JP7191071B2 - Positive electrode active material and secondary battery comprising the 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 the positive electrode active material, and a spinel crystal structure is known as one of typical structures. For example, in Patent Document 1, hydroxide particles having fine primary particles are baked, and the average particle size of a positive electrode active material (lithium-containing composite oxide) having a spinel crystal structure is set to a range of 2 μm to 8 μm. It is disclosed that a good battery output is realized with , and good cycle characteristics are realized by suppressing the spread of the particle size distribution.

JP 2012-246199 A

By the way, as a result of intensive studies by the present inventors, in the above-mentioned Patent Document 1, the specific surface area is increased by the fine primary particles, so the contact area with the electrolytic solution is increased, and good battery output is realized. On the other hand, it was found that manganese atoms tend to be easily eluted. That is, it was found that the resistance tends to increase with the elution of manganese atoms accompanying repeated charging and discharging.

Therefore, the present invention has been made in view of the above problems, and a positive electrode active material having a spinel crystal structure that can reduce the increase in resistance and the decrease in capacity retention rate due to repeated charging and discharging of a non-aqueous electrolyte secondary battery. The main purpose is to provide Another object of the present invention is to provide a non-aqueous electrolyte secondary battery comprising such a positive electrode active material.

In order to achieve the above object, a positive electrode active material for non-aqueous electrolyte secondary batteries having the following configuration is provided. That is, the positive electrode active material disclosed herein is composed of a lithium-manganese composite oxide having a spinel-type crystal structure containing at least one of aluminum atoms and magnesium atoms, and the lithium-manganese composite oxide is porous. The porous particles contain secondary particles in which a plurality of primary particles are aggregated, and in a cross-sectional SEM image of the secondary particles, the area of the voids relative to the area occupied by the secondary particles constituting the active material The average value of the ratio is 10% or more and 40% or less. Further, in the secondary particles constituting the active material, the average maximum particle diameter based on the SEM image of the primary particles constituting the secondary particles is 5 μm or more and less than 12 μm.
According to such a configuration, it is possible to provide a positive electrode active material that reduces the initial resistance and the resistance increase rate due to cycle tests (repeated charge-discharge tests) and realizes a non-aqueous electrolyte secondary battery having an excellent capacity retention rate. .

Moreover, as one preferred aspect of the positive electrode active material disclosed herein, the average particle size of the primary particles based on the SEM image is 2 μm or more and 5 μm or less.
According to such a configuration, it is possible to realize a more excellent resistance suppressing effect and capacity retention rate.

Further, in order to achieve the above object, there is provided a non-aqueous electrolyte secondary battery comprising the following positive electrode active material disclosed herein. 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 that has a low initial resistance and can reduce an increase in resistance and a decrease in capacity retention rate after a cycle test.

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. 3 is a cross-sectional view schematically showing the configuration of secondary particles that constitute a positive electrode active material according to one embodiment.

An embodiment of the present invention will be described below with reference to the drawings. 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.

As used herein, the term "secondary battery" refers to a general battery that can be repeatedly charged and discharged. ). In this 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 sealed type constructed by housing a flat-shaped electrode body 20 and a non-aqueous electrolyte (not shown) inside a battery case 30 . 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 42 a 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 . Similarly, the negative current collecting plate 44 a 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 .

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.

Examples of the negative electrode current collector 62 that constitutes the negative electrode 60 include copper foil. The negative electrode active material layer 64 includes 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.

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 positive electrode active material disclosed herein is composed of a lithium-manganese composite oxide having a spinel-type crystal structure. A lithium-manganese composite oxide having a spinel crystal structure is an oxide having a spinel crystal structure and containing at least lithium, manganese and oxygen as constituent elements. Also, the lithium-manganese composite oxide may contain one or more additional elements. In the positive electrode active material disclosed here, the lithium-manganese composite oxide contains at least one of aluminum atoms and magnesium atoms.
That is, as a typical spinel-type crystal structure lithium-manganese composite oxide, the general formula:
Li _1+z Mn _2-xyz M _y Me _x O ₄
(In the formula, M represents at least one of Al and Mg, Me represents no or at least one metal element, and x, y, and z are respectively 0 ≤ x ≤ 0.25, 0 < y ≤0.25, 0<z≤0.15, and x+y≤0.25). Examples of the additive element (Me) include transition metal elements such as Co, Ni, Ca, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn and Sn. or typical metal elements.
Incidentally, y in the above general formula is typically 0<y≤0.25, preferably 0.05≤y≤0.1. Within this range, it is possible to provide a lithium ion secondary battery that has an excellent capacity retention rate and exhibits an excellent reduction effect on the initial resistance and the resistance increase rate after the cycle test.
The average chemical composition of the positive electrode active material may be measured by a known method, for example, by inductively coupled plasma (ICP) emission spectrometry.

