Sodium supplementing coating based on helical iron-carbon based composite material and preparation method thereof
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- NORTHWESTERN POLYTECHNICAL UNIV
- Filing Date
- 2026-04-13
- Publication Date
- 2026-06-23
AI Technical Summary
Sodium-ion batteries suffer from irreversible sodium loss and poor interface stability during the initial cycle, leading to a decrease in energy density and a deterioration in cycle stability.
A sodium-supplementing coating based on a spiral iron-carbon composite material was adopted. Sodium oxalate was uniformly loaded onto the surface and pores of the Fe3C@CNS spiral carbon composite material through a preparation method. The Fe3C nanocrystals and sodium oxalate formed a heterogeneous interface, which optimized the interface structure and catalyzed the decomposition of sodium ions, thus forming a stable SEI film.
It significantly improved the first-week coulombic efficiency and cycle stability of sodium-ion batteries. The first-week coulombic efficiency increased from 56.37% to 82.70%, the capacity retention rate reached 92.73% after 100 cycles at 1C rate, and the discharge specific capacity remained at 145.41mAh/g.
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Abstract
Description
Technical Field
[0001] This invention relates to the field of sodium battery materials technology, and in particular to a sodium-supplementing coating based on a helical iron-carbon composite material and its preparation method. Background Technology
[0002] Lithium-ion batteries (LIBs) have become the mainstream energy storage technology in portable devices and automobiles due to their superior performance. However, lithium resources are scarce and geographically unevenly distributed, making it difficult to meet the explosive growth in global energy storage demand in the long term. Against this backdrop, sodium-ion batteries (SIBs), with their abundant sodium resources and significant cost-effectiveness, have become a highly promising alternative to lithium-ion batteries and have attracted widespread attention.
[0003] However, sodium-ion batteries have some problems in practical applications, which seriously restrict their energy density and cycle stability: (1) Irreversible sodium loss in the first cycle: In the initial cycle of sodium-ion batteries, the negative electrode (especially the hard carbon (HC) negative electrode used in mainstream applications) will irreversibly consume the sodium ions released by the positive electrode, resulting in a low initial coulombic efficiency (ICE) (the ICE of hard carbon negative electrode is usually only 70%-90%). This phenomenon directly causes a large loss of active sodium in the positive electrode, making it impossible for the whole cell to fully release the inherent capacity of the positive electrode material, ultimately leading to a significant drop in energy density, which becomes a key obstacle restricting the practical application of sodium-ion batteries. (2) Performance degradation in long-term cycling: After long-term cycling, sodium-ion batteries will be affected by multiple factors, resulting in continuous performance degradation. On the one hand, the active elements (such as vanadium) in the positive electrode material (such as NVP) may dissolve (in V 3+ V 4 + The dissolved vanadium ions migrate to the surface of the negative electrode, damaging the integrity of the solid electrolyte interphase (SEI) film and catalyzing the growth of sodium dendrites, leading to cross-contamination between the positive and negative electrodes. On the other hand, the migrating vanadium ions also trigger continuous side reactions inside the battery through the redox shuttle effect. In addition, the parasitic irreversible reaction between the active sodium exposed on the negative electrode surface and the organic solvent of the electrolyte, as well as the incomplete stripping of sodium ions, will further deplete the active sodium ion stockpile in the positive electrode lattice, resulting in obstructed ion transport and deterioration of battery cycle stability.
[0004] Therefore, there is an urgent need to provide a feasible solution to maximize the actual volumetric and gravimetric energy density of sodium-ion batteries. Summary of the Invention
[0005] The purpose of this invention is to provide a sodium-replenishing coating based on a spiral iron-carbon composite material and its preparation method, which solves the problems of irreversible sodium loss and poor interface stability in the first week of existing sodium-ion batteries, achieves efficient pre-sodiumization and synergistic interface regulation, and improves the overall performance of the battery.
[0006] To achieve the above objectives, the present invention provides a method for preparing a sodium-supplementing coating based on a helical iron-carbon composite material, comprising the following steps:
[0007] S1. Preparation of chiral surfactant: Glutamic acid was dissolved in an alkaline solution in an ice bath, stearoyl chloride was added and reacted, then acidified to precipitate, and the chiral surfactant was obtained by washing and vacuum drying.
[0008] S2, Helical polypyrrole template synthesis: Using the chiral surfactant of S1 as a template and ammonium persulfate as an oxidant, pyrrole monomers were polymerized at low temperature to prepare the helical polypyrrole template T-PPy;
[0009] S3, preparation of iron-loaded and sulfurized precursors: The polypyrrole template T-PPy of S2 was ultrasonically dispersed in methanol, and an iron salt solution was added. After ultrasonic loading and centrifugal drying, Fe@T-PPy was obtained. Then, it was refluxed with thiophene under an inert atmosphere to obtain the Fe-S@T-PPy precursor.
[0010] S4. Preparation of spiral carbon composite material: The Fe-S@T-PPy precursor obtained in S3 was pyrolyzed and carbonized in an inert atmosphere to obtain Fe3C@CNS spiral carbon composite material.
[0011] S5. Sodium oxalate loading and coating preparation: The Fe3C@CNS spiral carbon composite material of S4 was dispersed in a saturated sodium oxalate solution, and ethanol was added to induce sodium oxalate recrystallization loading to obtain Na2C2O4@Fe3C@CNS composite material. Then, the Na2C2O4@Fe3C@CNS composite material was mixed with conductive agent and binder to form a slurry and coated on the electrode surface. After drying, a sodium-replenishing coating was formed.
