Carbon nitride-coated pyrophosphoric acid sodium manganese phosphate-based positive electrode material, preparation method and application

Abstract

The application belongs to the technical field of sodium ion batteries, and discloses a carbon nitride-coated sodium manganese pyrophosphate-based positive electrode material, a preparation method and application. The positive electrode material comprises a core and a coating layer arranged on the surface of the core, the core is cerium-doped sodium manganese pyrophosphate, and the coating layer is carbon nitride; the chemical general formula of the positive electrode material is Na b‑a Ce a Mn 2.9‑a (PO4)2P2O7 / C3N4, wherein 0.001<=a<=0.3 and 3.95<=b<=4.1. The carbon nitride-coated sodium manganese pyrophosphate is used to improve the electrical conductivity, and the cerium element is used to improve the ion conductivity. The cerium element optimizes the bulk electron / ion conduction in the positive electrode material and stabilizes the skeleton of the positive electrode material. The carbon nitride provides a high-speed electron channel and physical / chemical protection outside the positive electrode material, and solves the problem of poor rate performance of the sodium manganese pyrophosphate material in the prior art.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, specifically relating to a carbon nitride-coated sodium manganese pyrophosphate-based cathode material, its preparation method, and its application. Background Technology

[0002] Sodium is abundant and widely distributed in the Earth's crust, and its production is simple, providing a raw material guarantee for the development of low-cost sodium-ion batteries. Due to the large ionic radius of sodium ions, they exhibit better rate performance compared to lithium-ion batteries. Furthermore, both the positive and negative electrode current collectors of sodium-ion batteries can use relatively inexpensive aluminum foil. The similar processing technology to lithium-ion batteries allows for compatibility with existing lithium-ion battery production equipment during industrialization, reducing technical bottlenecks and further lowering production costs. Sodium-ion batteries, as low-cost and high-safety products, have a very broad application prospect. Polyanionic compounds, as sodium storage materials, are simple to synthesize, have high structural stability, and are easy to mass-produce. As the most promising cathode material for developing low-cost sodium-ion batteries, research on their sodium storage performance is of great practical significance for promoting the industrialization of sodium-ion batteries.

[0003] Currently, polyanionic cathode materials still face many challenges in practical applications. Sodium iron pyrophosphate has a theoretical capacity of 129 mAh / g and an average voltage of 3.0 V. Although the capacity is high, the low voltage results in low energy density, which is significantly lower than that of layered oxide and Prussian blue sodium ion cathode materials, making it less competitive in terms of high power density. Compared to sodium iron pyrophosphate's average voltage of 3.0 V, sodium manganese pyrophosphate has an average voltage of 3.7 V with a similar theoretical capacity, increasing the energy density and providing higher energy at the same capacity, showing great development potential. However, due to the manganese ion's 3d orbital being half-filled, electrons are difficult to excite into free electrons, resulting in poor electronic conductivity. Furthermore, the one-dimensional diffusion channels for sodium ions in the structure lead to poor rate performance. Summary of the Invention

[0004] This invention provides a carbon nitride-coated sodium manganese phosphate pyrophosphate-based cathode material, its preparation method, and its application. It improves the electronic conductivity of the sodium manganese phosphate pyrophosphate system and solves the problem of poor rate performance of sodium manganese phosphate pyrophosphate materials in the prior art.

[0005] To achieve the above objectives, in a first aspect, the present invention provides a carbon nitride-coated sodium manganese pyrophosphate-based cathode material, the cathode material comprising a core and a coating layer disposed on the surface of the core, wherein the core is cerium-doped sodium manganese pyrophosphate, and the coating layer is carbon nitride; the general chemical formula of the cathode material is Na. b-a Ce a Mn 2.9-a(PO4)2P2O7 / C3N4, where 0.001≤a≤0.3, 3.95≤b≤4.1.

[0006] Preferably, the general chemical formula of the positive electrode material is Na. b-a Ce a Mn 2.9-a (PO4)2P2O7 / C3N4, where 0.01≤a≤0.05, 4.02≤b≤4.06.

[0007] Preferably, the main peak intensity at 33.38° of the XRD pattern obtained by XRD detection is shifted to the left by more than 0.1° compared with the main peak intensity at 33.38° of the standard sample with the same composition and no Ce doping, and the interplanar spacing is increased by more than 2%.

[0008] Secondly, the present invention provides a method for preparing a carbon nitride-coated sodium manganese phosphate-based cathode material, comprising the following steps:

[0009] S1. Dissolve sodium source, manganese source, phosphorus source, cerium source and carbon source in deionized water to obtain solution A, wherein the carbon source is at least one of urea and melamine;

[0010] S2. The solution from step S1 is milled to obtain a slurry; the particle size D50 of the slurry is ≤0.28μm;

[0011] S3. Spray dry the slurry from step S2 to obtain a precursor; the particle size D50 of the precursor is ≤18μm.

[0012] S4. The precursor from step S3 is sintered. The sintering includes a first stage sintering and a second stage sintering. The sintering temperature of the first stage sintering is 200~440℃, and the sintering temperature of the second stage sintering is 510~690℃, thus obtaining a carbon-coated cerium-doped sodium manganese pyrophosphate cathode material. The sintering atmosphere is nitrogen, the flow rate of the atmosphere is 10L / min~50L / min, and the pressure of the atmosphere is 0.101~0.12MPa.

[0013] Preferably, in step S1, the sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium pyrophosphate.

[0014] The manganese source is at least one of manganese carbonate, manganese oxalate, manganese acetate, and manganese nitrate.

[0015] The cerium source is at least one of cerium nitrate, cerium acetate, cerium oxalate, and cerium carbonate.

[0016] Preferably, in step S1, the molar ratio of the sodium source, manganese source, cerium source, and phosphorus source is (3.90~4.2):(2.9~3.0):(0.003~0.3):4; and the carbon source content is 3wt%~14.5wt% of the cathode material.

[0017] Preferably, in step S3, the spray drying process has an inlet air temperature of 183℃~248℃ and an outlet air temperature of 91℃~118℃; in step S4, the sintering heating rate is 1℃ / min~6℃ / min, and the sintering includes a first stage sintering and a second stage sintering. The sintering temperature of the first stage sintering is 200~440℃, and the holding time is 2~8h; the sintering temperature of the second stage sintering is 510~690℃, and the holding time is 5~20h, and then the temperature is cooled to room temperature.

[0018] Preferably, the sintering includes a first stage of sintering and a second stage of sintering, wherein the first stage of sintering is heated to 280~330℃ and the second stage of sintering is heated to 605~650℃.

[0019] Preferably, the sintering includes a first stage sintering and a second stage sintering, wherein the first stage sintering is heated to 280~290℃ and the second stage sintering is heated to 605~650℃.

