Preparation method of Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material for sodium ion battery and sodium ion battery

Abstract

The invention discloses a preparation method of a Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material for a sodium ion battery, and the preparation method is characterized in that a Mn3O4 / CoP2 heterostructure composite material grows in situ on the surface of NF (foamed nickel) by adopting a hydrothermal-high-temperature phosphorization two-step method to form a regular and ordered three-dimensional nano needle-shaped structure. The invention further discloses a sodium ion battery, the Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material is used as a working electrode, the regulation and control synergistic effect between the high theoretical specific capacity of Mn3O4 and the high conductivity of CoP2 is utilized, and an excellent result is achieved in the application of the sodium ion battery through the synergistic optimization effect between different compounds.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a method for preparing a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries and a sodium-ion battery. Background Technology

[0002] With the continuous development of society, the demand for energy is constantly increasing. Fossil fuels have drawbacks such as being non-renewable and causing high pollution emissions. Therefore, the demand for green and renewable energy sources is gradually increasing. Sodium-ion batteries, as a new type of battery, have advantages such as abundant and widely distributed sodium resources, significantly lower cost than lithium batteries, good thermal stability, low risk of thermal runaway under abuse conditions, high safety, strong adaptability to a wide temperature range, excellent capacity retention at low temperatures, and manufacturing processes compatible with lithium battery production lines, making them easy to scale up. However, sodium-ion batteries also have disadvantages such as lower energy density compared to ternary lithium batteries, significant volume expansion of some electrode materials during cycling, affecting lifespan, and the need for further optimization of high-rate charge and discharge performance.

[0003] In recent years, transition metal phosphides (TMPs) have become a class of functional materials with metal-like conductivity. In their crystal structure, metal and phosphorus atoms are bonded together through covalent or metallic bonds, resulting in excellent conductivity, good redox activity, and structural stability. Among numerous transition metal phosphides, CoP2 exhibits superior performance in energy storage and catalysis due to its unique monoclinic crystal structure, high electron transport rate, and low volume change rate during cycling. Studies have shown that the high conductivity of CoP2 stems from the metallic characteristics of the Co-Co bonds in its crystal structure, while the electronegativity difference of phosphorus atoms endows it with abundant active sites. Constructing a heterojunction structure by combining Mn3O4 and CoP2 can achieve complementary and synergistic optimization of the performance of the two materials: the high conductivity of CoP2 can construct a continuous conductive network, significantly reducing the charge transfer resistance of Mn3O4 and improving rate performance; at the same time, the rigid crystal structure of CoP2 can effectively suppress the volume expansion and structural collapse of Mn3O4 during cycling, enhancing the cycling stability of the electrode. In addition, the electronic interactions at the heterojunction interface may also regulate the surface electronic structure of the material, further enhancing redox activity.

[0004] Based on the above research background, this paper adopts the "hydrothermal-phosphating two-step method" to grow Mn3O4 / CoP2 heterostructure composite material on the surface of NF (nickel foam) in situ to construct sodium-ion battery electrodes. By systematically controlling process parameters such as hydrothermal temperature and phosphating temperature, the phase composition, microstructure and interfacial bonding state of the material are optimized, providing a new approach to improve the electrochemical performance of sodium-ion batteries. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries. By leveraging the synergistic optimization effect between two different compounds, Mn3O4 and CoP2, an electrode material with high specific capacity and high stability can be obtained.

[0006] The present invention also aims to provide a sodium-ion battery that uses the Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material as the working electrode, exhibiting excellent rate performance and cycle performance.

