Ternary positive electrode material, preparation method thereof and electrochemical device

Abstract

The ternary positive electrode material, the preparation method thereof and the electrochemical device are characterized in that the ternary positive electrode material provided by the embodiment of the invention comprises a base material and a coating layer located on the surface of the base material, the base material is doped with Y2O3, the coating layer contains yttria-stabilized zirconia, the chemical formula of the yttria-stabilized zirconia is (ZrO2) 1-x (Y2O3) x, 0 < = xlt; 1. The ternary positive electrode material has the characteristics of matrix material Y2O3 doping and surface yttria-stabilized zirconia coating, the stability of a coating layer is improved through cooperation of the matrix material Y2O3 doping and the surface yttria-stabilized zirconia coating, stress concentration between the coating layer and the matrix material is reduced, and the ionic conductivity, the structural stability and the cycle performance of the ternary positive electrode material are improved; therefore, the long-term cycling stability of the ternary positive electrode material under the conditions of high temperature and high voltage is improved.

Description

Technical Field

[0001] The present application relates to the technical field of new energy materials, and in particular to a ternary positive electrode material and a preparation method thereof, and an electrochemical device. Background Art

[0002] With the rapid development of the new energy vehicle market, consumers have put forward higher requirements for the energy density, cycle life and high temperature performance of power lithium-ion batteries. Ternary cathode materials are considered to be one of the most promising cathode materials for power batteries due to their high theoretical specific capacity and low cost advantages.

[0003] However, ternary positive electrode materials have problems of capacity attenuation and deterioration of cycle performance in long-term cycling scenarios at high voltage and high temperature, resulting in a decline in the performance of lithium-ion batteries. Summary of the invention

[0004] In view of this, in order to solve at least one of the above technical problems, an embodiment of the present application provides a ternary positive electrode material.

[0005] The embodiments of the present application also provide a method for preparing the aforementioned ternary positive electrode material, and an electrochemical device using the ternary positive electrode material.

[0006] In a first aspect, an embodiment of the present application provides a ternary positive electrode material, the ternary positive electrode material comprising a base material and a coating layer located on the surface of the base material, wherein the base material is doped with Y2O3, the coating layer contains yttria-stabilized zirconia, and the chemical formula of the yttria-stabilized zirconia is (ZrO2) 1-x (Y2O3) x , where 0≤x<1.

[0007] In some possible embodiments, the chemical formula of the yttria-stabilized zirconia is (ZrO2) 1-x (Y2O3) x , where 0 <x<1,所述基体材料中的钇元素与所述包覆层中的钇元素互扩散,形成钇元素的梯度过渡。

[0008] In some possible embodiments, the yttria-stabilized zirconia includes Zr 0.97 Y 0.06 O 2.03 、Zr 0.95 Y 0.1 O 2.05 and Zr 0.92 Y 0.16 O 2.08 At least one of .

[0009] In some possible embodiments, the coating layer has a thickness of 1 nm to 10 nm; and / or The average particle size D50 of the particles of the ternary positive electrode material is 3 μm to 4 μm.

[0010] In a second aspect, an embodiment of the present application provides a method for preparing a ternary positive electrode material, comprising: mixing a ternary positive electrode material precursor, a lithium source and a doping material containing Y2O3 and performing a primary sintering to obtain a matrix material doped with yttrium; and mixing the matrix material doped with yttrium with a coating material containing yttria-stabilized zirconia and performing a secondary sintering so that the coating material forms a coating layer on the surface of the matrix material to obtain the ternary positive electrode material, wherein the chemical formula of the yttria-stabilized zirconia is (ZrO2) 1-x (Y2O3) x , where 0≤x<1.

[0011] In some possible embodiments, during the secondary sintering process, the yttrium element in the base material and the yttrium element in the coating layer diffuse mutually to form a gradient transition of the yttrium element.

[0012] In some possible embodiments, the mass percentage of the yttrium element in the Y2O3 to the total mass of the ternary cathode material precursor and the lithium source is 0.001wt%≤N≤10wt%; and / or The doping material also includes at least one of Al2O3, SiO2, MgO and ZrO2.

[0013] In some possible embodiments, the mass percentage of the yttria-stabilized zirconia in the matrix material is 0.001wt%-5wt%; and / or The yttria-stabilized zirconia comprises Zr 0.97 Y 0.06 O 2.03 、Zr 0.95 Y 0.1 O 2.05 and Zr 0.92 Y 0.16 O 2.08 At least one of .

[0014] In some possible embodiments, the primary sintering temperature is 750° C. to 1000° C., and the primary sintering time is 6 h to 18 h; and / or The temperature of the secondary sintering is 300° C. to 500° C., and the time of the secondary sintering is 6 h to 18 h.

[0015] In a third aspect, an embodiment of the present application provides an electrochemical device, which includes a positive electrode plate, a negative electrode plate, a separator, and an electrolyte. The positive electrode plate includes a positive electrode active material, and the positive electrode active material is the aforementioned ternary positive electrode material.

