A fluorine-containing polymer solid electrolyte coated positive electrode material and its preparation method

Abstract

The present invention discloses a fluoropolymer solid electrolyte-coated positive electrode material and its preparation method. The method first reacts PFEC polymer monomer molecules with an initiator and a lithium salt at a certain temperature to form a mixture of FEC oligomers and lithium salts with active sites. The modified positive electrode material is then coated with a directing agent in an organic solvent and coated by liquid-phase heating and stirring, thereby obtaining a positive electrode material uniformly coated with the PFEC polymer solid electrolyte. The fluoropolymer solid electrolyte-coated positive electrode material obtained by this method exhibits high specific capacity utilization, good cycling performance, excellent electrochemical properties, and significantly improved thermal stability.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion battery positive electrode materials, and in particular to a fluorine-containing polymer solid electrolyte coated positive electrode material and a preparation method thereof. Background Art

[0002] In recent years, the electric vehicle industry has experienced rapid growth. With the increasing demand for electric vehicle range, NCM ternary materials have gradually become the mainstream of the market. However, the resulting safety issues of electric vehicles have also become increasingly prominent, with electric vehicle safety accidents occurring one after another. Therefore, the pursuit of a high-energy-density and highly safe battery system has become a constant goal for current battery manufacturers. Consequently, both industry and academia are very optimistic about the prospects of the semi-solid-state and all-solid-state battery industries, aiming to completely solve the energy density and safety issues of battery cells from a systemic perspective. To match the semi-solid-state and solid-state battery systems, the positive electrode material is generally a solid electrolyte-coated positive electrode material to improve the ionic conductivity and thermal stability of the positive electrode material.

[0003] In addition, uncoated ternary cathode materials have the following serious disadvantages: they cannot form a stable CEI film, produce a lot of gas, have serious side reactions with the electrolyte, have relatively low electronic and ionic conductivity, and for all-solid-state battery systems, the contact between the solid electrolyte and the cathode material is unstable, resulting in a large interfacial impedance. Therefore, coating the surface of the ternary cathode material with one or more layers of functional materials is an effective way to improve its performance.

[0004] Common coating methods include dry sintering and wet coating. High-temperature solid-phase sintering is an effective method for preparing all-solid-state batteries, achieving a close bond between the solid electrolyte and the cathode material. However, the disadvantage is that high-temperature heat treatment easily leads to element interdiffusion and interfacial reactions at the interface, resulting in high interfacial resistance. Furthermore, the limited solubility of polymer solid electrolytes in various polar aprotic organic solvents makes wet coating difficult. Summary of the Invention

[0005] Based on the technical problems existing in the background technology, the present invention proposes a fluorine-containing polymer solid electrolyte coated positive electrode material and a preparation method thereof.

[0006] The present invention provides a method for preparing a fluorine-containing polymer solid electrolyte coated positive electrode material, comprising the following steps:

[0007] S1, heating and stirring a precursor solution comprising fluoroethylene carbonate, a lithium salt, and an initiator to react to obtain a solution A comprising a FEC active oligomer and a lithium salt;

[0008] S2. Add the ternary cathode material and the silane coupling agent to a polar aprotic organic solvent, heat and stir to react, so that the silane coupling agent is coated on the surface of the ternary cathode material to obtain a mixture B, and then add the solution A to the mixture B, heat and stir to react, and obtain the product.

[0009] In S2, the added silane coupling agent can serve as a directional agent to form a certain hydrophobic directional layer on the surface of the positive electrode material, thereby enabling the hydrophobic polymer to be directionally coated on the surface of the positive electrode material.

[0010] Preferably, in S1, a precursor solution comprising fluoroethylene carbonate, lithium salt and initiator is heated and stirred at 60-100° C. for 4-12 hours to obtain solution A.

[0011] Preferably, in S2, the ternary cathode material and the silane coupling agent are added to a polar aprotic organic solvent, and heated and stirred at 60-100° C. for 4-12 h to obtain a mixture B.

[0012] In the present invention, the preparation of solution A and mixture B needs to be carried out under oxygen-free and water-free conditions.

[0013] Preferably, in S2, the solution A is added to the mixture B, and the mixture is heated and stirred at 60-100° C. for 12-24 hours to obtain a slurry containing a fluorine-containing polymer solid electrolyte coated positive electrode material.

[0014] Preferably, in S1, the precursor solution further comprises a polar aprotic organic solvent.

