FeOHSO₄@C Cathode with Low Strain and High Pseudocapacitance for Advanced Potassium‐Ion Batteries

Iron-based polyanionic cathode materials in potassium-ion batteries (KIBs) have appealed to an increasing number of interest due to these advantages of low cost, environmental friendliness and excellent structural stability. However, these inherent drawbacks of inferior electronic conductivity and terrible nanostructural stability hinder its practical application. Here, we report a novel low-strain iron-based polyanion-type cathode material FeOHSO₄@C for KIBs. In this work, the surface of FeOHSO₄ nanoparticles is well carbon encapsulated, carbon coating layer with large surface area and excellent electrical conductivity is ≈2.5 nm in thick, which can not only inhibit the aggregation and growth between FeOHSO₄@C nanoparticles during charging and discharging, but also provide a 3D electronic conductive framework that activates electrochemical reactivity. As a result, the FeOHSO₄@C cathode exhibits outstanding potassium storage capacity (capacity retention of 80.95% over 200 cycles at 20 mA g^-1) attributed to a low-strain mechanism for K⁺ uptake/removal, high pseudocapacitance, as well as 3D electronic conductive framework. Operando XRD and ex situ XPS analyses revealed a single-phase reaction route of orthorhombic FeOHSO₄@C during cycling.