Positive electrode material with element synergistic effect and preparation method of positive electrode material

Abstract

The invention discloses an element synergistic positive electrode material and a preparation method thereof, and belongs to the field of lithium ion battery electrode materials. In order to solve the technical problems that an existing doping type Co coating system lacks fine design, is poor in cycling stability and is limited in capacity, collaborative doping of multiple ions such as lanthanide is mainly adopted, and a composite design method of doping of a doping type Co coating structure and a lithium cobalt oxide body is constructed. By controlling the distribution of different ions in the body and the coating layer, stress regulation and performance optimization are realized, and the structural stability and capacity retention performance of lithium cobalt oxide under high voltage are remarkably improved.

Description

Technical Field

[0001] The invention relates to a surface element synergistic technology of a positive-valent material and a preparation method thereof, and belongs to the field of lithium-ion battery electrode materials. Background Art

[0002] The cut-off voltage of cathode materials for consumer electronics is getting higher and higher, and has been developed to 4.55V and 4.58V. Under the current voltage and future higher application voltage systems, the control of material surface modification needs to be more refined. Co additives are commonly used in surface modification to control the residual lithium content on the surface and improve electrochemical properties such as circulation and storage. At the same time, there is also the preparation of doped Co-coated additives. For example, the relevant patent "A lithium cobalt oxide cathode material for lithium-ion batteries above 4.45V and its preparation method" (patent application number: 201811215476.1) reports that the doped Co coating layer of doped elements such as Y, Ti, Sn, and Mn improves the high voltage performance of lithium cobalt oxide.

[0003] However, although the above patents prepare doped Co additive materials, the doped Co coating system lacks fine design, especially the synergistic effect of each doping element is poorly understood. Existing studies have shown that although doping with large ionic radius elements (such as Y, La, Ti, etc.) can improve the high-voltage performance of lithium cobalt oxide to a certain extent, the residual stress introduced by doping itself is not conducive to its cyclic stability under high voltage, and will produce cracks and accelerate degradation during high-voltage cycles. In addition, the current doped Co coating system ignores the connection with the bulk lithium cobalt oxide, so it is difficult to significantly improve the high-voltage performance of lithium cobalt oxide efficiently. For example, the related patent "A high-voltage lithium cobalt oxide positive electrode material, its preparation method and application" (application number: 202410798693.7) reported the effect of lanthanide metal Lu, Er, Dy, Eu, Nd and La lithium layer doping on inhibiting surface phase change, but this simple lanthanide doping will seriously inhibit the capacity. Summary of the invention

[0004] The purpose of the present invention is to solve the technical problems of the existing doped Co coating system lacking fine design, poor cycle stability and limited capacity, and to propose a positive electrode material with synergistic elements and a preparation method thereof, so as to achieve the fine design of the doped Co coating system and the doping of lithium cobalt oxide body, and utilize the synergistic effect of lanthanide ions with different ionic radii in different doping structures to greatly improve the comprehensive high voltage performance of lithium cobalt oxide.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] A positive electrode material with synergistic surface elements, the molecular formula of which is LizMAO2, M=Ni 1-x-y Mn xCo y where \(0 \lt y\leq1\), \(0.8 \lt z\leq1\), and the doping element A is at least two of F, Mg, Ti, Al, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Zr, Ni, Mn, Ca, Sn, Zn.

[0007] Furthermore, the structure of the positive electrode material includes an inner positive electrode material body \(Li_zMAO_2\) and an outer cobalt-based solid solution layer with a molecular formula mainly being \(CoO\), \(Co_3O_4\) or \(Li_xCoO_2\), where \(0 \lt x \lt 1\). There is an interface between the positive electrode material body and the cobalt-based solid solution layer, and the doping element is doped in the positive electrode material body and the cobalt-based solid solution layer.

[0008] Furthermore, the thickness of the cobalt-based solid solution layer is less than 200 nm, and it is a spinel structure or a tetrahedral or octahedral structure of a rock salt phase.

[0009] Furthermore, the positive electrode material body is a layered structure, and the doping element of the positive electrode material body is a small ionic radius element or a combination of it and other elements. The small ionic radius element is at least one lanthanide element selected from Tb, Dy, Ho, Er, Tm, Yb, Lu, and the other elements include one or more of Ca, Sn, Zn, Mg, Ti, Al.

