# Development on Solid Polymer Electrolytes for Electrochemical Devices
## 1 Introduction  
In any electrochemical devices, the electrolyte is viewed as the core constituent separating the cathode and anode, and it serves as the medium for charge transport. The electrolytes in electrochemical cells exist in three forms, viz liquid, gel and solid. Liquid electrolytes (LEs) are usually found to be highly conducting; thus, they are favored in all kinds of electrochemical devices, namely lithium-ion batteries, dye-sensitized solar cells (DSSCs), quantum dot sensitized solar cells (QDSSCs), fuel cells, electrochemical double layer capacitors (EDLCs), electrochromic devices (ECDs) and so on. However, LEs are sometimes volatile and vulnerable to evaporation, leakage and corrosion in the long-term. This will lead to stability and safety issues of the electrochemical devices. Moreover, LEs can also cause the formation of solid electrolyte interphase (SEI) at the anode surface not only in Li-ion batteries but also for all alkali-metal-ion batteries. This can bring adverse effects on their performance. On the other hand, gel-type electrolytes that inherit both characteristics from electrolytes in liquid and solid forms can reduce complications due to safety, but they suffer from structural instability and low mechanical property. Hence, electrolytes in the solid state are the better and more trustworthy choice to resolve the safety problems without compromising the structural and mechanical traits. Solid polymer electrolyte (SPE), which is fundamentally made up from dissolved conducting salt within the polymer host, was first discovered by three polymer chemists from Sheffield, UK, i.e., [PERSON], [PERSON] and [PERSON], about 48 years ago [1]. Then, in 1978, [PERSON] and co-workers [2] envisioned the promising implementation of SPE in solid-state batteries. Since then, research on SPE has improved tremendously. In order for SPE to be utilized in electrochemical devices, it must have reasonably good ionic conductivity, good chemical, and thermal and mechanical stabilities, as well as good interfacial contact with electrodes. SPE is considered safe, due to the absence of volatile and flammable solvents, as well as flexible, since it can accommodate volume changes in electrodesduring charging/discharging reactions of electrochemical cells (namely in Li-ion batteries). Furthermore, SPE can be prepared in various shapes and desired designs to meet the needs of electrochemical devices. It can be prepared as an ultra-thin membrane which is lightweight, thus offering promising high energy and power density. SPE is unique compared to liquid electrolyte in the sense that the polymer matrix is in charge for the solvation of ions, while the ions are located at sites which are flexible and can travel within the polymer segments. In order to be functional in electrochemical devices, the SPE must fulfil some prerequisites [3]:  
1. Good ambient ionic conductivity of the order between \(10^{-4}\) and \(10^{-2}\) S cm\({}^{-1}\). 2. Wide electrochemical stability window. 3. Mechanically stable in order to be safe and durable for devices operation
4. Thermally stable when in contact with electrode components. 5. Chemical and electrochemical compatibility with electrodes. 6. Obtainability--the raw materials should be abundantly found and low-priced. A basic SPE contains a polymer and a salt. The salt is dispersed in the polymer matrix, and the ions can conduct through the polymer chains. In order for the polymer to function as host in polymer electrolytes, the macromolecules must possess polar groups which have lone-pair electrons for dative bonding with the cation from the salt. Both synthetic and natural polymers can serve as polymer matrices in SPEs as long as they have functional group(s) to fulfil the complexation criterion.