Enhanced grain connection and ionic conductivity of Na3.3La0.3Zr1.7Si2PO12 ceramic electrolyte by adding Na2B4O7

NASICON-type Na3Zr2Si2PO12 (NZSP) is supposed to be one of the greatest feasible solid electrolytes due to its high electrochemical performance and safety. Herein, Na2B4O7 (NBO) additive is proposed to enhance the grain connection and ionic conductivities, and reduce the sintering temperature. The grain boundary microstructure of Na3.3La0.3Zr1.7Si2PO12 (NLZSP) is adjusted via introducing NBO additive to facilitate the densification sintering at 950 °C. NLZSP with 3 wt% NBO (NZSP(NBO3)) demonstrates an optimized total ionic conductivity (σt) of 1.81 mS cm−1 at 25 °C and low activation energy (Ea) of 0.22 eV. EIS and SEM analysis reveals that NBO additive promotes more intimate contact between NLZSP grains and extensively reduces the grain-boundary resistance (Rgb) from 135 Ω cm2 for NLZSP to 8 Ω cm2 for NLZSP(NBO3). Besides, wide electrochemical stability window of 5 V vs Na+/Na and galvanostatic sodium stripping-plating up to 400 h reconfirms the super stability of NLZSP(NBO3)/Na interface, while small contact angle of 82° will bring better wettability and interface contact of NLZSP(NBO3)/Na. Solid-state sodium metal batteries (SMBs) based reduced graphene oxide wrapped NaTi2(PO4)3 (NTP@rGO) 3D conductive cathode and NLZSP(NBO3) ceramic electrolyte shown excellent rate performance and cycle stability (91.4% of capacity retention after 200 cycles at 1C). Hence, NLZSP(NBO) ceramic electrolyte exhibits a significant advantage in enhancing grain connection, regulating ionic conductivity and reducing sintering temperature, and is anticipated to be used in all solid-state sodium ions batteries (ASSSIBs). [Display omitted] •Na2B4O7 additive can extensively reduce Rgb of NLZSP by enhancing the grain connection.•The optimum sintering temperature of NLZSP electrolyte can be reduced to 950 °C by introducing 3 wt% Na2B4O7 additive.•NZSP(NBO3) demonstrates an optimized σt of 1.81 mS cm−1 and low Ea of 0.22 eV at 25 °C.•NTP@rGO//NLZSP(NBO3)//Na solid state SMBs could maintain 91.4% of the initial capacity after 200 cycles at 1C.