Properties of Li4Ti5O12 as an anode material in non-flammable electrolytes

Two non-flammable electrolytes 1 M LiPF₆ in sulfolane (TMS) + 5 wt% VC and 0.7 M lithium bis(trifluoromethanesulphonyl)imide (LiNTf₂) in N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulphonyl)imide (MePrPyrNTf₂) + 10 wt% gamma-butyrolactone (GBL) were tested with Li₄Ti₅O₁₂ (LTO) as highly promising anode material for application in lithium-ion batteries. The results were compared for the titanium anode in the classic electrolyte: 1 M LiPF₆ in propylene carbonate + dimethyl carbonate (PC + DMC, 1:1). The performances of LTO/electrolyte/Li cell were tested using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge and scanning electron microscopy (SEM). SEM images of electrodes and those taken after electrochemical cycling showed changes which may be interpreted as a result of solid-state interface formation. Good charge/discharge capacities and low capacity loss at medium C rates preliminary cycling was obtained for the Li₄Ti₅O₁₂ anode. For LTO/1 M LiPF₆ in PC + DMC/Li system, the best capacity was obtained at C/10 and C/3 (145 and 154 mAh g^?1, respectively). In the case of a system working on the basis of a TMS solution (1 M LiPF₆ in TMS + 5 wt% VC) the best value was obtained at a C/5 current and an average of more than 150 mAh g^?1 (86 % of theoretical capacity). For the 0.7 M LiNTf₂ in MePrPyrNTf₂ + 10 wt% GBL electrolyte, the highest capacitance value (at C/20 current) of about 150 mAh g^?1 was observed. The 1 M LiPF₆ in TMS + 5 wt% VC and 0.7 M LiNTf₂ in MePrPyrNTf₂ + 10 wt% GBL electrolytes had a relatively broad thermal stability range and no decomposition peak was observed below 150 °C.     

Electrochemical performance of nanoporous Si as anode for lithium ion batteries in alkyl carbonate and ionic liquid-based electrolytes

Nanoporous Si was obtained by means of metal-assisted chemical etching. Li ion insertion–extraction was tested by voltammetric and galvanostatic electrochemical cycling in conventional 1 M LiPF₆ ethylene carbonate/dimethyl carbonate EC/DMC and in 1 M LiTFSI 1-butyl-1-methyl-pyrrolidinium bis (trifluoromethyl) sulfonylimide [BMP] [TFSI] electrolytes. The nanoporous Si demonstrated high reversibility when cycled in 1 M LiPF₆ EC/DMC electrolyte and showed superior activity compared to the non-structured sample. In contrast to the organic carbonate electrolyte, the material cycling in ionic liquid media showed reduced capacity and reversibility of the Li ion exchange. The latter results were discussed in terms of the high viscosity of the ionic liquid and ineffective cathodic passivation of the Si substrate in the ionic liquid-based electrolyte. Scanning electron microscopy imaging showed minor morphological changes due to the large volume change during Li insertion. No signs of crack formation and propagation were detected during the time span of the measurement.     

Effect of vinylene carbonate as additive to electrolyte for lithium metal anode

The cycling efficiencies and cycling performance of a lithium metal anode in a vinylene carbonate (VC)-containing electrolyte were evaluated using Li/Ni and LiCoO₂/Li coin type cells. The cycling efficiencies of deposited lithium on a nickel substrate in an EC + DMC (1:1) electrolyte containing LiPF₆, LiBF₄, LiN(SO₂CF₃)₂ (LiTFSI), or LiN(SO₂C₂F₅) (LiBETI) at 25 and 50 °C were improved by presence of VC. However, the lithium cycling efficiencies at low temperature (0 °C) decreased by adding VC to the EC+DMC (1:1) electrolyte. The deposited lithium at low temperature exhibited a dendritic morphology and a thicker surface film. The lithium ion conductivity of the VC derived surface film was lower than that of the VC-free surface film at low temperature. Therefore, we concluded that the cycling efficiency decreased with decreasing temperature. On the other hand, the cell containing VC additive has excellent performance at elevated temperature. The deposited lithium at 50 °C in the VC-containing electrolyte exhibited a particulate morphology and formed a thinner surface film. The VC derived surface film, which consists of polymeric species, suppressed the deleterious reaction between the deposited lithium and the electrolyte.     