[0020] Thirdly, the present invention provides a polyanion sodium-ion battery, comprising a positive electrode sheet, wherein the positive electrode sheet comprises a positive current collector and a positive active material layer disposed on the positive current collector, wherein the positive active material layer comprises the above-mentioned carbon nitride coated sodium manganese pyrophosphate phosphate-based positive electrode material or the carbon nitride coated sodium manganese pyrophosphate phosphate-based positive electrode material prepared by the above-mentioned preparation method.

[0021] The beneficial effects of this invention are as follows:

[0022] In this embodiment of the invention, the provided cathode material includes a core and a coating layer disposed on the surface of the core. The core is cerium-doped sodium manganese pyrophosphate, and the coating layer is carbon nitride. The general chemical formula of the cathode material is Na. b-a Ce a Mn 2.9-a (PO4)2P2O7 / C3N4, where 0.001≤a≤0.3, 3.95≤b≤4.1. The conductivity of sodium manganese pyrophosphate was improved by carbon nitride coating, and cerium doping further enhanced ionic conductivity. Cerium optimized bulk electron / ion conduction and stabilized the cathode material's framework within the cathode material, while carbon nitride provided a high-speed electron pathway and physical / chemical protection on the outside. A unique synergistic effect emerged between nitrogen and cerium ions, forming Ce-N bonds at the interface, strongly bridging the bulk phase and the coating layer, improving electronic conductivity, and ultimately significantly enhancing the rate performance of the sodium-ion battery.

[0023] The preparation method of this invention is simple, with low production cost and short production cycle. It produces a carbon nitride-coated cerium-doped sodium manganese pyrophosphate cathode material with simple synthesis process, high energy density, and excellent cycle performance. When applied to the cathode of sodium-ion batteries, the batteries exhibit excellent electrochemical performance, a high voltage plateau, and good capacity utilization. The prepared sodium-ion batteries achieve an initial discharge specific capacity of over 100 mAh / g at 0.2C rate (100 mAh / g) and a capacity retention rate of over 89% at 10C rate (100 mAh / g). Therefore, the carbon nitride-coated cerium-doped sodium manganese pyrophosphate cathode material of this invention has excellent application prospects. Attached Figure Description

[0024] Figure 1 Na prepared in Example 1 of this invention 4.02 Ce 0.03 Mn 2.87 Scanning electron microscope image of the microstructure of (PO4)2P2O7 / C3N4 cathode material.

[0025] Figure 2 The XRD patterns of the cathode materials prepared in Example 1 and Comparative Example 1 of this invention are shown.

[0026] Figure 3 Na prepared in Example 1 of this invention 4.02 Ce 0.03 Mn 2.87 Rate performance of a polyanion sodium-ion battery assembled with (PO4)2P2O7 / C3N4 cathode material. Detailed Implementation

[0027] In this invention, unless otherwise stated, directional terms such as "up," "down," "left," and "right" are generally understood in conjunction with the accompanying drawings and the directions shown in actual applications.

[0028] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0029] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0030] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. The terms "optional" and "discretionary" mean that they may or may not be included (or may or may not be present).

[0031] The applicant of this invention discovered that, compared with traditional lithium-ion battery cathode materials, such as lithium cobalt oxide, lithium manganese oxide, and ternary materials, sodium iron phosphate pyrophosphate (Na4Fe3(PO4)2P2O7, denoted as NFPP) exhibits superior performance due to its thermodynamic stability, higher theoretical capacity (~129 mAh / g), moderate average operating potential (above 3.0 V), and better performance in Na… + Sodium iron phosphate (SOP) is considered a promising cathode material for sodium-ion batteries (SIBs) due to its relatively small volume change (~4%) and low cost during insertion / extraction. However, its performance degrades during cycling due to structural changes, lattice defects, or the formation of impurity phases. Localized iron enrichment can lead to the formation of inert phases, reducing reversible capacity and cycle stability. Furthermore, physical properties such as compaction density, particle size distribution, and sphericity also affect its electrochemical performance. Despite its high theoretical capacity and voltage plateau, SOP still suffers from insufficient cycle performance and rate capability in practical applications. Especially at high charge / discharge rates, the ion diffusion rate and electronic conductivity may be limited, leading to capacity decay and increased polarization.

[0032] Sodium iron phosphate pyrophosphate has a theoretical capacity of 129 mAh / g and an average voltage of 3.0 V. Although the capacity is relatively high, the low voltage results in a low energy density, which is significantly lower than that of layered oxide and Prussian blue sodium ion cathode materials, making it less competitive in terms of high power density. Compared to sodium iron phosphate pyrophosphate's average voltage of 3.0 V, sodium manganese phosphate pyrophosphate has an average voltage of 3.7 V with a similar theoretical capacity, increasing the energy density and providing higher energy at the same capacity, showing great development potential. However, due to the manganese ion's 3d orbital being half-filled, electrons are difficult to excite into free electrons, resulting in poor electronic conductivity. Furthermore, the one-dimensional diffusion channels for sodium ions in the structure lead to poor rate performance of the cathode material.

[0033] Using only sodium manganese pyrophosphate results in poor electronic conductivity, poor cycling performance, and poor rate performance due to the characteristics of Mn. Furthermore, the significant Jan Taylor effect leads to greater volume changes during the divalent and trivalent transformations of manganese, resulting in poor structural stability. In particular, the direct coating of carbon and other layers with sodium manganese pyrophosphate results in poor interfacial bonding, hindering performance improvement. Using sodium iron manganese pyrophosphate, which contains a higher content of iron and is an essential component, provides relatively intermediate performance, but it also results in an average voltage increase to 3.7V (due to the presence of iron lowering the average voltage) when the theoretical capacity is similar.

[0034] To address the aforementioned problems, this invention provides a carbon nitride-coated sodium manganese pyrophosphate (Na2PO4)-based cathode material. The cathode material comprises a core and a coating layer disposed on the surface of the core. The core is cerium-doped sodium manganese pyrophosphate, and the coating layer is carbon nitride. The general chemical formula of the cathode material is Na2PO4. b-a Ce a Mn 2.9-a (PO4)2P2O7 / C3N4, where 0.001≤a≤0.3, 3.95≤b≤4.1. The conductivity of sodium manganese pyrophosphate was improved by coating it with carbon nitride, and the ionic conductivity was further enhanced by cerium doping. Cerium optimizes bulk electron / ion conduction and stabilizes the cathode material's framework within the cathode material, while carbon nitride provides a high-speed electron pathway and physical / chemical protection on the outside. A unique synergistic effect exists between nitrogen and cerium ions, forming a Ce-N bond synergistic effect at the interface, strongly bridging the bulk phase and the coating layer, improving electronic conductivity, and ultimately significantly improving the rate performance of the sodium-ion battery. The preparation method of this invention is simple, has low production cost, and a short production cycle.

