Organic mixed conductor for preparing positive electrode as well as preparation method and application of organic mixed conductor

Abstract

The invention provides an organic mixed conductor for preparing a positive electrode as well as a preparation method and application of the organic mixed conductor. The organic mixed conductor comprises poly (benzodifuran diketone) and an electrolyte salt. The organic mixed conductor provided by the invention has good cohesiveness and ion / electron conductivity, and can replace a common polyvinylidene fluoride binder material and a conductive carbon material in the prior art, so that a positive electrode material is prepared, and a solid secondary battery obtained by assembling has better cycle stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of conductor materials, and particularly relates to an organic mixed conductor for preparing a positive electrode, a preparation method thereof, and an application thereof. Background Art

[0002] For traditional liquid lithium-ion secondary batteries, it is difficult to meet the market's usage requirements for energy storage devices with higher energy density and safety only by optimizing the battery structure and preparation process. Therefore, in order to meet the above requirements, it is urgent to develop lithium-ion secondary batteries of other systems to further improve the electrochemical performance and safety of lithium-ion secondary batteries. Currently, a lithium metal secondary battery using lithium metal as the negative electrode and a positive electrode material with a high voltage is considered to be a high-energy-density energy storage device with great application prospects. However, traditional liquid electrolytes can cause lithium metal pulverization, and uncontrollably form lithium dendrites and produce dead lithium under electrochemical conditions.

[0003] In contrast, a solid-state lithium secondary battery using a solid electrolyte can well overcome the above problems. In the prior art, a conventional positive electrode sheet prepared by using polyvinylidene fluoride / conductive carbon / positive electrode active material belongs to a solid-state porous electrode. In a liquid lithium-ion battery, the positive electrode sheet can achieve good electron and ion transport effects only when it is fully infiltrated by the electrolyte. However, in a solid-state lithium secondary battery, the contact between the solid electrolyte and the positive electrode sheet is a solid-solid contact. Since the ionic conductivity of commonly used binders such as polyvinylidene fluoride is very low, except for the active material part in contact with the solid electrolyte on the surface layer of the positive electrode sheet, the lithium ion transport of most of the active materials in its inner layer is relatively difficult, which in turn affects the overall performance of the solid-state lithium secondary battery. In addition, directly adding an ionic conductive agent to the original positive electrode slurry in the prior art will not only reduce the loading of the active material in the positive electrode sheet, but also cannot ensure that the ionic conductivity of the positive electrode sheet can be significantly improved.

[0004] Therefore, for solid-state lithium secondary batteries, it is urgent to develop a conductor material suitable for preparing positive electrode sheets, so as to make the positive electrode sheet have higher ionic conductivity while ensuring a high loading of positive electrode active materials. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide an organic mixed conductor for preparing a positive electrode, a preparation method thereof, and an application thereof. The organic mixed conductor provided by the present invention has both good adhesiveness and ionic / electronic conductivity, can replace the commonly used polyvinylidene fluoride binder material and conductive carbon material in the prior art, and then prepare a positive electrode material, and the assembled solid-state secondary battery has better cycle stability.

[0006] To achieve the purpose of this invention, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides an organic mixed conductor for preparing a positive electrode, and the organic mixed conductor includes poly(benzodifurandione) and an electrolyte salt.

[0008] First, the organic mixed conductor provided by the present invention has good electron and ion conductivities, and excellent adhesiveness to both the positive electrode active material and the current collector. Therefore, it can replace the polyvinylidene fluoride binder and the conductive carbon material in the traditional positive electrode material, ensuring that the positive electrode active material has a high areal loading. Second, by adjusting the side chain length and the degree of polymerization of the main chain in the organic mixed conductor, the ion and electron conductivities of the organic mixed conductor can be regulated, thereby preparing solid-state secondary battery positive electrode materials with different performances. Finally, compared with the positive electrode material and the solid-state secondary battery prepared by using the traditional polyvinylidene fluoride / conductive carbon material, the organic mixed conductor provided by the present invention and the solid-state secondary battery prepared therefrom can significantly improve the cycling performance of the solid-state secondary battery.

