Single-crystal positive active material, preparation method thereof, positive plate and battery

Abstract

The invention provides a single-crystal positive electrode active material and a preparation method thereof, a positive plate and a battery, the single-crystal positive electrode active material comprises a positive electrode active material matrix, the positive electrode active material matrix comprises a bulk phase matrix located at the center and a surface layer matrix coating the outer surface of the bulk phase matrix, and the positive electrode active material matrix comprises a lithium-nickel mixed arrangement structure; element E is doped in the surface matrix, and the valence of the element E is larger than trivalence. The E element is doped in the surface matrix, the E element can occupy the sites of lithium, and part of lithium atoms occupying the sites can occupy the original sites of nickel atoms; the nickel atoms of which the sites are occupied by the E element can also be migrated to the positions of the lithium atoms. High-valence element doping increases the mixing degree between lithium and nickel. When lithium ions are separated out and migrated, nickel atoms and E atoms still exist in the crystal structure to support the crystal structure, so that the material is prevented from cracking due to excessive structural change, and the cycle performance of the material is improved.

Description

Technical Field

[0001] This invention relates to the field of batteries, and more particularly to a single-crystal positive electrode active material and its preparation method, a positive electrode sheet, and a battery. Background Technology

[0002] In recent years, with the widespread application of new energy vehicles, energy storage systems, and portable electronic devices, the lithium-ion battery market has developed rapidly. Lithium batteries, due to their high energy density, long cycle life, and good charge / discharge efficiency, have become the most mainstream energy storage technology.

[0003] During charging and discharging, lithium ions intercalate and deintercalate into the crystal structure of the cathode active material, causing changes in the material's crystal structure. Especially under high voltage and high rate operation, the large-scale intercalation and deintercalation of lithium ions leads to significant mechanical stress on the cathode active material. If this stress is not uniformly distributed, microcracks can easily form within the cathode material, and these cracks propagate with increasing cycle count, eventually causing the material to fracture. Currently, some studies use single-crystal cathode active materials to reduce the instability caused by grain boundaries. While this offers some improvement compared to secondary particles, cracks are still easily induced under high voltage, thus affecting the battery's cycle performance.

[0004] Therefore, improving the stability of positive electrode active materials, and thus improving the cycle performance of batteries, is an urgent problem to be solved. Summary of the Invention

[0005] This invention provides a single-crystal positive electrode active material and its preparation method, a positive electrode sheet, and a battery, which can improve the stability of the positive electrode active material and improve the cycle performance of the battery.

[0006] This invention provides a single-crystal positive electrode active material, comprising: a positive electrode active material matrix, wherein the positive electrode active material matrix includes a bulk phase matrix located at the center and a surface matrix covering the outer surface of the bulk phase matrix, and the positive electrode active material matrix includes a lithium-nickel hybrid structure;

[0007] The surface matrix is ​​doped with element E, and the oxidation state of element E is greater than trivalent.

[0008] Furthermore, the lithium-nickel mixture in the positive electrode active material matrix is ​​0.5-2%; and / or the thickness of the surface matrix is ​​10-100 nm.

[0009] Furthermore, in the surface matrix, the content of divalent nickel on the side away from the bulk matrix is ​​greater than the content of divalent nickel on the side closer to the bulk matrix.

[0010] Furthermore, the chemical composition of the positive electrode active material matrix is ​​Li a Nix Co y Mn z M m E e O2; and / or, the E element is doped at a site of at least one of nickel, cobalt, manganese and lithium; and / or, the E element includes one or more of W, Nb, Mo, Ta, Zr, Ti and Ce.

[0011] Furthermore, the bulk matrix is ​​doped with element M, wherein the oxidation state of element M is divalent or trivalent, and element M is doped at a site of at least one of nickel, cobalt, and manganese.

[0012] Furthermore, the M element includes one or more of Al, Mg, La, Y, and Sr.

[0013] Furthermore, the single-crystal positive electrode active material also includes a coating layer, which covers at least a portion of the surface of the positive electrode active material matrix, and the coating layer includes a lithium-containing fast ion conductor compound.

[0014] Furthermore, the coating layer thickness is 5~50nm; and or, the particle size D50 of the single crystal positive electrode active material is 4~7um; and or, the minimum particle size of the single crystal positive electrode active material is 1~2um, and the maximum particle size is 10~20um.

[0015] Secondly, this application provides a method for preparing the single-crystal positive electrode active material according to any one of the first aspects, the preparation method comprising:

[0016] The intermediate product was prepared by mixing the lithium source and the positive electrode active material precursor and sintering them in an oxygen atmosphere.

[0017] The intermediate product and the E-containing compound are mixed and sintered at 650-800℃ for 8-12 hours in an oxygen atmosphere. Then the temperature is lowered to 500-650℃ and held for 4-6 hours, and the material is naturally cooled to obtain a single-crystal positive electrode active material.

[0018] Furthermore, the intermediate product prepared by mixing the lithium source and the positive electrode active material precursor and sintering them in an oxygen atmosphere includes:

[0019] The lithium source, the positive electrode active material precursor, and the oxide containing element M are mixed and sintered at 400-600°C for 3-6 hours in an oxygen atmosphere. The temperature is then raised to 900-1000°C and held for 8-16 hours. The temperature is then lowered to 750-900°C and held for 4-8 hours. The intermediate product is obtained by natural cooling.