[0007] To achieve the above objectives, the solution of the present invention is: A method for preparing a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries includes the following steps: Step 1, Nickel foam (NF) pretreatment: First, cut the NF sheet to a size of 1 cm × 2 cm, then clean it with deionized water and ethanol alternately by ultrasonic cleaning several times to remove surface oxides and impurities of the NF sheet, and dry it for later use. Step 2: Hydrothermal synthesis of Mn-Co hydroxide precursor / NF precursor: 0.6-0.7 g cobalt nitrate, 0.14-0.25 g manganese nitrate, 0.15-0.25 g ammonium fluoride, and 0.77-1.20 g urea were completely dissolved in 30-40 mL of deionized water. The pretreated NF tablets from Step 1 were added, and the mixture was transferred to a 50 mL high-pressure reactor. The mixture was hydrothermally reacted at 120-150 °C for 6-8 hours. The reaction product was then washed with deionized water and ethanol to remove surface deposits, and dried to obtain the Mn-Co hydroxide precursor / NF precursor. Step 3, High-Temperature Phosphating: Finally, the Mn-Co hydroxide precursor / NF precursor and 0.7~1.5g of sodium hypophosphite were placed on both sides of a ceramic boat. Under a N2 atmosphere, the temperature was increased to 320~400℃ at a heating rate of 1~3℃ / min and held for 2~4 hours. After natural cooling, a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries was obtained, wherein the total mass of Mn3O4 and CoP2 loaded on NF was 3~5 mg / cm³. 2 ; When the Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material is used as the negative electrode material for the sodium-ion battery, the discharge capacity is 517.6 mAh / g at a current density of 0.1 A / g.

[0008] In step 1, the cleaning time is 10-30 minutes.

[0009] In step 1, the drying temperature is 60°C and the drying time is 2-3 hours.

[0010] In step 2, the cobalt nitrate is cobalt nitrate hexahydrate (Co(NO3)2·6H2O), and the manganese nitrate is manganese nitrate tetrahydrate (Mn(NO3)2·4H2O).

[0011] In step 2, the drying temperature is 60°C and the drying time is 5 to 8 hours.

[0012] A sodium-ion battery includes a working electrode, a counter electrode, a separator, and an electrolyte. The working electrode is made of the above-mentioned three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries. The counter electrode is made of sodium sheet. The separator is made of glass fiber. The electrolyte is sodium hexafluorophosphate electrolyte.

[0013] After adopting the above technical solution, the preparation method of the Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material for sodium-ion batteries of the present invention has the following beneficial effects: 1. Complementary components and highly efficient synergistic activity of active sites: Possesses abundant redox valence states (Mn 2+ / Mn 3+ / Mn 4+ CoP2 provides ample storage sites, resulting in high specific capacity for the material. CoP2, with its excellent electronic conductivity (far exceeding that of traditional metal oxides) and good structural stability, can not only accelerate charge transport within the electrode but also inhibit the aggregation and dissolution of Mn3O4 during cycling. In the heterostructure formed by the two, electrons are directionally transferred from CoP2 to Mn3O4 through the interface, optimizing the electron density of active sites, reducing the energy barrier of redox reactions, and significantly improving the kinetic rate of charge storage and catalytic reactions. 2. Heterogeneous interface regulation significantly reduces charge transfer resistance: At the interface of the tight heterojunction formed between Mn3O4 and CoP2, a unique electron transport channel and built-in electric field are formed, which effectively promotes the separation and transfer of interfacial charges and avoids the problem of increased interfacial impedance caused by poor contact between components in traditional composite materials. 3. Defect engineering synergy, dual enhancement of catalytic / energy storage activity: During the preparation process, a large number of lattice defects (such as oxygen vacancies and phosphorus vacancies) are introduced during the composite process of Mn3O4 and CoP2. These defects can not only serve as additional active sites, but also regulate the band structure of the material. 4. Rate performance is a core performance indicator for sodium-ion batteries. It is directly related to the insertion / extraction rate of sodium ions in the electrode material, the ionic conductivity of the electrolyte, and the stability of the SEI film. Excellent rate performance signifies rapid charge transfer and a stable interface structure, directly reflecting the rationality of the material system and manufacturing process. Test results of sodium-ion half-cells assembled with Mn3O4 / CoP2 / NF as the negative electrode show excellent rate performance across a wide current density range of 0.1–30 A / g. Furthermore, even when the current density recovers to 0.1 A / g, good electrochemical response is maintained. This phenomenon not only directly reflects the stable electrochemical performance of the electrode at low, medium, and high current densities, indicating a fast sodium ion insertion / extraction kinetic rate in the electrode material and low electrode-electrolyte interfacial impedance, making it suitable for diverse scenarios from low-power energy storage to high-power fast charging; but also demonstrates that the high-current charge / discharge process does not affect the crystal structure, morphology, or SEI of the electrode. The membrane causes irreversible damage, but the material possesses excellent structural integrity and cyclic reversibility. Attached Figure Description

