Positive active material, positive electrode and lithium-ion battery

Abstract

A positive active material characterized by comprising a core and a coating layer, the coating layer being disposed on the surface of the core, the core comprising lithium nickel manganese oxide, the coating layer comprising lithium nickel phosphate, and the particle size Dn50 of the core and the thickness “d” of the coating layer satisfy the following: 1.5nm / µm≤d / (Dn50)≤14nm / µm.

Description

TECHNICAL FIELD

[0001] The present invention relates to the technical field of secondary battery, and more particularly to a positive active material, a positive electrode and a lithium ion battery. BACKGROUND

[0002] The demand for high energy density and high power output in lithium-ion batteries has drawn attention to the research and development of high-voltage positive electrode materials. The spinel-structured lithium nickel manganese oxide (LNMO) material can provide a three-dimensional lithium ion passage channel, has good ionic conductivity, a high-voltage platform (close to 4.7 V), and a high theoretical specific capacity (147 mAh / g), thus offering good application potential in the lithium-ion battery sector.

[0003] However, lithium nickel manganese oxide (LNMO), as a positive active material, has poor electronic conductivity. Under high-voltage conditions, the side reaction between the cathode interphase and the electrolyte intensifies, and interphase byproducts increase, leading to poor stability of the cathode structure and a large amount of dissolved transition metal ions, thus reducing battery performance. SUMMARY OF THE INVENTION

[0004] In view of the disadvantages of the prior art, the object of the present application is to provide a positive active material, a positive electrode and a lithium ion battery.

[0005] The technical solution applied by the present application to achieve the above object is: The first aspect of the present application provides a positive active material including a core and a coating layer, wherein the coating layer is disposed on the surface of the core, the core includes lithium nickel manganese oxide, the coating layer includes lithium nickel phosphate, and 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.

[0006] Preferably 4 nm / µm≤d / (Dn50)≤6 nm / µm.

[0007] The particle size Dn50 of the core is preferably 3-10 µm; more preferably, the particle size Dn50 of the core is 3-8 µm.

[0008] The thickness “d” of the coating layer is preferably 8-60 nm; more preferably, the thickness “d” of the coating layer is 10-50 nm.

[0009] The particle size Dn10 of the core is preferably 0.5-5 µm; more preferably, the particle size Dn10 of the core is 0.5-4 µm.

[0010] The particle size Dn90 of the core is preferably 10-18 µm; more preferably, the particle size Dn90 of the core is 12-18 µm.

[0011] The particle strength of the positive active material is preferably 200-300 MPa.

[0012] The coating layer preferably comprises an element M, wherein the element M comprises at least one of Fe, Mn, Co, Mg, Ca, Al and Zn.

[0013] The second aspect of the present application provides a positive electrode comprising the positive active material as described in the first aspect.

[0014] The compaction density of the positive electrode coating layer is preferably 2.6-3.4 g / cm 3 .

[0015] The positive electrode preferably comprises a positive current collector, at least one surface of the positive current collector is coated with a positive active material layer, and the positive active material layer contains the positive active material.

[0016] The mass percentage of the positive active material in the positive active material layer is more preferably 90%~99%.

[0017] The third aspect of the present application provides a lithium ion battery comprising the positive electrode as described in the second aspect.

[0018] The lithium-ion battery preferably also comprises a negative electrode, an electrolyte and a separator.

[0019] Compared to the prior art, the advantageous effects of the present application are: The present application uses lithium nickel phosphate as the coating layer and lithium nickel manganese oxide as the core. By adjusting the particle size Dn50 of the core and the thickness d of the coating layer, d / (Dn50) is controlled within the range of 1.5–14 nm / µm, which can improve the stability of the material structure while inhibiting the side reaction between the cathode interphase and the electrolyte, reducing the dissolution of transition metal ions, and thus improving the cycling stability of the lithium-ion battery. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] To further explain the objects, technical solutions, and advantages of embodiments of the present invention, the technical solutions according to the embodiments of the present invention are described clearly and completely below. It should be understood that the described embodiments are part of the embodiments of the present invention, not all embodiments. Based on the embodiments of the present disclosure, all other embodiments obtained by those skilled in the art without creative effort are within the scope of the present invention.

