A solid capacity-enhancing material for a positive electrode of a vanadium flow battery and a preparation method thereof

Abstract

This invention proposes a solid capacity-enhancing material for the cathode of vanadium redox flow batteries and its preparation method, comprising the following steps: S1, preparing a porous carbon framework, performing surface activation treatment on the porous carbon framework, adding a coupling agent to obtain a modified carbon framework; S2, preparing a precursor solution containing transition metal salts and alkali metal salts, immersing the modified carbon framework in the precursor solution, then adding a ferricyanide solution to perform an in-situ precipitation reaction to obtain a Prussian blue analogue, the Prussian blue analogue undergoing a coordination reaction with the modified carbon framework to obtain a Prussian blue analogue intermediate; S3, immersing the Prussian blue analogue intermediate in an acidic solution containing conductive polymer monomers and an oxidant to form a conductive polymer buffer layer on the surface of the Prussian blue analogue intermediate, washing, drying, coating with a gradient porous sealing layer, and curing to obtain a solid capacity-enhancing material for the cathode of vanadium redox flow batteries, thus solving the problems of high interfacial resistance and short cycle life.

Description

Technical Field

[0001] This invention relates to the field of flow battery technology, and in particular to a solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery and its preparation method. Background Technology

[0002] Vanadium redox flow batteries are a typical flow battery system. Their working principle utilizes the reversible redox reaction between different valence states of vanadium ions to achieve the interconversion of electrical energy and chemical energy. This battery typically includes a positive electrolyte tank, a negative electrolyte tank, a stack, and a circulation pump. During charging and discharging, VO2 in the positive electrolyte... + / VO 2 + V in the redox couple and negative electrode electrolyte 3+ / V 2+ The redox couple reacts on the electrode surface, achieving charge balance through the ion exchange membrane, thus completing energy storage and release. Since both the positive and negative electrodes use vanadium ions as active materials, this system effectively avoids the irreversible capacity decay caused by cross-contamination of different active materials, thus exhibiting a long cycle life and good operational stability.

[0003] However, the energy density of traditional vanadium redox flow batteries is limited by the solubility of vanadium ions in the electrolyte, resulting in a low volumetric energy density and restricting their application in space-constrained scenarios. To address this issue, researchers have proposed a strategy of adding solid energy storage materials to the electrolyte storage tank. This strategy utilizes the redox-targeted reaction between the solid material and the active ions in the electrolyte to enable reversible charging and discharging of the solid material within the tank, thereby overcoming the limitation of vanadium ion solubility and increasing the overall capacity of the system.

[0004] In the prior art, solid energy storage materials are usually prepared by physically mixing Prussian blue analog powder with conductive carbon black and binder and then granulating them, and then directly adding them to the positive electrode electrolyte tank. However, this physical mixing and granulation scheme has the following problems: (1) The Prussian blue analog particles and the conductive additives are only in physical contact, and the electron transport path is discontinuous, resulting in low utilization of solid materials; (2) Under the long-term flushing of strong acid and strong oxidizing vanadium electrolyte, the particles are easy to break and pulverize, and the active material is seriously dissolved, resulting in rapid capacity decay; (3) The particle surface lacks a protective layer, and the Prussian blue analog grains are directly exposed to the electrolyte, resulting in poor structural stability and limited cycle life; (4) In order to maintain sufficient conductivity, a large amount of conductive agent needs to be added, which further reduces the effective energy storage density of the solid material. Summary of the Invention

[0005] In view of this, the present invention proposes a solid capacity-enhancing material for the cathode of vanadium redox flow batteries and its preparation method, which solves the technical problems of discontinuous electron transport and easy shedding of active material caused by physical mixing and granulation of existing solid capacity-enhancing materials, resulting in high interface resistance and short cycle life.

[0006] The technical solution of this invention is implemented as follows: A method for preparing a solid compatibilizing material for the positive electrode of a vanadium redox flow battery includes the following steps: S1, prepare a porous carbon framework, perform surface activation treatment on the porous carbon framework, add a coupling agent, and form a graft layer on the surface and pores of the porous carbon framework to obtain a modified carbon framework. S2, prepare a precursor solution containing transition metal salts and alkali metal salts, immerse the modified carbon skeleton in the precursor solution, add ferricyanide solution, and carry out an in-situ precipitation reaction to obtain a Prussian blue analog. The Prussian blue analog undergoes a coordination reaction with the modified carbon skeleton to obtain a Prussian blue analog intermediate. S3, Prussian blue analog intermediate is immersed in an acidic solution containing conductive polymer monomers and oxidants to carry out a polymerization reaction, forming a conductive polymer buffer layer on the surface of the Prussian blue analog intermediate, to obtain a composite material, which is washed, dried, coated with a gradient porous sealing layer on the surface of the composite material, and cured to obtain a solid capacity-enhancing material for the positive electrode of vanadium redox flow battery.

[0007] Based on this scheme, step S1 further includes: S1.1. A porous carbon framework is prepared by template method or hydrothermal method. The carbon source includes at least one of phenolic resin and biomass-based carbon. The porous carbon framework is immersed in an organic solvent, ultrasonically cleaned for 15-25 minutes, and dried to obtain a pretreated carbon framework. S1.2, add a pore-expanding agent, expand and activate the pores at room temperature for 0.5-1 hour, wash with deionized water, prepare a coupling agent solution and adjust the pH to 5-6, immerse the activated porous carbon skeleton in the coupling agent solution, soak for 2-4 hours to allow the coupling agent to penetrate into the pores of the porous carbon skeleton, and then pre-dry in an oven at 60-80℃ for 1-2 hours to form a graft layer on the surface and in the pores of the porous carbon skeleton, thus obtaining the modified carbon skeleton.

