English

• Ever-increasing energy and environmental problems, call for an urgent development of renewable energy sources including solar, wind, and hydropower to replace fossil fuels. However, the intermittency and variability of these renewable energy sources, resulting an inevitable surge in demand for energy storage devices [1,2]. So, various electrochemical energy storage devices have been developed, such as lithium-ion batteries (LIBs) [3-6], nickel-metal hydride batteries [7], lead-acid batteries [8], supercapacitors [9], and lithium-sulfur batteries [10]. Among them, LIBs with considerable energy density can meet the performance requirements of large-scale energy storage systems (LSESSs) [11-13]. However, the limited and uneven distribution of lithium resources hampered their further practical applications [14-16]. Sodium-ion batteries (SIBs) emerged as the most promising alternative battery systems to LIBs owing to their abundant raw materials reserves and similar electrochemical behaviors [17-24]. Because of the abundant material resources, low cost, and good safety, SIBs exhibited great application potential in low-speed electric vehicles and LSESSs [25]. Therefore, researchers have dedicated efforts to develop electrode materials to achieve high-performance, low-cost SIBs [26,27].

  Cathode materials, a crucial component of SIBs, play a critical role in determining the overall energy density of batteries [28-30]. Up to now, various SIBs cathode materials have been discovered, such as transition metal oxides [31-33], polyanionic compounds [34-40], Prussian blue analogs [41], and organic compounds [42,43]. Among the above materials, polyanionic compounds exhibit superior structural stability, excellent security, and negligible volume change, making them the most promising candidates for SIBs [2,44]. Because of this, various polyanionic compounds have been explored. Iron-based phosphates possess the advantages of low cost, high abundance, non-toxicity, and high structural stability emerging as the most suitable candidate. Unlike the commercialized LiFePO₄, the directly synthesized NaFePO₄ (NFP) [45-47], despite its excellent thermodynamic stability, lacks clear Na⁺ diffusion pathways exhibiting lower electrochemical activity. Therefore, researchers have shifted their attention to other iron-based phosphate compounds, including Na₂FeP₂O₇ [36,48,49], Na₂FePO₄F [50,51], Na₃Fe₂(PO₄)₃ [52,53], and so on [54,55]. Among them, Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) exhibits a high operating voltage of ~3.1 V and a high theoretical specific capacity of 129 mAh/g, along with a lower volume change of ~4% [56-61]. Additionally, three-dimensional Na⁺ channels of NFPP enable the possibility of fast charging and discharging [62-64]. Although NFPP demonstrates such promising potential, its practical application and actual performance are still limited by its low interface coupled with carbon materials has been proven a feasible way to enhance conductivity. Carbon coating could confine the electrical conductivity [20,65]. The low electrical conductivity originates from the intrinsic crystal structure. Introducing a heteromeric growth of NFPP nanoparticles, resulting heteromeric interface thereby enhancing the electronic conductivity [17,54,62]. However, most of the coating carbon is derived from the pyrolysis of organic compounds, which limits the improvement of electronic conductivity. Expanded graphite (EG) inherits superior conductivity and large surface area, which could achieve a good combination with NFPP particles to further improve the overall electronic conductivity.

  Herein, a double carbon modified NFPP encapsulated by in-situ pyrolytic carbon and coupled with EG (NFPP@C/EG) was prepared via a sol-gel route followed by a ball-mill procedure. The effects of each of these on the material were evaluated. The effects of these two types of carbons were analyzed through a series of characterization techniques. In comparison to NFPP@C, M-NFPP@C/EG (the optimized NFPP@C/EG) exhibits a superior specific capacity of 116.9 mAh/g at 0.1 C and excellent rate capability of 0.5 mAh/g at 50 C and remarkable cycle stability of 76.6 mAh/g over 11,000 cycles at 50 C with capacity retention of 89.85%. In addition, the low-temperature performance and pouch cells exhibit favorable electrochemical performance, which further substantiates its potential for application.

  As depicted in Fig. 1a, the H-NFPP@C, M-NFPP@C, and L-NFPP@C (H: high carbon content, M: medium carbon content, L: low carbon content) were prepared via a sol-gel method. To achieve a more homogeneous composite coating of NFPP with EG, ball milling and mixing as well as sintering processes were used, resulting in the preparation of H-NFPP@C/EG and M-NFPP@C/EG. The XRD patterns of H-NFPP@C, M-NFPP@C, L-NFPP@C, H-NFPP@C/EG, and M-NFPP@C/EG (Fig. 1b, Figs. S1a and b in Supporting information) appear similar shape and match well with the previously reported NFPP (crystal system: orthorhombic, space group: Pn21a) [57]. Comparison of XRD patterns of H-NFPP@C-1 (pristine), H-NFPP@C-2 (secondary calcination), and H-NFPP@C/EG-2 (not calcination) shows that sintering and ball milling did not destroy the original crystal structure. As exhibited in Fig. S1g (Supporting information), the diffraction peak of expanded graphite also appeared in M-NFPP@C/EG at 26.4° whereas it was not present in H-NFPP@C, indicating that M-NFPP@C/EG is encapsulated with EG. Fig. S1h (Supporting information) shows the Fourier transform infrared (FTIR) spectra of H-NFPP@C and M-NFPP@C/EG. The characteristic peaks at 400–700 and 975–1300 cm⁻¹ correspond to the O-P-O and P-O vibrations of PO₄, respectively, and the characteristic peaks at about 700–975 cm⁻¹ correspond to the symmetric and antisymmetric P-O-P vibrations of P₂O₇. To determine the carbon content of these materials, thermo gravimetric (TG) analysis was conducted in air from 40 ℃ to 700 ℃. The TG results (Fig. 1c and Fig. S1d in Supporting information) indicate that the mass losses for H-NFPP@C, M-NFPP@C, L-NFPP@C, H-NFPP@C/EG, and M-NFPP@C/EG between 30 ℃ and 700 ℃ were 18.62%, 14.92%, 8.43%, 25.13% and 17.30%, respectively. According to the reaction products of NFPP with oxygen are Na₃Fe₂(PO₄)₃ and Fe₂O₃ [57]. Therefore, the carbon content can be calculated using the carbon content can be calculated using the following reaction equations:

