Multilayer core-shell structure single-crystal lithium cobalt oxide cathode material, preparation method and application thereof

Abstract

The application discloses a kind of multilayer core-shell structure single-crystal lithium cobaltate positive electrode materials and preparation method and application, and the structure general formula of multilayer core-shell structure single-crystal lithium cobaltate positive electrode material is as follows: LiCo 1‑ a Al a Mg b O2‑NO x / MO y ‑LiF;N is any one of low diffusion rate element Ti, Zr, Ta, W;M is at least two kinds of Mg, Al, Co, W, Mo, Y, La, Ca;Preparation method includes the following steps: after being mixed with lithium source and doped element Mg source, first sintering is carried out to the doped Al three cobalt tetroxide, and pre-sintered material is obtained;After being mixed with low diffusion rate element coating agent N source, LiF and coating agent M source, second sintering is carried out to pre-sintered material, and multilayer core-shell structure single-crystal lithium cobaltate positive electrode material is obtained.The application can overcome the problems such as serious structure collapse of lithium cobaltate positive electrode material under high voltage, serious side reaction between surface interface and electrolyte, low capacity cycle retention rate and the like in the prior art.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery cathode materials, in particular to a kind of multilayer core-shell structure single crystal lithium cobaltate cathode material and its preparation method and application. BACKGROUND

[0002] Since commercialization in 1991, lithium cobaltate has become the first choice of cathode material for consumer electronics due to its excellent volume energy density and high operating voltage. Currently, commercial lithium cobaltate cathodes with a typical charge cut-off voltage of 4.4 V (vs Li + / Li) can only provide 170 mAh g –1 and 0.6 mol Li + extraction, leaving a lot of room for its theoretical capacity (274 mAh g –1 ). By increasing the cut-off voltage, more lithium ions can be extracted to achieve higher capacity and energy density. Increasing the upper cut-off voltage can effectively obtain higher reversible capacity, however, the highly delithiated state with higher Li + extraction rate can lead to significant decrease in cycle performance, accompanied by problems such as structural collapse, irreversible phase transition, electrolyte oxidation, lattice oxygen release and Co dissolution.

[0003] The structural stability at high voltage can be effectively improved by multiple element doping modification, but in the one-step co-doping process, some co-doped cations (heavy and / or high-valence cation dopants Ti 4+ , Zr 4+ and W 6+ , etc.) are easy to enrich on the surface of the crystal grains and the grain boundaries, and further induce the formation of polycrystalline particles due to the limited growth kinetics in the cathode material. During the charge and discharge process at high voltage, the anisotropic volume expansion / contraction of polycrystalline lithium cobaltate can cause cracks at the grain boundaries, leading to particle breakage and electrolyte penetration, accelerating capacity decay. Single crystal material, consisting of single grains, avoids the cycle stress concentration problem caused by random orientation of primary particles in polycrystalline material, and significantly reduces the risk of particle cracking by eliminating grain boundaries, while its denseness can withstand greater pressure during battery rolling process, maintaining structural integrity.

[0004] The diffusion rate of cations depends on the charge and radius of the cations, and the diffusion rates of different dopant ions vary greatly. Therefore, according to the different diffusion rates of dopant ions in lithium cobaltate crystals, the elements used for doping or coating are reasonably distributed, and a multilayer core-shell structure single crystal cathode is designed, which is the key to improving the electrical performance. SUMMARY

[0005] In view of the above deficiencies existing at present, the application provides a multi-layer core-shell structure single-crystal lithium cobaltate positive electrode material and a preparation method and application thereof, and the multi-layer core-shell structure single-crystal lithium cobaltate positive electrode material can overcome the problems of the (polycrystal) structure collapse of the lithium cobaltate positive electrode material under high voltage, serious side reactions between the surface interface and the electrolyte, and low capacity cycle retention rate in the prior art.

