Composite cathode active material, method for preparing same, and use thereof

Abstract

The embodiment of the present application provides a composite positive electrode active material and a preparation method and application thereof. The composite positive electrode active material comprises a positive electrode active material and nitrogen-doped carbon nanotubes, wherein the nitrogen-doped carbon nanotubes comprise graphite nitrogen. The composite positive electrode active material can realize higher electronic conductivity under lower carbon nanotube consumption, and can be used for providing a secondary battery with better comprehensive performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to composite positive electrode active materials, their preparation methods, and applications. Background Technology

[0002] The industry often improves the rate performance and low-temperature performance of secondary batteries by increasing the electronic conductivity of the positive electrode active material. For example, conductive carbon is coated on the surface of the positive electrode active material particles, or the positive electrode active material particles are mixed with conductive agents such as carbon nanotubes. However, the improvement of the electronic conductivity of the positive electrode active material by the above two methods is limited. Summary of the Invention

[0003] Therefore, embodiments of this application provide composite positive electrode active materials, their preparation methods, and applications. These composite positive electrode active materials can achieve high electronic conductivity with relatively low carbon nanotube content, and can be used to provide a secondary battery with superior overall performance.

[0004] The first aspect of this application provides a composite positive electrode active material, including a positive electrode active material and nitrogen-doped carbon nanotubes, wherein the nitrogen-doped carbon nanotubes include graphitic nitrogen.

[0005] Nitrogen doping can effectively improve the conductivity of carbon nanotubes. Further controlling the nitrogen content within the aforementioned range allows for the construction of sufficient electronic pathways within the nitrogen-doped carbon nanotubes. Graphite nitrogen is particularly beneficial for enhancing the conductivity of carbon nanotubes, thereby effectively improving the conductivity of the composite cathode active material. Furthermore, based on the structural properties of nitrogen-doped carbon nanotubes, they can form a conductive network within the composite cathode active material. Therefore, compared to related technologies, with a comparable amount of carbon nanotubes, the aforementioned composite cathode active material can achieve higher electronic conductivity, specifically resulting in a secondary battery with superior rate performance.

[0006] The second aspect of this application provides a method for preparing a composite positive electrode active material, including:

[0007] A composite positive electrode active material is obtained by mixing a nitrogen-containing carbon source and a positive electrode active material precursor and calcining the mixture; wherein the nitrogen-containing carbon source includes at least one of cyanamide, dicyandiamide, melamine and urea.

[0008] The composite positive electrode active material includes a positive electrode active material and nitrogen-doped carbon nanotubes, wherein the nitrogen-doped carbon nanotubes include graphitic nitrogen.

[0009] The above preparation method is simple and easy to implement, with strong process controllability and high production efficiency, making it suitable for large-scale industrial production.

[0010] A third aspect of this application provides a positive electrode, including the composite positive electrode active material provided in the first aspect of this application. Due to the inclusion of the composite positive electrode active material provided in this application, the resistivity of the positive electrode provided in this application is lower when the carbon content in the positive electrode is the same.

[0011] A fourth aspect of this application provides a secondary battery, including the positive electrode provided in the third aspect of this application. Because it incorporates the positive electrode provided in this application, the aforementioned secondary battery can also possess superior rate performance.

[0012] A fifth aspect of this application provides an electrical device including the secondary battery provided in the fourth aspect of this application. Because it incorporates the secondary battery provided in this application, the aforementioned electrical device has high market competitiveness. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of nitrogen atoms with different configurations in nitrogen-doped carbon nanotubes.

[0014] Figure 2 This is a simplified structural diagram of a composite positive electrode active material provided in an embodiment of this application;

[0015] Figure 3 This is a partial scanning electron microscope (SEM) image of the composite positive electrode active material prepared in Example 1 of this application.

[0016] Figure 4 The image shows the X-ray photoelectron spectroscopy (XPS) spectrum of nitrogen in the composite positive electrode active material prepared in Example 1 of this application. Detailed Implementation

[0017] This application provides a composite positive electrode active material, including a positive electrode active material and nitrogen-doped carbon nanotubes; wherein the nitrogen-doped carbon nanotubes include graphitic nitrogen.

