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

Abstract

The invention discloses a graphite composite material with a functional coating layer, and a preparation method and application thereof, and belongs to the technical field of lithium ion batteries. The graphite composite material with the functional coating layer is a secondary particle formed by bonding single graphite particles with a core-shell structure, wherein the single graphite particles comprise graphite particles and the functional coating layer coating the surfaces of the graphite particles; the functional coating layer comprises a fast ion conductor and a doped element carbon layer. According to the graphite composite material with the functional coating layer disclosed by the invention, secondary particles are formed by coating the surface of the graphite particles with an elemental carbon layer and a fast ion conductor layer, so that the Li < + > diffusion path and the interface diffusion resistance can be shortened, and a surface SEI layer is stabilized; therefore, the high-rate capacity exertion and the long cycle stability of the graphite composite material are remarkably improved.

Description

Technical Field

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

[0002] Graphite anodes mainly include artificial graphite, natural graphite and mesophase carbon microspheres. They are still the main anode materials for lithium-ion batteries and will remain the first choice for a long time in the future. Although graphite materials have advantages such as high energy density, good conductivity, low lithium insertion potential and low price, the two-dimensional layered structure of graphite determines the Li + It must be embedded from the end face of the material and gradually diffuse into the interior of the particle, resulting in Li + The solid phase diffusion path is longer, and its smaller interlayer spacing (0.335nm) also causes Li + The diffusion rate of Li is low, which limits its fast charging performance. Especially in the high-rate charging process, the interface Li + Aggregation, concentration increase, resulting in greater concentration polarization, Li + It will continue to deposit at the interface, forming uncontrolled surface lithium deposition and even dendrite propagation, which will not only cause the battery capacity to decay rapidly, but may even penetrate the diaphragm, causing local micro-short circuits and leading to thermal runaway, posing serious safety hazards.

[0003] It is generally believed that ion transport is the rate-limiting step for lithium insertion in graphite materials. This process mainly includes Li + Diffusion process in electrolyte and electrode materials. Interfaces and interfacial phases are common in LIBs. When the graphite negative electrode is charged and discharged at high rates, Li + The diffusion rate inside the particle does not match the rate of lithium insertion and removal on the particle surface, and various polarizations will inevitably occur, including ohmic drop, concentration overpotential, and charge transfer overpotential. Based on the above understanding, the optimization strategy of graphite negative electrode basically starts from particle structure design and interface modification. The first is the structural design of graphite particles, including particle size refinement and secondary granulation, which specifically shortens the Li + The bulk diffusion path and the expansion of lithium insertion channels can reduce the ion transmission resistance and improve the fast charging performance; the second is to construct a functional carbon coating layer on the graphite surface to make Li + Rapid transport from the base surface to the edge face of graphite enables rapid lithium insertion.

[0004] In the existing technical solutions, small-sized graphite single particles and binders (asphalt-based, resin-based, etc.) are usually used to granulate and prepare graphite secondary particles, and a functional carbon coating layer is constructed on the surface of the secondary particles. However, the asphalt-based and resin-based binders have poor dispersibility and self-agglomeration, resulting in reduced bonding effect. In addition, the carbon coating layer on the surface of the secondary particles is not suitable for Li +The improvement of diffusion path and interface impedance is limited, resulting in the Li + The diffusion resistance is large, making it difficult to bring into play the fast charging advantage of ultrafine graphite. Summary of the invention

[0005] The object of the present invention is to provide a graphite composite material with a functional coating layer that can shorten the solid-phase diffusion path of Li+; another object of the present invention is to provide a method for preparing the graphite composite material with a functional coating layer; another object of the present invention is to provide an application of the graphite composite material with a functional coating layer.

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

[0007] Furthermore, 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 particles is 50-500 nm.

[0008] The fast ion conductor is coated on the surface of graphite single particles to effectively improve the Li + The interface diffusion of Li + It quickly enters the interior of secondary particles and directly inserts lithium on the surface of single particles, giving full play to the fast charging advantage of small-sized graphite.

[0009] Fast ion conductors have the advantages of high ionic conductivity, good chemical stability, and compatibility 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 a chemical formula of LiAlTi2(PO4)3 and a NASICON (Na Super Ionic CONductor) structure. LLZO is an oxide-based lithium-ion conductor with a chemical formula of Li7La3Zr2O 12 , with a garnet structure. LLTO is an oxide-based lithium ion conductor with a chemical formula of Li 1+x Li 2-x TiO3 has a spinel structure.