[0009] Preferably, the weight-average molecular weight of the poly(benzodifurandione) is 25 - 70 KDa, preferably 45 - 60 KDa, and for example, it can be 25 KDa, 30 KDa, 35 KDa, 40 KDa, 45 KDa, 50 KDa, 55 KDa, 60 KDa, 65 KDa, 70 KDa, etc.

[0010] In the present invention, by regulating the weight-average molecular weight of the poly(benzodifurandione), the finally prepared positive electrode has the best ion conductivity and electron conductivity. If the weight-average molecular weight is too low, the electron conductivity of the positive electrode will be reduced, and vice versa, the ion conductivity of the positive electrode will be reduced.

[0011] Preferably, the mass ratio of the poly(benzodifurandione) to the electrolyte salt is (1 - 15):(1 - 15), preferably (1 - 10):(1 - 5), and for example, it can be 1:1, 1:2, 1:5, 1:8, 1:10, 1:12, 1:15, 2:1, 5:2, 8:5, 10:1, 10:12, 15:1, etc.

[0012] In the present invention, by regulating the mass ratio of the poly(benzodifurandione) to the electrolyte salt, the finally prepared positive electrode has the best ion conductivity and electron conductivity. If the mass ratio is too low, the electron conductivity of the positive electrode will be reduced, and vice versa, the ion conductivity of the positive electrode will be reduced.

[0013] Preferably, the electrolyte salt includes a lithium salt.

[0014] Preferably, the lithium salt includes any one or a combination of at least two of lithium difluorophosphate, lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroborate, lithium trifluoromethanesulfonate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(difluoromethylsulfonyl)imide, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, and lithium malonate oxaloborate.

[0015] In a second aspect, the present invention provides a method for preparing an organic mixed conductor for preparing a positive electrode according to the first aspect, the method comprising the following steps:

[0016] Mix poly(benzodifurandione) and an electrolyte salt to obtain the organic mixed conductor for preparing a positive electrode.

[0017] Preferably, the mass ratio of the poly(benzodifurandione) to the electrolyte salt is (1 - 15):(1 - 15), preferably (1 - 10):(1 - 5), and for example, it can be 1:1, 1:2, 1:5, 1:8, 1:10, 1:12, 1:15, 2:1, 5:2, 8:5, 10:1, 10:12, 15:1, etc.

[0018] Preferably, the mixing method includes a liquid-phase mixing method or a solid-phase grinding method.

[0019] In a third aspect, the present invention provides a positive electrode, the preparation raw materials of the positive electrode include a positive electrode active material and a conductor material, and the conductor material includes the organic mixed conductor for preparing a positive electrode according to the first aspect.

[0020] Preferably, based on the total mass of the preparation raw materials of the positive electrode being 100%, the mass percentage content of the organic mixed conductor for preparing a positive electrode is not higher than 40%, preferably 10%, and for example, it can be 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 30%, 35%, 40%.

[0021] In the present invention, by regulating the mass percentage content of the organic mixed conductor for preparing a positive electrode, the finally prepared positive electrode has the best ionic conductivity and electronic conductivity. If the mass percentage content is too low, the electronic conductivity of the positive electrode will be reduced, and vice versa, the ionic conductivity of the positive electrode will be reduced.

[0022] Preferably, the positive electrode active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium cobaltate, lithium manganate, nickel cobalt manganese ternary electrode material, nickel cobalt aluminum ternary electrode material, and lithium-rich manganese-based material.

[0023] In a fourth aspect, the present invention provides a solid-state secondary battery, the solid-state secondary battery includes a positive electrode, a negative electrode, a solid electrolyte, and a housing, and the positive electrode includes the positive electrode according to the third aspect.

[0024] Preferably, the solid electrolyte is selected from polymer solid electrolytes, inorganic solid electrolytes or organic-inorganic composite solid electrolytes.

[0025] Preferably, the active material in the negative electrode is one or more of carbon-based materials, silicon-based materials, boron-based materials, metallic lithium, metallic bismuth, nitrides, magnesium-based alloys, transition metal oxides and phosphides.