[0018] Nitrogen doping can effectively improve the conductivity of carbon nanotubes, and by further controlling the nitrogen content within the aforementioned range, sufficient electronic pathways can be constructed within the nitrogen-doped carbon nanotubes. The types of nitrogen doping in nitrogen-doped carbon nanotubes generally include graphitic nitrogen, pyridine nitrogen, pyrrole nitrogen, and nitrogen oxides. For more information, please refer to [link to relevant documentation]. Figure 1In this process, nitrogen-doped graphite replaces carbon atoms within the carbon framework of carbon nanotubes, forming a non-planar sp3 hybridization with three surrounding carbon atoms, which further enhances the electrical conductivity of the carbon nanotubes. Furthermore, based on the structural properties of nitrogen-doped carbon nanotubes, they can form a conductive network in composite cathode active materials. Therefore, compared to related technologies, with the same amount of carbon nanotubes, the aforementioned composite cathode active material can achieve higher electronic conductivity, specifically manifested as lower powder resistivity. Understandably, improved electronic conductivity is beneficial for optimizing the rate performance and low-temperature performance of the material.

[0019] In some embodiments of this application, the mass percentage of nitrogen in the nitrogen-doped carbon nanotubes is 20%-40%, and the graphitic nitrogen accounts for ≥20% of the total nitrogen content. Specifically, the mass percentage of nitrogen 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 nitrogen doping amount allows for a higher nitrogen content in the carbon nanotubes, reduces defect structures, and ensures sufficient carbon atoms to form the basic structure of the carbon nanotubes, thereby significantly improving their conductivity.

[0020] In some embodiments of this application, in nitrogen-doped carbon nanotubes, the proportion of graphite nitrogen in the total mass of nitrogen is ≥20%, which can significantly improve the electrical conductivity of nitrogen-doped carbon nanotubes. Specifically, the proportion of graphite nitrogen in the total mass of nitrogen can be, but is not limited to, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, etc. The higher the proportion of graphite nitrogen, the more beneficial it is to improving the electrical conductivity of carbon nanotubes. It should be noted that when the mass proportion of graphite nitrogen in nitrogen is <100%, other nitrogen elements may include one or more of pyridine nitrogen, pyrrole nitrogen, and nitrogen oxides; this application does not impose any limitations on this.

[0021] In this embodiment, X-ray fluorescence spectrometer (XRF) or energy dispersive spectrometer (EDS) can be used to test the mass percentage of nitrogen in carbon nanotubes.

[0022] In this embodiment, X-ray photoelectron spectroscopy (XPS) can be used to test the proportion of graphitic nitrogen in nitrogen. Specifically, the composite positive electrode active material is placed in XPS for testing, and the characteristic peaks of the obtained N element are fitted to calculate the mass proportion of graphitic nitrogen. The characteristic peak positions of pyridine nitrogen and pyrrole nitrogen are as follows: 398.2 eV; 399.5 eV; 400.8 eV; and 402.6 eV.

[0023] In some embodiments of this application, the proportion of graphite nitrogen in the total mass of nitrogen is 20%-72%. This ensures high conductivity of the composite positive electrode active material and facilitates its fabrication. Specifically, the proportion of graphite nitrogen in the total mass of nitrogen 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%, and 72%.

[0024] For some embodiments of this application, please refer to Figure 2 The composite positive electrode active material 1 includes secondary particles 10, which comprise multiple primary particles 11 of the positive electrode active material and multiple nitrogen-doped carbon nanotubes 12. The nitrogen-doped carbon nanotubes 12 are grown in situ on the surface of at least some of the primary particles 11. For example, when the positive electrode active material is lithium iron phosphate, the nitrogen-doped carbon nanotubes are grown in situ on the surface of the primary lithium iron phosphate particles. Thus, during the formation of the secondary particles, the carbon nanotubes grown in situ on the surface of the primary particles are dispersed within the secondary particles along with the aggregation of the primary particles. Compared to carbon nanotube-containing positive electrode active materials in related technologies, the risk of aggregation of the nitrogen-doped carbon nanotubes in this embodiment is extremely low, allowing for the construction of a highly uniform conductive network within the secondary particles. This significantly improves the electronic conductivity of the composite positive electrode active material, thereby greatly enhancing the electron transport capability within the secondary particles. Furthermore, the carbon nanotubes grown in situ on the surface of the primary particles have strong bonding with the positive electrode active material and good structural stability of the conductive network. The conductive network inside the secondary particles can remain stable in subsequent pulping processes and the final charge-discharge cycle.