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

[0011] The present invention also provides a method for preparing a graphite composite material having a functional coating layer, comprising the following steps:

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

[0013] S2: mixing the graphite particles and the dispersant to form a suspension;

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

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

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

[0017] Firstly, the original graphite is crushed and refined into ultrafine graphite particles by pulverizing, and then a carboxyl-containing polymer is used as a binder, and fast ion conductors and doping elements are added. The functional coating layer on the surface of the graphite particles and the granulation of 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 obtained through high-temperature sintering.

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

[0019] Ultrafine graphite particles shorten Li + The solid-phase diffusion path enables rapid lithium insertion.

[0020] Furthermore, in step S3, the homogenization temperature is 20-80°C; the carboxyl-containing polymer is selected from one or more of polyacrylic acid, polymethacrylic acid, polyvinyl acid, and maleic acid copolymer; the doping elements are nitrogen and phosphorus, and the nitrogen-containing substance is selected from one of melamine, pyrrole, urea, and ethylenediamine; the phosphorus-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] While serving as a carbon source, the carboxyl-containing polymer can also construct a uniform, thin and dense coating layer through the interaction with the hydroxyl groups on the graphite surface and the amino groups in the nitrogen source, and form a stable and dense nitrogen-phosphorus dual-doped carbon coating layer after high-temperature carbonization, which significantly improves the wettability of the graphite composite material and the electrolyte, induces the formation of a stable SEI layer, and enhances the stability of the lithium-ion battery cycle.

[0023] Furthermore, 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.

[0024] By adjusting the composition of the functional coating layer, the Li + The interfacial migration of graphite aggregates offsets the effect of anisotropic intercalation.

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

[0026] Furthermore, in step S4, the spray drying conditions include: an air inlet temperature of 150-250°C, an air outlet temperature of 75-150°C, and a feed flow rate of 0.1-10 L / h.

[0027] Furthermore, in the step S4, the particle size D50 of the precursor B is 8-15 μm.

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

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

[0030] The present invention discloses a graphite composite material with a functional coating layer, which is formed by coating the surface of graphite particles with an element carbon layer and a fast ion conductor layer and then granulating to form secondary particles, which helps to shorten the Li + The diffusion path and interface diffusion resistance stabilize the surface SEI layer, thereby significantly improving the high-rate capacity and long-cycle stability of graphite composites. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of the structure of the graphite composite material of the functional coating layer of Example 1 of the present invention;

[0032] Figure 2 This is a SEM image of the graphite composite material of the functional coating layer in Example 1 of the present invention. DETAILED DESCRIPTION

[0033] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] Example 1

[0035] Preparation of graphite composites with functional coating layers:

[0036] S1: The artificial graphite is ultra-finely pulverized by a jet mill to obtain graphite particles with an average particle size D50 of 3 μm.

[0037] S2: Place 1 kg of graphite particles prepared in step S1 in a high-speed homogenizer, add 4 kg of deionized water, control the solid content of the slurry to 20%, and mix and homogenize into a suspension.

[0038] S3: In a 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 mixed solution was heated to 80°C and stirred for 30 min, and after cooling to room temperature, 30 g of nano-LATP slurry with a D50 of 100 nm was added, and stirring was continued for 120 min to obtain a mixed solution A.

[0039] S4: The mixed solution A is sent into the spray dryer through a peristaltic pump at a flow rate of 1 L / h, the inlet air temperature is set to 200°C, the outlet air temperature is set to 150°C, and the atomizer aperture is adjusted to obtain a precursor B powder with a D50 of 10 μm.

[0040] S5: Precursor B powder is placed in a box furnace, nitrogen is introduced to exhaust the air in the furnace, the temperature is increased to 1100°C at a heating rate of 10°C / min, and constant temperature sintering is performed for 2 hours to obtain a graphite composite material with a functional coating layer.

[0041] like Figure 1 As shown, in the graphite composite material with a functional coating layer prepared in this embodiment, the fast ion conductor is coated on the surface of the ultrafine graphite particles, and the carbon layer is coated on the surface of the exposed graphite and the fast ion conductor.

[0042] like Figure 2 As shown, the graphite composite material with a functional coating layer prepared in this example exhibits the morphology of graphite secondary particles after coating.

