Positive electrode active material and preparation method thereof, positive plate and secondary battery

Abstract

The invention provides a positive active material and a preparation method thereof, a positive plate and a secondary battery. The positive electrode active material comprises a complex of a sodium vanadium phosphate phase, a sodium vanadium fluorophosphate phase and a sodium pyrophosphate phase, at least part of V elements in the sodium vanadium phosphate phase are substituted by active elements, or at least part of V elements in the sodium vanadium phosphate phase and the sodium vanadium fluorophosphate phase are substituted by the active elements, and the active elements comprise at least one of Cr, Co, Mn, Fe, Ni and Al. The positive electrode active material is a composite phase of a sodium vanadium phosphate phase, a sodium vanadium fluorophosphate phase and a sodium pyrophosphate phase, and a part of V elements in the sodium vanadium phosphate phase and the sodium vanadium fluorophosphate phase are replaced by active elements, so that hole carriers are generated, and meanwhile, the migration path of charge carriers is widened; mechanical stress can be dispersed, and structural degradation in the circulation process is inhibited; through the synergistic effect of the above aspects, the working voltage, the cycling stability and the rate capability of the secondary battery are improved.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery technology, and particularly relates to a positive electrode active material and its preparation method, a positive electrode sheet, and a secondary battery. Background Technology

[0002] In recent years, electrochemical energy storage technology based on lithium-ion batteries has developed rapidly, leading to a continuous increase in the demand for lithium resources. However, the limited global reserves and uneven geographical distribution of lithium resources severely restrict the large-scale application of lithium-ion batteries. Sodium-ion batteries, due to their similar structure and working principle to lithium-ion batteries, as well as the advantages of abundant and widely distributed sodium resources, have become a highly promising complementary electrochemical energy storage technology.

[0003] Sodium vanadium phosphate (Na3V2(PO4)3, abbreviated as NVP), as a polyanionic cathode material, possesses open three-dimensional sodium ion transport channels, which facilitates the rapid insertion and extraction of sodium ions. Simultaneously, vanadium exhibits multiple valence states, and upon activation, it can undergo multi-electron redox reactions, giving the material a high theoretical specific capacity. Nevertheless, the average operating voltage, cycle life, and rate performance of sodium-ion batteries using NVP as the cathode still require further improvement.

[0004] Therefore, developing novel cathode materials with higher operating voltage, excellent cycle stability, and good rate performance has become a key research direction for promoting the practical application of this technology. Summary of the Invention

[0005] This invention provides a positive electrode active material that can improve the operating voltage, cycle stability and rate performance of secondary batteries.

[0006] This invention provides a method for preparing a positive electrode active material, which can improve the operating voltage, cycle stability and rate performance of a secondary battery.

[0007] The present invention also provides a positive electrode sheet, comprising the above-mentioned positive electrode active material or the positive electrode active material prepared by the above-mentioned preparation method. Therefore, the secondary battery made from the positive electrode sheet has excellent operating voltage, cycle stability and rate performance.

[0008] The present invention also provides a secondary battery comprising the above-mentioned positive electrode active material, or the positive electrode active material prepared by the above-mentioned preparation method, or the above-mentioned positive electrode sheet. Therefore, the secondary battery has excellent operating voltage, cycle stability and rate performance.

[0009] The first aspect of the present invention provides a positive electrode active material comprising a composite of a sodium vanadium phosphate phase, a sodium vanadium fluorophosphate phase, and a sodium pyrophosphate phase, wherein at least a portion of the V element in the sodium vanadium phosphate phase is replaced by an active element, or at least a portion of the V element in both the sodium vanadium phosphate phase and the sodium vanadium fluorophosphate phase is replaced by the active element, wherein the active element comprises at least one of Cr, Co, Mn, Fe, Ni, and Al.

[0010] The positive electrode active material as described above, wherein the chemical composition of the sodium vanadium phosphate phase is as shown in Formula 1 and / or the chemical composition of the sodium vanadium fluorophosphate phase is as shown in Formula 2:

[0011] Na3M x V 2-x (PO4)3 Formula 1 Na3N y V 2-y (PO4)2F3 Equation 2;

[0012] Wherein, M and N are each independently selected from the active elements, 0 < x < 2, 0 ≤ y < 2.

[0013] In the positive electrode active material described above, the active element has a mass percentage content of 1-12% in the positive electrode active material.

[0014] In the above-mentioned positive electrode active material, the molar ratio of the sodium vanadium phosphate phase, the sodium vanadium fluorophosphate phase, and the sodium pyrophosphate phase is (5-8):(1-5):(1-3).

[0015] The positive electrode active material as described above is a polycrystalline particle with an average particle size of 1 μm ≤ D ≤ 3 μm.

[0016] A second aspect of the present invention provides a method for preparing the aforementioned positive electrode active material, comprising the following steps:

[0017] 1) The organic acid solution is mixed with the vanadium source and then subjected to heat treatment to obtain the first mixed solution;

[0018] 2) Mix the first mixed solution with the active element source, and add a phosphorus source under acidic conditions to obtain a second mixed solution;

[0019] 3) After mixing the second mixed solution with the sodium source and fluorine source, the pH is controlled to neutral to obtain the precursor solution;

[0020] 4) The precursor solution is subjected to heat preservation treatment, aging treatment and pre-calcination treatment in sequence, and then subjected to first calcination treatment, second calcination treatment and third calcination treatment in sequence to obtain the positive electrode active material; wherein, the aging treatment time is 8-16h, the temperature of the first calcination treatment is 450-550℃ and the time is 2-4h, the temperature of the second calcination treatment is 600-650℃ and the time is 2-4h, and the temperature of the third calcination treatment is 700-750℃ and the time is 2-4h.

[0021] In the preparation method described above, the temperature of the heat preservation treatment is 150-200℃, and the heat preservation treatment time is 18-24h; and / or, the temperature of the aging treatment is 50-80℃; and / or, the temperature of the pre-calcination treatment is 300-400℃, and the pre-calcination treatment time is 3-5h; and / or,

[0022] The heating rate of the pre-calcination treatment is 4-6℃ / min; and / or, the heating rate of the first calcination treatment is 4-6℃ / min; and / or, the heating rate of the second calcination treatment is 4-6℃ / min; and / or, the heating rate of the third calcination treatment is 4-6℃ / min.

[0023] In the preparation method described above, in step 1), the heat treatment temperature is 50-80℃ and the heat treatment time is 10-20 min; and / or,

[0024] Preferably, in step 2), when controlling the acidic conditions, a first buffer solution is added, and the dropping rate of the first buffer solution is 2-3 drops / min; and / or,

[0025] Preferably, in step 2), the acidic conditions are controlled during water bath heating treatment, and the temperature of the water bath heating treatment is 70-90°C; and / or,

[0026] Preferably, in step 2), the molar ratio of phosphorus atoms to vanadium ions in the second mixed solution is 1.5-1.6; and / or,

[0027] Preferably, in step 2), the phosphorus source is added at a dropping rate of 3-5 drops / min; and / or,

[0028] Preferably, in step 3), when mixing the second mixed solution with the sodium source and the fluorine source, the sodium source and the fluorine source are first mixed to obtain a sodium-fluorine mixture, and the sodium-fluorine mixture is added to the second mixed solution at a dropping rate of 3-5 drops / min.

[0029] A third aspect of the present invention provides a positive electrode sheet comprising the positive electrode active material described in the first aspect above, or the positive electrode active material prepared by the preparation method described in the second aspect above.

