Highly efficient sodium-ion capacitor enabled by mesoporous NaTi2(PO4)3/C anode and hydrogel-derived hierarchical porous activated carbon cathode

Abstract

Developing efficient anode materials to minimize the mismatch in the reaction kinetics of sodium-ion capacitors is highly desirable. Herein, mesoporous sodium titanium phosphate nanoparticles with highly sp2-coordinated carbon coatings (meso-NTP/C NPs) are successfully synthesized by utilizing cost-effective and moisture-stable precursors. Their crystalline structure, morphology, textural features, carbon nature and amount are systematically investigated. Following the electrochemical measurements, the synthesized meso-NTP/C NPs not only deliver comparable reversible capacity (94 mAh/g at 0.1 C) and superior rate capability (78 mAh/g at 2 C) but also achieve an excellent lifespan (capacity retention: 91 % after 4500 cycles at 1.5 C). Besides that, the asymmetric sodium-ion capacitors fabricating by meso-NTP/C anode and free-standing hydrogel-derived hierarchical porous activated carbon (H-HPAC) cathode exhibit remarkable electrochemical properties (i.e., 73 mAh/g at 0.76 A/g, 42 mAh/g at 7.6 A/g, 96 % of retention after 5000 cycles at 2.85 A/g). These attributes can be attributed to the distinctive mesostructure and open three-dimensional frameworks for facile sodium-ion insertion/extraction, and their continuous sp2-coordinated carbon coatings, which facilitate electronic conduction.