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

TL;DR

This study uses computational methods to identify metastable NaMnO₂ polymorphs with ultralow Na⁺ migration barriers and high voltages, promising for fast-charging sodium-ion batteries.

Contribution

The paper discovers and characterizes two metastable NaMnO₂ phases with exceptional ionic mobility and stability, advancing cathode material design for sodium-ion batteries.

Findings

Cmcm phase has record-low Na⁺ migration barriers (0.39 eV and 0.27 eV).

Both phases exhibit high thermodynamic stability and dynamic stability.

The phases deliver higher voltages than conventional materials.

Abstract

Using an ab initio evolutionary algorithm combined with first-principles calculations, two metastable NaMnO$_2$ polymorphs, $I4_1/amd$ 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 \textit{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 $C2/m$, while delivering a higher average voltage (3.19~V vs 2.88~V). The $I4_1/amd$ 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.