Positive electrode active material, positive electrode sheet, and electrochemical device

Abstract

This invention discloses a positive electrode active material, a positive electrode sheet, and an electrochemical device, relating to the field of electrochemical energy storage. The positive electrode active material includes lithium cobalt oxide in the O2 phase doped with Al and M, wherein the molar ratio of Co, Al, and M in the lithium cobalt oxide is (1-b-c):b:c, satisfying the relationship 0.00004≤c×b×(E M‑0 / 600)≤0.00085; M is a metallic element with a bond energy of 600-800 kJ / mol with O, and the ionic radius of the M ion in its six-coordinate structure is 0.6-0.75 Å. This application uses Al doping in lithium cobalt oxide to enhance interfacial and structural stability, and broadens the Li-O bond through the large ionic radius and high metal-oxygen bond energy of the cation M. + Migration channel, alleviate Li + The lattice stress during the insertion / extraction process enhances the structural robustness of O2-phase lithium cobalt oxide from multiple dimensions, including bulk material structure, ion transport kinetics, and surface stability, thereby improving its overall performance under high-pressure cycling.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage, and more particularly to positive electrode active materials, positive electrode sheets, and electrochemical devices. Background Technology

[0002] O2-phase lithium cobalt oxide has attracted considerable attention from researchers due to its significantly higher specific capacity (>240 mAh / g) compared to O3-phase lithium cobalt oxide (≈220 mAh / g). However, due to differences in oxygen layer stacking, the electrostatic repulsion between Li⁺ and Co³⁺ within the LiO6 and CoO6 octahedral layers in the O2-phase lithium cobalt oxide crystal structure is more pronounced than in the O3 phase, easily inducing interlayer slip and leading to particle cracking and pulverization. Furthermore, under high voltage (>4.5 V), deep delithiation results in a highly oxidized surface, making the O2-phase lithium cobalt oxide more prone to side reactions with the electrolyte than the O3 phase, causing problems such as surface oxygen evolution and cobalt dissolution.

[0003] Furthermore, the O2 phase lithium cobalt oxide is a thermodynamically metastable phase, and when the sintering temperature exceeds approximately 250°C, it undergoes an irreversible transformation to the O3 phase. The low-temperature sintering requirement of the O2 phase lithium cobalt oxide greatly limits the applicability of subsequent high-temperature coating processes, resulting in a very limited range of coating materials to choose from.

[0004] Therefore, although O2 phase lithium cobalt oxide has a higher initial efficiency under high pressure, its cycle stability under high pressure is worse than that of O3 phase lithium cobalt oxide, which seriously limits its promotion in practical applications. Summary of the Invention

[0005] This invention provides a positive electrode active material, a positive electrode sheet, and an electrochemical device. Al doping enhances the material's structural and interfacial stability and reduces side reactions between the material interface and the electrolyte. Simultaneously, by introducing large-radius cations M with high metal-oxygen bond energy at cobalt sites for doping, and synergistically increasing the M doping amount, the structural stability of the cobalt-oxygen layer is enhanced, the lithium-ion migration channels are broadened, and the Li-ion degradation rate is reduced. + With Co 3+ The electrostatic repulsion between them is reduced, and the lattice stress during the lithium-ion insertion / extraction process is effectively alleviated, thereby improving the structural robustness of the O2 phase lithium cobalt oxide material and ultimately achieving comprehensive performance optimization under high pressure, high temperature and long cycle conditions.

[0006] To address the aforementioned technical problems, one objective of this invention is to provide a positive electrode active material, including lithium cobalt oxide doped with Al and M, wherein the molar ratio of Co, Al, and M in the lithium cobalt oxide is (1-bc):b:c; M is a metallic element with a bond energy of 600-800 kJ / mol to O, and the ionic radius of the M ion in its six-coordinate structure is 0.6-0.75 Å. The conditions b and c satisfy the following relationship: 0.00004 ≤ c × b × (E M-0 / 600)≤0.00085; where, E M-0 The values ​​represent the bond energies of M and O, in kJ / mol. The positive electrode active material has an O2 phase structure.

[0007] In some embodiments, the positive electrode active material includes a core layer, an intermediate layer, and a shell layer, wherein the chemical formula of the core layer is Li. 1-a1 Na a1 Co 1-b1-c1 Al b1 M c1 O2, where 0.001≤a1≤0.010, 0<b1≤0.01, 0<c1<0.07; The chemical formula of the intermediate layer is Li. 1-a2 Na a2 Co 1-b2-c2 Al b2 M c2 O2, where 0.001≤a2≤0.010, 0<b2≤0.02, 0<c2<0.07; The chemical formula of the shell is Li 1-a3 Na a3 Co 1-b3-c3 Al b3 M c3 O 2-d3 F d3 Where 0.001≤a3≤0.010, 0<b3≤0.03, 0<c3<0.07, 0<d3≤0.015.

[0008] In some implementations, b1 / b2 < 1, b2 / b3 < 1.

[0009] In some implementations, b1:b2:b3 is (0.5-2):(2.5-4):(4.5-6).

[0010] In some embodiments, the chemical formula of the shell contains b3 and d3, which satisfy the following relationship: 0.2 ≤ d3 / b3 ≤ 0.9.

[0011] In some embodiments, the chemical formula of the shell contains b3 and d3, which satisfy the following relationship: 0.4 ≤ d3 / b3 ≤ 0.7.

[0012] In some embodiments, in the chemical formula of the core layer, 1 ≤ c1 / a1 ≤ 4; in the chemical formula of the intermediate layer, 1 ≤ c2 / a2 ≤ 4; and in the chemical formula of the shell layer, 1 ≤ c3 / a3 ≤ 4.

[0013] In some embodiments, M is at least one of Cr, Zr, Ti, Ta, and Nb.

[0014] In some embodiments, the Dv50 particle size of the positive electrode active material is L, the thickness of the intermediate layer is 0.2×L to 0.3×L, and the thickness of the shell layer is 0.05×L to 0.1×L.

[0015] In some embodiments, the Dv50 particle size L of the positive electrode active material is 6-14 μm.

[0016] In some embodiments, the Dv10 particle size of the positive electrode active material is 3-5 μm.

[0017] In some embodiments, the Dv90 particle size of the positive electrode active material is 12-15 μm.

[0018] In some implementations, 0.0013 ≤ b1 ≤ 0.006.

[0019] In some implementations, 0.003 ≤ b2 ≤ 0.012.

[0020] In some implementations, 0.0045 ≤ b3 ≤ 0.018.

[0021] In some embodiments, the space group of the positive electrode active material is P63mc.

[0022] To address the aforementioned technical problems, a second objective of this invention is to provide a positive electrode sheet, including a positive electrode active material.

[0023] To address the aforementioned technical problems, a third objective of this invention is to provide an electrochemical device, including a positive electrode.

[0024] Compared with the prior art, the present invention has the following beneficial effects: 1. This application utilizes a large ionic radius and high metal-oxygen bond energy cation M in synergistic Al doping. Al doping can better enhance the material structure and interface stability and reduce side reactions between the material interface and the electrolyte. Since the bond energy between Al and oxygen is low, it is difficult to cope with the lattice stress caused by deep lithium insertion / extraction of O2 lithium cobalt oxide. Simultaneously utilizing M can enhance the structural stability of the cobalt oxide layer, widen the lithium-ion migration channel, and alleviate the lattice stress during the lithium-ion insertion / extraction process. Furthermore, when 0.00004 ≤ c × b × (E) is satisfied... M-0 When ( / 600)≤0.00085, the structural robustness of O2 phase lithium cobalt oxide materials can be improved from three dimensions: bulk material structure, ion transport dynamics, and surface stability.