[0030] A third aspect of the present invention provides a secondary battery comprising the positive electrode active material described in the first aspect, or the positive electrode active material prepared by the preparation method described in the second aspect, or the positive electrode sheet described in the third aspect.

[0031] The positive electrode active material of the present invention is a composite phase of sodium vanadium phosphate phase, sodium vanadium fluorophosphate phase and sodium pyrophosphate phase, and some of the V elements in the sodium vanadium phosphate phase and sodium vanadium fluorophosphate phase are replaced by active elements, which not only generates hole carriers but also broadens the migration path of charge carriers; it can also disperse mechanical stress and suppress structural degradation during cycling. The above-mentioned aspects work together to improve the working voltage, cycle stability and rate performance of the secondary battery. Attached Figure Description

[0032] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0033] Figure 1 The X-ray diffraction pattern of the positive electrode active material provided in Embodiment 1 of the present invention;

[0034] Figure 2 This is a scanning electron microscope image of the positive electrode active material provided in Embodiment 1 of the present invention;

[0035] Figure 3 The energy dispersive X-ray spectrum of the positive electrode active material provided in Embodiment 1 of the present invention;

[0036] Figure 4 This is a transmission electron microscope (TEM) image of the positive electrode active material provided in Embodiment 1 of the present invention;

[0037] Figure 5 A schematic diagram of the cycle performance of a secondary battery prepared with the positive electrode active material provided in Example 1 of the present invention;

[0038] Figure 6 A schematic diagram of the cycle performance of a secondary battery prepared with the positive electrode active material provided in Example 3 of the present invention;

[0039] Figure 7 This is a schematic diagram of the cycle performance of a secondary battery prepared with the positive electrode active material provided in Comparative Example 3 of the present invention.

[0040] Figure 8 The X-ray diffraction pattern of the positive electrode active material provided in Comparative Example 5 of this invention;

[0041] Figure 9This is an X-ray diffraction pattern of the positive electrode active material provided in Comparative Example 6 of the present invention.

[0042] The accompanying drawings have illustrated specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to specific embodiments. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0044] The inventors conducted an in-depth study on the working mechanism of existing sodium vanadium phosphate cathode materials and found that the main reasons for the poor performance of secondary batteries using it as a cathode material in terms of operating voltage, cycle stability, and rate performance are twofold: First, sodium vanadium phosphate has a low intrinsic electronic conductivity, and the electron transport inside the electrode is slow during charging and discharging, which cannot effectively coordinate with the migration rate of sodium ions. This not only leads to severe electrode polarization but also damages the structural stability of the material, resulting in rapid capacity decay and shortened cycle life. Second, the operating voltage of this material (~3.4V) is too low, which limits the performance of the battery in high-power application scenarios and makes it difficult to meet the actual needs of efficient energy storage, becoming an important obstacle to its commercialization.

[0045] Therefore, the inventors attempted to prepare composite materials by combining sodium vanadium phosphate with sodium vanadium fluorophosphate (Na3V2(PO4)2F3) or sodium pyrophosphate (Na4P2O7). However, the inventors found that the above composite materials could not simultaneously improve voltage, capacity, and cycle stability.

[0046] Based on this, the first aspect of the present invention provides a positive electrode active material comprising a composite of sodium vanadium phosphate phase, sodium vanadium fluorophosphate phase and sodium pyrophosphate phase, wherein at least a portion of the V element in the sodium vanadium phosphate phase is replaced by an active element, or at least a portion of the V element in both the sodium vanadium phosphate phase and the sodium vanadium fluorophosphate phase is replaced by an active element, wherein the active element comprises at least one of Cr, Co, Mn, Fe, Ni and Al.

[0047] It should be noted that the active element can replace only part of the V element in the sodium vanadium phosphate phase, or it can simultaneously replace part of the V element in both the sodium vanadium phosphate and sodium fluorophosphate phases. The ionic radii of these active elements are close to those of V, making them easy to incorporate into the crystal and partially replace V sites; furthermore, the stable trivalent state of these active elements can effectively activate more electronic reactions of V (such as V...). 2+ / V 3+ V 3+ / V 4+ V 4+ / V 5+ Before activation, only V can occur. 2+ / V 3+ V 3+ / V 4+ (Electron reaction), enhances the reversibility of redox couples, and improves the specific capacity of positive electrode active materials.

[0048] The positive electrode active material provided by this invention can improve the operating voltage, cycle stability, and rate performance of secondary batteries. This is because: firstly, the sodium vanadium phosphate and sodium vanadium fluorophosphate phases of the positive electrode active material increase the operating voltage, the sodium pyrophosphate phase enhances ionic conductivity, and the doping of active elements optimizes electron transport; moreover, compared with simple physical mixing, the multiphase composite structure forms heterogeneous interfaces with higher interface energy and abundant defect states. These interfaces serve as highly active channels for ion migration and charge exchange, significantly improving the reaction kinetics of the positive electrode active material; the presence of numerous interfaces not only increases the density of electrochemical active sites and promotes interphase synergistic effects but also helps disperse mechanical stress and alleviate strain during cycling, thereby comprehensively improving the specific capacity, rate performance, and structural stability of the positive electrode material; secondly, in the positive electrode active material of this invention, some V elements in the sodium vanadium phosphate and sodium vanadium fluorophosphate phases are replaced by active elements, generating hole carriers through a charge compensation mechanism, improving electronic conductivity. Simultaneously, the doping of active elements broadens the migration path of charge carriers (such as sodium ions), thereby improving the rate performance of the secondary battery.

[0049] Furthermore, the sodium pyrophosphate phase (Na4P2O7) effectively absorbs volume changes during cycling, suppresses particle breakage, and improves the cycle stability of the secondary battery. Additionally, sodium pyrophosphate exhibits a low energy barrier. + Migration path to improve the rate performance of secondary batteries.

[0050] In one specific embodiment, the chemical composition of the sodium vanadium phosphate phase is shown in Formula 1 and / or the chemical composition of the sodium vanadium fluorophosphate phase is shown in Formula 2:

[0051] Na3M x V 2-x (PO4)3 Formula 1 Na3N y V2-y (PO4)2F3 Equation 2;

[0052] In this context, M and N are each independently selected from active elements, where 0 < x < 2 and 0 ≤ y < 2. When y = 0, the active element only replaces the vanadium element in the sodium vanadium phosphate phase; when y > 0, the active element replaces the vanadium element in both the sodium vanadium phosphate phase and the sodium fluorophosphate phase.

[0053] In one specific embodiment, the mass percentage of the active element in the positive electrode active material is 1-12%, preferably 3-8%. Within this range, the doping mass fraction of the active element can further balance the electronic conductivity and structural stability of the positive electrode active material. Within the preferred range, it further activates the voltage plateau of vanadium.

[0054] For example, the mass percentage of the active element in the positive electrode active material is 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or 12%, or a range of any two of these values.

[0055] In one specific embodiment, the molar ratio of sodium vanadium phosphate phase, sodium vanadium fluorophosphate phase, and sodium pyrophosphate phase is (5-8):(1-5):(1-3), preferably 6:3:1. Within this range, the molar ratio of sodium vanadium phosphate phase, sodium vanadium fluorophosphate phase, and sodium pyrophosphate phase can further balance the operating voltage, capacity, and cycle stability of the secondary battery. Within the preferred range, the operating voltage, capacity, and cycle stability of the secondary battery are even better.

[0056] For example, the molar ratio of sodium vanadium phosphate phase, sodium vanadium fluorophosphate phase and sodium pyrophosphate phase is 5:4:1, 5:3:2, 5:2:3, 6:3:1, 6:2:2, 6:1:3, 7:2:1, 7:1:2, 8:1:1, or any range of two of these values.