[0008] Based on this scheme, further, the organic solvent includes acetone and anhydrous ethanol, the pore-expanding agent includes at least one of hydrogen peroxide aqueous solution with a mass concentration of 3-10% and dilute nitric acid solution with a mass concentration of 2-8%, the coupling agent includes γ-mercaptopropyltriethoxysilane and γ-mercaptopropyltrimethoxysilane, and 0.01-0.05 mol / L boron trifluoride diethyl ether complex is added as a catalyst when preparing the coupling agent solution.

[0009] Based on this scheme, further step S2 includes: S2.1, Dissolve the transition metal salt and alkali metal salt in deionized water at a mass ratio of 1-3:1, stir until dissolved, prepare a precursor solution, filter through a 0.22 μm filter membrane, stir at 500-800 r / min for 5-10 minutes, then ultrasonically disperse for 10-20 minutes, immerse the modified carbon skeleton in the precursor solution, and soak for 1-2 hours. S2.2, Dissolve ferricyanide in deionized water and stir until dissolved to prepare a ferricyanide solution. Degas the solution with nitrogen for 10-15 minutes. Then, add the ferricyanide solution dropwise to the precursor solution at a rate of 3-8 mL / h and stir the reaction at 20-80℃ for 2-6 hours to obtain a Prussian blue analog. The Prussian blue analog is coordinated with the modified carbon skeleton through a graft layer to obtain a Prussian blue analog intermediate.

[0010] Based on this scheme, furthermore, the volume ratio of the precursor solution to the ferricyanide solution is 3-5:1, the concentration of the precursor solution is 0.05-0.2 mol / L, the transition metal salt includes at least one of ferric chloride, chromium chloride, copper chloride, manganese sulfate, and vanadium oxysulfate, the alkali metal salt includes at least one of lithium chloride, sodium chloride, and potassium chloride, and the ferricyanide includes at least one of potassium ferricyanide, sodium ferricyanide, and ammonium ferricyanide.

[0011] Based on this scheme, further step S3 includes: S3.1, Prepare an acidic solution containing conductive polymer monomers and an oxidant, adjust the pH of the acidic solution to 1-3, immerse the Prussian blue analog intermediate in the acidic solution at room temperature for 2-6 hours, so that the conductive polymer monomers undergo a polymerization reaction on the surface of the Prussian blue analog intermediate under the action of the oxidant to form a conductive polymer buffer layer, and obtain the composite material. Wash with deionized water until the washing solution is neutral to remove residual impurities, and vacuum dry at 60-80℃ for 4-8 hours. S3.2, Prepare a sealing agent, which includes silicone resin, curing agent, nano-silica, and solvent. The mass ratio of silicone resin to curing agent is 100:5-10, and the solvent is a mixture of acetone and anhydrous ethanol in a volume ratio of 1:1. A coating method is used to form a gradient porous sealing layer on the surface of the composite material. When coating the inner layer, the amount of nano-silica added is 0.5-1%, forming an inner layer with a porosity of 30-50%. When coating the surface layer, the amount of nano-silica added is 1.5-2%, forming a surface layer with a porosity of 5-10%. After coating, the material is placed in an oven at 80-120℃ for 20-60 minutes to complete the curing, thereby obtaining a solid capacity-enhancing material for the positive electrode of vanadium redox flow batteries.

[0012] Based on this scheme, the monomers of the conductive polymer further include aniline, pyrrole, and 3,4-ethylenedioxythiophene; the oxidant includes ammonium persulfate, ferric chloride, and ferric p-toluenesulfonate; the acidic solution includes dilute sulfuric acid, dilute hydrochloric acid, and dilute methanesulfonic acid solution; the solid content of the sealing agent is 5-20%; the organosilicon resin is methyl silicone resin or phenyl silicone resin; and the curing agent is ethylenediamine, diethylenetriamine, or phthalic anhydride.

[0013] In addition, the present invention also provides a solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery, which is prepared by the method for preparing the solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery according to any one aspect, comprising: Porous carbon framework; Grafted layers formed on the surface and within the pores of the porous carbon framework; Prussian blue analogues are coordinated and bonded to the surface of the graft layer via the graft layer. A conductive polymer buffer layer is coated on the surface of the Prussian blue analogue, the thickness of the conductive polymer buffer layer being 5-50 nm. And a gradient porous sealing layer coated with a conductive polymer buffer layer, wherein the gradient porous sealing layer has a gradient porous structure with an inner layer porosity of 30-50% and a surface porosity of 5-10%.

[0014] Based on this scheme, the general chemical formula of the Prussian blue analogue is further: N x AFe(CN)6, where N is an alkali metal element selected from one or more of Li, Na, and K, A is a transition metal element selected from one or more of Fe, Cr, Cu, Mn, Ni, Co, Zn, and V(VO), and x is 0-2; or NA x B 1-x Fe(CN)6, where A and B are different transition metal elements selected from one or more of Fe, Cr, Cu, Mn, Ni, Co, Zn, and V(VO), and x is 0-1.

[0015] Based on this scheme, furthermore, the solid capacity-enhancing material is used in the positive electrode electrolyte storage tank of the all-vanadium redox flow battery, and the positive electrode electrolyte includes 0.1-2 mol / L VO2. + / VO 2+ The solution contains at least one of sulfuric acid, hydrochloric acid, methanesulfonic acid, and phosphoric acid at a concentration of 0.1-4 mol / L, and the electrolyte additives contain at least one of sulfates, chlorides, methanesulfonates, and phosphates of Li, Na, and K at a concentration of 0.1-2 mol / L.

