Porous carbon material for zinc-iodine battery, iodine positive electrode material, preparation method and application

Abstract

The invention relates to a porous carbon material for a zinc-iodine battery, which is prepared by a method comprising the following steps: (1) preparing a ZIF-8 crystal by using zinc salt and 2-methylimidazole as raw materials; (2) soaking the ZIF-8 crystal in a trisodium citrate aqueous solution, and synthesizing an SC-ZIF-8 composite material; and (3) placing the SC-ZIF-8 composite material in an inert atmosphere, and carrying out heat treatment at 850-950 DEG C to obtain the porous carbon material. The invention also relates to an iodine positive electrode material which is prepared by a method comprising the following steps: mixing the porous carbon material with solid iodine, placing the mixture in a sealed container, and preparing the iodine positive electrode material by a melt diffusion method. When the porous carbon material is used as an iodine positive electrode carrier, efficient conversion and utilization of an iodine active material are realized, and the specific discharge capacity, the rate capability and the cycle life of an aqueous zinc-iodine battery based on the iodine positive electrode are improved.

Description

Technical Field

[0001] This invention relates to the field of battery material preparation, mainly to the field of aqueous zinc-iodine battery preparation, and specifically to a porous carbon material, an iodine cathode material, and their preparation and application for zinc-iodine batteries. Background Technology

[0002] With the rapid development of new energy vehicles, portable electronic devices, and other power-driven terminals, the demand for high-safety, low-cost, and long-life energy storage systems is becoming increasingly urgent. Currently, the most mature commercially available lithium-ion battery (LIB) is facing limitations in large-scale application due to the limited and uneven distribution of lithium metal resources and their continuously rising prices. Therefore, developing an energy storage system that can replace lithium-ion batteries has become a top priority. Among many candidate systems, metal-iodine batteries stand out due to the abundant and inexpensive availability of iodine and a high capacity of 211 mAh g / L. -1 With its theoretical specific capacity, it is considered one of the most promising next-generation energy storage technologies. In 1972, the lithium-iodine battery was first introduced as a primary battery; subsequently, sodium-iodine, potassium-iodine, and zinc-iodine systems were proposed and studied in depth. Among them, the aqueous zinc-iodine battery (AZIB) stands out due to the low redox potential (-0.76V vs. SHE) and high theoretical capacity (820mAh g / g) of its zinc anode. -1 Zinc-iodine batteries have attracted widespread attention in recent years due to their advantages such as good hydrogen evolution overpotential and intrinsic safety of aqueous electrolytes. However, they face two major challenges: the formation of soluble polyiodides (I3+) during charging and discharging. - I5 - (etc.) will cause a severe shuttle effect, leading to shortened cycle life, reduced coulombic efficiency (CE), and continuous corrosion of the zinc anode; elemental iodine has a low intrinsic conductivity (~1.3×10-7S cm). -1 This limits its rate performance and the utilization rate of active substances.

[0003] To address the aforementioned issues, researchers have used porous carbon materials with high conductivity, porosity, and specific surface area to load iodine, thereby improving electrochemical performance. For example, Pan et al. (H. Pan, B. Li, D. Mei, Z. Nie, Y. Shao, G. Li, XS Li, ​​KSHan, KTMueller, V. Sprenkle, J. Liu, ACS Energy Letters 2 (2017) 2674-2680) used activated carbon fibers (ACF) to load I₂ to absorb iodine as a cathode, achieving excellent electrochemical performance. Although these porous carbon materials can physically confine iodine within the pores, porous carbon mainly relies on van der Waals interactions for the physical adsorption of iodine molecules, resulting in limited fixation capacity and a need for further improvement in long-term lifetime. Introducing heteroelements into carbon materials and utilizing their surface polar groups for chemisorbent adsorption of iodine can enhance the performance of iodine cathodes. However, how to simultaneously regulate the pore structure and surface chemical environment of carbon materials to maximize the improvement of the electrochemical performance of iodine cathodes remains a current research hotspot in this field. Summary of the Invention

[0004] The technical problem solved by this invention is: in zinc-iodine batteries, iodine has low conductivity and produces polyiodide ions (I3) as intermediate products of iodine discharge. - Iodine dissolves, and the dissolved polyiodide ions shuttle to the negative electrode, causing self-discharge and leading to a decrease in capacity (short lifetime). Generally, porous carbon materials are used to coat iodine as the positive electrode, utilizing the pore binding effect and the adsorption of iodine by polar functional groups on the carbon. In existing technologies, when nonpolar carbon is used as an iodine carrier, it can only interact with polar iodine substances through weak van der Waals forces, resulting in limited suppression of the shuttle effect. Furthermore, the preparation of doped carbon materials often requires relatively complex additive introduction methods, consumes a large amount of additives, and makes it difficult to achieve reasonable pore structure design and simultaneous control.

