A sodium ion energy storage battery cathode material based on polyanion and a preparation method thereof

Abstract

This invention relates to the field of battery technology, specifically to a sodium-ion energy storage battery cathode material based on polyanion and its preparation method. The material uses a NASICON-type polyanion compound as a matrix, incorporating transition metal and non-metal composite dopants to achieve precise doping at lattice sites. A dual-coating synergistic structure is formed on the matrix surface: an inner ion-conductive oxide coating and an outer porous conductive carbon coating. The material morphology is a hierarchical porous core-shell nanorod structure with a preferred orientation. This invention addresses the core limitations of existing polyanion cathode materials. Composite doping optimizes the lattice structure, improving conductivity and ion diffusion performance. The dual coating constructs efficient transport pathways, and morphology and crystal facet control enhance structural stability. It exhibits good wide-temperature adaptability, and the preparation process is controllable and suitable for mass production, significantly improving the overall performance of sodium-ion batteries and facilitating industrial applications.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a sodium-ion energy storage battery cathode material based on polyanion and its preparation method. Background Technology

[0002] Sodium-ion energy storage batteries, due to the abundance of sodium resources, low cost, and excellent safety, have broad application prospects in large-scale energy storage, low-speed electric vehicles, and other fields, and have become one of the important alternatives to lithium-ion batteries. As the core component of sodium-ion batteries, the performance of the cathode material directly determines the battery's energy density, cycle stability, rate performance, and high and low temperature adaptability. Therefore, developing high-performance cathode materials is a key breakthrough for the industrialization of sodium-ion batteries.

[0003] Polyanionic cathode materials have become a research hotspot in sodium-ion battery cathode materials due to their open crystal structure, stable electrochemical performance, and good safety. Among them, NASICON-type polyanionic compounds have attracted much attention due to their three-dimensional sodium ion transport channels. However, existing NASICON-type polyanionic cathode materials still have many technical bottlenecks: First, the intrinsic conductivity and sodium ion diffusion rate are low, resulting in poor battery rate performance and difficulty in meeting the requirements of high-power scenarios; Second, lattice distortion and volume expansion are prone to occur during charge and discharge, and the material structure is prone to collapse after long-term cycling, resulting in rapid capacity decay and insufficient cycle stability; Third, doping modification technology is immature, and single-element doping is often used, which makes it difficult to simultaneously optimize electron transport and ion diffusion performance, and the doping element is prone to lattice occupancy disorder, which can damage the integrity of the crystal structure; Fourth, coating modification has defects. A single coating layer cannot simultaneously achieve ion conduction and electron conduction, and the bonding force between the coating layer and the substrate is weak, making it prone to peeling off during cycling and losing its protective function.

[0004] Furthermore, existing materials suffer from insufficient morphology control, often consisting of irregular particles with small specific surface areas and long ion transport paths, further limiting ion diffusion efficiency. Simultaneously, disordered crystal orientation fails to fully utilize the ion transport characteristics of dominant crystal planes. Under extreme temperature environments, the electrochemical reactivity of existing materials significantly decreases, exhibiting poor high- and low-temperature performance, thus limiting their application in wide-temperature range scenarios. In terms of fabrication processes, traditional methods struggle to achieve synergistic control over doping, coating, morphology, and crystal orientation, easily leading to poor material performance consistency and failing to meet industrial production requirements. These issues collectively constrain the performance improvement and industrialization of polyanionic sodium-ion battery cathode materials. Therefore, developing a polyanionic sodium-ion battery cathode material with high conductivity, fast ion diffusion, excellent cycle stability, and wide-temperature adaptability, along with its efficient and controllable fabrication method, has significant practical importance and application value. Summary of the Invention

[0005] (a) Technical problems to be solved

[0006] To address the shortcomings of existing technologies, this invention provides a sodium-ion energy storage battery cathode material based on polyanion and its preparation method.

[0007] (II) Technical Solution

[0008] A cathode material for sodium-ion energy storage batteries based on polyanion, wherein the cathode material uses a NASICON-type polyanion compound as a matrix, the matrix having the general chemical formula Na3M1-xTix(PO4)3; the matrix incorporates a composite dopant of transition metals and nonmetals, wherein the transition metals are one or more of chromium, cobalt, and nickel, and the nonmetals are one or more of nitrogen, boron, and fluorine, the total doping amount of the composite dopant is 0.1-5 at%, the transition metal elements occupy the M-site lattice sites in the matrix, and the nonmetal elements occupy the lattice sites in the matrix. The O-site lattice site of the group; the substrate surface is provided with a double coating layer synergistic structure, the inner layer is an ion-conductive oxide coating layer with a coating amount of 0.3-1.5wt%, and the outer layer is a porous conductive carbon coating layer with a coating amount of 0.5-3wt%; the cathode material is a hierarchical porous core-shell structure nanorod, the nanorod particle size is 80-300nm, the aspect ratio is 3-8, the nanorod contains mesopores and micropores in hierarchical channels, the mesopore pore size is 2-50nm, the micropore pore size is 0.5-2nm, and the total pore volume is 0.08-0.22cm³ / g.

[0009] Preferably, the NASICON-type polyanionic compound matrix is ​​selected from one or more composite matrices of sodium vanadium phosphate, sodium manganese titanium phosphate, and sodium iron vanadium phosphate. The composite matrix is ​​a heterojunction composite, and an electron-rich region is formed at the heterojunction interface. The room temperature conductivity of the matrix is ​​≥1.5×10-6S / cm.

[0010] Preferably, the composite doping element is one of chromium-nitrogen composite doping, cobalt-boron composite doping, and nickel-fluorine composite doping, with the transition metal doping amount being 0.05-3 at and the non-metal doping amount being 0.05-2 at.

