Cathode active material, preparation method thereof, secondary battery and device

Abstract

The invention relates to a positive electrode active material, a preparation method thereof, a secondary battery and a device. The chemical formula of the positive electrode active material is LiaNibCocMndAleAfOx, a is greater than or equal to 0.95 and less than or equal to 1.3, b is greater than or equal to 0.5 and less than or equal to 0.96, c is greater than or equal to 0 and less than or equal to 0.35, d is greater than or equal to 0 and less than or equal to 0.35, e is greater than 0 and less than or equal to 0.05, f is greater than 0 and less than or equal to 0.05, x is greater than or equal to 2 and less than or equal to 2.2, and b + c + d + e + f = 1; a comprises at least one of Zr, B, W, Ti, Y, Sr, Nb, Sb, Na, Mg, Ba, Si, Sn, V, P, Ca, In and Mo; the positive electrode active material comprises a core body, a first coating layer arranged on the surface of the core body and a second coating layer arranged on the surface of the first coating layer, the mass percentage content of the Al element on the surface of the positive electrode active material is X1 wt%, the mass percentage content of the Al element at the position 100 nm deep from the surface of the positive electrode active material is X2 wt% in the direction from the surface of the positive electrode active material to the center, the mass percentage content of the Al element at the position 200 nm deep from the surface of the positive electrode active material is X3 wt%, X1 is smaller than X2, and X3 is smaller than X2. Therefore, the positive electrode active material has excellent calendar life, cycle performance and storage performance.

Description

Technical Field

[0001] The present application relates to the field of energy storage, and in particular to a positive electrode active material and a preparation method thereof, a secondary battery, and a device. Background Art

[0002] Lithium-ion batteries are an essential energy storage solution for modern electronic devices and electric vehicles. Improving the energy density of lithium-ion battery cathodes primarily relies on three approaches: high voltage, high compaction, and high nickel content. High-nickel ternary cathode materials, due to their considerable electron transferability, exhibit high discharge capacity, thus meeting the demand for high energy density. However, structural degradation of high-nickel ternary cathode materials and interfacial instability with the electrolyte can lead to voltage decay. These detrimental surface behaviors include surface structural reconstruction, stress-induced cracking, electrolyte decomposition, transition metal (TM) dissolution, and surface chemical instability. The surface structure and chemical properties of cathode materials play an important role in determining structural and interfacial stability, regulating the reversibility of lithium ion insertion / deintercalation at the interface, and influencing the kinetics of interfacial reactions during charge / discharge. Therefore, coating cathode materials is often used to improve their structural stability and electrochemical performance by reducing the direct contact area between the cathode material and the electrolyte.

[0003] However, current positive electrode active materials and preparation methods thereof, secondary batteries and devices still need to be improved. Summary of the Invention

[0004] The inventors found that although the positive electrode material can be coated to reduce its contact with the electrolyte and thus improve the structural stability and electrochemical performance, it is necessary to consider the Li +The conduction rate is low, and the coating layer is mostly an unstable amorphous lithium compound, which will decompose in the later stage of the cycle. In order to solve the above problems, the present application provides a positive electrode active material and a preparation method thereof, a secondary battery and a device. The present application performs a double-layer coating on the core of the positive electrode active material to form an outer coating layer (second coating layer) of a fast ion conductor with a high coverage rate and an inner coating layer (first coating layer) containing Al elements. On the one hand, the outer coating layer reduces the direct contact area between the core and the electrolyte, thereby improving the surface stability and calendar life of the positive electrode active material; on the other hand, an inner coating layer (first coating layer) containing Al elements is arranged between the outer coating layer and the core, and the Al element in the positive electrode active material is controlled to be enriched on the surface of the inner coating layer (X1<X2, X3<X2). This structural design can enable the inner coating layer to have a high content of Al elements relative to the outer coating layer and the core. The inner coating layer can serve as a phase change stabilizer in the high-delithiation state near the surface layer, effectively improving the stability of the layered structure under the high-delithiation state, and significantly reducing the cycle impedance (DCR) growth rate. As a result, the cathode active material comprises a core with high specific capacity and a double-layer coating with high structural stability. Both the core and the double-layer coating are electrochemically active, achieving both high specific capacity and cycling stability. Based on the above improvements, the cathode active material of the present application has at least one of the following advantages: excellent material structural stability, calendar life, cycling performance, and storage performance.

[0005] The first aspect of the present application provides a positive electrode active material, wherein the chemical formula of the positive electrode active material is Li a Ni b Co c Mn d Al e A f O x , wherein, 0.95≤a≤1.3, 0.5≤b≤0.96, 0≤c≤0.35, 0≤d≤0.35, 0<e≤0.05, 0<f≤0.05, 2≤x≤2.2, b+c+d+e+f=1; A includes at least one of Zr, B, W, Ti, Y, Sr, Nb, Sb, Na, Mg, Ba, Si, Sn, V, P, Ca, In and Mo; the positive electrode active material includes: a core body, a first coating layer provided on the surface of the core body and a second coating layer provided on the surface of the first coating layer; wherein the mass percentage content of the Al element on the surface of the positive electrode active material is X1 wt%, and the mass percentage content of the Al element at a depth of 100 nm from the surface of the positive electrode active material along the direction from the surface of the positive electrode active material to the center is X2 wt%, and the mass percentage content of the Al element at a depth of 200 nm from the surface of the positive electrode active material is X3 wt%, wherein X1<X2, X3<X2.

[0006] The second aspect of the present application provides a method for preparing a positive electrode active material, which comprises: mixing a core body precursor, a lithium salt and a second compound containing an element M1, and performing a first calcination treatment to obtain a core body; wherein M1 comprises at least one of Al, Zr, B, W, Ti, Y, Sr, Nb, Sb, Na, Mg, Ba, Si, Sn, V, P, Ca, In and Mo; mixing the core body and a third compound containing the element Al, and performing a second calcination treatment to obtain a first product; mixing the first product, a fourth compound containing the element Al and a fifth compound containing the element M2, and performing a third calcination treatment to obtain the positive electrode active material; wherein M2 comprises at least one of B, W, Ti, Y, P, Zr and Si.

[0007] The present application also provides a secondary battery, which includes the positive electrode active material described above or the positive electrode active material formed by the preparation method described above.

[0008] The present application also provides a device, which includes the secondary battery described above.

