A long-life graphite composite material and its preparation method

Abstract

The present invention relates to the technical field of lithium-ion battery materials, and provides a long-life graphite composite material and a preparation method thereof. The graphite composite material has a core-shell structure, wherein the core is fluorine-containing graphite and the shell is an organic fast ion conductor; the mass of the shell is 1%-5% of the mass of the graphite composite material. Through the above technical solution, the problem in the prior art that the graphite composite material has more side reactions at high temperatures, resulting in poor cycle performance and affecting the battery life, is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery materials, and more specifically, to a long-life graphite composite material and a preparation method thereof. Background Art

[0002] With the increasing requirement for the cruising range of electric vehicles, the anode materials used in lithium-ion batteries are required to have excellent cycling performance. At present, the commercially available anode materials are mainly artificial graphite, which has become the preferred anode material due to its low cost, stable structure, good power performance, etc. However, due to the small interlayer spacing of artificial graphite itself, it causes the expansion of the material during the cycling process (theoretical expansion is about 10%), affecting its cycling performance and reducing the battery life. And the surface coating of amorphous carbon on the material has more side reactions at high temperatures, which will reduce its cycling performance and lead to a reduction in battery life. Therefore, it is necessary to modify the surface of the material to reduce the side reactions on the material surface, improve the cycling performance and life, and thus improve the comprehensive performance of the graphite composite material. Summary of the Invention

[0003] The present invention provides a long-life graphite composite material and a preparation method thereof, which solve the problem that the graphite composite material in the related art has more side reactions at high temperatures, resulting in poor cycling performance and affecting the battery life.

[0004] The technical solution of the present invention is as follows:

[0005] A long-life graphite composite material, the graphite composite material has a core-shell structure, the inner core is fluorine-containing graphite, and the outer shell is an organic fast ion conductor; the mass of the outer shell is 1%-5% of the mass of the graphite composite material.

[0006] As a further technical solution, the organic fast ion conductor includes one or more of lithium naphthenate, lithium acetate, lithium oxalate, lithium perfluorohexanesulfonate, lithium acetate, lithium stearate, lithium benzoate, lithium oxalate, and lithium stearate.

[0007] A preparation method of a long-life graphite composite material, comprising the following steps:

[0008] S1. Crush and shape isotropic coke to obtain a precursor material A;

[0009] S2. Add the precursor material A, inorganic fluoride, binder, and additive to an organic solvent, disperse evenly and soak to obtain a mixture;

[0010] S3. Grind and dry the mixture, and calcine to obtain a precursor material B;

[0011] S4. Modify the precursor material B in a fluorine gas and argon gas atmosphere to obtain a modified precursor material;

[0012] S5. Using the modified precursor material as the matrix and the organic fast ion conductor as the target, perform atomic vapor deposition to obtain a graphite composite material;

[0013] The additive in the step S2 includes one or more of lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, and lithium bis(trifluoromethanesulfonyl)imide.

[0014] As a further technical solution, the particle size of the isotropic coke after crushing in the step S1 is 8-10 μm; the particle size distribution after shaping is (D90-D10) / D50 = 0.9-1.2.

[0015] As a further technical solution, the mass ratio of the precursor material A, inorganic fluoride, binder, additive, and organic solvent in the step S2 is 100:1-5:1-5:0.5-2:500.

[0016] As a further technical solution, the inorganic fluoride in the step S2 includes one or more of sodium fluoride, calcium fluoride, magnesium fluoride, and potassium fluoride;

[0017] The organic solvent includes one or more of ethylene glycol, butanediol, 1,2-ethylene glycol, glycol, 1,3-butanediol, cyclohexanol, 1,4-butanediol, and n-hexanol.

[0018] As a further technical solution, the soaking in the step S2 is carried out under vacuum for 24-72 h.

[0019] As a further technical solution, the calcination in the step S3 is carried out by heating to 200-400 °C in an inert atmosphere, and the calcination time is 0.5-1 h.

[0020] As a further technical solution, the volume ratio of fluorine gas to argon gas in the step S4 is 1:10; the modification temperature is 700-1100 °C, and the modification time is 0.5-1 h.

