A multi-level carbon-coated ternary composite material and its preparation method

Abstract

This invention discloses a multi-level carbon-coated ternary composite material and its preparation method, comprising the following steps: S1: Nanoscale wet grinding of single-crystal NCM and expanded graphite, allowing NCM to be inserted between the expanded graphite layers, followed by drying to obtain mixture 1; S2: Mixing mixture 1 from S1 with medium-temperature pitch to obtain mixture 2; S3: Low-temperature sintering of mixture 2 obtained in S2, followed by crushing to obtain the multi-level carbon-coated ternary composite material. This invention embeds single-crystal NCM into the expanded graphite layers through grinding, improving the electronic conductivity of NCM and enhancing rate performance. The expanded graphite adapts to the volume expansion of single-crystal NCM during charge and discharge, maintaining close contact between materials, thereby improving cycle life. The secondary coating of the material with low-softening-point medium-temperature pitch further reduces the contact area between single-crystal NCM and the electrolyte, improving cycle stability. The medium-temperature pitch can complete carbonization at a lower temperature, avoiding the collapse of the NCM structure during high-temperature carbonization.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a multi-level carbon-coated ternary composite material and its preparation method. Background Technology

[0002] In recent years, ternary lithium-ion batteries have gained an increasingly important position in the power battery field due to their advantages such as high capacity and low-temperature resistance. However, ternary materials (lithium nickel cobalt manganese oxide, NCM) still have shortcomings in terms of air stability, cycle stability, and thermal stability. First, unstable lattice oxygen in ternary materials is released during charge and discharge, posing a serious safety hazard. Second, during oxygen release, the surface of ternary materials is also prone to conversion into an electrochemically inert salt rock phase, exacerbating the Li / Ni mixing problem, thus causing capacity loss and kinetic degradation. Third, the highly active Ni on the surface of ternary materials... 4+ It is easy to catalyze the decomposition of electrolyte, resulting in passivation of the material surface; finally, ternary materials are prone to abrupt volume changes during deep delithiation, leading to the generation of microcracks and reducing cycle stability.

[0003] To address the aforementioned issues, surface modification of ternary materials through coating is the primary approach. Carbon materials, with their superior conductivity, high electrochemical / thermal stability, and low cost, are commonly used coating materials for lithium batteries. CN202310223376.8 discloses a porous carbon-coated ternary cathode material, its preparation method, and its applications. This patent utilizes an initial wet impregnation method to coat the surface of a quinone-amine polymer onto the ternary material, followed by high-temperature sintering to obtain a porous carbon-coated ternary cathode material. The presence of the carbon coating layer effectively reduces side reactions between the ternary cathode material and the electrolyte during cycling, improving conductivity and thus increasing cycle stability and rate performance. CN201811289186.1 discloses a method for preparing a carbon-coated ternary cathode material and the resulting carbon-coated ternary cathode material. This patent involves mixing a ternary precursor and lithium salt and adding it to a carbon source solution. Through freeze-drying and high-temperature sintering, uniform carbon coating is achieved on the surface and inside of the ternary material, significantly improving its rate performance and cycle stability. However, current methods for coating ternary materials with carbon still suffer from problems such as weak bonding between the ternary material and the carbon layer, and the presence of carbon during high-temperature calcination easily leading to structural collapse of the ternary material, resulting in capacity loss. Summary of the Invention

[0004] This invention provides a multi-level carbon-coated ternary composite material and its preparation method, which enhances the bonding strength between carbon materials and ternary materials to improve the cycle stability and rate performance of ternary materials.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A method for preparing a multi-level carbon-coated ternary composite material includes the following steps:

[0007] S1: Single-crystal NCM and expanded graphite are subjected to nanoscale wet grinding to allow NCM to be inserted between the expanded graphite layers. After drying, a mixture 1 is obtained.

[0008] S2: Mix the mixture 1 obtained in step S1 with medium-temperature asphalt to obtain mixture 2;

[0009] S3: The mixture 2 obtained in step S2 is subjected to low-temperature sintering and then crushed to obtain a multi-stage carbon-coated ternary composite material.

[0010] Furthermore, in step S1, the wet grinding conditions are as follows: grinding is performed using a low-temperature planetary ball mill, the grinding media is zirconia balls, the grinding time is 20–50 hours, the grinding speed is 500–1000 rpm, the grinding atmosphere is an inert atmosphere, the grinding temperature is 5–10°C, and the ball milling solvent is one of ethanol, isopropanol, and N-methylpyrrolidone, with a solid content of 30–50%. This grinding process can also be performed using a horizontal sand mill.

[0011] Furthermore, in step S1, the amount of expanded graphite used is 1wt% to 4wt%.

[0012] Furthermore, the softening point of the medium-temperature asphalt in step S2 is 80–100°C; the amount of medium-temperature asphalt used is 0.5–2 wt%.

[0013] Furthermore, in step S1, the single-crystal NCM material includes any one of NCM811, NCM613, NCM622, and NCM523.

