Positive electrode lithium supplement additive, preparation method thereof, positive electrode sheet, and secondary battery

Abstract

This application belongs to the technical field of battery materials, and particularly relates to a cathode lithium supplement additive and a preparation method thereof, a cathode sheet, and a secondary battery. Among them, the cathode lithium supplement additive includes a lithium supplement material core and a packaging layer coated on the outer surface of the lithium supplement material core, and a liquid film covering layer is at least partially formed at the crack on the outer surface of the packaging layer. The cathode lithium supplement additive provided by this application seals the microscopic cracks on the surface packaging layer of the cathode lithium supplement additive through the surface tension of the liquid, so that harmful gas molecules in the external environment cannot pass through the microscopic cracks on the packaging layer, thereby isolating the influence of harmful gas molecules in the external environment on the stability of the core lithium supplement material, ensuring the stability of the core lithium supplement material, improving the lithium supplement stability of the cathode lithium supplement material, achieving a better lithium supplement effect in the cathode, effectively maintaining the active lithium in the battery system, and improving the capacity retention rate of the battery.

Description

Technical Field

[0001] The present application belongs to the technical field of battery materials, and in particular relates to a positive electrode lithium supplement additive and a preparation method thereof, as well as a positive electrode sheet and a secondary battery. Background Art

[0002] With the rapid development of energy storage technology, the use of portable digital devices and vehicle-mounted power supplies is increasing, and people have higher and higher requirements for the energy density of batteries. It is imperative to develop secondary batteries with large capacity, long life and high safety. During the first charge and discharge process of lithium-ion batteries, a SEI film will form at the interface of the negative electrode material. Studies have shown that the components of SEI are mainly lithium salt materials such as LiF, Li2CO3, R-COOLi, and R-CH2OLi. The formation of SEI is an irreversible process. The Li used to form SEI + During the discharge process, it can no longer be embedded in the positive electrode material, resulting in a loss of battery capacity.

[0003] The study found that the formation of SEI film consumes part of the Li in the positive electrode material. + , which in turn leads to irreversible capacity loss of electrode materials. Therefore, in order to further improve the energy density of lithium-ion batteries, this part of the capacity loss can be compensated by pre-replenishing lithium. There are two main types of pre-replenishing lithium technology. One is the negative electrode material lithium replenishment technology. This technology has high requirements for the operating environment. The lithium replenisher is generally metal lithium foil and inert lithium powder. The activity is too high and cannot be stored stably for a long time, which increases the difficulty of operation and production risks. The other is the positive electrode material lithium replenishment technology, which requires relatively safer and easier operation.

[0004] At present, although the commonly used lithium supplement materials have a high lithium content, these lithium supplement materials are not stable in the air. Affected by water, oxygen, carbon dioxide and other components in the atmosphere, the lithium supplement materials are easily deteriorated, so that the lithium supplement effect of the materials is significantly reduced or even directly destroyed, which not only affects the lithium supplement effect of the materials on the positive electrode, but also leads to the introduction of impurities in the positive electrode, reducing the capacity of the battery. Although the surface of the lithium supplement material can be coated to improve the stability of the lithium supplement material through the coating layer, there will inevitably be microscopic cracks in the surface coating layer of the lithium supplement material, resulting in a certain passage between the internal lithium supplement material and the external environment. The moisture, oxygen, carbon dioxide and other components in the atmosphere will still affect the stability of the lithium supplement material. Summary of the invention

[0005] The purpose of the present application is to provide a positive electrode lithium supplement additive and a preparation method thereof, as well as a positive electrode sheet and a secondary battery, aiming to solve the problem of poor stability of existing lithium supplement additives to a certain extent.

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

[0007] In a first aspect, this application provides a cathode lithium supplement additive, which includes a lithium supplement material core and a packaging layer coated on the outer surface of the lithium supplement material core, and a liquid film covering layer is at least partially formed at the cracks on the outer surface of the packaging layer.

[0008] Further, the liquid film covering layer is selected from a water film.

[0009] Further, hydrophilic groups are bonded to the outer surface of the packaging layer.

[0010] Further, hydrophobic groups are bonded to the inner surface of the packaging layer.

[0011] Further, the hydrophilic groups include at least one of hydroxyl, aldehyde, carboxyl, and amino groups.

[0012] Further, the hydrophobic groups include at least one of hydrocarbon groups, halogens, nitro groups, and ester groups.

[0013] Further, the moisture content on the outer surface of the packaging layer is 10 - 2000 ppm.

[0014] Further, the lithium supplement material core includes Li x A 1-y-4z / 3 B y C z O 3 / 2+x / 2 ; where A is selected from Fe or Ni, and B and C represent doped metal elements with different valence states; 1 ≤ x ≤ 6, 0 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.375, 0 ≤ 6y + 8z ≤ 3.

[0015] Further, the lithium supplement material core is primary particles and / or secondary particles.

[0016] Further, the packaging layer includes at least one of an isolation packaging layer, an ion conductor packaging layer, and an electron conductor packaging layer.

[0017] Further, the doped element B includes at least one of Al, Co, Ni, Fe, Ga, and La.

[0018] Further, the doped element C includes at least one of Mn, Ge, Sn, Ti, Si, V, and Zr.

[0019] Further, the particle size of the primary particles is 50 nm - 500 nm, and the particle size of the secondary particles is 0.1 μm - 10 μm.

[0020] In a second aspect, this application provides a preparation method for a cathode lithium supplement additive, including the following steps:

[0021] Prepare a lithium supplement material;

[0022] Prepare a packaging layer on the surface of the lithium supplement material;

[0023] Form a liquid film covering layer on the outer surface of the packaging layer, and the liquid film covering layer at least partially covers the cracks on the surface of the packaging layer to obtain a cathode lithium supplement additive.

[0024] Further, the step of forming a liquid film covering layer on the outer surface of the packaging layer includes: placing the lithium supplement material with the packaging layer formed on its surface in a constant temperature and humidity condition for storage treatment to form a water film on the outer surface of the packaging layer.

