JP7275092B2 - Positive electrode active material and non-aqueous electrolyte secondary battery comprising said positive electrode active material - Google Patents

Description

The present invention relates to a positive electrode active material and a non-aqueous electrolyte secondary battery comprising the positive electrode active 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. Lithium transition metal oxides (lithium transition metal composite oxides) are generally used as positive electrode active materials, and from the viewpoint of cost reduction, the use of transition metals, which are abundant in resources, is being studied. It is A typical transition metal is manganese. A lithium-manganese composite oxide with a predetermined element ratio has a spinel-type crystal structure, which is known as one of typical crystal structures, and can exhibit good lithium ion diffusibility. However, on the other hand, there are problems that need to be solved, such as manganese atoms eluting and depositing on the negative electrode, and the high-current discharge capacity being small due to low conductivity. Therefore, for example, Patent Literature 1 discloses a technique of modifying the surface of a lithium-manganese composite oxide having a spinel crystal structure with an oxide containing tungsten at a predetermined ratio. As a result, the large-current discharge capacity of the lithium manganese oxide can be dramatically improved.

JP 2005-320184 A

However, according to the study of the present inventors, in the technique disclosed in Patent Document 1, the particle size of the lithium-manganese composite oxide particles becomes small, and thus there is a possibility that manganese atoms are easily eluted from the lithium-manganese composite oxide. Therefore, it was found that there is a problem in cycle durability.

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 having a spinel-type crystal structure that can achieve excellent cycle durability. Another object of the present invention is to provide a non-aqueous electrolyte secondary battery comprising such a positive electrode active material.

The present inventors have made intensive studies to solve the above problems, and found that the average particle size of the lithium manganese composite oxide particles is made larger than before, and a predetermined lithium tungsten composite oxide is formed on the surface of the composite oxide particles. The present inventors have found that excellent cycle durability is achieved by allowing the presence of N in a ratio of
That is, the positive electrode active material disclosed herein is a positive electrode active material used in a non-aqueous electrolyte secondary battery, includes second particles in which a plurality of first particles are aggregated, and the SEM of the first particles The average particle size based on the image is 5 μm or more and 10 μm or less, the average particle size based on the SEM image of the second particles is 10 μm or more and 20 μm or less, and the first particles contain at least lithium atoms and manganese atoms. Contains spinel-type crystal structure lithium-manganese composite oxide.
Furthermore, at least part of the surface of the second particle is provided with lithium tungsten composite oxide particles, and the total of the lithium manganese composite oxide and the lithium tungsten composite oxide particles is 100 mol%. At that time, the ratio of the lithium-tungsten composite oxide particles is 3 mol % or less.
According to such a configuration, a positive electrode active material is provided that can reduce the initial resistance of the non-aqueous electrolyte secondary battery and impart excellent cycle durability.

Further, in a preferred aspect of the positive electrode active material disclosed herein, the proportion of the lithium tungsten composite oxide particles is 0.1 mol % or more and 3 mol % or less.
With such a configuration, lower initial resistance and better cycle durability can be achieved.

Further, in a preferred embodiment of the positive electrode active material disclosed herein, the spinel-type crystal structure lithium-manganese composite oxide has the following chemical formula:
Li _1+z Mn _2-x-z Mex _{O 4}
(In the above chemical formula, Me represents one or more metal elements containing at least one of Mg and Al, and x and z are positive real numbers, with x ≤ 0.1 and z ≤ 0.15. )
Including the compound represented by.
With such a configuration, even better cycle durability can be achieved.

Moreover, in a preferred embodiment of the positive electrode active material disclosed herein, Li ₂ WO ₄ is included as the lithium-tungsten composite oxide particles.
According to such a configuration, the effect of lowering the initial resistance and the effect of improving the cycle durability can be achieved at a higher level.

Moreover, in order to solve the above problems, a non-aqueous electrolyte secondary battery including the positive electrode active material disclosed herein is provided. That is, the non-aqueous electrolyte secondary battery disclosed herein includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode includes a positive electrode active material layer, and the positive electrode active material layer is The positive electrode active material disclosed in.
According to such a configuration, it is possible to provide a non-aqueous electrolyte secondary battery having a low initial resistance and excellent cycle durability.

