Ternary Polymer Electrolyte Research Articles

Physical and Chemical Properties of Poly(ethylene)oxide Ternary Solid Polymer Electrolytes Based on Ni/Al Layered Double Hydroxide

Physical and Chemical Properties of Poly(ethylene)oxide Ternary Solid Polymer Electrolytes Based on Ni/Al Layered Double Hydroxide

Combining ternary, ionic liquid-based, polymer electrolytes with a single-ion conducting polymer-based interlayer for lithium metal batteries

Among the many approaches to improve the performance of lithium-metal batteries, ternary polyethylene oxide/ionic liquid/lithium salt electrolytes offer several advantages such as low flammability, high conductivity (vs. polyethylene oxide/lithium salt electrolytes) and, to a large extent, limiting the growth of dendrites at moderate currents. However, they suffer from relatively low mechanical strength for lithium metal confinement. Besides, the lithium transport numbers are very low, which is conducive to lithium depletion during plating at high current densities at the lithium/electrolyte interface. Thus, we show here that the combination of a ternary solid polymer electrolyte with a single-ion polymer-based conducting interlayer allows for a significant improvement of the cyclability of the lithium metal anode. This results in a strong improvement of the electrochemical performance of lithium-metal batteries using solid polymer electrolytes at 80 °C, with an 85% capacity retention after 350 cycles (vs. 60% after 62 cycles for the uncoated anode). This is attributed, via focused ion beam-scanning electron microscopy and X-ray photoelectron spectroscopy, to a denser lithium deposit, better contact with the electrolyte and a reduced reactivity of electrolyte species with the interlayer.

Enhanced performance of solid polymer electrolytes combining poly(vinylidene fluoride-co-hexafluoropropylene), metal-organic framework and ionic liquid for advanced solid state lithium-ion batteries

The search for sustainable and high-performance materials for lithium-ion batteries is leading to significant advances in solid polymer electrolyte (SPE) technology. However, the current drawbacks of this approach prove the need for further research and development in the field. Herein, novel ternary solid polymer electrolytes have been developed using varying loads of MOF-808 metal-organic framework and [BMIM][SCN] ionic liquid (IL) incorporated in a poly(vinylidene fluoride-co-hexafluoropropylene) matrix. The solid polymer electrolytes were evaluated at morphological, structural, thermal, mechanical and electrochemical levels, and their performance in cycling battery testing was assessed. The results showed a homogeneous structure throughout all the samples and a good dispersion of the distinct components. The polymer polar phase and degree of crystallinity of the samples are increased with increasing IL content, and the thermal and mechanical properties are appropriate for battery application. The ionic conductivity of the samples reaches maximum values of 4.68 × 10−5 S‧cm−1 at room temperature, lithium transference numbers up to 0.65, and high electrochemical stability, making them well-suited for battery applications. The assembled stability after 50 cycles at C/10 with a discharge capacity value of 150 mAh‧g−1 at room temperature was tested/derived. The obtained results show the potential of this system for high performance room temperature solid polymer electrolytes.

Ternary Solid Polymer Electrolytes at the Electrochemical Interface: A Computational Study.

Polymer-based solid-like gel electrolytes have emerged as a promising alternative to improve battery performance. However, there is a scarcity of studies on the behavior of these media at the electrochemical interface. In this work, we report classical MD simulations of ternary polymer electrolytes composed of poly(ethylene oxide), a lithium salt [lithium bis(trifluoromethanesulfonyl)imide], and different ionic liquids [1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide] confined between two charged and uncharged graphene-like surfaces. The molecular solvation of Li+ ions and their diffusion as well as the polymer conformational picture were characterized in terms of the radial distribution functions, coordination numbers, number density profiles, orientations, displacement variance, polymer radius of gyration, and polymer end-to-end distance. Our results show that the layering behavior of the ternary electrolyte in the interfacial region leads to a decrease of Li+ mobility in the direction perpendicular to the electrodes and high energy barriers that hinder lithium cations from coming into direct contact with the graphene-like surface. The nature of the ionic liquid and its concentration were found to influence the structural and dynamic properties at the electrode/electrolyte interface, the electrolyte with low amounts of the pyrrolidinium-based ionic liquid being that with the best performance since it favors the migration of Li+ cations toward the negative electrode when compared to the imidazolium-based one.

