Low-cobalt positive electrode material and preparation method therefor

Abstract

The present application relates to the technical field of batteries, and in particular to a low-cobalt positive electrode material and a preparation method therefor, so as to improve the structural stability, rate capability and cycle performance of positive electrode materials. The positive electrode material comprises an inner core, a first coating layer wrapping the inner core, and a second coating layer wrapping the first coating layer; the inner core comprises a first metal oxide, the first coating layer comprises a second metal oxide, and the second coating layer is a passivation layer inert to an electrolyte; the general formula of the first metal oxide is Liy1[Nia1Mnb1Mc1]O2, wherein 0.6≤a1<1.0, 0<b1≤0.4, 0≤c1≤0.1, a1+b1+c1=1, 0.98≤y1≤1.10, and M is selected from at least one of tungsten, boron, aluminum, zirconium, titanium, magnesium, calcium, molybdenum, cerium and strontium; and the general formula of the second metal oxide is αLiy2Coa2Rb2O2·βCo3O4, wherein 0.5<a2≤1, 0≤b2≤0.2, 0.98≤y2≤1.10, α+β=1, 0<β<1, and R is selected from at least one of tungsten, aluminum, zirconium, titanium and magnesium.

Description

A low-cobalt positive electrode material and preparation method thereof

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on June 14, 2023, with application number 202310701569.X and application name “A low-cobalt positive electrode material and its preparation method”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of battery technology, and in particular to a low-cobalt positive electrode material and a preparation method thereof. Background Art

[0003] Currently, secondary batteries, such as lithium-ion batteries, are widely used in transportation, communications, portable electronic products, and power tools. With the development of society, the comprehensive performance requirements of lithium-ion secondary batteries are becoming increasingly stringent. As a key component of lithium-ion batteries, cathode materials determine the battery's capacity and cost. Among these, ternary cathode materials offer the distinct advantage of high energy density, making them the preferred material for lithium-ion batteries. Cobalt, a scarce resource, has the highest price and the most volatile of all the components in ternary materials. Currently, the Co content in ternary cathode materials is relatively high (typically greater than or equal to 10%), posing a potential risk to the future supply of cobalt raw materials. However, reducing the Co content in ternary cathode materials can lead to a decrease in their structural stability, rate performance, and cycling performance. Therefore, there is an urgent need to develop a low-cobalt ternary cathode material to reduce manufacturing costs. Such ternary cathode materials possess structural stability, high energy density, good rate performance, and high capacity retention.

[0004] Summary of the Invention

[0005] The present application discloses a low-cobalt cathode material and a preparation method thereof, in order to solve the problems of poor structural stability, rate performance and cycle performance of existing low-cobalt cathode materials.

[0006] To achieve the above objectives, this application provides the following technical solutions:

[0007] In the first aspect, the present application provides a low-cobalt positive electrode material, which includes a core, a first coating layer covering the core, and a second coating layer covering the first coating layer; the core includes a first metal oxide, the first coating layer includes a second metal oxide, and the second coating layer is a passivation layer inert to the electrolyte; the general formula of the first metal oxide is Li y1 [Ni a1 Mn b1 M c1O2, where 0.6 ≤ a1 < 1.0, 0 < b1 ≤ 0.4, 0 ≤ c1 ≤ 0.1, a1 + b1 + c1 = 1, 0.98 ≤ y1 ≤ 1.10, and M is selected from at least one of tungsten, boron, aluminum, zirconium, titanium, magnesium, calcium, molybdenum, cerium, and strontium; the general formula of the second metal oxide is αLi y2 Co a2 R b2 O2·βCo3O4, where 0.5 < a2 ≤ 1, 0 ≤ b2 ≤ 0.2, 0.98 ≤ y2 ≤ 1.10; α + β = 1, 0 < β < 1, and R is selected from at least one of tungsten, aluminum, zirconium, titanium, and magnesium.

[0008] Further, the molar proportion z of cobalt element in the cathode material is 0 < z ≤ 3%.

