WO2023236906A1 - 表面包覆正极材料及其制备方法、锂离子电池 - Google Patents

表面包覆正极材料及其制备方法、锂离子电池 Download PDF

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Publication number
WO2023236906A1
WO2023236906A1 PCT/CN2023/098360 CN2023098360W WO2023236906A1 WO 2023236906 A1 WO2023236906 A1 WO 2023236906A1 CN 2023098360 W CN2023098360 W CN 2023098360W WO 2023236906 A1 WO2023236906 A1 WO 2023236906A1
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source
cathode material
sintering
lithium
peak
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English (en)
French (fr)
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张博皓
刘亚飞
陈彦彬
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Beijing Easpring Material Technology Co Ltd
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Beijing Easpring Material Technology Co Ltd
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Priority to JP2023580763A priority Critical patent/JP7640758B2/ja
Priority to KR1020237044757A priority patent/KR102724244B1/ko
Priority to EP23819071.4A priority patent/EP4345947A4/en
Publication of WO2023236906A1 publication Critical patent/WO2023236906A1/zh
Priority to US18/400,312 priority patent/US11984586B1/en
Anticipated expiration legal-status Critical
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
    • H01M4/366—Composites as layered products
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G53/00—Compounds of nickel
    • C01G53/40—Complex oxides containing nickel and at least one other metal element
    • C01G53/42—Complex oxides containing nickel and at least one other metal element containing alkali metals, e.g. LiNiO2
    • C01G53/44—Complex oxides containing nickel and at least one other metal element containing alkali metals, e.g. LiNiO2 containing manganese
    • C01G53/50—Complex oxides containing nickel and at least one other metal element containing alkali metals, e.g. LiNiO2 containing manganese of the type (MnO2)n-, e.g. Li(NixMn1-x)O2 or Li(MyNixMn1-x-y)O2
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/04—Processes of manufacture in general
    • H01M4/0471—Processes of manufacture in general involving thermal treatment, e.g. firing, sintering, backing particulate active material, thermal decomposition, pyrolysis
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/485—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of mixed oxides or hydroxides for inserting or intercalating light metals, e.g. LiTi2O4 or LiTi2OxFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
    • H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
    • H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/70—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/021—Physical characteristics, e.g. porosity, surface area
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the invention relates to the technical field of lithium-ion batteries, and in particular to a surface-coated positive electrode material and a preparation method thereof, and a lithium-ion battery.
  • CN109065875A discloses a preparation method for coating high-nickel multi-component materials, in which ZrO 2 , TiO 2 , Al 2 O 3 , MgO and other materials are used as coating agents. After coating, although side reactions between the positive electrode and the electrolyte can be reduced, the improvement of Battery performance, but the coating agents used in this method are all inert materials, fail to participate in ion and electron transmission, and do not improve the thermal stability of high-nickel materials.
  • the purpose of the present invention is to overcome the problems existing in the prior art and provide a surface-coated cathode material and a preparation method thereof, as well as a lithium-ion battery.
  • the surface-coated cathode material includes a base body and a surface coating on the surface of the base body.
  • the characteristic peaks at 2 ⁇ of 31°-35° tested by XRD of the coating layer show a bimodal distribution, and the bimodal distribution has a specific peak intensity ratio between the main peak and the secondary peak, so that
  • the surface-coated cathode material has high ion and electron conductivity, which can accelerate the diffusion rate of ions and electrons in the cathode material, thereby significantly improving the rate performance of lithium-ion batteries containing the cathode material.
  • the surface-coated cathode material with the above-mentioned specific coating layer can avoid catalytic corrosion between the cathode material and the electrolyte, and improve the cycle stability of lithium-ion batteries containing the cathode material.
  • the thermal stability of the surface-coated cathode material with the above-mentioned specific coating layer is significantly improved.
  • a first aspect of the present invention provides a surface-coated cathode material, which is characterized in that the cathode material includes a base body and a coating layer covering the surface of the base body;
  • the characteristic peaks at 2 ⁇ of 31°-35° of the coating layer tested by XRD show a bimodal distribution; and the ratio of the peak intensity I b of the secondary peak in the bimodal distribution to the peak intensity I a of the main peak I b /I a is 0.8-1.
  • a second aspect of the present invention provides a method for preparing a surface-coated cathode material, characterized in that the method includes the following steps:
  • the coating agent and the cathode material matrix are mixed for a second time to obtain a second mixture, and the second mixture is subjected to a first heat treatment to obtain the surface-coated cathode material.
  • a third aspect of the present invention provides a method for preparing a surface-coated cathode material, characterized in that the method includes the following steps:
  • the lithium source, the La source, optionally the N 1 source, the N 2 source, the N 3 source, the optional N 4 source and the cathode material matrix are mixed for a third time to obtain a third mixture that is subjected to a second heat treatment to obtain the surface Covering the cathode material.
  • a fourth aspect of the present invention provides a surface-coated cathode material prepared by the above preparation method.
  • a fifth aspect of the present invention provides a lithium ion battery, characterized in that the above surface of the lithium ion battery is coated with a positive electrode material.
  • the surface-coated cathode material, its preparation method and application, and the lithium-ion battery provided by the present invention obtain the following beneficial effects:
  • the surface-coated cathode material provided by the present invention includes a substrate and a coating layer coated on the surface of the substrate.
  • the characteristic peaks at 2 ⁇ of 31°-35° of the coating layer tested by XRD show a bimodal distribution.
  • the bimodal distribution has a specific peak intensity ratio between the main peak and the secondary peak, which makes the surface-coated cathode material have high ion and electron conductivity and can accelerate the diffusion rate of ions and electrons in the cathode material. This significantly improves the rate performance of lithium-ion batteries containing this cathode material.
  • the surface-coated cathode material with the above-mentioned specific coating layer can avoid catalytic corrosion between the cathode material and the electrolyte, and improve the cycle stability of lithium-ion batteries containing the cathode material.
  • the thermal stability of the surface-coated cathode material with the above-mentioned specific coating layer is significantly improved.
  • the matrix and coating layer of the surface-coated cathode material provided by the present invention have a specific composition, which enables the cathode material to have electrochemical activity of lithium ion deintercalation, and the coating material used has high lithium ion transport. ability, and at the same time has a low oxygen hole formation energy, which can increase the electronic conductivity and accelerate the diffusion rate of ions and electrons in the cathode material, thus significantly improving the rate performance of the cathode material.
  • the coating layer with a specific composition can effectively suppress the interface side reaction between the surface of the cathode material particles and the electrolyte, significantly improve the cycle stability of the lithium-ion battery containing the cathode material, and effectively enhance the cathode Thermal stability of materials.
  • the preparation method of the cathode material provided by the present invention has a simple process, a simple introduction method of pollution-free doping elements and coating layers, a small amount, no special requirements for the heat treatment atmosphere, low production cost, and is suitable for large-scale industrial production.
  • Figure 1 is the XRD phase diagram of the coating agent in Example 1.
  • Figure 2 is an XRD phase diagram of the coated cathode material in Example 1.
  • Figure 3 is a charge-discharge curve at 0.1C for the half-cells prepared in Comparative Example 1, Example 1 and Example 4.
  • Figure 4 is a charge-discharge curve at 0.1C for the half-cells prepared in Comparative Example 2, Example 2 and Example 3.
  • Figure 5 is a DSC curve graph of Comparative Example 1, Example 1 and Example 4.
  • Figure 6 is a DSC curve graph of Comparative Example 2, Example 2 and Example 3.
  • a first aspect of the present invention provides a surface-coated cathode material, characterized in that the cathode material includes a base body and a coating layer covering the surface of the base body;
  • the characteristic peaks at 2 ⁇ of 31°-35° of the coating layer tested by XRD show a bimodal distribution; and the ratio of the secondary peak intensity I b in the bimodal distribution to the peak intensity I a of the main peak I b /I a is 0.8-1.
  • the surface-coated cathode material includes a substrate and a coating layer coating the surface of the substrate.
  • the characteristic peaks of the coating layer at 2 ⁇ of 31°-35° after XRD testing are bimodal. And this bimodal distribution has a specific peak intensity ratio between the main peak and the secondary peak, so that the surface-coated cathode material has high ion and electron conductivity, and can accelerate the diffusion rate of ions and electrons in the cathode material, thereby The rate performance of lithium-ion batteries containing this cathode material is significantly improved.
  • the surface-coated cathode material with the above-mentioned specific coating layer can avoid catalytic corrosion between the cathode material and the electrolyte, and improve the cycle stability of the lithium-ion battery containing the cathode material.
  • the thermal stability of the surface-coated cathode material with the above-mentioned specific coating layer is significantly improved.
  • the secondary peak intensity I b and the peak intensity I a of the main peak in the bimodal distribution are respectively measured by XRD.
  • the peak intensity of the characteristic peak (003) corresponding to the (003) crystal plane of the matrix tested by XRD is I (003) , 0.01% ⁇ I a /I (003) ⁇ 100% ⁇ 3.5%.
  • the thermal stability of the cathode material can be improved, causing the temperature corresponding to the exothermic peak of the cathode material to increase when tested by DSC, thereby making the cathode containing the Materials for lithium-ion batteries have higher safety characteristics.
  • the peak intensity I (003) of the characteristic peak (003) corresponding to the (003 ) crystal plane of the matrix is measured by XRD.
  • the peak area of the characteristic peak (003) corresponding to the (003) crystal plane of the substrate tested by XRD is A (003)
  • the 2 ⁇ of the coating layer tested by XRD is at 31°-35°.
  • the peak area of the main peak in the bimodal distribution of the characteristic peaks is A a , 0.01% ⁇ A a /A (003) ⁇ 100% ⁇ 6%.
  • the thermal stability of the cathode material can be improved, causing the temperature corresponding to the exothermic peak of the cathode material to increase when it is tested by DSC, thereby making it possible to include the Lithium-ion batteries with positive electrode materials have higher safety characteristics.
  • the main peak area in the bimodal distribution of the characteristic peaks of the coating layer at 31°-35° is A.
  • the peak area of the characteristic peak (003) corresponding to the (003) crystal plane of the matrix is A ( 003) measured by XRD.
  • the matrix has the composition shown in formula I: Li 1+a Ni x Co y Mn z M k O 2Formula I;
  • M is selected from Ga, At least one element among Sc, In, Y, Ce, Co, La, Cr, Mo, Mn, Fe, Hf, Zr, W, Nb, Sm and Al;
  • the coating layer has the composition shown in formula II: Li ⁇ - ⁇ - ⁇ - ⁇ La ⁇ N 1 ⁇ N 2 ⁇ N 3 1- ⁇ N 4 ⁇ O 3Formula II;
  • N 1 is selected from at least one of Y, Nd, Pr, Ce, Sm and Sc.
  • One element, N 2 is selected from at least one element among Sr, Ca, Mg, Si, Ge and Ru, N 3 is selected from at least one element among Ni, Mn and Co, N 4 is selected from Cr, Al, At least one element among V, Nb, Zr, Ti and Fe.
  • the matrix and coating layer of the surface-coated cathode material have a specific composition, which enables the cathode material to have electrochemical activity for lithium ion deintercalation, and the coating material used has a higher lithium ion transport capacity. , and at the same time has a low oxygen hole formation energy, which can increase the electronic conductivity and accelerate the diffusion rate of ions and electrons in the cathode material, thus significantly improving the rate performance of the cathode material.
