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

Abstract

The invention 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) performing high-pressure treatment on graphite under a heating condition to obtain thin-layer graphite; (2) carrying out plasma treatment on the thin-layer graphite to obtain pretreated graphite; and (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%. According to the preparation method disclosed by the invention, the obtained modified graphite can obtain a very good coating effect, and the battery performance such as the battery capacity and the capacity retention rate of the graphite negative electrode material can be greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, relates to the anode materials of lithium batteries, and specifically relates to a modified graphite and its preparation method and application, a battery anode sheet, and a lithium battery. Background Art

[0002] The anode material is one of the key materials of lithium batteries and has a crucial impact on the performance of lithium batteries. It plays a role in energy storage and release in lithium batteries and has a great influence on the first efficiency, cycle performance, energy density, charge-discharge rate, and low-temperature discharge performance of lithium batteries. Therefore, developing anode materials with excellent performance is an important way to obtain lithium batteries with excellent comprehensive performance.

[0003] Graphite has the advantages of high electrical conductivity, stable structure, low cost, good processability, and environmental friendliness, and is the mainstream anode material of lithium batteries. However, its theoretical capacity is limited, its volume expands during lithium intercalation, it is easy to react with the electrolyte at high temperatures, and lithium is easily deposited during fast charging, which greatly limits the improvement of the performance of graphite anode lithium batteries.

[0004] To solve the above problems existing in graphite anodes, the graphite anode materials are usually doped, surface-coated, etc. for modification. Currently, common graphite modification methods include forming a carbon layer on the graphite surface by chemical vapor deposition or pyrolysis, forming a metal oxide layer (Al2O3, TiO2) on the graphite surface by sol-gel method or atomic layer deposition, and forming a polymer layer on the graphite surface by solution impregnation or in-situ polymerization. However, these common methods have the defects that the coating layer is easy to fall off or the coating is not uniform enough, so it is difficult to truly solve the above problems existing in graphite anodes, which limits the effective improvement of the performance of graphite anode lithium batteries; and these methods have complex processes and high costs, and will use organic solvents or acid-base solutions, resulting in a large amount of waste liquid and polluting the environment. Summary of the Invention

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

[0006] The technical solutions to achieve the above purpose are as follows.

[0007] In the first aspect, the present invention provides a preparation method of modified graphite, including the following steps:

[0008] (1) Subjecting graphite to high-pressure treatment under heating conditions to obtain thin-layer graphite;

[0009] (2) Subjecting the thin-layer graphite to plasma treatment to obtain pretreated graphite;

[0010] (3) Coating a conductive agent on the surface of the pretreated graphite to obtain the modified graphite;

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

[0012] In a second aspect, the present invention provides modified graphite prepared by the preparation method of the present invention.

[0013] In a third aspect, the present invention provides the use of the modified graphite described in the present invention as a negative electrode active material in the preparation of a negative electrode sheet for a battery.

[0014] In a fourth aspect, the present invention provides the use of the modified graphite described in the present invention as a negative electrode active material in the preparation of a lithium battery.

[0015] In a fifth aspect, the present invention provides a battery negative electrode sheet, which is prepared using the modified graphite described in the present invention as a raw material.

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

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

[0018] The present invention provides a new graphite modification method, which firstly performs high-pressure treatment on graphite to dissociate the graphite layers, thin the graphite layers, increase the diameter-to-thickness ratio, and obtain a thin layer of graphite with a thickness of 10nm-100nm, which can greatly increase the specific surface area of ​​the graphite and expose more active sites, which is conducive to the uniform coating of the subsequent conductive agent; then, the graphite treated with high pressure is modified by plasma, the surface of the graphite particles is etched to increase the surface roughness of the particles, and the specific surface area of ​​the graphite is further increased; and CN and CO hydrophilic groups are introduced on the graphite surface, so that the subsequent coating liquid can easily wet the powder, thereby increasing the conductive agent The compatibility and dispersibility with graphite powder make the conductive agent coating more uniform, and at the same time, the conductive agent reacts chemically with the groups on the graphite surface to improve the bonding force between the conductive coating layer and the graphite particles and prevent the coating layer from falling off; coating the graphite after the above treatment with a conductive layer can increase the contact area between the graphite and the conductive agent, make the coating more uniform, and increase the interaction force between the graphite and the conductive agent, so that the bonding is more firm and not easy to fall off; and controlling the reasonable ratio of graphite and the conductive agent can make the obtained modified graphite obtain a very good coating effect, which can greatly improve the battery performance of the graphite negative electrode material, such as the battery capacity and capacity retention rate.

