JP7772049B2 - Positive electrode active material, method for producing positive electrode active material, and lithium ion secondary battery - Google Patents

Description

This disclosure relates to a positive electrode active material, a method for manufacturing a positive electrode active material, and a lithium-ion secondary battery.

Composite oxides of lithium and transition metals (lithium transition metal composite oxides) are known as positive electrode active materials for lithium-ion secondary batteries. To improve the properties of lithium transition metal composite oxides as positive electrode active materials, combinations of various different elements have been proposed. For example, Patent Document 1 describes a positive electrode active material in which the surface of a composite oxide containing nickel as a transition metal is modified with strontium.

International Publication No. 2021/152996

It is desirable to suppress deterioration of lithium ion secondary batteries that use lithium transition metal composite oxides as the positive electrode active material after repeated charge and discharge.
An object of the present disclosure is to provide a positive electrode active material that suppresses deterioration after repeated charging and discharging of a lithium ion secondary battery, a method for producing this positive electrode active material, and a lithium ion secondary battery including a positive electrode containing this positive electrode active material.

The means for solving the above problems include the following embodiments.
<1> A first phase made of a composite oxide belonging to R-3m and a second phase made of a composite oxide belonging to a tetragonal or hexagonal crystal,
The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li,
The composite oxide belonging to a tetragonal or hexagonal crystal system contains Sr and at least one element selected from Li, Ni, Co, and Mn.
<2> Shows an X-ray diffraction pattern including a peak corresponding to a composite oxide attributable to R-3m and a peak corresponding to a composite oxide attributable to a tetragonal or hexagonal crystal;
The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li,
The composite oxide belonging to a tetragonal or hexagonal crystal system contains Sr and at least one element selected from Li, Ni, Co, and Mn.
<3> The method for producing a positive electrode active material according to <1> or <2>, comprising a step of heating a mixture containing a compound containing at least one selected from Ni, Co, and Mn, a compound containing Li, and a compound containing Sr.
<4> The method for producing a positive electrode active material according to <3>, wherein the step of heating the mixture includes a step of heating the mixture at temperature A and a step of heating the mixture at temperature B higher than temperature A, in this order.
<5> A lithium ion secondary battery comprising a positive electrode containing the positive electrode active material according to <1> or <2>.

This disclosure provides a positive electrode active material that suppresses deterioration after repeated charging and discharging of a lithium ion secondary battery, a method for producing this positive electrode active material, and a lithium ion secondary battery equipped with a positive electrode containing this positive electrode active material.

In the present disclosure, a numerical range indicated using "to" means a range that includes the numerical values before and after "to" as the minimum and maximum values, respectively.
In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In the numerical ranges described in this disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the value shown in the examples.
In the present disclosure, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.
In the present disclosure, a combination of two or more preferred aspects is a more preferred aspect.
In the present disclosure, when there are multiple substances corresponding to each component, the amount of each component means the total amount of the multiple substances unless otherwise specified.

First Embodiment
The first embodiment of the present disclosure includes:
The first phase is made of a composite oxide belonging to R-3m, and the second phase is made of a composite oxide belonging to a tetragonal or hexagonal crystal,
The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li,
The composite oxide belonging to the tetragonal or hexagonal crystal system is a positive electrode active material containing Sr and at least one element selected from Li, Ni, Co, and Mn.

In this disclosure, a "positive electrode active material comprising a first phase made of a composite oxide belonging to R-3m and a second phase made of a composite oxide belonging to tetragonal or hexagonal crystal structure" refers to a positive electrode active material in which the first and second phases exist within the bulk (e.g., within the same particle). Therefore, for example, a mixture of particles made of a composite oxide belonging to R-3m and particles made of a composite oxide belonging to tetragonal or hexagonal crystal structure does not fall under the category of a "positive electrode active material comprising a first and second phase."

As shown in the examples described below, by using a positive electrode active material containing a first phase made of a specific complex oxide belonging to R-3m and a second phase made of a specific complex oxide belonging to a tetragonal or hexagonal crystal structure, deterioration of a lithium-ion secondary battery after repeated charge and discharge is suppressed.

