investigations

Mindemark J., Sun B., Törmä E., Brandell D.

Incorporation of carbonate repeating units in a poly(e-caprolactone) (PCL) backbone used as a host material in solid polymer electrolytes is found to not only suppress crystallinity in the polyester material, but also give higher ionic conductivity in a wide temperature range exceeding the melting point of PCL crystallites. Combined with high cation transference numbers, this electrolyte material has sufficient lithium transport properties to be used in battery cells that are operational at temperatures down to below 23 °C, thus clearly demonstrating the potential of using non-polyether electrolytes in high-performance all-solid lithium polymer batteries.

Kimura K., Matsumoto H., Hassoun J., Panero S., Scrosati B., Tominaga Y.

Poly(ethylene carbonate) (PEC) is known as an alternating copolymer derived from carbon dioxide (CO 2 ) and an epoxide as monomers. Here, we describe a new quaternary PEC-based composite electrolyte containing lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt, N - n -butyl- N -methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr 14 TFSI) ionic liquid, and an electrospun silica (SiO 2 ) fiber (SiF) with a submicron diameter in view of its possible applications in solid-state Li polymer batteries. A free-standing electrolyte membrane is prepared by a solvent casting method. The Pyr 14 TFSI ionic liquid enhances the ionic conductivity of the electrolyte as a result of its plasticizing effect. The electrochemical properties, such as ionic conductivity and Li transference number ( t Li+ ), as well as mechanical strength of the electrolyte, are further improved by the SiF. We show that the quaternary electrolyte has a conductivity of the order of 10 −7  S cm −1 at ambient temperature and a high t Li+ value of 0.36 with an excellent flexibility. A prototype Li polymer cell using LiFePO 4 as a cathode material is assembled and tested. We demonstrate that this battery delivers a reversible charge-discharge capacity close to 100 mAh g −1 at 75 °C and C/15 rate. We believe that this work may pave the road to utilize CO 2 as a carbon source for highly-demanded, functional battery materials in future.

Costa L.T., Sun B., Jeschull F., Brandell D.

This paper presents atomistic molecular dynamics simulation studies of lithium bis(trifluoromethane)sulfonylimide (LiTFSI) in a blend of 1-ethyl-3-methylimidazolium (EMIm)-TFSI and poly(ethylene oxide) (PEO), which is a promising electrolyte material for Li- and Li-ion batteries. Simulations of 100 ns were performed for temperatures between 303 K and 423 K, for a Li:ether oxygen ratio of 1:16, and for PEO chains with 26 EO repeating units. Li+ coordination and transportation were studied in the ternary electrolyte system, i.e., PEO16LiTFSI⋅1.0 EMImTFSI, by applying three different force field models and are here compared to relevant simulation and experimental data. The force fields generated significantly different results, where a scaled charge model displayed the most reasonable comparisons with previous work and overall consistency. It is generally seen that the Li cations are primarily coordinated to polymer chains and less coupled to TFSI anion. The addition of EMImTFSI in the electrolyte system enhances Li diffusion, associated to the enhanced TFSI dynamics observed when increasing the overall TFSI anion concentration in the polymer matrix.

Mindemark J., Törmä E., Sun B., Brandell D.

Random copolymers of trimethylene carbonate (TMC) and e-caprolactone (CL) were synthesized through bulk ring-opening polymerization for use as host materials for solid polymer electrolytes. Amorphous electrolytes were solution-cast from the copolymers together with LiTFSI salt and showed lower Tg and higher ionic conductivity as the CL content was increased. The best-performing electrolyte, with a TMC:CL ratio of 60:40 with 28 wt% of LiTFSI, was found to have a conductivity of 1.6 × 10−5 S cm−1 at 60 °C (7.9 × 10−7 S cm−1 at 25 °C) and a Tg of −26 °C. This electrolyte was used in all-solid-state LiFePO4 half-cells that showed high capacity and coulombic efficiency at rates up to and including C/5.

