Electrospun Flexible Cellulose Acetate-Based Separators for Sodium-Ion Batteries with Ultralong Cycle Stability and Excellent Wettability: The Role of Interface Chemical Groups

Na-ion batteries are one of the best technologies for large-scale applications depending on almost infinite and widespread sodium resources. However, the state-of-the-art separators cannot meet the engineering needs of large-scale sodium-ion batteries to match the intensively investigated electrode materials. Here, a kind of flexible modified cellulose acetate separator (MCA) for sodium-ion batteries was synthesized via the electrospinning process and subsequently optimizing the interface chemical groups by changing acetyl to hydroxyl partly. Upon the rational design, the flexible MCA separator exhibits high chemical stability and excellent wettability (contact angles nearly 0°) in electrolytes (EC/PC, EC/DMC, diglyme, and triglyme). Moreover, the flexible MCA separator shows high onset temperature of degradation (over 250 °C) and excellent thermal stability (no shrinkage at 220 °C). Electrochemical measurements, importantly, show that the Na-ion batteries with flexible MCA separator exhibit ultralong cycle life (93.78%, 10 000 cycles) and high rate capacity (100.1 mAh g^-1 at 10 C) in the Na/Na₃V₂(PO₄)₃ (NVP) half cell (2.5-4.0 V) and good cycle performance (98.59%, 100 cycles) in the Na/SnS₂ half cell (0.01-3 V), respectively. Moreover, the full cell (SnS₂/NVP) with flexible MCA separator displays the capacity of 98 mAh g^-1 and almost no reduction after 40 cycles at 0.118 A g^-1. Thus, this work provides a kind of flexible modified cellulose acetate separator for Na-ion batteries with great potential for practical large-scale applications.