Synthesis and characterization of two ionic liquids with emphasis on their chemical stability towards metallic lithium

Two room temperature ionic liquids (RTILs) without acidic protons, based on different cationic species (1-n-butyl-2,3-dimethylimidazolium) (BMMI) and N-n-butyl-N-methylpiperidinium (BMP) using (CF₃SO₂)₂N⁻ (TFSI) as anion, were prepared by quaternization of their respective amines with an appropriate alkyl halide, followed by ion exchange reaction. All relevant properties of these ionic liquids, such as, thermal stability, density, viscosity, electrochemical behavior, ionic conductivity and self-diffusion coefficients for both ionic species, were determined at different temperatures. In spite of their ionic conductivity being lower than 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonylimide) (BMITFSI), the absence of an acidic proton in both compounds is crucial to maintain their chemical stability towards metallic lithium and, thereby, to make possible the safe assembly of lithium ion batteries. Both ionic liquids without acidic protons do not react with metallic lithium; on the other hand, the formation of carbene species when BMITFSI was exposed to Li was confirmed by ¹H and ¹³C nuclear magnetic resonance (NMR) and gas chromatography-mass spectrometry (GC-MS).     

Physical and electrochemical properties of 1-(2-hydroxyethyl)-3-methyl imidazolium and N-(2-hydroxyethyl)-N-methyl morpholinium ionic liquids

Various ionic liquids (ILs) were prepared via metathesis reaction from two kinds of 1-(2-hydroxyethyl)-3-methyl imidazolium ([HEMIm]⁺) and N-(2-hydroxyethyl)-N-methyl morphorinium ([HEMMor]⁺) cations and three kinds of tetrafluoroborate ([BF₄]⁻), bis(trifluoromethanesulfonyl)imide ([TFSI]⁻) and hexafluorophosphate ([PF₆]⁻) anions. All the [HEMIm]⁺ derivatives were in a liquid state at room temperature. In particular, [HEMIm][BF₄] and [HEMIm][TFSI] showed no possible melting point from −150 °C to 200 °C by DSC analysis, and their high thermal stability until 380-400 °C was verified by TGA analysis. Also, their stable electrochemical property (electrochemical window of more than 6.0 V) and high ionic conductivity (0.002-0.004 S cm⁻¹) further confirm that the suggested ILs are potential electrolytes for use in electrochemical devices. Simultaneously, the [HEMMor]⁺ derivatives have practical value in electrolyte applications because of their easy synthesis procedures, cheap morpholinium cation sources and possibilities of high Li⁺ mobility by oxygen group in the morpholinium cation. However, [HEMMor]⁺ derivatives showing high viscosity usually had lower ionic conductivities than [HEMIm]⁺ derivatives.     

Recent developments in the ENEA lithium metal battery project

Solvent-free P(EO)₂₀LiTFSI + PYR₁₄TFSI polymer electrolyte films with PYR₁₄⁺/Li⁺ mole ratios ranging from 0.96 to 3.22 were prepared by hot-pressing mixtures composed of PEO, LiTFSI and PYR₁₄TFSI of selected stoichiometries. The PYR₁₄TFSI room temperature ionic liquid (RTIL) is homogeneously incorporated into the P(EO)₂₀LiTFSI membrane without phase separation. For a PYR₁₄⁺/Li⁺ mole ratio of 3.22, the ionic conductivity was about 2 × 10⁻⁴ S/cm at 20 °C, i.e., more than one order of magnitude higher than that of the RTIL-free electrolyte. The electrochemical stability window of the polymer electrolyte containing the RTIL was about 6 V (versus Ag/Ag⁺). Li/V₂O₅ cells with the polymer electrolyte (PYR₁₄⁺/Li⁺ = 1.92) showed a 60% capacity retention after 80 cycles at 40 °C (the initial capacity was 210 mA h/g). Li/V₂O₅ cells (PYR₁₄⁺/Li⁺ = 1.28) held at 30 °C delivered about 93 mA h/g (at 0.057 mA/cm²), which corresponds to approximately 34% utilization of the active material. These results suggest that the incorporation of the RTILs into PEO-based polymer electrolytes is very promising for the future realization of solid-state lithium metal polymer batteries operating near ambient temperatures.     

