Solvent-free Electrolytes Research Articles

Molten Salt Based High Entropy Electrolytes for Calcium Batteries

Calcium (Ca) is in principle a very attractive metal anode for rechargeable batteries due to the large abundance of calcium in the Earth’s crust, its low electrochemical potential, and its low cost1. Until rather recently2, however, stable plating and stripping of Ca and hence rechargeability of Ca-batteries was deemed impossible, due to the stable phases formed at the electrolyte/electrode interface, completely blocking Ca2+ transfer3.Since this pivotal breakthrough, Ca-batteries have attracted more interest, but they are still in their very infancy and new battery designs – such as using novel electrolyte concepts – are needed to achieve stable cycling of Ca metal anodes, and this preferably at room temperature.Here we report on the use of molten salt electrolytes, i.e. binary and multi-component systems of inorganic cations and anions, and more specifically on their physico-chemical and electrochemical properties as a function of composition with special emphasis on the compositional entropy. By using completely solvent-free electrolytes issues associated with organic electrolytes and solvents, such as the blocking of Ca2+ transfer and electrolyte flammability3, are avoided. Furthermore, they should/do not induce any large polarization, as opposed to ionic liquid-based electrolytes4.In more detail, using the Ca(FSI)2 salt in combination with the analogous Li-, Na-, and KFSI salts in equimolar compositions, all with melting temperatures (significantly) above 100 °C, we can create electrolytes melting at ca. 60-75 °C for the ternary systems, which is further reduced to ca. 55 °C for the quaternary system. These lowered melting temperatures are achieved by increased entropy of mixing, reducing the Gibbs free energy. This can for example be illustrated by Raman spectroscopy via comparing the spectra of “as cast” electrolytes with the spectra for the ones “aged” for one week (Figure 1). Here the FSI anion band at 772 cm-1 (νsS-N-S)5 shows that the electrolytes partially (re-)crystallize/micro-separate for the ternary systems, but stay more stable for the quaternary system, which we, at least partially, attribute to an entropy effect. Reference s M. E. Arroyo-De Dompablo, A. Ponrouch, P. Johansson, and M. R. Palacín, Chem . Rev., 120, 6331–6357 (2020).A. Ponrouch, C. Frontera, F. Bardé, and M. R. Palacín, Nat. Mater., 15, 169–172 (2016).D. Aurbach, R. Skaletsky, and Y. Gofer, J . Electrochem Soc., 138, 3536–3545 (1991).Q. Pang et al., Nature, 608, 704–711 (2022).K. Matsumoto, T. Oka, T. Nohira, and R. Hagiwara, Inorg . Chem ., 52, 568–576 (2013). Figure 1

Chemically and Physically Cross-Linked Inorganic–Polymer Hybrid Solvent-Free Electrolytes

Safe, self-standing, all-solid-state batteries with improved solid electrolytes that have adequate mechanical strength, ionic conductivity, and electrochemical stability are strongly desired. Hybrid electrolytes comprising flexible polymers and highly conductive inorganic electrolytes must be compatible with soft thin films with high ionic conductivity. Herein, we propose a new type of solid electrolyte hybrid comprising a glass–ceramic inorganic electrolyte powder (Li1+x+yAlxTi2−xSiyP3−yO12; LICGC) in a poly(ethylene)oxide (PEO)-based polymer electrolyte that prevents decreases in ionic conductivity caused by grain boundary resistance. We investigated the cross-linking processes taking place in hybrid electrolytes. We also prepared chemically cross-linked PEO/LICGC and physically cross-linked poly(norbornene)/LICGC electrolytes, and evaluated them using thermal and electrochemical analyses, respectively. All of the obtained electrolyte systems were provided with homogenous, white, flexible, and self-standing thin films. The main ionic conductive phase changed from the polymer to the inorganic electrolyte at low temperatures (close to the glass transition temperature) as the LICGC concentration increased, and the Li+ ion transport number also improved. Cyclic voltammetry using [Li metal|Ni] cells revealed that Li was reversibly deposited/dissolved in the prepared hybrid electrolytes, which are expected to be used as new Li+-conductive solid electrolyte systems.

