Carbon nanotube / vanadium dioxide composite positive electrode material, preparation method and application thereof

Abstract

This invention discloses a carbon nanotube / vanadium dioxide composite cathode material, its preparation method, and its applications. The carbon nanotube / vanadium dioxide composite cathode material comprises: a conductive framework, which includes a three-dimensional continuous conductive network constructed from interwoven carbon nanotubes; and vanadium dioxide nanoparticles, serving as the active material, uniformly loaded on the surface of the carbon nanotubes and filling the pores of the three-dimensional continuous conductive network. The vanadium dioxide nanoparticles and carbon nanotubes form a strong interfacial bond through C–O–V chemical bonds and π–π interactions. The carbon nanotube / vanadium dioxide composite cathode material of this invention constructs a three-dimensional continuous conductive network, significantly improving the electronic conductivity of the cathode material, reducing the internal resistance of the electrode, and enhancing the structural stability of VO2 during charge and discharge processes through structural design and interface control, suppressing volume changes and framework collapse, thus achieving the preparation of a high-capacity, long-cycle-life, and high-current-charge-discharge-suitable iron-ion battery cathode material.

Description

Technical Field

[0001] This invention relates to an aqueous iron-ion battery, and specifically to a novel cathode material with a carbon nanotube / vanadium dioxide composite structure, its preparation method, and its application. Background Technology

[0002] With the global energy structure transformation and the large-scale application of renewable energy, the development of low-cost, highly safe, and environmentally friendly energy storage technologies has become a research hotspot in the energy field. Aqueous iron-ion batteries (AFIBs) are considered an important supplement and potential alternative technology to lithium-ion batteries because they use abundant and inexpensive iron as a charge carrier and employ non-flammable aqueous electrolytes, demonstrating significant advantages in safety, cost, and environmental compatibility.

[0003] The working principle of iron-ion batteries is based on Fe 2+ / Fe 3+ The redox reaction has a theoretical specific capacity of up to 960 mAh / g and a volumetric capacity of up to 7557 mAh / cm³, demonstrating high energy density potential. However, the practical application of iron-ion batteries still faces several technical challenges.

[0004] Currently, research on cathode materials for iron-ion batteries mainly focuses on the following areas:

[0005] 1. Prussian blue materials

[0006] Prussian blue and its analogues have been extensively studied due to their open framework structure and tunable redox sites. However, pure-phase Prussian blue exhibits poor electrical conductivity (typically below 10). -6 The electron transport rate is slow (S / cm), resulting in high internal resistance. Furthermore, it suffers from dissolution and structural degradation in aquatic environments, leading to poor cycle stability.

[0007] 2. Vanadium dioxide-based materials

[0008] Vanadium dioxide (VO2), a typical transition metal oxide, possesses a layered or tunnel crystal structure, providing reversible insertion / extraction channels for iron ions. It has a high theoretical capacity (exceeding 400 mAh / g), and vanadium exhibits multi-valence transition capability (V2). 4+ / V 5+ (etc.), suitable as a redox active center. Pure-phase VO₂ was reported for use in iron-ion batteries in *Chem. Commun* (2023, 59, 8576). However, pure-phase VO₂ is a semiconductor material with low intrinsic conductivity (typically <10). -2The power density of VO2 (S / cm) is severely polarized at high-rate charge and discharge, limiting its performance. It also suffers from poor conductivity (a characteristic of semiconductors), large volume changes during cycling, and insufficient structural stability. During charge and discharge, VO2 undergoes lattice expansion / contraction during repeated insertion / extraction of iron ions, leading to accumulated structural stress, crystal framework collapse, pulverization of the active material, and rapid capacity decay.

[0009] 3. Polyaniline / carbon nanotube composite materials

[0010] A sandwich-type cathode structure based on polyaniline / carbon nanotube foam was reported in *Energy & Environmental Science* (2025, 18, 1428–1439). This cathode achieved a discharge specific capacity of 225 mAh / g at 5C and retained 82% of its capacity after 27,000 cycles at 15C. However, polyaniline has a relatively low theoretical capacity (approximately 300 mAh / g), and its energy storage mechanism differs from the intercalation / deintercalation mechanism of iron ions, thus failing to fully utilize the high capacity advantage of iron-ion batteries. Furthermore, VO2 typically has a low ion diffusion coefficient (approximately 10). -12 -10 -10 cm 2 Under high-rate charge and discharge conditions, ion transport becomes the rate-controlling step, and insufficient ion diffusion kinetics result in rapid capacity decay and poor rate performance.

[0011] Furthermore, existing composite methods for bonding VO2 with carbon materials largely rely on physical adsorption or weak chemical interactions, resulting in weak interfacial bonding. This leads to interfacial delamination and disruption of electron conduction pathways during long-term cycling. Existing VO2 / carbon composite material preparation processes are complex, often involving high-temperature treatments, toxic solvents, or sophisticated equipment, which hinders large-scale production and cost control. Summary of the Invention

[0012] The main objective of this invention is to provide a carbon nanotube / vanadium dioxide composite cathode material, its preparation method, and its application, thereby overcoming the shortcomings of the prior art.

[0013] To achieve the aforementioned objectives, the technical solution adopted by this invention includes:

[0014] The first aspect of this invention provides a carbon nanotube / vanadium dioxide composite cathode material, comprising:

[0015] A conductive framework comprising a three-dimensional continuous conductive network constructed from interwoven carbon nanotubes;

[0016] And, the active material, wherein the active material is vanadium dioxide nanoparticles, which are uniformly loaded on the surface of carbon nanotubes and fill the pores of a three-dimensional continuous conductive network, and the vanadium dioxide nanoparticles and carbon nanotubes form a strong interfacial bond through C–O–V chemical bonds and π–π interactions.

