Simultaneously enhancing ionic conductivity and interfacial stability by Fe2O3 for solid-state sodium metal batteries

NASICON-structured Na3Zr2Si2PO12 (NZSP) has been considered as one of the ideal electrolytes for all-solid-state sodium metal batteries (ASSSB). However, the practical application of NZSP-based ASSSB is hindered by the low ionic conductivity and large interfacial resistance caused by the poor contact between NZSP and Na metal. Herein, the introduction of Fe2O3 not only improves ionic conductivity and reduces activation energy by the doping of Fe3+ in the crystal structure of NZSP, but also reduces the interfacial resistance and enhances interface stability between NZSP and Na metal anode. The synergistic effects significantly enhance the cycling stability, rate capability, and critical current density of the symmetrical solid-state cells. The interfacial reaction mechanism indicates that Fe3+ in the interface is reduced Fe2+ by Na anode, which effectively even the electric-filed distribution and suppresses the dendrite growth. Consequently, the symmetric solid-state cells exhibit stable cycling performance for 1500 h at 0.1 mA·cm−1/0.1 mA·h·cm−1 and over 900 h at 0.2 mA·cm−1/0.2 mA·h·cm−1. The Na|NZSP-0.075%Fe2O3|Na2FePO4F solid-state full cells display high capacity retention of 94.2 % after 100 cycles at 0.5 C. The stable interface of NZSP/Na and improved ionic conductivity contribute to excellent electrochemical performance, which accelerates the practical application of ASSSB.