[0006] In order to achieve the above-mentioned purpose, the application provides a preparation method of a multi-layer core-shell structure single-crystal lithium cobaltate positive electrode material, and the structural general formula of the multi-layer core-shell structure single-crystal lithium cobaltate positive electrode material is as follows: LiCo 1-a Al a Mg b O2-NO x / MO y -LiF, 0.001≤a≤0.01, 0.001≤b≤0.01, 1

[0007] S1, the Al-doped tricobalt tetraoxide, a lithium source and a Mg-doped element source are uniformly mixed, and then first sintering is performed to obtain pre-sintered material; wherein the temperature of the first sintering is 900-1060 DEG C, and the time is 8-13 h;

[0008] S2, the pre-sintered material, a low-diffusivity element coating agent N source, LiF and a coating agent M source are uniformly mixed, and then second sintering is performed to obtain the multi-layer core-shell structure single-crystal lithium cobaltate positive electrode material; wherein the temperature of the second sintering is 700-900 DEG C, and the time is 9-14 h.

[0009] It should be noted that the sintering atmosphere of the first sintering and the second sintering is air.

[0010] According to one aspect of the application, in step S1, the doping amount of Al is 0.4wt%-0.6wt% of the tricobalt tetraoxide; the Mg-doped element source includes at least one of magnesium oxide and magnesium hydroxide; the doping amount of the Mg-doped element in the pre-sintered material is 800-1500 ppm of the mass of the pre-sintered material; and the lithium source includes one or more of lithium carbonate, lithium hydroxide and lithium acetate.

[0011] According to one aspect of the present application, in step S1, the first-time sintering includes first-time sintering A and first-time sintering B; the temperature of the first-time sintering A is 1000-1060℃, and the time is 8-12h; the temperature of the first-time sintering B is 900-960℃, and the time is 9-13h; the pre-sintered material includes large-particle pre-sintered material with D50 particle size of 18-21μm and small-particle pre-sintered material with D50 particle size of 4-8μm; the first-time sintering A obtains the large-particle pre-sintered material with D50 particle size of 18-21μm, and the first-time sintering B obtains the small-particle pre-sintered material with D50 particle size of 4-8μm.

[0012] According to one aspect of the present application, the mass ratio of the large-particle pre-sintered material to the small-particle pre-sintered material in the pre-sintered material is 3-5:1; the molar ratio of lithium source to cobalt lithium in the preparation of the large-particle pre-sintered material is 1.05-1.07; and the molar ratio of lithium source to cobalt lithium in the preparation of the small-particle pre-sintered material is 1.01-1.04.

[0013] It should be noted that the above lithium source is all excessive, and the lithium source is excessive to prevent lithium evaporation in the sintering process. The temperature for preparing the large-particle pre-sintered material is higher than that for preparing the small-particle pre-sintered material, and the lithium evaporation is more serious, so the molar ratio of lithium source to cobalt lithium in the preparation of the large-particle pre-sintered material is relatively high.

[0014] According to one aspect of the present application, in step S1, the preparation method of the Al-doped tricobalt tetraoxide is: slowly adding cobalt chloride and / or cobalt sulfate and aluminum chloride and / or aluminum sulfate into a precipitator to form a Co-Al precursor, and obtaining after calcining at 300-600℃ in air for 2-6h; wherein the precipitator is at least one of sodium hydroxide and ammonia water.

[0015] According to one aspect of the present application, in step S2, the N source of the low-diffusivity element coating agent is at least one of oxide of N and hydroxide of N, the particle size satisfies D50≤2μm, and the coating amount of the low-diffusivity element N in the pre-sintered material accounts for 500-1200ppm of the mass of the pre-sintered material.

[0016] According to one aspect of the present application, in step S2, the F element in the pre-sintered material accounts for 600-1200ppm of the mass of the pre-sintered material.

[0017] According to one aspect of the present application, in step S2, the M source of the coating agent is at least one of oxide of M and hydroxide of M, and the M element in the pre-sintered material accounts for 10000-30000ppm of the mass of the pre-sintered material.

[0018] Based on the same inventive concept, the present application also provides a multi-layer core-shell structure single-crystal lithium cobalt oxide positive electrode material prepared by the above preparation method.

[0019] Based on the same inventive concept, the application also provides application of the multi-layer core-shell structure single-crystal lithium cobalt oxide cathode material in a lithium ion battery.