[0016] The present invention has the following advantages over the prior art: By constructing a graft layer on the surface of a porous carbon framework and utilizing a precipitation reaction, Prussian blue analog grains are directly nucleated on the graft layer and form coordination chemical bonds with it. Unlike the discrete conductivity mode of physical mixing and binders in existing technologies, this allows electrons to be directly transferred to each Prussian blue analog grain through the three-dimensional continuous carbon framework, significantly reducing electron transport resistance. At the same time, the chemical bonds firmly anchor the grains to the carbon framework, effectively inhibiting the shedding of active materials during long-term cycling.

[0017] Secondly, a conductive polymer buffer layer and a gradient porous sealing layer are further coated on the surface of the Prussian blue analog grains. The redox potential of the conductive polymer buffer layer is between that of the Prussian blue analog and VO2. + / VO 2+ Between the redox pairs, a potential bridging effect can be achieved between the solid grains and the electrolyte, reducing the interfacial charge transfer resistance of the redox-targeted reaction and thus improving the utilization rate of the solid material. The gradient porous sealing layer ensures rapid electrolyte penetration through the high porosity of the inner layer and physically blocks grain dissolution through the low porosity of the outer layer. This effectively solves the problems of easy loss of active material and increased interfacial resistance caused by physical mixing and granulation, and significantly extends the cycle life.

[0018] Through the synergistic effect of chemical bonding anchoring, conductive polymer buffering, and gradient sealing protection, the solid capacity-enhancing material prepared in this invention can achieve a solid material utilization rate of over 90% in the positive electrode electrolyte storage tank of vanadium redox flow batteries, an active material dissolution rate of less than 1%, a capacity retention rate of no less than 95% after 1000 cycles, and a working temperature range of 5-70℃. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 The solid compatibilizing materials obtained in Examples 1 and 2 and VO2 + / VO 2+ Comparison of redox potentials of redox couples; Figure 2 The graphs show the changes in capacity-voltage curves of symmetrical batteries before and after the addition of the solid capacity-enhancing materials from Examples 1 and 2, respectively. Figure 3 A graph showing the change in symmetrical battery capacity before and after adding the solid capacity-enhancing material of Example 3. Detailed Implementation

[0021] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0022] As a core energy storage component, the solid-state capacity enhancement material for the cathode of vanadium redox flow batteries directly determines the overall energy storage performance of the battery through its electron transport efficiency, active material stability, and cycle life. Existing solid-state capacity enhancement materials mostly employ physical mixing and granulation processes, which suffer from problems such as easy detachment and dissolution of active materials, discontinuous electron transport paths, high interfacial impedance, and poor cycle stability. This invention proposes a method for preparing a solid-state capacity enhancement material for the cathode of vanadium redox flow batteries. Through a synergistic process of porous carbon framework activation grafting, in-situ coordination precipitation of Prussian blue analogues, coating with a conductive polymer buffer layer, and composite with a gradient porous sealing layer, the method achieves a comprehensive effect of increased capacity, enhanced stability, and extended cycle life for the solid-state capacity enhancement material.

[0023] In a preferred embodiment, a method for preparing a solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery includes the following specific steps: S1, preparing a porous carbon framework, subjecting the porous carbon framework to surface activation treatment, adding a coupling agent, and forming a graft layer on the surface and pores of the porous carbon framework to obtain a modified carbon framework; this step specifically includes: S1.1. A porous carbon framework is prepared by template method or hydrothermal method. The carbon source includes at least one of phenolic resin and biomass-based carbon. The porous carbon framework is immersed in an organic solvent, ultrasonically cleaned for 15-25 minutes, and dried to obtain a pretreated carbon framework. S1.2, add a pore-expanding agent, expand and activate the pores at room temperature for 0.5-1 hour, wash with deionized water, prepare a coupling agent solution and adjust the pH to 5-6, immerse the activated porous carbon skeleton in the coupling agent solution, soak for 2-4 hours to allow the coupling agent to penetrate into the pores of the porous carbon skeleton, and then pre-dry in an oven at 60-80℃ for 1-2 hours to form a graft layer on the surface and in the pores of the porous carbon skeleton, thus obtaining the modified carbon skeleton.

[0024] Pore-expanding agents such as hydrogen peroxide or dilute nitric acid can gently oxidize the carbon skeleton surface, increasing the density of oxygen-containing functional groups such as hydroxyl and carboxyl groups, while moderately expanding some micropores, which is beneficial for the penetration and anchoring of the coupling agent. Washing to neutral pH is to avoid residual oxidant damaging the active structure of the coupling agent. The coupling agent selected is γ-mercaptopropyltriethoxysilane or γ-mercaptopropyltrimethoxysilane. The trialkoxysilane at one end of the molecule condenses with the hydroxyl groups on the carbon skeleton surface to form a Si-OC covalent bond, while the thiol group at the other end provides a coordination active site. A pH of 5-6 can promote the hydrolysis and condensation reaction of silane while maintaining the thiol group in the form of free thiol, thus preserving its coordination ability. Soaking for 2-4 hours ensures that the coupling agent diffuses into the internal pores of the carbon skeleton, and pre-baking treatment stabilizes and solidifies the grafted layer, forming a uniform and firm thiol-containing interface.

[0025] The coupling agent solution preferably contains 0.01-0.05 mol / L of boron trifluoride diethyl ether complex as a catalyst. As a Lewis acid catalyst, the boron trifluoride diethyl ether complex accelerates the hydrolysis of silanes and their condensation reaction with the carbon skeleton, while suppressing the self-condensation side reaction between silanol groups. This increases the thiol grafting density by more than 50%, providing sufficient active sites for the subsequent coordination anchoring of Prussian blue analogs, thereby enhancing the binding strength between Prussian blue analogs and the carbon skeleton and reducing the risk of active material detachment during long-term cycling.

