POSITIVE ELECTRODE ACTIVE SUBSTANCE FOR LITHIUM ION SECONDARY BATTERY AND LITHIUM ION SECONDARY BATTERY

Abstract

The purpose of the present invention is to provide positive electrode active substance particles for a lithium ion secondary battery, such particles being capable of producing a lithium ion secondary battery having excellent high-speed discharge properties. The present invention is a granulated body of a positive electrode active substance for a lithium ion secondary battery, wherein the primary particle average diameter is 10 to 80 nm and the number of primary particles having a diameter of 100 nm or greater is no more than 5.0%.

Claims

1. A positive electrode active material for a lithium ion secondary battery, the positive electrode active material being a granulated body of positive electrode active material particles for a lithium ion secondary battery, wherein an average particle diameter of primary particles is 10 nm or more and 80 nm or less, and a number ratio of particles having a particle diameter of 100 nm or more is 5.0% or less.

2. The lithium ion battery secondary battery positive electrode according to claim 1, wherein the lithium ion secondary battery positive electrode active material particles are olivine-based positive electrode active material particles.

3. The positive electrode active material for a lithium ion secondary battery according to claim 2, wherein the primary particles have a carbon cover layer on surfaces.

4. The positive electrode active material for a lithium ion secondary battery according to claim 3, wherein a ratio of carbon contained in the granulated body is 2.0 wt % or more and 5.0 wt % or less.

5. The positive electrode active material for a lithium ion secondary battery according to claim 2, wherein the lithium ion secondary battery positive electrode active material particles are lithium manganese iron phosphate particles represented by Li.sub.αMn.sub.aFe.sub.bPO.sub.4 (0.9≤α≤1.1, 0.6≤a≤1.0, 0<b≤0.4, and 0.9≤a+b≤1.1).

6. The positive electrode active material for a lithium ion secondary battery according to claim 5, wherein a ratio I.sub.20/I.sub.29 of a peak intensity at 20° to a peak intensity at 29° obtained by X-ray diffraction is 0.88 or more and 1.05 or less, and a ratio I.sub.35/I.sub.29 of a peak intensity at 35° to the peak intensity at 29° is 1.05 or more and 1.20 or less.

7. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein an average diameter of pores is 10 nm or more and 60 nm or less, a sum of pore volumes of pores having a pore diameter of 1 nm or more and 60 nm or less is 0.100 cm.sup.3/g or more and 0.300 cm.sup.3/g or less, and a maximum value of a log differential pore volume of pores having a pore diameter of 1 nm or more and 60 nm or less is 0.30 cm.sup.3/g or more.

8. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein a pore specific surface area of pores having a pore diameter of 1 nm or more and 60 nm or less is 25 m.sup.2/g or more and 50 m.sup.2/g or less.

9. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein a specific surface area is 30 m.sup.2/g or more and 45 m.sup.2/g or less.

10. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein a volume resistivity is 10.sup.5 Ω.Math.cm or less.

11. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein an average particle diameter of the granulated body is 1.0 μm or more and 20.0 μm or less.

12. A lithium ion secondary battery obtained by using the positive electrode active material for a lithium ion secondary battery according to claim 1.

Description

EXAMPLES

[0065] Hereinafter, the present invention will be described specifically by means of Examples; however, the present invention is not limited only to these Examples. First, the evaluation method in each Example will be described.

[0066] [Measurement A] Composition Ratio of LMFP

[0067] For the LMFP granulated body, 15 mg of the LMFP granulated body used in each of Examples and Comparative Examples was decomposed by heating using perchloric acid and nitric acid, and the volume was adjusted to 100 mL using ultrapure water. With respect to this solution, Li was measured by atomic absorption spectrometry, and Mn, Fe, and P were measured by ICP emission spectrometry, and the content of each element in a sample was determined and converted into an atomic ratio.

