Graphite composite material having functional coating layer, preparation method therefor and use thereof

Abstract

The present invention relates to the technical field of lithium-ion batteries. Disclosed are a graphite composite material having a functional coating layer, a preparation method therefor and a use thereof. The graphite composite material having a functional coating layer is a secondary particle formed by bonding graphite single particles of a core-shell structure. Each graphite single particle comprises a graphite particle and a functional coating layer that coats the surface of the graphite particle. The functional coating layer comprises a fast ion conductor and a doped element carbon layer. In the graphite composite material having a functional coating layer disclosed in the present invention, by coating the surfaces of graphite particles with a doped element carbon layer and a fast ion conductor layer and performing re-granulation to form a secondary particle, the Li+ diffusion path is shortened and the interfacial diffusion resistance is reduced, and the surface SEI layer is stabilized, thereby significantly improving the high-rate capacity performance and long-cycle stability of the graphite composite material.

Description

Graphite composite material with functional coating layer, preparation method and application TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a graphite composite material with a functional coating layer, a preparation method and application. BACKGROUND

[0002] Graphite negative electrodes mainly include artificial graphite, natural graphite and mesocarbon microbeads, and are currently the main negative electrode material of lithium ion batteries, and will still be the preferred negative electrode in a long time in the future. Although graphite materials have advantages such as high energy density, good electrical conductivity, low lithium intercalation potential and low price, the two-dimensional layered structure of graphite determines that Li + must be inserted from the end face of the material and gradually diffuse into the particle interior, resulting in a long solid-phase diffusion path for Li + , and the small interlayer spacing (0.335 nm) of graphite also causes a low diffusion rate of Li + , resulting in limited fast-charging performance. Especially during high-rate charging, due to the limited internal diffusion, the interface Li + accumulates, the concentration increases, causing large concentration polarization, Li + continuously deposits on the interface, forming uncontrolled surface lithium precipitation and even dendrite propagation, not only causing rapid capacity decay of the battery, but also causing local micro-short circuits, leading to thermal runaway, and posing a serious safety hazard.

[0003] It is generally believed that ion transport is the rate-limiting step for lithium intercalation in graphite materials, and this process mainly includes the diffusion of Li + in the electrolyte and electrode material. Interfaces and interfacial phases are ubiquitous in LIBs, and the diffusion of Li + in the particle interior does not match the rate of lithium deintercalation on the particle surface during high-rate charging and discharging of the graphite negative electrode, and various polarizations, including ohmic drop, concentration overpotential and charge transfer overpotential, inevitably occur. Based on the above understanding, the optimization strategy for graphite negative electrodes is basically based on particle structure design and interface modification. First, the structure design of graphite particles, including particle size refinement and secondary granulation, specifically shortens the Li + solid-phase diffusion path and expands the lithium intercalation channel, reduces the ion transport resistance, and improves the fast-charging performance; second, a functional carbon coating layer is constructed on the surface of the graphite, allowing Li + to be rapidly transported from the basal plane to the edge of the graphite, achieving rapid lithium intercalation.

[0004] In the prior art, small particle size graphite single particles and binders (bitumen-based, resin-based, etc.) are usually used to prepare graphite secondary particles by granulation, and a functional carbon coating layer is constructed on the surface of the secondary particles. However, the bitumen-based and resin-based binders have poor dispersibility and exist in the form of self-agglomeration, resulting in reduced binding effect. In addition, the carbon coating layer on the surface of the secondary particles has limited improvement on the diffusion path and interface impedance of Li + , and the Li + diffusion resistance in the secondary particles is large, which makes it difficult to exert the fast charging advantage of superfine graphite. SUMMARY

[0005] The present application aims to provide a graphite composite material with a functional coating layer capable of shortening the solid-phase diffusion path of Li+; the present application also aims to provide a preparation method of a graphite composite material with a functional coating layer; and the present application also aims to provide an application of a graphite composite material with a functional coating layer.

[0006] The present application discloses a graphite composite material with a functional coating layer, which is a secondary particle formed by binding graphite single particles in a core-shell structure, wherein the graphite single particles comprise graphite particles and a functional coating layer coated on the surface of the graphite particles; and the functional coating layer comprises a fast ion conductor and a doped element carbon layer.

[0007] Further, the fast ion conductor is selected from at least one of LATP, LLZO and LLTO; wherein the particle size D50 of the fast ion conductor is 50-500 nm.

