Positive electrode material and preparation method thereof, electrochemical device and electronic equipment

Abstract

The invention provides a positive electrode material and a preparation method thereof, an electrochemical device and electronic equipment, and particularly relates to the technical field of batteries. The positive electrode material comprises an inner core and a protective layer, the chemical formula of the inner core is Li1 + xM1-xO2, in the formula, x is more than 0 and less than or equal to 0.2, and M is selected from one or more of Ni, Co and Mn; the surface of the inner core is coated with the protective layer, the main body structure of the protective layer and the crystal structure of the inner core are integrally and continuously distributed, and cations, anions and alkali metal ions are doped in the protective layer; wherein the cations and the anions are distributed in a doping concentration gradient increasing mode from inside to outside in the thickness direction of the protective layer, and the alkali metal ions are doped on the surface layer of the protective layer. Cations comprise B < 3 + > and Zr < 4 + >, anions comprise F <->, and alkali metal ions comprise Na < + >. According to the invention, the problems of low initial coulombic efficiency, serious voltage attenuation and poor safety of the lithium-rich layered positive electrode material can be effectively improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a positive electrode material, a preparation method thereof, an electrochemical device and an electronic device. BACKGROUND

[0002] With the rapid development of new energy technology, the market has put forward more stringent requirements for the energy density of lithium ion batteries. Lithium-rich layered positive electrode materials have become one of the core research directions for improving the energy density of lithium ion batteries due to their theoretical specific capacity far exceeding that of traditional ternary positive electrode materials, and have shown broad application potential in high-energy-density power batteries and energy storage fields.

[0003] However, the current lithium-rich layered positive electrode materials still have a series of technical defects in the actual application process: in the first charge-discharge process, irreversible oxygen loss on the surface layer and severe side reactions at the interface between the electrolyte and the positive electrode material lead to a large number of lithium ions failing to realize reversible deintercalation during the first charge process, and the first coulombic efficiency is less than 80%; during the cycle process, the migration of transition metal ions (such as Ni 2+ , Mn 3+ ) causes Li / Ni mixing, continuous loss of lattice oxygen, and phase transition from layered phase to spinel phase / rock salt phase, which not only blocks the Li + channel but also reduces the diffusion efficiency, and the microcracks of the electrode particles exacerbate the electrolyte side reactions, resulting in poor cycle stability, serious voltage decay; poor safety, low lattice oxygen binding energy, easy to separate and oxidize the electrolyte at high temperature, and the dissolved transition metal ions (such as Mn 2+ ) in the cycle catalyze the decomposition of the electrolyte to generate flammable and explosive substances, which together trigger the risk of thermal runaway. These defects limit the industrial application of lithium-rich layered positive electrode materials in electric vehicles, large-scale energy storage and other fields.

[0004] Therefore, it is urgent to develop a lithium-rich layered oxide positive electrode material that can simultaneously solve the above multi-dimensional defects to promote the further development of high-energy-density lithium ion batteries. SUMMARY

[0005] The present application provides a positive electrode material, a preparation method thereof, an electrochemical device and an electronic device to solve the technical problems of low first coulombic efficiency, serious voltage decay and poor safety of lithium-rich layered oxides.

[0006] To achieve the above and other related purposes, the present application provides a positive electrode material, which comprises: an inner core and a protective layer, the chemical formula of the inner core is Li 1+x M 1-xO2, wherein 0 < x ≤ 0.2, M is selected from one or more of Ni, Co, and Mn; the protective layer is coated on the surface of the core, the main structure of the protective layer is integrally and continuously distributed with the crystal structure of the core, and the protective layer is doped with cations, anions, and alkali metal ions; wherein the cations and the anions are distributed in a gradient increasing from inside to outside along the thickness direction of the protective layer, and the alkali metal ions are doped in the surface layer of the protective layer; the cations include B 3+ and Zr 4+ , the anions include F - , and the alkali metal ions include Na + .

[0007] In an embodiment of the present application, the gradient coefficient of the B 3+ doping is 0.1-0.5 at% µm -1 , and the atomic percentage of the B 3+ in the outermost layer of the protective layer is 0.5-3 at%.

[0008] In an embodiment of the present application, the gradient coefficient of the Zr 4+ doping is 0.05-0.3 at% µm -1 , and the atomic percentage of the Zr 4+ in the outermost layer of the protective layer is 0.2-2 at%.

[0009] In an embodiment of the present application, the gradient coefficient of the F - doping is 0.2-1 at% µm -1 , and the atomic percentage of the F - in the outermost layer of the protective layer is 2-10 at%.

[0010] In an embodiment of the present application, the atomic percentage of the Na + in the outermost layer of the protective layer is 0.8-5 at%.

[0011] In an embodiment of the present application, the thickness of the protective layer is 1-5 µm, and / or the doping depth of the Na + in the protective layer is 0-50 nm from the outer surface of the protective layer.

[0012] In an embodiment of the present application, the particle size distribution D50 of the positive electrode material is 5-15 µm.

[0013] The present application also provides a preparation method of the positive electrode material, which comprises the following steps: The precursor co-precipitation: configuring a salt solution containing transition metal elements according to the chemical formula of the positive electrode material, adding a cation source and an anion source in the salt solution, adjusting the pH value and the temperature for co-precipitation to obtain a doped precursor; The gradient diffusion sintering: mixing the doped precursor with a lithium source and then performing gradient diffusion sintering to obtain an intermediate doped with cations and anions in a gradient; The surface modification: introducing a sodium source into the intermediate to form a sodium enrichment on the surface layer of the intermediate to obtain the positive electrode material.

[0014] In an embodiment of the present application, the gradient diffusion sintering comprises: The first sintering: sintering at a temperature of 650-750 ℃ for 2-6 hours to form an interface layer rich in doped ions; The second sintering: sintering at a temperature of 780-850 ℃ for 10-20 hours to drive the diffusion of the doped ions in the interface layer to form a smooth doped concentration gradient.

[0015] In an embodiment of the present application, the surface modification comprises: introducing a sodium source vapor into the intermediate at 450-600 ℃ under an inert atmosphere for 30 minutes.

[0016] In an embodiment of the present application, the pH value of the precursor co-precipitation is 9.5-11 and the temperature is 40-60 ℃.

[0017] In an embodiment of the present application, the transition metal elements are selected from one or more of Ni, Co and Mn, and the salt solution containing transition metal elements comprises any one of a sulfate solution, a nitrate solution, an acetate solution and a chloride solution.

[0018] In an embodiment of the present application, the lithium source comprises one or more of lithium carbonate and lithium hydroxide.

[0019] In an embodiment of the present application, the cation source comprises a boron source and a zirconium source, the boron source comprises one or more of boric acid, ammonium borate and sodium borate, and the zirconium source comprises one or more of zirconium sulfate and zirconium tetrachloride.

[0020] In an embodiment of the present application, the anion source comprises a fluorine source, and the fluorine source comprises one or more of ammonium fluoride and hydrogen fluoride.

[0021] In an embodiment of the present application, the sodium source comprises one or more of sodium fluoride and sodium carbonate.

[0022] The present application also provides an electrochemical device, which comprises the positive electrode material according to any one of the above or prepared by the above preparation method.

[0023] The application also provides an electronic device comprising the above-mentioned electrochemical device.

[0024] The application has the following beneficial effects: the positive electrode material provided by the application forms a B 3+ / Zr 4+ dication and F - synergistic gradient and Na + The surface enrichment of the multi-dimensional protective layer improves the material performance from the aspects of structural stability, interface protection and ion transmission.

