Mass transport in lithium-battery solvents

We describe and implement a microelectrode procedure for the determination of important transport properties required for the evaluation of liquid electrolytes used in lithium-based batteries. Three solvents of interest (propylene carbonate, ethylene carbonate, and diethyl carbonate) and two lithium salts (lithium hexafluorophosphate and lithium perchlorate) are investigated. In addition, by combining microelectrode and radiometric analyses, we are able to characterize fully the transport phenomena in the nonaqueous solvent + salt systems. Thus a radiometric technique is used to monitor solvent transport, both under diffusion and current-passage conditions, and the solvent diffusion coefficient is reported as a function of salt concentration.     

Electrochemical measurement of transference numbers in polymer electrolytes

Electrochemical methods for the determination of transference numbers in polymer electrolytes are considered and a new technique which overcomes some of the problems associated with other methods in current use is described. Results are given of measurements of the transference numbers of lithium and trifluoromethanesulphonate ions in poly(ethylene oxide) at 90°C. A mean value of 0.46 ± 0.02 is reported for lithium.     

Effect of various plasticizers on the transport properties of hexanoyl chitosan‐based polymer electrolyte

Hexanoyl chitosan that exhibited solubility in tetrahydrofuran was prepared by acyl modification of chitosan. Films of hexanoyl chitosan‐based polymer electrolyte were prepared by the technique of solution casting. Ethylene carbonate, propylene carbonate, and diethyl carbonate with different dielectric constants were employed as the plasticizing solvents and lithium trifluoromethanesulfonate (LiCF₃SO₃) was used as the salt. The importance of dielectric constant affecting conductivity and transport properties of hexanoyl chitosan:LiCF₃SO₃ electrolytes have been examined in the present study. An enhancement in the ionic conductivity has been found on plasticization, and the magnitude of conductivity increment strongly depended on the dielectric constant of the plasticizer. Transport properties such as activation energy and charge carrier concentration have been calculated to obtain information that may be used to elucidate the mechanism of conductance. In addition to conductivity studies, thermal studies and transference number measurements were performed to correlate the phase structure and diffusion phenomena to the conductivity behavior of hexanoyl chitosan‐based polymer electrolyte. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101:4474–4479, 2006     

Heterogeneously doped polyethylene glycol as nano-composite soft matter electrolyte

We have applied the concept of heterogeneous doping [1] to prepare and examine composite electrolytes, consisting of silica particles, low molecular weight polyethylene glycol solvents and lithium perchlorate salt. These “soggy sand” electrolytes combine high ionic conductivities (on the order of mS cm⁻¹) and high Li transference numbers (typically 60–80%) with improved mechanical properties. They were characterized using differential scanning calorimetry, dc-polarization and ac-impedance spectroscopy, zeta potential measurements and viscosimetry. Oxide, size and concentration as well as solvent molecular weight were varied to better understand the influence of ceramic oxide fillers on the ion conduction in these systems. As regarding the filler content, we observe that both conductivity and transference number of Li⁺ start increasing already at low volume fractions of oxide particles, reach a maximum and subsequently decrease to low values. The percolating network is – after initial partial coarsening – found to be stable within the time periods of the measurements.     

Determination of lithium transference number in PEO-DME-LiClO4 modified with alumina powders of various surface acidity

The effect of alumina additives bearing various surface groups on conductivity and lithium cation transference numbers in poly(ethylene oxide) dimethyl ether (PEO-DME)-LiClO₄ electrolytes is examined. It is demonstrated that an increase in the conductivity and lithium transference number in composite electrolytes compared to pure PEO-DME-LiClO₄ electrolyte is observed in the limited salt concentration range. Both quantities seem to depend mostly on ionic species mobility. Also, their salt concentration dependence resembles that of viscosity of electrolytes studied. The conduction mechanism is discussed on the basis of conductivity, transference numbers and ionic association studies.     

Electrochemical characterization of polymer precoated lithium electrodes

The electrochemical behaviour of lithium electrodes in contact with solutions of propylene carbonate is investigated as a function of different polymer precoating materials (poly-(2-vinylpyridine) and poly-(ethylene oxide)). Impedance and polarization measurements show that the electrochemical behaviour of the surface is influenced markedly by precoating due to a modification of the passivating layer. Such information is important for lithium electrodes in batteries with organic electrolytes.     