[0026] Preferably, the solid secondary battery is a solid lithium secondary battery.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The present invention provides an organic mixed conductor for preparing a positive electrode. First, the organic mixed conductor provided by the present invention has good electronic and ionic conductivities and excellent adhesiveness to both the positive electrode active material and the current collector. Therefore, it can replace the polyvinylidene fluoride binder and conductive carbon material in the traditional positive electrode material, ensuring that the positive electrode active material has a high areal loading. Second, by adjusting the side chain length and the degree of polymerization of the main chain in the organic mixed conductor, the present invention can further control the ionic and electronic conductivities of the organic mixed conductor, thereby preparing positive electrode materials for solid secondary batteries with different performances. Finally, compared with the positive electrode materials and the solid secondary batteries prepared by using traditional polyvinylidene fluoride / conductive carbon materials, the organic mixed conductor provided by the present invention and the solid secondary battery prepared therefrom can significantly improve the cycling performance of the solid secondary battery. Detailed Embodiments

[0029] The technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be construed as specific limitations to the present invention.

[0030] Example 1

[0031] This example provides an organic mixed conductor for preparing a positive electrode, and the organic mixed conductor comprises poly(benzodifurandione) (PBFDO, weight average molecular weight is 45KDa) and lithium hexafluorophosphate (LiPF6) with a mass ratio of 10:1.

[0032] This example provides a preparation method of the above-mentioned organic mixed conductor for preparing a positive electrode, which comprises the following steps:

[0033] Dissolve 1 g of LiPF6 and 10 g of PBFDO in 10 mL of N-methylpyrrolidone solvent, stir to dissolve into a homogeneous solution; then evaporate the solvent to obtain the organic mixed conductor.

[0034] Example 2

[0035] This embodiment provides an organic mixed conductor for preparing a positive electrode. The organic mixed conductor comprises poly(benzodifurandione) (PBFDO, with a weight-average molecular weight of 52 KDa) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) with a mass ratio of 5.5:1.

[0036] This embodiment provides a preparation method of the above-mentioned organic mixed conductor for preparing a positive electrode, which comprises the following steps:

[0037] Dissolve 1 g of LiTFSI and 5.5 g of PBFDO in 6 mL of dimethyl sulfoxide, and stir to dissolve into a homogeneous solution; then evaporate the solvent to obtain the organic mixed conductor.

[0038] Example 3

[0039] This embodiment provides an organic mixed conductor for preparing a positive electrode. The organic mixed conductor comprises poly(benzodifurandione) (PBFDO, with a weight-average molecular weight of 56 KDa) and lithium bis(difluoromethanesulfonyl)imide (LiFSI) with a mass ratio of 4:1.

[0040] This embodiment provides a preparation method of the above-mentioned organic mixed conductor for preparing a positive electrode, which comprises the following steps:

[0041] Dissolve 1 g of LiFSI and 4 g of PBFDO in 5 mL of dimethyl sulfoxide, and stir to dissolve into a homogeneous solution; then evaporate the solvent to obtain the organic mixed conductor.

[0042] Example 4

[0043] The difference between this embodiment and Example 1 is that the organic mixed conductor comprises poly(benzodifurandione) and lithium hexafluorophosphate with a mass ratio of 1:15, and the others are the same as those in Example 1.

[0044] Example 5

[0045] The difference between this embodiment and Example 1 is that the organic mixed conductor comprises poly(benzodifurandione) and lithium hexafluorophosphate with a mass ratio of 15:1, and the others are the same as those in Example 1.

[0046] Example 6

[0047] The difference between this embodiment and Example 1 is that the organic mixed conductor comprises poly(benzodifurandione) and lithium hexafluorophosphate with a mass ratio of 0.5:20, and the others are the same as those in Example 1.

[0048] Example 7

[0049] The difference between this example and Example 1 is that the organic mixed conductor includes poly(benzodifurandione) and lithium hexafluorophosphate with a mass ratio of 20:0.5, and the others are the same as in Example 1.

