JP7275094B2 - Materials for preparing positive electrode active materials and their uses - Google Patents

Description

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

In recent years, secondary batteries such as lithium-ion secondary batteries have been used in portable power sources such as personal computers and mobile terminals, and power sources for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). is preferably used for

A positive electrode of a lithium ion secondary battery is generally provided with a positive electrode active material capable of intercalating and deintercalating 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. It is A narrow particle size distribution reduces the difference in voltage applied to the particles, thereby preventing selective deterioration of the particles and realizing excellent cycle characteristics. In addition, by having a hollow structure into which the electrolytic solution can penetrate, the reaction area with the electrolytic solution is increased, so excellent output characteristics can be realized.

JP 2018-104276 A

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 separated by sintering. Since bonding may occur, there is a risk of variation in the size of the positive electrode active material particles. As a result, deterioration of the positive electrode active material particles is likely to occur due to repeated charging and discharging, resulting in a problem of deterioration in cycle characteristics.

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

The present inventors conducted extensive studies to solve the above problems, and 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. Found it. Then, the inventors have found that the positive electrode active material manufactured using such a material has good cycle characteristics, and completed the present invention.
That is, the material for preparing a positive electrode active material disclosed herein includes a raw material particle having a core portion and a coating portion present on at least a part of the surface of the core portion, and the coating portion is formed on the raw material particle. In the SEM image, it is composed of small particles having an average particle diameter of 1/10 or less of the particle diameter of the raw material particles. Furthermore, 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 positive electrode active material preparation material for producing a positive electrode active material that imparts excellent cycle characteristics to a lithium ion secondary battery is provided.

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

In a preferred embodiment of the material for preparing a positive electrode active material disclosed herein, the SEM image of the raw material particles includes the raw material particles in which the coating portion occupies 10% or more of the surface of the core portion.
With such a configuration, better cycle characteristics and an effect of reducing electrical resistance can be achieved.

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

Further, in a preferred embodiment of the material for preparing a positive electrode active material disclosed herein, in a cross-sectional SEM image of the raw material particles, the small particles contained in the coating portion are denser than the core portion.
According to such a configuration, even higher levels of cycle characteristics and electrical resistance reduction effect can be achieved.

Moreover, 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 herein is provided. That is, the positive electrode active material manufacturing method disclosed herein includes mixing the positive electrode active material preparation material disclosed here with a lithium compound, and firing the mixture obtained by such mixing.
Such a configuration provides a positive electrode active material capable of imparting excellent cycle characteristics to a lithium ion secondary battery.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view schematically showing the configuration of a lithium ion secondary battery including a positive electrode active material manufactured from a positive electrode active material preparation material according to one embodiment; 1 is a schematic exploded view showing the configuration of a wound electrode body including a positive electrode active material manufactured from a material for preparing a positive electrode active material according to one embodiment; FIG. 1 is a rough flow chart for explaining a manufacturing process of a positive electrode active material according to one embodiment; 2 is an SEM image of the positive electrode active material preparation material of Example 2 observed at a magnification of 5000 times. 1 is an SEM image of the positive electrode active material preparation material of Example 2 observed at a magnification of 15000 times. 2 is a cross-sectional SEM image of the positive electrode active material preparation material of Example 2 observed at a magnification of 15,000. 1 is an SEM image of the positive electrode active material preparation material of Example 5 observed at a magnification of 15000 times.

An embodiment of the present invention will be described below with reference to the drawings. Since the positive electrode active material produced using the positive electrode active material preparation material disclosed herein can be suitably used as a positive electrode active material for a lithium ion secondary battery, first, the configuration of the lithium ion secondary battery 100 An example will be described. Matters other than those specifically referred to in this specification that are necessary for implementation can be grasped as design matters by those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and common general technical knowledge in the field. Further, in the drawings below, members and portions having the same function are denoted by the same reference numerals, and redundant description may be omitted or simplified. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships.
In addition, when the numerical range is described as A to B (where A and B are arbitrary numerical values) in this specification, it is the same as the general interpretation and means from A to B. .

In this specification, the term "secondary battery" refers to a battery that can be repeatedly charged and discharged, and in addition to so-called storage batteries such as lithium ion secondary batteries (i.e. chemical batteries), capacitors such as electric double layer capacitors (i.e. physical batteries ). In the present specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and charges and discharges by the transfer of charge associated with the lithium ions between the positive electrode and the negative electrode. In addition, the term “active material” as used herein refers to a material that reversibly absorbs and releases charge carriers.

