A modified polyvinylidene fluoride lithium-sulfur battery cathode binder and a preparation method thereof

Abstract

The application discloses a modified polyvinylidene fluoride lithium-sulfur battery positive electrode binder and a preparation method thereof, and belongs to the technical field of lithium-sulfur batteries. In the method, a modifier is connected to a polyvinylidene fluoride molecular chain through hydrogen bonds, cross-linking of the polyvinylidene fluoride molecular chain is realized, and the mechanical property of the polyvinylidene fluoride binder is improved. The introduction of the modifier reduces the crystallinity of the polyvinylidene fluoride, increases the porosity, promotes the swelling of the polyvinylidene fluoride binder in electrolyte, and increases the lithium ion conductivity. Meanwhile, the metallocycle connected to the iron atom in the modifier not only provides an adsorption site for lithium polysulfide, but also promotes the conversion of the lithium polysulfide. The modified polyvinylidene fluoride binder obtained by the method has the advantages of simple synthesis process and low cost, can significantly reduce sulfur positive electrode polarization when applied in a lithium-sulfur battery positive electrode, improves the cycle stability and rate performance of the battery, is suitable for traditional lithium battery electrode preparation processes, and is suitable for large-scale commercial application.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium-sulfur batteries, and particularly relates to a modified polyvinylidene fluoride lithium-sulfur battery positive electrode binder capable of improving lithium polysulfide adsorption in a lithium-sulfur battery, promoting lithium polysulfide conversion, and having good mechanical properties, and a preparation method thereof. BACKGROUND

[0002] Lithium-sulfur batteries have high theoretical energy density, low cost, and environmental friendliness, and are regarded as the next generation of secondary battery systems after the existing commercial lithium-ion batteries. However, before practical application, lithium-sulfur batteries face multiple technical challenges. On the positive electrode side, the "shuttle effect" of lithium polysulfide (LiPSs) and the huge volume change during the charging and discharging process of the sulfur positive electrode lead to the loss of active materials and the destruction of electrode integrity, thereby making the cycle stability of lithium-sulfur batteries very poor.

[0003] Polyvinylidene fluoride (PVDF) has the advantages of good electrochemical stability and thermal stability, and is the most commonly used electrode binder in commercial lithium-ion batteries. However, PVDF is not suitable for lithium-sulfur battery positive electrodes, for the following reasons: first, its bonding strength to sulfur is low, and it has poor ductility, making it difficult to maintain the integrity of the sulfur positive electrode, especially at high sulfur loadings; second, PVDF lacks LiPSs-philic functional groups and cannot play a role in inhibiting the "shuttle effect"; third, the electron and lithium ion insulation will increase the polarization of the electrode. Therefore, it is necessary to modify PVDF to improve its mechanical properties and endow it with adsorption and catalytic conversion effects on LiPSs.

[0004] In the prior art, the modification of PVDF-based binders mainly adopts copolymerization, grafting and other methods. For example, Chinese invention patent CN112175127A discloses a copolymerized PVDF resin for lithium battery binder, which improves the bonding performance of PVDF resin to positive electrode active materials and current collectors. For example, Chinese invention patent CN117089298A discloses a cross-linked polyvinylidene fluoride binder and its preparation method and an electrode using the cross-linked polyvinylidene fluoride binder. Compared with ordinary PVDF binders, this binder has stronger mechanical strength and toughness. During the battery charge and discharge cycle, it can buffer the stress generated by the volume change of the active material and enhance the overall strength of the electrode; the adhesion between the binder and the current collector is stronger, preventing the occurrence of active material breakage, peeling and other phenomena; and it also improves the conductive performance of the battery using the binder. For example, Chinese invention patent CN117625093A discloses a PVDF-based graft copolymer binder for lithium-sulfur batteries, its preparation method, and its application. The polycationic structure introduced into the polyvinylidene fluoride (PVDF) main chain adsorbs polysulfides produced during the electrochemical reaction of lithium-sulfur batteries, inhibiting the loss of positive electrode active materials and improving the cycle stability of lithium-sulfur batteries. The bistrifluoromethylsulfonyl imide anionic groups coordinated with the cationic main chain facilitate lithium ion transport in the positive electrode, improving the positive electrode's ionic conductivity. The branched binder structure provides enhanced adhesion and prevents the shedding of positive electrode active materials. However, a common drawback of these technologies is the complex preparation process and high production cost of the modified PVDF binder. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention provides a modified polyvinylidene fluoride (PVDF) binder for lithium-sulfur battery cathodes. Compared to unmodified PVDF binders, this binder improves the sulfur cathode's ability to resist volume changes during charge and discharge, particularly under high sulfur loading conditions. This allows the sulfur cathode material to adhere firmly to the current collector. Furthermore, it can effectively accelerate the LiPS conversion reaction in lithium-sulfur batteries, significantly inhibit the shuttling of soluble LiPSs, and improve the battery's long-term cycling stability.

