Composite negative electrode material and preparation method thereof, lithium ion battery and electric equipment

Abstract

The invention provides a composite negative electrode material and a preparation method thereof, a lithium ion battery and electric equipment, and relates to the technical field of lithium ion batteries, the composite negative electrode material comprises an inner core and a shell layer coating the inner core, the inner core is hard carbon doped with nitrogen element, and the hard carbon contains a plurality of mesopores and micropores; and the shell layer is a compact carbon layer. The invention also provides a preparation method of the composite negative electrode material, and a lithium ion battery and electric equipment based on the composite negative electrode material. According to the technical scheme provided by the invention, the problems of low first reversible capacity and low first coulombic efficiency of a hard carbon negative electrode material in the prior art are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a composite negative electrode material and a preparation method thereof, a lithium ion battery, and electrical equipment. Background Art

[0002] Carbon-based lithium-ion negative electrode materials mainly include soft carbon and hard carbon. Among them, hard carbon is used as a lithium-ion negative electrode material, which has a much higher Li + Diffusion coefficient, good low temperature performance. However, hard carbon is not widely used at present because it is difficult to develop hard carbon and its energy density is low, which makes it difficult to meet the capacity and first efficiency requirements of lithium-ion batteries. Therefore, how to improve the energy density of hard carbon has become the key to promoting the large-scale application of hard carbon in lithium-ion negative electrode materials. Summary of the invention

[0003] The object of the present invention is to provide a composite negative electrode material, which improves the problems of low first reversible capacity and low first coulombic efficiency of hard carbon negative electrode materials in the prior art.

[0004] Another object of the present invention is to provide a method for preparing a composite negative electrode material.

[0005] Another object of the present invention is to provide a lithium ion battery.

[0006] Another object of the present invention is to provide an electrical device.

[0007] In a first aspect, the present application provides a material including an inner core and a shell layer coated on the inner core, wherein the inner core is hard carbon doped with nitrogen and contains a plurality of mesopores and micropores; and the shell layer is a dense carbon layer.

[0008] Further, in some embodiments of the present application, the dense carbon layer closes the mesopores and / or the micropores; and / or

[0009] The dense carbon layer is a cracked carbon layer and / or an amorphous carbon layer.

[0010] Furthermore, in some embodiments of the present application, the porosity of the hard carbon is 10% to 50%; and the volume proportion of the micropores in the pores of the hard carbon is 1% to 10%.

[0011] The nitrogen content in the inner core is 0.01-10% by mass.

[0012] Furthermore, in some embodiments of the present application, the compacted density of the core is 1.05-1.15 g / cm 3 ; and / or

[0013] The thickness of the shell layer is 2 to 15 nm.

[0014] Further, in some embodiments of the present application, the D50 of the composite anode material is 8-10 μm, and the particle size of the composite anode material is not greater than 25 μm; and / or

[0015] The BET specific surface area of the composite anode material is 1-20 m 2 / g.

[0016] Further, in some embodiments of the present application, in the Raman test of the core, there is a D peak in the range of 1350 cm -2 ±20 cm -2 range, a G peak in the range of 1580 cm -2 ±20 cm -2 range, and the value of ID1 / IG1 is 1.0-1.1. In the Raman test of the composite anode material, there is a D peak in the range of 1580 cm -2 ±20 cm -2 range, a G peak in the range of 1580 cm -2 ±20 cm -2 range, and the value of ID2 / IG2 is 1.1-1.6.

[0017] Further, in some embodiments of the present application, the proportion of the shell layer in the composite anode material is 0.01-3% by mass fraction.

[0018] In a second aspect, the present application further provides a method for preparing a composite anode material, comprising the following steps:

[0019] Provide a carbon source and a nitrogen source;

[0020] Mix the carbon source and the nitrogen source to obtain a mixture, and perform pre-carbonization treatment on the mixture to obtain a primary product;

[0021] Perform primary carbonization treatment on the primary product under an inert gas atmosphere to obtain a secondary product;

[0022] Crush and screen the secondary product, and perform secondary carbonization treatment on the screened product to obtain a core;

[0023] Coat a dense carbon layer on the surface of the core by chemical vapor deposition to obtain a composite anode material.

[0024] Further, in some embodiments of the present application, in coating a dense carbon layer on the surface of the core by chemical vapor deposition, the atmosphere used includes a hydrocarbon and an inert gas, and the flow ratio of the hydrocarbon to the inert gas is (10-100):100.

[0025] Further, in some embodiments of the present application, during the process of coating a dense carbon layer on the surface of the core by chemical vapor deposition, the process temperature is 800 - 1100 °C, and the process time is 2 - 8 h.