[0057] To further improve the coating uniformity of the electrode slurry prepared from this positive electrode active material, the average particle size of the positive electrode active material can also be controlled.

[0058] In one specific embodiment, the positive electrode active material is a polycrystalline particle with an average particle size of 1 μm ≤ D ≤ 3 μm, preferably 1 μm ≤ D ≤ 2 μm. Within this range, the average particle size of the positive electrode active material further improves the coating uniformity of the electrode slurry, reduces electrode polarization, and thus enhances the cycle stability of the secondary battery. With the average particle size within the preferred range, the cycle stability of the secondary battery is even better.

[0059] For example, the average particle size of the positive electrode active material is 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2 μm, 2.2 μm, 2.4 μm, 2.6 μm, 2.8 μm and 3 μm, or a range of any two of these values.

[0060] A second aspect of the present invention provides a method for preparing a positive electrode active material, comprising the following steps:

[0061] 1) The organic acid solution is mixed with the vanadium source and then subjected to heat treatment to obtain the first mixed solution;

[0062] 2) Mix the first mixed solution with the active element source, and add the phosphorus source under acidic conditions to obtain the second mixed solution;

[0063] 3) After mixing the second mixed solution with the sodium source and fluorine source, the pH is controlled to neutral to obtain the precursor solution;

[0064] 4) After the precursor solution is subjected to heat preservation treatment, aging treatment and pre-calcination treatment in sequence, it is then subjected to first calcination treatment, second calcination treatment and third calcination treatment in sequence to obtain positive electrode active material; wherein, the aging treatment time is 8-16h, the temperature of the first calcination treatment is 450-550℃ and the time is 2-4h, the temperature of the second calcination treatment is 600-650℃ and the time is 2-4h, and the temperature of the third calcination treatment is 700-750℃ and the time is 2-4h.

[0065] This invention employs a hydrothermal-calcination coupled process. First, a porous precursor gel (third precursor) with a loose, porous structure is formed using a hydrothermal method. This porous structure shortens the diffusion path of charge carriers (such as sodium ions), improving the rate performance of the secondary battery. Then, during calcination, a three-stage programmed temperature control is used to precisely control the formation temperature of each phase, reducing fluorine escape or agglomeration caused by high temperatures and ensuring the high operating voltage of the secondary battery. Simultaneously, excess phosphate is converted into sodium pyrophosphate during high-temperature calcination, acting as a stress buffer phase to suppress volume changes during cycling, inhibit particle breakage, and improve the cycle stability of the secondary battery. Therefore, the positive electrode active material prepared by this method has uniformly distributed and tightly bound phases, and the synergistic effect of each phase achieves a simultaneous improvement in the operating voltage, cycle stability, and rate performance of the secondary battery.

[0066] This invention does not specifically limit the pH of the acidic conditions in 2). In one specific embodiment, the pH is 1.5-2.5, for example, 1.5, 1.7, 2, 2.3, or 2.5, or any range of two values ​​therein. Acidic conditions can inhibit the hydrolysis of active elements and avoid the occurrence of side reactions.

[0067] This invention does not specifically limit the pH value in 3). In one specific embodiment, the pH is 7-8, for example, 7, 7.2, 7.4, 7.6, 7.8, or 8, or a range consisting of any two of these values. The above acidic conditions are adjusted to neutral so that the subsequent reaction occurs under neutral conditions.

[0068] For example, the aging time is 8h, 10h, 12h or 16h, or a range of any two of these values.

[0069] For example, the temperature of the first calcination treatment in 4) is 450°C, 500°C or 550°C, or a range consisting of any two of these values.

[0070] For example, the time for one calcination treatment in 4) is 2h, 2.5h, 3h, 3.5h or 4h, or a range consisting of any two of these values.

[0071] Within this temperature and time range, a single calcination treatment can form a sodium vanadium phosphate phase in which some V elements are replaced by active elements.

[0072] For example, the temperature of the secondary calcination treatment in 4) is 600°C, 625°C or 650°C, or a range consisting of any two of these values.

[0073] For example, the time for the secondary calcination treatment in 4) is 2h, 3.5h or 4h, or a range consisting of any two of these values.

[0074] Within this temperature and time range, the secondary calcination treatment can form a sodium vanadium fluorophosphate phase in which some V elements are replaced by active elements.

[0075] For example, the temperature of the three calcination treatments in 4) is 700°C, 725°C or 750°C, or a range consisting of any two of these values.

[0076] For example, the time for the three calcination treatments in 4) is 2h, 3h or 4h, or a range consisting of any two of these values.

[0077] Within this temperature and time range, the sodium pyrophosphate phase can be formed during the three calcination treatments.

[0078] In this invention, the vanadium source refers to the raw material that provides vanadium, the phosphorus source refers to the raw material that provides phosphorus, the sodium source refers to the raw material that provides sodium, the fluorine source refers to the raw material that provides fluorine, and the active element source refers to the raw material that provides the above-mentioned active elements (Cr, Co, Mn, Fe, Ni, Al).

[0079] This invention does not specifically limit the types of vanadium sources, phosphorus sources, sodium sources, fluorine sources, and active element sources; they can be conventional materials in the art. For example, the vanadium source includes at least one of ammonium metavanadate, vanadium pentoxide, and vanadium oxysulfate; the phosphorus source includes at least one of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the sodium source includes at least one of sodium fluoride, sodium carbonate, sodium oxalate, sodium acetate, and sodium citrate; the fluorine source includes at least one of sodium fluoride, ammonium fluoride, and hydrofluoric acid; and the active element source includes at least one of chromium nitrate, cobalt nitrate, manganese nitrate, ferrous chloride, nickel chloride, and aluminum nitrate.

[0080] When the vanadium source, phosphorus source, sodium source, fluorine source and active element source are mixtures of the aforementioned substances, the present invention does not specifically limit the proportion of each specific substance in the mixture.

[0081] The present invention does not specifically limit the type of organic acid solution in 1), including but not limited to oxalic acid solution and citric acid solution.

[0082] The present invention does not specifically limit the molar ratio of organic acid to vanadium source in organic acid solution. In one embodiment, the molar ratio of organic acid to vanadium source is (1-1.5):1.

[0083] This invention does not specifically limit the preparation method of the organic acid solution. In one embodiment, oxalic acid is dissolved in deionized water, dispersed with ultrasonic assistance, and then magnetically stirred in a water bath until completely dissolved to form an organic acid solution. This invention does not specifically limit the water bath temperature and time. In one embodiment, the water bath temperature is 60°C and the time is 15 minutes.

[0084] To further improve the uniform distribution of each phase in the positive electrode active material, the heating rate of the pre-calcination treatment, the heating rate of the first calcination treatment, the heating rate of the second calcination treatment, and the heating rate of the third calcination treatment can be controlled.

[0085] In one specific embodiment, the heat treatment temperature is 150-200℃ and the heat treatment time is 18-24 hours. Within this temperature and time range, the heat treatment is more conducive to the full formation of the gel's three-dimensional network structure.

[0086] For example, the temperature of the insulation treatment is 150°C, 160°C, 170°C, 180°C, 190°C or 200°C, or a range of any two of these values.

[0087] For example, the heat preservation time is 18h, 20h, 22h or 24h, or a range of any two of these values.

[0088] In one specific embodiment, the aging temperature is 50-80°C. Within this temperature range, aging is more complete. Preferably, the aging conditions simultaneously meet the following requirements: aging time of 8-16 hours and aging temperature of 50-80°C.

