Composite binder applicable to wide-temperature-range hard carbon negative electrode and application of composite binder

Abstract

The invention relates to a composite binder suitable for a wide-temperature-range hard carbon negative electrode and application thereof, belongs to the technical field of sodium ion batteries, and aims to solve at least one of the problems that an existing sodium ion battery cannot meet the wide-temperature-range high-load requirement, the cycle performance of the battery is poor, and the discharge performance at low temperature is poor. The adhesive is formed by compounding PAA-MIBPA and SBR, one end of MIBPA is connected with carboxyl of PAA through an amido bond (keeping polarity), and an alkyl chain and amino at the other end of MIBPA have certain hydrophobicity, so that compatibility with SBR can be enhanced, PAA-MIBPA and SBR are promoted to form a more uniform interpenetrating network, and the glass-transition temperature of PAA can be reduced. Besides, the primary amino group (-NH2) of the MIBPA is coordinated with Na < + >, so that the ionic conductivity of the electrode can be remarkably improved, the charge transfer resistance of the battery is reduced, and the rate capability of a battery cell is further improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sodium ion batteries, in particular to a composite binder suitable for a hard carbon negative electrode in a wide temperature range and application thereof. BACKGROUND

[0002] Sodium ion batteries are becoming an important choice for multi-field energy due to their low cost, resource abundance and high safety, etc. The main application scenarios in the future include energy storage power stations, two-wheeled electric vehicles and some mobile devices in cold regions, etc.

[0003] However, there are some problems in the application of the current high-capacity sodium ion battery negative electrode: the sodium ion transmission network in the high surface density electrode sheet (≥18mg / cm 2 ) is not fully constructed, and the ion transmission is limited, resulting in a decrease in active material utilization rate; the hard carbon material has volume change during charging and discharging, and the traditional binder system is difficult to balance the mechanical strength and elastic modulus of the negative electrode sheet under low temperature conditions, limiting the low temperature electrochemical performance of the material.

[0004] PAA (polyacrylic acid) as a commonly used sodium ion battery negative electrode binder is prone to brittle transition under low temperature conditions, resulting in the expansion of micro-cracks in the electrode sheet and a sharp decrease in capacity retention rate. In the existing high surface density electrode sheet manufacturing technology, PAA is often used in combination with SBR (styrene-butadiene rubber), but the carboxyl group of PAA is prone to protonation under low temperature (-COO - +H + →-COOH, i.e. dissociation is inhibited), the proportion of negative groups (-COOH) decreases, further reducing the sodium ion conductivity, and limiting the rate performance of the high load electrode sheet. Under low temperature conditions, the brittleness of PAA itself increases, and the high load electrode sheet is prone to cracking due to volume change, SBR has a low glass transition temperature, but SBR alone cannot completely compensate for the low temperature stiffness problem of PAA, and the PAA / SBR system cannot meet the wide temperature range requirement under low temperature conditions due to the rupture of the hydrogen bond network. Therefore, it is crucial to develop a wide temperature range negative electrode sheet that can maintain structural stability and toughness under low temperature conditions. SUMMARY

[0005] In view of the above analysis, the present application aims to provide a composite binder suitable for a hard carbon negative electrode in a wide temperature range and application thereof, to solve at least one of the problems that the existing sodium ion battery cannot meet the high load demand in a wide temperature range, the battery has poor cycle performance, poor discharge performance under low temperature, etc.

[0006] In a first aspect, the present application provides a composite binder suitable for hard carbon negative electrode in a wide temperature range, wherein the binder comprises modified polyacrylic acid (PAA-MIBPA) and styrene-butadiene rubber (SBR), and the modified polyacrylic acid is prepared by grafting N,N-bis(3-aminopropyl)methylamine (MIBPA) onto the side chain of polyacrylic acid.

[0007] Further, the mass ratio of the modified polyacrylic acid to the styrene-butadiene rubber is 2.2-2.4:1.8-2.

[0008] Further, the modified polyacrylic acid is prepared by the following method:

[0009] The polyacrylic acid is added to water, the pH value is adjusted, EDC (1-ethyl-(3-dimethylaminopropyl) carbodiimide) and Sulfo-NHS (N-hydroxysulfosuccinimide) are added for activation, grafting stirring reaction is performed, MIBPA (N,N-bis(3-aminopropyl)methylamine) is added, stirring and dissolution are performed, and freeze-drying is performed to obtain the modified polyacrylic acid.

[0010] Further, the pH value is adjusted to 5-6.5.

[0011] Further, the molar ratio of the polyacrylic acid, EDC and Sulfo-NHS is 1:1.5-2:1-1.2, and the molar ratio of the polyacrylic acid to MIBPA is 7:1-8.5:1.