[0026] Further, in some embodiments of the present application, the treatment temperature of the pre-carbonization treatment is 150 - 250 °C, and the treatment time of the pre-carbonization treatment is 8 - 15 h; and / or

[0027] the treatment temperature of the primary carbonization treatment is 300 - 600 °C, and the treatment time of the primary carbonization treatment is 1 - 3 h; and / or

[0028] the treatment temperature of the secondary carbonization treatment is 1000 - 1200 °C, and the treatment time of the secondary carbonization treatment is 1 - 3 h.

[0029] Further, in some embodiments of the present application, the carbon source is selected from at least one of glucose, sucrose, starch, resin, and asphalt; and / or

[0030] the nitrogen source is selected from at least one of urea, ammonium chloride, ammonium bicarbonate, and aniline hydroazoamine; and / or

[0031] the mass ratio of the carbon source to the nitrogen source is 100:(10 - 40); and / or

[0032] the hydrocarbon is selected from at least one of methane, ethylene, and acetylene.

[0033] In a third aspect, the present application further provides a lithium-ion battery, including a negative electrode, and the negative electrode includes the composite negative electrode material described in the first aspect or the composite negative electrode material prepared by the preparation method of the composite negative electrode material described in the second aspect.

[0034] In a fourth aspect, the present application further provides an electrical device, including the lithium-ion battery described in the third aspect.

[0035] The embodiments of the present application provide a composite anode material, a preparation method thereof, a lithium-ion battery, and an electrical device. The nitrogen-doped hard carbon pyrolytic carbon composite anode material uses nitrogen-doped porous hard carbon as the core, and a dense pyrolytic carbon layer is coated on the porous hard carbon to improve the tap density, capacity, and initial Coulomb efficiency of the hard carbon. At the same time, the dense carbon layer on the surface of the porous hard carbon can also reduce the porosity of the porous hard carbon, improve the defect that the initial Coulomb efficiency of the composite anode material decreases due to the porous structure, ensure that the capacity, initial Coulomb efficiency, and tap density of the composite anode material are all optimized, and thus improve the energy density of the anode material based on hard carbon. In addition, both micropores and mesopores exist on the core of the composite anode material provided in the present application, and its initial reversible capacity and initial Coulomb efficiency are significantly improved. The initial reversible capacity can reach more than 480 mAh / g, and the initial Coulomb efficiency can reach more than 82%. Its energy density is close to or even higher than that of conventional graphite anodes. Description of the Drawings

[0036] Figure 1 It is a schematic structural diagram of the open pores and closed pores on the surface of the particles provided in the present application;

[0037] Figure 2 It is an SEM image of the core of the composite anode material before being coated provided in Example 1 of the present application.

[0038] Figure 3 It is an SEM image of the composite anode material particles formed after the core is coated provided in Example 1 of the present application.

[0039] Figure 4 It is a scatter plot of ID / IG of the core and the composite anode material particles provided in Example 1 of the present application.

[0040] Figure 5 It is a nitrogen adsorption and desorption curve of the composite anode material particles provided in Comparative Example 1 of the present application.

[0041] Figure 6 It is a nitrogen adsorption and desorption curve of the composite anode material particles provided in Example 1 of the present application;

[0042] Figure 7 It is a charge-discharge voltage-specific capacity curve of the battery using the composite anode material particles provided in Example 5 of the present application as the anode material. Detailed Embodiments

[0043] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the following will clearly and completely describe the technical solutions in the embodiments of this application with reference to the accompanying drawings in the embodiments of this application. Apparently, the described embodiments are only a part rather than all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts fall within the scope of protection of the present invention.

[0044] Hard carbon is carbon that cannot be graphitized after being heat-treated at temperatures above 2800°C. Its internal crystal arrangement is disorderly, with more pores, and lithium can be stored in the graphite sheet layers, closed micropores, surface, and defect sites. Therefore, it has a relatively high capacity and has the advantages of high charge capacity, excellent rate capacity, long cycle life, and good low-temperature performance compared to graphite anodes. However, as the anode material of lithium-ion batteries, hard carbon easily causes excessive consumption of lithium ions to form a SEI film, and it can also capture lithium, resulting in a large amount of charge "loss" during the first charge / discharge cycle, with low reversible capacity and first Coulomb efficiency. Moreover, there are usually some organic group fragments remaining in hard carbon, such as aromatic fragments, and lithium ions will bind to these sites, causing the potential to shift to a higher voltage when the lithium ions dissociate from these sites, resulting in voltage hysteresis. Therefore, in the prior art, the application of hard carbon as the anode material of lithium-ion batteries is not common.