[0035] In the above embodiments, by using a relatively high Na content in sodium manganese pyrophosphate, excessive reduction in discharge specific capacity is avoided. By using only sodium manganese pyrophosphate, the molecular proportion of Mn is limited to a certain level. While fully utilizing its high average voltage characteristics, the relatively low manganese proportion helps to mitigate the significant J. Taylor effect, where the volume change during divalent and trivalent transformations of manganese leads to poor structural stability. It also facilitates the improvement effect of doping with Ce and other substances. Carbon nitride coating of sodium manganese pyrophosphate enhances electronic conductivity, and cerium doping further improves ionic conductivity, thus improving rate performance. This method involves high-temperature processing, resulting in a simple preparation process and a short production cycle. Furthermore, all raw materials used in the preparation are commercially available, significantly reducing energy consumption and cost.

[0036] Optionally, in some embodiments, the general chemical formula of the above-mentioned cathode material is Na. b-a Ce a Mn 2.9-a (PO4)2P2O7 / C3N4, where 0.01≤a≤0.05, 4.02≤b≤4.06. The sodium content can be controlled by the cerium doping content. Cerium doping can construct an electron transport network, thereby improving the electronic conductivity of the cathode material. Furthermore, by reducing the iron content (introduced iron impurities), the purity of the phase is improved, the formation of impurity phases (sodium iron phosphate impurities) is suppressed, the cycle stability of the cathode material is improved, and the diffusion capacity of sodium ions is enhanced. This reduces interfacial side reactions and the dissolution of active materials into the electrolyte, improving the cycle performance of the battery. Cerium doping further improves the synthesis process of the cathode material, increasing the particle uniformity and compaction density. Introducing more Ce, with its high trivalent state, reduces the sodium ratio. Excessive reduction in sodium leads to an excessive decrease in discharge capacity; by increasing the sodium ratio, a relatively smaller Ce ratio is used, avoiding excessive reduction in discharge capacity.

[0037] Optionally, in some embodiments, the peak intensity at 33.38° obtained by XRD detection is shifted to the left by more than 0.1° compared to the peak intensity at 33.38° of the same undoped Ce standard sample, and the interplanar spacing is increased by more than 2%. By optimizing the amount of Ce doping and Ce occupancy, the Ce doping is controlled to moderately increase the lattice size, thereby improving the rate performance. At the same time, by controlling the detection performance of the above-mentioned XRD detection, problems such as lattice distortion and the generation of obvious impurity phases caused by excessive Ce doping are avoided.

[0038] This invention provides a method for preparing a carbon nitride-coated sodium manganese phosphate-based cathode material, comprising steps 201 to 204:

[0039] 201. Dissolve sodium source, manganese source, phosphorus source, cerium source and carbon source in deionized water to obtain solution A, wherein the carbon source is at least one of urea and melamine.

[0040] For example, in step 201, when adding the sodium source to deionized water, the temperature is heated to 40°C and stirred until completely dissolved; when adding the manganese source to deionized water, the temperature is heated to 40°C and stirred, wherein water-insoluble substances in the manganese source can be treated with acid; when adding the cerium source to deionized water, the temperature is maintained at 30~40°C; the mixture is slowly poured into a large magnetically stirred container while stirring at 80 rpm, and the pH is adjusted to 7.5~8.5 to help the metal ions form a uniform hydroxyl precursor, while avoiding excessive neutralization of phosphoric acid leading to premature precipitation; the mixture is heated to 80°C and maintained for 1 hour.

[0041] Optionally, in one embodiment, the carbon source is at least one of urea and melamine. Urea decomposes during high-temperature pyrolysis to produce ammonia and cyanate, and nitrogen atoms are in-situ incorporated into the generated carbon network, forming an N-doped carbon layer. N-doping can improve the conductivity of the carbon layer and increase surface active sites. Furthermore, urea is readily soluble in water and can be uniformly dispersed in the precursor slurry during hydrothermal or solid-phase synthesis, ensuring uniform coverage of the carbon layer on the particle surface. Melamine molecules contain three amino groups (C3N6H6), which release a large amount of nitrogen during pyrolysis, forming a highly nitrogen-doped carbon layer, further improving conductivity. At high temperatures (approximately 950°C), melamine co-pyrolyzes with organic acids, generating a uniform amorphous carbon layer on the particle surface, increasing surface porosity, which is beneficial for Na... + Rapid penetration. During pyrolysis, melamine also generates a small amount of nitrogen-carbon bonds, forming a carbon layer with a high degree of graphitization, which improves the structural stability of the material and inhibits phase transitions during cycling. The formation of the carbon-nitrogen layer acts as an "elastic shell" during battery charging and discharging, reducing particle cracking and active material shedding.

[0042] 202. The solution from step 201 is milled to obtain a slurry; the particle size D50 of the slurry is ≤0.28μm;

[0043] In step 202, a dispersant or grinding aid is added to the solution before sand milling, followed by wet milling. Commonly used dispersants or grinding aids are citric acid or ethanol, which help prevent particle agglomeration and improve grinding efficiency. Ceramic beads (zirconia) or cemented carbide balls are used as the grinding media. The diameter of the ceramic beads (zirconia) or cemented carbide balls is 0.3~1mm, and the ball-to-material ratio is 5~10:1. Deionized water or ethanol is used as the dispersion medium. The grinding speed is 1200~1500 r / min, and the time is 4~6 hours, with a short pause every hour for heat dissipation.

[0044] Optionally, in one embodiment, the particle size D50 of the slurry is ≤0.28 μm. Smaller particle size facilitates the uniform dispersion of Ce and the occupation of Mn, improving phase purity and mitigating the adverse effects of large phase transformation in pure sodium manganese pyrophosphate without added iron. Excessively large particle size leads to component inhomogeneity, causing bulk phase inhomogeneity and the formation of impurity phases. Optionally, the particle size of the slurry is any value selected from 0.28 μm, 0.25 μm, 0.22 μm, 0.19 μm, 0.16 μm, 0.13 μm, and 0.10 μm, or a range of both.

[0045] 203. Spray dry the slurry from step 202 to obtain the precursor; the particle size D50 of the precursor is ≤18μm.

[0046] In step 203, the slurry concentration (solid mass fraction) w / v is 4%~20%, and the viscosity is below 200 mPa·s. Excessive viscosity will lead to uneven atomization and particle agglomeration. The pre-dispersed particle size should be ≤20µm to help form uniform droplets and prevent large particles from clogging the nozzle. Water or a low-boiling-point organic solvent should be used as the solvent; the solvent's boiling point should be lower than the inlet air temperature to ensure rapid evaporation.

[0047] Optionally, in one embodiment, the particle size D50 of the precursor is ≤18μm. The precursor is nearly spherical, with uniform particles and a smooth surface, which can significantly reduce the adhesion and agglomeration of powder and improve the flowability during conveying, weighing, and pressing. Fine and narrowly distributed particles result in more uniform heat conduction during high-temperature calcination, leading to more complete solid-phase reaction. This allows for complete decomposition and crystal phase transformation at lower temperatures, avoiding particle melting or coarse grain formation caused by localized overheating. Optionally, the particle size of the slurry is any value selected from 18μm, 16μm, 14μm, 12μm, 10μm, 8μm, 6μm, 4μm, and 2μm, or a range of any two of these values.

