Positive electrode active material, lithium-ion secondary battery, and production method for positive electrode active material

Abstract

To provide a positive electrode active material containing nickel as a transition metal and exhibiting excellent capacity retention in a lithium-ion secondary battery, a lithium-ion secondary battery including a positive electrode containing the positive electrode active material, and a production method for the positive electrode active material.SOLUTION: A positive electrode active material has a crystal structure in which a transition metal layer containing nickel and a lithium layer are alternately arranged. An a / b axis length ratio calculated by Rietveld analysis of synchrotron X-ray diffraction is 0.8 or greater.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a positive electrode active material, a lithium ion secondary battery, and a method for manufacturing the positive electrode active material.

Background Art

[0002] A lithium transition metal composite oxide having a layered crystal structure in which a transition metal layer composed of an octahedral structure composed of a transition metal and oxygen and a lithium layer are alternately arranged is widely used as a positive electrode active material for a lithium ion secondary battery. As the lithium transition metal composite oxide having a layered crystal structure, those containing at least one selected from nickel, cobalt, and manganese as a transition metal are known (for example, see Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Among lithium transition metal composite oxides having a layered crystal structure, those containing nickel as a transition metal are suitable as positive electrode active materials for large-capacity lithium ion secondary batteries such as electric vehicle batteries. On the other hand, for lithium ion secondary batteries using a positive electrode active material containing nickel as a transition metal, improvement of the capacity retention rate is an issue. An object of the present disclosure is to provide a positive electrode active material containing nickel as a transition metal and having excellent capacity retention rate of a lithium ion secondary battery, a lithium ion secondary battery including a positive electrode containing this positive electrode active material, and a method for manufacturing this positive electrode active material.

Means for Solving the Problems

[0005] Means for solving the above problems include the following embodiments. <1>A cathode active material having a crystal structure in which a transition metal layer containing nickel and a lithium layer are alternately arranged, and an a / b axis length ratio calculated by Rietveld analysis of synchrotron radiation XRD is 0.8 or more. <2>The cathode active material according to <1>, which contains a doping element having an ionic radius larger than that of nickel. <3>The cathode active material according to <1> or <2>, wherein the a / b axis length ratio is 1.2 or less. <4>A lithium-ion secondary battery including a cathode comprising the cathode active material according to any one of <1> to <3>. <5>A method for producing a cathode active material having a crystal structure in which a transition metal layer containing nickel and a lithium layer are alternately arranged, The method for producing a cathode active material includes adding a doping element to the cathode active material so that an a / b axis length ratio calculated by Rietveld analysis of synchrotron radiation XRD of the cathode active material is 0.8 or more.

Advantages of the Invention

[0006] According to an embodiment of the present disclosure, there are provided a cathode active material containing nickel as a transition metal and excellent in the capacity retention rate of a lithium-ion secondary battery, a lithium-ion secondary battery including a cathode containing this cathode active material, and a method for producing this cathode active material.

Embodiments for Carrying Out the Invention

[0007] In the present disclosure, a numerical range indicated by using "~" means a range including the numerical values described before and after "~" as a minimum value and a maximum value, respectively. In the numerical ranges described stepwise in the present disclosure, an upper limit value or a lower limit value described in a certain numerical range may be replaced with an upper limit value or a lower limit value of another numerically described stepwise range. In the numerical ranges described in the present disclosure, an upper limit value or a lower limit value described in a certain numerical range may be replaced with a value shown in the examples. In the present disclosure, the term "step" includes not only an independent step but also a step that cannot be clearly distinguished from other steps, as long as the intended purpose of the step is achieved. In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In the present disclosure, the amount of each component means the total amount of a plurality of substances corresponding to each component, unless otherwise specified, when there are a plurality of substances corresponding to each component.

[0008] <Positive electrode active material> The positive electrode active material of the present disclosure is a crystal structure in which a transition metal layer containing nickel and a lithium layer are alternately arranged, The a / b axis length ratio calculated by the Rietveld analysis of synchrotron radiation XRD is 0.8 or more.

