JP6206227B2 - Positive electrode active material and non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a positive electrode active material used as a positive electrode material in a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery, and a non-aqueous electrolyte secondary battery using the positive electrode active material, specifically, as a positive electrode active material The present invention relates to a positive electrode active material composed of a mixture of a lithium cobalt composite oxide and a lithium transition metal composite oxide and a non-aqueous electrolyte secondary battery using the positive electrode active material.

  In recent years, with the widespread use of portable devices such as mobile phones and notebook computers, development of small and lightweight secondary batteries with high energy density is strongly desired. As such a battery, there is a lithium ion secondary battery using lithium, a lithium alloy, a metal oxide, or carbon as a negative electrode, and research and development are actively performed.

  A lithium ion secondary battery using a lithium composite oxide, particularly a lithium cobalt composite oxide that is relatively easy to synthesize as a positive electrode material, is expected as a battery having a high energy density because a high voltage of 4V class is obtained. Practical use is progressing. The lithium cobalt composite oxide is characterized by high filling properties compared to other positive electrode active materials such as lithium nickel composite oxide and lithium nickel cobalt manganese composite oxide.

  In general, to improve the filling property of the positive electrode active material, it is necessary to improve the sphericity of the particles serving as the positive electrode active material, to improve the density of the particles themselves, to give a proper width to the particle size distribution, It is effective to increase the diameter within an appropriate range. Moreover, the filling property of the positive electrode active material tends to reflect the filling property of hydroxides such as cobalt hydroxide and oxides such as cobalt oxide used as the precursor of the positive electrode active material.

Regarding the powder characteristics of the hydroxides and oxides described above, for example, in Patent Document 1, the particle shape of cobalt oxide is almost spherical, the 50% particle size (D50) is 1.5 to 15 μm, and D90 is D50. The cobalt oxide powder whose D10 is 1/5 or more of D50 and whose specific surface area is 2-15 m < ² > / g is disclosed.

Patent Document 2 discloses cobalt oxyhydroxide particles having a tapping density of 2.3 g / cm ³ or more, a substantially spherical shape, and an average particle size of 5 μm to 15 μm.

In addition, US Pat. No. 6,057,059 has a density of about 0.5 to 2.2 g / cm ³ , a particle size of greater than about 1 μm, typically about 1 to 20 μm, and about 0.5 to 20 m ² / g A hydroxide of cobalt hydroxide having a specific surface area or an alloy formed from cobalt and other metals is disclosed.

  However, the hydroxides and oxides described in Patent Documents 1 to 3 cannot be said to have sufficient comprehensive examination of the properties of the particles, and the resulting lithium cobalt composite oxide has sufficient filling properties. It's hard to say.

  On the other hand, Patent Document 4 discloses hexagonal columnar cobalt hydroxide particles having an average particle diameter of 1 to 30 μm at the bottom and an average particle height of 0.2 to 10 μm. A shape with a low ratio is disadvantageous for improving the filling property.

  Furthermore, in Patent Document 5, an aqueous solution of cobalt salt and a caustic solution are continuously supplied and stirred in the same tank, and the pH value in the tank is kept constant with the supply salt concentration, the supply salt flow rate, and the temperature in the tank being constant. Discloses a method for producing a cobalt hydroxide to obtain a cobalt hydroxide by controlling the value in the range of 11.0 to 13.5. In such a continuous method, it is considered that the particle size distribution is wide and effective in improving the filling property, but improvement of cycle characteristics is an issue, and it is difficult to say that the study on the filling property is sufficient.

Patent Document 6 discloses an active material A represented by a general formula Li _x M ¹ O ₂ and a different element solid solution active material B represented by a general formula Li _x M ² _y M ³ _1-y O _2. A positive electrode active material having a mixed composition is disclosed. With this positive electrode active material, it is said that the tap density and the electron conduction network can be improved to provide a non-aqueous electrolyte secondary battery having high capacity and high rate characteristics. However, only mixing of two kinds of active materials having different particle diameters has been studied, and examination of characteristics of the mixed powder is not sufficient, and it cannot be said that the filling property is satisfied.

  As described above, in order to improve the filling properties of the lithium cobalt composite oxide, it is necessary to examine the density and particle size distribution of the particles themselves, including the cobalt hydroxide particles that are precursors of the lithium cobalt composite oxide. However, the lithium cobalt composite oxide described in Patent Documents 1 to 6 or a precursor thereof cannot obtain sufficient filling properties. Therefore, the further improvement of the filling property of lithium cobalt complex oxide is desired.

JP 2001-354428 A JP 2007-001809 A Special table 2003-503300 gazette JP 11-292549 A JP 09-022692 A JP 2006-156004 A

  Therefore, in view of such problems, the present invention has a positive electrode active material for a non-aqueous electrolyte secondary battery that has high filling properties and excellent battery characteristics, and a non-aqueous electrolyte secondary material using the positive electrode active material. A secondary battery is provided.

