Positive electrode active material, preparation method thereof and battery

Abstract

The invention discloses a positive electrode active material, a preparation method thereof and a battery, the positive electrode active material comprises niobium-doped lithium-rich manganese-based oxide, and the positive electrode active material comprises a first region and a second region which are sequentially and adjacently arranged in a normal direction of a {010} crystal plane family, the minimum distance between the boundary of the second region and the surface of the positive electrode active material is smaller than the minimum distance between the boundary of the first region and the surface of the positive electrode active material; the Nb concentration of the second region is greater than the Nb concentration of the first region. Therefore, the positive electrode active material has component and structure gradient distribution along the {010} crystal plane family, the structural stability of the positive electrode active material is improved, the de-intercalation and diffusion rate of lithium ions is increased, and the capacity exertion, rate capability and cycle performance of the battery are further improved.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, specifically to positive electrode active materials and their preparation methods, and batteries. Background Technology

[0002] Lithium-ion batteries, as the core of modern energy storage systems, continue to drive the development of electric vehicles and portable electronic devices. To meet the ever-increasing demand for high energy density, research on cathode active materials has gradually shifted towards high-capacity, high-voltage layered oxides, particularly lithium-rich manganese-based layered oxides. These materials can achieve energies exceeding 250 mAh g⁻¹ through cation and anion redox reactions. -1 The specific capacity is significantly higher than that of traditional layered materials. However, lithium-rich manganese-based layered oxides face significant challenges in practical applications, including voltage decay, capacity degradation, oxygen evolution, and structural degradation during cycling. Therefore, the cathode active material still needs further improvement.

[0003] It should be noted that the above statements are only used to provide background information related to this application and do not necessarily constitute prior art. Summary of the Invention

[0004] In a first aspect, this application proposes a positive electrode active material comprising a niobium-doped lithium-rich manganese-based oxide. The positive electrode active material includes a first region and a second region arranged sequentially adjacent to each other along the normal direction of the {010} crystal plane family. The minimum distance between the boundary of the second region and the surface of the positive electrode active material is less than the minimum distance between the boundary of the first region and the surface of the positive electrode active material. The Nb concentration in the second region is greater than the Nb concentration in the first region. Therefore, the positive electrode active material has a compositional and structural gradient distribution along the {010} crystal plane family, which is beneficial for improving the structural stability of the positive electrode active material, accelerating the lithium-ion insertion / extraction and diffusion rate, and thus improving the battery's capacity utilization, rate performance, and cycle performance.

[0005] In some embodiments of this application, the positive electrode active material further includes a third region located between the first and second regions in the normal direction of the {010} crystal plane family of the positive electrode active material. The Nb concentration in the second region is greater than that in the third region, which is greater than that in the first region. Thus, the niobium gradient distribution of the {010} active crystal plane family helps to induce a continuous structural gradient from a bulk ordered layered structure to an intermediate spinel phase and then to a surface disordered rock salt phase. This allows the positive electrode active material to possess the high capacity and high ionic conductivity of a layered structure, the three-dimensional lithium-ion diffusion channels of the spinel phase, and the high stability of the disordered rock salt phase.

[0006] In some embodiments of the present application, the first region comprises an ordered layered structure, the third region comprises a spinel phase, and the second region comprises a disordered rock salt phase. Thus, the gradual change in the structure of the surface layer region helps to limit the variation of the lattice parameter of the positive electrode active material, relieve lattice strain, and enhance the mechanical structural stability.

[0007] In some embodiments of the present application, the thickness of the third region in the normal direction of the {010} crystal plane family of the positive electrode active material is 2 nm-8 nm. Thus, it helps to construct a three-dimensional lithium ion channel, and significantly improve the lithium ion transmission rate.

[0008] In some embodiments of the present application, the thickness h of the second region in the normal direction of the {010} crystal plane family of the positive electrode active material satisfies: 0 nm < h ≤ 3 nm. Thus, the interface can be effectively passivated, the occurrence of side reactions can be reduced, and the structural stability of the positive electrode active material at high voltage can be improved.

[0009] In some embodiments of the present application, the thickness of the first region in the normal direction of the {010} crystal plane family of the positive electrode active material is 1 μm-2 μm. Thus, it helps to improve the transmission rate of lithium ions and improve the capacity of the battery.

[0010] In some embodiments of the present application, the positive electrode active material satisfies: 0.003% ≤ |F WPPF - F HW | ≤0.01%, wherein F WPPF is the microstress measured by the full spectrum fitting method, and F HW is the microstress measured by the Halder-Wagner method. Thus, the stress distribution in the positive electrode active material is highly uniform, and the structure has long-range order characteristics, the crystal structure is more stable, the structural stress generated during high-voltage cycling can be effectively relieved, and the cycle stability is improved.

[0011] In some embodiments of the present application, the positive electrode active material satisfies the chemical formula: Li 1+a Ni b Co c Mn d Nb e O2, wherein 0 < a ≤ 0.33, 0 ≤ b < 1, 0 ≤ c < 1, 0 < d < 1, 0 < e ≤ 0.04, and a + b + c + d + e = 1. Thus, the Nb-O ionic bond with strong stability is constructed on the surface of the positive electrode active material by niobium doping, and the stability of the surface oxygen is improved.

[0012] In a second aspect of the present application, a method for preparing a positive electrode active material is provided, comprising: mixing a manganese source, a nickel source, a cobalt source, a precipitating agent, and a complexing agent to obtain a hydroxide precursor; mixing the hydroxide precursor, a sodium source, and a first lithium source and performing a first sintering treatment to obtain a first intermediate; mixing the first intermediate and a second lithium source and performing a second sintering treatment, and then performing water washing and drying to obtain a second intermediate; mixing the second intermediate and a niobium source and performing a third sintering treatment to obtain the positive electrode active material; and wherein the temperature of the third sintering treatment is 450-650 DEG C. Thus, the gradient distribution of the Nb element concentration can be achieved by precisely controlling the doping and sintering process, the process flow is simple and easy to scale up, and has good industrial application prospects.

