A positive electrode active material, a method for preparing the same, and an application thereof

Abstract

The application relates to the field of batteries, in particular to a positive electrode active material and a preparation method and application thereof, the positive electrode active material comprising a positive electrode active material base body and a coating layer coated on at least part of the surface of the positive electrode active material base body; in the direction away from the positive electrode active material base body, the coating layer comprises a hydrophobic layer, a cross-linking layer and an ion-conducting layer which are sequentially arranged. The purpose is to improve the electrochemical performance and material storage stability of the positive electrode active material of a sodium ion battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of batteries, in particular to a positive electrode active material and a preparation method and application thereof. BACKGROUND

[0002] With the increasing demand for energy storage, sodium-ion batteries as a potential energy storage technology have attracted much attention. Layered oxides are of interest due to their high specific capacity. Among the layered positive electrode oxides, P2 and O3 type oxides are representative. Compared with P2 type, the advantage of O3 type positive electrode is that more can be accommodated in the layered structure, thereby providing higher reversible capacity. However, most O3 type layered oxides will produce a large amount of residual alkali on the surface during preparation or exposure to air / humidity: i.e. the formation of and etc. on the surface. If the residual alkali content of the sodium-ion battery positive electrode active material is too high during the production process, it will cause the slurry to flocculate, increase the gas production during the cycle process of the battery, and greatly reduce the safety performance, electrochemical reaction performance and reaction kinetics of the battery.

[0003] In the prior art, the methods for reducing the residual alkali on the surface of the positive electrode active material mainly include water washing and acid treatment, but these methods have certain limitations. Water washing is used in the industrial production of lithium battery ternary positive electrode active materials to remove the residual alkali on the surface, but water washing cannot be used for sodium-ion battery layered positive electrodes because water washing not only removes the residual alkali on the surface, but also causes the sodium ions in the bulk phase to dissolve out, resulting in serious performance degradation of the positive electrode material. Acid treatment is also an effective measure to reduce the residual alkali on the surface of the sodium-ion battery positive electrode material. Researchers have tried various acids (such as oxalic acid, tartaric acid, citric acid, etc.) and optimized the parameters. Although acid treatment can effectively reduce the residual alkali and is simple to operate, improper control of the concentration, time and temperature of acid treatment may affect the material structure; and the residual acid may adversely affect the performance of the battery.

[0004] Therefore, how to improve the air stability of the sodium-ion positive electrode active material and further improve the electrical performance of the battery is a technical problem that needs to be solved by those skilled in the art. SUMMARY

[0005] The present application provides a positive electrode active material, which can not only reduce the residual alkali on the surface of the positive electrode active material, but also improve the air stability of the positive electrode active material and further improve the electrical performance of the positive electrode active material by forming a multilayer coating on the positive electrode active material.

[0006] The present application provides a preparation method of a positive electrode active material, which is used to prepare the above-mentioned positive electrode active material. The method is simple, has low energy consumption and uses non-toxic solvents, and is suitable for industrial production.

[0007] The application provides a positive electrode sheet, which comprises the positive electrode active material, and helps to improve the electrical performance of a battery.

[0008] The application also provides a sodium ion battery, which comprises the positive electrode sheet, and helps to improve the safety performance and electrochemical performance of the battery.

[0009] In a first aspect, the application provides a positive electrode active material, which comprises a positive electrode active material base and a coating layer coated on at least part of the surface of the positive electrode active material base.

[0010] In the direction away from the positive electrode active material base, the coating layer comprises a hydrophobic layer, a cross-linking layer and an ion-conducting layer which are sequentially stacked.

[0011] The positive electrode active material as described above, part of the cross-linking layer is embedded in the hydrophobic layer, part of the ion-conducting layer is embedded in the cross-linking layer, and the cross-linking layer is at least partially connected to the positive electrode active material base by a bond.

[0012] The positive electrode active material as described above, the mass ratio of the hydrophobic layer, the cross-linking layer and the ion-conducting layer is (50-80):(10-30):(5-30); and / or,

[0013] The mass of the coating layer is 3-8 wt% of the mass of the positive electrode active material; and / or,

[0014] The thickness of the coating layer is 8-20 nm.

[0015] The positive electrode active material as described above, the total amount of surface residual alkali of the positive electrode active material is not more than 1.0 wt% of the positive electrode active material.

[0016] The positive electrode active material as described above, the positive electrode active material base comprises a compound represented by Formula 1;

[0017] wherein, Formula 1, wherein M is a doping element, comprising at least one of Li, Fe, Cu, Mg, Zn, Co, Al, Zr, Ti, Te, Sr, B, Sn, Mo, Nb, Sb, 0.6≤x≤1, 0.3≤y≤0.95, 0

[0018] The positive electrode active material as described above, the hydrophobic layer comprises a carboxylic acid metal salt; and / or,

[0019] The cross-linking layer comprises at least one of an organic silicon compound, an organic titanium compound, and an organic aluminum compound; and / or,

[0020] The ion-conducting layer comprises a sodium salt compound.

[0021] As in the above-described positive electrode active material, the organosilicon compound includes siloxane compounds; the organotitanium compound includes titanate compounds; the organoaluminum compound includes aluminate compounds; and / or,

[0022] The sodium salt compound includes , , , , At least one of them.

[0023] Secondly, this application provides a method for preparing a positive electrode active material, comprising the following steps:

[0024] After saponification of a mixture comprising a carboxylic acid ester and a positive electrode active material matrix, a coupling agent is added to induce a crosslinking reaction. Subsequently, a conductive ion is added to the system to obtain the positive electrode active material.

[0025] As described above in the preparation method of the positive electrode active material, the carboxylic acid ester includes C 12 -C 18 Alkyl chain fatty acid esters; and / or,

[0026] The coupling agent includes at least one of fluorosilane coupling agents, octyltrimethoxysilane, titanate, vinyltrimethoxysilane, KH550, KH570, and aluminate coupling agents.

[0027] As described above in the preparation method of the positive electrode active material, the saponification reaction is carried out under stirring conditions at a reaction temperature of 40-60°C; and / or,

[0028] During the crosslinking reaction, the reaction temperature is 40-80℃, the stirring speed is 800-1200 rpm, and the stirring time is 1-3 hours; and / or,

[0029] When adding the conductive ions, the ions are added in steps, while ultrasonic and stirring treatments are performed simultaneously at 40-80°C for 40-60 minutes.

[0030] As described above in the preparation method of the positive electrode material, the method further includes preheating the conductive ions at 100-160°C before their addition; and / or,

[0031] Before carrying out the saponification reaction, the positive electrode active material matrix is ​​pretreated. The pretreatment includes vacuum drying the positive electrode active material matrix at 40-80°C for 8-15 hours.

[0032] This application provides a positive electrode sheet, comprising the above-described positive electrode active material or the positive electrode active material prepared by the above-described positive electrode active material preparation method.

[0033] The application provides a sodium ion battery, comprising the above positive electrode sheet.

