Positive electrode material, preparation method thereof, positive electrode sheet and secondary battery

Abstract

A cathode material, a preparation method thereof, a cathode electrode sheet and a secondary battery. The cathode material includes a core and a coating layer. The core is composed of a lithium-rich material; the coating layer is coated on the outer surface of the core, and the coating layer includes a carbon material and a lithium organic acid salt, and the lithium organic acid salt is in-situ reacted by residual alkali contained in the core and an organic acid. In this application, by reacting the organic acid with the surface residual alkali of the lithium-rich material, the surface residual alkali is converted into a lithium organic acid salt, thereby effectively reducing the content of the surface residual alkali, and thus effectively reducing the gas swelling phenomenon during the process of using the cathode material in a lithium battery, and improving the electrochemical performance and processing performance of the cathode material; and the remaining organic acid can also form a carbon material after sintering, which is used as an isolation encapsulation layer on the surface of the lithium-rich material core, thereby reducing the influence of the external environment on the lithium-rich material core and enhancing the electronic conductivity of the cathode material.

Description

Technical Field

[0001] The present application relates to the technical field of secondary batteries, and particularly relates to a positive electrode material, a preparation method thereof, a positive electrode sheet, and a secondary battery. Background Art

[0002] During the preparation of the lithium-rich positive electrode material, it is inevitable that alkaline impurities such as lithium carbonate remain on the surface of the material. However, when the residual alkali content on the surface of the positive electrode material is too high, it will bring many negative impacts on the electrochemical performance of the assembled secondary battery. The influence of the surface alkaline compound on the electrochemical performance is mainly reflected in increasing the irreversible capacity loss and deteriorating the cycle performance at the same time. In addition, the lithium carbonate on the surface decomposes to generate gas at high voltage, which is also one of the main reasons for the swelling of the secondary battery, thus bringing potential safety hazards. Therefore, how to reduce the residual alkali content on the surface of the positive electrode material is of great significance for the practical application of the positive electrode material in power batteries.

[0003] In addition, the lithium-rich positive electrode material is extremely likely to react with moisture and CO2 in the environment during the transfer to the next process after being synthesized in an air-free atmosphere, resulting in deterioration, which restricts their use. In addition, it has also been found in practical applications that the electronic conductivity and ionic conductivity of the lithium-rich positive electrode material are generally low. Due to the deficiencies such as unstable interface and low electronic conductivity of the existing lithium-rich positive electrode materials, the storage stability and processing stability of the existing lithium-rich positive electrode materials are not ideal, resulting in an unsatisfactory lithium compensation effect. Summary of the Invention

[0004] The purpose of the present application is to provide a positive electrode material, a preparation method thereof, a positive electrode sheet, and a secondary battery.

[0005] To achieve the purpose of the present application, the following technical solutions are provided:

[0006] In a first aspect, the present application provides a cathode material, comprising a core and a coating layer. The core is composed of a lithium-rich material; the coating layer coats the outer surface of the core, and the coating layer includes a carbon material and a lithium organic acid salt, and the lithium organic acid salt is in-situ reacted and generated from the residual alkali contained in the core. The cathode material has a core-shell structure, and the core is a lithium-rich material. The lithium-rich material can be cathode material particles for assembling a lithium battery in cooperation with an anode material; the lithium-rich material can also be lithium supplement material particles, serving as a "sacrificial agent" for supplementing the lithium source, so as to ensure the first charging efficiency. The coating layer can be prepared by mixing the lithium-rich material with a precursor. The lithium organic acid salt in the coating layer can be a product generated after the reaction of the precursor with the residual alkali on the surface of the core. The precursor of the coating layer can be an organic acid. By subjecting the organic acid powder and the lithium-rich material to a high-temperature sintering reaction, on the one hand, the surface residual alkali can be reacted with the organic acid, so that the surface residual alkali is converted into a lithium organic acid salt, thereby effectively reducing the content of the surface residual alkali, avoiding the loss of the secondary battery capacity caused by the residual alkali, and effectively reducing the gas swelling phenomenon during the use of the cathode material in the secondary battery, and improving the electrochemical performance and processing performance of the cathode material; on the other hand, the remaining organic acid can form a carbon material after sintering, serving as an isolation and encapsulation layer on the surface of the lithium-rich material core, thereby reducing the influence of the external environment (water and carbon dioxide) on the lithium-rich material core and improving the electronic conductivity of the cathode material.

[0007] In a possible implementation manner, the coating layer includes a conversion layer and an encapsulation layer. The conversion layer coats the outer surface of the core, and the encapsulation layer coats the outer surface of the conversion layer. The conversion layer includes the lithium organic acid salt, and the encapsulation layer includes the carbon material. Specifically, after using the precursor to coat the lithium-rich material, the precursor in contact with the lithium-rich material will first react with the residual alkali to form a lithium organic acid salt, and the precursor farther away from the core is more difficult to react with the residual alkali. Therefore, after the reaction of the precursor with the residual alkali, a conversion layer will be pre-formed to coat the outer surface of the core, and the sintered precursor will form an encapsulation layer on the outer surface of the conversion layer. It can be understood that there is no obvious interface between the conversion layer and the encapsulation layer to distinguish the two. When the precursor is less, the thickness of the encapsulation layer should be thinner, and the existence of the encapsulation layer is less obvious; when the precursor is more, the thickness of the encapsulation layer should be thicker, and the existence of the encapsulation layer is more obvious. By using an organic acid to coat the lithium-rich material, the sintered organic acid can form a two-layer structure of a conversion layer and an encapsulation layer; after the organic acid reacts with the residual alkali substance on the surface of the lithium-rich material, the residual alkalinity can be reduced. After further increasing the sintering temperature, a carbon coating can be formed without additional carbon raw materials, reducing the production cost; at the same time, the lithium organic acid salt in the conversion layer and the carbon material in the encapsulation layer have a synergistic effect, and the double-layer coating structure can more effectively inhibit the contact with the electrolyte and reduce the amount of gas generated due to the side reaction with the electrolyte.

