NCM composite positive electrode material, preparation method thereof and battery

Abstract

The invention provides an NCM composite positive electrode material, a preparation method thereof and a battery. The composite positive electrode material comprises a nickel-cobalt-manganese ternary material core and a nitrogen-doped carbon coating layer coating the surface of the nickel-cobalt-manganese ternary material core, the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 4.8-10.5 wt% by taking the mass of the nitrogen-doped carbon coating layer as 100%. The NCM composite positive electrode material provided by the invention has relatively good electronic conductivity, ion transmission performance, interface stability and structural stability at the same time; therefore, the battery prepared from the NCM composite positive electrode material has excellent rate capability and cycling stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries and relates to an NCM composite positive electrode material, and in particular to an NCM composite positive electrode material and a preparation method thereof and a battery. Background Art

[0002] NCM cathode materials (nickel-cobalt-manganese ternary cathode materials) have become one of the mainstream cathode materials for lithium-ion batteries due to their high energy density and good cycle performance. However, NCM cathode materials are prone to side reactions with the electrolyte at high voltages, leading to interfacial instability and capacity decay.

[0003] In order to solve this problem, the prior art has disclosed increasing the stability of NCM positive electrode materials by coating the surface with strong inorganic materials (such as metal oxides or fluorides); however, metal oxides are usually insulators, which will hinder lithium ion diffusion and electron transport, resulting in a decrease in rate performance; although fluorides can promote interfacial ion conduction, excessive coating will still increase the interfacial impedance, especially under low temperature or high rate conditions, which will have a greater adverse effect on the interfacial impedance. In addition, the fluoride coating layer also has a mismatch in thermal expansion coefficient, which leads to cracks in the NCM positive electrode material, and poor chemical compatibility with the electrolyte, thereby generating the side reaction product HF.

[0004] CN119153672A discloses a method for fully carbon-coating a cathode material. This method uses a water-soluble polymer as a carbon coating precursor and achieves full coating of the cathode material through a vacuum negative pressure method, effectively improving the conductivity of the cathode material interface. However, the harsh vacuum negative pressure conditions in this method make industrial production difficult. Furthermore, the fully carbon-coated cathode material produced by this method exhibits insufficient electronic conductivity and ion transport properties.

[0005] CN104051724A discloses a method for preparing a carbon-coated lithium nickel cobalt manganese oxide cathode material, which belongs to the technical field of lithium battery cathode material preparation. In this patent document, a chelating agent and a carbon source are added to a solution containing lithium salt, nickel salt, cobalt salt and manganese salt, and the solution is subjected to high-temperature spray pyrolysis to obtain a precursor powder, which is then vibrated or compacted to a density of 0.3 g / cm 3 ~3.2g / cm 3 The process involves calcining the precursor powder to ensure uniform dispersion and close contact of lithium, nickel, cobalt, and manganese ions within the powder. This precursor powder is then cooled to produce a carbon-coated lithium nickel cobalt manganese oxide cathode material. However, the interfacial transport performance and stability of this carbon-coated lithium nickel cobalt manganese oxide cathode material still struggle to meet the requirements of practical applications. Furthermore, the disclosed preparation method requires high-temperature spray pyrolysis, which is complex and requires high equipment requirements.

[0006] NCM cathode materials disclosed in the prior art all have certain drawbacks, including insufficient electronic conductivity and ion transport properties, insufficient interfacial stability, and insufficient structural stability. Consequently, batteries made with these materials suffer from insufficient rate capability and cycling stability. Therefore, the development and design of novel NCM composite cathode materials, their preparation methods, and batteries are crucial. Summary of the Invention

[0007] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide an NCM composite positive electrode material, a preparation method and a battery. The NCM composite positive electrode material provided by the present invention has good electronic conductivity, ion transport performance, interface stability and structural stability; therefore, the battery prepared with the NCM composite positive electrode material has excellent rate performance and cycle stability.

[0008] To achieve this object, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides an NCM composite cathode material, comprising a nickel-cobalt-manganese ternary material core and a nitrogen-doped carbon coating layer coated on the surface of the nickel-cobalt-manganese ternary material core;

[0010] Based on the mass of the nitrogen-doped carbon coating layer as 100%, the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 4.8-10.5 wt %.