[0019] Further, a powder coating instrument with an air atomization device is used to coat a conductive agent on the surface of the pretreated graphite. The convective spraying method is used to increase the collision intensity between particles. The nozzle in the powder coating instrument can atomize the conductive agent aqueous dispersion into small droplets of 20 μm - 50 μm, which can increase the contact area between the conductive agent and graphite particles, make the coating more uniform, and have a higher coating efficiency. With reasonable process conditions such as the carrier gas pressure, a better coating effect can be further obtained, and the battery performance such as the battery capacity and capacity retention rate of the modified graphite can be further improved.

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

[0021] The lithium battery prepared with the modified graphite of the present invention as the negative electrode active material has excellent rate performance and capacity retention rate. At the same time, its specific capacity, safety performance and low temperature performance are also improved to a certain extent, and the comprehensive performance is excellent. Specific Embodiments

[0022] To facilitate the understanding of the present invention, the present invention will be described more comprehensively below. The present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, these embodiments are provided so that the understanding of the disclosed content of the present invention is more thorough and comprehensive.

[0023] The experimental methods without specific conditions noted in the following embodiments are generally carried out under conventional conditions or according to the conditions recommended by the manufacturers. All kinds of commonly used chemical reagents used in the embodiments are commercially available products.

[0024] Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the description of the present invention are only for the purpose of describing specific embodiments and are not used to limit the present invention. The term "and / or" used in the present invention includes any and all combinations of one or more of the related listed items.

[0025] In addition, as used in the present invention, the term "or" is an inclusive "or" symbol and is equivalent to the term "and / or", unless the context clearly dictates otherwise. The term "based on" is not exclusive and allows for other factors not described, unless the context clearly dictates otherwise. Further, throughout the specification, the meanings of "a", "an" and "the" include plural referents. The meaning of "in..." includes "in..." and "on...".

[0026] In some of these embodiments, a preparation method of modified graphite is involved, including the following steps:

[0027] (1) Subject the graphite to high-pressure treatment under heating conditions to obtain thin-layer graphite;

[0028] (2) Subject the thin-layer graphite to plasma treatment to obtain pretreated graphite;

[0029] (3) Coat a conductive agent on the surface of the pretreated graphite to obtain the modified graphite;

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

[0031] Among them, subjecting the thin-layer graphite to plasma treatment generates C - O bonds and C - N bonds on the surface of the graphite.

[0032] In some preferred embodiments, the conditions of 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 of 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 of 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 time of the high-pressure treatment is 20 min - 60 min.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0060] In some of the preferred embodiments, the process conditions for coating the conductive agent on the surface of the pretreated graphite with a powder coater equipped with an air atomization device include: the carrier gas temperature is 25°C - 80°C, more preferably 25°C - 50°C; more preferably 25°C - 35°C.

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

[0062] In some of the preferred embodiments, the process conditions for coating the conductive agent on the surface of the pretreated graphite with a powder coater equipped with an air atomization device include: the rotational speed of the peristaltic pump is 10 RPM - 30 RPM, more preferably 15 RPM - 25 RPM; more preferably 18 RPM - 22 RPM.

[0063] In some of the preferred embodiments, the process conditions for coating the conductive agent on the surface of the pretreated graphite with a powder coater equipped with an air atomization device include: the carrier gas pressure is 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, more preferably 0.18 RPM - 0.22 RPM.

[0064] In some preferred embodiments, the process conditions for coating the conductive agent on the surface of the pretreated graphite using a powder coating instrument with an air atomization device include: the atomization pressure is 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 on the surface of the pretreated graphite using a powder coating instrument with an air atomization device include: the backflush frequency is 0 times / min - 10 times / min, more preferably 2 times / min - 10 times / min, even more preferably 4 times / min - 8 times / min, and even more preferably 4 times / min - 6 times / min.

[0066] In some preferred embodiments, the process conditions for coating the conductive agent on the surface of the pretreated graphite using a powder coating instrument with an air atomization device include: after the liquid feeding is completed, continue the gas flow purge 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 air atomization device, locking the reactor, adjusting the rotation speed of the peristaltic pump, setting the carrier gas temperature and the backflush frequency, then turning on the power supply and the gas source, adjusting the carrier gas pressure and the atomization pressure, then pumping in the conductive agent aqueous dispersion heated to the required temperature to start coating, after the liquid feeding is completed, continue the gas flow purge for a certain period of time, and finally turn off the power supply and the gas source. After the powder settles, take the powder to obtain the modified graphite.

