Composite sodium iron sulfate positive electrode material, and preparation method therefor and use thereof

Abstract

The present application belongs to the field of sodium batteries, and provides a composite sodium iron sulfate positive electrode material, and a preparation method therefor and the use thereof. The composite sodium iron sulfate positive electrode material comprises a core, the chemical formula of which is NaxMyFez(PO4)k(SO4)(0.4-0.6)xOt, wherein M comprises at least one of manganese, vanadium and titanium, 16≤x≤17, y=1, 4≤z≤5, 2≤k≤2.6, and y+z-0.1x-1.5k≤t≤y+z+0.1x-1.5k. In the present application, the decomposition of sulfate radicals is reduced, the material performance of the composite sodium iron sulfate positive electrode material is improved, and the performance aspects, such as the cycle performance, of a secondary battery in which the composite sodium iron sulfate positive electrode material is used are improved.

Description

A composite sodium ferrous sulfate cathode material, its preparation method and application

[0001] Cross-reference to related applications

[0002] This application claims priority to Chinese Patent Application No. 2024111909789, filed on August 27, 2024, entitled "A Composite Sodium Ferrous Sulfate Cathode Material and Its Preparation Method and Application", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This invention relates to the field of sodium battery technology, specifically to a composite sodium ferrous sulfate cathode material, its preparation method, and its application. Background Technology

[0004] With the development of lithium batteries, lithium has gradually become a bottleneck resource for the development of new energy. Sodium, as an element in the same group as lithium, has very similar electrochemical properties, is abundant (approximately 2.64% abundance in the Earth's crust), and is also much cheaper. Therefore, developing sodium-ion batteries to replace lithium has a very broad application prospect, and finding and developing suitable electrode materials has become one of the main tasks of sodium-ion battery research.

[0005] Polyanionic sodium battery materials possess advantages such as low cost and excellent cycle performance, and research on sodium-ion battery cathode materials has focused on polyanionic materials. Among numerous polyanionic compounds, SO4... 2- With higher electronegativity and stronger inductive effect, sulfate materials have higher operating voltages, thus attracting wider attention. Due to their lower sintering temperature and lower energy consumption, sulfate-based polyanionic materials are more cost-effective. When used as a cathode material in sodium-ion batteries, sodium ferrous sulfate provides a higher charge-discharge platform compared to LiFePO4. However, its lower electronic conductivity and sodium ion diffusion coefficient limit its practical applications.

[0006] In existing technologies, the ionic conductivity of polyanion sodium battery cathode materials is generally improved by doping with metal ions, and the electronic conductivity is improved by carbon coating. Conventional doping methods typically involve mixing and grinding a dopant metal salt and an iron salt, followed by sintering the mixture. However, the cathode materials obtained using this method have poor electrical performance because the sintering temperature cannot exceed 350°C to prevent the decomposition of sulfate, a necessary raw material. At this temperature, ion diffusion is slow, making it difficult to achieve a homogeneous eutectic mixture of the dopant metal and iron. Therefore, it is necessary to provide a solution that reduces sulfate decomposition and improves the material properties of composite ferrous sodium sulfate cathode materials.

[0007] Summary of the Invention

[0008] In view of the technical problems existing in the background art, this application provides a composite sodium ferrous sulfate cathode material, its preparation method and application, aiming to solve the technical problem of how to reduce sulfate decomposition and improve the material performance of the composite sodium ferrous sulfate cathode material.

[0009] In one aspect, embodiments of this application provide a composite sodium ferrous sulfate cathode material, comprising a core having the chemical formula Na. x M y Fe z (PO4) k (SO4)(0.4~0.6)xO t M includes at least one of manganese, vanadium, and titanium, 16≤x≤17, y=1, 4≤z≤5, 2≤k≤2.6, and y+z-0.1x-1.5k≤t≤y+z+0.1x-1.5k.

[0010] In the technical solution of this application embodiment, the composite sodium ferrous sulfate cathode material improves the ionic conductivity through element doping, ensures the sodium ion diffusion coefficient, and has a high degree of sulfate ion retention and uniform compound formation of iron and doped metals, thereby achieving further improvement in the performance of the composite sodium ferrous sulfate cathode material.

[0011] In some embodiments, the composite sodium ferrous sulfate cathode material further includes a carbon coating layer, which coats the surface of the core, wherein the molar ratio of carbon in the carbon coating layer to sodium in the core is greater than or equal to 0.5 and less than or equal to 1.

[0012] In this embodiment, the electronic conductivity of the material is improved by the carbon-coated core-shell structure.

[0013] In some embodiments, the sodium ion diffusion coefficient of the composite sodium ferrous sulfate cathode material is 1*10. -10 cm 2 / s-5*10 -10 cm 2 / s; Powder resistivity is 15Ω.cm-25Ω.cm.

[0014] In this embodiment, the sodium ion diffusion coefficient is high and the powder resistivity is low, resulting in good electrochemical performance of the composite sodium ferrous sulfate cathode material.

[0015] Secondly, this application provides a method for preparing a composite sodium ferrous sulfate cathode material, comprising the following steps:

[0016] S1. Mix the ferrous source, sodium source, phosphorus source, dopant, reducing agent and first solvent to obtain the first slurry;

[0017] S2. The first slurry is subjected to a hydrothermal reaction, followed by filtration and washing to obtain the washed material;

[0018] S3. Mix the washing material, sulfate compound, carbon source and second solvent to obtain the second slurry;

[0019] S4. The second slurry is dried and calcined, and then crushed and impurities are removed to obtain a composite sodium ferrous sulfate cathode material.

[0020] In the technical solution of this application embodiment, a uniform doped metal phase, iron, and sodium composite oxide precipitate is first synthesized under hydrothermal conditions, and the oxidation of ferrous ions is avoided by adding a reducing agent; after calcination, the carbon source is coated on the surface of the composite sodium ferrous sulfate, which effectively reduces the powder resistivity of the material and improves the electronic conductivity of the material; the preparation method of the composite sodium ferrous sulfate cathode material provided in this application reduces the decomposition of sulfate ions while carrying out metal element doping and anion doping, improving the sodium ion diffusion coefficient and improving the performance of the composite sodium ferrous sulfate cathode material.

[0021] In some embodiments, the dopant in the first slurry includes a dopant element, and the molar ratio of the dopant element to iron, sodium, and phosphorus is 1:(4-5):(16.0-17.0):(2-2.6); the molar ratio of iron in the ferrous source to the reducing agent is 1:(0.1-0.3); the solid content of the first slurry is 25%-35%; the molar ratio of sodium in the washing material to the carbon source is 1:(0.5-1); and the solid content of the second slurry is 20%-30%.