[0021] In the present invention, the technical features defined with open limitation include a closed technical solution composed of the mentioned features and also an open technical solution containing the mentioned features.

[0022] It should be noted that with respect to a numerical interval range in the present invention, unless otherwise stated, the above numerical interval range is considered continuous and includes the minimum and maximum values ​​of the defined range, as well as any value between such minimum and maximum values. Furthermore, when a range refers to an integer, any integer between the minimum and maximum values ​​of the range is included. When multiple ranges are arranged to describe ranges or characteristics, the ranges may be merged. In other words, all ranges disclosed herein, unless otherwise stated, are to be understood as including any and all subranges included therein.

[0023] In the present invention, there is no particular limitation on specific dispersing and stirring treatment methods.

[0024] The first aspect of the present application provides a positive active material comprising a core and a coating layer, wherein the coating layer is disposed on the surface of the core, the core comprises lithium nickel manganese oxide, the coating layer includes lithium nickel phosphate, and 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.

[0025] The positive active material of the present application comprises a core and a coating layer structure. By regulating the particle size Dn50 of the core and the thickness "d" of the coating layer, d / (Dn50) is controlled within the range of 1.5-14 nm / µm, which can improve the stability of the material structure while inhibiting the side reaction between the cathode interphase and the electrolyte, reducing the dissolution of transition metal ions, and thus improving the cycling stability of lithium-ion batteries.

[0026] Preferably, 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 formed by any two of the above values.

[0027] In particular, 4 nm / µm≤d / (Dn50)≤6 nm / µm, and controlling d / (Dn50) within the range of 4-6 nm / µm can achieve a better balance between the kinetics and stability of the positive active material.

[0028] The particle size Dn50 of the core is 3-10 µm in some embodiments.

[0029] The particle size Dn50 of the core is preferably 3-8 µm.

[0030] The inventors found that by controlling the particle size Dn50 of the core to within the range of 3-8 µm, the mechanical strength and dynamics of the positive active material can be taken into account, and the positive active material particles are not easily broken during the rolling process, which can better alleviate the side reactions between the cathode interphase and the electrolyte.

[0031] For example, in some embodiments, the particle size Dn50 of the core can 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 formed by any two of the above values.

[0032] The particle size Dn50 of the core is preferably 5-6 µm, within this range the mechanical strength and dynamics of the positive active material are better.

[0033] The particle size Dn10 of the core is 0.5-5 µm in some embodiments.

[0034] For example, Dn10 can 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 formed by any two of the above values.

[0035] The particle size Dn10 of the core is preferably 0.5-4 µm.

[0036] In some embodiments, the particle size Dn90 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 formed by any two of the above values.

[0037] The particle size Dn90 of the core is preferably 12-18 µm.

[0038] It should be understood that in the present application, the particle size Dn50 of the core refers to the particle size corresponding to when the percentage of the cumulative particle size distribution of the core reaches 50%; the particle size Dn90 of the core refers to the particle size corresponding to when the percentage of the cumulative particle size distribution of the core reaches 90%; the particle size Dn10 of the core refers to the particle size corresponding to when the percentage of the cumulative particle size distribution of the core reaches 10%.

[0039] In some embodiments, the thickness d of the coating layer is 8-60 nm. The thickness d of the coating layer may, for example, 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 a range formed by any two of the above values.

[0040] The thickness “d” of the coating layer is preferably 10-50 nm.

[0041] The inventors found that controlling “d” within the range of 10-50 nm can ensure the electronic conductivity of the positive active material while enhancing the inhibitory effect of the coating layer on the side reaction between the cathode interphase and the electrolyte.

[0042] The thickness “d” of the coating layer is particularly preferably 20-30 nm. The overall performance of the positive active material is better within this range.

[0043] The particle strength of the positive active material is 200-300 MPa in some embodiments. The particle strength of the positive active material may be, for example, 200 MPa, 210 MPa, 220 MPa, 230 MPa, 240 MPa, 250 MPa, 260 MPa, 270 MPa, 280 MPa, 300 MPa, or a range formed by any two of the above values.

[0044] In some embodiments, the chemical formula of the lithium nickel manganese oxide is LiNi x Mn 2-x O4, where 0 <x≤0,5.

