JP2022090426A - Cathode active material preparation material and utilization of them - Google Patents

Abstract

To provide a cathode active material preparation material for preparing a cathode active material capable of realizing an excellent cycle characteristic.SOLUTION: A cathode active material preparation material disclosed here, contains a raw material particle having a core part, and a coated part existed in at least one part of a front surface of the core part. The coated part is structured by small particles each having an average particle diameter of 1/10 or less of a particle diameter of the raw material particle in a SEM image of the raw material particle. In addition, the cathode active material preparation material contains a transition metal hydroxide comprising at least one kind of metal elements, selected from a group of Ni, Mn, and Co in the core part and the small particle.SELECTED DRAWING: Figure 5

Description

The present invention relates to a material for preparing a positive electrode active material and a method for producing a positive electrode active material using the material for preparing a positive electrode active material.

In recent years, secondary batteries such as lithium-ion secondary batteries have been used as portable power supplies for personal computers, mobile terminals, etc., and vehicle drive power supplies for electric vehicles (EV), hybrid vehicles (HV), plug-in hybrid vehicles (PHV), etc. It is suitably used for.

The positive electrode of a lithium ion secondary battery is generally provided with a positive electrode active material capable of storing and releasing lithium ions. For example, Patent Document 1 discloses lithium transition metal-containing composite oxide particles having a narrow particle size distribution and a hollow structure as a positive electrode active material in order to obtain a lithium ion secondary battery having excellent cycle characteristics and output characteristics. Has been done. Since the difference in voltage applied to the particles is small due to the narrow particle size distribution, it is possible to prevent selective deterioration of the particles and realize excellent cycle characteristics. Further, since the hollow structure through which the electrolytic solution can penetrate increases the reaction area with the electrolytic solution, excellent output characteristics can be realized.

Japanese Unexamined Patent Publication No. 2018-104276

By the way, in order to produce a positive electrode active material for a lithium ion secondary battery, when a positive electrode active material precursor powder (material for preparing a positive electrode active material) and a lithium compound are mixed and fired, the particles are sintered. Since bonding may occur, the size of the positive electrode active material particles may vary. As a result, deterioration of the positive electrode active material particles is likely to occur due to repeated charging and discharging, and there is a problem that the cycle characteristics are deteriorated.

Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a material for preparing a positive electrode active material for preparing a positive electrode active material capable of realizing excellent cycle characteristics. From another aspect, another object is to provide a method for producing a positive electrode active material using such a material for preparing a positive electrode active material.

As a result of diligent studies to solve the above problems, the present inventor has found that by firing a material for preparing a positive electrode active material having small particles on the surface, bonding between particles due to sintering can be suppressed. I found it. Then, they found that the positive electrode active material produced by using such a material had good cycle characteristics, and completed the present invention.
That is, the material for preparing a positive electrode active material disclosed here includes a raw material particle including a core portion and a coating portion existing on at least a part of the surface of the core portion, and the coating portion is the raw material particle. In the SEM image, it is composed of small particles having an average particle size of 1/10 or less of the particle size of the raw material particles. Further, the core portion and the small particles contain a transition metal hydroxide containing at least one metal element selected from the group consisting of Ni, Mn and Co.
According to such a configuration, a material for preparing a positive electrode active material for producing a positive electrode active material that imparts excellent cycle characteristics to a lithium ion secondary battery is provided.

Further, in a preferred embodiment of the material for preparing a positive electrode active material disclosed herein, the average aspect ratio of the small particles based on the SEM image is 1.7 or less.
According to such a configuration, the specific surface area is increased, and an excellent electric resistance reducing effect can be exhibited.

Further, in a preferred embodiment of the material for preparing a positive electrode active material disclosed herein, the raw material particles include the raw material particles in which the covering portion occupies 10% or more of the surface of the core portion in the SEM image of the raw material particles.
According to such a configuration, more excellent cycle characteristics and electric resistance reducing effect can be realized.

Further, in a preferred embodiment of the positive electrode active material preparing material disclosed herein, the average particle size of the raw material particles based on the laser diffraction / light scattering method is 4 μm or more and 6 μm or less.
According to such a configuration, the cycle characteristics and the electric resistance reducing effect can be realized at a higher level.

Further, in a preferred embodiment of the positive electrode active material preparing material disclosed herein, in the cross-sectional SEM image of the raw material particles, the fineness of the small particles contained in the covering portion is higher than that of the core portion.
According to such a configuration, the cycle characteristics and the electric resistance reducing effect can be realized at an even higher level.

Further, in order to solve the above problems, a method for producing a positive electrode active material using the material for preparing a positive electrode active material disclosed here is provided. That is, the method for producing a positive electrode active material disclosed herein includes mixing the material for preparing a positive electrode active material disclosed here with a lithium compound, and firing the mixture by such mixing.
According to such a configuration, a positive electrode active material capable of imparting excellent cycle characteristics to a lithium ion secondary battery is provided.

