Thin NASICON Electrolyte to Realize High Energy Density Solid‐State Sodium Metal Battery

The solid‐state electrolyte in a solid‐state battery acts as an electrons' barrier and an ions' bridge between the two electrodes. As solid‐state electrolyte does not store the mobile ions, it is necessary to achieve a thin solid‐state electrolyte to reduce the internal resistance and enhance the energy density. In this work, a thin NASICON solid‐state electrolyte, with a stoichiometry of Na₃Zr₂Si₂PO₁₂, is fabricated by the tape‐casting method and its thickness can be easily controlled by the gap between substrate and scraper. The areal‐specific resistance and the flexural strength increase with the electrolyte thickness. A solid‐state sodium metal battery with 86 μm thick Na₃Zr₂Si₂PO₁₂ exhibits a reversible specific capacity of 73–78 mAh g⁻¹ with a redox potential of 3.4 V at 0.2 C. This work presents the importance of electrolyte thickness to reduce internal resistance and achieve a high energy density for sodium batteries.