A dry electrode film, a method for preparing the same, and a lithium ion battery

Abstract

The present application relates to the technical field of battery, especially to a dry method electrode film, a preparation method thereof and a lithium ion battery, and provides a dry method electrode film, which comprises an active substance, a conductive agent, a binder and a polyionic liquid copolymer; the polyionic liquid copolymer contains a pyrrolidone ring structure unit and an alkyl imidazole cation group.The dry method electrode film of the present application uses the polyionic liquid copolymer as an auxiliary binder, anchors the metal base through the pyrrolidone ring structure unit, and combines the two-dimensional synergistic effect of the alkyl imidazole cation group and PTFE, thereby improving the interfacial bonding strength of the dry method electrode film and the metal base and the charge-discharge cycle stability.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a dry electrode film, its preparation method, and a lithium-ion battery. Background Technology

[0002] In the iterative development of lithium-ion battery technology, dry electrode technology has gradually emerged. It eliminates the solvent-based coating and drying processes required in traditional wet processes, instead directly rolling-forming films by physically and mechanically mixing active materials, conductive agents, and binders. Dry electrode technology offers significant advantages: it eliminates the need for solvent recovery, reducing energy consumption by over 30%; it enables the fabrication of thick electrodes with a thickness greater than 500 μm, thereby increasing energy density; and it avoids interfacial side reactions caused by solvent residue. However, the core challenge of this technology lies in achieving stable bonding and conductive network construction under solvent-free conditions. Here, polytetrafluoroethylene (PTFE) fiberization technology becomes a key breakthrough.

[0003] PTFE fiberization technology uses high shear force or rolling action to directionally stretch PTFE molecular chains into a micron-scale fiber network, playing a dual role in dry electrodes: firstly, the fiber network entangles with active particles to form a mechanically interlocking structure, replacing the molecular-level adhesion of traditional PVDF; secondly, the interstices between fibers naturally form pores, promoting electrolyte wetting and maintaining smooth ion transport channels. This technology can improve the tensile strength of the electrode, while effectively avoiding electrode cracking because it eliminates the need for solvents.

[0004] However, the PTFE dry electrode technology still has some shortcomings. The interaction force between PTFE and the metal substrate is insufficient, resulting in poor adhesion and making it impossible to ensure the structural stability of the electrode active layer during charge and discharge cycles.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] The primary objective of this invention is to provide a dry electrode membrane that uses a polyionic liquid copolymer as an auxiliary binder. Through the dual-dimensional synergistic effect of anchoring the metal substrate with pyrrolidone ring structural units and combining alkyl imidazole cationic groups with PTFE, the interfacial adhesion strength and charge-discharge cycle stability between the dry electrode membrane and the metal substrate are improved.

[0007] A second objective of this invention is to provide a method for preparing the aforementioned dry electrode film.

[0008] A third objective of this invention is to provide a lithium-ion battery.

[0009] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: This invention provides a dry electrode membrane, comprising: an active material, a conductive agent, a binder, and a polyionic liquid copolymer; The polyionic liquid copolymer contains pyrrolidone ring structural units and alkylimidazolium cationic groups.

[0010] Furthermore, the preparation method of the polyionic liquid copolymer includes the following steps: After reacting N-vinylpyrrolidone and 1-vinyl-3-butylimidazolium bromide, KPF6 is added for anion exchange to obtain the polyionic liquid copolymer.

[0011] Further, the molar ratio of the N-vinylpyrrolidone and the 1-vinyl-3-butylimidazolium bromide is 1:(0.5~1).

[0012] Further, the reaction includes: reacting at 70~75℃ for 10~12h, then raising the temperature to 80~85℃ and reacting for 1.5~3h.

[0013] Furthermore, the anion exchange time is 20-30 hours.

[0014] Furthermore, the content of polyionic liquid copolymer in the dry electrode membrane is 0.5wt%~2wt%.

