Positive electrode active material, preparation method thereof, positive electrode sheet, sodium ion battery and application

Abstract

This application provides a positive electrode active material, its preparation method, a positive electrode sheet, a sodium-ion battery, and its application. The positive electrode active material includes phosphate materials and sulfate materials; the chemical formula of the phosphate material is Na. x M y (P a O b ) α (X c O d ) β Z w Wherein, M is one or more of Fe, Mn, and Co; X is one of Si, P, B, and C; Z is one or more of F and OH; 0 < x ≤ 4, 1 ≤ y ≤ 3, 1 ≤ a ≤ 3, 4 ≤ b ≤ 12, 0 ≤ c ≤ 3, 0 ≤ d ≤ 12, 0 < α ≤ 3, 0 ≤ β ≤ 2, 0 ≤ w ≤ 2; the chemical formula of the sulfate material is Na. i Fe j (SO4) γ ·λ(H2O); where 2≤i≤6, 1≤j≤3, 2≤γ≤4, 0≤λ≤2. This positive electrode active material is used to achieve both high specific capacity and high voltage in sodium-ion batteries.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a positive electrode active material and its preparation method, a positive electrode sheet, a sodium-ion battery and its applications. Background Technology

[0002] Sodium and lithium metals have similar chemical properties and applications, and sodium metal is abundant. Therefore, sodium-ion batteries are considered the most promising next-generation energy storage technology. Among them, the composition of the positive electrode active material plays a key role in the battery performance of sodium-ion batteries.

[0003] In the existing technology, during the charging and discharging process of the battery, the positive electrode active material in the sodium-ion battery itself cannot simultaneously achieve high potential and high capacity, resulting in a lower specific capacity and voltage of the sodium-ion battery. Summary of the Invention

[0004] This application provides positive electrode active materials and their preparation methods, positive electrode sheets, sodium-ion batteries and their applications. While maintaining the capacity that the positive electrode active materials can perform, the potential of the positive electrode active materials can be increased, thereby enabling the sodium-ion battery to have both high specific capacity and high voltage.

[0005] In a first aspect, embodiments of this application provide a positive electrode active material, the positive electrode active material comprising phosphate materials and sulfate materials;

[0006] The chemical formula of the phosphate material is Na. x M y (P a O b ) α (X c O d ) β Z w Where M is one or more of Fe, Mn, and Co; X is one of Si, P, B, and C; Z is one or more of F and OH; 0 < x ≤ 4, 1 ≤ y ≤ 3, 1 ≤ a ≤ 3, 4 ≤ b ≤ 12, 0 ≤ c ≤ 3, 0 ≤ d ≤ 12, 0 < α ≤ 3, 0 ≤ β ≤ 2, 0 ≤ w ≤ 2;

[0007] The chemical formula of the sulfate material is Na. i Fe j (SO4) γ ·λ(H2O); where 2≤i≤6, 1≤j≤3, 2≤γ≤4, 0≤λ≤2.

[0008] In one possible embodiment, the phosphate material is selected from Na4Fe3(PO4)2(P2O7), Na4Mn3(PO4)2(P2O7), NaFePO4, Na3Fe2(PO4)3, Na2FeP2O7, Na2MnP2O7, β-Na2MnP2O7, Na2FePO4F, Na2MnPO4F, Na3FePO4CO3, Na3MnPO4CO3, Na4Fe 2 / 3 Mn 2 / 3 At least one of (PO4)2(P2O7) and Na4FeMnCo(PO4)2(P2O7);

[0009] The sulfate material is selected from Na. 6-2z Fe z At least one of (SO4)3, Na2Fe(SO4)2·2H2O, and Na6Fe(SO4)4; 1.5≤z≤2.

[0010] In one possible implementation, the mass percentage of phosphate materials in the positive electrode active material is (20~80)%; and the mass percentage of sulfate materials in the poly-positive electrode active material is (20~80)%.

[0011] In one possible implementation, the mass percentage of phosphate materials in the positive electrode active material is (40~60)%; the mass percentage of sulfate materials in the positive electrode active material is (40~60)%.

[0012] In one possible implementation, the phosphate material comprises primary particles and / or secondary particles composed of primary particles.

[0013] The sulfate material includes primary particles and / or secondary particles composed of primary particles.

[0014] In one possible implementation, the average particle size of the phosphate material is (0.05~30) μm.

[0015] In one possible implementation, the average particle size of the sulfate material is (0.05~15) μm.

[0016] Secondly, embodiments of this application provide a method for preparing the above-mentioned positive electrode active material, comprising:

[0017] The positive electrode active material is prepared by mixing phosphate materials and sulfate materials.

[0018] Thirdly, embodiments of this application provide a positive electrode sheet, the positive electrode sheet comprising the positive electrode active material described in the first aspect or the positive electrode active material described in the second aspect.

[0019] Fourthly, embodiments of this application provide a sodium-ion battery, including the aforementioned positive electrode sheet.

[0020] Fifthly, embodiments of this application provide an electrical device including the aforementioned sodium-ion battery.

[0021] The positive electrode active materials, their preparation methods, positive electrode sheets, sodium-ion batteries, and applications provided in this application include phosphate materials and sulfate materials; the chemical formula of the phosphate materials is Na. x M y (P a O b ) α (X c O d ) β Z w Wherein, M is one or more of Fe, Mn, and Co; X is one of Si, P, B, and C; Z is one or more of F and OH; 0 < x ≤ 4, 1 ≤ y ≤ 3, 1 ≤ a ≤ 3, 4 ≤ b ≤ 12, 0 ≤ c ≤ 3, 0 ≤ d ≤ 12, 0 < α ≤ 3, 0 ≤ β ≤ 2, 0 ≤ w ≤ 2; the chemical formula of the sulfate material is Na. i Fe j (SO4) γ ·λ(H2O); where 2≤i≤6, 1≤j≤3, 2≤γ≤4, 0≤λ≤2. By using a blend of phosphate materials with high capacity and sulfate materials with high potential, a positive electrode active material is obtained. While maintaining the capacity that the positive electrode active material can exert, its potential can be increased, thereby enabling the sodium-ion battery to have both high specific capacity and high voltage. Attached Figure Description

[0022] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0023] Figure 1 This is a schematic diagram showing the charge and discharge results of the sodium-ion half-cell provided in this application.

