Multifunctional composite binder for lithium-rich manganese-based positive electrode, and preparation method and application thereof

Abstract

The invention belongs to the technical field of lithium ion battery preparation, and particularly relates to a multifunctional composite adhesive for a lithium-rich manganese-based positive electrode and a preparation method and application thereof. The multifunctional composite adhesive is obtained by dissolving cellulose nanocrystals (CNC) and sodium carboxymethyl cellulose (CMC) in deionized water and then stirring and cross-linking. According to the lithium-rich manganese-based positive electrode material, the bonding and mechanical properties can be effectively improved, the conductivity of a lithium-rich manganese-based positive electrode can be enhanced, reaction kinetics can be accelerated, a transmission path can be optimized, the charge-discharge cycle stability, rate capability and the like of the battery can be greatly improved, and the battery with stable capacity and long cycle life can be obtained; and within the voltage of 2-4.8 V, the capacity retention ratio is 103.36% after 50 cycles under the current of 0.1 C, the capacity retention ratio is 90.47% after 400 cycles under the current of 1C, and the specific discharge capacity can reach 161.98 mAh / g under the current of 5C. In addition, the composite binder disclosed by the invention is simple in composition, simple in preparation process, easy in raw material acquisition, low in cost, non-toxic and safe, greatly reduces the production difficulty, and is suitable for large-scale industrial production.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-ion batteries, and particularly relates to a multifunctional composite binder for a lithium-rich manganese-based cathode and its preparation and application. Technical Background

[0002] In recent years, with the rapid development of the new energy electric vehicle industry, the demand for high energy density lithium-ion batteries has been increasing day by day. As one of the core components of lithium-ion batteries, the cathode material is the key to improving the energy density of the battery. However, the energy density of commercial cathode materials can no longer meet the future requirements for lithium-ion batteries. Therefore, the development of cathode materials with higher energy density is crucial for the large-scale development of electric vehicles.

[0003] The lithium-rich manganese-based cathode material xLi2MnO3·(1-x)LiMO2 (0 < x < 1, M = transition metals such as Ni, Co, Mn and their combinations, hereinafter referred to as LRMO) uses inexpensive manganese as the main transition metal element. Because of its high specific capacity (>250 mAhg -1 ), high energy density (>1000 Whkg -1 ), and high working voltage (average > 3.5V), it has attracted much attention. However, there are still serious problems in its practical application: (1) Poor electrode charge transfer kinetics performance, including: poor electronic conductivity of the Li2MnO3 component in the lithium-rich material, which in turn causes poor overall electronic conductivity of the material, and a large number of lithium vacancies and transition metal ion migrations will occur during the activation process of its structure. When the volume structure of the cathode material changes, the conductive carbon particles may lose contact; (2) The binder has a single function. In the preparation of traditional lithium battery cathodes, polyvinylidene fluoride (PVDF) is mostly used as the binder, which only plays a mechanical role in binding the active material, conductive agent and current collector, and has electrical insulation due to its non-conjugated structure, seriously hindering ion and electron transport; (3) Poor cycle stability. During the charge and discharge process, ion mixing is likely to occur in the layered structure part, resulting in unstable microstructure of the cathode material, and then the phenomenon that the discharge specific capacity gradually decays with the increase of the cycle number. To realize the commercialization of lithium-rich manganese-based cathode materials, the above problems must be solved.

[0004] In the prior art, the strategies for improving the above problems mostly focus on means such as surface modification, ion doping and morphology control, and a series of excellent results have been achieved. However, these results are all modifications of the cathode material itself, ignoring that the binder with a small proportion (<10 wt%) will also have a great impact on the material performance. Compared with the above strategies, it is easier to operate and easier to commercialize by simply changing the binder component to improve the problems existing in the electrode.

[0005] CN118853062A discloses a polymer PHMT binder, but its research focuses primarily on its mechanical and bonding properties. Its single functionality does not ideally improve battery charge-discharge cycle stability and rate performance, and it is unable to effectively address the practical application issues of lithium-rich manganese-based cathodes. Therefore, designing a multifunctional binder that, in addition to its bonding properties, also enhances electrochemical properties such as electrode charge transfer kinetics and cycle stability is an important step towards the commercialization of lithium-rich manganese-based cathode materials. Summary of the Invention

[0006] The present invention provides a multifunctional composite adhesive for lithium-rich manganese-based positive electrodes, a preparation method and an application thereof, in order to overcome the problems of the prior art in that the adhesive has a single functionality and is not ideal for improving the charge and discharge cycle stability and rate performance of the battery.

