Elucidating the pressure-induced enhancement of ionic conductivity in sodium closo-hydroborate electrolytes for all-solid-state batteries

Hydroborates are an emerging class of solid electrolytes for all-solid-state batteries. Here, we investigate the impact of pressure on the crystal structure and ionic conductivity of a close-hydroborate salt consisting of Na(2)B(10)H(10) and Na(2)B(12)H(12). Two Na(2)B(10)H(10):Na(2)B(12)H(12) ratios were studied, 1:1 and 1:3. The anions of the as-synthesized powder with 1:1 ratio crystallize in a single face-centered cubic phase, while the anions of the powder with 1:3 ratio crystallize in a single monoclinic phase. After applying pressure to densify the powder into a pellet, a partial phase transformation into a body-centered cubic (BCC) phase is observed for both ratios. The BCC content saturates at 50 weight percent (wt%) at 500 MPa for the 1:1 ratio and at 77 wt% at 1000 MPa for the 1:3 sample. The room temperature sodium-ion conductivity follows an analogous trend. For the 1:1 ratio, it increases from 2 × 10(–4) Scm(−1) at 10 wt% BCC content to about 1.0 × 10(–3) Scm(−1) at 50 wt% BCC content. For the 1:3 ratio, it increases from 1.3 × 10(–5) Scm(−1) at 11.9 wt% BCC to 8.1 × 10(–4) Scm(−1) at 71 wt% BCC content. Our results show that pressure is a prerequisite to achieve high sodium-ion conductivity by formation of the highly conductive BCC phase. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10853-022-08121-8.