[0014] Furthermore, in step S3, the sintering is a gradient sintering including stage I and stage II. The sintering conditions for stage I are: sintering at 180-250°C for 4-6 hours in an air atmosphere.

[0015] The sintering conditions for Stage II are: sintering at 400–550℃ for 2–4 hours in an inert atmosphere;

[0016] The heating rate for gradient sintering is 5–10 °C / min.

[0017] Furthermore, in step S1, the drying method is forced air drying at 80-100°C for 2-5 hours.

[0018] Furthermore, in step S3, the equipment used in the crushing process includes one or more of the following: jaw crusher, double roller mill, mechanical pulverizer, and air jet mill.

[0019] A multi-level carbon-coated ternary composite material, prepared by the preparation method disclosed in this invention.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] This invention discloses a method for preparing a multi-level carbon-coated ternary composite material. By embedding NCM single crystals into the interlayer of expanded graphite through a grinding process, a good conductive carbon network can be constructed, improving the electronic conductivity of NCM and enhancing rate performance. Simultaneously, the flexible expanded graphite can adapt to the volume expansion of the single-crystal NCM during charge and discharge, maintaining close contact between the two materials and achieving good structural stability, thereby improving cycle life. Furthermore, secondary coating of the above material with low-softening-point medium-temperature pitch further reduces the contact area between the single-crystal NCM and the electrolyte, thereby reducing interfacial side reactions and improving the cycle stability of the cathode. The lower softening point also improves the fluidity of the medium-temperature pitch during high-temperature mixing, thus improving coating uniformity. Simultaneously, the medium-temperature pitch can be carbonized at a lower temperature, avoiding the collapse of the NCM structure during high-temperature carbonization. Based on the above design of embedding single-crystal NCM into a multi-level carbon framework, the prepared composite material has good charge transport channels, with a 3C / 0.1C capacity retention rate of over 84%. At the same time, the composite material exhibits high structural stability, with a capacity retention rate of over 91% after 200 cycles. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1:

[0024] S1: Weigh 250g of ternary material (NCM811) and put it into a low-temperature planetary ball mill, add 2wt% expanded graphite, use ethanol as solvent (solid content 30%), use zirconia balls as grinding media, ball mill for 30h under nitrogen atmosphere, speed of 1000rpm, ball milling temperature of 7±2℃, after ball milling, dry in a forced-air drying oven for 4 hours to obtain mixture 1;

[0025] S2: Place mixture 1 into a high-speed shear mixer and add 1 wt% of medium-temperature asphalt with a softening point of 80℃. Then, stir at 110℃ and 30 Hz for 1 hour to obtain mixture 2.

[0026] S3: Place the mixture 2 into a tube furnace and perform pre-oxidation treatment in an air atmosphere to solidify the medium-temperature asphalt. The pre-oxidation heating rate is 5℃ / min, the heat treatment temperature is 250℃, and the holding time is 4h (Stage I sintering); then perform carbonization treatment on the mixture 2 in a nitrogen atmosphere. The carbonization heating rate is 5℃ / min, the heat treatment temperature is 450℃, and the holding time is 2h (Stage II sintering).

[0027] After the sintered mixture 2 is cooled down, the collected material is obtained. The collected material is washed repeatedly with deionized water 3 times. After drying, the collected material is crushed by a roller mill and a mechanical crusher to obtain a multi-stage carbon-coated ternary composite material.

[0028] Example 2:

[0029] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the ball milling time in step S1 is 20 hours.

[0030] Example 3

[0031] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the ball milling time in step S1 is 40 hours.

[0032] Example 4

[0033] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the ball milling time in step S1 is 50 hours.

[0034] Example 5

[0035] The only difference between this embodiment and Embodiment 1 is that, in this embodiment, the amount of expanded graphite used in step S1 is 4 wt%.

[0036] Example 6

[0037] The only difference between this embodiment and Embodiment 1 is that, in this embodiment, the amount of expanded graphite used in step S1 is 1 wt%.

[0038] Example 7

[0039] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the amount of medium-temperature asphalt used in step S2 is 2wt%.

[0040] Example 8

[0041] The only difference between this embodiment and embodiment 1 is that in this embodiment, the amount of medium-temperature asphalt used in step S2 is 0.5 wt%.

[0042] Example 9

[0043] The only difference between this embodiment and Embodiment 1 is that the carbonization temperature in step S3 is 550°C.

[0044] Comparative Example 1

[0045] The only difference between this comparative example and Example 1 is that step S1 is omitted in this comparative example, i.e., expanded graphite is not used as a carbon coating material to treat single-crystal NCM.

[0046] Comparative Example 2

[0047] The only difference between this comparative example and Example 1 is that step S2 is omitted in this comparative example, i.e., medium-temperature asphalt is not used as a carbon coating material to mix with the mixture 1 obtained in step S1.