  • the coating layer with a specific composition can effectively suppress the interface side reaction between the surface of the cathode material particles and the electrolyte, significantly improve the cycle stability of the lithium-ion battery containing the cathode material, and effectively enhance the cathode Thermal stability of materials.
  • M is selected from at least one element among Ce, Co, La, Cr, Mo, Y, Zr, W, Nb and Al.
  • N 1 is selected from at least one element among Nd, Pr, Ce, Sm and Sc.
  • N 2 is selected from at least one element among Sr, Ca, Mg, Si and Ru.
  • N 3 is selected from Ni and/or Mn.
  • N 4 is selected from at least one element among Cr, Al, Nb, Zr, Ti and Fe.
  • the content of the coating layer is 0.05-4wt%.
  • the surface of the cathode material when the content of the coating layer meets the above range, the surface of the cathode material has a relatively uniform coating layer, which can make the surface of the cathode material have higher ion and electron transmission characteristics, thereby making the lithium ions containing the cathode material
  • the battery has excellent electrochemical properties.
  • the content of the coating layer is 0.5-3.6 wt%.
  • the average particle size D 50 of the surface-coated cathode material is 2-17 ⁇ m.
  • the average particle size D 50 of the surface-coated cathode material is 3-12 ⁇ m.
  • the electronic conductivity of the coating layer is 1 ⁇ 10 -5 S/cm-8 ⁇ 10 -3 S/cm.
  • the electron conductivity of the coating layer meets the above range, it has a higher electron transmission capability and can form an electron conductive layer on the surface of the cathode material, thereby making the lithium ion battery containing the cathode material have excellent Electrochemical properties.
  • the electronic conductivity of the coating layer is 1.8 ⁇ 10 -5 S/cm-7.2 ⁇ 10 -3 S/cm.
  • the ion conductivity of the coating layer is 1 ⁇ 10 -6 S/cm-6 ⁇ 10 -4 S/cm.
  • the ion conductivity of the coating layer meets the above range, it has high ion transport performance, and can form an ion conductive layer on the surface of the cathode material, thereby making the lithium ion battery containing the cathode material have excellent Electrochemical properties.
  • the ion conductivity of the coating layer is 3 ⁇ 10 -6 S/cm-5 ⁇ 10 -4 S/cm.
  • the oxygen hole formation energy of the cladding layer is -2 eV to 4.5 eV.
  • the oxygen hole formation of the coating layer meets the above range, it has a higher electron transport capability and can form an electron conductive layer on the surface of the cathode material, thereby making the lithium ion battery containing the cathode material have excellent electrochemical properties.
  • oxygen hole formation energy of the cladding layer is -1.8eV to 4eV.
  • the temperature corresponding to the exothermic peak of the surface-coated cathode material tested by DSC is T1
  • the temperature corresponding to the exothermic peak tested by DSC of the substrate is T0
  • T1-T0 is 3-15°C.
  • the thermal stability of the cathode material can be significantly improved, thereby making the lithium ions containing the cathode material
  • the battery has high safety.
  • T1-T0 is 3-14°C.
  • a second aspect of the present invention provides a method for preparing a surface-coated cathode material, characterized in that the method includes the following steps:
  • the coating agent and the positive electrode are obtained by sintering a first mixture containing a lithium source, a La source, optionally a N 1 source, an optional N 2 source, an N 3 source, and an optional N 4 source.
  • a surface-coated cathode material including a matrix and a coating layer on the surface of the matrix can be obtained, and the coating layer has the characteristics described in the first aspect of the present invention.
  • the characteristic peaks at 2 ⁇ of 31°-35° measured by XRD of the coating layer show a bimodal distribution, and the bimodal distribution has specific main peaks and secondary peaks.
  • the ratio of peak intensity thus making the surface-coated cathode material have high ion and electron conductivity, which can accelerate the diffusion rate of ions and electrons in the cathode material, thereby significantly increasing the rate of lithium-ion batteries containing the cathode material. performance.
  • the surface-coated cathode material with the above-mentioned specific coating layer can avoid catalytic corrosion between the cathode material and the electrolyte, and improve the cycle stability of the lithium-ion battery containing the cathode material.
  • the thermal stability of the surface-coated cathode material with the above-mentioned specific coating layer is significantly improved.
  • the preparation method has a simple process, a pollution-free doping element and a simple introduction method of the coating layer, a small amount, no special requirements for the heat treatment atmosphere, low production cost, and is suitable for large-scale industrial production.
  • the lithium source, the La source, the N 1 source, the N 2 source, the N 3 source, and the N 4 source are used in amounts such that n(Li):n(La): n(N 1 ):n(N 2 ):n(N 3 ):n(N 4 ) is ( ⁇ - ⁇ - ⁇ - ⁇ ): ⁇ : ⁇ :(1- ⁇ ): ⁇ , where, 0.7 ⁇ 1.5, 0.2 ⁇ 1, 0 ⁇ 0.5, 0 ⁇ 0.5, 0 ⁇ 0.5.
  • the amount of the coating agent is 0.05-4wt%, preferably 0.1-3.6wt%.
  • the N 1 source is selected from compounds containing at least one element among Y, Nd, Pr, Ce, Sm and Sc.
  • the N source is selected from compounds containing at least one element among Sr, Ca, Mg, Si, Ge and Ru.
  • the N 3 source is selected from compounds containing at least one element among Ni, Mn and Co.
  • the N 4 source is selected from compounds containing at least one element among Cr, Al, V, Nb, Zr, Ti and Fe.
  • the specific type of the lithium source is not particularly limited, and it can be a conventional lithium source in the field, for example, selected from the group consisting of lithium carbonate, lithium hydroxide, lithium fluoride, lithium chloride, lithium nitrate and lithium acetate. of at least one.
  • the specific type of the La source is not particularly limited, and it can be a compound that is conventional in the art and can provide La element.
  • the conditions for the first mixing are not particularly limited, as long as the lithium source, the La source, the N 1 source, the N 2 source, the N 3 source and the N The 4 sources are fully mixed evenly.
  • the conditions for the first mixing include: the mixing speed is 400-1000 rpm, and the mixing time is 3-6 hours.
  • the conditions for the first sintering include: the sintering temperature is 500-1120°C, and the sintering time is 3-9 hours.
  • the first mixture containing the lithium source, the La source, optionally the N 1 source, the optional N 2 source, the N 3 source, and the optional N 4 source is sintered under the above conditions to obtain a product having A coating agent with higher ion and electron conductivity.
  • the conditions for the first sintering include: the sintering temperature is 600-1000°C, and the sintering time is 3-9 hours.
  • the average particle size D 50 of the coating agent is 30-200 nm.
  • the equipment used for crushing is not particularly limited.
  • at least one selected from flow mills, mechanical mills, colloid mills, high-energy pulverizers, stirring mills, sand mills, etc. is used to crush the sintered product.
  • the coating agent is obtained.
  • the average particle size D 50 of the coating agent is 50-180 nm.
  • the conditions for the second mixing are not particularly limited, as long as the coating agent and the cathode material matrix can be fully mixed evenly.
  • the conditions for the second mixing include: mixing speed The speed is 400-1000rpm, and the mixing time is 3-6h.
  • the equipment used for the first mixing and the second mixing is not particularly limited, and conventional mixing equipment in the field can be selected, such as a high-speed mixer.
  • the second sintering conditions include: the sintering temperature is 300-900°C, and the sintering time is 1-12 hours.
  • the surface of the cathode material can be provided with ion and electron transport properties, thereby enabling the lithium ion battery containing the cathode material to have excellent electrochemical performance.
  • the conditions for the second sintering include: the sintering temperature is 400-800°C, and the sintering time is 4-10 hours.
  • the coating agent is prepared according to the following steps:
  • the lithium source, the La source, optionally the N 1 source, the optional N 2 source, the N 3 source, and the optional N 4 source are mixed for a fourth time to obtain a fourth mixture, and the fourth mixture
  • the fourth sintering is performed, and after crushing, the coating agent is obtained.
  • the conditions for the fourth mixing are not particularly limited, as long as the lithium source, the La source, the N 1 source, the N 2 source, the N 3 source and the N The four sources are fully mixed evenly.
  • the conditions for the fourth mixing include: the mixing speed is 400-1000 rpm, and the mixing time is 3-6 hours.
  • the conditions of the fourth mixing and the first mixing may be the same or different.
  • the fourth sintering conditions include: the sintering temperature is 600-1000°C, and the sintering time is 3-9 hours.
  • the fourth sintering conditions include: the sintering temperature is 700-1000°C, and the sintering time is 5-9 hours.
  • the coating agent is prepared according to the following steps:
  • the conditions for the fifth mixing are not particularly limited, as long as the lithium source, the La source, the N 1 source, the N 2 source, the N 3 source, and the N 4.
  • the source and the solvent are fully mixed evenly.
  • the fifth mixing conditions include: the mixing speed is 400-1000 rpm, and the mixing time is 3-6 hours.
  • the type of the solvent is not particularly limited, as long as it can make the lithium source, the La source, the N 1 source, the N 2 source, the N 3 source, and the N 4 source Form a solution such as ethanol.
  • a conventional acidic solution in the art such as citric acid, can be used to adjust the pH of the fifth mixture.
  • an acidic solution is added so that the pH value of the fifth mixture is 9-12.
  • the drying conditions are not particularly limited, as long as the fifth mixture is fully dried.
  • the drying conditions include: a drying temperature of 40-120°C, and a drying time of 5-12 hours. .
  • the fifth sintering conditions include: the sintering temperature is 500-850°C, and the sintering time is 4-8 hours.
  • the fifth sintering conditions include: the sintering temperature is 550-800°C, and the sintering time is 5-8 hours.
  • a third aspect of the present invention provides a method for preparing a surface-coated cathode material, characterized in that the method includes the following steps:
  • the lithium source, the La source, optionally the N 1 source, the optional N 2 source, the N 3 source, the optional N 4 source and the cathode material matrix are mixed for a third time to obtain a third mixture and then the third sintering is performed to obtain The surface is coated with positive electrode material.
  • the types and amounts of other materials involved, the conditions and parameters of each step, etc. are the same as the preparation method of the second aspect.
  • certain characteristics of the substance such as optional types of substances, etc.
  • the lithium source, the La source, the N 1 source, the N 2 source, the N 3 source, and the N 4 source are used in amounts such that n(Li):n(La): n(N 1 ):n(N 2 ):n(N 3 ):n(N 4 ) is ( ⁇ - ⁇ - ⁇ - ⁇ ): ⁇ : ⁇ :(1- ⁇ ): ⁇ , where, 0.7 ⁇ 1.5, 0.2 ⁇ 1, 0 ⁇ 0.5, 0 ⁇ 0.5, 0 ⁇ 0.5.
  • the total amount of the lithium source, the La source, the N 1 source, the N 2 source, the N 3 source, and the N 4 source is such that, based on the total amount of the cathode material matrix,
  • the content of the products of lithium source, La source, N 1 source, N 2 source, N 3 source, and N 4 source is 0.05-4wt%, preferably 0.1-3.6wt%.
  • the third sintering conditions include: the sintering temperature is 300-900°C, and the sintering time is 1-12 hours.
  • the surface of the cathode material can be made to have ion and electron transport properties, thereby enabling the lithium ion battery containing the cathode material to have excellent electrochemical performance.
  • the third sintering conditions include: heat treatment temperature is 400-800°C, and heat treatment time is 4-10 hours.
  • the average particle size D 50 of the cathode material matrix is 2.3-12 ⁇ m.
