English

• 0.   Introduction

  As the global energy crisis intensifies and environmental protection awareness continues to rise, efficient and environmentally friendly energy storage technologies have become the focus of research and industry^[1-2]. Lithium-ion batteries (LIBs), as high-energy-density and long-cycle-life energy storage devices, have been widely used in various fields such as electric vehicles, smartphones, and renewable energy storage^[3-4]. However, with the continuous expansion of the market and technological advancements, the performance requirements for LIBs are also increasing, especially in terms of energy density, cycle stability, and safety^[5-6].

  Currently, research on LIBs mainly focuses on the innovation of electrode materials. Although traditional graphite anode materials possess good cycle stability, their theoretical specific capacity is relatively low, making it difficult to meet the demands of high-energy-density batteries^[7-10]. Therefore, the development of novel anode materials with high capacity and stability has become a hot topic in LIBs research.

  In recent years, nitrogen-doped graphene has demonstrated significant potential in the energy storage field due to its unique physicochemical properties, such as high electrical conductivity, large specific surface area, and excellent chemical stability^[11-13]. The introduction of nitrogen atoms not only improves the electronic structure of graphene, enhancing its electrochemical activity but also strengthens its interaction with lithium ions, thus improving its lithium storage performance. For example, Xie et al. prepared highly nitrogen-doped graphene carbon nanosheets using coffee grounds as a carbon and nitrogen source, calcium carbonate, and iron nitrate and its derivatives as structural templates, along with a graphitization catalyst^[14]. The removal of by-products CaO and Fe/Fe_xO_y/Fe₃C through diluted hydrochloric acid resulted in the formation of voids and nanopores. Compared to waste coffee grounds, the relatively high content of calcium carbonate led to the random cross-linking of the generated hollow graphitized carbon structural units, which entailed the intertwining of graphene-like nanosheets to form a 3D porous structure. Additionally, the gaseous by-product carbon dioxide generated during this process may further activate the carbon framework, creating additional nanopores. The battery maintained a high reversible capacity of 760 mAh·g^-1 after 100 cycles at 100 mA·g^-1. Additionally, carbon nanotubes, as a one-dimensional nanomaterial, also possess excellent electrical conductivity and mechanical properties, showing immense potential in the application of LIBs^[15-17]. The high specific surface area and electrical conductivity of carbon nanotubes enable the formation of continuous conductive networks in electrodes, which enables the promotion of electrical conductivity and electrochemical performance of electrodes, further enhancing the overall performance of batteries. To address the issue of uneven distribution of carbon nanotube (CNT)/reduced graphene oxide (RGO) composite materials and enhance their electrochemical energy storage performance, Jiang et al. adopted a modified Hummers method to fabricate CNT/RGO monoliths by partially oxidizing and exfoliating CNT^[18]. They partially exfoliated CNT to prepare a composite material of CNT and graphene oxide (GO), which was then reduced with the assistance of l-ascorbic acid, followed by freeze-drying and annealing processes. This resulted in CNT/RGO composite monoliths that could be directly assembled into electrochemical supercapacitors without the need for any binders. Characterization via scanning electron microscopy, X-ray diffraction, and laser Raman spectroscopy revealed a uniform distribution of CNT and RGO within the composite, and the monoliths possessed a large specific surface area and a high volume of mesopores. The specific capacitance of CNT/RGO reached 128 F·g^-1 (with organic electrolyte), exhibiting only a 4.37% decay rate after 2 000 cycles, demonstrating its superior potential in the field of electrochemical energy storage. Heteroatom-doped 3D graphene-carbon nanotube networks not only inherit the superior properties of 1D and 2D materials but also achieve rapid electron and ion transport through their 3D network structure, further enhancing the lithium storage performance and cycle stability of electrode materials.

  In this study, we successfully prepared a nitrogen-doped 3D graphene/multi-walled carbon nanotube (CS-GO-NCNT) cross-linked network by combining chitosan (CS) and GO with melamine and carbon nanotubes (CNTs). The GO and CNTs were self-assembled in CS solution to form a conductive network, and the additional melamine was used as a nitrogen source to obtain the precursor with a 3D structure through freeze-drying. The effects of various components in CS-GO-NCNT, such as CNT and graphene, on battery performance were investigated in detail. As expected, the CS-GO-NCNT electrode delivered a high reversible capacity over 500 mAh·g^-1, and even after 100 cycles, a specific capacity of 404 mAh·g^-1 was effectively maintained. This work provides strong support for the application of carbon-based materials in the field of energy storage, but also highlights the key role of the structural design of electrode materials in improving electrochemical performance, and provides a new idea for the design and development of novel electrode materials.

