Modified graphite, preparation method and application thereof, battery negative plate and lithium battery

Abstract

The application relates to modified graphite, a preparation method and application thereof, and a battery negative plate and a lithium battery containing the modified graphite. The preparation method of the modified graphite comprises the following steps: (1) high-pressure treatment of graphite under heating conditions to obtain thin-layer graphite; (2) plasma treatment of the thin-layer graphite to obtain pretreated graphite; and (3) coating of a conductive agent on the surface of the pretreated graphite to obtain the modified graphite; the mass ratio of the conductive agent to the pretreated graphite is 0.014% to 0.112%. The preparation method can make the obtained modified graphite have very good coating effect, and can greatly improve the battery capacity, capacity retention rate and other battery performances of the graphite negative electrode material.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery technology, and relates to lithium battery anode materials, specifically to a modified graphite and its preparation method and application, battery anode sheets, and lithium batteries. Background Technology

[0002] Anode materials are one of the key materials in lithium batteries and have a crucial impact on their performance. They play a role in energy storage and release in lithium batteries and have a significant impact on the initial efficiency, cycle performance, energy density, charge-discharge rate, and low-temperature discharge performance of lithium batteries. Therefore, developing high-performance anode materials is an important way to obtain lithium batteries with excellent overall performance.

[0003] Graphite has the advantages of high conductivity, stable structure, low cost, good processing performance and environmental friendliness, making it the mainstream anode material for lithium batteries. However, its limited theoretical capacity, volume expansion during lithium intercalation, easy reaction with electrolyte at high temperature, and easy lithium deposition during fast charging greatly limit the improvement of the performance of graphite anode lithium batteries.

[0004] To address the aforementioned problems with graphite anodes, modifications such as doping and surface coating are commonly employed. Common graphite modification methods include forming a carbon layer on the graphite surface through chemical vapor deposition or pyrolysis; forming a metal oxide layer (Al₂O₃, TiO₂) on the graphite surface through sol-gel methods or atomic layering; and forming a polymer layer on the graphite surface through solution impregnation or in-situ polymerization. However, these commonly used methods suffer from defects such as easy coating detachment or uneven coating, making it difficult to truly solve the aforementioned problems of graphite anodes and limiting the effective improvement of graphite anode lithium battery performance. Furthermore, these methods are complex and costly, and the use of organic solvents or acid / alkali solutions leads to the generation of large amounts of waste liquid, polluting the environment. Summary of the Invention

[0005] Based on this, the purpose of this invention is to provide a new graphite modification method to improve the battery performance of modified graphite anode materials, such as battery capacity and capacity retention.

[0006] The technical solutions for achieving the above objectives include the following.

[0007] In a first aspect, the present invention provides a method for preparing modified graphite, comprising the following steps:

[0008] (1) Graphite is subjected to high pressure under heating conditions to obtain thin-layer graphite;

[0009] (2) The thin-layer graphite is subjected to plasma treatment to obtain pretreated graphite;

[0010] (3) Coating the surface of the pretreated graphite with a conductive agent yields the modified graphite.

[0011] The mass ratio of the conductive agent to the pretreated graphite is 0.014%-0.112%.

[0012] Secondly, the present invention provides modified graphite prepared by the preparation method described in the present invention.

[0013] Thirdly, this invention provides the application of the modified graphite described herein as a negative electrode active material in the preparation of battery negative electrode sheets.

[0014] Fourthly, this invention provides the application of the modified graphite described herein as a negative electrode active material in the preparation of lithium batteries.

[0015] Fifthly, the present invention provides a battery negative electrode sheet prepared from modified graphite as described in the present invention.

[0016] In a sixth aspect, the present invention provides a lithium battery, wherein the active material in the negative electrode of the lithium battery is the modified graphite described in the present invention.

[0017] The present invention has the following beneficial effects:

[0018] This invention provides a novel graphite modification method. The method first subjects the graphite to high pressure treatment, causing interlayer dissociation, thinning of the graphite layers, and an increase in aspect ratio, obtaining thin-layer graphite with a thickness of 10nm-100nm. This significantly increases the specific surface area of ​​the graphite and exposes more active sites, which is beneficial for the uniform coating of subsequent conductive agents. Next, plasma modification of the high-pressure treated graphite is performed, etching the surface of the graphite particles to increase the surface roughness and further improve the specific surface area. Furthermore, CN and CO hydrophilic groups are introduced onto the graphite surface, making it easier for the subsequent coating liquid to wet the powder and improving the conductivity of the conductive agent. The compatibility and dispersibility with graphite powder make the conductive agent coating more uniform. At the same time, the conductive agent reacts chemically with the groups on the graphite surface, improving the bonding force between the conductive coating layer and the graphite particles and preventing the coating layer from falling off. Coating the graphite with the conductive layer after the above treatment increases the contact area between the graphite and the conductive agent, making the coating more uniform, the interaction force between the graphite and the conductive agent stronger, and the bonding more firm, making it less likely to fall off. By controlling the reasonable ratio of graphite to conductive agent, the modified graphite can achieve a very good coating effect, which can significantly improve the battery performance of graphite anode materials, such as battery capacity and capacity retention.