[0009] The beneficial effects of this application are:

[0010] The positive electrode active material of the present application is formed by double-layer coating of its core to form an outer coating layer (second coating layer) containing Al with high coverage and an inner coating layer (first coating layer) containing Al elements. On the one hand, the outer coating layer is a fast ion conductor coating layer, and reduces the direct contact area between the core and the electrolyte, reduces the side reaction between the core and the electrolyte, and, during the preparation of the outer coating layer, the Al source can react with the residual alkali (such as Li2CO3, LiOH) to reduce the amount of residual alkali on the surface. Further, the outer coating layer can consume HF in the electrolyte during the battery cycle, alleviate the gas production problem, and improve the positive electrode. The surface stability and calendar life of the active material improve the cycle performance; on the other hand, an inner coating layer (first coating layer) containing Al elements is provided between the outer coating layer and the core body, and the Al element in the positive electrode active material is controlled to be enriched on the surface of the inner coating layer (X1<X2, X3<X2). This structural design can make the inner coating layer have a high content of Al elements relative to the outer coating layer and the core body. The high-concentration Al slightly doped inner coating layer can serve as a phase change stabilizer in the high-delithiation state near the surface layer, effectively improving the stability of the layered structure under the high-delithiation state, inhibiting the surface structure phase change, and significantly reducing the cycle impedance (DCR) growth rate. As a result, the positive electrode active material has a core body with high specific capacity and a double-layer coating layer with high structural stability. Both the core body and the double-layer coating layer are electrochemically active, and have both high specific capacity and cycle stability. Based on the above improvements, the positive electrode active material of the present application has at least one of the following advantages: excellent material structure stability, calendar life, cycle performance and storage performance. Description of the drawings:

[0011] Figures 1-2 This is a scanning electron microscope image of the positive electrode active material in the prior art.

[0012] Figures 3-4 1 is a scanning electron microscope image of a positive electrode active material according to one embodiment of the present application.

[0013] Figure 5 4 is a scanning electron microscope image of a cross section of a positive electrode active material according to one embodiment of the present application.

[0014] Figure 6 is the mass percentage content of the Al element in the positive electrode active material according to one embodiment of the present application that varies with surface depth. DETAILED DESCRIPTION

[0015] For the sake of clarity, this application only specifically discloses certain numerical ranges. However, any lower limit may be combined with any upper limit to form an unspecified range; and any lower limit may be combined with other lower limits to form an unspecified range, and similarly, any upper limit may be combined with any other upper limit to form an unspecified range. In addition, each individually disclosed point or single value may itself serve as a lower limit or upper limit and be combined with any other point or single value, or with other lower limits or upper limits, to form an unspecified range.

[0016] Unless otherwise specified, the terms used in this application have the commonly understood meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values ​​of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application).

[0017] A list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0018] The present application will be further described below in conjunction with specific embodiments. It should be understood that these specific embodiments are only used to illustrate the present application and are not used to limit the scope of the present application.

[0019] 1. Cathode Active Materials

[0020] The first aspect of the present application provides a positive electrode active material, wherein the chemical formula of the positive electrode active material is Li a Ni b Co c Mn d Al e A f O x , wherein, 0.95≤a≤1.3, 0.5≤b≤0.96, 0≤c≤0.35, 0≤d≤0.35, 0<e≤0.05, 0<f≤0.05, 2≤x≤2.2, b+c+d+e+f=1; A includes at least one of Zr, B, W, Ti, Y, Sr, Nb, Sb, Na, Mg, Ba, Si, Sn, V, P, Ca, In and Mo; the positive electrode active material includes: a core body, a first coating layer provided on the surface of the core body and a second coating layer provided on the surface of the first coating layer; wherein the mass percentage content of the Al element on the surface of the positive electrode active material is X1 wt%, and the mass percentage content of the Al element at a depth of 100 nm from the surface of the positive electrode active material along the direction from the surface of the positive electrode active material to the center is X2 wt%, and the mass percentage content of the Al element at a depth of 200 nm from the surface of the positive electrode active material is X3 wt%, wherein X1<X2, X3<X2.

[0021] Thus, the positive electrode active material of the present application forms an outer coating layer (second coating layer) containing Al with high coverage and an inner coating layer (first coating layer) containing Al elements by double-coating its core. On the one hand, the outer coating layer is a fast ion conductor coating layer, and reduces the direct contact area between the core and the electrolyte, reduces the side reaction between the core and the electrolyte, and, during the preparation of the outer coating layer, the Al source can react with the residual alkali to reduce the amount of residual alkali on the surface. Furthermore, the outer coating layer can consume HF during the battery cycle, alleviate the gas production problem, and improve the surface stability of the positive electrode active material. Qualitative and calendar life, improve cycle performance; on the other hand, an inner coating layer (first coating layer) containing Al elements is provided between the outer coating layer and the core body, and the Al element in the positive electrode active material is controlled to be enriched on the surface of the inner coating layer (X1<X2, X3<X2). This structural design can make the inner coating layer have a high content of Al elements relative to the outer coating layer and the core body. The high-concentration Al slightly doped inner coating layer can be used as a phase change stabilizer in the high-delithiation state near the surface layer, effectively improving the stability of the layered structure under the high-delithiation state, inhibiting the surface structure phase change, and significantly reducing the cycle impedance (DCR) growth rate. As a result, the positive electrode active material has a core body with high specific capacity and a double-layer coating layer with high structural stability. Both the core body and the double-layer coating layer are electrochemically active, and have both high specific capacity and cycle stability. Based on the above improvements, the positive electrode active material of the present application has at least one of the following advantages: excellent material structure stability, calendar life, cycle performance and storage performance.

[0022] In some embodiments, the chemical formula of the positive electrode active material is Li a Ni b Co c Mn d Al e A f O x, wherein 0.95≤a≤1.3, 0.5≤b≤0.96, 0≤c≤0.35, 0≤d≤0.35, 0<e≤0.05, 0<f≤0.05, 2≤x≤2.2, b+c+d+e+f=1; A comprises at least one of Zr, B, W, Ti, Y, Sr, Nb, Sb, Na, Mg, Ba, Si, Sn, V, P, Ca, In, and Mo. In some embodiments, a is 0.95, 0.97, 0.99, 1, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, or any value therebetween. In some embodiments, b is 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 0.96, or any value therebetween. In some embodiments, 0.8≤b≤0.96. In some embodiments, c is 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, or any value therebetween, d is 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, or any value therebetween, e is 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, or any value therebetween, f is 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, or any value therebetween, and x is 2, 2.05, 2.1, 2.15, 2.2, or any value therebetween.

[0023] In some embodiments, the positive electrode active material includes: a core body, a first coating layer provided on the surface of the core body, and a second coating layer provided on the surface of the first coating layer. In some embodiments, the core body includes secondary particles formed by agglomeration of primary particles. Specifically, the first coating layer and the second coating layer may be located on the surface of the primary particles of the core body. More specifically, the first coating layer and the second coating layer may be located on the surface of the primary particles forming the outermost layer of secondary particles in the core body. More specifically, the first coating layer and the second coating layer may be located on the surface of the primary particles forming the outermost layer of secondary particles in the core body and at least a portion of the primary particles inside (i.e., the primary particles located in the non-outermost position of the matrix). It should be noted that, in this application, "primary particles (primary particles)" refers to crystals within a scale range of several μm, which do not contain grain boundaries, and the crystallographic orientation of the crystals inside is basically consistent, such as what is commonly referred to as single crystal materials. "Secondary particles" refers to a collection of primary particles of many oriented grains, and the interior of the secondary particles is based on a lattice-type periodic structure, but is isotropic, such as what is commonly referred to as polycrystalline materials.

