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 reduced resistance in a lithium-ion secondary battery, a lithium-ion secondary battery 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 arranged alternately and contains a dopant element M1 and a dopant element M2 whose ion radius ratio expressed by M1 / M2 is 1.03 or more and 2.2 or less.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 a 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 a 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 (see, for example, 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, a lithium-ion secondary battery using a positive electrode active material containing nickel as a transition metal has a problem of reducing resistance. An object of the present disclosure is to provide a positive electrode active material containing nickel as a transition metal and having reduced resistance 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 positive electrode active material having a crystal structure in which a transition metal layer containing nickel and a lithium layer are alternately arranged, and containing a doping element M1 and a doping element M2 with an ion radius ratio represented by M1 / M2 being 1.03 or more and 2.2 or less. <2>The positive electrode active material according to <1>, wherein the ion radius ratios of the doping element M1 and the doping element M2 to nickel are each 0.7 or more and 2.3 or less. <3>The positive electrode active material according to <1> or <2>, wherein the doping element M1 and the doping element M2 are each selected from the group consisting of Sn, Y, Pr, La, Sr, Ta, W, Fe, and Nb. <4>A lithium ion secondary battery including a positive electrode containing the positive electrode active material according to any one of <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 M1 and a doping element M2 with an ion radius ratio represented by M1 / M2 being 1.03 or more and 2.2 or less to the positive electrode active material.

Advantages of the Invention

[0006] According to one embodiment of the present disclosure, there are provided a positive electrode active material containing nickel as a transition metal and having a reduced resistance 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 producing this positive electrode 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 the minimum value and the maximum value, respectively. In the numerical ranges described step by step in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the upper limit value or the lower limit value of the numerical range described in other step-by-step descriptions. In the numerical ranges described in the present disclosure, the upper limit value or the 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 "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 It has a crystal structure in which a transition metal layer containing nickel and a lithium layer are alternately arranged, and contains a doping element M1 and a doping element M2 having an ion radius ratio represented by M1 / M2 of 1.03 or more and 2.2 or less.

[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 laminated 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 ion radius ratio represented by M1 / M2 means a value obtained by dividing the ion radius of the doping element M1 by the ion radius of the doping element M2.

[0010] As shown in the embodiments described below, a lithium-ion secondary battery using a cathode active material containing doping elements M1 and M2 with an ionic radius ratio represented by M1 / M2 of 1.03 or more and 2.2 or less has a reduced resistance compared to a lithium-ion secondary battery using a cathode active material that does not satisfy the above conditions. The reason is speculated as follows, for example. However, the present disclosure is not limited by the following speculation.

[0011] In a lithium-ion secondary battery using a lithium transition metal composite oxide having a layered crystal structure as a cathode active material, lithium ions are desorbed from the lithium layer disposed between the transition metal layers during charging, and lithium ions are inserted into the lithium layer during discharging. When the doping elements M1 and M2 with an ionic radius ratio of 1.03 or more and 2.2 or less are contained in the transition metal layer, the positions of oxygen in the transition metal layer change, the octahedral structure contracts, and the width of the lithium layer expands. As a result, it is considered that the desorption or insertion of lithium ions in the lithium layer becomes easy and the resistance of the battery is reduced.

[0012] From the viewpoint of effectively reducing the resistance of the battery, the ionic radius ratio represented by M1 / M2 is preferably 1.1 or more, more preferably 1.3 or more, and even more preferably 1.5 or more. From the viewpoint of effectively reducing the resistance of the battery, the ionic radius ratio represented by M1 / M2 is preferably 2.0 or less, more preferably 1.8 or less, and even more preferably 1.7 or less.

[0013] The types of doping elements contained in the transition metal layer are not particularly limited, and examples include Au, Bi, Hf, La, Mo, Nb, Pd, Pr, Rh, Pt, Sr, Ta, Tc, Ti, W, Y, Zr, etc. Preferred examples of the doping element 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), Pr (ionic radius: 0.99 Å), and Fe (ionic radius: 0.55 Å). The types of doping elements contained in the positive electrode active material may be only two types or three or more types.

[0014] From the viewpoint of effectively reducing the resistance of the battery, it is preferable that the ionic radius ratios (M1 or M2 / Ni) of the doping elements M1 and M2 to the ionic radius of Ni (0.56 Å) are each 0.7 or more and 2.3 or less.

