Sodium-ion battery aqueous binder and positive electrode slurry thereof

Abstract

The application belongs to the technical field of sodium ion battery materials, and particularly relates to a sodium ion battery water-based binder and a positive electrode slurry thereof. Poly[(methyl vinyl ether)-(maleic acid)] is hydrolyzed in a sodium hydroxide solution, and poly[(methyl vinyl ether)-(sodium maleate)] is obtained after vacuum drying, which is used as a water-based binder for a sodium ion battery positive electrode. The water-based binder has both high-polarity groups and low-polarity groups, thus solving the problem of easy settlement of conductive carbon in the water-based binder, ensuring high adhesion between components, and enhancing the volume expansion relief and repair capacity of the water-based binder during the charging and discharging process of the positive electrode sheet compared with PVDF and other binders. In general, the water-based binder has strong adhesion, repair force and interface adaptability, is environment-friendly and low in cost, and can make P2-Na x The MO2 positive electrode material has good energy density and stable cycle characteristics when used in a sodium ion battery.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery materials technology, specifically to a water-based binder for sodium-ion batteries and its positive electrode slurry. Background Technology

[0002] Since its commercialization in 1991, lithium-ion batteries have experienced rapid development and are widely used in handheld electronic devices, electric transportation equipment, and stationary energy storage systems. This rapid market expansion has led to energy shortages and rising costs. Therefore, there is a need to develop new energy storage batteries that are environmentally friendly and utilize abundant raw materials. Sodium-ion batteries (SIBs) have the potential for large-scale application due to the abundance of sodium in nature and their technological similarities to lithium-ion batteries. However, compared to lithium-ion batteries, sodium-ion batteries still suffer from low energy density and short lifespan. Layered transition metal oxides (Na₂O₃)... x MO2 (where M is a transition metal) has attracted widespread attention from researchers as a high-energy-density SIB cathode material. Based on the sodium content, it mainly has two structures: P2 phase and O3 phase. The P2 phase structure contains Na... x MO2 is relatively less sensitive to humidity, especially after being doped with transition metals such as Fe and Ni, which further enhances its resistance to the effects of water on Na. + Ion exchange. P2-Na x Fe 1 / 2 Mn 1 / 2 The O2 cathode material has a wide operating voltage range (1.5-4.3V) and a high specific capacity (190mAh g / g). -1 This type of cathode material exhibits energy density characteristics comparable to lithium-ion batteries. However, its cycle performance is unsatisfactory. The main reason is that during charge and discharge, sodium ions undergo extraction and insertion, causing the material to continuously switch between the O3 and P2 phases, resulting in a volume change exceeding 20%. This volume expansion leads to separation between the active material and the conductive agent, resulting in discontinuous electron transport and detachment of the electrode material from the current collector, ultimately leading to increased electrode impedance and decreased specific capacity. Another factor contributing to the unstable cycle performance of this type of material is that its high surface activity easily leads to electrolyte decomposition under high voltage, resulting in persistently low coulombic efficiency during cycling. To improve the efficiency of Na… x To improve the cycle stability of MO2-type cathode materials, researchers have employed heteroatoms (such as Cu and Ti) to dope the active material, reducing its expansion rate during cycling. Other methods include using electrolyte additives or solid electrolytes to mitigate electrolyte decomposition under high voltage.

[0003] Existing literature addresses the P2-Na problem from the perspective of binders. xThe problem of weak cycle stability in MO2-type cathode materials lies in the role of binders. The binder's function is to fully disperse and bind the active material and conductive carbon to the current collector, ensuring good adhesion and electron transport between components. It also needs to provide a compatible interface between the active material, conductive carbon, and electrolyte, reducing side reactions and ensuring good cycle characteristics. Currently, 90% of the binders used in sodium-ion batteries are polyvinylidene fluoride (PVDF), a technology inherited from lithium-ion batteries. The adhesive strength of PVDF mainly comes from mechanical interlocking and weak intermolecular forces. Therefore, when the electrode undergoes significant volume expansion during charging and discharging, the integrity of the electrode structure is compromised, leading to cracks. Furthermore, the solvent for PVDF is N-methylpyrrolidone (NMP), which is difficult to degrade, environmentally unfriendly, and expensive. Therefore, PVDF cannot meet the needs of different battery electrode slurry preparations, and the market urgently needs to develop new adhesive systems with strong adhesion, repair capabilities, interface adaptability, environmental friendliness, and low cost. Summary of the Invention

[0004] Therefore, the object of this invention is to provide an aqueous binder suitable for sodium-ion batteries. A common polymer material was screened and modified to effectively disperse carbon-based conductive agents, improve the bonding strength between the active material, conductive carbon, and current collector to resist volume expansion during charge and discharge, and facilitate the formation of a highly adaptable positive electrode interfacial electrolyte film (CEI) at the electrolyte and active material interface, thereby reducing side reactions in the electrolyte.

