Patent Application
20250105271
The present invention relates to techniques for a positive electrode for a secondary battery and a method for producing a positive electrode active material for a secondary battery.
As a positive electrode active material for a secondary battery such as a lithium ion secondary battery, for example, Patent Literature 1 discloses a positive electrode active material represented by a composition formula LixNi1-yCoy-zMzO2-aXb (M is at least Al), in which a lattice constant of an a-axis measured by X-ray diffraction is greater than or equal to 2.81 Å and less than or equal to 2.91 Å, a lattice constant of a c-axis measured by X-ray diffraction is greater than or equal to 13.7 Å and less than or equal to 14.4 Å, and a ratio of a diffraction peak intensity of a (104) plane to a peak intensity of a (003) plane is greater than or equal to 0.3 and less than or equal to 0.8.
An object of the present disclosure is to provide a positive electrode for a secondary battery and a method for producing a positive electrode active material for a secondary battery that can increase the capacity of the secondary battery.
A positive electrode for a secondary battery according to one aspect of the present disclosure includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, in which the positive electrode mixture layer contains a positive electrode active material containing a lithium-containing composite oxide composed of secondary particles in which primary particles are aggregated, a coating film containing a Nb element is formed on a surface of the primary particle of the lithium-containing composite oxide, and a Gini coefficient of the Nb element calculated from an element mapping image obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) of a cross section of the secondary particle is less than or equal to 0.5.
A method for producing a positive electrode active material for a secondary battery according to one aspect of the present disclosure includes a first step of baking a mixture containing a lithium compound and a metal element-containing compound other than lithium to obtain a lithium-containing composite oxide; and a second step of mixing the lithium-containing composite oxide obtained in the first step with a solution in which a Nb compound is dissolved, and baking the obtained mixture.
According to one aspect of the present disclosure, it is possible to provide a positive electrode for a secondary battery and a method for producing a positive electrode active material for a secondary battery that can increase the capacity of the secondary battery.
Hereinafter, an example of an embodiment will be described in detail. The drawings referred to in the description of embodiments are schematically illustrated, and dimensional ratios and the like of components drawn in the drawings may be different from actual ones.
The electrolyte is, for example, a non-aqueous electrolyte containing a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixtures of two or more thereof. The non-aqueous solvent may contain a halogen-substituted product in which at least a part of hydrogen in a solvent described above is substituted with a halogen atom such as fluorine. Examples of the electrolyte salt include lithium salts such as LiPF6. Note that the electrolyte is not limited to a liquid electrolyte, and may be a solid electrolyte in which a gel polymer or the like is used.
The case body 16 is, for example, a bottomed cylindrical metal container. A gasket 28 is provided between the case body 16 and the sealing assembly 17 to ensure the sealability inside the battery. The case body 16 has a projecting portion 22 in which, for example, a part of the side part of the case body 16 protrudes inward to support the sealing assembly 17. The projecting portion 22 is preferably formed in an annular shape along a circumferential direction of the case body 16, and supports the sealing assembly 17 on an upper surface thereof.
The sealing assembly 17 has a structure in which a filter 23, a lower vent member 24, an insulating member 25, an upper vent member 26, and a cap 27 are sequentially stacked from the electrode assembly 14. Each member included in the sealing assembly 17 has, for example, a disk shape or a ring shape, and the members excluding the insulating member 25 are electrically connected to each other. The lower vent member 24 and the upper vent member 26 are connected to each other at their central parts, and the insulating member 25 is interposed between the circumferential parts of the lower vent member 24 and the upper vent member 26. When the internal pressure of the secondary battery 10 increases due to heat generated by an internal short circuit or the like, for example, the lower vent member 24 deforms so as to push the upper vent member 26 up toward the cap 27 side and breaks, and thus the current pathway between the lower vent member 24 and the upper vent member 26 is cut off. When the internal pressure is further increased, the upper vent member 26 is broken, and gas is discharged through the opening of the cap 27.
