The interplay of collective electronic orders represents a central theme in condensed matter physics, and kagome lattices offer fertile ground for studying their competition and coexistence. Although metal-decorated two-dimensional boron kagome lattices can be stabilized, their quantum phases remain largely unexplored. Here, we report a family of inverse-sandwich monolayers, $M_2{\mathrm{B}}_3$ ($M$ = Mg, Ca, Sr, Ba), in which a boron kagome sheet is sandwiched between two alkaline earth metal layers, and identify them as charge density wave (CDW) materials. We found that the CDW in ${\mathrm{Mg}}_2{\mathrm{B}}_3$ originates from the combined effects of Fermi surface nesting and electron-phonon coupling, whereas in ${\mathrm{Ca}}_2{\mathrm{B}}_3, {\mathrm{Sr}}_2{\mathrm{B}}_3$, and ${\mathrm{Ba}}_2{\mathrm{B}}_3$, it is driven by strong electron-phonon coupling between B $\sigma$ orbitals and in-plane vibrational modes. These instabilities lead to a 2 × 2 CDW ground state in ${\mathrm{Mg}}_2{\mathrm{B}}_3$ and 3 × 3 reconstructions in the heavier analogs, all of which are suppressed at room temperature by anharmonic lattice effects. Remarkably, ${\mathrm{Mg}}_2{\mathrm{B}}_3$ hosts a coexisting 2 × 2 CDW and superconductivity with a $T{}_{\text{C}}$ of 10.6 K. Under biaxial compressive strain, superconductivity coexists with nontrivial topology in the heavier compounds. Moreover, the nearly unchanged $T{}_{\text{C}}$ between the CDW ground state and the strained kagome phase implies weak competition between CDW and superconductivity. Our results establish kagome-based boron monolayers as a promising class of materials for exploring intertwined CDW, superconducting, and topological states in two dimensions.

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• Charge density waves
• Fermi surface
• Superconductivity
• 2-dimensional systems
• Topological materials
• Density functional theory