[0005] To achieve the above objectives, this discovery employs the following technical solution:

[0006] This invention provides an aqueous binder for sodium-ion batteries and its preparation method. Poly[(methyl vinyl ether)-(maleic acid)] is hydrolyzed in sodium hydroxide solution and then vacuum dried to obtain poly[(methyl vinyl ether)-(sodium maleate)], which is then used as an aqueous binder for the positive electrode of sodium-ion batteries.

[0007] Furthermore, since the molecular weight of poly[(methyl vinyl ether)-(maleic acid)] affects its solubility in water, and thus its alkalization treatment and slurry mixing, the weight-average molecular weight of poly[(methyl vinyl ether)-(maleic acid)] is preferably 130,000 to 1,000,000, more preferably 200,000 to 500,000, and most preferably 216,000.

[0008] Furthermore, poly[(methyl vinyl ether)-(maleic acid)] was prepared at 0.1 mol·L⁻¹ -1 Hydrolysis in sodium hydroxide solution at 25-40℃ for no less than 15 hours.

[0009] Furthermore, the mass ratio of poly[(methyl vinyl ether)-(maleic acid)] to sodium hydroxide is 2.5 to 5:1.

[0010] The present invention provides a positive electrode slurry for sodium-ion batteries, comprising 2-10 parts of the above-mentioned aqueous binder solution, 80-90 parts of active material, and 3-15 parts of carbon-based conductive agent.

[0011] Furthermore, the concentration of the aqueous adhesive solution is 1wt%-10wt%. In order to reduce the occurrence of side reactions, the concentration of the aqueous adhesive solution can be slightly higher. To reduce the initial use of water, the concentration of the aqueous adhesive solution is preferably 5%-8%.

[0012] Furthermore, if too much aqueous binder solution is added, there will be too much inactive material, which will affect the energy density of the sodium-ion battery. If too little is added, it will affect the dispersibility of the active material and conductive carbon black, reduce the adhesion of the binder, and affect the performance. Therefore, it is necessary to reasonably control the amount of aqueous binder solution added. Preferably, the active material is 80-90 parts, the carbon conductive agent is 8-15 parts, and the aqueous binder solution is 4-8 parts. More preferably, the mass ratio is 80:15:5.

[0013] Furthermore, the active substance is P2 type Na. x MO2 (M-transition metal), including Na 0.67 MnO2, Na 0.5 Ni 0.25 Mn 0.75 O2, Na 2 / 3 Ni 1 / 3 Mn 2 / 3 O2, Na 2 / 3 Fe 1 / 2 Mn 1 / 2 O2, Na 2 / 3 Cu 1 / 12 Ni 1 / 4 Mn 2 / 3 One or more of O2.

[0014] Furthermore, the carbon-based conductive agent is one or more of conductive carbon black, graphene, carbon nanotubes, acetylene black, and carbon fiber.

[0015] Furthermore, the solid content of this positive electrode slurry is 30%-60%.

[0016] Furthermore, the positive electrode slurry was tested in 100 seconds. -1 Viscosity at shear rate is 1500-6000 mPa·s -1 .

[0017] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0018] 1) Water is used as a solvent, the material source is abundant, and the subsequent electrode drying and waste battery recycling are simple and energy-saving, so the production cost is greatly reduced and it is very environmentally friendly.

[0019] 2) Water-based binders contain both high-polarity and low-polarity groups, thus solving the problem of easy sedimentation of conductive carbon in water-based binders, while ensuring high adhesion between the components.

[0020] 3) PVDF relies on mechanical interlocking to generate adhesive force, while the aqueous binder provided by this invention relies on hydrogen bonds to generate adhesive force, thus resulting in stronger adhesive force and easier repair of hydrogen bonds. Therefore, this aqueous binder has a greater ability to alleviate and repair volume expansion of the positive electrode during charging and discharging than binders such as PVDF.

[0021] 4) Introduce a certain amount of Na during binder modification. + This can not only inhibit the Na+ content in the active substances during slurry preparation, but also... + The dissolution can also compensate for the Na+ leaching caused by the formation of the SEI film during charging and discharging. + The loss and irreversibility of [something]. Therefore, this aqueous binder can ensure that P2-type NaxMO2 for sodium-ion batteries achieves high energy density and cycle stability while maintaining low cost and environmental friendliness.

