**2. Properties and ion transport mechanism in PEO**

PEO has chemical structure H▬(O▬CH2▬CH2)n▬OH which is a polyether compound. Depending on the molecular weight of polymer, it is also known as polyethylene glycol (PEG). Generally, polymer having molecular weight above 20,000 g/ mol is called PEO while, below 20,000 g/mol is known as PEG [21]. PEO is a low toxic compound; therefore, it is widely used in many applications such as chemical, industrial, medical, biological etc. PEO has ethylene oxide unit which provides flexibility and high donor number for Li+ ions and promotes the ion transport. Also, it has high dielectric constant and Li+ ion solvating ability [22]. The ion transport mechanism in PEO occurs by the coordination of Li+ ions with the ether oxygen atom of PEO chain. This process occurs by formation and dissociation of Li▬O bond by local segmental motion of polymer chain (**Figure 2**) [23]. The ion transportation in polymer

electrolytes mainly depends on polymer chain segmental motion which becomes faster in amorphous region or above the glass transition temperature (Tg) of polymer (as shown in **Figure 2(a)**). Hence, ideally polymer should have lower Tg to remain in rubbery state or to promote the Li+ ion conductivity at RT [17]. In order to enhance Li+ ion conductivity of polymer electrolytes different strategies such as addition of nano fillers, use of plasticizers or gel polymer electrolytes has been proposed. On this basis, polymer electrolytes are classified into different categories as discussed below.
