Gel electrolyte

Gel electrolytes are attractive alternatives to the dry electrolytes, particularly with 

The solidity of gel electrolytes results from chain entanglements. At high temperatures they flow like liquids, but on cooling they show a small increase in the shear modulus at temperatures well above 

Addition of a plasticizing solvent to a polymer-salt system modifies the electrolyte by lowering the through an isothermal increase in the system s configurational entropy and this consequently increases the mobility of all particles. A suitable choice of organic solvent can lead to 

The conductivity of gelled electrolytes is determined primarily by the liquid and salt components. High liquid content, of the order of 40 percent, is required to attain conductivities comparable with those of the corresponding liquid electrolyte. At these liquid loading levels there is often insufficient mechanical strength, and although this effect may not be noticeable on 1-2 cm laboratory cells, it is certainly evident on scale-up [111]. Polymer blends such as PEO-MEEP are much more mechanically stable than MEEP itself and more conductive than PEO but there is little overall improvement of the room tern- 

NBR/SBR matrix containing 1 mol L LiC104 in /-butyrolactone exhibits a conductivity of -10 S cm .  

Gel electrolytes are attractive alternatives to the dry electrolytes, particularly with respect to higher, more practical, ionic conductivities. Two distinct methods can be used to achieve macroscopic immobilization of the Hquid solvent increase the viscosity of the liquid electrolyte by adding a soluble polymer, for example, PEO, poly(methyl methacrylate) (PMMA), polyacrylonitrile (PAN), poly(vinylidene fluoride) (PVdF), and so on. [32, 34] or load the liquid electrolyte into a microscopous matrix, for example, porous polyethylene [102,103]. 

The solidity of gel electrolytes results from chain entanglements. At high temperatures they flow like liquids, but on cooHng they show a small increase in the shear modulus at temperatures well above Tg. This is the liquid-to-mbber transition. The values of shear modulus and viscosity for mbbery soHds are considerably lower than those for glass-forming Hquids at an equivalent structural relaxation time. The local or microscopic viscosity relaxation time of the rubbery material, which is reflected in the Tg, obeys a VTF equation with a pre-exponential factor equivalent to that for smaU-molecule liquids. Above the hquid-to-mbber transition, the VTF equation is also obeyed, but the pre-exponential term for viscosity is much larger than is typical for small-molecule liquids and is dependent on the polymer molecular weight. 

PAN-based systems have been the focus of much recent interest. Although the polymer is assumed to be nonsolvating, NMR studies [107] suggest that there may be some competition (albeit small) for solvation between the polymer and plasticizing 

The majority of electrochemical cells to have been constructed are based on PEO, PAN, or PVdP [101]. Recently, the Yuasa Corporation has commercialized solid polymer electrolyte batteries, primarily for use in devices such as smart cards, ID cards, and so on. To date, the batteries which have been manufactured and marketed are primary lithium batteries based on a plasticized polymer electrolyte, but a similar secondary battery is expected [120]. 

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The interfacial properties of gel electrolytes containing ethylene carbonate immobilized in a polyacrylonitrile (PAN) matrix with a lithium (bis)trifluoromethane sulfonimide (LiTFSI) salt have been studied 1139]. SEI stability appeared to be strongly dependent on the LiTFSI concentration. A minimum value of / SE1 of about 1000 Qcm2 was obtained after 200h... 

A gel electrolyte, formed by dissolving a salt in a polar liquid and adding an inactive polymeric material to give the material mechanical stability. 

The preparation and properties of a novel, commercially viable Li-ion battery based on a gel electrolyte has recently been disclosed by Bellcore (USA) [124]. The technology has, to date, been licensed to six companies and full commercial production is imminent. The polymer membrane is a copolymer based on PVdF copolymerized with hexafluoropropylene (HFP). HFP helps to decrease the crystallinity of the PVdF component, enhancing its ability to absorb liquid. Optimizing the liquid absorption ability, mechanical strength, and processability requires optimized amorphous/crystalline-phase distribution. The PVdF-HFP membrane can absorb plasticizer up to 200 percent of its original volume, especially when a pore former (fumed silica) is added. The liquid electrolyte is typically a solution of LiPF6 in 2 1 ethylene carbonate dimethyl car-... 

Battery makers sometimes view separators with disdain the separator is needed but does not actively contribute to battery operation. Consequently, very little work (relative to that on electrode materials and electrolytes) is directed towards characterizing separators. In fact, development efforts are under way to displace microporous membranes as battery separators and instead to use gel electrolytes or polymer electrolytes. Polymer electrolytes, in particular, promise enhanced safety by elimi-... 

