阅读说明：本技术 正极活性材料、正极极片及锂离子二次电池 (Positive electrode active material, positive electrode plate and lithium ion secondary battery ) 是由 钭舒适 胡春华 蒋耀 吴奇 何金华 邓斌 于 2019-09-02 设计创作，主要内容包括：本发明公开了一种正极活性材料、正极极片及锂离子二次电池。正极活性材料的颗粒包括二次颗粒及包覆于二次颗粒外表面的包覆层,二次颗粒包括含掺杂元素M~1的锂过渡金属氧化物,包覆层包括M~2元素的氧化物,M~1选自Si、Ti、Cr、Mo、V、Ge、Se、Zr、Nb、Ru、Rh、Pd、Sb、Te、Ce及W中的一种或多种,M~2选自Mg、Al、Ca、Ce、Ti、Zr、Zn、Y及B中的一种或多种；二次颗粒中任意一点的M~1元素质量浓度偏差为20％以下；二次颗粒由颗粒核心至外表面包含多层沿二次颗粒的径向排布的一次颗粒,多层中的最外层含有一次颗粒的数量为5个/μm~2～50个/μm~2。采用本发明的正极活性材料和正极极片,能使锂离子二次电池同时兼顾较高的能量密度、高温存储性能及高温循环性能。(The invention discloses a positive active material, a positive pole piece and a lithium ion secondary battery. The particles of the positive electrode active material include secondary particles and a coating layer coated on the outer surface of the secondary particles, and the secondary particles include a doping element M 1 The coating layer comprises M 2 Oxides of the elements, M 1 One or more selected from Si, Ti, Cr, Mo, V, Ge, Se, Zr, Nb, Ru, Rh, Pd, Sb, Te, Ce and W, M 2 One or more selected from Mg, Al, Ca, Ce, Ti, Zr, Zn, Y and B; m at any point in the secondary particles 1 The deviation of the element mass concentration is less than 20%; the secondary particle comprises a plurality of layers of primary particles arranged in the radial direction of the secondary particle from the particle core to the outer surface, and the outermost layer of the plurality of layers contains the primary particles in an amount of 5 particles/. mu.m 2 50 pieces/mum 2 . The positive active material and the positive pole piece can simultaneously give consideration to higher energy density, high-temperature storage performance and high-temperature cycle performance of the lithium ion secondary battery.)

1. A positive electrode active material, characterized in that particles of the positive electrode active material comprise secondary particles and a coating layer coated on the outer surface of the secondary particles, the secondary particles comprise lithium transition metal oxide, and the coating layer comprises M²An oxide of an element, particles of the positive electrode active material being a compound represented by chemical formula (1),

Li_1+a[Ni_xCo_yMn_zM¹ _bM² _c]O_2-dX_dchemical formula (1)

In the chemical formula (1), M¹Doping the transition metal site of the lithium transition metal oxide with an element, M¹One or more selected from Si, Ti, Cr, Mo, V, Ge, Se, Zr, Nb, Ru, Rh, Pd, Sb, Te, Ce and W, X is an oxygen site doping element of the lithium transition metal oxide, X is one or more selected from F, Cl, Br, I, S, N and P, M²Is an element of the coating layer, M²Is selected from one or more of Mg, Al, Ca, Ce, Ti, Zr, Zn, Y and B, and x is more than or equal to 0.5<1，0<y≤0.3，0≤z≤0.3，-0.1<a<0.2，0<b<0.3，0<c<0.3，0≤d<0.2，0<b+c<0.3，x+y+z+b＝1；

M at any point in the secondary particles¹The deviation of the element mass concentration is less than 20%;

the secondary particle comprises a plurality of layers of primary particles arranged in the radial direction of the secondary particle from the particle core to the outer surface, and the outermost layer of the plurality of layers contains the primary particles in an amount of 5 particles/μm²50 pieces/mum²。

2. The positive electrode active material according to claim 1, wherein M in the positive electrode active material¹The content of element and M in the secondary particle¹The variation epsilon of the average mass concentration of the elements is 30% or less, preferably 20% or less.

3. The positive electrode active material according to claim 1Characterized in that the specific surface area of the positive electrode active material is 0.2m²/g～1m²A/g, preferably of 0.3m²/g～0.8m²/g。

4. The positive electrode active material according to claim 1, wherein the positive electrode active material has a compacted density of 3.0g/cm at a pressure of 5 tons³Above, preferably 3.1g/cm³～3.8g/cm³。

5. The positive electrode active material according to claim 1, wherein the average particle diameter D of the positive electrode active material_v50 is 6 to 20 μm, preferably 8 to 15 μm.

6. The positive electrode active material according to any one of claims 1 to 5, wherein the primary particles have a length of 100nm to 1000nm and a width of 50nm to 400 nm;

preferably, the aspect ratio of the primary particles is 2 to 20.

7. The positive electrode active material according to any one of claims 1 to 6, wherein M is in the positive electrode active material¹The content of the element is 200ppm to 6000ppm, and M in the positive active material¹Element and M²The total content of the elements is 1000 ppm-10000 ppm.

8. The positive electrode active material according to claim 1,

when the positive active material is in 78% delithiated state, M¹The valence of the element is +3 or more, preferably one or more of +4, +5, +6, +7, and + 8;

and/or, when the positive active material is in a 78% delithiated state, the M¹The element has more than two different valence states, and the M in the highest valence state²The valence of the element is one or more of +4 valence, +5 valence, +6 valence, +7 valence and +8 valence.

9. A positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode active material according to any one of claims 1 to 8.

10. A lithium-ion secondary battery characterized by comprising the positive electrode sheet according to claim 9.

Technical Field

The invention belongs to the technical field of secondary batteries, and particularly relates to a positive active material, a positive pole piece and a lithium ion secondary battery.

Background

A lithium ion secondary battery is a rechargeable battery that mainly operates by movement of lithium ions between a positive electrode and a negative electrode, and is a clean energy source that is currently widely used. The positive electrode active material is used as an important component of the lithium ion secondary battery and provides lithium ions which move in a reciprocating manner between the positive electrode and the negative electrode in the charging and discharging process of the battery, so that the positive electrode active material is very important for the performance of the battery.

As lithium ion secondary batteries are increasingly used in various electronic products, electric appliances, and electric vehicles, higher requirements are also placed on the energy density of the batteries. Therefore, how to further increase the energy density of the lithium ion secondary battery becomes a technical problem to be solved urgently.

Disclosure of Invention

The present inventors found that nickel-containing lithium transition metal oxide has a high theoretical capacity, and a lithium ion secondary battery using the nickel-containing lithium transition metal oxide as a positive electrode active material can achieve a high energy density, but studies have found that the high-temperature cycle performance of the lithium ion secondary battery is poor.

The present inventors have conducted extensive studies with a view to improving conventional positive active materials so that they have a high specific capacity and, at the same time, can improve their own stability, thereby providing a lithium ion secondary battery that can simultaneously achieve high energy density, high-temperature storage performance, and high-temperature cycle performance.

Accordingly, the present invention provides, in a first aspect, a positive electrode active material, particles of which include secondary particles including a lithium transition metal oxide, and a coating layer coated on outer surfaces of the secondary particles, the coating layer including M²An oxide of an element, particles of the positive electrode active material being a compound represented by chemical formula (1),

Li_1+a[Ni_xCo_yMn_zM¹ _bM² _c]O_2-dX_dchemical formula (1)

In the formula (1), M¹Doping element for transition metal site of lithium transition metal oxide, M¹One or more selected from Si, Ti, Cr, Mo, V, Ge, Se, Zr, Nb, Ru, Rh, Pd, Sb, Te, Ce and W, X is an oxygen site doping element of lithium transition metal oxide, X is one or more selected from F, Cl, Br, I, S, N and P, M²Is an element of the coating layer, M²Is selected from one or more of Mg, Al, Ca, Ce, Ti, Zr, Zn, Y and B, and x is more than or equal to 0.5<1，0<y≤0.3，0≤z≤0.3，-0.1<a<0.2，0<b<0.3，0<c<0.3，0≤d<0.2，0<b+c<0.3，x+y+z+b＝1；

M at any point in the secondary particles¹The deviation of the element mass concentration is less than 20%;

the secondary particle comprises a plurality of layers of primary particles arranged in the radial direction of the secondary particle from the particle core to the outer surface, and the outermost layer of the plurality of layers contains the primary particles in an amount of 5 particles/. mu.m²50 pieces/mum²。

The invention provides a positive electrode plate, which comprises a positive electrode current collector and a positive electrode active material layer arranged on the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode active material according to the first aspect of the invention.

The third aspect of the invention provides a lithium ion secondary battery comprising the positive electrode sheet according to the second aspect of the invention.