1 is a cross-sectional view schematically showing the configuration of a non-aqueous electrolyte secondary battery according to one embodiment; FIG. 1 is a schematic exploded view showing the configuration of a wound electrode body of a non-aqueous electrolyte secondary battery according to one embodiment; FIG. FIG. 2 is a schematic cross-sectional view schematically showing the structure of particles contained in a positive electrode active material according to one embodiment.

An embodiment of the present invention will be described below with reference to the drawings, taking a lithium ion secondary battery as an example. 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 constructed by housing a flat-shaped electrode body (wound electrode body) 20 and a non-aqueous electrolyte (not shown) inside a battery case 30 . It is a square-shaped 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 electrode body 20 is formed by laminating a long sheet-shaped positive electrode 50 and a long sheet-shaped negative electrode 60 with two long sheet-shaped separators 70 interposed therebetween. , is a wound electrode body 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.

FIG. 3 is a schematic cross-sectional view schematically showing the configuration of particles contained in the positive electrode active material disclosed herein. The positive electrode active material disclosed here includes second particles 10 in which a plurality of first particles 12 are aggregated. At least part of the surface of the second particles 10 is provided with lithium tungsten composite oxide particles 14 .

The average particle size of the first particles 12 based on the SEM image is preferably 5 μm or more and 10 μm or less. By setting the average particle size based on the SEM image of the first particles 12 to such a range (the average particle size is larger than the conventional one), the specific surface area of the first particles 12 can be reduced. , non-aqueous electrolyte) can be reduced. This can suppress the elution of manganese atoms contained in the positive electrode active material, thereby improving the cycle durability. Moreover, within such a range, the diffusion of lithium ions is good, so an increase in electrical resistance can be suppressed.

The average particle diameter of the second particles 10 based on the SEM image is preferably 10 μm or more and 20 μm or less. According to such a range, the mechanical strength of the second particles 10 can be improved, so cracks are less likely to occur in the second particles 10 . Moreover, since the diffusibility of lithium ions in the solid phase is improved, an increase in electrical resistance can be suppressed.

The average particle size of the second particles 10 and the average particle size of the first particles 12 of the positive electrode active material disclosed herein are measured based on images observed using a scanning electron microscope (SEM). can be done. Specifically, first, in order to use the average particle size of the second particles 10 as a reference, a laser diffraction particle size distribution measuring device based on a general laser diffraction/light scattering method was used to measure the volume of the positive electrode active material. The particle size distribution is measured, and the particle diameter (median diameter: D50) corresponding to the cumulative 50% by volume from the small particle side is measured. Next, the positive electrode active material is observed at a plurality of locations with an SEM, and after obtaining a plurality of SEM images, a plurality (for example, 30 or more) of second particles having a size corresponding to D50 are randomly selected. Next, the length of the longest diameter of each of the selected plurality of second particles is defined as the major diameter, and the length of the longest diameter among the lines perpendicular to the major diameter is defined as the minor diameter. The particle diameter of the second particles is calculated by converting the equivalent circle diameter using the area of the ellipse consisting of the major axis and the minor axis. Then, the average value of the particle diameters of the selected plurality of second particles can be calculated as the average particle diameter of the second particles. As used herein, "the average particle diameter of the second particles" refers to the average value calculated in this way.

In the present specification, the term "average particle diameter of the first particles" refers to, among the plurality of second particles selected above, among the first particles constituting each of the second particles, particles It means the average value of the particle diameters of the first particles whose entire diameter is visible (that is, there is no portion hidden by other first particles). One or more first particles whose particle diameters are to be calculated from each of the selected plurality of second particles is selected. The method for calculating the particle diameter of the first particles is the same as the method for calculating the particle diameter of the second particles described above. can be calculated by converting the equivalent circle diameter.

