Gel-based electrolyte

Another reported class of bio-ILs is the group of cholinium-based ILs (choline, an essential nutrient) with amino acids as the anion [16, 17], Reported applications of such ILs have focused on their activities for the pretreatment of lignocellulosic biomass and as catalysts [18, 19]. Recently gel-based electrolytes, choline chloride-based eutectic solvents, were also used as electrolyte for the fabrication of environmentally friendly supercapacitors on paper [20]. 

Silicone gel-based electrolyte is probably the most commonly used electrolyte in lead-acid batteries. Silicone gel is prepared using either a reaction between fumed silica dioxide and a base (sodium hydroxide, potassium hydroxide, etc.) or hydrolysis of silica dioxide. 

Solid polymer and gel polymer electrolytes could be viewed as the special variation of the solution-type electrolyte. In the former, the solvents are polar macromolecules that dissolve salts, while, in the latter, only a small portion of high polymer is employed as the mechanical matrix, which is either soaked with or swollen by essentially the same liquid electrolytes. One exception exists molten salt (ionic liquid) electrolytes where no solvent is present and the dissociation of opposite ions is solely achieved by the thermal disintegration of the salt lattice (melting). Polymer electrolyte will be reviewed in section 8 ( Novel Electrolyte Systems ), although lithium ion technology based on gel polymer electrolytes has in fact entered the market and accounted for 4% of lithium ion cells manufactured in 2000. On the other hand, ionic liquid electrolytes will be omitted, due to both the limited literature concerning this topic and the fact that the application of ionic liquid electrolytes in lithium ion devices remains dubious. Since most of the ionic liquid systems are still in a supercooled state at ambient temperature, it is unlikely that the metastable liquid state could be maintained in an actual electrochemical device, wherein electrode materials would serve as effective nucleation sites for crystallization. 

Gel polymer lithium-ion batteries replace the conventional liquid electrolytes with an advanced polymer electrolyte membrane. These cells can be packed in lightweight plastic packages as they do not have any free electrolytes and they can be fabricated in any desired shape and size. They are now increasingly becoming an alternative to liquid-electrolyte lithium-ion batteries, and several battery manufacturers. such as Sanyo. Sony, and Panasonic have started commercial production.Song et al. have recently reviewed the present state of gel-type polymer electrolyte technology for lithium-ion batteries. They focused on four plasticized systems, which have received particular attention from a practical viewpoint, i.e.. poly(ethylene oxide) (PEO). poly (acrylonitrile) (PAN). ° poly (methyl methacrylate) (PMMA). - and poly(vinylidene fluoride) (PVdF) based electrolytes. ... 

The electrochemical technique can be used also for direct synthesis of bimetallic alkoxides. For instance, the anodic dissolution of rhenium in the methanol-based electrolyte that already contained MoO(OMe)4, permitted to prepare with a good yield (60%) a bimetallic complex RevMov,02(OMe)7, with a single Re-Mo bond [904], Application of the same procedure permitted the preparation of complex alkoxide solutions with controlled composition for sol-gel processing of ferroelectric films [1777]. 

Ion conducting polymers may be preferable in these devices electrolytes because of their flexibility, moldability, easy fabrication and chemical stability (for the same reasons that they have been applied to lithium secondary batteries [19,48,49]). The gel electrolyte systems, which consist of a polymeric matrix, organic solvent (plasticizer) and supporting electrolyte, show high ionic conductivity about 10 5 S cnr1 at ambient temperature and have sufficient mechanical strength [5,7,50,51], Therefore, the gel electrolyte systems are superior to solid polymer electrolytes and organic solvent-based electrolytes as batteries and capacitor materials for ambient temperature operation. 

The gel polymer electrolytes studied were composed of PVdF or poly(vi-nylidene fluoride-hexafluoropropylene) (PVdF-HFP) as base materials with the addition of TEABF4/EC + PC as the plasticizer. 