[0008] In a possible implementation, the conversion layer further includes the carbon material, and the carbon material content in the conversion layer is less than the carbon material content in the encapsulation layer; and / or the encapsulation layer includes the organic lithium salt, and the organic lithium salt content in the encapsulation layer is less than the organic lithium salt content in the conversion layer. Specifically, when the residual alkali content on the surface of the core is small or unevenly distributed, the precursor near the surface of the core does not react with the residual alkali, so carbon materials are formed and retained in the conversion layer after sintering. Moreover, the carbon material content in the conversion layer is less than the carbon material content in the encapsulation layer to ensure that the encapsulation layer can have a sufficient carbon material content, thereby providing a good electronic conduction environment. It can be understood that when the provided precursor content is small or the residual alkali content on the surface of the core is large, the organic lithium salt will also be formed in the encapsulation layer.

[0009] In a possible implementation, the content of the organic lithium salt in the coating layer gradually decreases in the direction away from the core. Specifically, the precursor reacts with the residual alkali on the surface of the core, and the formed organic lithium salt will be concentrated in large quantities at the interface between the core and the coating layer. Then, as the coating layer thickens, the content of the organic lithium salt gradually decreases.

[0010] In a possible implementation, the precursors of the carbon material and the organic lithium salt are solid organic acid powders, and the solid organic acid powders include one or more of oxalic acid, citric acid, tartaric acid, malic acid, ascorbic acid, acetic acid, succinic acid, salicylic acid, and caffeic acid. Specifically, the advantage of selecting an organic acid as the precursor of the carbon material and the organic lithium salt is that the organic acid can not only react with the residual alkali to form an organic lithium salt, but also the excess organic acid can form carbon materials at high temperatures, thereby eliminating the step of re-coating carbon materials to prepare the isolation encapsulation layer. In this implementation, solid organic acid powders are selected as the precursors because during the preparation of the cathode material, the solid organic acid powder and the lithium-rich material can form a uniform solid precursor mixture, and the solid organic acid powder can uniformly coat the outer surface of the lithium-rich material. After sintering, the cathode material is tightly combined and the coating layer is not easily detached. This method not only shortens the production cycle but also ensures the uniformity of the coating. Secondly, compared with liquid-phase coating, during the firing process after liquid-phase coating, the coating layer is easily damaged, resulting in coating defects; while the solid-phase coating process has less impact on the material properties, and the overall process cycle of solid-phase coating is shorter and the production efficiency is higher.

[0011] In a possible implementation, the chemical formula of the lithium-rich material in the core is Li 1+x M y O z , where 1 ≤ x ≤ 6, 0 ≤ y ≤ 5, 2 ≤ Z ≤ 12, and M includes one or more of Ni, Fe, Mn, Co, Al, Cu, Zn, and Cr.

[0012] In a possible implementation, the thickness of the coating layer is 1 nm to 100 nm. Controlling the thickness of the coating layer within the above range is beneficial for adjusting the particle size of the cathode material, and at the same time can ensure the specific capacity and electron conduction environment of the cathode material. When the thickness of the coating layer is less than the above range, the coating layer does not completely coat the core, which is not conducive to constructing a good electron conduction environment, and it is very easy to have an incomplete residual alkali reaction; when the thickness of the coating layer is greater than the above range, it will cause the particle size of the cathode material to be too large, and since the coating layer does not contribute lithium ions, it will reduce the overall specific capacity of the cathode material.

[0013] In a possible implementation, the thickness of the conversion layer is 1 nm to 50 nm. The thickness of the conversion layer can be used to illustrate the reaction content of the organic acid and the residual alkali, and to control the thickness of the encapsulation layer. By controlling the thickness of the conversion layer and cooperating with the overall thickness of the coating layer mentioned above, the thickness of the encapsulation layer can be adjusted, thereby improving the electron conductivity. When the thickness of the conversion layer is less than the above range, it means that the reaction content of the organic acid and the residual alkali is less, and there is still more residual alkali in the coated cathode material, and the coating layer fails to play the role of removing the residual alkali; when the thickness of the conversion layer is greater than the above range, it is very easy to cause the thickness of the outermost encapsulation layer to be small, which is not conducive to constructing a good electron conduction environment.

[0014] In a possible implementation, the mass fraction of the coating layer in the cathode material is 1% to 20%. By controlling the mass ratio of the coating layer in the cathode material within the above range, it is more beneficial to adjust the coating thickness of the coating layer, realize the effective protection of the core by the coating layer; and make the lithium ions in the core have a moderate extraction rate.

[0015] In a possible implementation, in the coating layer, the mass ratio of the lithium organic acid salt to the carbon material is (0.1 to 0.5):1. In this embodiment, in the coating layer, the mass fraction of the carbon material is greater than that of the lithium organic acid salt to ensure that the coating layer can effectively improve the electron conductivity of the cathode material. In this embodiment, by controlling the mass ratio of the lithium organic acid salt and the carbon material within the above range, it is ensured that the residual alkalinity of the lithium-rich cathode material is effectively reduced, and at the same time, the role of double-layer coating is better exerted, and the contact between the lithium-rich cathode material and the electrolyte is more effectively inhibited, and the amount of gas generated due to the side reaction with the electrolyte is reduced.

