Mechanism of ionic transport in cesium-conducting solid electrolytes based on cesium orthophosphate

The earlier obtained data on the transport properties of cesium-conducting solid electrolytes based on cesium orthophosphate in the Cs_3–2xM_x^II PO₄ (M^II = Mg, Ca, Sr, Ba), Cs_3–3x PO₄ (M_x^III = Sc, Y, La, Sm, Nd) and Cs_3–xP_1–xZ_x^VI O₄ (Z = S, Cr, Mo, W) systems are analyzed. It is shown that, in addition to the conventional jump mechanism, the “paddle wheel” mechanism can play an important role in the ionic transport. This mechanism is associated with the orientation disordering of [PO₄] tetrahedrons at the elevated temperatures, which leads to their rotation promoting “pushing” cesium ions into the accessible neighboring positions.     

Optimization of ionic conductivity in solid electrolytes through dopant-dependent defect cluster analysis

Atomistic simulation based on an energy minimization technique has been carried out to investigate defect clusters of R(2)O(3) (R = La, Pr, Nd, Sm, Gd, Dy, Y, Yb) solid solutions in fluorite CeO(2). Defect clusters composed of up to six oxygen vacancies and twelve accompanied dopant cations have been simulated and compared. The binding energy of defect clusters increases as a function of the cluster size. A highly symmetric dumbbell structure can be formed by six oxygen vacancies, which is considered as a basic building block for larger defect clusters. This is also believed to be a universal vacancy structure in an oxygen-deficient fluorite lattice. Nevertheless, the accurate positions of associated dopants depend on the dopant radius. As a consequence, the correlation between dopant size and oxygen-ion conductivity has been elucidated based on the ordered defect cluster model. This study sheds light on the choice of dopants from a physical perspective, and suggests the possibility of searching for optimal solid electrolyte materials through atomistic simulations.     

Fluoride solid electrolytes

Relations between the structure and electrical properties of fluoride systems with different structures are presented. Physical properties of fluorite-structured (MF₂-RF₃, MF₂-AF, MF₂-M′F₂, M = Ba, Pb, R = La-Lu, Sc, Y, A = Li, Na, K, Rb, M′ = Ba, Cd, Mg), orthorhombic (RF₃, R = Tb-Er, Y), tysonite-structured (RF₃-MF₂, R = La-Nd, M = Sr), and monoclinic (BaR₂F₈, R = Ho-Yb, Y) fluoride single crystals or ceramics (ErF₃, MF₂-RF₃, M = Ca, Ba, R = La, Gd, Tb, Y), glasses (ZBLAN, PIBAL) and eutectic composites (LiF-RF₃, R = La-Gd, Y, PbF₂-RF₃, R = Ho, Yb, Y, Sc, PbF₂-AF, A = Li, Na, PbF₂-MgF₂) are compared. Anisotropy of electrical properties is explained. Models of aggregation of defects into clusters are proposed. In fluoritestructured crystals, the highest ionic conductivity was found for PbF₂: 7 mol % ScF₃ (at 500 K, σ₅₀₀ = 0.13 S/cm). In tysonite-structured crystals, the highest ionic conductivity was found for LaF 3: 3 mol % SrF₂ (σ₅₀₀ = 2.4 × 10⁻² S/cm). Different types of coordination polyhedrons and their different linking in orthorhombic and tysonite structures explain a large difference between conductivities in both structures. Published in Russian in Elektrokhimiya, 2009, Vol. 45, No. 6, pp. 668–678. The article is published in the original. Published by report at IX Conference “Fundamental Problems of Solid State Ionics”, Chernogolovka, 2008.     

Composite solid electrolytes

Studies of composite ionic conductors are overviewed. Mechanisms of defect formation at ionic crystal surfaces and at interphase boundaries in the composites are discussed; the Stern model that allows calculating surface potential has been involved. Methods for the calculating of the composite’s electrical conductance and other physicochemical characteristics are suggested. Thermodynamic stability of nanocomposites and the genesis of their morphology during sintering are analyzed. General regularities of changes in ionic-salt properties over wide range of the “ionic salt-oxide” systems, as well as size effects are discussed.     

Ionic liquid incorporated SPEEK/Chitosan solid polymer electrolytes: ionic conductivity and dielectric study

Journal of Solid State Electrochemistry - In this study, polymeric membranes composed of ionic liquid (IL), 1-ethyl-3-methylimidazolium tetrafluoroborate supported sulfonated poly(ether ether...     

