Modified graphite as well as preparation method and application thereof

Abstract

The invention relates to modified graphite and a preparation method and application thereof.The modified graphite comprises an inner core and a carbon coating layer coating the surface of the inner core, the inner core comprises graphite and a lithium phosphide layer coating the surface of the graphite, and the median particle size of the graphite is 5-20 micrometers. The modified graphite enables the prepared battery to have high rate performance, high first efficiency and long cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of anode materials, and particularly to modified graphite and its preparation method and application. Background Art

[0002] Fast-charging lithium-ion batteries can meet the growing demands for portable electronic products and electric vehicles. Graphite anodes are widely regarded as the limiting components for battery fast charging. On the one hand, fast charging is restricted by the slow electrochemical reaction process on the graphite surface, including Li + desolvation at the solid electrolyte interface (SEI) and Li + transport within the SEI; on the other hand, due to its low equilibrium potential (~0.1V vs Li / Li + +), graphite has a low overpotential tolerance, which means that once the anode potential drops below 0V during charging (vs Li / Li+), metallic lithium will be easily generated, resulting in poor cycle stability and even safety problems. Summary of the Invention

[0003] Based on this, it is necessary to provide a modified graphite and its preparation method and application to solve the above problems. The modified graphite can endow the prepared battery with high rate performance, high initial efficiency, and long cycle performance.

[0004] A modified graphite, comprising a core and a carbon coating layer coated on the surface of the core. Among them, the core includes graphite and a lithium phosphide layer coated on the surface of the graphite, and the median particle size of the graphite is 5μm - 20μm.

[0005] In one embodiment, the thickness of the lithium phosphide layer is 5nm - 20nm;

[0006] and / or, the thickness of the carbon coating layer is 1nm - 10nm.

[0007] In one embodiment, the mass ratio of the graphite to the lithium phosphide layer is 100:0.8 - 100:12.

[0008] In one embodiment, the lithium phosphide layer is coated on the end face of the graphite.

[0009] In one embodiment, the graphite is selected from at least one of artificial graphite, natural graphite, and mesocarbon microbeads;

[0010] and / or, the carbon coating layer material is selected from carbon nanotubes and / or graphene.

[0011] In the modified graphite of the present invention, by providing a core and a carbon coating layer, and defining that the core includes graphite of a specific size and a lithium phosphide layer coated on the surface of the graphite, taking advantage of the characteristics of lithium phosphide having high Li + ionic conductivity, electronic insulation, and a compact structure, using the lithium phosphide layer as the artificial SEI film of the graphite, and cooperating with the carbon coating layer, the modified graphite can have high ionic conductivity, high electronic conductivity, and excellent kinetic performance, effectively improving the rate performance of the battery. At the same time, it can effectively reduce the loss of irreversible lithium in the battery and improve the first charge-discharge efficiency of the battery. In addition, it can effectively avoid the contact between the negative electrode material and the electrolyte, significantly improving the cycle performance of the battery. Therefore, the modified graphite of the present invention enables the prepared battery to have both high rate performance, high first efficiency, and long cycle performance.

[0012] A preparation method of the modified graphite as described above includes the following steps:

[0013] Provide graphite with a median particle size of 5 μm - 20 μm;

[0014] Prepare a red phosphorus layer on the surface of the graphite by the sublimation-adsorption method;

[0015] Mix the graphite with the red phosphorus layer and a lithium source, and then place it in an inert atmosphere for a heating reaction to form a lithium phosphide layer, obtaining the core material. Among them, the heating reaction temperature is greater than the melting point temperature of lithium and less than the sublimation temperature of red phosphorus;

[0016] Prepare a carbon coating layer on the surface of the core material to obtain the modified graphite.

[0017] In one embodiment, in the step of preparing a red phosphorus layer on the surface of the graphite by the sublimation-adsorption method, the graphite and red phosphorus are mixed to obtain a mixture, and then the mixture is placed in an inert atmosphere for heating to sublime the red phosphorus in the mixture into phosphorus vapor, and then cooled to condense and deposit the phosphorus vapor on the surface of the graphite to form a red phosphorus layer.

[0018] In one embodiment, in the step of preparing a red phosphorus layer on the surface of the graphite by the sublimation-adsorption method, at least one of the following conditions is satisfied:

[0019] (1) The mass ratio of the graphite to the red phosphorus is 100:1.5 - 100:10;

[0020] (2) In the step of mixing the graphite and red phosphorus, the mixing speed of the mixer is 15 rpm - 25 rpm, and the mixing time is 60 min - 180 min;

[0021] (3) In the step of heating the mixture in an inert atmosphere, the mixture is placed in a rotary kiln device. First, nitrogen and / or an inert gas is introduced, and the volume fraction of oxygen is controlled to be less than 1 ppm. Then, it is heated to 420°C - 800°C at a heating rate of 2°C / min - 10°C / min and kept warm for 1 h - 5 h;

[0022] (4) In the cooling step, first cool down to 260°C - 300°C, keep warm for 20 h - 30 h, and then cool naturally.

[0023] In one embodiment, in the step of preparing the core material, at least one of the following conditions is satisfied:

[0024] (1) The mass ratio of the graphite with a red phosphorus layer to the lithium source is 100:1 - 100:7;

[0025] (2) The lithium source is selected from at least one of lithium foil, lithium powder, or molten lithium;

[0026] (3) The volume fraction of water vapor in the inert atmosphere is less than 0.1 ppm, and the volume fraction of oxygen is less than 0.2 ppm;

[0027] (4) The heating reaction time is 0.5 h - 3 h.