[0068] Some embodiments also relate to the modified graphite prepared by the preparation method of the present invention.

[0069] Some embodiments also relate to a battery negative electrode sheet prepared using the modified graphite of the present invention as a negative electrode active material, and a lithium battery including 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 examples are described as follows:

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

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

[0074] Example 1

[0075] The preparation method of the modified graphite provided by this embodiment is as follows:

[0076] (1) Weigh a certain mass of graphite and place it in a high-temperature reaction kettle. Set the temperature to 300 °C, the pressure to 30 MPa, and process for 30 min to obtain thin-layer graphite with a thickness of 100 nm;

[0077] (2) Transfer the above thin-layer graphite to a radio frequency plasma device. Use an oxygen / nitrogen mixed gas (volume ratio of oxygen to nitrogen is 1:3) as the plasma gas source, with a gas flow rate of 90 sccm. Process for 10 min under the conditions of 0.01 MPa, 80 °C, and 150 W. Take it out and store it sealed, denoted as pre-treated graphite for later use;

[0078] (3) Weigh 30 g of multi-walled carbon nanotube aqueous dispersion and heat it to 30 °C for later use. Weigh 1.5 kg of pre-treated graphite and place it in the reactor of a powder coating instrument with a gas atomization device. Then lock the reactor, adjust the flow pump speed to 20 RPM, set the carrier gas temperature to 30 °C, set it to non-backflush, then turn on the power supply and gas source, adjust the carrier gas pressure to 0.4 MPa and the atomization pressure to 0.5 MPa, and then start pumping the multi-walled carbon nanotube aqueous dispersion heated to 30 °C for coating. After the liquid feeding is completed, continue gas purging for 20 min. Finally, turn off the power supply and gas source. After the powder settles, start taking the powder, which is the modified graphite.

[0079] Example 2

[0080] The preparation method of the modified graphite provided by this embodiment is as follows:

[0081] (1) Weigh a certain mass of graphite and place it in a high-temperature reaction kettle. Set the temperature to 300 °C, the pressure to 30 MPa, and process for 30 min to obtain thin-layer graphite with a thickness of 100 nm;

[0082] (2) Transfer the above thin-layer graphite to a radio frequency plasma device. Use an oxygen / nitrogen mixed gas (volume ratio of oxygen to nitrogen is 1:3) as the plasma gas source, with a gas flow rate of 90 sccm. Process for 10 min under the conditions of 0.01 MPa, 80 °C, and 150 W. Take it out and store it sealed, denoted as pre-treated graphite for later use;

[0083] (3) Weigh 30 g of the multi-walled carbon nanotube aqueous dispersion and heat it to 30 °C for later use. Weigh 1.5 kg of the pretreated graphite and place it in the reactor of the powder coating instrument with an air atomization device. Then lock the reactor, adjust the flow pump speed to 20 RPM, set the carrier gas temperature to 30 °C, set the backflush frequency to 5 times / min. Then turn on the power supply and gas source, adjust the carrier gas pressure to 0.4 MPa and the atomization pressure to 0.5 MPa. Then start pumping in the multi-walled carbon nanotube aqueous dispersion heated to 30 °C for coating. After the liquid feeding is completed, continue the gas purge for 20 min. Finally, turn off the power supply and gas source. After the powder settles, start taking the powder, which is the modified graphite.

[0084] Example 3

[0085] The preparation method of the modified graphite provided in this example is as follows:

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

[0087] (2) Transfer the above thin-layer graphite to a radio frequency plasma device. Use a mixture of oxygen / nitrogen (volume ratio of oxygen to nitrogen is 1:3) as the plasma gas source, and the gas flow rate is 90 sccm. Treat it under the conditions of 0.01 MPa, 80 °C, and 150 W for 10 min. Take it out and store it sealed, denoted as pretreated graphite for later use.

[0088] (3) Weigh 30 g of the multi-walled carbon nanotube aqueous dispersion and heat it to 30 °C for later use. Weigh 1.5 kg of the pretreated graphite and place it in the reactor of the powder coating instrument with an air atomization device. Then lock the reactor, adjust the flow pump speed to 20 RPM, set the carrier gas temperature to 30 °C, set the backflush frequency to 5 times / min. Then turn on the power supply and gas source, adjust the carrier gas pressure to 0.2 MPa and the atomization pressure to 0.5 MPa. Then start pumping in the multi-walled carbon nanotube aqueous dispersion heated to 30 °C for coating. After the liquid feeding is completed, continue the gas purge for 20 min. Finally, turn off the power supply and gas source. After the powder settles, start taking the powder, which is the modified graphite.

