[2] It conducts like a metal, and has exceptional thermoelectric properties (for 0.5 ≤ x ≤ 0.75) combining a large Seebeck coefficient with low resistivity, as discovered in 1997 by Ichiro Terasaki's research group.

Layers of monovalent sodium cations (Na+) alternate with two-dimensional anionic sheets of cobalt and oxygen atoms.

The diffusion rate of the ions, plotted as a function of x, shows sharp dips (from about 10−7 to 10−10 cm2/s at ambient temperature) at values of x that correspond to particular regular arrangements, namely 1/3, 1/2, and 5/7.

The figure of merit Z = S/ρκ (where ρ is the in-plane resistivity and κ is the thermal conductivity) is maximum for x about 0.89 at about 65 K.[4] The fully reduced compound NaCoO2 can be prepared by dissolving stoichiometric amounts of sodium acetate C2H3O2Na and cobalt tartrate C4H4O6Co in ethanol with a gelling agent, drying and calcinating the resulting gel, and annealing it at 650 °C.

[6] The compound NaxCoO2 with x around 0.8 can be obtained by treating a mixture of sodium carbonate Na2CO3 and cobalt(II,III) oxide Co3O4 at 850–1050 °C.

[3] Higher values of x can be obtained by immersing thermally grown crystals of Na0.71CoO2 in a hot solution prepared from sodium metal and benzophenone in tetrahydrofuran for several days at 100 C.[4]