Electrochemical aspects of the deintercalation of layered AMO2 compounds

The electrochemical deintercalation of AMO2 compounds, where A = Li or Na, and M = Cr, Co or Fe, was investigated. The open circuit voltage of A/1M AClO4-propylene carbonate/AxMO2 cells and the chemical diffusion constant, D, of alkali in AxMO2 were measured using a current-pulse relaxation technique. The cells made from lithium compounds showed an OCV of about 4 V. The voltage increased continuously with charging. The cell OCV of sodium compounds increased discontinuously in the range 2.4 – 3.2 V. The discontinuity was related to the change of MO2 layer stacking in the deintercalation process. The highest diffusivity of D (Na) = 5 × 10−8 cm2 s−1 was observed in NaxCoO2 (0.5 < x < 0.6). Diffusion constants were larger for trigonal prismatically surrounded sodium than for that octahedrally coordinated. Lithium diffused faster than sodium in the compounds having the same kind of transition metal in an octahedral environment.