[0028] In one embodiment, in the step of preparing a carbon coating layer on the surface of the core material, at least one of the following conditions is satisfied:

[0029] (1) The carbon coating layer is prepared on the surface of the core material by ball milling or solvothermal method;

[0030] (2) The mass ratio of the core material to the carbon coating layer material is 100:0.5 - 100:2, wherein the carbon coating layer material is selected from carbon nanotubes and / or graphene.

[0031] In the preparation method of the modified graphite of the present invention, graphite with a specific median particle size is used as the matrix material of the core. At the same time, a red phosphorus layer is deposited on the graphite surface by sublimation - adsorption method, and then under specific conditions, red phosphorus reacts with lithium to in - situ form a lithium phosphide layer on the graphite surface. Finally, a carbon coating layer is prepared to obtain the modified graphite. This preparation method is simple and is conducive to large - scale production.

[0032] A negative electrode sheet, the negative electrode sheet includes a current collector and a negative electrode active layer provided on the surface of the current collector, and the components of the negative electrode active layer include the modified graphite as described above.

[0033] A secondary battery including the negative electrode sheet as described above. Detailed Description of the Invention

[0034] For the convenience of understanding the present invention, the present invention will be described in more detail below. However, it should be understood that the present invention can be implemented in many different forms and is not limited to the embodiments or examples described herein. On the contrary, these embodiments or examples are provided to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs. The terms used in the description of the present invention herein are only for the purpose of describing specific embodiments or examples and are not intended to limit the present invention. The optional scope of the term "and / or" used herein includes any one of two or more related listed items, and also includes any and all combinations of the related listed items, and the any and all combinations include any two related listed items, any more related listed items, or the combination of all related listed items.

[0036] The modified graphite provided by the present invention includes a core and a carbon coating layer coated on the surface of the core. Among them, the core includes graphite and a lithium phosphide layer coated on the surface of the graphite, and the median particle size of the graphite is 5 μm - 20 μm.

[0037] Specifically, in the present invention, graphite is used as the core body of the modified graphite, and a lithium phosphide layer is coated on the surface of the graphite. By using the characteristics of high Li + electrical conductivity, electron insulation, and compact structure of lithium phosphide, the lithium phosphide layer is used as the artificial SEI film of graphite. Among them, the characteristic of high Li + electrical conductivity can accelerate Li + desolvation and diffusion through the SEI, enhancing the kinetic performance of the modified graphite; the characteristic of electron insulation makes electrons unable to enter the graphite interlayer, inhibiting the self-discharge and chemical side reactions of the battery to a certain extent, and effectively reducing the probability of internal short circuit; and the characteristic of the compact structure can prevent the co-insertion of graphite and the solvent in the electrolyte, greatly reducing the probability of graphite layer peeling. Therefore, in the present invention, the lithium phosphide layer is used as the artificial SEI film of graphite, which can effectively avoid the process of forming the SEI film by the reaction of lithium and electrolyte during the first charge and discharge of the battery, reduce the irreversible lithium loss of the battery, improve the first charge and discharge efficiency of the battery, and improve the cycle performance of the battery. At the same time, by controlling the median particle size of graphite, the specific surface area of the modified graphite can be effectively controlled, which is beneficial to improving the first efficiency and cycle performance of the battery.

[0038] In addition, by setting the carbon coating layer, on the one hand, it can improve the electronic conductivity of the modified graphite and improve the characteristic of low electronic conductivity of lithium phosphide; on the other hand, it can further protect the lithium phosphide layer and prevent it from undergoing unstable changes or decomposition during the cycling process, further improving the cycling performance of the battery.

[0039] Therefore, for the modified graphite of the present invention, using graphite of a specific size, with the lithium phosphide layer as the artificial SEI film of the graphite, in cooperation with the carbon coating layer, the modified graphite can have high ionic conductivity, high electronic conductivity, and excellent kinetic performance, effectively improving the rate performance of the battery. At the same time, it can effectively reduce the loss of irreversible lithium in the battery and improve the first charge-discharge efficiency of the battery. In addition, it can effectively avoid the contact between the negative electrode material and the electrolyte, significantly improving the cycling performance of the battery, so that the battery prepared with the modified graphite has both high rate performance, high first efficiency, and long cycling performance.

[0040] Optionally, the thickness of the lithium phosphide layer is 5 nm - 20 nm. It can be understood that the thickness of the lithium phosphide layer is any value between 5 nm and 20 nm. Specifically, the thickness of the lithium phosphide layer includes but is not limited to 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 12 nm, 14 nm, 16 nm, 18 nm, 20 nm. Such a setting is beneficial to improving the first efficiency and cycling performance of the battery.

[0041] Optionally, the thickness of the carbon coating layer is 1 nm - 10 nm. It can be understood that the thickness of the carbon coating layer is any value between 1 nm and 10 nm. Specifically, the thickness of the carbon coating layer includes but is not limited to 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm. Such a setting is beneficial to improving the electronic conductivity of the modified graphite and further improving the rate performance and cycling performance of the battery.

[0042] Optionally, the mass ratio of the graphite to the lithium phosphide layer is 100:0.8 - 100:12. It can be understood that the mass ratio of the graphite to the lithium phosphide layer is any value between 100:0.8 and 100:12. Specifically, the mass ratio of the graphite to the lithium phosphide layer includes but is not limited to 100:0.8, 100:1, 100:2, 100:3, 100:4, 100:5, 100:6, 100:8, 100:10, 100:12. Such a setting can, by adjusting the mass ratio of the two, be beneficial to further improving the rate performance, first efficiency, and cycling performance of the battery.

[0043] In the present invention, the mass fraction of phosphorus in the modified graphite is 0.2%-3%, and the mass fraction of lithium in the modified graphite is 0.6%-9%. It can be understood that according to the stoichiometric ratio in the molecular formula of lithium phosphide (Li3P), the mass fractions of phosphorus and lithium in the modified graphite of the present invention fully conform to the molecular formula of lithium phosphide.

