Positive electrode material for sodium battery, preparation method thereof, positive electrode sheet and sodium battery

Abstract

This application provides a positive electrode material for sodium batteries, a method for preparing the same, a positive electrode sheet, and a sodium battery. The positive electrode material includes a core and a coating layer covering the surface of the core; the molecular formula of the core includes Na3V. 2‑x M x (PO4)2F3, where M represents a dopant element that can substitute for V, and the M element includes at least one selected from Fe, Cr, Mn, Co, Ti, Ni, Cu, Zn, Mo, Nb, Zr, La, and Ce, where 0 ≤ x < 0.2; the coating material includes carbon materials, and at least a portion of the Raman spectrum of the carbon materials exhibits Ig... D / I G The value is y, where 0.9 ≤ y < 1. By coating the surface of the core active component with the above-mentioned carbon material, the electronic conductivity and cycle performance of the material can be effectively improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to positive electrode materials for sodium batteries and their preparation methods, positive electrode sheets, and sodium batteries. Background Technology

[0002] NASICON-type materials are widely used as cathode materials for sodium batteries due to their three-dimensional ion diffusion channels and high charge / discharge voltage. Among them, Na3V2(PO4)2F3 cathode material has more significant advantages, such as its higher sodium ion diffusion coefficient (approximately 7.2 × 10⁻⁶). -10 cm 2 It has high ionic conductivity and an energy density of up to 507 Wh / kg. However, the electronic conductivity of Na3V2(PO4)2F3 material is extremely low (approximately 10 Wh / s). -12 The current density (S / cm) severely restricts the material's ability to co-transport electrons and ions at high current densities, thus affecting the full realization of its battery performance.

[0003] Currently, the carbothermal reduction method is a commonly used method to prepare carbon-coated Na3V2(PO4)2F3 to improve its electronic conductivity. However, the final carbon coating material is generally amorphous carbon, which has limited effect on improving the electronic conductivity of the material. Summary of the Invention

[0004] In view of this, this application provides a positive electrode material for sodium batteries, which is obtained by coating the surface of the core active component with I D / I G Carbon materials with values ​​in the range of 0.9≤y<1 can effectively improve the electronic conductivity and cycle performance of the material, thus enabling them to be used to provide a battery that combines high rate performance and good long cycle performance.

[0005] The first aspect of this application provides a positive electrode material for sodium batteries, comprising a core and a coating layer covering the surface of the core; the molecular formula of the core includes Na3V. 2-x M x (PO4)2F3, where M represents a dopant element that can substitute for V, and M includes at least one of Fe, Cr, Mn, Co, Ti, Ni, Cu, Zn, Mo, Nb, Zr, La, and Ce, where 0 ≤ x < 0.2; the coating material includes carbon materials, and the Raman spectrum of the carbon material has an Ig... D / I G The value is y, 0.9 ≤ y < 1; where, the I D / I G The peak intensity ratio of the D peak and the G peak in the Raman spectrum of the carbon material is given, wherein the Raman shift of the D peak is at 1300 cm⁻¹. -1 -1360cm -1Within the range, the Raman shift of the G peak is at 1580 cm⁻¹. -1 -1600cm -1 Within the range.

[0006] In the above coating layer, the carbon material I D / I G The values ​​are in the range of 0.9 ≤ y < 1, therefore the coating layer has high electronic conductivity. Coating the surface of the core material with this type of material can significantly improve the electronic conductivity of the cathode material, thereby enhancing the rate performance of the battery. Simultaneously, the carbon skeleton of the aforementioned carbon material exhibits good structural stability, which can more effectively suppress the Na3V in the core. 2-x M x The structural changes of (PO4)2F3 during repeated lithium insertion and extraction processes can significantly improve the cycle performance of cathode materials.

[0007] A second aspect of this application provides a method for preparing a positive electrode material for sodium batteries, comprising the following steps:

[0008] (1) The core raw material is mixed with a carbon source to obtain a precursor material; wherein the carbon source includes a substance containing aromatic hydrocarbons;

[0009] (2) The precursor material is calcined to obtain a cathode material; wherein the cathode material comprises a core and a coating layer covering the surface of the core; the general molecular formula of the core includes Na3V. 2-x M x (PO4)2F3, where M represents a dopant element that can substitute for V, and M includes at least one of Fe, Cr, Mn, Co, Ti, Ni, Cu, Zn, Mo, Nb, Zr, La, and Ce, where 0 ≤ x < 0.2; the coating material includes carbon materials, and the Raman spectrum of the carbon material has an Ig... D / I G The value is y, 0.9≤y<1.