[0089] Example 4

[0090] The preparation method of the modified graphite provided in this example is as follows:

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

[0092] (2) Transfer the above thin-layer graphite to a radio frequency plasma device. Use an oxygen / nitrogen mixed gas (volume ratio of oxygen to nitrogen is 1:3) as the plasma gas source, with a gas flow rate of 90 sccm. Treat it for 10 min under the conditions of 0.01 MPa, 80 °C, and 150 W. Take it out and store it sealed, denoted as pretreated graphite for later use;

[0093] (3) Weigh 15 g of multi-walled carbon nanotube aqueous dispersion and heat it to 30 °C for later use; weigh 1.5 kg 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 20 RPM, set the carrier gas temperature to 30 °C, set the backflush frequency to 5 times / min. Then turn on the power supply and gas source, adjust the carrier gas pressure to 0.2 MPa and the atomization pressure to 0.5 MPa. Then start pumping the multi-walled carbon nanotube aqueous dispersion heated to 30 °C for coating. After the liquid feeding ends, continue the gas purge for 20 min. Finally, turn off the power supply and gas source. After the powder settles, start taking the powder, which is the modified graphite.

[0094] Example 5

[0095] The difference between the preparation method of the modified graphite provided in this example and that in Example 3 is that after the liquid feeding ends, the time for continuous gas purge is 10 min. The specific method is as follows:

[0096] (1) Weigh a certain mass of graphite and place it in a high-temperature reaction kettle. Set the temperature to 300 °C and the pressure to 30 MPa, and treat it for 30 min to obtain a thin-layer graphite with a thickness of 100 nm;

[0097] (2) Transfer the above thin-layer graphite to a radio frequency plasma device. Use an oxygen / nitrogen mixed gas (volume ratio of oxygen to nitrogen is 1:3) as the plasma gas source, with a gas flow rate of 90 sccm. Treat it for 10 min under the conditions of 0.01 MPa, 80 °C, and 150 W. Take it out and store it sealed, denoted as pretreated graphite for later use;

[0098] (3) Weigh 30 g of multi-walled carbon nanotube aqueous dispersion and heat it to 30 °C for later use; weigh 1.5 kg 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 20 RPM, set the carrier gas temperature to 30 °C, set the backflush frequency to 5 times / min. Then turn on the power supply and gas source, adjust the carrier gas pressure to 0.2 MPa and the atomization pressure to 0.5 MPa. Then start pumping the multi-walled carbon nanotube aqueous dispersion heated to 30 °C for coating. After the liquid feeding ends, continue the gas purge for 10 min. Finally, turn off the power supply and gas source. After the powder settles, start taking the powder, which is the modified graphite.

[0099] Example 6

[0100] The difference between the preparation method of the modified graphite provided in this example and that in Example 3 is that the high-temperature and high-pressure treatment time is 20 min, 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 reaction kettle. Set the temperature to 300 °C, the pressure to 30 MPa, and treat for 20 min;

[0102] (2) Transfer the above-mentioned thin-layer graphite to a radio frequency plasma device. Use nitrogen as the plasma gas source, with a gas flow rate of 90 sccm. Treat it for 10 min under the conditions of 0.01 MPa, 80 °C, and 150 W. Take it out and store it sealed, denoted as pretreated graphite for later use;

[0103] (3) Weigh 30 g of multi-walled carbon nanotube aqueous dispersion and heat it to 30 °C for later use; Weigh 1.5 kg 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 20 RPM, set the carrier gas temperature to 30 °C, set the backflush frequency to 5 times / min. Then turn on the power supply and gas source, adjust the carrier gas pressure to 0.2 MPa and the atomization pressure to 0.5 MPa. Then start pumping the multi-walled carbon nanotube aqueous dispersion heated to 30 °C for coating. After the liquid feeding is completed, continue gas purging for 20 min. Finally, turn off the power supply and gas source. After the powder settles, start taking the powder, which is the modified graphite.

[0104] Comparative Example 1

[0105] Purchased graphite raw powder.

