Preparation method and application of ytterbium-doped sodium vanadium phosphate positive electrode material

Abstract

The invention discloses a preparation method and application of an ytterbium-doped sodium vanadium phosphate positive electrode material, and the method is characterized in that a sol-gel method is combined with a high-temperature sintering process, and a modified material with high ionic conductivity and electronic conductivity is successfully synthesized under specific temperature and atmosphere control. Ytterbium doping effectively widens a sodium ion migration channel and stabilizes a crystal structure, in-situ carbon compounding constructs a continuous conductive network, and the electrochemical performance of the material is remarkably improved through the synergistic effect of ytterbium doping and in-situ carbon compounding. The obtained material shows high specific capacity, low charge transfer resistance, high sodium ion diffusion coefficient and excellent long cycle stability in a sodium ion battery, is suitable for the field of high-power and long-life energy storage, is simple and convenient in preparation process and good in repeatability, and has a good industrialization prospect.

Description

Technical Field

[0001] This invention belongs to the field of battery material preparation technology, specifically relating to a method for preparing and applying a ytterbium-doped sodium vanadium phosphate cathode material. Background Technology

[0002] Sodium-ion batteries, as an important supplement to lithium-ion batteries, have shown broad application prospects in large-scale energy storage and low-speed electric vehicles due to the abundance, low cost, and wide distribution of sodium resources. Cathode materials, as the core components determining battery energy density, cycle life, and safety performance, have become a focus of industrialization and academic research. Among numerous cathode material systems, polyanionic compounds, especially sodium vanadium phosphate (Na3V2(PO4)3, or NVP for short) with a sodium superionic conductor (NASICON) structure, stand out due to their three-dimensional open framework and high operating voltage (approximately 3.4 V vs. Na2V2). + With its excellent thermal stability and NVP (Na+), it is considered one of the most promising materials for applications. The theoretical specific capacity of NVP can reach 117.6 mAh g⁻¹. -1 It also exhibits high structural stability and outstanding cycle performance.

[0003] However, the extremely low intrinsic electronic conductivity of NVP severely limits its rate performance and active material utilization. During high-current charge and discharge, significant electrode polarization occurs, leading to rapid capacity decay and making it difficult to meet the demands of high-power applications. Furthermore, the material also suffers from structural degradation and transition metal dissolution during long-cycle cycling, resulting in a gradual decrease in capacity.

[0004] To improve the electrochemical performance of NVPs, researchers have proposed various modification strategies, mainly including three categories: ion doping, surface composites, and nanostructure modulation. Regarding ion doping, common methods include partially substituting vanadium sites with cations such as magnesium and iron to expand sodium ion transport channels, or introducing anions such as fluorine to replace oxygen sites to enhance structural stability. For conductivity modification, carbon sources such as glucose and polypyrrole are typically used to coat the surface of NVP particles with an amorphous carbon layer through in-situ carbothermic reduction, or to be composited with one-dimensional / two-dimensional conductive agents such as carbon nanotubes and graphene to improve overall electronic conductivity. Furthermore, by controlling the synthesis process to prepare nanoscale NVP particles, ion diffusion paths can be effectively shortened, and reaction kinetics improved.

[0005] Although the above methods have achieved some success, they still have significant limitations: single-ion doping has a limited effect on improving electronic conductivity, and high-valence or large-radius doping ions may cause lattice distortion, which is detrimental to Na. +Migration; conventional carbon coating often suffers from problems such as uneven coating layers, weak bonding with active materials, and insufficient graphitization of some carbon structures during high-temperature processing, affecting the continuity and stability of the conductive network. While simple nano-sizing improves rate performance, it is prone to particle agglomeration and reduces electrode tap density, which is detrimental to practical applications. Therefore, developing a composite modification method that can synergistically improve electronic and ionic conductivity and enhance structural stability has become crucial for the practical application of NVP cathode materials. Summary of the Invention

[0006] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0007] In view of the problems existing in the above and / or prior art, the present invention is proposed.

[0008] Therefore, the purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing ytterbium-doped sodium vanadium phosphate cathode material.

