JP7225282B2 - Positive electrode active material and lithium ion secondary battery comprising the positive electrode active material - Google Patents

Description

The present invention relates to a positive electrode active material and a lithium ion 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, manganese atoms tend to be eluted during repeated charge/discharge cycles (especially repeated charge/discharge cycles in a high-temperature environment), so there is a problem that the battery life tends to be shortened. Therefore, Patent Document 1 discloses a positive electrode material (positive electrode active material) comprising lithium phosphate and a cubic metal oxide on the surface of a lithium-manganese composite oxide having a cubic spinel crystal structure. . With such a configuration, the elution of manganese atoms can be suppressed, so that the life of the secondary battery can be extended.

JP 2010-123466 A

However, according to the study of the present inventor, the specific surface area of the particles increases because the particle size of the particles made of the lithium-manganese composite oxide is not sufficiently increased in the technique disclosed in Patent Document 1. As a result, since the contact area between the electrolyte and the particles increases, the manganese atoms are easily eluted, and the effect of suppressing the elution of manganese atoms becomes insufficient. As a result, the effect of improving the cycle characteristics (for example, the reduction rate of the capacity retention rate and the resistance increase rate) is insufficient, and there is room for improvement in the 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 having a spinel structure that achieves excellent cycle characteristics. Another object of the present invention is to provide a lithium ion secondary battery including such a positive electrode active material.

In order to solve the above problems, the present inventor focused on the average particle size of spinel-type crystal structure particles containing a lithium-manganese composite oxide. As a result, it is possible to adjust the average particle size of such particles to a predetermined range that is larger than before, and to mix phosphorus-containing lithium transition metal composite oxide particles containing at least nickel or cobalt and perform heat treatment. and found that the cycle characteristics are improved. It was also found that the initial resistance is reduced in addition to the cycle characteristics.
That is, the positive electrode active material disclosed herein is a positive electrode active material used in a lithium ion secondary battery, includes second particles in which a plurality of first particles are aggregated, and an SEM image of the first particles. is 5 μm or more and 10 μm or less, the average particle diameter of the second particles based on the SEM image is 12 μm or more and 22 μm or less, and the first particles contain at least lithium atoms and manganese atoms a lithium-manganese composite oxide having a spinel-type crystal structure, wherein the surfaces of the second particles are composed of a phosphorus-containing lithium-transition metal composite oxide having transition metal atoms of at least one of nickel atoms and cobalt atoms. the third particles are present, and the ratio of the phosphorus-containing lithium-transition metal composite oxide constituting the third particles to the entire compound constituting the positive electrode active material is 0.3 mol% or more and 5 mol% or less and the transition metal atoms are unevenly distributed in the surface layer portion of 3 μm or less from the surface of the second particle.
With such a configuration, the specific surface area of the first particles is reduced, so the contact area between the electrolyte and manganese atoms is reduced, and the manganese atoms are less likely to be eluted. In addition, since at least one of nickel atoms and cobalt atoms is present in the surface layer portion of the second particle and the third particle, these atoms are preferentially eluted over manganese atoms, so the elution of manganese atoms is suppressed. can be suppressed. Furthermore, since the surface layer of the second particles, in which at least one of the nickel atoms and the cobalt atoms are unevenly distributed, changes in volume due to expansion and contraction due to charge and discharge is smaller than the inside of the second particles, cracking of the particles due to charge and discharge occurs. becomes less likely to occur. Thereby, excellent cycle characteristics can be realized.

Further, in a preferred aspect of the positive electrode active material disclosed herein, the ratio of the phosphorus-containing lithium-transition metal composite oxide that constitutes the third particles to the entire compound that constitutes the positive electrode active material is 0.3 mol. % or more and 3 mol % or less. With such a configuration, better cycle characteristics can be achieved.

Further, in a preferred aspect of the positive electrode active material disclosed herein, the surface layer portion of the second particles where the transition metal atoms are unevenly distributed is in the range of 2 μm or less from the surface of the second particles. With such a configuration, even better cycle characteristics can be achieved.

Moreover, in a preferred embodiment of the positive electrode active material disclosed herein, the lithium-manganese composite oxide includes at least one of aluminum atoms and magnesium atoms. According to such a configuration, it is possible to achieve excellent cycle characteristics at a higher level.

In a preferred embodiment of the positive electrode active material disclosed herein, LiMePO ₄ (Me includes at least one of Ni and Co) is included as the phosphorus-containing lithium transition metal composite oxide. According to such a configuration, it is possible to realize excellent cycle characteristics at a higher level.

