A low-cobalt positive electrode material for a lithium secondary battery and a method of manufacturing the same

Abstract

The present application provides a kind of positive electrode material for lithium secondary battery, its preparation method, the positive electrode comprising the positive electrode material and the lithium secondary battery comprising the positive electrode.The low cobalt positive electrode material for lithium secondary battery has core-shell structure, wherein the core layer is lithium nickel manganese aluminum ternary positive electrode material, the shell layer is three-dimensional conductive array network assembled by graphite carbon nanosheet dispersed with cobalt-rich low-nickel cluster, and the composition of the cobalt-rich low-nickel cluster is nickel-doped lithium cobaltate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium secondary battery cathode materials, and particularly relates to a low-cobalt cathode material with a three-dimensional conductive array network of graphite carbon nanosheet assemblies with nanocluster dispersion on the surface, a preparation method of the low-cobalt cathode material, a cathode containing the low-cobalt cathode material, and a lithium secondary battery comprising the cathode. BACKGROUND

[0002] Electric vehicles (EVs) have been invented long before the popularization of internal combustion engine vehicles (ICEVs) in the late 19th century. However, the manufacturing cost, power performance and cruising range hindered its development into the main mode of public and private transportation. Currently, lithium-ion batteries (LIBs) have become the main power source for electric vehicles due to their high energy density, excellent rate performance and long cycle life. Currently, LIBs are being used in an increasing number of applications, not only limited to electric vehicles, but also including electronic products, electric tools, smart grids, etc. As a high-energy-density portable energy source, LIBs are developing into a future energy carrier to replace traditional energy sources.

[0003] With the continuous expansion of the global rechargeable energy market, LIBs have posed a series of new challenges in applications, such as processing cost, charging behavior, cruising range, thermal runaway risk and battery life, etc. For example, the cruising range of the most advanced LIBs-driven electric vehicles in the world is also 600-800 kilometers, which cannot meet the driving range of ICEVs. Therefore, whether it is to reduce the processing cost or to improve the cruising pain point of LIBs, the performance of the LIBs cathode needs to be greatly improved. Among them, the cathode material accounts for as high as 20-25% of the cost of lithium batteries. In ternary materials, the crustal reserves of cobalt element are relatively scarce, and the price is expensive, which accounts for an important proportion in the cost of cathode materials. On the other hand, due to the electronic configuration of cobalt, it matches well with the oxygen electron cloud, so the lithium cobaltate material has good electronic conductivity. At the same time, cobalt mainly contributes to the capacity in the normal voltage and high voltage range of lithium-ion batteries, and plays an important role in stabilizing the crystal structure and reducing lithium-nickel mixing.

[0004] With the increasing demand of customers for capacity, cycle life and safety performance, in order to solve the above technical bottlenecks, the positive electrode material is undergoing iterative development towards high nickel and cobalt-free. The traditional cobalt-free and low cobalt material has low cobalt content, poor intrinsic conductivity of the material, high electrochemical impedance, high initial impedance during charge and discharge cycle, and often accompanied by DCR deterioration and other phenomena; the prior art solution often chooses Al, W, Mn, Ti, B, Mg, F, Cl and other doping or coating schemes, which is easy to cause the dissolution of the doped and coated elements in the electrolyte and the re-deposition in the negative electrode, resulting in battery performance deterioration and even safety hazards. Moreover, the traditional doping and coating scheme can only reduce the initial charge transfer impedance Rct of the material, and the direct current impedance DCIR and the overall impedance of the battery are not significantly reduced, and serious polarization phenomenon, rapid increase of battery impedance and deterioration of material cycle performance are easy to occur during the cycle process. SUMMARY

[0005] In view of the above difficulties and challenges, the technical problem to be solved by the present application is to provide a low-cobalt positive electrode material with low impedance and a preparation method thereof. The positive electrode material has a three-dimensional conductive network assembled by graphite carbon nanosheets with cobalt-rich low-nickel nanoclusters dispersed on the surface. The positive electrode material includes a lithium nickel manganese aluminum ternary positive electrode material as a core and a three-dimensional conductive array network assembled by graphite carbon nanosheets with cobalt-rich low-nickel clusters dispersed as a shell layer. The lithium secondary battery prepared from the positive electrode material has low impedance, high rate and long cycle performance.

[0006] According to a first aspect of the present application, a low-cobalt positive electrode material for a lithium secondary battery is provided, which has a core-shell structure, wherein the core layer is a lithium nickel manganese aluminum ternary positive electrode material, and the shell layer is a three-dimensional conductive array network assembled by graphite carbon nanosheets with cobalt-rich low-nickel clusters dispersed, and the composition of the cobalt-rich low-nickel clusters is lithium cobalt oxide doped with nickel.

[0007] Preferably, the particle size of the cobalt-rich low-nickel clusters is 1-50 nm, more preferably 1-20 nm.

[0008] Preferably, the low-cobalt positive electrode material is in the form of particles, and the particle size is 8-20 μm.

[0009] Preferably, the lithium nickel manganese aluminum ternary positive electrode material is LiNi 1-x-y Mn x Al y O2, wherein 0.01 < x < 0.05, 0 < y < 0.04, more preferably x = 0.03, y = 0.02.

[0010] Preferably, the nickel doping amount of the cobalt-rich low-nickel clusters is 0-30 mol%.

[0011] Preferably, the thickness of the graphite carbon nanosheet is 20-100 nm, more preferably 20-50 nm.

[0012] Preferably, the side diameter of the graphite carbon nanosheet is 20–500 nm.

[0013] Preferably, the cobalt-rich, low-nickel clusters are uniformly dispersed on the surface and subsurface of the graphite carbon nanosheets.