[0009] Further, the molar proportion of M in the cathode material is 0 - 8%; the molar proportion of R in the cathode material is 0 - 1%.

[0010] Further, the passivation layer includes the oxide of Q, the fluoride of Q, or the phosphide of Q, where Q is selected from at least one of tungsten, aluminum, zirconium, titanium, magnesium, molybdenum, cerium, strontium, or boron; the molar proportion of the constituent substances of the passivation layer in the cathode material is 0.5% - 1%.

[0011] Further, the cathode material further includes a first transition layer formed by the interpenetration of the core and the first coating layer.

[0012] Further, the thickness of the first transition layer is 0.02 μm - 0.2 μm.

[0013] Further, the cathode material further includes a second transition layer formed by the interpenetration of the first coating layer and the second coating layer.

[0014] Further, the thickness of the second transition layer is 0.01 μm - 0.1 μm.

[0015] Further, the lattice mismatch degrees between the material of the first transition layer and the material of the core, and between the material of the first transition layer and the material of the first coating layer are both less than or equal to 10%; the lattice mismatch degrees between the material of the second transition layer and the material of the first coating layer, and between the material of the second transition layer and the material of the second coating layer are both less than or equal to 10%.

[0016] In a second aspect, the present application provides a method for preparing the positive electrode material of the first aspect, the preparation method comprising the following steps: S1: mixing a cobalt-free precursor, a lithium source and a dopant and calcining the mixture to obtain a pretreated precursor; S2: mixing the pretreated precursor, a nano-cobalt-containing compound and a first coating agent and sintering the mixture for the first time to obtain a composite precursor; S3: mixing the composite precursor and the second coating agent and sintering the mixture for the second time to obtain a positive electrode material; wherein the dopant is selected from Al2O3, Al(OH)3, SrO, TiO2, ZrO2, MgO, CaCO3, Ca(OH)2, Y2O3, Nb2O5, MoO3, SnO2, WO3 , H2WO4, B2O3, H3BO3, Ta2O5, CeO2, La2O3; the first coating agent is selected from at least one of ZrO2, ZrF4, Al2O3, Al(OH)3, AlOOH, AlF3, WO3, H2WO4, TiO2, MgO; the second coating agent is selected from at least one of ZrO2, Al2O3, AlF3, AlPO4, ZrF4, MgO, MgF, WO3, H2WO4, TiO2, LATP, SiO2, H2SiO3, carbon nanotubes GTNs, EDOT-PEG, PPy, PANI, (NH4)2HPO4.

[0017] Furthermore, step S2 further includes a first annealing treatment after the first sintering to obtain a composite precursor, and the temperature of the first annealing treatment is 50° C.-100° C. lower than the temperature of the first sintering.

[0018] Furthermore, step S3 further includes a second annealing treatment after the second sintering to obtain the positive electrode material, and the temperature of the second annealing treatment is 50° C.-100° C. lower than the temperature of the second sintering.

[0019] The technical solution of this application has the following beneficial effects:

[0020] The present application provides a low-cobalt positive electrode material, wherein the core of the positive electrode material does not contain cobalt elements, and only contains cobalt elements in the first coating layer, thereby effectively reducing the content of cobalt elements. In addition, the first coating layer contains Li y2 Co a2 R b2O2 fast ion conductor, which can activate the residual lithium on the surface of the positive electrode material, thereby increasing the capacity of the positive electrode material, and constructing a three-dimensional channel for lithium ion conduction, stabilizing the crystal structure, can effectively reduce the internal resistance of the positive electrode material, and improve the cycle stability of the positive electrode material. Moreover, the fast ion conductor and Co3O4 spinel form a composite phase coating layer, which can stabilize the structure of the positive electrode material and reduce the internal resistance. The second coating layer is a passivation layer, which is inert to the electrolyte and can hinder the fast ion conductor from directly contacting the electrolyte, further improving the stability of the material. Therefore, the low-cobalt positive electrode material provided by the present application not only has a stable structure, but also has the advantages of high energy density, good rate performance and high cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG1 is a schematic structural diagram of a positive electrode material provided in one embodiment of the present application;