[0025] It should be noted that, Figure 2 This is just an example drawing. Figure 2 The size, quantity, and distribution of nitrogen-doped carbon nanotubes in secondary particles do not constitute any limitation on this application. Figure 2The number, size, and arrangement of the primary particles of the positive electrode active material in the illustration are merely exemplary and do not constitute any limitation on this application.

[0026] Furthermore, given the good conductivity of secondary particles, it is not necessary to control the particle size D50 of the secondary particles in the composite positive electrode active material within a small range to achieve superior rate performance and low-temperature performance. In some embodiments of this application, the particle size D50 of the secondary particles in the composite positive electrode active material is 5μm-10μm. This ensures superior rate performance, low-temperature performance, and power performance, and also facilitates the subsequent application of the composite positive electrode active material. For example, it is easy to disperse during the preparation of positive electrode slurry, resulting in a uniform and stable slurry. Specifically, the particle size D50 of the secondary particles can be, but is not limited to, 5μm, 6μm, 7μm, 8μm, 9μm, and 10μm. In the embodiments of this application, the particle size D50 refers to the particle size corresponding to a cumulative volume percentage of 50% of the composite positive electrode active material, which can be measured using a laser particle size analyzer. In this embodiment, the secondary particles can be spherical, quasi-spherical, ellipsoidal, etc. This application does not limit the shape of the secondary particles, nor does it limit their sphericity. Those skilled in the art can select according to the actual production situation.

[0027] In some embodiments of this application, the average particle size of the primary particles of the positive electrode active material is 200nm-300nm. This facilitates both the deintercalation / intercalation of active ions (lithium ions) and the formation of secondary particles with suitable particle sizes, and also facilitates the formation of a conductive network of carbon nanotubes within the secondary particles. In the embodiments of this application, the average particle size of the primary particles can be determined using a scanning electron microscope (SEM). Specifically, 200 secondary particles are randomly selected within the field of view, and one primary particle is randomly selected from each secondary particle. Their particle sizes are measured, and the average value is taken. The average particle size of the primary particles can be, but is not limited to, 200nm, 210nm, 220nm, 230nm, 240nm, 250nm, 260nm, 270nm, 280nm, 290nm, and 300nm. In the embodiments of this application, the primary particles can be spherical, quasi-spherical, ellipsoidal, etc. This application does not limit the shape of the primary particles, nor does it limit their sphericity. Those skilled in the art can select according to actual production conditions.

[0028] In some embodiments of this application, the mass percentage of nitrogen-doped carbon nanotubes in the composite positive electrode active material is 0.5%-3%. This ensures both good conductivity of the composite positive electrode active material and a sufficient content of the active material, thereby guaranteeing a high specific capacity. Therefore, it can be used to provide a secondary battery with superior rate performance, low-temperature performance, and high energy density. In the embodiments of this application, a sulfur-carbon analyzer can be used to determine the mass percentage of carbon in the composite positive electrode active material, and the mass percentage of nitrogen-doped carbon nanotubes in the composite positive electrode active material can be calculated by combining this with the mass percentage of nitrogen in the carbon nanotubes. Specifically, the mass percentage of 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%, and 3.0%.

[0029] In some cases, such as when the mass percentage of nitrogen-doped carbon nanotubes in the composite positive electrode active material is 1.5%-3%, no additional conductive agent needs to be added to the final positive electrode material layer when the composite positive electrode active material is applied.

