Poly(ethylene oxide)/cellulose-nanocrystal nanocomposites as polymer electrolyte membranes

Abstract

Over the past several decades battery technology has fallen behind the tremendous growth seen in other technological industries, such as microprocessors and computer memory. For today’s wireless technology to continue to improve, the need for batteries capable of storing a greater amount of charge in a small package is of paramount importance. One of the most promising routes to achieve this goal is to use lithium ions as the charge carrier. This is because lithium is lightweight and has a very high redox potential. However, the lifespan and safety of lithium based batteries have been problems since their inception, resulting in scientists constantly working to improve the mechanical properties of the electrolyte without greatly reducing its ionic conductivity.In the following thesis a system of poly(ethylene oxide) (PEO) with lithium perchlorate (LiClO4) salt and cellulose nanocrystals (CNCs) is investigated to determine its usefulness as a solid polymer electrolyte. It is seen that the addition of CNCs increase the crystallinity of the PEO-LiClO4 system, resulting in an enhancement of the mechanical properties. While increased crystallinity is also usually associated with a decrease in ionic conductivity this was not observed. Instead it is proposed that the CNCs act as a Lewis Acid, competing with lithium cations for ether oxygen atoms along the PEO chain. This competition results in a decrease in the strength of binding between the PEO and lithium ions, causing the lithium to diffuse much more easily and thus have higher conductivity when compared to a sample with no CNCs. It was also seen that the ionic conductivity of these solid polymer electrolytes improves with increasing temperature.To further improve the conductivity, dimethylformamide (DMF) was then used as a liquid plasticizer. The plasticizer allows the polymer chains to reptate more easily, improving the ionic conductivity of the sample. A three order of magnitude improvement in conductivity over the completely dry films was observed with the addition of up to 30% DMF. Unfortunately, with this improvement in conductivity comes a slight reduction in the modulus of the films. However this work does lay the foundation of using a nanofiller to improve the mechanical properties of an electrolyte at the expense of the conductivity and then add a plasticizer to somewhat reduce the mechanical properties while greatly improving the conductivity. When carefully balanced, these two additions can result in an electrolyte with moderately improved mechanical and conductivity properties.%%%%M.S., Materials Science and Engineering – Drexel University, 2012