Positive electrode active material, secondary battery, and electric device

Abstract

The application discloses a positive electrode active material, a secondary battery and a power utilization device. The positive electrode active material comprises a core material and a shell layer distributed on the surface of the core material, the shell layer comprises at least one of an aluminum-containing oxide and a boron-containing oxide, and a fluoride. The aluminum-containing oxide and / or the boron-containing oxide can react with residual lithium compounds on the surface of the core material, thereby preventing the residual lithium compounds from reacting with a binder in a positive electrode slurry during homogenization, and helping to maintain the stability of the structure of the positive electrode active material. The fluoride can enable the shell layer to resist the attack of hydrofluoric acid in a fluorine-containing electrolyte, not only can maintain the stability of the structure of the positive electrode active material, but also can improve the passing property of electrons and ions, and improve the performance of the secondary battery, such as the specific capacity, the high-temperature cycle capability and the like.

Description

Technical Field

[0001] This application belongs to the field of battery technology, specifically relating to a positive electrode active material, a secondary battery, and an electrical device. Background Technology

[0002] Secondary batteries, especially lithium-ion batteries, are widely used in electric vehicles due to their high energy density and long cycle life. Common positive electrode active materials for lithium-ion batteries include layered high-nickel ternary materials. However, the presence of residual alkalis (such as LiOH and Li₂CO₃) on the surface of these materials can easily react with the adhesives in the positive electrode, causing the adhesives to lose their bonding properties and affecting the transport of electrons and ions within the positive electrode. Furthermore, because layered high-nickel ternary materials are in direct contact with the electrolyte, they are susceptible to corrosion from fluorine-containing substances in the electrolyte, leading to capacity decay and reduced cycle performance in the secondary battery. Summary of the Invention

[0003] Objective of the Invention: This application provides a positive electrode active material, a secondary battery, and an electrical device. The positive electrode active material can reduce surface residual alkali and effectively resist corrosion from hydrofluoric acid in the electrolyte, solving the technical problems of battery capacity decay and cycle performance degradation.

[0004] Technical Solution: This application provides a positive electrode active material. The positive electrode active material includes a core material and a shell layer distributed on the surface of the core material. The shell layer includes at least one of aluminum oxide and boron oxide, and a fluoride.

[0005] Optionally, in embodiments of this application, the aluminum oxide includes at least one of Al2O3, LiAlO2, LiAl5O8, Li2Al4O7, Li3AlO3, and Li5AlO4.

[0006] Optionally, in the embodiments of this application, the boron-containing oxide includes at least one of B2O3, LiBO2, LiB5O8, Li2B4O7, Li3BO3, and Li5BO4.

[0007] Optionally, in embodiments of this application, the fluoride includes at least one of aluminum-containing fluorides and boron-containing fluorides, wherein the aluminum-containing fluoride includes AlF3, LiAlF4, and LiAl5F. 16 At least one of LiAl2F7, Li3AlF6, and Li5AlF8, and boron-containing fluorides including BF3, LiBF4, and LiB5F... 16 At least one of LiB2F7, Li3BF6 and Li5BF8.

[0008] Optionally, in embodiments of this application, the core material includes Lix Ni a Co b Mn c Me d O₂, where 0.8 ≤ x ≤ 1.1, 0.5 < a < 1, 0 ≤ b < 0.3, 0 < c < 0.4, 0 < d < 0.2, a + b + c + d = 1, and Me is selected from one or more of V, Al, Zr, Sr, Ti, B, Mg, Sn, and W.

[0009] Optionally, in the embodiments of the present application, based on the mass of the positive electrode active material, the content of the fluoride is 0.02% to 2%.

[0010] Optionally, in the embodiments of the present application, the thickness of the shell layer is 0.1 nm to 20 nm.

[0011] Optionally, in the embodiments of the present application, the positive electrode active material satisfies the following characteristics:

[0012] I = BET × TD × (Dv99 - Dv50), 2 ≤ I ≤ 20; where

[0013] BET is the specific surface area of the positive electrode active material, with the unit of m 2 / g, 0.2 ≤ BET ≤ 5.5; <00001​​​​​​​​​​​​​​​​​​Beneficial Effects: Compared with the prior art, the technical solution of this application embodiment achieves the following beneficial effects: The core material of the positive electrode active material in this application embodiment has a shell layer distributed on its surface. The shell layer includes at least one of aluminum oxide and boron oxide, as well as fluoride. The aluminum oxide and / or boron oxide can react with residual alkali on the surface of the core material to eliminate residual alkali as much as possible, thereby preventing residual alkali from reacting with the adhesive in the positive electrode sheet. This is beneficial to maintaining the stability of the positive electrode active material structure and also beneficial to the transport of lithium ions and electrons in the positive electrode sheet. The presence of fluoride enables the shell layer to resist the attack of hydrofluoric acid in the fluorinated electrolyte. While maintaining the stability of the positive electrode active material structure, it can also improve the conductivity of electrons and ions, and improve the specific capacity, high-temperature cycle capability, and gas generation suppression capability of the secondary battery. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a field emission scanning electron microscope image of the positive electrode active material provided in Example 1 of this application. Detailed Implementation

[0022] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0023] Ternary cathode materials (such as Li) that constitute the positive electrode of a secondary battery x Ni a Co b Mn c Me d O2, etc., are derived from ternary material precursors (such as Ni). a Co b Mn cThe ternary cathode material is obtained by reacting (OH)₂, etc., with lithium salts and metal sources (also known as Me sources). Therefore, residual alkali (such as LiOH and Li₂CO₃) will exist on the surface of this material. This residual alkali will react with the adhesive of the cathode slurry (used to coat the cathode current collector to form the cathode sheet) during the homogenization process, resulting in gelation. This affects the coating quality of the cathode slurry and thus the stability of the cathode sheet structure. Furthermore, the electrolyte in the battery system generally contains a small amount of fluorine, and can be called a fluorinated electrolyte. This fluorinated electrolyte is corrosive and will damage the high-nickel ternary cathode material, causing a decrease in the capacity and cycle performance of the secondary battery.

[0024] To address the aforementioned technical problems, the positive electrode active material provided in this application includes a core material and a shell layer distributed on the surface of the core material. The shell layer includes at least one of aluminum oxide and boron oxide, as well as a fluoride. The aluminum oxide and / or boron oxide are formed by reacting an aluminum source and / or a boron source with residual alkali at high temperatures (e.g., 300°C to 700°C). This reduces the residual alkali content on the surface of the core material, preventing the residual alkali from escaping from the shell layer, thereby inhibiting the reaction between the residual alkali and the adhesive of the secondary battery's positive electrode sheet. In some embodiments, the aluminum oxide and / or boron oxide include oxide-type fast ion conductors, giving the secondary battery better conductivity, specific capacity, and cycle performance. Furthermore, the fluoride in the shell layer allows the positive electrode active material to retain more lithium extraction / intercalation sites and pre-activates the surface of the positive electrode active material, reducing initial diffusion resistance and improving rate performance. Additionally, the fluoride in the shell layer further weakens the corrosive effect of hydrogen fluoride on the core material, reducing gas production in the secondary battery and improving its cycle performance. In some embodiments, the fluoride includes a fluoride-type fast ion conductor component, which resists hydrofluoric acid attack without affecting the permeability of electrons and ions and the conductivity of the shell itself, thereby further improving the electrochemical performance of the battery.