[0011] In the present invention, taking the mass of the nitrogen-doped carbon coating layer as 100%, the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 4.8-10.5wt%, for example, it can be 4.8wt%, 5.0wt%, 5.5wt%, 6.0wt%, 6.5wt%, 7.0wt%, 7.5wt%, 8.0wt%, 8.5wt%, 9.0wt%, 9.5wt%, 10.0wt% or 10.5wt%, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0012] In the present invention, the material of the nickel-cobalt-manganese ternary material core includes but is not limited to any one of NCM622, NCM811 or NCM523, or a combination of at least two of them.

[0013] In the NCM composite positive electrode material provided by the present invention, a nitrogen-doped carbon coating layer is coated on the outside of the nickel-cobalt-manganese ternary material core; first, due to the doping of nitrogen into the carbon coating layer, more defects and polarization are introduced into the nitrogen-doped carbon coating layer, a more continuous conductive network is constructed, and nitrogen atoms are embedded in the carbon skeleton in the form of pyridinic nitrogen and graphitic nitrogen, thereby improving the electronic conductivity of the NCM composite positive electrode material; secondly, there is Coulomb attraction between the nitrogen sites in the nitrogen-doped carbon coating layer and lithium ions, thereby improving the ion transport performance of the NCM composite positive electrode material; thirdly, the nitrogen atoms in the nitrogen-doped carbon coating layer can form hydrogen bonds with the electrolyte molecules, inhibiting the decomposition of the electrolyte, and can also coordinate with the transition metal ions in the nickel-cobalt-manganese ternary material core to reduce metal dissolution, thereby improving the interface stability of the NCM composite positive electrode material; thirdly, due to the doping of nitrogen into the carbon coating layer, a nitrogen-doped carbon coating layer with stronger structural stability is obtained, thereby improving the structural stability of the NCM composite positive electrode material.

[0014] In the NCM composite positive electrode material provided by the present invention, when the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 4.8 to 10.5 wt%, based on the mass of the nitrogen-doped carbon coating layer, it can be ensured that while the structural stability of the nitrogen-doped carbon coating layer is maintained, it is more conducive to forming a conductive network with stronger conductivity, more conducive to enhancing the Coulomb attraction of lithium ions, and more conducive to forming hydrogen bonds with the electrolyte to inhibit the decomposition of the electrolyte, thereby further improving the electronic conductivity, ion transport performance and interface stability of the NCM composite positive electrode material.

[0015] In summary, the NCM composite cathode material provided by the present invention has good electronic conductivity, ion transport performance, interface stability and structural stability; therefore, the battery prepared with the NCM composite cathode material has excellent rate performance and cycle stability.

[0016] Preferably, based on the mass of the nitrogen-doped carbon coating layer, the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 6 to 10 wt%, for example, it can be 6.0 wt%, 6.2 wt%, 6.4 wt%, 6.6 wt%, 6.8 wt%, 7.0 wt%, 7.2 wt%, 7.4 wt%, 7.6 wt%, 7.8 wt%, 8.0 wt%, 8.2 wt%, 8.4 wt%, 8.6 wt%, 8.8 wt%, 9.0 wt%, 9.2 wt%, 9.4 wt%, 9.6 wt%, 9.8 wt% or 10.0 wt%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0017] Preferably, the ratio of the D50 particle size of the nickel-cobalt-manganese ternary material core to the thickness of the nitrogen-doped carbon coating layer is (50-200):1, for example, it can be 50:1, 60:1, 70:1, 80:1, 90:1, 100:1, 110:1, 120:1, 130:1, 140:1, 150:1, 160:1, 170:1, 180:1, 190:1 or 200:1, but is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable, preferably (80-150):1.

[0018] Preferably, the thickness of the nitrogen-doped carbon coating layer is 3 to 10 nm, for example, it can be 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, 5.5 nm, 6 nm, 6.5 nm, 7 nm, 7.5 nm, 8 nm, 8.5 nm, 9 nm, 9.5 nm or 10 nm, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0019] Preferably, the porosity of the nitrogen-doped carbon coating layer is 35-55%, for example, it can be 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54% or 55%, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable, preferably 40-50%.

[0020] In a second aspect, the present invention provides a method for preparing the NCM composite positive electrode material according to the first aspect, the preparation method comprising:

[0021] A ternary nickel-cobalt-manganese material, polyacrylonitrile powder and a solvent are mixed to obtain a slurry, and the slurry is heat-treated in a protective atmosphere to obtain an NCM composite positive electrode material;

[0022] The holding temperature during the heat treatment is 700-780°C.