[0025] 2. This application adopts gradient Al 3+Doping technology to construct Al from core to shell 3+ The distribution pattern shows a gradual increase in concentration, with high Al concentration on the surface. 3+ The design can better enhance the stability of the material interface and reduce side reactions between the material interface and the electrolyte. The core layer is less prone to side reactions with the electrolyte, therefore, low-concentration Al 3+ It can maintain a high lithium-ion diffusion capacity in the core of the material, and on the basis of maintaining a high lithium-ion diffusion capacity in the core, gradually enhance the structural stability of the surface region, and improve the comprehensive performance of O2 phase lithium cobalt oxide under high pressure and high temperature long cycle conditions.

[0026] 3. The shell of the positive electrode active material in this application is doped with F, through F... - Replace O 2- The site forms a strong bonding structure, which inhibits the formation of oxygen vacancies on the material surface during cycling, enhances the structural integrity of the near-surface region of the material, and when the molar doping amount of Al and F in the shell layer satisfies 0.4≤d / b≤0.7, the specific capacity and high-temperature cycling capacity retention of the material are optimal, resulting in excellent overall performance. Attached Figure Description

[0027] Figure 1 The XRD results are for the O2 phase lithium cobalt oxide positive electrode active material synthesized in Example 1 of this invention and the O3 phase lithium cobalt oxide positive electrode active material synthesized in Comparative Example 6. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0030] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0031] As used in this article: In these embodiments, unless otherwise specified, the portions and percentages are all by weight.

[0032] "And / or" is used to indicate that one or both of the described situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0033] In the description of this invention, it should be understood that the terms "upper", "lower", "left", "right", "top", "bottom", etc., indicating orientation or positional relationship are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0034] This invention provides a positive electrode active material, comprising lithium cobalt oxide doped with Al and M, wherein the molar ratio of Co, Al and M in the lithium cobalt oxide is (1-bc):b:c; M is a metallic element with a bond energy of 600-800 kJ / mol to O, and the ionic radius of the M ion in its six-coordinate structure is 0.6-0.75 Å. b and c satisfy the following relationship: 0.00004 ≤ c × b × (E M-0 / 600)≤0.00085; where, E M-0 The values ​​represent the bond energies of M and O, in kJ / mol. The positive electrode active material has an O2 phase structure.

[0035] This application utilizes Al doping to enhance the material structure and interfacial stability, and reduce side reactions between the material interface and the electrolyte. Simultaneously, by introducing large-radius cations M with high metal-oxygen bond energy at cobalt sites, the temperature resistance and structural stability of the cobalt-oxygen layer are enhanced, lithium-ion migration channels are broadened, and Li-ion degradation is reduced. + With Co 3+ This effectively mitigates the electrostatic repulsion between ions and alleviates the lattice stress during lithium-ion insertion / extraction. Furthermore, when 0.00004 ≤ c × b × (E) M-0When ( / 600)≤0.00085, the structural robustness of O2 phase lithium cobalt oxide materials can be improved, thereby achieving comprehensive performance optimization under high pressure, high temperature and long cycle conditions.

[0036] In some embodiments, the positive electrode active material includes a core layer, an intermediate layer, and a shell layer, wherein the chemical formula of the core layer is Li. 1-a1 Na a1 Co 1-b1-c1 Al b1 M c1 O2, where 0.001≤a1≤0.010, 0<b1≤0.01, 0<c1<0.07; The chemical formula of the intermediate layer is Li 1-a2 Na a2 Co 1-b2-c2 Al b2 M c2 O2, where 0.001≤a2≤0.010, 0<b2≤0.02, 0<c2<0.07; The chemical formula of the shell is Li 1-a3 Na a3 Co 1-b3-c3 Al b3 M c3 O 2-d3 F d3 Where 0.001≤a3≤0.010, 0<b3≤0.03, 0<c3<0.07, 0<d3≤0.015.

[0037] The positive electrode active material of this application is designed as a multi-layer structure, with fluorine ion doping performed separately in the shell layer, through F... - Replace part of O 2- This process forms a high-bond-energy stable structure, suppressing the generation of surface oxygen vacancies during cycling and enhancing the structural integrity of the near-surface region. The absence of fluorine ion doping in the core and shell layers reduces the overall fluorine content, preventing deintercalation and intercalation that would increase the initial and cycling impedance of lithium ions, thus reducing the material's specific capacity and cycling performance. Simultaneously, the appropriate amount of electrochemically inert Na residue in the layered material further supports the material structure, enhancing the structural robustness of the O2-phase lithium cobalt oxide material from multiple dimensions.

[0038] In some implementations, 0 < b1 ≤ 0.001, 0 < b2 ≤ 0.015, and 0 < b3 ≤ 0.002.

[0039] In some implementations, 0.001≤a1≤0.005, 0.001≤a2≤0.005, and 0.001≤a3≤0.005.

[0040] In some implementations, 0 < c1 ≤ 0.35, 0 < c2 ≤ 0.35, and 0 < c3 ≤ 0.35.

[0041] In some implementations, b1 / b2 < 1, b2 / b3 < 1.

[0042] In some implementations, b1:b2:b3 is (0.5-2):(2.5-4):(4.5-6).

[0043] The positive electrode active material design in this application does not have a multilayer structure and adopts a gradient Al 3+ Doping technology to construct Al from core to shell 3+ The gradually increasing concentration distribution pattern, while maintaining the high lithium-ion diffusion capacity of the material core, progressively enhances the structural stability of the surface region. Simultaneously, the introduction of large-radius cations M with high metal-oxygen bond energy at cobalt sites for doping enhances the temperature resistance and structural stability of the cobalt-oxygen layer, widens the lithium-ion migration channels, and reduces Li-ion diffusion capacity. + With Co 3+ This effectively reduces the electrostatic repulsion between the lithium ions and alleviates the lattice stress during lithium ion insertion / extraction. Further, fluorine ion doping is performed on the shell layer, using F... - Replace part of O 2- By forming a high bond energy stable structure, suppressing the generation of surface oxygen vacancies during cycling, and strengthening the structural integrity of the near-surface region, the structural robustness of O2 phase lithium cobalt oxide materials is improved from three dimensions: bulk structure, ion transport dynamics, and surface stability, thereby achieving comprehensive performance optimization under high pressure, high temperature, and long cycling conditions.

[0044] It should be noted that the elements and molar amounts of the core layer, intermediate layer and shell layer in the positive electrode active material can be obtained by inductively coupled plasma spectroscopy (ICP).

[0045] In some embodiments, the bond energy of M and O is a range of any one or any two of the following: 600 kJ / mol, 610 kJ / mol, 620 kJ / mol, 630 kJ / mol, 640 kJ / mol, 650 kJ / mol, 660 kJ / mol, 670 kJ / mol, 680 kJ / mol, 690 kJ / mol, 700 kJ / mol, 710 kJ / mol, 720 kJ / mol, 730 kJ / mol, 740 kJ / mol, 750 kJ / mol, 760 kJ / mol, 770 kJ / mol, 780 kJ / mol, 790 kJ / mol, and 800 kJ / mol.

