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

Abstract

The invention relates to a positive electrode active material, a lithium ion secondary battery, and a method for manufacturing the positive electrode active material. The positive electrode active material has a crystal structure in which lithium layers and transition metal layers containing nickel are alternately arranged, and contains a dopant element M1 and a dopant element M2 having an ion radius ratio represented by M1 / M2 of 1.03 to 2.2 (inclusive).

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] As a positive electrode active material for a lithium ion secondary battery, a lithium transition metal composite oxide having a layered crystal structure in which a transition metal layer and a lithium layer are alternately arranged is widely used. In the transition metal layer, an octahedral structure is formed by a transition metal and oxygen.

[0003] As a lithium transition metal composite oxide having a layered crystal structure, a lithium transition metal composite oxide containing at least one selected from nickel, cobalt, and manganese as a transition metal is known (for example, see JP-A-2019-23149). Summary of the Invention

[0004] Among lithium transition metal composite oxides having a layered crystal structure, a composite oxide containing nickel as a transition metal is suitable as a positive electrode active material for a large-capacity lithium ion secondary battery such as a battery for a battery electric vehicle (BEV). On the other hand, in a lithium ion secondary battery using a positive electrode active material containing nickel as a transition metal, reduction of resistance has become a problem.

[0005] 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 the positive electrode active material, and a method for manufacturing the positive electrode active material.

[0006] Means for solving the above problems include the following embodiments.

[0007] 1. A positive electrode active material having a crystal structure in which a lithium layer and a transition metal layer containing nickel are alternately arranged, and containing a doping element M1 and a doping element M2 having an ion radius ratio M1 / M2 of 1.03 or more and 2.2 or less.

[0008] 2. The positive electrode active material according to 1, wherein the ion radius ratios of the doping element M1 and the doping element M2 with respect to nickel are each 0.7 or more and 2.3 or less.

[0009] 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 Sn, Y, Pr, La, Sr, Ta, W, Fe, and Nb.

[0010] 4. A lithium ion secondary battery including a positive electrode containing the positive electrode active material according to any one of 1 to 3.

[0011] 5. A method for manufacturing a positive electrode active material, the positive electrode active material having a crystal structure in which lithium layers and transition metal layers containing nickel are alternately arranged, the manufacturing method including 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.

[0012] According to an embodiment of the present disclosure, a positive electrode active material containing nickel as a transition metal and reducing the resistance of a lithium ion secondary battery, a lithium ion secondary battery including a positive electrode containing the positive electrode active material, and a method for manufacturing the positive electrode active material can be provided. Detailed embodiments

[0013] In the present disclosure, a numerical range specified by “A - B”, “between A and B”, “from A to B”, etc. herein represents a range including a minimum value A and a maximum value B.

[0014] In the numerical ranges described stepwise in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range can be replaced with the upper limit value or the lower limit value of other stepwise described numerical ranges. In the numerical ranges described in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range can be replaced with the value shown in the examples.

[0015] In the present disclosure, a term such as “process” not only refers to an independent process, but also includes the case where it cannot be clearly distinguished from other processes as long as the expected purpose of the process is achieved.

[0016] In the present disclosure, a combination of two or more preferred solutions is a more preferred solution.

[0017] In the present disclosure, when there are multiple substances corresponding to each component, the amount of each component refers to the total amount of the multiple substances as long as there is no special description.

[0018] Positive electrode active material

[0019] The positive electrode active material of the present disclosure has a crystal structure in which lithium layers and transition metal layers containing nickel 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.

[0020] The positive electrode active material of the present disclosure is a compound belonging to a lithium transition metal composite oxide having a crystal structure in which lithium layers and transition metal layers are alternately arranged (also referred to as a layered stacked structure or an R - 3m type crystal structure). The transition metal layer is composed of an octahedral structure formed by a transition metal and oxygen.

[0021] In the present disclosure, the lithium transition metal composite oxide refers to a composite oxide containing lithium and one or more transition metals.