[0020] Thirdly, this application provides a positive electrode sheet, which includes the single-crystal positive electrode active material described in any one of the first aspects, or the single-crystal positive electrode active material prepared by the preparation method described in the second aspect.

[0021] Fourthly, this application provides a battery comprising the positive electrode plate described in the third aspect.

[0022] The single-crystal cathode active material provided in this application includes a cathode active material matrix, which comprises a central bulk matrix and a surface matrix covering the outer surface of the bulk matrix. The cathode active material matrix includes a lithium-nickel hybrid structure. The surface matrix is ​​doped with element E, where the valence of E is greater than trivalent. When E is doped into the surface matrix, it can occupy lithium sites, and some of the occupied lithium atoms can occupy the original nickel sites. Nickel atoms at sites occupied by E can also migrate to lithium sites. On the one hand, because element E has a high valence (greater than trivalent), after replacing lithium or transition metal elements, it can form bonds with more negatively valent elements (such as oxygen ions), enhancing the internal bonding force of the single-crystal cathode active material and helping to improve its cycle performance during long-term charge-discharge processes. On the other hand, high-valence E element doping increases the degree of mixing between lithium and nickel. When lithium ions are deposited and migrate, nickel atoms and E atoms are still supporting the crystal structure, preventing the material from cracking due to excessive structural changes and improving the material's cycle performance. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of the single-crystal positive electrode active material provided in this application;

[0024] Figure 2 This is a SEM image of the single-crystal positive electrode active material of Example 1;

[0025] Figure 3 This is a SEM image of the single-crystal positive electrode active material in Comparative Example 1.

[0026] Figure 4 This is a cross-section of the single-crystal positive electrode active material of Example 1;

[0027] Figure 5 This is a Ce element distribution diagram within the cross-section of the single-crystal positive electrode active material of Example 1;

[0028] Figure 6 This is a cross-sectional diagram of the Mo element distribution in the single-crystal positive electrode active material of Example 1;

[0029] Figure 7 The image shows the X-ray photoelectron spectrum of the nickel valence state in the coating layer of the single-crystal cathode material in Example 1. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0031] Traditional polycrystalline cathode active material particles are prone to cracking and breakage during cycling, leading to shortened cycle life and deteriorated battery performance. To overcome these shortcomings, monocrystalline cathode active materials have been proposed. Monocrystalline materials have fewer grain boundaries, which can effectively alleviate strain. However, monocrystalline materials can still crack under high pressure, affecting battery performance.

[0032] To increase the stability of monocrystalline cathode active materials and improve the cycle performance of batteries, high-valence elements are introduced into the shallow surface layer of monocrystalline cathode active materials to increase the bonding force between materials. A lithium-nickel mixed structure is also introduced, which can support the crystal structure through the mixed nickel after lithium is desorbed, thereby increasing the stability of monocrystalline cathode active materials.

[0033] Figure 1 This is a schematic diagram of the structure of the single-crystal positive electrode active material provided in this application, as shown below. Figure 1 As shown, the single-crystal positive electrode active material includes: a positive electrode active material matrix, which includes a bulk matrix located at the center and a surface matrix covering the outer surface of the bulk matrix. The positive electrode active material matrix includes a lithium-nickel hybrid structure.

[0034] The surface matrix is ​​doped with element E, and the oxidation state of element E is greater than trivalent.

[0035] The cathode active material matrix can be a ternary lithium cathode active material, whose single crystal is composed of transition metals (such as nickel, cobalt, manganese; or nickel, cobalt, aluminum) and lithium, forming a stable crystal lattice structure.

[0036] High-valence element E is doped onto the surface layer of the positive electrode active material matrix to obtain a surface matrix. The E element is doped into the crystal structure of the surface matrix, occupying some lithium sites and transition metal sites in the original crystal structure.

[0037] Because of its high chemical valence, E can bond with more negatively valenced elements (such as oxygen ions) after replacing lithium or transition metal elements, thus enhancing the bonding force inside the single-crystal cathode active material and helping to improve the cycle performance of the single-crystal cathode active material during long-term charge and discharge.

[0038] After doping, lithium atoms occupy lithium sites, and some of these occupied lithium atoms can occupy the original nickel sites, or nickel atoms can migrate to the lithium sites. This increases the degree of mixing between lithium and nickel. When lithium ions are deposited and migrate, nickel and lithium atoms in the lithium-nickel mixed structure still support the crystal structure, preventing the material from cracking due to excessive structural changes and improving the material's cycle performance.

[0039] In some embodiments, the lithium-nickel mixing degree in the positive electrode active material matrix is ​​0.5% to 2%, for example, 0.5%, 1%, 1.5%, 2%, or any combination thereof. A lithium-nickel mixing degree greater than 0.5% can increase the stability of the material. When the mixing degree is greater than 0.5%, it is sufficient to provide a certain degree of stability support for the crystal lattice structure, thereby effectively improving the cycle performance of the material. When the lithium-nickel mixing degree exceeds 2%, it often alters the structure of ion channels in the crystal, affecting lithium-ion migration. Therefore, a lithium-nickel mixing degree between 0.5% and 2% can balance the stability and ionic conductivity of the material, improving the cycle performance of the battery.

[0040] In some embodiments, the thickness of the surface substrate is 10-100 nm. For example, 10 nm, 30 nm, 50 nm, 70 nm, 100 nm, or any combination thereof. A surface substrate with a lithium-nickel hybrid structure having a thickness of 10-100 nm can improve the structural stability of the single-crystal cathode active material and reduce its impact on the diffusion rate of lithium ions.

