Review on composite polymer electrolytes for lithium batteries

This paper reviews the state of the art of composite polymer electrolytes (CPE) in view of their electrochemical and physical properties for the applications in lithium batteries. This review mainly encompasses on composite polymer electrolyte hosts namely poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVdF) studied so far. Also the ionic conductivity, transference number, compatibility and the cycling behavior of poly(vinylidene fluoride-hexafluoro propylene) (PVdF-HFP)-[AlO(OH)]_n-LiPF₆/LiClO₄ composite electrolytes have been studied and the results are discussed.     

Crystallinity, morphology, mechanical properties and conductivity study of in situ formed PVdF/LiClO4/TiO2 nanocomposite polymer electrolytes

Nanocomposite polymer electrolytes composed of poly(vinylidene fluoride) (PVdF), lithium perchlorate (LiClO₄) and TiO₂ nanoparticles were prepared by a solution-cast method. The nanosized ceramic filler, TiO₂, was synthesized in situ by a sol-gel process. Differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) analysis revealed that the crystalline phase and crystallinity were slightly decreased with the addition of TiO₂ to the PVdF/LiClO₄ system. Scanning electron microscopy (SEM) micrographs showed that the PVdF/LiClO₄/TiO₂ solid polymer electrolyte (SPE) membranes had a porous structure to a certain extent, and that the pore size decreased with increasing TiO₂ content. The overfull nanoparticles tended to aggregate on the surface and inside the pores at TiO₂ content above 15 wt.% so that the porosity decreased. Regarding mechanical properties, the strength of the PVdF/LiClO₄/TiO₂ electrolytes decreased after the uptake of EC/PC solution. In contrast to the conductive behavior of wet PVdF/LiClO₄/TiO₂ membranes relative to the uptake of EC/PC solution, the conductive mechanism of the solid membranes, after the lithium ion of LiClO₄ had already been installed in the PVdF solid polymer network, was mainly influenced by the TiO₂ nanoparticles. At a TiO₂ content of 10 wt.%, the solid and wet PVdF/LiClO₄/TiO₂ systems had the maximum conductivity values of 7.1 × 10⁻⁴ and 1.8 × 10⁻³ S/cm, respectively.     

Mesoscale models of conductivity in polymeric electrolytes—A comparative study

The studies of conductivity behavior of composite polymeric electrolytes (CPE) have been widely realized for about 15 years. There is, however, a lack of a versatile and efficient model of conductivity in these systems. The effective medium approach was introduced to predict the conductivity of systems according to some previously defined mixing rules. The main limitation of this method, however, is the fact that each new composite microstructure forces a new mixing rule development. This paper presents the random resistor network approach as an alternative method to the effective medium theory based approach. The presented comparison of both methods is based on both dc and ac simulated conductivity data. Some experimental data are introduced to compare the models with the conductivity values obtained for real samples.     

Solid polymer electrolytes of blends of polyurethane and polyether modified polysiloxane and their ionic conductivity

Polymer electrolytes based on thermoplastic polyurethane (TPU) and polyether modified polysiloxane (PEMPS) blend with lithium salts were developed via an in-situ polymerization of TPU with the presence of PEMPS and salts. Morphological study of TPU/PEMPS electrolytes showed that TPU and PEMPS were immiscible and TPU/PEMPS electrolytes had a multiphase morphology. The lithium salt enhanced the interfacial compatibilization between TPU and PEMPS via the interaction of lithium ions with different phases. Three lithium salts with different interaction strengths with TPU and PEMPS were used to prepare TPU/PEMPS electrolytes with different levels of phase compatibilization: LiCl, LiClO₄, and LiN(SO₂CF₃)₂ (LiTFSI). The effect of PEMPS on ionic conductivity, dimensional stability and thermal stability of TPU/PEMPS electrolytes and their relationship with the blend morphology were investigated. TPU/PEMPS electrolytes showed good dimensional stability and thermal stability. The addition of PEMPS to TPU increased the ionic conductivity of TPU/PEMPS electrolytes. The room temperature ionic conductivity of TPU/PEMPS electrolytes with LiTFSI can reach up to 2.49 × 10⁻⁵ S/cm.     