[0106] Comparative Example 2

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

[0108] Weigh 30 g of multi-walled carbon nanotube aqueous dispersion and heat it to 30 °C for later use; Weigh 1.5 kg of graphite raw powder 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 20 RPM, set the carrier gas temperature to 30 °C, set the backflush frequency to 5 times / min. Then turn on the power supply and gas source, adjust the carrier gas pressure to 0.2 MPa and the atomization pressure to 0.5 MPa. Then start pumping the multi-walled carbon nanotube aqueous dispersion heated to 30 °C for coating. After the liquid feeding is completed, continue gas purging for 20 min. Finally, turn off the power supply and gas source. After the powder settles, start taking the powder, which is the modified graphite.

[0109] Comparative Example 3

[0110] The preparation method of the modified graphite provided in this comparative example is different from that of Example 3 in that the graphite is 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 mixed gas (volume ratio of oxygen to nitrogen is 1:3) as the plasma gas source, with a gas flow rate of 90 sccm, and treat it for 10 min under the conditions of 0.01 MPa, 80 °C, and 150 W. Take it out and store it sealed, denoted as pretreated graphite for later use;

[0112] (2) Weigh 30 g of multi-walled carbon nanotube aqueous dispersion and heat it to 30 °C for later use; weigh 1.5 kg 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 20 RPM, set the carrier gas temperature to 30 °C, set the backflush frequency to 5 times / min, then turn on the power supply and gas source, adjust the carrier gas pressure to 0.2 MPa and the atomization pressure to 0.5 MPa, and then start pumping the multi-walled carbon nanotube aqueous dispersion heated to 30 °C for coating. After the liquid feeding is completed, continue the gas purge for 20 min. Finally, turn off the power supply and gas source. After the powder settles, start taking the powder, which is the modified graphite.

[0113] Comparative Example 4

[0114] The preparation method of the modified graphite provided in this comparative example is different from that of Example 3 in that the graphite is 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 reaction kettle, set the temperature to 300 °C, the pressure to 30 MPa, and treat it for 30 min to obtain thin-layer graphite with a thickness of 100 nm for later use;

[0116] (2) Weigh 30 g of multi-walled carbon nanotube aqueous dispersion and heat it to 30 °C for later use; weigh 1.5 kg of thin-layer 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 20 RPM, set the carrier gas temperature to 30 °C, set the backflush frequency to 5 times / min, then turn on the power supply and gas source, adjust the carrier gas pressure to 0.2 MPa and the atomization pressure to 0.5 MPa, and then start pumping the multi-walled carbon nanotube aqueous dispersion heated to 30 °C for coating. After the liquid feeding is completed, continue the gas purge for 20 min. Finally, turn off the power supply and gas source. After the powder settles, start taking the powder, which is the modified graphite.

[0117] Comparative Example 5

[0118] The preparation method of the modified graphite provided in this comparative example is different from that of Example 3 in that the amount of the multi-walled carbon nanotube aqueous dispersion is 75 g. 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 °C, the pressure to 30 MPa, and process for 30 minutes to obtain thin-layer graphite with a thickness of 100 nm.

[0120] (2) Transfer the above thin-layer graphite to a radio frequency plasma device. Use a mixture of oxygen / nitrogen (volume ratio of oxygen to nitrogen is 1:3) as the plasma gas source, with a gas flow rate of 90 sccm. Process under the conditions of 0.01 MPa, 80 °C, and 150 W for 10 minutes, then take it out and store it sealed. Denote it as pretreated graphite for later use.

[0121] (3) Weigh 75 g of multi-walled carbon nanotube aqueous dispersion and heat it to 30 °C for later use. Weigh 1.5 kg 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 pump speed of the flow pump to 20 RPM, set the carrier gas temperature to 30 °C, set the backflush frequency to 5 times / min. Then turn on the power supply and gas source, adjust the carrier gas pressure to 0.2 MPa and the atomization pressure to 0.5 MPa. Then start pumping the multi-walled carbon nanotube aqueous dispersion heated to 30 °C for coating. After the liquid feeding is completed, continue gas purging for 20 minutes. Finally, turn off the power supply and gas source. After the powder settles, start taking the powder, which is the modified graphite.

[0122] Preparation of Coin Cell in Example 7

[0123] Use the modified graphite obtained in Examples 1 - 6 and Comparative Examples 2 - 5 and the graphite raw powder in Comparative Example 1 as the negative electrode active material respectively, and use lithium iron phosphate as the positive electrode active material to prepare a coin cell according to the following method.