Doyle et al. [40] used a mathematical model to examine the effect of separator thickness for the PVDF.HFP gel electrolyte system and found that decreasing separator thickness below 52 pm caused only a minor decrease in ohmic drop across the cell. The voltage drops in the electrodes were much more significant. They state that their model predictions were confirmed experimentally. 

Gel electrolytes were also prepared by Allcock [605] from co-substituted polyphosphazenes with various ratios of methoxyethoxyethoxy and trifluo-roethoxy side groups, lithium triflate and propylene carbonate. These gel electrolyte systems have a better mechanical stability than MEEP. The highest ionic conductivity obtained was 7.7x10" S cm" at 25 °C for a gel containing 37.5% of polymer with 80% and 20% of methoxyethoxyethoxy and trifluoro ethoxy... 

The mechanical properties of X depend partly on the length of the oli-goethyleneoxy groups. The polymers are solid for x=l, highly viscous gums for x=3 and elastomers for x=7.2. The maximum conductivity of these complexes at 30 °C are reported in Table 16. Gel electrolytes have also been obtained by adding propylene carbonate (PC) (10-50 wt%) to these polynorbornene de-... 

In any case, it is perceived from the above discussion that the problem of longterm chemical stability of polycrystalline semiconductor liquid junction solar cells is far from being solved. Still, as already pointed out in the early research, any practical photovoltaic and PEC device would have to be based on polycrystalline photoelectrodes. Novel approaches mostly involving specially designed PEC systems with alternative solid or gel electrolytes and, most importantly, hybrid/sensitized electrodes with properties dictated by nanophase structuring - to be discussed at the end of this chapter - promise new advances in the field. 

Figure 8.1 Beer s law-type plot of change in optical absorbance against charge density q for the cell WO3 polymer electrolyte Prussian Blue. Reprinted from Inaba, H Iwaka, M., Nakase, K., Yasukawa, H., Seo, I. and Oyama, N., Electrochromic display device of tungsten trioxide and Prussian Blue films using polymer gel electrolyte of methacrylate , Electrochim. Acta, 40, 227-232 (1995), Copyright 1995, with permission from Elsevier Science.

Lithium secondary batteries can be classified into three types, a liquid type battery using liquid electrolytes, a gel type battery using gel electrolytes mixed with polymer and liquid, and a solid type battery using polymer electrolytes. The types of separators used in different types of secondary lithium batteries are shown in Table 1. The liquid lithium-ion cell uses microporous polyolefin separators while the gel polymer lithium-ion cells either use a PVdF separator (e.g. PLION cells) or PVdF coated microporous polyolefin separators. The PLION cells use PVdF loaded with silica and plasticizer as separator. The microporous structure is formed by removing the plasticizer and then filling with liquid electrolyte. They are also characterized as plasticized electrolyte. In solid polymer lithium-ion cells, the solid electrolyte acts as both electrolyte and separator. 

The solid polymer electrolyte approach provides enhanced safety, but the poor ambient temperature conductivity excludes their use for battery applications. which require good ambient temperature performance. In contrast, the liquid lithium-ion technology provides better performance over a wider temperature range, but electrolyte leakage remains a constant risk. Midway between the solid polymer electrolyte and the liquid electrolyte is the hybrid polymer electrolyte concept leading to the so-called gel polymer lithium-ion batteries. Gel electrolyte is a two-component system, viz., a polymer matrix... 

To overcome the poor mechanical properties of polymer and gel polymer type electrolytes, microporous membranes impregnated with gel polymer electrolytes, such as PVdF. PVdF—HFP. and other gelling agents, have been developed as an electrolyte material for lithium batteries.Gel coated and/ or gel-filled separators have some characteristics that may be harder to achieve in the separator-free gel electrolytes. For example, they can offer much better protection against internal shorts when compared to gel electrolytes and can therefore help in reducing the overall thickness of the electrolyte layer. In addition the ability of some separators to shutdown... 

Kato T, Okazaki A, Hayase S (2005) Latent gel electrolyte precursors for quasi-solid dye sensitized solar cell. Chem Commun 363-364... 

Wang P, Zakeeruddin SM, Moser JE, Nazeeruddin MK, Sekiguchi T, Gratzel M (2003) A stable-quasi-solid state dye-sensitized solar cell with amphiphilic ruthenium sensitizer and polymer gel electrolyte. Nature Mater 2 402-406... 

The same authors [340] have proposed another polymer gel electrolyte for Zn-Mn02 cells. 

IONIC LIQUIDS AND GEL ELECTROLYTES BASED ON THE I /l3 REDOX COUPLE... 

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