[0013] In some embodiments of the present application, the temperature of the first sintering treatment is 700-850 DEG C, and the time is 6-20 h. Thus, the grain growth can be promoted, and the crystallinity can be improved.

[0014] In some embodiments of the present application, the temperature of the second sintering treatment is 400-800 DEG C, and the time is 5-15 h. Thus, the lithium-sodium ion exchange can be facilitated.

[0015] In some embodiments of the present application, the time of the third sintering treatment is 5-15 h. Thus, the thermal diffusion of the niobium element from the surface to the interior of the particles can be facilitated, and a radial concentration gradient gradually decreasing from the surface to the bulk phase can be formed.

[0016] In some embodiments of the present application, the manganese source comprises at least one of manganese sulfate, manganese carbonate, manganese nitrate, and trimanganese tetraoxide; and / or, the nickel source comprises at least one of a sulfate, a nitrate, a chloride, and an oxalate of the nickel element; and / or, the cobalt source comprises at least one of tricobalt tetroxide, cobalt hydroxide, cobalt monohydroxide, and cobalt oxyhydroxide; and / or, the sodium source comprises at least one of sodium carbonate, sodium hydroxide, and sodium bicarbonate; and / or, the first lithium source and the second lithium source each independently comprises at least one of lithium carbonate, lithium hydroxide, anhydrous lithium hydroxide, lithium oxide, and lithium chloride. Thus, the raw materials are widely available, the cost is low, and the large-scale promotion is facilitated.

[0017] In some embodiments of the present application, the niobium source comprises at least one of niobium ethoxide, niobium oxalate, di-niobium pentoxide, ammonium niobium oxalate, and niobium chloride. Thus, the niobium gradient doping can be facilitated, and a continuous structure gradient from the bulk ordered layered structure to the intermediate spinel phase and then to the surface disordered rock salt phase can be induced.

[0018] In a third aspect, the present application provides a battery comprising a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer on at least one side of the positive electrode current collector, and the positive electrode active material layer comprises the positive electrode active material according to the first aspect of the present application or the positive electrode active material prepared by the method according to the second aspect of the present application. Thus, the battery has all the features and advantages of the positive electrode active material described above, which will not be repeated here.

[0019] In some embodiments of the present application, the dQ / dV curve of the battery has a characteristic reduction peak at 2.5-2.7 V. Thus, the characteristic reduction peak corresponds to the lithium intercalation process in the spinel phase on the surface of the positive electrode active material. BRIEF DESCRIPTION OF DRAWINGS

[0020] The above and / or additional aspects and advantages of the present application will become apparent and be readily appreciated from the description of the embodiments, taken in conjunction with the following drawings in which: Figure 1 Nb 3d XPS spectra of the positive electrode active material prepared in Example 1 of the present application after etching for different times; Figure 2 STEM-EDS element mapping of the positive electrode active material prepared in Example 1 of the present application; Figure 3 FIG. 1a is a HAADF-STEM image of a surface region of the positive electrode active material prepared in Example 1 of the present application, FIG. 1b is a partial enlarged view of the orange region in FIG. 1a, FIG. 1c is a fast Fourier transform image of the orange region in FIG. 1a, and FIG. 1d is a fast Fourier transform image of the purple region in FIG. 1a; Figure 4 FIG. 2a is a STEM image of a surface region of the positive electrode active material prepared in Example 1 of the present application, FIG. 2b is the interlayer spacing variation ε xx c-axis direction of the geometric phase analysis of FIG. 2a, and FIG. 2c is the shear strain ε xy a-b plane of the geometric phase analysis of FIG. 2a; Figure 5 FIG. 3a is the initial activation charge-discharge curve of the battery of Example 1 and Comparative Example 1 of the present application, and FIG. 3b is the dQ / dV curve of the battery of Example 1 and Comparative Example 1 of the present application; Figure 6 FIG. 4a is a comparison of the cycle performance of the batteries of Example 1-3 and Comparative Example 1 of the present application, FIG. 4b is a comparison of the voltage decay of the batteries of Example 1-3 and Comparative Example 1 of the present application, and FIG. 4c is a comparison of the rate performance of the batteries of Example 1-3 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0021] Embodiments of the present application are described in detail below with reference to the attached drawing figures, but it is to be understood that the embodiments described are by way of illustration only and are not intended to limit the scope of the application. For example, the making and using of certain structures, materials, and acts are described herein, but it is understood that the features of each embodiment are not limited to the particular descriptions used herein, but are understood as being among other known equivalents within the skill in the art. In future claims, the indefinite article "a," or "an" can be used followed by "comprising" certain features or steps. Such phrasing should be antecedented by the characterization "as many as" or at least one. The phrase "one or more" followed by a list of elements and / or features should be understood to mean one, some, or all elements and / or features in the list; one set of elements and / or features selected from the list, and one or more additional sets of elements and / or features selected from the list; one element from a claim limiting the item and the elements in the list; or a combination of these. In addition, use of the "a" or "an" are open ended unless otherwise constrained by the context.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this application; and various parameters recited in the specification of the application can be measured using any method well known in the art (e.g., can be tested according to the methods given in the examples of the application), unless otherwise stated.

[0023] The terms "comprise" and "have" and any variations thereof used in the specification and claims of this application are open-ended transitional phrases, i.e., include but not limited to the recited elements.

[0024] In the description of the application, all numerical values disclosed herein are approximate. Each numerical value can vary up to 10% or other value deemed reasonable by one of skill in the art, such as 1%, 2%, 3%, 4%, or 5%.