  • the cathode material matrix used in the second aspect and the third aspect is not particularly limited, and can be prepared by conventional preparation methods in this field.
  • the cathode material matrix is prepared according to the following steps:
  • At least one of the nickel salt, cobalt salt, manganese salt and compound containing M element is added.
  • the type of nickel salt is not particularly limited and can be a conventional nickel salt in the art, such as at least one of nickel sulfate, nickel chloride, nickel nitrate and nickel acetate.
  • the type of cobalt salt is not particularly limited and can be conventional cobalt salts in the art, such as at least one of cobalt sulfate, cobalt chloride, cobalt nitrate and cobalt acetate.
  • the type of manganese salt is not particularly limited and may be conventional manganese salts in the art, such as at least one of manganese sulfate, manganese chloride, manganese nitrate and manganese acetate.
  • the type of the precipitating agent is not particularly limited, and can be a conventional precipitating agent in the field, for example, selected from the group consisting of ammonium carbonate, ammonium bicarbonate, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, and hydrogen carbonate. At least one of sodium oxide, potassium hydroxide and lithium hydroxide.
  • the types of the complexing agent and the second complexing agent are not particularly limited. They can be conventional complexing agents in the field, for example, selected from the group consisting of ammonia water, disodium ethylenediaminetetraacetate, ammonium nitrate, and chlorine. At least one of ammonium chloride and ammonium sulfate.
  • the nickel cobalt manganese hydroxide precursor, the lithium source and the compound containing M element are used in an amount such that n(Li)/[n(Ni)+n(Co)+n(Mn) +n(M)] is 0.95-1.5; n(M)/[n(Ni)+n(Co)+n(Mn)+n(M)] is 0-0.06.
  • the amounts of the nickel cobalt manganese hydroxide precursor, the lithium source and the compound containing the M element are such that n(Li)/[n(Ni)+n(Co)+n(Mn)+ n(M)] is 0.96-1.03; n(M)/[n(Ni)+n(Co)+n(Mn)+n(M)] is 0-0.05.
  • the compound containing M element is selected from the group consisting of at least one of Ga, Sc, In, Y, Ce, Co, La, Cr, Mo, Mn, Fe, Hf, Zr, W, Nb, Sm and Al.
  • a compound of elements is selected from the group consisting of at least one of Ga, Sc, In, Y, Ce, Co, La, Cr, Mo, Mn, Fe, Hf, Zr, W, Nb, Sm and Al.
  • the type of the lithium source is not particularly limited and can be a conventional lithium source in the field, such as one selected from lithium carbonate, lithium hydroxide, lithium fluoride, lithium chloride, lithium nitrate and lithium acetate. At least one.
  • the conditions for the co-precipitation reaction include: the reaction temperature is 40-120°C, and the pH value is 9-12.
  • the sixth mixing conditions are not particularly limited, as long as the nickel cobalt manganese hydroxide precursor, the lithium source and the compound containing M element can be fully mixed evenly, preferably , the sixth mixing conditions include: the mixing speed is 400-1000rpm, and the mixing time is 3-6h.
  • the sixth sintering conditions include: the sintering temperature is 500-1000°C; the sintering time is 3-18 hours.
  • the resulting cathode material when the sixth mixture is subjected to the sixth sintering under the above conditions, the resulting cathode material has no impurity phase.
  • the sixth sintering conditions include: the sintering temperature is 600-1000°C, and the sintering time is 5-9 hours.
  • a fourth aspect of the present invention provides a surface-coated cathode material prepared by the above preparation method.
  • a fifth aspect of the present invention provides a lithium-ion battery, characterized in that the lithium-ion battery includes the above-mentioned surface-coated positive electrode Material.
  • the lithium ion battery further includes a negative electrode and an electrolyte.
  • a negative electrode and an electrolyte There are no particular limitations on the types of the negative electrode and the electrolyte. Conventional negative electrodes and electrolytes in this field can be used.
  • the electrolyte is a common commercial electrolyte, consisting of 1 mol/L LiPF 6 and ethylene carbonate. An equal mixture of (EC) and diethyl carbonate (DEC).
  • the average particle size D 50 of the surface-coated cathode material was measured using a Malvern particle size analyzer.
  • composition of the surface-coated cathode material is calculated based on the molar ratio of the reactants.
  • phase structure of the coating agent and the cathode material after coating was measured by XRD.
  • the electronic conductivity of the coating agent was measured using the four-probe method.
  • the oxygen hole formation energy of the coating agent is calculated from first principles.
  • the surface coating cathode material, acetylene black and polyvinylidene fluoride (PVDF) are mixed according to the mass ratio of 95:2.5:2.5, coated on aluminum foil and dried, and punched into a shape with a diameter of 100MPa. 12mm, 120 ⁇ m thick positive electrode piece, and then put the positive electrode piece into a vacuum drying box to dry at 120°C for 12 hours.
  • PVDF polyvinylidene fluoride
  • the negative electrode uses a Li metal sheet with a diameter of 17mm and a thickness of 1mm; the separator uses a polyethylene porous membrane with a thickness of 25 ⁇ m; the electrolyte uses 1mol/L LiPF 6 , ethylene carbonate (EC) and diethyl carbonate (DEC). Mix equal amounts.
  • the thermal stability of the surface-coated cathode material is measured by DSC testing of the cathode sheet in the delithiated state (that is, when the battery is charged off).
  • buttons batteries The performance evaluation of button batteries is as follows:
  • Charge and discharge performance test The charge and discharge performance of the material is examined at a temperature of 25°C, a voltage range of 3.0-4.3V, and a rate of 0.1C.
  • Cycle performance test The temperature is 25°C, in the voltage range of 3.0-4.3V, and the capacity retention rate of the material is examined for 80 cycles at a rate of 1C.
  • Li 0.2 La 0.48 Nd 0.1 Sr 0.05 Mn 0.8 Ti 0.2 O 3 is prepared by the solid phase method, and the raw materials lithium hydroxide, lanthanum oxide, neodymium oxide, strontium oxide, manganese tetroxide, and titanium dioxide are weighed according to the metric ratio of each element in the chemical formula.
  • the XRD spectrum of the coating agent is shown in Figure 1. It can be seen from Figure 1 that the characteristic peaks at 31°-35° present a bimodal distribution.
  • the XRD spectrum of the surface-coated cathode material A1 is shown in Figure 2. It can be seen from Figure 2 that the double peaks belonging to the coating agent at 31°-35° still exist after coating.
  • the peak intensity I b of the secondary peak is the ratio of the peak intensity I a of the main peak.
  • Ratio I b /I a the peak intensity I a of the main peak between 31°-35° of the coating layer Li 0.2 La 0.48 Nd 0.1 Sr 0.05 Mn 0.8 Ti 0.2 O 3 and the peak of the P1 (003) crystal plane of the cathode material matrix
  • the ratio A a /A (003) of the peak area A (003) of the P(003) crystal plane was calculated, and the results are shown in Table 3.
  • the particle size, ionic conductivity, electronic conductivity and oxygen hole formation energy of the coating agent were tested. The results are shown in Table 3.
  • XRD tests were conducted on the coating agent, cathode material matrix and surface-coated cathode material. The results are shown in Table 3.
  • the coating material D 50 is 200nm.
  • Li 0.34 La 0.47 Ce 0.05 Mg 0.05 MnO 3 will be prepared by the solid phase method. According to the metric ratio of the elements in the chemical formula, the raw materials lithium carbonate, lanthanum oxide, cerium oxide (N 1 source), magnesium oxide (N 2 source), and tetracycline are weighed. Trimanganese oxide ( N source).
  • the peak intensity I b of the secondary peak is the ratio of the peak intensity I a of the main peak.
  • Ratio I b /I a the peak intensity I a of the main peak between 31°-35° of the coating layer Li 0.2 La 0.48 Nd 0.1 Sr 0.05 Mn 0.8 Ti 0.2 O 3 and the peak of the P1 (003) crystal plane of the cathode material matrix
  • the ratio A a /A (003) of the peak area A (003) of the P(003) crystal plane was calculated, and the results are shown in Table 3.
  • the particle size, ionic conductivity, electronic conductivity and oxygen hole formation energy of the coating agent were tested. The results are shown in Table 3.
  • XRD tests were conducted on the coating agent, cathode material matrix and surface-coated cathode material. The results are shown in Table 3.
  • the coating agent and the cathode material matrix P2 into a high-speed mixer and stir for 4 hours at 800 rpm, then sinter at 500°C for 8 hours. After cooling to room temperature, crush and sieve to obtain the surface-coated cathode material A3. .
  • the amount of the coating agent is 2.2 wt%.
  • the peak intensity I b of the secondary peak is the ratio of the peak intensity I a of the main peak.
  • Ratio I b /I a the peak intensity I a of the main peak between 31°-35° of the coating layer Li 0.2 La 0.48 Nd 0.1 Sr 0.05 Mn 0.8 Ti 0.2 O 3 and the peak of the P1 (003) crystal plane of the cathode material matrix
  • the ratio A a /A (003) of the peak area A (003) of the P(003) crystal plane was calculated, and the results are shown in Table 3.
  • the particle size, ionic conductivity, electronic conductivity and oxygen hole formation energy of the coating agent were tested. The results are shown in Table 3.
  • XRD tests were conducted on the coating agent, cathode material matrix and surface-coated cathode material. The results are shown in Table 3.
  • the coating agent and the cathode material matrix P1 into a high-speed mixer and stir for 6 hours at 800 rpm, then sinter at 450°C for 8 hours. After cooling to room temperature, crush and sieve to obtain the surface-coated cathode material A4. .
  • the amount of the coating agent is 0.8 wt%.
  • the peak intensity I b of the secondary peak is the ratio of the peak intensity I a of the main peak.
  • Ratio I b /I a the peak intensity I a of the main peak between 31°-35° of the coating layer Li 0.2 La 0.48 Nd 0.1 Sr 0.05 Mn 0.8 Ti 0.2 O 3 and the peak of the P1 (003) crystal plane of the cathode material matrix
  • the ratio A a /A (003) of the peak area A (003) of the P(003) crystal plane was calculated, and the results are shown in Table 3.
  • the particle size, ionic conductivity, electronic conductivity and oxygen hole formation energy of the coating agent were tested. The results are shown in Table 3.
  • XRD tests were conducted on the coating agent, cathode material matrix and surface-coated cathode material. The results are shown in Table 3.
  • the ratio of the secondary peak intensity I b to the peak intensity I a of the main peak I b /I a the peak intensity I a of the main peak of the coating layer Li 0.2 La 0.48 Nd 0.1 Sr 0.05 Mn 0.8 Ti 0.2 O 3 between 31°-35° and the peak intensity I of the P1(003) crystal plane of the cathode material matrix (003) ratio I a /I (003)
  • the ratio of the peak area A (003) of the crystal plane A a /A (003) was calculated, and the results are shown in Table 3.
  • the particle size, ionic conductivity, electronic conductivity and oxygen hole formation energy of the coating agent were tested. The results are shown in Table 3.
  • XRD tests were conducted on the coating agent, cathode material matrix and surface-coated cathode material. The results are shown in Table 3.
  • the ratio of the secondary peak intensity I b to the peak intensity I a of the main peak I b /I a the main peak intensity I a of the coating layer Li 0.2 La 0.48 Nd 0.1 Sr 0.05 Mn 0.8 Ti 0.2 O 3 between 31°-35° and the peak intensity I of the P1(003) crystal plane of the cathode material matrix I ( 003) ratio I a /I (003)
  • the ratio of the peak area A (003) of the crystal plane A a /A (003) was calculated, and the results are shown in Table 3.
