Optimizing the Na metal/solid electrolyte interface through a grain boundary design

Abstract

Poor compatibility between an alkaline metal electrode and solid electrolyte at interfaces is the critical issue for solid-state metal batteries. We propose a grain boundary sealing (GBS) design of the Na₃Zr₂Si₂PO₁₂ (denoted as GBS-NZSP) solid electrolyte to enhance interfacial contact with Na metal and realize stable Na plating/stripping cycles at room temperature. (ZnO)₂–(B₂O₃)₃ (ZBO) is selected to promote densification sintering of NZSP and seal the grain boundary from electrons, thus suppressing Na metal dendrite growth and maintaining interfacial stability during charge/discharge cycles. The optimal GBS-NZSP reaches an impressive interfacial resistance of 23 Ω cm², over 41 times lower than that of bare NZSP against Na metal at 25 °C. The corresponding symmetrical Na//Na cell preserves super cycling stability for 1400 h at 0.3 mA cm⁻². The excellence is attributed to the positive effect of the GBS design on enhanced ionic conductivity and reduced electron transfer at the grain boundary, which leads to steady high-flux Na⁺-ion migration across the solid electrolyte without dendrite formation. Moreover, a 4 V full cell of Na₃V_1.5Cr_0.5(PO₄)₃/GBS-NZSP/Na is assembled accordingly, exhibiting high-rate capability and delivering a capacity of 108 mA h g⁻¹ for 560 cycles with over 80% retention at 10C rate.