[0014] Preferably, the cobalt content in the low-cobalt cathode material is 500-5000 ppm of the total metal content, more preferably 1500-2500 ppm.

[0015] Preferably, the carbon content in the low-cobalt cathode material is 1 to 2 wt% relative to 100 wt% of the low-cobalt cathode material.

[0016] According to a second aspect of the present invention, a method for preparing the low-cobalt cathode material according to the present invention is provided, comprising the following steps:

[0017] 1) Preparation of lithium-nickel-manganese-aluminum ternary cathode material;

[0018] 2) Impregnation load

[0019] 2a) The lithium nickel manganese aluminum ternary cathode material obtained in step 1) is added to a solution containing zinc salt and cobalt salt, and stirred and dispersed to obtain a mixed solution 1, wherein the molar ratio of zinc element to cobalt element in the solution containing zinc salt and cobalt salt is 1 to 6:1.

[0020] 2b) Add the 2-methylimidazole solution to the mixture 1 obtained in step 2a), mix, impregnate to form nuclei, and allow to stand and age to obtain mixture 2;

[0021] 2c) The mixture 2 obtained in step 2b) is washed with pure water, filtered, centrifuged, and dried to obtain a positive electrode material supported on zinc cobalt nickel organic framework material;

[0022] 3) The cathode material loaded with zinc-cobalt-nickel organic framework material obtained in step 2c) is heat-treated until the reaction is complete to obtain the low-cobalt cathode material.

[0023] Preferably, the zinc salt and cobalt salt in step 2a) are selected from one or more of sulfates, nitrates, hydrochlorides (chlorides), acetates and acetylacetones, and more preferably nitrates.

[0024] Preferably, in step 2a), the solvent in the solution containing zinc salt and cobalt salt is one or more selected from water, ethanol and cyclohexane, more preferably water.

[0025] Preferably, the molar ratio of zinc element to cobalt element in the solution containing zinc salt and cobalt salt is 3-5:1, and the total concentration of zinc element and cobalt element in the solution containing zinc salt and cobalt salt is 0.03-0.07 mol / L, more preferably 0.04-0.06 mol / L.

[0026] Preferably, the mass ratio of the lithium nickel manganese aluminum ternary positive electrode material to the solution containing zinc salt and cobalt salt is 1:1-3, more preferably 1:1.5-2.5.

[0027] Preferably, the stirring rate in step 2a) is 3-7 Hz, more preferably 4-6 Hz, and the stirring time is 1-5 min, more preferably 2-4 min.

[0028] Preferably, the solvent in the solution of 2-methylimidazole in step 2b) is selected from one or more of water, ethanol and cyclohexane, more preferably water.

[0029] Preferably, the concentration of the solution of 2-methylimidazole is 0.2-0.6 mol / L, more preferably 0.3-0.5 mol / L.

[0030] The mass ratio of the solution of 2-methylimidazole to the solution containing zinc salt and cobalt salt is 0.5-1.5:1, more preferably 1:1.

[0031] Preferably, the solution of 2-methylimidazole is added by peristaltic pump, and the delivery flow rate is selected from 0.5-1.5 L / min; preferably, the stirring rate is 1-5 Hz, more preferably 2-4 Hz, and the stirring time is 0-4 min, more preferably 1-3 min; the immersion time in step 2b) is 10-70 min, more preferably 30-60 min.

[0032] Preferably, the atmosphere of the heat treatment in step 3) is selected from one or more of nitrogen, argon and vacuum, preferably vacuum negative pressure atmosphere, the programmed temperature rising rate is 0.5-1.5 ℃ / min, more preferably 0.75-1.25 ℃ / min, the programmed temperature rising is to 650-850 ℃, more preferably 700-800 ℃, the holding time is 1-3 h, more preferably 1.5-2.5 h, and the cooling rate is 50-150 ℃ / h, more preferably 75-125 ℃ / h.

[0033] The lithium nickel manganese aluminum ternary positive electrode material can be a lithium nickel manganese aluminum ternary positive electrode material commonly used in the art, for example, LiNi 1-x-y Mn x Al y O2, wherein 0.01 1-x-y Mn xAl y O2, can be prepared by the following steps:

[0034] 4) Dry ball-milling the nickel-manganese-aluminum hydroxide precursor Ni 1-x-y Mn x Al y (OH)2and a lithium source to obtain a mixture, wherein the definitions of x and y are the same as the foregoing definitions;

[0035] 5) Heat-treating the mixture obtained in step 4) to reaction completion to obtain a lithium nickel-manganese-aluminum ternary positive electrode material LiNi 1-x-y Mn x Al y O2.

[0036] Preferably, the nickel-manganese-aluminum hydroxide precursor in step 4) can be prepared by the following steps:

[0037] Dissolving nickel salt, manganese salt and aluminum salt in water to obtain a mixed salt solution, fully stirring and mixing the mixed solution with an alkali solution and a complexing agent, heating under an inert atmosphere to nucleate, aging and growing, after the reaction is completed, washing, filtering, drying and grinding to obtain the nickel-manganese-aluminum hydroxide precursor.

[0038] Preferably, the nickel-manganese-aluminum hydroxide precursor can be prepared by the following steps:

[0039] Preferably, the nickel-manganese-aluminum hydroxide precursor can be prepared by the following steps:

[0040] Preferably, the nickel salt, manganese salt and aluminum salt are each selected from one or more of sulfate, nitrate and hydrochloride (chloride).

[0041] Preferably, the alkali in the alkali solution is selected from one or more of sodium hydroxide, potassium hydroxide and lithium hydroxide.

[0042] Preferably, the complexing agent is selected from one or more of ammonia, ammonium sulfate, and sodium citrate.

[0043] Preferably, the gas of the inert atmosphere is one or more of argon, nitrogen.