[0010] The above preparation method is simple, highly controllable, and has high production efficiency, enabling large-scale industrial production.

[0011] A third aspect of this application provides a positive electrode sheet, comprising the positive electrode material for sodium batteries provided in the first aspect of this application or the positive electrode material for sodium batteries prepared by the preparation method provided in the second aspect of this application. This positive electrode sheet can be used to provide batteries with good rate performance and good cycle performance.

[0012] This application provides a sodium battery in a fourth aspect, including the positive electrode provided in the third aspect. This sodium battery has high energy density, good rate performance, and excellent long-cycle capability. Attached Figure Description

[0013] Figure 1 The X-ray diffraction (XRD) spectra of the cathode materials of Examples 1-3 of this application are shown below.

[0014] Figure 2 Thermogravimetric analysis curves of the cathode materials in Examples 1-3 of this application;

[0015] Figure 3A The image shows the Raman spectrum of the cathode material in Example 2 of this application.

[0016] Figure 3B The Raman spectrum of the cathode material of Comparative Example 2 of this application;

[0017] Figure 4 The image provided in Embodiment 2 of this application is a scanning electron microscope (SEM) image of the cathode material.

[0018] Figure 5 Transmission electron microscope (TEM) images of the cathode material provided in Embodiment 2 of this application at different magnifications;

[0019] Figure 6 The specific capacity-cycle count curves of batteries with the positive electrode materials of Examples 1-3 of this application at different rates;

[0020] Figure 7 The specific capacity-cycle count curves of batteries with cathode materials from Comparative Examples 1-3 at different rates are shown. Detailed Implementation

[0021] This application provides a positive electrode material for sodium batteries, comprising a core and a coating layer covering the surface of the core; the molecular formula of the core includes Na3V. 2-x M x (PO4)2F3, where M represents a dopant element that can substitute for V, and the M element includes at least one selected from Fe, Cr, Mn, Co, Ti, Ni, Cu, Zn, Mo, Nb, Zr, La, and Ce, where 0 ≤ x < 0.2; the coating material includes carbon materials, and the Raman spectrum of the carbon material has an Ig... D / I G The value is y, 0.9 ≤ y < 1; where, the I D / I G The peak intensity ratio of the D peak and the G peak in the Raman spectrum of the carbon material is given, wherein the Raman shift of the D peak is at 1300 cm⁻¹. -1 -1360cm -1 Within the range, the Raman shift of the G peak is at 1580 cm⁻¹.-1 -1600cm -1 Within the range.

[0022] As is well known, Raman spectroscopy is one of the most important characterization methods for carbon materials, specifically for determining the microstructure and the degree of order in the arrangement of carbon atoms. When wavelengths of 514 nm, 532 nm, and 633 nm are selected as excitation sources, carbon materials with ordered carbon atoms exhibit two characteristic peaks in their Raman spectra: the D peak and the G peak. The D peak is located at approximately 1300 cm⁻¹. -1 -1360cm -1 Nearby, it represents the presence of defective structures in carbon materials (i.e., the presence of disordered carbon), with the G peak located at 1580 cm⁻¹. -1 -1600cm -1 Nearby, representing the presence of sp in carbon materials 2 Hybridized carbon (that is, the presence of carbon atoms arranged in an ordered manner), and the peak intensity ratio of the D peak to the G peak is I. D / I G This can represent the degree of orderliness of the carbon atom arrangement in the carbon material. In the cathode material of this application, the coating material of the core active component includes carbon material, and at least a portion of the carbon material contains I... D / I G When the value is greater than or equal to 0.9 and less than 1, the ordered arrangement of carbon atoms is relatively high, and the electronic conductivity of carbon materials is high. This significantly improves the co-transport capability of electrons and ions at high current densities, thus giving the cathode material better rate performance. Simultaneously, satisfying the above-mentioned I... D / I G The carbon skeleton structure of the carbon material with the specified value has good stability, which can more effectively mitigate the structural changes of the core active components during repeated charging, resulting in better long-cycle stability of the cathode material.

[0023] Furthermore, replacing the V element in Na3V2(PO4)2F3 with the M element in the core material can effectively improve the specific capacity of the core material and further optimize the electrochemical performance of the cathode material. Controlling x to a range less than 0.2, that is, controlling the doping amount of the M element within a suitable range, is beneficial to ensuring the electrochemical activity of the cathode material.

[0024] In this application, by way of example, carbon material I D / I G The value y can be 0.90, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 0.995, etc.