[0015] Preferably, in S1, the precursor solution comprises the following raw materials in percentage by mass: 15-25% lithium salt, 0.5-2% initiator, 0-50% polar aprotic organic solvent, and the balance being fluoroethylene carbonate.

[0016] The precursor solution can be prepared by conventional methods in the art. Preferably, the precursor solution is prepared by mixing the raw materials except the initiator and dissolving them fully, then adding the initiator and dissolving them fully to obtain the precursor solution.

[0017] Preferably, in S2, the mass ratio of the silane coupling agent to the ternary positive electrode material is (0.005-0.015):1.

[0018] Preferably, in S2, the mass ratio of the solution A to the ternary positive electrode material is (0.01-0.05):1.

[0019] Preferably, the lithium salt is at least one of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium difluorooxalatoborate (LiDFOB), lithium tetrafluoroborate (LiBF4), lithium hydroxide (LiOH), and lithium nitrate.

[0020] Preferably, the initiator is at least one of tin isooctanoate (Sn(OCT)2) and aluminum trifluoromethanesulfonate (Al(OTf)3).

[0021] Preferably, the silane coupling agent is an alkyl silane coupling agent. Preferably, the silane coupling agent is at least one of silane coupling agent G-570, silane coupling agent KH560, silane coupling agent KH550, n-octyltriethoxysilane, and n-octyltrimethoxysilane.

[0022] Preferably, the ternary cathode material is LiCoO2, LiFePO4, Li2MnO3, LiNi 1-x-y Co x Mn y At least one of O2, wherein 1 / 20≤x≤1 / 3, 1 / 20≤y≤1 / 3.

[0023] Preferably, the polar aprotic organic solvent is NMP.

[0024] A fluorine-containing polymer solid electrolyte coated positive electrode material is prepared by the preparation method.

[0025] Preferably, the fluorine-containing polymer solid electrolyte coated positive electrode material includes a ternary positive electrode material and a fluorine-containing polymer solid electrolyte coated on the surface of the ternary positive electrode material, the fluorine-containing polymer solid electrolyte includes a lithium salt and a fluorine-containing polymer, and the molecular weight of the fluorine-containing polymer is 20,000-50,000.

[0026] In the present invention, in order to obtain a fluorine-containing polymer solid electrolyte coated positive electrode material, the preparation method of the fluorine-containing polymer solid electrolyte coated positive electrode material may further include a conventional post-processing step.

[0027] For example: in S2, the reaction product is filtered, washed, and dried to obtain a fluorine-containing polymer solid electrolyte-coated positive electrode material.

[0028] In the present invention, the method for preparing a fluorinated polymer solid electrolyte-coated positive electrode material can also be used directly to prepare a lithium-ion battery positive electrode slurry without any post-processing steps. For example, a conventional positive electrode slurry additive can be added to the reaction product described in S2 to obtain a lithium-ion battery positive electrode slurry. This is because when the solvent used in this method is NMP, NMP can be directly used in the subsequent slurrying step. As the solvent used in the slurrying process, this not only saves solvent usage but also solves the solvent recovery and pollution issues of traditional liquid-phase coating methods.

[0029] The present invention also provides a lithium ion battery positive electrode slurry, comprising the fluorine-containing polymer solid electrolyte coated positive electrode material, a conductive agent and a binder.

[0030] A method for preparing a positive electrode slurry for a lithium-ion battery comprises the following steps:

[0031] S1, heating and stirring a precursor solution comprising fluoroethylene carbonate, a lithium salt, and an initiator to react to obtain a solution A comprising a FEC active oligomer and a lithium salt;

[0032] S2, adding the ternary cathode material and the silane coupling agent to a polar aprotic organic solvent, heating and stirring to react, so that the silane coupling agent is coated on the surface of the ternary cathode material to obtain a mixture B, then adding the solution A to the mixture B, heating and stirring to react, and obtaining a reaction product;

[0033] S3. Add a conductive agent and a binder to the reaction product and mix them evenly to obtain the product.

[0034] In S3, the method of adding a conductive agent and a binder to the reaction product and mixing them uniformly is a conventional method, for example: adding a conductive agent to the reaction product, mixing and stirring uniformly, then adding a binder, mixing and stirring uniformly to obtain a lithium-ion battery positive electrode slurry.

[0035] The conductive agent and the binder may be conventional ones in the art, for example, the conductive agent may be SP, and the binder may be PVDF. The amounts of the conductive agent and the binder are conventional amounts.