[0022] In this embodiment, by controlling the molar ratio of dopant, iron, sodium, and phosphorus in the first slurry within this range, a uniform mixture and eutectic formation of iron, sodium, and dopant metal can be achieved through hydrothermal reaction. At the same time, if the ratio of doped phosphate is unbalanced, it will be detrimental to achieving the above objectives. By controlling the molar ratio of iron to reducing agent in the first slurry within this range, it is beneficial to avoid the oxidation of ferrous iron.

[0023] In some embodiments, the ferrous source includes at least one of ferrous acetate and ferrous phosphate; the sodium source includes at least one of sodium hydroxide, sodium oxide, sodium carbonate, sodium acetate, and sodium phosphate; the phosphorus source includes sodium phosphate; the reducing agent includes sodium phosphite; the dopant includes at least one of manganese, vanadium, and titanium; the carbon source includes at least one of graphene and carbon nanotubes; and the sulfate compound includes at least one of sulfuric acid and ammonium sulfate.

[0024] In some embodiments, the sulfate compound includes sulfuric acid and ammonium sulfate, and in the second slurry, the molar ratio of sodium to sulfuric acid in the washing material is 1:(0.3-0.4); the molar ratio of sodium to ammonium sulfate in the washing material is 1:(0.1-0.2).

[0025] In this embodiment, the introduction of manganese, vanadium, and titanium sources as dopants is beneficial to improving ionic conductivity.

[0026] In some embodiments, the hydrothermal reaction temperature is 200℃-300℃, the hydrothermal reaction time is 12h-15h, and the hydrothermal reaction pressure is 1.5MPa-2MPa; the hydrothermal reaction is carried out under stirring conditions.

[0027] In this embodiment, by controlling the temperature and time of the hydrothermal reaction within this range, the reaction can be completed while avoiding excessive energy consumption. If the temperature of the hydrothermal reaction is too high, the energy consumption will be high and the requirements for the equipment will be high. If the temperature of the hydrothermal reaction is too low, the reaction cannot occur completely. If the hydrothermal reaction time is too long, the energy consumption will be high and the production efficiency will be low. If the hydrothermal reaction time is too short, the reaction cannot occur completely.

[0028] In some embodiments, the hydrothermal reaction is carried out under stirring conditions at a stirring speed of 200 r / min to 300 r / min.

[0029] In some embodiments, in the second slurry, the molar ratio of sodium to sulfuric acid in the washing material is 1:(0.3-0.4); the molar ratio of sodium to ammonium sulfate in the washing material is 1:(0.1-0.2); the molar ratio of sodium to carbon source in the washing material is 1:(0.5-1); and the solid content of the second slurry is 20%-30%.

[0030] In this embodiment, by controlling the ratio of sulfate ions, some sulfate ions can be replaced by oxygen (from metal oxides formed by high-temperature hydrothermal method) and phosphate ions in the washing material, thus achieving both cation doping and anion doping and recombination.

[0031] In some embodiments, in the step of drying and calcining the second slurry, the calcination temperature is 300℃-350℃, the calcination time is 4h-8h, the heating rate is 1℃ / min-2℃ / min, the cooling rate is 2℃ / min-4℃ / min, and the calcination is carried out under a protective atmosphere.

[0032] In this embodiment, drying and calcining the second slurry facilitates coating the surface of the composite sodium ferrous sulfate with a carbon source without causing the decomposition of sulfate ions.

[0033] In some embodiments, the drying is carried out by spray drying, with the inlet air temperature being 150°C-350°C and the outlet air temperature being 70°C-95°C, in order to improve the efficiency of spray drying.

[0034] In some embodiments, the pulverization is air jet pulverization, using a protective gas as the gas source for pulverization, with a gas pressure of 0.5MPa-0.8MPa, which is more conducive to improving pulverization efficiency.

[0035] Thirdly, embodiments of this application provide a positive electrode sheet, including the composite sodium ferrous sulfate positive electrode material provided in this application or the composite sodium ferrous sulfate positive electrode material prepared by the preparation method of the composite sodium ferrous sulfate positive electrode material provided in this application.

[0036] In the technical solution of this application embodiment, the positive electrode sheet includes the above-mentioned composite sodium ferrous sulfate positive electrode material, and therefore, when applied to a secondary battery, it has superior electrical performance in terms of discharge capacity and cycle performance.

[0037] Fourthly, embodiments of this application provide a secondary battery, including the positive electrode sheet provided in the third aspect of this application.

[0038] In the technical solution of this application embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and therefore has superior advantages in terms of charging capacity, discharging capacity, and cycle performance.

[0039] Compared with the prior art, the beneficial effects of this application include: the composite sodium ferrous sulfate cathode material improves the ionic conductivity through element doping, ensures the sodium ion diffusion coefficient, and has a high degree of sulfate ion retention, with iron and doped metals forming a uniform compound, thereby achieving further improvement in the performance of the composite sodium ferrous sulfate cathode material.

[0040] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0041] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0042] Figure 1 is a process flow diagram of a method for preparing composite sodium ferrous sulfate cathode material provided in an embodiment of this application;

[0043] Figure 2 is a scanning electron microscope (SEM) image of the washing material obtained in Example 1 of this application;

[0044] Figure 3 is a scanning electron microscope (SEM) image of the composite sodium ferrous sulfate cathode material obtained in Example 1 of this application;

[0045] Figure 4 shows the charging and discharging curves of the composite sodium ferrous sulfate cathode material obtained in Example 1 of this application at 0.1C and 1C rates. Detailed Implementation

[0046] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0048] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0049] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0050] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0051] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0052] In the preparation of existing sodium ferrous sulfate cathode materials, the sintering temperature is relatively low, generally below 350℃, due to the easy decomposition of sulfate ions. At this temperature, the ion diffusion rate is slow, making it difficult to achieve a uniform mixture of doped metal and iron and the formation of a eutectic, resulting in insufficient performance of the final cathode material.