[0030] Given that lithium iron phosphate (LFP) cathode active materials, especially lithium manganese iron phosphate (LFP), possess high specific capacity and a higher voltage platform, they have the potential to become next-generation high-energy-density cathode materials. However, LFP has relatively weak electronic conductivity. Therefore, when the primary particle material of the cathode active material is LFP or doped / modified LFP, the composite cathode active material exhibits a greater improvement in electronic conductivity relative to its primary particle. Therefore, considering cost-effectiveness, in some embodiments of this application, the cathode active material includes Li... 1-a A a Mn x M y Fe 1-x-y (P 1-b E b O4, 0≤a<1, 0.4≤x+y≤0.9, 0≤b<1; A includes K + Ca + Na + and NH4 + At least one of the following, 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 is understood that it is also feasible for the positive electrode active material to be a ternary positive electrode material or other positive electrode active materials, and this application does not impose any limitations on this. In some specific embodiments, the positive electrode active material includes LiMn. x Fe 1-y PO4.

[0031] In some embodiments of this application, the resistivity of the composite positive electrode active material at room temperature under 200 MPa pressure is 20 Ω·cm-100 Ω·cm. In some specific embodiments, the positive electrode active material is Li. 1-a A a Mn x M y Fe 1-x-y (P 1-b E b When O4 is present, the resistivity of the composite positive electrode 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 this application, the room temperature is 25±2℃.

[0032] This application also provides a method for preparing a composite positive electrode active material, including:

[0033] S01. A mixture of nitrogen-containing carbon source and positive electrode active material precursor is calcined to obtain a composite positive electrode active material; wherein, the nitrogen-containing carbon source includes at least one of cyanamide (molecular formula CH2N2), dicyandiamide (molecular formula C2H4N4), melamine (molecular formula C3H6N6), and urea (molecular formula CH4N2O);

[0034] The composite positive electrode active material includes a positive electrode active material and nitrogen-doped carbon nanotubes, wherein the nitrogen-doped carbon nanotubes include graphitic nitrogen.

[0035] The above preparation method is simple and easy to implement, with strong process controllability and high production efficiency, making it 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 positive electrode active material particles, thereby avoiding the problems of finished carbon nanotubes being difficult to disperse and easily agglomerating in the positive electrode active material particles in related technologies. The composite positive electrode active material prepared by the above method can have a uniform and good conductive network, thus the composite positive electrode active material can have better conductivity.

[0036] In some embodiments of this application, in step S01, the precursor of the positive electrode active material can be prepared using a process well-known to those skilled in the art, but the difference lies in that the raw materials used to prepare the precursor do not contain a carbon source. This application does not impose any limitations on this. Taking lithium manganese iron phosphate as an example, a simple explanation is provided: A phosphorus source, a manganese source, an iron source, and a lithium source are dispersed in a solvent according to the stoichiometric ratio of each element in the lithium manganese iron phosphate to be prepared. The mixture is ground to obtain a homogeneous solution, subjected to a hydrothermal reaction, and the resulting solid is washed and dried to obtain the lithium manganese iron phosphate precursor. The phosphorus source includes, but is not limited to, phosphoric acid; the manganese source includes, but is not limited to, manganese sulfate; the iron source includes, but is not limited to, ferrous sulfate; and the lithium source includes, but is not limited to, lithium carbonate.

[0037] In some embodiments of this application, step S01, the calcination includes a first calcination and a second calcination performed sequentially. The first calcination causes the nitrogen-containing carbon source to form amorphous carbon doped with N element with a two-dimensional layered structure. The second calcination causes the carbon atoms in the amorphous carbon obtained from the first calcination to rearrange, forming nitrogen-doped carbon nanotubes.

[0038] In some embodiments of this application, the holding temperature for the first calcination (hereinafter referred to as the first holding temperature) is 500℃-600℃, and the holding time is 1h-3h. This facilitates the obtaining of nitrogen-doped amorphous carbon. Furthermore, it helps to control the nitrogen content in the final nitrogen-doped carbon nanotubes within the range of 20%-40%. It should be noted that since some nitrogen is released during the second calcination process (e.g., forming NH3), in order to control the nitrogen content in the carbon nanotubes within the range of 20%-40%, the nitrogen content in the material obtained after the first calcination will be slightly higher, for example, reaching 30%-50%. Specifically, the first holding temperature can be, for example, 500℃, 520℃, 550℃, 580℃, 600℃, etc., and the holding time for the first calcination can be, for example, 1h, 1.5h, 2h, 2.5h, 3h, etc.

