A composite conductive agent, its preparation method and application

Abstract

The present invention discloses a composite conductive agent and a preparation method and application thereof. The composite conductive agent includes a conductive agent and a lithium-conductive plastic crystal, and the lithium-conductive plastic crystal is coated on the surface of the conductive agent. The lithium-conductive plastic crystal is a mixture of plastic crystal and lithium salt, and the mass ratio of plastic crystal to the lithium salt is (2-6):1. The composite conductive agent of the present invention coats the surface of the conductive agent particles with plastic lithium-conductive plastic crystals, so that the conductive agent has the functions of conducting electrons and lithium ions at the same time, and is beneficial to improve the contact interface in the positive electrode sheet of the battery, thereby improving the first effect and charge and discharge efficiency of the positive electrode active material. The composite conductive agent is prepared by a melt mixing method. Compared with the traditional solvent method, it avoids the dissolution of lithium ions and the side reaction of the solvent and the sulfide electrolyte, and exhibits better first effect and charge and discharge performance. The first charge specific capacity of the all-solid-state battery is 225.15-233.13%, and the first coulomb efficiency is 81.05-89.41%.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and particularly relates to a composite conductive agent, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium batteries with liquid / gel electrolytes have been widely used in commercial electronic devices. However, in large-scale applications such as electric vehicles and smart grids, their disadvantages such as flammability, volatility, thermal instability, narrow electrochemical window, and difficulty in suppressing the growth of lithium dendrites limit the safety and energy density of traditional liquid batteries. Therefore, it is necessary to develop a new electrolyte for the next-generation batteries.

[0003] All-solid-state batteries use thermally stable solid electrolytes such as sulfides, halides, and oxides to replace liquid electrolytes. Among them, sulfides have an electrical conductivity close to that of liquid electrolytes and have advantages such as soft texture, non-corrosiveness to equipment, and environmental friendliness, thus attracting wide attention. However, there is still room for improvement in the first-cycle charge-discharge efficiency. The capacity loss of the cathode material mainly occurs due to the structural change after the first discharge, and the lithium-insertable positions in the material decrease.

[0004] In order to alleviate the pulverization of the high-nickel NCM (nickel, cobalt, manganese) cathode material caused by volume change during cycling, currently, increasing the test pressure or adding an oxide / halide interlayer is mainly used to solve this problem. However, the increased test pressure also promotes the growth of lithium dendrites to a certain extent and penetrates the electrolyte to cause battery short circuit. At the same time, oxides / halides also have problems such as insufficient ionic conductivity, which reduces the cathode potential to a certain extent.

[0005] Therefore, it is urgent to develop a new material or method to further improve the first-cycle charge-discharge efficiency (first efficiency) and rate performance of solid-state batteries. Summary of the Invention

[0006] The present invention aims to solve at least one of the technical problems existing in the above prior art. For this purpose, the present invention provides a composite conductive agent, a preparation method thereof, and an application thereof. The composite conductive agent uses a lithium-conducting plastic crystal to coat the conductive agent, which can greatly increase the contact area between the sulfide electrolyte and the cathode active material on the cathode side, thereby improving the mass transfer efficiency, and further improving the first efficiency and rate performance of the cathode.

[0007] The inventive concept of the present invention is as follows: The present invention coats the conductive agent particles with a plastic lithium-conducting plastic crystal, enabling the conductive agent to simultaneously possess the functions of conducting electrons and lithium ions, which is beneficial to improving the contact interface in the battery cathode sheet, thereby enhancing the initial efficiency and charge-discharge efficiency of the cathode active material, improving the cycle stability of the high-nickel NCM battery, as well as the structural stability and rate performance of the sulfide solid-state battery at high temperatures. At the same time, the composite conductive agent of the present invention coats the conductive agent with a plastic lithium-conducting plastic crystal. On the positive electrode side, it is beneficial to increase the contact area between the sulfide electrolyte and the positive electrode active material, improve the mass transfer efficiency, and further enhance the initial efficiency and rate performance of the positive electrode. In addition, the composite conductive agent of the present invention is prepared by a melt mixing method. Compared with the traditional solvent method, it avoids the dissolution of lithium ions and the side reaction between the solvent and the sulfide electrolyte. Therefore, it exhibits better initial efficiency and charge-discharge performance.