[0048] 204. The precursor in step 203 is sintered. The sintering includes a first stage sintering and a second stage sintering. The sintering temperature of the first stage sintering is 200~440℃, and the sintering temperature of the second stage sintering is 510~690℃, thus obtaining carbon-coated cerium-doped sodium manganese pyrophosphate cathode material. The sintering atmosphere is nitrogen, the flow rate of the atmosphere is 10L / min~50L / min, and the pressure of the atmosphere is 0.101~0.12 MPa.

[0049] In step 204, sintering includes a first stage of sintering and a second stage of sintering. The sintering temperature of the first stage of sintering is 200~440℃, and the sintering temperature of the second stage of sintering is 510~690℃.

[0050] The first stage of sintering removes volatiles and decomposes organic matter or moisture. Within the temperature range of 200~440℃, bound water, residual organic matter, and some basic carbonates in the precursor decompose and evaporate, resulting in a relatively clean precursor matrix. If sintering is directly performed at 510~690℃, rapid decomposition will generate a large amount of gas, easily forming "irreversible pores" inside the material, leading to material fragmentation. At the same time, during pre-firing, the highly reactive nitrogen in the coating layer can achieve "preliminary adhesion" to the material matrix. If pre-firing is skipped and high-temperature sintering is performed directly, the coating layer is easily blocked by impurities on the matrix surface, resulting in a loose bond between the coating layer and the matrix, which is prone to detachment in subsequent cycles. The low-temperature sintering in the early stage causes the metal oxides in the precursor to undergo solid-state reactions, generating preliminary intermediate phases or crystal nuclei, thus laying the framework for the subsequent crystal growth. The second-stage sintering process facilitates crystal formation and stabilization. Within the temperature range of 510–690℃, the intermediate phase in the precursor further reacts to generate the main crystalline phase of the target cathode material, completing the ordered crystal structure. This temperature range provides sufficient thermal energy to accelerate interfacial diffusion between particles, promoting grain growth and pore closure, thus increasing the material's density and mechanical strength. Furthermore, this temperature range ensures crystal integrity and a distinct layered structure while preventing structural damage or collapse of internal dendrites due to excessively high temperatures, thereby maintaining high discharge capacity and excellent electrochemical performance. Optionally, the sintering temperature of the first stage sintering is any one of 200℃, 240℃, 280℃, 320℃, 360℃, 400℃, 440℃ or a range of any two of the above; optionally, the sintering temperature of the second stage sintering is any one of 510℃, 520℃, 530℃, 540℃, 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, 660℃, 670℃, 680℃, 690℃ or a range of any two of the above.

[0051] Optionally, in some embodiments, the sintering atmosphere is nitrogen, with a flow rate of 10 L / min to 50 L / min and a pressure of 0.101 to 0.12 MPa. Nitrogen itself does not participate in chemical reactions and can displace the air in the furnace, preventing oxidizing gases such as oxygen and moisture from entering the sintering zone, thereby avoiding oxidation of the precursor at high temperatures or the formation of unfavorable oxide layers. At a suitable pressure of 0.101 to 0.12 MPa, the gas density increases, the thermal convection coefficient improves, and the heat distribution in the furnace becomes more uniform, further enabling the precursor to heat up rapidly and uniformly. A moderate nitrogen flow rate of 10 L / min to 50 L / min can form a stable airflow field, ensuring effective heat transfer on the sample surface and inside. This further accelerates the removal of volatile products and reduces the inhibition of decomposition temperature. During sintering, carbon nitride precursors may form micropores due to the removal of small molecules (such as NH3). Micro-positive pressure can promote the spreading and accumulation of carbon nitride molecules on the surface of sodium manganese phosphate pyrophosphate, reducing porosity, increasing the bonding force between the coating layer and the matrix, and thus improving ion transport efficiency at the interface. Under high-pressure nitrogen atmosphere, the sintering temperature window is expanded, promoting precursor densification and grain growth. Optionally, the flow rate of the atmosphere is any one of 10 L / min, 20 L / min, 30 L / min, 40 L / min, 50 L / min, or a range of both. Optionally, the pressure of the atmosphere is 0.101~0.12 MPa, and can be any one of 0.101 MPa, 0.102 MPa, 0.103 MPa, 0.104 MPa, 0.105 MPa, 0.106 MPa, 0.107 MPa, 0.108 MPa, 0.109 MPa, 0.111 MPa, 0.112 MPa, 0.113 MPa, 0.114 MPa, 0.115 MPa, 0.116 MPa, 0.117 MPa, 0.118 MPa, 0.119 MPa, 0.12 MPa, or a range of both.

[0052] In step 201 above, the sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium pyrophosphate; the role of the sodium source is to provide Na + It will enter the material lattice to form a layered or phosphate structure, ensuring that the final material is a sodium-based compound, and further suppressing Mn. 2+ Ce 3+ When metal ions hydrolyze in water to form hydroxyoxides, they more readily form a uniform precursor precipitate with phosphate ions. When carbon and sodium sources coexist, Na… + In the precursor, it can help the carbon-nitrogen precursor to be uniformly carbonized at high temperature, forming a dense carbon-nitrogen coating layer, thereby improving the electronic conductivity of the final material.

[0053] In step 201 above, the manganese source is at least one of manganese carbonate, manganese oxalate, manganese acetate, and manganese nitrate; after adding the phosphorus source, Mn 2+ With PO4 3- Coordination precipitation occurs, forming a manganese phosphate precursor with Mn-PO bonds. This further provides the framework for high-temperature solid-state reactions, Mn... 2+ The addition of [a specific ingredient] can adjust the lattice parameters, crystal size, and morphology of the precursor, resulting in finer particles and more uniform dispersion, which is beneficial for subsequent sintering or carbonitriding. The decomposition of the manganese source at high temperatures (e.g., MnCO3 → MnO + CO2) generates micropores or vacancies; the microporous structure facilitates [the process] for Li [a specific material]. + Rapid diffusion within the material improves rate performance.

[0054] In step 201 above, the cerium source is at least one selected from cerium nitrate, cerium acetate, cerium oxalate, and cerium carbonate. Cerium can react with water in Ce... 3+ / Ce 4+ They can rapidly transform into each other to form Ce 3+ This reversible redox pair provides a self-regulating redox environment within the system, further maintaining the concentration of manganese ions (Mn). 2+ / Mn 3+ / Mn 4+ The appropriate valence state of cerium prevents excessive reduction or oxidation of manganese. Cerium's coordination properties allow it to interact with other metal ions (Na+, Mn+, etc.) in the aqueous phase. n+ P 5+ This process forms complex or co-precipitate precursors, leading to more uniform Ce-Mn-P-Na-CN complexes in subsequent hydrothermal or heat treatment steps. The uniform metal distribution helps increase the specific surface area and active site density of the material. During high-temperature calcination, the redox cycle of cerium can inhibit excessive grain growth, maintain a fine particle structure, and thus improve the specific surface area and cycle stability of the material.

