Recently, different and unexpected electrochemical behaviours have been demonstrated for M x Sb electrodes (M = Li, Na) in Li/Na ion batteries. Despite a similar thermodynamic stability of the hexagonal and cubic polymorphs of Li3Sb, mostly cubic Li3Sb is observed at the end of discharge. In contrast, mostly the hexagonal Na3Sb polymorph is observed when cycling the Na/Sb, in agreement with its higher thermodynamic stability compared to the cubic polymorph. This polymorph selectivity is here investigated by means of simple thermodynamic and electrostatic considerations using first-principles Density Functional Theory (DFT) calculations. We show that the Na-based polymorphs are more ionic than their Li-based homologues, despite less ionic Na/Sb interactions. We establish a direct correlation between the relative compactness and stability of the M3Sb polymorphs to rationalize the preference of the hexagonal structure type for the most ionic compounds of the M3Sb series (M = Li, Na, K, Rb, Cs). The M−Sb interactions are further linked to the different electrochemical behaviours of the M x Sb electrodes through Madelung constant calculations. This method is based on the knowledge of only one given M x Sb composition and thus allows rationalizing the different intermediate compositions achieved through electrochemical cycling. To validate our method, we finally provide the first-principles computed phase stability diagrams which further reveal two new phases for both Li–Sb and Na–Sb systems.