Bilayer Mn-based Prussian blue cathode with high redox activity for boosting stable cycling in aqueous sodium-ion half/full batteries.

The Mn-based Prussian blue analogs (PBAs) have garnered significant attention due to their high specific capacity, stemming from the unique multi-electron reactions with Na. However, the structural instability caused by multi-ion insertion impacts the cycle life, thus limiting their further application in aqueous sodium-ion batteries (ASIBs). To address this issue, this work employed an in situ epitaxial solvent deposition method to homogeneously grow Ni hexacyanoferrate (NiHCF) on the surface of MnPBA, which can effectively overcome the de-intercalation instability. The resulting heterostructured MnPBA@NiHCF integrates the multiple redox-active centers of MnPBA with the confinement ability of the outer NiHCF layer, thereby maintaining overall structural stability. As a cathode material for ASIBs, MnPBA@NiHCF achieves a reversible specific capacity of 66.2 mAh/g after 200 cycles at 1 A/g, significantly outperforming the single-component MnPBA and NiHCF, respectively. Moreover, it demonstrates ultralong cycling stability with only 0.0002 % capacity fade per cycle over 20,000 cycles at 10 A/g. Extensive kinetic analyses further confirm its superior Na diffusion behaviors with disclosed redox mechanism through the comprehensive in situ Raman and ex situ analysis. A full cell built with a polyimide (PI) anode achieved an energy density of up to 59.9 Wh kg, displaying a power output of 1200.4 W kg and exceptional cycle stability. This work provides innovative insights for developing stable PBA cathodes for ASIB applications.