  1.   Experimental

  1.1   Chemicals

  Flake graphite, glacial acetic acid, and CS were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Sodium nitrate (NaNO₃), concentrated sulfuric acid (H₂SO₄), potassium permanganate (KMnO₄), and hydrogen peroxide (H₂O₂) were purchased from Sinopharm Reagent Co., Ltd. Melamine was purchased from Shanghai Titan Technology Co., Ltd. Multi-walled carbon nanotubes (CNTs) were purchased from Nanjing XFNANO Materials Tech Co., Ltd. All chemicals are analytical grade and require no further manipulation.

  1.2   Synthesis of GO

  GO was prepared by the Hummers method. The specific experimental process is as follows: 3 g flake graphite and 3 g NaNO₃ were mixed with 144 mL concentrated H₂SO₄ in an ice water bath, and 18 g KMnO₄ was added to the mixture in batches. The resulting paste was stirred in the ice water bath for 90 min and then stirred at 35 ℃ for 2 h. 120 mL of deionized water was added to the system drop by drop. After cooling, 15 mL of 30% H₂O₂ was added, and the reaction was violently stirred for 10 min. The obtained GO was washed by centrifugation until neutral. GO dispersion was obtained by ultrasonic stripping for 2 h.

  1.3   Synthesis of CS-GO-NCNT

  First, CS powder was dissolved with 1% glacial acetic acid solution to produce a 20 mg·mL^-1 CS solution. Next, 5 mL of deionized water, 15 mL of CS solution, and 10 mL of GO suspension were mixed to produce CS-GO suspension. The CS-GO-NCNT precursor was obtained by adding 0.5 g melamine and 0.15 g CNTs at the same time, which were evenly dispersed and frozen with liquid nitrogen for 15 min and freeze-dried for 48 h. The sample was put into a tube furnace and heated at 2 ℃·min^-1 to 500 ℃ for 1 h under a nitrogen atmosphere, and then heated at 5 ℃·min^-1 to 800 ℃ for 2 h to obtain the target sample. Comparison sample (CS-GO-N) were prepared by the same preparation method as above, without adding CNTs.

  1.4   Material characterization

  The surface morphology of CS-GO-NCNT was thoroughly examined using a field emission scanning electron microscope (FESEM, model Nova NanoSEM 450, America). The structural features and elemental composition of CS-GO-NCNT were carefully observed and accurately determined through transmission electron microscopy (TEM, model JEM-1400-plus, Japan) and energy dispersive X-ray spectroscopy (EDS) at acceleration voltages of 200 kV. Furthermore, to characterize the crystalline structure of the powder, X-ray diffractometry (XRD, PANalytical, Netherlands) was performed within the Cu Kα radiation (λ=0.154 16 nm) at an operating voltage of 40 kV and current of 40 mA with a range of 10° to 70°. Additionally, Raman spectroscopy with a 532 nm laser was employed to analyze the molecular structure of the prepared samples, and the obtained data were recorded on the HORIBA Scientific LabRAM HR Evolution Raman spectrometer system. To gain further insights into the surface chemistry of the product, monochromatic Al Kα source X-ray photoelectron spectroscopy (XPS, model ESCALAB 250Xi) was utilized. This technique provided valuable information regarding the surface elements and their chemical states. Moreover, the pore structure of CS-GO-NCNT was systematically analyzed using a multistation automatic micropore analyzer (model ASAP 2460 3.01). Specifically, the specific surface area was accurately determined by the Brunauer-Emmett-Teller (BET) method, while the pore size distribution was derived using the Barrett-Joyner-Halenda (BJH) method.

  1.5   Electrochemical characterization

  The electrochemical testing of CS-GO-NCNT was conducted in a CR2025 coin-type half-cell. The entire battery assembly process was conducted in a single- station glovebox filled with high-purity argon. The active material, conductive graphite (Super P), and polyvinylidene fluoride (PVDF) were mixed with N-methylpyrrolidone (NMP) as a solvent at a mass ratio of 8∶1∶1. The resulting mixture was stirred for 12 h to form a viscous liquid slurry. This slurry was then evenly coated onto copper foil using a spatula and subsequently dried in a vacuum oven at 80 ℃ for 10 h. The coated copper foil was then cut into discs with a diameter of 12 mm. Using a porous polyethylene separator (Celgard 2400), the electrolyte was composed of 1 mol·L^-1 LiPF₆ dissolved in a mixture of ethyl carbonate (EC), methyl ethyl carbonate (EMC), and dimethyl carbonate (DMC) with a volume ratio of 1∶1∶1. This electrolyte was paired with a lithium foil as the counter electrode.