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

[0125] Formula: Graphite: Conductive carbon black: PVDF = 90:5:5 (mass ratio), and the 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 disperse it ultrasonically for 10 minutes;

[0129] c) Add graphite in portions and stir with a homogenizer for 2 hours (avoiding bubbles);

[0130] d) Control the slurry viscosity at 3000–5000 mPa·s.

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

[0132] Formula: LiFePO4: Conductive carbon black: PVDF = 80:10:10 (by mass), and the solvent is NMP.

[0133] Preparation steps:

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

[0135] b) Add conductive carbon black and disperse it by ultrasonic wave for 10 minutes;

[0136] c) Add LiFePO4 in portions and stir with a homogenizer for 2 hours (avoiding air bubbles);

[0137] d) Control the slurry viscosity at 3000 - 5000 mPa·s.

[0138] (3) Electrode coating and drying:

[0139] a) Coating: Use a coater to uniformly coat the negative electrode slurry and the positive electrode slurry on the copper foil (negative electrode) and the aluminum foil (positive electrode) respectively, and the wet film thickness is about 100 - 150 μm;

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

[0141] (4) Electrode sheet punching:

[0142] Use a punching machine to punch the dried electrode in step (3) into round sheets with a diameter of 12 mm to obtain the corresponding negative electrode sheets and positive electrode sheets.

[0143] (5) Assemble a button cell in a glove box:

[0144] Assemble the battery in the glove box with a high-purity argon atmosphere in the order of negative electrode shell, negative electrode sheet, electrolyte LP40, polypropylene separator, electrolyte LP40, positive electrode sheet, gasket, spring piece, and positive electrode shell. Put it into a sealed plastic bag and use a manual button cell sealer to seal the battery to obtain a button cell for subsequent testing.

[0145] Performance test of Example 8

[0146] Perform specific surface area tests on the modified graphite obtained in Examples 1 - 6 and Comparative Examples 2 - 5 and the graphite raw powder of Comparative Example 1 by the BET method.

[0147] Perform 100-cycle charge and discharge tests on the button cell prepared in Example 7 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 can be seen from the above table, the specific surface area of the modified graphite prepared by the present invention is much larger than that of the purchased raw graphite powder. This may be because: firstly, the high-pressure and high-temperature treatment in the high-pressure reactor destroys the van der Waals force between the graphite molecular layers, resulting in the lateral slip of the graphite molecular layers, reducing the graphite molecular layer spacing, thinning the graphite molecules, thus exposing more active sites and significantly increasing its surface area; secondly, plasma modification not only introduces C-N bonds and C-O bonds, but also etches the surface of graphite particles, thereby increasing the surface roughness of the particles.

[0153] The button cell prepared with the modified graphite of the present invention as the negative electrode active material has a better capacity output rate at 2C rate and a better capacity retention rate after 100 cycles at 1C than the button cell prepared with the raw graphite powder of Comparative Example 1 as the negative electrode active material. This is because after uniformly coating a layer of conductive agent on the graphite, its conductivity increases, 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 raw graphite powder in Comparative Example 1. The increase in roughness and specific surface area can enhance the electrolyte penetration ability and reduce the interfacial impedance, thereby improving the rate performance and capacity retention rate of the battery.

[0154] Among them, the modified graphite prepared in Example 3 has the best coating effect, and the battery performance prepared with it as the negative electrode active material is the best. Because, in Example 3, back blowing was carried out during the gas atomization coating of the conductive agent, and at the same time, the carrier gas pressure was reduced. The present invention found that such treatment can not only blow up all the graphite to fully contact with the conductive agent droplets, but also avoid the problem of waste of the conductive agent and uneven coating caused by a large amount of graphite being blown to the filter element part. Therefore, the performance of Example 3 is the best.

[0155] In Example 1, back blowing was not carried out during the gas atomization coating of the conductive agent, and at the same time, the carrier gas pressure was higher than that in Example 3, which would cause the graphite at the bottom of the reactor to be blown to the filter element and stick to the filter element and could not return to the middle of the reactor, resulting in the conductive agent not being able to completely and uniformly coat on the graphite. Therefore, the coating effect of the modified graphite prepared in Example 1 is worse than that in Example 3, and its performance is worse than that in Example 3. After back blowing at a frequency of 5 times / min in Example 2, part of the graphite blown to the filter element part can be returned to the middle of the reactor to contact with the conductive agent droplets. Therefore, its coating effect is better than that in Example 1, but the carrier gas pressure in Example 2 is higher than that in Example 3, and its coating effect is still worse than that in Example 3.