(Phase 1)
The first phase contained in the positive electrode active material has a crystal structure attributed to R-3m and is composed of a composite oxide containing at least one transition metal selected from Ni, Co, and Mn, and Li. This composite oxide has a layered crystal structure in which transition metal layers composed of at least one transition metal selected from Ni, Co, and Mn and oxygen, and lithium layers are arranged alternately.
Specific examples of the composite oxide constituting the first phase include a compound represented by the composition LiMO ₂ (where M is at least one selected from Ni, Co, and Mn), and a compound obtained by adding a doping element to the above compound.
Specific examples of doping elements include Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, La, W, B, Ru, Cd, Ag, Y, Sc, As, Sb, Pt, Au, and Si.
The first phase may consist of only one type of composite oxide, or may consist of two or more types of composite oxides.
The composite oxide constituting the first phase may or may not contain Sr.

The composite oxide constituting the first phase may contain any one transition metal selected from Ni, Co and Mn, or may contain two or three transition metals.
The type of transition metal contained in the positive electrode active material may be selected depending on the application of the lithium ion secondary battery, etc. For example, when used as a positive electrode active material for a large-capacity lithium ion secondary battery such as an automotive battery, the first phase preferably contains at least Ni as a transition metal, more preferably contains Ni and at least one selected from Co and Mn, and even more preferably contains Ni, Co, and Mn (NCM, nickel cobalt manganese oxide).

The NCM may contain a high proportion of Ni (e.g., 50 mol% or more, 60 mol% or more, or 70 mol% or more of the total transition metals).

The molar ratios of Ni, Co, and Mn contained in the NCM may be, for example, selected from the range of 1:0.1 to 1:1 for the Ni to Co molar ratio (Ni:Co), and 1:0.1 to 1:1 for the Ni to Mn molar ratio (Ni:Mn).

The molar ratio of Ni to Co (Ni:Co) may be selected from the range of 1:0.1 to 1:0.5, 1:0.1 to 1:0.3, or 1:0.1 to 1:0.2.
The molar ratio of Ni to Mn (Ni:Mn) may be selected from the range of 1:0.1 to 1:0.5, 1:0.1 to 1:0.3, or 1:0.1 to 1:0.2.

(Phase 2)
The second phase contained in the positive electrode active material has a crystal structure belonging to a tetragonal or hexagonal system, and is made of a composite oxide containing Sr and at least one element selected from Li, Ni, Co, and Mn.
Specific examples of the composite oxide that constitutes the second phase include SrMnO ₃ , Sr ₂ MnO ₄ , and SrNiMnO ₄ .
The second phase may consist of only one type of composite oxide, or may consist of two or more types of composite oxides.

The composition ratio of the composite oxide constituting the first phase and the composite oxide constituting the second phase in the positive electrode active material is not particularly limited.
From the viewpoint of exhibiting the effect of suppressing battery degradation while maintaining the function as a positive electrode active material, the proportion of the complex oxide constituting the second phase in the total of the complex oxide constituting the first phase and the complex oxide constituting the second phase is preferably 0.01 mol % or more and 10 mol % or less, and more preferably 1 mol % or more and 5 mol % or less.

The composition ratio of the elements constituting the positive electrode active material is not particularly limited.
From the viewpoint of exhibiting the effect of suppressing battery degradation while maintaining the function as a positive electrode active material, the proportion of Sr in the total of Ni, Co, Mn, Li, and Sr among the elements constituting the positive electrode active material is preferably 0.01 mol % or more and 10 mol % or less, and more preferably 1 mol % or more and 5 mol % or less.

The positive electrode active material may be in a particulate form. The volume average particle diameter of the particulate positive electrode active material is not particularly limited and can be selected, for example, from the range of 5 μm to 30 μm. When the positive electrode active material is in the form of secondary particles that are an aggregate of multiple primary particles, the volume average particle diameter is the volume average particle diameter of the secondary particles.
The volume average particle size of the positive electrode active material particles is not particularly limited and can be selected, for example, from the range of 5 μm to 30 μm.

In this disclosure, the volume average particle size of particles is the particle size (D50) at which the cumulative volume reaches 50% in the volume-based particle size distribution. The volume-based particle size distribution can be obtained, for example, by laser diffraction/scattering.

