Site-Selective Substitution by Transition Metal Cations for NaCo2O4 via Ion-Exchange

Abstract

Incorporation of Ni ions into two different sites of the Na and Co sites of NaCo2O4 was carried out by the processes of conventional elemental substitution for Co ion and solid state ion exchange of mobile Na+ ion. The changes of the lattice parameters of “substituted” and “exchanged” samples were different, suggesting that the Ni ions occupy different sites. The Ni-substituted sample for Co showed a significant decrease of Seebeck coefficient keeping the electrical conductivity almost intact, while Ni-exchanged samples for Na showed a lower electrical conductivity than that of the non-substituted sample with keeping the Seebeck coefficient unchanged. These results suggest that Ni cations introduced to the Co site decouples the Co-Co spin exchange, whereas those at the Na site compensates positive holes by providing conduction electrons.