Ethylene carbonate—propylene carbonate mixed electrolytes for lithium batteries

Electrolytic characteristics of propylene carbonate (PC)ethylene carbonate (EC) mixed electrolytes were studied, compared with those in PC electrolytes. Conductivity and Li charge—discharge efficiency values increased with EC contents increasing. For example, 1 M LiClO₄ECPC (EC mixing molar ratio; [EC]/[PC] = 4) showed the conductivity of 8.5 ohm^?1 cm^?1, which value was 40% higher than that in PC. Also, 1 M LiClO₄ECPC([EC]/[PC] = 5) showed the Li charge—discharge efficiency of 90.5% at 0.5 mA cm^?2, 0.6 C cm^?2, which value was ca. 25% higher than that in PC. ECPC mixed electrolytes were considered to be practically available for ambient lithium batteries in regard to the high Li⁺ ion conductivity and also high Li charge—discharge efficiency.     

Electrolytic characteristics of mixed solvent electrolytes for lithium secondary batteries

Measurements on conductivity and Li charge-discharge efficiency in various propylene carbonate (PC)-based electrolytes were carried out to obtain electrolytes for Li secondary batteries. Among the electrolytes examined, 2 M LiClO₄—PC—THF (PC/THF volume ratio = $\text{4}\text{6}$) showed 1.6 times higher conductivity of 9.8 × 10^?3 Ω^?1 cm^?1 and also ca. 10% higher Li charge-discharge efficiency of 81.3% at 5 mA cm^?2 (0.3 C cm^?2) than those in 1 M LiClO₄—PC. Generally, Li cycling efficiency increases with increase in electrolyte conductivity. From the analysis made on electrolytic parameters, such as transport number of Li⁺ ion, it was concluded that conductivity and Li cycling efficiency increases were caused by the total effects of lower chemical reactivity of THF to Li and smaller practical Li⁺ ion radius based on Li⁺—THF complex formation.     

Lithium cycling efficiency and conductivity for high dielectric solvent/low viscosity solvent mixed systems

Lithium cycling efficiency on a lithium substrate (Li-on-Li cycling) and conductivity for various mixed solvent systems of high dielectric solvent (HDS) and low viscosity solvent (LVS) were examined for secondary lithium batteries. For the HDS, sulfolane, dimethylsulfoxide, γ-lactones, propylene carbonate (PC) and ethylene carbonate (EC) were used. For the LVS, tetrahydrofuran (THF), 2-methyl-THF, 1,2-dialkoxyethanes and 1,3-dioxolane (DOL) were used. For the solute, LiAsF₆, LiBF₄, LiCF₃CO₃ and LiClO₄ were used. Lithium cycling efficiencies newly measured on a Li substrate (E_a) for EC/LVS or PC/LVS were ca 5% or 15% higher than those previously obtained by simple cycling of Li on a Pt substrate, while the order of Li cycling efficiencies to LVS change is similar in both cases, except for EC/DOL or PC/DOL. The reasons seem to be that the Li-on-Li cycling minimizes the influence of electrochemical Li/Pt alloying and partial solvent oxidation during the cycle on Li cycling efficiency. The E_a values in HDS/LVS mixed systems incorporating LiAsF₆ or LiClO₄ tended to increase with a decrease in the reactivity to Li, of not only LVS but also HDS. EC/THF systems incorporating LiAsF₆ or LiClO₄ showed high E_a values of ca 95% even by Li-on-Li cycling, the value being higher than those (ca 92%) for LiBF₄ or LiCF₃SO₃ systems. In addition, for all the HDS/LVS mixed systems examined in this work, conductivities were higher than those for HDS or LVS single solvent systems. In regard to both conductivity and Li cycling efficiency, HDS/LVS mixed systems are considered to be effective in various lithium battery applications.     