[0019] Furthermore, a conductive agent is coated onto the surface of the pretreated graphite using a powder coating instrument equipped with an atomizing device. The convective spray method increases the collision intensity between particles. The nozzles in the powder coating instrument can atomize the conductive agent aqueous dispersion into small droplets of 20μm-50μm, which can increase the contact area between the conductive agent and the graphite particles, making the coating more uniform and the coating efficiency higher. With reasonable process conditions such as carrier gas pressure, better coating effect can be obtained, further improving the battery performance of modified graphite, such as battery capacity and capacity retention rate.

[0020] The modified graphite preparation method of the present invention is simple to operate, environmentally friendly, pollution-free, and can achieve continuous production to improve the efficiency of graphite modification.

[0021] The lithium battery prepared using the modified graphite of this invention as the negative electrode active material has excellent rate performance and capacity retention, while its specific capacity, safety performance and low temperature performance are also improved to a certain extent, resulting in excellent overall performance. Detailed Implementation

[0022] To facilitate understanding of the present invention, a more complete description will be provided below. The present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.

[0023] Unless otherwise specified, experimental methods in the following examples are generally performed under standard conditions or as recommended by the manufacturer. All commonly used chemical reagents used in the examples are commercially available products.

[0024] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. The term "and / or" as used in this invention includes any and all combinations of one or more of the associated listed items.

[0025] Furthermore, as used herein, the term "or" is an inclusive "or" sign and is equivalent to the term "and / or" unless the context clearly specifies otherwise. The term "based on" is not exclusive and allows for basing on other factors not described unless the context clearly specifies otherwise. Additionally, throughout the specification, the meanings of "an," "a," and "the" include plural indicators. The meaning of "in" includes both "in" and "on."

[0026] Some embodiments involve a method for preparing modified graphite, comprising the following steps:

[0027] (1) Graphite is subjected to high pressure under heating conditions to obtain thin-layer graphite;

[0028] (2) The thin-layer graphite is subjected to plasma treatment to obtain pretreated graphite;

[0029] (3) Coating the surface of the pretreated graphite with a conductive agent yields the modified graphite.

[0030] The mass ratio of the conductive agent to the pretreated graphite is 0.014%-0.112%.

[0031] In this process, the thin-layer graphite is subjected to plasma treatment to generate CO and CN bonds on the surface of the graphite.

[0032] In some preferred embodiments, the conditions for the high-pressure treatment include: a pressure of 10 MPa-50 MPa and a temperature of 200°C-400°C.

[0033] In some preferred embodiments, the conditions for the high-pressure treatment include: a pressure of 20 MPa-40 MPa and a temperature of 250°C-350°C.

[0034] In some preferred embodiments, the conditions for the high-pressure treatment include: a pressure of 25 MPa-25 MPa and a temperature of 280°C-320°C.

[0035] In some preferred embodiments, the high-pressure treatment time is 20-60 minutes.

[0036] In some preferred embodiments, the high-pressure treatment time is 25-50 minutes.

[0037] In some preferred embodiments, the high-pressure treatment time is 28-40 minutes.

[0038] In some preferred embodiments, the high-pressure treatment time is 28-35 minutes.

[0039] In some preferred embodiments, the thickness of the resulting thin graphite layer is 10nm-100nm, for example 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm.

[0040] In some preferred embodiments, the plasma source for the plasma treatment is a mixture of oxygen and nitrogen.

[0041] In some preferred embodiments, the volume ratio of oxygen to nitrogen is 1:2-5.

[0042] In some preferred embodiments, the volume ratio of oxygen to nitrogen is 1:2-4.

[0043] In some preferred embodiments, the volume ratio of oxygen to nitrogen is 1:2.5-3.5.

[0044] In some preferred embodiments, the conditions for plasma treatment include: a power of 50W-200W, preferably 100W-180W, more preferably 120W-180W, and even more preferably 140W-160W.

[0045] In some preferred embodiments, the plasma treatment conditions include a pressure of 0.005 MPa-0.015 MPa, preferably 0.008 MPa-0.012 MPa.

[0046] In some preferred embodiments, the conditions for plasma treatment include a temperature of 60°C-95°C, preferably 75°C-85°C.

[0047] In some preferred embodiments, the conditions for plasma treatment include a gas flow rate of 70 sccm-110 sccm, preferably 85 sccm-95 sccm, and more preferably 88 sccm-92 sccm.

[0048] In some preferred embodiments, the plasma treatment conditions include a treatment time of 5 min to 30 min, preferably 8 min to 15 min.

[0049] In some preferred embodiments, the conductive agent is a multi-walled carbon nanotube and / or a single-walled carbon nanotube.

[0050] In some preferred embodiments, the mass ratio of the conductive agent to the pretreated graphite is 0.028%-0.084%.

[0051] In some preferred embodiments, the mass ratio of the conductive agent to the pretreated graphite is 0.028%-0.07%.

[0052] In some preferred embodiments, the mass ratio of the conductive agent to the pretreated graphite is 0.05%-0.07%.

[0053] In some preferred embodiments, the mass ratio of the conductive agent to the pretreated graphite is 0.053%-0.062%.

[0054] In some preferred embodiments, the conductive agent is added in the form of an aqueous dispersion of the conductive agent and atomized to coat the surface of the pretreated graphite.

[0055] In some preferred embodiments, the conductive agent content in the aqueous dispersion is 1wt%-10wt%.

[0056] In some preferred embodiments, the conductive agent content in the aqueous dispersion is 2wt%-8wt%.

[0057] In some preferred embodiments, the conductive agent content in the aqueous dispersion is 2wt%-6wt%.