[0011] Preferably, the inner ion-conductive oxide coating layer is selected from lithium aluminum phosphate, lithium metavanadate, and lithium titanate, with a coating layer thickness of 5-20 nm and an ionic conductivity ≥2.0×10-4 S / cm; the outer porous conductive carbon coating layer is selected from porous amorphous carbon, graphene, and carbon nanotubes, with a coating layer thickness of 10-35 nm and an electronic conductivity ≥100 S / cm.

[0012] Preferably, in the Na3M1-xTix(PO4)3, M is one or more of vanadium, manganese, and iron, 0.05≤x≤0.35, the matrix cell parameters a are 8.12-8.25Å, c is 21.85-22.10Å, and the cell volume is 1380-1420ų.

[0013] Preferably, the cathode material has a (110) crystal plane preferred orientation structure with a preferred orientation degree ≥85%, and is accompanied by (310) crystal plane auxiliary orientation with an auxiliary orientation degree of 10-15%. The sodium ion bulk diffusion coefficient is ≥1.2×10-12cm² / s, and the interface diffusion coefficient is ≥8.5×10-14cm² / s.

[0014] Preferably, the preparation method of the sodium-ion energy storage battery cathode material based on polyanion includes the following steps:

[0015] S1 precursor preparation: Sodium source, phosphorus source, transition metal source, and titanium source were weighed according to stoichiometric ratio, and a complexing agent was added and dissolved in deionized water. The pH of the solution was adjusted to 3.5-5.5, and the mixture was stirred at room temperature for 2-4 hours. The total molar ratio of the complexing agent to the metal element was 1.2-2:1. Ultrasonic dispersion was used during the stirring process. The ultrasonic power was 150-300W and the ultrasonic time was 30-60 minutes to obtain a uniform and non-agglomerated precursor solution.

[0016] S2 composite doping element is introduced, and transition metal doping source and non-metal doping source are added to the precursor solution. The temperature is raised to 60-80℃ and stirred at a constant temperature for 1-2 hours. The stirring rate is 300-500 r / min. At the same time, inert gas is introduced for protection to achieve in-situ co-doping of doping elements and obtain doped precursor solution.

[0017] S3 hydrothermal morphology control: The doped precursor solution was transferred to a hydrothermal reactor with a filling degree of 60-80%. The temperature was raised to 120-180℃ and reacted at a constant temperature for 8-16 hours with a heating rate of 1-3℃ / min. After naturally cooling to room temperature, the precursor was washed 3-5 times by alternating centrifugation with deionized water and anhydrous ethanol. The precursor was then vacuum dried at 60-80℃ for 10-14 hours to obtain a hierarchical porous core-shell nanorod precursor.

[0018] The S4 double-coating process involves first uniformly mixing the core-shell nanorod precursor with the inner coating source, then introducing an inert atmosphere and heating to 250-320℃ for 1-2 hours at a heating rate of 1-2℃ / min to complete the inner coherent coating. Next, the precursor is uniformly mixed with the outer conductive carbon source and heated to 400-480℃ for 2-3 hours at a heating rate of 2-3℃ / min to complete the outer three-dimensional network conductive carbon coating, thus obtaining the double-coated precursor.

[0019] S5 segmented sintering involves placing the double-coated precursor in an inert atmosphere with a flow rate of 50-100 mL / min, pre-firing at 350-400℃ for 3-5 hours at a rate of 2-5℃ / min to remove impurities and residual organic matter, and then firing at 650-780℃ for 10-14 hours at a rate of 2-5℃ / min.

[0020] After S6 annealing and activation, the temperature is reduced to 200-280℃ at a rate of 1-2℃ / min and held for 2-4 hours. During the holding process, a mixture of hydrogen and inert atmosphere with a volume fraction of 2-5% is introduced. The mixture is then naturally cooled to room temperature and ground through a 300-400 mesh sieve to obtain the target cathode material.

[0021] Preferably, in S1, the sodium source is sodium carbonate or sodium hydroxide, the phosphorus source is ammonium dihydrogen phosphate or diammonium hydrogen phosphate, the transition metal source is the corresponding metal nitrate, the titanium source is tetrabutyl titanate or titanium oxysulfate, the complexing agent is citric acid or ethylene glycol, the mass ratio of deionized water to the total raw material is 5-10:1, and ultrasonic-assisted dispersion can disperse the raw material particle size to 100-200 nm.

[0022] Preferably, the inert atmosphere in S5 is argon or nitrogen with a purity ≥99.99%. The heating rate during the pre-firing stage is controlled at 2-3℃ / min, the heating rate during the main firing stage is controlled at 3-5℃ / min, and the holding time is extended by 1-2 hours when the main firing temperature is higher than 700℃.

[0023] Preferably, the hydrogen gas fraction in the mixed gas in step S6 is controlled to be 2-5%.

[0024] (iii) Beneficial technical effects

[0025] Compared with existing technologies, the beneficial effects of this invention are:

[0026] 1. Through precise transition metal and non-metal composite doping, the dopant elements are accurately positioned in the matrix lattice, effectively optimizing the crystal structure, reducing the ion migration barrier, and significantly improving the intrinsic conductivity and sodium ion diffusion rate of the material, thereby fundamentally improving the rate performance of the material. The dual-coating layer synergistic structure design features an inner layer that forms a coherent interface with the matrix, exhibiting strong bonding and promoting ion conduction, while the outer three-dimensional network conductive carbon layer enhances electron conduction efficiency. Together, they construct a dual ion-electron transport pathway, while effectively inhibiting electrolyte erosion and protecting the integrity of the matrix structure.

[0027] 2. The synergistic regulation of the morphology and preferred crystal orientation of hierarchical porous core-shell nanorods not only increases the specific surface area of ​​the material and shortens the sodium ion transport path, but also effectively alleviates volume expansion during charge and discharge, avoids structural collapse, and significantly improves cycle stability. The synergistic optimization of the overall material structure enables it to maintain excellent electrochemical activity over a wide temperature range, significantly improves its adaptability to high and low temperatures, and broadens its application scenarios. Attached Figure Description

[0028] Figure 1 This is a flowchart of a method for preparing a sodium-ion energy storage battery cathode material based on polyanion disclosed in this invention;

[0029] Figure 2 This is a line graph comparing the 5C discharge specific capacity and interface impedance of the embodiment and the comparative example;

[0030] Figure 3 This is a bar chart comparing the capacity retention rate of the examples and comparative examples after 2000 cycles at 1C and 1000 cycles at 60°C.