[0008] To solve the above technical problems, a first aspect of the present invention provides a composite conductive agent, which includes a conductive agent and a plastic lithium-conducting plastic crystal. The plastic lithium-conducting plastic crystal coats the surface of the conductive agent. The plastic lithium-conducting plastic crystal is a mixture of a plastic crystal and a lithium salt, and the mass ratio of the plastic crystal to the lithium salt is (2-6):1.

[0009] Research has found that the content of the lithium salt in the plastic lithium-conducting plastic crystal should not be too high. Excessive lithium salt will reduce the dissociation ability of the plastic crystal to the lithium salt, which is not conducive to the initial efficiency and charge-discharge performance of the battery.

[0010] Preferably, the plastic crystal is selected from at least one of N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (P 12 FSI), N-ethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P 12 TFSI), N-ethyl-N-methylpiperidinium bis(fluorosulfonyl)imide (PP 12 FSI), N-ethyl-N-methylpiperidinium bis(trifluoromethanesulfonyl)imide (PP 12 TFSI), N-propyl-N-methylpiperidinium bis(fluorosulfonyl)imide (PP 13 FSI), N-propyl-N-methylpiperidinium bis(trifluoromethanesulfonyl)imide (PP 13 TFSI), triethyl(methyl)phosphonium bis(fluorosulfonyl)imide (P 1222 FSI), triethyl(methyl)phosphonium bis(trifluoromethanesulfonyl)imide (P 1222 TFSI), N,N-dimethylpyrrolidinium bis(fluorosulfonyl)imide (P 11 FSI), N,N-dimethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P 11 TFSI).

[0011] Preferably, the lithium salt is lithium bis(trifluoromethanesulfonyl)imide and / or lithium bis(fluorosulfonyl)imide salt.

[0012] Preferably, the mass percentage of the lithium-conducting plastic crystal in the composite conductive agent is 5-95%; more preferably, the mass percentage of the lithium-conducting plastic crystal in the composite conductive agent is 50-90%; even more preferably, the mass percentage of the lithium-conducting plastic crystal in the composite conductive agent is 80-85.7%.

[0013] Preferably, the conductive agent is selected from at least one of vapor-grown carbon fibers, carbon black, conductive graphite, graphene, and carbon nanotubes.

[0014] The second aspect of the present invention provides a method for preparing the above composite conductive agent, comprising the following steps:

[0015] (1) Mix the plastic crystal with the lithium salt to form a lithium-conducting plastic crystal composite solution;

[0016] (2) Mix the lithium-conducting plastic crystal composite solution prepared in step (1) with the conductive agent, heat, and infiltrate to obtain the composite conductive agent.

[0017] Preferably, in step (2), the infiltration method is selected from any one of normal temperature vacuum pumping, heating vacuum pumping, solution soaking, and pressure infiltration.

[0018] Preferably, the heating is carried out under vacuum conditions at a temperature of 70-90 °C for 3-15 hours.

[0019] The third aspect of the present invention provides a positive electrode, which includes a positive electrode active material, an electrolyte, a binder, and the above composite conductive agent.

[0020] Preferably, the positive electrode active material is a nickel-cobalt-manganese ternary material.

[0021] Preferably, the nickel-cobalt-manganese ternary material is Li 0.83 Co 0.12 Mn 0.05 O 2 (NCM83).

[0022] Preferably, the electrolyte is a sulfide electrolyte.

[0023] Preferably, the sulfide electrolyte is Li 3 PS 4 .

[0024] Preferably, the binder is polytetrafluoroethylene (PTFE).

[0025] The fourth aspect of the present invention provides a all-solid-state battery, which includes the above positive electrode.

[0026] The above technical solution of the present invention has at least the following technical effects or advantages compared with the prior art:

[0027] (1) The composite conductive agent of the present invention coats the plastic lithium-conducting crystal with plasticity on the surface of the conductive agent particles, enabling the conductive agent to have the functions of conducting electrons and lithium ions at the same time, and is beneficial to improving the contact interface in the battery positive electrode sheet, thereby enhancing the first efficiency and charge-discharge efficiency of the positive active material. At the same time, the composite conductive agent of the present invention is beneficial to increasing the contact area between the sulfide electrolyte and the positive active material on the positive electrode side, improving the mass transfer efficiency, and further enhancing the first efficiency and rate performance of the positive electrode.

