Research

Nanotechnology for Green Rechargeable Batteries

Introduction

The demand for a rechargeable, environmentally benign, lightweight, flexible battery with high energy density is increasing rapidly with advancing technology. Millions of cell phones and laptops containing toxic, liquid batteries that require heavy casings are retired annually. Cadmium is an example of a material found in traditional batteries that is toxic both to the body and the environment. In order to contain this liquid, a heavy battery casing is required, resulting in a battery that accounts for a large portion of the device weight. For example, the battery accounts for approximately 20% of the total weight in a laptop manufactured in 2004. These disadvantages could be reduced by the development of a rechargeable, solid polymer battery composed of benign materials; however, existing electrolyte systems based on lithium ion transport are unable to conduct sufficiently at room temperature. PEO (Figure 1) is the polymer of choice for these electrolyte systems since it is flexible, biodegradable and characterized by a low glass transition temperature [Tg] of -53°C at high molecular weight. Lithium is used as the ion because it is the lightest solid element with the highest oxidation potential, resulting in batteries with more power and a higher energy density. This next-generation battery would not require heavy casings, thereby permitting applications where weight and flexibility are critical.

Current Research

Despite the advantages of PEO, room temperature conductivity is currently too low to be practical due to a sharp decrease in ionic conductivity below 60 ºC. The goal of this research is to combine both experimental and computational results to develop a nano-filled PEO electrolyte that will have sufficient conductivity to function in a rechargeable battery. Previous studies have shown that ionic conductivity depends directly on temperature, and adding nanoparticles increases ionic conductivity at all temperatures. The molecular mechanism responsible for the conductivity increase with nanoparticle addition is not well understood. This research aims to elucidate the mechanism by isolating one part of the nanoparticle–polymer system: the surface effect between the nanoparticle and the polymer electrolyte. It would be difficult to study the interface between a nanoparticle and the surrounding polymer electrolyte due to the geometric and spatial limitations inherent within the study of nano-sized systems; however the interaction can be equivalently represented by a thin film on a substrate of the same material as the nanaoprticle (Figure 2). Both experimental techniques and molecular dynamics simulation will be employed to study how the surface effect alters the ionic conductivity in a PEO:LiClO4 thin film system.

Susan in her clean-room gear

Molecular dynamics simulation of boron oxide (B2O3)

Introduction

Molecular dynamics simulation harnesses current computing technology in an effort to model the motion of molecules on an atomic level. Since the atomic mobility of some materials can be difficult to detect by direct measurements, molecular simulation provide a means to explore the microscopic world otherwise unobservable. Boron oxide, a network glass former, is an example of a material whose structure and atomic mobility are difficult to detect directly. While experimental methods have suggested that 50% to 85% of the structure is comprised of six-membered, planar boroxol rings (Figure 1), simulation results failed to verify this structure for many years. A model recently developed at Penn State accounting for polarizability effects has provided results consistent with experiment. The simulated structure is illustrated in Figure 2.

Current Research: Structural

The nanoparticle of boron oxide illustrated in Figure 2 is comprised of planar boroxol rings at fractions consistent with direct measurements. If the geometry of this nanoparticle was altered by changing the sphere into a cube, would the planar rings aggregate along the flat surfaces of the cube? This type of change in nano-shape has been shown to affect optical properties, mechanical properties, and the glass transition temperature. The objective of this research is to determine if structure is also a function of nano-shape by comparing a spherical nanoparticle (16Å diameter) to cubic nanoparticle (16Å x16Å x16Å) of the same density.

http://www.personal.psu.edu/users/s/k/skf118/index.htm

Publications

S. K. Fullerton and J. K, Maranas "A Molecular Dynamics Study of the Structural Dependence of Boron Oxide Nanoparticles on Shape" Nano Letters 5, 363 (2005).

S.K. Fullerton and J. K. Maranas, "A Molecular Interpretation of Vitreous Boron Oxide Dynamics" J. Chem. Phys. 121, 8562 (2004).