Power factor enhancement in Na_xCoO₂ doped by Bi

Polycrystalline materials of Na_0.74-xBi_xCoO ₂ (0 ≤ x ≤ 0.3) have been prepared by combining solid state reaction with sintering process. The effects of partial substitution of Bi for Na-site on the thermoelectric performance of Na_0.74CoO₂ have been investigated. The optimal doping level was achieved when x = 0.15, at which the power factor is 3.6 times as large as that of the non-substituted one. The results indicate that the electrical conductivity and Seebeck coefficient of Bi-doped sample (x = 0.15) increase dramatically (two times and 24.6%, respectively) compared with the free-doped sample, which may be attributed to the adjustment of crystal structure and carrier mobility.