Negative electrode binder composition, negative electrode plate and secondary battery

Abstract

The invention provides a negative electrode binder composition, a negative electrode plate and a secondary battery, aiming at the problem that an existing negative electrode binder cannot effectively inhibit volume expansion of a silicon negative electrode. The negative electrode binder composition comprises a polysaccharide compound, a compound containing a linear rigid chain and metal cations complexed with the polysaccharide compound, and the mass ratio of the polysaccharide compound to the compound containing the linear rigid chain to the metal cations is (0.5-10): (10-40): (0.02-0.5); the polysaccharide compound and the compound containing the linear rigid chain both contain carboxyl groups, the pH value of the negative electrode binder composition is 5-8, and at least parts of the carboxyl groups in the polysaccharide compound and the carboxyl groups in the compound containing the linear rigid chain are complexed with metal cations. According to the negative electrode binder composition provided by the invention, the polysaccharide compound and the compound containing the linear rigid chain form a self-recovery network structure, so that crushing of negative electrode material particles is effectively prevented, and a material interface is stabilized.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and in particular relates to a negative electrode binder composition, a negative electrode sheet, and a secondary battery. Background Art

[0002] Lithium-ion batteries have become the main power source for modern portable electronic devices, electric vehicles and energy storage systems due to their high energy density, long cycle life and low self-discharge rate. Current lithium-ion batteries mainly use graphite materials as negative electrodes. With the development of portable electronic devices and electric vehicles, people's requirements for the energy density of lithium-ion batteries are also increasing. The theoretical specific capacity of graphite negative electrode materials is only 372mAh / g, which can no longer meet the development needs of high energy density lithium-ion batteries.

[0003] At present, nano-silicon materials (~4200mAh / g), silicon-carbon composite materials (>1700mAh / g) and silicon oxide materials (>1500mAh / g) are gradually being developed and applied as negative electrode materials. However, the volume of silicon negative electrode will expand greatly during the charging and discharging process (up to more than 300% of its original volume). The huge volume expansion causes the structure of silicon negative electrode to break, and the active material and the current collector, and the active material and the active material lose electrical contact. The lithium ion deintercalation process cannot proceed smoothly, resulting in huge irreversible capacity and rapid capacity decay. Therefore, how to effectively control the volume expansion of silicon and improve its cycle stability is a research focus in the field of silicon-based negative electrodes. Among them, the cycle stability of the battery can be improved by buffering the expansion / contraction of the active material during the charging and discharging process with the negative electrode binder. However, the traditional negative electrode material binders, sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR), cannot cope well with the huge volume changes brought about by the silicon-based negative electrode, resulting in the destruction of the electrode structure during the cycle. However, binders such as polyacrylic acid (PAA) and polyacrylonitrile (PAN) have high glass transition temperatures and high hardness. The negative electrode sheets prepared are very brittle and easily lose powder when the electrode sheets are cut, affecting battery performance.

[0004] Therefore, a new type of negative electrode binder is urgently needed to inhibit the volume expansion of the silicon negative electrode, maintain the integrity of the electrode structure, achieve excellent electrochemical performance, and improve the negative electrode cycle stability. Summary of the invention

[0005] The present invention aims at the problem that the existing negative electrode binder cannot effectively inhibit the volume expansion of the silicon negative electrode. The present invention provides a negative electrode binder composition, a negative electrode sheet and a secondary battery.

[0006] In order to solve the above technical problems, the present invention provides a negative electrode binder composition, comprising a polysaccharide compound, a compound containing a linear rigid chain and a metal cation, wherein the mass ratio of the polysaccharide compound, the compound containing a linear rigid chain and the metal cation is (0.5-10): (10-40): (0.02-0.5); The polysaccharide compound and the compound containing a linear rigid chain both contain carboxyl groups, the pH value of the negative electrode binder composition is 5-8, and the carboxyl groups in the polysaccharide compound and the compound containing a linear rigid chain are at least partially complexed with the metal cations.