Influences of Bentonite on conductivity of composite solid alkaline polymer electrolyte PVA-Bentonite-KOH-H2O

The influence of the dopant Bentonite, on the ionic conductivity of the PVA-KOH-H₂O alkaline solid polymer electrolyte (ASPE) is studied. The results show that the addition of Bentonite has both positive and negative effects on the ionic conductivity of ASPE. At lower KOH and H₂O contents, the addition of Bentonite can break the continuous ion conducting phase of the ASPE, and therefore decrease the ASPE conductivity. However, the addition of Bentonite can also increase the KOH content in PVA matrix. This greatly increases the conductivity of the ASPE especially at higher water content. A highest ionic conductivity of 0.11 S cm⁻¹ is reached at room temperature. A maximum ionic conductivity value is observed at relative lower water content for different amount of Bentonite-doped ASPE. The temperature dependence of the ionic conductivity is of the Arrhenius type. The ion transfer activation energy Ea, in the order of 4-6 kJ mol⁻¹, heavily depends on the Bentonite content. XRD and SEM tests show that PVA in the Bentonite-doped ASPE is of amorphous structure, and there are lots of interspaces in the composite ASPE inner structure. The composite electrolyte has good electrochemical stability window and good charged-discharge property in secondary Zn-Ni cells at low charge-discharge rate.     

Characterization of solid polymer electrolytes based on poly(trimethylenecarbonate) and lithium tetrafluoroborate

The results of an investigation of a polymer electrolyte system based on the poly(trimethylene carbonate) host matrix, designated as p(TMC), with lithium tetrafluoroborate guest salt are described in this presentation. Electrolytes with lithium salt compositions with n between 3 and 80 (where n represents the number of (OCOCH₂CH₂CH₂O) units per lithium ion) were prepared by co-dissolution of salt and polymer in anhydrous tetrahydrofuran. The homogeneous solutions obtained by this procedure were evaporated, within a preparative glovebox and under a dry argon atmosphere, to form thin films of electrolyte.The solvent-free electrolyte films produced were obtained as very flexible, transparent, completely amorphous films and were characterized by measurements of total ionic conductivity, cyclic voltammetry, differential scanning calorimetry and thermogravimetry.     

Correlation between microstructure and electrical conductivity in composite electrolytes containing Gd-doped ceria and Gd-doped barium cerate

Composite electrolytes with nominal compositions, Ce_0.8Gd_0.2O_1.9 + xBaO (x = 0.2 and 0.3), have been synthesized through the citrate route. Formation of two phases, namely Gd-doped ceria and Gd-doped barium cerate, has been confirmed through XRD and SEM studies. The impedance spectra show three distinct semi-circles, all originating from the composite electrolytes. In the temperature range 175-350 °C, the activation energies for the conductivity values extracted from the high frequency and intermediate frequency parts of the impedance spectra remains the same, irrespective of compositional and micro-structural variation. On the other hand, the activation energies for the conductivity values associated with the low frequency impedance spectra show a significant change with micro-structural variation. Solid oxide fuel cells constructed using these composite electrolytes exhibit a higher open circuit voltage compared to those based on single phase 20 mol% Gd-doped ceria.     

Composite gel electrolytes based on poly(methylmethacrylate) and hydrophilic fumed silica

Transparent and highly conducting (10⁻³ S cm⁻¹) composite gel electrolytes (cges) based on poly(methylmethacrylate) (PMMA) with 1 M LiClO₄ in propylene carbonate (PC) and hydrophilic fumed silica (SiO₂) added in different proportions up to 5 wt.% as the constituents were prepared. The unique property of the aggregates of surface hydroxyl groups of hydrophilic fumed silica to link together through hydrogen bonding to form a network and the interactions between various constituents, probed by FTIR spectroscopic technique, have revealed to control both the transport and rheological properties of the cges. The high mechanical and electrochemical stability of these cges makes them potential candidates as electrolytes in “All Solid State Electrochromic Windows” (ECWs). Detailed room temperature conductivity and viscosity measurements correlated to FTIR spectroscopic analysis of cges along with their optical and electrochemical investigations is reported in the present paper.     