[0007] To achieve the above objectives, the present invention provides the following technical solutions:

[0008] A method for preparing a multifunctional composite binder for a lithium-rich manganese-based positive electrode comprises the following steps

[0009] Step 1: dissolve cellulose nanocrystals (CNC) in deionized water, mix well, and then add sodium carboxymethyl cellulose (CMC);

[0010] Step 2: Stir the solution obtained in the first step at room temperature to obtain a multifunctional composite adhesive.

[0011] Furthermore, the surface of the cellulose nanocrystal contains one or more functional groups selected from sulfonic acid, nitric acid, phosphoric acid, carboxylic acid and hydroxyl groups, and the particle width of the cellulose nanocrystal is 5-20 nm and the length is 60-300 nm; the molecular weight of the sodium carboxymethyl cellulose is 250,000.

[0012] Furthermore, the cellulose nanocrystal particles are preferably 10-12 nm in width and 200 nm in length.

[0013] Furthermore, the mass ratio of the cellulose nanocrystals to sodium carboxymethyl cellulose to deionized water is 3-9:9-3:988.

[0014] Furthermore, the stirring speed of the multifunctional composite binder for the lithium-rich manganese-based positive electrode is 30-40 r / min, and the stirring time is 8-15 hours.

[0015] Furthermore, the above preparation method produces a multifunctional composite binder for lithium-rich manganese-based positive electrodes.

[0016] Furthermore, the multifunctional composite binder for the lithium-rich manganese-based positive electrode is used in lithium-rich manganese-based positive electrodes, lithium cobalt oxide positive electrodes and ternary materials.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. Compared with traditional electrically insulating binders (such as PVDF), the multifunctional composite binder of the present invention is composed of cellulose nanocrystals and sodium carboxymethyl cellulose polymerized and cross-linked, and has multiple functions. When used in lithium-ion battery lithium-manganese-rich positive electrodes, in addition to improving bonding and mechanical properties, it can enhance the conductivity of the lithium-manganese-rich positive electrode, accelerate reaction kinetics, optimize transmission paths, and significantly improve the battery's charge and discharge cycle stability and rate performance, thereby obtaining a battery with stable capacity and long cycle life.

[0019] 2. The multifunctional composite binder of the present invention has high electronic conductivity and improves the dispersibility of carbon nanomaterials in the electrode. It establishes interconnected bridges and networks through random overlapping on the surface of active particles, accelerates lithium ion transmission, reduces positive electrode impedance, and thus improves the rate performance of the battery. The button battery finally prepared has a discharge capacity of 161.98 mAh / g at a voltage of 2-4.8 V and a current of 5C.

[0020] 3. The multifunctional composite binder of the present invention can improve ion mixing and stabilize the crystal structure of the positive electrode material, thereby making the prepared button battery exhibit excellent cycle stability. Within the voltage range of 2-4.8V, the capacity retention rate is 103.36% after 50 cycles at a current of 0.1C, and the capacity retention rate is 90.47% after 400 cycles at a current of 1C.

[0021] 4. The multifunctional composite binder of the present invention is composed of cross-linked sodium carboxymethyl cellulose and cellulose nanocrystals, and the raw materials are easy to obtain and the cost is low.

[0022] 5. The preparation method of the multifunctional composite binder provided by the present invention is to directly dissolve the material in deionized water and use it as a lithium-rich manganese-based positive electrode binder, which has the advantages of being non-toxic and environmentally friendly, highly safe, low-cost, and having a simple process. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a comparison chart of the first charge and discharge capacities of batteries assembled with samples from Example 1 and Comparative Example 1 at 0.1C;

[0024] Figure 2 This is a comparison chart of the first charge and discharge capacities of batteries assembled with samples from Example 2 and Comparative Example 1 at 0.1C;

[0025] Figure 3 This is a comparison chart of the cycle performance of batteries assembled with samples from Example 2 and Comparative Example 1 at 0.1C for 50 cycles;

[0026] Figure 4This is a comparison of the cycling performance of batteries assembled from samples of Example 1 and Comparative Example 1 at 1C for 400 cycles;