The lithium-ion secondary battery 100 shown in FIG. 1 is a rectangular battery constructed by housing a flat-shaped 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 terminal 42 and a negative terminal 44 for external connection. Further, a thin safety valve 36 is provided so as to release the internal pressure of the battery case 30 when the internal pressure rises above a predetermined level. Further, the battery case 30 is provided with an injection port (not shown) for injecting a non-aqueous electrolyte. The material of the battery case 30 is preferably a metallic material that has high strength, light weight, and good thermal conductivity. Examples of such metallic materials include aluminum and steel.

As shown in FIGS. 1 and 2 , the wound electrode assembly 20 includes a long sheet-shaped positive electrode 50 and a long sheet-shaped negative electrode 60 interposed between two long sheet-shaped separators 70 . It is an electrode assembly 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 on one side or both sides of the positive electrode current collector 52 in the longitudinal direction. At one edge of the positive electrode current collector 52 in the winding axial direction (that is, in the sheet width direction orthogonal to the longitudinal direction), the positive electrode active material layer 54 is not formed in a belt 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. The negative electrode 60 also includes a negative electrode current collector 62 and a negative electrode active material layer 64 formed on one or both surfaces of the negative electrode current collector 62 in the longitudinal direction. At the edge of the negative electrode current collector 62 on the side opposite to one side in the winding axial direction, there is a portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64 being formed in a belt shape along the edge ( That is, a 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 joined to the positive electrode current collector exposed portion 52a and the negative electrode current collector exposed portion 62a, respectively. The positive current collecting plate 42a is electrically connected to the positive terminal 42 for external connection, and realizes conduction between the inside and the outside of the battery case 30. As shown in FIG. Similarly, the negative current collecting plate 44a is electrically connected to the negative terminal 44 for external connection, thereby realizing conduction between the inside and the outside of the battery case 30. As shown in FIG.

Examples of the positive electrode current collector 52 that constitutes the positive electrode 50 include aluminum foil. The cathode active material layer 54 comprises the cathode active material disclosed herein. Moreover, the positive electrode active material layer 54 may contain a conductive material, a binder, and the like. As the conductive material, for example, carbon black such as acetylene black (AB) and other carbon materials (such as graphite) can be suitably used. As the binder, for example, polyvinylidene fluoride (PVDF) or the like can be used.
The positive electrode active material layer 54 is formed by dispersing a positive electrode active material and optionally used materials (conductive materials, binders, etc.) in an appropriate solvent (eg, N-methyl-2-pyrrolidone: NMP) to form a paste (or It can be formed by preparing a slurry composition), 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 that constitutes the negative electrode 60 include copper foil. The negative electrode active material layer 64 contains a negative electrode active material. Carbon materials such as graphite, hard carbon, and soft carbon can be used as the negative electrode active material. Moreover, 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 a thickening agent, for example, carboxymethyl cellulose (CMC) or the like can be used.
The negative electrode active material layer 64 is prepared by dispersing a negative electrode active material and optionally used materials (binder etc.) in an appropriate solvent (eg ion-exchanged water) to prepare a paste (or slurry) composition. can be formed by applying an appropriate amount of the composition to the surface of the negative electrode current collector 62 and drying it.

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

The same non-aqueous electrolyte as used in conventional lithium ion secondary batteries can be used, and typically an organic solvent (non-aqueous solvent) containing a supporting electrolyte can be used. Aprotic solvents such as carbonates, esters and ethers can be used as the non-aqueous solvent. Among them, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and the like can be preferably used. Alternatively, fluorine-based solvents such as fluorinated carbonates such as monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC) are preferably used. be able to. Such non-aqueous solvents can be used singly or in combination of two or more. Lithium salts such as LiPF ₆ , LiBF ₄ and LiClO ₄ can be suitably used as supporting salts. 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-described non-aqueous solvent and supporting salt as long as they do not significantly impair the effects of the present invention. Various additives such as thickeners may be included.

