Positive electrode active material, method for preparing the same, secondary battery, and electric device

Abstract

This application provides a positive electrode active material, a method for preparing the same, a secondary battery, and an electrical device. The positive electrode active material comprises a polyanionic compound having the general formula shown in Formula I. The crystallinity of the positive electrode active material is 0.8–1, wherein M includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, and Pb; 1≤x≤7, 1≤y1+y2≤4, 1≤z≤2, and 1≤k≤4. This positive electrode active material has a high degree of crystallinity, which is beneficial for improving the initial discharge capacity of the battery.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, and in particular to a positive electrode active material, its preparation method, a secondary battery, and an electrical device thereof. Background Technology

[0002] Sodium batteries have great potential for large-scale energy storage due to their abundant reserves, low price, and wide operating temperature range.

[0003] Iron-based polyanionic compounds are considered one of the most promising electrode materials due to their excellent structural stability, safety, and suitable voltage platform. However, sodium batteries using iron-based polyanionic compounds as cathode materials still have many problems and cannot meet the application needs of next-generation electrochemical systems. Summary of the Invention

[0004] This application is made in view of the above-mentioned issues, and its purpose is to provide a positive electrode active material with high crystallinity, which is beneficial to improving the first discharge capacity of the battery.

[0005] In a first aspect, this application provides a positive electrode active material comprising a polyanionic compound having the general formula shown in Formula I, wherein the crystallinity of the positive electrode active material is 0.8 to 1.

[0006] Na x Fe y1 M y2 (PO4) z (P2O7) k Formula I

[0007] Wherein, M includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W and Pb; 1≤x≤7, 1≤y1+y2≤4, 1≤z≤2, 1≤k≤4.

[0008] Positive electrode active materials with a crystallinity of 0.8 to 1 are beneficial to improving the initial discharge capacity of the battery, thereby enabling the battery to have a higher charging capacity and improving battery performance.

[0009] In any embodiment, the ratio Id of the intensity of the (602) peak to the intensity of the (022) peak in the XRD diffraction spectrum of the positive electrode active material is 0.9 to 1.5.

[0010] The applicant unexpectedly discovered that the higher the ratio Id of the intensity of the (602) peak to the intensity of the (022) peak in the XRD diffraction spectrum, the better the electrochemical performance of the positive electrode active material.

[0011] In any embodiment, the positive electrode active material further includes carbon material located on at least a portion of the surface of the primary particles of the polyanionic compound.

[0012] The presence of carbon materials on the surface of primary particles can significantly improve the poor conductivity of iron-based polyanionic compounds, thereby greatly improving the electrochemical performance of positive electrode active materials.

[0013] A second aspect of this application provides a method for preparing a positive electrode active material, comprising the following steps:

[0014] A mixed slurry is obtained by mixing raw materials containing sodium source, iron source and phosphorus source with solvent. Optionally, the raw materials also include M source.

[0015] The mixed slurry is dried and calcined to obtain a positive electrode active material, which includes a polyanionic compound having the general formula shown in Formula I, and the crystallinity of the positive electrode active material is 0.8 to 1.

[0016] Na x Fe y1 M y2 (PO4) z (P2O7) k Formula I

[0017] Wherein, M includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W and Pb; 1≤x≤7, 1≤y1+y2≤4, 1≤z≤2, 1≤k≤4.

[0018] Positive electrode active materials with a crystallinity of 0.8 to 1 are beneficial to improving the initial discharge capacity of the battery, thereby enabling the battery to have a higher charging capacity and improving battery performance.

[0019] In any embodiment, the step of mixing the raw materials containing sodium source, iron source and phosphorus source with a solvent to obtain a mixed slurry specifically includes:

[0020] Raw materials containing sodium source, ferrous oxalate, and phosphorus source are added to a solvent, stirred, and ground to obtain a mixed slurry. The homogeneous shear rate of the mixed slurry is 300 s⁻¹. -1 ~900S -1 .

[0021] The homogenization shear rate of the stirring was controlled at 300 s. -1 ~900S -1 A high homogeneous shear rate helps to avoid or reduce the formation of precursors from the reaction of sodium and phosphorus sources with ferrous oxalate that encapsulate unreacted ferrous oxalate, thus facilitating the full exposure of unreacted ferrous oxalate for complete reaction and improving the crystallinity of the positive electrode active material.

[0022] In any embodiment, the median particle size Dv50 of the particles in the mixed slurry is 0.05μm to 1.5μm, and can be selected as 0.1μm to 0.7μm.

[0023] Controlling the median particle size Dv50 of the mixed slurry within a suitable particle size range can avoid the need for long grinding time and the risk of physical gelation of the slurry due to excessively small particle size, thereby reducing the difficulty of subsequent processing and the requirements for production equipment. It can also reduce the low crystallinity and excessive impurities in the positive electrode active material caused by excessively large particle size, thus ensuring the kinetic performance of the positive electrode active material during charge and discharge.

[0024] In any embodiment, the solid content of the mixed slurry is 15% to 50%, optionally 25% to 40%.

[0025] Controlling the solids content of the mixed slurry within a suitable range can avoid both excessively low solids content, which would reduce production capacity and prevent large-scale production, and excessively high solids content, which would increase equipment wear and tear and the requirements for production equipment. This approach balances production costs and production scale, making it suitable for industrial applications.

[0026] In any embodiment, the mixed slurry contains a carbon source.

[0027] The addition of a carbon source helps improve the conductivity of the positive electrode active material and increase the initial discharge capacity of the battery.

[0028] In any embodiment, the step of adding raw materials containing sodium, iron, and phosphorus sources to a solvent to obtain a mixed slurry specifically involves:

[0029] Raw materials containing sodium source, ferrous oxalate and phosphorus source are added to solvent, stirred and ground until the median particle size Dv50 of the slurry is 0.05μm to 1.5μm, and then mixed with carbon source to obtain mixed slurry.