Solvent-free protic liquid enabling batteries operation at an ultra-wide temperature range

Nowadays, electrolytes for commercial batteries are mostly liquid solutions composed of solvent and salt to migrate the ions. However, solvents of the electrolyte bring several inherent limitations, either the electrochemical window, working temperature, volatility or flammability. Herein, we report polyphosphoric acid as a solvent-free protic liquid electrolyte, which excludes the demerits of solvent and exhibits unprecedented superiorities, including nonflammability, wider electrochemical stability window (>2.5 V) than aqueous electrolyte, low volatility and wide working temperature range (>400 °C). The proton conductive electrolyte enables MoO3/LiVPO4F rocking-chair battery to operate well in a wide temperature range from 0 °C to 250 °C and deliver a high power density of 4975 W kg−1 at a high temperature of 100 °C. The solvent-free electrolyte could provide a viable route for the stable and safe batteries working under harsh conditions, opening up a route towards designing wide-temperature electrolytes.

Plasticized and salt-doped single-ion conducting polymer electrolytes for lithium batteries.

Single-ion conducting polymer electrolytes created by plasticizing LiPSTFSI with PPO and LiTFSI are shown to both improve the ionic conductivity and alter the ion conduction mechanism. This correlates with both local and macroscopic properties, opening for rational design of solid-state, but yet pliable electrolytes.

Lithium Metal Deposition and Stripping in Porous Hosts Coupled with Viscous Polymer Electrolytes

High energy density rechargeable batteries are desired to meet global goals of increased use of renewable energy and vehicle electrification. Higher energy densities can be achieved by replacing graphite anodes with metal anodes. Among metal anode materials, lithium metal anodes are promising due to their high theoretical capacity and low reduction potential. However, obstacles such as dendrite growth and continuous electrolyte decomposition need to be addressed for successful commercialization of rechargeable batteries with lithium metal anodes. It is widely reported that porous 3D lithium metal hosts can mitigate dendrite growth by accommodating lithium deposition uniformly within the hosts during cycling. Continuous electrolyte decomposition can also be mitigated by replacing liquid electrolytes with solvent-free electrolytes. We incorporate both strategies in this work to address the critical issues described above by demonstrating lithium deposition/stripping cycling on 3D electrode hosts with viscous polymer electrolytes. First, lithium metal deposition/stripping cycling within non-conductive and conductive hosts were compared, where zinc oxide (ZnO) coated polyacrylonitrile (PAN) nanofibers was used as the non-conductive lithium metal host and ZnO coated copper nanowires were prepared as the conductive lithium metal host. The non-conductive host showed successful cycling of 5 mAh/cm2(25 µm thickness) of lithium metal without short circuiting. Further, deposition/stripping cycling with viscous star polymers (polystyrene-co-poly(polyethylene glycol) methyl ether acrylate (PS-co-PPEGMA)) and a linear oligomer polyethylene glycol dimethyl ether (PEGDME) were compared. The effects of the physical and electrochemical properties of the polymer electrolytes on lithium deposition and interfacial impedance will be discussed.

Redox-active binary eutectics: Preparation and their electrochemical properties

Eutectics are emerging as promising candidate for electrochemical energy storage. However, the eutectics usually show high viscosity that usually arises from strong intermolecular interactions greatly limits their application. In this communication, we design and prepare two redox-active molecules that both are able to form binary eutectics with TBMA-TFSI. Combined experimental and computational studies indicate the intermolecular interaction is weakened after eutectic formation, resulting in decreased viscosity and enhanced ionization ratio of TBMA-TFSI without affecting the redox behaviors of the individual molecules. Further, the redox-active binary eutectic is used as solvent-free electrolyte for battery application.

Synthesis of Reduced Quinone - Alkali Metal Cation Pairs

Quinone-based molecules are currently being widely studied for use as electrolytes in redox flow batteries. Upon discharge (if used as the positive electrolyte), quinones accept 2 electrons to form a doubly reduced species. In aqueous solutions, this reduced species is typically a hydroquinone, due to the abundance of protons in solution. However, in non-aqueous and/or solvent-free electrolytes, the counterion is not necessarily a proton - it depends on the identity of the negative electrolyte. We aim to synthesize these compounds so that they may be studied directly. We investigate synthesis routes for these reduced quinones paired with alkali metal cations (Na+ and K+), as would be produced in a flow battery with NaK (a eutectic liquid metal) as the negative electrolyte. An ion-exchange procedure based on an existing synthesis route [1], starting from the desired hydroquinone, has been developed to synthesize these compounds. The products of these syntheses have been characterized via 1H NMR and other techniques.