[0039] In some embodiments of this application, the holding temperature for the second calcination (hereinafter referred to as the second holding temperature) is 650℃-900℃, and the holding time is 1h-3h. This promotes carbon atom rearrangement to form a graphitic carbon configuration, reduces structural defects in the formed nitrogen-doped carbon nanotubes, and increases the proportion of graphitic nitrogen, for example, increasing the proportion of graphitic nitrogen to over 20%, thereby obtaining a composite cathode active material with superior conductivity. Specifically, the second holding temperature can be, for example, 650℃, 700℃, 750℃, 800℃, 850℃, 900℃, etc., and the holding time for the second calcination can be, for example, 1h, 1.5h, 2h, 2.5h, 3h, etc.

[0040] In this embodiment of the application, in step S01, after the first calcination is completed, the temperature can be directly increased from the first holding temperature to the second holding temperature.

[0041] In this embodiment, no specific limitation is made on the heating rate during the first and second calcinations. Those skilled in the art can determine the rate based on actual production conditions such as equipment specifications, for example, a heating rate of 5°C / min to 25°C / min. The heating rate of the first and second calcinations can be the same or different.

[0042] In some embodiments of this application, after step S01, step S02 is further included: 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 10h to 48h to remove impurities (e.g., elemental Fe, Fe3O4, elemental Mn, Mn3O4, etc.).

[0043] In some embodiments of this application, step S02 further includes washing and drying the material obtained after the impurity removal treatment. The washing and drying processes described above can be processes well known to those skilled in the art. Specifically, drying can be baking or drying, which can remove residual acid and moisture.

[0044] This application also provides a positive electrode, including the aforementioned composite positive electrode active material. Due to the inclusion of the composite positive electrode active material, the resistivity of the positive electrode provided in this application is lower when the carbon content in the positive electrode is the same. Therefore, the above-mentioned positive electrode can be used to provide a secondary battery that combines superior rate performance, good low-temperature performance, and high energy density.

[0045] In some embodiments of this application, the positive electrode includes a current collector and a positive electrode material layer disposed on the surface of the current collector, wherein the positive electrode material layer includes a composite positive electrode active material.

[0046] In some embodiments of this application, the positive electrode material layer further includes a binder. In the embodiments of this application, the aforementioned positive electrode can be any current collector known in the art for positive electrodes of lithium-ion batteries, such as aluminum foil. The aforementioned binder can be any binder known in the art for positive electrodes of lithium-ion batteries, and this application does not impose any limitation on it.

[0047] This application also provides a secondary battery, including the positive electrode provided in this application embodiment. Because it incorporates the positive electrode provided in this application embodiment, the secondary battery can possess superior rate performance, good low-temperature performance, and high energy density.

[0048] In some embodiments of this application, the secondary battery includes a positive electrode, a negative electrode, and an electrolyte located between the positive and negative electrodes. In some embodiments of this application, the secondary battery may be a liquid battery using a liquid electrolyte, in which case a separator is also provided between the positive and negative electrodes. In other embodiments, the secondary battery may be a solid-state battery using a solid electrolyte. In still other embodiments, the secondary battery may be a semi-solid-state battery using a gel electrolyte, etc. This application provides specific limitations on the form of the secondary battery.

[0049] In this embodiment, the negative electrode can be any negative electrode suitable for lithium-ion batteries within the field of application, as long as it can form electronic and ion pathways with the positive electrode and electrolyte provided in this embodiment. In this embodiment, when the secondary battery also includes a separator, the separator can be any separator known in the field.

[0050] This application also provides an electrical device including the aforementioned secondary battery. Because it incorporates the secondary battery provided in this application, the electrical device has high market competitiveness.

[0051] In some embodiments of this application, the aforementioned electrical equipment includes, but is not limited to, vehicles and consumer electronics. Among these, the aforementioned vehicles include, but are not limited to, new energy vehicles and electric bicycles.