[0025] The ranges disclosed herein are intended to be "open" ranges, i.e., the upper and lower limits of the range are not included. The ranges are also intended to include any and all sub-ranges of the range, i.e., all combinations of any two of the range limits, unless the context clearly dictates otherwise. For example, a range of "1 to 10" is intended to include any number from 1 to 10, including the end values in the range, e.g., 1 and 10, and any intervening combinations, e.g., 1-3, 4-7, 9-10, 3-8, etc. In other words, unless expressly provided otherwise, any numerical range is intended to include any and all sub-ranges of the same numerical range. Also, the phrases "ranging / ranges between" a first and a second number and "ranging / ranges from" a first number "to" a second number are used herein, each in the context of describing a range of values, and are intended to include the first and second values, and to also include any numerical value or range of values subordinate to, or within, the specified range of values. For example, a range of "1 to 10" indicates that the first value is 1 and the second value is 10, and also indicates that any numerical value or range of values subordinate to, or within, the specified range of values (i.e., any particular number or range of numerical values from 1 to 10, including 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.5, 5, 6, 7, 8, 9, and 10, as well as sub-ranges such as 2-4, 3.5-10.5, 100-1, etc.) are expressly stated to be contemplated.

[0026] In the description of the present application, the terms "first", "second", etc. are used only for the purpose of description, and are not to be interpreted as indicating or implying relative importance or a specific order of the technical features indicated. The "first feature", "second feature" can include one or more of the features.

[0027] In the description of the present application, "A and / or B" can include any one of the case of A alone, the case of B alone, and the case of A and B, where A and B are only for example, and can be any technical feature connected by "and / or" in the present application.

[0028] In the present application, the order of the steps written does not mean strict execution order and constitute any limitation on the implementation process, and the specific execution order of each step should be determined by its function and possible inherent logic. If not specifically stated, all the steps of the present application can be performed sequentially or randomly, and preferably sequentially. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed sequentially, or steps (b) and (a) performed sequentially. For example, the method also comprises step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0029] If there is no special indication, all the embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0030] If there is no special indication, all the technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0031] The oxygen atoms in the lithium-rich manganese-based positive electrode active material are unstable when undergoing redox reactions at high voltage (>4.5V), leading to surface reconstruction, phase change and mechanical stress accumulation of the positive electrode active material, and further causing particle cracking and interface side reactions. Related technologies can alleviate these problems to some extent through element doping and surface coating, but often cannot fundamentally change the structural nature of the material, especially at extreme voltage (≥4.5V), its performance is still not satisfactory. From the perspective of crystal structure, the (003) crystal plane in the layered material does not have a channel for lithium ion intercalation / deintercalation, and the structure is relatively stable; while the {010} crystal plane family is an active channel for lithium ion migration, which is prone to cracking along the layer during repeated cycles, becoming a weak link of structural failure. Therefore, precise gradient structure regulation of the {010} active crystal plane family can effectively improve the overall structural integrity and cycle life of the positive electrode active material. In the present application, the {010} active crystal plane family is doped with niobium gradient, realizing the continuous regulation of composition and structure from the bulk to the surface, and successfully constructing a gradual structure of "ordered layered → semi-disordered spinel → disordered rock salt", so as to not only utilize the advantage of high stability of disordered rock salt phase, but also improve the diffusion rate of lithium ions through spinel phase, effectively alleviate the rate performance decay caused by high proportion of disordered rock salt phase, while retaining the advantages of high capacity and high ionic conductivity of ordered layered structure, and synergistically improving the capacity release, rate performance and cycle performance of the positive electrode active material.

[0032] In the first aspect of the present application, the present application proposes a positive electrode active material, comprising a niobium-doped lithium-rich manganese-based oxide, in the normal direction of the {010} crystal plane family of the positive electrode active material, comprising a first region and a second region arranged in sequence, the minimum distance between the boundary of the second region and the surface of the positive electrode active material is less than the minimum distance between the boundary of the first region and the surface of the positive electrode active material; the Nb concentration of the second region is greater than the Nb concentration of the first region. Therefore, the positive electrode active material has a composition and structure gradient distribution along the {010} crystal plane family, ensuring excellent lattice matching between regions, effectively inhibiting the sudden change of lattice parameters, thereby significantly alleviating the strain accumulation during the cycle, and enhancing the mechanical integrity of the positive electrode active material. Therefore, it is beneficial to improve the capacity release ability, rate performance and cycle performance of the battery using the positive electrode active material.

[0033] In some embodiments of the present application, the positive electrode active material further comprises a third region, which is located between the first region and the second region in the normal direction of the {010} crystal plane family of the positive electrode active material, and the Nb concentration of the second region is greater than that of the third region, which is greater than that of the first region.

[0034] It can be understood that the Nb element presents a radial concentration gradient in the positive electrode active material, which gradually decreases from the surface to the bulk phase, and is significantly enriched in the region about 0 nm-3 nm away from the surface. This gradient distribution is derived from the thermal diffusion behavior of the niobium source from the surface to the interior of the particle during the third sintering process. Under a specific temperature and time window, niobium ions cannot reach an equilibrium distribution, thereby naturally forming this concentration gradient. Since the {010} crystal plane family has a higher surface energy and lithium ion migration activity, the doping and diffusion behavior of niobium in this crystal plane region is more significant, which makes the gradient structure preferentially and stably construct along these active crystal planes. Therefore, the gradient distribution of niobium through the {010} active crystal plane family helps to induce the formation of a continuous structure gradient from the ordered layered structure of the bulk phase to the intermediate spinel phase to the surface disordered rock salt phase, so that the positive electrode active material has high capacity, high ionic conductivity of the layered structure, three-dimensional lithium ion diffusion channels of the spinel phase, and high stability of the disordered rock salt phase.

[0035] In some embodiments of the present application, the first region comprises an ordered layered structure, the third region comprises a spinel phase, and the second region comprises a disordered rock salt phase. Among them, the ordered layered structure of the bulk phase is beneficial to improve the lithium ion deintercalation rate; the disordered rock salt phase of the surface layer effectively passivates the interface and inhibits the surface oxygen reactivity and surface side reaction; and the intermediate spinel phase constructs three-dimensional lithium ion diffusion channels, which together realize the significant improvement of lithium ion transmission rate.

[0036] The foregoing structure is different from the conventional simple core-shell configuration, which exhibits excellent structural continuity and interface compatibility on a macroscopic scale, and forms a stable exposed surface mainly composed of the {010} crystal plane family on a microscopic level, which is beneficial to the rapid deintercalation and diffusion of lithium ions.