Thermal analysis of a cylindrical lithium-ion battery

This work investigates the heat generation characteristics of a cylindrical lithium-ion battery. The battery consists of the graphite, LiPF₆ of the propylene carbonate/ethylene carbonate/dimethyl carbonate (PC/EC/DMC) solution, and spinal as anode, electrolyte and cathode, respectively. The coupled electrochemical–thermal model is developed with full consideration of electrolyte transport properties as functions of temperature and Li ion concentration. A truly conservative finite volume numerical method is employed for the spatial discretization of the model equations. Three types of heat generation sources including the ohmic heat, the active polarization heat and the reaction heat are quantitatively analyzed for the battery discharge process. The ohmic heat is found to be the largest contribution with around 54% in the total heat generation. About 30% of the total heat generation in average is ascribed to the electrochemical reaction. The active polarization contributes the least comparing to the ohmic heat and reactions heat. The results also show that the Li ion concentration and its gradient in electrolyte are the main factors giving the effect on the heat generations of active polarization and electrolyte electric resistance. The raised temperature in the battery discharge is positive related with the thickness of both separator and electrodes.     

Crystallinity,thermal properties,morphology and conductivity of quaternary plasticized PEO‐based polymer electrolytes

Quaternary plasticized solid polymer electrolyte (SPE) films composed of poly(ethylene oxide), LiClO₄, Li_1.3Al_0.3Ti_1.7(PO₄)₃, and either ethylene carbonate or propylene carbonate as plasticizer (over a range of 10–40 wt%) were prepared by a solution‐cast technique. X‐ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) indicated that components such as LiClO₄ and Li_1.3Al_0.3Ti_1.7(PO₄)₃ and the plasticizers exerted important effects on the plasticized quaternary SPE systems. XRD analysis revealed the influence from each component on the crystalline phase. DSC results demonstrated the greater flexibility of the polymer chains, which favored ionic conduction. SEM examination revealed the smooth and homogeneous surface morphology of the plasticized polymer electrolyte films. EIS suggested that the temperature dependence of the films' ionic conductivity obeyed the Vogel–Tamman–Fulcher (VTF) relation, and that the segmental movement of the polymer chains was closely related to ionic conduction with increasing temperature. The pre‐exponential factor and pseudo activation energy both increased with increasing plasticizer content and were maximized at 40 wt% plasticizer content. The charge transport in all polymer electrolyte films was predominantly reliant on lithium ions. All transference numbers were less than 0.5. Copyright © 2006 Society of Chemical Industry     

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.     

涤纶POY油剂单体的合成及性能研究

以碳酸二乙酯为原料,与丙二醇聚醚反应,合成了低级聚碳酸酯,再用自制的不同脂肪醇聚氧乙烯醚封端,合成了改性聚碳酸酯;将改性聚碳酸酯与其他单体复配,配制成涤纶POY油剂,考察脂肪醇聚氧乙烯醚的起始剂、环氧乙烷(EO)与环氧丙烷(PO)加成数,碳酸二乙酯与丙二醇聚醚的摩尔比对POY油剂性能的影响。结果表明:使用辛醇为起始剂,与EO按摩尔比1:8缩合合成脂肪醇聚氧乙烯醚,1,2-丙二醇与PO按摩尔比1:8缩合合成丙二醇聚醚,碳酸二乙酯与丙二醇聚醚按摩尔比4:3缩合成低级聚碳酸酯,低级聚碳酸酯与脂肪醇聚氧乙烯醚按摩尔比1:1合成的改性聚碳酸酯作为单体复配的涤纶POY油剂,其润湿性能、热性能较好,表面张力较高。     

Gel electrolytes with ionic liquid plasticiser for electrochromic devices

The comparative performance of conducting polymer electrochromic devices (ECDs) utilising gel polymer electrolytes (GPEs) plasticised with ethylene carbonate/propylene carbonate or (N-butyl-3-methylpyridinium trifluoromethanesulphonylimide (P₁₄TFSI) has been made. Lithium perchlorate and lithium trifluoromethanesulphonylimide salts were used in the GPEs to provide enhanced ionic conductivity and inhibit phase separation of the polyethyleneoxide (PEO) and plasticiser. ECDs were assembled from cathodically colouring, polyethylenedioxythiophene (PEDOT), and anodically colouring, polypyrrole (PPy), conducting polymer electrochromes deposited by vapour deposition. The photopic contrast switching over the visible light spectrum, switching speeds and device stability of the ECDs were obtained. These studies demonstrate that the ionic liquid (IL) plasticised GPEs are a suitable replacement for pure IL based devices and volatile organic solvent plasticisers based upon ethylene carbonate/propylene carbonate mixtures.     

