In situ x-ray absorption spectro-electrochemical study of LiNiO_2 cathode material

摘要

We have used X-ray Absorption Spectroscopy (XAS) to examine the oxidation-state and local structure of Ni in LiNiO_2 cathode material, as a function of state of charge during the charge and discharge cycles, in a non-aqueous cell. Analyses of the Ni K-edge XANES and EXAFS, reveal that the composition of the pristine material is more accurately described by the formula Li_(1-x)Ni_(1+z)O_2, where 2z/(1+z) is the fraction of Ni(II) equally residing in Li sites and the NiO_2 slabs. The value of z was determined to be near 0.14, on the basis of the Ni_O coordination numbers. Upon charging, our results are consistent with oxidation of Ni(II) to Ni(III) for x<=2z and then Ni(III) to Ni(IV) for 2z