Positive electrode active material, positive plate and lithium ion battery

Abstract

The invention discloses a positive active material, a positive plate and a lithium ion battery, and belongs to the technical field of secondary batteries. The positive electrode active material comprises an inner core and a coating layer, the coating layer is arranged on the surface of the inner core, the inner core comprises lithium nickel manganese oxide, the coating layer comprises lithium nickel phosphate, and the particle size Dn50 of the inner core and the thickness d of the coating layer meet the condition that 1.5 nm / [mu] m < = d / (Dn50) < = 14 nm / [mu] m. The side reaction between a positive electrode interface and electrolyte is inhibited, and the dissolution of transition metal ions is reduced, so that the cycling stability of the lithium ion battery is improved.

Description

Technical Field

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

[0002] The demand for high energy density and high output power in lithium ion power batteries has attracted people's attention to the research and development of high voltage positive electrode materials. The lithium nickel manganese oxide (LNMO) material with a spinel structure can provide a three-dimensional lithium ion transport channel, has good ionic conductivity, a high voltage plateau (close to 4.7V) and a high theoretical specific capacity (147 mAh / g). Therefore, it has good application potential in the field of lithium ion batteries.

[0003] However, lithium nickel manganese oxide (LNMO) has relatively poor electronic conductivity as a positive electrode active material. Under the use conditions of high voltage, the side reaction between the positive electrode interface and the electrolyte is intensified, and the interface by-products increase, resulting in poor stability of the positive electrode structure, a large amount of transition metal ions dissolve out, and the performance of the battery is reduced. Summary of the Invention

[0004] Based on the defects existing in the prior art, the purpose of the present application is to provide a positive electrode active material, a positive electrode sheet and a lithium ion battery.

[0005] In order to achieve the above purpose, the technical solution adopted by the present application is as follows:

[0006] In the first aspect, a positive electrode active material provided by the present application includes a core and a coating layer. The coating layer is disposed on the surface of the core. The core includes lithium nickel manganese oxide, and the coating layer includes lithium nickel phosphate. The particle size Dn50 of the core and the thickness d of the coating layer satisfy: 1.5 nm / μm ≤ d / (Dn50) ≤ 14 nm / μm.

[0007] Preferably, 4 nm / μm ≤ d / (Dn50) ≤ 6 nm / μm.

[0008] Preferably, the particle size Dn50 of the core is 3 - 10 μm; more preferably, the particle size Dn50 of the core is 3 - 8 μm.

[0009] Preferably, the thickness d of the coating layer is 8 - 60 nm; more preferably, the thickness d of the coating layer is 10 - 50 nm.

[0010] Preferably, the particle size Dn10 of the core is 0.5 - 5 μm; more preferably, the particle size Dn10 of the core is 0.5 - 4 μm.

[0011] Preferably, the particle size Dn90 of the core is 10-18 μm; more preferably, the particle size Dn90 of the core is 12-18 μm.

[0012] Preferably, the particle strength of the positive electrode active material is 200-300 MPa.

[0013] Preferably, the coating layer includes element M, and the element M includes at least one of Fe, Mn, Co, Mg, Ca, and Al.

[0014] In a second aspect, a positive electrode sheet provided by the present application includes the positive electrode active material as described in the first aspect.

[0015] Preferably, the coating compaction density of the positive electrode sheet is 2.6-3.4 g / cm 3 .

[0016] Preferably, the positive electrode sheet includes a positive electrode current collector, and at least one surface of the positive electrode current collector is coated with a positive electrode active material layer, and the positive electrode active material layer contains the positive electrode active material.

[0017] More preferably, the mass percentage of the positive electrode active material in the positive electrode active material layer is 90-99%.

[0018] In a third aspect, a lithium-ion battery provided by the present application includes the positive electrode sheet as described in the second aspect.

[0019] Preferably, the lithium-ion battery further includes a negative electrode sheet, an electrolyte, and a separator.

[0020] Compared with the prior art, the beneficial effects of the present application are as follows:

[0021] In the present application, lithium nickel phosphate is used as the coating layer and lithium nickel manganate is used as the core. By controlling the particle size Dn50 of the core and the thickness d of the coating layer, and controlling d / (Dn50) within the range of 1.5-14 nm / μm, it is possible to improve the structural stability of the material while suppressing the side reaction between the positive electrode interface and the electrolyte and reducing the dissolution of transition metal ions, thereby improving the cycle stability of the lithium-ion battery. Detailed Embodiments

[0022] In order to better illustrate the purpose, technical solution, and advantages of the present application, the present application will be further described below in conjunction with specific embodiments and comparative examples. The purpose is to understand the content of the present application in detail, rather than to limit the present application. All other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present application.