[0009] The positive electrode active material of the present disclosure belongs to a compound of a lithium transition metal composite oxide having a crystal structure (also referred to as a layered stacked structure or an R-3m type crystal structure) in which a transition metal layer composed of an octahedral structure composed of a transition metal and oxygen and a lithium layer are alternately arranged. In the present disclosure, the lithium transition metal composite oxide means a composite oxide containing lithium and one or more transition metals. In the present disclosure, the a / b axis length ratio means a value obtained by dividing the length of the a axis by the length of the b axis in the crystal structure of the positive electrode active material.

[0010] As shown in the examples described later, a lithium ion secondary battery using a positive electrode active material with an a / b axis length ratio calculated by the Rietveld analysis of synchrotron radiation XRD of 0.8 or more exhibits an excellent capacity retention rate compared to a lithium ion secondary battery using a positive electrode active material with an a / b axis length ratio of less than 0.8. The reason is speculated as follows, for example. However, the present disclosure is not limited by the following speculation.

[0011] In a lithium transition metal composite oxide having a layered crystal structure, a phenomenon called cation mixing occurs in which transition metal ions occupy lithium sites in the lithium layer. Also, cation mixing is likely to occur when nickel, which has an ionic radius close to that of lithium, is included as the transition metal. Cation mixing can inhibit the movement of lithium ions during charge and discharge of a lithium ion secondary battery and can cause a decrease in the capacity retention rate.

[0012] The positive electrode active material of the present disclosure has an a / b axis length ratio calculated by Rietveld analysis of synchrotron radiation XRD of 0.8 or more. When the a / b axis length ratio is 0.8 or more, the area of the triangle facing the lithium layer in the octahedral structure of the transition metal layer becomes small, and the movement of nickel ions from the transition metal layer to the lithium layer is less likely to occur. As a result, it is considered that the movement of nickel ions into the lithium layer is suppressed and the capacity retention rate is improved.

[0013] From the viewpoint of improving the capacity retention rate of a lithium ion secondary battery, the a / b axis length ratio of the positive electrode active material is preferably 0.85 or more, more preferably 0.90 or more, and even more preferably 0.95 or more. The upper limit of the a / b axis length ratio of the positive electrode active material is not particularly limited, but from the viewpoint of the balance of the characteristics of the positive electrode active material, it is preferably 1.2 or less, more preferably 1.15 or less, and even more preferably 1.1 or less.

[0014] In the present disclosure, the a / b axis length ratio of the positive electrode active material is calculated by Rietveld analysis of synchrotron radiation XRD (X-ray diffraction). The calculation of the a / b axis length ratio can be carried out by a known method. For example, it is carried out by the method described in the examples below.

[0015] A cathode active material with an a / b axis length ratio of 0.8 or more can be obtained, for example, by adding a doping element to the raw material of the cathode active material. When a part of the transition metal constituting the transition metal layer of the cathode active material layer is replaced by the doping element, the triangular area facing the lithium layer of the transition metal ions (especially nickel ions) is reduced under the influence of the doping element. As a result, the movement of the transition metal ions (especially nickel ions) into the lithium layer is suppressed, and it is considered that the capacity retention rate of the lithium ion secondary battery is improved.

[0016] The type of the doping element contained in the transition metal layer is not particularly limited, and examples thereof include Au, Bi, Hf, La, Mo, Nb, Pd, Pr, Rh, Pt, Sr, Ta, Tc, Ti, W, Y, Zr and the like. From the viewpoint of adjusting the a / b axis length ratio to 0.8 or more, the cathode active material preferably contains a doping element having an ionic radius larger than that of nickel (ionic radius: 0.56 Å). Preferred examples of the doping element having an ionic radius larger than that of nickel (ionic radius: 0.56 Å) include Y (ionic radius: 0.69 Å), La (ionic radius: 1.03 Å), Nb (ionic radius: 0.72 Å), W (ionic radius: 0.62 Å), Sr (ionic radius: 1.18 Å), and Pr (ionic radius: 0.99 Å). From the viewpoint of adjusting the a / b axis length ratio of the cathode active material to 0.8 or more, the ionic radius of the doping element is more preferably 0.6 Å or more, further preferably 0.7 Å or more, and still further preferably 0.8 Å or more. The ionic radius of the doping element contained in the cathode active material may be 2.0 Å or less, 1.5 Å or less, or 1.3 Å or less. The type of the doping element contained in the cathode active material may be only one kind or two or more kinds.

