Transition metal phosphate hydrate as well as preparation method and application thereof

Abstract

The invention discloses a transition metal phosphate hydrate and a preparation method and application thereof, and belongs to the technical field of batteries. The preparation method of the transition metal phosphate hydrate comprises the following steps: a premixed metal salt solution S1 is a mixed solution containing an element M and a doped metal element N, wherein the element N is selected from low-valence metal ions with the valence state being positive bivalence; the solution S2 is a solution containing a phosphate group; respectively introducing the premixed metal salt solution S1 and the solution S2 into a first inlet and a second inlet of a microreactor, and rapidly mixing the premixed metal salt solution S1 and the solution S2 through the microreactor to obtain a suspension; and a post-treatment process: curing the turbid liquid, and carrying out solid-liquid separation to obtain the transition metal phosphate hydrate. The method disclosed by the invention is simple and convenient to operate, and the phosphate material with higher stability and higher purity can be obtained.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a transition metal phosphate hydrate, its preparation method, and its application. Background Technology

[0002] Currently, the positive electrode materials used in the industrial production of batteries (such as lithium iron phosphate (LiFePO4) for lithium-ion batteries and sodium iron phosphate (Na4Fe3(PO4)2P2O7) for sodium-ion batteries) are typically iron phosphate and ferrous oxalate. However, the synthesis process often suffers from drawbacks such as cumbersome steps, high cost, and numerous byproducts. Research has found that using ferrous phosphate can reduce the use of reducing agents and the impurity removal process.

[0003] Therefore, synthesizing cathode materials using ferrous phosphate as a raw material has become one of the research directions in the new energy industry. Currently, commonly used methods for preparing ferrous phosphate include solid-phase synthesis, liquid-phase coprecipitation, and hydrothermal synthesis. However, the solid-phase method generates a large amount of toxic gases during synthesis, making it difficult to meet the environmental protection requirements of mass industrial production; the hydrothermal method, due to its high synthesis temperature, places high demands on the safety of production equipment, increasing production costs; the liquid-phase coprecipitation method typically has a long reaction time, easily causing oxidation and deterioration of ferrous ions. Furthermore, because the monoclinic crystal system is relatively more stable, ferrous phosphate synthesized under long reaction cycles or high-temperature environments is usually monoclinic.

[0004] Therefore, providing a new method for preparing phosphates (such as ferrous phosphate) to overcome the above-mentioned defects is an urgent problem to be solved. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention provides a transition metal phosphate salt hydrate, its preparation method, and its applications. This method is simple to operate and can yield phosphate materials with higher activity and purity.

[0006] The present invention achieves its objective through the following scheme: The first objective of this invention is to provide a method for preparing transition metal phosphate salt hydrates, wherein the molecular formula of the phosphate is M. 3-a N a (PO4)2, where M is selected from at least one of Fe, Mn, Ni, and Co, and N is a doped metal element different from M, 0 ≤ a ≤ 0.3, and the preparation method includes the following steps: Premixed metal salt solution S1 is prepared as follows: The premixed metal salt solution S1 is a mixed solution containing element M and doped metal element N, wherein element N is selected from low-valence metal ions with a valence state of +2. Prepare solution S2: Solution S2 is a solution containing phosphate groups; Microreactor mixing: The premixed metal salt solution S1 and the solution S2 are respectively introduced into the first inlet and the second inlet of the microreactor. The premixed metal salt solution S1 and the solution S2 are rapidly mixed in the microreactor to obtain a suspension. Post-processing: The suspension is aged, and the solid phase is separated from the liquid phase. The obtained solid phase is the transition metal phosphate salt hydrate.

[0007] In some embodiments of the present invention, the N element is selected from at least one of Mg, Ca, Cu and Zn.

[0008] In some embodiments of the present invention, the solutes in the premixed metal salt solution S1 include MSO4 and NSO4.

[0009] In some embodiments of the present invention, the M element is selected from Fe and / or Ni.

[0010] In some embodiments of the present invention, the molar ratio of the M element to the N element satisfies 100:(1-20); preferably 100:(3-15).

[0011] In some embodiments of the present invention, the concentration of the N element in the premixed metal salt solution S1 is in the range of 0.01-5 mol / L.

[0012] In some embodiments of the present invention, the flow rate V1 of the premixed metal salt solution S1 in the microreactor is in the range of 0.1-2 L / min; And / or, the flow rate V2 of the solution S2 in the microreactor is in the range of 0.1-2 L / min.

[0013] In some embodiments of the present invention, the premixed metal salt solution S1 and the solution S2 are respectively introduced into the first inlet and the second inlet of the microreactor by a multi-plunger pump.

[0014] In some embodiments of the present invention, the solute of solution S2 is selected from one or more of Na4P2O7, Na3HP2O7, Na2H2P2O7, NaH3P2O7, Na2HPO4, NaH2PO4, Na3PO4, H3PO4, NH4H2PO4, (NH4)2HPO4, and (NH4)3PO4.

[0015] A second objective of this invention is to provide a transition metal phosphate salt hydrate, prepared according to the aforementioned method; the molecular formula of the phosphate is M. 3-a N a(PO4)2, where M is selected from at least one of Fe, Mn, Ni and Co, and N is a doped metal element different from M, with 0 ≤ a ≤ 0.3; the crystal system of the transition metal phosphate hydrate is triclinic.

[0016] A third objective of this invention is to provide a phosphate-based cathode material precursor, comprising the aforementioned transition metal phosphate hydrate.

[0017] A fourth objective of this invention is to provide a cathode material, wherein the raw materials for the cathode material include phosphate-based cathode material precursors.

