Binder and its preparation method and application

Abstract

The present application relates to the technical field of battery binders, and in particular to binders and their preparation methods and applications. The binder includes a polymer and a one-dimensional carbon material, wherein the polymer contains at least two side chains, and the side chains are connected to the carbon material; wherein the carbon material also contains at least one of a phosphate group and a carboxyl group; and / or the polymer also contains at least one of a phosphate group and a carboxyl group. The phosphate group and the carboxyl group provide a strong bonding effect and also inhibit the dissolution of transition metals in the positive electrode active material. It can also optimize the solid electrolyte membrane. The polymer side chains are connected to the one-dimensional carbon material to form a framework, which is beneficial to enhance the uniformity of polymer distribution and the stretchability of the chain segments. On the one hand, it makes the binder act more fully and evenly on the various components of the electrode; on the other hand, these frameworks can relieve the stress generated by the phase change of the positive electrode active material and stabilize the crystal phase of the material; it can also form a polymer nanolayer coating on the surface of the positive electrode active material to reduce the erosion of the electrolyte on the positive electrode active material.

Description

Technical Field

[0001] The present application relates to the technical field of battery binders, and in particular to binders and their preparation methods and applications. Background Art

[0002] Cathode active materials are important components of sodium-ion batteries and lithium-ion batteries. Among them, layered oxides such as lithium transition metal oxides and sodium transition metal oxides are the most widely used cathode active materials. In order to meet people's demand for high-energy-density batteries, layered cathode materials need to have a higher cutoff voltage. However, higher voltages also make cathode active materials face challenges in bulk and interface, including bulk destruction caused by irreversible phase transitions or internal stress accumulation, corrosion at the interface between the cathode active material and the electrolyte, and surface damage caused by transition metal dissolution. These problems will cause problems such as increased battery resistance and capacity decay, greatly limiting their practical application.

[0003] To overcome the above problems, methods for designing high-performance layered cathode active materials, element doping, and surface coating have been widely reported. Taking sodium transition metal oxides as an example, by replacing the elements of the transition metal layer or sodium layer with other elements, the interlayer distance and local electron density between the transition metal layer and the sodium layer can be regulated, thereby suppressing the irreversible phase transition that occurs when sodium ions are inserted and removed. Alternatively, surface modification using materials such as oxides, fluorides, and phosphates can construct a more stable cathode / electrolyte interface (CEI), protecting the cathode surface from electrolyte corrosion and reducing the dissolution of transition metal ions. However, most of these methods are complex processes and increase battery cost, while also reducing battery capacity to a certain extent.

[0004] Other studies have optimized the binder composition, preparing a mixture of multiple active ingredients as a binder. The chelation effect of the active ingredients in the binder inhibits the dissolution of transition metal ions and promotes the formation of a CEI film on the surface of the positive electrode material, thereby improving the stability of the positive electrode material interface. However, this binder is a mixture, and when it is in the positive electrode slurry and when the slurry is coated on the current collector, the components in it are difficult to evenly act on the positive electrode active material, conductive agent, and current collector, and the degree of improvement in battery performance still needs to be improved. Summary of the Invention

[0005] The purpose of this application is to provide a binder and a preparation method and application thereof, aiming to solve the problem in the prior art that the bulk and interface of layered oxide-based positive electrode active materials are easily destroyed.

[0006] To achieve the above application objectives, the technical solutions adopted in this application are as follows:

[0007] In a first aspect, the present application provides a binder comprising a polymer and a one-dimensional carbon material, wherein the polymer comprises at least two side chains, and the side chains are connected to the carbon material;

[0008] Wherein, the carbon material further contains at least one of a phosphate group and a carboxyl group; and / or

[0009] The polymer also contains at least one of a phosphate group and a carboxyl group.

[0010] The phosphate and carboxyl groups in the binder of this application can form hydrogen bonds with the positive electrode active material, providing a strong adhesive effect. The phosphate and carboxyl groups also form bonds or chelates with the transition metals in the positive electrode active material, stabilizing the interface structure of the positive electrode active material and inhibiting transition metal dissolution. They also have a desolvation effect, optimizing the solid electrolyte membrane (CEI). At least two branches connect to the one-dimensional carbon material, enhancing the uniformity of polymer distribution and the stretchability of the chain segments. Furthermore, a framework is formed between the polymer, two adjacent branches, and the carbon material. The greater the number of branches, the greater the number of frameworks, which enhances the mechanical properties of the polymer segments. This not only allows the binder to more fully and uniformly bind the various electrode components, but also allows the binder to bind to the surface of the positive electrode active material. These frameworks can alleviate the stress generated by phase transitions in the positive electrode active material and stabilize the material's crystalline phase. Furthermore, the binder can form a polymer nanolayer coating on the surface of the positive electrode active material, reducing electrolyte erosion of the positive electrode active material. In summary, this application can significantly alleviate the problem of easily damaged bulk and interface structures in the positive electrode active material, promoting the stability of both the bulk and interface structures.