[0025] In a third aspect, the present application provides a cathode sheet, which contains the above-mentioned cathode lithium supplement additive, or contains the cathode lithium supplement additive prepared by the above method.

[0026] Further, the cathode sheet includes a current collector and an active material layer stacked and attached, and the mass percentage content of the cathode lithium supplement additive in the active material layer is 0.1-10%.

[0027] In a fourth aspect, the present application provides a secondary battery, which contains the above-mentioned cathode sheet.

[0028] The cathode lithium supplement additive provided in the first aspect of the present application includes a lithium supplement material core and a packaging layer coated on the surface, and a liquid film covering layer is at least partially formed at the cracks on the outer surface of the packaging layer. Since there will inevitably be microscopic crack distributions on the outer surface of the lithium supplement material core, there will be a certain passage between the core lithium supplement material and the external environment, and gaseous molecules such as water, oxygen, and carbon dioxide in the external environment will affect the stability of the core lithium supplement material. A liquid film covering layer is formed on the surface of the packaging layer. Through the surface tension of the liquid, the microscopic cracks on the surface packaging layer of the cathode lithium supplement additive are sealed, so that harmful gaseous molecules in the external environment cannot pass through the microscopic cracks on the packaging layer, thereby isolating the influence of harmful gaseous molecules in the external environment on the stability of the core lithium supplement material, ensuring the stability of the core lithium supplement material, effectively improving the lithium supplement stability of the cathode lithium supplement material, and playing a good lithium supplement effect in the cathode, making up for the active lithium ions consumed due to the formation of the SEI film during the first charge of the battery, thereby effectively maintaining the active lithium in the battery system and improving the capacity retention rate of the battery. In addition, during the use of this material, the liquid film on the surface of the cathode lithium supplement additive can be removed by processes such as baking during the production of the electrode sheet, without affecting the migration of ions.

[0029] The preparation method of the cathode lithium supplement additive provided by the second aspect of the present application has a simple process and is suitable for industrial large-scale production and application. Moreover, the prepared cathode lithium supplement additive has a stable structure and electrochemical performance, effectively isolating the influence of harmful gas-phase molecules in the environment on the stability of the core lithium supplement material, thereby ensuring the lithium supplement stability and effect of the additive. At the same time, it ensures the stability of lithium supplementation, the uniformity of dispersion, and good processing performance of the prepared lithium supplement additive in the electrode active slurry and the active layer.

[0030] The cathode sheet provided by the third aspect of the present application, due to containing the above-mentioned cathode lithium supplement additive, which includes a core lithium supplement material and a packaging layer coated on the outer surface, and at least partially forms a liquid film covering layer at the crack on the outer surface of the packaging layer. It not only has good lithium supplement capacity, but also because the liquid film covering layer contained on the outer surface of the packaging layer of the cathode lithium supplement additive forms an isolation layer, which can effectively isolate environmental gas-phase molecules such as moisture and carbon dioxide in the outside world. Therefore, it endows the cathode material with stable lithium supplement performance and stable performance in application. Thus, it has a good lithium supplement effect on the cathode sheet, high safety, and can effectively improve the electrochemical performance such as the capacity retention rate, cycle life, and safety of the cathode sheet.

[0031] The secondary battery provided by the fourth aspect of the present application, due to containing the above-mentioned cathode sheet, and the above-mentioned cathode lithium supplement additive is added to the cathode sheet, which can effectively make up for the active lithium ions consumed by the formation of the SEI film during the first charge of the battery, effectively maintain the active lithium in the battery system, and improve the capacity retention rate of the battery. Thus, the secondary battery provided by the present application has a high energy density and a good capacity retention rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for use in the embodiments or the description of 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, without creative efforts, other drawings can also be obtained based on these drawings.

[0033] Figure 1 is a schematic structural diagram of the cathode lithium supplement additive provided by the embodiment of the present application;

[0034] Figure 2 is a schematic diagram when the surface water content of the cathode lithium supplement additive provided by the embodiment of the present application is too high;

[0035] Figure 3 is a schematic diagram when the surface water content of the cathode lithium supplement additive provided by the embodiment of the present application is too low;

[0036] Figure 4 is a schematic flow diagram of the preparation method of the cathode lithium supplement material provided by the embodiment of the present application;

[0037] Figure 5 It is a test chart of the surface water content of the cathode lithium supplement materials provided in Embodiments 1-3 and Comparative Examples 1-2 of the present application;

[0038] Figure 6 It is a test chart of the lithium supplement gram capacity of the battery after applying the cathode lithium supplement materials provided in Embodiments 1-3 and Comparative Examples 1-2 of the present application to the battery. Detailed implementation manners

[0039] In order to make the technical problems to be solved, technical solutions and beneficial effects of the present application clearer, the present application will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0040] In the present application, the term "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may indicate: A exists alone, A and B exist simultaneously, and B exists alone. Wherein A and B may be singular or plural. The character " / " generally indicates that the associated objects before and after are in an "or" relationship.

[0041] In the present application, "at least one" means one or more, and "a plurality" means two or more. "At least one (item)" or its similar expression below refers to any combination of these items, including any combination of single item (item) or plural items (items). For example, "at least one (item) of a, b or c", or, "at least one (item) of a, b and c" can both represent: a, b, c, a-b (that is, a and b), a-c, b-c, or a-b-c, where a, b, and c can be single or multiple respectively.

[0042] It should be understood that in various embodiments of the present application, the size of the serial numbers of the above processes does not mean the order of execution. Some or all steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

[0043] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a" and "the" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0044] In the description of the embodiments of the present application, the weight of the relevant components mentioned not only can refer to the specific content of each component, but also can represent the proportional relationship between the weights of each component. Therefore, as long as the content of the relevant components in the description of the embodiments of the present application is enlarged or reduced in proportion, it is within the scope disclosed in the description of the embodiments of the present application. Specifically, the mass in the description of the embodiments of the present application can be mass units well-known in the chemical industry such as μg, mg, g, kg, etc.