[0026] S2, prepare a precursor solution containing transition metal salts and alkali metal salts, immerse the modified carbon skeleton in the precursor solution, add ferricyanide solution, and carry out an in-situ precipitation reaction to obtain a Prussian blue analog. The Prussian blue analog undergoes a coordination reaction with the modified carbon skeleton to obtain a Prussian blue analog intermediate. S2.1, Dissolve the transition metal salt and alkali metal salt in deionized water at a mass ratio of 1:1-3:1 and stir until completely dissolved to prepare a precursor solution. Filter through a 0.22 μm filter membrane, stir at 500-800 rpm for 5-10 minutes, and then ultrasonically disperse for 10-20 minutes. Immerse the modified carbon skeleton in the precursor solution for 1-2 hours to allow the transition metal ions and alkali metal ions to be fully adsorbed onto the thiol sites of the grafted layer.

[0027] No strong complexing agents such as EDTA or citrate are added to the precursor solution to ensure that metal ions can directly coordinate with thiol groups. Stirring and ultrasonic treatment make the solution uniform and remove bubbles. Soaking for 1-2 hours allows metal ions to be pre-enriched on the surface of the grafted layer through coordination, forming a local high-concentration ion atmosphere, which provides favorable conditions for the in-situ nucleation of Prussian blue analogs.

[0028] S2.2, Dissolve ferricyanide in deionized water and stir until completely dissolved to prepare a ferricyanide solution. After preparation, purge the ferricyanide solution with nitrogen for 10-15 minutes to remove dissolved oxygen and prevent oxidation side reactions. Then, add it dropwise to the precursor solution at a rate of 3-8 mL / h and stir at 20-80 °C for 2-6 hours to generate a Prussian blue analog. The Prussian blue analog is firmly anchored to the carbon skeleton surface through coordination bonds between the thiol groups on the grafted layer and the transition metal ions, yielding an intermediate loaded with the Prussian blue analog.

[0029] The preferred volume ratio of precursor solution to ferricyanide solution is 3:1-5:1, and the total concentration of the precursor solution is 0.05-0.2 mol / L. A lower dropping rate of 3-8 mL / h helps control the formation rate of crystal nuclei and avoids particle agglomeration and uneven size caused by rapid precipitation. The reaction temperature has a significant impact on the grain size and crystallinity: at low temperatures of 20-40℃, the grains grow slowly, the product has high crystallinity, and the reaction time requires 12-24 hours; at high temperatures of 60-80℃, the reaction rate is accelerated, and the reaction time can be shortened to 0.5-4 hours, but the grain size is slightly larger, and stirring for 2-6 hours ensures complete precipitation.

[0030] The core mechanism of this step lies in the coordination bonds between the thiol groups in the grafted layer and the transition metal ions, which anchor the metal ions to the surface of the carbon framework. Upon the addition of ferricyanide ions, an in-situ co-precipitation reaction occurs with the coordinated metal ions, generating Prussian blue analog grains. Because the grains nucleate and grow only at the thiol sites, a direct chemical bond is formed between the Prussian blue analog and the carbon framework, rather than simple physical adsorption. This chemically bonded interface significantly enhances the anchoring strength of the active material, maintaining structural integrity even under the scouring of strongly acidic electrolytes, thereby drastically reducing the dissolution rate of the Prussian blue analog.

[0031] S3, the Prussian blue analogue intermediate is immersed in an acidic solution containing conductive polymer monomers and an oxidant to carry out a polymerization reaction, forming a conductive polymer buffer layer on the surface of the Prussian blue analogue intermediate, resulting in a composite material. The composite material is washed, dried, coated with a gradient porous sealing layer, and cured to obtain a solid compatibilizer material for the positive electrode of a vanadium redox flow battery. S3.1, Prepare an acidic solution containing conductive polymer monomers and an oxidant, adjusting the pH to 1-3. Immerse the intermediate loaded with Prussian blue analogues in this acidic solution at room temperature for 2-6 hours, allowing the conductive polymer monomers to undergo in-situ polymerization on the surface of the Prussian blue analogues under the action of the oxidant, forming a conductive polymer buffer layer. Wash with deionized water until the washing solution is neutral to remove residual impurities, and vacuum dry at 60-80℃ for 4-8 hours to obtain the composite material.

[0032] The conductive polymer monomer is selected from aniline, pyrrole, or 3,4-ethylenedioxythiophene, and the oxidant is selected from ammonium persulfate, ferric chloride, or ferric p-toluenesulfonate. The monomer concentration is 0.05-0.5 mol / L, and the molar ratio of oxidant to monomer is 0.5:1-2:1. An acidic environment with a pH of 1-3 maintains the protonated state of the monomer, promotes the initiation of the polymerization reaction and chain growth, and the polymerization reaction is carried out in situ on the surface of the Prussian blue analogue to form a uniform buffer layer with a thickness of 5-50 nm.

[0033] The mechanism of action of the conductive polymer buffer layer can be understood from two aspects: electron transport and interfacial reaction. Firstly, the redox potential of the Prussian blue analogue is approximately 1.0V, and the VO2 in the positive electrode electrolyte... + / VO 2+ The redox potentials of the conductive polymer and the electrolyte are similar, and the polymer's redox potential matches that of the electrolyte, providing a low-impedance electron transfer channel between the Prussian blue analog grains and the electrolyte, thus acting as a potential bridge and significantly reducing the interfacial charge transfer resistance of the redox-targeted reaction. Secondly, the conductive polymer itself has high electronic conductivity, which can compensate for potential local contact defects between the Prussian blue analog and the carbon framework, further improving the electron transport efficiency of the entire electrode system. Experimental results show that after introducing the conductive polymer buffer layer, the interfacial charge transfer resistance is reduced by more than 50%, and the utilization rate of solid materials increases from less than 70% to over 90%.

