A lithium salt / organic phosphorus compound double-coated graphite material and a preparation method thereof

Abstract

The application discloses a kind of lithium salt / organic phosphorus compound double-shell coated graphite materials and preparation method.It presents core-shell structure, the core of composite material is artificial graphite, shell is inorganic lithium salt layer and organic phosphorus compound layer in turn from inside to outside.Lithium salt is deposited on the surface of artificial graphite by electrochemical deposition after adding oxidized artificial graphite into inorganic lithium salt, filtration, carbonization, and inorganic lithium salt coated artificial graphite material is obtained;After adding into organic phosphorus salt compound, it is uniformly dispersed, spray drying, and lithium salt / organic phosphorus compound double-shell coated artificial graphite composite material is obtained.The composite material uses inorganic lithium salt coating to reduce material surface defects, can prevent solvent co-embedding, improve the first efficiency of material, prolong cycle life;And outer layer is coated with organic phosphorus compound, when battery appears thermal runaway, impedance increases, prevents thermal runaway, and improves safety performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, specifically to a lithium salt / organophosphorus compound double-shell coated graphite material and its preparation method. Background Technology

[0002] With increasing market demands for fast charging, energy density, and safety performance of lithium-ion batteries, anode materials must not only possess fast charging capabilities but also ensure material safety. Currently, commercially available artificial graphite primarily improves fast charging performance by coating the material surface with amorphous carbon. However, due to the high reactivity of amorphous carbon, lithium plating occurs during high-rate charging, posing a safety hazard. While simply applying alumina to the material surface can improve safety, alumina itself lacks capacity and has high impedance, reducing energy density, fast charging performance, and cycle life. Therefore, it is impossible to simultaneously achieve improvements in safety, fast charging performance, and cycle life. Summary of the Invention

[0003] To improve the initial efficiency and safety performance of graphite materials, while also considering fast charging and cycle life, this invention reduces surface defects by coating the material surface with an inorganic lithium salt, thereby increasing initial efficiency and extending cycle life. Furthermore, coating the outer layer with an organophosphorus compound increases impedance during thermal runaway, preventing it and improving safety. A method for preparing a lithium salt / organophosphorus compound double-shell coated graphite material is also provided.

[0004] A lithium salt / organophosphorus compound double-shell coated graphite material is disclosed. The composite material exhibits a core-shell structure, with the core graphite being artificial graphite. The outer shell consists of a first outer shell inorganic lithium salt layer and a second outer shell organophosphorus compound layer, arranged from the inside out. Based on a composite material mass ratio of 100%, the first outer shell and the second outer shell account for 2-8% and 2-8% of the total mass, respectively.

[0005] A method for preparing a lithium salt / organophosphorus compound double-shell coated graphite material, characterized by comprising the following steps:

[0006] Step S1:

[0007] According to the mass ratio of artificial graphite: binder = 90-95: 5-10, the oxidized artificial graphite and binder are mixed evenly, heated and pressed onto a stainless steel plate, and used as a working electrode.

[0008] According to the mass ratio of coupling agent: organic solvent: inorganic lithium salt = 1-5: 1000: 50-200, the coupling agent is added to the organic solvent and dispersed evenly. Then, inorganic lithium salt is added to the mixture to obtain an inorganic lithium salt solution.

[0009] Then, by cyclic voltammetry, under the conditions of voltage range -2V to 2V and scan rate of 0.5-5mV / s, the working electrode was added to an inorganic lithium salt solution, with saturated calomel as the reference electrode, and lithium salt was deposited on the surface of the working electrode for 30-300 min. After washing with deionized water, it was vacuum dried at 80℃ for 24 h. The active material was then peeled off from the stainless steel plate, crushed, and then carbonized at 700-1000℃ for 1-6 h in an inert atmosphere to obtain inorganic lithium salt coated artificial graphite material.

