Positive electrode active material for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

A positive electrode active material for a non-aqueous electrolyte secondary battery according to one example of the present embodiment contains, as a main component, a lithium-cobalt composite oxide having a layered crystalline structure. 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), and preferably contains Mn.

Description

Positive electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

[0001] The present disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery using the positive electrode active material.

[0002] Non-aqueous electrolyte secondary batteries such as lithium-ion batteries are widely used as power sources for various electronic devices due to their light weight and high energy density. In recent years, with the increasing demand for longer operating times for electronic devices, there has been a demand for batteries with even higher capacities. Non-aqueous electrolyte secondary batteries are also widely used in automotive applications, power storage applications, and the like. The positive electrode active material, which is a major component of the positive electrode, significantly affects not only the battery capacity but also battery performance, such as charge / discharge cycle characteristics and thermal stability. Therefore, much research has been conducted on the positive electrode active material.

[0003] 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.

[0004] JP 2011-192541 A

[0005] In non-aqueous electrolyte secondary batteries that use lithium cobalt composite oxide (hereinafter sometimes referred to as "LCO") as the positive electrode active material and a carbon material such as graphite as the negative electrode active material, the end-of-charge voltage generally is 4.4 V or less, but higher 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 batteries that use conventional LCO causes the crystal structure of the LCO to collapse, shortening the charge-discharge cycle life and reducing thermal stability.

[0006] The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure is a positive electrode active material for a non-aqueous electrolyte secondary battery containing, as a main component, a lithium-cobalt composite oxide having a layered crystal structure, 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).

[0007] A non-aqueous electrolyte secondary battery according to the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the positive electrode contains the above-described positive electrode active material.

[0008] The positive electrode active material according to the present disclosure can increase the capacity of a non-aqueous electrolyte secondary battery while improving the cycle characteristics and thermal stability.

[0009] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment;

[0010] As described above, in a non-aqueous electrolyte secondary battery using an LCO-based composite oxide as a positive electrode active material, it is an important issue to increase the end-of-charge voltage and thereby achieve a high capacity while ensuring good cycle characteristics and thermal stability. As a result of intensive research to solve this issue, 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) is used, it has been found that excellent cycle characteristics and thermal stability can be ensured even when the end-of-charge voltage of the battery is increased.

[0011] Furthermore, as a result of the inventors' investigations, it has become clear that by adding a predetermined amount of Mn to the above composite oxide, the effect of improving cycle characteristics becomes more pronounced and low-temperature discharge characteristics are greatly improved.

[0012] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure and a non-aqueous electrolyte secondary battery using the positive electrode active material will be described in detail. Note that configurations obtained by selectively combining the respective components of the multiple embodiments and modifications described below are included within the scope of the present disclosure.

[0013] 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 (laminated 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.

[0014] 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.

[0015] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, 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 of these solvents are substituted with halogen atoms such as fluorine. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and mixed solvents thereof. Examples of the electrolyte salt include LiPF ６ The non-aqueous electrolyte is not limited to a liquid electrolyte, but may be a solid electrolyte.

[0016] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14 are all strip-shaped, long 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 deposition. That is, the negative electrode 12 is formed to be longer in the longitudinal direction and width direction (short direction) than the positive electrode 11. 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14, and in particular the positive electrode active material that constitutes the positive electrode 11, will be described in detail below.

[0021] [Positive Electrode] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer formed 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 active material is primarily composed of a lithium-cobalt composite oxide having a layered crystal structure. 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.

[0022] 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 salts thereof, and polyethylene oxide (PEO). The content of the binder is, for example, 0.5 to 5% by mass relative to the mass of the positive electrode mixture layer.

[0023] The conductive agent contained in the positive electrode mixture layer 31 is preferably a carbon material such as carbon black, such as acetylene black or ketjen black, graphite, or carbon nanotubes. The content of the conductive agent is, for example, 0.1 to 10 mass % relative to the mass of the positive electrode mixture layer.

[0024] As described above, the positive electrode active material is mainly composed of a lithium cobalt composite oxide having a layered crystal structure. The lithium cobalt composite oxide (hereinafter referred to as "LCO(A)") 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 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). LCO(A) 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(A) is, for example, 97.8 mol % or more based on the total number of moles of elements excluding Li and O.

[0025] 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(A) 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(A) alone. The composition of the composite oxide can be measured using an ICP optical emission spectrometer (iCAP6300, manufactured by Thermo Fisher Scientific).