  12 N a 4 F e 3 ( P O 4 ) 2 P 2 O 7 + 9 O 2 → 16 N a 3 F e 2 ( P O 4 ) 3 + 2 F e 2 O 3

  (1)

  C + O 2 → C O 2

  (2)

  Figure 1

  

  Figure 1.  (a) Schematic diagram of the synthesis of NFPP@C/EG. (b) XRD pattern, (c) the TG curve, (d) EPR spectra with g-factor, and (e) Raman spectra of H-NFPP@C and M-NFPP@C/EG. XPS spectra of C 1s of (f) M-NFPP@C/EG, (g) H-NFPP@C.

  DownLoad: Full-Size Img PowerPoint

  Based on Eqs. 1 and 2, the carbon contents of H-NFPP@C, M-NFPP@C, L-NFPP@C, H-NFPP@C/EG, and M-NFPP@C/EG were calculated to be 21.63%, 18.07%, 11.82%, 27.90%, and 20.36%, respectively. In this range of carbon content, the growth of NFPP particles can be effectively constrained, consequently increasing the electronic conductivity. H-NFPP@C and M-NFPP@C/EG samples exhibit noticeable peak profile changes in the electron paramagnetic resonance (EPR) spectrum (Fig. 1d). As for M-NFPP@C/EG, the sharp peaks transform into broadened profiles, which can be attributed to surface effects induced by coupling with EG. The coated EG could potentially enhance surface properties, such as conductivity, altering surface electronic structure and magnetic properties. To further investigate the intrinsic properties of coated carbon, Raman spectra of these samples were carried out. In the Raman spectra (Fig. 1e, Figs. S1e and f in Supporting informaion), the broad D and G bands at 1339 and 1587 cm⁻¹, respectively, demonstrate the existence of carbon in the samples. The D-band is related to the disordered and defective carbon, while the G-band is related to the graphitized carbon. The peak intensity ratios (I_D/I_G) of H-NFPP@C, M-NFPP@C, H-NFPP@C/EG, and M-NFPP@C/EG are 0.8108, 0.8052, 0.8777 and 0.8814, respectively. The ratio of peak intensities I_D/I_G typically reflects the defect density in carbon materials, which is generally associated with electrical conductivity [65]. Consequently, the impact of pyrolytic carbon on the degree of defects is not significant, whereas EG can noticeably increase the degree of defects and thereby enhance electrical conductivity. The full XPS spectrum (Figs. S2a-c in Supporting informaion) verified the coexistence of Fe, O, P, C, and Na in M-NFPP@C/EG, M-NFPP@C, and H-NFPP@C. The Fe 2p XPS spectra (Figs. S2d-f in Supporting informaion) of M-NFPP@C/EG, M-NFPP@C, and H-NFPP@C revealed the presence of both Fe²⁺/Fe³⁺ with a similar ratio. From the C 1s XPS spectra (Figs. 1f and g, Fig. S2g in Supporting informaion) of M-NFPP@C/EG, H-NFPP@C, and M-NFPP@C, it can be observed that the ratio of sp² peaks to sp³ peaks in M-NFPP@C and H-NFPP@C shows little variation, being 67.95% and 68.1% respectively. However, in M-NFPP@C/EG, the ratio of sp² peaks to sp³ peaks is 84.96%, significantly higher than that of the formers. Based on what is known, carbon with sp³ hybridization typically exhibits lower electrical conductivity, whereas carbon with sp² hybridization tends to have higher electrical conductivity [66]. The C 1s XPS spectrum further confirms that M-NFPP@C/EG exhibits higher electrical conductivity, which is consistent with the above Raman results.