[0043] Experimental Example 2

[0044] Preparation of graphite composites with functional coating layers:

[0045] S1: The natural flake graphite is ultrafinely crushed by a jet mill to obtain graphite particles with an average particle size D50 of 2 μm.

[0046] S2: Place 1 kg of graphite particles prepared in step S1 in a high-speed homogenizer, add 9 kg of deionized water, control the solid content of the slurry to 10%, and mix and homogenize into a suspension.

[0047] S3: In a high-speed homogenizer, add 20 g of poly(ethylene) acid, 10 g of ethylenediamine and 5 g of triphenylphosphine to the suspension obtained in step S2, heat the mixture to 40°C and stir for 30 min, cool to room temperature, add 20 g of nano-LLZO slurry with a D50 of 50 nm, and stir for 120 min to obtain a mixed solution A.

[0048] S4: The mixed liquid is sent into the spray dryer through a peristaltic pump at a flow rate of 5 L / h, the inlet air temperature is set to 200°C, the outlet air temperature is set to 150°C, and the atomizer aperture is adjusted to obtain the precursor B powder with a D50 of 12 μm.

[0049] S5: Precursor B powder is placed in a box furnace, argon gas is introduced to exhaust the air in the furnace, the temperature is increased to 1000°C at a heating rate of 10°C / min, and constant temperature sintering is performed for 2 hours to obtain a graphite composite material with a functional coating layer.

[0050] Experimental Example 3

[0051] Preparation of graphite composites with functional coating layers:

[0052] S1: The mesocarbon microspheres are ultrafinely pulverized by a jet mill to obtain graphite particles with an average particle size D50 of 5 μm.

[0053] S2: Place 1 kg of graphite particles prepared in step S1 in a high-speed homogenizer, add 4 kg of deionized water, control the solid content of the slurry to 10%, and mix and homogenize into a suspension.

[0054] S3: In a high-speed homogenizer, 20 g of maleic acid copolymer, 10 g of urea and 10 g of phytic acid were added to the suspension obtained in step S2, and the mixture was stirred at room temperature for 30 min. Then, 25 g of nano-LLTO slurry with a D50 of 200 nm was added, and the mixture was stirred for 60 min to obtain a mixed solution A.

[0055] S4: The mixed liquid is sent into the spray dryer through a peristaltic pump at a flow rate of 2L / h, the inlet air temperature is set to 220°C, the outlet air temperature is set to 180°C, and the atomizer aperture is adjusted to obtain the precursor B powder with a D50 of 15μm.

[0056] S5: Precursor B powder is placed in a box furnace, helium is introduced to exhaust the air in the furnace, and then the temperature is increased to 1200°C at a heating rate of 10°C / min, and constant temperature sintering is performed for 2 hours to obtain a graphite composite material with a functional coating layer.

[0057] Comparative Example 1

[0058] (1) Artificial graphite is ultrafinely pulverized using a jet mill to obtain graphite particles with an average particle size D50 of 3 μm.

[0059] (2) 1 kg of graphite particles 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%, 50 g of acrylic acid, 20 g of urea and 10 g of lithium dihydrogen phosphate were gradually added, and the mixture was heated to 80° C. and stirred for 120 min to obtain a suspension.

[0060] (3) The suspension was sent to a spray dryer at a flow rate of 3 L / h through a peristaltic pump, the inlet air temperature was set to 220° C., the outlet air temperature was set to 180° C., and the atomizer aperture was adjusted to obtain a precursor powder with a D50 of 8 μm.

[0061] (4) The obtained precursor powder is placed in a box furnace, helium is introduced to evacuate the air in the furnace, and the temperature is increased to 1100°C at a heating rate of 10°C / min, and constant temperature sintering is performed for 2 hours to obtain a graphite composite material.

[0062] Comparative Example 2

[0063] (1) Artificial graphite is ultrafinely pulverized using a jet mill to obtain graphite particles with an average particle size D50 of 5 μm.

[0064] (2) 1 kg of single graphite particles was placed in a high-speed homogenizer, and after adding 9 kg of deionized water, the solid content of the slurry was controlled to be 10%, and 30 g of maleic acid copolymer was gradually added. The mixture was stirred continuously for 30 min at room temperature, and then 30 g of nano-LATP slurry with a D50 of 100 nm was added. The mixture was stirred continuously for 60 min to obtain a suspension.

[0065] (3) The suspension was sent into a spray dryer at a flow rate of 5 L / h through a peristaltic pump, the inlet air temperature was set to 250° C., the outlet air temperature was set to 200° C., and the atomizer aperture was adjusted to obtain a precursor powder with a D50 of 10 μm.

