Nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery (10) includes a positive electrode (11), a negative electrode (12), and a nonaqueous electrolyte. A positive electrode mixture layer that constitutes the positive electrode (11) includes a positive electrode active material which is mainly composed of a lithium cobalt composite oxide that has a layered crystal structure, a carbon nanotube, and a carbon black. The lithium cobalt composite oxide is represented by general formula LiCo(1 – w – x – y − z)AlwMgxTiyMnzO2 (in the formula, 0.010 ≤ w ≤ 0.013, 0.003 ≤ x ≤ 0.006, 0.0004 ≤ y ≤ 0.0006, and 0 ≤ z ≤ 0.0015).

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

[0002] The positive electrode active material, which is a main component of a non-aqueous electrolyte secondary battery, has a large effect on battery performance, such as not only the battery capacity but also the charge / discharge cycle characteristics, thermal stability, and output characteristics. Therefore, much research has been conducted on the positive electrode active material. For example, Patent Document 1 discloses a positive electrode active material having the general formula LiNiCo （１－ｘ－ｙ） M1 ｙ O ２ (wherein 0.65≦x≦0.90, 0.02≦y≦0.06, and M1 is at least one metal element selected from the group consisting of Al, Mg, Ti, Mn, and Zr) and a lithium nickel cobalt composite oxide represented by the general formula LiCo （１－ｚ） M2 ｚ O ２ (0.005≦z≦0.03, M2 is at least one metal element selected from the group consisting of Al, Mg, Ti, Mn, and Zr) and a non-aqueous electrolyte secondary battery using the lithium-cobalt composite oxide.

[0003] JP 2011-192541 A

[0004] When a lithium-cobalt composite oxide (hereinafter sometimes referred to as "LCO") is used as the positive electrode active material of a nonaqueous electrolyte secondary battery, a high capacity can be achieved by increasing the end-of-charge voltage. However, as a result of studies by the present inventors, it was found that increasing the end-of-charge voltage in a battery using a conventional LCO results in a large decrease in capacity during charge and discharge and also in a decrease in thermal stability.

[0005] The nonaqueous electrolyte secondary battery according to the present disclosure is a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode has a positive electrode core and a positive electrode mixture layer provided on the positive electrode core, the positive electrode mixture layer includes a positive electrode active material mainly composed of a lithium cobalt composite oxide having a layered crystal structure, carbon nanotubes, and carbon black, and the lithium cobalt composite oxide is represented by the general formula LiCo(1-w-x-y-z)Al ｗ Mg ｘ Ti ｙ Mnｚ O ２ (wherein 0.010≦w≦0.013, 0.003≦x≦0.006, 0.0004≦y≦0.0006, 0≦z≦0.0015).

[0006] The nonaqueous electrolyte secondary battery according to the present disclosure can achieve high capacity while improving charge / discharge cycle characteristics and thermal stability.

[0007] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment;

[0008] In nonaqueous electrolyte secondary batteries using LCO-based composite oxides as the positive electrode active material, it is important to increase the end-of-charge voltage to achieve high capacity while maintaining good charge-discharge cycle characteristics and thermal stability. However, increasing the end-of-charge voltage causes crystal transition or decomposition of the LCO, releasing oxygen from the crystal structure and promoting the oxidative decomposition reaction between the positive electrode active material and the electrolyte. This is thought to result in a large capacity loss during charge and discharge and a decrease in thermal stability.

[0009] As a result of intensive research to solve the above problems, the present inventors have found that a positive electrode active material having the general formula LiCo(1-w-x-y-z)Al ｗ Mg ｘ Ti ｙ Mn ｚ O ２ (wherein 0.010≦w≦0.013, 0.003≦x≦0.006, 0.0004≦y≦0.0006, 0≦z≦0.0015) and the combined use of carbon nanotubes and carbon black as conductive agents has been found to form a good conductive path in the positive electrode mixture layer, ensuring excellent cycle characteristics and thermal stability even when the end-of-charge voltage of the battery is increased. Note that, although carbon nanotubes are excellent as conductive agents, sufficient effects cannot be obtained by using carbon nanotubes alone.

[0010] As a result of the inventors' investigations, it was found that the co-addition of Al, Mg, and Ti to an LCO-based composite oxide suppresses the transition or decomposition of the crystal structure that accompanies an increase in charging voltage, but that the co-addition of these elements increases the resistance of the positive electrode active material and reduces the output characteristics of the battery. It is believed that the coexistence of carbon nanotubes and carbon black significantly improves the output characteristics and ensures excellent cycle characteristics and thermal stability.