[0026] The positive electrode mixture layer may contain a lithium-containing composite oxide other than LCO(A) as a positive electrode active material, as long as the object of the present disclosure is not impaired. An example of a lithium-containing composite oxide other than LCO(A) is a lithium-cobalt composite oxide that is substantially free of Al, Mg, and Ti and has the general formula LiCoO ２ When LCO(B) is used in combination, the mass ratio of LCO(A) to LCO(B) is, for example, 60:40 to 95:5, and preferably 70:30 to 90:10.

[0027] As described above, LCO(A) 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, particularly 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).

[0028] The volume-based median diameter (D50) of LCO(A) is, for example, 5 to 50 μm, preferably 10 to 40 μm, and more preferably 15 to 30 μm. LCO(A) is, for example, a secondary particle formed by aggregation of primary particles, and the D50 of LCO(A) refers to the D50 of the secondary particles. D50 refers to the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the complex oxide can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrac-Bell Corporation) using water as the dispersion medium.

[0029] The D50 of LCO(B) is preferably smaller than the D50 of LCO(A), for example, 1 to 15 μm. By using two or more composite oxides with different D50s as the positive electrode active material, the particle packing density is improved, and a higher battery capacity can be achieved. When LCO(A) and LCO(B) are used in combination, by making the D50 of LCO(A) greater than the D50 of LCO(B), the battery capacity can be increased while improving the cycle characteristics and thermal stability.

[0030] The Al content in LCO(A) is 1.0 to 1.3 mol%, more preferably 1.1 to 1.2 mol%, 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%, the effect of improving the cycle characteristics and thermal stability cannot be obtained. Furthermore, if the amount of Al added is more than 1.3 mol%, the effect of improving the cycle characteristics and thermal stability cannot be obtained, and the capacity decrease becomes significant. Note that even if the amount of Al added is within the above range, if Mg and Ti are not added, the above-mentioned improvement effect cannot be obtained.

[0031] The Mg content in LCO(A) is 0.3 to 0.6 mol% 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 also 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 thermal stability cannot be obtained, and if the amount of Mg added is too large, the capacity also decreases significantly.

[0032] The Ti content in LCO(A) is 0.04 to 0.06 mol% 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 believed that the material resistance can be reduced, and cycle characteristics in particular are improved. If the amount of Ti added is less than 0.04 mol% or more than 0.06 mol%, the effect of improving cycle characteristics cannot be obtained, and if the amount of Ti added is too large, the capacity also decreases significantly.

[0033] LCO (A) preferably further contains Mn, and the Mn content (z) in the general formula is 0.0010≦z≦0.0015. It is believed that if the amount of Mn added is 0.10 to 0.15 mol% relative to the total number of moles of elements excluding Li and O, the material resistance can be reduced, and the effects of improving the battery's cycle characteristics and thermal stability become more pronounced, 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 low-temperature discharge characteristics may be adversely affected, and capacity may decrease significantly.

[0034] LCO (A) 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 calcining the mixture; and (3) a third step of washing the calcined product with water and drying it.

[0035] 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.

[0036] 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 mixture in the second step is fired, for example, in an oxygen stream at a temperature of 500 to 900° C. The firing temperature is preferably 600 to 800° C. The firing may be carried out in two or more stages with different firing conditions.

[0037] 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 LCO(A) 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 to 250°C.

[0038] [Negative Electrode] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer formed 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.

[0039] 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.

[0040] 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.

[0041] [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.

[0042] 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.

[0043] The present disclosure will be further explained below with reference to experimental examples, but the present disclosure is not limited to these experimental examples.

[0044] Experimental Example 1 Synthesis of Cathode Active Material A composite hydroxide containing Co, Al, Mg, and Ti 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 composite oxide (hereinafter referred to as "cathode active material (A)") having a layered crystal structure, containing predetermined amounts of Al, Mg, and Ti, and having a D50 of approximately 26 μm. The composition of the cathode active material (A) was determined by ICP to be LiCo. ０．９８ Al ０．０１ Mg ０．００６ Ti ０．０００４ O ２ It was confirmed that this is the case.

[0045] LiCoO having a D50 of about 6 μm was prepared by the same method as for the positive electrode active material (A). ２ (Hereinafter referred to as "positive electrode active material (B)") was obtained. The positive electrode active materials (A) and (B) were mixed in a mass ratio of 80:20 to obtain a positive electrode active material.

[0046] [Preparation of Positive Electrode] The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solids mass ratio of 97.6:1.2:1.2, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. 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.