[0025] Optionally, in some embodiments of this application, the aluminum-containing oxide includes, but is not limited to, at least one of Al2O3, LiAlO2, LiAl5O8, Li2Al4O7, Li3AlO3, and Li5AlO4. Al2O3 is formed by the oxidation of an aluminum source during high-temperature processing. Other aluminum-containing oxides besides Al2O3 belong to the lithium aluminum oxide type fast ion conductor, which can be generated by the reaction of a portion of Al2O3 with residual alkali on the surface of the core material. Taking LiAlO2 as an example, the reaction equation is shown below. The reaction equations for other lithium aluminum oxide type fast ion conductors are similar.

[0026] Al2O3+2LiOH=2LiAlO2+H2O↑;

[0027] Al2O3+Li2CO3=2LiAlO2+CO2↑.

[0028] In the embodiments of this application, lithium aluminum oxide type fast ion conductors can optimize the ion conduction performance of positive electrode active materials and reduce the impedance of positive electrode materials.

[0029] Optionally, in some embodiments of this application, the boron-containing oxides include, but are not limited to, at least one of B2O3, LiBO2, LiB5O8, Li2B4O7, Li3BO3, and Li5BO4. B2O3 is formed by the oxidation of a boron source during high-temperature processing. Other boron-containing oxides besides B2O3 belong to the lithium boron oxide type fast ion conductors, which can be generated by the reaction of a portion of B2O3 with residual alkali on the surface of the core material. Taking LiBO2 as an example, the reaction equation is as follows; the reaction equations for other lithium boron oxide type fast ion conductors are similar.

[0030] B₂O₃ + 2LiOH = 2LiBO₂ + H₂O↑;

[0031] B2O3+Li2CO3=2LiBO2+CO2↑.

[0032] In the embodiments of this application, the lithium boron oxide type fast ion conductor can accelerate the transmission rate of lithium ions through the positive electrode active material, optimize the impedance of the positive electrode active material, and is beneficial to the rate performance of the secondary battery.

[0033] In the embodiments of this application, aluminum oxide and boron oxide can exist separately in the positive electrode active material, or they can coexist in the positive electrode active material. Experiments have shown that the performance of secondary batteries made with positive electrode active materials containing both aluminum oxide and boron oxide is better than that with positive electrode active materials containing only aluminum oxide or only boron oxide. The reason may be that aluminum and boron belong to the same group of elements. The compounds formed by aluminum have stable interfacial properties and can resist the attack of HF, while the compounds formed by boron have faster ion conductivity. The two can play a synergistic role in improving the cycle stability and rate performance of the battery. In the embodiments of this application, fluorides include at least one of aluminum fluorides and boron fluorides. Fluorides can be generated by reacting aluminum oxide and / or boron oxide with a fluorine source, respectively. As one of the components of the shell, fluorides can act as fast ion conductors to improve the permeability of electrons and ions, and can also resist the corrosion of hydrofluoric acid in the fluorinated electrolyte after the positive electrode is assembled into the secondary battery, thereby maintaining the integrity of the positive electrode structure.

[0034] Aluminum-containing fluorides can be generated by reacting aluminum oxides with a fluorine source, including but not limited to AlF3, LiAlF4, and LiAl5F. 16, at least one of LiAl2F7, Li3AlF6 and Li5AlF8.

[0035] The boron-containing fluoride can be formed by reacting a boron-containing oxide with a fluorine source, including but not limited to BF3, LiBF4, LiB5F 16 , at least one of LiB2F7, Li3BF6 and Li5BF8.

[0036] In the embodiments of the present application, the core material includes Li x Ni a Co b Mn c Me d O2. Where 0.8 ≤ x ≤ 1.1, 0.5 < a < 1, 0 ≤ b < 0.3, 0 < c < 0.4, 0 < d < 0.2, and a + b + c + d = 1. Me is selected from one or more of V, Al, Zr, Sr, Ti, B, Mg, Sn and W.

[0037] In the embodiments of the present application, based on the mass of the positive electrode active material, the content of the aluminum-containing oxide or the boron-containing oxide is 0.02% to 1%, can also be 0.05% to 0.8%, can also be 0.1% to 0.5%, and can also be 0.2% to 0.3%. If the aluminum-containing oxide or the boron-containing oxide in the shell layer is too little, it cannot play the role of reducing residual alkali. If it is too much, the specific capacity of the positive electrode active material will be reduced.

[0038] In the embodiments of the present application, based on the mass of the positive electrode active material, the content of the fluoride can be 0.02% to 2%, can also be 0.1% to 1.5%, and can also be 0.5% to 1%. When the fluoride content in the shell layer of the positive electrode active material is within the above range, it can effectively resist the attack of HF in the fluoride-containing electrolyte, ensuring the structural stability of the positive electrode active material during the battery cycle. In addition, the fluoride content within the above range can ensure that the shell layer of the positive electrode active material has appropriate microporous channels, ensuring the permeability of electrons and ions, which is beneficial to improving the rate performance of the battery.

[0039] In the embodiments of this application, the thickness of the shell layer can be any value from 0.1 nm to 20 nm. In other embodiments, the thickness of the shell layer can be from 0.2 nm to 18 nm, or from 0.3 nm to 15 nm, or from 0.5 nm to 12 nm, or from 0.8 nm to 10 nm, or further from 1 nm to 9 nm, or from 2 nm to 8 nm, or from 5 nm to 6 nm. The thickness of the shell layer should not be too thin, otherwise the oxides in the shell layer cannot react completely with the residual alkali, and the unreacted residual alkali can easily escape from the shell layer and react with the adhesive in the positive electrode sheet, thereby damaging the structure of the positive electrode sheet. The thickness of the shell layer should also not be too thick, otherwise, although the shell layer can effectively reduce the content of residual alkali, the permeability of ions and electrons will be too poor, thus affecting the conductivity. Therefore, the thickness of the shell layer is preferably within the range of 0.1 nm to 20 nm.

[0040] In the embodiments of this application, after the reaction of oxides (Al2O3 and / or B2O3) with the residual alkali on the surface of the core material, the final LiOH content on the surface of the core material is less than 3000 ppm and the Li2CO3 content is less than 4000 ppm. This can significantly reduce the reaction between residual alkali and the adhesive in the positive electrode sheet, which is beneficial to maintaining the integrity of the positive electrode sheet structure.

[0041] In the embodiments of this application, the positive electrode active material is a porous material containing micropores, and the material satisfies the following characteristics: I=BET×TD×(Dv99-Dv50), 2≤I≤20.

[0042] Where BET is the specific surface area of ​​the positive electrode active material, in m². 2 / g, and 0.2≤BET≤5.5. For example, the endpoints of this range can also be 0.3, 0.45, 0.5, 0.54, 0.99, 1.0, 1.5, 2.0, 2.5, 3.0, 3.25, 3.5, 4.0, 4.5, or 5.0m. 2 The range of any one or two items in / g. The BET value can be tested using a specific surface area analyzer. By controlling the specific surface area of ​​the positive electrode active material, the contact area and degree of contact between the positive electrode active material and the fluorinated electrolyte can be controlled, thereby controlling the extent of the electrochemical reaction.

