A positive electrode material, a preparation method thereof and a battery

Abstract

This invention relates to the field of batteries, and more particularly to a positive electrode material, a method for preparing the same, and a battery. The positive electrode material includes a core and a coating layer covering at least a portion of the surface of the core; the core has the general chemical formula Na3V. 1.925 M 0.075 (PO4)3, where M is a transition metal element; the coating layer includes carbon materials, which include nitrogen-doped amorphous carbon and carbon nanotubes introduced by acidified carbon nanotubes. The cathode material provided by this invention can significantly improve the low-temperature, high-rate, and long-cycle electrochemical performance. Thanks to the synergistic effect of structural optimization and interface regulation, the polarization of the material is significantly reduced under low-temperature and high-current charge-discharge conditions, and the rate performance and cycle stability are significantly improved, enhancing the practicality and reliability of sodium-ion batteries in large-scale energy storage and wide-temperature applications.

Description

Technical Field

[0001] This invention relates to the field of batteries, and more particularly to a cathode material, its preparation method, and a battery. Background Technology

[0002] With the rapid development of sodium-ion batteries in large-scale and low-cost energy storage, cathode materials still face many challenges in terms of structural stability, rate performance, and low-temperature performance. Sodium vanadium phosphate [Na3V2(PO4)3, NVP] cathode material has advantages such as structural stability and moderate operating voltage, but it still has the following shortcomings in practical applications: On the one hand, the intrinsic electronic conductivity and sodium ion diffusion kinetics of the material are relatively slow, especially under low temperature or high current charge and discharge conditions, the polarization phenomenon is obvious and the capacity decay is aggravated. On the other hand, the uneven particle size distribution, discontinuous or uneven thickness of the carbon coating layer can easily lead to increased interfacial resistance and insufficient stability of the cathode-electrolyte interface (CEI), thereby affecting cycle life and rate performance.

[0003] Existing technologies typically improve material properties through single doping or simple carbon coating, but these methods suffer from limitations such as limited control dimensions and insufficient synergy between structure and interface. For example, while metal ion doping can stabilize the crystal structure to some extent, it is difficult to simultaneously improve interfacial charge transport; and simply introducing carbon materials or conductive additives often fails to regulate ion diffusion paths from within the material.

[0004] Therefore, there is an urgent need for a synergistic regulation strategy that takes into account both the internal structural stability of the material and the interfacial charge transport capability, so as to achieve an overall improvement in the structure, interface and electrochemical performance of sodium ion cathode materials. Summary of the Invention

[0005] This invention provides a cathode material, its preparation method, and a battery to solve the problem that existing coating and doping modified NVP cannot simultaneously achieve both internal structural stability and interfacial charge transport capability.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: This invention provides a cathode material comprising a core and a coating layer covering at least a portion of the surface of the core; The general chemical formula of the core is Na3V. 1.925 M 0.075 (PO4)3, where M is a transition metal element; The coating layer comprises a carbon material, which includes nitrogen-doped amorphous carbon and carbon nanotubes introduced by acidified carbon nanotubes.

[0007] In some specific embodiments, M is at least one of Zn, Ti, Fe and Mg.

[0008] In some specific embodiments, based on the mass of the cathode material, the carbon content in the carbon material is 4wt%~9wt%.

[0009] In some specific embodiments, the nitrogen content in the carbon material is 0.5wt%~1wt%, based on the mass of the cathode material.

[0010] In some specific embodiments, the positive electrode material is R. Rhombohedral phase structure of space group c.

[0011] In some specific embodiments, the specific surface area of ​​the positive electrode material is 35~60m². 2 g -1 .

[0012] A second aspect of the present invention also provides a method for preparing the above-mentioned positive electrode material, comprising the following steps: Sodium source, vanadium source, phosphorus source, transition metal source, nitrogen source, carbon source, acidified carbon nanotubes and solvent are mixed to obtain precursor slurry; The precursor slurry is dried to obtain the precursor material; The precursor material is calcined to obtain the cathode material; The nitrogen source includes polyquaternary ammonium salts.

[0013] In some specific embodiments, the molar ratio of sodium in the sodium source, vanadium in the vanadium source, phosphorus in the phosphorus source, metal in the transition metal source, and carbon in the carbon source is 3:1.925:3:0.075:(7~11).

[0014] In some specific embodiments, the mass ratio of the core, polyquaternary ammonium salt, and acidified carbon nanotubes is 1:2%~10%:0.5%~3%.

[0015] In some specific embodiments, the sodium source includes sodium salt.

[0016] In some specific embodiments, the vanadium source includes vanadium oxide.

[0017] In some specific embodiments, the phosphorus source includes phosphate.

[0018] In some specific embodiments, the transition metal source includes at least one of zinc salt, titanium salt, iron salt, and magnesium salt.

[0019] In some specific embodiments, the carbon source includes citric acid.

[0020] In some specific embodiments, the solvent includes water and / or alcohol solvents.

[0021] In some specific embodiments, the drying temperature is 80~110℃, and the drying time is 10~14h.

[0022] In some specific embodiments, the calcination is a staged calcination; The conditions for the first stage of the staged calcination are as follows: the heating rate is 3~5℃ / min, the final temperature is 300~350℃, the holding time at the final temperature is 4~8h, and the atmosphere is an inert atmosphere. The conditions for the second stage of calcination in the staged calcination are as follows: the heating rate is 3~5℃ / min, the final temperature is 700~850℃, the holding time at the final temperature is 8~10h, and the atmosphere is an inert atmosphere.

