Simultaneously enhanced thermoelectric power and reduced resistivity of NaxCo2O4 by controlling Na nonstoichiometry

Abstract

Electrical resistivity (ρ) and thermoelectric power $\mathrm{(S)}$ were measured for ${\mathrm{Na}}_x{\mathrm{Co}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}}$ samples with various Na contents $\mathrm{(x).}$ A highly precise control of Na nonstoichiometry was facilitated by optimizing the synthesis procedure. With increasing $\mathrm{x,}$ the absolute value of ρ monotonically decreased, while the value of $S$ increased. For the $\text{x=1.5}$ sample, $S$ reached a value as high as $\text{∼120\hspace{0.05em}μ}\mathrm{V/K}$ (at 300 K).