In the secondary battery 10 illustrated in
Hereinafter, the positive electrode 11, the negative electrode 12, and the separator 13 will be described in detail.
The positive electrode 11 includes a positive electrode current collector and a positive electrode mixture layer provided on the positive electrode current collector. The positive electrode mixture layer contains a positive electrode active material. In addition, the positive electrode mixture layer may contain a conductive agent, a binder, or the like. The positive electrode 11 can be obtained, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and the like onto a positive electrode current collector and drying the positive electrode mixture slurry to form a positive electrode mixture layer on the positive electrode current collector, and then rolling the positive electrode mixture layer. The positive electrode mixture layer may be provided on one surface of the positive electrode current collector, or may be provided on both surfaces of the positive electrode current collector.
As the positive electrode current collector, a foil of a metal stable in a potential range of the positive electrode, such as aluminum or an aluminum alloy, a film in which the metal is disposed on a surface layer, or the like can be used. The positive electrode current collector has a thickness of, for example, about greater than or equal to 10 μm and less than or equal to 100 μm.
The positive electrode active material includes a lithium-containing composite oxide composed of secondary particles in which primary particles are aggregated. The lithium-containing composite oxide is preferably a lithium-containing composite oxide represented by a composition formula: LixNiyM(1-y)O2 (wherein x and y satisfy 1<x<1.4 and 0.5<y≤1.0, and M contains at least one of Mn, Co, Al, and Fe) from the viewpoint of increasing the capacity of the battery. M in the composition formula may contain other elements as long as it contains at least one of Mn, Co, Al, and Fe. Examples of the other elements include Zr, B, Mg, Sc, Y, Ti, Cu, Zn, Cr, Pb, Sn, Na, K, Ba, Sr, Ca, Mo, Si, V, Hf, Ta, and Nb.
The composition of the lithium-containing composite oxide can be confirmed by Rietveld analysis of parameters obtained by inductively coupled plasma emission spectrometry and powder X-ray diffraction.
The Gini coefficient of the Nb element is determined as follows. First, the positive electrode 11 is embedded in a resin, and the cross section of the positive electrode mixture layer is prepared by cross section polisher (CP) processing or the like. Then, time-of-flight secondary ion mass spectrometry (TOF-SIMS) is performed on the cross section of the secondary particle contained in the cross section of the positive electrode mixture layer. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a method in which a sample surface is irradiated with an ion beam (primary ion), and ions (secondary ions) from which some molecules constituting the surface are ionized and released are mass-separated using the time of flight difference (the time of flight is proportional to the square root of the weight). Here, in order to obtain a profile in the thickness direction (depth direction) of the positive electrode mixture layer, the sample surface (cross section of the secondary particle) is alternately irradiated with sputtering ions (O2+) and primary ions (Bi3+), and detected secondary ions (Nb ions: NbO3−) are analyzed.
The measurement conditions of TOF-SIMS are as follows.
A color image obtained by mapping a region having a measurement area of 50 μm×50 μm is adjusted so that a portion where the Nb element is present appears bright and a portion where the Nb element is not present appears dark in color.
An average particle diameter of the primary particles 32 in the lithium-containing composite oxide is, for example, preferably less than or equal to 10 μm in diameter, and is more preferably greater than or equal to 0.1 μm and less than or equal to 5 μm, from the viewpoint of further improving the capacity of the secondary battery.
The average particle diameter of the primary particles 32 in the lithium-containing composite oxide is determined as follows. First, the positive electrode 11 is embedded in a resin, the cross section of the positive electrode mixture layer is prepared by cross section polisher (CP) processing or the like, and a cross section of the secondary particle 30 is exposed. Then, a cross section of the secondary particle 30 is imaged by a scanning electron microscope (SEM), and 30 primary particles 32 are randomly selected from the cross-sectional SEM image. Grain boundaries of the selected 30 primary particles 32 are observed, an outer shape of the primary particle 32 is specified, and then, a long diameter (maximum diameter) of each of the 30 primary particles 32 is measured. The average particle diameter of the primary particles 32 is determined using the long diameter as the particle diameter of the primary particle 32.