[0031] Figure 4 This is a radar comparison chart created by standardizing the dimensions of the performance comparison data of the examples and comparative examples. Detailed Implementation

[0032] according to Figures 1 to 4 The specific embodiments of the present invention are as follows:

[0033] I. Example 1

[0034] 1. Preparation of raw materials

[0035] Sodium source: Sodium carbonate, purity 99.8%, particle size 100-200 mesh; Phosphorus source: Ammonium dihydrogen phosphate, purity 99.7%, water solubility ≥95g / 100mL (20℃); Transition metal source: Vanadium nitrate, purity 99.5%, vanadium content ≥22%; Titanium source: Tetrabutyl titanate, purity 99.5%, viscosity 25mPa・s (25℃); Composite doping source: Chromium nitrate, purity 99.5%, chromium content ≥19%; Urea, purity 99.8%, nitrogen content ≥46.6%; Inner layer coating source: Lithium aluminum phosphate, purity 99.6%, ionic conductivity... The outer conductive carbon source is glucose, 99.9% pure, with a dehydration temperature of 150℃. The complexing agent is citric acid, 99.8% pure, with a water solubility of ≥59.2g / 100mL (20℃). The deionized water has a conductivity of ≤10μS / cm. The anhydrous ethanol has a purity of 99.9%. The argon gas has a purity of 99.99%. The hydrogen gas has a purity of 99.99%.

[0036] 2. Preparation steps

[0037] Preparation of S1 precursor: Sodium carbonate, ammonium dihydrogen phosphate, vanadium nitrate, and tetrabutyl titanate were weighed according to stoichiometric ratios. The corresponding matrix chemical formula is: The titanium doping ratio (x) was 0.2. Citric acid was added as a complexing agent, with a molar ratio of complexing agent to total metal element of 1.5:1. The above raw materials were dissolved in deionized water, with a deionized water to raw material mass ratio of 7:1. The pH of the solution was adjusted to 4.5, and the mixture was stirred at room temperature for 3 hours. During the stirring process, ultrasonic-assisted dispersion was performed simultaneously with an ultrasonic power of 200W for 45 minutes to obtain a homogeneous, non-agglomerated precursor solution.

[0038] S2 composite dopant introduction: Chromium nitrate and urea were added to the precursor solution, with chromium doping amount of 0.8 at, nitrogen doping amount of 0.3 at, and total composite doping amount of 1.1 at. The solution was heated to 70℃ and stirred at a constant temperature of 400 r / min for 1.5 hours. Argon gas was continuously introduced during the stirring process for protection, achieving in-situ co-doping of the dopant elements with precise site occupancy, resulting in a chromium-nitrogen composite doped precursor solution.

[0039] S3 Hydrothermal Morphology Control: The doped precursor solution was transferred to a hydrothermal reactor with a filling degree of 70%. The temperature was increased to 150℃ at a rate of 2℃ / min and held at that temperature for 12 hours. After the reaction, the solution was allowed to cool naturally to room temperature. The product was washed by centrifugation with deionized water and anhydrous ethanol alternately, four times. It was then vacuum dried at 70℃ for 12 hours to obtain a hierarchical porous core-shell structured nanorod precursor.

[0040] S4 Double Coating Treatment: Core-shell nanorod precursors were uniformly mixed with lithium aluminum phosphate powder, and argon gas was introduced as a protective atmosphere. The temperature was increased to 280℃ at a rate of 1.5℃ / min and held for 1.5 hours to complete the inner layer ion-conductive oxide coating, with an inner layer coating amount of 0.8wt%. Subsequently, glucose was added as the outer layer conductive carbon source, and the temperature was further increased to 450℃ at a rate of 2.5℃ / min and held for 2.5 hours to complete the outer layer porous amorphous carbon coating, with an outer layer coating amount of 1.5wt%, thus obtaining the double-coated precursor.

[0041] S5 Segmented Sintering: The double-coated precursor was placed in an argon atmosphere with an argon flow rate of 80 mL / min. The temperature was increased to 380°C at a rate of 3°C / min and pre-fired for 4 hours to remove impurities and residual organic matter from the raw material. Subsequently, the temperature was increased to 720°C at a rate of 4°C / min and sintered for 12 hours to promote ordered crystal growth and ensure that the preferred orientation of the crystal faces met the requirements.

[0042] S6 Annealing and Activation: After sintering, the temperature was lowered to 250℃ at a rate of 1.5℃ / min and held for 3 hours. During the holding process, a mixed gas was introduced, consisting of 3% hydrogen and 97% argon by volume. The mixture was then allowed to cool naturally to room temperature. The cooled product was ground and passed through a 350-mesh standard sieve to obtain the target cathode material.

[0043] 3. Performance Testing

[0044] The target positive electrode material was mixed with conductive carbon black and polyvinylidene fluoride in a mass ratio of 8:1:1, and N-methylpyrrolidone solvent was added. The mixture was stirred to form a uniform slurry, which was then coated onto an aluminum foil current collector and vacuum dried at 110°C for 12 hours to prepare the positive electrode sheet. Using metallic sodium as the counter electrode, glass fiber as the separator, and 1 mol / L NaPF6 / EC-DMC (volume ratio 1:1) as the electrolyte, a CR2032 button cell was assembled. Electrochemical performance tests were conducted at 25℃, -40℃, and 60℃, and the results are as follows: the preferred orientation degree of the (110) crystal plane is 88%, the sodium ion bulk diffusion coefficient is 1.5×10⁻¹²cm² / s; the discharge specific capacity at 0.1C rate is 138mAh / g, and the initial coulombic efficiency is 97%; the capacity retention rate after 2000 cycles at 1C rate is 93%; the discharge specific capacity at 5C rate is 108mAh / g; the capacity retention rate at 0.2C rate under -40℃ low temperature environment is 73%; the capacity retention rate after 1000 cycles at 1C rate under 60℃ high temperature environment is 90%; and the interface impedance is 65Ω.

