High‐Rate Polymeric Redox in MXene‐Based Superlattice‐Like Heterostructure for Ammonium Ion Storage

Achieving both high redox activity and rapid ion transport is a critical and pervasive challenge in electrochemical energy storage applications. This challenge is significantly magnified when using large‐sized charge carriers, such as the sustainable ammonium ion (NH4+). A self‐assembled MXene/n‐type conjugated polyelectrolyte (CPE) superlattice‐like heterostructure that enables redox‐active, fast, and reversible ammonium storage is reported. The superlattice‐like structure persists as the CPE:MXene ratio increases, accompanied by a linear increase in the interlayer spacing of MXene flakes and a greater overlap of CPEs. Concurrently, the redox activity per unit of CPE unexpectedly intensifies, a phenomenon that can be explained by the enhanced de‐solvation of ammonium due to the increased volume of 3 Å‐sized pores, as indicated by molecular dynamic simulations. At the maximum CPE mass loading (MXene:CPE ratio = 2:1), the heterostructure demonstrates the strongest polymeric redox activity with a high ammonium storage capacity of 126.1 C g−1 and a superior rate capability at 10 A g−1. This work unveils an effective strategy for designing tunable superlattice‐like heterostructures to enhance redox activity and achieve rapid charge transfer for ions beyond lithium. This work reports a superlattice‐like MXene@conjugated polyelectrolyte (CPE) heterostructure prepared by a facile self‐assembly method. The MXene@CPE heterostructure maintains ordered superlattice‐like structure with adjustable d‐spacing. The tunable structure enables enhanced polymeric redox owing to the de‐solvation of NH4+ ions facilitated by the 3 Å‐sized pores and shows a superior rate performance for NH4+ storage.