Boron-doped graphene anode coupled with microporous activated carbon cathode for lithium-ion ultracapacitors

Lithium-ion hybrid capacitors combine the advantages of both high energy of lithium-ion batteries and high-power of ultracapacitors by using one highly reversible battery-type electrode (e.g., graphite, carbon matrix, metal oxides, etc.) and another high surface area supercapacitor-type electrode (e.g., activated carbon) in one unit cell. This work reports the fabrication of a lithium-ion capacitor using boron-doped graphene nanosheets as anode and microporous carbon from activated rice husk as a cathode. The lithium-ion storage characteristics of boron-doped graphene nanomaterials and adsorption properties of microporous carbon leads to the realization of their usage in a lithium-ion capacitor electrode materials. The lithium-ion capacitor full cell with lithiated boron-doped graphene nanomaterial anode and activated rice husk cathode demonstrates energy densities of 162 and 83 W h kg−1 at power densities of 590, 14750 W kg−1, respectively. The mechanistic performance of the LIC full cell below and above the open circuit voltage (OCV) was analysed to understand the diffusion and adsorption control in the achievable energy. The cell cycles till 25,000 cycles with 65 % capacitance retention at 1 A g−1 current density. This study demonstrates a striking balance between the high energy and high power capabilities in a single unit lithium-ion capacitor cell through the utilization of adsorptive as well diffusive ion storage mechanism of the electrode active materials. The full cell is analysed for self discharge and leakage current to realise the practical feasibility of the electrode active materials. Also, the electrochemical impedance spectra reveal the probable reason behind the degradation of cycling performance.