[0055] In sodium manganese pyrophosphate composites, iron doping reduces the effectiveness of rare earth element doping by increasing sodium ion diffusion channels due to the small ionic radius of iron. Furthermore, iron lacks 4f orbitals, resulting in poorer electronic conductivity compared to the 4f orbital transitions of rare earth elements. The presence of iron also lowers the average voltage and energy density of the material. Therefore, direct Ce doping of sodium manganese pyrophosphate mitigates the adverse effects of iron doping. In this embodiment, sodium manganese pyrophosphate is coated with carbon nitride, forming nanoscale primary particle agglomerations. The resulting compact particle structure and rough surface provide more active sites. This is because Ce... 3+The ionic radius of sodium manganese phosphate (≈1.03 Å) differs significantly from that of Mn ions. After doping, it replaces Mn sites in the NMPP lattice, causing local lattice distortions (such as bond elongation and bond angle twisting). This distortion disrupts the isotropic nature of crystal growth, leading to an imbalance in growth rates across different crystal planes (e.g., growth is hindered in one direction while excessive growth occurs in another), resulting in numerous steps, pyramids, or depressions on the surface and increased roughness. The conductivity of sodium manganese phosphate was improved by carbon nitride coating, and ionic conductivity was further enhanced by cerium doping. Cerium doping in sodium manganese phosphate primarily involves adding Ce³⁺. + Introduced into the Mn site, the modulated band structure introduces shallow energy levels, Ce³ + The highly localized 4f orbitals in sodium manganese pyrophosphate narrow the effective band gap and provide electrons in Mn²⁺. + / Mn³ + Interion hopping provides a low-energy intermediate state channel, significantly reducing the activation energy of polaron hopping, while the charge compensation mechanism forcibly increases Mn³. + Concentration provides more charge carriers. Meanwhile, Ce³ + The presence of cerium alters the electron density and local potential field of the adjacent Mn-O bonds, further optimizing the electron hopping path. Therefore, cerium doping cleverly utilizes the unique 4f electron configuration and ionic properties of rare earth elements to comprehensively improve the electronic conductivity and structural stability of sodium manganese pyrophosphate, thereby optimizing the battery rate performance.

[0056] In this embodiment, when carbon nitride is coated onto the surface of cerium-containing sodium manganese pyrophosphate, a unique synergistic effect may occur between nitrogen and cerium ions. The pyridine and pyrrole nitrogen atoms in the coating layer have strong coordination abilities, while the cerium ions on the surface of sodium manganese pyrophosphate have high charge density and empty valence orbitals (4f / 5d orbitals), making them excellent Lewis acid sites. During heat treatment, the N atoms in carbon nitride readily react with the Ce³ atoms on the surface of sodium manganese pyrophosphate. + Ions coordinate with each other to form Ce-N covalent / coordinate bonds. The formation of Ce-N bonds changes the local electronic structure at the interface, creating new, lower-energy electron transport channels at the interface. This further lowers the energy barrier for electrons to transition or be injected from the matrix to the coating layer, greatly promoting electron transfer at the interface.

[0057] In this embodiment, cerium internally optimizes bulk electron / ion conduction and stabilizes the framework, while carbon nitride externally provides high-speed electron pathways and physical / chemical protection. The Ce-N bonds formed at the interface are the key chemical hub for the synergistic effect, strongly bridging the bulk phase and the coating layer, improving electronic conductivity, and ultimately significantly enhancing the rate performance of the sodium-ion battery.

[0058] In step 201 above, the molar ratio of the sodium source, manganese source, cerium source, and phosphorus source is (3.90~4.2):(2.9~3.0):(0.003~0.3):4, and may include, but is not limited to, 3.90:2.9:0.003:4, 3.95:2.92:0.005:4, 4.0:2.94:0.007:4, 4.05:2.97:0.3:4, 4.10:3.0:0.02:4, and 4.20:3.0:0.3:4. The main function of this solution is to act as a "metal-phosphorus precursor solution," enabling the conversion to Ce in the subsequent drying-calcination process. - The doped Na-Mn-P composite oxide achieved molecular-level homogeneous mixing, allowing metal ions (Na+, Mn-, P-) to be present in the aqueous phase. + Mn 2+ / Mn 4+ Ce 3+ / Ce 4+ ) and PO4 3- Complete coordination avoids local enrichment or phase separation during solid-phase mixing, thus obtaining a precursor with a uniform structure.

[0059] In step 201 above, the carbon source content is 3wt% to 14.5wt% of the cathode material, and can be, but is not limited to, any value from 3wt%, 4.5wt%, 6wt%, 7.5wt%, 9wt%, 10.5wt%, 12wt%, 13.5wt%, 14.5wt%, or any combination thereof. Within this range, the carbon source doping effect is better. Sodium manganese pyrophosphate is coated with a C source. Because the manganese-sodium structure is more unstable and the phase change is larger, it is easy to cause the carbon coating layer to fall off, resulting in a more deteriorated cycle performance. By introducing N, the CN coating structure is more stable, which helps to suppress its volume change. It can also form Mn-NC, Ce-NC, and their overlapping distribution, which greatly changes the surface composition distribution and forms a more stable structure than the manganese-carbon structure. This increases the interfacial energy, strongly bridges the bulk phase and the coating layer, and reduces the instability of the manganese-sodium structure, the larger phase change, and the easy cause of carbon coating layer falloff, which leads to a more deteriorated cycle performance.

[0060] In step 203 above, the spray drying process has an inlet air temperature of 183℃~248℃ and an outlet air temperature of 91℃~118℃. Within this range, it is beneficial for a more dispersed particle size distribution, more uniform carbon and nitrogen coating in the later stages, and reduces the adverse effects of the large phase transformation of pure sodium manganese pyrophosphate without added iron. Optionally, the inlet air temperature can be any value of 183℃, 188℃, 193℃, 198℃, 203℃, 208℃, 213℃, 218℃, 223℃, 228℃, 233℃, 238℃, 243℃, 248℃, or a range of any two of the above. Optionally, the outlet air temperature can be any value of 91℃, 96℃, 101℃, 106℃, 111℃, 118℃, or a range of any two of the above. If the inlet air temperature is <183℃, moisture removal is slow, the precursor is prone to agglomeration, and Ce is trapped inside the agglomeration, making it difficult to enter the crystal lattice during subsequent sintering. If the inlet air temperature is >248℃, Ce is easily oxidized, resulting in performance deterioration and increased energy consumption. A precursor outlet air temperature >90℃ can ensure low residual moisture in the precursor and prevent adhesion. An outlet air temperature <119℃ can prevent the precursor surface from "hardening," which would lead to uneven distribution of Ce inside the particles.