[0028] (2) The composite conductive agent of the present invention is prepared by the melt mixing method. Compared with the traditional solvent method, it avoids the dissolution of lithium ions and the side reaction between the solvent and the sulfide electrolyte. Therefore, it exhibits better first efficiency and charge-discharge performance. The first charge specific capacity of the all-solid-state battery is 225.15 - 233.13%, and the first Coulomb efficiency is 81.05 - 89.41%. Description of the Drawings

[0029] Figure 1 Charge-discharge specific capacity-voltage diagram of the 1st - 3rd cycles of the all-solid-state battery prepared in Application Example 3. Detailed Embodiments

[0030] The present invention will be specifically described below in conjunction with the embodiments for the understanding of those skilled in the art in the relevant technical field. It is necessary to specifically point out here that the embodiments are only used to further illustrate the present invention and should not be construed as limiting the protection scope of the present invention. Those skilled in the art, based on the above invention content, make non-essential improvements and adjustments to the present invention, which should still fall within the protection scope of the present invention. At the same time, for the raw materials not specifically described below, they are all commercially available products; for the process steps or preparation methods not specifically mentioned, they are all process steps or preparation methods known to those skilled in the art.

[0031] Example 1

[0032] A preparation method of a composite conductive agent, and this preparation method is the melt mixing method, including the following steps:

[0033] (1) Mix plastic crystal P 12 TFSI and lithium salt LiFSI in a mass ratio of 2.1:1, stir to form a lithium-conducting plastic crystal composite solution;

[0034] (2) Take 0.2 grams of the lithium-conducting plastic crystal composite solution prepared in step (1) and mix it with 0.05 grams of VGCF powder, stir evenly at 80°C, evacuate, and soak overnight to obtain the composite conductive agent P 12 TFSI / FSI-2.1@VGCF with the lithium-conducting plastic crystal coating VGCF.

[0035] A method for preparing a positive electrode, comprising the following steps:

[0036] Take 0.0125 grams of the composite conductive agent P prepared in this example 12 TFSI / FSI-2.1@VGCF, 0.4 grams of NCM83 positive electrode material, 0.09 grams of sulfide electrolyte Li 3 PS 4 and 0.005 grams of binder PTFE, dry grind to make a positive electrode sheet, and cut it into a disc with a diameter of 0.9 cm to obtain the positive electrode of this example.

[0037] Example 2

[0038] A method for preparing a composite conductive agent, the preparation method being a melt mixing method, comprising the following steps:

[0039] (1) Mix the plastic crystal P 12 TFSI and lithium salt LiFSI in a mass ratio of 2.1:1, stir to form a lithium-conducting plastic crystal composite solution;

[0040] (2) Take 0.3 grams of the lithium-conducting plastic crystal composite solution prepared in step (1) and mix it with 0.05 grams of VGCF powder, stir evenly at 80 °C, evacuate, and soak overnight to obtain a composite conductive agent P 12 TFSI / FSI-2.1@VGCF coated with lithium-conducting plastic crystals on VGCF.

[0041] A method for preparing a positive electrode, comprising the following steps:

[0042] Take 0.0175 grams of the composite conductive agent P prepared in this example 12 TFSI / FSI-2.1@VGCF, 0.4 grams of NCM83 positive electrode material, 0.085 grams of sulfide electrolyte Li 3 PS 4 and 0.005 grams of binder PTFE, dry grind to make a positive electrode sheet, and cut it into a disc with a diameter of 0.9 cm to obtain the positive electrode of this example.