[0017] The content rate of the doping element contained in the cathode active material is not particularly limited. From the viewpoint of sufficiently obtaining the effect of making the a / b axis length ratio of the cathode active material 0.8 or more, the content rate of the doping element may be 0.005 mol% or more with respect to the total of the transition metal and the doping element contained in the cathode active material. From the viewpoint of the balance of the characteristics of the positive electrode active material, the content rate of the doping element may be 1 mol% or less, 0.1 mol% or less, or 0.05 mol% or less with respect to the total of the transition metal and the doping element contained in the positive electrode active material.

[0018] The positive electrode active material of the present disclosure contains at least nickel as a transition metal. From the viewpoint of the balance of the characteristics of the positive electrode active material, it is more preferable that the positive electrode active material contains nickel as a transition metal and at least one selected from cobalt and manganese, and it is even more preferable that it contains nickel, cobalt, and manganese (NCM, nickel cobalt manganese oxide).

[0019] NCM may contain Ni at a high ratio (for example, 50 mol% or more, 60 mol% or more, or 70 mol% or more of the total transition metals).

[0020] The molar ratio of Ni, Co, and Mn contained in NCM may be selected, for example, from the range of the molar ratio of Ni to Co (Ni:Co) of 1:0.1 to 1:1, and the molar ratio of Ni to Mn (Ni:Mn) may be selected from the range of 1:0.1 to 1:1.

[0021] 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.

[0022] 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 a secondary particle that is an aggregate of a plurality of primary particles, the above volume average particle diameter is the volume average particle diameter of the secondary particle. The volume average particle diameter 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.

[0023] In the present disclosure, the volume-average particle diameter of the particles is the particle diameter (D50) at which the volume cumulative is 50% in the volume-based particle size distribution. The volume-based particle size distribution is obtained, for example, by the laser diffraction / scattering method.

[0024] <Lithium-ion secondary battery> The lithium-ion secondary battery of the present disclosure includes a positive electrode containing the above-described positive electrode active material. The positive electrode includes, for example, a current collector and a positive electrode layer disposed on the current collector, and the positive electrode layer contains the positive electrode active material of the present disclosure. The positive electrode layer may be disposed on one side or both sides of the current collector.

[0025] Examples of the material constituting the current collector of the positive electrode include aluminum, aluminum alloy, nickel, titanium, and stainless steel. Examples of the shape of the current collector include foil and mesh.

[0026] The positive electrode layer is disposed on the current collector, for example, by coating a slurry-like positive electrode material on one side or both sides of the current collector. If necessary, a pressure treatment may be performed to adjust the density of the positive electrode layer. The thickness of the positive electrode layer is not particularly limited and can be selected, for example, from the range of 10 μm to 100 μm.

[0027] The positive electrode material may be in a state of a mixture containing components other than the positive electrode active material, such as a conductive assistant and a binder. If necessary, a solvent may be added to the mixture to adjust the viscosity of the mixture.

[0028] Specific examples of the conductive assistant 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 a single type or two or more types.

[0029] Specific examples of the binder 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, polymethacrylate, and the like. The binder contained in the positive electrode material may be a single type or two or more types.

[0030] The lithium-ion secondary battery of the present disclosure 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 disposed on the current collector and containing a negative electrode active material. Examples of the type of the negative electrode active material include carbon materials such as graphite, hard carbon, soft carbon, and activated carbon, silicon, metallic lithium, lithium alloy, lithium titanate (LTO), and the like. Examples of the material constituting the current collector of the negative electrode include copper, copper alloy, nickel, titanium, stainless steel, and the like. Examples of the shape of the current collector of the negative electrode include foil, mesh, and the like.

[0031] The electrolyte may be either liquid or solid. As the liquid electrolyte (electrolyte solution), a known electrolyte such as LiPF6 dissolved in an organic solvent can be used without particular limitation. Specific examples of the organic solvent include cyclic or chain carbonates such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). 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 particular limitation.