[0044] Preferably, in step 4), the ball mill is first pre-mixed at 100-300 r / min for 2 min, more preferably 200 r / min, and then dry ball-mixed at 800-1300 r / min, more preferably 1100 r / min, for 30 min until the mixture is uniform.

[0045] Preferably, in step 4), the lithium source is one or more of lithium nitrate, lithium acetate, lithium oxalate, lithium chloride, lithium carbonate, and lithium hydroxide, more preferably lithium hydroxide; and the mass ratio of the nickel-manganese-aluminum hydroxide precursor and the lithium source is 1:1.02-1.06, more preferably 1:1.035-1.045.

[0046] Preferably, in step 5), the heat treatment conditions are: programmed heating at 1-3 ℃ / min to 720-750 ℃, more preferably 730-740 ℃, and holding for 6-10 h, more preferably 7-9 h, and the atmosphere is selected from one or more of oxygen, air, nitrogen, and vacuum, more preferably oxygen.

[0047] 2-methylimidazole forms a polymer in the lithium nickel-manganese-aluminum ternary positive electrode material, and in-situ adsorbs and fixes cobalt ions and zinc ions to form a metal-organic coordination framework material precursor. In the sintering process, the polymer film formed by 2-methylimidazole is carbonized into a network of graphite carbon nanosheets, and due to the interdiffusion effect of high-temperature solid-phase reaction, the Li ions in the surface residual alkali, the Ni ions on the surface of the positive electrode material, and the highly dispersed Co ions in the ligand react to generate cobalt-rich low-nickel clusters, and the Zn element volatilizes in the sintering process, resulting in that the low-cobalt positive electrode material of the present application almost does not contain zinc elements.

[0048] According to a third aspect of the present application, there is provided a positive electrode for a lithium secondary battery, which is prepared from the positive electrode material according to the present application.

[0049] According to a fourth aspect of the present application, there is provided a lithium secondary battery comprising the positive electrode according to the present application.

[0050] Preferably, the lithium secondary battery is a lithium ion battery or a lithium battery.

[0051] Advantages

[0052] The application adopts a novel three-dimensional conductive network of graphite carbon nanosheet assembled with cobalt-rich low-nickel clusters as a functional layer to coat traditional lithium nickel manganese aluminum ternary positive electrode materials, that is, a three-dimensional conductive array network assembled by graphite carbon nanosheets is wrapped on the surface of traditional lithium nickel manganese aluminum cobalt-free secondary balls, and cobalt-rich low-nickel lithium cobalt oxide nanoclusters doped with nickel are uniformly and dispersedly loaded on the graphite carbon nanosheets, which is beneficial to the rapid diffusion of lithium ions and the transfer of electrons, and reduces the initial and cyclic impedance of the material.

[0053] The existence of the cobalt-rich low-nickel nanoclusters provides efficient lithium ion intercalation and deintercalation sites, further improves the initial discharge capacity and coulombic efficiency, and reduces the electrochemical polarization.

[0054] The impregnation loading process with water as a solvent is simple, easy to operate, low in cost, and small in pollution, and can effectively reduce the surface residual alkali of the positive electrode material; the excess residual alkali can be used as a lithium source to promote the generation of lithium cobalt nickel clusters in the secondary heat treatment process.

[0055] The graphite carbon nanosheet assembled three-dimensional conductive array network coating layer is an electrochemically inert coating layer, which can effectively alleviate the oxygen production of the surface lattice and effectively improve the surface stability, and inhibit the deterioration of DCR and the capacity attenuation in the cycle process.

[0056] The preparation method provided by the application is simple, feasible and safe.

[0057] The application has the advantages that a three-dimensional conductive array network assembled by graphite carbon nanosheets is wrapped on the surface of traditional lithium nickel manganese aluminum cobalt-free secondary balls, and cobalt-rich low-nickel lithium cobalt nickel nanoclusters are uniformly and dispersedly loaded on the graphite carbon nanosheets, which is beneficial to the rapid diffusion of lithium ions and the transfer of electrons, and reduces the initial and cyclic impedance of the material; the electrochemical insulation of the particle surface and subsurface and the dynamic capacity loss caused by the electrochemical insulation are avoided, and the structural uniformity of the positive electrode material surface is maintained. BRIEF DESCRIPTION OF DRAWINGS

[0058] Figure 1 SEM photos of low-cobalt positive electrode materials prepared according to Example 1 of the application;

[0059] Figure 2 SEM photos of low-cobalt positive electrode materials prepared according to Example 2 of the application;

[0060] Figure 3 SEM photos of low-cobalt positive electrode materials prepared according to Example 3 of the application;

[0061] Figure 4 SEM photos of low-cobalt positive electrode materials prepared according to Comparative Example 2 of the application;

[0062] Figure 5 SEM photograph of the low-cobalt cathode material prepared according to Comparative Example 3 of the present application;

[0063] Figure 6 25℃ cycle curves of the wound soft pack battery prepared from the low-cobalt cathode material prepared according to Example 1 of the present application and the wound soft pack battery prepared from the conventional low-cobalt ternary cathode material in Comparative Example 2 at a test voltage of 2.5-4.3V;

[0064] Figure 7 XRD patterns of the low-cobalt cathode materials prepared according to Example 1, Example 2 and Example 3 of the present application. DETAILED DESCRIPTION

[0065] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0066] Example 1

[0067] The low-cobalt cathode material was prepared according to the following steps:

[0068] 1) Accurately weigh nickel nitrate, manganese nitrate and aluminum nitrate according to the stoichiometric ratio of 95:3:2, and then dissolve them in pure water to prepare a mixed solution with a total concentration of 1 mol / L of nickel ions, manganese ions and aluminum ions. Under the protection of inert gas, inject the 2 mol / L lithium hydroxide and 0.35 mol / L ammonia solution prepared in advance into the mixed solution at a flow rate of 5 L / min using a peristaltic pump, and stir until the mixture is uniform. Control the reaction temperature at 55℃ and the pH value at 11.25±0.25, and stir at a frequency of 15 Hz under the protection of nitrogen atmosphere for 4 h, and then stand for aging for 8 h. Wash the precipitate obtained by reaction with pure water, centrifugal filtration at least 3 times, and dry at 85℃. After grinding, the desired nickel-manganese-aluminum hydroxide precursor is obtained.

