Low Na-β′′-alumina electrolyte/cathode interfacial resistance enabled by a hydroborate electrolyte opening up new cell architecture designs for all-solid-state sodium batteries

Development of low-resistance electrode/electrolyte interfaces is key for enabling all-solid-state batteries with fast-charging capabilities. Low interfacial resistance and high current density were demonstrated for Na-β′′-alumina/sodium metal interfaces, making Na-β′′-alumina a promising solid electrolyte for high-energy all-solid-state batteries. However, integration of Na-β′′-alumina with a high-energy sodium-ion intercalation cathode remains challenging. Here, we report a proof-of-concept study that targets the implementation of a Na-β′′-alumina ceramic electrolyte with a slurry-casted porous NaCrO2 cathode with infiltrated sodium hydroborates as secondary electrolyte. The hydroborate Na4(B12H12)(B10H10) possesses similar sodium-ion conductivity of 1 mS cm−1 at room temperature as Na-β′′-alumina and can be fully densified by cold pressing. Using the Na4(B12H12)(B10H10) secondary electrolyte as interlayer between Na-β′′-alumina and NaCrO2, we obtain a cathode-electrolyte interfacial resistance of only 25 Ω cm2 after cold pressing at 70 MPa. Proof-of-concept cells with a sodium metal anode and a NaCrO2 cathode feature an initial discharge capacity of 103 mAh g−1 at C/10 and 42 mAh g−1 at 1 C with an excellent capacity retention of 88% after 100 cycles at 1 C at room temperature. Ion-milled cross-sections of the cathode/electrolyte interface demonstrate that intimate contact is maintained during cycling, proving that the use of hydroborates as secondary electrolyte and as an interlayer is a promising approach for the development of all-solid-state batteries with ceramic electrolytes.