[0016] Compared with the prior art, in the ternary positive electrode material provided by the embodiment of the present application, Y2O3 is doped in the matrix material, and yttria-stabilized zirconia (YSZ) is contained in the coating layer on the surface of the matrix material. Under the synergistic effect of the doping of Y2O3 and the coating of YSZ, the ionic conductivity, structural stability, and cycle stability of the ternary positive electrode material are improved. Among them, YSZ has good ionic conductivity and structural stability. By uniformly coating YSZ on the surface of the matrix material, it can promote the transport of lithium ions in the ternary positive electrode material, improve the rate performance of the ternary positive electrode material, and at the same time enhance the interfacial stability of the ternary positive electrode material under high-temperature and high-pressure cycling conditions. The yttrium element in the matrix material can diffuse into the coating layer during secondary sintering or subsequent high-temperature cycling, optimizing the lattice structure of YSZ in the coating layer and further improving the ionic conductivity and structural stability of YSZ. In addition, doping Y2O3 in the matrix material can slow down the volume change of the matrix material, effectively reduce the risk of coating layer shedding, and inhibit lithium-nickel mixing, crystal phase transformation, and lattice oxygen precipitation in the matrix material under high temperature and high pressure, which is beneficial to maintaining the stability of the crystal layered structure. Description of the Drawings

[0017] Figure 1 It is a process flow chart of the preparation method of the ternary positive electrode material provided by the embodiment of the present application.

[0018] Figure 2 It is a scanning electron microscope image of the ternary positive electrode material in Example 4 of the present application.

[0019] Figure 3 It is a scanning electron microscope image of the ternary positive electrode material in Comparative Example 1 of the present application.

[0020] Figure 4 It is a comparison chart of the first discharge capacity and the first efficiency of the coin cells prepared from the ternary positive electrode materials in Examples 1-4 and Comparative Example 1 of the present application.

[0021] Figure 5 It is a comparison chart of the high-temperature cycle performance of the coin cells prepared from the ternary positive electrode materials in Examples 1-4 and Comparative Example 1 of the present application. Detailed Embodiments

[0022] Embodiments of the present application will be described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present application and should not be construed as a limitation of the present application. It should be noted that unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this application belongs. Without conflict, the embodiments of the present application and the features in the embodiments can be combined with each other. In the following description, many specific details are set forth in order to fully understand the present application. The described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments.

[0023] The inventors of the present application have found through research that in the prior art, metal oxides or lithium compounds are usually used to form a coating layer on the surface of the ternary cathode material to improve the cycle stability and thermal stability of the material. However, these coating layers are still not stable enough during high-temperature cycling, and the stress concentration phenomenon between the coating layer and the substrate is relatively serious, resulting in the easy detachment of the coating layer and the inability to effectively improve the long-term high-temperature cycle stability of the ternary cathode material.

[0024] Therefore, the embodiments of the present application provide a new type of ternary cathode material, which includes a matrix material and a coating layer located on the surface of the matrix material. Among them, yttrium oxide (Y2O3) is doped in the matrix material, and yttria-stabilized zirconia (YSZ) is contained in the coating layer. The chemical formula of the yttria-stabilized zirconia is (ZrO2) 1-x (Y2O3) x , where 0 ≤ x < 1.

[0025] First, a YSZ-containing coating layer is formed on the surface of the matrix material. YSZ refers to a zirconia (ZrO2) material doped with yttrium oxide (Y2O3). x represents the doping degree of Y2O3, where 0 ≤ x < 1. Exemplarily, x can be 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 0.98, 0.99, 0.999, or any value within the numerical range composed of any two of the above values. It can be understood that when x = 0, YSZ represents ZrO2 without Y2O3 doping. ZrO2 has good chemical stability and mechanical strength, and can effectively enhance the thermal stability, oxidation resistance, and corrosion resistance of the ternary cathode material. The incorporation of yttrium elements in YSZ can stabilize the cubic phase of ZrO2, which helps to reduce the phase transformation of ZrO2 from the cubic phase to the tetragonal phase or monoclinic phase at high temperatures. YSZ has good ionic conductivity, which can promote the transport of lithium ions in the cathode material and improve the rate performance of the ternary cathode material. Moreover, YSZ exhibits excellent oxygen ion conductivity at high temperatures, and the cubic phase structure of YSZ maintains chemical stability within a wide oxygen partial pressure range. In addition, relying on the thermal expansion anisotropy of the cubic phase structure of YSZ, the coating layer regulates and optimizes the interfacial stress, effectively alleviating the problem of thermal stress concentration between the coating layer and the matrix material. Thus, YSZ can serve as an interfacial protective layer under high voltage and high temperature conditions, reducing the side reactions between the matrix material and the electrolyte, and enhancing the structural stability and cycling performance of the ternary cathode material under high temperature and high pressure conditions. However, yttrium element migration or segregation still occurs in YSZ at high temperatures or during long-term cyclic use, resulting in unstable structure and thus affecting the ionic conductivity.