[0020] The application has the following beneficial effects:

[0021] The application screens out high-diffusivity elements Mg and Al to dope Li sites and Co sites of the bulk phase, respectively, according to the difference in diffusivity of different ions in lithium cobalt oxide. Compared with some low-diffusivity doping elements, the high-diffusivity elements are easier to obtain single-crystal lithium cobalt oxide with higher mechanical strength and higher compaction density. A high-doping amount of Al greatly improves the structural stability and high-temperature performance of the material. A more uniform doping effect can be achieved through precursor pre-doping (doping of tricobalt tetroxide). The high-diffusivity element Mg acts as an interlayer 'pillar' to help inhibit adverse phase change and improve the electrical conductivity of the material. The second sintering is performed by grading large-particle pre-sintered material and small-particle pre-sintered material. A proper grading ratio helps to improve the compaction density of the material. The application realizes in-situ self-assembly of different functional layers through the difference in chemical reaction paths of the layers. The low-diffusivity elements (any one of Ti, Zr, Ta, and W) added in the second sintering coating process help to form a core-shell structure with a surface interface enrichment, thereby better isolating the electrolyte and protecting the inner core lithium cobalt oxide. In combination with some elements that can diffuse on the surface, a composite surface intermediate layer is constructed. The intermediate layer and the core layer are bonded by Co-O-Ti / Al, etc. to form a strong chemical bond, thereby enhancing the interfacial bonding force between the coating layer and the main phase and avoiding the peeling problem of traditional mechanical mixing coating. The outermost layer is matched with lithium fluoride to fill the pores and form a dense interface layer to inhibit electrolyte penetration and oxygen loss. The multi-layer core-shell structure single-crystal cathode lithium cobalt oxide realizes functional complementation, taking into account ion conduction, structural stability, and interface protection. The material exhibits excellent capacity, rate capability, and cycle performance at a high voltage of 3.0-4.48 V, breaks through the limitations of high-voltage stability and cycle life of lithium cobalt oxide cathode materials, and has excellent low-temperature and high-temperature performance in a wide temperature range. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 SEM images of the large-particle pre-sintered material, the small-particle pre-sintered material, and the lithium cobalt oxide cathode material prepared in Example 1 of the application are shown in FIG. 1. (a) is the large-particle pre-sintered material; (b) is the small-particle pre-sintered material; and (c) is the lithium cobalt oxide cathode material. DETAILED DESCRIPTION

[0023] In order to make the present application more easily understood, the present application is further described below in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present application and are not used to limit the scope of the present application. Obviously, the described examples are only a part of the examples of the present application, but not all the examples. Based on the examples in the present application, all other examples obtained by those skilled in the art without creative efforts are within the scope of protection of the present application. Unless otherwise defined, the professional terms used below are consistent with the meanings understood by those skilled in the art; unless otherwise specified, the raw materials and reagents involved in the present application can be purchased from the market or prepared by known methods.

[0024] In order to solve the problems of serious collapse of polycrystal structure under high voltage, serious side reaction between surface interface and electrolyte, and low capacity cycle retention rate in the prior art, the inventors of the present application provide a preparation method of a multi-layer core-shell structure single crystal lithium cobaltate positive electrode material, the structural general formula of the multi-layer core-shell structure single crystal lithium cobaltate positive electrode material is: LiCo 1- a Al a Mg b O2-NO x / MO y -LiF, 0.001≤a≤0.01, 0.001≤b≤0.01, 1

[0025] S1, the Al-doped tricobalt tetraoxide, a lithium source and a Mg-doped element source are uniformly mixed, and then first sintering is performed to obtain a pre-sintered material; wherein the temperature of the first sintering is 900-1060℃, and the time is 8-13h;

[0026] S2, the pre-sintered material, a low diffusion rate element coating agent N source, LiF and a coating agent M source are uniformly mixed, and then second sintering is performed to obtain a multi-layer core-shell structure single crystal lithium cobaltate positive electrode material; wherein the temperature of the second sintering is 700-900℃, and the time is 9-14h; wherein the sintering atmosphere of the first sintering and the second sintering is air.