[0046] In some embodiments, the ionic radius of the six-coordinate structure of the M ion is any one of 0.6 Å, 0.65 Å, 0.7 Å, 0.75 Å, or any value between any two.

[0047] Under high voltage conditions, cathode materials undergo deep charge-discharge cycles, resulting in severe lattice contraction during lithium insertion / extraction. In this application, the bond energies between M metal and O in the core, intermediate, and shell layers, as well as the ionic radius of the M ion's six-ligand structure, are within the aforementioned range. This enhances the structural stability of the cobalt-oxygen layer and expands the interlayer spacing, broadening the lithium-ion diffusion path and thus reducing the lithium-ion migration barrier. Conversely, excessively low bond energies are detrimental to crystal structure stability, while radii exceeding the aforementioned range can easily lead to material structural distortion, hindering lithium-ion migration.

[0048] It should be noted that the bond energy values ​​between M and O can be obtained from literature and the Langevin Chemical Handbook, and the ionic radius of the six-coordinate structure of the M ion can be obtained from literature.

[0049] In some embodiments, M is at least one of Cr, Zr, Ti, Ta, and Nb.

[0050] In some implementations, c×b×(E M-0 The value of / 600 is a range of any one of the following, or any two of the following values: 0.00004, 0.0001, 0.00015, 0.0002, 0.00025, 0.0003, 0.00035, 0.0004, 0.00045, 0.0005, 0.00055, 0.0006, 0.00065, 0.0007, 0.00075, 0.0008, and 0.00085.

[0051] Due to Al 3+ The low bond energy formed with O makes it difficult to cope with the lattice stress caused by deep lithium insertion / extraction of O2 lithium cobalt oxide. When aluminum is co-doped with M, a cation with a large ionic radius and high metal-oxygen bond energy, M can enhance the structural stability of the cobalt oxide layer, widen the lithium-ion migration channels, and reduce the Li-O bond energy. + With Co 3+ This effectively mitigates the electrostatic repulsion between ions and alleviates the lattice stress during lithium-ion insertion / extraction. Furthermore, when 0.00004 ≤ c × b × (E) M-0 When ( / 600)≤0.00085, it is beneficial to synergistically improve the stability of the crystal structure.

[0052] In some implementations, 0.001 ≤ b1 ≤ 0.018, 0.001 ≤ b2 ≤ 0.018, and 0.001 ≤ b3 ≤ 0.018. In some implementations, b1 is a range of any one or any two of the following values: 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.010, 0.011, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017, and 0.018.

[0053] In some implementations, b2 is a range of any one or any two of the following values: 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.010, 0.011, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017, and 0.018.

[0054] In some implementations, b3 is a range of any one or any two of the following values: 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.010, 0.011, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017, and 0.018.

[0055] In some implementations, 0.0013 ≤ b1 ≤ 0.006.

[0056] In some implementations, 0.003 ≤ b2 ≤ 0.012.

[0057] In some implementations, 0.0045 ≤ b3 ≤ 0.018.

[0058] This application employs gradient Al in the core layer, intermediate layer, and shell layer. 3+ Doping technology utilizes Al, an electrochemically inert element with an ionic radius comparable to cobalt. Its doping content has minimal impact on the bulk structure of materials, serving as a structural support. However, its low bond energy with oxygen makes it vulnerable to lattice stress caused by deep lithium insertion / extraction in lithium O2 cobalt oxide. This application controls the Al core... 3+ Lower doping levels can maintain a high lithium-ion diffusion capacity in the material core and construct a core-to-shell Al structure. 3+ A gradually increasing concentration distribution can enhance the structural stability of the surface region, such as the Al concentration in the core, intermediate, and shell layers. 3+ Excessive doping, excessive Al 3+ Doping reduces the lithium-ion diffusion capacity of a material, thereby reducing its specific capacity.

[0059] In some implementations, 0.0009 ≤ d3 ≤ 0.0054.

[0060] In some implementations, d3 is a range of any one or any two of 0.0009, 0.001, 0.0015, 0.002, 0.0025, 0.003, 0.0035, 0.004, 0.0045, 0.005, and 0.0054.

[0061] This application controls the F doping content in the shell layer of the positive electrode active material within the scope of this application, through F… - Replace part of O 2- It forms a high bond energy stable structure, suppresses the generation of surface oxygen vacancies during cycling, and enhances the structural integrity of the near-surface region; at the same time, it avoids excessive F doping, as excessive F doping in the shell layer will lead to an increase in the initial impedance and cycling impedance of lithium ion insertion and extraction, and a decrease in the material's specific capacity and cycling performance.

[0062] In some implementations, the chemical formulas of the shell, b3 and d3, satisfy the following relationship: 0.2 ≤ d3 / b3 ≤ 0.9.

[0063] In some implementations, the chemical formula of the shell contains d3 / b3, which is a range of any one or any two of the values ​​of 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9.

[0064] In some implementations, the chemical formulas of the shell, b3 and d3, satisfy the following relationship: 0.4 ≤ d3 / b3 ≤ 0.7.

[0065] The outermost shell of the positive electrode active material in this application uses F-doping for its anion sites. Al preferentially occupies cobalt sites, mainly acting as a support to enhance the surface structural stability of the material. However, it cannot resist the corrosion of HF acid. Therefore, F-doping is used... - Replace O 2- Strong bonding structures are formed at the sites. Appropriate amounts of F can replace surface anodes and partially react with residual lithium on the surface to form fluorides, inhibiting the formation of oxygen vacancies on the material surface during cycling, effectively resisting electrolyte corrosion, and enhancing the structural integrity of the near-surface region. However, excessive F will disrupt the layered structure of the material, leading to reduced surface dynamics. Furthermore, insufficient F content makes it difficult to stabilize the interface. Therefore, when the molar doping amounts of Al and F in the shell satisfy 0.4 ≤ d³ / b³ ≤ 0.7, a synergistic effect can be achieved, resulting in optimal specific capacity and high-temperature cycling capacity retention, and excellent overall performance.

[0066] In some embodiments, in the chemical formula of the core layer, 1 ≤ c1 / a1 ≤ 4; in the chemical formula of the intermediate layer, 1 ≤ c2 / a2 ≤ 4; and in the chemical formula of the shell layer, 1 ≤ c3 / a3 ≤ 4.

[0067] In the multilayer structure of the positive electrode active material of this application, an appropriate amount of electrochemically inert Na is dispersed within the layered material, which further supports the multilayer structure and improves the structural stability of the material. Simultaneously, by controlling the molar ratio of Na to M within the aforementioned range, M can broaden the lithium-ion migration channels and reduce Li-ion degradation. + With Co 3+ The electrostatic repulsion between them is effectively relieved, and the lattice stress during the lithium-ion insertion / extraction process is alleviated. Combined with the structural support of electrochemically inert Na, the overall long-cycle structural stability of the material is improved, and the cycle capacity retention rate of the material is further improved.

[0068] In some embodiments, the Dv50 particle size of the positive electrode active material is L, the thickness of the intermediate layer is 0.2×L to 0.3×L, and the thickness of the shell layer is 0.05×L to 0.1×L.

[0069] In some implementations, the thickness of the intermediate layer is 0.25×L and the thickness of the shell layer is 0.08×L.

