Novel Sodium Rare Earth Silicate Solid Electrolyte with Grain Boundary Electronic Insulation for Ultra‐Durable Solid‐State Sodium Metal Batteries

Abstract Sodium rare earth silicate is regarded as a novel and promising solid electrolyte (SE) for solid‐state sodium metal batteries (SSSMBs) because the merits of high ionic conductivity, low sintering temperature, and excellent chemical stability. However, impurity phases and voids induced by sintering always cause weak ion conduction and large electron conduction at grain boundaries, resulting in the growth of Na dendrites. Herein, a facile grain boundary modification strategy is conducted by introducing a second phase of Na 1.5 Y 2.5 F 9 into the grain boundaries of Na 5 YSi 4 O 12 . The Na 1.5 Y 2.5 F 9 phase can not only effectively impede the migration of electrons and reduce the bulk resistance and grain boundary resistance, but also improve the densification and mechanical strength of SE. Consequently, the Na/Na symmetric cell delivers a large critical current density of 0.7 mA cm −2 and an ultra‐long cycle life of 9000 h at 0.1 mA cm −2 and 0.05 mAh cm −2 without dendrite formation. Moreover, the assembled full cell achieves excellent cycling stability at 1 C for 750 cycles with a capacity retention of 70.2% and enhanced rate capability of 113.5 mAh g −1 at 2 C. This work lays a foundation to develop high‐performance sodium rare earth silicate‐based SE for ultra‐durable SSSMBs.