[0023] A third aspect of the present invention also provides a battery comprising a positive electrode, wherein the positive electrode comprises the above-described positive electrode material or a positive electrode material prepared according to the above-described preparation method.

[0024] Compared with the prior art, the present invention has the following beneficial effects: (1) The cathode material provided by the present invention can significantly improve the electrochemical performance at low temperature, high rate and long cycle. Thanks to the synergistic effect of structural optimization and interface regulation, the polarization of the material is significantly reduced under low temperature and high current charge and discharge conditions, and the rate performance and cycle stability are significantly improved, which enhances the practicality and reliability of sodium-ion batteries in large-scale energy storage and wide temperature range applications.

[0025] (2) The method for preparing the cathode material provided by the present invention, on the one hand, the (–N) in the polyquaternary ammonium salt + (CH3)3) By regulating the precursor through electrostatic interactions, uniform nucleation and restricted growth of NVP cathode materials are achieved, significantly improving particle size distribution and carbon layer thickness, and shortening the bulk diffusion path of sodium ions, thereby effectively improving ion transport efficiency, especially beneficial for electrochemistry under low temperature and high rate conditions. During calcination, the carbon layer is transformed in situ into a uniform and continuous nitrogen-doped carbon layer, reducing interfacial resistance and stabilizing the cathode-electrolyte interface (CEI), achieving simultaneous improvement in electronic conductivity and interfacial stability, overcoming the problem of insufficient interfacial regulation by traditional single carbon coating or doping strategies. On the other hand, combined with Mg ion doping to stabilize the crystal structure, it promotes rapid ion transport within the crystal and the long-range conductivity of CNTs, realizing synergistic electron / ion transport. Attached Figure Description

[0026] The above and other objects, features, and advantages of the invention will be apparent from the following description of preferred embodiments illustrating the gist of the invention and its use, and the accompanying drawings, in which: Figure 1 The diagram shows the formation mechanism of the cathode material in Examples 1-5.

[0027] Figure 2 The XRD patterns of the cathode materials prepared in Examples 1-3 and Comparative Example 1 are shown.

[0028] Figure 3 The XPS spectra of the cathode materials prepared in Example 1 and Comparative Example 1 are shown, where (a) is the full XPS spectrum, (b) is the N1s spectrum, and (c) is the Mg 1s spectrum.

[0029] Figure 4 The figure shows the conductivity diagrams of the cathode materials prepared in Examples 1-3 and Comparative Example 1.

[0030] Figure 5 The images shown are scanning electron microscope (SEM) images of the cathode materials prepared in Example 1 and Comparative Example 1, where (a) is the cathode material in Comparative Example 1 and (b) is the cathode material in Example 1.

[0031] Figure 6 The images shown are characterization diagrams of the cathode materials prepared in Example 1 and Comparative Example 1. (a) is a TEM image of the cathode material prepared in Example 1 with a scale of 50 nm. (b) is a TEM image of the cathode material prepared in Example 1 with a scale of 10 nm. (c) is a TEM image of the cathode material prepared in Comparative Example 1 with a scale of 50 nm. (d) is a TEM image of the cathode material prepared in Comparative Example 1 with a scale of 10 nm. (e) is an EDS image of the cathode material prepared in Example 1.

[0032] Figure 7 The figures shown are BET diagrams of the cathode materials prepared in Example 1 and Comparative Example 1, where (a) is the nitrogen adsorption-desorption isotherm and (b) is the pore size distribution curve.

[0033] Figure 8 The figures show the charge-discharge curves and rate performance test results of the batteries composed of the positive electrode materials prepared in Example 1 and Comparative Example 1. Among them, (a) is the charge-discharge curve of the battery composed of the positive electrode material prepared in Comparative Example 1 at -30~10℃ and 0.2C, (b) is the charge-discharge curve of the battery composed of the positive electrode material prepared in Example 1 at -30~10℃ and 0.2C, (c) is the rate performance test result of the batteries composed of the positive electrode materials prepared in Example 1 and Comparative Example 1 at 25℃ and different current densities (0.5~100C), and (d) is the rate performance test result of the batteries composed of the positive electrode materials prepared in Example 1 and Comparative Example 1 at -20℃ and different current densities (0.2~30C).

[0034] Figure 9The results show the cycle performance test results of the batteries composed of the cathode materials prepared in Example 1 and Comparative Example 1, where (a) is the cycle test at 80C (9.36A / g) at 25°C, and (b) is the cycle test at 10C at -20°C.

[0035] Figure 10 The figures shown are Gitt curves and diffusion coefficient diagrams of the batteries composed of the cathode materials prepared in Example 1 and Comparative Example 1.

[0036] Figure 11 The figures shown are AC impedance spectra of batteries composed of cathode materials prepared in Example 1 and Comparative Example 1 at 10 to -30°C, where (a) is the cathode material in Comparative Example 1 and (b) is the cathode material in Example 1.

[0037] Figure 12 The images shown are high-resolution transmission electron microscope images of the cathode materials in Example 1 and Comparative Example 1 after 500 cycles at -20°C and 5°C. (a) is the cathode material in Comparative Example 1, and (b) is the cathode material in Example 1. Detailed Implementation

[0038] The present invention will be described below through specific embodiments. Those skilled in the art will understand that the specific embodiments described below are for illustrative purposes only and do not limit the scope of the invention in any way. Furthermore, in the following embodiments, unless otherwise specified, the reagents and equipment used are commercially available. If specific processing conditions and methods are not explicitly described in the following embodiments, conditions and methods known in the art can be used for processing.