[0027] In one specific embodiment, in step S1, the doping amount of Al is 0.4wt%-0.6wt% of the tricobalt tetraoxide; the Mg-doped element source includes at least one of magnesium oxide and magnesium hydroxide; the doping amount of the Mg-doped element in the pre-sintered material is 800-1500ppm of the mass of the pre-sintered material; the lithium source includes one or more of lithium carbonate, lithium hydroxide and lithium acetate.

[0028] In a specific embodiment, in step S1, the first sintering comprises a first sintering A and a first sintering B; the first sintering A is performed at a temperature of 1000-1060℃ for 8-12h; the first sintering B is performed at a temperature of 900-960℃ for 9-13h; the pre-sintered material comprises large-particle pre-sintered material with a D50 particle size of 18-21μm and small-particle pre-sintered material with a D50 particle size of 4-8μm; the first sintering A obtains the large-particle pre-sintered material with a D50 particle size of 18-21μm; and the first sintering B obtains the small-particle pre-sintered material with a D50 particle size of 4-8μm.

[0029] In a specific embodiment, the mass ratio of the large-particle pre-sintered material to the small-particle pre-sintered material in the pre-sintered material is 3-5:1; the lithium cobalt molar ratio of the lithium source to the tricobalt tetraoxide in the preparation of the large-particle pre-sintered material is 1.05-1.07; and the lithium cobalt molar ratio of the lithium source to the tricobalt tetraoxide in the preparation of the small-particle pre-sintered material is 1.01-1.04.

[0030] In a specific embodiment, in step S1, the preparation method of the Al-doped tricobalt tetraoxide is as follows: slowly adding cobalt chloride and / or cobalt sulfate and aluminum chloride and / or aluminum sulfate into a precipitator to form a Co-Al precursor, and calcining at 300-600℃ in air for 2-6h to obtain; wherein the precipitator is at least one of sodium hydroxide and ammonia water.

[0031] In a specific embodiment, in step S2, the N source of the low-diffusivity element coating agent is at least one of an oxide of N and a hydroxide of N, the particle size satisfies D50≤2μm, and the coating amount of the low-diffusivity element N in the pre-sintered material accounts for 500-1200ppm of the mass of the pre-sintered material.

[0032] In a specific embodiment, in step S2, the F element in the pre-sintered material accounts for 600-1200ppm of the mass of the pre-sintered material.

[0033] In a specific embodiment, in step S2, the M source of the coating agent is at least one of an oxide of M and a hydroxide of M, and the M element in the pre-sintered material accounts for 10000-30000ppm of the mass of the pre-sintered material.

[0034] The application also provides a multi-layer core-shell structure single-crystal lithium cobalt oxide positive electrode material prepared by the above preparation method.

[0035] The application also provides application of the above multi-layer core-shell structure single-crystal lithium cobalt oxide positive electrode material in lithium ion batteries.

[0036] The application will be further described below in combination with specific examples and comparative examples.

[0037] Example 1

[0038] A preparation method of a multi-layer core-shell structure single crystal lithium cobaltate positive electrode material, comprising the following steps:

[0039] (1) 0.4wt% Al-doped tricobalt tetroxide, magnesium hydroxide and lithium carbonate with a particle size D50 of 16 μm are mixed, the Li:Co of lithium carbonate and Al-doped tricobalt tetroxide is 1.062, and the added amount of Mg is 1200 ppm of the weight of the large particle pre-sintered material. After mixing, the mixture is calcined at 1055°C for 10 hours in an air atmosphere to obtain a large particle primary synthesis material, and after mechanical grinding, the particle size of the large particle pre-sintered material is controlled to be about 19 μm. 0.4% Al-doped tricobalt tetroxide, magnesium hydroxide and lithium carbonate with a particle size D50 of 5 μm are mixed, the Li:Co of lithium carbonate and Al-doped tricobalt tetroxide is 1.032, and the added amount of Mg is 1200 ppm of the weight of the small particle pre-sintered material. After mixing, the mixture is calcined at 955°C for 12 hours in an air atmosphere to obtain a small particle primary synthesis material, and after mechanical grinding, the particle size of the small particle pre-sintered material is controlled to be about 5 μm. The preparation method of the Al-doped tricobalt tetroxide is as follows: cobalt chloride and aluminum chloride are slowly added to a precipitator (sodium hydroxide) to form a Co-Al precursor, which is calcined at 450°C in air for 3 hours to obtain the Al-doped tricobalt tetroxide.