[0036] The technical principle of the present invention is:

[0037] The solid electrolyte after the polymer PFEC of fluoroethylene carbonate FEC and salt composite has a high conductivity of 10-4S / cm, is resistant to high voltage to 5.5V, and is non-flammable. It is a very excellent polymer solid electrolyte material. Therefore, the positive electrode material is coated with this polymer solid electrolyte material, which can effectively improve the contact internal resistance between the composite positive electrode material and the polymer solid electrolyte in the all-solid-state battery system, and can effectively improve the conductivity of the composite solid electrolyte, which is convenient for the rapid diffusion of lithium ions. However, the solubility of this composite polymer solid electrolyte in various organic solvents is relatively low, which restricts its application in wet coating. Therefore, in order to solve this bottleneck problem, the present invention adopts a solution of a precursor (i.e., an active FEC oligomer) and a lithium salt containing PFEC, which is then added to the positive electrode material coated with a directing agent molecule (i.e., a silane coupling agent) surface, and the active FEC oligomer will uniformly carry out in-situ self-polymerization on the positive electrode material under the action of the directing agent, thereby synthesizing a uniformly coated composite positive electrode material.

[0038] The beneficial effects of the present invention are as follows:

[0039] (1) The present invention first reacts the monomer molecules of the PFEC polymer with an initiator and a lithium salt at a certain temperature to form a mixed solution of FEC oligomers and lithium salts having active sites, and then coats the modified positive electrode material with a surface coating of a directing agent in an organic solvent by a liquid phase heating and stirring method, wherein the active FEC oligomer can be uniformly coated in a slurry containing the positive electrode material, and the silane coupling agent, as a directing agent, can undergo a dehydration condensation reaction on the hydroxyl group on the surface of the ternary positive electrode material, and the alkyl end of the silane coupling agent can form a hydrophobic layer. Due to the presence of the highly hydrophobic layer on the surface of the positive electrode material, the active FEC oligomer can be directed on the surface of the positive electrode material to undergo in-situ self-polymerization coating; by the above method, a positive electrode material uniformly coated with a PFEC polymer solid electrolyte can be obtained;

[0040] (2) The present invention adopts wet liquid phase coating, which solves the problem that high temperature heat treatment easily leads to element interdiffusion and interface reaction at the interface, resulting in high interface resistance. In addition, this operation method is simple and convenient, and it is easy to form a uniformly coated positive electrode material, solving the problem of uneven coating in dry method;

[0041] (3) The polymer composite solid electrolyte coated positive electrode material prepared by the present invention has a relatively uniform coating effect, and the coating thickness can be controlled by the amount of active FEC oligomer added. After coating, the ionic conductivity of the positive electrode material is significantly improved, the gram capacity loss is small, the cycle performance is effectively improved, and the thermal stability is significantly enhanced, which can improve the safety of the ternary positive electrode material; this polymer composite solid electrolyte coated positive electrode material can also be used in combination with this polymer composite solid electrolyte to effectively reduce the interface impedance;

[0042] (4) The preparation method of the present invention has low energy consumption, simple and convenient process, and the solvent in the wet process is NMP, which can be directly used in the positive electrode material slurry, solving the problems of solvent recovery and pollution in the traditional liquid phase coating method. DETAILED DESCRIPTION

[0043] The technical solution of the present invention is described in detail below through specific embodiments.

[0044] Example 1

[0045] Preparation of fluorinated polymer solid electrolyte coated cathode material:

[0046] S1. 0.5 g LiDFOB, 1.5 g LiFSI, and 8 g FEC were mixed and dissolved by ultrasonication. Then, 0.1 g Sn(Oct)2 was added and dissolved by ultrasonication stirring to obtain a precursor solution. The precursor solution was heated and stirred at 80°C for 4 h to obtain solution A.

[0047] S2. Add 0.15 g of silane coupling agent KH560 and 10 g of high-nickel ternary 811 positive electrode material to 10 g of NMP solvent, heat and stir at 80° C. for 4 h to obtain mixture B, add 0.1 g of solution A to mixture B, heat and stir at 80° C. for 12 h to obtain a reaction product.

[0048] Preparation of lithium-ion battery cathode slurry:

[0049] 1.25 g of conductive agent SP was added to the reaction product obtained above, and the mixture was mixed and stirred evenly. Then, 1.25 g of binder PVDF was added, and the mixture was mixed and stirred evenly to obtain a positive electrode slurry for a lithium-ion battery.