[0006] The present invention also provides a method for preparing the modified PVDF lithium-sulfur battery cathode binder. This method uses commercial PVDF powder as the raw material, eliminating the need to modify existing PVDF powder preparation processes. The modifier is added directly to the electrode slurry as an additive. This method offers simple operation, a controllable process, and low production costs. It is well compatible with existing lithium-ion battery electrode preparation processes and therefore has promising prospects for industrial application.

[0007] In order to achieve the above technical objectives, the present invention adopts the following technical solutions.

[0008] The modified PVDF adhesive is prepared by dissolving PVDF powder and a modifier in an organic solvent according to a certain mass ratio, and then drying the organic solvent.

[0009] The modified PVDF lithium-sulfur battery positive electrode adhesive has a commercial PVDF powder as a base body, and the modifier is one or more of ferrocene boronic acid, ferrocene diboronic acid, ferrocene dicarboxylic acid, ferrocene dimethanol and diaminoferrocene, and the organic solvent is N-methyl pyrrolidone (NMP), N,N-dimethylformamide (DMF) or a mixture of the two.

[0010] The preparation method of the modified PVDF lithium-sulfur battery positive electrode adhesive sequentially comprises the following steps:

[0011] (1) PVDF powder and a modifier are added into an organic solvent according to a certain mass ratio, and a mixed solution of the PVDF and the modifier is obtained after stirring and dissolving;

[0012] (2) The organic solvent in the mixed solution obtained in step (1) is dried to obtain the modified PVDF lithium-sulfur battery positive electrode adhesive.

[0013] Further, in step (1), the mass ratio of the PVDF powder to the modifier is 1:0.05-0.25, too little modifier will result in too little hydrogen bond connection between the modifier and the PVDF, and thus the mechanical property improvement is not obvious, and too much modifier will result in the decrease of the adhesion and the electrochemical stability of the PVDF itself.

[0014] Further, in step (1), the stirring and dissolving time is 8-24h, and the preferred time is 12-16h.

[0015] Further, in step (2), the drying temperature is 60-150℃, and the preferred temperature is 80-120℃.

[0016] The mechanism of the present application is as follows:

[0017] The groups such as hydroxyl, carboxyl and amino in the modifier molecules can produce hydrogen bonds with fluorine atoms on different molecular chains of the PVDF, so as to uniformly anchor the modifier molecules on the PVDF molecular chains and cause the crosslinking effect between the PVDF molecular chains; the cyclopentadienyl ring in the modifier molecules has strong affinity for lithium ions, which is beneficial to inhibit the LiPSs shuttling and reduce the energy barrier of the reaction conversion step. Therefore, the modified PVDF lithium-sulfur battery positive electrode adhesive is reasonably constructed, aiming to improve the mechanical property of the PVDF adhesive, accelerate the process of the lithium-sulfur battery positive electrode LiPSs conversion and stabilize the positive electrode structure, so as to realize the excellent cycle performance of the lithium-sulfur battery.