  As a result of diligent research regarding the filling property of the positive electrode active material for the non-aqueous electrolyte secondary battery, the present inventor mixed active material particles having a narrow particle size distribution and active material particles having a smaller particle diameter than the active material particles. Thus, the present invention has been completed with the knowledge that a positive electrode active material having high filling properties and excellent battery characteristics can be obtained.

The positive electrode active material according to the present invention that achieves the above-described object is a positive electrode active material for a non-aqueous electrolyte secondary battery comprising a mixture of active material particles A and active material particles B, and the active material particles A are: It consists of spherical secondary particles in which primary particles are aggregated, and has an average particle size of 10 to 25 μm, and shows variation in particle size distribution (d90-d10) / MV (where MV is measured using a laser diffraction particle size distribution meter) , A lithium cobalt composite oxide particle having a volume average particle diameter (which can be measured by a laser diffraction scattering method) of 0.65 or less, and the cross-sectional major axis of the secondary particles is 3 μm or more in cross-sectional observation of the particles. The ratio (N1 / L) of the number of voids (N1) having a maximum major axis of 0.3 μm or more confirmed in the particles to the sectional major axis (L) of the secondary particles is 0.5 or less, and the maximum major axis is 0.5 μm. Number of voids above (N2) Ratio cross sectional length (L) of the secondary particles (N2 / L) is 0.2 or less, and the maximum diameter of the voids is not more than 25% of the cross sectional length of the secondary particles, the active material particles B Is a general formula Li _x M ¹ _1-y M ² _y O ₂ (where M ¹ is at least one element selected from Co, Ni and Mn, M ² is a transition metal element other than M1, a group 2 element, And at least one element selected from Group 13 elements, 0.95 ≦ x ≦ 1.1, 0 ≦ y ≦ 0.15), and a hexagonal crystal structure having a layered structure, and an average particle diameter Is smaller than the active material particle A, and is characterized by spherical lithium transition metal composite oxide particles.

  The nonaqueous electrolyte secondary battery according to the present invention that achieves the above-described object is characterized in that the positive electrode is formed of a positive electrode active material.

  The present invention can provide a positive electrode active material having a high filling property. In the present invention, the positive electrode active material filled per volume of the battery can be increased, the capacity can be increased, a non-aqueous electrolyte secondary battery having excellent battery characteristics can be obtained, and the industrial value is It can be said that it is extremely expensive.

2 is a cross-sectional SEM image of active material particles A produced in Example 1. FIG.

Hereinafter, a positive electrode active material for a non-aqueous electrolyte secondary battery to which the present invention is applied and a non-aqueous electrolyte secondary battery will be described in detail. Note that the present invention is not limited to the following detailed description unless otherwise specified. The embodiment according to the present invention will be described in the following order.
1. 1. Positive electrode active material Non-aqueous electrolyte secondary battery

<1. Cathode active material>
The cobalt hydroxide particles according to the embodiment of the present invention are precursors of a positive electrode active material of a non-aqueous electrolyte secondary battery, and are particularly precursors of a positive electrode active material of a lithium ion secondary battery.

  The positive electrode active material is composed of a mixture of active material particles A and active material particles B, and is suitable for a positive electrode active material of a non-aqueous electrolyte secondary battery.

  The active material particle A is composed of spherical secondary particles in which primary particles are aggregated, has an average particle size of 10 to 25 μm, and shows a variation in particle size distribution (d90−d10) / MV is 0.65 or less. Some lithium cobalt composite oxide particles.

The active material particle B has a general formula Li _x M ¹ _1-y M ² _y O ₂ (where M ¹ is at least one element selected from Co, Ni, and Mn, and M ² is a transition metal element other than M ^1. A hexagonal crystal structure having a layered structure represented by at least one element selected from group 2 elements and group 13 elements, 0.95 ≦ x ≦ 1.1, 0 ≦ y ≦ 0.15) In addition, the average particle size is smaller than that of the positive electrode active material A, and is a spherical lithium transition metal composite oxide particle.

  The positive electrode active material is prepared by mixing the active material particles A and the active material particles B having a particle size smaller than that of the active material particles A, thereby arranging the active material particles B in the gaps between the active material particles A. It is possible to improve the filling property of the battery and to realize a high-density positive electrode when used for the positive electrode of a battery. Furthermore, since the active material particles A have a narrow particle size distribution, a positive electrode active material having a higher packing property than that obtained by simply mixing active material particles having different particle sizes can be obtained.

[Active material particles A]
(Shape / average particle size)
The active material particles A are composed of spherical secondary particles in which primary particles are aggregated, and have an average particle size of 10 to 25 μm, preferably 15 to 25 μm. Here, the average particle diameter means MV (volume average particle diameter).

