Positive electrode active material, method for producing the same, and use thereof

Abstract

The application relates to a positive electrode active material and a preparation method and application thereof. The positive electrode active material comprises a matrix, a first coating layer and a second coating layer; the matrix comprises O2 phase lithium cobalt oxide, the first coating layer comprises T2 phase lithium cobalt oxide, and the second coating layer comprises a fast ion conductor layer and a nitrogen-doped carbon layer arranged in sequence from inside to outside; the fast ion conductor layer contains inorganic materials with lithium ion conductivity >= 10 ‑6 S / cm. In the scheme provided by the application, the positive electrode active material has a three-layer gradient structure of "O2 phase inner core-T2 phase interface layer-bifunctional coating layer", can significantly reduce the phase change stress of O2 phase lithium cobalt oxide in the cycle process, effectively inhibit the interface side reaction and cobalt dissolution, thereby improving the battery cycle life while maintaining high capacity and excellent ion transmission efficiency, electron transmission efficiency, improving the rate performance and long cycle performance of the battery at high voltage.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to a positive electrode active material, its preparation method, and its application. Background Technology

[0002] With the increasing demands for energy density in lithium-ion batteries from consumer electronics and electric vehicles, developing cathode materials that can operate stably at high voltages has become an important research direction for lithium-ion batteries.

[0003] Lithium cobalt oxide (LiCoO2) has long been the preferred cathode material for high-energy-density lithium-ion batteries due to its high volumetric energy density and excellent rate performance. Traditional O3-phase lithium cobalt oxide has an actual capacity of approximately 179 mAh / g at 4.45V, far below its theoretical capacity of 274 mAh / g. Therefore, increasing the charging voltage to above 4.5V can extract more lithium ions, significantly improving energy density. However, O3-phase lithium cobalt oxide is highly susceptible to irreversible and harmful phase transitions at high voltages (≥4.5V), leading to severe lattice contraction, structural instability, and increased surface lattice oxygen activity. This results in vigorous side reactions with the electrolyte, causing electrode interface instability, transition metal dissolution, and exacerbated interfacial side reactions, severely degrading battery cycle life.

[0004] O2-phase lithium cobalt oxide, due to its unique oxygen layer stacking sequence (ABBA), exhibits higher structural reversibility and stability in the high delithiation state, making it a promising candidate for next-generation high-energy-density cathode materials. However, the practical application of O2-phase lithium cobalt oxide still faces two core challenges: First, O2-phase lithium cobalt oxide is thermodynamically metastable, and during battery cycling, it tends to transform into other crystalline phases (such as the O3 phase). The resulting phase transformation stress easily leads to particle cracking, which in turn causes capacity decay. Second, O2-phase lithium cobalt oxide has poor interfacial compatibility with high-voltage electrolytes, easily triggering continuous interfacial side reactions, leading to increased cobalt dissolution and gas production, thus deteriorating the battery's cycle life and safety.

[0005] Therefore, developing an O2-phase lithium cobalt oxide cathode material with good structural and interfacial stability is of great significance for the practical application of high-voltage lithium cobalt oxide batteries. Summary of the Invention

[0006] To address or partially address the problems existing in related technologies, this application provides a positive electrode active material, its preparation method, and its application. This positive electrode active material constructs a three-layer gradient structure of "O2 phase core - T2 phase interface layer - bifunctional coating layer," which can significantly reduce the phase transformation stress of O2 phase lithium cobalt oxide during cycling, effectively suppress interfacial side reactions and cobalt dissolution, thereby improving battery cycle life while maintaining high capacity and excellent ion transport efficiency and electron transport efficiency, and improving the rate performance and long-cycle performance of the battery under high voltage.

[0007] The first aspect of this application provides a positive electrode active material, including a matrix, a first coating layer disposed on the surface of the matrix, and a second coating layer disposed on the surface of the first coating layer; The substrate comprises O2-phase lithium cobalt oxide, the first coating layer comprises T2-phase lithium cobalt oxide, and the second coating layer comprises a fast ion conductor layer and a nitrogen-doped carbon layer arranged sequentially from the inside out; the fast ion conductor layer contains a lithium-ion conductivity ≥10. -6 Inorganic materials with S / cm.

[0008] In some embodiments of this application, the fast ion conductor layer contains at least one of lithium phosphorus oxy nitrogen, amorphous Li3PO4, and amorphous lithium lanthanum zirconium oxide.

[0009] In some embodiments of this application, the first coating layer further includes O2 phase lithium cobalt oxide; in the first coating layer, the relative content ratio of T2 phase lithium cobalt oxide to O2 phase lithium cobalt oxide is ≥4:1.

[0010] In some embodiments of this application, the general chemical formula of the O2 phase lithium cobalt oxide is Li. x Co y M z O2; wherein 0.9≤x≤1.05, 0.98≤y≤1, 0≤z≤0.02, and M includes at least one of the elements Al, Mg, Ti, Zr, Ni, Mn, La, Ce, Y and W.

[0011] In some embodiments of this application, the average particle size Dv50 of the matrix is ​​1 μm to 20 μm, preferably 3 μm to 12 μm.

[0012] In some embodiments of this application, the total thickness of the first coating layer and the second coating layer is ≤200nm; preferably ≤100nm.

[0013] In some embodiments of this application, the thickness of the first coating layer is 10 nm to 100 nm, and the thickness of the second coating layer is 5 nm to 100 nm.

[0014] In some embodiments of this application, the thickness of the second coating layer is 5 nm to 50 nm; more preferably, in the second coating layer, the thickness of the fast ion conductor layer is 3 nm to 10 nm, and the thickness of the nitrogen-doped carbon layer is 2 nm to 40 nm.

[0015] In some embodiments of this application, the fast ion conductor layer is formed by atomic layer deposition (ALD).

[0016] In some embodiments of this application, the nitrogen-doped carbon layer is formed by chemical vapor deposition.

[0017] A second aspect of this application provides a method for preparing the above-mentioned positive electrode active material, comprising the following steps: The O2 phase lithium cobalt oxide is heat-treated to form the T2 phase lithium cobalt oxide on the surface of the O2 phase lithium cobalt oxide, thus obtaining a pretreated lithium cobalt oxide material. The fast ion conductor layer is deposited on the surface of the pretreated lithium cobalt oxide material using atomic layer deposition (ALD); the nitrogen-doped carbon layer is deposited on the surface of the fast ion conductor layer using chemical vapor deposition (CVD).

[0018] In some embodiments of this application, the heat treatment is carried out in an oxygen-containing atmosphere, at a temperature of 250°C to 400°C, for a time of 1 hour to 20 hours.

[0019] In some embodiments of this application, the fast ion conductor layer is formed by alternating deposition of a lithium-containing precursor and a doped element precursor using atomic layer deposition technology; wherein the doped element precursor includes at least one element: phosphorus, nitrogen, lanthanum, and zirconium.

[0020] In some embodiments of this application, the nitrogen-doped carbon layer is formed by depositing carbon source gas and nitrogen source gas on the surface of the fast ion conductor layer using chemical vapor deposition technology; wherein the volume flow rate ratio of the carbon source gas and the nitrogen source gas is (1~10):1.

[0021] A third aspect of this application provides a positive electrode sheet, including a positive current collector and a positive electrode material layer disposed on the surface of the positive current collector; the positive electrode material layer includes the aforementioned positive electrode active material.

[0022] The fourth aspect of this application provides a lithium-ion battery, including an electrolyte, a negative electrode, and a positive electrode as described above.

[0023] In some embodiments of this application, the electrolyte includes an additive, the additive being lithium difluorophosphate, and the mass percentage of the lithium difluorophosphate in the electrolyte is 0.05% to 5%; preferably 0.1% to 3%; more preferably 0.5% to 2%.

[0024] In some embodiments of this application, XPS analysis results of the positive electrode before cycling show that it simultaneously includes orbital characteristic peaks from the fast ion conductor layer and orbital characteristic peaks from the nitrogen-doped carbon layer; wherein, the orbital characteristic peaks from the fast ion conductor layer include at least one of P 2p orbital characteristic peaks, La 3d orbital characteristic peaks, and Zr 3d orbital characteristic peaks; and the orbital characteristic peaks from the nitrogen-doped carbon layer include N 1s orbital characteristic peaks, and the pyridine nitrogen content in the N 1s orbital characteristic peaks is not less than 40%.