It is sectional drawing which shows typically the structure of the lithium ion secondary battery which comprises the positive electrode active material manufactured from the positive electrode active material preparation material which concerns on one Embodiment. It is a schematic exploded view which shows the structure of the winding electrode body which comprises the positive electrode active material manufactured from the positive electrode active material preparation material which concerns on one Embodiment. It is a rough flowchart for demonstrating the manufacturing process of the positive electrode active material which concerns on one Embodiment. It is an SEM image which observed the material for preparing a positive electrode active material of Example 2 at a magnification of 5000 times. 6 is an SEM image obtained by observing the material for preparing the positive electrode active material of Example 2 at a magnification of 15,000 times. 6 is a cross-sectional SEM image obtained by observing the material for preparing the positive electrode active material of Example 2 at a magnification of 15,000 times. 6 is an SEM image obtained by observing the material for preparing the positive electrode active material of Example 5 at a magnification of 15,000 times.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Since the positive electrode active material produced by using the positive electrode active material preparation material disclosed herein can be suitably used as the positive electrode active material of the lithium ion secondary battery, first, the configuration of the lithium ion secondary battery 100 is configured. An example will be described. Matters other than those specifically mentioned in the present specification and necessary for implementation can be grasped as design matters of those skilled in the art based on the prior art in the art. The present invention can be carried out based on the contents disclosed in the present specification and the common general technical knowledge in the art. Further, in the following drawings, members / parts having the same function are designated by the same reference numerals, and duplicate explanations may be omitted or simplified. Further, the dimensional relations (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relations.
Further, when the numerical range is described as A to B (where A and B are arbitrary numerical values) in the present specification, it is the same as the general interpretation and means A or more and B or less. ..

As used herein, the term "secondary battery" generally refers to a battery that can be repeatedly charged and discharged, and includes a so-called storage battery (that is, a chemical battery) such as a lithium ion secondary battery and a capacitor (that is, a physical battery) such as an electric double layer capacitor. ) Is included. Further, in the present specification, the "lithium ion secondary battery" is a secondary battery that uses lithium ions as a charge carrier and charges and discharges between a positive electrode and a negative electrode by the transfer of electric charges associated with the lithium ions. Further, in the present specification, the “active material” refers to a material that reversibly stores and releases charge carriers.

The lithium ion secondary battery 100 shown in FIG. 1 is a square type formed by accommodating a flat wound electrode body 20 and a non-aqueous electrolyte (not shown) inside a battery case 30. It is a sealed battery. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection. Further, a thin-walled safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises to a predetermined level or higher is provided. Further, the battery case 30 is provided with a liquid injection port (not shown) for injecting a non-aqueous electrolyte. The material of the battery case 30 is preferably a metal material having high strength, light weight, and good thermal conductivity, and examples of such a metal material include aluminum and steel.

As shown in FIGS. 1 and 2, the wound electrode body 20 has a long sheet-shaped positive electrode 50 and a long sheet-shaped negative electrode 60 via two long sheet-shaped separators 70. It is an electrode body that is laminated and wound around a winding axis. The positive electrode 50 includes a positive electrode current collector 52 and a positive electrode active material layer 54 formed in the longitudinal direction on one or both sides of the positive electrode current collector 52. On one edge of the positive electrode current collector 52 in the winding axis direction (that is, the sheet width direction orthogonal to the longitudinal direction), the positive electrode active material layer 54 is not formed in a band shape along the edge. A portion where the positive electrode current collector 52 is exposed (that is, a positive electrode current collector exposed portion 52a) is provided. Further, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 formed in the longitudinal direction of one side or both sides of the negative electrode current collector 62. At the edge of the negative electrode current collector 62 on one side opposite to the winding axis direction, the negative electrode active material layer 64 is not formed in a band shape along the edge, and the negative electrode current collector 62 is exposed ( That is, the negative electrode current collector exposed portion 62a) is provided. A positive electrode current collector plate 42a and a negative electrode current collector plate 44a are bonded to the positive electrode current collector exposed portion 52a and the negative electrode current collector exposed portion 62a, respectively. The positive electrode current collector plate 42a is electrically connected to the positive electrode terminal 42 for external connection, and realizes conduction between the inside and the outside of the battery case 30. Similarly, the negative electrode current collector plate 44a is electrically connected to the negative electrode terminal 44 for external connection, and realizes conduction between the inside and the outside of the battery case 30.