[0045] II. Example 2

[0046] 1. Preparation of raw materials

[0047] Sodium source: Sodium hydroxide, purity 99.8%, particle size 100-200 mesh; Phosphorus source: Diammonium hydrogen phosphate, purity 99.7%, water solubility ≥72g / 100mL (20℃); Transition metal source: Manganese nitrate, purity 99.5%, manganese content ≥16%; Titanium source: Titanium oxysulfate, purity 99.5%, titanium content ≥19%; Composite doping source: Cobalt nitrate, purity 99.5%, cobalt content ≥20%; Boric acid, purity 99.8%, boron content ≥17%; Inner layer coating source: Lithium metavanadate, purity 99.6%, ionic conductivity... Outer conductive carbon source: graphene, purity 99.5%, number of layers 5-10, specific surface area 150-200 m² / g; Complexing agent: ethylene glycol, purity 99.9%, viscosity... Deionized water, conductivity ≤10μS / cm; anhydrous ethanol, purity 99.9%; nitrogen, purity 99.99%; hydrogen, purity 99.99%.

[0048] 2. Preparation steps

[0049] Preparation of S1 precursor: Sodium hydroxide, diammonium hydrogen phosphate, manganese nitrate, and titanium oxysulfate were weighed according to stoichiometric ratios. The corresponding matrix chemical formula is: The titanium doping ratio (x) was 0.1. Ethylene glycol was added as a complexing agent, with a total molar ratio of complexing agent to metal element of 1.2:1. The above raw materials were dissolved in deionized water, with a total mass ratio of deionized water to raw materials of 5:1. The pH of the solution was adjusted to 3.5, and the mixture was stirred at room temperature for 2 hours. During the stirring process, ultrasonic-assisted dispersion was performed simultaneously with an ultrasonic power of 150W for 30 minutes to obtain a homogeneous, non-agglomerated precursor solution.

[0050] S2 composite dopant introduction: Cobalt nitrate and boric acid were added to the precursor solution, with cobalt doping amount of 0.5 at and boron doping amount of 0.6 at, for a total composite doping amount of 1.1 at. The solution was heated to 60℃ and stirred at a constant temperature of 300 r / min for 2 hours. Nitrogen gas was continuously introduced during the stirring process for protection, achieving in-situ co-doping of the dopant elements with precise site occupancy, resulting in a cobalt-boron composite doped precursor solution.

[0051] S3 Hydrothermal Morphology Control: The doped precursor solution was transferred to a hydrothermal reactor with a filling degree of 60%. The temperature was increased to 130℃ at a rate of 1℃ / min and held at that temperature for 16 hours. After the reaction, the mixture was allowed to cool naturally to room temperature. The product was washed three times by alternating centrifugation with deionized water and anhydrous ethanol, and then vacuum dried at 60℃ for 14 hours to obtain a hierarchical porous core-shell nanorod precursor.

[0052] S4 Double Coating Treatment: Core-shell nanorod precursors were uniformly mixed with lithium metavanadate powder, and nitrogen gas was introduced as a protective atmosphere. The temperature was increased to 250°C at a rate of 1°C / min and held for 2 hours to complete the inner layer ion-conductive oxide coating, with an inner layer coating amount of 0.3 wt%. Subsequently, graphene was added as the outer layer conductive carbon source, and the temperature was further increased to 400°C at a rate of 2°C / min and held for 3 hours to complete the outer layer graphene coating, with an outer layer coating amount of 0.8 wt%, thus obtaining the double-coated precursor.

[0053] S5 Segmented Sintering: The double-coated precursor was placed in a nitrogen atmosphere with a nitrogen flow rate of 50 mL / min. The temperature was increased to 350°C at a rate of 2°C / min and pre-fired for 5 hours to remove impurities and residual organic matter from the raw material. Subsequently, the temperature was increased to 750°C at a rate of 3°C / min and main sintered for 10 hours to promote ordered crystal growth and ensure that the preferred orientation of the crystal faces met the standards.

[0054] S6 Annealing and Activation: After sintering, the temperature is lowered to 200℃ at a rate of 1℃ / min and held for 4 hours. During the holding process, a mixed gas is introduced, consisting of 2% hydrogen and 98% nitrogen by volume. The mixture is then allowed to cool naturally to room temperature. The cooled product is ground and passed through a 300-mesh standard sieve to obtain the target cathode material.

[0055] 3. Performance Testing

[0056] The test conditions were the same as in Example 1, and the test results are as follows: the preferred orientation degree of the (110) crystal plane of the material was 86%, and the bulk diffusion coefficient of sodium ions was... The discharge specific capacity at 0.1C rate is 136mAh / g, with an initial coulombic efficiency of 96.5%; the capacity retention after 2000 cycles at 1C rate is 92.5%; the discharge specific capacity at 5C rate is 106mAh / g; the capacity retention at 0.2C rate is 71% at -40℃; the capacity retention after 1000 cycles at 1C rate is 89% at 60℃; and the interface impedance is 70Ω.