[0041] In some embodiments, the divalent nickel content on the outer side of the surface substrate (i.e., the side furthest from the bulk substrate) is greater than the divalent nickel content on the inner side (the side closer to the bulk substrate). The higher divalent nickel content on the outer side of the surface substrate may be due to a greater degree of lithium-nickel mixing. After lithium ions are extracted, the outer lattice structure is more compact and stable, protecting the inner material and preventing it from peeling off.

[0042] In some embodiments, the chemical composition of the positive electrode active material matrix is ​​Li a Ni x Co y Mn z M m E e O2, 1.0<a<1.03, 0.8≤x<1, 0<y<0.1, 0<z<0.1, 0<m<0.1, 0<e<0.1, x+y+z+m+e =1.

[0043] In one implementation, element E is doped at at least one elemental site in nickel, cobalt, manganese, and lithium. The difference between the valence of element E and divalent nickel is ≥2. The higher valence of element E allows it to form chemical bonds or coordination bonds with transition metals or lithium ions. In the crystal, element E occupies at least one elemental site in nickel, cobalt, manganese, and lithium, and doping improves the cycle performance of the cathode active material.

[0044] In some embodiments, the chemical composition of the positive electrode active material matrix is ​​Li a Ni x Co y Al z M m E e O2, Li a Ni x Co y Mn z M m E e O2, 1.0<a<1.03, 0.8≤x<1, 0<y<0.1, 0<z<0.1, 0<m<0.1, 0<e<0.1, x+y+z+m+e =1.

[0045] E is doped at at least one elemental site in nickel, cobalt, aluminum, and lithium. Similarly, for nickel-cobalt-aluminum ternary active materials, E can also be doped into the surface matrix, replacing transition metal or lithium ion sites, which can improve the cycle performance of the cathode active material.

[0046] In some embodiments, element E includes one or more of W, Nb, Mo, Ta, Zr, Ti, and Ce. The valence of element E in the cathode active material matrix is ​​greater than trivalent, creating a valence difference with divalent nickel to achieve doping of transition metals and lithium sites, increasing lithium-nickel mixing.

[0047] In some embodiments, the bulk matrix is ​​doped with element M, which has a divalent or trivalent valence. Element M is doped at at least one elemental site of nickel, cobalt, and manganese. The valence difference between element M and divalent nickel is less than 2, for example, it is divalent or trivalent. Its valence state is similar to that of transition metals, making it easier to dope into transition metal sites and less likely to dope into lithium sites.

[0048] In some embodiments, element M includes one or more of Al, Mg, La, Y, and Sr. The valence of element M is divalent or trivalent to reduce the possibility of doping into lithium sites.

[0049] In some embodiments, the single-crystal cathode active material further includes a coating layer covering at least a portion of the surface of the cathode active material matrix. The coating layer comprises a lithium-containing fast-ion conductor compound, the ionic conductivity and / or electronic conductivity of which is greater than that of the cathode active material matrix. The presence of the coating layer provides an interface with higher ionic conductivity, thereby improving the ion transport rate and electronic conductivity of the entire cathode active material. The coating layer also provides mechanical support, increasing the surface strength and stability of the cathode active material, reducing the risk of material breakage or detachment during charge and discharge, and improving the cycle stability of the battery.

[0050] In addition, the coating layer can also isolate the electrolyte, reduce the corrosion of the positive electrode active material by the electrolyte, and extend the service life of the material.

[0051] In some embodiments, if the lithium-containing fast-ion conductor compound contains the element L, then the general formula of the positive electrode active material matrix is ​​Li. a Ni x Co y Mn z M m E e L n In O2, 1.0<a<1.03, 0.8≤x<1, 0<y<0.1, 0<z<0.1, 0<m<0.1, 0<e<0.1, 0<l<0.1, x+y+z+m+e+n=1.

[0052] In some embodiments, the coating thickness is 5–50 nm, for example, 5 nm, 15 nm, 25 nm, 35 nm, 45 nm, 50 nm, or any combination thereof. A coating thickness of 5–50 nm helps optimize ion and electron conduction, improving the stability and cycling performance of the material.

[0053] In some embodiments, the particle size D50 of the single-crystal positive electrode active material is 4~7 μm, for example, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, or any combination thereof. The positive electrode active material of this application is a single-crystal positive electrode material with a D50 within the range of 4~7 μm, which can effectively reduce the breakage or shedding of particles caused by volume expansion during charging and discharging, thereby improving the cycle stability of the material.

[0054] In some embodiments, the minimum particle size of the single-crystal positive electrode active material is 1~2 μm, for example, 1 μm, 1.25 μm, 1.5 μm, 1.75 μm, 2 μm, or any combination thereof. The maximum particle size is 10~20 μm, for example, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, or any combination thereof.

[0055] This application also provides a method for preparing a single-crystal positive electrode active material, the method comprising:

[0056] Step 1: Mix the lithium source and the positive electrode active material precursor, and sinter them in an oxygen atmosphere to prepare the intermediate product.

[0057] In one specific implementation, under an oxygen atmosphere, the mixed solid is heated to 400~600℃ at a heating rate of 2-5℃ and held for 3~6 hours, then the temperature is raised to 900~1000℃ and held for 8~16 hours, and then the temperature is lowered to 750~900℃ and held for 4~8 hours, and then cooled naturally to obtain the intermediate product.