[0043] Example 3

[0044] A method for preparing a composite conductive agent, the preparation method being a melt mixing method, comprising the following steps:

[0045] (1) Mix the plastic crystal P 12 TFSI and lithium salt LiFSI in a mass ratio of 5.68:1, stir to form a lithium-conducting plastic crystal composite solution;

[0046] (2) Take 0.2 g of the lithium-conductive plastic crystal composite solution prepared in step (1) and mix it with 0.05 g of VGCF powder. Stir evenly at 80 °C, evacuate, and soak overnight to obtain the composite conductive agent P with lithium-conductive plastic crystal-coated VGCF. 12 TFSI / FSI-5.68@VGCF.

[0047] A method for preparing a positive electrode, comprising the following steps:

[0048] Take 0.0125 g of the composite conductive agent P prepared in this example 12 TFSI / FSI-5.68@VGCF, 0.4 g of NCM83 positive electrode material, 0.09 g of sulfide electrolyte Li 3 PS 4 and 0.005 g of binder PTFE, and dry-grind to make a positive electrode sheet, and cut it into a disc with a diameter of 0.9 cm to obtain the positive electrode of this example.

[0049] Comparative Example 1

[0050] In Comparative Example 1, the solvent method was used to first coat the sulfide electrolyte and then prepare a composite positive electrode sheet.

[0051] A method for preparing a composite electrolyte, the preparation method being the solvent method, comprising the following steps:

[0052] (1) Mix plastic crystal P 12 TFSI and lithium salt LiFSI in a mass ratio of 2.1:1, stir to form a lithium-conductive plastic crystal composite solution;

[0053] (2) Take 0.2 g of the lithium-conductive plastic crystal composite solution prepared in step (1), disperse it in 20 mL of ethyl acetate solvent, after ultrasonic dispersion, add 1.8 g of sulfide electrolyte Li 3 PS 4 , stir, then rotary evaporate to remove the ethyl acetate solvent, and dry in vacuum to obtain the composite electrolyte P with lithium-conductive plastic crystal-coated SE 12 TFSI / FSI-2.1@SE.

[0054] A method for preparing a positive electrode, comprising the following steps:

[0055] Take 0.1 g of the composite electrolyte P prepared in this comparative example 12 TFSI / FSI-2.1@SE (the composite electrolyte contains 0.01 g of lithium-conductive plastic crystal and 0.09 g of sulfide), 0.4 g of NCM83 positive electrode material, 0.0025 g of VGCF and 0.005 g of binder PTFE, dry-grind to make a positive electrode sheet, and cut it into a disc with a diameter of 0.9 cm to obtain the positive electrode of this comparative example.

[0056] Comparative Example 2

[0057] The conductive agent of Comparative Example 2 is uncoated VGCF.

[0058] A method for preparing a positive electrode includes the following steps:

[0059] Take 0.0025 g of VGCF, 0.4 g of NCM83 positive electrode material, 0.1 g of sulfide electrolyte Li 3 PS 4 and 0.005 g of binder PTFE, grind them by dry method to make a positive electrode sheet, and cut it into a disc with a diameter of 0.9 cm to obtain the positive electrode of this comparative example.

[0060] Comparative Example 3

[0061] A method for preparing a composite conductive agent, which is a melt mixing method, includes the following steps:

[0062] (1) Mix plastic crystal P 12 TFSI and lithium salt LiFSI in a mass ratio of 1.42:1, stir to form a lithium-conducting plastic crystal composite solution;

[0063] (2) Take 0.2 g of the lithium-conducting plastic crystal composite solution prepared in step (1) and mix it with 0.05 g of VGCF powder, stir evenly at 80 °C, evacuate, and soak overnight to obtain a composite conductive agent P 12 TFSI / FSI-1.42@VGCF coated with lithium-conducting plastic crystal.

[0064] A method for preparing a positive electrode includes the following steps:

[0065] Take 0.0125 g of the composite conductive agent P 12 TFSI / FSI-1.42@VGCF prepared in this comparative example, 0.4 g of NCM83 positive electrode material, 0.09 g of sulfide electrolyte Li 3 PS 4 and 0.005 g of binder PTFE, grind them by dry method to make a positive electrode sheet, and cut it into a disc with a diameter of 0.9 cm to obtain the positive electrode of this comparative example.