[0021] As a further technical solution, the conditions for chemical deposition in the step S5 are a temperature of 150-250 °C and a pressure of 0.1-0.5 Torr.

[0022] The working principle and beneficial effects of the present invention are:

[0023] 1. In the present invention, fluoride is doped into the graphite core. During the carbonization process, fluorine forms a -C-F- chemical bond with carbon, which has the characteristic of stable structure, enabling the reduction of expansion during the charge and discharge process. Moreover, the doped fluorine has good compatibility with the lithium-containing additive in the electrolyte, reducing side reactions and improving the storage performance and cycling performance. On the outer layer, an organic fast ion conductor is deposited by atomic vapor deposition method, which has the advantages of high density, good consistency, and controllable deposition thickness. In addition, the organic fast ion conductor has good ionic conductivity, improving the rate of lithium ion insertion and extraction during the charge and discharge process and reducing side reactions. At the same time, the organic fast ion conductor has good compatibility with the organic electrolyte, which can improve the interface structure stability of the material and reduce the occurrence of side reactions, thus enhancing the cycling and storage performance.

[0024] 2. By jointly using an additive and a lithium salt anode film-forming additive in the present invention, a surface film with good shrinkage performance and high temperature resistance is formed on the anode surface, improving the structural stability of the anode material, enhancing the electrical contact performance between the active material and the current collector, and improving the stability of the battery internal resistance and the cycling performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be further described in detail below in conjunction with the drawings and specific embodiments.

[0026] Figure 1 SEM image of the graphite composite material prepared for Example 1. SPECIFIC EMBODIMENTS

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts fall within the scope of the present invention.

[0028] Example 1

[0029] A preparation method of a long-life graphite composite material includes the following steps:

[0030] S1. Crush isotropic coke to D50 = 9 μm, and after shaping, make its particle size distribution satisfy (D90 - D10) / D50 = 1 to obtain precursor material A;

[0031] S2. Add 100 g of precursor material A, 3 g of sodium fluoride, 3 g of pitch, and 1 g of bis(oxalato)borate lithium to 500 g of butanediol solution, disperse evenly, and then soak in vacuum for 48 h to obtain a mixture;

[0032] S3. Transfer the mixture to a ball mill for grinding and dispersion, then dry it under vacuum at 80 °C for 24 h, and heat it up to 300 °C under an argon atmosphere and hold for 1 h to obtain the precursor material B;

[0033] S4. Transfer the precursor material B to a tube furnace. First, introduce argon to exhaust the air in the tube, and then introduce a gas with a volume ratio of fluorine to argon of 1:10. Hold at a temperature of 950 °C for 1 h to obtain the modified precursor material;

[0034] S5. Through atomic vapor deposition, using the modified precursor material as the substrate and lithium naphthenate as the target, carry out chemical deposition at a temperature of 200 °C and a pressure of 0.3 Torr to obtain the graphite composite material.

[0035] The chemical deposition process in step S5 is as follows: ① Introduce the organic fast ion conductor for 1 s; ② Purge with nitrogen for 60 s; ③ Introduce the oxygen source for 5 s; ④ Purge with nitrogen for 5 s; ⑤ Introduce water for 0.05 s; ⑥ Purge with nitrogen for 50 s; Steps ① - ⑥ are cycled 50 times.

[0036] Example 2

[0037] A preparation method of a long - life graphite composite material, comprising the following steps:

[0038] S1. Crush the isotropic coke to D50 = 8 μm, and after shaping, make its particle size distribution satisfy (D90 - D10) / D50 = 0.9 to obtain the precursor material A;

[0039] S2. Add 100 g of the precursor material A, 1 g of calcium fluoride, 1 g of pitch, and 0.5 g of lithium tetrafluoroborate to 500 g of N - methylpyrrolidone, disperse evenly, and then soak under vacuum for 24 h to obtain a mixture;

