Effect of cobalt substitution on Li(3-2x)Co(x)N local structure: a XAS investigation.

Abstract

The influence of cobalt substitution on the local structural changes around Co atoms in the layered lithium nitridocobaltates Li(3-2x)Co(x)N for 0.05 ≤ x ≤ 0.44 is investigated using Co K-edge X-ray absorption spectroscopy (EXAFS and XANES). The Co-N bond length in Li(3-2x)Co(x)N compounds is obtained vs x by performing EXAFS fitting and found to be shorter (1.80 Å) than for x = 0 (Li3N), and its value does not change with x. A comparison of EXAFS data with XRD results is discussed. We show that the continuous decrease of interlayer distance versus Co content (x), described from XRD data, accounts for an average of the Co-N and Li-N distances, weighted by the number of these bond lengths. In addition, the present work supports the proposal that the Li1b-N bonds contract with x due to a significant increase of Coulombic attractive forces locally induced by the progressive Li(+)/Co(2+) substitution. XRD studies suggested that divalent Co ions bond to two nitrogen in Li(3-2x)Co(x)N. Although additional works are still needed to prove its valence, the present XAFS findings complements the local structure found by XRD, in good accord with the electrochemical properties previously reported.