Original Layered OP4-(Li,Na) x CoO2 Phase: Insights on Its Structure, Electronic Structure, and Dynamics from Solid State NMR

The OP4-(Li/Na)_xCoO₂ phase is an unusual lamellar oxide with a 1:1 alternation between Li and Na interslab spaces. In order to probe the local structure, electronic structure, and dynamics, ⁷Li and ²³Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was performed in complementarity to X-ray diffraction and electronic and magnetic properties measurements. ⁷Li MAS NMR showed that NMR shifts result from two contributions: the Fermi contact and the Knight shifts due to the presence of both localized and delocalized electrons, which is really unusual. ⁷Li MAS NMR clearly shows several Li environments, indicating that, moreover, Co ions with different local electronic structures are formed, probably due to the arrangement of the Na⁺ ions in the next cationic layer. ²³Na MAS NMR showed that some Na⁺ ions are located in the Li layer, which was not previously considered in the structural model. The Rietveld refinement of the synchrotron XRD led to the OP4-[Li_0.42Na_0.05]Na_0.32CoO₂ formula for the material. In addition, ⁷Li and ²³Na MAS NMR spectroscopies provide information about the cationic mobility in the material: Whereas no exchange is observed for ⁷Li up to 450 K, the ²³Na spectrum already reveals a single average signal at room temperature due to a much larger ionic mobility.