[0044] It should be noted that the surface structure of graphite is divided into end faces and basal planes. The basal plane is a conjugated large plane structure, while the end face is the edge of the large plane. It can be understood that the lithium phosphide layer is coated on the surface of the graphite, mainly on the end faces. That is, an artificial SEI film, namely the lithium phosphide layer, is coated at the end face defects of the graphite. This is because the carbon atoms at the end face defects have stronger chemical activity and are more likely to react, while the carbon with a complete lattice structure is difficult to react. Moreover, when no artificial SEI film is coated, the SEI film formed during the first charge and discharge process of the battery is also mainly formed at the end faces of the electrode material. Therefore, in the present invention, it is preferably that the lithium phosphide layer is coated on the end faces of the graphite, that is, an artificial SEI film, namely the lithium phosphide layer, is coated at the end face defects of the graphite. With such a setting, the first charge and discharge efficiency and cycle performance of the battery can be further improved.

[0045] Optionally, the graphite is selected from at least one of artificial graphite, natural graphite, and mesocarbon microbeads, preferably artificial graphite.

[0046] Optionally, the carbon coating layer material is selected from carbon nanotubes and / or graphene.

[0047] Meanwhile, the present invention also provides a preparation method of the modified graphite as described above, including the following steps:

[0048] S1. Provide graphite with a median particle size of 5 μm - 20 μm. It can be understood that by limiting the median particle size of the graphite, the particle size of the modified graphite can be effectively controlled, thereby controlling the specific surface area of the modified graphite, and it is beneficial to deposit a uniform red phosphorus layer and lithium phosphide layer on the surface of the graphite, especially on the end faces subsequently.

[0049] Optionally, the graphite is selected from at least one of artificial graphite, natural graphite, and mesocarbon microbeads, preferably artificial graphite.

[0050] In one embodiment, the preparation method of the artificial graphite includes the following steps: subjecting the coke raw material to conventional crushing, shaping, and pre-carbonization to obtain artificial graphite, wherein the coke raw material is selected from at least one of needle coke, petroleum coke, or pitch coke.

[0051] S2. Prepare a red phosphorus layer on the surface of the graphite by the sublimation-adsorption method.

[0052] In step S2, the graphite and red phosphorus are mixed to obtain a mixture, and then the mixture is placed in an inert atmosphere for heating to sublime the red phosphorus in the mixture into phosphorus vapor. Then, it is cooled to condense and deposit the phosphorus vapor on the surface of the graphite to form a red phosphorus layer. It can be understood that the phosphorus vapor obtained by sublimating red phosphorus occupies the active sites on the surface of the graphite, especially at the end face defects where the carbon atoms have higher activity, so that the phosphorus vapor adheres to the surface of the graphite, especially the end face of the graphite. Then, after cooling, white phosphorus is converted into red phosphorus, and then a red phosphorus layer is deposited on the surface of the graphite, mainly forming a red phosphorus layer at the end face defects of the graphite, because carbon with a complete lattice structure is not easily reactive with phosphorus.

[0053] Optionally, the mass ratio of the graphite to the red phosphorus is 100:1.5 - 100:10. Specifically, the mass ratio of the graphite to the red phosphorus includes but is not limited to 100:1.5, 100:2, 100:3, 100:4, 100:5, 100:6, 100:7, 100:8, 100:9, 100:10, and preferably 100:5 - 100:7. With such a setting, it is beneficial to uniformly form a red phosphorus layer on the surface of the graphite.

[0054] In the present invention, the graphite and red phosphorus are placed in a mixer for mixing to obtain a mixture. Among them, the mixing speed of the mixer is 15 rpm - 25 rpm, and the mixing time is 60 min - 180 min. With such a setting, the graphite and red phosphorus can be mixed more uniformly.

[0055] Optionally, in the step of heating the mixture in an inert atmosphere, the mixture is placed in a rotary kiln device. First, nitrogen and / or inert gas is introduced to control the volume fraction of oxygen to be less than 1 ppm. Then, it is heated at a heating rate of 2 °C / min - 10 °C / min to 420 °C - 800 °C and then held for 1 h - 5 h. Preferably, the heating rate is 2 °C / min - 5 °C / min, heated to 450 °C - 550 °C, and the holding time is 2 h - 4 h. With such a setting, it is beneficial to achieve the full sublimation of red phosphorus to form phosphorus vapor and make the phosphorus vapor adhere to the surface of the graphite.

[0056] Further, in the cooling step, it is first cooled to 260 °C - 300 °C and held for 20 h - 30 h, and then naturally cooled. With such a setting, it is beneficial to deposit a uniform red phosphorus layer on the surface of the graphite.

[0057] It should be noted that during the process of sublimating the mixture in the rotary kiln equipment and cooling to condense the phosphorus vapor and deposit it on the graphite surface to form a red phosphorus layer, the rotary kiln equipment is always in a rotating state. Among them, during the heat preservation stage of introducing nitrogen and / or inert gas to remove air and heating the equipment, the rotation speed of the rotary kiln equipment is 40 Hz - 60 Hz, and the flow rate of nitrogen and / or inert gas is 10 L / min - 20 L / min, preferably 15 L / min. During the cooling stage, the rotation speed of the rotary kiln equipment is 20 Hz - 40 Hz, and the flow rate of nitrogen and / or inert gas is 8 L / min - 15 L / min, preferably 10 L / min. With such settings, by controlling the rotary kiln equipment to be always in a rotating state, on the one hand, it can make the formed red phosphorus layer more uniform during the sublimation-adsorption process; on the other hand, it is beneficial to expel the oxygen-containing gas existing between the particles in the rotary kiln equipment with nitrogen and / or inert gas.