[0058] In some preferred embodiments, the conductive agent content in the aqueous dispersion is 2.5 wt%-3 wt%.

[0059] In some preferred embodiments, step (3) includes coating the surface of the pretreated graphite with a conductive agent using a powder coating instrument equipped with an atomizing device. The nozzle in the powder coating instrument can atomize the conductive agent aqueous dispersion into small droplets of 20μm-50μm, which can increase the contact area between the conductive agent and the graphite particles, making the coating more uniform and the coating efficiency higher.

[0060] In some preferred embodiments, the process conditions for coating the conductive agent onto the surface of the pretreated graphite using a powder coating instrument with a gas atomization device include: a carrier gas temperature of 25°C-80°C, more preferably 25°C-50°C; and even more preferably 25°C-35°C.

[0061] In some preferred embodiments, the process conditions for coating the surface of the pretreated graphite with a powder coating instrument equipped with an air atomization device include: the temperature of the conductive agent is 25℃-100℃, more preferably 25℃-50℃; even more preferably 25℃-35℃.

[0062] In some preferred embodiments, the process conditions for coating the conductive agent onto the surface of the pretreated graphite using a powder coating apparatus equipped with an air atomization device include: a flow pump speed of 10 RPM-30 RPM, more preferably 15 RPM-25 RPM; and even more preferably 18 RPM-22 RPM.

[0063] In some preferred embodiments, the process conditions for coating the conductive agent onto the surface of the pretreated graphite using a powder coating apparatus equipped with a gas atomization device include: a carrier gas pressure of 0.1 MPa-0.8 MPa, more preferably 0.1 MPa-0.4 MPa, more preferably 0.15 RPM-0.3 RPM, more preferably 0.15 RPM-0.25 RPM, and more preferably 0.18 RPM-0.22 RPM.

[0064] In some preferred embodiments, the process conditions for coating the conductive agent onto the surface of the pretreated graphite using a powder coating instrument with an air atomization device include: an atomization pressure of 0.4 MPa-0.8 MPa, more preferably 0.4 MPa-0.6 MPa, and even more preferably 0.45 MPa-0.55 MPa.

[0065] In some preferred embodiments, the process conditions for coating the conductive agent onto the surface of the pretreated graphite using a powder coating instrument with an air atomization device include: a backflushing frequency of 0-10 times / min, more preferably 2-10 times / min, more preferably 4-8 times / min, and even more preferably 4-6 times / min.

[0066] In some preferred embodiments, the process conditions for coating the conductive agent onto the surface of the pretreated graphite using a powder coating instrument with an air atomization device include: after the liquid injection is completed, continue airflow purging for 10 min-40 min, more preferably 15 min-25 min, and even more preferably 18 min-22 min.

[0067] In some preferred embodiments, step (3) includes: adding the pretreated graphite into the reactor of a powder coating instrument with an atomization device, locking the reactor, adjusting the flow pump speed, setting the carrier gas temperature and backflushing frequency, then turning on the power and gas source, adjusting the carrier gas pressure and atomization pressure, then pumping in a conductive agent aqueous dispersion heated to the required temperature to begin coating, after the liquid injection is completed, continuing the airflow to purge for a certain period of time, finally turning off the power and gas source, and taking the powder after it settles to obtain the modified graphite.

[0068] Some embodiments also involve modified graphite prepared using the preparation method described in this invention.

[0069] Some embodiments also relate to a battery negative electrode sheet prepared using the modified graphite described in this invention as the negative electrode active material, and a lithium battery comprising the battery negative electrode sheet.

[0070] The present invention will be further described in detail below with reference to specific embodiments.

[0071] The materials used in the following embodiments are described below:

[0072] Graphite: Purchased from BTR New Materials Group Co., Ltd., model number HRG-2.

[0073] Multi-walled carbon nanotube aqueous dispersion: model XFM07, carbon nanotube content 2.8wt%.

[0074] Example 1

[0075] The method for preparing modified graphite provided in this embodiment is as follows:

[0076] (1) Weigh a certain mass of graphite and place it in a high-temperature reactor. Set the temperature to 300℃ and the pressure to 30MPa. Process for 30min to obtain a thin layer of graphite with a thickness of 100nm.

[0077] (2) The above-mentioned thin-layer graphite was transferred to a radio frequency plasma device, and oxygen / nitrogen mixed gas (oxygen:nitrogen volume ratio of 1:3) was used as the plasma gas source with a gas flow rate of 90 sccm. It was treated for 10 min under the conditions of 0.01 MPa, 80℃ and 150 W. The graphite was then removed, sealed and stored, and recorded as pretreated graphite for later use.

[0078] (3) Weigh 30g of multi-walled carbon nanotube aqueous dispersion and heat it to 30℃ for later use; weigh 1.5kg of pretreated graphite and place it in the reactor of a powder coating instrument with an atomization device, then lock the reactor, adjust the flow pump speed to 20RPM, set the carrier gas temperature to 30℃, set it to no backflushing, then turn on the power and gas source, adjust the carrier gas pressure to 0.4MPa and the atomization pressure to 0.5MPa, then start pumping in the multi-walled carbon nanotube aqueous dispersion heated to 30℃ to begin coating. After the liquid injection is completed, continue airflow purging for 20min, and finally turn off the power and gas source. After the powder settles, start taking the powder. This is the modified graphite.

