NICKEL K-EDGE X-RAY-ABSORPTION FINE-STRUCTURE OF LITHIUM NICKEL OXIDES

Lithium nickel oxides, LixNi(1-x)O, with the lithium content x varying from 0.40 (close to the maximum value of 0.5) to 0 have been studied by using nickel K-edge X-ray absorption fine structure (XAFS) spectros copy. The XANES (X-ray absorption near-edge structure) shows several d istinct features whose energy and intensity evolve smoothly as a funct ion of x. Quantitative analysis of the EXAFS (extended XAFS) data conc lusively showed the presence of systematic structural trends with lith ium content, x. In particular, we observed two different Ni-O bond len gths (2.07(1) and 1.94(1) angstrom) with proportions changing with x a nd systematic variations (with x) of the outer-shell Ni-Ni distances. Overall, the structural trends deduced from the XAFS data were found t o be more consistent with an increased nickel oxidation state (Ni2+ to Ni3+) with increasing x rather than with an increased oxygen oxidatio n state (O2- to O-), although the true picture lies between these two extremes.