[0018] Compared with the prior art, the present application has the following beneficial effects:

[0019] (1) The modified PVDF lithium-sulfur battery positive electrode binder constructed by the application can well maintain the integrity of the positive electrode in the battery charging and discharging process, and exhibits good adsorption and catalytic conversion effect on soluble LiPSs, thereby improving the cycle stability of the lithium-sulfur battery;

[0020] (2) The binder obtained by the method has better electrochemical stability, more excellent catalytic activity and lower production cost than other types of cross-linked organic polymer binders;

[0021] (3) The method can be compatible with the existing lithium ion battery electrode preparation process, and is convenient for realizing large-scale production. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 X-ray photoelectron spectrum of the modified PVDF binder prepared in Example 1;

[0023] Figure 2 Fourier transform infrared spectrum of the modified PVDF binder prepared in Example 1;

[0024] Figure 3 The X-ray diffraction pattern of the binder prepared in Examples 1-3 and Comparative Example 1;

[0025] Figure 4 The Young's modulus column chart of the binder prepared in Examples 1-3 and Comparative Example 1;

[0026] Figure 5 Comparison of the cycle performance of the binder prepared in Example 1 and Comparative Example 1 in lithium-sulfur battery;

[0027] Figure 6 Comparison of the charge-discharge curve of the binder prepared in Examples 1-3 and Comparative Example 1 in lithium-sulfur battery;

[0028] Figure 7 Comparison of the rate performance of the binder prepared in Examples 1-3 and Comparative Example 1 in lithium-sulfur battery. DETAILED DESCRIPTION

[0029] The technical solutions of the application will be described in detail below in conjunction with the drawings and examples, and the described examples do not limit the application. Any modification or equivalent replacement of the technical solutions of the application without departing from the spirit and scope of the technical solutions of the application shall be covered in the protection scope of the application.

[0030] Unless otherwise specified, the experimental instruments and experimental chemicals described below can be obtained from commercial sources.

[0031] Example 1: PVDF lithium-sulfur battery cathode binder modified by ferrocene boronic acid (English name: Ferrocene boronic acid, abbreviation: FcBA, molecular formula: C 10 H 11 BFeO2) is prepared by the following steps:

[0032] (1) 80 mg of FcBA powder and 320 mg of PVDF powder are dissolved in 9.6 g of NMP and stirred for 12 h to obtain a mixed NMP solution of PVDF and FcBA with a solute mass fraction of 4%;

[0033] (2) The solution obtained in step (1) is dried at 100°C to obtain a modified PVDF binder with a FcBA mass fraction of 20%.

[0034] Example 2: The difference between Example 2 and Example 1 is that in step (1), the amount of FcBA used is 20 mg, and the amount of PVDF powder used is 380 mg; in step (2), a modified PVDF binder with a FcBA mass fraction of 5% is obtained.

[0035] Example 3: The difference between Example 3 and Example 1 is that in step (1), the amount of FcBA used is 40 mg, and the amount of PVDF powder used is 360 mg; in step (2), a modified PVDF binder with a FcBA mass fraction of 10% is obtained.

[0036] Comparative Example 1: Unmodified PVDF binder is prepared by the following steps:

[0037] (1) 400 mg of PVDF powder is dissolved in 9.6 g of NMP and stirred for 12 h to obtain a PVDF NMP solution with a solute mass fraction of 4%;

[0038] (2) The solution obtained in step (1) is dried at 100°C to obtain an unmodified PVDF binder.

[0039] The following analyzes the performance of the binders prepared in Examples 1-3 and Comparative Example 1:

[0040] Figure 1For the X-ray photoelectron spectroscopy of the modified PVDF binder with 20% FcBA mass fraction in Example 1, it is found from the figure that a new peak appears at the position of 292.4 eV in the C 1s spectrum of Example 1, which is because FcBA is dispersed in the PVDF multi-group linear arrangement molecular chain, causing the F atoms in the local PVDF molecular chain to not be directly opposite to the H atoms in the previously arranged molecular chain, resulting in the increase of the binding energy of the local C chain; similarly, a new peak is also found at the position of 688.3 eV in the F 1s spectrum, which is consistent with the result of C 1s, and the peak binding energy at 687.28 eV is reduced due to the hydrogen bond between the C-F chain and FcBA.