[0018] A fifth objective of the present invention is to provide a positive electrode sheet comprising a positive current collector and the aforementioned positive electrode material, wherein the positive electrode material is coated on the positive current collector.

[0019] A sixth objective of the present invention is to provide a battery comprising the aforementioned positive electrode, negative electrode, and separator.

[0020] A seventh object of the present invention is to provide an electrochemical device including the battery.

[0021] The beneficial effects of this invention are: This invention introduces a low-valence metal element into the process of synthesizing transition metal phosphate hydrates in a microreactor. Utilizing the fact that the ionic radius of the low-valence metal is close to that of the transition metal M, it can partially replace the active sites of the transition metal M. Therefore, it is possible to adjust the lattice parameters of the transition metal phosphate hydrate, improve its crystallinity, and enable a phase transition in the crystal system during the preparation process, successfully preparing triclinic transition metal phosphate hydrates. Attached Figure Description

[0022] Figure 1 The images show the XRD patterns of the transition metal ferrous phosphate hydrates obtained in Example 1 and Comparative Examples 1-2 of this invention.

[0023] Figure 2 This is a diagram of the microreactor device used in the embodiments of the present invention. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] 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 belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0026] In the description of this application, the terms "(1)", "(2)", "first", and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "(1)", "(2)", "first", or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified. In this application, when describing various components in the capacity layer, sodium supplement layer, and carbon layer, such as binders and conductive agents, the prefixes "first", "second", and "third" are used to define them, respectively. This is only to distinguish the components in the three layers in terms of description, and does not imply that the corresponding components in these three layers are necessarily different. The scope of protection of this application includes embodiments in which the same or different types of binders and conductive agents are used in the capacity layer, sodium supplement layer, and carbon layer.

[0027] In the description of this application, unless otherwise stated, the term "or" is inclusive. That is, the phrase "A or (or) B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0028] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the weights mentioned in the embodiments of this application can be well-known units of mass in the chemical industry, such as μg, mg, g, and kg.

[0029] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0030] Unless otherwise specified, all technical features and optional technical features of this application can be combined to form new technical solutions.

[0031] The structure of the microreactor used in this invention is as follows: Figure 2 The image shown is a conventional microreactor in this field.

[0032] The embodiments of this application will be described in detail below.

[0033] First aspect The first aspect of this application is to provide a method for preparing a transition metal phosphate salt hydrate, wherein the transition metal phosphate has the molecular formula M 3-a N a (PO4)2, where M is selected from at least one of Fe, Mn, Ni, and Co, and N is a doped metal element different from M, 0 ≤ a ≤ 0.3, and the preparation method includes the following steps: Premixed metal salt solution S1 is prepared as follows: The premixed metal salt solution S1 is a mixed solution containing element M and doped metal element N, wherein element N is selected from low-valence metal ions with a valence state of +2. Prepare solution S2: Solution S2 is a solution containing phosphate groups; Microreactor mixing: The premixed metal salt solution S1 and the solution S2 are respectively introduced into the first inlet and the second inlet of the microreactor. The premixed metal salt solution S1 and the solution S2 are rapidly mixed in the microreactor to obtain a suspension. Post-processing: The suspension is aged, and the solid phase is separated from the liquid phase. The obtained solid phase is the transition metal phosphate salt hydrate.

[0034] This invention introduces a low-valence dopant element, nitrogen (N), into the synthesis of transition metal phosphates in a microreactor. Utilizing the close ionic radius of N compared to the transition metal m, N can partially replace the active sites of m, thereby adjusting the lattice parameters of the transition metal phosphate product, increasing the crystallinity of the final product, and enabling the synthesis of high-purity, pure-phase triclinic transition metal phosphate products. Furthermore, since the dopant element N used in this application has the same valence state as m (both are divalent metal ions), side reactions caused by using high-valence dopant elements can be avoided, such as the oxidation and deterioration of divalent ferrous ions induced by the dopant ion (trivalent Al).

[0035] In a specific embodiment of the present invention, the N element is selected from at least one of Ca, Cu, Mg, and Zn. The present invention achieves a phase transformation of the final product, phosphate hydrate, by introducing low-valence metal elements such as Ca, Cu, Mg, and Zn into the premixed metal salt solution S1 during the synthesis process in a microreactor, thus successfully preparing triclinic phosphate hydrates with higher purity, activity, and crystallinity.

[0036] Taking Fe as an example, a monoclinic phosphate hydrate, Fe3(PO4)2·8H2O, was obtained using a microreactor preparation method (see Comparative Example 1 below for details). The researchers in this application discovered that introducing divalent low-valence metal ions into the premixed metal salt solution S1 during the reaction can achieve a phase transformation in the final product, the phosphate hydrate. During the reaction in the microreactor, the high-speed collisions between the premixed metal salt solution S1 and solution S2 within the narrow channel of the microreactor can generate phosphate precipitates with tiny particle sizes. Introducing divalent low-valence metal ions (such as Ca, Cu, Mg, and Zn) into the premixed metal salt solution S1, due to Ca… 2+ (1.00 Å), Cu 2+ (0.73 Å), Mg 2+ (0.72 Å), Zn 2+ (0.74 Å) and Fe 2+ With ionic radii close to (0.78 Å), these divalent low-valence metal ions react with Fe during the synthesis of ferrous phosphate in a microreactor. 2+ Simultaneously, it collides with phosphate ions, and the doped ions can partially replace Fe sites, thereby adjusting the lattice parameters of ferrous phosphate, causing ferrous phosphate to transform from a monoclinic crystal system to a triclinic crystal system, thus realizing the crystal system transformation of phosphate salt hydrates. A triclinic phosphate salt hydrate with higher purity, activity and crystallinity was successfully prepared.

