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Metastable Sodium Closo-hydridoborates for All Solid-state Batteries with Thick Cathodes Rapid cooling of sodium hydridoborate precursors from the high‐temperature formation window produced a metastable orthorhombic Na3(B12H12)(BH4) phase, confirmed by XRD and NMR. Slow cooling causes the material to revert to Na2B12H12 and NaBH4. The orthorhombic phase only persists when kinetically trapped. Faster cooling increased uniformity of the sodium environment and reduced the presence of precursor‐like signatures. Ionic conductivity measurements showed that conductivity increased with cooling rate, reaching 4.6 mS cm−1 at 30 °C for the quenched material, along with a lower activation energy. Electronic conductivity remains several orders of magnitude lower. Electrochemical testing of composite cathodes showed the metastable phase reduced tortuosity and lowered areal resistance compared with chloride‐only electrolytes. Thick NaCrO2 (45 mg cm−2) paired with Sn as the cathode and anode delivered reversible areal capacities up to 3.5 mAh cm−2. Cells maintained cycling stability when a chloride coating was used to prevent oxidation of the electrolyte, and full cells continued operating at temperatures down to –15 °C.
Kinetic stabilization of a metastable anion‐dynamic framework through quenching unlocks fast Na+ transport and enables high‐loading sodium solid‐state batteries.
Source: Oh et al., Joule, 2025.
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