Polymer electrolyte plasticized with PEG-borate ester having high ionic conductivity and thermal stability

We have focused on the PEG-borate ester as a new type of plasticizer for solid polymer electrolyte composed of poly(ethyleneglycol) methacrylate (PEGMA) and lithium bis-trifluoromethanesulfonimide (LiTFSI). The PEG-borate ester shows good thermal stability and high flash point. Ionic conductivity of the polymer electrolyte increases with increasing amount of the PEG-borate ester and exhibits values greater than 10⁻⁴ S cm⁻¹ at 30 °C and 10⁻³ S cm⁻¹ at 60 °C. Furthermore, PEG-borate ester has three EO chains whose lengths are variable, and various ionic conductivities are expected to depend on EO chain length. As a result, polymer electrolyte containing the PEG-borate ester whose EO chain length is n=3 shows highest ionic conductivity. Furthermore, polymer electrolytes containing PEG-borate esters show excellent thermal and electrochemical stability. The electrolytes are thermally stable up to 300 °C and electrochemically up to 4.5 V vs. Li⁺/Li.     

Ion conductive characteristics of cross-linked network polysiloxane-based solid polymer electrolytes

A series of cross-linked network polysiloxanes containing oligoethylene oxide units, (OCH₂CH₂)_n, as internal free chains have been synthesized by performing hydrosilylation of partially PEO-substituted polysiloxane precursor with , ω-diallyl terminated poly(ethylene glycol). The polymer electrolytes were formed by complexing with LiN(CF₃SO₂)₂ electrolyte salt and exhibited superior conductive property. The σ_RT of the network polymer electrolytes is in the range of 2.50×10⁻⁵ to 1.62×10⁻⁴ S/cm and depends on the cross-linking density (in terms of Si–H amount of the siloxane precursor), repeating unit number of internal oligoethylene oxide and chain length of the cross-linker. The significant enhancement of the conductivity was observed when low molecular weight dimethyl poly(ethylene glycol) was added as plasticizer. The temperature dependence of the ionic conductivity was also studied, following the Vogel–Tamman–Fulcher (VTF) equation.     

Effect of branching in base polymer on ionic conductivity in hyperbranched polymer electrolytes

Novel hyperbranched polymer, poly[bis(diethylene glycol)benzoate] capped with a 3,5-bis[(3′,6′,9′-trioxodecyl)oxy]benzoyl group (poly-Bz1a), was prepared, and its polymer electrolyte with LiN(CF₃SO₂)₂, poly-Bz1a/LiN(CF₃SO₂)₂ electrolyte, was all evaluated in thermal properties, ionic conductivity, and electrochemical stability window. The poly-Bz1a/LiN(CF₃SO₂)₂ electrolyte exhibited higher ionic conductivity compared with a polymer electrolyte based on poly[bis(diethylene glycol)benzoate] capped with an acetyl group (poly-Ac1a), and the ionic conductivity of poly-Bz1a/LiN(CF₃SO₂)₂ electrolyte was to be 7×10⁻⁴ S cm⁻¹ at 80 °C and 1×10⁻⁶ S cm⁻¹ at 30 °C, respectively. The existence of a 3,5-bis[(3′,6′,9′-trioxodecyl)oxy]benzoyl group as a branching unit present at ends in the base polymer improved significantly ionic conductivity of the hyperbranched polymer electrolytes. The polymer electrolyte exhibited the electrochemical stability window of 4.2 V at 70 °C and was stable until 300 °C.     

Lithium scandium phosphate-based electrolytes for solid state lithium rechargeable microbatteries

Li₃Sc₂(PO₄)₃ is a promising candidate for use as an electrolyte in solid state lithium rechargeable microbatteries due to its stability in air, ease of preparation, and resistance to dielectric breakdown. The room temperature ionic conductivity was optimized resulting in an increase of over two orders of magnitude to 3×10⁻⁶S/cm. The formation of Li₃(Sc_2−xM_x)(PO₄)₃, where M=Al³⁺ or Y³⁺, resulted in the decrease of porosity, greater sinterability, and considerable enhancement of the ionic conductivity. Yttrium substitutions enhanced the conductivity slightly while aluminum increased the room temperature ionic conductivity to 1.5×10⁻⁵S/cm for x=0.4. Preliminary electron beam evaporation of Li₃Sc₂(PO₄)₃ yielded amorphous thin films with ion ic conductivity as high as 5×10⁻⁵S/cm and a composition of Li_4.8Sc_1.4(PO₄)₃.     

