Solid polymer electrolytes are ideal candidates to replace liquid electrolytes in lithium-ion batteries, because the non-toxic solid polymer eliminates the need for a rigid casing resulting in increased design flexibility and decreased disposal problems. The characteristic that precludes effective application is low room-temperature conductivity, and thus many modifications have been attempted and their effects on conductivity investigated. One such modification is addition of nanoparticle fillers to lithium-doped poly(ethylene-oxide) [PEO], which increases conductivity in particular at low temperatures. Although this effect is well established, the mechanism through which it acts is not. Increased mobility of the polymer host is a frequently offered explanation: yet recent experiments prove that high conductivity may be obtained even in completely crystalline PEO, and simulation studies suggest that oxide nanoparticles slow dynamics of PEO. Interpretation of experiments measuring mobility or conductivity requires knowledge of nanoparticle aggregation, crystallization and humidity, yet these variables are frequently not reported. Aggregation is important because a dispersed system will be influenced by confinement of the polymer host. The kinetics of polymer host crystallization are influenced both by the lithium salt, and the nanoparticles. Since the desired operating temperature [room temperature] is below the melting point, time to crystallization is an important variable to monitor. It is equally important to eliminate differing humidity levels as the cause of increased conductivity with nanoparticle addition, because practical devices cannot operate in the presence of water. This project combines a variety of techniques: quasielastic neutron scattering, broadband dielectric spectroscopy, and small-angle neutron scattering, performed on the same samples, under the same conditions. The identities of the polymer (PEO), lithium salt (LiClO4), and nanoparticle (Al2O3) will be fixed, while the nanoparticle size and concentration will be varied around that established to provide optimum conductivity. The influence of nanoparticles on conductivity and PEO mobility will be tested in dry and ambient conditions, and before and after the time required to crystallize the PEO. The practical contribution of this study will be to isolate the effects of nanoparticle fillers on conductivity and PEO mobility, as a function of crystallization, particle aggregation, and water content.
NON-TECHNICAL SUMMARY:
New environmentally friendly and efficient energy sources include fuel cells, solar cells and long life rechargeable batteries. Lithium ion batteries, the focus of this project, are available commercially, with the material separating the two sides of the battery [the electrolyte] in the form of a liquid or gel. This requires a casing, and the solvents added to improve movement of lithium across the electrolyte are an end of life disposal problem. Using a solid polymer as the electrolyte alleviates these difficulties, but without solvents lithium movement is not sufficient to power a device. Using state of the art methods to characterize mobility of the various components, this project will determine the reasons that the addition of nano-sized fillers improves device performance. This will allow for lightweight, flexible and safe batteries to power computers, cell phones, and other devices. This project uses neutron scattering, an experimental technique performed at user facilities, such as the Center for Neutron Research at the National Institute of Standards and Technology. The US has invested considerably in neutron scattering facilities, including the Spallation Neutron Source at Oakridge National Laboratory, which is currently starting operation and will triple the number of neutron users that may be accommodated nationally. Part of this project is to educate US scientists in this technique, and begin to form this new user pool.