  By setting the average particle size to 10 to 25 μm, the filling property of the active material particles A can be made high, and by mixing with the active material particles B, a positive electrode active material having high filling properties can be obtained. it can. When the average particle size is less than 10 μm, the filling properties of the active material particles A themselves are lowered, so that even when mixed with the active material particles B having a small particle size, a positive electrode active material having high filling properties is not obtained. On the other hand, when the average particle size exceeds 25 μm, the particle size distribution of the active material particles A is widened, resulting in a problem that battery characteristics such as cycle characteristics and output characteristics are deteriorated.

  Moreover, since the active material particle A is spherical, it has a high filling property. Here, the spherical shape includes an elliptical shape and a spherical shape having irregularities on the particle surface.

(Average aspect ratio)
The active material particles A preferably have an average aspect ratio of 0.7 or more. By setting the average aspect ratio to 0.7 or more, particles having high sphericity and excellent filling properties are obtained. When the aspect ratio is less than 0.7, the spherical shape of the secondary particles is inferior, voids between the particles of the active material particles A are increased, and the filling property of the positive electrode active material may be lowered.

  Here, the average aspect ratio is an average value of aspect ratios obtained for individual particles. The aspect ratio of each individual particle is measured by measuring the distance between the point on the outer edge of the secondary particle and the point on the other outer edge that is the maximum length on the image of the secondary particle in the observation of the particle appearance with a scanning electron microscope. As described above, it can be obtained by measuring the ratio of the minimum measured particle size to the maximum measured particle size in the secondary particles. The average aspect ratio can be obtained by averaging the number of aspect ratios obtained for any 20 or more secondary particles from appearance observation of a scanning electron microscope.

(Variation of particle size distribution)
The active material particle A has a value of (d90−d10) / MV, which is an index indicating variation in the particle size distribution of the particles, of 0.65 or less, preferably 0.6 or less.

  When the value of (d90-d10) / MV is 0.65 or less, the spread of the particle size distribution of the active material particles A is suppressed, high particle size uniformity is obtained, and mixing of fine particles and coarse particles is suppressed. Therefore, a positive electrode active material having high filling properties can be obtained by controlling the particle size distribution when mixed with the active material particles B. In addition, deterioration of battery characteristics such as cycle characteristics and charge / discharge characteristics due to mixing of fine particles and coarse particles can be suppressed.

  On the other hand, when the value of (d90−d10) / MV exceeds 0.6, the mixture of fine particles and coarse particles increases, so that not only a positive electrode active material having high filling properties but also non-existence due to an increase in coarse particles. This causes a problem of short circuit in the aqueous electrolyte secondary battery.

  Here, d90 and d10 are particle sizes at which the volume cumulative distribution becomes 90% and 10%, respectively. The smaller the (d90-d10) / MV, the higher the uniformity of the particle size, but considering the manufacturing restrictions and the like, the lower limit of (d90-d10) / MV is usually about 0.3. In addition, d90, d10, and MV (volume average particle diameter) can be measured by a laser diffraction scattering method using a laser diffraction particle size distribution meter.

(N1 / L, N2 / L, maximum long diameter of void)
Furthermore, the active material particle A is a cross-section of secondary particles having the number of voids (N1) having a maximum major axis of 0.3 μm or more that is confirmed in a particle having a sectional major axis of 3 μm or more in the cross-sectional observation of the particles. The ratio (N1 / L) to the long diameter (L) is preferably 0.5 or less. Further, the ratio (N2 / L) of the number of voids (N2) having a maximum major axis of 0.5 μm or more to be confirmed in a particle having a sectional major axis of secondary particles of 3 μm or more to the sectional major axis (L) of the secondary particles. It is preferable that it is 0.2 or less. Furthermore, in addition to these conditions, it is preferable that the maximum major axis of the voids is 25% or less of the cross-sectional major axis of the secondary particles.

  As a result, the active material particles A become highly dense lithium cobalt composite oxide particles. When any one of N1 / L, N2 / L, and the maximum major axis of the void exceeds the above range, the void ratio in the particles becomes high, and the density of the active material particles A decreases.

  It should be noted that voids having a maximum major axis of less than 0.3 μm are excluded because it is difficult to accurately determine whether they are voids and the influence on the denseness of the particles is small. Further, particles having a cross-sectional major axis of less than 3 μm are excluded as particles for measuring the number or size of voids. This is because a particle whose cross-sectional major axis is less than 3 μm is a cross-section near the surface of the particle because the cross-sectional observation is a cross-section at an arbitrary position of the particle. This is because the dents on the surface are observed as voids and the voids inside the particles may not be accurately evaluated.

  The cross-sectional major axis and the maximum major axis of the void are the distance between the secondary particle to be measured on a scanning electron microscope or the point on the other outer edge that is the maximum length from the point of the outer edge of the void. It means the maximum gap length.

  Furthermore, it is preferable that the maximum major axis of the gap is 2 μm or less. The maximum major axis of the voids of the active material particles A is 25% or less of the cross-sectional major axis of the secondary particles, but the maximum major axis of the voids allowed for the secondary particles having a large particle size is relatively large. Therefore, by setting the maximum major axis of the voids to 2 μm or less, it is possible to make the denseness higher even for particles having a large particle size.

