Using an ab initio evolutionary algorithm combined with first-principles calculations, two metastable ${\mathrm{Na}\mathrm{Mn}\mathrm{O}}_2$ polymorphs, $I{4}_1/amd$ and Cmcm, are identified as promising cathode materials for sodium-ion batteries. Both phases exhibit excellent thermodynamic stability, lying within 35 meV/atom of the ground-state Pmmn phase across 0–50 GPa, and are dynamically and thermally stable under ambient conditions following high-pressure synthesis, as confirmed by phonon and ab initio molecular dynamics simulations. During desodiation, a Jahn-Teller-induced magnetic transition enhances $\mathrm{Mn}$-$\mathrm{O}$ hybridization, reduces the band gap, and promotes robust charge compensation and oxygen retention. Remarkably, the Cmcm phase achieves record low ${\mathrm{Na}}^{+}$ migration barriers (0.39 eV at high $\mathrm{Na}$ concentration; 0.28 eV at low concentration), representing 47% and 33% reductions, respectively, compared with conventional $C2/m$, while delivering a higher average voltage (3.19 vs 2.88 V). The $I{4}_1/amd$ phase exhibits concentration-dependent diffusion with a low-energy pathway (0.38 eV) and maintains a competitive voltage (2.94 V). These findings suggest that metastable ${\mathrm{Na}\mathrm{Mn}\mathrm{O}}_2$ polymorphs may offer viable alternatives to conventional cathode materials, particularly where fast ionic conduction is required.

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• Electrochemistry
• Phase behavior
• Pressure effects
• Batteries
• Crystal structures
• Energy materials
• First-principles calculations
• Genetic algorithms