[0017] A second aspect of this invention provides a method for preparing the aforementioned carbon nanotube / vanadium dioxide composite cathode material, comprising:

[0018] Carbon nanotubes were pretreated to obtain surface-functionalized carbon nanotubes.

[0019] A dispersion containing surface-functionalized carbon nanotubes is provided, vanadium dioxide nanoparticles are added and ultrasonically treated to form a uniform suspension, which is then formed into a film and post-treated to obtain a carbon nanotube / vanadium dioxide composite cathode material.

[0020] A third aspect of the present invention provides the application of the carbon nanotube / vanadium dioxide composite cathode material in an energy storage system, the energy storage system including an iron-ion battery.

[0021] A fourth aspect of the present invention provides an iron-ion battery, comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is made of the aforementioned carbon nanotube / vanadium dioxide composite positive electrode material.

[0022] Compared with the prior art, the advantages of the present invention include:

[0023] 1) The carbon nanotube / vanadium dioxide composite cathode material provided by the present invention constructs a three-dimensional continuous conductive network. Its electronic conduction path is better than that of traditional composite materials, and its overall conductivity can reach several times that of pure VO2. This excellent conductivity is due to the close contact between carbon nanotubes and the strong interfacial coupling between VO2 and carbon nanotubes.

[0024] 2) The carbon nanotube / vanadium dioxide composite cathode material provided by this invention adopts interface bonding control technology. Through surface functionalization treatment and heat treatment of carbon nanotubes, strong C–O–V metal chemical bonding between VO2 nanoparticles and carbon nanotubes is achieved. This covalent bond is a strong chemical bond with high bond energy, which can realize atomic-level bonding between carbon nanotubes and VO2, avoid the electron transport barrier at the interface during electrochemical reaction, and effectively suppress the interface delamination between the two during cycling, thereby improving the cycle stability of the battery.

[0025] 3) The carbon nanotube / vanadium dioxide composite cathode material provided by this invention can optimize the ion transport path, improve the diffusion kinetics of iron ions in the cathode material, and improve the rate performance; at the same time, the preparation process is simple, environmentally friendly, and scalable, reducing production costs and realizing the preparation of high-capacity, long-cycle-life iron-ion battery cathode materials suitable for high-current charge and discharge.

[0026] 4) When the carbon nanotube / vanadium dioxide composite cathode material provided by this invention is used in iron-ion batteries, Fe... 2+ The insertion and extraction of VO2 induces multivalent redox reactions, enabling iron-ion batteries to operate at 0.5 A·g⁻¹. -1 Achieving approximately 350 mAh·g at a current density -1 High capacity. Attached Figure Description

[0027] 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 recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a typical embodiment of the present invention, showing the preparation process and performance testing diagram of a VO2@CNT composite cathode material;

[0029] Figure 2 This is a schematic diagram illustrating the preparation of VO2 nanoparticles in a typical embodiment of the present invention;

[0030] Figure 3 This is a schematic diagram of the preparation process of a VO2@CNT composite cathode material in a typical embodiment of the present invention;

[0031] Figure 4 This is a SEM image of the VO2@CNT composite cathode material prepared in a typical embodiment of the present invention;

[0032] Figure 5 This is a CV comparison diagram of the VO2@CNT composite cathode material and VO2 cathode prepared in a typical embodiment of the present invention when used in an iron-ion battery;

[0033] Figure 6 This is a graph showing the long-cycle performance of the VO2@CNT composite cathode material prepared in a typical embodiment of the present invention in an iron-ion battery at a current density of 1A / g.

[0034] Figure 7This is a comparison chart of the long-cycle performance of VO2@CNT composite cathode material and VO2 cathode prepared in a typical embodiment of the present invention when used in iron-ion batteries at a current density of 0.5 A / g.

[0035] Figure 8 This is a comparison chart of the rate performance of VO2@CNT composite cathode material and VO2 cathode prepared in a typical embodiment of the present invention when used in iron-ion batteries.

[0036] Figure 9 This is a comparison of the CV curves of VO2@CNT composite cathode materials with different mass ratios prepared in a typical embodiment of the present invention when used in iron-ion batteries.

[0037] Figure 10 This is a comparison of the GCD curves of VO2@CNT composite cathode materials with different mass ratios prepared in a typical embodiment of the present invention when used in iron-ion batteries.

[0038] Figure 11 A schematic diagram of non-in-situ XPS analysis of the VO2@CNT composite cathode material prepared in a typical embodiment of the present invention at different sites during charging and discharging in an iron-ion battery. Detailed Implementation

[0039] In view of the shortcomings of the prior art, the inventors of this invention, through long-term research and extensive practice, have proposed the technical solution of this invention. It is the first to propose and realize the uniform loading of VO2 nanoparticles in a three-dimensional continuous carbon nanotube network, constructing an electron-ion dual continuous transport channel, and providing a novel cathode material based on a carbon nanotube / vanadium dioxide composite structure. This material uses a three-dimensional interconnected carbon nanotube network as a conductive framework and VO2 nanoparticles as the active material. Through a specific process, the distribution of VO2 nanoparticles on the surface of carbon nanotubes and strong interfacial bonding are achieved, forming a composite electrode structure with continuous electron conduction paths and open ion diffusion channels.

