Ultra‐Thin Single‐Particle‐Layer Sodium Beta‐Alumina‐Based Composite Polymer Electrolyte Membrane for Sodium‐Metal Batteries

Abstract Inorganic/organic composite polymer electrolytes (CPEs) with good flexibility and electrode contact have been pursued for solid−state sodium‐metal batteries. However, the application of CPEs for high energy density solid−state sodium‐metal batteries is still limited by the low Na⁺ conductivity, large thickness, and low ion transference number. Herein, an ultra‐thin single‐particle‐layer (UTSPL) composite polymer electrolyte membrane with a thickness of ≈20 µm straddled by a sodium beta−alumina ceramic electrolyte (SBACE) is presented. A ceramic Na⁺‐ion electrolyte that bridges or percolates across an ultra‐thin and flexible polymer membrane provides: 1) the strength and flexibility from the polymer membrane, 2) excellent electrolyte/electrode interfacial contact, and 3) a percolation path for Na⁺‐ion transfer. Owing to this novel design, the obtained UTSPL‐35SBACE membrane exhibits a high Na⁺‐ion conductivity of 0.19 mS cm⁻¹ and a transference number of 0.91 at room temperature, contributing to long−term cycling stability of symmetric sodium cells with a small overpotential. The assembled quasi‐solid‐state cell with the as−prepared UTSPL‐35SBACE membrane displays superior cycling performance with a discharge capacity of 105 mAh g⁻¹ at 0.5 °C rate after 100 cycles and excellent rate performance (82 mAh g⁻¹ at 5 °C rate) at room temperature with the potassium manganese hexacyanoferrate (KMHCF)@CNTs/CNFs cathode, where KMHCF refers to potassium manganese hexacyanoferrate.