[0043] TD is the tap density of the positive electrode active material at 60 MPa, expressed in g / cm³. 3The range is 1.2 ≤ TD ≤ 3.0. The endpoints of this range can also be any one or any two of 1.91, 1.97, 2.05, and 2.07. Dv99 is the particle size, in μm, corresponding to a cumulative volume distribution percentage of 99% for the positive electrode active material, and 3 ≤ Dv99 ≤ 15.5. For example, the endpoints of this range can also be any one or any two of 3.2, 5, 5.9, 6, 7, 8, 9, 10, 10.4, 11, 12, 13, 14, or 15.3. Dv50 is the particle size, in μm, corresponding to a cumulative volume distribution percentage of 50% for the positive electrode active material, and 1.5 ≤ Dv50 ≤ 7.5. For example, the end values ​​of this range can also be any one or any two of 2.0, 2.5, 2.6, 3.0, 3.5, 4.0, 4.5, 5.0, 5.1, 5.5, 6.0, or 7.0. Dv99 and Dv50 are tested using a Malvern laser particle size analyzer. Larger particle sizes result in smaller contact areas between particles, lower interparticle friction, and smaller interparticle volume, making it easier to achieve a higher tap density. This means more particles can be packed per unit volume. However, excessively high tap density can affect the wetting performance of the electrode and increase battery polarization.

[0044] This application also provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and a fluorinated electrolyte. The positive electrode includes a positive current collector and a positive active material layer formed on the surface of the positive current collector. The positive active material layer comprises the aforementioned positive active material.

[0045] The positive current collector in the positive electrode sheet is an aluminum foil with a thickness of 9 μm to 12 μm. The thickness of the aluminum foil can be 9.5 μm, 10 μm, 10.5 μm, 11 μm, or 11.5 μm. The preparation method of the positive electrode sheet includes: mixing the positive active material, adhesive, and conductive agent and homogenizing them to form a positive electrode slurry; coating the positive electrode slurry onto the surface of the positive current collector; and obtaining the positive electrode sheet after drying, rolling, slitting, and cutting.

[0046] The negative electrode material used in the negative electrode sheet can be one or more of the following: artificial graphite, natural graphite, mesophase carbon microspheres, amorphous carbon, lithium titanate, or silicon-carbon alloy.

[0047] The membrane can be a polypropylene membrane with a thickness of 9–18 μm. Any porous membrane with electrochemical and mechanical stability can be used as a membrane.

[0048] Fluorinated electrolytes include lithium salts and organic solvents. The lithium salt may include lithium hexafluorophosphate (LiPF6) or lithium tetrafluoroborate (LiBF4), etc. The organic solvent may contain linear esters and cyclic esters, with the linear esters comprising a greater mass percentage than the cyclic esters. Cyclic esters may contain ethylene carbonate (EC) and / or propylene carbonate (PP). Linear esters may contain at least one of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), or diethyl carbonate (DEC). In some embodiments, the solvent of the electrolyte includes EC, EMC, and DEC, and the mass ratio of EC, EMC, and DEC may be (10–25):(10–25):(51–75). In addition to the above components, fluorinated electrolytes also contain trace amounts of H2O and dissolved O2. Taking lithium hexafluorophosphate (LiPF6) as an example, LiPF6 decomposes into LiF and PF5. PF5 reacts with trace amounts of H2O to generate hydrofluoric acid (HF), as shown in the equation: PF5 + H2O → 2HF + PF3O. The reaction equation for lithium tetrafluoroborate (LiBF4) is similar. When the positive electrode is assembled into a secondary battery, since the shell of the positive electrode contains both oxides and fluorides, the fluorides can prevent the reaction between HF and the oxides in the shell, thus enabling the shell to resist HF corrosion. The oxides further act as a barrier, thereby helping to maintain the stability of the shell and the positive electrode structure.

[0049] The above-mentioned method for preparing a secondary battery includes: first, winding the above-mentioned positive electrode, negative electrode and separator, assembling them into a housing, sealing the housing with a cover plate, injecting a fluorine-containing electrolyte into the closed cavity formed by the housing and the cover plate, and then encapsulating it to make a secondary battery.

[0050] This application also provides an electrical device that includes the aforementioned secondary battery, which serves as the power supply for the device. The electrical device includes, but is not limited to, backup power supplies, electric vehicles, electric motorcycles, or electric bicycles.

[0051] This application also provides a method for preparing a secondary battery, which includes the following steps:

[0052] (1) The ternary material precursor, lithium salt and Me source are mixed and sintered in an oxygen atmosphere to obtain the first material;

[0053] (2) The first material, boron source and aluminum source are dispersed in a solvent, mixed and dried to obtain a precursor. The precursor is then subjected to high temperature treatment and reacted with a fluorine source to obtain a positive electrode active material.

[0054] (3) After mixing the positive electrode active material, adhesive and conductive agent, homogenize the mixture to make a positive electrode slurry. Coat the positive electrode slurry onto the surface of the positive electrode current collector, and then dry, roll, slit and cut the slab to obtain the positive electrode sheet.

[0055] (4) Assemble the positive electrode sheet, separator, and negative electrode sheet, inject a fluorine-containing electrolyte, and then seal to obtain a secondary battery.

[0056] Among them, in step (1), the components of the ternary material precursor include Ni a Co b Mn c (OH)2, where 0.5 < a < 1, 0 ≤ b < 0.3, 0 < c < 0.4. The lithium salt includes one or both of lithium carbonate and lithium hydroxide. The molar percentage of the lithium salt to the ternary material precursor is 95 - 110 mol%. The Me source is selected from one or more of the oxides or hydroxides of V, Al, Zr, Sr, Ti, B, Mg, Sn, and W. The molar percentage of the Me source to the ternary material precursor is 0.01 - 1 mol%.

[0057] In step (1), the sintering temperature is 600 - 1000 °C, the time is 8 - 24 hours, and the sintering is carried out in an oxygen atmosphere. The sintering process is a process in which the ternary material precursor, lithium salt, and Me source react to form the first material. The raw materials for producing the first material include the lithium salt and the Me source. After the reaction, residual alkali (such as LiOH and Li2CO3, etc.) will exist on the surface of the first material. Li2O will form LiOH and Li2CO3. This residual alkali can react with the adhesive in the positive electrode sheet, thus affecting the structure of the positive electrode sheet. Therefore, it is necessary to remove this residual alkali in the subsequent steps.

[0058] In step (1), the first material includes Li x Ni a Co b Mn c Me d O2, where 0.8 ≤ x ≤ 1.1, 0.5 < a < 1, 0 ≤ b < 0.3, 0 < c < 0.4, 0 < d < 0.2, and a + b + c + d = 1.