[0016] In a possible implementation, the D50 particle size of the cathode material is 0.5 μm to 25 μm.

[0017] In a possible implementation, in the cathode material, the mass fraction of the residual alkali is less than 5%.

[0018] In a second aspect, the present application also provides a method for preparing a cathode material, including: providing a lithium-rich material and an organic acid, mixing the lithium-rich material and the organic acid to obtain a precursor mixture; sintering the precursor mixture to obtain the cathode material, where the lithium-rich material forms a core, and the organic acid reacts to form a coating layer covering the outer surface of the core, and the coating layer includes a carbon material and a lithium organic acid salt.

[0019] In a possible implementation manner, the organic acid is a solid organic acid powder, and the lithium-rich material and the solid organic acid powder are mixed in a molar ratio of 1:y, where 0.02 ≤ y ≤ 0.2.

[0020] In a third aspect, the present application also provides a cathode electrode sheet, which includes a current collector and an active material layer disposed on the current collector. The active material layer includes the cathode material described in any one of the above, or the active material layer includes the cathode material obtained by the method for preparing the cathode material described in any one of the above.

[0021] In a fourth aspect, the present application also provides a secondary battery, which includes the cathode electrode sheet described in the above, or the battery includes the cathode material described in any one of the above, or the battery includes the cathode material obtained by the method for preparing the cathode material described in any one of the above. Description of the Drawings

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0023] Figure 1 is a cross-sectional schematic diagram of a cathode material in an embodiment;

[0024] Figure 2 is a cross-sectional schematic diagram of a cathode material in another embodiment;

[0025] Figure 3 is a cross-sectional schematic diagram of a cathode material in another embodiment;

[0026] Figure 4 is a process flow chart of the preparation of a cathode material in an embodiment.

[0027] Figure 5 is a TEM image of the cathode material prepared in Example 5. Detailed Embodiments

[0028] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0029] It should be noted that when a component is referred to as being "fixed to" another component, it can be directly on the other component or there may also be an intermediate component. When a component is considered to be "connected" to another component, it can be directly connected to the other component or there may be an intermediate component at the same time.

[0030] Unless otherwise defined, all technical and scientific terms used in the present application have the same meaning as commonly understood by those skilled in the technical field to which the present application belongs. The terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. The term "and / or" used in the present application includes any and all combinations of one or more of the related listed items.

[0031] Next, some embodiments of the present application will be described in detail in conjunction with the accompanying drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0032] In a first aspect, the present application provides a cathode material 100. Please refer to Figure 1 , the cathode material 100 includes a core 10 and a coating layer 20. The core 10 is composed of a lithium-rich material; the coating layer 20 is coated on the outer surface of the core 10, and the coating layer 20 includes a carbon material and a lithium organic acid salt, and the lithium organic acid salt is formed by an in-situ reaction of the residual alkali contained in the core 10.

[0033] Specifically, the cathode material 100 has a core-shell structure, and the core 10 is a lithium-rich material. The lithium-rich material can be cathode material 100 particles for assembling a lithium battery in cooperation with an anode material; the lithium-rich material can also be lithium-supplementing material particles as a "sacrificial agent" for supplementing the lithium source to ensure the first charging efficiency. The coating layer 20 can be prepared by mixing the lithium-rich material with a precursor. The lithium organic acid salt in the coating layer 20 can be a product formed by the reaction of the precursor with the residual alkali on the surface of the core 10. Optionally, the precursor of the coating layer 20 can be an organic acid. Taking oxalic acid as an example, after the core 10 is coated with oxalic acid, the following reaction will occur between oxalic acid and the residual alkali on the surface of the core 10:

[0034] Li2CO3 + H2C2O4 → Li2C2O4 + H2O + CO2

[0035] Among them, Li2CO3 is the residual alkali mentioned above. Of course, in other embodiments, other organic acids can also react with the residual alkali to form corresponding lithium organic acid salts.

[0036] In a possible embodiment, when the precursor is added in excess, the unreacted precursor can be sintered to form the carbon material mentioned above, and the carbon material and the lithium organic acid salt together form the coating layer 20.

[0037] In this application, by performing a high-temperature sintering reaction between the organic acid powder and the lithium-rich material, on the one hand, the surface residual alkali of the lithium-rich material can react with the organic acid, converting the surface residual alkali into a lithium organic acid salt, thereby effectively reducing the content of the surface residual alkali, avoiding the loss of the secondary battery capacity caused by the residual alkali, and effectively reducing the swelling phenomenon during the use of the cathode material in the secondary battery, improving the electrochemical performance and processing performance of the cathode material; on the other hand, the remaining organic acid can form a carbon material after sintering, which is used as an isolation and encapsulation layer on the surface of the lithium-rich material core, thereby reducing the influence of the external environment (water and carbon dioxide) on the lithium-rich material core and improving the electronic conductivity of the cathode material.

