Positive electrode lithium supplementing material, preparation method therefor and use thereof

Abstract

The present application relates to the technical field of batteries, and in particular to a positive electrode lithium supplementing material, a preparation method therefor and the use thereof, so as to improve the structural stability of positive electrode lithium supplementing materials and the cycle performance of batteries. The positive electrode lithium supplementing material comprises a positive electrode lithium supplementing base material and a shell layer present on at least part of the surface of the positive electrode lithium supplementing base material; the shell layer comprises a fluorine element, the content of the fluorine element in the shell layer tending to increase in the direction from the positive electrode lithium supplementing base material to the shell layer.

Description

A positive electrode lithium supplement material, a preparation method and application thereof TECHNICAL FIELD

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

[0002] In the field of battery manufacturing technology, the quality of the positive electrode active material directly affects the performance of the battery, such as energy density, power density, cycle life, etc. However, during the charging and discharging process of the battery, a solid-state electrolyte interface film will be formed on the surface of the positive electrode active material, which will consume part of the active lithium in the positive electrode active material and cannot be recovered, thereby causing problems such as low battery capacity, low first coulomb efficiency, and poor cycle life. The use of a positive electrode lithium supplement material can effectively reduce the irreversible loss of active lithium and simultaneously restore the structure of the positive electrode active material, thereby ensuring the normal performance of the material.

[0003] The commonly used positive electrode lithium supplement material has the problem of poor structural stability during the cycle process, thereby causing the capacity of the battery to decay quickly and the cycle performance to be poor. Therefore, there is an urgent need to develop a positive electrode lithium supplement material with high structural stability to improve the cycle performance of the battery.

[0004] SUMMARY

[0005] The present application discloses a positive electrode lithium supplement material, a preparation method and application thereof, to solve the problems of poor structural stability of the existing positive electrode lithium supplement material and poor cycle performance of the battery.

[0006] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0007] In a first aspect, the present application provides a positive electrode lithium supplement material, comprising a positive electrode lithium supplement base material and a shell layer existing on at least part of the surface of the positive electrode lithium supplement base material; the shell layer comprises fluorine elements, and the content of the fluorine elements in the shell layer shows an increasing trend along the direction from the positive electrode lithium supplement base material to the shell layer.

[0008] Further, the chemical formula of the shell layer is A a ZrF b wherein 0

[0009] And / or, the chemical formula of the positive electrode lithium supplement base material is Li x M y O z wherein 0

[0010] Further, the shell layer comprises a first shell layer existing on the surface of the positive electrode lithium supplement matrix material, a second shell layer existing on the side of the first shell layer away from the positive electrode lithium supplement matrix material, and a third shell layer existing on the side of the second shell layer away from the positive electrode lithium supplement matrix material.

[0011] The content of fluorine element in the first shell layer is less than that in the second shell layer, and the content of fluorine element in the second shell layer is less than that in the third shell layer.

[0012] Further, the thickness of the first shell layer is 0.05-0.6 μm, the thickness of the second shell layer is 0.3-0.75 μm, and the thickness of the third shell layer is 0.2-1 μm.

[0013] And / or, the mass percentage of fluorine element in the first shell layer is 0.01%-1.5% of the positive electrode lithium supplement material, the mass percentage of fluorine element in the second shell layer is 1%-3% of the positive electrode lithium supplement material, and the mass percentage of fluorine element in the third shell layer is 3%-5% of the positive electrode lithium supplement material.

[0014] Further, the D50 of the positive electrode lithium supplement matrix material is 8-10 μm, and the total thickness d of the shell layer is 1-2.1 μm.

[0015] And / or, through X-ray diffraction analysis, the half-peak width FWHM(003) of the diffraction peak corresponding to the (003) crystal face in the X-ray diffraction pattern of the positive electrode lithium supplement material is 0.25-0.42°; wherein FWHM(003)=0.0027d+0.13d+0.13, d is the total thickness of the shell layer. 2

[0016] And / or, the proportion of the area of the part of the surface of the positive electrode lithium supplement matrix material where the shell layer does not exist to the total area of the surface of the positive electrode lithium supplement matrix material is less than or equal to 5%.

[0017] Further, in the differential capacity curve of the battery prepared from the positive electrode lithium supplement material after 0-200 cycles, the fluctuation range of the phase transition peak peak position I1 in the interval of 4.1-4.3 V is 0%-2%, and the fluctuation range of the phase transition peak peak position I2 in the interval of 3.4-3.7 V is 0%-5%.

[0018] In a second aspect, the application provides a preparation method of the positive electrode lithium supplement material of the first aspect, comprising the following steps:

[0019] 1) Under the atmosphere of inert gas at 300-500℃, the raw material system comprising lithium source and M source is sintered for 8-16 h to obtain a first intermediate; ​

[0020] 2) performing secondary sintering on the first intermediate body at 600-800℃ under an inert gas atmosphere for 8-16h to obtain a second intermediate body;

[0021] 3) performing primary deposition of a redox medium on the surface of the second intermediate body to obtain a third intermediate body;

[0022] 4) performing secondary deposition of a redox medium on the surface of the third intermediate body to obtain a fourth intermediate body;

[0023] 5) performing tertiary deposition of a redox medium on the surface of the fourth intermediate body to obtain the positive electrode lithium supplement material.

[0024] Further, the temperature of the primary deposition is 400-450℃, the deposition rate is 0.01-0.02μm / min, and the deposition time is 2.5-30min;

[0025] And / or, the temperature of the secondary deposition is 350-400℃, the deposition rate is 0.02-0.03μm / min, and the deposition time is 10-25min;

[0026] And / or, the temperature of the tertiary deposition is 300-350℃, the deposition rate is 0.03-0.035μm / min, and the deposition time is 6-28min.

[0027] In a third aspect, the present application provides a positive electrode sheet, comprising the positive electrode lithium supplement material of the first aspect or the positive electrode lithium supplement material prepared by the preparation method of the second aspect.

[0028] Further, the positive electrode sheet comprises a positive electrode current collector, a positive electrode active material layer present on the surface of the positive electrode current collector, and a positive electrode lithium supplement layer present on the side of the positive electrode active material layer away from the positive electrode current collector.

