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

Abstract

The invention discloses a positive active material and a preparation method thereof, a positive pole piece and a battery. The positive electrode active material satisfies the chemical formula: Na < x > (1-u) + 4 Mn < y > (1-u) + 3 M < z > (1-u) Ti < 2-y-z > (1-u) (PO4) 3 (1-u) + 2 (P2O7) u, where 2 < = xlt; 1.2 < = y < = 1.42, 0 < = z < = 0.05, 0.06 < = u < = 0.10; the ion radius of the M element is larger than that of the Ti element, and / or the bond energy of the M-O bond is larger than that of the Ti-O bond; wherein in an X-ray diffraction pattern of the positive electrode active material, a first diffraction peak exists at a position where 2 theta is 31.3-31.8 degrees, and a second diffraction peak exists at a position where 2 theta is 32.7-33.0 degrees. Therefore, by increasing the manganese content and doping elements with larger radiuses or larger bond energy, the structural stability and sodium ion transmission rate of the positive electrode active material can be effectively improved, the gram volume of the positive electrode active material is high, and the cycle performance is excellent.

Description

Technical Field

[0001] The present application relates to the field of sodium ion batteries, and specifically to positive electrode active materials and preparation methods thereof, positive electrode sheets, and batteries. Background Art

[0002] As the global energy transition accelerates, electrochemical energy storage technology has become a core element supporting the development of new energy vehicles and smart grids. The current mainstream lithium-ion battery system faces significant challenges in terms of resource sustainability. Lithium is present in the Earth's crust at a mere 0.006%, posing a serious resource supply risk. In contrast, sodium, a congener alkali metal, has a high crustal abundance of 2.64%. Its global distribution and ease of industrial extraction make sodium-ion batteries a key breakthrough in overcoming the resource bottlenecks of traditional energy storage technologies.

[0003] In sodium-ion battery technology, polyanionic cathode active materials have attracted considerable attention due to their long cycle life, high thermal stability, and low cost. Strong X–O (X = P, S, Si, or B) covalent bonds provide high structural stability and relatively small volume changes during the insertion and extraction of sodium ions. Among them, the Na3MnTi(PO4)3 material with a NASICON three-dimensional ion channel structure has shown significant potential, with a theoretical capacity of 117 mAh / g in the voltage range of 2.5V-4.2V. However, many challenges remain in the practical application of this material, requiring further improvement.

[0004] It should be noted that the above statements are only used to provide background technical information related to this application and do not necessarily constitute prior art. Summary of the Invention

[0005] In the first aspect of the present application, a positive electrode active material is proposed, comprising: the positive electrode active material satisfies the chemical formula: Na x(1-u)+4u Mn y(1-u)+3u M z(1-u) Ti (2-y-z)(1-u) (PO4) 3(1-u)+2u (P2O7) u , where 2≤x<4,

[0006] 1.2≤y≤1.42, 0≤z≤0.05, 0.06≤u≤0.10; the ionic radius of the M element is greater than the ionic radius of the Ti element, and / or the bond energy of the MO bond is greater than the bond energy of the Ti-O bond; wherein, the X-ray diffraction pattern of the positive electrode active material has a first diffraction peak at a position of 31.3°-31.8° in 2θ, and a second diffraction peak at a position of 32.7°-33.0° in 2θ. Therefore, compared with the Mn element accounting for 50% of the total transition metal elements, by increasing the manganese content and doping elements with a larger radius or a larger bond energy within a limited range, the structural stability and sodium ion transfer rate of the positive electrode active material can be effectively improved, the specific capacity of the positive electrode active material is higher, and the cycle performance is better.

[0007] In some embodiments, the M element includes one or more of V, Cr, and Zr, thereby facilitating stable doping of the M element.

[0008] In some embodiments, the positive electrode active material is a composite material formed by a sodium superionic conductor material and sodium manganese pyrophosphate in a molar ratio of (1-u):u, wherein the sodium superionic conductor material satisfies the chemical formula Na x Mn y M z Ti 2-y-z (PO4)3, the sodium manganese pyrophosphate satisfies the chemical formula Na4Mn3(PO4)2(P2O7). As a result, the two phases coexist in the positive electrode active material, the interplanar spacing is large, and the sodium ion diffusion channel in the bulk phase is wide, which is beneficial to improving the kinetic performance of the positive electrode active material and thus improving the capacity of the positive electrode active material.

[0009] In some embodiments, 1.5<y / (2-yz)≤2.7; alternatively, 1.6<y / (2-yz)≤2.6. This can improve the specific capacity of the positive electrode active material.

[0010] In some embodiments, the peak intensity of the first diffraction peak is I(116), the peak intensity of the second diffraction peak is I(222), and 0.06≤I(222) / I(116)≤0.10. This can increase the sodium ion transmission rate, improve the kinetic properties of the positive electrode active material, and enhance the capacity.

[0011] In some embodiments, the first diffraction peak has a half-maximum width (FWHM) of 0.20-0.37, and the second diffraction peak has a half-maximum width (FWHM) of 0.22-0.37. Thus, the positive electrode active material has excellent crystallinity, a high sodium ion transport channel stability, and excellent cycling performance.

[0012] In some embodiments, the peak area of the first diffraction peak is S(116), the peak area of the second diffraction peak is S(222), and 0.057≤S(222) / S(116)≤0.065. Thus, the proportion of electrochemically active crystal planes is relatively high, which is beneficial for improving the kinetic properties of the positive electrode active material and enhancing the capacity.