[0023] In the present invention, the holding temperature in the heat treatment is 700-780°C, for example, it can be 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C or 780°C, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0024] In the present invention, the polyacrylonitrile (PAN) is a nitrogen-containing polymer, which forms a nitrogen-doped carbon coating layer containing nitrogen-doped carbon material on the surface of the nickel-cobalt-manganese ternary material after high-temperature cracking.

[0025] In the present invention, polyacrylonitrile (PAN) is used as the coating material. Since polyacrylonitrile contains abundant cyano groups (C≡N) in its molecular structure, it can simultaneously provide carbon and nitrogen sources during the pyrolysis process, realizing in-situ nitrogen doping. Nitrogen atoms are embedded in the carbon skeleton in the form of pyridinic nitrogen and graphitic nitrogen, which significantly improves the electronic conductivity and electrochemical activity of the carbon layer. While improving the performance of the NCM composite positive electrode material, it avoids the complex process of adding an external nitrogen source in the traditional method.

[0026] In the present invention, a solution method is used (nickel-cobalt-manganese ternary material, polyacrylonitrile powder and solvent are used to obtain a slurry) to uniformly coat PAN on the surface of NCM particles, and then high-temperature pyrolysis is performed to ensure the continuity and uniformity of the nitrogen-doped carbon layer, significantly improving the interfacial charge transfer efficiency.

[0027] In the present invention, in a protective atmosphere, the holding temperature during heat treatment is controlled to be 700-780°C, thereby avoiding destruction of the structure of the NCM ternary material while fully carbonizing the polyacrylonitrile and forming a highly conductive nitrogen-doped carbon coating layer, and ensuring that the mass fraction of nitrogen in the final obtained nitrogen-doped carbon coating layer is 4.8-10.5wt%.

[0028] Preferably, the mixing method includes ultrasonic dispersion, and the ultrasonic dispersion time is 30 to 60 minutes, for example, it can be 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes or 60 minutes, but it is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0029] Preferably, between the mixing and the heat treatment, the process further comprises: volatilizing the solvent by stirring and heating to obtain a PAN-coated NCM positive electrode material.

[0030] Preferably, the stirring and heating is carried out at a rotation speed of 300 to 600 rpm, a temperature of 130 to 150° C., and a time of 30 to 60 min.

[0031] In the present invention, the rotation speed of the stirring heating is 300-600 rpm, for example, it can be 300 rpm, 350 rpm, 400 rpm, 450 rpm, 500 rpm, 550 rpm or 600 rpm, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0032] In the present invention, the stirring and heating temperature is 130-150°C, for example, it can be 130°C, 132°C, 134°C, 136°C, 138°C, 140°C, 142°C, 144°C, 146°C, 148°C or 150°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0033] In the present invention, the stirring and heating time is 30 to 60 minutes, for example, it can be 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes or 60 minutes, but it is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0034] Preferably, the mass ratio of the nickel-cobalt-manganese ternary material to the polyacrylonitrile powder in the mixture is (5-20):1, for example, it can be 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1 or 20:1, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0035] Preferably, the heat treatment includes heating and keeping the temperature in a sequential manner.

[0036] Preferably, the heating rate is 2 to 5°C / min, for example, it can be 2°C / min, 2.5°C / min, 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min or 5°C / min, but is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable. The end temperature of the heating is the insulation temperature.

[0037] Preferably, the insulation temperature is 700-750° C. and the insulation time is 1-3 hours.

[0038] In the present invention, the insulation temperature is 700-750°C, for example, it can be 700°C, 705°C, 710°C, 715°C, 720°C, 725°C, 730°C, 735°C, 740°C, 745°C or 750°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0039] In the present invention, the insulation time is 1 to 3 hours, for example, it can be 1 hour, 1.2 hours, 1.4 hours, 1.6 hours, 1.8 hours, 2 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 hours or 3 hours, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0040] Preferably, the protective atmosphere comprises nitrogen and / or an inert gas.

[0041] Preferably, the preparation method further comprises cooling and grinding performed sequentially after the heat treatment.

[0042] As a preferred technical solution of the preparation method of the present invention, the preparation method comprises:

[0043] (1) A nickel-cobalt-manganese ternary material and polyacrylonitrile powder with a mass ratio of (5-20):1 were mixed with a solvent by ultrasonic dispersion for 30-60 min to obtain a slurry with a solid content of 10-20%, and then the solvent was evaporated by stirring and heating at a speed of 300-600 rpm, a temperature of 130-150°C, and a time of 30-60 min to obtain a PAN-coated NCM positive electrode material;

[0044] (2) The PAN-coated NCM cathode material obtained in step (1) is heated to 700-750° C. at a rate of 2-5° C. / min in a protective atmosphere, kept warm for 1-3 hours, and then cooled and ground in sequence to obtain an NCM composite cathode material.