The Influence of Polyethylene Oxide Degradation in Polymer‐Based Electrolytes for NMC and Lithium Metal Batteries

A multilayered ternary solid polymer electrolyte (TSPE) is presented. First, the influence of polyethylene oxide degradation on cell failure, development of subsequent volatile degradation products, and cell impedance is analyzed. The low electrochemical stability window/oxidative stability (≥3.8 V) results in side‐chain oxidation and loss of active material. Subsequently, electrolyte stability is improved and a thin‐film (≤50 μm) TSPE with three functional layers is developed to match the wide‐ranging electrolyte requirements toward Li metal anodes and different cathode materials like LiNi0.6Mn0.2Co0.2O2 and LiFePO4 (NCM622, LFP). The high‐voltage stability of ≥4.75 V makes the TSPE a promising candidate in high‐voltage applications. Because of high Coulombic efficiencies in NMC622‖Li metal (99.7%) and LFP‖Li metal (99.9%) cells, the presented electrolyte enables stable long‐term cycling with great capacity retention of 86% and 94%, respectively. The temperature stability of >300 °C and the capability to prevent high surface area Li and dendrite formation (even at an areal capacity utilization of >40 mAh cm−2) contribute to high safety under a wide range of conditions.

Solvent‐free Ternary Polymer Electrolytes with High Ionic Conductivity for Stable Sodium‐based Batteries at Room Temperature

AbstractInvited for this month's cover are the Electrochemistry for Batteries and Beyond Li: materials & interphases groups led by Prof. Stefano Passerini and Dr. Maider Zarrabeitia, respectively, at Helmholtz Institute Ulm (HIU). The cover picture shows a solvent‐free polymer electrolyte‐based sodium‐metal battery operating at room temperature for stationary applications. Read the full text of the Research Article at 10.1002/batt.202300092.

Cover Picture: Solvent‐free Ternary Polymer Electrolytes with High Ionic Conductivity for Stable Sodium‐based Batteries at Room Temperature (Batteries & Supercaps 9/2023)

Cover Picture: Solvent‐free Ternary Polymer Electrolytes with High Ionic Conductivity for Stable Sodium‐based Batteries at Room Temperature (Batteries & Supercaps 9/2023)

High Performance Ternary Solid Polymer Electrolytes Based on High Dielectric Poly(vinylidene fluoride) Copolymers for Solid State Lithium-Ion Batteries.

Renewable energy sources require efficient energy storage systems. Lithium-ion batteries stand out among those systems, but safety and cycling stability problems still need to be improved. This can be achieved by the implementation of solid polymer electrolytes (SPE) instead of the typically used separator/electrolyte system. Thus, ternary SPEs have been developed based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene), P(VDF-TrFE-CFE) as host polymers, clinoptilolite (CPT) zeolite added to stabilize the battery cycling performance, and ionic liquids (ILs) (1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN])), 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([PMPyr][TFSI]) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), incorporated to increase the ionic conductivity. The samples were processed by doctor blade with solvent evaporation at 160 °C. The nature of the polymer matrix and fillers affect the morphology and mechanical properties of the samples and play an important role in electrochemical parameters such as ionic conductivity value, electrochemical window stability, and lithium-transference number. The best ionic conductivity (4.2 × 10-5 S cm-1) and lithium transference number (0.59) were obtained for the PVDF-HFP-CPT-[PMPyr][TFSI] sample. Charge-discharge battery tests at C/10 showed excellent battery performance with values of 150 mAh g-1 after 50 cycles, regardless of the polymer matrix and IL used. In the rate performance tests, the best SPE was the one based on the P(VDF-TrFE-CFE) host polymer, with a discharge value at C-rate of 98.7 mAh g-1, as it promoted ionic dissociation. This study proves for the first time the suitability of P(VDF-TrFE-CFE) as SPE in lithium-ion batteries, showing the relevance of the proper selection of the polymer matrix, IL type, and lithium salt in the formulation of the ternary SPE, in order to optimize solid-state battery performance. In particular, the enhancement of the ionic conductivity provided by the IL and the effect of the high dielectric constant polymer P(VDF-TrFE-CFE) in improving battery cyclability in a wide range of discharge rates must be highlighted.

Solvent‐free Ternary Polymer Electrolytes with High Ionic Conductivity for Stable Sodium‐based Batteries at Room Temperature