[0034] The application adopting the above scheme also has at least the following beneficial effects:

[0035] In the application, by constructing a hydrophobic layer on the surface of the positive electrode active material matrix, not only the residual alkali content can be reduced, but also a hydrophobic layer with excellent hydrophobicity can be formed, so as to reduce the probability of further reaction of the surface of the positive electrode active material matrix with water to generate alkaline substances such as sodium carbonate and sodium hydroxide, and thus reduce the surface residual alkali content; then by constructing a cross-linked layer with a three-dimensional network structure, the cross-linked layer with a three-dimensional network structure is coated on the hydrophobic layer, so as to enhance the structural strength and interface bonding capacity of the coating; then by coating a sodium fast ionic conductor material to construct an ion conducting layer, a hydrophobic-reinforced-ion-conducting composite coating layer is constructed, which is beneficial to improve the electrochemical performance and material storage stability of the positive electrode active material. BRIEF DESCRIPTION OF DRAWINGS

[0036] The accompanying drawings, which are incorporated into and form part of the specification, illustrate embodiments consistent with the present application and, together with the specification, serve to explain the principles of the application.

[0037] Figure 1 It is a schematic diagram of the structure of the coating layer coated on the positive electrode active material matrix in Example 1.

[0038] Figure 2 It is an XRD graph of Example 1-3 and Comparative Example 1-2.

[0039] Figure 3 It is a SEM graph of Example 1 and Comparative Example 1.

[0040] Figure 4 It is a TEM graph of Example 1.

[0041] Figure 5 It is an XPS graph of Example 1.

[0042] Wherein, 1-positive electrode active material matrix; 2-hydrophobic layer; 3-cross-linked layer; 4-ion conducting layer. DETAILED DESCRIPTION

[0043] To make the objects, technical solutions and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below in combination with specific embodiments in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0044] In order to reduce the content of residual alkali on the surface of the positive active material, improve the safety performance, electrochemical reaction performance and reaction kinetics of the battery, the inventors have studied the method for reducing the residual alkali on the surface of the positive active material. It is found that coating modification is an effective method for improving the performance of the positive active material of the sodium ion battery. The principle is to form a coating layer on the surface of the positive active material of the sodium ion battery. This can not only avoid direct contact of the positive active material with water and carbon dioxide in the air and other substances, so as to improve the air stability of the material, but also slow down the corrosion of the electrolyte to the positive active material. In the prior art, the positive active material of the sodium ion battery is coated by high-energy ball milling, liquid mixing and other methods. However, the single coating method has the following problems: 1. Single hydrophobic coating will hinder ion transmission, resulting in a decrease in rate performance; 2. Single coating cannot solve the problem of residual alkali and has insufficient mechanical strength; and single coating will reduce the electrochemical performance of the positive active material.

[0045] Therefore, the embodiment of the present application proposes a positive active material, which comprises a positive active material substrate 1 and a coating layer coated on at least part of the surface of the positive active material substrate 1.

[0046] In the direction away from the positive active material substrate 1, the coating layer comprises a hydrophobic layer 2, a cross-linked layer 3 and an ion-conducting layer 4 arranged in sequence.

[0047] In the embodiment, by constructing the hydrophobic layer 2 on the surface of the positive active material substrate 1, the hydrophobic layer 2 has good hydrophobic performance, which avoids direct contact of the positive active material substrate 1 with water, oxygen and carbon dioxide in the environment, and helps to improve the air stability of the positive active material. Then, the cross-linked layer 3 is arranged on the hydrophobic layer 2 to improve the mechanical strength and interface bonding capacity of the coating. Then, the ion-conducting layer 4 is coated on the cross-linked layer 3 to establish a low-barrier transmission channel, thereby constructing a composite coating layer of “hydrophobic-reinforced-ion-conducting”, and the coating layer will not block ion transmission, thereby synergistically improving the comprehensive performance of the positive active material.

[0048] In a specific embodiment, as shown in Figure 1 part of the cross-linked layer 3 is embedded in the hydrophobic layer 2, and part of the ion-conducting layer 4 is embedded in the cross-linked layer 3. The cross-linked layer 3 is at least partially connected to the positive active material substrate 1 by a bond.

[0049] By partially penetrating and embedding the cross-linked layer 3 in the gap of the hydrophobic layer 2 and being connected to the surface of the positive active material substrate 1 by a bond, the mechanical strength and interface bonding capacity of the coating are improved. After forming the ion-conducting layer 4 on the cross-linked layer 3, part of the ion-conducting layer 4 is embedded in the cross-linked layer 3 with a three-dimensional network structure to realize ion conduction, thereby improving the electrochemical performance and material storage stability of the positive active material of the sodium ion battery.

[0050] In one embodiment, the mass ratio of the hydrophobic layer 2, the cross-linking layer 3 and the ion-conducting layer 4 is (50-80):(10-30):(5-30). Preferably, the mass ratio is (60-70):(15-25):(10-20). Exemplarily, it can be 60:15:10; 60:25:20; 60:20:15; 65:18:13; 65:19:16; 70:15:10; 70:20:10; 70:25:20, or a range between any two of the above.

[0051] In the present embodiment, by controlling the mass ratio of the hydrophobic layer 2, the cross-linking layer 3 and the ion-conducting layer 4, on the one hand, the effect of reducing the residual alkali content on the surface of the positive active material matrix 1 is achieved, and on the other hand, the best balance between the hydrophobic effect and the electrochemical performance of the positive active material is achieved. On this basis, the structural strength of the coating layer, the binding ability of different layers and the ion transmission rate are improved, and the problem of ion transmission rate caused by the formation of the hydrophobic layer 2 is avoided.

[0052] In one embodiment, the mass of the coating layer is 3-8wt% of the mass of the positive active material. Exemplarily, it can be 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, or a range between any two of the above.

[0053] In one embodiment, the thickness of the coating layer is 8-20nm. Exemplarily, it can be 8nm, 10nm, 12nm, 14nm, 16nm, 18nm, 20nm, or a range between any two of the above.

[0054] In the present embodiment, by controlling the mass ratio of the coating layer to the positive active material and the thickness of the coating layer, not only the dissolution of metal ions in the positive active material can be prevented, but also the side reaction of the positive active material with the electrolyte can be inhibited, and the structural stability of the positive active material can be improved, and the problems of affecting the ion transmission rate due to the over-thick coating or the inability to improve the air stability of the positive active material due to the over-thin coating can be avoided.

[0055] In one embodiment, the total amount of residual alkali on the surface of the positive active material is not more than 1.0wt% of the positive active material. Exemplarily, it can be 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 0.98wt%, or a range between any two of the above.

[0056] In the embodiment, by controlling the total amount of residual alkali on the surface of the positive electrode active material to be less than or equal to 1.0 wt% of the positive electrode active material, the pulp processing performance of the positive electrode active material can be improved, thereby improving the electrochemical performance of the positive electrode active material of the sodium-ion battery. The hydrophobic coating layer on the surface can improve the storage stability of the material in air.

[0057] In one specific embodiment, the positive electrode active material comprises at least one compound represented by Formula 1, Formula 2, Formula 3, and Formula 4.

[0058] wherein, Formula 1, wherein M is a doping element.