[0025] In some embodiments of this application, the TOF-SIMS analysis results of the positive electrode sheet before cycling show that the spatial distribution overlap between the active material region characterized by the first characteristic fragment and the second coating layer region characterized by the second characteristic fragment is not less than 90%; wherein, the first characteristic fragment is CoO2. - (m / z=91) fragment, the second characteristic fragment is PO2 - (m / z=63) fragments, LaO + Fragments, ZrO - Fragments or CN - (m / z=26) fragments.

[0026] In some embodiments of this application, the XPS analysis results of the positive electrode in the lithium-ion battery after 800 cycles show that the ratio of the LiF characteristic peak area at (684.5±0.2) eV to the PVDF characteristic peak area at (687.5±0.2) eV in the F 1s spectrum is not less than 1.5.

[0027] The technical solution provided in this application can include the following beneficial effects: a high-capacity lithium storage platform is provided through the O2 phase lithium cobalt oxide matrix, the T2 phase interface layer buffers the phase transition stress during the charging and discharging process and prevents the collapse of the core structure, and the dual-functional coating layer formed by the inner fast ion conductor layer and the outer nitrogen-doped carbon layer achieves synergistic complementarity of ion conduction and electron conduction, significantly reducing the phase transition stress of the O2 phase lithium cobalt oxide during cycling, effectively suppressing interface side reactions and cobalt dissolution, thereby improving the battery cycle life while maintaining high capacity and excellent ion / electron transport efficiency, and improving the rate performance and long-cycle stability of the battery at high voltage.

[0028] The nitrogen-doped carbon layer on the outer layer of the positive electrode active material can synergistically catalyze the formation of a stable LiF-rich CEI film with lithium difluorophosphate additive in the electrolyte. Through the synergistic effect of each layer and the optimization of thickness parameters (such as total coating thickness ≤200nm, T2 phase interface layer ratio ≥80%, pyridine nitrogen ratio ≥40%), the positive electrode active material can exhibit both excellent interfacial and structural stability at a high voltage of 4.6V. After 800 cycles, it maintains a capacity retention rate of ≥90% while keeping the interfacial impedance increase low, achieving synergistic optimization of structural stability and electrochemical performance.

[0029] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit this application. Detailed Implementation

[0030] To facilitate understanding of the present invention, it will be described in detail below. However, before describing the present invention in detail, it should be understood that the present invention is not limited to the specific embodiments described. It should also be understood that the terminology used herein is for describing specific embodiments only and is not intended to be restrictive.

[0031] The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The singular forms “a,” “the,” and “the” used in this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0032] Where numerical ranges are provided, it should be understood that every intermediate value between the upper and lower limits of the range and any other specified or intermediate value within the specified range is covered within the present invention. The upper and lower limits of these smaller ranges may be independently included in the smaller range and are also covered within the present invention, subject to any explicitly excluded limits within the specified range. Where a specified range includes one or two limits, the range excluding any or both of those included limits is also included within the present invention. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0033] Unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. While any methods and materials, or equivalents thereof, may be used in the practice or testing of this invention, preferred methods and materials are now described.

[0034] The practical application of O2 phase lithium cobalt oxide faces two core problems: First, O2 phase lithium cobalt oxide is thermodynamically metastable and tends to transform into other crystalline phases (such as O3 phase) during battery cycling. The resulting phase transformation stress can easily lead to particle cracking, which in turn leads to capacity decay. Second, O2 phase lithium cobalt oxide has poor interfacial compatibility with high-voltage electrolytes, which can easily trigger continuous interfacial side reactions, leading to increased cobalt dissolution and gas production, thus deteriorating the cycle life and safety of the battery.

[0035] To address these issues, two main improvement approaches are currently being explored: one is to surface-coat the O2 phase lithium cobalt oxide with metal oxides, carbon layers, or polymer derivatives; the other is to modify the crystal lattice of the O2 phase lithium cobalt oxide through bulk doping techniques. However, while conventional metal oxide coatings (such as Al2O3) can physically isolate the electrolyte, their poor ionic conductivity significantly increases interfacial impedance, affecting the transport kinetics of lithium-ion batteries. Although carbon coatings have high electronic conductivity, they cannot effectively suppress electrolyte penetration and cobalt dissolution, and the high-temperature carbonization process may disrupt the metastable structure of the O2 phase. While single bulk doping techniques can improve the intrinsic structural stability of the material, they cannot solve the problem of surface interfacial side reactions.

[0036] Therefore, this application provides a positive electrode active material with a unique "core-shell-shell" functional gradient structure, which solves the three core problems of structural stability of O2 phase lithium cobalt oxide, interfacial ion / electron transport, and interfacial stability through the synergistic effect of each level.

[0037] The positive electrode active material in this embodiment includes a substrate, a first coating layer disposed on the surface of the substrate, and a second coating layer disposed on the surface of the first coating layer. The substrate includes O2 phase lithium cobalt oxide, the first coating layer includes T2 phase lithium cobalt oxide, and the second coating layer is a bifunctional coating layer, which includes a fast ion conductor layer and a nitrogen-doped carbon layer disposed sequentially from the inside to the outside.

[0038] In this embodiment, a three-layer gradient structure of "O2 phase core - T2 phase interface layer - dual-functional coating layer" is constructed. The O2 phase core provides a high-capacity lithium storage platform; the T2 phase interface layer buffers phase transition stress during charging and discharging, preventing core structure collapse and transforming potentially harmful phase transition stress into a buffering structural design; the inner fast-ion conductor layer of the dual-functional coating layer provides an efficient lithium-ion conduction channel, ensuring rapid lithium-ion crossing of the interface while reducing interface impedance and inhibiting electrolyte penetration; the outer nitrogen-doped carbon layer provides an electron conduction channel, enhancing surface electron conductivity and acting as a physical barrier to prevent direct electrolyte contact with the active material, suppressing side reactions. Through the dual-functional coating layer, complementary synergy between ion and electron conduction is achieved, jointly constructing a composite interface layer with both ion and electron conduction capabilities. Therefore, through the synergistic effect of each layer, the structural and interface stability of the cathode active material under high voltage can be significantly improved, thereby enhancing the cycle life of the lithium-ion battery.

[0039] In this embodiment, the general chemical formula of O2 phase lithium cobalt oxide is Li x Co y M z O2; wherein 0.9≤x≤1.05, 0.98≤y≤1, 0≤z≤0.02, and M includes at least one of the elements Al, Mg, Ti, Zr, Ni, Mn, La, Ce, Y, and W. That is, the O2 phase lithium cobalt oxide can be a conventional LiCoO2 material or a bulk-doped O2 phase lithium cobalt oxide, whose XRD analysis results show a characteristic peak at (18.6±0.2)° and a c-axis interlayer spacing of 1.42 nm to 1.45 nm, specifically, for example, 1.43 nm.

[0040] XRD (X-ray Diffraction) analysis is a testing method that utilizes the interaction between X-rays and crystalline materials to obtain information such as the crystal structure, phase composition, and grain size of the material by analyzing the diffraction pattern. In this application, the crystal phase structure of lithium cobalt oxide can be confirmed through XRD analysis.

[0041] In this embodiment, the matrix is ​​synthesized using an ion exchange method. Specifically, the method for synthesizing O2-phase lithium cobalt oxide via ion exchange includes: 1. Precursor preparation: P2 phase Na was synthesized by co-precipitation or solid-state method. 0.7 CoO2 precursor. For example, sodium carbonate (Na2CO3) and cobalt tetroxide (Co3O4) in stoichiometric ratio are thoroughly mixed and ground, and then calcined in air at 800°C to 900°C for 10 to 20 hours to obtain sodium cobalt oxide powder with a P2 type layered structure.

[0042] 2. Ion exchange: The above P2 phase Na 0.7 CoO2 precursor powder is immersed in an excess of a eutectic salt mixture of lithium nitrate (LiNO3) or lithium chloride (LiCl) and potassium chloride (KCl) heated to a molten state, and undergoes an ion exchange reaction for 6 to 24 hours in an inert atmosphere (such as argon) at 250°C to 350°C.