An average particle diameter of the secondary particles 30 in the lithium-containing composite oxide is, for example, preferably 20 μm in diameter, and is more preferably greater than or equal to 3 μm and less than or equal to 15 μm, from the viewpoint of further improving the capacity of the secondary battery. The average particle diameter of the secondary particles 30 in the lithium-containing composite oxide is a volume average particle diameter measured by a laser diffraction method, and is a median diameter at which a volume integrated value is 50% in a particle diameter distribution. The average particle diameter of the secondary particles 30 in the lithium-containing composite oxide can be measured by a laser diffraction method using, for example, MT3000II manufactured by MicrotracBEL Corp.
The amount of the Nb element present in the positive electrode active material is, for example, preferably less than or equal to 0.1% and more preferably less than or equal to 0.5% with respect to the total amount of Ni and Mn in the lithium-containing composite oxide from the viewpoint of further improving the capacity of the secondary battery. Here, in a process of coating the surfaces of the primary particles 32 of the lithium-containing composite oxide with the coating film 34 containing the Nb element, a part of the Nb element may form a solid solution in the lithium-containing composite oxide. That is, a part of the Nb element may be incorporated into a crystal structure of the lithium-containing composite oxide and contained inside the particles. As described above, in a case where Nb solid-solved in the primary particles 32 of the lithium-containing composite oxide is present, the amount of the Nb element present in the positive electrode mixture layer includes not only the amount of Nb element present on the surfaces of the primary particles 32 of the lithium-containing composite oxide but also the amount of the Nb element solid-solved in the primary particles 32 of the lithium-containing composite oxide.
Hereinafter, an example of a method for producing a positive electrode active material of the present embodiment will be described.
The method for producing a positive electrode active material of the present embodiment includes a first step of baking a mixture containing a lithium compound and a metal element-containing compound other than lithium to obtain a lithium-containing composite oxide; and a second step of mixing the lithium-containing composite oxide obtained in the first step with a solution in which a Nb compound is dissolved, and baking the obtained mixture.
Examples of the lithium compound in the first step include lithium carbonate and lithium hydroxide. Examples of the metal element-containing compound other than lithium include a hydroxide and oxide containing a transition metal such as Ni or Mn. The metal element-containing compound other than lithium is preferably a composite hydroxide and composite oxide containing Ni and Mn. Note that the composite hydroxide containing Ni and Mn is obtained, for example, by a known coprecipitation method or the like.
The baking condition of the mixture in the first step is preferably, for example, a baking time of longer than or equal to 5 hours and shorter than or equal to 24 hours at a baking temperature in a range of higher than or equal to 600° C. and lower than or equal to 1,100° C. in the air.
The solution in which the Nb compound is dissolved in the second step is, for example, an alcohol solution in which niobium ethoxide is dissolved.
The baking condition of the mixture in the second step is preferably, for example, a baking time of longer than or equal to 1 hour and shorter than or equal to 10 hours at a baking temperature in a range of higher than or equal to 250° C. and lower than or equal to 650° C. in the air.
As described above, the lithium-containing composite oxide is produced, and then the solution in which the Nb compound is dissolved is added and baked, such that the Nb element can be dispersed over the entire surfaces of the primary particles of the lithium-containing composite oxide, and as a result, it is easy to control the Gini coefficient of the Nb element described above to less than or equal to 0.5. A cover rate of the Nb element can be controlled, for example, by adjusting the baking temperature or the baking time in the second step. In addition, a particle diameter of the lithium-containing composite oxide can be controlled by adjusting the baking temperature or the baking time in the first step.
Examples of the conductive agent contained in the positive electrode mixture layer include carbon-based particles such as carbon black (CB), acetylene black (AB), Ketjenblack, and graphite. These conductive agents may be used alone or in combination of two or more thereof.