[0059] In step (2), the boron source includes one or more of boric acid, boron trioxide, and boron hydroxide. The aluminum source includes one or more of aluminum nitrate, trimethylaluminum, aluminum carbonate, aluminum hydroxide, and lithium metaaluminate. The fluorine source includes lithium fluoride. The solvent includes alcohol solvents, such as methanol, absolute ethanol, propanol, etc. The purpose of dispersion and mixing is to make the ternary cathode material, boron source, and aluminum source mix evenly, which is beneficial for the subsequent high-temperature treatment reaction and reaction with the fluorine source.

[0060] In step (2), based on the mass (or molar amount) of the first material, the molar percentage of the boron source can be from 0.03 mol% to 1.61 mol%, the molar percentage of the aluminum source can be from 0.02 mol% to 1.28 mol%, and the molar percentage of the fluorine source can be from 0.07 mol% to 7.8 mol%. The relative molar ratio of the fluorine source needs to be less than the molar ratio of the boron source and the aluminum source (1 mol of Al or B consumes at least 3 mol of F) to prevent all the oxides in the positive electrode active material from being converted into fluorides. The shell of the positive electrode active material needs to have both oxides and fluorides to achieve better battery performance; if only oxides or fluorides are present, it will not be conducive to improving battery performance.

[0061] In step (2), based on the mass of the positive electrode active material, the content of aluminum oxide is 0.02% to 1%, the content of boron oxide is 0.02% to 1%, the content of fluoride is 0.02% to 2%, and the content of core material is 96% to 99.94%. The sum of the contents of aluminum oxide, boron oxide, fluoride, and core material is 100%.

[0062] In step (2), the solvent is an alcohol solvent, such as anhydrous ethanol.

[0063] In step (2), the high-temperature treatment temperature is 300℃ to 700℃. The high-temperature treatment time is 3h to 12h. The high-temperature treatment process is the process of forming a shell on the surface of the first material (as the core material). During the high-temperature treatment, the boron source reacts to generate B2O3, and the aluminum source is also decomposed to generate Al2O3. Since there is residual alkali on the surface of the core material, Al2O3 and B2O3 will further react with LiOH and Li2CO3 to generate oxide-type fast ion conductors (such as m1Li2O·n1Al2O3, m2Li2O·n2B2O3, etc.). These oxide-type fast ion conductors have more voids and interstitial ions and other defects. Interstitial spaces are atomic vacancies that appear where atoms should be filling, while interstitial ions are ions existing in the interstices of an ideal crystal lattice. Under the influence of an electric field, a large number of randomly arranged ions will move from one position to another, thus exhibiting good conductivity and overcoming the shortcomings of simple oxide coatings (such as Al2O3 and / or B2O3) in terms of poor electron and ion permeability. After generating oxide-type fast ion conductors, these conductors then react with added fluorine sources to obtain fluoride-type fast ion conductors (such as m3LiF·n3AlF3, m4LiF·n4BF3, etc.). Because the relative molar ratio of fluorine sources is less than that of boron and aluminum sources, the shell contains both oxide-type fast ion conductors and fluoride-type fast ion conductors. The coexistence of these two types enables the positive electrode active material to have suitable pore size and tap density, so as not to affect the conductivity. It also enables the positive electrode active material to resist the corrosion of hydrofluoric acid in the fluorine-containing electrolyte after being assembled into the secondary battery, thereby maintaining the integrity of the positive electrode structure.

[0064] In step (2), the temperature at which the fluorine source reacts after high-temperature treatment is 50℃±5℃ lower than that between 300℃ and 700℃. The reaction time with the fluorine source is 5-6 hours. The fluorine source includes ammonium fluoride and / or lithium fluoride.

[0065] In step (2), since the residual alkali reacts with the oxide to form an oxide-type fast ion conductor, the residual alkali on the surface of the core material is basically completely reacted. As a result, the content of LiOH on the surface of the core material is less than 3000 ppm, and the content of Li2CO3 on the surface is less than 4000 ppm. Such a low concentration of residual alkali will not react extensively with the adhesive of the positive electrode sheet. When coating the positive electrode current collector or after assembling into a finished battery, there will be no problem of LiOH and Li2CO3 reacting with the adhesive in the positive electrode sheet, causing the adhesive to coagulate and gel, thus preventing damage to the structure of the positive electrode sheet.

[0066] In step (3), the adhesive includes one or more of carboxymethyl cellulose, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl butyral, and polyvinyl alcohol. The conductive agent includes one or more of graphite, superconducting carbon, acetylene black, carbon black, carbon nanotubes, graphene, and carbon nanofibers.

[0067] The following description, in conjunction with specific embodiments and comparative examples, illustrates the process. The secondary battery products obtained from the embodiments and comparative examples of this application were subjected to the following tests, the specific test methods of which are as follows:

[0068] 1) Gas production performance test of secondary batteries at 70℃ under full charge. Since gas production causes volume expansion of the secondary battery, the gas production performance can be characterized by the volume expansion rate parameter. The specific test method includes the following steps: Take a secondary battery sample, charge it to 4.3V, and store it in a 70℃ constant temperature chamber. Measure the volume of the secondary battery every day using the water displacement method. Fully charge it with a 1C secondary current every 7 days. The calculation method is as follows: Measure the initial volume V0 using the water displacement method. Remove the battery from the constant temperature chamber every day, cool it, and measure the battery volume V using the water displacement method. (V / V0-1)×100% is the high-temperature storage volume expansion rate.

[0069] 2) Cyclic performance test of secondary batteries at 60℃ and 5C / 5C. The specific test method includes: after the secondary battery is placed in a constant temperature room at 60℃±2℃ for 120 minutes, it is discharged to 2.8V at 5C rate; after being placed for 5 minutes, it is charged to 4.2V at 5C rate with constant current and constant voltage, with a cutoff current of 0.05C, and placed for 5 minutes. The above steps are repeated until the capacity retention rate of the secondary battery is as low as 70%.

[0070] 3) The 1C discharge capacity test of the secondary battery is specifically conducted as follows: the secondary battery is placed in a constant temperature room at 25℃±2℃ and left to stand for 120 minutes before being charged to 4.2V at a constant current and constant voltage at a 1C rate, with a cutoff current of 0.05C; after standing for 5 minutes, it is discharged to 2.8V at a constant current at a 1C rate.

[0071] 4) The discharge capacity retention rate test of the secondary battery at a high rate of 30C is as follows: In a constant temperature room of 25℃±2℃, the secondary battery is charged to 4.2V at a constant current and constant voltage rate of 0.33C, and the cutoff current is 0.05C; after standing for 5 minutes, it is discharged to 2.8V at a constant current rate of 30C; the discharge capacity retention rate at a high rate of 30C = 30C discharge capacity / 0.33C discharge capacity * 100%.

[0072] The present application will be explained below with reference to specific embodiments.

[0073] Example 1

[0074] This embodiment provides a method for preparing a secondary battery, which includes the following steps:

[0075] (1) 91.99 g (1.0 mol) of ternary material precursor Ni 0.6 Co 0.2 Mn 0.2 (OH)2, 39.90g (0.54mol) lithium carbonate (Li2CO3, as lithium salt), 3.90g (0.0156mol) tungsten oxide (WO), and 1.31g (0.011mol) zirconium oxide (ZrO) (both as Me source) were thoroughly mixed and placed in a reactor containing an oxygen atmosphere and sintered at 900℃ for 12h to obtain 100g (1mol) of the first material.