[0024] In some embodiments, the mass percentage content of Al on the surface of the positive electrode active material is X1 wt %, the mass percentage content of Al at a depth of 100 nm from the surface of the positive electrode active material along the direction from the surface to the center of the positive electrode active material is X2 wt %, and the mass percentage content of Al at a depth of 200 nm from the surface of the positive electrode active material is X3 wt %, wherein X1 < X2 and X3 < X2. Thus, along the direction from the surface to the center of the positive electrode active material, the Al content at a depth of 100 nm is greater than the Al content at the surface of the positive electrode active material, and the Al content at a depth of 100 nm is greater than the Al content at a depth of 200 nm, that is, the Al element in the positive electrode active material is enriched at a depth of 100 nm. More specifically, the position at the depth of 100 nm is the surface of the first coating layer or the interior of the first coating layer (a position close to its surface), so the Al element is enriched on the surface of the first coating layer, and the first coating layer has a high content of Al element relative to the second coating layer and the core.

[0025] It should be noted that the values ​​X1, X2, and X3 of the Al element content at the above-mentioned different positions can be tested and obtained by the following method: using the above-mentioned positive electrode active material to prepare an electrode pole piece, cutting the electrode pole piece to form an electrode pole piece cross-section, bombarding and polishing the electrode pole piece cross-section with a plasma beam, and using a high-power scanning electron microscope (SEM) and an energy dispersive spectrometer (EDS) to analyze the types and contents of component elements in a micro area (a range of several μm below the electron beam bombardment point, and the result obtained is the average value within the volume) of the positive electrode active material cross-section in the cross-section of the electrode pole piece, for example, performing a point scan or line scan test along the surface of the cross-section to the center of the cross-section to obtain the content value of each element (such as Al) at the test point or test line in the positive electrode active material. Therefore, the design of the content of Al elements in different positions mentioned above is conducive to achieving effective protection of the ternary core without excessively reducing the capacity. The first coating layer is a high-concentration Al slightly doped coating layer, which can serve as a phase change stabilizer for the highly delithiated state near the surface layer, inhibiting the surface structure phase change. The second coating layer is a low-concentration lithium-aluminum related compound, which can serve as a fast lithium ion conductor and HF consumer.

[0026] In some embodiments, the mass percentage content of the Al element at the first position in the first coating layer is X4 wt%, and the value of X4 decreases as the distance from the first position to the surface of the positive electrode active material increases. In some embodiments, the mass percentage content of the Al element at the second position in the core body is X5 wt%, and the value of X5 decreases as the distance from the second position to the surface of the positive electrode active material increases. That is to say, along the direction from the surface of the positive electrode active material to the center, the mass percentage content of the Al element in the first coating layer gradually decreases, and the mass percentage content of the Al element in the core body gradually decreases. Similarly, the content change trend of the Al element at the first position in the first coating layer and the second position in the core body can be obtained by the above-mentioned EDS test. As a result, the Al element can be enriched on the surface of the first coating layer, and the first coating layer has a high content of Al element relative to the core body. The high-concentration Al-doped first coating layer can serve as a phase transition stabilizer for the high-delithiation state near the surface layer, effectively improving the stability of the layered structure under the high-delithiation state, inhibiting the surface structure phase change, and significantly reducing the cycle impedance (DCR) growth rate.

[0027] In some embodiments, X5<X4. Thus, the first coating layer can have a high Al content relative to the core. This high-concentration Al-doped first coating layer can serve as a phase transition stabilizer in the highly delithiated state near the surface layer, effectively improving the stability of the layered structure in the highly delithiated state, inhibiting surface structural phase transitions, and significantly reducing the growth rate of the direct current impedance (DCR).

[0028] In some embodiments, the mass ratio of the Al element in the first coating layer to the Al element in the second coating layer is (1.2 to 2):1. In some embodiments, the mass ratio of the Al element in the first coating layer to the Al element in the second coating layer is 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1 or any value therebetween. In some embodiments, the mass ratio of the Al element in the first coating layer to the Al element in the second coating layer is (1.4 to 1.8):1. This is beneficial to improving structural stability; when the above mass ratio is too large, the coating thickness of the first coating layer will be too thick, resulting in a longer lithium ion transmission channel, and ultimately leading to a decrease in the discharge capacity of the ternary positive electrode material; when the above mass ratio is too small, the thickness of the first coating layer will be too thin, and the harmful phase change in the surface area cannot be effectively suppressed, which will ultimately lead to an increase in the cycle internal resistance of the ternary positive electrode material and a decrease in the cycle performance.

[0029] In some embodiments, the core comprises Li a’ Ni b’ Co c’ Mn d’ A' f’ Ox’ , wherein 0.95≤a'≤1.3, 0.5≤b'≤0.96, 0≤c'≤0.35, 0≤d'≤0.35, 0<f'≤0.05, 2≤x≤2.2, b'+c'+d'+f'=1; A' comprises at least one of Al, Zr, B, W, Ti, Y, Sr, Nb, Sb, Na, Mg, Ba, Si, Sn, V, P, Ca, In, and Mo. In some embodiments, a' is 0.95, 0.97, 0.99, 1, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, or any value therebetween. In some embodiments, b' is 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 0.96, or any value therebetween. In some embodiments, 0.8≤b'≤0.96. In some embodiments, c' is 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, or any value therebetween, d' is 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, or any value therebetween, f' is 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, or any value therebetween, and x' is 2, 2.05, 2.1, 2.15, 2.2, or any value therebetween.

[0030] In some embodiments, the first coating layer includes at least one of LiAlO2 and Li5AlO4.

[0031] In some embodiments, the second coating layer includes Al2O3, LiAlO2 and a first compound, wherein the first compound includes at least one of lithium oxides of B, W, Ti, and Y. In some embodiments, the first compound includes B2O3, BO2, Li3BO3, WO3, Li2WO4, TiO2, Li4Ti5O 12 , at least one of Y2O3, LiYO2, Li3PO4, ZrO2, Li2ZrO3, SiO2 and Li2SiO3.

[0032] In some embodiments, the specific surface area of ​​the positive electrode active material is 0.1 m 2 / g~1.2m 2 In some embodiments, the specific surface area of ​​the positive electrode active material is 0.1 m 2 / g, 0.2m 2 / g, 0.4m 2 / g, 0.6m 2 / g, 0.8m 2 / g、1m 2 / g, 1.2m 2 / g or any value in between.

[0033] In some embodiments, the thickness of the first cladding layer is 50 nm to 400 nm. In some embodiments, the thickness of the first cladding layer is 50 nm, 80 nm, 100 nm, 200 nm, 300 nm, 400 nm, or any value therebetween. In some embodiments, the thickness of the second cladding layer is 10 nm to 100 nm. In some embodiments, the thickness of the second cladding layer is 10 nm, 20 nm, 50 nm, 80 nm, 100 nm, or any value therebetween.

[0034] In some embodiments, based on the total mass of the positive electrode active material, the mass percentage content of the first coating layer is 0.1 wt % to 4 wt %. In some embodiments, based on the total mass of the positive electrode active material, the mass percentage content of the first coating layer is 0.1 wt %, 0.3 wt %, 0.5 wt %, 0.7 wt %, 0.9 wt %, 1 wt %, 1.3 wt %, 1.5 wt %, 1.7 wt %, 1.9 wt %, 2 wt %, 2.5 wt %, 3 wt %, 3.5 wt %, 4 wt %, or any value therebetween.