[0010] Step S2:

[0011] According to the mass ratio of organophosphorus salt compound: dispersant: organic solvent: inorganic lithium salt coated artificial graphite = 1-5: 0.5-2: 100-500: 100, the organophosphorus salt compound and dispersant are added to the organic solvent and mixed evenly. Then, the inorganic lithium salt coated artificial graphite material is added, dispersed evenly, and spray-dried to obtain a lithium salt / organophosphorus compound double-shell coated graphite material.

[0012] In step S1, the inorganic lithium salt is one or more of lithium zirconate, lithium titanate, lithium niobate, lithium cerium oxide, and lithium borate; the solvent is one or more of ethanol, chloroform, methanol, acetone, tetrahydrofuran, trichloroethane, and carbon tetrachloride; and the coupling agent is one or more of 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, and γ-(2,3-epoxypropoxy)propyltrimethoxysilane (KH-560).

[0013] The adhesive in step S1 is one or more of sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyacrylonitrile.

[0014] In step S2, the organophosphate compound is one or more of triphenyl phosphate, tetraphenyl bisphenol A diphosphate, and tricresyl phosphate; the dispersant is one or more of sodium dodecyl sulfonate, polyethylene glycol, triethylhexyl phosphate, methylpentanol, polyacrylamide, and polyethylene glycol fatty acid ester; and the organic solvent is one or more of diethyl ether, benzene, acetone, and chloroform.

[0015] Beneficial effects:

[0016] 1) By coating the surface of the material with organophosphorus compounds, the flame-retardant properties of the organophosphorus compounds increase the impedance when the battery experiences thermal runaway, thereby preventing thermal runaway and improving safety performance; at the same time, the chemical method of coating the surface with organophosphorus compounds has the characteristics of good coating uniformity.

[0017] 2) Electrochemical deposition is used to deposit inorganic lithium salts on the graphite surface to improve the ionic conductivity and initial efficiency of the material, and has the advantages of good deposition uniformity and high efficiency. Attached Figure Description

[0018] Figure 1 The image shows a SEM image of the lithium salt / organophosphorus compound double-shell coated graphite material prepared in Example 1. Detailed Implementation

[0019] Example 1

[0020] A method for preparing a lithium salt / organophosphorus compound double-shell coated graphite material includes the following steps:

[0021] Step S1:

[0022] 92g of artificial graphite and 8g of sodium carboxymethyl cellulose were mixed evenly and pressed onto a stainless steel plate at a temperature of 80℃ and a pressure of 2 tons, and used as a working electrode.

[0023] Add 3g of 3-aminopropyltrimethoxysilane to 100g of chloroform organic solvent and disperse evenly. Then add 100g of lithium zirconate to the mixture to obtain an inorganic lithium salt solution.

[0024] Subsequently, using cyclic voltammetry, under conditions of -2V to 2V and a scan rate of 1mV / s, the working electrode was added to an inorganic lithium salt solution, with saturated calomel as the reference electrode. Lithium salt was deposited on the surface of the working electrode for 90 minutes. Afterwards, it was washed with deionized water and vacuum dried at 80℃ for 24 hours. The active material was then peeled off from the stainless steel plate, pulverized, and then carbonized at 850℃ for 3 hours under an argon inert atmosphere to obtain inorganic lithium salt-coated artificial graphite material; Step S2:

[0025] 3g of triphenyl phosphate and 1g of sodium dodecyl sulfonate were added to 300g of diethyl ether organic solvent and mixed evenly. Then, 100g of inorganic lithium salt-coated artificial graphite material was added, dispersed evenly, and spray-dried to obtain a lithium salt / organophosphorus compound double-shell coated graphite material.

[0026] Example 2

[0027] A method for preparing a lithium salt / organophosphorus compound double-shell coated graphite material includes the following steps:

[0028] Step S1:

[0029] 90g of artificial graphite and 10g of styrene-butadiene rubber were mixed evenly and pressed onto a stainless steel plate at a temperature of 80℃ and a pressure of 2 tons, and used as a working electrode.

[0030] 1g of 3-aminopropyltriethoxysilane was added to 1000g of methanol organic solvent and dispersed evenly. Then, 50g of lithium niobate was added to the mixture to obtain an inorganic lithium salt solution.

