A modified graphite material, a preparation method and application thereof

Abstract

The application provides a modified graphite material, a preparation method and application thereof. The modified graphite material comprises a plurality of composite particles, the composite particle comprises a graphite core and a composite coating layer arranged on the surface of the graphite core, the composite coating layer comprises a carbon coating layer and a titanium dioxide coating layer adhered to the surface of the carbon coating layer, the titanium dioxide coating layer comprises a plurality of nano titanium dioxide particles, and a part of the nano titanium dioxide particles are embedded in the carbon coating layer. In the modified graphite material, a part of the titanium dioxide particles are adhered to the carbon coating layer, and the other part of the titanium dioxide particles protrude from the carbon coating layer. The titanium dioxide and the carbon coating layer synergistically act to achieve excellent high-rate performance.

Description

Technical Field

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

[0002] The rapid development of new energy vehicles and digital products has placed higher demands on safety, fast charging, and other aspects, thus requiring various optimization strategies.

[0003] Existing technical documents:

[0004] CN106711417A discloses a method for preparing nano-titanium dioxide-coated graphite anode material. The total mass of the liquid raw materials used is several times that of the graphite material, and the amount of ethanol solvent used is also several times that of the graphite, resulting in high production costs. The drying process is detrimental to energy consumption and safety in industrial production. Furthermore, the solid content of the liquid material continuously increases during the drying process, which is unfavorable for controlling the uniformity of the product and thus affects the material's performance stability.

[0005] CN115440953A discloses a natural graphite anode material for lithium batteries and its preparation method. The method described herein involves a complex process route, requiring the extensive use of strong acid reagents. It requires the use of nano-alumina as a hard template reagent combined with carbon coating, followed by acid washing to remove the hard template and obtain a hollow carbon-coated material. Next, acid intercalation is used to micro-expand the graphite, and finally, it is immersed in a titanium source to obtain a composite graphite material with nano-titanium dioxide in the carbon coating layer.

[0006] CN115602845A discloses a modified graphene anode material, its preparation method, anode sheet, and battery. The method utilizes solvent-inducible phase separation and is suitable for preparing composite materials of graphene and other nano-carbon materials with titanium dioxide, but not for the composite modification of conventional micron-sized and graphite powder anode materials.

[0007] After in-depth research and development, the applicant of this invention made a surprising discovery:

[0008] In existing technologies, the total mass of liquid raw materials used is several times that of graphite, and the amount of ethanol solvent used is also several times that of graphite, resulting in high production costs. The drying process is detrimental to energy consumption and safety in industrial production. During the drying process, the solid content of liquid materials continuously increases, which is not conducive to the uniformity control of the product and thus affects the performance stability of the material. At the same time, the process route is complex and requires the use of a large amount of strong acid reagents. The introduction of acid washing process brings environmental problems such as acid mist and waste acid, as well as operational safety issues. Summary of the Invention

[0009] The purpose of this invention is to provide a modified graphite material, its preparation method, and its applications. In the modified graphite material of this invention, some titanium dioxide particles adhere to the carbon coating layer, while others protrude from the carbon coating layer. The synergistic effect of titanium dioxide and the carbon coating layer results in excellent high-rate performance when applied as a negative electrode material for lithium-ion batteries, and also improves the safety of lithium-ion batteries.

[0010] To achieve this objective, the present invention adopts the following technical solution:

[0011] In a first aspect, the present invention provides a modified graphite material comprising a plurality of composite particles, the composite particles comprising a graphite core and a composite coating layer disposed on the surface of the graphite core, the composite coating layer comprising a carbon coating layer and a titanium dioxide coating layer bonded to the surface of the carbon coating layer, the titanium dioxide coating layer comprising a plurality of nano-titanium dioxide particles, the nano-titanium dioxide particles being partially embedded in the carbon coating layer.

[0012] In the composite coating layer of the modified graphite material described in this invention, the first layer is a carbon coating layer, and the second layer is a nano-titanium dioxide coating layer. When a portion of the titanium dioxide particles is embedded in the carbon coating layer (and another portion protrudes from the carbon coating layer), the affinity between titanium dioxide and the electrolyte is beneficial to the rapid liquid-phase diffusion of the electrolyte between and on the surface of the composite graphite particles, as well as its small size and rapid lithium intercalation characteristics. The carbon coating layer is beneficial to accelerating the desolvation process of lithium ions, thereby synergistically achieving excellent high-rate performance.

[0013] Furthermore, because the coating layer of this invention contains a titanium dioxide coating layer, when the composite graphite modified material described in this invention is used as the negative electrode material of a lithium-ion battery, the occurrence of internal short circuits can be effectively reduced, that is, the occurrence of internal self-discharge of the battery can be reduced. When the battery undergoes extreme reactions, such as short circuits, overcharging, violent collisions, local high temperatures, etc., titanium dioxide itself has low thermal conductivity, and its thermal conductivity coefficient is much lower than that of graphite. Therefore, the titanium dioxide coating layer can play a heat insulation role between the composite graphite particles, prevent the rapid diffusion of heat, reduce the reaction intensity, thereby reducing the risk of thermal runaway and improving the safety of lithium-ion batteries.

[0014] In this invention, nano-titanium dioxide particles refer to particles that are less than 100 nm in at least one dimension, such as particles that are less than 80 nm in at least one dimension, particles that are less than 50 nm in at least one dimension, and so on.

[0015] Preferably, the coverage P' of the nano-titanium dioxide particles in the modified graphite material is 8-75%, more preferably 27-56%. The coverage P' is obtained by randomly selecting 15 composite particles from the modified graphite material, obtaining the surface nano-titanium dioxide particle coverage P of each composite particle, and calculating its average value. The surface titanium dioxide particle coverage P = (S2 / S1) × 100%, where S1 is the area of ​​the two-dimensional projection of the composite particle in the EDS-mapping image, and S2 is the total area occupied by the titanium element in the two-dimensional projection exhibiting false color.