[0050] Example 8

[0051] The difference between this example and Example 1 is that the weight-average molecular weight of poly(benzodifurandione) is 8 KDa, and the others are the same as in Example 1.

[0052] Example 9

[0053] The difference between this example and Example 1 is that the weight-average molecular weight of poly(benzodifurandione) is 82 KDa, and the others are the same as in Example 1.

[0054] Comparative Example 1

[0055] The difference between this comparative example and Example 1 is that poly(benzodifurandione) is replaced by polyvinylidene fluoride, and the others are the same as in Example 1.

[0056] Comparative Example 2

[0057] This comparative example provides an organic mixed conductor for preparing a positive electrode. The organic mixed conductor includes polyvinylidene fluoride and conductive carbon black with a mass ratio of 5:2.

[0058] Preparation Examples 1-9 and Comparative Preparation Examples 1-2

[0059] The positive electrode sheets and their solid-state lithium secondary batteries were prepared from the organic mixed conductors for preparing positive electrodes provided in Examples 1-9 and Comparative Examples 1 to 2. The preparation method is as follows:

[0060] The organic mixed conductors for preparing positive electrodes provided in Examples 1-9 and Comparative Examples 1 to 2, lithium iron phosphate positive electrode active material (mass ratio 90%) and N-methylpyrrolidone were stirred evenly to obtain a slurry; then the slurry was coated on aluminum foil, vacuum dried and roll-pressed to obtain positive electrode sheets;

[0061] The above positive electrode sheets, poly(ethylene oxide) solid electrolyte and lithium metal negative electrode were encapsulated to obtain solid-state lithium secondary batteries.

[0062] Preparation Examples 1-9 and Comparative Preparation Examples 1-2

[0063] The positive electrode sheets and their solid-state lithium secondary batteries were prepared from the organic mixed conductors for preparing positive electrodes provided in Examples 1-9 and Comparative Examples 1 to 2. The preparation method is as follows:

[0064] The organic mixed conductors for preparing the positive electrode, lithium cobaltate positive electrode active material (with a mass ratio of 85%), and N-methylpyrrolidone provided in Examples 1-9 and Comparative Examples 1 to 2 were stirred evenly to obtain a slurry; then the slurry was coated on an aluminum foil, vacuum-dried and roll-pressed to obtain a positive electrode sheet;

[0065] The above positive electrode sheet, LLZTO solid electrolyte, and lithium metal negative electrode were encapsulated to obtain a solid-state lithium secondary battery.

[0066] Preparation Examples 1-9 and Comparative Preparation Examples 1-2

[0067] The positive electrode sheets and their solid-state lithium secondary batteries were prepared from the organic mixed conductors for preparing the positive electrode provided in Examples 1-9 and Comparative Examples 1 to 2, and the preparation method was as follows:

[0068] The organic mixed conductors for preparing the positive electrode, NCM111 ternary positive electrode active material (with a mass ratio of 85%), and N-methylpyrrolidone provided in Examples 1-9 and Comparative Examples 1 to 2 were stirred evenly to obtain a slurry; then the slurry was coated on an aluminum foil, vacuum-dried and roll-pressed to obtain a positive electrode sheet;

[0069] The above positive electrode sheet, Li6PS5Cl solid electrolyte, and lithium metal negative electrode were encapsulated to obtain a solid-state lithium secondary battery.

[0070] Test conditions

[0071] The solid-state lithium secondary batteries with different systems provided in Preparation Examples 1 to 9 and Comparative Preparation Examples 1 to 2 were tested. The experimental batteries were placed in a constant temperature oven at 25°C and connected to a charge-discharge tester. First, they were charged at a constant current and constant voltage with a current of 0.5C to the corresponding working voltage, and the cut-off current was set to 0.05C; after standing for 5 minutes, they were discharged at a constant current of 0.5C to 2.8V. Such cyclic charge-discharge tests were carried out, and the discharge capacity of each time was recorded. The cell capacity retention rate at the 300th week was calculated respectively. Among them, the cycle capacity retention rate (%) of the lithium-ion cell in the Nth cycle = (discharge capacity in the Nth cycle / discharge capacity in the first cycle) × 100%.