[0022] In the present disclosure, the ionic radius ratio represented by M1 / M2 refers to the value obtained by dividing the ionic radius of the doping element M1 by the ionic radius of the doping element M2.

[0023] In the following embodiments, a lithium ion secondary battery using a positive electrode active material containing a doping element M1 and a doping element M2 with an ionic radius ratio represented by M1 / M2 of 1.03 or more and 2.2 or less is shown. This lithium ion secondary battery has a reduced resistance compared to a lithium ion secondary battery using a positive electrode active material that does not satisfy the above conditions. The reason is presumably as follows, for example. However, the present disclosure is not limited by the following speculation.

[0024] In a lithium ion secondary battery using a layered crystal structure lithium transition metal composite oxide as the positive electrode active material, during charging, lithium ions are detached from the lithium layer disposed between the transition metal layers. Moreover, in this lithium ion secondary battery, during discharging, lithium ions are inserted into the lithium layer.

[0025] If the transition metal layer contains a doping element M1 and a doping element M2 with an ionic radius ratio of 1.03 or more and 2.2 or less, the position of oxygen in the transition metal layer changes, the octahedral structure contracts, and the width of the lithium layer expands. As a result, it is considered that the detachment or insertion of lithium ions in the lithium layer becomes easier, and the resistance of the battery decreases.

[0026] From the viewpoint of effectively reducing the battery resistance, the ionic radius ratio represented by M1 / M2 is preferably 1.1 or more, more preferably 1.3 or more, and further preferably 1.5 or more.

[0027] From the viewpoint of effectively reducing the battery resistance, the ionic radius ratio represented by M1 / M2 is preferably 2.0 or less, more preferably 1.8 or less, and further preferably 1.7 or less.

[0028] There is no particular limitation on the type of doping element contained in the transition metal layer, and examples thereof include Au, Bi, Hf, La, Mo, Nb, Pd, Pr, Rh, Pt, Sr, Ta, Tc, Ti, W, Y, Zr, etc.

[0029] As a preferred example of the doping element, Y (ionic radius: )), La (ionic radius: )), Nb (ionic radius: )), W (ionic radius: )). In addition, as a preferred example of the doping element, Sr (ionic radius: )), Pr (ionic radius: )), and Fe (ionic radius: ) can be cited.

[0030] The types of doping elements contained in the positive electrode active material may be only two types, or may be three or more types.

[0031] From the viewpoint of effectively reducing the battery resistance, the ionic radii of the doping elements M1 and M2 relative to Ni The ionic radius ratio (M1 / Ni or M2 / Ni) is preferably 0.7 or more and 2.3 or less.

[0032] There is no particular limitation on the total content ratio of the doping element M1 and the doping element M2 contained in the transition metal layer.

[0033] From the viewpoint of sufficiently obtaining the effect of reducing the battery resistance, 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 relative to the total of the transition metal and the doping element contained in the positive electrode active material.

[0034] 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, or 0.1 mol% or less relative to the total of the transition metal and the doping element contained in the positive electrode active material. Or, 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 0.05 mol% or less relative to the total of the transition metal and the doping element contained in the positive electrode active material.

[0035] There is no particular limitation on the molar ratio of the doping element M1 and the doping element M2 contained in the transition metal layer. From the viewpoint of sufficiently obtaining the effect of reducing the battery resistance, the molar ratio of the doping element M1 to the doping element M2 (M1 / M2) is preferably in the range of 0.5 to 2.0.

[0036] The positive electrode active material of the present disclosure contains at least nickel as a transition metal.

[0037] 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 and at least one selected from cobalt and manganese as transition metals. In addition, from the viewpoint of the balance of the characteristics of the positive electrode active material, it is further preferable that the positive electrode active material contains nickel, cobalt, and manganese (NCM, nickel cobalt manganese oxide) as transition metals.

[0038] 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 whole transition metal).