[0043] 3. Washing and drying: After the reaction is complete, the product is repeatedly washed with deionized water or ethanol to thoroughly remove residual sodium and lithium salts. Then, it is vacuum dried at 80℃~120℃ for 6 hours~12 hours to obtain the target O2 phase LiCoO2 matrix material.

[0044] 4. Structural confirmation: The product was characterized by XRD. Its spectrum should be consistent with the structural characteristics of the O2 phase, that is: the main peak (003) is located at (18.6±0.2)° (Cu-Kα radiation), and the c-axis interlayer spacing calculated according to the Bragg equation is about 1.42 nm~1.45 nm.

[0045] Thus, the O2 phase lithium cobalt oxide matrix prepared by ion exchange refers to a layered lithium cobalt oxide crystal phase structure with an oxygen layer stacking sequence of ABBA and lithium ions located in octahedral positions. It is different from the O3 phase lithium cobalt oxide with an oxygen layer stacking sequence of ABCABC and has good structural reversibility and high voltage stability.

[0046] In the embodiments of this application, the average particle size Dv50 of the substrate is 1μm to 20μm, preferably 3μm to 12μm. Specifically, it includes, for example, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, or any range between these values. This particle size range ensures that the positive electrode active material has good tap density and electrode processing performance, while providing a suitable specific surface area for subsequent coating processing. Furthermore, if the particle size of the substrate is too small, the specific surface area of ​​the substrate will be too large, leading to increased side reactions between the positive electrode active material and the electrolyte; if the particle size is too large, the lithium-ion diffusion path will increase, resulting in a decrease in the rate performance of the battery.

[0047] Dv50 refers to the particle size value corresponding to the cumulative volume percentage in the material volume distribution reaching 50%, i.e., the median particle size.

[0048] In the embodiments of this application, the total thickness of the first coating layer and the second coating layer is ≤200nm; preferably ≤100nm. Therefore, the positive electrode active material obtained after coating with the T2 phase interface layer and the bifunctional coating layer has a final average particle size Dv50 that increases by no more than 200nm compared to the original matrix material. This keeps the final particle size of the positive electrode active material between 1μm and 20.2μm. The size change of the positive electrode active material caused by the coating with the T2 phase interface layer and the bifunctional coating layer is minimal, which means that while achieving multi-layer nanoscale coating, the bulk size of the positive electrode active material is hardly changed, which is beneficial to maintaining the pore structure and ion transport pathway of the positive electrode.

[0049] In this embodiment, the first coating layer, namely the T2 phase interface layer, is constructed by performing low-temperature heat treatment on the substrate. Specifically, the O2 phase lithium cobalt oxide (substrate) can be heat-treated in an oxygen-containing atmosphere at a temperature of 250°C to 400°C for 1 to 20 hours, thereby forming the T2 phase lithium cobalt oxide on the surface of the O2 phase lithium cobalt oxide. Specific heat treatment temperatures include, for example, 250°C, 280°C, 300°C, 320°C, 350°C, 380°C, 400°C, or any value within the above range. Specific heat treatment times include, for example, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, or any value within the above range.

[0050] T2 phase lithium cobalt oxide refers to a layered lithium cobalt oxide crystal phase structure with triclinic symmetry, which is reversibly generated during the charging process of O2 phase lithium cobalt oxide. In the embodiments of this application, by pre-constructing a T2 phase interface layer, which serves as a structural buffer zone connecting the O2 phase core and the surface coating layer, the phase transition characteristics of the O2 phase are actively utilized to transform the originally harmful phase transition stress into elements that can achieve structural buffering, thereby maintaining the integrity of the particle structure during charging and discharging.

[0051] In this embodiment, the first coating layer further includes O2 phase lithium cobalt oxide. That is, the first coating layer also includes a small amount of incompletely converted O2 phase lithium cobalt oxide, and the relative content ratio of T2 phase lithium cobalt oxide to O2 phase lithium cobalt oxide is ≥4:1. Specifically, the content of T2 phase lithium cobalt oxide and O2 phase lithium cobalt oxide in the intermediate product (powder) obtained after heat treatment of the matrix can be characterized by XRD. The XRD characterization results show that the content of T2 phase lithium cobalt oxide in the surface region of the matrix, i.e., the phase transition region, is ≥80%, meaning that the first coating layer is mainly composed of T2 phase lithium cobalt oxide, which can fully utilize its phase transition buffering function, while also possessing a certain degree of structural reversibility and stability, and maintaining high stability even in high-voltage environments.

[0052] In this embodiment, the thickness of the first coating layer is 10 nm to 100 nm. Specifically, it can be 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any range between the above values. If the thickness of the first coating layer is less than 10 nm, the effect of buffering phase transition stress is insufficient; if it is greater than 100 nm, it may affect the rapid insertion and extraction kinetics of lithium ions.

[0053] In this embodiment, the fast ion conductor layer contains lithium-ion conductivity ≥10. -6 Inorganic materials with a conductivity of S / cm. Specifically, they may include at least one of lithium phosphorus oxynitride (LiPON), amorphous Li3PO4, and amorphous lithium lanthanum zirconium oxide (LLZO). The high lithium-ion conductivity and wide electrochemical window of the fast-ion conductor layer enable them to maintain good ion conduction performance at high voltages and to be well-compatible with high-voltage charge-discharge conditions, i.e., they can still operate stably without decomposition at voltages of 4.6V and above.

[0054] The fast ion conductor layer is formed by atomic layer deposition (ALD) on the surface of the first cladding layer (T2 phase interface layer). Specifically, a lithium-containing precursor and a doped element precursor are deposited alternately using ALD; wherein the doped element precursor includes at least one element: phosphorus, nitrogen, lanthanum, and zirconium.

[0055] The lithium-containing precursors involved here include compounds such as lithium tert-butyl oxide, lithium bis(trimethylsilyl)amino, and lithium cyclopentadienyl, while the precursors containing doped elements include compounds such as tris(dimethylamino)phosphine, triethyl phosphate, tetra(diethylamino)zirconium, and tris(isopropylcyclopentadienyl)lanthanum.

[0056] Atomic Layer Deposition (ALD) technology involves alternately introducing gaseous precursor pulses into the reaction chamber and purging with an inert gas between each precursor pulse. In this way, precursor molecules are chemically adsorbed onto the substrate surface layer by layer, in units of single atomic layers, and react, thereby achieving precise, digital control of film thickness at the atomic scale.

[0057] The specific method includes: placing the lithium cobalt oxide positive electrode active material with the first coating layer in the ALD reaction chamber, and performing 20 to 200 ALD cycles on the lithium-containing precursor and the doped element precursor by alternating pulses at a deposition temperature of 100℃ to 300℃, thereby forming a continuous and dense fast ion conductor layer on the surface of the T2 phase interface layer. The thickness of the film deposited by ALD is precisely controllable and has excellent shape retention.

[0058] The resulting fast ion conductor layers, such as lithium phosphorus oxynitride (LiPON), amorphous Li3PO4, and amorphous lithium lanthanum zirconium oxide (LLZO), although different in composition, all possess excellent high-voltage oxidation stability, superior lithium-ion conductivity, and electronic insulation, and are perfectly compatible with ALD processes. As pure ion conductors (electronic insulators), they can conduct only Li... + It isolates electrons, effectively suppresses the side reaction of oxidative decomposition of electrolyte under high voltage at the positive electrode, and does not increase leakage current caused by electron tunneling. It also has the characteristics of "high voltage resistance, ultra-thin and interface compatibility".