[0011] In a second aspect, the present application provides a method for preparing the adhesive of the above application, comprising the following steps:

[0012] Performing a first cross-linking reaction between a one-dimensional carbon material raw material and a cross-linking agent to obtain a cross-linked carbon material;

[0013] performing a second cross-linking reaction between the cross-linked carbon material and the active polymer to bond the groups of the cross-linked carbon material to the groups of the active polymer to obtain a binder;

[0014] The carbon material raw material contains a phosphoric acid group and / or a carboxyl group; and the active polymer contains a phosphoric acid group and / or a carboxyl group.

[0015] The preparation method of the present application first undergoes a first crosslinking reaction between a carbon material raw material and a crosslinking agent, causing the crosslinking agent to partially crosslink with the phosphate groups and / or carboxyl groups of the carbon material raw material to obtain a crosslinked carbon material. The crosslinking reaction is then carried out with an active polymer containing phosphate groups and / or carboxyl groups. After bonding, side chains connecting the polymer backbone and the carbon material are formed, thereby obtaining the binder of the above application, which is beneficial to the stability of the bulk and interface of the positive electrode active material. The preparation method is process-controllable, and the structure and properties of the obtained binder are stable.

[0016] In a third aspect, the present application provides a positive electrode, comprising a current collector and a positive electrode active layer bonded to the current collector, wherein the positive electrode active layer contains the binder of the above application or contains the binder prepared by the preparation method of the above application.

[0017] The binder of the present application can strongly bond with the positive electrode active material, conductive agent, and current collector, and the action is fully and uniformly, which is more conducive to the bulk and interface stability of the positive electrode active material, and also has extremely high ionic conductivity and electronic conductivity. Therefore, the positive electrode of the present application containing this binder has a stable structure and high rate and cycle performance.

[0018] In a fourth aspect, the present application provides a secondary battery, which includes the positive electrode of the above-mentioned application.

[0019] The secondary battery of the present application includes the positive electrode of the above-mentioned application. Therefore, the positive electrode structure of the secondary battery is stable and has good rate performance and cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0021] Figure 1 Schematic diagram of the cross-linking reaction during the preparation of the adhesive in Example A1 of the present application;

[0022] Figure 2 This is an infrared spectrum of some raw materials and the prepared adhesive in Example A1 of the present application;

[0023] Figure 3 1 is a comparison chart of the cycle performance of Example B1, Comparative Example B1, and Comparative Example B2 of the present application;

[0024] Figure 4 It is a comparison chart of the rate performance of Example B1, Comparative Example B1, and Comparative Example B2 of the present application. DETAILED DESCRIPTION

[0025] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application more clearly understood, the present application 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 this application and are not intended to limit this application.

[0026] In this application, the term "and / or" describes the relationship between associated objects, indicating that three possible relationships exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0027] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items.

[0028] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0029] The weights of the relevant components mentioned in the examples of this application may not only refer to the specific content of each component, but also represent the weight ratio between the components. Therefore, as long as the content of the relevant components is proportionally increased or decreased according to the examples of this application, it is within the scope disclosed in the examples of this application. Specifically, the mass in the examples of this application may be a mass unit known in the chemical industry, such as μg, mg, g, kg, etc.

[0030] The terms "first" and "second" are used solely for descriptive purposes to distinguish objects, such as substances, from one another and should not be understood to indicate or imply relative importance or to implicitly specify the quantity of the technical features being referred to. For example, without departing from the scope of the embodiments of this application, a first XX may also be referred to as a second XX, and similarly, a second XX may also be referred to as a first XX. Thus, features defined as "first" or "second" may explicitly or implicitly include one or more of such features.

[0031] In a first aspect, an embodiment of the present application provides a binder comprising a polymer and a one-dimensional carbon material, wherein the polymer comprises at least two side chains, and the side chains are connected to the carbon material;

[0032] Wherein, the carbon material further contains at least one of a phosphate group and a carboxyl group; and / or

[0033] The polymer also contains at least one of a phosphate group and a carboxyl group.

[0034] The phosphate and carboxyl groups in the binder of the present invention can form hydrogen bonds with the positive electrode active material, providing a strong adhesive bond. The phosphate and carboxyl groups also form bonds or chelates with the transition metals in the positive electrode active material, stabilizing the interface structure of the positive electrode active material and inhibiting transition metal dissolution. They also have a desolvation effect, optimizing the solid electrolyte membrane (CEI). The polymer in the binder is connected to the carbon material via branches, forming a monolithic structure rather than a mixture. The carbon material is one-dimensional, and at least two polymer branches are connected to the one-dimensional carbon material, enhancing the uniformity of the polymer distribution and the stretchability of the chain segments. Furthermore, a framework is formed between the polymer, two adjacent branches, and the carbon material. The greater the number of branches, the greater the number of frameworks, which enhances the mechanical properties of the polymer segments. This not only allows the binder to more fully and uniformly bind the various electrode components (such as the positive electrode active material and the conductive agent), but also allows the binder to bind to the surface of the positive electrode active material. These frameworks can alleviate stress caused by phase transitions in the positive electrode active material and stabilize the material's crystalline phase. Furthermore, the binder can form a polymer nanolayer coating on the surface of the positive electrode active material, reducing electrolyte erosion of the positive electrode active material. In summary, the embodiments of the present application can greatly alleviate the problem of easy destruction of the bulk phase and interface of the positive electrode active material, which is beneficial to the stability of the bulk phase and interface.