[0027] Figure 5 This is a comparison chart of the cycle performance of batteries assembled from samples of Example 2 and Comparative Example 1 at 1C400 cycles;

[0028] Figure 6 This is a comparison chart of the cycle performance of batteries assembled from samples of Example 3 and Comparative Example 1 at 1C400 cycles;

[0029] Figure 7 XRD comparison diagram of the lithium-rich manganese positive electrode sheet of the battery assembled with the samples of Example 2 and Comparative Example 1 after 300 cycles at 1C;

[0030] Figure 8 A comparison chart of the rate performance of batteries assembled from samples of Example 1, Example 2, Example 3, and Comparative Example 1;

[0031] Figure 9 This is a comparison of EIS diagrams of batteries assembled from samples of Example 1, Example 2, Example 3 and Comparative Example 1 before cycling;

[0032] Figure 10 This is an EIS comparison chart of batteries assembled from samples of Example 1, Example 2, Example 3 and Comparative Example 1 after 300 cycles at 1C;

[0033] Figure 11 A comparison of ion diffusion coefficients of batteries assembled from samples of Example 1, Example 2, Example 3, and Comparative Example 1 before cycling;

[0034] Figure 12 This is a SEM comparison of the lithium-rich manganese positive electrode sheet of the battery assembled with the samples of Example 2 and Comparative Example 1 after 200 cycles at 1C;

[0035] Figure 13 This is an EDS comparison chart of the lithium-rich manganese positive electrode sheets prepared by the samples of Example 2 and Comparative Example 2. DETAILED DESCRIPTION

[0036] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood by those skilled in the art that the following examples are only for explaining and not for limiting the present invention.

[0037] Example 1, a method for preparing a multifunctional composite adhesive, comprising the following steps:

[0038] Step 1: Weigh 0.15g of cellulose nanocrystals and dissolve them directly in 49.4g of deionized water. Mix thoroughly and then add 0.45g of sodium carboxymethyl cellulose. The cellulose nanocrystals contain sulfonic acid and hydroxyl groups on their surfaces, and have a particle width of 5-10nm and a length of 60-140nm. The sodium carboxymethyl cellulose has a molecular weight of 250,000.

[0039] Step 2: Magnetic stirring was applied at room temperature at a rotation speed of 30 r / min for 15 h until the mixture was fully dissolved and polymerized, thereby obtaining a multifunctional composite adhesive with a mass concentration of 1.2%, which was recorded as NM13 for later use.

[0040] Example 2, a method for preparing a multifunctional composite adhesive, comprising the following steps:

[0041] Step 1: Weigh 0.3g of cellulose nanocrystals and dissolve them directly in 49.4g of deionized water. Mix thoroughly and then add 0.3g of sodium carboxymethyl cellulose. The cellulose nanocrystals contain sulfonic acid and hydroxyl groups on their surfaces, and have a particle width of 10-15nm and a length of 140-220nm. The sodium carboxymethyl cellulose has a molecular weight of 250,000.

[0042] Step 2: Magnetic stirring was applied at room temperature at a rotation speed of 35 r / min for 10 h until the mixture was fully dissolved and polymerized, thereby obtaining a multifunctional composite adhesive with a mass concentration of 1.2%, which was recorded as NM22 for later use.

[0043] Example 3, a method for preparing a multifunctional composite adhesive, comprising the following steps:

[0044] Step 1: Weigh 0.45g of cellulose nanocrystals and dissolve them directly in 49.4g of deionized water. Mix thoroughly and then add 0.15g of sodium carboxymethyl cellulose. The cellulose nanocrystals contain sulfonic acid and hydroxyl groups on their surfaces, and are 15-20nm wide and 220-300nm long. The sodium carboxymethyl cellulose has a molecular weight of 250,000.

[0045] Step 2: Magnetic stirring was applied at room temperature at a rotation speed of 40 / min for 8 hours until the mixture was fully dissolved and polymerized, thereby obtaining a multifunctional composite adhesive with a mass concentration of 1.2%, which was recorded as NM31 for later use.

[0046] Comparative Example 1, a method for preparing a binder, comprising the following steps:

[0047] Step 1: Weigh 0.6 g of sodium carboxymethyl cellulose and dissolve it directly in 49.4 g of deionized water. The molecular weight of the sodium carboxymethyl cellulose is 250,000.