[0057] III. Example 3

[0058] 1. Preparation of raw materials

[0059] Sodium source: Sodium carbonate, purity 99.8%, particle size 100-200 mesh; Phosphorus source: Ammonium dihydrogen phosphate, purity 99.7%, water solubility ≥95g / 100mL (20℃); Transition metal source: Ferric nitrate, purity 99.5%, iron content ≥17%; Titanium source: Tetrabutyl titanate, purity 99.5%, viscosity 25mPa・s (25℃); Composite doping source: Nickel nitrate, purity 99.5%, nickel content ≥21%; Ammonium fluoride, purity 99.8%, fluorine content ≥37%; Inner layer coating source: Lithium titanate, purity 99.6%, ionic conductivity... The outer conductive carbon source is carbon nanotubes, with a purity of 99.5%, a diameter of 10-20 nm, and a length of 1-5 μm. The complexing agent is citric acid, with a purity of 99.8% and a water solubility of ≥59.2 g / 100 mL (20℃). The deionized water has a conductivity of ≤10 μS / cm. The anhydrous ethanol has a purity of 99.9%. The argon gas has a purity of 99.99%. The hydrogen gas has a purity of 99.99%.

[0060] 2. Preparation steps

[0061] Preparation of S1 precursor: Sodium carbonate, ammonium dihydrogen phosphate, ferric nitrate, and tetrabutyl titanate were weighed according to stoichiometric ratios. The corresponding matrix chemical formula is: The titanium doping ratio (x) was 0.35. Citric acid was added as a complexing agent, with a molar ratio of complexing agent to total metal element of 2:1. The above raw materials were dissolved in deionized water, with a deionized water to raw material mass ratio of 10:1. The pH of the solution was adjusted to 5.5, and the mixture was stirred at room temperature for 4 hours. During the stirring process, ultrasonic-assisted dispersion was performed simultaneously at a power of 300W for 60 minutes to obtain a homogeneous, non-agglomerated precursor solution.

[0062] S2 composite dopant introduction: Nickel nitrate and ammonium fluoride were added to the precursor solution, with nickel doping amount of 2.0 at%, fluorine doping amount of 0.5 at%, and total composite doping amount of 2.5 at. The solution was heated to 80℃ and stirred at a constant temperature of 500 r / min for 1 hour. Argon gas was continuously introduced during the stirring process for protection, achieving in-situ co-doping of the dopant elements with precise site occupancy, resulting in a nickel-fluorine composite doped precursor solution.

[0063] S3 Hydrothermal Morphology Control: The doped precursor solution was transferred to a hydrothermal reactor with a filling degree of 80%. The temperature was increased to 170℃ at a rate of 3℃ / min and held at that temperature for 8 hours. After the reaction, the solution was allowed to cool naturally to room temperature. The product was washed by centrifugation with deionized water and anhydrous ethanol alternately, five times in total. Then, it was vacuum dried at 80℃ for 10 hours to obtain a hierarchical porous core-shell structured nanorod precursor.

[0064] S4 Double Coating Treatment: Core-shell nanorod precursors were uniformly mixed with lithium titanate powder, and argon gas was introduced as a protective atmosphere. The temperature was increased to 320°C at a rate of 2°C / min and held for 1 hour to complete the inner layer ion-conductive oxide coating, with an inner layer coating amount of 1.5 wt%. Subsequently, carbon nanotubes were added as the outer layer conductive carbon source, and the temperature was further increased to 480°C at a rate of 3°C / min and held for 2 hours to complete the outer layer carbon nanotube coating, with an outer layer coating amount of 3 wt%, resulting in a double-coated precursor.

[0065] S5 Segmented Sintering: The double-coated precursor was placed in an argon atmosphere with an argon flow rate of 100 mL / min. The temperature was increased to 400℃ at a rate of 5℃ / min and pre-fired for 3 hours to remove impurities and residual organic matter from the raw material. Subsequently, the temperature was increased to 680℃ at a rate of 5℃ / min and sintered for 14 hours to promote ordered crystal growth and ensure that the preferred orientation of the crystal faces met the requirements.

[0066] S6 Annealing and Activation: After sintering, the temperature was lowered to 280℃ at a rate of 2℃ / min and held for 2 hours. During the holding process, a mixed gas was introduced, consisting of 5% hydrogen and 95% argon by volume. The mixture was then allowed to cool naturally to room temperature. The cooled product was ground and passed through a 400-mesh standard sieve to obtain the target cathode material.

[0067] 3. Performance Testing

[0068] The test conditions were the same as in Example 1, and the test results are as follows: the preferred orientation degree of the (110) crystal plane of the material was 87%, and the sodium ion bulk diffusion coefficient was... The discharge specific capacity at 0.1C rate is 137mAh / g, with an initial coulombic efficiency of 96%; the capacity retention rate after 2000 cycles at 1C rate is 92%; the discharge specific capacity at 5C rate is 107mAh / g; the capacity retention rate at 0.2C rate is 72% at -40℃; the capacity retention rate after 1000 cycles at 1C rate is 88% at 60℃; and the interface impedance is 68Ω.

[0069] IV. Comparative Example 1

[0070] 1. Preparation of raw materials

[0071] The types, purity, and parameters of the raw materials are completely consistent with those in Example 1. Only the composite doping source is adjusted: urea is removed, and chromium nitrate is retained as the single doping source. The purity of chromium nitrate is 99.5%, and the chromium content is ≥19%.

[0072] 2. Preparation steps

[0073] Preparation of S1 precursor: The steps are exactly the same as in Example 1. Sodium carbonate, ammonium dihydrogen phosphate, vanadium nitrate, and tetrabutyl titanate are weighed according to the stoichiometric ratio. The corresponding matrix chemical formula is [missing information]. Citric acid was added as a complexing agent, with a total molar ratio of complexing agent to metal element of 1.5:1. It was dissolved in deionized water with a total mass ratio of deionized water to raw material of 7:1. The pH was adjusted to 4.5, and the mixture was stirred at room temperature for 3 hours. Simultaneously, it was dispersed with 200W ultrasonic assistance for 45 minutes to obtain a uniform precursor solution.

[0074] S2 Single Dopant Introduction: Chromium nitrate was added to the precursor solution, with a chromium doping amount of 1.1 at%, consistent with the total doping amount of composite doping in Example 1. The solution was heated to 70°C and stirred at a constant temperature of 400 r / min for 1.5 hours, with argon gas continuously introduced for protection during the stirring process, to obtain a single chromium-doped precursor solution.