FIG. 3 is a cross-sectional view schematically showing the configuration of secondary particles that constitute the positive electrode active material disclosed herein. As shown in FIG. 3 , the positive electrode active material disclosed herein is a porous particle containing secondary particles 10 in which a plurality of primary particles 12 are aggregated and having voids 14 .
In this specification, the term "primary particle" refers to the smallest unit of particles that constitute the positive electrode active material, specifically the smallest unit determined from the apparent geometrical form. Aggregates of such primary particles are called "secondary particles".

The average particle size of the primary particles 12 based on the SEM image is preferably 2 μm or more and 5 μm or less, and more preferably 2 μm or more and 4 μm or less. Particles smaller than this range have a large specific surface area, which may facilitate the elution of manganese atoms. In addition, if the particles are larger than this range, cracking of the particles is likely to occur during the cycle test, and manganese atoms are likely to be eluted. Therefore, by setting the average primary particle diameter within this range, the elution of manganese atoms can be suppressed at a high level, the initial resistance and the resistance increase rate after the cycle test are reduced, and the capacity retention rate is improved. I can do it.

Further, the average maximum particle size based on the SEM image of the primary particles 12 constituting the secondary particles 10 is preferably 5 μm or more. That is, it is preferable that the average maximum primary particle diameter of the primary particles 12 constituting the individual secondary particles 10 is 5 μm or more. The average maximum particle size of the primary particles 12 based on the SEM image is, for example, less than 12 μm, preferably 10 μm or less, more preferably 8 μm or less. When the secondary particles have primary particles of such a size, the diffusibility of lithium ions in the solid phase can be improved, and the electrical resistance can be reduced. Moreover, since the specific surface area is reduced, the elution of manganese atoms can be suppressed. Furthermore, by arranging other primary particles with a predetermined porosity around the primary particles of such a size, cracking of the particles is less likely to occur during the cycle test.

The primary particle size of the positive electrode active material disclosed herein can be measured based on an image observed using a scanning electron microscope (SEM). As a specific example, first, the volume-based particle size distribution is measured using a particle size distribution measuring device based on a general laser analysis/light scattering method, and the particle diameter corresponding to the cumulative 50% by volume from the small particle side (median diameter: D50) is measured. Next, the positive electrode active material is observed at multiple locations by SEM, and multiple SEM images are acquired. After that, a plurality of (for example, 30 or more) secondary particles having a size corresponding to D50 are randomly selected from the plurality of SEM images. Then, by using image analysis software (for example, Viewtrac (trademark), a product of Microtrack Bell Co., Ltd.), the average particle size of the primary particles constituting the selected secondary particles can be calculated. As used herein, the term "average primary particle size" refers to that calculated by such a method. The primary particles can be automatically identified by utilizing the difference in shade and color tone of the SEM image.
Further, in each of the selected plurality of secondary particles, the length of the longest diameter of the primary particle is defined as the major diameter with respect to the largest primary particle among the primary particles constituting the secondary particles, and the The length of the longest diameter among the lines perpendicular to the major diameter is defined as the minor diameter. By converting the equivalent circle diameter from the area of the ellipse defined by the major axis and the minor axis, the maximum particle size of the primary particles constituting the secondary particles can be calculated. In this specification, the average value of the maximum particle diameters calculated in this manner is referred to as "the average maximum particle diameter of the primary particles".

The voids 14 included in the secondary particles 10 may or may not be open. Moreover, when it is open, one gap may have two or more openings.

In the cross-sectional SEM image of the positive electrode active material disclosed here, the average value of the ratio (porosity) of the area of the voids 14 to the area occupied by the secondary particles 10 constituting the active material is 10% or more and 40%. % or less, more preferably 15% or more and 30% or less. If the porosity is too small, the initial resistance can be high because the surface area to react with the electrolyte is too small. On the other hand, if the porosity is too large, the surface area that reacts with the electrolyte becomes too large, which may facilitate the elution of manganese atoms. Therefore, by setting the porosity within the above range, it is possible to appropriately reduce the initial resistance value and appropriately suppress the elution of manganese atoms. In addition, by setting the porosity within this range, the mechanical strength of the secondary particles is also improved, so cracking of the particles that may occur during charging and discharging is appropriately suppressed.