[0025] In some embodiments of this application, 0.92 ≤ y ≤ 0.98. That is, the I of the carbon material D / IG The value is in the range of 0.92-0.98. During the preparation of the cathode material, a relatively low calcination temperature allows the carbon atoms in the carbon material to be arranged in an ordered manner within this range. At this temperature, the carbon material possesses good electronic conductivity, ensuring good rate performance of the cathode material; simultaneously, it allows the coating material to have a suitable thickness, thus exhibiting a strong ability to suppress core size changes. This controls the particle size of the cathode material within a suitable range, enabling the material to fully exert its electrochemical performance.

[0026] In some embodiments of this application, the average particle size of the cathode material is in the range of 50nm-2000nm. Exemplarily, the average particle size of the cathode material can be 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 200nm, 300nm, 500nm, 1000nm, 1500nm, 2000nm, etc. Controlling the average particle size of the cathode material within the above range helps to ensure a shorter transport path for sodium ions during charging and discharging, which is beneficial for rapid sodium ion insertion / extraction. Furthermore, it allows for control of the tap density of the cathode material within a suitable range, reducing the likelihood of agglomeration and ensuring a higher energy density in the final battery. Specifically, the average particle size refers to the average particle size of the primary particles of the cathode material. In some cases, multiple primary particles can aggregate to form secondary particles.

[0027] In this application, the aforementioned average particle size refers to the D50 particle size of the cathode material, specifically the particle size corresponding to when the cumulative percentage of cathode material reaches 50%. The aforementioned D50 particle size can be determined by observing the particle size of the cathode material under a scanning electron microscope (SEM) (the number of cathode material particles in the sample is generally above 500, preferably above 1000), and measuring the particle size corresponding to when the cumulative percentage of cathode material particles reaches 50%.

[0028] In some embodiments of this application, the thickness of the coating layer in the above-mentioned cathode material is in the range of 5nm-20nm. Exemplarily, the thickness of the coating layer can be 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, 14nm, 15nm, 16nm, 17nm, 18nm, 19nm, 20nm, etc. This effectively improves the electronic conductivity of the cathode material, effectively mitigates the structural changes (or structural collapse) of the core material during repeated charge-discharge processes, and does not affect the proportion of the core material in the cathode material, that is, it does not affect the electrochemical performance of the cathode material. Furthermore, a coating layer thickness within a suitable range can also significantly reduce particle agglomeration, which is beneficial to the electrochemical performance of the cathode material.

[0029] In some embodiments of this application, the mass percentage of carbon material in the cathode material is 7%-15%. Exemplarily, the mass percentage of carbon material in the cathode material can be 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 14.5%, 15%, etc. Controlling the mass percentage of carbon material within the above range is beneficial for improving the conductivity of the cathode material, ensuring good co-transport capacity of electrons and ions at high current densities, and preventing it from occupying a portion of the core active material, thus ensuring that the cathode material still has a high specific capacity. Understandably, a mass percentage of carbon material within the above range also helps to control the coating thickness of the cathode material appropriately, resulting in a shorter ion transport distance within the cathode material particles and reducing the likelihood of particle agglomeration. Specifically, the mass percentage of carbon material can be characterized using thermogravimetric analysis.

[0030] In some specific embodiments of this application, the mass percentage of carbon material is in the range of 10%-14%; correspondingly, the thickness of the coating layer is in the range of 14nm-18nm. At this point, the coating layer can provide enough free electrons to maintain the electronic conductivity of the material at a relatively optimal level without affecting the crystallinity of the core material. The crystallinity and crystal structure of the core material can still be maintained in an optimal state, resulting in better specific capacity and superior overall performance.

[0031] This application also provides a method for preparing a positive electrode material for sodium batteries, including the following steps:

[0032] (1) The core raw material is mixed with a carbon source to obtain a precursor material; wherein the carbon source includes a substance containing aromatic hydrocarbons;

[0033] (2) The precursor material is calcined to obtain a cathode material; wherein the cathode material comprises a core and a coating layer covering the surface of the core; the general molecular formula of the core includes Na3V.2-x M x (PO4)2F3, where M represents a dopant element that can substitute for V, and the M element includes at least one selected from Fe, Cr, Mn, Co, Ti, Ni, Cu, Zn, Mo, Nb, Zr, La, and Ce, where 0 ≤ x < 0.2; the coating material includes carbon materials, and at least a portion of the Raman spectrum of the carbon materials exhibits Ig... D / I G The value is y, 0.9 ≤ y < 1; where, the I D / I G The peak intensity ratio of the D peak and the G peak in the Raman spectrum of the carbon material is given, wherein the Raman shift of the D peak is at 1300 cm⁻¹. -1 -1360cm -1 Within the range, the Raman shift of the G peak is at 1580 cm⁻¹. -1 -1600cm -1 Within the range.