[0066] Application Example 1

[0067] A sulfide solid-state battery, which uses sulfide electrolyte Li 3 PS 4 as the intermediate electrolyte layer, and LiIn alloy (Li foil with a diameter of 6 mm and In sheet with a diameter of 9 mm) as the negative electrode, and is assembled with the positive electrode prepared in Example 1 of the present invention.

[0068] Application Example 2

[0069] A sulfide solid-state battery, which is composed of a sulfide electrolyte Li 3 PS 4 as the intermediate electrolyte layer, a graphite negative electrode (composed of VGCF, graphite, Li 3 PS 4 and PTFE mixed in a mass ratio of 0.005:0.2:0.3:0.005), and assembled with the positive electrode prepared in Example 1 of the present invention.

[0070] Application Example 3

[0071] A sulfide solid-state battery, which is composed of a sulfide electrolyte Li 3 PS 4 as the intermediate electrolyte layer, a graphite negative electrode (composed of VGCF, graphite, Li 3 PS 4 and PTFE mixed in a mass ratio of 0.005:0.2:0.3:0.005), and assembled with the positive electrode prepared in Example 2 of the present invention.

[0072] Application Example 4

[0073] A sulfide solid-state battery, which is composed of a sulfide electrolyte Li 3 PS 4 as the intermediate electrolyte layer, a LiIn alloy (Li foil with a diameter of 6 mm and In sheet with a diameter of 9 mm) as the negative electrode, and assembled with the positive electrode prepared in Example 3 of the present invention.

[0074] Comparative Application Example 1

[0075] A sulfide solid-state battery, which is composed of a sulfide electrolyte Li 3 PS 4 as the intermediate electrolyte layer, a LiIn alloy (Li foil with a diameter of 6 mm and In sheet with a diameter of 9 mm) as the negative electrode, and assembled with the positive electrode prepared in Comparative Example 1 of the present invention.

[0076] Comparative Application Example 2

[0077] A sulfide solid-state battery, which is composed of a sulfide electrolyte Li 3 PS 4 as the intermediate electrolyte layer, a LiIn alloy (Li foil with a diameter of 6 mm and In sheet with a diameter of 9 mm) as the negative electrode, and assembled with the positive electrode prepared in Comparative Example 2 of the present invention.

[0078] Comparative Application Example 3

[0079] A sulfide solid-state battery, which is composed of a sulfide electrolyte Li 3 PS 4is the intermediate electrolyte layer, and the LiIn alloy (Li foil with a diameter of 6 mm and In sheet with a diameter of 9 mm) is used as the negative electrode, which is assembled with the positive electrode prepared in Comparative Example 3 of the present invention.

[0080] Performance Test

[0081] The sulfide solid-state batteries assembled with Application Examples 1-4 and Comparative Application Examples 1-3 were respectively subjected to charge and discharge tests under the following conditions: the areal density of the active material of the electrode was 22-33 mg / cm -2 , the assembly pressure was 4 T, the test pressure was 5 N / m, 0.1C cycling was carried out at 25°C / 60°C, and the voltage range was 2.2-3.7 V (vs. Li + / Li), and 100 cycles were carried out.

[0082] The test results are shown in Table 1.

[0083] Table 1:

[0084]

[0085] As can be seen from Table 1, from the test results of Application Example 1, it can be concluded that when the same positive and negative electrodes are used and the test temperature is higher, the battery shows better charge-discharge specific capacity and initial efficiency, indicating that the battery has good high-temperature stability.

[0086] In Comparative Application Example 1, the lithium-conducting plastic crystal was coated on the electrolyte by the solvent method as in Comparative Example 1. Although it also showed good charge-discharge specific capacity and initial efficiency, during the solvent method process, the dissolution of lithium ions was inevitably generated, and at the same time, it was difficult to completely avoid the side reaction with the sulfide electrolyte for the solvent used. Therefore, the electrochemical performance of the battery decreased compared with Application Example 1.

[0087] Compared with Application Example 1, in Application Example 2, the LiIn alloy negative electrode was replaced with a graphite negative electrode, and the battery still had a discharge specific capacity and initial efficiency comparable to those of Application Example 1, further demonstrating the feasibility of the composite conductive agent prepared in the present invention.