In addition, in this specification, a porosity can be calculated|required as follows. First, using a scanning electron microscope (SEM), a plurality of cross-sectional SEM images are acquired in a plurality of fields of view. Next, in the cross-sectional SEM image, a plurality (for example, 30 or more) of secondary particles having a size corresponding to D50 are randomly selected. In each secondary particle cross section, using "ImageJ", which is an open source and well-known public domain image processing software, the part where the primary particle exists is colored white, and the void part where the primary particle does not exist is colored black. Perform binarization processing. Next, the area of the part where the primary particles are present (white part) is NW, the area of the void part (black part) is NB, and "NB / (NW + NB) × 100" is calculated to obtain the porosity. can be done. Then, the average porosity of the selected plurality of secondary particles is taken as the average porosity. Note that when the gap has an opening, the contour of the gap is not closed. In this case, both ends of the opening are connected by a straight line, and the area surrounded by the straight line and the outline of the primary particles is defined as the area of the void.

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 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”); It includes a step of obtaining a mixture with a lithium compound (hereinafter also referred to as a “mixing step”) and a step of firing the mixture to obtain a lithium-manganese composite oxide (hereinafter also referred to as a “firing 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 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 (for example, N ₂ gas, Ar gas, etc., but may contain a trace amount of oxygen), and an aqueous solution of an alkaline compound is added to adjust the pH (for example, 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). Thereby, a powder of precursor particles can be obtained.
The precursor particles obtained at this time have a porous structure with voids. The porosity of the precursor particles can be increased by changing the pH (for example, changing from the range of pH 11 to 13 to the range of pH 10 to 12) along with the formation of the precipitate. The porosity can be adjusted by adjusting the width of and the rate of change.

Next, the mixing process will be described. The precursor particle powder obtained in the precursor deposition step, the lithium compound powder, and the aluminum compound and/or magnesium compound powder are mixed. 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 powders to be mixed, it is possible to obtain a positive electrode active material having a desired elemental ratio.
Lithium compounds, aluminum compounds, and magnesium compounds used here may be compounds that are converted to oxides by firing. Examples of lithium compounds that can be used include lithium carbonate, lithium nitrate, and lithium hydroxide. 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.

Next, the firing process will be described. The mixture obtained in the mixing step 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 at 500° C. to 600° C. for 6 to 12 hours in an air atmosphere. It is then cooled and the pressure-molded mixture is pulverized. Next, a flux of a predetermined concentration (typically 0.1% by mass or more and 50% by mass or less relative to the mixture) is added to the pulverized mixture. By adding such a flux, the grain growth of the primary particles can be partially promoted, so that the particle diameter of some of the primary particles constituting the secondary 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 primary particles can be adjusted by adjusting the amount (concentration) of the mixed flux.
The fluxed mixture is then molded again. The reshaped mixture is fired at 800° C. to 1000° C. for 2 hours to 24 hours. After that, annealing treatment is performed, for example, at 700° C. for 12 to 48 hours. After the annealing treatment, it is cooled and the powder is pulverized again to obtain the positive electrode active material. The heating rate during firing can be, for example, 5° C./min to 40° C./min.
The particle size of the primary particles can also be adjusted by adjusting the firing temperature and firing time.

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>
(precursor preparation)
[Examples 1 to 16]
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 the pH was adjusted with the aqueous sodium hydroxide solution. Then, while controlling the pH, the manganese sulfate aqueous solution, the sodium hydroxide aqueous solution and the ammonia aqueous solution were added to obtain a coprecipitation product (hydroxide particles). At this time, the porosity of the hydroxide particles was adjusted by changing the controlled pH along with the formation of the precipitate. The resulting hydroxide particles were 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.

(Mixing process)
[Example 1]
The obtained hydroxide powder and lithium hydroxide powder were mixed so that the molar ratio (Li/Mn) of lithium (Li) to manganese (Mn) was 0.58 (that is, Li:Mg=1.1:1.9). ) was mixed.
[Examples 2 to 8]
The obtained hydroxide powder, magnesium hydroxide powder, and lithium hydroxide powder were mixed. At this time, the molar ratio of lithium (Li):magnesium (Mg):manganese (Mn)=1.1:0.05:1.85 was set.
[Example 9]
The obtained hydroxide powder, magnesium hydroxide powder, and lithium hydroxide powder were mixed. At this time, the molar ratio of lithium (Li):magnesium (Mg):manganese (Mn)=1.05:0.1:1.85.
[Example 10]
The obtained hydroxide powder, magnesium hydroxide powder, and lithium hydroxide powder were mixed. At this time, the molar ratio of lithium (Li):magnesium (Mg):manganese (Mn)=1.15:0.05:1.8 was set.
[Examples 11 to 16]
The obtained hydroxide powder, aluminum hydroxide powder, and lithium hydroxide powder were mixed. At this time, the molar ratio of lithium (Li):aluminum (Al):manganese (Mn)=1.1:0.1:1.8 was set.

