JP2019145204A - Positive electrode active material, positive electrode and nonaqueous electrolyte secondary battery - Google Patents

Abstract

To provide a positive electrode active material which is excellent in electrolyte permeability while suppressing reduction in particle strength.SOLUTION: A positive electrode active material 30 as an example of an embodiment consists mainly of a lithium-containing transition metal oxide and whose porosity calculated from a particle cross-sectional image of the positive electrode active material 30 is equal to or more than 20%. The positive electrode active material 30 has voids 32 communicated from the particle surface to the inside of a particle over a length corresponding to 1/6 of a particle diameter D. Here, the particle diameter D is the diameter of a circumcircle α of the particle in the particle cross-sectional image of the positive electrode active material 30.SELECTED DRAWING: Figure 4

Description

  The present disclosure relates to a positive electrode active material, a positive electrode, and a non-aqueous electrolyte secondary battery.

  Patent Document 1 discloses a lithium nickel composite oxide (positive electrode active material) which is a secondary particle having an average particle diameter of 5 μm to 30 μm formed by aggregation of primary particles having an average particle diameter of 1 μm to 8 μm. Yes. Patent Document 1 describes that the porosity of the positive electrode active material is preferably 10% to 20%, and the porosity increases as the particle size of the primary particles increases.

JP 2001-85006 A

  By the way, when aiming at the high output of a nonaqueous electrolyte secondary battery, it is preferable to use the positive electrode active material excellent in the permeability of electrolyte solution. However, it is not easy to improve the liquid permeability of the positive electrode active material. Even if the porosity of the positive electrode active material is simply increased, for example, the active material becomes brittle and the cycle characteristics of the battery are reduced. The permeability is not greatly improved. It is an important issue to improve liquid permeability while suppressing a decrease in particle strength of the positive electrode active material.

  A positive electrode active material that is one embodiment of the present disclosure is a positive electrode active material for a non-aqueous electrolyte secondary battery including a lithium-containing transition metal oxide as a main component, and is obtained from a particle cross-sectional image of the positive electrode active material. It has voids formed at a ratio of 20% or more with respect to the particle cross-sectional area, and the voids communicate from the surface of the positive electrode active material particle to the inside of the particle over a length corresponding to 1/6 of the particle size D. Where the particle diameter D is the diameter of the circumscribed circle of the particle in the particle cross-sectional image.

  A positive electrode that is one embodiment of the present disclosure is a positive electrode for a non-aqueous electrolyte secondary battery, and includes a positive electrode current collector, the positive electrode active material, a conductive material, and a binder. A positive electrode mixture layer formed on at least one surface, and a part of the conductive material is present in the voids of the positive electrode active material.

  A nonaqueous electrolyte secondary battery that is one embodiment of the present disclosure includes a positive electrode including the positive electrode active material, a negative electrode, and a nonaqueous electrolyte.

  According to one embodiment of the present disclosure, it is possible to provide a positive electrode active material having excellent electrolyte solution permeability while suppressing a decrease in particle strength. Moreover, the nonaqueous electrolyte secondary battery using the positive electrode active material has excellent output characteristics.

It is a perspective view of the nonaqueous electrolyte secondary battery which is an example of embodiment. It is sectional drawing of the positive electrode active material which is an example of embodiment. It is a SEM image of the positive electrode active material which is an example of embodiment. It is a SEM image of the particle section of the cathode active material which is an example of an embodiment. 3 is a SEM image of a positive electrode active material used in Comparative Example 1. 2 is a SEM image of a particle cross section of a positive electrode active material used in Comparative Example 1.

  In order to improve the output characteristics of the nonaqueous electrolyte secondary battery, it is effective to improve the liquid permeability of the positive electrode active material. As a result of intensive studies to improve the liquid permeability of the positive electrode active material, the present inventors have found that voids (long voids) communicating from the particle surface to the inside of the particles beyond the length corresponding to 1/6 of the particle size D. Has succeeded in developing a positive electrode active material in which a large number of particles are formed. According to the positive electrode active material that is one embodiment of the present disclosure, the electrolytic solution quickly permeates into the inside of the particles by the voids connected from the particle surface to the inside of the particles. For this reason, the nonaqueous electrolyte secondary battery excellent in output characteristics can be provided by using the said positive electrode active material. The positive electrode active material which is one embodiment of the present disclosure is suitable for a battery having a large electrode thickness.