Compared with the prior art, the embodiment of the invention at least has the following beneficial effects:

the positive active material provided by the invention comprises a lithium transition metal oxide containing nickel and cobalt, and M is uniformly doped in the lithium transition metal oxide¹The element enables the positive active material to have higher specific capacity, so that the battery adopting the positive active material has higher energy density. The secondary particles of the positive active material have proper primary particle size and distribution, and can improve the compaction density and capacity exertion of the positive active material, so that the specific capacity of the positive active material is further improved; and simultaneously, the mechanical strength of the positive active material is improved. Uniformly doped M¹The element can further improve the stability of the whole structure of the high-compaction-density positive active material in the high-temperature storage and high-temperature circulation processes, so that the positive active material is not easy to crack. Furthermore, M²The oxide coating layer of the element can play a role in protecting secondary particles, so that the positive active material is not corroded by the electrolyte, and the side reaction of the electrolyte on the surface of the positive active material is reduced. Therefore, the high-temperature storage performance and the high-temperature cycle performance of the battery can be obviously improved by adopting the positive active material.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments of the present invention will be briefly described below, and it is obvious that the drawings described below are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.

Fig. 1 is a Cross-section of a positive electrode active material according to an embodiment of the present invention prepared by using a Cross-section polishing apparatus (CP), and a doping element distribution image obtained by using an Energy Dispersive Spectroscopy (EDS), where bright spots indicate doping elements, and the doping elements are uniformly distributed in particles.

Fig. 2 is an SEM (Scanning Electron Microscope) image of the positive electrode active material according to the embodiment of the present invention.

FIG. 3 shows M at any point in a secondary particle according to the present invention¹Elemental mass concentration bias testSchematic diagram of the point-taking position of (1).

Detailed Description

In order to make the objects, technical solutions and advantageous technical effects of the present invention more clear, the present invention is further described in detail with reference to the following embodiments. It should be understood that the embodiments described in this specification are only for the purpose of explaining the present invention and are not intended to limit the present invention.

For the sake of brevity, only some numerical ranges are explicitly disclosed herein. However, any lower limit may be combined with any upper limit to form ranges not explicitly recited; and any lower limit may be combined with any other lower limit to form a range not explicitly recited, and similarly any upper limit may be combined with any other upper limit to form a range not explicitly recited. Also, although not explicitly recited, each point or individual value between endpoints of a range is encompassed within the range. Thus, each point or individual value can form a range not explicitly recited as its own lower or upper limit in combination with any other point or individual value or in combination with other lower or upper limits.

In the description herein, it is to be noted that, unless otherwise specified, "above" and "below" are inclusive, and "a plurality" of "one or more" means two or more, "and" a plurality "of" one or more "means two or more.

The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The following description more particularly exemplifies illustrative embodiments. At various points throughout this application, guidance is provided through a list of embodiments that can be used in various combinations. In each instance, the list is merely a representative group and should not be construed as exhaustive.

Positive electrode active material

The embodiment of the invention provides a positive electrode active material. The particles of the positive electrode active material include secondary particles and a coating layer coating the outer surfaces of the secondary particles.

The secondary particles are formed by aggregating primary particles. The secondary particle comprises a plurality of layers of primary particles from the particle core to the outer surface, and the primary particles in each layer are arranged in the radial direction of the secondary particle. Wherein the outermost layer of the plurality of layers contains primary particles in an amount of 5 particles/. mu.m²50 pieces/mum²Further, 10 pieces/μm²45 pieces/. mu.m²And further 20 pieces/mum²40 pieces/. mu.m²。

The secondary particles comprising a material doped with M¹Lithium transition metal oxide of the element, the coating layer comprising M²An oxide of the element. The particles of the positive electrode active material are a compound represented by chemical formula (1).

Li_1+a[Ni_xCo_yMn_zM¹ _bM² _c]O_2-dX_dChemical formula (1)

In the chemical formula (1), x is more than or equal to 0.5 and less than 1, y is more than or equal to 0 and less than or equal to 0.3, z is more than or equal to 0 and less than or equal to 0.3, a is more than 0.1 and less than 0.2, b is more than 0 and less than 0.3, c is more than 0 and less than 0.3, d is more than or equal to 0 and less than 0.2, b and c are more than 0.3, and x, y, z and b are equal to 1.

M¹Doping element for transition metal site of lithium transition metal oxide, M¹One or more selected from Si, Ti, Cr, Mo, V, Ge, Se, Zr, Nb, Ru, Rh, Pd, Sb, Te, Ce and W. Doping element M¹Are uniformly distributed in the secondary particles (as shown in fig. 1). Specifically, M at any point in the secondary particles¹The deviation of the element mass concentration is less than 20%.

X is an oxygen site doping element of the lithium transition metal oxide, and is selected from one or more of F, Cl, Br, I, S, N and P.

M²Is an element of the coating layer, M²One or more selected from Mg, Al, Ca, Ce, Ti, Zr, Zn, Y and B.

Herein, the coating layer on the outer surface of the secondary particle is a small amount of coating, and the profile of the primary particle on the outer surface of the secondary particle can be discriminated in the detection. The number of primary particles contained in the outermost layer of the secondary particles can be measured by the following method. The method comprises the following steps:

sampling positive electrode active material particles including secondary particles, and then samplingPerforming SEM (Scanning Electron Microscope) detection on the outer surface of the product at a magnification of 10K to obtain an SEM image (shown in FIG. 2), wherein the SEM image is full of primary particles; then, from the SEM image, the number of primary particles κ in the outermost layer of the secondary particles was calculated by the following formula (2) in units of one/μm²。

In the formula (2), x₁Represents the number of primary particles in the lateral direction of the lower edge in an SEM image of the positive electrode active material particles by a factor of 10K; x is the number of₂Represents the number of primary particles in the upper edge in the lateral direction in an SEM image of positive electrode active material particles by a factor of 10K; y is₁Represents the number of primary particles in the longitudinal direction of the left edge in an SEM image of positive electrode active material particles by a factor of 10K; y is₂Represents the number of primary particles in the vertical direction of the right edge in an SEM image of positive electrode active material particles by a factor of 10K; α represents a transverse actual measurement length in mm of the SEM image of the positive electrode active material particles by a factor of 10K; β represents a longitudinal actual measurement length in mm of the SEM image of the positive electrode active material particles by a factor of 10K; λ represents the corresponding actual measured length in mm/μm at a scale of 1 μm in the SEM image of the positive electrode active material particles at a magnification of 10K. When the number of primary particles in the SEM image of the positive electrode active material particles by a factor of 10K was calculated, as long as a part of the primary particles appeared, one primary particle was counted.

The outermost layer of the secondary particles contains the primary particles in an amount capable of reflecting the size and distribution of the primary particles in the secondary particles.

Here, M is present at any point in the secondary particles¹The mass concentration of the element is in the one-point minimum range, M¹The mass concentration of the elements in all elements can be determined by EDX (Energy Dispersive X-Ray Spectroscopy) or EDS (EDS elemental analysis) in combination with TEM (Transmission Electron Microscope) or SEM (Scanning Electron Microscope)Microscope) single point scan test element concentration profile or other similar means. Wherein the M in μ g/g at different sites in the secondary particles when measured by EDX or EDS elemental analysis in combination with TEM or SEM single point scanning¹The mass concentrations of the elements are respectively denoted as eta₁、η₂、η₃、…、η_nAnd n is a positive integer greater than or equal to 15 (as shown in FIG. 3).

M in the secondary particles¹The average mass concentration of the element being M within single or multiple secondary particles¹The mass concentration of the elements in all elements can be obtained by EDX or EDS element analysis combined with TEM or SEM surface scanning to test the element concentration distribution or other similar modes. Where the test surface includes all of the test sites in the above point test (as shown in figure 3) when tested in a combination of EDX or EDS elemental analysis with TEM or SEM plane scanning of the test element concentration distribution. M in the secondary particles¹The mean mass concentration of the elements is recorded asThe unit is μ g/g.

M at any point in the secondary particles¹The mass concentration deviation σ of the element is calculated according to equation (3):

the positive active material provided by the present invention includes a lithium transition metal oxide containing nickel cobalt, which has a higher theoretical capacity, thereby enabling a battery having a higher energy density to be obtained. Meanwhile, the lithium transition metal oxide is uniformly doped with M¹And (4) elements. M¹The element can have a larger valence and can contribute more electrons to the positive active material, thereby supporting the positive active material to release more lithium ions during the charging process of the battery. Thereby, the specific capacity of the positive electrode active material is further improved, and the energy density of the battery is further improved.

The inventor researches and discovers that the size and distribution of primary particles in secondary particles directly influence key indexes such as the transmission performance of lithium ions in the positive active material, the compaction density of the positive active material and the like. The secondary particles have improved primary particle distribution and size, and can improve the transmission performance of lithium ions in the positive active material particles, thereby improving the capacity exertion of the positive active material. In addition, the secondary particles also provide the positive electrode active material with a higher compacted density. The positive active material has higher compacted density and capacity exertion, and can further improve the specific capacity of the positive active material. Thereby, the energy density of the battery is further improved.

The secondary particles have proper primary particle distribution and size, the mechanical strength of the positive active material can be obviously improved, and the problem of particle breakage in the processes of pole piece rolling and high-temperature cyclic charge and discharge of the battery is avoided. In the battery adopting the positive active material, the side reaction generated by a large amount of fresh surfaces exposed by the broken positive active material particles is avoided, thereby reducing the gas production of the battery in the high-temperature storage and high-temperature circulation processes and inhibiting the impedance increase of the battery. The high-temperature storage performance and the high-temperature cycle performance of the battery can be improved by adopting the positive active material.