[0034] S3.2, Prepare sealing agents: Mix silicone resin and curing agent at a mass ratio of 100:5-10, and dilute with a mixed solvent of acetone and anhydrous ethanol at a volume ratio of 1:1 to a solid content of 5%-20%. Prepare two sealing agents with nano-silica addition amounts of 0.5%-1% and 1.5%-2%, respectively. The silicone resin can be selected from methyl silicone resin or phenyl silicone resin, and the curing agent can be selected from ethylenediamine, diethylenetriamine, or phthalic anhydride.

[0035] A coating method is used to form a gradient porous sealing layer on the surface of the composite material: first, the composite material is immersed in a sealing agent with a nano-silica content of 0.5%-1%, forming an inner layer with a porosity of 30%-50%; then, it is immersed in a sealing agent with a nano-silica content of 1.5%-2%, forming a surface layer with a porosity of 5%-10%. After coating, it is placed in an oven at 80-120℃ for 20-60 minutes to complete the curing, obtaining a solid capacity-enhancing material for the positive electrode of vanadium redox flow batteries.

[0036] The gradient porous sealing layer is designed based on the synergistic optimization of both ion transport and particle immobilization requirements. The high porosity of the inner layer (30%-50%) ensures the retention of VO2 in the electrolyte. + / VO 2+Ions can rapidly diffuse to the surface of Prussian blue analogues, maintaining the kinetic rate of the redox-targeted reaction. The low porosity of the surface layer (5%-10%) forms a physical barrier, effectively preventing Prussian blue analogue grains from detaching and dissolving from the carbon framework pores. Compared to a single uniform porous sealing layer with a porosity of less than 1%, the gradient structure avoids complete blockage of ion transport channels. Compared to a uniform porous sealing layer, the gradient structure provides stronger particle fixation on the surface. Therefore, this gradient sealing layer maintains efficient ion transport while controlling the dissolution rate of active materials to below 1%, resulting in a capacity retention rate of no less than 95% after 1000 cycles, achieving a balance between long-term cycle stability and high power performance.

[0037] Example 1: Biomass-based porous carbon frameworks were prepared by hydrothermal method. The porous carbon frameworks were immersed in anhydrous ethanol, ultrasonically cleaned for 20 minutes, and dried to obtain pretreated carbon frameworks. A 5% (w / w) hydrogen peroxide solution was added, and the frameworks were activated at room temperature for 0.8 hours to expand pores. The frameworks were then washed with deionized water until neutral.

[0038] A γ-mercaptopropyltriethoxysilane coupling agent solution containing 0.03 mol / L boron trifluoride diethyl ether complex was prepared, and the pH was adjusted to 5.5. The activated carbon skeleton was immersed in the coupling agent solution for 3 hours and pre-dried in an oven at 70°C for 1.5 hours to form a grafted layer, thus obtaining the modified carbon skeleton.

[0039] Ferric chloride and potassium chloride were dissolved in deionized water at a mass ratio of 2:1 to prepare a 0.1 mol / L precursor solution. The solution was filtered through a 0.22 μm filter membrane, stirred at 600 r / min for 8 minutes, and ultrasonically dispersed for 15 minutes. The modified carbon skeleton was then immersed in the precursor solution for 1.5 hours.

[0040] Potassium ferricyanide was dissolved in deionized water and degassed with nitrogen for 12 minutes. The solution was then added dropwise to the precursor solution at a rate of 5 mL / h. The volume ratio of the precursor to the ferricyanide solution was 4:1. The mixture was stirred at 50 °C for 4 hours to obtain the Prussian blue analog KFeFe(CN)6. The intermediate was obtained by coordinating the grafted layer with the modified carbon skeleton.

[0041] Prepare a dilute sulfuric acid solution containing aniline and ammonium persulfate, adjust the pH to 2, soak the intermediate at room temperature for 4 hours, polymerize in situ to form a conductive polymer buffer layer, wash with deionized water until neutral, and vacuum dry at 70°C for 6 hours.

[0042] A sealing agent with a solid content of 12% was prepared. Methyl silicone resin was selected as the silicone resin and ethylenediamine was selected as the curing agent. The mass ratio of silicone resin to curing agent was 100:7. 0.8% nano silica was added to the inner layer coating to form an inner layer structure with a porosity of 40%. 1.8% nano silica was added to the surface layer coating to form a surface layer structure with a porosity of 8%. The mixture was cured at 100℃ for 40 minutes to obtain a solid compatibilizer.

[0043] Example 2: Phenolic resin-based porous carbon frameworks were prepared using a template method. The porous carbon frameworks were immersed in acetone, ultrasonically cleaned for 15 minutes, and dried to obtain pretreated carbon frameworks. A 5% (w / w) dilute nitric acid solution was added, and the frameworks were activated at room temperature for 0.5 hours to expand pores. The frameworks were then washed with deionized water until neutral.

[0044] A γ-mercaptopropyltrimethoxysilane coupling agent solution containing 0.01 mol / L boron trifluoride diethyl ether complex was prepared, the pH was adjusted to 5, the activated carbon skeleton was immersed in the coupling agent solution for 2 hours, and pre-dried in an oven at 60°C for 2 hours to form a grafted layer, thus obtaining the modified carbon skeleton.