[0075] S3 Hydrothermal Morphology Control: The steps are exactly the same as in Example 1. The doped precursor solution is transferred to a hydrothermal reactor with a filling degree of 70%. The temperature is increased to 150°C at 2°C / min and reacted at a constant temperature for 12 hours. After naturally cooling to room temperature, it is washed 4 times by alternating centrifugation with deionized water and anhydrous ethanol. It is then vacuum dried at 70°C for 12 hours to obtain a multi-level porous core-shell structured nanorod precursor.

[0076] S4 Double Coating Treatment: The steps are exactly the same as in Example 1. The core-shell nanorod precursor is uniformly mixed with lithium aluminum phosphate powder, argon gas is introduced, and the temperature is raised to 280℃ at 1.5℃ / min and held for 1.5 hours, with an inner coating amount of 0.8wt%. Then glucose is added, and the temperature is raised to 450℃ at 2.5℃ / min and held for 2.5 hours, with an outer coating amount of 1.5wt%, to obtain the double-coated precursor.

[0077] S5 segmented sintering: The steps are exactly the same as in Example 1. The double-coated precursor is placed in an argon atmosphere with a flow rate of 80 mL / min, and pre-fired at 3°C / min to 380°C for 4 hours, and then pre-fired at 4°C / min to 720°C for 12 hours.

[0078] S6 Annealing and Activation: The steps are exactly the same as in Example 1. After sintering, the temperature is reduced to 250°C at 1.5°C / min, held for 3 hours, and a mixed gas of 3% hydrogen and argon is introduced. The mixture is then naturally cooled to room temperature and ground through a 350-mesh sieve to obtain the target cathode material.

[0079] 3. Performance Testing

[0080] The test conditions were the same as in Example 1, and the test results are as follows: the preferred orientation degree of the crystal plane of material (110) was 82%, the sodium ion bulk diffusion coefficient was 1.0×10⁻¹²cm² / s; the discharge specific capacity at 0.1C rate was 130mAh / g; the capacity retention rate after 2000 cycles at 1C rate was 88%; the discharge specific capacity at 5C rate was 98mAh / g; the capacity retention rate at 0.2C rate under -40℃ low temperature environment was 65%; the capacity retention rate after 1000 cycles at 1C rate under 60℃ high temperature environment was 82%; and the interface impedance was 85Ω.

[0081] V. Comparative Example 2

[0082] 1. Preparation of raw materials

[0083] The types, purity, and parameters of the raw materials were completely consistent with those in Example 1, with only the coating source being adjusted: lithium aluminum phosphate was removed, and glucose was retained as the sole coating source, with a glucose purity of 99.9% and a dehydration temperature of 150°C.

[0084] 2. Preparation steps

[0085] Preparation of S1 precursor: The steps are exactly the same as in Example 1. Sodium carbonate, ammonium dihydrogen phosphate, vanadium nitrate, and tetrabutyl titanate are weighed according to the stoichiometric ratio. The corresponding matrix chemical formula is [missing information]. Citric acid was added as a complexing agent, with a total molar ratio of complexing agent to metal element of 1.5:1. It was dissolved in deionized water with a total mass ratio of deionized water to raw material of 7:1. The pH was adjusted to 4.5, and the mixture was stirred at room temperature for 3 hours. Simultaneously, it was dispersed with 200W ultrasonic assistance for 45 minutes to obtain a uniform precursor solution.

[0086] S2 composite doping element introduction: The steps are exactly the same as in Example 1. Chromium nitrate and urea are added to the precursor solution. The chromium doping amount is 0.8at, the nitrogen doping amount is 0.3at, and the total doping amount is 1.1at. The temperature is raised to 70°C and stirred at 400r / min for 1.5 hours. Argon gas is introduced for protection to obtain the chromium-nitrogen composite doped precursor solution.

[0087] S3 Hydrothermal Morphology Control: The steps are exactly the same as in Example 1. The doped precursor solution is transferred to a hydrothermal reactor with a filling degree of 70%. The temperature is increased to 150°C at 2°C / min and reacted at a constant temperature for 12 hours. After naturally cooling to room temperature, it is washed 4 times by alternating centrifugation with deionized water and anhydrous ethanol. It is then vacuum dried at 70°C for 12 hours to obtain a multi-level porous core-shell structured nanorod precursor.

[0088] S4 Single Coating Treatment: The core-shell nanorod precursor was uniformly mixed with glucose, and argon gas was introduced as a protective atmosphere. The temperature was increased to 450°C at a heating rate of 2.5°C / min and held for 2.5 hours to complete the single porous amorphous carbon coating. The coating amount was 1.5wt%, which was consistent with the outer coating amount in Example 1, and a single coated precursor was obtained.

[0089] S5 segmented sintering: The steps are exactly the same as in Example 1. The single-coated precursor is placed in an argon atmosphere with a flow rate of 80 mL / min, and pre-fired at 3°C / min to 380°C for 4 hours, and then pre-fired at 4°C / min to 720°C for 12 hours.

[0090] S6 Annealing and Activation: The steps are exactly the same as in Example 1. After sintering, the temperature is reduced to 250°C at 1.5°C / min, held for 3 hours, and a mixed gas of 3% hydrogen and argon is introduced. The mixture is then naturally cooled to room temperature and ground through a 350-mesh sieve to obtain the target cathode material.

[0091] 3. Performance Testing

[0092] The test conditions were the same as in Example 1, and the test results are as follows: the preferred orientation degree of the crystal plane of material (110) was 83%, the sodium ion bulk diffusion coefficient was 1.1×10⁻¹²cm² / s; the discharge specific capacity at 0.1C rate was 131mAh / g; the capacity retention rate after 2000 cycles at 1C rate was 89%; the discharge specific capacity at 5C rate was 100mAh / g; the capacity retention rate at 0.2C rate under -40℃ low temperature environment was 66%; the capacity retention rate after 1000 cycles at 1C rate under 60℃ high temperature environment was 83%; the interface impedance was 80Ω.

