Lithium ion battery positive electrode material and preparation method thereof

Abstract

The invention discloses a lithium ion battery positive electrode material and a preparation method thereof. The positive electrode material comprises a positive electrode material matrix and a double-layer gradient composite nano coating layer coating the surface of the positive electrode material matrix, the double-layer gradient composite nano coating layer comprises an inner layer close to the substrate, the inner layer is an Nb-containing doped lithium iron phosphate layer, and the thickness of the inner layer is 3-20 nm; the outer layer covers the outer surface of the inner layer and is a porous lithium titanium aluminum phosphate layer, the thickness of the outer layer is 5-90 nm, and the aperture of the outer layer is 2-80 nm; wherein the concentration of phosphate groups in the double-layer gradient composite nano coating layer is gradually reduced from inside to outside. The positive electrode material provided by the invention has a double-layer gradient composite nano coating structure, and through the synergistic effect of the inner layer Nb-containing doped lithium iron phosphate and the outer layer porous lithium titanium aluminum phosphate, the thermal stability, high-voltage cycle performance and safety of the material are remarkably improved, and meanwhile, excellent ion conduction performance is kept.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery materials, and particularly relates to a lithium ion battery positive electrode material and a preparation method thereof. BACKGROUND

[0002] With the rapid development of electric vehicles, energy storage systems and portable electronic devices, higher requirements are put forward for the energy density, power density, cycle life and safety of lithium ion batteries. The performance of lithium ion batteries is largely dependent on the characteristics of the positive electrode material. The currently commercialized positive electrode materials mainly include lithium manganate (LiMn2O4), lithium cobaltate (LiCoO2), ternary material (LiNi x Co y Mn z O2) and the like. Although these materials have high theoretical capacity and working voltage, they still face many challenges in practical application.

[0003] Firstly, the thermal stability problem is a key factor restricting the safe application of lithium ion batteries. In a high temperature environment, the traditional positive electrode material is prone to structural collapse, oxygen release and metal ion dissolution. Especially in the charged state, the thermal stability of the delithiated positive electrode material is further reduced, which is prone to violent exothermic reaction with the electrolyte, resulting in thermal runaway phenomenon. Studies have shown that LiNi 0.8 Co 0.1 Mn 0.1 O2 after charging to 4.3V, its exothermic peak temperature is only 180-200℃, which is far below the safety requirements of practical application.

[0004] Secondly, the insufficient high-voltage cycle stability is also an important problem. In order to improve the energy density of the battery, it is necessary to work at a higher voltage, but the high voltage will aggravate the side reaction between the positive electrode material and the electrolyte, resulting in increased interface impedance, loss of active lithium and rapid capacity decay. At a high voltage of 4.4V or above, the cycle life of the traditional positive electrode material often cannot meet the actual application requirements.

[0005] Thirdly, the interface stability problem cannot be ignored. The positive electrode material will change in volume during charging and discharging, which will easily lead to particle cracking, poor interface contact and other problems after long-term cycling. At the same time, the side reaction on the surface of the positive electrode material will form an impedance layer, affecting the transmission efficiency of lithium ions.

[0006] To address the aforementioned problems, researchers have proposed various modification strategies, among which surface coating is one of the most effective methods. Traditional coating materials mainly include oxides (such as Al2O3 and ZrO2), phosphates (such as Li3PO4 and AlPO4), and polymers. However, these single coating layers often have the following shortcomings: 1) Poor lattice matching between the coating layer and the substrate material, resulting in insufficient bonding force and easy peeling after long-term cycling; 2) Difficulty in precisely controlling the coating layer thickness, as excessive thickness increases ion transport impedance while insufficient thickness limits the protective effect; 3) A single coating layer cannot simultaneously meet the multiple requirements of thermal stability, ion conductivity, and interfacial stability; 4) Complex coating processes, high costs, and difficulty in achieving industrial-scale production.

[0007] Therefore, developing a novel coating structure with reasonable structural design, simple preparation process, and excellent performance is of great significance for improving the thermal stability, high-voltage cycle performance, and safety of lithium-ion battery cathode materials. Summary of the Invention

[0008] The purpose of this invention is to address the shortcomings of existing technologies by providing a lithium-ion battery cathode material with a dual-layer gradient composite nano-coating structure and its preparation method. Through the synergistic effect of the inner Nb-doped lithium iron phosphate and the outer porous lithium titanium aluminum phosphate, the thermal stability, high-voltage cycle performance and safety of the material are significantly improved, while maintaining excellent ion conduction performance.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] In a first aspect, the present invention provides a lithium-ion battery cathode material, comprising a cathode material matrix and a double-layer gradient composite nanocoating layer covering the surface of the cathode material matrix;

[0011] The bilayer gradient composite nanocoating layer includes:

[0012] The inner layer, located close to the positive electrode material substrate, is an Nb-doped lithium iron phosphate layer with a thickness of 3–20 nm.

[0013] An outer layer, covering the outer surface of the inner layer, is a porous lithium aluminum titanium phosphate layer with a thickness of 5–90 nm and a pore size of 2–80 nm.

