Studies on lithium bis(oxalato)-borate/propylene carbonate-based electrolytes for Li-ion batteries

Lithium bis(oxalato)-borate (LiBOB) is a promising salt for Li-ion batteries owing to its various characteristics such as non-fluorine, non-toxicity, low cost, and safety. It has the unique merits such as the stability at high temperature and the film-forming characteristics in propylene carbonate (PC)-based electrolyte. In this work, the utilization of PC as the basal solvent and dimethyl carbonate, γ-butyrolactone and ethylene carbonate as co-solvents for LiBOB have been investigated. The results indicate that the co-solvent has conducive effects on the conductivities, viscosities, and battery performance. The conductivity and viscosity of 0.7 mol L⁻¹ LiBOB in PC+GBL+EC+DMC (1:1:1:1, v/v) are 6.22 mS cm⁻¹ and 3.74 mPa s, respectively, and it is very stable in 0–5 V range. The capacity of Li/LiFePO₄ battery is about 160 mAh g⁻¹ at 0.5 °C. Moreover, the battery has exhibited the excellent rate performance.     

Frequency dependence of conductivity of ethylene carbonate based electrolyte for Li-ion battery

Specific cell arrangement corresponding to four probe pouch cell with two reversible current and two indifferent potential electrodes is applied to study the existence of frequency dependence on conductivity of a medium between two potential electrodes, i.e., separator impregnated with electrolyte. It is established that frequency dependence of conductivity for carbonate-based electrolyte solution does exist. Series resistance is found to be in a good agreement by specific value with that reported earlier. Low frequency limit of impedance spectra is a few decimal orders higher than that of series resistance. Both low- and high-frequency resistances increase with the interelectrode distance in agreement with Ohm law.     

Ionic conductivity, thermal stability and FT-IR studies on plasticized PVC / PMMA blend polymer electrolytes complexed with LiAsF6 and LiPF6

The lithium salt (x) (x=LiAsF₆, LiPF₆) was complexed with a blend of poly(vinyl chloride) (PVC) / poly(methyl methacrylate)(PMMA) and plasticized with a combination of ethylene carbonate(EC) and propylene carbonate(PC). The electrolyte films were prepared using doctor blade method and subjected to ionic conductivity measurements at nine different temperatures viz.,-30, -15, 0, 15, 30, 40, 50, 60 and 70 °C. The films were also subjected to TG - DTA and FT-IR analysis. The effect of salt on ionic conductivity is discussed. A 75:25 PMMA/PVC blend at 60 % plasticizer content has been found to possess optimal properties in terms of ionic conductivity, thermal and electrochemical stability.     

Density functional theory calculations for ethylene carbonate-based binary electrolyte mixtures in lithium ion batteries

The density functional theory (DFT) calculations have been performed to investigate the interaction of Li⁺ with various organic solvents widely used as Li ion rechargeable battery electrolytes such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC); and their EC-based binary mixtures at the level of B3LYP/6-31G (d). The interaction of Li⁺ with these solvents has been calculated in terms of electronic structures of clusters of the mixtures of organic solvents including a lithium ion. The main objective of our investigation is to help in understanding a stable and enhancing ionic transfer at graphite/electrolyte interface assisted by the mixtures of the solvents. The calculated results favor the stability of EC-based binary mixtures and high EC-content binary mixture systems. In infrared (IR) vibrational spectra, the IR active modes of the solvent show significant changes due to the cation-solvent interaction.     

Ionic conductivity of electrolytes based on star-branched poly(ethylene oxide) with high concentration of lithium salts

Electrolytes based on star-branched poly(ethylene oxide) with lithium bis(trifluoromethanesulfone)imide LiTFSI and lithium iodide salts were prepared by casting from solution. The electrical properties of electrolytes subjected to various heating and cooling runs were studied by impedance spectroscopy and impedance spectroscopy simultaneous with optical microscope observation. Differential scanning calorimetry was used for additional characterization. The results indicate that in electrolytes with high content of salt, values of ionic conductivity comparable to that of dilute electrolytes can be achieved. Moreover, electrolytes with high amount of salt seem to show weaker temperature dependence of conductivity. Promising results in terms of ionic conductivity were obtained for mixture of LiTFSI and lithium iodide. A few problems which may decrease the performance of studied system as a solid electrolyte were also identified, from which changes of physical properties of samples subjected to thermal cycles and aging seem to be the most important ones.     