Also, in order to solve the above problems, a lithium ion secondary battery including the positive electrode active material disclosed herein is provided. That is, the lithium ion 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 It comprises the disclosed positive electrode active material.
According to such a configuration, it is possible to provide a lithium ion secondary battery imparted with excellent cycle characteristics.

1 is a cross-sectional view schematically showing the configuration of a lithium ion secondary battery according to one embodiment; FIG. 1 is a schematic exploded view showing the configuration of a wound electrode body of a lithium ion secondary battery according to one embodiment; FIG. 1 is a perspective view schematically showing the structure of a positive electrode active material particle according to one embodiment; FIG. FIG. 4 is a perspective view schematically showing the structure of second particles that constitute the positive electrode active material particles 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.
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 an electricity storage device that can be repeatedly charged and discharged, and is a term that includes so-called storage batteries and electricity storage elements such as electric double layer capacitors. In this specification, the term “lithium ion secondary battery” refers to a secondary battery that utilizes lithium ions as a charge carrier and is charged/discharged by the transfer of charge associated with the lithium ions between the positive and negative electrodes. In addition, the term “active material” as used herein refers to a material that reversibly absorbs and releases charge carriers. An embodiment of a lithium ion secondary battery including the positive electrode active material disclosed herein will be described below.

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. Furthermore, 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. , may be 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 positive electrode active material layer 54 includes a positive electrode active material disclosed herein, which will be described later. 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, for example, by dispersing a positive electrode active material and optionally used materials (a conductive material, a binder, etc.) in an appropriate solvent (eg, N-methyl-2-pyrrolidone: NMP) to form a paste. It can be formed by preparing a (or slurry) composition, applying an appropriate amount of the composition to the surface of the positive electrode current collector 52, and drying.

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 formed by, for example, 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. It can be formed by preparing, applying an appropriate amount of the composition to the surface of the negative electrode current collector 62, and drying.

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. 3A is a perspective view schematically showing the configuration of positive electrode active material particles 10 that constitute the positive electrode active material disclosed herein. 3B is a perspective view schematically showing the structure of the second particles 14 that constitute the positive electrode active material particles 10. As shown in FIG. The positive electrode active material particles 10 contain second particles 14 in which a plurality of first particles 12 are aggregated. Also, the third particles 16 are present on the surfaces of the second particles 14 .

The first particles 12 contain at least a lithium-manganese composite oxide having a spinel 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. Typically, the first particles 12 are composed only of a lithium-manganese composite oxide having a spinel crystal structure.

As a typical lithium-manganese composite oxide having a spinel-type crystal structure that constitutes the first particles 12,
General formula (chemical formula): Li _1+z Mn _2-xz M _x O ₄
(In the above chemical formula, M represents one or more metal elements, x and z are positive real numbers, and x≤0.25 and z≤0.15). . Here, M is, for example, Al, Mg, Ni, Co, Ca, Ti, V, Cr, Si, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, Sn, etc. could be. 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 mitigated. As a result, cracking of the positive electrode active material due to charging and discharging can be suppressed, so that cycle characteristics (for example, reduction rate of resistance increase rate, capacity retention rate, etc.) are improved. Moreover, as will be described later, at least one of nickel atoms and cobalt atoms is unevenly distributed in the surface layer portion of the second particles 14 . Therefore, at least the surface layer portions of the first particles 12 corresponding to the surface layer portions of the second particles 14 contain a lithium-manganese composite oxide having at least one of nickel atoms and cobalt atoms.
Also, the average chemical composition of the positive electrode active material can be measured by, for example, inductively coupled plasma (ICP) emission spectrometry.

The average particle diameter of the first particles 12 based on the SEM image is preferably 5 μm or more and 10 μm or less, more preferably 5 μm or more and 8 μm or less. The specific surface area of the first particles 12 increases as the average particle diameter of the first particles 12 decreases. As a result, the contact area between the first particles 12 and the non-aqueous electrolyte increases, and manganese atoms are easily eluted from the first particles 12, which is not preferable from the viewpoint of cycle characteristics. On the other hand, if the average particle diameter of the first particles 12 is too large, the diffusion of lithium ions in the solid phase is reduced, and cracks are likely to occur due to expansion and contraction due to charging and discharging, which is not preferable. Therefore, by setting the average particle diameter of the first particles 12 within the range described above, both the elution of manganese atoms and cracking due to expansion and contraction due to charging and discharging can be appropriately suppressed, so that cycle characteristics can be improved. be able to. Furthermore, within the range described above, the diffusion of lithium ions in the solid phase in the first particles 12 is favorably improved, and the diffusion resistance in the particles is reduced. Thereby, the initial resistance can be reduced.