[0040] (2) The large particle pre-sintered material and the small particle pre-sintered material obtained in (1) are mixed according to a weight ratio of 77:23 to obtain a mixed pre-sintered material, and then titanium dioxide (low-diffusivity element coating agent N source), aluminum hydroxide (coating agent M source), cobalt hydroxide (coating agent M source) and lithium fluoride are added and mixed uniformly in a high-speed mixer at 300 revolutions / 15 minutes. The added amount of Ti element is 800 ppm of the weight of the mixed pre-sintered material, the added amount of Al is 500 ppm of the weight of the mixed pre-sintered material, the weight of cobalt hydroxide is 18000 ppm of the weight of the mixed pre-sintered material, and the added amount of F element is 1100 ppm of the weight of the mixed pre-sintered material. The mixed material is calcined at 820°C for 12 hours in an air atmosphere to perform a second sintering, and after the second sintering, the material is mechanically ground by a pair of rollers to obtain a lithium cobaltate positive electrode material.

[0041] Example 2

[0042] The difference between this example and Example 1 is that the added amount of Ti element for the second sintering coating is 1200 ppm of the weight of the mixed pre-sintered material. The other steps and parameters are the same as those of Example 1.

[0043] Example 3

[0044] The difference between this example and Example 1 is that the Ti element for the second sintering coating is replaced by Zr element, i.e., the low-diffusivity element coating agent N source is replaced by zirconium oxide. The other steps and parameters are the same as those of Example 1.

[0045] Comparative Example 1

[0046] The difference between this comparative example and Example 1 is that no high diffusion doping element (Mg, Al) is added, i.e. the Al-doped tricobalt tetroxide in step (1) is not doped with Al, and no magnesium hydroxide is added. The other steps and parameters are the same as in Example 1.

[0047] Comparative Example 2

[0048] The difference between this comparative example and Example 1 is that no low diffusion rate element (Ti) is added, i.e. no titanium dioxide is added in step (2). The other steps and parameters are the same as in Example 1.

[0049] Comparative Example 3

[0050] The difference between this comparative example and Example 1 is that the high diffusion doping element Mg is replaced by the low diffusion rate element Ti. The other steps and parameters are the same as in Example 1.

[0051] The specific preparation steps are as follows:

[0052] (1) 0.4wt% Al-doped tricobalt tetroxide, titanium dioxide and lithium carbonate are mixed, with the Li:Co of lithium carbonate to Al-doped tricobalt tetroxide being 1.065, and the amount of Ti added being 900 ppm of the weight of the large particle pre-fired material. After mixing, calcination is performed at 1055°C for 10 hours in an air atmosphere to obtain a large particle primary synthesis material, and after roller air milling, the particle size of the large particle pre-fired material is controlled to be about 17 μm. 0.4% Al-doped tricobalt tetroxide, titanium dioxide and lithium carbonate are mixed, with the Li:Co of lithium carbonate to Al-doped tricobalt tetroxide being 1.032, and the amount of Ti added being 900 ppm of the weight of the small particle pre-fired material. After mixing, calcination is performed at 955°C for 12 hours in an air atmosphere to obtain a small particle primary synthesis material, and after roller air milling, the particle size of the small particle pre-fired material is controlled to be about 5 μm. The preparation method of the Al-doped tricobalt tetroxide is as follows: cobalt chloride and aluminum chloride are slowly added to a precipitant (sodium hydroxide) to form a Co-Al precursor, which is calcined at 450°C in air for 3 hours to obtain the Al-doped tricobalt tetroxide.