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort. Attached Figure Description

[0023] Figure 1 The data graphs show the peel strength of the positive electrode sheets prepared with the binders described in Examples 1-9 and Comparative Examples 1-2 of this invention.

[0024] Figure 2 The graphs show the charge transfer impedance data of sodium-ion batteries prepared from the positive electrode sheets prepared with the binders described in Examples 2, 5, 8 and Comparative Examples 1-2 of this invention, after the first cycle and after 300 cycles. Detailed Implementation

[0025] The above-described solution will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. The implementation conditions used in the embodiments may be further adjusted according to the conditions of specific manufacturers, and the implementation conditions not specified are generally those in routine experiments.

[0026] This invention is illustrated by way of example rather than limitation. It should be noted that "a" or "an" embodiment described in this disclosure does not necessarily refer to the same specific embodiment, but rather to at least one.

[0027] Various aspects of the invention will be described below. However, it will be apparent to those skilled in the art that the invention may be practiced according to only some or all of its aspects. For illustrative purposes, specific reference numerals, materials, and configurations are given herein to enable a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention can be practiced without specific details. In other instances, well-known features have been omitted or simplified so as not to obscure the invention.

[0028] The various operations are described sequentially as multiple discrete steps and in a manner most conducive to understanding the invention; however, the sequential description should not be construed as implying that these operations necessarily depend on the order.

[0029] Various embodiments will be described based on typical types of reactants. It will be apparent to those skilled in the art that the invention can be implemented using any number of different types of reactants, not just those given herein for illustrative purposes. Furthermore, it will also be apparent that the invention is not limited to any particular mixture example.

[0030] The low-polarity methyl vinyl ether group on the binder of this invention can adsorb onto the surface of carbon atoms, creating steric hindrance around the carbon atoms and thus preventing the carbon material from settling in an aqueous environment. Simultaneously, the other end of the binder is designed with a highly polar -COONa group, which can form hydrogen bonds. The strong adhesive force generated by these hydrogen bonds can resist the volume expansion during the charging and discharging process of sodium-ion batteries. This is consistent with the reported results that the high adhesive force of binders in Si anodes helps to mitigate their expansion and cracking. Furthermore, the hydrogen bonds are easily restored after breaking, thus the cracks between different parts caused by electrode expansion during charging and discharging have a self-healing function.

[0031] It is worth noting that, although P2 type Na x MO2 materials are relatively stable in water, but they will still undergo some degree of H2 oxidation. + / Na + Exchange reaction, Na x MO2 + yH2O → H y Na x-y MO2+yNa + +yOH - According to the principle of acid-base equilibrium, the presence of sodium ions in the binder increases the Na+ content in the product. +An increased content of Na is detrimental to the reaction proceeding towards the product side, thus inhibiting the activity of the active substance Na. x The decomposition of MO2 ensures the stability of its structure during the homogenization process.

[0032] In addition, according to the working mechanism of sodium-ion batteries, a portion of Na + During charging and discharging, it reacts with the electrolyte to form SEI, which no longer participates in the reversible insertion or extraction during subsequent charging and discharging processes and no longer contributes to energy storage. Therefore, the sodium ions in the binder provide an additional sodium source to compensate for the loss of active sodium and improve the coulombic efficiency of sodium-ion batteries.

[0033] Furthermore, impedance testing showed that the sodium-ion battery using poly[(methyl vinyl ether)-(sodium maleate)] as a binder exhibited low charge transfer impedance during the initial cycle and after 300 cycles. This indicates a lower polarization degree compared to batteries using PVDF or CMC-Na as binders, suggesting the formation of a stable CEI film during cycling and mitigating electrolyte decomposition. Therefore, the novel aqueous binder poly[(methyl vinyl ether)-(sodium maleate)] is beneficial for P2-type Na... x Improved cycle stability of cathode materials for MO2 sodium-ion batteries.

[0034] It is worth noting that the binder disclosed in this invention can fully disperse conductive carbon and can fully contact the active particles or current collector, with a strong binding effect, ensuring the structural stability of the electrode during the sodium insertion / extraction process and improving the cycle life of the battery.

[0035] Furthermore, the molecular weight of poly[(methyl vinyl ether)-(sodium maleate)] affects the length of the binder's molecular chains. Excessively long chains prevent the active material and carbon black from being dispersed and cross-linked in all dimensions, affecting dispersibility and viscosity. Conversely, excessively low molecular weight results in a lack of long-range adhesion, hindering subsequent efficient electron transport.