[0045] In some embodiments, the chemical formula of lithium nickel phosphate is LiNiPO4.

[0046] In some embodiments, the coating layer comprises an element M, wherein the element M comprises at least one of Fe, Mn, Co, Mg, Ca, Al, and Zn.

[0047] The present application dopes the element M into lithium nickel phosphate by physical or chemical doping, and a portion of the element M replaces nickel atoms in the crystal structure of lithium nickel phosphate, and a portion of the element M is distributed in the lattice defects of lithium nickel phosphate. The positive active material of the present application is detected by XRD (X-ray diffraction), and it was found that there is no alloy phase and metal phase in the crystal phase within the material. Thus, the present application maintains the original structure of lithium nickel phosphate while improving the conductivity of the coating layer by doping with the element M, thereby ensuring the structural stability of lithium nickel phosphate.

[0048] In some embodiments, the method for producing the core comprises the following steps: dissolving the first lithium source, the first nickel source and the manganese source in a solvent according to a certain mole, mixing them to obtain a mixture, and calcining the mixture to obtain a core.

[0049] In some embodiments, the molar ratio of the lithium element in the first lithium source, the nickel element in the first nickel source, and the manganese element in the manganese source is y:x:(2-x), where 0 <x≤0,5, 1≤y≤1,05.

[0050] In some embodiments, the first lithium source comprises, but is not limited to, a lithium source selected from at least one of lithium hydroxide, lithium carbonate, lithium oxalate, and lithium acetate; the first nickel source comprises, but is not limited to, at least one of nickel hydroxide, nickel carbonate, and nickel nitrate; the manganese source comprises, but is not limited to, at least one of manganese hydroxide, manganese oxide, and manganese sulfate; the solvent comprises at least one of ethanol, N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), acetonitrile, and dimethyl carbonate (DMC).

[0051] In some embodiments, in the process for preparing the inner core, the heating rate of calcination is 2-10 °C / min, the peak temperature of calcination is 650-850 °C, and the residence time of calcination is 12-24 h.

[0052] In some embodiments, the mixing process in the process for preparing the core includes at least one of, but is not limited to, milling and sol-gel processes.

[0053] In the core manufacturing process, in some embodiments, the mixed material is calcined, cooled, and ground. The grinding process includes, but is not limited to, at least one of ball milling and sand milling.

[0054] The process for producing the positive active material comprises, in some embodiments, the following steps: The second lithium source, the second nickel source, the M source and the phosphorus source are physically or chemically coated according to a certain molar ratio to form a coating layer on the surface of the core.

[0055] For example, in some embodiments, the physical or chemical coating process comprises at least one of atomic layer deposition, chemical vapor deposition, solid phase sintering, and solution combustion.

[0056] In some embodiments, the second nickel source comprises, but is not limited to, at least one of tetrakis(dimethylamino)nickel, tert-butylnickel, tetrakis(ethylmethylamino)nickel, tetrakis(ethylamino)nickel, and tetraethoxynickel.

[0057] In some embodiments, the second lithium source comprises, but is not limited to, at least one of lithium tert-butyl alcohol, lithium acetoacetate, tert-butyllithium, and lithium di(trimethylsilyl)amide.

[0058] In some embodiments, the phosphorus source comprises, but is not limited to, at least one of tetramethylmethylene diphosphate, trimethyl phosphate, tri(dimethylamine)phosphine, or trialkylphosphine oxide.

[0059] The M source comprises, in some embodiments, but is not limited to, the tert-butyl alcohol salt of the M element.

[0060] In some embodiments, the oxygen source comprises, but is not limited to, at least one of water, ozone, or hydrogen peroxide.

[0061] The second aspect of the present application provides a positive electrode comprising the positive active material as described in the first aspect.

[0062] The coating compaction density of the positive electrode is 2.6-3.4 g / cm 3 .

[0063] The present invention controls the compaction density of the positive electrode to within the range of 2.6-3.4 g / cm 3, which can reduce the risk of particle breakage of the positive active material during rolling, inhibit the side reaction between the positive electrode and the electrolyte, facilitate the complete infiltration of the electrolyte, and improve the capacity and energy density of the battery.