[0045] Chromium chloride and sodium chloride were dissolved in deionized water at a mass ratio of 1:1 to prepare a 0.05 mol / L precursor solution. The solution was filtered through a 0.22 μm filter membrane, stirred at 500 r / min for 5 minutes, and ultrasonically dispersed for 10 minutes. The modified carbon skeleton was then immersed in the precursor solution for 1 hour.

[0046] Sodium ferricyanide was dissolved in deionized water and degassed with nitrogen for 10 minutes. It was then added dropwise to the precursor solution at a rate of 3 mL / h, with a volume ratio of precursor to ferricyanide solution of 3:1. The mixture was stirred at 20 °C for 6 hours to obtain the Prussian blue analog KCrFe(CN)6. The intermediate was obtained by coordinating the grafted layer with the modified carbon skeleton.

[0047] Prepare a dilute hydrochloric acid solution containing pyrrole and ferric chloride, adjust the pH to 1, soak the intermediate at room temperature for 2 hours, polymerize in situ to form a conductive polymer buffer layer, wash with deionized water until neutral, and vacuum dry at 60°C for 4 hours.

[0048] A sealing agent with a solid content of 5% was prepared. The silicone resin used was phenyl silicone resin, and the curing agent was diethylenetriamine. The mass ratio of silicone resin to curing agent was 100:5. 0.5% nano-silica was added to the inner layer coating to form an inner layer structure with a porosity of 30%. 1.5% nano-silica was added to the surface layer coating to form a surface layer structure with a porosity of 5%. The mixture was cured at 80℃ for 20 minutes to obtain a solid compatibilizer.

[0049] Example 3: A phenolic resin composite biomass-based porous carbon framework was prepared by hydrothermal method. The porous carbon framework was immersed in anhydrous ethanol, ultrasonically cleaned for 25 minutes, and dried to obtain a pretreated carbon framework. A 10% hydrogen peroxide solution was added, and the framework was activated at room temperature for 1 hour to expand pores. It was then washed with deionized water until neutral. The carbon framework was then activated a second time by nitrogen and phosphorus doping at 120℃ for 1 hour. After activation, it was naturally cooled to room temperature.

[0050] A γ-mercaptopropyltriethoxysilane coupling agent solution containing 0.05 mol / L boron trifluoride diethyl ether complex was prepared, and the pH was adjusted to 6. The activated carbon skeleton was immersed in the coupling agent solution for 4 hours and pre-dried in an oven at 80°C for 1 hour to form a grafted layer, thus obtaining the modified carbon skeleton.

[0051] Vanadium oxysulfate and potassium chloride were dissolved in deionized water at a mass ratio of 3:1 to prepare a 0.2 mol / L precursor solution. The solution was filtered through a 0.22 μm filter membrane, stirred at 800 r / min for 10 minutes, and ultrasonically dispersed for 20 minutes. The modified carbon skeleton was then immersed in the precursor solution for 2 hours.

[0052] Ammonium ferricyanide was dissolved in deionized water, degassed with nitrogen for 15 minutes, and then added dropwise to the precursor solution at a rate of 8 mL / h. The volume ratio of precursor to ferricyanide solution was 5:1. The mixture was stirred at 80 °C for 2 hours to obtain the Prussian blue analogue K. 1.1 Fe 0.9 [VO] 0.1 Fe(CN)6 undergoes a coordination reaction with the modified carbon skeleton through a graft layer to obtain an intermediate.

[0053] Prepare a dilute methanesulfonic acid solution containing 3,4-ethylenedioxythiophene and ferric p-toluenesulfonate, adjust the pH to 3, soak the intermediate at room temperature for 6 hours, polymerize in situ to form a conductive polymer buffer layer, wash with deionized water until neutral, and vacuum dry at 80°C for 8 hours.

[0054] A sealing agent with a solid content of 20% was prepared. Methyl silicone resin was selected as the silicone resin and phthalic anhydride was selected as the curing agent. The mass ratio of silicone resin to curing agent was 100:10. 1% nano-silica was added to the inner layer coating to form an inner layer structure with a porosity of 50%. 2% nano-silica was added to the surface layer coating to form a surface layer structure with a porosity of 10%. The mixture was cured at 120℃ for 60 minutes to obtain a solid compatibilizer.

[0055] Comparative Example 1: The difference from Example 1 is that step S1, the coupling agent grafting step, was not performed to form the grafted layer.

[0056] Comparative Example 2: The difference from Example 1 is that step S2, in-situ precipitation and coordination bonding, was not performed; instead, a physical mixing method was used to load the Prussian blue analogue.

[0057] Comparative Example 3: The difference from Example 1 is that step S3, the preparation step of the conductive polymer buffer layer, was not performed.

[0058] Comparative Example 4: The difference from Example 1 is that a single content of nano-silica coating is used, and no gradient porous sealing layer is formed.

[0059] Comparative Example 5: The difference from Example 1 is that the pore-expanding activation step was not performed.

[0060] Comparative Example 6: The difference from Example 3 is that the nitrogen-phosphorus dual-doping secondary activation step was not performed; only conventional pore-expansion activation was performed.

[0061] Electrochemical performance testing: The Blue Battery testing system was used, with a concentration of 1.5 mol / L VOC. 2+ / H2SO4 was used as the simulated electrolyte, and a three-electrode system was used for testing: the working electrode was the prepared solid compatibilizer, the counter electrode was a platinum sheet electrode, and the reference electrode was a saturated calomel electrode; the discharge specific capacity at 0.1C rate and the capacity retention rate after 1000 cycles were tested.

[0062] Pore ​​structure testing: The porosity of the material was tested using the mercury intrusion porosimetry method.