[0040] S3. Transfer the mixture to a ball mill for grinding and dispersion, then dry it under vacuum at 80 °C for 24 h, and heat it up to 200 °C under an argon atmosphere and hold for 1 h to obtain the precursor material B;

[0041] S4. Transfer the precursor material B to a tube furnace. First, introduce argon to exhaust the air in the tube, and then introduce a gas with a volume ratio of fluorine to argon of 1:10. Hold at a temperature of 700 °C for 1 h to obtain the modified precursor material;

[0042] S5. Through atomic vapor deposition, using the modified precursor material as the substrate and lithium acetate as the target, carry out chemical deposition at a temperature of 150 °C and a pressure of 0.1 Torr to obtain the graphite composite material.

[0043] The chemical deposition process in step S5 is as follows: ① Introduce the organic fast ion conductor for 1 second; ② Purge with nitrogen for 60 seconds; ③ Introduce the oxygen source for 5 seconds; ④ Purge with nitrogen for 5 seconds; ⑤ Introduce water for 0.05 seconds; ⑥ Purge with nitrogen for 50 seconds; Steps ① - ⑥ are cycled 10 times.

[0044] Example 3

[0045] A preparation method of a long - life graphite composite material, comprising the following steps:

[0046] S1. Crush the isotropic coke to D50 = 10 μm, and after shaping, make its particle size distribution satisfy (D90 - D10) / D50 = 1.2 to obtain the precursor material A;

[0047] S2. Add 100 g of the precursor material A, 5 g of magnesium fluoride, 5 g of pitch, and 2 g of lithium bis(oxalato)borate to 500 g of cyclohexane, disperse evenly, and then soak in vacuum for 72 h to obtain a mixture;

[0048] S3. Transfer the mixture to a ball mill for grinding and dispersion, then dry in vacuum at 80 °C for 24 h, and heat up to 400 °C in an argon atmosphere and hold for 1 h to obtain the precursor material B;

[0049] S4. Transfer the precursor material B to a tube furnace, first introduce argon to exhaust the air in the tube, and then introduce a gas with a volume ratio of fluorine to argon of 1:10, and hold at a temperature of 1100 °C for 0.5 h to obtain a modified precursor material;

[0050] S5. Through atomic vapor deposition, using the modified precursor material as the substrate and the lithium perfluorooctanesulfonate target, carry out chemical deposition at a temperature of 250 °C and a pressure of 0.5 Torr to obtain the graphite composite material.

[0051] The chemical deposition process in step S5 is as follows: ① Introduce the organic fast ion conductor for 1 second; ② Purge with nitrogen for 60 seconds; ③ Introduce the oxygen source for 5 seconds; ④ Purge with nitrogen for 5 seconds; ⑤ Introduce water for 0.05 seconds; ⑥ Purge with nitrogen for 50 seconds; Steps ① - ⑥ are cycled 10 times.

[0052] Comparative Example 1

[0053] Compared with Example 1, in Comparative Example 1, sodium fluoride and lithium bis(oxalato)borate are not added, and the others are the same as in Example 1.

[0054] Comparative Example 2

[0055] Compared with Example 1, in Comparative Example 2, sodium fluoride is not added, and the others are the same as in Example 1.

[0056] Comparative Example 3

[0057] Compared with Example 1, Comparative Example 3 does not perform step S5, and the modified precursor material obtained in step S4 is a graphite composite material, and the rest is the same as Example 1.

[0058] Comparative Example 4

[0059] Compared with Example 1, in Comparative Example 4, the lithium bis(oxalate)borate in step S2 is replaced with an equal amount of lithium titanate, and the rest is the same as Example 1.

[0060] Test Example 1

[0061] Physical and chemical properties test

[0062] (1) SEM test

[0063] The graphite composite material prepared in Example 1 was subjected to SEM test, and the test results are as follows: Figure 1 As shown in the figure, it can be seen that the graphite composite material is a secondary particle structure with a particle size between (8-18) μm and less fine powder between the materials.