[0023] Among the technical features described in an open-ended manner in this application, there are also closed technical solutions composed of the listed features, as well as the listed open-ended technical solutions.

[0024] It should be understood that unless otherwise specified, the numerical ranges involved in this application are considered continuous, including the minimum and maximum values of the numerical range, as well as each value between the minimum and maximum values. Further, when the range refers to integers, it includes each integer between the maximum and minimum values of the range. In addition, when multiple ranges are provided to describe features or characteristics, these ranges can be combined. In other words, unless otherwise specified, all ranges disclosed in this application should be understood to include any or all of the sub-ranges subsumed therein.

[0025] It should be understood that this application does not particularly limit the specific dispersion and stirring treatment methods.

[0026] A cathode active material provided in the first aspect of this application includes a core and a coating layer. The coating layer is disposed on the surface of the core. The core includes lithium nickel manganese oxide, and the coating layer includes lithium nickel phosphate. The particle size Dn50 of the core and the thickness d of the coating layer satisfy: 1.5 nm / μm ≤ d / (Dn50) ≤ 14 nm / μm.

[0027] The cathode active material of this application has a core and coating layer structure. By regulating the particle size Dn50 of the core and the thickness d of the coating layer, and controlling d / (Dn50) within the range of 1.5 - 14 nm / μm, it is possible to improve the structural stability of the material while suppressing the side reactions between the cathode interface and the electrolyte, reducing the dissolution of transition metal ions, thereby improving the cycle stability of the lithium-ion battery.

[0028] Specifically, d / (Dn50) can be 1.5 nm / μm, 1.6 nm / μm, 1.9 nm / μm, 2 nm / μm, 2.5 nm / μm, 2.7 nm / μm, 2.8 nm / μm, 3 nm / μm, 3.1 nm / μm, 3.2 nm / μm, 3.5 nm / μm, 3.7 nm / μm, 3.8 nm / μm, 4 nm / μm, 4.2 nm / μm, 4.3 nm / μm, 4.4 nm / μm, 4.5 nm / μm, 4.6 nm / μm, 4.7 nm / μm, 4.8 nm / μm, 4.9 nm / μm, 5 nm / μm, 5.5 nm / μm, 5.6 nm / μm, 6 nm / μm, 7 nm / μm, 8 nm / μm, 8.3 nm / μm, 8.4 nm / μm, 8.5 nm / μm, 8.7 nm / μm, 9 nm / μm, 9.2 nm / μm, 9.3 nm / μm, 9.5 nm / μm, 9.8 nm / μm, 10 nm / μm, 10.3 nm / μm, 10.5 nm / μm, 10.7 nm / μm, 11 nm / μm, 11.3 nm / μm, 11.4 nm / μm, 11.5 nm / μm, 11.7 nm / μm, 12 nm / μm, 12.3 nm / μm, 12.5 nm / μm, 12.7 nm / μm, 13 nm / μm, 13.3 nm / μm, 13.5 nm / μm, 13.6 nm / μm, 13.8 nm / μm, 14 nm / μm, or a range composed of any two sets of these values.

[0029] Specifically, 4 nm / μm ≤ d / (Dn50) ≤ 6 nm / μm. By controlling d / (Dn50) within the range of 4 - 6 nm / μm, a better balance between the kinetics and stability of the positive electrode active material can be achieved.

[0030] In some embodiments, the particle size Dn50 of the core is 3 - 10 μm.

[0031] The particle size Dn50 of the core is preferably 3 - 8 μm.

[0032] The inventors have found through research that by controlling the particle size Dn50 of the core within the range of 3 - 8 μm, both the mechanical strength and kinetics of the positive electrode active material can be taken into account. The positive electrode active material particles are not easily broken during the rolling process, and the side reactions between the positive electrode interface and the electrolyte can be better alleviated.

[0033] For example, in some embodiments, the median particle size Dn50 of the core may be 3 μm, 3.1 μm, 3.3 μm, 3.5 μm, 3.7 μm, 4 μm, 4.3 μm, 4.5 μm, 5 μm, 5.1 μm, 5.2 μm, 5.3 μm, 5.5 μm, 5.9 μm, 6 μm, 6.5 μm, 7 μm, 7.2 μm, 7.5 μm, 7.7 μm, 7.8 μm, 7.9 μm, 8 μm, 9 μm, 9.5 μm, 9.9 μm, 10 μm, or a range composed of any two sets of these values.