AbstractTransitioning to solid‐state batteries using polymer electrolytes results in inherently safer devices and can facilitate the use of sodium metal anodes enabling higher energy densities. In this work, solvent‐free ternary polymer electrolytes based on cross‐linked polyethylene oxide (PEO), sodium bis(fluorosulfonyl) imide (NaFSI) or sodium bis(trifluoromethanesulfonyl) imide (NaTFSI) and N‐butyl‐N‐methyl‐pyrrolidinium‐based ionic liquids (ILs, Pyr14FSI or Pyr14TFSI) are developed. Synthesized polymer membranes are thoroughly characterized, verifying their good thermal and electrochemical stability, as well as a low glass transition and crystallinity, thus high segmental mobility of the polymer matrix. The latter results in good ionic conductivities around 1×10−3 S cm−1 at 20 °C. The polymer electrolytes are successfully employed in sodium‐metal battery (SMB) cells operating at room temperature (RT) and using P2‐Na2/3Ni1/3Mn2/3O2 layered oxide as cathode. The electrochemical performance strongly depends on the choice of anion in the conducting sodium salt and plasticizing IL. Furthermore, this solid‐state SMB approach mitigates capacity fading drivers for the P2‐Na2/3Ni1/3Mn2/3O2, resulting in high Coulombic efficiency (99.91 %) and high capacity retention (99 % after 100 cycles) with good specific capacity (140 mAh g−1).

Influence of ionic liquid characteristics on the performance of ternary solid polymer electrolytes with poly(vinylidene fluoride-co-hexafluoropropylene) and zeolite

Solid polymer electrolytes (SPEs) are the necessary step towards solid-state lithium-ion batteries (LIBs). Their function as separator and electrolyte allows to increase the safety of energy storage devices by the elimination of the liquid components. In this work, three-component SPEs based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as host polymer, clinoptilolite zeolite, and different ionic liquids (ILs) (1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]), 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]), 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][NCN2]) and 1-butyl-3-methylimidazolium dicyanamide ([BMIM][NCN2]) were produced and characterized. The influence of the nature of the IL anion and cation on the morphology, degree of crystallinity, mechanical properties, ionic conductivity, and battery cycling performance were studied. It is demonstrated that the [SCN]- anion is the most suitable if SPE applications are envisaged. The ionic conductivity depends on the IL type. At room temperature, a maximum value of 1.3 × 10−5 S cm−1 was obtained for the SPE doped with [BMIM][NCN2]. Regarding battery performance, the best value of discharge capacity was observed for the SPE based on [BMIM][SCN], which for an initial discharge capacity of 135 mAh.g−1 yielded a capacity loss below 30% after 30 cycles at room temperature. Thus, it is concluded that proper selection of the IL (cation chain length and anion size) allows tailoring battery performance of solid-state batteries based on three-component SPEs.

Single‐ion conducting polymer as lithium salt additive in polymerized ionic liquid block copolymer electrolyte

AbstractHerein, we describe the use of single‐ion conducting block copolymer (SIC) as an additional lithium salt additive to a ternary solid polymer electrolyte (SPE), consisting of a poly(styrene‐b‐1‐((2‐acryloyloxy)ethyl)‐3‐butylimidazolium bis(trifluoromethanesulfo‐nyl)imide) (S‐ImTFSI64‐16) block copolymer, a N‐propyl‐N‐methylpyrrolidinium bis(fluorosulfonyl)imide (C3mpyrFSI) ionic liquid (IL) and a lithium bis(fluorosulfonyl) imide (LiFSI) salt. For this purpose, the S‐ImTFSI64‐16 was substituted by a SIC, based on poly(styrene‐b‐((4‐styrenesulfonyl)(trifluoromethanesulfonyl)imide lithium salt)) (S‐STFSILi64‐16), at various molar ratios. The impact of the SIC concentration on the phase behavior and transport properties of the SPEs was investigated by means of differential scanning calorimetry, electrochemical impedance spectroscopy, and diffusion NMR. In addition, the electrochemical performance of the SPEs was assessed in lithium symmetrical cell at 50 and 80°C. Finally, the cycling performance of a selected SPE was also assessed at 80°C in a Li│NMC111 cell with capacity loading of 1.3 mAh.cm−2 at a C‐rate of 0.1 C. The Li│NMC111 full cell was able to deliver a stable capacity of 0.94 mAh.cm−2 after 20 cycles, corresponding to a capacity of 117 mAh.g−1. These results demonstrates that PIL block copolymer—IL—salt composites represent a promising choice of electrolyte for the next generation of solid‐state high energy density lithium metal batteries.

An ionic liquid- and PEO-based ternary polymer electrolyte for lithium metal batteries: an advanced processing solvent-free approach for solid electrolyte processing.

