Bifunctional Sn/NaF Interlayer Enables Stable Solid State Sodium Batteries

The practical application of sodium superionic conductor (NASICON) electrolytes in solid‐state sodium batteries is restricted by poor interfacial compatibility between the solid electrolyte and Na anode, as well as continuous dendrite growth. Herein, a bifunctional Sn/NaF interlayer is introduced at the Na/Na 3.4 Zn 0.1 Zr 1.9 Si 2.2 P 0.8 O 12 (NZZP) interface to improve wettability and suppress dendrite formation. Experimental and computational results confirm that NaF with a band gap of 6.1 eV can significantly enhance the electron transport barrier to prevent Na nucleation on the side of NZZP. Moreover, the in situ formed Na x Sn alloys at Na/NaF@NZZP interface possess excellent electronic conductivity, fast Na + migration kinetics, and low interfacial energy of 14.05 meV Å −2 with Na, effectively improving interfacial contact and facilitating uniform Na plating. Benefiting from the Na x Sn/NaF interphase, the critical current density of NZZP increased from 2.7 to 11.6 mA cm −2 under 60°C. The assembled Na 3 V 2 (PO 4 ) 3 @PEG/NZZP@NaF@Sn/Na solid state batteries show a high initial reversible discharge capacity of 92.0 mAh g −1 with a capacity retention of 88.9% at 0.5C after 1000 cycles. Even at 1C and 3C, the batteries still maintain stable cycling for 10 000 cycles with capacity retention of 68.8% and 67.6%, respectively.