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-cells^2,3. 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 carbon^1,3–5, 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 capacitors^7,8.

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].

Reference:

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