Conductivity and viscosity studies of ethylene carbonate based solutions containing lithium perchlorate

Specific conductivities and viscosities of lithium perchlorate at four different concentrations (0.5, 1.0, 1.5 and 2.0 M) in ethylene carbonate (EC) based binary mixed solvent systems at 25°C are reported. The co-solvents chosen were tetrahydrofuran (THF), 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL). Viscosity variations in all the three mixed solvent systems without electrolyte showed negative deviation from ideal behaviour thereby indicating the occurrence of a structure breaking effect in these three different binary systems. The increase in viscosity with increase in concentration of LiClO₄ is attributed to the structural enhancement through the formation of a solvated complex which occupies interstitials in the solvent mixtures. 1 M LiClO₄ solution shows maximum specific conductivity at 30 vol % EC for EC + DME and EC + DOL mixtures and at 50 vol % EC for EC + THF mixtures. Conductivity variations are explained on the basis of preferential solvation of lithium perchlorate by co-solvents (THF, DME and DOL) in their respective mixtures with ethylene carbonate.     

Ternary and quaternary mixed electrolytes for lithium cells

This paper reports the influence of composition of mixed solvent electrolyte composition on the discharge capacity and charge–discharge cycle life of lithium metal/amorphous V2O5–P2O5 (95:5 in molar ratio) cells. The solvents used were ethylene carbonate (EC), propylene carbonate (PC), 2-methyltetrahydrofuran (2MeTHF) and THF. LiAsF6 was used as the solute. The electrolyte solutions examined here contain ternary and quaternary mixed systems. The purpose of this work is to obtain an electrolyte solution which realizes a higher rate capability and/or a longer cycle life than the previously studied EC:PC:2MeTHF (15:70:15) ternary mixed system. Of the electrolyte systems examined here, the EC:PC:2MeTHF (30:40:30 in volume) ternary mixed solvent system showed the best cell performance. In addition, a heating test was carried out on an AA- size lithium cell with EC:PC:2MeTHF (30:40:30) as a fundamental abuse test to ensure cell safety.     

Lithium cycling efficiency and conductivity for γ-lactone-based electrolytes

Lithium cycling efficiency on a lithium substrate as well as conductivity were examined for-lactonebased electrolytes incorporating LiClO₄ for use in nonaqueous lithium secondary batteries.-butyrolactone (BL),-valerolactone and-octanoiclactone were used. Conductivity increased with a decrease in viscosity for lactone. Lithium cycling efficiency tended to increase with a decrease in reactivity between lithium and lactone, which would be expected from the oxidation potential for lactone. In order to decrease viscosity, tetrahydrofuran (THF) was mixed with lactone. Conductivity for lactone/THF was higher than those for systems using either lactone or THF alone. For example, 1 M LiClO₄-BL/THF (mixing volume ratio =11) showed conductivity of 13.0 × 10^–3 S cm^–1, approximately 20% higher than that for BL. Lithium cycling efficiency for BL/THF, which exceeded 90%, was also higher than that for BL. Morphology of the deposited lithium in BL/THF was smoother than that in BL and similar to that in THF, as observed with a scanning electron microscope. The reason for the enhancement of the lithium cycling efficiency for BL/THF seems to be the adsorption of THF or THF-Li⁺ around the deposited Li, which has lower reactivity to Li and higher solvation power to Li⁺ than BL.     

Cycling performance and safety of rechargeable lithium cells with binary and ternary mixed solvent electrolytes

The influence of electrolyte composition on the cycling performance and safety of AA rechargeable cells with a lithium metal anode, and an amorphous (a-) V₂O₅-P₂O₅ cathode was examined. The cells were cycled at a discharge current of 1000 mA and a charging current of 200 mA. The electrolytes were composed of ethylene carbonate (EC)/2-methyltetrahydrofuran (2MeTHF) binary and EC/propylene carbonate (PC)/2MeTHF ternary mixed solvents containing 40–70 vol% 2MeTHF to provide higher conductivity. The solute was 1.5mol dm^–3 LiAsF₆. The cycle life of the AA cells was evaluated by setting the end of cycle life at the cycle number where the discharge capacity fell to 50% of its maximum value. Cells with EC/2MeTHF (50:50) exhibited the longest cycle life among all the electrolytes examined here. Cells with EC/PC/2MeTHF (15:45:40) had the longest cycle life among the ternary mixed solvents systems. Fundamental abuse tests were also carried out on AA cells, which were cycled twice (fresh cells), cycled 100 times and cycled until the end of their cycle life. Neither the fresh nor the cycled cells with EC/PC/2MeTHF (15:45:40 ) smoked nor ignited in a 150 °C heating test or in an external short circuit test. However, the fresh cell with EC/2MeTHF (50:50) ignited in the 150 °C heating test. Summarizing the cycling and the abuse test results, the EC/PC/2MeTHF (15:45:40) ternary mixed systems exhibited the best performance. However, in terms of practical use, cell safety still requires further improvement.     