[0035] In some embodiments, based on the total mass of the positive electrode active material, the mass percentage content of the second coating layer is 0.05 wt % to 0.5 wt %. In some embodiments, based on the total mass of the positive electrode active material, the mass percentage content of the second coating layer is 0.05 wt %, 0.07 wt %, 0.09 wt %, 0.1 wt %, 0.15 wt %, 0.2 wt %, 0.25 wt %, 0.3 wt %, 0.35 wt %, 0.4 wt %, 0.45 wt %, 0.5 wt %, or any value therebetween.

[0036] In some embodiments, the Dv50 of the positive electrode active material is 2μm to 8μm. In some embodiments, the Dv50 of the positive electrode active material is 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8.0μm or any value therebetween. In some embodiments, the Dv50 of the positive electrode active material is 2.5μm to 4.5μm. It should be noted that the type of the positive electrode active material is a nickel-containing ternary material, and further a nickel-containing transition metal oxide. Dv50 is the particle size corresponding to when the cumulative volume distribution percentage of the positive electrode active material reaches 50%, for example, obtained by testing using a laser particle size instrument.

[0037] In summary, the positive electrode active material of the present application has at least one of the following advantages: excellent material structure stability, calendar life, cycle performance and storage performance.

[0038] 2. Preparation Method of Positive Electrode Active Materials

[0039] This application also provides a method for preparing a positive electrode active material. The positive electrode active material prepared by this method can be the positive electrode active material described above. Thus, this method can have all the characteristics and advantages of the positive electrode active material described above, and will not be described in detail again. The preparation method includes:

[0040] S100: mixing a core precursor, a lithium salt, and a second compound containing an element M1, and performing a first calcination treatment to obtain a core; wherein M1 includes at least one of Al, Zr, B, W, Ti, Y, Sr, Nb, Sb, Na, Mg, Ba, Si, Sn, V, P, Ca, In, and Mo;

[0041] S200: mixing the core body and a third compound containing elemental Al, and performing a second calcination process to obtain a first product;

[0042] S300: Mixing the first product, the fourth compound containing element Al, and the fifth compound containing element M2, and performing a third calcination treatment to obtain the positive electrode active material; wherein M2 includes at least one of B, W, Ti, Y, P, Zr, and Si.

[0043] Thus, the positive electrode active material prepared by this method is formed by double-layer coating of its core body to form an Al-containing outer coating layer (second coating layer) with high coverage and an inner coating layer (first coating layer) containing Al elements. On the one hand, the outer coating layer is a fast ion conductor coating layer, and reduces the direct contact area between the core body and the electrolyte, reduces the side reaction between the core body and the electrolyte, and, during the preparation of the outer coating layer, the Al source can react with the residual alkali to reduce the amount of residual alkali on the surface. Further, the outer coating layer can consume HF during the battery cycle, alleviate the gas production problem, and improve the surface of the positive electrode active material. Stability and calendar life, improve cycle performance; on the other hand, an inner coating layer (first coating layer) containing Al elements is set between the outer coating layer and the core body, and the Al element in the positive electrode active material is controlled to be enriched on the surface of the inner coating layer (X1<X2, X3<X2). This structural design can make the inner coating layer have a high content of Al elements relative to the outer coating layer and the core body. The high-concentration Al slightly doped inner coating layer can be used as a phase change stabilizer in the high-delithiation state near the surface layer, effectively improving the stability of the layered structure under the high-delithiation state, inhibiting the surface structure phase change, and significantly reducing the cycle impedance (DCR) growth rate. As a result, the positive electrode active material has a core body with high specific capacity and a double-layer coating layer with high structural stability. Both the core body and the double-layer coating layer are electrochemically active, and have both high specific capacity and cycle stability. In addition, this method can alleviate the Ni on the surface of the high-nickel ternary positive electrode active material. 3+ Instability leads to environmental sensitivity issues. Compared to lithium cobalt oxide coatings, double-layer Al-containing coatings offer lower cost and better storage performance. Based on the above improvements, the positive electrode active material of the present application has at least one of the following advantages: excellent material structural stability, calendar life, cycle performance, and storage performance.

[0044] In some embodiments, the temperature of the first calcination treatment is 650° C. to 800° C., and the time is 8 hours to 15 hours. In some embodiments, the temperature of the first calcination treatment is 650° C., 700° C., 750° C., 800° C., or any value therebetween, and the time is 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, or any value therebetween.

[0045] In some embodiments, the second calcination treatment is performed at a temperature of 600° C. to 700° C. for a time of 8 to 12 hours. In some embodiments, the second calcination treatment is performed at a temperature of 600° C., 620° C., 640° C., 650° C., 660° C., 680° C., 700° C., or any value therebetween, for a time of 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, or any value therebetween.

[0046] In some embodiments, the third calcination treatment is performed at a temperature of 250° C. to 400° C. for a time of 8 to 12 hours. In some embodiments, the third calcination treatment is performed at a temperature of 250° C., 260° C., 280° C., 300° C., 320° C., 340° C., 350° C., 360° C., 380° C., 400° C., or any value therebetween, for a time of 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, or any value therebetween.

[0047] It should be noted that when forming the first coating layer, under the high-temperature driving force, the Al element will penetrate into the core body. As the depth increases, due to the increase in doping diffusion resistance, the difficulty of Al element penetration increases, and the Al element content in the core body gradually decreases from the surface to the center. At the same time, when forming the first coating layer, the closer to the core body, the more Al elements in the first coating layer penetrate into the core body, and the less Al is left, and Al will be enriched on the surface of the first coating layer, and the Al element content in the first coating layer gradually decreases from the surface to the center. Similarly, when forming the second coating layer, the Al element in the second coating layer will also penetrate into the first coating layer, and Al will be enriched on the surface of the second coating layer. However, the Al content in the second coating layer is significantly less than the Al content in the first coating layer, so it still cannot affect the Al content in the first coating layer and its change trend. The Al element content in the first coating layer still gradually decreases from the surface to the center.

[0048] In some embodiments, the core precursor comprises Ni g Co h Mn j (OH)2, 0.5≤g≤0.96, 0≤h≤0.3, 0≤j≤0.4, g+h+j=1. In some embodiments, g is 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 0.96, or any value therebetween. In some embodiments, h is 0, 0.1, 0.15, 0.2, 0.25, 0.3, or any value therebetween.

[0049] In some embodiments, the lithium salt includes at least one of Li2CO3 and LiOH.

[0050] In some embodiments, the second compound includes at least one selected from the group consisting of carbonates, oxides, fluorides, and hydroxides of Al, Zr, B, W, Ti, Y, Sr, Nb, Sb, Na, Mg, Ba, Si, Sn, V, P, Ca, In, and Mo.

[0051] In some embodiments, the third compound includes at least one of Al2O3, Al(OH)3, and Al2(CO3)3.

[0052] In some embodiments, the fourth compound includes Al2O3.

[0053] In some embodiments, the fifth compound includes at least one selected from the group consisting of carbonates, oxides, fluorides, and hydroxides of B, W, Ti, Y, P, Zr, and Si.