[0050] Example 2

[0051] Preparation of fluorinated polymer solid electrolyte coated cathode material:

[0052] S1. 0.5 g LiDFOB, 1.5 g LiFSI, and 8 g FEC were mixed and dissolved by ultrasonication. Then, 0.1 g Sn(Oct)2 was added and dissolved by ultrasonication stirring to obtain a precursor solution. The precursor solution was heated and stirred at 80°C for 4 h to obtain solution A.

[0053] S2. Add 0.15 g of silane coupling agent KH560 and 10 g of high-nickel ternary 811 positive electrode material to 10 g of NMP solvent, heat and stir at 80°C for 4 h to obtain mixture B, add 0.3 g of solution A to mixture B, heat and stir at 80°C for 12 h to obtain a reaction product.

[0054] The method for preparing the positive electrode slurry for lithium-ion batteries is the same as that in Example 1.

[0055] Example 3

[0056] Preparation of fluorinated polymer solid electrolyte coated cathode material:

[0057] S1. 0.5 g LiDFOB, 1.5 g LiFSI, and 8 g FEC were mixed and dissolved by ultrasonication. Then, 0.1 g Sn(Oct)2 was added and dissolved by ultrasonication stirring to obtain a precursor solution. The precursor solution was heated and stirred at 80°C for 4 h to obtain solution A.

[0058] S2. Add 0.15 g of silane coupling agent KH560 and 10 g of high-nickel ternary 811 positive electrode material to 10 g of NMP solvent, heat and stir at 80° C. for 4 h to obtain mixture B, add 0.5 g of solution A to mixture B, heat and stir at 80° C. for 12 h to obtain a reaction product.

[0059] The method for preparing the positive electrode slurry for lithium-ion batteries is the same as that in Example 1.

[0060] Example 4

[0061] Preparation of fluorinated polymer solid electrolyte coated cathode material:

[0062] S1. 0.5 g LiDFOB, 1 g LiFSI, 3.45 g FEC, and 5 g NMP were mixed and dissolved by ultrasonication. Then, 0.05 g Sn(Oct)2 was added and dissolved by ultrasonication to obtain a precursor solution. The precursor solution was heated and stirred at 80°C for 4 h to obtain solution A.

[0063] S2. Add 0.05 g of silane coupling agent KH560 and 10 g of high-nickel ternary 811 positive electrode material to 10 g of NMP solvent, heat and stir at 60° C. for 12 h to obtain mixture B, add 0.5 g of solution A to mixture B, heat and stir at 60° C. for 24 h to obtain a reaction product.

[0064] The method for preparing the positive electrode slurry for lithium-ion batteries is the same as that in Example 1.

[0065] Comparative Example 1

[0066] 10 g of high-nickel ternary 811 cathode material was added to 10 g of NMP solvent to obtain a slurry containing the cathode material.

[0067] Preparation of lithium-ion battery cathode slurry:

[0068] 1.25 g of conductive agent SP was added to the slurry containing the positive electrode material prepared above, and the mixture was mixed and stirred evenly. Then, 1.25 g of binder PVDF was added, and the mixture was mixed and stirred evenly to obtain a positive electrode slurry for a lithium ion battery.

[0069] Test example

[0070] The lithium-ion battery cathode slurries prepared in Examples 1-3 and Comparative Example 1 were coated and roll-pressed to produce positive electrodes. These positive electrodes were then assembled with a separator (Cellgard 2400), a negative electrode (metallic lithium), and an electrolyte to produce CR2025 button cells. The batteries were tested at 25°C at rates of 0.1C, 0.2C, 0.5C, 1C, and 3C over a voltage range of 3.0-4.3V. The results are shown in Table 1.

[0071] Table 1 Electrical performance test results of button batteries

[0072]

[0073] The above assembled battery was subjected to DSC thermal stability test, and the results are shown in Table 2:

[0074] Table 2 DSC thermal stability test results of materials

[0075] sample Initial decomposition temperature ℃ Maximum decomposition temperature ℃ <![CDATA[Heat release amount / Jg -1 > Example 1 87.5 222 -249 Example 2 85.3 222 -191 Example 3 87.0 226 -187 Unwrapped 84.6 212 -538

[0076] It can be seen from the characterization results in Tables 1 and 2 that the present invention can fully solve the problem of polymer electrolyte insolubility in organic solvents by using active FEC oligomer precursors to carry out in-situ repolymerization coating on the positive electrode material, and can be coated by a wet method. In addition, the coating material of the present invention has good electrochemical performance, good rate performance, relatively high first effect, and small loss in gram capacity. And it can be seen from Table 2 that the thermal stability of the material can be significantly improved by coating the positive electrode material. And from the comprehensive performance evaluation, the thermal stability improvement of Example 3 is the most prominent, and the electrochemical performance is also relatively normal, which is the optimal coating amount.