[0045] Based on this, this application proposes a composite anode material suitable for lithium-ion batteries, which includes a core and a shell layer coated on the core. The core is hard carbon doped with nitrogen, and the hard carbon contains a plurality of mesopores and micropores; the shell layer is a dense pyrolytic carbon layer.

[0046] Among them, the dense carbon layer can close the mesopores and the micropores, especially the micropores, so that the opening rate of the mesopores and micropores on the composite anode material is not higher than 50%. It should be noted that when the dense carbon layer closes the micropores on the core, it should be understood that the dense pyrolytic carbon layer closes some / all of the micropores opened on the surface of the core. Both the micropore and mesopore structures can provide a large number of sites for lithium intercalation, and the porous structure is beneficial to improving the lithium intercalation capacity of the anode material. However, the micropores and mesopores will result in a relatively large specific surface area of the material, forming a large amount of SEI film structure during the first charge and discharge process, which reduces the first Coulombic efficiency of the anode material. Therefore, in this application, a dense pyrolytic carbon layer is used to close the micropores to form a closed pore structure. It can not only form a transport channel for lithium ions, improve the efficiency of lithium intercalation, and provide excellent sites for lithium storage, thereby improving the lithium intercalation capacity of the anode material; but also can improve the defects brought about by the large specific surface area, obvious formation of SEI film structure, and decline in the first Coulombic efficiency caused by micropores and mesopores. The doping of nitrogen element effectively improves the tap density of the material and the lithium intercalation capacity of the hard carbon material, providing a good matrix basis for preparing a hard carbon anode with high capacity and high first efficiency. It should be noted that the closed pores in this application should be understood as a pore structure closed on all sides, and the open pores should be understood as through holes communicating with the surface where the holes are opened. Figure 1 Schematic diagrams of the closed pores and open pores given.

[0047] It should be noted that the shell layer in this application should be a dense carbon layer rather than other graphene layers to increase the resistance to the interlayer diffusion of the electrolyte in the graphene layer and improve the first Coulombic efficiency of the composite anode material. The dense carbon layer provided in this application is an amorphous carbon layer formed by the pyrolysis of a carbon-containing compound.

[0048] In some embodiments, the porosity of the hard carbon is 10% - 50%, preferably 20% - 40%. The porosity of the hard carbon should not be too high or too low because a higher porosity increases the specific surface area of the material, forming an SEI film during the first charge and discharge, which reduces the first Coulombic efficiency of the material; while a lower porosity reduces the lithium ion storage capacity of the material, thereby reducing the capacity of the material.

[0049] In some embodiments, the volume ratio of the micropores in the pores of the hard carbon is 1% - 10%. Among them, the proportion of the micropores in the pores of the hard carbon should not be too high or too low because the micropores can adsorb a large number of lithium ions, improving the capacity of the material. An excessively high micropore rate will increase the specific surface area of the material, forming a large amount of SEI film, thereby reducing the first Coulombic efficiency of the material. Preferably, the volume ratio of the micropores in the pores of the hard carbon is 1% - 5%.

[0050] In some embodiments, the content of nitrogen element in the inner core is 0.01-10% by mass fraction. The content of nitrogen element in the inner core should not be too high or too low, because appropriate nitrogen doping can generate defects on the material surface, thereby improving the adsorption capacity for lithium ions.

[0051] In some embodiments, the tap density of the inner core is 1.05-1.15 g / cm 3 . The tap density of the composite anode material is 1.20-1.35 g / cm 3 , which is higher than that of hard carbon.

[0052] In some embodiments, the thickness of the shell layer is 2-15 nm. Preferably 2-10 nm, more preferably 2-8 nm.

[0053] In some embodiments, the D50 of the composite anode material is 8-10 μm, the particle size of the composite anode material is not greater than 25 μm, and the maximum value of the particle size of the composite anode material is 15-25 μm.

[0054] In some embodiments, the BET specific surface area of the composite anode material is 1-20 m 2 / g, preferably 1-10 m 2 / g, more preferably 1-8 m 2 / g.