The effect of desiccants on the cycling efficiency of the lithium electrode in propylene carbonate-based electrolytes

Cycling efficiencies of the Li electrode ,in propylene carbonate (PC) 1 M in either LiClO₄ or LiAsF₆ were assessed as a function of electrolyte purification procedure. The use of neutral alumina and galvanostatic pre-electrolysis resulted in the highest efficiency values to date. While cyclic voltammograms at Pt or vitreous C were insensitive to electrolyte impurities, voltammograms on Ni about the Li potential were very informative. Thus, the repeated deposition and subsequent removal of thin (2 mC/cm²) Li plates revealed enhanced nucleation but diminished rate of growth of the nuclei as cycling progresses. A model of Li encapsulation is proposed to account for the eventual failure of the Li electrode.     

A study of electrochemical kinetics of lithium ion in organic electrolytes

Limiting current densities equivalent to the transport-controlling step of lithium ions in organic electrolytes were measured by using a rotating disk electrode (RDE). The diffusion coefficients of lithium ion in the electrolyte of PC/LiClO₄, EC : DEC/LiPF₆ and EC : DMC/LiPF₆ were determined by the limiting current density data according to the Levich equation. The diffusion coefficients increased in the order of PC/LiClO₄<EC : DEC/LiPF₆<EC : DMC/ LiPF₆ with respect to molar concentration of lithium salt. The maximum value of diffusivity was 1.39x10-5cm²/s for 1M LiPF₆ in EC : DMC=1 : 1. Exchange current densities and transfer coefficients of each electrolyte were determined according to the Butler-Volmer equation.     

Li NMR, ionic conductivity and self-diffusion coefficients of lithium ion and solvent of plasticized organic-inorganic hybrid electrolyte based on PPG-PEG-PPG diamine and alkoxysilanes

Organic-inorganic hybrid electrolytes based on poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol) bis(2-aminopropyl ether) (D2000) complexed with LiClO₄ via the co-condensation of an epoxy trialkoxysilane and tetraethoxysilane have been prepared and plasticized by a solution of ethylene carbonate (EC)/propylene carbonate (PC) mixture (1:1 by weight). The cross-linked hybrid network shows no solvent exudation and retains a large amount of plasticizer over 70 wt.% in stable state. The in situ built in silica network provides the hybrid electrolytes with good mechanical properties. The ionic conductivity of the dry hybrid electrolyte films was enhanced by two orders of magnitude via plasticization, reaching a maximum conductivity value of 4.0 × 10⁻³ S/cm at 30 °C. Variable temperature ⁷Li-{¹H} magic angle spinning (MAS) NMR demonstrated that the Li⁺ cations can be complexed by the polymer network as well as by the plasticizing solvents, but not with the incorporated silica network. Furthermore, the ⁷Li chemical shift change indicated a progressive change in the lithium coordination from lithium-polymer to lithium-solvent with increasing temperatures. The role of the solvents and the mobility of the lithium ions were investigated by pulsed gradient spin echo (PGSE) NMR measurements to elucidate the behavior of the ionic conductivity.     