The inventors have found that the selection of a suitable doping element M¹The method can also effectively inhibit the positive active material from generating irreversible structural phase change from a layered phase to a spinel phase and then to a halite phase, thereby improving the structural stability of the positive active material in high-temperature storage and high-temperature circulation. And, M mentioned above¹The bonding bond energy of the element and the oxygen atom is very large, and the oxygen atom can be effectively bound, so that the oxygen release of the anode active material is not easy to occur in the high-temperature storage and high-temperature circulation processes. The gas generated by the anode active material is reduced, and meanwhile, the gas generated by the oxidation decomposition of the electrolyte caused by oxygen release is reduced, so that the battery flatulence phenomenon caused by the anode active material is effectively inhibited.

The inventors have also found that M is present at any point in the secondary particles¹The mass concentration deviation sigma of the element is below 20%, so that the properties of all parts in the secondary particles are consistent, the structural stability of all parts of the particles can be effectively improved, and oxygen release of all parts of the particles can be inhibited. M¹Secondary particles of homogeneous doping of elementsThe deformation resistance of each part inside the positive electrode active material is consistent, so that the stress distribution of each part inside the particles is uniform, the stability of the whole structure of the particles is high, and the positive electrode active material particles with high compaction density are not easy to crack, thereby further reducing the side reaction of electrolyte caused by the exposed fresh surface of the particles due to cracking. Furthermore, the lithium ion is in M¹The migration and diffusion capacities of different areas in the uniformly doped particles are in the same level, which is beneficial to improving the capacity exertion of the positive active material.

By selecting a suitable M¹The element uniformly dopes and modifies the secondary particles, so that the overall stability of the positive active material in high-temperature storage and high-temperature circulation of the battery is higher, and the positive active material has better lithium ion transmission performance. By adopting the positive active material, the gas production rate of the battery in the high-temperature storage and high-temperature circulation processes is less, the impedance is lower, and the capacity is better exerted, so that the energy density, the high-temperature storage performance and the high-temperature circulation performance of the battery are further improved.

In addition, the outer surface of the secondary particle is further coated with a coating layer comprising M²An oxide of the element. The coating can play the guard action to the secondary particle, and isolated secondary particle is not corroded by electrolyte with the contact of electrolyte, effective protection positive pole active material to reduce the side reaction on material surface, reduce battery gas production. Thereby, the high-temperature cycle performance and the high-temperature storage performance of the battery are effectively improved.

The lithium ion secondary battery with excellent comprehensive performance can be obtained by adopting the anode active material. The battery can simultaneously give consideration to higher energy density, high-temperature storage performance and high-temperature cycle performance.

In some preferred embodiments, M in the positive active material¹Content of element omega and M in secondary particles¹Average mass concentration of elementsHas a deviation of epsilon, epsilon satisfies epsilon<50 percent. Preferably, ε ≦ 30%. More preferably, ε ≦ 20%.

Active anodeIn the material M¹Content of element omega and M in secondary particles¹Average mass concentration of elementsThe deviation of (d) is calculated by the following formula (4):

omega is M in ppm in the positive electrode active material¹The overall mass concentration of the element, i.e., M in μ g per gram of the positive electrode active material¹The mass of the element. Wherein, ω represents M in the macroscopic positive electrode active material¹Bulk content of elements, including M, incorporated in secondary particles¹Element, M enriched in other phases on the surface of the secondary particles¹Element, and M embedded between particles of positive electrode active material¹And (4) elements. ω can be obtained by measurement of absorption spectrum of the positive electrode active material solution, such as ICP (inductively Coupled Plasma Emission Spectrometer), XAFS (X-ray absorption fine structure spectrum), and the like.

M in the positive electrode active material¹Content of element omega and M in secondary particles¹Average mass concentration of elementsThe deviation ε of (A) is in the above range, meaning that M is¹Elements can be smoothly doped into the secondary particles, doping elements distributed in other phases on the surface of the secondary particles and M embedded between gaps of the positive active material¹The content of the elements is less. The anode active material has good macro and micro consistency, uniform structure and high particle stability, and is beneficial to enabling the anode active material to have higher capacity exertion and high-temperature cycle performance.

In some embodiments, M in the positive active material¹The content of the element may be 200ppm to 8000ppm, preferably 200ppm to 6000ppm, more preferably 1000ppm to 5000 ppm.

Herein, ppm (parts per million) is a mass of a specific element in the positive electrode active material in parts per million of the mass of the positive electrode active material.

M in the positive electrode active material¹The element content is proper, and M can be effectively exerted¹The element has the functions of improving the stability of the positive active material and compensating charges, simultaneously can keep good layered structure of secondary particles, ensures that the positive active material provides good carriers for the de-intercalation of lithium ions, and prevents reversible lithium ions from being consumed on the surface of an electrode or in electrolyte. The positive active material can effectively reduce the irreversible capacity of the battery, so that the battery has higher initial capacity and cycle capacity retention rate.

Further, M in the positive electrode active material¹Element and M²The total content of elements may be 1000ppm to 12000ppm, preferably 1000ppm to 10000ppm, more preferably 3000ppm to 8000 ppm.

M in the positive electrode active material¹Element and M²The content of the element in the above range can better exert M¹Element and M²The foregoing effects of the elements, while allowing the battery to have a higher energy density.

In some preferred embodiments, M is 78% delithiated of the positive electrode active material¹The valence of the element is +3 or more, preferably one or more of +4, +5, +6, +7, and + 8.

Herein, the "78% delithiated state" refers to a state in which the molar content of lithium extracted from the positive electrode active material during charging of the battery is 78% of the theoretical lithium content. In the actual use process of the secondary battery, a "full charge state" is generally set, and a "charge cut-off voltage" is correspondingly set so as to ensure the safe use of the battery. The "fully charged State" means that the State of Charge (SOC) of the secondary battery is 100%, in other words, the secondary battery having the positive electrode composition including the positive electrode active material described above is charged to the Charge cut-off voltage within the range in which reversible Charge and discharge are performed. The "fully charged state" or "charge cut-off voltage" may vary depending on the positive electrode active material or the safety requirement. When a secondary battery prepared from the nickel-cobalt-containing lithium transition metal oxide positive active material is in a full charge state, the lithium removal state of the positive active material is about 78% lithium removal state generally, so that normal use is ensured.

In this context, the correspondence between "delithiated state" and charging voltage is combined to conduct a study to obtain a positive electrode active material in "78% delithiated state". Specifically, a series of batteries using the positive electrode active material are charged to 2.8V, 2.9V, 3.0V, 3.1V, 3.2V, 3.3V, …, 4.0V, 4.1V, 4.2V, 4.3V, 4.4V, 4.5V, 4.6V, and 4.7V (i.e., the charging voltage interval is 0.1V) at a rate of 0.1C, the positive electrode sheet of the battery is removed, the electrolyte is removed by washing, the positive electrode active material is digested, the mass concentrations of Li, transition metal, and O elements of the positive electrode active material are measured by an inductively coupled plasma-Emission spectrometer (ICP-OES), the stoichiometric ratio of each element of the positive electrode active material at the charging voltage is calculated, and the chemical formula of the positive electrode active material at the charging voltage is converted, and the lithium removal voltage corresponding to 78% is obtained.

And charging the battery containing the anode active material to be detected to the voltage corresponding to the 78% delithiation state, namely disassembling the battery to obtain the anode active material in the 78% delithiation state for further research. M in "78% delithiated" positive electrode active material¹The valence of the element can be obtained by X-ray photoelectron spectroscopy (XPS) test. More precisely, it can be obtained by Synchrotron radiation photoelectron spectroscopy (SRPES) test.

M in the positive active material in 78% delithiated state¹The valence of the element is higher when M¹The element can better bind oxygen atoms, so that the oxygen atoms are kept at the original lattice position, oxygen release of the anode active material after lithium removal in the heating and high-temperature circulation processes is prevented, and irreversible structural phase change is inhibited, so that the structural stability and the high-temperature circulation stability of the anode active material are further improved. And the M¹Element can be at the positive electrodeThe active material provides more electrons, so that the structure of the positive active material can be more stabilized, the surface activity of the positive active material is reduced, and the decomposition gas production of the electrolyte in the high-temperature circulation and high-temperature storage processes is reduced. Therefore, the high-temperature cycle performance and the high-temperature storage performance of the battery are improved. Furthermore, M¹The electrons contributed by the elements can support the positive active material to release more lithium ions, so that the energy density of the battery is further improved.

It can be understood that, in the positive electrode active material before and after delithiation, M is¹The valence of the element may be unchanged, M²The element does not participate in the redox reaction during charging of the battery. These M¹The element can stabilize the layered crystal structure of the positive electrode active material.

M in the positive electrode active material¹The element may also participate in redox reactions during battery charging, M¹The element has more than two stable valence states and is in a lower valence state in the positive active material before delithiation, and during the charging of the battery, M¹The element donates electrons to the positive electrode active material and the valence state is increased. During charging of the battery, M¹The electrons contributed by the elements enable charge compensation to occur in the material, and the quantity of lithium ions which can be extracted from the positive active material can be increased, so that the capacity performance and the energy density of the battery are improved. At the same time, M after valence state promotion¹The element can strengthen the constraint on oxygen atoms, improve the structural stability of the positive active material, reduce the surface activity of the positive active material and improve the high-temperature cycle performance and the high-temperature storage performance of the battery.