(Tap density)
The active material particles A preferably have a tap density of 2 to 3 g / ml. Thereby, the filling property of the positive electrode active material obtained by mixing the active material particles A and the active material particles B can be further enhanced. The higher the tap density of the active material particles A, the higher the filling property of the positive electrode active material mixed with the active material particles B. However, when the tap density exceeds 3 g / ml, fine particles and coarse particles mixed in the active material particles A This may increase the problem that the battery characteristics deteriorate.

(composition)
The active material particle A is composed of a lithium cobalt composite oxide as described above, and its composition can be expressed as LiCoO ₂ as a general formula, and is an element that is usually added to improve the characteristics of the nonaqueous electrolyte secondary battery May be included.

[Active material particles B]
(composition)
The active material particle B has a general formula LixM ¹ _1-y M ² _y O ₂ (where M ¹ is at least one element selected from Co, Ni, and Mn, M ² is a transition metal element other than M ¹ , 2 A group element and at least one element selected from group 13 elements, 0.95 ≦ x ≦ 1.1, 0 ≦ y ≦ 0.15) and a hexagonal crystal structure having a layered structure, It is a spherical lithium transition metal composite oxide having an average particle size smaller than that of the active material particle A.

  Since the lithium transition metal composite oxide having a hexagonal crystal structure with a layered structure in the composition represented by the general formula has good battery characteristics, when mixed with the active material particles A, the filling property may be increased. In addition, a positive electrode active material having good battery characteristics can be obtained.

M ² in the general formula improves battery characteristics, and is preferably at least one element selected from Al, Mg, Ca, Ti, Fe, V, Cr, Zr, Nb, Mo, and W. .

(Shape / average particle size)
The active material particles B have an average particle size smaller than that of the positive electrode active material A and a spherical particle shape. By making the average particle size of the active material particles B smaller than the average particle size of the active material particles A, it is possible to arrange the active material particles B between the particles of the active material particles A, and the filling property of the positive electrode active material Can be high. Further, by making the particle shape spherical, it becomes easy to arrange the active material particles B between the particles of the active material particles A, and the interval between the active material particles A can be reduced. Fillability can be increased.

  In order to further increase the filling property of the positive electrode active material, the average particle diameter of the active material particles B is preferably 3 to 10 μm, and more preferably 3 to 8 μm.

  When the average particle diameter of the active material particles B is less than 3 μm, the current at the time of charge / discharge concentrates on the active material particles B, so that the battery characteristics may be deteriorated. On the other hand, when the average particle size exceeds 10 μm, the active material particles B arranged between the particles of the active material particles A may decrease, and the filling property of the positive electrode active material may not be higher.

  Furthermore, the average particle diameter of the active material particles B is preferably 0.5 or less, more preferably 0.4 or less, as a ratio to the average particle diameter of the active material particles A. Thereby, it becomes possible to narrow the particle interval of the active material particles A and to arrange many active material particles B between the particles of the active material particles A, and it is possible to further improve the filling property of the positive electrode active material.

(Variation of particle size distribution)
The value of (d90-d10) / MV, which is an index indicating the variation in the particle size distribution of the active material particles B, is preferably 0.65 or less, and more preferably 0.60 or less.

  When the value of (d90-d10) / MV is 0.65 or less, mixing of fine particles and coarse particles can be suppressed, and the deterioration of the battery characteristics of the positive electrode active material is suppressed, and the filling property is further increased. Can be improved. When the value of (d90-d10) / MV exceeds 0.60, there may be a problem that battery characteristics are deteriorated due to mixing of fine particles and sufficient filling property cannot be obtained due to mixing of coarse particles.

[Mixing of active material particles A and active material particles B]
The mass mixing ratio (A: B) between the active material particles A and the active material particles B is preferably 70:30 to 90:10, and more preferably 75:25 to 90:10. Thereby, the amount of the active material particles B disposed between the active material particles A can be made sufficient, and the filling property of the positive electrode active material can be further improved.

  The positive electrode active material is obtained by mixing the particles so that the particles are not unevenly distributed while maintaining the properties of the active material particles A and the active material particles B as powder. For mixing, a general mixer can be used. For example, a shaker mixer, a Laedige mixer, a Julia mixer, a V blender, etc. can be used, and the properties of the particles before mixing are not destroyed. It is sufficient that each particle is sufficiently mixed.

[Method for Producing Active Material Particle A]
The active material particles A are produced using cobalt hydroxide particles as a precursor. While controlling the inside of the reaction vessel to a non-acidic atmosphere, the cobalt hydroxide particles are supplied with a chlorine-containing cobalt salt aqueous solution, an inorganic alkali aqueous solution and an ammonium ion-containing aqueous solution to the reaction vessel to obtain a reaction solution. A nucleation step for generating nuclei is performed by controlling the pH value at the reference to be 10.5 to 12.0. Next, the aqueous solution for particle growth containing nuclei formed in the reaction solution in the nucleation step has a pH value of 9.5 to 10.5 based on the liquid temperature of 25 ° C. and lower than the pH value in the nucleation step. A particle growth step is performed in which particle growth is performed under control. Thereby, cobalt hydroxide particles can be obtained.