[0034] The median particle size Dn50 of the core is more preferably 5 - 6 μm. Within this range, the comprehensive performance of the mechanical strength and kinetics of the positive electrode active material is better.

[0035] In some embodiments, the Dn10 particle size of the core is 0.5 - 5 μm.

[0036] For example, Dn10 may be 0.5 μm, 1 μm, 1.2 μm, 1.5 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 3 μm, 3.1 μm, 3.3 μm, 3.5 μm, 3.7 μm, 4 μm, 4.6 μm, 5 μm, or a range composed of any two sets of these values.

[0037] The Dn10 particle size of the core is preferably 0.5 - 4 μm.

[0038] In some embodiments, the Dn90 particle size of the core is 10 - 18 μm. For example, Dn90 may be 10 μm, 11.3 μm, 11.5 μm, 11.7 μm, 12 μm, 12.3 μm, 12.5 μm, 12.7 μm, 13 μm, 13.5 μm, 14 μm, 14.5 μm, 15 μm, 15.5 μm, 16 μm, 16.5 μm, 17 μm, 17.5 μm, 18 μm, or a range composed of any two sets of these values.

[0039] The Dn90 particle size of the core is preferably 12 - 18 μm.

[0040] It can be understood that in this application, the median particle size Dn50 of the core refers to the particle size corresponding to when the cumulative particle size distribution percentage of the core reaches 50%; the Dn90 particle size of the core refers to the particle size corresponding to when the cumulative particle size distribution percentage of the core reaches 90%; the Dn10 particle size of the core refers to the particle size corresponding to when the cumulative particle size distribution percentage of the core reaches 10%.

[0041] In some embodiments, the thickness d of the coating layer is 8 - 60 nm. For example, the thickness d of the coating layer can be 8 nm, 10 nm, 12 nm, 15 nm, 17 nm, 20 nm, 21 nm, 24 nm, 25 nm, 26 nm, 27 nm, 28 nm, 29 nm, 30 nm, 33 nm, 35 nm, 39 nm, 40 nm, 42 nm, 45 nm, 49 nm, 50 nm, 55 nm, 60 nm or the range composed of any two sets of these values.

[0042] The thickness d of the coating layer is preferably 10 - 50 nm.

[0043] The inventors have found through research that controlling d within the range of 10 - 50 nm can enhance the inhibitory effect of the coating layer on the side reactions between the cathode interface and the electrolyte while ensuring the electronic conductivity of the cathode active material.

[0044] Specifically, the thickness d of the coating layer is 20 - 30 nm, within which the comprehensive performance of the cathode active material is better.

[0045] In some embodiments, the particle strength of the cathode active material is 200 - 300 MPa. For example, the particle strength of the cathode active material can be 200 MPa, 210 MPa, 220 MPa, 230 MPa, 240 MPa, 250 MPa, 260 MPa, 270 MPa, 280 MPa, 280 MPa, 300 MPa or the range composed of any two sets of these values.

[0046] In some embodiments, the chemical formula of the lithium nickel manganate is LiNi x Mn 2-x O4, where 0 < x ≤ 0.5.

[0047] In some embodiments, the chemical formula of the lithium nickel phosphate is LiNiPO4.

[0048] In some embodiments, the coating layer includes element M, and the element M includes at least one of Fe, Mn, Co, Mg, Ca, Al, Zn.

[0049] In this application, element M is doped into the lithium nickel phosphate by physical or chemical doping methods. Part of element M replaces the nickel atoms in the crystal structure of the lithium nickel phosphate, and part of element M is distributed in the lattice defects of the lithium nickel phosphate. It is found by XRD (X-ray diffraction) detection of the cathode active material in this application that there are no alloy phases and metal phases in the internal crystal phase of the material. Therefore, in this application, by doping element M, while improving the conductivity of the coating layer, the original structure of the lithium nickel phosphate is maintained, ensuring the structural stability of the lithium nickel phosphate.

[0050] In some embodiments, the method for preparing the core includes the following steps: mixing a first lithium source, a first nickel source, and a manganese source in a certain molar ratio in a solvent to obtain a mixture, and calcining the mixture to obtain the core.

[0051] For example, in some embodiments, the molar ratio of lithium element in the first lithium source, nickel element in the first nickel source, and manganese element in the manganese source is y:x:(2 - x), where 0 < x ≤ 0.5 and 1 ≤ y ≤ 1.05.