Sun B., Mindemark J., Edström K., Brandell D.

This work describes effective approaches to achieve high cell performance of solid-state Li polymer batteries based on high-molecular-weight poly(trimethylene carbonate) (PTMC). The origin of a gradual capacity increase observed during passive storage and/or active cycling in LiFePO 4 |PTMC x LiTFSI|Li cells was investigated by SEM/EDX, indicating an obvious penetration of the polymer electrolyte through the porous composite electrode at elevated temperatures. Refining the interfacial contacts at the electrode/electrolyte interface by adding PTMC oligomer as an interfacial mediator led to significant capacity enhancement already during initial cycles. Optimized cell performance was achieved through this method rather than other approaches, such as casting electrolyte directly onto the electrode and using a polyether oligomer. Successful long-term cycling stability and rate capability tests also resulted from the suggested strategy.

Wang Y., Sokolov A.P.

Despite potential significant advantages of polymer based batteries, the poor ionic conductivity of dry polymer electrolytes at ambient and low temperatures has limited their application. This review describes the approach for improving conductivity by decoupling ionic transport from polymer segmental relaxation. It is emphasized that the decoupling approach is the key for design of superionic polymer electrolytes.

Tominaga Y., Yamazaki K., Nanthana V.

Poly(ethylene carbonate)-based polymer electrolytes with lithium salts (LiX; X=TFSI, ClO4, BF4 and PF6) were prepared and measured their lithium transference numbers (t +) for the comparison between different anion radius and salt concentrations. The LiTFSI electrolytes showed highest t + and Li-ion conductivities of all samples at 80 oC, and these values increased with increasing salt concentration. From the results of FT-IR measurements for all concentrated samples, it was revealed that the changes of a band fraction divided at around 1720 cm-1 for interacted carbonyl groups with Li+ (C=O --- Li+) strongly relate to the mobility of Li+.

Xu K.

Sun B., Mindemark J., Edström K., Brandell D.

Pressing demands for high power and high energy densities in novel electrical energy storage units have caused reconsiderations regarding both the choice of battery chemistry and design. Practical concerns originating in the conventional use of flammable liquid electrolytes have renewed the interests of using solvent-free polymer electrolytes (SPEs) as solid ionic conductors for safer batteries.In this thesis work, SPEs developed from two polymer host structures, polyethers and polycarbonates, have been investigated for all-solid-state Li- and Li-ion battery applications. In the first part, functional polyether-based polymer electrolytes, such as poly(propylene glycol) triamine based oligomer and poly(propylene oxide)-based acrylates, were investigated for 3D-microbattery applications. The amine end-groups were favorable for forming conformal electrolyte coatings onto 3D electrodes via self-assembly. In-situ polymerization methods such as UV-initiated and electro-initiated polymerization techniques also showed potential to deposit uniform and conformal polymer coatings with thicknesses down to nano-dimensions.Moreover, poly(trimethylene carbonate) (PTMC), an alternative to the commonly investigated polyether host materials, was synthesized for SPE applications and showed promising functionality as battery electrolyte. High-molecular-weight PTMC was first applied in LiFePO4-based batteries. By incorporating an oligomeric PTMC as an interfacial mediator, enhanced surface contacts at the electrode/SPE interfaces and obvious improvements in initial capacities were realized. In addition, room-temperature functionality of PTMC-based SPEs was explored through copolymerization of e-caprolactone (CL) with TMC. Stable cycling performance at ambient temperatures was confirmed in P(TMC/CL)-based LiFePO4 half cells (e.g., around 80 and 150 mAh g-1 at 22 °C and 40 °C under C/20 rate, respectively). Through functionalization, hydroxyl-capped PTMC demonstrated good surface adhesion to metal oxides and was applied on non-planar electrodes. Ionic transport behavior in polycarbonate-SPEs was examined by both experimental and computational approaches. A coupling of Li ion transport with the polymer chain motions was demonstrated.The final part of this work has been focused on exploring the key characteristics of the electrode/SPE interfacial chemistry using PEO and PTMC host materials, respectively. X-ray photoelectron spectroscopy (XPS) was used to get insights on the compositions of the interphase layers in both graphite and LiFePO4 half cells.