  • the particle size, ionic conductivity, electronic conductivity and oxygen hole formation energy of the coating agent were tested. The results are shown in Table 3.
  • XRD tests were conducted on the coating agent, cathode material matrix and surface-coated cathode material. The results are shown in Table 3.
  • the positive electrode material matrix P1 prepared in Preparation Example 1 was used as the positive electrode material D1.
  • XRD test was performed on the cathode material D1, and the results are shown in Table 3.
  • the positive electrode material matrix P2 prepared in Preparation Example 2 was used as the positive electrode material D2.
  • XRD test was performed on the cathode material D2, and the results are shown in Table 3.
  • the positive electrode material matrix P2 prepared in Preparation Example 3 was used as the positive electrode material D3.
  • XRD test was performed on the cathode material D3, and the results are shown in Table 3.
  • the cathode materials of the examples and comparative examples were assembled into lithium-ion batteries, and the thermal stability of the delithiated cathode materials was tested respectively. The results are shown in Table 4. The performance of lithium-ion batteries was tested, and the results are shown in Table 5.
  • the fully delithiated DSC peak position of the positive electrode material of Example 1 is 210°C
  • the fully delithiated DSC peak position of the positive electrode material of Example 4 is 207°C
  • the completely delithiated DSC peak position of the positive electrode material of Comparative Example 1 The location is 203°C. This shows that compared with Comparative Example 1, the cathode materials of Example 1 and Example 4 have better thermal stability.
  • Example 1 As shown in Figure 3, compared with Comparative Example 1, the discharge capacity of Example 1 and Example 4 increased at each rate, indicating that after coating, the rate performance was improved, and the DSC peak position moved backward. The thermal stability of the material is improved.
  • the DSC peak position of the cathode material in the fully delithiated state of Example 2 is 218°C
  • the DSC peak position of the cathode material in the fully delithiated state of Example 3 is 213°C
  • the DSC peak position of the cathode material in Comparative Example 2 is completely delithiated.
  • the peak position is 205°C. This shows that compared with Comparative Example 2, the cathode materials of Example 2 and Example 3 have better thermal stability.

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Abstract

本发明涉及锂离子电池领域，公开了一种表面包覆正极材料及其制备方法、锂离子电池。该正极材料包括基体和包覆在所述基体表面的包覆层；所述包覆层经XRD测试的2θ为31°-35°处的特征峰呈双峰分布；且双峰分布中的次峰峰强度Ib与主峰的峰强度Ia之比Ib/Ia为0.8-1。该表面包覆正极材料具有高的离子和电子传导能力，能够加快离子和电子在正极材料中的扩散率，从而显著提升了包含该正极材料的锂离子电池的倍率性能。同时，具有上述特定包覆层的表面包覆正极材料能够避免正极材料与电解质之间的催化腐蚀，提升包含该正极材料的锂离子电池的循环稳定性。此外，具有上述特定包覆层的表面包覆正极材料的热稳定性得到显著改善。

Description

表面包覆正极材料及其制备方法、锂离子电池
相关申请的交叉引用
本申请要求2022年06月30日提交的中国专利申请202210770763.9的权益，该申请的内容通过引用被合并于本文。
技术领域
本发明涉及锂离子电池技术领域，具体涉及一种表面包覆正极材料及其制备方法、一种锂离子电池。
背景技术
随着动力电池市场的快速发展，促使人们对锂离子电池的能量密度和安全性能提出更高的要求。近年来，为了进一步增加锂离子电池能量密度，降低成本，提镍降钴逐渐成为趋势。但随着镍含量的增加，材料的热稳定性变差，电池使用过程中的安全性能降低。此外，充放电过程中生成的四价镍氧化性较强，易于与电解液反应，致使循环性能变差，电池发生鼓胀。
因此，为了进一步提升材料热稳定性及循环稳定性，包覆改性技术被广泛应用在各种正极材料当中。CN109065875A公开一种包覆高镍多元材料的制备方法，其中使用ZrO2、TiO2、Al2O3、MgO等材料作为包覆剂，包覆后虽可以减少正极与电解液间副反应，提升电池性能，但该方法所使用包覆剂均为惰性材料，未能参与到离子和电子传输，且对高镍材料热稳定性没有改善。
发明内容
本发明的目的是为了克服现有技术存在的问题，提供一种表面包覆正极材料及其制备方法、一种锂离子电池，该表面包覆正极材料包括基体和包覆在所述基体表面的包覆层；该包覆层经XRD测试的2θ为31°-35°处的特征峰呈双峰分布，并且该双峰分布具有特定的主峰与次峰的峰强度之比，由此使得该表面包覆正极材料具有高的离子和电子传导能力，能够加快离子和电子在正极材料中的扩散率，从而显著提升了包含该正极材料的锂离子电池的倍率性能。同时，具有上述特定包覆层的表面包覆正极材料能够避免正极材料与电解质之间的催化腐蚀，提升包含该正极材料的锂离子电池的循环稳定性。此外，具有上述特定包覆层的表面包覆正极材料的热稳定性得到显著改善。
为了实现上述目的，本发明第一方面提供一种表面包覆正极材料，其特征在于，所述正极材料包括基体和包覆在所述基体表面的包覆层；
所述包覆层经XRD测试的2θ为31°-35°处的特征峰呈双峰分布；且双峰分布中次峰的峰强度Ib与主峰的峰强度Ia之比Ib/Ia为0.8-1。
本发明第二方面提供一种表面包覆正极材料的制备方法，其特征在于，所述方法包括以下步骤：
S1、将锂源、La源、可选地N1源、N2源、N3源、可选地N4源进行第一混合，得到第一混 合物，对所述第一混合物进行烧结，经破碎，得到包覆剂；
S2、将所述包覆剂与正极材料基体进行第二混合，得到第二混合物，对所述第二混合物进行第一热处理，得到所述表面包覆正极材料。
本发明第三方面提供一种表面包覆正极材料的制备方法，其特征在于，所述方法包括以下步骤：
将锂源、La源、可选地N1源、N2源、N3源、可选地N4源和正极材料基体进行第三混合，得到第三混合物进行第二热处理，得到所述表面包覆正极材料。
本发明第四方面提供一种上述制备方法制得的表面包覆正极材料。
本发明第五方面提供一种锂离子电池，其特征在于，所述锂离子电池上述表面包覆正极材料。
通过上述技术方案，本发明提供的表面包覆正极材料及其制备方法与应用、锂离子电池获得以下有益的效果：
(1)本发明提供的表面包覆正极材料包括基体和包覆在所述基体表面的包覆层，该包覆层经XRD测试的2θ为31°-35°处的特征峰呈双峰分布，并且该双峰分布具有特定的主峰与次峰的峰强度之比，由此使得该表面包覆正极材料具有高的离子和电子传导能力，能够加快离子和电子在正极材料中的扩散率，从而显著提升了包含该正极材料的锂离子电池的倍率性能。同时，具有上述特定包覆层的表面包覆正极材料能够避免正极材料与电解质之间的催化腐蚀，提升包含该正极材料的锂离子电池的循环稳定性。此外，具有上述特定包覆层的表面包覆正极材料的热稳定性得到显著改善。
(2)本发明提供的表面包覆正极材料的基体和包覆层具有特定的组成，能够使得该正极材料具有锂离子脱嵌的电化学活性，进而所用包覆材料具有较高的锂离子传输能力，同时具有较低的氧空穴形成能，可提高电子电导率，加快离子和电子在正极材料中的扩散速率，从而显著提升了包含该正极材料的倍率性能。与此同时，本发明中，具有特定组成的包覆层，能够有效抑制正极材料颗粒表面与电解液的界面副反应，明显改善包含该正极材料的锂离子电池的循环稳定性，并且有效提升正极材料的热稳定性。
(3)本发明提供的正极材料的制备方法工艺简单、无污染掺杂元素和包覆层引入方式简单，用量少，热处理气氛无特殊要求，生产成本低，适于大规模工业化生产。
附图说明
图1为实施例1中包覆剂的XRD物相图。
图2为实施例1中包覆后正极材料的XRD物相图。