Ionization condition of lithium ionic liquid electrolytes under the solvation effect of liquid and solid solvents

Ionization condition and ionic structures of the lithium ionic liquid electrolytes, LiTFSI/EMI-TFSI/(PEG or silica), were investigated through the measurements of ionic conductivity and diffusion coefficient. The size of the hydrodynamic lithium species (rLi) evaluated from the Stokes-Einstein equation was 0.90 nm before gelation with the PEG or silica. This reveals that the TFSI- anions from the solvent are coordinated on Li+ for solvation, forming, for example, Li(TFSI)4(3-) and Li(TFSI)2- in the electrolyte solution. By the dispersion of PEG for gelation, rLi increased up to 1.8 nm with the 10 wt % of PEG. This indicates that the lithium species is directly interacted with the oxygen sites on the polymer chains and the lithium species migrate, reflecting the polymer by hopping from site to site. In case of the silica dispersion, rLi decreased to 0.7 nm at 10 wt % silica. Although the silica surface with silanol groups fundamentally attracts Li+, the lithium does not migrate from site to site on the silica surface as in the gel of the polymer and follows random walk behavior in the network of the liquid-phase pathways in the two-phase gel. In the process, that solvated TFSI- anions are partially removed may be due to the attractive effect of H+, which was dissociated from the silanol group. It is concluded that the dispersed silica is effective to modify the hydrodynamic lithium species to be appropriate for charge transport as reducing the size and anionic charge of Li(TFSI)4(3-) by removing one or two TFSI- anions.     

High ionic conductivity P(VDF-TrFE)/PEO blended polymer electrolytes for solid electrochromic devices

Solid polymer electrolytes with excellent ionic conductivity (above 10(-4) S cm(-1)), which result in high optical modulation for solid electrochromic (EC) devices are presented. The combination of a polar host matrix poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) and a solid plasticized of a low molecular weight poly(ethylene oxide) (PEO) (M(w)≤ 20,000) blended polymer electrolyte serves to enhance both the dissolution of lithium salt and the ionic transport. Calorimetric measurement shows a reduced crystallization due to a better intermixing of the polymers with small molecular weight PEO. Vibrational spectroscopy identifies the presence of free ions and ion pairs in the electrolytes with PEO of M(w)≤ 8000. The ionic dissolution is improved using PEO as a plasticizer when compared to liquid propylene carbonate, evidently shown in the transference number analysis. Ionic transport follows the Arrhenius equation with a low activation energy (0.16-0.2 eV), leading to high ionic conductivities. Solid electrochromic devices fabricated with the blended P(VDF-TrFE)/PEO electrolytes and polyaniline show good spectroelectrochemical performance in the visible (300-800 nm) and near-infrared (0.9-2.4 μm) regions with a modulation up to 60% and fast switching speed of below 20 seconds. The successful introduction of the solid polymer electrolytes with its best harnessed qualities helps to expedite the application of various electrochemical devices.     

High ionic conductive PVDF-based fibrous electrolytes

The high ionic conductive polymer electrolytes were prepared based on poly(vinylidenefluoride) (PVDF) fibers modified via preirradiation grafting poly(methyl methacrylate) (PMMA). In these polymer electrolytes, the PVDF fibers served as the supporting phase providing dimensional stability, and PMMA acted as the gel phase helping for the trapping liquid electrolyte and substituting the nonconductive PVDF phase to provide contact with electrodes well thus increasing conductive area. The modified PVDF fibrous membranes were used as a polymer electrolyte in lithium ion battery after they were activated by uptaking 1 M LiPF₆/ethylene carbonate–dimethyl carbonate (1:1 vol) liquid electrolyte, which showed a much higher room-temperature ionic conductivity than the pristine PVDF fibrous membrane. The LiCoO₂-mesocarbon microbead coin cells containing the dual-phase fibrous membrane (degree of graft, 111.8%) demonstrated excellent rate performance, and the cell still retained about 86% of discharge capacity at 4C rate, as compared to that at 0.1C rate. The prototype cell showed good cycle performance.     