[0015] Furthermore, the thickness of the dry electrode film is 100~150μm.

[0016] The present invention also provides a method for preparing the dry electrode film as described above, comprising the following steps: After the active material and conductive agent are mixed evenly, a binder and a polyionic liquid copolymer are added for a first mixing to obtain a multi-component mixture; the multi-component mixture is then mixed a second time to obtain a granular coating; the granular coating is calendered into a film to obtain the dry electrode film.

[0017] Furthermore, it includes at least one of the following features (1) to (3); (1) The temperature of the first mixture is -5 to -15°C, and the time is 1.5 to 3 hours; (2) The temperature of the second mixing is 15~30℃ and the time is 15~30min; (3) The rolling temperature is 90~110℃.

[0018] The present invention also provides a lithium-ion battery comprising the dry electrode film as described above.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention introduces a polyionic liquid copolymer as an auxiliary binder into a dry electrode system. On one hand, it utilizes the hydrogen bonds formed between the carbonyl oxygen of the pyrrolidone ring structural unit in the polyionic liquid copolymer and the hydroxyl groups on the metal surface, and the van der Waals forces formed between the π-electron cloud of the pyrrolidone ring and the metal lattice, to enhance the interfacial adhesion between the dry electrode film and the metal substrate. On the other hand, it utilizes the electrostatic interaction between the alkyl imidazole cationic groups in the polyionic liquid copolymer and the fluorine atoms in PTFE to enhance its interfacial compatibility with PTFE. Through this dual-dimensional synergistic effect, the problem of poor adhesion between the dry electrode film and the metal substrate is effectively solved, resulting in better adhesion between the components in the dry electrode film and improved structural stability of the dry electrode film during charge-discharge cycles. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0021] Figure 1 The infrared spectrum of NVP-VBImPF6 in Embodiment 1 of the present invention is shown.

[0022] Figure 2 The thermogravimetric curve of NVP-VBImPF6 in Example 1 of this invention under nitrogen atmosphere.

[0023] Figure 3 The diagram shows the cycle performance of the lithium-ion half-cell prepared using the dry electrode film of Example 11 according to the present invention.

[0024] Figure 4 The diagram shows the cycle performance of the lithium-ion half-cell fabricated using the dry electrode film of Comparative Example 1. Detailed Implementation

[0025] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0026] In some embodiments of the present invention, a dry electrode membrane is provided, comprising: an active material, a conductive agent, a binder, and a polyionic liquid copolymer; The polyionic liquid copolymer contains pyrrolidone ring structural units and alkylimidazolium cationic groups.

[0027] The introduction of the auxiliary binder polyionic liquid copolymer in the dry electrode membrane of this invention can significantly improve the interfacial adhesion performance of the dry electrode membrane and the adhesion effect between the components through a two-dimensional synergistic effect; thereby improving the adhesion performance between the dry electrode membrane and the metal substrate and the charge-discharge cycle stability.

[0028] On the one hand, the pyrrolidone ring structural unit in the polyionic liquid copolymer structure can form multiple interactions with the oxides on the surface of the metal substrate (current collector, such as aluminum foil or copper foil) due to its strong polar amide group (-CON-) and the electron enrichment characteristics of the pyrrolidone ring. At the same time, the π electron cloud of the pyrrolidone ring can also form van der Waals forces with the metal lattice. This composite effect can significantly enhance the interfacial adhesion between it and the metal substrate and effectively suppress the interfacial delamination phenomenon during the cycling process.

[0029] On the other hand, the alkyl imidazole cationic groups in the polyionic liquid copolymer structure, through molecular design, introduce positively charged imidazole structures that can generate strong electrostatic interactions with the highly electronegative fluorine atoms (F) in the polytetrafluoroethylene (PTFE, binder) molecular chain. This specific "cation-fluorine" combination not only compensates for the wettability defects caused by the low surface energy of PTFE, but also enhances its interfacial compatibility with PTFE through the formation of dynamic coordination bonds.