[0012] Preferably, the glutamic acid in S1 includes one or more of L-glutamic acid, D-glutamic acid, and DL-glutamic acid; the molar ratio of glutamic acid to stearoyl chloride is 1:1 to 1.2, and the vacuum drying temperature is 60 to 80°C.
[0013] Preferably, in S1, the alkaline solution is an aqueous solution of sodium hydroxide, the acid used for acidification is hydrochloric acid, and the vacuum drying temperature is 60~80℃.
[0014] Preferably, in S2, the pyrrole monomer is N-substituted pyrrole, which includes one of N-methylpyrrole and N-ethylpyrrole. The mass ratio of the chiral surfactant to the pyrrole monomer is 1:100~150, the polymerization reaction temperature is 0~5℃, and the reaction time is 4~6 hours.
[0015] Preferably, in S3, the iron salt includes one of ferric nitrate nonahydrate, ferric chloride, and ferric sulfate, the mass ratio of the iron salt to the polypyrrole template T-PPy is 1:2~4, the vulcanization reaction temperature is 60~70℃, and the reaction time is 10~12 hours.
[0016] Preferably, in S4, the pyrolysis carbonization involves heating to 800-900°C at a heating rate of 3-5°C / min and maintaining the temperature at that rate for 1-2 hours.
[0017] Preferably, in S5, the mass loading ratio of Fe3C@CNS spiral carbon composite material to sodium oxalate is 1:0.5~2, and the mass ratio of Na2C2O4@Fe3C@CNS composite material to conductive agent and binder is (70-80):(10-20):(5-10).
[0018] Preferably, in S5, the ethanol is added dropwise during the recrystallization process at a flow rate of 0.5-2 mL / min while stirring.
[0019] Preferably, in S5, the coating thickness is 5~15μm, and the drying is performed by vacuum drying at 60~120℃ for 6~12 hours.
[0020] The present invention also provides a sodium-supplementing coating based on a spiral iron-carbon composite material, which is prepared by the above-described method for preparing a sodium-supplementing coating based on a spiral iron-carbon composite material. In the sodium-supplementing coating, sodium oxalate is uniformly loaded on the surface and pores of the Fe3C@CNS spiral carbon composite material, and Fe3C nanocrystals and sodium oxalate form a heterogeneous interface.
[0021] Mechanism of the invention:
[0022] This invention constructs Fe3C nanocrystal sites in situ within a helical carbon framework through structural regulation (resulting in a Fe3C@CNS helical carbon composite material), forming a highly efficient catalytically active network. The hollow structure and porous surface of the helical carbon material provide stable loading sites for sodium oxalate. High dispersion loading of sodium oxalate nanoparticles is achieved through recrystallization, effectively preventing aggregation, shortening the sodium ion diffusion path, mitigating volume changes during decomposition, and enhancing the material's structural stability, thus forming a structurally stable sodium oxalate-helical carbon (Na2C2O4@Fe3C@CNS) sodium-supplementing composite material.
[0023] The Fe3C nanocrystals in the Fe3C@CNS helical carbon composite material significantly reduce the decomposition potential of sodium oxalate, improving its electrochemical decomposition efficiency within the conventional voltage window and effectively avoiding electrolyte oxidation and side reactions caused by the excessively high decomposition potential of traditional sodium salts. Simultaneously, the three-dimensional network structure of the iron-spiral carbon and its surface catalytic active centers promote the efficient conversion of intermediate products such as oxalate ions generated during the decomposition of sodium oxalate, avoiding the formation of gaseous byproducts. The Na₂O₃ released after decomposition...+ It can be rapidly embedded into the negative electrode, synergistically regulate the solvation structure of the negative electrode interface, induce the formation of a dense SEI film with high ionic conductivity, effectively reduce interfacial impedance, and improve the first-cycle coulombic efficiency and cycle reversibility.
[0024] Therefore, the sodium-supplementing coating based on the above-mentioned helical iron-carbon composite material and its preparation method have the following beneficial effects:
[0025] (1) The spiral carbon synthesized by the chiral template method in this invention has abundant internal space and high specific surface area. It can highly disperse and confine sodium oxalate nanoparticles in its pores and surface by recrystallization, effectively preventing their aggregation, shortening the sodium ion diffusion path, and alleviating the volume change during the decomposition process.
[0026] (2) The Fe3C nanocrystals formed in situ during the pyrolysis carbonization process of this invention and the atomically dispersed Fe-Nx sites constitute a highly efficient composite catalytic center. These active sites can significantly reduce the electrochemical decomposition potential by optimizing the electron transfer path of sodium oxalate decomposition and lowering the reaction energy barrier, so that the sodium supplementation reaction can proceed efficiently and stably within a wider and safer voltage window, avoiding side reactions caused by high voltage.
[0027] (3) The Fe3C composite catalytic center of this invention significantly reduces the decomposition potential of sodium oxalate. Compared with commercial sodium oxalate (without obvious decomposition plateau) and recrystallized sodium oxalate (4.07V), the decomposition efficiency is greatly improved. Sodium ions can be efficiently released within the conventional voltage window, accurately compensating for the irreversible capacity loss in the first week, and increasing the coulombic efficiency of the battery from 56.37% to 82.70% in the first week. The released sodium ions can induce the formation of a stable SEI film on the negative electrode, reduce the interfacial impedance, inhibit the growth of sodium dendrites and cross-contamination between the positive and negative electrodes, and significantly improve the cycle stability of the battery. After 100 cycles at 1C rate, the capacity retention rate is still 92.73%, and the discharge specific capacity is maintained at 145.41mAh / g.
