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Traditional energy storage devices use the liquid electrolytes and that restricts the shape geometry of the device and hence practical applications. The enhanced demand for flexible devices motivated the scientific community to search for suitable electrolyte material that can serve the purpose of flexible devices. So, polymer electrolytes emerged as a suitable candidate due to intrinsic flexibility in polymer and it completely matches with the need of future devices. The first report that mentioned the ionic conduction in polymer electrolytes was published by Wright and co-workers in 1973. It shows that the polymer host polyethylene oxide (PEO) mixed with alkali metal salts (NaI) demonstrates the enhanced conductivity than the pure polymer, and it opened the door for researchers to develop new material and explore the ion transport mechanism. Later on, in the early 1980s, Prof. Armand demonstrated the technological importance of the polymer electrolytes over existing electrolytes [25]. So, a polymer electrolyte comprises polymer and metal salt. This metal salt get dissociated in cation and anions by interaction with solvent and polymer chain. The electron rich group in polymer chain provides coordinating sites to cation for migration and cation jump from one coordinating site to another on application of filed (hopping mechanism). Anion due to large radii remains in immobilized state and hangs with the polymer backbone. Figure 3.4 demonstrates the ion dynamics via hopping mechanism in polymer matrix [26].

The important component to the polymer electrolytes is the polymer host, salt, solvent, and nanoparticle. So, selection criteria need to be followed for developing suitable polymer electrolyte with optimum structural, electrical as well as mechanical properties [27]. In the following section, the important characteristics of the various components of the polymer electrolytes are discussed (Table 3.1) [16, 28, 29].

Figure 3.4 Mechanism of ion transport in PEO [Reproduced with permission from Ref. [26], © Royal Society of Chemistry 2015].

Table 3.1 Fundamental characteristics of various constituents of the polymer electrolyte matrix. [Reproduced with permission from Ref. [16], © IOP Publishing 2017].

Polymer Host	Plasticizer
Low glass transition temperature (Tg)High molecular weight, and Low ViscosityHigh degradation temperatureHigh Dielectric ConstantHave electron rich group (O, N)	Low Melting Point and High Boiling PointHigh dielectric constant, and Low viscosityInert and cost-effectiveGood SafetyNontoxic Nature
Solvent	Nanofiller
Abundant, and Non Aqueous in NatureLow Melting Point, and Low ViscosityLarge Flash PointHigh Dielectric ConstantGood Solubility for Polymer and Salt	High Polarity, Low Melting, & High Boiling PointSafe, cost-effective and Non-toxicEnvironmental friendly, and Inert to All Cell Components.High Dielectric Constant for better salt dissociation
Salt	Nanoclay
Low Lattice Energy, and High Ionic ConductivityHigh Mobility, and Broad Voltage Stability WindowSmaller cation radiiLarge Anion radiiHigh Thermal and Chemical StabilityLarge Transference NumberInert Towards Cell Components	Layered/unique structure with high aspect ratio (~1000).Greater ability for intercalation and swellingHigh swelling index & High cation exchange capacity (CEC) (~80 meq/100 g)High external/internal surface area (~31.82 m2 g-1)Appropriate interlayer charge (~0.55)Adjustable hydrophilic/hydrophobic balance
Ionic Liquid	Nanorod/Nanowire/Nanobelt
Good Thermal stability and broad Wide electrochemical stabilityLow Melting Point, and viscosityNegligible Volatility, Vapor PressureHigh Ionic ConductivityHigh Polarity, and High Dielectric ConstantNon-flammability	High aspect ratioEasily alignment perpendicular to electrodesOxygen vacancies on the surface for cationLess agglomeration at high contentHigh thermal stabilityBroad voltage stability windowBetter chemical stability

Polymer Host

The polymer host is a long chain of a polymer having an electron-rich group (oxygen, nitrogen) in the chain. These coordinating sites provide the path to the ion migration after the application of the electric field. The flexibility and number of sites influence the ion migration as well as salt dissociation.

Properties of Salt

The addition of suitable salt is important as it directly affects the ion migration. For better electrical properties, salt must be dissociated completely in the cation and anion. The salt interacts with the electron-rich group of the polymer chain as well as with hydrogen in the polymer backbone. This results in the availability of free ions that contributes the conductivity. The important properties of the salt are lattice energy, cation radii, and non-toxic nature. The low lattice energy suggests the easier salt dissociation, and hence more free charge carriers while smaller cation radii favor the faster migration via coordinating sites. Also, the ionic conductivity and the voltage stability window of the salt needs to be checked first. In general, smaller cation radii and larger anion radii salt are chosen for optimum electrical properties.

Properties of Solvent

Both polymer host and salt are to be added in some solvent for developing polymer salt matrix. So, the solvent needs to have high dielectric constant, low viscosity, inert toward device components. The polymer chain interacts with salt and gets stretched insolvent that makes it easier for an ion to interact with the polymer chain. The high dielectric constant favors the better salt dissociation and also suppress the ion-pair formation.

Properties of Ionic Liquid

Ionic liquids (ILs) are molten salts (of three types aprotic, protic and zwitter) with bulky anion that improves the salt dissociation in the polymer matrix. The ionic radii of anion influence the electrical properties of the polymer matrix. Also, by changing the cation and anion structure, IL can be modified as per requirement.

Properties of Plasticizer

The incorporation of the low molecular weight plasticizer (EC, DEC, PEG, DMF) is an innovative approach to suppress the polymer crystallinity and enhance the ionic conductivity as well as flexibility (i.e., low T_g; glass transition temperature). The plasticizer penetrates the polymer chains and reduces the cohesive forces between polymer chains which leads to enhanced segmental motion of the polymer chain. The enhanced segmental motion and improved free volume collectively enhance the ionic conductivity.

Properties of Nanofiller

Nanoparticles addition modifies the properties of the host matrix. So, the addition of nanoparticles of different morphology (spherical, wire, rod) has been examined for the development of suitable polymer electrolytes. The addition of nanoparticles enhances the salt dissociation due to surface group on the surface. The Lewis acid-base type indications with the salt and polymer results in enhanced polymer flexibility and more free charge carriers. The surface area is linked with the morphology of the nanofiller. The road and wire morphology provides a higher surface area for interaction and demonstrates faster ion migration than spherical morphology. The oxygen in the surface 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.