[0024] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0025] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this application as detailed in the appended claims.

[0026] The positive electrode active material in sodium-ion batteries mainly includes layered oxide materials, which are used to prepare the positive electrode sheet and the sodium-ion battery. Because the crystal structure of the layered oxide materials in the positive electrode active material is prone to change during charging and discharging or at high temperatures, it is susceptible to phase transitions or decomposition, reducing the cycle stability and safety of the positive electrode active material.

[0027] Based on this, the positive electrode active material provided in this application includes phosphate materials and sulfate materials; the chemical formula of the phosphate material is Na. x M y (P a O b ) α (X c O d ) β Z w Wherein, M is one or more of Fe, Mn, and Co; X is one of Si, P, B, and C; Z is one or more of F and OH; 0 < x ≤ 4, 1 ≤ y ≤ 3, 1 ≤ a ≤ 3, 4 ≤ b ≤ 12, 0 ≤ c ≤ 3, 0 ≤ d ≤ 12, 0 < α ≤ 3, 0 ≤ β ≤ 2, 0 ≤ w ≤ 2; the chemical formula of the sulfate material is Na. i Fe j (SO4) γ ·λ(H2O); where 2≤i≤6, 1≤j≤3, 2≤γ≤4, 0≤λ≤2.

[0028] For example, the positive electrode active material is a polyanionic positive electrode material, which includes phosphate materials and sulfate materials; the phosphate material includes one or more of phosphate materials, phosphate pyrophosphate materials, etc.

[0029] The chemical formula of this phosphate material is Na. x M y (P a O b ) α (X c O d ) β Z wWherein, M is one or more of Fe, Mn, and Co; X is one of Si, P, B, and C; Z is one or more of F and OH; 0 < x ≤ 4, 1 ≤ y ≤ 3, 1 ≤ a ≤ 3, 4 ≤ b ≤ 12, 0 ≤ c ≤ 3, 0 ≤ d ≤ 12, 0 < α ≤ 3, 0 ≤ β ≤ 2, 0 ≤ w ≤ 2; the chemical formula of this sulfate material is Na. i Fe j (SO4) γ ·λ(H2O); where 2≤i≤6, 1≤j≤3, 2≤γ≤4, 0≤λ≤2.

[0030] For example, phosphate materials can be sodium-containing phosphate pyrophosphate materials that include only Fe and Mn elements; or sodium-containing phosphate pyrophosphate materials that include mixed Fe and Mn elements.

[0031] Preferably, the phosphate material is selected from Na4Fe3(PO4)2(P2O7), Na4Mn3(PO4)2(P2O7), NaFePO4, Na3Fe2(PO4)3, Na2FeP2O7, Na2MnP2O7, β-Na2MnP2O7, Na2FePO4F, Na2MnPO4F, Na3FePO4CO3, Na3MnPO4CO3, Na4Fe 2 / 3 Mn 2 / 3 At least one of (PO4)2(P2O7) and Na4FeMnCo(PO4)2(P2O7); the sulfate material is selected from Na 6-2z Fe z At least one of (SO4)3, Na2Fe(SO4)2·2H2O, and Na6Fe(SO4)4; 1.5 ≤ z ≤ 2. It should be noted that Na4Fe3(PO4)2(P2O7) is a general formula, and the actual synthesized samples have a Na / Fe ratio lower than 4:3, for example, Na4Fe 2.87 (PO4)2(P2O7). Na 6-2z Fe z (SO4)3 is a general formula; actual synthesized samples include, for example, Na. 2.6 Fe 1.7 (SO4)3, Na 2.48 Fe 1.76 (SO4)3, Na 226 Fe 1.87 (SO4)3.

[0032] Preferably, the positive electrode active material includes a mixture of Na₄Fe₃(PO₄)₂(P₂O₇) and Na 6-2z Fe z(SO4)3. Among them, Na4Fe3(PO4)2(P2O7) has a voltage range of 2-3.6V and a specific capacity of 110mAh / g, while Na2Fe2(SO4)3 has a voltage range of 2-4.3V and a specific capacity of 95mAh / g. The capacity of Na4Fe3(PO4)2(P2O7) at 2-3.2V accounts for 90% of the total specific capacity. 6-2z Fe z (SO4)3 exhibits 90% of its total specific capacity at 3-4.3V. When the two materials are blended in a certain proportion, the specific capacity falls between that of the single components. Na... 6-2z Fe z The higher voltage of (SO4)3 has a boosting effect on the voltage of Na4Fe3(PO4)2P2O7. The median voltage of the mixed positive electrode active material is improved in the voltage range of 2-4.3V. This allows the active material to maintain voltage and capacity while taking into account material cost. At the same time, Na4Fe3(PO4)2P2O7 has better cycle stability and safety, which helps to maintain the overall cycle retention rate of the positive electrode active material.

[0033] Therefore, positive electrode active materials include phosphate materials and sulfate materials. By selecting suitable phosphate materials and sulfate materials as positive electrode active materials, on the one hand, by using a combination of phosphate materials with higher capacity and sulfate materials with higher potential, a polyanionic positive electrode material is obtained in the positive electrode active material. While maintaining the capacity that the positive electrode active material can exert, the potential of the positive electrode active material can be increased, thereby enabling the sodium-ion battery to have both higher specific capacity and higher voltage.

[0034] In one possible implementation, the mass percentage of phosphate materials in the positive electrode active material is (20~80)%; the mass percentage of sulfate materials in the positive electrode active material is (20~80)%.