[0059] Specifically, in Formula 1, M is a doping element, including at least one of Li, Fe, Cu, Mg, Zn, Co, Al, Zr, Ti, Te, Sr, B, Sn, Mo, Nb, Sb, 0.6≤x≤1, 0.3≤y≤0.95, 0

[0060] Specifically, when M is Fe and Cu, the expression is: Formula 2.

[0061] Specifically, when M is Fe and Ti, the expression is: Formula 3.

[0062] Specifically, when M is Cu and Ti, the expression is: Formula 4.

[0063] Specifically, in Formula 2, Formula 3, and Formula 4, 0.6≤x≤1, 0.3≤a≤0.9, 0

[0064] In one specific embodiment, the hydrophobic layer 2 comprises a carboxylic acid metal salt. By using a carboxylic acid metal salt as the hydrophobic layer 2, not only does it have good hydrophobic properties, but it can also avoid direct contact between water, oxygen, carbon dioxide and other substances in the environment and the positive electrode active material matrix 1, and it also helps to improve the air stability of the positive electrode active material.

[0065] In one specific embodiment, the cross-linking layer 3 comprises at least one of an organic silicon compound, an organic titanium compound, and an organic aluminum compound.

[0066] Specifically, the organic silicon compound comprises a siloxane compound.

[0067] Specifically, the organic titanium compound comprises a titanate compound.

[0068] Specifically, the organic aluminum compound includes an aluminate compound.

[0069] In a specific embodiment, the cross-linking layer 3 includes at least one of a siloxane compound, a titanate compound, and an aluminate compound; by using the above-mentioned material capable of forming a three-dimensional network structure as the cross-linking layer 3, the structural strength and stability of the coating layer can be improved, and the interface bonding capacity between different layers of materials can also be enhanced.

[0070] In a specific embodiment, the ion-conducting layer 4 includes a sodium salt compound.

[0071] Specifically, the sodium salt compound includes at least one of 、 、 、 、 By using 、 、 、 、 as the sodium fast ion conductor, it has the advantages of high ionic conductivity, wide electrochemical window, structural stability, environmental friendliness, etc.

[0072] Further, 、 、 、 、 are nanoparticles with a particle size of 20-100 nm; preferably, the particle size is 40-60 nm. Exemplarily, it can be 40 nm, 50 nm, 60 nm, or a range between any two of the above. By using the sodium fast ion conductor with the above-mentioned particle size, the best balance between dispersibility and functionality of the sodium fast ion conductor can be achieved.

[0073] In a second aspect, the embodiments of the present application provide a preparation method of a positive electrode active material, including the following steps:

[0074] After the mixture including the carboxylate and the positive electrode active material substrate 1 is subjected to a saponification reaction, a coupling agent is added to undergo a cross-linking reaction, and then an ion conductor is added to the system to obtain a positive electrode active material.

[0075] In the present embodiment, the residual alkali on the surface of the positive electrode active material substrate 1 is directly converted into a functional hydrophobic carboxylate layer in situ by saponification reaction of the carboxylate, realizing chemical conversion instead of physical coverage. The surface energy of this hydrophobic layer 2 is extremely low, the hydrophobic performance is excellent, the air stability is high, and the water can be effectively blocked, reducing the probability of reaction between sodium carbonate, sodium hydroxide and other substances in the positive electrode active material substrate 1 and water, and improving the air stability of the positive electrode active material.

[0076] Then, the coupling agent is used for self-condensation reaction to construct the three-dimensional network cross-linked layer 3, the network is interpenetrated in the gap of the hydrophobic layer 2, and is chemically bonded with the surface of the positive electrode active material matrix 1, which can improve the mechanical strength and interface bonding capacity of the coating; then, the ion-conducting coating and the partial adsorption and embedding of the cross-linked layer 3 are performed, so as to establish a low-barrier transmission channel, realize the function integration of environmental isolation-structure reinforcement-ion superconducting, and further improve the electrochemical performance and material storage stability of the positive electrode active material. Through the formation of the three-layer coating, the comprehensive performance of the positive electrode active material can be synergistically improved.

[0077] Moreover, the method is simple, low in energy consumption, and uses non-toxic solvents, and is suitable for industrial production.

[0078] In one specific embodiment, the preparation method of the positive electrode active material specifically comprises:

[0079] The carboxylate is subjected to saponification reaction with the alkali on the surface of the positive electrode active material matrix 1 to generate the hydrophobic layer 2 on the surface of the positive electrode active material matrix 1;

[0080] The coupling agent is added to the positive electrode active material matrix 1 in which the hydrophobic layer 2 is generated, and self-condensation reaction is performed to generate the cross-linked layer 3 on the hydrophobic layer 2, the cross-linked layer 3 is at least partially interpenetrated in the hydrophobic layer 2, and forms a covalent bond with the surface of the positive electrode active material matrix 1;

[0081] The sodium fast ionic conductor nanomaterial is introduced on the cross-linked layer 3 to form the ion-conducting layer 4, and part of the ion-conducting layer 4 is embedded in the cross-linked layer 3.

[0082] In this embodiment, the carboxylate is subjected to saponification reaction with the alkali on the surface of the positive electrode active material matrix 1 to generate the hydrophobic layer 2 on the surface of the positive electrode active material matrix 1; The hydrophobic layer 2 is combined with the surface of the positive electrode active material matrix 1 through the bond, and forms a coating, which can improve the pulping processing performance of the material. The coupling agent constructs a three-dimensional network through self-condensation, the network is interpenetrated in the gap of the hydrophobic layer 2, and forms a (M=Ni, Fe, Mn, etc.) covalent bond with the metal hydroxyl on the surface of the positive electrode active material matrix 1, thereby improving the mechanical strength and interface bonding capacity of the coating. The ion-conducting nanometer particles are partially embedded in the three-dimensional network through electrostatic adsorption and form a coating, thereby improving the electrochemical performance and material storage stability of the positive electrode active material.

[0083] In one specific embodiment, the carboxylate includes a C 12 -C 18 alkyl chain fatty acid ester.

[0084] Further, the C 12 -C 18 alkyl chain fatty acid ester includes at least one of isopropyl palmitate, methyl stearate, and ethylene laurate.

[0085] In the present embodiment, by selecting C 12 -C 18 The fatty acid ester with alkyl chain of less than 12 carbon atoms has insufficient hydrophobicity; the fatty acid ester with alkyl chain of more than 18 carbon atoms has too large impedance of the hydrophobic layer 2 formed thereby, which is not conducive to ion transmission.

[0086] In one specific embodiment, the coupling agent comprises at least one of fluorosilane coupling agent, octyltrimethoxysilane, titanate, vinyltrimethoxysilane, KH550, KH570, aluminate coupling agent. For example, when the fluorosilane coupling agent is selected, the general formula of the fluorosilane coupling agent is: wherein n = 6-12, R = C1-C4 alkyl.

[0087] In the present embodiment, the coupling agent can undergo hydrolysis and self-condensation reaction, thereby constructing a three-dimensional network, which is conducive to improving the mechanical strength and mutual binding capacity of the coating structure.

