Nano-size porous carbon spheres as a high-capacity anode with high initial coulombic efficiency for potassium-ion batteries

Hard carbon materials have been recognized as a promising family of anode materials for potassium ion batteries (PIBs), but their practical application is severely hindered due to the inferior initial coulombic efficiency (ICE) and low capacity. Herein, we report our findings in simultaneously improved potassium storage capacity and ICE through the design of nano-size and porous structure and the appropriate selection of electrolytes. Benefiting from the high specific surface area, stable electrode-electrolyte interface, and fast potassium ion and electron transfer, the optimized electrode exhibits a high ICE of up to 68.2% and an outstanding reversible capacity of 232.6 mA h g^-1 at 200 mA g^-1. In particular, superior cycling stability of 165.2 mA h g^-1 at 1000 mA g^-1 and 129.7 mA h g^-1 at 2000 mA g^-1 can be retained after 1500 cycles, respectively. Quantitative analysis reveals that this optimized structure leads to an enhanced surface-controlled contribution, resulting in fast potassiation kinetics and electronic-ionic conductivities, which are regarded as essential features for potassium storage. Our findings in this work provide an efficient strategy to significantly improve potassium storage capacity while maintaining a high ICE for hard carbon electrodes.