[0005] The purpose of this invention is to design a suitable host material through a simple method, and to jointly suppress the shuttle effect through the physical adsorption of hierarchical pore structure and the chemical interaction between surface polar functional groups and polyiodide ions.

[0006] Metal-organic framework (MOF)-derived porous carbon, with its very large surface area and numerous heteroatom doping, has been used in many fields. In view of this, to address the aforementioned technical problems, this invention introduces trisodium citrate as an additive. By altering the morphology, pore size distribution, and elemental composition of ZIF-8-derived porous carbon, its hierarchical pore structure provides abundant active sites for establishing chemical interactions with active materials. Furthermore, N and O are also introduced into the micro-mesoporous hierarchical carbon (CNC) scaffold to improve conductivity and provide abundant active sites for establishing chemical interactions with active materials. Thanks to the hierarchical pore structure, the CNC / I2 cathode exhibits excellent electrochemical performance; the Zn-I2 battery demonstrates good rate performance and an ultra-long lifespan of 10,000 cycles.

[0007] Specifically, in view of the shortcomings of the existing technology, the present invention provides the following technical solution:

[0008] A porous carbon material for zinc-iodine batteries, characterized in that it is prepared by a method comprising the following steps:

[0009] (1) ZIF-8 crystals were prepared using zinc salt and 2-methylimidazole as raw materials;

[0010] (2) ZIF-8 crystals were soaked in a trisodium citrate aqueous solution to synthesize SC-ZIF-8 composite material;

[0011] (3) The SC-ZIF-8 composite material was placed in an inert atmosphere and heat-treated at 850-950°C to obtain the porous carbon material.

[0012] Preferably, in the above-mentioned porous carbon material, the inert atmosphere is a nitrogen atmosphere.

[0013] Preferably, in the above porous carbon material, in step (2), the concentration of the trisodium citrate aqueous solution is 0.1-0.5 mol / L, and the mass-to-volume ratio of the ZIF-8 crystal to the trisodium citrate aqueous solution is 1 g: (20-30) mL.

[0014] Preferably, in the above-mentioned porous carbon material, the concentration of the trisodium citrate aqueous solution is 0.10-0.30 mol / L, more preferably 0.15-0.20 mol / L.

[0015] Preferably, in the above porous carbon material, step (2) includes the following steps:

[0016] ZIF-8 crystals were soaked in a trisodium citrate aqueous solution and stirred for 2.5–3.5 hours. The resulting mixture was allowed to stand for 12–14 hours and then filtered to obtain the SC-ZIF-8 composite material.

[0017] Preferably, in step (2), the stirring speed is 800-900 rpm.

[0018] Preferably, in the above porous carbon material, in step (3), the heat treatment time is 2 to 3 hours and the heating rate is 5 to 10 °C / min.

[0019] Preferably, step (1) includes the following steps: dissolving zinc salt and 2-methylimidazole separately in methanol, mixing, reacting at room temperature for 2-3 hours, standing for 12-14 hours, separating, and obtaining ZIF-8 crystals. The zinc salt is selected from zinc nitrate or zinc acetate, and the mass ratio of the zinc salt to 2-methylimidazole is 1:(2.0-2.5).

[0020] Preferably, in step (1), the reaction process is carried out under stirring at a speed of 800-900 rpm.

[0021] Preferably, the specific surface area of ​​the porous carbon material is 1000–1100 m². 2 g –1 The average pore size of the porous carbon material is 3.30 nm to 3.70 nm.

[0022] This invention also provides a method for preparing the above-mentioned porous carbon material for zinc-iodine batteries, characterized by comprising the following steps:

[0023] (1) ZIF-8 crystals were prepared using zinc salt and 2-methylimidazole as raw materials;

[0024] (2) ZIF-8 crystals were soaked in a trisodium citrate aqueous solution to synthesize SC-ZIF-8 composite material;

[0025] (3) The SC-ZIF-8 composite material was placed in an inert atmosphere and heat-treated at 850-950°C to obtain the porous carbon material.

[0026] The present invention also provides an iodine cathode material for zinc-iodine batteries, characterized in that it is prepared by a method comprising the following steps:

[0027] The porous carbon material was mixed with solid iodine and placed in a sealed container. Iodine cathode material was prepared by melt diffusion.

[0028] Preferably, in the above-mentioned iodine cathode material, the mass ratio of porous carbon material to solid iodine is 1:(0.8-1.2).

[0029] Preferably, in the above-mentioned iodine cathode material, the temperature of the melting and diffusion process is 110-120°C, and the time is 4-5 hours.

[0030] The present invention also provides an iodine cathode for zinc-iodine batteries, characterized in that it is prepared from raw materials comprising the above-mentioned iodine cathode material, sodium carboxymethyl cellulose binder and conductive carbon black.