(Baking process)
[Examples 1 to 16]
The mixture obtained by mixing the above powders was molded under pressure, heated at 550° C. for 12 hours in an air atmosphere, cooled, and then pulverized. After pulverization, B ₂ O ₃ was added as a flux at a predetermined concentration (0.1% by mass or more and 50% by mass or less with respect to the mixture) in order to partially promote grain growth of primary particles. After that, the mixture to which B ₂ O ₃ was added was remolded, baked at 800° C. to 1000° C. for 2 hours to 24 hours, and then annealed at 700° C. for 12 hours to 48 hours. The rate of temperature increase during the firing was set at 5° C./min to 40° C./min. After cooling, the obtained mixture was pulverized to obtain a positive electrode active material. The primary particle size was adjusted by adjusting the concentration of B ₂ O ₃ , the sintering temperature, and the sintering time.

<Production of lithium-ion secondary battery for evaluation>
As a lithium secondary battery for evaluation, the positive electrode active material prepared above, carbon black (CB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were combined into a positive electrode active material: CB: 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 the aluminum laminate type 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.

<Structure Evaluation of Positive Electrode Active Material>
The median diameter (D50) of each positive electrode active material prepared above was measured using a laser diffraction particle size distribution analyzer.
Next, a plurality of SEM images of the positive electrode active material prepared above were obtained. From the obtained SEM image, 30 secondary particles each having a size corresponding to D50 were arbitrarily (randomly) selected. The average primary particle size of the primary particles constituting the selected 30 secondary particles was calculated and shown in Table 1. In addition, the maximum particle size of the primary particles was measured for each of the 30 secondary particles selected above, and the average value is shown in Table 1 as the "average maximum primary particle size."
In addition, a cross-sectional SEM image of the positive electrode active material prepared above was obtained. From the cross-sectional SEM image, 30 secondary particles having a size corresponding to D50 were arbitrarily (randomly) selected, and the average porosity of the secondary particles was determined. The results are shown in Table 1.

<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.

<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 initial resistance of Example 1 was set to 1.00. 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 set to 1.00. Table 1 shows the results.

As shown in Table 1, it is composed of a lithium manganese composite oxide containing aluminum atoms or magnesium atoms, has an average maximum primary particle size of 5 μm or more and less than 12 μm, and has an average primary particle size of 2 μm or more and 5 μm or less. , Lithium ion secondary batteries (Examples 3 to 7, Examples 9 to 10, Examples 12 to 16) having a positive electrode active material having an average porosity of 10% or more and 40% or less do not contain aluminum atoms and magnesium atoms It was confirmed that the initial resistance and the rate of increase in resistance after endurance were reduced, and the capacity retention rate after endurance was improved, as compared with the lithium ion secondary battery (Example 1) having a positive electrode active material. In particular, Examples 4 to 6, Examples 9 to 10, and Examples 13 to 15 showed particularly excellent effects of reducing the initial resistance and resistance increase rate after endurance, and improving the capacity retention rate after endurance. That is, when the maximum primary particle size is 5 μm or more and 8 μm or less, the average primary particle size is 2 μm or more and 4 μm or less, and the average porosity is 15% or more and 30% or less, a particularly excellent non-aqueous electrolyte secondary battery is obtained. It can be seen that it is realized.

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 secondary particles 12 primary particles 14 void 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 (2)

A positive electrode active material used in a non-aqueous electrolyte secondary battery,
Consists of a spinel crystal structure lithium-manganese composite oxide containing at least one of aluminum atoms and magnesium atoms,
The lithium manganese composite oxide is porous particles,
The porous particles include secondary particles in which a plurality of primary particles are aggregated,
In the cross-sectional SEM image of the secondary particles, the average value of the ratio of the area of the voids to the area occupied by the secondary particles constituting the active material is 10% or more and 40% or less,
The average particle size based on the SEM image of the primary particles is 2 μm or more and 5 μm or less,
The secondary particles constituting the active material have the following characteristics:
A positive electrode active material, wherein the average maximum particle size based on an SEM image of primary particles constituting the secondary particles is 5 μm or more and less than 12 μm. 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 claim 1 ,
Non-aqueous electrolyte secondary battery.

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