Effect of ZrO2 on the conductivity of PVC-LiAsF6-DBP polymer electrolytes

The preparation and characterization of composite polymer electrolytes PVC-LiAsF₆-DBP for different concentrations of ZrO₂ have been investigated. X-ray and FTIR studies indicate complex formation between the polymer and salt and that the complex remains mainly in the amorphous phase. The electrical conductivity values measured by a.c impedance spectroscopy are found to depend upon the ZrO₂ concentration. The temperature dependence of the conductivity of the polymer films obeys the VTF relation. The conductivity values are presented and the results are discussed.     

MgAl2SiO6-incorporated poly(ethylene oxide)-based electrolytes for all-solid-state lithium batteries

Poly(ethylene oxide) (PEO)-based composite polymer electrolytes (CPEs), comprising various concentrations of lithium hexafluorophosphate and magnesium aluminium silicate, were prepared by hot-press technique. The membranes were characterised by scanning electron microscopy, tensile and thermal analyses. It has been demonstrated that the incorporation of the ceramic filler in the polymeric matrix has significantly enhanced the ionic conductivity, thermal stability and mechanical integrity of the membrane. It also improved the interfacial properties with lithium electrode. Finally, an all-solid-state lithium cell composed of Li/CPE/LiFePO₄ has been assembled and its cycling performance was analysed at 70 °C. The cell delivered a discharge capacity of 115 mAh g^?1 at 1 °C rate and is found to be higher than previous reports.     

PEG crosslinked poly(vinylbenzene boronic acid) polymer electrolytes for Li-ion batteries

Poly(4-vinylbenzeneboronic acid), PVBBA was synthesized via free-radical polymerization of 4-vinylbenzeneboronic acid (4-VBBA) and followed by crosslinking with polyethylene glycol (PEG) with different molecular weights to produce boron containing crosslinked polymers. Prior to crosslinking, the materials were doped with CF₃SO₃Li at several stoichiometric ratios to get PVBBAPEGX-Y where X is the molecular weight of PEG and Y is the EO/Li ratio. The materials were characterized by using Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC). The ionic conductivity of these novel crosslinked electrolytes was studied by dielectric-impedance spectroscopy. Li-ion conductivity of these polymer electrolytes depends on the length of the side units as well as the doping ratio. PVBBAPEG200-10 illustrated a satisfactory ionic conductivity of 3.1 × 10^?5 S/cm at 20 °C and 1.8 × 10^?3 S/cm at 100 °C.     

Effect of vinyl ethylene carbonate on the compatibility between graphite and the flame-retarded electrolytes containing dimethyl methyl phosphonate

Effect of the content of vinyl ethylene carbonate (VEC) on the compatibility between the graphite anode and the electrolytes containing 10–30% dimethyl methyl phosphonate (DMMP) was investigated by cyclic voltammetry measurement. Impact of the contents of VEC and DMMP on the formation of the solid electrolyte interface layer was discussed, and a competitive mechanism between the destructive effect of DMMP decomposition and the positive effect of VEC was proposed. In the LiCoO₂/graphite cells, the electrolytes modified by DMMP and VEC exhibited satisfying cell performances, especially for the electrolyte with 10% DMMP and 2% VEC.     