[0028] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description
[0029] Figure 1 These are TEM and SAED images of the Na2C2O4@Fe3C@CNS composite material prepared in Example 1 of this invention. Figure 1 Image (a) is a TEM image of the Na2C2O4@Fe3C@CNS composite material; Figure 1 (b) is the SAED diagram of the Na2C2O4@Fe3C@CNS composite material;
[0030] Figure 2 The XRD patterns are of recrystallized sodium oxalate of this invention and commercial sodium oxalate.
[0031] Figure 3 Comparison of the first charge-discharge test results of the Na2C2O4@Fe3C@CNS composite material prepared in Example 1 of this invention, recrystallized sodium oxalate (rc-Na2C2O4), and commercial sodium oxalate (Na2C2O4);
[0032] Figure 4 This is a comparison chart of the full-cell cycle stability test of the p-NVP cathode coated with Na2C2O4@Fe3C@CNS sodium supplement coating and the original NVP cathode without coating in Example 1 of the present invention. Detailed Implementation
[0033] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense by those skilled in the art. The features mentioned above or in the specific examples mentioned in this invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the invention and are not intended to limit the scope of the invention.
[0034] This invention provides a method for preparing a sodium-supplementing coating based on a helical iron-carbon composite material, comprising the following steps:
[0035] S1. Preparation of chiral surfactant: Glutamic acid was dissolved in an alkaline solution in an ice bath, stearoyl chloride was added and reacted, then acidified to precipitate, and the chiral surfactant was obtained by washing and vacuum drying.
[0036] S2, Helical polypyrrole template synthesis: Using the chiral surfactant C18-Glu from S1 as a template and ammonium persulfate as an oxidant, pyrrole monomers were polymerized at low temperature to prepare the helical polypyrrole template T-PPy;
[0037] S3, preparation of iron-loaded and sulfurized precursors: The polypyrrole template T-PPy of S2 was ultrasonically dispersed in methanol, and an iron salt solution was added. After ultrasonic loading and centrifugal drying, Fe@T-PPy was obtained. Then, it was refluxed with thiophene under an inert atmosphere to obtain the Fe-S@T-PPy precursor.
[0038] S4. Preparation of spiral carbon composite material: The Fe-S@T-PPy precursor obtained in S3 was pyrolyzed and carbonized in an inert atmosphere to obtain Fe3C@CNS spiral carbon composite material.
[0039] S5. Sodium oxalate loading and coating preparation: The Fe3C@CNS spiral carbon composite material of S4 was dispersed in a saturated sodium oxalate solution, and ethanol was added to induce sodium oxalate recrystallization loading to obtain Na2C2O4@Fe3C@CNS composite material. Then, the Na2C2O4@Fe3C@CNS composite material was mixed with conductive agent and binder to form a slurry and coated on the electrode surface. After drying, a sodium-replenishing coating was formed.
[0040] This invention achieves efficient and stable decomposition of sodium oxalate and controllable release of sodium ions through the confinement effect of the helical carbon skeleton and the catalytic effect of Fe3C active sites, while optimizing the negative electrode interface structure. The prepared sodium-replenishing coating, made by uniformly coating the surface of the positive electrode material with an iron-helical carbon / sodium oxalate composite material (Na2C2O4@Fe3C@CNS composite material), can serve as a catalytic sodium source. During the first charge, under the catalytic effect of the iron-based active sites, sodium oxalate can be efficiently electrochemically decomposed at a relatively low potential, accurately and controllably releasing active sodium ions to compensate for sodium loss on the positive electrode side. The catalytic decomposition path avoids violent gas generation, improving safety. The released sodium ions are embedded in the negative electrode, simultaneously optimizing the negative electrode interface structure and inducing the formation of an SEI film with high ionic conductivity and good mechanical stability, thereby constructing a sodium-ion battery with high initial efficiency and long cycle life.
[0041] Preferably, the glutamic acid in S1 includes one or more of L-glutamic acid, D-glutamic acid, and DL-glutamic acid; the molar ratio of glutamic acid to stearoyl chloride is 1:1 to 1.2, and the vacuum drying temperature is 60 to 80°C.
[0042] Preferably, in S1, the alkaline solution is an aqueous solution of sodium hydroxide, the acid used for acidification is hydrochloric acid, and the vacuum drying temperature is 60~80℃.
[0043] The chiral surfactant prepared in this invention provides a template for the subsequent formation of helical structures. Through its own asymmetric chiral centers, the chiral information is transferred from the molecular scale to the supramolecular assembly via non-covalent interactions during self-assembly. This chiral transfer process can directionally induce and control the helical direction (i.e., chiral bias) of the subsequently formed helical structure.
[0044] Preferably, in S2, the pyrrole monomer is N-substituted pyrrole, which includes one of N-methylpyrrole and N-ethylpyrrole. The mass ratio of the chiral surfactant to the pyrrole monomer is 1:100~150, the polymerization reaction temperature is 0~5℃, and the reaction time is 4~6 hours.
[0045] In a further preferred embodiment, in S2, a chiral surfactant is dissolved in methanol, then ultrapure water and pyrrole monomer are added, mixed thoroughly, and then a pre-cooled ammonium persulfate aqueous solution is quickly added. The reaction is carried out at low temperature. After the reaction is completed, the black solid is collected by filtration, washed successively with deionized water and ethanol, and then dried under vacuum to obtain a helical polypyrrole template T-PPy.