[0076] The molecular formula of the first material is Li. 1.08 Ni 0.585 Co 0.195 Mn 0.195 W 0.015 Zr 0.01 O2.

[0077] (2) 100 g (1.00 mol) of the first material (as the core material), 0.6 g (0.0097 mol) of boric acid (as the boron source), and 0.6 g (0.0077 mol) of aluminum hydroxide (as the aluminum source) were dispersed in 100 mL of anhydrous ethanol (as the solvent). After stirring at room temperature for 8 h, the mixture was dried in an oven at 80 °C for 12 h to obtain the precursor. The precursor was reacted at 450 °C for 6 h. After cooling, the above product was reacted with 0.37 g (0.01 mol) of ammonium fluoride and 0.78 g (0.03 mol) of lithium fluoride (together as the fluorine source) at 400 °C for 6 h. After cooling to room temperature, the mixture was ground and sieved to obtain the positive electrode active material.

[0078] The positive electrode active material comprises a core material and a shell layer distributed on the surface of the core material. The core material is composed of Li. 1.08 Ni 0.585 Co 0.195 Mn 0.195 W 0.015 Zr 0.01 O2. The shell consists of aluminum oxides (Al2O3 and LiAlO2), boron oxides (B2O3 and LiBO2), and fluorides (AlF3, LiAlF4, BF3, and LiBF4).

[0079] The positive electrode active material has a Dv50 of 2.6 μm, a Dv99 of 5.9 μm, and a specific surface area of ​​0.99 m². 2 / g, tap density is 2.07g / cm³ 3 .

[0080] The process of the precursor reacting at 450℃ is the process of shell formation on the surface of the core material. During this process, the boron source is oxidized to B₂O₃, and the aluminum source is oxidized to Al₂O₃. B₂O₃ reacts with residual alkalis (such as LiOH and Li₂CO₃) on the surface of the core material to form lithium boron oxide-type fast ion conductors (such as LiBO₂), and Al₂O₃ reacts with residual alkalis (such as LiOH and Li₂CO₃) on the surface of the core material to form lithium aluminum oxide-type fast ion conductors (LiAlO₂). A portion of the lithium boron oxides further reacts with a fluorine source to form lithium boron fluoride-type fast ion conductors (LiBF₄), while the remaining portion remains as lithium boron oxides. Similarly, a portion of the aluminum boron oxides further reacts with a fluorine source to form lithium aluminum fluoride-type fast ion conductors (LiAlF₄), while the remaining portion remains as lithium aluminum oxides. Therefore, the entire shell layer includes Al₂O₃, B₂O₃, LiAlO₂, LiBO₂, AlF₃, BF₃, LiAlF₄, and LiBF₄.

[0081] (3) 95g of positive electrode active material, 3g of polyvinylidene fluoride (as adhesive) and 2g of acetylene black (as conductive agent) are thoroughly mixed in a mass ratio of 95:3:2. After adding N-methylpyrrolidone solvent, the mixture is stirred evenly to obtain a positive electrode slurry. The positive electrode slurry is evenly coated onto the surface of the positive electrode aluminum foil (as the positive electrode current collector) using a coating machine. After coating, the positive electrode sheet is obtained by drying, rolling, slitting and cutting.

[0082] (4) The positive electrode, separator and negative electrode are stacked in sequence, so that the separator is between the positive electrode and the negative electrode and plays a role in isolation. The separator is wound into a square structure, injected with electrolyte and then sealed. After standing, hot and cold pressing, formation and capacity testing, the secondary battery product is obtained.

[0083] Examples 2 to 5

[0084] The preparation methods of the secondary batteries in Examples 2 to 5 differ from those in Example 1 in that the high-temperature treatment temperature is changed, while the remaining steps and parameters are basically the same as in Example 1, as detailed below:

[0085] In Example 1, the high-temperature treatment temperature of the precursor was 450℃; in Example 2, it was 700℃; in Example 3, it was 300℃; and in Example 4, it was 200℃. In Example 5, the precursor was treated at an ultra-high temperature of 900℃. According to the experimental data in Table 3, if the temperature for treating the coating precursor is too low (<300℃), Al2O3 and B2O3 will have difficulty reacting completely with the residual alkali on the surface of the core material. This residual alkali can easily cause the binder on the positive electrode to agglomerate and gel, affecting the transport of lithium ions and electrons in the positive electrode, thus reducing the 1C discharge capacity and the capacity retention rate at a high rate of 30C in the lithium secondary battery. If the temperature for treating the precursor is too high (>700℃), the boron source may volatilize, leading to uneven coating. Therefore, the high-temperature treatment temperature of the precursor should ideally be in the range of 300℃ to 700℃.

[0086] Examples 6 to 8

[0087] The preparation methods of the secondary batteries in Examples 6 to 8 differ from those in Example 1 in that the high-temperature treatment time is changed, while the remaining steps and parameters are basically the same as in Example 1, as detailed below:

[0088] In Example 1, the high-temperature treatment time of the precursor was 6 hours; in Example 6, it was 12 hours; in Example 7, it was 2 hours; and in Example 8, it was 14 hours. According to the experimental data in Table 3, if the treatment time of the boron-aluminum coated precursor is too short (<3 hours), Al2O3 and B2O3 will not react completely with the residual alkali on the surface of the core material, resulting in a decrease in the 1C discharge capacity and the capacity retention rate at a high rate of 30C for the lithium secondary battery. If the treatment time of the precursor is too long (>12 hours), since the residual alkali has already reacted almost completely, extending the high-temperature treatment time is not conducive to energy conservation. Therefore, a treatment time of 3 to 12 hours is preferable.

[0089] Examples 9 to 14

[0090] The preparation methods of the secondary batteries in Examples 9 to 14 differ from those in Example 1 in that the ratio of boron source to aluminum source is changed. The remaining steps and parameters are basically the same as in Example 1, as detailed below:

[0091] In Example 1, the first material (as the core material): boric acid (as the boron source): aluminum hydroxide (as the aluminum source): lithium fluoride (as the fluorine source) = 100g (1.00mol): 0.6g (0.0097mol): 0.6g (0.0077mol): 0.78g (0.03mol). The total molar amount of lithium fluoride and ammonium fluoride is 0.04mol. The shell thickness is 5nm, and the shell composition includes Al2O3, AlF3, lithium aluminum oxide (LiAlO2), lithium aluminum fluoride (LiAlO2), B2O3, BF3, lithium boron oxide (LiBO2), and lithium boron fluoride (LiBF4).

[0092] In Example 9, the first material (as the core material): boric acid (as the boron source): aluminum hydroxide (as the aluminum source): lithium fluoride (as the fluorine source) = 100g (1.00mol): 0.4g (0.0065mol): 0.8g (0.0103mol): 0.78g (0.03mol). The total molar amount of lithium fluoride and ammonium fluoride is 0.04mol. The shell thickness is 5nm, and the shell composition includes Al2O3, AlF3, lithium aluminum oxides (LiAlO2, LiAl5O8, Li3AlO3), and lithium aluminum fluorides (LiAlF4, LiAl5F4). 16 Lithium boron oxide (Li3AlF6), B2O3, BF3, lithium boron oxide (LiBO2), and lithium boron fluoride (LiBF4).

