Composite positive electrode active material and preparation method therefor and use thereof

Abstract

Embodiments of the present application provide a composite positive electrode active material and a preparation method therefor and a use thereof. The composite positive electrode active material comprises positive electrode active materials and nitrogen-doped carbon nanotubes, and the nitrogen-doped carbon nanotubes comprise graphite nitrogen. The composite positive electrode active material can achieve high electronic conductivity at a low carbon nanotube content, and can be used to provide a secondary battery having excellent overall performance.

Description

Composite positive electrode active material and preparation method and application thereof

[0001] Cross-reference to Related Applications

[0002] The present application claims priority to the Chinese patent application No. 202410641769.5, filed on May 20, 2024, and entitled "Composite positive electrode active material and preparation method and application thereof", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0003] The present application relates to the technical field of batteries, in particular to a composite positive electrode active material and a preparation method and application thereof. BACKGROUND

[0004] In the industry, the rate performance and low-temperature performance of secondary batteries are often optimized by improving the electronic conductivity of positive electrode active materials, for example, by coating the surface of positive electrode active material particles with conductive carbon, or by mixing positive electrode active material particles with conductive agents such as carbon nanotubes, but the improvement of the electronic conductivity of positive electrode active materials by the above two methods is limited.

[0005] SUMMARY

[0006] In view of this, the present application provides a composite positive electrode active material and a preparation method and application thereof. The composite positive electrode active material can achieve higher electronic conductivity at a lower carbon nanotube dosage, and can be used to provide a secondary battery with better comprehensive performance.

[0007] The first aspect of the embodiments of the present application provides a composite positive electrode active material, comprising a positive electrode active material and nitrogen-doped carbon nanotubes, wherein the nitrogen-doped carbon nanotubes comprise graphite nitrogen.

[0008] Nitrogen doping can effectively improve the electrical conductivity of carbon nanotubes, and further controlling the nitrogen content in the carbon nanotubes within the above range can build sufficient electronic paths inside the nitrogen-doped carbon nanotubes. Graphitic nitrogen is more conducive to improving the electrical conductivity of carbon nanotubes, thereby effectively improving the electrical conductivity of the composite positive electrode active material. Further, based on the structural properties of nitrogen-doped carbon nanotubes, they can form a conductive network in the composite positive electrode active material. Therefore, compared with related technologies, the composite positive electrode active material can achieve higher electronic conductivity at a comparable carbon nanotube dosage, and can provide a secondary battery with better rate performance.

[0009] The second aspect of the embodiments of the present application provides a preparation method of a composite positive electrode active material, comprising:

[0010] Mixing the nitrogen-containing carbon source and the positive electrode active material precursor, calcining to obtain the composite positive electrode active material; wherein the nitrogen-containing carbon source comprises at least one of monocyannamide, dicyannamide, melamine and urea.

[0011] The composite positive electrode active material comprises a positive electrode active material and nitrogen-doped carbon nanotubes, and the nitrogen-doped carbon nanotubes comprise graphitic nitrogen.

[0012] The preparation method is simple and easy to implement, has strong controllability, high production efficiency, and is suitable for large-scale industrial production.

[0013] The third aspect of the embodiments of the present application provides a positive electrode comprising the composite positive electrode active material provided by the first aspect of the embodiments of the present application. Since the composite positive electrode active material provided by the embodiments of the present application is used, the resistivity of the positive electrode is lower under the condition that the carbon content in the positive electrode is equal.

[0014] The fourth aspect of the embodiments of the present application provides a secondary battery comprising the positive electrode provided by the third aspect of the embodiments of the present application. Since the positive electrode provided by the embodiments of the present application is used, the secondary battery has a relatively good rate performance.

[0015] The fifth aspect of the embodiments of the present application provides an electric device comprising the secondary battery provided by the fourth aspect of the embodiments of the present application. Since the secondary battery provided by the embodiments of the present application is used, the electric device has high market competitiveness. BRIEF DESCRIPTION OF DRAWINGS

[0016] FIG. 1 is a schematic diagram of different configurations of nitrogen atoms in nitrogen-doped carbon nanotubes;

[0017] FIG. 2 is a schematic diagram of the structure of the composite positive electrode active material provided by an embodiment of the present application;

[0018] FIG. 3 is a partial scanning electron microscope (SEM) photograph of the composite positive electrode active material prepared in Example 1 of the present application;

[0019] FIG. 4 is an X-ray photoelectron spectroscopy (XPS) spectrum of the nitrogen element of the composite positive electrode active material prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0020] The embodiments of the present application provide a composite positive electrode active material comprising a positive electrode active material and nitrogen-doped carbon nanotubes; and the nitrogen-doped carbon nanotubes comprise graphitic nitrogen.