  The morphology and structure of these prepared materials were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and energy dispersive spectrum (EDS). The SEM images (Figs. 2a-c and Fig. S3 in Supporting information) show that all the samples are nano-sized and spherical. Among them, H-NFPP@C and M-NFPP@C have coated within a porous carbon framework originating from the pyrolysis of ascorbic acid, forming larger secondary particles. After mixing with EG by ball milling and subsequently sintering, M-NFPP@C/EG and H-NFPP@C/EG facilitate the fragmentation of the aforementioned large secondary particles. However, EG also allows the interconnectedness of the milled secondary particles, thereby generating larger pores and voids within the particles. In the TEM images (Fig. S4 in Supporting information), clear lattice fringes are evident within H-NFPP@C, exhibiting a lattice spacing of 0.904 nm, corresponding to the (200) crystal planes of NFPP. There are also a few lattice fringes with a spacing of 0.370 nm corresponding to the (111) crystal planes NFP [67]. In the TEM images of M-NFPP@C/EG (Figs. 2d and e), it is evident that small spherical particles are encapsulated by a carbon layer (~2.2 nm), while some block-like EG particles also exist between the spherical particles. In HRTEM (Fig. 1f), the microstructure and interface structure of M-NFPP@C/EG can be elucidated that a relatively uniform carbon layer on the surface of the matrix is attributed to the pyrolytic carbon and clear lattice spacing covered by the carbon layer is 0.351 nm, corresponding to the (411) crystal plane of NFPP [68]. In addition, the outside pyrolytic carbon layer coupled with a carbon block connecting various matrices with a measured lattice spacing of 0.332 nm corresponds to the (002) crystal plane of EG. Besides, the EDS results (Fig. 2g) demonstrate the coexistence and even distribution of Na, Fe, P, C, and O elements and are well overlapped, while the C element has a wider range compared to other elements, further confirming its dual carbon coated.

  Figure 2

  

  Figure 2.  (a-c) SEM images, (d, e) TEM images, (f) HRTEM image, (g) EDS mapping images of M-NFPP@C/EG.

  DownLoad: Full-Size Img PowerPoint

  To evaluate the Na⁺ storage performance of these cathodes, coin-type cells were assembled, and the electrochemical performance was measured within the voltage range of 1.7–4.3 V. The initial three charge-discharge profiles (Figs. 3a and b) of M-NFPP@C/EG at 0.1 C delivering a specific capacity of 116.33 mAh/g with initial Coulombic efficiency (ICE) of 100.85%, while for the H-NFPP@C exhibits a specific capacity of 103.11 mAh/g with an ICE of 107.66%. This phenomenon veried that after being coupled with EG, the ICE dramatically enhanced implying a more efficient utilization of active materials and improved cycling efficiency. At a current density of 1 C (Fig. 3c and Fig. S5a in Supporting information), the M-NFPP@C/EG exhibits a reversible capacity of 101.1 mAh/g after 700 cycles with a capacity retention rate of 95.5%. These performances are considerably higher than those observed for M-NFPP@C (capacity of 90.3 mAh/g and capacity retention of 93.71%) and H-NFPP@C (capacity of 96.6 mAh/g and capacity retention of 89.75%). Similarly, at high current densities of 10 C and 50 C (Fig. 3d, Figs. S5b and c in Supporting information), M-NFPP@C/EG also exhibits superior long-cycling stability than that of M-NFPP@C and H-NFPP@C. Particularly, M-NFPP@C/EG achieved a reversible capacity of 76.6 mAh/g after 11,000 cycles at 50 C, with a capacity retention of 89.85%, demonstrating ultra-long cycling stability. The rate performance of these cathodes is also evaluated and offered in Figs. S6a and b (Supporting information) and Fig. 3e. It can be observed that the rate performance of L-NFPP@C and M-NFPP@C is nearly identical. Compared to the above two, H-NFPP@C shows only a slight improvement in capacity at high charge/discharge rates. This suggests that the increase in the content of pyrolyzed carbon has no significant effect on enhancing the rate performance of NFPP. While the EG-coupled M-NFPP@C/EG and H-NFPP@C/EG exhibited superior rate performance. Even at 50 C, the capacity of M-NFPP@C/EG remains ~80.9 mAh/g, which is superior to that of H-NFPP@C (~60.3 mAh/g). This suggests that the EG is better than the pyrolytic carbon for rate performance enhancement at the same carbon content. Further comparison of the corresponding charge/discharge curves (Fig. 3f and Fig. S6c in Supporting information) showed that M-NFPP@C/EG exhibited significantly smaller overpotentials compared to H-NFPP@C and was more pronounced at higher current densities, corroborating the ultra-long cycling stability of M-NFPP@C/EG. Moreover, in the energy density plots at different rates (Fig. 3g), it is evident that M-NFPP@C/EG reaches an energy density of 317 Wh/kg at 0.5 C. As the rate increases, the gap in energy density between the two becomes wider. M-NFPP@C/EG still maintains an energy density of 226 Wh/kg at 50 C, which is significantly higher than that of H-NFPP@C (171.5 Wh/kg). The above results indicate that under the same carbon content, dual-carbon-modified NFPP with pyrolyzed carbon and EG exhibit superior cycling stability and rate performance. This phenomenon is attributed to the higher electrical conductivity of M-NFPP@C/EG achieved after coating with EG.

