Polymer electrolytes ionic conduction mechanism

For the composite polymer electrolytes, the conductive carbonaceous filler must be below the electrical percolation threshold, due to the need to obtain an electronically insulating material with suitable ionic conductivity. These fillers are also used to improve the thermal stabilization and serve as mechanical reinforcement to improve the electrolyte/ electrode compatibility. CNT/P(VDF-TrFE) composites showed higher porosity and electrolyte uptake compared to the pristine polymer. CNT also contributed to increase ionic conductivity (2.6 xlO S cm , 0.1 wt.% CNT) and diminished its variations with temperature. 

The motion of ions (i.e. conductivity) in polymer electrolytes appears to occur by a liquid-like mechanism in which the movement of ions through the polymer matrix is assisted by the large amplitude segmental motion of the polymer backbone. Ionic conductivity primarily occurs in the amorphous regions of the polymer [4,5]. The temperature dependence of the conductivity of polymer electrolytes is best related by the Vogel-Tamman-Fulcher (VTF) equation... 

Polymer gel is generally formed by incorporating liquid electrolyte into polymer matrix, which hints that the polymer should show excellent adsorption property toward the liquid electrolytes used in LIB. The ionic conduction mechanism in... 

Multivalent polymer electrolytes that use PEO as the polymer base are usually multiphase systems consisting of salt-rich crystalline phases, another crystalline phase of pure polymer, and amorphous phases with dissolved salts. Conductivity is thus often affected by factors such as slow crystallisation and salt redistribution processes between the phases, which result in values dependent on the thermal history, preparation methods, etc. The morphological and crystallographic structures of these polymeric systems are presented, and the influence of factors such as crystallisation of the pure polymer on ionic conduction, and the factors that influence them in turn, are highlighted. Discussion of conduction takes into account the two possible mechanisms of mobility of charged ionic clusters and exchange of... 

Noda and Watanabe [42] reported a simple synthetic procedure for the free radical polymerization of vinyl monomers to give conducting polymer electrolyte films. Direct polymerization in the ionic liquid gives transparent, mechanically strong and highly conductive polymer electrolyte films. This was the first time that ambient-temperature ionic liquids had been used as a medium for free radical polymerization of vinyl monomers. The ionic liquids [EMIM][BF4] and [BP][Bp4] (BP is N-butylpyridinium) were used with equimolar amounts of suitable monomers, and polymerization was initiated by prolonged heating (12 hours at 80 °C) with benzoyl... 

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... 

A method of characterising transport mechanisms in solid ionic conductors has been proposed which involves a comparison of a structural relaxation time, t, and a conductivity relaxation time, t . This differentiates between the amorphous glass electrolyte and the amorphous polymer electrolyte, the latter being a very poor conductor below the 7. A decoupling index has been defined where... 

Fig. 6.7 Average shear (mechanical) and conductivity (electric modules) relaxation times, T, and for ionically conductive materials, (a) For glassy materials r, r, above 7 and t T, below Tf. (b) For polymer electrolytes ((PP0),3 NaCF3S03) shown), x, > t, for T> T..

A polymer electrolyte with acceptable conductivity, mechanical properties and electrochemical stability has yet to be developed and commercialized on a large scale. The main issues which are still to be resolved for a completely successful operation of these materials are the reactivity of their interface with the lithium metal electrode and the decay of their conductivity at temperatures below 70 °C. Croce et al. found an effective approach for reaching both of these goals by dispersing low particle size ceramic powders in the polymer electrolyte bulk. They claimed that this new nanocomposite polymer electrolytes had a very stable lithium electrode interface and an enhanced ionic conductivity at low temperature. combined with good mechanical properties. Fan et al. has also developed a new type of composite electrolyte by dispersing fumed silica into low to moderate molecular weight PEO. 

The lithium polymer battery (LPB), shown schematically in Fig. 7.21, is an all-solid-state system which in its most common form combines a lithium ion conducting polymer separator with two lithium-reversible electrodes. The key component of these LPBs is the polymer electrolyte and extensive work has been devoted to its development. A polymer electrolyte should have (1) a high ionic conductivity (2) a lithium ion transport number approaching unity (to avoid concentration polarization) (3) negligible electronic conductivity (4) high chemical and electrochemical stability with respect to the electrode materials (5) good mechanical stability (6) low cost and (7) a benign chemical composition. 

Ionically conducting polymers and their relevance to lithium batteries were mentioned in a previous section. However, there are several developments which contain both ionically conducting materials and other supporting agents which improve both the bulk conductivity of these materials and the properties of the anode (Li)/electrolyte interface in terms of resistivity, passivity, reversibility, and corrosion protection. A typical example is a composite electrolyte system comprised of polyethylene oxide, lithium salt, and A1203 particles dispersed in the polymeric matrices, as demonstrated by Peled et al. [182], By adding alumina particles, a new conduction mechanism is available, which involved surface conductivity of ions on and among the particles. This enhances considerably the overall conductivity of the composite electrolyte system. There are also a number of other reports that demonstrate the potential of these solid electrolyte systems [183],... 

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. 

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. 

Conductivity within conducting electroactive polymers (CEPs) is a complex issue. A polymer that can exhibit conductivity across a range of some 15 orders of magnitude most likely utilizes different mechanisms under different conditions. In addition to the electronic conductivity exhibited by CEPs, they possess ionic conductivity because of the solvent or electrolyte incorporated during synthesis. The experimental parameters encountered during synthesis (as listed and discussed in Chapter 2) have an effect on the polymer conductivity. In particular, the electrochemical conditions, the solvent, the counterion, and monomers used during synthesis influence the electronic properties of the resulting polymer. 

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