[0022] FIG2 is a SEM image of the precursor in Example 1 of the present application;

[0023] FIG3 is a SEM image of the pretreated precursor in Example 1 of the present application;

[0024] FIG4 is a SEM image of the positive electrode material in Example 1 of the present application;

[0025] FIG5 is a SEM image of the positive electrode material in Example 2 of the present application;

[0026] FIG6 is a SEM image of the positive electrode material in Example 3 of the present application;

[0027] FIG7 is a SEM image of the positive electrode material in Comparative Example 1 of the present application;

[0028] FIG8 is a comparison of 200 cycle curves of soft-pack batteries prepared with the positive electrode materials in the examples of the present application and the comparative examples at 60° C.;

[0029] FIG9 is a comparison of DCR growth rate curves of soft-pack batteries prepared with the positive electrode materials in the examples of the present application and the comparative examples at 60° C. after 200 cycles.

[0030] Reference numerals: 10 - inner core; 20 - first cladding layer; 30 - second cladding layer; 40 - first transition layer; 50 - second transition layer. DETAILED DESCRIPTION

[0031] To make the objectives, technical solutions, and advantages of this application more clear, this application will be further described in detail below with reference to the accompanying drawings. It should be understood that the embodiments described herein are only some of the embodiments of this application, and not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of this application without inventive effort are intended to fall within the scope of protection of this application.

[0032] The application scenarios described in the embodiments of this application are intended to more clearly illustrate the technical solutions of the embodiments of this application and do not constitute a limitation on the technical solutions provided by the embodiments of this application. Persons skilled in the art will appreciate that, as new application scenarios emerge, the technical solutions provided by the embodiments of this application are equally applicable to similar technical problems. In the description of this application, unless otherwise specified, "multiple" means two or more.

[0033] FIG1 is a schematic diagram of the structure of a cathode material provided in one embodiment of the present application. Referring to FIG1 , the present application provides a low-cobalt cathode material comprising a core 10, a first coating layer 20 coating the core 10, and a second coating layer 30 coating the first coating layer 20. The core 10, the first coating layer 20, and the second coating layer 30 are explained below.

[0034] Kernel 10

[0035] The core 10 includes a first metal oxide, the general formula of which is Li y1 [Ni a1 Mn b1 M c1 ]O2, among which, 0.6≤a1<1.0, 0<b1≤0.4, 0≤c1≤0.1, a1+b1+c1=1, 0.98≤y1≤1.10.

[0036] Wherein, M is selected from at least one of tungsten, boron, aluminum, zirconium, titanium, magnesium, calcium, molybdenum, cerium, and strontium, and the molar ratio of M in the positive electrode material is 0-8%, and the molar ratio of M in the positive electrode material is, for example, 0.1%, 0.5%, 1%, 1.5%, 2.0%, 3%, 3.5%, 4%, 5%, 6%, 7% or 8%.

[0037] First coating layer 20

[0038] The first coating layer 20 includes a second metal oxide, the general formula of which is αLi y2 Co a2 R b2 O2·βCo3O4, of which 0.5 <a2≤1,0≤b2≤0.2,0.98≤y2≤1.10;α+β=1,0<β<1。

[0039] As an optional embodiment, R is selected from at least one of tungsten, aluminum, zirconium, and titanium, and the molar proportion of R in the positive electrode material is 0%-1%. The molar proportion of R in the positive electrode material is, for example, 0.1%, 0.2%, 0.3%, 0.5%, 0.8%, 0.9% or 1%.

[0040] Among them, Li in the first coating layer y2 Coa2 R b2 O2 fast ion conductor can activate the residual lithium on the surface of the positive electrode material, thereby increasing the capacity of the positive electrode material, and constructing a three-dimensional channel for lithium ion conduction, stabilizing the crystal structure, which can effectively reduce the internal resistance of the positive electrode material and improve the cycle stability of the positive electrode material. y2 Co a2 R b2 O2 and Co3O4 form a composite phase coating layer, which can stabilize the structure of the positive electrode material and reduce internal resistance.