[0045] In a third aspect, the present invention provides a battery comprising the NCM composite positive electrode material described in the first aspect.

[0046] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] (1) In the NCM composite positive electrode material provided by the present invention, a nitrogen-doped carbon coating layer is coated on the outside of the nickel-cobalt-manganese ternary material core; first, due to the doping of nitrogen into the carbon coating layer, more defects and polarization are introduced into the nitrogen-doped carbon coating layer, and a more continuous conductive network is constructed, thereby improving the electronic conductivity of the NCM composite positive electrode material; second, the nitrogen sites in the nitrogen-doped carbon coating layer have Coulomb attraction with lithium ions, thereby improving the ion transport performance of the NCM composite positive electrode material; third, the nitrogen atoms in the nitrogen-doped carbon coating layer can form hydrogen bonds with the electrolyte molecules, inhibiting the decomposition of the electrolyte, and can also coordinate with the transition metal ions in the nickel-cobalt-manganese ternary material core to reduce metal dissolution, thereby improving the interface stability of the NCM composite positive electrode material; third, due to the doping of nitrogen into the carbon coating layer, a nitrogen-doped carbon coating layer with stronger structural stability is obtained, thereby improving the structural stability of the NCM composite positive electrode material.

[0049] (2) In the NCM composite positive electrode material provided by the present invention, when the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 4.8 to 10.5 wt %, the structural stability of the nitrogen-doped carbon coating layer can be ensured, which is more conducive to forming a conductive network with stronger conductivity, more conducive to enhancing the Coulomb attraction to lithium ions, and more conducive to forming hydrogen bonds with the electrolyte to inhibit the decomposition of the electrolyte, thereby further improving the electronic conductivity, ion transport performance and interface stability of the NCM composite positive electrode material.

[0050] (3) The NCM composite cathode material provided by the present invention has good electronic conductivity, ion transport performance, interface stability and structural stability; therefore, the battery prepared with the NCM composite cathode material has excellent rate performance and cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 This is the EMPA diagram of the NCM composite positive electrode material provided in Example 1. DETAILED DESCRIPTION

[0052] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0053] Example 1

[0054] This embodiment provides an NCM composite positive electrode material, the composite positive electrode material comprising an NCM622 core and a nitrogen-doped carbon coating layer coated on the surface of the NCM622 core;

[0055] Based on the mass of the nitrogen-doped carbon coating layer as 100%, the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 7.6 wt %;

[0056] The porosity of the nitrogen-doped carbon coating layer is 45%;

[0057] The ratio of the D50 particle size of the NCM622 core to the thickness of the nitrogen-doped carbon coating layer is 120:1, and the thickness of the nitrogen-doped carbon coating layer is 6 nm;

[0058] The preparation method of the NCM composite positive electrode material comprises:

[0059] (1) NCM622 and polyacrylonitrile powders at a mass ratio of 12:1 were mixed with N,N-dimethylformamide by ultrasonic dispersion for 45 min to obtain a slurry with a solid content of 15%. The slurry was then heated at a speed of 450 rpm and a temperature of 140°C for 45 min to evaporate the solvent, thereby obtaining a PAN-coated NCM cathode material.

[0060] (2) The PAN-coated NCM positive electrode material obtained in step (1) was heated to 725°C at a rate of 3.5°C / min in a nitrogen atmosphere and kept warm for 2 hours, and then cooled and ground in sequence to obtain an NCM composite positive electrode material.

[0061] The NCM composite cathode material provided in this embodiment was tested using electron probe microanalysis technology, and the EMPA diagram of the NCM composite cathode material was obtained as shown in FIG. Figure 1 shown.