[0008] The fast ion conductor coated on the surface of the graphite single particles effectively improves the interface diffusion of Li + , and helps Li + to quickly enter the interior of the secondary particles and directly intercalate lithium on the surface of the single particles, thereby fully exerting the fast charging advantage of small-size graphite.

[0009] The fast ion conductor has high ionic conductivity, good chemical stability, and the advantage of being compatible with lithium metal negative electrodes. LATP, LLZO and LLTO are three commonly used lithium ion battery electrolyte materials. LATP is a phosphate-based lithium ion conductor with the chemical formula LiAlTi2(PO4)3 and NASICON (Na Super Ionic CONductor) type structure. LLZO is an oxide-based lithium ion conductor with the chemical formula Li7La3Zr2O 12 and garnet type structure. LLTO is an oxide-based lithium ion conductor with the chemical formula Li 1+x Li 2-x TiO3 and spinel type structure.

[0010] Further, the doping element is selected from one or more of nitrogen element, phosphorus element, sulfur element, boron element, fluorine element.

[0011] The application also provides a preparation method of the graphite composite material with the functional coating layer, comprising the following steps:

[0012] S1: crushing and refining the graphite to prepare graphite particles;

[0013] S2: mixing and homogenizing the graphite particles with a dispersant to prepare a suspension;

[0014] S3: mixing and homogenizing the suspension obtained in step S2 with a carboxyl-containing polymer, a doping element-containing substance, and then adding and homogenizing a fast ion conductor to obtain a mixed solution A;

[0015] S4: spray drying the mixed solution A to obtain a precursor B;

[0016] S5: high-temperature sintering the precursor B in a protective gas atmosphere to obtain the graphite composite material with the functional coating layer as described above.

[0017] First, the original graphite is crushed and refined into ultra-fine graphite particles by crushing, and then a carboxyl-containing polymer is used as a binder, a fast ion conductor and a doping element-containing substance are added, and the construction of the functional coating layer on the surface of the graphite particles and the granulation of the secondary particles are completed through a one-step liquid phase coating method and a spray drying process, and then a graphite composite material with a functional coating layer is prepared through high-temperature sintering.

[0018] Further, in step S1, the graphite is selected from one of artificial graphite, natural graphite, microcrystalline graphite and mesocarbon microbeads; the particle size D50 of the graphite particles is 2-8 μm; in step S2, the dispersant is water; and the solid content in the suspension is 5-20%.

[0019] The ultra-fine graphite particles shorten the solid-phase diffusion path of Li + , and realize fast lithium intercalation.

[0020] Further, in step S3, the homogenization temperature is 20-80℃; the carboxyl-containing polymer is selected from one or more of polyacrylic acid, polymethacrylic acid, polyvinyl acid and maleic acid copolymer; the doping element is nitrogen element and phosphorus element, the nitrogen element-containing substance is selected from one of melamine, pyrrole, urea and ethylenediamine; and the phosphorus element-containing substance is selected from one of phosphoric acid, triphenylphosphine, lithium dihydrogen phosphate and phytic acid.

[0021] The carboxyl groups in the carboxyl-containing polymer binder interact with the hydroxyl groups on the surface of the graphite particles, and can form a uniform, thin and dense functional coating layer on the surface of the graphite particles.

[0022] The carboxyl-containing polymer can not only be used as a carbon source, but also can construct a uniform, thin and dense coating layer through the interaction between the hydroxyl group on the surface of graphite and the amino group in the nitrogen source, and form a stable and dense nitrogen and phosphorus co-doped carbon coating layer after high-temperature carbonization, so that the wettability of the graphite composite and the electrolyte is significantly improved, a stable SEI layer is induced to form, and the stability of the lithium ion battery cycle is improved.

[0023] Further, in the step S3, the mass of the carboxyl-containing polymer added is 1-5% of the graphite particles, the mass of the nitrogen-containing substance added is 0.5-2% of the graphite particles, the mass of the phosphorus-containing substance added is 0.2-1% of the graphite particles, and the mass of the fast ion conductor added is 0.5-5% of the graphite particles.

[0024] Through the adjustment of the components of the functional coating layer, the interface migration of Li + Can be greatly strengthened, and the influence of the anisotropic intercalation of the graphite aggregate is offset.

[0025] The uniform and stable functional coating layer avoids the direct contact between the active material and the electrolyte, inhibits the irreversible side reaction, reduces the lithium consumption, improves the initial efficiency of the negative electrode material, and is helpful to further improve the cycle stability of the battery.

[0026] Further, in the step S4, the conditions of the spray drying include that the inlet air temperature is 150-250 DEG C, the outlet air temperature is 75-150 DEG C, and the feeding flow rate is 0.1-10 L / h.