[0061] In step 204 above, the heating rate of the sintering process is 1℃ / min to 6℃ / min. The heating rate is a key parameter for adjusting the densification degree, grain size, thermal stress, and production efficiency. A low rate of 1-2℃ / min is beneficial for obtaining high density, fine and uniform grains, and low crack risk; a medium rate of 3-4℃ / min improves process efficiency while maintaining a good microstructure; and a higher rate of 5-6℃ / min further shortens the sintering time. Optionally, the heating rate of the sintering process can be any value of 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, or 6℃ / min, or a range of any two of the above. A suitable heating rate allows Ce ions sufficient time to diffuse gradually with increasing temperature. Ce has a low diffusion coefficient, and a lower heating rate allows cerium to completely penetrate from the particle surface to the core, achieving uniform bulk doping and avoiding local concentration gradients. At the same time, the grains mainly grow in a "layered" manner (surface atoms are arranged layer by layer), resulting in fewer defects and higher crystallinity. When the heating rate is high, Ce diffusion lags behind. When the high temperature is reached rapidly, the NMPP grains have already grown rapidly, and Ce, unable to diffuse sufficiently, accumulates at the grain boundaries, forming impurity phases and losing its function of improving conductivity. Simultaneously, rapid heating leads to uncontrolled grain size, localized overheating causing recrystallization, and individual primary grains suddenly increasing to the micrometer scale, resulting in a wide grain size distribution and increased polarization. Furthermore, rapid heating causes a sharp increase in sodium volatilization rate, affecting the elemental ratio and easily forming impurity phases. Preferably, a moderate heating rate of 3-4 °C / min is used, which is beneficial for improving electrical performance while also increasing process efficiency.

[0062] In step 204 above, the sintering includes a first-stage sintering and a second-stage sintering. The sintering temperature of the first stage is 200~440℃, and the holding time is 2~8h; the sintering temperature of the second stage is 510~690℃, and the holding time is 5~20h, followed by cooling to room temperature. If the sintering temperature is too low, the cathode material will not sinter completely, resulting in low crystallinity and significantly reducing the discharge specific capacity and rate performance of the sodium-ion battery. If the temperature is too high, the structure will collapse, forming impurities and increasing energy consumption. Therefore, within this sintering temperature range, the cathode material obtained has good electronic conductivity and ionic conductivity, easily maximizing its capacity and further improving the rate performance of the sodium-ion battery. Low-temperature sintering first causes partial decomposition of the carbon source, generating a reducing atmosphere. The slower decomposition of the carbon source allows for a more complete reaction, which is beneficial for the formation of more Mn-NC and Ce-NC substances. The longer decomposition time at low temperatures promotes a more uniform distribution. The Mn-NC, Ce-NC, and their overlapping distribution significantly alter the surface composition distribution, reducing impurity phase formation, increasing phase purity, and preventing the destruction of Mn-NC and Ce-NC. Optionally, the holding time for the first sintering stage is any one of 2h, 4h, 6h, or 8h, or a range of both. Optionally, the holding time for the second sintering stage is any one of 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, or 20h, or a range of both.

[0063] This invention also provides a polyanion sodium-ion battery, including a positive electrode sheet, wherein the positive electrode sheet includes a positive current collector and a positive active material layer disposed on the positive current collector, wherein the positive active material layer includes the aforementioned carbon nitride-coated sodium manganese pyrophosphate-based positive electrode material or the carbon nitride-coated sodium manganese pyrophosphate-based positive electrode material prepared by the aforementioned preparation method.

[0064] In some embodiments, the positive electrode sheet is prepared as follows: the components used to prepare the positive electrode sheet, such as carbon nitride-coated sodium manganese phosphate-based positive electrode material, binder and any other components, are dispersed in a solvent such as N-methylpyrrolidone to form a positive electrode slurry; the positive electrode slurry is coated on a positive electrode current collector; after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0065] The polyanion sodium-ion battery provided in this embodiment of the invention also includes a negative electrode sheet, a separator, and an electrolyte.

[0066] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on the negative current collector. The negative active material layer may include at least one of artificial graphite, natural graphite, hard carbon, soft carbon, and carbon black.

[0067] In this embodiment, the electrolyte acts as a conductor of ions between the positive and negative electrodes. The electrolyte can be liquid, gel-like, or entirely solid. In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent, wherein the electrolyte salt is a lithium salt.

[0068] To make the inventive objectives, technical solutions, and beneficial effects of this invention clearer, the invention is further described below with reference to embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0069] The present invention will be described in detail below through embodiments.

[0070] Example 1

[0071] Carbon nitride-coated cerium-doped sodium manganese pyrophosphate Na 4.03 Ce 0.03 Mn 2.87 The preparation method of (PO4)2P2O7 / C3N4 includes the following steps:

[0072] (1) Weigh out 0.03 mol of sodium bicarbonate, 4 mol of sodium dihydrogen phosphate, 2.87 mol of manganese acetate tetrahydrate, 0.03 mol of cerium oxalate, and 37 g of melamine, add them to 5 L of deionized water, and stir evenly to obtain slurry A.

[0073] (2) Place slurry A in a sand mill and grind it until the particle size D50 reaches 0.27μm to obtain slurry B.

[0074] (3) Spray dry slurry B with an inlet air temperature of 248°C and an outlet air temperature of 95°C to obtain a precursor with a D50 of 17m.

[0075] (4) The precursor was placed in a tube furnace and held at 290℃ for 5 h and 610℃ for 12 h under a nitrogen atmosphere. The heating rate was 3℃ / min. The sintering atmosphere was nitrogen with a flow rate of 45 L / min and a pressure of 0.105 MPa. After cooling to room temperature, carbon nitride-coated cerium-doped sodium manganese pyrophosphate (Na) was obtained. 4.05 Ce 0.03 Mn 2.97 (PO4)2P2O7 / C3N4.

[0076] Example 2

[0077] Preparation of carbon nitride-coated cerium-doped sodium manganese pyrophosphate Na4Ce 0.1 Mn 2.8The preparation method of (PO4)2P2O7 / C3N4 differs from that in Example 1 mainly in that the molar ratio of the raw materials, sodium dihydrogen phosphate, manganese acetate tetrahydrate, and cerium oxalate, is changed to 4:2.8:0.1, yielding Na4Ce. 0.1 Mn 2.9 (PO4)2P2O7 / C3N4.

[0078] In Example 1, step (1) was changed to 4 mol sodium dihydrogen phosphate, 2.8 mol manganese acetate tetrahydrate, 0.1 mol cerium oxalate, and 62 g melamine. All other steps were the same as in Example 1.

[0079] Example 3

[0080] The main difference from Example 1 is the change in the type of raw materials.

