A composite sodium ferrous sulfate positive electrode material and its preparation method and application

Abstract

The present application provides a composite sodium ferrous sulfate positive electrode material and its preparation method and application, which belongs to the field of sodium batteries. The composite sodium ferrous sulfate positive electrode material includes a core, and the chemical formula of the core is Na x M y Fe z (PO4) k (SO4) (0.4～0.6)x O t , wherein M includes at least one of manganese, vanadium, and titanium, 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; the present application reduces sulfate decomposition, improves the material properties of the composite sodium ferrous sulfate positive electrode material, and improves the performance of the secondary battery using the composite sodium ferrous sulfate positive electrode material in terms of cycle performance and other aspects.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium batteries, and in particular to a composite sodium ferrous sulfate positive electrode material and a preparation method and application thereof. Background Art

[0002] Sodium, an element in the same main group as lithium, has very similar electrochemical properties and is abundant, with an abundance of approximately 2.64% in the Earth's crust. It is also much cheaper. Therefore, developing sodium-ion batteries (NIBs) using sodium as a replacement for lithium has broad application prospects, and finding and developing suitable electrode materials has become a major task in NIB research.

[0003] Polyanion sodium battery materials have the characteristics of low cost and excellent cycle performance. People's research on sodium ion battery positive electrode materials also focuses on polyanion materials. Among the many polyanion compounds, SO4 2- Sulfate materials have higher operating voltages due to their higher electronegativity and stronger inductive effect, thus attracting more widespread attention. Because sodium ferrous sulfate has a lower sintering temperature and lower energy consumption, sulfate-based polyanions are more cost-effective among polyanion materials. When used as a positive electrode material for sodium-ion batteries, sodium ferrous sulfate offers a higher charge and discharge platform than LiFePO4 positive electrode materials. However, drawbacks such as its lower electronic conductivity and sodium ion diffusion coefficient limit its practical application.

[0004] In the prior art, the ionic conductivity of polyanion sodium battery cathode materials is generally improved by doping with metal ions, and the electronic conductivity of the materials is improved by carbon coating. In the conventional doping method, the doping metal salt and the iron salt are generally mixed and ground first, and then the mixture is sintered. However, the electrical properties of the cathode materials obtained by the above scheme are poor. The reason is that in order to prevent the decomposition of the necessary raw material sulfate, the sintering temperature of the cathode material cannot exceed 350°C. However, at this temperature, the ion diffusion rate is slow, making it difficult to form a uniform mixed eutectic of the doped metal and iron. Therefore, it is necessary to provide a solution to reduce the decomposition of sulfate and improve the material properties of the composite sodium ferrous sulfate cathode material. Summary of the Invention

[0005] In view of the technical problems existing in the background technology, the present application provides a composite sodium ferrous sulfate positive electrode material and 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 positive electrode material.

[0006] In the first aspect, the present invention provides a composite sodium ferrous sulfate positive electrode material, comprising a core, the chemical formula of which is Na x M y Fe z (PO4)k (SO4) (0.4～0.6)x O t , wherein M includes at least one of manganese, vanadium, and titanium, 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.

[0007] In the technical solution of the embodiment of the present application, the composite sodium ferrous sulfate positive electrode material improves the ionic conductivity by element doping, ensures the sodium ion diffusion coefficient, and has a high degree of sulfate ion retention, and iron and doped metals form a uniform compound, thereby achieving further improvement in the performance of the composite sodium ferrous sulfate positive electrode material.

[0008] In some embodiments, the composite sodium ferrous sulfate positive electrode material further includes a carbon coating layer, which is coated on the surface of the core, and the molar ratio of the carbon element in the carbon coating layer to the sodium element in the core is greater than or equal to 0.5 and less than or equal to 1.

[0009] In this embodiment, the electronic conductivity of the material is improved by the carbon-coated core-shell structure.

[0010] In some embodiments, the sodium ion diffusion coefficient of the composite sodium ferrous sulfate positive electrode material is 1*10 -10 cm 2 / s-5*10 -10 cm 2 / s; powder resistivity is 15Ω.cm-25Ω.cm.

[0011] In this embodiment, the sodium ion diffusion coefficient is high, the powder resistivity is low, and the obtained composite sodium ferrous sulfate positive electrode material has good electrochemical performance.

[0012] In a second aspect, the present application provides a method for preparing a composite sodium ferrous sulfate positive electrode material, comprising the following steps:

[0013] S1, mixing a ferrous source, a sodium source, a phosphorus source, a dopant, a reducing agent, and a first solvent to obtain a first slurry;

[0014] S2. performing a hydrothermal reaction on the first slurry, followed by filtering and washing to obtain a washed material;

[0015] S3, mixing the washing material, the sulfate compound, the carbon source and the second solvent to obtain a second slurry;

[0016] S4. Dry and calcine the second slurry, and then crush and remove impurities to obtain a composite sodium ferrous sulfate positive electrode material.

[0017] In the technical solution of the embodiment of the present application, 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, effectively reducing the powder resistivity of the material and improving the electronic conductivity of the material; the preparation method of the composite sodium ferrous sulfate positive electrode material provided by the present application reduces the decomposition of sulfate while performing metal element doping and anion doping, thereby increasing the sodium ion diffusion coefficient and improving the performance of the composite sodium ferrous sulfate positive electrode material.