The average particle diameter of the second particles 14 based on the SEM image is preferably 12 μm or more and 22 μm or less, more preferably 12 μm or more and 16 μm or less. With this range, the solid-phase diffusibility of lithium ions in the second particles 14 is improved. Thereby, in addition to the effect obtained when the average particle diameter of the first particles 12 contained in the second particles 14 is set within the preferred range described above, the initial resistance can be further reduced.

The average particle size of the second particles 14 of the positive electrode active material disclosed here can be measured based on an image (SEM image) observed using a scanning electron microscope (SEM). Specifically, first, as a reference for the average particle size of the second particles 14, a volume-based particle size of the second particles is measured using a laser diffraction particle size distribution measuring device based on a laser diffraction/light scattering method. The 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 in the SEM image 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. Then, the equivalent circle diameter is converted using the area of the ellipse consisting of the major axis and the minor axis. Thereby, the particle diameters of the selected plurality of second particles are calculated. After that, the average value of the particle diameters of the selected plurality of second particles is calculated. The average value calculated in this manner is referred to as "the average particle size of the second particles" in this specification.

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.

At least one transition metal atom out of nickel atoms and cobalt atoms is preferably unevenly distributed in the surface layer portion of the second particles 14 . Specifically, the transition metal atoms are preferably unevenly distributed within a range (thickness) of 3 μm or less from the surface of the second particle 14, and more preferably within a range (thickness) of 2 μm or less. In the surface layer portion where such transition metals are unevenly distributed, the volume change due to expansion and contraction due to charging and discharging is smaller than in other regions. Therefore, the expansion and contraction of the second particles 14 due to charging and discharging are suppressed on the particle surface, and cracking of the particles due to such expansion and contraction can be suppressed. As a result, it is possible to suppress deterioration in cycle characteristics that may occur due to cracking of such particles (that is, to improve cycle characteristics). Moreover, such a configuration facilitates fixing the third particles 16 to the surfaces of the second particles 14 . As a result, the effect of the third particles 16, which will be described later, being present on the surfaces of the second particles 14 can be enhanced.
Further, although not particularly limited, if the thickness of the surface layer portion is too small, the above-described effect of suppressing expansion and contraction cannot be sufficiently obtained. Therefore, the thickness may be, for example, 1.0 μm or more, preferably 1.5 μm or more.

In this specification, the range (thickness) of the surface layer portion of the second particles 14 in which at least one of nickel atoms and cobalt atoms is unevenly distributed is measured as follows.
First, a positive electrode active material is embedded in a resin, and after forming a rough cross section by, for example, mechanical polishing, the cross section is processed by a cross section polisher (CP) method. Next, the cross section after cross section processing is observed with an SEM, and a plurality (at least 20 or more) of second particles having a size corresponding to D50 are randomly selected. Then, each of the plurality of selected second particles is photographed at a magnification of, for example, 3,000 to 5,000 times to obtain a cross-sectional SEM image. After that, each cross-sectional SEM image of a plurality of selected second particles is subjected to planar analysis using energy dispersive X-ray spectroscopy (EDX) to examine the intensity distribution of Ni and Co in the second particles. Then, in each cross-sectional SEM image, among the normal lines drawn from the surface of the second particle toward the inside, the normal line that maximizes the existence distance of Ni and Co is selected. Then, for each of the selected normals, the maximum intensity of Ni and Co and the intensity of 30% of the maximum intensity are determined. Then, along the normal line, the length from the second particle surface to the point where the intensity is 30% of the maximum intensity (the thickness of the surface layer portion where Ni and Co are unevenly distributed) is measured. In this way, the length of each of the selected plurality of second particles is measured. Then, the average value of such lengths is taken as the thickness of the surface layer portion of the second particles 14 in which at least one of nickel atoms and cobalt atoms is unevenly distributed.

The third particles 16 are present on the surface of the second particles 14 . In the present specification, "existing on the surface of the second particle" may be a state in which the third particle 16 is in contact with the surface of the second particle 14 (for example, a state in which it is fixed). , the third particles 16 may be arranged in the vicinity of the surface of the second particles 14 (for example, within 1 μm to 3 μm from the surface of the second particles 14).