[0040] The terms used in this invention are explained as follows:

[0041] VO2: Vanadium dioxide, a transition metal oxide with a layered or tunnel structure, can be used as a positive electrode active material for iron-ion batteries;

[0042] CNT: Carbon nanotubes, nano-carbon materials with high specific surface area, excellent electrical conductivity and mechanical strength;

[0043] AFIBs: Aqueous iron-ion batteries, which are secondary battery systems based on iron ions as charge carriers and aqueous electrolytes.

[0044] Composite cathode: An electrode structure formed by combining an active material (VO2) and a conductive framework (CNT) through a specific process;

[0045] Three-dimensional conductive network: a continuous, porous, highly conductive three-dimensional structure formed by interwoven carbon nanotubes.

[0046] The following will further explain the technical solution, its implementation process, and its principles. However, it should be understood that within the scope of this invention, the above-mentioned technical features of this invention and the technical features specifically described below (in embodiments) can be combined with each other to form new or preferred technical solutions. Due to space limitations, they will not be described in detail here.

[0047] As one aspect of the technical solution of this invention, a carbon nanotube / vanadium dioxide composite cathode material includes:

[0048] A conductive framework comprising a three-dimensional continuous conductive network constructed from interwoven carbon nanotubes;

[0049] And, the active material, wherein the active material is vanadium dioxide nanoparticles, which are uniformly loaded on the surface of carbon nanotubes and fill the pores of a three-dimensional continuous conductive network, and the vanadium dioxide nanoparticles and carbon nanotubes form a strong interfacial bond through C–O–V chemical bonds and π–π interactions.

[0050] In some preferred embodiments, the carbon nanotubes may be single-walled carbon nanotubes and / or multi-walled carbon nanotubes, etc.

[0051] In some preferred embodiments, the porosity of the three-dimensional continuous conductive network is 70-95%, the pore size is 5-500 nm, and the proportion of mesopores (2-50 nm) is ≥60%.

[0052] In some preferred embodiments, the vanadium dioxide nanoparticles are tunnel-shaped with a particle size of 20~100 nm.

[0053] In some embodiments, the carbon nanotube / vanadium dioxide composite cathode material has a continuous electron conduction path and an open ion diffusion channel.

[0054] In some embodiments, the content of vanadium dioxide nanoparticles in the carbon nanotube / vanadium dioxide composite cathode material is 33~66wt%.

[0055] In some more specific implementations, the novel iron-ion battery composite cathode material based on a carbon nanotube / vanadium dioxide composite structure has the following structural features:

[0056] 1) Three-dimensional CNT conductive network: an interconnected network structure formed by single-walled or multi-walled carbon nanotubes through van der Waals forces, entanglement or chemical cross-linking;

[0057] 2) VO2 nanoparticle loading: Vanadium dioxide is uniformly attached to the CNT surface in the form of nanoparticles;

[0058] 3) Interface binding mechanism: VO2 and CNT form a strong interface binding through C–O–V chemical bonds and π–π interactions, which is beneficial to electron transport and structural stability.

[0059] As another aspect of the technical solution of the present invention, it relates to the aforementioned method for preparing the carbon nanotube / vanadium dioxide composite cathode material, which includes:

[0060] Carbon nanotubes were pretreated to obtain surface-functionalized carbon nanotubes.

[0061] A dispersion containing surface-functionalized carbon nanotubes is provided, vanadium dioxide nanoparticles are added and ultrasonically treated to form a uniform suspension, which is then formed into a film and post-treated to obtain a carbon nanotube / vanadium dioxide composite cathode material.

[0062] In some embodiments, the surface-functionalized carbon nanotubes contain oxygen-containing active functional groups, which may include at least one of carboxyl groups, hydroxyl groups, etc.

[0063] The pretreatment in this invention serves two purposes: First, the carbon nanotubes undergo pretreatment, resulting in surface functionalization by introducing oxygen-containing active functional groups such as carboxyl groups (-COOH) and hydroxyl groups (-OH), providing reaction sites for chemical bonding. Second, the VO2 nanoparticles (VO2) have metallic hydroxyl groups (V–OH) on their surface. During subsequent ultrasonic composite and low-temperature heat treatment, the -COOH / -OH on the carbon nanotube surface undergoes a dehydration condensation reaction with the V–OH on the VO2 surface, forming a C–O–V covalent bond. This covalent bond is a strong chemical bond with high bond energy, enabling atomic-level bonding between the carbon nanotubes and VO2, avoiding the electron transport barrier at the interface, and effectively suppressing interfacial delamination during cycling.

[0064] Furthermore, CNTs themselves possess a conjugated π-electron system, and the crystal surface of VO2 nanoparticles contains unpaired electrons, forming localized π-electron clouds. When VO2 nanoparticles are tightly adsorbed onto the CNT surface, their π-electron clouds overlap and conjugate, forming π–π interactions. This interaction is a non-covalent bond; although the bond energy is lower than that of a covalent bond, it can further improve the uniformity of VO2 nanoparticle loading on the CNT surface, while also assisting in enhancing the interfacial bonding strength, forming a "master-auxiliary" bonding system with the C–O–V covalent bond. Existing technologies mostly involve simple physical adsorption / mechanical mixing, without the formation of chemical bonds, resulting in weaker bonding forces. Compared to pure-phase VO2, the composite cathode exhibits higher conductivity and a more stable structure.

[0065] In some implementations, the pretreatment can be plasma treatment to achieve carboxylation functionalization of the carbon nanotube surface, or other pretreatment methods such as concentrated acid oxidation. Both can introduce oxygen-containing active functional groups such as carboxyl and hydroxyl groups on the carbon nanotube surface, providing reaction sites for subsequent C–O–V chemical bond formation with VO2, without changing the subsequent composite process and material properties.