[0026] Therefore, in this application, by further doping Y2O3 into the matrix material, the electrochemical performance and cycling stability of the matrix material itself can be improved; in addition, during the high-temperature sintering process and subsequent high-temperature cycling process, the yttrium elements in the matrix can diffuse into the coating layer to improve the coating effect of the coating layer, thereby improving the conductivity and stability of the ternary cathode material during long-term cyclic use.

[0027] Specifically, from the perspective of improving the coating effect: on the one hand, through 3+ bulk doping to dynamically compensate for the lost yttrium elements in YSZ and optimize the lattice structure of the YSZ coating layer. 3+The introduced oxygen vacancy defects can significantly improve the lithium ion mobility of YSZ, while stabilizing the cubic zirconia crystal form of YSZ, which can inhibit the volume expansion and contraction of the coating layer caused by high-temperature phase change, and improve the thermomechanical stability of the coating layer, thereby further improving the ionic conductivity and structural stability of the YSZ coating layer, and improving the long-term cycle stability of the ternary positive electrode material at high temperature; on the other hand, since the matrix material is doped with Y2O3, the thermal stability of the matrix material is improved and the thermal expansion coefficient is reduced, which is beneficial to reduce the interface stress between the matrix material and the coating layer, and can effectively reduce the risk of the coating layer falling off the surface of the matrix material. In addition, from the perspective of improving the bulk phase of the matrix material, since the atomic radius of yttrium is larger than that of nickel, cobalt, manganese and other elements, iridium doping can increase the lattice spacing of the matrix material and introduce oxygen vacancy defects, thereby improving the electronic conductivity and ion diffusion rate of the ternary positive electrode material; Y2O3 doping can also reduce the lithium-nickel mixing in the matrix material and inhibit the lattice collapse induced by the phase change, thereby enhancing the ability of the layered structure of the ternary positive electrode material to resist lattice oxygen precipitation from the bulk phase level; at the same time, the starting temperature of the phase change of the matrix material is increased, the high temperature performance of the ternary positive electrode material is enhanced, and the long-term cycle stability of the ternary positive electrode material at high temperature is further improved.

[0028] In the present application, the ternary positive electrode material adopts YSZ, a coating material with high structural stability, and combines it with the core doped Y2O3. The two synergistically improve the stability and conductivity of the coating layer and reduce the risk of the coating layer falling off, thereby improving the long-term cycle stability of the ternary positive electrode material under high temperature and high pressure, and meeting the urgent needs of lithium-ion batteries for high energy density, long cycle life and excellent high temperature and high pressure performance.

[0029] In some embodiments, the chemical formula of yttria-stabilized zirconia is (ZrO2) 1-x (Y2O3) x , where 0 <x<1,基体材料中的钇元素扩散弥补所述包覆层中的钇元素,形成钇元素的梯度过渡,基体材料的热膨胀系数进一步降低,并缩小了包覆层和基体材料之间的热膨胀系数差距,提高了基体材料和包覆层的界面结合力,从而使基体材料与包覆层的界面剪切应力下降,有效缓解了包覆层与基材热应力集中问题,有效抑制电解液渗透引发的包覆层剥落。

[0030] In some embodiments, the yttria-stabilized zirconia may include Zr 0.97 Y 0.06 O 2.03 、Zr 0.95 Y 0.1 O 2.05 and Zr 0.92 Y 0.16 O 2.08At least one of them. Among them, Zr 0.97 Y 0.06 O 2.03 When x = 0.03, it is ZrO2 doped with 3 mol% Y2O3 (3YSZ); Zr 0.95 Y 0.1 O 2.05 When x = 0.05, it is ZrO2 doped with 5 mol% Y2O3 (5YSZ); Zr 0.92 Y 0.16 O 2.08 When x = 0.08, it is ZrO2 doped with 8 mol% Y2O3 (8YSZ). 3YSZ realizes the strengthening of the YSZ structure stability through martensitic transformation (tetragonal → monoclinic). With the increase of the Y2O3 doping amount, such as 5YSZ, 5YSZ obtains a higher oxygen vacancy concentration, thereby further optimizing the ionic conduction performance. The oxygen vacancy concentration in YSZ is positively correlated with the Y doping amount, but when the doping amount x > 0.08, it will cause a certain lattice distortion and the mechanical strength of YSZ will decrease slightly. Therefore, 8YSZ can balance good strength (improvement of biaxial flexural strength) and conductivity, and 8YSZ is preferably used in the coating layer.