[0093] VI. Comparative Example 3

[0094] 1. Preparation of raw materials

[0095] Sodium source: Sodium carbonate, purity 99.8%, particle size 100-200 mesh; Phosphorus source: Ammonium dihydrogen phosphate, purity 99.7%, water solubility ≥95g / 100mL; Transition metal source: Vanadium nitrate, purity 99.5%, vanadium content ≥22%; Titanium source: Tetrabutyl titanate, purity 99.5%, viscosity 25mPa・s; Conductive carbon source: Glucose, purity 99.9%, dehydration temperature 150℃; Complexing agent: Citric acid, purity 99.8%, water solubility ≥59.2g / 100mL; Deionized water, conductivity ≤10μS / cm; Anhydrous ethanol, purity 99.9%; Argon, purity 99.99%.

[0096] 2. Preparation steps

[0097] S1 Precursor Preparation: Sodium carbonate, ammonium dihydrogen phosphate, vanadium titanate, and tetrabutyl nitrate were weighed according to stoichiometric ratio, dissolved in deionized water with citric acid, and stirred at room temperature for 3 hours without ultrasonic-assisted dispersion to obtain a precursor solution. S2 Undoped Treatment: Without adding any dopant source, the precursor solution was directly stirred and aged for 2 hours to obtain an undoped precursor solution. S3 No Morphology Control: The precursor solution was directly evaporated and dried without hydrothermal reaction, and after grinding, irregular particle precursors were obtained. S4 Single Amorphous Carbon Coating: The particle precursor was mixed with glucose, heated to 450℃ and held for 2.5 hours to complete single carbon coating. S5 One-Step Sintering: The coated precursor was placed in an argon atmosphere and directly heated to 720℃ for sintering for 16 hours without segmented pre-sintering. S6 No Annealing Activation: After sintering, it was directly cooled to room temperature and ground through a 350-mesh sieve to obtain a cathode material produced using traditional processes.

[0098] 3. Performance Testing

[0099] The test conditions were the same as in Example 1, and the test results are as follows: (110) Preferred orientation degree of crystal plane 75%, sodium ion bulk diffusion coefficient 0.6×10⁻¹²cm² / s; 0.1C discharge specific capacity 122mAh / g, first coulombic efficiency 93%; 5C discharge specific capacity 85mAh / g; 1C cycle 2000 cycles capacity retention rate 81%; 60℃ high temperature 1C cycle 1000 cycles capacity retention rate 76%; -40℃ low temperature 0.2C capacity retention rate 58%; interface impedance 130Ω; 500 cycles crystal form retention rate 93.5%.

[0100] The basic electrochemical performance of the examples and comparative examples is compared in the table below:

[0101] Table 1

[0102] (110) Preferred orientation degree of crystal plane (%) 88 86 87 82 83 75 Sodium ion bulk diffusion coefficient (×10⁻¹²cm² / s) 1.5 1.3 1.4 1.0 1.1 0.6 0.1C discharge specific capacity (mAh / g) 138 136 137 130 131 122 First Coulomb efficiency (%) 97 96.5 96 95 95.5 93 5C discharge specific capacity (mAh / g) 108 106 107 98 100 85 Interface impedance (Ω) 65 70 68 85 80 130

[0103] The following table compares the cycling stability and high and low temperature performance of the examples and comparative examples:

[0104] Table 2

[0105] 1C cycle 2000 cycles capacity retention (%) 93 92.5 92 88 89 81 Capacity retention rate (%) after 1000 cycles at 60℃ 90 89 88 82 83 76 -40℃ low temperature 0.2C capacity retention (%) 73 71 72 65 66 58 Crystal form retention rate (%) after 500 cycles 98.5 98.2 98.0 96.0 96.5 93.5

[0106] The table comprehensively compares the performance of the examples and the comparative examples from the dimensions of basic electrochemical performance, cycle stability and wide temperature range adaptability. It can be seen that the performance of the examples is better than that of the comparative examples, which fully confirms the effectiveness of the core design of the present invention. The composite doping and double coating synergistic structure can simultaneously optimize the material crystal orientation, transport efficiency and structural stability, while single doping or single coating is difficult to take into account the comprehensive performance and the performance improvement is limited.

[0107] The comparison results clearly show that the embodiment performs better in terms of crystal plane selectivity, ion diffusion, rate performance, cycle stability, and high and low temperature adaptability, highlighting the synergistic advantages of precise lattice optimization by composite doping and the construction of efficient transport pathways by double coating layers. The comparative embodiment, due to the lack of composite doping or double coating structure, has obvious shortcomings in transport efficiency and structural stability, further proving that the technical solution of this invention has outstanding innovation and practicality.

[0108] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A sodium-ion energy storage battery cathode material based on polyanion, characterized in that, The cathode material uses a NASICON-type polyanionic compound as a matrix, and the general chemical formula of the matrix is: The matrix is ​​incorporating a composite dopant of transition metals and nonmetals. The transition metals are one or more of chromium, cobalt, and nickel, and the nonmetals are one or more of nitrogen, boron, and fluorine. The total doping amount of the composite dopant is 0.1-5 at%. The transition metal occupies the M-site lattice sites in the matrix, and the nonmetal occupies the... The O-site lattice site of the group; the substrate surface is provided with a double coating layer synergistic structure, the inner layer is an ion-conductive oxide coating layer with a coating amount of 0.3-1.5wt%, and the outer layer is a porous conductive carbon coating layer with a coating amount of 0.5-3wt%; the cathode material is a hierarchical porous core-shell structure nanorod, the nanorod particle size is 80-300nm, the aspect ratio is 3-8, the nanorod contains mesopores and micropores in hierarchical channels, the mesopore pore size is 2-50nm, the micropore pore size is 0.5-2nm, and the total pore volume is 0.08-0.22cm³ / g.

