A positive electrode material, a preparation method thereof, a positive electrode sheet, and a lithium ion battery

Abstract

This invention provides a cathode material, its preparation method, a cathode sheet, and a lithium-ion battery. The cathode material includes lithium cobalt oxide, the surface of which is coated with a zinc oxide-nitrogen-doped carbon coating layer. The cathode material provided by this invention, with the zinc oxide-nitrogen-doped carbon coating layer on the surface of the lithium cobalt oxide, can block direct contact between the electrolyte and the cathode active material under high voltage, inhibit the corrosion of the cathode by hydrofluoric acid generated from the oxidative decomposition of the electrolyte, and reduce lattice oxygen evolution and interfacial side reactions. Simultaneously, the zinc oxide sites can chemically passivate surface active defects of the cathode, and the nitrogen-doped carbon layer provides efficient ion transport channels and improves interfacial conductivity. The synergistic effect of these two elements can significantly inhibit cobalt ion dissolution, stabilize the cathode crystal structure and interfacial state, and alleviate the problems of increased battery impedance and capacity decay caused by interfacial instability and large-scale cobalt ion dissolution, thereby effectively improving the cycle stability and long cycle life of the lithium-ion battery under high voltage.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a cathode material and its preparation method, a cathode sheet, and a lithium-ion battery. Background Technology

[0002] Traditional lithium-ion batteries using O3-phase lithium cobalt oxide cathode materials suffer from irreversible phase transitions, lattice oxygen evolution, and intensified interfacial side reactions during high-voltage cycling at 4.5V and above, leading to rapid capacity decay and reduced safety. O2-phase lithium cobalt oxide, with its unique ABAB oxygen layer stacking sequence, possesses superior intrinsic structural stability and reversible phase transition capabilities, making it an ideal cathode material for high-voltage applications. However, O2-phase lithium cobalt oxide still presents certain challenges in practical cell applications. For instance, traditional electrolytes easily decompose under high pressure, producing hydrofluoric acid that corrodes the cathode surface and triggers massive cobalt ion dissolution, while simultaneously accelerating abnormal thickening of the solid electrolyte interfacial film at the anode. Oxidation side reactions occur between the active sites on the cathode surface and the electrolyte solvent, resulting in excessive gas production during high-temperature storage, affecting battery safety and lifespan. Existing coating and doping modification technologies have poor compatibility with electrolyte additives, lacking optimization of the cathode-electrolyte interface while improving bulk material stability. Conventional polymer coatings also suffer from narrow carbonization temperature windows and poor coating uniformity. Summary of the Invention

[0003] To address the problems of interface instability and large-scale dissolution of cobalt ions in the application of lithium cobalt oxide cathode materials in the prior art, which affect the cycle performance of lithium-ion batteries, this paper provides a cathode material, its preparation method, a cathode sheet, and a lithium-ion battery.

[0004] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: On one hand, the present invention provides a cathode material, including lithium cobalt oxide material, wherein the surface of the lithium cobalt oxide material is coated with a zinc oxide-nitrogen-doped carbon coating layer.

[0005] Optionally, the zinc oxide-nitrogen-doped carbon coating is obtained by carbonizing the ZIF-8 precursor.

[0006] Optionally, the lithium cobalt oxide material includes an O2-phase lithium cobalt oxide material, the general chemical formula of which is Li. x Co 1-z M z O2, where 0.95 ≤ x ≤ 1, 0 <z≤0.1; M includes at least one of Mg, Al, Ti, and Zr.

[0007] Optionally, in the zinc oxide-nitrogen-doped carbon coating layer, the atomic percentage of zinc is 1.5at% to 5at, and the atomic percentage of nitrogen is 3at% to 6at.

[0008] Optionally, the thickness of the zinc oxide-nitrogen-doped carbon coating layer is 5~40 nm.

[0009] Optionally, the lithium cobalt oxide material has an average particle size Dv50 of 3 μm to 10 μm and a specific surface area of ​​0.5 m². 2 / g~1.5m 2 / g.

[0010] Optionally, the method for preparing the cathode material includes the following steps: Lithium cobalt oxide material was mixed with ZIF-8 precursor, and then placed in an inert atmosphere for heat treatment to carbonize the ZIF-8 precursor, thereby obtaining a cathode material with a zinc oxide-nitrogen-doped carbon coating layer on its surface.

[0011] Optionally, the heat treatment temperature is 200~250℃ and the heat treatment time is 2~4h.

[0012] On the other hand, 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 a positive electrode active material, a binder, and a conductive agent. The positive electrode active material includes the positive electrode material described above, or a positive electrode material prepared by the method described above.

[0013] On the other hand, this application provides a lithium-ion battery, including a negative electrode, an electrolyte, and the aforementioned positive electrode.

[0014] Optionally, the lithium-ion battery electrolyte includes lithium salt, organic solvent, and additives; The lithium salt includes lithium bis(fluorosulfonyl)imide; The organic solvents include ethylene carbonate and ethyl methyl carbonate; The additives include lithium difluorophosphate and fluoroethylene carbonate.