[0039] This invention provides a cathode material comprising a core and a coating layer covering at least a portion of the surface of the core; The general chemical formula of the core is Na3V. 1.925 M 0.075 (PO4)3, where M is a transition metal element; The coating layer comprises a carbon material, which includes nitrogen-doped amorphous carbon and carbon nanotubes introduced by acidified carbon nanotubes.

[0040] In this invention, the coating layer is formed or covers at least a portion of the surface of the core, which can protect or improve the core. The coating layer being formed or covering at least a portion of the surface of the core means that the coating layer can completely encapsulate the core within the coating layer, or the coating layer can only cover a portion of the outer surface of the core; that is, the coating layer can completely cover the core, or it can cover a portion of the surface of the core, preferably completely covering it.

[0041] In some embodiments, M is at least one selected from Zn, Ti, Fe, and Mg. Preferably, M is Mg.

[0042] In this invention, Mg 2+ (0.72Å) and V 3+ / V 4+ With similar sizes (0.64-0.58 Å), Mg doping facilitates solid solution integration into the crystal lattice, maintaining the integrity of the NASICON framework. Mg is abundant, low-cost, environmentally friendly, non-toxic, and free of heavy metal pollution, making it suitable for large-scale energy storage systems. In contrast, Zn... 2+ The size (0.74 Å) is slightly large, which can easily cause local lattice expansion. Ti 4+ The valence state (0.61 Å) is too high, which can easily lead to charge imbalance. Fe 3+ The presence of multiple valence states at (0.65Å) affects the structural stability.

[0043] In some embodiments, the carbon content in the carbon material is 4 wt% to 9 wt% based on the mass of the cathode material. For example, the carbon content in the carbon material can be 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, and 9 wt%, etc., based on the mass of the cathode material.

[0044] In some embodiments, the nitrogen content in the carbon material is 0.5 wt% to 1 wt% based on the mass of the cathode material. As an example, the nitrogen content in the carbon material can be 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, and 1 wt%, etc., based on the mass of the cathode material.

[0045] In some embodiments, the positive electrode material is R Rhombohedral phase structure of space group c.

[0046] In this invention, the crystal form of the cathode material is consistent with that of sodium vanadium phosphate, indicating that the introduction of magnesium and the carbon material coating modification do not affect the crystal structure of the material and there are no impurities.

[0047] In some embodiments, the specific surface area of ​​the positive electrode material is 35~60m². 2 g -1 As an example, the specific surface area of ​​the cathode material can be 35 m². 2 g -1 40m 2 g -1 45m 2 g -1 50m 2 g -1 55m 2 g -1and 60m 2 g -1 wait.

[0048] A second aspect of the present invention also provides a method for preparing the above-mentioned positive electrode material, comprising the following steps: Sodium source, vanadium source, phosphorus source, transition metal source, nitrogen source, carbon source, acidified carbon nanotubes and solvent are mixed to obtain precursor slurry; The precursor slurry is dried to obtain the precursor material; The precursor material is calcined to obtain the cathode material; The nitrogen source includes polyquaternary ammonium salts.

[0049] The cathode material of this invention has a simple preparation process and can be achieved by conventional liquid-phase-drying-calcination methods. It is suitable for practical applications such as large-scale energy storage and wide temperature range, which is conducive to industrial production and promotion.

[0050] In this invention, using polyquaternary ammonium salts as the nitrogen source is advantageous because polyquaternary ammonium salts, with their stable positively charged backbones, are more conducive to achieving synergistic optimization of precursor regulation, nitrogen source introduction, and interface construction. Compared to other nitrogen sources, such as polyvinylpyrrolidone (PVP) and melamine, PPVP is a neutral polymer whose interaction with the precursor mainly relies on hydrogen bonds or weak coordination, resulting in limited precursor nucleation and dispersion regulation capabilities and difficulty in achieving molecular-level confined regulation. Melamine, on the other hand, is a small-molecule nitrogen source that is prone to local enrichment or volatilization during reaction and heat treatment, leading to poor nitrogen distribution uniformity and interface stability.

[0051] In this invention, compared with unacidified carbon nanotubes (CNTs), acidification introduces abundant oxygen-containing functional groups on their surface, significantly improving the dispersibility and interfacial activity of CNTs. Acidified CNTs can interact strongly with the cationic groups of polyquaternary ammonium salts and the surface of the core particles, thereby constructing a stable and continuous conductive network. Although the acidification process may introduce structural defects to some extent, its advantages in improving interfacial bonding, enhancing electron transport continuity, and improving low-temperature and high-rate electrochemical performance are more significant, making it more suitable for the interface engineering-based control strategy in this work.

[0052] In this invention, polyquaternium salts include, but are not limited to, polyquaternium salt-10 (PQ-10).

[0053] In this invention, the specific mixing process includes: A nitrogen source and a portion of the solvent are mixed, and then a sodium source, a vanadium source, a phosphorus source, a transition metal source, a nitrogen source, and a carbon source are added to obtain the first slurry. The acidified carbon nanotubes and the remaining solvent were mixed to obtain a second slurry; The first slurry and the second slurry are mixed to obtain the precursor slurry.

[0054] In some embodiments, the molar ratio of sodium in the sodium source, vanadium in the vanadium source, phosphorus in the phosphorus source, metal in the transition metal source, and carbon in the carbon source is 3:1.925:3:0.075:(7~11).