Lithium transference number measurements and complex abilities in anion trapping triphenyloborane-poly(ethylene oxide) dimethyl ether-lithium trifluoromethanesulfonate composite electrolyte

In this paper we report the combined, positive effect of triphenyloborane (BPh₃) additive on conductivity and lithium cation transference numbers in poly(ethylene oxide) dimethyl ether (PEODME)-lithium trifluoromethanesulfonate (LiCF₃SO₃, LiTf) electrolytes. The transport mechanism is discussed on the basis of impedance measurements, restricted diffusion t⁺ measurements, ionic association semi-empirical quantitative estimation and spectroscopic studies. A substantial increase in the lithium transference number values in triphenylborane enriched composite electrolytes was observed in comparison with the pure PEODME-LiCF₃SO₃ electrolyte. This effect is assisted by ionic conductivity enhancement.     

Ionic conductivity and interfacial properties of polymer electrolytes based on PEO and boroxine ring polymer

Blended polymer electrolytes based on poly(ethylene oxide) (PEO) and boroxine ring polymer (BP) solvated with lithium triflate were formulated and evaluated. Compared to PEO–salt polymer electrolyte, ionic conductivities of blended polymer electrolytes were two orders of magnitude higher in a low‐temperature range; as well, lithium transference numbers were increased to ～ 0.4. These were due to the increased mobility and anion trapping of boroxine rings. BP also exhibited the stabilizing effect on lithium–polymer electrolyte interface, and a reduced interfacial resistance between lithium metal and the polymer electrolyte was found with increasing of BP content. Polymer electrolytes based on PEO and BP are suitable for use in lithium secondary battery. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 17–21, 2002; DOI 10.1002/app.10090     

电池级碳酸酯类溶剂的一种提纯方法

研究了一种吸附与蒸馏相结合的提纯锂离子电池用碳酸酯类溶剂的方法,研究结果表明在蒸馏时加入金属钠可以有效减少含质子氢杂质,还对提纯原理进行了分析。     

Experimental and Kinetic Studies on a Homogeneous System for Diethyl Carbonate Synthesis by Transesterification

A synthesis of diethyl carbonate through transesterification of propylene carbonate and ethanol, coproducing useful propylene glycol, was carried out in the presence of sodium ethoxide as catalyst. The effects of reaction parameters such as catalyst concentration, mole ratio of ethanol to propylene carbonate and reaction temperature on the transesterfication reaction were investigated. The results showed that the reaction is reversible with a propylene carbonate equilibrium conversion of about 64 % at an ethanol to propylene carbonate mole ratio of 8.0 and a reaction temperature of 303 K. A kinetic model was proposed based on the reaction mechanism. The model parameters were estimated by the Runge‐Kutta method. The statistical test showed that the model calculation results were in good agreement with the experimental data.     

Fabrication and properties of crosslinked poly(propylene carbonate maleate) gel polymer electrolyte for lithium‐ion battery

The poly(propylene carbonate maleate) (PPCMA) was synthesized by the terpolymerization of carbon dioxide, propylene oxide, and maleic anhydride. The PPCMA polymer can be readily crosslinked using dicumyl peroxide (DCP) as crosslinking agent and then actived by absorbing liquid electrolyte to fabricate a novel PPCMA gel polymer electrolyte for lithium‐ion battery. The thermal performance, electrolyte uptake, swelling ratio, ionic conductivity, and lithium ion transference number of the crosslinked PPCMA were then investigated. The results show that the T_g and the thermal stability increase, but the absorbing and swelling rates decrease with increasing DCP amount. The ionic conductivity of the PPCMA gel polymer electrolyte firstly increases and then decreases with increasing DCP ratio. The ionic conductivity of the PPCMA gel polymer electrolyte with 1.2 wt % of DCP reaches the maximum value of 8.43 × 10⁻³ S cm⁻¹ at room temperature and 1.42 × 10⁻² S cm⁻¹ at 50°C. The lithium ion transference number of PPCMA gel polymer electrolyte is 0.42. The charge/discharge tests of the Li/PPCMA GPE/LiNi_1/3Co_1/3Mn_1/3O₂ cell were evaluated at a current rate of 0.1C and in voltage range of 2.8–4.2 V at room temperature. The results show that the initial discharge capacity of Li/PPCMA GPE/LiNi_1/3Co_1/3Mn_1/3 O₂ cell is 115.3 mAh g⁻¹. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanofiller on ion conductivity,transmittance, and glass transition temperature of PEI:LiTFSI:PC:EC polymer electrolytes