[0035] In addition to the aforementioned effects on the positive electrode active material, the phosphate and carboxyl groups in the positive electrode also form hydrogen bonds with the conductive agent and current collector, providing strong adhesion and improving the stability of the positive electrode sheet. The stretching of the binder's polymer chains also facilitates more complete and uniform interaction between the binder, the conductive agent, and the current collector. The phosphate groups also enhance the binder's ionic conductivity, such as lithium and sodium ions, while the carbon material enhances its electronic conductivity. As a result, the binder possesses extremely high ionic and electronic conductivity, which helps improve the positive electrode and battery's rate and cycling performance.

[0036] About polymers:

[0037] In some embodiments, the main chain of the polymer contains at least one of an alkyl group, an alkenyl group, an alkynyl group, an ether group, an ester group, a ketone group, an amide group, a phenyl group, and a sugar group, and the main chain can be optionally formed by an alkyl group.

[0038] In some embodiments, the side chains include at least one of amide group bonding, ester group bonding, carboxyl-carboxyl group hydrogen bonding, phosphate group bonding, phosphate group-carboxyl group hydrogen bonding, anhydride group bonding, and ether group bonding. Of course, the side chains may also include organic groups such as alkyl groups. These group bonding or hydrogen bonding within the side chains further enhances the stability of the side chains, facilitates the expansion of the polymer segments, and improves the strength and uniformity of the framework, thereby further facilitating the bulk and interfacial stability of the positive electrode active material. Ideally, the polymer backbone extends parallel to the carbon material, with the side chains connecting the polymer backbone and the carbon material, and the next side chain connecting to the next position of the polymer backbone and the carbon material, forming an orderly, periodically repeating framework that forms a quasi-quadrilateral structure. (Since neither the side chains nor the main chain are strictly straight lines, for ease of understanding, two adjacent side chains are considered as two edges, and the polymer backbone and carbon material between them are considered as the other two edges, representing a quasi-quadrilateral structure, rather than a strict geometrical quadrilateral.) The side chains can be short, with the number of carbon atoms in a side chain ≤ 100. Such short side chains facilitate the stretching of the polymer main chain parallel to the carbon material, facilitating the formation of frameworks one by one. This also increases the strength of the framework, thereby improving the binder's ability to stabilize the bulk and interface of the positive electrode active material. If the side chains are shorter or longer, they may become entangled, or even weaken the interaction between the polymer main chain and the carbon material, which is not conducive to the stretching of the polymer chain segments and the stability of the framework structure.

[0039] About carbon materials:

[0040] The carbon material is one-dimensional, i.e. linear, and in some embodiments, the carbon material includes at least one of carbon nanotubes, carbon fibers, and graphene nanoribbons. Among them, the carbon nanotubes can be single-walled carbon nanotubes or multi-walled carbon nanotubes. It should be noted that, since the carbon material is connected to the side chains of the polymer, these carbon materials do not exist in isolation in the binder and cannot be understood as having isolated carbon nanotubes, carbon fibers, and other materials, but rather forming a whole with the polymer. These types of carbon materials are beneficial to the stretching of the polymer chain segments and improving the strength of the framework, which is beneficial to the stability of the bulk and interface of the positive active material. At the same time, these one-dimensional carbon materials are also beneficial to improving the mechanical properties of the binder. The carbon material itself also has strong electrical conductivity, enriching the conductive network of the bond, which is beneficial to improving the conductivity of the binder.

[0041] The aspect ratio of the carbon material can be (10-1,000,000):1. In exemplary embodiments, the aspect ratio can include, but is not limited to, any ratio of 1,000:1, 5,000:1, 50,000:1, and 100,000:1, or a range between any two ratios. The length can be 10-1,000,000 nm, and the diameter can be 0.8-100 nm.

[0042] About the whole binder:

[0043] In some embodiments, the molar ratio of polymer to carbon material is (1-100):1. Examples include, but are not limited to, any ratio of 100:1, 10:1, 2:1, 1:1, or any range between two ratios. These molar ratios of polymer and carbon material are more compatible with the ratio of binding components to conductive components in the battery positive electrode material system. At these molar ratios, microscopically, one carbon material can connect to multiple polymer molecules, while one polymer molecule can connect to multiple carbon materials. The one-dimensional carbon material continuously guides the stretching of polymer chains, improving the binder's ability to stabilize the bulk and interface of the positive electrode active material.