  The chlorine-containing cobalt salt aqueous solution preferably has a chlorine content of 1 to 3 in terms of molar ratio with respect to the cobalt content, and more preferably 1.5 to 3. By setting the chlorine content to 1 to 3 in terms of molar ratio, the denseness of the cobalt hydroxide particles can be further improved.

  The concentration of the chlorine-containing cobalt salt aqueous solution is preferably 1 mol / L to 2.6 mol / L as cobalt, and more preferably 1.5 mol / L to 2.2 mol / L.

  As an aqueous solution containing ammonium ions, the ammonia concentration of the reaction solution is preferably adjusted to 5 g / L to 20 g / L, more preferably 7.5 g / L to 15 g / L. By adjusting the ammonia concentration to 5 g / L to 20 g / L, cobalt hydroxide particles having higher packing properties can be obtained. When the ammonia concentration is less than 5 g / L, the shape of the primary particles tends to be plate-like, and the filling property may be lowered. On the other hand, when the ammonia concentration exceeds 20 g / L, control of particle shape and structure such as the above-described particle shape, average aspect ratio, average particle size, index indicating the spread of particle size distribution, voids in the particle, tap density, etc. Is not effective, and there is a problem that the cost of chemicals and wastewater treatment increases and the cost increases.

  The inorganic alkaline aqueous solution is not particularly limited as long as the pH value of the reaction solution can be controlled to a predetermined value. For example, an aqueous alkali metal hydroxide solution such as sodium hydroxide or potassium hydroxide is appropriately used. Can do.

  The non-oxidizing atmosphere in the reaction vessel is preferably an inert gas mixed atmosphere having an oxygen concentration of 5% by volume or less, and more preferably an inert gas mixed atmosphere having an oxygen concentration of 2% by volume or less.

  The active material particles A can be produced in the same manner as in the ordinary method for producing a lithium cobalt composite oxide using the cobalt hydroxide particles obtained as described above as a precursor. That is, cobalt hydroxide particles and a lithium compound are mixed, adjusted to an oxidizing atmosphere, preferably an air atmosphere, and calcined at 800 ° C. to 1100 ° C., preferably 850 ° C. to 1000 ° C. Active material particles A composed of product particles can be obtained.

  The cobalt hydroxide particles may be heated to a temperature of 300 ° C. to 900 ° C. to remove moisture contained in the cobalt hydroxide particles, thereby forming cobalt oxide particles. Moreover, when aggregation is recognized after baking, you may crush.

[Method for Producing Active Material Particle B]
The active material particles B can be obtained using a known technique. However, when obtaining particles having a uniform particle size distribution, for example, the production method disclosed in International Publication No. 2011/067937 is used. Can be obtained.

  In the positive electrode active material as described above, active material particles A having a high filling property made of lithium cobalt composite oxide and active material particles B having a smaller particle diameter than active material particles A made of lithium transition metal composite oxide are further included. By being mixed, the active material particles B enter between the particles of the active material particles A, and the filling property is high. Therefore, this positive electrode active material can increase the packing density of the positive electrode in the non-aqueous electrolyte secondary battery, and can obtain high Coulomb efficiency. As a result, a high-capacity nonaqueous electrolyte secondary battery having a positive electrode with a high packing density can be obtained, and it is extremely useful as a positive electrode material for a nonaqueous electrolyte secondary battery.

<2. Non-aqueous electrolyte secondary battery>
The positive electrode active material described above is preferably used as a positive electrode active material for a non-aqueous electrolyte secondary battery. Hereinafter, the embodiment at the time of using for nonaqueous electrolyte secondary batteries is illustrated.

  A non-aqueous electrolyte secondary battery employs a positive electrode using the above-described positive electrode active material. Since the nonaqueous electrolyte secondary battery has substantially the same structure as a general nonaqueous electrolyte secondary battery except that the positive electrode active material described above is used as the positive electrode material, it will be briefly described.

  The nonaqueous electrolyte secondary battery has a structure including a case, and a positive electrode, a negative electrode, a nonaqueous electrolyte solution, and a separator housed in the case.

  The positive electrode is a sheet-like member, and can be formed by applying a positive electrode mixture paste containing a positive electrode active material to the surface of a current collector made of aluminum foil, for example, and drying it.

  The positive electrode mixture paste is formed by adding a solvent to the positive electrode mixture and kneading. The positive electrode mixture is formed by mixing the above-described positive electrode active material, a conductive material, and a binder.

  The conductive material is not particularly limited. For example, graphite such as natural graphite, artificial graphite, and expanded graphite, and carbon black materials such as acetylene black and ketjen black can be used.