[0010] Furthermore, the doping element of the cobalt-based solid solution layer is a large ionic radius element or a combination of it and other elements. The large ionic radius element is at least one lanthanide element selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, and the other elements include one or more of F, Mg, Ti, Al, Y, Zr, Ni, Mn, Sn, Zn.

[0011] Furthermore, there is an ionic radius difference in at least one group of lanthanide elements between the doping elements of the positive electrode material body and the cobalt-based solid solution layer. The radius ratio of the large radius to the small radius is greater than 1.1, and the doping molar ratio is less than 0.9.

[0012] A preparation method of a positive electrode material with synergistic action of surface elements includes the following steps:

[0013] Mix a Co precursor and a lithium source according to a molar ratio of lithium element to cobalt element of \((1 - 1.09):1\), then carry out a primary solid-phase sintering, then cool to room temperature, and perform a crushing treatment after sintering is completed to obtain a positive electrode material body;

[0014] Prepare one or more additives containing doping elements and grind them into powder;

[0015] Mix the cathode material body, cobalt-based solid solution cobalt source, and additive in the required proportions, then perform secondary solid-phase sintering, and then cool to room temperature. After sintering is completed, perform a crushing treatment to coat the cobalt-based solid solution layer on the cathode material body to obtain the final cathode material.

[0016] Further, the Co-containing precursor is Co3O4, the lithium source is Li2CO3, and the cobalt-based solid solution cobalt source is one or a combination of Co3O4, Co(OH)3, and CoO(OH).

[0017] Further, when performing the first solid-phase sintering, choose to add or not add an additive; if no additive is added, then when performing the second solid-phase sintering, add the additive of the cathode material body and the cobalt-based solid solution layer together.

[0018] Further, the doping elements contained in the additive required for the cathode material body are selected from at least one of Tb, Dy, Ho, Er, Tm, Yb, Lu or a combination of at least one of them with at least one element of Ca, Sn, Zn, Mg, Ti, Al. The doping element source is one or a combination of oxides, hydroxides, fluorides, and hydroxyoxides of Tb, Dy, Ho, Er, Tm, Yb, Lu, Ca, Sn, Zn, Mg, Ti, Al.

[0019] Further, the particle size of the oxides, hydroxides, fluorides, and hydroxyoxides is between 0 and 5 μm.

[0020] Further, the doping elements contained in the additive required for the cobalt-based solid solution layer are selected from at least one of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd or a combination of at least one of them with at least one element of F, Mg, Ti, Al, Y, Zr, Ni, Mn, Ca, Sn, Zn. The doping element source is one or a combination of oxides, hydroxides, fluorides, and hydroxyoxides of Mg, Ti, Al, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Y, Zr, Ni, Mn, Ca, Sn, Zn.

[0021] Further, the particle size of the oxides, hydroxides, fluorides, and hydroxyoxides is between 0 and 5 μm.

[0022] Further, the temperature of the first solid-phase sintering is controlled at 500 - 1100 °C, and the sintering time is controlled at 8 - 20 h.

[0023] Further, the temperature of the second solid-phase sintering is controlled at 500 - 1100 °C, and the sintering time is controlled at 8 - 20 h.

[0024] The advantages obtained by the present invention are as follows:

[0025] 1. The cathode material prepared by the present invention has a specific refined structure design, involving a specific surface structure and a combination of elements with different ionic radii. The specific structure includes the cathode material body and a cobalt-based solid solution coated on the surface layer. The specific combination of elements with different ionic radii includes the doping of small-radius ionic element lithium layer and the doping of large-radius ionic element cobalt-based solid solution. Through the directional doping of inert elements with different ionic radii, the synergistic effect between the Co coating layer and the lithium cobaltate body structure is realized, and the electrical performance of the cathode material is effectively improved.

