Thermoelectric properties of (Na,M)xCoO2 (M= Li, K, Cs)ceramics

The (Na,M)xCoO2(M= Li, K, Cs, x= 0.55, 0.89) ceramics was synthesized, and its phase composition, thermal expansion, electrical conductivity and thermo-EMF were investigated. The materials obtained were polyphase ones and theyconsisted of sodium cobaltite (?-NaxCoO2), Co3O4and of other alkaline metals cobaltites (LiCoO2, KCoO2, Cs2CoO3). The temperature dependences of electrical conductivity of the (Na,M)0.55CoO2(M= K, Cs) and (Na,M)0.89CoO2samples had semiconducting character, but (Na,Li)0.55CoO2ones had metallic character. The materials studied were p-type conductors, which thermo-EMF and power factor (P) values rose at temperature increasing, and values of linear thermal expansion coefficient varied within (11.1–14.9) · 10 –6 K –1 . Inserting into Na0.55CoO2ceramics the lithium or potassium oxides increased its power factor, thus, for Na0.28K0.27CoO2compound P1100= 361 ?W ?m –1 ?K –2 , which was 85% higher than for Na0.55CoO2.