[0007] Preferably, the polysaccharide compound is gellan gum.

[0008] Preferably, the compound containing a linear rigid chain is polyacrylic acid.

[0009] Preferably, the molar ratio of the carboxyl groups in the polysaccharide compound to the carboxyl groups in the compound containing the linear rigid chain is 0.008:1 to 0.4:1.

[0010] Preferably, the metal cation includes at least one of a divalent cation and a monovalent cation.

[0011] Preferably, the metal cation comprises Ca 2+ Mg 2+ 、Ni 2+ 、Na + , K + One or more of .

[0012] Preferably, the metal cation is selected from Ca 2+ Mg 2+ 、Ni 2+ One or more of .

[0013] Preferably, the negative electrode binder composition has a solid content of 2%-20% and a viscosity of 100-25000 cp.

[0014] In a second aspect, the present application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector, wherein the negative electrode active material layer comprises any one of the negative electrode binder compositions described above.

[0015] In a third aspect, the present application provides a secondary battery, comprising the negative electrode sheet as described above.

[0016] In the present application, a composition having a self-recovering network structure is formed by physically crosslinking polysaccharides, compounds containing linear rigid chains, and metal cations, and the compounds containing linear rigid chains provide rigid support for the negative electrode binder composition, maintain the electrode structure integrity of the silicon negative electrode during the charge and discharge cycle, and release stress to stabilize the interface. Under the action of metal cations, the cations can neutralize the electrostatic repulsion of the carboxylic acid groups in the polysaccharide compounds, promote the formation of the double helix structure of the polysaccharide compounds, and bring better tensile properties to the binder system. In addition, the cross-linked network formed by the helical structure of the polysaccharide compounds can improve the disadvantage that the compounds containing linear rigid chains are easy to slide, so that the polymer has a certain self-recovery ability. After the metal cations are complexed with the polysaccharide compounds, the binder network is strengthened, the stress dispersion path is increased, and the mechanical properties of the binder are further optimized. In particular, the double helix structure that can be stretched and recovered in the polysaccharide compounds can provide sufficient mechanical support to buffer and suppress the volume change of the negative electrode material particles under the synergistic effect of the compounds containing linear rigid chains, thereby effectively preventing the crushing of the negative electrode material particles to stabilize the material interface. DETAILED DESCRIPTION

[0017] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0018] An embodiment of the present application provides a negative electrode binder composition, comprising a polysaccharide compound, a compound containing a linear rigid chain, and a metal cation, wherein the mass ratio of the polysaccharide compound, the compound containing a linear rigid chain, and the metal cation is (0.5-10): (10-40): (0.02-0.5); The polysaccharide compound and the compound containing a linear rigid chain both contain carboxyl groups, the pH value of the negative electrode binder composition is 5-8, and the carboxyl groups in the polysaccharide compound and the compound containing a linear rigid chain are at least partially complexed with the metal cations.

[0019] In this embodiment, a composition having a self-recovering network structure is formed by physically crosslinking a polysaccharide compound, a compound containing a linear rigid chain, and a metal cation. The compound containing a linear rigid chain provides rigid support for the negative electrode binder composition, maintains the electrode structure integrity of the silicon negative electrode during the charge and discharge cycle, and releases stress to stabilize the interface. Under the action of metal cations, the polysaccharide compound can neutralize the electrostatic repulsion of the carboxylic acid group in the polysaccharide compound, promote the formation of the double helix structure of the polysaccharide compound, and bring better tensile properties to the binder system. In addition, the cross-linked network formed by the helical structure of the polysaccharide compound can improve the disadvantage that the compound containing a linear rigid chain is easy to slide, so that the polymer has a certain self-recovery ability. After the metal cation is complexed with the polysaccharide compound, the binder network is strengthened, the stress dispersion path is increased, and the mechanical properties of the binder are further optimized. In particular, the double helix structure that can be stretched and recovered in the polysaccharide compound can provide sufficient mechanical support to buffer and suppress the volume change of the negative electrode material particles under the synergistic action of the compound containing a linear rigid chain, thereby effectively preventing the crushing of the negative electrode material particles to stabilize the material interface.