The positive electrode active material (specifically, the first particles 12) disclosed herein contains a lithium-manganese composite oxide with a spinel-type crystal structure. A lithium-manganese composite oxide having a spinel-type crystal structure is an oxide having a spinel-type crystal structure and containing at least Li, Mn, and O as constituent elements.
As a typical lithium-manganese composite oxide with a spinel crystal structure,
General formula (chemical formula): Li _1+z Mn _2-x-z Me _x O ₄
(In the chemical formula, Me is absent or represents one or more metal elements including at least one of Mg and Al. When Me is absent, z is a positive real number, z ≤ 0.15 When Me is present, x and z are positive real numbers, and x≤0.1 and z≤0.15). In addition to Mg and Al, Me is, for example, Ni, Co, Ca, Ti, V, Cr, Si, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, Sn and the like. The lithium-manganese composite oxide preferably contains Mg and/or Al. Since Mg and Al do not cause a valence change during charge/discharge, expansion and contraction due to charge/discharge can be alleviated, and cracking of the positive electrode active material due to charge/discharge can be suppressed.
The above general formula indicates the lithium-manganese composite oxide contained in the positive electrode active material disclosed here when it is produced (in a new state). In other words, it shows the chemical composition of the lithium-manganese composite oxide before initial charging of the non-aqueous electrolyte secondary battery comprising such a positive electrode active material.
Also, the average chemical composition of the positive electrode active material can be measured by, for example, inductively coupled plasma (ICP) emission spectrometry.

The lithium-tungsten composite oxide particles 14 are provided on at least part of the surface of the second particles 10 . The lithium-tungsten composite oxide particles 14 refer to oxides containing at least Li, W and O. Li ₂ WO ₄ , LiHWO ₄ and the like are typically used, although they are not particularly limited, and Li ₂ WO ₄ is preferably used among these. By using Li ₂ WO ₄ , excellent initial resistance reduction effect and excellent cycle durability can be realized.

The average particle size of the lithium-tungsten composite oxide particles 14 is not particularly limited, but typically 1 μm to 3 μm can be used. Such an average particle size is obtained by measuring the volume-based particle size distribution using a laser diffraction particle size distribution measuring device based on a general laser diffraction/light scattering method, and the particles corresponding to the cumulative 50 volume% from the small particle side. It can be measured as a diameter (median diameter: D50).

When the total of the lithium manganese composite oxide and the lithium tungsten composite oxide particles 14 is 100 mol%, the proportion of the lithium tungsten composite oxide particles 14 is preferably 3 mol% or less, more preferably 2 mol% or less. Yes, more preferably 1.5 mol % or less. Moreover, the proportion of the lithium-tungsten composite oxide particles 14 is preferably 0.1 mol % or more, more preferably 0.3 mol % or more. Within this range, the lithium-tungsten composite oxide particles 14 can act well as an ionic conductor, so the battery resistance can be reduced. In addition, since the lithium tungsten composite oxide particles 14 can remove acidic components (for example, hydrogen fluoride, etc.) that may be contained in the non-aqueous electrolyte, elution of manganese atoms is suppressed and cycle durability is improved. I can let you.

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

A preferred method for producing the positive electrode active material particles disclosed herein includes a step of depositing hydroxide particles containing at least manganese as precursor particles (hereinafter also referred to as a “precursor deposition step”); and a lithium compound (hereinafter also referred to as a “mixing step”), a step of firing the mixture to obtain a lithium manganese composite oxide (hereinafter also referred to as a “firing step”), and the lithium manganese and a step of mixing the composite oxide and the lithium-tungsten composite oxide and firing the mixed oxide (hereinafter also referred to as a “coating step”).

First, the precursor deposition step will be described. The precursor deposition step may be the same as a known method in the production of normal positive electrode active materials. First, prepare an aqueous solution in which at least a manganese compound is dissolved. As the manganese compound, for example, water-soluble compounds such as manganese sulfate, manganese nitrate, and manganese halide can be used. Also, an aqueous solution of an alkaline compound is prepared. As the alkali compound, for example, lithium hydroxide, sodium hydroxide, potassium hydroxide, etc. can be used, and sodium hydroxide is preferably used. Prepare ammonia water.