[0012] In a second aspect, the present application provides a negative electrode slurry, characterized in that it comprises the above-mentioned composite binder.

[0013] Further, the negative electrode slurry comprises, in terms of mass ratio, hard carbon negative electrode material: conductive agent: modified polyacrylic acid: styrene-butadiene rubber = 94.2-94.6:1.3-1.5:2.2-2.4:1.8-2.0.

[0014] In a third aspect, the present application provides a preparation method of the above-mentioned negative electrode slurry, comprising the following steps:

[0015] (1) The modified polyacrylic acid and the conductive agent are mixed, first stirring treatment is performed, the hard carbon negative electrode material is added, second stirring treatment is performed, and a mixture is obtained.

[0016] (2) The solvent is added to the mixture, third stirring treatment is performed, the SBR (styrene-butadiene rubber) is added, fourth stirring treatment is performed, and the negative electrode slurry is obtained.

[0017] In a fourth aspect, the present application provides a negative electrode sheet comprising the above-mentioned composite binder.

[0018] In a fifth aspect, the present application provides a sodium ion battery comprising the negative electrode sheet described above.

[0019] Compared with the prior art, the present application can achieve at least one of the following beneficial effects:

[0020] 1. In the traditional PAA / SBR system, PAA is a high-polarity polymer (strongly hydrophilic), SBR is a weak-polarity elastomer (hydrophobic), and the compatibility of the two is poor. After compounding, phase separation easily occurs, and the network structure formed is loose and uneven. The binder of the present application is compounded from PAA-MIBPA and SBR. The MIBPA is connected to the carboxyl group of PAA through an amide bond at one end (retaining polarity), and the alkyl chain and amino group at the other end have certain hydrophobicity, which can enhance the compatibility with SBR, promote PAA-MIBPA and SBR to form a more uniform interpenetrating network, and reduce the glass transition temperature of PAA. In addition, the primary amino group (-NH2) of MIBPA can coordinate with Na + , significantly improving the ion conductivity of the electrode and reducing the charge transfer resistance of the battery, thereby improving the rate performance of the battery.

[0021] 2. The composite binder of the present application has significantly optimized low-temperature performance due to the reduced glass transition temperature and enhanced ion transport. The integrity of the electrode sheet at low temperature is significantly better than that of the PAA / SBR system (the synergistic effect of the flexible chain of MIBPA and SBR enables the electrode sheet to maintain good flexibility at low temperature and not easily crack when bent or the volume changes), significantly improving the low-temperature capacity retention rate and peel strength of the battery, ensuring the structural integrity of the electrode sheet in the range of -40℃ to 55℃, and effectively improving the low-temperature toughness and adaptability of the sodium ion battery negative electrode.

[0022] 3. The negative electrode slurry of the present application uses a PAA-MIBPA and SBR composite binder to form a bifunctional polymer that has a rigid skeleton (carboxyl group bonded to hard carbon) and a flexible segment (primary amino group of MIBPA promotes Na + transport). The carboxyl group (-COOH) of polyacrylic acid is a strong polar group that can combine with functional groups such as hydroxyl groups (-OH) and carboxyl groups on the surface of the hard carbon negative electrode through hydrogen bonds, coordination bonds, or covalent bonds, forming a stable interfacial bond and constructing a rigid skeleton. The flexible segment introduced by MIBPA (diamino monomer) can increase the flexibility of the polymer molecular chain and reduce the packing density between chains. The primary amino group (-NH2) is a polar group that can form weak interactions (similar to "ionic bridges") with Na + in the electrolyte, promoting Na +The migration in the binder network solves the problem of rigid pure polyacrylic acid segment and blocked ion transmission. The flexible segment of MIBPA reduces the glass transition temperature of the PAA polymer, and SBR can further supplement the low-temperature toughness of the pole piece to make up for the brittleness of PAA-MIBPA at extremely low temperatures. At the same time, SBR and PAA-MIBPA form an interpenetrating network at a specific mass ratio, dynamically buffer volume change stress, and improve the rate performance of the battery at low temperatures.

[0023] The various technical solutions described above can also be combined with each other in the present application to achieve more preferred combination solutions. Other features and advantages of the present application will be described in the subsequent specification, and some advantages will become apparent from the specification or will be understood by implementing the present application. The purpose and other advantages of the present application can be achieved and obtained from the specific indications in the specification and the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0024] The accompanying drawings are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and together with the description serve to explain the principles of the application, and are not intended to limit the scope of the application.

[0025] Figure 1 The schematic diagram of PAA structure conversion to PAA-MIBPA structure in the present application. DETAILED DESCRIPTION

[0026] The preferred embodiments of the present application will be specifically described below in conjunction with the accompanying drawings, wherein the drawings constitute a part of the present application and are used to illustrate the principles of the embodiments of the present application and are not intended to limit the scope of the present application.

