On the Li_xCo_(1-y)Mg_yO_2 system upon deintercalation: electrochemical, electronic properties and ~7Li MAS NMR studies

摘要

A detailed characterization of the structural modifications and redox processes occurring upon lithium deintercalation from the Li_(x0)Co_(1-y)Mg_yO_2 materials (x_0 = 1.0 and 1.10; y = 0.0, 0.03, 0.05 and 0.06) was performed in order to determine the effect of Mg doping on the cycling properties. Using electrochemical tests, X-ray diffraction (XRD), ~7Li MAS NMR and electrical properties measurements, we show that the Li_xCO_(1-y)Mg_yO_2 system exhibits a solid solution existing in the whole deintercalation range studied (0.30 ≤ x ≤ 1.0). These phases exhibit reversible capacities equivalent to that of LiCoO_2 upon cycling with a good structural stability. Moreover, the ~7Li MAS NMR study shows that the structural defects (O vacancies and intermediate spin Co~(3+) ions) which are present in the starting Mg-doped phases govern the electronic properties upon lithium deintercalation. Indeed, regardless of the presence of Mg ions in the structure, a behavior similar to that of the Li_xCoO_2 (1 ≤ x_0 ≤ 1-08) system is evidenced with an insulator to metal transition taking place at the microscopic scale.