[0064] In some embodiments, the positive electrode comprises a positive current collector, at least one surface of the positive current collector is coated with a positive active material layer, and the positive active material layer contains the positive active material.

[0065] The positive active material layer also includes a positive binder and a positive conductor.

[0066] The positive conductor may comprise a conductor conventional in the battery sector, for example, the positive conductor comprises at least one of conductive carbon black, conductive graphite, graphene, carbon nanotubes, and carbon fibers.

[0067] The positive binder is used to improve the adhesion between the positive active material particles and the adhesion between the positive active material and the positive current collector. The positive binder may include a binder conventionally used in the battery field. For example, the positive 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 carboxymethylcellulose (CMC), and sodium alginate.

[0068] In the present application, based on the total mass of the positive active material layer, the mass percentage of the positive active material is 90-99%, the mass percentage of the positive binder is 0.5-5%, and the mass percentage of the positive conductor is 0.5-5%. For example, the mass percentage of the positive active material may be 90%, 92%, 94%, 96%, 98%, 99%, or a range formed by any two of the above values ​​therein; the mass percentage of the positive binder may be 0.5%, 1%, 2%, 3%, 4%, 5%, or a range formed by any two of the above values ​​therein; and the mass percentage of the positive conductive agent may be 0.5%, 1%, 2%, 3%, 4%, 5%, or a range formed by any two of the above values ​​therein.

[0069] In the present application, the positive electrode can be prepared according to conventional methods in the art, for example, the positive active material, the positive conductive agent and the positive binder are dispersed in a solvent to obtain a positive slurry, the positive slurry is applied as a coating on at least one surface of the positive current collector, and the positive electrode is obtained after drying, rolling and cutting.

[0070] The third aspect of the present application provides a lithium ion battery comprising the positive electrode as described in the second aspect.

[0071] In some embodiments, the lithium-ion battery also includes a negative electrode.

[0072] In particular, the negative electrode comprises a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector. The negative active material layer comprises a negative active material, a negative conductive agent, and a negative binder. The negative conductive agent may include a conductive agent conventional in the battery sector. For example, the negative conductive agent comprises at least one of conductive carbon black, conductive graphite, graphene, carbon nanotubes, and carbon fibers.

[0073] The negative binder is used to improve the adhesion between the negative active material particles and the adhesion between the negative active material and the negative current collector. The negative binder may include a binder conventionally used in the battery industry. For example, the negative 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 carboxymethylcellulose (CMC), and sodium alginate.

[0074] In the present application, based on the total mass of the negative active material layer, the mass percentage of the negative active material is 90-99%, the mass percentage of the negative binder is 0.5-5%, and the mass percentage of the negative conductive agent is 0.5-5%. For example, the mass percentage of the negative active material may be 90%, 92%, 94%, 96%, 98%, 99%, or a range formed by any two of the above values ​​therein; the mass percentage of the negative binder may be 0.5%, 1%, 2%, 3%, 4%, 5%, or a range formed by any two of the above values ​​therein; and the mass percentage of the negative conductive agent may be 0.5%, 1%, 2%, 3%, 4%, 5%, or a range formed by any two of the above values ​​therein.

[0075] In the present application, the negative electrode can be prepared according to conventional methods in the art, for example, the negative active material, the negative conductive agent and the negative binder are dispersed in a solvent to obtain a negative slurry, the negative slurry is applied as a coating on at least one surface of the negative current collector, and the negative electrode is obtained after drying, rolling and cutting.

[0076] In some embodiments, in the formula, the lithium-ion battery further comprises a separator, wherein the separator comprises a substrate layer, and the substrate layer comprises at least one of polyethylene, polypropylene, polyamide, and aramid.

[0077] The separator may also comprise an adhesive layer and / or a ceramic layer. The material of the adhesive layer may comprise at least one of polyvinylidene fluoride, polymethyl methacrylate, aramid, polyethylene oxide, polyvinylidene fluoride-hexafluoropropylene copolymer, or polyaniline; the material of the ceramic layer may comprise at least one of boehmite, alumina, titanium dioxide, silicon dioxide, zirconium dioxide, tin dioxide, magnesium oxide, zinc oxide, barium sulfate, boron nitride, aluminum nitride, and magnesium nitride.