[0052] The technical solutions of the embodiments of this application are further described below through multiple examples.

[0053] Example 1

[0054] (1) Ferric sulfate, manganese sulfate, lithium carbonate and phosphoric acid are mixed and dispersed in deionized water in a certain proportion, and ground with a grinder to form a uniform mixture. The mixture is then subjected to hydrothermal reaction at 120°C for 6 hours. The positive electrode active material precursor is obtained by washing with deionized water and ethanol three times and drying.

[0055] The positive electrode active material precursor and a nitrogen-containing carbon source (specifically melamine, molecular formula C3H6N6, N element content 66.7%) were ground and mixed at a mass ratio of 100:5, and then transferred to a tube furnace for calcination. The calcination consisted of a first calcination: the tube furnace was heated from 25°C to a first holding temperature (specifically 550°C) at a rate of 5°C / min and held for 2 hours; followed by a second calcination: the tube furnace was heated from 550°C to a first holding temperature (specifically 800°C) at a rate of 5°C / min and held for 2 hours.

[0056] (2) The material obtained after the second calcination was placed in dilute hydrochloric acid for 12 hours to remove impurities and obtain composite positive electrode active material.

[0057] Example 2

[0058] The difference from Example 1 is that in step (1), the nitrogen-containing carbon source is replaced by dicyandiamide (molecular formula C2H4N4, N element content is 66.7%).

[0059] Example 3

[0060] The difference from Example 1 is that in step (1), the nitrogen-containing carbon source is replaced by urea (molecular formula CH4N2O, N element content is 46.7%).

[0061] Example 4

[0062] The difference from Example 1 is that in step (1), the second holding temperature of the second calcination is adjusted to 700°C.

[0063] Example 5

[0064] The difference from Example 1 is that in step (1), the second holding temperature of the second calcination is adjusted to 650°C.

[0065] Example 6

[0066] The difference from Example 1 is that in step (1), the nitrogen-containing carbon source is replaced by urea instead of melamine, and the second holding temperature of the second calcination is adjusted to 650°C.

[0067] Example 7

[0068] The difference from Example 1 is that in step (1), calcination only includes one calcination. Specifically, the positive electrode active material precursor and the nitrogen-containing carbon source (specifically melamine) are ground and mixed in a certain proportion, and then transferred into a tube furnace for calcination: the tube furnace is heated from 25°C to 800°C at 5°C / min and kept at that temperature for 2 hours.

[0069] To highlight the beneficial effects of the embodiments of this application, the following comparative examples are provided.

[0070] Comparative Example 1

[0071] 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 a first calcination: the tube furnace is heated from 25°C to a first holding temperature (specifically 550°C) at a rate of 5°C / min and held for 2 hours, and then a second calcination is performed: the tube furnace is heated from 550°C to a first holding temperature (specifically 800°C) at a rate of 5°C / min and held for 2 hours.

[0072] Comparative Example 2

[0073] The difference from Comparative Example 1 is that after obtaining the positive electrode active material, the positive electrode active material and carbon nanotubes were mixed at a mass ratio of 100:1.8.

[0074] Performance testing

[0075] Morphological testing:

[0076] The morphology of the materials prepared in each embodiment and comparative example was observed under SEM, and the parameters are summarized in Table 1. A partial SEM image of the secondary particles of the composite positive electrode active material in Example 1 is shown below. Figure 3 As shown.

[0077] Powder resistivity test:

[0078] The powder resistivity of the composite positive electrode active materials prepared in each embodiment and the materials prepared in each comparative embodiment was tested using a powder resistivity meter. At 25°C, 1g of each of the above materials was weighed and transferred to a sample container. A variable pressure test mode (pressure 0–200 MPa) was set, and the powder resistivity at the maximum pressure of 200 MPa was read. The results are summarized in Table 1.

[0079] Table 1

[0080]

[0081] It should be noted that in Comparative Example 2, the mass percentage of carbon nanotubes in the composite positive electrode active material is 1.80%.