Next, water (preferably ion-exchanged water) is added to the reactor at 30°C to 60°C and stirred. Stirring is continued while replacing the atmosphere with an inert gas (eg, N ₂ gas, Ar gas, etc.), and an aqueous solution of an alkaline compound is added to adjust the pH (eg, pH 10 to 13).
An aqueous solution of a manganese compound and aqueous ammonia are added to the reaction vessel while stirring is continued. At this time, since the pH in the reaction vessel is lowered by the addition of these, the pH in the reaction vessel is adjusted to the range of 10 to 13 with an aqueous solution of an alkaline compound. After that, the reaction vessel is allowed to stand for a predetermined time (for example, 1 to 3 hours) to sufficiently precipitate the precursor particles (hydroxide particles). Thereafter, the precursor particles are collected by suction filtration or the like, washed with water, and dried (for example, dried at 120° C. overnight) to obtain precursor powder.

Next, the mixing process will be described. As the lithium compound used here, for example, a compound that can be converted into an oxide by firing, such as lithium carbonate, lithium nitrate, or lithium hydroxide, can be used.
In addition to the obtained precursor powder and lithium compound powder, a flux having a predetermined concentration (typically 0.1% by mass or more and 50% by mass or less with respect to the mixture of the powders) is added and mixed. do. By adding and mixing the flux, the particle growth of the first particles can be promoted in the later-described firing step, so that the particle diameter of the first particles can be increased. The type of flux is not particularly limited, and a known flux (for example, B ₂ O ₃ powder) can be used. The particle size of the first particles can be adjusted by adjusting the amount (concentration) of the mixed flux.

For mixing, a mixture can be obtained by mixing according to a known method using a known mixing device (eg, shaker mixer, V blender, ribbon mixer, Julia mixer, Loedige mixer, etc.).
By adjusting the mixing ratio of the precursor powder and the lithium compound, a desired elemental ratio of the positive electrode active material can be obtained.

Further, for example, when the positive electrode active material contains aluminum atoms and/or magnesium atoms, in the mixing step, in addition to the precursor powder, the lithium compound powder, and the flux, the aluminum compound powder and / Or it can be manufactured by mixing a magnesium compound. Aluminum compounds and magnesium compounds are preferably compounds that are converted to oxides by firing. Examples of aluminum compounds that can be used include aluminum carbonate, aluminum nitrate, and aluminum hydroxide. Moreover, as a magnesium compound, magnesium carbonate, magnesium nitrate, magnesium hydroxide, etc. can be used, for example. Further, by adjusting the mixed amount of such compounds, a positive electrode active material having a desired elemental ratio can be obtained.

Next, the firing process 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. After pressure-molding the obtained mixture, it is heated, for example, at 500° C. to 600° C. for 6 to 12 hours in an air atmosphere. After cooling, the pressure-molded mixture is pulverized and molded again. The re-molded mixture is fired, for example, at 900° C. to 1000° C. for 6 hours to 24 hours. After that, annealing treatment is performed, for example, at 700° C. for 12 to 48 hours. After annealing, the mixture is cooled and pulverized again to obtain a lithium-manganese composite oxide. The heating rate during firing can be, for example, 5° C./min to 40° C./min (typically 10° C./min). Moreover, although there is no intention to limit it in particular, as a cooling method, the power of the electric furnace used for firing is turned off, and the firing can be carried out by allowing the firing to cool naturally.
The average particle size of the first particles can be adjusted by adjusting the concentration of the flux, the sintering temperature, and the sintering time.

Next, the coating process will be described. The lithium-manganese composite oxide obtained in the firing step and the lithium-tungsten composite oxide particles are mixed. Such mixing can be performed according to the above-described known mixing apparatus and known method. After mixing, such a mixture is heat-treated, for example, at 300° C. to 500° C. (typically 400° C.) in the atmosphere for 6 hours to 10 hours (typically 8 hours) to achieve the present disclosure. A positive electrode active material can be produced.
By adjusting such a mixing ratio, it is possible to obtain a positive electrode active material containing lithium-tungsten composite oxide particles in a desired proportion (mol %). Moreover, the lithium-tungsten composite oxide particles 14 may be purchased commercially, or may be produced by a conventionally known method.