[0053] To address the technical problem that conventional doping methods in existing technologies struggle to achieve uniform mixing of the doping metal and iron, as well as the formation of a eutectic, leading to insufficient performance of the resulting cathode material, this application provides a composite sodium ferrous sulfate cathode material, its preparation method, cathode electrode, and secondary battery. This application first achieves uniform mixing and eutectic formation of iron, sodium, and the doping metal under high-temperature hydrothermal conditions, while simultaneously doping with a phosphorus source. Then, through anion conversion at high temperature, the composite sodium ferrous sulfate cathode material is obtained. The composite sodium ferrous sulfate cathode material improves ionic conductivity through elemental doping, ensures sodium ion diffusion coefficient, and exhibits high sulfate ion retention and uniform compound formation of iron and the doping metal, thus further enhancing the performance of the composite sodium ferrous sulfate cathode material.

[0054] In one aspect, embodiments of this application provide a composite sodium ferrous sulfate cathode material, comprising a core having the chemical formula Na. x M y Fe z (PO4) k (SO4)(0.4~0.6)xO t M includes at least one of manganese, vanadium, and titanium sources, 16≤x≤17, y=1, 4≤z≤5, 2≤k≤2.6, and y+z-0.1x-1.5k≤t≤y+z+0.1x-1.5k.

[0055] In the technical solution of this application embodiment, the composite sodium ferrous sulfate cathode material is doped with M ions to improve ionic conductivity, ensure sodium ion diffusion coefficient, and has a high degree of sulfate ion retention and uniform iron and doped metal compound formation, thereby further improving the performance of the polyanion sodium battery cathode material with sulfate ions.

[0056] In some embodiments, the composite sodium ferrous sulfate cathode material further includes a carbon coating layer, which coats the surface of the core. The molar ratio of carbon in the carbon coating layer to sodium in the core is greater than or equal to 0.5 and less than or equal to 1. For example, the molar ratio of carbon in the carbon coating layer to sodium in the core is 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, or any value between 0.5 and 1.0.

[0057] In some embodiments, the mass fraction of the carbon coating layer in the composite sodium ferrous sulfate cathode material is 5% to 10%, for example, it can be any value between 5.5%, 5.8%, 6%, 7%, 7.5%, 8%, 9%, 9.5%, or 5% to 10%.

[0058] In this embodiment, the electronic conductivity of the material is improved by the carbon-coated core-shell structure.

[0059] In some embodiments, the composite sodium ferrous sulfate cathode material includes a core and a C coating layer, and the chemical formula of the composite sodium ferrous sulfate cathode material is Na. x M y Fe z (PO4) k (SO4)(0.4~0.6)xO t / C, where M includes at least one of manganese, vanadium, and titanium sources, 16≤x≤17, y=1, 4≤z≤5, 2≤k≤2.6, y+z-0.1x-1.5k≤t≤y+z+0.1x-1.5k.

[0060] In some embodiments, the chemical formula of the composite sodium ferrous sulfate cathode material is Na. 16.6 MnF e4.5 (PO4) 2.3 (SO4) 8.3 O 2.05 / C、Na 16 MnFe4(PO4)2(SO4) 6.4 O 3.6 / C、Na 17 MnFe5(PO4) 2.6 (SO4) 9.35 O 0.85 / C、Na 16.6 VF e4.5 (PO4) 2.3 (SO4) 8.3 O 2.05 / C、Na 16.6 TiF e4.5 (PO4) 2.3 (SO4) 8.3 O 3.05 / C or Na16.6 MnF e4.5 (PO4) 2.3 (SO4) 8.3 O 2.05 / C.

[0061] In some embodiments, the sodium ion diffusion coefficient of the composite sodium ferrous sulfate cathode material is 1*10. -10 cm 2 / s-5*10 -10 cm 2 / s, for example, the sodium ion diffusion coefficient of the composite sodium ferrous sulfate cathode material is 1*10. -10 cm 2 / s, 2*10 -10 cm 2 / s, 2.5*10 -10 cm 2 / s, 3*10 -10 cm 2 / s、4*10 -10 cm 2 / s、5*10 -10 cm 2 / s or 1*10 -10 cm 2 / s-5*10 -10 cm 2 Any value between / s. The powder resistivity of the composite sodium ferrous sulfate cathode material is 15Ω·cm-25Ω·cm, for example, the powder resistivity of the composite sodium ferrous sulfate cathode material is 15Ω·cm, 18Ω·cm, 20Ω·cm, 22Ω·cm, 25Ω·cm or any value between 15Ω·cm and 25Ω·cm.

[0062] In this embodiment, the composite sodium ferrous sulfate cathode material has a high sodium ion diffusion coefficient and low powder resistivity, which is beneficial to improving the electrochemical performance of the battery when applied to a secondary battery.

[0063] Please refer to Figure 1. This application provides a method for preparing a composite sodium ferrous sulfate cathode material, including the following steps:

[0064] S1. Mix the ferrous source, sodium source, phosphorus source, dopant, reducing agent and first solvent, and homogenize them to a slurry state to obtain the first slurry;

[0065] S2. The first slurry is subjected to a hydrothermal reaction, followed by filtration and washing to obtain the washed material;

[0066] In steps S1 and S2, a uniform doped metal phase, iron, and sodium composite oxide precipitate is first synthesized under hydrothermal conditions using ferrous, sodium, and phosphorus sources, dopants, and reducing agents. At the same time, the addition of reducing agents prevents the oxidation of ferrous ions. Steps S1 and S2 enable the materials to form a uniform compound.

[0067] S3. Mix the washing material, sulfate compound, carbon source and second solvent to obtain the second slurry;

[0068] Step S3 is an anion conversion process, the purpose of which is to introduce sulfate ions into the composite sodium ferrous sulfate cathode material; at the same time, by adding sulfate compounds (such as sulfuric acid and / or ammonium sulfate), not only sulfate ions can be introduced, but pH can also be adjusted, and ammonium ions and hydrogen ions are easily volatilized and will not leave residues.

[0069] S4. The second slurry is spray-dried and calcined, and then crushed and impurities are removed to obtain a composite sodium ferrous sulfate cathode material.

[0070] Step S4 involves spray drying, which enables uniform mixing of the carbon source and the iron / sodium / doped metal composite oxide. Calcination then coats the carbon source onto the surface of the composite sodium ferrous sulfate, effectively reducing the powder resistivity and improving the electronic conductivity of the material.

[0071] This application does not limit the type of carbon source, but those skilled in the art can impose restrictions based on the actual situation. For example, the carbon source can be graphene, carbon nanotubes (CNTs), etc.

[0072] Furthermore, in some embodiments, the molar ratio of the dopant element, iron element, sodium element and phosphorus element in the first slurry is 1:(4-5):(16.0-17.0):(2-2.6); suitable but not limiting, the molar ratio of the dopant element, iron element, sodium element and phosphorus element in the dopant is 1:4:16:2; 1:5:17:2.6; 1:4:17:2.3.

