Engineering a light, flexible and environmentally-friendly lithium-ion battery
We use neutron scattering to investigate solid polymer electrolytes for rechargeable lithium-ion batteries. The problem with solid polymer electrolytes is that room temperature conductivity is too low to power a portable device. [For more information on solid polymer electrolytes visit the student pages]. By understanding the molecular-level mechanism driving conductivity, we can modify the electrolyte to increase conductivity to a practical level.
Neutron scattering is a tool that allows us to investigate both the mobility of the polymer and structures within the electrolyte on time scales (picoseconds – nanoseconds) and length scales (angstroms – nanometers) relevant to this problem. Polymer mobility is thought to be important because it likely drives lithium-ion mobility - the mobility of the lithium-ion determines the conductivity of the electrolyte. The structure of the electrolyte is important because the presence of structures such as channels could possibly provide a pathway to speed-up lithium ion mobility.
Polymer mobility can be measured directly using neutron scattering because the hydrogen atoms in the polymer scatter neutrons much more strongly than any other element. The only species in the electrolyte that contains hydrogen is the polymer, therefore we are able to directly measure its mobility in the presence of other species. We use DCS and HFBS to measure polymer mobility.
The structure of the electrolyte is measured using SANS based on the neutron scattering contrast between the polymer and the other species in the system. The other species can include lithium salts, lithium ions, or in some cases, an oxide nanoparticle additive.
We are using Molecular Dynamics (MD) simulations to study how lithium ion transports in solid state electrolytes. MD simulations provide an evolution in time of the positions of all atoms. Although the mobility of Li+ may be directly measured using the dielectric and Li-NMR techniques, this does not reveal the exact path these ions take. Do they move continuously from one ion cluster to another? Do they leave an anion only to revisit it shortly thereafter? How long do Li+ ions coordinate with a given anion or ion cluster? All these questions are readily addressed by simulation, but not by any other method.
Protein aggregation: Alzheimer’s and Parkinson’s disease
There is a huge interest in deriving energy from plant-based biomaterials as the United States endeavors to achieve energy independence. Lignocellulose, the core structural material of plant bodies, is a cheap, sustainable and self-maintaining source of photosynthetically collected solar energy. Conversion of this trapped solar energy to portable fuels has attracted tremendous research interest. The Center for Lignocellulose Structure and Formation (CLSF) at The Pennsylvania State University was established to enhance the basic understanding of lignocellulose synthesis, assembly and structural properties. As a part of this center, we are working to determine the nano-scale structure of model cellulose systems prepared under various conditions using Small Angle Neutron Scattering (SANS). Also we are investigating the dynamics of confined water present in the cellulose matrix in attempt to understand the effect of cellulose structure and crystallinity on cellulose-water interactions. The dynamics of water in cellulose matrix is primarily being studied using Quasi-Elastic Neutron Scattering (QENS). Also, we are employing Dielectric Spectroscopy, Nuclear Magnetic Resonance (NMR) and Differential Scanning Calroimetry (DSC) methods to obtain complimentary information regarding the state and dynamics of water in cellulose matrices. The knowledge of cellulose structure and confined water dynamics shall help in linking its nano-scale structure to the macroscopic properties which will potentially help in engineering efficient pathways to decompose cellulosic biomass into biofuels. For more information visit www.lignocellulose.org
Computational Modeling of Electrochemical Interfaces
In the design and functionality of devices such as proton exchange membrane fuel cells (PEMFCs), the interaction between the electrode and electrolyte plays a critical role. It is at the electrode surface that the desired reactions, such as the oxygen reduction reaction (ORR), take place. Transport of reactive species to the surface and the overall reactions kinetics are greatly affected by the interfacial structure. However, there has been little research conducted in understanding the structure of this interface, and how it reacts to different environmental conditions. In our group, we are combining molecular dynamics (MD) simulations with quantum mechanical (QM) calculations to elucidate this structure. The goal is to design a new simulation technique capable of analyzing the important phenomena at electrochemical interfaces, such as chemical reactions and charge polarizability. With this understanding, reactions such as the ORR in PEMFCs can be analyzed more completely and experimental conditions can be suggested from a more fundamental standpoint. This research project has been supported by the National Science Foundation and is conducted in collaboration with the Mike Janik Research Group.
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Department of Chemical Engineering, The Pennsylvania State University