[0070] The thicknesses of the core layer, intermediate layer, and shell layer in the positive electrode active material of this application meet the requirements of this application and can be used in conjunction with Al. 3+ Gradient doping enhances structural stability from the inside out, while maintaining high lithium-ion diffusion capacity in the core and structural stability in the surface region, thereby improving overall performance under high-voltage, long-cycle conditions.

[0071] It should be noted that the thickness of the intermediate layer and the shell layer was tested using a thickness gauge (Mar C1202M).

[0072] In some embodiments, the particle size of the core layer is 0.15×L to 0.2×L.

[0073] In some embodiments, the Dv50 particle size L of the positive electrode active material is 6-14 μm, more preferably 6-10 μm.

[0074] In some embodiments, the Dv50 particle size L of the positive electrode active material is any one or a range between any two of 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, 10.5 μm, 11.5 μm, 12 μm, 12.5 μm, 13 μm, 13.5 μm, and 14 μm.

[0075] In some implementations, the Dv10 particle size of the positive electrode active material is 3-5 μm.

[0076] In some embodiments, the Dv10 particle size of the positive electrode active material is any one of 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, or a range between any two.

[0077] In some embodiments, the Dv90 particle size of the positive electrode active material is 12-15 μm.

[0078] In some embodiments, the Dv90 particle size of the positive electrode active material is any one or a range between any two of 12 μm, 12.5 μm, 13 μm, 13.5 μm, 14 μm, 14.5 μm, and 15 μm.

[0079] It should be noted that the particle size distribution of the core layer and the Dv10, Dv50, and Dv90 particles of the positive electrode active material were all measured using laser diffraction.

[0080] In some implementations, the space group of the positive electrode active material is P63mc.

[0081] It should be noted that the crystal structure of the positive electrode active material was determined using X-ray diffraction analysis.

[0082] This invention also provides a method for preparing a positive electrode active material, comprising the following steps: (1) Mix the cobalt source, aluminum source and M source according to the stoichiometric ratio of Co, Al and M in the core layer chemical formula and dissolve them in water to obtain the first cobalt salt solution; mix the cobalt source, aluminum source and M source according to the stoichiometric ratio of Co, Al and M in the intermediate layer chemical formula and dissolve them in water to obtain the second cobalt salt solution; mix the cobalt source, aluminum source, M source and fluorine source according to the stoichiometric ratio of Co, Al, M and F in the shell layer chemical formula and dissolve them in water to obtain the third cobalt salt solution; (2) Add the base liquid to the reaction equipment. The base liquid contains ammonium bicarbonate with a concentration of 30-90 g / L. Stir and heat to 30-40 °C. Add the first cobalt salt solution and ammonium bicarbonate solution to the reaction equipment in a co-flow manner to carry out a co-precipitation reaction to form a core layer of cobalt carbonate. (3) The second cobalt salt solution and the ammonium bicarbonate solution are added to the reaction equipment in a co-precipitation manner to form an intermediate layer of cobalt carbonate. The second cobalt salt solution is replaced with the third cobalt salt solution and added to the reaction equipment in a co-precipitation manner to form a shell layer of cobalt carbonate, thus obtaining a slurry containing doped cobalt carbonate. (4) The precipitate in the slurry was centrifuged, washed, and dried for 12 h, and then sintered to obtain cobalt tetroxide doped with it. (5) Mix doped cobalt tetroxide with sodium source and sinter in oxygen atmosphere to obtain doped sodium cobaltate, then mix with lithium source and heat to melt at 200-230 °C to prepare positive electrode active material.

[0083] In some embodiments, the cobalt source, aluminum source, M source, sodium source, and lithium source are each independent and include at least one of nitrate, carbonate, sulfate, and chloride.

[0084] In some embodiments, in step (1), the cobalt source includes at least one of cobalt sulfate, cobalt nitrate, cobalt chloride, and cobalt acetate.

[0085] In some embodiments, in step (1), the aluminum source includes at least one of aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum isopropoxide, and aluminum acetate.

[0086] In some embodiments, in step (1), the M source includes at least one of zirconium nitrate, titanium chloride, chromium chloride, tantalum chloride, and niobium chloride.

[0087] In some embodiments, in step (1), the fluorine source includes ammonium fluoride and / or ammonium hydrogen fluoride.

[0088] In some embodiments, in step (5), the sodium source includes sodium carbonate.

[0089] In some embodiments, in step (5), the lithium source includes lithium chloride and / or lithium nitrate.

[0090] In some embodiments, in step (5), the lithium source comprises lithium chloride and lithium nitrate in a mass ratio of (6-8):(2-4). This application employs a mixture of two lithium salts, which forms a eutectic mixture to lower the melting point of the lithium salts, thereby improving the low-temperature displacement efficiency of sodium and lithium.

[0091] In some embodiments, in steps (2) and (3), the concentration of the ammonium bicarbonate solution is 150-200 g / L.

[0092] In some embodiments, in step (2), the stirring rate of the coprecipitation reaction is 700-900 rpm, the temperature is 30-35 ℃, the flow rate of the first cobalt salt solution is 5-10 L / h, and the pH value of the system is controlled to be 7.5-8.5 by adjusting the flow rate of the ammonium bicarbonate solution.

[0093] In some embodiments, in step (3), the stirring rate of the coprecipitation reaction is 400-600 rpm, the temperature is 40-60 ℃, the flow rate of the second cobalt salt solution is 8-10 L / h, the flow rate of the second cobalt salt solution is 5-7 L / h, and the pH value of the system is controlled to be 7-8 by adjusting the flow rate of the ammonium bicarbonate solution.

[0094] In some implementations, the centrifugal speed in step (4) is 500-800 r / min.

[0095] In some embodiments, in step (4), the drying temperature is 90-110 °C and the time is 12-20 h.

[0096] In some embodiments, in step (4), the sintering temperature is 600-900 °C and the time is 10-15 h.

[0097] In some embodiments, in step (5), the molar ratio of cobalt in the doped cobalt tetroxide to sodium in the sodium source is (0.4-0.8):(0.98-1.2).

[0098] In some embodiments, in step (5), after mixing cobalt tetroxide with sodium source, the sintering temperature is 600-900 °C and the time is 35-38 h to obtain doped sodium cobaltate.

[0099] In some embodiments, in step (5), the molar ratio of lithium in the lithium source to sodium in the doped sodium cobaltate is (4-7):1.

[0100] The present invention also provides a positive electrode sheet, including a positive electrode active material.

[0101] In some implementations, the positive electrode includes a current collector and a positive active layer.

[0102] In some embodiments, the current collector is a metal foil, preferably an aluminum foil.

[0103] In some embodiments, the positive electrode active layer includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder.

[0104] In some embodiments, the positive electrode conductive agent includes at least one of carbon, carbon black, graphite, expanded graphite, graphene, graphene nanosheets, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon fibers, carbon nanofibers, graphitized carbon sheets, carbon tubes, carbon nanotubes, activated carbon, and mesoporous carbon.

