  Delithiation of the known layered LiBC compound was predicted to induce
conventional superconductivity at liquid nitrogen temperatures but extensive
experimental work over the past two decades has detected no signs of the
expected superconducting transition. Using a combination of first-principles
stability analysis and anisotropic Migdal-Eliashberg formalism, we have
investigated possible Li$_x$BC morphologies and established what particular
transformations of the planar honeycomb BC layers are detrimental to the
material's superconductivity. We propose that Li$_x$BC reintercalation with
select alkali and alkaline earth metals could lead to synthesis of otherwise
inaccessible metastable Li$_x$M$_y$BC superconductors with critical
temperatures ($T_{c}$) up to 73 K. The large-scale exploration of metal
borocarbides has revealed that NaBC and Li$_{1/2}$Na$_y$BC layered phases are
likely true ground states at low temperatures. The findings indicate that this
compositional space may host overlooked synthesizable compounds with potential
to break the $T_{c}$ record for conventional superconductivity in
ambient-pressure materials.