[0073] In the technical solution of this application embodiment, by controlling the molar ratio of dopant element, iron element, sodium element and phosphorus element in the first slurry within this range, the uniform mixing and eutectic of iron element, sodium element and dopant element can be achieved through hydrothermal reaction. At the same time, if the ratio of dopant phosphate is unbalanced, it is difficult to achieve this purpose.

[0074] Furthermore, in some embodiments, the molar ratio of iron to reducing agent in the first slurry is 1:(0.1-0.3); for example, the molar ratio of iron to reducing agent is 1:0.1, 1:0.2, or 1:0.3.

[0075] In the technical solution of this application embodiment, by controlling the molar ratio of iron to reducing agent in the first slurry within this range, the oxidation of ferrous iron can be avoided. If too much reducing agent is added, it will lead to an imbalance in the final product's formulation, resulting in a high phosphate content, which will ultimately affect the material's cycle performance and capacity.

[0076] Further, in some embodiments, the ferrous source includes at least one of ferrous acetate and ferrous phosphate; the sodium source includes at least one of sodium hydroxide, sodium oxide, sodium carbonate, sodium acetate, and sodium phosphate; the phosphorus source includes sodium phosphate; the reducing agent includes sodium phosphite; the dopant includes at least one of manganese, vanadium, and titanium sources; the manganese source includes at least one of manganese sulfate, manganese nitrate, and manganese acetate; the vanadium source includes at least one of ammonium metavanadate and sodium metavanadate; the titanium source includes titanium tetrachloride; the carbon source includes at least one of graphene and carbon nanotubes; the sulfate compound includes at least one of sulfuric acid and ammonium sulfate; it is understood that compounds containing both phosphorus and sodium elements can be used as both phosphorus and sodium sources, such as sodium phosphite, and phosphite is also used as a reducing agent. In some embodiments, the sodium source is sodium hydroxide, sodium oxide, or sodium carbonate, which provides a sodium source and also provides an alkaline environment for the reaction.

[0077] This application does not limit the type of dopant; those skilled in the art can select one according to the actual situation, as long as it can improve the ionic conductivity of the cathode material. For example, the dopant can be one or more of manganese, titanium, and vanadium sources.

[0078] In the technical solution of this application embodiment, by introducing a manganese source as a dopant, the ionic conductivity can be improved.

[0079] This application does not limit the type of manganese source, and those skilled in the art can choose according to the actual situation. For example, the manganese source is manganese acetate, etc.

[0080] Furthermore, in some embodiments, in the first slurry, the dopant is manganese acetate, the ferrous source is ferrous acetate, the sodium source is sodium hydroxide and sodium phosphate, the phosphorus source is sodium phosphate, and the reducing agent is sodium phosphite; the molar ratio of manganese acetate to ferrous acetate is 1:(4-5), the molar ratio of manganese acetate to sodium hydroxide is 1:(8-12), the molar ratio of manganese acetate to sodium phosphite is 1:(0.5-1), and the molar ratio of manganese acetate to sodium phosphate is 1:(1-2).

[0081] Furthermore, in some embodiments, in the first slurry, the dopant is ammonium metavanadate, the ferrous source is ferrous phosphate, the sodium source is sodium carbonate, the phosphorus source is sodium phosphate, and the reducing agent is sodium phosphite; the molar ratio of ammonium metavanadate to ferrous phosphate is 1:(4-5), the molar ratio of ammonium metavanadate to sodium carbonate is 1:(8-12), the molar ratio of ammonium metavanadate to sodium phosphite is 1:(0.5-1), and the molar ratio of ammonium metavanadate to sodium phosphate is 1:(1-2).

[0082] Furthermore, in some embodiments, in the first slurry, the dopant is titanium tetrachloride, the ferrous source is ferrous acetate, the sodium source is sodium carbonate, the phosphorus source is sodium phosphate, and the reducing agent is sodium phosphite; the molar ratio of titanium tetrachloride to ferrous acetate is 1:(4-5), the molar ratio of titanium tetrachloride to sodium carbonate is 1:(8-12), the molar ratio of titanium tetrachloride to sodium phosphite is 1:(0.5-1), and the molar ratio of titanium tetrachloride to sodium phosphate is 1:(1-2).

[0083] Furthermore, in some embodiments, the solid content (the solid content in the first slurry is the mass fraction of solids in the first slurry) is 25%-35%, including but not limited to 25%, 27%, 29%, 31%, 33%, 35%, etc., and this application does not limit it.

[0084] In the technical solution of this application embodiment, if the amount of the first solvent is too low, some raw materials may not be completely dissolved in water and crystallize out. If the amount of the first solvent is too high, the production efficiency will be low. Suitable but not limiting, the first solvent is selected from pure water, deionized water, distilled water, etc.

[0085] Furthermore, in some embodiments, the hydrothermal reaction temperature is 200℃-300℃, including but not limited to 200℃, 220℃, 240℃, 260℃, 280℃, 300℃, etc., which are not limited in this application; the hydrothermal reaction time is 12h-15h, including but not limited to 12h, 13h, 14h, 15h, etc., which are not limited in this application; the pressure is 1.5MPa-2MPa, including but not limited to 1.5MPa, 1.6MPa, 1.7MPa, 1.8MPa, 1.9MPa, 2MPa, etc., which are not limited in this application.

[0086] In the technical solution of this application embodiment, by controlling the temperature and time of the hydrothermal reaction within a certain range, the reaction can be completed and excessive energy consumption can be avoided. If the temperature of the hydrothermal reaction is too high, the energy consumption will be high and the requirements for the equipment will be high. If the temperature of the hydrothermal reaction is too low, the reaction cannot be completed. If the hydrothermal reaction time is too long, the energy consumption will be high and the production efficiency will be low. If the hydrothermal reaction time is too short, the reaction cannot be completed.

[0087] Furthermore, in some embodiments, the hydrothermal reaction is carried out under stirred conditions.

[0088] Furthermore, in some embodiments, during the hydrothermal reaction, the stirring speed is 200 r / min-300 r / min, including but not limited to 200 r / min, 220 r / min, 240 r / min, 260 r / min, 280 r / min, 300 r / min, etc., and this application does not limit it.

[0089] Furthermore, in some embodiments, washing is stopped when the conductivity of the washing water is ≤500μS / cm.