[0009] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for preparing a ytterbium-doped sodium vanadium phosphate cathode material, characterized in that it includes: Deionized water was heated to 70°C, citric acid (C6H8O7) was added and heated and stirred continuously, keeping the water temperature constant at 70°C until the citric acid was completely dissolved, thus obtaining a citric acid solution. Add vanadium source to citric acid solution and stir until completely dissolved. Then add ytterbium source, phosphorus source and sodium source in sequence and stir until a clear blue precursor solution is formed. The precursor solution was continuously heated until it reached a gel state, and then dried in a vacuum oven. The dried product was then ground to obtain a light green precursor powder. The precursor powder was preheated and sintered in a nitrogen atmosphere, and after cooling, a black powdery ytterbium-doped sodium vanadium phosphate cathode material was obtained.

[0010] In a preferred embodiment of the preparation method described in this invention, the vanadium source includes one of ammonium metavanadate (NH4VO3) and vanadium pentoxide (V2O5), the ytterbium source is ytterbium nitrate (Yb(NO3)3), the phosphorus source is ammonium dihydrogen phosphate (NH4H2PO4), and the sodium source includes one of sodium acetate (CH3COONa), sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), and sodium hydroxide (NaOH).

[0011] As a preferred embodiment of the preparation method described in this invention, the molar ratio of the sodium source, vanadium source, ytterbium source, phosphorus source, and citric acid is 3:2-X:X:3:2, wherein 0.01≤X≤0.04.

[0012] In a preferred embodiment of the preparation method described in this invention, the drying temperature is 100~120℃ and the drying time is 12h.

[0013] In a preferred embodiment of the preparation method described in this invention, the preheating temperature is 400~500℃ and the preheating time is 4h.

[0014] In a preferred embodiment of the preparation method described in this invention, the sintering temperature is 650~750℃ and the sintering time is 6h.

[0015] In a preferred embodiment of the preparation method described in this invention, the heating rate for preheating and sintering is 2~10 ℃ / min.

[0016] Another objective of this invention is to overcome the shortcomings of the prior art and provide a ytterbium-doped carbon composite sodium vanadium phosphate cathode material, characterized in that: the material has an initial discharge specific capacity ≥ 112.50 mAhg. -1 The first cycle coulombic efficiency is ≥98.27%, and the sodium ion diffusion coefficient is ≥1.29×10⁻⁶. -10 cm 2 S -1 .

[0017] Another objective of this invention is to overcome the shortcomings of the prior art and provide an application of ytterbium-doped sodium vanadium phosphate cathode material in the preparation of sodium-ion batteries for large-scale energy storage systems or electric vehicles.

[0018] In a preferred embodiment of the application described in this invention, the sodium-ion battery includes the positive electrode material, the hard carbon negative electrode, the electrolyte, and the separator.

[0019] Beneficial effects of this invention: This invention provides a method for preparing ytterbium-doped sodium vanadium phosphate cathode materials, enhancing electronic and ion conductivity through a ytterbium (Yb) doping modification strategy. Ytterbium doping introduces sodium vacancy defects and optimizes the vanadium valence state distribution, establishing more efficient small polaron transition channels and widening the sodium ion migration channels, thus lowering the energy barrier for ion migration and significantly improving the apparent electronic conductivity of the material. Simultaneously, the carbon coating layer constructs a three-dimensional continuous electron network on the surface of the active particles, effectively reducing the interfacial contact resistance. This simultaneous enhancement of ion and electron transport capabilities allows the material to maintain high capacity retention and a stable voltage plateau even under high-rate charge-discharge conditions. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein: Figure 1 The images show X-ray diffraction patterns and partial magnified views of the ytterbium-doped sodium vanadium phosphate cathode material and the undoped sodium vanadium phosphate cathode material prepared in Comparative Example 1 and Example 1 of this invention.

[0021] Figure 2 (a) is the Raman spectrum of the undoped sodium vanadium phosphate cathode material prepared in Comparative Example 1 of this invention. Figure 2 (b) is the Raman spectrum of the ytterbium-doped sodium vanadium phosphate cathode material prepared in Example 1 of the present invention.

[0022] Figure 3 (a, b) are high-resolution transmission electron microscope images of the ytterbium-doped sodium vanadium phosphate cathode material prepared in Example 1 of the present invention, and (c) is the fast Fourier transform spectrum of the ytterbium-doped sodium vanadium phosphate cathode material prepared in Example 1 of the present invention.