Effects of quinoneimine dyes on lithium cycling efficiency for LiClO4-propylene carbonate

Additive effects of quinoneimine dyes (QIDs) on Li cycling efficiency (E _ff) were examined in 1 M LiClO₄-propylene carbonate (PC). TheE _ff values were measured galvanostatically on a Pt working electrode. TheE _ff values for solutions with QID addition were higher than those for PC alone and theE _k values depended on cycling current density and on the amounts of QID added. For example, 1 M LiClO₄-PC with added methylene blue (MB) (10^–3 M addition) showedE _ff values exceeding 90% at 0.5mA cm^–2, 0.6 C cm^–2, while theE _ff values for PC alone were approximately 65%. From observation with a scanning electron microscope the morphology for the deposited Li in solution with MB added was found to be smoother than that in PC alone. TheE _ff values for solutions with QID added tended to increase with an increase in the reduction potential for QID vs Li-Li₊. The enhancement of the Li cycling efficiency by QID addition seems to be caused by Li₊ ion-conductive film formation on the deposited Li surface, resulting from the reaction between QID and the deposited Li, that suppresses the reaction between PC and the deposited Li. This film formation was strongly suggested by the measurement of the resistance on the Li surface.     

Dimethyl sulfoxide-based electrolytes for rechargeable lithium batteries

Mixed solutions of dimethyl sulfoxide (DMSO) and low viscosity solvents have been examined as a solvent of the electrolyte for rechargeable lithium (Li) batteries. The electrolytic conductivities of LiClO₄. LiBF₄ and LiPF₆ were measured as a function of the solvent composition. Maximum conductivities were observed in the DMSO concentration ranges of 60–80 mol% for LiClO₄ and LiBF₄, and 20–60 mol% for LiPF₆. The highest conductivity of all examined systems was 1.6 × 10⁻² S cm⁻¹ in the solution containing 1,2-dimethoxyethane (DME) and LiPF₆ as the co-solvent and the electrolyte, respectively. Polarization behavior and charge-discharge characteristics of the lithium electrode were investigated in the DMSO-based solutions. The cycling efficiency was markedly dependent not only on the co-solvent but also the Li salt. The highest efficiency on the nickel substrate was observed in LiPF₆ (1 mol dm⁻³)/DMSO-DME (1:1 by volume). High rechargeability of Li was also expected in the solution containing LiClO₄ or LiBF₄ when aluminum was used as the substrate.     

A novel electrospun TPU/PVdF porous fibrous polymer electrolyte for lithium ion batteries

Novel blend-based gel polymer electrolyte (GPE) films of thermoplastic polyurethane (TPU) and poly(vinylidene fluoride) (PVdF) (denoted as TPU/PVdF) have been prepared by electrospinning. The electrospun thermoplastic polyurethane-co-poly (vinylidene fluoride) membranes were activated with a 1M solution of LiClO₄ in EC/PC and showed a high ionic conductivity about 1.6 mS cm⁻¹ at room temperature. The electrochemical stability is at 5.0 V versus Li⁺/Li, making them suitable for practical applications in lithium cells. Cycling tests of Li/GPE/LiFePO4 cells showed the suitability of the electrospun membranes made of TPU/PVdF (80/20, w/w) for applications in lithium rechargeable batteries. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Ionic conductivity of hybrid films based on polyacrylonitrile and their battery application

The alternate current (AC) and direct current (DC) ionic conductivity of hybrid films composed of polyacrylonitrile (PAN), lithium perchlorate (LiClO₄), and a plasticizer was studied. Three kinds of the plasticizer [ethylene carbonate (EC), propylene carbonate (PC), N,N-dimethylformamide (DMF)] were used. Suitability of these hybrid films for lithium battery was investigated. The AC conductivity, which represents bulk ionic conductivity, was dependent on the component and the composition of the hybrid films, ranging from 10^?4?10^?8 Scm^?1. The AC conductivity was mainly determined by the molar ratio of [plasticizer]/[LiClO₄] in the hybrid films and increased with the increase in this ratio. The effect of the plasticizer on the enhancement in the AC conductivity was in the following order. DMF>EC>PC. The hybrid films with both electrodes of lithium showed the stable DC conductivity of about 1/10 of the AC conductivity, except for the hybrid films containing DMF. The hybrid films were found to be effective as a lithium ionic conductor. The galvanic cell. Li/sample/MnO₂, at the discharge current density of 90 μA/cm² showed the stable electromotive force of about 3 V for 70 h.     