1-Alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids as highly safe electrolyte for Li/LiFePO4 battery

Several 1-alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids (alkyl-DMimTFSI) were prepared by changing carbon chain lengths and configuration of the alkyl group, and their electrochemical properties and compatibility with Li/LiFePO₄ battery electrodes were investigated in detail. Experiments indicated the type of ionic liquid has a wide electrochemical window (−0.16 to 5.2 V vs. Li⁺/Li) and are theoretically feasible as an electrolyte for batteries with metallic lithium as anode. Addition of vinylene carbonate (VC) improves the compatibility of alkyl-DMimTFSI-based electrolytes towards lithium anode and LiFePO₄ cathode, and enhanced the formation of solid electrolyte interface to protect lithium anodes from corrosion. The electrochemical properties of the ionic liquids obviously depend on carbon chain length and configuration of the alkyl, including ionic conductivity, viscosity, and charge/discharge capacity etc. Among five alkyl-DMimTFSI-LiTFSI-VC electrolytes, Li/LiFePO₄ battery with the electrolyte-based on amyl-DMimTFSI shows best charge/discharge capacity and reversibility due to relatively high conductivity and low viscosity, its initial discharge capacity is about 152.6 mAh g⁻¹, which the value is near to theoretical specific capacity (170 mAh g⁻¹). Although the battery with electrolyte-based isooctyl-DMimTFSI has lowest initial discharge capacity (8.1 mAh g⁻¹) due to relatively poor conductivity and high viscosity, the value will be dramatically added to 129.6 mAh g⁻¹ when 10% propylene carbonate was introduced into the ternary electrolyte as diluent. These results clearly indicates this type of ionic liquids have fine application prospect for lithium batteries as highly safety electrolytes in the future.     

Electrochemical storage of polypyrrole-Fe2O3 nanocomposites in ionic liquids

Electroactive polypyrrole-Fe₂O₃ nanocomposite materials were prepared by chemical polymerization of pyrrole in aqueous Fe₂O₃ colloidal solution, using FeCl₃ as oxidant and tosylate anions (TS) as doping agent. The nanocomposite material named (PPyTSNC) was studied by X-ray diffraction analysis, Fourier Transform Infra-Red spectroscopy and thermogravimetric analysis. Their electrochemical storage properties were investigated on composite electrodes using 80% in weight of active materials in different immidazolium and pyrrolidinium based room temperature ionic liquids (RTILs) as electrolytes. Cyclic voltammetry and constant current charge discharge cycling showed high charge storage properties of the nanocomposite based electrodes in 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide (EMITFSI) and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) (respectively 72 mAh/g and 62 mAh/g at 1 mA/cm² discharge current) which are more than twice higher than the values obtained with pure PPy. These improvements in capacities have been attributed to the PPyTSNC morphology modification which ensures a large incorporation of the electrolyte inside the nanostructure. The specific capacitances of the nanocomposite electrodes reached 210 F/g and 190 F/g in EMITFSI and PYR₁₄TFSI and their cyclability has shown only 3-5% capacitance loss after one thousand cycles for both ionic liquids.     

Ionic liquids in electrochromic devices

The ionic liquid (PYR₁₄TFSI) has proved to be the key material to make a Li-ion conducting element of a complete electrochromic device, when interposed between transparent film electrodes like WO₃ and Li-charged V₂O₅. The key features of this ionic liquid and its mixtures with LiTFSI are the excellent transparency in the visible and NIR optical regions, the good ionic conductivity and the electrochemical compatibility with inorganic Li-intercalation oxide thin film electrodes used in electrochromic devices. The higher optical contrast found during WO₃ colouration with PYR₁₄TFSI-LiTFSI, compared to that in a conventional non-aqueous electrolyte like PC-LiTFSI, was attributed to the larger inertness of the former one (no decomposition reaction at the lowest electrode potential). This highly conductive ionic liquid has been incorporated into a polymer matrix (P(EO)₁₀LiTFSI), in order to obtain a transparent solid electrolyte with high Li ion conductivity and good mechanical stability. Finally this solid PYR₁₄TFSI-P(EO)₁₀LiTFSI transparent ion conductor was interposed between the same electrodes as above in order to yield a fully solid-state, Li-ion electrochromic device. This new solid electrolyte was able to transfer reversibly a Li ionic charge between 5 mC cm⁻² and 10 mC cm⁻² from the lithium storage electrode Li_xV₂O₅ to the WO₃ electrochromic electrode in less than 100 s at room T, darkening the device from an initial 80% to a final 30% transmittance (at 650 nm). Such a device has been tested first under various constant current conditions, and later under potentiostatic control using ±2 V steps. The latter method allows not only for a faster response of the electrochromic system, but provides also an easier life stability test of the device, which withstood 2000 cycles with little changes in its optical contrast.     