[0088] In one specific embodiment, when the saponification reaction is performed, the saponification reaction is performed under stirring condition, and the reaction temperature is 40-60°C. Illustratively, the temperature can be 40°C, 50°C, 60°C, or a range between any two of the above. When the saponification reaction is performed, the stirring speed can be 400 r / min, 500 r / min, 600 r / min, or a range between any two of the above. In the present embodiment, the specific stirring speed is not limited, and a person skilled in the art can adjust it according to the actual situation. The stirring time of the saponification reaction can be 1 h, 2 h, 3 h, or a range between any two of the above. In the present embodiment, the stirring time of the saponification reaction is not limited, and a person skilled in the art can adjust it according to the actual situation.

[0089] In the present embodiment, by performing the saponification reaction under the condition of low temperature and with stirring, the hydrophobic layer 2 formed by the saponification reaction can be as evenly coated as possible on the surface of the positive active material substrate 1, so that the production cost is reduced.

[0090] In one specific embodiment, when the cross-linking reaction is performed, the reaction temperature is 40-80℃, the stirring speed is 800-1200 rpm, and the stirring time is 1-3 h. Illustratively, the reaction temperature can be 40℃, 50℃, 60℃, 70℃, 80℃, or a range between any two of the above. The stirring speed can be 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, or a range between any two of the above. The stirring time can be 1 h, 2 h, 3 h, or a range between any two of the above.

[0091] Further, when the cross-linking reaction is performed, the reaction temperature is 60℃, the stirring speed is 1000 rpm, and the stirring time is 2 h.

[0092] In the present embodiment, by performing the cross-linking reaction under the condition of a lower temperature with stirring, not only is the energy consumption low, but also there is no need to add a chemical dispersant, which is conducive to the cross-linking layer 3 formed by the cross-linking reaction being coated on the hydrophobic layer 2, and is conducive to the cross-linking layer 3 being inserted into the gap between the hydrophobic layer 2 and then being bonded to the surface of the positive active material matrix 1.

[0093] In one specific embodiment, when the ion-conducting material is added, the ion-conducting material is added step by step, and at the same time, ultrasonic and stirring treatments are performed, at 40-80℃, for 40-60 min. Illustratively, the temperature can be 40℃, 50℃, 60℃, 70℃, 80℃, or a range between any two of the above. The treatment time can be 40 min, 50 min, 60 min, or a range between any two of the above.

[0094] Further, the ultrasonic power is 100-600 W, and the stirring speed is 800-1500 rpm. Illustratively, the ultrasonic power can be 100 W, 200 W, 300 W, 400 W, 500 W, 600 W, or a range between any two of the above. The stirring speed can be 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, 1300 rpm, 1400 rpm, 1500 rpm, or a range between any two of the above.

[0095] Further, when the ion-conducting material is added, the ion-conducting material is added step by step, and at the same time, ultrasonic and stirring treatments are performed, at 60℃, for 60 min.

[0096] In the present embodiment, by adding the ion-conducting material, i.e., the sodium fast ion conductor nanomaterial, step by step, and at the same time, continuously performing ultrasonic and stirring at a lower temperature, it is conducive to the sodium fast ion conductor nanomaterial being uniformly embedded in the cross-linking layer 3, and to the ion-conducting layer 4 formed being uniformly distributed on the cross-linking layer 3.

[0097] In the embodiment, during the preparation of the positive electrode active material, the coating is completed synchronously while the residual alkali content is reduced, the process is more efficient, and is performed at a lower temperature, thereby reducing energy consumption, and the process can be completed using only a conventional solvent, and the equipment is directly compatible with the existing battery production line, thereby greatly reducing the production cost.

[0098] In one specific embodiment, the ion-conducting material further comprises preheating the ion-conducting material at 100-160°C before being added. For example, the temperature can be 100°C, 120°C, 150°C, 160°C, or a range between any two of the above.

[0099] Further, the ion-conducting material is preheated at 150°C.

[0100] In the embodiment, the sodium fast ion conductor nanomaterial is preheated to remove moisture that can be adsorbed in the sodium fast ion conductor nanomaterial, thereby avoiding the negative effects of moisture on ion conduction performance, structural stability, and overall battery performance.

[0101] In one specific embodiment, the positive electrode active material substrate 1 is preheated before the saponification reaction, and the preheating comprises vacuum drying the positive electrode active material substrate 1 at 40-80°C for 8-15h. For example, the temperature can be 40°C, 50°C, 60°C, 70°C, 80°C, or a range between any two of the above. The vacuum drying time can be 8h, 10h, 12h, 15h, or a range between any two of the above.

[0102] Further, the preheating comprises vacuum drying the positive electrode active material substrate 1 at 60°C for 12h.

[0103] In the embodiment, the positive electrode active material substrate 1 is preheated to remove moisture, thereby improving the stability of the slurry.

[0104] In a third aspect, the embodiments of the present application provide a positive electrode sheet comprising the positive electrode active material described above, thereby helping to improve the electrical performance of the battery.

[0105] Specifically, the positive electrode sheet specifically comprises a positive electrode current collector and a positive electrode active layer formed of the positive electrode active material arranged on the surface of the positive electrode current collector.

[0106] Specifically, in the preparation of the positive electrode sheet, for example, the positive electrode active material of the present application can be dispersed with a conductive agent and a binder in an appropriate amount of N-methyl pyrrolidone (NMP) solvent, and the mixture is thoroughly stirred to form a uniform positive electrode slurry; the positive electrode slurry is uniformly coated on the positive electrode current collector, and then dried, rolled, and cut to obtain the positive electrode sheet.

[0107] In one specific embodiment, the positive electrode active layer comprises 70-99 wt% of the positive electrode active material, 0.5-15 wt% of the conductive agent, and 0.5-15 wt% of the binder in terms of mass percentage, and further comprises 80-98 wt% of the positive electrode active material, 1-10 wt% of the conductive agent, and 1-10 wt% of the binder.

[0108] The material of the positive electrode current collector can be at least one of an aluminum foil and a nickel foil; the conductive agent can be at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber; and the binder can be at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, an oxirane-containing polymer, polyvinylpyrrolidone, and polyurethane.

[0109] In a fourth aspect, the embodiments of the present application provide a sodium ion battery comprising the positive electrode sheet described above, which helps to improve the safety performance and electrochemical performance of the battery.

[0110] In the present application, unless otherwise specified, the coating, drying, rolling and other processes involved are conventional operations in the art, and the equipment used can be conventional equipment in the art, which is not particularly limited.

[0111] Generally, the battery comprises an electrolyte, a cell, and a shell encapsulating the cell, the electrolyte is injected into the cell in the shell, and the cell comprises a positive electrode sheet, a negative electrode sheet, and a separator between the positive electrode sheet and the negative electrode sheet. The cell can be a laminated cell, i.e., the cell is formed by interleaving and stacking the positive electrode sheet, the separator, and the negative electrode sheet; or the cell can also be a wound cell, i.e., the cell is formed by stacking and winding the positive electrode sheet, the separator, and the negative electrode sheet.