[0079] Example 2

[0080] The method for preparing modified graphite provided in this embodiment is as follows:

[0081] (1) Weigh a certain mass of graphite and place it in a high-temperature reactor. Set the temperature to 300℃ and the pressure to 30MPa. Process for 30min to obtain a thin layer of graphite with a thickness of 100nm.

[0082] (2) The above-mentioned thin-layer graphite was transferred to a radio frequency plasma device, and oxygen / nitrogen mixed gas (oxygen:nitrogen volume ratio of 1:3) was used as the plasma gas source with a gas flow rate of 90 sccm. It was treated for 10 min under the conditions of 0.01 MPa, 80℃ and 150 W. The graphite was then removed, sealed and stored, and recorded as pretreated graphite for later use.

[0083] (3) Weigh 30g of multi-walled carbon nanotube aqueous dispersion and heat it to 30℃ for later use; weigh 1.5kg of pretreated graphite and place it in the reactor of a powder coating instrument with an air atomization device, then lock the reactor, adjust the flow pump speed to 20RPM, set the carrier gas temperature to 30℃, set the backflushing frequency to 5 times / min, then turn on the power and air source, adjust the carrier gas pressure to 0.4MPa and the atomization pressure to 0.5MPa, and then start pumping in the multi-walled carbon nanotube aqueous dispersion heated to 30℃ to begin coating. After the liquid injection is completed, continue airflow purging for 20min, and finally turn off the power and air source. After the powder settles, start taking the powder. This is the modified graphite.

[0084] Example 3

[0085] The method for preparing modified graphite provided in this embodiment is as follows:

[0086] (1) Weigh a certain mass of graphite and place it in a high-temperature reactor. Set the temperature to 300℃ and the pressure to 30MPa. Process for 30min to obtain a thin layer of graphite with a thickness of 100nm.

[0087] (2) The above-mentioned thin-layer graphite was transferred to a radio frequency plasma device, and oxygen / nitrogen mixed gas (oxygen:nitrogen volume ratio of 1:3) was used as the plasma gas source with a gas flow rate of 90 sccm. It was treated for 10 min under the conditions of 0.01 MPa, 80℃ and 150 W. The graphite was then removed, sealed and stored, and recorded as pretreated graphite for later use.

[0088] (3) Weigh 30g of multi-walled carbon nanotube aqueous dispersion and heat it to 30℃ for later use; weigh 1.5kg of pretreated graphite and place it in the reactor of a powder coating instrument with an atomization device, then lock the reactor, adjust the flow pump speed to 20RPM, set the carrier gas temperature to 30℃, set the backflushing frequency to 5 times / min, then turn on the power and gas source, adjust the carrier gas pressure to 0.2MPa and the atomization pressure to 0.5MPa, and then start pumping in the multi-walled carbon nanotube aqueous dispersion heated to 30℃ to begin coating. After the liquid injection is completed, continue airflow purging for 20min, and finally turn off the power and gas source. After the powder settles, start taking the powder. This is the modified graphite.

[0089] Example 4

[0090] The method for preparing modified graphite provided in this embodiment is as follows:

[0091] (1) Weigh a certain mass of graphite and place it in a high-temperature reactor. Set the temperature to 300℃ and the pressure to 30MPa. Process for 30min to obtain a thin layer of graphite with a thickness of 100nm.

[0092] (2) The above-mentioned thin-layer graphite was transferred to a radio frequency plasma device, and oxygen / nitrogen mixed gas (oxygen:nitrogen volume ratio of 1:3) was used as the plasma gas source with a gas flow rate of 90 sccm. It was treated for 10 min under the conditions of 0.01 MPa, 80℃ and 150 W. The graphite was then removed, sealed and stored, and recorded as pretreated graphite for later use.

[0093] (3) Weigh 15g of multi-walled carbon nanotube aqueous dispersion and heat it to 30℃ for later use; weigh 1.5kg of pretreated graphite and place it in the reactor of a powder coating instrument with an atomization device, then lock the reactor, adjust the flow pump speed to 20RPM, set the carrier gas temperature to 30℃, set the backflushing frequency to 5 times / min, then turn on the power and gas source, adjust the carrier gas pressure to 0.2MPa and the atomization pressure to 0.5MPa, and then start pumping in the multi-walled carbon nanotube aqueous dispersion heated to 30℃ to begin coating. After the liquid injection is completed, continue airflow purging for 20min, and finally turn off the power and gas source. After the powder settles, start taking the powder. This is the modified graphite.

[0094] Example 5

[0095] The difference between the modified graphite preparation method provided in this embodiment and that in Example 3 is that the airflow purging time is continued for 10 minutes after the liquid injection is completed. The specific method is as follows:

[0096] (1) Weigh a certain mass of graphite and place it in a high-temperature reactor. Set the temperature to 300℃ and the pressure to 30MPa. Process for 30min to obtain a thin layer of graphite with a thickness of 100nm.

[0097] (2) The above-mentioned thin-layer graphite was transferred to a radio frequency plasma device, and oxygen / nitrogen mixed gas (oxygen:nitrogen volume ratio of 1:3) was used as the plasma gas source with a gas flow rate of 90 sccm. It was treated for 10 min under the conditions of 0.01 MPa, 80℃ and 150 W. The graphite was then removed, sealed and stored, and recorded as pretreated graphite for later use.