[0037] Similarly, when M is selected from Mn, the phosphate product obtained through the microreactor is Mn3(PO4)2·3H2O; when M is selected from Ni, the phosphate product obtained through the microreactor is Ni3(PO4)2·8H2O; and when M is selected from Co, the phosphate product obtained through the microreactor is Co3(PO4)2·8H2O. For the divalent low-valence metal ions of the aforementioned transition metal elements Mn, Ni, and Co (such as Ca, Cu, Mg, and Zn) and Mn... 2+ (0.67 Å), Ni 2+ (0.69 Å), Co 2+ (0.65 Å) is also similar and can also be used to adjust the lattice parameters of its phosphate to obtain triclinic phosphate products with high purity, activity and crystallinity.

[0038] Furthermore, Mg is preferred as the nitrogen element. By doping with Mg, magnesium can enhance the structural stability of ferrous phosphate, suppress the dissolution and oxidative deterioration of Fe during post-processing (such as aging), improve the crystallinity of phosphate salt hydrate products, and make the final product more stable. This is beneficial to improving the high-rate electrochemical performance (e.g., 10C) of the cathode material synthesized from its products.

[0039] Furthermore, the preferred N element is a combination of Mg and Cu. The variable valence state of Cu can introduce redox active sites and enhance the reactivity. In particular, during the synthesis of ferrous phosphate, it can inhibit the oxidation and deterioration of ferrous ions and improve the purity of the product. Cu doping can ensure the performance degradation rate of the material during long-term use. The synergistic doping of Mg and Cu can achieve a balance between "high rate + long cycle" in the synthesis of cathode materials from their products.

[0040] Furthermore, the solute containing doped metal N element in the premixed metal salt solution S1 can be one or more of the following: calcium nitrate (Ca(NO3)2), calcium bicarbonate (Ca(HCO3)2), copper chloride (CuCl2), copper nitrate (Cu(NO3)2), copper sulfate (CuSO4), copper acetate ((CH3COO)2Cu), magnesium chloride (MgCl2), magnesium nitrate (Mg(NO3)2), magnesium sulfate (MgSO4), zinc chloride (ZnCl2), zinc nitrate (Zn(NO3)2), zinc sulfate (ZnSO4), and zinc acetate ((CH3COO)2Zn).

[0041] Furthermore, the solute containing the transition metal element M in the premixed metal salt solution S1 can be one or more of the following: ferrous sulfate (FeSO4··7H2O), ferrous chloride (FeCl2··4H2O), ferrous nitrate (Fe(NO3)2··6H2O), manganese sulfate (MnSO4··4H2O), manganese chloride (MnCl2··4H2O), manganese nitrate (Mn(NO3)2··6H2O), nickel sulfate (NiSO4··6H2O), and cobalt sulfate (CoSO4··7H2O).

[0042] In any embodiment of the present invention, the solute of the premixed metal salt solution S1 includes MSO4 and NSO4. In the present invention, the solute NSO4 of the doped metal element is the same anionic compound as the solute MSO4 of the transition metal element M, which can avoid the introduction of additional impurity elements that may cause side reactions and lead to the formation of impurity phases in the material. In addition, the pH value of the sulfate aqueous solution is usually low, which can reduce the formation of other impurity precipitates (such as ferric hydroxide precipitate or ferrous hydroxide precipitate) due to pH rise.

[0043] Taking the synthesis of ferrous phosphate (Fe3(PO4)2··8H2O) as an example, the solute in the premixed metal salt solution S1 is composed of the metal compound ferrous sulfate and MSO4; the metal compound MSO4 is selected from at least one of CuSO4, MgSO4, and ZnSO4. In this invention, the metal compound MSO4 is the same anionic compound as ferrous sulfate, which can avoid the introduction of additional impurity elements that could cause side reactions and lead to the formation of impurity phases in the material.

[0044] Taking the synthesis of nickel phosphate (Ni3(PO4)2··8H2O) as an example, the solute of the premixed metal salt solution S1 is composed of the metal compound nickel sulfate and MSO4.

[0045] In a specific embodiment of the present invention, the solute of solution S2 is selected from one or more of Na4P2O7, Na3HP2O7, Na2H2P2O7, NaH3P2O7, Na2HPO4, NaH2PO4, Na3PO4, H3PO4, NH4H2PO4, (NH4)2HPO4, and (NH4)3PO4. It is understood that the solute of solution S2 can be selected from any of the above-mentioned solutes, as long as the aqueous solution formed contains a phosphate group, and a phosphate precipitate is formed by the phosphate group and the transition metal M element during the micro-reaction mixing process.

[0046] Preferably, the solute in solution S2 is NH4H2PO4, (NH4)2HPO4, or (NH4)3PO4. The aqueous solution prepared with the above solutes can dissociate to release ammonium ions (NH4). + This forms a pH buffer system, which can reduce the occurrence of side reactions caused by pH changes.

[0047] Preferably, the metal compound NSO4 is selected from MgSO4 and / or CuSO4.

[0048] In a specific embodiment of the present invention, the element M is selected from Fe and / or Ni. It can be understood that the solute in the premixed metal salt solution S1 may also consist of the metal compounds ferrous sulfate, nickel sulfate, and NSO4. Furthermore, the stoichiometric ratio of Fe and Ni can be adjusted to achieve multi-site doping of the phosphate.

[0049] In addition, it should be noted that the M element does not include both Mn and Fe. When the Mn element is present, it will induce the oxidation and deterioration of ferrous ions, resulting in impurities in the synthesized product, that is, it is impossible to synthesize pure phase ferrous phosphate / manganese phosphate.

