Promoting the activity of LiFePO4 (olivine) cathode of Li-ion battery by supramolecular complexes with single walled carbon nanotubes
L. Kavan, I. Exnar, M. Graetzel
J. Heyrovsky Institute of Physical Chemistry, CZ
Keywords: Li-ion batteries, olivine FePO4, carbon nanotubes
Abstract:The amphiphilic Ru-bipyridine complex, NaRu(4-carboxylic acid-4’-carboxylate-2, 2’bipyridine)(4,4’-dinonyl-2,2’bipyridine)(NCS)2 can be used as a surfactant for solubilization of single walled carbon nanotubes (SWNTs) in acetonitrile-t-butanol mixture. The Ru-complex represents a redox functionality of 3.5 V vs. Li/Li+, which is matching almost exactly the formal potential of LiFePO4/FePO4 couple. The supramolecular assembly of SWNT/Ru-bipyridine complex is adsorbed on the surface of LiFePO4 (olivine) via the free carboxylic groups at the bipyridine ligand. This provides a composite material with roughly monolayer coverage by the Ru-bipyridine complex. Electrodes fabricated from the Ru-complex/SWNT/LiFePO4 composite exhibited greatly enhanced activity for electrochemical Li+ extraction/insertion compared to the performance of electrodes made from commercial carbon-coated LiFePO4 or from LiFePO4 derivatized either by adsorption of sole Ru-bipyridine complex or by carbon nanotubes dispersed with the redox inactive pyrene butanoic acid. The SWNT backbone promotes the interfacial charge transfer between LiFePO4 and the Ru-complex, whose redox potentials are closely matching each other. The so called “nanotube wiring” effect is based on a synergic action of adsorbed redox-active molecule, which provides the interfacial charge transfer to the olivine surface and the SWNT, which transports charge to longer distances in the composite.