[0027] One specific embodiment of the present application discloses a composite binder suitable for a wide-temperature-range hard carbon negative electrode, which comprises modified polyacrylic acid (PAA-MIBPA) and butadiene styrene rubber (SBR), wherein the modified polyacrylic acid is prepared by grafting N,N-bis(3-aminopropyl)methylamine (MIBPA) to the side chain of polyacrylic acid.

[0028] In the traditional PAA / SBR system, PAA is a high-polarity polymer (strongly hydrophilic), SBR is a weak-polarity elastomer (hydrophobic), and the compatibility of the two is poor. After compounding, phase separation easily occurs, and the network structure formed is loose and uneven.

[0029] The binder of the present application is compounded from PAA-MIBPA and SBR. The alkyl chain and amino group at one end of MIBPA have certain hydrophobicity, which can enhance the compatibility with SBR, promote PAA-MIBPA and SBR to form a more uniform interpenetrating network, and reduce the glass transition temperature of PAA, such asFigure 1 In addition, the primary amino group (-NH2) of MIBPA can coordinate with Na + The coordination of the primary amino group (-NH2) of MIBPA with Na

[0030] In addition, the flexible chain segment of MIBPA reduces the glass transition temperature of PAA-MIBPA, effectively avoiding low-temperature embrittlement problems, and the use of SBR as a flexible binder ensures the structural integrity of the electrode sheet in the temperature range of -40℃ to 55℃ (i.e., a wide temperature range), effectively improving the low-temperature toughness and adaptability of the sodium-ion battery negative electrode. The amino group (-NH2) in MIBPA forms a reversible hydrogen bond network with the PAA carboxyl group (-COOH), dynamically buffers stress during charging and discharging, and inhibits the generation of negative electrode micro-cracks. Therefore, PAA-MIBPA can enhance sodium ion transfer within a high-load electrode and provide persistent mechanical strength during the cycling process. This modification effectively enhances the internal conduction of thick electrodes and helps maintain their integrity during low-temperature cycling. The modified polyacrylic acid of the present application has excellent sodium ion transmission rate and mechanical properties, significantly improving the rate performance, cycle stability, and low-temperature charging and discharging performance of the battery.

[0031] The composite binder of the present application has significantly optimized low-temperature performance due to the reduction in glass transition temperature and the enhancement of ion transmission, and the electrode sheet integrity at low temperature is significantly better than that of the PAA / SBR system (the synergistic effect of the flexible chain of MIBPA and SBR enables the electrode sheet to maintain good flexibility at low temperature and not easily crack when bent or the volume changes), significantly improving the low-temperature capacity retention rate and peel strength of the battery.

[0032] The primary amino group of MIBPA promotes the solvation and migration of Na + by weak coordination, and the flexible chain segment reduces the packing density of the PAA segment, making the ion transmission channel more open. Compared with the PAA / SBR system, the ion conductivity of the electrode interface of PAA-MIBPA / SBR can be improved by 1-2 orders of magnitude, improving the cycle performance of the battery at high rates.

[0033] It should be noted that the wide temperature range referred to in the present application refers to a temperature range of -40℃ to 55℃ (for example, -40℃, -35℃, -30℃, -25℃, -20℃, -15℃, -10℃, -5℃, 0℃, 5℃, 10℃, 15℃, 20℃, 25℃, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃), at which the battery has good capacity retention rate.

[0034] Specifically, the mass ratio of the modified polyacrylic acid and the styrene-butadiene rubber is 2.2-2.4 (for example, 2.2, 2.25, 2.3, 2.35, 2.4) : 1.8-2 (for example, 1.8, 1.85, 1.9, 1.95, 2).

[0035] The SBR and the PAA-MIBPA of the application form an interpenetrating network at a specific mass ratio, dynamically buffer volume change stress, and improve the rate performance of the battery at low temperature.

[0036] Specifically, the modified polyacrylic acid is prepared by the following method:

[0037] The polyacrylic acid is added to water, the pH value is adjusted, EDC (1-ethyl-(3-dimethylaminopropyl) carbodiimide) and Sulfo-NHS (N-hydroxysulfosuccinimide) are added for activation, grafting stirring reaction is performed, MIBPA (N,N-bis(3-aminopropyl)methylamine) is added, stirring and dissolution are performed, freeze-drying is performed, and the modified polyacrylic acid is obtained.

[0038] It should be noted that the -COOH of PAA in the application generates an unstable O-acyl isourea intermediate under the action of EDC; Sulfo-NHS further reacts with the intermediate to generate a more stable sulfosuccinimide ester (-COO-Sulfo-NHS), which significantly reduces the probability of hydrolysis of the intermediate and is beneficial to improving the grafting stability of MIBPA monomers.