[0023] Figure 4 The first charge-discharge curves at 0.1C are shown for the ytterbium-doped sodium vanadium phosphate cathode material and the undoped sodium vanadium phosphate cathode material prepared in Comparative Example 1 and Example 1 of this invention.

[0024] Figure 5 The rate performance of the ytterbium-doped sodium vanadium phosphate cathode material and the undoped sodium vanadium phosphate cathode material prepared in Comparative Example 1 and Example 1 of this invention at different rate ranges.

[0025] Figure 6 The graph shows the cycling performance of the ytterbium-doped sodium vanadium phosphate cathode material and the undoped sodium vanadium phosphate cathode material prepared in Comparative Example 1 and Example 1 of this invention at 1C.

[0026] Figure 7 The Nyquist plots and equivalent circuit models of the ytterbium-doped sodium vanadium phosphate cathode material and the undoped sodium vanadium phosphate cathode material prepared in Comparative Example 1 and Example 1 of this invention are shown.

[0027] Figure 8 The ytterbium-doped sodium vanadium phosphate cathode material and the undoped sodium vanadium phosphate cathode material Z prepared in Comparative Example 1 and Example 1 of this invention are shown. re With ω −0.5 Correspondence.

[0028] Figure 9The rate performance of the ytterbium-doped sodium vanadium phosphate cathode materials prepared in Examples 1-3 of this invention at different rate ranges are shown.

[0029] Figure 10 The rate performance of the ytterbium-doped sodium vanadium phosphate cathode materials prepared in Comparative Example 2 and Example 2 of this invention at different rate ranges is shown.

[0030] Figure 11 The rate performance of the ytterbium-doped sodium vanadium phosphate cathode materials prepared in Comparative Example 3 and Example 3 of this invention at different rate ranges is shown. Detailed Implementation

[0031] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0032] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0033] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0034] Unless otherwise specified, all raw materials used in this invention are commercially available in the field.

[0035] In this embodiment of the invention, X-ray diffraction (XRD, Bruker-AXS D8) was used to determine the crystal structure of the sample at a scanning speed of 5 μmin. -1 The obtained XRD patterns, ranging from 10 μm to 80 μm, were analyzed using Jade 9.0 software. Rietveld refinement of the material was performed using GSAS II software to obtain more accurate positional and structural information. Raman spectroscopy was conducted using a LabRam HR Evolution instrument, employing a 532 nm laser to determine the state of the carbon material. The crystal structure was studied using a high-resolution transmission electron microscope (HRTEM, JEM-2100 F).

[0036] Na3V in the embodiments of the present invention 2-x Yb xThe electrochemical properties of the (PO4)3 / C sample were evaluated using a 2032 type half-cell assembled in a nitrogen-filled glove box. The anode consisted of a 14 mm diameter pure sodium sheet, while the cathode electrode material comprised a mixture of active material, acetylene black, and polyvinylidene chloride (PVDF) in a mass ratio of 8:1:1. First, the weighed materials were thoroughly ground in a mortar, then an appropriate amount of N-methylpyrrolidone was added. Finally, the slurry was poured onto clean aluminum foil, evenly coated using a four-sided spreader, and dried in a vacuum drying oven at 120 °C for 12 h. Subsequently, the obtained electrode sheet was stamped into a 12 mm diameter disc with an average loading density of 1.5 mg / cm³. 2 The electrode mass was weighed. The electrolyte was 1.0 M NaClO4 (1.0 M) in a mixture of ethylene carbonate and propylene carbonate (1:1, v / v), with 5% fluoroethylene carbonate added. Electrochemical performance tests were performed at room temperature. First, the assembled battery was allowed to stand for 12 hours to stabilize. Electrochemical impedance spectroscopy was performed using an electrochemical workstation at a frequency range of 0.01 Hz to 100 kHz and an amplitude of 10 mV. Constant current charge-discharge tests were performed using the Xinwei Battery Testing System at 2.3 to 4.1 V (vs. NaClO4). + The test was completed within a voltage window of / Na, specifically including: one initial cycle, five cycles at different rates, and a long-cycle stability test of 500 cycles.