[0035] For example, in the positive electrode active material, the mass percentage of phosphate material is (20~80)% and the mass percentage of sulfate material is (20~80)%. For instance, the mass percentage of phosphate material can be one or any two of the following: 20%, 40%, 60%, 80%. Simultaneously, the mass percentage of sulfate material in the positive electrode active material can be one or any two of the following: 20%, 40%, 60%, 80%. By selecting appropriate mass ratios of phosphate and sulfate materials, the higher voltage of the sulfate material can increase the voltage of the phosphate material. This allows the potential of the positive electrode active material to be increased while maintaining its capacity, thereby enabling the sodium-ion battery to simultaneously possess both high specific capacity and high voltage.

[0036] In one possible implementation, the mass percentage of phosphate materials in the positive electrode active material is (40~60)%; the mass percentage of phosphate materials in the positive electrode active material is (40~60)%.

[0037] For example, in the positive electrode active material, the mass percentage of phosphate material is (40~60)% and the mass percentage of sulfate material is (40~60)%. For instance, the mass percentage of phosphate material can be one or any two of the following: 40%, 45%, 50%, 55%, 60%. Simultaneously, the corresponding mass percentage of sulfate material can be one or any two of the following: 40%, 45%, 50%, 55%, 60%. By further optimizing the mass ratio of phosphate and sulfate materials, the potential of the positive electrode active material can be further increased while maintaining its achievable capacity, thereby enabling the sodium-ion battery to simultaneously possess both high specific capacity and high voltage.

[0038] In one possible implementation, phosphate materials comprise primary particles and / or secondary particles composed of primary particles; sulfate materials comprise primary particles and / or secondary particles composed of primary particles.

[0039] For example, in the positive electrode active material, phosphate materials include primary particles and / or secondary particles composed of primary particles, and / or sulfate materials include primary particles and / or secondary particles composed of primary particles; for example, the positive electrode active material includes a mixture of Na4Fe3(PO4)2(P2O7) and Na 6-2z Fe z (SO4)3, Na4Fe3(PO4)2(P2O7) are secondary particles, Na 6-2z Fe z (SO4)3 is also a secondary particle. The distinction between primary and secondary particles is based on their particle size; smaller particles are primary particles, and larger particles are secondary particles. For example, particles with a diameter between 0.05 and 1 μm are primary particles, and particles with a diameter greater than 1 μm are secondary particles.

[0040] By rationally designing the particle structure of phosphate and sulfate materials in the positive electrode active material, and selecting the particle structure of primary and / or secondary particles, the specific surface area of ​​the positive electrode active material can be increased, thereby improving the energy density of the battery. At the same time, the optimized particle structure can reduce side reactions between the electrolyte and the positive electrode active material, extending the cycle life of sodium-ion batteries.

[0041] In one possible implementation, the phosphate material has an average particle size of (0.05~30) μm.

[0042] For example, when the phosphate material includes primary particles and / or secondary particles composed of primary particles, the average particle size of the phosphate material is (0.05~30) μm, for example, it can be one or any two of the following: 0.05 μm, 0.5 μm, 1.0 μm, 10 μm, 20 μm, 30 μm; preferably, the average particle size of the phosphate material is (0.1~30) μm, for example, it can be one or any two of the following: 0.1 μm, 10 μm, 20 μm, 30 μm. The average particle size is the volume average particle size, which is based on the average value of the volume size of each particle, to characterize the particle size distribution in the phosphate material. In this embodiment, a laser particle size analyzer can be used to test the particle size distribution of the phosphate material. The laser particle size analyzer detects the intensity of scattered light at different angles, and calculates the particle size distribution based on the angle and intensity distribution of the scattered light, generating a particle size distribution map, and calculating the volume average particle size of the phosphate material. By selecting phosphate materials with suitable average particle size, good interparticle contact is achieved in both phosphate and sulfate materials. This results in positive electrode active materials with good conductivity, specific surface area, and reactivity, improving the rate performance of sodium-ion batteries and enabling them to charge and discharge rapidly at high current densities.

[0043] In one possible implementation, the average particle size of the sulfate material is (0.05~15) μm.

[0044] For example, when the sulfate material includes primary particles and / or secondary particles composed of primary particles, the average particle size of the sulfate material is (0.05~15) μm, for example, it can be one or any combination of 0.05μm, 0.1μm, 1.0μm, 10μm, 13μm, 15μm, etc. Preferably, the average particle size of the sulfate material is (0.1~15) μm, for example, it can be one or any combination of 0.1μm, 1.0μm, 10μm, 13μm, 15μm, etc. Herein, the average particle size is the volume average particle size, which is based on the average value of the volume size of each particle to characterize the particle size distribution in the sulfate material. In this embodiment, a laser particle size analyzer can be used to test the particle size distribution of the sulfate material. The laser particle size analyzer detects the intensity of scattered light at different angles, calculates the particle size distribution based on the angle and intensity distribution of the scattered light, generates a particle size distribution map, and calculates the volume average particle size of the sulfate material. By selecting sulfate materials with suitable average particle size, good interparticle contact is achieved in both sulfate and phosphate materials. This results in positive electrode active materials with good conductivity, specific surface area, and reactivity, further improving the rate performance of sodium-ion batteries and enabling them to charge and discharge rapidly at high current densities.

[0045] This application also provides a method for preparing the above-mentioned positive electrode active material, including the following steps:

[0046] The positive electrode active material is prepared by mixing phosphate materials and sulfate materials.