[0039] Specifically, the pH value is adjusted to 5-6.5 (for example, 5, 5.2, 5.4, 5.6, 5.8, 6, 6.2, 6.4, 6.5), and preferably, hydrochloric acid is used to adjust the pH value. The pH value in the above range improves the reactivity and stability of EDC.

[0040] It should be noted that the mass ratio of the polyacrylic acid and water is 1-3:20; for example, 1:20, 1.5:20, 2:20, 2.5:20, 3:20, because the local concentration of the activated reagent and MIBPA is low due to the too large volume of the system; and the PAA may be wrapped due to the too high concentration, the carboxyl group is not completely activated due to the too low volume of the system.

[0041] Specifically, the molar ratio of polyacrylic acid, EDC and Sulfo-NHS is 1: 1.5-2 (for example, 1.5, 1.6, 1.7, 1.8, 1.9, 2): 1-1.2 (1, 1.02, 1.04, 1.06, 1.08, 1.10, 1.12, 1.14, 1.16, 1.18, 1.2), and the molar ratio of polyacrylic acid and MIBPA is 7:1-8.5:1 (for example, 7:1, 7.2:1, 7.4:1, 7.6:1, 7.8:1, 8:1, 8.2:1, 8.4:1, 8.5:1). Avoiding "multi-tooth crosslinking" caused by excess MIBPA.

[0042] Preferably, the activation time is 30-60 min, for example, 30 min, 32 min, 34 min, 36 min, 38 min, 40 min, 42 min, 44 min, 46 min, 48 min, 50 min, 52 min, 54 min, 56 min, 58 min, 60 min, and over-short activation is insufficient, and over-long activation hydrolyzes ester, the temperature of the stirring of the grafting is room temperature, and the reaction time is 1-2 h, for example, 1 h, 1.2 h, 1.4 h, 1.6 h, 1.8 h, 2.0 h, preventing over-crosslinking.

[0043] Specifically, the temperature of the freeze-drying is -80--50℃, for example, -80℃, -76℃, -72℃, -68℃, -64℃, -60℃, -56℃, -54℃, -50℃, and the time is 24-36 h, for example, 24 h, 25 h, 26 h, 27 h, 28 h, 29 h, 30 h, 31 h, 32 h, 33 h, 34 h, 35 h, 36 h.

[0044] Another specific embodiment of the present application discloses a negative electrode slurry comprising the above binder.

[0045] Specifically, the negative electrode slurry includes, in terms of mass ratio, hard carbon negative electrode material: conductive agent: modified polyacrylic acid: SBR = 94.2-94.6 (for example, 94.2, 94.25, 94.3, 94.35, 94.40, 94.45, 94.5, 94.55, 94.6): 1.3-1.5 (for example, 1.3, 1.32, 1.34, 1.36, 1.38, 1.40, 1.42, 1.44, 1.46, 1.48, 1.5): 2.2-2.4 (for example, 2.2, 2.22, 2.24, 2.26, 2.28, 2.3, 2.32, 2.34, 2.36, 2.38, 2.4): 1.8-2.0 (for example, 1.8, 1.82, 1.84, 1.86, 1.9, 1.92, 1.94, 1.96, 1.98, 2). Preferably, 2% of NMP (methyl pyrrolidone) solvent is added to the negative electrode slurry to improve the wettability of the slurry.

[0046] It should be noted that the negative electrode slurry of the present application uses PAA-MIBPA and SBR composite binder to form a bifunctional polymer with rigid skeleton (carboxyl bonded with hard carbon) and flexible segment (primary amino of MIBPA promotes Na + The carboxyl group (-COOH) of polyacrylic acid is a strong polar group, which can be combined with the functional groups such as hydroxyl group (-OH) and carboxyl group on the surface of hard carbon negative electrode through hydrogen bond, coordination bond or covalent bond to form stable interfacial bonding and build rigid skeleton; the flexible segment introduced by MIBPA (diamino monomer) can increase the flexibility of polymer molecular chain and reduce the packing density of the chain; the primary amino group (-NH2) is a polar group, which can form weak interaction (similar to "ionic bridge") with Na + in the electrolyte to promote the migration of Na + In the binder network, the migration solves the problem of rigid chain segment of pure polyacrylic acid and the problem of blocked ion transmission. The flexible segment of MIBPA reduces the glass transition temperature of PAA polymer, and SBR can further supplement the low-temperature toughness of the electrode sheet to make up for the brittleness of PAA-MIBPA at extremely low temperature. At the same time, SBR and PAA-MIBPA form an interpenetrating network at a specific mass ratio to dynamically buffer the stress of volume change and improve the rate performance of the battery at low temperature.