[0011] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that configurations obtained by selectively combining the components of the multiple embodiments and variations described below are included within the scope of the present disclosure.

[0012] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified, but the outer can is not limited to a cylindrical outer can and may be, for example, a prismatic outer can (prismatic battery) or a coin-shaped outer can (coin battery). It may also be an outer can (pouch-type battery) made of a laminate sheet including a metal layer and a resin layer. The electrode assembly is also not limited to a wound type and may be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0013] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1 , the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte (not shown), and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container with a bottom that is open on one axial side, and the opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience of explanation, the sealing member 17 side of the battery will be referred to as the top, and the bottom side of the outer can 16 will be referred to as the bottom.

[0014] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium precipitation. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in both the longitudinal direction and the width direction (short direction). The separator 13 is formed to be at least slightly larger than the positive electrode 11, and, for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0015] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0016] A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 that is crimped to the sealing body 17.

[0017] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0018] The end-of-charge voltage of the nonaqueous electrolyte secondary battery 10 is preferably 4.3 V or higher. The nonaqueous electrolyte secondary battery 10 can achieve excellent cycle characteristics and thermal stability while increasing the end-of-charge voltage and increasing the battery capacity. The upper limit of the end-of-charge voltage is not particularly limited, but is, for example, 4.8 V.

[0019] The positive electrode 11, the negative electrode 12, the separator 13, and the nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10 will be described in detail below.

[0020] [Positive Electrode] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer provided on the positive electrode core. The positive electrode core can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on the surface. The positive electrode mixture layer contains a positive electrode active material, a binder, and a conductive agent, and is preferably provided on both sides of the positive electrode core. The positive electrode 11 can be produced, for example, by applying a slurry of a positive electrode mixture containing a positive electrode active material, a binder, and a conductive agent onto the positive electrode core, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core.

[0021] The positive electrode active material is mainly composed of lithium cobalt composite oxide (hereinafter referred to as "LCO") having a layered crystal structure. The lithium cobalt composite oxide (hereinafter referred to as "LCO") constituting the main component of the positive electrode active material has the general formula LiCo(1-w-x-y-z)Al ｗ Mg ｘ Ti ｙMn ｚ O ２ (wherein, 0.010≦w≦0.013, 0.003≦x≦0.006, 0.0004≦y≦0.0006, 0≦z≦0.0015). LCO is a composite oxide in which predetermined amounts of Al, Mg, and Ti are added to a composite oxide containing Li and Co as metal elements. The content of Co in LCO is, for example, 97.8 mol % or more based on the total number of moles of elements excluding Li and O. Mn is an optional component, but by adding Mn, the effect of improving cycle characteristics becomes more pronounced.

[0022] In this specification, the term "main component of the positive electrode active material" refers to the component with the highest mass ratio among the materials constituting the positive electrode active material. The content of LCO is preferably 60 mass% or more, more preferably 70 mass% or more, of the mass of the positive electrode active material. The positive electrode active material may be substantially composed of LCO represented by the above general formula. The composition of the positive electrode active material can be measured using an ICP optical emission spectrometer (iCAP6300, manufactured by Thermo Fisher Scientific).

[0023] The positive electrode mixture layer may contain, as a positive electrode active material, a lithium-containing composite oxide other than LCO represented by the above general formula, as long as the object of the present disclosure is not impaired. An example of the other lithium-containing composite oxide is a lithium cobalt composite oxide that does not substantially contain Al, Mg, or Ti, and has the general formula LiCoO ２ Furthermore, a composite oxide other than the lithium-cobalt composite oxide may be used as long as it does not impair the object of the present disclosure.

[0024] As described above, LCO has a layered crystal structure. Specific examples include a layered structure belonging to the space group R-3m, or a layered structure belonging to the space group C2 / m. The addition of Al, Mg, and Ti, especially Al and Mg, is thought to significantly contribute to the stabilization of this crystal structure. The crystal structure of the composite oxide can be measured using a powder X-ray diffractometer (Rigaku Corporation, RINT2200, Cu-Kα source).