[0013] In some embodiments, the positive electrode active material further comprises a carbon coating layer, the carbon coating layer being located on at least a portion of the surface of the positive electrode active material, and the mass fraction of the carbon coating layer is 10% to 12% based on the total mass of the carbon coating layer and the positive electrode active material, thereby helping to improve the ionic conductivity and electronic conductivity of the positive electrode active material.

[0014] In some embodiments, the charge transfer impedance of the positive electrode active material is 900Ω-1200Ω. Thus, the positive electrode active material has high electronic conductivity.

[0015] In some embodiments, the porosity of the positive electrode active material is 1%-3%; and / or the compaction density of the positive electrode active material is 2 g / cm 3 -3g / cm 3 As a result, the contact area between the positive electrode active material and the electrolyte is larger, the sodium ion transmission distance is shorter, and the positive electrode active material layer using the positive electrode active material has a higher compaction density.

[0016] In the second aspect of the present application, the present application proposes a method for preparing the aforementioned positive electrode active material, comprising: dispersing a sodium source, a phosphorus source, a titanium source, a manganese source, and an M source in a solvent to form a gel according to the target chemical formula of the positive electrode active material, and drying to obtain a precursor; sintering the precursor in a non-oxidizing atmosphere to obtain the positive electrode active material, wherein the positive electrode active material satisfies the target chemical formula: Na x(1-u)+4u Mn y(1-u)+3u M z(1-u) Ti (2-y-z)(1-u) (PO4) 3(1-u)+2u (P2O7) u , where 2≤x<4, 1.2≤y≤1.42, 0≤z≤0.05, and 0.06≤u≤0.10; the ionic radius of the M element is larger than that of the Ti element, and / or the bond energy of the MO bond is larger than that of the Ti-O bond. This allows for low-cost preparation of the positive electrode active material, facilitating large-scale manufacturing.

[0017] In some embodiments, the sodium source, the phosphorus source, the titanium source, the manganese source, the M source, and the carbon source are dispersed in the solvent to form a gel, thereby simultaneously forming a positive electrode active material containing a carbon coating layer in a one-step process.

[0018] In some embodiments, the sodium source includes one or more of sodium carbonate, sodium hydroxide, sodium nitrate, sodium acetate, and sodium dihydrogen phosphate; and / or the manganese source includes one or more of manganese oxide, manganese phosphate, manganese sulfate, manganese chloride, manganese nitrate, manganese carbonate, and manganese acetate; and / or the titanium source includes one or more of titanium phosphate, titanium acetate, titanium sulfate, titanium chloride, titanium nitrate, titanium carbonate, and titanium organic matter; and / or the phosphorus source includes one or more of H3PO4, NH4H2PO4, (NH4)2HPO4, (NH4)3PO4, and P2O5; and / or the M source includes one or more of oxides, phosphates, sulfates, chlorides, nitrates, and carbonates corresponding to the M element; and / or the solvent includes one or more of deionized water, ethanol, and ethylene glycol. As a result, the raw material sources are wide-ranging and compatible with existing production lines, which is conducive to reducing preparation costs.

[0019] In some embodiments, the carbon source includes an organic carbon source and an inorganic carbon source, and the mass ratio of the organic carbon source to the inorganic carbon source is 1.8-2. Thus, the organic carbon source can form a three-dimensional electron transport channel from the bulk to the surface of the positive electrode active material, and combined with the high ionic conductivity of the inorganic carbon source, the electronic and ionic conductivity of the positive electrode active material can be improved.

[0020] In some embodiments, the organic carbon source includes one or more of citric acid, polypyrrole, and polyvinyl pyrrolidone; and / or the inorganic carbon source includes one or more of graphite, acetylene black, graphene, and carbon nanotubes. Thus, a wide range of carbon sources can be used to synergistically improve the electronic and ionic conductivity of the positive electrode active material.

[0021] In some embodiments, the sintering temperature is 500° C.-700° C., and the sintering time is 8 h-20 h. Thus, a two-phase composite positive electrode active material with good crystallinity can be formed.

[0022] In a third aspect, the present application provides a positive electrode sheet comprising: a positive electrode current collector and a positive electrode active material layer located on at least one side of the positive electrode current collector, wherein the positive electrode active material layer comprises the aforementioned positive electrode active material, or a positive electrode active material prepared using the aforementioned method. Thus, the positive electrode sheet possesses all the features and advantages of the aforementioned positive electrode active material and method for preparing the positive electrode active material, and no further details are given here.

[0023] In a fourth aspect of the present application, the present application provides a battery comprising the aforementioned positive electrode sheet. Thus, the battery has all the features and advantages of the aforementioned positive electrode sheet, which will not be described in detail here. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0025] Figure 1 This is a cycling performance curve of the battery of Example 1 of the present application at a 1C rate;

[0026] Figure 2 This is the X-ray diffraction pattern of the positive electrode active material of Example 1 of the present application. DETAILED DESCRIPTION

[0027] Unless otherwise defined, all technical and scientific terms used in this application have the same meanings as those commonly understood by technicians in the technical field to which this application belongs; the terms used in this application are only for the purpose of describing specific embodiments and are not intended to limit this application; unless otherwise specified, the numerical values of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the embodiments of this application).

[0028] The terms "include" and "have" in the description and claims of this application and any variations thereof are open expressions, that is, including the contents specified in this application but not excluding other contents.