  The binder used for the positive electrode mixture is not particularly limited. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorine rubber, ethylene propylene diene rubber, styrene butadiene, cellulose resin, poly Acrylic acid or the like can be used. In addition, activated carbon etc. may be added to a positive electrode compound material, and the electric double layer capacity | capacitance of a positive electrode can be increased by adding activated carbon etc.

  The solvent is not particularly limited, and for example, an organic solvent such as N-methyl-2-pyrrolidone can be used.

  The negative electrode is a sheet-like member formed by applying a negative electrode mixture paste to the surface of a metal foil current collector such as copper and drying it.

  As the negative electrode active material, for example, a material containing lithium, such as metallic lithium or a lithium alloy, or an occlusion material that can occlude and desorb lithium ions can be employed.

  The occlusion material is not particularly limited, and for example, natural graphite, artificial graphite, an organic compound fired body such as phenol resin, and a carbon material powder such as coke can be used.

  As the separator, for example, a thin film such as polyethylene or polypropylene and a film having many fine holes can be used. In addition, if it has a function of a separator, it will not specifically limit.

The nonaqueous electrolytic solution is obtained by dissolving a lithium salt as a supporting salt in an organic solvent. Examples of the organic solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, and dipropyl carbonate; tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxy Ether compounds such as ethane; sulfur compounds such as ethyl methyl sulfone and butane sultone; phosphorus compounds such as triethyl phosphate and trioctyl phosphate can be used alone or in admixture of two or more. . As the supporting salt, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , and complex salts thereof can be used.

  Since the non-aqueous electrolyte secondary battery having the above-described configuration has a positive electrode using the above-described positive electrode active material, the packing density of the positive electrode active material is high and a high electrode density can be obtained. Thereby, in a non-aqueous electrolyte secondary battery, a high initial discharge capacity and coulomb efficiency are obtained, and the capacity becomes high. Moreover, the nonaqueous electrolyte secondary battery has a high volume energy density. Furthermore, compared with the conventional positive electrode active material, since there is no mixing of fine particles, the cycle characteristics are excellent. Further, it has high thermal stability and is excellent in safety.

  Hereinafter, the present invention will be described in more detail by way of examples and comparative examples of the present invention, but the present invention is not limited to these examples. Various evaluation methods in each example and each comparative example are as follows.

(1) Volume average particle size and particle size distribution measurement It measured using the laser diffraction type particle size distribution meter (brand name Microtrack, Nikkiso Co., Ltd. product).

(2) Appearance and average aspect ratio of particles A value obtained by observing the appearance of particles with a scanning electron microscope (SEM, trade name S-4700, manufactured by Hitachi High-Technologies Corporation) and measuring 20 arbitrarily selected particles. From this, the average value was calculated.

(3) Denseness The cross section of the particles is observed at 1000 times using a scanning electron microscope (SEM, trade name S-4700, manufactured by Hitachi High-Technologies Corporation), and the particles whose entire cross section is observable are selected. Evaluation was made by measuring the major axis and the maximum major axis of the void.

(4) Analysis of metal component After the sample was dissolved, it was analyzed by ICP emission spectroscopy (ICP: Inductively Coupled Plasma).

(5) Identification of crystal structure It identified using the X-ray-diffraction pattern obtained by the X-ray-diffraction measuring apparatus (the product made by Panalical, X'Pert PRO).

(6) Battery evaluation A 2032 type coin battery was constructed using the positive electrode active material thus obtained, and the initial capacity was evaluated. Specifically, 20% by mass of acetylene black and 10% by mass of PTFE were added to and mixed with 70% by mass of the positive electrode active material powder, and 150 mg was weighed to produce pellets, which were used as the positive electrode. Moreover, lithium metal is used for the negative electrode, and an equivalent mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) (made by Toyama Pharmaceutical Co., Ltd.) using 1M LiClO ₄ as a supporting salt is used for the electrolyte. A 2032 type coin battery was produced in a glove box in an Ar atmosphere in which the dew point was controlled at −80 ° C.

The prepared coin battery is left for about 24 hours, and after the open circuit voltage (OCV) is stabilized, the current density with respect to the positive electrode is set to 0.5 mA / cm ² and charged to a cutoff voltage of 4.4 V to obtain an initial charge capacity. The capacity when discharging to a cut-off voltage of 3.0 V after 1 hour of rest was defined as the initial discharge capacity.

Example 1
First, the active material particles A were produced as follows.

  Into a crystallization reaction tank of 5 L with 4 baffle plates, 1.4 L of pure water and 90 ml of 25 mass% ammonia water were added and heated to 40 ° C. in a thermostatic chamber and a heating jacket, and 25 mass % Aqueous sodium hydroxide solution was added to adjust the pH of the reaction solution in the reaction vessel to 11.5 based on 25 ° C. Nitrogen gas was supplied into the reaction tank at 3 L / min, and the oxygen concentration in the reaction tank was controlled to 1% by volume or less.