[0031] Preferably, in the above-mentioned iodine cathode, the mass ratio of the iodine cathode material, sodium carboxymethyl cellulose binder, and conductive carbon black is 7:2:1.

[0032] Preferably, in the above-mentioned iodine cathode, the iodine mass loading is 0.6-0.7 mg / cm³. -2 .

[0033] The present invention also provides a zinc-iodine battery, characterized in that it comprises the above-mentioned iodine positive electrode.

[0034] The present invention also provides the application of the above-mentioned iodine cathode material, the above-mentioned iodine cathode or the above-mentioned zinc-iodine battery in the field of energy storage or electric power drive equipment.

[0035] This invention relates to a simple method for synthesizing carbon materials, achieving simultaneous control of the pore structure and surface chemical environment of zeolite imidazolium ester framework-8 (ZIF-8) derived carbon materials through a simple adsorption-combination mechanism. This not only expands the pore structure and increases the specific surface area but also introduces abundant oxygen, enabling the efficient conversion and utilization of iodine active materials when used as an iodine cathode support. Aqueous zinc-iodine batteries (AZIBs) based on this material exhibit significantly improved discharge specific capacity, excellent rate performance, and long cycle life.

[0036] The advantages of this invention are: (1) In the micro-mesoporous hierarchical carbon obtained by this invention, the micropores provide high specific surface area and defect sites, and chemically anchor I2 / I3. - (1) Reduce shuttle; (2) Mesoporous structures construct fast ion / electrolyte channels, reducing diffusion polarization; (3) In-situ generated ZnO-N active sites synergistically dope N / O heteroatoms, significantly improving charge transfer rate (R0). ct (From 178.9Ω to 105.2Ω). The CNC / I2 cathode-based battery at 0.1Ag... -1 The first cycle capacity is as high as 205.8mAh g. -1 It has a specific capacity close to that of iodine, with an initial capacity of 178.3 mAh g at 1.0 A / g. -1 It maintains a capacity of 113.1 mAh g after 10,000 cycles. -1 The capacity decay rate was only 0.0045% / cycle. In-situ Raman spectroscopy confirmed that the hierarchical pore structure effectively suppressed the accumulation of intermediate polyiodides and accelerated I-reduction. - / I2 reversibility. This work provides a new approach for designing highly stable, high-rate iodine hosts through the synergistic regulation of pore engineering and interfacial chemistry. Attached Figure Description

[0037] Figure 1 In the diagram, a is a scanning electron microscope (SEM) image of the ZIF-8 crystal obtained in Example 1, b is a scanning electron microscope (SEM) image of the SC-ZIF-8 composite material obtained in Example 1, c is a scanning electron microscope (SEM) image of the NC material obtained in Example 1, d is a scanning electron microscope (SEM) image of the CNC material obtained in Example 1, e is a transmission electron microscope (TEM) image of the NC material obtained in Example 1, and f is a TEM image of the CNC material obtained in Example 1.

[0038] Figure 2 Figure 1 shows the CNC / I2 composite material obtained in Example 1 and the EDS elemental mapping diagrams of each element. Figure 1(a) shows the CNC / I2 composite material, Figure 1(b) shows the carbon element, Figure 1(c) shows the nitrogen element, Figure 1(d) shows the oxygen element, Figure 1(e) shows the iodine element, and Figure 1(f) shows the zinc element.

[0039] Figure 3 The XPS full spectrum of the NC and CNC materials obtained in Example 1 is shown.

[0040] Figure 4 In the diagram, a represents the X-ray diffraction pattern of the ZIF-8 crystal and SC-ZIF-8 composite material obtained in Example 1, b represents the X-ray diffraction pattern of the NC and CNC materials obtained in Example 1, and c represents the X-ray diffraction pattern of the NC / I2 composite material and CNC / I2 composite material obtained in Example 1.

[0041] Figure 5 The images show the Raman spectra of the NC and CNC materials obtained in Example 1.

[0042] Figure 6 The N2 adsorption-desorption isotherms are those of the NC and CNC materials obtained in Example 1.

[0043] Figure 7 The thermogravimetric analysis curves of the NC / I2 composite material and the CNC / I2 composite material obtained in Example 1 are shown.

[0044] Figure 8 The CV curves are for the zinc-iodine batteries based on NC / I2 and CNC / I2 obtained in Example 1.

[0045] Figure 9 In the table, a represents the CV curves of the battery based on the NC electrode obtained in Example 1 at different scan rates, b represents the CV curves of the battery based on the CNC electrode obtained in Example 1 at different scan rates, and c represents the electrochemical active surface area test results.

[0046] Figure 10 The AC impedance spectra are those of the NC / I2 and CNC / I2 zinc-iodine batteries obtained in Example 1.

[0047] Figure 11In the figure, a represents the initial constant current charge-discharge curves of the NC / I2 and CNC / I2 zinc-iodine batteries obtained in Example 1, and b represents the rate charge-discharge performance of the NC / I2 and CNC / I2 zinc-iodine batteries obtained in Example 1.