[0030] Controlling the median particle size Dv50 of the mixed slurry within a suitable particle size range can avoid the need for long grinding time and the risk of physical gelation of the slurry due to excessively small particle size, thereby reducing the difficulty of subsequent processing and the requirements for production equipment. It can also reduce the low crystallinity and excessive impurities in the positive electrode active material caused by excessively large particle size, thus ensuring the kinetic performance of the positive electrode active material during charge and discharge.

[0031] In any embodiment, the preparation method includes:

[0032] Raw materials containing sodium source, ferrous oxalate and phosphorus source are added to solvent, stirred and ground until the median particle size Dv50 of the slurry is 0.1μm to 0.7μm, and then mixed with carbon source to obtain mixed slurry.

[0033] Further control the median particle size Dv50 of the mixed slurry to 0.1μm~0.7μm before mixing with carbon source to reduce the difficulty of initial grinding. The addition of carbon source will significantly increase the viscosity of slurry, increase grinding power and grinding time.

[0034] In any embodiment, the sodium source includes one or more of sodium carbonate, sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium bicarbonate, sodium oxalate, sodium acetate, sodium citrate, sodium nitrate, and sodium hydroxide.

[0035] In any embodiment, the phosphorus source includes one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium pyrophosphate, trisodium monohydrogen pyrophosphate, disodium dihydrogen pyrophosphate, monosodium trihydrogen pyrophosphate, phosphoric acid, and pyrophosphate.

[0036] In any embodiment, the solvent includes one or more of ethanol, water, and ethylene glycol.

[0037] The solvents and dispersants mentioned above are beneficial for the full solvent and dispersion of raw materials, forming a uniformly mixed slurry, which facilitates the control of particle size in the slurry.

[0038] In any embodiment, the carbon source includes an inorganic carbon source or an organic carbon source. The inorganic carbon source includes one or more of natural graphite, artificial graphite, carbon black, carbon nanotubes, and graphene. The organic carbon source includes one or more of sucrose, glucose, citric acid, starch, cyclodextrin, and pitch.

[0039] Both inorganic and organic carbon sources are beneficial for improving the conductivity of positive electrode active materials.

[0040] A third aspect of this application provides a secondary battery, including a positive electrode and a negative electrode, wherein the positive electrode comprises the positive active material described in the first aspect or the positive active material prepared by the preparation method described in the second aspect.

[0041] In any embodiment, the secondary battery is a sodium-free secondary battery without a negative electrode.

[0042] In any embodiment, the negative electrode sheet includes a negative current collector and a base coating disposed on at least one surface of the negative current collector, wherein the base coating includes one or more of carbon nanotubes, graphite, graphene, silver composite carbon nanoparticles, and tin composite carbon nanoparticles.

[0043] The aforementioned base coating not only has excellent conductivity, but also facilitates the uniform deposition of metal ions on the current collector surface, thereby improving the battery's cycle performance and safety.

[0044] In any embodiment, the areal density of the base coating is 5 g / m³. 2 ~50g / m 2 .

[0045] The surface density is 5 g / m³ 2 ~50g / m 2 The base coating facilitates the uniform distribution of nucleation sites, promotes uniform metal deposition, and does not affect electron transport behavior.

[0046] In any embodiment, the thickness of the base coating is 2 μm to 100 μm.

[0047] Controlling the thickness of the base coating to 2μm to 100μm can provide enough nucleation sites to facilitate the uniform deposition of metal ions and suppress dendrites.

[0048] A fourth aspect of this application provides an electrical device including a secondary battery as described in the third aspect of this application. Attached Figure Description

[0049] Figure 1a X-ray diffraction patterns of the positive electrode active material precursors in some embodiments and comparative examples of this application;

[0050] Figure 1b The standard X-ray diffraction pattern for ferrous oxalate;

[0051] Figure 2a The X-ray diffraction patterns of the positive electrode active materials in the embodiments and comparative examples of this application are shown below.

[0052] Figure 2b The standard X-ray diffraction pattern of sodium iron pyrophosphate;

[0053] Figure 3 This is a schematic diagram of a secondary battery according to one embodiment of this application;

[0054] Figure 4 yes Figure 3 An exploded view of a secondary battery according to an embodiment of this application is shown.

[0055] Figure 5 This is a schematic diagram of a battery module according to one embodiment of this application;

[0056] Figure 6 This is a schematic diagram of a battery pack according to one embodiment of this application;

[0057] Figure 7 yes Figure 6An exploded view of a battery pack according to one embodiment of this application is shown;

[0058] Figure 8 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0059] Explanation of reference numerals in the attached figures:

[0060] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0061] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, its preparation method, secondary battery, battery module, battery pack, and electrical device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for a full understanding of this application by those skilled in the art and are not intended to limit the subject matter of the claims.

[0062] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0063] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0064] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0065] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0066] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0067] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A 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).

[0068] In existing technologies, sodium iron pyrophosphate is commonly used as the cathode material for sodium batteries. However, current processes struggle to produce sodium iron pyrophosphate with high crystallinity, affecting its electrochemical performance. The applicant unexpectedly discovered that the difficulty in improving the crystallinity of sodium iron pyrophosphate is due to the fact that, during the mechanical solid-state synthesis process, intermediate products (such as precursors) easily coat the surface of the iron source, preventing the iron source from being exposed and continuing to participate in the reaction.

[0069] [Positive electrode active material]

[0070] Based on this, this application proposes a positive electrode active material, which includes a polyanionic compound having the crystallinity shown in Formula I, and the crystallinity of the positive electrode active material is 0.8 to 1.