[0089] For example, the aging temperature in 4) is 50°C, 60°C, 70°C or 80°C, or a range consisting of any two of these values.

[0090] In one specific embodiment, the pre-calcination temperature is 300-400℃ and the pre-calcination time is 3-5 hours. When the pre-calcination temperature and time are within the above range, nickel is more likely to volatilize adsorbed water and crystal water in the system after aging treatment.

[0091] For example, the temperature of the pre-calcination treatment is 300°C, 350°C, or 400°C, or a range of any two of these values.

[0092] For example, the pre-calcination time is 3h, 4h, or 5h, or a range of any two of these values.

[0093] In one specific embodiment, the heating rate of the pre-calcination treatment is 4-6 °C / min. Within this range, the stability of the porous structure of the third precursor is further improved.

[0094] For example, the heating rate of the pre-calcination treatment is 4°C / min, 4.5°C / min, 5°C / min, 5.5°C / min or 6°C / min, or a range of any two of these values.

[0095] In one specific embodiment, the heating rate for a single calcination treatment is 4-6 °C / min. Within this range, the stability and uniform distribution of Na3V2(PO4)3 are further ensured.

[0096] For example, the heating rate of a single calcination treatment is 4°C / min, 4.5°C / min, 5°C / min, 5.5°C / min or 6°C / min, or a range of any two of these values.

[0097] In one specific embodiment, the heating rate of the secondary calcination treatment is 4-6°C / min. Within this range, fluorine escape or agglomeration is further reduced, thereby increasing the operating voltage of the secondary battery.

[0098] For example, the heating rate of the secondary calcination treatment is 4℃ / min, 4.5℃ / min, 5℃ / min, 5.5℃ / min or 6℃ / min, or a range of any two of these values.

[0099] In one specific embodiment, the heating rate of the three calcination treatments is 4-6 °C / min. Within this range, the sodium pyrophosphate phase is further ensured to be uniformly distributed in the positive electrode active material, improving ionic conductivity and thus enhancing the rate performance of the secondary battery.

[0100] For example, the heating rate of the three calcination treatments is 4℃ / min, 4.5℃ / min, 5℃ / min, 5.5℃ / min or 6℃ / min, or a range of any two of these values.

[0101] In one specific embodiment, in step 1), the heat treatment temperature is 50-80°C, and the heat treatment time is 10-20 minutes. Within this temperature and time range, a first mixed solution can be rapidly formed.

[0102] For example, the heat treatment temperature is 50°C, 60°C, 70°C, or 80°C, or a range of any two of these values.

[0103] For example, the heat treatment time is 10 min, 15 min, or 20 min, or a range of any two of these values.

[0104] In one specific embodiment, in step 2), a first buffer solution is added when the acidic conditions are controlled, and the dropping rate of the first buffer solution is 2-3 drops / min.

[0105] The present invention does not specifically limit the type of the first buffer solution, including but not limited to dilute nitric acid solution.

[0106] In one specific embodiment, in step 2), water bath heating treatment is performed under controlled acidic conditions, and the temperature of the water bath heating treatment is 70-90°C. Within this temperature range, it is easier to achieve acidic conditions.

[0107] For example, the temperature of the water bath heating treatment is 70°C, 75°C, 80°C, 85°C or 90°C, or a range of any two of these values.

[0108] In one specific embodiment, in step 2), the molar ratio of phosphorus atoms to vanadium ions in the second mixed solution is 1.5-1.6. Within this range, the molar ratio of phosphorus atoms to vanadium ions further reduces the formation of impurity phases in the positive electrode active material.

[0109] For example, the molar ratio of phosphorus atoms to vanadium ions is 1.5, 1.52, 1.55, 1.56, 1.58 or 1.6, or a range of any two of these values.

[0110] In one specific embodiment, the phosphorus source is added at a dropping rate of 3-5 drops / min. This dropping rate further ensures that the phosphorus source is uniformly mixed with the first mixed solution and the active element source.

[0111] For example, the dropping rate when adding the phosphorus source is 3 drops / min, 4 drops / min, or 5 drops / min, or a range of any two of these values.

[0112] In one specific embodiment, when mixing the second mixed solution with the sodium source and fluorine source, the sodium source and fluorine source are first mixed to obtain a sodium-fluorine mixture. The sodium-fluorine mixture is then added to the second mixed solution at a dropping rate of 3-5 drops / min. Within this dropping rate range, fluoride ions and sodium ions mix more uniformly with the second mixed solution, resulting in a more uniform distribution of the sodium vanadium phosphate phase, sodium fluorophosphate phase, and sodium pyrophosphate phase in the positive electrode active material.

[0113] A third aspect of the present invention provides a positive electrode sheet comprising the positive electrode active material described in the first aspect, or the positive electrode active material prepared by the preparation method described in the second aspect. This positive electrode sheet can improve the operating voltage, cycle stability, and rate performance of a secondary battery.

[0114] The present invention does not specifically limit the structure of the positive electrode sheet. In one embodiment, the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least a portion of the surface of the positive current collector. The positive active layer includes the aforementioned positive active material, conductive agent and binder.

[0115] This invention does not specifically limit the material of the positive electrode current collector; it can be any material conventional in the art. For example, the material of the positive electrode current collector can be either aluminum foil or nickel foil.

[0116] This invention does not specifically limit the type of conductive agent; it can be any material conventional in the art. For example, the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber.

[0117] This invention does not specifically limit the type of adhesive, which can be a conventional material in the art. For example, the adhesive can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0118] This invention does not specifically limit the preparation method of the positive electrode sheet. In one embodiment, the positive electrode sheet can be prepared by a method including the following steps:

[0119] The positive electrode active material of the present invention is dispersed with a conductive agent and a binder in an N-methylpyrrolidone (NMP) solvent and thoroughly stirred to form a uniform positive electrode slurry. The positive electrode slurry is then uniformly coated onto a positive electrode current collector, and after drying, rolling and slitting, a positive electrode sheet is obtained.

[0120] This invention does not impose specific limits on the amount of positive electrode active material, conductive agent and binder, and can be adjusted according to the actual situation.

[0121] A fourth aspect of the present invention provides a secondary battery comprising the positive electrode active material described in the first aspect, or the positive electrode active material prepared by the method described in the second aspect, or the positive electrode sheet provided in the third aspect. Therefore, this secondary battery exhibits excellent operating voltage, cycle stability, and rate performance.

[0122] It is conceivable that, in addition to the aforementioned positive electrode, the secondary battery of the present invention also includes a negative electrode, an electrolyte, and a separator.

[0123] The present invention does not specifically limit the structure of the negative electrode sheet. In one embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least a portion of the surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, a binder, and a conductive agent.

[0124] This invention does not specifically limit the material of the negative electrode current collector; it can be any conventional material in the art. For example, the negative electrode current collector can be any of copper foil, nickel foam, or copper foam.

[0125] This invention does not specifically limit the type of negative electrode active material, and it can be any negative electrode active material commonly used in batteries. For example, the negative electrode active material can be selected from at least one of graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-based negative electrode materials (mainly including silicon suboxide and silicon-carbon negative electrodes), tin-based negative electrode materials (mainly including tin and tin alloys).

[0126] This invention does not specifically limit the type of binder; it can be any binder commonly used in battery negative electrodes. For example, the binder can be selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate.

[0127] This invention does not specifically limit the type of conductive agent; it can be any conductive agent commonly used in the negative electrode of batteries. For example, the conductive agent can be selected from at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, and graphene.

