Layered Organic–MXene Heterostructures for High‐Capacity and Long‐Life Aqueous Sodium‐Ion Storage in Seawater‐Based Hybrid Electrolytes

ABSTRACT Aqueous sodium‐ion batteries (ASIBs) are attractive for large‐scale storage due to intrinsic safety and low cost. However, their practical applications are often limited by sluggish kinetics and the dependence on purified electrolytes. Here, we report a layered organic–MXene heterostructured anode by direct thermal polymerization of a conjugated naphthalene‐based polyimide (NPI) in the presence of multilayer Ti 3 C 2 T x MXene, forming a tightly coupled 2D architecture (NPI@MXene). NPI provides reversible imide‐carbonyl redox sites, while MXene facilitates electron transport and interfacial ion‐transfer kinetics, thereby reducing polarization and accelerating Na + storage. Benefiting from the reversible carbonyl conversion, enhanced pseudocapacitive behavior, and improved Na + diffusion kinetics, the optimized NPI@MXene anode delivers a specific capacity of 124.5 mAh g − 1 at 1.00 A g − 1 and shows excellent durability with 100.0% retention over 65 000 cycles at 15.0 A g − 1 . For practical operation, seawater is used to prepare a 1.00 M sodium acetate electrolyte with the most favorable interfacial kinetics among the tested seawater‐based electrolytes. The assembled NPI@MXene//Cu‐PBA full cell operated stably over the 0–1.60 V range and maintained its performance for 15 000 cycles.