[0015] Optionally, the concentration of the lithium salt in the lithium-ion battery is 1.0~1.5M; The volume ratio of ethylene carbonate to methyl ethyl carbonate in the organic solvent is 3:7; The concentration of lithium difluorophosphate is 0.05M~0.2M, and the concentration of fluoroethylene carbonate is 0.1M~0.3M.

[0016] The beneficial effects of this application are as follows: The cathode material provided in this application includes lithium cobalt oxide material, wherein the surface of the lithium cobalt oxide material is coated with a zinc oxide-nitrogen-doped carbon coating layer. This zinc oxide-nitrogen-doped carbon coating layer can block direct contact between the electrolyte and the cathode active material under high voltage, inhibit the corrosion of the cathode by hydrofluoric acid generated from the oxidative decomposition of the electrolyte, and reduce lattice oxygen evolution and interfacial side reactions. Simultaneously, the zinc oxide sites can chemically passivate surface active defects of the cathode, and the nitrogen-doped carbon layer provides efficient ion transport channels and improves interfacial conductivity. The synergistic effect of these two elements can significantly inhibit cobalt ion dissolution, stabilize the cathode crystal structure and interfacial state, and alleviate the problems of increased battery impedance and capacity decay caused by interfacial instability and large-scale cobalt ion dissolution. This effectively improves the cycle stability and long cycle life of lithium-ion batteries under high voltage. Detailed Implementation

[0017] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0018] The present invention provides a cathode material, including lithium cobalt oxide material, wherein the surface of the lithium cobalt oxide material is coated with a zinc oxide-nitrogen-doped carbon coating layer.

[0019] Specifically, the zinc oxide-nitrogen-doped carbon coating layer on the surface of the lithium cobalt oxide material forms a physical isolation barrier, preventing direct contact between the electrolyte and the active surface of the lithium cobalt oxide under high voltage, and inhibiting the hydrofluoric acid corrosion of the positive electrode caused by the oxidative decomposition of the electrolyte; the zinc oxide sites chemically passivate oxygen defects and Co on the positive electrode surface. 3+ Active sites reduce lattice oxygen evolution; nitrogen-doped carbon provides continuous ion / electron transport channels and reduces interface impedance. The two work together to suppress interfacial side reactions and cobalt ion dissolution from both physical isolation and chemical passivation levels, stabilize the cathode interface and crystal structure, alleviate capacity decay and impedance rise, and thus effectively improve the cycle stability and long cycle life of lithium-ion batteries under high voltage.

[0020] Further physical characterization of the lithium cobalt oxide material and the cathode material obtained after coating was performed. The specific characterization operations were as follows: X-ray diffraction (XRD): used to confirm the structure of O2 phase lithium cobalt oxide, test cathode material powder, identify the main peak position (18.6±0.1°) and calculate the c-axis interlayer spacing.

[0021] Scanning / transmission electron microscopy (SEM / TEM): Observe the morphology of the cathode material and the coating layer, and measure the coating layer thickness (5-40 nm) using TEM.

[0022] X-ray photoelectron spectroscopy (XPS): Analyzes the chemical composition of the coating layer and interfacial film, and tests the positive and negative electrode plates before and after cycling (requires DMC cleaning). Key detection methods include: Cathode coating: Zn 2p3 / 2 binding energy (1021.5±0.3eV), pyridine nitrogen content in N 1s spectrum (≥40%).

[0023] Positive / negative electrode interface: F 1s (684.5±0.2eV, LiF), P 2p (133.2±0.3eV, LiF) x PO y ).

[0024] In some embodiments, the zinc oxide-nitrogen-doped carbon coating is obtained by carbonizing a ZIF-8 precursor.

[0025] The zeolite imidazole ester framework-8, i.e. ZIF-8, is used as a precursor and is carbonized by low-temperature pyrolysis to form a zinc oxide-nitrogen-doped carbon coating layer. Specifically, ZIF-8 contains Zn metal and imidazole organic ligands. During low-temperature carbonization, Zn is converted into ZnO, and the imidazole ring is pyrolyzed to generate nitrogen-doped carbon, forming a uniform composite coating layer in situ. The high specific surface area of ​​ZIF-8 enables the coating layer to uniformly cover the particle surface, avoiding local coating defects. Furthermore, the N atoms are doped in the form of pyridine nitrogen, which improves the interfacial chemical stability and ion conductivity, achieving atomic-level synergy between ZnO and NC.

[0026] In some embodiments, the lithium cobalt oxide material includes an O2-phase lithium cobalt oxide material, the general chemical formula of which is Li. x Co 1-z M z O2, where 0.95 ≤ x ≤ 1, 0 <z≤0.1; M includes at least one of Mg, Al, Ti, and Zr.