Bernin D., Koch V., Nydén M., Topgaard D.

The ability of lyotropic liquid crystals to form intricate structures on a range of length scales can be utilized for the synthesis of structurally complex inorganic materials, as well as in devices for controlled drug delivery. Here we employ magnetic resonance imaging (MRI) for non-invasive characterization of nano-, micro-, and millimeter scale structures in liquid crystals. The structure is mirrored in the translational and rotational motion of the water, which we assess by measuring spatially resolved self-diffusion tensors and spectra. Our approach differs from previous works in that the MRI parameters are mapped with spatial resolution in all three dimensions, thus allowing for detailed studies of liquid crystals with complex millimeter-scale morphologies that are stable on the measurement time-scale of 10 hours. The data conveys information on the nanometer-scale structure of the liquid crystalline phase, while the combination of diffusion and data permits an estimate of the orientational distribution of micrometer-scale anisotropic domains. We study lamellar phases consisting of the nonionic surfactant C10E3 in O, and follow their structural equilibration after a temperature jump and the cessation of shear. Our experimental approach may be useful for detailed characterization of liquid crystalline materials with structures on multiple length scales, as well as for studying the mechanisms of phase transitions.

Tominaga Y., Yamazaki K.

We have found remarkable ion-conductive properties in a novel polymer electrolyte composed of poly(ethylene carbonate) and Li bis-(fluorosulfonyl) imide. The self-diffusion coefficient of Li-ions exceeded 10(-7) cm(2) s(-1) and the Li transference number was estimated to be more than 0.8 in composites filled with only 1 wt% of TiO2 nanoparticles.

Hongyou K., Hattori T., Nagai Y., Tanaka T., Nii H., Shoda K.

Solvation/desolvation and the solid electrolyte interphase (SEI) formation at a graphite electrode during the initial charging process were investigated using in situ Fourier transform infrared spectroscopy (FTIR) measurements. These measurements were developed by applying a diamond attenuated total reflectance (ATR) crystal, which probed the electrolyte solvents at the surface of the graphite electrode and provided successive FTIR spectra with high signal-to-noise ratio. The charging process was performed in the Li(reference)/electrolyte/graphite(working)/Cu cell at a voltage ranging from 3.2 to 0.0001 V vs. Li/Li+. The measurement elucidated the change in the chemical bond of the electrolyte solvents. In an early stage, the amounts of solvated and desolvated solvents changed, providing evidence that the Li+ ions were intercalated into the graphite layer. The formation of the Li alkyl carbonate that forms the SEI layer was facilitated toward the end of the charging process. Measurements were also obtained of the electrolyte with a vinylene carbonate additive, and the contribution of the additive to the electrolyte solvent reduction was investigated.

Hiller M.M., Joost M., Gores H.J., Passerini S., Wiemhöfer H.-.

Lithium transference numbers, salt diffusion coefficients and effective lithium conductivities are investigated with the steady state polarization method for poly(ethylene oxide) (PEO) based polymer electrolytes containing lithium bis(oxalato)borate (LiBOB) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). We compare the commonly used evaluation technique with our approach. Here, the focus is laid on the potential relaxation of the polarized sample which delivers information about the salt diffusion and the potential distribution in symmetric Li/electrolyte/Li cell. The new approach circumvents the well-known initial current issue and thus increases the reliability of the experiment. The temperature dependence of the lithium transference numbers and the importance of data acquisition time are also discussed. The highest lithium transference numbers are 0.19 for P(EO) 60 ·LiTFSI at 90 °C.

Do C., Lunkenheimer P., Diddens D., Götz M., Weiß M., Loidl A., Sun X., Allgaier J., Ohl M.