The material for positive electrode active material preparation disclosed herein includes a raw material particle having a core portion and a coating portion present on at least part of the surface of the core portion.
FIG. 4 is an SEM image of the positive electrode active material preparation material of Example 2 in Examples described later, observed at a magnification of 5000 using a scanning electron microscope (SEM). A typical example of materials for FIG. 5 is an SEM image of the positive electrode active material preparation material of Example 2 observed at a magnification of 15000 times, showing an enlarged view of raw material particles contained in the positive electrode active material preparation material. FIG. 6 is a cross-sectional SEM image of the positive electrode active material preparation material of Example 2 observed at a magnification of 15000 times. As can be seen from the SEM images of FIGS. 4 to 6, the coating portion present 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 30% by number or more of raw material particles having the characteristics disclosed herein. It preferably contains 70% by number or more, more preferably 80% by number or more, and even more preferably 90% by number or more. 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 within the observation field are randomly obtained (for example, 10 or more), and each of the plurality of SEM images is obtained. , the ratio (% by number) of the raw material particles having the characteristics disclosed herein is calculated with respect to the particles in the entire SEM image, and the average value of the plurality of SEM images is calculated. can.

The positive electrode active material preparation material disclosed herein is typically a powdery powder material, and the powdery material may be in the form of a slurry or a paste dispersed in an appropriate solvent (for example, water or an organic solvent). 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. Also, it is preferably 4 μm or more and 9 μm or less, more preferably 4 μm or more and 6 μm or less, and still more preferably 4.1 μm or more and 5.8 μm or less. According to this range, it is possible to further suppress the raw material particles from being joined to each other by sintering, so that the cycle characteristics of the produced positive electrode active material can be improved.
In the present specification, the "average particle size of raw material particles" refers to a particle size D50 corresponding to a cumulative frequency of 50% by volume from the smaller particle size side in a volume-based particle size distribution based on a laser diffraction/light scattering method. (also referred to as “median diameter”).

The material for preparing a positive electrode active material disclosed herein contains raw material particles having a coating on at least a part of the surface of the core, preferably the coating occupies 10% or more of the surface of the core, More preferably 30% or more, more preferably 50% or more of raw material particles are included. Such a occupancy (coverage) suppresses the bonding of the raw material particles by sintering, and furthermore, it is possible to improve the specific surface area of the produced positive electrode active material, so that the variation in particle size distribution due to sintering is suppressed, In addition, it is possible to manufacture 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 with improved cycle characteristics and reduced electrical resistance.
In addition, the above occupation rate can be measured based on an SEM image. An SEM image of the raw material particles is obtained using an SEM, for example, using image analysis software "ImageJ", which is an open source and well-known image processing software in the public domain, the coating portion (small particles) is white, and the coating portion is A binarization process is performed to make the surface of the core portion where there is no (the portion where the small particles in the raw material particles do not exist) black. Then, where NW is the area of the portion (white portion) where the coating portion is present in the raw material particles, and NB is the area of the core portion (black portion) where the coating portion is not present and the surface is exposed, "NW / (NW + NB) × 100 , the occupancy rate (coverage rate) of the covered portion can be obtained.

The raw material particles having the above-mentioned suitable coverage (for example, 10% or more) are present in an SEM field of view (for example, a field of view with a magnification of 5000 times), preferably at 30% by number or more, more preferably at least 40% by number. , more preferably 50% by number or more. By existing in such a ratio, bonding between raw material particles due to sintering is 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 that 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 diameters can be performed based on SEM images. For example, by obtaining a SEM image at a magnification (for example, 15000 times) at which small particles can be visually observed, as shown in FIG. can be compared with the average particle size of a plurality of small particles of the raw material particles.
The particle sizes of raw material particles and small particles can be measured as follows. First, in the SEM image, the maximum diameter of each particle is determined and defined as the major diameter. Next, the longest diameter among the diameters perpendicular to the major diameter is determined and defined as the minor diameter. Then, by assuming an ellipse having such a major axis and a minor axis, and converting the area of the ellipse into a circle-equivalent diameter, the particle diameter of each particle can be obtained. Since a plurality of small particles can be provided for one raw material particle, the particle diameter of each of the plurality of small particles visible 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 coating 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 the specific surface area of the produced positive electrode active material can be increased. This can increase the contact area with the electrolyte and reduce the electrical resistance.