[0055] In some embodiments, the mass ratio of the core, polyquaternary ammonium salt, and acidified carbon nanotubes is 1:2%~10%:0.5%~3%. Here, the core refers to the theoretically generated core Na3V. 1.925 M 0.075 (PO4)3.

[0056] In this invention, by controlling the appropriate doping transition metal element and the appropriate amount of nitrogen source within the above range, it is helpful to achieve crystal structure stability and simultaneous improvement of electronic conductivity and interface stability.

[0057] In some embodiments, the sodium source includes, but is not limited to, sodium salts. Preferably, the sodium salt includes, but is not limited to, sodium carbonate.

[0058] In some embodiments, the vanadium source includes, but is not limited to, vanadium oxide. Preferably, the vanadium oxide includes, but is not limited to, vanadium pentoxide.

[0059] In some embodiments, the phosphorus source includes, but is not limited to, phosphates. Preferably, the phosphate includes, but is not limited to, ammonium dihydrogen phosphate.

[0060] In some embodiments, the transition metal source includes, but is not limited to, at least one of zinc salts, titanium salts, iron salts, and magnesium salts, preferably a magnesium salt. Preferably, the magnesium salt includes, but is not limited to, magnesium acetate.

[0061] In some embodiments, the carbon source includes, but is not limited to, citric acid.

[0062] In some embodiments, the preparation method of acidified carbon nanotubes includes the following steps: Carbon nanotubes and a mixed acid solution were mixed, dispersed, and then subjected to solid-liquid separation. The solid product after solid-liquid separation was washed with water until the pH reached 6-7, and finally vacuum dried to obtain acidified carbon nanotubes.

[0063] In some embodiments, the ratio of carbon nanotubes to mixed acid solution is 1g:200mL.

[0064] In some embodiments, the mixed acid solution includes a sulfuric acid solution and a nitric acid solution; the volume ratio of the sulfuric acid solution to the nitric acid solution is 3:1, the concentration of the sulfuric acid solution is 98 wt%, and the concentration of the nitric acid solution is 65~68 wt%.

[0065] In some embodiments, the vacuum drying temperature is 80°C and the vacuum drying time is 12 hours.

[0066] In some embodiments, the solvent includes water and / or alcohol solvents.

[0067] In some embodiments, the drying temperature is 80~110°C, and the drying time is 10~14h. As an example, the drying temperature can be 80°C, 90°C, 100°C, and 110°C, etc., and the drying time can be 10h, 11h, 12h, 13h, and 14h, etc.

[0068] In some embodiments, the calcination is staged calcination.

[0069] In some embodiments, the conditions for the first stage of calcination in the staged calcination are as follows: the heating rate is 3~5℃ / min, the final temperature is 300~350℃, the holding time at the final temperature is 4~8h, and the atmosphere is an inert atmosphere. As an example, the heating rate can be 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, and 5℃ / min, etc.; the final temperature can be 300℃, 310℃, 320℃, 330℃, 340℃, and 350℃, etc.; the holding time at the final temperature can be 4h, 5h, 6h, 7h, and 8h, etc.; and the inert atmosphere includes, but is not limited to, nitrogen, argon, etc.

[0070] In some embodiments, the conditions for the second stage of calcination in the staged calcination are as follows: a heating rate of 3~5℃ / min, a final temperature of 700~850℃, a holding time at the final temperature of 8~10h, and an inert atmosphere. As an example, the heating rate can be 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, and 5℃ / min, etc.; the final temperature can be 700℃, 720℃, 750℃, 800℃, and 850℃, etc.; the holding time at the final temperature can be 8h, 8.5h, 9h, 9.5h, and 10h, etc.; and the inert atmosphere includes, but is not limited to, nitrogen, argon, etc.

[0071] In some embodiments, after calcination, the process further includes: cooling the resulting product to room temperature, and then grinding and sieving it.

[0072] A third aspect of the present invention also provides a battery comprising a positive electrode, wherein the positive electrode comprises the above-described positive electrode material or a positive electrode material prepared according to the above-described preparation method.

[0073] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings and embodiments. The embodiments of this application are only examples, and all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0074] Example 1 The preparation of the cathode material includes the following steps: With 0.00505 mol Na3V 1.925 Mg 0.075 Based on (PO4)3 (2.29 g), according to the polyquaternary ammonium salt and the core (Na3V) 1.925 Mg 0.075 (PO4)3) Polyquaternium-10 (PQ-10) was added at a mass ratio of 1:6%, with an addition amount of approximately 0.137g.

[0075] (1) Dissolve 0.137 g of PQ-10 in 40 mL of water, stirring at 100 r·min. -1 The mixture was stirred at a certain speed for 0.5 h to obtain a gel-like solution. Then, 0.007575 mol sodium carbonate, 0.00486 mol vanadium pentoxide, 0.01515 mol ammonium dihydrogen phosphate, 0.00038 mol magnesium acetate tetrahydrate and 0.007575 mol citric acid monohydrate were dispersed into the gel-like solution and stirred at 100 r·min in a ball magnetic stirrer. -1 Stirring at a certain speed for 2 hours yielded a first slurry of yellow liquid; (2) Dissolve 1.0g of CNTs in 200mL of mixed acid solution (volume ratio of sulfuric acid solution (98wt%): nitric acid solution (65wt%) = 3:1), disperse by ultrasonication for 8h, filter, wash with distilled water until pH is 7, and then vacuum dry at 80℃ for 12h to obtain acidified CNTs. Dissolve 0.0458g of acidified CNTs in 20mL of distilled water, disperse by ultrasonication for 1h to obtain the second slurry.