[0054] In some embodiments, the molar ratio of the core precursor, the lithium salt, and the second compound is 1:(1.02-1.1):(0.001-0.005). In some embodiments, the molar ratio of the core precursor, the lithium salt, and the second compound is 1:1.02:0.001, 1:1.05:0.001, 1:1.1:0.001, 1:1.02:0.003, 1:1.05:0.003, 1:1.1:0.003, 1:1.02:0.005, 1:1.05:0.005, 1:1.1:0.005, or any value therebetween.

[0055] In some embodiments, the mass ratio of the core to the third compound is 1:(0.001-0.05). In some embodiments, the mass ratio of the core to the third compound is 1:0.001, 1:0.005, 1:0.01, 1:0.02, 1:0.03, 1:0.04, 1:0.05, or any value therebetween.

[0056] In some embodiments, the mass ratio of the first product, the fourth compound, and the fifth compound is 1:(0.0005-0.01):(0.0005-0.01). In some embodiments, the mass ratio of the first product, the fourth compound, and the fifth compound is 1:0.0005:0.0005, 1:0.0005:0.0005, 1:0.0005:0.0005, 1:0.01:0.005, 1:0.01:0.005, 1:0.05:0.005, 1:0.001:0.01, 1:0.01:0.01, 1:0.05:0.01, or any value therebetween.

[0057] 3. Secondary batteries

[0058] The present application also provides a secondary battery comprising the aforementioned positive electrode active material or the positive electrode active material formed by the aforementioned preparation method. Thus, the device can have all the features and advantages of the aforementioned positive electrode active material or preparation method, which are not further described here.

[0059] In some embodiments, the secondary battery includes a positive electrode sheet, a negative electrode sheet, and an electrolyte. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material disposed on the positive electrode current collector, wherein the positive electrode active material includes the positive electrode active material described above or the positive electrode active material prepared by the method described above.

[0060] In some embodiments, the positive electrode active material layer further includes a binder and a conductive material. The binder improves the bonding between the positive electrode active material particles and also improves the bonding between the positive electrode active material and the current collector. In some embodiments, the binder includes: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic (ester) styrene-butadiene rubber, epoxy resin or nylon, etc. In some embodiments, the conductive material includes: carbon-based materials, metal-based materials, conductive polymers and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0061] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer substrate.

[0062] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material disposed on the negative electrode current collector. The negative electrode active material includes at least one of a carbon-based material, a silicon-based material, a tin-based material, a phosphorus-based material, and metallic lithium. In some embodiments, the carbon-based material includes at least one of graphite, soft carbon, hard carbon, carbon nanotubes, and graphene; the silicon-based material includes at least one of silicon, a silicon alloy, a silicon oxide, and a silicon-carbon compound; the tin-based material includes at least one of tin, a tin oxide, and a tin alloy; and the phosphorus-based material includes phosphorus and / or a phosphorus-carbon complex. In some embodiments, the negative electrode active material includes a carbon-based material. In some embodiments, the negative electrode active material includes a carbon-based material and a silicon-based material, and the weight percentage of the silicon-based material is 0% to 30% based on the weight of the negative electrode active material. The weight percentage of the silicon-based material is 0%, 2%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 25%, 30%, or any value therebetween, based on the weight of the negative electrode active material. Based on the mass of the negative electrode active material, the mass percentage content of the silicon-based material is 0% to 5%.Based on the mass of the negative electrode active material, the mass percentage content of the silicon-based material is 0% to 15%.

[0063] In some embodiments, the negative electrode active material layer further comprises a binder and a conductive agent. In some embodiments, the binder comprises: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic (ester) styrene-butadiene rubber, epoxy resin or nylon, etc. In some embodiments, the conductive agent comprises: a carbon-based material, a metal-based material, a conductive polymer and a mixture thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0064] In some embodiments, the negative electrode current collector comprises: copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0065] In some embodiments, the electrolyte includes a lithium salt, a solvent, and an additive.

[0066] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluorosulfonyl (LiTf), lithium bis(fluorosulfonyl)imide (LiFSI), lithium trifluoromethanesulfonate, lithium (trifluoromethylsulfonyl)(perfluorobutylsulfonyl)imide (LiFNFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(pentafluoroethylsulfonyl)imide (LiBETI), lithium bis(fluoromalonate)borate (LiBFMB), lithium bisoxalatoborate (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium difluorobisoxalatophosphate, and lithium 4,5-dicyano-2-(trifluoromethyl)imidazolium (LiTDI).

[0067] In some embodiments, the weight percentage of the lithium salt is 4% to 25% based on the weight of the electrolyte. In some embodiments, the weight percentage of the lithium salt is 4%, 4.5%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, or any value therebetween. In some embodiments, the weight percentage of the lithium salt is 6% to 18%.

[0068] In some embodiments, the solvent includes at least one of a linear carbonate and a cyclic carbonate.

[0069] In some embodiments, linear carbonate is selected from at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, ethylpropyl carbonate and fluorinated linear carbonate. In some embodiments, cyclic carbonate includes at least one of ethylene carbonate, propylene carbonate and butylene carbonate. In some embodiments, organic solvent also includes non-fluorinated carboxylic acid ester, and non-fluorinated carboxylic acid ester is selected from at least one of methyl formate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate and gamma-butyrolactone.

[0070] In some embodiments, the weight percentage of the solvent is 40% to 80% based on the weight of the electrolyte. In some embodiments, the weight percentage of the solvent is 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any value therebetween. In some embodiments, the weight percentage of the solvent is 50% to 70%.

[0071] In some embodiments, the additive includes at least one of a cyclic carbonate containing a carbon-carbon double bond, a silyl-containing phosphate, and a nitrile compound salt. In some embodiments, the additive includes at least one of vinylene carbonate (VC), vinyl ethylene carbonate, tris(trimethylsilyl) phosphate (TMSP), tris(trimethylsilyl) borate (TMSB), succinonitrile, adiponitrile, glutaronitrile, and hexanetrinitrile. In some embodiments, the additive further includes at least one of methylene methyl disulfonate (MMDS), ethylene ethyl disulfonate, 1,3-propane sultone (1,3-PS), 1-propylene-1,3-sultone (PST), 1,4-butane sultone (1,4-BS), vinyl sulfate (DTD), 4-methylethylene sulfate (PCS), 4-ethylethylene sulfate (PES), 4-propylethylene sulfate (PEGLST), propylene sulfate (TS), ethylene sulfite (DTO), dimethyl sulfite (DMS) and diethyl sulfite (DES).

[0072] In some embodiments, the weight percentage of the additive is 0.05% to 10% based on the weight of the electrolyte. In some embodiments, the weight percentage of the additive is 0.05%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 6%, 7%, 8%, 9%, 10%, or any value therebetween. In some embodiments, the weight percentage of the additive is 0.1% to 5%.

[0073] In some embodiments, a separator is provided between the positive and negative electrode plates to prevent short circuits. The material and shape of the separator used in the embodiments of the present application are not particularly limited and may be any known prior art material. In some embodiments, the separator comprises a polymer or inorganic material, for example, formed from a material that is stable with the electrolyte of the present application.

[0074] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, film, or composite film having a porous structure, and the material of the substrate layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, polypropylene porous film, polyethylene porous film, polypropylene non-woven fabric, polyethylene non-woven fabric, or polypropylene-polyethylene-polypropylene porous composite film can be used.