[0068] [Measurement B1] Average Particle Diameter of Primary Particles, Number Ratio of Primary Particles Having Particle Diameter of 100 nm or More, and Average Particle Diameter of Granulated Body

[0069] The average particle diameter of the LMFP primary particles was calculated by magnifying and observing the LMFP granulated body used in each of Examples and Comparative Examples at a magnification of 200,000 using a scanning electron microscope S-5500 (manufactured by Hitachi High-Technologies Corporation), measuring the particle diameters of 200 primary particles randomly selected, and calculating a number average value. However, when the particle was not spherical, the particle diameter of a non-spherical particle was the average value of the major axis and the minor axis that can be measured in a two-dimensional image. When two or more particles were connected by sintering, these particles were regarded as one particle. When it was difficult to determine whether it was sintering or contact, an image was binarized into white and black, and when a line dividing a connection portion was obtained, it was determined as contact so that the particles were regarded as two particles, and when the line was not obtained, it was determined as sintering so that the particles were regarded as one particle.

[0070] The number of particles having a particle diameter of 100 nm or more among the measured 200 particles was counted, and the number ratio with respect to 200 particles was calculated.

[0071] Similarly, the average particle diameter of the granulated body was calculated by magnifying and observing the LMFP granulated body used in each of Examples and Comparative Examples at a magnification of 3,000 using a scanning electron microscope S-5500 (manufactured by Hitachi High-Technologies Corporation), measuring the particle diameters of 100 granulated bodies randomly selected, and calculating a number average value. However, when the granulated body was not spherical, the particle diameter of a non-spherical particle was the average value of the major axis and the minor axis that can be measured in a two-dimensional image.

[0072] [Measurement B2] Average Diameter of Pores of Granulated Body, Sum of Pore Volumes of Pores Having Pore Diameter of 1 nm or More and 60 nm or Less, Maximum Value of Log Differential Pore Volume of Pores Having Pore Diameter of 1 nm or More and 60 nm or Less, and Pore Specific Surface Area of Pores Having Pore Diameter of 1 nm or More and 60 nm or Less

[0073] In a 5 cc powder cell, 0.3 g of the LMFP granulated body used in each of Examples and Comparative Examples was taken, and measurement values were determined by a mercury injection method using a pore distribution measuring device AutoPore IV9520 Type (manufactured by SHIMADZU CORPORATION) under the condition of an initial air pressure of 7 kPa. The mercury parameter is set to a mercury contact angle of 130.0° and a mercury surface tension of 485.0 Dynes/cm. However, for distinguishing voids between granulated bodies from pores, the average diameter of pores was performed in a range of a pore diameter of 1 nm or more and 200 nm or less. As the average diameter of pores, the median value of the pore diameters was adopted.

[0074] [Measurement C] Specific Surface Area

[0075] The specific surface area of the LMFP granulated body used in each of Examples and Comparative Examples was measured by a BET flow method (adsorption gas N.sub.2) using a fully automatic specific surface area measuring apparatus Macsorb HM Model-1210 (manufactured by Mountech Co., Ltd.).

[0076] [Measurement D] Weight Ratio of Carbon Contained in LMFP Granulated Body

[0077] The weight ratio of carbon contained in the LMFP granulated body used in each of Examples and Comparative Examples was measured using a carbon/sulfur analyzer EMIA-810W (manufactured by HORIBA, Ltd.).

[0078] [Measurement E] Volume Resistivity

[0079] The volume resistivity at 25 MPa of 1.0 g of the positive electrode active material used in each of Examples and Comparative Examples was measured using a powder resistance measuring system MCP-PD51 (manufactured by Mitsubishi Chemical Analytech Co., Ltd.).

[0080] [Measurement F] Peak Intensity Ratio of X-Ray Diffraction

[0081] The peak intensity ratio of X-ray diffraction of the positive electrode active material used in each of Examples and Comparative Examples was measured using D8 ADVANCE manufactured by Bruker ASX K.K. Measurement conditions of 20=5° to 70°, a scan interval of 0.02°, and a scan speed of 20 seconds/deg were adopted. Using the analyzing software for powder X-ray diffraction EVA from (manufactured by Bruker ASX K.K.), the background removal (coefficient 1.77) was performed and peak intensities were read to calculate.