[0076] (3) Pour the second slurry into the first slurry, and stir at 150 r·min -1 Stirring at a certain speed for 2 hours yielded the precursor slurry.

[0077] (4) The precursor slurry was dried in a forced-air drying oven at 80°C for 10 hours, then ground in an agate mortar for 10 minutes and passed through a 400-mesh sieve to obtain the precursor material. (5) The precursor material was transferred to a tube furnace and sintered in two stages under argon protection. The temperature was increased to 350°C at a rate of 5°C / min and held at 350°C for 8 hours. Then the temperature was increased to 800°C at a rate of 5°C / min and held at 800°C for 8 hours. The temperature was then reduced to room temperature and ground through a 400-mesh sieve to obtain the cathode material Na3V. 1.925 M 0.075 (PO4)3 / CN@CNT, denoted as NVMP / CN@CNT-2; the cathode material was found to contain 6.9 wt% carbon and 0.74 wt% nitrogen.

[0078] Example 2 The preparation of the cathode material differs from that in Example 1 in that the amount of polyquaternium-10 used is 0.0687g, and the resulting cathode material is denoted as NVMP / CN@CNT-1.

[0079] Everything else is the same as in Example 1.

[0080] Example 3 The preparation of the cathode material differs from that in Example 1 in that the amount of polyquaternium-10 used is 0.206g, and the resulting cathode material is denoted as NVMP / CN@CNT-3.

[0081] Everything else is the same as in Example 1.

[0082] Example 4 The preparation of the cathode material differs from that in Example 1 in that the amount of CNTs used is 0.0229g.

[0083] Everything else is the same as in Example 1.

[0084] Example 5 The preparation of the cathode material differs from that in Example 1 in that the amount of CNTs used is 0.0687g.

[0085] Everything else is the same as in Example 1.

[0086] Example 6 The preparation of the positive electrode material differs from that in Example 1 in that magnesium acetate tetrahydrate was not added, and the amount of zinc acetate dihydrate was 0.00038 mol.

[0087] Everything else is the same as in Example 1.

[0088] Example 7 The preparation of the cathode material differs from that in Example 1 in that magnesium acetate tetrahydrate was not added, and the amount of tetrabutyl titanate was 0.00038 mol.

[0089] Everything else is the same as in Example 1.

[0090] Example 8 The preparation of the positive electrode material differs from that in Example 1 in that magnesium acetate tetrahydrate was not added, and the amount of ferric nitrate nonahydrate was 0.00038 mol.

[0091] Everything else is the same as in Example 1.

[0092] Comparative Example 1 The preparation of the cathode material differs from that in Example 1 in that magnesium acetate tetrahydrate, polyquaternium-10, and CNTs were not added, resulting in the cathode material Na3V2(PO4)3 / C, also known as NVP / C.

[0093] Everything else is the same as in Example 1.

[0094] Comparative Example 2 The preparation of the cathode material differs from that in Example 1 in that polyquaternium-10 was not added, and the amount of melamine used was 0.137g. Everything else is the same as in Example 1.

[0095] Comparative Example 3 The preparation of the cathode material differs from that in Example 1 in that: polyquaternium-10 was not added, and the amount of polyvinylpyrrolidone used was 0.137g. Everything else is the same as in Example 1.

[0096] Battery assembly and performance testing 1. Battery assembly The positive electrode materials prepared in the examples and comparative examples were used as positive electrode materials for sodium-ion batteries. They were mixed with a conductive agent (acetylene black) and a binder (polyvinylidene fluoride) in a 7:2:1 ratio in a methylpyrrolidone solvent to obtain a slurry. The slurry was coated onto an aluminum current collector to obtain the positive electrode. A coin cell was assembled in an argon glove box, using sodium metal as the counter electrode and glass fiber as the separator. Finally, two electrolytes were injected: the electrolyte for room temperature testing was a 1M NaClO4 solution of ethylene carbonate (EC) / propylene carbonate (PC) (1:1, v / v) containing 5wt% fluoroethylene carbonate (FEC) additive; the electrolyte for low-temperature testing was a 0.5M NaPF6 solution of DIGLYME.

[0097] 2. Performance Testing (1) Figure 1 The diagram shows the formation mechanism of the cathode materials in Examples 1-5. Figure 1 It is known that PQ-10 is an environmentally friendly, low-cost cellulose derivative containing hydroxyl (-OH) and quaternary ammonium (-N) groups. +(CH3)3) can enhance the hydrophilicity and impermeability of the membrane through hydrogen bonding and electrostatic interactions, while the quaternary ammonium cations (-N) in it... + (CH3)3) Its uniform distribution along the polymer chain enables it to react with anions (e.g., VO) in solution. 3- PO4 3- CO3 2- and CH3COO - Strong electrostatic interactions occur, promoting uniform nucleation, refining particle size, and forming a confined carbon layer, thus creating a thin and uniform electrode-electrolyte interface (CEI) and shortening the Na+-electrolyte transition time. + The transport path is optimized, reducing the interface resistance and enabling rapid charge transfer. This mechanism is illustrated in equation (1).

[0098] PQ-10 n+ +X m− →PQ-10....X (1) In this context, X represents anion, and "..." indicates multiple anions. During pyrolysis, PQ-10 forms a nitrogen-doped carbon layer, enhancing the interfacial electronic structure and charge transport. It works synergistically with carbon nanotubes to form a continuous ion-electron transport network, promoting long-range electron conduction and accelerating Na+ transport. + The interface spreads.