[0021] The nitrogen doping can effectively improve the electrical conductivity of the carbon nanotubes, and further controlling the nitrogen content in the carbon nanotubes within the above range can construct sufficient electronic paths inside the nitrogen-doped carbon nanotubes. The doping types of the nitrogen element in the nitrogen-doped carbon nanotubes generally include graphite nitrogen, pyridine nitrogen, pyrrole nitrogen and oxidized nitrogen. Referring to FIG. 1, the graphite nitrogen replaces the carbon atoms inside the carbon skeleton of the carbon nanotubes, which can form a non-planar sp3 hybrid with the three carbon atoms around it, and is more conducive to improving the electrical conductivity of the carbon nanotubes. Further, based on the structural properties of the nitrogen-doped carbon nanotubes, the nitrogen-doped carbon nanotubes can form a conductive network when being combined with the positive electrode active material. Therefore, compared with the related art, the above composite positive electrode active material can achieve higher electronic conductivity under the condition of the same amount of carbon nanotubes, which can be specifically manifested as lower powder resistivity. It can be understood that the improvement of the electronic conductivity is conducive to optimizing the rate performance and low-temperature performance of the material.

[0022] In some embodiments of the present application, the mass percentage of the nitrogen element in the nitrogen-doped carbon nanotubes is 20%-40%, and the mass percentage of the graphite nitrogen in the total amount of the nitrogen element is ≥20%. Specifically, the mass content of the nitrogen element in the nitrogen-doped carbon nanotubes can be but is not limited to 20%, 22%, 24%, 25%, 28%, 30%, 32%, 35%, 38%, or 40%. A suitable total amount of nitrogen doping can make there be more nitrogen elements in the carbon nanotubes, and there be less defect structure in the carbon nanotubes, so that there are sufficient carbon atoms to form the basic structure of the carbon nanotubes, thereby greatly improving the electrical conductivity of the carbon nanotubes.

[0023] In some embodiments of the present application, the mass percentage of the graphite nitrogen in the total mass of the nitrogen element in the nitrogen-doped carbon nanotubes is ≥20%, which can significantly improve the electrical conductivity of the nitrogen-doped carbon nanotubes. Specifically, the mass percentage of the graphite nitrogen in the total mass of the nitrogen element can be but is not limited to 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, etc. The higher the mass percentage of the graphite nitrogen, the more conducive to improving the electrical conductivity of the carbon nanotubes. It should be noted that when the mass percentage of the graphite nitrogen in the nitrogen element is <100%, the other nitrogen elements can include one or more of pyridine nitrogen, pyrrole nitrogen and oxidized nitrogen, which is not limited in the present application.

[0024] In the embodiments of the present application, the mass percentage of the nitrogen element in the carbon nanotubes can be tested by an X-ray fluorescence spectrometer (XRF) or an energy dispersive spectrometer (EDS).

[0025] In the embodiments of the present application, the proportion of graphite nitrogen in the nitrogen element can be tested by X-ray photoelectron spectroscopy (XPS). Specifically, the composite cathode active material is placed in XPS for testing, the N element characteristic peak obtained is subjected to peak fitting, and the mass proportion of graphite nitrogen is calculated. The peak position of the characteristic peak of pyridine nitrogen is 398.2 eV; the peak position of the characteristic peak of pyrrole nitrogen is 399.5 eV; the peak position of the characteristic peak of graphite nitrogen is 400.8 eV; and the peak position of the characteristic peak of oxidized nitrogen is 402.6 eV.

[0026] In some embodiments of the present application, the proportion of graphite nitrogen in the total mass of nitrogen element is 20%-72%. In this way, the conductivity of the composite cathode active material can be ensured to be relatively high, and the composite cathode active material is easy to prepare. Specifically, the proportion of graphite nitrogen in the total mass of nitrogen element can be, but is not limited to, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, 70%, 71%, 72%.