[0026] The present invention can realize the synergistic effect between the Co coating layer and the lithium cobaltate body structure, that is, the lattice strain energy introduced by the doping of cobalt-based solid solution with large ionic radius elements improves the dissolution stability of cobalt-based solid solution elements and compensates for the lithium diffusion inhibition effect brought by the doping of small-radius ionic element lithium layer. And the doping of small-radius ionic element lithium layer can not only effectively inhibit the lattice expansion of the cathode material in the deeply delithiated state but also inhibit the residual stress introduced by the diffusion of large-radius ionic elements into the bulk phase. Specifically, firstly, the stress difference brought by the ionic radius difference of the doping elements can effectively slow down the dissolution of bulk cobalt and the evolution of oxygen; secondly, the interlayer doping of small-radius ions will reduce the diffusion kinetics level of lithium ions, while the doping of large-radius ions can effectively make up for or even overall improve the diffusion kinetics level; finally, the doping of small ionic radius inert elements in the bulk phase of the layered cathode material can inhibit the diffusion of large-radius ions in the traditional doped Co coating layer into the bulk phase, thereby overall reducing the residual stress of the material. This surface element collaborative design can not only maintain the lithium ion transport performance of the cathode material of the lithium ion battery and inhibit the dissolution of elements compared with the traditional doped Co coating layer design, but also enhance the structural stability of the cathode material in the deeply delithiated state at high voltage.

[0027] 2. The operation process of the present invention is simple, the raw material cost is low, and it is easy to realize industrial production. Description of the Drawings

[0028] Figure 1 is the SEM image of the lithium ion battery cathode material sample prepared in Example 1.

[0029] Figure 2 is the SEM image of the lithium ion battery cathode material sample prepared in Comparative Example 1.

[0030] Figure 3 is the EPMA image of elements Er and Y in the cathode material sample prepared in Example 1.

[0031] Figure 4 is the EPMA image of elements Er and Y in the cathode material sample prepared in Comparative Example 1. Detailed Embodiments

[0032] In order to make the technical features, advantages or technical effects in the above technical solutions of the present invention more obvious and understandable, the following will be described in detail through embodiments.

[0033] Example 1

[0034] (1) Preparation of the cathode material matrix

[0035] Co3O4 was prepared by the co-precipitation method. Lithium carbonate and Co3O4 were taken and mixed according to the molar ratio of lithium to metal of 1.05. The mixed raw materials were heated from room temperature to the first holding section at a rate of 5 °C / min, with a holding temperature of 700 °C and a holding time of 1 h; then heated from the first holding section to the second holding section at a rate of 3 °C / min, with a holding temperature of 1050 °C and a holding time of 10 h; and then cooled to room temperature to obtain the cathode matrix material LiCoO2.

[0036] (2) Preparation of the coating additive

[0037] Raw material pretreatment: Weigh 10 g of high-purity (≥99.9%) raw materials according to the target molar ratio (La2O3:Er2O3 = 1:3), and dry to remove moisture.

[0038] After mixing the raw materials in proportion, add 5 mL of absolute ethanol and ball-mill in a planetary ball mill at a speed of 400 rpm for 6 hours, with a ball-to-material ratio of 10:1, to obtain a nanoscale uniformly dispersed powder. The average particle size of the powder is 50 nm, the specific surface area is 45 m 2 / g, and the element distribution is uniform.

[0039] (3) Preparation of the lithium-ion battery cathode material

[0040] The cathode material matrix obtained in step (1), the additive powder obtained in step (2) and Co3O4 were mixed evenly according to the mass ratio of 75:0.1:1. The mixed raw materials were heated from room temperature to the first holding section at a rate of 5 °C / min, with a holding temperature of 700 °C and a holding time of 1 h; then heated from the first holding section to the second holding section at a rate of 3 °C / min, with a holding temperature of 900 °C and a holding time of 10 h; and then cooled to room temperature and broken to obtain the cathode material.

[0041] The cathode material, conductive agent acetylene black and binder PVDF were mixed according to the mass ratio (95:2:10), N-methylpyrrolidone (NMP) solvent was added, and the mixture was stirred in a planetary stirrer at 200 rpm for 2 hours to form a uniform slurry; the slurry was evenly coated on the surface of the current collector aluminum foil, vacuum dried at 80 °C for 12 hours, and then rolled to a pole piece thickness of 50 μm and punched into a round piece with a diameter of 10 mm.