Stathatos E., Lianos R., Zakeeruddin S. M., Liska P. and Gratzel M. (2003), A qnasi-solid-state dye-sensitized solar cell based on a sol-gel nanocomposite electrolyte containing ionic hqnid , Chem. of Materials 15, 1825-1829. 

Polymer gels based on polymers such as poly(vinylidene fluoride), polyacrylonitrile, and aprotic solvents containing added alkali metal salts, gave appreciable room-temperature conductivity. However, solvent volatility and voltage stability of the electrolyte were serious problems. 

PANI-NFA 2O5 is promising nanocomposite material for utilization as a cathode for ion-Li batteries [292,293]. PANI-NFs have been used as a cathode material for rechargeable Li-polymer cells assembled with a gel polymer electrolyte [152], and in an aqueous PANI-Zn rechargeable battery [261]. Dispersions of dedoped PANI-NFs in poly(vinyhdene fluoride-hexafluoropropylene)-based gel polymers can be used as electrolyte membranes for rechargeable Li batteries [513]. PANI-NF and PANI-NT arrays, which show superior electrochemical properties to the bulk counterpart, can be applied to Li-polymer thin-film batteries, which are shape-flexible and specifically suitable for powering integrated circuit cards and microelectromechanical systems [514,515]. 

Ramesh, S. and K. Wong, Conductivity, dielectric behaviour and thermal stability studies of lithium ion dissociation in poly (methyl methacrylate)-based gel polymer electrolytes. Ionics, 2009.15(2) 249-254. 

SU, A., Sharma, R., Ray, S., 2015. Mechanical and thermal characteristics of PMMA-based nanocomposite gel polymer electrolytes with CNFs dispersion. Surf. Coat. Technol. 271,201-206. 

Kumar, Y, G. P. Pandey, and S. A. Hashmi. 2012. Gel polymer electrolyte based electrical double layer capacitors Comparative study with multiwalled carbon nanotubes and activated carbon electrodes. Journal of Physical Chemistry C 116 26118-26127. 

Sivaraman, P, A. Thakur, R. K. Kushwaha, D. Ratna, and A. B. Samui. 2006. Poly(3-methyl thiophene)-activated carbon hybrid supercapacitor based on gel polymer electrolyte. Electrochemical and Solid-State Letters 9 A435-A438. 

Pandey, G. P, and S. A. Hashmi. 2013. Ionic liquid l-ethyl-3-methylimidazolium tetracyanoborate-based gel polymer electrolyte for electrochemical capacitors. Journal of Materials Chemistry A 1 3372-3378. 

Suleman, M., Y. Kumar, and S. A. Hashmi. 2013. Structural and electrochemical properties of succinonitrile-based gel polymer electrolytes Role of ionic liquid addition. Journal of Physical Chemistry B 117 7436-7443. 

Pandey, G. P., A. C. Rastogi, and C. R. Westgate. 2013. Polyacrylonitrile and 1-ethyl-3-methyliniidazolium thiocyanate based gel polymer electrolyte for solid-state supercapacitors with graphene electrodes. Electrochemical Capacitors 50 145-151. 

Clearly, these gel-type electrolytes have quite promising properties in terms of conductivity, approaching that of liquid solutions. This can be seen in Figure 7.8, which shows the Arrhenius plots of some selected examples, and Figure 7.5 which compares the conductivity of gels with that of PEO-based membranes. 

One may then conclude that, the gel-type electrolytes, and the PAN-based ones in particular, have electrochemical properties that in principle make them suitable for application in versatile, high-energy lithium batteries. In practice, their use may be limited by the reactivity towards the lithium electrodes induced by the high content of the liquid component. Indeed, severe passivation phenomenon occurs when the lithium metal electrode is kept in contact with the gel electrolytes [60, 69]. This confirms the general rule that if from one side the wet-like configuration is essential to confer high conductivity to a given polymer electrolyte, from the other it unavoidably affects its interfacial stability with the lithium metal electrode. 

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