Recent progress in solid oxide and lithium ion conducting electrolytes research

Recent material developments of fast solid oxide and lithium ion conductors are reviewed. Special emphasis is placed on the correlation between the composition, structure, and electrical transport properties of perovskite-type, perovskite-related, and other inorganic crystalline materials in terms of the required functional properties for practical applications, such as fuel or hydrolysis cells and batteries. The discussed materials include Sr- and Mg-doped LaGaO₃, Ba₂In₂O₅, Bi₄V₂O₁₁, RE-doped CeO₂, (Li,La,)TiO₃, Li₃La₃La₃Nb₂O₁₂ (M=Nb, Ta), and Na super-ionic conductor-type phosphate. Critical problems with regard to the development of practically useful devices are discussed.     

Effect of molecular weight on conductivity of polymer electrolytes

The ionic conductivity, σ, of mixtures of poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfone)imide (LiTFSI) was measured as a function of molecular weight of the PEO chains, M, over the range 0.2-5000 kg/mol. Our data are consistent with an expression σ = σ₀ + K/M proposed by Shi and Vincent [Solid State Ionics 60 (1993)] where σ₀ and K are exponential and linear functions of inverse temperature respectively. Explicit expressions for σ₀ and K are provided.     

Solid polymer electrolytes based on poly(lithium carboxylate) salts

The ionic conductivity, lithium ion transference number, electrochemical stability, and thermal property of solid polymer electrolytes composed of poly(ethylene oxide) (PEO) and poly(lithium carboxylate)s, (poly(lithium acrylate) (Poly(Li-A)) or poly(lithium fumarate) (Poly(Li-F)), with and without BF₃·OEt₂ were investigated. The ionic conductivities of all solid polymer electrolytes were enhanced by one to two orders of magnitude with addition of BF₃·OEt₂ because the dissociation of lithium ion and carboxylate anion was promoted by the complexation with BF₃. The lithium ion transference number in the solid polymer electrolytes based on poly(lithium carboxylate)s showed relatively high values of 0.41–0.70, due to the suppression of the transport of counter anion by the use of a polymeric anion. The solid polymer electrolytes with addition of BF₃·OEt₂ showed good electrochemical stability.     

Solid polymer electrolytes based on squaric acid structure

Poly(squarate)s (PPS-1 and PPS-2) were synthesized by the reaction of squaryl dichloride with hydroquinone for PPS-1 and with 2,5-diethoxy-1,4-bis(trimethylsilyloxy)benzene for PPS-2, and the ionic conductivities, thermal properties, and electrochemical and thermal properties of their polymer electrolytes with LiN(CF₃SO₂)₂ were investigated. The ionic conductivity increased with increasing the lithium salt concentration for the PPS-1–LiN(CF₃SO₂)₂ electrolyte, and the highest ionic conductivities of 8.60 × 10⁻⁵ S/cm at 100 °C and 9.57 × 10⁻⁸ S/cm at 30 °C were found at the [Li] to [O] ratio of 2:1. And also, the ionic conductivity for the PPS-1–LiN(CF₃SO₂)₂ electrolyte increased with an increase in the lithium salt concentration, reached a maximum value at the [Li] to [O] ratio of 1:2, and then decreased. The highest ionic conductivity was to be 1.04 × 10⁻⁵ S/cm at 100 °C and 1.71 × 10⁻⁸ S/cm at 30 °C, respectively. Both polymer electrolytes exhibited relatively better electrochemical and thermal stabilities. Addition of the PPS-1 as a plasticizer into the poly(ethylene oxide) (PEO)–LiN(CF₃SO₂)₂ electrolyte system suppressed the crystallization of PEO, and improved the ionic conductivity at room temperature. Invited paper dedicated to Professor W. Weppner on his 65th birthday.     