[0055] In some embodiments, in the Raman test of the inner core, there is a D peak in the range of 1350 cm -2 ±20 cm -2 , a G peak in the range of 1580 cm -2 ±20 cm -2 , and the value of I D1 / I G1 is 1.0-1.1. In the Raman test of the composite anode material, there is a D peak in the range of 1580 cm -2 ±20 cm -2 , a G peak in the range of 1580 cm -2 ±20 cm -2 , and the value of I D2 / I G2 is 1.1-1.6, as shown in Figure 4 . The composite anode material and the surface of the inner core provided in this application have abundant defect sites, which can serve as adsorption sites for lithium ions, improve the conductivity of the composite anode material, and thus improve the slope region capacity. In addition, within the range of the value of I D1 / I G1 of the inner core and I D2 / I G2When the value is within the above range, it can also shorten the transmission distance of lithium ions, increase the charge transfer rate, and provide abundant reaction active sites on the surface of the composite anode material. However, the I of the core D1 / I G1 value and the I of the composite anode material D2 / I G2 value should not be too high either. An excessively high I D1 / I G1 value and I D2 / I G2 value will result in too high a capacity in the slope region, leading to a decrease in the initial Coulombic efficiency of the composite anode material. In some embodiments, the proportion of the shell layer in the composite anode material is 0.01-3% by mass fraction, preferably 1-3%.

[0056] Second, the present application also provides a preparation method of a composite anode material, including the following steps:

[0057] Provide a carbon source and a nitrogen source;

[0058] Mix the carbon source and the nitrogen source to obtain a mixture, and under an inert gas environment, such as an inert gas environment where the oxygen content drops to less than 50 ppm, pre-carbonize the mixture to obtain a primary product;

[0059] Carry out primary carbonization treatment on the primary product under an inert gas atmosphere to obtain a secondary product;

[0060] Crush and screen the secondary product, and carry out secondary carbonization treatment on the product obtained by screening to obtain a core;

[0061] Coat a dense carbon layer on the surface of the core by chemical vapor deposition to obtain a composite anode material.

[0062] In the present application, the carbon source can be any organic matter that can provide carbon elements, such as one or more of glucose, sucrose, starch, resin, and asphalt; and the nitrogen source can be any nitrogen-containing compound that can provide nitrogen elements, such as one or more of urea, ammonium chloride, ammonium bicarbonate, aniline hydrazoamine, and melamine. During the mixing of the carbon source and the nitrogen source, first configure the carbon source and the nitrogen source into carbon source solution and nitrogen source solution with a certain proportion respectively using a solvent, and then mix the carbon source solution and the nitrogen source solution to form a mixture. Among them, the solvent is a conventional solvent for precursor solutions, such as deionized water. It is also possible to directly mix the carbon source and the nitrogen source to obtain a mixture.

[0063] In addition, during mixing, it can adopt conventional mixing methods and mixing equipment, such as mechanical stirring and mixing using a mixer, with a rotation speed of 100-300 rmp / min and a mixing time of 0.5-3 h for uniform mixing of the carbon source and the nitrogen source.

[0064] The mass ratio of the carbon source to the nitrogen source is 100:(10 - 40), preferably 100 - 20.

[0065] The hydrocarbon is selected from at least one of methane, ethylene, and acetylene.

[0066] In some embodiments, in the process of coating a dense carbon layer on the surface of the inner core by chemical vapor deposition, the atmosphere used includes a hydrocarbon and an inert gas, and the flow rate ratio of the hydrocarbon to the inert gas is (10 - 100):100.

[0067] In some embodiments, in the process of coating a dense carbon layer on the surface of the inner core by chemical vapor deposition, the process temperature is 800 - 1100 °C, and the process time is 2 - 8 h; to obtain a dense carbon layer with a suitable thickness. Preferably, the process temperature is 900 - 1000 °C, and the process time is 2 - 4 h.

[0068] In some embodiments, the treatment temperature of the pre-carbonization treatment is 150 - 250 °C, and the treatment time of the pre-carbonization treatment is 8 - 15 h. The pre-carbonization temperature should not be too high or too low, because too high a temperature will cause the material to dehydrate too quickly, resulting in a sharp volume expansion; preferably, the treatment temperature of the pre-carbonization treatment is 180 - 215 °C, and the treatment time of the pre-carbonization treatment is 8 - 12 h.

[0069] In some embodiments, the treatment temperature of the primary carbonization treatment is 300 - 600 °C, and the treatment time of the primary carbonization treatment is 1 - 3 h. Adding a primary carbonization treatment between the pre-carbonization treatment and the high-temperature carbonization treatment further avoids volume expansion of the material.

[0070] In some embodiments, the treatment temperature of the secondary carbonization treatment is 1000 - 1200 °C, and the treatment time of the secondary carbonization treatment is 1 - 3 h. The treatment temperature of the secondary carbonization treatment should not be too high or too low, because the internal pore size of the material is related to the secondary carbonization temperature. Too high a temperature will cause the pore size to shrink, resulting in a decrease in pore volume and thus reducing the capacity; too low a temperature will cause the initial Coulomb efficiency of the material to decrease. Preferably, the treatment temperature of the secondary carbonization treatment is 1100 - 1150 °C, and the treatment time of the secondary carbonization treatment is 1 - 3 h.

