Dopant incorporation site in sodium cobaltate's host lattice: A critical factor for thermoelectric performance

TL;DR

This study uses density functional calculations to determine how Sr and Sb dopants incorporate into Na_xCoO_2, revealing their specific lattice sites and effects on thermoelectric properties, which is crucial for optimizing thermoelectric performance.

Contribution

It provides the first detailed computational analysis of dopant site preferences in Na_xCoO_2 and their impact on electronic and thermoelectric properties.

Findings

Sr dopants occupy Na layers, improving crystallinity and carrier mobility.

Sb dopants substitute Co ions, reducing electrical conductivity.

Dopant site determines the thermoelectric performance modifications.

Abstract

$Na_xCoO_2$ that comprises of alternating Na and $CoO_2$ layers has exotic magnetic and thermoelectric properties that could favorably be manipulated by adding dopants or varying Na concentration. In this work, we investigated the structural and electronic properties of Sr and Sb doped $Na_xCoO_2$ ($x$ = 0.50; 0.625; 0.75 and 0.875) through comprehensive density functional calculations. We found that Sr dopants always occupy a site in the Na layer while Sb dopants always substitute a Co ion in the host lattice regardless of Na concentration. This conclusion withstood when either generalized gradient approximation (GGA) or GGA+$U$ method was used. By residing on the Na layer, Sr dopants create charge and mass inertia against the liquid like Na layer, therefore, improving the crystallinity and decreasing the electrical resistivity through better carrier mobility. On the other hand, by substituting Co ions, Sb dopants reduce the electrical conductivity and therefore decrease the Seebeck coefficient.