[0042] In an inert atmosphere glove box, assemble the negative electrode case, metallic lithium sheet (thickness 0.2 - 0.6 mm), electrolyte, separator, working electrode sheet, gasket and spring sheet, and positive electrode case in sequence. Seal with a packaging machine at a pressure of 5 MPa to form a button-type half cell, and perform charge and discharge tests on the battery. The voltage range is 3.0 - 4.6 V, that is, 4.53 V under normal temperature conditions. Activate by charging and discharging at 0.2 C for 1 week, then charge from 0.7 C to 4.6 V in an oven at 45 °C and then discharge at 0.7 C for 100 cycles, and observe the capacity retention rate during the cycling process.

[0043] Example 2

[0044] This example provides a preparation method for a high-voltage lithium cobalt oxide cathode material. The difference between the preparation method and that of Example 1 is only that: the heat preservation temperature in step (1) is 1000 °C, and the heat preservation temperature in step (3) is 1000 °C.

[0045] Example 3

[0046] This example provides a preparation method for a high-voltage lithium cobalt oxide cathode material. The difference between the preparation method and that of Example 1 is only that: the Er2O3 added in step (3) is added in equal amount in step (1).

[0047] Example 4

[0048] (1) Preparation of the cathode material matrix

[0049] Prepare Co3O4 by the co-precipitation method. Take lithium carbonate and Co3O4, mix them according to the lithium and metal molar ratio of 1.05, and heat the mixed raw materials from room temperature to the first heat preservation section at a rate of 5 °C / min. The heat preservation temperature is 700 °C, and the heat preservation time is 1 h; heat from the first heat preservation section to the second heat preservation section at a rate of 3 °C / min. The heat preservation temperature is 1050 °C, and the heat preservation time is 20 h; then cool to room temperature to obtain the cathode matrix material LiCoO2.

[0050] (2) Preparation of the coating additive

[0051] Raw material pretreatment: Weigh 10 g of high-purity (≥99.9%) raw materials according to the target molar ratio (MgF:Lu2O3 = 1:1), dry to remove moisture, and record it as additive A; weigh 10 g of high-purity (≥99.9%) raw materials according to the target molar ratio (LaF:YF3 = 1:1), dry to remove moisture, and record it as additive B.

[0052] After mixing the raw materials in proportion, add 5 mL of anhydrous ethanol to additive A, and ball mill in a planetary ball mill at a rotation speed of 400 rpm for 6 hours. The ball-to-material ratio is 10:1 to obtain a nano-level uniformly dispersed powder. The average particle size of the powder is less than 50 nm, and the specific surface area is greater than 45 m 2 / g, the element distribution is uniform; after mixing the raw materials in proportion, 5 mL of absolute ethanol is added to additive B, and ball milling is carried out in a planetary ball mill at a rotation speed of 400 rpm for 6 hours, with a ball-to-material ratio of 10:1, to obtain a nano-level uniformly dispersed powder. The average particle size of the powder is less than 50 nm, and the specific surface area is greater than 45 m 2 / g, the element distribution is uniform.

[0053] (3) Preparation of the positive electrode material for lithium-ion batteries

[0054] Mix the positive electrode material matrix obtained in step (1), the ball-milled additive A powder obtained in step (2), and Co3O4 evenly according to a mass ratio of 75:0.1:0.5. Heat the mixed raw materials from room temperature to the first heat preservation section at a rate of 5 °C / min, with a heat preservation temperature of 700 °C and a heat preservation time of 1 h; heat from the first heat preservation section to the second heat preservation section at a rate of 3 °C / min, with a heat preservation temperature of 900 °C and a heat preservation time of 10 h; then cool to room temperature and then crush for use;

[0055] Mix the composite material to be used in the above steps, the ball-milled additive B powder obtained in step (2), and Co3O4 evenly according to a mass ratio of 75:0.1:0.5. Heat the mixed raw materials from room temperature to 900 °C at a rate of 5 °C / min, with a heat preservation time of 10 h; then cool to room temperature and then crush to obtain the positive electrode material;

[0056] Mix the positive electrode material, conductive agent acetylene black, and binder PVDF according to a mass ratio of (95:2:10), add N-methylpyrrolidone (NMP) solvent, and stir in a planetary stirrer at 200 rpm for 2 hours to form a uniform slurry; evenly coat the slurry on the surface of the current collector aluminum foil, vacuum dry at 80 °C for 12 hours, roll press to a pole piece thickness of 50 μm, and punch into circular pieces with a diameter of 10 mm.