  In the positive electrode active material that is one embodiment of the present disclosure, the lithium-containing transition metal oxide that is the main component contains, for example, at least nickel (Ni), cobalt (Co), and manganese (Mn), and lithium (Li). The ratio of Ni to the total number of moles of the metal element to be removed is 30 mol% or more. In this case, the above-mentioned gap is easily formed. Further, the capacity can be increased by increasing the Ni content.

  Hereinafter, an example of an embodiment will be described in detail with reference to the drawings. Note that the positive electrode active material, the positive electrode, and the nonaqueous electrolyte secondary battery of the present disclosure are not limited to the embodiments described below. Further, the drawings referred to in the description of the embodiments are schematically described, and the dimensions and the like of each component should be determined in consideration of the following description.

  In the embodiment described below, a rectangular battery in which a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated via separators is housed in a rectangular outer can is illustrated. The structure is not limited to the laminated structure, and may be a wound structure. Further, the battery case is not limited to a rectangular metal case (exterior can), and may be a metal case such as a coin shape or a cylindrical shape, or a resin case constituted by a resin film.

  FIG. 1 is a perspective view showing an appearance of a nonaqueous electrolyte secondary battery 10 which is an example of an embodiment. As illustrated in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes an outer can 11 that houses the electrode body and the nonaqueous electrolyte, and a sealing plate 12 that closes an opening of the outer can 11. The outer can 11 is a bottomed cylindrical metal container. The electrode body includes a plurality of positive electrodes, a plurality of negative electrodes, and at least one separator, and has a structure in which each positive electrode and each negative electrode are alternately stacked via the separator. A plurality of separators are provided, for example, and are respectively disposed on both sides of the positive electrode.

  The sealing plate 12 is provided with a positive external terminal 13, a negative external terminal 14, a gas discharge valve 15, and a liquid injection part 16. The positive electrode external terminal 13 and the negative electrode external terminal 14 are attached to the sealing plate 12 in a state of being electrically insulated from the sealing plate 12 using, for example, an insulating gasket. In addition, it is good also as a form which provides only the negative electrode external terminal as an external terminal in the sealing board 12, and uses the armored can 11 as a positive electrode external terminal. The liquid injection part 16 is generally composed of a liquid injection hole for injecting an electrolytic solution and a sealing plug for closing the liquid injection hole.

  Hereinafter, each component of the nonaqueous electrolyte secondary battery 10, particularly the positive electrode active material, will be described in detail.

[Positive electrode]
The positive electrode includes a positive electrode current collector such as a metal foil and a positive electrode active material layer formed on the positive electrode current collector. As the positive electrode current collector, a metal foil that is stable in the potential range of the positive electrode such as aluminum, a film in which the metal is disposed on the surface layer, or the like can be used. The positive electrode mixture layer includes a positive electrode active material, a conductive material, and a binder. For the positive electrode, for example, a positive electrode mixture slurry containing a positive electrode active material, a conductive material, a binder, and the like is applied onto a positive electrode current collector, the coating film is dried, and then rolled to collect a positive electrode mixture layer. It can be produced by forming on both sides of the body. The thickness of the positive electrode mixture layer is, for example, 100 μm or more, and the total of both surfaces of the current collector is 200 μm or more.

  Examples of the conductive material include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. These may be used alone or in combination of two or more.

  Examples of the binder include fluorine resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), and the like. These may be used alone or in combination of two or more.

  FIG. 2 is a cross-sectional view of a positive electrode active material 30 for a non-aqueous electrolyte secondary battery which is an example of an embodiment. FIG. 3 is a scanning electron microscope image (SEM image) of the positive electrode active material 30, and FIG. 4 is an SEM image of a cross section of the particle formed by the cross section polisher (CP) of the positive electrode active material 30. 3 and 4 are SEM images of the positive electrode active material produced in Example 1 described later. Hereinafter, the SEM image of the particle cross section of the positive electrode active material 30 formed by CP is referred to as a particle cross sectional image Z.

