Fe^HS vacancies in a Prussian white cathode leads to enhanced Fe^LS activity and electrode kinetics for boosted K⁺ storage

Fe-based Prussian white (PW) is considered a superior cathode material for potassium-ion batteries (PIBs) because of its three-dimensional open framework structure, high potassium content, and low cost; however, a dramatic distortion of low-spin (LS) Fe-C octahedra typically occurs upon the extraction of second K+, leading to a deteriorated structure as well as poor rate and cycling performance, thus limiting their practical applications. In this work, Fe-HS vacancies were successfully incorporated into the lattice structure of PW by simply controlling synthesis temperatures in the presence of the chelating agent potassium citrate, which was confirmed by a series of structural characterizations. A low reaction temperature is found to suppress the incorporation of Fe into the PW structure and therefore leads to the formation of more Fe-HS vacancies. As a result, a sample synthesized at the lowest temperature (0 degrees C) exhibits a more prominent charge-discharge plateau at a high potential region that corresponds to the enhanced activity of Fe-LS and best K+ storage properties, including a reversible capacity of 117.2 mA h g(-1) at a current rate of 30 mA g(-1), 80.7% capacity retention at 10C after 1000 cycles, and excellent rate capability. Kinetic analysis and impedance spectroscopy clearly revealed enhanced charge transfer/ion diffusion and a stable host structure upon cycling for the samples synthesized at lower temperatures, further demonstrating the critical role of Fe-HS vacancies for efficient K+ storage. In general, this work provides an efficient and simple strategy to create cation vacancies in the PW cathode, which enables the development of high-performance potassium-ion batteries and can be extended to other related energy storage systems.