[0029] In a fourth aspect, the present application provides a battery comprising the positive electrode sheet of the third aspect.

[0030] The technical solution of the present application has the following beneficial effects:

[0031] The positive electrode lithium supplement material provided by the present application can supplement lithium for the battery, make up for the loss of lithium ion concentration caused by the lithium ion battery during the cycle process, and the content of fluorine element in the shell layer of the positive electrode lithium supplement material in the present application increases in the direction from the positive electrode lithium supplement matrix material to the shell layer, so that a shell layer with gradually increasing density can be formed, the structural stability of the positive electrode lithium supplement material is effectively enhanced, and the cycle stability of the battery is improved. BRIEF DESCRIPTION OF DRAWINGS

[0032] FIG. 1 is a cross-sectional SEM image of the positive electrode lithium supplement material prepared in Example 1 of the present application.

[0033] FIG. 2 is a surface SEM image of the positive electrode lithium supplement material prepared in Example 1 of the present application;

[0034] FIG. 3 is a graph showing the change of the content of fluorine element in the positive electrode lithium supplement material prepared in Example 1 of the present application along the direction from the positive electrode lithium supplement matrix material to the shell layer;

[0035] FIG. 4 is a structural schematic diagram of a positive electrode sheet provided by the present application. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application will be further described in detail below with reference to the drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0037] The application scenarios described in the embodiments of the present application are for more clearly illustrating the technical scheme of the embodiments of the present application, and do not constitute a limitation on the technical scheme provided by the embodiments of the present application. Those skilled in the art can know that, with the appearance of new application scenarios, the technical scheme provided by the embodiments of the present application is also applicable to similar technical problems. In the description of the present application, unless otherwise specified, the meaning of "a plurality of" is two or more.

[0038] In a first aspect, the present application provides a positive electrode lithium supplement material, comprising a positive electrode lithium supplement matrix material and a shell layer existing on at least part of the surface of the positive electrode lithium supplement matrix material; the shell layer comprises fluorine element, and the content of the fluorine element in the shell layer shows an increasing trend along the direction from the positive electrode lithium supplement matrix material to the shell layer.

[0039] Specifically, the shell layer can be coated on the surface of the positive electrode lithium supplement matrix material, the content of fluorine element in the shell layer increases in the direction from the positive electrode lithium supplement matrix material to the shell layer, that is, in the direction from the positive electrode lithium supplement matrix material to the shell layer, a shell layer with gradually increasing density is formed, and the shell layer can be a redox medium layer. The gradual increase in the density strengthens the combination of the redox medium and the positive electrode lithium supplement matrix material, which is conducive to improving the transmission of electric charges, that is, the diffusion process of ions. The gradually increasing content of fluorine element can reduce the diffusion impedance of ions, effectively optimize the diffusion of ions, and the gradient distribution makes the diffusion of ions in the positive electrode lithium supplement material more smooth, reduces the diffusion resistance of ions, and at the same time, the outer high-density shell layer can increase the structural stability of the positive electrode lithium supplement material, reduce the degradation of the structure of the positive electrode lithium supplement material and the increase of the interface impedance in the charging and discharging process, thereby prolonging the cycle life of the battery. In addition, the redox medium can also spontaneously react with the dead lithium on the surface of the negative electrode. The reaction is a reversible reaction, which can effectively prolong the cycle life of the battery. Specifically, during the charging process, the oxidized redox medium can spontaneously react with the dead lithium on the surface of the metal lithium negative electrode to become a reduced redox medium, then diffuse to the positive electrode side, and lose electrons to be oxidized to an oxidized redox medium again; that is, it can effectively reduce the influence of dead lithium on the cycle performance of the battery without affecting the performance of the positive electrode lithium supplement matrix material.

[0040] In the present application, the content of fluorine element can be measured by X-ray energy spectrum (EDS) line scanning.

[0041] The positive electrode lithium supplement material of the present application can supplement lithium for the battery, make up for the loss of lithium ion concentration caused by the cycle process of the lithium ion battery, and the content of fluorine element in the shell layer of the positive electrode lithium supplement material in the present application increases in the direction from the positive electrode lithium supplement matrix material to the shell layer, which can form a shell layer with gradually increasing density, effectively enhance the structural stability of the positive electrode lithium supplement material, and further improve the cycle stability of the battery.

[0042] Specifically, the chemical formula of the shell layer is A a ZrF b wherein 0

[0043] In some embodiments, the chemical formula of the positive electrode lithium supplement matrix material is Li x M y O z wherein 0

[0044] Specifically, A aZrF b In some embodiments, a can be 0.1, 0.5, 1, 1.5, 2, or a range defined by any two of them, and b can be 4, 4.5, 5, 5.5, 6, or a range defined by any two of them. x M y O z In some embodiments, x can be 0.1, 0.5, 1, 1.5, 2, 3, 4, or a range defined by any two of them, y can be 0.1, 0.5, 1, 1.5, 2, or a range defined by any two of them, and z can be 0.1, 0.5, 1, 1.5, 2, 3, 4, or a range defined by any two of them.

[0045] The shell layer in the present application is fluorozirconate, which can adjust the kinetics of the reaction by changing the electron density on the surface of the electrode or changing the chemical environment on the surface of the electrode, which helps to achieve more efficient electron transfer and more uniform electrochemical reaction, thereby improving the energy density and rate performance of the battery. And the fluorozirconate shell layer can form a stable protective film on the surface of the positive lithium supplement matrix material, reduce the direct contact of the electrolyte with the positive lithium supplement matrix material, reduce the probability of side reactions, thereby improving the structural stability of the positive lithium supplement material, and further improving the cycle performance of the battery.

[0046] The use of the above positive lithium supplement matrix material is conducive to the function of lithium supplement, and the coating of fluorozirconate on the surface thereof can improve the structural stability, thereby improving the cycle performance of the battery.

[0047] In some embodiments of the present application, the shell layer includes a first shell layer present on the surface of the positive lithium supplement matrix material, a second shell layer present on the side of the first shell layer away from the positive lithium supplement matrix material, and a third shell layer present on the side of the second shell layer away from the positive lithium supplement matrix material.