In some embodiments, M in the "78% delithiated" positive electrode active material¹The element may have more than two different valences, and M in the highest valency state¹The valence of the element is one or more of +4 valence, +5 valence, +6 valence, +7 valence and +8 valence. M with higher valence state and with changed valence state¹The element can contribute more electrons in the positive active material, further stabilize the material structure and reduce the side reaction on the surface of the material, thereby further improving the high-temperature cycle performance and the high-temperature storage performance of the battery。

Further, M is present in the 78% delithiated state of the positive electrode active material¹The elements having more than two different valences, wherein M in the lower valences¹The element can further contribute electrons to support the release of more lithium ions from the positive electrode, thereby further improving the energy density of the battery.

The compacted density of the positive electrode active material of the present invention at a pressure of 5 tons is preferably 3.0g/cm³Above, more preferably 3.1g/cm³～3.8g/cm³. The positive active material has higher compaction density, and can improve the energy density of the battery.

The size and distribution of the primary particles in the secondary particles also directly affect the indexes such as the specific surface area of the positive electrode active material. The distribution and size of the primary particles in the secondary particles are proper, and the positive electrode active material has a proper specific surface area. The specific surface area of the positive active material is in a proper range, so that the electrochemical performance of the positive active material can be improved, and the capacity exertion of the battery can be improved; and the side reaction of the electrolyte on the surface of the positive active material is reduced, and the cycle life of the battery is prolonged. In addition, the specific surface area of the positive active material is proper, so that the particles are not easy to agglomerate in the processes of preparing slurry and charging and discharging, and the capacity performance and the cycle performance of the battery are improved.

In some embodiments, the specific surface area of the positive electrode active material may be 0.1m²/g～1.5m²(ii) in terms of/g. Preferably, the specific surface area of the positive electrode active material is 0.2m²/g～1m²G, more preferably 0.3m²/g～0.8m²/g。

In some embodiments, it is preferred that the primary particles have a length of 100nm to 1000nm and a width of 50nm to 400 nm. In this context, the length of the primary particle refers to the largest dimension of the primary particle. The extending direction of the maximum dimension is defined as a longitudinal direction, and the width of the primary particle refers to the maximum dimension in a direction perpendicular to the longitudinal direction thereof.

The size of the primary particles in the secondary particles is within the above range, so that the positive electrode active material has high lithium ion transport performance, high compaction density, appropriate specific surface area and high mechanical strength. The energy density, the high-temperature storage performance and the high-temperature cycle performance of the battery can be further improved by adopting the positive active material.

In some embodiments, the aspect ratio of the primary particles in the secondary particles is preferably from 2 to 20, such as from 5 to 15, and further such as from 8 to 12. The aspect ratio of the primary particles is the ratio of the length to the width of the primary particles. The aspect ratio of the aforementioned primary particles is suitable for giving the positive electrode active material particles a better overall performance, thereby improving the electrochemical performance of the battery.

In some embodiments, the average particle diameter D of the positive electrode active material_v50 may be 3 to 25 μm, preferably 6 to 20 μm, and more preferably 8 to 15 μm.

Average particle diameter D of positive electrode active material_v50 in a proper range, the lithium ion and electron transmission and diffusion performance in the positive active material is improved, and the side reaction of the electrolyte on the surface of the positive active material is reduced, so that the cycle performance and the rate performance of the battery are improved. In addition, the average particle diameter D of the positive electrode active material_v50 also enables a higher compacted density of the positive active material, thereby increasing the energy density of the battery.

In the cathode active material of some embodiments, the oxygen sites of the lithium transition metal oxide are also optionally doped with an element X, i.e., 0< d <0.2 in formula (1). The positive active material is doped with a preset amount of X element, so that the structural stability of the positive active material can be further improved, and the side reaction on the surface of particles can be reduced, thereby further improving the high-temperature cycle performance and the high-temperature storage performance of the battery.

In some preferred embodiments, 0.6. ltoreq. x <1, such as 0.7. ltoreq. x. ltoreq.0.95 in formula (1) of the lithium transition metal oxide. The lithium nickel cobalt manganese composite oxide with high nickel content has higher gram capacity, and can enable the battery to show higher energy density.

In some embodiments, the morphology of the secondary particles may be spherical or spheroidal.

Herein, the average particle diameter D of the positive electrode active material_v50The term "median particle diameter" is used in a well-known manner in the art to mean a particle diameter corresponding to 50% of the volume distribution of particles of the positive electrode active material. Average particle diameter D of positive electrode active material_v50 may be measured using apparatus and methods known in the art, for example conveniently using a laser particle size analyser, such as the Mastersizer 3000 laser particle size analyser from malvern instruments ltd, uk.

The specific surface area of the positive electrode active material is a known meaning in the art and can be measured by an apparatus and a method known in the art, for example, by a nitrogen adsorption specific surface area analysis test by a NOVA 2000e model specific surface area and pore size analyzer of corna, usa and can be calculated by a bet (brunauer Emmett teller) method. As a specific example, the test method is as follows: and taking 8.000-15.000G of the anode active material by using a weighed empty sample tube, uniformly stirring and weighing the anode active material, placing the sample tube into an NOVA 2000e degassing station for degassing, weighing the total mass of the degassed anode active material and the sample tube, and subtracting the mass of the empty sample tube from the total mass to calculate the mass G of the degassed anode active material. And (3) putting the sample tube into NOVA 2000e, measuring the adsorption quantity of nitrogen on the surface of the positive active material under different relative pressures, calculating the adsorption quantity of a monomolecular layer based on the Bronuore-Eltt-Taylor multilayer adsorption theory and the formula thereof, further calculating the total surface area A of the positive active material, and calculating the specific surface area of the positive active material through A/G.

The compacted density of the positive electrode active material can be conveniently determined using instruments and methods known in the art, for example, using an electronic pressure tester, such as an electronic pressure tester model UTM 7305.

M in the positive electrode active material¹Element, M²The content of both the element and the X element can be measured by an apparatus and a method known in the art, and can be measured, for example, by absorption spectroscopy of a solution of the positive electrode active material, such as ICP (inductively Coupled Plasma Emission Spectrometer), XAFS (X-ray absorption fine structure)Spectrum), etc.

Next, the present invention also provides a method for preparing the positive active material. Any one of the above positive electrode active materials can be prepared according to the preparation method. It is to be understood that the cathode active material of the present invention is not limited to being obtained by the preparation method.

The preparation method comprises the following steps:

and S10, providing a mixture, and sintering the mixture to obtain secondary particles. The mixture comprises a positive active material precursor, a lithium source and M¹Precursors of the elements.

S20, mixing the secondary particles with M²And mixing precursors of the elements, and sintering to obtain the positive active material.

In step S10, the positive active material precursor may be one or more of an oxide, a hydroxide, and a carbonate containing Ni, Co, and optionally Mn in a stoichiometric ratio, for example, a hydroxide containing Ni, Co, and Mn in a stoichiometric ratio.

The positive electrode active material precursor may be obtained by a method known in the art, for example, by a coprecipitation method, a gel method, or a solid phase method.

When the precursor of the positive electrode active material is prepared, the structure of the precursor of the positive electrode active material can be regulated and controlled in a plurality of theoretically feasible ways, so that the size and distribution of primary particles in secondary particles of the positive electrode active material are improved. For example, when the precursor of the positive electrode active material is prepared by a coprecipitation method, the structure of the precursor of the positive electrode active material can be controlled by adjusting, for example, the selection of reaction raw materials, the pH value of a reaction solution, the concentration of a metal salt, the concentration of a complexing agent, the reaction temperature, the reaction time, and the like in the preparation of the precursor of the positive electrode active material.

As an example of preparing a precursor of a positive active material by a coprecipitation method, a Ni source, a Co source, and optionally a Mn source are dispersed in a solvent to obtain a mixed solution; simultaneously pumping the mixed solution, the strong base solution and the complexing agent solution into a reaction kettle with stirring by adopting a continuous parallel flow reaction mode, controlling the pH value of the reaction solution to be 10-13, controlling the temperature in the reaction kettle to be 25-90 ℃, and introducing inert gas for protection in the reaction process; after the reaction is finished, the hydroxide containing Ni, Co and optional Mn is obtained after aging, filtration, washing and vacuum drying.

The Ni source may be a soluble nickel salt, such as one or more of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate, further such as one or more of nickel sulfate and nickel nitrate, further such as nickel sulfate. The Co source may be a soluble cobalt salt, such as one or more of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate, and cobalt acetate, such as one or more of cobalt sulfate and cobalt nitrate, such as cobalt sulfate. The Mn source may be a soluble manganese salt, such as one or more of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate and manganese acetate, further such as one or more of manganese sulfate and manganese nitrate, further such as manganese sulfate.

The strong base may be one or more of LiOH, NaOH, and KOH, for example NaOH.

The complexing agent can be one or more of ammonia water, ammonium sulfate, ammonium nitrate, ammonium chloride, ammonium citrate and disodium Ethylene Diamine Tetraacetate (EDTA), such as ammonia water.

