Film compositions, liquid electrolytes

Film-forming chemical reactions and the chemical composition of the film formed on lithium in nonaqueous aprotic liquid electrolytes are reviewed by Dominey [7], SEI formation on carbon and graphite anodes in liquid electrolytes has been reviewed by Dahn et al. [8], In addition to the evolution of new systems, new techniques have recently been adapted to the study of the electrode surface and the chemical and physical properties of the SEI. The most important of these are X-ray photoelectron spectroscopy (XPS), SEM, X-ray diffraction (XRD), Raman spectroscopy, scanning tunneling microscopy (STM), energy-dispersive X-ray spectroscopy (EDS), FTIR, NMR, EPR, calorimetry, DSC, TGA, use of quartz-crystal microbalance (QCMB) and atomic force microscopy (AFM). 

Perhaps more important than cost is the solution to the crucial problem of interfacial contacts that always plagues homogeneous GPE films prepared from traditional approaches. Since both cathode and anode composite materials are coated on their substrates with the same PVdF—HEP copolymer as the binder, the in situ gellification following the electrolyte activation effectively fuses the three cell components into an integrated multilayer wafer without physical boundaries, so that the interfaces between anode and electrolyte or cathode and electrolyte are well extended into the porous structures of these electrodes, with close similarity to the interfaces that a liquid electrolyte would access. 

Hashmi and Upadhyaya compared the electrochemical properties of the electrochemically synthesized MnO /PPy composite electrodes, fabricated with different electrolytes, namely polymer electrolyte film (polyvinyl alcohol [PVA]-HjPO aqueous blend), aprotic liquid electrolyte (LiClO -propylene carbonate [PC]), and polymeric gel electrolyte (poly methyl methacrylate [PMMA]-ethylene carbonate [EC]-PC-NaClO ) [60]. The cell with aqueous PVA-H PO showed non-capacitive behavior owing to some reversible chemical reaction of MnO with water, while the MnO / PPy composite was found to be a suitable electrode material for redox supercapacitors with aprotic (non-aqueous) electrolytes. The solid-state supercapacitor based on the MnO /PPy composite electrodes with gel... 

A catalyst that can prevent polysulfide anions from precipitating as solids would be highly desirable. While a catalyst still remains to be discovered, several approaches, common in catalysis, have been employed to improve the cycling performance of lithium-sulfur batteries. For example, new electrolytes [18-21], protective films [22], solubiHzed sulfides [23], and new cathodes [24] have been developed. However, the performance results have either not been reported or have been found to be inadequate for practical applications. For example, a disordered mesoporous carbon-sulfur composite in conjunction with ionic liquid electrolytes has been fabricated. This system achieves high initial capacity that deteriorates rapidly [20]. 

MMT is naturally hydrophilic and must be rendered hydrophobic or organophilic in order for it to be compatible with the polymer matrix. Gel nanocomposite polymer electrolytes were obtained by dispersing organophilic MMT in a PVDF or PVDF-hexafluoropropylene (PVDF-HFP) solution. The composite membranes were obtained by film-casting the film was dried and then the liquid electrolyte was added by swelling. The MMT was exfoliated and, in order to improve MMT delamination, the MMT was sonicated in dimethylformamide (DMF). The filled PVDF membrane shown excellent wettability with the electrolyte solution. Solvent and electrolyte uptake by the filled membrane was considerably greater than that of the unfilled membrane. This increase in electrolyte uptake could explain the improvement in ionic conductivity observed. 

Nanocomposites based on PMMA were synthesised via sol-gel transformation and in situ free radical polymerisation of MMA. Gel nanocomposite polymer electrolytes were obtained by the addition of PMMA-T102 or PMMA-SiOi to the PC-based liquid electrolyte. After 2 h at 55 °C, transparent films were obtained. The gel composite based on PMMA-T102 exhibited higher viscosity than the unfilled gel electrolyte and the PMMA-Si02-based electrolyte. The conductivity values of filled and unfilled gel electrolytes were very similar, but the nanocomposites exhibited better mechanical properties than the unfilled polymer electrolytes. 

To prepare the electrolyte an appropriate amount of mesoporous molecular sieve SBA-15 was dispersed in dimethylformamide (DMF) with vigorously stirring, then an adequate amount of PVdF-HFP powder was added and dissolved in the SBA-15/DMF suspension with further stirring. The resultant mixture was cast on a smooth and clean glass plate and was allowed to stand for several hours. After evaporating the solvent in vacuo, a PVdF-HFP/SBA-15 composite film was obtained, known as a dry film. After being further treated in vacuo to remove the solvent, the dry film was immersed into a liquid electrolyte. Activated CSPE films which contained liquid electrolyte were obtained known as wet films. The ion conductivity is 0.3 X 10 S cm at room temperature for the ratio of SBA-15 PVdF-HFP = 3 8. 

Thus, at temperatures lower than the liquid us temperature (usually above —20 °C for most electrolyte compositions).EC precipitates and drastically reduces the conductivity of lithium ions both in the bulk electrolyte and through the interfacial films in the system. During discharge, this increase of cell impedance at low temperature leads to lower capacity utilization, which is normally recoverable when the temperature rises. However, permanent damage occurs if the cell is being charged at low temperatures because lithium deposition occurs, caused by the high interfacial impedance, and results in irreversible loss of lithium ions. An even worse possibility is the safety hazard if the lithium deposition continues to accumulate on the carbonaceous surface. 

Continuous Phase Composition Emulsion liquid membrane properties can be significantly influenced by changing the composition of the external aqueous phase. Emulsion stability can be improved by an increase in the viscosity as a result of the decrease in the rate of fluid drainage between the liquid films [88]. An increase in ionic strength of the external phase has been shown to cause a decrease in entrainment phenomena during permeation. This has been attributed to an alteration of the stmcture of the interface between the emulsion and the external phase promoted by the presence of electrolytes in the external phase. A reduction in osmosis also occurs due to a reduction in the chemical potential difference between both sides of the membrane [94,98]. 

Many approaches have been developed for the production of ionic liquid-polymer composite membranes. For example, Doyle et al. [165] prepared RTILs/PFSA composite membranes by swelling the Nafion with ionic liquids. When 1-butyl, 3-methyl imidazolium trifluoromethane sulfonate was used as the ionic liquid, the ionic conductivity ofthe composite membrane exceeded 0.1 S cm at 180 °C. A comparison between the ionic liquid-swollen membrane and the liquid itself indicated substantial proton mobility in these composites. Fuller et al. [166] prepared ionic liquid-polymer gel electrolytes by blending hydrophilic RTILs into a poly(vinylidene fiuoridej-hexafluoropropylene copolymer [PVdF(HFP)] matrix. The gel electrolytes prepared with an ionic liquid PVdF(HFP) mass ratio of 2 1 exhibited ionic conductivities >10 Scm at room temperature, and >10 Scm at 100 °C. When Noda and Watanabe [167] investigated the in situ polymerization of vinyl monomers in the RTILs, they produced suitable vinyl monomers that provided transparent, mechanically strong and highly conductive polymer electrolyte films. As an example, a 2-hydroxyethyl methacrylate network polymer in which BPBF4 was dissolved exhibited an ionic conductivity of 10 S cm at 30 °C. 

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