[0048] The content of fluorine in the first shell layer is less than the content of fluorine in the second shell layer, and the content of fluorine in the second shell layer is less than the content of fluorine in the third shell layer.

[0049] In the present application, the shell layer includes a first shell layer, a second shell layer, and a third shell layer. Specifically, the first shell layer exists on the surface of the positive electrode lithium supplement matrix material, which can be part of the surface or the entire surface. The second shell layer exists on the side of the first shell layer away from the positive electrode lithium supplement matrix material, which can be part of the surface or the entire surface. The third shell layer exists on the side of the second shell layer away from the positive electrode lithium supplement matrix material, which can be part of the surface or the entire surface. The first shell layer can provide preliminary protection for the positive electrode lithium supplement matrix material, the second shell layer can further enhance the protection of the positive electrode lithium supplement matrix material, and the third shell layer can provide the strongest protection. This gradient structure can effectively prevent the corrosion of the electrolyte on the positive electrode lithium supplement matrix material and significantly improve the structural stability of the positive electrode lithium supplement material. Moreover, the gradient shell layer design can alleviate the stress concentration caused by the stress difference between different levels of materials, reduce the mechanical stress and lattice distortion of the material during charging and discharging, and thus improve the mechanical strength and structural stability of the material.

[0050] The content of fluorine elements in the first shell layer of the present application is less than that in the second shell layer, and the content of fluorine elements in the second shell layer is less than that in the third shell layer. This layer-by-layer increase in the content of fluorine elements forms a gradient protection structure. The lower content of fluorine elements in the first shell layer helps to maintain good electronic and ionic conductivity, ensuring efficient electrochemical reaction. With the increase in the content of fluorine elements, the second shell layer and the third shell layer can provide stronger chemical stability and protection, thereby improving the structural stability of the positive electrode lithium supplement material. In addition, the high content of fluorine elements in the third shell layer can form a stable protective film, reduce the side reaction of the electrolyte with the positive electrode lithium supplement material, reduce the decomposition of the electrolyte and the generation of electrode surface by-products, and thus improve the cycle life of the battery.

[0051] In some embodiments of the present application, the thickness of the first shell layer is 0.05-0.6 μm; the thickness of the second shell layer is 0.3-0.75 μm; and the thickness of the third shell layer is 0.2-1 μm.

[0052] In some embodiments, the content of fluorine elements in the first shell layer accounts for 0.01%-1.5% of the mass percentage of the positive electrode lithium supplement material; the content of fluorine elements in the second shell layer accounts for 1%-3% of the mass percentage of the positive electrode lithium supplement material; and the content of fluorine elements in the third shell layer accounts for 3%-5% of the mass percentage of the positive electrode lithium supplement material.

[0053] Exemplarily, the thickness of the first shell layer can be 0.05 μm, 0.1 μm, 0.15 μm, 0.2 μm, 0.25 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, or a range formed by any two of them, the thickness of the second shell layer can be 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.75 μm, or a range formed by any two of them, and the thickness of the third shell layer can be 0.2 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, or a range formed by any two of them. The mass percentage of fluorine in the first shell layer with respect to the positive electrode lithium supplement material can be 0.01%, 0.05%, 0.07%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.5%, or a range formed by any two of them, the mass percentage of fluorine in the second shell layer with respect to the positive electrode lithium supplement material can be 1%, 1.5%, 2%, 2.5%, 3%, or a range formed by any two of them, and the mass percentage of fluorine in the third shell layer with respect to the positive electrode lithium supplement material can be 3%, 3.5%, 4%, 4.5%, 5%, or a range formed by any two of them.

[0054] In the present application, the thickness of the first shell layer, the second shell layer and the third shell layer can be obtained by EDS line scanning to measure the content of fluorine in each shell layer. Specifically, after depositing the first shell layer on the positive electrode lithium supplement base material, EDS line scanning is performed, and the thickness of the first shell layer is obtained by testing the thickness of the region with a fluorine mass percentage greater than 0.5%; after depositing the second shell layer on the positive electrode lithium supplement base material, EDS line scanning is performed, and the first thickness is obtained by testing the thickness of the region with a fluorine mass percentage greater than 1.5%, and the second shell layer thickness is obtained by subtracting the first shell layer thickness from the first thickness; after depositing the third shell layer on the positive electrode lithium supplement base material, EDS line scanning is performed, and the second thickness is obtained by testing the thickness of the region with a fluorine mass percentage greater than 3%, and the third shell layer thickness is obtained by subtracting the first thickness from the second thickness.

[0055] In the present application, the thickness of the first shell layer, the second shell layer and the third shell layer, and the mass percentage of fluorine in the first shell layer, the second shell layer and the third shell layer with respect to the positive electrode lithium supplement material are in the above-mentioned ranges, which can further improve the structural stability of the positive electrode lithium supplement material, reduce the increase of interface impedance during charging and discharging, and thus improve the cycle performance of the battery.

[0056] In some embodiments of the present application, the D50 of the positive electrode lithium supplement base material is 8 μm-10 μm; and the total thickness d of the shell layer is 1 μm-2.1 μm.

[0057] In some embodiments, the positive electrode lithium supplement material has a half-peak width FWHM(003) of a diffraction peak corresponding to a (003) crystal face in an X-ray diffraction pattern of 0.25-0.42° by X-ray diffraction analysis; wherein FWHM(003) = 0.0027d 2 +0.13d+0.13, d is the total thickness of the shell layer.

[0058] In some embodiments, the proportion of the area of the part of the surface of the positive electrode lithium supplement matrix material where the shell layer is not present to the total area of the surface of the positive electrode lithium supplement matrix material is less than or equal to 5%.

[0059] Illustratively, the D50 of the positive electrode lithium supplement matrix material can be in a range consisting of 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, or any two thereof, the total thickness d of the shell layer can be in a range consisting of 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2 μm, 2.1 μm, or any two thereof, the half-peak width FWHM(003) of a diffraction peak corresponding to a (003) crystal face in an X-ray diffraction pattern of the positive electrode lithium supplement material can be in a range consisting of 0.25°, 0.3°, 0.32°, 0.35°, 0.38°, 0.4°, 0.42°, or any two thereof, and the proportion of the area of the part of the surface of the positive electrode lithium supplement matrix material where the shell layer is not present to the total area of the surface of the positive electrode lithium supplement matrix material can be in a range consisting of 1%, 2%, 3%, 4%, 5%, or any two thereof.