[0069] 2) Accurately weigh the nickel-manganese-aluminum hydroxide precursor obtained in step 1) and lithium hydroxide according to the stoichiometric ratio of 1:1.04, and then add them to a ball mill, and then add zirconium dioxide ball milling beads. First, pre-mix at 200 r / min for 2 min, and then mix at 1000 r / min for 30 min until the mixture is uniform.

[0070] 3) The mixture obtained in step 2) is subjected to a first sintering heat treatment, i.e. programmed heating to 730 DEG C at a rate of 1.8 DEG C / min in a pure oxygen atmosphere, and then natural cooling to obtain a lithium nickel manganese aluminum ternary positive electrode material, which is confirmed by XRD characterization to have a composition of LiNi 0.95 Mn 0.03 Al 0.02 O2.

[0071] 4) Zinc nitrate and cobalt nitrate are accurately weighed according to the stoichiometric ratio of 4:1 in terms of molar ratio, and then dissolved in pure water to prepare a mixed salt solution of 0.05 mol / L. The lithium nickel manganese aluminum ternary positive electrode material obtained in step 3) is rapidly added to the mixed solution according to a mass ratio of 1:2, and stirred at a frequency of 5 Hz for 3 min to obtain a mixed solution 1. Then, the 0.4 mol / L 2-methyl imidazole aqueous solution prepared in advance is injected into the mixed solution at a flow rate of 1 L / min using a peristaltic pump, and stirred at a frequency of 3 Hz for 2 min, and then aged for 45 min. The precipitate obtained by the reaction is washed with pure water, centrifuged and filtered at least 3 times, and dried at 60 DEG C. After grinding, a zinc-cobalt-nickel organic framework material loaded positive electrode material is obtained.

[0072] 5) The zinc-cobalt-nickel organic framework material loaded positive electrode material obtained in step 4) is subjected to a second sintering heat treatment, i.e. programmed heating to 800 DEG C at a rate of 1 DEG C / min in a nitrogen atmosphere, and then cooled to room temperature at a rate of 100 DEG C / h to obtain a low-cobalt positive electrode material with a shell of a three-dimensional conductive array network of graphite carbon nanosheet assembled with dispersed cobalt-rich low-nickel clusters.

[0073] Figure 1 The SEM photograph of the low-cobalt positive electrode material prepared can be seen that the three-dimensional conductive array conductive network coating layer is uniformly distributed, and the positive electrode material is a secondary spherical morphology with a particle size of 14 pm, and the surface is uniformly wrapped and grown with graphite carbon nanosheets with a thickness of 20-50 nm and a side diameter of 20-500 nm assembled into a three-dimensional conductive array network, and the cobalt-rich low-nickel nanoclusters are uniformly embedded and loaded on the graphite carbon nanosheets. The average particle size of the cobalt-rich low-nickel clusters is 9 nm (the core of the layered oxide cluster is a central metal ion, which is transformed during the heat treatment process by a metal complex morphology directing method, so it has morphology structure inheritance, so the average dispersed particle size dc=(w C / M C ) / (w MCo *10 -6 *100 / M) / 8*2.2nm*0.5, wherein w C is the mass fraction of carbon in the material, M C is the relative atomic mass of carbon, w MCo is the mass fraction of cobalt in the total metal, and M is the core material LiNi 0.95 Mn0.03 Al 0.02 O2, 2.2 nm is the distance between adjacent cobalt ions in the smallest repeating unit in the polymer precursor, 8 is the number ratio of carbon to cobalt in the smallest repeating unit in the polymer precursor. The cobalt content in the low-cobalt positive electrode material is 1913 ppm of the total metal content, and the carbon content is 1.534 wt% of the positive electrode material. The cobalt content is tested by inductively coupled plasma-atomic emission spectroscopy, and the carbon content is tested by a carbon-sulfur analyzer.

[0074] Example 2

[0075] The low-cobalt positive electrode material is prepared according to the following steps:

[0076] 1) Accurately weigh nickel nitrate, manganese nitrate and aluminum nitrate according to the stoichiometric ratio of 95:3:2 in terms of molar ratio, and then dissolve them in pure water to prepare a mixed solution with a total concentration of 1 mol / L of nickel ions, manganese ions and aluminum ions. Under the protection of an inert atmosphere, inject a previously prepared 2 mol / L lithium hydroxide and 0.35 mol / L ammonia solution into the mixed solution at a flow rate of 5 L / min using a peristaltic pump, and stir until the mixture is uniform. Control the reaction temperature at 55°C and the pH value at 11.25±0.25, and stir at a frequency of 15 Hz under the protection of an inert atmosphere and react for 4 h, and then stand for aging for 8 h. Wash the precipitate obtained by reaction with pure water, centrifuge and filter at least 3 times, dry at 85°C, and grind to obtain the desired nickel-manganese-aluminum hydroxide precursor.

[0077] 2) Accurately weigh the nickel-manganese-aluminum hydroxide precursor obtained in step 1) and lithium hydroxide according to the stoichiometric ratio of 1:1.04 in terms of molar ratio, and then add them to a ball mill, and then add zirconium dioxide ball milling beads. First, pre-mix at 200 r / min for 2 min, and then mix at 1000 r / min for 30 min until the mixture is uniform.