[0039] Regarding the molar ratios of Ni, Co, and Mn contained in NCM, for example, the molar ratio of Ni to Co (Ni:Co) can be selected from the range of 1:0.1 to 1:1, and the molar ratio of Ni to Mn (Ni:Mn) can be selected from the range of 1:0.1 to 1:1.

[0040] The molar ratio of Ni to Co (Ni:Co) can 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.

[0041] The molar ratio of Ni to Mn (Ni:Mn) can 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.

[0042] The positive electrode active material can be in particle form. There is no particular limitation on the volume average particle diameter of the particulate positive electrode active material. For example, it can be selected from the range of 5 μm to 30 μm. In the case where the positive electrode active material is an aggregate of a plurality of primary particles, i.e., secondary particles, the above volume average particle diameter is the volume average particle diameter of the secondary particles.

[0043] There is no particular limitation on the volume average particle diameter of the positive electrode active material particles. For example, it can be selected from the range of 5 μm to 30 μm.

[0044] In the present disclosure, the volume average particle diameter of the particles is the particle diameter (D50) at which the cumulative volume in the volume-based particle size distribution becomes 50%. The volume-based particle size distribution is obtained, for example, by the laser diffraction / scattering method.

[0045] Lithium ion secondary battery

[0046] The lithium ion secondary battery of the present disclosure includes a positive electrode containing the above positive electrode active material.

[0047] The positive electrode includes, for example, a current collector and a positive electrode layer disposed on the current collector. The positive electrode layer contains the positive electrode active material of the present disclosure.

[0048] The positive electrode layer can be disposed on one side of the current collector or on both sides of the current collector.

[0049] As the material of the current collector constituting the positive electrode, aluminum, aluminum alloy, nickel, titanium, stainless steel, etc. can be cited. As the shape of the current collector, foil, mesh, etc. can be cited.

[0050] The disposition of the positive electrode layer on the current collector is performed, for example, by coating a slurry-like positive electrode material on one side or both sides of the current collector. If necessary, a pressing treatment for adjusting the density of the positive electrode layer can be performed. There is no particular limitation on the thickness of the positive electrode layer. For example, it can be selected from the range of 10 μm to 100 μm.

[0051] The positive electrode material may be in a state of a mixture containing components other than positive electrode active materials such as a conductive aid and a binder. If necessary, a solvent may be added to the mixture to adjust the viscosity of the mixture.

[0052] Specific examples of the conductive aid include carbon materials such as carbon black (acetylene black, pyrolytic carbon black, furnace black, etc.), carbon nanotubes, and graphite.

[0053] The conductive material contained in the positive electrode material may be one kind alone or two or more kinds.

[0054] Specific examples of the binder include polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polyethylene terephthalate, cellulose, nitrocellulose, carboxymethyl cellulose, polyethylene oxide, etc. Additionally, specific examples of the binder include epichlorohydrin polymer, polyacrylonitrile, styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), polyacrylate, polymethacrylate, etc.

[0055] The binder contained in the positive electrode material may be one kind alone or two or more kinds.

[0056] The lithium ion secondary battery of the present disclosure includes, for example, a positive electrode, a negative electrode, and an electrolyte.

[0057] The negative electrode includes, for example, a current collector and a negative electrode layer containing a negative electrode active material disposed on the current collector.

[0058] Examples of the types of negative electrode active materials include carbon materials such as graphite, hard carbon, soft carbon, and activated carbon; silicon, metallic lithium, lithium alloys, lithium titanate (LTO), etc.

[0059] Examples of the material of the current collector constituting 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.

[0060] The electrolyte may be either liquid or solid. As the liquid electrolyte (electrolyte solution), a product obtained by dissolving a known electrolyte such as LiPF6 in an organic solvent can be used without particular limitation.

[0061] 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 and may be a mixture containing a cyclic carbonate and a chain carbonate.

[0062] The solvent may contain additives such as vinylene carbonate (VC).

[0063] 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.