[0041] Specifically, Li y2 Co a2 R b2 O2 forms a fast ion conductor layer and a discontinuous layer of Co3O4 particles on the surface of the core. The fast ion conductor layer promotes lithium ion diffusion, inhibits interfacial reactions, and constructs a three-dimensional lithium ion conduction channel. This stabilizes the crystal structure and helps improve electrode performance.

[0042] Second coating layer 30

[0043] The second coating layer 30 is a passivation layer that is inert to the electrolyte. The passivation layer can prevent the fast ion conductor from directly contacting the electrolyte, thereby preventing electrolyte corrosion and protecting the structure of the core 10, thereby further improving material stability.

[0044] As an optional embodiment, the passivation layer includes an oxide of Q, a fluoride of Q or a phosphate of Q, where Q is selected from at least one of tungsten, aluminum, zirconium, titanium, magnesium, molybdenum, cerium, strontium or boron; the molar proportion of the constituent substances of the passivation layer in the positive electrode material is 0.5%-1%, and the molar proportion of the constituent substances of the passivation layer in the positive electrode material is, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1%.

[0045] The passivation layer acts as a filler, making the coating on the core surface more uniform and preventing localized failures. It also helps evenly distribute the fast ion conductor. Compared to the first coating, the passivation layer is more structurally stable, hindering the dissolution of transition metals from the first coating and the core structure, preventing electrolyte corrosion of the core and the first coating, and improving material stability. This allows the positive electrode material to possess high capacity and high output power characteristics while also exhibiting excellent thermal stability.

[0046] In summary, the cathode material in the embodiment of the present application contains cobalt only in the first coating layer, effectively reducing the content of cobalt in the cathode material. Preferably, the molar ratio z of the cobalt element in the cathode material is 0<z≤3%. The molar ratio z of the cobalt element in the cathode material is, for example, 0.1%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 2.0%, 2.5% or 3%. Compared with existing cathode materials, the cobalt content of the cathode material in the present application is significantly reduced, reducing the amount of expensive precious metal cobalt, thereby reducing the cost of raw materials.

[0047] 1 , the positive electrode material further includes a first transition layer 40 formed by mutual penetration of the core 10 and the first coating layer 20 . The first transition layer 40 includes a first metal oxide and a second metal oxide to provide a stable interface layer between the first coating layer 20 and the core 10 .

[0048] The thickness of the first transition layer is 0.02 μm to 0.2 μm. The thickness of the first transition layer is typically, but not limited to, 0.02 μm, 0.03 μm, 0.04 μm, 0.05 μm, 0.06 μm, 0.08 μm, 0.10 μm, 0.15 μm, or 0.2 μm. The first transition layer having the above thickness range helps to provide two-phase compatibility between the first cladding layer and the core, while reducing internal forces and cladding defects.

[0049] 1 , the positive electrode material further includes a second transition layer 50 formed by mutual penetration of the first coating layer 20 and the second coating layer 30 . The second transition layer 50 includes a second metal oxide and a component of the passivation layer so that a stable interface layer is formed between the first coating layer 20 and the second coating layer 30 .

[0050] The thickness of the second transition layer is 0.01 μm-0.1 μm. The thickness of the second transition layer is typically, but not limited to, 0.02 μm, 0.03 μm, 0.04 μm, 0.05 μm, 0.06 μm, 0.07 μm, 0.08 μm, 0.09 μm, or 0.1 μm. The second transition layer having the above thickness range helps to provide two-phase compatibility between the first coating layer and the second coating layer, while reducing internal forces and coating defects.