Whether or not the positive electrode active material contains a composite oxide that constitutes the first phase and a composite oxide that constitutes the second phase can be confirmed using known analytical methods such as XRD (X-ray diffraction).

Second Embodiment
A second embodiment of the present disclosure is
It shows an X-ray diffraction pattern including a peak corresponding to a composite oxide attributable to R-3m and a peak corresponding to a composite oxide attributable to a tetragonal or hexagonal crystal;
The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li,
The composite oxide belonging to the tetragonal or hexagonal crystal system is a positive electrode active material containing Sr and at least one element selected from Li, Ni, Co, and Mn.

As shown in the examples described below, by using a positive electrode active material that exhibits an X-ray diffraction pattern that includes a peak corresponding to a specific complex oxide attributed to R-3m and a peak corresponding to a specific complex oxide attributed to tetragonal or hexagonal crystals, deterioration of a lithium-ion secondary battery after repeated charge and discharge is suppressed.

There are no particular restrictions on the method for obtaining the X-ray diffraction pattern of the positive electrode active material, and it can be performed using any known method.

The details and preferred aspects of the positive electrode active material of the second embodiment are the same as those of the positive electrode active material of the first embodiment.

Third Embodiment
A third embodiment of the present disclosure is a method for producing the positive electrode active material of the first or second embodiment described above,
The method includes a step of heating a mixture containing a compound containing at least one selected from Ni, Co, and Mn, a compound containing Li, and a compound containing Sr.

In the method of the present disclosure, examples of the compound containing at least one selected from Ni, Co, and Mn, the compound containing Li, and the compound containing Sr include hydroxides, carbonates, and oxides.
The compound containing at least one selected from Ni, Co, and Mn may be a composite compound containing two or more selected from Ni, Co, and Mn, or may be a compound containing Ni, a compound containing Co, a compound containing Mn, or a combination thereof.

In the method of the present disclosure, the step of heating a mixture containing a compound containing at least one selected from Ni, Co, and Mn, a compound containing Li, and a compound containing Sr more preferably includes, in this order, a step of heating the mixture at temperature A and a step of heating the mixture at temperature B higher than temperature A.
The mixture is heated at temperature A and then heated at temperature B higher than temperature A, thereby efficiently producing the positive electrode active material of the first embodiment or the second embodiment.

The temperature at which the heating step is carried out is not particularly limited as long as it is a temperature at which a desired composite oxide containing the elements in the mixture can be obtained. The heating temperature may be selected, for example, from the range of 600°C to 980°C.
When the heating step includes a step of heating the mixture at temperature A and a step of heating the mixture at temperature B higher than temperature A, in this order, temperature A is preferably selected from the range of 600°C to 800°C, more preferably selected from the range of 600°C to 750°C, and even more preferably selected from the range of 600°C to 700°C.
The time for the step of heating the mixture at temperature A is not particularly limited, and may be selected, for example, from between 1 hour and 10 hours.
When the heating step includes a step of heating the mixture at temperature A and a step of heating the mixture at temperature B higher than temperature A, in this order, temperature B is preferably selected from the range of 680°C to 950°C, more preferably selected from the range of 750°C to 920°C, and even more preferably selected from the range of 800°C to 900°C.
The time for the step of heating the mixture at temperature B is not particularly limited, and may be selected, for example, from between 1 hour and 10 hours.

The heating step can be carried out, for example, in an atmosphere having an oxygen content of 40% by volume to 100% by volume.
The oxygen content during the heating step may be constant from the start to the end of the heating step, or may be varied.

Fourth Embodiment
A fourth embodiment of the present disclosure is a lithium ion secondary battery including a positive electrode containing the positive electrode active material of the first or second embodiment described above.
The positive electrode includes, for example, a current collector and a positive electrode layer disposed on the current collector, and the positive electrode layer includes the positive electrode active material of the present disclosure.
The positive electrode layer may be disposed on one or both sides of the current collector.

Examples of materials that can be used to construct the positive electrode current collector include aluminum, aluminum alloys, nickel, titanium, and stainless steel. Examples of the current collector shape include foil and mesh.