[0062] Example 2

[0063] This embodiment provides an NCM composite positive electrode material, the composite positive electrode material comprising an NCM622 core and a nitrogen-doped carbon coating layer coated on the surface of the NCM622 core;

[0064] Based on the mass of the nitrogen-doped carbon coating layer as 100%, the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 6 wt %;

[0065] The porosity of the nitrogen-doped carbon coating layer is 50%;

[0066] The ratio of the D50 particle size of the NCM622 core to the thickness of the nitrogen-doped carbon coating layer is 80:1, and the thickness of the nitrogen-doped carbon coating layer is 8 nm;

[0067] The preparation method of the NCM composite positive electrode material comprises:

[0068] (1) NCM622 and polyacrylonitrile powders in a mass ratio of 8:1 were mixed with N,N-dimethylformamide by ultrasonic dispersion for 52 min to obtain a slurry with a solid content of 10%. The solvent was then evaporated by stirring and heating at a speed of 520 rpm and a temperature of 135°C for 52 min to obtain a PAN-coated NCM positive electrode material;

[0069] (2) The PAN-coated NCM positive electrode material obtained in step (1) was heated to 750°C at a rate of 4°C / min in a nitrogen atmosphere and kept warm for 1.5 hours, and then cooled and ground in sequence to obtain an NCM composite positive electrode material.

[0070] Example 3

[0071] This embodiment provides an NCM composite positive electrode material, the composite positive electrode material comprising an NCM622 core and a nitrogen-doped carbon coating layer coated on the surface of the NCM622 core;

[0072] Based on the mass of the nitrogen-doped carbon coating layer as 100%, the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 10 wt %;

[0073] The porosity of the nitrogen-doped carbon coating layer is 40%;

[0074] The ratio of the D50 particle size of the NCM622 core to the thickness of the nitrogen-doped carbon coating layer is 150:1, and the thickness of the nitrogen-doped carbon coating layer is 5 nm;

[0075] The preparation method of the NCM composite positive electrode material comprises:

[0076] (1) NCM622 and polyacrylonitrile powders at a mass ratio of 16:1 were mixed with N,N-dimethylformamide by ultrasonic dispersion for 38 min to obtain a slurry with a solid content of 20%. The solvent was then evaporated by stirring and heating at a speed of 380 rpm and a temperature of 145°C for 38 min to obtain a PAN-coated NCM positive electrode material;

[0077] (2) The PAN-coated NCM positive electrode material obtained in step (1) was heated to 715°C at a rate of 2.8°C / min in a nitrogen atmosphere and kept warm for 2.5 hours, and then cooled and ground in sequence to obtain an NCM composite positive electrode material.

[0078] Example 4

[0079] This embodiment provides an NCM composite positive electrode material, the composite positive electrode material comprising an NCM622 core and a nitrogen-doped carbon coating layer coated on the surface of the NCM622 core;

[0080] Based on the mass of the nitrogen-doped carbon coating layer as 100%, the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 4.8 wt %;

[0081] The porosity of the nitrogen-doped carbon coating layer is 35%;

[0082] The ratio of the D50 particle size of the NCM622 core to the thickness of the nitrogen-doped carbon coating layer is 50:1, and the thickness of the nitrogen-doped carbon coating layer is 10 nm;

[0083] The preparation method of the NCM composite positive electrode material comprises:

[0084] (1) NCM622 and polyacrylonitrile powders at a mass ratio of 5:1 were mixed with N,N-dimethylformamide by ultrasonic dispersion for 60 min to obtain a slurry with a solid content of 15%. The solvent was then evaporated by stirring and heating at a speed of 600 rpm and a temperature of 130°C for 60 min to obtain a PAN-coated NCM positive electrode material;

[0085] (2) The PAN-coated NCM positive electrode material obtained in step (1) was heated to 700°C at a rate of 2°C / min in a nitrogen atmosphere and kept warm for 3 hours, and then cooled and ground in sequence to obtain an NCM composite positive electrode material.

[0086] Example 5

[0087] This embodiment provides an NCM composite positive electrode material, the composite positive electrode material comprising an NCM622 core and a nitrogen-doped carbon coating layer coated on the surface of the NCM622 core;

[0088] Based on the mass of the nitrogen-doped carbon coating layer as 100%, the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 10.5 wt %;

[0089] The porosity of the nitrogen-doped carbon coating layer is 55%;

[0090] The ratio of the D50 particle size of the NCM622 core to the thickness of the nitrogen-doped carbon coating layer is 200:1, and the thickness of the nitrogen-doped carbon coating layer is 3 nm;

[0091] The preparation method of the NCM composite positive electrode material comprises:

[0092] (1) NCM622 and polyacrylonitrile powders at a mass ratio of 20:1 were mixed with N,N-dimethylformamide by ultrasonic dispersion for 30 min to obtain a slurry with a solid content of 15%. The solvent was then evaporated by stirring and heating at a speed of 300 rpm and a temperature of 150°C for 30 min to obtain a PAN-coated NCM positive electrode material;

[0093] (2) The PAN-coated NCM positive electrode material obtained in step (1) was heated to 780°C at a rate of 5°C / min in a nitrogen atmosphere and then kept warm for 1 hour, and then cooled and ground in sequence to obtain an NCM composite positive electrode material.

