Closo-borates represent a promising but yet under-explored alternative class of electrolytes for all-solid-state batteries. We recently reported ionic conductivities of 1 mS/cm and an electrochemical stability window of 3 V at room temperature for Na₄(B₁₂H₁₂)(B₁₀H₁₀).[1] The mixed-anion configuration stabilizes the ion-conducting high-temperature phase at room temperature.

We further demonstrated stable cycling for a 3V class all-solid-state battery based on this electrolyte consisting of a sodium metal anode and a NaCrO₂ cathode.[2] The cathode composite can be assembled by simple cold pressing, but cycling performance is enhanced through a preliminary solvent-based impregnation step of the cathode particles by a thin electrolyte coating. This coating guarantees intimate contact between cathode particles and electrolyte resulting in reversible and stable cycling with a capacity of 85 mAh/g at C/20 and 80 mAh/g at C/5 with more than 90% capacity retention after 20 cycles at C/20 and 85% after 250 cycles at C/5. We also investigated the effect of cycling outside the electrochemical stability window and observed that electrolyte decomposition leads to faster though not critical capacity fading.

We will further present routes to translate these results to lithium analogues and our efforts to establish low-cost synthesis routes for these non-toxic materials. Our results demonstrate that owing to their physical properties and processability, closo-borate-based electrolytes could play a significant role in the development of a competitive all-solid-state battery technology.

[1] L. Duchêne, R.-S. Kühnel, D. Rentsch, A. Remhof, H. Hagemann, C. Battaglia, Chem. Comm. 53, 4195 (2017)

[2] L. Duchêne, R.-S. Kühnel, E. Stilp, E. Cuervo Reyes, A. Remhof, H. Hagemann, C. Battaglia, Energy & Environmental Science 10, 2609 (2017)