[0082] [Measurement G] Average Particle Diameter of LMFP Primary Particles in Dispersion for Spray Drying

[0083] For the LMFP dispersion for spray drying, which was used in each of Examples and Comparative Examples, before adding glucose, the average particle diameter of the LMFP primary particles was measured using a dynamic light scattering particle size distribution measuring apparatus nanoPartica SZ-100V2 (manufactured by HORIBA, Ltd.).

[0084] [Measurement H] High-Speed Discharge Characteristics (Energy Density Measurement)

[0085] A 2032 coin battery was produced in which the electrode plate, produced in each of Examples and Comparative Examples, cut out to have a diameter of 15.9 mm was used as a positive electrode, a lithium foil cut out to have a diameter of 16.1 mm and a thickness of 0.2 mm was used as a negative electrode, “SETELA” (registered trademark) (manufactured by Toray Industries, Inc.) cut out to have a diameter of 20 mm was used as a separator, and a solution of ethylene carbonate:diethyl carbonate=3:7 (volume ratio) containing 1 M of LiPF.sub.6 was used as an electrolytic solution.

[0086] The obtained coin battery was charged and discharged three times at a cutoff voltage of 2.5 V and a maximum charge voltage of 4.3 V at a 0.1 C rate, and the energy density (Wh/kg) per positive electrode weight at a 0.1 C rate was measured from the third discharge. Subsequently, the battery was charged at a 0.1 C rate and discharged at a 4.0 C rate, and the energy density (Wh/kg) per positive electrode weight at a 4.0 C rate was measured. For evaluation of high-speed discharge characteristics, a ratio of the energy density during 4.0 C discharge to the energy density during 0.1 C discharge was determined.

[0087] [Measurement I] Cycle Resistance

[0088] A 2032 coin battery was produced in the same manner as in Measurement H, and was charged and discharged three times at a 0.1 C rate under an environment of 25° C. Subsequently, charging and discharging were performed once at a 1 C rate under an environment of 50° C., and the discharge energy density at this time was regarded as an initial energy density. Subsequently, charging and discharging were performed at a 1 C rate while maintaining an environment of 50° C., and the number of cycles when the discharge energy density was less than 80% of the initial energy density was determined and evaluated as cycle resistance.

[0089] In all the charging and discharging tests, charging was performed at a constant current until the maximum voltage reached 4.3 V, and after the maximum voltage was reached, charging was performed at the maximum voltage until the charge current fell below 0.01 C. Discharging was performed at a constant current until the discharge voltage fell below 2.5 V.

Example 1

[0090] In 25 g of pure water, 60 mmol of lithium hydroxide monohydrate was dissolved, after which 60 g of diethylene glycol was added thereto to prepare an aqueous lithium hydroxide/diethylene glycol solution. To the obtained aqueous lithium hydroxide/diethylene glycol solution stirred at 2000 rpm with a homo disper (HOMOGENIZING DISPER Model 2.5 manufactured by PRIMIX Corporation), an aqueous solution prepared by dissolving 20 mmol of phosphoric acid (an aqueous 85% solution), 16 mmol of manganese sulfate monohydrate, and 4 mmol of iron sulfate heptahydrate in 10 g of pure water was added to obtain a lithium manganese phosphate nanoparticle precursor. The obtained precursor solution was heated to 100° C. and held at the temperature for 2 hours to obtain LMFP nanoparticles as a solid content. The obtained LMFP was washed by adding pure water without drying and repeating solvent removal by a centrifuge, and the pH of the dispersion was set to 10.1. The solid content concentration of the obtained dispersion was adjusted to 50 wt %, and then subjected to a dispersion treatment under the conditions of 150 MPa and two passes using a wet jet mill Star Burst Mini (manufactured by Sugino Machine Limited).

[0091] To the obtained LMFP dispersion, glucose was added at a ratio of 0.15 g with respect to 1.0 g of LMFP and dissolved. Subsequently, the LMFP dispersion was dried and granulated by hot air at 200° C. using a spray dryer (MDL-050B manufactured by Fujisaki Electric Co. Ltd.). The obtained particles were heated at 700° C. for 4 hours in a nitrogen atmosphere using a rotary kiln (desktop rotary kiln manufactured by Takasago Industry Co., Ltd.) to obtain a granulated body of LMFP having a carbon cover layer.