[0029] In the description of this application, regardless of whether the word "about" or "approximately" is used, all numbers disclosed herein are approximate values. The value of each number may vary by less than 10% or by a reasonable difference considered by a person skilled in the art, such as 1%, 2%, 3%, 4% or 5%.

[0030] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0031] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly specifying the quantity of the technical features indicated. "First feature" and "second feature" may include one or more of the features.

[0032] In the description of this application, “plurality” means two or more.

[0033] In the description of this application, "A and / or B" may include the case of A alone, the case of B alone, or any of the cases of A and B, where A and B are only used for example, and may be any technical feature connected by "and / or" in this application.

[0034] In this application, the writing order of each step does not mean a strict execution order and does not constitute any limitation on the implementation process. The specific execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of this application can 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 steps (b) and (a) performed sequentially. For example, it is mentioned that the method may also include step (c), indicating that step (c) can be added to the method in any order. For example, the method may include steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0035] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0036] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0037] In the related art, the polyanionic positive electrode active material of the Na3MnTi(PO4)3 system has the problem of low intrinsic electronic conductivity and ionic conductivity, which leads to poor capacity performance of the positive electrode active material, and the hysteresis effect of the charge and discharge platform makes its energy efficiency low.

[0038] In the present application, the Mn content in the positive electrode active material is relatively high. Since the ionic radius of manganese is larger than that of titanium, the unit cell volume of the positive electrode active material increases after the manganese content is increased, the sodium ion transmission channel becomes wider, and the ionic conductivity is better. In addition, the higher content of manganese helps to increase the discharge voltage platform of the positive electrode active material, so that the positive electrode active material can release more capacity and the capacity is better. Furthermore, by using an M element with a larger ionic radius or stronger bond energy to dope a small amount of Ti in the positive electrode active material, the combination of the M element and the oxygen atom is stronger than the Ti-O bond, which can play a supporting role, improve the structural stability of the positive electrode active material, and improve the cycle performance of the positive electrode active material.

[0039] It should be noted that the applicant has found through a large number of creative experimental studies that when the chemical formula of the positive electrode material obtained by the preparation method described in this application meets the range defined in this application, a positive electrode active material with the Na3MnTi(PO4)3 system as the main phase will be formed, and at the same time, a substance with the chemical formula meeting the Na4Mn3(PO4)2(P2O7) will be formed. For example, when the positive electrode active material meets the chemical formula Na3Mn 1.20 Ti 0.80 (PO4)3, a Na4Mn3(PO4)2(P2O7) phase with certain electrochemical activity will appear in the positive electrode active material. 1.20 Ti 0.80 Compared with Na3MnTi(PO4)3, the (PO4)3 positive electrode material has a higher proportion of Mn element in the total amount of transition metal elements, and the formed Na4Mn3(PO4)2(P2O7) phase helps to improve the capacity and cycle performance of the positive electrode active material.

[0040] In the first aspect of the present application, the present application proposes a positive electrode active material, comprising: a positive electrode active material that satisfies the chemical formula: Na x(1-u)+4u Mn y(1-u)+3u M z(1-u) Ti(2-y-z)(1-u) (PO4) 3(1-u)+2u (P2O7) u , wherein, 2≤x<4, 1.2≤y≤1.42, 0≤z≤0.05, 0.06≤u≤0.10; the ionic radius of the M element is greater than the ionic radius of the Mn element, and / or the bond energy of the MO bond is greater than the bond energy of the Ti-O bond, wherein the X-ray diffraction pattern of the positive electrode active material has a first diffraction peak at a position of 31.3°-31.8° in 2θ, and a second diffraction peak at a position of 32.7°-33.0° in 2θ. Therefore, compared with the Na3MnTi(PO4)3 system in which the Mn element accounts for 50% of the total transition metal elements, by increasing the manganese content and doping elements with a larger radius or a larger bond energy, the structural stability and sodium ion transfer rate of the positive electrode active material can be effectively improved, the specific capacity of the positive electrode active material is higher, and the cycle performance is better.

[0041] As examples, x may be 2, 2.5, 3, 3.65, 3.8, 3.85, 3.9, or 4.

[0042] As an example, y may be 1, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, or 1.42.

[0043] As an example, z may be 0.001, 0.005, 0.01, 0.02, or 0.05.

[0044] As an example, u may be 0.06, 0.07, 0.08, 0.09, or 0.10.

[0045] As an example, the peak position 2θ of the first diffraction peak is 31.3°-31.8°, for example, 31.3°, 31.4°, 31.5°, 31.6°, 31.7°, or 31.8°.

[0046] The first diffraction peak corresponds to sodium superionic conductor type materials (NASICON type materials), such as Na x Mn y M z Ti 2-y-z Diffraction peak of the (116) crystal plane of (PO4)3.

[0047] As an example, the peak position 2θ of the second diffraction peak is 32.7°-33.0°, for example, 32.7°, 32.8°, 32.9°, or 33.0°.

[0048] The second diffraction peak corresponds to the diffraction peak of the (222) crystal plane of electrochemically active Na4Mn3(PO4)2(P2O7).

[0049] In some embodiments, the M element includes one or more of V, Cr, and Zr, thereby facilitating stable doping of the M element.

[0050] As an example, V 3+ Cr 2+ The ionic radius of Ti is larger than that of Ti.

[0051] As an example, the bond energy of a Zr—O bond is greater than the bond energy of a Ti—O bond.