[0047] For example, a positive electrode active material is prepared by mixing phosphate materials and sulfate materials in a certain mass ratio. The chemical formula of the phosphate material is Na. x M y (P a O b ) α (X c O d ) β Z w Wherein, M is one or more of Fe, Mn, and Co; X is one of Si, P, B, and C; Z is one or more of F and OH; 0 < x ≤ 4, 1 ≤ y ≤ 3, 1 ≤ a ≤ 3, 4 ≤ b ≤ 12, 0 ≤ c ≤ 3, 0 ≤ d ≤ 12, 0 < α ≤ 3, 0 ≤ β ≤ 2, 0 ≤ w ≤ 2; the chemical formula of the sulfate material is Na. i Fe j (SO4)γ·λ(H2O); among them, 2≤i≤6, 1≤j≤3, 2≤γ≤4, 0≤λ≤2.

[0048] For example, Na4Fe3(PO4)2(P2O7) and Na2Fe2(SO4)3 are weighed out according to a preset mass ratio of 40:60 and then mixed to obtain the positive electrode active material. By using a blend of phosphate materials with high capacity and sulfate materials with high potential, the positive electrode active material is obtained. While maintaining the capacity that the positive electrode active material can perform, its potential can be increased, thereby enabling the sodium-ion battery to have both high specific capacity and high voltage.

[0049] For example, phosphate materials and sulfate materials can be prepared separately first, and then the two can be mixed to obtain a positive electrode active material.

[0050] For the preparation process of phosphate materials, sodium source compounds, M source compounds, P source compounds, and optional X source compounds, optional Z source compounds, and solvents can be selected according to a certain mass ratio or stoichiometric ratio for mixed reaction treatment to obtain a first precursor solution. During the mixed reaction treatment, auxiliary agents such as grinding aids, embedding agents, and pH adjusters can be added. Grinding aids can effectively reduce the grinding time and increase the uniformity and consistency of particles. Maltodextrin, as an embedding agent, can reduce the phenomenon of powder sticking to the wall during spray drying, thereby improving the yield. pH adjusters help optimize the chemical properties of the solution and provide a more stable first precursor solution for subsequent post-processing. Phosphate materials are then prepared through post-processing, including drying and calcination. The drying process effectively removes water from the first precursor solution, giving the first precursor good dispersibility and uniformity, laying the foundation for subsequent calcination steps. Calcination in a strong reducing atmosphere not only promotes the phase transformation of the first precursor but also further optimizes its crystal structure and electrochemical performance.

[0051] For the preparation process of sulfate materials, sodium source compounds and ferric sulfate compounds can be mixed and reacted according to a certain stoichiometric ratio to obtain a second precursor solution. During the mixing and reaction process, auxiliary agents such as reducing agents and viscosity modifiers can be added. The reducing agent effectively prevents the oxidation of Fe ions, thus ensuring the stability of the valence state of iron during synthesis. The viscosity modifier helps optimize the physical properties of the solution, providing a more stable second precursor solution for subsequent post-processing. The second precursor solution is then post-processed, including drying, calcination, and cooling, to obtain the sulfate material. Drying effectively removes moisture from the second precursor solution, resulting in a second precursor with good dispersibility and uniformity, laying the foundation for the subsequent calcination step. The calcination process in a strong reducing atmosphere aims to thoroughly remove organic matter and residual moisture from the second precursor through higher calcination temperatures and extended holding times. Simultaneously, the strong reducing atmosphere promotes lattice reconstruction and stabilization of the sulfate material. This results in a high sodium content and stable crystal structure in the sulfate material, which significantly improves the electrochemical performance of the positive electrode active material.

[0052] Phosphate materials and sulfate materials with suitable chemical formulas are obtained through the above methods, and then mixed according to stoichiometric ratios to prepare positive electrode active materials. By using a combination of phosphate materials with high capacity and sulfate materials with high potential, positive electrode active materials can be obtained, enabling sodium-ion batteries to have both high specific capacity and high voltage.

[0053] This application also provides a positive electrode sheet, which includes the above-mentioned positive electrode active material.

[0054] For example, the positive electrode sheet is composed of a positive current collector and a positive active layer. The positive active layer includes the positive active material in the aforementioned embodiments. The positive active layer is located on at least one side surface of the positive current collector. For example, the positive active layer can be provided on one side surface of the positive current collector, or positive active layers can be provided on both the positive and negative surfaces of the current collector.

[0055] The active material layer also includes components such as conductive agents and binders. This invention does not specifically limit these materials; appropriate materials can be selected according to actual application requirements. Specifically, conductive agents include, but are not limited to, one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, and carbon nanotubes. Binders include, but are not limited to, one or more of polyvinylidene fluoride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

[0056] The embodiments of the present invention may employ conventional positive current collectors in the art, for example, positive current collectors may include aluminum foil.

[0057] In this embodiment, the positive electrode sheet has the aforementioned positive electrode active material. By using a combination of phosphate materials with high capacity and sulfate materials with high potential, a positive electrode active material is obtained, which enables the sodium-ion battery to have both high specific capacity and high voltage.

[0058] It is worth noting that the mass percentage and average particle size of each substance added during the preparation of the positive electrode sheet provided by this invention may deviate from the corresponding mass percentage and average particle size obtained from the positive electrode sheet obtained after disassembly from the battery, but within the error range. Therefore, the mass percentage and average particle size of each substance during the preparation of the positive electrode sheet are basically consistent with the mass percentage and average particle size of each substance in the positive electrode active material in the positive electrode sheet.

[0059] This application also provides a sodium-ion battery, including the aforementioned positive electrode sheet. In this embodiment, the positive electrode sheet of the sodium-ion battery contains the aforementioned positive electrode active material, and this sodium-ion battery has advantages corresponding to the aforementioned positive electrode sheet, which will not be elaborated further.

[0060] This invention also provides an electrical device, which includes the sodium-ion battery described in any of the above embodiments. Specifically, the electrical device can be an electric vehicle, electric motorcycle, electric bicycle, power bank, drone, mobile phone, computer, camera, power tool, smart home device, or wearable device.

[0061] The electrical device provided by the present invention contains the above-mentioned sodium-ion battery, which has both high specific capacity and voltage, enabling the electrical device to be used stably for a long time and improving the performance of the electrical device.