[0082] Phase characterization:

[0083] (1) Nitrogen content test: The mass percentage of nitrogen in nitrogen-doped carbon nanotubes was tested using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and the results are summarized in Table 2.

[0084] (2) XPS tests were performed on the composite positive electrode active materials prepared in each embodiment and the materials 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; N oxide: 402.6 eV). Based on the above characteristic peaks, the nitrogen peaks obtained by XPS were processed to separate the peaks and calculate the proportion of nitrogen element of each configuration in the total mass of nitrogen element in the nitrogen-doped carbon nanotubes. The XPS peak separation results of Example 1 are as follows. Figure 4 As shown in Table 2, the mass percentage of graphite nitrogen in nitrogen-doped nanotubes is calculated as: mass percentage of graphite nitrogen in the total mass of nitrogen element × mass percentage of nitrogen element in nitrogen-doped carbon nanotubes.

[0085] Table 2

[0086]

[0087] (3) Positive electrode resistivity test

[0088] The materials obtained in the various embodiments and comparative examples were used to prepare positive electrode slurries with a mass ratio of active material: conductive agent: PVDF: NMP = 100:1:3:70. These slurries were then coated onto the surface of the positive electrode current collector (specifically, aluminum foil), and subsequently dried and rolled to obtain a single-sided surface density of 200 g / m². 2 Both have a compacted density of 2.4 g / cm³. 3 The positive pole.

[0089] The resistivity of each positive electrode was tested. Specifically, the positive electrodes of each embodiment and comparative example were cut into 6cm×7cm samples. The samples were placed in an electrode resistivity meter for testing. The test pressure was set to 25MPa. The results are summarized in Table 3.

[0090] Table 3

[0091] Case Electrode resistivity / Ω·cm Example 1 14.8 Example 2 23.5 Example 3 37.4 Example 4 33.1 Example 5 57.2 Example 6 63.4 Example 7 43.1 Comparative Example 1 175.6 Comparative Example 2 70.8

[0092] (4) Battery performance test

[0093] ① Cut the above-mentioned positive electrodes into circular pieces with a diameter of 1.5 cm. Use a lithium metal sheet as the counter electrode and a polyethylene / polypropylene composite membrane as the separator. Stack the counter electrode, separator, and positive electrode to assemble a CR2016 coin cell. The electrolyte is a 1 mol / L lithium salt (specifically lithium hexafluorophosphate LiPF6) solution, with a solvent mass ratio of EC:EMC:DMC:DEC:VC of 23:25:24:28:5. Here, 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.

[0094] ② Initial Coulombic Efficiency Test: At 25±2℃, the coin cells of each embodiment and comparative example were charged to 4.3V at a constant current of 0.1C, and then held at 4.3V until the current cutoff of 0.05C. After resting for 10 minutes, they were discharged to 2.5V at 0.1C. The initial discharge capacity and initial charge capacity of the battery were recorded. The initial coulombic efficiency = initial discharge capacity / initial charge capacity. The results are summarized in Table 4.

[0095] ③ Charge-discharge cycle test: Specifically, at 25±2℃, charge at a constant current of 0.1C to 4.3V, then charge at a constant voltage of 0.05C to cut off; let stand for 10 minutes; discharge at a constant current of 0.1C to 2.5V, which constitutes one cycle. Repeat this step to test the battery capacity and capacity retention rate after 100 cycles. The results are summarized in Table 4.

[0096] ④ Rate performance test: At 25±2℃, charge at a constant current of 0.1C to 4.3V, then charge at a constant voltage to 0.05C (cutoff); let stand for 10 minutes; discharge at a constant current of 10C to 2.5V, and record the 10C discharge capacity. Capacity retention rate = 10C discharge capacity / 0.1C discharge capacity. The results are summarized in Table 4.