[0027] Further, in the step S4, the particle size D50 of the precursor B is 8-15 mu m.

[0028] Further, in the step S5, the protective gas is selected from one of nitrogen, argon and helium, and the high-temperature sintering temperature is 600-1200 DEG C.

[0029] The application further discloses an application of the graphite composite material with the functional coating layer, and the graphite composite material with the functional coating layer obtained by the preparation method is used for preparing a negative electrode sheet of a lithium ion battery.

[0030] The graphite composite material with the functional coating layer disclosed by the application is prepared by coating and regranulating the graphite particles with a doped element carbon layer and a fast ion conductor layer, which is helpful to shorten the diffusion path and interface diffusion resistance of Li + , stabilize the surface SEI layer, and significantly improve the high-rate capacity performance and long cycle stability of the graphite composite material. BRIEF DESCRIPTION OF DRAWINGS

[0031] Fig. 1 is a structure schematic view of the functional coating layer of the graphite composite material in Example 1 of the application.

[0032] Figure 2 is an SEM image of the graphite composite material with a functional coating layer according to Example 1 of the present application. DETAILED DESCRIPTION

[0033] To make the technical solutions of the present application clearer, the present application is further described in detail below in combination with the drawings and specific examples. EXAMPLE

[0034] Preparation of graphite composite material with a functional coating layer:

[0035] S1: The artificial graphite was superfine pulverized by using an airflow pulverizer to obtain graphite particles with an average particle size D50 of 3 μm.

[0036] S2: 1 kg of the graphite particles obtained in step S1 was placed in a high-speed homogenizer, 4 kg of deionized water was added, the solid content of the slurry was controlled to be 20%, and the mixture was homogenized into a suspension.

[0037] S3: In the high-speed homogenizer, 30 g of polyacrylic acid, 15 g of melamine and 10 g of phosphoric acid were added to the suspension obtained in step S2, the mixture was heated to 80°C for continuous stirring for 30 min, and then cooled to room temperature. Then, 30 g of nano-LATP slurry with a D50 of 100 nm was added, and the stirring was continued for 120 min to obtain a mixture A.

[0038] S4: The mixture A was sent into a spray dryer by using a peristaltic pump at a flow rate of 1 L / h, the inlet air temperature was set to be 200°C, the outlet air temperature was set to be 150°C, and the orifice diameter of the atomizer was adjusted, and thus a precursor B powder with a D50 of 10 μm was obtained.

[0039] S5: The precursor B powder was placed in a box furnace, the furnace was evacuated after the air in the furnace was replaced by nitrogen, and then the temperature was raised to 1100°C at a rate of 10°C / min, and the sintering was carried out at a constant temperature for 2 h, and thus the graphite composite material with a functional coating layer was obtained.

[0040] As shown in Figure 1, the graphite composite material with a functional coating layer prepared in the present example was coated with fast ion conductor on the surface of the superfine graphite particles, and the carbon layer was coated on the surface of the exposed graphite and fast ion conductor.

[0041] As shown in Figure 2, the graphite composite material with a functional coating layer prepared in the present example exhibited the morphology of the coated graphite secondary particles.

[0042] Preparation of graphite composite material with a functional coating layer:

[0043] S1: The natural flake graphite was superfine pulverized by using an airflow pulverizer to obtain graphite particles with an average particle size D50 of 2 μm.

[0044] S2: Put 1 kg of graphite particles prepared in step S1 into a high-speed homogenizer, add 9 kg of deionized water, control the solid content of the slurry to be 10%, and mix and homogenize into a suspension.

[0045] S3: In the high-speed homogenizer, add 20 g of polyvinyl acid, 10 g of ethylenediamine and 5 g of triphenylphosphine to the suspension obtained in step S2, heat the mixture to 40℃ and continue stirring for 30 min, then add nano-LLZO slurry with D50 of 50 nm after cooling to room temperature, the addition amount is 20 g, continue stirring for 120 min, and obtain mixture A.

[0046] S4: The mixture is sent into a spray dryer by a peristaltic pump at a flow rate of 5 L / h, the inlet air temperature is set to 200℃, the outlet air temperature is set to 150℃, and the orifice diameter of the atomizer is adjusted, and the precursor B powder with D50 of 12 μm is obtained.

[0047] S5: Put the precursor B powder into a box furnace, purge the air in the furnace with argon, heat to 1000℃ at a heating rate of 10℃ / min, and sinter at constant temperature for 2h, and the graphite composite material with functional coating layer is obtained.