[0036] The poly[(methyl vinyl ether)-(maleic acid)] used in the embodiments of this invention is the Hubei Xinjing AP series, with molecular weights of 130,000-200,000, 200,000-500,000, and 500,000-1,000,000 respectively. The binder system used in the comparative examples is PVDF / NMP and CMC-Na / H2O. All raw materials used are commercially available from a certain company.

[0037] Example 1

[0038] A method for preparing an aqueous slurry binder for sodium-ion batteries includes the following steps:

[0039] S1. Prepare 0.1 mol·L⁻¹ -1 Add 1.5g of poly[(methyl vinyl ether)-(maleic acid)] with a molecular weight of 130,000-200,000 to 100mL of sodium hydroxide solution and mix well.

[0040] S2. The mixed solution in S1 was subjected to hydrolysis at 25°C for 12 hours. After the reaction, it was vacuum dried for 24 hours to obtain the aqueous binder P13, which was then prepared into a 1 wt% aqueous solution.

[0041] Example 2

[0042] A water-based slurry binder for sodium-ion batteries is prepared in the same way as in Example 1, except that it is prepared as a 5wt% aqueous solution.

[0043] Example 3

[0044] A water-based slurry binder for sodium-ion batteries is prepared in the same way as in Example 1, except that it is prepared as an 8wt% aqueous solution.

[0045] Example 4

[0046] A water-based slurry binder for sodium-ion batteries is prepared using the same method as in Example 1, except that a poly[(methyl vinyl ether)-(maleic acid)] with a molecular weight of 200,000-500,000 is used, and the hydrolysis temperature is increased to 40°C to obtain an aqueous binder P20, which is then prepared as a 3wt% aqueous solution.

[0047] Example 5

[0048] A water-based slurry binder for sodium-ion batteries is prepared in the same way as in Example 4, except that it is prepared as a 5wt% aqueous solution.

[0049] Example 6

[0050] A water-based slurry binder for sodium-ion batteries is prepared in the same way as in Example 4, except that it is prepared as an 8wt% aqueous solution.

[0051] Example 7

[0052] A water-based slurry binder for sodium-ion batteries is prepared using the same method as in Example 1, except that a poly[(methyl vinyl ether)-(maleic acid)] with a molecular weight of 500,000-1,000,000 is used, and the hydrolysis temperature is increased to 40°C to obtain an aqueous binder of P50, which is then prepared as a 3wt% aqueous solution.

[0053] Example 8

[0054] A water-based slurry binder for sodium-ion batteries is prepared in the same way as in Example 7, except that it is prepared as a 5wt% aqueous solution.

[0055] Example 9

[0056] A water-based slurry binder for sodium-ion batteries is prepared in the same way as in Example 7, except that it is prepared as a 10wt% aqueous solution.

[0057] Comparative Example 1

[0058] A 3wt% NMP solution was prepared using commercially available polyvinylidene fluoride from a certain company as a binder for the positive electrode oily slurry.

[0059] Comparative Example 2

[0060] Sodium carboxymethyl cellulose, commercially available from a certain company, was used as the binder for the positive electrode aqueous slurry, and it was prepared into an aqueous solution with a mass fraction of 5 wt%.

[0061] To further demonstrate the beneficial effects of the present invention and to better understand the present invention, the binders of the above embodiments and comparative examples were used to prepare sodium-ion battery positive electrode sheets, and the performance of the sodium-ion water-based slurry binder disclosed in the present invention was further clarified based on their performance.

[0062] The positive electrode sheet was fabricated using the binders obtained in Examples 1-9 and Comparative Examples 1-2, as follows:

[0063] In the positive electrode slurry, the active material, conductive carbon black, and binder solution are mixed in a weight ratio of 80:15:5. First, binder solutions of different concentrations are mixed with conductive carbon black and stirred until homogeneous. Then, the active material is added to the mixture, and the stirring temperature and time are adjusted until homogeneous. Deionized water is added several times to adjust the viscosity, bringing the solid content of the material to 40%. The slurry is then evenly coated onto a 10μm thick aluminum foil, vacuum dried, and rolled to obtain the positive electrode sheet. The sheet is then sliced, weighed, and assembled into a sodium-ion button cell.