[0052] In some embodiments, the positive electrode active material is a composite material formed by a sodium superionic conductor type material (NASICON type material) and sodium manganese pyrophosphate in a molar ratio of (1-u):u, wherein the sodium superionic conductor type material satisfies the chemical formula Na x Mn y M z Ti 2-y-z (PO4)3, sodium manganese pyrophosphate satisfies the chemical formula Na4Mn3(PO4)2(P2O7).

[0053] As an example, the chemical formula of the positive electrode active material can be expressed as (1-u)Na x Mn y M z Ti 2-y-z (PO4)3·uNa4Mn3(PO4)2(P2O7).

[0054] 0.06≤u≤0.10, 0.9≤1-u≤0.94, indicating that the cathode active material is a composite phase with the Na3MnTi(PO4)3 system cathode material as the primary phase and Na4Mn3(PO4)2(P2O7) as the secondary phase. The coexistence of these two phases in the cathode active material results in a large interplanar spacing and wide sodium ion diffusion channels in the bulk phase, which is beneficial for improving the kinetic performance of the cathode active material and, in turn, its capacity.

[0055] In some embodiments, by controlling the molar content of each element, the proportion of manganese in the total amount of transition metal elements is increased, which helps to form electrochemically active Na4Mn3(PO4)2(P2O7) and Na x Mn y M z Ti 2-y-z The coexistence of the two phases (PO4)3 can reduce the charge transfer impedance of the positive electrode active material and improve the capacity and cycle performance of the positive electrode active material compared to the positive electrode material of the Na3MnTi(PO4)3 system; compared to the material directly mixing Na3MnTi(PO4)3 with Na4Mn3(PO4)2(P2O7), the Na4Mn3(PO4)2(P2O7) phase and Na in the composite phase material of the present application arex Mn y M z Ti 2-y-z The (PO4)3 phase is manganese valence, and the discharge voltage is higher.

[0056] In some embodiments, 1.5<y / (2-yz)≤2.7; alternatively, 1.6<y / (2-yz)≤2.6. This can improve the specific capacity of the positive electrode active material.

[0057] As an example, y / (2-yz) can be 1.51, 1.55, 1.6, 1.65, 1.7, 1.75, 1.8, 1.85, 1.9, 1.95, 2.0, 2.05, 2.1, 2.15, 2.2, 2.25, 2.3, 2.35, 2.4, 2.45, 2.5, 2.55, 2.6 or 2.65.

[0058] When y / (2-yz) is within the aforementioned range, the ratio of the Mn element and the Ti element content in the NASICON-type material in the composite phase of the positive electrode active material is high, so that the Mn element with a higher discharge voltage can exert a greater capacity, and the ionic radius of Mn is larger than the ionic radius of Ti. After increasing the Mn content, the unit cell volume is further increased, the space for lithium ion transmission is larger, the obstacles are less, the transmission rate is faster, and the capacity of the positive electrode active material is improved.

[0059] In some embodiments, the X-ray diffraction pattern of the positive electrode active material has a first diffraction peak within 2θ of 31.3°-31.8°, the peak intensity of the first diffraction peak is I(116), and a second diffraction peak within 2θ of 32.7°-33.0°, the peak intensity of the second diffraction peak is I(222), and 0.06≤I(222) / I(116)≤0.10. Thus, the sodium ion transmission rate can be increased, the kinetic properties of the positive electrode active material can be improved, and the capacity can be increased.

[0060] When the X-ray diffraction pattern of the positive electrode active material has both the second diffraction peak and the first diffraction peak within the aforementioned corresponding 2θ range, it indicates that the positive electrode active material contains both the (222) crystal plane representing the electrochemically active Na4Mn3(PO4)2(P2O7) and the diffraction peak of the (116) crystal plane representing the sodium superion conductor type material, that is, the positive electrode active material is a composite phase of the sodium superion conductor type material and Na4Mn3(PO4)2(P2O7) with certain electrochemical activity. The interplanar spacing of the positive electrode active material is large, and the sodium ion diffusion channel in the bulk phase of the positive electrode active material is wide, which is beneficial to improving the kinetic performance of the positive electrode active material and improving the capacity performance.

[0061] As an example, I(222) / I(116) may be 0.06, 0.07, 0.08, 0.09, or 0.10.

[0062] When I(222) / I(116) is within the aforementioned range, the (222) crystal plane strength of electrochemically active Na4Mn3(PO4)2(P2O7) increases, which is beneficial to the improvement of the kinetic properties of the positive electrode active material.

[0063] In some embodiments, the half-maximum width of the first diffraction peak is 0.20-0.37, and the half-maximum width of the second diffraction peak is 0.22-0.37. Therefore, the positive electrode active material has better crystallinity, higher sodium ion transport channel stability, and better cycle performance.

[0064] As an example, the half-value width of the first diffraction peak may be 0.20, 0.24, 0.28, 0.32, 0.36 or 0.37.

[0065] As an example, the half-value width of the second diffraction peak may be 0.22, 0.25, 0.28, 0.31, 0.34 or 0.37.

[0066] When the half-maximum widths of the first diffraction peak and the second diffraction peak are independently within the aforementioned ranges, Na4Mn3(PO4)2(P2O7) and Na x Mn y M z Ti 2-y-z The crystallinity of (PO4)3 is better, the crystallinity of the positive electrode active material is better, the proportion of the crystalline phase in the bulk phase is higher, the transmission channel of sodium ions in the bulk structure with better crystallinity is more stable, and the cycle performance of the positive electrode active material is better.