[0112] Specifically, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer on at least one side surface of the negative electrode current collector. Specifically, the negative electrode active layer can be arranged on one side surface of the negative electrode current collector, or the negative electrode active layer can be arranged on both side surfaces of the negative electrode current collector in the thickness direction.

[0113] Specifically, the negative active layer can include a negative active material, a conductive agent, and a binder, which can all be conventional materials in the art. For example, the negative active material can include one or more of carbon-based materials (hard carbon, soft carbon), titanium-based oxides (such as Na2Ti3O7, TiO2), alloy materials (such as Sn, P), organic compounds (such as polyimide), and metal sulfide / oxide based on conversion reaction (such as FeS2, MoS2), the conductive agent can include one or more of conductive carbon black, carbon nanotubes (CNT), acetylene black, graphene, ketjen black, carbon fiber, and the binder can include one or more of sodium carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, oxirane-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate.

[0114] The embodiments of the present application can employ a conventional negative current collector in the art, for example, the negative current collector includes an aluminum foil.

[0115] In the embodiments of the present application, the negative electrode sheet can be prepared by a conventional method in the art, for example, by a coating method. Specifically, the negative active material, the conductive agent, the binder, and the like, which are components for forming the negative active layer, can be dispersed in a solvent, for example, including water, to prepare a negative electrode slurry, which is then coated on the surface of the negative current collector, and after drying, rolling, and the like, the negative electrode sheet is prepared. The coating, drying, rolling, and the like, which are involved, are conventional operations for preparing the negative electrode sheet by the coating method, and are not particularly limited.

[0116] The electrolyte of the embodiments of the present application can be a conventional electrolyte in the art. For example, the electrolyte is a non-aqueous electrolyte, which can specifically include an organic solvent, an additive, and an electrolyte salt. The organic solvent can include one or more of ethylene carbonate (EC), diethyl carbonate (DEC), and propylene carbonate (PC), for example. The additive can include fluoroethylene carbonate (FEC), for example. The electrolyte salt can include a sodium salt, which can include NaClO4, NaPF6, NaBF4, NaAsF6, NaSCN, NaAlCl4, and the like, for example, but is not limited to the foregoing. 、 、 、 and the like, but is not limited to the foregoing.

[0117] In the embodiments of the present application, the diaphragm is used to separate the positive electrode sheet and the negative electrode sheet to avoid short circuit caused by contact between the positive electrode sheet and the negative electrode sheet. The diaphragm used in the embodiments of the present application can be a conventional diaphragm in the art, and no particular limitation is made thereto. For example, the diaphragm can be one of a glass fiber diaphragm, a polypropylene diaphragm (PP), a polyethylene diaphragm (PE), a polypropylene / polyethylene double-layer composite diaphragm (PP / PE), a polyimide electrospinning diaphragm (PI), a polypropylene / polyethylene / polypropylene three-layer composite diaphragm (PP / PE / PP), a cellulose non-woven diaphragm, and a diaphragm with a ceramic coating.

[0118] In the embodiments of the present application, a conventional shell material in the art can be used to package the battery cell. The shell can include a soft packaging material such as an aluminum plastic film, but is not limited thereto.

[0119] The battery can be prepared by a conventional method in the art. For example, during preparation of the battery, the positive electrode sheet, the diaphragm, and the negative electrode sheet are wound or laminated to obtain a bare battery cell, and the bare battery cell is packaged in an aluminum plastic film bag that is pre-punched and formed. After the packaged battery is dried at 85°C to remove moisture, electrolyte is injected into the dried battery. After the battery is left to stand, formed, and sealed for the second time, the preparation of the battery is completed.

[0120] In the following, the positive electrode active material of the present application is described in detail through specific embodiments.

[0121] Embodiment 1

[0122] The preparation of the positive electrode active material of the present embodiment includes the following steps:

[0123] Ion-conducting Preheating and removing water at 150°C.

[0124] 1) 10g The positive electrode active material matrix is dispersed in a 100mL ethanol solution containing 0.3g isopropyl palmitate, and a saponification reaction is carried out at 60°C with stirring at 500r / min for 1h to obtain a hydrophobic layer A.

[0125] 2) Then, 0.08g perfluorooctyltriethoxysilane is added, and stirring is carried out at 60°C with a stirring speed of 1000rpm for 2h to complete the self-condensation reaction, thereby obtaining a cross-linked layer B.

[0126] 3) 0.1g Nanoparticles are added in three portions, and at the same time, ultrasonic and mechanical stirring are applied, wherein the ultrasonic power is 200w, the mechanical stirring speed is 1000rpm, and the reaction is continuously carried out at 60°C for 1h. After the reaction is completed, an ion-conducting layer C is obtained. Then, centrifugal separation is carried out, and the unreacted substances are removed by washing with ethanol for three times. Then, vacuum drying is carried out at 60°C for 12h, and sieving is carried out to obtain the coated positive electrode active material sample.

[0127] The positive electrode active material matrix is an unmodified layered transition metal oxide material with a general chemical formula of wherein M is preferably Fe, x = 1, y = 1 / 3, and z = 1 / 3, and the specific chemical formula is .

[0128] The positive electrode active material matrix is prepared by the following method:

[0129] The nickel-manganese-iron precursor and sodium salt are mixed uniformly according to the stoichiometric ratio (5% excess sodium), heated to 950°C at a heating rate of 2°C / min, calcined at 950°C for 20 h, and then naturally cooled to room temperature to obtain NaNi 1 / 3 Mn 1 / 3Fe 1 / 3 O2positive electrode active material matrix. The positive electrode active material matrix is vacuum dried at 60°C for 12 h.

[0130] Example 2

[0131] The preparation method of this example is basically the same as that of Example 1, except that in step 1), vinyl laurate is used to replace isopropyl palmitate; and the saponification reaction is carried out at 40°C with stirring at 400 r / min for 2 h to obtain the hydrophobic layer A.

[0132] Example 3

[0133] The preparation method of this example is basically the same as that of Example 1, except that in step 1), methyl stearate is used to replace isopropyl palmitate; and the saponification reaction is carried out at 50°C with stirring at 300 r / min for 3 h to obtain the hydrophobic layer A.

[0134] Example 4

[0135] The preparation method of this example is basically the same as that of Example 1, except that in step 2), octyltrimethoxysilane is used to replace perfluorooctyltriethoxysilane; and the self-condensation reaction is completed by stirring at 40°C at a stirring speed of 1200 rpm for 3 h to obtain the crosslinked layer B.

[0136] Example 5

[0137] The preparation method of this example is basically the same as that of Example 1, except that in step 1), vinyl laurate is used to replace isopropyl palmitate;

[0138] In step 2), octyltrimethoxysilane is used to replace perfluorooctyltriethoxysilane; and the self-condensation reaction is completed by stirring at 80°C at a stirring speed of 800 rpm for 1 h to obtain the crosslinked layer B.

[0139] Example 6

[0140] The preparation method of this embodiment is basically the same as that of Example 1, except that in step 1), methyl stearate is used instead of isopropyl palmitate.

[0141] In step 2), octyltrimethoxysilane is used to replace perfluorooctyltriethoxysilane.