[0044] In some embodiments, the phosphate groups in the binder can be on the main chain of the polymer, on the side chains of the polymer, on other side chains of the polymer (the side chains are not connected to the carbon material), or can be bound to the surface of the carbon material; the carboxyl groups can be on the main chain of the polymer, on the side chains of the polymer, on the side chains of the polymer (the side chains are not connected to the carbon material), or can be bound to the surface of the carbon material. In some embodiments, the molar proportion of the phosphate groups in the binder is 20% to 50%, which may include but is not limited to any value of 10%, 20%, 30%, 40%, 50%, or any range between any two values; the molar proportion of the carboxyl groups in the binder is 30% to 70%, which may include but is not limited to any value of 30%, 40%, 50%, 60%, or 70%, or any range between any two values. These molar proportions of phosphate groups and carboxyl groups further facilitate the formation of hydrogen bonds between the binder and the positive electrode active material, the conductive agent, and the current collector, providing a strong bonding effect; they facilitate the formation of bonding or chelating effects with the transition metal of the positive electrode active material, stabilizing the interface structure of the positive electrode active material and inhibiting the dissolution of the transition metal; and they are more conducive to improving the ionic conductivity of the binder such as lithium ions and sodium ions.

[0045] Finally, the adhesive of the embodiment of the present application can use water and other common solvents as low-cost and environmentally friendly dispersants, getting rid of the problem of the traditional adhesive solvent N-methylpyrrolidone (NMP) being toxic and expensive.

[0046] A second aspect of the present application provides a method for preparing the adhesive of the above application embodiment, comprising the following steps:

[0047] S10: performing a first cross-linking reaction on the one-dimensional carbon material raw material and a cross-linking agent to obtain a cross-linked carbon material;

[0048] S20: performing a second cross-linking reaction on the cross-linked carbon material and the active polymer, so that the groups of the cross-linked material are bonded to the groups of the active polymer to obtain a binder.

[0049] The carbon material raw material in step S10 contains phosphate groups and / or carboxyl groups; and the active polymer in step S20 contains phosphate groups and / or carboxyl groups.

[0050] The preparation method of the embodiment of the present application first undergoes a first crosslinking reaction between the carbon material raw material and the crosslinking agent, causing the crosslinking agent to partially crosslink with the phosphate group and / or carboxyl group of the carbon material raw material to obtain a crosslinked material, and then undergoes a second crosslinking reaction with an active polymer containing phosphate groups and / or carboxyl groups. After bonding, a side chain connecting the polymer main chain and the carbon material can be formed, thereby obtaining the binder of the embodiment of the above text application, which is beneficial to the stability of the bulk and interface of the positive electrode active material. The preparation method process is controllable, and the structure and properties of the obtained binder are stable.

[0051] Step S10:

[0052] This step is the first crosslinking step. The carbon material raw material type and parameters such as aspect ratio can refer to the types and parameters of the carbon materials described above. The carbon material raw material contains phosphate groups and / or carboxyl groups. Therefore, the carbon material raw material can be phosphated carbon nanotubes, phosphated carbon fibers, carboxylated carbon nanotubes, carboxylated carbon fibers, etc. The carbon material raw material can also contain both phosphate groups and carboxyl groups. The mass percentage of phosphate groups to carboxyl groups in the carbon material raw material is 20% to 80%.

[0053] The crosslinking agent contains at least two functional groups. In some embodiments, the functional groups include at least one of a diimine, an isocyanate, a carboxyl group, a hydroxyl group, and an amino group. When the carbon material raw material contains carboxyl groups, the imine can react with the carboxyl group to form an amide group, the isocyanate can react with the carboxyl group to form an amide group, the carboxyl group can react with the carboxyl group to form an anhydride group, the hydroxyl group can react with the carboxyl group to form an ester group, and the amino group can react with the carboxylic acid to form an amide group. When the carbon material raw material contains a phosphate group, the diimine can react with the phosphate to form an amide group, the hydroxyl group can react with the phosphate to form a phosphate ester group, and the phosphate can react with the carboxylic acid to form a hydrogen bond.

[0054] The cross-linking agent may optionally be an aqueous cross-linking agent. In some embodiments, when the cross-linking agent contains a diimide, it may be polycarbodiimide. When the cross-linking agent contains an isocyanate, it may be an isocyanate. In an exemplary embodiment, it may include at least one of toluene diisocyanate and hexamethylene diisocyanate. When the cross-linking agent contains a carboxyl group, it may be at least one of citric acid, maleic acid, and malonic acid. When the cross-linking agent contains a hydroxyl group, it may be at least one of glycerol, ethylene glycol, and xylitol. When the cross-linking agent contains an amino group, it may be at least one of phenylenediamine and ethylenediamine. In addition, the cross-linking agent may also contain two or more different functional groups.

[0055] The first cross-linking reaction can be performed by mixing the raw materials in a solvent and continuously stirring. The solvent can be at least one of deionized water, ethanol, dichloromethane, and tetrahydrofuran. In some embodiments, the temperature of the first cross-linking reaction can be 20 to 160° C., and the reaction time can be 0.5 to 12 hours. After the first cross-linking reaction, it is ideal that some functional groups in the cross-linking agent have bonded to the carbon material raw materials, and other functional groups in the cross-linking agent have formed a large number of cross-linking points on the surface of the cross-linked carbon material, which can participate in the subsequent second cross-linking reaction.