[0016] In this invention, when the coverage of nano-titanium dioxide particles in the modified graphite material is too low (typically, the amount of nano-titanium dioxide particles is small), the improvement in electrolyte wetting and fast-charging performance is not significant. Furthermore, the low coverage also negatively impacts self-discharge and heat insulation. Conversely, when the coverage of nano-titanium dioxide particles in the modified graphite material is too high, although it improves electrolyte wetting, it hinders the diffusion of the electrolyte into the carbon coating layer and the desolvation process, which is detrimental to improving fast-charging performance. More preferably, the coverage P' of nano-titanium dioxide particles in the modified graphite material is 27-56%. This results in excellent wetting improvement and fast-charging performance, as well as good heat insulation properties.

[0017] When the coverage is too low, meaning the amount of nano-titanium dioxide is insufficient, the improvement in electrolyte wetting and further enhancement of fast-charging performance is not significant. Simultaneously, low coverage also negatively impacts self-discharge and heat insulation. Conversely, while high coverage improves electrolyte wetting, it hinders electrolyte diffusion into the carbon coating layer and the desolvation process, negatively impacting fast-charging performance. Preferably, the Pmax-Pmin of the nano-titanium dioxide particles in the modified graphite material is <5%. Fifteen composite particles are randomly selected from the modified graphite material, and the surface nano-titanium dioxide particle coverage P of each composite particle is obtained. Pmax-Pmin represents the range of surface nano-titanium dioxide particle coverage P for each composite particle. The surface titanium dioxide particle coverage P = (S2 / S1) × 100%, where S1 is the area of ​​the two-dimensional projection of the composite particle in the EDS-mapping diagram, and S2 is the total area occupied by the pseudo-color of titanium elements in the two-dimensional projection.

[0018] In this invention, when the Pmax-Pmin content of the nano-titanium dioxide particles in the modified graphite material is less than 5%, it can reduce the poor uniformity of titanium dioxide coating caused by insufficient or excessive titanium dioxide coverage on the surface of some graphite particles, thus preventing the overall performance degradation caused by performance differences among some graphite particles. When Pmax-Pmin is less than 5%, the coverage P' is 8-75%, thereby achieving controllable adjustment of performance.

[0019] Preferably, the particle size of the nano-titanium dioxide particles is 5-50 nm, for example: 5 nm, 8 nm, 10 nm, 20 nm or 50 nm, and more preferably 10-40 nm.

[0020] Preferably, the crystal structure of the nano-titanium dioxide particles in the titanium dioxide coating layer includes rutile, preferably anatase, and most preferably anatase.

[0021] EDS surface scanning revealed that the nanoparticles on the surface of the modified graphite material contain oxygen and titanium elements. Adding a metal oxide coating to the surface of the graphite anode active material significantly increases costs in terms of raw materials alone. Lowering the heat treatment temperature can drastically reduce manufacturing costs. Therefore, the anatase structure is preferred for titanium dioxide. Even though the rutile structure may be slightly superior to the anatase structure in a certain electrochemical system, the slight advantage outweighs the cost reduction, making the anatase titanium dioxide structure potentially superior.

[0022] Preferably, the ash content of the modified graphite material, by mass percentage, is 0.21% to 3.12%, for example: 0.21%, 0.45%, 0.5%, 1%, 2%, 3% or 3.12%, etc., preferably 0.45% to 3.12%.

[0023] In this invention, the ash content was determined according to the method described in YS / T 63.19-2012.

[0024] XRD analysis of the ash content revealed that it was primarily composed of titanium dioxide. Insufficient titanium dioxide content resulted in minimal improvement in fast charging and safety, while excessive titanium dioxide content led to a significant reduction in capacity and initial efficiency.

[0025] More preferably, the ash content of the modified graphite material is 0.45–3.12%. An ash content within this range, while ensuring a certain initial efficiency and capacity, can significantly improve safety. Even more preferably, the ash content of the modified graphite material is 0.45–2.01%. An ash content within this range can improve safety while achieving better capacity and initial efficiency.

[0026] Preferably, based on the mass of the modified graphite material as 100%, the mass fraction of the carbon coating layer is 0.5% to 3.0%, for example: 0.5%, 1.0%, 2.0%, 2.2% or 3.0%, etc., preferably 1% to 2.5%.

[0027] In this invention, the carbon coating layer enhances the desolvation process, prevents solvent co-intercalation, and improves rate performance. Furthermore, the carbon coating layer preparation process effectively embeds one end of the nano-titanium dioxide particles, thereby enhancing the synergistic effect with the carbon coating layer and effectively preventing the strong mechanical stirring force during the homogenization process of the modified graphite material in the battery cell electrode fabrication, which could cause the titanium dioxide nanoparticles to peel off from the graphite surface. Insufficient carbon coating can easily lead to weak embedding and peeling of the titanium dioxide, resulting in uneven distribution, while excessive carbon coating can cause all the titanium dioxide to be completely covered by the carbon coating layer.

[0028] Further preferably, based on the mass of the modified graphite material (100%), the mass fraction of the carbon coating layer is 1.0–2.5%. This range better achieves the goal of more firmly embedding one end of the nano-titanium dioxide particles without causing some nano-titanium dioxide particles to be coated with a carbon layer. The carbon coating layer of this invention can enhance the desolvation process, prevent solvent co-intercalation, and improve rate performance. In addition, the preparation process of the carbon coating layer can effectively embed one end of the titanium dioxide particles, thereby enhancing the synergistic effect between the particles and the carbon coating layer, and effectively preventing the strong mechanical stirring force during the homogenization process of the modified graphite material in the battery cell electrode sheet fabrication from causing the titanium dioxide nanoparticles to peel off from the graphite surface. Too little carbon coating can easily cause the titanium dioxide to be poorly embedded and peel off, resulting in uneven distribution, while too much carbon coating can cause all the titanium dioxide to be completely covered by the carbon coating layer. The carbon coating layer, with a mass fraction of 1.0–2.5%, allows one end of the nano-titanium dioxide particles to be more firmly embedded without causing some of the nano-titanium dioxide particles to be coated with a carbon layer.

[0029] Preferably, the BET specific surface area of ​​the modified graphite material is 1.3–3.7 m². 2 / g, for example: 1.3m 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2 / g or 3.7m 2 / g etc.