[0046] The helical polypyrrole template T-Ppy of the present invention creates a topological structure with high activity, high selectivity and structural stability by deeply integrating the physical exposure of catalytic sites with chiral chemical functions. This structure not only optimizes the accessibility of sites, but also achieves precise control of reaction stereochemistry through its intrinsic chirality.
[0047] Preferably, in S3, the iron salt includes one of ferric nitrate nonahydrate, ferric chloride, and ferric sulfate, the mass ratio of the iron salt to the polypyrrole template T-PPy is 1:2~4, the vulcanization reaction temperature is 60~70℃, and the reaction time is 10~12 hours.
[0048] The Fe-S@T-PPy precursor prepared in this invention provides a guarantee for the subsequent formation of Fe3C nanocrystals and iron single-atom sites. Fe-S species uniformly anchored in the framework are in situ encapsulated or isolated by carbon layers during pyrolysis, which restricts the long-range diffusion of Fe atoms and promotes the formation of highly dispersed Fe3C nanocrystals (~5-20nm) and atomic-level Fe-N4 sites.
[0049] Preferably, in S4, the pyrolysis carbonization involves heating to 800-900°C at a heating rate of 3-5°C / min and maintaining the temperature at that rate for 1-2 hours.
[0050] The Fe3C@CNS spiral carbon composite material of the present invention is a nitrogen / sulfur co-doped spiral carbon material rich in Fe3C nanocrystals and iron single atom sites. Its three-dimensional network structure and highly active catalytic sites are the key to achieving efficient decomposition of sodium oxalate.
[0051] Preferably, in S5, the mass loading ratio of Fe3C@CNS spiral carbon composite material to sodium oxalate is 1:0.5~2, and the mass ratio of Na2C2O4@Fe3C@CNS composite material to conductive agent and binder is (70-80):(10-20):(5-10).
[0052] Preferably, in S5, the ethanol is added dropwise at a flow rate of 0.5-2 mL / min during the recrystallization process with stirring.
[0053] Preferably, in S5, the coating thickness is 5~15μm, and the drying is performed by vacuum drying at 60~120℃ for 6~12 hours.
[0054] This invention utilizes a recrystallization method, based on the difference in solubility of sodium oxalate in water and ethanol, to highly disperse sodium oxalate nanoparticles on the surface and pores of a Fe3C@CNS helical carbon composite material, forming a structurally stable Na2C2O4@Fe3C@CNS composite material. The resulting sodium-replenishing coating combines sodium replenishment function with interface regulation.
[0055] The present invention also provides a sodium-supplementing coating based on a spiral iron-carbon composite material, which is prepared by the above-described method for preparing a sodium-supplementing coating based on a spiral iron-carbon composite material. In the sodium-supplementing coating, sodium oxalate is uniformly loaded on the surface and pores of Fe3C@CNS spiral carbon, and Fe3C nanocrystals and sodium oxalate form a heterogeneous interface.
[0056] Example 1
[0057] This invention provides a sodium-supplementing coating based on a helical iron-carbon composite material, and its preparation method includes the following steps:
[0058] S1. Preparation of the chiral surfactant C18-L-Glu: 3.53 g of L-glutamic acid was dissolved in 10 mL of 2 M sodium hydroxide aqueous solution and stirred in an ice bath for 30 min until completely dissolved. 6.05 g of stearoyl chloride (glutamic acid to stearoyl chloride molar ratio 1:1.1) was slowly added dropwise while continuously stirring (500 rpm) and the reaction was allowed to proceed at room temperature for 10 min. After the reaction was complete, 1 M hydrochloric acid solution was added dropwise to adjust the pH to 1, resulting in the precipitation of a white solid. The precipitate was repeatedly washed with deionized water until the pH of the filtrate reached 7, followed by washing three times with petroleum ether. The obtained solid was placed in a vacuum drying oven and dried at 75 °C for 12 h to obtain the chiral surfactant C18-L-Glu.
[0059] Synthesis of S2, Helical Polypyrrole Template (T-PPy): 24.5 mg of C18-L-Glu obtained from S1 was weighed and dissolved in 12.9 mL of methanol, and the solution was magnetically stirred for 10 min. 10 mL of ultrapure water and 3 g of N-methylpyrrole (C18-L-Glu to N-methylpyrrole mass ratio 1:122) were added and mixed thoroughly to obtain a homogeneous solution. 548 mg of ammonium persulfate was dissolved in 1.2 mL of pre-cooled ultrapure water and rapidly added to the above mixture under vigorous stirring. The mixture was transferred to an ice-water bath (0-5℃) and reacted for 5 h. After the reaction was complete, the black solid was collected by vacuum filtration, washed three times each with deionized water and ethanol, and then dried in a vacuum drying oven at 60℃ for 24 h to obtain a polypyrrole template with a distinct helical morphology, denoted as T-PPy.
[0060] Preparation of S3, iron-loaded and sulfide precursor (Fe-S@T-PPy): 30 mg of T-PPy obtained from S2 was dispersed in 30 mL of methanol and treated with an ultrasonic cell disruptor (300 W, 2 s operation followed by 1 s interval) for 30 min to obtain a T-PPy dispersion. 10 mg of ferric nitrate nonahydrate (iron salt to T-PPy mass ratio 1:3) was dissolved in 2 mL of methanol to prepare an iron salt solution. The iron salt solution was added dropwise to the T-PPy dispersion with stirring, and ultrasonic treatment was continued for 30 min. The precipitate was collected by centrifugation (8000 rpm, 10 min) and dried under vacuum at 60 °C for 12 h to obtain Fe@T-PPy. The obtained Fe@T-PPy was mixed with 200 μL of thiophene and placed in a three-necked flask. Under nitrogen protection, the mixture was refluxed and stirred in an oil bath at 65 °C for 12 h to carry out sulfidation. After the reaction was completed, the product was washed three times each with ultrapure water and ethanol, and then dried under vacuum at 60 °C overnight to obtain a black powder Fe-S@T-PPy precursor.