[0075] A surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic layer.

[0076] The inorganic layer includes inorganic particles and a binder, wherein the inorganic particles include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder includes at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0077] The polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride and poly(vinylidene fluoride-hexafluoropropylene).

[0078] In some embodiments, the method for preparing the secondary battery includes providing an electrode assembly, injecting liquid, packaging, and forming. In some embodiments, the forming temperature is 40° C. to 50° C., for example, 41° C., 42° C., 43° C., 44° C., 45° C., 46° C., 47° C., 48° C., or 49° C.

[0079] In some embodiments, the formation comprises: charging to 3.9V at 0.05C current and standing for 24 hours at a temperature of 40°C to 50°C, for example, 45°C, and a pressure of 150kgf to 750kgf, for example, 600kgf, followed by charging to 4.2V at 0.1C, and then discharging to 2.5V at 0.2C.

[0080] In some embodiments, the secondary battery is a lithium secondary battery or a sodium secondary battery. In some embodiments, the lithium secondary battery includes, but is not limited to: a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0081] In some embodiments, the secondary battery may include an outer packaging, which may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.

[0082] In some embodiments, the shape of the secondary battery is not particularly limited, and it can be cylindrical, square, or any other shape.

[0083] In some embodiments, the present application also provides a battery module. The battery module includes the aforementioned secondary battery. The battery module of the present application utilizes the aforementioned secondary battery and therefore has at least the same advantages as the aforementioned secondary battery. The battery module of the present application may include multiple secondary batteries, the specific number of which can be adjusted based on the application and capacity of the battery module.

[0084] In some embodiments, the present application further provides a battery pack comprising the above-mentioned battery module. The number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0085] 4. Device

[0086] The present application also provides a device comprising at least one of the aforementioned secondary battery, battery module, and battery pack. Thus, the device can have all the features and advantages of the aforementioned secondary battery, battery module, or battery pack, which are not further described here.

[0087] In some embodiments, the device includes, but is not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, etc. To meet the device's requirements for high power and high energy density of secondary batteries, a battery pack or battery module may be used.

[0088] In other embodiments, the device may be a mobile phone, a tablet computer, a laptop computer, etc. The device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0089] Examples and Comparative Examples

[0090] Example 1

[0091] The preparation steps of the positive electrode active material are as follows: Step (1), using the intermittent coprecipitation method to prepare Ni with an average particle size of 12 μm 0.9 Co 0.05 Mn 0.05 (OH)2 precursor, weigh Ni in molar ratio 0.9 Co 0.05 Mn 0.05(OH)2: LiOH·H2O: ZrO2 = 1: 1.04: 0.0011 were mixed evenly, and the temperature was raised to 500°C at 3.5°C / min under an oxygen atmosphere for 2h, and then raised to 730°C at 2°C / min, and sintered at a constant temperature for 11h to obtain a core body; step (2), weighing the core body in step (1): Al2O3 = 1: 0.0028 according to the mass ratio, mixing in an industrial mixer for 20min, and placing the mixture under oxygen atmosphere at 2°C / min in is heated to 650° C. and sintered for 10 hours to obtain a first product having a first coating layer coated on the surface of the core body; step (3), weighing the first product in step (2) according to the mass ratio: Al2O3:H3BO3=1:0.0019:0.0057, mixing in an industrial mixer for 20 minutes, placing the mixture in an oxygen atmosphere and heating it to 300° C. at a rate of 2° C. / min, and sintering for 10 hours to obtain a positive electrode active material with a double-layer coated core body.

[0092] The preparation steps of the positive electrode sheet are as follows: the above-obtained positive electrode active material, CNT (conductive agent carbon nanotube) / Super-P (conductive carbon black), and binder polyvinylidene fluoride PVDF are mixed in N-methylpyrrolidone NMP at a weight ratio of positive electrode active material: CNT / Super-P:PVDF=95:(1.0 / 2.0):2, and after being fully homogenized, they are coated on a 12μm thick aluminum current collector, and then dried, rolled, hot pressed, and other steps to obtain the positive electrode sheet.

[0093] The preparation steps of the negative electrode plate are as follows: the negative electrode active material silicon oxide (SiOx, 0.5≤x≤1.5)-graphite composite (Si / C=5:95), the conductive agent acetylene black, the binder styrene-butadiene rubber SBR, the thickener sodium carboxymethyl cellulose CMCNa, and the polyacrylic acid PAA are added to deionized water in a weight ratio of 95:2:1.5:1:0.5, and after sufficient homogenization, they are coated on an 8μm thick copper current collector, and then the negative electrode plate is obtained through drying, rolling, hot pressing and other steps.

[0094] Preparation of electrolyte: In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), the lithium salt LiPF6 was fully dissolved in a mixed solution of EC / DEC / EMC (ethylene carbonate / diethyl carbonate / ethyl methyl carbonate) = 25 / 20 / 55 to prepare a 1 mol / L solution.

[0095] Isolation film: PP / PE / PP (polypropylene / polyethylene / polypropylene) three-layer composite isolation film is used.

[0096] Preparation of lithium-ion secondary battery: The prepared positive electrode sheet, separator (PP / PE / PP three-layer composite film), and negative electrode sheet are overlapped in sequence, with the separator located between the positive electrode sheet and the negative electrode sheet to obtain a bare cell. The bare cell is placed in a punched aluminum-plastic film soft package shell. After sufficient drying, the prepared electrolyte is injected. The battery is placed at 45°C for 48 hours and formed in a high-temperature fixture (the formation conditions are: temperature 45°C, pressure 600kgf, 0.05C current charging to 3.9V and standing for 24 hours, then 0.1C charging to 4.2V, and then 0.2C discharge to 2.5V. After secondary sealing, conventional capacity division is carried out.

[0097] Examples 2 to 9

[0098] Examples 2 to 9 are implemented on the basis of Example 1 by adjusting the core material, the feeding type and feeding amount of the coating material corresponding to the first coating layer, the feeding type and feeding amount of the coating material corresponding to the second coating layer, etc. Specific adjustment measures and detailed data are shown in Table 1.

[0099] Comparative Example 1

[0100] The other steps in Comparative Example 1 are the same as those in Example 1, except that the steps for preparing the positive electrode active material are different, as follows:

[0101] The preparation steps of the positive electrode active material are as follows: Step (1), using the intermittent coprecipitation method to prepare Ni with an average particle size of 12 μm 0.9 Co 0.05 Mn 0.05 (OH)2 precursor, weigh Ni in molar ratio 0.9 Co 0.05 Mn 0.05 (OH)2: LiOH·H2O: ZrO2 = 1: 1.04: 0.0011, mix uniformly, increase the temperature to 500°C at a rate of 3.5°C / min in an oxygen atmosphere and keep constant for 2h, then increase the temperature to 730°C at a rate of 2°C / min, and sinter at a constant temperature for 11h to obtain a core body; step (2), weigh the core body in step (1) according to the mass ratio: Al2O3: H3BO3 = 1: 0.0019: 0.0057, mix in an industrial mixer for 20min, place the mixture in an oxygen atmosphere, increase the temperature to 300°C at a rate of 2°C / min, and sinter for 10h to obtain a single-layer coated core body positive electrode active material.