[0059] Specifically, lithium phosphorus oxynitride (LiPON) is essentially nitrogen-doped Li3PO4. The introduction of nitrogen significantly broadens its electrochemical window, enabling it to reach voltages above 5.5V. It exhibits perfect chemical and electrochemical inertness to O2-phase lithium cobalt oxide at voltages of 4.6V and higher, and is not easily oxidized or decomposed. Amorphous Li3PO4, due to its denser amorphous structure and lack of grain boundary defects, effectively blocks the electrolyte. Although its window is slightly lower than LiPON, it remains stable at 4.6V. Amorphous LLZO, with its garnet structure, possesses extremely high voltage stability. Amorphous LLZO films avoid grain boundary defects, giving them better flexibility to adapt to volume changes. More importantly, these fast ion conductor layers do not contain variable-valence transition metal elements (such as Ti, Ge, etc.), and are not easily oxidized under high voltage, thus ensuring the stability of the material structure and avoiding side reactions such as decreased conductivity and oxygen release. They are also less likely to undergo adverse interfacial reactions with active materials or electrolytes. Compared with titanium-containing fast ion conductors such as LATP, the material selected in this application can simultaneously construct a positive electrode interface with both high ion conductivity and chemical stability.

[0060] Moreover, the amorphous lithium phosphorus oxynitride (LiPON) layer, amorphous Li3PO4 layer, or amorphous lithium lanthanum zirconium oxide (LLZO) layer formed by ALD technology has the characteristics of nanoscale thickness, uniformity, density, and excellent shape preservation compared to multi-component complex materials LATP, which can avoid the ion conduction inhomogeneity and defect sites caused by grain boundaries.

[0061] In this embodiment, the nitrogen-doped carbon layer is formed by deposition on the surface of the fast ion conductor layer using chemical vapor deposition (CVD). Specifically, the nitrogen-doped carbon layer is formed by depositing carbon source gas and nitrogen source gas on the surface of the fast ion conductor layer using CVD; wherein the volumetric flow rate ratio of the carbon source gas to the nitrogen source gas is (1-10):1.

[0062] The carbon sources involved here include compounds such as ethylene and acetylene. The nitrogen sources involved here include compounds such as ammonia, pyridine vapor, and ethylenediamine.

[0063] Chemical vapor deposition (CVD) is a technique that uses gaseous precursors to react chemically on a substrate surface to form a solid film. CVD is typically a continuous process in which the precursor is simultaneously or continuously introduced into the reaction chamber, where it undergoes thermal decomposition or chemical reaction on or near the heated substrate surface to form a deposit.

[0064] More specifically, without disrupting the vacuum or inert atmosphere, chemical vapor deposition can be used to deposit a nitrogen-doped carbon layer on the surface of the fast ion conductor layer: the chamber temperature is raised to 500℃~800℃, and carbon source gas and nitrogen source gas are introduced with a volume flow ratio of (1~10):1. The reaction is carried out at atmospheric pressure or low pressure (0.1kPa~10kPa) for 5min~60min, thereby growing a nitrogen-doped carbon layer in situ on the surface of the fast ion conductor layer.

[0065] The nitrogen-doped carbon layer formed by introducing nitrogen atoms can effectively regulate the electronic structure and surface chemical properties of carbon materials, enhancing conductivity and chemical compatibility with interfaces. Simultaneously, the active sites such as pyridine nitrogen in the nitrogen-doped carbon layer can synergistically interact with electrolyte additives (such as lithium difluorophosphate) to catalyze the formation of a stable and LiF-rich CEI film.

[0066] In this embodiment, the total thickness of the second coating layer is 5nm to 100nm; preferably 5nm to 50nm. Specific examples include 5nm, 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm, 55nm, 60nm, 65nm, 70nm, 75nm, 80nm, 85nm, 90nm, 95nm, 100nm, etc., or any value within the above range.

[0067] In the second coating layer, the thickness of the fast ion conductor layer is 3 nm to 10 nm, and the thickness of the nitrogen-doped carbon layer is 2 nm to 40 nm. Specifically, the thickness of the fast ion conductor layer can be 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or any value within the aforementioned range. The thickness of the nitrogen-doped carbon layer can be 2 nm, 5 nm, 8 nm, 10 nm, 12 nm, 15 nm, 18 nm, 20 nm, 22 nm, 25 nm, 28 nm, 30 nm, 32 nm, 35 nm, 38 nm, 40 nm, or any value within the aforementioned range.

[0068] The fast ion conductor layer and the nitrogen-doped carbon layer are constructed sequentially through a series of processes (atomic layer deposition and chemical vapor deposition), forming a nanoscale, ultrathin, continuous multilayer composite multifunctional coating system. This allows the second coating layer to possess good ionic conductivity, electrical conductivity, and physical shielding capabilities, thereby suppressing side reactions without affecting the rapid transport of lithium ions.

[0069] The method for preparing the positive electrode active material provided in this application includes the following steps: 1) Heat-treat the O2 phase lithium cobalt oxide to form the T2 phase lithium cobalt oxide on the surface of the O2 phase lithium cobalt oxide, and obtain the pretreated lithium cobalt oxide material; 2) A fast ion conductor layer is deposited on the surface of pretreated lithium cobalt oxide material using atomic layer deposition (ALD) technology; 3) A nitrogen-doped carbon layer is deposited on the surface of a fast ion conductor layer using chemical vapor deposition (CVD).

[0070] Specifically, the heat treatment in step 1) is carried out in an oxygen-containing atmosphere at a temperature of 250℃ to 400℃ for 1 hour to 20 hours. The oxygen-containing atmosphere can be, for example, an oxygen atmosphere or an air atmosphere. Low-temperature heat treatment of O2-phase lithium cobalt oxide within the above temperature range can transform its surface portion into a T2 phase structure without damaging the main O2 phase structure. If the temperature is too low, the T2 phase lithium cobalt oxide cannot be effectively formed; if the temperature is too high, the main O2 phase structure may be destroyed.

[0071] Step 2) involves the fast-ion conductor layer formed by alternating deposition of lithium-containing precursors and doped element precursors using atomic layer deposition (ALD). The doped element precursors include at least one element: phosphorus, nitrogen, lanthanum, and zirconium. The lithium-containing precursors include compounds such as lithium tert-butyl oxide, lithium bis(trimethylsilyl)amino, and lithium cyclopentadienyl, while the doped element precursors include compounds such as tris(dimethylamino)phosphine, triethyl phosphate, tetra(diethylamino)zirconium, and tris(isopropylcyclopentadienyl)lanthanum.

[0072] In step 3), the nitrogen-doped carbon layer is formed by depositing carbon source gas and nitrogen source gas on the surface of the fast ion conductor layer using chemical vapor deposition (CVD). The volumetric flow rate ratio of the carbon source gas to the nitrogen source gas is (1–10):1. Within this range, a carbon layer with moderate nitrogen doping and a uniform structure can be formed. The carbon source includes compounds such as ethylene and acetylene, and the nitrogen source includes compounds such as ammonia, pyridine vapor, and ethylenediamine.

[0073] This application also provides a positive electrode sheet, which includes a positive current collector and a positive electrode material layer disposed on the surface of the positive current collector. The positive electrode material layer includes the above-described positive electrode active material or a positive electrode active material prepared by the above-described preparation method. The positive current collector is a conventional metal foil or a composite current collector, such as aluminum foil.

[0074] In this embodiment, the positive electrode material layer is formed by coating a positive electrode slurry onto the surface of the positive electrode current collector. The positive electrode slurry includes the aforementioned positive electrode active material, as well as a conductive agent and a binder. In some embodiments, the mass percentage of each component in the positive electrode material layer is: 80wt%~99wt% of positive electrode active material, 0.3wt%~10wt% of conductive agent, and 0.5wt%~15wt% of binder.

[0075] This application does not specifically limit the types of conductive agents and binders, and they can be selected according to actual needs. As an example, conductive agents include, but are not limited to, at least one of superconducting carbon, acetylene black, conductive carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; binders include, but are not limited to, at least one of polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins.

[0076] The positive electrode active material, conductive agent and binder are dispersed in a solvent, which can be N-methylpyrrolidone or deionized water, to obtain a uniform positive electrode slurry. The positive electrode slurry is then uniformly coated onto the positive electrode current collector, and the positive electrode sheet is obtained through processes such as drying, rolling and compaction.

[0077] The positive electrode sheet manufactured in the embodiments of this application has unique characteristics that can be detected by standard analytical methods.