[0066] (4) The precursor powder is placed in a box furnace, helium is introduced to exhaust the air in the furnace, the temperature is increased to 1100°C at a heating rate of 10°C / min, and constant temperature sintering is performed for 2 hours to obtain a graphite composite material.

[0067] Performance Testing:

[0068] The graphite composite materials described in Examples 1-3 and Comparative Examples 1 and 2 were slurried and coated and assembled into button half-cells for electrochemical performance testing.

[0069] The specific plan is as follows:

[0070] The graphite composite material, conductive agent SP, conductive agent VGCF, and aqueous binder LA136 were mixed evenly in a mass ratio of 75:10:5:10, and deionized water was used as a dispersant to prepare a slurry, which was coated on the surface of the copper foil, and then placed in a vacuum oven for drying. After cutting, a graphite electrode sheet was obtained. The counter electrode in the half-cell assembly used a metal lithium sheet, and the diaphragm used a Celgard 2400 microporous PP film. The electrochemical window in the cycle test was set to 0.005-1.5V, where the discharge process was first discharged to 0.005V at 0.1C, and then discharged to 0.005V at 0.02C, and the charging process was charged to 1.5V at 0.1C. The test results are shown in Table 1 below.

[0071] Table 1: Test results of power-off of Examples 1-3 and Comparative Examples 1-2

[0072]

[0073] As shown in Table 1, Examples 1-3 help to shorten the Li + The diffusion path and interface diffusion resistance stabilize the surface SEI layer, thereby significantly improving the high-rate capacity and long-cycle stability of graphite composites.

[0074] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A graphite composite material having a functional coating layer, characterized in that: The secondary particles are formed by bonding single graphite particles with a core-shell structure. The single graphite particles include graphite particles and a functional coating layer coated on the surface of the graphite particles; the functional coating layer includes a fast ion conductor and an element-doped carbon layer.

2. The graphite composite material with a functional coating layer according to claim 1, characterized in that: 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 particles is 50-500 nm.

3. The graphite composite material with a functional coating layer according to claim 1, characterized in that: The doping element is selected from one or more of nitrogen, phosphorus, sulfur, boron and fluorine.

4. A method for preparing a graphite composite material having a functional coating layer, characterized in that: The following steps are involved: S1: crushing and refining graphite to prepare graphite particles; S2: mixing the graphite particles and the dispersant to form a suspension; S3: the suspension obtained in step S2 is mixed with the carboxyl-containing polymer and the substance containing the doping element, and then a fast ion conductor is added and mixed to obtain a mixed solution A; S4: spray drying the mixed solution A to obtain a precursor B; S5: Sintering the precursor B at a high temperature in a protective gas atmosphere to obtain the graphite composite material with a functional coating layer as claimed in any one of claims 1 to 3.

5. The method for preparing a graphite composite material having a functional coating layer according to claim 4, characterized in that: In the step S1, the graphite is selected from one of artificial graphite, natural graphite, microcrystalline graphite, and mesophase carbon microspheres; the particle size D50 of the graphite particles is 2-8 μm; in the step S2, the dispersant is water; and the solid content of the suspension is 5-20%.

6. The method for preparing a graphite composite material having a functional coating layer according to claim 4, characterized in that: In the step S3, the homogenization temperature is 20-80°C; the carboxyl-containing polymer is selected from one or more of polyacrylic acid, polymethacrylic acid, polyvinyl acid, and maleic acid copolymer; the doping elements are nitrogen and phosphorus, and the nitrogen-containing substance is selected from one of melamine, pyrrole, urea, and ethylenediamine; the phosphorus-containing substance is selected from one of phosphoric acid, triphenylphosphine, lithium dihydrogen phosphate, and phytic acid.

7. The method for preparing a graphite composite material having a functional coating layer according to claim 6, characterized in that: 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 for preparing a graphite composite material having a functional coating layer according to claim 4, characterized in that: In step S4, the spray drying conditions include: an air inlet temperature of 150-250° C., an air outlet temperature of 75-150° C., and a feed flow rate of 0.1-10 L / h.

9. The method for preparing a graphite composite material having a functional coating layer according to claim 4, characterized in that: 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°C.

10. An application of a graphite composite material having 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 to 9 is used for preparing a negative electrode sheet for a lithium ion battery.

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