Influence of O-Co-O layer thickness on the thermal conductivity of Na_xCo₂O₄ studied by positron annihilation

Nominal stoichiometric Na_xCo₂O₄ (x = 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0) polycrystals were synthesized by a solid-state reaction method. They were further pressed into pellets by the spark plasma sintering. The crystal structure and morphology of Na_xCo₂O₄ samples were characterized by X-ray diffraction and scanning electron microscopy measurements. Good crystallinity and layered structures were observed for all the samples. Positron annihilation measurements were performed for Na_xCo₂O₄ as a function of Na content. Two lifetime components are resolved. τ₁ is attributed mainly to positron annihilation in the O-Co-O layers and shifts to Na layers only in the H3 phase. The second lifetime τ₂ is due to positron annihilation in vacancy clusters which may exist in the Na layers or grain boundary region. The size of vacancy clusters grow larger but their concentration decreases with increasing Na content in the range of 1.0 < x < 1.8. The thickness of O-Co-O layer also shows continuous increase with increasing Na content, which is reflected by the increase of τ₁. The thermal conductivity κ, on the other hand, shows systematic decrease with increasing Na content. This suggests that the increasing spacing of O-Co-O layer could effectively reduce the thermal conductivity of Na_xCo₂O₄