The solvent of the mixed solution, the strong alkali solution and the complexing agent solution is not particularly limited, for example, the solvent of the mixed solution, the strong alkali solution and the complexing agent solution is one or more of deionized water, methanol, ethanol, acetone, isopropanol and n-hexanol, such as deionized water.

The inert gas introduced during the reaction is, for example, one or more of nitrogen, argon and helium.

The lithium source may be lithium oxide (Li)₂O), lithium phosphate (Li)₃PO₄) Lithium dihydrogen phosphate (LiH)₂PO₄) Lithium acetate (CH)₃COOLi), lithium hydroxide (LiOH), lithium carbonate (Li)₂CO₃) And lithium nitrate (LiNO)₃) One or more of (a). Further, the lithium source is one or more of lithium carbonate, lithium hydroxide and lithium nitrate. Further, the lithium source is lithium carbonate.

M¹Precursor of an elementThe body may be M¹One or more of oxides, nitric acid compounds, carbonic acid compounds, hydroxide compounds and acetic acid compounds of the elements, e.g. M¹An oxide of the element. M¹The oxide of the element is, for example, silicon oxide (e.g., SiO)₂SiO, etc.), titanium oxide (e.g., TiO)₂TiO, etc.), chromium oxide (e.g., CrO)₃、Cr₂O₃Etc.), molybdenum oxide (e.g., MoO)₂、MoO₃Etc.), vanadium oxide (e.g. V)₂O₅、V₂O₄、V₂O₃VO, etc.), germanium oxide (e.g., GeO)₂Etc.), selenium oxide (e.g., SeO)₂Etc.), zirconia (e.g., ZrO)₂Etc.), niobium oxide (e.g., Nb)₂O₅、NbO₂Etc.), ruthenium oxide (e.g., RuO)₂Etc.), rhodium oxide (e.g., Rh)₂O₃Etc.), palladium oxide (e.g., PdO, etc.), antimony oxide (e.g., Sb)₂O₅、Sb₂O₃Etc.), tellurium oxide (e.g., TeO)₂Etc.), cerium oxide (e.g., CeO)₂Etc.) and tungsten oxide (e.g., WO)₂、WO₃Etc.).

In step S10, sintering may be performed in an atmosphere sintering furnace. The sintering atmosphere is an oxygen-containing atmosphere, for example, an air atmosphere or an oxygen atmosphere. The oxygen content of the sintering atmosphere is preferably 80% to 100%. The sintering temperature may be from 500 ℃ to 1000 ℃, for example from 600 ℃ to 1000 ℃. Preferably, the sintering temperature is 700 ℃ to 900 ℃. This is advantageous for M¹The elements have higher distribution uniformity. The sintering time can be adjusted according to the actual situation, and is, for example, 5h to 35h, further, for example, 5h to 25h, further, for example, 10h to 20 h.

It should be noted that, in the preparation of the positive electrode active material, there are a plurality of theoretically feasible ways to regulate and control the doping element M in the secondary particles¹The distribution uniformity of the positive electrode active material, such as sintering temperature, sintering time, sintering times, doping element types, doping amount and the like in the preparation of the positive electrode active material. In the application, some measures of solid-phase sintering doping mode are listed, and M in secondary particles is obtained by adjusting sintering times, doping elements in batches, controlling the overall sintering time and sintering temperature and the like¹The element is uniformly doped in the anode active material. It should be understood that the methods described herein are illustrative only and are not limiting.

As an example, M may be¹The element precursor is divided into L batches for doping the doping elements, wherein L can be 1-5, such as 2-3. In these embodiments, the method of preparing the positive active material may include the steps of: a positive electrode active material precursor, a lithium source and a 1 st batch M¹Mixing element precursors, and performing the 1 st sintering treatment; then the product of the 1 st sintering treatment is mixed with the 2 nd batch M¹Mixing element precursors, and performing 2 nd sintering treatment; and so on until the product of the L-1 sintering treatment and the L batch M¹And mixing the element precursors, and carrying out the L-time sintering treatment to obtain secondary particles.

Wherein, M may be¹The precursor of the element is equally divided into L parts or is arbitrarily divided into L parts, and L batches of doping are carried out. For example, in some embodiments, M may be¹Precursors of the elements are doped in 2 batches. 1 st batch M¹Elemental precursors and batch 2M¹The mass ratio of the element precursor can be 40-60: 60-40, such as 45-55: 55-45.

The temperature of each sintering process may be the same or different. The time for each sintering process may be the same or different. The sintering temperature and time can be adjusted by those skilled in the art according to the kind and amount of the doping element. For example, the temperature of each sintering treatment can be from 500 ℃ to 1000 ℃, e.g., from 600 ℃ to 1000 ℃, e.g., from 700 ℃ to 900 ℃, e.g., from 800 ℃ to 850 ℃; the time of each sintering treatment can be 2-25 h, such as 5-20 h; the total sintering time may be from 5h to 35h, such as from 5h to 25h, for example from 10h to 20 h.

For M difficult to dope¹Elements, e.g. M, having a large atomic radius¹Elements, M can be increased by increasing the sintering temperature and/or by increasing the sintering time¹Uniformity of doping of the elements.

In some embodiments, at step S10,optionally, the mixture also comprises a precursor of the X element. Thus, the secondary particles can simultaneously contain the doping element M¹And X.

The precursor of the X element can be added into the mixture at one time, or the precursor of the X element can be divided into more than two batches for doping the X element.

The precursor of the element X may be chosen, for example, from NH₄F、NH₄Cl、NH₄Br、NH₄I、(NH₄)₂S、(NH₄)₃PO₄、(NH₄)₂HPO₄、NH₄H₂PO₄、LiF、LiCl、LiBr、LiI、Li₃N、Li₃P and Li₂S, but is not limited thereto.

In step S20, M²The precursor of the element may be M²One or more of element chloride, sulfate, nitrate, oxide, hydroxide, fluoride, carbonate, bicarbonate, acetate, phosphate, dihydrogen phosphate and organic compound, but not limited thereto.

In step S20, the secondary particles may be mixed with M²And adding the mixed material of the element precursors into an atmosphere sintering furnace for sintering. The sintering atmosphere is an oxygen-containing atmosphere, for example, an air atmosphere or an oxygen atmosphere. The oxygen content of the sintering atmosphere is preferably 80% to 100%.

The sintering temperature is, for example, 200 ℃ to 700 ℃, such as 200 ℃ to 500 ℃. The sintering time may be 2 to 10 hours, such as 5 to 8 hours. Due to the lower sintering temperature, M²The oxide of the element is not easy to diffuse into the secondary particles, and forms a coating layer coated on the outer surface of the secondary particles. M²The surface crystal lattices of the elements and the secondary particles are matched, so that the coating layer is tightly combined with the secondary particles, the structure of the secondary particles cannot be damaged by the coating layer, and the coating layer can reliably protect the secondary particles.

In some embodiments, the secondary particles are mixed with M²Before the precursors of the elements are mixed, the secondary particles can also be crushed and sieved to obtain particles with an optimized sizeDistribution and specific surface area of the positive electrode active material. The crushing mode is not particularly limited, and may be selected according to actual requirements, for example, a particle crusher is used.

In the preparation of the positive electrode active material, the mixing of the materials may be performed using a ball mill mixer or a high speed mixer. For example, the materials are added into a high-speed mixer for mixing, and the mixing time can be 0.5h to 3 h.

Positive pole piece

The second aspect of the embodiment of the invention provides a positive pole piece, wherein the positive pole piece adopts any one or more positive active materials.

Because the positive pole piece adopts the positive active material, the battery adopting the positive pole piece can simultaneously give consideration to higher energy density, high-temperature storage performance and high-temperature cycle performance.

Specifically, the positive electrode plate comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector. For example, the positive electrode current collector includes two opposite surfaces in a thickness direction thereof, and the positive electrode active material layer is stacked on either or both of the two surfaces of the positive electrode current collector.

The positive electrode active material layer includes any one or more of the positive electrode active materials of the present invention.

In addition, the positive electrode active material layer may further include a conductive agent and a binder. The types of the conductive agent and the binder of the positive electrode active material layer are not particularly limited, and can be selected according to actual requirements.

As an example, the conductive agent may be one or more of graphite, superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the binder may be one or more of Styrene Butadiene Rubber (SBR), water-based acrylic resin (water-based acrylic resin), carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), Polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), ethylene-vinyl acetate copolymer (EVA), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorine-containing acrylic resin, and polyvinyl alcohol (PVA).

The positive current collector can be made of metal foil or porous metal plate with good conductivity and mechanical property, and the material of the positive current collector can be one or more of aluminum, copper, nickel, titanium, silver and their respective alloys. The positive electrode collector is, for example, an aluminum foil.

The positive pole piece can be prepared according to the conventional method in the field. For example, the positive electrode active material, the conductive agent and the binder are dispersed in a solvent, wherein the solvent can be N-methyl pyrrolidone (NMP) or deionized water, so as to form uniform positive electrode slurry, the positive electrode slurry is coated on a positive electrode current collector, and the positive electrode pole piece is obtained after the procedures of drying, rolling and the like.