[0027] Specifically, the O2 phase lithium cobalt oxide is an ABAB oxygen layer stack, which has better structural stability than the traditional O3 phase, stronger reversible phase transition capability under high pressure, and suppresses irreversible phase transition; doping elements such as Mg, Al, Ti, and Zr occupy cobalt lattice sites, enhance Li-O bond energy, suppress lattice oxygen release, and support the layered structure, reducing cell distortion during cycling, thereby improving the structural stability of the cathode from the bulk phase level, and achieving bulk-interface dual stability in conjunction with surface coating.

[0028] Furthermore, the main peak of the X-ray diffraction (XRD) pattern of the O2 phase lithium cobalt oxide is located at 18.6±0.1° (Cu-Kα rays), and the interlayer spacing c-axis is 1.42~1.45nm, which is significantly different from the traditional O3 phase structure (c-axis≈1.40nm).

[0029] In some embodiments, the atomic percentage of zinc is 1.5 at% to 5 at, and the atomic percentage of nitrogen is 3 at% to 6 at.

[0030] Specifically, in this application, the atomic percentage of zinc is in the range of 1.5 at% to 5 at%, which can sufficiently passivate surface activity defects without hindering Li. + diffusion; When the nitrogen doping amount is in the range of 3at% to 6at%, it can react in situ with electrolyte additives to form a stable CEI film, while improving the ionic conductivity of the carbon layer. The range of zinc oxide mass and nitrogen doping amount in the zinc oxide-nitrogen-doped carbon coating layer is beneficial to balancing the chemical passivation effect and ion transport efficiency, avoiding excessive zinc oxide mass and insufficient nitrogen doping amount in the coating layer that could lead to increased impedance or protection failure.

[0031] Furthermore, the atomic percentage content of the zinc element includes, but is not limited to, 1.5at%, 2at%, 2.5at%, 3at%, 3.5at%, 4at%, 4.5at%, or 5at%. The nitrogen doping amount includes, but is not limited to, 3at%, 4at%, 5at%, or 6at.

[0032] In some embodiments, the thickness of the zinc oxide-nitrogen-doped carbon coating is 5~40 nm.

[0033] Specifically, the zinc oxide-nitrogen-doped carbon coating layer, with a thickness of 5-40 nm, can completely cover the positive electrode particles, forming a complete physical barrier to block electrolyte erosion. If the thickness is too thin (less than 5 nm), pores are likely to appear, resulting in insufficient protection; if it is too thick (greater than 40 nm), it will increase the Li... + Diffusion path, increasing interface impedance; this range balances isolation protection and ion transport, achieving a balance between interface stability and kinetic performance.

[0034] In some embodiments, the average particle size Dv50 of the lithium cobalt oxide material is 3 μm to 10 μm, and the specific surface area of ​​the lithium cobalt oxide material is 0.5 m². 2 / g~1.5m 2 / g.

[0035] The volume average particle size Dv50 of the lithium cobalt oxide material is controlled in the range of 3μm~10μm, which can balance the particle processing performance and the lithium ion solid-phase diffusion efficiency. It can avoid the problem of too many side reaction sites and aggravated interfacial side reactions caused by excessively fine particle size, and improve the defects of slow lithium ion diffusion kinetics and reduced rate performance caused by excessively large particle size.

[0036] The lithium cobalt oxide material 0.5m2 / g~1.5m 2 The specific surface area of ​​ / g can effectively reduce the contact area between the electrolyte and the positive electrode active surface under high voltage, weaken the interfacial side reactions such as solvent oxidation decomposition, hydrofluoric acid corrosion and lattice oxygen evolution, and at the same time enable the ZIF-8 precursor to be uniformly coated on the surface of lithium cobalt oxide particles, forming a continuous and defect-free zinc oxide-nitrogen-doped carbon coating layer, improving the coating uniformity and interface protection consistency, and ultimately achieving the technical effects of inhibiting cobalt ion dissolution, stabilizing the interface structure, reducing impedance growth and improving high-voltage cycle stability.

[0037] Furthermore, the average particle size Dv50 of the lithium cobalt oxide material includes, but is not limited to, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm or 10μm; The specific surface area of ​​the lithium cobalt oxide material includes, but is not limited to, 0.5 m². 2 / g、1m 2 / g, 1.2m 2 / g, 1.5m 2 / g.

[0038] In some embodiments, the method for preparing the positive electrode material includes the following steps: Lithium cobalt oxide material was mixed with ZIF-8 precursor, and then placed in an inert atmosphere for heat treatment to carbonize the ZIF-8 precursor, thereby obtaining a cathode material with a zinc oxide-nitrogen-doped carbon coating layer on its surface.

[0039] The method for preparing the cathode material specifically includes the following operations: Lithium cobalt oxide of the O2 phase was mechanically mixed with ZIF-8 precursor at a mass ratio of (95:5) to (85:15). After mixing, the mixture was placed in an argon atmosphere and heat-treated at 200 to 250°C for 2 to 4 hours to carbonize the ZIF-8 precursor, thereby obtaining a cathode material with a zinc oxide-nitrogen-doped carbon coating layer on its surface.