Behaviour of highly crystalline graphitic materials in lithium-ion cells with propylene carbonate containing electrolytes: An in situ Raman and SEM study

The microcrystalline flaked graphites SFG6 and SFG44 were evaluated with regard to their compatibility with propylene carbonate (PC) by in situ Raman microscopy and postmortem scanning electron microscopy (SEM) study. PC is employed as electrolyte component in lithium-ion batteries. However, when used with certain types of graphitic materials, exfoliation occurs. To compare the effects of exfoliation, the first lithium insertion properties of these graphitic materials were measured with in situ Raman microscopy. Lithium half-cells containing either 1 M LiClO₄ 1:1 (w/w) ethylene carbonate (EC):dimethyl carbonate (DMC) or 1:1 (w/w) EC:PC were investigated. The commencement of the exfoliation process was detected in SFG44 EC:PC by the appearance of a shoulder band at 1597 cm⁻¹ on the G-band (1584 cm⁻¹) below 0.9 V versus Li/Li⁺. The band (assigned as the exfoliation or E-band) at higher wavenumbers (1597 cm⁻¹) corresponded to solvated lithium ions intercalated into graphite. The in situ Raman spectra of SFG6 in EC:DMC or EC:PC and SFG44 in EC:DMC did not show the E-band and instead displayed regular lithium intercalation spectra.In situ Raman microscopy and SEM were further employed to study the exfoliation process observed for SFG44 in 1:1 (w/w) EC:PC, when the potential was held under steady-state conditions at 0.8, 0.6 and 0.3 V, respectively. A blue-shift in the E-band from 1597 to 1607 cm⁻¹ was observed as the potential was lowered. SEM images showed dissimilar degrees of exfoliation at these three potentials.     

Study of lithiated Nafion ionomer for lithium batteries

Lithiated Nafion 112 ionomer was characterized by FT-IR spectroscopy, AC impedance, and cyclic voltammetry. The ionomer swollen with mixed solvents of propylene carbonate (PC) and ethylene carbonate shows ionic conductivity of 8.18×10^–5Scm^–1 at 25°C and good electrochemical stability to allow operation in Li/ionomer/LiCoO₂ cells. The discharge capacity of the first cycle is 126mAhg^–1. Significant capacity loss occurs during cycling due to the presence of PC. AC impedance shows that the passive layer formed at the Li/ionomer interface dominates the cycling performance of the cell.     

Blending poly(methyl methacrylate) and poly(styrene‐co‐acrylonitrile) as composite polymer electrolyte

A blend of poly(methyl methacrylate) (PMMA) and poly(styrene‐co‐acrylonitrile) (PSAN) has been evaluated as a composite polymer electrolyte by means of differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, ac impedance measurements, and linear sweep voltammetry (LSV). The blends show an interaction with the Li⁺ ions when complexed with lithium perchlorate (LiClO₄), which results in an increase in the glass‐transition temperature (T_g) of the blends. The purpose of using PSAN as another component of the blend is to improve the poor mechanical properties of PMMA‐based plasticized electrolytes. The mechanical property is further improved by introducing fumed silica as inert filler, and hence the liquid electrolyte uptake and ionic conductivity of the composite systems are increased. Room‐temperature conductivity of the order of 10^?4 S/cm has been achieved for one of the composite electrolytes made from a 1/1 blend of PSAN and PMMA containing 120% liquid electrolyte [1M LiClO₄/propylene carbonate (PC)] and 10% fumed silica. These systems also showed good compatibility with Li electrodes and sufficient electrochemical stability for safe operation in Li batteries. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1319–1328, 2001     

Electrochemical analyses of ZrO2 dispersoid incorporated poly (styrene-methyl methacrylate) blend gel electrolytes for lithium-ion battery