[0062] The present invention will be further described below through specific embodiments.

[0063] Example 1

[0064] 1. Preparation of phosphate materials

[0065] First, accurately weigh each raw material according to the molar ratio (0.018:0.0087:0.04:0.015:0.005:0.02:0.02): ferric phosphate (FePO4), ferric nitrate nonahydrate (Fe(NO3)3·9H2O), sodium carbonate (Na2CO3), sodium dihydrogen phosphate (NaH2PO4·2H2O), ammonium dihydrogen phosphate (NH4H2PO4), citric acid monohydrate (C6H8O7·H2O), and glucose (C6H... 12O6), the above raw materials are added sequentially to a reaction vessel equipped with a stirrer, and deionized water is added until the total volume of the existing solution in the reaction vessel is 30%. The mixture is stirred at 500 r / min for 2 hours to ensure that all raw materials are completely dissolved and form a homogeneous mixed solution. The chemical reaction formula for this process can be expressed as:

[0066] FePO4+Fe(NO3)3·9H2O+Na2CO3+NaH2PO4·2H2O+NH4H2PO4+C6H 12 O6→Na4Fe 2.87 (PO4)2(P2O7)+CO2↑+H2O↑+NH3↑+NO x ↑,

[0067] In this process, citric acid monohydrate, acting as a complexing agent and carbon source, decomposes into CO2 and H2O during the subsequent calcination stage and does not directly participate in the main reaction. Then, ethylene glycol (a grinding aid) and maltodextrin (an encapsulating agent) are added to the mixed solution to 10% of the total reaction solution volume, and the mixture is stirred at 500 rpm for 30 minutes. Finally, a pH adjuster (25%–28% ammonia solution) is added to 5% of the current total solution volume to adjust the pH to neutral (pH≈7), and stirring is maintained for 15 minutes to obtain the first precursor solution.

[0068] The prepared first precursor solution was transferred to a spray dryer, and the inlet temperature was set to 110°C and the outlet temperature to 80°C. Spray drying was performed to obtain a yellow powdery first precursor.

[0069] The first precursor obtained by spray drying was placed in an alumina crucible and then placed in a tube furnace. Under an Ar-H2 atmosphere with a volume ratio of 90:10, it was heated to 300°C at a heating rate of 2°C / min and held for 3 hours. This low-temperature calcination removed organic matter and moisture from the first precursor, while simultaneously allowing it to initially form the desired crystal structure in a mild reducing atmosphere. Subsequently, the temperature was further increased to 550°C and held for 10 hours to complete the high-temperature calcination process. During this stage, the first precursor underwent complex chemical reactions under the influence of high temperature and an enhanced reducing atmosphere (Ar-H2=90:10) to form the target product Na4Fe. 2.87 (PO4)2(P2O7). After natural cooling to room temperature, the final phosphate material Na4Fe was obtained. 2.87(PO4)2(P2O7)(NFPP). Throughout the synthesis process, parameters at each step were strictly controlled to ensure that the purity of the raw materials was not less than 99%, thus avoiding any impact of impurities on the final product performance. Simultaneously, thorough stirring was performed during solution preparation to ensure uniform dispersion of the components; precise temperature control was applied during spray drying to prevent precursor powder agglomeration; and the Ar-H2 atmosphere ratio (90:10) and heating rate were precisely controlled during high-temperature calcination to prevent insufficient oxidation or reduction. This precise synthesis process allows for the preparation of high-quality NFPP samples suitable for use as cathode materials in sodium-ion batteries.

[0070] 2. Preparation of sulfate materials

[0071] First, based on the stoichiometric ratio (Na:Fe:SO4) 2- Sodium sulfate (Na₂SO₄): 2.48:1.76:3 was accurately weighed, with a molar ratio of 1.24:1.76 (i.e., 1.24 mol Na₂SO₄ corresponds to 1.76 mol FeSO₄·7H₂O). The above raw materials were sequentially added to a reactor equipped with a stirrer, and deionized water was added to 40% of the total volume of the solution in the reactor. The mixture was stirred at 600 rpm for 1.5 hours to ensure complete dissolution and the formation of a homogeneous solution. During stirring, the components in the solution gradually formed a homogeneous mixture through chemical reactions, providing a foundation for subsequent spray drying and high-temperature calcination steps.

[0072] Subsequently, ascorbic acid was added to the mixed solution to 0.1% of the total solution mass, and stirring was continued at 600 rpm for 20 minutes. Finally, a viscosity modifier (30% ethanol) was added to 8% of the total solution volume to adjust the viscosity of the current solution to make it more suitable for spray drying, and stirring was continued for 10 minutes.

[0073] The prepared solution was transferred to a spray dryer, with the inlet temperature set at 220℃ and the outlet temperature at 120℃, and spray drying was performed to obtain a light yellow powdery second precursor. During the spray drying process, the solution was atomized into fine droplets through the nozzle and rapidly dried in the hot airflow to form uniform powder particles.

[0074] The second precursor obtained by spray drying was placed in an alumina crucible (to avoid reaction between graphite and the product at high temperatures), and then placed in a tube furnace. Under an Ar-H2 atmosphere with a volume ratio of 90:10, it was heated to 400°C at a heating rate of 5°C / min and held at that temperature for 15 hours. After natural cooling to room temperature, the target product, sulfate material Na, was finally obtained. 2.48 Fe 1.76(SO4)3. Throughout the synthesis process, parameters at each step are strictly controlled to ensure that the purity of the raw materials is not less than 99%, thus avoiding the impact of impurities on the performance of the final product. Simultaneously, thorough stirring is performed during solution preparation to ensure uniform dispersion of all components; precise temperature control is maintained during spray drying to prevent precursor powder agglomeration; and special attention must be paid to the temperature resistance of the alumina crucible during high-temperature calcination, with precise control of the Ar-H2 atmosphere ratio and heating rate to prevent insufficient oxidation or reduction. If trace amounts of unreacted Na2SO4 remain in the product, impurity phases can be removed by water washing and centrifugation. Through the above precise synthesis process, high-purity Na2SO4 can be prepared. 2.48 Fe 1.76 (SO4)3 sample.