[0067] In some embodiments, the peak area of the first diffraction peak is S(116), the peak area of the second diffraction peak is S(222), and 0.057≤S(222) / S(116)≤0.065. Thus, the proportion of electrochemically active crystal planes is relatively high, which is beneficial for improving the kinetic properties of the positive electrode active material and enhancing the capacity.

[0068] As an example, the ratio of the peak area of the second diffraction peak to the peak area of the first diffraction peak, S(222) / S(116), may be 0.057, 0.058, 0.059, 0.060, 0.061, 0.062, 0.063, 0.064, or 0.065.

[0069] As an example, the diffraction peak intensity, half-width, and peak area of different crystal planes of the positive electrode active material can be measured using an X-ray diffractometer. Specifically, the operating voltage is 40 kV, the operating current is 250 mA, and continuous scanning is used at a scanning speed of 4° / min, a step size of 0.02°, and a scanning angle of 10°-80°.

[0070] In some embodiments, the positive electrode active material further comprises a carbon coating layer, the carbon coating layer being located on at least a portion of the surface of the positive electrode active material, and having a mass fraction of 10% to 12% based on the total mass of the carbon coating layer and the positive electrode active material, thereby helping to improve the ionic conductivity and electronic conductivity of the positive electrode active material.

[0071] As an example, the mass fraction of the carbon coating layer may be 10%, 10.5%, 11%, 11.5% or 12% based on the total mass of the carbon coating layer and the positive electrode active material.

[0072] When the mass fraction of the carbon coating layer is within the aforementioned range, a mixture of an organic carbon source and an inorganic carbon source can be used to form the carbon coating layer. On the one hand, the organic carbon source can act as a complexing agent during the formation of the coating layer, allowing the functional groups of the inorganic carbon to bind to the transition metal, which not only uniformly disperses the transition metal but also forms a three-dimensional electron transmission channel from the bulk phase to the surface of the positive electrode active material, thereby increasing the outward transmission speed of bulk electrons and improving the bulk conductivity. On the other hand, the high ionic conductivity of the inorganic carbon source can be used to form an electron transmission network between the positive electrode active material particles, thereby increasing the electron transmission efficiency of the positive electrode active material in the positive electrode sheet and improving the charge and discharge efficiency.

[0073] In some embodiments, increasing the content of the Mn element to form two electrochemically active phases, doping the M element in the positive electrode active material, and providing a carbon coating layer on at least part of the surface of the positive electrode active material can help to exert a synergistic effect, thereby obtaining a positive electrode active material with high specific capacity, high ionic conductivity and electronic conductivity, more stable structure, and fewer side reactions.

[0074] In some embodiments, when an inorganic carbon source is used as the carbon source, the inorganic carbon source may form interlayer gaps during the coating process, thereby increasing the porosity of the positive electrode active material.

[0075] In some embodiments, the charge transfer resistance of the positive electrode active material is 900Ω-1200Ω. Therefore, the positive electrode active material has high electronic conductivity.

[0076] As an example, the charge transfer resistance of the positive active material may be 900Ω, 1000Ω, 1100Ω, or 1200Ω.

[0077] In some embodiments, the porosity of the positive electrode active material is 1%-3%.

[0078] As an example, the porosity of the positive electrode active material may be 1%, 2%, or 3%.

[0079] Based on the provision of the carbon coating layer, the porosity of the positive electrode active material can be within the aforementioned range, thereby increasing the contact area between the positive electrode active material and the electrolyte and increasing the number of electrochemical reaction active sites.

[0080] In some embodiments, the compacted density of the positive electrode active material is 2 g / cm 3 -3g / cm 3 .

[0081] As an example, the compacted density of the positive electrode active material may be 2 g / cm 3 , 2.5g / cm 3 or 3g / cm 3 .

[0082] When the compaction density of the positive electrode active material is within the aforementioned range, the positive electrode active material layer using the positive electrode active material has a higher compaction density, which helps to improve the energy density of the battery.

[0083] In a second aspect of the present application, the present application proposes a method for preparing the aforementioned positive electrode active material by a sol-gel method, which reduces the preparation cost of the positive electrode active material and facilitates large-scale production. Specifically, the method comprises:

[0084] S1: Disperse the sodium source, phosphorus source, titanium source, manganese source, and M source in a solvent to form a gel, and dry to obtain a precursor.

[0085] In some embodiments, in this step, the sodium source, phosphorus source, titanium source, and manganese source in the required stoichiometric amounts are mixed in proportion according to the target chemical formula of the positive electrode active material, dispersed in a solvent, and heated and stirred to form a gel, which is then dried to obtain a precursor, so that the positive electrode active material can be obtained through subsequent sintering treatment.

[0086] As an example, the positive electrode active material satisfies the target chemical formula: Na x(1-u)+4u Mn y(1-u)+3u M z(1-u) Ti (2-y-z)(1-u) (PO4) 3(1-u)+2u (P2O7) u , wherein, 2≤x<4, 1.2≤y≤1.42, 0≤z≤0.05, 0.06≤u≤0.10; the ionic radius of the M element is larger than the ionic radius of the Ti element, and / or the bond energy of the MO bond is larger than the bond energy of the Ti-O bond.

[0087] In some embodiments, in this step, the desired stoichiometric amounts of the sodium source, phosphorus source, titanium source, manganese source, M source, and carbon source can be mixed in a proportion according to the target chemical formula of the positive electrode active material, dispersed in a solvent, and heated and stirred to form a gel. Thus, the positive electrode active material containing the carbon coating layer can be simultaneously formed in a one-step process.