[0050] When element M is Fe, the following solutes should be avoided in preparing premixed metal salt solutions S1: calcium chlorate (Ca(ClO3)2) and magnesium chlorate (Mg(ClO3)2). These substances are oxidizing and will cause the ferrous ions to oxidize and deteriorate. Calcium chloride (CaCl2) generates heat when dissolved, and temperature changes will also cause the element M to oxidize and deteriorate.

[0051] In a specific embodiment of the present invention, the molar ratio of the M element and the N element satisfies 100:(1-20); for example, it can be 100:1, 100:2, 100:3, 100:4, 100:5, 100:6, 100:7, 100:8, 100:9, 100:10, 100:11, 100:12, 100:13, 100:14, 100:15, 100:16, 100:17, 100:18, 100:19, 100:20, etc., or any range of values ​​between these numbers. Further, the preferred range is 100:(3-15). Excessive nitrogen (N) content not only causes severe XRD peak shift in ferrous phosphate, leading to significant lattice distortion, but also generates N3(PO4)2 impurities (such as Ca3(PO4)2, Cu3(PO4)2, Mg3(PO4)2, and Zn3(PO4)2). Conversely, insufficient N content fails to alter the crystal phase. Therefore, appropriately controlling the nitrogen (M) content is crucial for synthesizing high-purity triclinic phosphate materials.

[0052] In a specific embodiment of the present invention, the concentration of nitrogen element in the premixed metal salt solution S1 ranges from 0.01 to 5 mol / L. Exemplarily, it can be 0.01, 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.8, 0.85, 0.9, 0.95, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0 mol / L, or any range between any two values. If the concentration of nitrogen element is too high, during the reaction process in the microreactor, nitrogen element will react with phosphate ions in solution S2 to form precipitated impurities of N3(PO4)2.

[0053] In a specific embodiment of the present invention, the molar concentration S1 of the transition metal M element in the premixed metal salt solution S1 is... mol The concentration is 0.05-10 mol / L. For example, it can be 0.05, 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10, etc., with a preferred range of 0.1-5 mol / L, or any interval between any two values.

[0054] In solution S2, the molar concentration of phosphorus P is S2. molThe concentration is 0.05-10 mol / L, and for example, it can be 0.05, 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10, etc., preferably 0.1-2 mol / L, or any range between any two values. Understandably, the molar ratio in the synthesis of phosphate salt hydrates is calculated based on the transition metal M and phosphorus elements. If the molar concentration is too high, the suspension concentration obtained by the microreactor will be too high, and the excessively high concentration of the suspension will clog the channels of the microreactor; if the molar concentration is too low, the synthesis rate will be slow.

[0055] In a specific embodiment of the present invention, the flow rate V1 of the premixed metal salt solution S1 in the microreactor ranges from 0.1 to 2 L / min. In a specific embodiment of the present invention, the flow rate V2 of the solution S2 in the microreactor is in the range of 0.1-2 L / min.

[0056] Furthermore, the premixed metal salt solution S1 and the solution S2 are respectively introduced into the first inlet and the second inlet of the microreactor by a multi-plunger pump.

[0057] The purity of the product can be improved by properly controlling the flow rates of the premixed metal salt solutions S1 and S2. When the flow rate is below the specified range, the reactants will not mix evenly, resulting in localized high or low concentrations, triggering side reactions and generating precipitates such as Fe(OH)2, Fe(OH)3, and FePO4. When the flow rate is above the specified range, the residence time of the reactants in the microreactor is too short, preventing complete reaction and also triggering side reactions. Furthermore, excessively high flow rates can cause localized heating in the reaction vessel, leading to oxidative deterioration of the reaction solution; for example, ferrous ions are more easily oxidized to ferric ions under high temperatures.

[0058] In a specific embodiment of the present invention, the premixed metal salt solution S1 and the solution S2 are respectively introduced into the first inlet and the second inlet of the microreactor via a multi-plunger pump. The multi-plunger pump includes a two-plunger pump, a three-plunger pump, etc. Using a multi-plunger pump can reduce the pulse difference between solutions S1 and S2 entering the microreactor, reducing the error in the feed ratio between solutions S1 and S2 caused by an excessively high pulse difference, which could affect the purity of the final synthesized product, thus avoiding the formation of impurity phases.

[0059] Furthermore, the pressure pulsation of the multi-plunger pump is less than or equal to 0.3 MPa, and / or the flow pulsation is less than or equal to 8%. Within these ranges, the multi-plunger pump exhibits low pulsation, enabling stable delivery of solutions S1 and S2 for the reaction, thus avoiding incomplete reactions and side reactions caused by unstable flow rates during solution input to the microreactor.

[0060] Further, V1 / V2 is 1:(0.95-1.05); even further, it is preferred that V1 / V2 is 1:1.

[0061] Understandably, one of the key factors of this invention is the precise control of the M element and phosphate group entering the microreactor, so that the ratio of the M element and P element in the instantaneous reaction is as close as possible to the stoichiometric ratio of the product to 3:2, thus avoiding incomplete reaction and side reactions caused by imbalance of proportions.

[0062] The purpose of controlling the flow rate ratio and pulse difference of premixed metal salt solutions S1 and S2 in the microreactor is to control the proportion of elements participating in the reaction.

[0063] Furthermore, the phosphate salt hydrate prepared by the above method has a higher product conversion rate, which can significantly reduce production costs.

[0064] In a specific embodiment of the present invention, the aging of the suspension is carried out by placing the suspension in a container and stirring it for 0.2-40 hours. The aging temperature is room temperature.