[0057] In an inert atmosphere glove box, assemble the negative electrode shell, metallic lithium sheet (thickness 0.2 - 0.6 mm), electrolyte, separator, working electrode pole piece, gasket and spring piece, and positive electrode shell in sequence. Seal with a packaging machine at a pressure of 5 MPa to form a button-type half-cell, and perform charge and discharge tests on the battery, with a voltage range of 3.0 - 4.6 V.

[0058] Comparative Example 1

[0059] (1) Preparation of the positive electrode material matrix

[0060] Co3O4 was prepared by the co-precipitation method. Lithium carbonate and Co3O4 were taken and mixed according to the molar ratio of lithium to metal of 1.05. The mixed raw materials were heated from room temperature to the first holding section at a rate of 5 °C / min, with a holding temperature of 700 °C and a holding time of 1 h; then heated from the first holding section to the second holding section at a rate of 3 °C / min, with a holding temperature of 1050 °C and a holding time of 10 h; and then cooled to room temperature to obtain the positive electrode matrix material LiCoO2.

[0061] (2) Preparation of the coating additive

[0062] Raw material pretreatment: Weigh 10 g of high-purity (≥99.9%) raw materials according to the target molar ratio (La2O3:Y2O3 = 1:1), and dry to remove moisture.

[0063] After mixing the raw materials in proportion, add 5 mL of absolute ethanol, and ball-mill in a planetary ball mill at a rotation speed of 400 rpm for 6 hours, with a ball-to-material ratio of 10:1, to obtain a nanoscale uniformly dispersed powder. The average particle size of the powder is 50 nm, the specific surface area is 45 m 2 / g, and the element distribution is uniform.

[0064] (3) Preparation of the positive electrode material for lithium-ion batteries

[0065] The positive electrode material matrix obtained in step (1), the additive powder obtained in step (2), and Co3O4 were mixed uniformly according to a mass ratio of 75:0.1:1. The mixed raw materials were heated from room temperature to the first holding section at a rate of 5 °C / min, with a holding temperature of 700 °C and a holding time of 1 h; then heated from the first holding section to the second holding section at a rate of 3 °C / min, with a holding temperature of 900 °C and a holding time of 10 h; and then cooled to room temperature and broken to obtain the positive electrode material.

[0066] The positive electrode material, conductive agent acetylene black, and binder PVDF were mixed according to a mass ratio of (95:2:10), N-methylpyrrolidone (NMP) solvent was added, and the mixture was stirred in a planetary mixer at 200 rpm for 2 hours to form a uniform slurry; the slurry was uniformly coated on the surface of the current collector aluminum foil, vacuum dried at 80 °C for 12 hours, rolled to a pole piece thickness of 50 μm, and punched into circular pieces with a diameter of 10 mm.

[0067] In an inert atmosphere glove box, the negative electrode shell, metallic lithium sheet (thickness 0.2 - 0.6 mm), electrolyte, separator, working electrode pole piece, gasket and spring piece, and positive electrode shell were assembled in sequence. A sealing machine was used to seal at a pressure of 5 MPa to form a button-type half-cell, and the battery was subjected to charge and discharge tests within a voltage range of 3.0 - 4.6 V.

[0068] Comparative Example 2

[0069] (1) Preparation of the positive electrode material matrix

[0070] Co3O4 was prepared by the co-precipitation method. Lithium carbonate and Co3O4 were taken and mixed according to the molar ratio of lithium to metal of 1.05. The mixed raw materials were heated from room temperature to the first heat preservation section at a rate of 5 °C / min, with the heat preservation temperature of 700 °C and the heat preservation time of 1 h; then heated from the first heat preservation section to the second heat preservation section at a rate of 3 °C / min, with the heat preservation temperature of 1050 °C and the heat preservation time of 10 h; and then cooled to room temperature to obtain the cathode matrix material lithium cobaltate.

[0071] (2) Preparation of the coating additive

[0072] Raw material pretreatment: Weigh 10 g of high-purity Er2O3 (≥99.9%) raw material, dry to remove moisture, add 5 mL of absolute ethanol, and ball mill in a planetary ball mill at a speed of 400 rpm for 6 hours, with the ball-to-material ratio of 10:1, to obtain a nanoscale uniformly dispersed powder. The average particle size of the powder is 50 nm, the specific surface area is 45 m 2 / g, and the element distribution is uniform.

