Si₃N₄-Assisted Densification Sintering of Na₃Zr₂Si₂PO₁₂ Ceramic Electrolyte toward Solid-State Sodium Metal Batteries

The solid-state metal battery with solid-state electrolytes has been considered the next generation of energy storage technology owing to its superior safety and high energy density. But, unfavorable ionic conductivity and interfacial problems make it difficult to widely use in practice. In this work, Si₃N₄ was rationally introduced into the NASICON matrix as a sintering aid, and the influence of Si₃N₄ on the crystal phase, microstructure, electrochemical and electrical performance of Na₃Zr₂Si₂PO₁₂ (NZSP) ceramic was systematically studied. The results demonstrate that the introduction of Si₃N₄ can effectively lower the densification sintering temperature of Na₃Zr₂Si₂PO₁₂ electrolyte and enhance the room temperature ionic conductivity of the NZSP to 3.82 × 10⁻⁴ S cm⁻¹. In addition, since Si₃N₄ has a high thermal conductivity and can inhibit the transmission of electrons between the grains of the electrolyte matrix, it will effectively hinder the generation of sodium metal dendrites and relieve the concentration of the heat source. Moreover, owing to the desirable interface compatibility of the Na and NZSP-Si₃N₄ electrolyte, the Na/NZSP-1150-1%Si₃N₄/Na symmetric battery exhibits excellent stability, and the electrode/electrolyte interface still maintains good integrity even after long-term cycling. The assembled Na/NZSP-1150-1%Si₃N₄/Na_3.5V_0.5Mn_0.5Fe_0.5Ti_0.5(PO₄)₃ cell manifests an initial specific capacity of 152.5 mA h g⁻¹, together with an initial Coulombic efficiency of 99.8%. Furthermore, after 200 cycles, the battery displays a capacity retention rate of 82%.