[0051] Preferably, the lattice mismatch between the material of the first transition layer and the material of the core, and between the material of the first transition layer and the material of the first cladding layer are both less than or equal to 10%, or the interface stress between the first transition layer and the core, and between the first transition layer and the first cladding layer are both less than or equal to 2%.

[0052] Preferably, the lattice mismatch between the material of the second transition layer and the material of the first coating layer, and between the material of the second transition layer and the material of the second coating layer are both less than or equal to 10%, or the interface stress between the second transition layer and the first coating layer, and between the second transition layer and the second coating layer are both less than or equal to 2%.

[0053] It can be understood that the interface stress between the first transition layer and the two adjacent phases is small or the lattice mismatch between the two is low, which helps the transmission of lithium ions between the core and the first coating layer, so that the lithium ion transmission efficiency is better than that of directly coating the surface of the core with the first coating layer.

[0054] Similarly, the second transition layer facilitates the transfer of lithium ions between the second coating layer and the first coating layer, so that the transfer efficiency of lithium ions is better than that of the second coating layer directly coating the first coating layer.

[0055] The structure and composition of the positive electrode material are explained above. The preparation method of the positive electrode material of the present application will be described in detail below.

[0056] The preparation method of the positive electrode material in the embodiment of the present application includes the following steps:

[0057] S1: mixing a cobalt-free precursor, a lithium source, and a dopant, calcining the mixture, and then jet milling to obtain a pretreated precursor;

[0058] S2: mixing the pretreated precursor, the nano-cobalt-containing compound, and the first coating agent and performing a first sintering to obtain a composite precursor;

[0059] S3: mixing the composite precursor and the second coating agent and sintering them for the second time to obtain a positive electrode material;

[0060] Wherein, the dopant is selected from at least one of Al2O3, Al(OH)3, SrO, TiO2, ZrO2, MgO, CaCO3, Ca(OH)2, Y2O3, Nb2O5, MoO3, SnO2, WO3, H2WO4, B2O3, H3BO3, Ta2O5, CeO2, and La2O3; the nano-cobalt-containing compound is selected from at least one of Co(OH)2, CoOOH, Co2O3, and Co3O4, and the above compounds are nano-compounds; the first coating agent is selected from at least one of ZrO2, ZrF4, Al2O3, Al(OH)3, AlOOH, AlF3, WO3, H2WO4, TiO2, and MgO; the second coating agent is selected from at least one of ZrO2, Al2O3, AlF3, AlPO4, ZrF4, MgO, MgF, WO3, H2WO4, TiO2, At least one of LATP, SiO2, H2SiO3, carbon nanotubes GTNs, EDOT-PEG, PPy, PANI, and (NH4)2HPO4.

[0061] In one embodiment of the present application, step S2 further includes a first annealing treatment after the first sintering to obtain a composite precursor, and the temperature of the first annealing treatment is 50° C.-100° C. lower than the temperature of the first sintering.

[0062] Specifically, the temperature of the first sintering is 500° C.-800° C., and the holding time is 8 h-15 h.

[0063] It is understood that, in order to control the temperature of the first sintering, a flux can be added during the first sintering process so that the first coating layer contains spinel phase Co3O4 particles, thereby forming a fast ion conductor and spinel composite coating layer. The flux includes but is not limited to LiOH, Li2CO3, Li2O, and SrO.

[0064] In one embodiment of the present application, step S3 further includes a second annealing treatment after the second sintering to obtain the positive electrode material, and the temperature of the second annealing treatment is 50° C.-100° C. lower than the temperature of the second sintering.

[0065] Specifically, the temperature of the second sintering is 200° C.-500° C., and the holding time is 8 h-15 h.

[0066] In one embodiment of the present application, the molar ratio of the cobalt-free precursor, the lithium source, and the dopant in step S1 is 1:(1.04-1.20):(0.01-0.04).

[0067] In one embodiment of the present application, the molar ratio of the pretreated precursor, the nano-cobalt-containing compound, and the first coating agent in step S2 is 1:(0.005-0.03):(0.005-0.02).

