XAS investigation of inhomogeneous metal-oxygen bond covalency in bulk and surface for charge compensation in li-ion battery cathode Li[Ni1/3Co1/3Mn1/3]O-2 material

Layered Li[Ni1/3Co1/3Mn1/3]O-2 was prepared by ultrasonic spray pyrolysis. The discharge capacity increases linearly with increases of the upper cutoff voltage limit. Layered Li[Ni1/3Co1/3Mn1/3]O-2 attained a high discharge capacity of over 200 mAh g(-1) between 2.8 and 4.6 V with an excellent cyclability. An in situ transition metal (TM) K-edge X-ray absorption spectroscopic (XAS) study was carried out to investigate the electronic and the local structures of the delithiated-relithiated Li[Ni1/3Co1/3Mn1/3]O-2 system compared with surface-sensitive soft XAS of oxygen K-edge and TM L-edges. The bulk-sensitive TM K-edge XAS analysis shows that the charge compensation is made through the formation of Ni2+<-> Ni4+ and Co3+<-> Co4+ during the delithiation-relithiation process. However, the soft TM L-edges XAS indicate that the Ni2+ in the electrochemical untreated sample prefers to be oxidized to Ni3+, not Ni4+, in the surface region of delithiated material. The bond covalency between TM and oxygen, especially the oxygen 2p hole state by ligand-to-metal charge transfer, plays an important role in the charge compensation. The TM K- and L-edge XAS results suggest that an inhomogeneous chemical state was induced along direction of electrochemical reaction. (c) 2005 The Electrochemical Society. All rights reserved.