[0060] In the present application, the D50 of the positive electrode lithium supplement matrix material is in the above range, which can provide a good specific surface area, help to improve the electrochemical reaction activity and efficiency, and also can balance the electrical conductivity and ion transference rate, ensure that the positive electrode lithium supplement material has good rate performance and energy density, and also help to improve the mechanical strength and structural stability of the positive electrode lithium supplement material, reduce the pulverization and breakage phenomenon in the charging and discharging process. The total thickness d of the shell layer is in the above range, which can provide effective protection for the positive electrode lithium supplement matrix material, prevent the electrolyte from corroding the positive electrode lithium supplement matrix material, reduce the occurrence of side reactions, also help to improve the structural stability of the positive electrode lithium supplement material, reduce the increase of interface impedance in the charging and discharging cycle, and thus improve the cycle performance and rate performance of the battery.

[0061] In the present application, the total thickness of the shell layer closely affects the uniformity of the lithium ion deintercalation channel, and the total thickness of the shell layer changing with the gradient can effectively regulate the uniformity of the lithium ion deintercalation channel, and thus regulate the charging and discharging uniformity. A moderate total thickness of the shell layer can reduce the polarization phenomenon of the electrode material in the charging and discharging process, reduce the internal resistance of the battery, and thus improve the energy density and rate performance of the battery, while an excessively thick surface coating can inhibit the reverse deintercalation of lithium ions.

[0062] The proportion of the area of the part of the surface of the positive electrode lithium supplement matrix material without the shell layer in the total area of the surface of the positive electrode lithium supplement matrix material in the present application is less than or equal to 5%, which means that the shell layer has a high coverage rate, can provide comprehensive protection for the positive electrode lithium supplement matrix material, can keep the structure stability of the positive electrode lithium supplement material in the charging and discharging process, prevent the direct contact of the electrolyte and the positive electrode lithium supplement matrix material, reduce the occurrence of side reactions, reduce the increase of the interface impedance in the charging and discharging process, and improve the cycle performance and rate performance of the battery.

[0063] In some embodiments of the present application, the differential differential capacity curve of the battery prepared by the positive electrode lithium supplement material in 0-200 cycles is as follows: in the interval of 4.1V-4.3V, the fluctuation range of the phase transition peak I1 is 0%-2%; in the interval of 3.4V-3.7V, the fluctuation range of the phase transition peak I2 is 0%-5%.

[0064] Specifically, the preparation method of the battery in the present application can include the following steps: under the conditions of 25°C and normal pressure (0.1 MPa), the positive electrode active material NCM622, the conductive agent Super P and the binder polyvinylidene fluoride (PVDF) are mixed uniformly in N-methyl pyrrolidone solvent at a mass ratio of 8:1:1 to obtain a positive electrode slurry with a solid content of 70%. The positive electrode lithium supplement material, conductive carbon black and binder polyvinylidene fluoride (PVDF) are mixed uniformly in N-methyl pyrrolidone solvent at a mass ratio of 4:2:2 to obtain a positive electrode lithium supplement slurry with a solid content of 20%. The positive electrode lithium supplement slurry is coated on the current collector Al foil by transfer coating, and the face density of the positive electrode supplement slurry is controlled in the range of 13-15 mg / cm 2 -15mg / cm 2 The coating speed is 1 mL / min. After the positive electrode slurry is pre-coated for 400 mm, the slurry nozzle is opened, and the positive electrode lithium supplement slurry is sprayed, with a spraying pressure of 0.5 MPa and a spraying speed of 10 mL / min. After coating, the positive electrode sheet is dried in an oven at 110°C-130°C, and the negative electrode is made of graphite, together with the separator polypropylene PP and the electrolyte (the electrolyte composition is 1 mol / L LiPF6, and the volume ratio of EC / DMC / DEC is 1:1:1) to form a full battery.

[0065] Under the condition of 25°C, the battery is charged at a charge rate of 1C to 4.35V, and then charged at a constant voltage of 4.35V until the current rate is 0.5C. Then, the battery is discharged at a discharge rate of 1C to 3.0V, and the above process is repeated for 200 times. The collected data is processed to calculate the voltage change (dV) and the charge change (dQ) in each cycle. The ratio of dQ / dV (the ratio of charge change to voltage change) is taken as the vertical coordinate, and the voltage or capacity is taken as the horizontal coordinate, so as to obtain the standard differential differential capacity (dQ / dV) curve.

[0066] Exemplarily, the fluctuation range of the phase transition peak peak position I1 can be 0%, 0.5%, 1%, 1.5%, 2%, or a range formed by any two of them, and the fluctuation range of the phase transition peak peak position I2 can be 0%, 1%, 2%, 3%, 4%, 5%, or a range formed by any two of them.

[0067] In the present application, the fluctuation range of the phase transition peak peak position I1 in the interval of 4.1V-4.3V and the phase transition peak peak position I2 in the interval of 3.4V-3.7V is in the above range, the surface battery has stable phase change behavior in the cycle process, and the positive electrode lithium supplement material maintains good structural stability and electrochemical stability in the charge and discharge process; the fluctuation range of the phase transition peak peak position is smaller, indicating that the battery has lower capacity attenuation in the cycle process, and can maintain higher capacity and energy density in the long-term use process, thereby prolonging the cycle life of the battery.

[0068] In a second aspect, the present application provides a preparation method of the positive electrode lithium supplement material of the first aspect, comprising the following steps:

[0069] 1) Sintering the raw material system comprising a lithium source and an M source once at 300-500℃ under an inert gas atmosphere for 8-16h to obtain a first intermediate;

[0070] 2) Sintering the first intermediate twice at 600-800℃ under an inert gas atmosphere for 8-16h to obtain a second intermediate;

[0071] 3) Depositing a redox medium on the surface of the second intermediate once to obtain a third intermediate;

[0072] 4) Depositing a redox medium on the surface of the third intermediate twice to obtain a fourth intermediate;

[0073] 5) Depositing a redox medium on the surface of the fourth intermediate three times to obtain the positive electrode lithium supplement material.