[0068] In one embodiment of the present application, the molar ratio of the composite precursor to the second coating agent in step S3 is 1:(0.01-0.02).

[0069] The positive electrode material in the present application and the battery composed of the positive electrode material will be further described in detail below with reference to specific embodiments and comparative examples.

[0070] Example 1

[0071] This embodiment is a positive electrode material, which includes a core, a first coating layer covering the core, and a second coating layer covering the first coating layer. The general formula of the first metal oxide constituting the core is Li y [Ni a Mn b M c ]O2, the general formula of the second metal oxide constituting the first coating layer is αLi y2 Coa2 R b2 O2·Co3O4, the second coating layer is X a O 2-α F α The preparation process of the positive electrode material includes the following steps:

[0072] S1: Ni 0.75 Mn 0.25 (OH)2 and LiOH were mixed in a molar ratio of 1:1.08, and then 3000ppm ZrO2, 1000ppm Al2O3 and 500ppm MoO3 were added and mixed at a high speed of 800rpm / min for 30min. The mixture was calcined at a high temperature in an oxygen atmosphere by increasing the temperature to 900℃ at 15℃ / min and holding for 12h. After natural cooling, it was air flow milled to control D50=3.5μm to obtain a pretreated precursor.

[0073] S2: The pretreated precursor was mixed with 6000ppm Co(OH)2 and 1000ppm Al(OH)3 and sintered for the first time in an oxygen atmosphere. The temperature was raised to 700℃ at 15℃ / min, maintained for 10h, and then lowered to 600℃ and maintained for 3h to complete the annealing homogenization process to obtain a composite precursor.

[0074] S3: The composite precursor and 1000ppm Al2O3 were mixed and sintered for the second time in an oxygen atmosphere. The temperature was raised to 400℃ at 15℃ / min, maintained for 10h, and then cooled to 350℃ and maintained for 3h to complete the annealing homogenization process. After natural cooling and sieving, the positive electrode material was obtained.

[0075] Figure 2 is a SEM image of the precursor in Example 1 of the present application, Figure 3 is a SEM image of the pretreated precursor in Example 1 of the present application, and Figure 4 is a SEM image of the positive electrode material in Example 1 of the present application. Referring to Figures 2 to 4, the polycrystalline precursor in Example 1 becomes a pretreated precursor with a single crystal morphology after high-temperature calcination. After the pretreated precursor undergoes step S1, a first coating layer with obvious granular point coating (spinel structure Co3O4) is formed on its surface.

[0076] Examples 2-6 and Comparative Examples 1-2

[0077] Examples 2-6 and Comparative Examples 1-2 are respectively a type of positive electrode material, and their preparation processes can refer to the preparation in Example 1, except for the differences in reaction reagents and reaction conditions. The specific compositions are listed in Table 1.

[0078] Table 1

[0079] Wherein, the first coating agent in Example 4 includes 6000 ppm Co(OH)2, 1000 ppm Al(OH)3 and 2000 ppm ZrO2.

[0080] Figure 5 is an SEM image of the positive electrode material in Example 2 of the present application, Figure 6 is an SEM image of the positive electrode material in Example 3 of the present application, and Figure 7 is an SEM image of the positive electrode material in Comparative Example 1 of the present application. Referring to Figures 4 to 7 together, the number of point-like particles on the surface of the positive electrode materials in Figure 5 (Example 2), Figure 4 (Example 1), Figure 6 (Example 3), and Figure 7 (Comparative Example 1) gradually decreases. It can be found that the sintering temperature has a great influence on the surface morphology of the positive electrode material. As the sintering temperature increases, the granularity gradually weakens.

[0081] Comparative Examples 3-4

[0082] Comparative Examples 3-4 are respectively a positive electrode material, and their preparation process can refer to the preparation in Example 1, except that there is no step S2 in Comparative Example 3, and the obtained positive electrode material does not have the first coating layer; there is no step S3 in Comparative Example 4, and the obtained positive electrode material does not have the second coating layer.