[0058] Step 2: Mix the intermediate product with the E-containing compound, sinter at 650~800℃ for 8~12h in an oxygen atmosphere, then lower the temperature to 500~650℃ and hold for 4~6h, and allow it to cool naturally to obtain the single-crystal positive electrode active material.

[0059] Specifically, compounds containing element E can be oxides of element E.

[0060] Through the above preparation process, element E can be doped onto the surface of the positive electrode active material substrate, causing lithium-nickel mixing.

[0061] In step 1, when preparing the intermediate product, element M can also be doped into the raw material.

[0062] The lithium source, positive electrode active material precursor, and oxide containing element M are mixed and sintered at 400~600℃ for 3~6h in an oxygen atmosphere. The temperature is then raised to 900~1000℃ and held for 8~16h. The temperature is then lowered to 750~900℃ and held for 4~8h. The intermediate product is obtained by natural cooling.

[0063] In some embodiments, the product obtained in step 2 is used as the positive electrode active material matrix, and then the outer side of the positive electrode active material matrix is ​​coated with a lithium-containing fast ion conductor compound.

[0064] Specifically, the nitrate and / or chloride salts of the fast ion conductor compound are dissolved in anhydrous ethanol or dimethyl methylamine, stirred evenly, and then stirred in an environment of 80~120℃ until dry. Then, under an oxygen atmosphere, the mixture is kept at 300~700℃ for 8~12h with the positive electrode active material matrix to obtain a single crystal positive electrode active material.

[0065] This application also provides a positive electrode sheet, which includes any of the above-mentioned single-crystal positive electrode active materials, or includes single-crystal positive electrode active materials prepared by any of the above-mentioned preparation methods.

[0066] The positive electrode sheet of this application specifically includes a positive current collector and a positive active layer formed of positive active material disposed on the surface of the positive current collector.

[0067] In the specific preparation of the positive electrode sheet, for example, the positive electrode active material of the present invention can be dispersed with a conductive agent and a binder in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and thoroughly stirred and mixed to form a uniform positive electrode slurry; the positive electrode slurry is uniformly coated on the positive electrode current collector, and after drying, rolling and slitting, the positive electrode sheet is obtained. In one specific embodiment, the positive electrode active layer comprises, by weight percentage, 70-99 wt% of positive electrode active material, 0.5-15 wt% of conductive agent, and 0.5-15 wt% of binder, and more specifically, 80-98 wt% of positive electrode active material, 1-10 wt% of conductive agent, and 1-10 wt% of binder.

[0068] The positive current collector can be made of at least one of aluminum foil or nickel foil; the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber; and the binder can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0069] This application also provides a battery comprising a positive electrode sheet prepared from the above-mentioned single-crystal positive electrode active material.

[0070] It is conceivable that, in addition to the aforementioned positive electrode, the lithium-ion battery of the present invention also includes a negative electrode, an electrolyte, and a separator.

[0071] This invention does not strictly limit the negative electrode active material in the negative electrode sheet. It can be at least one of the negative electrode active materials commonly used in lithium-ion batteries, such as graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-based negative electrode materials (mainly including silicon suboxide and silicon-carbon negative electrode), and tin-based negative electrode materials (mainly including tin and tin alloy).

[0072] This invention does not strictly limit the choice of electrolyte, and may include one or more solvents commonly used in lithium-ion battery electrolytes, as well as lithium salts commonly used in lithium-ion electrolytes. For example, the solvent may be ethylene carbonate, propylene carbonate, butene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, γ-butyrolactone, etc.; the lithium salt may be one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0073] This invention does not strictly limit the choice of separator material. It can be one of the separator materials commonly used in lithium-ion batteries, such as polypropylene separator (PP), polyethylene separator (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun separator (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven separator, and separator with ceramic coating.

[0074] In the preparation of lithium-ion batteries, the positive electrode, separator, and negative electrode are wound or stacked to obtain a bare cell, which is then packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery is dried at 85°C, the electrolyte is injected into the dried battery. The battery undergoes resting, formation, and secondary sealing to complete the preparation of the lithium-ion battery.

[0075] The present invention will be further described below through specific embodiments.

[0076] Example 1

[0077] The method for preparing the single-crystal positive electrode active material in this embodiment includes the following steps:

[0078] S1: Weigh out lithium hydroxide monohydrate and Ni at a molar ratio of Li:(Ni+Co+Mn)=1.04:1. 0.90 Co 0.05 Mn 0.05 The (OH)2 precursor was uniformly mixed in a high-speed mixer. The mixed raw material was then loaded into a sagger and sintered in a high-temperature atmosphere box furnace at an oxygen flow rate of 1.5 m³ / s. 3 Under the condition of / h, the temperature was increased to 500℃ at a heating rate of 2℃ / min and held for 4h; then the temperature was increased to 930℃ and held for 10h, then the temperature was decreased to 780℃ and held for 4h, and then allowed to cool naturally to obtain the intermediate product.

[0079] S2: Add 1000 ppm MoO3 and 2000 ppm CeO2 to the intermediate product, mix thoroughly in a high-speed mixer, load the mixed raw material into a sagger, and sinter in a high-temperature atmosphere box furnace with an oxygen flow rate of 1.5 m³ / min. 3 Under the condition of / h, the temperature was increased to 500℃ at a heating rate of 2℃ / min and held for 4h; then the temperature was increased to 850℃ and held for 10h, then the temperature was decreased to 780℃ and held for 4h, and then allowed to cool naturally to obtain the single crystal positive electrode active material.