In the cross-sectional SEM image of the raw material particles, it is preferable that the small particles contained in the coating portion are denser 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 the small particles have no internal voids. The high density of the small particles facilitates sintering of the small particles alone during sintering of the material for preparing a positive electrode active material disclosed herein. This can suppress variation in the particle size distribution of the produced positive electrode active material particles. Moreover, since the surface area of the positive electrode active material particles can be increased, an excellent battery resistance reduction effect and excellent cycle characteristics can be achieved.
A cross-sectional SEM image can be obtained according to a known method. The density of the core portion and the small particles can be measured by measuring the cross-sectional SEM image, and by analyzing the difference in density in the cross-sectional SEM image visually or with image analysis software (eg, ImageJ), the density of the core portion and the small particles can be determined. Compactness can be compared.

The raw material particles contained in the positive electrode active material-preparing 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, the core portion and the covering portion (small particles) of the raw material particles preferably 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.

In this specification, the term "nickel-cobalt-manganese-based composite hydroxide" refers to hydroxides containing Ni, Co, Mn, O, and H as constituent elements, as well as one or more of them. The term also includes hydroxides containing additional 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 and Sn, and typical A metal element etc. are mentioned. This also applies to the above-described nickel-based composite hydroxide, cobalt-based composite hydroxide, manganese-based composite hydroxide, nickel-manganese-based composite hydroxide, and the like.

As the nickel-cobalt-manganese composite hydroxide, one having a composition represented by the following formula (I) is preferable.
Ni _x Co _y Mn _z M _(1-xyz) (OH) ₂ (I)
(In formula (I), x, y, and z are 0.1<x<0.9, 0.1<y<0.4, 0.1<z<0.9, 1≧x+y+z and 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 the energy density and thermal stability of the positive electrode active material to be produced, x, y and z are 0.3≦x≦0.5, 0.2≦y<0.4 and 0.2≦ It is preferred to have z<0.4.)

Next, a suitable method for producing the positive electrode active material-preparing material disclosed herein will be described. In addition, the manufacturing method of the material for positive electrode active material preparation disclosed here is not restricted to the following.

A preferred method for producing the positive electrode active material preparation material disclosed herein includes a step of depositing transition metal hydroxide particles (hereinafter also referred to as a “precipitation step”), and adjusting the aqueous solution to a pH range of 9 to 11. and growing the transition metal hydroxide particles (hereinafter also referred to as "particle growth step").

First, the deposition step will be explained. The deposition 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. Examples of transition metal compounds that can be used include water-soluble compounds such as transition metal sulfates, transition metal nitrates, and transition metal halides. At least one element selected from the group consisting of Ni, Mn, and Co is preferably used as the transition metal element. Also, an aqueous solution of an alkaline compound is prepared. Examples of alkaline compounds that can be used include lithium hydroxide, sodium hydroxide, potassium hydroxide, and the like, preferably sodium hydroxide. Also, prepare ammonia water.

On the other hand, water (typically ion-exchanged water) is added to the reaction vessel, and the atmosphere in the reaction vessel is replaced with an inert gas (eg, _N2 gas, Ar gas, etc.) while stirring. Next, the aqueous solution of the alkali 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 dropped into 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 the aqueous solution of the alkaline compound.

While dropping the aqueous solution of the transition metal compound and the aqueous ammonia, the reaction vessel is stirred for a predetermined time (for example, 0.5 to 1.5 hours). This allows the transition metal hydroxide particles to grow moderately.

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

Next, the particle growth process will be explained. By adding the aqueous solution of the alkaline compound while stirring the inside of the reaction vessel, the pH inside the reaction vessel is adjusted to fall within the range of 11-13. Thereafter, the reaction vessel is maintained under agitation for a predetermined period of time (eg, 0.5 to 1.5 hours). Thereby, the transition metal hydroxide particles and the particles generated on the surfaces of the transition metal hydroxide particles can be grown.

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

In the precipitation step and the particle growth step, when adjusting the pH in the reaction vessel to within the range of 11 to 13, by changing the pH maintained within this range, materials for preparing positive electrode active materials with different average particle sizes 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 herein will be described.

As shown in FIG. 3, the positive electrode active material manufacturing method disclosed herein includes a step of mixing the positive electrode active material preparation material disclosed herein with a lithium compound (hereinafter also referred to as “mixing step S10”). and a step of firing the mixture mixed in this step (hereinafter also referred to as “firing step S20”).