[0040] Furthermore, mechanical mixing allows ZIF-8 to be uniformly attached to the surface of the O2 phase lithium cobalt oxide. An inert atmosphere prevents the material from oxidizing. Heat treatment drives the low-temperature pyrolysis of ZIF-8, carbonizing the organic ligands into nitrogen-doped carbon and converting Zn into ZnO, forming a tightly bonded coating layer in situ. The coating layer obtained by this operation has strong adhesion to the substrate, preventing it from falling off, while retaining the O2 phase crystal structure and not initiating a phase transformation.

[0041] In some embodiments, the heat treatment temperature is 200~250℃ and the heat treatment time is 2~4h.

[0042] Specifically, the heat treatment temperature of 200~250℃ can completely decompose ZIF-8 to generate a zinc oxide-nitrogen-doped carbon coating layer, while avoiding the irreversible transformation of the O2 phase to the O3 phase and retaining the high-pressure advantageous structure; the heat treatment time of 2~4h can ensure sufficient pyrolysis, moderate crystallization of the coating layer, and prevent particle agglomeration caused by high-temperature sintering, maintain the stability of the material particle size and specific surface area, and ensure the uniformity of the coating layer.

[0043] Another embodiment 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 a positive electrode active material, a binder, and a conductive agent. The positive electrode active material includes the aforementioned positive electrode material, or a positive electrode material prepared by the aforementioned method for preparing positive electrode materials.

[0044] Specifically, the positive current collector provides an electron conduction substrate, the conductive agent constructs a three-dimensional conductive network to compensate for the electron conduction of the coating layer; the binder fixes the active particles and maintains the stability of the electrode structure; the zinc oxide-nitrogen doped carbon coating layer coats the positive electrode material as an active core, suppresses side reactions at the electrode interface, and ensures the integrity of the electrode and the efficiency of electron / ion transport during cycling.

[0045] X-ray photoelectron spectroscopy (XPS) characterization of the cathode before cycling showed that the binding energy of the Zn 2p3 / 2 orbital was 1021.5±0.3eV, and the proportion of pyridine nitrogen (N-6) in the N 1s spectrum was not less than 40%.

[0046] Another embodiment of this application provides a lithium-ion battery, including a negative electrode, an electrolyte, and the aforementioned positive electrode.

[0047] Specifically, with the positive electrode as the core, the negative electrode and electrolyte are matched to form a synergistic system at the full cell interface; the positive electrode coating layer inhibits cobalt dissolution and electrolyte oxidation, reduces the thickening of the SEI film and cobalt deposition at the negative electrode, and avoids the deactivation of the negative electrode; at the full cell level, it blocks cobalt ion shuttle, stabilizes the positive and negative electrode interface, reduces the increase in cycle impedance, and improves cycle life and capacity retention at high voltage.

[0048] It should be noted that in uncoated or conventionally coated O2 phase lithium cobalt oxide batteries, the amount of cobalt deposited on the negative electrode is usually higher than 600 ppm or even more than 1000 ppm. The large amount of cobalt ion dissolution is due to the hydrofluoric acid produced by the decomposition of the electrolyte under high pressure, which corrodes the positive electrode and the intense interfacial side reaction, which leads to the dissolution, migration and deposition of cobalt on the negative electrode, causing abnormal thickening of the SEI film, increased impedance and rapid capacity decay. To verify that the lithium-ion battery provided in this application can suppress interface failure under high voltage, the lithium-ion battery after 800 cycles was disassembled, the negative electrode was collected, and after being cleaned with dimethyl carbonate (DMC), all solid components were detected by inductively coupled plasma mass spectrometry (ICP-MS). The cobalt content was ≤600ppm based on the percentage of cobalt element mass in the total mass of the negative electrode, which proves that the system has a high efficiency in suppressing cobalt dissolution from the positive electrode.

[0049] In some embodiments, the lithium-ion battery electrolyte includes lithium salt, organic solvent, and additives; The lithium salt includes lithium bis(fluorosulfonyl)imide; The organic solvents include ethylene carbonate and ethyl methyl carbonate; The additives include lithium difluorophosphate and fluoroethylene carbonate.

[0050] Specifically, using lithium bis(fluorosulfonyl)imide (LiFSI) as the main lithium salt, lithium difluorophosphate (LiDFP) and fluoroethylene carbonate (FEC) as key functional additives, and a mixture of ethylene carbonate (EC) and methyl ethyl carbonate (EMC) as the organic solvent, during battery formation and cycling, the electrolyte interacts with the zinc oxide-nitrogen-doped carbon coating layer on the positive electrode surface, generating a stable composite positive electrode electrolyte interface (CEI) film in situ. This composite positive electrode electrolyte interface film mainly consists of LiF and Li x PO y It is composed of components such as Zn3(PO4)2; Its specific mechanism of action is as follows: LiDFP decomposes on the surface of the positive electrode, producing PO4. 3- The species react with ZnO in the coating layer to generate Zn3(PO4)2, and at the same time decompose to generate LiF; FEC and nitrogen-doped carbon layer synergistically promote the formation of dense LiF layer.