[0034] The aforementioned aromatic hydrocarbon-containing substances, used as a carbon source, can be incorporated into the newly formed core material Na3V during the calcination process. 2-x M x In situ generation of a coating layer with high atomic order and good framework structure on the outer surface of (PO4)2F3 or the outer surface of existing core materials; and the above carbon source can achieve atomic ordering temperature and core material synthesis Na3V 2-x M x The temperature of (PO4)2F3 is consistent, which can ensure a high degree of reaction of each element source and the formation of Na3V. 2-x M x (PO4)2F3 has fewer crystal defects and avoids the decomposition of the core material (including newly formed cores and existing core materials) caused by high-temperature calcination. This results in a core-shell structured cathode material for sodium batteries with good electronic conductivity and strong long-cycle stability. The above preparation method is simple to operate, highly controllable, and efficient, making it suitable for large-scale industrial production.

[0035] In this application, the carbon source contains at least one aromatic hydrocarbon. The aromatic hydrocarbon can be a monocyclic aromatic hydrocarbon or a polycyclic aromatic hydrocarbon. A polycyclic aromatic hydrocarbon refers to a substance containing two or more aromatic rings; for example, it can be a biphenyl compound or a fused-ring compound.

[0036] In some embodiments of this application, the core material may be Na3V. 2-x M x The core material is (PO4)2F3. In other embodiments, the core material may be Na3V. 2-x M xThe elemental sources for (PO4)2F3 are (specifically, sodium, vanadium, phosphorus, fluorine, and optional M source). In this application, considering that alkali metal elements such as sodium and fluorine will be lost during the preparation process, each sodium source and fluorine source may be prepared in excess as appropriate. For example, the sodium source may be prepared in excess of 10 wt.%.

[0037] In some embodiments of this application, the carbon source includes a mixture of monocyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic hydrocarbons (PAHs). That is, the aromatic hydrocarbon-containing substance is preferably rich in both PAHs and PAHs. In some specific embodiments, the aromatic hydrocarbon-containing substance can be a byproduct of petroleum or coal refining, such as at least one of sulfonated asphalt, asphalt phenolates, oxidized asphalt, asphalt resin, and emulsified asphalt; it can also be a substance rich in PAHs produced by coking or incomplete combustion of coal and biomass, such as tar or coke. In some cases, the aromatic hydrocarbon-containing substance can be sulfonated asphalt, which is a powder obtained by sulfonating a high-viscosity liquid like asphalt. Sulfonated asphalt is not only rich in both PAHs and PAHs, making it easy to produce carbon materials with a high degree of ordered carbon atom arrangement that can uniformly coat the surface of the aforementioned core material, but it also has a wide availability and low price, which is very beneficial for reducing the raw material cost of cathode materials.

[0038] In some embodiments of this application, based on the Na element in the core raw material, when the aromatic hydrocarbon-containing substance is sulfonated pitch, the mass ratio of sulfonated pitch to the sodium source in the core raw material is in the range of (1-1.67):(2.46-7.33); when the aromatic hydrocarbon-containing substance is pitch phenolate, the mass ratio of pitch phenolate to the sodium source in the core raw material is in the range of (1-1.67):(3.51-10.43); when the aromatic hydrocarbon-containing substance is oxidized pitch, the oxygen... The mass ratio of emulsified asphalt to sodium source in the core material is within the range of (1-1.67):(3.73-11.11); when the above-mentioned aromatic-containing substance is asphalt resin, the mass ratio of asphalt resin to sodium source in the core material is within the range of (1-1.67):(3.31-9.84); when the above-mentioned aromatic-containing substance is emulsified asphalt, the mass ratio of emulsified asphalt to sodium source in the core material is within the range of (1-1.67):(3.17-9.44).

[0039] In this application, in step (2), the calcination process is carried out under a protective atmosphere. The protective gas can be nitrogen, argon, helium, etc.

[0040] In some embodiments of this application, the method for mixing sodium source, vanadium source, M source, phosphorus source, fluorine source and carbon source in step (1) includes, but is not limited to, solid phase mixing method or sol-gel method.