[0080] Example 2

[0081] The preparation method in this embodiment is basically the same as that in Example 1, except that the amount of CeO2 added in step S2 is 3000 ppm.

[0082] Example 3

[0083] The preparation method in this embodiment is basically the same as that in Example 1, except that the amount of CeO2 added in step S2 is 1000 ppm.

[0084] Example 4

[0085] The preparation method of this embodiment is basically the same as that of Example 1, except that the amount of MoO3 and CeO2 added in step S2 is 800 ppm.

[0086] Example 5

[0087] The preparation method of this embodiment is basically the same as that of Example 1, except that the amount of CeO2 added in step S2 is 5000ppm and the maximum temperature is 870℃.

[0088] Example 6

[0089] The preparation method of this embodiment is basically the same as that of Example 1, except that the amount of MoO3 and CeO2 added in step S2 is 800 ppm and the maximum temperature is 830°C.

[0090] Example 7

[0091] The preparation method in this embodiment is basically the same as that in Example 1, except that the amount of MoO2 added in step S2 is 2000ppm, the amount of CeO2 added is 5000ppm, and the maximum temperature is 850℃.

[0092] Example 8

[0093] The preparation method in this embodiment is basically the same as that in Example 1, except that in step S1, lithium hydroxide monohydrate and Ni are weighed in a molar ratio of Li:(Ni+Co+Mn)=1.04:1. 0.90 Co 0.05 Mn 0.05 (OH)₂ precursor was added, along with 500 ppm SrO and 1000 ppm Al₂O₃. The mixture was thoroughly mixed in a high-speed mixer, and the resulting material was loaded into a sagger and sintered in a high-temperature atmosphere box furnace at an oxygen flow rate of 1.5 m³ / s. 3 Under the condition of / h, the temperature was increased to 500℃ at a heating rate of 2℃ / min and held for 4h; then the temperature was increased to 930℃ and held for 10h, then the temperature was decreased to 780℃ and held for 4h, and then allowed to cool naturally to obtain the intermediate product.

[0094] Example 9

[0095] The preparation method in this embodiment is basically the same as that in Example 1, except that step S3 is included after step S2.

[0096] S3. Dissolve the product from step S2, along with 1.5%wt aluminum nitrate and titanium nitrate, in anhydrous ethanol, stir until homogeneous, and then stir at 80°C until dry. Place the resulting powder in a high-temperature furnace under an oxygen atmosphere and sinter at 670°C for 10 hours. After natural cooling, the final single-crystal positive electrode active material is obtained.

[0097] Example 10

[0098] The preparation method in this embodiment is basically the same as that in Example 9, except that the 1000 ppm MoO3 added in step S2 is replaced with 1000 ppm Nb2O5.

[0099] Example 11

[0100] The preparation method of this embodiment is basically the same as that of Example 9. The difference is that in step S3, the product of step S2, along with 1.5%wt of niobium oxalate and ammonium paratungstate, is dissolved in anhydrous ethanol, stirred evenly, and then stirred at 80°C until dry. The resulting powder is then placed in a high-temperature furnace under an oxygen atmosphere and sintered at 700°C for 10 hours. After natural cooling, the final single-crystal positive electrode active material is obtained.

[0101] Example 12

[0102] The preparation method of this embodiment is basically the same as that of Example 9. The difference is that in step S3, the product of step S2 and 1.5%wt aluminum nitrate and zirconium nitrate are dissolved in anhydrous ethanol, stirred evenly, and stirred in an environment of 80°C until dry. Then the obtained powder is placed in a high-temperature furnace in an oxygen atmosphere and sintered at 700°C for 10 hours. After natural cooling, the final single-crystal positive electrode active material is obtained.

[0103] Example 13

[0104] The preparation method in this embodiment is basically the same as that in Example 9, except that in S1, lithium hydroxide monohydrate and Ni are weighed in a molar ratio of Li:(Ni+Co+Mn)=1.04:1. 0.90 Co 0.05 Mn 0.05 The (OH)2 precursor was uniformly mixed in a high-speed mixer. The mixed raw material was then loaded into a sagger and sintered in a high-temperature atmosphere box furnace at an oxygen flow rate of 1.5 m³ / s. 3 Under the condition of / h, the temperature was increased to 500℃ at a heating rate of 2℃ / min and held for 4h; then the temperature was increased to 920℃ and held for 10h, then the temperature was decreased to 780℃ and held for 4h, and then allowed to cool naturally to obtain the intermediate product.

[0105] Example 14

[0106] The preparation method in this embodiment is basically the same as that in Example 9, except that in S1, lithium hydroxide monohydrate and Ni are weighed in a molar ratio of Li:(Ni+Co+Mn)=1.04:1. 0.90 Co 0.05 Mn 0.05 The (OH)2 precursor was uniformly mixed in a high-speed mixer. The mixed raw material was then loaded into a sagger and sintered in a high-temperature atmosphere box furnace at an oxygen flow rate of 1.5 m³ / s. 3 Under the condition of / h, the temperature was increased to 500℃ at a heating rate of 2℃ / min and held for 4h; then the temperature was increased to 960℃ and held for 10h, then the temperature was decreased to 780℃ and held for 4h, followed by natural cooling to obtain the intermediate product. The amount of CeO2 added to S2 was 4000ppm.

[0107] Example 15

[0108] The preparation method of this embodiment is basically the same as that of Example 9, except that the weight ratio of aluminum nitrate and titanium nitrate added in S3 is 5wt%.

