Enhanced processability and electrochemical cyclability of metallic sodium at elevated temperature using sodium alloy composite

Na metal batteries have attracted great attention owing to their considerable energy density, abundance of Na resources, and potentially low cost. However, Na metal anode suffers from poor processability and high reactivity, which inhibit its practical applications. Herein, we introduce a cross-linked sodium-tin alloy (Na15Sn4) network host for metallic Na and fabricated a Na15Sn4/Na composite foil using a simple cold calendaring approach via spontaneous reaction between metallic Na and metallic Sn, which markedly mitigated the above-mentioned challenges of Na metal anode. Due to its unique structure, the as-fabricated Na15Sn4/Na composite exhibited exceptional processability in contrast to the soft and sticky pure metallic Na, and can be easily fabricated into foils with small thickness (e.g., 100 μm). Na15Sn4/Na||Na15Sn4/Na symmetric cell exhibited stable electrochemical stripping/plating cycling for 100 cycles with constant overpotential of less than 15 mV at 1 mA cm−2 and 1 mAh cm−2 at 60°C. Even at harsh 90°C, the symmetric cell showed stable cycling with a low overpotential of around 3 mV at 1 mA cm−2 and 1 mAh cm−2. Furthermore, Na0.9[Cu0.22Fe0.30Mn0.48]O2 (NCFMO)||Na15Sn4/Na cell demonstrated high rate capability (98 mAh g−1 at 2 C) and cyclability (88% capacity retention for 100 cycles) in comparison to the counterpart with pure Na metal anode (92 mAh g−1 at 2 C, 80% capacity retention for 100 cycles). The concept of introducing metal alloy in sodium using cold calendaring to improve processability and electrochemical properties at elevated temperature can be expanded to other alkali metal electrodes in the future.