  The discharge-charge tests were performed on the Neware battery test system (Shenzhen, China) using a constant current within a voltage range of 0.01-3 V. Additionally, electrochemical impedance spectroscopy (EIS) was recorded on an electrochemical workstation (CHI660E, Shanghai, China) across the entire frequency range from 100 kHz to 0.01 Hz. Finally, cyclic voltammetry (CV) tests were conducted on the CHI660E electrochemical workstation at a scan rate of 0.1 mV·s^-1.

  2.   Results and discussion

  The entire preparation process is illustrated in Fig. 1. First, a 1% acetic acid solution was prepared and CS powder was added and dissolved under agitation to form 20 mg·mL^-1 CS solution. Then, the prepared GO solution was added to the above-configured CS solution and stirred evenly to obtain the CS-GO dispersion solution. Then melamine and CNTs were added, and the precursor solution was obtained under agitation and ultrasonic dispersion. The precursor solution was frozen with liquid nitrogen and dried for 48 h to obtain the CS-GO-NCNT precursor. The final sample CS-GO-NCNT was obtained by heat treatment of the above precursor under a nitrogen atmosphere.

  Figure 1

  

  Figure 1.  Schematic illustration of the preparation of CS-GO-NCNT

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  As depicted in Fig. 2, SEM and TEM techniques were employed to observe the morphology of CS-GO-NCNT. Fig. 2a shows that the prepared CS-GO-NCNT presented a large 3D cross-linked network after the assembly of graphene and CNTs, and the surface had rich pores, which facilitate the storage of lithium ions and thus increase the capacity. Fig. 2b clearly shows that the sample still retained the crumpled structure of graphene, which is also the reason for the high specific surface area and the large number of surface pores of the sample. These folds may form channels during the lithium embedding/exiting process, thereby shortening the diffusion path of lithium ions and improving the electrochemical performance of the battery. Fig.S1 (Supporting information) is the element mapping result of CS-GO-NCNT, showing the presence of C, N, and O elements. Fig. 2c is the TEM image of CS-GO-NCNT, from which the tubular structure of (CNTs) is closely combined with graphene to form a cross-linked network, and the CNT speeds up the transmission rate of lithium ions to a certain extent, and thus improves the rate performance of the assembly battery. Fig. 2d is a high-resolution TEM image of CS-GO-NCNT, from which it can be observed that the lattice fringes of graphene with a lattice spacing (d-spacing) of about 0.36 nm correspond to the (002) crystal faces of graphene, which is consistent with the literature^[19].

  Figure 2

  

  Figure 2.  (a, b) SEM images and (c, d) TEM images of CS-GO-NCNT

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  The crystal phase of CS-GO-NCNT was further analyzed by XRD. As shown in Fig. 3a, a wide diffraction peak near 22.5° corresponds to the (002) plane of carbon, while a weak and wide diffraction peak around 44° corresponds to the (100) plane of carbon. In comparison to conventional graphite, the peak position of the CS-GO-NCNT (002) plane was notably shifted towards a lower value, signifying an enlargement in d-spacing. This finding aligns well with the outcomes derived from TEM observations^[20]. Concurrently, the broadened peak width of CS-GO-NCNT further validates the ultra-thin nature of its structure. The Raman spectrum of CS-GO-NCNT is shown in Fig. 3b, with two obvious peaks around 1 355 and 1 597 cm^-1, corresponding to the D band (defect) and G band (crystalline graphite) respectively^[21-22]. The peak intensity ratio (I_D/I_G) between D and G bands was about 0.943, indicating that CS-GO-NCNT had a high degree of defect, which is conducive to ion adsorption and rapid charge transfer^[23]. Moreover, compared with GO (I_D/I_G=0.845) (Fig.S2), it should be noted that the I_D/I_G of CS-GO-NCNT was slightly higher, signifying the defect reduction, which is related to the addition of CNT. In addition, the N₂ adsorption-desorption isotherm and pore size distribution of CS-GO-NCNT are shown in Fig. 3c and 3d respectively. The results of Fig. 3c showed that the sample presented a type Ⅳ isotherm with a lag loop in the relative pressure (p/p₀) range of 0.15-1.0, confirming the presence of medium and large pores in CS-GO-NCNT^[24-25]. The BET surface area of CS-GO-NCNT was calculated to be 341 m²·g^-1. The aperture distribution in Fig. 3d shows that it is mainly micropores and mesoporous.