[0083] The cobalt content of the positive electrode materials in Examples 1-6 and Comparative Examples 1-4 was measured, and the results are shown in Table 2 below.

[0084] Table 2

[0085] Referring to the data in Table 2, it can be seen that the molar ratio of cobalt element in the positive electrode materials prepared in the embodiments of the present application is lower than 1.2%, which is significantly lower than the molar ratio of cobalt element in existing ternary positive electrode materials (usually greater than or equal to 10%).

[0086] The positive electrode materials in Examples 1-6 and Comparative Examples 1-4 were used to form 0.8AH soft-pack batteries. The batteries were charged at a current of 0.1C within a voltage range of 3.0V-4.35V and discharged at different currents to evaluate capacity and rate performance. The specific results are shown in Table 3 below.

[0087] Table 3

[0088] Note: The battery's state of charge (SOC), that is, the remaining power; the battery's direct current resistance (DCR). The smaller the DCR, the greater the power capacity and the better the battery's rate performance.

[0089] FIG8 is a comparison of 200 cycle curves of soft-pack batteries prepared with the positive electrode materials in the examples of the present application and the comparative examples at 45° C., and FIG9 is a comparison of 200 cycle DCR growth rate curves of soft-pack batteries prepared with the positive electrode materials in the examples of the present application and the comparative examples at 45° C. Referring to FIG8 and FIG9 , and in combination with Table 3, it can be seen that step S2 is not included in comparative example 3, and the obtained positive electrode material does not have a first coating layer. The discharge capacity and cycle performance of comparative example 3 are lower than those of examples 1-3 and 5, and the DCR growth rate is significantly higher than that of examples 1-3 and 5, indicating that the fast ion conductor layer generated by the first coating layer can significantly improve the capacity, rate performance and cycle stability of the positive electrode material. Moreover, by comparing the data of Examples 1-3, it can be seen that the temperature of the first sintering affects the structure and relative content of the fast ion conductor and Co3O4 spinel phase in the first coating layer. When the temperature is low, the amount of fast ion conductor generated is small, the Co3O4 content increases, and the capacity and DCR of the positive electrode material are improved; but when the temperature is too high, the high temperature has a greater effect on the surface doping of the positive electrode material, affecting the surface microstructure of the positive electrode material, resulting in a decrease in the capacity of the positive electrode material and an increase in the DCR.

[0090] 8-9 and Table 3, there is no step S3 in Comparative Example 4, and the obtained positive electrode material does not have a second coating layer. The discharge capacity of Comparative Example 4 is higher than that of Examples 1-3 and 5, and the cycle performance is lower than that of Examples 1-3 and 5. The DCR growth rate is significantly higher than that of Examples 1-3 and 5. By comparing the data of Example 1 and Example 5, it can be seen that the increase in the temperature of the second sintering will reduce the discharge capacity and increase the DCR value.

[0091] In Example 4, ZrO2 is added to the first coating agent. At high temperatures, Zr ions cause surface micro-doping, which expands the lithium ion diffusion channel and improves its rate performance.

[0092] Compared with Example 1, the first annealing treatment was omitted in Comparative Example 1, resulting in the absence of the first transition layer, which led to a decrease in discharge capacity and rate performance, an increase in DCR, and a decrease in cycle performance.

[0093] Compared with Example 1, the second annealing treatment was omitted in Comparative Example 2, resulting in the absence of the second transition layer, which led to a decrease in discharge capacity and rate performance, an increase in DCR, and a decrease in cycle performance.