[0142] Example 7

[0143] The preparation method in this embodiment is basically the same as that in Example 1, except that in step 3), a different method is used. Nanoparticle replacement Nanoparticles were prepared by ultrasonic power of 100W, mechanical stirring speed of 1500rpm, and continuous reaction at 80℃ for 40min. After the reaction was completed, ion-conducting layer C was obtained.

[0144] Example 8

[0145] The preparation method of this embodiment is basically the same as that of Example 1, except that in step 1), vinyl laurate is used instead of isopropyl palmitate.

[0146] In step 3), the following is adopted: Nanoparticle replacement Nanoparticles were prepared by ultrasonic power of 600 W, mechanical stirring speed of 800 rpm, and continuous reaction at 40 °C for 50 min. After the reaction was completed, ion-conducting layer C was obtained.

[0147] Example 9

[0148] The preparation method of this embodiment is basically the same as that of Example 1, except that in step 1), methyl stearate is used instead of isopropyl palmitate.

[0149] In step 3), the following is adopted: Nanoparticle replacement Nanoparticles.

[0150] Example 10

[0151] The preparation method of this embodiment is basically the same as that of Example 1, except that in step 1), the amount of isopropyl palmitate used is 0.1g.

[0152] Example 11

[0153] The preparation method of this embodiment is basically the same as that of Example 1, except that in step 1), the amount of isopropyl palmitate used is 0.323g.

[0154] Example 12

[0155] The preparation method of this example is basically the same as that of Example 1, except that in step 2), the amount of perfluorooctyltriethoxysilane is 0.06 g.

[0156] Example 13

[0157] The preparation method of this example is basically the same as that of Example 1, except that in step 2), the amount of perfluorooctyltriethoxysilane is 0.12 g.

[0158] Example 14

[0159] The preparation method of this example is basically the same as that of Example 1, except that in step 3), The amount of nanoparticles is 0.067 g.

[0160] Example 15

[0161] The preparation method of this example is basically the same as that of Example 1, except that in step 3), The amount of nanoparticles is 0.13 g.

[0162] Example 16

[0163] The preparation method of this example is basically the same as that of Example 1, except that in step 1), the positive electrode active material matrix is .

[0164] Example 17

[0165] The preparation method of this example is basically the same as that of Example 1, except that in step 1), the positive electrode active material matrix is .

[0166] Example 18

[0167] The preparation method of this example is basically the same as that of Example 1, except that in step 1), the positive electrode active material matrix is .

[0168] Example 19

[0169] The preparation method of this example is basically the same as that of Example 1, except that in step 2), perfluorooctyltriethoxysilane is replaced by a titanate.

[0170] Comparative Example 1

[0171] The positive electrode active material of this comparative example is the positive electrode active material matrix in Example 1.

[0172] 10 g of positive electrode active material was directly vacuum dried at 60°C for 12 h without any coating treatment, and a positive electrode active material sample was obtained.

[0173] Comparative Example 2

[0174] The positive electrode active material of this comparative example is the positive electrode active material matrix of Example 1.

[0175] 10 g of the positive electrode active material was stirred with 0.3 g of isopropyl palmitate in ethanol at 60°C for 3 h, washed by centrifugation and dried to obtain a sample of the positive electrode active material.

[0176] Comparative Example 3

[0177] The positive electrode active material of this comparative example is the positive electrode active material matrix of Example 1.

[0178] 10 g of the positive electrode active material was stirred with 0.08 g of perfluorooctyltriethoxysilane in ethanol at 60°C for 3 h, washed by centrifugation and dried to obtain a sample of the positive electrode active material.

[0179] Comparative Example 4

[0180] The positive electrode active material of this comparative example is the positive electrode active material matrix of Example 1.

[0181] 10 g of the positive electrode active material was stirred with 0.1 g of isopropyl palmitate in ethanol at 60°C for 3 h, washed by centrifugation and dried to obtain a sample of the positive electrode active material.

[0182] Comparative Example 5

[0183] The positive electrode active material of this comparative example is the positive electrode active material matrix of Example 1.

[0184] 10 g of the positive electrode active material was stirred with 0.08 g of perfluorooctyltriethoxysilane in ethanol at 60°C for 2 h to complete the self-condensation reaction.

[0185] 0.1 g of perfluorooctyltriethoxysilane was added in three portions nanoparticles, with simultaneous application of ultrasound and mechanical stirring, and the reaction was continued at 60°C for 1 h. After the reaction was complete, the unreacted material was removed by washing with ethanol three times by centrifugation, and the coated positive electrode active material sample was obtained by vacuum drying at 60°C for 12 h and sieving.

[0186] Comparative Example 6

[0187] The positive electrode active material of this comparative example is the positive electrode active material matrix of Example 1.

[0188] 10 g of the positive electrode active material was dispersed in 100 mL of an ethanol solution containing 0.3 g of isopropyl palmitate and stirred at 60°C for 1 h to perform the saponification reaction.

[0189] 0.1 g of perfluorooctyltriethoxysilane was added in three portions The nanoparticles were reacted for 1 h at 60°C with simultaneous application of ultrasonic and mechanical stirring. After the reaction was completed, centrifugal separation was performed, and the unreacted substances were removed by washing with ethanol three times. Subsequently, vacuum drying was performed at 60°C for 12 h, and sieving was performed to obtain the coated positive electrode active material sample.

[0190] Comparative Example 7

[0191] The positive electrode active material of the present comparative example was the positive electrode active material matrix of Example 1.

[0192] The saponification reaction was performed by dispersing 10 g of the positive electrode active material in 100 mL of an ethanol solution containing 0.3 g of isopropyl palmitate and stirring at 60°C for 1 h. Subsequently, 0.08 g of perfluorooctyltriethoxysilane was added, and stirring was maintained at 60°C for 2 h to complete the self-condensation reaction. After the reaction was completed, centrifugal separation was performed, and the unreacted substances were removed by washing with ethanol three times. Subsequently, vacuum drying was performed at 60°C for 12 h, and sieving was performed to obtain the coated positive electrode active material sample.

[0193] Test Example

[0194] The sample materials obtained in Examples 1 to 19 and Comparative Examples 1 to 7 were subjected to the following tests.

[0195] 1. The residual alkali content was tested by potentiometric titration. 1.0 g of the sample material was dispersed in 50 mL of de-carbonated water, and after stirring magnetically for 30 min, filtration was performed. The filtrate was titrated with a 0.1 mol / L hydrochloric acid standard solution, and the first end point of 8.3 and the second end point of 4.4 were monitored with a pH meter. According to the volume of the consumed hydrochloric acid, the residual alkali content and the residual lithium content were calculated, respectively. and the residual lithium content.

[0196] 2. The mass ratio of the coating layer of the sample material was tested by thermogravimetric analysis (TGA) combined with a blank control method. First, the TGA curve of the uncoated positive electrode material was tested as a benchmark (no significant weight loss within 800°C). Then, the TGA curve of the coated sample material was tested. According to the characteristic decomposition temperature, the layers were distinguished: the hydrophobic layer (carboxylate salt) decomposed and lost weight at 300-400°C, the crosslinked layer (fluorosilicone) decomposed and lost weight at 400-500°C, and the ion-conducting layer was stable without weight loss within 800°C.