[0078] XPS analysis of the positive electrode revealed simultaneous detection of orbital characteristic peaks from both the fast ion conductor layer and the nitrogen-doped carbon layer within a depth range of 5 nm to 50 nm from the surface of the positive electrode material layer towards the bulk phase. The orbital characteristic peaks from the fast ion conductor layer included at least one of the P 2p, La 3d, and Zr 3d orbitals. This indicates the simultaneous presence of a fast ion conductor layer and a nitrogen-doped carbon layer within a depth range of 5 nm to 20 nm on the surface of the positive electrode active material, confirming the existence of a bilayer coating structure.

[0079] For example, when the fast ion conductor layer is lithium phosphorus oxynitride or amorphous Li3PO4, a characteristic peak of the P 2p orbital with a binding energy in the range of (133.5±0.5) eV can be detected. This means that the phosphorus-containing component in the fast ion conductor layer exists stably on the cathode surface and has not been detached or decomposed in subsequent processes.

[0080] The orbital characteristic peaks from the nitrogen-doped carbon layer include the N 1s orbital characteristic peak, and the proportion of pyridine nitrogen (N-6) in the N 1s orbital characteristic peak is not less than 40%; preferably 40% to 60%. This means that there are sufficient active sites in the nitrogen-doped carbon layer, which is conducive to the interaction with the electrolyte to form a stable interface.

[0081] Understandably, pyridine nitrogen is one of the various nitrogen configurations in nitrogen-doped carbon materials. By controlling the volumetric flow rate ratio of carbon source gas to nitrogen source gas in the range of (1 to 10):1 during the chemical vapor deposition step, the proportion of pyridine nitrogen can be controlled in the range of 40% to 60%. This avoids the excessive number of defect sites in the nitrogen-doped carbon layer due to the excessive proportion of pyridine nitrogen, which would lead to a decrease in the chemical stability of the coating layer.

[0082] Among them, XPS (X-ray Photoelectron Spectroscopy) analysis is a surface analysis technique based on the photoelectric effect. It is a testing method that analyzes the elemental composition, valence state and chemical environment by detecting the photoelectron energy emitted by atoms on the surface of a material after being excited by X-rays.

[0083] TOF-SIMS analysis of the positive electrode revealed that, in the secondary ion image, the spatial distribution overlap between the active material region characterized by the first characteristic fragment and the second coating layer region characterized by the second characteristic fragment was no less than 90%. The first characteristic fragment was CoO2. - (m / z=91) fragment, the second characteristic fragment is PO2. - (m / z=63) fragments, LaO + Fragments, ZrO - Fragments or CN - (m / z=26) fragments. This means that the coating has extremely high coverage and good continuity on the surface of the active particles, with no large exposed areas.

[0084] For example, when the fast ion conductor layer is lithium phosphorus oxynitride or amorphous Li3PO4, it is possible to detect the presence of CoO2. - (m / z=91) The region characterized by fragments and PO2 - The signal distribution overlap in the fragment characterization region (m / z=63) is no less than 90%, indicating that the phosphorus-containing fast ion conductor layer is uniformly and completely coated on the surface of the active particles.

[0085] TOF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) analysis is a highly sensitive surface analysis technique that uses a high-energy primary ion beam to bombard the sample surface, causing surface atoms or molecules to ionize. The mass-to-charge ratio (m / z) of the secondary ions is determined by measuring their flight time, and the spatial distribution information of the elements is obtained.

[0086] This application also provides a lithium-ion battery, which includes the above-described positive electrode sheet.

[0087] In this embodiment, the lithium-ion battery further includes an electrolyte, a separator, and a negative electrode. The negative electrode, separator, and positive electrode are sequentially stacked and then formed into a cell using a winding or stacking process. The cell is then placed into a pre-formed aluminum-plastic film, baked, and then the electrolyte is injected into the baked and dried cell, immersing it in the electrolyte. Following vacuum sealing, settling, and formation processes, the lithium-ion battery is complete.

[0088] In the embodiments of this application, the electrolyte includes an organic solvent, a lithium salt, and additives. The solvent may be selected from one or more of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, ethyl propionate, ethyl acetate, and propyl propionate; preferably ethylene carbonate and methyl ethyl carbonate. The lithium salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorosulfonyl imide, lithium difluoromethyl imide, lithium difluorooxalate phosphate, and lithium perchlorate; preferably lithium difluorosulfonyl imide. The concentration of the lithium salt in the electrolyte is 0.9 mol / L to 1.3 mol / L.

[0089] In the embodiments of this application, the additive includes lithium difluorophosphate (LiDFP).

[0090] In the embodiments of this application, the mass percentage of lithium difluorophosphate in the electrolyte is 0.05% to 5%; preferably 0.1% to 3%; more preferably 0.5% to 2%. Specifically, it can be 0.05%, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, etc., or any value within the above range.

[0091] Lithium difluorophosphate (LiDFP) can synergistically interact with the aforementioned positive electrode surface at high voltages to catalyze the formation of a stable and LiF-rich cathode electrolyte interphase (CEI) film. Specifically, during the initial charging of the battery to a high voltage, LiDFP undergoes an (electro)chemical reaction at the positive electrode surface earlier than the main salt (lithium salt) and solvent, and its decomposition products (such as Li...) x PO y F2, LiF, etc., will deposit on the surface of the positive electrode active material, forming a dense solid-state interfacial film (CEI). LiF in the CEI film is an excellent electronic insulator but has moderate Li content. + Its conductivity effectively prevents continuous oxidation of the electrolyte while allowing Li to... + Furthermore, the CEI membrane can physically isolate the active material from direct contact with the electrolyte, significantly inhibiting side reactions such as cobalt ion dissolution, oxygen release, and gas production.

[0092] When conventional carbon coatings or bare cathode surfaces are used in conjunction with electrolytes containing LiDFP additives, the decomposition of LiDFP may be uneven or the resulting CEI film may be unstable. However, in the cathode electrode fabricated in this application, the outermost layer of the positive electrode active material has a nitrogen-doped carbon layer, which provides a good electronic conductivity network, facilitating uniform electrochemical oxidation and decomposition of LiDFP. Furthermore, the fast ion conductor layer (such as a LiPON fast ion conductor layer) possesses unique surface chemical properties that catalyze the decomposition pathway of LiDFP, promoting the formation of more and more stable LiF, and resulting in a more uniform, thinner, and lower impedance CEI film.

[0093] Furthermore, the lithium-ion battery after 800 cycles was disassembled, and its positive electrode was analyzed by XPS. The results showed that in the F 1s spectrum, the ratio of the characteristic peak area of ​​LiF at (684.5±0.2) eV to that of PVDF at (687.5±0.2) eV was not less than 1.5. This indicates that the relative content of LiF in the interfacial film is high, thus enabling the formation of a more stable CEI interface and stronger protection of the positive electrode interface. This is precisely because the synergistic effect of lithium difluorophosphate in the electrolyte and the positive electrode leads to the generation of abundant LiF on the positive electrode surface, thereby significantly improving interfacial stability and thus significantly improving the cycle life of the lithium-ion battery.

[0094] In the embodiments of the present application, the additive further includes fluoroethylene carbonate (FEC). The mass percentage of fluoroethylene carbonate in the electrolyte is 0.05% to 5%; preferably 1% to 3%. Specifically, it can be 0.05%, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, etc., or any value within the above range. As a film-forming additive in the electrolyte, FEC can form a stable SEI film on the surface of the negative electrode, while LiDFP can protect the high-voltage positive electrode interface. The two work together to achieve the ultra-long cycle life of the lithium-ion battery at high voltages.

[0095] The separator and the negative electrode tab can be arbitrarily selected from known materials, and the present application does not limit this.

[0096] In some embodiments, the separator described in the present application can be arbitrarily selected from porous structure separators with good chemical stability and mechanical stability that are well-known. The material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0097] In some embodiments, the negative electrode tab can be a lithium metal sheet. In some other embodiments, the negative electrode tab can also include a negative electrode current collector and a negative electrode material layer provided on the negative electrode current collector. Among them, the negative electrode current collector uses a conventional metal foil or a composite current collector, for example, a copper foil can be used. In some specific embodiments, the negative electrode material layer is formed by coating a negative electrode slurry on the surface of the negative electrode current collector. The negative electrode slurry includes a negative electrode active material, a conductive agent, and a binder. The embodiments of the present application do not specifically limit the types of the negative electrode active material, the conductive agent, and the binder, and can be selected according to actual needs. As an example, the negative electrode active material can be natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO m (0 < m < 2, such as m = 1), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12 , Li-Al alloy, and metallic lithium; the conductive agent can be one or more of graphite, superconducting carbon, acetylene black, conductive carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the binder can be one or more of styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, and carboxymethyl cellulose.

