In situ XAS study of LixNi0.7Fe0.15Co0.15O2 cathode material

We have examined the oxidation states and local atomic structures of Ni, Fe , and Co in LixNi0.7Fe0.15Co0.15O2 as a function of Li content during the f irst charge in a Li//LixNi0.7Fe0.15Co0.15O2 nonaqueous cell. We show that t he composition of the material in the pristine state is more accurately des cribed by Li0.95Ni(II)(0.09)Ni(III)(0.66)Fe(III)(0.15)Co(III)(0.15)O-2. Hal f of the Ni(II) resides in Li-vacant sites. Both Fe and Co substitute for N i 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 consiste nt 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 cont ent 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 han d, is unchanged during the whole range of Li deintercalation indicating tha t no significant change in the oxidation state of Co occurs upon the comple te removal of Li.