[0047] Another specific embodiment of the present application discloses a preparation method of the above-mentioned negative electrode slurry, which comprises the following steps:

[0048] (1) mixing the modified polyacrylic acid and the conductive agent, performing first stirring treatment, adding the hard carbon negative electrode material, performing second stirring treatment, and obtaining a mixture;

[0049] (2) adding a solvent into the mixture, performing a third stirring treatment, adding SBR, performing a fourth stirring treatment, and obtaining the negative electrode slurry.

[0050] Specifically, in step (1), the rotation speed of the first stirring treatment is 2000-2500 rpm, for example, 2000 rpm, 2100 rpm, 2200 rpm, 2300 rpm, 2400 rpm, 2500 rpm, and the time is 60-70 min, for example, 60 min, 61 min, 62 min, 63 min, 64 min, 65 min, 66 min, 67 min, 68 min, 69 min, 70 min. Under the above stirring conditions, the conductive agent can be mixed uniformly with the modified polyacrylic acid, preventing the conductive agent from floating or agglomerating and reducing the generation of air bubbles.

[0051] Specifically, in step (1), the rotation speed of the second stirring treatment is 2500-3000 rpm, for example, 2500 rpm, 2600 rpm, 2700 rpm, 2800 rpm, 2900 rpm, 3000 rpm, and the time is 90-100 min, for example, 90 min, 91 min, 92 min, 93 min, 94 min, 95 min, 96 min, 97 min, 98 min, 99 min, 100 min. Under the above stirring treatment, the particles can be uniformly dispersed, avoiding particle sedimentation in the later stage.

[0052] Preferably, in step (2), the amount of the solvent added is 2-3% of the total mass of the negative electrode slurry, for example, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%.

[0053] Preferably, the solvent is NMP (methyl pyrrolidone).

[0054] Specifically, in step (2), the rotation speed of the third stirring treatment is 2500-3000 rpm, for example, 2500 rpm, 2600 rpm, 2700 rpm, 2800 rpm, 2900 rpm, 3000 rpm, and the time is 30-40 min, for example, 30 min, 31 min, 32 min, 33 min, 34 min, 35 min, 36 min, 37 min, 38 min, 39 min, 40 min.

[0055] Specifically, in step (2), the stirring speed of the fourth stirring treatment is 400-700 rpm, for example, 400 rpm, 420 rpm, 440 rpm, 460 rpm, 480 rpm, 500 rpm, 520 rpm, 540 rpm, 560 rpm, 580 rpm, 600 rpm, 620 rpm, 640 rpm, 660 rpm, 680 rpm, 700 rpm, and the time is 20-30 min, for example, 20 min, 21 min, 22 min, 23 min, 24 min, 25 min, 26 min, 27 min, 28 min, 29 min, 30 min. Stirring for too long or at too high a speed can cause the SBR molecular chain to break.

[0056] Another specific embodiment of the present application discloses the negative pole piece.

[0057] Specifically, the preparation method of the negative pole piece is as follows: drying and compacting the current collector, cutting to obtain a current collector meeting the requirements, and coating the negative pole paste on the current collector to obtain the negative pole piece.

[0058] Another specific embodiment of the present application discloses a sodium ion battery comprising the negative pole piece.

[0059] The technical solutions of the present application are further explained and described below in combination with specific embodiments.

[0060] Embodiment 1

[0061] The composite binder suitable for the hard carbon negative pole in a wide temperature range in this embodiment has a mass ratio of 2.3:1.8 of the modified polyacrylic acid to the butadiene-styrene rubber.

[0062] The modified polyacrylic acid is prepared by the following method:

[0063] Polyacrylic acid is added to water, the mass ratio of polyacrylic acid to water is 1:20, hydrochloric acid is used to adjust the pH value to 5, EDC and Sulfo-NHS are added, the activation time is 30 min, the stirring reaction is carried out at room temperature for 1 h, the molar ratio of polyacrylic acid, EDC and Sulfo-NHS is 1:2:1, MIBPA is further added, the molar ratio of polyacrylic acid to MIBPA is 7:1, stirring and dissolving are carried out, freeze-drying is carried out, the freeze-drying temperature is-50℃, and the freeze-drying time is 24 h, to obtain the modified polyacrylic acid.

[0064] Embodiment 2

[0065] The composite binder suitable for the hard carbon negative pole in a wide temperature range in this embodiment has a mass ratio of 2.4:2 of the modified polyacrylic acid to the butadiene-styrene rubber.