[0032] The lithium-ion secondary battery may include a separator disposed between the positive electrode and the negative electrode. Examples of the separator include non-woven fabrics, cloths, and microporous films mainly composed of polyolefins such as polyethylene and polypropylene.

[0033] <Method for manufacturing a positive electrode active material> The method for manufacturing a positive electrode active material of the present disclosure is a method for manufacturing a positive electrode active material having a crystal structure in which a transition metal layer containing nickel and a lithium layer are alternately arranged, including adding a doping element to the positive electrode active material so that the a / b axis length ratio calculated from the Rietveld analysis of the synchrotron radiation XRD of the positive electrode active material is 0.8 or more.

[0034] In the method of the present disclosure, a doping element is added to the positive electrode active material so that the a / b axis length ratio calculated from the Rietveld analysis of the synchrotron radiation XRD of the positive electrode active material is 0.8 or more. That is, the a / b axis length ratio of the positive electrode active material manufactured by the method of the present disclosure is 0.8 or more. As shown in the examples described later, a lithium-ion secondary battery using a positive electrode active material having an a / b axis length ratio of 0.8 or more calculated from the Rietveld analysis of synchrotron radiation XRD exhibits an excellent capacity retention rate compared to a lithium-ion secondary battery using a positive electrode active material having an a / b axis length ratio of less than 0.8.

[0035] The conditions for implementing the method of the present disclosure are not particularly limited except for adding a doping element to the positive electrode active material so that the a / b axis length ratio is 0.8 or more, and known conditions can be used. The method of the present disclosure may be, for example, a method including a step of firing a mixture including a compound containing a transition metal as a raw material of the positive electrode active material, a compound containing lithium, and a compound containing a doping element. Examples of the compound containing a transition metal, lithium, or a doping element include hydroxides, carbonates, oxides, etc. The compound containing a transition metal may be a composite compound containing two or more transition metals.

[0036] The temperature during the firing process is not particularly limited and can be selected from known firing conditions. The firing temperature may be selected, for example, from the range of 600°C to 850°C. The temperature during the firing process may be constant or variable from the start to the end of the firing process. The firing process can be carried out, for example, in an atmosphere with an oxygen content of 40% to 100% by volume. The temperature or oxygen content during the firing process may be constant or variable from the start to the end of the firing process. The firing process may be carried out in one step or divided into two or more steps.

[0037] The positive electrode active material produced by the method of the present disclosure may be the positive electrode active material of the present disclosure described above. That is, the details and preferred embodiments of the positive electrode active material produced by the method of the present disclosure may be the same as the details and preferred embodiments of the positive electrode active material of the present disclosure described above.

Examples

[0038] Hereinafter, the present disclosure will be described in more detail with reference to examples, but the invention of the present disclosure is not limited to these examples.

[0039] <Preparation of Positive Electrode Active Material> NiSO4, CoSO4 and MnSO4 were dissolved in ion-exchanged water to obtain a raw material solution with a concentration of 30% by mass. The molar ratios of Ni, Co and Mn in the raw material solution were set to the values shown in Table 1.

[0040] An aqueous NH3 solution was introduced into the reaction vessel, and nitrogen substitution was carried out while stirring. Then, NaOH was added to the reaction vessel to adjust the aqueous solution to be alkaline. While controlling the pH in the reaction vessel to be maintained constant, the raw material solution and NH3 were dropped to precipitate hydroxides of Ni, Co and Mn. The obtained precipitate was taken out by filtration 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, and a transition metal hydroxide as a precursor of the positive electrode active material was obtained.

[0041] Lithium hydroxide and a compound containing a doping element shown in Table 1 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 lithium relative to 1 mol of the total transition metals (Ni, Co, and Mn) in the precursor was 1 mol. The amount of the compound containing the doping element was adjusted so that the amount of the doping element relative to the total of the transition metals (Ni, Co, and Mn) and the doping element in the precursor was 0.01 mol%.