[0056] Step S20:

[0057] This step is a step of further cross-linking the active polymer and the cross-linked carbon material. The cross-linked carbon material contains functional groups in the cross-linking agent. Since the active polymer contains phosphate groups and / or carboxyl groups, these functional groups can be bonded to the active polymer. The specific bonding method can refer to the statement in step S10 above that the phosphate groups and / or carboxyl groups form amide group bonds, ester bonds, anhydride bonds, phosphate ester bonds, etc. with the cross-linking agent. Therefore, in the binder obtained by the first cross-linking reaction and the second cross-linking reaction, a morphology is formed in which the polymer main chain and the carbon material are connected through the polymer side chains, which is conducive to the stretching of the polymer chain segments and the formation of a framework, and is conducive to the stability of the bulk and interface of the positive electrode active material.

[0058] The second cross-linking reaction can involve mixing the raw materials and continuously stirring. In some embodiments, the temperature of the second cross-linking reaction can be 20-160°C, and the reaction time can be 0.5-12 hours. These temperature and reaction time can be the same as or different from those of the first cross-linking reaction. In some embodiments, the mass ratio of the carbon material raw material, cross-linking agent, and active polymer can be (1-20): (1-50): (30-70). In exemplary embodiments, this ratio can include, but is not limited to, any ratio or range between any two ratios of (1 or 5 or 10 or 20): (1 or 5 or 10 or 20 or 50): (30 or 40 or 50 or 60 or 70). The amount of cross-linking agent used can be adjusted to adjust the cross-linking density of the resulting binder, thereby regulating the physical and chemical properties of the binder.

[0059] In some embodiments, the active polymer includes a carboxyl-containing polymer, and a polyphosphoric acid material is further added to the second cross-linking reaction, so that the carboxyl-containing polymer, the cross-linking agent, and the polyphosphoric acid material undergo a third cross-linking reaction. In this way, the carboxyl-containing polymer undergoes a second cross-linking reaction with the cross-linked carbon material, while the polyphosphoric acid material also undergoes a third cross-linking reaction with the carboxyl-containing polymer via the cross-linking agent. Furthermore, the polyphosphoric acid material can also directly form phosphate-carboxyl hydrogen bonds with the carboxyl-containing polymer, which facilitates the introduction of abundant carboxyl and phosphate groups into the resulting binder. This further facilitates hydrogen bonding between the binder and the positive electrode active material, the conductive agent, and the current collector, thereby improving bonding performance. It also further facilitates stabilizing the interface structure of the positive electrode active material, inhibiting transition metal dissolution, and further improving the ionic conductivity of the bond.

[0060] In some embodiments, the carboxyl-containing polymer includes at least one of polyacrylic acid, polyacrylic acid salt, hyaluronic acid, hyaluronate, alginic acid, and alginates (lithium, sodium, potassium, magnesium, calcium, zinc, or aluminum salts). The carboxyl group content of the carboxyl-containing polymer can range from 20% to 65% by weight. These high carboxyl-containing polymers can improve the binding properties of the binder and enhance the bulk and interfacial stability of the positive electrode active material. In exemplary embodiments, the weight-average molecular weight of the carboxyl-containing polymer can be >300,000.

[0061] In some embodiments, the polyphosphoric acid material includes at least one of polyphosphoric acid, phytic acid, polymetaphosphoric acid, alkyl polyphosphoric acid, phytates (lithium, sodium, potassium, magnesium, calcium, zinc, or aluminum salts), aminotrimethylphosphonic acid, diethylenetriaminepentamethylphosphonic acid, polyphosphates, polymetaphosphates, and alkyl polyphosphates. These polyphosphoric acid materials contain abundant phosphate groups. After undergoing a third crosslinking reaction with a carboxyl-containing polymer and a crosslinking agent, these phosphate groups increase the phosphate group content in the resulting binder, thereby improving the binder's binding properties, enhancing the bulk and interfacial stability of the positive electrode active material, and increasing the binder's ionic conductivity.

[0062] In some embodiments, the mass ratio of the carboxyl-containing polymer to the polyphosphoric acid material is (30-70): (20-50). In exemplary embodiments, the mass ratio may include but is not limited to any ratio of (30 or 40 or 50 or 60 or 70): (20 or 30 or 40 or 50) or a range between any two ratios.

[0063] In some embodiments, after the second cross-linking reaction, a pH adjustment step is also included. The pH adjustment is primarily to meet the pH requirements of the positive electrode slurry and current collector. The pH adjustment method can be to add a pH adjustment agent to the product of the second cross-linking reaction. The pH adjustment agent can be at least one of LiOH, NaOH, and KOH, so that the final pH of the resulting binder is 4 to 8 at room temperature. The final binder concentration is 1 to 50 wt%.

[0064] A third aspect of the embodiments of the present application provides a positive electrode, including a current collector and a positive electrode active layer bonded to the current collector, wherein the positive electrode active layer contains the binder of the above embodiments of the application or contains the binder prepared by the preparation method of the above embodiments of the application.