[0066] The modified polyacrylic acid is prepared by the following method:

[0067] The polyacrylic acid is added to water, the mass ratio of the polyacrylic acid to the water is 1:10, hydrochloric acid is used to adjust the pH value to 5.75, EDC and Sulfo-NHS are added, the activation time is 45 min, the stirring reaction is carried out at room temperature for 1.5 h, the molar ratio of the polyacrylic acid, the EDC and the Sulfo-NHS is 1:1.75:1.1, MIBPA is further added, the molar ratio of the polyacrylic acid to the MIBPA is 7.75:1, stirring and dissolving are carried out, and freeze-drying is carried out at a temperature of -70 DEG C for 30 h, so that the modified polyacrylic acid is obtained.

[0068] Example 3

[0069] The composite binder suitable for the hard carbon negative electrode in this example has a mass ratio of the modified polyacrylic acid to the butadiene-styrene rubber of 2.2:1.9.

[0070] The modified polyacrylic acid is prepared by the following method:

[0071] The polyacrylic acid is added to water, the mass ratio of the polyacrylic acid to the water is 3:20, hydrochloric acid is used to adjust the pH value to 6.5, EDC and Sulfo-NHS are added, the activation time is 60 min, the stirring reaction is carried out at room temperature for 2 h, the molar ratio of the polyacrylic acid, the EDC and the Sulfo-NHS is 1:1.5:1.2, MIBPA is further added, the molar ratio of the polyacrylic acid to the MIBPA is 8.5:1, stirring and dissolving are carried out, and freeze-drying is carried out at a temperature of -80 DEG C for 36 h, so that the modified polyacrylic acid is obtained.

[0072] Example 4

[0073] The negative electrode slurry in this example comprises, according to the mass ratio, hard carbon negative electrode material: conductive agent: modified polyacrylic acid prepared in Example 1: SBR = 94.4:1.5:2.3:1.8.

[0074] The negative electrode slurry is prepared by the following method:

[0075] (1) The modified polyacrylic acid and the conductive agent SP are mixed, and then a first stirring treatment is carried out at a rotation speed of 2000 rpm for 60 min, the hard carbon negative electrode material is added, and then a second stirring treatment is carried out at a rotation speed of 2500 rpm for 90 min, so that a mixture is obtained;

[0076] (2) 2.5% of solvent NMP in total slurry mass was added to the mixture, and third stirring treatment was carried out at 2500 rpm for 30 min, SBR was added, and fourth stirring treatment was carried out at 400 rpm for 20 min, to obtain the negative electrode slurry.

[0077] Example 5

[0078] A negative electrode slurry of the present example comprises, in terms of mass ratio: hard carbon negative electrode material: conductive agent: modified polyacrylic acid prepared in Example 2: SBR = 94.2: 1.4: 2.4: 2.0.

[0079] The negative electrode slurry was prepared by the following method:

[0080] (1) The modified polyacrylic acid and the conductive agent SP were mixed, and first stirring treatment was carried out at 2250 rpm for 65 min, the hard carbon negative electrode material was added, and second stirring treatment was carried out at 2750 rpm for 95 min, to obtain a mixture;

[0081] (2) 2% of solvent NMP in total slurry mass was added to the mixture, and third stirring treatment was carried out at 2750 rpm for 35 min, SBR was added, and fourth stirring treatment was carried out at 550 rpm for 25 min, to obtain the negative electrode slurry.

[0082] Example 6

[0083] A negative electrode slurry of the present example comprises, in terms of mass ratio: hard carbon negative electrode material: conductive agent: modified polyacrylic acid prepared in Example 3: SBR = 94.6: 1.5: 2.7: 2.1.

[0084] The negative electrode slurry was prepared by the following method:

[0085] (1) The modified polyacrylic acid and the conductive agent SP were mixed, and first stirring treatment was carried out at 2500 rpm for 70 min, the hard carbon negative electrode material was added, and second stirring treatment was carried out at 3000 rpm for 100 min, to obtain a mixture;

[0086] (2) 3% of solvent NMP in total slurry mass was added to the mixture, and third stirring treatment was carried out at 3000 rpm for 40 min, SBR was added, and fourth stirring treatment was carried out at 700 rpm for 30 min, to obtain the negative electrode slurry.

[0087] Comparative Example 1

[0088] The negative electrode slurry raw material and preparation method of the present comparative example are similar to those of example 4, except that the molar ratio of polyacrylic acid to MIBPA in the preparation process of the modified polyacrylic acid is 10:1.

[0089] Comparative example 2

[0090] The negative electrode slurry raw material and preparation method of the present comparative example are similar to those of example 4, except that the molar ratio of polyacrylic acid to MIBPA in the preparation process of the modified polyacrylic acid is 5:1.

[0091] Comparative example 3

[0092] The negative electrode slurry raw material and preparation method of the present comparative example are similar to those of example 4, except that the mass ratio of the modified polyacrylic acid to the butadiene-styrene rubber is 2:3.

[0093] Comparative example 4

[0094] The negative electrode slurry raw material and preparation method of the present comparative example are similar to those of example 4, except that the modified polyacrylic acid is replaced by unmodified polyacrylic acid.

