Charging of lithium cobalt oxide battery cathodes studied by means of magnetometry

• Magnetic measurements on LixCoO2 cathodes with precise background correction
• Battery charging/discharging monitored by in-operando magnetometry.
• Anderson-type of nonmetal–metal transition deduced from χ0 − x variation.
• Structural reordering for x ≤ 0.55 observed by magnetometry and positron annihilation.
• Complex oxidation behavior involving O deduced from observed Co-oxidation state.

The variation of the structural and electronic properties of LixCoO2 upon electrochemical Li-extraction was studied over wide range concentration of 1 ≥ x ≥ 0.20 by means of SQUID magnetometry. The ex-situ measurements performed for 13 different compositions were supplemented by operando measurements of the magnetic moment during repetitive electrochemical in-situ cycling of the Li-concentration. From the temperature-dependent measurements an effective magnetic susceptibility with Curie–Weiss behavior and an additional temperature-independent part due to Pauli and Van Vleck magnetism is derived. The increase of the temperature-independent susceptibility with Li-extraction reflects a concomitant increase of the electronic density of states and, in addition, indicates an Anderson-type of the occurring nonmetal–metal transition. The effective magnetic moment reveals that only a fraction of 30% of the charge is transferred to Co upon Li-extraction indicating a complex oxidation behavior involving oxygen. Exposure to ambient atmosphere gives rise to a complete oxidation of Co. The results on the structural variation with Li-concentration are compared with accompanying measurements by X-ray diffraction and by our recent defect studies by positron annihilation.