[0030] In a second aspect, the present invention provides a method for preparing the modified graphite material as described in the first aspect, the method comprising the following steps:

[0031] Gel-formed materials containing titanium dioxide precursors;

[0032] Graphite and liquid carbon source are mixed to obtain coated graphite;

[0033] The coated graphite and the gelled material are mixed and heat-treated to obtain the modified graphite material.

[0034] This invention uses mechanical force to coat the gelation system onto the surface of the material, and then further heat treatment to form a second coating layer. This mixing method can improve the uniformity of the coating and avoid the agglomeration of titanium dioxide nanoparticles.

[0035] Preferably, the gelled material containing the titanium dioxide precursor comprises: mixing a titanium source, a weak acid, and a solvent, and then heating to react.

[0036] Preferably, the titanium source includes tetrabutyl titanate.

[0037] Preferably, the weak acid includes any one or a combination of at least two of glacial acetic acid, oxalic acid, or boric acid.

[0038] Preferably, the mass ratio of the titanium source, the weak acid, and the solvent is (1-4):(2-4):(3-5), for example: 1:4:5, 2:2:3, 2:4:3, or 2:4:5, etc.

[0039] Preferably, the solvent includes alcohol and water.

[0040] Preferably, the mass ratio of the alcohol to water is (1-3):2, for example: 1:2, 1.8:2, 2:2 or 3:2, etc.

[0041] Preferably, the temperature of the heating reaction is 40 to 60°C, for example: 40°C, 45°C, 50°C, 55°C or 60°C.

[0042] Preferably, the heating reaction time is 20 to 30 hours, for example: 20 hours, 22 hours, 25 hours, 28 hours or 30 hours.

[0043] Preferably, the liquid-phase carbon source includes tar, and / or liquid-phase resin, and / or liquid-phase pitch. More preferably, the liquid-phase carbon source includes tar.

[0044] In this invention, the advantage of using tar as a liquid carbon source is that the liquid carbon coating process avoids the use of organic solvents, reducing costs and improving the safety of the material manufacturing process. When tar is used as the liquid carbon source, the mixing temperature of graphite and tar is higher than the softening point of tar. The mixing temperature is usually carried out at room temperature (higher than the softening point of tar). In addition, because the mixing of materials is carried out by mechanical stirring, the friction generated between the materials during stirring will raise the temperature of the materials, typically reaching 40-50°C.

[0045] Preferably, the mass ratio of graphite to liquid carbon source is 100:(5-30), for example: 100:5, 100:10, 100:15, 100:18, 100:20, 100:25 or 100:30, etc.

[0046] Preferably, the mass ratio of the coated graphite to the gelled material is 100:(10-30), for example: 100:10, 100:15, 100:18, 100:20, 100:22, 100:25 or 100:30, etc.

[0047] Preferably, the coated graphite and the gelled material are mixed by stirring at a speed of 600-900 rpm, such as 600 rpm, 650 rpm, 700 rpm, 800 rpm or 900 rpm, and for a mixing time of 0.5-1 h, such as 0.5 h, 0.7 h, 0.8 h or 1 h.

[0048] In this invention, the ratio of graphite to gelling material affects the size and uniformity of nano-titanium dioxide particles. A higher proportion of gelling material easily leads to agglomeration of nano-titanium dioxide particles, resulting in increased size and deteriorated uniformity. Furthermore, the mixing process of graphite and gelling material also has an impact. If the gelling material is not evenly mixed with graphite, i.e., if the gelling material is not uniformly coated on the graphite surface, the titanium dioxide coating on the graphite surface will be uneven. On the other hand, if the gelling material is not uniformly dispersed on the graphite surface, when the gelling material is converted into titanium dioxide through heat treatment, the agglomeration of the gelling material will lead to increased size and poor uniformity of the final titanium dioxide, as well as agglomeration of the titanium dioxide particles. Too low a mixing speed or too short a mixing time can also cause agglomeration of nano-titanium dioxide particles. Too high a mixing speed or too long a mixing time can easily lead to the breakage of graphite particles, thereby increasing BET (Brightness Effect), reducing first-time efficiency, and deteriorating other powder properties.

[0049] Preferably, the heat treatment is used to carbonize or partially carbonize the liquid-phase carbon source.

[0050] Preferably, the heat treatment temperature is 500–1200°C, for example: 500°C, 600°C, 620°C, 650°C, 700°C, 750°C, 900°C, 1000°C, or 1200°C. More preferably, the heat treatment temperature is 600–750°C.

[0051] Preferably, the heat treatment heating rate is 1 to 10 °C / min, for example: 1 °C / min, 2 °C / min, 5 °C / min, 8 °C / min or 10 °C / min, etc.

[0052] Preferably, the heat treatment time is 1 to 10 hours, for example: 1 hour, 2 hours, 5 hours, 8 hours or 10 hours.

[0053] Preferably, the atmosphere for the heat treatment includes nitrogen.

[0054] Thirdly, the present invention provides a negative electrode sheet comprising the modified graphite material as described in the first aspect.

[0055] Fourthly, the present invention provides a lithium-ion battery comprising a modified graphite material as described in the first aspect, and / or a negative electrode as described in the third aspect.

[0056] Compared with the prior art, the present invention has the following beneficial effects:

[0057] (1) The present invention coats a carbon precursor, i.e., the first coating layer, on the surface of graphite, and then coats a titanium dioxide precursor, i.e., the second coating layer. After heat treatment, the first and second coating layers form a carbon coating layer and a nano-titanium dioxide coating layer, respectively. Due to simultaneous carbonization, the nano-titanium dioxide is firmly adhered to the carbon coating layer. Therefore, when the composite graphite modified material is used as a negative electrode material for lithium-ion batteries, it not only has the desolvation effect of the carbon coating layer and the effect of improving the rate performance, but also the excellent fast charging performance of the nano-titanium dioxide and the adsorption and interface improvement effect due to the small size of the nano-titanium dioxide. This makes the composite graphite modified material have excellent wettability, liquid absorption and liquid retention capacity with electrolyte, so that the composite graphite modified material can exhibit high rate characteristics when applied to lithium-ion battery negative electrode materials.