[0105] In some embodiments, the positive electrode binder includes, but is not limited to, at least one of polyvinylidene fluoride, polyacrylic acid, polymethacrylic acid, polyacrylate, polymethacrylate, polyacrylamide, styrene-butadiene rubber, acrylic styrene-butadiene rubber, acrylic acid-acrylonitrile-acrylamide copolymer, acrylic acid-acrylonitrile-acrylate copolymer, acrylonitrile-butadiene rubber, nitrile rubber, acrylonitrile-styrene-butadiene copolymer, acryloyl rubber, butyl rubber, fluororubber, polytetrafluoroethylene, polyvinyl alcohol, polyvinyl acetate, polyepoxychloropropane, polyphosphazene, polyacrylonitrile, polystyrene, latex, acrylic resin, phenolic resin, epoxy resin, carboxymethyl cellulose, hydroxypropyl cellulose, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl cellulose, carboxymethyl chitosan, polyester, polyamide, polyether, polyimide, polycarboxylic acid ester, polycarboxylic acid, polyurethane, alginate, fluorinated polymer, chlorinated polymer, polyvinylidene fluoride, and poly(vinylidene fluoride)-hexafluoropropylene.

[0106] The present invention also provides an electrochemical device, including a positive electrode.

[0107] In some embodiments, the electrochemical device also includes a negative electrode, a diaphragm, and an electrolyte.

[0108] In some implementations, the negative electrode is a lithium metal sheet.

[0109] In some embodiments, the diaphragm is a porous substrate, which includes, but is not limited to, at least one of polyolefins, polyesters, polyacetals, polyamides, polyethylene terephthalate, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene ether, polyphenylene sulfide, polyacrylonitrile, polyvinylidene fluoride, polyoxymethylene, polyoxymethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, polysulfone, and polymethyl methacrylate. Some non-limiting examples of polyolefins include at least one of polyethylene (PE), ultra-high molecular weight polyethylene (UHMWPE), high-density polyethylene (HDPE), polypropylene (PP), polyethylene-polypropylene copolymer (PE-PP), and polyethylene-polypropylene-polyethylene copolymer.

[0110] In some embodiments, the electrolyte includes a non-aqueous organic solvent and a lithium salt.

[0111] In some embodiments, the lithium salt includes at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, LiBOB, and lithium difluoroborate.

[0112] In some embodiments, the non-aqueous organic solvent includes at least one of carbonate compounds, carboxylic acid ester compounds, and ether compounds.

[0113] In some embodiments, the electrochemical device includes any apparatus in which an electrochemical reaction occurs to interconvert chemical energy into electrical energy, and specific, non-limiting examples include all types of primary batteries, secondary batteries, fuel cells, solar cells, or capacitors. In particular, the electrochemical device is a lithium secondary battery, including lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0114] The application of the electrochemical device in this application is not particularly limited, and it can be used in any electronic device known in the prior art. In some embodiments, the electrochemical device includes, but is not limited to, mobile phones, smartphones, laptops, tablets, wearable devices, smartwatches, smart bracelets, smart glasses, power banks, televisions, game consoles, game controllers, digital cameras, smart speakers, headphones, keyboards, mice, monitors, drones, audio equipment, home appliances, toys, power tools, automobiles, motorcycles, electric bicycles, bicycles, robots, robot dogs, industrial robots, android robots, etc.

[0115] To further illustrate the present invention, detailed descriptions are provided below with reference to embodiments, but these should not be construed as limiting the scope of protection of the present invention. Unless otherwise specified, the raw materials used in the following embodiments and comparative examples are all commercially available, and the same raw materials were used in parallel experiments.

[0116] Example 1 The positive electrode active material has an O2 phase structure with space group P63mc, specifically consisting of a core layer, an intermediate layer, and a shell layer. The general chemical formula of the core layer, intermediate layer, and shell layer is Li. 1-a Na a Co 1-b-c Al b M c O 2-d F d In this embodiment, M in the chemical formulas of the core, intermediate layer and shell is specifically Zr, and the ionic radius in the six-coordinate structure of its ion is between 0.72 Å. Specifically, the chemical formula of the core layer is Li. 1-a1 Na a1 Co 1-b1-c1 Al b1 M c1 O2, core layer chemical formula Li 1- a Na a Co 1-b-c Al b Mc O 2-d F d In the formula, a is 0.005, b is 0.002, c is 0.0193, and d is 0, and in the general formula of the nuclear layer chemistry, a = a1, b = b1, c = c1, and d = d1. Specifically, the chemical formula of the intermediate layer is Li. 1-a2 Na a2 Co 1-b2-c2 Al b2 M c2 O2, intermediate layer chemical formula Li 1- a Na a Co 1-b-c Al b M c O 2-d F d In the formula, a is 0.005, b is 0.004, c is 0.0097, and d is 0, and in the general chemical formula of the intermediate layer, a = a2, b = b2, c = c2, and d = d2. Specifically, the chemical formula of the shell is Li 1-a3 Na a3 Co 1-b3-c3 Al b3 M c3 O 2-d3 F d3 The shell chemical formula is Li 1- a Na a Co 1-b-c Al b M c O 2-d F d In the formula, a is 0.005, b is 0.006, c is 0.0064, d is 0.003, and d / b is 0.5. In the general chemical formula of the shell, a=a3, b=b3, c=c3, d=d3. The ratio of b in the general chemical formula of the core, intermediate layer and shell is: H1:H2:H3. In this embodiment, H1:H2:H3 is 2:4:6. In this embodiment, the c and b in the general chemical formulas of the core layer, intermediate layer, and shell layer all satisfy the following relationship: c × b × (E M-0 / 600)=0.00005, where E M-0 Here are the bond energies between elements M and O, where M is Zr and E is O. M-0 Specifically, it is 776 kJ / mol; In this embodiment, the a and c in the chemical formulas of the core layer, intermediate layer and shell layer all satisfy the following relationship: 1.28≤c / a≤3.86; In this embodiment, the chemical formula of the core layer is specifically Li. 0.995 Na 0.005 Co 0.9787 Al 0.002 Zr 0.0193 O2, the chemical formula of the intermediate layer is Li 0.995 Na 0.005 Co 0.9863 Al 0.004 Zr 0.0097 O2, the specific chemical formula of the shell is Li 0.995 Na 0.005 Co 0.9876 Al 0.006 Zr 0.0064 O 1.997 F 0.003 ; The positive electrode active material has a D50 particle size of L, an intermediate layer thickness of 0.25×L, and a shell layer thickness of 0.08×L. In this embodiment, L is specifically 8 μm.