[0081] In Example 1, the sodium source weighed in step (1) was changed to sodium carbonate, the manganese source to manganese oxalate dihydrate, the phosphorus source to ammonium dihydrogen phosphate, and the carbon source to urea. All other aspects remained the same as in Example 1. This yielded sodium manganese phosphate (Na₂O₃) coated with carbon nitride and doped with cerium. 4.03 Ce 0.03 Mn 2.87 (PO4)2P2O7 / C3N4.

[0082] Example 4

[0083] The main difference from Example 1 is the change in sintering temperature: the holding time was changed from 290℃ for 5 hours to 350℃ for 4 hours, and the holding time was changed from 610℃ for 12 hours to 650℃ for 8 hours. All other aspects are the same as in Example 1, yielding carbon nitride-coated cerium-doped sodium manganese pyrophosphate (Na). 4.03 Ce 0.03 Mn 2.87 (PO4)2P2O7 / C3N4.

[0084] Comparative Example 1

[0085] The main difference from Example 1 is that cerium doping was removed, resulting in Na. 4.06 Mn 2.9 (PO4)2P2O7 / C3N4: Step (1) in Example 1 is modified by weighing 0.06 mol of sodium carbonate, 4 mol of sodium dihydrogen phosphate, 2.9 mol of manganese acetate tetrahydrate, and 37 g of melamine. All other steps remain the same as in Example 1, yielding sodium manganese phosphate coated with carbon nitride. 4.06 Mn 2.9 (PO4)2P2O7 / C3N4. Calculations based on the Bragg equation show that cerium doping increases the interplanar spacing by 5.8%, indicating that the incorporation of cerium atoms increases the lattice size and improves rate performance. Furthermore, the XRD pattern reveals that the sample is a highly crystalline orthorhombic crystal with no obvious impurities.

[0086] Comparative Example 2

[0087] The main difference from Example 1 is that the carbon nitride coating was removed, resulting in Na. 4.03 Ce 0.03 Mn 2.87 (PO4)2P2O7: Step (1) in Example 1 is modified by weighing 0.03 mol of sodium carbonate, 4 mol of sodium dihydrogen phosphate, 2.87 mol of manganese acetate tetrahydrate, and 0.03 mol of cerium oxalate. All other steps remain the same as in Example 1, yielding cerium-doped sodium manganese pyrophosphate (Na). 4.03 Ce 0.03 Mn 2.87 (PO4)2P2O7.

[0088] Comparative Example 3

[0089] The main difference from Example 1 is the change in sintering temperature, from 600℃ to 450℃, which was changed to 190℃. Everything else is the same as in Example 1, yielding carbon nitride-coated fluorine-doped sodium manganese pyrophosphate (Na). 4.05 Ce 0.03 Mn 2.87 (PO4)2P2O7 / C3N4.

[0090] Comparative Example 4

[0091] The difference between this comparative example and Example 1 is that melamine is replaced by glucose by mass in this comparative example, thus producing carbon-coated cerium-doped sodium manganese pyrophosphate (Na). 4.03 Ce 0.03 Mn 2.87 (PO4)2P2O7 / C; Specifically, the steps are as follows: Weigh 0.03 mol of sodium carbonate, 4 mol of sodium dihydrogen phosphate, 2.87 mol of manganese acetate tetrahydrate, 0.03 mol of cerium oxalate, and 37 g of glucose, add them to 5 L of deionized water, and stir evenly to obtain slurry A.

[0092] (2) Place slurry A in a sand mill and grind it until the particle size D50 reaches 0.30μm to obtain slurry B.

[0093] (3) Spray dry slurry B with an inlet air temperature of 250°C and an outlet air temperature of 90°C to obtain a precursor with a D50 of 18.5 μm.

[0094] (4) The precursor was placed in a tube furnace and held at 320℃ for 5 hours and 600℃ for 12 hours under a nitrogen atmosphere. The heating rate was 3℃ / min. After cooling to room temperature, cerium-doped sodium manganese pyrophosphate coated with carbon nitride was obtained. 4.03 Ce 0.03 Mn 2.87(PO4)2P2O7 / C.

[0095] Comparative Example 5

[0096] The difference between this comparative example and Example 1 is that melamine is replaced with glucose by mass in this comparative example, and cerium doping is removed, thus obtaining carbon-coated sodium manganese pyrophosphate (Na). 4.06 Mn 2.9 (PO4)2P2O7 / C; specifically, the following steps are included:

[0097] 1) Weigh out 0.06 mol of sodium bicarbonate, 4 mol of sodium dihydrogen phosphate, 2.9 mol of manganese acetate tetrahydrate, and 37 g of glucose, add them to 5 L of deionized water, and stir well to obtain slurry A.

[0098] (2) Place slurry A in a sand mill and grind it until the particle size D50 reaches 0.30μm to obtain slurry B.

[0099] (3) Spray dry slurry B with an inlet air temperature of 250°C and an outlet air temperature of 90°C to obtain a precursor with a D50 of 18.5 μm.

[0100] (4) The precursor was placed in a tube furnace and held at 320℃ for 5 hours and 600℃ for 12 hours under a nitrogen atmosphere. The heating rate was 3℃ / min. After cooling to room temperature, cerium-doped sodium manganese pyrophosphate coated with carbon nitride was obtained. 4.05 Mn3(PO4)2P2O7 / C.

[0101] Comparative Example 6

[0102] The difference between this comparative example and Example 1 is that slurry A was placed in a sand mill and milled until the particle size D50 reached 0.50 μm to obtain slurry B. Slurry B was then spray-dried to obtain a precursor with a D50 of 18.5 μm.

[0103] Comparative Example 7

[0104] The main difference from Example 1 is that the sintering atmosphere is nitrogen, the flow rate of the atmosphere is 2L / min, and the pressure of the atmosphere is 0.1MPa.

[0105] Application examples

[0106] (1) Preparation of the positive electrode sheet of sodium-ion battery

[0107] The positive electrode materials prepared in each embodiment and comparative example were mixed with conductive carbon (Super P) and binder polyvinylidene fluoride (PVDF) at a mass ratio of 7:2:1. After grinding, an appropriate amount of 1-methyl-2-pyrrolidone (NMP) solvent was added, and the mixture was placed in a homogenizer and vibrated for 10 minutes to uniformly coat the slurry onto carbon-coated aluminum foil. After drying, the aluminum foil coated with black slurry was punched into a circular electrode sheet with a diameter of 14 mm using a slicing machine. Then, a certain pressure was applied to the electrode sheet using a pressing machine to press it into a sheet. Finally, the electrode sheet was placed in a vacuum oven and vacuum dried at 120°C for 12 hours to obtain the positive electrode sheet for sodium-ion batteries.

[0108] (2) Preparation of sodium-ion batteries

[0109] The prepared sodium-ion battery positive electrode sheet was used as the working electrode, and metallic sodium was used as the counter electrode. A 1 mol / L NaClO4 / PC:EMC:FEC (49:49:2) organic electrolyte was used to assemble a coin cell in a glove box filled with argon atmosphere.