[0020] Specifically, the mass ratio of the polysaccharide compound, the linear rigid chain-containing compound and the metal cation includes but is not limited to 0.5:10:0.02, 10:40:0.5, 5:25:0.0.2, 5:30:0.5 or 5:40:0.5.

[0021] In some embodiments, the polysaccharide compound is gellan gum. The carboxyl groups on gellan gum provide cross-linking active sites and provide active sites for metal cation complexation. In addition, the carboxyl groups are beneficial to Li + transmission, improving rate performance.

[0022] In some embodiments, the compound containing a linear rigid chain is polyacrylic acid (PAA). The carboxyl groups in PAA and the carboxyl groups in the polysaccharide compound are respectively complexed with metal cations, so that polyacrylic acid and gellan gum are physically cross-linked under the action of metal cations. In addition, the carboxyl groups in PAA form hydrogen bonds with the hydroxyl groups on the surface of the silicon negative electrode particles, which can stabilize the electrode.

[0023] In some embodiments, the molar ratio of the carboxyl groups in the polysaccharide compound to the carboxyl groups in the linear rigid chain-containing compound is 0.008:1 to 0.4:1.

[0024] In some embodiments, the metal cation includes at least one of a divalent cation and a monovalent cation. The formation of the gel is promoted by adding metal cations, while strengthening the binder network, increasing the stress dispersion path, and further optimizing the mechanical properties of the binder. Specifically, the metal cation is obtained by adding a soluble metal salt. The metal salt includes but is not limited to calcium chloride, calcium nitrate, magnesium chloride, magnesium nitrate, nickel chloride, nickel sulfate, nickel nitrate, nickel acetate, sodium chloride, sodium sulfate, sodium carbonate, sodium nitrate, potassium chloride, potassium sulfate, potassium nitrate, and potassium carbonate.

[0025] Furthermore, the mass ratio of the polysaccharide compound, the compound containing a linear rigid chain, and the soluble metal salt is (0.5-10):(10-40):(0.1-1).

[0026] In some embodiments, the metal cation comprises Ca 2+ Mg 2+ 、Ni 2+ 、Na + , K + One or more of .

[0027] In some embodiments, the metal cation is selected from Ca 2+ Mg 2+ 、Ni 2+ By selecting divalent cations, the complexation with polysaccharides is enhanced, which helps to form a stable gel structure. Specifically, Mg 2+ Because of its small ionic radius and strong coordination and complexing ability, it can enhance the relaxation tendency of the contracted and tightened molecular chains.

[0028] In some embodiments, the negative electrode binder composition has a solid content of 2%-20% and a viscosity of 100-25000 cp.

[0029] An embodiment of the present invention further provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector, wherein the negative electrode active material layer comprises any one of the above-mentioned negative electrode binder compositions.

[0030] Furthermore, the negative electrode active material layer further comprises a negative electrode active material, a negative electrode conductive agent and a thickener. Preferably, the peel strength of the negative electrode sheet is greater than 10N / m.

[0031] Preferably, when the active material in the negative electrode active material layer is silicon carbon or silicon, the binder of the above embodiment can provide sufficient mechanical support to buffer and suppress the volume change of silicon negative electrode material particles, thereby effectively preventing the breakage of silicon negative electrode material particles to stabilize the silicon material interface.

[0032] An embodiment of the present invention also provides a secondary battery, comprising the negative electrode sheet as described above. The silicon negative electrode using the negative electrode binder composition of the above embodiment can have excellent electrical contact integrity and surface stability, and the electrochemical performance of the battery such as rate and cycle stability is significantly improved.

[0033] The present invention is further described below by way of examples.

[0034] The negative electrode binder, negative electrode and secondary battery disclosed in the present invention are specifically described.

