In Situ Forming Na─Sn Alloy/Na₂S Interface Layer for Ultrastable Solid State Sodium Batteries

Abstract The poor interfacial compatibility between sodium superionic conductor (NASICON) electrolyte and metallic sodium anode will lead to severe dendrite penetration, impeding the application of NASICON electrolytes for solid state sodium batteries. Herein, a homogeneous SnS₂ coating layer is sputtered on the surface of Na_3.4Zn_0.1Zr_1.9Si_2.2P_0.8O₁₂ electrolyte to in situ construct a kinetically stable Na─Sn alloy/Na₂S interlayer, possessing superior Na affinity, low diffusion barrier, and electronic insulating character to suppress dendrite growth, which is confirmed by experiments and density‐functional theory calculations. Benefiting from the Na─Sn alloy/Na₂S interphase, the critical current density of Na_3.4Zn_0.1Zr_1.9Si_2.2P_0.8O₁₂ increases from 2.4 to 9.4 mA cm⁻². In addition, the obtained Na₃V₂(PO₄)₃/Na_3.4Zn_0.1Zr_1.9Si_2.2P_0.8O₁₂@SnS₂/Na solid state batteries exhibit a high initial reversible discharge capacity of 115.1 mAh g⁻¹ at 0.1 °C with an initial Coulombic efficiency of 93.6%, and a capacity retention rate of 88.1% after 1000 cycles at 1 C.