[0015] The total content ratio of the doping element M1 and the doping element M2 contained in the transition metal layer is not particularly limited. From the viewpoint of sufficiently obtaining the effect of reducing the resistance of the battery, the total content ratio of the doping element M1 and the doping element M2 contained in the transition metal layer may be 0.005 mol% or more with respect to the total of the transition metal and the doping element contained in the positive electrode active material. From the viewpoint of the balance of the characteristics of the positive electrode active material, the total content ratio of the doping element M1 and the doping element M2 contained in the transition metal layer 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. The molar ratio of the doping element M1 and the doping element M2 contained in the transition metal layer is not particularly limited. From the viewpoint of sufficiently obtaining the effect of reducing the resistance of the battery, the molar ratio (M1 / M2) of the doping element M1 to the doping element M2 is preferably in the range of 0.5 to 2.0.

[0016] 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).

[0017] 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 metal).

[0018] The molar ratios of Ni, Co, and Mn contained in the NCM may be selected, for example, such that the molar ratio of Ni to Co (Ni:Co) is in the range of 1:0.1 to 1:1, and the molar ratio of Ni to Mn (Ni:Mn) is in the range of 1:0.1 to 1:1.

[0019] 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.

[0020] The positive electrode active material may be in 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 secondary particles that are an aggregate of a plurality of primary particles, the above volume average particle diameter is the volume average particle diameter of the secondary particles. 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.

[0021] In the present disclosure, the volume average particle diameter of the particles is the particle diameter (D50) when the volume cumulative is 50% in the volume-based particle size distribution. The volume-based particle size distribution can be obtained, for example, by the laser diffraction / scattering method.

[0022] <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.

[0023] Examples of the material constituting the current collector of the positive electrode include aluminum, aluminum alloy, nickel, titanium, stainless steel, etc. Examples of the shape of the current collector include foil, mesh, etc.

[0024] The arrangement of the positive electrode layer on the current collector is carried out, for example, by coating a slurry-like positive electrode material on one or both sides of the current collector. If necessary, a pressing 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.

[0025] The positive electrode material may be in the 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.

[0026] 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.

[0027] 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, etc. The binder contained in the positive electrode material may be a single type or two or more types.

[0028] 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, and lithium titanate (LTO). Examples of the material constituting the current collector of the negative electrode include copper, copper alloy, nickel, titanium, stainless steel, etc. Examples of the shape of the current collector of the negative electrode include foil, mesh, etc.

[0029] 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 a mixture containing a cyclic carbonate and a chain carbonate. The solvent may contain additives 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.

[0030] 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.

[0031] <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 M1 and a doping element M2 having an ionic radius ratio represented by M1 / M2 of 1.03 or more and 2.2 or less to the positive electrode active material.

[0032] In the method of the present disclosure, a doping element M1 and a doping element M2 represented by M1 / M2 and having an ion radius ratio of 1.03 or more and 2.2 or less are added to the positive electrode active material. That is, the positive electrode active material produced by the method of the present disclosure contains a doping element M1 and a doping element M2 represented by M1 / M2 and having an ion radius ratio of 1.03 or more and 2.2 or less. As shown in the examples described later, a lithium-ion secondary battery using a positive electrode active material containing a doping element M1 and a doping element M2 represented by M1 / M2 and having an ion radius ratio of 1.03 or more and 2.2 or less has a reduced resistance compared to a lithium-ion secondary battery using a positive electrode active material not containing a doping element M1 and a doping element M2 represented by M1 / M2 and having an ion radius ratio of 1.03 or more and 2.2 or less.

[0033] The conditions for carrying out the method of the present disclosure are not particularly limited except for adding a doping element M1 and a doping element M2 represented by M1 / M2 and having an ion radius ratio of 1.03 or more and 2.2 or less to the positive electrode active material, 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 containing 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.

[0034] The temperature at the time of carrying out the firing step 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 at the time of carrying out the firing step may be constant from the start to the end of the firing step or may be changed. The firing step can be carried out, for example, in an atmosphere having an oxygen content of 40% by volume to 100% by volume. The temperature or oxygen content at the time of carrying out the firing step may be constant from the start to the end of the firing step or may be changed. The firing step may be carried out in one stage or may be carried out in two or more stages.

