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 Ca 0.5 Ti 2 (PO 4 ) 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 Ca 0.5 Ti 2 (PO 4 ) 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 Ca 0.5 Ti 2 (PO 4 ) 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 Ca 0.5 Ti 2 (PO 4 ) 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.