Examples of the positive electrode current collector 52 constituting the positive electrode 50 include aluminum foil. The positive electrode active material layer 54 includes the positive electrode active material disclosed here. Further, the positive electrode active material layer 54 may contain a conductive material, a binder, or the like. As the conductive material, for example, carbon black such as acetylene black (AB) or other carbon material (graphite or the like) can be preferably used. As the binder, for example, polyvinylidene fluoride (PVDF) or the like can be used.
In the positive electrode active material layer 54, the positive electrode active material and a material (conductive material, binder, etc.) used as needed are dispersed in an appropriate solvent (for example, N-methyl-2-pyrrolidone: NMP) to form a paste (or It can be formed by preparing a composition (in the form of a slurry), applying an appropriate amount of the composition to the surface of the positive electrode current collector 52, and drying the composition.

Examples of the negative electrode current collector 62 constituting the negative electrode 60 include copper foil and the like. The negative electrode active material layer 64 contains a negative electrode active material. As the negative electrode active material, a carbon material such as graphite, hard carbon, or soft carbon can be used. Further, the negative electrode active material layer 64 may further contain a binder, a thickener and the like. As the binder, for example, styrene butadiene rubber (SBR) or the like can be used. As the thickener, for example, carboxymethyl cellulose (CMC) or the like can be used.
In the negative electrode active material layer 64, a negative electrode active material and a material (binder or the like) used as needed are dispersed in an appropriate solvent (for example, ion-exchanged water) to prepare a paste-like (or slurry-like) composition. , An appropriate amount of the composition can be applied to the surface of the negative electrode current collector 62 and dried.

As the separator 70, various microporous sheets similar to those conventionally used for lithium ion secondary batteries can be used, and for example, a microporous resin made of a resin such as polyethylene (PE) or polypropylene (PP). The sheet is mentioned. The microporous resin sheet may have a single-layer structure or a multi-layer structure having two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). Further, the surface of the separator 70 may be provided with a heat resistant layer (HRL).

As the non-aqueous electrolyte, the same one as that of the conventional lithium ion secondary battery can be used, and typically, an organic solvent (non-aqueous solvent) containing a supporting salt can be used. As the non-aqueous solvent, an aprotic solvent such as carbonates, esters, ethers and the like can be used. Among them, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and the like can be preferably adopted. Alternatively, a fluorinated solvent such as a fluorinated carbonate such as monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC) and trifluorodimethylcarbonate (TFDMC) is preferably used. be able to. As such a non-aqueous solvent, one kind may be used alone, or two or more kinds may be used in combination as appropriate. As the supporting salt, for example, lithium salts such as LiPF ₆ , LiBF ₄ , and LiClO ₄ can be preferably used. The concentration of the supporting salt is not particularly limited, but is preferably about 0.7 mol / L or more and 1.3 mol / L or less.
The non-aqueous electrolyte may contain components other than the above-mentioned non-aqueous solvent and supporting salt as long as the effects of the present invention are not significantly impaired. It may contain various additives such as thickeners.

The material for preparing a positive electrode active material disclosed herein includes a core portion and raw material particles having a coating portion present on at least a part of the surface of the core portion.
FIG. 4 is an SEM image obtained by observing the material for preparing the positive electrode active material of Example 2 in Examples described later using a scanning electron microscope (SEM) at a magnification of 5000 times, and the positive electrode active material preparation disclosed here. A typical example of a material is shown. FIG. 5 is an SEM image obtained by observing the material for preparing the positive electrode active material of Example 2 at a magnification of 15,000 times, and shows the raw material particles contained in the material for preparing the positive electrode active material in an enlarged manner. Further, FIG. 6 is a cross-sectional SEM image of the material for preparing the positive electrode active material of Example 2 observed at a magnification of 15,000 times. As can be seen from the SEM images of FIGS. 4 to 6, the coating portion existing on the surface of the core portion of the raw material particles is composed of small particles.
Although not particularly limited, the material for preparing a positive electrode active material disclosed herein contains, for example, 30% by number or more, preferably 50% by number or more, more preferably, the raw material particles having the characteristics disclosed here. It is preferably contained in an amount of 70% by number or more, more preferably 80% by number or more, and even more preferably 90% by number or more. For this ratio, for example, as shown in FIG. 4, a plurality of SEM images (for example, a magnification of 5000 times or less) in which a plurality of raw material particles are in the observation field of view are randomly acquired (for example, 10 or more), and each of the plurality of SEM images is obtained. In the above, the abundance ratio (number%) of the raw material particles having the characteristics disclosed here to the particles in the entire SEM image is calculated, and the average value in the plurality of SEM images can be calculated. can.

The material for preparing the positive electrode active material disclosed herein is typically a powder material in the form of a powder, but the powder material may be in the form of a slurry or paste in which the powder material is dispersed in a suitable solvent (for example, water or an organic solvent). You may.