[0035] Example 1 Negative electrode binder composition S1: Weigh 5 parts of gellan gum GG and deionized water, heat and stir to dissolve; S2: After GG is completely dissolved, add 0.5 parts of calcium nitrate and stir until it is completely dissolved; S3: Continue to add 25 parts of polyacrylic acid PAA, continue heating and stirring until completely dissolved; S4: Cool to 25°C, add deionized water to adjust the solid content and viscosity of the glue solution, add 20mM Tris buffer to make the pH value of the system between 5 and 8 to ensure that the calcium ions of PAA and gellan gum are complexed to obtain a negative electrode binder composition.

[0036] Among them, the amount of carboxyl groups in the polysaccharide compound gellan gum is n1, and the amount of carboxyl groups in the compound containing linear rigid chains is n2, and n1:n2=0.08.

[0037] Negative plate 96% silicon carbon, 1% thickener sodium carboxymethyl cellulose, and 1% conductive carbon black were stirred at low speed, and water was added to adjust the solid content to 50%. After continued stirring, kneading was performed, and 2% negative electrode binder was added. After medium-high speed dispersion, vacuum defoaming was performed and the material was discharged. Then the negative electrode slurry was coated on both sides of the negative electrode current collector Cu foil, and the single-sided coating surface density was 120g / m 2 , dried, cold pressed (compacted density 1.7g / cm 3 ), slitting and other processes to obtain the negative electrode sheet.

[0038] Positive electrode The positive electrode active material nickel-cobalt-manganese ternary, conductive carbon black, and positive electrode binder are mixed in a mass ratio of 96.8:2:1.2, and NMP is added to prepare positive electrode slurry. The positive electrode slurry is coated on aluminum foil and dried to obtain a positive electrode sheet.

[0039] Secondary battery preparation The negative electrode sheet, separator (PE film), and positive electrode sheet are wound in sequence so that the separator is placed between the positive and negative electrodes to play an isolating role, and the electrode assembly is made. The electrode assembly is placed in an outer package, and commercially available electrolyte is injected and packaged. After that, a secondary battery is obtained through processes such as liquid injection, formation, and exhaust.

[0040] Embodiments 2 to 8 Most of the steps of Examples 2 to 8 are the same as those of Example 1, except that the formula in Table 1 is used.

[0041] Comparative Examples 1 to 8 Most of the steps of Comparative Examples 1 to 8 are the same as those of Example 1, except that the formulation in Table 1 is used.

[0042] Table 1

[0043] The negative electrode sheets and secondary batteries prepared in the above examples and comparative examples were subjected to the following tests.

[0044] 1) Battery charge and discharge cycle 100 cycles of pole piece expansion rate test: The constant current method is used for charge and discharge cycles. After 100 charge and discharge cycles, when the electrode plate is in the lithium-embedded state, the ratio of the increase in the plate thickness to the plate thickness before charge and discharge is recorded as the plate expansion rate %.

[0045] 2) Normal temperature cycle test After the battery is placed in a constant temperature test room at 25℃±2℃ for 1h, it is charged to 3.65V at 1C constant current and constant voltage, with a cut-off current of 0.05C; it is discharged to 2.5V at 1C constant current, and the discharge capacity is recorded; the above steps are repeated 500 times, and the capacity retention rate is calculated.

[0046] 3) Battery rate performance test: After the battery is placed in a constant temperature test room at 25℃±2℃ for 1h, it is charged to 3.65V at 1C constant current and constant voltage, with a cut-off current of 0.05C; it is discharged to 2.5V at 0.5C, 2C, and 5C currents, and the discharge time, discharge capacity, and discharge platform voltage are recorded; the above steps are repeated 500 times, and the capacity retention rate is calculated.

[0047] 4) Negative electrode peel strength test: The coated single-sided negative electrode sheet was heated at 2.4 g / cm 3 After compaction, a tensile testing machine with a range of 20N was used. The electrode was cut into 20cm long and 3cm wide. 3M double-sided tape was attached to the steel plate. The electrode coating was fixed on the tape on the steel plate with the surface facing down. After rolling back and forth 6 times with a 2.5kg roller, the coating and copper foil were torn off. The upper plate clamped the copper foil side and stretched at a speed of 50mm / min and 180°C. The data of the stable tension section was recorded as the peel strength (N / m).