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.     

Effect on properties of PVDF-HFP based composite polymer electrolyte doped with nano-SiO2

Various kinds of nano-SiO₂ using different catalysts were obtained and characterized by scanning electron microscope (SEM) technique. The results showed that the nano-SiO₂ using NH₃·H₂O as catalyst presented the best morphology. Poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) based composite polymer electrolyte (CPE) membranes doped with different contents of nano-SiO₂ were prepared by phase inversion method. The as-prepared CPE membranes were immersed into 1.0 M LiPF₆-EC/DMC/EMC electrolytes for 0.5 h to be activated. The physicochemical and electrochemical properties of the CPEs were characterized by SEM, X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV) techniques. The results indicate that the CPEs doped with 10 % nano-SiO₂ exhibit the best performance. SEM micrographs showed that the CPE membranes have uniform surface with abundant interconnected micro-pores, and the uptake ratio was up to 104.4 wt%. EIS and LSV analysis also showed that the ionic conductivity at room temperature and electrochemical stability window of the modified membrane can reach 3.372 mS cm^?1 and 4.7 V, respectively. The interfacial resistance R _i was 670 Ω cm^?2 in the first day, then increased to a stable value of about 850 Ω cm^?2 in 10 days storage at room temperature. The Li/As-fabricated CPEs/LiCoO₂ cell also showed good charge–discharge performance, which suggested that the prepared CPE membranes can be used as potential electrolytes for lithium ion batteries.     

Synthesis and Electrochemical Performance of Spheroid LiNi<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> in the Electrolyte Modified by Ethylene Sulfate and Methylene Methanedisulfonate

In this study, spheroid LiNi_1/3Co_1/3Mn_1/3O₂ (NCM111) cathode material were synthesized using LiOH with Ni_0.5Co_0.2Mn_0.3(OH)₂ precursor by a simple solid-state reaction, and characterized by X-ray diffraction and scanning electron microscopy. Electrochemical behavior of NCM111 was investigated by electrochemical impedance spectroscopy (EIS) combining with cyclic voltammogram (CV) and charge/discharge test in the 1 M LiPF₆-EC:EMC electrolyte with ethylene sulfate (DTD) and methylene methanedisulfonate (MMDS) additives either singly or in combination with high cutoff voltage of 3.0–4.5 V at room temperature of 25 °C or elevated temperature of 55 °C. It was found that DTD additive can increase the initial coulombic efficiency of NCM111, and the spheroid NCM111 can obtain the maximum initial discharge capacity of 177.81 mAh/g with the 2 wt% DTD, and keep 92.29% capacity retention after 80 cycles. The MMDS additives would decrease the initial discharge capacity of the NCM111, and enhance significantly long cycle life of the NCM111 with the capacity retention of 99.23% over 80 cycles at high voltage of 4.5 V. The additive combination 2 wt% DTD?+?1 wt% MMDS was an optimal additive combination, demonstrating the 102.2% capacity retention over 80 cycles at room temperature and the 94.2% capacity retention over 70 cycles at elevated temperature of 55 °C. EIS results revealed that the additive blend of 2 wt% DTD?+?1 wt% MMDS can drastically lower the kinetics impedance and suppress the growth rate of R _ct for the NCM111 electrode.     

Cold sintering process of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte

The recently developed technique of cold sintering process (CSP) enables densification of ceramics at low temperatures, i.e., <300°C. CSP employs a transient aqueous solvent to enable liquid phase‐assisted densification through mediating the dissolution‐precipitation process under a uniaxial applied pressure. Using CSP in this study, 80% dense Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) electrolytes were obtained at 120°C in 20 minutes. After a 5 minute belt furnace treatment at 650°C, 50°C above the crystallization onset, Li‐ion conductivity was 5.4 × 10^?5 S/cm at 25°C. Another route to high ionic conductivities ~10^?4 S/cm at 25°C is through a composite LAGP ‐ (PVDF‐HFP) co‐sintered system that was soaked in a liquid electrolyte. After soaking 95, 90, 80, 70, and 60 vol% LAGP in 1 M LiPF₆ EC‐DMC (50:50 vol%) at 25°C, Li‐ion conductivities were 1.0 × 10^?4 S/cm at 25°C with 5 to 10 wt% liquid electrolyte. This paper focuses on the microstructural development and impedance contributions within solid electrolytes processed by (i) Crystallization of bulk glasses, (ii) CSP of ceramics, and (iii) CSP of ceramic‐polymer composites. CSP may offer a new route to enable multilayer battery technology by avoiding the detrimental effects of high temperature heat treatments.     

