**2.1 Random copolymers**

In the first study of poly(ethylene oxide) (PEO) with various inorganic lithium salts, PEO showed low conductivity at ambient temperature due to the higher degree of crystallization. To overcome this obstacle, reduced crystallinity or amorphous polyether–based host architectures have been focused. Random copolymerization is one of routes to produce amorphous host polymers. Booth and co–workers had successfully synthesized random oxyethylene–oxymethylene polymer structures in year 1990. Ethylene oxide monomers are randomly interspersed with methylene oxide groups. Thus, the methylene oxide would

Characterization of High Molecular Weight Poly(vinyl chloride) –

**2.4 Mixed salt system** 

**2.5 Mixed solvent system** 

**2.6 Addition of inorganic fillers** 

(Osinska et al., 2009).

**2.7 Plasticization** 

Lithium Tetraborate Electrolyte Plasticized by Propylene Carbonate 169

The conductivity of the mixed salts in polymer electrolyte is higher than single salt electrolyte. It is due to the addition of second salt may prevent the formation of aggregates and clusters. Therefore, it increases the mobility of ion carriers (Gray, 1997b). An approach had been done by Ramesh and Arof (2000). In this research, we synthesized poly (vinyl chloride) (PVC)–based polymer electrolytes with lithium trifluoromethanesulfonate (LiTf) and lithium tetrafluoroborate (LiBF4) as doping salts. The ionic conductivity is increased by four orders of magnitude in comparison with single salt system. It is attributed to the

On the other hand, the increase of conductivity in binary solvent system is proven by Deepa et al. (2002). In this study, poly(methyl methacrylate) (PMMA)–based polymer electrolytes containing lithium perchlorate (LiClO4), with a mixture of solvents of propylene carbonate (PC) and ethylene carbonate (EC) were prepared. The maximum ionic conductivity of 10-3 S cm-1 was obtained and it was increased by two orders of magnitude as compared to polymer electrolyte system with single solvent. Synergistic effect is the major factor to increase the ionic conductivity in this mixed solvent system. Different physicochemical properties of the individual solvents come into play and contribute to high ionic conductivity in the presence of the effect. The preparation on EC/PC/2–methyl–tetrahydrofuran (2MeTHF) ternary mixed solvent electrolyte had been done by Tobishima and co–workers. They found out that the discharge capacity of Li/amorphous V2O5–P2O5 cells with a ternary mixed solvent electrolyte are slightly better than for cells with EC/PC binary mixed solvent electrolytes.

Utilization of common additives such as inorganic fillers and plasticizers is the effective and efficient approach to enhance the ionic conductivity. Fillers (also known as reinforcing fillers) are divided into two types: inorganic and organic. Variety types of inorganic fillers have been used, including mica, clay, titania (TiO2), fumed silica (SiO2) and alumina (Al2O3). On the other hand, graphite fibre and aromatic polyamide are some examples of organic fillers. The main objectives of dispersion of inorganic filler are to alter the properties of the polymer, enhance processability and improve the mechanical stability in the polymer electrolyte system. Dispersion of inorganic fillers can also improve the ionic conductivity in the polymer electrolyte. Besides improving the lithium transport properties, the inclusion of ceramic filler has been found to enhance the interfacial stability of polymer electrolytes

A number of attempts have been made on plasticized–polymer electrolytes in order to rise up the ionic conductivity greatly. Plasticization is generally recognized as one of the effective and efficient methods available for decreasing the crystalline region of polymer electrolytes (Suthanthiraraj et al., 2009). Plasticizer is a non–volatile and low molecular weight aprotic organic solvent which has a �� in the vicinity of –50 °C. Carbonate ester such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC) and

increase in the mobility of charge carriers by avoiding the aggregation process.

break up the regular helical structure of PEO and suppress the crystalline region of polymer matrix. A similar random copolymer had been synthesized by replacing the methylene oxide with dimethyl siloxy units. This resultant polymer matrix illustrated higher flexibility which assists in ionic conduction (Gray, 1997b).
