The high-pressure phase diagram of the Ca-Li system remains controversial, particularly regarding the stability of ${\mathrm{CaLi}}_2$ and its possible decomposition pathways under compression. Intermetallic electrides under high pressure have emerged as a promising platform for accomplishing unconventional charge transfer and superconducting behaviors due to their unique interplay between interstitial anionic electrons (IAEs) and metal cations. Motivated by these challenges and opportunities, we perform an extensive first-principles structure search and identify P4/mnc ${\mathrm{Ca}}_4\mathrm{Li}$ as the only thermodynamically stable compound under pressure. This compound adopts a tetragonal host-guest framework with a two-dimensional windmill-shaped IAE network. X-ray diffraction simulations indicate that ${\mathrm{Ca}}_4\mathrm{Li}$ could be a viable candidate for the experimentally observed Ca-Li phase. Notably, it is a rare bimetal-donor electride at low pressure, but upon compression it undergoes a successive enhancement of Ca $s\to d$ transition/Li $2s\to \mathrm{Ca}3d$ charge transfer and a progressive reduction of IAEs, transforming Ca from an electron donor to an acceptor. This unique charge redistribution enhances the coupling between Ca $3d$ electrons and low-frequency phonons associated with the Ca framework, leading to a continuous increase in superconducting critical temperature, reaching 17.8 K at 150 GPa. Our work expands the understanding of charge transfer and superconductivity in intermetallic electrides, enabling design of novel electride superconductors with tunable properties.

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• Electronic structure
• High-pressure studies
• Phase transitions
• Superconductivity
• Density functional calculations