In the present study, ion conductivity, optical properties, and glass transition temperatures are characterized for polymer electrolytes composed of poly(ethyleneimine) (PEI), lithium bis(trifluoromethane)sulfonylimide (LiTFSI) salt, propylene carbonate (PC), and ethylene carbonate (EC). It was doped with nanoceramic particles in different ratio (0–15 wt.%) to see the effect of ceramic particles. The salt concentration was fixed as 1.04 mol.kg^?1. Although valuable improvement in ion conductivity could not be achieved due to nano-Al₂O₃ fillers, ion conductivity results are placed between 10^?2 and 10^?4 S/cm. Differential scanning calorimetry (DSC) measurements and optical measurements of all electrolytes were performed between ?80 and 140 °C, in the wavelength range between 400 and 700 nm for sample with 80 μm thickness, respectively. The results showed that transmittance of electrolytes decreased monotonically for increasing Al₂O₃ contents. In particular, its transmittance value at 550 nm where human sight is at its greatest sensitivity went from 100% without nanoparticles to 50% for 15 wt% of Al₂O₃.     

Reprint of “Suppressing propylene carbonate decomposition by coating graphite electrode foil with silver”

A method has been developed to suppress the decomposition of propylene carbonate (PC) by coating graphite electrode foil with a layer of silver. Results from electrochemical impedance measurements show that the Ag-coated graphite electrode presents lower charge transfer resistance and faster diffusion of lithium ions in comparison with the virginal one. Cyclic voltammograms and discharge-charge measurements suggest that the decomposition of propylene carbonate and co-intercalation of solvated lithium ions are prevented, and lithium ions can reversibly intercalate into and deintercalate from the Ag-coated graphite electrode. These results indicate that Ag-coating is a good way to improve the electrochemical performance of graphitic carbon in PC-based electrolyte solutions.     

Suppressing propylene carbonate decomposition by coating graphite electrode foil with silver

A method has been developed to suppress the decomposition of propylene carbonate (PC) by coating graphite electrode foil with a layer of silver. Results from electrochemical impedance measurements show that the Ag-coated graphite electrode presents lower charge transfer resistance and faster diffusion of lithium ions in comparison with the virginal one. Cyclic voltammograms and discharge-charge measurements suggest that the decomposition of propylene carbonate and co-intercalation of solvated lithium ions are prevented, and lithium ions can reversibly intercalate into and deintercalate from the Ag-coated graphite electrode. These results indicate that Ag-coating is a good way to improve the electrochemical performance of graphitic carbon in PC-based electrolyte solutions.     

Synthesis and electrochemical properties of lithium methacrylate-based self-doped gel polymer electrolytes

In this study, a strategy for synthesizing lithium methacrylate (LiMA)-based self-doped gel polymer electrolytes was described and the electrochemical properties were investigated by impedance spectroscopy and linear sweep voltammetry. LiMA was found to dissolve in ethylene carbonate (EC)/diethyl carbonate (DEC) (3/7, v/v) solvent after complexing with boron trifluoride (BF₃). This was achieved by lowering the ionic interactions between the methacrylic anion and lithium cation. As a result, gel polymer electrolytes consisting of BF₃-LiMA complexes and poly(ethylene glycol) diacrylate were successfully synthesized by radical polymerization in an EC/DEC liquid electrolyte. The FT-IR and AC impedance measurements revealed that the incorporation of BF₃ into the gel polymer electrolytes increases the solubility of LiMA and the ionic conductivity by enhancing the ion disassociations. Despite the self-doped nature of the LiMA salt, an ionic conductivity value of 3.0 × 10⁻⁵ S cm⁻¹ was achieved at 25 °C in the gel polymer electrolyte with 49 wt% of polymer content. Furthermore, linear sweep voltammetry measurements showed that the electrochemical stability of the gel polymer electrolyte was around 5.0 V at 25 °C.