[0197] 3. Fourier transform infrared spectroscopy (FTIR) was used for characterization. After the sample of Example 1 was mixed with potassium bromide (KBr) and pressed into a tablet, the Fourier transform infrared spectrometer was used to scan in the range of 400-4000 cm⁻¹ for testing, and the results were compared with those of the uncoated NaNi 1 / 3 Mn 1 / 3 Fe 1 / 3 O2matrix material.

[0198] 4. X-ray photoelectron spectroscopy (XPS) was used for analysis. The test was conducted under ultra-high vacuum conditions using monochromatic Al Kα rays as the excitation source, and a charge neutralization system was used to compensate for the charging effect on the sample surface caused by photoelectron emission. All spectra were corrected based on the contaminated carbon C 1s peak at 284.8 eV. Compared with the uncoated matrix, the full surface spectrum of the coated sample (Example 1) clearly shows key elements such as C, F, Si, Zr, and P. Figure 5 As shown, through detailed analysis of the high-resolution narrow spectrum, direct evidence can be provided for the successful construction of the three-layer structure: First, the characteristic peak at approximately 289.0 eV in the C 1s spectrum can be clearly attributed to the carboxylate group in the hydrophobic layer A. The presence of palmitate was confirmed by two factors: firstly, the peak around 103 eV in the Si 2p spectrum corresponds to the Si-O bond formed by silane hydrolysis condensation; secondly, the strong peak at approximately 688 eV in the F 1s spectrum clearly indicates the presence of the perfluoroalkyl chain. Finally, the presence of the ion-conducting layer C was confirmed by the characteristic peaks in the Zr 3d spectrum (approximately 182 eV) and the P 2p spectrum (approximately 133.5 eV), and its binding energy was consistent with... The (NZSP) standard is consistent, indicating that the positive electrode active material in Example 1 is coated with a three-layer structure.

[0199] 5. Perform XRD, scanning electron microscopy, and transmission electron microscopy on the positive electrode active material samples, including:

[0200] The sample materials prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to XRD analysis, and the test results are as follows: Figure 2 As shown. From Figure 2 As can be seen from the data, the sample materials in Examples 1-3 and Comparative Examples 1-2 all exhibit an O3-type layered oxide structure, indicating that the coating treatment does not affect the bulk structure of the material.

[0201] The sample materials prepared in Example 1 and Comparative Example 1 were examined by scanning electron microscopy, and the test results are as follows: Figure 3 As shown. From Figure 3 As can be seen from the SEM images, Comparative Example 1 has a rough surface with many obvious small particles, while Example 1 has a smooth surface with only a few particles attached, indicating that the coating treatment is uniform and helps to reduce residual alkali on the material surface.

[0202] The sample material prepared in Example 1 was examined by transmission electron microscopy, and the test results are as follows: Figure 4 As shown. From Figure 4 As can be seen in TEM, a thin amorphous layer, namely the coating layer, can be observed on the surface of the sample material. The thickness of the coating layer is about 12 nm.

[0203] 6. Air stability test was performed on the positive active material sample, which was placed in a constant temperature and humidity chamber at 60% humidity for 24 hours. After removal, the first circle capacity was tested by assembling a half-cell, and the retention rate was calculated by comparing the capacity of the positive active material sample with the same batch that was not treated by humidity.

[0204] 7. The battery assembly steps were as follows: 0.8 g of the positive electrode material prepared in Examples 1-19 and Comparative Examples 1-7 was weighed, 0.1 g of Super P was added as a conductive agent, and 0.1 g of PVDF (polyvinylidene fluoride) was added as a binder. After mixing uniformly, the positive electrode sheet was prepared by coating on an aluminum foil and drying. In an argon atmosphere glove box, a metal sodium sheet was used as the negative electrode, glass fiber (Whatman, GF / F, Ф = 90 mm) was used as the separator, 1 mol / L LiPF6+PC / EC (1:1) + 5% FEC was used as the electrolyte, and CR-2032 type button cells were assembled. The prepared slurry was tested for storage stability, and the viscosity was tested every hour using a rotational rheometer at 25°C. The change rate of viscosity was recorded, and the failure point was taken as the change rate exceeding 50%.

[0205] 8. Electrochemical performance tests were performed in the range of 2-4.2V, and the capacity retention rate after 100 cycles at 1C current rate was determined. After 100 cycles, the electrochemical workstation (VERSASTAT4) was used for AC impedance (EIS) test, the frequency range was 10 mHz-10 kHz, the perturbation voltage was 10 mV, and the increase of charge transfer resistance Rct was calculated.

[0206] The test results are shown in Tables 1, 2, 3 and 4, wherein the test items in Table 2 are the same as those in Table 1. The test items in Table 4 and Table 3 are the same.

[0207] Table 1

[0208]

[0209] Table 2

[0210]

[0211] ​In Table 1-2, the chemical composition of the positive electrode material and the composition of the hydrophobic layer, the composition of the cross-linking layer, and the composition of the ion-conducting layer in Examples 1-19 and Comparative Examples 1-7 are counted, and the entire thickness of the coating layer of the positive electrode material and the mass ratio of the hydrophobic layer, the cross-linking layer, and the ion-conducting layer are counted, which shows that the positive electrode active material in the present application is coated with a three-layer structure, and the hydrophobic layer, the cross-linking layer, and the ion-conducting layer have different thicknesses under different mass ratios, which can construct a composite coating layer of "hydrophobic-strengthening-ion-conducting", and the coating layer does not block ion transmission, which is beneficial to synergistically improving the comprehensive performance of the positive electrode active material.

[0212] In Examples 2-3, when the carboxylic acid ester is changed, the nanoparticles can still construct a three-layer coating structure, and the temperature, stirring speed, and stirring time of the saponification reaction do not affect the construction of the three-layer coating structure. Compared with Example 1, changing the type of carboxylic acid ester and the temperature, stirring speed, and stirring time of the saponification reaction has a synergistic effect on the 1C / 100 cycle capacity retention rate, slurry storage stability, air stability, and charge transfer resistance growth.

[0213] In Examples 4-5, when the coupling agent is changed, the nanoparticles can still construct a three-layer coating structure, and the temperature, stirring speed, and stirring time of the self-condensation reaction do not affect the construction of the three-layer coating structure. Compared with Example 1, changing the type of coupling agent and the temperature, stirring speed, and stirring time of the self-condensation reaction has a synergistic effect on the 1C / 100 cycle capacity retention rate, slurry storage stability, air stability, and charge transfer resistance growth.

[0214] In Examples 7-8, when the ion-conducting agent is changed, the nanoparticles can still construct a three-layer coating structure, and the ultrasonic power, temperature, stirring speed, and stirring time of the reaction do not affect the construction of the three-layer coating structure. Compared with Example 1, changing the type of ion-conducting agent and the ultrasonic power, reaction temperature, stirring speed, and stirring time has a synergistic effect on the 1C / 100 cycle capacity retention rate, slurry storage stability, air stability, and charge transfer resistance growth.