[0075] 3. Preparation of positive electrode sheet

[0076] Polyvinylidene fluoride (PVDF) binder was prepared into a PVDF adhesive solution. Conductive carbon black was added to the PVDF adhesive solution and premixed for 30 minutes to obtain a first premix. Positive electrode active material was added to the first premix to obtain a mixed slurry. The remaining PVDF adhesive solution was added to the mixed slurry and mixed again to obtain a positive electrode slurry. The positive electrode slurry was then coated onto aluminum foil, and subsequently vacuum dried in a vacuum drying oven at 120°C for 12 hours, and then rolled to form a positive electrode sheet. The mass ratio of the positive electrode active material, binder, and conductive carbon black in the positive electrode sheet was 94:3:3. The positive electrode active material included a phosphate material, Na4Fe. 2.87 (PO4)(P2O7) and sulfate material Na 2.48 Fe 1.76 (SO4)3; the mass percentage of phosphate material in the positive electrode active material is 20%, and the mass percentage of sulfate material in the positive electrode active material is 80%; the phosphate material includes primary particles, and the sulfate material includes secondary particles; the average particle size of the phosphate material is 0.05 μm, and the average particle size of the sulfate material is 14.98 μm.

[0077] 4. Preparation of sodium-ion batteries

[0078] Using the aforementioned positive electrode, a sodium sheet as the negative electrode, a polypropylene separator, a 1 mol / L NaPF6 electrolyte, and a mixed solution of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:1, a CR-2025 coin cell sodium-ion battery is assembled in an argon-filled glove box to obtain a sodium-ion coin cell half-cell.

[0079] Example 2

[0080] The preparation method of the sodium-ion battery in Example 2 is basically the same as that in Example 1, except that the mass ratio of phosphate material in the positive electrode active material is 30% and the mass ratio of sulfate material in the positive electrode active material is 70%; see Table 1 for details.

[0081] Example 3

[0082] The preparation method of the sodium-ion battery in Example 3 is basically the same as that in Example 1, except that the mass ratio of phosphate material in the positive electrode active material is 40% and the mass ratio of sulfate material in the positive electrode active material is 60%; the phosphate material includes secondary particles; the average particle size of the phosphate material is 4.66 μm; and the average particle size of the sulfate material is 3.27 μm, as detailed in Table 1.

[0083] Example 4

[0084] The preparation method of the sodium-ion battery in Example 4 is basically the same as that in Example 1, except that the mass ratio of phosphate material in the positive electrode active material is 50%, and the mass ratio of sulfate material in the positive electrode active material is 50%; the phosphate material includes secondary particles; the average particle size of the phosphate material is 4.66 μm; and the average particle size of the sulfate material is 3.27 μm, as detailed in Table 1.

[0085] Example 5

[0086] The preparation method of the sodium-ion battery in Example 5 is basically the same as that in Example 1, except that the mass ratio of phosphate material in the positive electrode active material is 60% and the mass ratio of sulfate material in the positive electrode active material is 40%; the phosphate material includes secondary particles; the average particle size of the phosphate material is 4.66 μm; and the average particle size of the sulfate material is 3.27 μm, as detailed in Table 1.

[0087] Example 6

[0088] The preparation method of the sodium-ion battery in Example 6 is basically the same as that in Example 1, except that the chemical formula of the sulfate material in the positive electrode active material is Na. 2.6 Fe 1.7 (SO4)3; where Na 2.6 Fe 1.7 In the preparation of (SO4)3, according to the stoichiometric ratio (Na:Fe:SO4) 2-The molar ratio of sodium sulfate (Na2SO4) and ferrous sulfate (FeSO4·7H2O) is accurately weighed (2.6:1.7:3), with a ratio of 1.3:1.7 (i.e., 1.3 mol Na2SO4 corresponds to 1.7 mol FeSO4·7H2O). The phosphate material accounts for 70% of the mass of the positive electrode active material, and the sulfate material accounts for 30% of the mass of the positive electrode active material. The phosphate material includes secondary particles with an average particle size of 7.26 μm. The sulfate material includes primary particles with an average particle size of 0.34 μm. See Table 1 for details.

[0089] Example 7

[0090] The preparation method of the sodium-ion battery in Example 7 is basically the same as that in Example 1, except that the chemical formula of the sulfate material in the positive electrode active material is Na. 2.6 Fe 1.7 (SO4)3; where Na 2.6 Fe 1.7 In the preparation of (SO4)3, according to the stoichiometric ratio (Na:Fe:SO4) 2- The molar ratio of sodium sulfate (Na2SO4) and ferrous sulfate (FeSO4·7H2O) is accurately weighed (2.6:1.7:3), with a ratio of 1.3:1.7 (i.e., 1.3 mol Na2SO4 corresponds to 1.7 mol FeSO4·7H2O). The phosphate material accounts for 80% of the mass of the positive electrode active material, and the sulfate material accounts for 20%. The phosphate material includes secondary particles with an average particle size of 7.26 μm. The sulfate material includes primary particles with an average particle size of 0.34 μm. See Table 1 for details.

[0091] Example 8

[0092] The preparation method of the sodium-ion battery in Example 8 is basically the same as that in Example 1, except that the chemical formula of the sulfate material in the positive electrode active material is Na. 2.6 Fe 1.7 (SO4)3; where Na 2.6 Fe 1.7 In the preparation of (SO4)3, according to the stoichiometric ratio (Na:Fe:SO4) 2-Sodium sulfate (Na2SO4) and ferrous sulfate (FeSO4·7H2O) were accurately weighed in a molar ratio of 1.3:1.7 (i.e., 1.3 mol Na2SO4 corresponds to 1.7 mol FeSO4·7H2O). The phosphate material accounted for 90% of the mass of the positive electrode active material, and the sulfate material accounted for 10%. The phosphate material included secondary particles with an average particle size of 29.98 μm. The sulfate material included primary particles with an average particle size of 0.34 μm. See Table 1 for details.