[0074] In the present application, through three times of deposition treatment, the positive electrode lithium supplement material of the first aspect of the present application can be finally prepared.

[0075] Specifically, in step 1), a lithium source, such as lithium hydroxide, and a M source can be mixed in a high-speed mixer in a certain ratio for 3h-5h to obtain sample A1; then sample A1 is placed in a ball mill for ball milling at a speed of 500 rpm to obtain sample A2; and the sample A2 is sintered once under an inert gas atmosphere for 8h-16h to obtain a first intermediate. The M source is a compound containing at least one element selected from C, Ni, Co, Mn and Fe. The temperature of the first sintering can be 300°C, 350°C, 400°C, 450°C, 500°C, or a range formed by any two of them, and the time of the first sintering can be 8h, 10h, 12h, 14h, 16h, or a range formed by any two of them.

[0076] In step 2), the first intermediate is cooled and then ball milled again, and then the milled first intermediate is sintered twice under an inert gas atmosphere at 600°C-800°C for 8h-16h to obtain a second intermediate. The temperature of the second sintering can be 600°C, 650°C, 700°C, 750°C, 800°C, or a range formed by any two of them, and the time of the second sintering can be 8h, 10h, 12h, 14h, 16h, or a range formed by any two of them.

[0077] In step 3), the oxidation-reduction medium is pretreated with an inert atmosphere N2, the flow rate of N2 is 20 mL / min, the oxidation-reduction medium is gasified under vacuum, the gasification load of the oxidation-reduction medium solution is 5L-10L, and a stepwise gradient deposition is performed. The oxidation-reduction medium is first deposited on the surface of the second intermediate to obtain a third intermediate with a first shell.

[0078] In step 4), a second deposition is performed on the surface of the third intermediate to obtain a fourth intermediate with a second shell.

[0079] In step 5), a third deposition is performed on the surface of the fourth intermediate to obtain the above-mentioned positive electrode lithium supplement material.

[0080] In the present application, by performing three deposition treatments of the oxidation-reduction medium on the surface of the second intermediate, the positive electrode lithium supplement material with gradient distribution of fluorine elements provided in the first aspect of the present application can be obtained, and the structural stability of the positive electrode lithium supplement material is improved, thereby improving the cycle stability of the battery.

[0081] In some embodiments of the present application, the temperature of the first deposition is 400-450°C, the deposition rate is 0.01-0.02 μm / min, and the deposition time is 2.5-30 min.

[0082] In some embodiments, the temperature of the second deposition is 350-400°C, the deposition rate is 0.02-0.03 μm / min, and the deposition time is 10-25 min.

[0083] In some embodiments, the temperature of the third deposition is 300-350℃, the deposition rate is 0.03-0.035 μm / min, and the deposition time is 6-28 min.

[0084] Exemplarily, the temperature of the first deposition can be 400℃, 410℃, 420℃, 430℃, 440℃, 450℃, or a range consisting of any two of them, the rate of the first deposition can be 0.01 μm / min, 0.012 μm / min, 0.015 μm / min, 0.018 μm / min, 0.02 μm / min, or a range consisting of any two of them, and the time of the first deposition can be 2.5 min, 5 min, 10 min, 20 min, 30 min, or a range consisting of any two of them. The temperature of the second deposition can be 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, or a range consisting of any two of them, the rate of the second deposition can be 0.02 μm / min, 0.022 μm / min, 0.025 μm / min, 0.028 μm / min, 0.03 μm / min, or a range consisting of any two of them, and the time of the second deposition can be 10 min, 15 min, 20 min, 22 min, 25 min, or a range consisting of any two of them. The temperature of the third deposition can be 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, or a range consisting of any two of them, the rate of the third deposition can be 0.03 μm / min, 0.031 μm / min, 0.032 μm / min, 0.033 μm / min, 0.034 μm / min, 0.035 μm / min, or a range consisting of any two of them, and the time of the third deposition can be 6 min, 10 min, 15 min, 20 min, 25 min, 28 min, or a range consisting of any two of them.

[0085] The preparation process of the positive electrode lithium supplement material in the present application is simple, and the reaction condition is mild, thereby expanding the use range of the shell. Different deposition temperatures and deposition rates can form a shell with gradually increasing density along the direction from the positive electrode lithium supplement matrix material to the shell. The gradual transition of the density strengthens the combination of the oxidation-reduction medium and the positive electrode lithium supplement matrix material, is beneficial to improving the charge transmission and ion diffusion process, and effectively enhances the stability of the positive electrode lithium supplement material, thereby improving the cycle performance of the battery.

[0086] In a third aspect, the present application provides a positive electrode sheet comprising the positive electrode lithium supplement material of the first aspect or the positive electrode lithium supplement material prepared by the preparation method of the second aspect.

[0087] In some embodiments of the present application, as shown in Figure 4, the positive electrode sheet comprises a positive electrode current collector, a positive electrode active material layer present on the surface of the positive electrode current collector, and a positive electrode lithium supplement layer present on the side of the positive electrode active material layer away from the positive electrode current collector.

[0088] The positive electrode sheet of the present application can be prepared by a preparation method comprising the following steps:

[0089] 1) coating a positive electrode slurry comprising a positive electrode active material, a conductive agent, and a binder on at least one functional surface of a positive electrode current collector to obtain a positive electrode sheet precursor;

[0090] 2) coating a positive electrode lithium supplement slurry comprising a positive electrode lithium supplement material, a conductive agent, and a binder on the side of the positive electrode sheet precursor away from the positive electrode slurry to obtain the positive electrode sheet.

[0091] The present application does not make specific limitations on the specific types of conductive agents and binders, and the components such as conductive agents and binders can be selected from conventional materials in the art. For example, the conductive agent can be selected from one or more of conductive carbon black, carbon nanotubes, conductive graphite, and graphene, and the binder can be selected from one or more of polyvinylidene fluoride (PVDF), acrylic modified PVDF, polyacrylate polymer, polyimide, butadiene-styrene rubber, and styrene-butadiene rubber.