The average particle size of the raw material particles is not particularly limited, but is, for example, 0.1 μm or more and 20 μm or less, and typically 1 μm or more and 10 μm or less. Further, it is preferably 4 μm or more and 9 μm or less, more preferably 4 μm or more and 6 μm or less, and further preferably 4.1 μm or more and 5.8 μm or less. According to such a range, since the raw material particles are further suppressed from being bonded to each other by sintering, the cycle characteristics of the produced positive electrode active material can be improved.
In the present specification, the "average particle size of the raw material particles" is the particle size D50 corresponding to the cumulative frequency of 50% by volume from the smaller particle size side in the volume-based particle size distribution based on the laser diffraction / light scattering method. (Also called "median diameter").

The material for preparing a positive electrode active material disclosed herein contains raw material particles having a coating portion on at least a part of the surface of the core portion, but the coating portion preferably occupies 10% or more of the surface of the core portion. It contains raw material particles that occupy 30% or more, more preferably 50% or more. With this occupancy rate (coverage), it is possible to suppress the bonding of raw material particles due to sintering and further improve the specific surface area of the positive electrode active material to be produced, so that variations in the particle size distribution due to sintering are suppressed. Moreover, it is possible to produce a positive electrode active material having an increased contact area with the electrolyte. As a result, it is possible to manufacture a positive electrode active material for realizing a lithium ion secondary battery having improved cycle characteristics and reduced electrical resistance.
The occupancy rate can be measured based on the SEM image. The SEM image of the raw material particles is acquired using SEM, and for example, using the image analysis software "ImageJ", which is an open source and is well-known as image processing software in the public domain, the covering part (small particles) is white and the covering part is covered. A binarization treatment is performed to make the surface of the core portion (the portion of the raw material particles in which the small particles do not exist) black. Then, the area of the portion (white portion) of the raw material particles where the coating portion exists is NW, and the area of the core portion (black portion) where the coating portion does not exist and the surface is exposed is NB, and “NW / (NW + NB) × 100”. ], The occupancy rate (coverage rate) of the covering portion can be obtained.

The raw material particles having the above-mentioned suitable coverage (for example, 10% or more) are preferably present in the SEM visual field (for example, a visual field with a magnification of 5000 times) by 30% by number or more, and more preferably 40% by number or more. , More preferably, it is present in an amount of 50% or more. By being present in such a ratio, bonding of raw material particles due to sintering can be further suppressed, and excellent cycle characteristics can be realized at a higher level.

The coating portion of the raw material particles is composed of small particles. Such small particles have an average particle size smaller than the particle size of the raw material particles. The average particle size of the small particles is not particularly limited, but is typically 1/10 or less, for example, 1/12 or less or 1/15 or less of the particle size of the raw material particles. obtain. Such a comparison of particle sizes can be performed based on an SEM image. For example, by acquiring an SEM image (for example, 15,000 times) at a magnification at which small particles can be visually recognized as shown in FIG. 5, one raw particle is obtained. It is possible to compare the particle size of the above with the average particle size of a plurality of small particles contained in the raw material particles.
The particle size of the raw material particles and the small particles can be measured as follows. First, in the SEM image, the maximum diameter of each particle is determined, and this is defined as the major diameter. Next, the longest diameter orthogonal to the major axis is determined, and this is defined as the minor axis. Then, assuming an ellipse having such a major axis and a minor axis, the particle diameter of each particle can be obtained by converting the area of the ellipse into a circle-equivalent diameter. As for the small particles, since a plurality of small particles can be provided for each raw material particle, the particle size of each of the plurality of small particles visually observable in the SEM image is calculated, and the average value is obtained.

The average aspect ratio (major axis / minor axis) based on the SEM image of the small particles constituting the covering portion is not particularly limited, but is preferably 1.7 or less. With such an aspect ratio, the specific surface area of the small particles increases, so that the specific surface area of the produced positive electrode active material can be increased. As a result, the contact area with the electrolyte can be increased and the electrical resistance can be reduced.

In the cross-sectional SEM image of the raw material particles, it is preferable that the small particles contained in the covering portion have higher density than the core portion. In other words, it is preferable that the internal voids of the small particles are smaller than the internal voids of the core portion, and it is more preferable that there are no voids inside the small particles. Due to the high density of the small particles, when the material for preparing the positive electrode active material disclosed here is fired, the sintering can easily proceed with the small particles alone, so that the bonding of the raw material particles due to the sintering can be suppressed. As a result, variations in the particle size distribution of the produced positive electrode active material particles can be suppressed. Further, since the surface area of the positive electrode active material particles can be increased, an excellent battery resistance reducing effect and excellent cycle characteristics can be realized.
The cross-sectional SEM image can be obtained according to a known method. The density of the core part and the small particles can be measured by measuring the cross-sectional SEM image, and by analyzing the difference in shading in the cross-sectional SEM image visually or by using image analysis software (eg, ImageJ), the core part and the small particles can be measured. The precision can be compared.