[0014] The chemical formula of the lithium iron phosphate layer is LiFe. 1-x Nb x PO4, where 0.005 ≤ x ≤ 0.05; the chemical formula of the lithium aluminum titanium phosphate layer is LiTi 2-y Al y (PO4)3, where 0 <y≤1.5;

[0015] The concentration of phosphate groups in the bilayer gradient composite nanocoating layer gradually decreases from the inside to the outside. The concentration gradient was determined by XPS depth profiling, and the concentration of phosphate groups in the inner layer was 10% to 30% higher than that in the outer layer.

[0016] The core innovation of this invention lies in the design of a bilayer gradient composite nano-coating structure, which achieves multiple functions through the synergistic effect of the inner and outer layers:

[0017] 1) Inner layer containing Nb-doped lithium iron phosphate: Nb doping can stabilize the crystal structure of lithium iron phosphate and improve its thermal stability and ion conductivity. 5+ Ionic radius (0.64 Å) and Fe 3+ With a similar ionic radius (0.65 Å), it can partially substitute Fe. 3 + Formation of solid solution, while Nb 5+ The high valence state of Nb enhances the covalent nature of the PO bond and improves the stability of the olivine structure. Lithium iron phosphate (LFP) exhibits excellent thermal stability; its olivine structure is not easily decomposed at high temperatures. Furthermore, LFP has good lattice matching with most cathode materials, enabling the formation of stable POM bonds (where M is a metal ion in the matrix material), thus strengthening the adhesion between the coating layer and the matrix. Nb doping creates defects in the LFP lattice, providing additional lithium-ion transport channels and improving ion conductivity.

[0018] 2) Outer porous lithium titanium aluminum phosphate layer: The porous structure provides channels for lithium-ion transport, reduces transport impedance, and can accommodate volume changes during charging and discharging, thus acting as a stress buffer and preventing the coating layer from cracking and peeling off. 4+ And Al 3+ The synergistic effect can modulate the electrochemical window of the material, Ti 4+ Having multiple valence states can provide additional redox activity, while Al 3+ This helps stabilize the structure. Lithium titanium aluminum phosphate has a NASICON-type structure with three-dimensional ion transport channels, which is beneficial for rapid ion transport.

[0019] 3) Gradient Concentration Distribution: The design of gradually decreasing phosphate group concentration from the inside to the outside creates a concentration gradient driving force, which is beneficial for the directional transport of lithium ions and reduces interfacial impedance. Simultaneously, the gradient distribution avoids stress concentration at the interface, improving interfacial stability. The formation mechanism is as follows: during rapid plasma sintering, P ions in the inner layer diffuse to the outer layer, while Ti and Al ions in the outer layer diffuse to the inner layer, spontaneously forming a concentration gradient.

[0020] Preferably, the cathode material matrix is ​​selected from lithium manganese oxide and ternary material LiNi. x Co yMn z At least one of O2, wherein x + y + z = 1 and 0.3 ≤ x ≤ 0.8, 0.1 ≤ y ≤ 0.2, 0.1 ≤ z ≤ 0.6. These materials are currently the most commercially available cathode materials, possessing high capacity and operating voltage, and are ideal matrices for the coating modification of this invention. Simultaneously, these compositional ranges ensure the stoichiometric balance and structural stability of the ternary material.

[0021] Preferably, the molar ratio x of Nb doping in the inner layer satisfies: 0.01 ≤ x ≤ 0.03. Within this doping range, the electrochemical performance of lithium iron phosphate can be effectively improved without excessively damaging its crystal structure. Too low a doping amount results in insignificant modification, while too high a doping amount may lead to impurity phases or structural instability.

[0022] Preferably, the porosity of the outer layer is 10–35%, determined by the BET method; and / or, the molar ratio of Ti to Al in the outer layer (2-y):y is 2:0–1.5:0.5. Appropriate porosity ensures ion transport channels without excessively reducing the material's mechanical strength. Optimizing the Ti / Al ratio can balance the material's electrochemical activity and structural stability.

[0023] Preferably, the coverage of the bilayer gradient composite nanocoating layer on the cathode material matrix is ​​≥95%, as determined by SEM statistical analysis. High coverage maximizes the protection of the matrix material, preventing direct contact with the electrolyte and reducing side reactions.

[0024] Preferably, the inner layer has a thickness of 5–15 nm, and the outer layer has a thickness of 10–40 nm. A suitable thickness provides effective protection without excessively increasing transmission impedance.

[0025] Preferably, in the bilayer gradient composite nanocoating layer, the inner layer is bonded to the cathode material matrix via POM bonds, where M represents a metal ion in the matrix material, and the outer layer is bonded to the inner layer via PO-Ti / Al bridging bonds. This chemical bonding method enhances the bonding force between the layers and prevents peeling during cycling.

[0026] Preferably, the porous structure of the outer layer includes mesopores and macropores, wherein the pore size of the mesopores is 2–50 nm, the pore size of the macropores is 50–80 nm, and the volume ratio of mesopores to macropores is (2–5):1. This hierarchical porous structure can optimize ion transport pathways; mesopores provide rapid transport channels, while macropores facilitate electrolyte penetration.

[0027] Secondly, the present invention provides a method for preparing the above-mentioned lithium-ion battery cathode material, comprising the following steps:

[0028] S1. Prepare the cathode material substrate;

[0029] S2. A lithium iron phosphate precursor layer containing Nb is deposited on the surface of the cathode material substrate by low-temperature liquid phase deposition.