[0098] (3) Weigh 30g of multi-walled carbon nanotube aqueous dispersion and heat it to 30℃ for later use; weigh 1.5kg of pretreated graphite into the reactor of a powder coating instrument with an atomization device, then lock the reactor, adjust the flow pump speed to 20RPM, set the carrier gas temperature to 30℃, set the backflushing frequency to 5 times / min, then turn on the power and gas source, adjust the carrier gas pressure to 0.2MPa and the atomization pressure to 0.5MPa, then start pumping in the multi-walled carbon nanotube aqueous dispersion heated to 30℃ to begin coating. After the liquid injection is completed, continue airflow purging for 10min, and finally turn off the power and gas source. After the powder settles, start taking the powder. This is the modified graphite.

[0099] Example 6

[0100] The difference between the modified graphite preparation method provided in this embodiment and that in Embodiment 3 is that the high-temperature and high-pressure treatment time is 20 minutes, and the radio frequency plasma gas source is nitrogen. The specific method is as follows:

[0101] (1) Weigh a certain mass of graphite and place it in a high-temperature reactor. Set the temperature to 300℃ and the pressure to 30MPa and process for 20min.

[0102] (2) The above-mentioned thin-layer graphite was transferred to a radio frequency plasma device, with nitrogen as the plasma gas source and a gas flow rate of 90 sccm. It was treated for 10 min at 0.01 MPa, 80 °C and 150 W. The graphite was then removed, sealed and stored as pretreated graphite for later use.

[0103] (3) Weigh 30g of multi-walled carbon nanotube aqueous dispersion and heat it to 30℃ for later use; weigh 1.5kg of pretreated graphite and place it in the reactor of a powder coating instrument with an atomization device, then lock the reactor, adjust the flow pump speed to 20RPM, set the carrier gas temperature to 30℃, set the backflushing frequency to 5 times / min, then turn on the power and gas source, adjust the carrier gas pressure to 0.2MPa and the atomization pressure to 0.5MPa, and then start pumping in the multi-walled carbon nanotube aqueous dispersion heated to 30℃ to begin coating. After the liquid injection is completed, continue airflow purging for 20min, and finally turn off the power and gas source. After the powder settles, start taking the powder. This is the modified graphite.

[0104] Comparative Example 1

[0105] Purchased raw graphite powder.

[0106] Comparative Example 2

[0107] The preparation method of the modified graphite provided in this comparative example differs from that in Example 3 in that the graphite was not subjected to high-temperature, high-pressure, and radio-frequency plasma treatment. The specific method is as follows:

[0108] Weigh 30g of multi-walled carbon nanotube aqueous dispersion and heat it to 30℃ for later use. Weigh 1.5kg of graphite powder and place it in the reactor of a powder coating instrument equipped with an air atomization device. Then lock the reactor, adjust the flow pump speed to 20RPM, set the carrier gas temperature to 30℃, and set the backflushing frequency to 5 times / min. Then turn on the power and air source, adjust the carrier gas pressure to 0.2MPa and the atomization pressure to 0.5MPa, and then start pumping in the multi-walled carbon nanotube aqueous dispersion heated to 30℃ to begin coating. After the liquid injection is completed, continue airflow purging for 20min. Finally, turn off the power and air source, and start collecting the powder after it settles. This is the modified graphite.

[0109] Comparative Example 3

[0110] The method for preparing the modified graphite provided in this comparative example differs from that in Example 3 in that the graphite was not subjected to high-temperature and high-pressure treatment. The specific method is as follows:

[0111] (1) Place the graphite in a radio frequency plasma device, use an oxygen / nitrogen mixture (oxygen:nitrogen volume ratio of 1:3) as the plasma gas source, and the gas flow rate is 90 sccm. Treat it for 10 min under the conditions of 0.01 MPa, 80℃ and 150 W, take it out, seal and store it, and record it as pretreated graphite for later use.

[0112] (2) Weigh 30g of multi-walled carbon nanotube aqueous dispersion and heat it to 30℃ for later use; weigh 1.5kg of pretreated graphite and place it in the reactor of a powder coating instrument with an atomization device, then lock the reactor, adjust the flow pump speed to 20RPM, set the carrier gas temperature to 30℃, set the backflushing frequency to 5 times / min, then turn on the power and gas source, adjust the carrier gas pressure to 0.2MPa and the atomization pressure to 0.5MPa, and then start pumping in the multi-walled carbon nanotube aqueous dispersion heated to 30℃ to begin coating. After the liquid injection is completed, continue airflow purging for 20min, and finally turn off the power and gas source. After the powder settles, start taking the powder. This is the modified graphite.

[0113] Comparative Example 4

[0114] The method for preparing the modified graphite provided in this comparative example differs from that in Example 3 in that the graphite was not subjected to radio frequency plasma treatment. The specific method is as follows:

[0115] (1) Weigh a certain mass of graphite and place it in a high-temperature reactor. Set the temperature to 300℃ and the pressure to 30MPa. Process for 30min to obtain a thin layer of graphite with a thickness of 100nm, and set it aside for use.

[0116] (2) Weigh 30g of multi-walled carbon nanotube aqueous dispersion and heat it to 30℃ for later use; weigh 1.5kg of thin-layer graphite and place it in the reactor of the powder coating instrument with gas atomization device, then lock the reactor, adjust the flow pump speed to 20RPM, set the carrier gas temperature to 30℃, set the backflushing frequency to 5 times / min, then turn on the power and gas source, adjust the carrier gas pressure to 0.2MPa and the atomization pressure to 0.5MPa, and then start pumping in the multi-walled carbon nanotube aqueous dispersion heated to 30℃ to begin coating. After the liquid injection is completed, continue airflow purging for 20min, and finally turn off the power and gas source. After the powder settles, start taking the powder. This is the modified graphite.

