Polymer electrolytes plasticised

Therefore, the ideal solution in this field would he the use of solid plasticisers , namely of solid additives which would promote amorphicity at ambient temperature without affecting the mechanical and the interfacial properties of the electrolyte. A result that approaches this ideal condition has been obtained by dispersing selected ceramic powders, such as Ti02, AI2O3 and Si02, at the nanoscale particle size, in the PEO-LiX matrix [35-41]. The conductivity behaviour of a selected example of these nanocomposite polymer electrolytes is shown in Figure 7.5. 

An alternative method of achieving high ionic conductivity whilst retaining the useful properties of polymer electrolytes in device applications such as lithium batteries, fuel cells, actuators and dye sensitised solar cells, is to use ionic liquids (ILs) either as the main conductivity medium supported in a polymer membrane or as a plasticising component in polymer electrolytes. In this section we provide some background of ILs and their properties in the context of device applications followed by their use in polymer electrolyte systems. 

It is found that simple salts, such as LiCl, do not provide high conductivity. In general, the bulkier the anion, the higher is the conductivity. For example, Armand (1994) synthesised lithium bis(trifluoromethylsulphonyl)imide, LiN(CF3S02)2, and observed high conductivity when dissolved in PEO-type polymer electrolytes. The high conductivity is the result of a more plasticised polymer. 

The third generation of polymer electrolytes is variously described as containing a plasticiser, as dilute solvent or as gelled (Andrieu et al, 1995 Benrabah et al, 1995). Each of these terms may have had precise meaning at one time or another, but recently the terms seem to be used indiscriminately. 

Polymer-based ion conducting materials have been of great interest to researchers in the field of lithium batteries since Armand et al proposed the use of poly(ethylene oxide) (PEO)-Li salts as a solid polymer electrolyte (SPE). In this application, the polymer electrolyte functions as a mechanical separator between the two electrodes and also as the ionic conductor. Polymer electrolytes are used in the form of thin films and may be either dry (organic solvent-free) or plasticised. A high specific energy density can be reached at medium temperature using a dry polymer electrolyte and lithium metal as the negative electrode. 

Many approaches have been used to improve the conductivity of polymer electrolytes. Studies have focused on the development of copolymers " and cross-linked polymers""" to create completely amorphous electrolytes with enhanced conductivity at room temperature. One method involves the addition of various plasticisers, such as ethylene carbonate (EC), propylene carbonate (PC) and tetraethylene glycol (TEG), which have been added to... 

Usually, both crystalline and amorphous phases are present in polymer electrolytes. Since only amorphous phases present high conductivity, plasticiser solvents are usually added to enhance the amorphous phase and thus the ionic conductivity. Here, propylene carbonate (PC), EC,y-butyrolactone (y-BL) and their binary mixtures are usually used as plasticiser solvents. Hie stereo structures of the polymer matrix molecules are given in Fig. 12.3, including linear, comb, crossing, star, hyper-branched and comb crosslinking. 

The star network polymer was dissolved adequately in acetonitrile, then LiC104 and plasticisers of EC/PC (mass ratio = 1 1) were added to the polymeric solution and stirred vigorously at 40 °C. The acetonitrile was driven out finally under vacuum at 60°C.The resulting polymer electrolytes were stored in a nitrogen-filled drybox. 

In the example above, a short-chain poly(ethylene glycol) was added to a rigid polyelectrolyte to plasticise the material and thereby increase polymer-solvent motion in the vicinity of mobile ions. This strategy has been widely explored as a means of improving ion transport in electrolytes. 

Most ISEs are based on purely physicochemical and non-catalytic recognition elements solid membranes with fixed ionic sites (e.g. the glass pH electrode), ion-exchange polymer membranes or plasticised hydrogel membranes incorporating ionophores [9], Silicon oxide or metal oxides act as the recognition element in pH-ISFETs, gas-sensitive FETs, solid-state electrolyte, solid-state semiconductor and many conductometric gas sensors. 

Anastasova et al. [49] have described solid contact reference electrode based on conducting polymer and traditional plasticised PVC membrane doped with lipophilic electrolyte. Here, Au/PEDOT, TBA-TBB, o-NPOE and PVC are parts of a pen-like multisensor probe (diameter 13 mm with seven integrated electrodes each of which has a diameter of 1 mm). The response time is of ca. 2 min due to kinetic hindrance of the surface ion-exchange process the electrodes showed good long-term stability and no significant deviation in response after 3 months. 

Ion conducting polymers may be preferable in these devices electrolytes because of their flexibility, malleability, easy manufacture and chemical stability (for the same reasons that they have been applied to lithium secondary batteries). The gel electrolyte systems, which consist of a polymeric matrix, organic solvent (plasticiser) and supporting electrolyte, show high ionic conductivity of about 10 Scm at room temperature and have sufficient mechanical strength (Reiche et al, 1995). Therefore, the gel electrolyte systems are superior to SPEs and organic solvent-based electrolytes as materials for batteries and capacitors, for room temperature operation (Choudhury et al, 2009 Kalpana et al, 2006 Kumar and Bhat, 2009 Zhang et al, 2009). 

Methyl methacrylate (MMA) (7 g, M 120000) was dissolved in dry acetonitrile (70 g). LiC104 (3 g) was added to the solution as supporting electrolyte. Finally, PC (20 g) was added as plasticiser. The whole mixture was then slowly heated until gelation. The GPE was spread on the polymer-coated side of the electrode, and the electrodes were sandwiched under atmospheric conditions. 

The GPE was composed of a PEG-based acrylate macromonomer, photoinitiator, plasticiser, glass beads and the electrolyte to compensate for the charge injected into or extracted from the conducting polymer. The ionic conductivity of lithium-PEO has been reported to be at a maximum for a Li/O ratio of 0.04 and hence the composition of the GPE with no plasticiser was prepared with this ratio. The different compositions of the GPEs used in this study are shown in Table 12.1. 

The properties of the electrolytes can be modified by changes in the composition of the polymer matrix and accordingly the plasticiser used and fraction of H3PO4. Utilisation of GMA-MMA copolymers leads to an increase in the conductivity, up to 10 S cm at RT, compared with that of pure PMMA-based systems as well as to an improvement in the mechanical properties and adhesion to the glass substrate. 

The properties of the GPE are affected by the polymer matrix content, type and concentration of the acidic component, i.e. AMPS or EGMP as well as the type of solvent used as plasticiser. The highest conductivities reached 1.5 and 0.67 x 10 Scm respectively at room temperature for AMPSA-and EGMP-based electrolytes. All EGMP-based systems had lower conductivities than AMPS-based systems. The use of PC-DMF, EC-DMF or PC-EC-DMF solvent mixtures, on the other hand, improves adhesion to a glass substrate and resistance to the influence of traces of water. It is found that the poly(AMPSA-co-MMA)-PC-DMF (30 wt% polymer) and poly(AMPS-co-HEMA)-PC-EC-DMF (20 wt% polymer) systems are the most promising for ECD applications. 

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