Effects of plasticizer and nanofiller on the dielectric dispersion and relaxation behaviour of polymer blend based solid polymer electrolytes

Solid polymer electrolytes consisted of poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) blend (50:50 wt/wt%) with lithium triflate (LiCF₃SO₃) as a dopant ionic salt at stoichiometric ratio [EO + (CO)]:Li⁺ = 9:1, poly(ethylene glycol) (PEG) as plasticizer (10 wt%) and montmorillonite (MMT) clay as nanofiller (3 wt%) have been prepared by solution cast followed by melt–pressing method. The X–ray diffraction study infers that the (PEO–PMMA)–LiCF₃SO₃ electrolyte is predominantly amorphous, but (PEO–PMMA)–LiCF₃SO₃–10 wt% PEG electrolyte has some PEO crystalline cluster, whereas (PEO–PMMA)–LiCF₃SO₃–10 wt% PEG–3 wt% MMT electrolyte is an amorphous with intercalated and exfoliated MMT structures. The complex dielectric function, ac electrical conductivity, electric modulus and impedance spectra of these electrolytes have been investigated over the frequency range 20 Hz to 1 MHz. These spectra have been analysed in terms of the contribution of electrode polarization phenomenon in the low frequency region and the dynamics of cations coordinated polymer chain segments in the high frequency region, and also their variation on the addition of PEG and MMT in the electrolytes. The temperature dependent dc ionic conductivity, dielectric relaxation time and dielectric strength of the plasticized nanocomposite electrolyte obey the Arrhenius behaviour. The mechanism of ions transportation and the dependence of ionic conductivity on the segmental motion of polymer chain, dielectric strength, and amorphicity of these electrolytes have been explored. The room temperature ionic conductivity values of the electrolytes are found ∼10⁻⁵ S cm⁻¹, confirming their use in preparation of all-solid-state ion conducting devices.     

An investigation on PAN-PVC-LiTFSI based polymer electrolytes system

Polymer-salt complex with poly(vinyl chloride) (PVC) and poly(acrylonitrile) (PAN) as host polymers blended with lithium bis-(trifluoro methanesulfonyl)imide, LiTFSI [LiN(CF₃SO₂)₂] as dopant salt were prepared in the form of thin film. Fourier transform infrared (FTIR) studies show the evidence of the complexation between PVC, PAN and LiTFSI. Ionic conductivity studies reveal that polymer electrolyte with 30 wt.% LiTFSI has the highest ionic conductivity of 4.39 × 10^− 4 S/cm at room temperature. The polymer electrolytes are also found to be stable up to 315 °C before they decompose. Thermal stability of the polymer electrolytes was also found to increase with increase in salt content. This was proven through thermogravimetric studies.     

An investigation of PVdF/PVC-based blend electrolytes with EC/PC as plasticizers in lithium battery applications

Solid polymer electrolytes (SPEs) composed of poly(vinylidene fluoride) (PVdF)-poly(vinyl chloride) (PVC) complexed with lithium perchlorate (LiClO₄) as salt and ethylene carbonate (EC)/propylene carbonate (PC) as plasticizers were prepared using solvent-casting technique, with different weight ratios of EC and PC. The amorphicity and complexation behavior of the polymer electrolytes were confirmed using X-ray diffraction (XRD) and FTIR studies. TG/DTA and scanning electron microscope (SEM) studies explained the thermal stability and surface morphology of electrolytes, respectively. The prepared thin films were subjected to AC impedance measurements as a function of temperature ranging from 302 to 373 K. The temperature-dependence conductivity of polymer films seems to obey VTF relation.     

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.     

High performance PEO-based polymer electrolytes and their application in rechargeable lithium polymer batteries

Solvent-free, lithium-ion-conducting, composite polymer electrolytes have been prepared by a double dispersion of an anion trapping compound, i.e., calyx(6)pyrrole, CP and a ceramic filler, i.e., super acid zirconia, S-ZrO₂ in a poly(ethylene oxide)-lithium bis(oxalate) borate, PEO–LiBOB matrix. The characterization, based on differential thermal analysis and electrochemical analysis, showed that while the addition of the S-ZrO₂ has scarce influence on the transport properties of the composite electrolyte, the unique combination of the anion-trapping compound, CP, with the large anion lithium salt, LiBOB, greatly enhances the value of the lithium transference number without depressing the overall ionic conductivity. These unique properties make polymer electrolytes, such as PEO₂₀LiBOB(CP)_0.125, of practical interest, as in fact confirmed by tests carried out on lithium battery prototypes.     