[0071] Na x Fe y1 M y2 (PO4) z (P2O7) k Formula I

[0072] Wherein, M includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W and Pb; 1≤x≤7, 1≤y1+y2≤4, 1≤z≤2, 1≤k≤4.

[0073] In some embodiments, M includes Mg. In some embodiments, M includes Al. In some embodiments, M includes V. In some embodiments, M includes Mn.

[0074] In some implementations, x can be selected as 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, or a value within the range formed by any two of the above points; y1 and y2 can be selected as 1, 1.5, 2, 2.5, 3, 3.5, 4, or a value within the range formed by any two of the above points; z can be selected as 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, or a value within the range formed by any two of the above points; and k can be selected as 1, 1.5, 2, 2.5, 3, 3.5, 4, or a value within the range formed by any two of the above points.

[0075] The crystallinity of the positive electrode active material can be measured by any known means, including but not limited to X-ray diffraction. As an example, in the X-ray diffraction spectrum, the area (Ag) of the strongest diffraction peak is compared with the integrated area of ​​that peak in the standard material card as a parameter to measure crystallinity, where crystallinity = As / Ag × 100%. For the sodium iron pyrophosphate provided in this application, the crystallinity is calculated using the area of ​​peak (222) and its integrated area in the standard card as parameters. The integrated area of ​​the diffraction peak can be directly obtained using Jade.

[0076] Positive electrode active materials with a crystallinity of 0.8 to 1 are beneficial to improving the initial discharge capacity of the battery, thereby enabling the battery to have a higher charging capacity and improving battery performance.

[0077] In some embodiments, the ratio Id of the intensity of the (602) peak to the intensity of the (022) peak in the XRD diffraction spectrum of the positive electrode active material is 0.9 to 1.5.

[0078] The ratio Id of the intensity of the (602) peak to the intensity of the (022) peak in the XRD diffraction spectrum of the positive electrode active material can be measured by any known means, including but not limited to X-ray diffractometer. As an example, the XRD pattern of the positive electrode active material is tested by X-ray method, and the ratio of the intensity of the (602) peak to the intensity of the (022) peak is calculated, i.e., Id = I (602) / I (022)Peak intensity can be directly determined using Jade. In the standard card of Na4Fe3(PO4)2(P2O7)4, the ratio of the peak intensity of (602) to that of (022) is 1.15.

[0079] In some embodiments, the ratio Id of the intensity of the (602) peak to the intensity of the (022) peak in the XRD diffraction spectrum of the positive electrode active material can be selected as 0.9, 1, 1.1, 1.2, 1.3, 1.4 or 1.5.

[0080] The applicant unexpectedly discovered that the higher the ratio Id of the intensity of the (602) peak to the intensity of the (022) peak in the XRD diffraction spectrum, the better the electrochemical performance of the positive electrode active material.

[0081] In some embodiments, the positive electrode active material also includes carbon material located on at least a portion of the surface of the primary particles of the polyanionic compound.

[0082] The presence of carbon materials on the surface of primary particles can significantly improve the poor conductivity of iron-based polyanionic compounds, thereby greatly improving the electrochemical performance of positive electrode active materials.

[0083] In some embodiments, the carbon material is distributed in particulate form between the primary particles of the polyanionic compound.

[0084] In some embodiments, the carbon material is inorganic carbon, which has high crystallinity, high particle size, and strong conductivity, and can effectively improve the conductivity of the positive electrode active material.

[0085] In some embodiments, carbon material is coated on the surface of primary particles of the polyanionic compound in the form of a carbon film.

[0086] In some embodiments, the carbon material is amorphous carbon, which is coated on the surface of the polyanionic compound to increase the contact area with the active material and improve electronic conductivity.

[0087] Introducing carbon materials into the positive electrode active material is beneficial to improving conductivity and optimizing the coulombic efficiency and cycle performance of the battery.

[0088] This application also provides a method for preparing a positive electrode active material, comprising the following steps:

[0089] A mixed slurry is obtained by adding raw materials containing sodium, iron and phosphorus sources to a solvent. Optionally, the raw materials also include source M.

[0090] The mixed slurry is dried and calcined to obtain a positive electrode active material. The positive electrode active material includes a polyanionic compound with formula I. The crystallinity of the positive electrode active material is 0.8–1. x Fe y1 M y2 (PO4) z(P2O7) k Formula I

[0091] Wherein, M includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W and Pb; 1≤x≤7, 1≤y1+y2≤4, 1≤z≤2, 1≤k≤4.

[0092] In some embodiments, M includes Mg. In some embodiments, M includes Al. In some embodiments, M includes V. In some embodiments, M includes Mn.

[0093] In some implementations, x can be selected as 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, or a value within the range formed by any two of the above points; y1 and y2 can be selected as 1, 1.5, 2, 2.5, 3, 3.5, 4, or a value within the range formed by any two of the above points; z can be selected as 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, or a value within the range formed by any two of the above points; and k can be selected as 1, 1.5, 2, 2.5, 3, 3.5, 4, or a value within the range formed by any two of the above points.

[0094] Positive electrode active materials with a crystallinity of 0.8 to 1 are beneficial to improving the initial discharge capacity of the battery, thereby enabling the battery to have a higher charging capacity and improving battery performance.

[0095] In some embodiments, the raw materials containing sodium, iron, and phosphorus sources are added to a solvent to obtain a mixed slurry, specifically as follows:

[0096] Raw materials containing sodium source, ferrous oxalate, and phosphorus source were added to a solvent, stirred, and ground to obtain a mixed slurry. The homogeneous shear rate of the mixed slurry was 300 s⁻¹. -1 ~900S -1 .

[0097] In some implementations, the homogeneous shear rate can be selected as 300 s. -1 350S -1 400S -1 450S -1 500S -1 550S -1 600S -1 650S -1 700S -1 750S -1 800S -1 850S -1 900S-1 The value within the range formed by any two of the above points.

