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Besides the size, the high energy density of battery can also be promise. In other words, the solid electrolyte especially their film can be physically sandwiched between electrodes, the anode and the cathode formed out of thin sheets and then the cell is contained in laminated foil and sealed at the edges to form an entire battery. The resulting cell is extremely thin and flexible, spill proof, more resilient under pressure, and capable of being engineered into any shape. In shorts, SPE are lighter, flexible and leak resistant than GPE.
Other advantages of SPEs are wide operating temperature range, no vapour pressure, low volatility ,ease of handling and manufacturing, high energy density and high ionic conductivity at ambient temperature (Baskaran et al., 2006; Rajendran, et al., 2004). In addition, the electrochemical, structural, thermal, photochemical and chemical stabilities can be enhanced for SPE in comparison to conventional liquid electrolyte (Adebahr et al., 2003; Nicotera et al., 2006). Polymer electrolytes are attractive because they can be cast as thin films. Thin films can minimize the resistance of the electrolyte and reduce its volume and weight. Although an electrolyte is essential for a battery, but it adds weight and occupies space. Therefore it reduces the energy stored per unit weight and volume that might otherwise be devoted to the active electrodes.
Limitation of Polymer Electrolyte
There are several limitations of polymer electrolytes when reviewed as industrial applications. The profound shortcoming is attributed to its low ionic conductivity which is not compatible for practical applications (Song et al.,1999). This limitation completely drags out the utilisation of polymer electrolytes as the separator since it is a crucial aspect of battery electrolyte. Besides that, this major deficiency of SPE not only poor conductivity, but the low transference number which leading to poor discharge and charge rate and short life cycle. So, there is a need for novel electrolyte that has high conductivity and high ion transference number with wide electrochemical stability windows (Venkatasetty H.V., 1996). In addition, due to the ambient politics are being extremely rigid in the control of the use of materials that "attack" the environment. The battery industry represents one important and growing sector where the use of non- toxic and hazardous substitute materials has not rapidly grown (Fonseca et al., 2007). Therefore, a biodegradable polymer electrolyte has to investigate to reduce environmental problem.
Ionic conductionÂ is the movement of anÂ ionÂ from one site to another throughÂ defectsÂ in theÂ crystal latticeÂ of a solid. The idea that ions can diffuse as rapidly in a solid as in an aqueous solution or in a molten salt may seem impossible. However, since the 1960s, a variety of solids that include crystalline compounds, glasses, polymers, and composite materials with exceptionally high ionic conductivities have been discovered. Materials that conduct anions (e.g. Fâˆ’ and O2âˆ’) and cations including monovalent (e.g. H+, Li+, Na+, Cu+, Ag+), divalent, and even trivalent and tetravalent ions have been synthesized. The electrical conductivities of several common substances and representative SPE at the temperature where the materials have potential application are shown in figure 2.2. The SPE have conductivities that fall between those of a typical semiconductor, silicon, and a typical aqueous electrolyte, sodium chloride.
Figure 2.:Electrical conductivities of selected common substances and representative SPE (Bruce King, 2005).
The single phase of polymer electrolyte can be expressed its electrical conductivity by:
where ni is the number of charge carriers type of i per unit volume, qi is the charge of ions type of i , and ðœ‡i is the mobility of ions type of i which is a measure of the drift velocity in a constant electric field.
Guideline to attain high ionic conductivity
There are 6 guidelines for SPE to attain high ionic conductivity:
1. A large number of the ions of one species should be mobile.
2. There should be a large number of empty sites for the mobile ions to jump into.
3. The empty and occupied sites should have similar potential energies with a low activation energy barrier for jumping between neighbouring sites. There is useless to have a large number of available empty sites if the ion does not have enough energy to get into them.
4. The structure should have a framework such as a rigid subarray or 'skeleton' with interconnected interstitial space in open channels in which ions move preferably in three dimensions.
5. The anion framework should be highly polarisable.
6. Retain a negligible electronic conductivity Ïƒ at the operating temperature. (Bruce King, 2005)
Guideline to attain high ionic conductivity
Although SPE should have high ionic conductivity than liquid electrolyte and GPE, but the SPE could not completely satisfy the requirement of properties such as good mechanical strength and dielectric constant, high ionic conductivity, high tensile strength and good abrasion resistance. Therefore, methods to enhance these properties of polymer were investigated. There are several methods to increase the ionic conductivity of polymer electrolyte such as polymer blending, mixed salt, mixed solvent, addition of plasticizer and ceramic inorganic solvent.
A polymer blend or polymer mixture is a member of a class of materials corresponding to metal alloys, in which at least two polymers are blended together to create a new material with different physical properties. A polymer blend is used as a matrix polymer to attain high ionic conductivity. The ionic conductivity enhancement can be attributed to the increase of amorphous regions responsible for the ionic conduction. The lithium ions migrate primarily through the film as in the GPE which in turn contribute to ionic conductivity enhancement (Choi et al., 2000).Polymer blending is useful because it is ease of preparation and control of the properties of polymer electrolytes by changing the composition of blended polymer (Subramania et al., 2006).According to Subramania, the ionic conductivity of PVA-PAN blend for blend ration 60 wt% of PVA and 40 wt% of PAN is the highest and sufficient mechanical strength. The ionic conductivity increase due to the increase of PAN content in PVA and the mechanical strength is decrease. Besides that, the linear sweep and cyclic voltammogram studies also show that this polymer blend electrolyte has a good electrochemical stability up to 4.7 V besides very good reversibility. PVA is chosen because of its high tensile strength and good abrasion resistance and it can also be easily blended with other polymer and PAN is chosen because of its high uptake of the electrolyte solution which may result in swelling or gelation of polymer rather than dissolution (Subramania et al., 2006).