[0048] Figure 12 In the figure, 'a' represents the zinc-iodine battery based on NC / I2 and CNC / I2 obtained in Example 1 at 0.1 Ag. –1 The graph shows the charge-discharge cycle efficiency performance at 1.0Ag, where b represents the efficiency obtained in Example 1 based on NC / I2 and CNC / I2 zinc-iodine batteries. –1 The charge / discharge cycle efficiency performance graph.

[0049] Figure 13 The CNC-2 / I2 and CNC-3 / I2 zinc-iodine batteries obtained in Examples 2 and 3 were used in 0.1Ag –1 The charge / discharge cycle efficiency performance graph. Detailed Implementation

[0050] Aqueous zinc-iodine batteries (AZIBs) have attracted much attention due to the high safety and low cost of zinc anodes. However, the polyiodide shuttle and poor conductivity of iodine have long limited their cycle life and rate performance. Traditional carbon-based hosts rely solely on van der Waals forces for physical confinement, making it difficult to suppress the loss of active iodine at high temperatures or high rates. Therefore, this invention utilizes the high-temperature pyrolysis of a metal-organic framework material (ZIF-8) to prepare carbon materials, proposing the use of sodium citrate as a pore-forming agent and functionalizing agent, thus optimizing the carbon material preparation method. This yielded N- and O-doped carbon materials with higher specific surface area, more mesopores, and more O doping, exhibiting better battery performance than without the introduction of this material—higher specific capacity, greater capacity retention after cycling, and greater capacity at high rates.

[0051] In a preferred embodiment, the present invention uses ZIF-8 as a precursor and introduces trisodium citrate (SC) to control the yield of micro-mesoporous hierarchical carbon (CNC). SC reacts with Zn at high temperature. 2+ The C / N reaction induces framework collapse and releases Zn vapor, simultaneously increasing the ratio of mesopores (3.6 nm) to micropores, thereby raising the specific surface area to 1039 m². 2 g -1 It is far higher than that of unmodified NC (923m). 2 g -1 ).

[0052] The mechanism of action of this invention is: (1) pore structure optimization (increased specific surface area, increased mesopores, more reaction sites and ion channels, resulting in higher iodine utilization, faster reaction kinetics, and better rate performance); (2) surface modification (more O doping, resulting in better contact between the iodine electrode and the electrolyte, better adsorption of polyiodide ions, and greater capacity retention after long cycles).

[0053] The following specific embodiments further illustrate the porous carbon material, iodine cathode material, preparation method, and application of the present invention for zinc-iodine batteries.

[0054] In the following examples, all reagents used were purchased from Sinopharm Reagent Co., Ltd. Information on the instruments used in the examples is shown in the table below:

[0055] Table 1 Instrument Information Sheet

[0056] Reagents / Instruments Specifications / Model Manufacturer / Source Scanning electron microscope SU8600 Hitachi X-ray diffraction SmartLab Japanese Neo-Confucianism X-ray photoelectron spectroscopy ESCALAB 250Xi Thermo Fisher Confocal Raman Spectrometer Xplora plus HORIBA Aperture Analyzer ASAP2460 Micromeritics Thermal analyzer STA 8000 PerkinElmer UV-Vis spectrophotometer TU-1950 Puxi

[0057] Example 1

[0058] 1. Preparation of porous carbon

[0059] (1) Preparation of ZIF-8 crystals: Zn(NO3)2·6H2O (1.2 g) and 2-methylimidazole (2.6 g) were dissolved in 40 mL of methanol respectively. The two solutions were mixed and stirred at room temperature for 2 hours, then allowed to stand for 12 hours. The resulting white turbid liquid was separated by centrifugation (8000 rpm), washed three times with methanol, and dried at 60 °C for 12 h to obtain ZIF-8 crystals.

[0060] (2) Synthesis of NC: ZIF-8 was ground to 30-80 mesh, placed in a quartz boat, and placed in a tube furnace. It was heat-treated at 900℃ for 2 hours under a nitrogen atmosphere with a heating rate of 5℃ / min. The resulting porous carbon polyhedron was named NC.

[0061] (3) Synthesis of CNC: 0.5 g of synthesized ZIF-8 was soaked in trisodium citrate aqueous solution (SC, 20 mL, 0.15 mol / L). -1 In a quartz boat, the mixture was stirred at room temperature for 3 hours, allowed to stand for 12 hours, and then filtered to obtain the SC-ZIF-8 composite material. The SC-ZIF-8 composite material was ground to 30-80 mesh, transferred to a quartz boat, and heat-treated at 900℃ for 2 hours under a nitrogen atmosphere with a heating rate of 5℃ / min. The resulting porous carbon polyhedron was named CNC.

