Fast Potassium Storage in Hierarchical Ca0.5Ti2(PO4)3@C Microspheres Enabling High-Performance Potassium-Ion Capacitors

Hybrid potassium-ion capacitors (KICs) show great promise for large-scale storage on the power grid because of cost advantages, the weaker Lewis acidity of K+ and low redox potential of K+/K. However, a huge challenge remains for designing high-performance K+ storage materials since K+ ions are heavier and larger than Li+ and Na+. Herein, the synthesis of hierarchical Ca0.5Ti2(PO4)(3)@C microspheres by use of the electrospraying method is reported. Benefiting from the rich vacancies in the crystal structure and rational nanostructural design, the hybrid Ca0.5Ti2(PO4)(3)@C electrode delivers a high reversible capacity (239 mA h g(-1)) and superior rate performance (63 mA h g(-1) at 5 A g(-1)). Moreover, the KIC employing a Ca0.5Ti2(PO4)(3)@C anode and activated carbon cathode, affords a high energy/power density (80 W h kg(-1) and 5144 W kg(-1)) in a potential window of 1.0-4.0 V, as well as a long lifespan of over 4000 cycles. In addition, in situ X-ray diffraction is used to unravel the structural transition in Ca0.5Ti2(PO4)(3), suggesting a two-phase transition above 0.5 V during the initial discharge and solid solution processes during the subsequent K+ insertion/extraction. The present study demonstrates a low-cost potassium-based energy storage device with high energy/power densities and a long lifespan.