[0099] (2) The cathode materials prepared in Examples 1-3 and Comparative Example 1 were characterized, and the results are as follows: Figures 2-7 As shown.

[0100] Figure 2 The image shows the XRD patterns of the cathode materials prepared in Examples 1-3 and Comparative Example 1. Figure 2 It can be seen that the positions of all diffraction peaks of the cathode material NVMP / CN@CNT in Examples 1-3 and the cathode material NVP / C in Comparative Example 1 are consistent with those of the standard card PDF#053-0018, and all belong to R. The crystal structure of NVP is rhombohedral with c space group and no impurity peaks were detected. At the same time, the amplified peaks of (211) and (116) indicate that Mg doping and PQ-10 modulation did not affect the crystal structure of NVP.

[0101] Figure 3 The figure shows XPS images of the cathode materials prepared in Example 1 and Comparative Example 1, wherein... Figure 3 Image (a) is a full XPS image, revealing the presence of C, N, Na, O, Mg, and V in the cathode material NVMP / CN@CNT-2 in Example 1, indicating the successful introduction of Mg and N. Figure 3In Figure (b), the N1s spectrum shows peaks at 403.2, 401.5, and 399.2 eV, which can be attributed to graphite N, pyrrole N, and pyridine N, respectively. No N peaks were detected in the NVP / C cathode material sample of Comparative Example 1, confirming that N elements in PQ-10 are doped into the carbon layer structure as an N source. Figure 3 In Figure (c), the 1s spectrum of Mg is shown. The peak at 1303.8 eV of NVMP / CN@CNT-2 proves the successful doping of Mg, which was not observed in Comparative Example 1.

[0102] Figure 4 The diagram shows the conductivity of the cathode materials prepared in Examples 1-3 and Comparative Example 1. The conductivity of the cathode material NVMP / CN@CNT in Examples 1-3 is significantly higher than that of the cathode material NVP / C in Comparative Example 1. Specifically, the conductivity of the cathode material NVMP / CN@CNT-2 in Example 1 is 5.34 × 10⁻⁶. -3 S cm -1 This is higher than the 1.16 × 10⁻⁶ NVP / C of the cathode material in Comparative Example 1. -3 S cm -1 Since Mg doping mainly regulates ion transport and reaction pathways in this system, its contribution to electronic conductivity is much weaker than that of the construction of continuous carbon phase. Therefore, it is not considered a dominant factor in conductivity analysis. This significant enhancement of conductivity can be attributed to the addition of nitrogen-doped carbon and acidified carbon nanotubes, which increases the conductivity of the material.

[0103] Figure 5 The image shown is a scanning electron microscope (SEM) image of the cathode material prepared in Example 1 and Comparative Example 1. Figure 5 In Figure (a), the cathode material is shown in Comparative Example 1. Figure 5 (b) shows the positive electrode material in Example 1. Figure 5 It can be seen that the cathode material NVMP / CN@CN-2 in Example 1 has a more uniform particle size, indicating that PQ-10 plays a key regulatory role in suppressing particle growth and obtaining uniformly distributed particles.

[0104] Figure 6 The figure shown is a characterization diagram of the cathode materials prepared in Example 1 and Comparative Example 1, wherein... Figure 6 (a) is a TEM image of the cathode material prepared in Example 1 with a scale of 50 nm; (b) is a TEM image of the cathode material prepared in Example 1 with a scale of 10 nm; (c) is a TEM image of the cathode material prepared in Comparative Example 1 with a scale of 50 nm; (d) is a TEM image of the cathode material prepared in Comparative Example 1 with a scale of 10 nm; and (e) is an EDS image of the cathode material prepared in Example 1. Figure 6As can be seen from (a) to (d), both the cathode material NVMP / C-CNT-2 in Example 1 and the cathode material NVMP / C in Comparative Example 1 are coated with a thin carbon layer. However, the carbon layer in Example 1 is more uniform and thinner (about 10 nm, compared to about 12 nm for NVP / C), and the CNTs attached to its surface form a more continuous conductive network, which helps to improve the interface electron transport efficiency. Figure 6 (e) EDS elemental analysis showed that Na, V, P, O, C, Mg and N were uniformly distributed in NVMP / C-CNT-2, confirming the uniform coating of N-doped carbon layers on the particle surface and uniform Mg doping.

[0105] Figure 7 The figures shown are BET diagrams of the cathode materials prepared in Example 1 and Comparative Example 1, where (a) is the nitrogen adsorption-desorption isotherm and (b) is the pore size distribution curve. Figure 7 It can be seen that NVMP / CN@CNT-2 has better performance than NVP / C (39.7m). 2 g -1 Larger surface area (47.6m²) 2 g -1 The particles exhibit distinct mesoporous characteristics. This indicates that the synergistic regulation of PQ-10 and acidified CNTs effectively inhibits particle aggregation and constructs a more open hierarchical porous structure, which is beneficial for electrolyte permeation and Na+. + Rapid transport enhances the dynamic properties of materials, especially at high rates and low temperatures.

[0106] (3) The batteries composed of the cathode materials prepared in Example 1 and Comparative Example 1 were subjected to performance testing, and the results are as follows: Figures 8-12 As shown.