[0018] In some embodiments, in the first slurry, the dopant includes a doping element, and the molar ratio of the doping element to the iron element, the sodium element, and the phosphorus element is 1:(4-5):(16.0-17.0):(2-2.6); the molar ratio of the iron element 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 the sodium element in the washing material to the carbon source is 1:(0.5-1); and the solid content of the second slurry is 20%-30%.

[0019] In this embodiment, by controlling the molar ratio of the doping element, iron element, sodium element, and phosphorus element in the first slurry within this range, uniform mixing and eutectic of iron, sodium, and doped metal can be achieved through hydrothermal reaction, and at the same time, if the ratio is unbalanced, it will be detrimental to achieving the above purpose; 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.

[0020] 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 a manganese source, a vanadium source, and a titanium source; 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; in the second slurry, the molar ratio of the sodium element in the washing material to the sulfuric acid is 1:(0.3-0.4); the molar ratio of the sodium element in the washing material to the ammonium sulfate is 1:(0.1-0.2).

[0021] In this embodiment, the introduction of a manganese source, a vanadium source, and a titanium source as dopants is beneficial for improving ion conductivity.

[0022] In some embodiments, the temperature of the hydrothermal reaction is 200° C.-300° C., the time of the hydrothermal reaction is 12 h-15 h, the pressure of the hydrothermal reaction is 1.5 MPa-2 MPa; and the hydrothermal reaction is carried out under stirring.

[0023] 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 is high and the equipment requirements are also 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 is high and the production efficiency is low. If the hydrothermal reaction time is too short, the reaction cannot be completed.

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

[0025] In this embodiment, by controlling the ratio of sulfate ions, part of the sulfate ions can be replaced by oxygen (from metal oxides formed by high-temperature hydrothermal method) and phosphate ions in the washing material, thereby achieving anion doping and recombination while performing cation doping.

[0026] In some embodiments, in the steps of drying and calcining the second slurry, the calcination temperature is 300°C-350°C, the calcination time is 4h-8h, the heating rate is 1°C / min-2°C / min, the cooling rate is 2°C / h-4°C / h, and the calcination is carried out under a protective atmosphere.

[0027] In this embodiment, drying and calcining the second slurry is beneficial for coating the carbon source as a coating layer on the surface of the composite sodium ferrous sulfate without causing decomposition of sulfate radicals.

[0028] In a third aspect, an embodiment of the present application provides a positive electrode plate, comprising the composite sodium ferrous sulfate positive electrode material provided in the present 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 the present application.

[0029] In the technical solution of the embodiment of the present application, the positive electrode plate includes the above-mentioned composite sodium ferrous sulfate positive electrode material, so when applied to a secondary battery, it has more excellent electrical performance in terms of discharge capacity, cycle performance, etc.

[0030] In a fourth aspect, an embodiment of the present application provides a secondary battery, comprising the positive electrode sheet provided in the third aspect of the present application.

[0031] In the technical solution of the embodiment of the present application, the secondary battery includes the above-mentioned positive electrode plate, and thus has more excellent advantages in terms of charging capacity, discharging capacity, cycle performance, etc.

[0032] Compared with the prior art, the beneficial effects of the present application include: the composite sodium ferrous sulfate positive electrode material improves the ionic conductivity by element doping, ensures the sodium ion diffusion coefficient, and has a high degree of sulfate ion retention, and iron and doped metals form a uniform compound, thereby achieving further improvement in the performance of the composite sodium ferrous sulfate positive electrode material.

[0033] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] To more clearly illustrate the technical solution of this application, the following is a brief introduction to the drawings used in this application. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be derived from these drawings without inventive effort.

[0035] Figure 1 A process flow chart of a method for preparing a composite sodium ferrous sulfate positive electrode material provided in an embodiment of the present application;

[0036] Figure 2 This is a scanning electron microscope (SEM) image of the washing material obtained in Example 1 of the present application;

[0037] Figure 3 This is a scanning electron microscope (SEM) image of the composite sodium ferrous sulfate cathode material obtained in Example 1 of the present application;

[0038] Figure 4 These are the charge curve and discharge curve of the composite sodium ferrous sulfate positive electrode material obtained in Example 1 of the present application at 0.1C rate and 1C rate. DETAILED DESCRIPTION

[0039] The following embodiments of the technical solution of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only examples and are not intended to limit the scope of protection of the present application.

[0040] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0041] In the description of the embodiments of this application, the technical terms "first," "second," etc. are used only to distinguish different objects and should not be understood to indicate or imply relative importance or to implicitly indicate the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise specifically defined.

[0042] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0043] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0044] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0045] In the preparation of existing sodium ferrous sulfate positive electrode materials, due to the easy decomposition of sulfate ions, the sintering temperature is relatively low, generally below 350°C. At this temperature, the ion diffusion rate is slow, making it difficult to achieve uniform mixing of doped metals and iron and the formation of a eutectic, resulting in insufficient performance of the final positive electrode material.