The positive electrode layer is disposed on the current collector by, for example, applying a slurry of positive electrode material to one or both sides of the current collector. If necessary, pressure treatment may be performed to adjust the density of the positive electrode layer. There are no particular restrictions on the thickness of the positive electrode layer, and it can be selected, for example, from the range of 10 μm to 100 μm.

The positive electrode material may be in the form of a mixture containing components other than the positive electrode active material, such as a conductive additive or binder. If necessary, a solvent may be added to the mixture to adjust the viscosity of the mixture.

Specific examples of the conductive aid include carbon materials such as carbon black (acetylene black, thermal black, furnace black, etc.), carbon nanotubes, and graphite.
The conductive material contained in the positive electrode material may be one type alone or two or more types.

Specific examples of binders include polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polyethylene terephthalate, cellulose, nitrocellulose, carboxymethyl cellulose, polyethylene oxide, polyepichlorohydrin, polyacrylonitrile, styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), polyacrylate, and polymethacrylate.
The binder contained in the positive electrode material may be one type alone or two or more types.

A lithium ion secondary battery includes, for example, a positive electrode, a negative electrode, and an electrolyte.
The negative electrode includes, for example, a current collector and a negative electrode layer that is disposed on the current collector and contains a negative electrode active material.
Examples of the negative electrode active material include carbon materials such as graphite, hard carbon, soft carbon, and activated carbon, silicon, metallic lithium, lithium alloys, and lithium titanate (LTO).
Examples of materials constituting the negative electrode current collector include copper, copper alloy, nickel, titanium, stainless steel, etc. Examples of the shape of the negative electrode current collector include foil, mesh, etc.

The electrolyte may be either liquid or solid. As the liquid electrolyte (electrolytic solution), a known electrolyte such as _LiPF6 dissolved in an organic solvent can be used without any particular limitation.
Specific examples of the organic solvent include cyclic or chain carbonates such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. The solvent may be a mixture of two or more solvents, or may be a mixture containing a cyclic carbonate and a chain carbonate.
The solvent may contain an additive such as vinylene carbonate (VC).
As the solid electrolyte, known solid electrolytes such as sulfide solid electrolytes, oxide solid electrolytes, and halide solid electrolytes can be used without any particular limitation.

The lithium-ion secondary battery may include a separator placed between the positive electrode and the negative electrode. Examples of separators include nonwoven fabrics, cloths, and microporous films primarily composed of polyolefins such as polyethylene and polypropylene.

The present disclosure will be explained in more detail below using examples, but the invention of the present disclosure is not limited to these examples.

<Preparation of Positive Electrode Active Material>
NiSO ₄ , CoSO ₄ , and MnSO ₄ were dissolved in ion-exchanged water to obtain a raw material solution with a concentration of 30 mass %. The molar ratios of Ni, Co, and Mn in the raw material solution were as shown in Table 1.

An aqueous _NH3 solution was added to the reaction vessel, and nitrogen substitution was performed while stirring. Next, NaOH was added to the reaction vessel to adjust the aqueous solution to alkaline.
While controlling the pH in the reaction vessel to maintain a constant value, the raw material solution and _NH3 were added dropwise to precipitate hydroxides of Ni, Co, and Mn. The resulting precipitate was filtered and dispersed in ion-exchanged water. The precipitate dispersed in ion-exchanged water was filtered and dried at 120°C for 16 hours to remove moisture, yielding transition metal hydroxides as precursors of the positive electrode active material.

Lithium hydroxide as a compound containing Li and strontium oxide as a compound containing Sr were added to the obtained precursor and mixed to obtain a raw material for the positive electrode active material.
The amount of lithium hydroxide was adjusted so that the amount of Li was 1 mole per mole of the total of the transition metals (Ni, Co, and Mn) in the precursor.
The amount of strontium oxide was adjusted so that the Sr content relative to the total of the transition metals (Ni, Co, and Mn) and Sr in the precursor would be the value (mol %) shown in Table 1.

The raw materials for the positive electrode active material were subjected to a heat treatment at 620°C for 2 hours. Then, they were subjected to a heat treatment at 850°C for 5 hours. Through these steps, the positive electrode active material was obtained.