  In the crystallization reaction of the nucleation step, a cobalt chloride aqueous solution having a molar concentration of cobalt of 1.2 mol / L is supplied at a rate of 5 ml / min using a metering pump while stirring the reaction solution, and 25 mass% ammonia water is also added. Was supplied at a rate of 0.8 ml / min, and a 25% by mass aqueous sodium hydroxide solution was intermittently added, and the pH at 25 ° C. was controlled to be 11.5. After the prescribed amount of reaction was completed, an appropriate amount of 35 mass% hydrochloric acid was added, and the pH of the reaction solution was adjusted to 10.0 based on 25 ° C.

  The crystallization reaction in the particle growth process is performed by adding a cobalt chloride aqueous solution, ammonia water, and sodium hydroxide aqueous solution in the same manner as in the nucleation process using a metering pump while stirring the reaction solution (particle growth aqueous solution). The pH of the liquid was controlled to be 10.0 at 25 ° C. The chlorine content in the cobalt chloride aqueous solution was 2.1 in terms of molar ratio with respect to cobalt. The ammonia concentration in the liquid was 15 g / L. The particle growth step was performed for 8 hours, and the resulting slurry containing cobalt hydroxide particles was solid-liquid separated, washed with water, and dried to obtain powdered cobalt hydroxide.

  The obtained cobalt hydroxide particles were mixed with lithium carbonate using a shaker mixer apparatus (Willy et Bacofen (WAB)) so that the molar ratio of lithium to cobalt (Li / Co) was 1.00. Firing was performed at 990 ° C. for 10 hours in an air stream. After cooling, it was crushed to obtain active material particles A.

The crystal structure of the obtained active material particles A was confirmed to be a layered compound represented by LiCoO ₂ . Moreover, the average particle diameter, (d90-d10) / MV, and tap density were measured.

  Moreover, when the density of the secondary particles was evaluated from the cross-sectional SEM image shown in FIG. 1, when the cross-section of the secondary particles was observed, the maximum long-diameter confirmed in the particles having a cross-sectional major axis of 3 μm or more was 0. Secondary particles having a ratio (N1 / L) of the number of voids (N1) of 3 μm or more to the cross-sectional major axis (L) of the secondary particles of 0.5 or less and the number of voids (N) having a maximum major axis of 0.5 μm or more It was confirmed that the ratio (N2 / L) to the cross-sectional major axis (L) was 0.2 or less, and the maximum major axis of the voids was 25% or less of the sectional major axis of the secondary particles. The maximum long diameter of the void was 1.8 μm.

  Next, active material particles B were produced as follows.

  While putting 17 L of water in the reaction vessel (34 L) and stirring, the temperature in the vessel was set to 40 ° C., and nitrogen gas was passed through the reaction vessel to create a nitrogen gas atmosphere. By adding appropriate amounts of 25% aqueous sodium hydroxide and 25% aqueous ammonia to the water in the reaction tank, the pH at the liquid temperature of 25 ° C. was adjusted to 12.6. Further, the ammonium ion concentration in the reaction solution was adjusted to 15 g / L.

  Next, a mixed aqueous solution of 1.8 mol / L prepared by dissolving nickel sulfate and cobalt sulfate in water so that Ni: Co = 0.82: 0.15 was added to the reaction solution in the reaction tank at 88 ml / min. Added in. At the same time, 25 mass% aqueous ammonia and 25 mass% sodium hydroxide aqueous solution are also added to the reaction liquid in the reaction tank at a constant rate so that the ammonium ion concentration in the obtained aqueous solution for nucleation is 15 g / L. In this state, crystallization was performed for 2 minutes and 30 seconds while controlling the pH to 12.6 to perform nucleation.

  Thereafter, until the nucleation aqueous solution pH at a liquid temperature of 25 ° C. reached 11.6 (particle growth pH), only the supply of the 25 mass% sodium hydroxide aqueous solution was temporarily stopped to obtain an aqueous solution for particle growth.

  After the pH of the reaction solution reached 11.6 as the pH at 25 ° C. of the obtained particle growth aqueous solution, the supply of the 25 mass% sodium hydroxide aqueous solution was resumed, and the pH was adjusted to 11.6. In a controlled state, the particles were grown for 2 hours. When the reaction tank became full, the supply of the sodium hydroxide solution was stopped and the stirring was stopped and the mixture was allowed to stand to promote precipitation of the product. Thereafter, after removing half of the supernatant from the reaction vessel, the supply of the sodium hydroxide solution was resumed, and after crystallization for 2 hours (4 hours in total), the particle growth was terminated. And the particle | grains were collect | recovered by washing with water, filtering, and drying the obtained product.

  The obtained particles were transferred to another reaction vessel and mixed with water at room temperature to form a slurry. To this mixed aqueous solution, an aqueous solution of sodium aluminate and sulfuric acid were added with stirring to adjust the pH of the slurry to 9.5. . Thereafter, stirring was continued for 1 hour to coat the surface of nickel cobalt composite hydroxide particles with aluminum hydroxide. At this time, the aqueous solution of sodium aluminate was added such that the molar ratio of metal elements in the slurry was Ni: Co: Al = 0.82: 0.15: 0.03. After the stirring was stopped, the aqueous solution was filtered, and the particles coated with aluminum hydroxide were washed with water to obtain a composite hydroxide.