[0037] The disordered rock salt structure does not rely on the change of lithium content to maintain stability, so it can maintain structural integrity even in a high de-lithiation state, thereby significantly improving the surface structure stability of the positive electrode active material, effectively inhibiting the surface reconstruction, phase transition and mechanical stress accumulation of the positive electrode active material, and greatly relieving the occurrence of particle cracking and interface side reaction.

[0038] It should be noted that the formation of the gradient structure is induced by the Nb 5+ The concentration distribution on the {010} crystal plane family is directly induced. In the outermost Nb enrichment region, high concentration of Nb 5+ (d0 The electronic configuration of Nb (4d15) has no crystal field stabilization energy, which significantly promotes the cationic mixing between Li and TM layers, and thus preferentially forms a stable disordered rock-salt phase on the structurally fragile {010} family of planes. In the intermediate transition region, with the decrease of Nb concentration, the ability of Nb to induce disorder is weakened, which induces a moderate Li / TM mixing, and together promotes the transformation of the layered structure into a spinel phase with three-dimensional ion channels in the {010} plane direction. In the inner bulk phase, the extremely low Nb content makes its influence negligible, and thus the positive active material maintains its intrinsic high-capacity layered structure. Therefore, this composition and structure double-gradient design ensures excellent lattice matching between the phases, effectively inhibiting the structural instability caused by the sudden change of lattice parameters, achieving precise strengthening and protection of the main channel for lithium ion migration, effectively inhibiting the oxygen loss and surface side reactions during the cycle process, improving the reversibility of the oxygen redox, relieving the strain accumulation, and providing a three-dimensional lithium ion diffusion path.

[0039] In some embodiments of the present application, with reference to b in the formula (I), the thickness w of the third region in the normal direction of the {010} family of planes of the positive active material is 2 nm-8 nm. Figure 3

[0040] For example, the thickness of the third region can be 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, or any numerical range between any two of them.

[0041] The thickness of the third region in the above range helps to build a three-dimensional lithium ion channel, which significantly improves the lithium ion transmission rate.

[0042] In some embodiments of the present application, with reference to b in the formula (I), the thickness h of the second region in the normal direction of the {010} family of planes of the positive active material satisfies: 0 nm Figure 3

[0043] For example, the thickness of the second region can be 0.01 nm, 0.05 nm, 0.1 nm, 0.5 nm, 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, or any numerical range between any two of them.

[0044] The thickness of the second region in the above range can effectively passivate the interface, reduce the occurrence of side reactions, and be conducive to improving the structural stability of the positive active material at high voltage.

[0045] In some embodiments of the present application, the thickness of the first region in the normal direction of the {010} family of planes of the positive active material is 1 μm-2 μm.

[0046] ​​As an example, the thickness of the first region can be 1 pm, 1.1 pm, 1.2 pm, 1.3 pm, 1.4 pm, 1.5 pm, 1.6 pm, 1.7 pm, 1.8 pm, 1.9 pm, 2 pm, or a numerical range between any two of them.

[0047] The thickness of the first region in the above range is beneficial to improve the transmission rate of lithium ions and improve the capacity of the battery.

[0048] As an example, the thickness of the first region, the second region, and the third region in the normal direction of the {010} plane family of the positive electrode active material can be measured by: embedding the positive electrode active material sample in a conventional manner, slicing and ion thinning to obtain a thin slice that meets the requirements of electron microscopic analysis. A spherical aberration-corrected scanning transmission electron microscope (Cs-STEM) is used to observe the obtained thin slice to obtain a high-resolution annular dark field phase (HAADF-STEM) of the radial cross section of the particle. According to the contrast difference of different crystal structures in the HAADF image, the layered phase, spinel phase, and disordered rock salt phase of the particle from the surface to the center are identified, and the interface position between each structure phase is determined. After the interface is clear, line scanning measurement is performed along the normal direction of the {010} plane family of the particle, and the continuous thickness of the second region, the third region, and the first region appearing from the surface to the center is recorded. Repeat the measurement on not less than 5 particles, and take the average of the obtained thickness as the statistical result of each region. The value measured by the above method can be used as the thickness data of the corresponding region in the embodiments of the present application.

[0049] In some embodiments of the present application, the positive electrode active material satisfies: 0.003% ≤ |F WPPF - F HW | ≤0.01%, wherein F WPPF is the microstress measured by the full spectrum fitting method, and F HW is the microstress measured by the Halder-Wagner method.

[0050] As an example, |F WPPF - F HW | can be 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, 0.001%, or a numerical range between any two of them.

[0051] |F WPPF - F HW| Within the above range, the internal stress distribution of the positive electrode active material is highly uniform, and has a long-range ordered structure characteristic, and the crystal structure is more stable, which can effectively relieve the structural stress generated in the high-pressure cycle process, and improve the cycle stability. This further confirms that the "gradual" design of the double gradient structure promotes the formation of a continuous transition of the phase interface, rather than a sharp interface, which fundamentally realizes the perfect matching of the lattice parameters. Such uniform stress distribution helps to relieve the anisotropic volume change generated during charging and discharging, which is crucial for inhibiting particle cracking and improving structural integrity.

[0052] F WPPF It can be tested by the following method: XRD device model: Smart Lab 9KW, powder X-ray diffraction determination using Cu Kα ray, detector: graphite monochromator+SC, continuous scanning, scanning range 10°-80°, scanning speed 2° / min. Among them, the WPPF method is processed by using the Pearson VII peak shape function through the instrument's own data processing software, and the initial model is Li2MnO3, belonging to the C2 / m space group.

[0053] F WH It can be tested by the following method: XRD device model: Smart Lab 9KW, powder X-ray diffraction determination using Cu Kα ray, detector: graphite monochromator+SC, continuous scanning, scanning range 10°-80°, scanning speed 2° / min. Among them, the WH method is fitted by selecting all characteristic peaks of 10°-80° through the instrument's own data processing software, including 003, 101, 006, 012, 104, 015, 107, 018, 110, 113, 021, 1010, 116 peaks.