[0078] In some embodiments, the lithium-ion battery further comprises an electrolyte, and the electrolyte may comprise an electrolyte conventional in the battery sector.

[0079] The electrolyte preferably comprises a solvent and a lithium salt.

[0080] For example, in some embodiments, the solvent comprises at least one of a carbonate solvent, a carboxylate solvent, an ether solvent, a sulfone solvent, a nitrile solvent, and a phosphate solvent. The carbonates include dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, and propylene carbonate; the carboxylates 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.

[0081] In some embodiments, the lithium salt comprises at least one selected from lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium dioxalate borate, lithium difluorooxalate borate, lithium trifluoromethanesulfonate, lithium bisfluoromethanesulfonylimide, lithium bistrifluoromethanesulfonylimide, lithium difluorobisoxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0082] The concentration of the lithium salt in the electrolyte is 0.5-2 mol / L in some embodiments.

[0083] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application are described clearly and completely below. Of course, the described embodiments are only a portion of the embodiments of the present application, not all of them.

[0084] Unless otherwise stated, the experimental reagents and instruments involved in the implementation of this application are all commonly used reagents and instruments. Unless otherwise specified, the components and raw materials used in each embodiment and the comparative example of this application are all commercially available raw materials, and the components and raw materials used in each parallel experiment are all of the same type. Example 1

[0085] According to the embodiment of the lithium-ion battery described in the present application, the method for manufacturing the lithium-ion battery comprises the following steps: S1. Production of the positive active material S11. The raw materials Li2CO3, NiCO3, and MnO2 are mixed by ball milling in a molar ratio of 0.5:0.5:1.5 to obtain a mixture. The mixture is placed in a muffle furnace and heated to 850 °C at a rate of 8 °C / min, calcined at 850 °C, allowed to cool naturally to room temperature, and then sand-ground to obtain lithium nickel manganese oxide particles. The calcination time and sand grinding time are shown in Table 1. S12. The lithium nickel manganese oxide particles are placed in the reaction chamber of the atomic layer deposition device, the reaction temperature is set to 150 °C, and the chamber is evacuated to ensure the correct reaction. S13. Under the condition of an air pressure of 10 -2 Pa, steps S131 to S133 are carried out in sequence. S131. Nickel tert-butoxide gas is introduced into the chamber for 0.2 s, followed by argon gas purging for 30 s, and the argon purge flow rate is 20 ml / min. Ozone is then introduced into the chamber for 0.2 s, followed by argon gas purging for 30 s, and the argon purge flow rate is 20 ml / min. S132. Trimethyl phosphate gas is introduced into the chamber for 0.2 s, purged with argon gas for 30 s, and the argon purge flow rate is 20 ml / min. Then, ozone is introduced into the chamber for 0.2 s, purged with argon gas for 30 s, and the argon purge flow rate is 20 ml / min. S133. Lithium tert-butoxide gas is introduced into the chamber for 0.2 s, followed by argon gas purging for 30 s at a flow rate of 20 ml / min; then ozone is introduced into the chamber for 0.2 s, followed by argon gas purging for 30 s at a flow rate of 20 ml / min.

[0086] In step S13, the molar ratio of Li in lithium tert-butoxide, Ni in nickel tert-butoxide, M in the tert-butoxide salt of element M and P in trimethyl phosphate is Li:Ni:P=1:0.9:1.

[0087] The above step S13 is repeated, and step S13 is completed once to complete one deposition cycle. The number of deposition cycles is shown in Table 1, and the positive active material is obtained. S2. Preparation of the positive electrode

[0088] The positive active material, the conductive agent (multi-walled carbon nanotubes), and the binder (PVDF) are mixed in a mass ratio of 97:1:2. NMP solvent is added and stirred under the action of a vacuum mixer until the system is uniform, resulting in a positive slurry. The positive slurry is evenly coated onto the two opposite surfaces of the positive current collector (aluminum foil) with a surface density of 400 g / m². 2 applied, dried in a vacuum oven at 100 °C, and then divided into strips and compacted with a density of 2.2 g / cm 3 rolled to obtain a positive electrode. S3. Manufacturing of the separator