Effect of water and oxygen traces on the cathodic stability of N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide

Although research in the field of ionic liquids for electrochemical applications has led to a deeper knowledge in their electrochemical properties, doubts in the interpretation of the experimental results are still encountered in the literature due to the poor control of the experimental conditions and/or to the limited number of experiments conducted. In this work, the effect of water and oxygen traces on the cathodic stability window of hydrophobic, air-stable ionic liquids composed of N-alkyl-N-methylpyrrolidinium (PYR_1A⁺) cations and bis(trifluoromethanesulfonyl)imide (TFSI⁻) anion, is reported. The extensive investigation performed by linear sweep voltammetry (LSV) and cyclic voltammetry (CV) indicates that the TFSI⁻ anion is cathodically stable if the ionic liquid is pure and dry. The N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids investigated showed featureless cathodic linear sweep voltammetry curves before the massive cation decomposition took place at very low potentials.     

Ionic liquid electrolytes based on multi-methoxyethyl substituted ammoniums and perfluorinated sulfonimides: Preparation, characterization, and properties

New functionalized ionic liquids (ILs), comprised of multi-methoxyethyl substituted quaternary ammonium cations (i.e. [N(CH₂CH₂OCH₃)_4−n(R)_n]⁺; n = 1, R = CH₃OCH₂CH₂; n = 1, R = CH₃, CH₂CH₃; n = 2, R = CH₃CH₂), and two representative perfluorinated sulfonimide anions (i.e. bis(fluorosulfonyl)imide (FSI⁻) and bis(trifluoromethanesulfonyl)imide (TFSI⁻)), were prepared. Their fundamental properties, including phase transition, thermal stability, viscosity, density, specific conductivity and electrochemical window, were extensively characterized. These multi-ether functionalized ionic liquids exhibit good capability of dissolving lithium salts. Their binary electrolytes containing high concentration of the corresponding lithium salt ([Li⁺] >1.6 mol kg⁻¹) show Li⁺ ion transference number (tLi⁺) as high as 0.6-0.7. Their electrochemical stability allows Li deposition/stripping realized at room temperature. The desired properties of these multi-ether functionalized ionic liquids make them potential electrolytes for Li (or Li-ion) batteries.     

Rechargeable Li/LiFePO4 cells using N-methyl-N-butyl pyrrolidinium bis(trifluoromethane sulfonyl)imide-LiTFSI electrolyte incorporating polymer additives

We have incorporated polymer additives such as poly(ethylene glycol) dimethyl ether (PEGDME) and tetra(ethylene glycol) dimethyl ether (TEGDME) into N-methyl-N-butylpyrrolidinium bis(trifluoromethane sulfonyl)imide (PYR₁₄TFSI)-LiTFSI mixtures. The resulting PYR₁₄TFSI + LiTFSI + polymer additive ternary electrolyte exhibited relatively high ionic conductivity as well as remarkably low viscosity over a wide temperature range compared to the PYR₁₄TFSI + LiTFSI binary electrolytes. The charge/discharge cyclability of Li/LiFePO₄ cells containing the ternary electrolytes was investigated. We found that Li/PYR₁₄TFSI + LiTFSI + PEGDME (or TEGDME)/LiFePO₄ cells containing the two different polymer additives showed very similar discharge capacity behavior, with very stable cyclability at room temperature (RT). Li/PYR₁₄TFSI + LiTFSI + TEGDME/LiFePO₄ cells can deliver about 127 mAh/g of LiFePO₄ (74.7% of theoretical capacity) at 0.054 mA/cm² (0.2C rate) at RT and about 108 mAh/g of LiFePO₄ (63.4% of theoretical capacity) at 0.023 mA/cm² (0.1C rate) at −1 °C for the first discharge. The cell exhibited a capacity fading rate of approximately 0.09-0.15% per cycle over 50 cycles at RT. Consequently, the PYR₁₄TFSI + LiTFSI + polymer additive ternary mixture is a promising electrolyte for cells using lithium metal electrodes such as the Li/LiFePO₄ cell reported here. These cells showed the capability of operating over a significant temperature range (∼0-∼30 °C).     