[0066] Furthermore, the plasma treatment power is 80~120W, and the time is 2~5min.

[0067] In some embodiments, the concentrated acid oxidation method includes: placing carbon nanotubes in a mixed solution of concentrated nitric acid and concentrated sulfuric acid, and refluxing at 80~100℃ for 2~4 hours to achieve surface functionalization modification of carbon nanotubes.

[0068] In some preferred embodiments, the concentrated acid oxidation method specifically includes: placing commercial-grade single-walled / multi-walled carbon nanotubes in a mixed solution of concentrated nitric acid and concentrated sulfuric acid (volume ratio of 1:3), refluxing at 80~100℃ for 2~4h, cooling, filtering, washing until neutral, and vacuum drying at 60℃ to achieve carboxylation modification of the carbon nanotube surface.

[0069] In some embodiments, the preparation method includes: dispersing surface-functionalized carbon nanotubes in a solvent to form a dispersion, adding vanadium dioxide nanoparticles, and then ultrasonically treating the dispersion to form a uniform suspension.

[0070] Furthermore, the solvent includes ethanol, water, and a mixed solution of ethanol, or a mixed solution of water and isopropanol, wherein the volume ratio of water to ethanol or isopropanol is in the range of 1:1 to 1:2, which ensures that the pretreated CNT and VO2 nanoparticles are fully dispersed without agglomeration. The dispersion effect is even better when water and ethanol or isopropanol are added simultaneously.

[0071] In some implementations, the mass ratio of the surface-functionalized carbon nanotubes to vanadium dioxide nanoparticles can be adjusted in the range of 1:2 to 2:1, which takes into account both the electron transport capability of the conductive framework and the capacity contribution of the active material.

[0072] In some implementations, the power of the ultrasonic treatment can be adjusted from 400 to 600 W, and the ultrasonic time can be adjusted from 1 to 3 hours. This parameter range can achieve sufficient contact between VO2 and CNT, laying the foundation for interfacial bonding.

[0073] In some embodiments, the preparation method includes: preparing the vanadium dioxide nanoparticles by at least a hydrothermal synthesis method, wherein the core reaction system is V2O5 and citric acid monohydrate, and different raw material ratios and reaction parameters within the scope defined by the present invention can all produce VO2 nanoparticles with uniform particle size and good crystallinity.

[0074] In some more preferred embodiments, the preparation method specifically includes: subjecting a hydrothermal reaction system containing vanadium pentoxide, a reducing agent, and water to a hydrothermal reaction, followed by drying, to obtain the vanadium dioxide nanoparticles.

[0075] Furthermore, the mass ratio of vanadium pentoxide to reducing agent can be adjusted within the range of 1:1.5 to 1:2.0. This ratio range can ensure that V2O5 is fully reduced to VO2 without the generation of by-products.

[0076] Furthermore, the reducing agent includes, but is not limited to, citric acid monohydrate.

[0077] Furthermore, the hydrothermal reaction temperature is 160~200℃ and the hydrothermal reaction time is 10~16h. This parameter range can ensure the full growth of VO2 nanocrystals, and the particle size of the obtained particles meets the small size requirements for composite with CNTs.

[0078] Furthermore, the drying is vacuum drying. The vacuum drying temperature of the hydrothermal product can be adjusted from 50 to 70°C, and the vacuum drying time can be adjusted from 10 to 14 hours. Both methods can achieve full drying of the product without changing the crystal structure of VO2.

[0079] In some embodiments, the film formation method can be one or more combinations of vacuum filtration film formation, vacuum freeze-drying film formation, etc., but is not limited to these methods. These methods can all form continuous, porous VO2@CNT composite electrode materials without the need for binders and conductive agents, thus preserving the three-dimensional conductive network structure of the material.

[0080] Furthermore, the various film-forming methods of the present invention preferentially employ ultrasonic composite-vacuum filtration to achieve the composite and film formation of VO2 and CNT, which can ensure the uniform loading of VO2 nanoparticles on the CNT surface and in the three-dimensional network pores, without damaging the interfacial bonding between the two.

[0081] Furthermore, the vacuum filtration membrane formation includes: under vacuum, passing a uniform suspension through an aqueous microporous membrane to form a membrane, wherein the pore size of the aqueous microporous membrane is 0.2~0.3μm.

[0082] More specifically, the vacuum filtration membrane formation includes: placing an aqueous microporous filter membrane (pore size of 0.2~0.3μm) in a Buchner funnel, slowly pouring in a uniform suspension, and forming a membrane under vacuum. After filtration, the filter membrane with the attached membrane is transferred to a vacuum drying oven and dried at 50~70℃ for 10~14 hours to completely remove residual solvent. The composite membrane can then naturally detach from the filter membrane substrate.

[0083] Furthermore, the vacuum freeze-drying film formation specifically includes: pouring a uniform suspension containing VO2 and CNT into a mold, pre-freezing at -40℃ to -20℃ for 2 to 4 hours, and then freeze-drying at a vacuum of 10 to 30 Pa for 12 to 24 hours to obtain a porous VO2@CNT composite membrane.

[0084] In some embodiments, the post-treatment may include vacuum drying, or one or more combinations of other post-treatment methods such as drying in an inert gas atmosphere, low-temperature heat treatment, etc., but is not limited to these. These post-treatment methods can all achieve sufficient drying of the film layer, while promoting the formation of C–O–V chemical bonds and π–π interactions between VO2 and CNTs, thereby enhancing the interfacial bonding strength.