The dynamics of Li(+) transport in polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl)imde mixtures are investigated by combining neutron spin-echo (NSE) and dielectric spectroscopy with molecular dynamics (MD) simulations. The results are summarized in a relaxation time map covering wide ranges of temperature and time. The temperature dependence of the dc conductivity and the dielectric α relaxation time is found to be identical, indicating a strong coupling between both. The relaxation times obtained from the NSE measurements at 0.05 Å(-1)

Stolwijk N.A., Kösters J., Wiencierz M., Schönhoff M.

The degree of ion association in polymer electrolytes is often characterized by the Nernst–Einstein deviation parameter Δ, which quantifies the relative difference between the true ionic conductivity directly measured by electrical methods and the hypothetical maximum conductivity calculated from the individual ionic self-diffusion coefficients. Despite its unambiguous definition, the parameter Δ is a global quantity with limited explanatory power. Similar is true for the cation transport number tcat*, which relies on the same ionic diffusion coefficients usually measured by nuclear magnetic resonance or radiotracer methods. Particularly in cases when neutral ion pairs dominate over higher-order aggregates, more specific information can be extracted from the same body of experimental data that is used for the calculation of Δ and tcat*. This information concerns the pair contributions to the diffusion coefficient of cations and anions. Also the true cation transference number based on charged species only can be deduced. We present the basic theoretical framework and some pertinent examples dealing with ion pairing in polymer electrolytes.

Farina C., Bernard L., Landa M., Leconte N., Picard L.

Ji J., Yan S., Zhou Z., Gu Y., Liu C., Yang S., Wang D., Xue Y., Tang C.

Gudla H., Hockmann A., Brandell D., Mindemark J.

Sharma S.

Elbouazzaoui K., Mahun A., Shabikova V., Rubatat L., Edström K., Mindemark J., Brandell D.

AbstractPoor ionic conductivity, low Li+ transference number, and limited electrochemical stability plague all‐solid‐state Li‐metal batteries based on solid polymer electrolytes (SPEs). One strategy to overcome these hurdles is the insertion of ceramic fillers to generate composite polymer electrolytes (CPEs). These are based either on active (ion‐conductive) fillers like Li7La3Zr2O12 or passive (non‐conductive) fillers like Al2O3. In this work, the effect of passive Li‐containing fillers is showcased, exemplified by a CPE platform of poly(trimethylene carbonate) (PTMC:LiTFSI) with LiAlO2 particles. The inclusion of such fillers shows a strikingly positive effect. The ionic conductivity is greatly improved by one order of magnitude at 20 wt% of LiAlO2 compared to the pristine PTMC SPE. Moreover, the Li+ transference number is significantly boosted and reaches values close to unity (T + = 0.97 at 20 wt% of LiAlO2), effectively rendering the material a single‐ion conductor. The CPEs show outstanding cycling stability vs Li‐metal, and electrochemical stability of up to 5 V vs Li+/Li. When implemented in a solid‐state battery cell with LiNi0.33Mn0.33Co0.33O2 (NMC111) and Li‐metal, a stable cycling performance for over 100 cycles is observed. This demonstrates the potential of using microsized and cost‐effective LiAlO2 fillers in CPEs for applications in all‐solid‐state Li‐metal batteries.

Lin Z., Li Y., Ding P., Lin C., Chen F., Yu R., Xia Y.

ABSTRACTPolymer electrolytes (PEs) compatible with NCM cathodes in solid‐state lithium metal batteries (SSLMBs) are gaining recognition as key candidates for advanced electrochemical storage, offering significant safety and stability. Nevertheless, the inherent properties of PEs and interactions at the interface with NCM cathodes are pivotal in influencing SSLMBs' overall performance. This review offers an in‐depth examination of PEs, focusing on design strategies that leverage electron‐group electronegativity for molecular structure adjustments. Furthermore, it delves into the challenges presented by the interface between PEs and NCM cathodes, including issues like poor interface contact, interface reactions, and elevated resistance. The review also discusses a range of strategies aimed at stabilizing these interfaces, such as applying surface coatings to NCM, optimizing the structure of PEs, and employing in situ polymerization techniques to improve compatibility and battery efficiency. The conclusion offers insights into future developments, highlighting the importance of electron‐group optimization and the adoption of effective methods to enhance interface stability and contact, thus advancing the practical implementation of high‐performance SSLMBs.