[0014] Figure 1 For Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode materials, Mn x O n Electron micrographs of Mn3O4 / NF electrode material and CoP2 / NF electrode material, where (a) and (b) are electron micrographs of the Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material at 1 μm and 500 nm, respectively, and (c) and (d) are Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material at 1 μm and 500 nm, respectively. x O n / NF electrode material electron microscope images at 1 μm and 500 nm, where (e) and (f) are electron microscope images of CoP2 / NF electrode material at 1 μm and 500 nm, respectively, and (g)~(i) are lattice analysis diagrams of Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material; Figure 2 For Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode materials, Mn x O n Characterization images of Mn3O4 / CoP2 / NF electrode materials and CoP2 / NF electrode materials, where (a) is the XRD pattern, (b)~(e) are the XPS fine spectra, and (f) is the characterization image of the Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material and Mn3O4 / CoP2 / NF electrode material. x O n / NF electrode material, CoP2 / NF electrode material N2 adsorption-desorption isotherms and specific surface area; Figure 3Rate performance of Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material at different current densities; Figure 4 For Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode materials, Mn x O n / NF electrode material, CoP2 / NF electrode material in 1A g -1 The discharge curve after 100 cycles at the current density; Figure 5 For Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode materials at 10 A g -1 Charge-discharge curves after 1000 cycles at current density; Figure 6 For Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode materials, Mn x O n EIS curves of / NF electrode materials and CoP2 / NF electrode materials. Detailed Implementation

[0015] To further explain the technical solution of the present invention, the present invention will be described in detail below through specific embodiments.

[0016] I. Preparation of Electrode Materials Example 1 A method for preparing a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries includes the following steps: Step 1, Nickel foam (NF) pretreatment: Cut the NF sheet to a size of 1 cm × 2 cm, then clean it with deionized water and ethanol alternately by ultrasonic cleaning multiple times, 30 minutes each time, alternating twice, to remove surface oxides and impurities of the NF sheet. Dry it at 60℃ for 2 hours for later use. Step 2: Hydrothermal synthesis of Mn-Co hydroxide precursor / NF precursor: 0.69 g cobalt nitrate hexahydrate, 0.23 g manganese nitrate tetrahydrate, 0.17 g ammonium fluoride and 0.8 g urea were completely dissolved in 30 mL deionized water, and the pretreated NF tablets from Step 1 were added. The mixture was then transferred to a 50 mL high-pressure reactor and hydrothermally reacted at 120 °C for 8 hours. The reaction product was then washed with deionized water and ethanol to remove surface deposits, and dried at 60 °C for 5 hours to obtain the Mn-Co hydroxide precursor / NF precursor. Step 3, High-Temperature Phosphating: Finally, the Mn-Co hydroxide precursor / NF precursor and 1g of sodium hypophosphite were placed on both sides of a ceramic boat. Under a N2 atmosphere, the temperature was increased to 350℃ at a heating rate of 2℃ / min and held for 2 hours. After natural cooling, a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries was obtained, in which the total mass of Mn3O4 and CoP2 loaded on NF was 4 mg / cm³. 2 .