[0094] Example 6

[0095] This embodiment provides an NCM composite cathode material, wherein the ratio of the D50 particle size of the NCM622 core to the thickness of the nitrogen-doped carbon coating layer is 20:1;

[0096] That is, in step (1) of the method for preparing the NCM composite positive electrode material, the mass ratio of the nickel-cobalt-manganese ternary material to the polyacrylonitrile powder is 2:1, and the rest is the same as in Example 1.

[0097] Example 7

[0098] This embodiment provides an NCM composite cathode material, wherein the ratio of the D50 particle size of the NCM622 core to the thickness of the nitrogen-doped carbon coating layer is 250:1;

[0099] That is, in step (1) of the method for preparing the NCM composite positive electrode material, the mass ratio of the nickel-cobalt-manganese ternary material to the polyacrylonitrile powder is 25:1, and the rest is the same as in Example 1.

[0100] Example 8

[0101] This embodiment provides an NCM composite positive electrode material, which is the same as Example 1 except that step (1) of the preparation method of the NCM composite positive electrode material is replaced by "ball milling the nickel-cobalt-manganese ternary material with a mass ratio of 12:1 to obtain a PAN-coated NCM positive electrode material."

[0102] Comparative Example 1

[0103] This comparative example provides an NCM composite positive electrode material, wherein the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 4 wt %, and the porosity of the nitrogen-doped carbon coating layer is 40%;

[0104] That is, except for heating to 670° C. and then maintaining the temperature in step (2) of the method for preparing the NCM composite positive electrode material, the rest is the same as in Example 1.

[0105] Comparative Example 2

[0106] This comparative example provides an NCM composite positive electrode material, wherein the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 11.5 wt % divided by the mass of the nitrogen-doped carbon coating layer, and the porosity of the nitrogen-doped carbon coating layer is 60%;

[0107] That is, except for heating to 800° C. and then keeping the temperature in step (2) of the method for preparing the NCM composite positive electrode material, the rest is the same as in Example 1.

[0108] Comparative Example 3

[0109] This comparative example provides an NCM composite cathode material, except that the nitrogen-doped carbon coating layer is replaced by a carbon coating layer;

[0110] That is, the preparation method of the NCM composite positive electrode material is the same as that of Example 1, except that the polyacrylonitrile powder in step (1) is replaced by glucose.

[0111] Comparative Example 4

[0112] This comparative example provides a NCM positive electrode material, which is the NCM622 positive electrode material in Example 1.

[0113] The NCM composite positive electrode material and NCM positive electrode material provided in the above embodiments and comparative examples are used to prepare positive electrode sheets. The method for preparing the positive electrode sheets is as follows: the NCM composite positive electrode material or NCM positive electrode material, conductive carbon black and polyvinylidene fluoride are mixed in a mass ratio of 92:4:4 to obtain a mixture, and the obtained mixture is then slurried with NMP (N-methylpyrrolidone) at a liquid-solid ratio of 1:1.3; the slurry obtained after slurrying is poured on aluminum foil, and kept warm in a vacuum oven at 40°C for 6 hours, and then kept warm at 110°C for 12 hours to remove NMP to obtain a positive electrode sheet.

[0114] The obtained positive electrode sheet is used to prepare a button half-cell. The method for preparing the button half-cell is as follows: the positive electrode sheet is punched into a disc with a diameter of 14 mm on a sheet punching machine as the positive electrode sheet, 1 mol / L LiPF6 is dissolved in a mixed solution of EC / EMC / EFC with a volume ratio of 1:1:1 as the electrolyte, and a polyethylene porous film is used as the separator; then, in an argon-protected glove box, the positive electrode shell, the positive electrode sheet, the separator, the electrolyte, the lithium negative electrode, and the negative electrode shell are assembled in the following order to obtain the button half-cell.

[0115] The obtained button half-cell was subjected to a rate performance test. The rate performance test method was as follows: the obtained button half-cell was charged and discharged at 0.1C / 0.1C, and the charge and discharge test was performed in a voltage window of 2.8-4.2V. The 0.1C rate performance of the button cell obtained by the test is shown in Table 1.