[0062] 2. Preparation of iodine cathode and assembly of battery

[0063] Preparation of iodine cathode: Porous carbon material (CNC) and solid iodine were mixed at a mass ratio of 1:1, placed in a sealed container, and heated at 120°C for 4 hours to obtain a CNC / I₂ carbon-iodine composite material. Using NC instead of CNC, NC / I₂ carbon-iodine composite materials were prepared using the same method.

[0064] The above-mentioned carbon-iodine composite material (NC / I2 or CNC / I2), sodium carboxymethyl cellulose (CMC) binder, and conductive carbon black were thoroughly mixed in deionized water at a mass ratio of 7:2:1 to obtain a mixed slurry. This slurry was then coated onto graphite paper and left to stand overnight at room temperature. The average iodine mass loading of both the NC / I2 and CNC / I2 electrodes was 0.6-0.7 mg / cm². -2 All specific capacity values ​​are calculated based on the iodine loading mass.

[0065] Battery Assembly: Electrochemical measurements were performed using CR2025 coin cells, with iodine as the working cathode, Zn as the working anode, and Olegee glass fiber as the separator. 2 mol L... –1 Using ZnSO4 as the electrolyte, zinc-iodine batteries based on NC / I2 and CNC / I2 were assembled.

[0066] Alternatively, NC or CNC, sodium carboxymethyl cellulose binder and conductive carbon black are thoroughly mixed in deionized water at a mass ratio of 7:2:1 to obtain a mixed slurry, which is then coated onto graphite paper and left overnight at room temperature to obtain an NC electrode or a CNC electrode.

[0067] Using an NC or CNC electrode as the working cathode, a Zn anode as the working anode, and Olegee glass fiber as the diaphragm, 2 mol / L –1 ZnSO4 was used as the electrolyte to assemble batteries based on either NC or CNC electrodes.

[0068] The obtained samples were characterized as follows:

[0069] (1) The microstructure of the sample was observed using field emission scanning electron microscopy.

[0070] The scanning electron microscope (SEM) images of ZIF-8 crystal, SC-ZIF-8 composite material, NC, and CNC are shown below. Figure 1 As shown in a, b, c, and d, Figure 1In the figures, e and f are transmission electron microscopy (TEM) images of NC and CNC, respectively. As shown, both ZIF-8 and SC-ZIF-8 exhibit a rhombic dodecahedral structure, indicating that SC adsorption did not alter the rhombic dodecahedral structure of ZIF-8. During carbonization, the coordination centers of ZIF-8 are oxidized to ZnO, which then transforms into Zn vapor at high temperatures. However, in the presence of SC, due to the presence of oxygen, more ZnO is generated with the coordination centers of ZIF-8. Zinc oxide is then used to generate Zn and CO along with the carbon matrix (ZnO + C → Zn + CO). Simultaneously, these oxygen elements also combine with C and N in the ZIF-8 framework, disrupting the dodecahedral structure and causing collapse. TEM images reveal that, compared to NC, CNC not only exhibits surface depressions but also internal hollowing.

[0071] (2) The elemental composition and valence states of the material surface were analyzed using an Al Ka ​​X-ray source X-ray photoelectron spectroscopy (EDS) instrument. The EDS elemental mapping diagram of the CNC / I2 composite material obtained in this embodiment is shown below. Figure 2 As shown in the figure, C, N, O, Zn, and I elements are uniformly distributed in the CNC, which also indicates that the melt diffusion method successfully adsorbs iodine into the pores.

[0072] Figure 3 The XPS full spectrum of NC and CNC obtained in this embodiment shows that the surface of the materials contains the four elements C, N, O and Zn. Compared with NC, the zinc content in CNC is reduced, which will increase the mesoporous content of carbon materials, which is conducive to the exchange of iodine ions, increases the number of active sites, and thus improves the performance of the battery during the charging and discharging process.

[0073] (3) The material structure was tested by Cu Kα radiation using an X-ray diffractometer.

[0074] The X-ray diffraction characteristic peaks of the ZIF-8 crystal obtained in this embodiment correspond one-to-one with the simulated standard card of ZIF-8, indicating that the ZIF-8 crystal was successfully synthesized. Figure 4 a) The addition of SC did not change the position of the characteristic peaks of ZIF-8, indicating that the surface adsorption of SC did not alter the crystal structure of ZIF-8. Figure 4 b shows the X-ray diffraction patterns of NC and CNC obtained in this embodiment. As can be seen from the figure, the sample has two broad diffraction peaks at 2θ = 25° and 45°, corresponding to the (002) and (101) planes of amorphous carbon. This means that both products are converted into porous carbon without any impurities. In addition, with the addition of SC as an additive, the peak of the 101 plane becomes more obvious compared with NC, indicating that the addition of SC has a significant impact on the crystal structure of the sample. When iodine is loaded onto CNC, the XRD pattern ( Figure 4c) No characteristic diffraction peaks of iodine were observed, indicating that iodine exists in the CNC in an amorphous state. Amorphous iodine can be better dispersed in the carbon matrix of the CNC, thereby reducing the dissolution and shuttle effect of iodine.