Enhanced lithium transference numbers in ionic liquid electrolytes

Ion transport processes in mixtures of N-butyl- N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) were characterized by ac impedance spectroscopy and pulsed field gradient NMR. Molar ratios x = n Li-TFSI/( n Li-TFSI + n BMP-TFSI) up to 0.377 could be achieved without crystallization. From the bulk ionic conductivity and the individual diffusion coefficients of cations and anions we calculate the Haven ratio and the apparent lithium transference number. Although the Haven ratio exhibits typical values for ionic liquid electrolytes, the maximal apparent lithium transference number is higher than found in other recent studies on ionic liquid electrolytes containing lithium ions. On the basis of these results we discuss strategies for further improving the lithium transference number of such electrolytes.     

Influence of thermal history and humidity on the ionic conductivity of nanoparticle‐filled solid polymer electrolytes

The crystallinity and conductivity of nanoparticle‐filled solid polymer electrolytes (SPEs) are investigated as a function of thermal history and water content. Our objective is to evaluate how performance is affected by the conditions under which the SPEs are handled and tested. The samples consist of polyethylene oxide (PEO), LiClO₄, and Al₂O₃ nanoparticles. At low humidity, SPEs at ether oxygen to lithium ratios of 8:1 do not crystallize immediately; instead, 3 days are required for crystallization to occur, and this does not depend strongly on the presence of nanoparticles. The conductivity is improved by the addition of nanoparticles at low humidity, but only at an ether oxygen to lithium ratio of 10:1, which corresponds to the eutectic concentration. At high humidity, the recrystallization time is delayed for 3 weeks, and the conductivity increases in both filled and unfilled SPEs beyond that of the low humidity samples. Although we observe that water amplifies the influence of nanoparticles on conductivity, we also find that nanoparticles inhibit water uptake—but only in the presence of lithium. Because Li⁺ strongly absorbs water, this result suggests that nanoparticles may interact directly with Li⁺ ions to prevent water uptake. In filled samples at the eutectic concentration (10:1), more water is absorbed compared to the nanoparticle‐filled 8:1 samples, even though less lithium is present. This suggests that nanoparticles may segregate to lithium‐poor regions in the 10:1 samples, and this scenario is supported by the morphology that would be expected at the eutectic concentration. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1496–1505, 2011     

An analysis of ionic conductivity in polymer electrolytes

Conductivities for a wide variety of ionically conducting polymer electrolytes with a range of salt compositions have been investigated over the temperature region T_g to 370 K. When the conductivity data are analyzed as a function of temperature using the empirical Vogel-Tammann-Fulcher (VTF) equation a common trend is observed in that deviations in the fits to the data invariably occur in the temperature range 1.2 T_g to 1.4 T_g for all of the samples investigated. This deviation is interpreted as a decoupling of the ions from polymer segmental motion. Recent ²³Na NMR and ²²Na positron annihilation studies of sodium salt-based polymer electrolytes support this interpretation with evidence of a change in dynamics at about 1.2T_g. © 1994 John Wiley & Sons, Inc.     

Potassium-ionic conduction in the gallate-ferrite solid electrolytes

New potassium-conducting solid electrolytes in the mixed gallate-ferrite systems (1 − x)Ga₂O₃ · xFe₂O₃ · 0.25TiO₂ · K₂O and 1.5[(1 − x)Ga₂O₃ · xFe₂O₃] · TiO₂ · 2K₂O are synthesized and studied. The electrolytes exhibit high ionic conductivity in the test temperature range of 300 to 750°C (above 10⁻² S/cm at 300°C and above 10⁻¹ S/cm at 700°C). An increase in the conductivity with increasing concentration of iron in the specimens is a general tendency. Possible reasons for the effect of Ga/Fe ratio in the structure of solid electrolytes on their transport properties are discussed.     

