STRUCTURAL AND ELECTROCHEMICAL PROPERTIES OF INTERCALATED AND DISPERSED TYPE POLYMER NANOCOMPOSITE FILMS

Abstract

The renewable and green source of energy now becomes the burning topic for worldwide research among the scientists. The demand for such energy resources is increasing day by day and it becomes the lifeblood of modern society. Global warming, finite fossil-fuel supplies, and city pollution conspire to make the use of environmentfriendly energy sources. Due to the enlarged dependency of a modern human being on energy resources in every sector and a limited supply of fossil fuels, leads to two main problematic consequences: (1) vulnerability of nation-states to fossil-fuel imports and (2) CO2 emissions that are acidifying our oceans and creating global warming. The controlled environment/climate has drawn the attention of the scientific community towards development and replacement of fossil fuels by an alternative/efficient energy resources. The prospective renewable energy resources are solar, tidal, hydro, wind energy etc. Next challenge comes to store it and could be supplied as per the demand. The said challenge could be overcome through the electrochemical storage/conversion devices (ESCDs) like supercapacitor, Lithium-ion batteries (LIB) and fuel cells. Especially, LIB having the ability to the portability of stored energy and to deliver it as and when required without gaseous exhaust, unlike fossil fuels. A secondary battery converts chemical energy into electrical energy and vice versa. Its structure is composed of a positive electrode as a cathode, a negative electrode as an anode, and electrolyte. Simultaneous movement of ions and electrons occurs in the battery system; ions flow through the electrolyte while electrons are generated at the anode and flow towards the cathode via an external circuit. The heart of the battery is the electrolyte as it is sandwiched between both electrodes and participate in charging/discharging. Although all the three components affect the overall cell performance, the electrolyte is dominating in nature and deciding the specific capacity, energy density, working voltage and the lifespan of the battery. Various types of electrolytes are liquid electrolytes, semi-gel and gel electrolytes. However, safety issues with lithium-metal anodes, the reaction of volatile/flammable organic solvents and the leakage of electrolytes have hindered the commercialization of any lithium-ion battery based on such electrolyte. The drawbacks associated with the battery comprising of above-mentioned electrolyte pushes us to develop new generation solid state polymer nanocomposite films (PNC films) which could possess inherent safety and good compatibility with electrodes as compared with liquid, semi-gel and gel electrolytes. v PNC films have numerous advantages like they are light in weight, flexible, have interfacial compatibility, no leakage issue and are very processable. Most importantly, they are very safe. SPEs are prepared by dissolving lithium salts in a high-molecularweight polymer matrix. The polymer acts as the host for the transmission of lithium ions through the motion of polymer segments. Solid polymer based electrolytes appear to be attractive as they can compensate for the volume changes of electrodes by elastic and plastic deformation. A PEO-based SPE is the most preferred polymer host in the research system due to its flexible backbone and ability to solvate lithium ions, with the coordination number dependent upon the salt concentration and identity of the anion. The main advantage of a PEO is its high solvation power. Hence it can form a complex easily with many alkali salts and provides a direct path for cation migration due to the presence of the ether group in the polymer backbone. But the low conductivity value (10–10 S cm−1 ) and poor mechanical properties of PEOs at ambient temperatures limit their use in devices. Many approaches have been explored to improve the ionic conductivity in order to suppress the concentration polarization and desirable electrochemical properties such as polymer blending, cross-linking, the addition of nano-sized fillers etc. Out of these approaches at host polymer level blending seems more appropriate and justified. Further to scale the relevant properties parameters by minimizing the concentration polarization, two novel approach i.e. (i) nanofiller dispersed polymer nanocomposites, and (ii) intercalated polymer nanocomposites have been adopted. The idea behind using nanofiller was the expectation to get percolation pathways composed of inorganic fillers grains through a flexible polymeric matrix. Such a phenomenon could lead to an increase in ionic conductivity followed, possibly, by an enhancement of the cation transport number while preserving mechanical properties and flexibility of the composite electrolyte prepared in the thin film configuration. The second one is also a thought-provoking approach and plays key role in (i) suppressing the concentration polarization by avoiding anion mobility, (ii) enhancement of the ion migration by allowing the cation coordinated polymer chain confinement in clay galleries, and (iii) negative surface charge on clay acts as Lewis acid and competes with Li+ cation to form complex with polymer which reduces ion coupling. An attempt has been made to understand the role of nanofiller and nanoclay in dispersed and intercalated blend polymer nanocomposites prepared by solution cast technique. A strong correlation exists between crystallinity, conductivity, free ion area, the number density of charge carriers, ion mobility, diffusion coefficient, activation energy, and glass transition temperature. Polymer-intercalated polymer nanocomposites display remarkable higher ionic conductivity, broad voltage stability window, high specific capacity and open-circuit voltage than the dispersed based polymer nanocomposites. Here we provide a cumulative account of an efficient polymer nanocomposite materials to identify their importance in the energy storage/conversion devices.