[0030] S3. A porous lithium titanium aluminum phosphate precursor layer is formed on the surface of the Nb-doped lithium iron phosphate precursor layer by in-situ liquid phase reaction.

[0031] S4. The double-layer gradient composite nanocoating layer is formed by sintering at 400-500℃ for 3-5 minutes using rapid plasma sintering (SPS), wherein the gradient concentration distribution is achieved by controlling the element diffusion during the sintering process.

[0032] The formation mechanism of the bilayer gradient composite nanocoating layer is as follows:

[0033] 1) Inner layer formation: In a low-temperature liquid phase environment, Li + Fe 3+ 、Nb 5+ and PO4 3- Ions undergo a co-precipitation reaction on the substrate surface, and due to the lattice-induced effect of the substrate surface, an Nb-doped lithium iron phosphate precursor layer matching the substrate lattice is formed.

[0034] 2) Outer layer formation: In-situ reaction occurs on the inner layer surface, Ti 4+ Al 3+ The ions react with phosphate ions to form lithium titanium aluminum phosphate precursor, and by controlling the addition rate of the precipitant, a hierarchical porous structure is formed.

[0035] 3) Gradient formation: During SPS sintering, due to the differences in diffusion coefficients of different elements: P ions (with a larger diffusion coefficient) diffuse from the inner layer to the outer layer; Ti and Al ions (with smaller diffusion coefficients) diffuse in small amounts from the outer layer to the inner layer; within a limited sintering time, a gradient distribution of phosphate group concentration is formed, which decreases from the inside to the outside; this gradient can be characterized and quantitatively analyzed by XPS depth profiling and EDS line scanning.

[0036] The unique feature of the preparation method of the present invention is that:

[0037] 1) Low-temperature liquid phase deposition: Compared with the traditional high-temperature solid phase method, liquid phase deposition can be carried out at a lower temperature, which is beneficial to maintain the structural integrity of the substrate material, while allowing for precise control of the thickness and composition of the coating layer.

[0038] 2) In-situ reaction: The outer layer is formed by in-situ reaction, which can ensure good bonding with the inner layer and form a continuous interface; by controlling the addition rate of the precipitant (0.1-1 mL / min), the hierarchical porous structure can be prepared in a controllable manner.

[0039] 3) Rapid Plasma Sintering (SPS): SPS technology can achieve rapid heating and short-time sintering, avoiding grain growth and excessive element diffusion caused by long-term high-temperature treatment; element diffusion during the sintering process forms a concentration gradient distribution.

[0040] Preferably, in step S2, the reaction conditions for the low-temperature liquid phase deposition are: a reaction temperature of 40–80°C, a reaction time of 20–60 minutes, and a solution pH controlled at 7–9. These conditions are conducive to uniform deposition of the precursor and an appropriate reaction rate.

[0041] Preferably, in step S2, the Nb-doped lithium iron phosphate precursor layer is prepared using the following raw materials: a lithium source, an iron source, a phosphorus source, and a niobium source, wherein the lithium source is selected from at least one of Li2CO3, LiOH, and LiNO3, the iron source is selected from at least one of FeC2O4·2H2O, FeCl3, and Fe(NO3)3, the phosphorus source is selected from at least one of H3PO4, NH4H2PO4, and (NH4)2HPO4, and the niobium source is selected from at least one of NbCl5, Nb2O5, and NbF5.

[0042] Preferably, in step S3, the porous titanium aluminum lithium phosphate precursor layer is prepared from the following raw materials: titanium source, aluminum source, phosphorus source and lithium source, wherein the titanium source is selected from at least one of Ti(SO4)2, TiCl4, and Ti(OC4H9)4, and the aluminum source is selected from at least one of Al(NO3)3, AlCl3, and Al2(SO4)3.

[0043] Preferably, in step S3, the porous structure is achieved by controlling the addition rate of the precipitant, which is 0.1–1 mL / min. A slow addition rate is beneficial for forming a uniform porous structure.

[0044] Preferably, in step S4, the heating rate of the rapid plasma sintering is 50–200 °C / min, the sintering pressure is 20–50 MPa, and the sintering atmosphere is an inert gas or a vacuum. Optimization of these parameters can achieve densification while maintaining a porous structure.

[0045] The gradient concentration distribution is formed in the following way: during the sintering process, P ions in the inner layer diffuse to the outer layer, and Ti and Al ions in the outer layer diffuse to the inner layer, forming a concentration gradient.

[0046] Preferably, the method further includes step S5: surface treatment of the sintered product, the surface treatment including acid washing, water washing and drying, wherein the acid washing uses a dilute hydrochloric acid solution with a concentration of 0.1 to 0.5 mol / L and a treatment time of 5 to 30 minutes.

[0047] Thirdly, the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode active material of the positive electrode is the aforementioned lithium-ion battery positive electrode material.

[0048] Preferably, the lithium-ion battery retains no less than 95% of its capacity after 200 1C charge-discharge cycles, and its exothermic peak temperature is at least 60°C higher than that of the uncoated material in a 250°C DSC test.