[0094] The above are only specific embodiments of the present application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. A low-cobalt cathode material, wherein: It comprises a core, a first coating layer covering the core, and a second coating layer covering the first coating layer; The core includes a first metal oxide, the first coating layer includes a second metal oxide, and the second coating layer is a passivation layer inert to the electrolyte; The general formula of the first metal oxide is Li y1 [Ni a1 Mn b1 M c1 ]O2, wherein 0.6≤a1<1.0, 0<b1≤0.4, 0≤c1≤0.1, a1+b1+c1=1, 0.98≤y1≤1.10, and M is selected from at least one of tungsten, boron, aluminum, zirconium, titanium, magnesium, calcium, molybdenum, cerium, and strontium; The general formula of the second metal oxide is αLi y2 Co a2 R b2 O2·βCo3O4, where 0.5 < a2 ≤ 1, 0 ≤ b2 ≤ 0.2, 0.98 ≤ y2 ≤ 1.10; α + β = 1, 0 < β < 1, and R is selected from at least one of tungsten, aluminum, zirconium, titanium, and magnesium.

2. The positive electrode material according to claim 1, wherein The molar proportion z of the cobalt element in the positive electrode material is 0<z≤3%.

3. The positive electrode material according to claim 1, wherein The molar proportion of M in the positive electrode material is 0-8%, and the molar proportion of R in the positive electrode material is 0-1%.

4. The positive electrode material according to claim 1, wherein The passivation layer includes an oxide of Q, a fluoride of Q or a phosphate of Q, wherein Q is selected from at least one of tungsten, aluminum, zirconium, titanium, magnesium, molybdenum, cerium, strontium or boron; The molar proportion of the constituent material of the passivation layer in the positive electrode material is 0.5%-1%.

5. The positive electrode material according to any one of claims 1 to 4, wherein The positive electrode material further includes a first transition layer formed by mutual penetration of the core and the first coating layer.

6. The positive electrode material according to claim 5, wherein The thickness of the first transition layer is 0.02 μm-0.2 μm.

7. The positive electrode material according to claim 5, wherein The positive electrode material further includes a second transition layer formed by mutual penetration of the first coating layer and the second coating layer.

8. The positive electrode material according to claim 7, wherein The thickness of the second transition layer is 0.01 μm-0.1 μm.

9. The positive electrode material according to claim 7 or 8, wherein The lattice mismatch between the material of the first transition layer and the material of the core, and between the material of the first transition layer and the material of the first cladding layer are all less than or equal to 10%; The lattice mismatch between the material of the second transition layer and the material of the first cladding layer, and between the material of the second transition layer and the material of the second cladding layer are both less than or equal to 10%.

10. A method for preparing a positive electrode material according to any one of claims 1 to 9, wherein: The steps include: S1: mixing a cobalt-free precursor, a lithium source and a dopant and calcining the mixture to obtain a pretreated precursor; S2: mixing the pretreated precursor, the nano-cobalt-containing compound, and the first coating agent, and then sintering them for the first time to obtain a composite precursor; S3: mixing the composite precursor and the second coating agent and sintering them for a second time to obtain the positive electrode material; Wherein, the dopant is selected from at least one of Al2O3, Al(OH)3, SrO, TiO2, ZrO2, MgO, CaCO3, Ca(OH)2, Y2O3, Nb2O5, MoO3, SnO2, WO3, H2WO4, B2O3, H3BO3, Ta2O5, CeO2, and La2O3; The first coating agent is selected from at least one of ZrO2, ZrF4, Al2O3, Al(OH)3, AlOOH, AlF3, WO3, H2WO4, TiO2, and MgO; The second coating agent is selected from at least one of ZrO2, Al2O3, AlF3, AlPO4, ZrF4, MgO, MgF, WO3, H2WO4, TiO2, LATP, SiO2, H2SiO3, carbon nanotubes GTNs, EDOT-PEG, PPy, PANI, and (NH4)2HPO4.

11. The preparation method according to claim 10, wherein: In step S2, after the first sintering, a first annealing treatment is further included to obtain the composite precursor, and the temperature of the first annealing treatment is 50° C.-100° C. lower than the temperature of the first sintering.

12. The preparation method according to claim 10 or 11, wherein: In step S3, after the second sintering, a second annealing treatment is further included to obtain the positive electrode material, and the temperature of the second annealing treatment is 50° C.-100° C. lower than the temperature of the second sintering.

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