[0017] Comparative Example 1 A sodium-ion battery using Mn x O n The preparation method of / NF electrode material includes the following steps: Step 1, Nickel foam (NF) pretreatment: Cut the NF sheet to a size of 1 cm × 2 cm, then clean it with deionized water and ethanol alternately by ultrasonic cleaning multiple times, 30 minutes each time, alternating twice, to remove surface oxides and impurities of the NF sheet. Dry it at 60°C for 2 hours for later use. Step 2: Hydrothermal synthesis of manganese hydroxide precursor / NF precursor: Then, 0.23 g of manganese nitrate tetrahydrate, 0.17 g of ammonium fluoride and 0.8 g of urea were completely dissolved in 30 mL of deionized water, and the pretreated NF tablets from Step 1 were added. The mixture was transferred to a 50 mL high-pressure reactor and hydrothermally reacted at 120 °C for 8 hours. The reaction product was then washed with deionized water and ethanol to remove surface deposits, and dried at 60 °C for 5 hours to obtain the manganese hydroxide precursor / NF precursor. Step 3, Annealing: Finally, the manganese hydroxide precursor / NF precursor is heated to 350℃ in a N2 atmosphere at a heating rate of 2℃ / min and held at that temperature for 2 hours. After natural cooling, the Mn for sodium-ion batteries is obtained. x O n / NF electrode material, in which Mn x O n The mass of the load on NF is 4 mg / cm³. 2 .

[0018] Comparative Example 2 A method for preparing a CoP2 / NF electrode material for sodium-ion batteries includes the following steps: Step 1, Nickel foam (NF) pretreatment: Cut the NF sheet to a size of 1 cm × 2 cm, then clean it with deionized water and ethanol alternately by ultrasonic cleaning multiple times, 30 minutes each time, alternating twice, to remove surface oxides and impurities of the NF sheet. Dry it at 60℃ for 2 hours for later use. Step 2: Hydrothermal synthesis of cobalt hydroxide precursor / NF precursor: 0.69 g cobalt nitrate hexahydrate, 0.17 g ammonium fluoride and 0.8 g urea were completely dissolved in 30 mL deionized water, and the pretreated NF tablets from Step 1 were added. The mixture was transferred to a 50 mL high-pressure reactor and hydrothermally reacted at 120 °C for 8 hours. The reaction product was then washed with deionized water and ethanol to remove surface deposits and dried at 60 °C for 5 hours to obtain the cobalt hydroxide precursor / NF precursor. Step 3, High-Temperature Phosphating: Finally, the cobalt hydroxide precursor / NF precursor and 1g of sodium hypophosphite were placed on both sides of a ceramic boat. Under a N2 atmosphere, the temperature was increased to 350℃ at a heating rate of 2℃ / min and held for 2 hours. After natural cooling, the CoP2 / NF electrode material for sodium-ion batteries was obtained, wherein the mass of CoP2 loaded on NF was 4 mg / cm³. 2 .

[0019] II. Characterization of Electrode Materials 1. SEM and TEM characterization: Morphology was observed using FE-SEM (NOVANanoSEM450). Figure 1 (a) and Figure 1 (b) are electron micrographs of the Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material at 1 μm and 500 nm, respectively. The Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material has a needle-like structure, which helps to alleviate the problem of decreased cycling performance caused by the volume expansion of the material during cycling. The needle-like "slender shape" can also build a continuous ion transport channel. In the electrolyte, the reactants can quickly reach the surface active sites of the needle-like structure.

[0020] Figure 1 (c) and Figure 1 (d) respectively show Mn x O n / Electron micrographs of NF electrode material at 1 μm and 500 nm. This bulk structure has a large volume and few internal pores, making it difficult for electrolyte / reactants to diffuse into the interior of the grains. The long transport paths of ions / electrons lead to slow reaction kinetics. Figure 1 (e) and Figure 1 (f) shows electron micrographs of the CoP2 / NF electrode material at 1 μm and 500 nm. It can be observed that CoP2 exhibits a nanosheet structure. These sheet-like grains are "densely stacked" and have high surface energy, which leads to a significant reduction in effective specific surface area.

[0021] Figure 1(g) shows a TEM image of the Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material, revealing locally oriented mixed lattice fringes, with dashed lines indicating individual Mn3O4 and CoP2 phases. These features confirm the crystallinity of the monomer material. Figure 1 (h) and Figure 1 (i) is a high-resolution interplanar lattice pattern of this heterostructure. Magnification shows that the interplanar lattice spacing is 0.269 nm, corresponding to the (020) Miller surface reflection of the CoP2 phase. The interplanar spacing in the blue box is 0.209 nm, related to the (111) Miller surface reflection of the Mn3O4 phase. The presence of polylattice orientation further confirms the good crystallinity of the single phase in the hybrid structure. In addition, Figure 1 A clear interface between Mn3O4 and CoP2 nanoparticles can be observed in (g), confirming the successful hybridization of the two components.