[0075] (4) Raman analysis was performed using a 532 nm laser excitation on a confocal Raman spectrometer.

[0076] Figure 5 The figures show the Raman spectra of the NC and CNC materials obtained in this embodiment. As can be seen from the figures, two main characteristic peaks appear in the Raman spectra: the D band (approximately 1351.2 cm⁻¹). –1 ) and G-band (approximately 1587.3 cm) –1 The D band corresponds to defects and disordered structures in carbon materials, while the G band corresponds to sp in graphitic carbon. 2 The vibrational modes of hybrid carbon atoms, especially when the intensity ratio of the D band to the G band is (I D :I G A value greater than 1.0 directly indicates the presence of defect structures in the carbon sample. In Raman spectroscopy, the IC of the Raman signal, NC, and CNC... D / I G The calculation results are 1.29 and 1.31, indicating that the introduction of SC forms a rich defect structure. The existence of this defect structure is of great significance for improving the electrochemical performance of the material. It can provide more active sites to facilitate the shuttle of iodine ions and enhance the interaction between the material and the electrolyte.

[0077] (5) Nitrogen adsorption-desorption isotherms were measured at 77 K using a Micromeritics ASAP2460 instrument to investigate the specific surface area and pore size distribution of the carbon support material.

[0078] Figure 6 The figures show the N2 adsorption-desorption isotherms of the NC and CNC materials obtained in this embodiment. As can be seen from the figures, NC exhibits a Type I isotherm, while CNC shows both Type I and Type IV isotherms. This indicates that NC is composed of micropores, while CNC is composed of both micropores and mesopores. These micropores and mesopores can significantly increase the specific surface area of ​​the material, thereby exposing more active sites. According to BJH desorption analysis, the average pore sizes of NC and CNC are 2.84 nm and 3.58 nm, respectively. Furthermore, the addition of SC binds to the C, N, and Zn elements in NC, disrupting the dodecahedral structure of NC, thus increasing the specific surface area of ​​CNC (1039.28 nm). 2 g –1 The specific surface area is higher than that of NC (923.51 m²). 2 g –1In the resulting micro-mesoporous carbon material CNC, the presence of micropores enhances ion storage capacity and generates diffusion resistance, while the presence of mesopores facilitates electrolyte wetting and ion diffusion. The rich hierarchical porous structure provides ample space for iodine loading and enables rapid mass transfer, thereby improving battery performance during charge and discharge processes.

[0079] (6) Thermogravimetric analysis (TGA) was performed using STA8000 to assess the iodine loading in the cathode material. The concentration of iodine species (I2, I3-, I5-…) was determined using ultraviolet-visible spectroscopy (TU-1950 spectrophotometer).

[0080] To evaluate the mass ratio of iodine in CNC / I2 and NC / I2 and their adsorption capacity for iodine and iodides, thermogravimetric analysis and immersion experiments were conducted, followed by ultraviolet characterization. Figure 7 As shown, the mass ratio of iodine in CNC / I2 is approximately 44.1%, slightly higher than the 42.2% in NC / I2. After immersing equal masses of NC and CNC samples in a saturated iodine solution for 48 hours, the iodine content in the supernatant was measured using a UV-Vis spectrophotometer. The results indicate that iodine decolorizes more quickly in CNC, suggesting that CNC has a stronger adsorption capacity for iodine and iodides than NC. This may be because the abundant hierarchical porous structure of CNC allows for faster capture of iodine and iodides.

[0081] (7) Electrochemical performance testing

[0082] EIS curves, cyclic voltammetry (CV) curves, and linear sweep voltammetry curves were measured in the frequency range of 0.01–100 kHz using a CHI 660E electrochemical workstation. All tests were performed at room temperature. Constant current charge-discharge was conducted using a LAND battery testing system at 25 °C.

[0083] First, with 1.0mV s –1 CV measurements were performed on NC / I2 zinc-iodine and CNC / I2 zinc-iodine batteries at a scan rate of [value missing]. A pair of redox peaks were observed in the CV curves within the range of 1.0–1.4 V. Figure 8 This corresponds to I2 and I. - In the redox reaction between NC and CNC electrodes, the CNC electrode exhibits a smaller peak difference and better reversibility compared to the NC / I2 electrode, indicating that the iodine conversion reaction kinetics of the CNC material are faster. Cells based on NC and CNC electrodes were subjected to different scan rates (1.0 mV s⁻¹). –1 -10mV s –1 CV (0.6-1.1V) test () Figure 9 a and Figure 9 b), and calculated the electrochemically active surface area (ECSA) of the two materials. Figure 9c) It can be concluded that the ECSA of the cell with NC electrode and the cell with CNC electrode are 94.7 and 103.5 F g, respectively. –1 This means that CNC has more active sites.