[0063] Table 1: Test results of the examples and comparative examples.

[0064] As shown in Table 1, Example 3 exhibits the best electrochemical performance, structural stability, and electrolyte compatibility. It boasts the highest discharge specific capacity, capacity retention after 1000 cycles, and porosity. This is likely due to the optimization of raw material ratios and process parameters, particularly the nitrogen-phosphorus dual-doping secondary activation step, which is key to further performance improvement. Simultaneously, the gradient sealing layer creates a superior synergistic effect between the conductive buffer layer and the gradient sealing layer. The nitrogen-phosphorus dual-doping component, acting as a structural stabilizer and electron conduction aid, effectively enhances the structural stability of the support framework, preventing the active material from detaching due to structural collapse. It also protects the active sites from electrolyte erosion and deactivation, significantly improving discharge specific capacity and cycle stability. Furthermore, the nitrogen-phosphorus dual-doping component forms a strong interaction with the carbon support, further enhancing the loading stability of the active material, reducing active material detachment, and extending the material's lifespan. This is also the core reason for the significant improvement in cycle capacity retention in Example 3.

[0065] Comparative Examples 1 and 4, lacking sufficient active material loading and efficient electron conduction pathways, exhibited significantly lower discharge specific capacities than Example 3. Comparative Examples 2 and 3, limited by the stability of the active material and carrier compatibility, showed significantly lower cycle stability and lifespan compared to Example 3. Comparative Example 6, lacking gradient sealing and precise activation, suffered from uneven contact of the active material and excessively rapid structural decay, failing to achieve the electrochemical performance of Example 3. Comparative Example 5, lacking the synergistic protective effect of the conductive buffer layer, showed a significant decrease in cycle stability compared to Example 3. These comparisons demonstrate that Example 3, through comprehensive optimization of raw material ratios, process parameters, and nitrogen-phosphorus dual-doping secondary activation, achieved a high-efficiency performance enhancement and strengthened electrochemical stability of the vanadium redox flow battery solid capacity-enhancing material.

[0066] Application Example 1: Use 20mL 0.60MVO 2+ Dissolved in 2 mol / L sulfuric acid as the positive electrode electrolyte, 20 mL of 0.60 MVO 2+ With 40mL 0.60MVO2 + The solid compatibilizer material prepared in Example 1 was dissolved in 2H₂SO₄ as the negative electrode electrolyte to construct a symmetrical battery. After the battery stabilized and cycled, 0.64g of the solid compatibilizer material prepared in Example 1 was added to the positive electrode storage tank.

[0067] like Figure 1 As shown in (a) and (b), the results show that the battery capacity is significantly improved, the material specific capacity reaches 78.5 mAh / g, the voltage plateau is more stable, and the cycle stability is significantly improved.

[0068] Application Example 2: In the same symmetric battery system as Application Example 1, 0.95 g of the solid compatibilizer material prepared in Example 2 was added. The results are as follows: Figure 2 As shown in (c) and (d), the test results show that the battery capacity is significantly improved, with a specific capacity of 72.1 mAh / g and a significantly extended voltage plateau.

[0069] Application Example 3: In the same symmetric battery system as Application Example 1, 0.8 g of the solid compatibilizer material prepared in Example 3 was added. The results are as follows: Figure 3 As shown, the test results indicate that the battery capacity is significantly improved, with a material specific capacity of 83.2 mAh / g. The structural stability and cycle life are further optimized, demonstrating that the hybrid transition metal system also has a good capacity-enhancing effect.

[0070] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery, characterized in that, Includes the following steps: S1, prepare a porous carbon framework, perform surface activation treatment on the porous carbon framework, add a coupling agent, and form a graft layer on the surface and pores of the porous carbon framework to obtain a modified carbon framework. S2, prepare a precursor solution containing transition metal salts and alkali metal salts, immerse the modified carbon skeleton in the precursor solution, add ferricyanide solution, and carry out an in-situ precipitation reaction to obtain a Prussian blue analog. The Prussian blue analog undergoes a coordination reaction with the modified carbon skeleton to obtain a Prussian blue analog intermediate. S3, Prussian blue analog intermediate is immersed in an acidic solution containing conductive polymer monomers and oxidants to carry out a polymerization reaction, forming a conductive polymer buffer layer on the surface of the Prussian blue analog intermediate, to obtain a composite material, which is washed, dried, coated with a gradient porous sealing layer on the surface of the composite material, and cured to obtain a solid capacity-enhancing material for the positive electrode of vanadium redox flow battery.

2. The method for preparing the solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery as described in claim 1, characterized in that, Step S1 includes: S1.

1. A porous carbon framework is prepared by template method or hydrothermal method. The carbon source includes at least one of phenolic resin and biomass-based carbon. The porous carbon framework is immersed in an organic solvent, ultrasonically cleaned for 15-25 minutes, and dried to obtain a pretreated carbon framework. S1.2, add a pore-expanding agent, expand and activate the pores at room temperature for 0.5-1 hour, wash with deionized water, prepare a coupling agent solution and adjust the pH to 5-6, immerse the activated porous carbon skeleton in the coupling agent solution, soak for 2-4 hours to allow the coupling agent to penetrate into the pores of the porous carbon skeleton, and then pre-dry in an oven at 60-80℃ for 1-2 hours to form a graft layer on the surface and in the pores of the porous carbon skeleton, thus obtaining the modified carbon skeleton.

