**1.4 Gel polymer electrolytes (GPEs)**

166 Recent Advances in Plasticizers

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

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

SPEs are of great interests in the technology field, especially the area of electrical power generation and storage systems. It is primarily due to their wider range of applications, ranging from small scale production of commercial secondary lithium ion batteries (also known as the rechargeable batteries) to advanced high energy electrochemical devices, such as chemical sensors, fuel cells, electrochromic windows (ECWs), solid state reference electrode systems, supercapacitors, thermoelectric generators, analog memory devices and solar cells (Gray, 1991; Rajendran et al., 2004). As for the commercial promises of lithium rechargeable batteries, there is a wide range of applications which ranges from portable electronic and personal communication devices such as laptop, mobile phone, MP3 player, PDA to hybrid electrical vehicle (EV) and start–light–ignition (SLI) which serves as traction

A force had been driven in the development of SPEs to replace conventional liquid electrolytes due to its attractive advantages. These features include the elimination of the problems of corrosive solvent leakage and harmful gas production, ease of processability due to elimination of liquid component, and suppression of lithium dendrite growth as well (Rajendran et al., 2004; Ramesh et al., 2010). Besides safety performance, SPE is also a promising candidate because of its intrinsic characteristics, such as ease to configure in any shape due to its high flexibility of polymer matrix, high automation potential for electrode application and no new technology requirement as well as light in weight (Xu and Ye, 2005; Gray, 1991). The other advantages of SPEs are viz., negligible vapor pressure, ease of handling and manufacturing, wide operating temperature range, low volatility, high energy density and high ionic conductivity at ambient temperature (Baskaran et al., 2007; Rajendran et al., 2004). In addition, they exhibit excellent electrochemical, structural, thermal, photochemical and chemical stabilities without the combustible reaction products at the electrode surface by

for the migration of these two ions.

as well (Gray, 1991; Kang, 2004).

**1.2 Applications of solid polymer electrolytes** 

power source for electricity (Gray, 1997a; Ahmad et al., 2005).

**1.3 Advantages of solid polymer electrolytes** 

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 crystalline or glass electrolytes (Gray, 1997a).