  The obtained composite hydroxide particles were heat-treated at 700 ° C. for 6 hours in an air stream (oxygen concentration: 21 vol%) to obtain composite oxide particles. Lithium hydroxide is weighed so that Li / Me = 1.02 (atomic ratio), and this lithium hydroxide and the obtained composite oxide particles are mixed using a shaker mixer device to obtain a mixture. It was.

The obtained mixture was calcined at 500 ° C. for 4 hours in an oxygen stream (oxygen concentration: 100% by volume), then calcined at 730 ° C. for 24 hours, cooled and crushed to obtain active material particles B. It was. It was confirmed that the crystal structure of the obtained active material particles B was a layered compound represented by Li _1.017 Ni _0.82 Co _0.15 Al _0.03 O ₂ . And the average particle diameter of the active material particle B, (d90-d10) / MV, was measured.

  The obtained active material particles A and active material particles B were weighed to a mass ratio of 80:20 and mixed using a shaker mixer device to obtain a positive electrode active material. The tap density and initial discharge capacity of the obtained positive electrode active material were measured.

(Example 2)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the active material particles A and the active material particles B were weighed and mixed so that the mass ratio was 85:15.

(Comparative Example 1)
The active material particles A were not mixed with the active material particles B and evaluated as the positive electrode active material in the same manner as in Example 1.

(Comparative Example 2)
In the nucleation process of the active material particles A, a positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the pH of the reaction solution was adjusted to 12.5 based on 25 ° C. When the denseness of the active material particles A was evaluated, the N1 / L observed in the observed particles was 0.8 at the maximum in the particles whose entire cross section was observed and the cross sectional major axis of the secondary particles was 3 μm or more. L was 0.5 at the maximum, and the maximum long diameter of the voids was 27% of the cross-sectional long diameter of the secondary particles. The maximum long diameter of the voids was 3.8 μm, and it was confirmed that there were many voids in the particles.

Claims (8)

A positive electrode active material for a non-aqueous electrolyte secondary battery comprising a mixture of active material particles A and active material particles B,
The active material particle A is composed of spherical secondary particles in which primary particles are aggregated, has an average particle size of 10 to 25 μm, and shows variation in particle size distribution (d90-d10) / MV (where MV is a laser diffraction type) using a particle size distribution meter, a lithium cobalt composite oxide particles value is 0.65 or less of the volume average particle diameter) which can be measured by a laser diffraction scattering method, the cross-sectional observation of the particles, the secondary particles The ratio (N1 / L) of the number of voids (N1) having a maximum major axis of 0.3 μm or more to be confirmed in particles having a sectional major axis of 3 μm or more to the sectional major axis (L) of the secondary particles is 0.5 or less, The ratio (N2 / L) of the number of voids (N2) having a maximum major axis of 0.5 μm or more to the sectional major axis (L) of the secondary particles is 0.2 or less, and the maximum major axis of the voids is the secondary 25% or less of the cross-sectional major axis of the particles,
The active material particle B has a general formula Li _x M ¹ _1-y M ² _y O ₂ (where M ¹ is at least one element selected from Co, Ni, and Mn, and M ² is a transition metal element other than M ^1. A hexagonal crystal structure having a layered structure represented by at least one element selected from group 2 elements and group 13 elements, 0.95 ≦ x ≦ 1.1, 0 ≦ y ≦ 0.15) And a positive electrode active material having a mean particle size smaller than that of the active material particles A and spherical lithium transition metal composite oxide particles. The average particle diameter of the active material particles B is 3 to 10 μm,
2. The positive electrode active material according to claim 1 , wherein a mass mixing ratio of the active material particles A and the active material particles B is 70:30 to 90:10. The positive electrode active material according to claim 1 or 2 , wherein a ratio of the average particle diameter of the active material particles B to the average particle diameter of the active material particles A is 0.5 or less. (D90-d10) / MV (where MV is a volume average particle diameter that can be measured by a laser diffraction scattering method using a laser diffraction particle size distribution meter ) indicating a variation in the particle size distribution of the active material particles B the positive electrode active material according to any one of claims 1 to 3 values, characterized in that it is 0.65 or less. The positive electrode active material according to any one of claims 1 to 4 average aspect ratio of the active material particles A is equal to or less than 0.7. The positive electrode active material according to any one of claims 1 to 5 tap density of the active material particles A is characterized in that it is a 2 to 3 g / ml. Wherein ^{M 2} are both Al, Mg, Ca, Ti, Fe, V, Cr, Zr, Nb, Mo, of claims 1 to 6, characterized in that at least one element selected from W 2. The positive electrode active material according to claim 1. A non-aqueous electrolyte secondary battery, wherein the positive electrode is formed of the positive electrode active material according to any one of claims 1 to 7 .