[0052] In some embodiments, the first lithium source includes but is not limited to at least one selected from lithium hydroxide, lithium carbonate, lithium oxalate, and lithium acetate; the first nickel source includes but is not limited to at least one selected from nickel hydroxide, nickel carbonate, and nickel nitrate; the manganese source includes but is not limited to at least one selected from manganese hydroxide, manganese oxide, and manganese sulfate; the solvent includes but is not limited to at least one selected from ethanol, N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), acetonitrile, and dimethyl carbonate (DMC).

[0053] In some embodiments, in the method for preparing the core, the heating rate of the calcination is 2 - 10 °C / min, the peak temperature of the calcination is 650 - 850 °C, and the holding time of the calcination is 12 - 24 h.

[0054] In some embodiments, in the method for preparing the core, the mixing method includes but is not limited to at least one of the grinding method and the sol-gel method.

[0055] In some embodiments, in the method for preparing the core, after calcining the mixture, it is cooled and ground. The grinding method includes but is not limited to at least one of ball milling and sand milling.

[0056] In some embodiments, the method for preparing the positive electrode active material includes the following steps:

[0057] Forming a coating layer on the surface of the core by physical or chemical coating method with a second lithium source, a second nickel source, an M source, and a phosphorus source in a certain molar ratio.

[0058] For example, in some embodiments, the physical or chemical coating method includes but is not limited to at least one of atomic layer deposition method, chemical vapor deposition method, solid-phase sintering method, and solution combustion method.

[0059] In some embodiments, the second nickel source includes but is not limited to at least one of nickel bis(dimethylamide), nickel tert-butoxide, nickel tetrakis(ethylmethylamino), nickel tetrakis(ethylamino), and nickel tetraethoxide.

[0060] In some embodiments, the second lithium source includes, but is not limited to, at least one of lithium tert-butoxide, lithium acetoacetate, lithium tert-butyl, and lithium bis(trimethylsilyl)amide.

[0061] In some embodiments, the phosphorus source includes, but is not limited to, at least one of tetramethyl methylene diphosphate, trimethyl phosphate, tris(dimethylamine)phosphine, or trialkylphosphine oxide.

[0062] In some embodiments, the M source includes, but is not limited to, the tert-butoxide of the M element.

[0063] In some embodiments, the oxygen source includes, but is not limited to, at least one of water, ozone, or hydrogen peroxide.

[0064] A positive electrode sheet provided by the second aspect of the present application includes the positive electrode active material as described in the first aspect.

[0065] In some embodiments, the coating compaction density of the positive electrode sheet is 2.6 - 3.4 g / cm 3 .

[0066] The present invention controls the compaction density of the positive electrode sheet within the range of 2.6 - 3.4 g / cm 3 to reduce the risk of particle breakage of the positive electrode active material during the rolling process, inhibit the side reaction between the positive electrode sheet and the electrolyte, facilitate the full infiltration of the electrolyte, and improve the capacity and energy density of the battery.

[0067] In some embodiments, the positive electrode sheet includes a positive electrode current collector, and at least one surface of the positive electrode current collector is coated with a positive electrode active material layer, and the positive electrode active material layer contains the positive electrode active material.

[0068] The positive electrode active material layer further contains a positive electrode binder and a positive electrode conductive agent.

[0069] The positive electrode conductive agent may include a conductive agent conventional in the battery field. For example, the positive electrode conductive agent includes at least one of conductive carbon black, conductive graphite, graphene, carbon nanotubes, and carbon fibers.

[0070] The positive electrode binder is used to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. The positive electrode binder may include a binder conventional in the battery field. For example, the positive electrode binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin, sodium carboxymethyl cellulose (CMC), and sodium alginate.

[0071] In the present application, based on the total mass of the positive electrode active material layer, the mass percentage of the positive electrode active material is 90-99%, the mass percentage of the positive electrode binder is 0.5-5%, and the mass percentage of the positive electrode conductive agent is 0.5-5%. For example, the mass percentage of the positive electrode active material can be 90%, 92%, 94%, 96%, 98%, 99% or the range composed of any two sets of these values, the mass percentage of the positive electrode binder can be 0.5%, 1%, 2%, 3%, 4%, 5% or the range composed of any two sets of these values, and the mass percentage of the positive electrode conductive agent can be 0.5%, 1%, 2%, 3%, 4%, 5% or the range composed of any two sets of these values.

[0072] In the present application, the positive electrode sheet can be prepared by conventional methods in the art. For example, the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder are dispersed in a solvent to obtain a positive electrode slurry, and the positive electrode slurry is coated on at least one surface of the positive electrode current collector, and after drying, rolling, and cutting, a positive electrode sheet is obtained.