[0109] Example 16

[0110] The preparation method of this embodiment is basically the same as that of Example 9, except that the weight ratio of aluminum nitrate and titanium nitrate added in S3 is 0.5wt%.

[0111] Example 17

[0112] The preparation method in this embodiment is basically the same as that in Example 1, except that in S2, 1000 ppm of MoO3 is added to the intermediate product, and the mixture is homogeneous in a high-speed mixer. The mixed raw material is then loaded into a sagger and sintered in a high-temperature atmosphere box furnace at an oxygen flow rate of 1.5 m³ / min. 3 Under the condition of / h, the temperature was increased to 500℃ at a heating rate of 2℃ / min and held for 4h; then the temperature was increased to 850℃ and held for 10h, then the temperature was decreased to 780℃ and held for 4h, and then allowed to cool naturally to obtain the single crystal positive electrode active material.

[0113] Example 18

[0114] The preparation method in this embodiment is basically the same as that in Example 1, except that in S2, 2000 ppm of CeO2 is added to the intermediate product, and the mixture is homogeneous in a high-speed mixer. The mixed raw material is then loaded into a sagger and sintered in a high-temperature atmosphere box furnace at an oxygen flow rate of 1.5 m³ / s. 3 Under the condition of / h, the temperature was increased to 500℃ at a heating rate of 2℃ / min and held for 4h; then the temperature was increased to 850℃ and held for 10h, then the temperature was decreased to 780℃ and held for 4h, and then allowed to cool naturally to obtain the single crystal positive electrode active material.

[0115] Comparative Example 1

[0116] The preparation method of the single-crystal positive electrode active material in this comparative example includes the following steps:

[0117] Weigh out lithium hydroxide monohydrate and Ni according to the molar ratio Li:(Ni+Co+Mn)=1.04:1. 0.90 Co 0.05 Mn 0.05 The (OH)2 precursor was uniformly mixed in a high-speed mixer. The mixed raw material was then loaded into a sagger and sintered in a high-temperature atmosphere box furnace at an oxygen flow rate of 1.5 m³ / s. 3 Under the condition of / h, the temperature was increased to 500℃ at a heating rate of 2℃ / min and held for 4h; then the temperature was increased to 930℃ and held for 10h, then the temperature was decreased to 780℃ and held for 4h, and then allowed to cool naturally to obtain the single crystal positive electrode active material.

[0118] Comparative Example 2

[0119] The preparation method in this embodiment is basically the same as that in Example 1, except that:

[0120] In S2, 3000 ppm of Al2O3 is added to the intermediate product and mixed evenly in a high-speed mixer. The mixed raw material is then loaded into a sagger and sintered in a high-temperature atmosphere box furnace with an oxygen flow rate of 1.5 m³ / min. 3 Under the condition of / h, the temperature was increased to 500℃ at a heating rate of 2℃ / min and held for 4h; then the temperature was increased to 850℃ and held for 10h, then the temperature was decreased to 780℃ and held for 4h, and then allowed to cool naturally to obtain the single crystal positive electrode active material.

[0121] Comparative Example 3

[0122] The preparation method in this embodiment is basically the same as that in Example 1, except that:

[0123] In S2, 2000 ppm SrO and 1000 ppm Al2O3 are added to the intermediate product and mixed evenly in a high-speed mixer. The mixed raw material is then loaded into a sagger and sintered in a high-temperature atmosphere box furnace with an oxygen flow rate of 1.5 m³ / min. 3 Under the condition of / h, the temperature was increased to 500℃ at a heating rate of 2℃ / min and held for 4h; then the temperature was increased to 850℃ and held for 10h, followed by cooling to 780℃ and holding for 4h, and then allowed to cool naturally to obtain a single-crystal positive electrode active material. And added

[0124] Test Example 1

[0125] Lithium-nickel mixing degree test of positive electrode active material matrix: The products obtained in step S2 of each example and comparative example were subjected to X-ray diffraction (XRD) test to obtain XRD patterns. The degree of lithium-nickel mixing was obtained by refinement (fitting XRD patterns).

[0126] By refining the XRD pattern, the location of the E element can also be determined, which occupies the sites of transition metals or lithium.

[0127] Test method for surface substrate thickness: Linear scanning of the cross-section of the positive electrode active substrate using EDS to determine the thickness of the layer containing element E.

[0128] Nickel valence state testing in the surface substrate: XPS was used to perform a line scan of the surface substrate region of the positive electrode active material to measure the proportion of nickel elements in different valence states at various locations. XPS testing revealed that the content of divalent nickel on the outer side (away from the bulk substrate) of the surface substrate was higher than that on the inner side (closer to the bulk substrate). The higher content of divalent nickel on the outer side of the surface substrate may be due to increased lithium-nickel mixing caused by high-valence electrons.

[0129] Testing of doping sites in the bulk matrix: The distribution of each element within the single crystal is confirmed by performing EPMA (electron probe microanalysis) or EDS surface scanning on the cross-section of the single crystal. Figure 4 This is a cross-section of the single-crystal positive electrode active material of Example 1; in Figure 4 A surface scan was performed on the cross-section to test the distribution of Ce and Mo elements. Figure 5 This is a Ce element distribution diagram within the cross-section of the single-crystal positive electrode active material of Example 1; Figure 6 The image shows the distribution of Mo in the cross-section of the single-crystal cathode active material in Example 1. Analysis of the three images reveals that... Figure 4 At the cross-section of medium to large bulk crystals, corresponding Figure 5 and Figure 6 In the bulk crystal, Ce and Mo elements are mainly distributed in the surface region, while there are almost no elements distributed in the middle region. Therefore, it can be determined that the doping elements are mainly distributed in the surface region of the single crystal.