[0053] (2) The large particle pre-fired material and the small particle pre-fired material obtained in (1) are mixed according to a weight ratio of 77:23 to obtain a mixed pre-fired material, and then titanium dioxide (low-diffusivity element coating agent N source), aluminum hydroxide (coating agent M source), cobalt hydroxide (coating agent M source), and lithium fluoride are added and mixed uniformly in a high-speed mixer at 300 rpm for 15 min. The amount of Ti added is 800 ppm of the mixed pre-fired material, the amount of Al added is 500 ppm of the mixed pre-fired material, the amount of cobalt hydroxide is 18,000 ppm of the mixed pre-fired material, and the amount of F added is 1,100 ppm of the mixed pre-fired material. The mixed material is calcined at 820°C for 12 hours in an air atmosphere to perform a second sintering, and then the material after the second sintering is mechanically ground by a pair of rollers to obtain a lithium cobaltate positive electrode material.

[0054] Comparative Example 4

[0055] The difference between this comparative example and Example 1 is that the low-diffusivity element (Ti) is replaced by a high-diffusivity doping element (Zn), and the other steps and parameters are the same as those in Example 1.

[0056] The specific preparation steps are as follows:

[0057] (1) 0.4wt% Al-doped tricobalt tetraoxide, magnesium hydroxide, and lithium carbonate with a particle size D50 of 16 μm are mixed, the Li:Co of lithium carbonate and Al-doped tricobalt tetraoxide is 1.062, and the amount of Mg added is 1,200 ppm of the weight of the large particle pre-fired material. After mixing, the large particle primary synthesis material is obtained by calcining at 1,055°C for 10 hours in an air atmosphere, and then the particle size is controlled to be about 19 μm (D50) after the large particle pre-fired material is ground by a pair of rollers. 0.4% Al-doped tricobalt tetraoxide, magnesium hydroxide, and lithium carbonate with a particle size D50 of 5 μm are mixed, the Li:Co of lithium carbonate and Al-doped tricobalt tetraoxide is 1.032, and the amount of Mg added is 900 ppm of the weight of the small particle pre-fired material. After mixing, the small particle primary synthesis material is obtained by calcining at 955°C for 12 hours in an air atmosphere, and then the particle size is controlled to be about 5 μm (D50) after the small particle pre-fired material is ground by a pair of rollers. The preparation method of the Al-doped tricobalt tetraoxide is as follows: cobalt chloride and aluminum chloride are slowly added to a precipitator (sodium hydroxide) to form a Co-Al precursor, and then the precursor is calcined at 450°C for 3 hours in air to obtain the Al-doped tricobalt tetraoxide.

[0058] (2) The large-particle pre-sintered material and small-particle pre-sintered material obtained in (1) were weighed at a ratio of 80:20 to obtain a mixed pre-sintered material. Zinc oxide (a high-diffusion element), aluminum hydroxide (coating agent M source), cobalt hydroxide (coating agent M source), and lithium fluoride were added and mixed evenly in a high-speed mixer at 300 rpm for 15 min. The amount of Zn added was 1000 ppm of the weight of the pre-sintered material mixture, the amount of Al added was 500 ppm of the weight of the pre-sintered material mixture, the weight of cobalt hydroxide was 18000 ppm of the weight of the pre-sintered material mixture, and the amount of F added was 1100 ppm of the weight of the pre-sintered material mixture. The mixture was calcined at 820℃ for 12 hours in air atmosphere for a second sintering. The material after the second sintering was mechanically ground by roller mill to obtain lithium cobalt oxide cathode material.

[0059] Performance testing and results analysis:

[0060] The large-particle pre-sintered material, small-particle pre-sintered material, and lithium cobalt oxide cathode material prepared in Example 1 were analyzed by scanning electron microscopy (SEM), and the results are as follows: Figure 1 As shown. By Figure 1 It can be seen that the large-particle pre-sintered material and the small-particle pre-sintered material prepared in Example 1 both have a single crystal morphology and a smooth surface; after gradation and second sintering, the surface of the lithium cobalt oxide cathode material becomes rough and is covered with some particles and island-like coatings; indicating that Example 1 of this application successfully prepared a core-shell structured single-crystal lithium cobalt oxide cathode material.