[0098] The homogeneous shear rate is a parameter related to stirring parameters, fluid viscosity, and volume, and it can be calculated using any known method. The homogeneous shear rate for controlling stirring is 300 s⁻¹. -1 ~900S -1 A high homogeneous shear rate helps to avoid or reduce the formation of precursors from the reaction of sodium and phosphorus sources with ferrous oxalate that encapsulate unreacted ferrous oxalate, thus facilitating the full exposure of unreacted ferrous oxalate for complete reaction and improving the crystallinity of the positive electrode active material.

[0099] In some embodiments, the median particle size Dv50 of the particles in the mixed slurry is 0.05 μm to 1.5 μm, and can be selected as 0.1 μm to 0.7 μm.

[0100] In some embodiments, the median particle size Dv50 of the particles in the mixed slurry can be selected as 0.05μm, 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, or a value within the range formed by any two of the above points.

[0101] The median particle size (Dv50) of the mixed slurry can be measured by any known method, including but not limited to centrifugation sedimentation and laser particle size analyzer testing. As an example, the sample is first centrifuged, deposited, and filtered to obtain a particle sample from the initial mixed slurry. Then, 0.1g–0.13g of the particle sample to be tested is weighed into a 50mL beaker, 5g of acetone is added, a stir bar of approximately 2.5mm is placed inside, and the beaker is sealed with plastic wrap. After ultrasonic treatment for 5 minutes, the sample is transferred to a magnetic stirrer and stirred at 500 rpm for at least 20 minutes. Two samples are taken from each batch for testing. The tests are performed using a Mastersizer 2000E laser particle size analyzer from Malvern Instruments Ltd., UK.

[0102] Controlling the median particle size Dv50 of the mixed slurry within a suitable particle size range can avoid the need for long grinding time and the risk of physical gelation of the slurry due to excessively small particle size, thereby reducing the difficulty of subsequent processing and the requirements for production equipment. It can also reduce the low crystallinity and excessive impurities in the positive electrode active material caused by excessively large particle size, thus ensuring the kinetic performance of the positive electrode active material during charge and discharge.

[0103] In some embodiments, the solid content of the mixed slurry is 15% to 50%, optionally 25% to 40%.

[0104] In some embodiments, the solid content of the mixed slurry may be selected as 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a value within the range of any two of the above.

[0105] Controlling the solids content of the mixed slurry within a suitable range can avoid both excessively low solids content, which would reduce production capacity and prevent large-scale production, and excessively high solids content, which would increase equipment wear and tear and the requirements for production equipment. This approach balances production costs and production scale, making it suitable for industrial applications.

[0106] In some implementations, the mixed slurry contains a carbon source.

[0107] The addition of a carbon source helps improve the conductivity of the positive electrode active material and increase the initial discharge capacity of the battery.

[0108] In some embodiments, the mixed slurry further includes a dispersant, which includes one or more of polyethylene glycol and polyvinyl alcohol.

[0109] Dispersants help to ensure the uniform dispersion of carbon sources in the mixed slurry, which facilitates the subsequent preparation of positive electrode active materials.

[0110] In some embodiments, the raw materials containing sodium, iron, and phosphorus sources are added to a solvent to obtain a mixed slurry, specifically as follows:

[0111] Raw materials containing sodium source, ferrous oxalate and phosphorus source are added to solvent, stirred and ground until the median particle size Dv50 of the slurry is 0.05μm to 1.5μm, and then mixed with carbon source to obtain mixed slurry.

[0112] Controlling the median particle size Dv50 of the mixed slurry within a suitable particle size range can avoid the need for long grinding time and the risk of physical gelation of the slurry due to excessively small particle size, thereby reducing the difficulty of subsequent processing and the requirements for production equipment. It can also reduce the low crystallinity and excessive impurities in the positive electrode active material caused by excessively large particle size, thus ensuring the kinetic performance of the positive electrode active material during charge and discharge.

[0113] In some embodiments, the preparation method includes:

[0114] Raw materials containing sodium source, ferrous oxalate and phosphorus source are added to solvent, stirred and ground until the median particle size Dv50 of the slurry is 0.1μm to 0.7μm, and then mixed with carbon source to obtain mixed slurry.

[0115] Further control the median particle size Dv50 of the mixed slurry to 0.1μm~0.7μm before mixing with carbon source to reduce the difficulty of initial grinding. The addition of carbon source will significantly increase the viscosity of slurry, increase grinding power and grinding time.

[0116] In some embodiments, the sodium source includes one or more of sodium carbonate, sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium bicarbonate, sodium oxalate, sodium acetate, sodium citrate, sodium nitrate, and sodium hydroxide.

[0117] In some embodiments, the phosphorus source includes one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium pyrophosphate, trisodium monohydrogen pyrophosphate, disodium dihydrogen pyrophosphate, monosodium trihydrogen pyrophosphate, phosphoric acid, and pyrophosphate.

[0118] In some embodiments, the solvent is selected from one or more of ethanol, water, and ethylene glycol.

[0119] The solvents and dispersants mentioned above are beneficial for fully dissolving the raw materials, forming a uniformly mixed slurry, and facilitating the control of particle size in the slurry.

[0120] In some embodiments, the carbon source includes an inorganic carbon source or an organic carbon source. The inorganic carbon source includes one or more of natural graphite, artificial graphite, carbon black, carbon nanotubes, and graphene. The organic carbon source includes one or more of sucrose, glucose, citric acid, starch, cyclodextrin, and pitch.

[0121] Both inorganic and organic carbon sources are beneficial for improving the conductivity of positive electrode active materials.

[0122] [Positive electrode plate]

[0123] The positive electrode includes a positive current collector and a positive active material layer formed on at least a portion of the surface of the positive current collector, the positive active material layer including the positive active material in some embodiments.