  Figure 3

  

  Figure 3.  The charge-discharge profiles of (a) M-NFPP@C/EG and (b) H-NFPP@C at 0.1 C. Long-term cycling performance of M-NFPP@C/EG and H-NFPP@C at (c) 1 C and (d) 50 C. (e) Rate performance of H-NFPP@C and M-NFPP@C/EG. (f) Charge/discharge curves of M-NFPP@C/EG at different rates. (g) The energy density of M-NFPP@C/EG and H-NFPP@C at different rates.

  DownLoad: Full-Size Img PowerPoint

  To gain in-depth insight into the outstanding electrochemical performance of M-NFPP@C/EG, cyclic voltammetry (CV) characterizations at different scan rates were investigated. With the increase in scanning rate, the oxidation–reduction peak currents of both H-NFPP@C and M-NFPP@C/EG noticeably increase (Figs. 4a and d). Additionally, the current of the secondary oxidation–reduction peak between the two pairs of oxidation–reduction peaks also increases accordingly. With the increasing scanning rate, the peak current of the secondary oxidation–reduction of M-NFPP@C/EG increases more pronounced. The peak ratio of Peak1/Peak2 for M-NFPP@C/EG at scanning rates of 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0 mV/s are 0.469, 0.507, 0.595, 0.657, 0.693 and 0.722, respectively. This is significantly higher than the peak variation of Peak1/Peak2 for H-NFPP@C, indicating that M-NFPP@C/EG involves more charge in the redox reaction within this range [69]. For in-depth disclosure of the charge storage mechanism of the H-NFPP@C and M-NFPP@C/EG, the CV curves at different scan rates are analyzed on the following equations [66]:

  $ i=a v^{\mathrm{b}} $

  (3)

  $ \log (i)=b \log (v)+\log (a) $

  (4)

  Figure 4

  

  Figure 4.  Kinetics analysis of H-NFPP@C and M-NFPP@C/EG: CV curves of (a) H-NFPP@C and (d) M-NFPP@C/EG at different scan rates. log(v) vs. log(i) plots for the different peaks in the CV curves of (c) H-NFPP@C and (f) M-NFPP@C/EG, where i is the peak current and v is the scan rate. Capacitive capacity contribution ratios of (c) H-NFPP@C and (f) H-NFPP@C/EG at different scan rates. In-situ EIS of (g) H-NFPP@C and (h) M-NFPP@C/EG. (i) Relationships between the specific capacity, rate and cycle number in half-cells for NFPP.

  DownLoad: Full-Size Img PowerPoint

  where i represents the peak current, v denotes the scan rate, a and b are adjustable parameters. According to the relationship between the peak current (i) and scan rate (log(i)), the b-valves can be simply determined. The b-value around 1 indicates the charge current occupied by pseudocapacitance behavior, whereas the b-value around 0.5 represents the diffusion-dominated process. By linear fitting of log(i) versus log(v) (Figs. 4b and e), the b values of Peak1, Peak1', Peak2, and Peak2' for M-NFPP@C/EG are 1.000, 1.021, 0.821 and 0.793, respectively, which are all higher than that of H-NFPP@C (0.998, 0.989, 0.772, and 0.746, respectively). Notably, all these values are close to 1, indicating pseudocapacitive behavior overwhelming the Na⁺ storage process. In addition, the diffusion-controlled and capacitive capacity contributions of the material at a fixed potential (V) were calculated according to the following equation [57]:

  i ( V ) = k 1 v + k 2 v 1 / 2

  (5)

  where the k₁v and k₂v_1/2 components represent the capacitive and diffusion-controlled capacity contributions, respectively. The calculated capacitive capacity contribution of M-NFPP@C/EG increases from 92% to 97% when the scan rate changes from 0.1 mV/s to 1.0 mV/s (Fig. 4f), while that of H-NFPP@C increases from 86% to 95% (Fig. 4c). As observed, M-NFPP@C/EG demonstrates a higher initial capacitive contribution and a smaller variation in capacitive contribution, which further explains its higher specific capacity and superior rate performance displayed. To deeply investigate the reaction kinetics of these cathodes, in situ electrochemical impedance spectroscopy (EIS) was conducted (Figs. 4g and h). M-NFPP@C/EG exhibited charge transfer impedances of 70.49, 44.01, 35.98, 37.56, 137.19, and 164.46 Ω during charging to 2.95, 3.25 and 4.3 V, and discharging to 3.2, 2.85 and 1.7 V, respectively. Under the same conditions, H-NFPP@C displayed charge transfer impedances of 93.94, 57.35, 45.51, 53.04, 267.06, and 246.31 Ω, respectively. Across different charge-discharge states, M-NFPP@C/EG consistently demonstrated lower charge transfer impedance, indicating that M-NFPP@C/EG possesses better electrical conductivity and faster charge transfer rates. Both M-NFPP@C/EG and H-NFPP@C exhibit consistently reduced charge transfer impedance after 1000 and 2500 cycles, respectively. It is evident that the charge transfer impedance of M-NFPP@C/EG decreases more significantly, indicating superior cycling stability of M-NFPP@C/EG than that of H-NFPP@C. The higher pseudocapacitance contribution rate and stable changes, along with the lower charge resistance of M-NFPP@C/EG, further demonstrate its superiority in enhancing electrochemical performance. The EG dual-carbon modified NFPP also demonstrates advantages in the ultra-stable cycling performance outperforming most other modification methods (Fig. 4i and Table S1 in Supporting information). This strategy offers a more straightforward and controllable manufacturing process for polyanionic cathode materials.