Effect of the concentration of diphenyloctyl phosphate as a flame-retarding additive on the electrochemical performance of lithium-ion batteries

We selected diphenyloctyl phosphate (DPOF) as a flame-retardant and plasticizer, and studied the influence of different amounts of the DPOF additive on the electrochemical performance of lithium-ion batteries. The electrochemical cell performances of the additive-containing electrolytes in combination with a cell comprising an LiCoO₂ cathode and mesocarbon microbeads (MCMB) anode were tested in coin cells. The cyclic voltammetry (CV) results showed that the oxidation potential of the electrolyte containing DPOF in the concentration range from 10 to 30 wt.% is about 4.75-5.5 V versus Li/Li⁺. In the present work, a DPOF content of 10 wt.% in the 1.15 M LiPF₆/EC:EMC (4:6 by vol.%) electrolyte turned out to be the optimum condition for the improvement of the electrochemical cell performance, due to the decrease of the irreversible capacity during the first cycle and decrease of the charge-transfer resistance after 40 cycles.     

γ-Butyrolactone-ethylene carbonate based electrolytes for lithium-ion batteries

-Butyrolactone-ethylene carbonate (BL-EC) mixtures have been investigated as electrolytes for Li-ion batteries using LiPF₆ and LiBF₄ as lithium salt. The thermal stability of the electrolytes in a large range of temperatures (–90 °C to 40 °C) have been studied by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). From the results of these experiments, the phase diagram of the BL-EC system has been determined. It is characterised by a eutectic point at –56.3 °C and a molar fraction in EC: x _EC = 0.1. A metastable compound has been demonstrated below –90 °C at x _EC = 0.4. Conductivity measurements of BL-EC solutions, in the presence of LiPF₆ and LiBF₄, indicate that LiPF₆ in the eutectic mixture is the most conducting electrolyte in the range of temperatures investigated (–30 °C to room temperature). Nevertheless, at low temperature, LiBF₄ based electrolytes compete well with LiPF₆, especially when the amount of EC in the mixture is as high as x _EC = 0.5. Moreover, recrystallisation of the salt below –20 °C is avoided when LiBF₄ is used as salt. A large increase in viscosity of the solvent mixture is observed when a salt is added, but the increase is lower for LiBF₄ than LiPF₆. When EC is added to BL at constant salt concentration (1 M), the conductivity of LiPF₆ solutions decreases more rapidly than LiBF₄ solutions. This has been attributed, at least partially, to the dissociating power of EC. The electrochemical windows of BL-EC (equimolar) mixtures in the presence of LiPF₆ and LiBF₄ are comparable but it is shown that the solvents oxidation rate at high potentials is lower when LiBF₄ is used.     

LiPF6/LiBOB blend salt electrolyte for high-power lithium-ion batteries

LiPF₆/LiBOB blend salt-based electrolytes were investigated as potential candidates for high-power lithium-ion batteries, especially for transportation applications. It was demonstrated that both the power capability and the cycling performance of the lithium-ion cells could be attenuated by controlling the concentration of LiBOB in blend salt electrolytes. The power capability of the lithium-ion cells decreases with the concentration of LiBOB, while the capacity retention of the cells at 55 °C increases with the LiBOB concentration. When electrolytes with no more than 0.1 M LiBOB was used, the MCMB/LiMn_1/3Ni_1/3Co_1/3O₂ cells have excellent capacity retention at 55 °C, while their impedance meets the requirement set by the FreedomCar Partnership. The similar performance improvement on the MCMB/LiMn₂O₄ cells was also observed with the blend salt electrolyte.     