[0124] The positive electrode active material layer may also include a conductive agent to improve the conductivity of the positive electrode. The conductive agent may be one or more of SuperP, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphite, graphene, and carbon nanofibers.

[0125] The positive electrode active material layer may also include a binder to firmly bond the positive electrode active material and optional conductive agent to the positive electrode current collector. The binder may be at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), polyvinyl alcohol (PVA), ethylene-vinyl acetate copolymer (EVA), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), sodium alginate (SA), polymethacrylic acid (PMA), and carboxymethyl chitosan (CMCS).

[0126] The positive electrode current collector can be made of conductive carbon sheet, metal foil, carbon-coated metal foil, porous metal plate, or composite current collector. The conductive carbon material of the conductive carbon sheet can be selected from one or more of Super P, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphite, graphene, and carbon nanofibers. The metal material of the metal foil, carbon-coated metal foil, and porous metal plate can be independently selected from at least one of copper, aluminum, nickel, and stainless steel. The composite current collector can be a composite current collector formed by combining metal foil with a polymer base film.

[0127] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0128] [Negative electrode plate]

[0129] The negative electrode may consist only of the negative current collector and not contain the negative active material. Alternatively, a metallic phase may be pre-deposited on the negative current collector.

[0130] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil or copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0131] In some embodiments, the negative electrode sheet includes a negative current collector and a base coating disposed on at least one surface of the negative current collector, the base coating including one or more of carbon nanotubes, graphite, graphene, silver composite carbon nanoparticles, and tin composite carbon nanoparticles.

[0132] The aforementioned base coating not only has excellent conductivity, but also facilitates the uniform deposition of metal ions on the current collector surface, thereby improving the battery's cycle performance and safety.

[0133] In some embodiments, the areal density of the base coating is 5 g / m³. 2 ~50g / m 2 .

[0134] In some embodiments, the areal density of the base coating may be selected as 5 g / m³. 2 10g / m 2 15g / m 2 20g / m 2 25g / m 2 30g / m 2 35g / m 2 40g / m 2 45g / m 2 50g / m 2 The value within the range formed by any two of the above points.

[0135] The surface density is 5 g / m³ 2 ~50g / m 2 The base coating facilitates the uniform distribution of nucleation sites, promotes uniform metal deposition, and does not affect electron transport behavior.

[0136] In some embodiments, the thickness of the base coating is 2 μm to 100 μm.

[0137] In some embodiments, the thickness of the base coating can be selected as 2μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, 85μm, 90μm, 95μm, 100μm, or a value within the range formed by any two of the above points.

[0138] Controlling the thickness of the base coating to 2μm to 100μm can provide enough nucleation sites to facilitate the uniform deposition of metal ions and suppress dendrites.

[0139] [Isolation membrane]

[0140] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0141] In some embodiments, the material of the separator can be selected from at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0142] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0143] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0144] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0145] [Rechargeable Battery]

[0146] A secondary battery includes a positive electrode and a negative electrode. The positive electrode includes a positive active material in some embodiments or a positive active material prepared by a preparation method in some embodiments.

[0147] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 3 This is an example of a square-structured secondary battery 5.

[0148] In some embodiments, the secondary battery also includes a separator and an electrolyte.

[0149] In some implementations, the secondary battery is a sodium-ion secondary battery without a negative electrode.

[0150] A negative electrode-less sodium secondary battery refers to a battery in which no negative electrode active material layer is actively placed on the negative electrode side during the battery manufacturing process. For example, a sodium metal or carbonaceous active material layer is not formed at the negative electrode through coating or deposition processes. During the first charge, sodium ions gain electrons on the anode side and deposit metallic sodium on the current collector surface to form a sodium metal phase. During discharge, the metallic sodium can be converted back into sodium ions and return to the positive electrode, achieving cyclic charging and discharging. Compared to other sodium secondary batteries, a negative electrode-less sodium secondary battery can achieve a higher energy density due to the absence of a negative electrode active material layer.

[0151] In some implementations, to improve battery performance, the negative electrode side of a sodium-free secondary battery can be provided with conventional materials that can be used as negative electrode active materials, such as carbonaceous materials, metal oxides, and alloys. Although these materials have a certain capacity, because the amount of these materials is small, they are not used as the main negative electrode active materials in the battery, and therefore are not considered to form a negative electrode active material layer that intercalates sodium. In this way, the sodium secondary battery can still be regarded as a sodium-free secondary battery.

[0152] In some implementations, the CB value of the sodium-free secondary battery is less than or equal to 0.1.

[0153] The CB value is the capacity per unit area of ​​the negative electrode in a secondary battery divided by the capacity per unit area of ​​the positive electrode. Since batteries without a negative electrode contain little or no negative electrode active material, the capacity per unit area of ​​the negative electrode is relatively small, and the CB value of the secondary battery is less than or equal to 0.1.

[0154] In some implementations, refer to Figure 4 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 via a winding or stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. A non-Newtonian fluid electrolyte composition is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0155] [Battery Module]

[0156] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0157] Figure 5 This is battery module 4, used as an example. (See reference...) Figure 5 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0158] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0159] [Battery Pack]

[0160] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0161] Figure 6 and Figure 7 This is battery pack 1 as an example. (See reference...) Figure 6 and Figure 7 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0162] [Electrical appliances]

[0163] In one embodiment of this application, an electrical device is provided, including at least one of a secondary battery, a battery module, or a battery pack according to any embodiment.

[0164] Electrical devices include at least one of the secondary batteries, battery modules, or battery packs provided in this application. The secondary batteries, battery modules, or battery packs can be the power source of the electrical device or the energy storage unit of the electrical device. Electrical devices may include, but are not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0165] As an electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0166] Figure 8 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0167] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0168] Example

[0169] 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.