[0037] Example 1 This embodiment provides a method for preparing a ytterbium-doped sodium vanadium phosphate cathode material. In this embodiment, the molar ratio of sodium source, vanadium source, ytterbium source, phosphorus source, and citric acid is 3:1.97:0.03:3:2, specifically: (1) After heating 120 mL of deionized water to 70 °C, 10 mmol of C6H8O7 was dissolved in 120 mL of deionized water and stirred continuously. Then, the water temperature was monitored with a thermometer and kept constant at 70 °C. After stirring thoroughly to dissolve the solution, 9.85 mmol of NH4VO3 was added. After stirring for more than ten minutes, the final solution became a clear blue precursor solution.

[0038] (2) Add 0.15 mmol Yb(NO3)3 to the precursor solution. After it is fully dissolved, add 15 mmol NH4H2PO4, and finally add 15 mmol CH3COONa to the precursor solution. After stirring for several hours, the blue precursor solution will turn into a light green gel. Then, put the gel into a vacuum oven and dry it at 120°C for 12 hours to obtain a light green solid. Then, grind the dried solid into precursor powder.

[0039] (3) The precursor powder was preheated at 450°C for 4 hours in N2 atmosphere (heating rate 5°C / min), and then sintered at 700°C for 6 hours in N2 atmosphere (heating rate 5°C / min). After natural cooling, ytterbium-doped sodium vanadium phosphate cathode material was obtained.

[0040] Comparative Example 1 Comparative Example 1 is based on Example 1. The difference between Comparative Example 1 and Example 1 is that in step (2), 10 mmol of NH4VO3 is added, and in step (3), Yb(NO3)3 is not added. The remaining steps are the same as in Example 1, and the undoped sodium vanadium phosphate cathode material of Comparative Example 1 is obtained.

[0041] Test case (1) such as Figure 1 As shown, the XRD patterns of the prepared Comparative Example 1 and Example 1 samples are displayed, along with magnified views of the crystal planes corresponding to the (012) orientation. The rare earth ion Yb was investigated. 3+ The effect of doping on the crystal structure of Na3V2(PO4)3 / C. The results show that the optimally doped sample has similar diffraction peaks to the undoped Na3V2(PO4)3 / C, and these peaks match well with the diffraction peaks of the rhombic NVP phase (PDF #97-024-8140) in the R-3c space group. Furthermore, no impurity phase peaks were detected in the prepared samples, indicating that no impurity phase was formed during the doping process. In addition, the diffraction peaks of each sample are very narrow, indicating that each modified sample has high crystallinity. Clearly, compared with the undoped Na3V2(PO4)3 / C, the optimally doped sample has stronger peak intensities, indicating that the optimally doped sample has higher crystallinity, which is consistent with subsequent experimental results.

[0042] (2) To further determine the properties of the carbon materials in the samples, Raman spectroscopy was performed on the composite materials of Comparative Example 1 and Example 1. like Figure 2 As shown in a and b, analysis of the Raman spectra of the modified composite materials of Comparative Example 1 and Example 1 clearly reveals a pair of important characteristic peaks specific to carbon materials. Among them, the peak at 1350.12 cm⁻¹ is particularly prominent. -1 The peak at 1585.93 cm corresponds to the D peak, while the peak at 1585.93 cm corresponds to the D peak. -1 The peak at point D corresponds to the G peak. The D peak is a vibrational mode caused by disorder, mainly related to boundaries and defects. It reflects defects, impurities, or edge effects in the sample. The G peak, on the other hand, is caused by the symmetric phonon vibrations of carbon atoms in the sp² hybridization state, reflecting the stretching vibrations of carbon-carbon bonds. For the composite material of Example 1, the intensity ratio of the D and G peaks (I... D / I GThe value is 0.76, indicating that the carbon present in the material is graphitized carbon and disordered carbon. The lower I... D / I G The ratio indicates a higher degree of graphitization of the carbon layer coated on the material surface, fewer defects, and a more ordered structure. The full width at half maximum (FWHM) of the D peak of the composite material in Comparative Example 1 is equal to that of the composite material in Example 1, indicating that both composite materials in Comparative Example 1 and Example 1 contain the same defects, disorder, or impurities. However, the full WHM of the G peak of the composite material in Comparative Example 1 is greater than that of the composite material in Example 1, indicating that the carbon layer of the composite material in Example 1 has a higher degree of graphitization than that of the composite material in Comparative Example 1. Therefore, the composite material in Example 1 exhibits much better electronic conductivity than the composite material in Comparative Example 1.

