Hollow Porous α-Fe₂O₃ Nanoparticles as Anode Materials for High-Performance Lithium-Ion Capacitors

The lithium-ion capacitor (LIC) is a novel energy storage device, pairing battery-type anodes with capacitor-type cathodes, capable of delivering high energy and power densities. The anode materials of excellent high-rate capability, however, are required to resolve the common kinetics imbalance issue for high-performance LICs. A simple one-step solvothermal, metal–organic framework (MOF) evolved process was developed to synthesize hollow porous α-Fe₂O₃ nanoparticles (α-Fe₂O₃ HPNPs) as an anode material of excellent high-rate capability for high-performance LICs. The α-Fe₂O₃ HPNP anode achieved an excellent high-rate capability and cycling stability, through accelerating lithium-ion diffusions with the porous shell and shortening lithium-ion diffusion paths and buffering large volume variations during cycling with the confined hollow space. The quantitative kinetic analyses showed that capacitive processes are the main contributor to the capacity generation of the α-Fe₂O₃ HPNP anode, making the α-Fe₂O₃ HPNP an excellent match with capacitor-type cathodes, glucose-derived carbon nanospheres (GCNS) of high specific surface areas, for the assembly of LICs. The α-Fe₂O₃ HPNP//GCNS LIC delivered a high energy density of 107 Wh kg–¹ at 0.24 W kg–¹ and maintained an adequate energy density of 86 Wh kg–¹ at an extremely high power density of 9.68 kW kg–¹. Moreover, it exhibited a high capacity retention of 84% after 2500 cycle operations at 1 A g–¹. Both materials and nanostructure of electrodes play a key role for high-performance LICs, and the hollow porous nanoparticulate structure is proven to be an advantageous nanostructure for the anode materials of LICs.