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 Na2B10H10 and Na2B12H12. Two Na2B10H10:Na2B12H12 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 x 10(-4) Scm(-1) at 10 wt% BCC content to about 1.0 x 10(-3) Scm(-1) at 50 wt% BCC content. For the 1:3 ratio, it increases from 1.3 x 10(-5) Scm(-1) at 11.9 wt% BCC to 8.1 x 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.

Automated summary

Hydroborates are investigated as solid electrolytes for all-solid-state batteries, and the impact of pressure on their crystal structure and ionic conductivity is studied. Different ratios of Na2B10H10 and Na2B12H12 result in different crystal phases. Applying pressure to densify the powder leads to a partial phase transformation into a body-centered cubic (BCC) phase. The sodium-ion conductivity increases with increasing BCC content for both ratios.