图3为对比例1、实施例1和实施例4制备的半电池在0.1C的充放电曲线图。
图4为对比例2、实施例2和实施例3制备的半电池在0.1C的充放电曲线图。
图5为对比例1、实施例1和实施例4的DSC曲线图。
图6为对比例2、实施例2和实施例3的DSC曲线图。
具体实施方式
在本文中所披露的范围的端点和任何值都不限于该精确的范围或值，这些范围或值应当理解 为包含接近这些范围或值的值。对于数值范围来说，各个范围的端点值之间、各个范围的端点值和单独的点值之间，以及单独的点值之间可以彼此组合而得到一个或多个新的数值范围，这些数值范围应被视为在本文中具体公开。
本发明第一方面提供一种表面包覆正极材料，其特征在于，所述正极材料包括基体和包覆在所述基体表面的包覆层；
所述包覆层经XRD测试的2θ为31°-35°处的特征峰呈双峰分布；且双峰分布中的次峰强度Ib与主峰的峰强度Ia之比Ib/Ia为0.8-1。
本发明中，所述表面包覆正极材料包括基体和包覆在所述基体表面的包覆层，该包覆层经XRD测试的2θ为31°-35°处的特征峰呈双峰分布，并且该双峰分布具有特定的主峰与次峰的峰强度之比，由此使得该表面包覆正极材料具有高的离子和电子传导能力，能够加快离子和电子在正极材料中的扩散率，从而显著提升了包含该正极材料的锂离子电池的倍率性能。
进一步地，具有上述特定包覆层的表面包覆正极材料能够避免正极材料与电解质之间的催化腐蚀，提升包含该正极材料的锂离子电池的循环稳定性。此外，具有上述特定包覆层的表面包覆正极材料的热稳定性得到显著改善。
本发明中，双峰分布中的次峰强度Ib与主峰的峰强度Ia分别通过XRD测得。
进一步地，Ib/Ia为0.97-1。
根据本发明，所述基体经XRD测试的(003)晶面对应的特征峰(003)的峰强度为I(003)，0.01％≤Ia/I(003)×100％≤3.5％。
本发明中，当所述包覆层在31°-35°处的特征峰的双峰分布中主峰的峰强度Ia与所述基体的(003)晶面对应的特征峰(003)的峰强度I(003)之比Ia/I(003)满足上述范围时，能够改善正极材料的热稳定性，使得正极材料经DSC测试时放热峰对应的温度升高，进而使得包含该正极材料的锂离子电池具有更高的安全特性。
本发明中，所述基体的(003)晶面对应的特征峰(003)的峰强度I(003)通过XRD测得。
进一步地，0.08％≤Ia/I(003)×100％≤3.1％。
根据本发明，所述基体经XRD测试的(003)晶面对应的特征峰(003)的峰面积为A(003)，所述包覆层经XRD测试的2θ为31°-35°处的特征峰双峰分布中主峰的峰面积为Aa，0.01％≤Aa/A(003)×100％≤6％。
本发明中，当所述包覆层在31°-35°处的特征峰的双峰分布中的主峰面积为Aa与所述基体的(003)晶面对应的特征峰(003)的峰面积为A(003)之比Aa/A(003)满足上述范围时，能够改善正极材料的热稳定性，使得正极材料经DSC测试时放热峰对应的温度升高，进而使得包含该正极材料的锂离子电池具有更高的安全特性。
本发明中，包覆层在31°-35°处的特征峰的双峰分布中的主峰面积为Aa与基体的(003)晶面对应的特征峰(003)的峰面积为A(003)通过XRD测得。
进一步地，0.5％≤Aa/A(003)×100％≤4.8％。
根据本发明，所述基体具有式I所示的组成：
Li1+aNixCoyMnzMkO2   式I；
其中，-0.05≤a≤0.5，0≤x≤1，0≤y≤1，0≤z≤1，0≤k≤0.06，0＜x+y+z+k≤1，M选自Ga、Sc、In、Y、Ce、Co、La、Cr、Mo、Mn、Fe、Hf、Zr、W、Nb、Sm和Al中的至少一种元素；
所述包覆层具有式II所示的组成：
Liα-β-γ-δLaβN1 γN2 δN3 1-λN4 λO3   式II；
其中，0.7＜α＜1.5，0.2＜β＜1，0≤γ＜0.5，0≤δ＜0.5，0≤λ＜0.5；N1选自Y、Nd、Pr、Ce、Sm和Sc中的至少一种元素，N2选自Sr、Ca、Mg、Si、Ge和Ru中的至少一种元素，N3选自Ni、Mn和Co中的至少一种元素，N4选自Cr、Al、V、Nb、Zr、Ti和Fe中的至少一种元素。
本发明中，所述表面包覆正极材料的基体和包覆层具有特定的组成，能够使得该正极材料具有锂离子脱嵌的电化学活性，进而所用包覆材料具有较高的锂离子传输能力，同时具有较低的氧空穴形成能，可提高电子电导率，加快离子和电子在正极材料中的扩散速率，从而显著提升了包含该正极材料的倍率性能。与此同时，本发明中，具有特定组成的包覆层，能够有效抑制正极材料颗粒表面与电解液的界面副反应，明显改善包含该正极材料的锂离子电池的循环稳定性，并且有效提升正极材料的热稳定性。
进一步地，式I中，M选自Ce、Co、La、Cr、Mo、Y、Zr、W、Nb和Al中的至少一种元素。
进一步地，式II中，0.9＜α＜1.3，0.35＜β＜0.7，0.04≤γ＜0.4，0＜δ＜0.3，0≤λ＜0.46。
进一步地，式II中，N1选自Nd、Pr、Ce、Sm和Sc中的至少一种元素。
进一步地，式II中，N2选自Sr、Ca、Mg、Si和Ru中的至少一种元素。
进一步地，式II中，N3选自Ni和/或Mn。
进一步地，式II中，N4选自Cr、Al、Nb、Zr、Ti和Fe中的至少一种元素。
根据本发明，以所述基体的总重量为基准，所述包覆层的含量为0.05-4wt％。
本发明中，当包覆层的含量满足上述范围时，正极材料表面具有较为均匀的包覆层，能够使得正极材料表面具有较高的离子、电子传输特性，进而使得包含该正极材料的锂离子电池具有优异的电化学特性。
进一步地，以所述基体的总重量为基准，所述包覆层的含量为0.5-3.6wt％。
根据本发明，所述表面包覆正极材料的平均粒径D50为2-17μm。
进一步地，所述表面包覆正极材料的平均粒径D50为3-12μm。
根据本发明，所述包覆层的电子电导率为1×10-5S/cm-8×10-3S/cm。
本发明中，当所述包覆层的电子电导率满足上述范围时，具有较高电子传输能力，并且能够在正极材料表面形成电子导电层，进而使得包含该正极材料的锂离子电池具有优异的电化学性能。
进一步地，所述包覆层的电子电导率为1.8×10-5S/cm-7.2×10-3S/cm。
根据本发明，所述包覆层的离子电导率为1×10-6S/cm-6×10-4S/cm。
本发明中，当所述包覆层的离子电导率满足上述范围时，具有较高离子传输性能，并且能够在正极材料表面形成离子导电层，进而使得包含该正极材料的锂离子电池具有优异的电化学性能。
进一步地，所述包覆层的离子电导率为3×10-6S/cm-5×10-4S/cm。
根据本发明，所述包覆层的氧空穴形成能为-2eV至4.5eV。
本发明中，当所述包覆层的氧空穴形成能满足上述范围时，具有较高电子传输能力，能够在正极材料表面形成电子导电层，进而使得包含该正极材料的锂离子电池具有优异的电化学性能。
进一步地，所述包覆层的氧空穴形成能为-1.8eV至4eV。
根据本发明，所述表面包覆正极材料经DSC测试放热峰对应的温度为T1，所述基体经DSC测试放热峰对应的温度为T0，T1-T0为3-15℃。
本发明中，所述表面包覆正极材料和所述基体经DSC测试放热峰对应的温度之差满足上述范围时，能够显著改善正极材料的热稳定性，进而使得包含该正极材料的锂离子电池具有较高安全性。
进一步地，T1-T0为3-14℃。
本发明第二方面提供一种表面包覆正极材料的制备方法，其特征在于，所述方法包括以下步骤：
S1、将锂源、La源、可选地N1源、可选地N2源、N3源、可选地N4源进行第一混合，得到第一混合物，对所述第一混合物进行第一烧结，经破碎，得到包覆剂；
S2、将所述包覆剂与正极材料基体进行第二混合，得到第二混合物，对所述第二混合物进行第二烧结，得到所述表面包覆正极材料。
本发明中，通过将包含锂源、La源、可选地N1源、可选地N2源、N3源、可选地N4源的第一混合物进行烧结得到的包覆剂与正极材料基体混合，并进行热处理后，能够获得包括基体以及所述基体表面含有包覆层的表面包覆正极材料，并且该包覆层具有本发明第一方面所述的特征，具体地：由本发明提供的制备方法制得的表面包覆正极材料中，包覆层经XRD测试的2θ为31°-35°处的特征峰呈双峰分布，并且该双峰分布具有特定的主峰与次峰的峰强度之比，由此使得该表面包覆正极材料具有高的离子和电子传导能力，能够加快离子和电子在正极材料中的扩散率，从而显著提升了包含该正极材料的锂离子电池的倍率性能。
进一步地，具有上述特定包覆层的表面包覆正极材料能够避免正极材料与电解质之间的催化腐蚀，提升包含该正极材料的锂离子电池的循环稳定性。此外，具有上述特定包覆层的表面包覆正极材料的热稳定性得到显著改善。
更进一步地，所述制备方法工艺简单、无污染掺杂元素和包覆层引入方式简单，用量少，热处理气氛无特殊要求，生产成本低，适于大规模工业化生产。
根据本发明，所述锂源、所述La源、所述N1源、所述N2源、所述N3源、所述N4源的用量使得n(Li)∶n(La)∶n(N1)∶n(N2)∶n(N3)∶n(N4)为(α-β-γ-δ)∶β∶γ∶δ∶(1-λ)∶λ，其中，0.7＜α＜1.5，0.2＜β＜1，0≤γ＜0.5，0≤δ＜0.5，0≤λ＜0.5。
进一步地，0.9＜α＜1.3，0.35＜β＜0.7，0.04≤γ＜0.4，0＜δ＜0.3，0≤λ＜0.46。
根据本发明，以所述正极材料基体的总重量，所述包覆剂的用量为0.05-4wt％，优选为0.1-3.6wt％。
根据本发明，所述N1源选自含有Y、Nd、Pr、Ce、Sm和Sc中的至少一种元素的化合物。
根据本发明，所述N2源选自含有Sr、Ca、Mg、Si、Ge和Ru中的至少一种元素的化合物。
根据本发明，所述N3源选自含有Ni、Mn和Co中的至少一种元素的化合物。
根据本发明，所述N4源选自含有Cr、Al、V、Nb、Zr、Ti和Fe中的至少一种元素的化合物。
本发明中，对于所述锂源的具体种类没有特别限定，可以为本领域中常规的锂源，例如选自碳酸锂、氢氧化锂、氟化锂、氯化锂、硝酸锂和乙酸锂中的至少一种。
本发明中，对于所述La源的具体种类没有特别限定，可以为本领域中常规的能够提供La元素的化合物。
本发明中，对于所述第一混合的条件没有特别限定，只要能使得所述锂源、所述La源、所述N1源、所述N2源、所述N3源和所述N4源充分混合均匀即可，优选地，所述第一混合的条件包括：混合转速为400-1000rpm，混合时间为3-6h。
根据本发明，所述第一烧结的条件包括：烧结温度为500-1120℃，烧结时间为3-9h。
本发明中，在上述条件下对包含锂源、La源、可选地N1源、可选地N2源、N3源、可选地N4源的第一混合物进行烧结，能够得到具有较高离子、电子导的包覆剂。
进一步地，所述第一烧结的条件包括：烧结温度为600-1000℃，烧结时间为3-9h。
根据本发明，所述包覆剂的平均粒径D50为30-200nm。
本发明中，对于破碎所用的设备没有特别限定，例如采用选自流磨、机械磨、胶体磨、高能粉碎机、搅拌磨、砂磨机等中的至少一种对烧结后的产物进行破碎，得到所述包覆剂。
进一步地，所述包覆剂的平均粒径D50为50-180nm。
本发明中，对于所述第二混合的条件没有特别限定，只要能使得所述包覆剂和所述正极材料基体充分混合均匀即可，优选地，所述第二混合的条件包括：混合转速为400-1000rpm，混合时间为3-6h。
本发明中，对于所述第一混合和所述第二混合所用设备没有特别限定，可以选用本领域中常规的混合设备，例如高速混料机。