[0058] (2) When the graphite modified material described in this invention is used as the negative electrode material of lithium-ion battery, it can effectively reduce the occurrence of internal short circuits, that is, reduce the occurrence of self-discharge inside the battery. When the battery undergoes extreme reactions, such as short circuits, overcharging, violent collisions, local high temperatures, etc., titanium dioxide itself has low thermal conductivity and its thermal conductivity coefficient is much lower than that of graphite. Therefore, the titanium dioxide coating layer can play a heat insulation role between composite graphite particles, prevent the rapid diffusion of heat, reduce the reaction intensity, thereby reducing the risk of thermal runaway and improving the safety of lithium-ion batteries. Attached Figure Description

[0059] Figure 1 This is a schematic diagram of the structure of the composite particles of the modified graphite material described in Example 1.

[0060] Figure 2 This is a SEM image of the modified graphite material described in Example 1.

[0061] Figure 3 This is a magnified SEM image of the modified graphite material described in Example 1.

[0062] Figure 4 This is a SEM image of the modified graphite material described in Example 1 (sample tested with EDS).

[0063] Figure 5This is the EDS diagram of carbon in the modified graphite material described in Example 1.

[0064] Figure 6 This is the EDS diagram of titanium in the modified graphite material described in Example 1.

[0065] Figure 7 This is the EDS diagram of oxygen in the modified graphite material described in Example 1.

[0066] Figure 8 This is a high-magnification SEM image of the ash content of the modified graphite material described in Example 1 after ash content testing.

[0067] Figure 9 This is the XRD pattern of the ash content of the modified graphite material described in Example 1 after ash content testing.

[0068] Figure 10 This is a SEM image of the modified graphite material described in Comparative Example 1 (sample tested with EDS).

[0069] Figure 11 This is the EDS diagram of carbon in the modified graphite material described in Comparative Example 1.

[0070] Figure 12 This is the EDS diagram of titanium in the modified graphite material described in Comparative Example 1.

[0071] Figure 13 This is the EDS diagram of oxygen in the modified graphite material described in Comparative Example 1.

[0072] Figure 14 This is a SEM image of the modified graphite material described in Comparative Example 2.

[0073] Figure 15 This is a magnified SEM image of the modified graphite material described in Comparative Example 2.

[0074] Figure 16 This is a SEM image of the modified graphite material described in Comparative Example 3 (sample tested with EDS).

[0075] Figure 17 This is the EDS diagram of carbon in the modified graphite material described in Comparative Example 3.

[0076] Figure 18 This is the EDS diagram of titanium in the modified graphite material described in Comparative Example 3.

[0077] Figure 19 This is the EDS diagram of oxygen in the modified graphite material described in Comparative Example 3.

[0078] Figure 20 This is a SEM image of the titanium dioxide described in Comparative Example 4 (sample tested with EDS).

[0079] Figure 21 This is the EDS diagram of titanium in titanium dioxide as described in Comparative Example 4.

[0080] Figure 22 This is the EDS diagram of oxygen in titanium dioxide described in Comparative Example 4.

[0081] Figure 23 This is the XRD pattern of the titanium dioxide described in Comparative Example 4.

[0082] Figure 24 This is the XRD pattern of the titanium dioxide described in Comparative Example 5. Detailed Implementation

[0083] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0084] Example 1

[0085] This embodiment provides a modified graphite material, which is prepared by the following method:

[0086] (1) Tetrabutyl titanate, glacial acetic acid, ethanol and water were mixed in a mass ratio of 2:4:1:2, the stirring speed was 300 rpm and the stirring time was 1 h to obtain a mixed solution, and the solution was kept at 50°C for 24 h to obtain a gelled material.

[0087] (2) Graphite and ethylene tar with a softening point of 5°C are added to a mixer at a mass ratio of 100:15 and stirred at a stirring speed of 600 rpm for 1 hour to obtain ethylene tar coated graphite material.

[0088] (3) The ethylene tar-coated graphite material and the gelled material were mixed at a mass ratio of 100:20 at 600 rpm for 1 h, and then placed in a heat treatment furnace to heat up to 750°C at 5°C / min and held for 2 h. Nitrogen gas was passed through the furnace for protection during the heat treatment process. After the material cooled down, the modified graphite material was obtained.

[0089] The modified graphite material contains a carbon coating layer comprising 1.5% of its mass.

[0090] The structural diagram of the single composite particle of the modified graphite material is shown below. Figure 1 As shown.

[0091] The SEM image of the modified graphite material is as follows: Figure 2-3 As shown, by Figure 2-3It can be seen that the nano-titanium dioxide particles uniformly coated on the surface of the modified graphite material, when the size of 100 titanium dioxide particles were randomly measured, showed that the particle size of the samples was all less than 50 nm, 98% of the sample particle sizes were in the range of 5 to 50 nm, and 90% of the sample particle sizes were in the range of 10 to 40 nm.

[0092] The SEM images of the modified graphite material tested by EDS are shown below. Figure 4 As shown, the EDS diagram of the modified graphite material is as follows: Figure 5-7 As shown, by Figure 4 and Figure 5-7 The comparison shows that the modified graphite material described in this invention contains oxygen and titanium elements on its surface, and the titanium dioxide is rutile titanium dioxide and / or anatase titanium dioxide, preferably anatase titanium dioxide. Adding a metal oxide coating layer to the surface of the graphite anode active material significantly increases costs in terms of raw materials alone. Lowering the temperature can greatly reduce manufacturing costs. Therefore, the anatase structure is preferred for titanium dioxide. Even though the rutile structure may be slightly superior to the anatase structure in a certain electrochemical system, the anatase structure is more suitable considering the overall advantages and cost.

[0093] Example 2

[0094] This embodiment provides a modified graphite material, which is prepared by the following method:

[0095] (1) Tetrabutyl titanate, oxalic acid, ethanol and water were mixed in a mass ratio of 2:4:3:2, the stirring speed was 300 rpm and the stirring time was 1 h to obtain a mixed solution, and the solution was kept at 40℃ for 30 h to obtain a gelled material.

[0096] (2) Graphite and ethylene tar with a softening point of 5°C are added to a mixer at a mass ratio of 100:10 and stirred at a stirring speed of 600 rpm for 1 hour to obtain ethylene tar coated graphite material.

