Rational design of ultrafine FeSe2 nanocrystals embedded within hollow mesoporous carbon bowls for potassium-ion batteries with long-term cycling stability and high volumetric capacity.

• Bowl-like carbon/FeSe 2 composites are successfully synthesized by a facile method. • Ultrafine FeSe 2 nanocrystals are well embedded in bowl-like carbon matrix. • Bowl structure has a higher packing density than the conventional hollow structure. • Compared with hollow structure, the volumetric capacity of sample increased by 60%. • The KIB full cell composed of FeSe 2 @HMCB//PB has been demonstrated. Hollow structures are commonly used to alleviate the mechanical stress on electrode materials and to provide more active sites in potassium-ion batteries (KIBs). Nevertheless, the excessive internal voids within these structures significantly reduce the packing density of particles, resulting in a relatively low volumetric energy density of the fabricated electrodes, which is undesirable for practical use. We designed a hollow mesoporous carbon bowl embedded with ultrafine bis(selanylidene)iron (FeSe 2) nanocrystals (FeSe 2 @HMCB) via a controllable impregnation method and subsequent selenization process for high-performance KIBs. The as-obtained FeSe 2 @HMCB can inherit the advantages of conventional hollow carbon-based composites, such as alleviation of volume variation in active materials, abundant ion storage sites, and high electrical conductivity. Simultaneously, the bowl structure has a higher packing density than the conventional hollow structure, resulting in a significant increase in the volumetric energy density of the fabricated electrodes. Because of these advantages, the FeSe 2 @HMCB exhibits a high, stable reversible capacity of 326 mA h g⁻¹ even after 1000 cycles at 0.5 A g⁻¹, and excellent rate capacities (182 mA h g⁻¹ at 3.0 A g⁻¹). Compared with the hollow structured counterpart, the volumetric capacity (mA h cm⁻³) of FeSe 2 @HMCB increased by 60%. Furthermore, a full cell consisting of FeSe 2 @HMCB//Prussian blue (PB) exhibits excellent electrochemical performance (99 mA h g⁻¹ after 100 cycles at 0.1 A g⁻¹). [Display omitted] [ABSTRACT FROM AUTHOR]