[0090] Furthermore, in some embodiments, the molar ratio of sodium to sulfuric acid in the washing material in the second slurry is 1:(0.3-0.4), including but not limited to 1:0.3, 1:0.32, 1:0.34, 1:0.36, 1:0.38, 1:0.4, etc., which are not limited in this application; the molar ratio of sodium to ammonium sulfate in the washing material is 1:(0.1-0.2), including but not limited to 1:0.1, 1:0.12, 1:0.14, 1:0.16, 1:0.18, 1:0.2, etc., which are not limited in this application.

[0091] In the technical solution of this application embodiment, when introducing sulfate ions, the ratio of sulfate ions is controlled so that some sulfate ions can be replaced by oxygen (from metal oxides formed by high temperature hydrothermal method) and phosphate ions. While performing cation doping, anion doping and recombination are also achieved.

[0092] Furthermore, in some embodiments, the molar ratio of sodium to carbon source in the washing material is 1:(0.5-1), including but not limited to 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, etc., and this application does not limit it.

[0093] Furthermore, in some embodiments, the solid content (the solid content in the second slurry is the mass fraction of solids in the second slurry) is 20%-30%, including but not limited to 20%, 22%, 24%, 26%, 28%, 30%, etc., and this application does not limit it.

[0094] In the technical solutions of this application embodiment, if the solid content of the second slurry is too high, it is easy to clog the material during drying (e.g., spray drying); if the solid content of the second slurry is too low, the energy consumption required for drying is too high. In some embodiments, the second solvent used for the second slurry is selected from pure water, deionized water or distilled water, and the carbon source is CNT (carbon nanotubes).

[0095] In step S4, the second slurry is first dried and calcined; then it is pulverized and impurities removed to obtain a composite sodium ferrous sulfate cathode material.

[0096] In some embodiments, drying is performed using spray drying, with the following parameters: inlet air temperature is 150℃-350℃, including but not limited to 150℃, 200℃, 250℃, 300℃, 350℃, etc., which are not limited in this application; outlet air temperature is 70℃-95℃, including but not limited to 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, etc., which are not limited in this application. During spray drying, a protective gas is used as the gas source. The particle size of the spray-dried material is 3μm-10μm, including but not limited to 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, etc., which are not limited in this application.

[0097] In some embodiments, the parameters of the calcination process are as follows: the calcination temperature is 300℃-350℃, including but not limited to 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, etc., which are not limited in this application; the calcination time is 4h-8h, including but not limited to 4h, 5h, 6h, 7h, 8h, etc., which are not limited in this application; the calcination is carried out under a protective gas.

[0098] This application does not limit the type of protective gas; those skilled in the art can select it according to the actual situation, such as nitrogen or argon. During the calcination process, the heating rate is 1℃ / min-2℃ / min, the cooling rate is 2℃ / min-4℃ / min, and the material is discharged after cooling to a temperature ≤70℃.

[0099] The pulverization and impurity removal process includes air jet milling, sieving, iron removal, and packaging. In some embodiments, a protective gas is used as the pulverizing source during air jet milling, with a pressure of 0.5 MPa-0.8 MPa. The pulverized material is ground to a particle size of 1 μm-3 μm. Under these conditions, the pulverization is more thorough, facilitating subsequent sieving. In some embodiments, a 100-200 mesh ultrasonic vibrating screen is used for sieving, which helps to ensure uniform particle size. In some embodiments, an electromagnetic iron separator is used for iron removal. In some embodiments, packaging is carried out in a constant temperature and humidity chamber with humidity ≤10% and temperature of 25℃±5℃.

[0100] The application provides a positive electrode sheet, including the composite sodium ferrous sulfate positive electrode material provided in this application or the composite sodium ferrous sulfate positive electrode material prepared by the preparation method of the composite sodium ferrous sulfate positive electrode material provided in this application.

[0101] In the technical solution of this application embodiment, the positive electrode sheet includes the above-mentioned composite sodium ferrous sulfate positive electrode material, and therefore, when applied to a secondary battery, it has superior electrical performance in terms of discharge capacity and cycle performance.

[0102] This application provides a secondary battery, including the positive electrode provided in the third aspect of this application.

[0103] In the technical solution of this application embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and therefore has superior advantages in terms of charging capacity, discharging capacity, and cycle performance.

[0104] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0105] I. Preparation Method

[0106] Example 1

[0107] The preparation of the composite sodium ferrous sulfate cathode material in this embodiment is as follows:

[0108] (1) Manganese acetate, ferrous acetate, sodium hydroxide, sodium phosphite and sodium phosphate are mixed in a molar ratio of 1:4.5:10.5:0.8:1.5, and then pure water is added to make the solid content 29%. The mixture is mixed and slurried to obtain the first slurry. In the first slurry, the molar ratio of manganese to iron, sodium and phosphorus is 1:4.5:16.6:2.3, and the molar ratio of iron to reducing agent sodium phosphite in ferrous acetate is 1:0.178.

[0109] (2) The first slurry was added to a high-pressure hydrothermal reactor and reacted at 260°C for 13 hours. The pressure was 1.8 MPa and the stirring speed was 250 r / min. Then, it was washed with pure water. The washing was stopped when the conductivity of the washing water of the first slurry was ≤500 μS / cm, and the washed material was obtained.

[0110] (3) Add sulfuric acid, ammonium sulfate and CNT to the washing material. The molar ratio of sodium in the washing material to added sulfuric acid, ammonium sulfate and CNT is 1:0.35:0.15:0.7. Then add pure water and stir to slurry so that the solid content is 25% and the second slurry is obtained.

[0111] (4) The second slurry is spray-dried with an inlet air temperature of 150℃ and an outlet air temperature of 70℃. A protective gas is used as the gas source. The resulting spray-dried material has a particle size of 7μm. It is then calcined. During the calcination process, the heating rate is 1.5℃ / min, the temperature is raised to 330℃, the calcination time is 7h, the cooling rate is 3℃ / min, and the material is discharged after the temperature is ≤70℃. The calcined material is then pulverized by airflow using 0.7MPa nitrogen as the gas source until the particle size is 1.7μm. It is then screened using a 100-mesh ultrasonic vibrating screen. The undersize material is removed by an electromagnetic iron remover and then packaged in a constant temperature and humidity room with a humidity ≤10% and a temperature of 25±5℃ to obtain a composite sodium ferrous sulfate cathode material.

[0112] The chemical formula of the composite sodium ferrous sulfate cathode material finally obtained in this embodiment is: Na 16.6 MnFe 4.5 (PO4) 2.3 (SO4) 8.3 O 2.05 / C, with a final carbon content of 7.26%.