[0088] In some embodiments, the sodium source includes one or more of sodium carbonate, sodium hydroxide, sodium nitrate, sodium acetate, and sodium dihydrogen phosphate.

[0089] In some embodiments, the manganese source includes one or more of a manganese-containing oxide, a manganese-containing phosphate, a manganese-containing sulfate, a manganese-containing chloride, a manganese-containing nitrate, a manganese-containing carbonate, and a manganese-containing acetate.

[0090] In some embodiments, the titanium source includes one or more of titanium-containing phosphates, titanium-containing acetates, titanium-containing sulfates, titanium-containing chlorides, titanium-containing nitrates, titanium-containing carbonates, and titanium-containing organics.

[0091] In some embodiments, the phosphorus source includes one or more of H3PO4, NH4H2PO4, (NH4)2HPO4, (NH4)3PO4, and P2O5.

[0092] In some embodiments, the M source includes one or more of oxides, phosphates, sulfates, chlorides, nitrates, and carbonates corresponding to the M element.

[0093] In some embodiments, the solvent includes one or more of deionized water, ethanol, and ethylene glycol. Thus, the raw materials are widely available and compatible with existing production lines, which helps reduce production costs.

[0094] In some embodiments, the carbon source includes an organic carbon source and an inorganic carbon source, with the mass ratio of the organic carbon source to the inorganic carbon source being 1.8-2. Thus, the organic carbon source can form a three-dimensional electron transport channel from the bulk to the surface of the positive electrode active material, which, combined with the high ionic conductivity of the inorganic carbon source, can improve the electronic and ionic conductivity of the positive electrode active material.

[0095] As an example, the feed mass ratio of the organic carbon source to the inorganic carbon source can be 1.8, 1.85, 1.9, 1.95 or 2.

[0096] When the carbon source includes an organic carbon source and an inorganic carbon source, in the process of forming the carbon coating layer, on the one hand, the organic carbon source can play a chelating role in the process of forming the coating layer, forming a three-dimensional electron transmission channel from the bulk phase to the surface of the positive electrode active material, thereby increasing the outward transmission speed of bulk electrons and increasing the bulk conductivity; on the other hand, utilizing the high ionic conductivity characteristics of the inorganic carbon source, an electron transmission network can be formed between the positive electrode active material particles, thereby increasing the electron transmission efficiency of the positive electrode active material in the positive electrode sheet and improving the charge and discharge efficiency.

[0097] In some embodiments, the organic carbon source includes one or more of citric acid, polypyrrole, and polyvinylpyrrolidone.

[0098] The organic carbon source will combine with the transition metal in the positive electrode active material during the process of forming the coating layer, thereby improving the dispersion uniformity of the transition metal in the positive electrode active material.

[0099] In some embodiments, the inorganic carbon source includes one or more of graphite, acetylene black, graphene, and carbon nanotubes. Thus, the carbon source is widely available and can synergistically improve the electronic and ionic conductivity of the positive electrode active material.

[0100] S2: Sintering the precursor in a non-oxidizing atmosphere

[0101] In some embodiments, in this step, the precursor is sintered in a non-oxidizing atmosphere to obtain a positive electrode active material. Sintering the precursor in a non-oxidizing atmosphere not only facilitates controlled carbonization of the organic carbon source, but also maintains the Mn and M elements in a low chemical valence state, thereby increasing the specific capacity of the positive electrode active material and the capacity of the positive electrode active material during sodium intercalation and deintercalation.

[0102] As an example, the non-oxidizing atmosphere may be nitrogen or argon.

[0103] In some embodiments, the sintering temperature is 500° C. to 700° C., and the sintering time is 8 h to 20 h. Thus, a two-phase composite positive electrode active material with good crystallinity can be formed.

[0104] As an example, the temperature of the sintering process may be 500°C, 600°C or 700°C.

[0105] As an example, the sintering treatment time may be 8 hours, 12 hours, 16 hours or 20 hours.

[0106] When the temperature and time of the sintering treatment are within the above ranges, a positive electrode active material in which two phases coexist can be obtained.

[0107] In some embodiments, a dispersant may be added when dispersing the raw materials to further improve the dispersion uniformity.

[0108] In other embodiments, the method for preparing the above-mentioned polyanion positive electrode active material can be prepared by a spray drying method, specifically, comprising mixing and dispersing a Na source, a Ti source, a Mn source, a P source, an M source, and a carbon source in a solvent, and obtaining a precursor after dispersion and spray drying; sintering the precursor in a non-oxidizing atmosphere, and the sintering temperature is 500°C-700°C to obtain the positive electrode active material.

[0109] As an example, the inlet air temperature of the spray drying is 180° C.-210° C., the outlet air temperature is 80° C.-110° C., and the feed speed is 20 rpm-23 rpm.

[0110] It should be noted that the preparation method of the above-mentioned positive electrode active material can refer to part of the above-mentioned preparation method, and the relevant parameters of the above-mentioned positive electrode active material can refer to part or all of the technical features in the above-mentioned embodiment. The parts not described in the preparation method embodiment can also refer to the above-mentioned embodiment and related drawings, and will not be repeated here.

[0111] The above description of the various embodiments tends to emphasize the differences between the various embodiments. The same or similar aspects can be referenced with each other and will not be repeated herein for the sake of brevity.