  To further test the potential of the material in practical applications, we evaluated its electrochemical performance in a low-temperature environment and manufactured pouch cells. As shown in Fig. 5a, the specific capacity of M-NFPP@C/EG is 86.4 mAh/g at 0 ℃ (still maintaining 84.45% capacity of its room temperature), while the specific capacity is 70.8 mAh/g at -10 ℃ (69.2% capacity of its room temperature). Even at 0 ℃ (Fig. 5b), the M-NFPP@C/EG cycled at 0.5, 1, 2, 5, 8, 10, 15 and 20 C rates achieved 86.4, 83.4, 79.8, 74.5, 71.5, 69.5, 65.6 and 61.57 mAh/g, respectively. These values are significantly better than those of H-NFPP@C under the same conditions, demonstrating its superior rate performance at low temperatures. As shown in Fig. 5c, the capacity of H-NFPP@C at 1 C exhibited a slight capacity decay after 250 cycles at 0 ℃, while M-NFPP@C/EG showed almost no capacity decay. At 5 C and 10 C current density (Figs. 5d and e), it can be found that the capacity of M-NFPP@C/EG is significantly higher than that of H-NFPP@C. Furthermore, H-NFPP@C undergoes obvious capacity degradation (with capacity retention of 83.42% after 4500 cycles at 5 C and 73.14% after 8800 cycles at 10 C) while M-NFPP@C/EG is not obvious (with almost no capacity degradation after4500 cycles at 5 C and with capacity retention of 95.81% after 8800 cycles at 10 C), which indicates that the low-temperature tolerance was significantly improved by EG-coated. The temperature was further lowered to -10 ℃ (Fig. S8 in Supporting information), and M-NFPP@C/EG still showed excellent cycling performance at current densities of 1, 5, and 10 C. This M-NFPP@C/EG cycled in pouch cell, the charge-discharge curve at 0.5 C (Fig. 5f) was not significantly different from the half-cell and still achieved a specific capacity of 92 mAh/g. The assembled pouch cell also cycled at 1 C and 2 C after 150 and 190 times (Figs. 5g and h), respectively, without any signs of delay, demonstrating its excellent cycling stability. The excellent performance demonstrated on low temperature and pouch cells indicates the outstanding application potential of M-NFPP@C/EG, laying a solid foundation for the commercialization of SIBs.

  Figure 5

  

  Figure 5.  (a) The charge-discharge curves of M-NFPP@C/EG at 0.5 C at 30 ℃, 0 ℃ and -10 ℃. (b) Rate performance of H-NFPP@C and M-NFPP@C/EG at 0 ℃. (c) Long-term cycling performance of H-NFPP@C and M-NFPP@C/EG at (d) 5 C and (e) 10 C at 0 ℃. Electrochemical performance of M-NFPP@C/EG, (f) the charge-discharge profiles at 0.5 C, long-term cycling performance at (g) 1 C, and (h) 2 C at room temperature.

  DownLoad: Full-Size Img PowerPoint

  In summary, we successfully synthesized in-situ pyrolyzed carbon and expanded graphite double-carbon coated NFPP through the sol-gel method followed by and ball-mill procedure. A series of characterizations were employed to demonstrate that M-NFPP@C/EG synthesized by this method possesses higher conductivity. The M-NFPP@C/EG showed a high specific capacity (116 mAh/g at 0.1 C), excellent rate performance (80.5 mAh/g at 50 C), and outstanding cycling stability (capacity retention of 89.85% after 11, 000 cycles at 50 C). The kinetic analysis results reveal that the EG coating improves the Na-ion diffusivity and reduces charge transfer resistance. In addition, the sodium storage process of M-NFPP@C/EG is dominated by the intercalation pseudocapacitive mechanism, and the proportion of capacitive capacity is improved by EG coating. The enhanced electrochemical kinetics and intercalation pseudocapacitance of M-NFPP@C/EG is responsible for the enhanced rate performance and ultra-long cycling life. In low-temperature testing (0 ℃), M-NFPP@C/EG also demonstrated excellent multiplier performance (~61.57 mAh/g at 20 C) and excellent cycling performance (with capacity retention of 95.81% after 8800 cycles at 10 C). It has also demonstrated excellent cycle stability in pouch batteries, further validating its application potential. This work validates the feasibility of the double carbon modification strategy, which aids in developing low-cost, high-performance SIBs.