[0170] I. Preparation Method

[0171] Example 1

[0172] 1) Preparation of positive electrode active materials

[0173] Sodium pyrophosphate, ferrous oxalate, and ammonium dihydrogen phosphate were dissolved in deionized water at a molar ratio of 0.998:3:2. The mixture was stirred continuously at room temperature for 30 minutes, and the homogenization shear rate was 630 s⁻¹. -1 After ball milling to an average particle size of 0.2 μm, a carbon source was added and mixed to obtain a mixed slurry with a solid content of 35%. The mixed slurry was spray-dried at an inlet air temperature of 220°C and an outlet air temperature of 109°C to obtain a powdered positive electrode active material precursor. Finally, in a N2 atmosphere, the temperature was increased to 320°C at a heating rate of 2°C and held for 4 hours, then increased to 550°C at a heating rate of 2°C and held for 10 hours. The sintered product was crushed and sieved to obtain Na. 3.98 Fe3(PO4)2P2O7 positive electrode active material.

[0174] 2) Preparation of positive electrode sheet

[0175] 2.5 wt% polyvinylidene fluoride binder was fully dissolved in N-methylpyrrolidone (NMP), and then 2.0 wt% Super P, 1.0 wt% carbon nanotubes, and 94.5 wt% of the above-mentioned positive electrode active material were added and stirred until homogeneous to obtain a positive electrode slurry. The slurry was uniformly coated on the surface of the current collector aluminum foil and then transferred to a vacuum drying oven for complete drying. The dried electrode sheet was then rolled and punched to obtain the positive electrode sheet.

[0176] 3) Preparation of negative electrode sheet

[0177] Single-walled carbon nanotubes and carboxymethyl cellulose (CMC) were added to deionized water at a mass ratio of 1:0.4 and stirred to form a uniform slurry. The slurry was then coated onto the surface of the copper foil of the negative electrode current collector, and then transferred to a vacuum drying oven for complete drying. Finally, it was punched to obtain a negative electrode sheet without a negative electrode structure.

[0178] 4) Electrolyte

[0179] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), sodium hexafluorophosphate (NaPF6) was dissolved in the organic solvent dimethyl ethylene glycol (DME) and stirred until homogeneous to obtain an electrolyte with a sodium salt concentration of 1.5 mol / L.

[0180] 5) Separating membrane

[0181] Polypropylene film is used as the separator.

[0182] 6) Battery manufacturing

[0183] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The resulting bare cell is then wound, tabs are welded onto it, and the cell is placed in an aluminum casing. It is then baked at 80°C to remove moisture, followed by the injection of electrolyte and sealing to obtain a non-charged battery. This non-charged battery then undergoes a series of processes including settling, hot and cold pressing, formation, shaping, and capacity testing to obtain the sodium metal battery product without a negative electrode as described in Example 1.

[0184] Examples 2-11

[0185] The batteries in Examples 2 to 11 are prepared using methods similar to those in Example 1, but the preparation parameters of the positive electrode active material have been adjusted. The specific parameters are shown in Table 1.

[0186] Comparative Examples 1-4

[0187] The battery preparation methods in Comparative Examples 1-4 are basically similar to those in Example 1, but the homogeneous shear rate is adjusted to be less than 300 s⁻¹. -1 The specific parameters are shown in Table 1.

[0188] II. Performance Testing

[0189] 1. Mixed slurry

[0190] 1) Initial mixed slurry particle size test

[0191] Centrifugal sedimentation and laser particle size analyzer were used for testing. First, the sample was centrifuged, sedimented, and filtered to obtain the initial particle sample from the mixed slurry. Then, 0.1g–0.13g of the sample to be tested was weighed into a 50mL beaker, 5g of acetone was added, a stir bar of approximately 2.5mm was placed inside, and the beaker was sealed with plastic wrap. After ultrasonic treatment for 5 minutes, the sample was transferred to a magnetic stirrer and stirred at 500 rpm for at least 20 minutes. Two samples were taken from each batch for testing. A Mastersizer 2000E laser particle size analyzer (Malvin Instruments Ltd., UK) was used for testing.

[0192] 2) Homogeneous shear rate test

[0193] The average shear rate is used to measure the homogeneous shear rate of a fluid, which is the sum of the axial average shear rate and the radial average shear rate.

[0194] The formula for calculating the radial average shear rate is:

[0195]

[0196] In the formula, S is the radial average shear rate. -1 K is the ratio of the rotor's outer diameter to the stator's outer diameter, and ω is the rotor's angular velocity.

[0197] The formula for calculating the axial average shear rate is:

[0198]

[0199] In the formula, S is the axial average shear rate. -1 R3 is the stator outer diameter in meters (m); η is the liquid viscosity in Pa·s; L is the length (the length of the shaft touched by the liquid) in meters; ΔP is the inlet and outlet pressure, which is positively correlated with the slurry volumetric flow rate in Pa.

[0200] 2. Positive electrode active material slurry

[0201] 1) Crystallinity test

[0202] Crystallinity is determined by X-ray diffraction: The area (As) obtained by integrating the strongest diffraction peak (i.e., the (222) peak) is compared with the area (Ag) obtained by integrating the standard substance, and used as an indicator for calculating crystallinity. Crystallinity = As / Ag × 100%. The XRD abscissa position 2θ of the (222) peak of the standard substance is at 33.304°.

[0203] 2) Id value test -- (602) crystal plane orientation growth

[0204] The XRD pattern of the positive electrode active material was tested using the X-ray diffraction method, and the ratio of the intensity of the (602) peak to that of the (022) peak was calculated, i.e., Id = I (602) / I (022) The ratio of the intensity of peak (602) (the XRD abscissa position of peak (602) is 34.304°) to the intensity of peak (022) (the XRD abscissa position of peak (022) is 32.218°) in the standard card of Na4Fe3(PO4)2(P2O7)4 is 1.15.