[0022] 2. XRD and XPS characterization: like Figure 2 As shown in (a), X-ray diffraction (XRD) was used to analyze the samples prepared in the examples and comparative examples (i.e., Mn3O4 / CoP2 / NF, CoP2 / NF, and Mn3O4 / CoP2 / NF). x O n The crystal structure and phase purity of Mn3O4 / CoP2 / NF were analyzed. Due to the presence of the NF substrate, the diffraction peaks at 44.51, 51.85, and 76.37 correspond to the (111), (200), and (220) crystal planes of nickel (PDF#04-0850). Observing the crystal structure of the Mn3O4 / CoP2 / NF sample, it can be observed that the diffraction peaks at 17.547 and 41.583 correspond to the (111) and (400) crystal planes of Mn3O4 (PDF#04-0732), and the diffraction peaks at 24.448, 32.004, 35.838, and 52.672 correspond to the (-111), (020), (200), and (-213) crystal planes of CoP2 (PDF#04-0481), successfully proving the successful synthesis of this heterostructure of Mn3O4 / CoP2.

[0023] Figure 2 (b) The three-dimensional emission spectra of Co in the mixed Mn3O4 / CoP2 / NF heterostructure and the single CoP2 / NF were compared. It can be observed that the binding energy of the mixed Mn3O4 / CoP2 / NF heterostructure is significantly improved compared with that of the single CoP2 / NF. Figure 2 (c) A comparison was made between the mixed Mn3O4 / CoP2 / NF heterostructure and the single Mn. x O n The three-dimensional emission spectrum of Mn in / NF shows that the mixed Mn3O4 / CoP2 / NF heterostructure is different from that of single Mn.x O n The binding energy of / NF decreased significantly. Figure 2 (a) Observation of Mn x O n The / NF crystal structure reveals the simultaneous presence of both MnO2 and Mn3O4. Combined with... Figure 2 (b) and Figure 2 (c) It can be inferred that the Mn element underwent interfacial electron transfer to the Co element, resulting in the Mn element being affected. 4+ The valence state decreases, transforming into Mn 2+ and Mn 3+ The interfacial electron transfer from Co to Mn at the Mn3O4 / CoP2 heterostructure interface optimizes the electronic structure and improves the electrode's conductivity, thereby enhancing capacity retention at high rates. It also strengthens redox reactivity, increasing the electrode's specific capacity and charge-discharge efficiency. The strong interaction formed by this interfacial electron transfer enhances the bonding force between the Mn3O4 and CoP2 phases, mitigating volume expansion and structural collapse caused by ion insertion / extraction during charge-discharge, thus improving the electrode's cycle stability and extending its cycle life.

[0024] Figure 2 (d) It can be observed that the binding energy of P in Mn3O4 / CoP2 / NF is slightly lower than that in CoP2 / NF. This may be because P has a higher electronegativity than oxygen, and some P forms PO bonds with oxygen, thus leading to a decrease in binding energy. Figure 2 (e) A metal oxidation peak can be observed in the O 1s of Mn3O4 / CoP2 / NF, which further confirms the presence of Mn3O4. Figure 2 (f) is Mn3O4 / CoP2 / NF, Mn x O n The N2 adsorption-desorption isotherms of three electrode materials, Mn3O4 / NF and CoP2 / NF, show that all three materials exhibit HIV-type isotherms with a significant hysteresis loop. The specific surface area of ​​Mn3O4 / CoP2 / NF is 16.4042 m². 2 / g) is much higher than Mn x O n / NF(10.1101 m 2 / g) and CoP2 / NF (8.7754 m 2 / g). The high specific surface area of ​​Mn3O4 / CoP2 / NF may be due to its three-dimensional needle-like morphology, which is conducive to exposing more specific surface area. This structural advantage provides abundant active sites for the adsorption, diffusion and electrochemical reactions of electrolyte ions.