Single-ion conductivity enhancement for the composite polymer electrolytes based on Li(PVAOB)/PPEGMA for lithium-ion batteries

In the present work, a series of single-ion conducting composite polymer electrolytes based on lithium polyvinyl alcohol oxalate borate (Li(PVAOB)) and poly(polyethylene glycol methacrylate) (PPEGMA) were produced. PEGMA was polymerized into PPEGMA, and the Li(PVAOB) was prepared from poly (vinyl alcohol) (PVA), oxalic acid, and boric acid. Li(PVAOB) was blended with PPEGMA at different stoichiometric ratios to obtain a single-ion conducting system. All the electrolytes were characterized by Fourier transformation infrared spectroscopy (FT-IR), thermogravimetry analysis (TGA), differential scanning calorimeter (DSC), and scanning electron microscopy (SEM) techniques. These results verified the interaction between host and guest polymers, sufficient thermal stability within the measured conductivity domain, and the homogeneity of the composite electrolytes. The effect of PPEGMA onto the ionic conductivity was investigated using impedance spectroscopy. The Li(PVAOB)-60PPEGMA is the optimum content, and this sample has a maximum ionic conductivity of 3 × 10^?4 S/cm at 100 °C which is approximately five orders of magnitude higher than neat Li(PVAOB). Activation energy (E _a ) of ionic transport decreased from 11.9 to 0.27 kJ/mol, suggesting a much faster ionic mobility for higher PPEGMA-containing samples.     

Effect of chitosan on deposition of LiCoO2 thin film for Li-ion batteries

Deposition of LiCoO₂ thin film using chitosan-added precursor solution was found to be a cost-effective way to fabricate cathode for Li-ion thin film batteries. The structures and electrochemical performance of such LiCoO₂ cathode were characterized by using an X-ray diffracotmeter (XRD), FTIR and charge–discharge tests. After annealing at ca. 500 °C, the results of XRD showed that the LiCoO₂ gel started to crystallize and showed hexagonal phase with a space group of R3?m. The enhanced stability of the precursor solution by the addition of chitosan is attributed to the complexation between metal ions and the ?NH₂ groups of chitosan.The electrochemical behaviour for the deposited films calcined at 700 °C for 4 h was also characterized by charge–discharge test. The result revealed that the film deposited from chitosan-containing precursor solution possesses an initial discharge capacity of 129 mAh g^? 1.     

Effect of molecular weight on conductivity of polymer electrolytes

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

Polymer electrolytes based on poly(vinylidene fluoride-co-hexafluoropropylene) with crosslinked poly(ethylene glycol) for lithium batteries

The combination of a poly(ethylene glycol) (PEG) network and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) copolymer chains is one of the most efficient means for modifying PVDF-HFP gel electrolytes. Previous preparations tend to introduce contamination into the polymer gel electrolyte because of irradiation, high temperature or the initiator needed for crosslinking which might result in the electrochemical degradation. In order to overcome the above disadvantages, a new method has been developed to successfully prepare the semi-interpenetrating polymer networks of PVDF-HFP based electrolytes with crosslinked diepoxy polyethylene glycol (DIEPEG). In this process, impurities are avoided because of a moderate reaction temperature at 50 °C and poly(ethylenimine) (PEI) as the crosslinking agent. Microporous films with various compositions are prepared and characterized. Thermal, mechanical, swelling and electrochemical properties, as well as microstructures of the prepared polymer electrolytes have been investigated using thermogravimetric analysis, electrochemical impedance spectroscopy, linear sweep voltammetry, and scanning electron microscopy. The results show that the blend polymer electrolyte with PVDF-HFP/PEI + DIEPEG (60:40 w/w) has an ionic conductivity of 2.3 mS cm^? 1 at room temperature in the presence of 1 M LiPF₆ in EC and DMC (1:1 w/w). All the blend electrolytes are electrochemically stable up to 4.8 V versus Li/Li⁺. The results reveal that this new method may be very promising for improving PVDF-HFP based electrolytes.     