[0042] The raw material for the positive electrode active material was calcined at 600 °C for 4 hours in an atmosphere with an oxygen content of 40% by volume. Then, the obtained calcined product was crushed, and further calcined at 700 °C for 10 hours in an atmosphere with an oxygen content of 100% by volume. Through the above steps, a positive electrode active material was obtained.

[0043] <Evaluation of the capacity retention rate of the battery> A positive electrode active material (88 parts by mass), acetylene black as a conductive material (10 parts by mass), and polyvinylidene fluoride as a binder (2 parts by mass) were mixed, and the viscosity was adjusted with a solvent to obtain a positive electrode mixture. The positive electrode mixture was coated on an aluminum foil and dried at 80 °C for 5 minutes to obtain a positive electrode. An electrode body in which the obtained positive electrode, a separator (a polyethylene microporous film), and a negative electrode containing graphite as an active material were laminated in this order, and an electrolytic solution were used to fabricate a laminated evaluation battery. As the electrolytic solution, a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (the volume ratio of EC / DMC / EMC was 3 / 4 / 3) in which LiPF6 (concentration: 1 M) was dissolved was used. For the evaluation battery, the discharge capacity when performing CCCV charging (1.5 to 4.1 V) at a current value of 0.1 C was measured and used as the initial capacity. Then, charge and discharge were performed 100 cycles under the conditions of 1 C and 1.5 to 4.1 V, and the discharge capacity after 100 cycles was measured. The capacity retention rate of the battery was obtained by the following formula. The results are shown in Table 1. Capacity retention rate (%) = (Discharge capacity after 100 cycles / Initial capacity) × 100

[0044] <Calculation of a / b axis length ratio> The a / b axis length ratio of the positive electrode active material was calculated by performing Rietveld analysis of synchrotron radiation XRD. The synchrotron radiation XRD data was obtained using Rigaku Corporation's SmartLab, and the measurement angle (2θ) was set to 10° to 120°. Using the analysis software nano-lab, a structural model in a state where a doping element was doped into the R-3m type crystal structure was created. By measuring the changes in the axis lengths of the crystal axes a and b due to the change in the position of oxygen in the state where Ni forms an octahedron, the a / b axis length ratio was calculated. The results are shown in Table 1.

[0045] <Calculation of Ni mixing amount> The amount of Ni ions moving into the lithium layer (Ni mixing) in the battery after 100 cycles of charge and discharge was calculated by performing Rietveld analysis of synchrotron radiation XRD under the above-mentioned conditions. Specifically, the Ni mixing amount (%) was calculated from the fitting results of the diffraction pattern using the analysis software Fullprof. The results are shown in Table 1.

[0046]

Table 1

[0047] As shown in Table 1, the batteries of Examples 1 to 5 fabricated using a positive electrode active material with an a / b axis length ratio of 0.8 or more had a better capacity retention rate after the cycle test than the battery of Comparative Example 1 fabricated using a positive electrode active material with an a / b axis length ratio of less than 0.8. Also, as shown in Table 1, the batteries of Examples 1 to 5 had less Ni mixing amount after the cycle test than the battery of Comparative Example 1. The above results suggest that when the a / b axis length ratio of the positive electrode active material is 0.8 or more, the mixing of Ni ions from the transition metal layer into the lithium layer is suppressed, and the capacity retention rate of the battery is improved.

Claims

1. It has a crystal structure in which a transition metal layer containing nickel and a lithium layer are alternately arranged, and the a / b axis length ratio calculated from the Rietveld analysis of synchrotron radiation XRD is 0.8 or more. A positive electrode active material.

2. The positive electrode active material according to claim 1, which contains a doping element having an ionic radius larger than that of nickel.

3. The positive electrode active material according to claim 1, wherein the a / b axis length ratio is 1.2 or less.

4. A lithium-ion secondary battery comprising a positive electrode containing the positive electrode active material according to any one of claims 1 to 3.

5. A method for producing a positive electrode active material having a crystal structure in which a transition metal layer containing nickel and a lithium layer are alternately arranged, The method for producing a positive electrode active material includes adding a doping element to the positive electrode active material so that the a / b axis length ratio calculated from the Rietveld analysis of synchrotron radiation XRD of the positive electrode active material is 0.8 or more.

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