[0027] In some embodiments of the present application, referring to FIG. 2, the composite cathode active material 1 comprises secondary particles 10, the secondary particles 10 comprise a plurality of primary particles 11 of the cathode active material and a plurality of nitrogen-doped carbon nanotubes 12, and the nitrogen-doped carbon nanotubes 12 are in-situ grown on the surface of at least some of the primary particles 11 of the cathode active material. For example, when the cathode active material is lithium iron phosphate, the nitrogen-doped carbon nanotubes are in-situ grown on the surface of the lithium iron phosphate primary particles. In this way, during the formation of the secondary particles, the carbon nanotubes in-situ grown on the surface of the primary particles are dispersed in the secondary particles along with the agglomeration of the primary particles. Compared with the cathode active material containing carbon nanotubes in the related art, the risk of agglomeration of the nitrogen-doped carbon nanotubes in the embodiments of the present application is extremely small, a relatively high degree of uniformity of the conductive network can be constructed in the secondary particles, and therefore the electronic conductivity of the composite cathode active material can be significantly improved, so that the electron transport capacity in the secondary particles can be greatly improved. In addition, the carbon nanotubes in-situ grown on the surface of the primary particles have strong adhesion to the cathode active material, and the structural stability of the conductive network is good. In the subsequent pulping process and the final charging and discharging cycle process, the conductive network in the secondary particles can stably exist.

[0028] It should be noted that FIG. 2 is only an exemplary drawing. The size, number and distribution of the nitrogen-doped carbon nanotubes in FIG. 2 do not constitute any limitation on the present application; the number, size and arrangement of the primary particles of the cathode active material in FIG. 2 are also exemplary drawings, and do not constitute any limitation on the present application.

[0029] In addition, based on the good electrical conductivity of the secondary particles, the rate performance and low-temperature performance can be achieved without controlling the particle size D50 of the secondary particles of the composite positive electrode active material in a small range. In some embodiments of the present application, the particle size D50 of the secondary particles of the composite positive electrode active material particles is 5-10 μm. In this way, the rate performance, low-temperature performance and power performance can be ensured, and the subsequent application of the composite positive electrode active material is also facilitated, for example, the composite positive electrode active material is easy to disperse when preparing a positive electrode slurry, and a uniform and stable slurry is obtained. Specifically, the particle size D50 of the secondary particles can be, but is not limited to, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm. In the embodiments of the present application, the particle size D50 refers to the particle size corresponding to the cumulative volume percentage of 50% of the composite positive electrode active material, which can be measured by a laser particle size analyzer. In the embodiments of the present application, the secondary particles can be spherical, spheroidal, ellipsoidal or the like, and the present application does not limit the shape of the secondary particles or the sphericity thereof, which can be selected by a person skilled in the art according to the actual production situation.

[0030] In some embodiments of the present application, the average particle size of the primary particles of the positive electrode active material is 200-300 nm. In this way, the deintercalation / intercalation of active ions (lithium ions) is facilitated, the secondary particles with a suitable particle size are formed, and the carbon nanotubes form a conductive network inside the secondary particles. In the embodiments of the present application, the average particle size of the primary particles can be measured by a scanning electron microscope (SEM). Specifically, 200 secondary particles are selected in the field of view, 1 primary particle is selected for each secondary particle, the particle size thereof is measured, and the average value is taken. The average particle size of the primary particles can be, but is not limited to, 200 nm, 210 nm, 220 nm, 230 nm, 240 nm, 250 nm, 260 nm, 270 nm, 280 nm, 290 nm or 300 nm. In the embodiments of the present application, the primary particles can be spherical, spheroidal, ellipsoidal or the like, and the present application does not limit the shape of the primary particles or the sphericity thereof, which can be selected by a person skilled in the art according to the actual production situation.

[0031] In some embodiments of the present application, the mass percentage of the nitrogen-doped carbon nanotubes in the composite positive electrode active material is 0.5%-3%. In this way, on the one hand, the conductivity of the composite positive electrode active material can be ensured to be good, and on the other hand, the content of the positive electrode active material can be ensured, so that the specific capacity of the composite positive electrode active material is high, and therefore the secondary battery with good rate performance, low-temperature performance and high energy density can be provided. In the embodiments of the present application, the mass percentage of carbon elements in the composite positive electrode active material can be determined by using a sulfur-carbon analyzer, and the mass percentage of the nitrogen-doped carbon nanotubes in the composite positive electrode active material can be calculated in combination with the mass percentage of nitrogen elements in the carbon nanotubes. Specifically, the mass percentage of the nitrogen-doped carbon nanotubes in the composite positive electrode active material can be, but is not limited to, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.8%, 3.0%.