Oligo(ethylene oxide)-functionalized trialkoxysilanes as novel electrolytes for high-voltage lithium-ion batteries

Oligo(ethylene oxide)-functionalized trialkoxysilanes were synthesized through hydrosilylation reaction by reacting trialkoxysilane with oligo(ethylene oxide) allyl methyl ether using PtO₂ as a catalyst. The physical properties of these compounds, such as viscosity, dielectric constant, and ionic conductivity, were characterized. Among them, [3-(2-(2-methoxyethoxy)ethoxy)-propyl]triethoxysilane (TESM2) exhibited a commercial viable ionic conductivity of 1.14 mS cm^?1 and a wide electrochemical window of 5.2 V. A preliminary investigation was conducted by using TESM2 as an electrolyte solvent for high-voltage applications in lithium-ion batteries. Using 1 M LiPF₆ in TESM2 with 1 vol% vinyl carbonate as an electrolyte, LiCoO₂/Li half-cell delivered a specific capacity of 153.9 mAh g^?1 and 90 % capacity retention after 80 cycles (3.0–4.35 V, 28 mA g^?1); Li_1.2Ni_0.2Mn_0.6O₂/Li₄Ti₅O₁₂ full cell exhibited the initial capacity of 161.3 mAh g^?1 and 86 % capacity retention after 30 cycles (0.5–3.1 V, 18 mA g^?1).     

An investigation of the salt dissociation effects on solid electrolyte interface (SEI) formation using linear carbonate-based electrolytes in lithium ion batteries

Lithium salts possess dissociating properties that are useful for passivation layer formation. In this study, these properties are investigated in the context of three kinds of linear carbonate electrolytes using several techniques, such as physical properties, AC impedance, electrochemical quartz crystal microbalance (EQCM), and nuclear magnetic resonance (NMR). The lithium salts are completely dissociated to the lithium ions by the unsymmetrical linear carbonate structure; therefore, the transference number and diffusion coefficient of the cations show that the lithium ions are important and dominate the ionic transfer and the passivation layer properties that are important and relate to battery performance. These data suggest that battery performance is influenced by ionic transfer properties, lithium salt and electrolyte structure.     

Influence of fluoroethylene carbonate on the solid electrolyte interphase of silicon anode for Li-ion batteries:A scanning force spectroscopy study

Silicon is an important high capacity anode material for the next generation Li-ion batteries.The electrochemical performances of the Si anode are influenced strongly by the properties of the solid electrolyte interphase(SEI).It is well known that the addition of flouroethylene carbonate(FEC)in the carbonate electrolyte is helpful to improve the cyclic performance of the Si anode.The possible origin is suggested to relate to the modification of the SEI.However,detailed information is still absent.In this work,the structural and mechanical properties of the SEI on Si thin film anode in the ethylene-carbonate-based(EC-based)and FEC-based electrolytes at different discharging and charging states have been investigated using a scanning atomic force microscopy force spectroscopy(AFMFS)method.Single-layered,double-layered,and multi-layered SEI structures with various Young’s moduli have been visualized three dimensionally at nanoscale based on the hundreds of force curves in certain scanned area.The coverage of the SEI can be obtained quantitatively from the two-dimensional(2D)project plots.The related analysis indicates that more soft SEI layers are covered on the Si anode,and this could explain the benefits of the FEC additive.     

Tuning hybrid liquid/solid electrolytes by lowering Li salt concentration for lithium batteries

Hybrid liquid/solid electrolytes(HLSEs) consisting of conventional organic liquid electrolyte(LE), polyacrylonitrile(PAN), and ceramic lithium ion conductor Li_(1.5)Al_(0.5)Ge_(1.5)(PO_4)_3(LAGP) are proposed and investigated. The HLSE has a high ionic conductivity of over 2.25 × 10~(-3) S/cm at 25?C, and an extended electrochemical window of up to 4.8 V versus Li/Li+. The Li|HLSE|Li symmetric cells and Li|HLSE|Li FePO_4 cells exhibit small interfacial area specific resistances(ASRs) comparable to that of LE while much smaller than that of ceramic LAGP electrolyte, and excellent performance at room temperature. Bis(trifluoromethane sulfonimide) salt in HLSE significantly affects the properties and electrochemical behaviors. Side reactions can be effectively suppressed by lowering the concentration of Li salt. It is a feasible strategy for pursuing the high energy density batteries with higher safety.     