Effects of solvents and salts on the thermal stability of LiC6

Accelerating rate calorimetry (ARC) was used to study the thermal stability of Li_0.81C₆ in dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC), and an EC/DEC mixture as well as in LiPF₆- and LiBOB-based electrolytes. ARC results show that linear carbonates like DMC or DEC react strongly with Li_0.81C₆ and that robust passivating layers do not form. By contrast, the cyclic carbonate, EC, creates a robust passivating film that limits the rate of reaction between Li_0.81C₆ and EC as the temperature increases. X-ray diffraction shows that the addition of LiPF₆ to EC/DEC changes the surface film that forms on Li_0.81C₆ at elevated temperature to one dominated by LiF instead of lithium-alkyl carbonate or lithium carbonate. This increases the thermal stability of Li_0.81C₆ in LiPF₆ electrolyte compared to pure EC/DEC solvent. By an apparently similar mechanism, the addition of only 0.2 M LiBOB to EC/DEC greatly improves the thermal stability of Li_0.81C₆. ARC results for Li_0.81C₆ in pure and mixed salt LiPF₆ and LiBOB EC/DEC electrolytes of various molarities shed light on the reasons for the beneficial effect of the salts.     

Limiting factors for low-temperature performance of electrolytes in LiFePO4/Li and graphite/Li half cells

Low-temperature performance of LiBF₄ and LiPF₆-based electrolytes in LiFePO₄/Li and graphite/Li half cells was investigated. In the temperature range from 0 °C to ?40 °C, electrochemical impedance spectroscopy (EIS) results show that the charge-transfer resistance (R_ct) of graphite/Li cell decreases, the R_ct of LiFePO₄/Li cell increases, and sum resistance of LiFePO₄/Li and graphite/Li cell decreases when replacing LiPF₆ with LiBF₄. In the temperature range from 25 °C to ?40 °C, energy barrier (W) for Li-ion jump at the solid electrolyte interface (SEI) alters slightly from 16.04 kJ/mol to 13.60 kJ/mol in LiFePO₄/Li cells, but declines greatly from 46.47 kJ/mol to 19.81 kJ/mol in graphite/Li cells when using LiBF₄ instead of LiPF₆, meanwhile, activation energy (ΔG) of electrode reaction is approximately the same (～60 kJ/mol). The above results indicate that the ionic conductivity is the main limiting factor for low-temperature performance of electrolytes in LiFePO₄/Li cell, while factors related with electrolyte-interface are more crucial in graphite/Li cell than in LiFePO₄/Li cell.     

The effects of growth conditions on the surface properties and photocatalytic activities of anatase TiO<Subscript>2</Subscript> films prepared via electrochemical anodizing and annealing methods

In the present work, nanostructured TiO₂ films were prepared by electrochemical anodization process of titanium in fluoride-containing electrolytes using an innovative approach. After anodization, the TiO₂ films were annealed at 480?°C for 2 h in air in order to acquire anatase phase transformation and increase its crystallinity. The effects of anodization voltage, electrolyte concentration and anodization time on the formation of TiO₂ films and the photocatalytic degradation of methylene blue (MB) were discussed in details. The phase structure and surface morphology of the samples characterized by means of X-ray diffraction and scanning electron microscope. The as-prepared nanostructured TiO₂ film anodized in 0.5% HF electrolyte at 15 V for 240 min showed excellent photocatalytic degradation of MB and is promising for environmental purification.     

Electrokinetic Properties of Acid-Activated Montmorillonite Dispersions

In this study, the influence of pH, electrolyte concentration, and type of ionic species on the electrokinetic properties (zeta potential and electrokinetic charge density) of the acid-activated montmorillonite mineral have been investigated using the microelectrophoresis method. The electrokinetic properties of acid-activated montmorillonite dispersions have been determined in aqueous solutions of mono-, di-, and trivalent salts and divalent heavy metal salts. Zeta potential experiments have been performed to determine the point of zero charge (pzc) and potential determining ions (pdi). The zeta potential values of the acid-activated montmorillonite particles were negative and did not vary significantly within the pH range studied. Acid-activated montmorillonite dispersions do not have point of zero charge (pzc). The valence of the electrolytes has a great influence on the electrokinetic behavior of the suspension. A gradual decrease in the zeta potential (from ?25 mV to ?5 mV) occurs with the monovalent electrolytes when concentration increased. Divalent and heavy metal electrolytes have less negative z-potentials due to the higher valence of ions. A sign reversal of z-potential has been observed at AlCl₃, FeCl₃, and CrCl₃ electrolytes (potential determining ions) and zeta potential values have had a positive sign at high electrolyte concentrations.