[0031] Subsequently, using cyclic voltammetry, under conditions of -2V to 2V and a scan rate of 0.5mV / s, the working electrode was added to an inorganic lithium salt solution, with saturated calomel as the reference electrode. Lithium salt was deposited on the surface of the working electrode for 30 minutes, followed by washing with deionized water and vacuum drying at 80℃ for 24 hours. The active material was then peeled off from the stainless steel plate, pulverized, and then carbonized at 700℃ for 6 hours under an argon inert atmosphere to obtain inorganic lithium salt-coated artificial graphite material; Step S2:

[0032] 1g of tetraphenylbisphenol A diphosphate and 0.5g of polyethylene glycol were added to 100g of chloroform organic solvent and mixed evenly. Then, 100g of inorganic lithium salt-coated artificial graphite material was added, dispersed evenly, and spray-dried to obtain a lithium salt / organophosphorus compound double-shell coated artificial graphite composite material.

[0033] Example 3

[0034] A method for preparing a lithium salt / organophosphorus compound double-shell coated graphite material includes the following steps:

[0035] Step S1:

[0036] 95g of artificial graphite and 5g of polyacrylonitrile were mixed evenly and pressed onto a stainless steel plate at a temperature of 80℃ and a pressure of 2 tons, and used as a working electrode.

[0037] 5g of N-2-(aminoethyl)-3-aminopropyltrimethoxysilane was added to 1000g of tetrahydrofuran organic solvent and dispersed evenly. Then, 200g of lithium cerate was added to the mixture to obtain an inorganic lithium salt solution.

[0038] Then, by cyclic voltammetry, under the conditions of voltage range -2V to 2V and scan rate of 5mV / s, the working electrode was added to the inorganic lithium salt solution, and lithium salt was deposited on the surface of the working electrode for 300 min. After washing with deionized water, it was vacuum dried at 80℃ for 24 h. Then, the active material was peeled off from the stainless steel plate, crushed, and then carbonized at 1000℃ for 1 h under an argon inert atmosphere to obtain inorganic lithium salt coated artificial graphite material.

[0039] Step S2:

[0040] 5g of tricresyl phosphate and 2g of triethylhexyl phosphate were added to 500g of acetone organic solvent and mixed evenly. Then, 100g of inorganic lithium salt-coated artificial graphite material was added, dispersed evenly, and spray-dried to obtain a lithium salt / organophosphorus compound double-shell coated artificial graphite composite material.

[0041] Comparative Example 1:

[0042] Unlike Example 1, the artificial graphite material is not coated with inorganic lithium salt; the detailed preparation process is as follows: 5g of tricresyl phosphate and 2g of triethylhexyl phosphate are added to 500g of acetone organic solvent and mixed evenly, then 100g of artificial graphite material is added, dispersed evenly, and spray-dried to obtain an organophosphorus compound coated artificial graphite composite material.

[0043] Comparative Example 2:

[0044] Unlike Example 1, phosphorus coating is not performed; instead, the inorganic lithium salt-coated artificial graphite material from step S2 of Example 1 is used as the negative electrode.

[0045] Experimental Example

[0046] (1) SEM testing

[0047] The lithium salt / organophosphorus compound double-shell coated graphite material prepared in Example 1 was subjected to SEM testing, and the results are as follows: Figure 1 As shown. By Figure 1 It can be seen that the material exhibits a secondary granular structure with a particle size between 10-15 μm.

[0048] (2) Physicochemical performance testing

[0049] The conductivity, tap density, specific surface area, and particle size D50 of the graphite composite anode materials in Examples 1-3 and Comparative Examples 1-2 were tested according to the test methods in standard GB / T-24533-2019 "Graphite Anode Materials for Lithium-ion Batteries". The test results are shown in Table 1.

