Through systematic analysis of electron–phonon calculations across more than 20,000 metallic systems, this work first establishes the fundamental Tc ceiling for conventional superconductors at ambient pressure, revealing an inherent trade-off between the logarithmic average phonon frequency and the electron–phonon coupling constant λ, and demonstrating that room-temperature conventional superconductivity at ambient pressure is extremely unlikely. Li₂AgH₆ and Li₂AuH₆ are identified as the materials most closely approaching this practical limit.

Against this physical backdrop, two targeted material studies explore how high Tc can be pushed in practice. In the X₄H₁₅ hydride family, hole doping with X³⁺ cations is shown to enhance Tc to approximately 50 K at ambient pressure through stronger coupling to both cation and hydrogen phonon modes, with a computationally validated synthesis route proposed via controlled doping of YZr₃H₁₅. A high-throughput screening of boron–carbon cages compounds then identifies RbSrB₆C₆ and TlB₂C₈ as particularly promising candidates, reaching predicted Tc values of 57 K and 76 K respectively and substantially advancing the known Tc record for this materials class.

Together, these three works provide both a fundamental upper bound and concrete material-specific design strategies for the realisation of high-temperature conventional superconductivity at ambient pressure.