Doping is considered to be the main method for improving the thermoelectric performance of layered sodium cobaltate (Na$_{1-x}$CoO$_2$). However, in the vast majority of past reports, the equilibrium location of the dopant in the Na$_{1-x}$CoO$_2$s complex layered lattice has not been confidently identified. Consequently, a universal strategy for choosing a suitable dopant for enhancing Na$_{1-x}$CoO$_2$s figure of merit is yet to be established. Here, by examining the formation energy of Gd and Yb dopants in Na$_{0.75}$CoO$_2$ and Na$_{0.50}$CoO$_2$, we demonstrate that in an oxygen poor environment, Gd and Yb dopants reside in the Na layer while in an oxygen rich environment these dopants replace a Co in CoO$_2$ layer. When at Na layer, Gd and Yb dopants reduce the carrier concentration via electron-hole recombination, simultaneously increasing the Seebeck coefficient ($S$) and reducing electric conductivity ($sigma$). Na site doping, however, improves the thermoelectric power factor (PF) only in Na$_{0.50}$CoO$_2$. When replacing a Co, these dopants reduce $S$ and PF. The results demonstrate how thermoelectric performance critically depends on the synthesis environment that must be fine-tuned for achieving any thermoelectric enhancement.
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