[0073] (3) Preparation of the lithium-ion battery cathode material

[0074] The cathode material matrix obtained in step (1), the additive powder obtained in step (2) and Co(OH)2 were mixed evenly according to the mass ratio of 75:0.1:1. The mixed raw materials were heated from room temperature to the first heat preservation section at a rate of 5 °C / min, with the heat preservation temperature of 700 °C and the heat preservation time of 1 h; then heated from the first heat preservation section to the second heat preservation section at a rate of 3 °C / min, with the heat preservation temperature of 900 °C and the heat preservation time of 10 h; and then cooled to room temperature and broken to obtain the cathode material.

[0075] The cathode material, conductive agent acetylene black and binder PVDF were mixed according to the mass ratio of (95:2:10), N-methylpyrrolidone (NMP) solvent was added, and the mixture was stirred in a planetary stirrer at 200 rpm for 2 hours to form a uniform slurry; the slurry was evenly coated on the surface of the current collector aluminum foil, vacuum dried at 80 °C for 12 hours, rolled to a pole piece thickness of 50 μm, and punched into circular pieces with a diameter of 10 mm.

[0076] In an inert atmosphere glove box, the negative electrode shell, lithium metal sheet with a thickness of 0.2 - 0.6 mm, electrolyte, separator, working electrode pole piece, gasket and spring piece, and positive electrode shell were assembled in sequence. A button-type half-cell was sealed with a packaging machine at a pressure of 5 MPa, and the battery was subjected to charge and discharge tests with a voltage range of 3.0 - 4.6 V.

[0077] The results are shown in Figures 1 to 4 . By comparison, it can be seen that there is no enrichment of Er element on the surface of the cathode material prepared in Example 1, while there is a dot-like coating with a certain regular shape of Y element on the surface of the cathode material prepared in Comparative Example 1.

[0078] The test results of the above Example 1 and Comparative Examples 1-2 are shown in Table 1:

[0079] Table 1 Data of sample discharge specific capacity and 45 °C high-temperature cycle capacity retention rate

[0080] Example Initial week discharge specific capacity mAh / g Capacity retention rate % after 50 cycles at 45°C, 4.6V, 0.7C Example 1 204.8 88 Comparative Example 1 202.1 71 Comparative Example 2 201.3 69

[0081] It can be seen from the test results of the above examples and comparative examples that the special design of the surface structure of lithium cobaltate and the reasonable combination of ions with specific radii have a significant synergistic effect, breaking the conventional understanding that the capacity cannot be maintained in the bulk cycle. The reason lies in the specific refined structure design of the present invention. Through the directional doping of inert elements with different ionic radii, the synergistic effect between the Co coating layer and the lithium cobaltate bulk structure is realized, which effectively improves the electrical properties of the cathode material. The interlayer doping of small-radius ions will reduce the diffusion kinetics level of lithium ions, while the doping of large-radius ions can effectively compensate for and even overall improve the diffusion kinetics level; the doping of inert elements with small ionic radii in the bulk phase of the layered cathode material can inhibit the diffusion of large-radius ions in the traditional doping Co coating layer into the bulk phase, thereby overall reducing the residual stress of the material. Compared with the traditional doping Co coating layer design, this surface element synergistic design can not only maintain the lithium ion transport performance of the lithium ion battery cathode material and inhibit the dissolution of elements, but also enhance the structural stability of the cathode material in the high-voltage deep delithiation state.

[0082] Although the present invention has been disclosed above by way of examples, it is not intended to limit the present invention. Any appropriate modifications or equivalent replacements made by those of ordinary skill in the art to the technical solutions of the present invention shall be covered within the protection scope of the present invention. The protection scope of the present invention shall be defined by the claims.