[0097] (3) The ethylene tar-coated graphite material and the gelled material were mixed at a mass ratio of 100:10 at 600 rpm for 1 h, and then placed in a heat treatment furnace to heat treatment at 5℃ / min to 680℃ and held for 5 h. Nitrogen gas was passed through for protection during the heat treatment process. After the material cooled down, the modified graphite material was obtained.

[0098] The carbon coating layer in the modified graphite material accounts for 1% of the total mass.

[0099] Example 3

[0100] This embodiment provides a modified graphite material, which is prepared by the following method:

[0101] (1) Tetrabutyl titanate, boric acid, ethanol and water are mixed in a mass ratio of 2:2:1:2, the stirring speed is 300 rpm and the stirring time is 1 h to obtain a mixed solution, and the solution is kept at 60℃ for 20 h to obtain a gelled material.

[0102] (2) Graphite and ethylene tar with a softening point of 5°C are added to a mixer at a mass ratio of 100:25 and stirred at a stirring speed of 600 rpm for 1 hour to obtain ethylene tar-coated graphite material.

[0103] (3) The ethylene tar-coated graphite material and the gelled material were mixed at a mass ratio of 100:25 at 600 rpm for 1 h, and then placed in a heat treatment furnace to heat treatment at 5℃ / min to 600℃ and held for 10 h. Nitrogen gas was passed through for protection during the heat treatment process. After the material cooled down, the modified graphite material was obtained.

[0104] The modified graphite material contains a carbon coating layer comprising 2.5% by mass.

[0105] Example 4

[0106] This embodiment provides a modified graphite material, which is prepared by the following method:

[0107] (1) Tetrabutyl titanate, glacial acetic acid, ethanol and water were mixed in a mass ratio of 2:4:1:2, the stirring speed was 300 rpm and the stirring time was 1 h to obtain a mixed solution, and the solution was kept at 50°C for 24 h to obtain a gelled material.

[0108] (2) Graphite and coal tar with a softening point of 10℃ are added to a mixer at a mass ratio of 100:5 and stirred at a stirring speed of 600 rpm for 1 hour to obtain a material coated with coal tar.

[0109] (3) The coal tar-coated graphite material and the gelled material were mixed at 600 rpm for 1 h at a mass ratio of 100:20. The mixture was placed in a heat treatment furnace and heated to 750 °C at 1 °C / min for 2 h. Nitrogen gas was passed through the furnace for protection during the heat treatment process. After the material cooled down, the modified graphite material was obtained.

[0110] The modified graphite material contains a carbon coating layer comprising 0.5% of its mass.

[0111] Example 5

[0112] This embodiment provides a modified graphite material, which is prepared by the following method:

[0113] (1) Tetrabutyl titanate, glacial acetic acid, ethanol and water were mixed in a mass ratio of 2:4:1:2, the stirring speed was 300 rpm and the stirring time was 1 h to obtain a mixed solution, and the solution was kept at 50°C for 24 h to obtain a gelled material.

[0114] (2) Graphite and coal tar with a softening point of 10℃ are added to a mixer at a mass ratio of 100:30 and stirred at a stirring speed of 600 rpm for 1 hour to obtain a material coated with coal tar.

[0115] (3) The coal tar-coated graphite material and the gelled material were mixed at 600 rpm for 1 h at a mass ratio of 100:20. The mixture was placed in a heat treatment furnace and heated to 750 °C at 1 °C / min for 2 h. Nitrogen gas was passed through the furnace for protection during the heat treatment process. After the material cooled down, the modified graphite material was obtained.

[0116] The modified graphite material contains a carbon coating layer comprising 3.0% of its mass.

[0117] Example 6

[0118] This embodiment provides a modified graphite material, which is prepared by the following method:

[0119] (1) Tetrabutyl titanate, oxalic acid, ethanol and water were mixed in a mass ratio of 1:4:3:2, the stirring speed was 300 rpm and the stirring time was 1 h to obtain a mixed solution, and the solution was kept at 40℃ for 30 h to obtain a gelled material.

[0120] (2) Graphite and liquid resin with a softening point of 15℃ are added to a mixer at a mass ratio of 100:10 and stirred at a stirring speed of 600 rpm for 1 hour to obtain graphite coated with liquid resin.

[0121] (3) The material coated with liquid resin and the gelled material are mixed at 600 rpm for 1 h at a mass ratio of 100:10. The mixture is placed in a heat treatment furnace and heated to 680°C at 5°C / min for 5 h. Nitrogen gas is passed through the furnace for protection during the heat treatment process. After the material cools down, the modified graphite material is obtained.

[0122] The carbon coating layer in the modified graphite material accounts for 1% of the total mass.

[0123] Example 7

[0124] This embodiment provides a modified graphite material, which is prepared by the following method:

[0125] (1) Tetrabutyl titanate, boric acid, ethanol and water were mixed in a mass ratio of 4:2:1:2, the stirring speed was 300 rpm and the stirring time was 1 h to obtain a mixed solution, and the solution was kept at 60℃ for 20 h to obtain a gelled material.

[0126] (2) Graphite and liquid resin with a softening point of 15℃ are added to a mixer at a mass ratio of 100:25 and stirred at a stirring speed of 600 rpm for 1 hour to obtain a material coated with liquid resin.

[0127] (3) The material coated with liquid resin and the gelled material are mixed at 600 rpm for 1 h at a mass ratio of 100:30. The mixture is placed in a heat treatment furnace and heated to 600 °C at 5 °C / min for heat treatment and held for 10 h. Nitrogen gas is passed through the furnace for protection during the heat treatment process. After the material cools down, the modified graphite material is obtained.

[0128] The modified graphite material contains a carbon coating layer comprising 2.5% by mass.

[0129] Example 8

[0130] This embodiment provides a modified graphite material, which is prepared by the following method:

[0131] (1) Tetrabutyl titanate, glacial acetic acid, ethanol and water were mixed in a mass ratio of 2:4:1:2, the stirring speed was 300 rpm and the stirring time was 1 h to obtain a mixed solution, and the solution was kept at 50°C for 24 h to obtain a gelled material.