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 an example, the prismatic non-aqueous electrolyte lithium ion secondary battery having the flat wound electrode assembly has been described above. However, this is only an example and is not limiting. For example, instead of the wound electrode body, a non-aqueous electrolyte secondary battery may be provided with a laminated electrode body in which sheet-like positive electrodes and sheet-like negative electrodes are alternately laminated via separators. 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>
[Example 1]
Manganese sulfate was dissolved in ion-exchanged water to prepare an aqueous manganese sulfate solution having a predetermined concentration. Also, an aqueous sodium hydroxide solution and an aqueous ammonia solution were prepared using ion-exchanged water.
The ion-exchanged water was stirred while being kept within the range of 30° C. to 60° C., and adjusted to a predetermined pH (pH 10 to 13) with the aqueous sodium hydroxide solution. Then, the manganese sulfate aqueous solution, the sodium hydroxide aqueous solution and the ammonia aqueous solution were added while controlling the pH to the predetermined value to obtain a coprecipitation product (hydroxide). The resulting hydroxide was filtered, washed with water, and dried in an oven at 120° C. to obtain hydroxide powder (positive electrode active material precursor powder). The ion-exchanged water used here was used after removing dissolved oxygen by passing an inert gas in advance.
The obtained hydroxide powder and lithium hydroxide powder were prepared and mixed so that the molar ratio (Li/Mn) of lithium (Li) and manganese (Mn) was 1.1:1.9. The mixture was pressure-molded, heated at 550° C. for 12 hours in an air atmosphere, cooled, and pulverized. The pulverized mixture was remolded, fired at 900° C. to 1000° C. for 6 hours to 24 hours, and then annealed at 700° C. for 12 hours to 48 hours. The rate of temperature rise during the firing was set at 10° C./min. After the annealing treatment, the positive electrode active material of Example 2 was obtained by cooling and pulverizing the obtained mixture.

[Examples 2 to 8]
A hydroxide powder was obtained in the same manner as in Example 1. The obtained hydroxide powder, lithium hydroxide powder, and magnesium hydroxide powder were mixed so that the molar ratio of lithium, magnesium, and manganese (Li:Mg:Mn) was 1.1:0.05:1.85. prepared to be Furthermore, B ₂ O ₃ was added and mixed so as to be 0.1% by mass to 50% by mass with respect to the entire prepared powder. The mixture was pressure-molded, heated at 550° C. for 12 hours in an air atmosphere, cooled, and pulverized. The pulverized mixture was remolded, fired at 900° C. to 1000° C. for 6 hours to 24 hours, and then annealed at 700° C. for 12 hours to 48 hours. The rate of temperature rise during the firing was set at 10° C./min. After such annealing treatment, it was cooled and the resulting mixture was pulverized. The average particle size of the first particles was adjusted by changing the concentration of B ₂ O ₃ , the sintering temperature, and the sintering time.
Next, the mixture pulverized after the annealing treatment and Li ₂ WO ₄ powder (manufactured by Alfa Aesar) were prepared, and the molar ratio of Li ₂ WO ₄ powder to the total of the mixture and Li ₂ WO ₄ powder was 0. .1 mol % to 3.5 mol %. The positive electrode active materials of Examples 2 to 8 were obtained by heating the mixture at 400° C. for 8 hours in an air atmosphere.