Lithium ion secondary battery

The third aspect of the embodiments of the present invention provides a lithium ion secondary battery, which includes a positive electrode plate, a negative electrode plate, an isolation film and an electrolyte, wherein the positive electrode plate is any positive electrode plate in the present invention.

By adopting the positive pole piece, the lithium ion secondary battery can simultaneously give consideration to higher energy density, high-temperature storage performance and high-temperature cycle performance.

The negative electrode plate can be a metallic lithium plate.

The negative electrode sheet may further include a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector includes two opposite surfaces in a thickness direction thereof, and the negative electrode active material layer is stacked on either or both of the two surfaces of the negative electrode current collector.

The anode active material layer includes an anode active material. The kind of the negative electrode active material in the embodiment of the present invention is not particularly limited, and may be selected according to actual needs. As an example, the negative active material may be natural graphite, artificial graphite, mesophase micro carbon spheres (MCMB), hard carbon, soft carbon, silicon-carbon composite, SiO_m(0<m<2, e.g. m ═ 1), Li-Sn alloys, Li-Sn-O alloys, Sn、SnO、SnO₂Lithium titanate Li of spinel structure₄Ti₅O₁₂One or more of Li-Al alloy and metallic lithium.

The anode active material layer may further include a conductive agent and a binder. The embodiment of the present invention does not specifically limit the types of the conductive agent and the binder of the negative electrode active material layer, and may be selected according to actual needs. As an example, the conductive agent is one or more of graphite, superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the binder is one or more of Styrene Butadiene Rubber (SBR), polyvinylidene fluoride (PVDF), Polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), water-based acrylic resin (water-based acrylic resin) and carboxymethyl cellulose (CMC).

The negative electrode active material layer may further optionally include a thickener such as carboxymethyl cellulose (CMC).

The negative current collector can be made of metal foil or porous metal plate with good conductivity and mechanical property, and the material of the negative current collector can be one or more of copper, nickel, titanium, iron and their respective alloys. The negative electrode collector is, for example, a copper foil.

The negative pole piece can be prepared according to the conventional method in the field. For example, a negative electrode active material, a conductive agent, a binder and a thickening agent are dispersed in a solvent, wherein the solvent can be N-methyl pyrrolidone (NMP) or deionized water, so as to form uniform negative electrode slurry, the negative electrode slurry is coated on a negative electrode current collector, and the negative electrode pole piece is obtained after the working procedures of drying, rolling and the like.

In the lithium ion secondary battery according to the embodiment of the present invention, the electrolyte may be a solid electrolyte, such as a polymer electrolyte, an inorganic solid electrolyte, or the like, but is not limited thereto. The electrolyte may be an electrolytic solution. The electrolyte solution includes a solvent and a lithium salt dissolved in the solvent.

The solvent may be a non-aqueous organic solvent, for example, one or more, preferably two or more, of Ethylene Carbonate (EC), Propylene Carbonate (PC), Ethyl Methyl Carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), Methyl Propyl Carbonate (MPC), Ethyl Propyl Carbonate (EPC), Methyl Formate (MF), Methyl Acetate (MA), Ethyl Acetate (EA), Propyl Acetate (PA), Methyl Propionate (MP), Ethyl Propionate (EP), Propyl Propionate (PP), Methyl Butyrate (MB), and Ethyl Butyrate (EB).

The lithium salt may be LiPF₆(lithium hexafluorophosphate), LiBF₄Lithium tetrafluoroborate (LiClO), LiClO₄(lithium perchlorate) LiAsF₆(lithium hexafluoroarsenate), LiFSI (lithium bis (fluorosulfonylimide)), LiTFSI (lithium bis (trifluoromethanesulfonylimide)), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalato borate), LiBOB (lithium bis (oxalato borate)), LiPO₂F₂One or more of (lithium difluorophosphate), LiDFOP (lithium difluorooxalate phosphate) and LiTFOP (lithium tetrafluorooxalate phosphate), for example LiPF₆(lithium hexafluorophosphate), LiBF₄(lithium tetrafluoroborate), LiBOB (lithium bis (oxalato) borate), LiDFOB (lithium difluoro (oxalato) borate), LiTFSI (lithium bis (trifluoromethanesulfonyl) imide), and LiFSI (lithium bis (fluorosulfonyl) imide).

The electrolyte may optionally contain other additives such as Vinylene Carbonate (VC), ethylene carbonate (VEC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFPC), Succinonitrile (SN), Adiponitrile (ADN), Glutaronitrile (GLN), Hexanetricarbonitrile (HTN), 1, 3-propanesultone (1,3-PS), vinyl sulfate (DTD), Methylene Methanedisulfonate (MMDS), 1-propene-1, 3-sultone (PST), 4-methyl-ethylene sulfate (PCS), 4-ethyl-ethylene sulfate (PES), 4-propyl-ethylene sulfate (PEGLST), propylene sulfate (TS), 1, 4-butane sultone (1,4-BS), ethylene sulfite (DTO), dimethyl sulfite (DMS), One or more of Diethylsulfite (DES), cyclic quaternary ammonium sulfonate, tris (trimethylsilane) phosphate (TMSP), and tris (trimethylsilane) borate (TMSB), but is not limited thereto.

The lithium ion secondary battery according to the embodiment of the present invention is not particularly limited to the separator, and any known separator having a porous structure with electrochemical stability and mechanical stability, such as a single-layer or multi-layer film of one or more of glass fiber, non-woven fabric, Polyethylene (PE), polypropylene (PP), and polyvinylidene fluoride (PVDF), may be used.

The positive pole pieces and the negative pole pieces are alternately stacked, and the isolating film is arranged between the positive pole pieces and the negative pole pieces to play an isolating role, so that the battery cell is obtained, or the battery cell can be obtained after winding. And placing the battery cell in a shell, injecting electrolyte, and sealing to obtain the lithium ion secondary battery.

Examples

The present disclosure is more particularly described in the following examples that are intended as illustrations only, since various modifications and changes within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise indicated, all parts, percentages, and ratios reported in the following examples are on a weight basis, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further treatment, and the equipment used in the examples is commercially available.

Example 1

Preparation of positive electrode active material

1. Preparing a precursor of the positive active material: preparing a precursor [ Ni ] of the positive electrode active material by adopting the coprecipitation method_0.8Co_0.1Mn_0.1](OH)₂. The positive electrode active material precursor includes aggregate secondary particles formed by aggregation of primary particles, the secondary particles including a plurality of layers of the primary particles arranged in a radial direction of the particles from a core to an outer surface of the particles. Average particle diameter D of positive electrode active material precursor_v50 is 13 μm. The length of the primary particles in the precursor of the positive electrode active material is 200 nm-800 nm, the width of the primary particles is 50 nm-400 nm, and the length-width ratio of the primary particles is 2-10.

2. Preparation of secondary particles: doping element Sb, and oxidizing antimony Sb as precursor of the doping element₂O₃Doping was carried out approximately equally divided into two batches. The method comprises the following steps:

preparing a positive active material precursor, lithium hydroxide LiOH and antimony oxide Sb of the 1 st batch₂O₃Adding intoAnd mixing for 1h in a high-speed mixer to obtain a mixture, wherein the molar ratio Li/Me of the precursor of the positive active material to lithium hydroxide is 1.05, and Me represents the total molar amount of Ni, Co and Mn in the precursor of the positive active material. Placing the mixture into an atmosphere sintering furnace for sintering for the 1 st time, wherein the sintering temperature is 820 ℃, and the sintering atmosphere is O₂The concentration is 90 percent, and the sintering time is 10 hours.

And adding the product of the 1 st sintering treatment and the 2 nd batch of antimony oxide into a high-speed mixer for mixing for 1h, and performing the 2 nd sintering, wherein the sintering atmosphere is the same as that of the 1 st sintering, the sintering temperature is 820 ℃, and the sintering time is 6h, namely the total sintering time is 16 h.

And crushing and sieving the product obtained after the 2 nd sintering treatment to obtain secondary particles.

3. Preparation of a coating layer: mixing the secondary particles with alumina Al₂O₃Adding the mixture into a high-speed mixer for mixing for 1 h. Putting the mixed materials into an atmosphere sintering furnace for sintering, wherein the sintering temperature is 345 ℃, and the sintering atmosphere is O₂Oxygen-containing atmosphere with concentration of 90%, sintering time of 7h, and forming Al on outer surface of secondary particles₂O₃And coating the layer to obtain the high-nickel ternary positive active material.

Wherein, the addition amount of the antimony oxide ensures that the content of Sb in the positive active material is 3500ppm, and Al₂O₃Is added so that the content of Al in the clad layer is 3000ppm, which is the content in the positive electrode active material. The doping element Sb is uniformly doped in the bulk structure of the secondary particles.

Preparation of the electrolyte

Mixing EC, DEC and DMC according to the volume ratio of 1:1:1 to obtain a solvent, and then adding lithium salt LiPF₆Dissolving in the above solvent to obtain an electrolyte solution, wherein LiPF₆The concentration of (2) is 1 mol/L.

