# Restoration of long range order of Na ions in $Na_xCoO_2$ at high temperatures by sodium site doping

## TL;DR

This study uses density functional theory to analyze how doping with various elements affects the sodium ion order and thermoelectric properties of Na_xCoO_2, revealing that dopant site choice influences electrical performance.

## Contribution

It provides a systematic theoretical investigation of dopant effects on Na_xCoO_2, highlighting the impact of dopant site substitution on thermoelectric performance and resistivity.

## Key findings

- Dopants like Sn, W, Bi substitute Co; Au substitutes Na.
- Doping at Na sites enhances thermoelectric performance.
- Resistivity decreases with Na-site doping despite lower hole concentration.

## Abstract

We have systematically investigated the $Na_xCoO_2$ system doped with Cu, Y, Sn, W, Au and Bi for $x$ = 0:5; 0:75 and 1.00 using density functional theory. Sn, W, and Bi always substitute a Co while Au always substitutes a Na regardless of Na concentration. However, for Cu and Y, the substitution site depends on Na concentration. When compared to the available experimental data, we find that thermoelectric performance is enhanced when the dopants substitute a Na site. In this case, surprisingly, resistivity decreases despite the reduced hole concentration caused by carrier recombination. We propose improved carrier mobility to be the cause of observed reduced resistivity.