A processing solvent-free manufacturing process for cross-linked ternary solid polymer electrolytes (TSPEs) is presented. Ternary electrolytes (PEODA, Pyr14TFSI, LiTFSI) with high ionic conductivities of >1 mS cm-1 are obtained. It is shown that an increased LiTFSI content in the formulation (10 wt% to 30 wt%) decreases the risk of short-circuits by HSAL significantly. The practical areal capacity increases by more than a factor of 20 from 0.42 mA h cm-2 to 8.80 mA h cm-2 before a short-circuit occurs. With increasing Pyr14TFSI content, the temperature dependency of the ionic conductivity changes from Vogel-Fulcher-Tammann to Arrhenius behavior, leading to activation energies for the ion conduction of 0.23 eV. In addition, high Coulombic efficiencies of 93% in Cu‖Li cells and limiting current densities of 0.46 mA cm-2 in Li‖Li cells were obtained. Due to a temperature stability of >300 °C the electrolyte guarantees high safety in a broad window of conditions. In LFP‖Li cells, a high discharge capacity of 150 mA h g-1 after 100 cycles at 60 °C was achieved.

Highly-concentrated bis(fluorosulfonyl)imide-based ternary gel polymer electrolytes for high-voltage lithium metal batteries

Polymer electrolytes (PEs) have arisen as the most promising candidates for being used in rechargeable lithium metal batteries (LMBs). However, classic gel polymer electrolytes (GPEs) with volatile plasticizers show poor inherent safety, despite their higher ionic conductivities vs. dry PE systems, which largely hampers the practical deployment of GPE-based LMBs. Herein, we report the electrochemical performances of a highly-concentrated bis(fluorosulfonyl)imide (FSI)-based ternary gel polymer electrolyte (TGPE) comprising a plasticized polymerized ionic liquid as polymer scaffold. The as-prepared FSI-TGPEs show high lithium-ion conductivities at room temperature, wide electrochemical stability both at oxidative and reductive potentials, and superior compatibility with lithium metal electrode. These unique properties of FSI-TGPE enable stable Li plating and stripping processes, as well as high discharge capacity of a Li||Li(Ni0.6Mn0.2Co0.2)O2 cell at ambient temperatures. The present work offers not only safe and highly conductive PEs but also a better understanding on the determining role of FSI− anions in high-energy LMBs.

Coordinating Anions “to the Rescue” of the Lithium Ion Mobility in Ternary Solid Polymer Electrolytes Plasticized With Ionic Liquids

AbstractLithium salts with low coordinating anions such as bis(trifluoromethanesulfonyl)imide (TFSI) have been the state‐of‐the‐art for polyethylene oxide (PEO)‐based “dry” polymer electrolytes for 3 decades. Plasticizing PEO with TFSI‐based ionic liquids (ILs) to form ternary solid polymer electrolytes (TSPEs) increases conductivity and Li+ diffusivity. However, the Li+ transport mechanism is unaffected compared to their “dry” counterparts and is essentially coupled to the dynamics of the polymer host matrix, which limits Li+ transport improvement. Thus, a paradigm shift is hereby suggested: the utilization of more coordinating anions such as trifluoromethanesulfonyl‐N‐cyanoamide (TFSAM), able to compete with PEO for Li+ solvation, to accelerate the Li+ transport and reach a higher Li+ transference number. The Li–TFSAM interaction in binary and ternary TFSAM‐based electrolytes is probed by experimental methods and discussed in the context of recent computational results. In PEO‐based TSPEs, TFSAM drastically accelerates the Li+ transport (increases Li+ transference number by a factor 6 and the Li+ conductivity by 2–3) and computer simulations reveal that lithium dynamics are effectively re‐coupled from polymer to anion dynamics. Last, this concept of coordinating anions in TSPEs is successfully applied in LFP||Li metal cells leading to enhanced capacity retention (86% after 300 cycles) and an improved rate performance at 2C.

Revealing the Anion Chemistry Effect on Transport Properties of Ternary Gel Polymer Electrolytes

Revealing the Anion Chemistry Effect on Transport Properties of Ternary Gel Polymer Electrolytes

Polymer‐Induced Inversion of the Li+ Drift Direction in Ionic Liquid‐Based Ternary Polymer Electrolytes

AbstractWhile salt‐in‐ionic liquid (IL) electrolytes provide a promising alternative to organic electrolytes, they suffer from large ionic clusters impeding lithium ion transport. Here, a lithium‐coordinating polymer is added and the transition from salt‐in‐IL to IL‐based ternary polymer gel electrolytes is followed, investigating the influence of the composition on Li transport mechanisms. The electrolyte is composed of the salt lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), the IL 1‐butyl‐1‐methylpyrrolidinium (Pyr14) TFSA, and a variable amount of poly(ethylene oxide) (PEO). Impedance spectroscopy, multinuclear (1H, 7Li, and 19F) diffusion NMR and electrophoretic NMR (eNMR) yield electrophoretic mobilities, transference numbers, partial conductivities, and effective charges of all ionic species. As the polymer fraction increases, the lithium ion drift direction is inverted. The sign reversal of Li+ mobility and transference number occurs at the optimum coordination of 5–6 ethylene oxide units per Li+ ion. The dominant Li+ transport mechanism shifts from a vehicular transport via anionic clusters to chain‐dominated transport mechanisms. It is concluded that the lithium transport properties of IL‐based electrolytes can be vastly improved by the addition of PEO. Tuning the composition of the ternary electrolyte reveals optimal Li+ transport properties at moderately high polymer contents as well as high salt concentrations.