[0112] In a third aspect, the present application provides a positive electrode sheet comprising: a positive electrode current collector and a positive electrode active material layer located on at least one side of the positive electrode current collector, wherein the positive electrode active material layer comprises the aforementioned positive electrode active material, or a positive electrode active material prepared using the aforementioned method. Thus, the positive electrode sheet possesses all the features and advantages of the aforementioned positive electrode active material and method for preparing the positive electrode active material, and no further details are given here.

[0113] In a fourth aspect of the present application, the present application provides a battery comprising the aforementioned positive electrode sheet. Thus, the battery has all the features and advantages of the aforementioned positive electrode sheet, which will not be described in detail here.

[0114] In some embodiments, the electrical device may use the aforementioned battery for power supply or energy storage.

[0115] As examples, electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as 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., but are not limited to these.

[0116] The present invention will be described below by way of specific examples. It should be noted that the following examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. Where specific techniques or conditions are not specified in the examples, they are determined according to the techniques or conditions described in the literature in this area or according to the product specifications. Reagents or instruments not specified by manufacturer are all commercially available conventional products.

[0117] Example 1

[0118] 1. Preparation of positive electrode active materials

[0119] (1) 6.4 mmol of citric acid was dissolved in 160 ml of deionized water and stirred for 0.5 h to obtain a transparent solution. 10.5 mmol of sodium acetate, 3.9 mmol of manganese acetate tetrahydrate, 0.05 mmol of ammonium vanadate, 2.1 mmol of di(2-hydroxypropionic acid)diammonium dihydroxide titanium and 9 mmol of ammonium dihydrogen phosphate were added in sequence and stirred for 0.5 h to obtain a homogeneous solution. The mixture was heated in a water bath at 80°C with stirring until a gel was obtained. The mixture was dried in an oven at 110°C overnight and ground to obtain a precursor.

[0120] (2) The precursor obtained in step (1) was placed in a tube furnace for sintering at a heating rate of 1.5°C / min, a sintering temperature of 550°C, a sintering time of 10 h, and a sintering atmosphere of nitrogen to obtain the positive electrode active material 0.94Na 3.45 Mn 1.2 Ti 0.75 V 0.05 (PO4)3·0.06Na4Mn3(PO4)2(P2O7) sample.

[0121] The differences between the remaining embodiments and comparative examples and embodiment 1 are shown in Table 1. When the doping element M is Cr, Zr, Al, and Zn, the M source is the nitrate corresponding to the corresponding element:

[0122]

[0123]

[0124] The following tests were performed on the positive electrode active materials in the examples and comparative examples. The test results are shown in Table 2:

[0125] XRD test: operating voltage 40 kV, operating current 250 mA, continuous scanning, scanning speed 4° / min, step size 0.02°, scanning angle 10°-80°.

[0126] Table 2

[0127]

[0128] Preparation of the battery:

[0129] Preparation of positive electrode sheets: The positive electrode active material, acetylene black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 80:10:10, coated on aluminum foil and dried, and stamped into positive electrode sheets with a diameter of 12 mm and a thickness of 120 μm using a pressure of 100 MPa. The positive electrode sheets were then placed in a vacuum drying oven and dried at 120°C for 12 hours.

[0130] Negative electrode: Use a Na metal sheet with a diameter of 17 mm and a thickness of 1 mm.

[0131] Separator membrane: A Celgard 2400 porous membrane with a thickness of 25 μm was used.

[0132] Electrolyte: A NaPF6 solution with a concentration of 1 mol / L was used, and the solvent was a mixture of equal volumes of ethylene carbonate (EC) and diethyl carbonate (DEC).

[0133] Assemble the battery: Assemble the positive electrode sheet, separator, negative electrode sheet and 4 μL electrolyte into a 2025 button cell in an Ar gas glove box with a water content and an oxygen content of less than 5 ppm.

[0134] The following tests were performed on the batteries in the examples and comparative examples. The test results are shown in Table 3:

[0135] Initial efficiency: Test the battery in a battery test cabinet. First, charge the battery to 4.3V at a constant current and constant voltage at a rate of 0.1C. After ten minutes, discharge the battery to 2.5V at a constant current at a rate of 0.1C. Divide the specific capacity obtained during the discharge process by the specific capacity obtained during the charging process. The ratio is the initial efficiency.

[0136] Discharge specific capacity: The battery is tested in a battery test cabinet, charged to 4.3V at a constant current and constant voltage at a rate of 1C. After ten minutes, it is discharged to 2.5V at a constant current at a rate of 1C. The specific capacity obtained during the discharge process is the 1C discharge specific capacity.

[0137] 3. Cycle capacity retention rate

[0138] The battery was tested in a battery test cabinet, charged to 4.3V at a constant current and constant voltage at a rate of 1C. After standing for ten minutes, it was discharged to 2.5V at a constant current at a rate of 1C. The above process was repeated 100 times. The ratio of the specific capacity obtained in the 100th discharge process to the specific capacity obtained in the first discharge process is the 100-cycle retention rate.

[0139] Table 3

[0140]

[0141]

[0142] See also Figure 2 In addition to the corresponding diffraction peak in the standard spectrum of Na3MnTi(PO4)3 (PDF#00-069-0231), the positive electrode active material in Example 1 has a diffraction peak at 2θ of 32.96°, which is the characteristic peak of Na4Mn3(PO4)2(P2O7), indicating that the positive electrode active material in Example 1 has a two-phase composite structure.

