XAS investigation of inhomogeneous metal-oxygen bond covalency in bulk and surface for charge compensation in Li-ion battery cathode Li[Ni _1/3Co_1/3Mn_1/3]O₂ Material

Layered Li[Ni_1/3Co_1/3Mn_1/3]O₂ was prepared by ultrasonic spray pyrolysis. The discharge capacity increases linearly with increases of the upper cutoff voltage limit. Layered Li[Ni _1/3Co_1/3Mn_1/3]O₂ 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[Ni_1/3Co _1/3Mn_1/3]O₂ 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 Ni ⁴⁺, 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.