[0041] Figure 2 For the Fourier transform infrared spectroscopy of the modified PVDF binder with 20% FcBA mass fraction in Example 1, it can be seen from the figure that the PVDF-FcBA binder film has a new absorption peak at 3250-3500 cm -1 The vibration contraction of O-H is narrowed and right-shifted relative to the absorption peak of pure FcBA powder, indicating that there is a hydrogen bond between the PVDF and FcBA molecular chains, and the F-C-F absorption peak of the corresponding PVDF-FcBA film at 1230-1400 cm -1 The F-C-F dipole moment is significantly reduced, which is also caused by the hydrogen bond between the PVDF and FcBA molecules.

[0042] Figure 3 For the grazing incidence X-ray diffraction patterns of the binders prepared in Examples 1-3 and Comparative Example 1, it can be found from the figure that the diffraction peaks of the modified PVDF binders in Examples 1-3 match those of the unmodified PVDF binder in Comparative Example 1, and the diffraction peak intensities of the (100), (020), (110) and (021) crystal planes of PVDF are continuously reduced with the increase of the content of FcBA additive, indicating that the addition of FcBA reduces the crystallinity of PVDF and increases the porosity, which can promote the absorption of electrolyte by the PVDF binder.

[0043] Figure 4 For the Young's modulus column chart of the binders prepared in Examples 1-3 and Comparative Example 1, it can be seen from the figure that the average Young's modulus of the unmodified PVDF binder in Comparative Example 1 is only 0.45 GPa, while the average Young's modulus of the modified PVDF binder with 20% FcBA mass fraction in Example 1 can reach 2.12 GPa, indicating that the modified PVDF binder has higher toughness and mechanical properties, which is beneficial to buffer the volume expansion of high-load sulfur cathode and reduce the shedding of cathode material during the cycle process.

[0044] Figure 5For the comparison of the cycle performance of the binders prepared in Example 1 and Comparative Example 1 in lithium-sulfur batteries, it can be seen that, under a current density of 2C, long cycle of 600 cycles, the battery using the binder in Example 1 exhibits excellent cycle stability, and the capacity retention rate is above 83%, and the coulombic efficiency is maintained at about 98.5%, indicating that the modified binder has excellent chemical stability in the battery, and the sulfur positive electrode maintains better integrity; while the battery using the unmodified PVDF binder in Comparative Example 1 exhibits low capacity and continuously decreasing coulombic efficiency in long cycle, which may be due to the continuous shuttling and accumulation of LiPSs or the corrosion of the negative metal lithium.

[0045] Figure 6 For the comparison of the charge-discharge curves of the binders prepared in Example 1-3 and Comparative Example 1 in lithium-sulfur batteries, it can be seen from the figure that the battery using the modified PVDF binder with 20% FcBA content in Example 1 has the lowest voltage polarization.

[0046] Figure 7 For the comparison of the rate curves of the binders prepared in Example 1-3 and Comparative Example 1 in lithium-sulfur batteries, it can be seen from the figure that, when the current density is 0.2C-3C, whether it is small current density or large current density, the battery using the modified PVDF binder with 20% FcBA content in Example 1 has the highest capacity, exhibiting good rate performance.

Claims

1. A method for preparing a modified polyvinylidene fluoride lithium-sulfur battery positive electrode binder, characterized in that: The following steps are involved: (1) adding polyvinylidene fluoride powder and ferrocenyl boric acid modifier into an organic solvent at a mass ratio of 1: 0.05-0.25, stirring and dissolving to obtain a mixed solution of polyvinylidene fluoride and ferrocenyl boric acid; (2) The organic solvent in the mixed solution obtained in step (1) is dried to obtain the modified polyvinylidene fluoride lithium-sulfur battery positive electrode binder.

2. The method for preparing a modified polyvinylidene fluoride lithium-sulfur battery positive electrode binder according to claim 1, wherein: The organic solvent in step (1) is N-methylpyrrolidone (NMP) or N,N-dimethylformamide (DMF) or a mixture of the two.

3. A modified polyvinylidene fluoride lithium-sulfur battery positive electrode binder prepared according to any one of the preparation methods of claims 1 to 2.

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