[0107] Figure 8 The figures show the charge-discharge curves and rate performance test results of the batteries composed of the positive electrode materials prepared in Example 1 and Comparative Example 1. (a) is the charge-discharge curve of the battery composed of the positive electrode material prepared in Comparative Example 1 at -30~10℃ and 0.2C. (b) is the charge-discharge curve of the battery composed of the positive electrode material prepared in Example 1 at -30~10℃ and 0.2C. (c) is the rate performance test result of the batteries composed of the positive electrode materials prepared in Example 1 and Comparative Example 1 at 25℃ and different current densities (0.5~100C). (d) is the rate performance test result of the batteries composed of the positive electrode materials prepared in Example 1 and Comparative Example 1 at -20℃ and different current densities (0.2~30C).

[0108] Depend on Figure 8As shown in (a) and (b), compared with Comparative Example 1, the discharge specific capacity of the cathode material NVMP / C-CNT-2 in Example 1 at -30~10℃ and 0.2C current density is 100.5, 98.5, 94.3, 87.3 and 76.8 mAh / g, respectively, which are all higher than those of the cathode material NVP / C (96.4, 92.5, 85.1, 74.5 and 62.6 mAh / g) prepared in Comparative Example 1. Figure 8 As shown in (c), the average discharge specific capacity of the cathode material NVMP / C-CNT-2 in Example 1 at different current densities was 99.8, 98.8, 96.8, 95.6, 91.7, 89.4, 83.2, and 81.3 mAh / g. However, at the same current density, the cathode material NVP / C prepared in Comparative Example 1 only had 97.7, 96.6, 93.6, 91.3, 82.9, 78, 72.8, and 68.3 mAh / g. The cathode material NVMP / C-CNT-2 in Example 1, especially at 100C (11.7A g), showed a significantly higher discharge specific capacity. -1 Even at extremely high current densities, this electrode still maintains a high reversible capacity, indicating the synergistic effect between CNTs and the PQ-10 derived carbon layer, as well as the effect of Mg. 2+ Doping effectively constructs a stable interface and a multi-scale conductive network, thereby significantly improving Na + / Electron transport dynamics enable excellent rate performance. Figure 8 As shown in Figure (d), the average discharge specific capacity of the cathode material NVMP / C-CNT-2 in Example 1 at different current densities was 92.2, 86.3, 82.2, 79.4, 77.0, 75.7, 71.5, 65.3, and 59.5 mAh / g. However, at the same current density, the cathode material NVP / C prepared in Comparative Example 1 only had 82.7, 78.3, 75.6, 73.2, 68.5, 66.9, 62.4, and 85.5 mAh / g. The cathode material NVMP / C-CNT-2 in Example 1 still maintained high rate performance at low temperatures, indicating that Mg... 2+ The synergistic effect between CNTs and PQ-10 derived carbon layers enhances interfacial ion transport, thus improving structural stability.

[0109] Figure 9 The results show the cycle performance test results of the batteries composed of the cathode materials prepared in Example 1 and Comparative Example 1, where (a) is the cycle test at 80C (9.36A / g) at 25°C and (b) is the cycle test at 10C at -20°C.

[0110] Depend on Figure 9As shown in Figure (a), the cathode material NVMP / C-CNT-2 in Example 1 exhibits significant fast charging capability (approximately 45 s / cycle) and provides a capacity of 83.2 mAh / g; even after 5000 cycles, it retains 74.4% of its capacity. In contrast, the cathode material NVP / C prepared in Comparative Example 1 only achieved a capacity of 49.4 mAh / g after 2491 cycles at the same rate. These results demonstrate that ion doping and interface modulation effectively and significantly improve conductivity, rate performance, and long-cycle stability. Figure 9 As shown in (b), at -20°C, the cathode material NVMP / C-CNT-2 in Example 1 retains 100% capacity retention after 5000 cycles at a current density of 10C. This indicates that the cathode material NVMP / C-CNT-2 in Example 1 maintains high lifetime and rate performance at low temperatures. This further demonstrates that synergistic modification achieves both ionic and electronic conduction, and also improves the Na+ performance at low temperatures. + / Electron transfer efficiency to achieve high-rate performance and low-temperature cycling stability.

[0111] The cathode materials prepared in Example 1 and Comparative Example 1 were subjected to galvanostatic intermittent titration (GITT) to determine the Na content under low-temperature conditions. + Transmission behavior, results as follows Figure 10 As shown. By Figure 10 It can be seen that the cathode material NVMP / C-CNT-2 in Example 1 maintains a high Na content across the entire charge and discharge voltage range. + Diffusion coefficient (10) -7 -10 -12 cm 2 ( / s) further confirms that the interface regulation strategy effectively enhanced Na + The coupling transport dynamics of electrons support its excellent rate capability and cycling performance.

[0112] Figure 11 The figures show the AC impedance spectra of batteries composed of the cathode materials prepared in Example 1 and Comparative Example 1 at temperatures ranging from 10 to -30°C, where (a) is the cathode material in Comparative Example 1 and (b) is the cathode material in Example 1. Figure 11 It can be seen that, compared with the cathode material NVP / C prepared in Comparative Example 1, the cathode material NVMP / CN@CNT-2 in Example 1 maintains a lower charge transfer resistance (Rct) in a temperature range of 10 to -30°C, indicating that its electrode / electrolyte interface is stable and beneficial to Na + The insertion and extraction of these molecules improve low-temperature electrochemical performance.