  CRediT authorship contribution statement

  Zheng Li: Writing – original draft, Investigation, Data curation. Fangkun Li: Writing – original draft, Supervision, Conceptualization. Xijun Xu: Writing – review & editing, Supervision, Conceptualization. Jun Zeng: Methodology. Hangyu Zhang: Investigation, Formal analysis. Lei Xi: Formal analysis. Yiwen Wu: Methodology, Formal analysis. Linwei Zhao: Formal analysis, Data curation. Jiahe Chen: Formal analysis. Jun Liu: Supervision, Funding acquisition, Conceptualization. Yanping Huo: Resources. Shaomin Ji: Writing – review & editing, Supervision.

  Acknowledgments

  This work was supported by the National Key Research and Development Program of China (No. 2022YFB2502000), the National Natural Science Foundation of China (Nos. U21A20332, 51771076, U21A200970, 52301266), and the Science and Technology Planning Project of Guangzhou (No. 2024A04J3332). We would like to thank the Analysis and Test Center of Guangdong University of Technology for the characterization.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110390.

  
  
• 1. [1]

     Y. Wu, X. Xu, S. Shi, et al., Chin. Chem. Lett. 36 (2025) 110062. doi: 10.1016/j.cclet.2024.110062

  2. [2]

     H. Li, M. Xu, Z. Zhang, et al., Adv. Funct. Mater. 30 (2020) 2000473. doi: 10.1002/adfm.202000473

  3. [3]

     W. Liang, X. Zhou, B. Zhang, et al., Angew. Chem. Int. Ed. 63 (2024) e202320149. doi: 10.1002/anie.202320149

  4. [4]

     F. Li, Z. Liu, C. Liao, et al., ACS Energy Lett. 8 (2023) 4903–4914. doi: 10.1021/acsenergylett.3c02072

  5. [5]

     H. Li, X. Xu, F. Li, et al., Chem. Eur. J. 29 (2023) e202204035. doi: 10.1002/chem.202204035

  6. [6]

     S. Zhao, Q. Shi, W. Feng, et al., Chin. Chem. Lett. 35 (2024) 108606. doi: 10.1016/j.cclet.2023.108606

  7. [7]

     L. Cassayre, B. Guzhov, M. Zielinski, et al., Renew. Sus. Energy Rev. 170 (2022) 112983. doi: 10.1016/j.rser.2022.112983

  8. [8]

     P. Lopes, V. Stamenkovic, Science 369 (2020) 923–924. doi: 10.1126/science.abd3352

  9. [9]

     Q. Zhu, D. Zhao, M. Cheng, et al., Adv. Energy Mater. 9 (2019) 1901081. doi: 10.1002/aenm.201901081

  10. [10]

      Z. Liang, J. Shen, X. Xu, et al., Adv. Mater. 34 (2022) 2200102. doi: 10.1002/adma.202200102

  11. [11]

      Y. Guo, Q. Niu, F. Pei, et al., Energy Environ. Sci. 17 (2024) 1330–1367. doi: 10.1039/d3ee04183b

  12. [12]

      M. Zhao, H. Peng, B. Li, et al., Acc. Chem. Res. 57 (2024) 545–557. doi: 10.3390/systems12120545

  13. [13]

      J. Frith, M. Lacey, U. Ulissi, Nat. Commun. 14 (2023) 420. doi: 10.1038/s41467-023-35933-2

  14. [14]

      F. Degen, M. Winter, D. Bendig, et al., Nat. Energy 8 (2023) 1284–1295. doi: 10.1038/s41560-023-01355-z

  15. [15]

      Z. Lin, T. Liu, X Ai, et al., Nat. Commun. 9 (2018) 5262. doi: 10.1038/s41467-018-07599-8

  16. [16]

      Y. Cao, M. Li, J. Lu, et al., Nat. Nanotechnol. 14 (2019) 200–207. doi: 10.1038/s41565-019-0371-8

  17. [17]

      G. Yao, X. Zhang, Y. Yan, et al., J. Energy Chem. 50 (2020) 387–394. doi: 10.1016/j.jechem.2020.03.047

  18. [18]

      M. Chen, L. Chen, Z. Hu, et al., Adv. Mater. 29 (2017) 1605535. doi: 10.1002/adma.201605535

  19. [19]

      K. Ha, S. Woo, D. Mok, et al., Adv. Energy Mater. 3 (2013) 770–776. doi: 10.1002/aenm.201200825

  20. [20]

      H. Kim, I. Park, D. Seo, et al., J. Am. Chem. Soc. 134 (2012) 10369–10372. doi: 10.1021/ja3038646

  21. [21]

      P. Nayak, L. Yang, W. Brehm, et al., Angew. Chem. Int. Ed. 57 (2018) 102–120. doi: 10.1002/anie.201703772

  22. [22]

      X. Xu, F. Li, D. Zhang, et al., Carbon Neutral. 2 (2023) 54–62. doi: 10.1002/cnl2.40

  23. [23]

      N. Kosova, A. Shindrov, Energy Storage Mater. 42 (2021) 570–593. doi: 10.1016/j.ensm.2021.08.016

  24. [24]

      T. Jin, H. Li, K. Zhu, et al., Chem. Soc. Rev. 49 (2020) 2342–2377. doi: 10.1039/c9cs00846b

  25. [25]