Zirconium oxide (ZrO₂) filler is successfully synthesized with spherical morphology (particle size 170 nm) by co-precipitation technique earlier. The as-prepared ZrO₂ -(bare, 3, 6, 9, and 12 wt%) is spread into the augmented poly(styrene-co- methyl methacrylate) P(S-MMA)- poly vinylidene fluoride (PVdF) (25:75 of 27 wt%)-LiClO₄ (8 wt%)- ethylene carbonate and propylene carbonate (EC + PC) (1: 1 of 65 wt%) system. The solution casting technique is employed throughout the process. The structural, morphology, thermal and ionic conducting behavior of sample are examined. The highest conductivity is 1.2 × 10⁻² S cm⁻¹ at 303 K for P(S-MMA)-PVdF (25:75 of 27 wt%) LiClO₄ (8 wt%)- EC + PC (65 wt%) +6 wt% ZrO₂ system. The linear sweep voltammetry and the cyclic voltammetry tests are performed and the results are discussed. The optimized electrolyte is used to make the LiFePO₄/CGPE/Li confined 2032 coin cell couple. It holds the discharge capacity of 144 mAh g⁻¹ at rate of 0.1 C with 88% coulombic efficiency.     

Reaction of dinaphthyl and diphenyl ethers at liquefaction conditions

The reactions of 2,2′-dinaphthyl ether and diphenyl ether were studied at 375–425°C using 6.9 MPa (cold) hydrogen or nitrogen, 9,10-dihydrophenanthrene (DHP) and decalin as solvents, and a molybdenum sulfide catalyst. We chose to examine these compounds as models for the cleavage of diaryl ether bridges during coal liquefaction. The molybdenum sulfide was added to the reaction as MoS₃, which should transform to the active MoS₂ catalyst. Cleavage of the C_arO in 2,2′-dinaphthyl ether, at reaction temperatures of 375 and 400°C, proceeded in the sequence H₂ < DHPN₂ < DHPH₂ < DHPMoS₃N₂ < DHPMoS₃H₂ < MoS₃H₂ < Dec.MoS₃H₂. At 425°C, the MoS₃H₂ and Dec.MoS₃H₂ systems exchange places in this order. Diphenyl ether is less reactive than dinaphthyl ether toward hydrogenolysis reactions under these conditions. The conversion rate of diphenyl ether increases in the order H₂ < DHPH₂ < DHPMoS₃N₂ < DHPMoS₃H₂ < Dec.MoS₃H₂ < MoS₃H₂. Although the rates of conversion of the two ethers are different, the relative effects of using a reactive gaseous atmosphere, donor solvent, catalyst - or some combination of these factors - are the same for both compounds. In liquefaction experiments, hydrogen donor solvent or hydrogen shuttling solvent seems necessary to reduce retrogressive reactions. However, a solvent interacting strongly with catalyst and scavenging hydrogen atoms can reduce the activity of catalysts in hydrocracking reactions.     

A highly conductive organic-inorganic hybrid electrolyte based on co-condensation of di-ureasil and ethylene glycol-containing alkoxysilane

Organic-inorganic hybrid electrolytes based on di-ureasil backbone structures by reacting poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol) bis(2-aminopropyl ether) (ED2000) with 3-(triethoxysilyl)propyl isocyanate (ICPTES), followed by co-condensation with methoxy(polyethylenoxy)propyl trimethoxysilane (MPEOP) in the presence of LiClO₄ were prepared and characterized by a variety of techniques. The hybrid electrolytes showed good resistance to crystallization and excellent conductivity for use in lithium-ion batteries, as determined by differential scanning calorimetry (DSC) and impedance measurements, respectively. The temperature dependence of the ionic conductivity exhibited a VTF (Vogel-Tamman-Fulcher)-like behavior for all the compositions studied and a maximum ionic conductivity value of 6.9 × 10⁻⁵ S cm⁻¹, a relatively high value for solid polymer electrolytes, was achieved at 30 °C for the hybrid electrolyte with a [O]/[Li] ratio of 16. A microscopic view of the dynamic behavior of the polymer chains (¹³C) and the ionic species (⁷Li) was provided by the ¹H and ⁷Li line widths measured from 2D ¹H-¹³C WISE (Wideline Separation) and variable temperature ⁷Li static NMR, respectively, to elucidate the influence of the mobility of the polymer chains and the charge carriers on the observed ionic conductivity. The present salt-free hybrid electrolyte after plasticization with 1 M LiClO₄ in EC/PC solution exhibited a swelling ratio of 275% and reached an ionic conductivity value up to 8.3 × 10⁻³ S cm⁻¹ at 30 °C, which make it a good candidate for the further development of advanced rechargeable lithium-ion batteries.