Soft X-ray absorption spectroscopic study of a LiNi0.5Mn0.5O2 cathode during charge

Soft X-ray (200 to 1000 eV) absorption spectroscopy at the O K-edge and the metal L-II,L-III-edges, in both the fluorescence yield (FY) and the partial electron yield (PEY) mode, has been used to probe the electronic structure of electrochemically deintercalated Li1-xNi0.5Mn0.5O2. FY and PEY spectra of the transition metal L-II,L-III-edges, indicated that Mn ions remain mostly unchanged in the Mn4+ state at all levels of charge. However, the Ni FY L-edge spectra show a continuous shift to higher energy during charge, but remain mostly unchanged in the PEY data. The results of the FY data show that the Ni ions in the bulk are oxidized form Ni2+ to Ni4+ during charge. The difference between the surface-sensitive PEY data and the bulk-sensitive FY data indicates that the surface of Li1-xNi0.5Mn0.5O2 has a different electronic structure than the bulk. The shift in the O K-edge to lower energies and the development of a shoulder on the low energy side of the first pre-edge peak indicates that the holes compensating the lithium ion deintercalation are located in O 2p states as well as Ni 3d states. These results show that soft X-ray absorption is a powerful technique for studying the electronic structure of new battery materials and it provides unique complementary information to that obtained from hard X-ray (above 1000 eV) absorption studies at the transition metal K-edges. (C) 2004 The Electrochemical Society.