Unlock the Capacity of MnO ₂ Cathodes in Aqueous Zn‐Ion Storage

Abstract Structural degradation and rapid capacity decay of MnO 2 cathodes significantly limit the battery lifespan of aqueous zinc‐ion batteries (AZIBs). Herein, an ion‐enriched and multifunctional composite film is developed through a controllable and mature layer‐by‐layer (LbL) self‐assembly technique for stabilizing MnO 2 cathodes. The multilayer consists of two bio‐macromolecules of chitosan (CS) and sodium alginate (SA). It is shown that this LbL film can optimize the ion distribution on the MnO 2 cathode due to the presence of a large amount of polar functional groups. This can promote the Mn 2+ ↔ Mn 3+ ↔ Mn 4+ two‐step redox process and the reversible deposition of Zn 4 (OH) 6 SO 4 ·xH 2 O. It is also shown that the dissolved Mn 2+ can be immobilized to prevent capacity decay. Full cells with the double layer of SA/CS exhibit maximum reversible capacity of 604.3 mAh g −1 at 0.3 A g −1 . The cathode maintains the capacity retention of 86.96% after 1000 stable cycles at 2 A g −1 . Given the facile and cost‐effective material and production of the LbL films, this cathode‐modified strategy may boost the industrial application of high‐capacity AZIBs.