<Battery evaluation>
A positive electrode active material (88 parts by mass), acetylene black (10 parts by mass) as a conductive material, and polyvinylidene fluoride (2 parts by mass) as a binder were mixed, and the viscosity was adjusted with a solvent to obtain a positive electrode mixture. The positive electrode mixture was coated on aluminum foil and dried at 80°C for 5 minutes to obtain a positive electrode.
A laminated test battery was fabricated using an electrode assembly formed by laminating the obtained positive electrode, a separator (a polyethylene microporous film), and a negative electrode containing graphite as an active material in this order, and an electrolyte solution.
The electrolyte used was a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (volume ratio of EC/DMC/EMC: 3/4/3) in which LiPF ₆ (concentration: 1 M) was dissolved.

The evaluation battery was subjected to CCCV charging (1.5 to 4.1 V) at a current value of 0.1 C, and the discharge capacity was measured and used as the initial capacity. Next, 50 cycles of charge and discharge were performed under conditions of 1 C and 1.5 to 4.1 V, and the discharge capacity after 50 cycles was measured. The capacity retention rate of the battery was calculated using the following formula. The results are shown in Table 1.
Capacity retention rate (%) = (discharge capacity after 50 cycles/initial capacity) x 100
The battery of Comparative Example 1 was fabricated in the same manner as the evaluation battery of the Examples, except that strontium oxide was not mixed into the raw material of the positive electrode active material and the heat treatment was carried out in one step at 700°C for 10 hours.

<XRD analysis>
The X-ray diffraction patterns of the positive electrode active materials obtained in Examples 1 to 5 were obtained using a fully automated multipurpose X-ray diffractometer, SmartLab, manufactured by Rigaku Holdings Corporation. As a result, a peak corresponding to the complex oxide attributed to R-3m as the complex oxide constituting the first phase and a peak corresponding to Sr ₂ MnO ₄ as the complex oxide constituting the second phase were observed in the XRD patterns. From these results, it was found that the positive electrode active materials obtained in Examples 1 to 5 contained a specific complex oxide constituting the first phase and a specific complex oxide constituting the second phase.

As shown in Table 1, the batteries of Examples 1 to 5, which were fabricated using a cathode active material containing a specific complex oxide that constitutes the first phase and a specific complex oxide that constitutes the second phase, had a higher capacity retention rate after repeated charge/discharge and showed more suppressed degradation than the battery fabricated using the cathode active material of Comparative Example 1, which did not contain the specific complex oxide that constitutes the second phase. The reason for this is not clear; however, it is thought that, for example, while the complex oxide that constitutes the first phase functions as an active material, the complex oxide that constitutes the second phase inhibits the reaction between the cathode active material and the electrolyte near the surface of the particles.

Claims (7)

The first phase is made of a composite oxide belonging to R-3m, and the second phase is made of a composite oxide belonging to a tetragonal or hexagonal crystal,
The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li,
The composite oxide belonging to a tetragonal or hexagonal crystal system contains Sr and at least one element selected from Li, Ni, Co, and Mn. It shows an X-ray diffraction pattern including a peak corresponding to a composite oxide attributable to R-3m and a peak corresponding to a composite oxide attributable to a tetragonal or hexagonal crystal;
The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li,
The composite oxide belonging to a tetragonal or hexagonal crystal system contains Sr and at least one element selected from Li, Ni, Co, and Mn. 2. The positive electrode active material according to claim 1, wherein a proportion of Sr in the total of Ni, Co, Mn, Li and Sr among elements constituting the positive electrode active material is 0.01 mol % or more and 10 mol % or less. 3. The positive electrode active material according to claim 2, wherein a proportion of Sr in the total of Ni, Co, Mn, Li and Sr among elements constituting the positive electrode active material is 0.01 mol % or more and 10 mol % or less. 5. The method for producing a positive electrode active material according to claim 1 , comprising a step of heating a mixture containing a compound containing at least one selected from Ni, Co, and Mn, a compound containing Li, and a compound containing Sr. 6. The method for producing a positive electrode active material according to claim 5 , wherein the step of heating the mixture includes a step of heating the mixture at a temperature A and a step of heating the mixture at a temperature B higher than the temperature A, in this order. A lithium ion secondary battery comprising a positive electrode containing the positive electrode active material according to any one of claims 1 to 4 and an electrolyte solution .