  Designing high potential cathodes for Na-ion batteries, which are comparable
in performance to Li-ion cathodes, remains a challenging task. Through
comprehensive density functional calculations, we disentangle the relationship
between the cathode potential and the ionicity of $TM-O$ bonds in $O3$
$NaTMO_2$ compounds in which TM ions is a fourth- or fifth-row transition
metal. We demonstrate that the magnetic exchange interaction and the local
distortions in the coordination environment of TM ions play more significant
roles in determining the cathode potential of the $TM^{3+}$ $\to$ $TM^{4+}+
e^-$ reaction than the ionicity of the $TM-O$ bonds in these compounds. These
results indicate that designing cathode materials solely based on empirical
electronegativity values to achieve high potential may not be a feasible
strategy without taking into account a detailed structural assessment.
