Hybrid Li-Ion Electrochemical Capacitor Enabled By Highly Crumpled Nitrogen-Doped Graphene
Authors: AlZahrani Atif Saeed , Yi Ran , Song Jiangxuan , Wang Donghai 
Published: February 2020

Energy storage systems are vital components to the success of emerging technologies such as renewable resources harvesting technologies and electric vehicles. Commercially, rechargeable Li-ion batteries (LIBs) and electrochemical capacitors (ECs) share the majority of energy storage systems markets. While LIBs are known for high energy density (≈ 240 Wh/kg) and ordinary power density (≈ 1000 W/kg), ECs have high power density (>10000 W/kg) and cyclability (>100000 cycles) but low energy density (≈ 5 Wh/kg)1.
For past two decades, a new energy storage device has been studied to hybridize the two technologies by combining an EC cathode and a rechargeable battery's anode and achieve a balanced performance2. The hybridized device is commonly known as hybrid electrochemical capacitor (h-EC) or asymmetrical electrochemical capacitors which is intended to have a power density higher than rechargeable batteries and energy density higher than ECs. The initial study of the designed system comprised of nanostructured Li4Ti5O12 negative electrode accompanied with activated carbon as positive electrode. The performance showed similar cyclability to non-aqoues EDLC electrodes and 500% energy storage improvement2.
Unlike the negative electrode materials, positive electrodes are believed to be more sensitive in determining the overall performance of the h-ECs due to the huge gap between the capacity of electrodes half-cells2,3. Thus, higher capacitance positive electrodes are perused using higher surface area and/or pseudocapacitance properties. The primary material for the positive electrode is carbonaceous materials with by high surface areas, such as biomass-derived doped and un-doped activated carbon1,3–5, 3D graphene/carbon composite6. Nonetheless, heteroatom-doped graphene, which has shown to increase the conductivity and activate the pseudocapacitance feature to enhance the capacitance, has rarely been reported as a cathode for hybrid electrochemical capacitors7,8.
Herein, we demonstrate a high energy density of 106.2 Wh/kg at high power density 14160 Wh/kg hybrid electrochemical cell by hybridizing highly crumpled nitrogen-doped graphene with carbon-coated nano-sized silicon [Figure 1a, b]. Furthermore, the crumpled nitrogen-doped graphene was synthesized by a two-step method which led to a surface area of 1200 m2/g and high pore volume 4.5 cm3/g9. In addition, the long cycling performance has shown a capacity retention of 78% after 5000 cycles at high rate 1A/g [Figure 1c]. Carbon coating silicon anode proved to be highly advantageous for both cycling the h-EC and storing energy at higher power density. While 88% was of the initial capacity was preserved for carbon-coated silicon, only 63% was maintained for bare nano-sized silicon after 1000 cycles [Figure 1d].

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Figure 1