[0025] B 3+ and Zr 4+ Firstly, the synergy is formed in the main body structure stability, B 3+ builds the oxygen stable foundation of the layered structure by inhibiting the release of lattice oxygen, and Zr 4+ inhibits lithium / nickel mixing by supporting the layered skeleton, and the skeleton support of Zr 4+ strengthens the stable oxygen environment of B 3+ , both of which slow down voltage decay and structural degradation, and can also effectively resist the lattice stress generated by repeated ion intercalation / deintercalation, reduce the generation and expansion of microcracks, and lay a structural foundation for the improvement of the cycle life of the material.

[0026] F - On the one hand, it is enriched on the surface of the material and reacts with transition metal ions to generate a dense and stable metal fluoride layer, which can passivate the positive electrode surface activity, inhibit the oxidative decomposition of the electrolyte, and reduce the dissolution of transition metal ions, thereby constructing a more stable positive electrode-electrolyte interface (CEI), and on the other hand, F - can induce the electronic cloud reconstruction of surrounding metal cations (M) in the lattice, thereby enhancing the M-O bond strength, which forms a bond energy-oxygen stability double guarantee with the effect of B 3+ inhibiting the release of lattice oxygen, and together inhibits the irreversible phase transition of the layered phase to the spinel phase / rock salt phase.

[0027] The Na + enriched on the outermost layer forms a synergistic protection system with the inner layer protection formed by F - , B 3+ , Zr 4+ , and Na + can efficiently capture HF acid generated by trace water in the electrolyte, avoid HF attack on the positive electrode material, and cause metal ion dissolution, and Na + changes the surface energy and charge distribution of the interface, which is helpful to form a thin and stable CEI layer, which can not only reduce the interface impedance and reduce the blockage of the Li + conduction channel by the interface byproducts, but also synchronously improve the rate performance and the initial coulombic efficiency of the material.

[0028] In summary, B 3+ / Zr 4+ / F - / Na + Through the multi-dimensional coordination of structure stability, interface protection, environmental adaptation and ion conduction, the synchronous optimization of material cycle life, safety and initial coulomb efficiency is realized. BRIEF DESCRIPTION OF DRAWINGS

[0029] The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate embodiments consistent with the present application and, together with the description, further serve to explain the principles of the application. It is to be understood that the drawings are designed solely for purposes of illustration to be used in conjunction with the description in

[0030] In the drawings: Figure 1 The preparation method flowchart of the positive electrode material provided by an embodiment of the present application is shown. DETAILED DESCRIPTION

[0031] The above and other advantages and features of the present application will become apparent from the following description of the embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the application. This description is given for the sake of example and the details are not intended to limit the present application. Various embodiments of the present application will be described with reference to the attached drawings to provide an overall understanding of the present application. The present application can be practiced with or without these specific details. In other instances, well known methods have not been described in detail in order to not unnecessarily obscure the present application.

[0032] It is to be understood that the drawings are to be used only for illustrating the specific embodiments and that the actual implementation of the present application can be different from the specific examples shown in the drawings.

[0033] 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 in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0034] As used herein, the terms or phrases have the following meanings: As used herein, the terms or phrases have the following meanings:

[0035] In the present text, "preferably", "preferred" and the like are used as terms of description and do not pose a limitation on the scope of protection of the present application.

[0036] In the present text, in relation to a range of values, unless specifically stated otherwise, the distribution of values obtainable within the range is considered to be continuous, and the endpoints of the ranges (that is, the minimum and maximum values) are included in the range, as well as every value between the minimum and maximum values. When providing a plurality of ranges of values to describe a feature or a characteristic, these ranges of values can be combined.

[0037] In the present text, atomic percentage (at%): is calculated based on the total number of atoms in the system, the number of atoms of a certain element accounts for the percentage of the total number of atoms in the system, which is a unit to measure the relative concentration of elements.

[0038] Li-rich layered cathode material (Li 1+x M 1-x O2, M is a transition metal) high energy density is the core of "rich lithium structure" driven by "lithium ion reserves-charge compensation-voltage platform" triple synergistic effect: first, the "rich lithium" feature reserves more lithium ions that can participate in deintercalation in unit mass of material, which significantly improves the initial lithium ion supply compared with traditional layered cathode (such as LiCoO2, NCM), and lays the foundation for high capacity; second, it breaks through the limitation of traditional materials relying only on single redox of transition metal, and amplifies specific capacity through "transition metal redox and lattice oxygen redox coordination" mechanism—transition metal ions (such as Ni 2+ →Ni 4+ , Co 3+ →Co 4+ ) contribute to the basic capacity through valence change, while O 2- in the lattice can be reversibly oxidized to low-valence oxygen species (such as O2 2- ) at high voltage, releasing additional capacity, making the specific capacity of the material far exceed that of traditional materials; third, the activation process of active substances (transition metal, lattice oxygen) at high voltage further raises the voltage platform, and energy density is the product of specific capacity and voltage platform, ultimately realizing high energy density breakthrough through the "multiple lithium ion supply → double redox energy supply → high voltage synergistic effect chain".

[0039] However, the material still has some defects in practical application: the first coulombic efficiency (FCE) is low, and during the first charge and discharge process, the surface layer of the material is prone to irreversible oxygen loss reaction, and the interface between the electrolyte and the positive electrode material will produce severe side reactions (such as electrolyte decomposition, transition metal ion dissolution and reaction with electrolyte components to generate inert products), resulting in a large number of lithium ions that cannot be reversibly deintercalated during discharge during the first charge process, and the first coulombic efficiency is usually less than 80%, which significantly reduces the energy output utilization rate of the battery. Poor cycle stability and severe voltage decay, during long-term charge and discharge cycling, the layered crystal structure is prone to irreversible phase transition - with repeated intercalation and deintercalation of lithium ions, transition metal ions (such as Ni 2 + , Mn 4+ ) in the material will migrate and occupy lithium ion vacancies, causing the layered structure to transform into spinel or rock salt phase, accompanied by continuous oxygen loss, further damaging the integrity of the crystal structure; the above structural degradation directly causes the continuous decay of the battery voltage, and the capacity retention rate is usually less than 80% after 100 cycles, which cannot meet the use requirements of power batteries for long cycle life. The lack of thermal safety limits its application in power batteries, the lattice oxygen binding energy is low, which is easy to separate at high temperature and react with the electrolyte through oxidation and reduction, releasing flammable and explosive substances, causing battery thermal runaway, which brings serious safety hazards to the use of the battery, especially difficult to adapt to the safety requirements of power batteries in high temperature environment. These defects limit the application of lithium-rich layered positive electrode materials.

[0040] To improve the above problems, the present application provides a positive electrode material and a preparation method thereof, an electrochemical device and an electronic equipment, by constructing a B 3+ / Zr 4+ double cation and F - ion gradient and Na + surface enrichment multi-dimensional protective layer on the surface of the lithium-rich layered positive electrode material in situ, from the three dimensions of chemical bond stability, physical structure support and interface passivation, the electrochemical and thermal stability problems of lithium-rich materials are systematically solved.

[0041] The present application provides a positive electrode material, which comprises an inner core and a protective layer, wherein the inner core is a lithium-rich layered oxide with a chemical formula of Li 1+x M 1-xO2, wherein 0 < x < 0.2, and M is selected from one or more of Ni, Co, and Mn. Exemplarily, x can be 0.05, 0.1, 0.15, or 0.2, etc.; M can be selected from any one of the transition metal elements listed above, such as Ni, or Mn, or Co; or M can be a combination of any two or more of the transition metal elements listed above, such as M being Ni and Mn, or Ni and Co, or Mn and Co, or Ni, Co, and Mn. The protective layer is coated on the surface of the core, and the main structure of the protective layer is in an integrated and continuous distribution with the crystal structure of the core, i.e., there is no obvious interface fracture or crystal structure mutation between the protective layer and the core, and the two are seamlessly connected at the atomic scale. The protective layer is doped with cations B 3+ and Zr 4+ , anions F - , and alkali metal Na + . That is, the core is undoped Li 1+x M 1-x O2, and the protective layer is doped Li 1+x M 1-x O2, and the doping ions include B 3+ , Zr 4+ , F - , and Na + .