[0130] The thickness of the coating layer was measured by high-resolution transmission electron microscopy.

[0131] Coating material testing: The valence states of the coating elements in the coating layer were obtained by X-ray photoelectron spectroscopy. The sample surface was sputtered layer by layer using an argon ion gun, and X-ray photoelectron spectroscopy was performed alternately to obtain the chemical composition and element valence state information from the surface to the interior. The valence state distribution of Ni element in the coating layer and the formation of lithium-containing fast ion conductor compound on the surface were determined. Combined with the rate performance test in Table 2, the rate performance of Examples 9-16 is better than that of Example 1, further confirming that the coating layer has the performance of a fast ion conductor compound. Figure 7 The image shows the X-ray photoelectron spectrum of the nickel valence state in the coating layer of the single-crystal cathode material in Example 1, combined with... Figure 7 It can be determined that the divalent nickel content on the outer side (the side away from the bulk matrix) of the surface matrix is ​​greater than the divalent nickel content on the inner side (the side closer to the bulk matrix) of the surface matrix.

[0132] Particle size test of positive electrode active material: The single crystal positive electrode active materials of each example and comparative example were subjected to external ultrasonic treatment for 30 minutes, and then the particle size was tested using a Malvern 2000 laser particle size analyzer to obtain particle size data. Figure 2 This is a SEM side view of the single-crystal positive electrode active material of Example 1. Figure 3 This is a SEM side view of the single-crystal positive electrode active material of Comparative Example 1. Figure 2 and Figure 3 The particle size and particle size tests for single-crystal cathode active materials are basically the same.

[0133] Table 1. Structure of the single-crystal positive electrode active materials of each embodiment and comparative example

[0134]

[0135] Test case

[0136] Preparation of coin cells: At 25°C and atmospheric pressure (0.1 MPa), the single-crystal positive electrode active material, conductive carbon black, and binder polyvinylidene fluoride (PVDF) of each example and comparative example were weighed at a mass percentage ratio of 95:3:2. After thorough mixing in N-methylpyrrolidone solvent, a positive electrode slurry was obtained. The positive electrode slurry was coated onto aluminum foil, dried, and cold-pressed to obtain a positive electrode sheet containing a 100 μm thick positive electrode active layer. The areal density of the positive electrode sheet was 4.12 g / 100 cm³. 2 The compacted density is 3.5 g / cm³. 3 Then, the positive electrode is punched into small round pieces with a diameter of 12mm using a film punch. After drying and weighing, the positive electrode is assembled into a coin cell using a 2025 coin cell case, a Li metal disc as the negative electrode, and conventional high-voltage lithium cobalt oxide electrolyte in a glove box under Ar protective atmosphere.

[0137] Button cell capacity test: The button cells of the examples and comparative examples were charged at a constant current rate of 0.2C, with a charging cutoff voltage of 4.3V. Under this cutoff voltage condition, the cells were charged at a constant voltage until the current was less than 0.05C. The charging capacity at this point was recorded as the first-cycle charging capacity. After resting for 5 minutes, the cells were discharged at a constant current rate of 0.1C until the voltage reached 2.5V. The discharge capacity at this point was recorded as the battery's first-cycle discharge specific capacity, which is also the initial capacity. The test results are detailed in Table 2.

[0138] Rate performance: Coin cells were prepared according to the method described in the coin cell capacity test. They were charged at a constant current rate of 0.1C until the cutoff voltage was reached. Then, under the cutoff voltage condition, they were charged at a constant voltage until the current was less than 0.05C. After resting for 5 minutes, they were discharged at a constant current rate of 0.1C until the voltage reached 2.5V. The capacity at this point was recorded as the discharge capacity at the 0.1C rate. After resting for 10 minutes, they were charged at a constant current rate of 2C until the cutoff voltage was reached. Then, under the cutoff voltage condition, they were charged at a constant voltage until the current was less than 0.05C. After resting for 5 minutes, they were discharged at a constant current rate of 2C until the voltage reached 2.5V. The capacity at this point was recorded as the discharge capacity at the 2C rate. The ratio of the discharge capacity at the 2C rate to the discharge capacity at the 0.1C rate is the 2C rate performance. The test results are detailed in Table 2.

[0139] Preparation of full cells: After fabricating the positive electrode active materials from all examples and comparative examples into positive electrode sheets, they were assembled with negative electrode sheets, electrolytes, and separators according to the following method to obtain lithium-ion batteries. The method includes:

[0140] 1) The positive electrode active materials from the examples and comparative examples were mixed with conductive carbon black and PVDF at a weight ratio of 95%:3%:2%, respectively, and dispersed to obtain a positive electrode slurry. This slurry was coated onto an aluminum foil current collector, with a positive electrode areal density of 4.12 g / cm³. 3The positive electrode sheet is prepared by rolling.

[0141] 2) Artificial graphite, styrene-diene rubber (SBR), sodium carboxymethyl cellulose, and conductive carbon black are mixed in a weight ratio of 94%:3%:2%:1%. The mixture is dispersed in water and then mixed using a double planetary mixer to obtain a negative electrode slurry. This slurry is coated onto a copper current collector, followed by rolling and drying to obtain a negative electrode sheet.