[0061] Preparation of S4 spiral carbon composite material (Fe3C@CNS): The Fe-S@T-PPy precursor powder obtained in S3 was placed in a quartz boat and then placed in a tube furnace. Under the protection of an argon atmosphere (flow rate 100 mL / min), the temperature was programmed to rise to 900℃ at a heating rate of 5℃ / min, and held at this temperature for 1.5 h for carbonization. Subsequently, it was naturally cooled to room temperature to obtain a black powdery carbon composite material, denoted as Fe3C@CNS spiral carbon composite material.
[0062] S5. Preparation of Sodium Oxalate-Loaded and Sodium-Supplementing Coating: The Fe3C@CNS spiral carbon composite material obtained in S4 was dispersed in a saturated aqueous solution of sodium oxalate, with a mass ratio of Fe3C@CNS spiral carbon composite material to sodium oxalate of 1:1. Ethanol was slowly added dropwise at a flow rate of 0.5 mL / min under stirring. Utilizing the principle that the solubility of sodium oxalate in ethanol is significantly reduced, sodium oxalate was induced to recrystallize and precipitate on the surface and in the pores of the spiral carbon. The solid material was collected by filtration, washed with ethanol, and then vacuum dried to obtain the sodium oxalate-loaded spiral carbon composite material, denoted as Na2C2O4@Fe3C@CNS composite material. Na2C2O4@Fe3C@CNS composite material, carbon black conductive agent, and polyvinylidene fluoride binder were weighed at a mass ratio of 80:10:10. N-methylpyrrolidone was added, and the mixture was ground and stirred to remove bubbles, forming a uniform slurry. The slurry was coated onto the surface of the NVP positive electrode, with a coating thickness controlled at 15 μm. The coating was then vacuum dried at 80℃ for 8 h to form a sodium-supplementing coating.
[0063] Example 2
[0064] S1. Preparation of the chiral surfactant C18-D-Glu: 3.53 g of D-glutamic acid was dissolved in 10 mL of 2 M sodium hydroxide aqueous solution and stirred in an ice bath for 30 min until completely dissolved. 5.5 g of stearoyl chloride (glutamic acid to stearoyl chloride molar ratio 1:1) was slowly added dropwise while continuously stirring (500 rpm) and the reaction was allowed to proceed at room temperature for 10 min. After the reaction was complete, 1 M hydrochloric acid solution was added dropwise to adjust the pH to 1, resulting in the precipitation of a white solid. The precipitate was repeatedly washed with deionized water until the pH of the filtrate reached 7, followed by washing three times with petroleum ether. The obtained solid was placed in a vacuum drying oven and dried at 60 °C for 12 h to obtain the chiral surfactant C18-D-Glu.
[0065] Synthesis of S2, Helical Polypyrrole Template (T-PPy): 24.5 mg of C18-D-Glu obtained from S1 was weighed and dissolved in 12.9 mL of methanol, and the solution was magnetically stirred for 10 min. 10 mL of ultrapure water and 2.7 g of N-ethylpyrrole (C18-D-Glu to N-ethylpyrrole mass ratio 1:112) were added and mixed thoroughly to obtain a homogeneous solution. 548 mg of ammonium persulfate was dissolved in 1.2 mL of pre-cooled ultrapure water and rapidly added to the above mixture under vigorous stirring. The mixture was transferred to an ice-water bath (0-5℃) and reacted for 4 h. After the reaction was complete, the black solid was collected by vacuum filtration and washed three times each with deionized water and ethanol. The product was placed in a vacuum drying oven and dried at 60℃ for 24 h to obtain a polypyrrole template with a distinct helical morphology, denoted as T-PPy.
[0066] Preparation of S3, iron-loaded and sulfide precursor (Fe-S@T-PPy): 30 mg of T-PPy obtained from S2 was dispersed in 30 mL of methanol and treated with an ultrasonic cell disruptor (300 W, 2 s operation followed by 1 s interval) for 30 min to obtain a T-PPy dispersion. 7.5 mg of ferric chloride (iron salt to T-PPy mass ratio 1:4) was dissolved in 2 mL of methanol to prepare an iron salt solution; the iron salt solution was added dropwise to the T-PPy dispersion with stirring, and ultrasonic treatment was continued for 30 min. The precipitate was collected by centrifugation (8000 rpm, 10 min) and dried under vacuum at 60 °C for 12 h to obtain Fe@T-PPy. The obtained Fe@T-PPy was mixed with 200 μL of thiophene and placed in a three-necked flask. Under nitrogen protection, the mixture was refluxed and stirred in an oil bath at 60 °C for 10 h to carry out sulfidation. After the reaction was completed, the product was washed three times each with ultrapure water and ethanol, and then dried under vacuum at 60 °C overnight to obtain a black powder Fe-S@T-PPy precursor.
[0067] Preparation of S4 spiral carbon composite material (Fe3C@CNS): The Fe-S@T-PPy precursor powder obtained in S3 was placed in a quartz boat and then placed in a tube furnace. Under nitrogen atmosphere (flow rate 100 mL / min), the temperature was programmed to rise to 800℃ at a rate of 3℃ / min, and held at this temperature for 2 hours for carbonization. Subsequently, it was naturally cooled to room temperature to obtain a black powdered carbon composite material, denoted as Fe3C@CNS spiral carbon composite material.