[0078] 3) Perform a one-time sintering heat treatment on the mixture obtained in step 2), i.e. program the temperature to rise to 730°C at a rate of 1.8°C / min under a pure oxygen atmosphere, and keep the temperature for 8 h, and then naturally cool to obtain the lithium-nickel-manganese-aluminum ternary positive electrode material LiNi 0.95 Mn 0.03 Al 0.02 O2.

[0079] 4) The zinc nitrate and cobalt nitrate were accurately weighed according to the stoichiometric ratio of 4:1 in molar ratio, and then dissolved in pure water to prepare a mixed salt solution with a total concentration of 0.05 mol / L. The lithium nickel manganese aluminum ternary positive electrode material obtained in step 3) was rapidly added to the mixed solution according to a mass ratio of 1:1.75, and stirred at a frequency of 5 Hz for 3 min to obtain a mixed solution 1. Then, the 0.4 mol / L 2-methylimidazole aqueous solution prepared in advance was injected into the mixed solution at a flow rate of 1 L / min using a peristaltic pump. After stirring at a frequency of 3 Hz for 2 min, it was left to stand for 45 min. The precipitate obtained by reaction was washed with pure water, centrifuged and filtered at least 3 times, and dried at 60°C. After grinding, the zinc-cobalt-nickel organic framework material loaded positive electrode material was obtained.

[0080] 5) The zinc-cobalt-nickel organic framework material loaded positive electrode material obtained in step 4) was subjected to secondary sintering heat treatment, i.e. heating to 800°C at a rate of 1°C / min under a nitrogen atmosphere, and then cooling to room temperature at a rate of 100°C / h to obtain a low-cobalt positive electrode material with a shell of a three-dimensional conductive array network assembled by graphite carbon nanosheets dispersing cobalt-rich low-nickel clusters.

[0081] The difference between this embodiment and embodiment 1 is that the mass ratio of the ternary positive electrode material to the mixed salt solution during the impregnation process is slightly lower, as shown in Figure 2 The positive electrode material has a particle size of 14 μm, and the surface is uniformly wrapped with graphite carbon nanosheets with a thickness of 20-30 nm and a side diameter of 20-300 nm assembled into a three-dimensional conductive array network. The low-nickel cobalt-rich nanoclusters are uniformly embedded on the graphite carbon nanosheets. The average particle size of the cobalt-rich low-nickel clusters is 7 nm. The cobalt content in the low-cobalt positive electrode material accounts for 1733 ppm of the total metal content, and the carbon content accounts for 1.103 wt% of the positive electrode material.

[0082] Example 3

[0083] The low-cobalt positive electrode material was prepared according to the following steps:

[0084] 1) Nickel nitrate, manganese nitrate and aluminum nitrate were accurately weighed according to the stoichiometric ratio of 95:3:2 in molar ratio, and then dissolved in pure water to prepare a mixed solution of 1 mol / L of nickel ions, manganese ions and aluminum ions. Under the protection of inert atmosphere, the mixed solution was injected with a 2 mol / L lithium hydroxide and 0.35 mol / L ammonia solution prepared in advance at a flow rate of 5 L / min using a peristaltic pump, and stirred until mixed uniformly. The reaction temperature was controlled at 55°C, and the pH value was 11.25±0.25. Under the protection of inert atmosphere, the mixture was stirred at a frequency of 15 Hz for 4 h, and then left to stand for 8 h. The precipitate obtained by reaction was washed with pure water, centrifuged and filtered at least 3 times, and dried at 85°C. After grinding, the desired nickel-manganese-aluminum hydroxide precursor was obtained.

[0085] 2) The nickel-manganese-aluminum hydroxide precursor obtained in step 1) and lithium hydroxide are accurately weighed in a stoichiometric ratio of 1:1.04, and then added to a ball mill, followed by the addition of zirconium dioxide ball milling beads; first pre-mixing at 200 r / min for 2 min, and then mixing at 1000 r / min for 30 min until uniform.

[0086] 3) The mixture obtained in step 2) is subjected to a first sintering heat treatment, i.e. programmed heating to 730℃ at a rate of 1.8℃ / min under a pure oxygen atmosphere, holding for 8h, and then naturally cooling to obtain a lithium nickel-manganese-aluminum positive electrode material LiNi 0.95 Mn 0.03 Al 0.02 O2.

[0087] 4) Zinc nitrate and cobalt nitrate are accurately weighed in a stoichiometric ratio of 4:1, and then dissolved in pure water to prepare a mixed salt solution of 0.05 mol / L; the lithium nickel-manganese-aluminum positive electrode material obtained in step 3) is rapidly added to the mixed salt solution in a mass ratio of 1:2.25, and stirred at a frequency of 5Hz for 3 min to obtain a mixed solution 1; then a 0.4 mol / L aqueous solution of 2-methylimidazole of the same mass prepared in advance is injected into the mixed solution at a flow rate of 1L / min using a peristaltic pump, and stirred at a frequency of 3Hz for 2 min, and then left to stand for 45 min; the precipitate obtained by reaction is washed with pure water, centrifuged and filtered at least 3 times, and dried at 60℃, and then ground to obtain a zinc-cobalt-nickel organic framework material loaded positive electrode material.

[0088] 5) The zinc-cobalt-nickel organic framework material loaded positive electrode material obtained in step 4) is subjected to a second sintering heat treatment, i.e. programmed heating to 800℃ at a rate of 1℃ / min under a nitrogen atmosphere, holding for 2h, and then cooling to room temperature at a rate of 100℃ / h to obtain a lithium nickel-cobalt cluster monodisperse carbon conductive network low-cobalt high-nickel positive electrode material.