Preparation and characterization of a lithium ion conducting electrolyte based on poly(trimethylene carbonate)

In this paper, the results of preliminary studies of two new solvent-free polymer electrolytes based on poly(trimethylene carbonate), p(TMC), with lithium trifluoromethanesulphonate, (triflate), and lithium perchlorate are described. Thin films of these electrolytes were obtained by evaporation of solvent from homogeneous mixtures of known masses of host polymer and salt. Electrolytes with compositions of n between 1.5 and 85, where n represents the molar ratio of (O=COCH₂CH₂CH₂O) units per lithium ion, have been prepared. These solvent-free electrolytes were characterized by measurements of total ionic conductivity, differential scanning calorimetry (DSC) and thermogravimetry (TGA). As expected from previous studies with these salts in poly(ethylene oxide), PEO, the triflate-based system showed superior thermal stability but with a lower total ionic conductivity than that of the perchlorate-containing electrolyte. The highest conductivity (approximately 3×10⁻⁴ Ω⁻¹ cm⁻¹) was found at 95°C with the electrolyte composition of (TMC)₂LiClO₄.     

Investigation on PVDF-HFP microporous membranes prepared by TIPS process and their application as polymer electrolytes for lithium ion batteries

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) microporous membranes were prepared via thermally induced phase separation (TIPS) process. Then they were immersed in a liquid electrolyte to form polymer electrolytes. The effects of polymer content in casting solution on the morphology, crystallinity, and porosity of the membranes were studied by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and a mercury porosimeter, respectively. Ionic conductivity, lithium-ion transference number, and electrochemical stability window of corresponding polymer electrolytes were characterized by AC impedance spectroscopy, DC polarization/AC impedance combination method, and linear sweep voltammetry, respectively. The results showed that spherulites and “net-shaped” structure coexisted for the membranes. Polymer content had no effect on crystal structure of the membranes. The maximum transference number was 0.55. The temperature dependence of ionic conductivity followed the Vogel–Tammann–Fulcher (VTF) relation. The maximum ionic conductivity was 2.93 × 10⁻³ Scm⁻¹ at 20 °C. Electrochemical stability window was stable up to 4.7 V (vs. Li⁺/Li).     

Effect of microstructure on the conductivity of a NASICON-type lithium ion conductor

Sintering temperature is used to control the microstructure of Li_{1 + x + y}Al_xTi_2 − xSi_yP_3 − yO₁₂ (x = 0.3, y = 0.2), a NASICON-type glass-ceramic. Scanning Electron Microscope imaging, X-Ray Diffraction, and Electrochemical Impedance Spectroscopy are employed to show that increase in sintering temperature increases conductivity while generating secondary crystalline phases. Total conductivity is as high as 3.81 × 10⁻⁴ S cm⁻¹ for sintering temperatures above 1000 °C. Crystallization of dielectric phases places the optimal sintering temperature in the 900 °C to 1000 °C range. Thermal analysis of the glass precursor reveals the glass transition, and crystallization temperatures.     

Characterization of solid electrolytes prepared from ionic glass and ionic liquid for all-solid-state lithium batteries

Glassy solid electrolytes were prepared by combining the 50Li₂SO₄·50Li₃BO₃ (mol%) ionic glass and the 1-ethyl-3-methyl-imidazolium tetrafluoroborate ([EMI]BF₄) ionic liquid. High-energy ball milling was carried out for the mixture of the inorganic ionic glass and the organic ionic liquid. The ambient temperature conductivity of the glass electrolyte with 10 mol% [EMI]BF₄ was 10⁻⁴ S cm⁻¹, which was three orders of magnitude higher than that of the 50Li₂SO₄·50Li₃BO₃ glass. The addition of [EMI]BF₄ to the ionic glass decreased glass transition temperature (T_g) of the glass and the decrease of T_g is closely related to the enhancement of conductivity of the glass. Morphology and local structure of the glass electrolyte was characterized. The dissolution of an ionic liquid in an ionic glass with Li⁺ ion conductivity is a novel way to developing glass electrolytes for all-solid-state lithium secondary batteries.     