[0065] In a specific embodiment of the present invention, the curing process further includes: preparing a stabilizing liquid S3, wherein the stabilizing liquid S3 and the suspension are mixed together in the curing tank for curing.

[0066] In a specific embodiment of the present invention, the solute of the stabilized liquid S3 includes a dispersant. The dispersant is selected from one or more of sodium dodecyl sulfate, polyethylene glycol, polyvinyl alcohol, and polypropylene glycol. During the maturation process, the suspension after mixing in the microreactor has high kinetic energy and a large specific surface area, making it prone to agglomeration, which in turn leads to excessive particle growth. Increasing the amount of dispersant can inhibit particle agglomeration.

[0067] Preferably, the mass concentration of the solute in the stabilized liquid S3 is ≤5%.

[0068] Preferably, the dispersant is selected from sodium dodecyl sulfate. This invention has found that slurries made with sodium dodecyl sulfate exhibit higher stability compared to alcohol-based dispersants, as alcohol solvents induce the oxidation of ferrous ions. Sodium dodecyl sulfate possesses strong dispersion stability and can provide ultra-long steric hindrance, which is beneficial for the long-term storage of nanoscale ferrous phosphate.

[0069] More preferably, the mass concentration of sodium dodecyl sulfate is ≤5%. Sodium dodecyl sulfate is easily adsorbed on the surface of materials and is difficult to remove. Excessive sodium dodecyl sulfate content can also carbonize and generate impurities during the preparation of cathode materials (e.g., sintering process).

[0070] In a specific embodiment of the present invention, the solute of the stabilizing solution S3 further includes an antioxidant. The antioxidant is selected from one or more of sodium sulfite, ascorbic acid, vitamin C, citric acid, and vanillin. Adding an antioxidant to the stabilizing solution S3 can inhibit the oxidation of ferrous ions.

[0071] Preferably, the antioxidant is selected from sodium sulfite. Understandably, the premixed metal salt solution S1 is a sulfate solution; when the antioxidant is selected from the same type of phosphate, the introduction of additional impurity elements due to the addition of the antioxidant can be avoided.

[0072] In a specific embodiment of the present invention, the solute of the stabilized liquid S3 further includes a pH adjuster. The pH adjuster is selected from at least one of H3PO4, NH4H2PO4, (NH4)2HPO4, NaH2PO4, or Na2HPO4. As the suspension matures, the pH value of the suspension gradually increases. Maintaining the pH value of the suspension within a relatively low range can inhibit the formation of impurities such as M(OH)2 or M(OH)3. Since the selected pH adjuster is an acidic adjuster containing phosphorus (P), it not only maintains the acidic environment of the suspension and prevents deterioration, but also serves as a supplementary phosphorus source.

[0073] Second aspect A second aspect of this application is to provide a transition metal phosphate salt hydrate, prepared according to the preparation method described above; the molecular formula of the transition metal phosphate is M. 3-a N a (PO4)2, where element M is selected from at least one of Fe, Mn, Ni, and Co, and element N is a doping metal element different from element M, with 0 ≤ a ≤ 0.3; the crystal system of the transition metal phosphate hydrate is triclinic. The number of hydrates in the phosphate hydrate depends on the different transition metal M and can be any positive integer between 1 and 8, preferably 3 or 8; when M is selected from Fe, Co, or Ni, the number of hydrates is 8; when M is selected from Mn, the number of hydrates is 3.

[0074] Third aspect A third aspect of this application provides a phosphate-based cathode material precursor, comprising the aforementioned transition metal phosphate hydrate.

[0075] Fourth aspect The fourth aspect of this application provides a method for preparing a cathode material, wherein the cathode material has the molecular formula Na4M3(PO4)2P2O7, the M element is selected from at least one of Fe, Mn, Ni and Co, and is prepared from a phosphate-based cathode material precursor. The method for preparing the cathode material includes the following steps: Mixing step: Weigh a certain amount of raw material X and raw material Y and place them in a container filled with water and mix and stir to obtain slurry A. Then, grind them to obtain precursor B, wherein raw material X is the transition metal phosphate salt hydrate prepared by the first aspect of this application. Sintering step: Precursor B is placed in a sintering device for sintering treatment to obtain the cathode material.

[0076] Understandably, when M is selected from Fe, the raw material X is Fe3(PO4)2·8H2O; when M is selected from Mn, the raw material X is Mn3(PO4)2·3H2O; when M is selected from Ni, the raw material X is Ni3(PO4)2·8H2O; when M is selected from Co, the raw material X is Co3(PO4)2·8H2O.

[0077] Since the phosphate salt hydrate prepared in this application is triclinic, it has a higher number of active sites compared to the monoclinic system. During the synthesis of cathode materials, it can enhance the adsorption and activation capacity of Y material on the surface of X material, accelerate the uniform distribution of sodium ions on the surface of X raw material in the mixing step, and reduce the energy required to form cathode materials in the sintering step, thereby improving the efficiency of industrial mass production.

[0078] In any embodiment of this application, raw material Y is selected from one or more of NaOH, Na2C2O4, CH3COONa, Na2CO3, NaHCO3, Na4P2O7, Na3HP2O7, Na2H2P2O7, NaH3P2O7, H4P2O7, Na2HPO4, NaH2PO4, Na3PO4, H3PO4, NH4H2PO4, (NH4)2HPO4, and (NH4)3PO4. It is understood that the above compounds include both their corresponding anhydrous compounds and compounds containing different amounts of water of crystallization.