[0065] The binder in the embodiment of the present application can strongly bond with the positive electrode active material, conductive agent, and current collector, and the action is fully and uniformly, which is more conducive to the bulk and interface stability of the positive electrode active material, and also has extremely high ionic conductivity and electronic conductivity. Therefore, the positive electrode in the embodiment of the present application contains this binder, and the positive electrode has a stable structure and high rate and cycle performance.

[0066] In some embodiments, the positive electrode active layer contains a positive electrode active material, which is a lithium ion layered positive electrode material, which can be selected from at least one of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, lithium fluorinated vanadium phosphate, lithium titanate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide. The positive electrode active material can also be a sodium ion layered positive electrode material that satisfies the chemical formula Na x At least one of TMO2 (TM = one or more of Co, Mn, Ni, Li, Ti, Fe, Cr, Cu, V).

[0067] Compared with the prior art, since the binder in the above-mentioned application embodiment has extremely high electronic conductivity, a conductive agent may not be added to the positive electrode active layer, or a conductive agent may be appropriately added, which may include at least one of conductive graphite, carbon black, acetylene black, graphene, carbon fiber, C60 and carbon nanotubes.

[0068] In an embodiment, the positive electrode sheet preparation process may include: mixing the positive electrode active material, a conductive agent (optional), and a binder, and adjusting the solids content of the system by adding water or other solvents (ethanol, NMP) to obtain a positive electrode slurry. The binder ratio (excluding the weight of the binder solvent) in the slurry is 1-20wt%, the conductive agent ratio is 0-20wt%, and the slurry solids content is 0.3-1 g / mL. The positive electrode slurry is then coated on a current collector. Furthermore, the current collector may be aluminum foil or other widely used lithium-ion or sodium-ion battery positive electrode current collectors. Finally, the positive electrode sheet is prepared through drying, roll pressing, and die-cutting.

[0069] A fourth aspect of the embodiments of the present application provides a secondary battery, which includes the positive electrode of the embodiments of the present application.

[0070] The secondary battery of the present embodiment includes the positive electrode of the above embodiment. Therefore, the positive electrode structure of the secondary battery is stable and has good rate performance and cycle performance. The secondary battery also includes necessary components such as the negative electrode, separator, and electrolyte, and of course other necessary or auxiliary components.

[0071] The following describes the details in conjunction with specific embodiments.

[0072] Example A1

[0073] This embodiment provides a binder and a preparation method thereof, please refer to Figure 1 As shown, the preparation method includes the following steps S1 to S2:

[0074] S1: First cross-linking reaction

[0075] First, 0.5 g of carboxylated multi-walled carbon nanotubes (carbon material raw material) containing 50% carboxyl groups by mass was dissolved in 100 mL of deionized water, and then 2.5 g of polycarbodiimide (cross-linking agent, excess) was added to carry out the first step of cross-linking reaction. The cross-linking temperature was 80 ° C, and stirring was continued for 6 h to obtain cross-linked carbon nanotubes.

[0076] S2: Second cross-linking reaction

[0077] 5 g of polyacrylic acid (carboxyl-containing polymer) and 2 g of sodium polyphosphate (polyphosphoric acid material) were added to the cross-linked carbon nanotubes obtained in step S1 to carry out a second cross-linking reaction. During the process, not only the cross-linked carbon nanotubes and polyacrylic acid were cross-linked, but also the polyacrylic acid, the cross-linking agent, and the sodium polyphosphate were cross-linked. The cross-linking temperature was 80°C, and stirring was continued for 6 h to obtain a uniform, transparent, viscous liquid. 1 M sodium hydroxide was added to the uniformly dispersed system to adjust the pH to 7 at room temperature to obtain a binder.

[0078] Example A2

[0079] This embodiment provides a binder and a preparation method thereof, which differs from Embodiment A1 only in that polycarbodiimide in step S1 is replaced with propylene glycol, and all other aspects are the same.

[0080] Example A3

[0081] This embodiment provides a binder and a preparation method thereof, which differs from Embodiment A1 only in that the polyacrylic acid in step S2 is replaced with hyaluronic acid, and all other aspects are the same.

[0082] Example A4

[0083] This embodiment provides a binder and a preparation method thereof, which differs from Embodiment A1 only in that the sodium polyphosphate in step S2 is replaced with aminotri(methylene)phosphoric acid, and all other aspects are the same.

[0084] Example A5

[0085] This embodiment provides a binder and a preparation method thereof, which differs from embodiment A1 only in that the carboxylated multi-walled carbon nanotubes in step S1 are replaced with phosphorylated carbon nanotubes, and all other aspects are the same.

[0086] Example A6

[0087] This embodiment provides a binder and a preparation method thereof, which differs from embodiment A6 only in that polycarbodiimide in step S1 is replaced with propylene glycol, and all other aspects are the same.

[0088] Comparative Example A1

[0089] This comparative example provides a binder, which is polyvinylidene fluoride (PVDF) commonly used in positive electrodes.

[0090] Comparative Example A2

[0091] This comparative example provides a binder, which is a mixture. The raw materials are the raw materials other than the cross-linking agent in Example A1. 0.5 g of carboxylated single-walled carbon nanotubes, 5 g of polyacrylic acid, and 2 g of sodium polyphosphate are directly dissolved in 100 mL of deionized water and stirred for 10 h to obtain a mixed binder.