[0038] In a possible embodiment, please refer to Figure 2 , the coating layer includes a conversion layer 21 and an encapsulation layer 22. The conversion layer 21 is coated on the outer surface of the core 10, and the encapsulation layer 22 is coated on the outer surface of the conversion layer 21. The conversion layer 21 includes a lithium organic acid salt, and the encapsulation layer 22 includes a carbon material. Specifically, after using the precursor to coat the lithium-rich material, the precursor in contact with the lithium-rich material will first react with the residual alkali to form a lithium organic acid salt, and the precursor farther away from the core 10 is more difficult to react with the residual alkali. Therefore, after the precursor reacts with the residual alkali, a conversion layer 21 will be pre-formed and coated on the outer surface of the core 10, and the sintered precursor will form an encapsulation layer 22 on the outer surface of the conversion layer 21. It can be understood that there is no obvious interface between the conversion layer 21 and the encapsulation layer 22 to distinguish the two. When the precursor is less, the thickness of the encapsulation layer 22 should be thinner, and the existence of the encapsulation layer 22 is less obvious; when the precursor is more, the thickness of the encapsulation layer 22 should be thicker, and the existence of the encapsulation layer 22 is more obvious. By using an organic acid to coat the lithium-rich material, the sintered organic acid can form a two-layer structure of the conversion layer 21 and the encapsulation layer 22; after the organic acid reacts with the residual alkali substance on the surface of the lithium-rich material, the residual alkalinity can be reduced. After further increasing the sintering temperature, a carbon coating can be formed without additional carbon raw materials, reducing the production cost; at the same time, the lithium organic acid salt in the conversion layer 21 and the carbon material in the encapsulation layer 22 have a synergistic effect, and the double-layer coating structure can more effectively inhibit the contact with the electrolyte and reduce the amount of gas generated due to the side reaction with the electrolyte.

[0039] In a possible implementation, the conversion layer further includes a carbon material, and the carbon material content in the conversion layer is less than that in the encapsulation layer. Specifically, when the residual alkali content on the surface of the core is low or unevenly distributed, the precursor near the surface of the core does not react with the residual alkali, so carbon material is formed and retained in the conversion layer after sintering. Moreover, the carbon material content in the conversion layer is less than that in the encapsulation layer to ensure that the encapsulation layer can have sufficient carbon material content to provide a good electron conduction environment. In a possible implementation, the encapsulation layer includes an organic acid lithium salt, and the organic acid lithium salt content in the encapsulation layer is less than that in the conversion layer. Specifically, when the provided precursor content is low or the residual alkali content on the surface of the core is high, the organic acid lithium salt will also be formed in the encapsulation layer.

[0040] In a possible implementation, the content of the organic acid lithium salt in the coating layer gradually decreases in the direction away from the core. Specifically, the precursor reacts with the residual alkali on the surface of the core, and the formed organic acid lithium salt will be concentrated in large amounts at the interface between the core and the coating layer. Then, as the coating layer thickens, the content of the organic acid lithium salt gradually decreases.

[0041] In a possible implementation, the chemical formula of the lithium-rich material in the core is Li 1+x M y O z , where 0 ≤ x ≤ 5, 1 ≤ y ≤ 6, 2 ≤ Z ≤ 12, and M includes one or more of Ni, Fe, Mn, Co, Al, Cu, Zn, Cr. Specifically, the molecular formula of the lithium-rich material can be, but is not limited to, Li2NiO2, Li2CuO2, Li6CoO4, Li5FeO4, Li5AlO4, Li6Co 0.5 Mn 0.5 O4, Li6Ni 0.5 Co 0.5 O4.

[0042] In a possible implementation, the precursors of the carbon material and the organic acid lithium salt are solid organic acid powders, and the solid organic acid powders include one or more of oxalic acid, citric acid, tartaric acid, malic acid, ascorbic acid, acetic acid, succinic acid, salicylic acid, and caffeic acid. Specifically, the advantage of using an organic acid as the precursor of the carbon material and the organic acid lithium salt is that the organic acid can not only react with the residual alkali to form an organic acid lithium salt, but also the excess organic acid can form a carbon material at high temperature, thus eliminating the step of re-coating the carbon material to prepare the isolation encapsulation layer. In this implementation, solid organic acid powders are also selected as the precursors because during the preparation of the cathode material, the solid organic acid powders can form a uniform solid precursor mixture with the lithium-rich material, and the solid organic acid powders can uniformly coat the outer surface of the lithium-rich material. After sintering, the cathode material is tightly bonded and the coating layer is not easily detached. This method not only shortens the production cycle but also ensures the uniformity of the coating. Secondly, compared with liquid-phase coating, during the firing process after liquid-phase coating, the coating layer is easily damaged, resulting in coating defects; while the solid-phase coating process has less impact on the material properties, and the overall process cycle of solid-phase coating is shorter and the production efficiency is higher.

[0043] Of course, in other implementations, the precursors of the carbon material and the organic acid lithium salt can also be liquid organic acid solutions.

[0044] In a possible implementation, the thickness of the coating layer is 1 nm to 100 nm. Specifically, the thickness of the coating layer can be, but is not limited to, 1 nm, 5 nm, 10 nm, 20 nm, 50 nm, 80 nm, 100 nm. Controlling the thickness of the coating layer within the above range is beneficial for adjusting the particle size of the cathode material and ensuring the specific capacity and electron conduction environment of the cathode material. When the thickness of the coating layer is less than the above range, the coating layer does not completely coat the core, which is not conducive to constructing a good electron conduction environment, and it is very easy to have an incomplete residual alkali reaction; when the thickness of the coating layer is greater than the above range, it will cause the particle size of the cathode material to be too large, and since the coating layer does not contribute lithium ions, it will reduce the overall specific capacity of the cathode material.