Eutectic Mixing As a Knob for Next-Generation Flow Batteries

Flow batteries can offer scalability, long cycle-life, and power-to-energy tunability, however, they have low energy density. An under-explored “knob” for increasing the energy density of flow batteries is eutectic mixing for the depression of melting points. This knob could enable high energy-density flow batteries by achieving solvent-free electrolytes. We demonstrate the ability of eutectic mixing to decrease melting points in the case of n-component mixtures of benzoquinone derivatives, which are redox-active molecules. Predicting the properties of mixtures of more than two components is not commonly done, but we show that, in this case, a regular solution model has predictive power for finding the eutectic melting temperatures and compositions for mixtures of multiple components.

A comparative analysis on the morphology and electrochemical performances of solution-casted and electrospun PEO-based electrolytes: The effect of fiber diameter and surface density

Solvent-free polymer electrolytes have been prepared through standard electrospinning and solution casting techniques, respectively. A series of nanofibrous electrolytes with various average fiber diameters and different surface densities were electrospun. In addition, polymeric films were casted with several thicknesses. It was shown that crystalline regions as well as fraction of free lithium ions were enhanced by fabrication of the finer fibers. The solution-casted electrolytes with the higher thicknesses displayed lower free ions and less amorphous phases. The nanofibrous mats illustrated the ionic conductivity from 0.062 to 0.172 mS.cm−1.While, the solution-casted ones presented lower ionic conductance in the range of 0.006 to 0.008 mS.cm−1. As surface density of the nanofibrous and solution-casted mats increased, the bulk resistance was improved. Moreover, it was identified that the prepared solvent-free electrolytes obeyed the Arrhenius behavior. The nanofibrous and solution-casted electrolytes with the highest ionic conductivities showed the highest values for the dielectric parameters. Furthermore, the optimum as-spun membrane illustrated higher specific capacity and Faradic current density compared with the solution-casted one. The results implied that the electrospun membranes are good candidates for solvent-free electrolyte application in the Li-ion batteries.

Electrospun core‐shell nanofibers based on polyethylene oxide reinforced by multiwalled carbon nanotube and silicon dioxide nanofillers: A novel and effective solvent‐free electrolyte for lithium ion batteries

Insertion of conductive fillers into solvent-free polymer electrolytes enhances electrochemical behavior of the electrolyte membranes leading to higher ionic conductivity, lower capacity fading, and so on. Although, the presence of the conductive fillers in the polymer matrixes increases the risk of electrical shorting, herein, polyethylene oxide (PEO)-based core-shell nanofibers were prepared via a simple electrospinning method. In the core-shell electrospun fibers, ethylene carbonate (EC) and lithium perchlorate (LiClO4) were used as a plasticizer and as a lithium salt, respectively. The core component was enwrapped by the PEO/EC/LiClO4 shell part incorporated with SiO2 nanoparticles. Various properties of the fabricated membranes were evaluated by changing the ratio of multiwall carbon nanotubes (MWCNTs) in the core part of the nanofibers. The morphology and core-shell structure of the electrospun fibers were studied by FESEM and TEM images. According to FTIR and XRD results, addition of the EC plasticizer and the fillers into the as-spun fibers increased the fraction of free ions and the amorphous regions. From electrochemical impedance spectroscopy studies, the ionic conductivity enhanced by insertion of the plasticizer molecules and the filler particles into the core-shell structures. The highest ionic conductivities of 0.09 and 0.21 mS.cm−1 were obtained for the free-filler and the filler-loaded nanofibrous membranes, respectively. The prepared mats obeyed the Arrhenius behavior ( R2~1). Dielectric studies confirmed the obtained data from the ionic conductivities. Furthermore, the capacity residual was enhanced from 69% to 85% by incorporation of the MWCNTs filler into the core component of the electrospun nanofibers. The presented results may facilitate development of versatile nanofibrous membranes embedded with the conductive fillers as solvent-free electrolytes applicable in lithium-ion batteries.