[0071] In a third aspect, the present application further provides a lithium-ion battery, including a negative electrode, and the negative electrode includes the composite negative electrode material described in the first aspect or the composite negative electrode material prepared by the preparation method of the composite negative electrode material described in the second aspect. This lithium-ion battery can use the above negative electrode material to prepare a negative electrode sheet, and then use the negative electrode sheet containing the above negative electrode material to prepare a battery.

[0072] Fourthly, the present application also provides an electrical device, including the lithium-ion battery described in the third aspect. The electrical device can be any electrical device that can be powered by a lithium-ion battery, such as an electric vehicle, an electric bicycle, an energy storage system, etc.

[0073] To facilitate those skilled in the art to understand the innovative points of the nitrogen-doped hard carbon pyrolytic carbon composite anode material, its preparation method, lithium-ion battery, and electrical device provided by the present application, some preferred embodiments are provided below with reference to the accompanying drawings to illustrate the above technical solutions by way of example.

[0074] Example 1

[0075] This example provides a preparation method of a composite anode material, including the following steps:

[0076] Step 1:

[0077] Take 100 parts by mass of starch and 20 parts by mass of urea and place them in a mixer. Mix them at a rotation speed of 200 rad / min for 120 min to obtain a mixture. Place the mixture in a box furnace, and introduce argon into the box furnace. After the oxygen content drops below 50 ppm, heat it up to 150 °C and perform pre-carbonization treatment for 10 h to obtain a primary product;

[0078] Step 2:

[0079] Introduce argon and heat it up to 500 °C, and perform carbonization treatment for 3 h to obtain a secondary product;

[0080] Step 3:

[0081] Crush the secondary product obtained in Step 2, pass it through a 350-mesh sieve to obtain secondary particles with a D50 of 8.5 μm, and continue to perform carbonization treatment at a temperature of 1100 °C for 3 h in a box furnace under an argon atmosphere to obtain hard carbon, as Figure 2 shown;

[0082] Step 4:

[0083] Take the hard carbon obtained in Step 3 as the core, place it in a rotary furnace, and at 900 °C, introduce a mixed gas of methane and nitrogen with a gas ratio of 1:10 to deposit a dense pyrolytic carbon layer on the surface of the core to coat the core, and the deposition time is 2 h to obtain a composite anode material, as Figure 3 shown.

[0084] Example 2

[0085] Take 100 parts by mass of starch and 10 parts by mass of obtained urea and place them in a mixer. Mix them at a rotation speed of 200 rad / min for 120 min to obtain a mixture. Place the mixture in a box furnace and introduce argon. After the oxygen content drops below 50 ppm, heat it up to 180 °C and perform pre-carbonization treatment for 8 h to obtain a primary product;

[0086] Step Two:

[0087] Keep introducing argon and heat it up to 400 °C. Perform carbonization treatment for 3 h to obtain a secondary product;

[0088] Step Three:

[0089] Crush the secondary product obtained in Step Two, sieve it through a 350-mesh sieve to obtain secondary particles with a D50 of 8.9, and continue to perform carbonization treatment at a temperature of 1100 °C for 3 h in a box furnace under an argon atmosphere to obtain hard carbon;

[0090] Step Four:

[0091] Take the hard carbon obtained in Step Three as the core, place it in a rotary furnace, and at 900 °C, introduce a mixed gas of methane and nitrogen with a gas ratio of 1:5. Deposit a dense graphene layer on the surface of the core to coat the core, and the deposition time is 2 h to obtain a composite negative electrode material.

[0092] Example 3

[0093] Take 100 parts by mass of starch and 10 parts by mass of obtained urea and place them in a mixer. Mix them at a rotation speed of 100 rad / min for 150 min to obtain a mixture. Place the mixture in a box furnace and introduce argon. After the oxygen content drops below 50 ppm, heat it up to 150 °C and perform pre-carbonization treatment for 15 h to obtain a primary product;

[0094] Step Two:

[0095] Introduce argon and heat it up to 400 °C. Perform carbonization treatment for 3 h to obtain a secondary product;

[0096] Step Three:

[0097] Crush the secondary product obtained in Step Two, sieve it through a 350-mesh sieve to obtain secondary particles with a D50 of 8.3 μm, and continue to perform carbonization treatment at a temperature of 1100 °C for 3 h in a box furnace under an argon atmosphere to obtain hard carbon;

[0098] Step Four:

[0099] Take the hard carbon obtained in Step 3 as the core, place it in a rotary furnace, and at 1000 °C, introduce a mixed gas of methane and nitrogen with a gas ratio of 1:10. Deposit a dense pyrolytic carbon layer on the surface of the core to coat the core. The deposition time is 4 h to obtain a composite negative electrode material.