The raw material particles contained in the material for preparing a positive electrode active material disclosed herein preferably contain a transition metal hydroxide containing at least one metal element selected from the group consisting of Ni, Mn, and Co. That is, it is preferable that the core portion and the coating portion (small particles) included in the raw material particles contain such a transition metal hydroxide. Specific examples include nickel-based composite hydroxides, cobalt-based composite hydroxides, manganese-based composite hydroxides, nickel-manganese-based composite hydroxides, nickel-cobalt-manganese-based composite hydroxides, and iron-nickel-manganese-based composite hydroxides. Things etc. can be mentioned.

In the present specification, the term "nickel-cobalt-manganese-based composite hydroxide" refers to one or more of other hydroxides having Ni, Co, Mn, O, and H as constituent elements. It is a term that also includes hydroxides containing additive elements. Examples of such additive elements include transition metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, Sn and the like. Examples include metal elements. This also applies to the above-mentioned nickel-based composite hydroxide, cobalt-based composite hydroxide, manganese-based composite hydroxide, nickel-manganese-based composite hydroxide, and the like.

The nickel-cobalt-manganese-based composite hydroxide preferably has a composition represented by the following formula (I).
Ni _{x Coy} Mn _z M _(1-x-yz) (OH) ₂ (I)
(In the formula (I), x, y, z comprises 0.1 <x <0.9, 0.1 <y <0.4, 0.1 <z <0.9, 1 ≧ x + y + z. M is at least one element selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn and Sn. From the viewpoint of energy density and thermal stability of the positive electrode active material produced, x, y and z are 0.3 ≦ x ≦ 0.5, 0.2 ≦ y <0.4, 0.2 ≦, respectively. It is preferable to have z <0.4.)

Next, a suitable manufacturing method of the material for preparing the positive electrode active material disclosed here will be described. The method for producing the material for preparing the positive electrode active material disclosed here is not limited to the following.

Suitable methods for producing the material for preparing the positive electrode active material disclosed herein include a step of precipitating transition metal hydroxide particles (hereinafter, also referred to as “precipitation step”) and adjusting the above aqueous solution to a pH range of 9 to 11. This step (hereinafter, also referred to as “weak alkali treatment step”) and the growth of the transition metal hydroxide particles (hereinafter, also referred to as “particle growth step”) are included.

First, the precipitation step will be described. The precipitation step may be the same as a known method for crystallizing transition metal hydroxide particles in the production of a normal positive electrode active material.

For example, first, an aqueous solution of a transition metal compound is prepared. As the transition metal compound, for example, a water-soluble compound such as a sulfate of a transition metal, a nitrate of a transition metal, or a halide of a transition metal can be used. As the element of the transition metal, it is preferable to use at least one selected from the group consisting of Ni, Mn and Co. Also, prepare an aqueous solution of the alkaline compound. As the alkaline compound, for example, lithium hydroxide, sodium hydroxide, potassium hydroxide and the like can be used, and sodium hydroxide is preferable. Also, prepare ammonia water.

On the other hand, water (typically ion-exchanged water) is added to the reaction vessel, and the atmosphere is replaced with an inert gas (for example, _N2 gas, Ar gas, etc.) while stirring the inside of the reaction vessel. Next, an aqueous solution of the alkaline compound is added to the reaction vessel to adjust the pH (for example, pH 11 to 13).

While stirring the inside of the reaction vessel, the aqueous solution of the transition metal compound and the aqueous ammonia are added dropwise to the reaction vessel. At this time, since the pH in the reaction vessel is lowered, the pH in the reaction vessel is adjusted within the range of 11 to 13 by appropriately adding an aqueous solution of the alkaline compound.

The reaction vessel is stirred for a predetermined time (for example, 0.5 hour to 1.5 hours) while dropping the aqueous solution of the transition metal compound and aqueous ammonia. As a result, the transition metal hydroxide particles can be appropriately grown.

Next, the weak alkali treatment step will be described. The pH is adjusted to the range of 9 to 11 by adding an acidic aqueous solution to the reaction vessel or stopping the supply of the alkaline compound aqueous solution. The acidic aqueous solution is not particularly limited, but it is preferable to use an aqueous solution containing an anion bonded to the transition metal in the transition metal compound (for example, an aqueous solution of a sulfate of the transition metal is used). If so, use sulfuric acid, etc.).
Then, stirring is maintained for, for example, 1 hour or less (typically 10 minutes or less) while maintaining the pH range. By the treatment for such a time, new nuclei of particles are generated on the surface of the transition metal hydroxide particles.