[0073] A lithium-ion battery provided in the third aspect of the present application includes a positive electrode sheet as provided in the second aspect.

[0074] In some embodiments, the lithium-ion battery further includes a negative electrode sheet.

[0075] Specifically, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder. The negative electrode conductive agent can include conventional conductive agents in the battery field. For example, the negative electrode conductive agent includes at least one of conductive carbon black, conductive graphite, graphene, carbon nanotubes, and carbon fibers.

[0076] The negative electrode binder is used to improve the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector. The negative electrode binder can include conventional binders in the battery field. For example, the negative electrode binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin, sodium carboxymethyl cellulose (CMC), and sodium alginate.

[0077] In the present application, based on the total mass of the negative electrode active material layer, the mass percentage of the negative electrode active material is 90-99%, the mass percentage of the negative electrode binder is 0.5-5%, and the mass percentage of the negative electrode conductive agent is 0.5-5%. For example, the mass percentage of the negative electrode active material can be 90%, 92%, 94%, 96%, 98%, 99% or the range composed of any two sets of values among them. The mass percentage of the negative electrode binder can be 0.5%, 1%, 2%, 3%, 4%, 5% or the range composed of any two sets of values among them. The mass percentage of the negative electrode conductive agent can be 0.5%, 1%, 2%, 3%, 4%, 5% or the range composed of any two sets of values among them.

[0078] In the present application, the negative electrode sheet can be prepared by conventional methods in the art. For example, the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder are dispersed in a solvent to obtain a negative electrode slurry. The negative electrode slurry is coated on at least one surface of the negative electrode current collector, and after drying, rolling, and cutting, a negative electrode sheet is obtained.

[0079] In some embodiments, the lithium ion battery further includes a separator. The separator includes a substrate layer, and the substrate layer includes at least one of polyethylene, polypropylene, polyamide, and aramid.

[0080] The separator may further include an adhesive layer and / or a ceramic layer. The material of the adhesive layer may include at least one of polyvinylidene fluoride, polymethyl methacrylate, aramid, polyethylene oxide, polyvinylidene fluoride-hexafluoropropylene copolymer, or polyaniline; the material of the ceramic layer may include at least one of boehmite, aluminum oxide, titanium dioxide, silicon dioxide, zirconium dioxide, tin dioxide, magnesium oxide, zinc oxide, barium sulfate, boron nitride, aluminum nitride, and magnesium nitride.

[0081] In some embodiments, the lithium ion battery further includes an electrolyte, and the electrolyte may include a conventional electrolyte in the battery field.

[0082] Specifically, the electrolyte includes a solvent and a lithium salt.

[0083] For example, in some embodiments, the solvent includes at least one of carbonate solvents, carboxylate solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate solvents. The carbonate solvents include dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, and propylene carbonate; the carboxylate solvents include at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; the ether solvents include at least one of ethylene glycol dimethyl ether and tetrahydrofuran.

[0084] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium trifluoromethanesulfonate, lithium bis(fluoromethanesulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorodioxalate phosphate, lithium tetrafluorooxalate phosphate, etc.

[0085] In some embodiments, the concentration of the lithium salt in the electrolyte is 0.5 - 2 mol / L.

[0086] To enable those skilled in the art to better understand the solutions of this application, the technical solutions in the embodiments of this application will be clearly and completely described below. Apparently, the described embodiments are only a part of the embodiments of this application, rather than all of the embodiments.

[0087] The experimental reagents and instruments involved in the implementation of this application are all common ordinary reagents and instruments unless otherwise specified. The component raw materials used in each embodiment and comparative example of this application are all commercially available raw materials unless otherwise specified, and the component raw materials used in each parallel experiment are the same.

[0088] Example 1

[0089] For the embodiment of the lithium-ion battery described in this application, the preparation method of the lithium-ion battery includes the following steps:

[0090] S1. Preparation of the positive electrode active material

[0091] S11. Mechanically mix the raw materials Li2CO3, NiCO3, and MnO2 in a molar ratio of 0.5:0.5:1.5 by ball milling to obtain a mixture; put the mixture into a muffle furnace and heat it to 850°C at a rate of 8°C / min, calcine it at 850°C, and cool it to room temperature naturally and then perform sanding to obtain lithium nickel manganate particles; the calcination time and sanding time are shown in Table 1;

