In electrides, interstitial anionic electrons (IAEs) constitute a key electronic degree of freedom whose spatial organization governs the electronic structure and emergent quantum properties. However, controlled realization of IAEs with complex topologies remains a central challenge. Here we propose a heterometallic competitive polarization strategy to engineer IAEs in a high-pressure Li-Mg-P system, leading to the stabilization of a ${\mathrm{Li}}_6{\mathrm{Mg}}_2\mathrm{P}$ electride. The intrinsic chemical inequivalence between Li and Mg induces divergent coordination responses, reconstructing the interstitial confinement potential into inequivalent Mg-centered cavities. This dual-cavity landscape stabilizes two electronically distinct black-phosphorene-like IAEs with different charge densities and localization strengths. Among them, the more delocalized IAE(2) dominates the electronic states near the Fermi level, thereby governing low-energy electronic excitations. This interstitial-state-dominated electronic structure strongly couples to low-frequency lattice vibrations of the metal framework, with IAE(2) playing the primary role in mediating electron-phonon interactions. As a result, enhanced coupling between interstitial electrons and lattice dynamics gives rise to pressure-enhanced superconductivity. These results establish heterometallic competitive polarization as an effective route for engineering nontrivial IAE topologies and provide a design principle for emergent quantum phenomena in high-pressure electrides.

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• Electrical properties
• Pressure effects
• Superconductivity
• Density functional calculations
• First-principles calculations