Next, the particle growth process will be described. The pH in the reaction vessel is adjusted to be in the range of 11 to 13 by adding an aqueous solution of the alkaline compound while stirring the inside of the reaction vessel. Then, the reaction vessel is maintained with stirring for a predetermined time (for example, 0.5 hour to 1.5 hours). Thereby, the transition metal hydroxide particles and the particles generated on the surface of the transition metal hydroxide particles can be grown.

Then, the transition metal hydroxide particles are collected by suction filtration or the like, washed with water, and then dried. Thereby, the material for preparing the positive electrode active material disclosed here can be obtained.

When adjusting the inside of the reaction vessel to the range of pH 11 to 13 in the precipitation step and the particle growth step, by changing the pH maintained within this range, a material for preparing a positive electrode active material having a different average particle diameter can be obtained. Can be manufactured.

Next, a suitable method for producing a positive electrode active material using the material for preparing a positive electrode active material disclosed here will be described.

As shown in FIG. 3, the method for producing a positive electrode active material disclosed here is a step of mixing the material for preparing a positive electrode active material disclosed here with a lithium compound (hereinafter, also referred to as “mixing step S10”). And a step of firing the mixture mixed by such a step (hereinafter, also referred to as “firing step S20”).

First, the mixing step S10 will be described. In this step, first, the material for preparing the positive electrode active material disclosed here and the lithium compound are prepared. Such a material for preparing a positive electrode active material can be prepared, for example, by the method described above. As the lithium compound, for example, a compound that is converted into an oxide by firing, such as lithium carbonate, lithium nitrate, and lithium hydroxide, can be used.

The obtained transition metal hydroxide particles and the lithium compound are mixed according to a known method using a known mixing device (eg, Shaka mixer, V blender, ribbon mixer, Julia mixer, Lady Gemixer, etc.) to obtain a mixture. Obtainable.
The mixing amount of the material for preparing the positive electrode active material and the lithium compound may be according to the elemental ratio of lithium and the transition metal contained in the desired positive electrode active material.

Next, the firing step S20 will be described. Firing of the obtained mixture can be performed using, for example, a batch type electric furnace, a continuous type electric furnace, or the like. The firing temperature may be, for example, 400 ° C. or higher and 1000 ° C. or lower, and the firing time may be, for example, 2 hours or more and 10 hours or less.

The lithium ion secondary battery 100 can be used for various purposes. For example, it can be suitably used as a high-output power source (driving power source) for a motor mounted on a vehicle. The type of vehicle is not particularly limited, but typically examples thereof include automobiles, for example, plug-in hybrid automobiles (PHVs), hybrid automobiles (HVs), electric vehicles (EVs), and the like. The lithium ion secondary battery 100 can also be used in the form of an assembled battery in which a plurality of lithium ion secondary batteries 100 are electrically connected.

As described above, as an example of a lithium ion secondary battery provided with a positive electrode active material manufactured by the positive electrode active material manufacturing method disclosed herein, a square non-aqueous electrolytic solution lithium ion secondary battery provided with a flat wound electrode body. The next battery was explained. However, this is only an example and is not limited. For example, instead of the wound electrode body, a lithium ion secondary battery may be provided in which a laminated electrode body in which a sheet-shaped positive electrode and a sheet-shaped negative electrode are alternately laminated via a separator is provided. Further, a polymer battery using a polymer electrolyte as the electrolyte may be used. Further, a cylindrical battery case, a coin-shaped battery case, or the like may be used instead of the square battery case, or a laminated secondary battery using a laminated film instead of the battery case may be used.

Hereinafter, examples relating to the present invention will be described, but the present invention is not intended to be limited to those shown in such examples.

<Preparation of positive electrode active material>
(Examples 1 to 4)
A raw material solution was prepared by dissolving nickel sulfate (NiSO ₄ ), cobalt sulfate (CoSO ₄ ), and manganese sulfate (MnSO ₄ ) in ion-exchanged water so as to have a molar ratio of 1: 1: 1. In addition, aqueous ammonia and aqueous sodium hydroxide solution were prepared.
Ion-exchanged water was added to the reaction vessel, and the atmosphere inside the reaction vessel was replaced with an inert gas while stirring to create an inert atmosphere. Next, an aqueous NaOH solution was added into the reaction vessel to adjust the pH to be in the range of 11-13. Then, the above raw material solution and ammonia water were added dropwise to the reaction vessel in fixed amounts. At this time, in order to keep the pH within the above range, an aqueous NaOH solution was appropriately added. The mixture was stirred for 0.5 to 1.5 hours while dropping the above raw material solution and aqueous ammonia. After such stirring, the pH was adjusted to be in the range of 9 to 11 by stopping the supply of the NaOH aqueous solution, and stirring was maintained for 0.1 to 10 minutes. Then, the supply of the NaOH aqueous solution was restarted, the pH was adjusted again in the range of 11 to 13, and stirring was maintained for 0.5 to 1.5 hours to obtain a precipitate. The precipitate was collected by suction filtration, washed with ion-exchanged water, and dried under reduced pressure at 40 ° C to 80 ° C for 6 to 8 hours to obtain a powder of hydroxide particles as a material for preparing a positive electrode active material. ..
In Examples 1 to 4, the inside of the reaction vessel was maintained at a pH of different values in the range of pH 11 to 13.