[0116] The obtained button half-cell was subjected to a cycle performance test. The cycle performance test method is as follows: the test temperature is 25°C, the voltage range is 2.8-4.2V, the test process is to first cycle for 5 cycles at a current density of 0.1C, then cycle for 5 cycles at 0.3C, 0.5C, 1C, 2C, 5C, and 0.1C respectively, and finally perform a long cycle test at 0.5C for 100 cycles. The capacity retention rate of the button half-cell after 100 cycles at a rate of 0.5C is shown in Table 1.

[0117] Table 1

[0118] 0.1C rate (mAh / g) Capacity retention rate (%) Example 1 201.5 90.9 Example 2 199.8 90.4 Example 3 203.2 92.5 Example 4 195.6 88.7 Example 5 197.3 89.2 Example 6 185.4 84.3 Example 7 187.9 85.1 Example 8 190.2 86.2 Comparative Example 1 178.6 80.5 Comparative Example 2 176.9 79.8 Comparative Example 3 182.4 82.1 Comparative Example 4 175.3 82.5

[0119] From Table 1, we can get:

[0120] (1) In the present invention, the batteries prepared using the NCM composite cathode materials provided in Examples 1 to 3 exhibit good rate performance and excellent cycle stability;

[0121] (2) By comparing Example 1 with Examples 4 and 5, it can be seen that, based on the mass of the nitrogen-doped carbon coating layer as a percentage, the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 4.8-10.5wt%; and when the porosity of the nitrogen-doped carbon coating layer is 35-55%, the NCM composite positive electrode material and the battery have better performance; when the nitrogen content is insufficient, it will lead to fewer electronic defect sites in the carbon layer and a discontinuous conductive network. When the porosity is low, it is not conducive to the penetration of the electrolyte, the ion diffusion path is limited, and the insufficient nitrogen sites at the interface make it difficult to inhibit side reactions. The structural buffering capacity also decreases due to insufficient porosity; when the nitrogen content is high, the excessive nitrogen content will lead to excessive distortion of the carbon skeleton, reduce the degree of graphitization, and increase the resistance. At the same time, the excessively high porosity will also reduce the mechanical strength of the nitrogen-doped carbon coating layer, and it will easily collapse during the cycle, thereby causing the performance of the battery to deteriorate;

[0122] (3) By comparing Example 1 with Examples 6 and 7, it can be seen that the ratio of the D50 particle size of the nickel-cobalt-manganese ternary material core to the thickness of the nitrogen-doped carbon coating layer will affect the performance of the NCM composite positive electrode material and the battery; when the ratio of the D50 particle size of the nickel-cobalt-manganese ternary material core to the thickness of the nitrogen-doped carbon coating layer is (50-200):1, the NCM composite positive electrode material and the battery have better performance. This thickness ratio can ensure that the nitrogen-doped carbon coating layer is evenly covered on the surface of the nickel-cobalt-manganese ternary material core to form a continuous conductive network, and avoid the decrease in the proportion of active materials or the increase in resistance caused by the excessive thickness of the nitrogen-doped carbon coating layer; at the same time, the appropriate thickness can balance the porosity and mechanical strength, buffer the stress during the charge and discharge volume change, and do not hinder the penetration of the electrolyte, thereby improving the performance of the battery;

[0123] (4) By comparing Example 1 with Example 8, it can be seen that in the present invention, PAN is uniformly coated on the surface of NCM particles by a solution method (nickel-cobalt-manganese ternary material, polyacrylonitrile powder and solvent are used to obtain a slurry), and then pyrolyzed at high temperature, thereby ensuring the continuity and uniformity of the nitrogen-doped carbon layer and significantly improving the interfacial charge transfer efficiency;

[0124] (5) By comparing Example 1 with Comparative Examples 1 to 4, it can be seen that in the NCM composite positive electrode material provided by the present invention, a nitrogen-doped carbon coating layer is coated on the outside of the nickel-cobalt-manganese ternary material core; first, due to the doping of nitrogen to the carbon coating layer, more defects and polarization are introduced into the nitrogen-doped carbon coating layer, a more continuous conductive network is constructed, and nitrogen atoms are embedded in the carbon skeleton in the form of pyridinic nitrogen and graphitic nitrogen, thereby improving the electronic conductivity of the NCM composite positive electrode material; secondly, there is Coulomb attraction between the nitrogen sites in the nitrogen-doped carbon coating layer and lithium ions, thereby improving the ion transport performance of the NCM composite positive electrode material; thirdly, the nitrogen atoms in the nitrogen-doped carbon coating layer can form hydrogen bonds with the electrolyte molecules, inhibit the decomposition of the electrolyte, and can also coordinate with the transition metal ions in the nickel-cobalt-manganese ternary material core to reduce metal dissolution, thereby improving the interface stability of the NCM composite positive electrode material; thirdly, due to the doping of nitrogen to the carbon coating layer, a nitrogen-doped carbon coating layer with stronger structural stability is obtained, thereby improving the structural stability of the NCM composite positive electrode material;