Ionic liquid plasticizers comprising solvating cations for lithium metal polymer batteries

Ternary solid polymer electrolytes (TSPEs) with ionic liquids (ILs) including alkyl-based ammonium cations and low coordinating anions suffer from the lack of Li+ ion coordination by the ILs compared to the immobile polymer backbone, in terms of Li+ ion transport. Thus, solvating ionic liquids (SILs) with an oligo(ethylene oxide) side chain attached onto the cation were prepared to improve the interaction between Li+ and the IL and accelerate Li+ transport in TSPEs. A variety of methods, such as pulsed field gradient nuclear magnetic resonance spectroscopy, Li metal plating/stripping and measurements of Sand's times were used to show that Li+ ion transference numbers increase with the oligo(ethylene oxide) side chain length in SIL-based TSPEs, which results in faster Li+ ion transport and translates into much slower lithium depletion at a given current, thereby delaying the onset of fast dendrite growth of lithium metal.

Beyond fluorine: sustainable ternary polymer electrolytes for lithium batteries

A non-fluorinated solid polymer electrolyte including an ionic liquid has achieved a higher transference number and similar lithium transport as compared to its fluorinated analog and allows the cycling of LiFePO4||Li metal cells for 300 cycles.

Interfacial Structures in Ionic Liquid-Based Ternary Electrolytes for Lithium-Metal Batteries: A Molecular Dynamics Study

Lithium-metal batteries are promising candidates to fulfill the future performance requirements for energy storage applications. However, the tendency to form metallic dendrites and the undesirable side reactions between the electrolyte and the Li electrode lead to poor performance and safety issues in these batteries. Therefore, understanding the interfacial properties and the Li-metal surface/electrolyte interactions is crucial to resolve the remaining obstacles and make these devices feasible. Here, we report a computational study on the interface effects in ternary polymer electrolytes composed by poly(ethylene oxide) (PEO), lithium salts, and different ionic liquids (ILs) confined between two Li-metal slabs. Atomistic simulations are used to characterize the local environment of the Li+ ions and the transport properties in the bulk and at the interface regions. Aggregation of ions at the metal surface is seen in all investigated systems; the structure and composition are directly correlated to the IL components. The strong interactions between the electrolyte species and the Li-metal atoms result in the structuring of the electrolyte at the interface region, in which comparatively small and flat ions result in a well-defined region with extensive Li+ populations and high self-diffusion coefficients. In contrast, large ions such as [P222mom]+ increase the PEO density in the bulk due to large steric effects at the interface. Therefore, the choice of specific ILs in ternary polymer electrolytes can tune the structure-dynamic properties at the Li-metal surface/electrolyte interface, controlling the SEI formation at the electrode surface, and thereby improve battery performance.

Influence of ionic interactions on lithium diffusion properties in ionic liquid-based gel polymer electrolytes

In this study, we characterize a ternary gel polymer electrolyte (GPE) composed of bisphenol A ethoxylate dimethacrylate (BEMA) and N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr13TFSI) ionic liquid and we particularly study the influence of salt content and polymer network composition. The effect of salt concentration in liquid solutions (1-x)Pyr13TFSI-xLiTFSI (molar ratio R = x/(1-x)) is finely characterized by Arrhenius measurements, PFG-NMR and Raman spectroscopy. They highlight ions interactions evolution with the salt content. For lower salt concentrations, ions pairs with bidentate coordinations [Li(TFSI)2]- are preferentially formed. A salt content rise leads to the formation of ionic aggregates where several Li+ are bound by TFSI− to form [Liz(TFSI)y](z-y) large clusters. These ionic arrangements changes modify the mean charge of the complexes involving Li. It leads to better cycling performances with higher salt concentration contrary to the ionic conductivity evolution. When the binary solution is added to the GPE, a dual transport mechanism, by diffusion in the liquid phase and hops assisted by segmental motion, is confirmed. The Li transference number is improved with success by enhancing this segmental motion with the addition of a linear polymer (poly(ethylene glycol) methyl ether methacrylate, me-PEGMA). Cycling with 70% of theoretical capacity and improved high current ability is achievable at 25 °C with a dual me-PEGMA/BEMA polymer network.