[0025] III. Application of Electrode Materials Application Example 1 A 2016 coin-type sodium-ion half-cell was assembled using a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries as the working electrode, a sodium sheet as the counter electrode, glass fiber as the separator, and sodium hexafluorophosphate electrolyte.

[0026] Application Example 2 Mn for sodium-ion batteries x O n The / NF electrode material is used as the working electrode, sodium sheet as the counter electrode, glass fiber as the separator, and sodium hexafluorophosphate electrolyte to assemble a 2016 coin cell sodium-ion half cell.

[0027] Application Example 3 A 2016 coin cell sodium-ion half-cell was assembled using CoP2 / NF electrode material (used in sodium-ion batteries) as the working electrode, sodium sheet as the counter electrode, glass fiber as the separator, and sodium hexafluorophosphate electrolyte.

[0028] IV. Performance Testing of Electrode Materials 1. Using a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries, a 2016 coin-type sodium-ion half-cell was assembled and subjected to charge-discharge tests. The test voltage window was 0.01~3 V, and the test currents were 0.1 A / g, 0.2 A / g, 0.5 A / g, 1 A / g, 2 A / g, 10 A / g, 20 A / g, and 30 A / g, respectively. Finally, the current was restored to 0.1 A / g. The rate performance of the sodium-ion battery was then tested. Figure 3 As shown, the Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material exhibits excellent rate performance at all current densities. The discharge capacities at 0.1, 0.2, 0.5, 1.0, 2.0, 10.0, 20.0 and 30.0 A / g are 517.6, 475.5, 418.4, 365.8, 311.4, 283.6, 254.3 and 220.5 mAh / g, respectively.

[0029] 2. Using Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode materials and Mn x O n Using / NF electrode material and CoP2 / NF electrode material as working electrodes, a 2016 coin-type sodium-ion half-cell was assembled and electrochemically tested. The test voltage window was 0.01~3V, the test current was 1 A / g, and the cycle was 100 times. Using Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material as working electrode, the cell was cycled 1000 times at a high current density of 10A / g.

[0030] Figure 4 For Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode materials, Mn x O n The long cycling curves of Mn3O4 / CoP2 / NF electrode materials after 100 cycles at a current density of 1 A / g clearly show that the Mn3O4 / CoP2 / NF electrode material is superior to the Mn3O4 / CoP2 / NF electrode material. x O n / NF electrode materials and CoP2 / NF electrode materials, Figure 5 The Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material was cycled 1000 times at a high sweep rate and current density of 10 A / g. This is because the three-dimensional vacuum nanoneedle structure of Mn3O4 / CoP2 / NF effectively alleviated the decrease in specific capacity caused by volume expansion during cycling.

[0031] 3. Figure 6 Mn3O4 / CoP2 / NF, Mn x O n The EIS test curves of three electrode materials, Mn3O4 / CoP2 / NF and CoP2 / NF, show that the interfacial impedance of the Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material is lower than that of the other two materials. This is because the hollow structure of Mn3O4 / CoP2 / NF effectively reduces the diffusion path of ions and lowers the activation energy barrier of the reaction.