[0045] The terms "first" and "second" are only used for descriptive purposes to distinguish objects such as substances from each other, and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX can also be referred to as the second XX, and similarly, the second XX can also be referred to as the first XX. Thus, the features defined with "first" and "second" can explicitly or implicitly include one or more of such features.

[0046] The first aspect of the embodiments of the present application provides a cathode lithium supplement additive, including a lithium supplement material core and a packaging layer coated on the outer surface of the lithium supplement material core, and a liquid film covering layer is at least partially formed at the cracks on the outer surface of the packaging layer.

[0047] The cathode lithium supplement additive provided by the first aspect of the embodiments of the present application includes a lithium supplement material core and a packaging layer coated on the surface, and a liquid film covering layer is at least partially formed at the cracks on the outer surface of the packaging layer. Since there will inevitably be microscopic crack distributions on the outer surface of the packaging layer of the lithium supplement material core, a certain passage exists between the core lithium supplement material and the external environment, and gaseous molecules such as water, oxygen, and carbon dioxide in the external environment will affect the stability of the core lithium supplement material. The liquid film covering layer formed on the surface of the packaging layer in the embodiments of the present application can form a certain protective effect on the surface of the cathode lithium supplement additive material, such as Figure 1 As shown, the microscopic cracks on the surface packaging layer of the cathode lithium supplement additive are sealed by the surface tension of the liquid, so that the harmful gaseous molecules in the external environment cannot pass through the microscopic cracks on the packaging layer, thereby isolating the influence of the harmful gaseous molecules in the external environment on the stability of the core lithium supplement material, ensuring the stability of the core lithium supplement material, effectively improving the lithium supplement stability of the cathode lithium supplement material, achieving a good lithium supplement effect in the cathode, compensating for the active lithium ions consumed by the formation of the SEI film during the first charge of the battery, thereby effectively maintaining the active lithium in the battery system and improving the capacity retention rate of the battery. In addition, during the use of this material, the liquid film on the surface of the cathode lithium supplement additive can be removed by processes such as baking during the production of the electrode sheet, without affecting the migration of ions.

[0048] In some embodiments, the liquid film covering layer is selected from a water film. The water content on the surface of the cathode lithium supplement additive plays a crucial role in the stability of the additive, and the level of water content will also greatly affect the stability of the additive. In some preferred embodiments, the water content on the outer surface of the encapsulation layer is 10 to 2000 ppm. As shown in the appendix Figure 2 If the water content on the surface of the cathode lithium supplement additive is too high, it will cause interference to other moisture-sensitive components in the cathode sheet during actual application, affecting the performance. At the same time, when the cathode lithium supplement additive material is soaked in a high-moisture environment for a long time, due to the large moisture concentration difference, moisture will slowly diffuse into the inner layer of the material, causing the core material to absorb moisture and deteriorate. As shown in the appendix Figure 3 If the water content on the surface of the cathode lithium supplement additive is too low, the formed water film cannot effectively seal the microscopic cracks on the surface encapsulation layer, resulting in some microscopic cracks on the surface encapsulation layer being directly exposed to the external environment. Some harmful gas molecules in the external environment can pass through the microscopic cracks through the surface encapsulation layer and come into contact with the lithium supplement material in the core, causing side reactions and deterioration of the core lithium supplement material. In some preferred embodiments, the water content on the outer surface of the encapsulation layer is 10 to 2000 ppm, further 10 to 1500 ppm, still further 10 to 1000 ppm, and still further 10 to 500 ppm.

[0049] In some embodiments, the lithium supplement material core includes Li x A 1-y-4z / 3 B y C z O 3 / 2+x / 2 ; wherein, A is selected from Fe or Ni, and B and C represent doped metal elements with different valence states; 1≤x≤6, 0≤y≤0.5, 0≤z≤0.375, 0≤6y + 8z≤3; this doped lithium supplement material can provide abundant lithium, and can effectively release lithium during the first cycle charging process and supplement the irreversible lithium ions consumed by forming the SEI film on the negative electrode, thereby increasing the lithium content of the lithium positive electrode material in the electrode, and thus improving the capacity and cycling performance of the electrode. Among them, the doped elements B and C contained in the lithium supplement material can alleviate the reaction of lithium and metal A with air, thereby inhibiting the generation of residual alkali by the lithium supplement additive and making the lithium supplement additive have good stability.

[0050] In some embodiments, Li x A 1-y-4z / 3 B y C z O 3 / 2+x / 2The presence of the doped elements B and C can increase the crystal defect positions, broaden the ion transport channels of the core lithium supplementing material, and make the lithium in the core more easily removed. In some embodiments, the doped element B includes at least one of Al, Co, Ni, Fe, Ga, and La; in other embodiments, the doped element C includes at least one of Mn, Ge, Sn, Ti, Si, V, and Zr. That is, by controlling the types of the doped elements B and / or C, the above effects of the doped elements can be further improved.

[0051] In some embodiments, the core of the lithium supplementing material is a primary particle, which is understood as a crystal particle and an unagglomerated particle in the embodiments of the present application. In some embodiments, the particle size of the primary particle is 50 nm - 500 nm, further 50 - 250 nm, further 50 - 150 nm, further 50 - 100 nm, etc. In specific embodiments, the particle size of the primary particle can be typical but non-limiting particle sizes such as 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, etc.

[0052] In other embodiments, the core of the lithium supplementing material can also be a secondary particle, which is understood as an agglomerated particle formed by the aggregation of primary particles in the embodiments of the present application. In some embodiments, the particle size of the secondary particle is 0.1 μm - 10 μm, further 0.1 - 8 μm, further 0.1 - 5 μm, further 0.1 - 3 μm, further 0.1 - 1 μm, etc. In specific embodiments, the particle size of the secondary particle is 0.1 μm - 10 μm, further 0.1 μm - 5 μm. In specific embodiments, the particle size of the secondary particle can be typical but non-limiting particle sizes such as 0.1 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 91 μm, 10 μm, etc. By controlling the particle size of the core of the lithium supplementing material in the embodiments of the present application, the content of the lithium supplementing additive in the cathode can be controlled, thereby optimizing the lithium supplementing effect of the cathode lithium supplementing additive.