[0025] The volume-based median diameter (D50) of LCO is, for example, 5 μm or more and 50 μm or less, preferably 10 μm or more and 40 μm or less, and more preferably 15 μm or more and 30 μm or less. LCO is, for example, a secondary particle formed by aggregation of primary particles, and the D50 of LCO means the D50 of the secondary particle. D50 means the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size. The particle size distribution of the complex oxide can be measured using a laser diffraction particle size distribution measuring device (e.g., MT3000II manufactured by Microtrac-Bell Co., Ltd.) using water as a dispersion medium.

[0026] The Al content in LCO is 1.0 mol% or more and 1.3 mol% or less, more preferably 1.0 mol% or more and 1.2 mol% or less, based on the total number of moles of elements excluding Li and O. If the amount of Al added is within this range, the stability of the crystal structure is improved, and the cycle characteristics and thermal stability of the battery are improved. If the amount of Al added is less than 1.0 mol% or more than 1.3 mol%, the improvement effect of cycle characteristics, etc. is not obtained, and the capacity decrease is also significant. Note that even if the amount of Al added is within the above range, if Mg and Ti are not added, the above improvement effect is not obtained.

[0027] The Mg content in the LCO is 0.3 mol % or more and 0.6 mol % or less, based on the total number of moles of elements excluding Li and O. If the amount of Mg added is within this range, the stability of the crystal structure is improved, and the cycle characteristics and thermal stability are improved. If the amount of Mg added is less than 0.3 mol % or more than 0.6 mol %, the effect of improving the cycle characteristics and the like cannot be obtained, and if the amount of Mg added is too large, the capacity also decreases significantly.

[0028] The Ti content in the LCO is 0.04 mol % or more and 0.06 mol % or less, based on the total number of moles of elements excluding Li and O. If the amount of Ti added is within this range, it is thought that the material resistance can be reduced, and in particular, the cycle characteristics are improved. If the amount of Ti added is less than 0.04 mol % or more than 0.06 mol %, the effect of improving the cycle characteristics, etc. cannot be obtained, and if the amount of Ti added is too large, the capacity decrease becomes significant.

[0029] LCO further contains Mn, and the Mn content (z) in the general formula is preferably 0.0004≦z≦0.0015, and more preferably 0.0010≦z≦0.0015. In particular, it is believed that material resistance can be effectively reduced if the amount of Mn added is 0.10 mol % or more and 0.15 mol % or less relative to the total number of moles of elements excluding Li and O. As a result, the battery's cycle characteristics and thermal stability are more significantly improved, and discharge characteristics in low-temperature environments are also improved. Note that if the amount of Mn added is too large, for example, cycle characteristics and the like may be degraded, and capacity loss may be significant.

[0030] LCO is produced, for example, through the following steps: (1) a first step of obtaining a composite hydroxide or composite oxide containing Co, Al, Mg, Ti, and Mn; (2) a second step of mixing the composite hydroxide or composite oxide with a lithium compound and firing the mixture; and (3) a third step of washing the fired product with water and drying it.

[0031] In the first step, for example, an alkaline solution such as sodium hydroxide is added dropwise to a solution of a metal salt containing Co, Al, Mg, Ti, and Mn while stirring the solution to adjust the pH to the alkaline side, thereby precipitating (co-precipitating) a composite hydroxide. Note that in the first step, for example, the composite hydroxide may be calcined at a temperature lower than that of the calcination in the second step.

[0032] In the second step, the composite hydroxide or composite oxide obtained in the first step is mixed with a lithium compound. An example of the lithium compound is Li ２ CO ３ , LiOH, Li ２ O ２ , Li ２ O, LiNO ３ , LiNO ２ , Li ２ SO ４ , LiOH·H ２ Examples of suitable firing materials include O, LiH, and LiF. The firing of the mixture in the second step is carried out, for example, under an oxygen stream at a temperature of 500° C. or higher and 900° C. or lower. The firing temperature is preferably 600° C. or higher and 800° C. or lower. The firing may be carried out in two or more stages with different firing conditions.

[0033] In the third step, the fired product is washed with water to remove impurities such as excess Li, and the washed fired product is dried. If necessary, the fired product is pulverized, classified, or the like, to adjust the D50 of the LCO to a desired range. The washed fired product may be dried in a vacuum or in the air. An example of the drying temperature is 150°C or higher and 250°C or lower.