[0085] Furthermore, the present invention employs a post-treatment process of vacuum drying at 50~70℃ for 10~14h on the composite membrane after filtration.

[0086] Furthermore, the drying temperature in an inert gas atmosphere is 70–90°C, and the drying time is 6–8 hours. The inert gas includes nitrogen and / or argon, etc. The inert atmosphere can prevent the functional groups on the CNT surface from being oxidized, further improving the interfacial bonding stability.

[0087] Furthermore, the low-temperature heat treatment is carried out under vacuum conditions, with a temperature of 100~150℃ and a time of 2~4h. The low-temperature heat treatment can promote the dehydration condensation reaction between VO2 and CNT, strengthen the C–O–V covalent bonding, and does not change the bulk structure of VO2 and CNT.

[0088] In summary, this invention employs interfacial bonding control technology and achieves strong C–O–V metal-to-carbon nanotube bonding between VO2 nanoparticles and carbon nanotubes through surface functionalization and heat treatment processes, significantly improving interfacial stability and electron transport efficiency.

[0089] As another aspect of the technical solution of this invention, it relates to the application of the carbon nanotube / vanadium dioxide composite cathode material in energy storage systems. This invention specifically focuses on structural optimization and performance improvement of iron-ion battery cathode materials under high-current charge-discharge scenarios, making it suitable for electrochemical energy storage devices with high power density and long cycle life.

[0090] Furthermore, in addition to coin cells and pouch cells, the positive electrode material of the electrochemical energy storage device can also be applied to flexible cells, micro cells, or to construct hybrid cells with other battery systems (such as iron-ion cells, zinc-ion cells, etc.).

[0091] Accordingly, as another aspect of the technical solution of the present invention, it also relates to an iron-ion battery, including a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode adopts the aforementioned carbon nanotube / vanadium dioxide composite positive electrode material.

[0092] Furthermore, the electrolyte can be an aqueous electrolyte, but can be replaced with a gel electrolyte or a solid electrolyte to improve battery safety and voltage window. The gel electrolyte can be based on a PVA or PEG polymer matrix with added FeSO4 salt; the solid electrolyte can employ a NASICON-type fast ion conductor.

[0093] By employing the above technical solutions, the carbon nanotube / vanadium dioxide composite cathode material of this invention constructs a three-dimensional continuous conductive network, significantly improving the electronic conductivity of the cathode material and reducing the internal resistance of the electrode. Through structural design and interface control, the structural stability of VO2 during charge and discharge is enhanced, suppressing volume changes and framework collapse. The ion transport path is optimized, improving the diffusion kinetics of iron ions in the cathode material and enhancing rate performance. It can be used as the cathode in aqueous iron-ion batteries, particularly for high-current charge and discharge scenarios. Simultaneously, this invention develops an integrated process from material synthesis to electrode forming, avoiding the addition of binders and conductive agents in traditional electrode preparation, improving the overall electrode performance, and enabling the preparation of high-capacity, long-cycle-life iron-ion battery cathode materials suitable for high-current charge and discharge.

[0094] To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. It should be noted that the following embodiments are intended to facilitate understanding of this invention and are not intended to limit it in any way. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or according to the conditions recommended by the manufacturer.

[0095] The electrochemical performance indicators of the examples under specific test conditions include specific capacity, rate performance, cycle life, etc.

[0096] Example 1

[0097] Please see Figure 1 As shown, a method for preparing a VO2@CNT composite cathode material mainly includes the following steps:

[0098] Step 1: Carbon nanotube (CNT) pretreatment (surface functionalization)

[0099] Commercial-grade single-walled carbon nanotubes were subjected to plasma treatment (100W power, 3min time) to perform surface carboxylation modification.

[0100] Step 2: Preparation of vanadium dioxide nanoparticles (hydrothermal method)

[0101] like Figure 2 As shown, VO2 nanoparticles were prepared by hydrothermal synthesis: 0.4728 g of vanadium pentoxide (V2O5) was dissolved in 43 mL of deionized water and stirred for 15 minutes. Then, 0.8195 g of citric acid monohydrate was added to the solution and stirring was continued for 60 minutes. The mixed solution was transferred to a 100 mL high-pressure reactor lined with polytetrafluoroethylene and reacted at 180 °C for 12 hours. After the reaction was completed, the black precipitate was collected, washed three times alternately with deionized water and ethanol, and dried under vacuum at 60 °C for 12 hours to obtain VO2 nanoparticles.

[0102] Step 3: Preparation of VO2@CNT composite cathode material (ultrasonic composite-vacuum filtration)

[0103] like Figure 3 As shown, 12.8 mg of pretreated CNTs were dispersed in a mixed solution of 10 mL water and 10 mL ethanol. Then, 12.8 mg of VO2 nanoparticles were added to the dispersion at a mass ratio of CNT:VO2 = 1:1. The mixture was ultrasonically treated at 500 W for 2 hours to form a homogeneous suspension. The suspension was filtered to form a membrane, which was then vacuum dried at 60 °C for 12 hours to obtain an electrode active material loading of 1 mg / cm³. 2 VO2@CNT composite cathode material.

[0104] Step 4: Button cell assembly (CR2032 type)

[0105] A CR2032 coin cell was assembled using VO2@CNT composite cathode material as the cathode, iron foam (0.5 mm thick, porosity > 95%) as the anode, glass fiber membrane (Whatman GF / D) as the separator, and 1 M FeSO4 and 0.1 M H2SO4 aqueous solution as the electrolyte.