[0053] In some embodiments, the core of the lithium supplementing material can also be a mixture particle formed by the above primary particles and secondary particles.

[0054] In some embodiments, a hydrophilic group is bonded to the outer surface of the encapsulation layer, which is beneficial to the binding of moisture on the outer surface of the encapsulation layer to form a water film covering layer at the crack. In some specific embodiments, the hydrophilic group includes at least one of a hydroxyl group, an aldehyde group, a carboxyl group, and an amino group.

[0055] In some embodiments, a hydrophobic group is bonded to the inner surface of the encapsulation layer. Through the hydrophobic group bonded to the inner surface of the encapsulation layer, the moisture on the outer surface of the encapsulation layer and in the environment is further prevented from infiltrating into the core to damage the lithium supplement material. In some specific embodiments, and / or, the hydrophobic group includes at least one of a hydrocarbon group, a halogen, a nitro group, and an ester group.

[0056] In some embodiments, the encapsulation layer includes at least one of an isolation encapsulation layer, an ion conductor encapsulation layer, and an electronic conductor encapsulation layer. These encapsulation layers can effectively improve the electron and ion conduction properties of the lithium supplement material in the core, and improve the extraction of lithium during charging; they can also play a certain role in isolating moisture, improve the stability of the cathode lithium supplement additive, and achieve a stable lithium supplement effect. In addition, it can also ensure the stability of the cathode lithium supplement additive during lithium supplementation in the electrode active slurry and the active layer, the uniformity of dispersion, and good processing performance.

[0057] In some embodiments, the encapsulation layer can be a separate isolation encapsulation layer that completely covers the core of the lithium supplement material to play a protective role and improve the stability of the core of the lithium supplement material. It can also be a composite laminated structure of an isolation encapsulation layer and an electronic conductor encapsulation layer. The preferred structure is that the isolation encapsulation layer covers the outer surface of the core of the lithium supplement material, and the electronic conductor encapsulation layer covers the outer surface of the isolation encapsulation layer. It can also be a composite laminated structure of an isolation encapsulation layer and an ion conductor encapsulation layer. The preferred structure is that the isolation encapsulation layer covers the outer surface of the core of the lithium supplement material, and the ion conductor encapsulation layer covers the outer surface of the isolation encapsulation layer. It can also be a composite laminated structure of an isolation encapsulation layer, an electronic conductor encapsulation layer, and an ion conductor encapsulation layer. The preferred structure is that the isolation encapsulation layer covers the outer surface of the core of the lithium supplement material, the ion conductor encapsulation layer covers the outer surface of the isolation encapsulation layer, and the electronic conductor encapsulation layer covers the outer surface of the ion conductor encapsulation layer; or, the isolation encapsulation layer covers the outer surface of the core of the lithium supplement material, the electronic conductor encapsulation layer covers the outer surface of the isolation encapsulation layer, and the ion conductor encapsulation layer covers the outer surface of the electronic conductor encapsulation layer

[0058] The isolation encapsulation layer in the embodiments of the present application completely covers the core of the lithium supplement material to play a protective role and avoid contact with water and carbon dioxide in the environment. In some embodiments, the material of the isolation encapsulation layer includes at least one of ceramics, polymer materials, or carbon materials. In some specific embodiments, the ceramics include at least one of Al2O3, SiO2, boehmite, Si3N4, SiC, and BN. In some specific embodiments, the polymer includes an organic polymer with the structure of [C6H7O6Na] n as the structure, an organic polymer with the structure of [C6H7O2(OH)2OCH2COONa] n as the structure, an organic polymer with the structure of [C3H4O2] n as the structure, an organic polymer with the structure of [C3H3O2M a ​n organic polymers with a [C3H3N] structure, n organic polymers with a [CH2-CF2] structure, n organic polymers with a -[NHCO]- structure, organic polymers with an imide ring -[CO-N-CO]- structure in the main chain, and one or more of polyvinylpyrrolidone, where M a is an alkali metal element. Specifically, the polymers include one or more of polyvinylidene fluoride, sodium alginate, sodium carboxymethyl cellulose, polyacrylic acid, polyacrylates, polyacrylonitrile, polyamides, polyimides, polyvinylpyrrolidone, polyethylene oxide (PEO), polypyrrole (PPy), polytetrafluoroethylene (PTFE), and polyurethane (PU). Further, the polymers include one or more of sodium carboxymethyl cellulose and polyacrylic acid. Sodium carboxymethyl cellulose and polyacrylic acid are two-dimensional planar high-molecular polymers with good adhesion, which can effectively coat the core of the lithium-rich material, thus avoiding the contact between the core of the lithium-rich material and air and improving the stability of the lithium supplement additive. In the embodiments of the present application, the molecular weight of the polymer is greater than or equal to 100,000. The molecular weight of the polymer can specifically but is not limited to 100,000, 150,000, 200,000, 300,000, 500,000, or 1,000,000. The larger the molecular weight of the polymer, the higher the density and structural strength of the polymer layer, which is more conducive to protecting the core of the lithium-rich material. In some specific embodiments, the carbon material includes at least one of graphene, carbon nanotubes, amorphous carbon, graphite, and carbon black.

[0059] In some embodiments, the thickness of the isolation encapsulation layer is 5 - 200 nm; more preferably 5 - 50 nm. By adjusting the material and thickness of the isolation encapsulation layer in the embodiments of the present application, the contact between water, carbon dioxide and the lithium source core can be further blocked, and the stability of the lithium source core can be improved.