[0125] In the NCM composite positive electrode material provided by the present invention, when the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 4.8 to 10.5 wt %, based on the mass of the nitrogen-doped carbon coating layer, it is possible to ensure that while the structural stability of the nitrogen-doped carbon coating layer is maintained, it is more conducive to forming a conductive network with stronger conductivity, more conducive to enhancing the Coulomb attraction of lithium ions, and more conducive to forming hydrogen bonds with the electrolyte to inhibit the decomposition of the electrolyte, thereby further improving the electronic conductivity, ion transport performance and interface stability of the NCM composite positive electrode material;

[0126] The NCM composite cathode material provided by the present invention has good electronic conductivity, ion transport performance, interface stability and structural stability; therefore, the battery prepared with the NCM composite cathode material has excellent rate performance and cycle stability.

[0127] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and disclosure scope of the present invention.

Claims

1. An NCM composite cathode material, characterized in that: The composite positive electrode material comprises a nickel-cobalt-manganese ternary material core and a nitrogen-doped carbon coating layer coated on the surface of the nickel-cobalt-manganese ternary material core; Based on the mass of the nitrogen-doped carbon coating layer as 100%, the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 4.8-10.5 wt %.

2. The NCM composite cathode material according to claim 1, characterized in that Based on the mass of the nitrogen-doped carbon coating layer as 100%, the mass fraction of nitrogen in the nitrogen-doped carbon coating layer is 6-10 wt%.

3. The NCM composite cathode material according to claim 1, characterized in that The ratio of the D50 particle size of the nickel-cobalt-manganese ternary material core to the thickness of the nitrogen-doped carbon coating layer is (50-200):1, preferably (80-150):1; Preferably, the thickness of the nitrogen-doped carbon coating layer is 3 to 10 nm.

4. The NCM composite cathode material according to claim 1, characterized in that The porosity of the nitrogen-doped carbon coating layer is 35-55%, preferably 40-50%.

5. A method for preparing the NCM composite cathode material according to any one of claims 1 to 4, characterized in that: The preparation method comprises: A ternary nickel-cobalt-manganese material, polyacrylonitrile powder and a solvent are mixed to obtain a slurry, and the slurry is heat-treated in a protective atmosphere to obtain an NCM composite positive electrode material; The holding temperature during the heat treatment is 700-780°C.

6. The preparation method according to claim 5, characterized in that The mixing method includes ultrasonic dispersion, and the ultrasonic dispersion time is 30 to 60 minutes; Preferably, between the mixing and the heat treatment, the process further comprises: volatilizing the solvent by stirring and heating to obtain a PAN-coated NCM positive electrode material.

7. The preparation method according to claim 5, characterized in that The mass ratio of the nickel-cobalt-manganese ternary material to the polyacrylonitrile powder in the mixture is (5-20):

1.

8. The preparation method according to claim 5, characterized in that The heat treatment includes heating and keeping warm in sequence; Preferably, the heating rate is 2-5°C / min, and the end temperature of the heating is the holding temperature; Preferably, the insulation temperature is 700-750° C. and the insulation time is 1-3 hours.

9. The preparation method according to claim 5, characterized in that The preparation method comprises: (1) A nickel-cobalt-manganese ternary material and polyacrylonitrile powder with a mass ratio of (5-20):1 were mixed with a solvent by ultrasonic dispersion for 30-60 min to obtain a slurry with a solid content of 10-20%, and then the solvent was evaporated by stirring and heating at a speed of 300-600 rpm, a temperature of 130-150°C, and a time of 30-60 min to obtain a PAN-coated NCM positive electrode material; (2) The PAN-coated NCM cathode material obtained in step (1) is heated to 700-750° C. at a rate of 2-5° C. / min in a protective atmosphere, kept warm for 1-3 hours, and then cooled and ground in sequence to obtain an NCM composite cathode material.

10. A battery, characterized in that: The battery comprises the NCM composite positive electrode material according to any one of claims 1 to 4.

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