Role of Ductile Materials in Improving Interfacial Contact Between Solid-State Electrolytes and Hard Carbon Anodes in Sodium-Ion Batteries (Supporting Information)

There is considerable interest in all-solid-state sodium-ion batteries from the perspectives of safety and resource efficiency. However, it is known that the interfacial resistance of oxide-based solid electrolytes such as Na_1+xZr₂Si_xP_3−xO₁₂ (NASICON), which is used in all-solid-state sodium-ion batteries, is high because of poor contact at the solid/solid interface. In this study, to reduce the interfacial resistance, we conceived of introducing a ductile electrolyte, Na(CB₉H₁₀)_0.7(CB₁₁H₁₂)_0.3 (NaCBH), Na_2.25Y_0.25Zr_0.75Cl₆ (NYZC), and a polymer electrolyte (PE), into the NASICON/hard carbon interface. The ionic conductivity of NaCBH was the highest among three ductile electrolytes. In impedance measurements using a symmetric cell, the interfacial resistance was high for NYZC but low for NaCBH and the PE. This suggests that good contact was formed at the NaCBH/NASICON and NaCBH/PE interfaces and that NaCBH is an excellent electrolyte as an intermediate layer. In addition, when NaCBH and hard carbon were combined, charged, and discharged, a capacity comparable to that of organic electrolytes was obtained. Furthermore, the interfacial resistance between the solid electrolyte and hard carbon was reduced by introduction of NaCBH, proving that NaCBH is an effective intermediate layer on the anode side for use in all-solid-state sodium-ion batteries.