[0106] Performance testing and structural characterization

[0107] 1. Electrochemical performance testing: Constant current charge-discharge (GCD) tests were performed using a LAND CT2001A battery testing system, with a voltage window of 0–1 V (vs. Fe / Fe). 2+ Cyclic voltammetry (CV) tests (scan rate 0.5–10 mV / s) and electrochemical impedance spectroscopy (EIS) tests (frequency range 0.01 Hz–100 kHz, amplitude 5 mV) were performed using a CHI660E electrochemical workstation.

[0108] 2. Structural Characterization: The morphology, crystal structure, and chemical state of the material were analyzed using scanning electron microscopy (SEM, FEI Quanta 400 FEG), transmission electron microscopy (TEM, FEI Tecnai G2 F20 S-Twin), X-ray diffraction (XRD, Bruker D8 Advance), and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi). In this embodiment, the volume change of the composite cathode during charging and discharging is effectively suppressed through the mechanical support of the CNT network and the small size effect of VO2 nanoparticles. Figure 4 As shown, SEM and TEM observations indicate that the CNT network can confine VO2 nanoparticles, preventing their aggregation and pulverization.

[0109] The VO2@CNT composite cathode material in this embodiment constructs a three-dimensional continuous conductive network, whose electron conduction path is superior to that of traditional composite materials. Experiments show that the overall conductivity of this composite cathode material can reach several times that of pure VO2. This excellent conductivity stems from the tight contact between CNTs and the strong interfacial coupling between VO2 and CNTs.

[0110] Fe was calculated by GITT test. 2+ The diffusion coefficient in this composite cathode material is several times higher than that of pure VO2, which is mainly attributed to the short-range ion diffusion path and open diffusion channels.

[0111] Figure 5 This is a CV comparison graph showing the application of the VO2@CNT composite cathode material and the VO2 cathode in iron-ion batteries. Figure 6 This figure shows the long-cycle performance of the VO2@CNT composite cathode material used in iron-ion batteries at a current density of 1 A / g. Tests showed that after 500 cycles at 1 A / g, the VO2@CNT composite cathode material prepared in this embodiment retained 83% of its capacity, exhibiting a significantly extended cycle life. Its excellent cycle stability is attributed to the synergistic effect of multiple factors: the mechanical support of the CNT network, the volume effect of the VO2 nanoparticles, stable interfacial bonding, and optimized pore structure.

[0112] Testing showed that the VO2@CNT composite cathode material prepared in this embodiment has a specific capacity of 425 mAh / g at a current density of 0.1 A / g, and can still maintain a capacity of 210 mAh / g at a high rate of 2 A / g. This excellent rate performance is attributed to rapid electron conduction and ion diffusion.

[0113] like Figure 7The figure shows a comparison of the long-cycle performance of the VO2@CNT composite cathode material and the VO2 cathode in iron-ion batteries at a current density of 0.5 A / g. Figure 8 This is a comparison chart of the rate performance of corresponding iron-ion batteries.

[0114] like Figure 11 As shown, when this carbon nanotube / vanadium dioxide composite cathode material is used in iron-ion batteries, Fe... 2+ The VO2 multivalent redox process can be fully utilized. Through non-in-situ XPS verification, the V2p of the VO2@CNT composite cathode material in three states—original state, charged to 1 V, and discharged to 0 V—was demonstrated. 3 / 2 XPS spectra (reflecting changes in V valence state); in the pristine state, V is the dominant valence state. 4+ Mainly, with a small amount of V 3+ It conforms to the intrinsic valence state of VO2, with a small amount of V 3+ This is a minor defect that occurred during the manufacturing process. A V value appears when charged to 1 V. 5+ Characteristic peaks, and V 4+ Peak intensity decreases, charging is an oxidation process, V 4+ It loses electrons and is oxidized to V 5+ To compensate Fe 2+ The positive charge imbalance resulting from the deintercalation / extraction from the one-dimensional tunnel of VO2. V at discharge to 0 V. 5+ The peak disappears, V 3+ The peak intensity is significantly improved, and the discharge is a reduction process, V 5+ The electron is first reduced to V 4+ Further restored to V 3+ To compensate Fe 2+ The positive charge imbalance caused by embedding the VO2 tunnel. During charging and discharging, the V valence state achieves "V 4+ (V) 3+ (small amount) — V 4+ / V 5+ ——V 4+ / V 3+ The reversible transformation of V2@CNT composite cathode material verifies the core iron storage mechanism. Simultaneously with directional insertion / deintercalation along the one-dimensional tunnel of V2, V2 is coupled with V2. 3+ / V 4+ / V 5+ The reversible redox reaction enables charge compensation, and this reversible valence state change is the essential reason why VO2@CNT composite cathode materials possess excellent electrochemical reversibility.

[0115] Example 2

[0116] Steps 1 and 2 in this embodiment are the same as in embodiment 1, except that:

[0117] Step 3: Preparation of VO2@CNT composite cathode materials with different mass ratios (ultrasonic composite-vacuum filtration)

[0118] 12.8 mg of pretreated CNTs were dispersed in a mixed solution of 10 mL water and 10 mL ethanol. Then, 12.8 mg of VO2 nanoparticles were added to the dispersion at a mass ratio of CNT:VO2 = 1:1. The mixture was ultrasonically treated at 500 W for 2 hours to form a uniform suspension. The suspension was filtered to form a film and then vacuum dried at 60 °C for 12 hours to obtain a VO2@CNT composite cathode material with an electrode active material loading of 1 mg / cm².