[0060] The electronic conductor encapsulation layer in the embodiments of the present application can enhance the electronic conductivity of the encapsulation layer, thereby enhancing the electronic conductivity of the lithium supplement additive and being beneficial to reducing the impedance inside the electrode. In some embodiments, the material of the electronic conductor encapsulation layer includes at least one of carbon materials, conductive polymers, or conductive oxides. In some specific embodiments, the carbon materials include at least one of mesoporous carbon, carbon nanotubes, graphite, carbon black, graphene, etc., the conductive polymers can be but are not limited to the conductive polymers contained in the above isolation encapsulation layer, and the conductive oxides include at least one of In2O3, ZnO, and SnO2.

[0061] In some embodiments, the thickness of the electronic conductor encapsulation layer is 5 - 200 nm; more preferably 5 - 50 nm. By adjusting the thickness of the electronic conductor encapsulation layer in the embodiments of the present application, the electronic conductivity of the cathode lithium supplement additive can be further improved.

[0062] In the embodiment of the present application, the ionic conductor encapsulation layer can enhance the ionic conductivity of the cathode lithium supplement additive, thereby enhancing the ionic conductivity of the lithium supplement additive, which is beneficial to the outward transport of lithium ions in the core of the lithium supplement material. In some embodiments, the material of the ionic conductor encapsulation layer includes at least one of perovskite-type, NASICON-type, garnet-type or polymer-type solid electrolytes. In some specific embodiments, the perovskite type includes Li 3x La 2 / 3-x TiO3 (LLTO), specifically such as Li 0.5 La 0.5 TiO3, Li 0.33 La 0.57 TiO3, Li 0.29 La 0.57 TiO3, Li 0.33 Ba 0.25 La 0.39 TiO3, (Li 0.33 La 0.56 ) 1.005 Ti 0.99 Al 0.01O3 , Li 0.5 La 0.5 Ti 0.95 Zr 0.05 O3, etc., at least one of them; the NASICON type is such as but not limited to Li 1.4 Al 0.4 Ti 1.6 (PO4)3 (LATP), the garnet type includes Li7La3Zr2O 12 (LLZO), Li 6.4 La3Zr 1.4 Ta 0.6 O 12 , Li 6.5 La3Zr 1.5 Ta 0.5 O 12 at least one of them; the polymer-type solid electrolyte includes at least one of PEO / PPO / PVDF, etc. that dissolves lithium salts.

[0063] In some embodiments, the thickness of the ionic conductor encapsulation layer is 5 - 200 nm; more preferably 5 - 50 nm. By adjusting the thickness and material of the ionic conductor encapsulation layer in the embodiment of the present application, the ionic conductivity of the cathode lithium supplement additive can be further improved.

[0064] In some embodiments, the specific surface area of the cathode lithium supplement additive is 0.1 - 5.0 m 2 / g. Controlling the specific surface area of the positive electrode lithium supplement additive within this range, on the one hand, a smaller specific surface area can reduce the contact area between the positive electrode lithium supplement additive and air, thereby reducing the sensitivity of the material to the outside world. On the other hand, a smaller specific surface area can reduce the overall viscosity of the electrode slurry and improve the processability.

[0065] The positive electrode lithium supplement additive of the embodiment of the present application can be prepared by the following example methods.

[0066] As shown in the attached Figure 4 figure, the second aspect of the embodiment of the present application provides a preparation method of a positive electrode lithium supplement additive, including the following steps:

[0067] S10. Prepare a lithium supplement material;

[0068] S20. Prepare a packaging layer on the surface of the lithium supplement material;

[0069] S30. Form a liquid film covering layer on the outer surface of the packaging layer, and the liquid film covering layer at least partially covers the cracks on the surface of the packaging layer to obtain the positive electrode lithium supplement additive.

[0070] For the preparation method of the positive electrode lithium supplement additive provided in the second aspect of the embodiment of the present application, after preparing the lithium supplement material, a packaging layer is prepared on its surface to obtain a positive electrode lithium supplement material with a core-shell structure; then a liquid film covering layer is formed at the cracks on the surface of the packaging layer to obtain the positive electrode lithium supplement additive. The preparation method provided in the embodiment of the present application has a simple process, is suitable for industrial large-scale production and application, and the prepared positive electrode lithium supplement additive has stable structure and electrochemical performance, effectively isolating the influence of harmful gas molecules in the environment on the stability of the inner core lithium supplement material, thereby ensuring the lithium supplement stability and effect of the additive. At the same time, it ensures the stability and uniform dispersion of the prepared lithium supplement additive in the electrode active slurry and the active layer, as well as good processability.

[0071] The lithium supplement material prepared in step S10 in the embodiment of the present application is the inner core of the lithium supplement material of the additive in the previous embodiment. In some embodiments, the chemical formula of the lithium supplement material is Li x A 1-y-4z / 3 B y C z O 3 / 2+x / 2 ; wherein, A is selected from Fe, Ni, and B and C represent doped metal elements with different valence states; 1≤x≤6, 0≤y≤0.5, 0≤z≤0.375, 0≤6y + 8z≤3; its preparation method can be carried out according to the existing method for preparing Li x A 1-y-4z / 3 B y C z O 3 / 2+x / 2 materials.

[0072] The lithium supplement material prepared in the embodiments of the present application has the characteristics of being sensitive to water and carbon dioxide. The prepared lithium supplement material is stored as follows to improve the stability of the lithium supplement performance of the lithium supplement material: the relative humidity of the storage environment (25°C) ≤ 5%, that is, the dew point temperature ≤ -15°C; preferably, the environmental relative humidity ≤ 1%, that is, the dew point temperature ≤ -30°C; more preferably, the environmental relative humidity ≤ 0.1%, that is, the dew point temperature ≤ -50°C; the carbon dioxide content in the storage environment (25°C) ≤ 200 ppm; preferably, the carbon dioxide content ≤ 100 ppm; more preferably, the carbon dioxide content ≤ 50 ppm.