[0132] (2) Graphite and ethylene tar with a softening point of 5°C are added to a mixer at a mass ratio of 100:15 and stirred at a stirring speed of 900 rpm for 0.5 h to obtain ethylene tar coated graphite material.

[0133] (3) The ethylene tar-coated graphite material and the gelled material were mixed at a mass ratio of 100:20 at 900 rpm for 0.5 h, and then placed in a heat treatment furnace to heat treatment at 10℃ / min to 500℃ and held for 5 h. Nitrogen gas was passed through during the heat treatment process for protection. After the material cooled down, the modified graphite material was obtained.

[0134] The modified graphite material contains a carbon coating layer comprising 1.5% of its mass.

[0135] Example 9

[0136] This embodiment provides a modified graphite material, which is prepared by the following method:

[0137] (1) Tetrabutyl titanate, glacial acetic acid, ethanol and water were mixed in a mass ratio of 2:4:1:2, the stirring speed was 300 rpm and the stirring time was 1 h to obtain a mixed solution, and the solution was kept at 50°C for 24 h to obtain a gelled material.

[0138] (2) Graphite and ethylene tar with a softening point of 5°C are added to a mixer at a mass ratio of 100:15 and stirred at a stirring speed of 900 rpm for 0.5 h to obtain ethylene tar coated graphite material.

[0139] (3) The ethylene tar-coated graphite material and the gelled material were mixed at a mass ratio of 100:20 at 900 rpm for 0.5 h, and then placed in a heat treatment furnace to heat treatment at 10℃ / min to 1200℃ and held for 5 h. Nitrogen gas was passed through for protection during the heat treatment process. After the material cooled down, the modified graphite material was obtained.

[0140] The modified graphite material contains a carbon coating layer comprising 1.5% of its mass.

[0141] Comparative Example 1

[0142] The only difference between this comparative example and Example 1 is that a liquid carbon source (ethylene tar) is not added; all other conditions and parameters are exactly the same as in Example 1. The SEM and EDS images of the obtained modified graphite materials are shown below. Figure 10-13 As shown, by Figure 10-13 It can be seen that without the addition of a carbon source, the coating uniformity of the material is poor.

[0143] Comparative Example 2

[0144] The only difference between this comparative example and Example 1 is that a titanium source is not added; all other conditions and parameters are exactly the same as in Example 1. The SEM image of the obtained modified graphite material is shown below. Figure 14-15 As shown, by Figure 14-15 It can be seen that the surface of the prepared material is smooth and there are no nanoparticle coatings.

[0145] Comparative Example 3

[0146] In this comparative example, graphite material was coated directly through liquid-phase mixing. The preparation method is as follows:

[0147] (1) Mix graphite and anhydrous ethanol at a mass ratio of 1:2 to obtain a dispersion;

[0148] (2) Add tetrabutyl titanate to the above dispersion containing graphite and stir evenly at a stirring speed of 300 rpm. The mass ratio of tetrabutyl titanate to graphite is 0.1:1. Then dry at 70°C for 24 h.

[0149] (3) Finally, the dried material is placed in a heat treatment furnace and heated to 750°C at 5°C / min and kept at that temperature for 2 hours to obtain the modified graphite material.

[0150] SEM and EDS images of the modified graphite materials were obtained as follows: Figure 16-19 As shown, by Figure 16-19 It can be seen that direct liquid-phase mixing and coating results in uneven coating of titanium dioxide.

[0151] Comparative Example 4

[0152] In this comparative example, material B described in Example 1 was heated to 750°C at a rate of 5°C / min in a heat treatment furnace and held at that temperature for 2 hours. Nitrogen was used as the protective gas throughout the heat treatment process. After the material was cooled, titanium dioxide was obtained.

[0153] The SEM and EDS images of the prepared titanium dioxide are as follows: Figure 20-22 As shown, the XRD pattern is as follows Figure 23 As shown, by Figure 23 It can be seen that the obtained sample is anatase titanium dioxide.

[0154] Comparative Example 5

[0155] In this comparative example, material B described in Example 1 was heat-treated in a heat treatment furnace at a rate of 5°C / min to 1000°C and held at that temperature for 2 hours. Nitrogen was used as the protective gas throughout the heat treatment process. After cooling, titanium dioxide was obtained. The XRD pattern of the obtained sample is shown below. Figure 24 As shown, analysis software such as Jade indicates that the sample is rutile titanium dioxide.

[0156] The ash content of the sample obtained in Example 1 and the XRD characterization of the samples obtained in Comparative Examples 4-5 show that the surface coating of the composite graphite modified material obtained in Example 1 is anatase titanium dioxide. Titanium dioxide undergoes a crystal structure transformation at excessively high temperatures. The ash structure of the sample obtained in Example 1 indicates that titanium dioxide transforms from anatase to rutile structure at higher temperatures.

[0157] Performance testing:

[0158] The graphite materials prepared in Examples 1-9 and Comparative Examples 1-3 were subjected to performance testing:

[0159] (1) The ash content of the sample was determined according to the method described in YS / T 63.19-2012 (after the ash content test of the modified graphite material described in Example 1, the high-magnification SEM image of the ash content is shown in Figure 1). Figure 8 As shown, the XRD pattern of the obtained ash content is as follows. Figure 9 As shown, by Figure 9 It can be seen that the ash is rutile titanium dioxide;

[0160] (2) The specific surface area (BET) of the sample shall be determined in accordance with the provisions of GB / T 19587;

[0161] (3) In the EDS-mapping diagram of a single composite graphite modified particle, the pseudo-color corresponding to the titanium element represents the area occupied by the titanium dioxide particle in the two-dimensional projection of the composite graphite modified material; the two-dimensional projection of the composite graphite modified particle is defined as area S1, the total area occupied by the titanium element in the two-dimensional projection is S2, and the coverage of titanium dioxide on the surface of the composite graphite modified particle is expressed as P, P = S2 / S1 × 100%. 15 randomly selected particles are statistically analyzed, and their arithmetic mean P' (i.e. coverage) is calculated.

[0162] The test method for the coating uniformity of the modified graphite material is as follows: For different composite graphite modified particles, the above coverage rate is statistically analyzed for 15 particles, and the range is recorded as Pmax-Pmin. The test results are shown in Table 1.

