Ceramic polymer electrolytes conductive fillers

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. 

Pitawala et al., (2007) studied the combined effect of both plasticizer and nano-ceramic filler on the thermal behavior and conductivity of (PEO)g-lithium trifiuoromethanesulphonate (LiCFjSOj or LiTf) composite polymer electrolyte. The formula (PE0)gLiCFjS03 denotes the chemical composition of the polymer-salt complex in which 9 is the molar ratio of (ethylene oxide (EOl/LiCF SOj). According to their work, addition of 15 wt% Aip lowered the and T of (PEO)gLiTf from 58 °C and °Cto 51 °C and -50 °C, respectively and the con-... 

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. 

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). 

Li et al (2001, 2003) prepared a kind of hyperbranched solid polymer electrolyte with a low ion conductivity of about 10 S/cm at room temperature. Later, they added 10 wt% BaTiOs as ceramic fillers into this SPE, making the ion conductivity rise to 1.4 x 10 S/cm. 

Features of ceramic particles such as the type, morphology, particle size, particle content in the CPE and so on are important in improving its ion conductivity. In general, ceramic fillers used for the polymer electrolyte matrix are classified into active and passive species. The active one participates in the ion conduction process, e.g. li2N and LiAl203,while the inactive ones, such as AI2O3, Si02, MgO, are not involved in the lithium ion transport process. The selection of filler between active and passive components is quite arbitrary. 

An alternative approach to conductivity enhancement by crystallinity supression is by the incorporation of inert fillers such as ceramic composites [77]. Another class of materials in which both polymer and organic materials are present are the so-called Ormocers [78] or Ormolytes [79]. These are produced by a sol-gel process in which amino-alkylsilanes are hydrolysed and condensed, and triflic acid (for proton electrolytes) or lithium perchlorate complexed with ethylene glycol diglycidyl ether (for a Li electrolyte) is incorporated. 

The last but not the least approach is addition of ceramic fillers. There is a variety of ceramic materials that offer relatively high ionic conductivities at room temperature (Goodenough et al. 1976 Hong 1976 Hooper 1977 Kafalas and Hong 1978 Sebastian and Gopalakrishnan 2003), however, applying them directly as electrolytes in commercial cells for mobile applications is not possible because they are brittle, difficult to process, and they cannot provide contact with the entire surface of porous electrodes made of powders as easily as polymers and liquids can do. Thus the first approach was using the polymeric electrolytes as conductive binders for the ceramic ionic conductors added to the polymer in a form of powder. Then inert materials were used. This will be described in detail in the next section. 

Evidently, both SPEs and GPEs are not yet perfect, since, for example, the former suffers from low ion conductivity and the latter is impaired by its liquid components. However, recent studies have revealed that the addition of ceramic fillers into the SPE can improve the ion conductivity of polymer hosts and their interfacial properties in contact with the lithium electrode. The rise in the ion conductivity is explained by means of enhancing the amorphous phase of the polymer or hindering recrystaUization. In all the cases the particle size and characteristics of the ceramic fillers are the key factors to improve the electrochemical properties of the electrolytes. In a pioneering work,Weston and Steele (1982) first demonstrated the effectiveness of incorporating inert filler (a-alumina) into the PEG system. The mechanical strength and ion conductivity are significantly enhanced upon... 

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