[0117] The preparation method of the above-mentioned positive electrode active material includes the following steps: (1) Cobalt chloride, aluminum sulfate, M source and ammonium fluoride are mixed according to the stoichiometric ratio of Co, Al, M and F in the core layer chemical formula, and dissolved in deionized water at a solid-liquid ratio of 2:98 to obtain the first cobalt salt solution; cobalt chloride, aluminum sulfate, M source and ammonium fluoride are mixed according to the stoichiometric ratio of Co, Al, M and F in the intermediate layer chemical formula, and dissolved in deionized water at a solid-liquid ratio of 2:98 to obtain the second cobalt salt solution; cobalt chloride, aluminum sulfate, M source and ammonium fluoride are mixed according to the stoichiometric ratio of Co, Al, M and F in the shell layer chemical formula, and dissolved in deionized water at a solid-liquid ratio of 2:98 to obtain the third cobalt salt solution; and an ammonium bicarbonate solution with a concentration of 180 g / L is prepared. (2) Crystal nucleus preparation: Deionized water and ammonium bicarbonate solution were added to the reactor (15 m³) as the base liquid. The volume of the base liquid accounted for 15% of the reactor volume. The concentration of ammonium bicarbonate in the base liquid was 60 g / L. The reactor was heated and stirred at 800 rpm. When the temperature of the base liquid reached 35 °C, the first cobalt salt solution and ammonium bicarbonate solution were added to the reactor in a co-current manner. The flow rate of the first cobalt salt solution was kept constant at 7 L / h. The pH value of the system was controlled to 8 by adjusting the flow rate of the ammonium bicarbonate solution. Co-precipitation reaction was carried out to form a core layer of cobalt carbonate. When the cobalt carbonate particle size reached D1 (D1=2 μm in this embodiment), the precipitation ended. (3) Crystal nucleus growth stage: The stirring speed of the reactor is adjusted to 500 rpm and the temperature is raised to 45 ℃. The second cobalt salt solution and the ammonium bicarbonate solution are added to the reactor in a co-current manner. The flow rate of the second cobalt salt solution is kept constant at 9 L / h. The pH value of the system is controlled to 7.5 by adjusting the flow rate of the ammonium bicarbonate solution. Co-precipitation reaction is carried out to form the intermediate layer of cobalt carbonate. When the cobalt carbonate particle size reaches D2 (D2=5 μm in this embodiment), the second cobalt salt solution is replaced with the third cobalt salt solution and added to the reactor in a co-current manner with the ammonium bicarbonate solution. The flow rate of the third cobalt salt solution is kept constant at 6 L / h. The pH value of the system is controlled to 7.5 by adjusting the flow rate of the ammonium bicarbonate solution. Co-precipitation reaction is carried out to form the shell layer of cobalt carbonate. When the cobalt carbonate particle size reaches D3 (D3=6 μm in this embodiment), the precipitation ends and doped cobalt carbonate is obtained. (4) The slurry was centrifuged and washed using a centrifuge at a speed of 400 r / min. After precipitation and washing, it was dried in an oven at 120 ℃ for 12 h, and then sintered at 680 ℃ for 13 h to obtain cobalt tetroxide doped. (5) Cobalt tetroxide and Na2CO3 were uniformly mixed in a molar ratio of Co to Na of 0.70:0.99. Oxygen was introduced and sintered at 780 °C for 40 h to obtain sodium cobalt oxide. Then, it was mixed with a lithium source and heated to melt at 220 °C. In this embodiment, the melting time was 12 h. The lithium source included LiCl and LiNO3 in a mass ratio of 7:3. The molar ratio of Li in the lithium source to Na in the sodium cobalt oxide was 6. Lithium cobalt oxide was prepared, which is the positive electrode active material.

[0118] Examples 2-4 The positive electrode active material differs from that in Example 1 in that the content of cobalt chloride, aluminum sulfate, M source and ammonium fluoride in the first cobalt salt solution, the second cobalt salt solution and the third cobalt salt solution in step (1) is adjusted to achieve different molar amounts of Al in the core layer, the intermediate layer and the shell layer.

[0119] Examples 5-6 The positive electrode active material differs from that in Example 1 in that the content of cobalt chloride, aluminum sulfate, M source and ammonium fluoride in the first cobalt salt solution, the second cobalt salt solution and the third cobalt salt solution in step (1) is adjusted to achieve different molar amounts of Al and H1:H2:H3 ratios in the core layer, intermediate layer and shell layer.

[0120] Examples 7-8 The positive electrode active material differs from that in Example 1 in that the content of cobalt chloride and M source in the first, second, and third cobalt salt solutions in step (1) is adjusted to achieve c×b×(E) in the core, intermediate, and shell layers. M-0The / 600) value is different.

[0121] Examples 9-12 The positive electrode active material differs from that in Example 1 in that the content of cobalt chloride and ammonium fluoride in the third cobalt salt solution in step (1) is adjusted to achieve a different molar ratio of F and Al in the shell.

[0122] Examples 13-16 The positive electrode active material differs from that in Example 1 in that the content of the M source, cobalt chloride, and the M source in the first, second, and third cobalt salt solutions in step (1) is adjusted to achieve different M values ​​in the chemical formulas of the core, intermediate, and shell layers. Specifically, the M source is Ta, Cr, Ti, or Nb, the ionic radius of the Ta ion in its six-coordinate structure is 0.60 Å, and the bond energy E between Ta and O is... M-0 The energy density is 726 kJ / mol; the ionic radius of the Cr ion in its six-coordinate structure is 0.615 Å, and the bond energy E between Cr and O is... M-0 The energy density is 610 kJ / mol; the ionic radius of the six-coordinate Ti ion is 0.605 Å, and the bond energy E between Ti and O is... M-0 The energy density is 672 kJ / mol; the ionic radius of the Nb ion in its six-coordinate structure is 0.69 Å, and the bond energy E between Nb and O is... M-0 It is 726 kJ / mol.

[0123] Examples 17-18 The difference between the positive electrode active material and Example 1 is that ammonium fluoride is added to the first cobalt salt solution and the second cobalt salt solution in step (1), and the content of cobalt chloride and ammonium fluoride in the first cobalt salt solution, the second cobalt salt solution and the third cobalt salt solution is adjusted to achieve different molar amounts of F in the core layer, the intermediate layer and the shell layer.

[0124] Example 19 The positive electrode active material differs from that in Example 1 in that the content of cobalt chloride and ammonium fluoride in the third cobalt salt solution is adjusted in step (1) to achieve a molar amount of F in the core and shell layers of 0.

[0125] Example 20 The difference between the positive electrode active material and that in Example 1 is that the amount of the second cobalt salt solution added in step (3) is adjusted to achieve a thickness of 0 for the intermediate layer, that is, no intermediate layer is provided.

[0126] Examples 21-22 The positive electrode active material differs from that in Example 1 in that the heating and melting time at 220 °C in step (5) is adjusted to achieve different molar amounts of Na in the core layer, intermediate layer and shell layer, as well as different molar ratios of M and Na.

[0127] Example 23 The positive electrode active material differs from that in Example 1 in that the chemical formula of the shell is Li. 0.995 Na 0.005 Co 0.9876 Al 0.006 Zr 0.0064 O 1.997 Cl 0.003 ; In step (1) of the preparation method, cobalt chloride, aluminum sulfate, and M source are mixed according to the stoichiometric ratio of Co, Al, and M in the core layer chemical formula, and dissolved in deionized water at a solid-liquid ratio of 2:98 to obtain the first cobalt salt solution; cobalt chloride, aluminum sulfate, and M source are mixed according to the stoichiometric ratio of Co, Al, and M in the intermediate layer chemical formula, and dissolved in deionized water at a solid-liquid ratio of 2:98 to obtain the second cobalt salt solution; cobalt chloride, aluminum sulfate, M source, and ammonium chloride are mixed according to the stoichiometric ratio of Co, Al, M, and Cl in the shell layer chemical formula, and dissolved in deionized water at a solid-liquid ratio of 2:98 to obtain the third cobalt salt solution; and an ammonium bicarbonate solution with a concentration of 180 g / L is prepared.

[0128] Examples 24-25 The difference between the positive electrode active material and Example 1 is that the co-precipitation reaction time of the first cobalt salt solution in step (2) and the co-precipitation reaction time of the second and third cobalt salt solutions in step (3) are adjusted to achieve different D50 particle sizes L of the positive electrode active material.

[0129] Examples 26-27 The positive electrode active material differs from that in Example 1 in that the co-precipitation reaction time of the first cobalt salt solution in step (2) and the co-precipitation reaction time of the second and third cobalt salt solutions in step (3) are adjusted to achieve different thicknesses of the core layer, intermediate layer and shell layer.