[0092] The differences between Examples A1 to A6 and Comparative Examples A1 to A2 are shown in Table 1.

[0093] Table 1

[0094]

[0095] Sodium ion battery example:

[0096] The binders provided in Examples A1 to A6 and Comparative Examples A1 to A2 were assembled into positive electrodes and sodium ion batteries according to the following methods:

[0097] Positive electrode: The positive electrode active material is sodium-ion P2 type layered positive electrode material Na 0.7 Li 0.1 Cu 0.1 Mg 0.1 Mn 0.7 The positive electrode active material, binder, and super P conductive carbon were placed in a mixing tank at a mass ratio of 80:10:10. Zirconium beads were added to improve mixing efficiency. Deionized water was added to the mixing tube at a solid content of 0.4 g / mL, and the mixture was stirred into a uniform electrode slurry using a micro-mixer.

[0098] The prepared electrode slurry was applied to the surface of aluminum foil using a coating machine to form a 0.2 mm thick film. The electrode sheet was immediately transferred to a 60°C forced air drying oven and dried for 6 hours. The baked electrode sheet was then transferred to a vacuum oven and baked at 110°C for 12 hours to remove any residual moisture. The electrode sheet was then rolled using a roller press and cut into 12 mm diameter circular pieces using a cutting machine.

[0099] Assembly: Assemble 2032 button cells in an argon glove box in the order of positive electrode shell, positive electrode sheet, 50 μL electrolyte, glass fiber, 50 μL electrolyte, sodium sheet, gasket, spring sheet, and negative electrode shell.

[0100] Among them, the binder of Example A1 is used to prepare the sodium ion battery of Example B1, the positive electrode material of Example A2 is used to prepare the sodium ion battery of Example B2, and so on, until the binder of Comparative Example A2 is used to prepare the sodium ion battery of Comparative Example B2.

[0101] Related performance tests and result analysis

[0102] 1. Reaction process

[0103] like Figure 1 As shown, the cross-linking agent in Example A1 first undergoes a first cross-linking reaction with the carboxylated carbon nanotubes. The carboxyl groups on the surface of the carboxylated carbon nanotubes react with the carbodiimide groups of the cross-linking agent to form amide groups, generating a large number of cross-linking points on the carbon nanotubes. Subsequently, the polyphosphoric acid material and the carboxyl-containing polymer are added to carry out a second cross-linking reaction. The carboxyl-containing polymer with a high degree of polymerization reacts with the carbon nanotubes containing a large number of cross-linking points to form an ordered series of quadrilateral-like (closer to a trapezoid in observation) frameworks. The polyphosphoric acid material also reacts with the cross-linking agent to generate amide groups, which are then cross-linked to the carboxyl-containing polymer. Alternatively, the polyphosphoric acid material is directly bonded to the carboxyl-containing polymer through hydrogen bonds, and is then incorporated into the entire binder.

[0104] The resulting crosslinker comprises carbon nanotubes and a polymer, with several polymer branches connecting the carbon nanotubes. The binder also contains abundant phosphate and carboxyl groups. Guided by the carbon nanotubes, the polymer chains in the binder are further stretched. This synergistic framework ensures uniform and effective binding of the binder to the cathode active material. These frameworks also mitigate stress caused by phase transitions in the cathode active material, stabilizing the material's crystalline phase. They also act as a desolvating agent, optimizing the solid electrolyte interface (CEI). Furthermore, they form a polymer nanolayer coating on the surface of the cathode active material, reducing electrolyte erosion.

[0105] 2. Structural Analysis

[0106] Figure 2 The infrared spectra of polyacrylic acid, carboxylated carbon nanotubes, polycarbodiimide, polyphosphoric acid and the prepared binder in Example A1 show that polyacrylic acid and carboxylated carbon nanotubes have an infrared spectrum of 1700 cm -1 There is an obvious infrared absorption peak of C=O near the polycarbodiimide at 1600 cm -1 The vibration peak of the C=N functional group is shown, and the final composite binder has a peak at 1500 cm -1A new infrared absorption peak of the CN functional group appears nearby, indicating that the carboxylic acid functional group successfully reacts with the cross-linker to form an amide bond.

[0107] 3. Cycle performance

[0108] At 25°C, 1.5-4.5 V charge and discharge voltage conditions, 1C charge and discharge rate, and a nominal specific capacity of 100 mAh / g, a cycle stability test was conducted. The results of Example B1, Comparative Example B1, and Comparative Example B2 are shown in FIG. Figure 3 As shown, Figure 3 The "conductive and ion-conducting ladder binder" is the sodium ion battery of Example B1 prepared by the binder of Example A1, "PVDF" corresponds to the corresponding ratios A1 and B1, and the "mixed binder" corresponds to the corresponding ratios A2 and B2. Figure 2 It can be seen that the performance of Comparative Example B2 is better than that of Comparative Example B1. However, compared with the two comparative examples, the cycle stability of the sodium ion battery of Example B1 is significantly higher.