The third particles 16 are composed of a phosphorus-containing lithium transition metal composite oxide having transition metal atoms of at least one of nickel atoms and cobalt atoms. Here, the phosphorus-containing lithium-transition metal composite oxide is an oxide containing at least Li, P, and O as constituent elements and at least one of Ni and Co. A typical phosphorus _- containing lithium transition metal composite oxide contained in the third particles 16 includes a compound represented by LiMePO ₄ (Me includes at least one of Ni and Co). and _LiCoPO4 are preferably employed. Also, the phosphorus-containing lithium-transition metal composite oxide may contain elements such as Fe and F. Although the crystal structure of the third particles 16 is not particularly limited, it may have, for example, an olivine type crystal structure.

The phosphorus-containing lithium-transition metal composite oxide that constitutes the third particles 16 is present at a ratio of 0.3 mol% or more and 5 mol% or less when the total of the compounds that constitute the positive electrode active material disclosed herein is 100 mol%. preferably. According to this ratio, the nickel atoms and/or cobalt atoms contained in the third particles 16 are preferentially eluted rather than the manganese atoms contained in the second particles 14, so the elution of manganese atoms is suppressed. can do. Thereby, cycle characteristics can be improved. On the other hand, if the proportion of the third particles 16 is higher than the above range, the cost of the active material increases and the capacity decreases, which is undesirable. Moreover, when the above ratio of the third particles 16 is lower than the above range, the effect of suppressing the elution of manganese atoms cannot be sufficiently obtained. Therefore, the above ratio is preferably within an appropriate range, for example, it is more preferably 0.3 mol % or more and 3 mol % or less, and further preferably 2 mol % or more and 3 mol % or less.

The average particle diameter of the third particles 16 is not particularly limited, but may be, for example, about 1 μm to 10 μm. Such an average particle size can be measured based on a laser diffraction/light scattering method.

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 lithium-manganese composite oxide particles (hereinafter also referred to as a “sintering step”), and the lithium A step of mixing manganese composite oxide particles with particles containing a lithium phosphorus composite oxide having transition metal atoms of at least one of nickel atoms and cobalt atoms (hereinafter also referred to as a “second mixing step”); and a step of heat-treating the mixture prepared in the second mixing step (hereinafter referred to as “heat treatment 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 a reaction vessel at a predetermined temperature (for example, 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 left still 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 lithium-manganese composite oxide particles. The heating rate during firing can be, for example, 5° C./min to 40° C./min (typically 10° C./min). Although not intended to be particularly limited, as a cooling method, for example, the electric furnace used for firing may be turned off and allowed 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 second mixing step will be explained. Lithium-manganese composite oxide particles and phosphorus-containing lithium-transition metal composite oxide particles having transition metal atoms of at least one of nickel atoms and cobalt atoms are mixed. _LiNiPO4 and _LiCoPO4 , for example, can be suitably used as such phosphorus-containing lithium transition metal composite oxides. The mixing method should just follow a well-known mixing apparatus and a well-known method similarly to the above-mentioned mixing process. The mixing ratio of the lithium manganese composite oxide particles and the phosphorus-containing lithium transition metal composite oxide can be adjusted to a desired ratio.

Next, the heat treatment process will be described. By heat-treating the mixture obtained in the second mixing step, at least one of nickel atoms and cobalt atoms contained in the phosphorus-containing lithium transition metal composite oxide is added to the surface layer portion of the lithium manganese composite oxide particles. It can be substituted with a transition metal atom. The heat treatment can be performed using, for example, a batch-type electric furnace, a continuous electric furnace, or the like. The heat treatment conditions can be, for example, 400° C. to 800° C. for 2 hours to 20 hours in an air atmosphere. By adjusting the mixing amount of the phosphorus-containing lithium transition metal composite oxide, the heat treatment temperature, and the heat treatment time, the thickness of the surface layer of the lithium manganese composite oxide particles substituted with nickel atoms and/or cobalt atoms is changed. can do.

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 lithium ion secondary battery including the flattened 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 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. Also, a gel electrolyte may be used instead of the non-aqueous 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.

<Production 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 so that the molar ratio of lithium (Li) to manganese (Mn) was Li:Mn=1.1:1.9. Next, B ₂ O ₃ was added in an amount of 0.1% by mass to 50% by mass with respect to the entire prepared powder and mixed. 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 1 was obtained by cooling and pulverizing the obtained mixture.