[0045] In a possible implementation, the thickness of the conversion layer is 1 nm to 50 nm. Specifically, the thickness of the conversion layer can be, but is not limited to, 1 nm, 5 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm. The thickness of the conversion layer can be used to illustrate the reaction content of the organic acid and the residual base, and to control the thickness of the encapsulation layer. By controlling the thickness of the conversion layer and coordinating with the overall thickness of the encapsulation layer described above, the thickness of the encapsulation layer can be adjusted, thereby improving the electronic conductivity. When the thickness of the conversion layer is less than the above range, it indicates that the reaction content of the organic acid and the residual base is small, and there is still a large amount of residual base remaining in the coated cathode material, and the encapsulation layer fails to play a role in removing the residual base; when the thickness of the conversion layer is greater than the above range, it is extremely likely to cause the thickness of the outermost encapsulation layer to be small, which is not conducive to constructing a good electronic conduction environment.

[0046] In a possible implementation, the mass fraction of the encapsulation layer in the cathode material is 1% to 20%. Specifically, the mass fraction of the encapsulation layer in the cathode material can be, but is not limited to, 1%, 2%, 3%, 5%, 10%, 15%, 20%. By controlling the mass ratio of the encapsulation layer in the cathode material within the above range, it is more conducive to adjusting the coating thickness of the encapsulation layer and realizing the effective protection of the encapsulation layer for the core; and enabling the lithium ions in the core to have a moderate extraction rate.

[0047] In a possible implementation, the mass ratio of the lithium organic acid salt to the carbon material is (0.1 to 0.5):1. Specifically, the mass ratio of the lithium organic acid salt to the carbon material can be, but is not limited to, 0.1:1, 0.12:1, 0.15:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1. In this embodiment, in the encapsulation layer, the mass fraction of the carbon material is greater than that of the lithium organic acid salt to ensure that the encapsulation layer can effectively improve the electronic conductivity of the cathode material. In this embodiment, by controlling the mass ratio of the lithium organic acid salt and the carbon material within the above range, it is ensured that the residual alkalinity of the lithium-rich cathode material is effectively reduced, while better playing the role of double-layer coating, and more effectively inhibiting the contact between the lithium-rich cathode material and the electrolyte, and reducing the amount of gas generated due to the side reaction with the electrolyte.

[0048] In a possible implementation, the D50 particle size of the cathode material is 0.5 μm to 25 μm. Specifically, the D50 particle size of the cathode material can be, but is not limited to, 0.5 μm, 1 μm, 5 μm, 15 μm, 20 μm, 25 μm.

[0049] In a possible implementation, the core includes primary particles and / or secondary particles, and the secondary particles are stacked by a plurality of primary particles. Specifically, when the core is a primary particle, its structural schematic diagram is as Figure 1 shown; when the core is a secondary particle, and the structural schematic diagram is as Figure 3As shown. In a possible implementation, the particle size D50 of the core is 0.1 μm to 20 μm.

[0050] In a possible implementation, in the positive electrode material, the mass ratio of the residual alkali is less than 5%. Specifically, the mass ratio of the residual alkali can be, but is not limited to, 0%, 0.2%, 0.5%, 0.8%, 1.0%, 1.5%, 2.0%, 5.0%. By fully reacting the organic acid with the residual alkali on the surface of the lithium-rich material in this application, the surface residual alkali is converted into organic acid lithium salt, which can effectively reduce the surface residual alkali content and thus avoid the loss of the secondary battery capacity caused by the residual alkali.

[0051] In a second aspect, this application also provides a preparation method for a positive electrode material. Please refer to Figure 4 , which is specifically used for the preparation of the positive electrode material in the first aspect. The preparation method includes the following steps:

[0052] Step S10: Provide a lithium-rich material and an organic acid, and mix the lithium-rich material and the organic acid to obtain a precursor mixture.

[0053] Step S20: After sintering the precursor mixture, a positive electrode material is obtained. The lithium-rich material forms a core, and the organic acid reacts to form a coating layer covering the outer surface of the core. The coating layer includes a carbon material and an organic acid lithium salt.

[0054] The preparation method for the positive electrode material provided by the implementation manner of this application pre-synthesizes a lithium-rich material. The chemical formula of the lithium-rich material is Li x M y O z , where 1 ≤ x ≤ 6, 1 ≤ y ≤ 6, 2 ≤ Z ≤ 12, and M includes one or more of Ni, Fe, Mn, Co, Al, Cu, Zn, Cr. And there is no limitation on the precursor for synthesizing the lithium-rich material. The synthesized lithium-rich material is uniformly mixed with the organic acid so that the organic acid can coat the outer surface of the lithium-rich material. Then, the precursor mixture of the organic acid and the lithium-rich material is placed in an inert atmosphere for sintering, so that the organic acid can react with the residual alkali on the surface of the lithium-rich material to form an organic acid lithium salt, and the unreacted organic acid can be carbonized at high temperature to form a carbon material. The carbon material and the organic acid lithium salt together form a coating layer and cover the core formed by the lithium-rich material.