[0089] The difference between this example and examples 1 and 2 is that the mass ratio of the positive electrode material to the mixed salt solution during the impregnation process is slightly higher, as shown in Figure 3 The positive electrode material has a particle size of 14μm, and a three-dimensional conductive array network is uniformly grown on the surface of the positive electrode material, with a thickness of 20-200nm and a side diameter of 20-1500nm graphite carbon nanosheet assembled into a three-dimensional conductive array network. The low-nickel cobalt-rich nanoclusters are uniformly embedded on the graphite carbon nanosheet. The average particle size of the cobalt-rich low-nickel clusters is 10nm (same as example 1). The cobalt content in the low-cobalt positive electrode material accounts for 2094ppm of the total metal content, and the carbon content accounts for 1.864wt% of the positive electrode material (same as example 1).

[0090] Comparative Example 1

[0091] A cobalt-free positive electrode material is prepared by the following steps:

[0092] 1) Accurately weigh the nickel nitrate, manganese nitrate and aluminum nitrate according to the stoichiometric ratio of 95:3:2, and then dissolve them in pure water to prepare a mixed solution with a total concentration of 1 mol / L of nickel ions, manganese ions and aluminum ions. Under the protection of an inert atmosphere, inject the prepared 2 mol / L lithium hydroxide and 0.35 mol / L ammonia solution into the mixed solution at a flow rate of 5 L / min using a peristaltic pump, and stir until the mixture is uniform. Control the reaction temperature at 55°C and the pH value at 11.25±0.25, and stir at a frequency of 15 Hz under the protection of an inert atmosphere for 4 h, and then stand for aging for 8 h. Wash the precipitate obtained by reaction with pure water, centrifugal filtration at least 3 times, and dry at 85°C. After grinding, the desired nickel-manganese-aluminum hydroxide precursor is obtained.

[0093] 2) Accurately weigh the nickel-manganese-aluminum hydroxide precursor obtained in step 1) and lithium hydroxide according to the stoichiometric ratio of 1:1.04, and then add them to a ball mill, and then add zirconium dioxide ball milling beads. First, pre-mix at 200 r / min for 2 min, and then mix at 1000 r / min for 30 min until the mixture is uniform.

[0094] 3) Perform a one-time sintering heat treatment on the mixture obtained in step 2), i.e. program the temperature to rise to 730°C at a rate of 1.8°C / min under a pure oxygen atmosphere, and then heat for 8 h, and then naturally cool to obtain the lithium nickel-manganese-aluminum cobalt-free positive electrode material LiNi 0.95 Mn 0.03 Al 0.02 O2.

[0095] The difference between the present comparative example and the above-mentioned examples is that there is no impregnation loading and secondary sintering step, and the product obtained is a cobalt-free lithium nickel-manganese-aluminum positive electrode material.

[0096] Comparative Example 2

[0097] A low-cobalt positive electrode material is prepared by the following steps:

[0098] 1) Accurately weigh the nickel nitrate, manganese nitrate, aluminum nitrate and cobalt nitrate according to the stoichiometric ratio of 95:2.5:1.5:1, and then dissolve them in pure water to prepare a mixed solution with a total concentration of 1 mol / L of metal ions. Under the protection of an inert atmosphere, inject the prepared 2 mol / L lithium hydroxide and 0.35 mol / L ammonia solution into the mixed solution at a flow rate of 5 L / min using a peristaltic pump, and stir until the mixture is uniform. Control the reaction temperature at 55°C and the pH value at 11.25±0.25, and stir at a frequency of 15 Hz under the protection of an inert atmosphere for 4 h, and then stand for aging for 8 h. Wash the precipitate obtained by reaction with pure water, centrifugal filtration at least 3 times, and dry at 85°C. After grinding, the desired nickel-manganese-aluminum-cobalt hydroxide precursor is obtained.

[0099] 2) The nickel-manganese-aluminum hydroxide precursor obtained in step 1) and lithium hydroxide are accurately weighed in a stoichiometric ratio of 1:1.04 and then added to a ball mill, followed by the addition of zirconium dioxide ball milling beads; first pre-mix at 200 r / min for 2 min, and then mix at 1000 r / min for 30 min until uniform.

[0100] 3) The mixture obtained in step 2) is subjected to a one-time sintering heat treatment, i.e., programmed heating to 730°C at a rate of 1.8°C / min under a pure oxygen atmosphere, holding for 8 h, and then naturally cooling to obtain a lithium nickel-manganese-aluminum-cobalt low-cobalt positive electrode material.

[0101] The difference between the present comparative example and the above-described examples is that a low-cobalt lithium nickel-manganese-aluminum positive electrode material is prepared by a traditional method, as shown in FIG. 1, and the surface of the primary particles is smooth. Figure 4

[0102] Comparative Example 3

[0103] A low-cobalt positive electrode material is prepared by the following steps:

[0104] 1) Nickel nitrate, manganese nitrate, and aluminum nitrate are accurately weighed in a stoichiometric ratio of 95:3:2 and dissolved in pure water to prepare a mixed solution with a total concentration of 1 mol / L of nickel ions, manganese ions, and aluminum ions. Under inert atmosphere protection, a 2 mol / L lithium hydroxide and 0.35 mol / L ammonia solution prepared in advance is injected into the mixed solution at a flow rate of 5 L / min using a peristaltic pump, and stirred until uniform. The reaction temperature is controlled at 55°C, and the pH value is 11.25±0.25. Stirring is performed at a frequency of 15 Hz under inert atmosphere protection for 4 h, and then left to stand for 8 h. The precipitate obtained by reaction is washed with pure water, centrifuged and filtered at least 3 times, and dried at 85°C. After grinding, the desired nickel-manganese-aluminum hydroxide precursor is obtained.

