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

In this study, a Na3.1Zr1.95Mg0.05(SiO4)2(PO4) (NZMSP) solid electrolyte is successfully densified at 400 °C by employing NaBO2·4H2O (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 Na3V2(PO4)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.