[0048] Step 2: magnetic stirring at room temperature at a speed of 35 r / min for 10 hours until fully dissolved, to obtain a binder with a mass concentration of 1.2%, which is recorded as CMC for later use.

[0049] Comparative Example 2, a method for preparing a binder, comprising the following steps:

[0050] Step 1: Weigh 0.6 g of cellulose nanocrystals (CNC) and directly dissolve them in 49.4 g of deionized water. The cellulose nanocrystals contain sulfonic acid groups and hydroxyl groups on their surfaces, and the particle width is 10-15 nm and the length is 140-220 nm.

[0051] Step 2: Magnetic stirring was performed at room temperature at a rotation speed of 35 r / min for 10 h until the mixture was fully dissolved, to obtain a binder with a mass concentration of 1.2%, which was recorded as CNC for later use.

[0052] The adhesives obtained in Examples 1-3 and Comparative Examples 1 and 2 were prepared by the following method:

[0053] Weigh 1.25g of 1.2% binder, add 0.253g of lithium-rich manganese-based positive electrode material LRMO (Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O2) and 0.032g of conductive agent Super P were mixed and ball-milled to prepare electrode slurry, which was then coated on aluminum foil and vacuum-dried at 110°C for 20h to obtain a lithium-rich manganese-based positive electrode.

[0054] The lithium-rich manganese-based positive electrodes prepared in Examples 1-3 and Comparative Examples 1-2 were cut and vacuum-dried at 115°C for 10 hours. The cells were then placed in a glove box to prepare button batteries. The test results are as follows:

[0055] like Figure 1 、 2 As shown, the charge-discharge curves of Examples 1 and 2 show no abnormal plateaus at a voltage of 2.0-4.8 V and a current of 0.1 C, indicating that the composite binder NM is electrochemically stable in the 2.0-4.8 V range. The first-cycle discharge specific capacity of Example 1 is 300 mAh / g, that of Example 2 is 302.90 mAh / g, and that of Comparative Example 1 is 292.44 mAh / g. The first-cycle discharge specific capacity of Examples 1 and 2 is higher than that of Comparative Example 1. This is because the CNC in the composite binder NM is a conductive crystal, overcoming the low electronic conductivity of CMC alone and giving the NM itself high electronic conductivity.

[0056] like Figure 3As shown, the capacity retention rate of Example 2 tends to be stable after 5 cycles at a voltage of 2.0-4.8V and a current of 0.1C, and gradually increases after 35 cycles. The capacity retention rate is 103.36% after 50 cycles. Under the same conditions, the capacity retention rate of the control group continues to decrease. This shows that compared with the pure CMC binder, the composite binder NM after polymerization with CNC improves the cycle stability of the battery. This is because the NM composite binder after polymerization and cross-linking can establish an interconnected electron transfer network within the electrode by adsorbing carbon nanomaterial particles, avoiding the problem of poor contact caused by microscopic changes in the volume of the active particle structure of the carbon material during the cycle, thereby improving its electronic conductivity. In addition, the CNC component is a rod-like structure with a high aspect ratio. This special structure helps the composite binder NM to establish interconnected bridges and networks on the surface of the LRMO particles through random overlapping, further improving the kinetic performance of the electrode, reducing polarization and thus stabilizing the electrochemical performance.

[0057] like Figure 4 、 5 As shown in Figures 6 and 7, after 400 cycles at a voltage of 2.0-4.8V and a current of 1C, the discharge specific capacity of Example 1 is 183.29mAh / g, and the capacity retention rate is 79.39%. The discharge specific capacity of Example 2 is 203.42mAh / g, and the capacity retention rate is 90.47%. The discharge specific capacity of Example 3 is 154.24mAh / g, and the capacity retention rate is 69.06%. Under the same conditions, the discharge specific capacity of Comparative Example 1 is 124.55mAh / g, and the capacity retention rate is 56.21%. Examples 1-3 are all higher than Comparative Example 1. This shows that the battery using the composite binder NM has more stable cycle performance.

[0058] like Figure 7 As shown, by comparing the XRD patterns of Example 2 and Comparative Example 1 after 300 cycles at a current of 1C, it can be seen that the peak intensity of Example 2 is higher, reflecting the crystal structure integrity and electrochemical activity of the lithium-rich manganese-based positive electrode, indicating that the positive electrode of the battery prepared by the composite binder NM is more structurally stable during the charge and discharge process, and can more effectively transport ions and electrons, thereby providing a longer battery life and higher energy density.