  As shown in FIGS. 2 to 4, the positive electrode active material 30 is secondary particles formed by agglomerating primary particles 31 mainly composed of a lithium-containing transition metal oxide. The positive electrode active material 30 has voids 32 formed at a ratio of 20% or more with respect to the particle cross-sectional area A obtained from the particle cross-sectional image Z (see FIG. 4). The portion that appears black in the particle cross-sectional image Z is the void 32. The particle cross-sectional area A is the area of the portion surrounded by the outline drawn along the particle surface of the positive electrode active material 30 in the particle cross-sectional image Z, and includes the area of the void 32. Hereinafter, the ratio of the area of the void 32 to the particle cross-sectional area A, that is, (area of the void 32 / particle cross-sectional area A) × 100 (%) may be referred to as the porosity.

  The positive electrode active material 30 is composed mainly of a lithium-containing transition metal oxide. The main component means a material having the largest content among materials constituting the positive electrode active material 30. The content rate of the lithium containing transition metal oxide in the positive electrode active material 30 is 90 mass% or more, for example, and may be 100 mass% substantially. An example of a suitable lithium-containing transition metal oxide contains at least nickel (Ni), cobalt (Co), and manganese (Mn), and the ratio of Ni to the total number of moles of metal elements excluding lithium (Li) is 30. It is an oxide of mol% or more. By using a composite oxide containing Ni, Co, and Mn, it is easy to form a favorable void 32, and by increasing the Ni content, the capacity of the positive electrode can be increased.

The lithium-containing transition metal oxide is, for example, a composition formula Li _a Ni _x M ₍₁ - _x) O ₂ (0.95 ≦ a ≦ 1.2, 0.3 ≦ x <1.0, M is Li, Ni It is an oxide represented by other metal elements. The Ni content may be 0.5 mol% or more, and may be 0.5 mol% to 0.8 mol%. As described above, Co and Mn are preferable as metallic elements other than Li and Ni contained in the lithium-containing transition metal oxide, but magnesium (Mg), aluminum (Al), calcium (Ca), and scandium are also preferable. (Sc), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), gallium (Ga), germanium (Ge), yttrium (Y), zirconium At least one selected from (Zr), tin (Sn), antimony (Sb), lead (Pb), and bismuth (Bi) may be included.

  The positive electrode active material 30 is, for example, secondary particles formed by aggregating primary particles 31 having an average particle diameter of 1 μm or less, and the positive electrode active material 30 has grain boundaries of the primary particles 31. The void 32 is formed between the primary particles 31. The grain boundaries of the primary particles 31 can be observed by SEM as shown in FIGS. In other words, the portion partitioned by the grain boundary is the primary particle 31. The positive electrode active materials 30 that are secondary particles may also aggregate, but the aggregation of secondary particles can be separated from each other by ultrasonic dispersion. On the other hand, even if the secondary particles are ultrasonically dispersed, the particles are not separated into the primary particles 31.

  The average particle diameter of the primary particles 31 is preferably 2 μm or less, for example, 0.5 μm to 2 μm. If the average particle diameter is within the range, the porosity is easily increased, and the long voids 33 are easily formed. Many primary particles 31 are, for example, ellipsoidal or rod-shaped particles, and have an aspect ratio that is a ratio of a minor axis to a major axis (major axis / minor axis) of 2 times or more. Of the primary particles 31 constituting the positive electrode active material 30, for example, 50% or more of the primary particles 31 have an aspect ratio of twice or more. The average particle size of the primary particles 31 is calculated based on the major axis. The short diameter of the primary particle 31 is, for example, 0.2 μm to 1 μm.

The average particle diameter of the primary particles 31 can be measured using SEM.
Specific measurement methods are as follows.
(1) Ten particles are randomly selected from a particle image obtained by observing the particles of the positive electrode active material 30 with SEM (2000 times).
(2) The grain boundaries of the primary particles 31 are observed for the 10 selected particles, and the primary particles 31 are determined respectively.
(3) The major axis (longest diameter) of each primary particle 31 is obtained, and the average value of the selected ten particles is defined as the average particle diameter of the primary particles 31.

  The average particle diameter of the positive electrode active material 30 (secondary particles) is, for example, 5 μm to 30 μm, and preferably 7 μm to 20 μm. The average particle diameter of the positive electrode active material 30 means a median diameter (volume basis) measured by a laser diffraction method, and can be measured using, for example, a laser diffraction scattering type particle size distribution measuring apparatus manufactured by Horiba.

