Ceramic polymer electrolytes properties

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. 

Another approach extensively apphed in recerrt years to improve the ion conductivity ((, lithiirm ion transference number (C), mechanical properties, and the electrode-electrolyte interfacial stability of a polymer electrolyte is the addition of inorganic or ceramic fillers into the polymer-salt complexes (Capiglia et al., 1999 Kim et al., 2003 Chen-Yang et al., 2008 Croce et al., 2001 Rahman et al., 2009 Shen et al., 2009 Zhang et al., 2011 Munichandratah et al., 1995 Wiec-zorek, 1992). Micro and nano-sized inorganic filler such as silicone oxide (SiO ), alumina (AI2O3), ceria (CeO ), and so on are incorporated into PEO-salt complex in an effort to improve the mechanical, thermal stabihty, and ion conductivity of PEO-based polymer electrolytes. The effect of nano-fillers on the thermal properties of the PEO-based polymer complex varies with the type of nano-particles as well as the polymer-salt complex host matrix. 

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. 

The ceramic fillers (e.g., AI2O3, SiOa, TiOa) can greatly influence the characteristics and properties of polymer electrolyte by enhancing the mechanical stability and the conductivity [135, 175-178]. Prosini et al. [179] in a PVdF-HFP polymer matrix used y-LiAlOa, AI2O3, and MgO as fillers to form self-standing, intrinsically porous separators for lithium-ion batteries. The MgO-based separators showed the best anode and cathode compatibilities. 

While a battery separator s materials are usually inert and do not influence electrical energy storage or output, its properties can have an important influence on safety. There are three types commonly used [46] (i) high-temperature sohd-polymer electrolytes (SPEs) such as poly(ethylene oxide) (PEO), (ii) microporous shutdown separators, which are composed of poly(ethylene) (PE) or laminates of poly(propylene) (PP) and PE, (iii) gel polymers such as poly(vinyhdene fluoride), PVdF, and (iv) ceramic separators. Table 27.2 shows the types of separators used in secondary Hthium-based batteries. 

The general concept of adding ceramic powders to PEO-LiX polymer electrolytes dates back to the early 1980s when this procedure was successfully employed to improve their mechanical properties, their interface with the lithium electrode and their ionic conductiv-ity [66,67] it is only recently that the role of the dispersed... 

On the basis of the model described above, one would expect that the enhancement of the transport properties should depend upon the degree of acidity of the ceramic s surface states. This is indeed the case as demonstrated by the behaviour of PEO-based polymer electrolytes using ceramic fillers with a high surface acidity, e.g. the sulfate-promoted superacid zirconia, S-ZrOi. The results show that this ceramic filler considerably enhances the transport properties of the electrolyte. 

Some extended works have been devoted to studies on the influence of highly acidic fillers (both soluble such as AlBrs or AlCh and ceramic ones) on the properties of gel polymer electrolytes based on PVdF. It was found that the conductivity and cation transference numbers increase on the addition of these strong Lewis acids. Also the stability of the interface with lithium metal is enhanced however, in the case of aluminium halides the reactions with organic carbonates can take place and affect the properties of such systems on prolonged storage (Stolarska et al. 2007 Walkowiak et al. 2006,2007). 

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