[0049] Preferably, the lithium-ion battery does not exhibit thermal runaway under overcharge conditions up to 5.0V, and the thermal runaway temperature increases to over 300°C.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] 1) Significantly improved thermal stability: The double-layer composite structure effectively blocks oxygen release from direct contact with the electrolyte. The high thermal stability of the inner lithium iron phosphate layer and the structural stability of the outer lithium titanium aluminum phosphate layer work synergistically to increase the exothermic peak temperature of the material by at least 60°C in the 250°C DSC test, which greatly improves safety.

[0052] 2) Excellent high-voltage cycling performance: The double-layer coating structure effectively suppresses interfacial side reactions under high voltage. The capacity retention rate is ≥95% after 200 charge-discharge cycles at 1C, which is far superior to the cycling performance of uncoated materials.

[0053] 3) Significantly enhanced interface stability: The inner layer of Nb-doped lithium iron phosphate has good lattice matching with the matrix material, forming a stable chemical bond; the outer layer of porous lithium titanium aluminum phosphate provides a flexible buffer, effectively relieving the volume change stress during cycling and preventing the coating layer from peeling off.

[0054] 4) Excellent ion transport performance: The gradient concentration distribution and porous structure design form an efficient ion transport network, which reduces the interfacial impedance and maintains excellent rate performance.

[0055] 5) Simple and controllable preparation process: The preparation process combines low-temperature liquid phase deposition and rapid plasma sintering. The reaction conditions are mild and the process parameters are easy to control, making it suitable for industrial production.

[0056] 6) Significantly improved safety: No thermal runaway occurs under overcharge to 5.0V conditions, and the thermal runaway temperature is increased to over 300℃, providing a material basis for the application of high-safety batteries. Detailed Implementation

[0058] To make the technical problem to be solved, the technical solution, and the beneficial effects of the present invention clearer, the present 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 present invention and are not intended to limit the present invention.

[0059] In this invention, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0060] In this invention, "at least one" means one or more, and "more" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of a single item or a plurality of items.

[0061] It should be understood that in various embodiments of the present invention, the order of the above-mentioned processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0062] In this invention, "gradient concentration distribution" refers to the continuous decreasing distribution of phosphate group concentration from the inside to the outside of the coating layer. Through XPS depth analysis, the concentration of phosphate groups in the inner layer is 10-30% higher than that in the outer layer.

[0063] In this invention, "porous structure" refers to a structure with interconnected pores, with pore sizes distributed in the range of 2-80 nm, including mesopores (2-50 nm) and macropores (50-80 nm).

[0064] Example 1

[0065] 1. Matrix Preparation

[0066] Select commercial LiNi 0.5 Co 0.2 Mn 0.3 O2 ternary materials are used as the cathode material matrix, with an average particle size of 8-12 μm.

[0067] 2. Inner layer deposition

[0068] 0.02 mol / L Li₂CO₃, 0.02 mol / L FeC₂O₄·2H₂O, 0.04 mol / L H₃PO₄, and 0.0006 mol / L NbCl₅ were dissolved in deionized water, and the pH was adjusted to 8.0 with ammonia. The mixture was stirred at 60°C for 30 minutes to ensure complete dissolution of the precursors. 10 g of matrix powder was added to the above solution, and the reaction was continued with stirring for 45 minutes. After the reaction was complete, the product was collected by centrifugation, washed three times successively with deionized water and ethanol, and vacuum dried at 80°C for 12 hours to obtain a powder coated with Nb-doped LiFePO₄ precursor.

[0069] Among them, the deposition reaction mechanism is as follows: under weakly alkaline conditions, Li + Fe 3+ 、Nb 5+ and PO4 3- Ions undergo co-precipitation reaction: Li + + (1-x)Fe 3+ + xNb 5+ + PO4 3- → LiFe 1-x Nb x PO4↓.

[0070] 3. In-situ formation of the outer layer

[0071] The powder obtained in step 2 was added to a mixed aqueous solution containing 0.01 mol / L Ti(SO4)2, 0.015 mol / L Al(NO3)3, 0.025 mol / L H3PO4, and 0.025 mol / L Li2CO3, and stirred at room temperature for 10 minutes. Then, a 3 mol / L NH3·H2O solution was added dropwise at a rate of 0.5 mL / min until the pH reached 9.0. The reaction was continued with stirring for 2 hours to form porous LiTi. x Al y The outer layer of the (PO4)3 precursor. After the reaction was complete, the product was collected by centrifugation, washed with deionized water until neutral, and dried under vacuum at 80°C for 8 hours.

[0072] The key to forming a hierarchical porous structure is the slow addition of the precipitant (NH3·H2O), while rapid precipitation will form a dense structure.

[0073] Among them, the formation mechanism of porous structure:

[0074] 1) Rapid nucleation stage: The slow addition of NH3·H2O gradually increases the local pH, Ti 4+ Al 3+ Ions first form small-sized nuclei;

[0075] 2) Aggregation and growth stage: Small nuclei gradually aggregate to form large particles, and mesopores are formed during the aggregation process;

[0076] 3) Template effect: NH3 molecules and water molecules act as soft templates, forming macropores during the drying process.