根据本发明，所述第二烧结的条件包括：烧结温度为300-900℃，烧结时间为1-12h。
本发明中，在上述条件下对所述第二混合物进行第二烧结时，能够使得正极材料表面具有离子、电子传输特性，进而使得包含该正极材料的锂离子电池具有优异的电化学性能。
进一步地，所述第二烧结的条件包括：烧结温度为400-800℃，烧结时间为4-10h。
本发明的一个具体实施方式中，所述包覆剂按照以下步骤制备：
(a)将锂源、La源、可选地N1源、可选地N2源、N3源、可选地N4源进行第四混合，得到第四混合物，对所述第四混合物进行第四烧结，经破碎，得到包覆剂。
本发明中，对于所述第四混合的条件没有特别限定，只要能使得所述锂源、所述La源、所述N1源、所述N2源、所述N3源和所述N4源充分混合均匀即可，优选地，所述第四混合的条件包括：混合转速为400-1000rpm，混合时间为3-6h。本发明中，所述第四混合和所述第一混合的条件可以相同，也可以不同。
根据本发明，所述第四烧结的条件包括：烧结温度为600-1000℃，烧结时间为3-9h。
进一步地，所述第四烧结的条件包括：烧结温度为700-1000℃，烧结时间为5-9h。
本发明的另一个具体实施方式中，所述包覆剂按照以下步骤制备：
(b)将锂源、La源、可选地N1源、可选地N2源、N3源、可选地N4源和溶剂进行第五混合，得到第五混合物，调节所述第五混合物的pH、干燥后，进行第五烧结，经破碎，得到包覆剂。
本发明中，对于所述第五混合的条件没有特别限定，只要能使得所述锂源、所述La源、所述N1源、所述N2源、所述N3源、所述N4源和所述溶剂充分混合均匀即可，优选地，所述第五混合的条件包括：混合转速为400-1000rpm，混合时间为3-6h。
本发明中，对于所述溶剂的种类没有特别限定，只要能够使得所述锂源、所述La源、所述N1源、所述N2源、所述N3源、所述N4源形成溶液，例如乙醇。
本发明中，可以采用本领域中常规的酸性溶液，例如柠檬酸调节所述第五混合物的pH，优选地，添加酸性溶液，使得所述第五混合物的pH值为9-12。
本发明中，对于所述干燥的条件没有特别限定，只要使得所述第五混合物充分干燥即可，优选地，所述干燥的条件包括：干燥温度为40-120℃，干燥时间为5-12h。
根据本发明，所述第五烧结的条件包括：烧结温度为500-850℃，烧结时间为4-8h。
进一步地，所述第五烧结的条件包括：烧结温度为550-800℃，烧结时间为5-8h。
本发明第三方面提供一种表面包覆正极材料的制备方法，其特征在于，所述方法包括以下步骤：
将锂源、La源、可选地N1源、可选地N2源、N3源、可选地N4源和正极材料基体进行第三混合，得到第三混合物进行第三烧结，得到所述表面包覆正极材料。
在本发明第三方面所述的表面包覆正极材料的制备方法中，所涉及的其他物料种类以及用量、各步骤的条件、参数等均与前述第二方面的制备方法相同，为了避免重复，本发明在第二方面中对物质的某些特征(例如物质的可选种类等)不再重复叙述。
根据本发明，所述锂源、所述La源、所述N1源、所述N2源、所述N3源、所述N4源的用量使得n(Li)∶n(La)∶n(N1)∶n(N2)∶n(N3)∶n(N4)为(α-β-γ-δ)∶β∶γ∶δ∶(1-λ)∶λ，其中，0.7＜α＜1.5，0.2＜β＜1，0≤γ＜0.5，0≤δ＜0.5，0≤λ＜0.5。
根据本发明，所述锂源、所述La源、所述N1源、所述N2源、所述N3源、所述N4源的总用量使得，以所述正极材料基体的总重量，经第三烧结后，锂源、La源、N1源、N2源、N3源、N4源的产物的含量为0.05-4wt％，优选为0.1-3.6wt％。
根据本发明，所述第三烧结的条件包括：烧结温度为300-900℃，烧结时间为1-12h。
本发明中，当在上述条件下对所述第三混合物进行第三烧结时，能够使得正极材料表面具有离子、电子传输特性，进而使得包含该正极材料的锂离子电池具有优异的电化学性能。
进一步地，所述第三烧结的条件包括：热处理温度为400-800℃，热处理时间为4-10h。
本发明中，所述正极材料基体的平均粒径D50为2.3-12μm。
本发明中，对于第二方面以及第三方面所用的正极材料基体没有特别限定，可以采用本领域常规的制备方法制备，为了进一步改善表面包覆正极材料的锂离子脱嵌的电化学活性，使得包含该表面包覆正极材料的锂离子电池具有优异的倍率性能，优选地，所述正极材料基体按照以下步骤制备：
(1)将镍盐、钴盐、锰盐按照n(Ni)∶n(Co)∶n(Mn)为x∶y∶z的摩尔比配置混合盐溶液；将沉淀剂和络合剂分别配置为沉淀剂溶液和络合剂溶液；
(2)将所述混合盐溶液、所述沉淀剂溶液和所述络合剂溶液流加入到反应釜中，进行共沉淀 反应，得到前驱体浆料，所述前驱体浆料经过固液分离、洗涤、烘干、筛分后，得到镍钴锰氢氧化物前驱体；
(3)将所述镍钴锰氢氧化物前驱体、锂源和可选地含有M元素的化合物进行混合，得到第六混合物，在含氧气气氛中，对所述第六混合物进行第六烧结，经破碎、筛分，得到所述正极材料基体。
根据本发明0≤x≤1，0≤y≤1，0≤z≤1。
本发明中，所述正极材料基体的制备方法中，至少添加所述镍盐、钴盐、锰盐和含有M元素的化合物中的至少一种。
本发明中，对镍盐的种类没有特别限定，可以为本领域中常规的镍盐，例如硫酸镍、氯化镍、硝酸镍和醋酸镍中的至少一种。
本发明中，对钴盐的种类没有特别限定，可以为本领域中常规的钴盐，例如硫酸钴、氯化钴、硝酸钴和醋酸钴中的至少一种。
本发明中，对锰盐的种类没有特别限定，可以为本领域中常规的锰盐，例如硫酸锰、氯化锰、硝酸锰和醋酸锰中的至少一种。
本发明中，对所述沉淀剂的种类没有特别限定，可以为本领域中常规的沉淀剂，例如选自碳酸铵、碳酸氢铵、碳酸钠、碳酸氢钠、碳酸钾、碳酸氢钾、氢氧化钠、氢氧化钾和氢氧化锂中的至少一种。
本发明中，对所述络合剂和第二络合剂的种类没有特别限定，可以为本领域中常规的络合剂，例如选自氨水、乙二胺四乙酸二钠、硝酸铵、氯化铵和硫酸铵中的至少一种。
根据本发明，所述镍钴锰氢氧化物前驱体、所述锂源和所述含有M元素的化合物的用量使得n(Li)/[n(Ni)+n(Co)+n(Mn)+n(M)]为0.95-1.5；n(M)/[n(Ni)+n(Co)+n(Mn)+n(M)]为0-0.06。
进一步地，所述镍钴锰氢氧化物前驱体、所述锂源和所述含有M元素的化合物的用量使得n(Li)/[n(Ni)+n(Co)+n(Mn)+n(M)]为0.96-1.03；n(M)/[n(Ni)+n(Co)+n(Mn)+n(M)]为0-0.05。
根据本发明，所述含有M元素的化合物选自含有Ga、Sc、In、Y、Ce、Co、La、Cr、Mo、Mn、Fe、Hf、Zr、W、Nb、Sm和Al中至少一种元素的化合物。
本发明中，对于所述锂源的种类没有特别限定，可以为本领域中常规的锂源，例如选自碳酸锂、氢氧化锂、氟化锂、氯化锂、硝酸锂和乙酸锂中的至少一种。
根据本发明，所述共沉淀反应的条件包括：反应温度为40-120℃，pH值为9-12。
本发明中，对于所述第六混合的条件没有特别限定，只要能使得所述镍钴锰氢氧化物前驱体、所述锂源和所述含有M元素的化合物充分混合均匀即可，优选地，所述第六混合的条件包括：混合转速为400-1000rpm，混合时间为3-6h。
根据本发明，所述第六烧结的条件包括：烧结温度为500-1000℃；烧结时间为3-18h。
本发明中，当在上述条件下对所述第六混合物进行第六烧结时，所得正极材料无杂相。
进一步地，所述第六烧结的条件包括：烧结温度为600-1000℃，烧结时间为5-9h。
本发明第四方面提供一种由上述制备方法制得的表面包覆正极材料。
本发明第五方面提供一种锂离子电池，其特征在于，所述锂离子电池包括上述表面包覆正极 材料。
本发明中，所述锂离子电池还包括负极和电解液。对于所述负极和所述电解液的种类没有特别限定，可以采用本领域常规种类的负极和电解液，例如所述电解液为普通商用电解液，组成为1mol/L的LiPF6、碳酸乙烯酯(EC)和碳酸二乙酯(DEC)的等量混合液。
以下将通过实施例对本发明进行详细描述。以下实施例中，
表面包覆正极材料的平均粒径D50采用马尔文粒度仪测得。
表面包覆正极材料的组成根据反应物的摩尔比计算得到。
包覆剂及包覆后正极材料的物相结构由XRD测得。
包覆剂的电子电导率采用四探针法测得。
包覆剂的离子电导率采用烧制陶瓷片进行阻塞电极交流阻抗测试，通过公式σ＝L/RS计算的到，L为陶瓷片厚度，R为阻抗值，S为有效电极面积。
包覆剂的氧空穴形成能由第一性原理计算得到。
液态扣式电池的组装：
首先，将表面包覆正极材料、乙炔黑及聚偏二氟乙烯(PVDF)按照质量比95∶2.5∶2.5进行混合，涂覆在铝箔上并进行烘干处理，用100MPa的压力冲压成直径为12mm、厚120μm的正极极片，然后将正极极片放入真空烘干箱中120℃烘干12h。
负极使用直径为17mm，厚度为1mm的Li金属片；隔膜使用厚度为25μm的聚乙烯多孔膜；电解液使用1mol/L的LiPF6、碳酸乙烯酯(EC)和碳酸二乙酯(DEC)的等量混合液。
将正极极片、隔膜、负极极片及电解液在水含量与氧含量均小于5ppm的Ar气手套箱内组装成2025型扣式电池。
表面包覆正极材料的热稳定性通过对脱锂态(即电池充电截止时)的正极极片进行DSC测试测得。
扣式电池的性能评价如下：
(1)充放电性能测试：温度25℃，在3.0-4.3V的电压区间，0.1C的倍率下考察材料的充放电性能。
(2)循环性能测试：温度25℃，在3.0-4.3V的电压区间，分别以1C的倍率下循环80圈考察材料的容量保持率。
制备例1
正极材料基体
(1)按照Ni、Co、Mn摩尔比为93∶2∶5的比例，配制2mol/L硫酸镍、硫酸钴、硫酸锰混合盐溶液。配制7mol/L氢氧化钠碱溶液；配制4mol/L氨水络合剂溶液。
(2)将混合盐溶液、碱溶液、氨水络合剂溶液以并流的方式连续加入搅拌器中进行反应，搅拌转速为120rpm。同时控制混合盐溶液进液流量为40L/h，碱溶液进液流量为20L/h，络合剂溶液进液流量为7L/h，pH为11.5，温度为60℃。当反应完成，将得到的镍钴锰氢氧化物浆料进行固液 分离、洗涤，滤饼100℃烘干10h后筛分，将产物进行水洗、过滤与干燥，得到前驱体。
(3)将上述前驱体、氢氧化锂和三氧化铝以Li/(Ni+Co+Mn+Al)＝1.02、Al/(Ni+Co+Mn+Al)＝0.01的比例混合均匀，在900℃烧结10h，将产物进行过破碎、过筛处理，得到正极材料基体P1，平均粒径D50为3.4μm。
对正极材料基体的组成以及粒径进行测试，结果如表1所示。
制备例2
(1)按照Ni、Co、Mn摩尔比为90∶3∶7的比例配制2mol/L硫酸镍、硫酸钴、硫酸锰混合盐溶液。配制7mol/L氢氧化钠碱溶液；配制6mol/L氨水络合剂溶液。
(2)将混合盐溶液、碱溶液、氨水络合剂溶液以并流的方式连续加入搅拌器中进行反应，搅拌转速为100rpm。同时控制混合盐溶液进液流量为35L/h，碱溶液进液流量为17L/h，络合剂溶液进液流量为9L/h，控制pH为11.6，温度为60℃。当反应完成，将得到的镍钴锰氢氧化物浆料进行固液分离、洗涤，滤饼100℃烘干10h后筛分，将产物进行水洗、过滤与干燥，得到前驱体。
(3)将上述前驱体、碳酸锂和氧化锆以Li/(Ni+Co+Mn+Zr)＝1.03以及Zr/(Ni+Co+Mn+Zr)＝0.02的比例混合均匀，在750℃烧结10h，将产物进行过破碎、过筛处理，得到正极材料基体P2，平均粒径D50为3.7μm。对正极材料基体的组成以及粒径进行测试，结果如表1所示。
