PEO-LiX membranes

The PEO-LiX membranes are typically prepared by casting an acetonitrile solution of the two components or by directly hot-pressing their intimate mixture. The polar oxygen atoms in the sequential oxyethylene groups of the PEO chains coordinate the Li+ cations, separating them from their X counteranions. Accordingly, the structure of the PEO-LiX complexes may be broadly discussed as a sequence of polymer chains coiled around the lithium ions while the anions are more loosely coordinated [14]. A pictorial model of this structural sequence is shown in Figure 7.1. 

Another important feature of the composite membranes is that their improved mechanical properties are not affected by a decrease in conductivity, provided that the particle size of the filler is maintained sufficiently small (i.e. of the order of 1 pm). Figure 6.19 compares the conductivity of composite PEO-LiX membranes with that of a pure PEO-LiX one [39]. In the temperature range above the transition, and in particular around 100°C,... 

PEO-LiX, membrane Solvent-free, fully solid configuration low cost Temperature-dependent conductivity low lithium ion transfer number and limited oxidation stability E ... 

Figure 6.1 Schematic illustration of the casting fabrication procedure of a polymer ionic membrane formed by PEO-LiX.

Figure 6.3 Photograph of the LPB components. From the rear the lithium foil anode, the PEO-LiX electrolytic membrane and the IC, PEO-LiX, C composite cathode.

The active cathodic material is an intercalation compound, namely a compound with an open structure which allows reversible insertion-deinsertion of lithium ions from-to the electrolyte medium. This makes intercalation compounds very suitable electrodes for rechargeable lithium batteries [10] and indeed they are currently used for this purpose in liquid electrolyte sytems [11]. In the case of the LPBs of interest here, the electrochemical discharge process may be described as the dissolution of lithium at the anode, the migration of the Li" ions across the PEO-LiX electrolytic membrane and its insertion within the structure of the hosting intercalation compound IC ... 

Figure 6.17 Conductivity trends of various types of polymer ionic membranes, such as the PEO-LiX complex (first generation), the MEEP-LiX complex and the PUE-LiX complex (second generation), and the PAN-PC-EC-LiX system (third generation).

Therefore, the identification of the most suitable electrolyte media, where the passivation rates remain confined to a limited extent, is essential for the development of long-life LPBs. The answer may again be provided by replacing the simple PEO-LiX system with the composite membranes of the type of those previously described. By finely dispersing in the polymer structure ceramic additives which have high affinity for the electrolyte impurities, and in particular for water impurities, a network of trapping centres for the impurities themselves can be provided and thus ensure their removal from the interface with the final result of controlling the corrosion rate of the electrode. 

The PEO-LiX polymer membranes, due to their fully solid nature, are stable in contact with lithium metal that can then be cycled several hundred times without dendrite formation. In addition, the PEO-LiX electrolytes are of low cost and easily manufactured for the fabrication of plastic-like batteries (see Figure 3.10). 

The composite cathode (e.g. a membrane cast from a slurry mixture formed by the active cathodic IC intercalation compounds, acetylene black and the PEO and LiX electrolyte components) is laminated onto a metal (e.g. nickel or aluminium) foil current collector (positive terminal) ... 

Systems based on the simple combination of high molecular weight polymer hosts (e.g. poly(ethylene oxide), PEO) and lithium salts (LiX), which may be classified as first generation polymer ionic membranes ... 

Historically, the first type of ionicaUy conducting membranes to be considered for battery appKcations were those formed by blending high molecular weight poly(ethylene oxide) PEO with a lithium salt, LiX where X is preferably a large soft anion. These solvent-free membranes... 

For this reason, battery manufacturers are reluctant to fabricate batteries based on lithium metal. The only way to safely use lithium as an electrode is by coupling it with a stable electrolyte. The most common examples are sol-vent-free, polymer membranes formed by the combination of a poly(ethylene oxide) (PEO) matrix and a lithium salt, LiX [5,6]. The excess of negative charge on the oxygen in the PEO chains coordinates by coulombic attraction of the Li+ ions, thus separating them from the anions. By this process, the lithium salt is dissolved in the PEO matrix, analogous to the process of salt dissolution in liquid solvents [5]. The main difference is that while the ions can move with their solvation shell in liquids, this is not possible in the PEO complexes due to the large size and encumbrance of the chains. Therefore, ion transport in the polymer electrolytes requires flexibility of the PEO chains so... 

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