[0102] Comparative Examples 2-3

[0103] The other steps in Comparative Example 2 are the same as those in Example 1, except that the second coating layer is not provided.

[0104] The other steps in Comparative Example 2 are the same as those in Example 1, except for the type and amount of the coating material corresponding to the first coating layer. Detailed data are shown in Table 1.

[0105] Test Method

[0106] 1. Gram capacity test

[0107] After standing for 4 hours at 25°C, perform the initial charge and discharge capacity test on the lithium-ion secondary battery. The test conditions are: charge at 0.1C to 4.2V, charge at constant voltage to 0.05C, then stand for 5 minutes, and then discharge at 0.1C to 2.5V. Record the initial charge capacity (CC0 mAh / g) and the initial discharge capacity (DC0 mAh / g). Calculate the initial coulombic efficiency (CC0 / CC0) × 100%. DC0 is the 4.2V-0.1C capacity.

[0108] 2. Capacity recovery rate at 60℃

[0109] Before storage, charge the battery to 4.2V at a constant current rate of 1C, then charge it at a constant voltage until the current is less than 0.05C. After standing for 5 minutes, discharge it to 2.5V at a rate of 1C, and record the discharge capacity C0. After storing it at 60°C for 30 days, remove the battery and place it at room temperature (more than 4 hours), discharge it to 2.5V at a rate of 1C, record the holding capacity C1, and then charge it to 4.2V at a constant current rate of 1C, then charge it to 2.5V at a constant voltage until the current is less than 0.05C. After standing for 5 minutes, discharge it to 2.5V at a rate of 1C, and record the discharge capacity. Repeat this charge and discharge cycle 3 times, take the average value of the three discharge capacities C2, and the capacity recovery rate after storing it at 60°C for 30 days is (C0-C2) / C0.

[0110] 3. Cycle life at 45℃

[0111] The lithium-ion secondary battery was cycled at a constant temperature of 45°C (cell capacity 150Ah, charged to 4.2V at a current of 150A, then discharged to 2.5V at a current of 150A, and so on) until the capacity retention rate decayed to 80%, and the total number of cycles was recorded.

[0112] 4. Cross-sectional scanning electron microscope test

[0113] The electrode pieces are cut to form electrode section, the electrode section is bombarded and polished by a plasma beam, and a cross-sectional image of the material is observed by imaging using a high-magnification scanning electron microscope (SEM).

[0114] Among them, SEM-CP (Cross Section Polisher-Scanning Electron Microscope) is a cross-section polishing-scanning electron microscope. Cross-section polishing (CP) uses an argon ion beam to polish the sample. Its principle is as follows: a high-voltage electric field is used to ionize argon gas to produce an ion state. The generated argon ions bombard the sample surface at high speed under the action of the acceleration voltage, and the sample is eroded layer by layer to achieve the polishing effect.

[0115] 5. Energy Dispersive Spectrometer (EDS) test

[0116] The electrode pieces are cut to form electrode piece cross sections, which are bombarded and polished by a plasma beam, and used in conjunction with SEM to analyze the types and contents of component elements in a micro area (a range of several μm below the electron beam bombardment point, and the result obtained is the average value within the volume) of the material section in the cross section of the electrode piece using EDS. For example, a point scanning test is performed along the surface of the cross section to the center of the cross section to test the mass percentage content of the Al element on the surface of the positive electrode active material, the mass percentage content of the Al element at a depth of 100 nm from the surface of the positive electrode active material, and the mass percentage content of the Al element at a depth of 200 nm from the surface of the positive electrode active material, so as to obtain the element content value of the Al element in the positive electrode active material that changes with the surface depth.

[0117] Test results

[0118] Table 1

[0119]

[0120]

[0121] Table 2

[0122]

[0123]

[0124] Note: The Al content is the mass percentage of Al in the total of other transition metals (such as Ni, Co, Mn, Al, etc.) except Li as determined by EDS testing.

[0125] It can be seen from Examples 1 to 8 and Comparative Examples 1 to 3 that the present application forms an outer coating layer (second coating layer) containing Al with a high coverage rate and an inner coating layer (first coating layer) containing Al elements by double-coating the core body. On the one hand, the outer coating layer is a fast ion conductor coating layer, and reduces the direct contact area between the core body and the electrolyte, reduces the side reaction between the core body and the electrolyte, and, during the preparation of the outer coating layer, the Al source can react with the residual alkali to reduce the amount of residual alkali on the surface. Further, the outer coating layer can consume HF during the battery cycle, alleviate the gas production problem, and improve the positive On the other hand, an inner coating layer (first coating layer) containing Al elements is provided between the outer coating layer and the core body, and the Al elements in the positive electrode active material are enriched on the surface of the inner coating layer. This structural design enables the inner coating layer to have a high content of Al elements relative to the outer coating layer and the core body. The high-concentration Al-doped inner coating layer can serve as a phase change stabilizer for the high-delithiation state near the surface layer, effectively improving the stability of the layered structure under the high-delithiation state, inhibiting the surface structure phase change, and significantly reducing the cycle impedance (DCR) growth rate. As a result, the positive electrode active material has a core body with high specific capacity and a double-layer coating layer with high structural stability. Both the core body and the double-layer coating layer are electrochemically active, and have both high specific capacity and cycle stability.

[0126] Comparing Comparative Example 1 with Examples 1 to 6, it can be seen that the first coating layer is not provided in Comparative Example 1, and a double-layer coated positive electrode active material in which the Al element is enriched on the surface of the first coating layer cannot be formed, resulting in low capacity recovery rate, poor cycle life, and a significantly increased DCR growth rate.

[0127] Comparing Comparative Example 2 with Examples 1 to 6, it can be seen that in Comparative Example 2, no second coating layer is provided, and a double-layer coated positive electrode active material in which Al element is enriched on the surface of the first coating layer is not formed, that is, no fast ion conductor coating layer is formed, resulting in a high residual alkali amount, a low final capacity recovery rate, and a low gram capacity.

[0128] Comparing Comparative Example 3 with Examples 1 to 6, it can be seen that although Comparative Example 3 has a double-layer coating, the mass percentage content of the Al element along the direction from the surface of the positive electrode active material to the core body is decreasing. Furthermore, along the direction from the surface of the positive electrode active material to the center, the Al element content at a depth of 100 nm from the surface is less than the Al element content on the surface, while in Example 1, the Al element content at a depth of 100 nm from the surface is greater than the Al element content on the surface. As a result, the harmful phase change in the surface region cannot be effectively suppressed in Comparative Example 3, and the cycle life is ultimately poor and the DCR growth rate increases.

[0129] Figures 1-2 This is a scanning electron microscope image of the positive electrode active material in Comparative Example 1.

[0130] Figures 3-4 is a scanning electron microscope image of the positive electrode active material according to Example 1 of the present application. Figures 1-2 In this application, due to the formation of a double coating layer, the grain boundaries of the single crystal (primary particle) in the figure are relatively blurred, and Figures 1-2 If only a single-layer cladding is formed without providing the first cladding layer, the grain boundaries of the single crystal will be obvious.