制备例3
(1)按照Ni、Co、Mn摩尔比为90∶5∶5的比例配制2mol/L硫酸镍、硫酸钴、硫酸锰混合盐溶液。配制7mol/L氢氧化钠碱溶液；配制5mol/L氨水络合剂溶液。
(2)将混合盐溶液、碱溶液、氨水络合剂溶液以并流的方式连续加入搅拌器中进行反应，搅拌转速为100rpm。同时控制混合盐溶液进液流量为40L/h，碱溶液进液流量为19L/h，络合剂溶液进液流量为9L/h，控制pH为11.6，温度为60℃。当反应完成，将得到的镍钴锰氢氧化物浆料进行固液分离、洗涤，滤饼100℃烘干10h后筛分，将产物进行水洗、过滤与干燥，得到前驱体。
(3)将上述前驱体、碳酸锂和氧化锆以Li/(Ni+Co+Mn+Zr)＝1.03以及Zr/(Ni+Co+Mn+Zr)＝0.02的比例混合均匀，在750℃煅烧10h，将产物进行过破碎、过筛处理，得到正极材料基体P3，平均粒径D50为3.5μm。对正极材料基体的组成以及粒径进行测试，结果如表1所示。
表1
实施例1
S1、将锂源、La源、N1源、N2源、N3源、N4源加入高速混料机中，在1000rpm转速下搅拌3h得到第一混合物，对第一混合物在空气气氛下800℃烧结8h，待冷却至室温后，粉碎、过筛得 到包覆剂Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3，D50为170nm。
通过固相法制备Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3，根据化学式中个元素计量比称取原料氢氧化锂、氧化镧、氧化钕、氧化锶、四氧化三锰、二氧化钛。
S2、将所述包覆剂与正极材料基体P1加入高速混料机中在800rpm转速下搅拌6h，随后450℃烧结8h，待冷却至室温后，粉碎、过筛，得到表面包覆正极材料A1。其中，以所述正极材料基体P1的总重量为基准，所述包覆剂的用量3wt％。
制备过程的各物料的种类、用量以及具体操作条件如表2所示。
包覆剂的XRD谱图如1所示，由图1可以看出，31°-35°处的特征峰呈双峰分布。表面包覆正极材料A1的XRD谱图如图2所示，由图2可以看出，包覆后在31°-35°处属于包覆剂的双峰仍然存在。
根据包覆剂以及表面包覆正极材料的XRD谱图，对包覆剂的31°-35°处的特征峰的双峰分布中，次峰的峰强度Ib与主峰的峰强度Ia之比Ib/Ia、包覆层Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3在31°-35°之间主峰的峰强度Ia与正极材料基体P1(003)晶面的峰强度I(003)之比Ia/I(003)、包覆层Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3在31°-35°之间主峰的峰面积Aa与正极材料基体P(003)晶面的峰面积A(003)之比Aa/A(003)进行计算，结果如表3所示。
对包覆剂的粒径、离子电导率、电子电导率以及氧空穴形成能进行测试，结果如表3所示。对包覆剂、正极材料基体以及表面包覆正极材料进行XRD测试，结果如表3所示。
实施例2
S1、将锂源、La源、N1源、N2源、N3源加入高速混料机中，加入高速混料机中，在1200rpm转速下搅拌2h后，在空气气氛下750℃烧结6h，待冷却至室温后，粉碎、过筛得到包覆料Li0.34La0.47Ce0.05Mg0.05MnO3。包覆料D50为200nm。
将通过固相法制备Li0.34La0.47Ce0.05Mg0.05MnO3，根据化学式中个元素计量比称取原料碳酸锂、氧化镧、氧化铈(N1源)、氧化镁(N2源)、四氧化三锰(N3源)。
S2、将所述包覆剂与正极材料基体P2加入高速混料机中在950rpm转速下搅拌4h，随后500℃烧结5h，待冷却至室温后，粉碎、过筛，得到表面包覆正极材料A2。其中，以所述正极材料基体P2的总重量为基准，包覆剂的用量为1.5wt％。
制备过程的各物料的种类、用量以及具体操作条件如表2所示。
根据包覆剂以及表面包覆正极材料的XRD谱图，对包覆剂的31°-35°处的特征峰的双峰分布中，次峰的峰强度Ib与主峰的峰强度Ia之比Ib/Ia、包覆层Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3在31°-35°之间主峰的峰强度Ia与正极材料基体P1(003)晶面的峰强度I(003)之比Ia/I(003)、包覆层Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3在31°-35°之间主峰的峰面积Aa与正极材料基体P(003)晶面的峰面积A(003)之比Aa/A(003)进行计算，结果如表3所示。
对包覆剂的粒径、离子电导率、电子电导率以及氧空穴形成能进行测试，结果如表3所示。对包覆剂、正极材料基体以及表面包覆正极材料进行XRD测试，结果如表3所示。
实施例3
S1、将锂源、La源、N1源、N3源、N4源加入高速混料机中，在1000rpm转速下搅拌3h得到第一混合物，对第一混合物在空气气氛下800℃烧结8h，待冷却至室温后，粉碎、过筛得到包覆剂Li0.5La0.4Sm0.2Ni0.7Cr0.3O3，D50为110nm。
通过固相法制备Li0.5La0.4Sm0.2Ni0.7Cr0.3O3，根据化学式中个元素计量比称取原料碳酸锂、氧化镧、氧化钐(N1源)、氧化镍(N3源)、三氧化二铬(N4源)。
S2、将所述包覆剂与正极材料基体P2加入高速混料机中在800rpm转速下搅拌4h，随后500℃烧结8h，待冷却至室温后，粉碎、过筛，得到表面包覆正极材料A3。其中，以所述正极材料基体P2的总重量为基准，所述包覆剂的用量2.2wt％。
制备过程的各物料的种类、用量以及具体操作条件如表2所示。
根据包覆剂以及表面包覆正极材料的XRD谱图，对包覆剂的31°-35°处的特征峰的双峰分布中，次峰的峰强度Ib与主峰的峰强度Ia之比Ib/Ia、包覆层Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3在31°-35°之间主峰的峰强度Ia与正极材料基体P1(003)晶面的峰强度I(003)之比Ia/I(003)、包覆层Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3在31°-35°之间主峰的峰面积Aa与正极材料基体P(003)晶面的峰面积A(003)之比Aa/A(003)进行计算，结果如表3所示。
对包覆剂的粒径、离子电导率、电子电导率以及氧空穴形成能进行测试，结果如表3所示。对包覆剂、正极材料基体以及表面包覆正极材料进行XRD测试，结果如表3所示。
实施例4
S1、将锂源、La源、N1源、N2源、N3源、N4源和乙醇溶液进行混合，随后向上述溶液中加入柠檬酸形成pH＝9.6的混合溶液，60℃、500rpm搅拌5h，转移至120℃烘箱中烘干8h，在空气气氛下800℃烧结8h，待冷却至室温后，粉碎、过筛得到包覆剂，D50为80nm。
通过溶胶凝胶法制备Li0.43La0.48Pr0.08Ca0.1Mn0.7Fe0.3O3，根据化学式中个元素计量比称取原料硝酸锂、硝酸镧、氯化镨(N1源)、氯化钙(N2源)、氯化锰(N3源)、氯化铁(N4源)。
S2、将所述包覆剂与正极材料基体P1加入高速混料机中在800rpm转速下搅拌6h，随后450℃烧结8h，待冷却至室温后，粉碎、过筛，得到表面包覆正极材料A4。其中，以所述正极材料基体P1的总重量为基准，所述包覆剂的用量0.8wt％。
制备过程的各物料的种类、用量以及具体操作条件如表2所示。
根据包覆剂以及表面包覆正极材料的XRD谱图，对包覆剂的31°-35°处的特征峰的双峰分布中，次峰的峰强度Ib与主峰的峰强度Ia之比Ib/Ia、包覆层Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3在31°-35°之间主峰的峰强度Ia与正极材料基体P1(003)晶面的峰强度I(003)之比Ia/I(003)、包覆层Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3在31°-35°之间主峰的峰面积Aa与正极材料基体P(003)晶面的峰面积A(003)之比Aa/A(003)进行计算，结果如表3所示。
对包覆剂的粒径、离子电导率、电子电导率以及氧空穴形成能进行测试，结果如表3所示。对包覆剂、正极材料基体以及表面包覆正极材料进行XRD测试，结果如表3所示。
实施例5
S1、将锂源、La源、N1源、N2源、N3源、N4源加入高速混料机中，在1000rpm转速下搅拌3h得到第一混合物，对第一混合物在空气气氛下800℃烧结8h，待冷却至室温后，粉碎、过筛得到包覆剂Li0.5La0.4Sm0.2Ni0.7Cr0.3O3，D50为110nm。
通过固相法制备Li0.5La0.4Sm0.2Ni0.7Cr0.3O3，根据化学式中个元素计量比称取原料碳酸锂、氧化镧、氧化钐(N1源)、氧化镍(N3源)、三氧化二铬(N4源)。
S2、将所述包覆剂与正极材料基体P2加入高速混料机中在800rpm转速下搅拌4h，随后500℃烧结8h，待冷却至室温后，粉碎、过筛，得到表面包覆正极材料A3。其中，以所述正极材料基体P2的总重量为基准，所述包覆剂的用量0.2wt％。
制备过程的各物料的种类、用量以及具体操作条件如表2所示。
根据包覆剂以及表面包覆正极材料的XRD谱图，对包覆剂的31°-35°处的特征峰的双峰分布中，次峰强度Ib与主峰的峰强度Ia之比Ib/Ia、包覆层Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3在31°-35°之间主峰的峰强度Ia与正极材料基体P1(003)晶面的峰强度I(003)之比Ia/I(003)、包覆层Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3在31°-35°之间的主峰面积Aa与正极材料基体P(003)晶面的峰面积A(003)之比Aa/A(003)进行计算，结果如表3所示。
对包覆剂的粒径、离子电导率、电子电导率以及氧空穴形成能进行测试，结果如表3所示。对包覆剂、正极材料基体以及表面包覆正极材料进行XRD测试，结果如表3所示。
实施例6
S1、将锂源、La源、N1源、N2源、N3源、N4源加入高速混料机中，在1000rpm转速下搅拌3h得到第一混合物，对第一混合物在空气气氛下900℃烧结8h，待冷却至室温后，粉碎、过筛得到包覆剂Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3，D50为220nm。
通过固相法制备Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3，根据化学式中个元素计量比称取原料氢氧化锂、氧化镧、氧化钕(N1源)、氧化锶(N2源)、四氧化三锰(N3源)、二氧化钛(N4源)。
S2、将所述包覆剂与正极材料基体P3加入高速混料机中在800rpm转速下搅拌6h，随后300℃烧结8h，待冷却至室温后，粉碎、过筛，得到表面包覆正极材料A1。其中，以所述正极材料基体P3的总重量为基准，所述包覆剂的用量8wt％。
制备过程的各物料的种类、用量以及具体操作条件如表2所示。