[0093] In Example 10, the first material (as the core material): boric acid (as the boron source): aluminum hydroxide (as the aluminum source): lithium fluoride (as the fluorine source) = 100g (1.00mol): 0.2g (0.0032mol): 1.0g (0.0128mol): 0.78g (0.03mol). The total molar amount of lithium fluoride and ammonium fluoride is 0.04mol. The shell thickness is 5nm, and the shell composition includes Al2O3, AlF3, lithium aluminum oxides (LiAlO2, LiAl5O8, Li3AlO3, Li5AlO4, Li2Al4O7), and lithium aluminum fluorides (LiAlF4, LiAl5F4). 16 Li3AlF6, Li5AlF8, LiAl2F7), B2O3, BF3, lithium boron oxide (LiBO2), and lithium boron fluoride (LiBF4).

[0094] In Example 11, the first material (as the core material): boric acid (as the boron source): aluminum hydroxide (as the aluminum source): lithium fluoride (as one of the fluorine sources) = 100g (1.00mol): 0.8g (0.0129mol): 0.4g (0.0051mol): 0.78g (0.03mol). The total molar amount of lithium fluoride and ammonium fluoride is 0.04mol. The shell thickness is 5nm, and the shell composition includes Al2O3, AlF3, lithium aluminum oxide (LiAlO2), lithium aluminum fluoride (LiAlF4), B2O3, BF3, lithium boron oxide (LiBO2, LiB5O8, Li3BO3), and lithium boron fluoride (LiBF4, LiB5F4). 16 (Li3BF6).

[0095] In Example 12, the first material (as the core material): boric acid (as the boron source): aluminum hydroxide (as the aluminum source): lithium fluoride (as the fluorine source) = 100g (1.00mol): 1.0g (0.0162mol): 0.2g (0.0026mol): 0.78g (0.03mol). The total molar amount of lithium fluoride and ammonium fluoride is 0.04mol. The shell thickness is 5nm, and the shell composition includes Al2O3, AlF3, lithium aluminum oxide (LiAlO2), lithium aluminum fluoride (LiAlF4), B2O3, BF3, lithium boron oxide (LiBO2, LiB5O8, Li3BO3, Li5BO4, Li2B4O7), and lithium boron fluoride (LiBF4, LiB5F4). 16 , Li3BF6, Li5BF8, LiB2F7).

[0096] Experimental data show that Examples 9 to 12, compared to Example 1, adjusted the ratio of boron and aluminum sources while keeping the ratio of fluorine source unchanged, and the performance of the resulting secondary batteries was not significantly different from that of Example 1. This is because aluminum and boron are elements in the same group, and their functions are: boron increases ionic conductivity, and aluminum increases interfacial stability. Therefore, they can be substituted for each other, thus achieving a synergistic effect.

[0097] Example 13 only added aluminum and fluorine sources, without adding any form of boron source; the remaining steps and parameters were the same as in Example 1. In Example 13, the first material (as the core material): aluminum hydroxide (as the aluminum source): lithium fluoride (as the fluorine source) = 100g (1.00mol): 1.2g (0.0154mol): 0.78g (0.03mol). The total molar amount of lithium fluoride and ammonium fluoride was 0.04mol. The shell thickness was 5nm, and the shell composition included Al2O3, AlF3, lithium aluminum oxides (LiAlO2, LiAl5O8, Li3AlO3, Li5AlO4, Li2Al4O7), and lithium aluminum fluorides (LiAlF4, LiAl5F4). 16 , Li3AlF6, Li5AlF8, LiAl2F7).

[0098] Example 14 only added boron and fluorine sources, without adding any form of aluminum source; the remaining steps and parameters were the same as in Example 1. In Example 14, the first material (as the core material): boric acid (as the boron source): lithium fluoride (as the fluorine source) = 100g (1.00mol): 1.2g (0.0194mol): 0.78g (0.03mol). The total molar amount of lithium fluoride and ammonium fluoride was 0.04mol. The shell thickness was 5nm, and the shell composition included B2O3, BF3, lithium boron oxides (LiBO2, LiB5O8, Li3BO3, Li5BO4, Li2B4O7), and lithium boron fluorides (LiBF4, LiB5F). 16 , Li3BF6, Li5BF8, LiB2F7).

[0099] The experimental data shows that the discharge capacity retention rate at a high rate of 30C and the capacity retention rate after 3000 cycles of the secondary batteries obtained in Examples 13 and 14 are lower than those of the secondary batteries in Examples 1, 9 to 12. This indicates that the shell containing boron or aluminum alone is not as effective as the shell containing both boron and aluminum in improving the performance of the secondary battery. Therefore, boron and aluminum have a synergistic effect in improving the battery performance of the secondary battery.

[0100] Examples 15 to 17

[0101] The preparation methods of the secondary batteries in Examples 15 to 17 differ from those in Example 1 in that the specific types of boron and aluminum sources are changed. The remaining steps and parameters are basically the same as in Example 1, as detailed below:

[0102] In Example 15, the first material (as the core material): boron hydroxide (as the boron source): trimethylaluminum (as the aluminum source): lithium fluoride (as the fluorine source) = 100g (1.00mol): 0.6g (0.0097mol): 0.6g (0.0083mol): 0.78g (0.03mol). The total molar amount of lithium fluoride and ammonium fluoride is 0.04mol. The shell thickness is 5nm, and the shell composition includes Al2O3, AlF3, lithium aluminum oxide (LiAlO2), lithium aluminum fluoride (LiAlO2), B2O3, BF3, lithium boron oxide (LiBO2), and lithium boron fluoride (LiBF4).

[0103] In Example 16, the first material (as the core material): boron trioxide (as the boron source): lithium aluminate (as the aluminum source): lithium fluoride (as the fluorine source) = 100g (1.00mol): 0.4g (0.0057mol): 0.8g (0.0121mol): 0.78g (0.03mol). The total molar amount of lithium fluoride and ammonium fluoride is 0.04mol. The shell thickness is 5nm, and the shell composition includes Al2O3, AlF3, lithium aluminum oxides (LiAlO2, LiAl5O8, Li3AlO3), and lithium aluminum fluorides (LiAlF4, LiAl5F4). 16 Lithium boron oxide (Li3AlF6), B2O3, BF3, lithium boron oxide (LiBO2), and lithium boron fluoride (LiBF4).

[0104] In Example 17, the first material (as the core material): boric acid (as the boron source): aluminum nitrate (as the aluminum source): lithium fluoride (as the fluorine source) = 100g (1.00mol): 0.8g (0.0130mol): 0.4g (0.0019mol): 0.78g (0.03mol). The total molar amount of lithium fluoride and ammonium fluoride is 0.04mol. The shell thickness is 4nm, and the shell composition includes Al2O3, AlF3, lithium aluminum oxide (LiAlO2), lithium aluminum fluoride (LiAlF4), B2O3, BF3, lithium boron oxide (LiBO2, LiB5O8, Li3BO3), and lithium boron fluoride (LiBF4, LiB5F4). 16 (Li3BF6).