[0030] In some embodiments of the present invention, the preparation method of the polyionic liquid copolymer includes the following steps: Following the reaction of N-vinylpyrrolidone (NVP) and 1-vinyl-3-butylimidazolium bromide, anion exchange was performed by adding sufficient saturated KPF6 solution to obtain a polyionic liquid copolymer, denoted as NVP-VBImPF6. The amount of saturated KPF6 solution added was sufficient to ensure the Br... - Fully exchanged into PF6 - That's all.

[0031] Polyionic liquid copolymers are a two-dimensional interface reinforcement system. They are copolymerized by molecular design of N-vinylpyrrolidone (NVP) structural units and imidazole-based ionic liquid segments (1-vinyl-3-butylimidazolium bromide), forming a two-dimensional synergistic mechanism. In the first dimension, the NVP structural units enhance the interfacial adhesion strength with the metal substrate. In the second dimension, the imidazole-based ionic liquid segments modify the PTFE interface to improve the adhesion performance.

[0032] This invention introduces a pyrrolidone ring structure into the dry electrode film system, utilizing its electron enrichment characteristics to directionally anchor the metal current collector, overcoming the limitation that traditional PTFE cannot form chemical bonds with metals.

[0033] An interfacial modification method for polymer segments containing imidazole cations with fluorinated polymers (PTFE) employs an interface engineering strategy of specific "cation-fluorine" binding; the positive charge density of the imidazole groups is tunable to optimize the electrostatic interaction strength with PTFE; anions (PF6) - The size effect creates steric hindrance, preventing excessive entanglement of PTFE fibers.

[0034] Polyionic liquid copolymers are copolymer structures with surface energy gradient regulation capabilities. The surface energy gradient distribution is achieved through copolymer molecular chain design: the metal-loving end is an NVP segment, which is used to reduce the contact angle of the metal interface and promote wetting; the fluorine-repellent end is an imidazole segment, which is used to match the low surface energy characteristics of PTFE and reduce phase separation.

[0035] Polyionic liquid copolymers are copolymer molecular topologies with stress-buffering functions, achieving stress dispersion through random copolymerization topologies. In these topologies, NVP rigid rings and imidazole flexible chains are arranged alternately to absorb volumetric strain during charging and discharging, preventing electrode cracking caused by polymer chain entanglement.

[0036] In some embodiments of the invention, the molar ratio of N-vinylpyrrolidone to 1-vinyl-3-butylimidazolium bromide is 1:(0.5~1); typically, but not limitingly, for example, the molar ratio of N-vinylpyrrolidone to 1-vinyl-3-butylimidazolium bromide can be 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, and any value between any two thereof.

[0037] In some embodiments of the present invention, the reaction includes: reacting at 70~75°C for 10~12 hours, then raising the temperature to 80~85°C and reacting for 1.5~3 hours; preferably, the reaction includes: reacting at 75°C for 11 hours, then raising the temperature to 80°C and reacting for 2 hours.

[0038] In some embodiments of the present invention, the reaction further includes adding a solvent; preferably, the solvent includes, but is not limited to, methanol.

[0039] In some embodiments of the present invention, the anion exchange time is 20-30 hours; preferably 24 hours; and the anion exchange temperature is 10-30°C, i.e., room temperature.

[0040] In some embodiments of the present invention, the content of the polyionic liquid copolymer in the dry electrode membrane is 0.5wt% to 2wt%; typically, but not limitingly, for example, the content of the polyionic liquid copolymer in the dry electrode membrane can be 0.5wt%, 1wt%, 1.5wt%, 2wt%, and any value between any two thereof.

[0041] In some embodiments of the present invention, the active material includes LiNi. 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 At least one of O2, LiFePO4 and LiCoO2.

[0042] In some embodiments of the present invention, the conductive agent includes Super P and / or CNTs.