[0041] The specific steps of the solid-phase mixing method can be as follows: Add the aforementioned sodium source, vanadium source, M source, phosphorus source, fluorine source, and carbon source to a ball milling apparatus and mix to obtain a precursor material. For example, the ball milling apparatus can be a planetary ball mill. Specifically, a certain amount of the aforementioned sodium source, vanadium source, M source, phosphorus source, fluorine source, and carbon source can be weighed, dispersed in acetone, thoroughly stirred, and then transferred to a ceramic ball milling jar. The ceramic ball milling jar is then placed in a planetary ball mill and ball-milled at a speed of 400 r / min-800 r / min for 4 h-10 h. Subsequently, the ceramic ball milling jar is placed in a vacuum drying oven and dried at 50℃-80℃ for 1 h-5 h to remove the aforementioned acetone. After grinding, the precursor material is obtained.

[0042] The specific steps of the sol-gel method can be as follows: add the sodium source, vanadium source, M source, phosphorus source, fluorine source and carbon source to the solvent, heat to 60℃-85℃ and stir thoroughly to generate a gel, continue heating and stirring to evaporate the solvent in the obtained gel to obtain the above precursor material.

[0043] In this application, the solvent is a volatile solvent, specifically one or more of water, ethanol, ethylene glycol, and acetone.

[0044] In this application, the sodium source can be at least one of sodium fluoride, sodium acetate, sodium formate, sodium carbonate, sodium nitrate, sodium oxalate, sodium sulfate, sodium citrate, and sodium acetylacetonate.

[0045] In this application, the aforementioned vanadium source includes, but is not limited to, at least one of vanadium sources in which vanadium is trivalent, tetravalent, or pentavalent in the compound. For example, the aforementioned vanadium source may be at least one of vanadium pentoxide, vanadium tetroxide, vanadium trioxide, ammonium metavanadate, sodium metavanadate, oxysulfate, vanadium acetylacetonate, and vanadium acetylacetonate oxyacetate. When a tetravalent or pentavalent vanadium source is used, the aforementioned carbon source can also act as a reducing agent to reduce the high-valence V element to V. 3+ .

[0046] In this application, the source M is a compound familiar to those skilled in the art. For example, it is at least one of the organometallic salts, metal salts, and metal oxides of the specific element corresponding to element M. For example, when element M is Ti, the source M can be at least one of tetrabutyl titanate, isopropyl titanate, and tetraethyl titanate. When element M is Zr, the source M can be at least one of zirconium oxynitrate, zirconium acetate, zirconium acetylacetone, and zirconium oxychloride.

[0047] In this application, the phosphorus source can be at least one of phosphoric acid, sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate.

[0048] In this application, the fluorine source can be at least one of sodium fluoride, ammonium fluoride, hydrofluoric acid, and sodium hydrogen fluoride.

[0049] In some embodiments of this application, in step (2), the calcination temperature is 600℃-800℃ and the calcination time is 10min-480min during the calcination process.

[0050] The above-mentioned calcination treatment method can be microwave calcination. When using microwave calcination, the holding time is 10-25 minutes. Exemplarily, the holding time can be 10 minutes, 15 minutes, 20 minutes, 25 minutes, etc. In some specific embodiments of this application, the power of the microwave calcination equipment is 800W-1500W. The microwave calcination equipment can be a microwave tube furnace. Microwave calcination has a short processing time, which allows the carbon source to quickly rearrange and coat the surface of the core active component, forming a uniform coating layer, and also avoids significant loss of fluorine elements during excessively long calcination holding times.

[0051] In some embodiments of this application, the coating layer may be subjected to carbon composite treatment or the cathode material may be nano-sized to further improve the electrochemical performance of the material. Specifically, carbon composite treatment refers to the addition of carbon sources such as citric acid and oxalic acid, which can form amorphous carbon, to the precursor material for calcination.

[0052] This application also provides a positive electrode sheet, including the positive electrode material for sodium batteries provided in this application embodiment or the positive electrode material for sodium batteries prepared by the preparation method in this application embodiment. This positive electrode sheet can be used to provide batteries with good rate performance and good cycle performance.

[0053] In this application, the aforementioned positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector. The positive active material contains the aforementioned positive electrode material for sodium batteries, a binder, and an optional conductive agent.

[0054] Binders and conductive agents are common choices in the battery field. The aforementioned positive electrode current collector can be any material suitable for use as a current collector in the positive electrode sheet, including but not limited to elemental metal foils, alloy foils, metal-plated polymer films, or the aforementioned materials with carbon coatings on their surfaces. Specifically, elemental metal foils can be aluminum foils, alloy foils can be aluminum alloy foils, and the metal plated on the surface of the polymer film can be an elemental aluminum layer or an aluminum alloy layer.