[Example 9]
Among the methods for producing the positive electrode active materials of Examples 2 to 8, the molar ratio of Li:Mg:Mn was changed to 1.05:0.1:1.85, and the molar ratio of the Li ₂ WO ₄ powder was The ratio was changed to 1 mol %. A positive electrode active material of Example 9 was obtained in the same manner as the positive electrode active material production methods of Examples 2 to 8 above, except for such changes.
[Example 10]
Among the methods for producing the positive electrode active materials of Examples 2 to 8, the molar ratio of Li:Mg:Mn was changed to 1.15:0.05:1.8, and the molar ratio of the Li ₂ WO ₄ powder was The ratio was changed to 1.2 mol %. Except for such changes, the positive electrode active material of Example 10 was obtained by carrying out the same manufacturing method of the positive electrode active material of Examples 2 to 8 above.
[Examples 11 to 15]
In the method for producing the positive electrode active material of Examples 2 to 8 above, magnesium hydroxide powder is changed to aluminum hydroxide powder, and the molar ratio of Li:Al:Mn is 1.1:0.1:1.8. changed to Except for such changes, the positive electrode active materials of Examples 11 to 15 were obtained in the same manner as the positive electrode active materials of Examples 2 to 8 above.

<Production of lithium-ion secondary battery for evaluation>
As a lithium secondary battery for evaluation, the positive electrode active material prepared above, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were combined into a positive electrode active material: AB: PVDF = 90: 8. :2 in N-methyl-2-pyrrolidone to prepare a paste for forming a positive electrode active material layer. This paste was applied to an aluminum foil current collector, dried and then pressed to produce a sheet-like positive electrode.
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 = 98: 1: 1. 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 a copper foil current collector, dried, and then pressed to prepare a sheet-like negative electrode.
Also, a porous polyolefin sheet having a three-layer structure of PP/PE/PP was prepared as a separator.
The produced sheet-like positive electrode and negative electrode were laminated so as to face each other with a separator interposed therebetween to produce a laminated electrode body. A collector terminal was attached to the laminated electrode assembly, and the assembly was placed in an aluminum laminate bag. Then, the laminated electrode body was impregnated with a non-aqueous electrolyte, and the opening of an aluminum laminate bag was sealed to produce a lithium ion secondary battery for evaluation. As the non-aqueous electrolyte, 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. A solution dissolved at a concentration of 0 mol/L was used.

<Calculation of Average Particle Size of First Particles and Second Particles>
Using a laser diffraction particle size distribution analyzer, the median diameter (D50) of the particles before mixing the Li ₂ WO ₄ powder in each positive electrode active material of each example was measured. Next, an SEM image of the positive electrode active material (second particles) was obtained. From the obtained SEM image, 30 second particles each having a size corresponding to D50 were arbitrarily (randomly) selected. Then, the particle diameters of the 30 second particles were calculated by the method described above, and the average value thereof was taken as the average particle diameter of the second particles.
Further, in each of the 30 second particles selected above, among the first particles contained in the second particles, the entire particle can be visually recognized in the SEM image (that is, the portion hidden by other particles The first particles were selected, the particle diameters of the first particles were calculated by the method described above, and the average value thereof was taken as the average particle diameter of the first particles.

<Composition analysis of positive electrode active material>
Inductively coupled plasma (ICP) emission spectroscopic analysis was performed on the positive electrode active material prepared above to measure the content of each element.

<Activation treatment and measurement of initial discharge capacity>
After performing constant current charging to 4.2 V at a current value of 0.1 C, each lithium ion secondary battery for evaluation prepared above was subjected to constant voltage charging until the current value during constant voltage charging became 1/50 C. , fully charged. Thereafter, each evaluation lithium ion secondary battery was discharged to 3.0 V at a current value of 0.1 C by a constant current method, and the discharge capacity at this time was taken as the initial discharge capacity. The charge/discharge operation was performed at 25°C. Here, "1 C" refers to the amount of current that can charge the SOC (state of charge) from 0% to 100% in one hour.

<Measurement of initial battery resistance>
Each lithium ion secondary battery for evaluation was adjusted to a state of 50% of the battery capacity (SOC=50%). Next, various current values were applied in an environment of −10° C., and the battery voltage was measured after 2 seconds. The applied current and voltage change were linearly interpolated, and the battery resistance (initial battery resistance) was calculated from the gradient. Then, the relative value of the initial resistance was calculated when the period resistance of Example 1 was set to 1.0. Table 1 shows the results.