[0068] S5. Preparation of Sodium Oxalate-Loaded and Sodium-Supplementing Coating: The Fe3C@CNS spiral carbon composite material obtained in S4 was dispersed in a saturated aqueous solution of sodium oxalate, with a mass ratio of Fe3C@CNS spiral carbon composite material to sodium oxalate of 1:0.5. Ethanol was slowly added dropwise at a flow rate of 1.0 mL / min under stirring. Utilizing the principle that the solubility of sodium oxalate in ethanol is significantly reduced, sodium oxalate was induced to recrystallize and precipitate on the surface and in the pores of the spiral carbon. The solid was collected by filtration, washed with ethanol, and then vacuum dried to obtain the sodium oxalate-loaded spiral carbon composite material, denoted as Na2C2O4@Fe3C@CNS composite material. Na2C2O4@Fe3C@CNS composite material, acetylene black conductive agent, and polyvinylidene fluoride binder were weighed at a mass ratio of 80:10:10. N-methylpyrrolidone was added and ground and mixed, then stirred to degas and form a uniform slurry. The slurry was coated onto the surface of the NVP positive electrode, with a coating thickness controlled at 5 μm. Vacuum drying at 60℃ for 12 h formed a sodium-supplementing coating.
[0069] Example 3
[0070] S1. Preparation of the chiral surfactant C18-DL-Glu: 3.53 g of DL-glutamic acid was dissolved in 10 mL of 2 M sodium hydroxide aqueous solution and stirred in an ice bath for 30 min until completely dissolved. 6.65 g of stearoyl chloride (glutamic acid to stearoyl chloride molar ratio 1:1.2) was slowly added dropwise while continuously stirring (500 rpm) and reacted at room temperature for 10 min. After the reaction was complete, 1 M hydrochloric acid solution was added dropwise to adjust the pH to 1, resulting in the precipitation of a white solid. The precipitate was repeatedly washed with deionized water until the pH of the filtrate reached 7, followed by washing three times with petroleum ether. The obtained solid was placed in a vacuum drying oven and dried at 80 °C for 12 h to obtain the chiral surfactant C18-DL-Glu.
[0071] Synthesis of S2, Helical Polypyrrole Template (T-PPy): 24.5 mg of C18-DL-Glu obtained from S1 was weighed and dissolved in 12.9 mL of methanol, and the solution was magnetically stirred for 10 min. 10 mL of ultrapure water and 2.7 g of N-methylpyrrole (C18-DL-Glu to N-methylpyrrole mass ratio 1:112) were added and mixed thoroughly to obtain a homogeneous solution. 548 mg of ammonium persulfate was dissolved in 1.2 mL of pre-cooled ultrapure water and rapidly added to the above mixture under vigorous stirring. The mixture was transferred to an ice-water bath (0-5℃) and reacted for 6 h. After the reaction was complete, the black solid was collected by vacuum filtration and washed three times each with deionized water and ethanol. The product was placed in a vacuum drying oven and dried at 60℃ for 24 h to obtain a polypyrrole template with a distinct helical morphology, denoted as T-PPy.
[0072] Preparation of S3, iron-loaded and sulfide precursor (Fe-S@T-PPy): 30 mg of T-PPy obtained from S2 was dispersed in 30 mL of methanol and treated with an ultrasonic cell disruptor (300 W, 2 s operation followed by 1 s interval) for 30 min to obtain a T-PPy dispersion. 15 mg of ferric sulfate (iron salt to T-PPy mass ratio 1:2) was dissolved in 2 mL of methanol to prepare an iron salt solution. The iron salt solution was added dropwise to the T-PPy dispersion with stirring, and ultrasonic treatment was continued for 30 min. The precipitate was collected by centrifugation (8000 rpm, 10 min) and dried under vacuum at 60 °C for 12 h to obtain Fe@T-PPy. The obtained Fe@T-PPy was mixed with 200 μL of thiophene and placed in a three-necked flask. Under nitrogen protection, the mixture was refluxed and stirred in an oil bath at 70 °C for 11 h to carry out sulfidation. After the reaction was completed, the product was washed three times each with ultrapure water and ethanol, and then dried under vacuum at 60 °C overnight to obtain a black powder Fe-S@T-PPy precursor.
[0073] Preparation of S4 spiral carbon composite material (Fe3C@CNS): The Fe-S@T-PPy precursor powder obtained in S3 was placed in a quartz boat and then placed in a tube furnace. Under the protection of an argon atmosphere (flow rate 100 mL / min), the temperature was programmed to rise to 850℃ at a heating rate of 4℃ / min, and held at this temperature for 1.5 h for carbonization. Subsequently, it was naturally cooled to room temperature to obtain a black powdery carbon composite material, denoted as Fe3C@CNS spiral carbon composite material.
[0074] S5. Preparation of Sodium Oxalate-Loaded and Sodium-Supplementing Coating: The Fe3C@CNS spiral carbon composite material obtained in S4 was dispersed in a saturated aqueous solution of sodium oxalate, with a mass ratio of Fe3C@CNS spiral carbon composite material to sodium oxalate of 1:2. Ethanol was slowly added dropwise at a flow rate of 2.0 mL / min under stirring. Utilizing the principle that the solubility of sodium oxalate in ethanol is significantly reduced, sodium oxalate was induced to recrystallize and precipitate on the surface and in the pores of the spiral carbon. The solid material was collected by filtration, washed with ethanol, and then vacuum dried to obtain the sodium oxalate-loaded spiral carbon composite material, denoted as Na2C2O4@Fe3C@CNS composite material. Na2C2O4@Fe3C@CNS composite material, carbon black conductive agent, and polyvinylidene fluoride binder were weighed at a mass ratio of 80:10:10. N-methylpyrrolidone was added, and the mixture was ground and mixed, stirred, and degassed to form a uniform slurry. The slurry was coated onto the surface of the NVP positive electrode, with a coating thickness controlled at 10 μm. The coating was then vacuum dried at 120℃ for 6 h to form a sodium-supplementing coating.