[0113] Example 2

[0114] The preparation of the composite sodium ferrous sulfate cathode material in this embodiment is as follows:

[0115] (1) Manganese acetate, ferrous acetate, sodium hydroxide, sodium phosphite and sodium phosphate are mixed in a molar ratio of 1:4:10.4:0.4:1.6, and then pure water is added to make the solid content 29%. The mixture is mixed and slurried to obtain the first slurry. In the first slurry, the molar ratio of the doped metal to iron, sodium and phosphorus is 1:4:16:2; the molar ratio of iron to reducing agent is 1:0.1.

[0116] (2) The first slurry was added to a high-pressure hydrothermal reactor and reacted at 200℃ for 15 hours. The pressure was 1.5MPa and the stirring speed was 250r / min. Then, it was washed with pure water. The washing was stopped when the conductivity of the washing water of the first slurry was ≤500μS / cm, and the washed material was obtained.

[0117] (3) Add sulfuric acid, ammonium sulfate and CNT to the washing material. The molar ratio of sodium in the washing material to added sulfuric acid, ammonium sulfate and CNT is 1:0.3:0.1:0.5. Then add pure water and stir to make the slurry so that the solid content is 25% to obtain the second slurry.

[0118] (4) The second slurry was spray-dried using nitrogen gas at 230℃ as the gas source. The resulting spray-dried material had a particle size of 3μm. It was then calcined at 300℃ for 8 hours with a heating rate of 1℃ / min and a cooling rate of 2℃ / min. The material was discharged after cooling to a temperature ≤70℃. Calcination was carried out under a protective gas. During the air jet milling process, a protective gas was used as the gas source for milling at a pressure of 0.5MPa until the particle size of the material was 1μm. During the sieving process, a 200-mesh ultrasonic vibrating screen was used. During the iron removal process, an electromagnetic iron remover was used. The packaging process was carried out in a constant temperature and humidity room with a humidity ≤10% and a temperature of 25±5℃ to obtain a composite sodium ferrous sulfate cathode material.

[0119] The chemical formula of the composite sodium ferrous sulfate cathode material finally obtained in this embodiment is: Na 16 MnFe4(PO4)2(SO4) 6.4 O 3.6 / C, with a final carbon content of 5.86%.

[0120] Example 3

[0121] The preparation method of the composite sodium ferrous sulfate cathode material in this embodiment is the same as that in Example 1. The difference is that the molar ratio of the doped metal to iron, sodium and phosphorus in the first slurry is 1:5:17:2.6; the molar ratio of iron to reducing agent is 1:0.3; and the solid content of the first slurry is 35%.

[0122] The hydrothermal reaction was carried out at a temperature of 300℃ for 12 hours and a pressure of 2 MPa. The hydrothermal reaction was conducted under stirring conditions.

[0123] In the second slurry, the molar ratio of sodium to sulfuric acid in the washing material is 1:0.35; the molar ratio of sodium to ammonium sulfate in the washing material is 1:0.2; the solid content of the second slurry is 30%; and the molar ratio of sodium to carbon source CNT in the washing material is 1:1.

[0124] During spray drying, the inlet air temperature is 350℃ and the outlet air temperature is 95℃, using a protective gas as the air source. The particle size of the spray-dried material is 10μm. During calcination, the calcination temperature is 350℃, the calcination time is 4 hours, the heating rate is 2℃ / min, the cooling rate is 4℃ / min, and the material is discharged after cooling to a temperature ≤70℃. Calcination is carried out under a protective gas. During air jet milling, a protective gas is used as the air source for milling at a pressure of 0.8MPa, and the particle size of the material is 3μm. During sieving, a 100-mesh ultrasonic vibrating screen is used. During iron removal, an electromagnetic iron separator is used. The packaging process is carried out in a constant temperature and humidity chamber with a humidity ≤10% and a temperature of 25±5℃.

[0125] The chemical formula of the composite sodium ferrous sulfate cathode material finally obtained in this embodiment is: Na 17 MnFe5(PO4) 2.6 (SO4) 9.35 O 0.85 / C, with a final carbon content of 9.78%.

[0126] Example 4

[0127] The preparation method of the composite sodium ferrous sulfate cathode material in this embodiment is the same as that in Example 1, except that manganese acetate is replaced with sodium metavanadate.

[0128] The chemical formula of the composite sodium ferrous sulfate cathode material finally obtained in this embodiment is: Na 16.6 VFe 4.5 (PO4) 2.3 (SO4) 8.3 O 2.05 / C, with a final carbon content of 7.75%.

[0129] Example 5

[0130] The preparation method of the composite sodium ferrous sulfate cathode material in this embodiment is the same as that in Example 1, except that manganese acetate is replaced with titanium tetrachloride.

[0131] The chemical formula of the composite sodium ferrous sulfate cathode material finally obtained in this embodiment is: Na 16.6 TiFe 4.5 (PO4) 2.3 (SO4) 8.3 O 3.05 / C, with a final carbon content of 7.78%.

[0132] Example 6

[0133] The preparation method of the composite sodium ferrous sulfate cathode material in this embodiment is the same as in Example 1, except that the solid content of the first slurry is 25%, the solid content of the second slurry is 20%, and the carbon source is graphene. The chemical formula of the final composite sodium ferrous sulfate cathode material obtained in this embodiment is: Na 16.6 MnFe 4.5 (PO4) 2.3 (SO4) 8.3 O 2.05 / C, with a final carbon content of 7.21%.

[0134] Comparative Example 1

[0135] The preparation of the composite sodium ferrous sulfate cathode material is as follows:

[0136] (1) Ferrous acetate, sodium hydroxide, sodium phosphite and sodium phosphate are mixed in a molar ratio of 5.5:10.5:0.98:1.32, and then pure water is added to make the solid content 29%. The mixture is mixed and slurried to obtain the first slurry. In the first slurry, the molar ratio of iron, sodium and phosphorus is 5.5:16.6:2.3, and the molar ratio of iron to reducing agent sodium phosphite in ferrous acetate is 1:0.178.

[0137] (2) The first slurry was added to a high-pressure hydrothermal reactor and reacted at 260°C for 13 hours. The pressure was 1.8 MPa and the stirring speed was 250 r / min. Then, it was washed with pure water. The washing was stopped when the conductivity of the washing water of the first slurry was ≤500 μS / cm, and the washed material was obtained.