Examples of the binder contained in the positive electrode mixture layer include a fluorine resin such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, an acrylic resin, a polyolefin, a cellulose derivative such as carboxymethyl cellulose (CMC) or a salt thereof, and polyethylene oxide (PEO). These binders may be used alone or in combination of two or more thereof.
The negative electrode 12 includes, for example, a negative electrode current collector such as a metal foil and a negative electrode mixture layer formed on the negative electrode current collector. The negative electrode current collector may be, for example, a foil of a metal, such as copper, that is stable in a potential range of the negative electrode, a film in which the metal is disposed on its surface layer. The negative electrode mixture layer contains, for example, a negative electrode active material, a binder, and the like.
The negative electrode 12 is obtained by, for example, applying a negative electrode mixture slurry containing a negative electrode active material, a binder, and the like onto a negative electrode current collector and drying the slurry to form a negative electrode mixture layer on the negative electrode current collector, and rolling the negative electrode mixture layer. The negative electrode mixture layer may be provided on one surface of the negative electrode current collector, or may be provided on both surfaces of the negative electrode current collector.
The negative electrode active material may be a material capable of occluding and releasing lithium ions, and specific examples thereof include metal lithium, lithium alloys such as a lithium-aluminum alloy, a lithium-lead alloy, a lithium-silicon alloy, and a lithium-tin alloy, carbon materials such as graphite, coke, and an organic substance baked body, and metal oxides such as SnO2, SnO, and TiO2. These negative electrode active materials may be used alone or in combination of two or more thereof.
The binder may be, for example, a fluororesin, PAN, a polyimide, an acrylic resin, a polyolefin, or the like as in the case of the positive electrode 11, and may be styrene-butadiene rubber (SBR), CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like.
As the separator 13, for example, a porous sheet having an ion permeation property and an insulation property is used. Specific examples of the porous sheet include fine porous thin films, woven fabrics, and nonwoven fabrics. As a material for the separator, olefin-based resins such as polyethylene and polypropylene, cellulose, and the like are suitable. The separator 13 may be a laminate including a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. The separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and a separator may be used that has a surface to which a material such as an aramid-based resin or a ceramic is applied.
Hereinafter, the present disclosure will be further described with reference to Examples. However, the present disclosure is not limited to the following Examples.
Lithium hydroxide and a composite hydroxide containing Ni and Mn were mixed so that a molar ratio of Li to the total amount of Ni and Mn was 1.07:1. The mixture was baked in the air at 900° C. for 10 hours and then pulverized to obtain a lithium-containing composite oxide. Next, to the obtained lithium-containing composite oxide, 400 μl of an ethanol solution in which niobium ethoxide was dissolved so that a proportion was 0.25% with respect to the total amount of Ni and Mn in the oxide was added and mixed. The mixture was baked in the air at 300° C. for 3 hours and then pulverized to obtain a lithium-containing composite oxide in which a coating film containing a Nb element is formed on a surface of a primary particle. The lithium-containing composite oxide was used as a positive electrode active material. In addition, as a result of analysis by inductively coupled plasma (ICP) atomic emission spectroscopy, the amount of the Nb element present in the positive electrode active material was 0.25 mol % with respect to the total amount of Ni and Mn in the lithium-containing composite oxide.
After mixing 95 parts by mass of the positive electrode active material, 5 parts by mass of acetylene black (AB) as a conductive agent, and 3 parts by mass of polyvinylidene fluoride as a binder, an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied onto both surfaces of a positive electrode current collector formed of an aluminum foil, and a coating film was dried and then rolled using a rolling roller. In this way, a positive electrode in which a positive electrode mixture layer was formed on both surfaces of the positive electrode current collector was manufactured.
From the obtained positive electrode, a cross section of the positive electrode mixture layer was prepared by the cross section polisher (CP) processing described above. Then, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was performed on the cross section of the secondary particle under the conditions described above, and a Gini coefficient of the Nb element was calculated from an element mapping image obtained by the analysis, and as a result the Gini coefficient of the Nb element was 0.33.
