Lithium deintercalation in LiNi0.30Co0.70O2: Redox processes, electronic and ionic mobility as characterized by7Li MAS NMR and electrical properties

Abstract

ABSTRACTLiNi1-yCoyO2 materials are particularly interesting as positive electrode in lithium-ion batteries. Their 7Li MAS NMR spectra are sensitive to the presence of paramagnetic Ni3+ cations as nearest and next-nearest neighbors. These contact shift (or Fermi contact) interactions are used to yield information on the lithium environment in terms of paramagnetic cations.Electrochemically deintercalated LixNi0.30Co0.70O2 phases have been characterized by XRD, 7Li MAS NMR, electronic conductivity and thermoelectronic power. Ni3+ oxidation to Ni4+ occurs at the beginning of deintercalation, leading to a Ni3+/Ni4+ hopping which causes an exchange of the 7Li NMR signals of the Li ions interacting with nickel. For higher deintercalation amounts, the signal due to Li ions with only cobalt as first and second neighbor is also involved in the exchange upon heating, showing the onset of ionic hopping. For x close to 0.70, an increase of the NMR shift with temperature is observed, which is assigned to a hopping between Ni4+ and Co3+. Therefore, the question as to which ion is actually oxidized during deintercalation is somewhat irrelevant around this composition. Finally, for x < 0.70, the presence of Ni hinders a true long-range electronic delocalisation between Co4+/Co3+ (LS) as it happens in LixCoO2(x < 0.70). However, electrons are clearly delocalized in restricted zones, as seen from NMR and thermoelectronic power coefficient.