[0119] To investigate the effect of different mass ratios of carbon nanotubes (CNTs) and vanadium dioxide on electrode performance, while keeping the CNT mass constant at 12.8 mg, the vanadium dioxide mass was adjusted to 6.4 mg and 25.6 mg, respectively, to achieve mass ratios of 2:1 and 1:2. Subsequently, slurries were prepared using the same solvent ratio (10 mL deionized water + 10 mL anhydrous ethanol) and ultrasonic dispersion process, and then deposited as films via vacuum filtration. The final composite cathode film had an active material loading of approximately 0.5 mg·cm⁻¹. -2 and 2 mg·cm -2 This provides samples with different structural features for subsequent comparative studies of electrochemical performance.

[0120] Step 4: Button cell assembly (CR2032 type)

[0121] CR2032 coin cells were assembled using VO2@CNT composite cathode materials with different mass ratios as cathodes, iron foam (0.5 mm thick, porosity > 95%) as anodes, glass fiber membrane (Whatman GF / D) as separators, and 1 M FeSO4 and 0.1 M H2SO4 aqueous solutions as electrolytes.

[0122] The steps for performance testing and structural characterization are the same as in Example 1.

[0123] like Figure 9As shown, the CV curves of composite electrodes with different VO2:CNT mass ratios (1:1, 1:2, 2:1) used in iron-ion batteries are compared at a scan rate of 1 mV / s, clearly revealing the key influence of the ratio on electrode performance. Among them, when VO2:CNT=1:1, the CV curve has the largest enclosed area, the highest redox peak current, and the best symmetry, indicating that the electrode has the highest specific capacity and the best charge-discharge reversibility at this ratio. Analysis shows that when VO2:CNT=1:1, the active material and the conductive agent achieve the optimal balance: ensuring sufficient VO2 to provide charge storage capacity, and having enough CNTs to build a continuous conductive network, improving electron transport efficiency.

[0124] like Figure 10 As shown, when composite electrodes with different VO2 to CNT mass ratios (1:2, 1:1, 2:1) are used in iron-ion batteries, the results are shown at 1 A·g -1 The GCD curves at current density visually reflect the effect of the VO2:CNT ratio on electrochemical performance. The highest discharge capacity (approximately 240 mAh·g) is achieved when VO2:CNT = 1:1. -1 At this point, the active material and the conductive agent reach optimal balance, resulting in the most complete capacity release. In terms of kinetic performance, a VO2:CNT ratio of 1:1 exhibits a smooth charge-discharge plateau, good curve symmetry, and a small polarization voltage difference, demonstrating excellent electron transport efficiency and charge-discharge reversibility. This result is consistent with previous CV test conclusions, verifying that a VO2:CNT mass ratio of 1:1 is the optimal ratio, balancing high capacity with excellent kinetic performance.

[0125] Example 3

[0126] The difference between this embodiment and Embodiment 1 is that:

[0127] Step 1: Carbon nanotube (CNT) pretreatment (surface functionalization)

[0128] Commercial-grade single-walled carbon nanotubes were subjected to plasma treatment (80W power, 5min time) to perform surface carboxylation modification.

[0129] Example 4

[0130] The difference between this embodiment and Embodiment 1 is that:

[0131] Step 1: Carbon nanotube (CNT) pretreatment (surface functionalization)

[0132] Commercial-grade single-walled carbon nanotubes were subjected to plasma treatment (120 W for 2 min) to perform surface carboxylation modification.

[0133] Example 5

[0134] The difference between this embodiment and Embodiment 1 is that:

[0135] Step 3: Preparation of VO2@CNT composite cathode material (ultrasonic composite-vacuum filtration)

[0136] 12.8 mg of pretreated CNTs were dispersed in a mixed solution of 10 mL water and 20 mL ethanol. Then, 12.8 mg of VO2 nanoparticles were added to the dispersion at a mass ratio of CNT:VO2 = 1:1. The mixture was ultrasonically treated at 400 W for 3 hours to form a uniform suspension. The suspension was filtered to form a film and then vacuum dried at 50 °C for 14 hours to obtain a VO2@CNT composite cathode material with an electrode active material loading of 1 mg / cm².

[0137] Example 6

[0138] The difference between this embodiment and Embodiment 1 is that:

[0139] Step 3: Preparation of VO2@CNT composite cathode material (ultrasonic composite-vacuum filtration)

[0140] 12.8 mg of pretreated CNTs were dispersed in a mixed solution of 10 mL water and 20 mL ethanol. Then, 12.8 mg of VO2 nanoparticles were added to the dispersion at a mass ratio of CNT:VO2 = 1:1. The mixture was ultrasonically treated at 600 W for 1 hour to form a uniform suspension. The suspension was filtered to form a film and then vacuum dried at 70 °C for 10 hours to obtain a VO2@CNT composite cathode material with an electrode active material loading of 1 mg / cm².

[0141] Furthermore, the final vacuum drying step can be replaced by drying in an inert gas atmosphere (70~90℃), low-temperature heat treatment (100~150℃), etc.

[0142] Example 7

[0143] The difference between this embodiment and Embodiment 1 is that:

[0144] Step 3: The VO2@CNT composite cathode material was prepared by vacuum freeze-drying. A uniform suspension containing VO2 and CNT was poured into a mold and pre-frozen at -40℃ to -20℃ for 2 to 4 hours. Then, it was freeze-dried at a vacuum of 10 to 30 Pa for 12 to 24 hours to obtain a porous VO2@CNT composite membrane.

[0145] Tests showed that the microstructure and performance of the VO2@CNT composite cathode materials prepared in Examples 3-7 were basically the same as those in Example 1.