[0142] 3) Assemble the positive electrode, negative electrode, and separator into a lithium-ion battery and inject a non-aqueous electrolyte. The electrolyte is prepared by mixing ethylene carbonate, dimethyl carbonate, and propylene carbonate in a mass ratio of 2:5:3, then adding 5% fluoroethylene carbonate and 13% lithium hexafluorophosphate by mass of the total electrolyte.

[0143] Cyclic performance testing: The capacity retention of each battery was tested. The specific test method was as follows: at 45℃, the battery was charged at a constant current rate of 1C to 4.30V, then charged at a constant voltage rate of 0.05C to 4.30V, and then discharged at a discharge rate of 1C to 2.8V. This charge-discharge cycle was repeated 300 times. The discharge capacity Q1 at the first cycle and the discharge capacity Q at the 500th cycle were measured. 500 The capacity retention rate Q after 300 cycles is calculated using the following formula: Capacity retention rate Q = Q 300 / Q1*100%.

[0144] Table 2. Performance tests of each embodiment and comparative example

[0145]

[0146] As shown in Table 2, compared to the comparative example, the battery prepared using the positive electrode active material of the examples has higher capacity and better cycle performance. This is mainly because the high-valence element E is doped into the surface matrix, increasing the mixing of lithium-nickel cations on the outer side. This results in a higher content of divalent nickel on the side away from the surface matrix than on the side closer to the matrix, forming a lattice-coherent Li... + / Ni 2+ In the disordered region, Ni-O bonding mitigates the degradation of oxygen-containing phases, fixes lattice oxygen, and can improve the structural stability of the material, as well as its charge-discharge capacity and cycle performance.

[0147] In the preparation methods of Examples 9-16, a thin layer of lithium-containing fast ion conductor compound with high ionic conductivity and / or electronic conductivity is coated on the surface of the positive electrode active material, which improves the lithium-ion diffusion capability of the large-size high-nickel single crystal material and improves the rate performance of the battery.

[0148] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and 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 the present invention.

Claims

1. A single-crystal positive electrode active material, characterized in that, include: A positive electrode active material matrix, the positive electrode active material matrix comprising a bulk phase matrix located at the center and a surface matrix covering the outer surface of the bulk phase matrix, the positive electrode active material matrix comprising a lithium-nickel hybrid structure; The surface matrix is ​​doped with element E, and the oxidation state of element E is greater than trivalent.

2. The single-crystal positive electrode active material according to claim 1, characterized in that, In the positive electrode active material matrix, the degree of lithium-nickel mixing is 0.5-2%; and / or, the thickness of the surface matrix is ​​10-100 nm.

3. The single-crystalline cathode active material of claim 1, wherein In the surface matrix, the content of divalent nickel on the side away from the bulk matrix is ​​greater than the content of divalent nickel on the side closer to the bulk matrix.

4. The single-crystalline cathode active material of claim 1, wherein The chemical composition of the positive electrode active material matrix is ​​Li a Ni x Co y Mn z M m E e O2; and / or, the E element is doped at a site of at least one of nickel, cobalt, manganese and lithium; and / or, the E element includes one or more of W, Nb, Mo, Ta, Zr, Ti and Ce.

5. The single-crystalline cathode active material according to any one of claims 1 to 4, characterized in that The bulk matrix is ​​doped with element M, wherein element M has a divalent or trivalent valence.

6. The single-crystal positive electrode active material according to claim 5, characterized in that, The M element includes one or more of Al, Mg, La, Y and Sr; and / or, the M element is doped at a site of at least one of nickel, cobalt and manganese.

7. The single-crystal positive electrode active material according to any one of claims 1-4, characterized in that, The single-crystal positive electrode active material further includes a coating layer, which covers at least a portion of the surface of the positive electrode active material matrix, and the coating layer includes a lithium-containing fast ion conductor compound.

8. The single-crystalline cathode active material of claim 7, wherein The coating layer thickness is 5~50nm; and or, the particle size D50 of the single crystal positive electrode active material is 4~7um; and or, the minimum particle size of the single crystal positive electrode active material is 1~2um, and the maximum particle size is 10~20um.

9. A method of producing the single-crystal positive electrode active material according to any one of claims 1 to 8, characterized by, The preparation method includes: The intermediate product was prepared by mixing the lithium source and the positive electrode active material precursor and sintering them in an oxygen atmosphere. The intermediate product and the E-containing compound are mixed and sintered at 650-800℃ for 8-12 hours in an oxygen atmosphere. Then the temperature is lowered to 500-650℃ and held for 4-6 hours, and the material is naturally cooled to obtain a single-crystal positive electrode active material.

10. The method of claim 9, wherein, The intermediate product prepared by mixing a lithium source and a positive electrode active material precursor and sintering them in an oxygen atmosphere includes: The lithium source, the positive electrode active material precursor, and the oxide containing element M are mixed and sintered at 400-600°C for 3-6 hours in an oxygen atmosphere. The temperature is then raised to 900-1000°C and held for 8-16 hours. The temperature is then lowered to 750-900°C and held for 4-8 hours. The intermediate product is obtained by natural cooling.

11. A positive electrode sheet characterized by comprising: The positive electrode sheet includes the single-crystal positive electrode active material according to any one of claims 1-8, or includes the single-crystal positive electrode active material prepared by the preparation method according to claim 9 or 10.

12. A battery, characterized by The battery includes the positive electrode as described in claim 11.

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