First, the mixing step S10 will be described. In this step, first, the positive electrode active material preparation material disclosed herein and a lithium compound are prepared. Such a positive electrode active material preparation material can be prepared, for example, by the method described above. As the lithium compound, for example, a compound that is converted to 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 apparatus (eg, shaker mixer, V blender, ribbon mixer, Julia mixer, Loedige mixer, etc.) to give a mixture. Obtainable.
The amount of the material for preparing the positive electrode active material and the lithium compound to be mixed may be in accordance with 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. The obtained mixture can be fired using, for example, a batch-type electric furnace, a continuous-type electric furnace, or the like. The firing temperature is, for example, 400° C. or more and 1000° C. or less, and the firing time is, 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. Although the type of vehicle is not particularly limited, typical vehicles include plug-in hybrid vehicles (PHV), hybrid vehicles (HV), electric vehicles (EV), 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 batteries are electrically connected.

As described above, as an example of a lithium ion secondary battery provided with a positive electrode active material produced by the method for producing a positive electrode active material disclosed herein, a rectangular non-aqueous electrolyte lithium ion secondary battery provided with a flat wound electrode assembly was described. Next battery was explained. However, this is only an example and is not limiting. For example, instead of the wound electrode body, a lithium ion secondary battery may be provided with a laminated electrode body in which a sheet-like positive electrode and a sheet-like negative electrode are alternately laminated via a separator. Moreover, it may be a polymer battery using a polymer electrolyte as the electrolyte. Further, instead of the rectangular battery case, a cylindrical battery case, a coin-shaped battery case, or the like may be used, and a laminated secondary battery using a laminate film may be used instead of the battery case.

EXAMPLES Examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.

<Preparation of positive electrode active material>
(Examples 1-4)
Nickel sulfate (NiSO ₄ ), cobalt sulfate (CoSO ₄ ), and manganese sulfate (MnSO ₄ ) were dissolved in deionized water at a molar ratio of 1:1:1 to prepare a raw material solution. In addition, ammonia water and sodium hydroxide aqueous solution were prepared.
Ion-exchanged water was added to the reactor, and the atmosphere in the reactor was replaced with an inert gas while stirring to create an inert atmosphere. Next, an aqueous NaOH solution was added to the reaction vessel to adjust the pH to a range of 11-13. Then, constant amounts of the raw material solution and ammonia water were added dropwise to the reaction vessel. At this time, an aqueous NaOH solution was appropriately added to maintain the pH within the above range. The raw material solution and ammonia water were added dropwise while stirring for 0.5 to 1.5 hours. After such stirring, the supply of the aqueous NaOH solution was stopped to adjust the pH to a range of 9 to 11, and the stirring was maintained for 0.1 to 10 minutes. After that, the supply of the NaOH aqueous solution was restarted, the pH was again adjusted to 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 deionized 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 reaction vessel was maintained at different pH values within the pH range of 11 to 13.

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

(Example 5)
In the step of obtaining a powder of hydroxide particles of Example 1, the same steps were performed except that the step of adjusting the pH to be in the range of 9 to 11 was not performed. got

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

As the negative electrode active material, natural graphite (C), styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed at 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 such that This paste was applied to both sides of a copper foil current collector using a film applicator manufactured by Allgood Co., Ltd., and dried at 80° C. for 5 minutes to prepare a negative electrode sheet.

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

The produced positive electrode sheet, the produced negative electrode sheet, and the prepared separator sheet were superimposed and wound to produce a cylindrical wound electrode assembly. Electrode terminals were respectively attached by welding to the positive electrode sheet and the negative electrode sheet of the wound electrode body thus produced, and housed in a battery case having a liquid inlet.

Next, a non-aqueous electrolyte was filled from the liquid inlet of the battery case, and the liquid inlet was airtightly sealed. As the non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3 was mixed with LiPF ₆ as a supporting salt in an amount of 1. A solution dissolved at a concentration of 0 mol/L was used. As described above, lithium ion secondary batteries for evaluation of each example and each comparative example were obtained.

<Measurement of average particle size>
The median diameter (D50) was measured using a laser diffraction particle size distribution analyzer for each of the positive electrode active material-preparing materials prepared above. The results are shown in Table 1 as "raw material particle average particle size (μm)". Also, the median diameter (D50) was similarly measured for each positive electrode active material of each example. The results are shown in Table 1 as "positive electrode active material average particle size (μm)".