[0088] In Application Example 3, a all-solid-state battery was assembled with the positive electrode prepared in Example 2 and the graphite negative electrode. The initial discharge specific capacity at 60°C reached 201.44 mAh / g; at the same time, the discharge specific capacities of 188.82 mAh / g and 186.02 mAh / g were still achieved at 0.3C in the third cycle (as Figure 1 shown), further indicating that the battery prepared in the present invention has good high-temperature stability.

[0089] In Application Example 4, different mass ratios of PP 12The composite of TFSI / LiTFSI coats VGCF, and the prepared positive electrode sheet also has a high discharge specific capacity and a high initial efficiency. Comparing with Application Example 3, the content of the lithium salt is further increased, and the charge-discharge specific capacity and the initial efficiency of the prepared electrode sheet are significantly reduced, which may be because the excessive lithium salt concentration reduces the dissociation ability of the plastic crystal to the lithium salt.

[0090] Therefore, the composite conductive agent prepared by coating VGCF with the lithium-ion conducting plastic crystal of the present invention has both the functions of conducting lithium ions and electrons, can significantly improve the first-cycle discharge specific capacity of the battery, and has good high-temperature stability.

[0091] For those of ordinary skill in the art to which the present invention pertains, several simple deductions or substitutions can be made without departing from the concept of the present invention, without the need for creative labor. Therefore, all simple improvements made by those skilled in the art based on the disclosure of the present invention should fall within the protection scope of the present invention. The above embodiments are the preferred embodiments of the present invention, and all processes similar to the present invention and equivalent changes made thereto shall fall within the protection scope of the present invention.

Claims

1. A composite conductive agent, characterized in that: The composite conductive agent includes a conductive agent and a lithium conductive plastic crystal, wherein the lithium conductive plastic crystal is coated on the surface of the conductive agent, and the lithium conductive plastic crystal is a mixture of plastic crystal and lithium salt, wherein the mass ratio of the plastic crystal to the lithium salt is (2-6): 1, and the mass ratio of the lithium conductive plastic crystal in the composite conductive agent is 80-85.7%; The plastic crystal is selected from at least one of N-ethyl-N-methylpyrrole bis(fluorosulfonyl)imide, N-ethyl-N-methylpyrrole bis(trifluoromethanesulfonyl)imide, N-ethyl-N-methylpiperidine bis(fluorosulfonyl)imide, N-ethyl-N-methylpiperidine bis(trifluoromethanesulfonyl)imide, N-propyl-N-methylpiperidine bis(fluorosulfonyl)imide, N-propyl-N-methylpiperidine bis(trifluoromethanesulfonyl)imide, triethyl(methyl)phosphorus bis(fluorosulfonyl)imide, triethyl(methyl)phosphorus bis(trifluoromethanesulfonyl)imide, N,N-dimethylpyrrole bis(fluorosulfonyl)imide, and N,N-dimethylpyrrole bis(trifluoromethanesulfonyl)imide; The composite conductive agent is prepared by a preparation method comprising the following steps: (1) mixing the plastic crystal with the lithium salt to form a lithium-conducting plastic crystal composite solution; (2) The lithium conductive plastic crystal composite solution obtained in step (1) is mixed with a conductive agent, heated, and infiltrated to obtain the composite conductive agent.

2. The composite conductive agent according to claim 1, characterized in that: The lithium salt is lithium bis(trifluoromethanesulfonyl imide) and / or lithium bis(fluorosulfonyl imide) salt.

3. The composite conductive agent according to claim 1 or 2, characterized in that: The conductive agent is selected from at least one of vapor-grown carbon fibers, carbon black, conductive graphite, graphene, and carbon nanotubes.

4. The composite conductive agent according to claim 1, characterized in that: In step (2), the infiltration method is selected from any one of room temperature vacuuming, heated vacuuming, solution immersion, and pressure infiltration.

5. The composite conductive agent according to claim 1, characterized in that: In step (2), the heating temperature is 70-90°C.

6. A positive electrode, characterized in that The positive electrode comprises a positive electrode active material, an electrolyte, a binder and the composite conductive agent according to any one of claims 1 to 5.

7. An all-solid-state battery, characterized in that: The all-solid-state battery comprises the positive electrode according to claim 6.

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