PEM Fuel Cell Bipolar Plates.- Basic Applications of the Analysis of Variance and Covariance in Electrochemical Science and Engineering.- Nanomaterials in Li-Ion Battery Electrode Design.- Direct Methanol Fuel Cells: Fundamentals, Problems and Perspectives.- Review of Direct Methanol Fuel Cells.- Direct Numerical Simulation of Polymer Electrolyte Fuel Cell Catalyst Layers.
"3 Nanomaterials in Li-Ion Battery Electrode Design (p. 75-76)
Charles R. Sides and Charles R. Martin*
I. INTRODUCTION
Li-ion batteries have generated great interest as lightweight, portable, rechargeable power sources over the last decade. Their introduction in 1990 by T. Nagaura and K. Tozawa of SonyTec Inc. fueled the explosion of personal electronic devices. Li-ion batteries are now the power source of choice for laptops, cell phones, and digital cameras. The public has quickly embraced this technology, which accounts for an approximately $3 billion annual market. 2 Despite (or perhaps as a result of) the commercial success of these batteries, a global research initiative exists to improve the existing design.
The goal of which is to apply this technology to more demanding and exotic uses, such as the electric component of hybrid vehicles, low-temperature applications, and power supplies for MEMs. However, the current design cannot adequately satisfy the power requirements of such systems, due to the inability to deliver a sufficient quantity of charge at high discharge currents. 3 This chapter will detail the efforts of laboratories, ours in particular, to incorporate the field of nanomaterials to improve upon Li-ion batteries.
Li-ion batteries operate by reversibly intercalating charge in each of two electrodes. Intercalation is the process by which a guest species (Li+) is able to reversibly enter/exit a host structure, causing little or no difference to the lattice of the host. These electrodes are separated by an ion-conductive electrolyte. Upon discharge, the Li-ions deintercalate from the low-potential electrode, migrate through the electrolyte, and insert into the highpotential electrode. The ions then rely on solid-state diffusion to fill the non-surface intercalation sites.
Obeying the governing laws of charge neutrality, electrons compensate for the movement of the ions. If current flow is reversed (from cathode to anode), Li-ions insert into the low-potential electrode and the system is charged. The low-potential electrode is the anode and the high-potential electrode is the cathode. This convention (adopted from the discharge process) is obeyed regardless of the direction of current flow. In the analysis of a battery system, both the ionic conductivity and electronic conductivity must be considered. Nanomaterials are advantageous in both regards.
The Martin research group has pioneered the nanofabrication strategy of template synthesis." This general method has been used to synthesize nanostructures of a variety of materials such as gold 5-8 carbon 9-11 semiconductors 12,13 polymers 14,15 and Li-ion battery electrodes,II,13,16-28 our focus here. In general, this method involves deposition of a precursor material into a micro- or nanoporous template. This template is typically commercially available track-etch polymer filters or anodized alumina, though others have been demonstrated. Depending on both the porediameter and the specific chemical interactions between the pore wall and the precursor, the resulting structures may be tubes (hollow) or wires (solid). These structures are referred to as "nano", if one or more of their dimensions are on the nanoscale « 100 nm). The aspect ratio (length / width), though, is often on the order of 10.
In this embodiment of Li-ion electrodes, a precursorimpregnated polycarbonate template membrane is attached to a section of metal foil. The foil has dual-functionality as it serves as a substrate during synthesis and as a common current collector during electrochemical characterization. The precursor is processed (typically, by aging or heating) into the desired product. Often in the case of battery materials, the template is then removed by plasma etching or dissolution. The result is an electrode that consists of structures that mirror the geometry (length, diameter, and number density) of the pores of the template."