[0047] [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.

[0048] [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.

[0049] [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 solution 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, and 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, 4.9 mm in thickness, and 3150 mAh in rated capacity.

[0050] The nonaqueous electrolyte secondary battery was subjected to a cycle test, a thermal test, and a low-temperature discharge test under the following conditions. Each test was performed on two batteries, and the measured values ​​were averaged to obtain the evaluation results. The evaluation results are shown in Table 1, along with the composition of the positive electrode active material (A).

[0051] [Cycle Test] The nonaqueous electrolyte secondary battery was charged at a constant current of 2205 mA in a room temperature (25° C.) environment until the battery voltage reached 4.45 V, and then discharged at a constant current of 1575 mA until the battery voltage reached 3.0 V. This charge / discharge cycle was repeated 500 times, and the ratio of the 500th discharge capacity to the first discharge capacity was calculated as the capacity retention rate.

[0052] [Thermal Test] The nonaqueous electrolyte secondary battery described above was charged until the battery voltage reached 4.50 V, and then placed in a thermostatic chamber. The temperature was raised from room temperature to 130°C at a rate of 5°C / min. The battery voltage, battery surface temperature, and thermostatic chamber temperature were measured during the test, and the state of the battery after the test was confirmed.

[0053] [Low-Temperature Discharge Test] The nonaqueous electrolyte secondary battery was charged at a constant current of 2205 mA in a −20° C. environment until the battery voltage reached 4.45 V, and then discharged at a constant current of 1575 mA until the battery voltage reached 3.0 V. The discharge capacity at this time (−20° C. capacity) was determined, and the ratio (−20° C. capacity / 25° C. capacity) to the discharge capacity in a room temperature environment (25° C. capacity) was calculated.

[0054] Experimental Examples 2 to 14 Non-aqueous electrolyte secondary batteries were fabricated in the same manner as in Experimental Example 1, except that composite oxides having the compositions shown in Table 1 were used as the positive electrode active material (A), and the above-mentioned tests were carried out.

[0055]

[0056] As shown in Table 1, the batteries of Experimental Examples 1 to 6 all had higher capacity retention rates in cycle tests and superior cycle characteristics compared to the batteries of Experimental Examples 7 to 13. Furthermore, the batteries of Experimental Examples 1 to 6 all had small temperature rises in thermal tests and superior thermal stability. The battery of Experimental Example 14 had good cycle characteristics, but temperature rises were observed in the thermal test. Furthermore, when a predetermined amount of Mn was contained in addition to predetermined amounts of Al, Mg, and Ti (see Experimental Examples 3 to 6), the improvement in cycle characteristics became more pronounced, and low-temperature discharge characteristics were also significantly improved (see Experimental Examples 1, 3, and 6 in particular).

[0057] A comparison of Experimental Example 1 and Experimental Example 7 shows that the addition of a predetermined amount of Al significantly improves the cycle characteristics and thermal stability. Furthermore, a comparison of Experimental Example 1 and Experimental Example 8 shows that the addition of a predetermined amount of Mg significantly improves the cycle characteristics and thermal stability. The addition of predetermined amounts of Al and Mg is believed to stabilize the crystal structure of the LCO. A comparison of Experimental Example 1 and Experimental Example 9 shows that the addition of a predetermined amount of Ti significantly improves the cycle characteristics. The addition of a predetermined amount of Ti is believed to reduce the material resistance of the composite oxide, resulting in improved cycle characteristics.

[0058] 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 positive electrode active material for a non-aqueous electrolyte secondary battery, comprising as a main component a lithium-cobalt composite oxide having a layered crystal structure, The lithium cobalt composite oxide has the general formula LiCo （１－ｗ－ｘ－ｙ－ｚ） A 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, and 0≦z≦0.0015).

2. 2. The positive electrode active material according to claim 1, wherein the lithium-cobalt composite oxide contains Mn, and the Mn content (z) in the general formula satisfies 0.0010≦z≦0.0015.

3. The lithium-cobalt composite oxide is a mixture of a first composite oxide represented by the general formula and a lithium-cobalt composite oxide represented by the general formula LiCoO ２ and a second composite oxide represented by The positive electrode active material according to claim 1 , wherein the content of the first composite oxide is 60 mass % or more of the mass of the positive electrode active material.

4. A positive electrode, a negative electrode, and a non-aqueous electrolyte, The positive electrode of a non-aqueous electrolyte secondary battery comprises the positive electrode active material according to any one of claims 1 to 3.