[0128] This invention does not specifically limit the preparation method of the negative electrode sheet. In one embodiment, the negative electrode sheet can be prepared by a method including the following steps:

[0129] The negative electrode active material, conductive agent, and binder are dispersed in deionized water and thoroughly stirred to form a uniform negative electrode slurry. The negative electrode slurry is then uniformly coated onto the negative electrode current collector, and after drying, rolling, and slitting, a negative electrode sheet is obtained.

[0130] This invention does not impose specific limits on the amount of negative electrode active material, conductive agent, and binder, and these amounts can be adjusted according to actual conditions.

[0131] This invention does not specifically limit the composition of the electrolyte, which may include one or more solvents commonly used in current battery electrolytes, as well as lithium salts commonly used in current battery electrolytes. For example, the solvent may include at least one of ethylene carbonate, propylene carbonate, butene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, and γ-butyrolactone; the lithium salt may include at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0132] This invention does not specifically limit the material of the separator; it can be any separator material commonly used in batteries. For example, the separator can be selected from any of the following: polypropylene separator (PP), polyethylene separator (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun separator (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven fabric separator, and separator with ceramic coating.

[0133] This invention does not specifically limit the method of battery preparation. In one embodiment, the battery can be prepared by a method including the following steps:

[0134] The above-mentioned positive electrode sheet, separator, and negative electrode sheet are wound or stacked to obtain a bare cell, which is then packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery is dried, electrolyte is injected into the dried battery. After the battery is placed, formed, and resealed, the secondary battery is completed.

[0135] The present invention will be further described below through specific embodiments.

[0136] Example 1

[0137] The method for preparing the positive electrode active material in this embodiment includes the following steps:

[0138] 1) Weigh 1.2510 g of oxalic acid, add 60 mL of deionized water, and disperse by sonication at 40 kHz for 15 min. Then heat in a water bath at 60 °C for 15 min to obtain an oxalic acid aqueous solution. Under stirring, add 0.8774 g of ammonium metavanadate to the oxalic acid aqueous solution, sonicate at 40 kHz for 15 min, and then heat in a water bath at 60 °C with continuous stirring for 15 min to fully dissolve it, forming a blue solution, which is the first mixed solution.

[0139] 2) Add 0.2632 g of chromium nitrate to the first mixed solution, disperse by sonication at 40 kHz for 15 min to ensure uniform mixing, then adjust the pH to 2 by adding dilute nitric acid dropwise, heat to 80 ℃ in a water bath and stir for 30 min until the solid is completely dissolved; then add sodium dihydrogen phosphate solution dropwise to obtain the second mixed solution. In the second mixed solution, the molar ratio of phosphorus atoms to vanadium ions is 1.55; the dropping rate of dilute nitric acid is 3 drops / min; and the dropping rate of sodium dihydrogen phosphate solution is 4 drops / min.

[0140] 3) Dissolve 0.1890g of sodium fluoride in 15 mL of deionized water to obtain a sodium fluoride solution. Add the sodium fluoride solution dropwise slowly to the second mixed solution while stirring. The addition rate is controlled at 3 drops / min. Then adjust the pH value to 7 with ammonia water to form a precursor solution.

[0141] 4) a. Transfer the precursor solution to a polytetrafluoroethylene-lined hydrothermal reactor, with the amount of precursor solution added being 80% of the reactor volume. Heat the reactor to 180°C and maintain the temperature for 24 hours, then allow it to cool naturally to room temperature to obtain a translucent gel (i.e., the first precursor). Place the obtained translucent gel in a constant temperature drying oven and age it at 60°C for 12 hours. Then, pre-freeze it at -20°C for 12 hours and transfer it to a freeze dryer for dehydration and solidification for 24 hours to obtain a fluffy dry gel, i.e., the second precursor.

[0142] b. Grind the dry gel into a fine powder, pass it through a 2800-mesh sieve, place it in a crucible, put the crucible in a tube furnace, introduce an inert gas (such as argon), and heat it to 350 ℃ at a heating rate of 5 ℃ / min for pre-calcination treatment. After holding at this temperature for 4 hours, the third precursor is obtained.

[0143] c. After grinding the third precursor, pass it through a 2800-mesh sieve, raise the temperature to 500℃ at a heating rate of 5℃ / min, and hold it at this temperature for 2.5 h (i.e., the first calcination treatment); then raise the temperature to 600℃ at a heating rate of 5℃ / min, and hold it at this temperature for 3.5 h (i.e., the second calcination treatment); then raise the temperature to 700℃ at a heating rate of 5℃ / min, and hold it at this temperature for 2 h (i.e., the third calcination treatment), to obtain the positive electrode active material.

[0144] Example 1-1

[0145] The method for preparing the positive electrode active material in this embodiment includes the following steps:

[0146] 1) Weigh 1.2510 g of oxalic acid, add 60 mL of deionized water, and disperse by sonication at 40 kHz for 15 min. Then heat in a water bath at 50 °C for 10 min to obtain an oxalic acid aqueous solution. Under stirring, add 1.0177 g of ammonium metavanadate to the oxalic acid aqueous solution, sonicate at 40 kHz for 15 min, and then heat in a water bath at 50 °C with continuous stirring for 10 min to fully dissolve it, forming a blue solution, which is the first mixed solution.

[0147] 2) Add 0.1310 g of cobalt nitrate to the first mixed solution, disperse by sonication at 40 kHz for 15 min to ensure uniform mixing, then adjust the pH to 1.5 by adding dilute nitric acid dropwise, heat to 70 ℃ in a water bath and stir for 30 min until the solid is completely dissolved; then add sodium dihydrogen phosphate solution dropwise to obtain the second mixed solution. In the second mixed solution, the molar ratio of phosphorus atoms to vanadium ions is 1.5; the dropping rate of dilute nitric acid is 3 drops / min; and the dropping rate of sodium dihydrogen phosphate solution is 3 drops / min.

[0148] 3) Dissolve 0.1890g of sodium fluoride in 15mL of deionized water to obtain a sodium fluoride solution. Add the sodium fluoride solution dropwise to the second mixed solution while stirring. The addition rate is controlled at 4 drops / min. Then adjust the pH value to 7 with ammonia water to form a precursor solution.

[0149] 4) a. The precursor solution was transferred to a polytetrafluoroethylene-lined hydrothermal reactor, with the amount of precursor solution added being 80% of the reactor volume. The reactor was heated to 150 °C in an oven and kept at that temperature for 18 h. After being naturally cooled to room temperature, a translucent gel (i.e., the first precursor) was obtained. The translucent gel was placed in a constant temperature drying oven and aged at 50 °C for 8 h. After being pre-frozen at -20 °C for 12 h, the gel was transferred to a freeze dryer for dehydration and solidification for 24 h to obtain a fluffy dry gel, i.e., the second precursor.

[0150] b. Grind the dry gel into a fine powder, pass it through a 2800-mesh sieve, place it in a crucible, put the crucible in a tube furnace, introduce an inert gas (such as argon), and heat it to 300 ℃ at a heating rate of 4 ℃ / min for pre-calcination treatment. After holding at this temperature for 3 hours, the third precursor is obtained.

[0151] c. After grinding the third precursor, pass it through a 2800-mesh sieve, raise the temperature to 450℃ at a heating rate of 4℃ / min, and hold it at this temperature for 2 hours (i.e., the first calcination treatment); then raise the temperature to 600℃ at a heating rate of 4℃ / min, and hold it at this temperature for 3 hours (i.e., the second calcination treatment); then raise the temperature to 700℃ at a heating rate of 4℃ / min, and hold it at this temperature for 2 hours (i.e., the third calcination treatment) to obtain the positive electrode active material.