  As described above, the voids 32 are formed at a ratio of 20% or more with respect to the particle cross-sectional area A of the positive electrode active material 30. The porosity of the positive electrode active material 30 is preferably 50% or less of the particle cross-sectional area A, for example, 20% to 45%, or 25% to 40%. When the porosity is within the range, it becomes easy to achieve both the particle strength of the positive electrode active material 30 and the permeability of the electrolytic solution. The average value (N = 100) of the porosity of the positive electrode active material 30 having an average particle diameter of 7 μm to 20 μm is, for example, 20% to 50%, or 25% to 40%, or 25% to 35%. The air gap 32 is maintained even after the battery is charged and discharged, and the porosity is hardly changed.

The porosity of the positive electrode active material 30 can be measured using SEM.
Specific measurement methods are as follows.
(1) In the SEM image (particle cross-sectional image Z) of the particle cross section of the positive electrode active material 30, an outline is drawn along the particle surface, and a particle cross-sectional area A that is the area of the portion surrounded by the outline is obtained.
(2) The area of the gap 32 in the portion surrounded by the outline is obtained.
(3) The porosity (%) is calculated by the formula: (area of void 32 / particle cross-sectional area A) × 100.

  The void 32 includes a long void 33 that communicates from the particle surface of the positive electrode active material 30 to the inside of the particle over a length corresponding to 1/6 of the particle diameter D. By forming the long gap 33, the electrolytic solution quickly penetrates into the particles of the positive electrode active material 30. Here, the particle diameter D is the diameter of the circumscribed circle α of the particles of the positive electrode active material 30 in the particle cross-sectional image Z. The particle surface is a position on the outline. In the present specification, a void having a length exceeding 1/6 of the particle diameter D from the particle surface toward the center X of the circumscribed circle α is defined as a long void 33. In other words, a closed void that does not have an opening (inlet) on the particle surface, and a void that is equal to or less than 1/6 of the particle diameter D is not the long void 33.

  The long voids 33 may extend substantially straight from the particle surface toward the center X, or may meander. Further, the long gap 33 may be branched, and in one continuous long gap 33, there may be a plurality of at least one of the entrance and the abutment. The long gap 33 formed by meandering may have a length exceeding the particle diameter D.

  The entrance of the long gap 33 is preferably formed uniformly on the entire particle surface of the positive electrode active material 30. The long gap 33 communicates from the particle surface toward the center X over the length corresponding to 2/6 (1/3) or 3/6 (1/2) of the particle diameter D to the inside of the particle. Also good. FIG. 2 illustrates a circle β that is concentric with the circumscribed circle α and has a diameter that is 5/6 of the diameter D. When the particle cross section of the positive electrode active material 30 has a substantially perfect circle shape, the long voids 33 formed from the surface of each particle communicate with the inside of the particle beyond the circle β.

  The ratio of the long gaps 33 occupying the gaps 32 (hereinafter sometimes referred to as “long void ratio”) is, for example, 20% or more, 30% or more, or 50% or more. Here, the ratio of the long gap 33 to the gap 32 is calculated by the formula: (area of the long gap 33 / area of the gap 32) × 100. The average value (N = 100) of the long porosity of the positive electrode active material 30 having an average particle diameter of 7 μm to 15 μm is, for example, 20% to 80%, or 30% to 70%, or 30% to 60%.

  Part of the conductive material included in the positive electrode mixture layer may exist in the gap 32. A part of the conductive material enters, for example, the void 32 opened on the particle surface of the positive electrode active material 30 when preparing the positive electrode mixture slurry or forming the positive electrode mixture layer. A part of the conductive material may exist in the long gap 33 and may enter the inside of the particle beyond the length corresponding to 1/6 of the particle diameter D. By the presence of the conductive material in the gap, for example, a good conductive path is formed in the positive electrode mixture layer, and the output characteristics are further improved.

The positive electrode active material 30 is obtained, for example, by mixing and firing a transition metal compound such as nickel cobalt manganese hydroxide synthesized by a coprecipitation method, a lithium compound, and a sintering inhibitor. Firing is performed in an oxygen stream at a temperature of 900 ° C. to 1000 ° C., for example. Various compounds can be used as the transition metal compound. However, as described above, when a material containing Ni, Co, or Mn is used, a favorable void 32 is easily formed. For the transition metal compound, for example, a material having a tap density (consolidation density) of 1.8 g / cc or less, preferably 1 g / cc to 1.8 g / cc, measured by a powder tester (PT-X manufactured by Hosokawa Micron) is used. . Examples of the lithium compound include lithium hydroxide (LiOH) and lithium carbonate (Li ₂ CO ₃ ). As the firing inhibitor, for example, an oxide containing tungsten, niobium, molybdenum or the like, a phosphate such as lithium phosphate, or the like is used.

