group (--OH) of nanofiller also provides additional conducting sites for ion migration along with sites provided by polymer chains. The nanoclay addition is also a beneficial approach and nanoclay with high cation exchange capacity is effective in enhancing ion dynamics. The nanoclay having a negative surface charge on clay layers allows polymer penetration inside it and accommodates cation coordinates polymer chain inside. It allows the cation migration by stopping the anions outside clay gallery owing to large anion size. It also suppresses the ion-pair formation tendency.
3.1.4 Modification Strategies for Polymer Electrolytes
Depending upon the addition of ionic liquid, plasticizer and nanofiller polymer electrolytes are categorized in three types, (i) ionic liquid-based polymer electrolyte, (ii) gel polymer electrolyte and (iii) solid/composite polymer electrolyte. The ionic liquid-based polymer electrolytes have polymer salt matrix with ILs incorporated in it. The IL increases the polymer flexibility and helps in salt dissociation. The gel polymer electrolytes (GPE) consist of plasticizer (EC, PC, DMF) in the polymer salt matrix. This type provides improved shape flexibility, enhanced electrical property and safety as compared to the ionic liquid. It enables the simultaneous presence of polymer matrix cohesive properties and liquid electrolytes type ion diffusion. The solid polymer electrolyte (SPE) is a solvent-free system and comprises polymer host, salt and nanofiller. The electron rich group in polymer matrix provides the coordinating sites for cation migration and nanofiller enhances salt dissociation along with modifying the polymer chain via Lewis acid-base interactions. The solid (dispersed) polymer electrolytes are prepared by the addition of inorganic insulating nanofiller (Active and passive) such as LiO, Al2O3, SiO2, and TiO2.
The high surface area with OH group of nanofiller effectively alters the polymer chain arrangement and also supports salt dissociation. The nanoparticle of various morphology such as nanorod, nanowire is effective due to higher surface area. The addition of nanoclay has also emerged as an attractive approach termed as a solid (intercalated) polymer electrolyte [30]. Figure 3.5 shows the different types of polymer electrolytes. Table 3.2 shows some important polymer electrolytes and their characteristics.
Figure 3.5 Types of additive based polymer electrolytes.
Table 3.2 Properties of mostly used polymer host in polymer electrolytes.
| Abbreviation | Polymer name | Tg | Tm | Formula |
| PEO | Poly(ethylene) Oxide | -67 | 65 | (-CE2CE2O-)n |
| PMMA | Poly (methyl methacrylate) | 105 | 160 | (C5O2H8)n |
| PAN | Poly(acrylonitrile) | 125 | 317 | (C3H3N)n |
| PVA | Poly(vinyl alcohol) | 85 | 230 | (C2H4O) |
| PVC | Poly(vinyl chloride) | 81 | 160 | (C2H3Cl)n |
| PDMS | Poly(dimethylsiloxane) | -127 | -40 | -[SiO(-CH3)2]n |
| PVdF | Poly(vinylidene fluoride) | -40 | 171 | -(CE2CF2)n- |
| PVdF-HFP | Poly(vinylidene fluoridehexafluoropropylene) | -65 | 135 | -(CE2CF2)n [CF2CF(CF3)]m - |
| PEMA | poly (ethyl methacrylate) | 66 | 160 | [CH2C(CH3)(CO2C2H5)]n |
3.2 Preparation and Characterization Techniques
The preparation of polymer electrolytes (PE) and characterization is an important part. The polymer electrolytes are prepared by solution cast technique, in-situ polymerization, Phase separation/inversion method, electrospinning technology, spin coating, melt intercalation, and hot press method. The structural and morphological examination is done by X-ray diffraction, Field emission scanning electron microscopy, and transmission electron microscopy. The electrical properties of PE like ionic conductivity, voltage stability window, and ion/cation transport number are evaluated from the impedance spectroscopy technique. The thermal properties of the PE are examined by thermogravimetric analysis and differential scanning calorimetry. The characterizations are summarized in Table 3.3.
Performance Parameters for Supercapacitors
The suitability of the prepared polymer electrolyte as an electrolyte in the supercapacitor cell is examined by evaluating the characteristic parameter. These parameters play a significant role and are specific capacitance, resistance (bulk, charge transfer), energy density, power density, capacity retention, and coulombic efficiency. The important techniques are complex impedance spectroscopy (CIS), cyclic voltammetry (CV), and galvanostatic charge/discharge (GCD).
The overall capacitance of the cell is
Table 3.3 Selected separator characterization techniques with examples for extracted parameters [Reprinted with permission from Ref. [31], © Springer Nature 2019].
| Type of analysis | Parameters extracted | |