[0105] 2) The nickel-manganese-aluminum hydroxide precursor obtained in step 1) and lithium hydroxide are accurately weighed in a stoichiometric ratio of 1:1.04 and then added to a ball mill, followed by the addition of zirconium dioxide ball milling beads; first pre-mix at 200 r / min for 2 min, and then mix at 1000 r / min for 30 min until uniform.

[0106] 3) The mixture obtained in step 2) is subjected to a one-time sintering heat treatment, i.e., programmed heating to 730°C at a rate of 1.8°C / min under a pure oxygen atmosphere, holding for 8 h, and then naturally cooling to obtain a lithium nickel-manganese-aluminum positive electrode material LiNi 0.95 Mn 0.03 Al 0.02 O2.

[0107] ​4) Accurately weigh the zinc nitrate and cobalt nitrate according to the stoichiometric ratio of 2:1 in molar ratio, and then dissolve them in pure water to prepare a mixed salt solution of 0.05 mol / L. Add the lithium nickel manganese aluminum positive electrode material obtained in step 3) and the mixed salt solution into the mixed solution according to a mass ratio of 1:2, and stir at a frequency of 5 Hz for 3 min to obtain a mixed solution 1. Then, use a peristaltic pump to inject the 0.4 mol / L 2-methylimidazole aqueous solution prepared in advance into the mixed solution at a flow rate of 1 L / min. Stir at a frequency of 3 Hz for 2 min, and then stand for aging for 45 min. Wash the precipitate obtained by reaction with pure water, centrifuge and filter at least 3 times, and dry at 60°C. After grinding, the zinc-cobalt-nickel organic framework material loaded positive electrode material is obtained.

[0108] 5) The zinc-cobalt-nickel organic framework material loaded positive electrode material obtained in step 4) is subjected to secondary sintering heat treatment, i.e. heating to 800°C at a rate of 1°C / min under a nitrogen atmosphere, and then cooling to room temperature at a rate of 100°C / h to obtain a lithium-nickel-cobalt nanocrystalline carbon conductive network low-cobalt high-nickel positive electrode material.

[0109] The difference between the present comparative example and the above-mentioned examples is that the molar ratio of zinc salt and cobalt salt in the mixed salt solution during the impregnation process is lower, so that the dispersion degree of cobalt in the loaded zinc-cobalt-nickel organic framework material is low, as shown in FIG. 2, lithium cobaltate clusters and graphite carbon nanosheet clusters form small particles. Figure 5

[0110] Experimental Examples

[0111] The same negative electrode, electrolyte, and separator are used to prepare 3 Ah wound soft pack lithium ion batteries of the same specification for Examples 1-3 and Comparative Examples 1-3, but the nickel-cobalt-manganese ternary positive electrode material in the present application is not limited to the preparation of 3 Ah wound soft pack lithium ion batteries. The electrical performance is tested under the following conditions: ① 0.33C charge-discharge test; ② 25°C direct current impedance test at 90% SOC; ③ -10°C direct current impedance test at 90% SOC; ④ 25°C alternating current impedance spectrum test at 90% and 50% SOC; and ⑤ 25°C 4A / 20A cycle test. The test results are shown in Table 1.

[0112] Table 1: Performance of lithium ion batteries

[0113]

[0114] ​As can be seen from the data in Table 1, the low-cobalt positive electrode material prepared by the method of the present application has higher energy utilization, lower direct-current impedance at normal temperature and low temperature, lower surface charge transfer impedance, and higher cycle performance. The cobalt-rich low-nickel nanocluster reduces the material surface polarization, cooperates with the three-dimensional conductive array network to effectively increase the electronic conductivity of the material, and reduces the impedance between the interface and the particles, thereby significantly reducing the internal resistance of the battery. However, too many nanoclusters are easy to agglomerate, and too thick conductive graphite carbon structure will shield the exposure of the active component, hindering the diffusion of lithium ions, so the thickness of the nanocluster dispersed conductive network layer needs to be reasonably controlled.

[0115] Figure 6 As can be seen from the 25℃ normal temperature cycle curves of the wound soft pack batteries respectively prepared by using the low-cobalt positive electrode material of Example 1 and the conventional method low-cobalt lithium nickel manganese aluminum positive electrode material of Comparative Example 2 at a test voltage of 2.75-4.3V, the capacity retention rate of the material 400 with the nanocluster monodisperse conductive network structure is close to 90% under high-voltage long cycle, while the capacity retention rate of the conventional low-cobalt material is only about 78%, and the capacity attenuation is serious.

[0116] The above examples are only used to illustrate the technical solutions of the present application, but not limit it; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions recorded in the foregoing examples can be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A low cobalt cathode material for lithium secondary batteries, having a core-shell structure, wherein the core layer is a lithium nickel manganese aluminum ternary cathode material, and the shell layer is a three-dimensional conductive array network assembled by graphite carbon nanosheets dispersed with cobalt-rich low-nickel clusters, the composition of the cobalt-rich low-nickel clusters is lithium cobalt oxide doped with nickel, wherein the cobalt content in the low cobalt cathode material accounts for 500-5000 ppm of the total metal content, the nickel doping amount of the cobalt-rich low-nickel clusters is 0-30 mol%. 2.The low cobalt cathode material for lithium secondary batteries according to claim 1, wherein, the cobalt content in the low cobalt cathode material accounts for 1500-2500 ppm of the total metal content; and / or the content of carbon in the low cobalt cathode material is 1-2 wt% relative to 100 wt% of the low cobalt cathode material. 3.The low cobalt cathode material for lithium secondary batteries according to claim 1 or 2, wherein, the particle size of the cobalt-rich low-nickel clusters is 1-50 nm; and / or the low cobalt cathode material is in the form of particles, and the particle size is 8-20 μm; and / or the thickness of the graphite carbon nanosheets is 20-100 nm; and / or the side diameter of the graphite carbon nanosheets is 20-500 nm; and / or The lithium nickel manganese aluminum ternary positive electrode material is LiNi 1-x-y Mn x Al y O2, wherein 0.01 < x < 0.05, 0 < y < 0.