      A. Cresce, S. Russell, O. Borodin, et al., Phys. Chem. Chem. Phys. 19 (2017) 574–586. doi: 10.1039/C6CP07215A

  26. [26]

      C. Zhao, Y. Lu, Y. Li, et al., Small Methods 1 (2017) 1600063. doi: 10.1002/smtd.201600063

  27. [27]

      H. Liu, X. Zheng, Y. Du, et al., Adv. Mater. 36 (2024) 2307645. doi: 10.1002/adma.202307645

  28. [28]

      C. Peng, X. Xu, F. Li, et al., Small Struct. 4 (2023) 2300150. doi: 10.1002/sstr.202300150

  29. [29]

      C. Delmas, Adv. Energy Mater. 8 (2018) 1703137. doi: 10.1002/aenm.201703137

  30. [30]

      K. Song, C. Liu, L. Mi, et al., Small 17 (2021) 1903194. doi: 10.1002/smll.201903194

  31. [31]

      Z. Liu, J. Wu, J. Zeng, et al., Adv. Energy Mater. 13 (2023) 2301471. doi: 10.1002/aenm.202301471

  32. [32]

      J. Wang, Y. Zhu, Y. Su, et al., Chem. Soc. Rev. 53 (2024) 4230–4301. doi: 10.1039/d3cs00929g

  33. [33]

      Q. Wang, S. Chu, S. Guo, Chin. Chem. Lett. 31 (2020) 2167–2176. doi: 10.1016/j.cclet.2019.12.008

  34. [34]

      T. Watcharatharapong, J. T-Thienprasert, S. Chakraborty, et al., Nano Energy 55 (2019) 123–134. doi: 10.1016/j.nanoen.2018.10.038

  35. [35]

      J. Zhang, X. Zhou, Y. Wang, et al., Small 15 (2019) 1903723. doi: 10.1002/smll.201903723

  36. [36]

      J. Wang, W. Zeng, J. Zhu, et al., Nano Energy 116 (2023) 108822. doi: 10.1016/j.nanoen.2023.108822

  37. [37]

      R. Rajagopalan, B. Chen, Z. Zhang, et al., Adv. Mater. 29 (2017) 1605694. doi: 10.1002/adma.201605694

  38. [38]

      W. Pan, W. Guan, S. Liu, et al., J. Mater. Chem. 7 (2019) 13197–13204. doi: 10.1039/c9ta02188d

  39. [39]

      S. Jiang, H. Wang, T. Wang, et al., Battery Energy 3 (2024) 20230071. doi: 10.1002/bte2.20230071

  40. [40]

      L. Tao, C. Peng, F. Bin, et al., Chin. Chem. Lett. 35 (2024) 109267. doi: 10.1016/j.cclet.2023.109267

  41. [41]

      J. Qian, C. Wu, Y. Cao, et al., Adv. Energy Mater. 8 (2018) 1702619. doi: 10.1002/aenm.201702619

  42. [42]

      R. Zeng, Y. Wu, S. Qian, et al., ACS Appl. Mater. Interfaces 14 (2022) 11448–11456. doi: 10.1021/acsami.1c24012

  43. [43]

      L. Wang, Y. Ni, X. Hou, et al., Angew. Chem. Int. Ed. 59 (2020) 22126–22131. doi: 10.1002/anie.202008726

  44. [44]

      N. Yabuuchi, K. Kubota, M. Dahbi, et al., Chem. Rev. 114 (2014) 11636–11682. doi: 10.1021/cr500192f

  45. [45]

      M. Avdeev, Z. Mohamed, C. Ling, et al., Inorg. Chem. 52 (2013) 8685–8693. doi: 10.1021/ic400870x

  46. [46]

      M. Galceran, D. Saurel, B. Acebedo, et al., Phys. Chem. Chem. Phys. 16 (2014) 8837–8842. doi: 10.1039/C4CP01089B

  47. [47]

      J. Kim, D. Seo, H. Kim, et al., Energy Environ. Sci. 8 (2015) 540–545. doi: 10.1039/C4EE03215B

  48. [48]

      Y. Zhang, J. Zhang, T. Shao, et al., ACS Appl. Mater. Interfaces 14 (2022) 14253–14263. doi: 10.1021/acsami.2c00821

  49. [49]

      Y. Zhang, J. Zhang, X. Li, et al., Chem. Eng. J. 430 (2022) 132708. doi: 10.1016/j.cej.2021.132708

  50. [50]

      Y. Gong, X. Gao, L. Sheng, et al., Electrochim. Acta 439 (2023) 141670. doi: 10.1016/j.electacta.2022.141670

  51. [51]

      M. Lu, R. Guo, W. Li, et al., J. Electroanal. Chem. 922 (2022) 116772. doi: 10.1016/j.jelechem.2022.116772

  52. [52]

      C. Xu, J. Zhao, E. Wang, et al., Adv. Energy Mater. 11 (2021) 2100729. doi: 10.1002/aenm.202100729

  53. [53]

      R. Essehli, H. Ben Yahia, R. Amin, et al., Energy Storage Mater. 24 (2020) 343–350. doi: 10.1016/j.ensm.2019.07.040

  54. [54]