[0043] (3) To further characterize the microstructure, thickness distribution and Yb doping effect on the lattice structure of the carbon coating, high-resolution transmission electron microscopy (HRTEM) was used for analysis, and the crystal structure changes were analyzed by fast Fourier transform (FFT).

[0044] like Figure 3 As shown in figure a, high-resolution transmission electron microscopy (HRTEM) images reveal a uniform coating layer on the particle surface, approximately 5 nm thick, corresponding to a carbon coating structure. Notably, the particles exhibit distinct lattice fringes, indicating good crystallinity of the sample. Meanwhile, as... Figure 3 As shown in b and c, the high-resolution images also reveal a distance of 0.3848 nm between two adjacent lattice fringes, attributed to the (024) crystal plane. This value is significantly larger than that of the original NVP crystal (0.31 nm), consistent with XRD analysis.

[0045] (4) Electrochemical performance testing, specifically: ① All composite materials underwent their first constant current charge-discharge (GCD) test at a current density of 0.1C, specifically as follows: Figure 4 As shown, it is clear that a distinct flat voltage plateau appears at 3.4 V, which corresponds to V 3+ / V 4+ The redox reaction occurs between the samples. Furthermore, compared to the sample in Comparative Example 1, the capacity of the improved sample is significantly increased. It can be clearly seen that the sample in Example 1 has the optimal capacity, with an initial charging capacity of 114.48 mAhg. -1 The discharge specific capacity reached 112.50 mAhg. -1 It also boasts an impressive first-cycle coulombic efficiency of 98.27%, while the initial charge capacity of Comparative Example 1 sample was only 86.04 mAhg. -1 The discharge specific capacity is 84.93 mAh g.-1 This indicates that Yb 3+ The amorphous carbon layer, integrated into the NVP sample framework, expands ion channels. It also protects the structural stability of the sample, enhances electron conduction, and improves interfacial stability, significantly improving the electrochemical performance of the NVP material. Notably, irreversible capacity was observed in all prepared samples, which can be attributed to electrolyte decomposition and the formation of a solid electrolyte interphase (SEI) film at the interface between the solid material and the electrolyte.

[0046] ② Rate performance testing at different current densities, specifically: such as Figure 5 As shown, the capacity of both Comparative Example 1 and Example 1 samples decreased with increasing current density. Example 1 sample exhibited the best rate performance, displaying capacities of 106.64, 105.02, 102.97, 100.27, 97.09, 93.68, and 106.28 mAhg at current densities of 1, 2, 4, 6, 8, 10, and 1 C, respectively. -1 High reversible specific capacity. In contrast, the reversible discharge capacities of the Comparative Example 1 sample at 1, 2, 4, 6, 8, 10, and 1C were only 74.21, 71.1, 67.38, 64.84, 62.71, 60.76, and 74.43 mAhg, respectively. -1 Compared to the sample in Comparative Example 1, the rate performance of the sample in Example 1 was significantly improved, which is attributed to the lower charge transfer resistance and higher sodium ion diffusion coefficient of the modified sample in Example 1.

[0047] ③ Long-cycle stability test, specifically: Figure 6 The results of the long-term stability test of the prepared Comparative Example 1 and Example 1 samples at 1C rate for 500 cycles show that the optimal doped Example 1 achieved a discharge capacity of 106.64 mAh g after the first charge. -1 It exhibited a high retention rate of 92.47% after 500 cycles, while the first discharge capacity of the Comparative Example 1 sample was only 78.86 mAhg. -1 The retention rate was 86.9%, and the modified sample of Example 1 showed certain improvements in capacity and long-cycle stability.

