Advances in free-standing electrodes for sodium ion batteries

Sodium-ion batteries (SIBs) have recently reemerged as a promising technology in the fields of large-scale energy storage systems and low-speed electric vehicles, owing to the abundance and even distribution of sodium resources. Moreover, the similarity in working principles between SIBs and lithium-ion batteries (LIBs) further accelerates their development. However, the development of SIBs still faces challenges, such as the limited availability of electrode materials that demonstrate both satisfactory cycling stability and high-rate performance. Typically, common electrodes utilize specific binders to integrate the active materials with conductive additives. Unfortunately, frequently used binders are often dielectric and mechanically unstable, leading to a decrease in specific capacity and poor cycling stability. In addition, strongly electronegative groups within binders can irreversibly capture Na⁺ ions, resulting in an increase in irreversible capacity. Therefore, the use of binder-free, free-standing electrodes is crucial for the development of high-performance SIBs due to their enhanced electronic conductivity and reversible electrochemical reactions. This review provides a comprehensive overview of the recent advancements in free-standing electrodes for SIBs and flexible SIBs. It examines the challenges specific to free-standing electrodes and flexible SIBs and proposes effective strategies to overcome these obstacles. By addressing these challenges, this review aims to stimulate significant progress in the development of flexible energy storage devices, fostering their extensive utilization across diverse applications.