[0048] Preparation of graphite composite material with functional coating layer:

[0049] S1: The mesocarbon microbeads are superfine pulverized by an airflow pulverizer to obtain graphite particles with an average particle size D50 of 5 μm.

[0050] S2: Put 1 kg of graphite particles prepared in step S1 into a high-speed homogenizer, add 4 kg of deionized water, control the solid content of the slurry to be 10%, and mix and homogenize into a suspension.

[0051] S3: In the high-speed homogenizer, add 20 g of maleic acid copolymer, 10 g of urea and 10 g of phytic acid to the suspension obtained in step S2, continue stirring at room temperature for 30 min, then add nano-LLTO slurry with D50 of 200 nm, the addition amount is 25 g, continue stirring for 60 min, and obtain mixture A.

[0052] S4: The mixture is sent into a spray dryer by a peristaltic pump at a flow rate of 2 L / h, the inlet air temperature is set to 220℃, the outlet air temperature is set to 180℃, and the orifice diameter of the atomizer is adjusted, and the precursor B powder with D50 of 15 μm is obtained.

[0053] S5: Put the precursor B powder into a box furnace, purge the air in the furnace with helium, heat to 1200℃ at a heating rate of 10℃ / min, and sinter at constant temperature for 2h, and the graphite composite material with functional coating layer is obtained.

[0054] (1) The artificial graphite is ultra-finely pulverized by using an air flow pulverizer to obtain graphite particles with an average particle size D50 of 3 μm.

[0055] (2) 1 kg of graphite particles is placed in a high-speed homogenizer, 4 kg of deionized water is added, the solid content of the slurry is controlled to be 20%, 50 g of acrylic acid, 20 g of urea and 10 g of lithium dihydrogen phosphate are gradually added, and after being heated to 80℃, the stirring is continued for 120 min to obtain a suspension.

[0056] (3) The suspension is sent into a spray dryer by a peristaltic pump at a flow rate of 3 L / h, the inlet air temperature is set to be 220℃, the outlet air temperature is set to be 180℃, and the orifice diameter of the atomizer is adjusted to obtain a precursor powder with a D50 of 8 μm.

[0057] (4) The obtained precursor powder is placed in a box furnace, helium gas is introduced to exhaust the air in the furnace, the temperature is raised to 1100℃ at a rate of 10℃ / min, and isothermal sintering is carried out for 2 h to obtain a graphite composite material.

[0058] (1) The artificial graphite is ultra-finely pulverized by using an air flow pulverizer to obtain graphite particles with an average particle size D50 of 5 μm.

[0059] (2) 1 kg of graphite single particles is placed in a high-speed homogenizer, 9 kg of deionized water is added, the solid content of the slurry is controlled to be 10%, 30 g of maleic acid copolymer is gradually added, stirring is continued at room temperature for 30 min, then 30 g of nano-LATP slurry with a D50 of 100 nm is added, and stirring is continued for 60 min to obtain a suspension.

[0060] (3) The suspension is sent into a spray dryer by a peristaltic pump at a flow rate of 5 L / h, the inlet air temperature is set to be 250℃, the outlet air temperature is set to be 200℃, and the orifice diameter of the atomizer is adjusted to obtain a precursor powder with a D50 of 10 μm.

[0061] (4) The precursor powder is placed in a box furnace, helium gas is introduced to exhaust the air in the furnace, the temperature is raised to 1100℃ at a rate of 10℃ / min, and isothermal sintering is carried out for 2 h to obtain a graphite composite material.

[0062] The graphite composite materials described in Examples 1-3 and Comparative Examples 1 and 2 are homogenously coated and assembled into button half-cells for electrochemical performance testing.

[0063] The specific scheme is as follows:

[0064] The graphite composite material, the conductive agent SP, the conductive agent VGCF and the water-based binder LA136 are mixed uniformly at a mass ratio of 75:10:5:10, a slurry is prepared by taking deionized water as a dispersant, and is coated on the surface of a copper foil, and then is placed in a vacuum oven for drying, and a graphite electrode sheet is obtained after cutting. In the half-cell assembly, a metal lithium sheet is used as a counter electrode, and a Celgard 2400 microporous PP membrane is used as a separator. In the cycle test, the electrochemical window is set to 0.005-1.5 V, wherein the discharge process is first discharged to 0.005 V at 0.1C, and then discharged to 0.005 V at 0.02C, and the charging process is charged to 1.5 V at 0.1C. The test results are shown in Table 1 below.