[0048] ④ For example Figure 7 As shown, Nyquist curves for the samples of Comparative Example 1 and Example 1 were plotted in the frequency range of 0.01–100 kHz, and the fitted equivalent circuit diagram of the electrochemical process was interpolated. Each curve in the semi-circular high-frequency region corresponds to the charge transfer resistance (Rc). ct The amplitude curve corresponds to the Warburg impedance (Z) representing the sodium ion diffusion process in the diagonal low-frequency region. re )diagonal.

[0049] Based on the fitted equivalent circuit, the charge transfer resistance of the modified Example 1 sample can be calculated to be 152.9 Ω, which is significantly lower than the 557.8 Ω of the NVP. Clearly, the significant reduction in charge transfer resistance of the modified sample is mainly due to the Yb 3+ Introducing Na + The expansion of the transshipment channel will benefit Na + Embedding and de-embedding.

[0050] pass Figure 8 In the low-frequency region Z re The D of the sample is calculated from the fitting result of the square root of the reciprocal frequency. Na+ D can be calculated using the following equations (Eq.2) (Eq.3). Na+ : Z re = R s + R ct +σ ω⁻ 0.5 (2) D Na+ = (3) in, R It is the gas constant; T It is absolute temperature; A It is the surface area of ​​the active cathode; N It is the number of electrons in each molecule during the redox process; F It is Faraday's constant; C σ is the concentration of sodium ions in the electrolyte; σ is related to... Z re The relevant Warburg coefficient.

[0051] Table 1. D values ​​of Comparative Example 1 and Example 1 samples calculated using EIS. Na+ .

[0052] The calculation and fitting results are shown in Table 1. The D of the sample in Example 1... Na+ It is 1.29 × 10 -10 cm 2 S -1 The result is an order of magnitude higher than that of control sample 1. Experimental results show that, through Yb... 3+The doping modification strategy significantly improves the charge transfer kinetics of pure NVP / C cathode materials. On the one hand, ytterbium doping introduces sodium vacancy defects and optimizes the vanadium valence state distribution, establishing more efficient small polaron transition channels and broadening the migration channels of sodium ions, thus lowering the energy barrier for ion migration and significantly improving the ion / electron conduction efficiency of the material. On the other hand, the carbon coating enhances electronic conduction and structural stability. This results in excellent electrochemical performance.

[0053] Example 2 The difference between this embodiment and embodiment 1 is that the molar ratio of sodium source, vanadium source, ytterbium source, phosphorus source and citric acid in this embodiment is 3:1.98:0.02:3:2. Specifically, in step (1), 9.9 mmol NH4VO3 is added, and in step (2), 0.1 mmol Yb(NO3)3 is added. The remaining steps are the same as in embodiment 1, and the ytterbium-doped sodium vanadium phosphate cathode material of this embodiment is obtained.

[0054] Example 3 The difference between this embodiment and embodiment 1 is that the molar ratio of sodium source, vanadium source, ytterbium source, phosphorus source and citric acid in this embodiment is 3:1.965:0.035:3:2. Specifically, in step (1), 9.825 mmol NH4VO3 is added, and in step (2), 0.175 mmol Yb(NO3)3 is added. The remaining steps are the same as in embodiment 1, and the ytterbium-doped sodium vanadium phosphate cathode material of this embodiment is obtained.

[0055] Comparative Example 2 The difference between this embodiment and Example 1 is that the molar ratio of sodium source, vanadium source, ytterbium source, phosphorus source and citric acid in this comparative example is 3:1.99:0.01:3:2. Specifically, in step (1), 9.95 mmol NH4VO3 is added, and in step (2), 0.05 mmol Yb(NO3)3 is added. The remaining steps are the same as in Example 1, and the ytterbium-doped sodium vanadium phosphate cathode material of this embodiment is obtained.

[0056] Comparative Example 3 The difference between this embodiment and Example 1 is that the molar ratio of sodium source, vanadium source, ytterbium source, phosphorus source and citric acid in this comparative example is 3:1.96:0.04:3:2. Specifically, in step (1), 9.8 mmol NH4VO3 is added, and in step (2), 0.2 mmol Yb(NO3)3 is added. The remaining steps are the same as in Example 1, and the ytterbium-doped sodium vanadium phosphate cathode material of this embodiment is obtained.