[0079] Furthermore, raw material Y is selected from one or more of Na4P2O7 and Na2HPO4. The applicant discovered that Na4P2O7 and Na2HPO4 are not only soluble in water, but also have a strong complexing ability with transition metal elements, which helps to uniformly mix with raw material X when water is used as a medium, and helps to generate cathode materials with high purity.

[0080] In any embodiment of this application, the raw materials further include a carbon source in the mixing step.

[0081] Specifically, the carbon source can be selected from one or more inorganic carbon materials and / or organic carbon materials.

[0082] Furthermore, the organic carbon material is selected from one or more of citric acid, glucose, sucrose, polyethylene glycol, starch, lignin, and asphalt.

[0083] Furthermore, the inorganic carbon material is selected from one or more of graphite, activated carbon, carbon nanotubes, and graphene.

[0084] In any embodiment of this application, the sintering temperature range is 300-800°C during the sintering step. The sintering time is 3-20 hours; and / or the heating rate of the sintering equipment is 1-10°C / min.

[0085] In some embodiments, the preparation method of the cathode material further includes a pretreatment step, which is usually located before the sintering step. The pretreatment step includes treating the precursor B in an ammonia solution.

[0086] In this application, when the X raw material is the phosphate salt hydrate prepared by the first aspect of this application, the above-mentioned pretreatment step is not included in the preparation process of the cathode material, that is, the precursor powder C does not need to be treated in an ammonia solution.

[0087] During the experiment, the researchers of this application discovered that the transition metal phosphate salts obtained by the preparation method of the first aspect of this application have high purity and crystallinity, and the triclinic phosphates have higher active sites. Even without using the above pretreatment method, it is possible to obtain cathode materials with better electrochemical performance.

[0088] Therefore, the transition metal phosphate salts obtained by the preparation method of the first aspect of this application can not only obtain pure-phase cathode materials with excellent electrochemical performance, but also simplify the production steps of cathode materials and improve production efficiency.

[0089] Fifth aspect A fifth aspect of this application is to provide a positive electrode sheet comprising the aforementioned positive electrode material.

[0090] The positive electrode sheet includes a positive current collector and a positive active material layer. The positive active material layer includes the positive electrode material, as well as a conductive agent and a binder. The molecular formula of the positive electrode material is Na4M3(PO4)2P2O7, and the M element is selected from at least one of Fe, Mn, Ni and Co. The positive electrode material is prepared from the transition metal phosphate salt hydrate.

[0091] Sixth aspect A sixth aspect of this application is to provide a battery comprising the aforementioned positive electrode, electrolyte, separator, and negative electrode.

[0092] The separator can be any material suitable for separators in electrochemical energy storage devices, for example, including but not limited to at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers.

[0093] The electrolyte can be a common electrolyte for sodium-ion batteries, such as an electrolyte formed by dissolving 1M NaPF6 in EC:DEC (50:50 vol / vol).

[0094] The negative electrode sheet is formed by coating a negative electrode active material onto a negative electrode current collector. The negative electrode active material can be common metallic sodium, hard carbon, soft carbon, etc.

[0095] Seventh aspect A seventh aspect of this application is to provide an electrochemical device including the battery.

[0096] The electronic devices include, but are not limited to: computers, media players, telephones, fax machines, copiers, printers, headphones, video recorders, televisions, calculators, memory cards, radios, backup power supplies, motors, automobiles, motorcycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, large household batteries, energy storage or lithium-ion capacitors, etc.

[0097] Example The following describes embodiments of this application. 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 are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0098] Example 1 This embodiment provides a method for synthesizing ferrous phosphate, as detailed below: (1) Solution preparation: Using water as solvent, prepare a premixed metal salt solution S1 with a ferrous ion concentration of 0.56 mol / L FeSO4·7H2O and a magnesium ion concentration of 0.04 mol / L MgSO4; and a phosphorus element solution S2 with a phosphorus element concentration of 0.4 mol / L (NH4)3PO4·3H2O. (2) Microreactor mixing: Premixed metal salt solution S1 and solution S2 are introduced into the first inlet and the second inlet of the microreactor, respectively. The flow rates of the first inlet and the second inlet are both set to 1 L / min. After mixing through the microreactor, the suspension is collected at the outlet of the microreactor. (3) Maturation process: Place the collected suspension in a maturation container (such as a beaker) and mature at room temperature for 1 hour; (4) Post-processing: The slurry in the maturation container was washed three times using a centrifuge (using distilled water as the detergent) and then dried in a vacuum oven at 60°C for 10 hours, finally obtaining a blue-gray solid. The obtained product was characterized, and the results are shown in […]. Figure 1 As can be seen from the XRD pattern, the blue-gray powder obtained by this invention is pure phase Fe3(PO4)2·8H2O (ferrous phosphate octahydrate), and the triclinic Fe3(PO4)2·8H2O with space group P-1(2) is 84-0341.

[0099] Example 2 This embodiment provides a method for synthesizing ferrous phosphate, as detailed below: The preparation method is the same as in Example 1, except that the ferrous ion concentration of FeSO4·7H2O in step (1) is 0.58 mol / L and the magnesium ion concentration of MgSO4 is 0.02 mol / L.

[0100] Example 3 This embodiment provides a method for synthesizing ferrous phosphate, as detailed below: The preparation method is the same as in Example 1, except that the ferrous ion concentration of FeSO4·7H2O in step (1) is 0.54 mol / L and the magnesium ion concentration of MgSO4 is 0.06 mol / L.

[0101] Example 4 This embodiment provides a method for synthesizing ferrous phosphate, as detailed below: The preparation method is the same as in Example 1, except that in step (1), MgSO4 is replaced with NiSO4·6H2O.