[0113] After cycling the batteries composed of the cathode materials prepared in Example 1 and Comparative Example 1 at -20°C and 5C for 500 cycles, high-resolution transmission electron microscopy images of the cathode materials in Example 1 and Comparative Example 1 are shown below. Figure 12 As shown, (a) is the cathode material in Comparative Example 1, and (b) is the cathode material in Example 1. Figure 12 As can be seen, in Example 1, the cathode material NVMP / CN@CNT-2 formed a continuous and dense solid electrolyte interface (CEI) layer with a thickness of approximately 1 nm, while the CEI layer of the cathode material NVP / C prepared in Comparative Example 1 was thicker and more uneven. A thin and uniform CEI layer can shorten the Na... + While a thick CEI layer can improve transport paths and reduce impedance, an excessively thick CEI layer can lead to particle breakage, increased interfacial resistance, and reduced ionic conductivity, thereby impairing cycling performance and stability. Considering morphological characteristics, PQ-10 modulates particle size and carbon layer uniformity during precursor preparation, resulting in a stable and consistent CEI layer that enhances interfacial kinetics and improves low-temperature cycling reversibility.

[0114] (4) The following electrochemical performance tests were conducted on the batteries assembled with the cathode materials in Examples 1-8 and Comparative Examples 1-3: The assembled batteries were placed in a 40℃ oven for 12 hours and then subjected to constant current charge-discharge tests. The charge-discharge voltage window was 2.5-4.0V. The specific capacity, rate performance, and long-cycle performance of the sodium-ion battery cathode were measured at room temperature (25℃) and -20℃. The results are shown in Tables 1 and 2.

[0115] Table 1. Electrochemical performance test results of batteries assembled with cathode materials in Examples 1-8 and Comparative Examples 1-3 at room temperature (25°C).

[0116] Table 2. Electrochemical performance test results of batteries assembled with cathode materials in Examples 1-8 and Comparative Examples 1-3 at -20°C and 0.2C.

[0117] As shown in Tables 1 and 2, among the examples and comparative examples, Example 1 exhibits a higher cycle life at room temperature and a high current density of 80C compared to the other comparative examples. At a low temperature of -20°C, it demonstrates a higher initial capacity of 10C and a higher cycle life. This indicates that the performance of the modification method provided by this invention, which combines ion doping and interface synergistic regulation, is superior to that of single doping and other interface regulator modification methods. Furthermore, the doping amount, dopant ions, and type of interface regulator also affect the material's performance. This further demonstrates that the preparation method of this invention, employing specific steps, conditions, and parameters, achieves better overall interaction, resulting in products with superior discharge specific capacity and cycle performance, thereby effectively improving the product's electrochemical performance and low-temperature performance.

[0118] Although preferred embodiments of the invention have been shown and described, it is conceivable that those skilled in the art can devise various modifications to the invention within the spirit and scope of the appended claims.

Claims

1. A positive electrode material, characterized in that, The positive electrode material includes a core and a coating layer covering at least a portion of the surface of the core; The general chemical formula of the core is Na3V. 1.925 M 0.075 (PO4)3, where M is a transition metal element; The coating layer comprises a carbon material, which includes nitrogen-doped amorphous carbon and carbon nanotubes introduced by acidified carbon nanotubes.

2. The cathode material according to claim 1, characterized in that, M is at least one of Zn, Ti, Fe and Mg.

3. The cathode material according to claim 1, characterized in that, Based on the mass of the cathode material, the carbon content in the carbon material is 4wt%~9wt%; Based on the mass of the cathode material, the nitrogen content in the carbon material is 0.5wt%~1wt%.

4. The cathode material according to claim 1, characterized in that, The positive electrode material is R. rhombohedral phase structure of space group c; The specific surface area of ​​the cathode material is 35~60m². 2 g -1 .

5. A method for preparing the cathode material according to any one of claims 1 to 4, characterized in that, Includes the following steps: Sodium source, vanadium source, phosphorus source, transition metal source, nitrogen source, carbon source, acidified carbon nanotubes and solvent are mixed to obtain precursor slurry; The precursor slurry is dried to obtain the precursor material; The precursor material is calcined to obtain the cathode material; The nitrogen source includes polyquaternary ammonium salts.

6. The method for preparing the cathode material according to claim 5, characterized in that, The molar ratio of sodium in the sodium source, vanadium in the vanadium source, phosphorus in the phosphorus source, metal in the transition metal source, and carbon in the carbon source is 3:1.925:3:0.075:(7~11). The mass ratio of the core, polyquaternary ammonium salt, and acidified carbon nanotubes is 1:2%~10%:0.5%~3%.

7. The method for preparing the cathode material according to claim 5, characterized in that, The sodium source includes sodium salt; The vanadium source includes vanadium oxide; The phosphorus source includes phosphate; The transition metal source includes at least one of zinc salt, titanium salt, iron salt, and magnesium salt; The carbon source includes citric acid; The solvent includes water and / or alcohol solvents.

8. The method for preparing the cathode material according to claim 5, characterized in that, The drying temperature is 80~110℃, and the drying time is 10~14h.

9. The method for preparing the cathode material according to claim 5, characterized in that, The calcination is a staged calcination; The conditions for the first stage of the staged calcination are as follows: the heating rate is 3~5℃ / min, the final temperature is 300~350℃, the holding time at the final temperature is 4~8h, and the atmosphere is an inert atmosphere. The conditions for the second stage of calcination in the staged calcination are as follows: the heating rate is 3~5℃ / min, the final temperature is 700~850℃, the holding time at the final temperature is 8~10h, and the atmosphere is an inert atmosphere.

10. A battery comprising a positive electrode, characterized in that the positive electrode comprises the positive electrode material according to any one of claims 1 to 4 or the positive electrode material prepared by the preparation method according to any one of claims 5 to 9.

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