[0146] The preparation method employed in this invention does not require high temperature and high pressure, avoids the use of toxic solvents, and most steps can be carried out at room temperature and pressure. The raw materials are inexpensive (CNTs and VO2 are both commercially available products), the process is simple, and it is easy to scale up for production. The composite cathode material prepared using this method costs more than 30% less than existing high-end cathode materials.

[0147] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A carbon nanotube / vanadium dioxide composite cathode material, characterized in that, include: A conductive framework comprising a three-dimensional continuous conductive network constructed from interwoven carbon nanotubes; And, the active material, wherein the active material is vanadium dioxide nanoparticles, which are uniformly loaded on the surface of carbon nanotubes and fill the pores of a three-dimensional continuous conductive network, and the vanadium dioxide nanoparticles and carbon nanotubes form a strong interfacial bond through C–O–V chemical bonds and π–π interactions.

2. The carbon nanotube / vanadium dioxide composite cathode material according to claim 1, characterized in that: The porosity of the three-dimensional continuous conductive network is 70-95%, the pore size is 5-500 nm, and the proportion of mesopores is ≥60%. And / or, the carbon nanotubes include single-walled carbon nanotubes and / or multi-walled carbon nanotubes.

3. The carbon nanotube / vanadium dioxide composite cathode material according to claim 1, characterized in that: The vanadium dioxide nanoparticles are tunnel-shaped with a particle size of 20~100 nm. And / or, the carbon nanotube / vanadium dioxide composite cathode material has a continuous electron conduction path and an open ion diffusion channel; And / or, the content of vanadium dioxide nanoparticles in the carbon nanotube / vanadium dioxide composite cathode material is 33~66wt%.

4. The method for preparing the carbon nanotube / vanadium dioxide composite cathode material according to any one of claims 1 to 3, characterized in that, include: Carbon nanotubes were pretreated to obtain surface-functionalized carbon nanotubes. A dispersion containing surface-functionalized carbon nanotubes is provided, vanadium dioxide nanoparticles are added and ultrasonically treated to form a uniform suspension, which is then formed into a film and post-treated to obtain a carbon nanotube / vanadium dioxide composite cathode material.

5. The preparation method according to claim 4, characterized in that: The surface-functionalized carbon nanotubes contain oxygen-containing active functional groups, including at least one of carboxyl and hydroxyl groups. And / or, the pretreatment method includes one or more combinations of plasma treatment and concentrated acid oxidation; Preferably, the plasma treatment power is 80~120W and the time is 2~5min; Preferably, the concentrated acid oxidation method includes: placing carbon nanotubes in a mixed solution of concentrated nitric acid and concentrated sulfuric acid, and refluxing at 80~100℃ for 2~4 hours to achieve surface functionalization modification of carbon nanotubes.

6. The preparation method according to claim 4, characterized in that, include: Surface-functionalized carbon nanotubes are dispersed in a solvent to form a dispersion, then vanadium dioxide nanoparticles are added, followed by ultrasonic treatment to form a uniform suspension. Preferably, the solvent includes ethanol, water, and a mixed solution of ethanol, or a mixed solution of water and isopropanol. More preferably, the volume ratio of water to ethanol or isopropanol is 1:1 to 1:

2. And / or, the mass ratio of the surface-functionalized carbon nanotubes to vanadium dioxide nanoparticles is 1:2 to 2:1; And / or, the ultrasonic treatment power is 400~600W, and the time is 1~3h.

7. The preparation method according to claim 4, characterized in that, include: The vanadium dioxide nanoparticles were prepared by at least a hydrothermal synthesis method. Preferably, the preparation method includes: subjecting a hydrothermal reaction system containing vanadium pentoxide, a reducing agent and water to a hydrothermal reaction, followed by drying, to obtain the vanadium dioxide nanoparticles; Preferably, the mass ratio of vanadium pentoxide to reducing agent is 1:1.5 to 1:2.0; Preferably, the reducing agent comprises citric acid monohydrate; Preferably, the hydrothermal reaction is carried out at a temperature of 160~200℃ for a time of 10~16h; Preferably, the drying is vacuum drying, with a temperature of 50~70℃ and a time of 10~14h.

8. The preparation method according to claim 4, characterized in that, The film formation method includes one or more combinations of vacuum filtration film formation and vacuum freeze-drying film formation; Preferably, the vacuum filtration membrane formation includes: under vacuum, passing a uniform suspension through an aqueous microporous filter membrane to form a membrane, wherein the pore size of the aqueous microporous filter membrane is 0.2~0.3μm; Preferably, the vacuum freeze-drying film formation includes: pre-freezing a uniform suspension at -40℃ to -20℃ for 2 to 4 hours, and then freeze-drying it under a vacuum of 10 to 30 Pa for 12 to 24 hours; And / or, the post-processing method includes one or more combinations of vacuum drying, drying in an inert gas atmosphere, and low-temperature heat treatment; Preferably, the vacuum drying temperature is 50~70℃ and the time is 10~14h; Preferably, the drying temperature in an inert gas atmosphere is 70~90℃, and the drying time is 6~8h. The inert gas includes nitrogen and / or argon. Preferably, the low-temperature heat treatment is carried out under vacuum conditions, with a temperature of 100~150℃ and a time of 2~4h.

9. The application of the carbon nanotube / vanadium dioxide composite cathode material according to any one of claims 1 to 3 in an energy storage system, wherein the energy storage system includes an iron-ion battery.

10. An iron-ion battery, comprising a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, and an electrolyte, characterized in that: The cathode is made of the carbon nanotube / vanadium dioxide composite cathode material as described in any one of claims 1 to 3.

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