(Example 2)
A positive electrode active material of Example 2 was obtained in the same manner as in the method for producing the positive electrode active material of Example 1 described above, except that B ₂ O ₃ was not used.
(Example 3)
In the manufacturing method of the positive electrode active material of Example 1 described above, the steps of preparing the hydroxide powder and the lithium hydroxide powder were changed. In Example 3, hydroxide powder, lithium hydroxide powder, and aluminum hydroxide powder were prepared in a molar ratio of Li:Al:Mn=1.1:0.05:1.85. . Other steps were performed in the same manner as in Example 1 to obtain a positive electrode active material of Example 3.
(Examples 4-11)
A positive electrode active material was obtained in the same manner as in the manufacturing method of the positive electrode active material of Example 3 described above. LiCoPO ₄ powder (manufactured by Toyoshima Seisakusho Co., Ltd.) was mixed with this positive electrode active material. At this time, the LiCoPO ₄ powder was adjusted to be 0.1 mol % to 6 mol % with respect to the entire mixture. The mixture was heat-treated in the air at 400° C. to 800° C. for 2 hours to 20 hours to obtain positive electrode active materials of Examples 4-11. In each example, by changing the heat treatment temperature and time, the range (thickness) of the surface layer portion of the second particles substituted with Co atoms was changed. Further, by changing the amount of B ₂ O ₃ added, the firing temperature, and the firing time described in Example 1 in each example, the average particle sizes of the first particles and the second particles were changed.

(Example 12)
In the methods for producing the positive electrode active materials of Examples 4 to 11 described above, the molar ratio of the hydroxide powder, the lithium hydroxide powder, and the aluminum hydroxide powder was changed. In Example 12, the above powder was prepared in a molar ratio of Li:Al:Mn=1.05:0.1:1.85. Other steps were performed in the same manner as in Examples 4 to 11 to obtain a positive electrode active material of Example 12.
(Example 13)
In the methods for producing the positive electrode active materials of Examples 4 to 11 described above, the molar ratio of the hydroxide powder, the lithium hydroxide powder, and the aluminum hydroxide powder was changed. In Example 12, the powder was prepared to have a molar ratio of Li:Al:Mn=1.15:0.05:1.8. Other steps were performed in the same manner as in Examples 4 to 11 to obtain a positive electrode active material of Example 13.
(Example 14)
LiCoPO ₄ powder was changed to LiNiPO ₄ powder (manufactured by Toyoshima Seisakusho Co., Ltd.) in the method for producing the positive electrode active materials of Examples 4 to 11 described above, and the LiNiPO ₄ powder was 2 mol % of the entire mixture. . Other steps were performed in the same manner as in Examples 4 to 11 to obtain a positive electrode active material of Example 14.
(Example 15)
In the method for producing the positive electrode active material of Examples 4 to 11 described above, the aluminum hydroxide powder was changed to magnesium hydroxide powder. A hydroxide powder, a magnesium hydroxide powder, and a lithium hydroxide powder were prepared so that the molar ratio of Li:Mg:Mn=1.1:0.05:1.85. Other steps were performed in the same manner as in Examples 4 to 11 to obtain a cathode active material of Example 15.
(Example 16)
A positive electrode active material was obtained in the same manner as in the manufacturing method of the positive electrode active material of Example 1 described above. LiCoPO ₄ powder was mixed with this positive electrode active material. At this time, the LiCoPO ₄ powder was adjusted to 2 mol % with respect to the entire mixture. The mixture was heat-treated in the air at 400° C. to 800° C. for 2 hours to 20 hours to obtain a positive electrode active material of Example 16.

<Calculation of Average Particle Size of First Particles and Second Particles>
In each positive electrode active material of each example produced above, the _median diameter ( _D50 ) was measured with a laser diffraction particle size distribution analyzer. SEM images of such particles were then 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 particles of each positive electrode active material prepared above before mixing the LiCoPO ₄ powder or LiNiPO ₄ powder, and the content of each element was measured.

<Measurement of Transition Metal Me (Me: Ni or Co) Containing Thickness in Second Particle Surface Layer>
Each of the positive electrode active materials prepared above was embedded in a resin, and after preparing a rough cross section, the cross section was processed by a cross section polisher (CP) method. A plurality of such cross sections were observed with an SEM at a magnification of 1000 times, and 20 second particles having a size corresponding to D50 were randomly selected. Selected 20 second particles were photographed at 3000x or 5000x magnification to obtain cross-sectional SEM images. Then, the intensity distribution of Ni or Co was measured for each of the selected second particles by planar analysis using energy dispersive X-ray spectroscopy (EDX). Then, the normal line that maximizes the existence distance of Ni or Co was selected. The point at which the Ni or Co intensity is 30% of the maximum value of the Ni or Co intensity measured by EDX along this normal line was determined. Then, the length from the surface of the secondary particles to the point was calculated. The average value of the lengths thus obtained for each of the selected second particles is shown in Table 1 as "Me unevenly distributed surface layer portion (μm)".