[0064] The lithium-ion secondary battery may include a separator (partition) disposed between the positive electrode and the negative electrode. As the separator, non-woven fabrics, fabrics, microporous membranes, etc. mainly composed of polyolefins such as polyethylene and polypropylene can be cited.

[0065] Manufacturing method of positive electrode active material

[0066] The manufacturing method of the positive electrode active material of the present disclosure is a manufacturing method of a positive electrode active material having a crystal structure in which a lithium layer and a transition metal layer containing nickel are alternately arranged, and the manufacturing method includes 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.

[0067] In the method of the present disclosure, 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 are added to the positive electrode active material. That is, the positive electrode active material manufactured by the method of the present disclosure 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.

[0068] In the following examples, a lithium-ion secondary battery using 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 is shown. This lithium-ion secondary battery has a reduced resistance compared to a lithium-ion secondary battery using a positive electrode active material that does not contain 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.

[0069] The conditions for implementing the method of the present disclosure are not particularly limited except for 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, and can be set as known conditions.

[0070] The method of the present disclosure may include, for example, the following steps: a step of firing a mixture containing a transition metal-containing compound, a lithium-containing compound, and a doping element-containing compound as raw materials for the positive electrode active material.

[0071] Examples of the compound containing a transition metal, lithium, or a doping element include hydroxides, carbonates, oxides, etc. The transition metal-containing compound may be a composite compound containing two or more transition metals.

[0072] There is no particular limitation on the temperature during the firing process, and it can be selected from known firing conditions. For example, the firing temperature can be selected in the range of 600°C to 850°C.

[0073] The temperature during the firing process can be a fixed temperature or can vary from the start to the end of the firing process.

[0074] The firing process can be carried out, for example, in an atmosphere with an oxygen content of 40% by volume to 100% by volume.

[0075] The temperature or oxygen content during the firing process can be fixed or can vary from the start to the end of the firing process.

[0076] The firing process can be carried out in one stage or can be divided into two or more stages.

[0077] The positive electrode active material manufactured by the method of the present disclosure can be the positive electrode active material of the present disclosure as described above. That is, the detailed information and preferred modes of the positive electrode active material manufactured by the method of the present disclosure can be the same as the detailed information and preferred modes of the positive electrode active material of the present disclosure as described above.

[0078] The present disclosure will be described in more detail below using examples, and the invention of the present disclosure is not limited to these examples.

[0079] Preparation of Positive Electrode Active Material

[0080] 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 the values shown in Table 1.

[0081] An NH3 aqueous solution was introduced into the reaction vessel, and nitrogen replacement was carried out while stirring. Next, NaOH was added to the reaction vessel to adjust the aqueous solution to be alkaline.

[0082] While controlling to maintain the pH in the reaction vessel at a constant (fixed) value, the raw material solution and NH3 were dropped to precipitate the 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 the moisture, and a transition metal hydroxide as a precursor of the positive electrode active material was obtained.

[0083] 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 of the positive electrode active material.

[0084] Regarding the amount of lithium hydroxide, it was adjusted so that the amount of lithium relative to 1 mole in total of the transition metals (Ni, Co, and Mn) in the precursor became 1 mole.

[0085] Regarding the amounts of the compounds containing the doping elements, the amounts of the doping elements M1 and M2 are adjusted such that the amounts of the doping elements M1 and M2 each become 0.01 mol% relative to the total of the transition metals (Ni, Co, and Mn) and the doping elements (M1 and M2) in the precursor.

[0086] For the raw material of the positive electrode active material, firing was carried out at 700 °C for 3 hours. Then, the obtained fired product was crushed, and further, firing was carried out at 850 °C for 10 hours. Through the above processes, the positive electrode active material was obtained.

[0087] Evaluation of battery resistance

[0088] The 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 composite material. The positive electrode composite material was coated on an aluminum foil and dried at 80 °C for 5 minutes to obtain a positive electrode.