[0077] 4. Rapid plasma sintering

[0078] The dried powder was placed in a graphite mold and sintered using an SPS-2080 rapid plasma sintering equipment. The heating rate was set to 100℃ / min, the sintering temperature to 450℃, the holding time to 4 minutes, the sintering pressure to 30MPa, and the sintering atmosphere to argon. After sintering, the material was naturally cooled to room temperature to obtain a double-layer gradient composite nano-coated cathode material.

[0079] The gradient formation process includes:

[0080] 1) Heating stage (25-450℃): The precursor gradually decomposes and begins to crystallize;

[0081] 2) Isothermal stage (450℃, 4 minutes): At high temperature, P ions diffuse from the inner layer to the outer layer (diffusion coefficient DP ≈ 10-12 cm² / s), while Ti and Al ions diffuse in small amounts from the outer layer to the inner layer (diffusion coefficient DTi, Al ≈ 10-14 cm² / s).

[0082] 3) Cooling stage: The gradient distribution is solidified in the final product.

[0083] 5. Characterization and Testing

[0084] 1) Structural characterization:

[0085] TEM analysis: Transmission electron microscopy revealed that the material formed a clear double-layer coating structure, with an inner layer thickness of approximately 8-10 nm and an outer layer thickness of approximately 15-20 nm. The outer layer exhibited a distinct porous structure.

[0086] HRTEM analysis: High-resolution transmission electron microscopy showed good lattice matching between the inner and outer layers and the substrate. Continuous lattice fringes could be observed at the inner layer / substrate interface, proving that chemical bonding had been formed.

[0087] XRD analysis: The material retains the main crystal phase structure of the matrix (R-3m space group). Characteristic peaks of LiFePO4 appeared at 2θ = 20.8°, 25.6°, and 29.8°, and characteristic peaks of LiTi2(PO4)3 appeared at 2θ = 24.1° and 31.2°. The moderate peak intensity indicates that the coating thickness is appropriate.

[0088] 2) Characterization of gradient concentration distribution:

[0089] XPS depth profile: Ion etching was performed sequentially from the surface inwards, with each layer having an etching depth of approximately 2 nm. Results show:

[0090] Surface (0-5nm): P 2p peak intensity is relatively low, while Ti 2p and Al 2p peak intensities are relatively high;

[0091] Intermediate layer (5-15nm): P 2p peak intensity gradually increases, while Ti 2p and Al 2p peak intensities gradually decrease;

[0092] Inner layer (15-25nm): P 2p peak intensity reaches the highest, while Ti 2p and Al 2p peak intensities are the lowest;

[0093] The concentration of phosphate groups decreases by approximately 25% from the inner layer to the outer layer, which meets the design requirements.

[0094] 3) Characterization of porous structure:

[0095] BET analysis: Specific surface area is 45.2 m² / g, and porosity is 25.3%;

[0096] Pore ​​size distribution: Mesopores (2-50nm) account for 78% of the total pore volume, macropores (50-80nm) account for 22%, and the mesopore / macropore volume ratio is 3.5:1;

[0097] Pore ​​structure type: The N2 adsorption-desorption isotherm exhibits type IV isotherm characteristics, confirming the existence of a mesoporous structure.

[0098] 4) Coverage measurement: SEM statistical analysis of 200 particles showed that 97.8% of the substrate surface was covered by the coating layer, meeting the requirement of ≥95%.

[0099] Example 2

[0100] The material preparation method is basically the same as in Example 1, except that:

[0101] The matrix material is LiMn2O4;

[0102] The Nb doping concentration in the inner layer was adjusted to 0.0004 mol / L NbCl5;

[0103] The molar ratio of Ti to Al in the outer layer was adjusted to 1:0.5;

[0104] The SPS sintering temperature was adjusted to 420℃, and the holding time was 3 minutes.

[0105] TEM analysis revealed that the material also formed a uniform bilayer coating structure, with an inner layer thickness of approximately 6-8 nm and an outer layer thickness of approximately 12-18 nm.

[0106] Example 3

[0107] The material preparation method is basically the same as in Example 1, except that:

[0108] The Nb doping concentration in the inner layer was increased to 0.001 mol / L NbCl5;

[0109] The drop rate of the outer precipitant was adjusted to 0.2 mL / min;

[0110] The SPS sintering temperature was increased to 480℃, and the holding time was 5 minutes.

[0111] The material prepared in this embodiment has a thicker coating layer, with an inner layer thickness of about 12-15 nm and an outer layer thickness of about 25-30 nm, and a richer porous structure.

[0112] Example 4

[0113] The material preparation method is basically the same as in Example 1, except that:

[0114] The matrix material is LiNi 0.8 Co 0.1 Mn 0.1 O2;

[0115] The inner layer deposition temperature was adjusted to 45℃;

[0116] The molar ratio of Ti to Al in the outer layer was adjusted to 1:1.5;

[0117] The precipitant drop rate was adjusted to 0.8 mL / min.

[0118] Example 5

[0119] The material preparation method is basically the same as in Example 1, except that:

[0120] The inner layer deposition time has been extended to 60 minutes;

[0121] The outer layer reaction time has been extended to 3 hours;

[0122] The SPS sintering pressure was adjusted to 40 MPa.

[0123] Example 6

[0124] The material preparation method is basically the same as in Example 1, except that:

[0125] Pure LiFePO4 inner layer was prepared without adding Nb dopant.