<Analysis of raw material particles based on SEM image>
SEM images (magnifications of 5,000 and 15,000) of the positive electrode active material-preparing material (hydroxide particle powder) of each example prepared above were obtained. Based on the SEM image, the ratio of the average particle size of the small particles forming 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".
Also, 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 representative SEM images of the positive electrode active material preparation material disclosed herein, SEM images of the positive electrode active material preparation material of Example 2 observed at a magnification of 5,000 and 15,000 are shown in FIGS. 4 and 5, respectively. . FIG. 6 shows a cross-sectional SEM image of the positive electrode active material preparation material of Example 2 observed at a magnification of 15000 times.
As a representative example of the prior art, FIG. 7 shows an SEM image of the positive electrode active material preparation material of Example 5 observed at a magnification of 15000 times.

<Activation treatment and measurement of initial discharge capacity>
Using a constant current-constant voltage system, each lithium ion secondary battery for evaluation prepared above was charged at a current value of 1/3 C to 4.1 V at a constant current, and then a constant voltage until the current value became 1/50 C. The battery was charged and brought to a fully charged state. After that, each lithium ion secondary battery for evaluation was subjected to constant current discharge to 3.0 V at a current value of 1/3C. The discharge capacity at this time was defined as the initial discharge capacity. The charge/discharge operation was performed at 25°C. Also, "1 C" here means the magnitude of current that can charge the SOC (state of charge) from 0% to 100% in one hour.

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

<Measurement of Cycle Capacity Retention Rate>
Each of the activated lithium ion secondary batteries for evaluation was placed in an environment of 60°C. Each lithium ion secondary battery for evaluation was subjected to constant current charging to 4.2 V at 2 C and then to constant current discharging to 3.0 V at 2 C, which was defined as one cycle, and was repeated 200 cycles. After that, the discharge capacity after 200 cycles was measured by the same method as the initial discharge capacity described above. Then, the cycle capacity retention rate (%) is expressed by the following formula 1:
(discharge capacity after 200 cycles) / (initial discharge capacity) × 100 Equation 1
Calculated by The results are shown in Table 1 as "cycle capacity retention rate (%)".

As shown in FIGS. 4 and 5, the raw material particles contained in the positive electrode active material-preparing material of Example 2 had a coating portion made of small particles 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 of small particles were also observed.

As shown in Table 1, 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 Manufactured from a material for preparing a positive electrode active material Compared with the positive electrode active material of Example 5, the positive electrode active materials of Examples 1 to 4 can realize better cycle characteristics (cycle capacity retention rate) for lithium ion secondary batteries. Furthermore, it can be seen that Examples 1 to 4 reduce the initial resistance more than Example 5.
Furthermore, in Examples 1 and 2, in which the raw material particles have an average particle size of 4 μm or more and 6 μm or less, it can be seen that particularly excellent cycle characteristics and an initial resistance reduction effect can be achieved.
Therefore, according to the material for positive electrode active material preparation disclosed here, it turns out that the initial resistance of a lithium ion secondary battery can be reduced and the positive electrode active material which gives an excellent cycling characteristic can be manufactured.

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 technology described in the claims includes various modifications and changes of the specific examples illustrated above.

20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector 50 Positive electrode 52 Positive electrode current collector 52a Positive electrode current collector exposed portion 54 Positive electrode active material layer 60 Negative electrode 62 Negative electrode collector Current collector 62a Negative electrode current collector exposed portion 64 Negative electrode active material layer 70 Separator 100 Lithium ion secondary battery

Claims (5)

A material for preparing a positive electrode active material for a lithium ion secondary battery,
A raw material particle comprising a core portion and a coating portion present on at least a part of the surface of the core portion,
The coating part is composed of small particles having an average particle diameter of 1/10 or less of the particle diameter of the raw material particles in the SEM image of the raw material particles,
Here, 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,
In the cross-sectional SEM image of the raw material particles, the small particles are denser than the core portion,
Materials for preparing positive electrode active materials. 2. The material for preparing a positive electrode active material according to claim 1, wherein the small particles have an average aspect ratio of 1.7 or less based on an SEM image. 3. The material for preparing a positive electrode active material according to claim 1, wherein, in an SEM image of said raw material particles, said coating portion contains said raw material particles in which 10% or more of the surface of said core portion is occupied. 4. The material for preparing a positive electrode active material according to claim 1, wherein the raw material particles have an average particle size of 4 μm or more and 6 μm or less based on a laser diffraction/light scattering method. Mixing the positive electrode active material preparation material according to any one of claims 1 to 4 with a lithium compound, and
calcining the mixed mixture;
A method for producing a positive electrode active material, comprising:

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