  Figure 3

  

  Figure 3.  (a) XRD patterns, (b) Raman spectra, (c) N₂ adsorption-desorption isotherm, and (d) pore size distribution of CS-GO-NCNT

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  The elemental composition of CS-GO-NCNT was subjected to a detailed analysis using XPS, as depicted in Fig. 4. Fig. 4a exclusively features three distinct peaks, corresponding precisely to O1s, N1s, and C1s. Upon evaluating the peak areas, we ascertain that the nitrogen content (atomic fraction) in CS-GO-NCNT amounts to 6.21%. In Fig. 4b, the high-resolution C1s spectrum is discernibly segmented into three peaks, centered at 283.7, 284.6, and 287.5 eV, representative of the C—C/C=C, C—N, and C=O bonds, respectively^[26-27]. Similarly, Fig. 4c exhibited a high-resolution N1s spectrum with three notable peaks, positioned at 397.5, 399.7, and 402.6 eV, corresponding to pyridinic-N, pyrrolic-N, and graphitic-N, respectively^[28-29]. The estimated ratio of these nitrogen species stands at 1∶1.87∶0.55. Reports indicate that pyridinic- and pyrrolic-N exhibit robust chemical activity, enabling reversible binding with Li, thereby augmenting the Li storage capacity. Additionally, graphitic N, characterized by three sp² carbon atoms, significantly contributes to enhancing the electrical conductivity of graphite carbon. Hence, in the context of this study, while considering the capacity and magnification performance of the CS-GO-NCNT electrode, a favorable balance is achieved in the content of different types of nitrogen. Lastly, Fig. 4d portrays the high-resolution XPS spectrum of O1s, primarily divided into two distinct peaks, centered at 530.4 and 531.9 eV, representing the C=O and C—O bonds, respectively^[30].

  Figure 4

  

  Figure 4.  (a) Survey, (b) C1s, (c) N1s, and (d) O1s XPS spectra of CS-GO-NCNT

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  To assess the lithium storage capacity of the synthesized CS-GO-NCNT material, we evaluated its electrochemical performance by employing a CR2025 coin-type half-cell with a pure Li foil serving as the counter electrode. The comprehensive battery assembly procedure is detailed in the experimental section. Fig. 5a presents the initial three CV curves of the CS-GO-NCNT electrode, scanned at a rate of 0.1 mV·s^-1 within a voltage range of 0 to 3 V. The broad peaks observed within the 0.5-1 V range diminish in subsequent cycles, attributed to the irreversible capture of Li⁺ in the sample and the formation of a solid electrolyte interface (SEI) layer on the electrode surface^[31]. Notably, the disappearance of these peaks in later cycles, coupled with the nearly identical peaks in the second and third cycles, suggests the CS-GO-NCNT electrode possessed excellent stability. Furthermore, a distinct reduction peak at approximately 0 V represented the insertion of lithium ions into the primary material, while the anode peaks at ca. 0.2 and 1.2 V are attributed to the extraction of lithium ions from the electrode^[32].

  Figure 5

  

  Figure 5.  (a) CV curves of CS-GO-NCNT from 0 to 3 V at a scan rate of 0.1 mV s^-1; (b) Voltage profiles of CS-GO-NCNT at 1st, 3rd, and 5th cycles

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  Fig. 5b depicts the characteristic charge and discharge voltage profiles of the CS-GO-NCNT electrode during the 1st, 3rd, and 5th cycles, with a current density of 0.1 A·g^-1. A prominent voltage plateau was observed below 1.0 V during the initial discharge phase, aligning with the CV findings. Regarding the electrode's performance, during the first cycle, the specific discharge and charge capacities were recorded as 1 160 and 929 mAh·g^-1, respectively, resulting in an initial Coulombic efficiency of 80.1%. Notably, the Coulombic efficiency underwent a rapid increase, reaching 88% in the third cycle, further rising to 91.5% in the fifth cycle, and maintaining stability above 95% in subsequent cycles. It is hypothesized that the volume loss encountered during the initial cycle is primarily attributed to the formation of the SEI film, which is corroborated by detailed CV curve analyses.

  The evaluation of the rate performance of the CS-GO-NCNT negative electrode is presented in Fig. 6a, demonstrating a capacity decrease with rising current density. Before testing, the electrodes underwent activation cycles at a low current density. Specifically, the CS-GO-NCNT electrodes exhibited specific capacities of 501, 485, 421, 365, 322, and 291 mAh·g^-1 at current densities of 0.1, 0.2, 0.4, 0.6, 0.8, and 1 A·g^-1, respectively. Notably, upon returning to a current density of 0.1 A·g^-1, the specific discharge capacity reverted to its initial value. In comparison, the CS-GO-N and CNT electrodes delivered significantly lower specific capacities of 332, 214, 138, 107, 91, 80 mAh·g^-1 and 251, 193, 154, 138, 128, 123 mAh·g^-1, respectively, under the same current densities. This underscores the exceptional reversibility and high-capacity retention of the CS-GO-NCNT electrode at high current densities, indicating its potential for applications demanding high energy density, such as plug-in hybrid vehicles and aerospace.