[0058] In one embodiment, the inert gas is selected from argon, helium, neon, etc.

[0059] S3. Mix the graphite with the red phosphorus layer and the lithium source, and then place it in an inert atmosphere for heating reaction to form a lithium phosphide layer, obtaining the core material. Among them, the heating reaction temperature is higher than the melting point temperature of lithium and lower than the sublimation temperature of red phosphorus. Preferably, the heating reaction temperature is 200°C - 250°C. It can be understood that by controlling the heating reaction temperature to be higher than the melting point temperature of lithium and lower than the sublimation temperature of red phosphorus, it can ensure that the lithium source reacts fully with the red phosphorus on the graphite surface in the form of molten lithium and in-situ generates lithium phosphide, that is, a uniform lithium phosphide layer is formed on the graphite surface. Among them, the reaction formula of red phosphorus and lithium is as follows: P + 3Li = Li3P.

[0060] Optionally, the mass ratio of the graphite with the red phosphorus layer to the lithium source is 100:1 - 100:7. Specifically, the mass ratio of the graphite with the red phosphorus layer to the lithium source includes but is not limited to 100:1, 100:2, 100:3, 100:4, 100:5, 100:6, 100:7, preferably 100:3 - 100:5. With such settings, it is beneficial to form a uniform lithium phosphide layer on the graphite surface.

[0061] Optionally, the lithium source is selected from at least one of lithium foil, lithium powder, or molten lithium, preferably molten lithium.

[0062] Optionally, the volume fraction of water vapor in the inert atmosphere is less than 0.1 ppm, and the volume fraction of oxygen is less than 0.2 ppm. With such settings, the yield of phosphorus and lithium can be further improved, and the generation of side reactions and by-products can be effectively avoided. For example, the generation of oxides of phosphorus and / or lithium can be avoided.

[0063] In one embodiment, the inert atmosphere is selected from a nitrogen atmosphere and / or an inert gas atmosphere, wherein the inert gas atmosphere is selected from an argon atmosphere, a helium atmosphere, a neon atmosphere, etc.

[0064] Optionally, the heating reaction time is 0.5 h - 3 h. Specifically, the heating reaction time includes but is not limited to 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, and is preferably 1 h - 1.5 h. With such a setting, the sufficient reaction between phosphorus and lithium and the complete conversion into lithium phosphide can be further ensured.

[0065] In one embodiment, after mixing graphite with a red phosphorus layer and a lithium source, the mixture is then placed in an inert atmosphere and heated under continuous stirring to form a lithium phosphide layer.

[0066] S4. A carbon coating layer is prepared on the surface of the core material to obtain modified graphite. It can be understood that by setting the carbon coating layer, on the one hand, the overall electronic conductivity of the modified graphite can be improved, and on the other hand, it can serve as a protective layer for the core material.

[0067] In step S4, the carbon coating layer is prepared on the surface of the core material by a ball milling method or a solvothermal method, preferably the ball milling method.

[0068] In one embodiment, in the ball milling method, the ball milling speed is 50 rpm - 150 rpm, and the ball milling time is 20 min - 40 min.

[0069] Optionally, the mass ratio of the core material to the carbon coating layer material is 100:0.5 - 100:2. Specifically, the mass ratio of the core material to the carbon coating layer material includes but is not limited to 100:0.5, 100:1, 100:1.5, 100:2. With such a setting, it is beneficial to form a uniformly coated carbon coating layer on the core surface.

[0070] Optionally, the carbon coating layer material is selected from carbon nanotubes and / or graphene.

[0071] In the preparation method of the modified graphite of the present invention, graphite with a specific median particle size is used as the matrix material of the core. At the same time, a red phosphorus layer is deposited on the graphite surface by a sublimation - adsorption method, and then under specific conditions, red phosphorus reacts with lithium to in - situ form a lithium phosphide layer on the graphite surface. Finally, a carbon coating layer is prepared to obtain modified graphite. This preparation method is simple and is conducive to large - scale production.

[0072] A negative electrode sheet, the negative electrode sheet includes a current collector and a negative electrode active layer provided on the surface of the current collector, and the components of the negative electrode active layer include the modified graphite as described above.

[0073] A secondary battery including the negative electrode sheet as described above.

[0074] In one embodiment, the secondary battery may be a lithium battery.

[0075] Hereinafter, the modified graphite, its preparation method and application will be further described through the following specific examples. However, those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. For those not specified in the examples, they are carried out under conventional conditions or conditions recommended by the manufacturer. For reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0076] It should be noted that the median particle size of the artificial graphite or natural graphite involved in the examples and comparative examples of the present invention is measured by a Malvern, Master Size 3000 laser particle size analyzer, and the thickness of the lithium phosphide layer and the thickness of the carbon coating layer are both measured by a high-resolution transmission electron microscope.

[0077] Example 1

[0078] The needle coke is subjected to conventional crushing, shaping, pre-carbonization, and graphitization processes to obtain artificial graphite with a median particle size of about 12 μm.

[0079] The artificial graphite and red phosphorus are placed in a ribbon mixer for mixing to obtain a mixture. Among them, the mass ratio of artificial graphite to red phosphorus is 100:6, the rotation speed of the mixer is 20 rpm, and the mixing time is 120 min. Then, the obtained mixture is placed in a rotary kiln device. First, nitrogen is introduced. When the oxygen content in the rotary kiln device is lower than 1 ppm, heating is started. The temperature is raised to 500 °C at a heating rate of 5 °C / min, and after holding for 3 h, the rotary kiln device is cooled to 280 °C and held for 25 h to form a uniform red phosphorus layer on the surface of the artificial graphite. During this period, the rotary kiln device is always in a rotating state. Among them, in the heating and holding stage, the rotation speed of the rotary kiln device is 50 Hz, and the nitrogen flow rate is 15 L / min. In the cooling stage, the rotation speed is 30 Hz, and the nitrogen flow rate is 10 L / min.

