  The development of high-performance sodium (Na) ion batteries requires
improved electrode materials. The energy and power densities of Na superionic
conductor (NaSICON) electrode materials are promising for large-scale energy
storage applications. However, several practical issues limit the full
utilization of the theoretical energy densities of NaSICON electrodes. A
pressing challenge lies in the limited sodium extraction in low Na content
NaSICONs, e.g., $\rm Na_1V^{IV}V^{IV}(PO_4)_3 \leftrightarrow
V^{V}V^{IV}(PO_4)_3 + 1e^- + 1Na^+$. Hence, it is important to quantify the
Na-ion mobility in a broad range of NaSICON electrodes. Using a kinetic Monte
Carlo approach bearing the accuracy of first-principles calculations, we
elucidate the variability of Na-ion transport vs. Na content in three important
NaSICON electrodes, Na$_{\rm x}$Ti$_{2}$(PO$_{4}$)$_{3}$, Na$_{\rm
x}$V$_{2}$(PO$_{4}$)$_{3}$, and Na$_{\rm x}$Cr$_{2}$(PO$_{4}$)$_{3}$. Our study
suggests that Na$^+$ transport in NaSICON electrodes is almost entirely
determined by the local electrostatic and chemical environment set by the
transition metal and the polyanionic scaffold. The competition with the
ordering-disordering phenomena of Na-vacancies also plays a role in influencing
Na-transport. We link the variations in the Na$^+$ kinetic properties by
analyzing the competition of ligand field stabilization transition metal ions
and their ionic radii. We interpret the limited Na-extraction at $x = 1$
observed experimentally by gaining insights into the local Na-vacancy
interplay. We propose that targeted chemical substitutions of transition metals
disrupting local charge arrangements will be critical to reducing the
occurrence of strong Na$^+$-vacancy orderings at low Na concentrations, thus,
expanding the accessible capacities of these electrode materials.