[0093] Example 9

[0094] The preparation method of the sodium-ion battery in Example 9 is basically the same as that in Example 1, except that the positive electrode active material includes the phosphate material Na4Mn3(PO4)2(P2O7) and the sulfate material Na2Fe(SO4)2·2H2O; wherein, in the preparation process of Na4Mn3(PO4)2(P2O7), the raw materials are accurately weighed according to the molar ratio (0.018:0.0087:0.04:0.015:0.005:0.02:0.02): manganese phosphate (Mn3(PO4)2), manganese nitrate hexahydrate (Mn(NO3)2·6H2O), sodium carbonate (Na2CO3), sodium dihydrogen phosphate (NaH2PO4·2H2O), ammonium dihydrogen phosphate (NH4H2PO4), citric acid monohydrate (C6H8O7·H2O) and glucose (C6H 12 In the preparation of O6); Na2Fe(SO4)2·2H2O, according to the stoichiometric ratio (Na:Fe:SO4) 2- Sodium sulfate (Na2SO4) and ferrous sulfate (FeSO4·7H2O) were accurately weighed in a ratio of 2:1:2, with a molar ratio of 1:1 (i.e., 1 mol of Na2SO4 corresponds to 1 mol of FeSO4·7H2O). The mass percentage of phosphate materials in the positive electrode active material was 50%, and the mass percentage of sulfate materials in the positive electrode active material was 50%. The average particle size of the phosphate materials was 0.11 μm, and the average particle size of the sulfate materials was 4.68 μm, as detailed in Table 1.

[0095] Comparative Example 1

[0096] The preparation method of the sodium-ion battery in Comparative Example 1 is basically the same as that in Example 1, except that the positive electrode active material only includes the phosphate material Na4Fe. 2.87 (PO4)2(P2O7), the phosphate material includes secondary particles, and the average particle size of the phosphate material is 29.98 μm, as shown in Table 1.

[0097] Comparative Example 2

[0098] The preparation method of the sodium-ion battery in Comparative Example 2 is basically the same as that in Example 1, except that the positive electrode active material only includes the sulfate material Na. 2.48 Fe 1.76 The average particle size of the (SO4)3 sulfate material is 3.47 μm, as detailed in Table 1.

[0099] Comparative Example 3

[0100] The preparation method of the sodium-ion battery in Comparative Example 3 is basically the same as that in Example 1, except that the positive electrode active material only includes the phosphate material Na4Mn3(PO4)2(P2O7). In the preparation of Na4Mn3(PO4)2(P2O7), the raw materials are accurately weighed according to the molar ratio (0.018:0.0087:0.04:0.015:0.005:0.02:0.02): manganese phosphate (Mn3(PO4)2), manganese nitrate hexahydrate (Mn(NO3)2·6H2O), sodium carbonate (Na2CO3), sodium dihydrogen phosphate (NaH2PO4·2H2O), ammonium dihydrogen phosphate (NH4H2PO4), citric acid monohydrate (C6H8O7·H2O), and glucose (C6H2PO4·2H2O). 12 The average particle size of the phosphate materials (O6) and phosphates is 0.048 μm, as detailed in Table 1.

[0101] Comparative Example 4

[0102] The preparation method of the sodium-ion battery in Comparative Example 4 is basically the same as that in Example 1, except that the positive electrode active material only includes the sulfate material Na2Fe(SO4)2·2H2O. During the preparation of Na2Fe(SO4)2·2H2O, according to the stoichiometric ratio (Na:Fe:SO4... 2- The ratio of sodium sulfate (Na2SO4) to ferrous sulfate (FeSO4·7H2O) is 2:1:2, and their molar ratio is 1:1 (i.e., 1 mol of Na2SO4 corresponds to 1 mol of FeSO4·7H2O). The phosphate material includes primary particles, and the average particle size of the sulfate material is 0.049 μm, as shown in Table 1.

[0103] The performance of the positive electrode and sodium-ion battery of each embodiment and comparative example was tested according to the following process, and the results are shown in Table 2 and 2. Figure 1 ,in, Figure 1 This is a schematic diagram of the charge and discharge results of the sodium-ion half-cell provided in this application, including the charge and discharge results of sodium-ion batteries in various embodiments and comparative examples.

[0104] Test case

[0105] 1. Average particle size

[0106] By sampling the prepared positive electrode sheet or powder sample, during the test, the powder of the material to be tested is first mixed with an inert dispersion medium (such as anhydrous ethanol), and a dispersant is added for ultrasonic treatment to eliminate agglomeration and ensure that the particles are fully dispersed into individual particles. Then, the scattering model parameters are set according to the optical properties of the material, the instrument is calibrated and injected into the sample cell, and the suspension is kept dynamically uniform by a circulation pump. The laser beam penetrates the sample and records the full-angle scattered light signal. Finally, the volume particle size distribution is calculated by combining the theoretical model, and the obtained volume D50 is taken as the final average particle size of the material to be tested. This value is listed in Table 2.