[0051] Further analysis of the cycled cathode using time-of-flight secondary ion mass spectrometry (TOF-SIMS) revealed the presence of PO2. - The signal strength of the fragment (m / z=63) is not less than 1×10 4 counts, and PO2 - With Zn - The signal strength ratio of the fragment (m / z=64) to (PO2) - / Zn - The ratio should be no less than 1.5:1; XPS analysis revealed characteristic peaks of LiF (F 1s binding energy 684.5±0.2eV) and phosphorus oxides (P 2p binding energy 133.2±0.3eV) in the composite cathode electrolyte interface film.

[0052] To match the high-voltage positive electrode system, the negative electrode uses graphite or silicon-carbon composite materials. LiDFP and FEC in the electrolyte also participate in the formation of a stable solid electrolyte interphase (SEI) film on the surface of the negative electrode.

[0053] Surface analysis of the cycled negative electrode revealed a uniformly distributed phosphorus signal on the electrode surface, with an atomic percentage content of 0.5–1.5 at; further XPS analysis of the C 1s and P 2p spectra showed characteristic peaks of the ROCO₂Li component at ~290.0 eV and Li at 134.0 ± 0.5 eV. x PO y The characteristic peaks of phosphorus-containing lithium salts such as F2 further verified that phosphorus has been successfully embedded in the SEI membrane structure.

[0054] In some embodiments, the concentration of the lithium salt in the lithium-ion battery is 1.0~1.5M; The volume ratio of ethylene carbonate to methyl ethyl carbonate in the organic solvent is 3:7; The concentration of lithium difluorophosphate is 0.05M~0.2M, and the concentration of fluoroethylene carbonate is 0.1M~0.3M.

[0055] Specifically, the lithium salt used is 1.0~1.5M lithium bisfluorosulfonyl imide (LiFSI), which can provide sufficient lithium ion concentration in the electrolyte while maintaining suitable viscosity, ensuring high ionic conductivity and wettability, and achieving a balance between ion transport capacity and electrolyte processing performance; the organic solvent is a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7, which can optimize the lithium ion solvation structure, reduce the tendency of solvent molecules to oxidize and decompose at high potentials, and improve the high-voltage stability of the electrolyte; the additive is 0.05~0.2M lithium difluorophosphate (LiDFP), which can be used at the positive electrode interface. A stable phosphorus-containing passivation layer is generated in situ on the surface, and within this concentration range, the chemical stability of the electrolyte is not compromised. The additive used is 0.1~0.3M fluoroethylene carbonate (FEC), which can form a dense and stable solid electrolyte interface (SEI) film on the negative electrode surface. The amount of additive can be controlled to avoid excessive interface thickness leading to abnormal impedance increase. The above electrolyte components and ratios are highly compatible with the zinc oxide-nitrogen-doped carbon coating layer on the positive electrode surface, and can synergistically construct a stable positive and negative electrode interface film, maximizing the synergistic effect of interface protection and ion transport, and significantly improving the cycle stability and long cycle life of the battery under high voltage.

[0056] The present invention will be further illustrated by the following examples.

[0057] Example 1 This embodiment illustrates the cathode material, its preparation method, cathode sheet, and lithium-ion battery disclosed in this invention, and includes the following operational steps: Preparation of positive electrode sheet O2 phase Li was synthesized by solid-state method 0.98 Co 0.96 Mg 0.04 O2, XRD showed that its main peak was located at 18.6° and the c-axis interlayer spacing was 1.43 nm; Lithium cobalt oxide of the O2 phase was mechanically mixed with ZIF-8 precursor at a mass ratio of 90:10. After mixing, the mixture was placed in an argon atmosphere and heat-treated at 250°C for 3 hours to carbonize the ZIF-8 precursor, thus obtaining a cathode material with a zinc oxide-nitrogen-doped carbon coating layer on its surface. Among them, TEM observations showed that its microstructure was 20 nm thick; Characterization of zinc oxide-nitrogen-doped carbon coating (XPS): The atomic percentage of zinc is 2.9 at, the nitrogen doping amount is 4.3 at, and the proportion of pyridine nitrogen (N-6) in the N 1s spectrum is 45%. The coated positive electrode material, conductive agent (such as Super P), and binder (PVDF) are mixed in N-methylpyrrolidone (NMP) at a mass ratio of 96:2:2 to form a slurry, which is then coated onto aluminum foil and dried and cold-pressed to obtain the positive electrode sheet. The compaction density of the positive electrode sheet is controlled at 3.8 g / cm³. 3 The areal density is 32.5 mg / cm³. 2 .

[0058] diaphragm A 10μm thick ceramic-coated polyethylene diaphragm (the ceramic layer is Al2O3, with a thickness of 2μm) is used.