Ionic liquid/polymer composite electrolytes by in situ photopolymerization and their application in dye-sensitized solar cells

A new kind of quasi-solid state electrolytes for dye-sensitized solar cells (DSCs) has been prepared by in situ photopolymerization from the precursor 1,6-hexanediol diacrylate (HDDA) in 1-hexyl-3-methyl imidazolium iodide (HMII). The optimal ratio of polymer/ionic liquid is determined by the conductivities of the electrolytes. In order to further increase the miscibility between ionic liquid and the polymer, oligomer polyethylene glycol dimethyl ether (PEGDME) is introduced. By optimization of the amount of PEGDME in the electrolyte, the DSCs using this kind of solid-state electrolytes can present 6.5% of light-to-electricity conversion efficiency under 41 mW cm⁻². In the meantime, the influence of PEGDME additive is detailedly investigated by electrochemical impedance spectrum (EIS) and intensity modulated photovoltage spectroscopy (IMVS) techniques. Preliminary long-term stability test revealed that this in situ photopolymerized electrolyte exhibits good stability after 1000 h thermal test.     

Study on the conductivity of recast Nafion/montmorillonite and Nafion/TiO2 composite membranes

Nafion^® composite membranes were formed from a recast procedure already applied by the authors. Montmorillonite (MMT) and titanium dioxide (TiO₂) were used separately as fillers in the recast process and dimethylformamide (DMF) was used as the casting solvent. Addition of 1 wt.% MMT or 1 wt.% TiO₂ to the ionomer dispersions prior to the heat treatment demonstrated that there is an increase in water content of the recast membranes. For both the samples, it was verified that the tangential conductivity increases with increasing relative humidity (RH) of the environment. Fuel cell tests carried out with recast membranes showed that the best performances are seen when the anode and cathode humidification temperatures are low. With a ΔT of −35 °C between cell temperature and anode humidification, the conductivity of additive-containing samples is 10% higher than that of additive-free membranes.     

ZrB_2－刚玉莫来石质复合材料显微结构及力学性能的研究

通过对不同刚玉－莫来石含量的ＺｒＢ２材料力学性能及显微结构的研究，探讨了刚玉莫来石对ＺｒＢ２强度和韧性的影响，同时也分析了强化和韧化机制。     

Detailed studies on the fillers modification and their influence on composite, poly(oxyethylene)-based polymeric electrolytes

Ever since composite polymeric electrolytes (CPEs) were developed and studied authors publishing their results used to emphasize the role of new materials they used and not the details of the preparation conditions. In this work it is proven that the later can influence markedly the former and shown that the comparison of results obtained for different materials by different authors can be made only in limited number of cases.Herein results of the detailed studies on the composite, PEO-based polymeric electrolytes with surface modified fillers are shown. Extensive studies on the influence of different powders and preparation conditions on the final properties of composite electrolytes are described. The modification of fillers of various type and grain size was studied together with their influence on the functional parameters of electrolytes. Multiple research approaches and analysis methods were utilized in order to obtain as complete image of the materials as possible.     

Electrooxidation and dischargeability of transition-metal borides as possible anodic materials in neutral aqueous electrolytes

A number of transition-metal borides were studied as anodic materials for neutral aqueous batteries. These borides are shown to have considerably high electrochemical activities in neutral electrolytes. The discharge capacities for TiB₂ reach 1,350 mAh g⁻¹ at a constant current density of 50 mA g⁻¹, exceeding those for all the metal electrodes reported so far. Amorphous CoB_x can deliver a discharge capacity of >650 mAh g⁻¹, and even simply ball-milled FeB_x can also give a discharge capacity of >200 mAh g⁻¹. These results suggest the possible use of boride compounds as a large family of new anodic materials for constructing neutral aqueous batteries with high electrochemical capacity and rate capability.     

Synthesis, structure characterization and ionic conductivity of star-branched organic-inorganic hybrid electrolytes based on cyanuric chloride, diamine-capped poly(oxyalkylene) and alkoxysilane

A synthesis route for preparing highly conductive solid organic-inorganic hybrid electrolytes has been developed by using cyanuric chloride as the coupling core to react with diamino-terminated poly(oxyalkylene) triblock copolymers, followed by cross-linking with an epoxy alkoxysilane 3-glycidyloxypropyl trimethoxysilane via a sol-gel process. The present hybrid electrolyte with a [O]/[Li] ratio of 32 was found to be the most conductive, reaching a maximum lithium ion conductivity of 6.8 × 10⁻⁵ Scm⁻¹ at 30 °C. The Li-ion mobility was determined from ⁷Li static NMR line width measurements and correlated with their ionic conductivities. The onset of ⁷Li line narrowing was closely related to the T_g of the hybrid electrolytes as measured by DSC experiments. Thus, the motions of the lithium cations are strongly coupled with the segmental motion of the polymer chains, which is in line with the Vogel-Tamman-Fulcher behavior as observed in ionic conductivity.     