[0034] As described above, the positive electrode mixture layer contains a binder and two types of conductive agents. Examples of binders contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, and polyethylene oxide (PEO). The content of the binder is, for example, 0.5% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

[0035] The positive electrode mixture layer contains carbon nanotubes (CNTs) and carbon black (CB) as conductive agents. The combined use of CNTs and CB with the above-mentioned positive electrode active material significantly improves the battery's cycle characteristics, output characteristics (load characteristics), and high-temperature storage characteristics. The stabilization effect of the crystal structure due to the co-addition of Al, Mg, and Ti to the LCO-based composite oxide, and the combined use of CNTs and CB suppresses resistance increases, which are thought to significantly improve these battery performances.

[0036] The content of the conductive agent is, for example, 0.5% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer. The mass ratio of CNT to CB is not limited to a specific value, but is preferably CNT:CB=1:12 to 1:1.5. That is, it is preferable that the positive electrode mixture layer contains more CB than CNT on a mass basis.

[0037] CNTs are conductive carbon fibers with a tube outer diameter (fiber diameter) of, for example, several tens of nanometers or less, and have a large aspect ratio. The aspect ratio of CNTs is the ratio of the fiber length to the fiber diameter. The average aspect ratio of CNTs is preferably 20 times or more, more preferably 50 times or more. The fiber length refers to the length of a CNT when it is stretched linearly, and the fiber diameter refers to the length perpendicular to the fiber length direction.

[0038] The average fiber diameter of the CNTs is, for example, 20 nm or less, preferably 15 nm or less, and more preferably 10 nm or less. The lower limit of the average fiber diameter of the CNTs is not particularly limited, but an example is 1 nm. The average fiber diameter of the CNTs is determined by image analysis using a transmission electron microscope (TEM). The average fiber diameter of the CNTs is determined by measuring the fiber diameters of 100 randomly selected CNTs and calculating the arithmetic average of the measured values.

[0039] The average fiber length of the CNTs is, for example, 0.5 μm or more, and may be 1 μm or more. The upper limit of the average fiber length of the CNTs is not particularly limited, but an example is 100 μm. The average fiber length of the CNTs is determined by image analysis using a scanning electron microscope (SEM). The average fiber length of the CNTs is determined by measuring the lengths of 100 randomly selected CNTs and arithmetically averaging the measured values. The CNTs present in the positive electrode mixture layer may be present in the form of a bundle of multiple CNTs. The length of a single CNT present in the bundle of CNTs is used to calculate the above average fiber length.

[0040] The CNT may be either a single-walled CNT (SWCNT) or a multi-walled CNT (MWCNT), or a combination of SWCNT and MWCNT may be used. SWCNT has a structure in which one layer of graphite sheet is formed into a tubular shape, while MWCNT has a structure in which multiple layers of graphite sheet are formed into a tubular shape. An example of the MWCNT is a double-walled CNT having a double-walled structure. An example of the BET specific surface area of ​​the CNT is 200 m ２ / g or more, 2000m ２ The BET specific surface area is measured in accordance with the BET method (nitrogen adsorption method) described in JIS R1626.

[0041] The CNT content is preferably 0.05% by mass or more and 0.5% by mass or less, more preferably 0.1% by mass or more and 0.4% by mass or less, and particularly preferably 0.1% by mass or more and 0.3% by mass or less, relative to the mass of the positive electrode active material. Even a small amount of CNT forms a good conductive path in the positive electrode mixture layer, but the lower limit of the content is, for example, 0.01% by mass, more preferably 0.05% by mass. If the CNT content is within this range, the effects of improving cycle characteristics, output characteristics, and high-temperature storage characteristics become more pronounced.

[0042] CB is a particulate conductive carbon material, examples of which include acetylene black and ketjen black. The average particle size of CB is, for example, 50 nm or less. The lower limit of the average particle size is not particularly limited, but an example is 1 nm. CB is composed of primary particles called domains or particles, primary aggregates formed by gathering these primary particles, and secondary aggregates formed by gathering the primary aggregates. The average particle size of CB means the average particle size of the primary particles.

[0043] The aspect ratio of the CB is, for example, 1.1 times or more and 2.5 times or less, or 1.2 times or more and 2.0 times or less. In this specification, the aspect ratio of the CB means the ratio of the major axis to the minor axis of the primary aggregate. The average particle size of the CB is determined by image analysis using a TEM. The average particle size of the CB is determined by randomly selecting 100 CBs, measuring the diameter of the circumscribed circle of the primary particle, and arithmetically averaging the measured values.