[0143] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode active material, characterized in that include: The positive electrode active material satisfies the chemical formula: Na x(1-u)+4u Mn y(1-u)+3u M z(1-u) Ti (2-y-z)(1-u) (PO4) 3(1-u)+2u (P2O7) u , where 2≤x<4, 1.2≤y≤1.42, 0≤z≤0.05, 0.06≤u≤0.10; the ionic radius of the M element is greater than the ionic radius of the Ti element, and / or the bond energy of the MO bond is greater than the bond energy of the Ti-O bond; wherein, The positive electrode active material has an X-ray diffraction pattern with a first diffraction peak at a position of 31.3°-31.8° 2θ and a second diffraction peak at a position of 32.7°-33.0° 2θ.

2. The positive electrode active material according to claim 1, characterized in that The M element includes one or more of V, Cr, and Zr.

3. The positive electrode active material according to claim 1, characterized in that The positive electrode active material is a composite material formed by a sodium super ion conductor material and sodium manganese pyrophosphate in a molar ratio of (1-u):u. The sodium super ion conductor material satisfies the chemical formula Na x Mn y M z Ti 2-y-z (PO4)3, the sodium manganese pyrophosphate satisfies the chemical formula Na4Mn3(PO4)2(P2O7).

4. The positive electrode active material according to claim 3, characterized in that 1.5<y / (2-yz)≤2.7; Optionally, 1.6<y / (2-yz)≤2.

6.

5. The positive electrode active material according to claim 3, characterized in that The peak intensity of the first diffraction peak is I(116), the peak intensity of the second diffraction peak is I(222), 0.06≤I(222) / I(116)≤0.10; and / or, the half-value width of the first diffraction peak is 0.20-0.37, and the half-value width of the second diffraction peak is 0.22-0.37; And / or, the peak area of the first diffraction peak is S(116), the peak area of the second diffraction peak is S(222), and 0.057≤S(222) / S(116)≤0.

065.

6. The positive electrode active material according to any one of claims 1 to 5, characterized in that Further comprising a carbon coating layer, the carbon coating layer being located at least on a portion of the surface of the positive electrode active material, and the mass fraction of the carbon coating layer being 10%-12% based on the total mass of the carbon coating layer and the positive electrode active material; And / or, the charge transfer impedance of the positive electrode active material is 900Ω-1200Ω.

7. The positive electrode active material according to any one of claims 1 to 5, characterized in that The porosity of the positive electrode active material is 1%-3%; And / or, the compaction density of the positive electrode active material is 2g / cm 3 -3g / cm 3 .

8. A method for preparing the positive electrode active material according to any one of claims 1 to 7, characterized in that: include: According to the target chemical formula of the positive electrode active material, a sodium source, a phosphorus source, a titanium source, a manganese source, and an M source are dispersed in a solvent to form a gel, and dried to obtain a precursor; The precursor is sintered in a non-oxidizing atmosphere to obtain the positive electrode active material. Wherein, the positive electrode active material satisfies the target chemical formula: Na x(1-u)+4u Mn y(1-u)+3u M z(1-u) Ti (2-y-z)(1-u) (PO4) 3(1-u)+2u (P2O7) u , where 2≤x<4, 1.2≤y≤1.42, 0≤z≤0.05, 0.06≤u≤0.10; the ionic radius of the M element is larger than the ionic radius of the Ti element, and / or the bond energy of the MO bond is larger than the bond energy of the Ti-O bond.

9. The method according to claim 8, characterized in that The sodium source, the phosphorus source, the titanium source, the manganese source, the M source, and the carbon source are dispersed in the solvent to form a gel.

10. The method according to claim 8 or 9, characterized in that The sodium source includes one or more of sodium carbonate, sodium hydroxide, sodium nitrate, sodium acetate, and sodium dihydrogen phosphate; and / or, The manganese source includes one or more of manganese oxides, manganese phosphates, manganese sulfates, manganese chlorides, manganese nitrates, manganese carbonates, and manganese acetates; and / or, The titanium source includes one or more of titanium-containing phosphates, titanium-containing acetates, titanium-containing sulfates, titanium-containing chlorides, titanium-containing nitrates, titanium-containing carbonates, and titanium-containing organic matter; and / or, The phosphorus source includes one or more of H3PO4, NH4H2PO4, (NH4)2HPO4, (NH4)3PO4, P2O5; and / or, The M source includes one or more of oxides, phosphates, sulfates, chlorides, nitrates, and carbonates corresponding to the M element; and / or, The solvent includes one or more of deionized water, ethanol, and ethylene glycol.

11. The method according to claim 9, characterized in that The carbon source includes an organic carbon source and an inorganic carbon source, and the feeding mass ratio of the organic carbon source to the inorganic carbon source is 1.8-2; Preferably, the organic carbon source includes one or more of citric acid, polypyrrole, and polyvinyl pyrrolidone; and / or the inorganic carbon source includes one or more of graphite, acetylene black, graphene, and carbon nanotubes.

12. The method according to claim 8, characterized in that The sintering temperature is 500° C.-700° C., and the sintering time is 8 h-20 h.

13. A positive electrode plate, characterized in that: include: A positive electrode current collector and a positive electrode active material layer located on at least one side of the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode active material according to any one of claims 1 to 7, or the positive electrode active material prepared by the method according to any one of claims 8 to 12.

14. A battery, characterized in that: Including the positive electrode sheet according to claim 13.

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