[0105] According to the experimental data in Table 3, boric acid, boron trioxide, and boron hydroxide can all be used as boron sources, while aluminum nitrate, trimethylaluminum, aluminum carbonate, aluminum hydroxide, and lithium aluminate can all be used as aluminum sources. When the shell contains both boron and aluminum, the electrical performance of the secondary battery does not fluctuate significantly.

[0106] Examples 18 to 21

[0107] The preparation methods of the secondary batteries in Examples 18 to 21 differ from those in Example 1 in that the value of lithium fluoride is different; the remaining steps and parameters are basically the same as in Example 1, as detailed below:

[0108] In Example 1, the first material (as the core material): boric acid (as the boron source): aluminum hydroxide (as the aluminum source): lithium fluoride (as one of the fluorine sources) = 100g (1.00mol): 0.6g (0.0097mol): 0.6g (0.0078mol): 0.78g (0.03mol). The total molar amount of lithium fluoride and ammonium fluoride is 0.04mol. The shell thickness is 5nm, and the shell composition includes Al2O3, AlF3, lithium aluminum oxide (LiAlO2), lithium aluminum fluoride (LiAlF4), B2O3, BF3, lithium boron oxide (LiBO2), and lithium boron fluoride (LiBF4).

[0109] In Example 18, the first material (as the core material): boric acid (as the boron source): aluminum hydroxide (as the aluminum source): lithium fluoride (as the fluorine source alone) = 100g (1.00mol): 0.6g (0.0097mol): 0.6g (0.0078mol): 0.02g (0.00077mol). The shell thickness is 5nm, and the shell composition includes Al2O3, AlF3, lithium aluminum oxide (LiAlO2), lithium aluminum fluoride (LiAlF4), B2O3, BF3, lithium boron oxide (LiBO2), and lithium boron fluoride (LiBF4).

[0110] In Example 19, the first material (as the core material): boric acid (as the boron source): aluminum hydroxide (as the aluminum source): lithium fluoride (as the fluorine source alone) = 100g (1.00mol): 0.6g (0.0097mol): 0.6g (0.0078mol): 2g (0.078mol), the shell thickness is 5nm, and the shell composition includes Al2O3, AlF3, lithium aluminum oxide (LiAlO2), lithium aluminum fluoride (LiAlF4), B2O3, BF3, lithium boron oxide (LiBO2), and lithium boron fluoride (LiBF4).

[0111] According to the experimental data in Table 3, although the amount of lithium fluoride added was changed in Examples 18 to 19, the performance of the secondary battery was not significantly affected. This indicates that the shell containing both fluoride and oxide can enable the positive electrode to resist the corrosive effect of hydrofluoric acid in the electrolyte, thereby maintaining good battery performance.

[0112] In Example 20, the first material (as the core material): boric acid (as the boron source): aluminum hydroxide (as the aluminum source): lithium fluoride (as the fluorine source alone) = 100g (1.00mol): 0.6g (0.0097mol): 0.6g (0.0078mol): 0.01g (0.00039mol). The shell thickness was 5nm, and the shell composition included Al2O3, AlF3, lithium aluminum oxide (LiAlO2), lithium aluminum fluoride (LiAlF4), B2O3, BF3, lithium boron oxide (LiAlO2), and lithium boron fluoride (LiBF4). Experimental results show that when the amount of lithium fluoride added is small, the shell thickness is relatively thin, the fluoride content is low, and the oxide content is high, making it difficult to resist the corrosion of hydrofluoric acid in the electrolyte, thus causing a decrease in the performance of the secondary battery.

[0113] In Example 21, the first material (as the core material): boric acid (as the boron source): aluminum hydroxide (as the aluminum source): lithium fluoride (as the fluorine source alone) = 100g (1.00mol): 0.6g (0.0097mol): 0.6g (0.0078mol): 4g (0.1542mol). The shell thickness was 5nm, and the shell composition included Al2O3, AlF3, lithium aluminum oxide (LiAlO2), lithium aluminum fluoride (LiAlF4), B2O3, BF3, lithium boron oxide (LiAlO2), and lithium boron fluoride (LiBF4). Experimental results showed that when the amount of lithium fluoride added was large, the shell contained almost no oxides but entirely fluorides. While this could resist the corrosion of hydrofluoric acid in the electrolyte, the performance of the secondary battery decreased. This indicates that oxides play a role in electron and ion conduction, and a shell entirely containing fluorides is insufficient to meet the battery performance requirements. Therefore, the molar ratio of lithium fluoride is suitable within the following range: the ratio of first material: boron source: aluminum source: fluorine source (including lithium fluoride and / or hydrofluoric acid) is 1.00: (0.03%-1.61%): (0.02%-1.28%): (0.07%-7.8%).

[0114] Examples 22 to 24

[0115] The preparation methods of the secondary batteries in Examples 22 to 24 differ from those in Example 1 in that the composition of the core materials is different. The remaining steps and parameters are basically the same as in Example 1, as detailed below:

[0116] The core material of the positive electrode active material in Example 22 is composed of Li. 1.08 Ni 0.585 Co 0.195 Mn 0.195 V 0.025O2. The core material of the positive electrode active material in Example 23 is composed of Li. 1.08 Ni 0.585 Co 0.195 Mn 0.195 Sn 0.015 Ti 0.01 O2. The core material of the positive electrode active material in Example 24 is composed of Li. 1.08 Ni 0.585 Co 0.195 Mn 0.195 Mg 0.025 O2. The experimental results show that the minor adjustments to the core material composition in the above embodiments do not significantly affect the performance of the secondary battery, and the overall performance still meets the requirements.

[0117] Examples 25 to 29

[0118] The preparation methods of the secondary batteries in Examples 25 to 29 differ from those in Example 1 in that the Dv50, Dv99, BET, and TD parameters are different. These parameters can be changed by controlling parameters such as the particle size of the precursor raw materials, sintering time, and temperature. The remaining steps and parameters are basically the same as in Example 1, as detailed below:

[0119] The particle size distribution of the positive electrode active material in Example 1 meets the following requirements: Dv50 is 2.6 μm, Dv99 is 5.9 μm, and BET is 0.99 μm. 2 / g, TD satisfies 2.07g / cm 3 Its material properties satisfy I=BET×TD×(Dv99-Dv50), 2≤I≤20.

[0120] The particle size distribution of the positive electrode active material in Example 25 meets the following requirements: Dv50 is 5.1 μm, Dv99 is 10.4 μm, and BET is 0.54 μm. 2 / g, TD satisfies 1.97g / cm 3 Its material properties satisfy I=BET×TD×(Dv99-Dv50), 2≤I≤20.

[0121] The particle size distribution of the positive electrode active material in Example 26 meets the following requirements: Dv50 ≥ 7.5 μm, Dv99 ≥ 15.3 μm, and BET ≥ 0.45 μm. 2 / g, TD satisfies 2.05g / cm 3 Its material properties satisfy I=BET×TD×(Dv99-Dv50), 2≤I≤20.