[0152] Examples 1-2

[0153] The method for preparing the positive electrode active material in this embodiment includes the following steps:

[0154] 1) Weigh 1.2510 g of oxalic acid, add 60 mL of deionized water, and disperse by sonication at 40 kHz for 15 min. Then heat in an 80°C water bath for 20 min to obtain an oxalic acid aqueous solution. Under stirring, add 0.6317 g of ammonium metavanadate to the oxalic acid aqueous solution, sonicate at 40 kHz for 15 min, and then heat in an 80°C water bath with continuous stirring for 20 min to fully dissolve it, forming a blue solution, which is the first mixed solution.

[0155] 2) Add 1.2554 g of manganese nitrate to the first mixed solution, disperse by sonication at 40 kHz for 15 min to ensure uniform mixing, then add dilute nitric acid dropwise to adjust the pH to 2.5 and stir for 30 min until the solid is completely dissolved; then add sodium dihydrogen phosphate solution dropwise to obtain the second mixed solution. In the second mixed solution, the molar ratio of phosphorus atoms to vanadium ions is 1.6; the dropping rate of dilute nitric acid is 3 drops / min; and the dropping rate of sodium dihydrogen phosphate solution is 5 drops / min.

[0156] 3) Dissolve 0.1890 g of sodium fluoride in 15 mL of deionized water to obtain a sodium fluoride solution. Add the sodium fluoride solution dropwise to the second mixed solution while stirring. The addition rate is controlled at 5 drops / min. Then adjust the pH value to 8 with ammonia water to form a precursor solution.

[0157] 4) a. The precursor solution was transferred to a polytetrafluoroethylene-lined hydrothermal reactor, with the amount of precursor solution added being 80% of the reactor volume. The reactor was heated to 200 °C in an oven and kept at that temperature for 24 h. After being naturally cooled to room temperature, a translucent gel (i.e., the first precursor) was obtained. The translucent gel was placed in a constant temperature drying oven and aged at 80 °C for 16 h. After being pre-frozen at -20 °C for 12 h, the gel was transferred to a freeze dryer for dehydration and solidification for 24 h to obtain a fluffy dry gel, i.e., the second precursor.

[0158] b. Grind the dry gel into a fine powder, pass it through a 2800-mesh sieve, place it in a crucible, put the crucible in a tube furnace, introduce an inert gas (such as argon), and heat it to 400 ℃ at a heating rate of 6 ℃ / min. After holding it at this temperature for 5 h, the third precursor is obtained.

[0159] c. After grinding the third precursor, pass it through a 2800-mesh sieve, raise the temperature to 550℃ at a heating rate of 6℃ / min, and hold it at this temperature for 4 h (i.e., the first calcination treatment); then raise the temperature to 650℃ at a heating rate of 6℃ / min, and hold it at this temperature for 4 h (i.e., the second calcination treatment); then raise the temperature to 750℃ at a heating rate of 6℃ / min, and hold it at this temperature for 4 h (i.e., the third calcination treatment) to obtain the positive electrode active material.

[0160] Example 2

[0161] This embodiment is basically the same as Embodiment 1, except that:

[0162] The amount of chromium nitrate added in 2) is increased to 0.7896g; that is, the mass percentage of the active element in the positive electrode active material is 15%.

[0163] Example 3

[0164] This embodiment is basically the same as Embodiment 1, except that:

[0165] Replace chromium nitrate in 2) with nickel nitrate.

[0166] Example 4

[0167] This embodiment is basically the same as Embodiment 1, except that:

[0168] Replace chromium nitrate in 2) with ferrous nitrate.

[0169] Example 5

[0170] This embodiment is basically the same as Embodiment 1, except that:

[0171] Replace chromium nitrate in 2) with aluminum nitrate.

[0172] Example 6

[0173] This embodiment is basically the same as Embodiment 1, except that:

[0174] Ammonium metavanadate 0.8109g, chromium nitrate 0.2632g, sodium fluoride 0.1638g, with a molar ratio of phosphorus atoms to vanadium ions of 1.55, resulting in a molar ratio of sodium vanadium phosphate phase, sodium vanadium fluorophosphate phase, and sodium pyrophosphate phase of 5:3:2.

[0175] In 4), c means that the temperature of the first calcination treatment is 500℃ and the holding time is 3 h.

[0176] Example 7

[0177] This embodiment is basically the same as Embodiment 1, except that:

[0178] Ammonium metavanadate 0.8146g, chromium nitrate 0.2444g, sodium fluoride 0.0559g, with a molar ratio of phosphorus atoms to vanadium ions of 1.55, make the molar ratio of sodium vanadium phosphate phase, sodium vanadium fluorophosphate phase, and sodium pyrophosphate phase 8:1:1.

[0179] Comparative Example 1

[0180] The method for preparing the positive electrode active material in this comparative example includes the following steps:

[0181] 1) Weigh 1.2510 g of oxalic acid, add 60 mL of deionized water, sonicate for 15 min to aid dispersion, then heat in a 60 °C water bath for 15 min to obtain an oxalic acid aqueous solution. Under stirring, add 1.0530 g of ammonium metavanadate to the oxalic acid aqueous solution, sonicate for 15 min, then heat in a 60 °C water bath and stir continuously for 15 min to fully dissolve it, forming a blue solution, which is the first mixed solution.

[0182] 2) Then, sodium dihydrogen phosphate solution is added dropwise to the first mixed solution at a rate of 3 drops / min to form a precursor solution. The molar ratio of phosphorus atoms to vanadium ions in the precursor solution is 1.5.

[0183] 3) a. Transfer the precursor solution to a polytetrafluoroethylene-lined hydrothermal reactor, filling it to 80%. Heat the reactor to 180°C and maintain the temperature for 24 hours. Then, allow it to cool naturally to room temperature to obtain a translucent gel (i.e., the first precursor). Place the obtained translucent gel in a constant temperature drying oven and age it at 60°C for 12 hours. Then, pre-freeze it at -20°C for 12 hours and transfer it to a freeze dryer for dehydration and solidification for 24 hours to obtain a fluffy dry gel, i.e., the second precursor.

[0184] b. Grind the dry gel into a fine powder, pass it through a 2800-mesh sieve, place it in a crucible, put the crucible in a tube furnace, introduce an inert gas (such as argon), and heat it to 350 ℃ at a heating rate of 5 ℃ / min for pre-calcination treatment. After holding at this temperature for 4 hours, the third precursor is obtained.

[0185] c. After grinding the third precursor, pass it through a 2800-mesh sieve, raise the temperature to 700℃ at a heating rate of 5℃ / min, and keep it at this temperature for 5 hours to obtain sodium vanadium phosphate, which is the positive electrode active material.

[0186] Comparative Example 2

[0187] The method for preparing the positive electrode active material in this comparative example includes the following steps:

[0188] 1) Weigh 1.2510 g of oxalic acid, add 60 mL of deionized water, sonicate for 15 min to aid dispersion, then heat in a 60 °C water bath for 15 min to obtain an oxalic acid aqueous solution. Under stirring, add 1.0530 g of ammonium metavanadate to the oxalic acid aqueous solution, sonicate for 15 min, then heat in a 60 °C water bath and stir continuously for 15 min to fully dissolve it, forming a blue solution, which is the first mixed solution.