[0156] Compared with Example 3, in Example 4, the mass ratio of the multi-walled carbon nanotube aqueous dispersion to graphite was adjusted from 2% to 1%. The reduction of the conductive agent would lead to a decrease in the rate performance and capacity retention rate of the battery. In Comparative Example 5, compared with Example 3, the mass ratio of the multi-walled carbon nanotube aqueous dispersion to graphite was adjusted from 2% to 5%. The excessive proportion of the conductive agent caused serious wetting of the graphite, resulting in agglomeration and caking of the graphite, and the conductive agent could not be uniformly coated. This would lead to a large local impedance and uneven current density of the battery prepared with the obtained modified graphite as the negative active material, thus greatly reducing the rate performance and capacity retention rate of the battery. It can be seen that only by controlling the mass ratio of the conductive agent (multi-walled carbon nanotube aqueous dispersion) to graphite within a reasonable range can the performance of the modified graphite as the negative electrode material of the battery be effectively improved.

[0157] Gas flow mixing can cause friction between particles and make the particles move and tumble continuously, which can further uniformly mix the conductive agent among the graphite particles to achieve the effect of uniform coating. Compared with Example 3, in Example 5, the time of gas flow mixing after the liquid feeding ended was reduced, resulting in poorer uniformity of the conductive agent coating than that in Example 3, and there was a phenomenon of local accumulation of the conductive agent, which would lead to a large local impedance and uneven current density of the battery, etc. Therefore, its performance was inferior to that of Example 3.

[0158] In Comparative Example 2, high-pressure treatment in a high-temperature autoclave and plasma modification were not carried out, but the graphite raw powder was directly used for gas atomization coating, and the coating method and process parameters were the same as those in Example 3. Compared with Example 3, in Comparative Example 3, no high-temperature and high-pressure pretreatment was carried out. Compared with Example 3, in Comparative Example 4, no plasma treatment was carried out. It can be seen that the specific surface area of the modified graphite prepared in Comparative Example 2 was basically the same as that of the graphite raw powder in Comparative Example 1, far lower than that of the modified graphite in Example 3. The specific surface area of the modified graphite prepared in Comparative Example 3 was slightly larger than that in Comparative Example 1, but still much lower than that in Example 3. The specific surface area of the modified graphite prepared in Comparative Example 4 was larger than that in Comparative Examples 1-3, but still smaller than that in Example 3. This shows that high-pressure treatment is the main reason for increasing the specific surface area of graphite, and plasma modification can further increase the specific surface area of graphite. The increase in the specific surface area can expose more active sites, which is beneficial to the uniform coating of the conductive agent. The modified graphite in Example 3 contains hydrophilic groups such as C-N bonds and C-O bonds, which can improve the dispersion of the conductive agent in graphite, and is also conducive to the uniform coating of the conductive agent on the graphite. At the same time, it can cause a chemical reaction between the conductive agent and the surface groups of the graphite, improving the bonding force between the coating layer and the graphite particles and avoiding the shedding of the coating layer. That is, the synergistic effect of high-temperature and high-pressure treatment and plasma treatment can make the modified graphite have a large specific surface area and many hydrophilic groups, and the two cooperate synergistically to significantly improve the comprehensive performance of the modified graphite in Example 3 as the negative electrode material of the battery, far superior to Comparative Examples 2-4.

[0159] Compared with Example 3, in Example 6, the high-temperature and high-pressure treatment time was shortened from 30 min to 20 min, and the gas source of the plasma was nitrogen. It can be seen that the comprehensive performance of the modified graphite in Example 6 as the battery anode material is inferior to that in Example 3. This may be because the high-temperature and high-pressure treatment time is insufficient, resulting in insufficient interlayer slip of graphite molecules, so that the specific surface area of the modified graphite prepared in Example 6 is smaller than that of the modified graphite prepared in Example 3; and using nitrogen as the plasma gas source makes there be only C-N bonds on the graphite surface and lack of C-O bonds, which weakens the dispersion of the conductive agent in the graphite and also reduces the binding force between the coating layer and the graphite substrate, leading to uneven coating of the conductive agent. It can be seen that both the high-temperature and high-pressure treatment time and the type of plasma gas source have a great influence on the performance of the modified graphite as the battery anode material.