[0138] (3) Add sulfuric acid, ammonium sulfate and CNT to the washing material. The molar ratio of sodium in the washing material to added sulfuric acid, ammonium sulfate and CNT is 1:0.35:0.15:0.7. Then add pure water and stir to slurry so that the solid content is 25% and the second slurry is obtained.

[0139] (4) The second slurry is spray-dried with an inlet air temperature of 150℃ and an outlet air temperature of 70℃. A protective gas is used as the gas source. The resulting spray-dried material has a particle size of 7μm. It is then calcined. During the calcination process, the heating rate is 1.5℃ / min, the temperature is raised to 330℃, the calcination time is 7h, the cooling rate is 3℃ / h, and the material is discharged after cooling down to a material temperature ≤70℃ to obtain calcined material. The obtained calcined material is then pulverized by airflow using 0.7MPa nitrogen as the gas source until the particle size of the material is 1.7μm. It is then screened using a 100-mesh ultrasonic vibrating screen. The undersize material is removed by an electromagnetic iron remover and then packaged in a constant temperature and humidity room with a humidity ≤10% and a temperature of 25±5℃ to obtain composite sodium ferrous sulfate cathode material.

[0140] The chemical formula of the composite sodium ferrous sulfate cathode material finally obtained in this embodiment is: Na 16.6 Fe 5.5 (PO4) 2.3 (SO4) 8.3 O 2.05 / C.

[0141] Comparative Example 2

[0142] The preparation method of the composite sodium ferrous sulfate cathode material in this embodiment is as follows:

[0143] Manganese acetate, ferrous acetate, sodium hydroxide, sodium phosphite, and sodium phosphate were mixed in a molar ratio of 1:4.5:10.5:0.8:1.5 to obtain a mixture. Sulfuric acid, ammonium sulfate, and CNT were then added to the mixture, with the molar ratio of sodium to added sulfuric acid, ammonium sulfate, and CNT being 1:0.35:0.15:0.7. The mixture was stirred and slurried to achieve a solid content of 25%. It was then spray-dried with an inlet air temperature of 150℃ and an outlet air temperature of 70℃, using a protective gas source. The resulting spray-dried material had a particle size of 7μm and was subsequently calcined. During the calcination process, the heating rate was 1.5℃ / min, the temperature was raised to 330℃, the calcination time was 7h, the cooling rate was 3℃ / min, and the material was discharged after cooling to a temperature ≤70℃ to obtain calcined material. The obtained calcined material was then subjected to air jet milling using 0.7MPa nitrogen as the gas source to mill until the particle size was 1.7μm. Then, it was sieved using a 100-mesh ultrasonic vibrating screen, and the undersize material was removed with an electromagnetic iron separator. The material was then packaged in a constant temperature and humidity chamber with a humidity ≤10% and a temperature of 25±5℃ to obtain composite sodium ferrous sulfate cathode material.

[0144] II. Testing Methods

[0145] Property testing of composite sodium ferrous sulfate cathode material:

[0146] 1. SEM tests were performed on the above reactants and the composite sodium ferrous sulfate cathode material;

[0147] 2. The contents of Na, Fe, and S were determined by inductively coupled plasma atomic emission spectrometry.

[0148] 3. The carbon content was tested using a carbon-sulfur analyzer;

[0149] 4. D10, D50, and D90 were measured using a laser particle size analyzer;

[0150] 5. Specific surface area was determined using the gas adsorption BET method;

[0151] 6. Powder resistivity was tested using the four-probe method at a pressure of 10 MPa.

[0152] 7. PD (Compacted Density): Tested using a compaction density meter, with a test pressure of 3T and a compaction time of 30S;

[0153] 8. pH and free sodium element test: The method for pH value determination shall refer to GB / T 9724, General Rules for pH Value Determination of Chemical Reagents; the free sodium element content shall be determined by potentiometric titration.

[0154] 9. Moisture content was determined using the KF moisture test method;

[0155] 10. Ferrous iron (Fe3+) is measured using a colorimetric method;

[0156] 11. The sodium ion diffusion coefficient was obtained by electrochemical impedance spectroscopy (EIS).

[0157] 2. Properties of secondary batteries

[0158] The composite sodium ferrous sulfate cathode materials obtained in the examples and comparative examples were respectively prepared into cathode sheets and applied to batteries. The battery assembly and testing process is as follows:

[0159] 1. Preparation of the positive electrode sheet: In the positive electrode slurry, the mass ratio of composite sodium ferrous sulfate positive electrode material, acetylene black (as a conductive agent), and polyvinylidene fluoride (PVDF, as a binder) is 88:6:6, with a mass accuracy to 0.001g. Aluminum foil is used as the current collector for the positive electrode sheet. The composite sodium ferrous sulfate positive electrode material, acetylene black, PVDF, and N-methylpyrrolidone are stirred to obtain the positive electrode slurry. The positive electrode slurry is uniformly coated onto the aluminum foil, dried in an oven at 100℃, and then cut into electrode sheets with a diameter of 18mm and a thickness of 0.10mm. The electrode sheets are weighed, with a mass accuracy to 0.0001g. The mixing and coating processes of the positive electrode slurry are strictly controlled to ensure consistent electrode area and thickness.

[0160] 2. Battery Assembly: In an inert gas glove box with water and oxygen contents ≤0.0005%, a sodium-ion battery electrolyte consisting of 1 mol / L sodium perchlorate (NaClO4) and a mixed carbonate-based organic solvent [ethylene carbonate (EC) / diethyl carbonate (DEC) / fluoroethylene carbonate (FEC) (the volume ratio of EC, DEC, and FEC is 1:1:0.05)] was assembled and sealed to form a test battery.

[0161] 3. Battery Testing: The test battery was subjected to charge-discharge cycles at 25°C using a sodium-ion battery electrochemical performance tester (Wuhan Landian CT2001A battery testing system).

[0162] (a) Charging at 0.1C and 1C rates, with a charging limit voltage of 4.5V;

[0163] (b) Discharge at 0.1C and 1C rates, with a discharge termination voltage of 2.0V.

[0164] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0165] Table 1. Physicochemical properties of the cathode materials obtained in the examples and comparative examples.

[0166] Referring to Figure 2, the washing material in Example 1 is in the form of flake particles, indicating that the reactants form a uniform and stable eutectic material. Referring to Figure 3, it can be seen that the composite sodium ferrous sulfate cathode material prepared in Example 1 is in the form of particles, indicating that carbon coating has been achieved.

