Crystallization of closo-borate electrolytes from solution enabling infiltration into slurry-casted porous electrodes for all-solid-state batteries

The direct crystallization of a closo-borate solid electrolyte from solution is demonstrated and applied to infiltrate porous battery electrodes fabricated by traditional slurry casting methods. Employing isopropanol as a solvent, we show that Na₄(B₁₂H₁₂)(B₁₀H₁₀) crystallizes inside the open porosity of the electrodes with the appropriate crystal structure yielding high ionic conductivity (1 ​mS ​cm^-1 at 25 ​°C) and providing efficient contact to the active materials. Very stable cycling is demonstrated at 30 ​°C for NaCrO₂|Na-Sn half cells employing infiltrated NaCrO₂ cathodes with mass loadings of ~7.7 ​mg ​cm^-2 at rates up to 5C (4.5 ​mA ​cm^-2). The all-solid-state cells exhibit remarkable cycling stability at moderate (70 ​MPa), and low (3 ​MPa) applied external pressure, retaining 95.6% and 85.9% of the initial capacity after 100 cycles at 0.5C respectively. A NaCrO₂|Sn rocking-chair all-solid-state full cell with capacity-balanced infiltrated electrodes is also demonstrated. The more pronounced fading of the full cell is characterized with a three-electrode measurement and can be attributed to the poorer cyclability of Sn. This work bridges the gap between the attractive materials properties of closo-borate electrolytes and their integration into all-solid-state batteries facilitating their adoption in industrial cell production at scale.