[0092] S12. Put the lithium nickel manganate particles into the reaction chamber of an atomic layer deposition device, set the reaction temperature to 150°C, and evacuate the chamber to reach the reaction vacuum degree;

[0093] S13. Under the condition of a pressure of 10 -2 Pa, successively perform steps S131 to S133:

[0094] S131. Introduce nickel tert-butoxide gas into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min; then, introduce ozone into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min;

[0095] S132. Introduce trimethyl phosphate gas into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min; then, introduce ozone into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min;

[0096] S133. Introduce lithium tert-butoxide gas into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min; then, introduce ozone into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min;

[0097] In step S13, the molar ratio of Li in lithium tert-butoxide, Ni in nickel tert-butoxide, M in the tert-butoxide of element M, and P in trimethyl phosphate is Li:Ni:P = 1:0.9:1;

[0098] Repeat the above step S13. When step S13 is completed once, one deposition cycle is completed. The number of cycles of the deposition cycle is shown in Table 1 to obtain the positive electrode active material.

[0099] S2. Preparation of the positive electrode sheet

[0100] Mix the positive electrode active material, the conductive agent multi-walled carbon nanotubes, and the binder PVDF in a mass ratio of 97:1:2, add the solvent NMP, and stir under the action of a vacuum mixer until the system becomes homogeneous to obtain the positive electrode slurry; coat the positive electrode slurry evenly on two opposite surfaces of the positive electrode current collector aluminum foil, dry it in a vacuum furnace at 100 °C, then cut it into strips, and roll it at a compaction density of 2.2 g / cm 2 to obtain the positive electrode sheet; 3

[0101] S3. Preparation of the separator

[0102] The separator is selected from a commercial PE polyethylene separator with a thickness of 15 μm;

[0103] S4. Preparation of the negative electrode sheet

[0104] Mix the negative electrode active material artificial graphite, the conductive agent acetylene black, the thickener CMC, and the binder SBR in a mass ratio of 96.4:1:1.2:1.4, add the solvent deionized water, and stir under the action of a vacuum mixer until the system becomes homogeneous to obtain the negative electrode slurry; coat the negative electrode slurry evenly on two opposite surfaces of the negative electrode current collector copper foil, dry it in a vacuum furnace at 100 °C, then cut it into strips, and cold press it at a compaction density of 1.62 g / cm 2 to obtain the negative electrode sheet; 3

[0105] S5. Preparation of the electrolyte

[0106] Ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed in a volume ratio of 1:1 to obtain an organic solvent. Then, the fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte solution, and the concentration of LiPF6 in the electrolyte solution is 1 mol / L;

[0107] S6. Preparation of lithium-ion battery

[0108] The above positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, and the separator is placed between the positive electrode sheet and the negative electrode sheet to play a role in isolation, and then wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, dried and then injected with the electrolyte solution, and after vacuum packaging and standing for 24 h, it is ready for use.

[0109] Examples 2-7

[0110] The differences between Examples 2-7 and Example 1 are that Examples 2-7 change the sanding time in step S11, as shown in Table 1 specifically.

[0111] Examples 8-14

[0112] The differences between Examples 8-14 and Example 1 are that Examples 8-14 change the number of deposition cycles, as shown in Table 1 specifically.

[0113] Examples 15-22

[0114] The differences between Examples 15-22 and Example 1 are that in Examples 15-22, step S13 is as follows:

[0115] S13. Under the condition of a pressure of 10 -2 Pa, steps S131 to S134 are carried out in sequence:

[0116] S131. Introduce nickel tert-butoxide gas into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min; then, introduce ozone into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min;

[0117] S132. Introduce trimethyl phosphate gas into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min; then, introduce ozone into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min;

[0118] S133. Introduce lithium tert-butoxide gas into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min; then, introduce ozone into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min;

[0119] S134. Introduce the tert-butoxide gas of element M (element M is shown in Table 1) into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min; then, introduce ozone into the chamber for 0.2 s, purge with argon for 30 s, and the purge flow rate of argon is 20 ml / min;

[0120] In step S13, the molar ratio among Li in lithium tert-butoxide, Ni in nickel tert-butoxide, M in the tert-butoxide of element M, and P in trimethyl phosphate is Li:Ni:M:P = 1:0.9:0.1:1;

[0121] Repeat the above step S13. Completing step S13 once finishes one deposition cycle, and the number of cycles of the deposition cycle is shown in Table 1, to obtain the positive electrode active material.

[0122] Examples 23 - 27

[0123] The differences between Examples 23 - 27 and Example 1 are that Examples 23 - 27 change the sanding time and the number of cycles of the deposition cycle, as specifically shown in Table 1.