2. The sodium-ion energy storage battery cathode material based on polyanion as described in claim 1, characterized in that, The NASICON-type polyanionic compound matrix is ​​selected from one or more composite matrices of sodium vanadium phosphate, sodium manganese titanium phosphate, and sodium iron vanadium phosphate. The composite matrix is ​​a heterojunction composite, and an electron-rich region is formed at the heterojunction interface. The room temperature conductivity of the matrix is ​​≥1.5×10-6S / cm.

3. The sodium-ion energy storage battery cathode material based on polyanion as described in claim 1, characterized in that, The composite doping element is one of chromium-nitrogen composite doping, cobalt-boron composite doping, and nickel-fluorine composite doping, with a transition metal doping amount of 0.05-3 at and a non-metal doping amount of 0.05-2 at.

4. The sodium-ion energy storage battery cathode material based on polyanion according to claim 1, characterized in that, The inner ion-conductive oxide coating layer is selected from one of lithium aluminum phosphate, lithium metavanadate, and lithium titanate, with a coating layer thickness of 5-20 nm and an ionic conductivity ≥2.0×10-4 S / cm; the outer porous conductive carbon coating layer is selected from one of porous amorphous carbon, graphene, and carbon nanotubes, with a coating layer thickness of 10-35 nm and an electronic conductivity ≥100 S / cm.

5. The sodium-ion energy storage battery cathode material based on polyanion according to claim 1, characterized in that, The M represents one or more of vanadium, manganese, and iron, with 0.05 ≤ x ≤ 0.35, matrix cell parameters a = 8.12-8.25 Å, c = 21.85-22.10 Å, and cell volume = 1380-1420 ų.

6. The sodium-ion energy storage battery cathode material based on polyanion according to claim 1, characterized in that, The cathode material has a (110) crystal plane preferred orientation structure with a preferred orientation degree ≥85%, and is accompanied by (310) crystal plane auxiliary orientation with an auxiliary orientation degree of 10-15%. The sodium ion bulk diffusion coefficient is ≥1.2×10-12cm² / s, and the interface diffusion coefficient is ≥8.5×10-14cm² / s.

7. A method for preparing the sodium-ion energy storage battery cathode material based on polyanion as described in claim 1, characterized in that, Includes the following steps: S1 precursor preparation: Sodium source, phosphorus source, transition metal source, and titanium source were weighed according to stoichiometric ratio, and a complexing agent was added and dissolved in deionized water. The pH of the solution was adjusted to 3.5-5.5, and the mixture was stirred at room temperature for 2-4 hours. The total molar ratio of the complexing agent to the metal element was 1.2-2:

1. Ultrasonic dispersion was used during the stirring process. The ultrasonic power was 150-300W and the ultrasonic time was 30-60 minutes to obtain a uniform and non-agglomerated precursor solution. S2 composite doping element is introduced, and transition metal doping source and non-metal doping source are added to the precursor solution. The temperature is raised to 60-80℃ and stirred at a constant temperature for 1-2 hours. The stirring rate is 300-500 r / min. At the same time, inert gas is introduced for protection to achieve in-situ co-doping of doping elements and obtain doped precursor solution. S3 hydrothermal morphology control: The doped precursor solution was transferred to a hydrothermal reactor with a filling degree of 60-80%. The temperature was raised to 120-180℃ and reacted at a constant temperature for 8-16 hours with a heating rate of 1-3℃ / min. After naturally cooling to room temperature, the precursor was washed 3-5 times by alternating centrifugation with deionized water and anhydrous ethanol. The precursor was then vacuum dried at 60-80℃ for 10-14 hours to obtain a hierarchical porous core-shell nanorod precursor. The S4 double-coating process involves first uniformly mixing the core-shell nanorod precursor with the inner coating source, then introducing an inert atmosphere and heating to 250-320℃ for 1-2 hours at a heating rate of 1-2℃ / min to complete the inner coherent coating. Next, the precursor is uniformly mixed with the outer conductive carbon source and heated to 400-480℃ for 2-3 hours at a heating rate of 2-3℃ / min to complete the outer three-dimensional network conductive carbon coating, thus obtaining the double-coated precursor. S5 segmented sintering involves placing the double-coated precursor in an inert atmosphere with a flow rate of 50-100 mL / min, pre-firing at 350-400℃ for 3-5 hours at a rate of 2-5℃ / min to remove impurities and residual organic matter, and then firing at 650-780℃ for 10-14 hours at a rate of 2-5℃ / min. After S6 annealing and activation, the temperature is reduced to 200-280℃ at a rate of 1-2℃ / min and held for 2-4 hours. During the holding process, a mixture of hydrogen and inert atmosphere with a volume fraction of 2-5% is introduced. The mixture is then naturally cooled to room temperature and ground through a 300-400 mesh sieve to obtain the target cathode material.

8. The method for preparing the sodium-ion energy storage battery cathode material based on polyanion according to claim 7, characterized in that, In S1, the sodium source is sodium carbonate or sodium hydroxide, the phosphorus source is ammonium dihydrogen phosphate or diammonium hydrogen phosphate, the transition metal source is the corresponding metal nitrate, the titanium source is tetrabutyl titanate or titanium oxysulfate, the complexing agent is citric acid or ethylene glycol, the mass ratio of deionized water to the total raw material is 5-10:1, and ultrasonic-assisted dispersion can disperse the raw material particle size to 100-200nm.

9. The method for preparing the sodium-ion energy storage battery cathode material based on polyanion according to claim 7, characterized in that, The inert atmosphere in S5 is argon or nitrogen with a purity ≥99.99%. The heating rate during the pre-firing stage is controlled at 2-3℃ / min, and the heating rate during the main firing stage is controlled at 3-5℃ / min. When the main firing temperature is higher than 700℃, the holding time is extended by 1-2 hours.

10. The method for preparing the sodium-ion energy storage battery cathode material based on polyanion according to claim 7, characterized in that, The hydrogen gas fraction in the mixed gas in S6 is controlled to be 2-5%.

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