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 Fe 3 O 4 nanocrystallites mechanically anchored among the layers of three-dimensional arrays of graphene (Fe 3 O 4 –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 Fe 3 O 4 –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 Fe 3 O 4 –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.