[Negative electrode]
A negative electrode is comprised with the negative electrode collector which consists of metal foil etc., for example, and the negative electrode compound-material layer formed on the said collector. As the negative electrode current collector, a metal foil that is stable in the potential range of a negative electrode such as copper, a film in which the metal is disposed on the surface layer, or the like can be used. The negative electrode mixture layer includes a negative electrode active material and a binder. For example, the negative electrode is prepared by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc. on a negative electrode current collector, drying the coating film, and rolling the negative electrode mixture layer on both sides of the current collector. It can be manufactured by forming.

  The negative electrode active material is not particularly limited as long as it can reversibly store and release lithium ions. For example, carbon materials such as natural graphite and artificial graphite, lithium and alloys such as silicon (Si) and tin (Sn), etc. Or an alloy containing a metal element such as Si or Sn, a composite oxide, or the like can be used. A negative electrode active material may be used independently and may be used in combination of 2 or more types.

  As the binder, as in the case of the positive electrode, fluororesin, PAN, polyimide, acrylic resin, polyolefin and the like can be used. When preparing a mixture slurry using an aqueous solvent, it is preferable to use CMC or a salt thereof, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like.

[Separator]
As the separator, a porous sheet having ion permeability and insulating properties is used. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. The separator is made of, for example, polyolefin such as polyethylene or polypropylene, cellulose, or the like. The separator may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as polyolefin. Moreover, the multilayer separator containing a polyethylene layer and a polypropylene layer may be sufficient as a separator, and it may have a surface layer containing the surface layer comprised by an aramid resin, or an inorganic filler.

[Nonaqueous electrolyte]
The nonaqueous electrolyte includes a nonaqueous solvent and a solute (electrolyte salt) dissolved in the nonaqueous solvent. As the non-aqueous solvent, for example, esters, ethers, nitriles, amides such as dimethylformamide, isocyanates such as hexamethylene diisocyanate, and a mixed solvent of two or more of these can be used. The non-aqueous solvent may contain a halogen-substituted product in which at least a part of hydrogen in these solvents is substituted with a halogen atom such as fluorine.

  Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate, dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), and methyl propyl carbonate. Chain carbonates such as ethyl propyl carbonate and methyl isopropyl carbonate, cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone, methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate, etc. And a chain carboxylic acid ester.

  Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4 -Cyclic ethers such as dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineol, crown ether, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether , Dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxy toluene, benzyl ethyl ether, diphenyl ether, dibenzyl Ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, tri Examples thereof include chain ethers such as ethylene glycol dimethyl ether and tetraethylene glycol dimethyl.

  Examples of the nitriles include acetonitrile, propionitrile, butyronitrile, valeronitrile, n-heptanitrile, succinonitrile, glutaronitrile, adionitrile, pimeonitrile, 1,2,3-propanetricarbonitrile, 1,3. , 5-pentanetricarbonitrile and the like.

  Examples of the halogen-substituted product include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylates such as methyl fluoropropionate (FMP). .

Examples of the electrolyte _{salt, LiBF 4, LiClO 4, LiPF} 6, LiAsF 6, LiSbF 6, LiAlCl 4, LiSCN, LiCF 3 SO 3, LiCF 3 CO 2, Li (P (C 2 O 4) F 4), LiPF _6-x (C _n F _{2n + 1} ) _x (1 <x <6, n is 1 or 2), LiB ₁₀ Cl ₁₀ , LiCl, LiBr, LiI, chloroborane lithium, lower aliphatic lithium carboxylate, Li Borates such as ₂ B ₄ O ₇ and Li (B (C ₂ O ₄ ) F ₂ ), LiN (SO ₂ CF ₃ ) ₂ , LiN (C _l F _{2l + 1} SO ₂ ) (C _m F _{2m + 1} SO ₂ ) and imide salts such as {1, m is an integer of 1 or more}. These electrolyte salts may be used alone or in combination of two or more. The concentration of the electrolyte salt is, for example, 0.8 to 1.8 mol per liter of the nonaqueous solvent.

  Hereinafter, although this indication is further explained by an example, this indication is not limited to these examples.