[0126] All other conditions remain unchanged.

[0127] Comparative Example 1

[0128] The traditional single-layer Li3PO4 coating method is used: the matrix material LiNi is coated with Li3PO4. 0.5 Co 0.2 Mn 0.3 O2 and Li3PO4 were mixed at a mass ratio of 100:3, ball-milled for 6 hours, and then calcined at 600℃ for 5 hours to obtain Li3PO4-coated cathode material.

[0129] Comparative Example 2

[0130] Uncoated matrix material: Direct use of commercially available LiNi 0.5 Co 0.2 Mn 0.3 O2 was used as a control sample and no surface treatment was performed.

[0131] Comparative Example 3

[0132] The traditional double-layer coating method is adopted: first, Al2O3 is used for coating, and then Li3PO4 is used for coating.

[0133] The specific method is as follows: First, the matrix material is mixed with the Al2O3 precursor and calcined at 500°C for 3 hours to form the inner layer; then it is mixed with the Li3PO4 precursor and calcined at 600°C for 5 hours to form the outer layer.

[0134] Comparative Example 4

[0135] Preparation of inner-layer coated materials only: Nb-doped materials were prepared according to the method in Example 1. The inner layer is not coated with an outer layer; instead, SPS sintering is performed directly.

[0136] Comparative Example 5

[0137] Materials prepared only for the outer coating: Porous LiTi directly coated onto the substrate surface. x Al y (PO4) 3 layers, without preparing an inner layer, other conditions are the same as in Example 1.

[0138] Performance tests were performed on the examples and comparative examples respectively, and the performance testing methods are as follows:

[0139] 1. Electrochemical performance testing

[0140] Battery Assembly: The prepared positive electrode material was mixed with conductive agent Super P and binder PVDF at a mass ratio of 8:1:1, and an appropriate amount of NMP solvent was added to form a slurry. This slurry was then coated onto an aluminum foil current collector and vacuum dried at 120°C for 12 hours. Using lithium metal as the negative electrode, Celgard 2400 as the separator, and 1M LiPF6 EC / DEC (1:1, vol%) as the electrolyte, CR2032 button cells were assembled in an argon-filled glove box.

[0141] Charge / discharge test: Constant current charge / discharge test was conducted at 25°C using the NEWARE battery testing system. The charging termination voltage was 4.4V, the discharging termination voltage was 2.8V, and the current density range was 0.1C-5C (1C=200mAh / g).

[0142] Cyclic performance test: Long-term cycle test was conducted at 4.4V and 1C current density, and the capacity change was recorded for 200-500 cycles.

[0143] Rate performance test: The material was charged and discharged five times in sequence at current densities of 0.1C, 0.2C, 0.5C, 1C, 2C and 5C, and then returned to 0.1C to evaluate its rate performance.

[0144] 2. Thermal stability test

[0145] DSC test: Using a differential scanning calorimeter (DSC), the positive electrode material charged to 4.4V was heated from 50℃ to 400℃ in an argon atmosphere at a heating rate of 10℃ / min, and the exothermic peak temperature and exothermic heat were recorded.

[0146] Thermogravimetric analysis (TGA): The material is heated from room temperature to 800°C at a heating rate of 10°C / min in an argon atmosphere, and the mass change is recorded.

[0147] 3. Structural Characterization

[0148] Transmission electron microscopy (TEM): Using a JEOL JEM-2100F transmission electron microscope with an accelerating voltage of 200kV, the microstructure and coating structure of the material were observed.

[0149] X-ray diffraction (XRD): Rigaku D / max-2500PC X-ray diffractometer, Cu Kα rays, scanning range 10-80°, scanning rate 5° / min.

[0150] X-ray photoelectron spectroscopy (XPS): The surface elemental composition and valence state were analyzed using the Thermo Fisher Scientific K-Alpha XPS system.

[0151] 4. Electrochemical impedance spectroscopy

[0152] Using a CHI660E electrochemical workstation, a 5mV sinusoidal perturbation was applied at an open-circuit voltage, with a frequency range of 10mHz-100kHz, to test the electrochemical impedance spectrum of the material.

[0153] Test Results and Analysis

[0154] The test results are shown in Tables 1 and 2.

[0155] Table 1. Electrochemical performance test results of each example and comparative example.

[0156]

[0157] Table 2 Structural parameters and coating features of each embodiment

[0158]

[0159] The analysis of the above test results is as follows:

[0160] 1. Electrochemical performance analysis

[0161] As can be seen from the test results in Table 1, the bilayer gradient composite nano-coated cathode material of this invention is significantly superior to the comparative example in all electrochemical performance indicators:

[0162] Capacity performance: The initial discharge capacity of Examples 1-5 remained at a high level, with Example 4 reaching 185.3 mAh / g. This was attributed to the double-layer coating structure, which protected the substrate material while maintaining good ion transport performance. In contrast, the capacity of conventional coating methods (Comparative Examples 1 and 3) decreased due to the higher impedance of the coating layer.

[0163] Cyclic stability: Under a voltage window of 4.4V, the capacity retention rate of all embodiments exceeded 95% after 200 cycles, with Embodiment 3 reaching 97.2%. In contrast, the capacity retention rate of Comparative Example 2 (uncoated) was only 85.3%, indicating that the double-coated structure provides significant protection under high voltage.