[0102] Example 5 This embodiment provides a method for synthesizing ferrous phosphate, as detailed below: The preparation method is the same as in Example 1, except that CuSO4 is replaced in step (1).

[0103] Example 6 This embodiment provides a method for synthesizing ferrous phosphate, as detailed below: The preparation method is the same as in Example 1, except that MgSO4 is replaced with ZnSO4 in step (1).

[0104] Example 7 This embodiment provides a method for synthesizing ferrous phosphate, as detailed below: The preparation method is the same as in Example 1, except that MgSO4 is replaced with CaSO4 in step (1).

[0105] Example 8 This embodiment provides a method for synthesizing ferrous phosphate, as detailed below: The preparation method is the same as in Example 1, except that in step (1), the magnesium ion concentration of 0.04 mol / L MgSO4 is replaced with 0.02 mol / L MgSO4 and 0.02 mol / L CuSO4.

[0106] Example 9 This embodiment provides a method for synthesizing manganese phosphate, as detailed below: The preparation method is the same as in Example 1, except that FeSO4·7H2O is replaced with MnSO4·H2O in step (1).

[0107] Comparative Example 1 This comparative example provides a method for synthesizing ferrous phosphate, as detailed below: Similar to Example 1, the difference lies in the preparation of the solution in step (1): using water as the solvent, a solution S1 of FeSO4·7H2O with a ferrous ion concentration of 0.6 mol / L and a solution S2 of (NH4)3PO4·3H2O with a phosphorus element concentration of 0.4 mol / L were prepared respectively.

[0108] A bluish-gray solid was finally obtained. The resulting bluish-gray solid was characterized, and the results are shown below. Figure 1 As can be seen from the XRD pattern, when no doping element is introduced into the original solution, the pure phase of monoclinic ferrous phosphate octahydrate Fe3(PO4)2·8H2O is prepared (corresponding card number 83-2453, space group C2 / m(12)).

[0109] Comparing Example 1 and Comparative Example 1, it can be seen that introducing dopant element N into the original reaction solution can achieve the transformation of ferrous phosphate from a monoclinic to a triclinic crystal system. Furthermore, the XRD comparison shows that the ferrous phosphate prepared by the method of Example 1 has a stronger peak intensity, indicating that doping can also improve the crystallinity of the final product.

[0110] Comparative Example 2 This comparative example provides a method for synthesizing ferrous phosphate (doping in a suspension, i.e., using Mg as the base solution), as shown below: (1) Using water as a solvent, prepare a FeSO4·7H2O solution S1 with a ferrous ion concentration of 0.56 mol / L and a (NH4)3PO4·3H2O solution S2 with a phosphorus concentration of 0.4 mol / L. (2) Prepare the base solution: Prepare a MgSO4 base solution with a magnesium ion concentration of 0.04 mol / L and transfer the base solution to a beaker; (3) Microreactor mixing: Solution S1 and solution S2 are introduced into the first inlet and the second inlet of the microreactor respectively. The flow rate of the first inlet and the second inlet is set to 1L / min. After mixing through the microreactor, the suspension flowing out of the outlet is collected into a beaker containing MgSO4 bottom liquid. (4) Maturation process: Place the collected suspension in a maturation container (such as a beaker) and mature at room temperature for 1 hour; (5) Post-processing: The slurry in the maturation container was washed three times using a centrifuge (using distilled water as the detergent) and then dried in a vacuum oven to obtain a blue-gray solid. The obtained blue-gray solid product was characterized, and the results are as follows: Figure 1 As shown in the XRD pattern, the ferrous phosphate obtained by this invention exhibits two different crystal forms (monoclinic and triclinic), and also produces numerous impurity phase peaks. When Mg is used as the substrate, the ferrous phosphate precipitate particles synthesized by the microreactor enter the substrate. Due to the high kinetic energy of the ferrous phosphate precipitate particles, some Mg can penetrate into the interior of the ferrous phosphate precipitate particles, altering their crystal phase. However, since the amount of Mg in the substrate is much greater than that of the ferrous phosphate precipitate collected in the substrate during the initial stage of the reaction, Mg will also undergo side reactions with other elements in the reaction solution, such as reacting with P to produce magnesium phosphate precipitate.

[0111] Comparative Example 3 This comparative example provides a method for synthesizing ferrous phosphate (directly mixing solutions S1 and S2 without setting a flow rate), as shown below: (1) Solution preparation: Using water as solvent, prepare a premixed metal salt solution S1 with a ferrous ion concentration of 0.56 mol / L FeSO4·7H2O and a magnesium ion concentration of 0.04 mol / L MgSO4; and a phosphorus element solution S2 with a phosphorus element concentration of 0.4 mol / L (NH4)3PO4·3H2O. (2) Directly mix solutions S1 and S2 in a beaker to obtain a suspension; (3) Maturation process: Place the collected suspension in a maturation container (such as a beaker) and mature at room temperature for 1 hour; (4) Post-processing: The slurry in the maturation container was washed three times using a centrifuge (using distilled water as the detergent) and then dried in a vacuum oven at 60°C for 10 hours, ultimately yielding a mixed solid of grayish-blue and light yellow. It is evident that pure-phase ferrous phosphate cannot be obtained using the above method. This is mainly because the direct mixing method cannot precisely control the iron-to-phosphorus ratio involved in the reaction, leading to side reactions.

[0112] In the subsequent preparation of the positive electrode sheet, after the light yellow solid was removed by sieving, the remaining material was assumed to be ferrous phosphate.