[0098] The negative electrode sheet can be prepared according to conventional methods in the art. For example, the negative electrode active material, conductive agent, and binder are dispersed in a solvent, which can be N-methylpyrrolidone (NMP) or deionized water, to form a uniform negative electrode slurry. The negative electrode slurry is then coated onto the negative electrode current collector, and after drying, rolling, and compaction, the negative electrode sheet is obtained.

[0099] The lithium-ion battery of this application embodiment is suitable for high-voltage systems, such as high-voltage systems with a cutoff voltage of 4.6V.

[0100] This application does not impose any particular restrictions on the application fields of lithium-ion batteries, and they can be used in consumer electronics products, new energy vehicle-grade energy storage, and other fields.

[0101] To make the present invention easier to understand, the present application will be further described in detail below with reference to embodiments. These embodiments are for illustrative purposes only and are not limited to the scope of application of the present application. Unless otherwise specified, the raw materials or components used in the present application can be obtained commercially or by conventional methods.

[0102] Example 1: Preparation of Positive Electrode Active Material 1) Synthesis of O2 phase lithium cobalt oxide matrix Sodium carbonate (Na₂CO₃) and cobalt tetroxide (Co₃O₄) were thoroughly mixed and ground, then calcined in air at 850°C for 15 hours to obtain sodium cobalt oxide powder with a P₂-type layered structure. The P₂-phase sodium cobalt oxide powder was immersed in a lithium-containing molten salt system composed of LiNO₃ and LiCl (mass ratio 3:7), and a molten salt ion exchange reaction was carried out at 280°C under an inert atmosphere (argon) for 15 hours. After the reaction, the product was washed three times with deionized water and dried under vacuum at 80°C for 12 hours to obtain the LiCoO₂ matrix material.

[0103] XRD analysis showed that the main peak was located at (18.6±0.1)° and the c-axis interlayer spacing was 1.43 nm, confirming it as an O2 phase structure.

[0104] 2) Place the above-mentioned O2 phase lithium cobalt oxide matrix in a box furnace and heat treat it at 300°C for 5 hours in an air atmosphere, then allow it to cool naturally to room temperature.

[0105] HRTEM observation revealed a T2 phase interface layer with a thickness of approximately 50 nm on the particle surface. XRD refinement showed that the T2 phase content on the particle surface was greater than 80%.

[0106] 3) A LiPON layer was deposited on the surface of the above-mentioned material (containing a T2 phase interface layer) as an inorganic fast ion conductor layer using thermal atomic layer deposition (ALD) technology. The deposition process was as follows: lithium tert-butyl oxide (LiOtBu) was used as the lithium source precursor, tris(dimethylamino)phosphine (TDMAP) was used as the phosphorus and nitrogen co-source precursor, and high-purity water (H2O) was used as the oxygen source precursor. The deposition temperature was 200℃, and the cycle was 50 times.

[0107] HRTEM observation revealed that an amorphous LiPON film with a thickness of approximately 5 nm was formed on the particle surface.

[0108] 4) Subsequently, a nitrogen-doped carbon layer was deposited on the LiPON layer surface using chemical vapor deposition (CVD). The deposition process was as follows: acetylene (C2H2) was used as the carbon source gas, and ammonia (NH3) was used as the nitrogen source gas, with a volumetric flow rate ratio of C2H2:NH3 = 3:1 (sccm). The deposition temperature was 700℃, and the deposition time was 15 minutes. HRTEM observation showed that a nitrogen-doped carbon layer with a thickness of approximately 5 nm was grown in situ on the particle surface, ultimately obtaining the positive electrode active material.

[0109] The test methods for material physics and structural characterization of the positive electrode active material prepared in Example 1 are as follows: (1) Crystal structure analysis - X-ray diffraction (XRD) Test equipment: X-ray diffractometer (Cu-Kα radiation source) Sample preparation: Take an appropriate amount of positive electrode material powder and fill it evenly into the sample cell. Test parameters: scanning range (10°~80°, 2θ), scanning speed (2° / min), step size (0.02°).

[0110] (2) Microscopic morphology and coating layer observation - scanning / transmission electron microscopy Testing equipment: Field emission scanning electron microscope (FESEM), high resolution transmission electron microscope (HRTEM) Sample preparation: SEM: Powder sample dispersed in conductive adhesive; TEM: The powder sample was ultrasonically dispersed and then dropped onto an ultrathin carbon film.

[0111] Example 2: The same preparation method as Example 1 was used, except that the thickness of the second coating layer was adjusted by changing the processing time of ALD and CVD. Specifically, the thickness of the crystalline LiPON film formed in step 3) was 8 nm, the thickness of the nitrogen-doped carbon layer formed in step 4) was 12 nm, and the total thickness of the second coating layer was 20 nm.

[0112] Example 3: The same preparation method as Example 1 was used, except that the O2 phase lithium cobalt oxide matrix formed in step 1) was modified with Al doping, i.e., the matrix was O2 phase Li. 0.98 Co 0.92 Al 0.08 O2 materials.

[0113] Example 4: The difference from Example 1 is that the ALD process in step 3) is adjusted as follows: Amorphous Li3PO4 layers were deposited as fast ion conductors on the surface of the aforementioned material (containing a T2 phase interface layer) using thermal atomic layer deposition (ALD). The deposition process was as follows: lithium tert-butyl oxide (LiOtBu) was used as the lithium source precursor, trimethyl phosphate (TMPO) as the phosphorus source precursor, and high-purity water (H2O) as the oxygen source precursor. The deposition temperature was 180℃, and the cycle was 60 times to obtain an amorphous Li3PO4 film with a thickness of approximately 6 nm.

[0114] Example 5: The difference from Example 1 is that the ALD process in step 3) is adjusted as follows: Amorphous LLZO layer was deposited as a fast ion conductor layer on the surface of the aforementioned material (containing a T2 phase interface layer) using thermal atomic layer deposition (ALD) technology. The deposition process was as follows: lithium tert-butyl oxide (LiOtBu) was used as the lithium source precursor, tetratetra(diethylamino)zirconium (TDEAZ) was used as the zirconium source, tris(diethylamino)lanthanum (TDEAL) was used as the lanthanum source, and high-purity water (H2O) was used as the oxygen source precursor. The deposition temperature was 200℃, and the cycle was 80 times to obtain an amorphous LLZO film with a thickness of approximately 8 nm.

[0115] Example 6: The difference from Example 1 lies in the process adjustments in steps 3) and 4: The powder material obtained in step 2) was dispersed in an ethanol solution containing lithium acetate and ammonium dihydrogen phosphate, stirred, and evaporated to dryness. Then, it was sintered in air at 400°C for 2 hours to generate a Li3PO4 coating layer. The above material was redispersed in a polyacrylonitrile (PAN) solution, evaporated to dryness, and then carbonized in argon at 600°C for 2 hours to generate a nitrogen-doped carbon coating layer.

[0116] Comparative Example 1: The difference from Example 1 is that no first and second coating layers are applied. That is, the O2 phase LiCoO2 matrix material obtained in step 1) is used as the positive electrode active material.

[0117] Comparative Example 2: The difference from Example 1 is that no second coating layer is applied. That is, the material obtained in step 2) is used as the positive electrode active material.

[0118] Comparative Example 3: The difference from Example 1 is that only nitrogen-doped carbon coating is performed. The specific steps include: A nitrogen-doped carbon layer was deposited on the surface of the O2-phase LiCoO2 substrate material prepared in step 1) using chemical vapor deposition (CVD). The deposition process was as follows: acetylene (C2H2) was used as the carbon source gas, and ammonia (NH3) was used as the nitrogen source gas. The volumetric flow rate ratio of the carbon source gas to the nitrogen source gas was C2H2:NH3 = 3:1 (sccm). The deposition temperature was 700℃, and the deposition time was 15 minutes, resulting in the in-situ growth of a nitrogen-doped carbon layer with a thickness of approximately 5 nm on the surface, ultimately obtaining the positive electrode active material.