[0092] The present application does not make specific limitations on the coating method, and any one of gravure coating, extrusion coating, spraying, and screen printing can be used to realize the coating of the positive electrode active layer slurry.

[0093] The conventional positive electrode lithium supplement material is used by being uniformly mixed with the positive electrode active material and then coated, and the present application uses the positive electrode lithium supplement material and the positive electrode active material to be uniformly mixed respectively and then coated in a double-layer coating manner, which can not only fully exert the capacity of the positive electrode lithium supplement material, but also can not destroy the conductive network of the positive electrode slurry, ensure the stability of lithium ion kinetics, ensure the adhesion between the current collector and the positive electrode slurry, avoid electrical isolation, and ensure the normal transportation of electrons.

[0094] In a fourth aspect, the present application provides a battery comprising the positive electrode sheet of the third aspect.

[0095] The battery of the present application comprises, in addition to the positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte. The composition of the negative electrode sheet can refer to the conventional negative electrode sheet in the art, and the separator can also use the conventional separator in the art, such as a PP film and a PE film.

[0096] The battery of the present application can be prepared by conventional methods in the art. Specifically, the positive electrode sheet, the separator, and the negative electrode sheet can be sequentially stacked and placed, and then a battery cell can be obtained by a stacking or winding process. Then, the above-mentioned battery can be obtained by the processes of baking, liquid injection, formation, and packaging.

[0097] The technical solutions of the present application are further illustrated below in combination with specific examples.

[0098] Example 1

[0099] The preparation method of the positive electrode sheet of the present example comprises the following steps:

[0100] 1) Lithium hydroxide and oxalic acid containing C element were put into a high-speed mixer at a mass ratio of 1.2:1 and mixed for 5 h to obtain sample A1.

[0101] 2) The sample A1 was placed in a ball mill and ball-milled at a speed of 500 rpm to obtain sample A2.

[0102] 3) The sample A2 was sintered once under an inert gas N2 atmosphere, the sintering temperature was 500℃, and the holding time was 8h, to obtain a first intermediate.

[0103] 4) After cooling, the first intermediate was ball-milled again, and the ball-milled first intermediate was sintered twice under an inert gas N2 atmosphere, the sintering temperature was 800℃, and the holding time was 10h, to obtain a second intermediate.

[0104] 5) The redox medium K2ZrF6 was pretreated in an inert atmosphere N2, the N2 flow rate was 20mL / min, the redox medium was vaporized in vacuum, the vaporization load of the redox medium solution was 6L, and the stepwise gradient deposition was carried out. The first deposition temperature was 450℃, the deposition rate was 0.02μm / min, and the deposition time was 15min; the second deposition temperature was 400℃, the deposition rate was 0.03μm / min, and the deposition time was 16min; the third deposition temperature was 350℃, the deposition rate was 0.035μm / min, and the deposition time was 28min, to obtain a positive electrode lithium supplement material.

[0105] ​The chemical formula of the positive electrode lithium supplement matrix material is Li2C2O4, the D50 of the positive electrode lithium supplement matrix material is 9.65 μm, the chemical formula of the shell layer existing on the surface of the positive electrode lithium supplement matrix material is K2ZrF6, and the total thickness d of the shell layer is 1.8 μm. The thickness of the first shell layer existing on the surface of the positive electrode lithium supplement matrix material is 0.3 μm, the mass percentage of fluorine in the first shell layer is 1% of the positive electrode lithium supplement material, which is denoted as ratio 1; the thickness of the second shell layer existing on the side of the first shell layer away from the positive electrode lithium supplement matrix material is 0.5 μm, the mass percentage of fluorine in the second shell layer is 2.5% of the positive electrode lithium supplement material, which is denoted as ratio 2; the thickness of the third shell layer existing on the side of the second shell layer away from the positive electrode lithium supplement matrix material is 1 μm, the mass percentage of fluorine in the third shell layer is 4% of the positive electrode lithium supplement material, which is denoted as ratio 3. Through X-ray diffraction analysis, the half-peak width FWHM(003) of the positive electrode lithium supplement material corresponding to the diffraction peak of the (003) crystal face in the X-ray diffraction pattern is 0.214°; wherein FWHM(003) = 0.0027d 2 + 0.13d + 0.13, d is the total thickness of the shell layer. The area of the part of the surface of the positive electrode lithium supplement matrix material where no shell layer exists accounts for 5% of the total area of the surface of the positive electrode lithium supplement matrix material, which is denoted as ratio 4.

[0106] 6) At 25°C and normal pressure (0.1 MPa), the positive electrode active material NCM622, conductive carbon black Super P, and binder polyvinylidene fluoride (PVDF) were mixed uniformly in N-methylpyrrolidone solvent at a mass ratio of 8:1:1 to obtain a positive electrode slurry with a solid content of 70%. The positive electrode lithium supplement material, conductive carbon black, and binder polyvinylidene fluoride (PVDF) in step 5) were mixed uniformly in N-methylpyrrolidone solvent at a mass ratio of 4:2:2 to obtain a positive electrode lithium supplement slurry with a solid content of 20%. The positive electrode lithium supplement slurry was coated on the current collector Al foil using a transfer coating method, and the positive electrode slurry loading surface density was controlled to be 15 mg / cm 2 2. The positive electrode slurry was pre-coated for 400 mm, and then the slurry nozzle was opened to spray the positive electrode lithium supplement slurry, with a spraying pressure of 0.5 MPa and a spraying speed of 10 mL / min. The coated positive electrode sheet was dried in an oven at 110°C to obtain a positive electrode sheet. As shown in FIG. 4, the positive electrode sheet includes a positive electrode current collector, a positive electrode active material layer existing on the surface of the positive electrode current collector, and a positive electrode lithium supplement layer existing on the side of the positive electrode active material layer away from the positive electrode current collector.

[0107] Example 2

[0108] Example 2 and the preparation method of the positive electrode sheet of Example 1 are basically the same, except that the one-time sintering temperature in step 5) is changed to 300°C.

[0109] Example 3

[0110] Example 3 was prepared in substantially the same manner as the positive electrode sheet of Example 1, except that the time of the first sintering in step 5) was changed to 16 h.

[0111] Example 4

[0112] Example 4 was prepared in substantially the same manner as the positive electrode sheet of Example 1, except that the temperature of the second sintering in step 5) was changed to 600°C.

