|Cyclic voltammograms (CV) of BMABP-derivatized LiFePO4 (red, blue) and mesoscopic Al2O3 (green) electrodes. The black line shows the curve of a bare LiFePO4 electrode. Click to enlarge.|
Michael Grätzel (of the Grätzel Cell, a dye-sensitized photoelectrochemical cell) and colleagues at the Swiss Federal Institute of Technology in Lausanne (École Polytechnique Fédérale de Lausanne, EPFL), are researching the use of a molecular charge transport layer to reduce the amount of carbon required in lithium-ion battery cathodes and open up the possibility of much improved energy storage density.
Their work will be published in the 4 April print issue of the Journal of the American Chemical Society, and is currently published online.
he cathodes (positive electrodes) in lithium-ion batteries usually contain large amounts of carbon, graphite or other conductive material to improve conductivity. In some cells, the conductive additives occupy practically half the volume of the active materials, greatly decreasing the energy density of the cell.
This situation is even more severe for the new generation cathodic materials, olivine-type LiMPO4 (M = Fe, Co, Ni, Mn), because of their extremely low electronic conductivity (s, ~10-9 S cm-1). For this reason, great endeavors are being made to increase the conductivity of the cathode materials, for example, carbon coating and supervalent cation doping. However, so far the success of these strategies has been very limited, a large amount of carbon still being needed to afford reasonable battery performance.
The EPFL team had been working with self-assembled molecular charge transport layers on mesoscopic oxide films. In this work, they applied such molecular charge transport layers to electrochemically address insulating battery materials.
The EPFL team took a lithium iron phosphate cathode material (LiFePO4)—one of the most thermally stable li-ion cathode materials, but hampered by lower voltage and lower energy—and created a derivative material with a monolayer of 4-[bis(4-methoxyphenyl)amino]benzylphosphonic acid (BMABP).
The findings demonstrated that a single molecular layer of a suitable redox-active molecule alone can provide the desired electronic charge transport while still permitting lithium-ion exchange to occur rapidly across the solid/electrolyte interface.
As compared to the total electrode size, the space occupied by the molecular charge transport layer is negligibly small, which greatly reduces the volume of conductive additive, opening up the possibility to increase substantially the energy storage density and rate capability at equal amounts of loading of conductive additives.
“Molecular Wiring of Insulators: Charging and Discharging Electrode Materials for High-Energy Lithium-Ion Batteries by Molecular Charge Transport Layers”; Qing Wang, Nick Evans, Shaik M. Zakeeruddin, Ivan Exnar, and Michael Grätzel; J. Am. Chem. Soc., ASAP Article 10.1021/ja066260j S0002-7863(06)06260-3