[0075] Comparative Example 1
[0076] Commercial sodium oxalate powder is used as a sodium supplement, and it is directly mixed with conductive agent and binder in the same proportion to form a slurry for coating without carrier loading.
[0077] Performance testing
[0078] The coated aluminum foils obtained in Examples 1-3 and Comparative Example 1 were dried respectively (the content of solid matter on the dried coated aluminum foil was 3.5 mg / cm³). -2 The 12mm round discs were used as the positive electrode, the sodium sheet as the negative electrode, Whatman GF / A as the separator between the positive and negative electrodes, and NP-035 as the electrolyte. The CR2016 button cell was assembled and its electrochemical performance was tested.
[0079] The Na2C2O4@Fe3C@CNS composite material prepared in Example 1 of this invention was characterized and analyzed using high-resolution transmission electron microscopy (TEM) and selected area electron diffraction (SAED).
[0080] like Figure 1 As shown in (a), well-crystallized nanoparticles (Na2C2O4) are clearly attached to the surface of the Fe3C@CNS helical carbon skeleton, indicating that sodium oxalate was successfully loaded onto the surface and pores of the carbon skeleton.
[0081] Further selected area electron diffraction (SAED) analysis was performed on the loaded region, such as... Figure 1As shown in (b), the diffraction pattern displays multiple sets of clear diffraction rings, some of which can be attributed to characteristic crystal planes of sodium oxalate crystals and Fe3C crystals, respectively. The continuity of the diffraction rings indicates that both the supported sodium oxalate and the Fe3C nanocrystals in the support have good crystallinity, and that they form a uniform and compact composite structure at the nanoscale.
[0082] The microstructural features indicate that a tight composite system of sodium oxalate and Fe3C@CNS support was successfully constructed via recrystallization. The helical carbon framework with a high curvature topology provides stable loading sites and ion transport channels for sodium oxalate, while the highly dispersed Fe3C nanocrystals, acting as catalytic active centers, effectively modulate the electronic structure of sodium oxalate through a tight interface, reducing the reaction barrier for sodium ion insertion / extraction. This integrated support-active center-sodium source structure facilitates the efficient and controllable release of sodium ions during electrochemical processes, thereby significantly improving sodium replenishment efficiency and battery electrochemical performance.
[0083] Ethanol was slowly added dropwise to a saturated aqueous solution of sodium oxalate under stirring. Taking advantage of the significantly reduced solubility of sodium oxalate in ethanol, recrystallization of sodium oxalate was induced, yielding recrystallized sodium oxalate. The recrystallized sodium oxalate and commercial sodium oxalate were characterized using X-ray diffraction.
[0084] The results are as follows Figure 2 As shown, after recrystallization, no new phase characteristic diffraction peaks other than Na2C2O4 appeared, and the diffraction peaks of sodium oxalate were significantly broadened after recrystallization. This is mainly attributed to the significant reduction in its grain size during the recrystallization process. This indicates that the recrystallization process only refines the particle size of sodium oxalate and makes it uniformly dispersed on the surface and within the pores of the support.
[0085] The Na2C2O4@Fe3C@CNS composite material prepared in Example 1, recrystallized sodium oxalate (rc-Na2C2O4), and commercial sodium oxalate (Na2C2O4) were used as working electrodes, and metallic sodium was used as the counter / reference electrode. They were assembled into a half cell and the first charge-discharge test was carried out at a charge-discharge rate of 0.1C.
[0086] like Figure 3 As shown, commercial sodium oxalate, within the test voltage range (up to 4.5V vs. Na), +The absence of a clear electrochemical decomposition plateau in the Na2C2O4@Fe3C@CNS composite material indicates its inability to effectively decompose and release sodium ions in conventional electrolyte systems. In contrast, recrystallized sodium oxalate exhibits a identifiable decomposition plateau at approximately 4.07 V, but its voltage window remains within the high-voltage range where the electrolyte is unstable, and the plateau shows significant polarization. The Na2C2O4@Fe3C@CNS composite material prepared in Example 1, however, displays a clear and stable decomposition plateau at approximately 3.84 V, corresponding to a significant increase in reversible capacity. Compared to recrystallized sodium oxalate, the decomposition voltage of this composite material is further reduced by approximately 0.23 V; compared to commercial sodium oxalate, its decomposition behavior changes from being difficult to occur to occurring efficiently under moderate voltage. This result confirms that the Fe3C@CNS support not only provides a good dispersion and conductive environment for sodium oxalate, but its surface Fe3C active centers can also significantly catalyze the decomposition reaction of sodium oxalate, lowering the reaction energy barrier and enabling controllable and efficient sodium ion release within a safer voltage range (<4.0 V) that better matches the stability window of conventional electrolytes.