[0032] In some cases, for example, when the mass percentage of the nitrogen-doped carbon nanotubes in the composite positive electrode active material is 1.5%-3%, the composite positive electrode active material does not need to add an additional conductive agent in the positive electrode material layer of the final positive electrode when it is applied.

[0033] In view of the lithium iron phosphate-based positive electrode active material, especially the lithium manganese iron phosphate, has the potential to become the next generation of high-energy-density positive electrode material due to its high specific capacity and high voltage platform. However, the electronic conductivity of the lithium manganese iron phosphate is weak, so when the material of the primary particles of the positive electrode active material is the lithium manganese iron phosphate or the doped modified lithium manganese iron phosphate, the composite positive electrode active material has a greater improvement in the relative electronic conductivity. Therefore, considering the cost-effectiveness, in some embodiments of the present application, the positive electrode active material includes Li1-aAaMnxMyFe1-x-y(P1-bEb)O4, 0≤a<1, 0.4≤x+y≤0.9, 0≤b<1; A includes at least one of K+, Ca+, Na+ and NH4+, M includes at least one of Ti, V, Co, Ni, Cu, Zn, Mg, Ca, Al, Nb and Mo, and E includes at least one of N, Si and B. It can be understood that the positive electrode active material is a ternary positive electrode material or other positive electrode active material, which is also feasible, and the embodiments of the present application do not make any limitation thereon. In some specific embodiments, the positive electrode active material includes LiMnxFe1-yPO4.

[0034] In some embodiments of the present application, the room temperature powder resistivity of the composite positive electrode active material under a pressure of 200 MPa is 20 Ω·cm-100 Ω·cm. In some specific embodiments, the positive electrode active material is

[0035] When Li1-aAaMnxMyFe1-x-y(P1-bEb)O4, the powder resistivity of the composite cathode active material at room temperature under a pressure of 200 MPa can be 20 Ω·cm, 30 Ω·cm, 40 Ω·cm, 50 Ω·cm, 60 Ω·cm, 70 Ω·cm, etc. In the embodiments of the present application, room temperature is 25±2°C.

[0036] The embodiments of the present application also provide a preparation method of the composite cathode active material, comprising:

[0037] S01, mixing a nitrogen-containing carbon source and a cathode active material precursor, calcining to obtain a composite cathode active material; wherein the nitrogen-containing carbon source includes at least one of monocyannamide (molecular formula CH2N2), dicyannamide (molecular formula C2H4N4), melamine (molecular formula C3H6N6), and urea (molecular formula CH4N2O);

[0038] The composite cathode active material includes a cathode active material and nitrogen-doped carbon nanotubes, and the nitrogen-doped carbon nanotubes include graphite nitrogen.

[0039] The above preparation method is simple and easy to implement, has strong controllability, and has high production efficiency, and is suitable for large-scale industrial production. In particular, the above preparation method can grow nitrogen-doped carbon nanotubes in situ on the surface of the cathode active material particles, thereby avoiding the problems that the finished carbon nanotubes are difficult to disperse in the cathode active material particles and are prone to agglomeration in the related art. The composite cathode active material prepared by the above preparation method can have a uniform and good conductive network, so that the composite cathode active material can have better conductivity.

[0040] In some embodiments of the present application, in step S01, the cathode active material precursor can be prepared by a process known to those skilled in the art, but the difference is that the raw materials used to prepare the cathode active material precursor do not contain a carbon source. This is not limited by the present application. Taking the lithium manganese iron phosphate to be prepared as an example, a brief description is as follows: disperse the phosphorus source, manganese source, iron source, phosphorus source, and lithium source in a solvent according to the stoichiometric ratio of each element in the lithium manganese iron phosphate to be prepared, grind to obtain a uniform mixture, perform a hydrothermal reaction, wash and dry the obtained solid substance, and obtain the lithium manganese iron phosphate precursor. The above-mentioned phosphorus source includes but is not limited to phosphoric acid; the above-mentioned manganese source includes but is not limited to manganese sulfate; the above-mentioned iron source includes but is not limited to ferrous sulfate; and the above-mentioned lithium source includes but is not limited to lithium carbonate.

[0041] In some embodiments of the present application, the calcining in step S01 comprises a first calcining and a second calcining performed in sequence. The first calcining can make the nitrogen-containing carbon source form amorphous carbon doped with N elements with a two-dimensional layered structure, and the second calcining can make the carbon atoms in the amorphous carbon obtained by the first calcining rearrange to form nitrogen-doped carbon nanotubes.