[0055] This application also provides a sodium battery, including the positive electrode sheet provided in this application embodiment. Because this sodium battery uses the aforementioned positive electrode material as the positive electrode active material, it has high energy density, excellent rate performance, and strong long-cycle operation capability. This sodium battery can be used in 3C electronic products (such as mobile phones, tablets, etc.), transportation vehicles (such as cars, ships, etc.) or other electrical equipment to improve the performance and market competitiveness of electrical equipment.

[0056] The sodium battery can be a liquid battery using a liquid electrolyte, or a semi-solid or solid battery using a semi-solid or solid electrolyte. In some embodiments, the secondary battery may include the aforementioned positive and negative electrode plates, as well as a separator and electrolyte disposed between the positive and negative electrode plates. In other embodiments, the sodium battery may include a positive and negative electrode plate, as well as a semi-solid or solid electrolyte disposed between the positive and negative electrode plates. Furthermore, when using a semi-solid or solid electrolyte, the positive and negative electrode plates may also contain semi-solid or solid electrolyte materials.

[0057] The technical solution of this application will be further described in detail below with reference to several embodiments.

[0058] Example 1

[0059] (1) Take a certain amount of sodium source, fluorine source (specifically NaF), vanadium source (specifically NH4VO3), and phosphorus source (specifically NH4H2PO4) so ​​that the molar ratio of sodium:vanadium:phosphorus:fluorine is 3:2:2:3; weigh 66.7g of carbon source (specifically sulfonated pitch) for every 1mol of Na3V2(PO4)2F3 synthesized. Disperse the above raw materials in the solvent-acetone, stir thoroughly, transfer to a ceramic ball mill jar, and place the ceramic ball mill jar in a planetary ball mill. Ball mill at 500r / min for 6h. Then place the ceramic ball mill jar in a vacuum drying oven and dry at 60℃ for 3h to remove the aforementioned acetone. After grinding, obtain the precursor material.

[0060] (2) After placing the above precursor material in a ceramic boat, the ceramic boat was transferred to a microwave tube furnace and heated to a holding temperature of 700°C under a flowing argon atmosphere. The temperature was held at 700°C for 15 minutes, cooled, and then ground to obtain the cathode material Na3V(PO4)2F3@C. The coating thickness of the cathode material was 16.0 nm, and the average particle size of the cathode material was 1000 nm.

[0061] Example 2

[0062] The only difference from Example 1 is that in step (1), the mass of sulfonated asphalt weighed for every 1 mol of Na3V2(PO4)2F3 synthesized is 50 g. The coating thickness of the cathode material is 11.2 nm, and the average particle size of the cathode material is 1500 nm.

[0063] Example 3

[0064] The only difference from Example 1 is that in step (1), the mass of sulfonated asphalt added for every 1 mol of synthesized Na3V2(PO4)2F3 is 83.3 g. The coating thickness of the cathode material is 20.4 nm, and the average particle size of the cathode material is 400 nm.

[0065] Example 4

[0066] The only difference from Example 1 is that in step (2), the heat preservation temperature is 650℃. The coating thickness of the positive electrode material is 18.1nm, and the average particle size of the positive electrode material is 800nm.

[0067] Example 5

[0068] The only difference from Example 1 is that in step (2), the heat preservation temperature is 750℃. The coating thickness of the positive electrode material is 13.3nm, and the average particle size of the positive electrode material is 1500nm.

[0069] Example 6

[0070] The only difference from Example 1 is that in step (2), the heat preservation temperature is 900℃ and the heat preservation time is 30min. The coating thickness of the positive electrode material is 7.5nm and the average particle size of the positive electrode material is 2800nm.

[0071] Example 7

[0072] The only difference from Example 1 is that in step (1), a certain amount of sodium source and fluorine source (specifically NaF), vanadium source (specifically NH4VO3), M source (specifically zirconium oxynitrate), and phosphorus source (specifically NH4H2PO4) are taken, so that the molar ratio of sodium element: vanadium element: M element: phosphorus element: fluorine element is 3:1.95:0.05:2:3; for every 1 mol of Na3V synthesized 1.95 Zr 0.05 66.7 g of carbon source (specifically sulfonated pitch) was weighed out as raw material to obtain the cathode material Na3V. 1.95 Zr 0.05 (PO4)2F3@C. The coating thickness of the cathode material is 16.1 nm, and the average particle size of the cathode material is 1100 nm.