[0048] Test example:

[0049] (1) Test Method: Lithium-ion battery cathodes were prepared using the ternary materials obtained in Examples 1-9 and Comparative Examples 1-2, and their electrochemical performance was tested. Specifically, the composite material was mixed uniformly with conductive carbon black and PVDF in NMP at a mass ratio of 80:10:10, and then coated, baked, and rolled to prepare the lithium-ion battery cathode. A commercial lithium-ion battery electrolyte (1M LiPF6 in EC / DMC (vol. 1:1)) was used to assemble button cells, and constant current charge-discharge tests were conducted. The counter electrode was lithium metal, and the separator was Celgard 2400. The constant current charge-discharge current density was 1C = 172 mA / g, and the voltage range during the test was set to 2.5–4.2V vs. Li / Li. + The test environment was 25℃ and 45% RH.

[0050] (2) The test results are shown in Table 1:

[0051] Table 1: Electrochemical performance test results of multi-level carbon-coated ternary composite materials

[0052]

[0053] Examples 1-4 illustrate that as the ball milling time of single-crystal NCM and expanded graphite is extended (from 20 hours to 50 hours), the rate performance (3C / 0.1C capacity retention) of the composite material increases from 85.2% to 88.2%, and the capacity retention after 200 cycles increases from 91.1% to 94.2%. This is because extending the ball milling time allows the single-crystal NCM particles to be fully embedded between the expanded graphite layers under mechanical shearing, extrusion, and friction, improving the uniformity of the composite and thus enhancing cycle stability and rate performance. Examples 1, 5, 6, 7, and 8 demonstrate that increasing the amount of coated carbon material with high electrical conductivity can increase its proportion in the composite material, thereby improving electronic conductivity and enhancing rate performance. The 3C / 0.1C capacity retention increased from 84.5% to 89.2%. Simultaneously, increasing the amount of electrochemically inert carbon material further reduces the contact area between the single-crystal NCM electrode and the electrolyte, thereby reducing interfacial side reactions and improving cycle stability. However, since the carbon material does not contribute capacity, it leads to a loss in the specific capacity of the composite material (the 0.1C specific capacity decreased from 202.7 mAh / g to 195.2 mAh / g). Examples 1 and 9 illustrate that increasing the carbonization temperature of the medium-temperature pitch can improve rate performance because high temperature helps reduce the defect content of the carbon material, thereby increasing electronic conductivity. However, increasing the carbonization temperature slightly reduces specific capacity and cycle stability, possibly because the reducing products of the carbon material at high temperatures cause the collapse of the ternary structure. Examples 1 and Comparative Examples 1 and 2 demonstrate that the cycle stability and rate performance of the single-crystal NCM ternary material can be effectively improved using the multi-level carbon material framework design of this invention.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a multi-level carbon-coated ternary composite material, characterized in that, Includes the following steps: S1: Single-crystal NCM and expanded graphite are subjected to nanoscale wet grinding to allow NCM to be inserted between the expanded graphite layers. After drying, a mixture 1 is obtained. The wet grinding conditions are as follows: grinding is performed using a low-temperature planetary ball mill; the grinding media is zirconia balls; the grinding time is 20-50 hours; the grinding speed is 500-1000 rpm; the grinding atmosphere is an inert atmosphere; the grinding temperature is 5-10℃; the ball milling solvent is one of ethanol, isopropanol, and N-methylpyrrolidone; the solid content is 30-50%; and the amount of expanded graphite used is 1-4 wt%. S2: The mixture 1 obtained in step S1 is mixed with medium-temperature asphalt to obtain mixture 2; the softening point of the medium-temperature asphalt is 80~100℃; the amount of medium-temperature asphalt used is 0.5~2wt%; S3: The mixture 2 obtained in step S2 is subjected to low-temperature sintering, followed by crushing to obtain a multi-stage carbon-coated ternary composite material; The sintering is a gradient sintering including stage I and stage II. The sintering conditions for stage I are: sintering at 180~250℃ for 4~6 hours in an air atmosphere. The sintering conditions for Stage II are: sintering at 400~550℃ for 2~4 hours in an inert atmosphere; The heating rate for gradient sintering is 5~10℃ / min.

2. The method for preparing a multi-level carbon-coated ternary composite material according to claim 1, characterized in that, In step S1, the single-crystal NCM material includes any one of NCM811, NCM613, NCM622, and NCM523.

3. The method for preparing a multi-level carbon-coated ternary composite material according to claim 1, characterized in that, In step S1, the drying method is forced air drying at 80~100℃ for 2~5 hours.

4. The method for preparing a multi-level carbon-coated ternary composite material according to claim 1, characterized in that, In step S3, the equipment used in the crushing process includes one or more of the following: jaw crusher, double roll mill, and air jet mill.

5. A multi-level carbon-coated ternary composite material, characterized in that, The multi-level carbon-coated ternary composite material prepared by the preparation method according to any one of claims 1-4.

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