[0065] Table 1: Discharge test results of examples 1-3 and comparative examples 1-2

[0066] First lithium extraction capacity (mAh / g) First efficiency (%) 1000-week capacity retention rate (%) 2C lithium extraction capacity (mAh / g) 5C lithium extraction capacity (mAh / g) Example 1 353.2 94.3 95.1 322.5 199.6 Example 2 352.5 93.8 93.2 318.8 187.5 Example 3 353.4 94.0 94.5 312.1 185.0 Comparative example 1 353.7 93.1 88.7 254.0 123.6 Comparative example 2 355.8 92.5 90.8 287.3 147.7

[0067] As shown in Table 1, examples 1-3 are coated with a nitrogen and phosphorus double-doped carbon layer and a fast ion conductor layer on the surface of the ultrafine graphite, which helps to shorten the Li + diffusion path and the interface diffusion resistance, stabilize the surface SEI layer, and thus significantly improve the high-rate capacity of the graphite composite material and the long cycle stability.

[0068] The above-described examples only express several embodiments of the present application, and the description is more specific and detailed, but it cannot be understood as limiting the scope of the patent of the present application. It should be noted that for ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A graphite composite material having a functional coating layer, characterized by, The secondary particle is formed by bonding graphite single particles with core-shell structure, wherein the graphite single particle comprises a graphite particle and a functional coating layer covering the surface of the graphite particle; the functional coating layer comprises a fast ion conductor and a doped element carbon layer.

2. The graphite composite material having a functional coating layer according to claim 1, characterized by, The fast ion conductor is selected from at least one of LATP, LLZO and LLTO; wherein the particle size D50 of the fast ion conductor is 50-500 nm.

3. The graphite composite material having a functional coating layer according to claim 1, wherein The doped element is selected from one or more of nitrogen element, phosphorus element, sulfur element, boron element and fluorine element.

4. A method for producing a graphite composite material having a functional coating layer, characterized by, The method comprises the following steps: S1: crushing and refining graphite to prepare graphite particles; S2: mixing the graphite particles with a dispersant to form a homogeneous suspension; S3: mixing the suspension obtained in step S2 with a carboxyl-containing polymer, a substance containing a doped element, and then adding a fast ion conductor to obtain a mixture A; S4: spray drying the mixture A to obtain a precursor B; S5: sintering the precursor B in a protective gas atmosphere at a high temperature to obtain the graphite composite material with a functional coating layer according to any one of claims 1-3.

5. The method of claim 4, wherein the graphite composite having a functional coating layer is prepared by the steps of: In step S1, the graphite is selected from one of artificial graphite, natural graphite, microcrystalline graphite and mesocarbon microbeads; the particle size D50 of the graphite particles is 2-8 μm; in step S2, the dispersant is water; and the solid content in the suspension is 5-20%.

6. The method of claim 4, wherein the graphite composite having a functional coating layer is prepared by the steps of: In step S3, the homogenization temperature is 20-80 ℃; the carboxyl-containing polymer is selected from one or more of polyacrylic acid, polymethacrylic acid, polyvinyl acid and maleic acid copolymer; the doped element is nitrogen element and phosphorus element; the nitrogen-containing substance is selected from one of melamine, pyrrole, urea and ethylenediamine; and the phosphorus-containing substance is selected from one of phosphoric acid, triphenylphosphine, lithium dihydrogen phosphate and phytic acid.

7. The method of claim 6, wherein the graphite composite material having a functional coating layer is prepared by the steps of: In step S3, the mass of the carboxyl-containing polymer added is 1-5% of the graphite particles; the mass of the nitrogen-containing substance added is 0.5-2% of the graphite particles; the mass of the phosphorus-containing substance added is 0.2-1% of the graphite particles; and the mass of the fast ion conductor added is 0.5-5% of the graphite particles.

8. The method of claim 4, wherein the graphite composite having a functional coating layer is prepared by the steps of: In step S4, the spray drying conditions include: an inlet air temperature of 150-250 ℃, an outlet air temperature of 75-150 ℃, and a feed flow rate of 0.1-10 L / h.

9. The method of claim 4, wherein the graphite composite material having a functional coating layer is prepared by the steps of: In step S5, the protective gas is selected from one of nitrogen, argon and helium; and the high-temperature sintering temperature is 600-1200 ℃.

10. Use of a graphite composite with a functional coating layer, characterized in that The graphite composite material with a functional coating layer obtained by the preparation method according to any one of claims 4-9 is used for preparing a lithium ion battery negative electrode sheet.

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