[0113] application: Preparation of positive electrode sheet The Fe3(PO4)2·8H2O obtained in Examples 1-9 and the Fe3(PO4)2·8H2O obtained in Comparative Examples 1-3 were used to prepare positive electrode sheets, as shown in the following specific steps: Methods for preparing cathode materials: 100.32 g of phosphate (Fe3(PO4)2·8H2O) prepared by the methods of Examples 1-7 and Comparative Examples 1-3 were weighed as raw material X. Separately, 53.18 g of raw material Y (Na4P2O7), 20 g of carbon source (sucrose), and 15.74 g of dispersant (PEG, molecular weight 1675) were weighed. The mixture was stirred in an aqueous medium at 500 r / min for 30 min to obtain slurry A. Slurry A was then milled to obtain slurry B. After slurry B was dried, it was placed in a nitrogen atmosphere and calcined at 500℃ for 10 h to obtain the cathode material Na4Fe3(PO4)2P2O7 / C.

[0114] For Example 9, the difference is that the weighed raw material X is 81.80 g of Mn3(PO4)2·3H2O, and the raw material Y is 53.18 g of Na4P2O7.

[0115] Take 0.8 g of the above positive electrode material, mix it with 0.1 g of conductive carbon black and 0.1 g of PVDF, grind it with NMP as the medium, coat the resulting slurry onto an aluminum foil current collector, and dry it to obtain the positive electrode sheet.

[0116] Electrochemical performance testing Preparation of button cells The positive electrode obtained above was cut into a square electrode with a side length of 1 cm and used as the working electrode. The sodium metal sheet was used as the counter electrode. The electrolyte was 1M NaPF6 dissolved in EC:DEC (50:50 vol / vol). The coin cell was assembled under an inert atmosphere (2032).

[0117] The electrochemical performance of the batteries obtained in the examples and comparative examples was tested, and the experimental results are shown in Table 1.

[0118] Table 1 As shown in Table 1 above, comparing Example 1 and Comparative Example 1, it can be seen that the secondary battery prepared with the cathode material synthesized in Example 1 has better high-rate (1C and 10C) performance.

[0119] Comparing Example 1 and Comparative Example 2, it can be seen that Example 1 exhibits superior electrochemical performance compared to directly introducing dopant element M into the suspension. This is mainly due to the uneven mixing of dopant element M in the suspension, leading to a non-uniform material composition. Premixing dopant element N with ferrous ions to form a premixed metal salt solution S1, followed by microreactor precipitation, can improve the uniformity of dopant element N distribution, allowing the dopant element to penetrate the interior of the phosphate, thereby altering the crystal phase and increasing crystallinity, while preventing the formation of impurities.

[0120] Comparing Examples 1-3, it can be seen that appropriate doping with phosphate helps to synthesize cathode materials with better electrochemical performance.

[0121] It should be noted that, although embodiments of the present invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing transition metal phosphate salts, wherein the molecular formula of the transition metal phosphate is M 3-a N a (PO4)2, where M is selected from at least one of Fe, Mn, Ni, and Co, and N is a doped metal element different from M, 0 ≤ a ≤ 0.3, characterized in that... The preparation method includes the following steps: Premixed metal salt solution S1 is prepared as follows: The premixed metal salt solution S1 is a mixed solution containing element M and doped metal element N, wherein element N is selected from low-valence metal ions with a valence state of +2. Prepare solution S2: Solution S2 is a solution containing phosphate groups; Microreactor mixing: The premixed metal salt solution S1 and the solution S2 are respectively introduced into the first inlet and the second inlet of the microreactor. The premixed metal salt solution S1 and the solution S2 are rapidly mixed in the microreactor to obtain a suspension. Post-processing: The suspension is aged, and the solid phase is separated from the liquid phase. The obtained solid phase is the transition metal phosphate salt hydrate.

2. The method for preparing transition metal phosphate hydrates according to claim 1, characterized in that, The N element is selected from at least one of Ca, Cu, Mg and Zn.

3. The method for preparing transition metal phosphate hydrates according to claim 1, characterized in that, The solutes in the premixed metal salt solution S1 include MSO4 and NSO4.

4. The method for preparing transition metal phosphate hydrates according to claim 1, characterized in that, The element M is selected from Fe and / or Ni.

5. The method for preparing transition metal phosphate hydrates according to claim 1, characterized in that, The molar ratio of the M element to the N element is 100:(1-20).

6. The method for preparing transition metal phosphate hydrates according to claim 1, characterized in that, The concentration of N element in the premixed metal salt solution S1 is 0.01-5 mol / L.

7. The method for preparing transition metal phosphate hydrates according to claim 1, characterized in that, The flow rate V1 of the premixed metal salt solution S1 in the microreactor is 0.1-2 L / min; And / or, the flow rate V2 of the solution S2 in the microreactor is 0.1-2 L / min.

8. The method for preparing transition metal phosphate hydrates according to claim 1, characterized in that, The solute in solution S2 is selected from one or more of Na4P2O7, Na3HP2O7, Na2H2P2O7, NaH3P2O7, Na2HPO4, NaH2PO4, Na3PO4, H3PO4, NH4H2PO4, (NH4)2HPO4, and (NH4)3PO4.

9. The method for preparing transition metal phosphate hydrates according to claim 1, characterized in that, The premixed metal salt solution S1 and the solution S2 are respectively introduced into the first inlet and the second inlet of the microreactor by a multi-plunger pump.

10. A transition metal phosphate hydrate, characterized in that, Prepared by the method according to any one of claims 1 to 8; the molecular formula of the phosphate is M. 3-a N a (PO4)2, where the M element is selected from at least one of Mn, Fe, Ni and Co, and the N element is a doped metal element different from the M element, with 0≤a≤0.3; the crystal system of the transition metal phosphate hydrate is triclinic.

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