[0089] The separator is selected from commercial PE polyethylene separator with a thickness of 15 µm. S4. Preparation of the negative electrode

[0090] The negative active material (artificial graphite), the conductive agent (acetylene black), the thickener (CMC), and the binder (SBR) are mixed in a mass ratio of 96.4:1:1.2:1.4. The solvent (deionized water) is added, and the mixture is stirred under the action of a vacuum mixer until the system is uniform, resulting in a negative slurry. The negative slurry is evenly coated onto the two opposite surfaces of the negative current collector (copper foil) with a surface density of 200 g / m². 2 applied and then dried in a vacuum oven at 100 °C and then divided into strips and with a compaction density of 1.62 g / cm 3 cold-pressed to obtain a negative electrode. S5. Preparation of the electrolyte

[0091] Ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed in a volume ratio of 1:1 to obtain an organic solvent, and then completely dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte, and the concentration of LiPF6 in the electrolyte is 1 mol / L. S6. Manufacturing of the lithium-ion battery

[0092] The above-mentioned positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide insulation, and then wound to form a bare cell. The bare cell is placed in an outer packaging case, and the electrolyte is injected after drying. It is vacuum-packed and allowed to stand for 24 hours. Example 2-7

[0093] The difference between Example 2-7 and Example 1 is that in Example 2-7, the sanding time is changed in step S11, as shown in Table 1. Example 8-14

[0094] The difference between Example 8-14 and Example 1 is that in Example 8-14 the number of deposition cycles is changed as shown in Table 1. Example 15-22

[0095] The difference between Example 15-22 and Example 1 is that step S13 in Example 15-22 is as follows: S13. Under the condition of a gas pressure of 10-2 Pa, steps S131 to S134 are carried out in sequence: S131. Nickel tert-butylate gas is introduced into the chamber for 0.2 seconds, followed by 30 seconds of purging with argon gas at a purge flow rate of 20 ml / min. Ozone is then introduced into the chamber for 0.2 seconds, followed by 30 seconds of purging with argon gas at a purge flow rate of 20 ml / min. Ozone is then introduced into the chamber for 0.2 seconds, followed by 30 seconds of purging with argon gas at a purge flow rate of 20 ml / min. Purging is continued for 30 seconds, with an argon purge flow rate of 20 ml / min. S132. Trimethyl phosphate gas is introduced into the chamber for 0.2 seconds, followed by 30 seconds of purging with argon at a purge flow rate of 20 ml / min. Ozone is then introduced into the chamber for 0.2 seconds, followed by 30 seconds of purging with argon at a purge flow rate of 20 ml / min. S133. Lithium tert-butylate gas is introduced into the chamber for 0.2 s, followed by argon purge for 30 s, and the argon purge flow rate is 20 ml / min. Ozone is then introduced into the chamber for 0.2 s, followed by argon purge for 30 s, and the argon purge flow rate is 20 ml / min. S134. Tert-butyl alcohol salt of element M (element M is shown in Table 1) is introduced into the chamber as a gas for 0.2 s, purged with argon gas for 30 s, and the argon purge flow rate is 20 ml / min. Then, ozone is introduced into the chamber for 0.2 s, purged with argon gas for 30 s, and the argon purge flow rate is 20 ml / min.

[0096] In step S13, the molar ratio between Li in lithium tert-butyl alcohol, Ni in nickel tert-butyl alcohol, M in the tert-butyl alcohol salt of element M and P in trimethyl phosphate is Li:Ni:M:P=1:0.9:0.1:1;

[0097] The above step S13 is repeated, step S13 is completed once, one deposition cycle is completed, the number of deposition cycles is shown in Table 1, and a positive active material is obtained. Example 23-27

[0098] Examples 23-27 differ from Example 1 in that in Examples 23-27 the sanding time and the number of deposition cycles are changed, as shown in Table 1. Comparison example 1

[0099] Comparative Example 1 differs from Example 1 in that in Example 1 the number of deposition cycles is changed, as shown in Table 1. Comparison example 2

[0100] Example 2 differs from Example 1 in that in Example 2 the sanding time was changed, as shown in Table 1. Comparison example 3

