7Li MAS NMR study of electrochemically deintercalated LixNi0.30Co0.70O2 phases:: evidence of electronic and ionic mobility, and redox processes

LixNi0.30Co0.70O2 phases obtained by electrochemical deintercalation have been characterised by XRD, Li-7 MAS NMR, electronic conductivity and thermoelectronic power measurements. Ni3+ oxidation to Ni4+ occurs at the onset of deintercalation, leading to Ni3+/Ni4+ hopping which causes an exchange of the Li-7 NMR signals, characteristic of a contact shift due to paramagnetic low spin Ni3+. Depending on composition and temperature, the presence or absence of lithium ions, which only interact with diamagnetic Co3+ ions, allows discrimination between an electronic and an ionic exchange phenomenon. For x close to 0.70, Ni/Co hopping (Ni4+ + Co3+ <----> Ni3+ + Co4+) is evidenced, which makes the question as to which ion is actually oxidised during deintercalation somewhat irrelevant. Finally, for x < 0.70, the presence of Ni ions hinders true long range electronic delocalisation, but low spin Co4+ ions are clearly not localised, as evidenced by the NMR data and the thermoelectronic power coefficient.