[0044] The CB content is preferably 0.5% by mass or more and 1.5% by mass or less, more preferably 0.6% by mass or more and 1.2% by mass or less, and particularly preferably 0.7% by mass or more and 1.1% by mass or less, relative to the mass of the positive electrode active material. The CB content is preferably greater than the CNT content. When the CB content is greater than the CNT content and the CB content is within this range, the effects of improving cycle characteristics, output characteristics, and high-temperature storage characteristics become more pronounced.

[0045] [Negative Electrode] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer provided on the negative electrode core. The negative electrode core can be a foil of a metal, such as copper or a copper alloy, that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface layer. The negative electrode mixture layer contains a negative electrode active material, a binder, and, if necessary, a conductive agent such as carbon nanotubes, and is preferably provided on both sides of the negative electrode core. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the negative electrode core, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core.

[0046] The negative electrode mixture layer generally contains, as the negative electrode active material, a carbon material that reversibly absorbs and releases lithium ions. Suitable examples of the carbon material include natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). Furthermore, as the negative electrode active material, an active material containing at least one of an element that alloys with Li, such as Si or Sn, and a material containing such an element may be used. A suitable example of the active material is SiO ２ The negative electrode active material may be a silicon-containing material in which silicon particles are dispersed in a silicon phase, a silicate phase such as lithium silicate, or an amorphous carbon phase. Graphite and a silicon-containing material may be used in combination as the negative electrode active material.

[0047] As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer can be a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but it is preferable to use styrene-butadiene rubber (SBR). The negative electrode mixture layer preferably further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. Among these, it is preferable to use SBR in combination with CMC or a salt thereof, or PAA or a salt thereof.

[0048] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0049] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides and phosphate compounds containing metal elements such as Ti, Al, Si, and Mg. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0050] [Non-aqueous electrolyte] The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0051] The liquid electrolyte (electrolytic solution) contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0052] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0053] 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-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane ethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, 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, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0054] The electrolyte salt is preferably a lithium salt. The concentration of the lithium salt is, for example, 0.5 mol or more and 3 mol or less, and preferably 0.8 mol or more and 1.5 mol or less, per 1 L of the non-aqueous solvent. One type of lithium salt may be used alone, or multiple types may be used in combination.

[0055] An example of a lithium salt is 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 (CnF ２ｎ＋１ ) ｘ (1<x<6, n is 1 or 2), LiB １０ Cl １０ , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, Li[B(C ２ O ４ ) ２ ], Li ２ B ４ O ７ , Li(B(C ２ O ４ ) F ２ ), lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ ), LiN(C １ F ２ｌ＋１ SO ２ ) (C ｍ F ２ｍ＋１ SO ２) {l and m are integers of 0 or more}. Among them, LiPF ６ is preferred.

[0056] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As the inorganic solid electrolyte, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0057] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0058] Example 1 Synthesis of Positive Electrode Active Material A composite hydroxide containing Co, Al, Mg, Ti, and Mn obtained by coprecipitation was mixed with lithium hydroxide so that the molar ratio of the total amount of metal elements to Li was 1.0:1.03. The mixture was fired at 850°C for 20 hours under an oxygen stream. The fired product was then washed with water and vacuum dried at 180°C for 2 hours to obtain a lithium-cobalt composite oxide having a layered crystal structure, containing predetermined amounts of Al, Mg, Ti, and Mn, and having a D50 of approximately 26 μm. In Example 1, the lithium-cobalt composite oxide was used as the positive electrode active material. The composition of the positive electrode active material was determined by ICP to be LiCo ０．９８２１ Al ０．０１ Mg ０．００６ Ti ０．０００４ Mn ０．００１５ O ２ Table 1 shows the amounts (mol %) of metal elements other than Li and Co added.

[0059] [Preparation of Positive Electrode] The above positive electrode active material, carbon nanotubes (CNT), carbon black (CB), and polyvinylidene fluoride were mixed in a solids mass ratio of 100:0.2:0.9:1.2, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Multi-walled CNTs with an average fiber diameter of 8 nm and an average fiber length of 15 μm were used for the CNTs, and Ketjen Black with an average particle size of 30 nm was used for the CBs. Next, the positive electrode mixture slurry was applied to a positive electrode core made of aluminum foil, the coating was dried and compressed, and then the positive electrode core was cut to a predetermined electrode size to obtain a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode core. An aluminum lead was welded to the exposed portion of the positive electrode core.