Novel electrospun polymer electrolytes incorporated with Keggin‐type hetero polyoxometalate fillers as solvent‐free electrolytes for lithium ion batteries

AbstractIn this study, solvent‐free nanofibrous electrolytes were fabricated through an electrospinning method. Polyethylene oxide (PEO), lithium perchlorate and ethylene carbonate were used as polymer matrix, salt and plasticizer respectively in the electrolyte structures. Keggin‐type hetero polyoxometalate (Cu‐POM@Ru‐rGO, Ni‐POM@Ru‐rGO and Co‐POM@Ru‐rGO (POM, polyoxometalate; rGO, reduced graphene oxide)) nanoparticles were synthesized and inserted into the PEO‐based nanofibrous electrolytes. TEM and SEM analyses were carried out for further evaluation of the synthesized filler structures and the electrospun nanofibre morphologies. The fractions of free ions and crystalline phases of the as‐spun electrolytes were estimated by obtaining Fourier transform infrared and XRD spectra, respectively. The results showed a significant improvement in the ionic conductivity of the nanofibrous electrolytes by increasing filler concentrations. The highest ionic conductivity of 0.28 mS cm−1 was obtained by the introduction of 0.49 wt% Co‐POM@Ru‐rGO into the electrospun electrolyte at ambient temperature. Compared with solution‐cast polymeric electrolytes, the electrospun electrolytes present superior ionic conductivity. Moreover, the cycle stability of the as‐spun electrolytes was clearly improved by the addition of fillers. Furthermore, the mechanical strength was enhanced with the insertion of 0.07 wt% fillers to the electrospun electrolytes. The results implied that the prepared nanofibres are good candidates as solvent‐free electrolytes for lithium ion batteries. © 2020 Society of Chemical Industry

Electrospun PEO nanofibrous membrane enable by LiCl, LiClO4, and LiTFSI salts: a versatile solvent-free electrolyte for lithium-ion battery application

In this study, the effect of type of lithium salts on the main properties of the nanostructure electrolytes was studied. Electrospinning process was applied to production of solvent-free PEO-based nanofibrous electrolytes containing various lithium salts, i.e., LiCl, LiClO4, and LiTFSI. Then, the characteristics of the electrospun nanofibers were evaluated by various techniques. The fraction of free ions was estimated by the FTIR spectrum. Also, to investigate the crystalline phases of the as-spun electrolytes, the samples were subjected to X-ray analysis. The highest room temperature ionic conductivity of the fabricated electrolytes was obtained as 0.33 mS cm−1 by the addition of 1.5 wt% LiClO4 into the nanofibers. Furthermore, the cycling stability of the as-spun structures was enhanced by increasing the amount of LiClO4 and LiCl salts in the produced nanofibers. The results implied that the prepared nanofibers are good candidates as solvent-free electrolytes for Li-ion batteries.

Electrospun Polyethylene Oxide-Based Membranes Incorporated with Silicon Dioxide, Aluminum Oxide and Clay Nanoparticles as Flexible Solvent-Free Electrolytes for Lithium-Ion Batteries

In this study, thin electrospun solvent-free electrolytes based on polyethylene oxide (PEO) incorporated with silicon dioxide (SiO2), aluminum oxide (Al2O3) and clay nanoparticles were prepared and characterized. Lithium perchlorate and ethylene carbonate were used as salt and plasticizer, respectively. The morphological properties were investigated using scanning electron microscopy, energy-dispersive spectroscopy, Fourier transform infrared spectroscopy and x-ray diffraction patterns. The obtained results confirmed an increment of the fraction of free ions and amorphous regions by incorporation of the fillers into the electrospun electrolytes. Introduction of the fillers into the PEO matrix significantly improved the ion conductivity. The highest ion conductivities of 0.033 mS cm−1, 0.059 mS cm−1 and 0.016 mS cm−1 were obtained by the addition of SiO2, Al2O3 and clay nanoparticles into the as-spun electrolytes, respectively. The electrospun electrolytes showed superior ion conductivities compared with polymeric electrolytes synthesized through a standard solution-casting method. In addition, the activation energy decreased with the addition of fillers into the electrospun fibres. The as-spun electrolytes displayed low cycling durability. Furthermore, tensile properties implied that tensile strength could be improved by loading an optimum ratio of the fillers. This investigation presents the great potential of electrospun membranes as electrolytes applicable for solid-state lithium-ion batteries.