[0113] Example 5

[0114] Example 5 was prepared in substantially the same manner as the positive electrode sheet of Example 1, except that the time of the second sintering in step 5) was changed to 16 h.

[0115] Example 6

[0116] Example 6 was prepared in substantially the same manner as the positive electrode sheet of Example 1, except that in step 6) the positive electrode slurry and the positive electrode lithium supplement slurry were mixed uniformly at a mass ratio of 8:2, and were coated by transfer coating, with a loading area density of 15 mg / cm 2 , and a coating speed of 1 mL / min. The obtained positive electrode sheet included a positive electrode active material layer, and the positive electrode lithium supplement material was present in the positive electrode active material layer.

[0117] Examples 7-25

[0118] Examples 7-25 were prepared in substantially the same manner as the positive electrode sheet of Example 1, except that one or more of the preparation conditions were changed.

[0119] Comparative Example 1

[0120] Comparative Example 1 was prepared in substantially the same manner as the positive electrode sheet of Example 1, except that the three times of deposition treatment in step 5) was not performed, and the chemical formula of the obtained positive electrode lithium supplement material was Li2C2O4.

[0121] Comparative Example 2

[0122] Comparative Example 2 was prepared in substantially the same manner as the positive electrode sheet of Example 5, except that the three times of deposition treatment in step 5) was not performed, and the chemical formula of the obtained positive electrode lithium supplement material was Li2C2O4.

[0123] Comparative Example 3

[0124] The preparation method of the positive electrode sheet of Comparative Example 3 is basically the same as that of Example 1, except that in step 5), the first deposition temperature is 350℃, the deposition rate is 0.035 μm / min; the second deposition temperature is 400℃, the deposition rate is 0.03 μm / min; the third deposition temperature is 450℃, the deposition rate is 0.02 μm / min, and the positive electrode lithium supplement material is obtained after the third deposition.

[0125] Comparative Example 4

[0126] The preparation method of the positive electrode sheet of Comparative Example 4 is basically the same as that of Example 1, except that in step 5), the first deposition temperature is 450℃, the deposition rate is 0.02 μm / min, the deposition time is 90 min, and the second and third depositions are not performed.

[0127] Comparative Example 5

[0128] The preparation method of the positive electrode sheet of Comparative Example 5 is basically the same as that of Example 1, except that in step 5), the first deposition temperature is 400℃, the deposition rate is 0.03 μm / min, the deposition time is 60 min, and the second and third depositions are not performed.

[0129] Comparative Example 6

[0130] The preparation method of the positive electrode sheet of Comparative Example 6 is basically the same as that of Example 1, except that in step 5), the first deposition temperature is 350℃, the deposition rate is 0.035 μm / min, the deposition time is 51 min, and the second and third depositions are not performed.

[0131] Test Example:

[0132] 1. Fluorine element content: The fluorine element content is measured by linear scanning from the center of the sphere to the outside using an energy dispersive spectrometer (EDS).

[0133] 2. Shell thickness: The shell thickness is measured by linear scanning from the center of the sphere to the outside using an energy dispersive spectrometer (EDS). Specifically, after depositing the first shell on the positive electrode lithium supplement substrate material, EDS line scanning is performed, and the thickness of the region where the fluorine element mass percentage is greater than 0.5% is measured to obtain the first shell thickness; after depositing the second shell on the positive electrode lithium supplement substrate material, EDS line scanning is performed, and the thickness of the region where the fluorine element mass percentage is greater than 1.5% is measured to obtain the first thickness, and the second shell thickness is obtained by subtracting the first shell thickness from the first thickness; after depositing the third shell on the positive electrode lithium supplement substrate material, EDS line scanning is performed, and the thickness of the region where the fluorine element mass percentage is greater than 3% is measured to obtain the second thickness, and the third shell thickness is obtained by subtracting the first thickness from the second thickness.

[0134] 3. D50: The particle size of the material is measured using a laser particle size analyzer.

[0135] 4. The half-peak width FWHM(003) of the diffraction peak corresponding to the (003) crystal face in the XRD pattern: the XRD pattern of the material is tested by X-ray diffraction, and the half-peak width FWHM(003) of the diffraction peak corresponding to the (003) crystal face is derived.

[0136] 5. The ratio of the area of the part of the surface of the positive electrode lithium supplement matrix material without a shell layer to the total area of the surface of the positive electrode lithium supplement matrix material: the surface coating ratio is obtained by counting the gray value of all the particles identified by the SEM particle statistical analysis software, and the uncoated ratio is 100% minus the surface coating ratio.

[0137] 6. The phase transition peak position: the phase transition peak positions I1 and I2 are obtained from the differential capacity (dQ / dV) curve derived from the cycle test of the prepared battery.

[0138] 7. Impedance: the impedance value is obtained by testing the impedance of the prepared battery on a Princeton impedance tester.

[0139] 8. Cycle performance: the positive electrode sheet in each example and the comparative example is used, the negative electrode is graphite, the separator is polypropylene, and the electrolyte (composition: 1 mol / L LiPF6, solvent: EC / DMC / DEC = v / v / v: 1 / 1 / 1) is used to form a full battery, which is charged at a charge rate of 1C to 4.35V at 25°C, and then charged at a constant voltage of 4.35V to a current rate of 0.5C, and then discharged at a discharge rate of 1C to 3V, and the process is repeated for 300 times, the discharge capacity Q1 at the first cycle and the discharge capacity Q300 at the 300th cycle are measured. 300 The capacity retention rate Q after 300 cycles is Q300 / Q1*100%. 300

[0140] Figure 1 is a cross-sectional SEM image of the positive electrode lithium supplement material prepared in Example 1 of the present application.

[0141] As can be seen from Figure 1, the content of fluorine element in the positive electrode lithium supplement material prepared in Example 1 of the present application is distributed in a gradient.

[0142] Figure 2 is a surface SEM image of the positive electrode lithium supplement material prepared in Example 1 of the present application.

[0143] As can be seen from Figure 2, the positive electrode lithium supplement material prepared in Example 1 of the present application has a uniformly coated shell layer on the surface.