[0060] [Preparation of Negative Electrode] Graphite was used as the negative electrode active material. The negative electrode active material, the sodium salt of CMC, and the dispersion of SBR were mixed in a solid mass ratio of 97.8:1.2:1.2, and water was used as the dispersion medium to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to a negative electrode core made of copper foil, and the coating was dried and rolled. The negative electrode core was then cut to a predetermined electrode size to obtain a negative electrode having a negative electrode mixture layer formed on both sides of the negative electrode core. A nickel lead was welded to the exposed portion of the negative electrode core.

[0061] [Preparation of non-aqueous electrolyte] LiPF 6 was added to a solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70 (at 25°C). ６ was dissolved in the solution to a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte.

[0062] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] The positive electrode and the negative electrode were spirally wound with a separator interposed therebetween, and the wound body was then pressed flat to obtain an electrode assembly. A laminate sheet was processed to form an outer casing, and the electrode assembly and non-aqueous electrolyte were housed inside the outer casing in an inert atmosphere. The pressure inside the outer casing was then reduced to impregnate the separator with the electrolyte solution. The opening of the outer casing was sealed to produce a non-aqueous electrolyte secondary battery measuring 69 mm in length, 56 mm in width, and 4.9 mm in thickness, with a rated capacity of 3150 mAh.

[0063] Examples 2 to 5 Non-aqueous electrolyte secondary batteries were fabricated in the same manner as in Example 1, except that a lithium-cobalt composite oxide having the composition shown in Table 1 was used as the positive electrode active material.

[0064] Examples 6 and 7 Non-aqueous electrolyte secondary batteries were fabricated in the same manner as in Example 2, except that the amounts of CNT and CB added were changed to those shown in Table 1.

[0065] Comparative Example 1 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that CNTs were not added.

[0066] Comparative Example 2 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that CB was not added.

[0067] Comparative Example 3 A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that no CNTs were added.

[0068] Comparative Example 4 A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that CB was not added.

[0069] Comparative Examples 5 to 7 Non-aqueous electrolyte secondary batteries were fabricated in the same manner as in Example 1, except that a lithium-cobalt composite oxide having the composition shown in Table 1 was used as the positive electrode active material.

[0070] The nonaqueous electrolyte secondary batteries of the Examples and Comparative Examples were evaluated for cycle characteristics, discharge load characteristics, and high-temperature charge storage characteristics under the following conditions. The evaluation results are shown in Table 1, along with the composition of the positive electrode active material and the amounts of CNT and CB added.

[0071] [Evaluation of Cycle Characteristics] Each nonaqueous electrolyte secondary battery of the Examples and Comparative Examples was charged at a constant current of 0.7 C in a temperature environment of 23° C. until the battery voltage reached 4.45 V, and then constant-voltage charged at 4.45 V until the current value reached 1 / 50 C. Thereafter, the battery was discharged at a constant current of 0.5 C until the battery voltage reached 3.0 V. This charge / discharge cycle was repeated 500 and 1000 times, and the capacity retention rate after each charge / discharge cycle was calculated according to the following formula: Capacity retention rate = (discharge capacity at the 500th or 1000th cycle / discharge capacity at the 1st cycle) × 100

[0072] [Evaluation of Discharge Load Characteristics] Each of the nonaqueous electrolyte secondary batteries of the Examples and Comparative Examples was charged at a constant current of 0.7 C in a temperature environment of 23° C. until the battery voltage reached 4.45 V, and then constant voltage charged at 4.45 V until the current value reached 1 / 50 C. Thereafter, the batteries were discharged at constant currents of 0.2 C, 1.0 C, and 2.0 C until the battery voltage reached 3.0 V, and the discharge load characteristics were calculated based on the following formulas: Load characteristic (1 C) = (discharge capacity at 1.0 C / discharge capacity at 0.2 C) × 100 Load characteristic (2 C) = (discharge capacity at 2.0 C / discharge capacity at 0.2 C) × 100

[0073] [Evaluation of High-Temperature Charge Storage Characteristics] Each nonaqueous electrolyte secondary battery of the Examples and Comparative Examples was charged at a constant current of 0.7 C in a temperature environment of 23°C until the battery voltage reached 4.45 V, and then constant-voltage charged until the current value at 4.45 V reached 1 / 50 C. The charged battery was then left to stand in a temperature environment of 60°C for 20 days. Discharge was performed at a constant current of 1.0 C-0.2 C until the battery voltage reached 3.0 V, and the remaining capacity was determined. Furthermore, the battery was charged at a constant current of 0.7 C in a temperature environment of 23°C until the battery voltage reached 4.45 V, and then discharged at a constant current of 1.0 C-0.2 C until the battery voltage reached 3.0 V, and the recovery capacity was determined.