[0097] Table 4

[0098]

[0099]

[0100] As can be seen from the data in Tables 1-4, the composite positive electrode active material provided in this application embodiment can be used to provide a battery with better rate performance. Compared with Comparative Example 1, it can be found that the initial charge and discharge specific capacity and initial coulombic efficiency of the battery in the embodiment are significantly improved, which is attributed to the improved electronic conductivity of the composite positive electrode active material. In addition, comparing the data between the embodiments, it can be found that in the positive electrode composite active material, when the content of nitrogen-doped carbon nanotubes is similar and the total amount of nitrogen in the nitrogen-doped carbon nanotubes is similar (for example, comparing Embodiment 3 and Embodiment 6, or comparing Embodiment 4 and Embodiment 5), the higher the proportion of graphite nitrogen, the better the conductivity of the composite positive electrode active material.

[0101] Furthermore, it should be noted that although the nitrogen content in the N-doped carbon nanotubes in Example 7 is similar to that in 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 because only one calcination was used. Therefore, the electrochemical performance of the positive electrode composite active material of Example 7 is weaker.

[0102] The above are exemplary embodiments of this application. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principles of this application, and these improvements and modifications are also considered to be within the scope of protection of this application.

Claims

1. A composite positive electrode active material, characterized by, The composite positive electrode active material comprises positive electrode active material and nitrogen-doped carbon nanotubes, the nitrogen-doped carbon nanotubes comprise graphite nitrogen, the mass percentage of nitrogen elements in the nitrogen-doped carbon nanotubes is 30-40%, and the mass percentage of the graphite nitrogen in the total mass of nitrogen elements is greater than or equal to 25%.

2. The composite cathode active material according to claim 1, characterized in that, The mass percentage of the graphite nitrogen in the total mass of nitrogen elements is 25-72%.

3. The composite cathode active material according to any one of claims 1-2, characterized in that, The composite positive electrode active material comprises secondary particles, 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 grow in situ on surfaces of at least some of the primary particles of the positive electrode active material.

4. The composite cathode active material according to claim 3, characterized in that, The particle size D50 of the secondary particles is 5-10 microns, and the average particle size of the primary particles of the positive electrode active material is 200-300 nanometers.

5. The composite cathode active material according to any one of claims 1 to 4, characterized in that, The mass percentage of the nitrogen-doped carbon nanotubes in the composite positive electrode active material is 0.5-3%.

6. The composite cathode active material according to any one of claims 1 to 5, characterized in that, The positive electrode active material includes Li 1-a A a Mn x M y Fe 1-x-y (P 1-b E b )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.

7. The composite cathode active material according to claim 5 or 6, characterized in that, The powder resistivity of the composite positive electrode active material at room temperature under a pressure of 200 MPa is 20-100 ohm-centimeter.

8. A method for producing a composite positive electrode active material, characterized by, The composite positive electrode active material comprises positive electrode active material and nitrogen-doped carbon nanotubes, the nitrogen-doped carbon nanotubes comprise graphite nitrogen, the mass percentage of nitrogen elements in the nitrogen-doped carbon nanotubes is 30-40%, and the mass percentage of the graphite nitrogen in the total mass of nitrogen elements is greater than or equal to 25%. The composite positive electrode active material comprises positive electrode active material and nitrogen-doped carbon nanotubes, the nitrogen-doped carbon nanotubes comprise graphite nitrogen, the mass percentage of nitrogen elements in the nitrogen-doped carbon nanotubes is 30-40%, and the mass percentage of the graphite nitrogen in the total mass of nitrogen elements is greater than or equal to 25%. The calcination comprises first calcination and second calcination performed in sequence; 9. The preparation method according to claim 8, characterized in that, The holding temperature of the first calcination is 500-600 degrees Celsius, and the holding time is 1-3 hours; the holding temperature of the second calcination is 650-900 degrees Celsius, and the holding time is 1-3 hours. The composite positive electrode active material comprises positive electrode active material and nitrogen-doped carbon nanotubes, the nitrogen-doped carbon nanotubes comprise graphite nitrogen, the mass percentage of nitrogen elements in the nitrogen-doped carbon nanotubes is 30-40%, and the mass percentage of the graphite nitrogen in the total mass of nitrogen elements is greater than or equal to 25%.

10. A positive electrode, characterized by comprising: The positive electrode comprises the positive electrode active material.

11. A secondary battery characterized by comprising: The secondary battery comprises the positive electrode.

12. An electrical device, characterized by ​

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