[0043] In some embodiments of the present invention, the adhesive includes polytetrafluoroethylene (PTFE).

[0044] In some embodiments of the present invention, the mass ratio of the active material, conductive agent, binder and polyionic liquid copolymer is: (91~97); (2~5): (0.5~2): (0.5~2).

[0045] In some embodiments of the present invention, the thickness of the dry electrode film is 100-150 μm; typically, but not limitingly, for example, the thickness of the dry electrode film can be 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm and any value between any two of these.

[0046] In some embodiments of the present invention, a method for preparing the above-mentioned dry electrode film is also provided, comprising the following steps: After the active material and conductive agent are mixed, a binder and a polyionic liquid copolymer are added for the first mixing to obtain a multi-component mixture; the multi-component mixture is then mixed for the second mixing to obtain a granular coating; the granular coating is calendered into a film to obtain a dry electrode film.

[0047] In some embodiments of the present invention, mixing the active material and the conductive agent includes mixing at 10~30°C (room temperature) for 0.5~1.5 h.

[0048] In some embodiments of the present invention, the temperature of the first mixing is -5 to -15°C, and the time is 1.5 to 3 hours; preferably, the temperature of the first mixing is -10°C, and the time is 2 hours. After the first mixing, a multi-component mixture with no deformation of the binder is formed.

[0049] In some embodiments of the present invention, the temperature of the second mixing is 15-30°C and the time is 15-30 min; preferably, the temperature of the second mixing is 20°C and the time is 20 min. The multi-component mixture is then subjected to the second mixing to form a granular coating.

[0050] In some embodiments of the present invention, the calendering temperature is 90~110°C; preferably, the calendering temperature is 100°C.

[0051] In some embodiments of the present invention, a lithium-ion battery is also provided, comprising the above-described dry electrode film.

[0052] In some embodiments of the present invention, the lithium-ion battery includes an electrode, the electrode including a current collector and a dry electrode film disposed on the surface of the current collector; preferably, the lithium-ion battery includes a positive electrode, the positive electrode including a positive electrode current collector and a dry electrode film disposed on the surface of the positive electrode current collector.

[0053] Example 1 The method for preparing the polyionic liquid copolymer provided in this embodiment includes the following steps: 13.5 mmol of N-vinylpyrrolidone was placed in a two-necked flask, and 16 mL of methanol and 6.75 mmol of 1-vinyl-3-butylimidazolium bromide were added. The mixture was stirred at a low speed. After the addition was complete, the flask was sealed and argon gas was introduced. After 30 min of purging, heating was started and the temperature was maintained at 75 °C. The reaction was continued for 11 h, and then the temperature was adjusted to 80 °C. The solution gradually boiled and became viscous. After 2 h, the solution was discharged into acetone, and a milky white precipitate was formed. This step was repeated to ensure complete precipitation, and the copolymer was obtained. Sufficient KPF6 saturated solution was added to the aqueous solution of the above copolymer and stirred at room temperature (25°C) for 24 h to obtain a precipitate; the precipitate was washed three times with water and dried under vacuum at room temperature (25°C) to obtain NVP-VBImPF6.

[0054] Example 2 The method for preparing the polyionic liquid copolymer provided in this embodiment includes the following steps: 13.5 mmol of N-vinylpyrrolidone was placed in a two-necked flask, and 16 mL of methanol and 13.5 mmol of 1-vinyl-3-butylimidazolium bromide were added. The mixture was stirred at a low speed. After the addition was complete, the flask was sealed and aeration was started. After 30 min of aeration, heating was started and the temperature was maintained at 75 °C. The reaction was continued for 11 h. The temperature was then adjusted to 80 °C. The solution gradually boiled and became viscous. After 2 h, the solution was discharged into acetone, and a milky white precipitate was formed. This step was repeated to ensure complete precipitation, and the copolymer was obtained. Sufficient KPF6 saturated solution was added to the aqueous solution of the above copolymer and stirred at room temperature (25°C) for 24 h to obtain a precipitate; the precipitate was washed three times with water and dried under vacuum at room temperature (25°C) to obtain NVP-VBImPF6.