[0117] Comparative Example 5

[0118] The difference between the modified graphite preparation method provided in this comparative example and Example 3 is that the amount of the multi-walled carbon nanotube aqueous dispersion is 75g. The specific method is as follows:

[0119] (1) Weigh a certain mass of graphite and place it in a high-temperature reactor. Set the temperature to 300℃ and the pressure to 30MPa. Process for 30 minutes to obtain a thin layer of graphite with a thickness of 100nm.

[0120] (2) The above-mentioned thin-layer graphite was transferred to a radio frequency plasma device, and oxygen / nitrogen mixed gas (oxygen:nitrogen volume ratio of 1:3) was used as the plasma gas source with a gas flow rate of 90 sccm. It was treated for 10 min at 0.01 MPa, 80℃ and 150 W. The graphite was then removed, sealed and stored as pretreated graphite for later use.

[0121] (3) Weigh 75g of multi-walled carbon nanotube aqueous dispersion and heat it to 30℃ for later use; weigh 1.5kg of pretreated graphite and place it in the reactor of a powder coating instrument with an atomization device, then lock the reactor, adjust the flow pump speed to 20RPM, set the carrier gas temperature to 30℃, set the backflushing frequency to 5 times / min, then turn on the power and gas source, adjust the carrier gas pressure to 0.2MPa and the atomization pressure to 0.5MPa, and then start pumping in the multi-walled carbon nanotube aqueous dispersion heated to 30℃ to begin coating. After the liquid injection is completed, continue airflow purging for 20min, and finally turn off the power and gas source. After the powder settles, start taking the powder. This is the modified graphite.

[0122] Example 7: Preparation of coin cell batteries

[0123] Using the modified graphite obtained in Examples 1-6 and Comparative Examples 2-5, and the graphite powder in Comparative Example 1 as the negative electrode active material, respectively, and lithium iron phosphate as the positive electrode active material, coin cells were prepared according to the following method.

[0124] (1) Preparation of negative electrode slurry:

[0125] Formula: Graphite: Conductive carbon black: PVDF = 90:5:5 (mass ratio), solvent is NMP.

[0126] Preparation steps:

[0127] a) Slowly add PVDF to NMP and stir magnetically at 60°C until completely dissolved;

[0128] b) Add conductive carbon black and ultrasonically disperse for 10 minutes;

[0129] c) Add graphite in batches and stir in a homogenizer for 2 hours (to avoid air bubbles);

[0130] d) The viscosity of the slurry is controlled at 3000–5000 mPa·s.

[0131] (2) Preparation of positive electrode slurry:

[0132] Formula: LiFePO4: conductive carbon black: PVDF = 80:10:10 (mass ratio), solvent is NMP.

[0133] Preparation steps:

[0134] a) Slowly add PVDF to NMP and stir magnetically at 60°C until completely dissolved;

[0135] b) Add conductive carbon black and ultrasonically disperse for 10 minutes;

[0136] c) Add LiFePO4 in batches and stir in a homogenizer for 2 hours (to avoid air bubbles);

[0137] d) The viscosity of the slurry is controlled at 3000-5000 mPa·s.

[0138] (3) Electrode coating and drying:

[0139] a) Coating: Using a coating machine, the negative electrode slurry and the positive electrode slurry are uniformly coated on copper foil (negative electrode) and aluminum foil (positive electrode), respectively, with a wet film thickness of about 100-150μm;

[0140] b) Drying: Pre-dry at 80℃ for 30 minutes, then transfer to 120℃ for vacuum drying for 12 hours.

[0141] (4) Electrode sheet punching:

[0142] The dried electrode from step (3) is punched into a circular sheet with a diameter of 12 mm using a punching machine to obtain the corresponding negative electrode sheet and positive electrode sheet.

[0143] (5) Assemble the button batteries inside the glove box:

[0144] In a glove box under a high-purity argon atmosphere, the battery was assembled in the following order: negative electrode shell, negative electrode sheet, electrolyte LP40, polypropylene separator, electrolyte LP40, positive electrode sheet, gasket, spring sheet, and positive electrode shell. The battery was then placed into a sealed bag and sealed using a manual button cell sealing machine to obtain button cells for subsequent testing.

[0145] Example 8 Performance Test

[0146] The specific surface area of ​​the modified graphite obtained in Examples 1-6 and Comparative Examples 2-5, as well as the graphite raw powder of Comparative Example 1, was tested using the BET method.

[0147] The coin cells prepared in Example 7 were subjected to 100 charge-discharge cycles at 1C and 2C current densities in the range of 2.50V-3.65V.

[0148] The test results are shown in Table 1.

[0149] Table 1

[0150]

[0151]

[0152] As shown in the table above, the specific surface area of ​​the modified graphite prepared by this invention is much larger than that of the purchased graphite powder. This may be because: firstly, the high-pressure and high-temperature treatment in the high-pressure reactor destroys the van der Waals forces between graphite molecular layers, causing the graphite molecular layers to slide laterally, reducing the interlayer spacing, making the graphite molecules thinner, thus exposing more active sites and significantly increasing their surface area; secondly, plasma modification introduces CN and CO bonds while also etching the surface of graphite particles, thereby increasing the surface roughness of the particles.

