Coupled Ion/Electron Hopping in LixNiO2: A 7Li NMR Study

Abstract

Deintercalated “Li_xNiO₂” materials (x = 0.25, 0.33, 0.50, 0.58, and 0.65) were obtained using the electrochemical route from the Li_0.985Ni_1.015O₂ and Li_0.993Ni_1.007O₂ compounds. Refinements of X-ray diffraction data using the Rietveld method show a good agreement with the phase diagram of the Li_xNiO₂ system studied earlier in this laboratory. Electronic conductivity measurements show a thermally activated electron-hopping process for the deintercalated Li_0.5NiO₂ phase. In the Li_xNiO₂ materials investigated (x = 0.25, 0.33, 0.50, and 0.58), ⁷Li NMR shows mobility effects leading to an exchanged signal at room temperature. A clear tendency for Li to be surrounded mainly by Ni³⁺ ions with the 180° configuration is observed, particularly, for strongly deintercalated materials with smaller Li⁺ and Ni³⁺ contents, even upon heating, when this mobility becomes very fast in the NMR time scale. This suggests that Li/vacancy hopping does occur on the NMR time scale but that Ni³⁺/Ni⁴⁺ hopping does not occur independently. The position of Li seems to govern the oxidation state of the Ni around it at any time; the electrons follow the Li ions to satisfy local electroneutrality and minimal energy configuration. The observed NMR shifts are compatible with the Li/vacancy and Ni³⁺/Ni⁴⁺ ordering patterns calculated by Arroyo y de Dompablo et al. for x = 0.25 and x = 0.50, but not for x = 0.33 and x = 0.58.