[0130] Comparative Examples 1-2 The positive electrode active material differs from that in Example 1 in that the content of cobalt chloride and M source in the first, second, and third cobalt salt solutions in step (1) is adjusted to achieve c×b×(E) in the core, intermediate, and shell layers. M-0 The / 600) value is different.

[0131] Comparative Example 3 The positive electrode active material differs from that in Example 1 in that the content of cobalt chloride and M source in the first cobalt salt solution and the second cobalt salt solution in step (1) is adjusted to achieve a molar amount of M of 0 in the core layer and the intermediate layer.

[0132] Comparative Example 4 The positive electrode active material differs from that in Example 1 in that the content of M source, cobalt chloride and M source in the first cobalt salt solution, the second cobalt salt solution and the third cobalt salt solution in step (1) is adjusted to achieve different M in the chemical formulas of the core layer, the intermediate layer and the shell layer.

[0133] Comparative Example 5 The positive electrode active material differs from that in Example 1 in that the content of M source, cobalt chloride, and M source in the first, second, and third cobalt salt solutions in step (1) is adjusted to achieve the chemical formulas M and c×b×(E) in the core, intermediate, and shell layers. M-0 The / 600) value is different; M specifically refers to Mn. The ionic radius of the Mn ion in its six-coordinate structure is 0.53 Å. The bond energy E between Mn and O is... M-0 It is 494 kJ / mol.

[0134] Comparative Example 6 The difference between the positive electrode active material and that in Example 1 is that the positive electrode active material contains an O3 phase structure, and in step (5), the heating melting temperature is 300 °C.

[0135] The particle size L of the positive electrode active material, the thickness of the intermediate layer, the thickness of the shell layer, the source M in step (1), the D1 in step (2), the D2 and D3 in step (3), and the heating and melting time in step (5) in the above embodiments and comparative examples are all shown in Table 1 below.

[0136] In the above embodiments and comparative examples, the chemical formulas of the core layer, intermediate layer, and shell layer of the positive electrode active material include the values ​​of a, b, c, d, M element, the ratio of b in the three layers H1:H2:H3, the d / b value in the shell layer, the c / a value in the three layers, and the c×b×(E) value in the three layers. M-0 The / 600) value and the chemical formulas of the three layers are shown in Table 2 below; among them, the chemical formulas of the core layer, intermediate layer and shell layer are Li 1- a Na a Co 1-b-c Al b M c O 2-d F d In Table 2, the a values ​​of the core, intermediate layer, and shell are independent and are shown as a1, a2, and a3, respectively; the b values ​​of the core, intermediate layer, and shell are independent and are shown as b1, b2, and b3, respectively; the c values ​​of the core, intermediate layer, and shell are independent and are shown as c1, c2, and c3, respectively; and the d values ​​of the core, intermediate layer, and shell are independent and are shown as d1, d2, and d3, respectively.

[0137] Table 1 - Process settings for the preparation methods of positive electrode active materials in the embodiments and comparative examples of this application. Table 2 - Parameter settings of positive electrode active materials in the embodiments and comparative examples of this application The positive electrode active materials prepared in the above embodiments and comparative examples are used to prepare positive electrode sheets and assemble electrochemical devices, including the following steps: (1) The positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 95:3:2 and added to the N-methylpyrrolidone solvent system and stirred thoroughly to obtain a positive electrode slurry with a solid content of 78%. The slurry is then coated on aluminum foil, dried, cold-pressed, slit, and the tabs are welded to obtain the positive electrode sheet. (2) Lithium salt LiPF6 was mixed with a non-aqueous organic solvent at a mass ratio of 8:92. The non-aqueous organic solvent included ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), propyl propionate (PP), and ethylene carbonate (VC) in a mass ratio of 20:30:20:28:2 to prepare an electrolyte. (3) In a glove box filled with argon, the positive electrode, separator and negative electrode are stacked and wound. The negative electrode is a lithium metal sheet and the separator is a polyethylene (PE) porous polymer film. After being put into the shell, the electrolyte is injected. After vacuum sealing, standing and formation, CR2032 button battery is prepared.

[0138] Performance testing 1. The positive electrode active materials prepared in Example 1 and Comparative Example 6 were analyzed using an X-ray diffraction analyzer. The XRD results are as follows: Figure 1 As shown, it can be observed that the main peak of the O3 phase lithium cobalt oxide in Comparative Example 6 is approximately 18.9 degrees, while the main peak of the O2 phase lithium cobalt oxide in Example 1 is approximately 18.5 degrees.

[0139] 2. Specific capacity: The specific capacity was tested using the constant current charge-discharge method at room temperature. The batteries of the examples and comparative examples were tested using the LAND CT3001A battery testing system at an environment of (25±2)℃. The test procedure was as follows: the battery was charged to 4.60 V at a constant current and constant voltage of 0.1 C, the cutoff current was 0.02 C, and then discharged to 3.0 V at a constant current of 0.1 C. The specific capacity was calculated using the following formula: Specific capacity (mAh / g) = [discharge capacity of half cell (mAh)] / [mass of positive electrode active material contained in the electrode sheet (g)]. The test results are shown in Table 3 below.

[0140] 3. Cycle capacity retention rate: The batteries assembled in the examples and comparative examples were subjected to cycle charge-discharge tests at temperatures of 45 ℃ and 60 ℃, respectively. The charging mode was 1.0 C constant current and constant voltage charging to 4.60 V, and the cutoff current was 0.05 C; the discharging mode was 1.0 C constant current discharging to 3.0 V. The capacity retention rate after 100cls of cycles was recorded. The test results are shown in Table 2 below.

[0141] Table 3 - Performance test results of batteries prepared from positive electrode active materials in the embodiments and comparative examples of this application As shown in Table 3, Example 1 of this application constructs a core-to-shell Al structure using lithium cobalt oxide material. 3+ The increasing concentration gradient distribution can gradually enhance the structural stability of the surface region and reduce the high lithium-ion diffusion capability of the material core; simultaneously, fluorine ion doping is performed on the shell layer, through F... - Replace part of O 2- This process forms a high-bond-energy stable structure, suppresses the generation of surface oxygen vacancies during cycling, and enhances the structural integrity of the near-surface region. Furthermore, by introducing large-radius Zr cations with high metal-oxygen bond energy at cobalt sites, the structural stability of the cobalt-oxygen layer is enhanced, lithium-ion migration channels are broadened, and lattice stress during lithium-ion insertion / extraction is effectively alleviated, while satisfying 0.00004 ≤ c × b × (E M-0 The formula range of / 600) ≤0.00085 achieves a comprehensive improvement in the structural robustness of O2 phase lithium cobalt oxide materials from three dimensions: bulk structure, ion transport kinetics, and surface stability, resulting in high specific capacity and capacity retention under high-pressure, long-cycle conditions. Compared to Example 1, Comparative Example 1 lithium cobalt oxide has a higher c×b×(E) value. M-0The / 600) value is too low, indicating that the Zr doping amount is too low. Zr is mainly a high bond energy metal element, which plays a role in stabilizing the material structure. If the doping amount is too low, the improvement in the stability of the material structure is not significant enough, and Al-O is insufficient to resist lattice stress, resulting in a sharp decrease in high-temperature cycling capacity. Compared with Example 8, the c×b×(E) value of Comparative Example 2 lithium cobalt oxide is lower. M-0 The Zr doping level is too high ( / 600), which distorts the layered structure and hinders lithium-ion insertion / extraction, thus reducing the specific capacity and cycle capacity retention.

