Engineering the NASICON Electrolyte/Na Anode Interface by Tuning the Phase of Electrolyte for Solid-State Sodium Battery

The commercialization of all-solid-state sodium batteries is currently mainly inhibited by the poor interface between the solid-state electrolyte and electrode. Herein, the interface between NASICON electrolyte and Na anode is engineered by tuning the phase of NASICON with the composition of Na_3.36Zr_1.64Sc_0.36Si₂PO₁₂ (NZSSP), which was prepared by the solid-state reactive sintering method. The phase is adjusted by varying the sintering temperature (900-1150 °C) and investigated by X-ray diffraction Rietveld refinement. The sintered NZSSP samples are dominated by the rhombohedral NASICON phase, and its content increases with the sintering temperature, 97% for the 1150 °C-sintered sample, whereas the Na₃Zr₂Si₂PO₁₂ (NZSP) sample is dominated by the monoclinic NASICON. The scanning electron microscopy and X-ray photoelectron spectroscopy results reveal that the surface microstructure and composition are similar. However, the interfacial impedance (R_inter) between solid electrolyte and Na anode and the critical current density (CCD) are quite different; R_inter decreases and CCD increases with the sintering temperature. The 1150 °C-sintered NZSSP has significantly lower R_inter (4.7 vs 316.4 Ω cm²) and higher CCD (0.85 vs 0.1 mA cm^-2) compared with NZSP at 30 °C. The full battery Na/NZSSP/Na₃V₂(PO₄)₃ can be stably cycled at 1C for 350 cycles. The contact angle measurement and adsorption energy calculation show that the adhering property of NASICON with Na plays a dominating role. The rhombohedral NASICON exhibits much greater adsorption energy to Na and lower contact angle, which is beneficial for the interfacial property and thus the Na plating/stripping processes. This work demonstrates that engineering the phase of the NASICON electrolyte is an effective strategy to optimize the interfacial property between NASICON and the Na anode.