Motivated by recent experimental breakthroughs in mixed-anion solid-state Li-ion electrolytes [Z. Liu et al., Nat. Chem. 16, 1584 (2024); F. Zhao et al., Science 390, 199 (2025)], we propose a previously unexplored mixed-halide rare-earth Na-ion solid electrolyte, ${\mathrm{Na}}_3{\mathrm{GdCl}}_4{\mathrm{Br}}_2$ (NGCB), and systematically establish its structure-transport-stability relationships using density functional theory, ab initio molecular dynamics, and grand-potential phase diagram analysis. Our computational simulations suggest that NGCB is stable dynamically, thermally, thermodynamically, and mechanically, with a wide electronic band gap of 5.73 eV and a broad electrochemical stability window of 0.57–3.65 V. Moreover, we reveal that anion mixing in NGCB generates tetrahedral anion vacancies that percolate into continuous low-barrier tetrahedral-tetrahedral $\mathrm{N}{\mathrm{a}}^{+}$ migration networks. Consequently, NGCB exhibits a high room-temperature ionic conductivity of 5.05 $\mathrm{mS}\mathrm{c}{\mathrm{m}}^{-1}$ with a low activation energy of 0.23 eV, supported by low migration barriers of 0.38–0.46 eV in two-dimensional channels and 0.40–0.45 eV in three-dimensional channels. Beyond fast ion transport, we show that anion mixing simultaneously enhances interfacial electrochemical compatibility. NGCB displays low interfacial reaction energies with high-voltage cathodes, reaching −32 $\mathrm{meV}\mathrm{ato}{\mathrm{m}}^{-1}$ with ${\mathrm{Na}}_3{\mathrm{V}}_2{\mathrm{P}}_2{\mathrm{O}}_8{\mathrm{F}}_3$. These results demonstrate a clear advantage of mixed-anion sodium solid-state electrolytes over conventional single-anion halides.

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