[0100] Example 4

[0101] This example also provides a method for preparing a composite negative electrode material. Compared with Example 1, the carbon source in Step 1 is sucrose, the nitrogen source is melamine, and the remaining preparation steps are the same as those in Example 1 to obtain a hard carbon negative electrode material.

[0102] Example 5

[0103] Take 100 parts by mass of starch and 40 parts by mass of urea and place them in a mixer. Mix at a rotation speed of 200 rad / min for 90 min to obtain a mixture. Place the mixture in a box furnace and introduce argon. After the oxygen content drops below 50 ppm, heat up to 150 °C and carry out pre-carbonization treatment for 8 h to obtain a primary product;

[0104] Step 2:

[0105] Introduce argon and heat up to 600 °C. Carry out carbonization treatment for 1 h to obtain a secondary product;

[0106] Step 3:

[0107] Crush the secondary product obtained in Step 2, pass it through a 350-mesh sieve to obtain secondary particles with a D50 of 8.2 μm, and continue to carry out carbonization treatment at a temperature of 1200 °C for 3 h in a box furnace under an argon atmosphere to obtain hard carbon;

[0108] Step 4:

[0109] Take the hard carbon obtained in Step 3 as the core, place it in a rotary furnace, and at 800 °C, introduce a mixed gas of methane and nitrogen with a gas ratio of 1:5. Deposit a dense pyrolytic carbon on the surface of the core to coat the core. The deposition time is 3 h to obtain a composite negative electrode material. The charge-discharge performance of the battery formed by the composite negative electrode material as a component of the negative electrode material is as Figure 7 shown.

[0110] Comparative Example 1

[0111] Compared with Example 2, this comparative example does not have Step 4; Steps 1, 2, and 3 are the same as those in Example 1 to obtain a comparative sample.

[0112] Comparative Example 2

[0113] Compared with Example 1, this comparative example does not have Step 2; Steps 1, 3, and 4 are the same as those in Example 1, and the comparative sample is obtained.

[0114] Comparative Example 3

[0115] Compared with Example 1, in Step 1 of this comparative example, 100 parts by mass of starch and 20 parts by mass of obtained urea are placed in a mixer and mixed at a rotation speed of 200 rad / min for 120 min to obtain a mixture. The mixture is placed in a box furnace, and argon is introduced to reduce the oxygen content to below 50 ppm; the pre-carbonization treatment process is omitted; then, through Steps 2, 3, and 4, the comparative sample is obtained, and Steps 2, 3, and 4 are the same as those in Example 1.

[0116] Comparative Example 4

[0117] Compared with Example 1, in Step 1 of this comparative example, the pre-carbonization temperature is 140 °C and the pre-carbonization treatment time is 8 h. Steps 2, 3, and 4 are the same as those in Example 1, and the comparative sample is obtained.

[0118] Comparative Example 5

[0119] Compared with Example 1, in Step 2 of this comparative example, the carbonization temperature is 800 °C and the carbonization treatment time is 1 h. Steps 1, 3, and 4 are the same as those in Example 1, and the comparative sample is obtained.

[0120] Comparative Example 6

[0121] Compared with Example 1, in Step 3 of this comparative example, the carbonization temperature is 1250 °C and the carbonization treatment time is 1 h. Steps 1, 2, and 4 are the same as those in Example 1, and the comparative sample is obtained.

[0122] Comparative Example 7

[0123] Compared with Example 1, in Step 1 of this comparative example, the mass ratio of the carbon source to the nitrogen source is 2:1, and the remaining preparation steps are the same as those in Example 1, and the comparative sample is obtained.

[0124] Comparative Example 8

[0125] Compared with Example 1, in Step 4 of this comparative example, the volume ratio of the hydrocarbon to the inert gas is 1:20, and the remaining preparation steps are the same as those in Example 1, and the comparative sample is obtained.

[0126] Comparative Example 9

[0127] Compared with Example 1, in Step 4 of this comparative example, the deposition temperature is 700 °C and the deposition time is 2 h, and the remaining preparation steps are the same as those in Example 1, and the comparative sample is obtained.