[0042] Specifically, B 3+ and Zr 4+ mainly exist in the form of substitutional doping in the layered lattice of the protective layer. Since their ionic radii are similar to that of lithium ions (Li + ) or transition metal ions (such as Ni 2+ , Mn 4+ ), they will replace part of the positions of Li + or transition metal (M 3+ / M 4+ ). Among them, B 3+ has high electronegativity and strong covalent bond characteristics, and it tends to form strong and tough B-O covalent bonds, and the bond energy is much higher than that of transition metal-oxygen bonds (such as Ni-O, Mn-O). A key failure mode of lithium-rich layered oxide materials in the cycle process is the loss of lattice oxygen and the irreversible phase transition from the layered structure to the spinel / rock salt phase. The introduction of B 3+ effectively inhibits the release of lattice oxygen by enhancing the covalence of M-O bonds, thereby stabilizing the main layered structure, slowing down the voltage decay and structural degradation. However, if the content of B 3+ inside the crystal structure is too high, it will affect the transport of Li + , and therefore, in the protective layer of the present application, the content of B 3+The doping concentration gradient increases from inside to outside (from the inner core to the outer core) along the thickness direction of the protective layer, so that the interface stability can be improved without affecting the Li + Zr 4+ is an electrochemically inert ion with a large ionic radius. When Zr 4+ occupies part of the lithium site or the transition metal site, its stable chemical properties and large volume can effectively support the skeleton of the layered structure, inhibit lithium / nickel mixing, and especially maintain the stability of the structure in the deep delithiation state. This physical barrier effectively slows down the lattice stress and micro-cracks caused by the repeated insertion / extraction of lithium ions, thereby improving the cycle life of the material. In the protective layer of the present application, Zr 4+ has a doping concentration gradient that increases from inside to outside along the thickness direction of the protective layer, Zr 4+ The inside is less, which can reduce the influence on the insertion / extraction capacity of lithium ions, and the outside gradually increases, which can improve the stability of the structure. The skeleton support of Zr 4+ can strengthen the B 3+ stable oxygen environment, both of which slow down voltage decay and structural degradation, and also effectively resist the lattice stress generated by repeated insertion / extraction of ions, reducing the generation and expansion of micro-cracks, laying a structural foundation for improving the cycle life of the material.

[0043] F - mainly replaces part of O 2- in the lattice. F - has a higher electronegativity than O 2- , and its reaction with transition metal ions can form stable metal fluorides (such as NiF2, MnF2). This dense fluoride layer can effectively passivate the positive electrode surface, inhibit the oxidative decomposition of the electrolyte and the dissolution of transition metal ions, thereby constructing a more stable positive electrode-electrolyte interface (CEI). In addition, F - will induce the electron cloud of surrounding metal cations (M) in the lattice, enhancing the strength of the M-O bond, which forms a bond energy-oxygen stability double guarantee with B 3+ inhibiting the release of lattice oxygen, together inhibiting the irreversible phase transition of the layered phase to the spinel phase / rock salt phase and reducing side reactions. In the protective layer of the present application, F - has a doping concentration gradient that increases from inside to outside along the thickness direction of the protective layer.

[0044] The gradient distribution of B 3+ , Zr 4+ and F - solves the fundamental contradiction between surface stability and bulk electrochemical activity. The high concentration area is concentrated at the interface, ensuring excellent interface stability and thermal stability; and the low concentration area smoothly transitions to the inner core, ensuring Li +on the other hand, also reduces the interfacial impedance and lattice stress: the core-shell interface with abrupt composition change will generate huge physical stress due to lattice parameter mismatch, and form a high energy barrier for Li + shuttling. The smooth concentration gradient structure means that the lattice parameter and chemical potential are also gradually changed, forming a buffer transition zone. This greatly relieves the mechanical stress during cycling, avoids coating peeling and particle cracking, and provides a smooth, low impedance migration path for Li + , thereby significantly improving the cycle life and rate performance.

[0045] Na + is a larger alkali metal ion, which mainly exists in the form of surface enrichment in the surface layer of the protective layer, and its form may be sodium oxide or fluoride. Na + enrichment in the surface layer of the protective layer can effectively capture HF generated by trace water in the electrolyte, reduce the dissolution of metal ions in the positive electrode material, and improve the long-term stability and safety of the material. In addition, the surface enrichment of Na + can also change the surface energy and charge distribution of the interface, which helps to form a thin and stable CEI layer that is more conducive to Li + conduction, thereby improving the rate performance and initial coulombic efficiency.

[0046] The gradient coefficient of the doping ion refers to the average rate of change of the concentration of the doping ion with depth in the entire thickness of the protective layer. B 3+ , Zr 4+ and F - ion doping gradient coefficient has a great influence on the performance of the positive electrode material. If the gradient coefficient is too small, it means that the gradient is too flat, and the concentration difference of the doping elements inside and outside the protective layer is small: if the concentration of the doping elements inside and outside the protective layer is small, it will greatly reduce the protective function and structural support of the outermost layer, and it is difficult to form a high concentration of active substances in the surface area that needs the most protection, so as to effectively inhibit the oxidation of the electrolyte and the dissolution of transition metals. If the concentration of the doping elements inside and outside the protective layer is large, the bulk electrochemical activity of the inner layer is not guaranteed, and the high concentration of doping elements in the inner layer will block the fast transport channel of Li + , affecting the high capacity of the material body.

[0047] A too large gradient coefficient means that the chemical composition and lattice parameter of the material change dramatically within a very short distance, which forms a sharp interface between the core and the protective layer, resulting in a huge lattice mismatch stress. This stress can become a structural weakness, leading to particle cracking, coating peeling during electrochemical cycling, and accelerating material failure. At the same time, the steep chemical potential gradient forms a high-energy barrier at the interface, hindering the transport of lithium ions, which significantly increases the interfacial impedance, reduces the first coulombic efficiency and rate performance of the material.

[0048] In an embodiment, B 3+ has a gradient coefficient of 0.1-0.5 at% per µm -1 , i.e., the doping concentration of B 3+ decreases by 0.1-0.5 at% per 1 µm towards the core along the thickness direction of the protective layer. Exemplarily, the gradient coefficient of B 3+ doping can be 0.1 at% per µm -1 , 0.3 at% per µm -1 , or 0.5 at% per µm -1 , etc.

[0049] Zr 4+ has a gradient coefficient of 0.05-0.3 at% per µm -1 , i.e., the doping concentration of Zr 4+ decreases by 0.05-0.3 at% per 1 µm towards the core along the thickness direction of the protective layer. Exemplarily, the gradient coefficient of Zr 4+ doping can be 0.05 at% per µm -1 , 0.1 at% per µm -1 , 0.2 at% per µm -1 , or 0.3 at% per µm -1 , etc.

[0050] F - has a gradient coefficient of 0.2-1 at% per µm -1 , i.e., the doping concentration of F - decreases by 0.2-1 at% per 1 µm towards the core along the thickness direction of the protective layer. Exemplarily, the gradient coefficient of F - doping can be 0.2 at% per µm -1 , 0.5 at% per µm -1 , 0.8 at% per µm -1 , or 1 at% per µm -1 , etc.