[0163] (4) The graphite materials described in Examples 1-9 and Comparative Examples 1-3 were prepared into a slurry according to the ratio of modified graphite anode material:CMC:SP:SBR=93.5:1.5:2:3, then coated, dried, and rolled to obtain anode sheets. The compacted density of the rolled sheets was 1.65 g / cm³. 3 After contact angle testing using pure water as the test liquid medium, a CR2430 coin cell was prepared using a negative electrode sheet, with a lithium metal sheet as the counter electrode. The initial delithiation capacity, initial coulombic efficiency, and rate performance of the composite graphite-modified negative electrode material were tested. The coin cell was discharged at 0.1C to 0.005V, allowed to stand for 10 min, discharged at 0.01C to 0.005V, allowed to stand for 10 min, and then charged at 0.1C to 2V. This yielded the lithium insertion capacity and delithiation capacity, respectively. The ratio of the delithiation capacity to the lithium insertion capacity is the initial efficiency. The lithium intercalation capacity was obtained under the following conditions: discharge at 0.2C to 0.01V, constant voltage discharge at 0.01V to 0.01C, and rest for 10s; charge at 0.2C to 1.5V, rest for 10s, discharge at 2C to 0.01V, constant voltage discharge at 0.01V to 0.01C, and charge at 2C to 1.5V. The test results are shown in Table 2.

[0164] (5) Contact angle test: Take the above negative electrode sheet, and the compacted density of the sheet after rolling is 1.65 g / cm³. 3 The test was conducted using pure water as the test liquid medium.

[0165] (6) The XRD spectrum of the sample was scanned using a copper target with a wavelength of 0.154056 nm and a scanning speed of about 4 degrees / min. Spectral information, such as crystal structure, was obtained using analysis software such as HighScore Plus and Jade.

[0166] (7) In accordance with GB 31241-2014 Safety Requirements for Lithium-ion Batteries and Battery Packs for Portable Electronic Products, Section 7.8 Thermal Abuse, the modified graphite obtained in the examples and comparative examples was used to manufacture batteries. After the batteries were fully charged according to the test method specified in 4.5.1, they were placed in a test chamber. The test chamber was heated at a temperature rise rate of 5℃ / min. When the temperature inside the chamber reached 130℃, it was kept constant until the battery thermally ran away. The constant temperature time T was recorded.

[0167] (8) Test method for carbon coating content: Take 10 mg of modified graphite in the sample crucible of the comprehensive thermal analyzer (NETZSCH 449F3), and under oxygen atmosphere, the gas flow rate is 50 ml / min, the temperature is increased from room temperature to 700℃ at 5℃ / min, and the temperature is kept constant for 60 min. The weight loss ratio is the mass percentage of carbon coating.

[0168] Table 1

[0169] Example 1 1.03 1.7 45 3 Example 2 0.45 1.3 27 4 Example 3 2.01 2.6 56 4 Example 4 1.02 1.6 37 5 Example 5 1.04 1.5 43 4 Example 6 0.21 1.5 8 5 Example 7 3.12 3.7 75 5 Example 8 1.03 1.9 46 3 Example 9 1.04 1.6 44 3 Comparative Example 1 1.03 2.2 25 9 Comparative Example 2 0.00 1.1 0 / Comparative Example 3 1.04 1.9 23 13

[0170] Table 2

[0171]

[0172]

[0173] As shown in Table 1-2, and as demonstrated in Examples 1-3, with the increase of titanium dioxide coating amount, safety improves, titanium dioxide coverage increases, and contact angle decreases. On the other hand, since the carbon coating layer and titanium dioxide have a synergistic effect on improving rate performance, when the carbon coating amount and titanium dioxide coating amount increase simultaneously, the kinetic performance shows a pattern of first increasing and then decreasing. This indicates that the carbon coating amount and titanium dioxide coating amount need to be matched to achieve better rate performance.

[0174] Comparing Examples 2 and 6, and Examples 3 and 7, it can be seen that during the preparation of the modified graphite material of the present invention, the amount of titanium source added affects the amount of titanium dioxide coating, thereby affecting the performance of the modified graphite material. Too little titanium dioxide coating results in insignificant improvement in rate performance and safety performance; too much titanium dioxide coating will also lead to a significant decrease in capacity and first-time efficiency, which is closely related to the fact that the capacity and first-time efficiency of titanium dioxide are lower than those of pure graphite. The higher the mass proportion of titanium dioxide in the modified graphite material, the more significant the decrease in capacity and first-time efficiency of the modified graphite material.

[0175] A comparison of Examples 1 and 4-5 shows that the mass ratio of graphite to liquid carbon source affects the performance of the modified graphite material during preparation. When the carbon coating mass ratio is controlled at 1-2.5%, the modified graphite material has better performance. If the carbon coating amount is too low, it is not conducive to the bonding of titanium dioxide, resulting in uneven coating and poor rate performance. If the carbon coating amount is too high, the first-efficiency is reduced due to the high proportion of amorphous carbon.

[0176] A comparison of Examples 1-3 and Examples 8-9 shows that the heat treatment temperature affects the performance of the modified graphite material during preparation. Controlling the heat treatment temperature between 600 and 750°C yields a modified graphite material with better performance. Too low a heat treatment temperature results in poor carbonization of the carbon coating, significantly reducing initial efficiency and causing poor rate performance. Too high a heat treatment temperature not only leads to high energy consumption but also causes excessive shrinkage of the carbon coating, resulting in weak titanium dioxide adhesion and poor rate performance.

[0177] As can be seen from the comparison between Example 1 and Comparative Example 1, the method of directly coating the graphite particles with titanium dioxide precursor without coating the graphite particles with carbon precursor is prone to causing the titanium dioxide to peel off from the surface of the graphite particles after heat treatment. At the same time, it is also difficult to coat evenly, thus exhibiting poor rate performance.

[0178] A comparison of Example 1 and Comparative Example 2 shows that without coating with the titanium dioxide precursor, the rate performance improvement is not good.

