**1. Introduction**

164 Recent Advances in Plasticizers

[42] Moraru C. I., Lee T.-C., Karwe M. V., Kokini J. L., Plasticizing and Antiplasticizing

Science 2002, Vol. 67, Nr. 9, p. 3396-3401.

Effects of Water and Polyols on a Meat-Starch Extruded Matrix, Journal of Food

An electrolyte is a substance consisting of free ions and acts as a medium channel for transferring the charges between a pair of electrodes. Sometimes, they are also referred as "lytes" which is derived from the Greek word, "lytos", which means "it may be dissolved". Electrolyte is comprised of positively charged species which is called as cation and negatively charged species, anion. The properties of an electrolyte can be exploited via electrolysis process: separation of chemically bonded element or compounds by applying the electrical current. In the early stage, liquid electrolytes have been discovered and investigated. Liquid electrolyte is a substance that conducts the electricity in an aqueous solution by migrating both cations and anions to the opposite electrodes through an electrically conducting path as a useful electric current. However, it faces problem of leakage of hazardous liquids or gases. Other drawbacks are formation of lithium dendrite, electrolytic degradation of electrolyte and uses of ammable organic solvent (Ramesh et al., 2011a). Apart from that, it exhibits poor long–term stability due to the evaporation of the liquid phase in the cells (Yang et al., 2008). Therefore, solid polymer electrolytes (SPEs) were synthesized to prevail over the limitations of conventional liquid electrolytes.

#### **1.1 Solid polymer electrolytes**

The development on SPEs was initiated by the pioneering work of Wright et al. three decades ago (Lee and Wright, 1982). A polymer electrolyte (PE) is defined as a solvent–free system whereby the ionically conducting pathway is generated by dissolving the low lattice energy metal salts in a high molecular weight polar polymer matrix with aprotic solvent (Gray, 1997a). The fundamental of ionic conduction in the polymer electrolytes is the covalent bonding between the polymer backbones and ions. Initially, the electron donor group in the polymer would form solvation onto the cation component in the doping salt

Characterization of High Molecular Weight Poly(vinyl chloride) –

**1.4 Gel polymer electrolytes (GPEs)** 

crystalline or glass electrolytes (Gray, 1997a).

plasticizers.

**2.1 Random copolymers** 

**2. Methods to improve ionic conductivity** 

Lithium Tetraborate Electrolyte Plasticized by Propylene Carbonate 167

comparing with conventional liquid electrolyte (Adebahr et al., 2003; Nicotera et al., 2006; Stephan, 2006). Moreover, they produce miniaturized structures via fabrication methods and

SPEs possess high mechanical integrity; however they exhibit low ionic conductivity. Therefore, gel polymer electrolytes (GPEs), sometimes known as gelionic solid polymer electrolytes are yet to be developed to substitute SPEs because of its inherent characteristics (Stephan et al., 2000a). Such features are low interfacial resistance, decrease in reactivity, improved safety and exhibit better shape flexibility as well as significant increases in ionic conductivity with a small portion of plasticizers (Ahmad et al., 2008; Pandey and Hashmi, 2009). GPE is obtained by dissolving the host polymer along with a metal dopant salt in a polar organic solvent (more commonly known as plasticizer) (Osinska et al., 2009; Rajendran et al., 2008a). In other words, it is an immobilization of a liquid electrolyte in a polymer matrix (Han et al., 2002). As a unique characteristic, GPEs possess both cohesive properties of solids and the diffusive property liquids. Even though they are in a solid state, but at atomic level, the local relaxations provide liquid–like degree of freedom which is comparable to those conventional liquid electrolytes. Moreover, GPEs show better mechanical and electrochemical properties within a wide operational temperature range in comparison with that of liquid electrolytes (Ahmad et al., 2005; Stephan et al., 2000a). Other attractive advantages are leak proof construction, lighter, cheaper and easy fabrication into desired shape and size (Zhang et al., 2011). They also maintain the interfacial contacts under stresses such as the changing of volume associated with cell charging and discharging. GPEs could form good interfacial contacts with electrode materials as they are not brittle as solid

Ionic conductivity is the main aspect to be concerned in the solid polymer electrolytes. Ionic conductivity is defined as ionic transportation from one site to another through defects in the crystal lattice of a solid under the influence of an external electric field. Much effort has been devoted for developing highly conducting polymer electrolytes. Several ways have been done to modulate the ionic conductivity of polymer electrolytes, for instance, random and comb–like copolymer of two polymers, polymer blending, mixed salt and mixed solvent systems as well as impregnation of additives such as ceramic inorganic fillers and

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

provide longer shelf–lives with no internal shorting (Gray, 1997a; Stephan, 2006).

and thus facilitate the ion separation, leading to ionic hopping mechanism. Hence, it generates the ionic conductivity. In other words, the ionic conduction of PE arises from rapid segmental motion of polymer matrix combined with strong Lewis–type acid–base interaction between the cation and donor atom (Ganesan et al., 2008). However, the well separated ions might be poor conductors if the ions are immobile and unable for the migration. Therefore, the host polymer must be sufficiently flexible to provide enough space for the migration of these two ions.

SPE serves three principal roles in a lithium rechargeable battery. Firstly, it acts as the electrode separator that insulates the anode from the cathode in the battery which removes the requirement of inclusion of inert porous membrane between the electrolytes and electrodes interface. Besides, it plays the role as medium channel to generate ionic conductivity which ions are transported between the anode and cathode during charging and discharging. This induces to enhancement of energy density in the batteries with formation of thin film. In addition, it works as binders to ensure good electrical contact with electrodes. Thus, high temperature process for conventional liquid electrolytes is eliminated as well (Gray, 1991; Kang, 2004).