[0046] In order to solve the technical problem that conventional doping methods in the prior art are difficult to achieve uniform mixing of doped metals and iron and the formation of eutectics, resulting in insufficient performance of the final positive electrode material, the present application provides a composite sodium ferrous sulfate positive electrode material and a preparation method thereof, a positive electrode plate and a secondary battery. The present application first achieves uniform mixing and eutectic of iron, sodium and doped metals under high-temperature hydrothermal conditions, and simultaneously dopes part of the phosphate, and then obtains the composite sodium ferrous sulfate positive electrode material by converting the anions at high temperature. The composite sodium ferrous sulfate positive electrode material improves the ionic conductivity of the composite sodium ferrous sulfate positive electrode material through element doping, ensures the sodium ion diffusion coefficient, and has a high degree of sulfate ion retention, and iron and the doped metal form a uniform compound, thereby achieving further improvement in the performance of the composite sodium ferrous sulfate positive electrode material.

[0047] In the first aspect, the present invention provides a composite sodium ferrous sulfate positive electrode material, comprising a core, the chemical formula of which is Na x M y Fe z (PO4) k (SO4) (0.4～0.6)x O t , wherein 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.

[0048] In the technical solution of the embodiment of the present application, the composite sodium ferrous sulfate positive electrode material is doped with M ions to improve the ionic conductivity and ensure the sodium ion diffusion coefficient. In addition, the sulfate ion retention degree is high, and iron and doped metals form a uniform compound, thereby further improving the performance of the polyanion sodium battery positive electrode material with sulfate ions.

[0049] In some embodiments, the composite sodium ferrous sulfate positive electrode material further includes a carbon coating layer, which is coated on the surface of the core, and the molar ratio of the carbon element in the carbon coating layer to the sodium element in the core is greater than or equal to 0.5 and less than or equal to 1; for example, the molar ratio of the carbon element in the carbon coating layer to the sodium element in the core is 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, or any value between 0.5-1.0.

[0050] In some embodiments, the mass fraction of the carbon coating layer in the composite sodium ferrous sulfate positive electrode material is 5% to 10%, for example, it can be 5.5%, 5.8%, 6%, 7%, 7.5%, 8%, 9%, 9.5%, or any value between 5% and 10%.

[0051] In this embodiment, the electronic conductivity of the material is improved by the carbon-coated core-shell structure.

[0052] In some embodiments, the composite sodium ferrous sulfate positive electrode material includes a core and a C coating layer. The chemical formula of the composite sodium ferrous sulfate positive electrode material is Na x M y Fe z (PO4) k (SO4) (0.4～0.6)x O t / C, wherein 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.

[0053] In some embodiments, the chemical formula of the composite sodium ferrous sulfate positive electrode material is Na 16.6MnF 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 Na 16.6 MnF e4.5 (PO4) 2.3 (SO4) 8.3 O 2.05 / C.

[0054] In some embodiments, the sodium ion diffusion coefficient of the composite sodium ferrous sulfate positive electrode 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 positive electrode 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 The powder resistivity of the composite sodium ferrous sulfate positive electrode material is 15Ω.cm-25Ω.cm, for example, the powder resistivity of the composite sodium ferrous sulfate positive electrode material is 15Ω.cm, 18Ω.cm, 20Ω.cm, 22Ω.cm, 25Ω.cm, or any value between 15Ω.cm and 25Ω.cm.

[0055] In this embodiment, the composite sodium ferrous sulfate positive electrode material has a high sodium ion diffusion coefficient and a low powder resistivity, which is beneficial to improving the electrochemical performance of the battery when applied to a secondary battery.

[0056] Please refer to Figure 1 The present invention provides a method for preparing a composite sodium ferrous sulfate positive electrode material, comprising the following steps:

[0057] S1, mixing a ferrous source, a sodium source, a phosphorus source, a dopant, a reducing agent, and a first solvent, and uniformly mixing them into a slurry state to obtain a first slurry;

[0058] S2. performing a hydrothermal reaction on the first slurry, followed by filtering and washing to obtain a washed material;

[0059] In steps S1 and S2, a uniform doped metal phase, iron, and sodium composite oxide precipitate is synthesized using a ferrous source, a sodium source, a phosphorus source, a dopant, and a reducing agent under hydrothermal conditions, while simultaneously adding a reducing agent to prevent oxidation of the ferrous ions. Steps S1 and S2 enable the materials to form a uniform compound.

[0060] S3, mixing the washing material, the sulfate compound, the carbon source and the second solvent to obtain a second slurry;

[0061] Step S3 is an anion conversion process, the purpose of which is to introduce sulfate into the composite sodium ferrous sulfate cathode material; by adding sulfuric acid and ammonium sulfate, not only can sulfate be introduced, but also the pH can be controlled, and the ammonium ions and hydrogen ions are easily volatilized and will not cause residue;

[0062] S4, spray drying and calcining the second slurry, and then crushing and removing impurities to obtain a composite sodium ferrous sulfate positive electrode material;

[0063] In step S4, spray drying can achieve uniform mixing of the carbon source and the iron / sodium / doped metal composite oxide. After calcination, the carbon source can be coated on the surface of the composite sodium ferrous sulfate, thereby effectively reducing the powder resistivity of the material and improving the electronic conductivity of the material.

[0064] The present application does not limit the type of carbon source, and those skilled in the art may limit it according to actual circumstances. For example, the carbon source may be graphene, carbon nanotubes (CNTs), etc.