[0059] Preparation of electrolyte In a dry environment with a dew point ≤ -40℃, LiFSI, LiDFP and FEC were dissolved in EC / EMC (3:7, v / v) solvent and stirred until homogeneous to obtain an electrolyte. The concentrations of LiFSI, FEC, and LiDFP were 1.2 M, 0.2 M, and 0.1 M, respectively.

[0060] Preparation of negative electrode sheet The negative electrode material (artificial graphite) is mixed with binder (SBR-CMC) and conductive agent (carbon black) in a mass ratio of 95:3.5:1.5, and then added to water to make a negative electrode slurry. The negative electrode slurry is coated on copper foil, and after drying and cold pressing, a negative electrode sheet is obtained.

[0061] Assembly of lithium-ion batteries The positive electrode, separator, and negative electrode are stacked in sequence, wound, packed into an aluminum-plastic film shell, baked to remove moisture, and then injected with the above-mentioned electrolyte. After vacuum sealing, standing, formation (0.05C constant current charging to 4.6V), aging and other processes, a soft-pack battery with a rated capacity of 5000mAh is finally produced.

[0062] Example 2 This embodiment illustrates the cathode material and its preparation method, cathode sheet, and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: Lithium cobalt oxide of the O2 phase was mechanically mixed with ZIF-8 precursor at a mass ratio of 90:15. After mixing, the mixture was placed in an argon atmosphere and heat-treated at 250°C for 5 hours to carbonize the ZIF-8 precursor, thus obtaining a cathode material with a zinc oxide-nitrogen-doped carbon coating layer on the surface. Among them, TEM observation showed that its microstructure was 35 nm thick; Characterization of zinc oxide-nitrogen-doped carbon coating (XPS): The atomic percentage of zinc is 4.1 at, the nitrogen doping amount is 5.2 at, and the proportion of pyridine nitrogen (N-6) in the N 1s spectrum is 42%.

[0063] Example 3 This embodiment illustrates the cathode material and its preparation method, cathode sheet, and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: Lithium cobalt oxide of the O2 phase was mechanically mixed with ZIF-8 precursor at a mass ratio of 90:5. After mixing, the mixture was placed in an argon atmosphere and heat-treated at 250°C for 2.5 hours to carbonize the ZIF-8 precursor, thus obtaining a cathode material with a zinc oxide-nitrogen-doped carbon coating layer on its surface. Among them, TEM observation showed that its microstructure was 8 nm thick; Characterization of zinc oxide-nitrogen-doped carbon coating (XPS): The atomic percentage of zinc is 1.4 at, the nitrogen doping amount is 2 at, and the proportion of pyridine nitrogen (N-6) in the N 1s spectrum is 48%.

[0064] Example 4 This embodiment illustrates the cathode material and its preparation method, cathode sheet, and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: Lithium cobalt oxide material is Li 0.98 Co 0.90 Mg 0.10 O2; Among them, TEM observation showed that its microstructure was 8 nm thick; Characterization of zinc oxide-nitrogen-doped carbon coating (XPS): The atomic percentage of zinc is 2.8 at, the nitrogen doping amount is 4.1 at, and the proportion of pyridine nitrogen (N-6) in the N 1s spectrum is 44%.

[0065] Example 5 This embodiment illustrates the cathode material and its preparation method, cathode sheet, and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: Zinc oxide is mixed with nitrogen-containing organic matter (polyacrylonitrile), and then mechanically mixed with O2 phase lithium cobalt oxide material at a mass ratio of 90:10. After mixing, the mixture is placed in an argon atmosphere and heat-treated at 250°C for 3 hours to carbonize the zinc oxide precursor, thereby obtaining a cathode material with a zinc oxide-nitrogen-doped carbon coating layer on the surface. TEM and XPS characterization revealed that the surface coating of the cathode material showed obvious ZnO particle agglomeration, uneven nitrogen distribution, and no nitrogen doping signal in some areas; the binding energy of the Zn2p3 / 2 orbitals deviated from 1021.5±0.3eV. Compared with the coating obtained by carbonization of ZIF-8 precursor in other embodiments, the coating formed by zinc oxide and nitrogen-containing organic matter (polyacrylonitrile) in this embodiment has poor uniformity of zinc oxide and nitrogen distribution.

[0066] Among them, TEM observation of its microstructure showed that the coating layer thickness was uneven, with an average thickness of about 15 nm (the thickness of local areas was 5~40 nm). XPS characterization (multi-point average): The atomic percentage of zinc is 3.2 at%, the nitrogen doping content is 3.5 at%, and the proportion of pyridine nitrogen (N-6) in the N1s spectrum is 38%.

[0067] Example 6 This embodiment illustrates the cathode material and its preparation method, cathode sheet, and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: The electrolyte is a standard high-voltage system used in the industry. Lithium salt LiPF6 and additive FEC were dissolved in EC / EMC (3:7, v / v) solvent and stirred until homogeneous to obtain the electrolyte. The concentration of LiPF6 was 1.2 M and the concentration of FEC was 2 wt%.