Improved ionic conductivity of nitrile rubber/ionic liquid composites

Polymer electrolytes with high ionic conductivity and good elasticity were prepared by mixing nitrile rubber (poly(acrylonitrile-co-butadiene) rubber; NBR) with ionic liquid, N-ethylimidazolium bis(trifluoromethanesulfonyl)imide (EImTFSI). The NBR/EImTFSI composites were obtained as homogeneous and transparent films when the ionic liquid content was less than 60 wt%. Raman spectroscopy suggested the interaction between nitrile group of NBR and TFSI anion. Sample with ionic liquid content of 50 wt% showed the ionic conductivity of 1.2×10⁻⁵ S cm⁻¹ at 30 °C. Addition of lithium salt to this NBR/EImTFSI composite further enhanced the ionic conductivity to about 10⁻⁴ S cm⁻¹ without spoiling mechanical properties. DSC studies showed two glass transition temperatures for composites indicating microphase separation.     

Molten lithium sulfonimide salt having poly(propylene oxide) tail

Poly(propylene oxide) (PPO) tailed lithium(trifluoromethyl sulfonylimide)s (TFSI-PPO) were prepared as non-onium type ionic liquid polymers. Introduction of PPO chain to the TFSI salt group resulted in lower the glass transition temperature (T_g) and induce the salt dissociation. The TFSI-PPO showed relatively high ionic conductivity owing to the high dissociation degree of the TFSI salt group. The maximum ionic conductivity of 3.3×10⁻⁶ S cm⁻¹ was observed at 30 °C for TFSI salt having PPO tail with number average molecular weight of 850. On the other hand, PPOs having the same salt moiety on both chain ends ((TFSI)₂-PPO) showed higher T_g than that of TFSI-PPOs. The lithium transference number of the (TFSI)₂-PPO with PPO chain length of Mn=2000 was 0.74 in spite of slightly lower ionic conductivity.     

Electrochemical properties of novel ionic liquids for electric double layer capacitor applications

An aliphatic quaternary ammonium salt which has a methoxyethyl group on the nitrogen atom formed an ionic liquid (room temperature molten salt) when combined with the tetrafluoroborate (BF₄⁻) and bis(trifluoromethylsulfonyl)imide [TFSI; (CF₃SO₂)₂N⁻] anions. The limiting oxidation and reduction potentials, specific conductivity, and some other physicochemical properties of the novel ionic liquids, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEME-BF₄) and DEME-TFSI have been evaluated and compared with those of 1-ethyl-3-methylimidazolium tetrafluoroborate. DEME-BF₄ is a practically useful ionic liquid for electrochemical capacitors as it has a quite wide potential window (6.0 V) and high ionic conductivity (4.8 mS cm⁻¹ at 25 °C). We prepared an electric double layer capacitor (EDLC) composed of a pair of activated carbon electrodes and DEME-BF₄ as the electrolyte. This EDLC (working voltage ∼2.5 V) has both, a higher capacity above room temperature and a better charge-discharge cycle durability at 100 °C when compared to a conventional EDLC using an organic liquid electrolyte such as a tetraethylammonium tetrafluoroborate in propylene carbonate.     

Electropolymerization of poly(3-methylthiophene) in pyrrolidinium-based ionic liquids for hybrid supercapacitors

The ionic liquids (ILs) N-butyl-N-methyl-pyrrolidinium trifluoromethanesulfonate (PYR₁₄Tf) and N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide (PYR₁₃FSI) are investigated as electropolymerization media for poly(3-methylthiophene) (pMeT) in view of their use in carbon/IL/pMeT hybrid supercapacitors. Data on the viscosity, solvent polarity, conductivity and electrochemical stability of PYR₁₄Tf and PYR₁₃FSI as well as the effect of their properties on the electropolymerization and electrochemical performance of pMeT, which features >200 Fg⁻¹ at 60 °C when prepared and tested in such ILs, are reported and discussed; the results of the electrochemical characterization in N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide of the so-obtained pMeT are also given, for comparison.     