[0031] In some embodiments, the thickness of the coating layer can be 1 nm to 10 nm, which is beneficial to forming effective protection for the matrix material, enhancing the bonding force with the matrix material, and at the same time will not have an adverse impact on the energy density of the ternary cathode material. Exemplarily, the thickness of the coating layer can be 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm or any value within the numerical range composed of any two of the above values. Further, the thickness of the coating layer can be 3 nm to 8 nm.

[0032] In some embodiments, the average particle size D50 of the particles of the ternary cathode material is 3 μm to 4 μm, which is beneficial. Exemplarily, the average particle size D50 of the particles of the cathode material can be 3 μm, 3.2 μm, 3.5 μm, 3.8 μm, 4 μm or any value within the numerical range composed of any two of the above values.

[0033] In some embodiments, the mass percentage of YSZ in the ternary cathode material can be 0.001 wt% to 5 wt%, which is beneficial to uniformly coating YSZ on the surface of the matrix material, forming the coating layer with the above thickness, and improving the protection effect on the matrix material and the improvement effect on the cycle performance and structure stability of the ternary cathode material.

[0034] In some embodiments, in the Y2O3 doped in the matrix material, the mass percentage of yttrium element in the ternary cathode material can be 0.001wt% - 10wt%, which is beneficial to the moderate doping of Y2O3, thereby further improving the long-term cycle stability of the ternary cathode material at high temperatures.

[0035] In some embodiments, the ternary cathode material is a single-crystalline ternary cathode material. The crystal structure of the single-crystalline ternary cathode material is complete and ordered, without the presence of grain boundaries as in polycrystalline materials, and has fewer lattice defects. Therefore, it has higher structural stability and high-temperature stability.

[0036] Compared with the prior art, the ternary cathode material provided by the embodiments of the present application has the following beneficial effects: 1. The surface of the matrix material is coated with YSZ. YSZ has good ionic conductivity and stability, which can effectively improve the long-term cycle stability and rate performance of the ternary cathode material at high temperatures and high pressures.

[0037] 2. The matrix material is doped with Y2O3 to reduce the degree of cation mixing in the ternary cathode material and improve the stability of the crystal layer structure; at the same time, the yttrium element in the matrix material can diffuse into the coating layer to optimize the lattice structure of the YSZ coating layer, thereby further improving the stability and conductivity of the coating layer and reducing the risk of coating layer shedding.

[0038] 3. The surface of the matrix material is coated with YSZ and the matrix is doped with Y2O3 at the same time. The two act synergistically to jointly improve the stability of the coating layer, reduce the stress concentration between the coating layer and the matrix material, improve the ionic conductivity, structural stability and cycle performance of the ternary cathode material, thereby improving the long-term cycle stability of the ternary cathode material under high temperature and high voltage conditions.

[0039] Based on the same inventive concept, please refer to Figure 1 As shown, the embodiments of the present application provide a preparation method for a ternary cathode material, which specifically includes the following steps: Step S1, mixing a ternary cathode material precursor, a lithium source and a doping material containing Y2O3 and performing a first sintering to obtain a matrix material doped with yttrium element.

[0040] Specifically, the ternary cathode material precursor Ni x Co y Mn 1-x-y (OH)2 (0.5 ≤ x ≤ 0.7, 0 ≤ y ≤ 0.1) is selected. The ternary cathode material precursor, the lithium source and the doping material containing Y2O3 are added to a high-speed mixer in a certain proportion for uniform mixing to obtain a mixed material, and then the mixed material is placed in a certain sintering atmosphere for the first sintering. After cooling, it is subjected to crushing treatment to obtain the matrix material Li1Ni xCo y Mn 1-x-y O2 (0.5 ≤ x ≤ 0.7, 0 ≤ y ≤ 0.1).

[0041] In some embodiments, the mass percentage of yttrium element in Y2O3 in the total mass of the ternary cathode material precursor and the lithium source is 0.001 wt% to 10 wt%, which is beneficial to the moderate doping of Y2O3 in the matrix material.

[0042] In some embodiments, the doping material may further include at least one of Al2O3, SiO2, MgO, and ZrO2, etc., which is beneficial to further improve the structural stability and cycling performance of the matrix material, etc.

[0043] In some embodiments, the temperature of the first sintering is 750 °C to 1000 °C. A higher first sintering temperature can promote grain growth, reduce the number of grain boundaries and grain boundary defects, which is beneficial to the formation of a matrix material with good dispersion in single crystal form, and is also beneficial to the doping of Y2O3 into the matrix material and uniform dispersion. The temperature of the first sintering can be, for example, 750 °C, 800 °C, 900 °C, 915 °C, 930 °C, 935 °C, 950 °C, 975 °C, 1000 °C or any value within the range composed of any two of the above values. The temperature of the first sintering can further be 900 °C to 1000 °C.

[0044] In some embodiments, the time of the first sintering can be 6 h to 18 h, which is beneficial to the full sintering to form the matrix material. The time of the first sintering can further be 12 h to 16 h, and can be, for example, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, 18 h or any value within the range composed of any two of the above values.