[0128] Test

[0129] 1. Nitrogen adsorption and desorption curve

[0130] The nitrogen adsorption and desorption test mainly tests the specific surface area and pore size distribution of porous materials, and can obtain parameters such as pore size, pore volume, and specific surface area. The specific surface area is measured by multi-point BET, and the pore size distribution of microporous materials is calculated by the nonlinear density functional theory NLDFT (Non-liner Density Function Theory) method. The test equipment is the ASAP 2000 nitrogen adsorption mass tester from Micromeritics, USA.

[0131] 2. Small angle X-ray scattering (SAXS) test

[0132] SAXS test is used to characterize the closed pore volume and other parameters of the material. The test equipment is XenocsNano-InXider tester with Cu Kα source.

[0133] 3. Raman test

[0134] Raman spectroscopy is used to analyze the graphitization and disorder of carbon fiber. The test equipment is the Invia RM200 confocal Raman spectrometer from Renishaw, UK. The test temperature is room temperature, and the test light source is an argon ion exciter with a wavelength of 514.5nm. During the test, a 50x lens is used to focus the sample.

[0135] 4. Particle size D50

[0136] Laser diffraction method, Malvern MS3000 particle size analyzer, UK. Laser dispersion conditions: 1% Nonidet p40 dispersant and the substance to be tested are ultrasonicated for 1 minute.

[0137] 5. Nitrogen content

[0138] X-ray Photoelectron Spectroscopy (XPS) is a quantitative spectroscopy technique that uses X-rays as the excitation light source to determine the chemical composition of the material surface, the chemical state of the elements, and the electronic state. It is an important test method for qualitative and semi-quantitative analysis of the sample surface. This paper mainly uses XPS to determine the relative content and existence form of nitrogen in ammonia-doped carbon nanofibers.

[0139] The XPS test was conducted using a PHI5802 X-ray electron spectrometer from Physical Electronics, with a Mg target as the monochromatic light source (Ka 1253.6 eV) and the binding energy of the Cs main peak at 284.8 eV as the reference. The XPS spectrum was fitted and analyzed using XPSPeak software.

[0140] 6. Compaction density

[0141] It is measured by using a 300 kN microcomputer-controlled electronic compressive testing machine.

[0142] 7. Coin cell performance

[0143] Half-cell tests are adopted. For CR2032 coin cells, metal lithium sheets are used as the counter electrode and reference electrode. In this patent, three-stage discharging is carried out at a certain current density, and charging is carried out at a certain current density. Combining the charge-discharge curves, electrochemical parameters such as the capacity and initial Coulomb efficiency of the material can be obtained. A Blue-Energy multi-channel battery test system is used, and the coin cells are assembled in a Labstar (1200 / 780) glove box of Braun GmbH, Germany. The composite anode materials obtained in Examples 1-5 and the comparative samples obtained in Comparative Examples 1-9 are respectively subjected to the above tests, and the surface morphologies of the cores, composite anode materials obtained in Examples 1-5 and the hard carbon / comparative samples obtained in Comparative Examples 1-9 are characterized by scanning electron microscopy. The test results are shown in Table 1 and Table 2.

[0144] Table 1

[0145]

[0146] Table 2

[0147]

[0148]

[0149] It can be seen from Table 1 and Table 2 that the pre-carbonization process can effectively reduce the specific surface area of the finished hard carbon. The primary carbonization and secondary carbonization temperatures have a great influence on the capacity and initial Coulomb efficiency. An excessively high secondary carbonization temperature will cause the material capacity to decrease. The coating process can effectively improve the capacity and initial Coulomb efficiency of the material. Too low a temperature will make it difficult for the gas to crack, thus losing the coating effect. Too high a hydrocarbon content in the mixed gas will also make it difficult to improve the initial efficiency.

[0150] In addition, in order to verify that the hard carbon provided in this application contains both mesopores and micropores, the nitrogen adsorption-desorption curves of the hard carbon and composite anode material obtained in Example 1 are respectively tested in this application, as shown in Figure 5 、 Figure 6As shown, it can be seen from the figure that the nitrogen adsorption-desorption curve model is accompanied by a mesoporous hysteresis loop, indicating that the material contains both micropores and mesopores; while the nitrogen adsorption-desorption curve of the composite anode material obtained by coating with a dense pyrolytic carbon layer shows a significant reduction in adsorption in the low-pressure region. This phenomenon indicates that the carbon material grown on the surface of the material by CVD closes the micropore entrances, resulting in a reduction in micropores. At the same time, this application also provides hard carbon containing only mesopores and the composite anode material (comparative sample 1) prepared by using this hard carbon in step four of Example 1. The coin cell performance and tap density of the two composite anode materials were tested respectively, and the test results are shown in Table 3.