Mishra A.K., Parmar J., Mukhopadhyay I.

Julien C., Mauger A.

This chapter is a review of polymers used either as electrolytes in lithium batteries or in the form of films at the electrode interface. It includes polymers with various organic and inorganic cores, PILs, and LCs. The role of different composites obtained through blendingBlending, graftingGrafting, cross-linkingCross-linking copolymerizationCopolymerization is studied, as well as that of structure (branched, cyclic, chain architecture effects). The focus is on the state-of-the-art inBatteriesall-solid-state all-solid-state batteriesAll-solid-state batteries. Nevertheless, some notable results are also mentioned for lithium batteries equipped with gel electrolytes, when the amount of liquid added to the polymer (and duly mentioned) is so low that it does not compromise safety. The effects of various components related to polymers on battery performance are also detailed, such as the introduction of plasticizersPlasticizers, fillers, and the choice of lithium salts.

Emilsson S., Albuquerque M., Öberg P., Brandell D., Johansson M.

Andersson R., Emilsson S., Hernández G., Johansson M., Mindemark J.

AbstractIn the development of polymer electrolytes, the understanding of the complex interplay of factors that affect ion transport is of importance. In this study, the strongly coordinating and flexible poly (ethylene oxide) (PEO) is compared to the weakly coordinating and stiff poly (trimethylene carbonate) (PTMC) as opposing model systems. The effect of molecular weight (Mn) and end group chemistry on the physical properties: glass transition temperature (Tg) and viscosity (η) and ion transport properties: transference number (T+), ion coordination strength and ionic conductivities were investigated. The cation transference number (T+) showed the opposite dependence on Mn for PEO and PTMC, decreasing at low Mn for PTMC and increasing for PEO. This was shown to be highly dependent on the ion coordination strength of the system regardless of whether the end group was OH or if the chains were end‐capped. Although the coordination is mainly of the cations in the systems, the differences in T+ were due to differences in anion rather than cation conductivity, with a similar Li+ conductivity across the polymer series when accounting for the differences in segmental mobility.

Zhour K., Heuer A., Diddens D.

Tawalbeh M., Ali A., Aljawrneh B., Al-Othman A.

Sodium ion batteries (SIBs) have resurfaced into the spotlight, given the supply chain uncertainties and the soaring demand for lithium-ion batteries (LIBs). Although, even now, their lower energy density may stall their commercialization in the portable sector, they are considered prime candidates for large scale electrochemical energy storage applications. Accordingly, advancing, establishing, and maintaining the safety of SIBs is crucial to prevent catastrophic thermal runaways and colossal financial losses to garner the trust of concerned authorities. Electrolytes play a pivotal role in the safety of batteries. Considering the above, this paper presents a comprehensive review of the progress in safe electrolytes for SIBs. It explains the various approaches employed to enhance the safety of high-risk based electrolytes and the electrochemical performance of intrinsically safe electrolytes. Moreover, a state-of-the-art review of the assembled cells/half cells employing different classes of electrolytes is also presented. Particular attention has been devoted to specifying the techniques and results, if available, of thermal stability and safety tests besides highlighting the electrochemical characteristics and performance, such as the cell capacity and cyclability, and electrolyte ionic conductivity and electrochemical stability window (ESW) of the electrolyte. Finally, challenges and future research directions have been summarized and recommended. This review concludes that solid state electrolytes with high conductivity are among the practical and safe electrolytes for SIBs.

Foran G.Y., St‐Antoine C., Lepage D., Cui M., Zheng R., Prébé A., Goward G.R., Dollé M.