[0160] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A method for preparing modified graphite, characterized in that: The steps include: (1) subjecting graphite to high pressure treatment under heating conditions to obtain thin-layer graphite; (2) subjecting the thin layer graphite to plasma treatment 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%.

2. The method for preparing modified graphite according to claim 1, characterized in that: The conditions of the high pressure treatment include: a pressure of 10MPa-50MPa and a temperature of 200°C-400°C; And / or, the high pressure treatment time is 20 min-60 min.

3. The method for preparing modified graphite according to claim 2, characterized in that: The conditions of the high pressure treatment include: a pressure of 20MPa-40MPa and a temperature of 250°C-350°C; And / or, the high pressure treatment time is 28 min-35 min.

4. The method for preparing modified graphite according to claim 1, characterized in that: The thickness of the thin layer of graphite is 10nm-100nm.

5. The method for preparing modified graphite according to claim 1, characterized in that: The plasma gas source of the plasma treatment is a mixed gas of oxygen and nitrogen.

6. The method for preparing modified graphite according to claim 5, characterized in that: The volume ratio of oxygen to nitrogen is 1:2-5.

7. The method for preparing modified graphite according to claim 1, characterized in that: The conditions of the plasma treatment include: power of 50W-200W; and / or, the pressure is 0.005MPa-0.015MPa; and / or, the temperature is 60°C-95°C; and / or, the gas flow rate is 70 sccm-110 sccm; And / or, the processing time is 5min-30min.

8. The method for preparing modified graphite according to claim 7, characterized in that: The conditions of the plasma treatment include: power of 140W-160W; and / or, the pressure is 0.008MPa-0.012MPa; and / or, the temperature is 75°C-85°C; and / or, the gas flow rate is 85 sccm-95 sccm; And / or, the processing time is 8min-15min.

9. The method for preparing modified graphite according to claim 1, characterized in that: The conductive agent is a multi-walled carbon nanotube and / or a single-walled carbon nanotube; And / or, the mass ratio of the conductive agent to the pretreated graphite is 0.028%-0.084%; And / or, the conductive agent is added in the form of a conductive agent aqueous dispersion and is atomized to coat the pretreated graphite.

10. The method for preparing 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 a conductive agent using a powder coating apparatus with an aerosol device, wherein the process conditions include: The carrier gas temperature is 25℃-80℃; and / or, the conductive agent temperature is 25°C-100°C; and / or, the flow pump speed is 10RPM-30RPM; and / or, the carrier gas pressure is 0.1MPa-0.8MPa; and / or, the atomization pressure is 0.4MPa-0.8MPa; and / or, the backflush frequency is 0 times / min-10 times / min; And / or, after the liquid is added, continue to purge with airflow for 10 minutes to 40 minutes.

11. The method for preparing modified graphite according to claim 10, characterized in that: The process conditions for coating the conductive agent on the surface of the pretreated graphite using an aerosol coating apparatus include: The carrier gas temperature is 25℃-35℃; and / or, the conductive agent temperature is 25°C-35°C; and / or, the flow pump speed is 15RPM-25RPM; and / or, the carrier gas pressure is 0.1MPa-0.4MPa; and / or, the atomization pressure is 0.4MPa-0.6MPa; and / or, the backflush frequency is 4 times / min to 8 times / min; And / or, after the liquid is added, continue to purge with airflow for 15 minutes to 25 minutes.

12. The method for preparing modified graphite according to claim 10, characterized in that: Step (3) comprises: adding the pretreated graphite into a reactor of a powder coating apparatus with an atomization device, locking the reactor, adjusting the speed of the flow pump, setting the carrier gas temperature and the back-blowing frequency, then turning on the power supply and the gas source, adjusting the carrier gas pressure and the atomization pressure, and then pumping in a conductive agent aqueous dispersion heated to a desired temperature to start coating. After the liquid is added, the air flow is continued for a certain period of time, and finally the power supply and the gas source are turned off. After the powder is settled, the powder is taken out to obtain the modified graphite.

13. Modified graphite prepared by the preparation method according to any one of claims 1 to 12.

14. Use of the modified graphite according to claim 13 as a negative electrode active material in the preparation of a negative electrode sheet for a battery.

15. Use of the modified graphite according to claim 13 as a negative electrode active material in the preparation of a lithium battery.

16. A battery negative electrode sheet, characterized in that: The modified graphite is prepared using the modified graphite as claimed in claim 13 as raw material.

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

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