The electrokinetic charge density of acid-activated montmorillonite has shown similar trends for variation in mono- and divalent electrolyte solutions. Up to concentrations of ca. 10^?3 M, it has remained practically constant at approximately 0.5 × 10^?3 C m^?2 For higher concentrations of monovalent electrolytes more negative values (?16 × 10^?3 C m^?2) were observed. It has less negative values in divalent electrolyte concentrations according to monovalent electrolytes (?5 × 10^?3 C m^?2). For low concentrations of trivalent electrolytes, the electrokinetic charge density of montmorillonite particles is constant, but at certain concentrations it rapidly increased and changed its sign to positive.     

Electrochemical performances of electric double layer capacitor with UV-cured gel polymer electrolyte based on poly[(ethylene glycol)diacrylate]-poly(vinylidene fluoride) blend

Poly[(ethylene glycol)diacrylate]-poly(vinylidene fluoride), a gel polymer blend with ethylene carbonate:dimethyl carbonate:ethylmethyl carbonate (EC:DMC:EMC, 1:1:1 volume ratio) and containing 1.0 M of lithium hexafluoro phosphate (LiPF₆) as liquid components, is employed as a gel polymer electrolyte for an electric double layer capacitor (EDLC). Its electrochemical characteristics is compared with that of liquid organic electrolyte mixture of ethylene carbonate, dimethyl carbonate and ethylmethyl carbonate in a 1:1:1 volume ratio containing 1.0 M LiPF₆ salt. The specific surface area of the activated carbon powder as an active material is 1908 m²/g. Liquid poly[(ethylene glycol)diacrylate] (PEGDA) oligomer with a high retention capability of liquid electrolytes is cured by UV irradiation and poly(vinylidene fluoride)-hexafluoropropylene (PVdF-HFP) copolymer with a porous structure endows polymer matrix with high mechanical strength.The specific capacitance of EDLC using the gel polymer electrolyte (GPE-EDLC) shows 120 F/g, which is better than the liquid organic electrolyte. Good cycling efficiency is observed for a GPE-EDLC with high retention capability of liquid components. The high specific capacitance and good cycling efficiency are most likely due to the polarization resistance of EDLC with the gel polymer electrolyte, which is lower than the liquid organic electrolyte. This may result from the distinguished adhesion between the activated carbon electrode and the gel polymer electrolyte, as well as high retention capability of liquid components.Power densities of GPE-EDLC and LOE-EDLC shows 1.88 kW/kg and 1.21 kW/kg, respectively. However, the energy densities are low in both electrolytes.The GPE-EDLC exhibits rectangular cyclic voltammogram similar to an ideal EDLC within operating voltage range of 0 V-2.5 V. It should be noted that a region of electric double layer means a wide voltage and a rapid formation. Redox currents of both EDLCs are not observed in the sweep region and the cyclic voltammograms are unchanged on repeated runs. The observed leakage current shows 49 μA after 720 s at a constant voltage of 2.5 V, due to the high ionic conductivity of 1.5 × 10⁻³ S cm⁻¹ during storage time. Swelling and well-developed pore structures of the GPE blend films allow ions and solvents to move easily.     

Optimization of EC-based multi-solvent electrolytes for low temperature applications of lithium-ion batteries

The ionic conductivities of EC-based multi-component electrolytes in various solvent compositions were measured over a wide temperature range of +40 to −40 °C, and the factors affecting the low temperature conductivities of the electrolytes were discussed. It is revealed from the experimental results that the co-solvents with high dielectric constant and low viscosity can improve the ionic conductivity at room temperature, whereas, only the co-solvents which possess low melting points can effectively expand the operating temperature range of the electrolyte. The Li-ion batteries using the optimized electrolyte of 1 M LiPF₆/EC-DMC-EMC (8.3:25:66.7) show the capacity retentions about 90.3% of their nominal capacities when discharged to 2.0 V at −40 °C at 0.1 C, demonstrating excellent low temperature performances.     