[0051] The above B 3+ , Zr 4+ and F -The gradient coefficient of the dopant can be obtained by focused ion beam-time of flight secondary ion mass spectrometry (FIB-TOF-SIMS) depth profiling, which is performed as follows: Sample preparation: A flat analysis area (e.g., 5 pm x 5 pm) is cut on a secondary spherical particle (e.g., a particle with a diameter of 10 pm) using FIB, ensuring that the analysis starting point is located at the outermost surface of the particle.

[0052] Depth profiling: TOF-SIMS is used for depth profiling. A sputtering ion beam (e.g., Cs + ) vertically bombards the analysis area at a constant rate, layer by layer, to strip the material. At the same time, an analysis ion beam (e.g., Bi + ) excites secondary ions, and the TOF analyzer detects the mass-to-charge ratio of the ions according to the difference in their flight times, thereby obtaining the elemental composition information of each depth point.

[0053] Data calibration: The signal intensity of the secondary ions is converted to atomic percentage (at%) by a standard sample, and the sputtering time is converted to depth (pm). In this way, the original data point graph of concentration (at%) vs. depth (pm) is obtained.

[0054] Calculation of the gradient coefficient: On the concentration vs. depth curve, the region representing the gradient protective layer is identified (e.g., from a depth of 0.1 pm to 4.9 pm, avoiding the Na + enriched layer on the surface and the junction between the innermost and the core).

[0055] Linear regression fitting is performed on the data points in this region, and the slope of the fitting straight line, i.e., the gradient coefficient of the dopant ion, is at% pm -1 .

[0056] In addition to the doping gradient coefficient of the dopant ion having a significant impact on the performance improvement of the positive electrode material, the doping concentration also has an impact on the improvement of the positive electrode material. Since B 3+ , Zr 4+ , and F - are distributed in a gradient decreasing manner from the outside to the inside along the thickness direction of the protective layer, the doping concentration of the outermost layer and the doping gradient coefficient determine the total amount of doping of the dopant ion. If the doping concentration of the outermost layer is too low, the total amount of doping is insufficient to form an effective protective layer, thereby failing to achieve performance improvement; if the doping concentration of the outermost layer is too high, it will occupy too many lithium ion or transition metal sites, blocking the transmission channel of lithium ions, thereby significantly reducing the capacity and rate performance of the material.

[0057] In an embodiment, B 3+The atomic percentage of the outermost layer of the protective layer is 0.5-3 at%, wherein the outermost layer refers to a region extending 0-0.1 μm from the outside to the inside along the thickness direction of the protective layer. Exemplarily, the atomic percentage of B 3+ The atomic percentage of the outermost layer of the protective layer can be 0.5 at%, 1 at%, 2 at% or 3 at%, etc.

[0058] Zr 4+ The atomic percentage of the outermost layer of the protective layer is 0.2-2 at%, and the atomic percentage of Zr 4+ The atomic percentage of the outermost layer of the protective layer can be 0.2 at%, 0.8 at%, 1.5 at% or 2 at%, etc.

[0059] F - The atomic percentage of the outermost layer of the protective layer is 2-10 at%, and the atomic percentage of F - The atomic percentage of the outermost layer of the protective layer can be 2 at%, 5 at%, 8 at% or 10 at%, etc.

[0060] In an embodiment, the atomic percentage of Na + The atomic percentage of the outermost layer of the protective layer is not less than 0.8 at%, and the atomic percentage of Na + The atomic percentage of the outermost layer of the protective layer is 0.8-5 at%, and can be 0.8 at%, 1 at%, 3 at%, 5 at%, etc. The atomic percentage of Na + Doping in the outermost layer can improve the transport of Li + , but if the doping concentration is too high, a too thick and non-conductive sodium salt layer (such as NaF) will be formed, which will hinder the shuttling of lithium ions, increase the total impedance of the interface, and reduce the electrochemical performance of the material. + Doping within the above range can ensure sufficient HF removal and interface optimization capability, and avoid the problem of increased impedance caused by too high concentration.

[0061] In an embodiment, the thickness of the protective layer is 1-5 μm, i.e. the thickness of the lithium-rich layered oxide with doped ions coated outside the core is 1-5 μm, which can be 1 μm, 3 μm or 5 μm, etc. Further, the atomic percentage of Na + The doping depth in the protective layer is 0-50 nm from the outer surface of the protective layer to the inside.

[0062] In an embodiment, the particle size distribution D50 of the positive electrode material is 5-15 μm, which can be 5 μm, 10 μm or 15 μm, etc., to meet the use requirements of the particle size distribution composite positive electrode material. In the present application, D50 refers to the particle size corresponding to the cumulative volume particle size distribution percentage of 50% of a sample, which can be measured by a laser particle size distribution instrument.

[0063] See Figure 1 The application further provides a preparation method of the positive electrode material, which comprises the following steps: S1, configuring a salt solution containing transition metal elements according to the chemical formula of the positive electrode material, and adding a cation source and an anion source in the salt solution, adjusting the pH value and the temperature for co-precipitation to obtain a doping precursor; S2, mixing the doping precursor with a lithium source and then performing gradient diffusion sintering to obtain an intermediate doped with cations and anions in gradient; S3, introducing a sodium source into the intermediate to form sodium enrichment on the surface layer of the intermediate to obtain the positive electrode material.

[0064] Specifically, the chemical formula of the positive electrode material in step S1 is Li 1+x M 1-x O2, wherein 0 < x ≤ 0.2, and M is selected from one or more of Ni, Co and Mn. Exemplarily, x can be 0.05, 0.1, 0.15 or 0.2, etc., and M can be selected from any one or more of the transition metal elements listed above. Exemplarily, the chemical formula of the positive electrode material is Li 1.2 Ni 0.6 Mn 0.2 O2, Li 1.2 Ni 0.8 O2, Li 1.2 Mn 0.8 O2 or Li 1.2 Ni 0.6 Co 0.1 Mn 0.1 O2, etc.

[0065] Step S1, i.e., the precursor co-precipitation step, according to the chemical formula and stoichiometric ratio of the positive electrode material to be prepared, a compound containing a corresponding transition metal element and an amount are selected to configure a salt solution containing the transition metal element. The transition metal element is selected from one or more of Ni, Co and Mn according to the chemical formula. The salt solution can be any one of a sulfate solution, a nitrate solution, an acetate solution and a chloride solution. That is, the sulfate containing the transition metal element, the nitrate containing the transition metal element, the acetate containing the transition metal element or the chloride containing the transition metal element is dissolved in deionized water to obtain a salt solution containing the transition metal element. Further, the sulfate containing the transition metal element is used, which has low cost, good solubility and few impurities.

[0066] Next, a doped ion source, such as a boron source, a zirconium source, or a fluorine source, is added to the salt solution containing a transition metal, and the mixture is stirred until the doped ion source is completely dissolved to obtain a mixture. The boron source includes one or more of boric acid (H3BO3), ammonium borate ((NH4)3O3), sodium borate (Na3BO3), and other soluble borates. For example, the boron source can be boric acid, or amine borate, or a combination of boric acid and sodium borate. The zirconium source includes one or more of zirconium sulfate (Zr(SO4)2) and zirconium tetrachloride (ZrCl4), for example, zirconium sulfate, or zirconium tetrachloride, or a combination of zirconium sulfate and zirconium tetrachloride, etc. The fluorine source includes one or more of ammonium fluoride (NH4F) and hydrogen fluoride (HF), for example, ammonium fluoride, or hydrogen fluoride, or a combination of amine fluoride and hydrogen fluoride, etc. It should be noted that the amount of doped ion source added is determined based on the doping concentration of the doping element in the protective layer of the cathode material.

