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

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 Na3V1.5Cr0.5(PO4)3/GBS-NZSP/Na is assembled accordingly, exhibiting high-rate capability and delivering a capacity of 108 mA h g^-1for 560 cycles with over 80% retention at 10C rate.