[0054] The XRD sample preparation method is: the positive electrode active material powder after passing through a 400 mesh sieve is spread on the entire glass slide surface, the excess powder is removed, and it is lightly pressed and flattened on the sample stage.

[0055] In some embodiments of the present application, the positive electrode active material satisfies the chemical formula: Li 1+a Ni b Co c Mn d Nb e O2, wherein 0

[0056] As an example, a can be 0.01, 0.03, 0.05, 0.07, 0.1, 0.13, 0.15, 0.17, 0.2, 0.23, 0.25, 0.27, 0.3, 0.33, or any numerical range between any two of them.

[0057] As an example, b can be 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or any numerical range between any two of them.

[0058] As an example, c can be 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or any numerical range between any two of them.

[0059] As an example, d can be 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.99, or any numerical range between any two of them.

[0060] As an example, e can be 0.01, 0.02, 0.03, 0.04, or any numerical range between any two of them.

[0061] The gradient structure construction strategies in the related art, such as multi-step core-shell construction or precise co-precipitation, generally have problems such as complex process and low repeatability, and the interface between different phases is often poor due to lattice mismatch, which is prone to interface degradation in long-term cycling and is difficult to achieve stable long-term performance.

[0062] In a second aspect of the present application, a method for preparing a positive electrode active material is provided, which can realize the gradient distribution of the Nb element concentration by precisely controlling the doping and sintering process, has a simple process flow and is easy to scale up, and has good industrial application prospects. Specifically, the method comprises: S1: mixing a manganese source, a nickel source, a cobalt source, a precipitant, and a complexing agent to obtain a hydroxide precursor.

[0063] In some embodiments of the present application, the manganese source comprises at least one of manganese sulfate, manganese carbonate, manganese nitrate, and trimanganese tetraoxide; and / or the nickel source comprises at least one of a sulfate, a nitrate, a chloride, and an oxalate corresponding to the nickel element; and / or the cobalt source comprises at least one of tricobalt tetraoxide, cobalt hydroxide, cobalt hydroxide, and cobalt oxyhydroxide. Thus, the raw materials are widely sourced and have low cost, facilitating large-scale promotion.

[0064] S2: mixing the hydroxide precursor, a sodium source, and a first lithium source and performing a first sintering treatment to obtain a first intermediate.

[0065] As an example, Na 0.9 Li a Ni b′ Co c′ Mn d′ O2 required hydroxide precursor, sodium source and the first lithium source, after mixing well, the first sintering treatment to obtain the first intermediate (P2 phase intermediate).

[0066] In some embodiments of the present application, the sodium source includes at least one of sodium carbonate, sodium hydroxide, sodium bicarbonate; and / or, the first lithium source includes at least one of lithium carbonate, lithium hydroxide, anhydrous lithium hydroxide, lithium oxide. Thus, the raw material source is extensive, the cost is lower, and it is convenient for large-scale promotion.

[0067] In some embodiments of the present application, the temperature of the first sintering treatment is 700-850℃, and the time is 6-20h. Thus, the grain growth can be promoted, and the crystallinity is improved.

[0068] As an example, the temperature of the first sintering treatment can be 700℃, 730℃, 760℃, 790℃, 820℃ or 850℃, and the time can be 6h, 8h, 10h, 12h, 14h, 16h, 18h or 20h, etc.

[0069] S3: mixing the first intermediate with a second lithium source and performing a second sintering treatment to obtain a second intermediate.

[0070] As an example, the obtained first intermediate is subjected to a second sintering treatment in the second lithium source to realize lithium-sodium ion exchange, and after cooling, it is subjected to water washing and drying treatment to obtain the second intermediate.

[0071] In some embodiments of the present application, the second lithium source includes at least one of lithium carbonate, lithium hydroxide, anhydrous lithium hydroxide, lithium oxide, lithium chloride. Thus, the raw material source is extensive, the cost is lower, and it is convenient for large-scale promotion.

[0072] In some embodiments of the present application, the temperature of the second sintering treatment is 400-800℃, and the time is 5-15h. Thus, it is beneficial to realize lithium-sodium ion exchange.

[0073] As an example, the temperature of the second sintering treatment can be 400℃, 450℃, 500℃, 550℃, 600℃, 700℃, 750℃ or 800℃, and the time can be 5h, 7h, 9h, 11h, 13h or 15h, etc.

[0074] S4: mixing the second intermediate with a niobium source and performing a third sintering treatment under an air or oxygen atmosphere, and after water washing and drying, obtaining the positive electrode active material.

[0075] In some embodiments of the present application, the third sintering treatment has a temperature of 450-650℃ and a time of 5-15h. In this way, the thermal diffusion of the niobium element from the surface of the particles to the interior is facilitated, forming a radial concentration gradient that gradually decreases from the surface to the bulk phase.

[0076] By way of example, the third sintering treatment can have a temperature of 450℃, 480℃, 500℃, 530℃, 550℃, 580℃, 600℃, 630℃ or 650℃, etc., and a time of 5h, 7h, 9h, 11h, 13h or 15h, etc.

[0077] In some embodiments of the present application, the niobium source comprises at least one of niobium ethoxide, niobium oxalate, niobium pentoxide, ammonium niobium oxalate, niobium chloride. In this way, the formation of a niobium gradient doping is facilitated, inducing the formation of a continuous structural gradient from the bulk ordered layered structure to the intermediate spinel phase and then to the surface disordered rock salt phase.

[0078] The present application realizes the dual gradient synergistic regulation of composition and structure through a single Nb 5+ Doping process, realizing the dual gradient synergistic regulation of composition and structure, and constructing a continuous gradient structure gradually transitioning from layered structure to spinel phase and then to disordered rock salt phase. 5+ (d 0 Electronic configuration) due to the lack of crystal field stabilization energy, the selectivity of the coordination environment is weak, thereby promoting the cationic mixing between the transition metal and the lithium layer, and facilitating the formation of the disordered rock salt phase.

[0079] In a third aspect of the present application, a battery is provided, comprising a positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer located on at least one side of the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode active material of the first aspect of the present application or the positive electrode active material prepared by the method of the second aspect of the present application. In this way, the battery has all the features and advantages of the aforementioned positive electrode active material, which will not be described here again.