[0042] In some embodiments of the present application, the holding temperature of the first calcining (hereinafter referred to as the first holding temperature) is 500-600°C, and the holding time is 1-3h. In this way, it is easy to obtain nitrogen-doped amorphous carbon. Moreover, it is beneficial to control the proportion of nitrogen elements in the finally obtained nitrogen-doped carbon nanotubes to be within the range of 20%-40%. It should be noted that, due to the escape of part of the nitrogen elements (for example, forming NH3, etc.) during the second calcining, in order to control the proportion of nitrogen elements in the carbon nanotubes to be within the range of 20%-40%, the nitrogen element content in the material obtained after the first calcining will be slightly higher, for example, reaching 30%-50%. Specifically, the first holding temperature can be, for example, 500°C, 520°C, 550°C, 580°C, 600°C, etc., and the holding time of the first calcining can be, for example, 1h, 1.5h, 2h, 2.5h, 3h, etc.

[0043] In some embodiments of the present application, the holding temperature of the second calcining (hereinafter referred to as the second holding temperature) is 650-900°C, and the holding time is 1-3h. In this way, it can promote the rearrangement of carbon atoms to form graphite carbon configuration, reduce the structural defects of the formed nitrogen-doped carbon nanotubes, and increase the proportion of graphite nitrogen, for example, increase the proportion of graphite nitrogen to more than 20%, thereby obtaining a composite positive electrode active material with better electrical conductivity. Specifically, the second holding temperature can be, for example, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C, etc., and the holding time of the second calcining can be, for example, 1h, 1.5h, 2h, 2.5h, 3h, etc.

[0044] In the embodiments of the present application, after the first calcining is completed in step S01, the temperature can be directly increased from the first holding temperature to the second holding temperature.

[0045] In the embodiments of the present application, the heating rate during the first calcining and the second calcining is not specifically limited, and can be determined by the actual production conditions such as the equipment conditions by those skilled in the art, for example, the temperature can be increased at a rate of 5-25°C / min. The heating rate of the first calcining and the heating rate of the second calcining can be the same or different.

[0046] In some embodiments of the present application, after step S01, the process further comprises step S02: removing impurities from the material obtained in step S01. In some specific embodiments, the material obtained in step S01 is etched in dilute hydrochloric acid for 10-48 hours to remove impurities (e.g., Fe, Fe3O4, Mn, Mn3O4, etc.).

[0047] In some embodiments of the present application, step S02 further comprises cleaning and drying the material obtained after removing impurities. The cleaning and drying can be a process known to those skilled in the art. Specifically, the drying can be baking and the like, and the drying can remove residual acid and moisture.

[0048] The present application also provides a positive electrode comprising the aforementioned composite positive electrode active material provided by the present application. Due to the composite positive electrode active material provided by the present application, the positive electrode has a lower resistivity under the same carbon content in the positive electrode. Therefore, the positive electrode can be used to provide a secondary battery with better rate performance, good low-temperature performance, and higher energy density.

[0049] In some embodiments of the present application, the positive electrode comprises a current collector and a positive electrode material layer disposed on the surface of the current collector, and the positive electrode material layer comprises the composite positive electrode active material.

[0050] In some embodiments of the present application, the positive electrode material layer further comprises a binder. In the present application, the positive electrode can be any current collector known in the art and suitable for a positive electrode of a lithium ion battery, for example, an aluminum foil. The binder can be any binder known in the art and suitable for a positive electrode of a lithium ion battery, and the present application does not limit the binder.

[0051] The present application also provides a secondary battery comprising the aforementioned positive electrode provided by the present application. Due to the positive electrode provided by the present application, the secondary battery can have better rate performance, good low-temperature performance, and higher energy density.

[0052] In some embodiments of the present application, the secondary battery comprises the aforementioned positive electrode, a negative electrode, and an electrolyte between the positive electrode and the negative electrode. In the present application, the secondary battery can be a liquid battery using a liquid electrolyte, in which case, the positive electrode and the negative electrode further comprise a separator. In other embodiments, the secondary battery can also be a solid-state battery using a solid-state electrolyte. In yet other embodiments, the secondary battery can also be a semi-solid battery using a gel-state electrolyte, and the present application does not specifically limit the form of the secondary battery.