[0101] Comparative Example 3 differs from Example 1 in that in Example 3, steps S12 to S13 are not performed and the lithium nickel manganese oxide obtained in step S11 is directly used as the positive active material. Comparison example 4

[0102] Comparative Example 4 differs from Example 1 in that the nickel tert-butoxide is replaced by iron tert-butoxide in step S131. Performance Test 1

[0103] The positive active material obtained in step S1 of the above embodiment and comparative example is tested as follows: (1) Composition of the coating layer: First, a layer of conductive adhesive is applied to the sample holder, the sample is adhered to the sample holder, and then a conductive film is plated. The tool and sample stage are wiped with anhydrous ethanol, and a certain amount of the sample to be tested is placed on the sample stage. The elemental composition of the coating layer is determined under a scanning electron microscope (SEM) at a magnification of 10Kx, combined with an EDS energy spectrum scan analysis.

[0104] The sample is fixed in the test mold with conductive tape and then tested with FTIR. The test conditions are: FTIR450 infrared spectrometer, scan range 400-2000 cm -1 , instrument resolution 4 cm -1 ; 500-700 cm -1 in the infrared spectrum represents the stretching and bending vibration of OPO, 900-1200 cm -1 represents the stretching vibration of PO4 3-, confirming the existence of phosphate; the absorption peak at 525 cm -1 is the stretching vibration of the Ni-O bond in the Ni-O octahedron. The composition of the coating layer is determined by combining the infrared spectrum and the elemental composition of the coating layer, determined by EDS energy spectrum scanning analysis.

[0105] (2) Particle size Dn50, Dn10, and Dn90 of the core: first, a layer of conductive adhesive is applied to the sample holder, the sample is attached to the sample holder, CP argon ion polishing is performed, and then the conductive film is coated. The tools and sample stage are wiped with anhydrous ethanol, and a certain amount of the sample to be tested is placed on the sample stage; it is observed under a scanning electron microscope (SEM). The SEM is used to adjust the magnification to 5Kx. Three areas are selected for image acquisition, the particle size of all particles in each area is measured, Dn50 is calculated, which counts for 50% of the numerical distribution, Dn10 is calculated, which counts for 10% of the numerical distribution, and Dn90 is calculated, which counts for 90% of the numerical distribution. The corresponding average values ​​of Dn50, Dn10, and Dn90 of the three areas are taken.

[0106] (3) Thickness of coating layer “d”: Take a small amount of sample and place it in a sample tube, add anhydrous ethanol and sonicate for 40 min; use a pipette to take a small amount of solution and drop it onto the copper mesh; finally, the sample is completely dried in a vacuum drying oven; place the sample on a test table and transfer it into the cavity, and use TEM to magnify the coating thickness to a suitably high magnification according to different coating thicknesses; select five areas for measurement, and take the average value.

[0107] (4) Particle strength: Refer to GB / T 43091-2023 for the powder compression strength test. The test results are shown in Table 1. Performance Test 2

[0108] The lithium-ion batteries obtained in the above embodiments and comparative examples are used as test objects, and the battery capacity retention rate and DCR kinetic growth rate after 100 cycles are tested: After standing, the LAND system is used to charge to 4.8V at a constant current of 0.33C and then discharge to 3.5V at a constant current of 0.33C, performing two cycles. Afterward, the battery is removed from the charger for standby. After the battery is activated, it is charged to 4.8V at a constant current rate of 1C at 45°C, then charged at a constant voltage to a current of less than 0.05C; then discharged to a voltage of 3.5V at a rate of 1C. A complete charge and discharge process is considered one cycle repeated 100 times. The capacity retention rate is calculated according to the following formula: Capacity retention rate = discharge capacity of the 100th cycle / discharge capacity of the first cycle * 100%.

[0109] The DCR value after 100 cycles is recorded as R1, and the initial DCR value before the cycle was recorded is recorded as R0; then the DCR growth rate after 100 cycles = (R1-R0) / R0*100%.