[0080] In some embodiments of the present application, the dQ / dV curve of the battery has a characteristic reduction peak at 2.5-2.7V. In this way, the characteristic reduction peak corresponds to the lithium intercalation process in the spinel phase on the surface of the positive electrode active material.

[0081] The above description of each embodiment tends to emphasize the differences between each embodiment, and the same or similar parts can be mutually referred to for brevity, which will not be described here again.

[0082] The application will be described in detail below through specific examples. It should be noted that the examples below are only used to illustrate the application and should not be regarded as limiting the scope of the application. If the specific technology or conditions are not specified in the examples, the technology or conditions described in the literature in the art or according to the product manual are used. If the reagent or instrument is not specified by the manufacturer, it is a conventional product that can be obtained by purchase.

[0083] Example 1 Step 1, weigh 0.3375 mol of manganese sulfate, 0.08125 mol of nickel sulfate and 0.01825 mol of cobalt sulfate compound, add an appropriate amount of deionized water to prepare 500 ml of salt solution to obtain solution A. Weigh 2 mol of sodium hydroxide, ammonia water containing 1.2 mol of ammonia gas as a complexing agent, and dissolve it into 500 ml of alkali solution, marked as solution B. Before the reaction starts, 500 ml of deionized water is added as the bottom liquid in a 3 L reaction kettle, and the temperature in the reaction kettle is controlled at 55℃. Then adjust the pH to 10.2 by adding ammonia water, then seal the reaction kettle, and pass argon for 30 min to exhaust the air in the reaction kettle. Then, adjust the stirring paddle speed to 1000 r / min, and add solution A to the reaction kettle at a rate of 1.5 mL / min, and solution B is automatically added to the reaction kettle by pH self-feedback adjustment. The pH is maintained at about 10.2 throughout the reaction. After the addition of solution A is completed, the speed is reduced to 700 r / min, and the product is aged for 12 h before being discharged. Then, it is washed with deionized water and suction filtered 3 times, and dried in a vacuum oven at 150℃ for 12 h. After taking out, the target precursor is obtained and stored in a glove box.

[0084] Step 2, prepare the P2 phase intermediate of the chemical formula Na0.9[Li0.2Mn0. 54 Ni0. 13 Co0. 13 ]O2. Weigh 71.55 g of sodium carbonate, 11.64 g of lithium carbonate, and 108.26 g of the transition metal hydroxide obtained in step 1, mix thoroughly, and then place in a corundum boat for sintering. The sintering process is as follows: increase the temperature to 450℃ at a rate of 5℃·min -1 , and keep it for 4 h; then continue to increase the temperature to 800℃ at a rate of 5℃·min -1 , and keep it for 15 h. After the first sintering treatment is completed, the furnace is cooled to room temperature, and the P2 phase intermediate, i.e. the first intermediate, is obtained.

[0085] Step 3, weigh 100 g of the P2 phase intermediate obtained in step 2, mix with 200 g of lithium chloride, and grind uniformly before sintering. The sintering process is as follows: increase the temperature to 450℃ at a rate of 5℃·min -1The reaction product was taken out after the second sintering process, washed with deionized water and suction filtered three times, and finally dried in a 100℃ air oven for 15h to obtain a lithium-rich manganese-based positive electrode active material (LRM) that can be used for subsequent surface treatment, i.e., a second intermediate.

[0086] Step 4, 100g of the LRM powder prepared in step 3 was weighed and added to an ethanol solution with a metered amount of niobium ethoxide (so that the final product has a Nb doping amount of 1% in terms of moles relative to the total amount of transition metals). The ethanol was evaporated to dryness on a magnetic stirrer with the water bath temperature controlled at 80℃. The dried mixture was sintered at a temperature rising rate of 5℃·min -1 -1 to 550℃ in an air atmosphere and held for 10 hours for a third sintering process. After natural cooling to room temperature, it was ground and sieved to obtain S1.

[0087] The differences between other examples and the comparative example and example 1 are shown in Table 1.

[0088] Table 1

[0089] " / " indicates that the step is not performed or the substance is not added.

[0090] The S1 sample was systematically characterized by etching X-ray photoelectron spectroscopy (XPS), atomic resolution scanning transmission electron microscopy (STEM) and its corresponding energy dispersive spectroscopy (EDS). The results are as follows: From Figure 1 it can be seen that as the etching time increases, the signal peak of the Nb element gradually weakens and broadens, indicating that the Nb content decreases from the surface to the inside in a gradient distribution, which is closely related to the decrease in diffusion rate of high-valence ions as the diffusion distance increases. The spatial distribution of Nb can be clearly observed by STEM-EDS Figure 2 ), and there is obvious enrichment in the region about 1nm thick on the surface, which is highly consistent with the EDS results. It is worth noting that the Nb-enriched crystal face is mainly the active crystal face of lithium ion intercalation / deintercalation—{010} crystal face family, clearly revealing the gradient distribution characteristics of Nb on the {010} crystal face family.

[0091] To further refine the structure evolution analysis, the present application uses high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to perform atomic-level characterization of the surface structure. As shown in Figure 3 Figure a, the granular phase region shows a typical layered structure of atomic arrangement; while Figure 3Fig. 2b shows that the structure gradually densifies from the bulk to the surface. In the bulk region, no bright spots are observed in the lithium layer, which is consistent with the standard layered structure characteristics; while in the transition region close to the surface, obvious but discontinuous bright spots appear in the lithium layer, corresponding to the projection of spinel phase along the

[110] zone axis, indicating that the transition region is a spinel phase layer with a thickness of about 5.3 nm. In contrast, the bright spots in the outermost lithium layer are continuously distributed, representing a typical disordered rock salt structure, with a thickness of about 1 nm, which is highly consistent with the Nb-rich region detected by EDS, indicating that the formation of the disordered rock salt phase is induced by high concentration of Nb doping. Further fast Fourier transform (FFT) analysis results show that the surface region (Fig. 2c) has an additional set of diffraction spots derived from the spinel phase compared to the bulk region (Fig. 2d), further confirming the existence of the spinel phase. The spinel phase is located between the layered phase and the disordered rock salt phase, serving as a transition layer of structure and composition, which is highly consistent with the gradient decreasing trend of Nb doping concentration. Figure 3 Figure 3

[0092] In summary, the gradient doping of Nb 5+ achieves the continuous evolution of the positive electrode active material from the internal layered structure to the surface disordered rock salt phase through the spinel phase, and forms a composition and structure double gradient distribution from the surface to the interior on the {010} family of crystal planes.