Influence of Water on the Interface between Hydrophilic Ionic Liquid and Single Crystal Bismuth Electrodes

Ionic liquids (ILs) as solvent-free electrolytes are new favorites for interfacial chemistry as well as for many other potential applications in electrochemical systems, such as supercapacitors, solar cells, fuel cells and secondary ion batteries. One of the main advantages of ILs is the good electrochemical stability. Differently from the hygroscopic, bulky ILs, many ILs are completely miscible with water, conventionally defined as hydrophilic. As an inevitable impurity, water has a strong effect on the surface of ILs within the electrochemical stability region where condensation of water molecules is dependent on applied potential [1-2]. The electrochemical faradic processes at the interface affect both charge transfer and energy storage processes. Those prompt us to focus on the interaction of ILs and water at electrified electrode interface, and the formation kinetics of the electrical double layer (EDL). Adsorption of particles, such as molecules or ions, contributes to the variety of EDL structure at ionic liquid│electrode interface. Halide ions, namely chloride, bromide and iodide, have been studied as part of a group of strongly adsorbing anions to form a dense layer at Bi(hkl) electrodes [3-4]. Electrosorption of water on electrode surface depends not only on the water-ion interaction, but also on the water-eletrode interaction [2]. To further explore the interaction mechanism of water molecules at electrified electrode interface, classical electrochemical methods (cyclic voltammetry and electrochemical impedance spectroscopy) as well as in situ scanning tunneling microscopy (STM) method have been applied to study the effect of water on the interfacial structure of ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate (EMImOTf)│Bi(hkl) interface. Single crystal bismuth electrodes as working electrodes have variable electronic properties and surface structure. Electrical equivalent circuit analysis derived from impedance data has been applied to characterize the interfacial processes [3]. The fitted data double layer capacitance (Cdl ) has a good agreement with the series capacitance (Cs ) at lower frequency, suggesting relative sluggishness of dried IL system. Compared with dry EMImOTf, systems with up to 0.1wt% water additive have a pronounced peak in Cdl vs. potential curves, observed within less negative potential region, showing that water has a much stronger interaction with IL anions than with the cations. OTf⁻ anions combined with aliphatic imidazolium cations are generally thought specifically adsorb at metallic electrodes due to the presence of sulfonyl groups. The different metallic properties of bismuth single crystal planes also show a difference in capacitive behavior. Other parameter, such as charge transfer resistance (Rct ) reveals how water adsorption process varies with applied electrode potential. The in situ STM results show that there are no surface reconstruction processes at Bi(111) surface within the studied polarization range. It should be noted, however, that for other single crystal planes surface reconstruction process are observed by varying the applied potential value, being affected by both anion adsorption and water content. Acknowledgements This work was supported by the Estonian Ministry of Education and Research (projects no. IUT 20-13, PUT1033, PUT1107 and PSG 249) and Estonian Centres of Excellence project TK141.

Screening Lengths in Ionic Fluids.

The decay of correlations in ionic fluids is a classical problem in soft matter physics that underpins applications ranging from controlling colloidal self-assembly to batteries and supercapacitors. The conventional wisdom, based on analyzing a solvent-free electrolyte model, suggests that all correlation functions between species decay with a common decay length in the asymptotic far field limit. Nonetheless, a solvent is present in many electrolyte systems. We show using an analytical theory and molecular dynamics simulations that multiple decay lengths can coexist in the asymptotic limit as well as at intermediate distances once a hard sphere solvent is considered. Our analysis provides an explanation for the recently observed discontinuous change in the structural force across a thin film of ionic liquid-solvent mixtures as the composition is varied, as well as reframes recent debates in the literature about the screening length in concentrated electrolytes.