[0089] The obtained positive electrode, a separator (a polyethylene microporous membrane), and a negative electrode containing graphite as an active material were laminated in sequence, and a laminated evaluation battery was fabricated using the laminated electrode body and an electrolyte solution.

[0090] As the electrolyte solution, a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (the volume ratio of EC / DMC / EMC is 3 / 4 / 3) in which LiPF6 (concentration: 1 M) was dissolved was used.

[0091] The charge rate (SOC) of the evaluation battery was adjusted to 50%, and the temperature was adjusted to -10 °C. Next, based on the differences in voltage and current during 0.2C discharge and the voltage and current 10 seconds after the start of 1C discharge, the resistance of the battery was measured. The obtained measured value was converted to an index when the measured value of the standard battery was set to 100. The results are shown in Table 1.

[0092] Regarding the standard battery, except that no doping elements were added to the raw material of the positive electrode active material and the firing process was carried out in one stage at 750 °C for 10 hours, it was fabricated in the same manner as the above-described evaluation battery.

[0093] Measurement of the width of the lithium layer

[0094] Rietveld analysis of synchrotron radiation XRD was performed to calculate the width of the lithium layer of the positive electrode active material.

[0095] Regarding synchrotron radiation XRD, measurements were carried out using the powder X-ray diffractometer BL5S2 at the Aichi Synchrotron Radiation Center under the conditions of measurement energy: 15 keV, threshold: 7.5 to 10 keV, 2θ range: 10 to 90°. The results are shown in Table 1.

[0096] For the synchrotron radiation XRD data obtained, Rietveld analysis was performed using the Application Fullprof for Rietveld analysis.

[0097] Specifically, the c-axis length (C h ) and the z-coordinate of oxygen (Z oxy ) were determined when the Chi2 value was at its minimum, and the width of the lithium layer (D Li ) was calculated according to the following formula.

[0098] D TM = 2{(1 / 3) - Z oxy}C h

[0099] D Li = C h / 3 - D TM

[0100] The Chi2 value is the value of the convergence index obtained by using the least squares method for fitting the diffraction data. The Chi2 value is at its minimum when the deviation between the diffraction data and the profile fitting is the smallest.

[0101] Table 1

[0102]

[0103] As shown in Table 1, the batteries of Examples 1 to 6 were fabricated using a positive electrode active material doped with doping elements M1 and M2 having an ionic radius ratio represented by M1 / M2 of 1.03 or more and 2.2 or less. Compared with the batteries of Comparative Examples 1 and 2 in which the doping elements added to the positive electrode active material did not satisfy the above conditions, the resistance of the batteries was reduced.

[0104] In addition, as shown in Table 1, the width of the lithium layer in the positive electrode active materials obtained in Examples 1 to 5 was wider than that in the positive electrode active materials obtained in Comparative Examples 1 and 2. The above results suggest that by including doping elements M1 and M2 having an ionic radius ratio represented by M1 / M2 of 1.03 or more and 2.2 or less in the positive electrode active material, the width of the lithium layer becomes wider and the resistance of the battery is reduced.

Claims

1. A positive electrode active material having a crystal structure in which lithium layers and transition metal layers containing nickel are alternately arranged, and comprising a doping element M1 and a doping element M2, wherein the ion radius ratio represented by M1 / M2 is 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 greater than or equal to 0.7 and less than or equal to 2.

3.

3. The positive electrode active material according to claim 1, wherein The doping element M1 and the doping element M2 are each selected from Sn, Y, Pr, La, Sr, Ta, W, Fe and Nb. 4 . A lithium ion secondary battery comprising a positive electrode, wherein the positive electrode comprises the positive electrode active material according to claim 1 .

5. A method for manufacturing a positive electrode active material, wherein the positive electrode active material has a crystal structure in which lithium layers and transition metal layers containing nickel are alternately arranged, and the manufacturing method comprises adding an impurity element M1 and an impurity element M2 whose ion radius ratio represented by M1 / M2 is greater than 1.03 and less than 2.2 to the positive electrode active material.

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