[0164] Rate performance: The 5C / 0.1C rate capacity ratio of the examples generally reached over 84%, and Example 4 even reached 88.9%. This is mainly attributed to the efficient ion transport channels provided by the outer porous structure and the ion transport driving force generated by the gradient concentration distribution.

[0165] Nb doping effect: Comparing Examples 1 and 6, it can be found that Nb doping significantly improves various properties of the material, especially high voltage cycling stability (96.8% vs 95.1%) and thermal stability (315℃ vs 308℃), demonstrating the important role of Nb doping in stabilizing the coating structure.

[0166] 2. Thermal stability analysis

[0167] DSC test results show that the exothermic peak temperature of the material of this invention is significantly higher than that of the uncoated material (Comparative Example 2, 248℃), and even reaches 318℃ in Example 3. This significant improvement in thermal stability mainly stems from:

[0168] 1) The intrinsic high thermal stability of the inner lithium iron phosphate layer;

[0169] 2) The outer layer of lithium titanium aluminum phosphate blocks oxygen release;

[0170] 3) The double-layer structure effectively prevents direct contact between the substrate material and the electrolyte.

[0171] Thermal runaway temperature tests show that the thermal runaway temperatures of the materials in the examples are all above 300°C, which is 80-95°C higher than that of the uncoated materials, significantly improving the safe operating temperature range of the battery.

[0172] 3. Structural Characterization Analysis

[0173] TEM analysis: All embodiments formed a clear bilayer coating structure with distinct and tightly bonded inner and outer layers. The inner layer exhibits a dense layered structure with good lattice matching with the substrate material; the outer layer exhibits a distinct porous structure with pore sizes distributed in the range of 2-80 nm, meeting the design requirements.

[0174] XRD analysis showed that the material maintained the main crystal phase structure of the matrix, and the introduction of the coating layer did not disrupt the crystal structure of the matrix. The characteristic peaks of the coating layer appearing at specific angles had moderate intensities, indicating that the coating layer thickness was appropriate.

[0175] XPS analysis: Surface elemental analysis confirmed the successful doping of Nb into the lithium iron phosphate lattice, while the presence of Ti and Al confirmed the formation of the outer layer of lithium titanium aluminum phosphate. Valence state analysis showed that Nb mainly exists in the +5 valence state, Ti in the +4 valence state, and Al in the +3 valence state, consistent with the expected chemical environment.

[0176] 4. Effects of different preparation parameters

[0177] Effect of coating thickness: Comparing Examples 1 and 3, it can be found that appropriately increasing the coating thickness can further improve thermal stability and high-voltage cycling performance, but an excessively thick coating will slightly reduce rate performance. The coating thickness (23-30 nm) in Example 1 shows the best performance balance.

[0178] Influence of matrix material: The coating effect varies with different matrix materials. High-nickel ternary materials (Example 4) show more significant performance improvement after coating modification due to their higher capacity and more active surface chemistry.

[0179] Effect of Ti / Al ratio: The lower Ti / Al ratio (1:0.5) in Example 2 resulted in a slight decrease in the stability of the outer structure and slightly inferior cycling performance compared to other examples. The Ti / Al ratio (1:1.5) in Example 1 achieved a good balance between structural stability and ion transport performance.

[0180] 5. Mechanism Analysis

[0181] The dual-layer synergistic effect: The inner layer of Nb-doped lithium iron phosphate mainly plays a role in structural and thermal stability. Its olivine structure is not easily decomposed at high temperatures, and Nb doping further enhances structural stability. The outer porous lithium titanium aluminum phosphate is mainly responsible for ion transport and interface protection. Its NASICON-type structure provides a three-dimensional ion transport channel.

[0182] Gradient effect: The gradient distribution of phosphate group concentration creates a concentration-driving force between the inner and outer layers, which is beneficial for the directional transport of lithium ions and reduces interfacial impedance. This gradient design is an advantage that traditional uniform coating does not possess.

[0183] Porous structure effect: The hierarchical porous structure (mesoporous + macroporous) of the outer layer not only provides efficient ion transport channels, but also can accommodate a certain volume change, playing a buffering role and preventing the coating layer from cracking and peeling off during cycling.

[0184] 6. Process optimization analysis

[0185] Low-temperature liquid phase deposition: Compared with the traditional high-temperature solid-state method, liquid phase deposition can achieve uniform coating at a lower temperature, avoiding the damage to the matrix material structure caused by high temperature. In the examples, the reaction temperature of 40-80℃ ensured that the reaction proceeded while maintaining the structural integrity of the matrix material.

[0186] In-situ reaction: The in-situ formation of the outer layer ensures a tight bond with the inner layer, avoiding interface problems that may arise from stepwise coating. The slow addition of the precipitant (0.1-1 mL / min) is key to forming a uniform porous structure.

[0187] Rapid Plasma Sintering: The rapid heating and short sintering of SPS technology avoids grain growth and element diffusion that may be caused by prolonged high-temperature treatment, and maintains the nanoscale and gradient distribution of the coating layer.