根据包覆剂以及表面包覆正极材料的XRD谱图，对包覆剂的31°-35°处的特征峰的双峰分布中，次的峰强度Ib与主峰的峰强度Ia之比Ib/Ia、包覆层Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3在31°-35°之间主峰强度Ia与正极材料基体P1(003)晶面的峰强度I(003)之比Ia/I(003)、包覆层Li0.2La0.48Nd0.1Sr0.05Mn0.8Ti0.2O3在31°-35°之间主峰的峰面积Aa与正极材料基体P(003)晶面的峰面积A(003)之比Aa/A(003)进行计算，结果如表3所示。
对包覆剂的粒径、离子电导率、电子电导率以及氧空穴形成能进行测试，结果如表3所示。对包覆剂、正极材料基体以及表面包覆正极材料进行XRD测试，结果如表3所示。
对比例1
以制备例1制得的正极材料基体P1作为正极材料D1。对正极材料D1进行XRD测试，结果如表3所示。
对比例2
以制备例2制得的正极材料基体P2作为正极材料D2。对正极材料D2进行XRD测试，结果如表3所示。
对比例3
以制备例3制得的正极材料基体P2作为正极材料D3。对正极材料D3进行XRD测试，结果如表3所示。
表2
表3

表3(续)
测试例
将实施例以及对比例的正极材料组装成锂离子电池，分别对脱锂态正极材料的热稳定性进行测试，结果如表4所示。对锂离子电池的性能进行测试，结果如表5所示。
表4
表5

结合表4和表5，能够看出，相对于对比例1-3，实施例1-6制得的正极材料完全脱锂后DSC峰对应的温度更高，表明实施例1-6的正极材料具有更为优异的热稳定性。与此同时，由实施例1-6制得的正极材料组装得到的锂离子电池的倍率性能以及循环性能均得到提升。
如图5所示，实施例1的正极材料完全脱锂态DSC峰位置为210℃，实施例4的正极材料完全脱锂DSC峰位置为207℃，对比例1的正极材料完全脱锂DSC峰位置为203℃。由此表明，相对于对比例1而言，实施例1和实施例4的正极材料具有更为优异的热稳定性。
如图3所示，实施例1和实施例4与对比例1相比，各倍率下放电容量均有所升高，说明包覆后，倍率性能得到了改善，且DSC出峰位置后移，材料热稳定性得到提升。
如图6所示，实施例2的正极材料完全脱锂态DSC峰位置为218℃，实施例3的正极材料完全脱锂态DSC峰位置为213℃，对比例2的正极材料完全锂态DSC峰位置为205℃。由此表明，相对于对比例2而言，实施例2和实施例3的正极材料具有更为优异的热稳定性。
如图4所示，实施例2和实施例3与对比例2相比较，循环保持率升高，说明包覆后正极材料与电解液间副反应减少。DSC峰位置升高，说明材料本身热稳定性提升。

Claims (12)

  1. 一种表面包覆正极材料，其特征在于，所述正极材料包括基体和包覆在所述基体表面的包覆层；
    所述包覆层经XRD测试的2θ为31°-35°处的特征峰呈双峰分布；且双峰分布中的次峰峰强度Ib与主峰的峰强度Ia之比Ib/Ia为0.8-1，优选为0.97-1。
  2. 根据权利要求1所述的表面包覆正极材料，其中，所述基体经XRD测试的(003)晶面对应的特征峰(003)的峰强度为I(003)，0.01％≤Ia/I(003)×100％≤3.5％，优选地，0.08％≤Ia/I(003)×100％≤3.1％；
    优选地，所述基体经XRD测试的(003)晶面对应的特征峰(003)的峰面积为A(003)，所述包覆层经XRD测试的2θ为31°-35°处的特征峰双峰分布中主峰的峰面积为Aa，0.01％≤Aa/A(003)×100％≤6％，优选地，0.5％≤Aa/A(003)×100％≤4.8％。
  3. 根据权利要求1或2所述的表面包覆正极材料，其中，所述基体具有式I所示的组成：
    Li1+aNixCoyMnzMkO2   式I；
    其中，-0.05≤a≤0.5，0≤x≤1，0≤y≤1，0≤z≤1，0≤k≤0.06，0＜x+y+z+k≤1，M选自Ga、Sc、In、Y、Ce、Co、La、Cr、Mo、Mn、Fe、Hf、Zr、W、Nb、Sm和Al中的至少一种元素；
    所述包覆层具有式II所示的组成：
    Liα-β-γ-δLaβN1 γN2 δN3 1-λN4 λO3   式II；
    其中，0.7＜α＜1.5，0.2＜β＜1，0≤γ＜0.5，0≤δ＜0.5，0≤λ＜0.5；N1选自Y、Nd、Pr、Ce、Sm和Sc中的至少一种元素，N2选自Sr、Ca、Mg、Si、Ge和Ru中的至少一种元素，N3选自Ni、Mn和Co中的至少一种元素，N4选自Cr、Al、V、Nb、Zr、Ti和Fe中的至少一种元素；
    优选地，式I中，M选自Ce、Co、La、Cr、Mo、Y、Zr、W、Nb和Al中的至少一种元素；
    优选地，式II中，0.9＜α＜1.3，0.35＜β＜0.7，0.04≤γ＜0.4，0＜δ＜0.3，0≤λ＜0.46；
    优选地，式II中，N1选自Nd、Pr、Ce、Sm和Sc中的至少一种元素；
    优选地，式II中，N2选自Sr、Ca、Mg、Si和Ru中的至少一种元素；
    优选地，式II中，N3选自Ni和/或Mn；
    优选地，式II中，N4选自Cr、Al、Nb、Zr、Ti和Fe中的至少一种元素；
    优选地，以所述基体的总重量为基准，所述包覆层的含量为0.05-4wt％，优选为0.5-3.6wt％。
  4. 根据权利要求1-3中任意一项所述的表面包覆正极材料，其中，所述表面包覆正极材料的平均粒径D50为2-17μm，优选为3-12μm；
    优选地，所述包覆层的电子电导率为1×10-5S/cm-8×10-3S/cm，优选为1.8×10-5S/cm-7.2×10-3S/cm；
    优选地，所述包覆层的离子电导率为1×10-6S/cm-6×10-4S/cm，优选为3×10-6S/cm-5×10-4S/cm；
    优选地，所述包覆层的氧空穴形成能为-2eV至4.5eV，优选为-1.8eV至4eV。
  5. 根据权利要求1-4中任意一项所述的表面包覆正极材料，其中，所述表面包覆正极材料经DSC测试放热峰对应的温度为T1，所述基体经DSC测试放热峰对应的温度为T0，T1-T0为3-15℃，优选为3-14℃。
  6. 一种表面包覆正极材料的制备方法，其特征在于，所述方法包括以下步骤：
    S1、将锂源、La源、可选地N1源、可选地N2源、N3源、可选地N4源进行第一混合，得到第一混合物，对所述第一混合物进行第一烧结，经破碎，得到包覆剂；
    S2、将所述包覆剂与正极材料基体进行第二混合，得到第二混合物，对所述第二混合物进行第二烧结，得到所述表面包覆正极材料；
    优选地，所述锂源、所述La源、所述N1源、所述N2源、所述N3源、所述N4源的用量使得n(Li):n(La):n(N1):n(N2):n(N3):n(N4)为(α-β-γ-δ):β:γ:δ:(1-λ):λ，其中，0.7＜α＜1.5，0.2＜β＜1，0≤γ＜0.5，0≤δ＜0.5，0≤λ＜0.5；
    优选地，以所述正极材料基体的总重量，所述包覆剂的用量为0.05-4wt％，优选为0.1-3.6wt％。
  7. 一种表面包覆正极材料的制备方法，其特征在于，所述方法包括以下步骤：
    将锂源、La源、可选地N1源、可选地N2源、N3源、可选地N4源和正极材料基体进行第三混合，得到第三混合物进行第三烧结，得到所述表面包覆正极材料；
    优选地，所述锂源、所述La源、所述N1源、所述N2源、所述N3源、所述N4源的用量使得n(Li):n(La):n(N1):n(N2):n(N3):n(N4)为(α-β-γ-δ):β:γ:δ:(1-λ):λ，其中，0.7＜α＜1.5，0.2＜β＜1，0≤γ＜0.5，0≤δ＜0.5，0≤λ＜0.5；
    优选地，所述锂源、所述La源、所述N1源、所述N2源、所述N3源、所述N4源的总用量使得，以所述正极材料基体的总重量，经第三烧结后，锂源、La源、N1源、N2源、N3源、N4源的 产物的含量为0.05-4wt％，优选为0.1-3.6wt％。
  8. 根据权利要求6或7所述的制备方法，其中，所述N1源选自含有Y、Nd、Pr、Ce、Sm和Sc中的至少一种元素的化合物；
    优选地，所述N2源选自含有Sr、Ca、Mg、Si、Ge和Ru中的至少一种元素的化合物；
    优选地，所述N3源选自含有Ni、Mn和Co中的至少一种元素的化合物；
    优选地，所述N4源选自含有Cr、Al、V、Nb、Zr、Ti和Fe中的至少一种元素的化合物；
    优选地，所述第一烧结的条件包括：烧结温度为500-1120℃，烧结时间为3-9h；
    优选地，所述包覆剂的平均粒径D50为30-200nm；
    优选地，所述第二烧结的条件包括：烧结温度为300-900℃，烧结时间为1-12h；
    优选地，所述第三烧结的条件包括：烧结温度为300-900℃，烧结时间为1-12h。
  9. 根据权利要求6或8所述的制备方法，其中，所述步骤S1包括：
    (a)将锂源、La源、可选地N1源、可选地N2源、N3源、可选地N4源进行第四混合，得到第四混合物，对所述第四混合物进行第四烧结，经破碎，得到包覆剂；
    或者，(b)将锂源、La源、可选地N1源、可选地N2源、N3源、可选地N4源和溶剂进行第五混合，得到第五混合物，调节所述第五混合物的pH、干燥后，进行第五烧结，经破碎，得到包覆剂；
    优选地，所述第四烧结的条件包括：烧结温度为600-1000℃，烧结时间为3-9h；
    优选地，所述第五烧结的条件包括：烧结温度为500-850℃，烧结时间为4-8h。
  10. 根据权利要求6-9中任意一项所述的制备方法，其中，所述正极材料基体按照以下步骤制备：
    (1)将镍盐、钴盐、锰盐按照n(Ni):n(Co):n(Mn)为x:y:z的摩尔比配置混合盐溶液；将沉淀剂和络合剂分别配置为沉淀剂溶液和络合剂溶液；
    (2)将所述混合盐溶液、所述沉淀剂溶液和所述络合剂溶液流加入到反应釜中，进行共沉淀反应，得到前驱体浆料，所述前驱体浆料经过固液分离、洗涤、烘干、筛分后，得到镍钴锰氢氧化物前驱体；
    (3)将所述镍钴锰氢氧化物前驱体、锂源和可选地含有M元素的化合物进行混合，得到第六混合物，在含氧气气氛中，对所述第六混合物进行第六烧结，经破碎、筛分，得到所述正极材料基体；
    优选地，0≤x≤1，0≤y≤1，0≤z≤1；
    优选地，所述镍钴锰氢氧化物前驱体、所述锂源和所述含有M元素的化合物的用量使得n(Li)/[n(Ni)+n(Co)+n(Mn)+n(M)]为0.95-1.5；n(M)/[n(Ni)+n(Co)+n(Mn)+n(M)]为0-0.06；
    优选地，所述含有M元素的化合物选自含有Ga、Sc、In、Y、Ce、Co、La、Cr、Mo、Mn、Fe、Hf、Zr、W、Nb、Sm和Al中至少一种元素的化合物；
    优选地，所述共沉淀反应的条件包括：反应温度为40-120℃，pH值为9-12；
    优选地，所述第六烧结的条件包括：烧结温度为500-1000℃；烧结时间为3-18h。
  11. 由权利要求6-10中任意一项所述的制备方法制得的表面包覆正极材料。
  12. 一种锂离子电池，其特征在于，所述锂离子电池包括权利要求1-5和11中任意一项所述的表面包覆正极材料。
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