      S. Li, X. Hou, Z. Gu, et al., New J. Chem. 45 (2021) 4854–4859. doi: 10.1039/d1nj00262g

  55. [55]

      H. Ben Yahia, R. Essehli, R. Amin, et al., J. Power Sources 382 (2018) 144–151. doi: 10.1016/j.jpowsour.2018.02.021

  56. [56]

      A. Zhao, T. Yuan, P. Li, et al., Nano Energy 91 (2022) 106680. doi: 10.1016/j.nanoen.2021.106680

  57. [57]

      F. Xiong, J. Li, C. Zuo, et al., Adv. Funct. Mater. 33 (2022) 2211257.

  58. [58]

      N. Wang, J. Ma, Z. Liu, et al., Chem. Eng. J. 433 (2022) 133798. doi: 10.1016/j.cej.2021.133798

  59. [59]

      A. Gezović, M. Vujković, M. Milović, et al., Energy Storage Mater. 37 (2021) 243–273. doi: 10.1016/j.ensm.2021.02.011

  60. [60]

      Z. Fan, W. Song, N. Yang, et al., Angew. Chem. Int. Ed. 63 (2024) e202316957. doi: 10.1002/anie.202316957

  61. [61]

      X. Li, Y. Meng, D. Xiao, Chem. Eur. J. 29 (2023) e202203381. doi: 10.1002/chem.202203381

  62. [62]

      X. Wu, G. Zhong, Y. Yang, J. Power Sources 327 (2016) 666–674. doi: 10.1016/j.jpowsour.2016.07.061

  63. [63]

      W. Ren, M. Qin, Y. Zhou, et al., Energy Storage Mater. 54 (2023) 776–783. doi: 10.1016/j.ensm.2022.11.018

  64. [64]

      H. Kim, I. Park, S. Lee, et al., Chem. Mater. 25 (2013) 3614–3622. doi: 10.1021/cm4013816

  65. [65]

      M. Chen, W. Hua, J. Xiao, et al., Nat. Commun. 10 (2019) 1480. doi: 10.1038/s41467-019-09170-5

  66. [66]

      T. Yuan, Y. Wang, J. Zhang, et al., Nano Energy 56 (2019) 160–168. doi: 10.1016/j.nanoen.2018.11.011

  67. [67]

      X. Ma, Z. Pan, X. Wu, et al., Chem. Eng. J. 365 (2019) 132–141. doi: 10.1016/j.cej.2019.01.173

  68. [68]

      C. Xu, L. Zhou, T. Gao, et al., J. Am. Chem. Soc. 146 (2024) 9819–9827. doi: 10.1021/jacs.3c14452

  69. [69]

      X. Zhao, X. Wang, Z. Gu, et al., Adv. Funct. Mater. 34 (2024) 2402447. doi: 10.1002/adfm.202402447

• 

  Figure 1  (a) Schematic diagram of the synthesis of NFPP@C/EG. (b) XRD pattern, (c) the TG curve, (d) EPR spectra with g-factor, and (e) Raman spectra of H-NFPP@C and M-NFPP@C/EG. XPS spectra of C 1s of (f) M-NFPP@C/EG, (g) H-NFPP@C.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a-c) SEM images, (d, e) TEM images, (f) HRTEM image, (g) EDS mapping images of M-NFPP@C/EG.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  The charge-discharge profiles of (a) M-NFPP@C/EG and (b) H-NFPP@C at 0.1 C. Long-term cycling performance of M-NFPP@C/EG and H-NFPP@C at (c) 1 C and (d) 50 C. (e) Rate performance of H-NFPP@C and M-NFPP@C/EG. (f) Charge/discharge curves of M-NFPP@C/EG at different rates. (g) The energy density of M-NFPP@C/EG and H-NFPP@C at different rates.

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Kinetics analysis of H-NFPP@C and M-NFPP@C/EG: CV curves of (a) H-NFPP@C and (d) M-NFPP@C/EG at different scan rates. log(v) vs. log(i) plots for the different peaks in the CV curves of (c) H-NFPP@C and (f) M-NFPP@C/EG, where i is the peak current and v is the scan rate. Capacitive capacity contribution ratios of (c) H-NFPP@C and (f) H-NFPP@C/EG at different scan rates. In-situ EIS of (g) H-NFPP@C and (h) M-NFPP@C/EG. (i) Relationships between the specific capacity, rate and cycle number in half-cells for NFPP.

  下载: 全尺寸图片 幻灯片
  

  Figure 5  (a) The charge-discharge curves of M-NFPP@C/EG at 0.5 C at 30 ℃, 0 ℃ and -10 ℃. (b) Rate performance of H-NFPP@C and M-NFPP@C/EG at 0 ℃. (c) Long-term cycling performance of H-NFPP@C and M-NFPP@C/EG at (d) 5 C and (e) 10 C at 0 ℃. Electrochemical performance of M-NFPP@C/EG, (f) the charge-discharge profiles at 0.5 C, long-term cycling performance at (g) 1 C, and (h) 2 C at room temperature.

  下载: 全尺寸图片 幻灯片
•