Material design of new lithium ionic conductor, thio-LISICON, in the Li2S–P2S5 system

A new lithium ionic conductor of the thio-LISICON (LIthium SuperIonic CONductor) family was found in the binary Li₂S–P₂S₅ system; the new solid solution with the composition range 0.0≤x≤0.27 in Li_3+5xP_1−xS₄ was synthesized at 700 °C and characterized by X-ray diffraction measurements. Its electrical and electrochemical properties were studied by ac impedance and cyclic voltammetry measurements, respectively. The solid solution member at x=0.065 in Li_3+5xP_1−xS₄ showed the highest conductivity value of 1.5×10⁻⁴ S cm⁻¹ at 27 °C with negligible electronic conductivity and the activation energy of 22 kJ mol⁻¹ which is characteristic of high ionic conduction state. The extra lithium ions in Li₃PS₄ created by partial substitution of P⁵⁺ for Li⁺ led to the large increase in ionic conductivity. In the solid solution range examined, the minimum conductivity was obtained for the compositions, Li₃PS₄ (x=0.0 in Li_3+5xP_1−xS₄) and Li₄P_0.8S₄ (x=0.2 in Li_3+5xP_1−xS₄); this conductivity behavior is similar to other thio-LISICON family with the general formula, Li_xM_1−yM_y′S₄ (M=Si, Ge, and M′=P, Al, Zn, Ga, Sb). Conduction mechanism and the material design concepts are discussed based on the conduction behavior and the structure considerations.     

Thermal and complex impedance analysis of amorphous and crystalline lithium salt mixtures

Amorphous electrolytes consisting of the lithium salts, Li[R-NSO₂CF₃] were prepared and the attendant low ionic conductivities of the lithium salt mixtures (1×10⁻⁶ S cm⁻¹ at room temperature) are attributed to high glass transition temperatures. An example is the novel amorphous salt, Li[18-C-6NSO₂CF₃] which produces an amorphous salt mixture with Li[N(SO₂CF₃)₂] (LiTFSI).     

Characterization of pTMCnLiPF6 solid polymer electrolytes

In this study the results of our characterization of a solid polymer electrolyte based on poly(trimethylene carbonate), henceforth designated as p(TMC), and lithium hexafluorophosphate (LiPF₆) are described. Samples of solvent-free electrolytes were prepared with a range of concentration of guest salt using solvent casting from tetrahydrofuran and characterized by conductivity measurements, thermal analysis and electrochemical stability. Electrolytes based on this host polymer, with LiPF₆, were obtained as mechanically robust, flexible, transparent and completely amorphous films.     

High temperature transport processes in lithium niobate

The electrical conductivity of single crystal lithium niobate (LiNbO₃) was determined as a function of temperature for various oxygen partial pressures. The electrical conductivity is proportional to P_o2^−1/4 which can be explained by a defect equilibrium involving singly ionized oxygen vacancies and electrons.

Measurements of electrical transport numbers at 1000°K show the electrical conductivity of LiNbO₃ to be ionic at one atmosphere of oxygen and electronic at low oxygen partial pressures.

Thermoelectric measurements indicate that LiNbO₃ at low oxygen partial pressures is n-type and that the concentration of electrons at 1000°K and in an atmosphere of 50% C0/50% CO₂a is 4 × 10¹⁷cm³ with a mobility of 1.7 cm²V sec.