[0067] Then, the pH and temperature of the mixture are adjusted to allow the transition metal and dopant ions to co-precipitate and obtain the doped precursor. The pH and temperature can be selected based on the type of salt solution. For example, when using a sulfate solution, the pH of the mixture is 9.5–11, such as 9.5, 10, or 11, etc. If the pH is too low, Ni / Co / Mn precipitation will be incomplete; if the pH is too high, the precipitation rate will be too fast, leading to uneven particle morphology or agglomeration. The temperature is controlled at 40–60°C, such as 40°C, 50°C, or 60°C, etc. A coprecipitation temperature of 40–60°C provides a moderate reaction rate, which is beneficial for forming spherical precursors with good crystallinity and uniform particle size.

[0068] Step S2, the gradient diffusion step, involves thoroughly mixing the doped precursor obtained in step S1 with a lithium source. The lithium source includes, but is not limited to, lithium carbonate (Li₂CO₃) and lithium hydroxide (LiOH). The amount of lithium source added is adjusted according to the stoichiometry of the cathode material's chemical formula. The mixing method can be conventional in the art, such as using a mixer. After the mixture is homogeneous, it undergoes two sintering processes. The first sintering is performed at 650-750°C for 2-6 hours to form a doped ion-rich interface layer in the mixture. This sintering, conducted at a relatively low temperature and for a short time, allows for the initial reaction between the lithium salt and the doped precursor, with limited diffusion of B, Zr, and F ions only near the particle surface, forming a thin and enriched interface layer. For example, the sintering temperature can be 650°C, 700°C, or 750°C, and the sintering time can be 2 hours, 4 hours, or 6 hours, etc. Too low a temperature or too short a time can lead to insufficient diffusion; too high a temperature or too long a time may result in excessive element diffusion, affecting subsequent gradient formation.

[0069] The second sintering is performed at a temperature of 780-850℃ for 10-20 hours to drive the diffusion of the doped ions in the interface layer to form a smooth doped concentration gradient. The second sintering is performed at a higher temperature and for a longer time to drive the diffusion of the B, Zr and F elements to the interior by using the difference in ion diffusion kinetics at high temperatures to form a smooth concentration gradient. Specifically, after the first sintering, the temperature is raised at a rate of 2℃ / min to the temperature of the second sintering. A too fast temperature raising rate will cause a temperature difference between the interior and the exterior of the material, which may cause uneven stress or non-uniform diffusion, and a too slow temperature raising rate will prolong the preparation period. Exemplarily, the sintering temperature of the second sintering can be 780℃, 810℃ or 850℃, and the sintering time can be 10 hours, 15 hours or 20 hours. The sintering temperature can control the diffusion rate, thereby affecting the diffusion degree and the gradient smoothness.

[0070] The step S3, i.e. the surface modification step, after the second sintering, the temperature is cooled to 450-600℃ in an inert atmosphere such as nitrogen atmosphere, and sodium source vapor is introduced, and the gas is maintained for 30 minutes to form a sodium enrichment on the surface layer. In this step, the sodium source includes sodium fluoride (NaF), sodium carbonate (Na2CO3) and the like, and the temperature can be 450℃, 500℃ or 600℃ and the like. At this temperature, part of the sodium source will sublimate or decompose to generate sodium-containing vapor, while the stability of the main crystal structure is ensured. The amount of sodium source can be determined according to the target concentration of Na on the surface of the final product. +

[0071] After the step S3, the secondary spherical particles with a particle size D50 of 5-15µm are selected by sieving as the positive electrode material.

[0072] The application also provides an electrochemical device comprising the positive electrode material described above or prepared by the preparation method described above.

[0073] In an embodiment, the electrochemical device is a lithium ion battery, which can be a liquid lithium ion battery (the electrolyte is a non-aqueous electrolyte) or a solid-state lithium ion battery (the electrolyte is a solid-state electrolyte), which is not limited herein.

[0074] Hereinafter, the structure of the liquid lithium ion battery will be described in detail: the lithium ion battery comprises a positive electrode sheet, a negative electrode sheet, a non-aqueous electrolyte and a separator.

[0075] ​The positive electrode tab includes a positive electrode current collector and a positive electrode active material layer disposed on at least one side surface of the positive electrode current collector. The positive electrode current collector is, for example, an aluminum foil or a carbon-coated aluminum foil, and has two surfaces disposed opposite to each other along the thickness direction thereof. The positive electrode active material layer can be disposed on one side surface of the positive electrode current collector or on both side surfaces thereof. The positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. The positive electrode active material is the positive electrode material described above. The positive electrode binder includes any one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, polyethylene oxide, polyamide, polyacrylonitrile, polyacrylate, polyvinyl ether, polymethyl methacrylate, ethylene-propylene-diene terpolymer, polyhexafluoropropylene, and the like. The positive electrode binder can be, for example, polyvinylidene fluoride or polytetrafluoroethylene. The positive electrode conductive agent includes, but is not limited to, one or more of conductive carbon black (SP), acetylene black, carbon nanotube, carbon fiber, and graphite. The positive electrode conductive agent can be, for example, conductive carbon black, a combination of carbon fiber and conductive carbon black, a combination of carbon nanotube and graphene, or the like. The proportions of the positive electrode active material, the conductive agent, and the binder can be set according to conventional settings in the art.

[0076] The positive electrode tab can be prepared as follows. The positive electrode active material, the positive electrode conductive agent, and the positive electrode binder are mixed in a solvent such as N-methylpyrrolidone (NMP) in a certain ratio to form a positive electrode slurry. The positive electrode slurry is then coated on the positive electrode current collector, and the coated positive electrode current collector is subjected to drying, rolling, and cutting to obtain the positive electrode tab.

[0077] The negative electrode tab includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector. The negative electrode current collector is, for example, a copper foil or a carbon-coated copper foil, and has two surfaces disposed opposite to each other along the thickness direction thereof. The negative electrode active material layer can be disposed on one side surface of the negative electrode current collector or on both side surfaces thereof. The negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder, and a thickening agent. The specific types of the negative electrode active material, the negative electrode conductive agent, the negative electrode binder, and the thickening agent are not specifically limited, and materials known in the art that can be used in lithium ion batteries can be used. The skilled person can select the materials according to actual needs.

[0078] Exemplarily, the negative active material includes, but is not limited to, artificial graphite, natural graphite, silicon-carbon material, etc. The negative conductive agent is selected from one or more of acetylene black, conductive carbon black (Super P), carbon fiber (VGCF), carbon nanotube (CNT), ketjen black, graphite flake, etc., for example, can be acetylene black, or conductive carbon black, or a combination of carbon fiber and carbon nanotube, etc. The negative binder is selected from any one or a combination of several in any proportion of vinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, butadiene-styrene rubber, etc.; for example, can be vinylidene fluoride, or polyvinylidene fluoride-hexafluoropropylene, or butadiene-styrene rubber, etc.; the thickening agent is selected from sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0079] The preparation process of the negative electrode sheet is as follows: the negative active material, the negative conductive agent, the negative binder and the thickening agent are mixed in a solvent such as deionized water in a certain ratio to form a negative slurry, and then the negative slurry is coated on the negative current collector, and after drying, rolling, sheet cutting and other processes, the negative electrode sheet is obtained.

[0080] The separator is arranged between the positive electrode sheet and the negative electrode sheet to separate the positive electrode sheet and the negative electrode sheet, prevent short circuit in the battery, and enable lithium ions to move between the positive electrode and the negative electrode to realize the charging and discharging process of the battery. The separator can be selected from porous materials such as polyethylene film (PE), polypropylene film (PP), glass fiber film or composite film.