[0080] The artificial graphite with a red phosphorus layer is mixed with molten lithium. Among them, the mass ratio of the artificial graphite with a red phosphorus layer to molten lithium is 100:4. Then, in an argon gas atmosphere with a humidity lower than 0.1 ppm and an oxygen content lower than 0.2 ppm, a heating reaction is carried out under continuous mechanical stirring for 1.3 h to form a lithium phosphide layer coated on the surface of the artificial graphite. Among them, the heating reaction temperature is 210 °C to obtain a core material, and the thickness of the lithium phosphide layer is 15.4 nm.

[0081] Mix the obtained core material and carbon nanotubes, where the mass ratio of the core material to the carbon nanotubes is 100:1. Ball mill at a speed of 100 rpm for 30 min in a ball mill to form a carbon coating layer on the surface of the core material, obtaining modified graphite, where the thickness of the carbon coating layer is 5.3 nm.

[0082] After testing, the modified graphite prepared in this example has a core-shell structure.

[0083] Example 2

[0084] Perform conventional crushing, shaping, pre-carbonization, and graphitization processes on needle coke to obtain artificial graphite with a median particle size of approximately 11 μm.

[0085] Place the artificial graphite and red phosphorus in a ribbon mixer for mixing to obtain a mixture, where the mass ratio of the artificial graphite to the red phosphorus is 100:5, the rotation speed of the mixer is 15 rpm, and the mixing time is 180 min. Then place the obtained mixture in a rotary kiln equipment. First, introduce nitrogen. When the oxygen content in the rotary kiln equipment is lower than 1 ppm, start heating. Heat at a heating rate of 5 °C / min to 450 °C, keep the temperature for 2 h, then cool the rotary kiln equipment to 275 °C, and keep the temperature for 28 h to form a uniform red phosphorus layer on the surface of the artificial graphite. During this period, the rotary kiln equipment is always in a rotating state. Among them, during the heating and holding stage, the rotation speed of the rotary kiln equipment is 50 Hz, and the nitrogen flow rate is 15 L / min. During the cooling stage, the rotation speed is 30 Hz, and the nitrogen flow rate is 10 L / min.

[0086] Mix the artificial graphite with red phosphorus layer and molten lithium, where the mass ratio of the artificial graphite with red phosphorus layer to the molten lithium is 100:3. Then, in an argon gas atmosphere with a humidity lower than 0.1 ppm and an oxygen content lower than 0.2 ppm, carry out a heating reaction under continuous mechanical stirring for 1 h to form a lithium phosphide layer on the surface of the artificial graphite. Among them, the heating reaction temperature is 200 °C, obtaining the core material, where the thickness of the lithium phosphide layer is 13.1 nm.

[0087] Mix the obtained core material and carbon nanotubes, where the mass ratio of the core material to the carbon nanotubes is 100:0.8. Ball mill at a speed of 90 rpm for 25 min in a ball mill to form a carbon coating layer on the surface of the core material, obtaining modified graphite, where the thickness of the carbon coating layer is 4.6 nm.

[0088] Example 3

[0089] Perform conventional crushing, shaping, pre-carbonization, and graphitization processes on needle coke to obtain artificial graphite with a median particle size of approximately 13 μm.

[0090] Artificial graphite and red phosphorus are placed in a ribbon mixer for mixing to obtain a mixture. Among them, the mass ratio of artificial graphite to red phosphorus is 100:7, the rotation speed of the mixer is 25 rpm, and the mixing time is 60 min. Then, the mixture obtained above is placed in a rotary kiln equipment. First, nitrogen is introduced. When the oxygen content in the rotary kiln equipment is lower than 1 ppm, heating starts. It is heated to 550 °C at a heating rate of 5 °C / min, and after holding for 4 h, the rotary kiln equipment is cooled to 285 °C and held for 22 h to form a uniform red phosphorus layer on the surface of the artificial graphite. During this period, the rotary kiln equipment is always in a rotating state. Among them, in the heating and holding stage, the rotation speed of the rotary kiln equipment is 50 Hz, and the nitrogen flow rate is 15 L / min. In the cooling stage, the rotation speed is 30 Hz, and the nitrogen flow rate is 10 L / min.

[0091] The artificial graphite with a red phosphorus layer is mixed with molten lithium. Among them, the mass ratio of the artificial graphite with a red phosphorus layer to molten lithium is 100:5. Then, in an argon gas atmosphere with a humidity lower than 0.1 ppm and an oxygen content lower than 0.2 ppm, a heating reaction is carried out under continuous mechanical stirring for 1.5 h to form a lithium phosphide layer coated on the surface of the artificial graphite. Among them, the heating reaction temperature is 220 °C to obtain a core material. Among them, the thickness of the lithium phosphide layer is 17.3 nm.

[0092] The core material obtained above is mixed with carbon nanotubes. Among them, the mass ratio of the core material to carbon nanotubes is 100:1.2, and ball milling is carried out in a ball mill at a rotation speed of 110 rpm for 35 min to form a carbon coating layer coated on the surface of the core material to obtain modified graphite. Among them, the thickness of the carbon coating layer is 6.2 nm.

[0093] Example 4

[0094] Compared with Example 1, Example 4 is only different in that the mass ratio of artificial graphite to red phosphorus is 100:1.5, and the other conditions are the same. Modified graphite is obtained. Among them, the thickness of the lithium phosphide layer is 5.1 nm, and the thickness of the carbon coating layer is 5.2 nm.

