  We have systematically investigated the $Na_xCoO_2$ 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 substitutes 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.