<Example 1>
[Preparation of positive electrode active material]
A transition metal hydroxide represented by a composition formula Ni _0.33 Co _0.33 Mn _0.33 (OH) ₂ and having a tap density of 1.5 g / cc, LiOH, and a sintering inhibitor are mixed, and 935 ° C. for 50 hours. The lithium-containing transition metal oxide was synthesized by firing in an oxygen stream. Tungsten oxide (WO ₃ ) was used as the sintering inhibitor, and the amount added was 0.3 mol%. The oxide was classified to obtain a positive electrode active material A1 having an average particle size of 10 μm. The average particle diameter (median diameter / volume basis) of the positive electrode active material A1 was measured using a laser diffraction / scattering particle size distribution analyzer (“LA950” manufactured by Horiba, Ltd.).

The positive electrode active material A1 was analyzed by a powder X-ray diffraction method using a powder X-ray diffractometer (“D8ADVANCE” manufactured by Bruker AXS, source Cu-Kα). As a result, it was attributed to a layered rock salt type crystal structure. . Further, the composition of the positive electrode active material A1 was analyzed using an ICP emission spectroscopic analyzer (“iCAP6300” manufactured by Thermo Fisher Scientific). As a result, it was Li _1.05 Ni _0.33 Co _0.33 Mn _0.33 O ₂ .

  FIG. 3 shows an SEM image of the positive electrode active material A1, and FIG. 4 shows an SEM image of a particle cross section of the positive electrode active material A1 formed by CP. 3 and 4, the positive electrode active material A1 has a large number of voids, and the voids include long voids that extend to the inside of the particles beyond the length corresponding to 1/6 of the particle diameter D. I understand that. The average particle size of the primary particles determined by the above method was 0.5 μm. In addition, 100 particles of the positive electrode active material A1 were randomly selected from the particle cross-sectional image, the porosity was determined for each particle by the above-described method, and the average value was calculated to be 31%. For the selected 100 particles, the ratio of long voids (long void ratio) in the voids was determined, and the average value was calculated. As a result, it was 45%.

[Production of positive electrode]
Mix so that the positive electrode active material A1 is 95.8% by mass, the carbon powder is 3% by mass, and the polyvinylidene fluoride powder is 1.2% by mass, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) is added. A positive electrode mixture slurry was prepared. The slurry is applied to both sides of a current collector made of aluminum foil by the doctor blade method, and after the coating film is dried, the coating film is rolled by a rolling roller to form a positive electrode mixture layer on both sides of the positive electrode current collector. A positive electrode was produced. A portion where the composite material layer was not formed was provided in the central portion in the longitudinal direction of the current collector, and a positive electrode tab was attached to the portion. The thickness of the positive electrode mixture layer was about 100 μm, and the total of both sides of the current collector was about 200 μm.

[Production of negative electrode]
A graphite was mixed so that 98.2% by mass, styrene-butadiene rubber was 0.7% by mass, and sodium carboxymethylcellulose was 1.1% by mass, and this was mixed with water to prepare a slurry. The slurry is applied to both sides of a current collector made of copper foil by the doctor blade method, and after the coating film is dried, the coating film is rolled by a rolling roller to form a negative electrode mixture layer on both sides of the negative electrode current collector. A negative electrode was prepared. The part which does not form a compound-material layer in the longitudinal direction both ends of the electrical power collector was provided, and the negative electrode tab was attached to the said part. The thickness of the negative electrode mixture layer was about 100 μm, and the total of both sides of the current collector was about 200 μm.

[Preparation of non-aqueous electrolyte]
LiPF ₆ was dissolved at a concentration of 1.6 mol / L in an equal volume mixed non-aqueous solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) to obtain a non-aqueous electrolyte.

[Production of non-aqueous electrolyte secondary battery]
Using the positive electrode, the negative electrode, the non-aqueous electrolyte, and the separator, a battery A1 was produced according to the following procedure.
(1) The positive electrode and the negative electrode were wound through a separator to produce a wound structure electrode body.
(2) Insulating plates were arranged above and below the electrode body, and the wound electrode body was housed in a cylindrical battery outer can having a diameter of 18 mm and a height of 65 mm.
(3) The current collecting tab of the negative electrode was welded to the bottom inner surface of the battery outer can, and the current collecting tab of the positive electrode was welded to the bottom plate of the sealing body.
(4) A non-aqueous electrolyte was injected from the opening of the battery outer can, and then the battery outer can was sealed with a sealing body to obtain a battery A1.