[0122] The particle size distribution of the positive electrode active material in Example 27 meets the following requirements: Dv50 is 1.5 μm, Dv99 is 3.2 μm, and BET is 3.25 μm. 2 / g, TD satisfies 1.91g / cm 3 Its material properties satisfy I = BET × TD × (Dv99 - Dv50), 2 ≤ I ≤ 20. According to experimental data, the parameters of the positive electrode active material are suitable within the following ranges: 0.2m 2 / g≤BET≤5.5m 2 / g, 1.2g / cm 3 ≤TD≤3g / cm 3 , 1.5μm≤Dv50≤7.5μm, 3μm≤Dv99≤15.5μm, I=BET×TD×(Dv99-Dv50), 2≤I≤20.

[0123] The particle size distribution of the positive electrode active material in Example 28 meets the following requirements: Dv50 is 0.8 μm, Dv99 is 2.5 μm, and BET is 10.8 μm. 2 / g, TD satisfies 1.40g / cm 3 Its material properties do not satisfy 2≤I=BET×TD×(Dv99-Dv50)≤20.

[0124] The particle size distribution of the positive electrode active material in Example 29 meets the following requirements: Dv50 ≥ 9.5 μm, Dv99 ≥ 14.4 μm, and BET ≥ 0.15 μm. 2 / g, TD satisfies 2.19g / cm 3 Its material properties do not satisfy 2≤I=BET×TD×(Dv99-Dv50)≤20.

[0125] As shown in Table 3, if the particle size distribution is too small, the ionic and electronic conductivity is poor, resulting in poor performance of the secondary battery. If the particle size distribution is too large, the structure of the positive electrode is unstable. If the specific surface area (BET) of the positive electrode active material is too small, the specific capacity decreases. If the specific surface area (BET) of the positive electrode active material is too large, gas generation in the battery is severe. If the tap density (TD) of the positive electrode active material is too small, the pores are easily filled by inactive electrolyte material, which is detrimental to the overall energy density of the battery. If the tap density (TD) of the positive electrode active material is too large, it is detrimental to ion transport within the electrode.

[0126] Comparative Example 1

[0127] This embodiment provides a method for preparing a secondary battery, which differs from Example 1 in that: in step (2) of Comparative Example 1, the precursor is reacted at 450°C for 6 hours, and after cooling, ammonium fluoride and lithium fluoride are not added; the remaining steps are the same as in Example 1. The resulting positive electrode active material's shell does not contain fluorides, but only aluminum oxides (Al2O3 and LiAlO2) and boron oxides (B2O3 and LiBO2). Experimental results show that if the shell does not contain fluorides, its resistance to hydrofluoric acid attack in the fluorinated electrolyte is reduced, the stability of the shell structure is lower, and the high-temperature cycling capability of the secondary battery is reduced.

[0128] Comparative Example 2

[0129] This embodiment provides a method for preparing a secondary battery, which differs from Example 1 in that: in step (2) of Comparative Example 1, excess ammonium fluoride and lithium fluoride are added to react the oxides in the shell to produce fluorides; the remaining steps are the same as in Example 1. The resulting positive electrode active material does not contain oxides in its shell, but only fluorides (AlF3, LiAlF4, BF3, and LiBF4). Experimental results show that if the shell does not contain oxides, the porous structure of the shell will be affected, thus affecting the migration paths of ions and electrons, and also affecting the stability of the shell structure, resulting in a decrease in the rate performance of the secondary battery.

[0130] Table 1 is a comparison table of material and condition parameters for various embodiments and comparative examples of this application.

[0131]

[0132]

[0133]

[0134] Table 2 is a comparison table of the positive electrode active material parameters of various embodiments and comparative examples of this application.

[0135]

[0136]

[0137] Table 3 shows the test performance of the lithium batteries in the various embodiments and comparative examples of this application.

[0138]

[0139]

[0140] The embodiments of this application have been described in detail above. Specific examples have been used in this application to illustrate the principles and implementation methods of this application. The description of the above embodiments is only for the purpose of helping to understand the technical solutions and core ideas of this application. Those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A positive electrode active material, characterized in that, include: The core material and a shell layer distributed on the surface of the core material; the shell layer includes aluminum oxide and boron oxide, and fluoride, wherein the fluoride includes at least one of aluminum fluoride and boron fluoride; the aluminum oxide and the boron oxide include oxide-type fast ion conductors; the fluoride includes fluoride-type fast ion conductors; The positive electrode active material satisfies: I = BET × TD × (Dv99 - Dv50), 2 ≤ I ≤ 20; where... BET is the specific surface area of ​​the positive electrode active material, in m². 2 / g, 0.2≤BET≤5.5; TD is the tap density of the positive electrode active material at 60 MPa, in g / cm³. 3 1.2≤TD≤ 3.0; Dv99 is the particle size corresponding to a cumulative volume distribution percentage of 99% for the positive electrode active material, in μm. Dv50 is the particle size corresponding to the cumulative volume distribution percentage of the positive electrode active material reaching 50%, in μm, where 1.5 ≤ Dv50 ≤ 7.

5.

2. The positive electrode active material according to claim 1, characterized in that, The aluminum oxide includes at least one of LiAlO2, LiAl5O8, Li2Al4O7, Li3AlO3, and Li5AlO4.

3. The positive electrode active material according to claim 1, characterized in that, The aluminum-containing oxide also includes Al2O3.

4. The positive electrode active material according to claim 1, characterized in that, The boron-containing oxides include at least one of LiBO2, LiB5O8, Li2B4O7, Li3BO3, and Li5BO4.

5. The positive electrode active material according to claim 1, characterized in that, The boron-containing oxides also include B2O3.

6. The positive electrode active material according to claim 1, characterized in that, The aluminum-containing fluoride includes LiAlF4 and LiAl5F. 16 At least one of LiAl2F7, Li3AlF6, and Li5AlF8, wherein the boron-containing fluoride includes LiBF4 and LiB5F6. 16 At least one of LiB2F7, Li3BF6 and Li5BF8.

7. The positive electrode active material according to claim 1, characterized in that, The core material includes Li x Ni a Co b Mn c Me d O2, where 0.8 ≤ x ≤ 1.1, 0.5 < a < 1, 0 ≤ b < 0.3, 0 < c < 0.4, 0 < d < 0.2, a + b + c + d = 1, and Me is selected from one or more of V, Al, Zr, Sr, Ti, B, Mg, Sn, and W.

8. The positive electrode active material according to claim 1, characterized in that, Based on the mass of the positive electrode active material, the fluoride content is from 0.02% to 2%.

9. The positive electrode active material according to claim 1, characterized in that, The thickness of the shell layer is from 0.1 nm to 20 nm.

10. The positive electrode active material according to claim 1, characterized in that, The Dv99 satisfies: 3≤Dv99≤15.

5.

11. A secondary battery, characterized in that, It includes a positive electrode sheet, wherein the positive electrode sheet comprises the positive electrode active material according to any one of claims 1 to 10.

12. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 11, wherein the secondary battery serves as the power supply for the electrical device.

Images