[0153] The coin cell battery prepared using the modified graphite of the present invention as the negative electrode active material exhibits better capacity output rate at 2C and better capacity retention rate after 100 cycles at 1C than the coin cell battery prepared using graphite powder of Comparative Example 1 as the negative electrode active material. This is because the conductivity of graphite increases after uniformly coating it with a conductive agent, which can improve the rate performance of the battery. At the same time, the roughness and specific surface area of ​​the modified graphite in the examples are much larger than those of the graphite powder of Comparative Example 1. The increase in roughness and specific surface area can enhance the electrolyte penetration ability and reduce the interfacial resistance, thereby improving the rate performance and capacity retention rate of the battery.

[0154] Among them, the modified graphite prepared in Example 3 showed the best coating effect, and the battery prepared using it as the negative electrode active material had the best performance. This is because in Example 3, backflushing was performed during the atomization coating of the conductive agent, and the carrier gas pressure was also reduced. This invention found that this treatment can both blow all the graphite up to fully contact the conductive agent droplets and avoid a large amount of graphite being blown to the filter element, which would lead to waste of conductive agent and uneven coating. Therefore, Example 3 has the best performance.

[0155] In Example 1, no backflushing was performed during the atomization coating of the conductive agent, and the carrier gas pressure was higher than in Example 3. This caused the graphite at the bottom of the reactor to be blown to the filter element and stick to it, preventing it from returning to the middle of the reactor. Consequently, the conductive agent could not be completely and uniformly coated on the graphite. Therefore, the coating effect of the modified graphite prepared in Example 1 was worse than that in Example 3, and its performance was inferior. In contrast, Example 2, after backflushing at a frequency of 5 times / min, was able to return some of the graphite blown to the filter element to the middle of the reactor to contact the conductive agent droplets. Therefore, its coating effect was better than that in Example 1. However, the carrier gas pressure in Example 2 was higher than that in Example 3, and its coating effect was still worse than that in Example 3.

[0156] Compared to Example 3, in Example 4, the mass ratio of multi-walled carbon nanotube aqueous dispersion to graphite was adjusted from 2% to 1%. This reduction in conductive agent led to a decrease in the battery's rate performance and capacity retention. In Comparative Example 5, compared to Example 3, the mass ratio of multi-walled carbon nanotube aqueous dispersion to graphite was adjusted from 2% to 5%. This excessively high proportion of conductive agent resulted in severe wetting of the graphite, causing graphite agglomeration and clumping. This prevented the conductive agent from uniformly coating the graphite, leading to increased local impedance and uneven current density in the battery prepared using the modified graphite as the negative electrode active material. Consequently, the battery's rate performance and capacity retention were significantly reduced. Therefore, controlling the mass ratio of conductive agent (multi-walled carbon nanotube aqueous dispersion) to graphite within a reasonable range is crucial to effectively improve the performance of modified graphite as a battery negative electrode material.

[0157] Airflow mixing causes friction between particles, resulting in continuous movement and tumbling. This further promotes uniform mixing of the conductive agent among the graphite particles, achieving a uniform coating effect. Compared to Example 3, Example 5 reduced the airflow mixing time after liquid injection, leading to poorer uniformity of the conductive agent coating and localized accumulation of the conductive agent. This results in increased local impedance and uneven current density in the battery, thus its performance is inferior to Example 3.

[0158] Comparative Example 2 did not undergo high-pressure treatment in a high-temperature reactor or plasma modification; instead, it directly used graphite powder for atomized coating. The coating method and process parameters were consistent with Example 3. Compared to Example 3, Comparative Example 3 did not undergo high-temperature and high-pressure pretreatment. Compared to Example 3, Comparative Example 4 did not undergo plasma treatment. It can be seen that the specific surface area of ​​the modified graphite prepared in Comparative Example 2 is basically the same as that of the graphite powder in Comparative Example 1, but much lower than that of the modified graphite in Example 3. The specific surface area of ​​the modified graphite prepared in Comparative Example 3 is slightly larger than that of Comparative Example 1, but still much smaller than that of Example 3. The specific surface area of ​​the modified graphite prepared in Comparative Example 4 is larger than that of Comparative Examples 1-3, but still smaller than that of Example 3. This indicates that high-pressure treatment is the main reason for the increase in the specific surface area of ​​graphite. Plasma modification can further increase the specific surface area of ​​graphite. The increase in specific surface area exposes more active sites, which is beneficial for the uniform coating of conductive agents. The modified graphite in Example 3 contains hydrophilic groups such as CN and CO bonds, which can improve the dispersibility of the conductive agent in the graphite and facilitate the uniform coating of the conductive agent on the graphite. Simultaneously, it allows the conductive agent to chemically react with the graphite surface groups, increasing the bonding force between the coating layer and the graphite particles and preventing the coating layer from detaching. In other words, the synergistic effect of high-temperature and high-pressure treatment and plasma treatment results in modified graphite with a larger specific surface area and more hydrophilic groups. This synergistic combination significantly improves the overall performance of the modified graphite in Example 3 as a battery anode material, far surpassing that of Comparative Examples 2-4.

