High Performance Lithium-Ion Hybrid Capacitors Employing Fe₃O₄-Graphene Composite Anode and Activated Carbon Cathode

Lithium-ion capacitors (LICs) are considered as promising energy storage devices to realize excellent electrochemical performance, with high energy-power output. In this work, we employed a simple method to synthesize a composite electrode material consisting of Fe₃O₄ nanocrystallites mechanically anchored among the layers of three-dimensional arrays of graphene (Fe₃O₄-G), which exhibits several advantages compared with other traditional electrode materials, such as high Li storage capacity (820 mAh g^-1 at 0.1 A g^-1), high electrical conductivity, and improved electrochemical stability. Furthermore, on the basis of the appropriated charge balance between cathode and anode, we successfully fabricated Fe₃O₄-G//activated carbon (AC) soft-packaging LICs with a high energy density of 120.0 Wh kg^-1, an outstanding power density of 45.4 kW kg^-1 (achieved at 60.5 Wh kg^-1), and an excellent capacity retention of up to 94.1% after 1000 cycles and 81.4% after 10 000 cycles. The energy density of the Fe₃O₄-G//AC hybrid device is comparable with Ni-metal hydride batteries, and its capacitive power capability and cycle life is on par with supercapacitors (SCs). Therefore, this lithium-ion hybrid capacitor is expected to bridge the gap between Li-ion battery and SCs and gain bright prospects in next-generation energy storage fields.