[0095] Comparative example 5

[0096] The negative electrode slurry raw material and preparation method of the present comparative example are similar to those of example 4, except that no SBR is added to the negative electrode slurry, and is replaced by the same mass of modified polyacrylic acid.

[0097] Comparative example 6

[0098] The negative electrode slurry raw material and preparation method of the present comparative example are similar to those of example 4, except that the preparation method of the modified polyacrylic acid is as follows:

[0099] The polyacrylic acid is added to water, and the pH value is adjusted to 5. EDS (1-ethyl-(3-dimethylaminopropyl) carbonyldiimide) and NHS (N-hydroxysuccinimide) are added, the activation time is 30 min, the reaction is stirred at room temperature for 1 h, the molar ratio of polyacrylic acid, EDS and NHS is 1:2:1, AEP (aminoethyl piperazine) is further added, the molar ratio of polyacrylic acid to AEP is 7:1, and then stirring and dissolving are performed. The temperature for freeze-drying is -50°C, and the time for freeze-drying is 24 h, to obtain the modified polyacrylic acid.

[0100] Experimental example 1

[0101] Sodium ion batteries are prepared using the negative electrode slurries prepared in examples 4-6 and comparative examples 1-6, respectively, and the specific method is as follows:

[0102] (1) Preparation of negative electrode sheet: the negative electrode slurries are respectively coated on the copper foil according to a single-sided area density of 8.5 mg / cm 2Coating on aluminum foil current collector, getting the pole piece, drying the pole piece, rolling (the compaction density is 0.95g / cm 3 ), and after the cutting and cutting process, the size of the negative pole piece is 52mm*240mm.

[0103] (2) Positive pole piece preparation: the layered oxide (NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2), PVDF, Super-P and CNT conductive agent are added into NMP and stirred uniformly, wherein the mass ratio of each substance in NMP is 96%:2.1%:1.5%:0.5%. Coating on aluminum foil current collector according to the surface density of 17mg / cm 2 , drying by oven and rolling (the compaction density is 3.1g / cm 3 ), after the cutting and cutting process, the size of the positive pole piece is 50mm*200mm.

[0104] (3) Battery assembly: including positive and negative pole pieces, diaphragm PP, electrolyte and aluminum plastic film shell: winding in the order of diaphragm→negative→diaphragm→positive→diaphragm, ensuring that the positive and negative poles are aligned, and the edge of the negative pole piece exceeds the positive pole piece by 0.5-1mm. After winding, the battery is flattened by a hot press and placed in an aluminum plastic film pit, dried and then injected with electrolyte to seal and stand for 48h.

[0105] Among them, the electrolyte is NaPF6 dissolved in a solvent with a volume ratio of EC (ethylene carbonate): PC (polycarbonate): EMC (methyl ethyl carbonate) = 1:1:3, the concentration of NaPF6 is 1.0mol / L, and the electrolyte contains 2% FEC (fluorinated ethylene carbonate) and 1% VC (vinyl carbonate) by mass fraction.

[0106] The performance test results of each negative pole piece and battery are shown in Table 1, and the test method is as follows:

[0107] (a) Negative pole piece peeling strength test: using a tension machine with a range of 20N, cutting the pole piece into a size of 20cm*2.5cm and fixing it on a steel plate, taking the data of the stable tension section as the peeling strength.

[0108] (b) Rate performance test: at room temperature 25℃, charge to 3.95V with 0.5C constant current and constant voltage, cut-off current 0.05C, then discharge to 2V with 0.5C constant current, repeat 3 times, take the 3rd discharge capacity as standard capacity Co. Again charge to 3.95V with 0.5C constant current and constant voltage, cut-off current 0.05C, discharge to 2V with 1C, record 1C discharge capacity. Again charge to 3.95V with 0.5C constant current and constant voltage, cut-off current 0.05C, discharge to 2V with 2C, record 2C discharge capacity. Finally calculate the discharge capacity retention rate of 1C and 2C of the battery cell.

[0109] (c) High and low temperature test: at 25℃, charge to 3.95V with 0.5C constant current and constant voltage, cut-off current 0.05C, then discharge to 2V with 0.5C constant current, repeat 3 times, take the 3rd discharge capacity as standard capacity Co. Again charge to 3.95V with 0.5C constant current and constant voltage, cut-off current 0.05C, adjust the temperature to -20℃ and stand for 4h, then discharge to 2V with 0.5C, record the discharge capacity at -20℃. The charge and discharge steps at -40℃ and 55℃ are consistent with those at -20℃ except for the temperature change. Finally calculate the capacity retention rate of the battery cell at low temperature.