[0159] Compared to Example 3, Example 6 reduced the high-temperature and high-pressure treatment time from 30 min to 20 min, and used nitrogen as the plasma source. It can be seen that the overall performance of the modified graphite in Example 6 as a battery anode material is inferior to that in Example 3. This may be because the insufficient high-temperature and high-pressure treatment time leads to inadequate interlayer slippage of graphite molecules, resulting in a smaller specific surface area of ​​the modified graphite prepared in Example 6 compared to that prepared in Example 3. Furthermore, using nitrogen as the plasma source results in the graphite surface having only CN bonds and lacking CO bonds, which weakens the dispersion of the conductive agent in the graphite and also reduces the bonding force between the coating layer and the graphite substrate, leading to uneven coating of the conductive agent. Therefore, the high-temperature and high-pressure treatment time and the type of plasma source both significantly affect the performance of modified graphite as a battery anode material.

[0160] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for producing a modified graphite, characterized by, The method comprises the following steps: (1) treating graphite under high pressure with heating to obtain thin-layer graphite; (2) treating the thin-layer graphite by plasma to obtain pretreated graphite; (3) coating the surface of the pretreated graphite with a conductive agent to obtain the modified graphite; The mass ratio of the conductive agent to the pretreated graphite is 0.014%-0.112%; The plasma gas source for the plasma treatment is a mixed gas of oxygen and nitrogen; The conductive agent is multi-walled carbon nanotubes and / or single-walled carbon nanotubes; The conductive agent is added in the form of a conductive agent aqueous dispersion and is coated on the pretreated graphite after atomization.

2. The method for producing modified graphite according to claim 1, characterized by, The high-pressure treatment is performed under the conditions of a pressure of 10-50 MPa and a temperature of 200-400℃. And / or, the high-pressure treatment is performed for 20-60 min.

3. The method of producing modified graphite according to claim 2, characterized by, The high-pressure treatment is performed under the conditions of a pressure of 20-40 MPa and a temperature of 250-350℃. And / or, the high-pressure treatment is performed for 28-35 min.

4. The method of producing modified graphite according to claim 1, characterized by, The thickness of the thin-layer graphite is 10-100 nm.

5. The method of producing modified graphite according to claim 1, characterized by, The volume ratio of oxygen to nitrogen is 1:2-5.

6. The method of producing modified graphite according to claim 5, characterized by, The volume ratio of oxygen to nitrogen is 1:2.5-3.

5.

7. The method of producing modified graphite according to claim 1, characterized by, The plasma treatment is performed under the conditions of a power of 50-200 W; And / or, a pressure of 0.005-0.015 MPa; And / or, a temperature of 60-95℃; And / or, a gas flow rate of 70-110 sccm; And / or, a treatment time of 5-30 min.

8. The method of producing modified graphite according to claim 7, characterized by, The plasma treatment is performed under the conditions of a power of 140-160 W; And / or, a pressure of 0.008-0.012 MPa; And / or, a temperature of 75-85℃; And / or, a gas flow rate of 85-95 sccm; And / or, a treatment time of 8-15 min.

9. The method of producing modified graphite according to claim 1, characterized by, The mass ratio of the conductive agent to the pretreated graphite is 0.028%-0.084%.

10. The method of producing modified graphite according to any one of claims 1 to 9, characterized in that, Step (3) comprises coating the surface of the pretreated graphite with the conductive agent by using a powder coating instrument with an air atomization device, and the process conditions comprise: The carrier gas temperature is 25-80℃; And / or, the conductive agent temperature is 25-100℃; And / or, the flow pump rotation speed is 10-30 RPM; And / or, the carrier gas pressure is 0.1-0.8 MPa; And / or, the atomization pressure is 0.4-0.8 MPa; And / or, the back-blowing frequency is 0-10 times / min; And / or, after the liquid feeding is completed, the gas flow is continued to be blown for 10-40 min.

11. The method of producing modified graphite according to claim 10, characterized by, The process conditions for coating the surface of the pretreated graphite with the conductive agent by using the air atomization coating instrument comprise: The carrier gas temperature is 25-35℃; And / or, the conductive agent temperature is 25-35℃; And / or, the flow pump rotation speed is 15-25 RPM; And / or, the carrier gas pressure is 0.1-0.4 MPa; And / or, the atomization pressure is 0.4-0.6 MPa; And / or, the back-blowing frequency is 4-8 times / min; And / or, after the liquid is finished, continue to blow for 15-25 minutes.

12. The method of producing modified graphite according to claim 10, wherein Step (3) comprises: adding the pretreated graphite into the reactor of the powder coating instrument with gas atomization device, locking the reactor, adjusting the rotation speed of the flow pump, setting the carrier gas temperature and back flushing frequency, then turning on the power and gas source, adjusting the carrier gas pressure and atomization pressure, then pumping the conductive agent water dispersion liquid heated to the required temperature to start coating, after the liquid is finished, continue to blow for a certain time, finally turn off the power and gas source, take the powder after the powder is settled, and the modified graphite is obtained.

13. The modified graphite prepared by the preparation method of any one of claims 1-12.

14. The application of the modified graphite of claim 13 as a negative active material in the preparation of battery negative electrode sheets.

15. The application of the modified graphite of claim 13 as a negative active material in the preparation of lithium batteries.

16. A battery negative electrode sheet, characterized by, The lithium battery is prepared from raw materials comprising the modified graphite of claim 13.

17. A lithium battery, characterized by The active material in the negative electrode sheet of the lithium battery is the modified graphite of claim 13.

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