[0064] Electrode peel strength test: The rolled electrode was cut into strips 30mm wide and 60mm long. Double-sided tape was used to fix one side of the electrode material layer to the substrate. The strip was then transferred to a tensile testing machine and peeled at a speed of 100mm / min in a 90° direction. The peel strength was measured. The strength at which the current collector was peeled from the material layer was also measured. The results are as follows: Figure 1 As shown.

[0065] Electrochemical performance testing: Electrochemical impedance spectroscopy (EIS) was performed on the sodium-ion battery under the following conditions: 25℃, 50% SOC, voltage amplitude 5mV, and frequency range from 10... -2 -10 5 Hz. The measured results are as follows: Figure 2 As shown.

[0066] The cycle stability of the sodium-ion battery was tested as follows: the battery's charge-discharge cycle performance was tested within a voltage range of 2.0-4.5V and a rate of 0.1C. The coulombic efficiency diagram for the first cycle and the capacity retention rate after 300 cycles are shown in Table 1 below.

[0067] Results analysis: From Figure 1 It can be seen that using poly[(methyl vinyl ether)-(sodium maleate)], which has both low and high polarity groups, as a binder can significantly improve the bonding strength between particles and current collectors. When the binder chain length is appropriate, the ideal peel strength is more than twice that of PVDF and CMC-Na.

[0068] from Figure 2 As shown in Table 1, the binder provided by this invention has low charge transfer impedance at the start of the cycle and after 300 cycles, and improved cycle stability after 300 cycles. This indicates that the binder can help form a highly adaptable CEI film at the interface between the electrolyte and the active material, alleviate the decomposition of the electrolyte, and ensure the structural integrity of the active material during the cycling process.

[0069] As can be seen from the first-cycle coulombic efficiency data in Table 1, the sodium carboxyl group in the binder provided by this invention can compensate for the loss of sodium ions during the formation of the SEI film during cycling, thereby ensuring high initial charge-discharge efficiency.

[0070] Table 1. Specific capacity test results and initial coulombic efficiency of sodium-ion batteries.

[0071]

Claims

1. A method for preparing an aqueous binder for sodium-ion batteries, characterized in that, Poly[(methyl vinyl ether)-(maleic acid)] with a weight-average molecular weight of 216,000 was subjected to a reaction at 0.1 mol·L⁻¹ -1 The poly[(methyl vinyl ether)-(maleic acid)] was hydrolyzed in sodium hydroxide solution at 25-40℃ for no less than 15 hours. The mass ratio of poly[(methyl vinyl ether)-(maleic acid)] to sodium hydroxide was 2.5-5:

1. After vacuum drying, poly[(methyl vinyl ether)-(sodium maleate)] was obtained and used as a water-based binder for the positive electrode of sodium-ion batteries.

2. The aqueous adhesive prepared by the method of claim 1.

3. A positive electrode slurry for a sodium-ion battery, characterized in that, The solution comprises 2-10 parts by weight of the aqueous binder solution as described in claim 2, 80-90 parts by weight of the active substance, and 3-15 parts by weight of the carbon-based conductive agent, wherein the concentration of the aqueous binder solution is 1wt%-10wt%.

4. A positive electrode slurry for a sodium-ion battery, characterized in that, The solution comprises 4-8 parts by weight of the aqueous binder solution as described in claim 2, 80-90 parts by weight of the active substance, 8-15 parts by weight of the carbon-based conductive agent, and the concentration of the aqueous binder solution is 5%-8%.

5. A positive electrode slurry for a sodium-ion battery, characterized in that, It includes 5 parts by weight of the aqueous binder solution as described in claim 2, 80 parts by weight of the active substance, and 15 parts by weight of the carbon-based conductive agent.

6. The positive electrode slurry according to any one of claims 3 to 5, characterized in that, The active substance is Na 0.67 MnO2, Na 0.5 Ni 0.25 Mn 0.75 O2, Na 2 / 3 Ni 1 / 3 Mn 2 / 3 O2, Na 2 / 3 Fe 1 / 2 Mn 1 / 2 O2, Na 2 / 3 Cu 1 / 12 Ni 1 / 4 Mn 2 / 3 One or more of O2; the carbon-based conductive agent is one or more of conductive carbon black, graphene, carbon nanotubes, acetylene black, and carbon fiber.

7. The positive electrode slurry according to any one of claims 3 to 5, characterized in that, The solid content of this positive electrode slurry is 30%-60%.

8. The positive electrode slurry according to any one of claims 3 to 5, characterized in that, The positive electrode slurry was in 100s -1 Viscosity at shear rate is 1500-6000 mPa·s -1 .

Images