[0110] Table 1

[0111]

[0112] From Table 1, it can be seen that the electrode strip peeling force of the negative electrode strip of the application is 11.3-12N, the 2C cycle 200 cycle capacity retention rate is 95.6-96.8%, the -20℃ capacity retention rate is 83.9-98.3%, the -40℃ capacity retention rate is 75.4-79.4%, and the 55℃ capacity retention rate is 101.5-102.7%.

[0113] From the test results of Examples 4-6 and Comparative Examples 1-2, it can be seen that the negative electrode adhesive of the application, through the synergistic effect of MIBPA modified PAA and SBR, takes into account rigidity and flexibility, so that the negative electrode strip of the application has high peeling strength even at high area density, and the prepared sodium ion battery has good cycle performance and low temperature performance. Excess MIBPA (5:1) leads to excessive crosslinking, and the flexibility and ion transmission decrease, and the electrode strip is too rigid. Insufficient MIBPA (10:1) leads to weak modification effect, and the incomplete bonding network leads to reduced peeling force. When the molar ratio of PAA:MIBPA is controlled at (7-8.5):1, the electrode strip has the best performance.

[0114] At the same time, from the test results of Example 4 and Comparative Example 4, it can be seen that the low temperature retention rate decreases when using unmodified PAA / SBR adhesive, which is because it cannot form an elastic network to buffer volume expansion, and the interfacial bonding force is weak.

[0115] From the test results of Example 4 and Comparative Examples 3 and 5, it can be seen that the SBR ratio is too high, the peeling force is significantly reduced, the compaction density is affected, the electrode sheet is too soft, and the high-temperature swelling causes the capacity to be falsely high. Compared with the negative electrode sheet added with SBR, the electrode sheet without SBR has low peeling force and poor low-temperature performance, and the lack of flexible phase causes the electrode sheet to be brittle and hard, resulting in particle shedding.

[0116] From the test results of Example 4 and Comparative Example 6, it can be seen that the comprehensive performance of PAA modified by AEP is lower than that of MIBPA modification described in the application, especially the low-temperature performance has a significant decline. This proves that the unique structure of MIBPA not only provides flexibility, but also forms a powerful three-dimensional conductive network through the amino group, which can maintain excellent interface stability and ion conductivity in a wide temperature range.

[0117] The above merely describes preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical scope disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A composite binder suitable for wide temperature range hard carbon anodes, characterized in that, The binder comprises modified polyacrylic acid and styrene-butadiene rubber, wherein the modified polyacrylic acid is prepared by grafting N,N-bis(3-aminopropyl)methylamine onto the side chain of polyacrylic acid. 2.The composite binder suitable for hard carbon anode with wide temperature range according to claim 1, characterized in that, The mass ratio of the modified polyacrylic acid to the styrene-butadiene rubber is 2.2-2.4:1.8-2. 3.The composite binder suitable for hard carbon anode with wide temperature range according to claim 1 or 2, characterized in that, The modified polyacrylic acid is prepared by the following method: The polyacrylic acid is added to water, the pH value is adjusted, 1-ethyl-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide are added for activation, grafting stirring reaction is performed, N,N-bis(3-aminopropyl)methylamine is added, stirring and dissolution are performed, and freeze-drying is performed to obtain the modified polyacrylic acid. 4.The composite binder suitable for hard carbon anode with wide temperature range according to claim 3, characterized in that, The pH value is adjusted to 5-6.

5.

5. The composite binder for hard carbon anodes suitable for wide temperature range according to claim 3, characterized in that, The molar ratio of polyacrylic acid, 1-ethyl-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide is 1:1.5-2:1-1.2, and the molar ratio of polyacrylic acid and N,N-bis(3-aminopropyl)methylamine is 7:1-8.5:

1.

6. A negative electrode slurry, characterized by, The composite binder of any one of claims 1-5.

7. The negative electrode slurry of claim 6, wherein The negative electrode slurry comprises, by mass ratio, hard carbon negative electrode material: conductive agent: modified polyacrylic acid: styrene-butadiene rubber = 94.2-94.6:1.3-1.5:2.2-2.4:1.8-2.

0.

8. A method for producing the negative electrode slurry according to claim 6 or 7, characterized by, The method comprises the following steps: (1) The modified polyacrylic acid and the conductive agent are mixed, first stirring treatment is performed, the hard carbon negative electrode material is added, second stirring treatment is performed, and a mixture is obtained; (2) A solvent is added to the mixture, third stirring treatment is performed, the styrene-butadiene rubber is added, fourth stirring treatment is performed, and the negative electrode slurry is obtained.

9. A negative electrode sheet characterized by comprising: The composite binder of any one of claims 1-5.

10. A sodium-ion battery, characterized in that, The negative electrode sheet of claim 9.

Images