[0084] The AC impedance spectra of zinc-iodine batteries based on NC / I2 and CNC / I2 are as follows: Figure 10 As shown, electrochemical impedance spectroscopy (EIS) measurements were performed at the onset potential (when iodine reduction has just begun), and the CNC electrode exhibited the lowest charge transfer resistance (105.2 Ω) compared to the NC (178.9 Ω). Figure 11 Figure 11a shows the initial constant current charge-discharge GCD curves of NC / I2 and CNC / I2. As can be seen from the figure, the potential gap of the CNC / I2 electrode at 50% charge (32.1mV) is lower than that of the NC / I2 electrode (42.5mV), which confirms that the zinc-iodine electrode of the CNC / I2 electrode has low voltage polarization. Figure 11 In b, the CNC / I2 electrode is between 0.1 and 2.0 Ag. –1 (Carbon-iodine mass) exhibits satisfactory rate performance at different current densities, providing higher specific capacity at 0.1, 0.2, 0.5, 1.0, and 2.0 Ag. –1 They are available in 205.8, 191.8, 183.6, 175.0, and 161.0 mAh g values ​​respectively. –1 This is higher than the capacity of the NC / I2 electrode at different current densities, when the current density returns to 0.1Ag. –1 At that time, the NC electrode could still maintain 191.1 mAh g. –1 This may stem from the capacitance contribution brought about by the rich, multi-level porous structure of the CNC material itself. The wrinkled porous structure promotes the interaction between I2 and I. - The efficient conversion between them allows the loaded iodine to be fully utilized. Figure 12 a represents NC / I2 and CNC / I2 zinc-iodine batteries at 0.1Ag –1 The charge-discharge cycle efficiency performance graph at that time. Figure 12 Figure b shows the charge-discharge cycle efficiency performance of NC / I2 and CNC / I2 zinc-iodine batteries at 1.0 A / g. As can be seen from the figure, at 0.1 A / g... –1 At low current densities, the first-cycle specific capacities of the CNC / I2 zinc-iodine battery and the NC / I2 zinc-iodine battery are 205.8 and 170 mAh g, respectively. –1 After 200 cycles, the CNC / I2 electrode exhibited excellent cycling stability, with a capacity retention of approximately 87.00%, higher than the 82.35% of the NC / I2 electrode. (The last sentence appears to be incomplete and possibly refers to a 1.0Ag...) –1 At high current densities, it exhibits a first-cycle specific capacity of 178.3 mAh g.–1 This is significantly higher than the 153.7 mAh g of the NC / I2 electrode. –1 It still maintains 113.1 mAh g even after 10,000 cycles. –1 The specific capacity of the NC / I2 electrode is much lower in comparison.

[0085] Therefore, this embodiment uses ZIF-8 as a precursor and derives nitrogen / oxygen multi-doped polyhedral carbon (CNC) with micro- and mesoporous coexistence through trisodium citrate regulation, constructing a hierarchical pore-active site synergistic iodine host. Micropores confine and chemically anchor I2 / I3-, while mesopores accelerate electrolyte wetting and ion diffusion; heteroatoms and ZnO further enhance conductivity, reducing charge transfer impedance to 105.2 Ω. The resulting CNC / I2 cathode operates at 0.1 Ag... -1 Downward reversible capacity 205.8mAh g -1 1Ag -1 After 10,000 cycles, it still has 113.1 mAh g. -1 The decay rate is only 0.0045% per lap.

[0086] Example 2

[0087] 1. Preparation of porous carbon

[0088] (1) ZIF-8 crystals were prepared according to the method in Example 1.

[0089] (2) Synthesis of CNC: 0.5 g of synthesized ZIF-8 crystals were soaked in trisodium citrate aqueous solution (SC, 20 mL, 0.1 mol / L). -1 In a quartz boat, the mixture was stirred at room temperature for 3 hours, allowed to stand for 12 hours, and then filtered to obtain the SC-ZIF-8 composite material. The SC-ZIF-8 composite material was ground to 30-80 mesh, transferred to a quartz boat, and heat-treated at 900℃ for 2 hours under a nitrogen atmosphere with a heating rate of 5℃ / min. The resulting porous carbon polyhedron was named CNC-2.

[0090] 2. Preparation of iodine cathode and assembly of battery

[0091] Preparation of iodine cathode:

[0092] The CNC-2 / I2 carbon-iodine composite material was prepared according to the method of Example 1, and the iodine cathode was prepared according to the method of Example 1.

[0093] Battery assembly: Electrochemical measurements were performed using CR2025 button cells. An iodine cathode was used as the working electrode, a Zn anode as the working electrode, NKK as the separator, and 2M ZnSO4 as the electrolyte to assemble a zinc-iodine battery.