<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 uniformly applied to an aluminum foil current collector and dried by heating to prepare a coated sheet. Then, the coated sheet was roll-pressed to increase the density, thereby producing 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 = 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 uniformly applied to a copper foil current collector and dried by heating to prepare a coated sheet. Then, a negative electrode sheet was produced by roll-pressing the coated sheet. A porous polyolefin sheet having a three-layer structure of PP/PE/PP was prepared as a separator.
The positive electrode sheet and the negative electrode sheet thus prepared were laminated so as to face each other with a separator interposed therebetween to prepare 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.

<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 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. A resistance value (initial resistance) was calculated from the slope of the linear interpolation of the applied current and voltage change. Then, the relative value of the initial resistance of each example was calculated when the initial resistance of Example 1 was set to 1.0. Table 1 shows the results.

<Measurement of resistance increase rate after high temperature endurance and capacity retention rate after high temperature 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 resistance after 50 cycles were measured by the same method as above. Then, the capacity retention rate after high temperature 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 high temperature endurance is expressed by the following equation 2:
(Resistance after 50 cycles)/(Initial resistance)×100 Expression 2
Calculated by In each of the calculated capacity retention rate after high temperature endurance and resistance increase rate after high temperature endurance, the relative value of each example was calculated when Example 1 was taken as 1.0. Table 1 shows the results. A higher value of the capacity retention rate after high temperature endurance indicates better cycle characteristics, and a lower value of the resistance increase rate after high temperature endurance indicates better cycle characteristics.

As shown in Table 1, Examples 5 to 8 and Examples 12 to 16 were superior to Examples 1 and 3 in terms of cycle characteristics, and their initial resistance was reduced. In Examples 1, 3, 5 to 8, and 12 to 16, the average particle size of the first particles is 5 μm or more and 10 μm or less, and the average particle size of the second particles is 12 μm or more and 22 μm or less. However, in Examples 5 to 8 and Examples 12 to 16, the proportion of the third particles (here, LiMeO ₄ particles) is 0.3 mol% or more and 5 mol% or less, and the Me thickness is 3 μm. It has the following. That is, it can be seen that the presence of the third grains in such a proportion and the uneven distribution of Me in the surface layer portion of the second grains in such a thickness can improve the cycle characteristics and reduce the initial resistance. Among them, Examples 5 to 7 and Examples 12 to 16 exhibited better cycle characteristics. From this, it can be seen that when the abundance ratio of the third particles (LiMePO ₄ ) is 0.3 mol % or more and 3 mol % or less and the Me unevenly distributed surface layer portion is 2 μm or less, even better cycle characteristics are 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 Positive electrode active material particles 12 First particles 14 Second particles 16 Third 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 Body 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 (6)

A positive electrode active material used in a lithium ion 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 12 μm or more and 22 μm or less,
The first particles comprise a spinel crystal structure lithium-manganese composite oxide containing at least lithium atoms and manganese atoms,
Third particles composed of a phosphorus-containing lithium-transition metal composite oxide having transition metal atoms of at least one of nickel atoms and cobalt atoms are present on the surfaces of the second particles,
The ratio of the phosphorus-containing lithium-transition metal composite oxide constituting the third particles to the entire compound constituting the positive electrode active material is 0.3 mol% or more and 5 mol% or less,
The transition metal atoms are unevenly distributed in a surface layer portion of 3 μm or less from the surface of the second particle,
Positive electrode active material. 2. The positive electrode active material according to claim 1, wherein the ratio of said phosphorus-containing lithium-transition metal composite oxide constituting said third particles to the whole compound constituting said positive electrode active material is 0.3 mol % or more and 3 mol % or less. material. 3 . The positive electrode active material according to claim 1 , wherein the surface layer portion of the second particles where the transition metal atoms are unevenly distributed is within a range of 2 μm or less from the surface of the second particles. The positive electrode active material according to any one of claims 1 to 3, wherein the lithium-manganese composite oxide comprises at least one of aluminum atoms and magnesium atoms. 5. The positive electrode active material according to claim 1, containing LiMePO ₄ (Me includes at least one of Ni and Co) as the phosphorus-containing lithium transition metal composite oxide. A lithium ion 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 5,
Lithium-ion secondary battery.

Images