[0057] like Figure 9As shown in the figure, the specific capacity of samples from Examples 1-3 decreases with increasing current density. Example 1 exhibits the best rate performance, with capacities of 106.64, 105.02, 102.97, 100.27, 97.09, 93.68, and 106.28 mAhg at rates of 1, 2, 4, 6, 8, and 10C, respectively. -1 The reversible discharge capacities of Examples 2 and 3 at 1, 2, 4, 6, 8, 10, and 1C were 101.95, 100.61, 97.79, 93.54, 89.31, 85.11, and 101.52 mAhg, respectively. -1 And 103.55, 100.61, 97.29, 90.68, 85.31, 80.67 and 105.21 mAhg -1 The comparison shows that the modified sample exhibits a significantly higher reversible capacity than NVP, with the sample showing the best performance when the ytterbium doping concentration is 0.03%. The excellent rate performance indicates that the structure of the doped sample possesses superior stability; its spatial structure remains stable after testing at different rate rates, which is beneficial for the cycling of the cathode material at high current densities.

[0058] like Figure 10 and 11 As shown in the figure, the rate performance of Example 2 and Comparative Example 2, and Example 3 and Comparative Example 3, under different current densities are compared. Figure 9 The trend is consistent: the specific capacity of all samples decreases significantly with increasing current density. However, the example samples maintain a higher overall capacity retention, and the decreasing trend is significantly slower than that of the corresponding comparative samples. The comparison shows that, at different rates, the reversible discharge capacity of the example samples is significantly better than that of the corresponding comparative samples, indicating that the superior doping level effectively improves the structural stability and electrochemical performance at high rates.

[0059] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the present invention.

Claims

1. A method for preparing a ytterbium-doped sodium vanadium phosphate cathode material, characterized in that: include, Deionized water was heated to 70°C, citric acid (C6H8O7) was added and heated and stirred continuously, keeping the water temperature constant at 70°C until the citric acid was completely dissolved, thus obtaining a citric acid solution. Add vanadium source to citric acid solution and stir until completely dissolved. Then add ytterbium source, phosphorus source and sodium source in sequence and stir until a clear blue precursor solution is formed. The precursor solution was continuously heated until it reached a gel state, and then dried in a vacuum oven. The dried product was then ground to obtain a light green precursor powder. The precursor powder was preheated and sintered in a nitrogen atmosphere, and after cooling, a black powdery ytterbium-doped sodium vanadium phosphate cathode material was obtained.

2. The preparation method according to claim 1, characterized in that: The vanadium source includes one of ammonium metavanadate (NH4VO3) and vanadium pentoxide (V2O5), the ytterbium source is ytterbium nitrate (Yb(NO3)3), the phosphorus source is ammonium dihydrogen phosphate (NH4H2PO4), and the sodium source includes one of sodium acetate (CH3COONa), sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), and sodium hydroxide (NaOH).

3. The preparation method according to claim 1, characterized in that: The molar ratio of sodium source, vanadium source, ytterbium source, phosphorus source and citric acid is 3:2-X:X:3:2, where 0.01≤X≤0.

04.

4. The preparation method according to claim 1, characterized in that: The drying temperature is 100~120℃, and the drying time is 12h.

5. The preparation method according to claim 1, characterized in that: The preheating temperature is 400~500℃, and the preheating time is 4h.

6. The preparation method according to claim 1, characterized in that: The sintering temperature is 650~750℃, and the sintering time is 6h.

7. The preparation method according to claim 1, characterized in that: The heating rate for preheating and sintering is 2~10 ℃ / min.

8. The ytterbium-doped sodium vanadium phosphate cathode material prepared by any one of the preparation methods described in claims 1 to 7, characterized in that: The material has an initial discharge specific capacity ≥112.50 mAhg. -1 The first cycle coulombic efficiency is ≥98.27%, and the sodium ion diffusion coefficient is ≥1.29×10⁻⁶. -10 cm 2 S -1 .

9. The application of the ytterbium-doped sodium vanadium phosphate cathode material as described in claim 8 in the preparation of sodium-ion batteries for large-scale energy storage systems or electric vehicles.

10. The application as described in claim 9, characterized in that: The sodium-ion battery includes the positive electrode material, the hard carbon negative electrode, the electrolyte, and the separator.

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