[0119] The positive electrode active materials obtained in the above embodiments and comparative examples were used to make positive electrode sheets, and then combined with negative electrode sheets, electrolytes, separators, etc. to make lithium-ion batteries. The electrochemical performance of the lithium-ion batteries was tested to observe the influence of the positive electrode active materials on the electrochemical performance of the batteries.

[0120] Application Example 1 1. Preparation of positive electrode sheet The positive electrode active material prepared in Example 1 above was mixed with the conductive agent Super-P and the binder PVDF (polyvinylidene fluoride) at a mass ratio of 96:2:2. The mixture was thoroughly stirred in N-methylpyrrolidone solvent to form a uniform positive electrode slurry. The positive electrode slurry was uniformly coated onto the positive electrode current collector Al foil, and then dried, rolled, and cut to obtain the positive electrode sheet.

[0121] 2. Preparation of negative electrode sheet The negative electrode active material graphite, conductive agent conductive carbon black, binder SBR (styrene-butadiene rubber), and thickener CMC (carboxymethyl cellulose) are mixed in a mass ratio of 95:1.5:2.5:1.0. Deionized water is added and stirred to disperse the mixture evenly, thus obtaining the negative electrode slurry. The negative electrode slurry is uniformly coated onto the negative electrode current collector copper foil, and after drying, rolling, and cutting, the negative electrode sheet is obtained.

[0122] 3. Electrolyte preparation In an argon-filled glove box (water content <10 ppm), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed thoroughly at a volume ratio of 3:7 to obtain a mixed solvent. Lithium bis(fluorosulfonyl)imide (LiFSI) was added to the mixed solvent until the concentration reached 1.2 M. Then, 2% (by mass) of fluoroethylene carbonate (FEC) and 1% (by mass) of lithium difluorophosphate (LiDFP) were added as additives. The mixture was stirred until completely dissolved to obtain the electrolyte.

[0123] 4. Battery assembly A microporous single-layer polypropylene (PP) membrane was used as the separator. The positive electrode, separator, and negative electrode were sequentially stacked and wound into a cell, which was then placed in an aluminum-plastic film casing. After vacuum baking at 80℃ for 24 hours to remove moisture, electrolyte was injected, and the casing was sealed. After standing for 24 hours, it was charged at a constant current of 0.1C to 4.6V for formation. Following aging and capacity testing, a soft-pack lithium-ion battery with a rated capacity of 5000 mAh was obtained.

[0124] The surface chemical composition and interface of the positive electrode and lithium-ion battery prepared in the above application examples were analyzed, and their electrochemical performance was tested. The test methods are shown below.

[0125] (1) Chemical valence state analysis - X-ray photoelectron spectroscopy (XPS) Testing equipment: X-ray photoelectron spectrometer (monochromatic Al Kα source) Sample preparation: Before cycling: Take a sample of the positive electrode directly. After cycling: Vacuum transfer after DMC cleaning Test parameters: Binding energy calibrated to C 1s = 284.8 eV throughout.

[0126] (2) Elemental distribution analysis - Time-of-flight secondary ion mass spectrometry (TOF-SIMS) Testing equipment: TOF-SIMS V-type instrument Test conditions: Bi3+ primary ion source, static SIMS mode.

[0127] (3) Charge-discharge cycle test Using the Blue Electric testing system, charge and discharge were performed at 25℃ with a 0.1C rate within a voltage range of 3.0V to 4.6V. The initial charge capacity and initial discharge capacity were recorded, and the initial efficiency was calculated as (initial discharge capacity / initial charge capacity) × 100%. Then, 800 charge-discharge cycles were performed, and the discharge capacity on the 800th cycle was recorded. The cycle retention rate was calculated as (800th discharge capacity / initial discharge capacity) × 100%.

[0128] (4) Interfacial impedance testing - electrochemical impedance spectroscopy (EIS) After 800 cycles of battery cycling, EIS testing was performed using a Solartron electrochemical workstation. Test conditions: constant temperature 25°C, battery state of charge 50%, frequency range 100 kHz to 10 mHz, signal amplitude 5 mV. Charge transfer impedance (Rct) was obtained through equivalent circuit fitting. The increase in Rct (%) was calculated as: [(Rct after cycling - initial Rct) / initial Rct] × 100%.

[0129] The test results of the electrochemical performance are shown in Table 1.

[0130] Table 1

[0131] XPS testing was performed on the positive electrode sheet prepared according to Example 1 (the positive electrode active material of Example 1). The results showed that the material had a P 2p (133.5±0.5 eV) characteristic peak and an N 1s characteristic peak, and the proportion of pyridine nitrogen (N-6) in the N 1s characteristic peak was ≥40%. TOF-SIMS testing (for the uncycled electrode sheet) showed that CoO 2- (m / z=91) Signal region and PO 2- (m / z=63) overlap of signal regions and CoO 2- (m / z=91) Signal region and CN - The overlap of the signal regions (m / z=26) is ≥90%, indicating that the coating layer is continuous and intact.

[0132] The electrochemical performance test results in Table 1 show that: Compared to Comparative Example 1 (uncoated), the first-cycle efficiency of Example 1 increased from 88.2% to 93.8%, the cycle capacity retention significantly increased from 68.4% to 92.5%, the LiF / PVDF peak area ratio at the cathode interface after cycling increased from 0.5 to 1.8, and the increase in charge transfer impedance (Rct) after cycling significantly decreased from 120% to 28%. It is evident that the cathode active material obtained after the first and second coating layer treatments described in this application forms a continuous and complete coating layer and a LiF-rich interface film, significantly improving its cycle capacity retention and interface stability. In contrast, the unmodified O2-phase lithium cobalt oxide material 9 (Comparative Example 1) undergoes severe side reactions with the electrolyte interface under high voltage, leading to deterioration of structural stability and failing to meet the long-life requirements.

[0133] Compared to Comparative Example 2 (which only constructed the first coating layer (T2 phase interface layer coating)), Example 1 showed an improvement in first-cycle efficiency from 90.5% to 93.8%, a significant increase in cycle capacity retention from 75.1% to 92.5%, an increase in the LiF / PVDF peak area ratio at the cathode interface after cycling from 0.8 to 1.8, and a significant decrease in the increase in charge transfer impedance (Rct) after cycling from 85% to 28%. This demonstrates that while the introduction of the T2 phase interface layer in Comparative Example 2 provides some bulk structural stability, its surface remains directly exposed to the electrolyte, failing to effectively suppress interfacial side reactions and leading to increased impedance. This comparison proves that the T2 phase interface layer alone is insufficient to construct a stable interface and must work synergistically with a bifunctional coating layer.

[0134] Compared to Comparative Example 3 (traditional carbon coating), Example 1 showed an improvement in first-cycle efficiency from 89.8% to 93.8%, a significant increase in cycle capacity retention from 78.5% to 92.5%, an increase in the LiF / PVDF peak area ratio at the cathode interface after cycling from 0.9 to 1.8, and a significant decrease in the increase in charge transfer impedance (Rct) after cycling from 95% to 28%. It is evident that while the traditional carbon coating layer in Comparative Example 3 provides a certain physical barrier, it lacks an efficient ion conduction pathway and cannot effectively catalyze the formation of a stable LiF-rich CEI film. Therefore, the bifunctional coating layer formed by the "inner fast ion conductor layer + outer nitrogen-doped carbon layer" of this application can balance ion conduction, electronic conduction performance, and interfacial catalysis, representing a significant improvement over a single electronic conductor coating.

[0135] Compared to Example 6 (liquid phase coating method), Example 1 shows an improvement in initial efficiency from 89.0% to 93.8%, a significant increase in cycle capacity retention from 79.5% to 92.5%, an increase in the LiF / PVDF peak area ratio at the cathode interface after cycling from 0.7 to 1.8, and a significant decrease in the increase in charge transfer impedance (Rct) after cycling from 110% to 28%. It is evident that Example 1 outperforms Example 6, which uses the traditional liquid phase coating method, in all aspects. The "ALD+CVD" continuous process described in this application allows for atomic-level thickness control and perfect shape preservation, while CVD enables the uniform growth of a nitrogen-doped carbon layer on complex surfaces, resulting in an ultra-thin, dense, and complete gradient coating structure on the cathode active material surface. In contrast, the liquid phase method cannot achieve nanoscale precision control, and the high temperature during sintering may damage the O2 phase structure, leading to a significant deterioration in battery performance.