[0144] Figure 3 is a graph showing the change of the content of fluorine element in the positive electrode lithium supplement material along the distance in the direction from the positive electrode lithium supplement matrix material to the shell layer.

[0145] ​As can be seen from FIG. 3, the content of the fluorine element in the positive electrode lithium supplement material prepared in Embodiment 1 of the present application increases in the direction from the positive electrode lithium supplement matrix material to the shell.

[0146] Table 1

[0147] Table 2

[0148] As can be seen from Tables 1-2, compared with the comparative examples, the positive electrode lithium supplement material of the present application can supplement lithium for the battery, make up for the loss of lithium ion concentration caused by the lithium ion battery in the cycle process, and the content of the fluorine element in the shell of the positive electrode lithium supplement material of the present application increases in the direction from the positive electrode lithium supplement matrix material to the shell, which can form a shell with gradually increasing compactness, effectively enhance the structural stability of the positive electrode lithium supplement material, and thus improve the cycle stability of the battery.

[0149] As can be seen from the comparison between Example 1 and Comparative Example 1, the positive electrode lithium supplement material without a shell has poor structural stability and fails to effectively activate dead lithium in the cycle process.

[0150] As can be seen from the comparison between Example 1 and Comparative Example 2, the positive electrode lithium supplement material without a shell blended with the positive electrode slurry has poor homogenizing and coating cycle stability, and the positive electrode lithium supplement material fails to play a role.

[0151] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical scope disclosed in the present application, which should be covered in the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A positive electrode lithium supplementing material, characterized by, The positive electrode lithium supplement matrix material comprises a shell layer existing on at least part of the surface of the positive electrode lithium supplement matrix material; the shell layer comprises fluorine elements, and the content of the fluorine elements in the shell layer has an increasing trend along the direction from the positive electrode lithium supplement matrix material to the shell layer.

2. The positive electrode lithium supplement material according to claim 1, wherein, The chemical formula of the shell layer is A a ZrF b wherein 0 < a ≤ 2, 4 ≤ b ≤ 6, A is selected from at least one of K, Na, NH4; and / or, the positive electrode lithium supplement matrix material has a chemical formula of Li x M y O z wherein, 0 3.The positive-electrode lithium supplementing material of claim 1 or 2, characterized in that, the shell layer comprises a first shell layer existing on the surface of the positive electrode lithium supplement matrix material, a second shell layer existing on the side of the first shell layer away from the positive electrode lithium supplement matrix material, and a third shell layer existing on the side of the second shell layer away from the positive electrode lithium supplement matrix material; the content of the fluorine elements in the first shell layer is less than that in the second shell layer, and the content of the fluorine elements in the second shell layer is less than that in the third shell layer. 4.The positive-electrode lithium supplementing material of claim 3, characterized in that, the thickness of the first shell layer is 0.05-0.6 μm, the thickness of the second shell layer is 0.3-0.75 μm, and the thickness of the third shell layer is 0.2-1 μm; and / or, the mass percentage of the fluorine elements in the first shell layer in the positive electrode lithium supplement material is 0.01%-1.5%, the mass percentage of the fluorine elements in the second shell layer in the positive electrode lithium supplement material is 1%-3%, and the mass percentage of the fluorine elements in the third shell layer in the positive electrode lithium supplement material is 3%-5%.

5. The positive-electrode lithium supplementing material according to any one of claims 1 to 4, characterized in that, the D50 of the positive electrode lithium supplement matrix material is 8-10 μm, and the total thickness d of the shell layer is 1-2.1 μm; And / or, by X-ray diffraction analysis, the positive electrode lithium supplementing material has a half-peak width FWHM(003) of 0.25-0.42° of the diffraction peak corresponding to the (003) crystal face in the X-ray diffraction pattern; wherein, FWHM(003)=0.0027d 2 +0.13d+0.13, d is the total thickness of the shell layer; and / or, the proportion of the area of the part of the surface of the positive electrode lithium supplement matrix material where the shell layer does not exist in the total area of the surface of the positive electrode lithium supplement matrix material is less than or equal to 5%. 6.The positive-electrode lithium supplementing material of any one of claims 1-5, characterized in that, In the differential capacity curve of the battery prepared from the positive electrode lithium supplement material after 0-200 cycles, the fluctuation range of the phase transition peak I1 in the interval of 4.1-4.3 V is 0%-2%, and the fluctuation range of the phase transition peak I2 in the interval of 3.4-3.7 V is 0%-5%.

7. A method for preparing the positive-electrode lithium supplementing material according to any one of claims 1 to 6, characterized in that, The method comprises the following steps: 1) performing primary sintering on a raw material system comprising a lithium source and an M source under the atmosphere of an inert gas at 300-500 ℃ for 8-16 h to obtain a first intermediate; 2) performing secondary sintering on the first intermediate under the atmosphere of an inert gas at 600-800 ℃ for 8-16 h to obtain a second intermediate; 3) performing primary deposition of a redox medium on the surface of the second intermediate to obtain a third intermediate; 4) performing secondary deposition of a redox medium on the surface of the third intermediate to obtain a fourth intermediate; 5) performing tertiary deposition of a redox medium on the surface of the fourth intermediate to obtain the positive electrode lithium supplement material.

8. The production method according to claim 7, characterized by, The temperature of the primary deposition is 400-450 ℃, the deposition rate is 0.01-0.02 μm / min, and the deposition time is 2.5-30 min; and / or, the temperature of the secondary deposition is 350-400 ℃, the deposition rate is 0.02-0.03 μm / min, and the deposition time is 10-25 min; And / or, the temperature of the third deposition is 300-350 DEG C, the deposition rate is 0.03-0.035 mu m / min, and the deposition time is 6-28 min.

9. A positive electrode sheet characterized by comprising: The positive electrode lithium supplement material prepared by the preparation method of claim 7 or 8.

10. The positive electrode sheet according to claim 9, characterized by The positive electrode sheet comprises a positive electrode current collector, a positive electrode active material layer present on the surface of the positive electrode current collector, and a positive electrode lithium supplement layer present on the side of the positive electrode active material layer away from the positive electrode current collector.

11. A battery, characterized by The positive electrode sheet of claim 9 or 10.

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