Oxide Solid-State Sodium Batteries Sintered at 400 °C Using an NaBO₂·4H₂O Sintering Aid

In this study, a Na 3.1 Zr 1.95 Mg 0.05 (SiO 4 ) 2 (PO 4 ) (NZMSP) solid electrolyte is successfully densified at 400 °C by employing NaBO 2 ·4H 2 O (NBO2) as a sintering aid. The NZMSP solid electrolyte containing 20 wt % NBO2 shows a total ionic conductivity of 3.3 × 10 –4 S cm –1 at 25 °C and demonstrates more stable Na deposition/stripping cycles compared with samples without NBO2. Although the Na 3 V 2 (PO 4 ) 3 (NVP) active material sintered with NZMSP containing 20 wt % NBO2 decomposes after sintering, NVP with 1 or 5 wt % NBO2 remains mostly intact. An Na/NZMSP/NVP battery containing 5 wt % NBO2 exhibits an initial discharge capacity of 171 mAh g –1 at 1.0–4.0 V vs Na + /Na, which is comparable to the theoretical capacity, 176.4 mAh g –1 . Additionally, a symmetric NVP/NZMSP/NVP battery containing 5 wt % NBO2 is operated with an initial discharge capacity of 70.4 mAh g –1 . This study demonstrates that the NBO2 sintering aid enables cost-effective, low-temperature fabrication of oxide solid-state sodium batteries.