[0189] 2) Add sodium dihydrogen phosphate solution dropwise to the first mixed solution at a rate of 3 drops / min to obtain the second mixed solution. In the second mixed solution, the molar ratio of phosphorus atoms to vanadium ions is 1.

[0190] 3) Dissolve 0.7644g of sodium fluoride in 15mL of deionized water to obtain a sodium fluoride solution. Add the sodium fluoride solution dropwise to the second mixed solution while stirring. The addition rate is controlled at 3 drops / min. Then adjust the pH value to 7 with ammonia water to form a precursor solution.

[0191] 4) a. Transfer the precursor solution to a polytetrafluoroethylene-lined hydrothermal reactor, filling it to 80%. Heat the reactor to 180°C and maintain the temperature for 24 hours. Then, allow it to cool naturally to room temperature to obtain a translucent gel (i.e., the first precursor). Place the obtained translucent gel in a constant temperature drying oven and age it at 60°C for 12 hours. Then, pre-freeze it at -20°C for 12 hours and transfer it to a freeze dryer for dehydration and solidification for 24 hours to obtain a fluffy dry gel, i.e., the second precursor.

[0192] b. Grind the dry gel into a fine powder, pass it through a 2800-mesh sieve, place it in a crucible, put the crucible in a tube furnace, introduce an inert gas (such as argon), and heat it to 350 ℃ at a heating rate of 5 ℃ / min for pre-calcination treatment. After holding at this temperature for 4 hours, the third precursor is obtained.

[0193] c. After grinding the third precursor, pass it through a 2800-mesh sieve, raise the temperature to 600℃ at a heating rate of 5℃ / min, and keep it at this temperature for 5 hours to obtain sodium vanadium fluorophosphate, which is the positive electrode active material.

[0194] Comparative Example 3

[0195] This comparative example is basically the same as Example 1, except that:

[0196] 2) No chromium nitrate is added.

[0197] Comparative Example 4

[0198] This comparative example is basically the same as Example 1, except that:

[0199] 3) No sodium fluoride is added.

[0200] In step 4), after grinding the third precursor, the temperature is increased to 500℃ at a heating rate of 5℃ / min and held at this temperature for 2.5h; then the temperature is increased to 700℃ at a heating rate of 5℃ / min and held at this temperature for 2h to obtain the positive electrode active material.

[0201] Comparative Example 5

[0202] This comparative example is basically the same as Example 1, except that:

[0203] In 4), the aging time is 6 hours.

[0204] Comparative Example 6

[0205] This comparative example is basically the same as Example 1, except that:

[0206] In 4), the aging time is 20 hours.

[0207] Test case

[0208] 1. The positive electrode active material of Example 1 was subjected to X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray diffraction, and transmission electron microscopy tests, see [the results are described in the original text]. Figure 1-4 The active electrode materials of the examples and comparative examples were tested for the mass percentage of active elements (abbreviated as active element content), the molar ratio of sodium vanadium phosphate phase, sodium vanadium fluorophosphate phase, and sodium pyrophosphate phase, and the average particle size. The results are shown in Table 1. X-ray diffraction was performed on the active electrode materials prepared in Comparative Examples 5 and 6, and the results are shown in Table 1. Figure 8 , 9 .

[0209] 2. Testing Methods

[0210] 1) X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray energy, and transmission electron microscopy tests

[0211] X-ray diffraction testing: The powder sample (positive electrode active material of Example 1, Comparative Example 5, or Comparative Example 6) was uniformly filled into the sample holder and flattened. The sample was then placed in the instrument, and Cu target Kα rays were used for scanning within an angle range of 10-70°, recording the diffraction intensity variation with angle. Finally, the obtained diffraction patterns were compared with a standard card database on Jade to complete phase identification and structural analysis. The results are shown below. Figure 1 , Figure 8 , Figure 9 .

[0212] Depend on Figure 8 , 9 It is known that if the aging time is not within 8-16 hours, the positive electrode active material of this invention cannot be obtained.

[0213] Scanning electron microscopy (SEM) testing: The gold-sprayed sample (the positive electrode active material of Example 1) was fixed on the sample stage and placed in the microscope tube after vacuuming. Observation was performed by adjusting electron beam parameters (such as accelerating voltage and beam spot size). Clear images at different magnifications were acquired by receiving secondary electron and backscattered electron signals. The results are shown in [Figure number missing]. Figure 2 .

[0214] Energy-dispersive X-ray energy measurement: After locating the area to be measured using SEM, select the point, line, or surface analysis mode, excite the sample with an electron beam, and collect the generated characteristic X-rays. The software automatically identifies the characteristic peaks in the energy spectrum to determine the elemental species, and calculates the weight percentage (wt%) or atomic percentage (at%) of each element based on its intensity. The results are shown in […]. Figure 3 .

[0215] Transmission electron microscopy (TEM) testing: The powder sample (the positive electrode active material of Example 1) needs to be ultrasonically dispersed and then dropped onto a copper mesh with a carbon film. After mounting, bright-field / dark-field imaging can be performed by adjusting the electron beam, selected area electron diffraction can be used to analyze the crystal structure, and high-resolution imaging can be performed to directly resolve lattice fringes. The results are shown in [Figure number missing]. Figure 4 .

[0216] 2) Test method for the mass percentage of active elements in positive electrode active materials

[0217] The content of active elements in the positive electrode active material was determined using ICP-OES. The sample was accurately weighed, microwave-digested with a strong acid (such as aqua regia or a hydrofluoric acid system), and then brought to a final volume after complete dissolution. A calibration curve was established using a series of standard solutions. The spectral intensity of the test solution was measured and substituted into the curve to calculate the concentration. The final concentration was calculated using the formula: Content (%) = (C × V × D × 10⁻⁶) / (C × V × D × 10⁻⁶) -4The mass percentage content is calculated as C / m. Where C is the instrument reading (mg / L), V is the final volume (mL), D is the dilution factor, and m is the sample mass (g).

[0218] 3) Test method for the molar ratio of sodium vanadium phosphate phase, sodium vanadium fluorophosphate phase, and sodium pyrophosphate phase.

[0219] XRD coupled with ICP-OES and ion chromatography (IC) was employed. First, XRD Rietveld refinement was used for phase identification and to obtain the mass fractions of the three phases. Then, ICP was used to accurately determine the total molar amounts of Na, V, and P in the digested sample, and IC was used to determine the total molar amount of F. Finally, a system of equations was established based on the molar conservation laws of sodium, vanadium, phosphorus, and fluorine to calculate the precise molar ratios of the three phases.

[0220] 4) Test method for average particle size

[0221] Two-dimensional images of particles are acquired using an electron microscope. Image analysis software is then used to automatically identify the outline of individual particles and calculate thousands of parameters, including their projected area, equivalent diameter, length, and width. After analyzing a large number of particles, the average particle size and distribution can be obtained.

[0222] Table 1

[0223]

[0224] 3. The positive electrode active materials prepared in the above examples and comparative examples were assembled into secondary batteries. Voltage plateau testing, discharge specific capacity testing, cycle stability testing, and rate performance testing were performed on the secondary batteries. The test results are shown in Table 2. A schematic diagram of the cycle performance of the secondary batteries prepared with the positive electrode active materials of Examples 1, 3, and Comparative Example 3 is shown in [Table 2]. Figure 5-7 .