[0093] In addition, the geometric phase analysis (GPA) is used to quantitatively evaluate the interlayer spacing change (ε xx ) in the c-axis direction and the shear strain (ε xy ) in the a-b plane. Figure 4 The results show that the strain distribution in the near-surface region of the S1 sample is close to that at the interface of the bulk layered structure. In addition, the difference between F WWPF and F HW of the S1-S7 samples calculated by XRD is small, indicating that the gradient change of the surface structure effectively reduces the lattice mismatch between the phase interfaces, showing good lattice compatibility. The excellent lattice matching between the phases effectively suppresses the sudden change of the lattice parameters, significantly alleviating the strain accumulation during the cycle process, thereby improving the mechanical integrity of the material. At the same time, this highly compatible lattice interface helps to promote the efficient transport of lithium ions and effectively alleviate the concentration of stress and strain during charging and discharging.

[0094] The above positive electrode active materials are assembled into button cells for electrochemical performance testing.

[0095] ​​Preparation of positive electrode sheet: according to the mass ratio of positive active material: acetylene black: polyvinylidene fluoride = 92:5:3, the mixture was weighed and mixed, an appropriate amount of NMP was added and stirred uniformly to form a positive electrode slurry, which was coated on an aluminum foil and dried. After rolling, the sheet was cut into a small round piece with a diameter of 12 mm, and the positive electrode sheet was obtained after drying in a vacuum oven at 120°C overnight.

[0096] The negative electrode sheet was a metal lithium sheet with a diameter of 17 mm and a thickness of 1 mm.

[0097] Preparation of electrolyte: a mixed solvent prepared from ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1, and 1 mol / L LiPF6 was dissolved.

[0098] The positive electrode sheet, separator, negative electrode sheet and electrolyte were assembled into a 2025 type button cell, and the charge and discharge test was carried out on a Neware test system.

[0099] The capacity test conditions were as follows: the assembled battery was placed for 6 hours, and then tested at room temperature with a current density of 1C = 250 mA / g. A stepwise activation system was used: the cutoff voltage was gradually increased from 4.5V to 4.6V, and finally to 4.8V. The test was carried out in the voltage window of 2.1V-4.5V, 2.1V-4.6V and 2.1V-4.8V at 0.1C rate, respectively. The discharge capacity obtained by voltage test of 2.1V-4.8V was taken as the discharge specific capacity of the battery.

[0100] Rate performance test: the voltage range was 2.5V-4.8V. Five cycles were carried out in turn at 0.1C (2.1V-4.5V), 0.1C (2.1V-4.6V), 0.1C (2.1V-4.8V), followed by 5 cycles at 0.2C, 0.33C, 0.5C, 1C, 2C and 5C rate, and finally 5 cycles at 0.1C rate, to measure the specific capacity at different currents. The first discharge specific capacity at 5C rate was taken as the 5C specific capacity of the battery.

[0101] Cycle stability test: after activation in the voltage range of 2.1V-4.5V, 2.1V-4.6V and 2.1V-4.8V at 0.1C rate, 300 cycles were carried out at 1C rate in the range of 2.1V-4.6V, and the discharge capacity of the 300th cycle was divided by the discharge capacity of the first cycle to obtain the cycle retention rate of the battery.

[0102] Voltage degradation: The battery was subjected to 300 cycle tests under conditions of 2.1V-4.6V and a current ratio of 1C. The average discharge voltage of each cycle was calculated by dividing the discharge specific energy by the discharge specific capacity. The difference between the average voltage of the 1st cycle and the 300th cycle was taken as the total voltage degradation value; then, this difference was divided by 300 to obtain the average voltage degradation per cycle.

[0103] The physicochemical parameters of the above-mentioned positive electrode active materials, as well as the battery capacity, cycle retention rate, voltage decay and rate performance are shown in Tables 2-1 and 2-2.

[0104] Table 2-1

[0105] In both Comparative Example 1 and Comparative Example 4, the positive electrode active materials were ordered layered structures, and in Comparative Example 4, niobium was uniformly doped.

[0106] Table 2-2

[0107] Both Sample 1 (S1) and Comparative Sample 1 (D1) exhibited typical redox characteristics of lithium-rich materials. Figure 5 As shown, the voltage plateau at approximately 4.5 V corresponds to the oxygen oxidation process (reflected in the O2 peak); during discharge, oxygen reduction and transition metal reduction correspond to peaks R1 and R2, respectively; while peak R3 reflects the hysteresis phenomenon in the oxygen reduction process. Notably, the S1 sample exhibits a unique R4 peak near approximately 2.6 V. This peak originates from the lithiation reaction of the spinel phase and displays typical spinel phase characteristics in the electrochemical curve, further confirming the analysis of the spinel structure in the HAADF-STEM results.

[0108] After full activation, the discharge capacity of the S1 positive electrode active material reaches 308.9 mAh·g. -1 It is higher than the 302.3 mAh·g of the D1 cathode. -1 This indicates that it has superior oxygen redox reversibility. Figure 6 As shown in Table 2-2, Examples 1-7 (S1-S7) exhibit higher cycling stability and voltage retention compared to Comparative Examples 1-2 (D1-D2): S1 maintained a capacity retention of 87.3% after 300 cycles, significantly higher than D1's 67.4%; in terms of voltage decay, S1 was only 0.85 mV / cycle, far lower than D1's 1.24 mV / cycle. This indicates that the composition-structure dual gradient can effectively stabilize the {010} active crystal plane family, suppress transition metal ion migration and spinel phase transformation during cycling, thereby significantly mitigating voltage decay. Among these, sample D2, compared to samples S1-S7, |F WPPF -F HW| Greater than 0.01%, the cycle is slightly poor, indicating that the lattice parameter matches, which will slow down the structural stress inside the positive active material, and is beneficial to improve the cycle performance.