[0136] Data from Examples 1 and 2 (thick coating) show that the cycle capacity retention of Example 2 decreased from 92.5% to 89.8%, and the post-cycle charge transfer impedance (Rct) increased from 28% to 35%. While increasing the thickness of the second coating to 20 nm may provide stronger protection, it begins to have a significant negative impact on lithium-ion diffusion kinetics. Therefore, limiting the total thickness of the second coating to the range of 5 nm to 50 nm achieves the optimal balance between protection and ion transport performance, ensuring both high capacity and long cycle life.

[0137] Data from Examples 1 and 3 (Al doping) show that the cycling capacity retention of Example 3 increased from 92.5% to 93.2%, while the post-cycle charge transfer impedance (Rct) increase decreased from 28% to 25%. This indicates that bulk element doping (such as Al, Mg, Ti, La, Ce, Y, Zr, etc.) can produce a positive synergistic effect with the first and second coating layers of this application. This enhances the intrinsic structural strength of the material while effectively isolating it from external erosion, achieving a stability improvement from the inside out, resulting in a "1+1>2" effect.

[0138] Data from Examples 1 and 4 (Li3PO4) and Example 5 (LLZO) show that the three different fast-ion conductor layers (LiPON, Li3PO4, and LLZO) described in this application all achieved excellent cycle performance and interfacial stability, maintaining a cycle capacity retention of over 90%, and the LiF / PVDF peak area ratio at the cathode interface remained above 1.6 after cycling. This indicates that the fast-ion conductor layer described in this application meets the requirements for fast ion conduction and high-voltage stability, and the second coating layer (bifunctional coating layer) formed synergistically with the nitrogen-doped carbon layer can achieve a synergistic effect of ion conduction, electronic conduction, and interfacial catalysis, significantly improving the high-voltage cycle stability of the battery.

[0139] It should be noted that the embodiments described above are only for explaining this application and do not constitute any limitation on this application. This application has been described with reference to typical embodiments, but it should be understood that the terms used therein are descriptive and explanatory terms, not limiting terms. Modifications and revisions can be made to this application within the scope of the claims as prescribed, and without departing from the scope and spirit of this application. Although the application described herein relates to specific methods, materials, and embodiments, it does not mean that this application is limited to the specific examples disclosed herein; on the contrary, this application can be extended to all other methods and applications with the same function.

Claims

1. A positive electrode active material, characterized in that, It includes a substrate, a first coating layer disposed on the surface of the substrate, and a second coating layer disposed on the surface of the first coating layer; The substrate comprises O2-phase lithium cobalt oxide, the first coating layer comprises T2-phase lithium cobalt oxide, and the second coating layer comprises a fast ion conductor layer and a nitrogen-doped carbon layer arranged sequentially from the inside out; the fast ion conductor layer contains a lithium-ion conductivity ≥10. -6 Inorganic materials with S / cm.

2. The positive electrode active material according to claim 1, characterized in that, The fast ion conductor layer contains at least one of lithium phosphorus oxy nitrogen, amorphous Li3PO4, and amorphous lithium lanthanum zirconium oxide.

3. The positive electrode active material according to claim 1, characterized in that, The first coating layer further includes O2 phase lithium cobalt oxide; in the first coating layer, the relative content ratio of T2 phase lithium cobalt oxide and O2 phase lithium cobalt oxide is ≥4:

1.

4. The positive electrode active material according to claim 1, characterized in that, The general chemical formula of the O2 phase lithium cobalt oxide is Li x Co y M z O2; wherein 0.9≤x≤1.05, 0.98≤y≤1, 0≤z≤0.02, and M includes at least one of the elements Al, Mg, Ti, Zr, Ni, Mn, La, Ce, Y and W.

5. The positive electrode active material according to claim 1, characterized in that, The average particle size Dv50 of the matrix is ​​1μm to 20μm, preferably 3μm to 12μm; And / or, The total thickness of the first coating layer and the second coating layer is ≤200nm; preferably ≤100nm. Preferably, the thickness of the first coating layer is 10 nm to 100 nm, and the thickness of the second coating layer is 5 nm to 100 nm; Preferably, the thickness of the second coating layer is 5 nm to 50 nm; more preferably, in the second coating layer, the thickness of the fast ion conductor layer is 3 nm to 10 nm, and the thickness of the nitrogen-doped carbon layer is 2 nm to 40 nm.

6. The positive electrode active material according to claim 1, characterized in that, The fast ion conductor layer is formed by atomic layer deposition (ALD) technology; and / or, The nitrogen-doped carbon layer was formed by chemical vapor deposition.

7. A method for preparing a positive electrode active material as described in any one of claims 1 to 6, characterized in that, Includes the following steps: The O2 phase lithium cobalt oxide is heat-treated to form the T2 phase lithium cobalt oxide on the surface of the O2 phase lithium cobalt oxide, thus obtaining a pretreated lithium cobalt oxide material. The fast ion conductor layer was deposited on the surface of the pretreated lithium cobalt oxide material using atomic layer deposition (ALD) technology. The nitrogen-doped carbon layer was deposited on the surface of the fast ion conductor layer using chemical vapor deposition.

8. The preparation method according to claim 7, characterized in that, The heat treatment is carried out in an oxygen-containing atmosphere at a temperature of 250℃ to 400℃ for 1 hour to 20 hours. And / or, The fast ion conductor layer is formed by alternating deposition of a lithium-containing precursor and a doped element precursor using atomic layer deposition technology; wherein the doped element precursor includes at least one element: phosphorus, nitrogen, lanthanum, and zirconium. And / or, The nitrogen-doped carbon layer is formed by depositing carbon source gas and nitrogen source gas on the surface of the fast ion conductor layer using chemical vapor deposition technology; wherein the volume flow rate ratio of the carbon source gas and the nitrogen source gas is (1~10):

1.

9. A positive electrode sheet, characterized in that, It includes a positive current collector and a positive electrode material layer disposed on the surface of the positive current collector; the positive electrode material layer includes the positive electrode active material as described in any one of claims 1 to 6 or the positive electrode active material prepared by the preparation method as described in claim 7 or 8.

10. A lithium-ion battery, characterized in that, It includes an electrolyte, a negative electrode, and a positive electrode as described in claim 7.

11. The lithium-ion battery according to claim 10, characterized in that, The electrolyte includes an additive, which includes lithium difluorophosphate, and the mass percentage of lithium difluorophosphate in the electrolyte is 0.05% to 5%; preferably 0.1% to 3%; more preferably 0.5% to 2%.

12. The lithium-ion battery according to claim 11, characterized in that, XPS analysis of the positive electrode before cycling showed that it simultaneously included orbital characteristic peaks from the fast ion conductor layer and orbital characteristic peaks from the nitrogen-doped carbon layer; wherein, the orbital characteristic peaks from the fast ion conductor layer included at least one of P 2p orbital characteristic peaks, La 3d orbital characteristic peaks, and Zr 3d orbital characteristic peaks; the orbital characteristic peaks from the nitrogen-doped carbon layer included N1s orbital characteristic peaks, and the pyridine nitrogen content in the N 1s orbital characteristic peaks was not less than 40%; And / or, TOF-SIMS analysis of the positive electrode before cycling showed that the spatial distribution of the active material region characterized by the first characteristic fragment overlapped with that of the second coating layer region characterized by the second characteristic fragment by no less than 90%; wherein, the first characteristic fragment was CoO2. - (m / z=91) fragment, the second characteristic fragment is PO2 - (m / z=63) fragments, LaO + Fragments, ZrO - Fragments or CN - (m / z=26) fragments; and / or, XPS analysis of the positive electrode in the lithium-ion battery after 800 cycles showed that the ratio of the LiF characteristic peak area at (684.5±0.2) eV to the PVDF characteristic peak area at (687.5±0.2) eV in the F 1s spectrum was not less than 1.5.

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