Asymmetrical dicationic ionic liquids based on both imidazolium and aliphatic ammonium as potential electrolyte additives applied to lithium secondary batteries

Asymmetrical dicationic ionic liquids based on the combination of imidazolium and aliphatic ammonium cations with TFSI anion, MIC_nN₁₁₁-TFSI₂, have been synthesized for the first time, wherein MI represents imidazolium cation, N₁₁₁ represents trimethylammonium cation, and C_n represents spacer length. The physical and electrochemical properties of this family of ionic liquids were studied. 1-(3-Methylimidazolium-1-yl)ethane-(trimethylammonium) bi[bis(trifluoromethane-sulfonyl) imide] (MIC₂N₁₁₁-TFSI₂) shows solid-solid transition characteristics. 1-(3-Methylimidazolium-1-yl)pentane-(trimethylammonium) bi[bis(trifluoromethan-esulfonyl)imide] (MIC₅N₁₁₁-TFSI₂) has one of the lowest solid-liquid transformation temperatures among analogues, and belongs to the greatest thermal stable ionic liquids. Additionally, it has an order of conductivity of 10⁻¹ ms cm⁻¹, and electrochemical window of about 3.7 V at room temperature. To evaluate the potential of MIC₅N₁₁₁-TFSI₂ as an additive of electrolyte for lithium secondary batteries, cells composed of LiMn₂O₄ cathode/1 M LiPF₆ in EC:DMC (1:1, v/v) electrolytic solution containing 5 wt% of MIC₅N₁₁₁-TFSI₂/lithium metal anode have been prepared. The charge-discharge cycling test reveals that unlike the cases of Li/LiMn₂O₄ cells employing a conventional electrolyte with a monocationic ionic liquid, such as 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EtMeImTFSI) as an additive, the performances of Li/LiMn₂O₄ cells do not drop with the addition of MIC₅N₁₁₁-TFSI₂ at 1C rate, moreover, the cell exhibits better discharge capacity and cycle durability compared with the cell using the conventional electrolyte.     

Ionic liquids containing an alkyl sulfate group as potential electrolytes

Various types of ionic liquid (IL) containing an alkyl sulfate group are synthesized and their physical and electrochemical properties are investigated. The temperature dependency of dynamic viscosity and ionic conductivity are measured for these ILs. The low temperature phase behaviour of the ethylsulfate salts is investigated using differential scanning calorimetry. Three ethylsulfate-containing ionic liquids exhibit wide electrochemical windows of about 5.0 V, and one pyrrolidinium-containing ionic liquid shows a conductivity of 3.8 mS cm⁻¹. The various cations of alkylsulfate-containing ionic liquids are shown to greatly influence viscosity, density, and conductivity. Absorbance solvatochromic probe, Nile Red is used to investigate the relative polarity of alkylsulfate base ionic liquids compared with several organic solvents. The electrochemical and thermal stabilities of these ILs make them promising electrolytes for use in electrochemical devices.     

Spectroscopic study of the electrochemical behaviour of tantalum(V) chloride and oxochloride species in 1-butyl-1-methylpyrrolidinium chloride

FTIR spectroscopy was used to identify the oxochloride species of tantalum(V) in ionic liquids and to confirm the correlations between their presence in electrolytes and the changes in the route of electrochemical reduction of tantalum(V). Electrochemical behaviour of the mixtures (x)1-butyl-1-methyl-pyrrolidinium chloride-(1 − x)TaCl₅ at x = 0.80, 0.65, and 0.40 was investigated over the temperature range 90-160 °C with respect to the electrochemical deposition of tantalum and was discussed in terms of spectroscopic data. The mechanism of electrochemical reduction of tantalum(V) in the basic and acidic electrolytes depends strongly on the structure and composition of the electro active species of tantalum(V) defined by the molar composition of ionic liquids and on the competition between tantalum(V) chloride and oxochloride species. In the basic mixture at x = 0.80, with octahedral [TaCl₆]⁻ ions as the electrochemically active species only the first reduction step Ta⁵⁺ → Ta⁴⁺ at −0.31 V was observed. The competitive reduction of tantalum(V) oxochloride species occurs at more anodic potential (−0.01 V) than the reduction of the chloride complexes and can restrict the further reduction of tantalum(IV). In the basic ionic liquid at x = 0.65, the cyclic voltammograms exhibit reduction peaks at −0.31 V and −0.51 V attributed to the diffusion controlled process as [TaCl₆]⁻ + e⁻ → [TaCl₆]²⁻ and [TaCl₆]²⁻ + e⁻ → [TaCl₆]³⁻. The further irreversible reduction of tantalum(III) to metallic state may occur at −2.1 V. In the acidic ionic liquids, at x = 0.40 the electrochemical reduction of two species occurs, TaCl₆⁻ and Ta₂Cl₁₁⁻ and it is limited by two electron transfer for both of them at −0.3 V and −1.5 V, respectively.     