[0065] Further, in some embodiments, in the first slurry, the molar ratio of the doping element, iron element, sodium element, and phosphorus element in the dopant is 1:(4-5):(16.0-17.0):(2-2.6); suitably but not restrictively, the molar ratio of the doping 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.

[0066] In the technical solution of the embodiment of the present application, by controlling the molar ratio of the doping element, iron element, sodium element and phosphorus element in the first slurry within this range, uniform mixing and eutectic of iron element, sodium element and doping element can be achieved through hydrothermal reaction, while part of the phosphate is doped at the same time. If the ratio is unbalanced, it will be difficult to achieve this goal.

[0067] Furthermore, in some embodiments, in the first slurry, the molar ratio of the iron element to the reducing agent is 1:(0.1-0.3); for example, the molar ratio of the iron element to the reducing agent is 1:0.1, 1:0.2, or 1:0.3.

[0068] In the technical solution of the embodiment of the present application, by controlling the molar ratio of iron to reducing agent in the first slurry within this range, oxidation of ferrous iron can be avoided. If too much reducing agent is added, the final product will have an unbalanced ratio and high phosphate content, which will ultimately affect the material's cycling performance and capacity.

[0069] Furthermore, 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 a manganese source, a vanadium source, and a titanium source; 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 a compound having both phosphorus and sodium elements can be used as both a phosphorus source and a sodium source, such as sodium phosphite, with phosphorous acid also used as a reducing agent. In some embodiments, the sodium source is sodium hydroxide, sodium oxide, or sodium carbonate, which, while providing a sodium source, also provides an alkaline environment for the reaction.

[0070] The present application does not limit the type of dopant, and those skilled in the art can select it according to actual conditions, as long as it can improve the ionic conductivity of the positive electrode material through doping. For example, the dopant can be one or more of a manganese source, a titanium source, and a vanadium source.

[0071] In the technical solution of the embodiment of the present application, by introducing a manganese source as a dopant, the improvement of ionic conductivity can be achieved.

[0072] The present application does not limit the type of manganese source, and those skilled in the art can select it according to actual conditions. For example, the manganese source is manganese acetate.

[0073] 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).

[0074] 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).

[0075] 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).

[0076] Furthermore, in some embodiments, in the first slurry, the solid content (the solid content in the first slurry is the mass fraction of the solid in the first slurry) is 25%-35%, including but not limited to 25%, 27%, 29%, 31%, 33%, 35%, etc., which is not limited in this application.

[0077] In the technical solution of the embodiment of the present application, if the amount of the first solvent is too low, it is easy to cause some raw materials to be unable to completely dissolve in water and crystallize out; if the amount of the first solvent is too high, the production efficiency is low; suitably but not restrictively, the first solvent is selected from pure water, deionized water, distilled water, etc.

[0078] Further, in some embodiments, the temperature of the hydrothermal reaction is 200°C-300°C, including but not limited to 200°C, 220°C, 240°C, 260°C, 280°C, 300°C, etc., which is not limited in this application; the time of the hydrothermal reaction is 12h-15h, including but not limited to 12h, 13h, 14h, 15h, etc., which is 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 is not limited in this application.

[0079] In the technical solution of the embodiment of the present application, 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 is high and the equipment requirements are also 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 is high and the production efficiency is low. If the hydrothermal reaction time is too short, the reaction cannot be completed.

[0080] Furthermore, in some embodiments, the hydrothermal reaction is carried out under stirring conditions.

[0081] Furthermore, in some embodiments, during the hydrothermal reaction, the stirring speed is 200r / min-300r / min, including but not limited to 200r / min, 220r / min, 240r / min, 260r / min, 280r / min, 300r / min, etc., and this application does not impose any restrictions on this.

[0082] Furthermore, in some embodiments, washing is stopped when the conductivity of the washing water becomes ≤500 μS / cm.

[0083] Further, 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), 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 is 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 is not limited in this application.

[0084] In the technical solution of the embodiment of the present application, when introducing sulfate ions, the ratio of sulfate ions is controlled so that part of the sulfate ions can be replaced by oxygen (from metal oxides formed by high-temperature hydrothermal method) and phosphate ions, thereby achieving anion doping and recombination while performing cation doping.

[0085] 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., which is not limited in this application.

[0086] Furthermore, in some embodiments, in the second slurry, the solid content (the solid content in the second slurry is the mass fraction of the solid in the second slurry) is 20-30%, including but not limited to 20%, 22%, 24%, 26%, 28%, 30%, etc., which is not limited in this application.

[0087] In the technical solutions of the embodiments of the present application, if the solid content of the second slurry is too high, clogging is likely to occur during drying (e.g., spray drying); if the solid content of the second slurry is too low, excessive energy consumption is required for drying. In some embodiments, the second solvent used in the second slurry is selected from pure water, deionized water, or distilled water, and the carbon source is CNTs (carbon nanotubes).

[0088] In step S4, the second slurry is first dried and calcined; then crushed and impurities are removed to obtain a composite sodium ferrous sulfate positive electrode material.

[0089] In some embodiments, drying is performed by spray drying, and the parameters of spray drying are as follows: the inlet air temperature is 150°C-350°C, including but not limited to 150°C, 200°C, 250°C, 300°C, 350°C, etc., which is not limited in this application; the outlet air temperature is 70°C-95°C, including but not limited to 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, etc., which is not limited in this application. During the spray drying process, 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 is not limited in this application.