[0073] Example 8

[0074] The only difference from Example 1 is that in step (2), the process parameters are fine-tuned, the heat preservation temperature is 600℃, the heat preservation time is 15min, and the average particle size of the positive electrode material is 710nm.

[0075] To highlight the beneficial effects of the embodiments of this application, the following comparative examples are provided.

[0076] Comparative Example 1

[0077] The difference from Example 1 is that in step (2), the holding temperature is 500℃ and the holding time is 10min. The final cathode material has a coating thickness of 24.7nm, and the coating carbon material I... D / I G The value is 1.0, and the average particle size of the cathode material is 500 nm.

[0078] Comparative Example 2

[0079] The only difference from Example 1 is that 1.6 mol of citric acid is added as the carbon source for every 1 mol of Na3V2(PO4)2F3 synthesized. The coating thickness of the cathode material is 4.5 nm, and the average particle size of the cathode material is 580 nm.

[0080] Comparative Example 3

[0081] The only difference from Example 1 is that 1.9 mol of citric acid is added as the carbon source for every 1 mol of Na3V2(PO4)2F3 synthesized. The coating thickness of the cathode material is 7.0 nm, and the average particle size of the cathode material is 420 nm.

[0082] Comparative Example 4

[0083] The only difference from Example 1 is that 2.2 mol of citric acid is added as the carbon source for every 1 mol of Na3V2(PO4)2F3 synthesized. The coating thickness of the cathode material is 9.0 nm, and the average particle size of the cathode material is 300 nm.

[0084] Comparative Example 5

[0085] The only difference from Example 1 is that in step (2), the holding temperature is 900℃ and the holding time is 40min. The carbon material in the coating layer of the finally obtained cathode material is I... D / I G The value is 0.87, and the average particle size of the cathode material is 3000 nm.

[0086] Performance testing

[0087] (1) XRD tests were performed on the cathode materials prepared in Examples 1-3, and the results are summarized in... Figure 1 middle.

[0088] (2) The cathode materials prepared in the examples and comparative examples were subjected to thermogravimetric analysis (TG) tests. Specifically, 8 mg of sample was taken and thermogravimetric analysis was performed in an air or oxygen atmosphere (all samples were tested in an oxygen atmosphere, or all samples were tested in an air atmosphere) at a heating rate of 5 °C / min to determine the carbon content in the cathode material. The TG curves of some test samples are summarized in […]. Figure 2 middle.

[0089] (3) SEM tests were performed on the cathode materials prepared in some of the embodiments. The SEM image of the cathode material prepared in Example 2 is shown below. Figure 4 As shown.

[0090] (4) TEM tests were performed on the cathode materials prepared in some of the embodiments. Figure 5 In the image, a and b are TEM images of the cathode material prepared in Example 2 at two different magnifications.

[0091] (5) Raman spectroscopy tests were performed on the cathode materials of the examples and comparative examples. The wavelength of the Raman spectroscopy excitation source was 514 nm, and the resolution was 2 cm⁻¹. -1 The Raman spectrum of Example 2 is shown below. Figure 3A As shown, the Raman spectrum of Comparative Example 2 is as follows: Figure 3B As shown.

[0092] (6) Electrochemical performance testing

[0093] a) The positive electrode material, conductive agent (carbon black), and LA-132 binder provided in the above embodiments and comparative examples were added to the solvent (deionized water) at a mass ratio of 8:1:1, and then stirred in a vacuum mixer to form a stable and uniform positive electrode slurry. The above positive electrode slurry was coated onto the current collector (aluminum foil) and dried at 105°C for 6 hours to obtain the positive electrode sheet;

[0094] b) Cut a sodium metal sheet of suitable size to serve as the negative electrode sheet;

[0095] c) Dissolve 1 mol of sodium salt-NaClO4 in 1 L of organic solvent (the volume ratio of ethylene carbonate, diethyl carbonate and methyl ethyl carbonate is 1:1:1), and 2 wt.% of fluoroethylene carbonate to obtain the electrolyte.

[0096] d) In a glove box, under an Ar atmosphere, the positive electrode, separator-glass fiber separator and negative electrode obtained in step (2) are alternately stacked and injected with electrolyte to assemble a CR2032 button cell.

[0097] e) After assembling the above button cells, let them stand at 25°C for 12 hours and then test them on the Land-2001A battery testing system.