[0107] 2. Specific capacity

[0108] First, the prepared positive electrode sheet is ultrasonically cleaned with an inert solvent (such as carbonate) to remove electrolyte residue. Then, it is dried in a vacuum drying oven at 80-120℃ for 12 hours. The total mass of the electrode sheet is accurately weighed and recorded. The treated electrode sheet is used as the working electrode, and the sodium metal sheet is used as the counter electrode. A half-cell is assembled in an argon glove box. The electrolyte must be consistent with the original battery system to avoid interference from side reactions. During testing, the dynamic voltage window is set at 1.5-4.2V. After determining the actual polarization range through pre-cycling, the cutoff potential is adjusted. The first discharge capacity data is obtained by constant current charge and discharge at 20mA / g. The area specific capacity is calculated by measuring the electrode coating areal density, dividing the first discharge capacity by the measured electrode coating areal density (mass of active material per unit area), and combining the discharge capacity. The final specific capacity is obtained and listed in Table 2.

[0109] 3. Median voltage

[0110] At 25°C, the sodium-ion battery was charged to 3.6V at a constant current density of 20mA / g and then discharged to 1.5V at a constant current density of 20mA / g. During the constant current charge and discharge test of the sodium-ion battery, the voltage value corresponding to 50% of the total specific capacity (i.e., half of the total specific capacity of the battery) was recorded. This voltage value was defined as the median voltage and is listed in Table 2.

[0111] 4. Specific energy

[0112] First, the prepared positive electrode sheet is ultrasonically cleaned with an inert solvent (such as carbonate) to remove electrolyte residue. Then, it is dried in a vacuum drying oven at 80-120℃ for 12 hours. The total mass of the electrode sheet is accurately weighed and recorded. The treated electrode sheet is used as the working electrode, and the sodium metal sheet is used as the counter electrode. A half-cell is assembled in an argon glove box. The electrolyte must be consistent with the original battery system to avoid interference from side reactions. During testing, the dynamic voltage window is set at 1.5-4.2V. After determining the actual polarization range through pre-cycling, the cutoff potential is adjusted. The first discharge energy data is obtained by constant current charge and discharge at 20mA / g. The area specific energy is calculated by measuring the electrode coating surface density, dividing the first discharge energy by the measured electrode coating surface density (mass of active material per unit area), and combining the discharge energy. The final specific energy is listed in Table 2.

[0113] Table 1

[0114]

[0115] Table 2

[0116]

[0117] From Table 1, Table 2 and Figure 1 The following conclusions can be drawn from the analysis:

[0118] 1) Compared with Comparative Examples 1 to 4, in the positive electrode active materials of Examples 1 to 9, phosphate materials and sulfate materials with appropriate chemical formulas are used as positive electrode active materials. While maintaining the capacity that the positive electrode active material can exert, the potential of the positive electrode active material can be increased, thereby enabling the sodium-ion battery to have both higher specific capacity and higher voltage.

[0119] 2) Compared to Examples 1-2 and Examples 6-8, the positive electrode active material in Examples 3-5 was selected as a phosphate material, Na4Fe. 2.87 (PO4)2(P2O7) and sulfate material Na 2.48 Fe 1.76 When the mass percentage of the phosphate material is (40~60)% and the mass percentage of the sulfate material is (40~60)%, it is beneficial to ensure a higher specific capacity of the sodium-ion battery while enabling the sodium-ion battery to have a higher voltage.

[0120] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode active material, characterized in that, The positive electrode active material includes phosphate materials and sulfate materials; The chemical formula of the phosphate material is Na. x M y (P a O b ) α (X c O d ) β Z w Where M is one or more of Fe, Mn, and Co; X is one of Si, P, B, and C; Z is one or more of F and OH; 0 < x ≤ 4, 1 ≤ y ≤ 3, 1 ≤ a ≤ 3, 4 ≤ b ≤ 12, 0 ≤ c ≤ 3, 0 ≤ d ≤ 12, 0 < α ≤ 3, 0 ≤ β ≤ 2, 0 ≤ w ≤ 2; The chemical formula of the sulfate material is Na. i Fe j (SO4) γ ·λ(H2O); where 2≤i≤6, 1≤j≤3, 2≤γ≤4, 0≤λ≤2.

2. The positive electrode active material according to claim 1, characterized in that, The phosphate materials are selected from Na4Fe3(PO4)2(P2O7), Na4Mn3(PO4)2(P2O7), NaFePO4, Na3Fe2(PO4)3, Na2FeP2O7, Na2MnP2O7, β-Na2MnP2O7, Na2FePO4F, Na2MnPO4F, Na3FePO4CO3, Na3MnPO4CO3, Na4Fe 2 / 3 Mn 2 / 3 At least one of (PO4)2(P2O7) and Na4FeMnCo(PO4)2(P2O7); The sulfate material is selected from Na. 6-2z Fe z At least one of (SO4)3, Na2Fe(SO4)2·2H2O, and Na6Fe(SO4)4; 1.5≤z≤2.

3. The positive electrode active material according to claim 1 or 2, characterized in that, The mass percentage of phosphate materials in the positive electrode active material is (20~80)%; the mass percentage of sulfate materials in the positive electrode active material is (20~80)%.

4. The positive electrode active material according to claim 3, characterized in that, The phosphate material in the positive electrode active material has a mass ratio of (40~60)%; the sulfate material in the positive electrode active material has a mass ratio of (40~60)%.

5. The positive electrode active material according to any one of claims 1-4, characterized in that, The phosphate material includes primary particles and / or secondary particles composed of primary particles; The sulfate material includes primary particles and / or secondary particles composed of primary particles.

6. The positive electrode active material according to claim 5, characterized in that, The average particle size of the phosphate material is (0.05~30) μm.

7. The positive electrode active material according to claim 5, characterized in that, The average particle size of the sulfate material is (0.05~15) μm.

8. A method for preparing a positive electrode active material according to any one of claims 1-7, characterized in that, include: The positive electrode active material is prepared by mixing phosphate materials and sulfate materials.

9. A positive electrode plate, characterized in that, The positive electrode sheet comprises the positive electrode active material as described in any one of claims 1-7 or the positive electrode active material prepared by the preparation method described in claim 8.

10. A sodium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 9.

11. An electrical appliance, characterized in that, Including the sodium-ion battery of claim 10.

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