Restoration of long range order of Na ions in NaxCoO2 at high temperatures by sodium site doping

• We studied the formation energy of Cu, Y, Sn, W, Bi and Au dopants in NaxCoO2.
• Thermoelectric performance is best enhanced for dopants substituting Na.
• This is a result of improved carrier mobility caused by dopants substituting Na.

We have systematically investigated the NaxCoO2 system doped with Cu, Y, Sn, W, Au and Bi for x = 0.5, 0.75 and 1.00 using density functional theory. Sn, W and Bi always substitute a Co while Au always substitute a Na regardless of Na concentration. However, for, Cu and Y, the substitution site depends on Na concentration. When compared to the available experimental data, we find that thermoelectric performance is enhanced when the dopants substitute a Na site. In this case, surprisingly, resistivity decreases despite the reduced hole concentration caused by carrier recombination. We propose improved carrier mobility to be the cause of observed reduced resistivity.

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