Carboxymethyl Cellulose–Polyaniline (CMC–PANI) Hydrogels as Multifunctional Binders for Sustainable Sodium-ion Batteries: Structure, Mechanism, and Performance Insights

The growing global demand for energy and the urgent transition toward sustainable technologies have accelerated the search for efficient, low-cost, and environmentally benign energy storage systems. Sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs) due to the natural abundance and comparable electrochemical properties of sodium. However, the larger ionic radius of Na⠺ results in significant volume fluctuations during cycling, leading to rapid capacity fading and poor mechanical stability of electrodes. In this context, the binder—though a minor component by weight—plays a crucial role in maintaining electrode integrity and electrochemical stability. This review highlights the synthesis, structure, and function of carboxymethyl cellulose–polyaniline (CMC–PANI) hydrogel binders, a new class of multifunctional materials for SIBs. The CMC component offers strong adhesion, mechanical flexibility, and water processability, while PANI contributes electrical conductivity and electrochemical stability. The in-situ polymerization approach produces a semi-interpenetrating polymer network (semi-IPN) that synergistically integrates ionic and electronic conduction pathways. This composite binder enhances electrode cohesion, facilitates stable solid electrolyte interphase (SEI) formation, and improves rate capability and cycle life. The review also discusses structure–property relationships, mechanisms of performance enhancement, and the potential of CMC–PANI hydrogels as sustainable binders for next-generation sodium-ion batteries.