[0053] In the embodiments of the present application, the negative electrode can be any negative electrode suitable for lithium ion batteries in the art, and can form an electron and ion passage with the positive electrode and the electrolyte provided by the embodiments of the present application. In the embodiments of the present application, when the secondary battery further comprises a separator, the separator can be any separator known in the art.

[0054] The embodiments of the present application also provide a power consuming device comprising the aforementioned secondary battery provided by the embodiments of the present application. Due to the secondary battery provided by the embodiments of the present application, the power consuming device has higher market competitiveness.

[0055] In some embodiments of the present application, the power consuming device includes but is not limited to a vehicle, a consumer electronic product, etc. The vehicle includes but is not limited to a new energy vehicle, a power-assisted bicycle, etc.

[0056] The technical solutions of the embodiments of the present application are further described below in multiple embodiments.

[0057] Embodiment 1

[0058] (1) Iron sulfate, manganese sulfate, lithium carbonate and phosphoric acid are mixed and dispersed in deionized water in a certain proportion, and a grinder is used to grind to form a uniform mixed solution. The mixed solution is hydrothermally reacted at 120℃ for 6h, and then washed with deionized water and ethanol for three times and dried to obtain a positive electrode active material precursor.

[0059] The positive electrode active material precursor and the nitrogen-containing carbon source (specifically melamine, molecular formula C3H6N6, N element content 66.7%) are mixed by grinding in a mass ratio of 100:5, and then moved into a tube furnace for calcination. The calcination includes first calcination: the tube furnace is heated from 25℃ to a first holding temperature (specifically 550℃) at a rate of 5℃ / min and held for 2h, and then second calcination: the tube furnace is heated from 550℃ to a first holding temperature (specifically 800℃) at a rate of 5℃ / min and held for 2h.

[0060] (2) The material obtained after the second calcination is placed in dilute hydrochloric acid for etching for 12h to remove impurities, and a composite positive electrode active material is obtained.

[0061] Embodiment 2

[0062] The difference from Embodiment 1 is that in step (1), the nitrogen-containing carbon source is replaced from melamine to dicyandiamide (molecular formula C2H4N4, N element content 66.7%).

[0063] Embodiment 3

[0064] The difference from Embodiment 1 is that in step (1), the nitrogen-containing carbon source is replaced from melamine to urea (molecular formula CH4N2O, N element content 46.7%).

[0065] Example 4

[0066] The difference from Example 1 is that in step (1), the second holding temperature of the second calcination is adjusted to 700℃.

[0067] Example 5

[0068] The difference from Example 1 is that in step (1), the second holding temperature of the second calcination is adjusted to 650℃.

[0069] Example 6

[0070] The difference from Example 1 is that in step (1), the nitrogen-containing carbon source is replaced by urea, and the second holding temperature of the second calcination is adjusted to 650℃.

[0071] Example 7

[0072] The difference from Example 1 is that in step (1), the calcination only includes one calcination, specifically, the positive electrode active material precursor and the nitrogen-containing carbon source (specifically, melamine) are ground and mixed according to a certain proportion, and then moved into a tube furnace for calcination: the tube furnace is heated from 25℃ to 800℃ at a rate of 5℃ / min and held for 2h.

[0073] In order to highlight the beneficial effects of the embodiments of the present application, the following comparative examples are set.

[0074] Comparative Example 1

[0075] The difference from Example 1 is that in step (1), the positive electrode active material precursor is moved into a tube furnace for calcination. The calcination includes first calcination: the tube furnace is heated from 25℃ to a first holding temperature (specifically, 550℃) at a rate of 5℃ / min and held for 2h, and then second calcination: the tube furnace is heated from 550℃ to a first holding temperature (specifically, 800℃) at a rate of 5℃ / min and held for 2h.

[0076] Comparative Example 2

[0077] The difference from Comparative Example 1 is that after obtaining the positive electrode active material, the positive electrode active material and carbon nanotubes are mixed according to a mass ratio of 100:1.8.

[0078] Performance Test

[0079] Morphology Test:

[0080] The morphology of the materials prepared in each example and comparative example was observed under SEM, and the parameters are summarized in Table 1. Among them, the local SEM photo of the secondary particles of the composite positive electrode active material of Example 1 is shown in Figure 3.

[0081] Powder resistivity test:

[0082] The powder resistivity of the composite positive electrode active material prepared in each example and the material prepared in each comparative example was tested using a powder resistivity tester. 1 g of each of the above materials was weighed into a sample loader at 25°C. A variable pressure test mode (pressure 0-200 MPa) was set, and the powder resistivity at a maximum pressure of 200 MPa was read. The results are summarized in Table 1.

