**3.1 Poly(vinyl chloride) PVC**

Apart from PEO, poly(vinyl alcohol) (PVA), poly(acrylonitrile) (PAN), poly(ethyl methacrylate) (PEMA), poly(vinyl chloride) (PVC), poly(vinylidene fluoride) (PVdF) have also been used as polymer host materials. PVC is a thermoplastic polymer where its IUPAC name is poly(chloroethanediyl). It consists of numerous repeating units of monomers called vinyl chloride. It is a vinyl polymer composing of numerous repeating units of CH2–CHCl.

Fig. 1. Chemical structure of PVC.

Characterization of High Molecular Weight Poly(vinyl chloride) –

solute as it shows multiple merits than other lithium salts.

propylene glycol and carbonic acid as illustrated as below.

Fig. 3. Chemical structure of propylene carbonate.

**3.4 Tetrahydrofuran (THF)** 

**3.3 Propylene carbonate (PC)** 

Lithium Tetraborate Electrolyte Plasticized by Propylene Carbonate 173

solvent which caused by its low dissociation constant in low dielectric media and its moderate ion mobility, is the major shortcoming of LiTf as compared with other lithium salts (Kang, 2004). Therefore, it can be concluded that Li2B4O7 is an indispensable electrolyte

As aforesaid, plasticizer could enhance the ionic conductivity of polymer electrolytes. Propylene carbonate (PC) is used in this study. PC is an organic, colourless and odourless organic compound. It is also well known as highly polar and aprotic solvent. Furthermore, it is a byproduct of the synthesis of polypropylene carbonate from propylene oxide and carbon dioxide. It can be obtained from the synthesis of urea and polypropylene glycol in the presence of zinc–iron double oxide catalysis. It is composed of twofold ester of

O O

PC is a preferred solvent as it exhibits many unique characteristics. These properties are static stability with lithium, wide liquid range and low freezing point (Kang, 2004). Many researchers have drawn interests onto this plasticizer due to its high dielectric constant (ε= 64.9). This high dielectric constant could help in the dissociation of the charge carriers, especially the cations from the dopant salt. The electrical performance would be improved as it manifests excellent plasticizing effect (Tobishima and Yamaji, 1984). Cations are more readily to be obtained as the salt is dissolved in PC through electrolysis process, because of its high molecular dipole moment of 4.81 D (Jorné and Tobias, 1975). Moreover, high polarity of this plasticizer creates an effective solvation shell around the cations and hence forms a conductive electrolyte. PC displays the highest dielectric constant in comparison with *DEC (*ε= 2.8), *ethyl methyl carbonate (EMC*) *(*ε= 2.9), *DMC (*ε= 3.1), DOA *(*ε= 4–5), DBP *(*ε= 6.4), DMP *(*ε= 8.5) and benzyl acetate (BC) *(*ε= 53) (Kang, 2004; Gu et al., 2006). Despite EC illustrates higher dielectric constant than PC, however PC exhibits wider range of liquidity than EC as its melting point is up to –48.8 °C. In contrast, high melting temperature of 36.4 °C is the main shortcoming of EC. High dipole moment of PC than EC (4.61 D) with low vapor pressure also compensates the obstacle of lower dielectric constant of PC (Kang, 2004). Therefore, PC is becoming an attractive prospect as plasticizer compared to other plasticizers.

Tetrahyrdofuran (THF) is a colorless, water–miscible organic liquid with low viscosity at standard temperature and pressure. It is one of the most polar ethers with a wide liquid range and is widely been used as solvent. In addition, it is an aprotic and highly volatile solvent with

O

PVC is mainly produced by radical polymerization (Endo K, 2002). In this polymerization, it associates the vinyl chloride molecules and thus forms the polymeric chains of macromolecules. From the scientific point of view, lone pair of electrons at the chlorine atoms is the main reason for choosing PVC as host polymer. Thus, it can form solvation onto lithium salts easily (Ramesh and Chai, 2007). PVC is chosen due to its high compatibility with the liquid electrolyte, good ability to form homogeneous hybrid film, commercially available and inexpensive (Li et al., 2006). Other unique characteristics are easy processability and well compatible with a large number of plasticizers (Ramesh and Ng, 2009). It plays an important role as mechanical stiffener because of the dipole–dipole interactions between the hydrogen and chlorine atoms (Ramesh and Chai, 2007).

#### **3.2 Lithium tetraborate (Li2B4O7)**

Several types of lithium salt has widely been used, such as lithium hexaflurorophosphate (LiPF6), lithium hexafluoroarsenate (LiAsF6), lithium bis(trifluoromethanesulfonyl) imide (LiTFSI), LiTf, LiClO4 and LiBF4. Lithium tetraborate (also known as boron lithium oxide or dilithium borate) (LBO or more commonly known as Li2B4O7) was employed in this study. This compound is generally defined as one type of dopant used to provide lithium cations in the preparation of polymer electrolytes. It is constructed of lithium cations and tetraborate anions, where its stoichiometric ratio is two cations to one anion, as illustrated as below.

Fig. 2. Chemical structure of lithium tetraborate.

In addition, it is a new non–ferroelectric piezoelectric substrate material with a congruent melting point of 917 °C. LBO single crystal is also a superior substrate for surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices as it has a specific crystallographic plane (110). Thus, it provides a zero temperature coefficient of frequency and a fairly high electromechanical coupling factor. According to Byrappa and Shekar (1992), LBO occurs naturally as diomignite, i.e. colourless crystals in fluid inclusions in the mineral spodumene. It appears as a loosely attached crystal with a very fine–grained structure. These intrinsic and unique properties make it as excellent material by comparing with other lithium salt. The fine structure of Li2B4O7 would enhance the solubility in the polymer matrix and eventually, speed up the salt dissociation process. Other factors to choose it as doping salt are abundant availability of raw materials and no environmental pollution (Xu et al., 2004). As aforementioned, Li2B4O7 is naturally obtained from mineral. Therefore, it is a cost effective material compared to those synthetic lithium salts, for instance LiBF4, LiTFSI and LiTf. High toxicity of LiAsF6 and poor chemical and thermal stabilities of LiPF6 are not good choices as doping salt in polymer electrolyte. Likewise, LiClO4 reacts with most organic species readily in violent ways under certain conditions such as high temperature and high current charge because of high oxidation state of chlorine (VII) in perchlorate. Moreover, the corrosion of a key component of the cell by TFSI anions restricts the possible application of LiTFSI greatly in the polymer electrolytes. Poor ionic conductivity of LiTf in non–aqueous solvent which caused by its low dissociation constant in low dielectric media and its moderate ion mobility, is the major shortcoming of LiTf as compared with other lithium salts (Kang, 2004). Therefore, it can be concluded that Li2B4O7 is an indispensable electrolyte solute as it shows multiple merits than other lithium salts.