[0110] The DCR test requires the battery to be at 50% SOC. The procedure for the DCR test is as follows: A1. The battery is left at room temperature for 10 minutes. A2. The battery is charged with constant current and constant voltage, with a constant current rate of 0.33 C and a voltage of 4.80 V. The constant voltage process continues until the current is ≤0.05 C, and then it is discharged with constant current to 3.5 V, with a current rate of 0.33 C. A3. Steps A1-A2 are repeated three times, and the discharge capacity of the last step A2 in the process is recorded as C1. A4. Repeat steps A1-A2 until the battery is fully charged. A5. Constant current discharge until the capacity is 0.5 C1, the discharge rate is 0.33 C, and the battery is in a state of 50% SOC. A6. The battery is charged while standing for 120 minutes, and the voltage V0 is recorded at the end of the standing period. A7. Discharge at a constant current for 18 seconds, with the current rate being 1 C, and the voltage V1 is recorded at the end of the discharge. A8. DCR is calculated using the formula: DCR=(V0-V1) / C1.

[0111] The results are shown in Table 1.

[0112] As can be seen from Table 1, each embodiment of the present invention controls parameters such as "d", Dn50, and d / (Dn50) within an appropriate range, so that the positive active material has higher particle strength and is not easily fractured during the rolling process. The kinetic performance and stability of the positive active material can also be improved, so that the battery capacity retention rate after 100 cycles is greater than 80% and the DCR kinetic growth rate is less than 10%. When 4 nm / µm≤d / (Dn50)≤6 nm / µm holds, the balance between kinetics and stability of the positive active material is better.

[0113] Compared with Example 1 and Examples 8-14, d / (Dn50) in Comparative Example 1 is too small, and the coating layer easily falls off with the expansion and contraction of the positive electrode volume during the charging and discharging process, and the side reaction between the cathode interphase and the electrolyte cannot be well suppressed, resulting in a significant decrease in the battery capacity retention rate and DCR kinetic growth rate after 100 cycles.

[0114] Compared with Examples 1-7, Dn50 in Comparative Example 2 is relatively small, which makes d / (Dn50) too large, and the electron transport performance of the core is poor, resulting in the aggravation of the side reaction between the cathode interphase and the electrolyte, resulting in a significant decrease in the battery capacity retention rate after 100 cycles and the DCR kinetic growth rate.

[0115] The positive active material in Comparative Example 3 has no coating layer compared with each embodiment, which leads to the deterioration of the kinetics and stability of the positive active material.

[0116] It is evident from Example 1 and Comparative Example 4 that the use of lithium nickel phosphate as a coating layer can improve the kinetics and stability of the positive active material compared with lithium iron phosphate.

[0117] Finally, it should be noted that the above embodiments are used only to illustrate the technical solution of this article, rather than limiting the scope of this article. Although the present application has been described in detail with reference to the preferred embodiments, one skilled in the art should understand that the technical solution of this article may be modified or replaced with equivalents without deviating from the essence and scope of the technical solution of this article.

Claims

[1] Positive active material, characterized by that it comprises a core and a coating layer, wherein the coating layer is arranged on the surface of the core, wherein the core comprises lithium nickel manganese oxide, the coating layer comprises lithium nickel phosphate, and the particle size Dn50 of the core and the thickness "d" of the coating layer satisfy the following: 1.5nm / µm≤d / (Dn50)≤14nm / µm. [2] Positive active material according to claim 1, characterized by that 4 nm / µm≤d / (Dn50)≤6nm / µm is met. [3] Positive active material according to claim 1, characterized by that the particle size Dn50 of the core is 3-10 µm. [4] Positive active material according to claim 1, characterized by that the thickness “d” of the coating layer is 8-60 nm. [5] Positive active material according to claim 1, characterized by that 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] Positive active material according to claim 1, characterized by that the particle strength of the positive active material is 200-300 MPa. [7] Positive active material according to claim 1, characterized by that the coating layer comprises an element M, wherein the element M comprises at least one of Fe, Mn, Co, Mg, Ca, Al and Zn. [8] Positive electrode, characterized by that it comprises the positive active material according to any one of claims 1 to 7. [9] Positive electrode according to claim 8, characterized by that the compaction density of the coating layer of the positive electrode is 2.6-3.4 g / cm 3 amounts. [10] Lithium-ion battery, characterized by that it comprises the positive electrode according to one of claims 8 to 9.