Effects of cathode impedance on the performances of power-oriented lithium ion batteries

Since power batteries have different requirements than traditional energy-oriented lithium ion batteries (LIBs) and their design concept is also different than energy-type cells, some new problems not encountered in energy-oriented LIBs must be carefully considered. This study illustrates that cathode impedance, a contributor to total cell impedance which can be ignored in the traditional energy-type LIBs, plays a very important role in power cells. This study uses 18650 cylindrical power cells consisting of a LiMn₂O₄ cathode and graphite anode with a basic electrolyte of PC/EC/DMC = 1/3/6 by weight containing 1.2 M LiPF₆ as model power LIBs. This study also investigates the charge–discharge performance of these model batteries made from cathodes with the same recipe but dried at different oven temperatures. The high impedance cathode produced under a high drying temperature causes the cell to fail during high-power applications. Cell heating during extreme high rate discharging periods not only causes pore closure in the porous separator, but also cathode peeling from the substrate. These phenomena, increasing the cell resistance and reducing the transfer rate of charged species, are believed to be the main causes for the poor cycle life of model batteries in high rate discharge tests.     

Electrochemical characteristics of LiMxFe1−xPO4 cathode with LiBOB based electrolytes

The conductivity of LiBOB based electrolyte, which we formed in our lab, has been compared with commercialized LiPF₆ based electrolyte firstly. The charge and discharge capacity of the LiFePO₄/Li half-cell with two kind of electrolyte has compared both at room temperature and elevated temperature. LiBOB cell presented better charge/discharge stability at elevated temperature than the counterpart. ICP method was adopted to analyze the reaction of electrolytes with cathode material at high temperature. Cyclic voltammogram was conducted to analyze the charge/discharge process of the cell in both LiBOB based electrolyte and LiPF₆ based electrolyte at different temperature.     

Novel amphiphilic polymer gel electrolytes based on (PEG‐b‐GMA)‐co‐MMA

Amphiphilic conetwork–structured copolymers containing different lengths of ethylene oxide (EO) chains as ionophilic units and methyl methacrylate (MMA) chains as ionophobic units were prepared by free radical copolymerization and characterized by FTIR and thermal analysis. Polymer gel electrolytes based on the copolymers complexed with liquid lithium electrolytes (dimethyl carbonate (DMC) : diethyl carbonate (DEC) : ethylene carbonate (EC) = 1 : 1 : 1 (W/W/W), LiPF₆ 1.0M) were characterized by differential scanning calorimetry and impedance spectroscopy. A maximum ion conductivity of 4.27 × 10^?4 S/cm at 25^oC was found for the polymer electrolyte based on (PEG2000‐b‐GMA)‐co‐MMA with long EO groups. Moreover, the effect of temperature on conductivity of the amphiphilic polymer electrolytes obeys the Arrhenius equation. The good room temperature conductivity of the polymer electrolytes is proposed to relate to the enhancement in the amorphous domain of the copolymers due to their amphiphilic conetwork structure. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Glyme-based nonaqueous electrolytes for rechargeable lithium cells

Poly(ethylene glycol)dimethyl ethers [(CH₃O(CH₂CH₂O)_nCH₃, n = 1, 2, 3, and 4)] are generally known as “glymes”. This study examines the conductivity, lithium ion solvation state and charge-discharge cycling efficiency of lithium metal anodes in glyme-based electrolytes for rechargeable lithium cells. 1 M (M: mol l⁻¹) LiPF₆ was used as the solute. The properties of the glymes were investigated by using a ternary mixed solvent consisting of n-glyme, ethylene carbonate (EC) and methylethylcarbonate (MEC). This was because the solubility of LiPF₆ is far less than 1 M in an n-glyme single solvent. The glyme solutions exhibited higher conductivity and higher lithium cycling efficiency than EC/MEC. The conductivity tended to increase with decreases in ethylene oxide chain number (n) and solution viscosity. The decrease in the solution viscosity resulted from the change in the lithium ion solvation structure that occurred when a glyme was added to EC/MEC. The selective solvation of the glyme with respect to lithium ions was clearly demonstrated by -NMR measurements. The lithium cycling efficiency value depended on the charge-discharge current (I_ps). When n increased there was an increase in lithium cycling efficiency at a low I_ps and a decrease in the reduction potential of the glymes. When the conductivities including those at low temperature (below 0 °C), and charge-discharge cycling at a high current are taken into account, di- or tri-glyme is superior to the other glymes tested here.