[0142] Compared to Example 1, the core and intermediate layers of Comparative Example 3 were not doped with Zr. This is because the lithium cobalt oxide core requires high lithium-ion diffusion capability, while the core and intermediate layers are doped with Al. 3+ The low bond energy formed with O makes it difficult to cope with the lattice stress caused by the deep lithium insertion and extraction of O2 lithium cobalt oxide. It is necessary to dope with Zr to broaden the lithium ion migration channel and alleviate the lattice stress during the lithium ion insertion and extraction process, which leads to a decrease in the lithium ion diffusion energy of the core and a significant decrease in the specific capacity of lithium cobalt oxide.

[0143] Compared to Example 1, in Comparative Examples 4-5, Zr doping in lithium cobalt oxide was replaced by Mn doping. Since the bond energy of Mn is significantly lower than that of Zr, its effect on improving the structural stability of the cobalt oxide layer and widening the lithium-ion migration channel is poor. It is difficult to alleviate the lattice stress during the lithium-ion insertion and extraction process, which in turn leads to a decrease in the lithium-ion diffusion capacity and structural stability of lithium cobalt oxide, resulting in low specific capacity and cycle capacity retention.

[0144] Compared to Example 1, the sintering temperature of step (5) in the preparation process of lithium cobalt oxide in Comparative Example 6 exceeds 250°C, which leads to an irreversible transformation to the O3 phase, resulting in a decrease in the specific capacity of lithium cobalt oxide, which is not as good as the specific capacity of the O2 phase structure lithium cobalt oxide in Example 1.

[0145] Compared to Example 1, the overall Al of Example 4 is different. 3+ The concentration is high. Since Al is an electrochemically inert element, if the doping content of Al in the positive electrode active material is high, it will lead to a decrease in the capacity of the active material. Therefore, the specific capacity of the assembled battery is not as good as that of Example 1.

[0146] Compared to Example 1, Example 6 has Al doped with lithium cobalt oxide in its core, intermediate, and shell layers. 3+ The concentrations were all consistent, without forming a gradient increasing distribution. This is because the core has high requirements for lithium ion diffusion; if the Al in the core... 3+ The concentration is not low enough, Al 3+The low bond energy formed with O makes it difficult to cope with the lattice stress caused by deep lithium insertion / extraction of O2 lithium cobalt oxide, leading to a decrease in the material's lithium-ion diffusion capacity and a reduction in specific capacity. Furthermore, the outer shell is prone to side reactions with the electrode solution; if the outer shell Al... 3+ Insufficient concentration leads to reduced interfacial stability of the outer shell material and insufficient structural stability of lithium cobalt oxide, thus affecting its cycling performance under high-pressure conditions. Because Example 6 struggles to balance internal lithium-ion diffusion energy and outer interface structural stability, its overall performance under high-pressure, long-term cycling conditions is reduced.

[0147] Compared to Example 1, the lithium cobalt oxide in Example 17 is doped with F only in the core and intermediate layers, while the lithium cobalt oxide in Example 19 is not doped with F in the core, intermediate, and shell layers. Since the shell layers of Examples 17 and 19 are not doped with F, they cannot suppress the generation of surface oxygen vacancies during cycling, nor can they enhance the structural integrity of the near-surface region. This makes the material susceptible to damage from high voltage and corrosion from electrolyte decomposition products (HF), ultimately reducing structural stability and cycle capacity retention under high voltage and high temperature conditions. In Example 18, the lithium cobalt oxide core, intermediate, and shell layers are all doped with F, resulting in excessive F doping in the positive electrode active material. This leads to increased initial impedance and cycling impedance for lithium ion insertion / extraction, and a decrease in specific capacity and cycle performance.

[0148] Compared to Example 1, the lithium cobalt oxide in Example 20 did not have an intermediate layer, resulting in Al in the core layer... 3+ Concentration and Al in the shell 3+ The concentration did not transition smoothly, Al 3+ The concentration fluctuates greatly, and the intermediate layer can improve the structural stability of the material to maintain a high cycle life. Therefore, the overall performance of Example 20 is not as good as that of Example 1.

[0149] Compared to Example 1, the lithium cobalt oxide shell of Example 23 uses Cl doping instead of F doping. Since the Cl-O bond energy is not as high as the FO bond energy, the electrolyte is easily decomposed to generate HF during cycling. The LiCl in the material cannot effectively resist HF corrosion, resulting in reduced structural stability and ultimately a lower cycle capacity retention rate than in Example 1.

[0150] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention for those skilled in the art.

Claims

1. A positive electrode active material, characterized in that, This includes lithium cobalt oxide doped with Al and M, wherein the molar ratio of Co, Al and M in the lithium cobalt oxide is (1-bc):b:c; M is a metallic element with a bond energy of 600-800 kJ / mol to O, and the ionic radius of the M ion in its six-coordinate structure is 0.6-0.75 Å. The conditions b and c satisfy the following relationship: 0.00004 ≤ c × b × (E M-0 / 600)≤0.00085; where, E M-0 The values ​​represent the bond energies of M and O, in kJ / mol. The positive electrode active material has an O2 phase structure.

2. The positive electrode active material as described in claim 1, characterized in that, The positive electrode active material comprises a core layer, an intermediate layer, and a shell layer, wherein the chemical formula of the core layer is Li. 1-a1 Na a1 Co 1-b1-c1 Al b1 M c1 O2, where 0.001≤a1≤0.010, 0<b1≤0.01, 0<c1<0.07; The chemical formula of the intermediate layer is Li. 1-a2 Na a2 Co 1-b2-c2 Al b2 M c2 O2, where 0.001≤a2≤0.010, 0<b2≤0.02, 0<c2<0.07; The chemical formula of the shell is Li 1-a3 Na a3 Co 1-b3-c3 Al b3 M c3 O 2-d3 F d3 Where 0.001≤a3≤0.010, 0<b3≤0.03, 0<c3<0.07, 0<d3≤0.

015.

3. The positive electrode active material as described in claim 2, characterized in that, b1 / b2 < 1, b2 / b3 < 1.

4. The positive electrode active material as described in claim 3, characterized in that, b1:b2:b3 is (0.5-2):(2.5-4):(4.5-6).

5. The positive electrode active material as described in claim 2, characterized in that, In the chemical formula of the shell, b3 and d3 satisfy the following relationship: 0.2≤d3 / b3≤0.

9.

6. The positive electrode active material as described in claim 2, characterized in that, In the chemical formula of the core layer, 1 ≤ c1 / a1 ≤ 15; in the chemical formula of the intermediate layer, 1 ≤ c2 / a2 ≤ 15; in the chemical formula of the shell layer, 1 ≤ c3 / a3 ≤ 15.

7. The positive electrode active material as described in claim 1, characterized in that, M is at least one of Cr, Zr, Ti, Ta, and Nb.

8. The positive electrode active material as described in claim 1, characterized in that, The positive electrode active material has a Dv50 particle size of L, the thickness of the intermediate layer is 0.2×L to 0.3×L, and the thickness of the shell layer is 0.05×L to 0.1×L.

9. A positive electrode sheet, characterized in that, Includes the positive electrode active material as described in any one of claims 1-8.

10. An electrochemical device, characterized in that, Including the positive electrode sheet as described in claim 9.

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