[0092] Acetylene black (Li-400 manufactured by Denka Company Limited) and a binder (KF POLYMER L #9305 manufactured by KUREHA CORPORATION) were mixed, then the obtained Li LMFP granulated body was added, and the resulting mixture was solid-kneaded in a mortar. At that time, the mass ratio of each material contained, the granulated body:acetylene black:the binder, was set to 90:5:5. Then, the solid content concentration was adjusted to 48 wt % by adding N-methylpyrrolidinone to obtain a slurry electrode paste. N-methylpyrrolidinone was added to the obtained paste until the paste became flowable, and the paste was treated for 30 seconds under a stirring condition of 40 m/sec using a thin-film spin-type high-speed mixer (manufactured by PRIMIX Corporation “FILMIX” (registered trademark) 40-L type).

[0093] The resulting electrode paste was applied to an aluminum foil (thickness: 18 μm) using a doctor blade (300 μm), dried at 80° C. for 30 minutes, and then pressed to produce an electrode plate.

Example 2

[0094] An electrode plate was produced in the same manner as in Example 1, except that the amount of diethylene glycol at the time of LMFP synthesis was set to 80 g.

Example 3

[0095] An electrode plate was produced in the same manner as in Example 1, except that the amount of diethylene glycol at the time of LMFP synthesis was set to 120 g.

Example 4

[0096] An electrode plate was produced in the same manner as in Example 1, except that the amount of glucose to be added was set to 0.07 g with respect to 1.0 g of LMFP.

Example 5

[0097] An electrode plate was produced in the same manner as in Example 1, except that the amount of glucose to be added was set to 0.22 g with respect to 1.0 g of LMFP.

Example 6

[0098] An electrode plate was produced in the same manner as in Example 1, except that the amount of glucose to be added was set to 0.11 g with respect to 1.0 g of LMFP, and the temperature during firing was set to 600° C.

Example 7

[0099] An electrode plate was produced in the same manner as in Example 1, except that the dispersion treatment of LMFP was performed using a shear mixer (Model AX5 Head manufactured by Silverson Nippon K. K.: emulsifying screen) at 5000 rpm for 5 minutes instead of a wet jet mill.

Comparative Example 1

[0100] An electrode plate was produced in the same manner as in Example 1, except that the dispersion treatment was not performed using a wet jet mill.

Comparative Example 2

[0101] An electrode plate was produced in the same manner as in Example 1, except that the pH of the LMFP dispersion was adjusted not by washing with pure water but by adding LiOH.

Comparative Example 3

[0102] To 40 g of pure water, 60 mmol of lithium hydroxide monohydrate, 20 mmol of phosphoric acid (an aqueous 85% solution), 16 mmol of manganese sulfate monohydrate, and 4 mmol of iron sulfate heptahydrate were added, the mixture was placed in a pressure-resistant container, heated to 180° C., and held for 8 hours to obtain LMFP particles as a solid content.

[0103] Pure water was added to the obtained LMFP, the LMFP was washed by repeating solvent removal by a centrifuge five times, and the obtained LMFP dispersion was dried with a hot plate to obtain a powder. The average particle diameter of the obtained LMFP primary particles was measured in the same manner as in Measurement Example B, and found to be 281 nm.

[0104] The obtained LMFP powder was subjected to a pulverization treatment using a planetary ball mill P5 (manufactured by Fritsch GmbH). The container used for the pulverization treatment was a 45 ml container made of zirconia, 18 zirconia beads having a size of 10 mm were used as beads, and the treatment conditions were a rotation speed of 300 rpm and 6 hours.

[0105] Water was added to the obtained LMFP to obtain a dispersion, and glucose was further added thereto at a ratio of 0.15 g with respect to 1.0 g of LMFP and dissolved. Subsequently, the LMFP dispersion was dried and granulated by hot air at 200° C. using a spray dryer (MDL-050B manufactured by Fujisaki Electric Co. Ltd.). The obtained particles were heated at 700° C. for 4 hours in a nitrogen atmosphere using a rotary kiln (desktop rotary kiln manufactured by Takasago Industry Co., Ltd.) to obtain a granulated body of LMFP having a carbon cover layer.