Preparation of button cell

Dispersing the prepared positive electrode active material, conductive carbon black and binder PVDF into solvent N-methylpyrrolidone (NMP) according to the weight ratio of 90:5:5, and uniformly mixing to obtain positive electrode slurry; and uniformly coating the positive electrode slurry on a positive current collector aluminum foil, and drying and cold pressing to obtain the positive electrode piece.

And in the button cell, sequentially stacking the positive pole piece, the isolating membrane and the metal lithium piece, injecting the electrolyte, and assembling to obtain the button cell.

Preparation of full cell

Dispersing the prepared positive electrode active material, conductive agent acetylene black and binder PVDF into solvent NMP according to the weight ratio of 94:3:3, and uniformly mixing to obtain positive electrode slurry; and uniformly coating the positive electrode slurry on a positive current collector aluminum foil, and drying and cold pressing to obtain the positive electrode piece.

Dispersing a negative electrode active material artificial graphite, hard carbon, a conductive agent acetylene black, a binder Styrene Butadiene Rubber (SBR) and a thickening agent sodium carboxymethyl cellulose (CMC) into deionized water according to a weight ratio of 90:5:2:2:1, and uniformly mixing to obtain negative electrode slurry; and uniformly coating the negative electrode slurry on a negative current collector aluminum foil, and drying and cold pressing to obtain a negative electrode plate.

Polyethylene (PE) porous polymeric films were used as separators. And stacking the positive pole piece, the isolating membrane and the negative pole piece in sequence to obtain a bare cell, placing the bare cell in an external package, injecting the electrolyte, packaging, and performing procedures such as formation to obtain the full cell.

Examples 2 to 36 and comparative examples 1 to 10

Different from the embodiment 1, the relevant parameters in the steps 1 to 3 of preparing the positive active material are changed, including adjusting the type, the content of each batch, the sintering temperature, the sintering time and the like when the doping elements are mixed, so as to obtain different positive active materials, and the specific parameters are detailed in tables 1-1 and tables 1-2.

Wherein the precursors of the positive electrode active materials of examples 2 to 34 and comparative examples 1 to 7 are all [ Ni ]_0.8Co_0.1Mn_0.1](OH)₂(ii) a The precursors of the positive electrode active materials of examples 35 to 36 and comparative examples 8 to 10 were all [ Ni ]_0.5Co_0.2Mn_0.3(OH)₂。

M of examples 2 and 30 to 32¹The element precursor is TeO₂(ii) a M of examples 3, 33 and 34¹The element precursor is WO₂(ii) a M in examples 4, 21 to 25¹The element precursor is NbO₂(ii) a M of examples 6, 19 and comparative example 6¹The element precursor is TiO₂(ii) a M of examples 17 to 18, 36 and comparative example 7¹The element precursor is RuO₂(ii) a M of example 20 and comparative example 5¹The element precursor is VO; m of examples 28 and 29¹The element precursor is SiO₂(ii) a M of example 7¹The element precursor is Sb₂O₃And WO₂And the contents of the two precursors are basically the same; m of comparative examples 3 to 4 and 10¹The element precursor is Al₂O₃。

The remainder of the coating elements M which are different from those of example 1²The precursor is selected from Y₂O₃、ZrO₂、TiO₂、MgO、B₂O₃And CaO; two kinds of M in example 8²The content of the precursor was substantially the same.

In tables 1-1 and 1-2,

M¹content of (A) and M¹And M²The total content of (b) is the content in the positive electrode active material;

σ is M at any point in the secondary particles¹Deviation of mass concentration of elements;

D_v50 is the volume average particle diameter of the positive electrode active material;

the specific surface area is a specific surface area of the positive electrode active material;

the compaction density is the compaction density of the positive electrode active material powder under the pressure of 5 tons;

κ is the number of primary particles contained in the outermost layer of the secondary particles;

mass ratio of 1 st batch M¹Mass of elemental precursor 2 batch M¹Mass of elemental precursor.

Test section

1) M at any point in the secondary particles¹Elemental mass concentration bias test

Weighing 2g of positive active material powder sample, uniformly sprinkling the sample on a sample table adhered with conductive adhesive, slightly pressing to fix the powder, or cutting out a pole piece of 1cm multiplied by 1cm from a battery positive pole piece, and adhering the pole piece to the sample table to serve as a sample to be detected. The sample stage was placed in a vacuum sample chamber and fixed, a cross section of the secondary particle was prepared using an IB-09010CP cross section polisher by JEOL (JEOL) and the mass concentration of the 17 site doping elements was measured using an X-Max type energy spectrometer (EDS) by Oxford instruments, England, in combination with a Sigma-02-33 type Scanning Electron Microscope (SEM) by ZEISS, Germany, with reference to 17 site spotting of the cross section of the secondary particle shown in FIG. 3, by using the following method: selecting Li, O, Ni, Co, Mn and doping elements as detection elements, setting SEM parameters of 20kV acceleration voltage, 60 mu m light bar, 8.5mm working distance and 2.335A current, stopping testing when the spectrogram area reaches more than 250000cts (controlled by acquisition time and acquisition rate) during EDS testing, and acquiring data to obtain the mass concentration of the doping elements at each position, which is respectively marked as eta₁、η₂、η₃、…、η₁₇。

Average mass concentration of doping element in secondary particleThe determination method comprises the following steps: by the EDS-SEM test method, the test area covers all points of the secondary particle scan and does not exceed the cross section of the secondary particle, as shown by the dashed box in FIG. 3.

Then calculating any point M in the secondary particles according to the formula (3) described above¹The mass concentration deviation σ of the element.

In order to test the positive active material in the battery, the battery can be disassembled in a drying room, the middle part of a positive pole piece is taken out and put into a beaker, a proper amount of high-purity anhydrous dimethyl carbonate (DMC) is poured, the DMC is replaced every 8 hours, the continuous cleaning is carried out for 3 times, then the positive pole piece is put into a vacuum standing box of the drying room, the vacuumizing state (-0.096MPa) is kept, the drying is carried out for 12 hours, a pole piece sample with the size of more than 1cm multiplied by 1cm is cut after the drying, and the pole piece sample is adhered on a sample table adhered with conductive adhesive; or 2g of the positive active material powder was scraped off in a dry room with a blade as a test sample, and the test was performed according to the above-described method.

2) Number of primary particles in outermost layer of secondary particles

Taking a positive electrode active material particle sample, and detecting the outer surface of the sample by adopting a Sigma-02-33 SEM of a Germany ZEISS company, wherein the magnification is 10K to obtain an SEM image; then, from the SEM image, the number of primary particles κ in units of one/μm in a unit area of 1 μm × 1 μm of the outermost layer of the secondary particles was calculated by the aforementioned formula (2)²。

3) M in the positive electrode active material¹Element and M²Content test of elements

Weighing about 5g of positive active material powder and filling into a digestion tank; or, discharging the full battery to 2.80V according to 0.33C, detaching the battery core with scissors in a drying room, taking out the middle part of the whole positive pole piece, putting the middle part into a beaker, pouring a proper amount of high-purity anhydrous dimethyl carbonate (DMC), replacing the DMC every 8 hours, continuously cleaning for 3 times, then putting the positive pole piece into a vacuum standing box of the drying room, keeping a vacuumizing state (-0.096MPa), and drying for 12 hours; and (3) drying the finished positive pole piece, scraping powder in a drying room by using a blade, weighing about 5g of positive active material powder, and filling the powder into a digestion tank.

Weighing the digestion tank filled with the positive electrode active material sample to be accurate to 0.0001 g; slowly adding 10mL of aqua regia (a mixture of concentrated hydrochloric acid and concentrated nitric acid in a volume ratio of 3: 1) as a digestion reagent, placing the digestion reagent into a MARS6 type microwave digestion instrument of CEM company of America, and digesting a sample by adjusting the microwave emission frequency to 2450 Hz; transferring the digested liquid sample into an atomization chamber of an Optima 7000DV inductively coupled plasma-emission spectrometer (ICP-OES) of PerkinElmer company (PE for short), setting the radio frequency to be 40.68MHz, the argon pressure to be 0.6MPa and the radio frequency power to be 1300W, testing the sample to obtain a characteristic spectral line, determining the element type according to the characteristic spectral line wavelength of the element to be detected, and determining the element content according to the characteristic spectral line intensity.

4) M in "78% delithiated" positive electrode active material¹Element(s)Valence state distribution test of

a. Determination of 78% delithiation state

At 25 ℃, 8 button cells are respectively charged to the upper limit of the charge-discharge cut-off voltage by a constant current of 1C, then are charged to the current of less than or equal to 0.05mA by a constant voltage, and are placed for 2 minutes, and then are discharged to the lower limit of the charge-discharge cut-off voltage by the constant current of 1C.