[0072] The test results are shown in Table 1:

[0073] Table 1

[0074]

[0075]

[0076] As can be seen from Table 1, the organic mixed conductors for preparing the positive electrode in the present invention can significantly improve the cycle performance of the solid-state lithium secondary batteries prepared therefrom because they have excellent ionic conductivity and electronic conductivity themselves.

[0077] From the comparison between Preparation Example 1 and Preparation Examples 6-7, it can be seen that by regulating the preferred mass ratio range of poly(benzodifurandione) and lithium hexafluorophosphate, the prepared positive electrode has the best ionic conductivity and electronic conductivity.

[0078] From the comparison between Preparation Example 1 and Preparation Examples 8-9, it can be seen that by regulating the preferred weight-average molecular weight of poly(benzodifurandione), the prepared positive electrode has the best ionic conductivity and electronic conductivity.

[0079] From the comparison between Preparation Example 1 and Comparative Preparation Examples 1-2, it can be seen that the polyvinylidene fluoride binder and the combination of polyvinylidene fluoride and conductive carbon material disclosed in the prior art cannot achieve all the technical effects of the present invention.

[0080] The applicant declares that the present invention uses the above-mentioned embodiments to illustrate the process method of the present invention, but the present invention is not limited to the above-mentioned process steps, that is, it does not mean that the present invention must rely on the above-mentioned process steps to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent substitution of the raw materials selected for the present invention, the addition of auxiliary components, the selection of specific methods, etc. all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. An organic mixed conductor for preparing a positive electrode, characterized in that, The organic mixed conductor includes poly(benzodifurandione) and an electrolyte salt.

2. The organic hybrid conductor according to claim 1, characterized in that, The poly(benzodifurandione) has a weight-average molecular weight of 25 - 70 KDa, preferably 45 - 60 KDa.

3. The organic mixed conductor according to claim 1 or 2, characterized in that, The mass ratio of the poly(benzodifurandione) to the electrolyte salt is (1 - 15):(1 - 15), preferably (1 - 10):(1 - 5).

4. The organic mixed conductor according to any one of claims 1-3, characterized in that The electrolyte salt includes a lithium salt.

5. The organic hybrid conductor according to claim 4, characterized in that, The lithium salt includes any one or a combination of at least two of lithium difluorophosphate, lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroborate, lithium trifluoromethanesulfonate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(difluoromethylsulfonyl)imide, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, and lithium malonate oxaloborate.

6. A method for preparing an organic mixed conductor for preparing a positive electrode according to any one of claims 1-5, characterized in that, The method includes the following steps: Mix the poly(benzodifurandione) and the electrolyte salt to obtain the organic mixed conductor for preparing the positive electrode.

7. The method according to claim 6, characterized in that, The mass ratio of the poly(benzodifurandione) to the electrolyte salt is (1 - 15):(1 - 15), preferably (1 - 10):(1 - 5); Preferably, the mixing method includes a liquid-phase mixing method or a solid-phase grinding method.

8. A positive electrode, characterized in that, The raw materials for preparing the positive electrode include a positive electrode active material and a conductor material, and the conductor material includes the organic mixed conductor for preparing the positive electrode according to any one of claims 1 - 5.

9. The positive electrode according to claim 8, characterized in that, Based on the total mass of the raw materials for preparing the positive electrode being 100%, the mass percentage content of the organic mixed conductor for preparing the positive electrode is not higher than 40%, preferably 10%. Preferably, the positive electrode active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium cobaltate, lithium manganate, nickel cobalt manganese ternary electrode material, nickel cobalt aluminum ternary electrode material, and lithium-rich manganese-based material.

10. A solid-state secondary battery, characterized in that, The solid-state secondary battery includes a positive electrode, a negative electrode, a solid electrolyte, and a housing, and the positive electrode includes the positive electrode according to claim 8 or 9.

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