Properties of gel polymer electrolytes based on poly(butyl acrylate) semi-interpenetrating polymeric networks toward Li-ion batteries

A new type of gel polymer electrolyte (GPE) based on poly(butyl acrylate) (PBA) semi-interpenetrating polymer networks (IPNs) and polyvinylidene fluoride (PVDF) was prepared in different molar ratios ranging from 1:0.5 to 1:1. A series of structure characterizations of PBA/PVDF had been measured using FTIR, XRD, and SEM. The electrolyte uptake test revealed that when the semi-IPNs were swollen with the commercial liquid electrolyte solutions, they showed an outstanding electrolyte uptake of 120% with a chemically cross-linked structure. All results indicated that the GPE exhibited the best performance when the molar ratio of BA and PVDF was 1:0.5. The prototype cell assembled with LiFePO₄ as cathode, lithium metal as anode, and GPE as the electrolyte as well as separator retained 94% of its initial specific capacity after 100 charge-discharge cycles, showing an excellent cycling stability and a high electrochemical window (up to 4.5 V against Li⁺/Li) at room temperature. Compared with the liquid electrolyte, the GPE exhibited a similar stable cycling performance and was suitable for practical application in Li-ion batteries.     

Nanopowders of spinel-type electrode materials for Li-ion batteries

Nanomaterials are becoming important for use in Li-ion battery electrodes as these can deliver increased capacity and improved power performance. Our work is focused on Mg-doped high-voltage spinel materials, such as LiNi_0.5Mn_1.5O₄, in order to improve its stability. LiMg_δNi_0.5−δMn_1.5O₄ with δ = 0.05, having the cubic spinel structure (P4₃32) were made via four different synthesis routes – a solid-state route, a sol–gel method, a xerogel route and an auto ignition method.The powders were investigated with SEM and TEM analysis. XRD was used to determine the crystallographic structure. Electrochemical tests were performed in CR2320 coin cells built with 1 M LiPF₆ in EC/EMC/DMC 1:2:2 as electrolyte and metallic Li as negative electrode – cells were measured with a MACCOR cycler.LiMg_0.05Ni_0.45Mn_1.5O₄ made via the sol–gel and xerogel routes revealed agglomerated nanoparticles with sizes ranging from 10 to 200 nm, whereas the auto ignition method gives particle sizes between 10 and 50 nm. Although agglomerated, often residual LiMn₂O₄ is observed, with increasing concentration going from solid-state, sol–gel, xerogel to auto ignition.Hence, thanks to these different synthesis routes, we are able to obtain particle sizes reaching from 10 to 200 nm, with a narrow particle size distribution. The electrochemical tests of the xerogel particles showed promising results. The auto ignition method show also promising results, however, the impurity phase needs to be suppressed significantly. The sol–gel method, the xerogel route and the auto ignition method show increased capacity retention at high power rates compared to the solid state method.     

Effect of ethylene carbonate plasticizer on agar-agar: NH<Subscript>4</Subscript>Br-based solid polymer electrolytes

Proton-conducting polymer electrolytes based on biopolymer, agar-agar as the polymer host, ammonium bromide (NH₄Br) as the salt and ethylene carbonate (EC) as the plasticizer have been prepared by solution casting technique with dimethylformamide as solvent. Addition of NH₄Br and EC with the biopolymer resulted in an increase in the ionic conductivity of polymer electrolyte. EC was added to increase the degree of salt dissociation and also ionic mobility. The highest ionic conductivity achieved at room temperature was for 50 wt% agar/50 wt% NH₄Br/0.3% EC with the conductivity 3.73?×?10^?4 S cm^?1. The conductivity of the polymer electrolyte increases with the increase in amount of plasticizer. The frequency-dependent conductivity, dielectric permittivity (ε′) and modulus (M′) studies were carried out.