Computational Discovery of Metastable NaMnO${}_2$ Polymorphs as High-Performance Cathodes with Ultralow Na${}^{+}$ Migration Barriers

Using an ab initio evolutionary algorithm combined with first-principles calculations, two metastable NaMnO${}_2$ polymorphs, $I{4}_1\mathrm{/}amd$ 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 $\mathit{\text{Pmmn}}$ 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 Mn–O hybridization, reduces the bandgap, and promotes robust charge compensation and oxygen retention. Remarkably, the Cmcm phase achieves record-low Na${}^{+}$ migration barriers (0.39~eV at high Na concentration; 0.27~eV at low concentration), representing 47% and 36% reductions respectively compared to conventional $C2\mathrm{/}m$, while delivering a higher average voltage (3.19~V vs 2.88~V). The $I{4}_1\mathrm{/}amd$ phase exhibits concentration-dependent diffusion with a low-energy pathway (0.38~eV) and maintains competitive voltage (2.94~V). These findings suggest that metastable NaMnO${}_2$ polymorphs may offer viable alternatives to conventional cathode materials, particularly where fast ionic conduction is required.