[0045] In some embodiments, the atmosphere of the first sintering can be air atmosphere, oxygen-rich atmosphere or pure oxygen atmosphere, which is beneficial to keeping the metal ions in the matrix material in the oxidized state. The atmosphere of the first sintering can further be oxygen-rich atmosphere or pure oxygen atmosphere, which is beneficial to the formation of a stable single crystal structure.

[0046] In some embodiments, the lithium source may include at least one of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium sulfate, etc.

[0047] In some embodiments, the ratio of the number of moles of lithium in the lithium source to the total number of moles of transition metals in the ternary cathode material precursor can be (1.02 to 1.3):1, which is beneficial to the full incorporation of lithium ions during the formation of the matrix material, promotes the uniform distribution of lithium ions in the matrix material, reduces the degree of cation mixing, and improves the electrochemical performance and structural stability of the ternary cathode material.

[0048] The method of mixing the ternary cathode material precursor, the lithium source and the doping material may include mixing with a high-speed mixer, mixing with a plowshare mixer, mixing with a ribbon mixer, stirring or grinding, etc., which can effectively mix the materials. It is understandable that the mixing method includes but is not limited to the above method, and any method that can achieve mixing of the lithium source, the ternary cathode material precursor and the doping material is possible.

[0049] Step S2, mixing the yttrium-doped base material with the coating material containing yttria-stabilized zirconia and performing secondary sintering, so that the coating material forms a coating layer on the surface of the base material to obtain a ternary positive electrode material, wherein the chemical formula of the yttria-stabilized zirconia is (ZrO2) 1-x (Y2O3) x , where 0≤x<1.

[0050] Specifically, the obtained yttrium-doped matrix material is evenly mixed with a coating material containing YSZ, and then the mixed material is subjected to secondary sintering to form a ternary positive electrode material doped with yttrium and coated with YSZ. The coating layer can reduce the direct contact between the matrix material and the electrolyte, effectively reduce side reactions, enhance the interface stability of the ternary positive electrode material under high temperature and high pressure cycle conditions, and can also promote the transmission of lithium ions in the ternary positive electrode material, thereby improving the rate performance of the ternary positive electrode material.

[0051] In some embodiments, the mass percentage of YSZ in the matrix material is 0.001wt%~5wt%, which is conducive to forming a coating layer of suitable thickness, improving the protection effect of the matrix material and the improvement effect of the cycle performance and structural stability of the ternary positive electrode material. The mass percentage of YSZ in the matrix material can be 0.001wt%, 0.01wt%, 0.05wt%, 0.1wt%, 0.5wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt% or any value within the numerical range composed of any two of the above values. The mass percentage of YSZ in the matrix material can further be 0.05wt%~2wt%.

[0052] In some embodiments, YSZ includes Zr 0.97 Y 0.06 O 2.03 、Zr 0.95 Y 0.1 O 2.05 and Zr 0.92 Y 0.16 O 2.08 At least one of the above.

[0053] In some embodiments, the mixing method can be solid-phase dry mixing, such as mixing in a high-speed mixer, a plowshare mixer, a ribbon mixer, stirring or grinding, etc.; it can also be liquid-phase wet mixing, such as wet grinding or spray coating, etc. A reasonable mixing method and mixing parameters can further effectively improve the coating effect. Specifically, in this embodiment, the solid-phase dry mixing method can be adopted, and a high-speed mixer is used to uniformly mix and coat the matrix material and the coating material. Compared with wet spraying, dry mixing is beneficial to reducing the introduction of solvents and improving the stability of the mixture, so as to achieve a better coating effect.

[0054] During the secondary sintering process, the yttrium element in the matrix material and the yttrium element in the coating layer diffuse mutually to form a gradient transition of the yttrium element.

[0055] During the secondary sintering process, the temperature of the secondary sintering is 300°C to 500°C. Within the above temperature range, it is beneficial for the yttrium element in YSZ and the matrix material to undergo elemental mutual diffusion at the interface to form a gradient transition, reduce the thermal expansion coefficient of the matrix material, and improve the interfacial bonding force between the matrix material and the coating layer. Thus, the interfacial shear stress between the matrix material and the coating layer is reduced, effectively alleviating the problem of thermal stress concentration between the coating layer and the matrix material, and improving the structural stability of the ternary cathode material. In addition, the diffusion of the yttrium element in the matrix into the coating layer can also improve the ionic conductivity, structural stability and high-temperature performance of YSZ. Exemplarily, the temperature of the secondary sintering can be 300°C, 310°C, 320°C, 330°C, 340°C, 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, 500°C or any value within the range composed of any two of the above values. Further, the temperature of the secondary sintering can be 350°C to 500°C.

[0056] The time of the secondary sintering can be 6h to 18h, which is beneficial to the full diffusion of the yttrium element and the formation of a coating layer with strong adhesion. Further, the time of the secondary sintering can be 12h to 16h. Exemplarily, it can be 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h or any value within the range composed of any two of the above values.