The diffusion of oxygen in LiNbO₃ was determined as a function of temperature at an oxygen partial pressure of 70 Torr. by measuring O¹⁸/O¹⁶ isotope exchange with the gas phase as a function of time. The diffusion data may be represented by D = 3.03 × 10⁻⁶ exp (−29.4 kcal mole⁻¹/RT)cm²sec. Consideration of the Nernst-Einstein relation for oxygen and the variation in conductivity with Li₂O activity indicate that the ionic conduction is caused by transport of lithium ions.     

Studies of lithium intercalation in 3R-WS2

The lithium intercalation into the layered dichalcogenide 3R-WS₂ has been investigated by electrochemical reduction and by chemical reaction in n-butyl lithium solution. Essential results are (a) a charge transfer of nearly 0.6e⁻/W in Li_xWS₂, (b) a small increase of the c-axis parameter of about 0.6%, and (c) a high mobility of the Li⁺-ions. The chemical diffusion coefficient of Li⁺-ions is estimated to be 8 × 10⁻⁹ cm² s⁻¹ in the composition range 0 ≤ x ≤ 0.25. The appearance of a structural transformation from 3R-WS₂ to 2H-Li_xWS₂ is interpreted on grounds of instabilities in the interlayer structure.     

Blended lithium ion conducting polymer electrolytes based on boroxine polymers

A new series of blended polymer electrolytes based on a boroxine polymer (BP) with poly(ethylene oxide) (PEO), an ethylene oxide–propylene oxide copolymer or poly(methyl methacrylate) were prepared. Good room temperature mechanical properties were exhibited by electrolytes containing in excess of 30% PEO. Cationic transference number measurements indicated that a slight improvement in lithium ion conductivity could be achieved by using a mixture of LiCF₃SO₃ and LiN(CF₃SO₂)₂ as the electrolyte salt. Electrolytes incorporating significant proportions of BP exhibited reduced lithium–polymer electrolyte interfacial resistance.     

The microscopic features of (Li0.5La0.5)TiO3

In ionic conducting materials, the crystal structure is closely related to the ionic conductivity. In this research we studied the microscopic features of Li_0.5La_0.5TiO₃ which exhibited a lithium ionic conductivity as high as 1×10⁻³ Scm⁻¹ at room temperature by XRD, TEM and SIMS. It was found that the superstructure was caused by the ordering of La⁺³ and vacancy, producing the 2a_p×2a_p×2a_p unit cell. This ordering was found to be regular in microscopic region, but became irregular in macroscopic region. Li⁺ showed a random distribution which meet the needs for the fast ionic conduction. The second phase was found to be Li₂TiO₃ which existed in the grain boundary junctions.     

The study of hydronium NASICON conductivity with deuterium

The ionic conductivity of the bulk phase of bonded hydronium NASICON (Hyceram^TM) was measured at equilibrium with an H₂O/N₂ and then a D₂O/N₂ atmosphere, each at 100% relative humidity and 75% relative humidity over the temperature range 25°C to 50°C. At 100% relative humidity and 25°C, the protonic system had a bulk conductivity of 5.0×10⁻⁴ S/cm and an activation energy of 17.3kJ/mole; the same sample, when deuterated, had a bulk conductivity of 2.2×10⁻⁴ S/cm and an activation energy of 19.3kJ/mole. At 75% relative humidity and 25°C, the conductivity of the protonated system decreased to 1.4×10⁻⁴S/cm with an activation energy of 24.1 kJ/mole. The deuterated sample at 75% relative humidity had a bulk conductivity of 5.4×10⁻⁵ S/cm with an activation energy of 26.0 kJ/mole. The isotope effect suggested a proton hopping (Grotthus) mechanism as the means by which the protons pass through the lattice.     

Poly(ethylene oxide-co-epichlorohydrin)/NaI: a promising polymer electrolyte for photoelectrochemical cells

We have investigated the thermal and ionic conductivity properties of the elastomer poly(ethylene oxide-co-epichlorohydrin) filled with NaI and I₂. The reason for using this composition is its potential application as electrolyte in photoelectrochemical cells. This copolymer was characterized as a function of NaI concentration, temperature and relative humidity. According to the data obtained, the Na⁺ ion interacts with the ethylene oxide repeating units by means of Lewis type acid–base interactions. The empirical Vogel–Tamman–Fulcher equation was used to model the conductivity and temperature relationships, indicating that the conduction occurs in the amorphous phase of the copolymer. The sample with 9.0% (w/w) of NaI presents a conductivity of 1.5×10⁻⁵ S cm⁻¹ in a dry atmosphere (30°C, [H₂O]<1 ppm) and 2.0×10⁻⁴ S cm⁻¹ at 86% relative humidity (22°C).     