AbstractSolid Polymer electrolytes are versatile, highly processible and electrochemically compatible with solid electrode materials. The versatility of these materials is a result of the existence of many possible conductive polymer‐salt, polymer‐polymer and salt‐salt combinations. Despite the wide array of available lithium salts, most polymer electrolyte materials are made using lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) due to its long history of achieving relatively high ionic conductivities in polymer electrolyte systems with the most famous being poly(ethylene oxide) (PEO). It is however possible that better ionic conductivities can be achieved with different salts and/or in polymer matrices containing different functional groups. This is because ionic conductivity in polymer electrolytes is partially based on the ability of the polymer matrix to dissolve and bond to the salt. These interactions impact local‐scale ion mobility which can be measured via NMR spectroscopy using pulsed field gradient experiments. In this work, polymer electrolytes are prepared using PEO, hydrogenated nitrile butadiene rubber and poly(propylene) carbonate. Ion mobility, lithium conductivity and salt‐polymer interactions are investigated to compare interactions between LiTFSI and lithium cyano(trifluorosulfonyl)imide in polymers with common salt‐dissociating functional groups such as ethyl, nitrile and carbonate to determine the impact of these interactions on ionic conductivity.

Jayanthi S., Parangusan H., babu A., Balakrishnan S., Ponnamma D.

AbstractFree standing nanocomposite polymer electrolytes (NCPEs) based on the polymer host poly(vinyl) chloride (PVC) were successfully prepared using the solution casting technique. Lithium nitrate (LiNO3) and nano-sized silica (SiO2) (< 100 nm) were employed as the electrolyte and filler, respectively. Impedance studies revealed a maximum ionic conductivity value of 1.226 × 10−4 S/cm at room temperature for the PVC/LiNO3 with 5 wt.% nano-SiO2. X-ray diffraction (XRD) analysis verified the sample’s amorphous nature. Dielectric permittivity and relaxation time values were consistent with impedance results. Additionally, parameters such as diffusion coefficient, mobile concentration, and mobility were evaluated for the prepared samples. Differential scanning calorimetry (DSC) studies confirmed a change in glass transition temperature (Tg) of PVC/LiNO3/SiO2 sample. The scanning electron micrograph (SEM) images revealed a honeycomb morphology, indicating ease of Li+ ion transportation.

Samsudin A.S., Ghazali N.M., Mazuki N.F., Aoki K., Nagao Y.

This study presents the synthesis and characterization of solid polymer blend electrolytes (SPBEs) using alginate (Alg) and polyvinyl alcohol (PVA) as host polymers, incorporating lithium bis(trimethanesulfonyl)imide (LiTFSI) as the ion-providing salt for potential application in EDLCs. The surface morphology of the SPBEs was revealed using scanning electron microscopy (SEM), while thermal gravimetric analysis (TGA) demonstrates enhanced thermal stability, characterized by reduced weight loss and a shift toward higher decomposition temperature. Complexation between Alg-PVA and LiTFSI was indicated by Fourier-transform infrared spectroscopy (FTIR), as evident by the transitions and intensity changes in FTIR bands corresponding to functional groups. Increasing LiTFSI content reduces bulk resistance, with Alg-PVA containing 20 wt% LiTFSI (Li-20) showing maximum room temperature ionic conductivity (3.31 × 10-4 S cm−1) and the lowest activation energy (0.05 eV). Transport properties, analyzed using the Arof-Noor (A-N) method, reveal that ionic conductivity in SPBEs is governed by ionic mobility and ions' diffusion coefficient. Sample Li-20 displays predominantly ionic transport with a transference number (tion) 0.98 and electrochemical stability up to 2.55 V. The EDLC, employing activated carbon electrodes and the most conductive electrolyte, demonstrates notable performance features, including specific capacitance (87.51F/g at 2 mV/s, assessed from CV), energy density (25.17 Wh kg−1), and power density (1038.92 W kg−1). Testing at various current densities reveals the highest specific capacitance values associated with the lowest current density, measuring 51.15F/g for the EDLC cell based on the Li-20 sample.