[0094] Example 3

[0095] (1) ZIF-8 crystals were prepared according to the method in Example 1.

[0096] (2) Synthesis of CNC: 0.5 g of synthesized ZIF-8 crystals were soaked in a trisodium citrate aqueous solution (SC, 20 mL, 0.3 mol / L). -1 In a quartz boat, the mixture was stirred at room temperature for 3 hours, allowed to stand for 12 hours, and then filtered to obtain the SC-ZIF-8 composite material. The SC-ZIF-8 composite material was ground to 30-80 mesh, transferred to a quartz boat, and heat-treated at 900℃ for 2 hours under a nitrogen atmosphere with a heating rate of 5℃ / min. The resulting porous carbon polyhedron was named CNC-3.

[0097] 2. Preparation of iodine cathode and assembly of battery

[0098] Iodine cathode preparation: CNC-3 / I2 carbon-iodine composite material was prepared according to the method of Example 1, and iodine cathode was prepared according to the method of Example 1.

[0099] Battery assembly: Electrochemical measurements were performed using CR2025 button cells. An iodine cathode was used as the working electrode, a Zn anode as the working electrode, NKK as the separator, and 2M ZnSO4 as the electrolyte to assemble a zinc-iodine battery.

[0100] The electrochemical performance of the zinc-iodine batteries obtained in Examples 2 and 3 was tested according to the method in Example 1. Figure 13 The graph shows the charge-discharge cycle efficiency performance of CNC-2 / I2 and CNC-3 / I2 zinc-iodine batteries at 0.1 A / g. As can be seen from the graph, at 0.1 A / g... –1 At low current density, the zinc-iodine battery obtained in Example 2 has a first-cycle specific capacity of 149.37 mAh g. -1 The zinc-iodine battery obtained in Example 3 had a first-cycle specific capacity of 163.39 mAh g. -1 .

[0101] In summary, this invention simultaneously regulates the pore structure and surface chemistry of ZIF-8 derived carbon materials. When the resulting porous carbon material is used as an iodine cathode support, it achieves efficient conversion and utilization of iodine active materials. The discharge specific capacity, rate performance, and cycle life of aqueous zinc-iodine batteries based on this iodine cathode are all improved.

Claims

1. A porous carbon material for zinc-iodine batteries, characterized in that, Prepared by a method comprising the following steps: (1) ZIF-8 crystals were prepared using zinc salt and 2-methylimidazole as raw materials; (2) ZIF-8 crystals were soaked in a trisodium citrate aqueous solution to synthesize SC-ZIF-8 composite material; (3) The SC-ZIF-8 composite material was placed in an inert atmosphere and heat-treated at 850-950°C to obtain the porous carbon material.

2. The porous carbon material according to claim 1, wherein, In step (2), the concentration of the trisodium citrate aqueous solution is 0.1-0.5 mol / L, and the mass-to-volume ratio of the ZIF-8 crystals to the trisodium citrate aqueous solution is 1 g: (20-30) mL.

3. The porous carbon material according to claim 2, wherein, Step (2) includes the following steps: ZIF-8 crystals were soaked in a trisodium citrate aqueous solution and stirred for 2.5–3.5 hours. The resulting mixture was allowed to stand for 12–14 hours and then filtered to obtain the SC-ZIF-8 composite material.

4. The method for preparing the porous carbon material for zinc-iodine batteries according to claim 1, characterized in that, Includes the following steps: (1) ZIF-8 crystals were prepared using zinc salt and 2-methylimidazole as raw materials; (2) ZIF-8 crystals were soaked in a trisodium citrate aqueous solution to synthesize SC-ZIF-8 composite material; (3) The SC-ZIF-8 composite material was placed in an inert atmosphere and heat-treated at 850-950°C to obtain the porous carbon material.

5. An iodine cathode material for zinc-iodine batteries, characterized in that, Prepared by a method comprising the following steps: The porous carbon material described in claim 1 is mixed with solid iodine, placed in a sealed container, and iodine cathode material is prepared by melt diffusion method.

6. The iodine cathode material according to claim 5, wherein, The mass ratio of porous carbon material to solid iodine is 1:(0.8~1.2).

7. The iodine cathode material according to claim 5, wherein, The temperature of the melting and diffusion process is 110–120°C.

8. An iodine positive electrode for use in zinc-iodine batteries, characterized in that, It is prepared from raw materials comprising the iodine cathode material as described in claim 5, sodium carboxymethyl cellulose binder, and conductive carbon black.

9. A zinc-iodine battery, characterized in that, It includes the iodine positive electrode as described in claim 8.

10. The application of the iodine cathode material of claim 5, the iodine cathode of claim 8, or the zinc-iodine battery of claim 9 in the field of energy storage or electric drive equipment.

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