04. 4.The low cobalt cathode material for lithium secondary batteries according to claim 3, wherein, the particle size of the cobalt-rich low-nickel clusters is 1-20 nm; and / or the thickness of the graphite carbon nanosheets is 20-50 nm; and / or The lithium nickel manganese aluminum ternary positive electrode material is LiNi 1-x-y Mn x Al y O2, wherein x=0.03, y=0.

02. 5.A preparation method of the low cobalt cathode material according to any one of claims 1 to 4, comprising the following steps: 1) preparing a lithium nickel manganese aluminum ternary cathode material; 2) immersing the lithium nickel manganese aluminum ternary cathode material in a solution containing zinc salt and cobalt salt to obtain a mixture 1; 2a) adding the lithium nickel manganese aluminum ternary positive electrode material obtained in step 1) to a solution containing a zinc salt, a cobalt salt, stirring and dispersing to obtain a mixed solution 1, wherein, the molar ratio of zinc element to cobalt element in the solution containing zinc salt and cobalt salt is 1-6:1; 2b) adding a solution of 2-methylimidazole into the mixture 1 obtained in step 2a) to immerse and nucleate, and then standing to age to obtain a mixture 2; 2c) washing, filtering, centrifuging and drying the mixture 2 obtained in step 2b) to obtain a zinc-cobalt-nickel organic framework material loaded cathode material; 3) heat treating the zinc-cobalt-nickel organic framework material loaded cathode material obtained in step 2c) until the reaction is completed to obtain the low cobalt cathode material. 6.The preparation method according to claim 5, wherein, the zinc salt and the cobalt salt in step 2a) are each selected from one or more of sulfate, nitrate, hydrochloride, acetate and acetylacetone salt; and / or in step 2a), the solvent in the solution containing zinc salt and cobalt salt is selected from one or more of water, ethanol and cyclohexane; and / or the molar ratio of zinc element to cobalt element in the solution containing zinc salt and cobalt salt is 3-5:1, and the total concentration of zinc element and cobalt element in the solution containing zinc salt and cobalt salt is 0.03-0.07 mol / L; and / or the mass ratio of the lithium nickel manganese aluminum ternary cathode material to the solution containing zinc salt and cobalt salt is 1:1-3; and / or the solvent in the solution of 2-methylimidazole in step 2b) is selected from one or more of water, ethanol and cyclohexane; and / or the concentration of the solution of 2-methylimidazole is 0.2-0.6 mol / L.

7. The preparation method according to claim 6, wherein, the zinc salt and the cobalt salt in step 2a) are each selected from nitrates; and / or the total concentration of zinc element and cobalt element in the solution containing zinc salt and cobalt salt is 0.04-0.06 mol / L; and / or the mass ratio of the lithium nickel manganese aluminum ternary positive electrode material to the solution containing zinc salt and cobalt salt is 1:1.5-2.5; and / or the solvent in the solution of 2-methylimidazole in step 2b) is water; and / or the concentration of the solution of 2-methylimidazole is 0.3-0.5 mol / L.

8. The preparation method according to any one of claims 5-7, wherein, the mass ratio of the solution of 2-methylimidazole to the solution containing zinc salt and cobalt salt is 0.5-1.5:1; and / or the solution of 2-methylimidazole is added by a peristaltic pump, and the delivery flow rate is selected from 0.5-1.5 L / min; the soaking time in step 2b) is 10-70 min.

9. The preparation method according to claim 8, wherein, the mass ratio of the solution of 2-methylimidazole to the solution containing zinc salt and cobalt salt is 1:1; and / or the soaking time in step 2b) is 30-60 min.

10. The preparation method according to any one of claims 5-7, wherein, the atmosphere of the heat treatment in step 3) is selected from one or more of nitrogen, argon and vacuum; the programmed temperature rising rate is 0.5-1.5 ℃ / min; the programmed temperature rising is to 650-850 ℃; the holding time is 1-3 h; the cooling rate is 50-150 ℃ / h.

11. The preparation method according to claim 10, wherein, the atmosphere of the heat treatment in step 3) is a vacuum negative pressure atmosphere; and / or the programmed temperature rising rate is 0.75-1.25 ℃ / min; and / or the programmed temperature rising is to 700-800 ℃; and / or the holding time is 1.5-2.5 h; and / or the cooling rate is 75-125 ℃ / h.

12. The preparation method according to any one of claims 5-7, wherein, The lithium nickel manganese aluminum ternary positive electrode material is LiNi 1-x-y Mn x Al y O2, wherein 0.01 < x < 0.05, 0 < y < 0.

04.

13. The preparation method according to claim 12, wherein, The lithium nickel manganese aluminum ternary positive electrode material is LiNi 1-x-y Mn x Al y O2, wherein x=0.03, y=0.

02.

14. A positive electrode for a lithium secondary battery, which is prepared from the positive electrode material according to any one of claims 1-4 or the positive electrode material prepared by the preparation method according to any one of claims 5-13.

15. A lithium secondary battery comprising the positive electrode according to claim 14.

16. The lithium secondary battery according to claim 15, wherein, The lithium secondary battery is a lithium ion battery.

17. The lithium secondary battery according to claim 15, wherein, The lithium secondary battery is a lithium battery. The lithium secondary battery is a lithium battery.

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