[0064] (2) Powder conductivity test

[0065] The graphite composite materials in Examples 1-3 and Comparative Examples 1-4 were tested for powder conductivity. The test method for powder conductivity was as follows: the powder was pressed into a block structure on a powder compaction density meter at a pressure of 2 T, and then the powder conductivity was tested using a four-probe tester. The test results are shown in Table 1.

[0066] (3) Conductivity, specific surface area, particle size, and tap density tests

[0067] The conductivity, specific surface area, particle size and tap density were tested in accordance with GB / T 24533-2019 "Graphite Anode Materials for Lithium-ion Batteries". The test results are shown in Table 1.

[0068] Table 1 Physical and chemical properties test results

[0069]

[0070] As can be seen from Table 1, compared with Example 1, sodium fluoride and lithium bis(oxalato)borate are not added in Comparative Example 1, and chemical deposition is not carried out in Comparative Example 3. The conductivity of the prepared graphite composite material is significantly lower than that of Example 1. The reason is that the surface of the material is coated with a fast ion conductor with higher conductivity, which improves the electron transport rate and the tap density of the material. Compared with Example 1, in Comparative Example 4, an inorganic fast ion lithium salt is used to replace the organic fast ion conductor. The conductivity of the prepared graphite composite material is lower than that of Example 1 because the organic fast ion conductor in the example has a large deformation under pressure, resulting in good contact between its materials, thereby improving the conductivity of the powder material, while the inorganic fast ion lithium salt cannot achieve such an effect. Compared with Example 1, the conductivity and tap density of the graphite composite material prepared in Comparative Example 2 without adding inorganic fluoride are lower than those of Example 1.

[0071] Experimental Example 2 Coin Cell Test

[0072] The graphite materials in Examples 1-3 and Comparative Examples 1-4 were assembled into coin cells respectively. The assembly method is as follows: Add a binder, a conductive agent and a solvent to the negative electrode material, stir to make a slurry, coat it on a copper foil, and obtain a negative electrode sheet after drying and rolling. The binder used is LA132 binder, the conductive agent is SP, the negative electrode materials are the graphite composite materials in Examples 1-3 and Comparative Examples 1-4 respectively, and the solvent is secondary distilled water. The ratio of each component is: negative electrode material: SP: LA132: secondary distilled water = 95 g: 1 g: 4 g: 220 mL; the electrolyte is LiPF6 / EC+DEC (the concentration of LiPF6 is 1.2 mol / L, and the volume ratio of EC and DEC is 1:1), the metal lithium sheet is the counter electrode, and the separator uses a polyethylene (PE), polypropylene (PP) or polyethylene-propylene (PEP) composite film. The assembly of the coin cell is carried out in a glove box filled with hydrogen, and the electrochemical performance test is carried out on a Wuhan Blue Electric CT2001A battery tester. The charge-discharge voltage range is 0.005 V to 2.0 V, and the charge-discharge rate is 0.1C. The test results are shown in Table 2.

[0073] Table 2 Coin Cell Test Results

[0074] Initial discharge capacity (mAh / g) Initial efficiency (%) Example 1 361.3 97.1 Example 2 360.4 96.8 Example 3 359.5 95.7 Comparative Example 1 353.4 94.2 Comparative Example 2 355.6 94.1 Comparative Example 3 359.4 92.3 Comparative Example 4 360.4 93.5

[0075] As can be seen from Table 2, the initial discharge capacity and the initial charge-discharge efficiency of the lithium ion battery using the composite negative electrode materials obtained in Examples 1-3 are significantly higher than those of Comparative Examples 1-4. The reason is that the surface of the graphite is coated with an organic fast ion conductor composite material. The high lithium ion conductivity of its fast ion conductor is used to improve the intercalation and deintercalation of lithium ions. Moreover, the organic fast ion conductor has good compatibility with the organic lithium compound, which improves the formation quality of the SEI film, reduces the loss of the irreversible capacity of the material, and improves the initial efficiency.