[0083] Table 1

[0084] It should be noted that the mass fraction of carbon nanotubes in the composite positive electrode active material in Comparative Example 2 was 1.80%.

[0085] Phase characterization:

[0086] (1) Nitrogen content test: The mass fraction of N element in the nitrogen-doped carbon nanotubes was tested using a scanning electron microscope-energy dispersive spectrometer (SEM-EDS). The results are summarized in Table 2.

[0087] (2) XPS test was performed on the composite positive electrode active material prepared in each example and the material prepared in each comparative example to test the content of different nitrogen configurations (pyridine N: 398.2 eV; pyrrole N: 399.5 eV; graphite N: 400.8 eV; oxidized N: 402.6 eV). The nitrogen element peak obtained by XPS was processed by peak separation according to the above characteristic peaks, and the mass fraction of each configuration of nitrogen element in the total mass of nitrogen element in the nitrogen-doped carbon nanotubes was calculated. The XPS peak separation results of Example 1 are shown in Figure 4. The results are summarized in Table 2. The mass fraction of graphite nitrogen in the nitrogen-doped nanotubes = the mass fraction of graphite nitrogen in the total mass of N element x the mass fraction of N element in the nitrogen-doped carbon nanotubes. The results are summarized in Table 2.

[0088] Table 2

[0089] (3) Positive electrode resistivity test

[0090] The materials prepared in each example and comparative example were prepared into a positive electrode slurry with a mass ratio of active material: conductive agent: PVDF: NMP = 100: 1: 3: 70, and coated on the surface of a positive electrode current collector (specifically, an aluminum foil). After drying and rolling, a positive electrode with a single-sided area density of 200 g / m2 and a compacted density of 2.4 g / cm3 was obtained.

[0091] The resistivity of each of the above positive electrodes was tested. Specifically, the positive electrodes of each example and comparative example were cut into a 6 cm x 7 cm sample, and the sample was placed in a sheet resistivity tester for testing. The test pressure was set to 25 MPa. The results are summarized in Table 3.

[0092] Table 3

[0093] (4) Battery performance test

[0094] ① The positive electrode was cut into a circular piece with a diameter of 1.5 cm, lithium metal was used as the counter electrode, and a polyethylene / polypropylene composite film was used as the separator. The counter electrode, the separator, and the positive electrode were stacked to assemble a CR2016 button cell. The electrolyte was a lithium salt (specifically lithium hexafluorophosphate LiPF6) solution with a concentration of 1 mol / L, and the solvent was EC:EMC:DMC:DEC:VC with a mass ratio of 23:25:24:28:5. Among them, EC refers to ethylene carbonate, EMC refers to ethyl methyl carbonate, DMC refers to dimethyl carbonate, DEC refers to diethyl carbonate, and VC refers to vinylene carbonate.

[0095] ② First coulombic efficiency test: the button cell of each example and comparative example was charged at a current of 0.1C to 4.3V at 25±2°C, and then kept at 4.3V until the current cutoff was 0.05C. After 10 min, it was discharged at 0.1C to 2.5V. The first discharge capacity and the first charge capacity of the battery were recorded, the first coulombic efficiency = the first discharge capacity / the first charge capacity, and the results were summarized in Table 4.

[0096] ③ Charge-discharge cycle test, specifically: at 25±2°C, charge at 0.1C to 4.3V, and charge at constant voltage to 0.05C cutoff; stand for 10 min; discharge at 0.1C to 2.5V, which is 1 cycle. Repeat the steps to test the capacity and capacity retention rate of the battery after 100 cycles, and the results are summarized in Table 4.

[0097] ④ Rate performance test: at 25±2°C, charge at 0.1C to 4.3V, and charge at constant voltage to 0.05C cutoff; stand for 10 min; discharge at 10C to 2.5V, record the 10C discharge capacity. Capacity retention rate = 10C discharge capacity / 0.1C discharge capacity. The results are summarized in Table 4.