[0151] Table 3

[0152]

[0153] It can be seen from the table that after partial micropores are closed, the contact area between the material and the electrolyte decreases, thereby reducing the formation of the SEI film and improving the first Coulombic efficiency of the material; in addition, the pyrolytic carbon layer increases the degree of defects of the material, can adsorb more lithium ions, and improves the capacity of the material. At the same time, since the dense carbon layer covers the micropores, some micropores are closed, effectively improving the first Coulombic efficiency of the material.

[0154] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A composite negative electrode material, comprising a core and a shell layer coated on the core, characterized in that, The core is hard carbon doped with nitrogen element, and there are multiple mesopores and micropores on the hard carbon; the shell layer is a dense carbon layer.

2. The composite anode material according to claim 1, wherein The dense carbon layer closes the mesopores and / or the micropores; and / or The dense carbon layer is a pyrolytic carbon layer and / or an amorphous carbon layer.

3. The composite anode material according to claim 1, characterized in that The porosity of the hard carbon is 10% - 50%; the volume ratio of the micropores in the pores of the hard carbon is 1% - 10%; and / or In the composite anode material, the content of the nitrogen element in the core is 0.01 - 10% by mass fraction; and / or The tap density of the core of the composite anode material is 1.05 - 1.15 g / cm 3 ; and / or The thickness of the shell layer is 2 - 15 nm; and / or In the composite anode material, the D50 of the composite anode material is 8 - 10 μm, and the particle size of the composite anode material is not greater than 25 μm; and / or The BET specific surface area of the composite anode material is 1 to 20 m 2 / g.

4. The composite anode material according to any one of claims 1 to 3, characterized in that, In the Raman test of the core, there is a D peak in the range of 1350 cm -2 ±20 cm -2 , and there is a G peak in the range of 1580 cm -2 ±20 cm -2 . And the value of I D1 / I G1 is 1.0 - 1.

1. In the Raman test of the composite negative electrode material, there is a D peak in the range of 1350 cm -2 ±20 cm -2 , and there is a G peak in the range of 1580 cm -2 ±20 cm -2 . And the value of I D2 / I G2 is 1.1 - 1.

6.

5. The composite negative electrode material according to claim 1, characterized in that, The proportion of the shell layer in the composite anode material is 0.01 - 3% by mass fraction.

6. A preparation method of a composite anode material, characterized in that, It includes the following steps: Provide a carbon source and a nitrogen source; Mix the carbon source and the nitrogen source to obtain a mixture, and perform pre-carbonization treatment on the mixture to obtain a primary product; Perform primary carbonization treatment on the primary product under an inert gas atmosphere to obtain a secondary product; Crush and screen the secondary product, and perform secondary carbonization treatment on the product obtained by screening to obtain the core; Coat a dense carbon layer on the surface of the core by chemical vapor deposition to obtain a composite anode material.

7. The preparation method of the composite negative electrode material according to claim 6, wherein, When coating a dense carbon layer on the surface of the core by chemical vapor deposition, the atmosphere used includes a hydrocarbon and an inert gas, and the flow ratio of the hydrocarbon to the inert gas is (10 - 100):100; and / or When coating a dense carbon layer on the surface of the core by chemical vapor deposition for the composite anode material, the process temperature is 800 - 1100 °C, and the process time is 2 - 8 h.

8. The preparation method of the composite negative electrode material according to claim 6, characterized in that, The treatment temperature of the pre-carbonization treatment is 150 - 250 °C, and the treatment time of the pre-carbonization treatment is 8 - 15 h; and / or The treatment temperature of the primary carbonization treatment is 300 - 600 °C, and the treatment time of the primary carbonization treatment is 1 - 3 h; and / or The treatment temperature of the secondary carbonization treatment is 1000 - 1200 °C, and the treatment time of the secondary carbonization treatment is 1 - 3 h; and / or The carbon source in the composite anode material is selected from at least one of glucose, sucrose, starch, resin, and asphalt; and / or The nitrogen source is selected from at least one of urea, ammonium chloride, ammonium bicarbonate, aniline hydroazoamine, and melamine; and / or The mass ratio of the carbon source to the nitrogen source is 100:(10 - 40); and / or The hydrocarbon is selected from at least one of methane, ethylene, and acetylene.

9. A lithium-ion battery, characterized in that, It includes a negative electrode, and the negative electrode includes the composite anode material described in any one of claims 1 - 5 or the composite anode material prepared by the preparation method of the composite anode material described in any one of claims 6 - 8.

10. An electrical device, characterized in that, It includes the lithium-ion battery described in claim 9.

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