[0095] Example 5

[0096] Compared with Example 1, Example 5 is only different in that the mass ratio of the artificial graphite with a red phosphorus layer to molten lithium is 100:1, and the other conditions are the same. Modified graphite is obtained. Among them, the thickness of the lithium phosphide layer is 7.8 nm, and the thickness of the carbon coating layer is 5.1 nm.

[0097] Example 6

[0098] Example 6 is different from Example 1 only in that the reaction time between artificial graphite with a red phosphorus layer and molten lithium is 0.5 h, and the other conditions are the same, obtaining modified graphite, wherein the thickness of the lithium phosphide layer is 13.9 nm and the thickness of the carbon coating layer is 5.3 nm.

[0099] Example 7

[0100] Example 7 is different from Example 1 only in that the mass ratio of the core material to the carbon nanotubes is 100:0.5, and the other conditions are the same, obtaining modified graphite, wherein the thickness of the lithium phosphide layer is 15.5 nm and the thickness of the carbon coating layer is 2.7 nm.

[0101] Example 8

[0102] Example 8 is different from Example 1 only in that the mass ratio of artificial graphite with a red phosphorus layer to molten lithium is 100:7, and the other conditions are the same, obtaining modified graphite, wherein the thickness of the lithium phosphide layer is 15.2 nm and the thickness of the carbon coating layer is 5.1 nm.

[0103] Example 9

[0104] Example 9 is different from Example 1 only in that natural graphite with a median particle size of about 12 μm is used to replace the artificial graphite in Example 1, lithium foil is used to replace molten lithium, and graphene is used to replace carbon nanotubes, and the other conditions are the same, obtaining modified graphite, wherein the thickness of the lithium phosphide layer is 16.7 nm and the thickness of the carbon coating layer is 5.3 nm.

[0105] Comparative Example 1

[0106] Comparative Example 1 is different from Example 1 only in that the median particle size of the artificial graphite is about 2 μm, and the other conditions are the same, obtaining modified graphite, wherein the thickness of the lithium phosphide layer is 15.6 nm and the thickness of the carbon coating layer is 4.8 nm.

[0107] Comparative Example 2

[0108] Comparative Example 2 is different from Example 1 only in that the step of preparing a carbon coating layer on the surface of the core material is not included, and the other conditions are the same, obtaining modified graphite, wherein the thickness of the lithium phosphide layer is 15.3 nm.

[0109] Comparative Example 3

[0110] Comparative Example 3 is different from Example 1 only in that the step of placing the above-obtained mixture in a rotary kiln device is not included, that is, the above-obtained mixture is directly mixed with molten lithium, wherein the mass ratio of the mixture to molten lithium is 100:4, and the other conditions are the same, obtaining modified graphite, wherein the thickness of the carbon coating layer is 5.4 nm.

[0111] Comparative Example 4

[0112] Comparative Example 4 is different from Example 1 only in that it does not contain the step of depositing a red phosphorus layer on the surface of artificial graphite. That is, artificial graphite with a median particle size of about 12 μm is directly mixed with molten lithium. The mass ratio of artificial graphite to molten lithium is 100:4, and the other conditions are the same, obtaining modified graphite, where the thickness of the carbon coating layer is 5.2 nm.

[0113] Comparative Example 5

[0114] Comparative Example 5 is different from Example 1 only in that it does not contain the step of forming a lithium phosphide layer on the surface of artificial graphite. That is, the mixture obtained above is placed in a rotary kiln equipment. First, nitrogen is introduced. When the oxygen content in the rotary kiln equipment is lower than 1 ppm, heating is started, and it is heated to 550 °C at a heating rate of 5 °C / min. After holding for 4 h, with the other conditions being the same, a uniform red phosphorus layer is formed on the surface of artificial graphite, obtaining artificial graphite with a red phosphorus layer; the artificial graphite with a red phosphorus layer and carbon nanotubes are mixed, where the mass ratio of the artificial graphite with a red phosphorus layer to carbon nanotubes is 100:1, and they are ball-milled in a ball mill at a rotation speed of 100 rpm for 30 min to form a carbon coating layer covering the surface of the artificial graphite with a red phosphorus layer, obtaining modified graphite, where the thickness of the carbon coating layer is 5.1 nm.

[0115] Comparative Example 6

[0116] Comparative Example 6 is different from Example 1 only in that the median particle size of artificial graphite is about 25 μm, and the other conditions are the same, obtaining modified graphite, where the thickness of the lithium phosphide layer is 15.1 nm and the thickness of the carbon coating layer is 5.5 nm.

[0117] Comparative Example 7

[0118] Comparative Example 7 is different from Example 9 only in that the heating reaction temperature of natural graphite with a red phosphorus layer and lithium foil is 170 °C, and the other conditions are the same, obtaining modified graphite, where the thickness of the lithium phosphide layer is 0 nm and the thickness of the carbon coating layer is 5.2 nm.

[0119] An inductively coupled plasma optical emission spectrometer (ICP-OES) was used to test the phosphorus and lithium contents in the modified graphite prepared in Examples 1 to 9 and Comparative Examples 1 to 7 respectively. The test results are shown in Table 1.

[0120] Table 1

[0121]

[0122] The modified graphite prepared in Examples 1 to 9 and Comparative Examples 1 to 7 was respectively used to prepare negative electrode sheets, and the corresponding lithium-ion button cells were prepared as follows: The above samples were respectively mixed with conductive agent carbon black (Surper P) and CMC (carboxymethyl cellulose) binder according to a mass ratio of 97:2:1 to obtain a slurry, and then coated on a copper foil to form a negative electrode sheet; The prepared negative electrode sheet was assembled with a lithium metal electrode sheet into a lithium-ion button cell, and LiPF6 was dissolved in an electrolyte of ethylene carbonate / diethyl carbonate / ethyl methyl carbonate = 2:3:1 at a concentration of 1 mol / L, and the button cell assembly was completed.