The Influence of Water on the Double-Layer Capacitance of an Ionic Liquid

Solvent-free electrolytes, room temperature ionic liquids (RTILs) have properties in between solid electrolytes and electrolyte solutions with a number of potential applications in electrochemical systems. One of their main advantages is their large potential window, in some cases 4.5-6V 1,2. The presence of impurities, water in particular, may reduce the width of this potential window. The adsorption of water at electrodes has only recently been investigated theoretically3 and experimentally2,4. Feng et al. studied humid imidazolium-based ionic liquids and the adsorption of water at a carbon-like electrode using molecular dynamics simulations3. They found that the adsorption of water increases with potential and that the electrosorption effect is stronger at potentials positive of the potential of zero charge (p.z.c.), due to the stronger interaction of water with the anions. Motobayahsi and Osawa used Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS) as a tool to detect the presence of water in the electrochemical double layer (ECDL) of a humid imidazolium-based RTIL at an Au electrode 4. They confirmed that water accumulates at the electrode even at low concentrations and that the adsorption is more pronounced at potentials positive of the p.z.c.. In this study we employ electrochemical impedance spectroscopy to determine the electrochemical double layer capacity CDL of 1-butyl-1-methy-pyrrolidinium bis[(trifluoromethyl)sulfonyl]imide (PyrTFSI) in contact with an Au(111) electrode. The water content of PyrTFSI is carefully adjusted and monitored from very dry (from 1.4 water molecules for 1000 RTIL anions (1000 : 1.4) to 1000 : 87.4). Recorded Nyquist spectra are fitted to three equivalent circuits and we give a detailed physical explanation for the single circuit elements. Electrochemical Scanning Tunnelling Microscopy is used to monitor the Au(111) surface to ensure that the electrode does not change during the electrochemical measurements. Our experimentally obtained CDL vs. potential curves are compared to curves calculated by mean-field theory (Fig. 1). Together, experiment and theory yield a conclusive understanding of the influence of water on the electrode-RTIL interface: 1.Our results show potential-dependent agglomeration of water in the ECDL, as found by AFM 4; 2.Theoretical predictions regarding the sensitivity of CDL of RTILs towards water are confirmed 3; 3.Theoretical tools such as mean-field theory calculations with the compacity parameter γ of RTILs 5 are shown to be able to reproduce experimental data. From the parameters used in the calculation properties of RTILs, like the preferred interaction of water with anions, can be deducted. (1) Buzzeo, M. C.; Hardacre, C.; Compton, R. G. ChemPhysChem 2006, 7 (1), 176–180. (2) Friedl, J.; Markovits, I. I. E.; Herpich, M.; Feng, G.; Kornyshev, A. A.; Stimming, U. ChemElectroChem 2016, 71 (111), 311–315. (3) Feng, G.; Jiang, X.; Qiao, R.; Kornyshev, A. A. ACS Nano 2014, 8 (11), 11685–11694. (4) Motobayashi, K.; Osawa, M. Electrochem. commun. 2016, 65, 14–17. (5) Kornyshev, A. A. J. Phys. Chem. B 2007, 111 (20), 5545–5557. Figure 1: (left) Normalized double layer capacitance over potential of 1-butyl-1- methyl-pyrrolidinium bis[(trifluoromethyl)sulfonyl]imide with various contents of water obtained from electrochemical impedance spectroscopy. (right) Double layer capacitance over potential calculated from mean field theory with the driest ionic liquid in red and the wettest in teal. with Graph adapted from 2 . Figure 1

Quinone Redox-active Ionic Liquids

Simple ionic liquids exhibit unique physical and chemicalproperties that make them very useful for deployment in electrochemicaldevices such as solvent-free electrolytes in capacitors and batteries.However, incorporating redox functionality into ionic liquidstructures opens up in situ faradaic electrochemistry which allows accessto a large array of new electrochemical applications reliant uponheterogeneous or homogenous electron-transfer processes. This paperpresents and discusses the opportunities and challenges for these typesof electro-materials across a myriad of applications by consideringexemplar quinone-functionalised ionic liquids.

