In situ XAS study of Li_xNi_0.7Fe_0.15Co_0.15O₂ cathode material

We have examined the oxidation states and local atomic structures of Ni, Fe, and Co in Li_xNi_0.7Fe_0.15Co_0.15O₂ as a function of Li content during the first charge in a Li//Li_xNi_0.7Fe_0.15Co_0.15O₂ nonaqueous cell. We show that the composition of the material in the pristine state is more accurately described by Li_0.95Ni(II)_0.09Ni(III)_0.66Fe(III)_0.15Co(III)_0.15O₂. Half of the Ni(II) resides in Li-vacant sites. Both Fe and Co substitute for Ni within the NiO₂ slabs with no significant amounts of Fe or Co that can be attributed to Li-vacant sites. The local structure parameters are consistent with oxidation states observed on the basis of the XANES data. The Ni K-edge energy continuously shifts to a higher energy with decrease in Li content due to oxidation of Ni(II) to Ni(III) and Ni(III) to Ni(IV). After the complete oxidation of Ni(III) to Ni(IV), the Fe K-edge energy begins to increase with further decrease in Li content indicating the oxidation of Fe(III) to Fe(IV). The Co K-edge energy at half-height, on the other hand, is unchanged during the whole range of Li deintercalation indicating that no significant change in the oxidation state of Co occurs upon the complete removal of Li.