[0057] In some embodiments, the atmosphere of the secondary sintering can be air atmosphere, oxygen-rich atmosphere or pure oxygen atmosphere, which is beneficial to keeping the metal ions in the ternary cathode material in the oxidized state. Further, the atmosphere of the primary sintering can be oxygen-rich atmosphere or pure oxygen atmosphere.

[0058] Compared with the prior art, the preparation method of the ternary cathode material provided by the embodiment of the present application has the following beneficial effects: 1. By mixing a ternary cathode material precursor, a lithium source, and a doping material containing Y2O3 and performing a primary sintering, bulk doping of yttrium elements is achieved, which is beneficial to reducing lithium-nickel mixing in the matrix material and inhibiting lattice collapse induced by phase transformation, enhancing the oxygen evolution resistance of the layered structure of the ternary cathode material from the bulk level.

[0059] 2. By mixing the matrix material and a coating material containing YSZ and performing a secondary sintering to form a coating layer, YSZ has good conductivity and stable structure, which is beneficial to improving the ionic conductivity and structural stability of the coating layer, improving the long-term cycle stability of the ternary cathode material at high temperatures, and meeting the urgent requirements of power lithium-ion batteries for high energy density, long cycle life, and excellent high-temperature performance. In addition, performing the secondary sintering in the temperature range of 300°C to 500°C is beneficial to the elemental interdiffusion of YSZ and yttrium elements in the matrix material at the interface, forming a gradient transition, reducing the thermal expansion coefficient of the matrix material, thereby reducing the interfacial shear stress between the matrix material and the coating layer, effectively alleviating the problem of thermal stress concentration between the coating layer and the substrate material, and further improving the structural stability of the ternary cathode material.

[0060] 3. The yttrium elements doped in the bulk can optimize the lattice structure of YSZ in the coating layer through interdiffusion. The introduced oxygen vacancy defects can significantly improve the lithium-ion mobility of the coating layer. At the same time, by stabilizing the cubic zirconia crystal form of the coating layer, the volume expansion and contraction caused by high-temperature phase transformation of the coating layer are inhibited, further improving the ionic conductivity and thermomechanical stability of the coating layer.

[0061] 4. This preparation method is simple and efficient, which is beneficial to the large-scale production of ternary cathode materials and has excellent commercial prospects.

[0062] The embodiment of the present application also provides an electrochemical device (such as a battery), which includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet includes a positive electrode active material, and the positive electrode active material is the ternary cathode material as described above.

[0063] The electrochemical device prepared by using the aforementioned ternary cathode material has the advantages of high electrochemical performance, high voltage tolerance, high structural stability, high cycle stability, high safety, and long life, etc., and can be applied to long-term high-temperature cycling use, meeting the requirements of high energy density, excellent high-temperature performance, and long cycle life for electrochemical devices.

[0064] The aforementioned ternary cathode material, its preparation method, and the electrochemical device are further described below through specific examples.

[0065] Example 1 Step S1: Select a ternary cathode material precursor Ni 0.68 Co 0.12 Mn 0.2(OH)2, the ternary cathode material precursor, lithium hydroxide, and the doping material Y2O3 are added to a high-speed mixer and mixed evenly at a speed of 1400 rpm. Among them, the ratio of the number of moles of lithium in the lithium source to the total number of moles of transition metals in the ternary cathode material precursor is 1.05:1, and the mass percentage of yttrium element in Y2O3 in the total mass of the ternary cathode material precursor and lithium hydroxide is 0.1 wt%; the evenly mixed mixture is sintered once in a pure oxygen atmosphere, the temperature of the first sintering is 950 °C, the time is 16 h, and after cooling and pulverizing, a matrix material doped with yttrium element is obtained.

[0066] Step S2: The matrix material and the coating material YSZ are put into a high-speed mixer and mixed evenly. Among them, the chemical formula of YSZ is (ZrO2) 1-x (Y2O3) x , x = 0, and the added YSZ accounts for 0.2 wt% of the matrix material; the evenly mixed mixture is sintered twice in a pure oxygen atmosphere, the temperature of the second sintering is 450 °C, and the time is 10 h, to obtain a single-crystalline ternary cathode material with yttrium element doping and a YSZ coating layer.

[0067] Example 2 The difference from Example 1 is that: in Step S2, the chemical formula of the coating material YSZ is (ZrO2) 1-x (Y2O3) x , x = 0.03, that is, Zr 0.97 Y 0.06 O 2.03 , ZrO2 doped with 3 mol% Y2O3 (3YSZ). Other steps are basically the same as those in Example 1, please refer to Example 1.

[0068] Example 3 The difference from Example 1 is that: in Step S2, the chemical formula of the coating material YSZ is (ZrO2) 1-x (Y2O3) x , x = 0.05, that is, Zr 0.95 Y 0.1 O 2.05 , ZrO2 doped with 5 mol% Y2O3 (5YSZ). Other steps are basically the same as those in Example 1, please refer to Example 1.