[0205] 3. Battery performance test

[0206] 1) Initial discharge capacity test

[0207] The first discharge capacity test process is as follows: At 25°C, the prepared battery is charged to 3.75V with a constant current of 1C, then charged at a constant voltage of 3.75V until the current drops to 0.05C, and then discharged to 1.5V with a constant current of 1C to obtain the first discharge capacity (Cd1).

[0208] 2) Coulomb efficiency test

[0209] The coulombic efficiency test procedure is as follows: At 25°C, the prepared battery is charged to 3.75V with a constant current of 1 / 5C, and then charged at a constant voltage of 3.75V until the current drops to 0.05C, obtaining the initial charge capacity (Cc1); then discharged to 1.5V with a constant current of 1 / 5C, obtaining the initial discharge capacity (Cd1), and the battery coulombic efficiency is calculated according to the following formula: Battery coulombic efficiency = Initial discharge capacity (Cd1) / Initial charge capacity (Cc1). The test procedures for the comparative example and other embodiments are the same as above.

[0210] 3) Battery cycle capacity retention test

[0211] The battery capacity retention rate test process is as follows: At 25°C, the prepared battery is charged to 3.75V with a constant current of 1C, then charged with a constant voltage of 3.75V until the current drops to 0.05C, and then discharged to 1.5V with a constant current of 1C. The resulting capacity is recorded as the initial capacity (C0). The above steps are repeated for the same battery, and the discharge capacity (Cn) of the battery after the nth cycle is recorded. The battery capacity retention rate after each cycle is Pn = Cn / C0 × 100%. A curve is obtained by plotting the 100 points P1, P2...100 as the vertical axis and the corresponding cycle number as the horizontal axis. In this test, the first cycle corresponds to n=1, the second cycle to n=2, ..., the 100th cycle to n=100. The battery capacity retention rate data corresponding to the examples or comparative examples in Table 2 are the data measured after 100 cycles under the above test conditions, i.e., the value of P100. The test process for the comparative examples and other examples is the same as above.

[0212] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0213] Batteries for each embodiment and comparative example were prepared according to the above method, and various performance parameters were measured. The results are shown in the table below.

[0214] Table 1

[0215]

[0216] Table 2

[0217]

[0218] The precursor obtained after spray drying of the mixed slurry was characterized by XRD, and the results are as follows: Figure 1a As shown. With Figure 1b The comparison of the standard cards of ferrous oxalate shown in the example clearly shows that the precursor component in Comparative Example 1 still has highly crystalline ferrous oxalate remaining. In Example 5, by increasing the shear rate, the crystallinity of ferrous oxalate in the precursor is significantly reduced. In Example 3, the shear rate is further increased, and the precursor exhibits a completely amorphous state, indicating that the raw material ferrous oxalate participates in the reaction.

[0219] The positive electrode active material obtained after calcination was characterized by XRD, and the results are as follows: Figure 2a As shown, the XRD pattern of the standard card for sodium ferric pyrophosphate is compared with... Figure 2b As shown. By Figure 2a It can be seen that as the shear rate increases, the ratio Id of the intensity of the (602) peak to the intensity of the (022) peak in the XRD diffraction spectrum of the positive electrode active material continuously increases. This is because the precursor coated with ferrous oxalate restricts the growth of the (602) crystal plane. As the raw material components completely enter the precursor, the crystallinity of the positive electrode active material after calcination is significantly improved.

[0220] Examples 1 through 11 all include a positive electrode active material, which comprises a polyanionic compound having the crystallinity shown in Formula I, and the crystallinity of the positive electrode active material is 0.8 to 1.

[0221] Na x Fe y1 M y2 (PO4) z (P2O7) k

[0222] Wherein, M includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W and Pb; 1≤x≤7, 1≤y1+y2≤4, 1≤z≤2, 1≤k≤4.

[0223] As can be seen from the comparison between Examples 1-11 and Comparative Examples 1-4, controlling the crystallinity of the positive electrode active material to be 0.8-1 is beneficial to improving the first discharge capacity of the battery and improving the battery performance.

[0224] A comparison of Examples 1-11 with Comparative Examples 1-4 shows that controlling the homogeneous shear rate to 300 s⁻¹ is effective. -1 ~900S -1 This is beneficial for improving the crystallinity of the positive electrode active material and the first discharge capacity of the battery, thereby improving the battery performance.

[0225] As can be seen from the comparison of Examples 1-4, the presence of sodium or iron vacancies in sodium iron pyrophosphate helps to further improve the battery capacity and cycle performance.

[0226] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A positive electrode active material, characterized in that, The positive electrode active material includes a polyanionic compound having the general formula shown in Formula I, and the crystallinity of the positive electrode active material is 0.8~1. Na x Fe y1 M y2 (PO4) z (P2O7) k Formula I Wherein, M includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W and Pb; 1≤x≤7, 1≤y1+y2≤4, 1≤z≤2, 1≤k≤4.

2. The positive electrode active material according to claim 1, characterized in that, The ratio Id of the intensity of the (602) peak to the intensity of the (022) peak in the XRD diffraction spectrum of the positive electrode active material is 0.9~1.

5.

3. The positive electrode active material according to claim 1 or 2, characterized in that, The positive electrode active material also includes carbon material located on at least a portion of the surface of the primary particles of the polyanionic compound.

4. The positive electrode active material according to any one of claims 1 to 3, characterized in that, 1≤x≤7, 1≤y1+y2≤4, 1≤z≤2, 1≤k≤4, and 0≤y2<4.

5. The positive electrode active material according to any one of claims 1 to 4, characterized in that, 1≤x≤4, 1≤y1+y2≤3, 1≤z≤2, 1≤k≤2, and y2 is 0.