[0087] Using an NVP positive electrode coated with a 15μm thick Na2C2O4@Fe3C@CNS sodium-supplemented coating (denoted as p-NVP) from Example 1 and an uncoated original NVP positive electrode (control group, denoted as NVP) as the positive electrode, and hard carbon (HC) as the negative electrode, a sodium-ion full cell was assembled; a carbon-coated copper foil was also used as the counter electrode. The cycle stability of the full cell was tested with the following parameters: charge / discharge rate of 1C, 100 cycles, focusing on monitoring changes in discharge specific capacity and capacity retention.
[0088] like Figure 4 As shown, the p-NVP electrode exhibits excellent cycle stability. In a full cell without a negative electrode assembled with carbon-coated copper foil as the counter electrode, the control group without sodium replenishment showed significant capacity decay after only 29 cycles. However, after applying Na2C2O4@Fe3C@CNS as a pre-sodium coating to the NVP positive electrode surface, the full cell showed a significant performance improvement, maintaining a discharge specific capacity of 145.41 mAh / g after 100 cycles, demonstrating excellent capacity retention and cycle stability. This result fully verifies that the functional coating constructed by combining sodium oxalate and Fe3C@CNS support can not only effectively reduce the decomposition potential of sodium oxalate, but also achieve controlled release and efficient utilization of sodium ions through optimized coating structure, thereby significantly improving the cycle life and electrochemical stability of a negative electrode-free sodium-ion battery.
[0089] Therefore, this invention employs the above-mentioned sodium-supplementing coating based on helical iron-carbon composite material and its preparation method. Through the synergistic effect of catalysis-confinement-interface regulation, it achieves the integration of pre-sodiumization and interface regulation, providing a new strategy and key technology for the development of high-performance sodium-ion batteries, and has broad application prospects.
[0090] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.
Claims
1. A method for preparing a sodium-supplementing coating based on a helical iron-carbon matrix composite material, characterized in that: Includes the following steps: S1. Preparation of chiral surfactant: Glutamic acid was dissolved in an alkaline solution in an ice bath, stearoyl chloride was added and reacted, then acidified to precipitate, and the chiral surfactant was obtained by washing and vacuum drying. S2, Helical polypyrrole template synthesis: Using the chiral surfactant of S1 as a template and ammonium persulfate as an oxidant, pyrrole monomers were polymerized at low temperature to prepare the helical polypyrrole template T-PPy; S3, preparation of iron-loaded and sulfurized precursors: The polypyrrole template T-PPy of S2 was ultrasonically dispersed in methanol, and an iron salt solution was added. After ultrasonic loading and centrifugal drying, Fe@T-PPy was obtained. Then, it was refluxed with thiophene under an inert atmosphere to obtain the Fe-S@T-PPy precursor. S4. Preparation of spiral carbon composite material: The Fe-S@T-PPy precursor obtained in S3 was pyrolyzed and carbonized in an inert atmosphere to obtain Fe3C@CNS spiral carbon composite material. S5. Sodium oxalate loading and coating preparation: The Fe3C@CNS spiral carbon composite material of S4 was dispersed in a saturated sodium oxalate solution, and ethanol was added to induce sodium oxalate recrystallization loading to obtain Na2C2O4@Fe3C@CNS composite material. Then, the Na2C2O4@Fe3C@CNS composite material was mixed with conductive agent and binder to form a slurry and coated on the electrode surface. After drying, a sodium-replenishing coating was formed. The glutamic acid in S1 includes one or more of L-glutamic acid, D-glutamic acid, and DL-glutamic acid; the molar ratio of glutamic acid to stearoyl chloride is 1:1~1.2, and the vacuum drying temperature is 60~80℃; In S2, the pyrrole monomer is N-substituted pyrrole, which includes one of N-methylpyrrole and N-ethylpyrrole. The mass ratio of the chiral surfactant to the pyrrole monomer is 1:100~150, the polymerization temperature is 0~5℃, and the reaction time is 4~6 hours. In S3, the iron salt includes one of ferric nitrate nonahydrate, ferric chloride, and ferric sulfate. The mass ratio of the iron salt to the polypyrrole template T-PPy is 1:2~4. The vulcanization reaction temperature is 60~70℃ and the reaction time is 10~12 hours. In S4, the pyrolysis carbonization involves heating to 800-900℃ at a heating rate of 3-5℃ / min and maintaining the temperature at that rate for 1-2 hours. In S5, the mass loading ratio of Fe3C@CNS spiral carbon composite material to sodium oxalate is 1:0.5~2, and the mass ratio of Na2C2O4@Fe3C@CNS composite material to conductive agent and binder is (70-80):(10-20):(5-10).
2. The method for preparing a sodium-supplementing coating based on a spiral iron-carbon composite material according to claim 1, characterized in that: In S1, the alkaline solution is an aqueous solution of sodium hydroxide, the acid used for acidification is hydrochloric acid, and the vacuum drying temperature is 60~80℃.
3. The method for preparing a sodium-supplementing coating based on a spiral iron-carbon composite material according to claim 1, characterized in that: In S5, ethanol is added dropwise at a flow rate of 0.5-2 mL / min during the recrystallization process with stirring.
4. The method for preparing a sodium-supplementing coating based on a spiral iron-carbon composite material according to claim 1, characterized in that: In S5, the coating thickness is 5~15μm, and the drying is carried out under vacuum at 60~120℃ for 6~12 hours.
5. A sodium-supplementing coating based on a helical iron-carbon matrix composite material, characterized in that: The sodium-supplemented coating is prepared by the method for preparing a helical iron-carbon composite material according to any one of claims 1-4. In the sodium-supplemented coating, sodium oxalate is uniformly loaded on the surface and pores of the Fe3C@CNS helical carbon composite material, and Fe3C nanocrystals and sodium oxalate form a heterogeneous interface.