From Jackfruit Rags to Hierarchical Porous N-Doped Carbon: A High-Performance Anode Material for Sodium-Ion Batteries
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Renewable biomass-derived carbon materials have attracted increasing research attention as promising electrode materials for electrochemical energy storage devices, such as sodium-ion batteries (SIBs), due to their outstanding electrical conductivity, hierarchical porous structure, intrinsic heteroatom doping, and environmental friendliness. Here, we investigate the potential of hierarchical N-doped porous carbon (NPC) derived from jackfruit rags through a facile pyrolysis as an anode material for SIBs. The cycling performance of NPC at 1 A/g for 2000 cycles featured a stable reversible capacity of 122.3 mA·h/g with an outstanding capacity retention of 99.1%. These excellent electrochemical properties can be attributed to the unique structure of NPC; it features hierarchical porosity with abundant carbon edge defects and large specific surface areas. These results illuminate the potential application of jackfruit rags-derived porous carbon in SIBs.
KeywordsPorous carbon N-doped carbon Sodium-ion battery Anode Jackfruit rags Energy storage and conversion
The serious issues of ever-increasing global warming and resource shortage have stimulated increasing attention on exploring novel energy storage and conversion technologies . New energy sources such as solar, water, wind, and geothermal energy have been largely developed to solve the problems. However, these energy sources are intermittent and unpredictable and thus cannot provide a stable and continuous power supply. Storing the generated electrical energy in electrochemical energy storage devices is a promising route to maximize renewable energy utilization. Currently, rechargeable lithium-ion batteries (LIBs) with high energy density, long lifetime, and environmental benignity are the most promising portable energy storage devices [2, 3, 4]. However, the high cost and uneven distribution of lithium minerals will restrict the large-scale commercial markets of electronic products and electric vehicles. In recent years, sodium-ion batteries (SIBs) have been widely revived as an intriguing alternative to LIBs for energy storage owing to the abundant distribution of sodium (Na) resources in the earth and the similarity of sodium with lithium in physical/chemical properties [5, 6, 7]. However, the poor kinetics of the Na+ insertion/de-insertion reaction caused by the larger radius of Na+ (0.102 nm) compared to Li+ (0.076 nm) limits the development and practical applications of SIBs [8, 9]. Therefore, seeking appropriate and efficient anode materials that are capable of reversible insertion/de-insertion of Na+ is essential to achieving the practical application of SIBs.
Carbonaceous materials represent the most widely used anode materials for various rechargeable batteries. One example is the commercial graphite; however, in the previous research, Na+ could not be inserted into graphite in SIBs, indicating the thermodynamic electrochemical insertion of Na+ into graphite is not possible [10, 11]. Research on other carbonaceous materials has found that Na+ is more inclined to be inserted into disordered layers of carbon [12, 13]. Moreover, it is worth noting that the electrochemical performance of carbonaceous materials can be enhanced through doping heteroatom elements and manufacturing porous structures [14, 15], which provides higher conductivity and extra Na-ion storage sites in SIBs [16, 17, 18, 19, 20, 21]. Hence, in recent years, biomass-derived porous carbon has been extensively investigated as anode materials for SIBs  because of their advantages of high abundance, low cost, easy accessibility, environmental friendliness, hierarchical porous structure, and intrinsic heteroatom doping behavior. In addition, their high electrical conductivity and large electrolyte/electrode contact area are favorable to the reversible insertion and extraction of Na+, resulting in excellent and stable electrochemical performance . Many types of biomass such as bamboo , water hyacinths , waste tea , corn stalk , wood [28, 29], lotus , kelp , and human hair  have been investigated as a precursor for producing porous carbon materials for various energy storage devices; however, it is still a challenge to prepare high-performance porous carbon from a cost-efficient resource.
Jackfruit (Artocarpus heterophyllus) is a tropical fruit that is widely planted in South and Southeast Asian countries. Jackfruit rags are latex-like filaments surrounding the edible part (fruit/aril), but they are typically deserted as a waste. Therefore, to reduce the waste and develop new value-added products, herein we employ the jackfruit rags as a precursor to produce porous carbon. Hierarchical N-doped porous carbon (NPC) was synthesized through a simple one-step carbonization of jackfruit rags without using any chemical or physical activation process. Moreover, jackfruit rags contain numerous alkaline earth elements (Na+, K+, Ca2+, Mg2+) that could lead to self-activation during pyrolysis, which makes the carbon production process more environmentally friendly and low cost [33, 34]. We evaluated the effectiveness of as-synthesized NPC as anode materials for SIBs, and they showed excellent electrochemical performance.