[0123] Subsequently, the assembled button cells were respectively tested for capacity, rate performance and cycle performance, and the test results are shown in Table 2. The specific test methods are as follows:

[0124] Test steps for the first charge-discharge efficiency at 0.1C: 0.1C discharge specific capacity / 0.1C charge specific capacity.

[0125] Among them, the test steps for the charge-discharge specific capacity at 0.1C are as follows: 1. Stand for 5 h; 2. Discharge at 0.05C to 0.005V; 3. Constant current discharge at 0.05 mA to 0.005V; 4. Stand for 5 min; 5. Constant current discharge at 0.01 mA to 0.005V; 6. Stand for 5 min; 7. Charge at 0.1C to 2V; 8. Stand for 5 min.

[0126] Test steps for the 2C discharge specific capacity: 1. Stand for 5 h; 2. Discharge at 0.05C to 0.005V; 3. Constant current discharge at 0.05 mA to 0.005V; 4. Stand for 5 min; 5. Constant current discharge at 0.01 mA to 0.005V; 6. Stand for 5 min; 7. Charge at 0.1C to 2V; 8. Stand for 5 min; 9. Discharge at 2C to 0.005V; 10. Stand for 30 min; 11. Discharge at 0.1C to 0.005V; 12. Stand for 30 min; 13. Discharge at 0.05C to 0.005V; 14. Stand for 30 min; 15. Charge at 2C to 2V.

[0127] Test steps for the 5C discharge specific capacity: 1. Stand for 5 h; 2. Discharge at 0.05C to 0.005V; 3. Constant current discharge at 0.05 mA to 0.005V; 4. Stand for 5 min; 5. Constant current discharge at 0.01 mA to 0.005V; 6. Stand for 5 min; 7. Charge at 0.1C to 2V; 8. Stand for 5 min; 9. Discharge at 5C to 0.005V; 10. Stand for 30 min; 11. Discharge at 0.1C to 0.005V; 12. Stand for 30 min; 13. Discharge at 0.05C to 0.005V; 14. Stand for 30 min; 15. Charge at 5C to 2V.

[0128] 0.1C 800-cycle capacity retention test procedure: The same as the 0.1C discharge specific capacity test procedure. Set the program to stop after 800 cycles and compare its capacity retention rate.

[0129] Data processing: Select 4 data points with a range within 3 mAh. Remove the maximum and minimum values and calculate the average.

[0130] Table 2

[0131]

[0132]

[0133] Combining the data in Table 1 and Table 2, it can be seen that compared with the data of Examples 1 to 4, among which, in Examples 1 to 3, the percentage content of lithium is approximately three times that of phosphorus, and in Example 4, the percentage content of lithium is approximately six times that of phosphorus. Thus, it can be known that when phosphorus exists on the surface of graphite, molten lithium first reacts with phosphorus to form Li3P, and the reaction formula is P + 3Li = Li3P; when there is no excess phosphorus, prelithiation will occur.

[0134] At the same time, compared with the data of Examples 1, 5, and 8, it can be seen that controlling the mass ratio of artificial graphite with a red phosphorus layer to molten lithium within a suitable range is beneficial to improving the initial efficiency and cycle performance of the battery. Among them, from Example 5, it can be seen that due to the insufficient addition amount of molten lithium, phosphorus is not fully converted into lithium phosphide Li3P, and it needs to react with Li in the electrolyte during the first charge and discharge + to form Li3P, that is, the SEI film, which affects the initial efficiency and cycle performance of the battery. In Example 8, due to the high content of lithium, part of the lithium enters the graphite interior to form prelithiation, and the excessive lithium will consume part of the active material and hinder the normal insertion and extraction of lithium ions inside the battery, resulting in capacity loss of the battery, affecting the rate performance and cycle performance of the battery, and at the same time increasing the cost.

[0135] In Comparative Example 1, the median particle size D of artificial graphite 50 is 2 μm. Its specific surface area is too large and there are many small particles, which will cause an uncontrollable decrease in the first charge and discharge efficiency and cycle performance of the battery. Moreover, due to the small particle size of artificial graphite, its end faces will be more, and the deposited phosphorus content increases by 30.3% compared with Example 1. Therefore, it can be proved from the side that the red phosphorus layer is mainly deposited on the end faces of graphite; in Comparative Example 6, due to the large median particle size of artificial graphite, the channels through which lithium ions are inserted and extracted are long, resulting in poor rate performance of the battery. In addition, the deposited phosphorus content once again proves that the red phosphorus layer is mainly deposited on the end faces of graphite.

[0136] In Comparative Example 2, without the carbon coating layer, the Li3P has excellent ionic conductivity but poor electronic conductivity, which affects the overall rate performance of the battery and cannot protect the SEI film, thus affecting the partial cycle performance of the battery.

[0137] In Comparative Example 3, without using the sublimation-adsorption method, it is impossible to deposit a red phosphorus layer on the surface of graphite, especially on the end face, and thus impossible to form a lithium phosphide SEI film, which affects the rate performance of the battery. Moreover, during the first charge-discharge cycle of the battery, an SEI film needs to be formed, which increases the loss of irreversible lithium and affects the first charge-discharge efficiency and cycle performance of the battery. In addition, the lithium content in the modified graphite is very low, which further proves that molten lithium preferentially reacts with phosphorus to form Li3P.

