<p>Potassium-ion capacitors (PICs) have received increasing attention because of their high energy/power densities and the abundance of potassium. However, achieving high-capacity anodes for PICs is still a great challenge. Herein, FePS₃ nanoflakes encased in carbon (C-FePS₃) have been constructed via a nanospace-confinement solid-phase reaction that synergistically utilizes an ultrathin precursor and carbon to constrain the layer stacking of FePS₃ while improving electrical conductivity and structural stability. The potassium storage mechanism of C-FePS₃, which involves initial intercalation followed by a conversion-alloying reaction with a nine-electron transfer, has been elucidated. Theoretical calculations further confirm the thermodynamic feasibility of this electrochemical reaction, high K-adsorption capability, and rapid transfer kinetics of C-FePS₃. Owing to the few-layered structure, nine-electron transfer, and fast kinetics, the C-FePS₃ achieves an impressive specific capacity of 607.2 mAh g⁻¹ at 0.1 A g⁻¹ and 179.4 mAh g⁻¹ at 5 A g⁻¹, coupled with long-term stability of 500 cycles, positioning it at the forefront among anode materials. Additionally, the C-FePS₃ in PICs yields superior energy power density. This study gives new design strategies for achieving high-level electrodes for potassium storage and provides an in-depth understanding of the storage mechanism of metal phosphorous chalcogenide families.</p>