[0098] Table 4

[0099] It can be seen from the data in Tables 1-4 that the composite positive electrode active material provided by the embodiments of the present application can be used to provide a battery with good rate performance. Compared with Comparative Example 1, it can be found that the first charge and discharge specific capacity and the first coulombic efficiency of the battery of the embodiments are all significantly improved, which is due to the improvement of the electronic conductivity of the composite positive electrode active material. In addition, by comparing the data between the embodiments, it can be found that when the content of the nitrogen-doped carbon nanotubes in the composite positive electrode active material is similar, and the total amount of nitrogen in the nitrogen-doped carbon nanotubes is similar (for example, comparing Example 3 and Example 6, or comparing Example 4 and Example 5), the higher the proportion of graphite nitrogen, the better the conductivity of the composite positive electrode active material.

[0100] In addition, it should be noted that although the nitrogen content in the N-doped carbon nanotubes of Example 7 is similar to that of Example 3, the content of nitrogen-doped carbon nanotubes in the composite positive electrode active material of Example 7 is lower than that of Example 3 due to only one calcination, so the electrochemical performance of the positive electrode composite active material of Example 7 is weak.

[0101] The above is an exemplary embodiment of the present application. It should be noted that those skilled in the art can make several improvements and refinements without departing from the principles of the present application, and these improvements and refinements are also considered within the scope of protection of the present application.

Claims

1. A composite positive electrode active material, characterized by, The composite positive electrode active material comprises a positive electrode active material and nitrogen-doped carbon nanotubes, and the nitrogen-doped carbon nanotubes comprise graphite nitrogen.

2. The composite cathode active material according to claim 1, characterized in that, The mass percentage of nitrogen in the nitrogen-doped carbon nanotubes is 20%-40%, and the mass percentage of the graphite nitrogen in the total mass of nitrogen is greater than or equal to 20%.

3. The composite cathode active material according to claim 2, characterized in that, The mass percentage of the graphite nitrogen in the total mass of nitrogen is 20%-72%.

4. The composite cathode active material according to any one of claims 1 to 3, characterized in that, The composite positive electrode active material comprises secondary particles, and the secondary particles comprise a plurality of primary particles of the positive electrode active material and a plurality of the nitrogen-doped carbon nanotubes, and the nitrogen-doped carbon nanotubes are grown in situ on the surfaces of at least some of the primary particles of the positive electrode active material.

5. The composite cathode active material according to claim 4, characterized in that, The particle size D50 of the secondary particles is 5 μm-10 μm, and the average particle size of the primary particles of the positive electrode active material is 200 nm-300 nm.

6. The composite cathode active material according to any one of claims 1 to 5, characterized in that, The mass percentage of the nitrogen-doped carbon nanotubes in the composite positive electrode active material is 0.5%-3%.

7. The composite cathode active material according to any one of claims 1 to 6, characterized in that, The positive electrode active material comprises Li1-aAaMnxMyFe1-x-y(P1-bEb)O4, 0≤a<1, 0.4≤x+y≤0.9, 0≤b<1; A comprises at least one of K+, Ca+, Na+ and NH4+, M comprises at least one of Ti, V, Co, Ni, Cu, Zn, Mg, Ca, Al, Nb and Mo, and E comprises at least one of N, Si and B.

8. The composite cathode active material according to claim 6 or 7, characterized in that, The powder resistivity of the composite positive electrode active material at room temperature under a pressure of 200 MPa is 20 Ω·cm-100 Ω·cm.

9. A method for producing a composite positive electrode active material, characterized by, The composite positive electrode active material comprises a positive electrode active material and nitrogen-doped carbon nanotubes, and the nitrogen-doped carbon nanotubes comprise graphite nitrogen. The composite positive electrode active material comprises a positive electrode active material and nitrogen-doped carbon nanotubes, and the nitrogen-doped carbon nanotubes comprise graphite nitrogen. The calcination comprises first calcination and second calcination performed in sequence.

10. The method of claim 9, wherein, The holding temperature of the first calcination is 500°C-600°C, and the holding time is 1h-3h; the holding temperature of the second calcination is 650°C-900°C, and the holding time is 1h-3h. The composite positive electrode active material comprises a positive electrode active material and nitrogen-doped carbon nanotubes, and the nitrogen-doped carbon nanotubes comprise graphite nitrogen.

11. A positive electrode, characterized by comprising: The composite positive electrode active material comprises a positive electrode active material and nitrogen-doped carbon nanotubes, and the nitrogen-doped carbon nanotubes comprise graphite nitrogen.

12. A secondary battery characterized by comprising: The composite positive electrode active material comprises a positive electrode active material and nitrogen-doped carbon nanotubes, and the nitrogen-doped carbon nanotubes comprise graphite nitrogen.

13. An electrical device, characterized by ​

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