[0188] Industrialization Prospect Analysis

[0189] The preparation process of this invention has good prospects for industrialization:

[0190] 1) Raw materials are readily available: The raw materials used are all common inorganic salts, which are low in cost and have a stable supply.

[0191] 2) Simple process: It mainly adopts liquid phase reaction and SPS sintering, with low equipment requirements and easy scale-up.

[0192] 3) Low energy consumption: The reaction temperature is low, the sintering time is short, and the energy cost is controllable.

[0193] 4) Environmentally friendly: The preparation process does not produce any toxic or harmful substances and meets environmental protection requirements.

[0194] In summary, this invention, through innovative dual-layer gradient composite nano-coating structure design and optimized fabrication process, successfully developed a lithium-ion battery cathode material with high thermal stability, excellent high-voltage cycling performance, and significantly improved safety. While maintaining high capacity, this material achieves a capacity retention rate of over 95% after 200 cycles at a high voltage of 4.4V, and an DSC exothermic peak temperature increase of over 60℃, providing an important material foundation for the development of next-generation high-safety, high-energy-density lithium-ion batteries.

[0195] It should be noted that the contents not described in detail in this specification are existing technologies known to those skilled in the art, and will not be elaborated here.

[0196] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments described above, and any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. A lithium-ion battery cathode material, characterized in that, It includes a positive electrode material matrix and a double-layer gradient composite nanocoating layer covering the surface of the positive electrode material matrix; The bilayer gradient composite nanocoating layer includes: The inner layer, located close to the positive electrode material substrate, is an Nb-doped lithium iron phosphate layer with a thickness of 3–20 nm. An outer layer, covering the outer surface of the inner layer, is a porous lithium aluminum titanium phosphate layer with a thickness of 5–90 nm and a pore size of 2–80 nm. The chemical formula of the lithium iron phosphate layer is: Where 0.005 ≤ x ≤ 0.05; the chemical formula of the lithium titanium aluminum phosphate layer is , of which 0 <y≤1.5; The concentration of phosphate groups in the bilayer gradient composite nanocoating layer gradually decreases from the inside to the outside.

2. The lithium-ion battery cathode material according to claim 1, characterized in that, The cathode material matrix is ​​selected from lithium manganese oxide and ternary materials. At least one of the following, where x+y+z=1 and 0.3≤x≤0.8, 0.1≤y≤0.2, 0.1≤z≤0.

6.

3. The lithium-ion battery cathode material according to claim 1, characterized in that, The porosity of the outer layer is 10–35%.

4. The lithium-ion battery cathode material according to claim 1, characterized in that, The dual-layer gradient composite nanocoating layer has a coverage rate of ≥95% over the cathode material matrix; and / or, The thickness of the inner layer is 5–15 nm, and the thickness of the outer layer is 10–40 nm.

5. The lithium-ion battery cathode material according to claim 1, characterized in that, In the bilayer gradient composite nanocoating layer, the inner layer forms a chemical bond with the positive electrode material matrix, and the outer layer is connected to the inner layer through phosphate groups.

6. The lithium-ion battery cathode material according to claim 1, characterized in that, The porous structure of the outer layer includes mesopores and macropores, wherein the pore size of the mesopores is 2-50 nm, the pore size of the macropores is 50-80 nm, and the volume ratio of mesopores to macropores is (2-5):

1.

7. A method for preparing a lithium-ion battery cathode material as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Prepare the cathode material substrate; S2. A lithium iron phosphate precursor layer containing Nb is deposited on the surface of the cathode material substrate by low-temperature liquid phase deposition. S3. A porous lithium titanium aluminum phosphate precursor layer is formed on the surface of the Nb-doped lithium iron phosphate precursor layer by in-situ liquid phase reaction. S4. Rapid plasma sintering is performed at 400-500°C for 3-5 minutes to form the double-layer gradient composite nano-coating layer.

8. The preparation method according to claim 7, characterized in that, In step S2, the reaction conditions for the low-temperature liquid phase deposition are: reaction temperature 40–80°C, reaction time 20–60 minutes, and solution pH controlled at 7–9; and / or, The Nb-doped lithium iron phosphate precursor layer is prepared using the following raw materials: lithium source, iron source, phosphorus source, and niobium source, wherein the lithium source is selected from... , , At least one of the following, wherein the iron source is selected from At least one of the following, wherein the phosphorus source is selected from , , At least one of the following, wherein the niobium source is selected from , , At least one of them.

9. The preparation method according to claim 7, characterized in that, In step S3, the porous lithium titanium aluminum phosphate precursor layer is prepared using the following raw materials: titanium source, aluminum source, phosphorus source, and lithium source, wherein the titanium source is selected from... At least one of the following, wherein the aluminum source is selected from At least one of them; and / or, The porous structure is achieved by controlling the addition rate of the precipitant, which is 0.1–1 mL / min. The precipitant is selected from… At least one of them.

10. The preparation method according to claim 7, characterized in that, In step S4, the heating rate of the rapid plasma sintering is 50-200℃ / min, the sintering pressure is 20-50MPa, and the sintering atmosphere is an inert gas or a vacuum. The gradient concentration distribution is formed in the following way: during the sintering process, P ions in the inner layer diffuse to the outer layer, and Ti and Al ions in the outer layer diffuse to the inner layer, forming a concentration gradient.

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