[0098] Cyclic performance testing: Each battery was subjected to charge-discharge cycle testing at 25℃ with a current rate of 5C, and a voltage range of 2.0V-4.5V. The specific capacity of the first discharge cycle and the capacity retention rate after 500 cycles were recorded for each battery. The specific capacity of the first discharge cycle is equal to the ratio of the first discharge capacity of each coin cell to the mass of the positive electrode material in the battery; the capacity retention rate after 500 cycles is equal to the ratio of the discharge capacity after 500 cycles to the discharge capacity of the first cycle. The relevant results are summarized in Table 1.

[0099] Rate performance: The discharge capacity of each battery was tested at 25°C at different rates (0.2C, 0.5C, 1C, 2C, 5C, and 10C) as a function of the number of cycles, with a voltage range of 2V-4.5V. Rate performance curves for some examples and comparative examples are shown below. Figures 6-7 As shown in Table 1, the ratio of the first-cycle discharge capacity at 10C to the first-cycle discharge specific capacity at 0.2C is summarized.

[0100] Table 1 Summary of test parameters for cathode materials in each embodiment and comparative example

[0101]

[0102]

[0103] Figure 1 The images show the XRD patterns of the cathode materials in Examples 1-3. The characteristic peaks in the images belong to the core material Na3V2(PO4)2F3. As can be seen from the (002) crystal plane, the crystallinity of the core material in each example remains at a high level. Correspondingly, Figure 5 In the middle, the lattice stripes with a spacing of about 0.531 nm correspond to the (002) crystal plane. The high degree of lattice order indicates that the core material is well crystallized.

[0104] As can be seen from the data in Table 1, when the carbon content of the cathode material is similar, the I of the coating carbon material... D / I G With different values, the electrochemical performance of the cathode materials is more comparable. Table 1 shows that the capacity retention and rate performance of the cathode materials in examples with similar carbon material content are significantly better than those in the corresponding comparative examples. When the calcination temperature is further increased to 900℃, the final coating carbon material I... D / I GWhen the value dropped to 0.87 (Comparative Example 5), the rate performance of the cathode material deteriorated significantly, especially compared to Example 6 with a similar carbon content. The cycle performance of Comparative Example 5 was also weaker than that of the Example 6. This indicates that the I-value of the coated carbon material... D / I G A value that is too small is also detrimental to the final battery performance.

[0105] The above description is an exemplary embodiment of this application. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principles of this application, and these improvements and modifications are also considered to be within the scope of protection of this application.

Claims

1. A method for preparing a positive electrode material for sodium batteries, characterized in that, Includes the following steps: (1) The core raw material is mixed with a carbon source to obtain a precursor material; wherein the carbon source includes a substance containing aromatic hydrocarbons, and the core raw material includes the Na3V to be prepared. 2-x M x The elemental sources of (PO4)2F3, where M represents a dopant element that can substitute for V, and M includes at least one of Fe, Cr, Mn, Co, Ti, Ni, Cu, Zn, Mo, Nb, Zr, La and Ce, where 0 ≤ x < 0.

2. (2) The precursor material is calcined to obtain a cathode material; the calcination temperature is 650℃-750℃, and the calcination time is 10min-480min, or the calcination method includes microwave calcination; the holding time of the microwave calcination is 10min-25min; wherein, the cathode material includes a core and a coating layer covering the surface of the core; the general molecular formula of the core includes Na3V. 2-x M x (PO4)2F3; the coating material includes carbon material, and the Raman spectrum of the carbon material is I D / I G The value is y, 0.92≤y≤0.98; Wherein, the I D / I G The peak intensity ratio of the D peak and the G peak in the Raman spectrum of the carbon material is given, wherein the Raman shift of the D peak is at 1300 cm⁻¹. -1 -1360cm -1 Within the range, the Raman shift of the G peak is at 1580 cm⁻¹. -1 -1600cm -1 Within the range.

2. The preparation method according to claim 1, characterized in that, The aromatic hydrocarbons include monocyclic aromatic hydrocarbons and polycyclic aromatic hydrocarbons.

3. The preparation method according to claim 1 or 2, characterized in that, The aromatic hydrocarbon-containing substances include at least one of sulfonated asphalt, asphalt phenolates, oxidized asphalt, asphalt resin, and emulsified asphalt.

4. The method for preparing the positive electrode material for sodium batteries according to claim 1 or 2, characterized in that, The average particle size of the cathode material is between 50 nm and 2000 nm.

5. The method for preparing the positive electrode material for sodium batteries according to claim 1 or 2, characterized in that, The carbon material comprises 7%-15% by mass in the cathode material.

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