Fabrication of free-standing thin Na/Na2In composite anode with superior processability for high energy density batteries

The development of thin sodium (Na) metal anodes (e.g., ≤ 40 μm) with favorable electrochemical performance is crucial for the practical implementation of high energy density batteries. However, producing such thin Na foils remains challenging because of metallic Na’s poor processability under conventional mechanical rolling techniques. This study reports the successful fabrication of free-standing Na/Na 2 In (NaIn61) composite foils with tunable thicknesses from 40 to 20 μm. They were successfully fabricated via repeated mechanical rolling, using metallic Na and indium as precursors. In-situ generated Na 2 In particles were uniformly distributed within the Na matrix, and strong interfacial bonding between Na 2 In and Na effectively suppressed dislocation motion during extrusion, thereby significantly enhancing the mechanical processability of the composite foil. Furthermore, Na 2 In reduced the nucleation barrier for Na deposition and stabilized the solid electrolyte interphase, improving Na plating/stripping behavior. A full-cell assembled with a 40 μm-thick NaIn61 composite anode and a NaFe 1/3 Ni 1/3 Mn 1/3 O 2 cathode achieved a high energy density of approximately 172 Wh kg -1 at 0.2 C with a low negative-to-positive capacity ratio of ∼2.0. These findings demonstrate the effectiveness of rational composite design in enabling the production of free-standing, thin Na metal anodes with improved electrochemical performance.