Solid-state electrolytes based on ionic network polymers

Interpenetrating and semi-interpenetrating polymer networks are synthesized with the use of cationic and anionic ionic monomers: N-[3-(methacryloyloxy)propyl]-N-methylpyrrolidinium bis(trifluoromethane-sulfonyl)imide, N-[2-(2-(2-(methacryloyloxy)ethoxy)ethoxy)ethyl]-N-methylpyrrolidinium bis(fluorosulfonyl)imide, and (N-butyl-N-methylpyrrolidinium 1-[3-(methacryloyloxy)propylsulfonyl] (trifluoromethanesulfonyl) imide. Their ionic conductivities, electrochemical stabilities, heat resistances, thermal stabilities, and mechanical properties and the swelling of the films in ionic liquid/lithium salt mixtures were studied. The copolymerization of N-[2-(2-(2-(methacryloyloxy)ethoxy)ethoxy)ethyl]-N-methylpyrrolidinium bis(fluorosulfonyl)imide and poly(ethylene glycol dimethacrylate) and poly(ethylene glycol methacrylate) in the presence of butadiene-acrylonitrile rubber and a solution of Li(CF₃SO₂)₂N in N-(methoxymethyl)-N-methylpyrrolidinium bis(fluorosulfonyl)imide yielded a solid-state electrolyte with a set of properties optimum among the studied films: an ionic conductivity of 1.3 × 10^?4S/cm (25°C), a tensile strength of 80 kPa, and an elongation at break of 60%.     

Versatile heat resistant solid electrolytes with performances of liquid electrolytes

The confinement of ionic liquids (room temperature molten salts) within a porous silica matrix was performed by a one-step non-hydrolytic sol–gel route, leading to hybrid materials featuring both the mechanic and transparency properties of silica gels and the high ionic conductivity of ionic liquids, as well as a thermal stability up to around 550 K. Butylmethylimidazolium [BMI] or butylpyridinium [BPy] as cations with bis(trifluorosulfonyl)imide [TFSI] or tetrafluoroborate [BF₄] as anions along with a silica matrix showed similar properties.     

Field-dependent ion transport in disordered solid electrolytes

We have carried out experimental and theoretical studies of the electric field-dependent ion transport in disordered materials and in disordered potential landscapes, respectively. In our experiments, we work in an electric field range up to 100 kV cm(-1), which is characterised by a weak nonlinear response of the mobile ions. We detect remarkable differences between different ion-conducting glasses regarding the temperature dependence of the nonlinear response. Theoretically, we study one-dimensional hopping models and continuous disordered potential models, respectively. When comparing theoretical and experimental data, we find both analogies and discrepancies.     

Thin solid state reference electrodes for use in solid polymer electrolytes

This paper reports two low-profile (~ 10 μm thick) solid state reference electrodes for use in solid polymer electrolytes. The thin, open geometry of the electrodes enables close positioning between the working and counter electrodes. The first electrode uses the palladium hydride (Pd|PdH_x) couple (PHRE), and the second utilises the hydrous iridium oxide (IrO_x·yH₂O|IrO_a·bH₂O) couple (IORE). To our knowledge this is the first use of the latter as a reference electrode. The PHRE had a stable potential of + 70 mV vs RHE with a 4 mV h^− 1 drift and two hour lifetime, whilst the IORE gave a potential of + 847 mV vs RHE with a drift of 0.3 mV h^− 1 and no deterioration after 24 h of use. The use of these reference electrodes in a three-electrode solid state cell and a fuel cell is demonstrated.     

DSC studies of solid polymeric electrolytes

Relations are demonstrated between the conductivity, phase structure and thermal history of some solid polymeric electrolytes. The results obtained for systems based on commercially available polymers, e.g. (ethylene oxide), and for specially synthesized materials are presented. Special emphasis is placed on the correlation between the crystallinity, glass transition temperature, melting temperature and conduction properties of the polymeric electrolytes.This work was supported financially by the Rector of Warsaw University of Technology according to research program 503/164/220/1.     

Capacitance response of carbons in solvent-free ionic liquid electrolytes

Ionic liquids (IL) are very promising “solvent-free” electrolytes for high-voltage double-layer supercapacitors (EDLCs) and to this purpose they are generally selected on the basis of their bulk properties, such as electrochemical stability and ion conductivity, without taking into account those of the electrified electrode-IL interface. This interface, which has yet to be well characterized, has features that notably affect electrode capacitance, and our paper for the first time highlights the importance of the molecular chemistry and structure of the ions for the double-layer capacitive response of carbonaceous electrodes in IL. The double-layer capacitive responses of negatively charged electrodes based on activated carbons and aero/cryo/xerogel carbons in two ILs featuring the same anion and different cations of almost the same size, i.e. the N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) and 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI) are reported. The porosity, structure and surface chemistry of the carbons are compared to their capacitive response to evince the role played by these carbon properties and by the chemistry and structure of the IL ions in the electric double-layer.