Electronic structure of Li-intercalated oligopyridines: a comparative study by photoelectron spectroscopy

The role of nitrogen in the charge transfer and storage capacity of lithium-intercalated heterocyclic oligophenylenes was investigated using photoelectron spectroscopy. The development of new occupied states at low binding energies in the valence band region, as well as core level chemical shifts at both carbon and nitrogen sites, demonstrates partial charge transfer from lithium atoms to the organic component during formation of the intercalated compound. In small compounds, i.e., biphenyl and bipyridine derivatives, the position of the nitrogen heteroatom significantly affects the spacing between gap states in the Li-intercalated film; yet it has minimal effects on the charge storage capacity. In larger, branched systems, the presence of nitrogen in the aromatic system significantly enhances the charge storage capacity while the Li-N bond strength at high intercalation levels is significantly weakened relative to the nitrogen-free derivative. These observations have strong implications towards improved deintercalation processes in organic electrodes in lithium-ion batteries.