Effect of SiC nanowires on the mechanical properties and thermal conductivity of 3D-SiCf/SiC composites prepared via precursor infiltration pyrolysis

In this study, SiC nanowires (SiC_NWS) were grown in situ on the surface of PyC interface through chemical vapor infiltration (CVI) to improve the mechanical characteristics and thermal conductivity of three-dimensional SiC_f/SiC composites fabricated via precursor infiltration pyrolysis (PIP). The effect of SiC_NWS on the properties of the obtained composites was investigated by comparing them with conventional SiC_f/PyC/SiC composites. After the deposition of SiC_NWS, the flexural strength of the SiC_f/SiC composites was found to increase by 46 %, and the thermal conductivity showed an obvious increase at 25?1000 °C. The energy release of the composites in the damage evolution process was analysed by acoustic emission. The results indicated that the damage evolution process was delayed owing to the decrease in porosity, the crack deflection and bridging of the SiC_NWS. Furthermore, the excellent thermal conductivity was attributed to the thermally conductive pathways formed by the SiC_NWS in the dense structure.     

Synthesis of functionalized copolymer electrolytes based on polysiloxane and analysis of their conductivity

Functionalized siloxane-based solid polymer electrolytes were synthesized using a platinum-catalyzed silylation reaction. The ionic conductivities of these solid polymer electrolytes were measured as a function of the concentration of lithium bis(trifluoromethylsulfonyl)imide (LiTFSi) salt. The highest ionic conductivity and lowest activation energy of solid polymer electrolytes were observed to be 1.15 × 10⁻⁴ S cm⁻¹ (25 °C) and 3.85 kJ mol⁻¹, respectively. The interface property between electrolyte and electrode and thermal stability of the polymer electrolytes were found to enhance after they were functionalized with acrylate, and the functionalized electrolytes were observed to maintain a glass transition temperature as low as that of other siloxane compounds. Thus, modifications involving acrylate with ethylene oxide group substitution provide a route for carrier ions and enhance both the ionic conductivity and mechanical properties of the siloxane structure.     

Role of fumed silica on ion conduction and rheology in nanocomposite polymeric electrolytes

The electrochemical, rheological, calorimetric, spectroscopic and morphological investigations have been used to examine poly(methyl methacrylate), PMMA based electrolytes dispersed with nano-sized fumed silica (SiO₂). The observed ionic conductivity was one of the highest and is of the order ∼mS/cm at ambient temperature which was studied as a function of concentration of fumed silica nano-particles. It was further found that the fumed silica acted as a passive filler and played a predominant role in controlling the rheological properties while ion transport properties were least effected. The differential calorimetry studies revealed single glass transition temperature pointing towards homogeneous nature of the composite polymeric electrolytes (CPEs). At an optimum concentration of fumed silica (2 wt%) the observed maximum conductivity and morphology was attributed to the presence of a strong network structure, while at a higher concentration the elastic behavior was more pronounced which impeded ion transport. This contention was supported by spectroscopic data.     

Creating polymer templates and their use in the in-situ synthesis of biodegradable composite networks

A biodegradable polyethyleneoxide–polycaprolactone–polyethyleneoxide (PEO–PCL–PEO or PECE) triblock polymer was synthesized as a structure-directing agent for high-surface area silica formation. Systematic end-group functionalization of the triblock polymer, prior to its use as a structure-directing agent for high-surface area silica growth, imparts an additional reactive function that is exploited to grow a continuous organic phase within and around the silica particles. Hence, the biodegradable triblock also acts as a macromer whose dual function leads to in-situ generation of intimately mixed biopolymer/nanoporous–silica composite networks. This composite can be both biodegradable and biocompatible, and the modulus is comparable to other non-biodegradable materials.