<Comparative Example 1>
In the synthesis of the lithium-containing transition metal oxide, a positive electrode active material was prepared in the same manner as in Example 1 except that a transition metal hydroxide having a tap density of 2.5 g / cc was used and no sintering inhibitor was added. Material B1 and battery B1 were made.

  FIG. 5 shows an SEM image of the positive electrode active material B1, and FIG. 6 shows an SEM image of a particle cross section of the positive electrode active material B1 formed by CP. 5 and 6, it can be seen that the positive electrode active material B1 has voids inside the particles, but the porosity is lower than that of the positive electrode active material A1, and the voids do not include long voids. As in the case of Example 1, the average value of the porosity of the positive electrode active material B1 was calculated to be 5%.

  About each battery of an Example and a comparative example, the output characteristic was evaluated with the following method, and the evaluation result was shown in Table 1 with the average porosity of the positive electrode active material, and the average long porosity.

[Evaluation of output characteristics (measurement of discharge capacity at −20 ° C., discharge rates 1C and 2C)]
Each battery was charged at a constant current of up to 4.2 V at 0.1 C in an environment of −20 ° C., and then charged at a constant voltage of 4.2 V until the current value was equivalent to 0.01 C to complete the charging. After resting for 10 minutes, constant current discharge was performed until the voltage became 2.5 V at 0.5C. From the discharge curve at this time, the discharge capacity per mass of the positive electrode active material was determined.

  From Table 1, in the battery A1 of Example 1, the discharge capacity under the conditions of the discharge rates 1C and 2C is greatly improved as compared with the battery B1 of Comparative Example 1, and the output characteristics (low-temperature output characteristics) are greatly improved. It turns out that it is improving. Note that the discharge capacity of each battery was approximately the same under the condition of a discharge rate of 0.25C. According to the positive electrode active material A1 having a large number of long voids, it is considered that the electrolyte quickly penetrates into the particles. That is, it is considered that the result of the output characteristic evaluation of the battery A1 is caused by the excellent liquid permeability of the positive electrode active material A1.

  DESCRIPTION OF SYMBOLS 10 Nonaqueous electrolyte secondary battery, 11 Exterior can, 12 Sealing plate, 13 Positive electrode external terminal, 14 Negative electrode external terminal, 15 Gas discharge valve, 16 Injection part, 30 Positive electrode active material, 31 Primary particle, 32 Void, 33 Length Void, D particle size, α circumscribed circle

Claims (8)

A positive electrode active material for a non-aqueous electrolyte secondary battery composed mainly of a lithium-containing transition metal oxide,
Having voids formed at a ratio of 20% or more with respect to the particle cross-sectional area obtained from the particle cross-sectional image of the positive electrode active material,
The void includes a long void that extends from the particle surface of the positive electrode active material to the inside of the particle beyond a length corresponding to 1/6 of the particle size D, where the particle size D is the particle size. A positive electrode active material that is a diameter of a circumscribed circle of particles in a cross-sectional image.   The positive electrode active material according to claim 1, wherein the lithium-containing transition metal oxide contains at least nickel, cobalt, and manganese, and the ratio of nickel to the total number of moles of metal elements excluding lithium is 30 mol% or more. .   The positive electrode active material according to claim 1, wherein the positive electrode active material is a secondary particle formed by aggregating primary particles having an average particle diameter of 1 μm or less.   The positive electrode active material according to claim 3, wherein the primary particle has a ratio of a minor axis to a major axis that is twice or more.   The positive electrode active material according to any one of claims 1 to 4, wherein a ratio of the long voids in the voids is 20% or more. A positive electrode for a non-aqueous electrolyte secondary battery,
A positive electrode current collector;
A positive electrode mixture layer composed of the positive electrode active material according to any one of claims 1 to 5, a conductive material, and a binder, and formed on at least one surface of the positive electrode current collector;
With
The positive electrode, wherein a part of the conductive material is present in the gap of the positive electrode active material.   The positive electrode according to claim 6, wherein the positive electrode mixture layer has a thickness of 75 μm or more. A positive electrode comprising the positive electrode active material according to claim 1;
A negative electrode,
A non-aqueous electrolyte,
A non-aqueous electrolyte secondary battery.