[0073] The encapsulation layer prepared in step S20 of the embodiments of the present application is the encapsulation layer in the previous embodiments. In some embodiments, the step of preparing the encapsulation layer on the surface of the lithium supplement material includes: the step of forming at least one of an isolation encapsulation layer, an ion conductor encapsulation layer, and an electron conductor encapsulation layer on the surface of the lithium supplement material.

[0074] In some embodiments, active groups such as hydroxyl groups, aldehyde groups, carboxyl groups, and amino groups are bonded to the outer surface of the encapsulation layer formed on the surface of the lithium supplement material, which is beneficial for liquids such as moisture to adhere to the outer surface of the encapsulation layer and form a liquid film encapsulation layer at the cracks of the encapsulation layer.

[0075] In some embodiments, groups such as hydrocarbon groups, halogens, nitro groups, and ester groups are bonded to the inner surface of the encapsulation layer formed on the surface of the lithium supplement material. These groups have hydrophobic properties and can prevent moisture from the outer surface of the encapsulation layer and the environment from infiltrating into the core to damage the lithium supplement material, thereby improving the stability of the cathode lithium supplement additive.

[0076] In some embodiments, the material for preparing the isolation encapsulation layer on the surface of the lithium supplement material includes at least one of ceramics, polymer polymers, or carbon materials. Further, the thickness of the isolation encapsulation layer is 5 - 200 nm.

[0077] In some specific embodiments, when the material of the isolation encapsulation layer is a ceramic layer, the ceramic target can be sputtered and deposited on the surface of the lithium supplement material to form a ceramic isolation encapsulation layer by magnetron sputtering, but not limited to this method. Among them, the conditions of magnetron sputtering are adjusted according to the specific properties of the target material.

[0078] In another specific embodiment, when the material of the isolation encapsulation layer is a polymer polymer layer, the step of forming the polymer polymer isolation encapsulation layer can be: dispersing the lithium supplement material in a solution containing the polymer polymer, and then vacuum drying to form a dense polymer isolation encapsulation layer on the surface of the lithium supplement material. Among them, the solvent of the solution is a solvent that can uniformly disperse or dissolve the polymer polymer, such as including one or more of N-methylpyrrolidone, methanol, ethanol, isopropanol, acetone, tetrahydrofuran, and ether.

[0079] In another specific embodiment, when the material of the isolation encapsulation layer is a carbon material layer, the method for forming the carbon material isolation encapsulation layer includes the following steps: dispersing the lithium supplement material in a solution containing a carbon source, performing carbonization treatment after drying, and forming a dense carbon isolation encapsulation layer on the surface of the lithium supplement material. Among them, the carbon source can be but not limited to PEO, and other carbon sources can also be used. As long as it can form a coated carbon source layer on the surface of the lithium supplement material, it is suitable for the present invention. Specifically, for example, the lithium supplement material and PEO are mixed evenly. At 300 °C, PEO reaches its melting point and uniformly coats the surface of the lithium supplement material. The coated material is sintered in an inert atmosphere at 600 °C for 16 hours, and a dense carbon layer is formed after sintering is completed.

[0080] In some embodiments, the materials for preparing the electronic conductor encapsulation layer include at least one of a carbon material, a conductive polymer, or a conductive oxide; the carbon material, the conductive polymer, and the conductive oxide are all the materials of the electronic conductor encapsulation layer contained in the lithium supplement additive as described above; the thickness of the electronic conductor encapsulation layer is 5 - 200 nm. The methods and conditions for forming the electronic conductor encapsulation layers of the carbon material, the conductive polymer, and the conductive oxide are specifically formed according to the methods for forming the carbon material, the conductive polymer, or the conductive oxide. In some specific embodiments, the method for forming the electronic conductor encapsulation layer can be formed by chemical deposition, magnetron sputtering, or atomic layer deposition to form an isolation conductive encapsulation layer.

[0081] In some embodiments, the materials for preparing the ion conductor encapsulation layer include at least one of perovskite-type, NASICON-type, garnet-type, or polymer-type solid electrolytes; the thickness of the ion conductor encapsulation layer is 5 - 200 nm; the methods and conditions for forming the ion conductor encapsulation layer are specifically formed according to the methods for forming perovskite-type, NASICON-type, garnet-type, or polymer-type solid electrolytes.

[0082] In some embodiments, in the above step S30, the step of forming a liquid film covering layer on the outer surface of the encapsulation layer includes: placing the lithium supplement material with the encapsulation layer formed on its surface in a constant temperature and humidity condition for storage treatment to form a water film on the outer surface of the encapsulation layer. In the embodiment of the present application, the lithium supplement material with the encapsulation layer formed on its surface is stored in a constant temperature and humidity condition, so that a liquid film is formed on the outer surface of the encapsulation layer. By adjusting the dew point temperature of the storage environment, the water content on the surface can be controlled. In some specific embodiments, it is stored at a constant temperature of 25 °C and a dew point temperature of -60 °C to form a water film with a water content of about 10 ppm on the outer surface of the encapsulation layer. In some other specific embodiments, it is stored at a constant temperature of 25 °C and a dew point temperature of -10 °C to form a water film with a water content of about 2000 ppm on the outer surface of the encapsulation layer. Further preferably, a water film with a water content of 10 - 2000 ppm is formed on the outer surface of the encapsulation layer.

[0083] The third aspect of the embodiments of the present application provides a positive electrode sheet, which contains the above-mentioned positive electrode lithium supplement additive or the positive electrode lithium supplement additive prepared by the above method.