[0055] In this application, by subjecting the organic acid powder and the lithium-rich material to a high-temperature sintering reaction, on the one hand, the surface residual alkali of the lithium-rich material can react with the organic acid, converting the surface residual alkali into lithium organic acid salt, thereby effectively reducing the surface residual alkali content, avoiding the loss of the secondary battery capacity caused by the residual alkali, and effectively reducing the gas swelling phenomenon during the use of the cathode material in the secondary battery, improving the electrochemical performance and processing performance of the cathode material; on the other hand, the remaining organic acid can form a carbon material after sintering, which is used as an isolation encapsulation layer on the surface of the lithium-rich material core, thereby reducing the influence of the external environment (water and carbon dioxide) on the lithium-rich material core and enhancing the electronic conductivity of the cathode material.

[0056] In a possible implementation manner, the organic acid is a solid organic acid powder. Specifically, the type of the organic acid can refer to the above implementation manner, and the advantages of using the solid organic acid powder can also refer to the above implementation manner, which will not be elaborated here.

[0057] In a possible implementation manner, the lithium-rich material and the solid organic acid powder are mixed in a molar ratio of 1:y, where 0.02 ≤ y ≤ 0.2. Specifically, the molar ratio of the core to the solid organic acid powder can be but is not limited to 1:0.02, 1:0.05, 1:0.1, 1:0.15, 1:0.2. By mixing the lithium-rich material and the solid organic acid powder according to the molar ratio provided in the above range, the mixing amount of the lithium-rich material and the organic acid can be better controlled, so that the thickness and mass ratio of the finally formed coating layer can be controlled within a suitable range. After ensuring that the organic acid reacts with the residual alkali, part of the organic acid can be retained to form a carbon material, thereby forming an encapsulation layer with excellent conductive performance.

[0058] In a possible implementation manner, in the above step S20, the sintering temperature of the precursor mixture can be 550°C - 700°C, and the sintering time can be 30 min - 90 min.

[0059] In a possible implementation manner, in the above step S20, the precursor mixture can be carbonized through two-step sintering. Specifically, first sinter at a low temperature of 150 - 200°C for 60 - 120 min to enable the residual alkali and the organic acid powder to fully react; then, sinter at a high temperature of 500 - 700°C for 200 - 300 min to enable the remaining organic acid powder on the surface to be carbonized to form an encapsulation layer.

[0060] In a possible implementation manner, in the above step S20, the sintering environment of the precursor mixture can be an atmosphere formed by any one of the protective gases such as nitrogen, argon, and nitrogen-argon mixed gas.

[0061] In a third aspect, the present application also provides a positive electrode plate, which includes a current collector and an active material layer disposed on the current collector. The active material layer includes the positive electrode material according to any one of the second aspect. The positive electrode material of the present application can not only be used as a lithium supplement additive to supplement the active lithium consumed in forming the SEI film during the first charge of the battery, but also participate in the cycle as a positive electrode active material, having good application prospects. In some embodiments of the present application, the current collector includes any one of copper foil and aluminum foil. In some embodiments, the active material layer includes an electrode active material, a lithium-rich material, a binder, and a conductive agent. In some embodiments, the active material layer includes a lithium-rich material, a binder, and a conductive agent, that is, the lithium-rich material directly serves as the active material. In the embodiments of the present application, the binder includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives. In the embodiments of the present application, the conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60, and carbon nanotubes. In the embodiments of the present application, the electrode active material includes one or more of lithium cobaltate, lithium manganate, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, lithium fluorovanadate, lithium titanate, lithium nickel cobalt manganate, and lithium nickel cobalt aluminate.

[0062] In a fourth aspect, the present application also provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte. Among them, the positive electrode includes the positive electrode plate provided by the present application. Since the secondary battery provided by the present application uses the positive electrode material of the present application, it has good cycle performance and safety performance, which is beneficial to the application of secondary batteries in various fields.

[0063] The technical solution of the present invention is described in detail below through specific examples.

[0064] Example 1

[0065] This example provides a positive electrode material and a preparation method thereof. The positive electrode material includes a core and a coating layer coated on the outer surface of the core. The coating layer includes a carbon material and lithium oxalate salt.

[0066] The preparation method of the positive electrode material includes the following steps:

[0067] (1) Lithium-rich material Li2NiO2 and solid oxalic acid powder are fully mixed in a molar ratio of 1:0.02 to obtain a uniform solid precursor mixture.

[0068] (2) The precursor mixture is heated to 150 °C and kept warm for 60 min under a nitrogen atmosphere, then heated to 550 °C and calcined for 200 min, and cooled with the furnace to obtain a positive electrode material coated with lithium oxalate and carbon material.

[0069] The obtained cathode material through the above preparation method has a coating layer thickness of 9 nm, a conversion layer thickness of 6 nm, the mass fraction of the coating layer in the cathode material is 3.1%, the mass ratio of lithium oxalate salt to carbon material is 10%, and the residual alkali content is 3.5%.

[0070] Example 2

[0071] This example provides a cathode material and its preparation method. The cathode material includes a core and a coating layer coated on the outer surface of the core. The coating layer includes a carbon material and lithium citrate salt.

[0072] The preparation method of the cathode material includes the following steps:

[0073] (1) Mix the lithium-rich material Li2MnO2 and solid citric acid powder in a molar ratio of 1:0.02 with an ethanol solvent and mix well. Evaporate the ethanol solvent under a nitrogen atmosphere to obtain a uniform mixture.

[0074] (2) Under a nitrogen atmosphere, heat the mixture to 150 °C and keep it warm for 60 min, then heat it to 550 °C and calcine it for 200 min, and cool it with the furnace to obtain a lithium-rich cathode material coated with lithium citrate salt and carbon material.