[0055] Example 3 The dry electrode film preparation method provided in this embodiment includes the following steps: Active material (LiNi) 0.8 Co 0.1 Mn 0.1 O2) and conductive agent (Super P) were mixed at room temperature (25°C) for 1 hour, and then PTFE and NVP-VBImPF6 prepared in Example 1 were added and mixed at -10°C for 2 hours to obtain a multi-component mixture; the multi-component mixture was mixed at 20°C for 20 minutes to obtain a granular coating; the granular coating was calendered at 100°C to form a self-supporting film with a thickness of 100 μm, thus obtaining a dry electrode film; The mass ratio of the active material, conductive agent, PTFE and NVP-VBImPF6 is 95:3:1:1.

[0056] Example 4 The dry electrode film preparation method provided in this embodiment is the same as in Embodiment 3, except that the active material is LiNi. 0.6 Co 0.2 Mn 0.2 O2.

[0057] Example 5 The dry electrode film preparation method provided in this embodiment is the same as in Embodiment 3, except that the active material is LiFePO4.

[0058] Example 6 The dry electrode film preparation method provided in this embodiment is the same as in Embodiment 3, except that the active material is LiCoO2.

[0059] Example 7 The dry electrode film preparation method provided in this embodiment is the same as in Embodiment 3, except that the conductive agent is CNTs.

[0060] Example 8 The dry electrode film preparation method provided in this embodiment is the same as in Embodiment 4, except that the conductive agent is CNTs.

[0061] Example 9 The dry electrode film preparation method provided in this embodiment is the same as in Embodiment 5, except that the conductive agent is CNTs.

[0062] Example 10 The dry electrode film preparation method provided in this embodiment is the same as in Embodiment 6, except that the conductive agent is CNTs.

[0063] Example 11 The dry electrode film preparation method provided in this embodiment is the same as that in Embodiment 3, except that the thickness of the self-supporting film is 150 μm.

[0064] Example 12 The dry electrode film preparation method provided in this embodiment is the same as that in Embodiment 4, except that the thickness of the self-supporting film is 150 μm.

[0065] Example 13 The dry electrode film preparation method provided in this embodiment is the same as that in Embodiment 5, except that the thickness of the self-supporting film is 150 μm.

[0066] Example 14 The dry electrode film preparation method provided in this embodiment is the same as that in Embodiment 6, except that the thickness of the self-supporting film is 150 μm.

[0067] Example 15 The dry electrode film preparation method provided in this embodiment is the same as in Embodiment 3, except that the active material is LiNi. 0.8 Co 0.1 Mn 0.1 O2, CNTs as conductive agent; the thickness of the self-supporting film is 150 μm.

[0068] Example 16 The dry electrode film preparation method provided in this embodiment is the same as in Embodiment 3, except that the active material is LiNi. 0.6 Co 0.2 Mn 0.2 O2; conductive agent is CNTs; the thickness of the self-supporting film is 150 μm.

[0069] Example 17 The dry electrode film preparation method provided in this embodiment is the same as that in Example 3, except that the active material is LiFePO4, the conductive agent is CNTs, and the thickness of the self-supporting film is 150 μm.

[0070] Example 18 The dry electrode film preparation method provided in this embodiment is the same as that in Example 3, except that the active material is LiCoO2, the conductive agent is CNTs, and the thickness of the self-supporting film is 150 μm.

[0071] Example 19 The dry electrode film preparation method provided in this embodiment is the same as that in Example 3, except that the NVP-VBImPF6 prepared in Example 2 is used.

