By means of first-principles calculations and crystal structure predictions, we identify a series of potassium-aluminum (K-Al) compounds under high pressure, which are absent in ambient conditions. Among them, the P4/nmm KAl phase, stable under pressure from 15 to 70 GPa, exhibits a range of intriguing electronic properties, including a three-dimensional (3D) flat band near $\mathrm{\Gamma }$, a composite nodal network, and 3D interstitial anionic electrons (IAEs). The 3D flat band of the highest valence band leads to a Van Hove singularity (VHS) with two saddle points near $\mathrm{\Gamma }$. In addition, there exists a visible band inversion between the K-$d$- and the Al-$p$-dominant bands, which generates a composite nodal network without spin-orbit coupling (SOC), and evolves into a Dirac semimetal once SOC is included. Furthermore, calculations of local charge distributions indicate the existence of IAEs. Notably, the ${\mathrm{IAE}}_{2c}$ and ${\mathrm{IAE}}_{8i}$ states make a significant contribution to the 3D flat band and composite nodal network. Thus, our study not only enriches the phase diagram of a K-Al system, but also proposes a potential platform for studying the interplay among a 3D flat band, a 3D flat-band-induced VHS, and a 3D topological electride behavior.

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• Flat bands
• Pressure effects
• Topological materials
• Density functional calculations