[0110] (3) Electrical performance testing

[0111] The electrochemical performance of the prepared sodium-ion battery was tested using conventional methods in the art, with a test voltage range of 2.0–4.3 V. The test results are shown in Table 1.

[0112] Table 1

[0113]

[0114] Table 1 shows the average charge-discharge and discharge voltages of the cathode materials prepared in Examples 1, 2, 3, 4, Comparative Examples 1, 2, and 3 at a rate of 0.2C.

[0115] As can be seen from the comparison between Example 1 and Comparative Example 1, cerium doping can significantly improve the rate performance of sodium-ion batteries.

[0116] As can be seen from the comparison between Example 1 and Comparative Example 2, the carbon nitride coating can significantly improve the capacity utilization and rate performance of sodium-ion batteries.

[0117] As can be seen from the comparison between Example 1 and Comparative Example 3, when the sintering temperature is too low, the positive electrode material will not be sintered completely, which will significantly reduce the discharge specific capacity and rate performance of the sodium-ion battery.

[0118] As can be seen from the comparison between Example 1 and Comparative Example 4, carbon nitride coating alone and cerium doping alone cannot effectively improve the capacity and rate performance of sodium-ion batteries. However, when carbon nitride and cerium work together, the capacity and rate performance can be significantly improved.

[0119] As shown in Examples 1-3, the types and molar ratios of sodium, iron, phosphorus, and cerium sources have little impact on the discharge specific capacity and rate performance of sodium-ion batteries. Therefore, the present invention uses carbon nitride-coated cerium-doped sodium manganese pyrophosphate with the chemical formula Na. b-a Ce a Mn 2.9-a (PO4)2P2O7 / C3N4, where 0.001≤a≤0.3 and 3.95≤b≤4.1, exhibits good discharge specific capacity and rate performance.

[0120] like Figure 1 As shown in the SEM image of Example 1, the product consists of spherical particles of varying sizes. These particles are formed by the aggregation of nanoscale primary particles, resulting in a compact particle structure and a rough surface, which can provide more active sites.

[0121] like Figure 2 As shown in the XRD patterns of Example 1 and Comparative Example 1, analysis reveals that the main peak intensity at 33° in Example 1 is higher, and its peak position shifts to the left compared to Comparative Example 1. This indicates that the incorporation of cerium atoms increases the lattice size and improves the rate performance. Furthermore, the figure shows that the sample of Example 1 is a highly crystalline orthorhombic phase crystal, and is a pure phase without any other impurities detectable by XRD.

[0122] like Figure 3 As shown in the figure, the capacity retention rate at different expansion rates of Example 1 is shown. It can be seen from the figure that the sample has excellent expansion performance, and the capacity retention rate is still as high as 91.2% at the 10C expansion rate.

[0123] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A carbon nitride-coated sodium manganese phosphate-based cathode material, characterized in that, The positive electrode material includes a core and a coating layer disposed on the surface of the core. The core is cerium-doped sodium manganese pyrophosphate, and the coating layer is carbon nitride. The general chemical formula of the positive electrode material is Na. b-a Ce a Mn 2.9-a (PO4)2P2O7 / C3N4, where 0.001≤a≤0.3, 3.95≤b≤4.

1.

2. The carbon nitride-coated manganese phosphate sodium-based cathode material according to claim 1, characterized in that, The general chemical formula of the cathode material is Na. b-a Ce a Mn 2.9-a (PO4)2P2O7 / C3N4, where 0.01≤a≤0.05, 4.02≤b≤4.

06.

3. The carbon nitride-coated manganese phosphate sodium-based cathode material according to claim 1, characterized in that, XRD analysis revealed that the main peak intensity at 33.38° was shifted to the left by more than 0.1° compared to the main peak intensity at 33.38° of the same undoped Ce standard sample, and the interplanar spacing increased by more than 2%.

4. A method for preparing a carbon nitride-coated sodium manganese phosphate-based cathode material, characterized in that, Includes the following steps: S1. Dissolve sodium source, manganese source, phosphorus source, cerium source and carbon source in deionized water to obtain solution A, wherein the carbon source is at least one of urea and melamine; S2. The solution from step S1 is milled to obtain a slurry; the particle size D50 of the slurry is ≤0.28μm; S3. Spray dry the slurry from step S2 to obtain a precursor; the particle size D50 of the precursor is ≤18μm. S4. The precursor from step S3 is sintered. The sintering includes a first stage sintering and a second stage sintering. The sintering temperature of the first stage sintering is 200~440℃, and the sintering temperature of the second stage sintering is 510~690℃, thus obtaining a carbon-coated cerium-doped sodium manganese pyrophosphate cathode material. The sintering atmosphere is nitrogen, the flow rate of the atmosphere is 10L / min~50L / min, and the pressure of the atmosphere is 0.101~0.12 MPa.

5. The preparation method according to claim 4, characterized in that, In step S1, the sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium pyrophosphate. The manganese source is at least one of manganese carbonate, manganese oxalate, manganese acetate, and manganese nitrate. The cerium source is at least one of cerium nitrate, cerium acetate, cerium oxalate, and cerium carbonate.

6. The preparation method according to claim 4, characterized in that, In step S1, the molar ratio of the sodium source, manganese source, cerium source, and phosphorus source is (3.90~4.2):(2.9~3.0):(0.003~0.3):4; the carbon source content is 3wt%~14.5wt% of the cathode material.

7. The preparation method according to claim 4, characterized in that, In step S3, the spray drying process has an inlet air temperature of 183℃~248℃ and an outlet air temperature of 91℃~118℃. In step S4, the sintering heating rate is 1℃ / min~6℃ / min. The sintering includes a first stage sintering and a second stage sintering. The sintering temperature of the first stage sintering is 200~440℃, and the holding time is 2~8h. The sintering temperature of the second stage sintering is 510~690℃, and the holding time is 5~20h, and then the temperature is cooled to room temperature.

8. The preparation method according to claim 4, characterized in that, The sintering includes a first stage sintering and a second stage sintering. The first stage sintering is heated to 280~330℃, and the second stage sintering is heated to 605~650℃.

9. The preparation method according to claim 4, characterized in that, The sintering includes a first stage sintering and a second stage sintering, wherein the first stage sintering is heated to 280~290℃ and the second stage sintering is heated to 605~650℃.

10. A polyanion sodium-ion battery, characterized in that, The material includes a positive electrode sheet, which includes a positive current collector and a positive active material layer disposed on the positive current collector. The positive active material layer includes a carbon nitride-coated sodium manganese pyrophosphate-based positive electrode material as described in any one of claims 1 to 3, or a carbon nitride-coated sodium manganese pyrophosphate-based positive electrode material prepared by the preparation method described in any one of claims 4 to 9.

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