[0081] The non-aqueous electrolyte plays a role in conducting lithium ions during the charging and discharging process of the battery. The non-aqueous electrolyte includes an organic solvent and a lithium salt, and the lithium salt can be selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonylimide (LiFSI), lithium bis-trifluoromethanesulfonylimide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluoro oxalate borate (LiDFOB), lithium difluoro oxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluoro dioxalate phosphate (LiDFOP) and lithium tetrafluoro oxalate phosphate (LiTFOP). Further, the lithium salt is selected from lithium hexafluorophosphate or a combination of lithium hexafluorophosphate and other lithium salts, for example, a combination of lithium hexafluorophosphate and lithium bisfluorosulfonylimide, which has better comprehensive performance. The organic solvent can be selected from one or more of fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC) and ethyl propyl carbonate (EPC).

[0082] The non-aqueous electrolyte can further include functional additives, such as fluoroethylene carbonate (FEC), propylene-1,3-sultone (PST), tetraethenylsilane (TVSI), vinylene carbonate (VC), vinyl sulfate (DTD), etc. The additives can be added according to the actual production needs.

[0083] Battery assembly: the prepared positive electrode sheet, separator, and negative electrode sheet are sequentially placed, with the separator between the positive and negative electrode sheets to play a role of isolation, and a bare cell is obtained through winding or stacking. The bare cell is loaded into a battery shell, and the water content is reduced to below 450 ppm after sufficient baking. After processes such as liquid injection, formation, sealing, and inspection, a lithium ion battery is obtained.

[0084] In other embodiments, the lithium ion battery is a solid-state lithium ion battery, and the electrolyte of the solid-state lithium ion battery is solid. Common solid-state electrolytes include oxide solid-state electrolytes, halide solid-state electrolytes, sulfide solid-state electrolytes, etc., which will not be described here. The skilled person in the art can select according to the actual production needs.

[0085] It should be noted that the structures not described in detail in the above lithium ion battery can be set according to the prior art, and will not be described here.

[0086] The present application also provides an electronic device comprising the above lithium ion battery. The lithium ion battery can be used in the form of a single battery, a battery module, or a battery pack for an electronic device to provide power for it.

[0087] In some embodiments, the electronic device includes but is not limited to a mobile phone, a tablet, a notebook computer, an electric toy, an electric vehicle, a new energy vehicle, a ship, a spacecraft, etc. The electric toy can include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric plane toys, etc. The spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc. The new energy vehicle can be a pure electric vehicle, a hybrid electric vehicle, or an extended-range vehicle, etc.

[0088] The technical solutions of the present application will be described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art, and the instruments used in the examples are commercially available.

[0089] Example 1 The present embodiment provides a positive electrode material, which comprises an inner core and a protective layer, wherein the chemical formula of the inner core is Li 1.2 Ni 0.6 Mn 0.2O2, the main structure of the protective layer is Li 1.2 Ni 0.6 Mn 0.2 O2, and B is doped in the protective layer 3+ , Zr 4+ , F - and Na + , wherein B 3+ , Zr 4+ and F - are distributed in a gradient increasing direction along the thickness direction of the protective layer from the direction close to the core to the direction away from the core, and Na + is doped in the surface layer of the protective layer. The doping concentration and concentration gradient of the outermost layer of each element are shown in Table 1.

[0090] The preparation method of the positive electrode material of the embodiment includes the following steps: (1) Precursor co-precipitation: In a sulfate solution containing Ni and Mn (molar ratio 0.6:0.2), 0.2 mol / L H3BO3, 0.05 mol / L Zr(SO4)2 and 0.15 mol / L NH4F are added. After all the doping sources are dissolved, the pH of the mixed solution is adjusted to 10, and co-precipitation is carried out at 50°C to obtain a preliminary doped precursor doped with B / Zr / F. -1 -1 -1 NH4F, after all the doping sources are dissolved, the pH of the mixed solution is adjusted to 10, and co-precipitation is carried out at 50°C to obtain a preliminary doped precursor doped with B / Zr / F. (2) Gradient diffusion sintering: The doped precursor is mixed with Li2CO3 (molar ratio of Li to transition metals (Ni and Mn) =1.5:1) and sintered twice: The first sintering is sintered at 700°C for 4h. This step makes the lithium salt react preliminarily and forms a B / Zr / F-rich interface layer.

[0091] The second sintering is sintered at 820°C for 20h. This step utilizes the difference in ion diffusion kinetics at high temperature to drive the migration of B, Zr and F elements to the core, forming a smooth concentration gradient. The heating rate is controlled at 2°C / min -1 .

[0092] (3) Surface modification: When cooled to 500°C, the steam generated by passing heated NaF powder into the N2 atmosphere for 30min completes the surface Na + enrichment, wherein 0.5g of NaF is added per 100g of sintered powder.

[0093] (4) Screen the secondary spherical particles with D50=10 µm.

[0094]

Embodiments 2-4

[0095] [Examples 5-8] The difference between each embodiment and embodiment 2 is that the doping concentration of the outermost layer B 3+ of the protective layer changes, and the rest remains unchanged; the specific changes are shown in Table 1 and Table 2. 3+ The gradient coefficient changes accordingly, and the rest remains unchanged; the specific changes are shown in Table 1 and Table 2.

[0096] [Examples 9-12] The difference between each embodiment and embodiment 2 is that the doping concentration of the outermost layer Zr 4+ of the protective layer changes, and the rest remains unchanged; the specific changes are shown in Table 1 and Table 2. 4+ The gradient coefficient changes accordingly, and the rest remains unchanged; the specific changes are shown in Table 1 and Table 2.

[0097] [Examples 13-16] The difference between each embodiment and embodiment 2 is that the doping concentration of the outermost layer F - of the protective layer changes, and the rest remains unchanged; the specific changes are shown in Table 1 and Table 2. - The gradient coefficient changes accordingly, and the rest remains unchanged; the specific changes are shown in Table 1 and Table 2.

[0098] [Examples 17-20] The difference between each embodiment and embodiment 2 is that the doping concentration of Na + in the protective layer changes, and the rest remains unchanged; the specific changes are shown in Table 1 and Table 2.

[0099] Example 21 The difference between this embodiment and embodiment 2 is that the core material is Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O2, and the parameters for the second sintering are: sintering at 810°C for 16h.

[0100] Example 22 The difference between this embodiment and embodiment 2 is that the core material is Li 1.2 Mn 0.6 Ni 0.2 O2.

[0101] Comparative Example 1 The positive electrode material of this comparative example is Li 1.2 Ni 0.6 Mn 0.2 O2, without protective layer coating treatment.

[0102] Comparative Example 2 The difference between this comparative example and Example 2 is that each doping element is uniformly doped, and the amount of each doping source added is unchanged during preparation.

[0103] Comparative Example 3 The difference between this comparative example and Example 2 is that only B 3+ is doped in the protective layer.

[0104] Comparative Example 4 The difference between this comparative example and Example 2 is that only B 3+ and Zr 4+ are doped in the protective layer.

[0105] Comparative Example 5 The difference between this comparative example and Example 2 is that only F - is doped in the protective layer.

[0106] Comparative Example 6 The difference between this comparative example and Example 2 is that only Na + is doped in the protective layer.

[0107] Comparative Example 7 The difference between this comparative example and Example 2 is that B 3+ is replaced by Al 3+ , Zr 4+ is replaced by Ti 4+ , F - is replaced by Cl - , and Na + is replaced by K + , and the amount of the corresponding element added during preparation is the same.

[0108] Comparative Example 8 The difference between this comparative example and Example 21 is that the positive electrode material is Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O2, and no protective layer is coated.