[0131] Figure 5 1 is a cross-sectional scanning electron microscope image of the positive electrode active material according to Example 1 of the present application.

[0132] Figure 6 is the Al element mass percentage content value in the positive electrode active material according to Example 1 of the present application as the surface depth of the positive electrode active material changes. Specifically, based on Figure 5 On the line indicated by the middle arrow, EDS is used to analyze the types and contents of the component elements in the cross section of the positive electrode active material to obtain the element content value of the Al element in the positive electrode active material that changes with the surface depth. Figure 6 The abscissa represents depth from the surface, and the ordinate represents the mass percentage of Al at the corresponding depth. As can be seen from the figure, in Example 1, the Al content at a depth of 100 nm, along the direction from the surface to the core of the positive electrode active material, is greater than that at the surface, and the Al content at a depth of 100 nm is greater than that at a depth of 200 nm. In contrast, in Comparative Example 1, the Al content at a depth of 100 nm is less than that at the surface.

[0133] Although some exemplary embodiments of the present application have been illustrated and described, the present application is not limited to the disclosed embodiments. On the contrary, those skilled in the art will recognize that some modifications and changes may be made to the described embodiments without departing from the spirit and scope of the present application as described in the appended claims.

Claims

1. A positive electrode active material, characterized in that The chemical formula of the positive electrode active material is Li a Ni b Co c Mn d Al e A f O x , wherein, 0.95≤a≤1.3, 0.5≤b≤0.96, 0≤c≤0.35, 0≤d≤0.35, 0<e≤0.05, 0<f≤0.05, 2≤x≤2.2, b+c+d+e+f=1; A includes at least one of Zr, B, W, Ti, Y, Sr, Nb, Sb, Na, Mg, Ba, Si, Sn, V, P, Ca, In and Mo; The positive electrode active material comprises: a core body, a first coating layer arranged on the surface of the core body, and a second coating layer arranged on the surface of the first coating layer; The mass percentage content of the Al element on the surface of the positive electrode active material is X1 wt%, the mass percentage content of the Al element at a depth of 100 nm from the surface of the positive electrode active material along the direction from the surface of the positive electrode active material to the center is X2 wt%, and the mass percentage content of the Al element at a depth of 200 nm from the surface of the positive electrode active material is X3 wt%, wherein X1<X2, X3<X2.

2. The positive electrode active material according to claim 1, characterized in that The mass percentage content of the Al element at the first position in the first coating layer is X4 wt%, and the value of X4 decreases as the distance between the first position and the surface of the positive electrode active material increases; and / or The mass percentage content of the Al element at the second position in the core body is X5 wt %, and the value of X5 decreases as the distance between the second position and the surface of the positive electrode active material increases.

3. The positive electrode active material according to claim 2, characterized in that The mass ratio of Al element in the first coating layer to Al element in the second coating layer is (1.2-2):1; and / or X5<X4.

4. The positive electrode active material according to claim 1, characterized in that The core body includes Li a’ Ni b’ Co c’ Mn d’ A' f’ O x’ , wherein, 0.95≤a'≤1.3, 0.5≤b'≤0.96, 0≤c'≤0.35, 0≤d'≤0.35, 0<f'≤0.05, 2≤x'≤2.2, b'+c'+d'+f'=1; A' comprises at least one of Al, Zr, B, W, Ti, Y, Sr, Nb, Sb, Na, Mg, Ba, Si, Sn, V, P, Ca, In and Mo; and / or The core body includes secondary particles formed by agglomeration of primary particles.

5. The positive electrode active material according to claim 1, characterized in that The first coating layer comprises at least one of LiAlO2 and Li5AlO4; and / or The second coating layer includes Al2O3, LiAlO2 and a first compound, wherein the first compound includes B2O3, BO2, Li3BO3, WO3, Li2WO4, TiO2, Li4Ti5O 12 , at least one of Y2O3, LiYO2, Li3PO4, ZrO2, Li2ZrO3, SiO2 and Li2SiO3.

6. The positive electrode active material according to claim 1, characterized in that The Dv50 of the positive electrode active material is 2 μm to 8 μm; and / or The specific surface area of ​​the positive electrode active material is 0.1 m 2 / g~1.2m 2 / g; and / or The thickness of the first coating layer is 50 nm to 400 nm; and / or The thickness of the second coating layer is 10 nm to 100 nm; and / or The mass ratio of the Al element in the first coating layer to the Al element in the second coating layer is (1.4-1.8):1; and / or Based on the total mass of the positive electrode active material, the mass percentage content of the first coating layer is 0.1wt% to 4wt%; and / or Based on the total mass of the positive electrode active material, the mass percentage content of the second coating layer is 0.05 wt % to 0.5 wt %.

7. A method for preparing a positive electrode active material, characterized in that: include: A core precursor, a lithium salt, and a second compound containing an element M1 are mixed and subjected to a first calcination process to obtain a core; wherein M1 includes at least one of Al, Zr, B, W, Ti, Y, Sr, Nb, Sb, Na, Mg, Ba, Si, Sn, V, P, Ca, In, and Mo; The core body and a third compound containing elemental Al are mixed and subjected to a second calcination process to obtain a first product; The first product, the fourth compound containing element Al and the fifth compound containing element M2 are mixed and subjected to a third calcination treatment to obtain the positive electrode active material; wherein M2 includes at least one of B, W, Ti, Y, P, Zr and Si.

8. The preparation method according to claim 7, characterized in that The temperature of the first calcination treatment is 650° C. to 800° C., and the time is 8 hours to 15 hours; and / or The temperature of the second calcination treatment is 600° C. to 700° C., and the time is 8 h to 12 h; and / or The temperature of the third calcination treatment is 250° C. to 400° C., and the time is 8 hours to 12 hours; and / or The core precursor includes Ni g Co h Mn j (OH)2, 0.5≤g≤0.96, 0≤h≤0.3, 0≤j≤0.4, g+h+j=1; and / or The lithium salt comprises at least one of Li2CO3 and LiOH; and / or The second compound comprises at least one selected from the group consisting of carbonates, oxides, fluorides and hydroxides of Al, Zr, B, W, Ti, Y, Sr, Nb, Sb, Na, Mg, Ba, Si, Sn, V, P, Ca, In and Mo; and / or The third compound comprises at least one of Al2O3, Al(OH)3 and Al2(CO3)3; and / or The fourth compound comprises Al2O3; and / or The fifth compound comprises at least one selected from the group consisting of carbonates, oxides, fluorides and hydroxides of B, W, Ti, Y, P, Zr and Si; and / or The molar ratio of the core precursor, the lithium salt and the second compound is 1:(1.02-1.1):(0.001-0.005); and / or The mass ratio of the core body to the third compound is 1:(0.001-0.05); and / or The mass ratio of the first product, the fourth compound and the fifth compound is 1:(0.0005-0.01):(0.0005-0.01).

9. A secondary battery, characterized in that: The positive electrode active material comprises the positive electrode active material according to any one of claims 1 to 6 or the positive electrode active material formed by the preparation method according to any one of claims 7 to 8.

10. A device, characterized in that: The secondary battery according to claim 9 is included.

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