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

Abstract

Li_xNi_0.30Co_0.70O₂ phases obtained by electrochemical deintercalation have been characterised by XRD, ⁷Li 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 ⁷Li 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.