[0032] This invention reveals the intrinsic mechanism by which the Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material exhibits excellent performance through material characterization and electrochemical testing: 1. Built-in electric field and directional charge migration at the interface: Mn3O4 is a typical p-type semiconductor, and CoP2 is a highly conductive compound with metallic properties. After the two are combined, a built-in electric field is formed at the interface due to the difference in work function. The electric field drives the directional migration of electrons from Co to Mn. This charge separation effect optimizes the electron density of active sites on the one hand, and lowers the energy barrier for charge transfer on the other hand, which greatly improves the charge transport rate at the electrode / electrolyte interface. This is reflected in the significant reduction of charge transfer resistance (Rct) in the electrochemical impedance spectroscopy. 2. Enhanced interfacial interactions through chemical bonding: During the preparation of composite materials, oxygen atoms on the surface of Mn3O4 and Co atoms on the surface of CoP2 readily form Mn-O-Co covalent bonds, rather than simple physical contact. This chemical bonding not only enhances the structural stability of the interface (inhibiting the aggregation and shedding of components during cycling), but also modulates the band structure of the material, causing the conduction band and valence band positions of Mn3O4 to shift, narrowing the electron transition gap, improving electron conduction efficiency, and providing more active sites for pseudocapacitive reactions and catalytic reactions. 3. Complementary and synergistic effects of components: Mn3O4 provides abundant redox valence states (Mn 2+ / Mn 3+ / Mn 4+ Mn3O4 (Mn3O4) is the core source of high pseudocapacitance and can store a large amount of charge through reversible redox reactions; however, its intrinsic conductivity is poor, and it is severely polarized at high current densities, limiting its rate performance. CoP2 has metal-like high conductivity (2-3 orders of magnitude higher than Mn3O4), which can serve as a "high-speed channel" for electron transport, alleviating the polarization problem of Mn3O4; at the same time, CoP2 itself has excellent structural stability and catalytic activity (HER / OER dual function), which can synergistically improve the cycle life and catalytic performance of composite materials. The complementarity of the two eliminates the performance shortcomings of single components, achieving a synergistic effect of "1+1>2".

[0033] The above embodiments and figures are not intended to limit the product form and style of the present invention. Any appropriate changes or modifications made by those skilled in the art should be considered as not departing from the patent scope of the present invention.

Claims

1. A method for preparing a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries, characterized in that: Includes the following steps: Step 1, NF pretreatment: First, cut the NF sheet to a size of 1 cm × 2 cm, then clean it with deionized water and ethanol alternately by ultrasonic cleaning several times to remove surface oxides and impurities of the NF sheet, and dry it for later use. Step 2: Hydrothermal synthesis of Mn-Co hydroxide precursor / NF precursor: 0.6-0.7 g cobalt nitrate, 0.14-0.25 g manganese nitrate, 0.15-0.25 g ammonium fluoride, and 0.77-1.20 g urea were completely dissolved in 30-40 mL of deionized water. The pretreated NF tablets from Step 1 were added, and the mixture was transferred to a 50 mL high-pressure reactor. The mixture was hydrothermally reacted at 120-150 °C for 6-8 hours. The reaction product was then washed with deionized water and ethanol to remove surface deposits, and dried to obtain the Mn-Co hydroxide precursor / NF precursor. Step 3, High-Temperature Phosphating: Finally, the Mn-Co hydroxide precursor / NF precursor and 0.7~1.5g of sodium hypophosphite were placed on both sides of a ceramic boat. Under a N2 atmosphere, the temperature was increased to 320~400℃ at a heating rate of 1~3℃ / min and held for 2~4 hours. After natural cooling, a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries was obtained, wherein the total mass of Mn3O4 and CoP2 loaded on NF was 3~5 mg / cm³. 2 ; When the Mn3O4 / CoP2 / NF three-dimensional heterostructure electrode material is used as the negative electrode material for the sodium-ion battery, the discharge capacity is 517.6 mAh / g at a current density of 0.1 A / g.

2. The method for preparing a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries according to claim 1, characterized in that: In step 1, the cleaning time is 10-30 minutes.

3. The method for preparing a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries according to claim 1, characterized in that: In step 1, the drying temperature is 60°C and the drying time is 2-3 hours.

4. The method for preparing a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries according to claim 1, characterized in that: In step 2, the cobalt nitrate is cobalt nitrate hexahydrate, and the manganese nitrate is manganese nitrate tetrahydrate.

5. The method for preparing a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries according to claim 1, characterized in that: In step 2, the drying temperature is 60°C and the drying time is 5-8 hours.

6. A sodium-ion battery, comprising a working electrode, a counter electrode, a separator, and an electrolyte, wherein the working electrode is made of a three-dimensional heterostructure electrode material of Mn3O4 / CoP2 / NF for sodium-ion batteries as described in claim 1, the counter electrode is made of a sodium sheet, the separator is made of a glass fiber separator, and the electrolyte is a sodium hexafluorophosphate electrolyte.

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