6. The positive electrode active material according to any one of claims 1 to 5, characterized in that, M includes at least one of Mg, Al, V, and Mn.

7. The positive electrode active material according to any one of claims 1 to 6, characterized in that, The positive electrode active material includes Na. 3.98 Fe3(PO4)2P2O7, Na4Fe 2.8 (PO4)2P2O7, Na 3.98 Fe 2.8 At least one of (PO4)2P2O7 and Na4Fe3(PO4)2P2O7.

8. A method for preparing a positive electrode active material, characterized in that, Includes the following steps: A mixed slurry is obtained by mixing raw materials containing sodium source, iron source and phosphorus source with solvent, wherein the raw materials also include M source; The mixed slurry is dried and calcined to obtain a positive electrode active material, which includes a polyanionic compound having the general formula shown in Formula I, and the crystallinity of the positive electrode active material is 0.8~1. Na x Fe y1 M y2 (PO4) z (P2O7) k Formula I Wherein, M includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W and Pb; 1≤x≤7, 1≤y1+y2≤4, 1≤z≤2, 1≤k≤4.

9. The method for preparing the positive electrode active material according to claim 8, characterized in that, 1≤x≤7, 1≤y1+y2≤4, 1≤z≤2, 1≤k≤4, and 0≤y2<4.

10. The method for preparing the positive electrode active material according to claim 8 or 9, characterized in that, 1≤x≤4, 1≤y1+y2≤3, 1≤z≤2, 1≤k≤2, and y2 is 0.

11. The method for preparing the positive electrode active material according to any one of claims 8 to 10, characterized in that, M includes at least one of Mg, Al, V, and Mn.

12. The method for preparing the positive electrode active material according to any one of claims 8 to 11, characterized in that, The positive electrode active material includes Na. 3.98 Fe3(PO4)2P2O7, Na4Fe 2.8 (PO4)2P2O7, Na 3.98 Fe 2.8 At least one of (PO4)2P2O7 and Na4Fe3(PO4)2P2O7.

13. The method for preparing the positive electrode active material according to any one of claims 8 to 12, characterized in that, The process of mixing raw materials containing sodium, iron, and phosphorus sources with a solvent to obtain a mixed slurry specifically includes: Raw materials containing sodium source, ferrous oxalate, and phosphorus source are added to a solvent, stirred, and ground to obtain a mixed slurry. The homogeneous shear rate of the mixed slurry is 300 s⁻¹. -1 ~900 S -1 .

14. The method for preparing the positive electrode active material according to any one of claims 8 to 13, characterized in that, The median particle size Dv50 of the particles in the mixed slurry is 0.05 μm to 1.5 μm.

15. The method for preparing the positive electrode active material according to any one of claims 8 to 14, characterized in that, The median particle size Dv50 of the particles in the mixed slurry is 0.1 μm to 0.7 μm.

16. The method for preparing the positive electrode active material according to any one of claims 8 to 15, characterized in that, The solid content of the mixed slurry is 15% to 50%.

17. The method for preparing the positive electrode active material according to claim 16, characterized in that, The solid content of the mixed slurry is 25% to 40%.

18. The preparation method according to any one of claims 8 to 17, characterized in that, The mixed slurry contains a carbon source.

19. The method for preparing the positive electrode active material according to claim 18, characterized in that, The preparation method includes: Raw materials containing sodium source, ferrous oxalate and phosphorus source are added to solvent, stirred and ground until the median particle size Dv50 of the slurry is 0.1 μm~1.3 μm, and then mixed with carbon source to obtain mixed slurry.

20. The method for preparing the positive electrode active material according to claim 19, characterized in that, The preparation method includes: Raw materials containing sodium source, ferrous oxalate and phosphorus source are added to solvent, stirred and ground until the median particle size Dv50 of the slurry is 0.1 μm~0.7 μm, and then mixed with carbon source to obtain mixed slurry.

21. The preparation method according to any one of claims 18 to 20, characterized in that, The sodium source includes one or more of sodium carbonate, sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium bicarbonate, sodium oxalate, sodium acetate, sodium citrate, sodium nitrate, and sodium hydroxide.

22. The preparation method according to any one of claims 18 to 21, characterized in that, The phosphorus source includes one or more of the following: sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium pyrophosphate, trisodium monohydrogen pyrophosphate, disodium dihydrogen pyrophosphate, monosodium trihydrogen pyrophosphate, phosphoric acid, and pyrophosphate.

23. The preparation method according to any one of claims 18 to 22, characterized in that, The solvent includes one or more of ethanol, water, and ethylene glycol.

24. The preparation method according to any one of claims 21 to 23, characterized in that, The carbon source includes inorganic carbon source or organic carbon source. The inorganic carbon source includes one or more of natural graphite, artificial graphite, carbon black, carbon nanotubes, and graphene. The organic carbon source includes one or more of sucrose, glucose, citric acid, starch, cyclodextrin, and pitch.

25. A secondary battery, characterized in that, It includes a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode active material prepared by any one of claims 1 to 7 or any one of claims 8 to 24.

26. The secondary battery according to claim 25, characterized in that, The secondary battery is a sodium-free secondary battery without a negative electrode.

27. The secondary battery according to claim 25 or 26, characterized in that, The negative electrode sheet includes a negative current collector and a base coating disposed on at least one surface of the negative current collector, wherein the base coating includes one or more of carbon nanotubes, graphite, graphene, silver composite carbon nanoparticles, and tin composite carbon nanoparticles.

28. The secondary battery according to claim 27, characterized in that, The areal density of the base coating is 5 g / m³. 2 -50g / m 2 .

29. The secondary battery according to claim 27 or 28, characterized in that, The thickness of the base coating is 2 μm to 100 μm.

30. An electrical device, characterized in that, The secondary battery includes any one of claims 25 to 29.

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