[0076] Experimental Example 3 Soft Pack Battery Test

[0077] Using the graphite composites in Examples 1 - 3 and Comparative Examples 1 - 4 as the negative electrode materials, negative electrode sheets were prepared. Using a ternary material (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2) as the positive electrode, a LiPF6 solution (with a solvent of EC + DEC, volume ratio 1:1, LiPF6 concentration 1.3 mol / L) as the electrolyte, and celegard 2400 as the separator, 5 Ah soft pack batteries were respectively prepared. Then, the cycle performance and rate performance of the soft pack batteries were tested.

[0078] Cycle performance test conditions: charge - discharge current 1C / 1C, voltage range 2.8 - 4.2V, number of cycles 500 times.

[0079] The test results are shown in Table 3.

[0080] Table 3 Test Results of Soft Pack Batteries

[0081]

[0082]

[0083] It can be seen from Table 3 that the cycle performance of the graphite composites prepared in Examples 1 - 3, as the negative electrode of the soft pack battery, is better than that of Comparative Examples 1 - 4. The reason is that in terms of the 1C / 1C rate cycle performance, relying on the organic fast ion conductor on the graphite surface to provide sufficient lithium ions to improve the cycle performance, and at the same time, the fluorine gas modification forms a - C - F - chemical bond with the characteristic of structural stability, which improves the cycle performance, thereby increasing the service life of the soft pack battery.

[0084] In summary, the graphite composite prepared by the present invention has a high conductivity and tap density, and the graphite composite has a high initial discharge capacity and initial efficiency in the coin cell, and has good cycle performance in the soft pack battery.

[0085] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A long-life graphite composite material, characterized in that, The graphite composite material has a core-shell structure, with a fluorine-containing graphite as the core and an organic fast ion conductor as the shell; the mass of the shell is 1%-5% of the mass of the graphite composite material; The preparation method of the long-life graphite composite material includes the following steps: S1. Crush and shape the isotropic coke to obtain the precursor material A; S2. Add the precursor material A, inorganic fluoride, binder, and additive to the organic solvent, disperse evenly, and soak to obtain a mixture; S3. Grind and dry the mixture, and calcine to obtain the precursor material B; S4. Modify the precursor material B under a fluorine gas and argon gas atmosphere to obtain a modified precursor material; S5. Use the modified precursor material as the matrix and the organic fast ion conductor as the target material to perform atomic vapor deposition to obtain the graphite composite material; The additive in the step S2 includes one or more of lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, and lithium bis(trifluoromethanesulfonyl)imide.

2. The long-life graphite composite material according to claim 1, characterized in that The organic fast ion conductor includes one or more of lithium naphthenate, lithium acetate, lithium oxalate, lithium perfluorohexanesulfonate, lithium acetate, lithium stearate, lithium benzoate, lithium oxalate, and lithium stearate.

3. A long-life graphite composite material according to claim 1, characterized in that The particle size of the isotropic coke after crushing in the step S1 is 8-10 µm; the particle size distribution after shaping is (D90-D10) / D50 = 0.9-1.

2.

4. A long-life graphite composite material according to claim 1, characterized in that, The mass ratio of the precursor material A, inorganic fluoride, binder, additive, and organic solvent in the step S2 is 100:1-5:1-5:0.5-2:

500.

5. A long-life graphite composite material according to claim 1, characterized in that, The inorganic fluoride in the step S2 includes one or more of sodium fluoride, calcium fluoride, magnesium fluoride, and potassium fluoride; The organic solvent includes one or more of ethylene glycol, butanediol, cyclohexanol, and n-hexanol.

6. A long-life graphite composite material according to claim 1, characterized in that, The soaking in the step S2 is carried out under vacuum for 24-72 h.

7. A long-life graphite composite material according to claim 1, characterized in that, The calcination in the step S3 is carried out by heating to 200-400 °C in an inert atmosphere, and the calcination time is 0.5-1 h.

8. A long-life graphite composite material according to claim 1, characterized in that, The volume ratio of fluorine gas to argon gas in the step S4 is 1:10; the modification temperature is 700-1100 °C, and the modification time is 0.5-1 h.

9. A long-life graphite composite material according to claim 1, wherein, The conditions for chemical deposition in the step S5 are a temperature of 150-250 °C and a pressure of 0.1-0.5 Torr.

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