Study of effects of Mn in CdS nanocrystals

Mn²⁺-doped CdS nanocrystals have been synthesized by adopting an aqueous solution precipitation method. These nanocrystals have been studied using X-ray diffraction (XRD), X-ray fluorescence (XRF), optical absorbance, photoluminescence (PL), DC electrical conductivity measurements and positron annihilation lifetime spectroscopy (PALS). The system has been found to be in the hexagonal phase. PL spectra have been studied on most prominent exciton peaks within the wavelength range (586–731 nm). The emission intensity is found to increase on increasing Mn²⁺ ion concentration (0–5%). Electrical conductivity lies within 0.819×10⁻⁶ to 1.69×10⁻⁶ Ω⁻¹ m⁻¹ and the system shows power law dependence for n=3–3.77. The Cd vacancies concentration has been found to decrease on increasing Mn%.     

An electrochemical investigation of Li intercalation in the sol-gel LiMn2O4 spinel oxide

An alternative approach for obtaining the LiMn₂O₄ spinel phase is provided by the use of the sol-gel method in aqueous solution. The main electrochemical properties of the sol-gel LiMn₂O₄ phase are reported. In addition to chronopotentiometric and voltammetric experiments, the kinetics of the electrochemical insertion–extraction of lithium in Li_xMn₂O₄ (0.25<x<1) has been investigated using ac impedance spectroscopy. The strong variation of the chemical diffusion coefficient D_Li vs x, in the range 10⁻⁸–10⁻¹¹ cm² s⁻¹ (D_Li is found to be maximum for x=0.55) is critically discussed.     

Impedance studies on polyacrylonitrile–CuX2–DMSO (X=Cl, Br, CF3SO3) solid polymer electrolyte

A series of polyacrylonitrile–dimethylsulfoxide–CuX₂ (X=CF₃SO₃⁻, Cl⁻, Br⁻), films (foils) were prepared by means of the solution cast technique. The thickness of the foils was between 0.04 and 0.09 cm and they contained 70–80 wt.% of the solvent. Conductivities of the solid electrolytes were obtained from impedance measurements. The conductivity increases with the increase of the salt content up to 8 wt.%; at higher concentrations (>8 wt.%) the conductivity is more or less stable, and reaches, in the case of Cu(CF₃SO₃)₂ and CuCl₂, the level of ca. 10⁻³ Ω⁻¹ cm⁻¹ at room temperature. The foils based on the CuBr₂ show even higher conductivity, close to 10⁻² Ω⁻¹ cm⁻¹ at room temperature, a value comparable to that characteristic for liquid solutions. The temperature variation of the conductivity for all the systems studied is of the Arrhenius type. The activation energy, determined from linear plots lnσ=f(T⁻¹), is of the order ca. 14 kJ mol⁻¹ for the PAN/CuBr₂/DMSO and of ca. 21 kJ mol⁻¹ for the PAN/CuCl₂/DMSO and the PAN/Cu(CF₃SO₃)₂/DMSO systems.     

Studies on strontium substituted rare earth manganites

Sintering, electrical conductivity and thermal expansion behaviour of combustion synthesised strontium substituted rare earth manganites with the general formula Ln_1−xSr_xMnO₃ (Ln=Pr, Nd and Sm; x=0, 0.16 and 0.25) have been investigated as solid oxide fuel cell cathode materials. The combustion derived rare earth manganites have surface area in the range of 13–40 m²/g. Strontium substitution increases the electrical conductivity values in all the rare earth manganites. With the decreasing ionic radii of rare earth ions, the conductivity value decreases. Among the rare earth manganites studied, (Pr/Nd)_0.75Sr_0.25MnO₃ show high electrical conductivity (>100 S/cm). The thermal expansion coefficients of Pr_0.75Sr_0.25MnO₃ and Nd_0.75Sr_0.25MnO₃ were found to be 10.2×10⁻⁶ and 10.7×10⁻⁶ K⁻¹ respectively, which is very close to that of the electrolyte (YSZ) used in solid oxide fuel cells.     