Then, the 8 button cells after the charge and discharge were charged to 4.0V, 4.1V, 4.2V, 4.3V, 4.4V, 4.5V, 4.6V, and 4.7V at 0.1C rate, respectively. Taking each charged button cell, disassembling a positive pole piece in a drying room to be used as a sample, weighing and recording the mass of the sample, putting the sample into a digestion tank, slowly adding 10mL of aqua regia to be used as a digestion reagent, assembling the solution, putting the solution into a CEM-MARS6 microwave digestion instrument, and digesting the solution at 2450Hz microwave emission frequency; transferring the digested sample solution into a volumetric flask, shaking uniformly, sampling, putting into a 7000DV type inductively coupled plasma-emission spectrometer (ICP-OES) sample introduction system of the American PE company, and testing the mass concentration of Li, O, Ni, Co, Mn and doping elements of the positive active material at the argon pressure of 0.6MPa and the radio frequency power of 1300W; and converting based on the mass concentration of each element to obtain a chemical formula under each voltage, and further obtaining a lithium removal state under each voltage. The chemical formula of the positive electrode active material converted under the voltage of 4.3V is Li_0.22Ni_0.8Co_0.1Mn_0.1O₂The corresponding delithiated state is (1-0.22) × 100% ═ 78% delithiated state, i.e., the cell voltage corresponding to 78% delithiated state is 4.3V.

The button cell is charged to a voltage corresponding to a 78% delithiation state at 25 ℃ by 0.1C multiplying power to obtain a 78% delithiation state sample, and then the following operations are carried out:

XPS test of elemental valence states

Firstly, disassembling 78% of a lithium-removed battery cell in a drying room, taking out the whole positive pole piece, putting the positive pole piece into a beaker, pouring a proper amount of high-purity anhydrous dimethyl carbonate DMC, replacing DMC every 8 hours, continuously cleaning for 3 times, then putting the positive pole piece into a vacuum standing box of the drying room, keeping a vacuumizing state (-0.096MPa), and drying for 12 hours; and scraping and grinding the dried positive pole piece in a drying room by using a blade, and weighing about 50mg of positive active material powder.

Wiping the surface of an aluminum foil of about 2cm multiplied by 2cm by acetone, cutting a double-sided adhesive tape of about 1cm multiplied by 1cm to be attached to the center of the aluminum foil, spreading a powder sample on the double-sided adhesive tape, and uniformly spreading the powder on the whole adhesive tape by using a clean stainless steel sampling spoon. And (3) taking another piece of aluminum foil which is wiped cleanly by acetone to cover the sample, integrally placing the aluminum foil between two flat stainless steel modules, and then tabletting by using a tabletting machine, wherein the pressure is about 10MPa, and keeping for 15 seconds.

③ adopting escalab 250Xi type X-ray photoelectron spectrometer of America Seimer Feishale (Thermo) science and technology company, putting the pressed sample into the sample cavity, setting monochromatic Al Ka (hv) 1486.6eV excitation source, X-ray power 150W, focusing spot 500 μ M, collecting M¹The 2p or 3d spectra of the elements were processed by XPSpeak software for peak separation and M was determined¹The valence distribution of the element.

5) Initial gram capacity test for button cell

And charging the battery to the upper limit of a charge-discharge cut-off voltage at a constant current of 0.1 ℃ at 25 ℃, then charging the battery to a constant voltage until the current is less than or equal to 0.05mA, standing for 2 minutes, and then discharging the battery to the lower limit of the charge-discharge cut-off voltage at a constant current of 0.1 ℃, wherein the discharge capacity at this time is the initial gram capacity of the button battery.

6) Initial gram capacity test of full cell

Charging the battery to the upper limit of a charge-discharge cut-off voltage at a constant current of 1/3 ℃ at 25 ℃, then charging the battery to a constant voltage until the current is less than or equal to 0.05mA, standing the battery for 5 minutes, and then discharging the battery to the lower limit of the charge-discharge cut-off voltage at a constant current of 1/3 ℃, wherein the discharge capacity at this time is the initial gram capacity of the whole battery.

7) High temperature cycle performance testing of full cells

Charging the battery at a constant current of 1C to the upper limit of the cut-off voltage of charge and discharge at 45 ℃, then charging the battery at a constant voltage until the current is less than or equal to 0.05mA, standing the battery for 5 minutes, and then discharging the battery at a constant current of 1C to the lower limit of the cut-off voltage of charge and discharge, which is a charge and discharge cycle, wherein the discharge capacity of the cycle is recorded as the discharge specific capacity D of the 1 st cycle₁. The battery is circularly charged for 400 times according to the methodDischarging test, recording the specific discharge capacity D of the 400 th cycle₄₀₀。

Capacity retention (%) of the full cell at 45 ℃ and 400 cycles of 1C/1C₄₀₀/D₁×100％

8) High temperature storage performance testing of full cells

Charging the battery at 25 deg.C with constant current of 1C rate to the upper limit of cut-off voltage, charging at constant voltage until the current is less than or equal to 0.05mA, and measuring the volume of the battery and recording as V₀(ii) a The cells were then placed in an 80 ℃ incubator and the volume of the cells after 10 days of storage was measured and recorded as V₁. In this test, the volume of the cell was tested using a drainage method.

Volume expansion rate Δ V (%) (V) after 10 days of storage at 80 ℃ of the full-cell₁-V₀)/V₀×100％

In the tests 4) to 8),

in examples 1 to 34 and comparative examples 1 to 7, the charge-discharge cut-off voltage of the button cell battery is 2.8V to 4.25V, and the charge-discharge cut-off voltage of the full cell battery is 2.8V to 4.2V;

in examples 35 to 36 and comparative examples 8 to 10, the charge-discharge cut-off voltage of the button cell battery was 2.8V to 4.35V, and the charge-discharge cut-off voltage of the full cell was 2.8V to 4.3V.

TABLE 1-1

Tables 1 to 2

TABLE 2

As can be seen from the comparison of examples 1 to 34 with comparative examples 1 to 7 and examples 35 to 36 with comparative examples 8 to 10, lithium was addedThe transition metal oxide secondary particles are uniformly doped with M¹Elements, the outer surface of the secondary particles is coated with M²Oxide coating of elements, M¹Element, M²The elements are selected from specific element types, and the secondary particles have proper primary particle size and distribution, so that the lithium ion secondary battery has high energy density, high-temperature storage performance and high-temperature cycle performance.

As can be seen from comparison of examples 17 to 18 with comparative example 7, M at any point in the secondary particles¹When the deviation of the element mass concentration is below 20%, the doping uniformity is good, and the comprehensive performance of the positive active material is good, so that the capacity performance, the high-temperature storage performance and the high-temperature cycle performance of the battery are obviously improved; when the mass concentration deviation of any point of doping elements in the secondary particles is more than 20%, lithium ion diffusion channels and barriers at all positions in the positive active material particles are inconsistent, structural stability and deformation resistance of all regions are different, so that stress distribution in the material is uneven, the regions with large internal stress are easy to break, the positive active material is exposed out of a fresh surface, impedance is increased, and the capacity performance, high-temperature storage performance and high-temperature cycle performance of the battery are further deteriorated.

As can be seen from comparison of examples 19 to 20 with comparative examples 5 to 6, the outermost layer of secondary particles contained primary particles in an amount of 5 particles/. mu.m²50 pieces/mum²The size and distribution of the primary particles in the secondary particles are better, and the specific capacity of the positive active material can be further improved, so that the energy density of the battery is improved. And the number of primary particles contained in the outermost layer of the secondary particles is 5 particles/. mu.m²50 pieces/mum²In the case of the positive electrode active material, the mechanical strength is high, and the high-temperature cycle performance and the high-temperature storage performance of the battery can be improved.

As can be seen from the results of examples 9 to 16, M is contained in the positive electrode active material¹Element and M²When the content of the element is small, the improvement effect on the capacity performance, the high-temperature storage performance and the high-temperature cycle performance of the battery is small. When M is in the positive electrode active material¹The element content exceeds 6000ppm, and the positive electrode activityIn the material M¹Element and M²When the total content of the elements exceeds 10000ppm, the capacity performance, the high-temperature storage performance and the high-temperature cycle performance of the positive active material are also affected due to the structural damage of the positive active material body.

From the results of examples 30 to 32, it can be seen that the compacted density of the powder of the positive electrode active material at a pressure of 5 tons is 3.0g/cm³In the above case, the battery can have a higher energy density, and the high-temperature cycle performance and the high-temperature storage performance of the battery can be improved.

From the results of examples 3, 33 and 34, it can be seen that the element M was doped in the positive electrode active material¹When the deviation epsilon between the mass concentration of (B) and the average mass concentration of the doping element in the secondary particles is 30% or less, M is¹The content of elements entering the interior of the secondary particles is higher, thereby fully exerting M¹The improvement effect of the elements enables the battery to have higher energy density, high-temperature cycle performance and high-temperature storage performance.

From the results of examples 21 to 25, it can be seen that the average particle diameter D of the positive electrode active material_vThe thickness of 50 is less than 6 μm, the surface of the positive active material has more side reactions, the gas production of the battery core is larger, the high-temperature cycle performance and the high-temperature storage performance of the battery are poorer, and the capacity performance of the battery is also influenced; average particle diameter D of positive electrode active material_vAnd 50 is greater than 20 μm, the diffusion migration performance of lithium ions in the particles is poor, adversely affecting the performance of the battery.

It is seen from the results of examples 26 to 29 that the capacity performance, high-temperature storage performance and high-temperature cycle performance of the battery are adversely affected by the excessively large or small specific surface area of the positive electrode active material.

While the invention has been described with reference to specific embodiments, the invention is not limited thereto, and various equivalent modifications and substitutions can be easily made by those skilled in the art within the technical scope of the invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.