[0061] The lithium cobalt oxide cathode materials prepared in Examples 1-3 and Comparative Examples 1-4 were used to assemble half-cells (3.0V-4.48V) and their corresponding electrochemical performance was tested. The key parameters are as follows: cathode material: SP:PVDF = 90:5:5; anode: lithium sheet; electrolyte: 1.0M LiPF6 (DMC:EC:EMC = 1:1:1 Vol%). The experimental results of their electrochemical performance are shown in Table 1.

[0062] Table 1. Electrochemical performance results of lithium cobalt oxide cathode materials prepared in Examples 1-3 and Comparative Examples 1-4

[0063]

[0064] As can be seen from the comparison between Example 1 and Comparative Example 1 in Table 1, the half-cell prepared with the lithium cobalt oxide cathode material in Example 1 has a slightly lower capacity, but a slightly improved rate capability, and significantly improved cycle performance at both room temperature and high temperature. This indicates that the uniform lattice doping of Al and Mg ions with high diffusivity can effectively suppress the transfer of O3 to H2O. 1-3 Phase transformation reduces lattice defects and microstrain, thereby improving cycling performance. In particular, Al has a significant impact on high-temperature performance.

[0065] As can be seen from the comparison of Example 1 and Comparative Example 2 in Table 1, the capacity of the half battery prepared from the lithium cobalt oxide cathode material of Example 1 is close to that of the half battery prepared from the lithium cobalt oxide cathode material of Comparative Example 2, and the rate and cycle performance of Example 1 are better than those of Comparative Example 2; it is indicated that the low-diffusivity element is enriched at the surface interface and reacts with the residual lithium on the surface, forming a complex interconnected network in the lithium cobalt oxide, which is divided into multiple sub-domains, and is beneficial to improving the Li + diffusion and enhancing the cycle stability.

[0066] As can be seen from the comparison of Example 1 and Comparative Example 3 in Table 1, the capacity, rate and cycle of the half battery prepared from the lithium cobalt oxide cathode material of Example 1 are all better than those of the half battery prepared from the lithium cobalt oxide cathode material of Comparative Example 3; it is indicated that the low-diffusivity element is added in the form of doping into the bulk phase, which causes the deformation of the lithium cobalt oxide lattice, forms lattice defects such as bending, twisting and d-spacing heterogeneity, and easily forms a polycrystalline agglomerate state, which affects the compaction density of the finished product and increases the grain boundary of the material, and the cracking is more likely to occur at high voltage.

[0067] As can be seen from the comparison of Example 1 and Comparative Example 4 in Table 1, the capacity, rate and cycle of the half battery prepared from the lithium cobalt oxide cathode material of Example 1 all have certain advantages, it is indicated that after the high-diffusivity ion replaces the low-diffusivity ion in the coating layer, the covering layer lacking the surface interface segregation is generated, the grain boundary is most likely to crack at high voltage, which causes the electrolyte to erode into the lithium cobalt oxide, and affects the long cycle of the battery.

[0068] In summary, the multilayer coating structure is generated in situ at high temperature by the difference in chemical paths of the bulk phase high-diffusivity element doping and the material, a multilayer core-shell structure lithium cobalt oxide cathode material is formed, which has an inner core doped with a high-diffusivity element and a dense interface layer filled with a composite surface interface conductive ion layer and molten lithium fluoride, has excellent discharge specific capacity and rate performance at high voltage of 3.0-4.48 V, and has excellent capacity retention at high temperature.

[0069] The above merely illustrates the specific embodiments of the present application, but the protection scope of the present application is not limited thereto, any changes or replacements easily thought of by those skilled in the art within the technical range disclosed by the present application should be covered in the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for preparing a multi-layer core-shell structured single-crystal lithium cobalt oxide cathode material, characterized in that, The structure general formula of the multilayer core-shell structure single crystal lithium cobaltate positive electrode material is: LiCo 1-a Al a Mg b O2-NO x / MO y -LiF, 0.001≤a≤0.01, 0.001≤b≤0.01, 1 1-a Al a Mg b O2, a composite surface interface conductive ion layer NO x / MO y and a dense interface layer LiF; the preparation method comprises the following steps: S1, the Al-doped tricobalt tetraoxide, lithium source and Mg-doped element source are mixed and uniformly mixed, and then first sintering is performed to obtain pre-sintered material; wherein the temperature of the first sintering is 900-1060℃, and the time is 8-13h; S2, the pre-sintered material is mixed with a low-diffusivity element coating agent N source, LiF and a coating agent M source, and then second sintering is performed to obtain a multi-layer core-shell structure single-crystal lithium cobalt oxide positive electrode material; wherein the temperature of the second sintering is 700-900℃, and the time is 9-14h.