[0072] Comparative Example 1 The dry electrode film preparation method provided in this comparative example includes the following steps: Active material (LiNi) 0.8 Co 0.1 Mn 0.1 O2) and conductive agent (Super P) are mixed at room temperature (25℃) for 1 h, then PTFE is added and mixed at -10℃ for 2 h to obtain a multi-component mixture; the multi-component mixture is mixed at 20℃ for 20 min to obtain a granular coating; the granular coating is calendered at 100℃ into a self-supporting film with a thickness of 150 μm to obtain a dry electrode film. The mass ratio of the active material, conductive agent, and PTFE is 95:3:2.

[0073] Test case The infrared spectrum of NVP-VBImPF6 prepared in Example 1 is as follows: Figure 1 As shown, the thermogravimetric curve under nitrogen is as follows: Figure 2 As shown.

[0074] Lithium-ion half-cells were fabricated using the dry-process electrode films of Example 11 and Comparative Example 1, respectively, and their cycle performance was tested. The results are shown in Table 1. The cycle performance of the lithium-ion half-cell fabricated using the dry-process electrode film of Example 11 is as follows: Figure 3 As shown; the cycle performance of the lithium-ion half-cell fabricated using the dry electrode film of Comparative Example 1 is as follows. Figure 4 As shown.

[0075] The preparation of a lithium-ion half-cell includes: using a dry electrode membrane as the positive electrode, 1M LiPF6 in DMC:EC=1:1Vol% as the electrolyte, Celgard 2500 as the separator, and a metal Li sheet as the negative electrode to assemble the battery.

[0076] Cyclic performance: Test voltage range is 1.2~4.2V.

[0077] Table 1

[0078] From Table 1, Figure 3 and Figure 4 It can be seen that, under the same conditions, the introduction of NVP-VBImPF6 improves the cycle performance of the battery compared to the ordinary dry electrode system.

[0079] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A dry electrode film, characterized in that, include: Active substances, conductive agents, binders, and polyionic liquid copolymers; The polyionic liquid copolymer contains pyrrolidone ring structural units and alkylimidazolium cationic groups.

2. The dry electrode film according to claim 1, characterized in that, The preparation method of the polyionic liquid copolymer includes the following steps: After reacting N-vinylpyrrolidone and 1-vinyl-3-butylimidazolium bromide, KPF6 is added for anion exchange to obtain the polyionic liquid copolymer.

3. The dry electrode film according to claim 2, characterized in that, The molar ratio of the N-vinylpyrrolidone to the 1-vinyl-3-butylimidazolium bromide is 1:(0.5~1).

4. The dry electrode film according to claim 2, characterized in that, The reaction includes reacting at 70-75℃ for 10-12 hours, followed by heating to 80-85℃ and reacting for 1.5-3 hours.

5. The dry electrode film according to claim 2, characterized in that, The anion exchange time is 20-30 hours.

6. The dry electrode film according to claim 2, characterized in that, The content of polyionic liquid copolymer in the dry electrode membrane is 0.5wt%~2wt%.

7. The dry electrode film according to claim 2, characterized in that, The thickness of the dry electrode film is 100~150μm.

8. The method for preparing a dry electrode film according to any one of claims 1 to 7, characterized in that, Includes the following steps: After the active material and conductive agent are mixed evenly, a binder and a polyionic liquid copolymer are added for a first mixing to obtain a multi-component mixture; the multi-component mixture is then mixed a second time to obtain a granular coating; the granular coating is calendered into a film to obtain the dry electrode film.

9. The method for preparing a dry electrode film according to claim 8, characterized in that, Includes at least one of the following features (1) to (3); (1) The temperature of the first mixture is -5 to -15°C, and the time is 1.5 to 3 hours; (2) The temperature of the second mixing is 15~30℃ and the time is 15~30min; (3) The rolling temperature is 90~110℃.

10. A lithium-ion battery, characterized in that, Includes the dry electrode film as described in any one of claims 1 to 7.

Images