[0068] Comparative Example 1 This comparative example is used to illustrate the cathode material and its preparation method, cathode sheet and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: Lithium cobalt oxide material without coating: O2 phase Li was synthesized by solid-state method 0.98 Co0.96 Mg 0.04 O2 is used as the positive electrode material.

[0069] Comparative Example 2 This comparative example is used to illustrate the cathode material and its preparation method, cathode sheet and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: O2 phase material was mixed with polyacrylonitrile (PAN) at a mass ratio of 100:2 and carbonized at 300°C for 4 hours under an argon atmosphere to form a nitrogen-doped carbon layer. XPS showed that its nitrogen doping level was 4.0 at%, but there was no zinc signal.

[0070] Comparative Example 3 This comparative example is used to illustrate the cathode material and its preparation method, cathode sheet and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: Polyacrylonitrile was used as a precursor and mechanically mixed with O2 phase lithium cobalt oxide material at a mass ratio of 90:10. After mixing, the mixture was placed in an argon atmosphere and heat-treated at 250°C for 3 hours to carbonize the polyacrylonitrile precursor, resulting in a cathode material with only a nitrogen-doped carbon coating layer on its surface.

[0071] Among them, TEM observation of its microstructure showed that the coating layer was of uniform thickness, approximately 15 nm. XPS characterization: No zinc signal, nitrogen doping level of 4.2 at%, pyridine nitrogen (N-6) accounted for 40% in the N 1s spectrum.

[0072] Comparative Example 4 This comparative example is used to illustrate the cathode material and its preparation method, cathode sheet and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: Zinc oxide and a nitrogen-free organic polymer (polypropylene) were used as precursors and mechanically mixed with O2 phase lithium cobalt oxide material at a mass ratio of 90:10. After mixing, the mixture was placed in an argon atmosphere and heat-treated at 250°C for 3 hours to carbonize the zinc oxide precursor, resulting in a cathode material with only a zinc oxide-doped carbon coating layer on its surface.

[0073] TEM observations revealed that the coating layer was approximately 20 nm thick, and that ZnO particles were locally aggregated on the surface. XPS characterization: The atomic percentage of zinc is 3.5 at%, the nitrogen doping is 0%, and there is no pyridine nitrogen signal in the N 1s spectrum.

[0074] Performance testing The following performance tests were performed on Examples 1-6 and Comparative Examples 1-4 prepared above: Interfacial chemical composition analysis Time-of-flight secondary ion mass spectrometry (TOF-SIMS): Deep profiling of the interface layer of the cathode electrode after cycling. Detection of PO₂⁻ and Zn⁻ fragment signals, calculation of their intensity ratio (PO₂⁻...). - / Zn - ≥1.5:1).

[0075] Inductively coupled plasma mass spectrometry (ICP-MS): Quantitatively detects the amount of cobalt dissolved in the negative electrode after cycling. The negative electrode is completely digested after cleaning, and the results are expressed as the percentage of Co mass in ppm of the total mass of the negative electrode (≤600ppm).

[0076] Energy dispersive spectroscopy (EDS): Surface elemental scanning is performed on the negative electrode sheet after cycling and cleaning to semi-quantitatively analyze the atomic percentage content of phosphorus (P) (0.5~1.5 at%).

[0077] Electrochemical performance testing Charge-discharge cycle test: Performed at 25°C using a battery testing system.

[0078] Voltage range: 3.0-4.6V.

[0079] First-time efficiency and specific capacity: Calculate the first coulombic efficiency and specific capacity by performing the first charge and discharge at a rate of 0.1C.

[0080] Cycle life: 800 continuous charge-discharge cycles at 1C rate, with capacity retention calculated (≥90%).

[0081] Electrochemical impedance spectroscopy (EIS): Measured at 50% charge state after the battery has been cycled to a specific period (e.g., 800 cycles). Frequency range: 100 kHz to 10 mHz. Analyze the change in charge transfer impedance (Rct) before and after cycling (increase ≤ 30%).

[0082] The test results are entered into Table 1.