  Figure 6

  

  Figure 6.  (a) Rate performance, (b) EIS, cycling performance at (c) 0.1 A·g^-1 and (d) 1 A·g^-1 of CS-GO-NCNT, CS-GO-N, and CNT electrodes

  Inset: the corresponding equivalent circuit diagram, where R₁ stands for solution resistance, R₂ stands for charge transfer resistance, Z_W stands for diffusion resistance, and CPE1 stands for capacitive reactance.

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  Furthermore, Fig. 6b depicts an EIS test, exploring the structural influence on electron conductivity. As evident in Fig. 6b, the semicircle diameter of CS-GO-NCNT was notably smaller than that of CS-GO-N and CNT, confirming the enhanced electron mobility attributed to its 3D cross-linked network. The outstanding rate performance and low impedance of the CS-GO-NCNT electrode are attributed to its ingenious design, where CNTs serve as a skeleton, preventing graphene agglomeration, while the stacked folds on the graphene surface create a porous structure, expanding the specific surface area. Additionally, the nitrogen doping in the material is provided by the rich amino groups in the CS and melamine precursors, significantly enhancing the performance of the CS-GO-NCNT battery.

  To evaluate and contrast the cyclic stability of CS-GO-NCNT, CS-GO-N, and CNT, a series of long-term cycle tests were conducted at 0.1 and 1 A·g^-1, as depicted in Fig. 6c and 6d. Specifically, at a current density of 0.1 A·g^-1, after enduring 150 cycles, the CS-GO-NCNT electrode retained a substantial specific capacity of 360 mAh·g^-1, whereas the CS-GO-N and CNT electrodes exhibited significantly lower values of 82 and 216 mAh·g^-1, respectively. In stark contrast, at a higher current density of 1 A·g^-1, the CS-GO-NCNT electrode demonstrated remarkable durability, maintaining a high specific capacity of 268 mAh·g^-1 and a Coulombic efficiency of 99% after 300 cycles. This underscores the effectiveness of CNTs in mitigating structural changes during charge-discharge cycles, thus confirming that the 3D cross-linked network architecture significantly enhances the electrochemical performance of the electrode.

  3.   Conclusions

  In summary, using GO as a conductive network, multi-walled carbon nanotubes as a rigid skeleton, and melamine and CS as nitrogen and carbon sources, a nitrogen-doped 3D graphene/multi-walled carbon nanotubes cross-linked network material (CS-GO-NCNT) was prepared as an efficient anode for LIBs. The obtained CS-GO-NCNT electrode showed a reversible capacity of up to 360 mAh·g^-1 after 150 discharge-charge cycles at 0.1 A·g^-1 and 268 mAh·g^-1 after 300 discharge-charge cycles at a current density of 1 A·g^-1. The 3D cross-linked network assembled by graphene and multi-walled carbon nanotubes has high porosity, high specific surface area, good electrical conductivity, and abundant active sites, which greatly improve the cycle and rate performance of the battery, and provide a new idea for the structural design of carbon nanomaterials.

  Supporting information is available at http://www.wjhxxb.cn

  Conflicts of interest: There are no conflicts to declare.

  
  
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  Figure 1  Schematic illustration of the preparation of CS-GO-NCNT

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  Figure 2  (a, b) SEM images and (c, d) TEM images of CS-GO-NCNT

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  Figure 3  (a) XRD patterns, (b) Raman spectra, (c) N₂ adsorption-desorption isotherm, and (d) pore size distribution of CS-GO-NCNT

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  Figure 4  (a) Survey, (b) C1s, (c) N1s, and (d) O1s XPS spectra of CS-GO-NCNT

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  Figure 5  (a) CV curves of CS-GO-NCNT from 0 to 3 V at a scan rate of 0.1 mV s^-1; (b) Voltage profiles of CS-GO-NCNT at 1st, 3rd, and 5th cycles

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  Figure 6  (a) Rate performance, (b) EIS, cycling performance at (c) 0.1 A·g^-1 and (d) 1 A·g^-1 of CS-GO-NCNT, CS-GO-N, and CNT electrodes

  Inset: the corresponding equivalent circuit diagram, where R₁ stands for solution resistance, R₂ stands for charge transfer resistance, Z_W stands for diffusion resistance, and CPE1 stands for capacitive reactance.

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