The increasing demand for sustainable, high-power energy storage systems necessitates the development of advanced energy storage devices. While lithium-ion batteries (LIBs) offer high energy density, their power density is limited. Conversely, supercapacitors (SCs) provide high power but suffer from low energy density. Lithium-ion capacitors (LICs), combining the advantages of both LIBs and SCs, emerge as a promising solution. However, a major challenge in LICs is the kinetic mismatch between capacitive cathodes and faradaic anodes. To address this, improving electron and ion transport in both electrodes is crucial through structural design and chemical doping. Graphene, with its high surface area and conductivity, is an attractive anode material. However, pure graphene suffers from poor rate performance and cycling stability due to difficulties in Li⁺ stabilization and diffusion. Therefore, strategies like creating porosity to enhance ionic conduction and introducing heteroatoms (e.g., nitrogen) to improve electrical conductivity and increase active sites are being explored. This paper proposes a new method for fabricating NGFs via a magnesiothermic combustion synthesis, aiming to overcome limitations of existing N-doped graphene synthesis methods and provide a synergistic enhancement of both ionic and electronic conductivity for high-power LIC applications.

Existing literature highlights the use of various anode materials in LICs, including alloy materials (Si and Sn-based), conversion materials (MnO₂, VN, MoS₂, Fe₃O₄), and intercalation materials (Li₄Ti₅O₁₂, V₂O₅, TiO₂, Nb₂O₅). However, these materials face challenges such as large volume changes (alloys and conversion anodes) or low Li⁺ storage ability (intercalation anodes). Graphene, an ideal candidate due to its high surface area and conductivity, has shown promise, but its performance is hampered by challenges in Li⁺ stabilization and diffusion. Studies show that porous graphene and heteroatom-doped graphene (particularly nitrogen-doped graphene, NG) can overcome these limitations. While various methods such as solvothermal treatment and annealing of graphite oxide (GO) or chemical vapor deposition (CVD) exist for producing N-doped graphene, they often involve complex procedures and high energy consumption. This research aims to address these shortcomings by proposing a simpler, scalable synthesis method.

Nitrogen-enriched graphene frameworks (NGFs) were synthesized via a magnesiothermic combustion synthesis using CO₂ as the carbon source, Mg powder as the reducing agent, and melamine (C₃H₆N₆) as the nitrogen source. MgO powder acted as a template. The reaction was initiated by applying a current to a tungsten coil embedded in the precursor mixture, leading to a self-sustaining combustion reaction. The resulting product was then washed with diluted HCl to remove the MgO template and freeze-dried. Different melamine amounts were used to control nitrogen doping levels, resulting in NGF-1, NGF-2, and NGF-3 samples, with NGF-0 representing undoped graphene. The synthesized materials were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Brunauer-Emmett-Teller (BET) surface area analysis, and four-probe conductivity measurements. Electrochemical performance was evaluated using 2032-type coin cells with Li foil as the counter and reference electrodes, and a soft-packaged all-graphene LIC full cell was assembled using NGFs as both cathode and anode. Electrochemical measurements included cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). Density functional theory (DFT) calculations were performed to understand the interaction between Li and graphene.

The magnesiothermic combustion synthesis yielded NGFs with a crosslinked porous structure and uniformly distributed nitrogen doping. SEM and TEM images revealed the porous, interconnected morphology of the NGFs, with the degree of crystallinity increasing with higher melamine content. XRD analysis confirmed the enhanced graphitic structure of NGFs compared to NGF-0. Raman spectroscopy showed an increase in the I_G/I_D ratio with increasing nitrogen doping, indicating improved graphitic order. The NGFs exhibited excellent electrochemical performance, with NGF-2 displaying capacities of 1361 mA h g⁻¹ at 0.1 A g⁻¹ and 827 mA h g⁻¹ at 3 A g⁻¹. The LIC using NGF-2 achieved a high energy density of 151 Wh kg⁻¹ and retained 86 Wh kg⁻¹ at an ultrahigh power density of 49 kW kg⁻¹. DFT calculations confirmed that pyridinic and pyrrolic nitrogen doping increased Li adsorption energy and carrier density, leading to enhanced kinetics.

The superior electrochemical performance of the NGFs can be attributed to the synergistic effect of the porous structure and nitrogen doping. The porous structure facilitates efficient ion diffusion, while the nitrogen doping enhances electrical conductivity and provides additional active sites for lithium storage. The high energy and power densities achieved by the LIC demonstrate the effectiveness of the magnesiothermic combustion synthesis in producing high-performance electrode materials for energy storage applications. These findings highlight the advantages of this facile, scalable, and cost-effective synthesis method for producing advanced carbon materials for energy storage applications.

This study successfully demonstrates a large-scale, low-cost synthesis of nitrogen-enriched graphene frameworks via magnesiothermic combustion of CO₂. The resulting NGFs exhibit enhanced electrochemical performance due to a synergistic effect of porosity and nitrogen doping. The LIC fabricated using this material shows promising performance for high-power applications. Future work could explore optimizing the synthesis parameters for further improved performance and investigating other heteroatom doping strategies.

While the study demonstrates excellent performance, further investigation is needed to assess long-term cycling stability at high current densities. The study primarily focuses on the anode material; future work should explore the full cell performance optimization by considering cathode materials. The scalability of the method needs further testing for commercial application.