2. The method for preparing the multi-layer core-shell structured single-crystal lithium cobalt oxide cathode material according to claim 1, characterized in that, In step S1, the doping amount of Al is 0.4wt%-0.6wt% of the tricobalt tetraoxide; the Mg-doped element source includes at least one of magnesium oxide and magnesium hydroxide; the doping amount of the Mg-doped element in the pre-sintered material is 800-1500ppm of the mass of the pre-sintered material; and the lithium source includes one or more of lithium carbonate, lithium hydroxide and lithium acetate.

3. The method for preparing the multi-layer core-shell structured single-crystal lithium cobalt oxide cathode material according to claim 1, characterized in that, In step S1, the first sintering includes first sintering A and first sintering B; the temperature of the first sintering A is 1000-1060℃, and the time is 8-12h; the temperature of the first sintering B is 900-960℃, and the time is 9-13h; the pre-sintered material includes large-particle pre-sintered material with a D50 particle size of 18-21mm and small-particle pre-sintered material with a D50 particle size of 4-8mm; the first sintering A obtains the large-particle pre-sintered material with a D50 particle size of 18-21mm; and the first sintering B obtains the small-particle pre-sintered material with a D50 particle size of 4-8mm.

4. The method for preparing the multi-layer core-shell structured single-crystal lithium cobalt oxide cathode material according to claim 3, characterized in that, The mass ratio of the large-particle pre-sintered material to the small-particle pre-sintered material in the pre-sintered material is 3-5:1; the molar ratio of the lithium source to cobalt in the preparation of the large-particle pre-sintered material is 1.05-1.07; and the molar ratio of the lithium source to cobalt in the preparation of the small-particle pre-sintered material is 1.01-1.

04.

5. The method for preparing the multi-layer core-shell structured single-crystal lithium cobalt oxide cathode material according to claim 1, characterized in that, In step S1, the preparation method of the Al-doped tricobalt tetraoxide is: slowly adding cobalt chloride and / or cobalt sulfate and aluminum chloride and / or aluminum sulfate into a precipitator to form a Co-Al precursor, and then calcining at 300-600℃ in air for 2-6h to obtain the Al-doped tricobalt tetraoxide; wherein the precipitator is at least one of sodium hydroxide and ammonia water.

6. The method for preparing the multi-layer core-shell structured single-crystal lithium cobalt oxide cathode material according to claim 1, characterized in that, In step S2, the low-diffusivity element coating agent N source is at least one of an oxide of N and a hydroxide of N, and the particle size satisfies D50≤2mm; and the coating amount of the low-diffusivity element N in the pre-sintered material accounts for 500-1200ppm of the mass of the pre-sintered material.

7. The method for preparing the multi-layer core-shell structured single-crystal lithium cobalt oxide cathode material according to claim 1, characterized in that, In step S2, the F element in the pre-sintered material accounts for 600-1200ppm of the mass of the pre-sintered material.

8. The method for preparing the multi-layer core-shell structured single-crystal lithium cobalt oxide cathode material according to claim 1, characterized in that, In step S2, the coating agent M source is at least one of an oxide of M and a hydroxide of M, and the M element in the pre-sintered material accounts for 10000-30000ppm of the mass of the pre-sintered material.

9. A multi-layer core-shell structure single-crystal lithium cobalt oxide positive electrode material prepared by the preparation method of the multi-layer core-shell structure single-crystal lithium cobalt oxide positive electrode material according to any one of claims 1-8.

10. Application of the multi-layer core-shell structure single-crystal lithium cobalt oxide positive electrode material according to claim 9 in lithium ion batteries.

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