[0179] A comparison of Example 1 and Comparative Example 3 shows that the conventional liquid-phase coating method for titanium dioxide, due to the required drying process, leads to uneven coating in the resulting material. This results in poor rate performance and low initial coulombic efficiency when the composite graphite-modified material is used as a lithium-ion battery anode material. After in-depth research, the inventors of this invention speculate that the uneven coating is likely caused by liquid-phase mixing, with the unevenness primarily stemming from the drying process.

[0180] The ash content of the sample obtained in Example 1 and the XRD characterization of the samples obtained in Comparative Examples 4-5 show that the surface coating of the composite graphite modified material obtained in Example 1 is anatase titanium dioxide. When the temperature is too high, titanium dioxide will undergo a crystal structure transformation, changing from anatase to rutile.

[0181] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A modified graphite material, characterized in that, The modified graphite material includes multiple composite particles, each composite particle including a graphite core and a composite coating layer disposed on the surface of the graphite core. The composite coating layer includes a carbon coating layer and a titanium dioxide coating layer bonded to the surface of the carbon coating layer. The titanium dioxide coating layer includes multiple nano-titanium dioxide particles, a portion of which are embedded in the carbon coating layer. The coverage P' of the modified graphite material is 8~75%. The coverage P' is obtained by randomly selecting 15 composite particles from the modified graphite material, obtaining the surface coverage P of the composite particles, and calculating their average value. The surface coverage P of the titanium dioxide particles is P=(S2 / S1)×100%, where S1 is the area of ​​the two-dimensional projection of the composite particles in the EDS-mapping diagram, and S2 is the total area occupied by the titanium element in the two-dimensional projection showing a false color. The modified graphite material has a Pmax-Pmin ratio of less than 5% for the nano-titanium dioxide particles. Fifteen composite particles are randomly selected from the modified graphite material, and the surface nano-titanium dioxide particle coverage P of each composite particle is obtained. Pmax-Pmin is the range of the surface nano-titanium dioxide particle coverage P of each composite particle. The surface titanium dioxide particle coverage P = (S2 / S1) × 100%, where S1 is the area of ​​the two-dimensional projection of the composite particle in the EDS-mapping diagram, and S2 is the total area occupied by the pseudo-color of titanium elements in the two-dimensional projection.

2. The modified graphite material as described in claim 1, characterized in that, The coverage rate P' of the modified graphite material is 27-56%. The coverage rate P' is obtained by randomly selecting 15 composite particles from the modified graphite material, obtaining the surface coverage rate P of the composite particles of each composite particle, and calculating their average value. The surface coverage rate P = (S2 / S1) × 100%, where S1 is the area of ​​the two-dimensional projection of the composite particle in the EDS-mapping image, and S2 is the total area occupied by the titanium element in the two-dimensional projection showing a false color.

3. The modified graphite material as described in claim 1, characterized in that, The particle size of the nano-titanium dioxide particles is 5~50nm; And / or, the crystal structure of the nano-titanium dioxide particles in the titanium dioxide coating layer includes rutile and / or anatase type; And / or, by mass percentage, the ash content of the modified graphite material is 0.21~3.12%.

4. The modified graphite material as described in claim 3, characterized in that, The particle size of the nano-titanium dioxide particles is 10~40nm.

5. The modified graphite material as described in claim 3, characterized in that, The nano-titanium dioxide particles in the titanium dioxide coating have anatase crystal structure.

6. The modified graphite material as described in claim 3, characterized in that, The modified graphite material has an ash content of 0.45~3.12%.

7. The modified graphite material as described in claim 3, characterized in that, The ash content of the modified graphite material is 0.45~2.01%.

8. The modified graphite material as described in claim 1, characterized in that, Based on the mass of the modified graphite material being 100%, the mass fraction of the carbon coating layer is 0.5% to 3.0%. And / or, the BET specific surface area of ​​the modified graphite material is 1.3~3.7m². 2 / g.

9. The modified graphite material as described in claim 8, characterized in that, Based on the mass of the modified graphite material being 100%, the mass fraction of the carbon coating layer is 1~2.5%.

10. A method for preparing the modified graphite material according to any one of claims 1-9, characterized in that, The preparation method includes the following steps: Gel-formed materials containing titanium dioxide precursors; Graphite and liquid carbon source are mixed to obtain coated graphite; The coated graphite and the gelled material are mixed and heat-treated to obtain the modified graphite material.

11. The preparation method according to claim 10, characterized in that, The gelled material containing the titanium dioxide precursor comprises: mixing a titanium source, a weak acid, and a solvent, and then heating the reaction. And / or, the titanium source includes tetrabutyl titanate; And / or, the weak acid includes any one or a combination of at least two of glacial acetic acid, oxalic acid, or boric acid; And / or, the mass ratio of the titanium source, weak acid and solvent is (1~4): (2~4): (3~5); And / or, the solvent includes alcohol and water; And / or, the mass ratio of the alcohol to water is (1~3):2; And / or, the temperature of the heating reaction is 40~60°C; And / or, the heating reaction time is 20-30 hours.

12. The preparation method according to claim 10, characterized in that, The liquid carbon source includes tar, and / or liquid resin, and / or liquid pitch; And / or, the mass ratio of the graphite to the liquid carbon source is 100:(5~30).

13. The preparation method according to claim 10, characterized in that, The mass ratio of the coated graphite to the gelled material is 100:(10~30); And / or, the coated graphite and gelled material are mixed by stirring at a speed of 600~900 rpm for a time of 0.5~1 h; And / or, the heat treatment is used to carbonize or partially carbonize the liquid-phase carbon source; And / or, the temperature of the heat treatment is 500~1200℃; And / or, the heating rate of the heat treatment is 1~10℃ / min; And / or, the heat treatment time is 1~10h; And / or, the atmosphere for the heat treatment includes nitrogen.

14. The preparation method according to claim 13, characterized in that, The heat treatment temperature is 600~750℃.

15. A negative electrode sheet, characterized in that, The negative electrode sheet comprises the modified graphite material as described in any one of claims 1-9.

16. A lithium-ion battery, characterized in that, The lithium-ion battery comprises a modified graphite material as described in any one of claims 1-9, and / or the lithium-ion battery comprises a negative electrode as described in claim 15.

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