Effect of Mechanical Pressure on the Ionic Conductivity of Sodium Dodecahydrido-closo-dodecaborate

To optimize the performance of solid-state batteries, careful control of the material properties is essential. Solid-state batteries require significant mechanical pressure during assembly and cycling to ensure intimate contact and stability of the electrochemical cell. Here we show that mechanical pressure can induce a polymorphic transition (P2₁/c → Pbca) of sodium dodecahydrido-closo-dodecaborate, Na₂B₁₂H₁₂, which increases both the boron dynamics and the ionic conductivity. Heating mechanically pressed samples above 65 °C results in full conversion back to the initial polymorph (P2₁/c), along with a reduction of one order of magnitude in ionic conductivity, which is consistent with the as-synthesized Na₂B₁₂H₁₂. This work emphasizes the important role that postsynthesis treatment can have on the ionic conductivity, the activation energy, and the texture of solid electrolytes.