Effects of novel benzotriazole based zwitterionic salt as electrolyte additive for lithium ion batteries

A novel zwitterionic lithium-benzotriazole sulfobetaine is fabricated by grafting 1,3– propanesultone onto benzotriazole and then lithiating it. The resultant lithium-benzotriazole-sulfobetaine additive is used as an electrolyte additive in lithium ion batteries in 1 M LiPF₆ (ethylene carbonate/dimethyl carbonate = 1:1). The electrolytes with the lithium-benzotriazole sulfobetaine shows higher ionic conductivities (2.18 $\times$ 10⁻² S cm⁻¹) compared to the bare electrolyte (1.07 $\times$ 10⁻² S cm⁻¹) and greater electrochemical stability (anodic limit at ~5.5 V vs. Li/Li⁺) than the pure electrolyte (anodic limit at ~4.6 V vs. Li/Li⁺). The discharge capacity of the lithium cobalt oxide/graphite cells is improved at higher C-rates with the addition of lithium-benzotriazole sulfobetaine due to increased ionic conductivity. The lithium cobalt oxide/graphite cells with the lithium-benzotriazole sulfobetaine additive also show stable cycling performance. These findings warrant the use of lithium-benzotriazole sulfobetaine as an electrolyte additive in lithium ion batteries.     

Polymer electrolytes based on ethylene oxide–epichlorohydrin copolymers

In this work we studied the ionic conductivity for three copolymers of the title co-monomers as a function of LiClO₄ content, temperature and ambient relative humidity. We also investigated the interactions between the salt and the co-monomer blocks in the copolymers and its effect on the morphology and thermal properties of the copolymer/salt complexes. Our data indicate that the Li⁺ ion predominantly interacts with the ethylene oxide repeating units of the copolymers. The copolymer with the highest ionic conductivity was obtained with an ethylene oxide/epichlorohydrin ratio of 84/16 containing 5.5% (w/w) of LiClO₄. It showed a conductivity of 4.1×10⁻⁵ S cm⁻¹ (30°C, humidity< 1 ppm) and 2.6×10⁻⁴ S cm⁻¹ at 84% relative humidity (24°C). The potential stability window of the copolymer/salt complex is 4.0 V, as measured by cyclic voltammetry. For comparison, we also prepared a blend of the corresponding homopolymers containing LiClO₄; it showed higher crystallinity and lower ionic conductivity.     

Studies on PEO-based sodium ion conducting composite polymer films

A sodium ion conducting composite polymer electrolyte (CPE) prepared by solution-caste technique by dispersion of an electrochemically inert ceramic filler (SnO₂) in the PEO–salt complex matrix is reported. The effect of filler concentration on morphological, electrical, electrochemical, and mechanical stability of the CPE films has been investigated and analyzed. Composite nature of the films has been confirmed from X-ray diffraction and scanning electron microscopy patterns. Room temperature d.c. conductivity observed as a function of filler concentration indicates an enhancement (maximum) at 1–2 wt% filler concentration followed by another maximum at ∼10 wt% SnO₂. This two-maxima feature of electrical conductivity as a function of filler concentration remains unaltered in the CPE films even at 100 °C (i.e., after crystalline melting), suggesting an active role of the filler particles in governing electrical transport. Substantial enhancement in the voltage stability and mechanical properties of the CPE films has been noticed on filler dispersion. The composite polymer films have been observed to be predominantly ionic in nature with t _ion ∼ 0.99 for 1–2 wt% SnO₂. However, this value gets lowered on increasing addition of SnO₂ with t _ion ∼ 0.90 for 25 wt% SnO₂. A calculation of ionic and electronic conductivity for 25 wt% of SnO₂ film works out to be ∼2.34 × 10⁻⁶ and 2.6 × 10⁻⁷ S/cm, respectively.     