[0109] Comparative Example 9 The difference between this comparative example and Example 22 is that the positive electrode material is Li 1.2 Mn 0.6 Ni 0.2 O2, and no protective layer is coated.

[0110] Table 1: Various parameters of Examples 1-22 and Comparative Examples 1-9

[0111] Table 2: Amount of doping source added in each example

[0112] To verify the performance of the positive electrode material of the present application, the positive electrode materials of each example and comparative example were assembled into a battery, and the specific composition of the battery was as follows: (1) Preparation of positive electrode sheet The above positive electrode material, binder polyvinylidene fluoride, and conductive agent carbon black (Super P) were mixed in a mass ratio of 97:1:2, N-methyl pyrrolidone (NMP) was added, and the mixture was stirred uniformly under the action of a vacuum stirrer to obtain a positive electrode slurry; the positive electrode slurry was uniformly coated on an aluminum foil, dried at room temperature, then transferred to an oven for drying, and then subjected to cold pressing, slitting and other processes to obtain a positive electrode sheet.

[0113] (2) Negative electrode sheet: metal lithium sheet.

[0114] (3) Separator: PP / PE / PP three-layer composite porous separator, manufacturer and model Celgard® 2325.

[0115] (4) Preparation of electrolyte In an argon atmosphere glove box with a water content of <10 ppm, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1 to obtain an organic solvent, then the fully dried lithium salt LiPF6 and the additive fluoroethylene carbonate (FEC) were dissolved in the organic solvent, and the mixture was stirred uniformly to obtain an electrolyte; wherein the lithium salt concentration was 1 mol / L, and the amount of FEC added was 0.5% of the total mass of the electrolyte.

[0116] (5) Battery assembly A metal lithium sheet was used as the counter electrode, and the above positive electrode sheet was used as the positive electrode to assemble a button-type half-cell (CR 2025 type), and the nominal capacity of the button-type half-cell was 6 mAh.

[0117] Then, the performance of each button-type half-cell was tested, and the test results are shown in Table 3, and the test method is as follows: (1) First coulombic efficiency test The assembled button cell was placed in a constant temperature oven at 25°C for 2-4 hours, and then a battery test system (Blue Electricity) was used for the first charge-discharge.

[0118] Charging process: charge from open circuit voltage to 4.8 V at constant current 0.1 C, record the charge capacity.

[0119] Discharge process: discharge to 2.0 V at the same constant current, record the discharge capacity.

[0120] Calculation formula: the first coulombic efficiency FCE = (the first discharge capacity / the first charge capacity) x 100%.

[0121] (2) Cycle test process: Test method: after the freshly prepared battery is activated (activation process: the freshly prepared battery is left for 1 hour, then charged to 3.5 V at a constant current of 0.3 mA, and then left for 1 hour), multiple charge-discharge cycles are performed.

[0122] Charging and discharging system: constant current-constant voltage (CC-CV) charging and constant current (CC) discharging are adopted.

[0123] Charging: charge to 4.6 V at a constant current of 0.5 C (3 mA), and then convert to constant voltage charging until the current decreases to 0.05 C.

[0124] Discharging: discharge to 2.0 V at a constant current of 0.5 C.

[0125] Cycle number: 200 cycles.

[0126] Capacity retention rate calculation: capacity retention rate = (discharge capacity of the 200th cycle / first discharge capacity) x 100% (3) Voltage decay test: Test method: during the cycle test process, the average discharge voltage of each cycle is recorded.

[0127] Average discharge voltage: the average discharge voltage of each cycle is calculated by integrating the discharge curve of each cycle.

[0128] Voltage decay percentage = ((first cycle average discharge voltage-200th cycle average discharge voltage) / first cycle average discharge voltage) x 100%; Take the voltage decay value of the prior art sample (Comparative Example 1) as the benchmark to calculate the decrease of the voltage decay value of each group of batteries: for example, "the voltage decay decreases by 50% after 200 cycles", that is, the voltage decay value of the sample of the present application is half of that of the prior art sample (Comparative Example 1).

[0129] (5) Differential scanning calorimetry (DSC) analysis test: Sample preparation: The positive electrode material particles were taken out from the battery (battery after 200 cycles of test) in the glove box (ultra-dry argon atmosphere).

[0130] Mixing: The removed positive electrode material was mixed with electrolyte at a mass ratio of 1:1, and sealed in a specially designed pressure-resistant DSC crucible.

[0131] Test: The crucible was placed in the DSC instrument, and heated from room temperature to above 300 ℃ at a constant heating rate of 5 ℃ / min under inert atmosphere.

[0132] Analysis results: Record the onset temperature, peak temperature and total heat release of the DSC exothermic peak. The higher the onset temperature, the higher the peak temperature, and the smaller the total heat release, the better the thermal stability of the material.

[0133] Table 3: Test results of examples 1-22 and comparative examples 1-9

[0134] As can be seen from Table 1 and Table 3: compared with comparative example 1, examples 1-4 form a multi-ion gradient doped protective layer, and the first coulombic efficiency, cycle performance and thermal stability of the battery are obviously improved, and the voltage decay is improved.

[0135] In addition, in examples 1-4, the concentration of the outermost doped ions of the protective layer remains unchanged, and by adjusting the second sintering temperature, the thickness of the protective layer and the gradient coefficients of B 3+ , Zr 4+ and F - can be controlled. The higher the temperature and the longer the time, the faster and farther the ion diffusion, the more gentle the gradient, and the thicker the protective layer. On the contrary, the slower and closer the ion diffusion, the steeper the gradient, and the thinner the protective layer. The test results show that: when the thickness of the protective layer is 1-5 μm, the gradient coefficient of B 3+ doping is 0.1-0.5 at% μm -1 , the gradient coefficient of Zr 4+ doping is 0.05-0.3 at% μm -1 , and the gradient coefficient of F - doping is 0.2-1 at% μm -1 , the improvement effect on the positive electrode material is better.

[0136] Examples 2, 5-8, the rest of the conditions remain unchanged, the doping concentration of B 3+ in the outermost layer of the protective layer changes, and B 3+The gradient coefficient changes accordingly, and it is found that the higher the doping concentration of the outermost layer, the higher the starting point of the gradient curve, and under the same diffusion conditions, the steeper the final gradient. The doping concentration of the outermost layer of Example 7 is relatively high, and the gradient coefficient formed is relatively large, which means that the chemical composition and lattice parameters of the material change greatly within a very short distance. This will form a sudden interface between the core and the shell layer, produce a large lattice mismatch stress, cause the particle to crack and the coating to peel off during the cycle process, accelerate the failure of the material, and in addition, increase the interface impedance, resulting in a decrease in the initial coulomb efficiency and cycle capacity retention rate of the material. The doping concentration of the outermost layer of Example 8 is too low, and the total amount of doping is too small, so the improvement effect is relatively poor.

[0137] Examples 2, 9-12, with the rest unchanged, the doping concentration of the outermost layer of the protective layer is changed 4+ , and the gradient coefficient of Zr 4+ changes accordingly; it is found that the higher the doping concentration of the outermost layer, the higher the starting point of the gradient curve, and under the same diffusion conditions, the steeper the final gradient. The doping concentration of the outermost layer of Example 11 is relatively high, and the gradient coefficient formed is relatively large, which means that the chemical composition and lattice parameters of the material change greatly within a very short distance. This will form a sudden interface between the core and the shell layer, produce a large lattice mismatch stress, cause the particle to crack and the coating to peel off during the cycle process, accelerate the failure of the material, and in addition, increase the interface impedance, resulting in a decrease in the initial coulomb efficiency and cycle capacity retention rate of the material. The doping concentration of the outermost layer of Example 12 is relatively low, and the total amount of doping is relatively small, so the improvement effect is relatively poor.