[0050] Table 1

[0051] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Electrical conductivity (cm / S) <![CDATA[4.2*10 -9 ]]> <![CDATA[2.1*10 -9 ]]> <![CDATA[7.5*10 -9 ]]> <![CDATA[3.7*10 -10 ]]> <![CDATA[1.2*10 -10 ]]> <![CDATA[Tap density (g / cm 3 )]]> 1.00 0.98 1.03 0.90 0.93 <![CDATA[Specific surface area (m 2 / g)]]> 2.4 2.1 2.6 1.3 1.5 Particle size D50 (μm) 13.2 13.1 13.5 14.1 14.3

[0052] As can be seen from Table 1, the conductivity of the lithium salt / organophosphorus compound double-shell coated graphite materials prepared in Examples 1-3 is significantly better than that of the comparative examples. This is because the inorganic lithium salt is deposited on the graphite surface by electrochemical deposition to improve the ionic conductivity of the material. At the same time, phosphorus doping creates pores to increase the specific surface area of ​​the material, increases the lithium ion insertion and extraction channels during charging and discharging, and improves the rate performance.

[0053] (3) Button cell battery test

[0054] The lithium salt / organophosphorus compound double-shell coated graphite materials prepared in Examples 1-3 and the graphite composite anode materials of the comparative examples were assembled into coin cells according to the following method: The graphite composite anode materials prepared in Examples 1-3 and Comparative Examples 1-2 were used as anodes and assembled into coin cells with lithium sheets, electrolytes, and separators in a glove box with argon and water contents both below 0.1 ppm. The separator was Celgard 2400; the electrolyte was a LiPF6 solution with a LiPF6 concentration of 1 mol / L, and the solvent was a mixed solution obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DMC) in a weight ratio of 1:1.

[0055] The fabricated coin cells were labeled A-1, B-1, C-1, D-1, and E-1, respectively. The performance of the coin cells was then tested using a blue-light battery tester under the following conditions: 0.1C charge / discharge rate, voltage range of 0.005-2V, cycling for 3 cycles, followed by testing the discharge specific capacity at 1C. The 1C / 0.1C rate performance and cycle performance (25±3℃, 0.2C / 0.2C, 100 cycles) were calculated. The test results are shown in Table 2.

[0056] Table 2

[0057]

[0058] As shown in Table 2, the coin cells using the lithium salt / organophosphorus compound double-shell coated artificial graphite composite material of Examples 1-3 exhibit significantly higher discharge capacity and initial efficiency than those of Comparative Examples 1-2. The experimental results indicate that this is because coating the material surface with lithium salt reduces its irreversible capacity and improves the initial efficiency, while phosphorus doping creates pores to enhance the material's lithium storage performance and increase its specific discharge capacity.

[0059] (4) Performance testing of pouch batteries

[0060] Using the lithium salt / organophosphorus compound double-shell coated artificial graphite composite materials of Examples 1-3 and Comparative Examples 1-2 as negative electrode active materials, and the positive electrode active material ternary material (LiNi) 1 / 3 Co 1 / 3 Mn 1 / 3 A 5Ah pouch cell was assembled from O2, electrolyte, and separator. The separator was Celgard 2400, and the electrolyte was a LiPF6 solution (a 1:1 volume ratio mixture of EC and DEC, with a LiPF6 concentration of 1.3 mol / L). The fabricated pouch cells were labeled A-2, B-2, C-2, D-2, and E-2, respectively, and their cycle life, rate performance, and safety performance were tested. The test results are detailed in Table 3.

[0061] 1) Cyclic performance: The battery's 500-cycle performance was tested at a charge / discharge rate of 1C / 1C, a voltage range of 2.8V-4.2V, and a temperature of 25±3℃.

[0062] 2) Rate performance: At a rate of 2C, the battery is charged to 100% SOC using constant current + constant voltage mode. Then, the constant current ratio is calculated as constant current capacity / (constant current capacity + constant voltage capacity).

[0063] 3) Safety performance: In accordance with the method of the FreedomCAR Battery Test Manual, 10 batteries were taken respectively to prepare batteries for the example and comparative examples, and nail penetration test was carried out to test the nail penetration pass rate of the batteries.