[0075] The obtained cathode material through the above preparation method has a coating layer thickness of 10 nm, a conversion layer thickness of 7 nm, the mass fraction of the coating layer in the cathode material is 3.2%, the mass ratio of lithium citrate salt to carbon material is 10.6%, and the residual alkali content is 3.3%.

[0076] Example 3

[0077] This example provides a cathode material and its preparation method. The cathode material includes a core and a coating layer coated on the outer surface of the core. The coating layer includes a carbon material and lithium citrate salt.

[0078] The preparation method of the cathode material includes the following steps:

[0079] (1) Mix the lithium-rich material Li2NiO2 and solid citric acid powder in a molar ratio of 1:0.05 to obtain a uniform solid precursor mixture.

[0080] (2) Under a nitrogen atmosphere, heat the precursor mixture to 160 °C and keep it warm for 90 min, then heat it to 580 °C and calcine it for 200 min, and cool it with the furnace to obtain a lithium-rich cathode material of lithium citrate salt and carbon material.

[0081] The positive electrode material obtained by the above preparation method has a coating layer thickness of 15nm, a conversion layer thickness of 8nm, a coating layer mass fraction of 5.4% in the positive electrode material, a lithium citrate to carbon material mass ratio of 15%, and a residual alkali content of 3.1%.

[0082] Example 4

[0083] This embodiment provides a positive electrode material and a preparation method thereof. The positive electrode material includes a core and a coating layer coated on the outer surface of the core, and the coating layer includes a carbon material and lithium citrate.

[0084] The preparation method of the positive electrode material comprises the following steps:

[0085] (1) The lithium-rich material Li2NiO2 and solid citric acid powder are fully mixed in a molar ratio of 1:0.1 to obtain a uniform solid drive mixture.

[0086] (2) The precursor mixture is heated to 200° C. in a nitrogen atmosphere and kept at this temperature for 90 minutes, then heated to 600° C. and calcined for 250 minutes, and then cooled in the furnace to obtain a lithium-rich positive electrode material coated with lithium citrate salt and carbon material.

[0087] The positive electrode material obtained by the above preparation method has a coating layer thickness of 19nm, a conversion layer thickness of 10nm, a coating layer mass fraction of 8.3% in the positive electrode material, a lithium citrate to carbon material mass ratio of 21%, and a residual alkali content of 1.3%.

[0088] Example 5

[0089] This embodiment provides a positive electrode material and a preparation method thereof. The positive electrode material includes a core and a coating layer coated on the outer surface of the core, and the coating layer includes a carbon material and lithium citrate.

[0090] The preparation method of the positive electrode material comprises the following steps:

[0091] (1) The lithium-rich material Li2NiO2 and solid citric acid powder are fully mixed in a molar ratio of 1:0.2 to obtain a uniform solid drive mixture.

[0092] (2) The precursor mixture is heated to 200° C. in a nitrogen atmosphere and kept at this temperature for 90 minutes, then heated to 600° C. and calcined for 200 minutes, and then cooled in the furnace to obtain a lithium-rich positive electrode material coated with lithium citrate salt and carbon material.

[0093] The positive electrode material obtained by the above preparation method has a coating layer thickness of 31nm, a conversion layer thickness of 15nm, a coating layer mass fraction of 10.2% in the positive electrode material, a lithium citrate to carbon material mass ratio of 36%, and a residual alkali content of 0.6%.

[0094] Comparative Example 1

[0095] This comparative example provides a lithium-rich cathode material, and the lithium-rich cathode material does not include a coating layer.

[0096] The lithium-rich material is Li2NiO2.

[0097] The cathode materials provided in the above Examples 1 to 5 and Comparative Example 1 were respectively subjected to TEM analysis. Among them, the TEM of Example 5 is as Figure 5 shown. It can be seen from Figure 5 that the coating layer formed by the lithium organic acid salt and the carbon material can uniformly coat the surface of the cathode material, indicating that a cathode material with a core-shell structure can be obtained by mixing and sintering the lithium-rich material with the solid organic acid powder.

[0098] The cathode materials provided in the above Examples 1 to 5 and the lithium-rich cathode material provided in Comparative Example 1 were respectively assembled into a positive electrode and a lithium-ion secondary battery according to the following method:

[0099] Positive electrode: The cathode material is mixed and ball-milled with polyvinylidene fluoride and SP-Li in a mass ratio of 93:3:4 to obtain a cathode slurry. The cathode slurry is coated on the surface of the aluminum foil and vacuum-dried overnight at 110 °C, and then roll-pressed to obtain a positive electrode sheet;

[0100] Negative electrode: Graphite is mixed and ball-milled with carboxymethyl cellulose (CMC), SBR and SP in a mass ratio of 95.8:1.2:2:1 to obtain a negative electrode slurry. The negative electrode slurry is coated on the surface of the copper foil and vacuum-dried overnight at 110 °C to obtain a negative electrode sheet;

[0101] Electrolyte: Ethylene carbonate and ethyl methyl carbonate are mixed in a volume ratio of 3:7, and LiPF6 is added to form an electrolyte, and the concentration of LiPF6 is 1 mol / L;

[0102] Separator: Polypropylene microporous separator;

[0103] Assembly of lithium-ion secondary battery: A button-type lithium-ion full battery is assembled in an inert gas glove box in the order of graphite negative electrode sheet - separator - electrolyte - positive electrode sheet.

[0104] Test conditions: The battery is tested in a cabinet at 25 °C. The test voltage range is set to 2.00 to 4.25 V, and the first charge and discharge are carried out at a test current of 0.1C, and the cycle test is carried out at a test current of 0.2C charge and discharge.