[0084] The positive electrode sheet provided by the third aspect of the embodiments of the present application contains the above-mentioned positive electrode lithium supplement additive. This additive includes a lithium supplement material core and a packaging layer coated on the outer surface, and a liquid film covering layer is also formed at the cracks on the outer surface of the packaging layer. It not only has good lithium supplement capacity, but also, due to the liquid film covering layer formed on the outer surface of the packaging layer of the positive electrode lithium supplement additive, can effectively isolate environmental gas-phase molecules such as moisture and carbon dioxide in the outside world. Therefore, it endows the positive electrode material with stable lithium supplement performance and stable performance in application. Thus, it has a good lithium supplement effect on the positive electrode sheet, high safety, and can effectively improve the electrochemical properties such as the capacity retention rate, cycle life, and safety of the positive electrode sheet.

[0085] In some embodiments, the positive electrode sheet includes a current collector and an active material layer stacked and attached. The mass percentage content of the positive electrode lithium supplement additive in the active material layer is 0.1-10%, and this ratio can just make up for the loss of active lithium during the first charging process of the battery. Since most of the lithium provided by the positive electrode lithium supplement cannot be recycled during the operation of the battery, if the addition amount of the positive electrode lithium supplement additive in the positive electrode sheet is too high, too much lithium will cause lithium ions to precipitate on the surface of the negative electrode during the operation of the battery, forming lithium dendrites; if the addition amount of the positive electrode lithium supplement additive in the positive electrode sheet is too low, the lost active lithium in the positive electrode material cannot be completely replenished, which is not conducive to improving the energy density and capacity retention rate of the battery. In some specific embodiments, the mass percentage content of the positive electrode lithium supplement additive in the active material layer of the positive electrode sheet includes but is not limited to 0.1-1%, 1-2%, 2-5%, 5-8%, 8-10%, etc.

[0086] In some embodiments, the positive electrode active material in the positive electrode sheet includes but is not limited to at least one of lithium iron phosphate, lithium cobaltate, lithium manganese iron phosphate, lithium manganate, lithium nickel cobalt manganate, and lithium nickel manganate. These positive electrode materials have a high specific capacity, which is beneficial to improving the energy density of the battery.

[0087] In some embodiments, the positive electrode current collector includes but is not limited to any one of copper foil and aluminum foil.

[0088] In some embodiments, the positive electrode active layer further includes components such as a conductive agent and a binder. The embodiments of the present application do not make specific limitations on these materials, and appropriate materials can be selected according to actual application requirements.

[0089] In some embodiments, the content of the binder in the positive electrode active layer is 2 wt% - 4 wt%. In specific embodiments, the content of the binder can be typical but non-limiting contents such as 2 wt%, 3 wt%, 4 wt%, etc. In specific embodiments, 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.

[0090] In some embodiments, the content of the conductive agent in the positive electrode active layer is 3 wt% - 5 wt%. In specific embodiments, the content of the conductive agent can be typical but non-limiting contents such as 3 wt%, 4 wt%, 5 wt%, etc. In specific embodiments, the conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60, and carbon nanotubes.

[0091] In some embodiments, the preparation process of the positive electrode sheet is as follows: Mix the positive electrode active material, positive electrode lithium supplement additive, conductive agent, and binder to obtain an electrode slurry, coat the electrode slurry on a current collector, and prepare the positive electrode sheet through steps such as drying, rolling, and die-cutting.

[0092] In the fourth aspect of the embodiments of the present application, a secondary battery is provided, and the positive electrode sheet described above is included in the secondary battery.

[0093] For the secondary battery provided in the fourth aspect of the embodiments of the present application, since the positive electrode sheet is included, and the positive electrode lithium supplement additive is added to the positive electrode sheet, it can effectively make up for the active lithium ions consumed due to the formation of the SEI film during the first charge of the battery, effectively maintain the active lithium in the battery system, and improve the capacity retention rate of the battery. Therefore, the secondary battery provided in the embodiments of the present application has a high energy density and a good capacity retention rate.

[0094] The secondary battery in the embodiments of the present application can be a lithium-ion battery or a lithium metal battery.

[0095] The negative electrode sheet, electrolyte, separator, etc. of the secondary battery in the embodiments of the present application are not specifically limited and can be applicable to any battery system.

[0096] To enable those skilled in the art to clearly understand the above-mentioned implementation details and operations of the present application, and to significantly reflect the improved performance of the positive electrode lithium supplement additive and its preparation method, positive electrode sheet, and secondary battery in the embodiments of the present application, the following technical solutions are illustrated through multiple embodiments.

[0097] Example 1

[0098] A cathode lithium supplement additive, the preparation of which includes the steps of: drying the Li2NiO2 cathode lithium supplement additive coated with a carbon layer in a vacuum at 100 °C for 24 h, cooling to room temperature, then placing it in an air environment at 25 °C with a dew point temperature of -60 °C, and standing still for 24 h to obtain the cathode lithium supplement additive of Example 1.

[0099] Example 2

[0100] A cathode lithium supplement additive, the preparation of which includes the steps of: drying the Li2NiO2 cathode lithium supplement additive coated with a carbon layer in a vacuum at 100 °C for 24 h, cooling to room temperature, then placing it in an air environment at 25 °C with a dew point temperature of -55 °C, and standing still for 24 h to obtain the cathode lithium supplement additive of Example 2.

[0101] Example 3

[0102] A cathode lithium supplement additive, the preparation of which includes the steps of: drying the Li2NiO2 cathode lithium supplement additive coated with a carbon layer in a vacuum at 100 °C for 24 h, cooling to room temperature, then placing it in an air environment at 25 °C with a dew point temperature of -10 °C, and standing still for 24 h to obtain the cathode lithium supplement additive of Example 3.

[0103] Comparative Example 1

[0104] A cathode lithium supplement additive, the preparation of which includes the steps of: drying the Li2NiO2 cathode lithium supplement additive coated with a carbon layer in a vacuum at 100 °C for 24 h, cooling to room temperature, then placing it in an air environment at 25 °C with a dew point temperature of 0 °C, and standing still for 24 h to obtain the cathode lithium supplement additive of Comparative Example 1.