[0109] As shown in Table 2-2, the rate performance of S1-S7 samples is significantly higher than that of D1 material, highlighting its excellent high-rate performance, which is attributed to the three-dimensional lithium ion diffusion channel formed by the surface spinel structure, which accelerates the lithium ion diffusion kinetics. At the same time, this also shows that the disordered rock salt phase structure with moderate surface thickness and good lattice parameter matching does not hinder the formation and transmission of lithium ion diffusion channel, so as to still maintain excellent electrochemical reaction reversibility under high-rate conditions.

[0110] The positive active material in the embodiment can realize a higher discharge specific capacity under a 4.8 V cutoff voltage, showing significant high-energy characteristics; still has a higher capacity retention rate after 300 cycles, showing excellent cycle stability; and the average voltage decay rate is much lower than that of conventional lithium-rich manganese-based materials; still has excellent capacity release under 5C high rate, embodying excellent rate performance. The positive active material in the application has enhanced oxygen redox reversibility, faster Li + diffusion kinetics, and good mechanical integrity, and comprehensively realizes the performance balance of high energy density and long life, high rate compatibility. The application successfully constructs a lithium-rich manganese-based positive active material with composition-structure double gradient through {010} active crystal plane family Nb doping, which effectively improves the problems of high-voltage instability and poor lithium ion diffusion kinetics in the long cycle process, thereby overcoming the defects of traditional lithium-rich materials such as fast voltage decay, poor cycle stability, and insufficient rate performance, and showing excellent comprehensive electrochemical performance and structural stability, having a broad application prospect in the field of high-energy-density lithium-ion batteries.

[0111] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and playing the same role and effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications of the embodiments that can be thought of by those skilled in the art, and other ways constructed by combining part of the constituent elements of the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode active material, characterized in that, Including niobium-doped lithium-rich manganese-based oxides, In the normal direction of the {010} crystal plane family of the positive electrode active material, there are a first region and a second region arranged sequentially adjacent to each other. The minimum distance between the boundary of the second region and the surface of the positive electrode active material is less than the minimum distance between the boundary of the first region and the surface of the positive electrode active material. The Nb concentration in the second region is greater than that in the first region.

2. The positive electrode active material according to claim 1, characterized in that, It further includes a third region located between the first region and the second region in the normal direction of the {010} crystal plane family of the positive electrode active material, wherein the Nb concentration in the second region is greater than the Nb concentration in the third region, which is greater than the Nb concentration in the first region.

3. The positive electrode active material according to claim 2, characterized in that, The first region comprises an ordered layered structure, the third region comprises a spinel phase, and the second region comprises a disordered rock salt phase.

4. The positive electrode active material according to claim 3, characterized in that, In the normal direction of the {010} crystal plane family of the positive electrode active material, the thickness of the third region is 2nm-8nm; and / or, In the normal direction of the {010} crystal plane family of the positive electrode active material, the thickness h of the second region satisfies: 0 nm < h ≤ 3 nm; and / or, In the normal direction of the {010} crystal plane family of the positive electrode active material, the thickness of the first region is 1μm-2μm.

5. The positive electrode active material according to claim 1, characterized in that, The positive electrode active material satisfies: 0.003% ≤ |F WPPF - F HW | ≤ 0.01%, where F WPPF For the micro-stress measured by the full-spectrum fitting method, F HW The micro-stress was measured using the Halder-Wagner method.

6. The positive electrode active material according to any one of claims 1-5, characterized in that, Satisfies the chemical formula: Li 1+a Ni b Co c Mr d No e O2, Among them, 0 <a≤0.33,0≤b<1,0≤c<1,0<d<1,0<e≤0.04,a+b+c+d+e=1。 7. A method for preparing the positive electrode active material according to any one of claims 1-6, characterized in that, include: Manganese source, nickel source, cobalt source, precipitant, and complexing agent are mixed to obtain hydroxide precursor; The hydroxide precursor, sodium source, and first lithium source are mixed and subjected to a first sintering treatment to obtain a first intermediate. The first intermediate is mixed with a second lithium source and subjected to a second sintering process to obtain a second intermediate; The second intermediate is mixed with a niobium source and subjected to a third sintering treatment, followed by washing and drying to obtain the positive electrode active material. The temperature of the third sintering treatment is 450℃-650℃.

8. The method according to claim 7, characterized in that, The first sintering treatment is performed at a temperature of 700℃-850℃ for a time of 6h-20h; and / or, The second sintering treatment is performed at a temperature of 400℃-800℃ for 5 hours to 15 hours and / or... The third sintering process takes 5-15 hours.

9. The method according to claim 7 or 8, characterized in that, The manganese source includes at least one of manganese sulfate, manganese carbonate, manganese nitrate, and manganese tetroxide; and / or, The nickel source includes at least one of the following: nickel sulfate, nitrate, chloride, and oxalate; and / or, The cobalt source includes at least one of cobalt tetroxide, cobalt hydroxide, cobalt sulfide, and cobalt hydroxyl oxide; and / or, The sodium source includes at least one of sodium carbonate, sodium hydroxide, and sodium bicarbonate; and / or, The first lithium source and the second lithium source each independently include at least one of lithium carbonate, lithium hydroxide, anhydrous lithium hydroxide, lithium oxide, and lithium chloride; and / or, The niobium source includes at least one of niobium ethanol, niobium oxalate, niobium pentoxide, ammonium niobium oxalate, and niobium chloride.

10. A battery, characterized in that, The invention includes a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer located on at least one side of the positive current collector, the positive active material layer comprising the positive active material according to any one of claims 1-6 or the positive active material prepared by the method according to any one of claims 7-9.

11. The battery according to claim 10, characterized in that, The battery's dQ / dV curve shows a characteristic reduction peak in the 2.5V-2.7V range.

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