[0077] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing a fluorine-containing polymer solid electrolyte coated positive electrode material, characterized in that: The following steps are involved: S1, heating and stirring a precursor solution comprising fluoroethylene carbonate, a lithium salt, and an initiator to react to obtain a solution A comprising a FEC active oligomer and a lithium salt; S2. Add the ternary cathode material and the silane coupling agent to a polar aprotic organic solvent, heat and stir to react, so that the silane coupling agent is coated on the surface of the ternary cathode material to obtain a mixture B, and then add the solution A to the mixture B, heat and stir to react, and obtain a reaction product.

2. The method for preparing a fluorine-containing polymer solid electrolyte coated positive electrode material according to claim 1, characterized in that: In S1, a precursor solution comprising fluoroethylene carbonate, a lithium salt, and an initiator is heated and stirred at 60-100° C. for 4-12 hours to obtain a solution A; The precursor solution comprises the following raw materials in percentage by mass: 15-25% of lithium salt, 0.5-2% of initiator, 0-50% of polar aprotic organic solvent, and the balance being fluoroethylene carbonate.

3. The method for preparing a fluorine-containing polymer solid electrolyte coated positive electrode material according to claim 1, characterized in that: In S2, the ternary cathode material and the silane coupling agent are added to a polar aprotic organic solvent, and heated and stirred at 60-100° C. for 4-12 hours to obtain a mixture B.

4. The method for preparing a fluorine-containing polymer solid electrolyte coated positive electrode material according to claim 1, characterized in that: In S2, the solution A is added to the mixture B, and the mixture is heated and stirred at 60-100° C. for 12-24 hours to obtain a reaction product.

5. The method for preparing a fluorine-containing polymer solid electrolyte coated positive electrode material according to claim 1, characterized in that: In S2, the mass ratio of the silane coupling agent to the ternary positive electrode material is (0.005-0.015):1; in S2, the mass ratio of the solution A to the ternary positive electrode material is (0.01-0.05):

1.

6. The method for preparing a fluorine-containing polymer solid electrolyte coated positive electrode material according to claim 1, characterized in that: The lithium salt is at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorooxalatoborate, lithium tetrafluoroborate, lithium hydroxide, and lithium nitrate; the initiator is at least one of tin isooctanoate and aluminum trifluoromethanesulfonate; the silane coupling agent is at least one of silane coupling agent G-570, silane coupling agent KH560, silane coupling agent KH550, n-octyltriethoxysilane, and n-octyltrimethoxysilane; the ternary positive electrode material is LiNi 1-x-y Co x Mn y O2, wherein 1 / 20≤x≤1 / 3, 1 / 20≤y≤1 / 3; the polar aprotic organic solvent is NMP.

7. A fluorine-containing polymer solid electrolyte coated positive electrode material, characterized in that: Prepared by the preparation method according to any one of claims 1 to 6.

8. The fluorine-containing polymer solid electrolyte coated positive electrode material according to claim 7, characterized in that: The invention comprises a ternary positive electrode material and a fluorine-containing polymer solid electrolyte coated on the surface of the ternary positive electrode material. The fluorine-containing polymer solid electrolyte comprises a lithium salt and a fluorine-containing polymer. The molecular weight of the fluorine-containing polymer is 20,000-50,000.

9. A positive electrode slurry for a lithium ion battery, characterized in that: The invention comprises the fluorine-containing polymer solid electrolyte coated positive electrode material according to claim 7, a conductive agent and a binder.

10. A method for preparing a positive electrode slurry for a lithium-ion battery according to claim 9, characterized in that: The following steps are involved: S1, heating and stirring a precursor solution comprising fluoroethylene carbonate, a lithium salt, and an initiator to react to obtain a solution A comprising a FEC active oligomer and a lithium salt; S2, adding the ternary cathode material and the silane coupling agent to a polar aprotic organic solvent, heating and stirring to react, so that the silane coupling agent is coated on the surface of the ternary cathode material to obtain a mixture B, then adding the solution A to the mixture B, heating and stirring to react, and obtaining a reaction product; S3. Add a conductive agent and a binder to the reaction product and mix them evenly to obtain the product.

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