[0105] Comparative Example 2 A cathode lithium supplement additive, the preparation of which includes the steps of: drying the Li2NiO2 cathode lithium supplement additive coated with a carbon layer in a vacuum at 100 °C for 24 h, cooling to room temperature, then placing it in the daily environment at 25 °C, and standing still for 24 h to obtain the cathode lithium supplement additive of Comparative Example 2.

[0106] Furthermore, in order to verify the progressiveness of the embodiments of the present application, a Karl Fischer moisture analyzer was used to test the moisture content of the cathode lithium supplement additives prepared in Examples 1 to 3 and Comparative Examples 1 to 2, and the test results are as follows Figure 5 shown. Among them, the moisture content of Example 1 is 10.58 ppm, the moisture content of Example 2 is 32.26 ppm, the moisture content of Example 3 is 2000 ppm, the moisture content of Comparative Example 1 is 6023 ppm, and the moisture content of Comparative Example 2 is 10735 ppm.

[0107] In addition, according to the ratio of the positive electrode lithium supplement additive: SP: PVDF = 95:2:3, the positive electrode lithium supplement additives provided in the above Examples 1 to 3 and Comparative Examples 1 to 2 were respectively mixed with SP: PVDF. The mixing method was ball milling, and the ball milling time was 60 min; the rotation speed was set at 30 HZ. After operations of homogenization - coating - drying - sheet cutting, positive electrode sheets were respectively prepared. Coin - type lithium - ion batteries were assembled in an inert - atmosphere glove box in the order of lithium metal sheet - separator - electrolyte - positive electrode sheet. For the lithium - ion batteries prepared in Examples 1 to 3 and Comparative Examples 1 to 2, they were charged at 0.05 C to 4.3 V and held at 4.3 V until the current was less than 0.01 C; their initial charge specific capacity was tested.

[0108] The test results are as shown in the Figure 6 appendix. Among them, the lithium - supplement specific capacity of the positive - electrode lithium - supplement additive in Example 1 was 389 mAh / g, the lithium - supplement specific capacity of the positive - electrode lithium - supplement additive in Example 2 was 403 mAh / g, the lithium - supplement specific capacity of the positive - electrode lithium - supplement additive in Example 3 was 400 mAh / g, the lithium - supplement specific capacity of the positive - electrode lithium - supplement additive in Comparative Example 1 was 366 mAh / g, and the lithium - supplement specific capacity of the positive - electrode lithium - supplement additive in Comparative Example 2 was only 309 mAh / g. It can be seen that when the water content on the surface of the positive - electrode lithium - supplement additive is too high (i.e., the liquid - film covering layer is too thick), the lithium - supplement effect of the material will be reduced; when the water content on the surface of the positive - electrode lithium - supplement additive is 10 - 2000 ppm, the positive - electrode lithium - supplement additive has a better lithium - supplement effect.

[0109] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present application shall be included within the protection scope of the present application.

Claims

1. A cathode lithium supplement additive, characterized in that, The positive electrode lithium supplement additive includes a lithium supplement material core and a packaging layer coated on the outer surface of the lithium supplement material core, and a liquid film covering layer is at least partially formed at the crack on the outer surface of the packaging layer.

2. The cathode lithium supplement additive according to claim 1, characterized in that, The liquid film covering layer is selected from a water film.

3. The cathode lithium supplement additive according to claim 2, characterized in that, The water content on the outer surface of the packaging layer is 10-2000 ppm; and / or, hydrophilic groups are bonded to the outer surface of the packaging layer; and / or, hydrophobic groups are bonded to the inner surface of the packaging layer.

4. The cathode lithium supplement additive according to claim 3, characterized in that, The hydrophilic groups include at least one of hydroxyl group, aldehyde group, carboxyl group, and amino group; and / or, the hydrophobic groups include at least one of hydrocarbon group, halogen, nitro group, and ester group.

5. The cathode lithium supplement additive according to any one of claims 1 to 4, characterized in that, The lithium supplement material core includes Li x A 1-y-4z / 3 B y C z O 3 / 2+x / 2 ; wherein, A is selected from Fe or Ni, and B and C represent doped metal elements with different valence states; 1≤x≤6, 0≤y≤0.5, 0≤z≤0.375, 0≤6y + 8z≤3; and / or, the lithium supplement material core is primary particles and / or secondary particles; and / or, the packaging layer includes at least one of an isolation packaging layer, an ion conductor packaging layer, and an electron conductor packaging layer.

6. The cathode lithium supplement additive according to claim 5, characterized in that, The doping element B includes at least one of Al, Co, Ni, Fe, Ga, and La; and / or, the doping element C includes at least one of Mn, Ge, Sn, Ti, Si, V, and Zr; and / or, the particle size of the primary particles is 50 nm - 500 nm, and the particle size of the secondary particles is 0.1 μm - 10 μm.

7. A preparation method of a cathode lithium supplement additive, characterized in that, It includes the following steps: Prepare the lithium supplement material; Prepare a packaging layer on the surface of the lithium supplement material; Form a liquid film covering layer on the outer surface of the packaging layer, and the liquid film covering layer at least partially covers the crack on the surface of the packaging layer to obtain the positive electrode lithium supplement additive.

8. The preparation method of the cathode lithium supplement additive according to claim 7, characterized in that, The step of forming a liquid film covering layer on the outer surface of the packaging layer includes: placing the lithium supplement material with the packaging layer formed on its surface in a constant temperature and humidity condition for preservation treatment to form a water film on the outer surface of the packaging layer.

9. A positive electrode sheet, characterized in that, The positive electrode sheet contains the positive electrode lithium supplement additive according to any one of claims 1-6, or contains the positive electrode lithium supplement additive prepared by the method according to any one of claims 7-8.

10. The positive electrode sheet according to claim 9, characterized in that, The positive electrode sheet includes a current collector and an active material layer which are laminated and adhered, and the mass percentage content of the positive electrode lithium supplement additive in the active material layer is 0.1-10%.

11. A secondary battery, characterized in that, The secondary battery contains the positive electrode sheet according to any one of claims 9-10.

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