[0090] In some embodiments, the parameters of the calcination process are as follows: the calcination temperature is 300°C-350°C, including but not limited to 300°C, 310°C, 320°C, 330°C, 340°C, 350°C, etc., which is not limited in this application; the calcination time is 4h-8h, including but not limited to 4h, 5h, 6h, 7h, 8h, etc., which is not limited in this application; the calcination is carried out under protective gas.

[0091] The present application does not limit the type of protective gas, and those skilled in the art can select it according to actual conditions, for example, nitrogen, argon, etc. During the calcination process, the heating rate is 1°C / min-2°C / min, and the cooling rate is 2°C / h-4°C / h. The material is discharged after cooling to a temperature of ≤70°C after calcination.

[0092] Crushing and impurity removal include air flow crushing, screening, iron removal, and packaging processes; in some embodiments, during the air flow crushing process, protective gas is used as the gas source for crushing, and the air pressure is 0.5MPa-0.8MPa; the material is crushed to a particle size of 1μm-3μm. Under this condition, the degree of crushing is more sufficient, which is convenient for subsequent screening. In some embodiments, the screening process uses a 100-200 mesh ultrasonic vibrating screen, which is conducive to the uniformity of the particle size obtained. In some embodiments, an electromagnetic iron remover is used for iron removal. In some embodiments, packaging is carried out in a constant temperature and humidity room with a humidity of ≤10% and a temperature of 25±5°C.

[0093] An embodiment of the application provides a positive electrode plate, comprising the composite sodium ferrous sulfate positive electrode material provided in the present 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 the present application.

[0094] In the technical solution of the embodiment of the present application, the positive electrode plate includes the above-mentioned composite sodium ferrous sulfate positive electrode material, so when applied to a secondary battery, it has more excellent electrical performance in terms of discharge capacity, cycle performance, etc.

[0095] An embodiment of the present application provides a secondary battery, including the positive electrode plate provided in the third aspect of the present application.

[0096] In the technical solution of the embodiment of the present application, the secondary battery includes the above-mentioned positive electrode plate, and thus has more excellent advantages in terms of charging capacity, discharging capacity, cycle performance, etc.

[0097] Some specific examples are listed below. It should be noted that the examples described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. Where specific techniques or conditions are not specified in the examples, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used without manufacturer's indication are all commercially available conventional products.

[0098] 1. Preparation method

[0099] Example 1

[0100] The preparation of the composite sodium ferrous sulfate positive electrode material in this embodiment is as follows:

[0101] (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, pure water is added to make the solid content 29%, and the mixture is mixed and slurried to obtain a 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 in ferrous acetate to the reducing agent sodium phosphite is 1:0.178.

[0102] (2) The first slurry is added to a high-pressure hydrothermal autoclave, reacted at a temperature of 260°C for 13 hours, a pressure of 1.8 MPa, and a stirring speed of 250 r / min, and then filtered and washed with pure water. Washing is stopped when the conductivity of the washing water of the first slurry is ≤500 μS / cm to obtain a washed material.

[0103] (3) adding sulfuric acid, ammonium sulfate, and CNT to the washing material, wherein the molar ratio of sodium in the washing material to the added sulfuric acid, ammonium sulfate, and CNT is 1:0.35:0.15:0.7, and then adding pure water and stirring to slurry so that the solid content is 25%, thereby obtaining a second slurry.

[0104] (4) The second slurry is spray-dried with an air inlet temperature of 150°C and an air outlet temperature of 70°C. Protective gas is used as the gas source to obtain a spray-dried material with a particle size of 7 μm. The material is then calcined. During the calcination process, the temperature is increased at a rate of 1.5°C / min to a temperature of 330°C. The calcination time is 7 hours and the temperature is cooled at a rate of 3°C / h. The material is cooled to a temperature of ≤70°C and then discharged to obtain a calcined material. The calcined material is subjected to air flow pulverization using 0.7MPa nitrogen as the gas source to pulverize the material until the particle size is 1.7 μm. The material is then sieved using a 100-mesh ultrasonic vibrating screen. The material under the sieve is deironed using an electromagnetic iron remover and then packaged. The packaging is carried out in a constant temperature and humidity room with a humidity of ≤10% and a temperature of 25±5°C to obtain a composite sodium ferrous sulfate positive electrode material.

[0105] The chemical formula of the composite sodium ferrous sulfate positive electrode material finally obtained in this embodiment is: Na 16.6 MnFe 4.5 (PO4) 2.3 (SO4) 8.3 O 2.05 / C, the final carbon content is 7.26%.

[0106] Example 2

[0107] The preparation of the composite sodium ferrous sulfate positive electrode 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 1:4:10.4:0.4:1.6, pure water is added to make the solid content 29%, and the mixture is mixed and slurried to obtain a first slurry. In the first slurry, the molar ratio of the doping metal to iron, sodium, and phosphorus is 1:4:16:2; and the molar ratio of iron to the reducing agent is 1:0.1.