[0109] 4. Rate performance

[0110] At 25°C and a charge-discharge voltage of 1.5 to 4.5 V, the rate performance at a charge-discharge rate of 1C was tested. The nominal specific capacity was 100 mAh / g. The charge-discharge currents were set at 0.1C, 0.2C, 0.5C, 1C, 2C, 3C, 10C, 20C, and 30C, respectively. After the rate cycle, the half-cell was cycled at 0.5C to recover. The results of Example B1, Comparative Example B1, and Comparative Example B2 are shown in FIG. Figure 4 As shown, Figure 4 The "conductive and ion-conducting ladder binder" is the sodium ion battery of Example B1 prepared by the binder of Example A1, "PVDF" corresponds to the corresponding ratios A1 and B1, and the "mixed binder" corresponds to the corresponding ratios A2 and B2. Figure 4 It can be seen that the performance of Comparative Example B2 is better than that of Comparative Example B1. However, compared with the two comparative examples, the rate performance of the sodium ion battery of Example B1 is significantly higher.

[0111] Therefore, compared to existing commonly used PVDF binders and optimized physical mixed binders, the binder in this application can fully interact with the positive electrode active material when used in the positive electrode, alleviating the stress generated by the phase transition of the positive electrode active material, stabilizing the material's crystal phase, inhibiting the dissolution of transition metals, and further optimizing the solid electrolyte interphase (CEI). It can also reduce electrolyte erosion and promote the stability of the positive electrode active material's bulk phase and interface. The binder also has high ionic and electronic conductivity, which is beneficial for improving the battery's rate and cycle performance.

[0112] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A binder, characterized in that: The invention comprises a polymer and a one-dimensional carbon material, wherein the polymer comprises at least two side chains, and the side chains are connected to the carbon material; The preparation method of the adhesive comprises the following steps: providing a cross-linking agent, wherein the cross-linking agent contains at least two functional groups; Performing a first cross-linking reaction on a one-dimensional carbon material raw material and an excess of a cross-linking agent to obtain a cross-linked carbon material, wherein the cross-linked carbon material contains the functional group; performing a second cross-linking reaction between the cross-linked carbon material and the active polymer, so that the functional groups of the cross-linked carbon material are bonded to the groups of the active polymer to obtain a binder; The active polymer includes a carboxyl-containing polymer, and a polyphosphoric acid material is further added to the second cross-linking reaction, so that the carboxyl-containing polymer, the remaining cross-linking agent, and the polyphosphoric acid material undergo a third cross-linking reaction; Wherein, the carbon material raw material includes a phosphorylated carbon material raw material and / or a carboxylated carbon material raw material; and the carboxyl-containing polymer includes at least one of polyacrylic acid, hyaluronic acid, and alginic acid.

2. The adhesive according to claim 1, wherein: The side chains include at least one of amide group bonding, ester group bonding, carboxyl-carboxyl hydrogen bonding, phosphate group bonding, phosphate group-carboxyl hydrogen bonding, anhydride group bonding, and ether group bonding.

3. The adhesive according to claim 1 or 2, characterized in that: The molar ratio of the polymer to the carbon material is (1-100):1; And / or, the carbon material includes at least one of carbon nanotubes, carbon fibers, and graphene nanoribbons; And / or, the aspect ratio of the carbon material is (10-1,000,000):

1.

4. The adhesive according to claim 1, wherein: The functional group includes at least one of diimide, isocyanate, hydroxyl and amino; and / or, the cross-linking agent comprises at least one of polycarbodiimide, isocyanate, sodium tricyanomethanide, citric acid, maleic acid, malonic acid, glycerol, ethylene glycol, xylitol, phenylenediamine, and ethylenediamine; And / or, the mass ratio of the carbon material raw material, the cross-linking agent, and the active polymer is (1-20): (1-50): (30-70).

5. The adhesive according to claim 1, wherein: The mass ratio of the carboxyl group-containing polymer to the polyphosphoric acid material is (30-70): (20-50); and / or, the polyphosphoric acid material comprises at least one of polyphosphoric acid, phytic acid, polymetaphosphoric acid, alkyl polyphosphoric acid, phytate, amino trimethylene phosphoric acid, diethylene triamine penta (methylene phosphonic acid), polyphosphate, polymetaphosphate, and alkyl polyphosphate; and / or, the weight average molecular weight of the carboxyl group-containing polymer is greater than 300,000; And / or, the mass proportion of the carboxyl groups in the carboxyl-containing polymer is 20% to 65%.

6. The adhesive according to any one of claims 1, 4 or 5, characterized in that: The temperature of the first cross-linking reaction and the second cross-linking reaction is the same or different and is 20 to 160° C., and the reaction time is the same or different and is 0.5 to 12 h; And / or, after the second cross-linking reaction, a step of adjusting pH is further included.

7. A positive electrode, characterized in that: The invention comprises a current collector and a positive electrode active layer bonded to the current collector, wherein the positive electrode active layer contains the binder according to any one of claims 1 to 6.

8. A secondary battery, characterized in that: The secondary battery includes the positive electrode according to claim 7.

Images