According to Ramesh S. and Arof A.K., plasticized electrolytes are in a more amorphous state compared with the unplasticized electrolytes due to plasticized polymer electrolytes have lower value of the glass transition temperature, Tg due to a lubricating effect. The plasticizer can behave as solvent when mixed with polymer. This plasticization effect is related to a weakening of the dipole-dipole interactions due to the presence of plasticizer molecules between polymer chains. The decrease in Tg helps to soften the polymer backbone and increase its segmental motion and such motion produces voids, which enables the easy flow of ions through the material when there is an applied electric field (Ramesh S. and Arof A.K., 2001).However, one of the important drawbacks in plasticized polymer electrolytes is poor mechanical property at high degree of plasticization. According to Pita et al., the addition of a plasticizer to a PVC formulation decreases many mechanical properties of the PVC product (Pita et al., 2002). However, low temperature flexibility, elongation and the ease of processing are all improved. Although many polymer gel electrolytes are fabricated as free-standing films, their mechanical strength needs further enhancement when it comes to practical applications. In practical lithium polymer cells, inorganic fillers are frequently added to improve the mechanical strength of electrolyte films (Pita et al., 2002).
An electrolyte studies based on double or mixed salt system is also another technique of enhancing the ionic conductivity of polymer electrolytes. Even though this study does not involve direct mixing of two dopant salts, but this method serves as an alternative of further enhancing the performance of polymer electrolytes after the optimum ratio of polymer to dopant salt has been identified. Mixed salt systems containing mixed cations and mixed anions are found to give better ionic conductivity than single salt systems (Ramesh S. and Arof A.K., 2001). It can be explained as the addition of the second component hinders the formation of aggregates and clusters, thus increasing the mobility of ionic carriers. However, there are only certain composition of salt systems can get the good conductivity which is reported by Chowdari et al. (Chowdari et al.,1992). This research reported that there are two competing factors between segmental mobility and mobile charge carrier concentration compromised or optimised at particular concentration, which resulted in higher conductivity. On the other hand, choice of salts having low lattice energy is important as it promotes greater dissociation that give rise to more mobile ions.
Ceramic Inorganic Filler
Nano scale ceramic additives have two important common features: large specific surface areas and grains covered with various Lewis acidic or basic groups. The interactions between these surface groups and the ionic species of the salt are responsible for the observed conductivity enhancement. Wang et al. reported that by the combination of filler vinyl ethylene carbonate (VEC) into propylene carbonate (PC) based electrolyte will helps the dissolution of the lithium salt and dissociation of Li+ ion. Besides that, the addition of VEC can prevent the co-intercolation of solvent and the solvated Li+ ions into graphitic electrolyte (Wang et al., 2005).
By the addition of ceramic fillers, having the right particles size and composition, prevents crystallization and favours the amorphous, highly conducting structure of PE. The addition of nano filler SiO2 will enhances the ionic conductivity of both PVC-PMMA and PVC-PEO-blend-based PE. This would be attributed to the increase in mobility and number of charge carriers upon addition of SiO2 (Ramesh S. and Arof A.K., 2009).The interest in using SiO2 comes into several distinct advantage of this ceramic filler which are from its 'starburst' particulate shape enables it to effectively disturb the order packing tendency of the host polymer chains at low solid loadings, its large surface area leads to an open network structure that support high ionic mobility and its surface groups can be modified to tailor the interfacial properties for a specific need.
In 2002, Venkatasetty also reported that by addition of nanoscale particle titanium dioxide (TiO2) were added to the composite to get better mechanical property. The adding of lithium salts can reduce crystallinity but the adding of TiO2 can further reduce the crystallinity of SPE films (Venkatasetty & Jeong, 2002).
Biodegradable Polymer Electrolyte
In this study we are involved in the development of polymer electrolytes utilising biodegradable polymers. The biodegradable nature of the conducting matrix formed makes it more appealing since it is greener to the environment. In spite of that, the employment of biodegradable polymers as the host polymer potential in reducing the cost of production in commercial wise which are very much desired by the manufacturers. The cost of production is reduced since it is a renewable resource that naturally available in the surroundings. The utilisation of bio based materials in suppressing the high degree of crystallinity in biodegradable polymer sustains the biodegradable nature. Ghufira et al. reported that the use of bentonite in PVC-LiClO4 polymer electrolyte in different weight percent and manage to found that 10 % of bentonite in PVC-LiClO4 polymer electrolyte has the highest conductivity which is 4.86 x 10-3 S cmâˆ’1. This already shows that bentonite is potential natural filler for polymer electrolyte (Ghufira et al., 2012).Besides that, Fonseca et al. also reported that the use of biodegradable GPE and Poly (Æ-caprolactone) (PCL) as polymer host. The PCL with 10% Propylene Carbonate (PC) and 12 % LiClO4 has ionic conductivity that 2.26 x 10-6 S cm-1 which showed this electrolyte could be attributed to the higher amorphicity and increased concentration of the charge carrier in the electrolyte, because of the increase of the degree of dissociation of the lithium salt due to presence of plasticizer agent and polymer-salt complexation. The electrochemical stability window had a potential range of up to a potential range of 5.5 V vs Li/Li+. This novel GPE demonstrated the property of biodegradation, and it is a promising candidate to be one more component to green batteries (Fonseca et al., 2007).