[0109] (2) The first slurry is added to a high-pressure hydrothermal autoclave, reacted at a temperature of 200°C for 15 hours, a pressure of 1.5 MPa, and a stirring speed of 250 r / min, and then filtered and washed with pure water. Washing is stopped when the conductivity of the washing water of the first slurry is ≤500 μS / cm to obtain a washed material.

[0110] (3) Add sulfuric acid, ammonium sulfate, and CNT to the washing material, with the molar ratio of sodium in the washing material to the added sulfuric acid, ammonium sulfate, and CNT being 1:0.3:0.1:0.5, and then add pure water and stir to slurry so that the solid content is 25%, thereby obtaining a second slurry.

[0111] (4) The second slurry is spray-dried, and nitrogen at a temperature of 230°C is used as the gas source for spray drying. The obtained spray-dried material has a particle size of 3 μm, and is then calcined. During the calcination process, the calcination temperature is 300°C, the calcination time is 8 hours, the heating rate is 1°C / min, the cooling rate is 2°C / h, and the material is discharged after cooling to a material temperature of ≤70°C. The calcination is carried out under protective gas; during the air flow pulverization process, protective gas is used as the gas source for pulverization, the gas pressure is 0.5 MPa, and the material is pulverized to a particle size of 1 μm; during the screening process, a 200-mesh ultrasonic vibrating screen is used; during the iron removal process, an electromagnetic iron remover is used for iron removal; the packaging process is carried out in a constant temperature and humidity room with a humidity of ≤10% and a temperature of 25±5°C to obtain a composite sodium ferrous sulfate positive electrode material.

[0112] The chemical formula of the composite sodium ferrous sulfate positive electrode material finally obtained in this embodiment is: Na 16 MnFe4(PO4)2(SO4) 6.4 O 3.6 / C, the final carbon content is 5.86%.

[0113] Example 3

[0114] The preparation method of the composite sodium ferrous sulfate positive electrode material in this embodiment is the same as that in Example 1, except that: in the obtained first slurry, the molar ratio of the doping metal to iron, sodium, and phosphorus is 1:5:17:2.6; the molar ratio of iron to the reducing agent is 1:0.3; and the solid content of the first slurry is 35%.

[0115] The temperature of the hydrothermal reaction is 300° C., the time of the hydrothermal reaction is 12 h, and the pressure of the hydrothermal reaction is 2 MPa; the hydrothermal reaction is carried out under stirring conditions.

[0116] In the second slurry, the molar ratio of sodium in the washing material to sulfuric acid is 1:0.35; the molar ratio of sodium in the washing material to ammonium sulfate is 1:0.2; the solid content of the second slurry is 30%; and the molar ratio of sodium in the washing material to carbon source CNT is 1:1.

[0117] During the spray drying process, the air inlet temperature is 350°C, the air outlet temperature is 95°C, protective gas is used as the gas source, and the particle size of the spray dried material is 10μm; during the calcination process, the calcination temperature is 350°C, the calcination time is 4h, the heating rate is 2°C / min, the cooling rate is 4°C / h, and the material is discharged after cooling to a material temperature of ≤70°C. The calcination is carried out under protective gas; during the air flow pulverization process, protective gas is used as the gas source for pulverization, the gas pressure is 0.8MPa, and the material is pulverized to a particle size of 3μm; during the screening process, a 100-mesh ultrasonic vibrating screen is used; during the iron removal process, an electromagnetic iron remover is used for iron removal; the packaging process is carried out in a constant temperature and humidity room with a humidity of ≤10% and a temperature of 25±5°C.

[0118] The chemical formula of the composite sodium ferrous sulfate positive electrode material finally obtained in this embodiment is: Na 17 MnFe5(PO4) 2.6 (SO4) 9.35 O 0.85 / C, the final carbon content is 9.78%.

[0119] Example 4

[0120] The preparation method of the composite sodium ferrous sulfate positive electrode material in this embodiment is the same as that in Example 1, except that manganese acetate is replaced by sodium metavanadate.

[0121] The chemical formula of the composite sodium ferrous sulfate positive electrode material finally obtained in this embodiment is: Na 16.6 VFe 4.5 (PO4) 2.3 (SO4) 8.3 O 2.05 / C, the final carbon content is 7.75%.

[0122] Example 5

[0123] The preparation method of the composite sodium ferrous sulfate positive electrode material in this embodiment is the same as that in Example 1, except that manganese acetate is replaced by titanium tetrachloride.

[0124] The chemical formula of the composite sodium ferrous sulfate positive electrode material finally obtained in this embodiment is: Na 16.6 TiFe 4.5 (PO4) 2.3 (SO4) 8.3 O 3.05 / C, the final carbon content is 7.78%.

[0125] Example 6

[0126] The preparation method of the composite sodium ferrous sulfate positive electrode material in this embodiment is the same as that 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 composite sodium ferrous sulfate positive electrode material finally obtained in this embodiment is: Na 16.6 MnFe 4.5 (PO4) 2.3 (SO4) 8.3 O 2.05 / C, the final carbon content is 7.21%.