[0225] Methods for preparing secondary batteries:

[0226] The positive electrode active materials prepared in the above examples and comparative examples were mixed and ground with acetylene black and polyvinylidene fluoride, respectively. Then, 1-methyl-2-pyrrolidone was added and stirred into a slurry. The slurry was then uniformly coated onto aluminum foil to form an electrode sheet, which was then vacuum dried at 80°C to obtain the positive electrode sheet. The mass ratio of the positive electrode active material, acetylene black, polyvinylidene fluoride, and 1-methyl-2-pyrrolidone was 7:2:1:30.

[0227] In an anhydrous and oxygen-free glove box, the prepared sodium-ion battery positive electrode, sodium sheet, glass fiber membrane, and a mixture of sodium perchlorate, dimethyl carbonate, ethylene carbonate, ethyl methyl carbonate, and fluoroethylene carbonate were assembled to obtain a sodium-ion battery.

[0228] Test method:

[0229] 1) Voltage plateau test

[0230] Using the LAND battery testing system, the electrode materials under test were assembled into coin cells. The coin cells were then subjected to charge-discharge cycles at a constant current and a charging rate of 0.2C at 25°C. The charging cutoff voltage was set to 4.2V, and the discharging cutoff voltage to 1.5V. The voltage versus time / capacity curves were recorded throughout the process. From the charge-discharge curves, the segment with the smoothest voltage change and the longest duration was identified; its average voltage is the plateau voltage.

[0231] 2) Discharge specific capacity test

[0232] Using the LAND battery testing system, the electrode materials to be tested were assembled into coin cells. The coin cells were charged at a constant current of 0.2C until the cutoff voltage was reached. After a short rest, they were discharged at the same constant current until the discharge cutoff voltage was reached. The instrument automatically recorded the specific capacity (mAh g⁻¹) during the discharge process. -1 ).

[0233] 3) Cyclic stability test

[0234] At 25°C, the capacitor was charged to 4.2V at a constant current rate of 0.2C, and then discharged to 1.5V at a discharge rate of 0.2C. This charge-discharge cycle was repeated 50 times. The discharge capacity Q1 at the first cycle and the discharge capacity Q at the 50th cycle were measured. 50 .

[0235] Capacity retention rate Q = Q 50 / Q1*100%.

[0236] 4) Ratio Performance Test

[0237] The battery was charged and discharged at 25°C using a battery charge and discharge tester. The charge and discharge regime was as follows: constant current charging at 0.2C to 4.2V, then constant voltage charging at 4.2V until the current decreased to 0.02C, followed by resting for 5 minutes, and then constant current discharging at 0.2C to 1.5V. The discharge capacity Q was recorded. 0.2c After resting for 5 minutes, charge with a constant current of 0.2C to 4.2V, then switch to a constant voltage of 4.2V to charge until the current decreases to 0.02C. After resting for 5 minutes, discharge with a constant current of 3C to 1.5V, and record the discharge capacity Q. 5c The capacity retention rate can be calculated using the following formula:

[0238] 3C discharge rate capacity retention rate = Q 5c / Q 0.2c ×100%.

[0239] Table 2

[0240]

[0241] Note: " / " indicates that relevant data could not be measured.

[0242] As shown in Table 2, compared with the comparative example, the embodiments of the present invention exhibit higher specific capacity, more stable cycle performance, and superior rate performance. Finally, it should be noted that other embodiments of the invention will readily conceive of those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A positive electrode active material, characterized in that, A composite comprising a sodium vanadium phosphate phase, a sodium vanadium fluorophosphate phase, and a sodium pyrophosphate phase, wherein at least a portion of the V element in the sodium vanadium phosphate phase is replaced by an active element, or at least a portion of the V element in both the sodium vanadium phosphate phase and the sodium vanadium fluorophosphate phase is replaced by the active element, wherein the active element includes at least one of Cr, Co, Mn, Fe, Ni, and Al.

2. The positive electrode active material according to claim 1, characterized in that, The chemical composition of the sodium vanadium phosphate phase is shown in Formula 1 and / or the chemical composition of the sodium fluorophosphate phase is shown in Formula 2: Na3M x V 2-x (PO4)3 formula 1 Na3N y V 2-y (PO4)2F3 formula 2; Wherein, M and N are each independently selected from the active elements, 0 < x < 2, 0 ≤ y < 2.

3. The positive electrode active material according to claim 1 or 2, characterized in that, The active element has a mass percentage content of 1-12% in the positive electrode active material.

4. The positive electrode active material according to any one of claims 1-3, characterized in that, The molar ratio of the sodium vanadium phosphate phase, the sodium vanadium fluorophosphate phase, and the sodium pyrophosphate phase is (5-8):(1-5):(1-3).

5. The positive electrode active material according to any one of claims 1-4, characterized in that, The positive electrode active material is a polycrystalline particle, and the average particle size of the positive electrode active material is 1μm≤D≤3μm.

6. A method for preparing the positive electrode active material according to any one of claims 1-5, characterized in that, Includes the following steps: 1) The organic acid solution is mixed with the vanadium source and then subjected to heat treatment to obtain the first mixed solution; 2) Mix the first mixed solution with the active element source, and add a phosphorus source under acidic conditions to obtain a second mixed solution; 3) After mixing the second mixed solution with the sodium source and fluorine source, the pH is controlled to neutral to obtain the precursor solution; 4) The precursor solution is subjected to heat preservation treatment, aging treatment and pre-calcination treatment in sequence, and then subjected to first calcination treatment, second calcination treatment and third calcination treatment in sequence to obtain the positive electrode active material; wherein, the aging treatment time is 8-16h, the temperature of the first calcination treatment is 450-550℃ and the time is 2-4h, the temperature of the second calcination treatment is 600-650℃ and the time is 2-4h, and the temperature of the third calcination treatment is 700-750℃ and the time is 2-4h.

7. The preparation method according to claim 6, characterized in that, The heat preservation treatment is performed at a temperature of 150-200℃ for 18-24 hours; and / or, the aging treatment is performed at a temperature of 50-80℃; and / or, the pre-calcination treatment is performed at a temperature of 300-400℃ for 3-5 hours; and / or, The heating rate of the pre-calcination treatment is 4-6℃ / min; and / or, the heating rate of the first calcination treatment is 4-6℃ / min; and / or, the heating rate of the second calcination treatment is 4-6℃ / min; and / or, the heating rate of the third calcination treatment is 4-6℃ / min.

8. The preparation method according to claim 6 or 7, characterized in that, In step 1), the heat treatment temperature is 50-80℃, and the heat treatment time is 10-20 min; and / or, Preferably, in step 2), when controlling the acidic conditions, a first buffer solution is added, and the dropping rate of the first buffer solution is 2-3 drops / min; and / or, Preferably, in step 2), the acidic conditions are controlled during water bath heating treatment, and the temperature of the water bath heating treatment is 70-90°C; and / or, Preferably, in step 2), the molar ratio of phosphorus atoms to vanadium ions in the second mixed solution is 1.5-1.6; and / or, Preferably, in step 2), the phosphorus source is added at a dropping rate of 3-5 drops / min; and / or, Preferably, in step 3), when mixing the second mixed solution with the sodium source and the fluorine source, the sodium source and the fluorine source are first mixed to obtain a sodium-fluorine mixture, and the sodium-fluorine mixture is added to the second mixed solution at a dropping rate of 3-5 drops / min.

9. A positive electrode plate, characterized in that, It includes the positive electrode active material according to any one of claims 1-5, or the positive electrode active material prepared by the preparation method according to claims 6-8.

10. A secondary battery, characterized in that, It includes the positive electrode active material according to any one of claims 1-5, or the positive electrode active material prepared by the preparation method according to claims 6-8, or the positive electrode sheet according to claim 9.

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