A 6Li, 7Li and 59Co MAS NMR study of rock salt type Li xCoO 2 (0.48≤ x≤1.05)

Abstract

Low temperature (LT, 400°C) and high temperature (HT, 900°C) Li x CoO 2 samples were synthesised with Li/Co ratios between 0.8 and 1.05. These samples were characterised by XRD, chemical analysis, 6Li, 7Li and 59Co MAS NMR. 7Li MAS NMR of the Li x CoO 2-T 400 samples (LT) showed a single resonance line at −0.9 ppm, characteristic of lithium in an octahedral coordination. A somewhat narrower resonance at the same position was observed in the 7Li MAS NMR spectra of the Li x CoO 2-T 900 samples synthesised with a Li/Co ratio ≤0.95. The Li x CoO 2-T 900 samples synthesised with a Li/Co ratio >0.95 showed several new lithium resonances, containing up to 35% of the signal intensity. The resonance positions of these new resonances indicate the presence of a paramagnetic cobalt species in the material. The 59Co MAS NMR spectra of the Li x CoO 2-T 900 samples revealed a spinning sideband manifold centred at ≈14 250 ppm for x ≤0.95 and three overlapping spinning sideband manifolds at ≈14 285, ≈14 260 and ≈14 230 ppm for Li/Co ratios >0.95. Compared to the Li x CoO 2-T 900 samples the Li x CoO 2-T 400 samples all showed one resonance with an increased line width. These resonance positions are typical for Co 3+ octahedrally coordinated by oxygen. No quadrupole induced shift is observed as a function of the external field (measurements performed at 14.1 and 7.05 T), indicating an undistorted octahedral coordination of cobalt in the materials studied. Chemical extraction of lithium with H 2SO 4 leads to the formation of lithium that can not be detected with NMR. Electrochemical extraction of lithium, on the other hand, leads to a low field shift of the lithium resonance of 60–120 ppm, depending on the amount of lithium removed. Electrochemical intercalation (discharging) of the samples results in the reappearance of the original lithium spectra, showing the excellent reversibility of the charge/discharge process. Partly charged samples are characterised by signals in the range of 120–60 ppm and a signal at −0.9 ppm, indicating that part of the lithium remains in an unchanged environment. The dependence of the chemical shift on the amount of lithium removed is due to an rapid exchange process.