[0124] Comparative Example 1

[0125] The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 changes the number of cycles of the deposition cycle, as specifically shown in Table 1.

[0126] Comparative Example 2

[0127] The difference between Comparative Example 2 and Example 1 is that Comparative Example 2 changes the sanding time, as specifically shown in Table 1.

[0128] Comparative Example 3

[0129] The difference between Comparative Example 3 and Example 1 is that Comparative Example 3 does not perform steps S12 to S13, and directly uses the lithium nickel manganate obtained in step S11 as the positive electrode active material.

[0130] Comparative Example 4

[0131] The difference between Comparative Example 4 and Example 1 is that nickel tert-butoxide in step S131 is replaced with iron tert-butoxide.

[0132] Performance Test 1

[0133] The positive electrode active materials obtained in the above examples and comparative examples in step S1 are tested as follows:

[0134] (1) Composition of the coating layer:

[0135] First, stick a layer of conductive adhesive on the sample holder, stick the sample on the sample holder, then coat a conductive film, wipe the tools and the sample stage with anhydrous ethanol, and place a certain amount of the sample to be measured on the sample stage; observe under a scanning electron microscope (SEM); at a magnification of 10Kx, combined with EDS energy spectrum surface scanning analysis, determine the elemental composition of the coating layer;

[0136] Fix the sample in the test mold with conductive tape, and then perform a test with FTIR. The test conditions for FTIR are: model FTIR450 infrared spectrometer, scanning range 400 - 2000 cm -1 , the instrument resolution is 4 cm -1 ; in the infrared spectrum, 500 - 700 cm -1 represents the stretching and bending vibrations of O - P - O, and 900 - 1200 cm -1 represents the stretching vibration of PO4 3- , confirming the presence of phosphate; the absorption peak at 525 cm -1 is the stretching vibration of the Ni - O bond within the Ni - O octahedron;

[0137] Combined with the infrared spectrum and the elemental composition of the coating layer determined by EDS energy spectrum surface scanning analysis, determine the composition of the coating layer.

[0138] (2) Particle size Dn50, Dn10, Dn90 of the core: First, stick a layer of conductive adhesive on the sample holder, stick the sample on the sample holder, perform CP argon ion polishing, then coat a conductive film, wipe the tools and the sample stage with anhydrous ethanol, and place a certain amount of the sample to be measured on the sample stage; observe under a scanning electron microscope (SEM); adjust the magnification to 5Kx with SEM, select three regions to take pictures, measure the particle sizes of all particles within each region, calculate Dn50 with a quantity distribution of 50%, Dn10 with a quantity distribution of 10%, and Dn90 with a quantity distribution of 90%, and take the corresponding average values of Dn50, Dn10, and Dn90 for the three regions respectively.

[0139] (3) Thickness d of the coating layer: Take a small amount of the sample and place it in a sample tube, add anhydrous ethanol, and ultrasonicate for 40 min; use a pipette to take a small amount of the solution and drop it on a copper mesh; finally, dry the sample completely in a vacuum drying oven; place the sample on the test stage and transfer it to the cavity, and use TEM to measure the thickness of the coating layer at an appropriate high magnification corresponding to different coating layer thicknesses; select 5 regions to measure and take the average value.

[0140] (4) Particle strength: Refer to GB / T 43091 - 2023 for the powder compressive strength test.

[0141] The test results are shown in Table 1.

[0142] Performance Test 2

[0143] Using the lithium-ion batteries obtained from the above examples and comparative examples as the detection objects, the battery capacity retention rate and the growth rate of kinetic DCR were tested after 100 cycles:

[0144] After standing still, use the LAND system to charge the battery at a constant current to 4.8V with a current of 0.33C, and then discharge it at a constant current to 3.5V with a current of 0.33C for 2 cycles. After that, remove the battery from the charging device and set it aside for use. After activating the battery, at 45°C, charge it at a constant current of 1C to 4.8V, and then charge it at a constant voltage until the current is less than 0.05C; then discharge it at a constant current of 1C until the voltage reaches 3.5V. One complete charge and discharge process is regarded as one cycle, and cycle 100 times. Calculate the capacity retention rate according to the following formula: Capacity retention rate = Discharge capacity of the 100th cycle / Discharge capacity of the first cycle * 100%;

[0145] The DCR value after 100 cycles is denoted as R1, and the initial DCR value before cycling is denoted as R0; then the growth rate of DCR after 100 cycles = (R1 - R0) / R0 * 100%.