[0059] like Figure 8 As shown, by comparing the discharge specific capacities of Example 1, Example 2, Example 3 and Comparative Example 1 at currents of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C and 10C, it can be seen that the battery made therefrom has better rate performance due to the improved kinetic effect of the composite binder NM.

[0060] like Figure 9 、 10As shown, the semicircle in the high-frequency region is related to the charge transfer resistance Rct. The radius of the semicircle in the high-frequency region of Example 1, Example 2 and Example 3 before and after the cycle is smaller than that of Comparative Example 1, indicating that the composite binder NM improves the migration ability of lithium ions in the electrode and improves the conductivity.

[0061] like Figure 11 As shown, the lithium ion diffusion coefficients of Example 1, Example 2, and Example 3 are 9.89143*10 - 11 cm 2 / s、1.61222*10 -10 cm 2 / s、1.14210*10 -10 cm 2 / s are higher than the lithium ion diffusion coefficient of comparative example 1 8.10619*10 -11 cm 2 / s, wherein the lithium ion diffusion coefficient of Example 2 is about twice that of Comparative Example 1, indicating that the composite binder NM with a suitable ratio of CMC and CNC has a better improvement on the kinetic performance of the electrode.

[0062] like Figure 12 As shown, by comparing the SEM images of Example 2 and Comparative Example 1 after 200 cycles at a current of 1C, it can be seen that the carbon material is more evenly dispersed in Example 2 and more agglomerated in Comparative Example 1, which proves that the composite binder NM can improve the dispersion of carbon nanoparticle materials in the electrode, thereby improving the electronic conductivity of the electrode.

[0063] like Figure 13 As shown, the carbon material of Example 2 is evenly dispersed, while the carbon material of Comparative Example 2 is poorly dispersed and agglomerated. This indicates that a properly proportioned composite binder NM improves the dispersibility of the carbon nanoparticle material, but an unbalanced composite binder inhibits the dispersion of the carbon material, thereby suppressing the electronic conductivity of the electrode.

[0064] The cycle performance tests and lithium ion diffusion coefficients of the above Examples 1-3 and Comparative Example 1 are listed in Table 1, and the rate performance test results are listed in Table 2.

[0065] Table 1

[0066]

[0067] Table 2

[0068]

[0069] In summary, embodiment 2 is the best embodiment.

[0070] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for preparing a multifunctional composite binder for a lithium-rich manganese-based positive electrode, characterized in that: The following steps are involved: Step 1: dissolve cellulose nanocrystals in deionized water, mix well, and then add sodium carboxymethyl cellulose; Step 2: Stir the solution obtained in the first step at room temperature to obtain a multifunctional composite adhesive.

2. The method for preparing a multifunctional composite binder for a lithium-rich manganese-based positive electrode according to claim 1, characterized in that: The surface of the cellulose nanocrystal contains one or more functional groups selected from sulfonic acid, nitric acid, phosphoric acid, carboxylic acid and hydroxyl groups, and the particle width of the cellulose nanocrystal is 5-20 nm and the length is 80-300 nm; the molecular weight of the sodium carboxymethyl cellulose is 250,000.

3. The method for preparing a multifunctional composite binder for a lithium-rich manganese-based positive electrode according to claim 2, characterized in that: The cellulose nanocrystal particles preferably have a width of 10-12 nm and a length of 200 nm.

4. The method for preparing a multifunctional composite binder for a lithium-rich manganese-based positive electrode according to claim 3, characterized in that: The mass ratio of the cellulose nanocrystals to sodium carboxymethyl cellulose to deionized water is 3-9:3-9:

988.

5. The method for preparing a multifunctional composite binder for a lithium-rich manganese-based positive electrode according to claim 4, characterized in that: The multifunctional composite adhesive is stirred at a speed of 30-40 r / min for 8-15 hours.

6. A multifunctional composite binder for lithium-rich manganese-based positive electrodes prepared by the preparation method according to claim 1.

7. Use of the multifunctional composite binder for lithium-rich manganese-based positive electrode according to claim 6 in lithium-rich manganese-based positive electrode, lithium cobalt oxide positive electrode and ternary positive electrode.

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