[0035] 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

[0036] 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.

[0037] <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.

[0038] An aqueous NH3 solution was introduced into the reaction vessel, and nitrogen substitution was carried out while stirring. Next, 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.

[0039] Lithium hydroxide and a compound containing the doping elements 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 mole of the total transition metals (Ni, Co and Mn) in the precursor was 1 mole. The amount of the compound containing the doping elements was adjusted so that the amounts of the doping elements M1 and M2 relative to the total of the transition metals (Ni, Co and Mn) and the doping elements (M1 and M2) in the precursor were each 0.01 mol%.

[0040] The raw material of the positive electrode active material was calcined at 700 °C for 3 hours. Then, the obtained calcined product was crushed and further calcined at 850 °C for 10 hours. Through the above steps, a positive electrode active material was obtained.

[0041] <Evaluation of battery resistance> The 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) (volume ratio of EC / DMC / EMC is 3 / 4 / 3) in which LiPF6 (concentration: 1 M) was dissolved was used. The charge rate (SOC) of the evaluation battery was adjusted to 50% and the temperature was adjusted to -10 °C. Then, the resistance of the battery was measured from the difference between the voltage and current during 0.2C discharge and the voltage and current 10 seconds after the start of 1C discharge. The obtained measured values were converted into an index when the measured value of the reference battery was set to 100. The results are shown in Table 1. The reference battery was fabricated in the same manner as the above evaluation battery except that no doping element was added to the raw material of the positive electrode active material and the firing process was carried out in one step at 750 °C for 10 hours.

[0042] <Measurement of the width of the lithium layer> The width of the lithium layer of the positive electrode active material was calculated by performing Rietveld analysis of synchrotron radiation XRD. The synchrotron radiation XRD was carried out using the powder X-ray diffraction equipment BL5S2 at the Aichi Synchrotron Center under the conditions of measurement energy: 15 keV, threshold: 7.5 - 10 keV, 2θ range: 10 - 90°. The results are shown in Table 1. For the obtained synchrotron radiation XRD data, Rietveld analysis was performed using the Rietveld analysis application Fullprof. Specifically, when the Chi2 value takes the minimum value, the c-axis length (C h ) and the z coordinate of oxygen (Z oxy ) were determined, and the width of the lithium layer (D Li ) was calculated by the following formula. D TM = 2{(1 / 3) - Z oxy}C h D Li = C h / 3 - D TM The Chi2 value is the value of the convergence index obtained by fitting the diffraction data using the least squares method. The Chi2 value takes the minimum value when the deviation between the diffraction data and the profile fitting is minimized.

[0043]

Table 1

[0044] As shown in Table 1, the batteries of Examples 1 to 6 prepared using the cathode active material to which the doping elements M1 and M2 with an ionic radius ratio represented by M1 / M2 of 1.03 or more and 2.2 or less were added had a reduced battery resistance compared to the batteries of Comparative Examples 1 and 2 in which the doping elements added to the cathode active material did not satisfy the above conditions. Also, as shown in Table 1, the cathode active materials obtained in Examples 1 to 5 had a wider lithium layer width than the cathode active materials obtained in Comparative Examples 1 and 2. The above results suggest that the inclusion of the doping elements M1 and M2 with an ionic radius ratio represented by M1 / M2 of 1.03 or more and 2.2 or less in the cathode active material expands the width of the lithium layer and reduces the battery resistance.

Claims

1. It has a crystal structure in which a transition metal layer containing nickel and a lithium layer are alternately arranged, A positive electrode active material containing a doping element M1 and a doping element M2 having an ion radius ratio represented by M1 / M2 of 1.03 or more and 2.2 or less.

2. The positive electrode active material according to claim 1, wherein the ion radius ratios of the doping element M1 and the doping element M2 to nickel are each 0.7 or more and 2.3 or less.

3. The positive electrode active material according to claim 1, wherein the doping element M1 and the doping element M2 are each selected from the group consisting of Sn, Y, Pr, La, Sr, Ta, W, Fe, and Nb.

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, A method for producing a positive electrode active material, comprising adding a doping element M1 and a doping element M2 having an ion radius ratio represented by M1 / M2 of 1.03 or more and 2.2 or less to the positive electrode active material.

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