This project addresses solid polymer electrolytes [SPEs], a class of materials with the potential for use in lithium ion batteries. The specific aims are to produce nanocomposites with controllable particle dispersion, determine the nanoscale morphology of coexisting crystalline phases, and determine the relationship between polymer dynamics and increased conductivity. Nanocomposites are relevant to SPEs because they have been demonstrated to increase conductivity into the practical range. However, the mechanism by which nanoparticles increase conductivity in SPEs is not well understood. One of the problems is aggregation of the particles, which is significant using standard preparation methods and leads to irreproducible samples and results. The project will apply a technique recently reported for controlling the length scale of phase separation in a polymer blend. This technique encapsulates the polymers in a nano-emulsion, followed by evaporation of the polymer phase solvent, leaving a polymer nano-sphere with controllable size suspended in a non-solvent by use of a surfactant. Two such preparations are mixed to form the blend. The current project will replace one of the polymers with a nanoparticle, but otherwise follow the same procedure. Production of nanocomposites with well controlled spacing will allow for study of crystallization, polymer mobility and conductivity under reproducible conditions, thus providing a mechanism for enhanced conductivity, as well as fundamental understanding of the behavior of confined polymer/salt complexes.
NON-TECHNICAL SUMMARY: This project seeks to improve the properties of a material with potential use as a flexible and lightweight battery in portable and other devices. The required material is a solid plastic, and its use in batteries improves with the addition of nanometer-sized particles. The reason the particles improve properties is not well understood, because dispersion of the particles in the plastic matrix is difficult. The project will use a new method for dispersion of the particles, producing well characterized materials that can be used to gain the needed understanding. The materials will be studied using neutron scattering, a technique measuring both existence and movement of structures over nanometer length scales. The US has recently invested significantly in neutron scattering facilities: for example the Spallation Neutron Source at Oakridge National Laboratories. These expanded facilities require additional investigators to use the instruments. Thus outreach activities targeted towards scientific areas where neutron scattering is under-utilized is a part of this project.
We investigate the use of polymer electrolytes (SPEs), a safer, lighter, and more flexible alternative to the electrolytes currently used in rechargeable lithium ion batteries. These batteries are used to power cell phones, laptops, and more recently fully electric and hybrid electric vehicles. The importance of safety was highlighted in January 2012 when a Li-ion battery overheated and caught fire in an empty Japan Airlines Boeing 787. SPEs have the potential to increase the miles per charge in electric vehicles from 100 to 500 miles. Despite their advantages, SPEs have an important limitation: they cannot move Li ions fast enough to generate sufficient power. This ionic conductivity can be improved in several ways. One of them was the target of this project: the use of very small [nanoscale] fillers. Spherical ceramic fillers improve SPE conductivity, but have another equally important effect on the mechanical properties. They make the electrolyte stiffer, which is required to realize the potential of 500 miles per charge. Although these fillers improve conductivity and increase stiffness, the reason for this effect is not known. Also not understood is the highly desirable decoupling of conductivity and stiffness. This is important because improving conductivity almost always decreases stiffness. We investigated spherical nanofillers with different surface chemistry, and the importance of aspect ratio, specifically when the nanofiller is longer than it is wide. We identified a salt concentration that is crucial to increasing conductivity. This eutectic composition occurs when the liquid and two solid states of the electrolyte are equally favored. It offers the highest conductivity in general, and the largest increase when filled with nanofillers. We propose that a highly conductive solid state is stabilized at the nanofiller surface even when the temperature is high enough to cause melting. The higher aspect ratio nanowhiskers begin to align this solid state and increase conductivity more than spherical nanoparticles. The use of neutrons to determine structure and dynamics at nanoscales was an important part of this project. These experiments must be performed at user facilities. We used facilities at the NIST Center for Neutron Research and the Spallation Neutron Source at Oak Ridge National Laboratory. This project trained two PhD students, one MS student, and one undergraduate researcher. The principal investigator addressed the need for clear scientific writing and speaking through a graduate elective course [taught once and scheduled again in Spring 2015], a summer writing workshop, and talks on hypothesis development and testing, the process of publishing scientific research, and presentations to undergraduates about graduate school and neutron science.