[0138] In Comparative Example 4, without adding red phosphorus and only reacting with molten lithium, it is equivalent to pre-lithiation of graphite. Although it can improve the first charge-discharge efficiency and cycle performance of the battery to some extent, the rate performance drops severely.

[0139] In Comparative Example 5, without adding a lithium source, no lithium phosphide SEI film is formed, so that the phosphorus deposited on the surface of graphite, especially at the end face, needs to react with Li in the electrolyte during the first charge-discharge cycle + to form a lithium phosphide SEI film, which affects the first efficiency and cycle performance of the battery.

[0140] In Comparative Example 7, since the heating reaction temperature is lower than the melting point temperature of the lithium foil, it is difficult for lithium to react with phosphorus, and no lithium phosphide SEI film can be formed, which affects the first efficiency and cycle performance of the battery. And compared with Comparative Example 5, it can be seen that compared with artificial graphite, natural graphite has more surface defects and a larger specific surface area, which has a greater impact on the cycle performance and rate performance.

[0141] Therefore, in the present invention, by using graphite with a specific median particle size as the matrix material of the core, and at the same time using the sublimation-adsorption method to deposit a red phosphorus layer on the surface of graphite, and then making red phosphorus react with lithium under specific conditions to in-situ form a lithium phosphide layer on the surface of graphite, and finally preparing a carbon coating layer, the modified graphite of the present invention, under the synergistic effect of the lithium phosphide layer and the carbon coating layer, enables the prepared battery to have high rate performance, high first efficiency and long cycle performance.

[0142] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0143] The above-described embodiments merely represent several implementation manners of the present invention. The description thereof 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 fall within 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 modified graphite, characterized in that, The modified graphite includes a core and a carbon coating layer coated on the surface of the core. Among them, the core includes graphite and a lithium phosphide layer coated on the surface of the graphite, and the median particle size of the graphite is 5μm - 20μm.

2. The modified graphite according to claim 1, wherein The thickness of the lithium phosphide layer is 5nm - 20nm; and / or, the thickness of the carbon coating layer is 1nm - 10nm.

3. The modified graphite according to claim 1, wherein, The mass ratio of the graphite to the lithium phosphide layer is 100:0.8 - 100:

12.

4. The modified graphite according to claim 1, wherein, The lithium phosphide layer is coated on the end face of the graphite.

5. The modified graphite according to any one of claims 1 to 4, characterized in that, The graphite is selected from at least one of artificial graphite, natural graphite, and mesophase carbon microspheres; and / or, the carbon coating layer material is selected from carbon nanotubes and / or graphene.

6. A method for preparing the modified graphite according to any one of claims 1 to 5, characterized in that, It includes the following steps: Provide graphite with a median particle size of 5μm - 20μm; Prepare a red phosphorus layer on the surface of the graphite by the sublimation-adsorption method; Mix the graphite with a red phosphorus layer with a lithium source, and then place it in an inert atmosphere for heating reaction to form a lithium phosphide layer, obtaining core material. Among them, the heating reaction temperature is greater than the melting point temperature of lithium and less than the sublimation temperature of red phosphorus; Prepare a carbon coating layer on the surface of the core material to obtain modified graphite.

7. The preparation method of the modified graphite according to claim 6, wherein In the step of preparing a red phosphorus layer on the surface of the graphite by the sublimation-adsorption method, mix the graphite and red phosphorus to obtain a mixture, and then place the mixture in an inert atmosphere for heating to sublime the red phosphorus in the mixture into phosphorus vapor, and then cool it to condense and deposit the phosphorus vapor on the surface of the graphite to form a red phosphorus layer.

8. The preparation method of the modified graphite according to claim 7, wherein, In the step of preparing a red phosphorus layer on the surface of the graphite by the sublimation-adsorption method, at least one of the following conditions is satisfied: (1) The mass ratio of the graphite to the red phosphorus is 100:1.5 - 100:10; (2) In the step of mixing the graphite and red phosphorus, the mixing speed of the mixer is 15rpm - 25rpm, and the mixing time is 60min - 180min; (3) In the step of heating the mixture in an inert atmosphere, place the mixture in a rotary kiln device, first introduce nitrogen and / or inert gas, control the volume fraction of oxygen to be less than 1ppm, and then heat it at a heating rate of 2℃ / min - 10℃ / min to 420℃ - 800℃ and keep it warm for 1h - 5h; (4) In the cooling step, first cool it to 260℃ - 300℃, keep it warm for 20h - 30h, and then cool it naturally.

9. The preparation method of the modified graphite according to claim 6, wherein, In the step of preparing the core material, at least one of the following conditions is satisfied: (1) The mass ratio of the graphite with a red phosphorus layer to the lithium source is 100:1 - 100:7; (2) The lithium source is selected from at least one of lithium foil, lithium powder, or molten lithium; (3) The volume fraction of water vapor in the inert atmosphere is less than 0.1ppm, and the volume fraction of oxygen is less than 0.2ppm; (4) The heating reaction time is 0.5h - 3h.

10. The preparation method of the modified graphite according to claim 6, wherein, In the step of preparing a carbon coating layer on the surface of the core material, at least one of the following conditions is satisfied: (1) Prepare a carbon coating layer on the surface of the core material by the ball milling method or the solvothermal method; (2) The mass ratio of the core material to the carbon coating layer material is 100:0.5 - 100:2, wherein the carbon coating layer material is selected from carbon nanotubes and / or graphene.

11. A negative electrode sheet, characterized in that, The negative electrode sheet includes a current collector and a negative electrode active layer provided on the surface of the current collector, and the components of the negative electrode active layer include the modified graphite according to any one of claims 1 to 5.

12. A secondary battery including the negative electrode sheet according to claim 11.

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