[0146] The DCR test state is that the battery is at 50% SOC. The specific DCR test method:

[0147] A1. Let the battery stand still at room temperature for 10 minutes;

[0148] A2. First, charge the battery at a constant current and constant voltage, with a constant current rate of 0.33C and a voltage of 4.80V. During the constant voltage process until the current ≤ 0.05C, and then discharge it at a constant current to 3.5V with a current rate of 0.33C;

[0149] A3. Repeat steps A1 - A2 three times. The discharge capacity during the last step A2 is denoted as C1;

[0150] A4. Continue to repeat steps A1 - A2 until the battery is fully charged;

[0151] A5. Discharge at a constant current until the capacity is 0.5C1, with a discharge rate of 0.33C. At this time, the battery is in a 50% SOC state;

[0152] A6. Let the battery stand still for 120 min, and record the voltage at the end of standing still as V0;

[0153] A7. Discharge at a constant current for 18 s, with a current rate of 1C, and record the voltage at the end of discharge as V1;

[0154] A8. Calculate DCR. The calculation formula for DCR is: DCR = (V0 - V1) / C1.

[0155] The results are shown in Table 1.

[0156] Table 1

[0157]

[0158]

[0159] As can be seen from Table 1, in each embodiment of the present invention, parameters such as d, Dn50, and d / (Dn50) are respectively controlled within appropriate ranges, so that the positive electrode active material has high particle strength, is not easily broken during the rolling process, can also improve the kinetic performance and stability of the positive electrode active material, and enables the battery capacity retention rate to be greater than 80% after 100 cycles, and the kinetic DCR growth rate to be less than 10%. When 4 nm / μm ≤ d / (Dn50) ≤ 6 nm / μm, the balance between the kinetics and stability of the positive electrode active material is better.

[0160] Compared with Example 1 and Examples 8-14, in Comparative Example 1, d / (Dn50) is too small, and the coating layer is likely to fall off during charge and discharge with the expansion and contraction of the positive electrode volume, and cannot effectively inhibit the side reaction between the positive electrode interface and the electrolyte, thereby resulting in a significant decrease in the battery capacity retention rate and the kinetic DCR growth rate after 100 cycles.

[0161] Compared with Examples 1-7, in Comparative Example 2, Dn50 is relatively too small, making d / (Dn50) too large, and the electron transport performance of the core is poor, resulting in an aggravation of the side reaction between the positive electrode interface and the electrolyte, and a significant decrease in the battery capacity retention rate and the kinetic DCR growth rate after 100 cycles.

[0162] Compared with each embodiment, the positive electrode active material in Comparative Example 3 has no coating layer, resulting in the deterioration of the kinetics and stability of the positive electrode active material.

[0163] As can be seen from Example 1 and Comparative Example 4, compared with lithium iron phosphate, using lithium nickel phosphate as the coating layer can improve the kinetics and stability of the positive electrode active material.

[0164] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application rather than to limit the protection scope of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present application can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A positive electrode active material, characterized in that, It includes a core and a coating layer. The coating layer is disposed on the surface of the core. The core includes lithium nickel manganese oxide, and the coating layer includes lithium nickel phosphate. The particle size Dn50 of the core and the thickness d of the coating layer satisfy: 1.5 nm / μm ≤ d / (Dn50) ≤ 14 nm / μm.

2. The positive electrode active material according to claim 1, wherein, 4 nm / μm ≤ d / (Dn50) ≤ 6 nm / μm.

3. The positive electrode active material according to claim 1, characterized in that, The particle size Dn50 of the core is 3 - 10 μm.

4. The positive electrode active material according to claim 1, wherein The thickness d of the coating layer is 8 - 60 nm.

5. The positive electrode active material according to claim 1, wherein The particle size Dn10 of the core is 0.5 - 5 μm; and / or, the particle size Dn90 of the core is 10 - 18 μm.

6. The cathode active material according to claim 1, wherein The particle strength of the positive electrode active material is 200 - 300 MPa.

7. The positive electrode active material according to claim 1, wherein The coating layer includes element M, and the element M includes at least one of Fe, Mn, Co, Mg, Ca, and Al.

8. A positive electrode sheet, characterized in that, It includes the positive electrode active material according to any one of claims 1 - 7.

9. The positive electrode sheet according to claim 8, wherein, The coating compaction density of the positive electrode sheet is 2.6 - 3.4 g / cm 3 .

10. A lithium-ion battery, characterized in that, It includes the positive electrode sheet according to any one of claims 8 - 9.

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