Electronic Structure of Lithium Battery Materials

Abstract

Lithium batteries are important as the power source for portable electronic devices and could also be used in hybrid vehicles with improvements in capacity. We have used first principles calculations of electronic structure to determine how charge is redistributed as Li is added or removed. In the final of year of the project we have examined Lix(NiMn)0.5O2 and Lix(NiMnCo)0.333O2 cathode materials in more detail. As lithium is removed electrons are removed from the valence band which is mainly Oxygen 2p states at the top of the band. There is very little change in the charge state of the transition element ions in either case. These results are confirmed by electron energy loss spectroscopy which shows a pre-peak on the oxygen K edge as lithium is removed, but no changes in the transition metal L edges. We have also investigated the LixFePO4 cathode material which is less costly than the LixCoO2 used at present and is also less damaging to the environment. In this case we find that as lithium is removed there is a change in charge state of iron while electrons are removed from oxygen 2p states at the top of the valence band. Again this is confirmed by the pre-peak on the oxygen K edge observed in electron energy loss spectroscopy and by the high resolution multiplet structure observed on the iron L edge which agrees with our calculations.