<Measurement of resistance increase rate and capacity retention rate after endurance>
A cycle test was performed in an environment of 60° C. for each evaluation lithium ion secondary battery whose initial resistance was measured. Specifically, 50 cycles of constant current charging to 4.2 V at 1 C followed by constant current discharging to 3.0 V at 1 C were repeated. Then, the discharge capacity and battery resistance after 50 cycles were measured by the same method as above. Then, the capacity retention rate after endurance is expressed by the following formula 1:
(discharge capacity after 50 cycles) / (initial discharge capacity) × 100 Equation 1
Calculated by Also, the resistance increase rate after endurance is expressed by the following equation 2:
(Battery resistance after 50 cycles) / (initial battery resistance) × 100 Equation 2
Calculated by For each of the calculated post-endurance capacity retention rate and post-endurance resistance increase rate, relative values were calculated when Example 1 was taken as 1.0. Table 1 shows the results.

As shown in Table 1, the average particle size of the first particles is 5 μm or more and 10 μm or less, the average particle size of the second particles is 10 μm or more and 20 μm or less, and lithium tungsten is formed on the surface of the second particles. Examples 2 to 7, Examples 9 to 10, and Examples 12 to 15, in which the composite oxide (specifically Li ₂ WO ₄ ) is 3 mol% or less, have the average particle diameter of the first particles and the second particles It showed better cycle durability than the smaller Example 1. Specifically, an increase in resistance increase rate after a cycle test in a 60° C. environment was suppressed, and the capacity retention rate was also improved. Further, Examples 2 to 7, Examples 9 to 10, and Examples 12 to 15 showed lower values than Example 1 in terms of initial resistance. Furthermore, among these, Examples 2 to 6, Examples 9 to 10, and Examples 12 to 15 in which the proportion of lithium tungsten composite oxide (specifically, Li ₂ WO ₄ ) is 2.0 mol% or less exhibit superior cycle durability and It showed an initial resistance reduction effect. Examples 3 to 5, Examples 9 to 10, and Examples 13 to 14, in which the proportion of the lithium-tungsten composite oxide is 0.3 mol% or more and 1.5 mol% or less, have particularly excellent cycle durability and initial resistance reduction effect. It can be seen that
Therefore, by using the positive electrode active material disclosed herein, it is possible to provide a non-aqueous electrolyte secondary battery with low initial resistance and excellent cycle durability.

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.

10 Second particles 12 First particles 14 Lithium-tungsten composite oxide particles 20 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 current collector 62a Negative electrode current collector exposed portion 64 Negative electrode active material layer 70 Separator 100 Lithium ion secondary battery

Claims (4)

A positive electrode active material used in a non-aqueous electrolyte secondary battery,
Including second particles in which a plurality of first particles are aggregated,
The average particle size based on the SEM image of the first particles is 5 μm or more and 10 μm or less,
The average particle diameter based on the SEM image of the second particles is 10 μm or more and 20 μm or less,
The first particles comprise a spinel crystal structure lithium-manganese composite oxide containing at least lithium atoms and manganese atoms,
Lithium-tungsten composite oxide particles are provided on at least part of the surface of the second particles ,
Here, when the total of the lithium manganese composite oxide and the lithium tungsten composite oxide particles is 100 mol%, the proportion of the lithium tungsten composite oxide particles is 0.1 mol% or more and 3 mol% or less.
Positive electrode active material. The spinel-type crystal structure lithium-manganese composite oxide has the following chemical formula:
Li _1+z Mn _2-x-z Mex _{O 4}
(In the chemical formula, Me represents one or more metal elements including at least one of Mg and Al, x and z are positive real numbers, and x≤0.1 and z≤0.15. )
The positive electrode active material according to claim 1 , comprising a compound represented by The positive _electrode active material according to claim 1 or 2 , containing _Li2WO4 as the lithium-tungsten composite oxide particles. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte,
The positive electrode comprises a positive electrode active material layer,
The positive electrode active material layer comprises the positive electrode active material according to any one of claims 1 to 3 ,
Non-aqueous electrolyte secondary battery.

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