Super soft elastomers as ionic conductors

Melts of linear brush polymers with PEO side chains attached at each repeat unit of the backbones have been doped with CF₃SO₃⁻Li⁺. Mechanical properties and ionic conductivity of such systems have been analyzed using mechanical and dielectric spectroscopies. Mechanical spectra indicated a presence of super soft states for samples with long backbones or for systems which have been slightly cross-linked (G′<10⁴ Pa). In the case of the polymer with longer crystallizing PEO side chains (MW_av=1100 g/mol), the ionic conductivity reaching the 10⁻³ S/cm level at the optimum CF₃SO₃⁻Li⁺ concentration (EO/Li⁺=10:1) have been detected at temperatures not far above the room temperature. The presence of lithium ions suppresses completely the crystallization of PEO side chains.     

Ionic liquids based on guanidinium cations and TFSI anion as potential electrolytes

Sixteen new guanidinium salts based on small cations and TFSI⁻ anion were prepared and characterized. Physical and electrochemical properties of these products, including melting point, thermal stability, viscosity, conductivity and electrochemical window were investigated. Reducing symmetry of cations can reduce the melting points, and 12 products are liquids at room temperature. The viscosities of cg22TFSI, cg12TFSI and cg13TFSI were 45, 46 and 52 mPa s at 25 °C, respectively. Electrochemical and thermal stabilities of these ILs permitted them to become promising electrolytes used in electrochemical devices.     

The influence of air and its components on the cathodic stability of N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide

Although water- and air-stable ionic liquids have been in use for some years, experiments found in the literature are still performed in inert gas with ppm levels of oxygen and water. In this study, the influence of different environments (vacuum, argon, nitrogen, air and oxygen and water) on the cathodic electrochemical window of the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) is reported and compared with investigations and processes found in the literature. The investigation indicates that this ionic liquid is highly stable in a vacuum and under argon flow. However, its cathodic stability is reduced in nitrogen and dry air. The simultaneous presence of water and air strongly affected the useful electrochemical window, as seen previously for imidazolium-based ionic liquids.     

Increasing ionic conductivity and mechanical strength of a plastic electrolyte by inclusion of a polymer

In this contribution we present a soft matter solid electrolyte which was obtained by inclusion of a polymer (polyacrylonitrile, PAN) in LiClO₄/LiTFSI-succinonitrile (SN), a semi-solid organic plastic electrolyte. Addition of the polymer resulted in considerable enhancement in ionic conductivity as well as mechanical strength of LiX-SN (X = ClO₄, TFSI) plastic electrolyte. Ionic conductivity of 92.5%-[1 M LiClO₄-SN]:7.5%-PAN (PAN amount as per SN weight) composite at 25 °C recorded a remarkably high value of 7 × 10⁻³ Ω⁻¹ cm⁻¹, higher by few tens of order in magnitude compared to 1 M LiClO₄-SN. Composite conductivity at sub-ambient temperature is also quite high. At −20 °C, the ionic conductivity of (100 − x)%-[1 M LiClO₄-SN]:x%-PAN composites are in the range 3 × 10⁻⁵-4.5 × 10⁻⁴ Ω⁻¹ cm⁻¹, approximately one to two orders of magnitude higher with respect to 1 M LiClO₄-SN electrolyte conductivity. Addition of PAN resulted in an increase of the Young's modulus (Y) from Y → 0 for LiClO₄-SN to a maximum of 0.4 MPa for the composites. Microstructural studies based on X-ray diffraction, differential scanning calorimetry and Fourier transform infrared spectroscopy suggest that enhancement in composite ionic conductivity is a combined effect of decrease in crystallinity and enhanced trans conformer concentration.