3. The method for preparing the solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery as described in claim 2, characterized in that, The organic solvent includes acetone and anhydrous ethanol; the pore-expanding agent includes at least one of hydrogen peroxide aqueous solution with a mass concentration of 3-10% and dilute nitric acid solution with a mass concentration of 2-8%; the coupling agent includes γ-mercaptopropyltriethoxysilane and γ-mercaptopropyltrimethoxysilane; and 0.01-0.05 mol / L boron trifluoride diethyl ether complex is added as a catalyst when preparing the coupling agent solution.

4. The method for preparing the solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery as described in claim 2, characterized in that, Step S2 includes: S2.1, Dissolve the transition metal salt and alkali metal salt in deionized water at a mass ratio of 1-3:1, stir until dissolved, prepare a precursor solution, filter through a 0.22 μm filter membrane, stir at 500-800 r / min for 5-10 minutes, then ultrasonically disperse for 10-20 minutes, immerse the modified carbon skeleton in the precursor solution, and soak for 1-2 hours. S2.2, Dissolve ferricyanide in deionized water and stir until dissolved to prepare a ferricyanide solution. Degas the solution with nitrogen for 10-15 minutes. Then, add the ferricyanide solution dropwise to the precursor solution at a rate of 3-8 mL / h and stir the reaction at 20-80℃ for 2-6 hours to obtain a Prussian blue analog. The Prussian blue analog is coordinated with the modified carbon skeleton through a graft layer to obtain a Prussian blue analog intermediate.

5. The method for preparing the solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery as described in claim 4, characterized in that, The volume ratio of the precursor solution to the ferricyanide solution is 3-5:1, the concentration of the precursor solution is 0.05-0.2 mol / L, the transition metal salt includes at least one of ferric chloride, chromium chloride, copper chloride, manganese sulfate, and vanadium oxysulfate, the alkali metal salt includes at least one of lithium chloride, sodium chloride, and potassium chloride, and the ferricyanide includes at least one of potassium ferricyanide, sodium ferricyanide, and ammonium ferricyanide.

6. The method for preparing the solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery as described in claim 4, characterized in that, Step S3 includes: S3.1, Prepare an acidic solution containing conductive polymer monomers and an oxidant, adjust the pH of the acidic solution to 1-3, immerse the Prussian blue analog intermediate in the acidic solution at room temperature for 2-6 hours, so that the conductive polymer monomers undergo a polymerization reaction on the surface of the Prussian blue analog intermediate under the action of the oxidant to form a conductive polymer buffer layer, and obtain the composite material. Wash with deionized water until the washing solution is neutral to remove residual impurities, and vacuum dry at 60-80℃ for 4-8 hours. S3.2, Prepare a sealing agent, which includes silicone resin, curing agent, nano-silica, and solvent. The mass ratio of silicone resin to curing agent is 100:5-10, and the solvent is a mixture of acetone and anhydrous ethanol in a volume ratio of 1:

1. A coating method is used to form a gradient porous sealing layer on the surface of the composite material. When coating the inner layer, the amount of nano-silica added is 0.5-1%, forming an inner layer with a porosity of 30-50%. When coating the surface layer, the amount of nano-silica added is 1.5-2%, forming a surface layer with a porosity of 5-10%. After coating, the material is placed in an oven at 80-120℃ for 20-60 minutes to complete the curing, thereby obtaining a solid capacity-enhancing material for the positive electrode of vanadium redox flow batteries.

7. The method for preparing the solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery as described in claim 6, characterized in that, The monomers of the conductive polymer include aniline, pyrrole, and 3,4-ethylenedioxythiophene; the oxidizing agent includes ammonium persulfate, ferric chloride, and ferric p-toluenesulfonate; the acidic solution includes dilute sulfuric acid, dilute hydrochloric acid, and dilute methanesulfonic acid solution; the solid content of the sealing agent is 5-20%; the organosilicon resin is methyl silicone resin or phenyl silicone resin; and the curing agent is ethylenediamine, diethylenetriamine, or phthalic anhydride.

8. A solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery, prepared by the method for preparing the solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery according to any one of claims 1-7, characterized in that, include: Porous carbon framework; Grafted layers formed on the surface and within the pores of the porous carbon framework; Prussian blue analogues are coordinated and bonded to the surface of the grafted layer via the grafted layer. A conductive polymer buffer layer is coated on the surface of the Prussian blue analogue, the thickness of the conductive polymer buffer layer being 5-50 nm. And a gradient porous sealing layer coated with a conductive polymer buffer layer, wherein the gradient porous sealing layer has a gradient porous structure with an inner layer porosity of 30-50% and a surface porosity of 5-10%.

9. The solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery as described in claim 8, characterized in that, The general chemical formula of the Prussian blue analogue is: N x AFe(CN)6, where N is an alkali metal element selected from one or more of Li, Na, and K, A is a transition metal element selected from one or more of Fe, Cr, Cu, Mn, Ni, Co, Zn, and V(VO), and x is 0-2; or NA x B 1-x Fe(CN)6, where A and B are different transition metal elements selected from one or more of Fe, Cr, Cu, Mn, Ni, Co, Zn, and V(VO), and x is 0-1.

10. The solid capacity-enhancing material for the positive electrode of a vanadium redox flow battery as described in claim 9, characterized in that, The solid compatibilizing material is used in the positive electrode electrolyte storage tank of a vanadium redox flow battery, the positive electrode electrolyte comprising 0.1-2 mol / L VO2. + / VO 2+ The solution contains at least one of sulfuric acid, hydrochloric acid, methanesulfonic acid, and phosphoric acid at a concentration of 0.1-4 mol / L, and the electrolyte additives contain at least one of sulfates, chlorides, methanesulfonates, and phosphates of Li, Na, and K at a concentration of 0.1-2 mol / L.

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