[0138] Examples 2, 13-16, with the rest unchanged, the doping concentration of the outermost layer of the protective layer is changed - , and the gradient coefficient changes accordingly. It is found that the higher the doping concentration of the outermost layer, the higher the starting point of the gradient curve, and under the same diffusion conditions, the steeper the final gradient. The doping concentration of the outermost layer of Example 15 is relatively low, and the total amount of doping is relatively small, so the improvement effect is not obvious; the doping concentration of the outermost layer of Example 16 is relatively high, and the gradient coefficient formed is relatively large, which means that the chemical composition and lattice parameters of the material change greatly within a very short distance. This will form a sudden interface between the core and the shell layer, produce a large lattice mismatch stress, cause the particle to crack and the coating to peel off during the cycle process, accelerate the failure of the material, and in addition, increase the interface impedance, resulting in a decrease in the initial coulomb efficiency and cycle capacity retention rate of the material.

[0139] Examples 2, 17-20, with the rest unchanged, the doping concentration of Na + is changed, and the test results show that when the Na + doping amount is relatively high, the initial coulomb efficiency, cycle performance and voltage decay of the battery decrease, which is because the Na +, the doping concentration is too high, forming an over-thick, non-conductive sodium salt layer (such as NaF). This over-thick "insulating" layer hinders the shuttling of lithium ions, increases the total impedance of the interface, and reduces the electrochemical performance of the material. When the doping concentration of Na + , the doping concentration is too low, and the HF acid produced by the trace water in the electrolyte cannot be completely removed, and the HF acid causes metal ion dissolution, affecting the capacity of the positive electrode material, and causing the electrochemical performance of the battery to be slightly poor.

[0140] Examples 21 and 22 replace the main body material, respectively, and compared with Comparative Examples 8 and 9, the performance of the battery is significantly improved, which shows that the protective layer formed by the application is suitable for various lithium-rich layered oxide materials.

[0141] Comparative Example 2 compared with Example 2: B 3+ , Zr 4+ and F - are uniformly doped in the protective layer, and the performance improvement effect of the material is not good. This is because the internal doping concentration is too high, which will block the fast transmission channel of Li + , affecting the high capacity of the main body of the material, thereby affecting the electrochemical performance of the battery.

[0142] Comparative Examples 3-6 compared with Example 2 are respectively partially ion-doped, and the test results show that: only B 3+ , Zr 4+ , F - and Na + four kinds of ions are doped at the same time, the performance of the battery is best.

[0143] Comparative Example 7 compared with Example 2, other doping ions with the same valence state as Example 2 are replaced, and the test results show that compared with the positive electrode material without protective layer coating (Comparative Example 1), the first coulombic efficiency, cycle performance, voltage decay and thermal stability of the battery are improved to a certain extent, but compared with Example 2, the improvement effect is not good.

[0144] The positive electrode material provided by the application constructs a B 3+ / Zr 4+ double cation and F - coordinated gradient and Na + face enrichment multi-dimensional protective layer on the surface of the lithium-rich layered positive electrode material in situ, which improves the material performance from the aspects of structural stability, interface protection, ion transmission, etc. Therefore, the application effectively overcomes some practical problems in the prior art, thereby having high utilization value and use significance.

[0145] The above embodiments are only illustrative of the principles of the present application and its efficacy, and are not intended to limit the present application. Any modification or change made by any person skilled in the art without departing from the spirit and scope of the present application shall be covered by the claims of the present application.

Claims

1. A positive electrode material, characterized in that, include: The core, the chemical formula of which is Li 1+x M 1-x O2, where 0 < x ≤ 0.2, and M is selected from one or more of Ni, Co, and Mn; A protective layer is wrapped around the surface of the core. The main structure of the protective layer is integrated and continuously distributed with the crystal structure of the core. The protective layer is doped with cations, anions and alkali metal ions. The cations and anions are distributed along the thickness direction of the protective layer in an increasing doping concentration gradient from the inside to the outside, and the alkali metal ions are doped on the surface of the protective layer; the cations include B. 3+ and Zr 4+ The anion includes F - The alkali metal ions include Na + .

2. The cathode material according to claim 1, characterized in that, The cathode material satisfies one or more of the following conditions: The B 3+ The doping gradient coefficient is 0.1~0.5 at% µm. -1 The B 3+ The atomic percentage of the outermost layer of the protective layer is 0.5~3 at%, wherein the outermost layer refers to the region of the protective layer that extends from the outside to the inside along the thickness direction of 0~0.1 μm; The Zr 4+ The doping gradient coefficient is 0.05~0.3 at% µm. -1 The Zr 4+ The atomic percentage of the outermost layer of the protective layer is 0.2~2 at%; The F - The doping gradient coefficient is 0.2~1 at% µm -1 The F - The atomic percentage of the outermost layer of the protective layer is 2-10 at% The Na + The atomic percentage of the outermost layer of the protective layer is 0.8~5 at%.

3. The cathode material according to claim 1, characterized in that, The thickness of the protective layer is 1~5μm, and / or, the Na + The doping depth in the protective layer is in the range of 0~50nm extending inward from the outer surface of the protective layer.

4. The cathode material according to claim 1, characterized in that, The particle size distribution D50 of the cathode material is 5~15µm.

5. A method for preparing the positive electrode material according to any one of claims 1 to 4, characterized in that, Includes the following steps: Precursor co-precipitation: Prepare a salt solution containing transition metal elements according to the chemical formula of the cathode material, add a cation source and an anion source to the salt solution, adjust the pH value and temperature to carry out co-precipitation, and obtain the doped precursor; Gradient diffusion sintering: The doped precursor is mixed with a lithium source and then subjected to gradient diffusion sintering to obtain an intermediate doped with cations and anions. Surface modification: A sodium source is introduced into the intermediate to form sodium enrichment on the surface of the intermediate, thereby obtaining the cathode material.

6. The method for preparing the cathode material according to claim 5, characterized in that, The gradient diffusion sintering includes: The first sintering is carried out at a temperature of 650~750℃ for 2~6 hours to form an interface layer rich in doped ions; The second sintering is carried out at a temperature of 780~850℃ for 10~20 hours to drive the diffusion of dopant ions in the interface layer to form a smooth dopant concentration gradient.

7. The method for preparing the cathode material according to claim 5, characterized in that, The surface modification includes: introducing sodium source vapor into the intermediate at a temperature of 450~600°C under an inert atmosphere and maintaining it for 30 minutes; and / or, The precursor co-precipitation occurs at a pH of 9.5-11 and a temperature of 40-60℃.

8. The method for preparing the cathode material according to claim 5, characterized in that, Includes one or more of the following items A through E: A. The transition metal element is selected from one or more of Ni, Co, and Mn, and the salt solution containing the transition metal element includes any one of sulfate solution, nitrate solution, acetate solution, and chloride solution; B. The lithium source includes one or more of lithium carbonate and lithium hydroxide; C. The cation source includes a boron source and a zirconium source, wherein the boron source includes one or more of boric acid, ammonium borate, and sodium borate, and the zirconium source includes one or more of zirconium sulfate and zirconium tetrachloride. D. The anion source includes a fluorine source, which includes one or more of ammonium fluoride and hydrogen fluoride. E. The sodium source includes one or more of sodium fluoride and sodium carbonate.

9. An electrochemical device, characterized in that, It includes the cathode material according to any one of claims 1 to 4, or the cathode material prepared by any one of claims 5 to 8.

10. An electronic device, characterized in that, Includes the electrochemical device as described in claim 9.

Images