Claims

1. A cathode material with synergistic effect of surface elements, characterized in that The molecular formula is LizMAO2, where M = Ni 1-x- y Mn x Co y , 0 < y ≤ 1, 0.8 < z ≤ 1, and the doping element A is at least two of F, Mg, Ti, Al, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Zr, Ni, Mn, Ca, Sn, Zn 2. The cathode material according to claim 1, characterized in that, The structure of the positive electrode material includes a positive electrode material body LizMAO2 inside and a cobalt-based solid solution layer outside. The molecular formula of the cobalt-based solid solution layer is mainly CoO, Co3O4 or LixCoO2, where 0 < x < 1. There is an interface between the positive electrode material body and the cobalt-based solid solution layer, and the doping element is doped in the positive electrode material body and the cobalt-based solid solution layer.

3. The cathode material according to claim 2, characterized in that, The thickness of the cobalt-based solid solution layer is less than 200 nm, and it is a spinel structure or a tetrahedral or octahedral structure of a rock salt phase.

4. The cathode material according to claim 2, characterized in that, The positive electrode material body has a layered structure. The doping element of the positive electrode material body is an element with a small ionic radius or a combination of it and other elements. The element with a small ionic radius is selected from at least one lanthanide element among Tb, Dy, Ho, Er, Tm, Yb, Lu, and the other elements include one or more of Ca, Sn, Zn, Mg, Ti, Al; The doping element of the cobalt-based solid solution layer is an element with a large ionic radius or a combination of it and other elements. The element with a large ionic radius is selected from at least one lanthanide element among La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, and the other elements include one or more of F, Mg, Ti, Al, Y, Zr, Ni, Mn, Sn, Zn; 5. The cathode material according to claim 4, characterized in that, There is an ionic radius difference in at least one group of lanthanide elements among the doping elements of the positive electrode material body and the cobalt-based solid solution layer. The radius ratio of the large radius to the small radius is greater than 1.1, and the doping molar ratio is less than 0.

9.

6. A preparation method of a cathode material with synergistic effect of surface elements, characterized in that, It includes the following steps: Mix the Co precursor and the lithium source according to the molar ratio of lithium element to cobalt element of (1 - 1.09):1, then carry out a primary solid-phase sintering, and then cool to room temperature. After the sintering is completed, carry out a crushing treatment to obtain the positive electrode material body; Prepare one or more additives containing the doping element and ball-mill them into powder; Mix the positive electrode material body, the cobalt-based solid solution cobalt source, and the additives in the required proportions, then carry out a secondary solid-phase sintering, and then cool to room temperature. After the sintering is completed, carry out a crushing treatment to coat the cobalt-based solid solution layer on the positive electrode material body to obtain the final positive electrode material.

7. The preparation method according to claim 6, wherein The Co precursor is Co3O4, the lithium source is Li2CO3, and the cobalt-based solid solution cobalt source is one or a combination of Co3O4, Co(OH)3, CoO(OH).

8. The preparation method according to claim 6, characterized in that, When carrying out the primary solid-phase sintering, choose to add or not add additives; if no additives are added, then when carrying out the secondary solid-phase sintering, add the additives of the positive electrode material body and the cobalt-based solid solution layer together.

9. The preparation method according to claim 8, wherein The doping element contained in the additives required for the positive electrode material body is selected from at least one of Tb, Dy, Ho, Er, Tm, Yb, Lu or a combination of it and at least one element of Ca, Sn, Zn, Mg, Ti, Al. The doping element source is one or a combination of oxides, hydroxides, fluorides, hydroxyoxides of Tb, Dy, Ho, Er, Tm, Yb, Lu, Ca, Sn, Zn, Mg, Ti, Al; the particle size of the oxides, hydroxides, fluorides, hydroxyoxides is between 0 - 5 μm; The doping elements contained in the additives required for the cobalt-based solid solution layer are selected from at least one of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd or a combination thereof with at least one element of F, Mg, Ti, Al, Y, Zr, Ni, Mn, Ca, Sn, Zn. The doping element source is one or a combination of oxides, hydroxides, fluorides, hydroxyoxides of Mg, Ti, Al, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Y, Zr, Ni, Mn, Ca, Sn, Zn; the particle size of the oxides, hydroxides, fluorides, hydroxyoxides is between 0 and 5 μm.

10. The preparation method according to claim 6, characterized in that, The temperature of the first solid-phase sintering is controlled at 500-1100 °C, and the sintering time is controlled at 8-20 h; the temperature of the second solid-phase sintering is controlled at 500-1100 °C, and the sintering time is controlled at 8-20 h.

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