[0127] Comparative Example 1

[0128] The preparation of the composite sodium ferrous sulfate positive electrode material is as follows:

[0129] (1) Ferrous acetate, sodium hydroxide, sodium phosphite, and sodium phosphate were mixed in a molar ratio of 5.5:10.5:0.98:1.32, and pure water was added to make the solid content 29%. The mixture was mixed and slurried to obtain a first slurry. In the first slurry, the molar ratio of iron, sodium, and phosphorus was 5.5:16.6:2.3, and the molar ratio of iron in ferrous acetate to the reducing agent sodium phosphite was 1:0.178.

[0130] (2) The first slurry is added to a high-pressure hydrothermal autoclave, reacted at a temperature of 260°C for 13 hours, a pressure of 1.8 MPa, and a stirring speed of 250 r / min, and then filtered and washed with pure water. Washing is stopped when the conductivity of the washing water of the first slurry is ≤500 μS / cm to obtain a washed material.

[0131] (3) adding sulfuric acid, ammonium sulfate, and CNT to the washing material, wherein the molar ratio of sodium in the washing material to the added sulfuric acid, ammonium sulfate, and CNT is 1:0.35:0.15:0.7, and then adding pure water and stirring to slurry so that the solid content is 25%, thereby obtaining a second slurry.

[0132] (4) The second slurry is spray-dried with an air inlet temperature of 150°C and an air outlet temperature of 70°C. Protective gas is used as the gas source to obtain a spray-dried material with a particle size of 7 μm. The material is then calcined. During the calcination process, the temperature is increased at a rate of 1.5°C / min to a temperature of 330°C. The calcination time is 7 hours and the temperature is cooled at a rate of 3°C / h. The material is cooled to a temperature of ≤70°C and then discharged to obtain a calcined material. The calcined material is subjected to air flow pulverization using 0.7MPa nitrogen as the gas source to pulverize the material until the particle size is 1.7 μm. The material is then sieved using a 100-mesh ultrasonic vibrating screen. The material under the sieve is deironed using an electromagnetic iron remover and then packaged. The packaging is carried out in a constant temperature and humidity room with a humidity of ≤10% and a temperature of 25±5°C to obtain a composite sodium ferrous sulfate positive electrode material.

[0133] The chemical formula of the composite sodium ferrous sulfate positive electrode material finally obtained in this embodiment is: Na 16.6 Fe 5.5 (PO4) 2.3 (SO4) 8.3 O 2.05 / C.

[0134] Comparative Example 2

[0135] The preparation method of the composite sodium ferrous sulfate positive electrode material in this embodiment is:

[0136] 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 to obtain a mixture, sulfuric acid, ammonium sulfate, and CNT are added to the mixture, and the molar ratio of sodium in the mixture to the sulfuric acid, ammonium sulfate, and CNT is 1:0.35:0.15:0.7. The mixture is stirred and slurried to make a solid content of 25%, and then spray-dried with an air inlet temperature of 150°C and an air outlet temperature of 70°C. Protective gas is used as a gas source to obtain a spray-dried material with a particle size of 7 μm, which is then calcined. During the calcination process, the temperature is increased at a rate of 1.5°C / min to 330°C, the calcination time is 7h, the temperature is cooled at a rate of 3°C / h, and the material is discharged after cooling to a material temperature of ≤70°C to obtain a calcined material; the obtained calcined material is subjected to air flow crushing, using 0.7MPa nitrogen as a gas source for crushing, and the material is crushed to a particle size of 1.7μm, and then sieved with a 100-mesh ultrasonic vibration screen, and the screened material is iron-removed with an electromagnetic iron remover, and then packaged. The packaging is carried out in a constant temperature and humidity room with a humidity of ≤10% and a temperature of 25±5°C to obtain a composite sodium ferrous sulfate positive electrode material.

[0137] 2. Test Method

[0138] 1. Property test of composite sodium ferrous sulfate positive electrode material:

[0139] 1. Perform SEM test on the above reaction materials and composite sodium ferrous sulfate positive electrode material;

[0140] 2. The contents of Na, Fe and S were determined by inductively coupled plasma atomic emission spectrometry;

[0141] 3. C content is tested using a carbon-sulfur analyzer;

[0142] 4.D10, D50 and D90 are tested by laser particle size analyzer;

[0143] 5. Specific surface area was determined by gas adsorption BET method;

[0144] 6. The powder resistivity is tested by the four-probe method with a test pressure of 10 MPa;

[0145] 7.PD (compacted density): tested with a compaction density meter, the test pressure is 3T, and the pressing time is 30S;

[0146] 8. pH and free sodium test: pH value shall be determined according to GB / T 9724, General Rules for Determination of pH Value of Chemical Reagents; free sodium content shall be determined by potentiometric titration;

[0147] 9. Moisture content is determined using the KF moisture test method;

[0148] 10. Ferric iron is measured by colorimetry;

[0149] 11. The sodium ion diffusion coefficient was obtained by electrochemical impedance spectroscopy (EIS).

[0150] 2. Property test of secondary batteries

[0151] The composite sodium ferrous sulfate positive electrode materials obtained in the embodiment and the comparative example were respectively prepared into positive electrode sheets and applied to batteries. The battery assembly and testing process was as follows:

[0152] 1. Preparation of the positive electrode sheet: In the positive electrode slurry, the mass ratio of the 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 of 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 and slurried to obtain a positive electrode slurry. The positive electrode slurry is evenly coated on aluminum foil, oven-dried at 100°C, and cut into electrode sheets with a diameter of 18mm and a thickness of 0.10mm. The electrode sheets are weighed to an accuracy of 0.0001g. The mixing and coating processes of the positive electrode slurry are strictly controlled to ensure that the area and thickness of the measured electrode sheets remain consistent.