[0215] Table 3

[0216]

[0217] Table 4

[0218]

[0219] ​​As can be seen from the data in Tables 3 and 4, in terms of residual alkali removal, the residual alkali content of the three-layer coating structure was successfully reduced to <1 wt%, which is much lower than that of the control group (e.g., 3.2 wt% of Comparative Example 1, 1.5 wt% of Comparative Example 2, 2.3 wt% of Comparative Example 3, and 3.0 wt% of Comparative Example 4).

[0220] In Example 10, when the amount of isopropyl palmitate was reduced, a three-layer coating structure could still be constructed with the crosslinking layer and the ion-conducting layer. However, the mass ratio of the hydrophobic layer, the crosslinking layer, and the ion-conducting layer did not fall within the range of (60~70):(15~25):(10~20), resulting in a decrease in the 1C / 100 cycle capacity retention rate and a decrease in the slurry storage stability, while the charge transfer resistance increased by ΔR. ct It has increased somewhat.

[0221] In Example 13, when the amount of perfluorooctyltriethoxysilane was increased, a three-layer coating structure could still be constructed in conjunction with the crosslinking layer and the ion-conducting layer. However, the mass ratio of the hydrophobic layer, the crosslinking layer, and the ion-conducting layer did not fall within the range of (60~70):(15~25):(10~20), and the mass proportion of the coating layer increased significantly, resulting in a decrease in the 1C / 100-cycle capacity retention rate and an increase in charge transfer resistance ΔR. ct The increase is somewhat evident. Data from Examples 10 and 13 show that when the amounts of raw materials used in the hydrophobic layer and crosslinking layer are in the range of (60~70):(15~25), they can synergistically improve the 1C / 100-cycle capacity retention of the positive electrode active material and reduce the charge transfer resistance increase ΔR. ct .

[0222] In terms of cycle stability, Example 1 achieved a 1C cycle retention rate of up to 92%, which is more than 10% higher than that of single-layer coating (Comparative Examples 2, 3, and 4) and more than 5% higher than that of double-layer coating (Comparative Examples 5, 6, and 7), fully demonstrating the necessity of the three-step collaborative design.

[0223] The three-layer coating structure is also excellent in environmental tolerance. The slurry storage time is more than 72 h, which is much better than the 9 h gel failure of Comparative Example 1, and after humidity aging, the capacity retention rate is still 98.5%, which is significantly higher than the 82.3% of Comparative Example 1 after attenuation; more importantly, this process improves the ion transport efficiency while effectively inhibiting the interface side reaction. The charge transfer resistance increment (ARct) of Example 1 after cycling is only 18%, which is much lower than the 95% increment of Comparative Example 1, indicating that the coating layer significantly improves the Rct affected by ion transport kinetics and interface side reaction, and has a blocking effect on the corrosion of the coating layer on the electrolyte. These excellent performances are due to the unique synergistic mechanism of gradient coating: in summary, through the synergistic effect of residual alkali conversion into hydrophobic shielding, coupling agent to strengthen the structural strength and ion conduction, the comprehensive performance of the positive active material is comprehensively improved.

[0224] It should be particularly pointed out that: if the specific experimental steps or conditions are not specified in the examples, the operation or conditions can be carried out according to the conventional experimental steps described in the literature in the art. If the reagents or instruments used are not specified by the manufacturer, they are all conventional reagent products that can be obtained by purchase.

[0225] Finally, it should be pointed out that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit it; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A positive electrode active material, characterized in that, The positive electrode active material includes a positive electrode active material matrix and a coating layer covering at least a portion of the surface of the positive electrode active material matrix; In the direction away from the positive electrode active material matrix, the coating layer includes a hydrophobic layer, a crosslinking layer and an ion-conducting layer stacked sequentially; The mass ratio of the hydrophobic layer, the crosslinking layer and the ion-conducting layer is (50~80):(10~30):(5~30); The positive electrode active material matrix includes the compound shown in Formula 1; Formula 1, wherein M is a doping element, including at least one of Li, Fe, Cu, Mg, Zn, Co, Al, Zr, Ti, Te, Sr, B, Sn, Mo, Nb, and Sb, 0.6≤x≤1, 0.3≤y≤0.95, and 0<z≤0.5; The hydrophobic layer comprises a metal carboxylic acid salt; The crosslinking layer includes at least one of organosilicon compounds, organotitanium compounds, and organoaluminum compounds; The ion-conducting layer comprises sodium salt compounds.

2. The positive electrode active material according to claim 1, characterized in that, Part of the crosslinked layer is embedded in the hydrophobic layer, and part of the ion-conducting layer is embedded in the crosslinked layer. The crosslinked layer is at least partially connected to the positive electrode active material matrix by bonds.

3. The positive electrode active material according to claim 1, characterized in that, The mass of the coating layer is 3-8 wt% of the mass of the positive electrode active material; and / or, The thickness of the coating layer is 8~20nm.

4. The positive electrode active material according to claim 1, characterized in that, The total amount of residual alkali on the surface of the positive electrode active material shall not exceed 1.0 wt% of the positive electrode active material.

5. The positive electrode active material according to any one of claims 1-4, characterized in that, The organosilicon compound includes siloxane compounds; the organotitanium compound includes titanate compounds; the organoaluminum compound includes aluminate compounds; and / or, The sodium salt compound includes , , , , At least one of them.

6. A method for preparing a positive electrode active material as described in any one of claims 1-5, characterized in that, Includes the following steps: After saponification of a mixture comprising a carboxylic acid ester and a positive electrode active material matrix, a coupling agent is added to induce a crosslinking reaction. Subsequently, a conductive ion is added to the system to obtain the positive electrode active material.

7. The method for preparing the positive electrode active material according to claim 6, characterized in that, The carboxylic acid ester includes C 12 -C 18 Alkyl chain fatty acid esters; and / or, The coupling agent includes at least one of fluorosilane coupling agents, octyltrimethoxysilane, titanate, vinyltrimethoxysilane, KH550, KH570, and aluminate coupling agents.

8. The method for preparing the positive electrode active material according to claim 6 or 7, characterized in that, The saponification reaction is carried out under stirring conditions at a temperature of 40-60°C; and / or, During the crosslinking reaction, the reaction temperature is 40-80℃, the stirring speed is 800-1200 rpm, and the stirring time is 1-3 hours; and / or, When adding the conductive ions, the ions are added in steps, while ultrasonic and stirring treatments are performed simultaneously at 40-80°C for 40-60 minutes.

9. The method for preparing the positive electrode active material according to claim 6 or 7, characterized in that, Before the ions are added, the process further includes preheating the ions at 100-160°C; and / or, Before carrying out the saponification reaction, the positive electrode active material matrix is ​​pretreated. The pretreatment includes vacuum drying the positive electrode active material matrix at 40-80°C for 8-15 hours.

10. A positive electrode plate, characterized in that, The positive electrode active material includes the positive electrode active material according to any one of claims 1-5 or the positive electrode active material according to any one of claims 6-9.

11. A sodium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 10.

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