Synthesis and electrophysical properties of novel lithium ion conducting oxides

Novel lithium ion conducting oxides with perovskite structure have been synthesized and studied. It has been found that the La_2/3−xLi_3xTiO₃ and La_2/3−xLi_3xNb₂O₆ compounds have a perovskite structure at 1/24 < x < 1/6 and 0 < x < 3/10, respectively. The results of investigating the electrical properties of both groups of perovskites indicate a high lithium ion conductivity.     

Ionic conduction in plasticized PVC/PAN blend polymer electrolytes

Blended polymer electrolytes with poly(vinyl chloride) (PVC)–poly(acrylonitrile) (PAN) were prepared with different plasticizer concentrations and constant lithium perchlorate (LiClO₄) ratio by the solution-casting technique. The structure and complexation of the prepared films were studied by X-ray diffraction and Fourier transform infrared spectroscopy. The effect of the plasticizer on the ionic conduction in these electrolytes was investigated using alternating current impedance measurement and discussed. The temperature-dependant ionic conductivity was carried out in the range 302–373 K. The prepared films were also examined by thermogravimetry/differential thermal analysis to determine their thermal stability.     

A study on the blending effect of polyvinyledene fluoride in the ionic transport mechanism of plasticized polyvinyl chloride + lithium perchlorate gel polymer electrolytes

The gel polymer electrolytes composed of the blend of polyvinylchloride (PVC) and polyvinylidene fluoride (PVdF) as host polymers, the mixture of ethylene carbonate (EC) and propylene carbonate (PC) as a plasticizer, and LiClO₄ as a salt was studied. An attempt was made to investigate the effect of PVdF in the plasticized PVC + LiClO₄ system in three blend ratios. The differential scanning calorimetry study confirms the formation of polymer–salt complex and miscibility of the PVC and PVdF. The X-ray diffraction results of plasticized PVC (S1, S2, S3) and PVdF-blended films (S4, S5, S6) were compared, in that an increase in PVC concentration decreases the degree of crystallinity for S1 and S3, respectively, but drastically increases for PVC (S2). The increase in PVC content has not accounted in the conductivity studies also noted. However, the blending effect of PVdF showed decreases in crystallinity homogeneously for (S6 > S5 > S4), which were reflected in ionic conductivity measurements. The surface morphology of the films were also studied by scanning electron microscope, and it corroborates the same. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, Dec. 7–9, 2006.     

Star-shaped oligo(ethylene glycol) ethers (OEGE) as a new type of plasticizer for polymer gel electrolytes

Star-shaped oligo(ethylene glycol) methyl ethers (three and four arm molecules of various molecular weights) were synthesized and characterized with regard to viscosity, thermal transitions, ability to solvate the electrolyte LiCF₃SO₃ and the ionic conductivity of their LiCF₃SO₃ solutions in comparison to linear oligo(ethylene glycol) methyl ethers. Polymer gel electrolytes were prepared by photopolymerization of tri(ethylene glycol) dimethacrylate (EG₃DMA) or its copolymerization with the polar comonomer cyanomethyl methacrylate (CyMA) in the presence of the oligo(ethylene glycol) ethers mentioned above and of the electrolyte LiCF₃SO₃. The gels were characterized concerning their thermal transitions, thermo-mechanical properties, their ability to solvate the electrolyte and their ionic conductivity. In comparison to the linear plasticizers the star-shaped ones show a distinctly lower tendency to crystallize, which is even completely suppressed in several cases. Intensified ion association was found in LiCF₃SO₃ solutions of the star-shaped plasticizers, if the number of the ethylene glycol units per arm was lower than 4. Therefore, the conductivity of the solutions and the gels was lower than that with linear plasticizers at room temperature. The modification of the polymer matrix by copolymerization with 20 mol% CyMA resulted in a maximum of the ionic conductivity σ≈1×10⁻⁵ S/cm of gels with star-shaped plasticizers at 25 °C.