Selecting the suitable dopants: electronic structures of transition metal and rare earth doped thermoelectric sodium cobaltate

Engineered Na0.75CoO2 is considered a prime candidate to achieve high efficiency thermoelectric systems to regenerate electricity from waste heat. In this work, three elements with outmost electronic configurations, (1) an open d shell (Ni), (2) a closed d shell (Zn), and (3) an half fill f shell (Eu) with a maximum unpaired electrons, were selected to outline the dopants' effects on electronic and crystallographic structures of Na0.75CoO2. Systematic ab initio density functional calculations showed that the formation energy of these dopants was found to be lowest when residing on sodium layer and ranked as -1.1 eV, 0.44 eV and 3.44 eV for Eu, Ni and Zn respectively. Furthermore Ni was also found to be stable when substituting Co ion. As these results show great harmony with existing experimental data, they provide new insights into the fundamental principle of dopant selection for manipulating the physical properties in the development of high performance sodium cobaltate based thermoelectric materials.