[0105] The electrochemical performances of the lithium-ion secondary batteries assembled in the above lithium-ion secondary battery examples were respectively subjected to the performance tests shown in Table 1, and the test results are shown in Table 1 below:

[0106] Table 1

[0107]

[0108] It can be seen from the test results of Examples 1-5 in Table 1 and Comparative Example 1 that the cathode material coated with lithium organic acid salt and carbon material has more excellent electrochemical performance, and the initial charge capacity can be as high as 444 mAh / g. Since dry and wet coating techniques are respectively used in Example 1 and Example 2, their electrochemical performances are different. The solid organic acid coating technique of the present invention has higher initial charge capacity and capacity retention rate, indicating that the binding force between the solid organic acid and the lithium-rich material is stronger before sintering, and the coating material can be well distributed on the surface of the lithium-rich material.

[0109] In summary, coating with organic acid on the outer surface with the lithium-rich material as the core can not only remove the residual alkali on the outer surface of the lithium-rich material core, but also effectively prevent the swelling phenomenon of the lithium battery during operation; it can also use the carbon material formed after the carbonization of the organic acid to achieve the function of an isolation encapsulation layer, reduce the influence of the external environment on the lithium-rich material core, and improve the conductivity of the lithium-rich material.

[0110] In the description of the embodiments of the present application, it should be noted that the orientation or positional relationship indicated by terms such as "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship in the drawings, and is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or component referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present application.

[0111] What is disclosed above is only a preferred embodiment of the present application. Of course, the scope of rights of the present application cannot be limited thereby. Those of ordinary skill in the art can understand all or part of the processes of implementing the above embodiments, and the equivalent changes made according to the claims of the present application still fall within the scope covered by the present application.

Claims

1. A cathode material, characterized in that, Comprising: a core composed of a lithium-rich material; a coating layer coated on the outer surface of the core, the coating layer includes a carbon material and a lithium organic acid salt, the precursors of the carbon material and the lithium organic acid salt are solid organic acid powders, and the solid organic acid powders react with the residual alkali contained in the core to in-situ generate the lithium organic acid salt; the coating layer includes a conversion layer and a encapsulation layer, the conversion layer is coated on the outer surface of the core, the encapsulation layer is coated on the outer surface of the conversion layer, the conversion layer includes the lithium organic acid salt, and the encapsulation layer includes the carbon material.

2. The positive electrode material according to claim 1, characterized in that, The conversion layer further includes the carbon material, and the carbon material content in the conversion layer is less than the carbon material content in the encapsulation layer; and / or the encapsulation layer includes the lithium organic acid salt, and the lithium organic acid salt content in the encapsulation layer is less than the lithium organic acid salt content in the conversion layer.

3. The cathode material according to claim 1, characterized in that, The lithium organic acid salt content in the coating layer gradually decreases in the direction away from the core.

4. The cathode material according to claim 1, characterized in that, The solid organic acid powder includes one or more of oxalic acid, citric acid, tartaric acid, malic acid, ascorbic acid, succinic acid, and caffeic acid.

5. The cathode material according to claim 1, characterized in that, The chemical formula of the lithium-rich material in the core is Li 1+ x M y O z , where 0 < x ≤ 5, 1 ≤ y ≤ 6, 2 ≤ Z ≤ 12, and M includes one or more of Ni, Fe, Mn, Co, Al, Cu, Zn, and Cr.

6. The cathode material according to claim 1, characterized in that, The thickness of the coating layer is 1 nm to 100 nm; and / or the thickness of the conversion layer is 1 nm to 50 nm; and / or the D50 particle size of the positive electrode material is 0.5 μm to 25 μm.

7. The cathode material according to claim 1, wherein The mass fraction of the coating layer in the positive electrode material is 1% to 20%; and / or, in the coating layer, the mass ratio of the lithium organic acid salt to the carbon material is (0.1 to 0.5):

1.

8. The cathode material according to claim 1, wherein In the positive electrode material, the mass proportion of the residual alkali is less than 5%.

9. A method for preparing a cathode material, characterized in that The preparation method is used to prepare the positive electrode material according to any one of claims 1-8, and the preparation method includes: providing a lithium-rich material and an organic acid, mixing the lithium-rich material and the organic acid to obtain a precursor mixture, and the organic acid is a solid organic acid powder; sintering the precursor mixture to obtain a positive electrode material, the lithium-rich material forms a core, and the organic acid reacts to form a coating layer coated on the outer surface of the core, and the coating layer includes a carbon material and a lithium organic acid salt; wherein, the precursor mixture is sintered under an inert atmosphere, and the unreacted organic acid is carbonized during sintering to form the carbon material.

10. The method for preparing the cathode material according to claim 9, wherein, The lithium-rich material and the solid organic acid powder are mixed in a molar ratio of 1:y, wherein 0.02 ≤ y ≤ 0.

2.

11. A positive electrode plate, characterized in that, The positive electrode sheet includes a current collector and an active material layer provided on the current collector, the active material layer includes the positive electrode material according to any one of claims 1-8, or the active material layer includes the positive electrode material obtained by the preparation method of the positive electrode material according to claim 9 or 10.

12. A secondary battery, characterized in that, Comprising the positive electrode sheet according to claim 11, or the battery includes the positive electrode material according to any one of claims 1-8, or the battery includes the positive electrode material obtained by the preparation method of the positive electrode material according to claim 9 or 10.

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