In addition, the cross section of the positive electrode mixture layer was observed by SEM, and the particle diameters of the primary particles of the positive electrode active material were measured under the conditions described above. As a result, the average particle diameter of the primary particles of the positive electrode active material was 0.5 μm.
Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 20:75:5. LiPF6 was dissolved in the mixed solvent so as to have a concentration of 1.3 mol/L to prepare a non-aqueous electrolyte.
Electrode leads were attached to the metal lithium as the positive electrode and the negative electrode, respectively. Then, an electrode assembly with a separator interposed between the positive electrode and the negative electrode was produced, the electrode assembly was housed in an aluminum laminate film, and the non-aqueous electrolyte was injected and sealed. This battery was used as a secondary battery of Example.
Lithium hydroxide and a composite hydroxide containing Ni and Mn were mixed so that a molar ratio of Li to the total amount of Ni and Mn was 1.07:1. The mixture was baked in the air at 900° C. for 10 hours and then pulverized to obtain a lithium-containing composite oxide. Next, the obtained lithium-containing composite oxide and a niobium oxide (Nb2O5) powder were mixed, and the mixture was baked in air at 600° C. for 3 hours and then pulverized to obtain a lithium-containing composite oxide in which a coating film containing a Nb element is formed on a surface of a primary particle. The lithium-containing composite oxide was used as a positive electrode active material.
In addition, as a result of analysis by inductively coupled plasma (ICP) atomic emission spectroscopy, the amount of the Nb element present in the positive electrode active material was 0.25 mol % with respect to the total amount of Ni and Mn in the lithium-containing composite oxide.
A positive electrode was manufactured using the positive electrode active material in the same manner as in Example. From the obtained positive electrode, a cross section of the positive electrode mixture layer was prepared by the cross section polisher (CP) processing described above. Then, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was performed on the cross section of the secondary particle under the conditions described above, and a Gini coefficient of the Nb element was calculated from an element mapping image obtained by the analysis, and as a result the Gini coefficient of the Nb element was 0.6.
In addition, the cross section of the positive electrode mixture layer was observed by SEM, and the particle diameters of the primary particles of the positive electrode active material were measured under the conditions described above. As a result, the average particle diameter of the primary particles of the positive electrode active material was 1.0 μm.
Then, a secondary battery was manufactured using the positive electrode of Comparative Example 1 in the same manner as in Example.
Lithium hydroxide and a composite hydroxide containing Ni and Mn were mixed so that a molar ratio of Li to the total amount of Ni and Mn was 1.07:1. The mixture was baked in the air at 900° C. for 10 hours and then pulverized to obtain a lithium-containing composite oxide. A secondary battery was manufactured in the same manner as in Example, except that the lithium-containing composite oxide was used as a positive electrode active material of Comparative Example 2.
Each of the secondary batteries of Example and Comparative Examples was subjected to constant current charge at 0.1 C until a cell voltage reached 4.7 V, and then was subjected to constant voltage charge at 4.7 V until a current value reached 0.05 C, under a temperature environment at 25° C. Next, constant current discharge was performed at 0.1 C until a cell voltage reached 2.5 V. The discharge capacity at this time was defined as the capacity of the secondary battery.
The capacity of the secondary battery of Example was 226.45 mAh/g, the capacity of the secondary battery of Comparative Example 1 was 222.82 mAh/g, and the capacity of the secondary battery of Comparative Example 2 was 218.1 mAh/g. From these results, it can be said that the capacity of the secondary battery can be improved by using a positive electrode in which a positive electrode mixture layer contains a positive electrode active material containing a lithium-containing composite oxide in which a coating film containing a Nb element is formed on a surface of a primary particle, and a Gini coefficient of the Nb element calculated from an element mapping image obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) of a cross section of a secondary particle is less than or equal to 0.5.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-214450 | Dec 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/040743 | 10/31/2022 | WO |