[0083] Table 1 As shown in Table 1, the test results of Examples 1-4 show an initial efficiency of 91.5%-94.1%, an initial capacity of 172-178 mAh / g, a capacity retention of 85.2%-93.2% after 800 cycles, a cobalt leaching amount of 265-480 ppm at the negative electrode, and a charge transfer impedance increase of only 25%-45% after cycling. The PO2 content at the positive electrode interface after cycling is also low. - / Zn -The ratio also meets the requirement of not less than 1.5:1. It is speculated that the reason is that the zinc oxide-nitrogen-doped carbon coating layer formed on the surface of the cathode material can play a good role in physical isolation and chemical passivation. Combined with the electrolyte to construct a stable CEI / SEI film in situ, it can significantly suppress cobalt dissolution and interfacial side reactions. Compared to Examples 1-4, the cycle retention rate in Example 5 was only 78.5%, the Co leaching amount was as high as 720 ppm, and the Rct increase reached 82%. This is because the direct physical mixing of zinc oxide and nitrogen-containing organic matter resulted in an uneven coating layer, which affected the synergistic effect of zinc oxide and nitrogen-doped carbon coating in the coating layer. In Example 6, a conventional LiPF6 electrolyte (LiPF6 lithium salt, 2wt% FEC) was used. Compared with the high-voltage electrolyte used in Examples 1-4, which consisted of LiFSI lithium salt, 0.05-0.2M LiDFP, and 0.1-0.3M FEC, the conventional LiPF6 electrolyte lacked the synergy of LiDFP and FEC. As a result, a dense CEI film containing Zn3(PO4)2 could not be formed in situ on the cathode surface, leading to a significant decrease in capacity retention, a significant increase in cobalt dissolution, and a significant increase in impedance. The cathode material in Comparative Example 1 had no coating layer (cycle retention rate of 68.4%, Co dissolution of 1250 ppm), the cathode materials in Comparative Examples 2 and 3 had only nitrogen-doped carbon coating on the surface, and the cathode material in Comparative Example 4 had only zinc oxide coating. All of them lacked the synergistic effect of zinc oxide and nitrogen-doped carbon, which could not effectively block the corrosion of the electrolyte and inhibit cobalt dissolution, resulting in reduced capacity retention, large amount of cobalt deposition on the anode, and high impedance increase. In summary, through the comparison of the above specific embodiments and comparative examples, it can be seen that the zinc oxide-nitrogen-doped carbon coating layer on the surface of the cathode material lithium cobalt oxide can block the direct contact between the electrolyte and the cathode active material under high voltage, inhibit the corrosion of the cathode by hydrofluoric acid generated by the oxidative decomposition of the electrolyte, and reduce lattice oxygen evolution and interfacial side reactions. At the same time, the zinc oxide sites can chemically passivate the active defects on the cathode surface, and the nitrogen-doped carbon layer provides efficient ion transport channels and improves interfacial conductivity. The synergistic effect of the two can significantly inhibit cobalt ion dissolution, stabilize the cathode crystal structure and interfacial state, and alleviate the increase in battery impedance caused by interfacial instability and large-scale dissolution of cobalt ions.

[0084] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A positive electrode material, characterized in that, The material includes lithium cobalt oxide, the surface of which is coated with a zinc oxide-nitrogen-doped carbon coating layer.

2. The cathode material according to claim 1, characterized in that, The zinc oxide-nitrogen-doped carbon coating is obtained by carbonization of the ZIF-8 precursor.

3. The cathode material according to claim 1, characterized in that, The lithium cobalt oxide material includes O2-phase lithium cobalt oxide material, the general chemical formula of which is Li. x Co 1-z M z O2, 0.95≤x≤1, 0 <z≤0.1; M includes at least one of Mg, Al, Ti, and Zr.

4. The cathode material according to claim 1, characterized in that, In the zinc oxide-nitrogen-doped carbon coating, the atomic percentage of zinc is 1.5at% to 5at, and the atomic percentage of nitrogen is 3at% to 6at.

5. The positive electrode material according to claim 1, characterized in that, The thickness of the zinc oxide-nitrogen-doped carbon coating is 5~40 nm.

6. The cathode material according to claim 1, characterized in that, The average particle size Dv50 of the lithium cobalt oxide material is 3μm~10μm, and the specific surface area of ​​the lithium cobalt oxide material is 0.5m². 2 / g~1.5m 2 / g.

7. The method for preparing the cathode material according to any one of claims 1 to 6, characterized in that, The following steps are included: Lithium cobalt oxide material was mixed with ZIF-8 precursor, and then placed in an inert atmosphere for heat treatment to carbonize the ZIF-8 precursor, thereby obtaining a cathode material with a zinc oxide-nitrogen-doped carbon coating layer on its surface.

8. The method for preparing the cathode material according to claim 7, characterized in that, The heat treatment temperature is 200~250℃, and the heat treatment time is 2~4h.

9. A positive electrode plate, characterized in that, The device 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 a positive electrode active material, a binder, and a conductive agent. The positive electrode active material includes the positive electrode material according to any one of claims 1 to 6, or the positive electrode material prepared by the method for preparing the positive electrode material according to any one of claims 7 to 8.

10. A lithium-ion battery, characterized in that, It includes a negative electrode, an electrolyte, and a positive electrode as described in claim 9.

11. The lithium-ion battery according to claim 10, characterized in that, The electrolyte includes lithium salt, organic solvent, and additives; The lithium salt includes lithium bis(fluorosulfonyl)imide; The organic solvents include ethylene carbonate and ethyl methyl carbonate; The additives include lithium difluorophosphate and fluoroethylene carbonate.

12. The lithium-ion battery according to claim 11, characterized in that, The concentration of the lithium salt is 1.0~1.5M; The volume ratio of ethylene carbonate to methyl ethyl carbonate in the organic solvent is 3:7; The concentration of lithium difluorophosphate is 0.05M~0.2M, and the concentration of fluoroethylene carbonate is 0.1M~0.3M.

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