The obtained hydroxide particle powder and lithium carbonate (Li ₂ CO ₃ ) are mixed so that the molar ratio of lithium to the total of nickel, cobalt and manganese contained in the hydroxide particles is 1: 1. Mixed in a dairy pot. The mixture was transferred to an alumina crucible and fired in a muffle furnace at 400 ° C. to 1000 ° C. for 2 to 10 hours. In this way, lithium composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ : NCM) particles, which are the positive electrode active materials of Examples 1 to 4, were obtained.

(Example 5)
In the step of obtaining the powder of the hydroxide particles of Example 1 above, the same steps were carried out except that the step of adjusting the pH to be in the range of 9 to 11 was not carried out, and the positive electrode active material of Example 5 was carried out. Got

<Manufacturing of lithium-ion secondary battery for evaluation>
As the lithium ion secondary battery for evaluation, the positive electrode active material (NCM) produced above, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were used as NCM: AB: PVDF = 87. A paste for forming a positive electrode active material layer was prepared by mixing in N-methyl-2-pyrrolidone so as to have a mass ratio of: 10: 3. This paste was applied to both sides of an aluminum foil current collector using a film applicator manufactured by Allgood, and dried at 80 ° C. for 5 minutes to prepare a positive electrode sheet.

Natural graphite (C) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener are used in a mass ratio of C: SBR: CMC = 96: 2: 2. A paste for forming a negative electrode active material layer was prepared by mixing in ion-exchanged water so as to be. This paste was applied to both sides of a copper foil current collector using a film applicator manufactured by Allgood, and dried at 80 ° C. for 5 minutes to prepare a negative electrode sheet.

Further, as a separator sheet, a porous polyolefin sheet having a three-layer structure of PP / PE / PP was prepared.

The prepared positive electrode sheet, the negative electrode sheet, and the prepared separator sheet were superposed and wound to prepare a cylindrical wound electrode body. Electrode terminals were attached to the positive electrode sheet and the negative electrode sheet of the manufactured wound electrode body by welding, respectively, and housed in a battery case having a liquid injection port.

Next, the non-aqueous electrolytic solution was sealed from the injection port of the battery case, and the injection port was hermetically sealed. As a non-aqueous electrolytic solution, LiPF ₆ as a supporting salt was added to a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of 3: 4: 3. The one dissolved at a concentration of 0.0 mol / L was used. As described above, a lithium ion secondary battery for evaluation of each Example and each Comparative Example was obtained.

<Measurement of average particle size>
The median diameter (D50) was measured using a laser diffraction type particle size distribution measuring device for each of the positive electrode active material preparation materials prepared above. The results are shown in Table 1 as "raw material particle average particle size (μm)". In addition, the median diameter (D50) was measured in the same manner for each of the positive electrode active materials of each example. The results are shown in Table 1 as "average particle size of positive electrode active material (μm)".

<Analysis of raw material particles based on SEM image>
SEM images (magnification of 5000 times and 15000 times) of each of the positive electrode active material preparation materials (powder of hydroxide particles) of each of the above-mentioned prepared examples were obtained. Based on the SEM image, the ratio of the average particle size of the small particles constituting the coating portion of the raw material particles to the particle size of the raw material particles was measured. The results are shown in Table 1 as "ratio of small particle size to raw material particle size".
In addition, the average aspect ratio of the small particles was measured based on the SEM image. The results are shown in Table 1 as "small particle aspect ratio".

As typical SEM images of the positive electrode active material preparation material disclosed here, SEM images obtained by observing the positive electrode active material preparation material of Example 2 at a magnification of 5000 times and a magnification of 15000 times are shown in FIGS. 4 and 5, respectively. .. Further, FIG. 6 shows a cross-sectional SEM image when observed at a magnification of 15,000 times that of the material for preparing the positive electrode active material of Example 2.
Further, as a representative example of the prior art, FIG. 7 shows an SEM image observed at a magnification of 15,000 times that of the material for preparing the positive electrode active material of Example 5.