[0167] Please refer to Table 1. The data in Table 1 shows that the composite sodium ferrous sulfate cathode material prepared in Example 1 has a high compaction density, low powder internal resistance, and relatively low pH and free sodium. Compared to Comparative Example 1, which did not introduce Mn, the material obtained in Example 1 of this application has a high sodium ion diffusion coefficient and low powder resistivity, indicating that Mn doping is beneficial to improving electrochemical performance. Comparative Example 2 directly obtained the composite sodium ferrous sulfate cathode material through mixed calcination. The raw materials did not form a uniform and stable eutectic, but were directly spray-dried and calcined. Compared to Comparative Example 2, the material obtained in Example 1 of this application has a high sodium ion diffusion coefficient and significantly low powder resistivity, which is beneficial to improving electrochemical performance.

[0168] Table 2. Material property test results of the examples and comparative examples.

[0169] Figure 4 shows the 0.1C and 1C charge-discharge curves of the composite sodium ferrous sulfate cathode material obtained in Example 1 of this application. Combined with Table 2 and Figure 4, it can be seen that, compared to Comparative Examples 1 and 2, the composite sodium ferrous sulfate cathode materials prepared in Examples 1-6 have very high capacity and excellent electrical properties such as rate performance and cycle performance. Compared to the composite sodium ferrous sulfate cathode materials prepared in the comparative examples, the composite sodium ferrous sulfate cathode material prepared in this application has superior electrical performance and better cycle performance compared to pure sodium ferrous sulfate and sodium ferrous sulfate prepared by conventional processes.

[0170] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A composite sodium ferrous sulfate cathode material, characterized in that, The composite sodium ferrous sulfate cathode material includes a core, the chemical formula of which is Na. x M y Fe z (PO4) k (SO4)(0.4~0.6)xO t M includes at least one of manganese, vanadium, and titanium, 16≤x≤17, y=1, 4≤z≤5, 2≤k≤2.6, and y+z-0.1x-1.5k≤t≤y+z+0.1x-1.5k.

2. The composite sodium ferrous sulfate cathode material according to claim 1, characterized in that, The composite sodium ferrous sulfate cathode material further includes a carbon coating layer, which coats the surface of the core. The molar ratio of carbon in the carbon coating layer to sodium in the core is greater than or equal to 0.5 and less than or equal to 1.

3. The composite sodium ferrous sulfate cathode material according to claim 1, characterized in that, The sodium ion diffusion coefficient of the composite sodium ferrous sulfate cathode material is 1*10. -10 cm 2 / s-5*10 -10 cm 2 / s; the resistivity of the composite sodium ferrous sulfate cathode material is 15Ω·cm-25Ω·cm.

4. A method for preparing a composite sodium ferrous sulfate cathode material, characterized in that, Includes the following steps: The ferrous source, sodium source, phosphorus source, dopant, reducing agent and first solvent are mixed to obtain the first slurry; The first slurry is subjected to a hydrothermal reaction, followed by filtration and washing to obtain the washed material; The washing material, sulfate compound, carbon source, and second solvent are mixed to obtain a second slurry; The second slurry was dried and calcined, and then pulverized and impurities removed to obtain a composite sodium ferrous sulfate cathode material.

5. The method for preparing the composite sodium ferrous sulfate cathode material according to claim 4, characterized in that, In the first slurry, the dopant includes dopant elements, wherein the molar ratio of the dopant elements to iron, sodium, and phosphorus is 1:(4-5):(16.0-17.0):(2-2.6); and / or The molar ratio of iron in the ferrifluorine source to the reducing agent is 1:(0.1-0.3); and / or The solids content of the first slurry is 25%-35%; and / or The molar ratio of sodium to carbon source in the washing material is 1:(0.5-1); and / or The solids content of the second slurry is 20%-30%.

6. The method for preparing the composite sodium ferrous sulfate cathode material according to claim 4, characterized in that, The ferrous source includes at least one of ferrous acetate and ferrous phosphate; and / or The sodium source includes at least one of sodium hydroxide, sodium oxide, sodium carbonate, sodium acetate, and sodium phosphate; and / or The phosphorus source includes sodium phosphate; and / or The reducing agent includes sodium phosphite; and / or The dopant includes at least one of manganese source, vanadium source, and titanium source; and / or The carbon source includes at least one of graphene and carbon nanotubes; and / or The sulfate compound includes at least one of sulfuric acid and ammonium sulfate.

7. The method for preparing the composite sodium ferrous sulfate cathode material according to claim 4, characterized in that, The sulfate compounds include sulfuric acid and ammonium sulfate; In the second slurry, the molar ratio of sodium to sulfuric acid in the washing material is 1:(0.3-0.4); and / or The molar ratio of sodium to ammonium sulfate in the washing material is 1:(0.1-0.2).

8. The method for preparing the composite sodium ferrous sulfate cathode material according to claim 4, characterized in that, The hydrothermal reaction temperature is 200℃-300℃, the hydrothermal reaction time is 12h-15h, and the hydrothermal reaction pressure is 1.5MPa-2MPa.

9. The method for preparing the composite sodium ferrous sulfate cathode material according to claim 4, characterized in that... The hydrothermal reaction is carried out under stirring conditions at a stirring speed of 200 r / min-300 r / min.

10. The method for preparing the composite sodium ferrous sulfate cathode material according to claim 4, characterized in that, In the step of drying and calcining the second slurry, the calcination temperature is 300℃-350℃, the calcination time is 4h-8h, and the calcination is carried out under a protective atmosphere.

11. The method for preparing the composite sodium ferrous sulfate cathode material according to claim 4, characterized in that, During the calcination process, the heating rate is 1℃ / min-2℃ / min, and the cooling rate is 2℃ / min-4℃ / min.

12. The method for preparing the composite sodium ferrous sulfate cathode material according to claim 4, characterized in that, The drying process employs spray drying, with an inlet air temperature of 150℃-350℃ and an outlet air temperature of 70℃-95℃.

13. The method for preparing the composite sodium ferrous sulfate cathode material according to claim 4, characterized in that, The pulverization is an airflow pulverization process, using a protective gas as the gas source, with a gas pressure of 0.5MPa-0.8MPa.

14. A positive electrode plate, characterized in that, The positive electrode includes the composite sodium ferrous sulfate positive electrode material according to any one of claims 1-3 or the composite sodium ferrous sulfate positive electrode material obtained by the preparation method according to any one of claims 4-13.

15. A secondary battery, characterized in that, The secondary battery includes the positive electrode sheet as described in claim 14.

Images