Preparation of N-Doped Porous Carbon
Thermogravimetric analysis (TGA, Netzsch STA 449) was performed to optimize the pyrolysis temperature of the biomass to obtain pyrolytic carbons. X-ray diffraction (XRD, MiniFlex600, Rigaku), X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi), and Raman spectroscopy (LabRAM HR, 532 nm laser) were used to examine the ingredients, element valence state environment, degree of crystallinity, and the intensity ratio of defect to graphitic carbon (ID/IG). Porous characteristics such as the specific surface area, pore volume, and pore size distribution were analyzed on a Micromeritics ASAP 2020 adsorption analyzer. Pore size and distribution were determined by the Brunauer–Emmett–Teller (BET) method. Scanning electron microscopy (SEM, Nova NanoSEM450) and transmission electron microscopy (TEM, JEM-2010) were employed to investigate the microscopic shape and structure of carbon materials. The content of nitrogen element was determined using an elemental analyzer (EA, Vario EL Cube, Elementar).
The electrochemical performance of the biomass-derived carbons was investigated by assembling CR2032 coin cells. The anode materials of SIBs were made by mixing 80% NPC-700 (as well as NPC-800 and NPC-900) with 10% Super P carbon black and 10% polyvinylidene fluoride to form a slurry and coating on a copper foil with an active mass loading of ~ 1.0 mg/cm2 and then dried at 80 °C for 12 h under vacuum. Sodium metal foil was used as the counter and reference electrodes. Glass microfiber membrane (Grade GF/D, Whatman) was used as separator, and 1.0 mol/L NaClO4 dissolved in a mixture of propylene carbonate, ethylene carbonate, and fluoroethylene carbonate (1:1:0.05 V/V/V) was used as electrolyte. All coin cells were assembled under argon atmosphere in a glovebox (H2O, O2 < 0.1 ppm, MBraun, Germany). The galvanostatic charge/discharge tests were carried out on a Land CT2001A tester between 3.00 and 0.01 V. Cyclic voltammetry was conducted on an electrochemical workstation (CHI760E, China) at a scan rate of 0.1 mV/s. Electrochemical impedance spectroscopy (EIS, Biologic, VSP) measurements were implemented in a frequency range of 0.01 Hz–100 kHz.
Results and Discussion
Structure and Composition Characterization
Result of nitrogen adsorption analysis and the mass of nitrogen obtained from elemental analysis
The morphology of as-prepared NPCs was detected using SEM analysis. As depicted in Fig. 4, the NPCs presented an interconnected porous network structure. As the pyrolysis temperature rose from 700 to 900 °C, the etching of jackfruit rags became increasingly critical, so that the pore size grew larger and the surface became rougher. Particularly, the NPC-900 holes became channel-like; and this can explain the decrease in the specific surface area as the temperature increased from 800 to 900 °C. The NPC-800 showed a large set of nanoscale pores, forming a 3D well-connected hierarchical porous structure (Fig. 4d). These hierarchical pores will serve as pathways for electrolyte permeation, shortening the diffusion, and transfer distance of sodium ions, thus favoring sodium storage performance.
Figure 6b shows the 1st, 2nd, 3rd, 4th, and 5th charge/discharge profiles of NPC-800 at a constant current density of 100 mA/g. The initial discharge/charge curves presented a discharge and charge specific capacities of 559.6 mA·h/g and 255.6 mA·h/g, respectively. According to the result, the initial coulombic efficiency was calculated to be 45.5%. In the subsequent cycles, the coulombic efficiency ascended enormously to 87.3%. The initial irreversible capacity loss was mainly due to the formation of a solid electrolyte interface layer and the irreversible trapping of Na ions in the porous structure .
In summary, hierarchical porous N-doped carbon materials were synthesized from inexpensive, environmentally friendly jackfruit rags by a facile calcination process under argon atmosphere without any chemical or physical activation. The calcination temperature and resulting sample were systematically investigated. The hierarchical NPC obtained from jackfruit rags at 800 °C (NPC-800) possessed the best cycling performance and rate capability when used as the anode of SIBs. This excellent electrochemical performance can be attributed to the presence of the hierarchical porous structure and N doping of carbon. The outstanding Na-ion storage performance combined with the sustainable biomass source and scalable synthesis method makes jackfruit rags-derived NPC-800 a potential competitive SIBs anode material.
This work was financially supported by National Natural Science Foundation of China (Nos. 21875253, 21703249) and the 1000 Plan Professorship for Young Talents.
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