Investigation of dibutyl phthalate as plasticizer on poly(methyl methacrylate)–lithium tetraborate based polymer electrolytes

Polymer electrolyte membranes, comprising of poly(methyl methacrylate) (PMMA), lithium tetraborate (Li₂B₄O₇) as salt and dibutyl phthalate (DBP) as plasticizer were prepared using a solution casting method. The incorporation of DBP enhanced the ionic conductivity of the polymer electrolyte. The polymer electrolyte containing 70 wt.% of poly(methyl methacrylate)–lithium tetraborate and 30 wt.% of DBP presents the highest ionic conductivity of 1.58 × 10⁻⁷ S/cm. The temperature dependence of ionic conductivity study showed that these polymer electrolytes obey Vogel–Tamman–Fulcher (VTF) type behaviour. Thermogravimetric analysis (TGA) was employed to analyse the thermal stability of the polymer electrolytes. Fourier transform infrared (FTIR) studies confirmed the complexation between poly(methyl methacrylate), lithium tetraborate and DBP.     

Conductivity anisotropy in directionally solidified CaZrO3---CaSZ and MgO-MgSZ eutectics

The results of the impedance spectroscopy measurements on eutectic samples based on zirconium oxide are presented here. Samples of CaZrO₃---ZrO₂(cubic) and MgO---ZrO₂(cubic) have been grown by a directional solidification procedure such that the different phases appear nearly oriented along the growth direction (lamellae in the system of CaZrO₃-ZrO₂(cubic) and fibers of MgO in a ZrO₂ matrix in the other system). The DC electrical conductivity has been measured by impedance spectroscopy along and across the growth axis. For CaZrO₃---ZrO₂ the coductivity is clearly anisotropic. The following values for σT have been obtained: the conductivity at 600 °C equals 2.0 × 10⁻⁶ Ω⁻¹ cm⁻¹ perpendicular to the fiber axis and 1.4 × 10⁻⁵ Ω⁻¹ cm⁻¹ parallel to it and with an activation energy of 1.3 eV for σT. For MgO---ZrO₂(cubic) the isotropic value of the conductivity at 600 °C is 10⁻⁴ Ω⁻¹ cm⁻¹ with activation energy for σT of 1.5 eV. The anisotropic conductivity in the CaZrO₃---ZrO₂ (cubic) system has been explained by a model of an ordered stacking of oxygen conducting (cubic ZrO₂) and non-conducting (CaZrO₃ or MgO) phases.     

Determination of different oxygen transport mechanisms and measurement of the oxygen ion conductivity of Y1Ba2Cu3O7−x

By undertaking AC electrochemical impedance experiments on yttria stabilised zirconia electrolytes with polished Y₁Ba₂Cu₃O_7−x electrodes, the activation energy for oxygen ion transport within the bulk of Y₁Ba₂Cu₃O_7−x, in air, over the temperature range 823 K–1043 K, was determined to be 1.50 ± 0.05 eV. At 1000 K the oxygen ionic conductivity was calculated to be around one order of magnitude lower than that in yttria stabilised zirconia. Typical calculated values were σ=5×10⁻⁵ (ω cm)⁻¹ and 6×10⁻³ (ω cm)⁻¹ at the respective temperatures 823 K and 1043 K. By employing a similar cell but with Y₁Ba₂Cu₃O_7−x paste electrodes, oxygen transfer between the Y₁Ba₂Cu₃O_7−x and the electrolyte was found to occur via a surface diffusional processes. Over the temperature range 873 K–1098 K, in air, the activation energy for in-diffusion at the surface was found to be 1.4±0.1 eV and that for out-diffusion at the surface to be 1.76±0.05 eV.