[0048] The above test results are shown in Table 2 below.

[0049] Table 2

[0050] From the test results of Example 1 and Comparative Examples 1 and 3, it can be seen that when the polysaccharide compound is pectin, it cannot form a double helix structure under the action of cations, the negative electrode binder composition cannot inhibit the volume change of the silicon negative electrode material particles, the expansion rate of the pole piece increases, the peel strength decreases, and the cycle performance and rate performance deteriorate. When the negative electrode binder composition does not contain gellan gum, the expansion rate of the pole piece increases, the peel strength decreases, and the cycle performance and rate performance deteriorate.

[0051] It can be seen from the test results of Examples 1 to 8 and Comparative Examples 4 to 8 that when the mass ratio of gellan gum, polyacrylic acid and soluble metal salt is (0.5-10):(10-40):(0.1-1), the electrode has good rate performance. When the mass ratio of gellan gum, polyacrylic acid and soluble metal salt is not in the range of (0.5-10):(10-40):(0.1-1), the expansion rate of the electrode increases, the peel strength decreases, and the cycle performance and rate performance deteriorate.

[0052] It can be seen from the test results of Example 1 and Comparative Example 2 that when the negative electrode binder composition does not contain metal cations, polyacrylic acid and gellan gum cannot form physical crosslinks and a self-recovering network structure, which reduces the bonding performance of the binder, increases the expansion rate of the electrode, reduces the peel strength, and deteriorates the cycle performance and rate performance.

[0053] In summary, the binder composition prepared by the present invention is applied to silicon negative electrode plates, which can inhibit the volume change of silicon negative electrode material particles, significantly reduce the expansion rate of the plates, and is beneficial to the improvement of cycle performance and rate performance.

[0054] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A negative electrode binder composition, characterized in that: The negative electrode binder composition comprises a polysaccharide compound, a compound containing a linear rigid chain and a metal cation, wherein the mass ratio of the polysaccharide compound, the compound containing a linear rigid chain and the metal cation is (0.5-10): (10-40): (0.02-0.5); The polysaccharide compound and the compound containing a linear rigid chain both contain carboxyl groups, the pH value of the negative electrode binder composition is 5-8, and the carboxyl groups in the polysaccharide compound and the compound containing a linear rigid chain are at least partially complexed with the metal cations.

2. The negative electrode binder composition according to claim 1, characterized in that The polysaccharide compound is gellan gum.

3. The negative electrode binder composition according to claim 1 or 2, characterized in that: The compound containing a linear rigid chain is polyacrylic acid.

4. The negative electrode binder composition according to claim 1, characterized in that: The molar ratio of the carboxyl groups in the polysaccharide compound to the carboxyl groups in the compound containing a linear rigid chain is 0.008:1 to 0.4:

1.

5. The negative electrode binder composition according to claim 1, characterized in that: The metal cation includes at least one of a divalent cation and a monovalent cation.

6. The negative electrode binder composition according to claim 5, characterized in that: The metal cations include Ca 2+ Mg 2+ 、Ni 2+ 、Na + , K + One or more of .

7. The negative electrode binder composition according to claim 6, characterized in that: The metal cation is selected from Ca 2+ Mg 2+ 、Ni 2+ One or more of .

8. The negative electrode binder composition according to claim 1, characterized in that: The negative electrode binder composition has a solid content of 2%-20% and a viscosity of 100-25000cp.

9. A negative electrode sheet, characterized in that: The invention comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one side of the negative electrode current collector, wherein the negative electrode active material layer comprises the negative electrode binder composition according to any one of claims 1 to 8.

10. A secondary battery, characterized in that: Including the negative electrode sheet as claimed in claim 9.

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