[0153] 2. Battery assembly: In an inert gas glove box with water and oxygen contents ≤0.0005%, a sodium metal sheet was used as the negative electrode material, a polypropylene microporous film was used as the separator, and a sodium ion battery electrolyte composed 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 cell.

[0154] 3. Battery testing: The test battery was subjected to charge-discharge cycles at 25°C on a sodium ion battery electrochemical performance tester (Wuhan Blue Power CT2001A battery testing system).

[0155] (a) 0.1C and 1C current charging, charging limit voltage 4.5V;

[0156] (b) 0.1C and 1C rate current discharge, discharge termination voltage 2.0V.

[0157] 3. Analysis of test results of various embodiments and comparative examples

[0158] Table 1 Physical and chemical indicators of the positive electrode materials obtained in the examples and comparative examples

[0159]

[0160]

[0161] Please refer to Figure 2 , the washing material of Example 1 is flaky particles, indicating that the reaction material forms a uniform and stable eutectic material; please refer to Figure 3 ,pass Figure 3 It can be seen that the composite sodium ferrous sulfate positive electrode material prepared in Example 1 is in a granular form, indicating that carbon coating has been achieved.

[0162] Please refer to Table 1. It can be seen from the data in Table 1 that the composite sodium ferrous sulfate positive electrode material prepared in Example 1 has a high compaction density, low powder internal resistance, and relatively low pH and free sodium. Compared with Comparative Example 1 in which the Mn element is not introduced, the material obtained in Example 1 of the present application has a high sodium ion diffusion coefficient and a low powder resistivity, indicating that the doping of the Mn element is beneficial to the improvement of electrochemical performance; Comparative Example 2 directly obtains the composite sodium ferrous sulfate positive electrode material by mixing and calcining. The raw materials do not form a uniform and stable eutectic, but are directly spray-dried and calcined. Compared with Comparative Example 2, the material obtained in Example 1 of the present application has a high sodium ion diffusion coefficient and a significantly low powder resistivity, which is beneficial to the improvement of electrochemical performance.

[0163] Table 2 Material performance test results of the embodiments and comparative examples

[0164]

[0165]

[0166] Figure 4 The 0.1C and 1C charge-discharge curves of the composite sodium ferrous sulfate positive electrode material obtained in Example 1 of this application are shown in Table 2 and Figure 4 It can be seen that compared with Comparative Examples 1 and 2, the composite sodium ferrous sulfate positive electrode materials prepared in Examples 1-6 have high capacity and excellent electrical properties such as rate performance and cycle performance. Compared with the composite sodium ferrous sulfate positive electrode materials prepared in the comparative examples, the composite sodium ferrous sulfate positive electrode materials prepared in the present application have better electrical properties, and the cycle performance is better than that of pure sodium ferrous sulfate and sodium ferrous sulfate prepared by conventional processes.

[0167] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A composite sodium ferrous sulfate positive electrode material, characterized in that: The composite sodium ferrous sulfate positive electrode material includes a core, the chemical formula of which is Na x M y Fe z (PO4) k (SO4) (0.4～0.6)x O t , wherein M includes at least one of manganese, vanadium, and titanium, 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.

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

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

4. A method for preparing a composite sodium ferrous sulfate positive electrode material, characterized in that: The following steps are involved: Mixing a ferrous source, a sodium source, a phosphorus source, a dopant, a reducing agent, and a first solvent to obtain a first slurry; performing a hydrothermal reaction on the first slurry, followed by filtering and washing to obtain a washed material; mixing the washing material, the sulfate compound, the carbon source and the second solvent to obtain a second slurry; The second slurry is dried and calcined, and then crushed and impurities are removed to obtain a composite sodium ferrous sulfate positive electrode material.

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

6. The method for preparing the composite sodium ferrous sulfate positive electrode 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 comprises at least one of sodium hydroxide, sodium oxide, sodium carbonate, sodium acetate, and sodium phosphate; and / or The phosphorus source comprises sodium phosphate; and / or The reducing agent comprises sodium phosphite; and / or The dopant comprises at least one of a manganese source, a vanadium source, and a 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; and / or In the second slurry, the molar ratio of sodium element in the washing material to sulfuric acid is 1:(0.3-0.4); and / or The molar ratio of sodium element to ammonium sulfate in the washing material is 1:(0.1-0.2).

7. The method for preparing the composite sodium ferrous sulfate positive electrode material according to claim 4, characterized in that: The temperature of the hydrothermal reaction is 200° C.-300° C., the time of the hydrothermal reaction is 12 h-15 h, and the pressure of the hydrothermal reaction is 1.5 MPa-2 MPa.

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

9. A positive electrode plate, characterized in that: The positive electrode plate comprises the composite sodium ferrous sulfate positive electrode material according to any one of claims 1 to 3 or the composite sodium ferrous sulfate positive electrode material obtained by the preparation method according to any one of claims 4 to 8.

10. A secondary battery, characterized in that: The secondary battery comprises the positive electrode sheet according to claim 9.

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