<Activation treatment and measurement of initial discharge capacity>
A constant current-constant voltage method is used, and after constant current charging of each evaluation lithium ion secondary battery manufactured above to 4.1 V with a current value of 1 / 3C, a constant voltage is applied until the current value becomes 1 / 50C. It was charged and fully charged. Then, each evaluation lithium ion secondary battery was constantly discharged to 3.0 V at a current value of 1 / 3C. The discharge capacity at this time was taken as the initial discharge capacity. The charge / discharge operation was performed at 25 ° C. Further, “1C” here means the magnitude of the current that can charge the SOC (state of charge) from 0% to 100% in one hour.

<Initial resistance measurement>
Each of the activated lithium ion secondary batteries for evaluation was adjusted to an open circuit voltage of 3.70 V. This was placed in a temperature environment of 25 ° C. Discharging was performed for 10 seconds at a current value of 1C, and the amount of voltage change ΔV was determined. Battery resistance was calculated using the current value and ΔV. The ratio of the initial resistance of the evaluation lithium ion secondary battery of each example was obtained when the initial resistance of the evaluation lithium ion secondary battery of Example 5 was 1.00. The results are shown in Table 1 as "initial resistance".

<Measurement of cycle capacity maintenance rate>
Each of the activated lithium ion secondary batteries for evaluation was placed in an environment of 60 ° C. Each evaluation lithium-ion secondary battery was charged at a constant current of 4.2 V at 2C, and then discharged at a constant current of 3.0 V at 2C, which was repeated for 200 cycles. Then, the discharge capacity after 200 cycles was measured by the same method as the above-mentioned initial discharge capacity. Then, the cycle capacity retention rate (%) is calculated by the following equation 1:
(Discharge capacity after 200 cycles) / (Initial discharge capacity) x 100 ... Equation 1
Calculated by The results are shown in Table 1 as "cycle capacity retention rate (%)".

As shown in FIGS. 4 and 5, in the raw material particles contained in the material for preparing the positive electrode active material of Example 2, a coating portion composed of small particles was present on at least a part of the surface of the core portion. Although not shown, in Examples 1, 3 and 4, raw material particles having a coating portion composed of small particles were also observed.

As shown in Table 1, it is produced from a material for preparing a positive electrode active material having a characteristic that the ratio of the average particle size of the small particles constituting the coating portion of the raw material particles to the particle size of the raw material particles is 1/10 or less. It can be seen that the positive electrode active materials of Examples 1 to 4 can realize excellent cycle characteristics (cycle capacity retention rate) by the lithium ion secondary battery as compared with the positive electrode active materials of Example 5. Further, it can be seen that in Examples 1 to 4, the initial resistance is reduced as compared with Example 5.
Further, it can be seen that particularly excellent cycle characteristics and initial resistance reducing effect can be realized in Examples 1 and 2 in which the average particle size of the raw material particles is 4 μm or more and 6 μm or less.
Therefore, according to the material for preparing the positive electrode active material disclosed here, it can be seen that the positive electrode active material can be produced by reducing the initial resistance of the lithium ion secondary battery and imparting excellent cycle characteristics.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The techniques described in the claims include various modifications and modifications of the specific examples exemplified above.

20 Winding electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector plate 44 Negative electrode terminal 44a Negative electrode current collector plate 50 Positive electrode 52 Positive electrode collector 52a Positive electrode current collector exposed part 54 Positive electrode active material layer 60 Negative electrode 62 Negative electrode collection Electrode 62a Negative electrode current collector Exposed part 64 Negative electrode active material layer 70 Separator 100 Lithium ion secondary battery

Claims (6)

A material for preparing positive electrode active materials for lithium-ion secondary batteries.
Contains raw material particles comprising a core portion and a coating portion present on at least a portion of the surface of the core portion.
The covering portion is composed of small particles having an average particle size of 1/10 or less of the particle size of the raw material particles in the SEM image of the raw material particles.
Here, the core portion and the small particles include a transition metal hydroxide having at least one metal element selected from the group consisting of Ni, Mn, and Co.
Material for preparing positive electrode active material. The material for preparing a positive electrode active material according to claim 1, wherein the average aspect ratio of the small particles based on the SEM image is 1.7 or less. The material for preparing a positive electrode active material according to claim 1 or 2, which comprises the raw material particles in which the covering portion occupies 10% or more of the surface of the core portion in the SEM image of the raw material particles. The material for preparing a positive electrode active material according to any one of claims 1 to 3, wherein the raw material particles have an average particle diameter of 4 μm or more and 6 μm or less based on a laser diffraction / light scattering method. The material for preparing a positive electrode active material according to any one of claims 1 to 4, wherein the small particles contained in the covering portion have higher density than the core portion in the cross-sectional SEM image of the raw material particles. Mixing the material for preparing a positive electrode active material according to any one of claims 1 to 5 with a lithium compound, and
Firing the mixed mixture,
A method for producing a positive electrode active material, which comprises.

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