[0069] Example 4 The difference from Example 1 is that: in Step S2, the chemical formula of the coating material YSZ is (ZrO2) 1-x (Y2O3) x , x = 0.08, that is, Zr 0.98 Y 0.16 O 2.08, 8 mol% Y2O3-doped ZrO2 (8YSZ). Other steps are basically the same as those in Example 1, please refer to Example 1.

[0070] Comparative Example 1 The difference from Example 1 is that in step S2, the coating material YSZ is not added. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0071] Comparative Example 2 The difference from Example 1 is that in step S1, the doping material Y2O3 is not added. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0072] The following tests were carried out on the ternary cathode materials obtained in Examples 1-4 and Comparative Examples 1-2.

[0073] Test method: 1. Scanning electron microscopy (SEM) test: A JSM-IT210 model scanning electron microscope was used. This model of instrument has high-resolution imaging ability and can clearly observe the microscopic morphology and structural characteristics of the cathode material. The acceleration voltage was 2.00 kV and the magnification was 10,000 times.

[0074] 2. Electrochemical performance test: Battery preparation: The ternary cathode material, conductive agent SP and PVDF were mixed evenly at a mass ratio of 90:5:5. The rotation speed during pasting was 2000 rpm for 90 min. After coating and cutting, it was vacuum dried at 120 °C for 12 h, transferred to a glove box, a lithium metal sheet was used as the negative electrode, and the battery case was CR2032 to make a button cell.

[0075] Performance test: A LAND battery test system was used for constant current charge and discharge tests. During the test, the working voltage range for the first discharge capacity test was 2.8 V to 4.4 V, the temperature was 25 °C, and at a current density of 0.1 C, the first discharge capacity and the first efficiency (1st Eff) of the battery were measured. The working voltage range for the cycle performance test was 2.8 V to 4.4 V, the temperature was 45 °C, and it was cycled 400 times at a current density of +1 C / -1 C to measure the capacity retention rate of the battery.

[0076] 3. Particle size test: Mastersizer 3000 laser diffraction technology was used to measure the particle size. After 5 min of internal ultrasonic dispersion, the particle size distribution was tested. When the laser beam passed through the dispersed particle sample, the particle size measurement was completed by measuring the intensity of the scattered light. Then the data was used to analyze and calculate the particle size distribution of the particles forming the scattered spectrogram. D50: is the median particle size, the particle size corresponding to when the cumulative particle size distribution percentage of a sample reaches 50%.

[0077] The relevant test results of Examples 1-4 and Comparative Examples 1-2 are shown in Table 1.

[0078] The above results show that: As Figure 2 and Figure 3 shown, the ternary cathode material in Example 4 is single crystal, with a dense coating layer on the surface. The coating layer particles adhere to the surface of the matrix material to form an obvious rough and dense outer surface of the coating layer. In contrast, the surface of the ternary single crystal cathode material in Comparative Example 1 is bare and smooth, indicating that the 8YSZ coating layer has good adhesion and forms a good combination with the matrix of the ternary single crystal cathode material.

[0079] Combined with Table 1 and Figure 4 it can be seen that the first discharge capacity and the first efficiency of the ternary cathode materials in Examples 1-4 are both at a relatively high level. Among them, the first charge capacity of the battery made of the ternary cathode material in Example 4 reached 205.72 mAh / g, and the first efficiency reached 91.63%. While the first charge capacity of Comparative Example 1 was only 201.29 mAh / g, and the first efficiency was only 89.47%. The first charge capacity of Comparative Example 2 was only 199.53 mAh / g, and the first efficiency was only 89.04%. From the perspective of rate performance, compared with Comparative Examples 1-2, in Example 1, Y2O3 was doped in the bulk and ZrO2 was coated on the surface. The yttrium element in the bulk could diffuse into the coating layer to optimize the lattice structure of the YSZ coating layer, thereby further improving the conductivity of the coating layer. In Examples 2-4, Y2O3 was doped in the bulk and YSZ was coated on the surface. YSZ has good ionic conductivity, which can promote the transport of lithium ions in the ternary cathode material and improve the electrochemical performance of the ternary cathode material. The Y2O3 in the bulk can further increase the oxygen vacancy defects in YSZ and improve the ionic transport rate of YSZ. Therefore, the rate performance of the batteries in Examples 1-4 has been improved. In addition, in Examples 1-4, as the content of Y2O3 in the YSZ coating layer increased, YSZ obtained a higher oxygen vacancy concentration, thereby optimizing the ionic conduction performance. The oxygen vacancy concentration in YSZ was positively correlated with the Y doping amount, but excessive doping (Y2O3 > 8 mol%) would cause lattice distortion and reduce the mechanical strength. Therefore, 8YSZ can balance the coating layer strength (improvement of biaxial bending strength) and conductivity, and has high application prospects.