[0106] An electrode plate was produced in the same manner as in Example 1 using the obtained LMFP granulated body.

Comparative Example 4

[0107] An electrode plate was produced in the same manner as in Comparative Example 3, except that the treatment conditions of the planetary ball mill were set to 200 rpm and 2 hours.

[0108] Table 1 and Table 2 show the evaluation results of each of Examples and Comparative Examples.

TABLE-US-00001 TABLE 1 Positive electrode active material Maximum value Average Number ratio of Average of log Pore particle particles particle differential specific Composition diameter of having particle diameter of Average Sum of pore pore volume surface area ratio in primary diameter of 100 granulated diameter volumes of of of Li.sub.αMn.sub.aFe.sub.bPO.sub.4 particles nm or more body of pores micropores*.sup.1 micropores*.sup.1 micropores*.sup.1 α a b (nm) (%) (μm) (nm) (cm.sup.3/g) (cm.sup.3/g) (m.sup.2/g) Example 1 1.00 0.79 0.21 55 1.2 18.2 26 0.193 0.93 31.9 Example 2 1.02 0.78 0.20 41 3.9 17.5 28 0.198 0.95 32.1 Example 3 1.00 0.81 0.20 26 4.8 17.4 25 0.205 0.88 33.4 Example 4 1.00 0.79 0.21 75 4.4 18.9 45 0.153 0.51 28.8 Example 5 1.00 0.79 0.21 52 1.5 15.5 38 0.158 0.67 29.1 Example 6 1.00 0.79 0.21 70 1.5 13.9 31 0.188 0.89 30.2 Example 7 1.01 0.79 0.21 50 1.9 18.9 28 0.205 0.89 32.1 Comparative 1.00 0.79 0.21 69 8.9 18.1 31 0.191 0.25 27.5 Example 1 Comparative 1.04 0.81 0.19 77 9.8 15.2 47 0.162 0.21 22.9 Example 2 Comparative 0.98 0.82 0.20 52 10.5 9.4 27 0.187 0.22 29.2 Example 3 Comparative 0.98 0.82 0.20 153 97.1 22.8 88 0.091 0.15 18.2 Example 4 *.sup.1Pores having pore diameter of 1 nm or more and 60 nm or less

TABLE-US-00002 TABLE 2 Dispersion for spray Positive electrode active material drying High-speed discharge characteristics Specific Average Energy surface Carbon Volume X-ray diffraction particle Energy density density Cycle area ratio resistivity peak ratio diameter 0.1 C 4.0 C ratio resistance (cc/g) (wt %) (Ω .Math. cm) I20/I29 I35/I29 (nm) (Wh/kg) (Wh/kg) 4.0 C/0.1 C (times) Example 1 39 2.8 1.2 × 10.sup.3 0.91 1.11 89 598 536 6.72 466 Example 2 35 2.8 1.5 × 10.sup.3 0.91 1.11 82 592 510 7.22 472 Example 3 49 2.9 8.2 × 10.sup.3 0.93 1.12 50 580 483 11.60 451 Example 4 38 1.7 4.3 × 10.sup.4 0.92 1.11 102 578 477 5.67 431 Example 5 35 6.8 7.8 × 10.sup.2 0.91 1.13 61 585 485 9.59 442 Example 6 36 2.1 5.2 × 10.sup.5 0.92 1.14 108 583 476 5.40 439 Example 7 39 2.7 2.1 × 10.sup.3 0.91 1.11 95 595 522 6.26 452 Comparative 32 2.6 1.1 × 10.sup.3 0.91 1.15 240 580 433 2.42 389 Example 1 Comparative 31 3.1 6.6 × 10.sup.4 0.93 1.12 198 550 398 2.78 367 Example 2 Comparative 42 2.9 3.9 × 10.sup.3 0.81 0.99 355 561 421 1.58 372 Example 3 Comparative 10 2.8 3.3 × 10.sup.3 0.83 0.98 421 422 251 1.00 339 Example 4