Highly Efficient Gel Polymer Electrolyte Operating at Room Temperature for All Solid State Battery Applications

Numerous incidents occurred in the past decades involving lithium batteries, from the explosion of car battery pack to the smartphones burning, highlighting the security issues of this kind of battery. The main danger comes from the liquid electrolyte which is thermally unstable and represents a weak barrier against lithium dendrite growth upon cycling. Polymer electrolytes are an interesting alternative to current liquid ones. Solid polymer electrolytes (SPE) are solvent free electrolyte and are composed of a salt and a polymer which gives mechanical properties. These SPE present an important safety increase but the electrochemical performance, especially at room temperature, remains very low. Another material family is under development: the Gel Polymer electrolyte (GPE). GPE are composed of a lithium salt, a polymer and a solvent which improves the electrochemical performance1,2. In this work, we want to develop an original ‘’wet’’ chemical process to realize the electrolyte thin film. The chosen gel polymer electrolyte (GPE) is composed of an acrylate polymer matrix which gives the mechanical behavior and a liquid phase which gives the electrochemical properties. The liquid phase is a mixture of an ionic liquid and a lithium salt. The electrolyte is polymerized by UV-curing (Fig.1). In a first step, the GPE was characterized with an original in-situ impedance spectroscopy to monitor the polymerization reaction (Fig.2). Conductivity measurements reveal very high ionic conductivity (> 1 mS/cm), whatever the electrolyte composition, which was expected considering the high ionic content in the liquid phase. However, the lithium transference number is very low (< 0.1) leading to low lithium conductivity (~10-12S/cm). Raman spectroscopy was used to determine Li-TFSI coordination (Fig.3) which, could partially explain the low transference number. Electrolyte composition was improved to increase the transference number (up to 0.29). The as-obtained electrolyte was integrated in an all solid state battery with LiCoO2 cathode. Galvanostatic cycles of batteries integrating a GPE or a standard liquid electrolyte are presented in Fig.4. Cycling was done at ambient temperature with a high 1C C-rate and electrochemical results are very similar despite a slightly higher polarization with the GPE. [1] M. Armand, Solid State lonics 69 (1994) 309-319. [2] A. Arya, Ionics (2017). Figure 1

Quantitative Information about Electrosorption of Ionic Liquids in Carbon Nanopores from Electrochemical Dilatometry and Quartz Crystal Microbalance Measurements

Electrochemical energy storage using nanoporous carbons and ionic liquids enables large cell voltages and is a promising way to increase the energy density of electrical double-layer capacitors. The structure of the double layer in solvent-free electrolytes is fundamentally different from other systems with organic or aqueous solvents. In our study, we investigate the physical behavior of nanoporous carbon electrodes in contact with ionic liquids with a multilength scale approach by combining electrochemical quartz-crystal microbalance and electrochemical dilatometry. Synergistic combination of both in situ methods allows one to correlate system properties on particle and electrode level. We find that the charging mechanism at low charge is characterized by the exchange of more smaller ions by fewer larger ions. At higher charges, the system is changing to preferred counterion adsorption, which is resulting in a strong increase in the electrode volume. The maximum linear strain for a bulk electrode is 2% ...

Functional Electro-materials Based on Ferricyanide Redox-active Ionic Liquids

The unique physical and chemical properties of conventional room temperature ionic liquids (RTILs) render them highly deployable materials in electrochemical devices performing functions such as solvent-free electrolytes in capacitors, batteries and sensors. However, these non-faradaic applications can be complimented by incorporating faradaic redox functionality into the ionic liquid structure which facilitates access to a large array of new electrochemical applications such as dye sensitised solar cells, redox batteries, hydrid capacitors and selective amperometric sensor applications which are all reliant on heterogeneous or homogenous electron-transfer processes. This paper presents and discuses some examples of redox active ionic liquids base on the ferri-/ferro- functionality. These functional electro-materials which are already known [Ref. [18]] exhibit simple reversible one-electron electrochemistry at very negative potentials (by at least −1V relative to aqueous systems) in anhydrous media. Glass transition temperatures lower than −50°C were also observed along with an overall thermal stability up to at least 400°C under dry N2 atmosphere conditions. Opportunities and challenges for these types of electro-materials are discussed.