[0074]

[0075] As shown in Table 1, all of the batteries of the Examples had higher capacity retention rates in cycle tests and superior cycle characteristics compared to the batteries of the Comparative Examples. The batteries of the Experimental Examples also had superior load characteristics and high-temperature storage characteristics. In other words, the batteries of the Examples were able to achieve high capacity by increasing the end-of-charge voltage while ensuring good charge-discharge cycle characteristics, output characteristics, and thermal stability.

[0076] The evaluation results of Example 1 and Comparative Examples 3 and 4, and Example 2 and Comparative Examples 1 and 2, show that using CNT and CB together with the LCO represented by the general formula above significantly improves cycle characteristics, output characteristics, and thermal stability. Adding Mn in addition to Al, Mg, and Ti further enhances the improvement. The batteries of Examples 3 to 5, in which the amounts of Al, Mg, and Ti added were changed compared to the battery of Example 2, and the batteries of Examples 6 and 7, in which the amounts of CNT and CB added were changed, also exhibited superior cycle characteristics, output characteristics, and thermal stability compared to the batteries of the comparative examples. Furthermore, Example 2 and Comparative Examples 5 to 7 show that even when CNT and CB were used, the battery performance was significantly reduced when a positive electrode active material to which any of Al, Mg, and Ti was not added was used.

[0077] The present disclosure will be further described by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode has a positive electrode core and a positive electrode mixture layer provided on the positive electrode core, the positive electrode mixture layer includes a positive electrode active material mainly composed of a lithium-cobalt composite oxide having a layered crystal structure, carbon nanotubes, and carbon black, and the lithium-cobalt composite oxide has a general formula of LiCo(1-w-x-y-z)Al ｗ Mg ｘ Ti ｙ Mn ｚ O ２ (wherein 0.010≦w≦0.013, 0.003≦x≦0.006, 0.0004≦y≦0.0006, 0≦z≦0.0015). Configuration 2: The nonaqueous electrolyte secondary battery according to Configuration 1, wherein the content of the carbon nanotubes is 0.05% by mass or more and 0.5% by mass or less, relative to the mass of the positive electrode active material. Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the content of the carbon black is greater than the content of the carbon nanotubes and is 0.5% by mass or more and 1.5% by mass or less, relative to the mass of the positive electrode active material. Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the lithium-cobalt composite oxide contains Mn, and the content (z) of Mn in the general formula satisfies 0.0010≦z≦0.0015.

[0078] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode has a positive electrode core and a positive electrode mixture layer provided on the positive electrode core, and the positive electrode mixture layer contains a positive electrode active material mainly composed of a lithium cobalt composite oxide having a layered crystal structure, carbon nanotubes, and carbon black, and the lithium cobalt composite oxide is a composite oxide represented by the general formula LiCo(1−w−x−y−z)Al ｗ Mg ｘ Ti ｙ Mn ｚ O ２ (where 0.010 ≤ w ≤ 0.013, 0.003 ≤ x ≤ 0.006, 0.0004 ≤ y ≤ 0.0006, 0 ≤ z ≤ 0.0015), the non-aqueous electrolyte secondary battery.

2. The content of the carbon nanotubes is 0.05% by mass or more and 0.5% by mass or less based on the mass of the positive electrode active material, and the non-aqueous electrolyte secondary battery according to claim 1.

3. The content of the carbon black is more than the content of the carbon nanotubes and is 0.5% by mass or more and 1.5% by mass or less based on the mass of the positive electrode active material, and the non-aqueous electrolyte secondary battery according to claim 2.

4. The lithium cobalt composite oxide contains Mn, and the content (z) of Mn in the general formula is 0.0010 ≦ z ≦ 0.0015, and the non-aqueous electrolyte secondary battery according to any one of claims 1 to 3.

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