[0064] Table 3

[0065]

[0066] Table 3 shows the cycle performance curves of the soft-pack batteries prepared from the obtained graphite composite materials. As can be seen from the table, the cycle performance of the batteries in the examples is significantly better than that of the comparative examples. This is because the inorganic lithium salt coating on the surface of the graphite composite materials in the examples reduces lithium ion consumption during charging and discharging and has high liquid retention properties, thus improving their cycle performance. Simultaneously, the high electronic conductivity and good rate performance of the materials in the examples enhance their rate performance. Furthermore, the organic phosphorus compound coating on the surface of the materials in the examples, relying on its inherent flame-retardant properties, increases impedance when the battery experiences short-circuit thermal runaway, preventing thermal runaway and improving safety performance such as nail penetration resistance.

[0067] Although preferred embodiments of the present invention have been described above in conjunction with the accompanying drawings, they are not intended to limit the scope of protection of the present invention. Various modifications and variations can be made to the present invention by those skilled in the art, and any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention are permitted.

Claims

1. A method for preparing a lithium salt / organophosphorus compound double-shell coated graphite material, characterized in that, The lithium salt / organophosphorus compound double-shell coated graphite material exhibits a core-shell structure. The core graphite is artificial graphite, and the outer shell consists of a first inorganic lithium salt layer and a second organophosphorus compound layer, arranged from the inside out. Based on a composite material mass percentage of 100%, the first and second outer shells account for 2-8% and 2-8% respectively. The preparation method includes the following steps: Step S1: Artificial graphite and binder are mixed evenly according to the mass ratio of artificial graphite: binder = 90-95: 5-10, and then heated and pressed onto a stainless steel plate to serve as a working electrode. The coupling agent is added to organic solvent A at a mass ratio of 1-5:1000:50-200 and dispersed evenly. Then, inorganic lithium salt is added to the mixture to obtain an inorganic lithium salt solution. The inorganic lithium salt is one or more of lithium zirconate, lithium titanate, lithium niobate, lithium cerium oxide, and lithium borate. Subsequently, using cyclic voltammetry, under conditions of -2V to 2V and a scan rate of 0.5-5mV / s, the working electrode was added to an inorganic lithium salt solution, with saturated calomel as the reference electrode. Lithium salt was deposited on the surface of the working electrode for 30-300 min. After washing with deionized water, the electrode was vacuum dried at 80℃ for 24 h. The active material was then peeled off from a stainless steel plate, pulverized, and carbonized at 700-1000℃ for 1-6 h under an inert atmosphere to obtain inorganic lithium salt-coated artificial graphite material. Step S2: According to the mass ratio of organophosphorus compound: dispersant: organic solvent B: inorganic lithium salt coated artificial graphite material = 1-5: 0.5-2: 100-500: 100, the organophosphorus compound and dispersant are added to organic solvent B and mixed evenly. Then, the inorganic lithium salt coated artificial graphite material is added and dispersed evenly, followed by spray drying to obtain a lithium salt / organophosphorus compound double-shell coated artificial graphite composite material. The organophosphorus compound is one or more of triphenyl phosphate, tetraphenylbisphenol A diphosphate, and tricresyl phosphate.

2. The method for preparing a lithium salt / organophosphorus compound double-shell coated graphite material according to claim 1, characterized in that, In step S1, the organic solvent A is one or more of ethanol, chloroform, methanol, acetone, tetrahydrofuran, trichloroethane, and carbon tetrachloride; the coupling agent is one or more of 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, and γ-(2,3-epoxypropoxy)propyltrimethoxysilane.

3. The method for preparing a lithium salt / organophosphorus compound double-shell coated graphite material according to claim 1, characterized in that, The adhesive in step S1 is one or more of sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyacrylonitrile.

4. The method for preparing a lithium salt / organophosphorus compound double-shell coated graphite material according to claim 1, characterized in that, In step S2, the dispersant is one or more of sodium dodecyl sulfonate, polyethylene glycol, methyl pentanol, polyacrylamide, and polyethylene glycol fatty acid ester; the organic solvent B is one or more of ether, benzene, acetone, and chloroform.

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