[0080] Combined with Table 1 and Figure 5It can be seen that in Examples 1-4, the ternary cathode material has yttrium element doping and YSZ coating. During the high-temperature cycling process, the yttrium elements in the matrix material and the coating layer in Examples 2-4 diffuse mutually, alleviating the thermal stress concentration between the coating layer and the matrix material. The layered structure of the ternary cathode material is well maintained, and there are fewer side reactions with the electrolyte. The cycling stability of the prepared battery at high temperature is better than that of Comparative Examples 1-2. After 400 cycles, the capacity retention rates of Examples 1-4 are still at a relatively high level, all higher than 76%, while that of Comparative Example 1 is only 74.03% after 400 cycles. Among them, the battery prepared from the ternary cathode material coated with 8YSZ in Example 4 has the highest cycling retention rate, and its capacity retention rate is increased by 10.17% compared with the comparative example, significantly improving the long-term high-temperature life of the material.

[0081] Therefore, in Examples 1-4 of the present application, by doping yttrium elements in the matrix material and coating YSZ on the surface of the matrix material, the two act synergistically to improve the ionic conductivity, structural stability and cycling performance of the ternary cathode material, endowing the battery with good rate performance and enabling it to stably cycle at high temperature for a long time.

[0082] It can be understood that the above embodiments are merely exemplary embodiments adopted to illustrate the principle of the present invention. However, the present invention is not limited thereto. For those of ordinary skill in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also regarded as the protection scope of the present invention.

Claims

1. A ternary cathode material, characterized in that, It includes a matrix material and a coating layer located on the surface of the matrix material. Among them, Y2O3 is doped in the matrix material, and yttria-stabilized zirconia is contained in the coating layer. The chemical formula of the yttria-stabilized zirconia is (ZrO2) 1-x (Y2O3) x , where 0 ≤ x < 1.

2. The ternary cathode material according to claim 1, wherein The chemical formula of the yttrium-stabilized zirconia is (ZrO2) 1-x (Y2O3) x , where 0 < x < 1, and the yttrium elements in the matrix material and the yttrium elements in the coating layer diffuse into each other to form a gradient transition of yttrium elements.

3. The ternary cathode material according to claim 1, wherein The yttria-stabilized zirconia includes Zr 0.97 Y 0.06 O 2.03 、Zr 0.95 Y 0.1 O 2.05 and Zr 0.92 Y 0.16 O 2.08 and at least one of them.

4. The ternary cathode material according to claim 1, wherein The thickness of the coating layer is 1 nm to 10 nm; and / or The average particle size D50 of the particles of the ternary cathode material is 3 μm to 4 μm.

5. A method for preparing a ternary cathode material, characterized in that Comprising: Mixing a ternary cathode material precursor, a lithium source, and a doping material containing Y2O3 and performing a first sintering to obtain a matrix material doped with yttrium element; And Mix the matrix material doped with yttrium with the coating material containing yttria-stabilized zirconia and perform secondary sintering so that the coating material forms a coating layer on the surface of the matrix material to obtain the ternary cathode material, where the chemical formula of the yttria-stabilized zirconia is (ZrO2) 1-x (Y2O3) x , where 0 ≤ x < 1.

6. The preparation method of the ternary cathode material according to claim 5, characterized in that, During the second sintering, the yttrium element in the matrix material and the yttrium element in the coating layer diffuse mutually to form a gradient transition of the yttrium element.

7. The preparation method of the ternary cathode material according to claim 5, characterized in that, The mass percentage of the yttrium element in Y2O3 in the total mass of the ternary cathode material precursor and the lithium source is 0.001 wt% to 10 wt%; and / or The doping material further comprises at least one of Al2O3, SiO2, MgO, and ZrO2.

8. The preparation method of the ternary cathode material according to claim 5, characterized in that, The mass percentage of the yttria-stabilized zirconia in the matrix material is 0.001 wt% to 5 wt%; and / or The yttria-stabilized zirconia includes Zr 0.97 Y 0.06 O 2.03 、Zr 0.95 Y 0.1 O 2.05 and Zr 0.92 Y 0.16 O 2.08 and at least one of them.

9. The preparation method of the ternary cathode material according to claim 5, wherein, The temperature of the first sintering is 750 °C to 1000 °C, and the time of the first sintering is 6 h to 18 h; and / or The temperature of the second sintering is 300 °C to 500 °C, and the time of the second sintering is 6 h to 18 h.

10. An electrochemical device, characterized in that, The electrochemical device includes a positive electrode plate, a negative electrode plate, a separator, and an electrolyte, wherein the positive electrode plate includes a positive electrode active material, and the positive electrode active material is the ternary cathode material according to any one of claims 1 to 4 or the ternary cathode material prepared by the preparation method of the ternary cathode material according to any one of claims 5 to 9.

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