JP6101583B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a positive electrode material for a non-aqueous electrolyte secondary battery using a lithium composite oxide as a positive electrode active material, and a non-aqueous electrolyte secondary battery.

  For example, in Patent Document 1, for the purpose of increasing the voltage resistance of a lithium ion secondary battery, the positive electrode active material includes lithium cobalt oxide to which at least one of Mg, Al, Ti, and Zr is added. By adding lithium phosphate to the substance, the reaction between lithium cobaltate and non-aqueous electrolyte is suppressed, and the high-temperature storage characteristics and cycle characteristics of batteries with a positive electrode charge potential of 4.3 V or more on the lithium basis are improved. It has been shown to be possible.

  Further, in Patent Document 2, by providing a coating layer similar to magnesium oxide by mechanochemical treatment on the surface of lithium cobalt oxide, it is possible to achieve both high capacity and battery characteristics with less cycle deterioration and gas generation. It is shown.

JP 2005-71641 A JP 2011-138718

  As shown in Patent Document 1, when lithium phosphate is added to the positive electrode active material, lithium phosphate and hydrofluoric acid react to form phosphoric acid, lithium fluoride, and the like. The concentration of hydrofluoric acid is reduced, thereby suppressing adverse effects on the positive electrode active material and the positive electrode binder. Moreover, since the crystal structure stability is improved by adding Mg, Al, Ti, and Zr, a nonaqueous electrolyte secondary battery excellent in high-temperature heat retention characteristics and cycle characteristics can be obtained. However, when the charge potential of the positive electrode is increased to 4.3 V or higher on the basis of lithium, the effect of suppressing the decomposition of the non-aqueous electrolyte is insufficient. For example, when stored at a high temperature in a 4.5 V continuous charge state on the basis of lithium, As a result, cobalt is eluted and deposited on the negative electrode surface, resulting in a high resistance.

  The elution of cobalt from lithium cobaltate is triggered by the occurrence of a disproportionation reaction due to the nucleophilic reaction by hydrogen fluoride decomposed from the electrolyte at the interface between the non-aqueous electrolyte and lithium cobaltate. In addition to this disproportionation reaction with hydrofluoric acid, a nucleophilic electrophilic reaction with a nonaqueous electrolytic solution causes the divalent cobalt ions to elute together with the decomposition of the electrolytic solution. Therefore, the chemical state of the interface in contact with the non-aqueous electrolyte is extremely important as well as the crystal structure stability of lithium cobalt oxide being high.

  As a solution to the above problem, there is a method in which the surface of the positive electrode is coated with a metal oxide as shown in Patent Document 2, but since the coating inhibits the diffusion of lithium ions, the charge / discharge current value is There is a problem that a sufficient capacity cannot be obtained as the size increases. Thus, by covering the surface of the positive electrode active material, the cycle and storage characteristics can be improved, but on the other hand, the battery capacity tends to decrease. Moreover, there is still room for improvement in the battery characteristics at high voltage of the battery using the positive electrode active material obtained by the above method.

  Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art, and uses a positive electrode of a nonaqueous electrolyte secondary battery whose positive electrode active material surface is coated with a metal oxide. Even when the charge / discharge current value in the practical range is sufficient, sufficient capacity is obtained, and further, the electrolyte solution decomposition suppression effect is excellent when charged until the positive electrode potential is 4.3 V or more on the basis of lithium, It aims at providing the nonaqueous electrolyte secondary battery which suppressed the elution of cobalt at the time of continuous charge under high temperature.

  In order to solve the above problems, for example, the configuration described in the claims is adopted.

  The present invention includes a plurality of means for solving the above-described problems. For example, the present invention includes a positive electrode having lithium cobaltate as a positive electrode active material, a negative electrode having a negative electrode active material, and a nonaqueous electrolyte. In the non-aqueous electrolyte secondary battery, the positive electrode has a part of the surface of the lithium cobalt oxide coated with a metal oxide, a part of the surface is exposed without being coated with the metal oxide, and a phosphorous Lithium phosphate is contained, and the lithium phosphate is dispersed over the entire surface of the lithium cobaltate.

  According to the non-aqueous electrolyte secondary battery of the present invention, a sufficient capacity can be obtained with the charge / discharge current value in the practical range, and further, the electrolyte solution when charged until the positive electrode potential is 4.3 V or higher on the basis of lithium. It is excellent in the effect of inhibiting decomposition and can suppress the elution of cobalt during continuous charging at high temperature. Problems, configurations, and effects other than those described above will be clarified by the following description of the embodiments.

It is an example of the block diagram of the battery assembled in the Example. It is an example of the structure of the surface of the positive electrode active material produced in Example 1

  Hereinafter, the present invention will be described in more detail on the basis of examples. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications without departing from the scope of the present invention. It is.

A conventional lithium ion secondary battery uses lithium cobaltate (LiCoO ₂ ) as a positive electrode. LiCoO _{2 has} a theoretical capacity of 274 mAh / g, but cannot use all lithium ions, and is usually used at a voltage of about 4.2 V and a capacity of about 160 mAh / g. That is, only about 60% of lithium ions in the positive electrode were utilized.

  Lithium ion secondary batteries can provide high capacity when charged and discharged at a voltage exceeding 4.2 V, but due to structural instability of the positive electrode material at high voltage and reaction of the electrolyte, life characteristics and reliability , Safety is reduced. In particular, when the charging voltage is higher than 4.3 V on the basis of lithium, the non-aqueous electrolyte solution is easily decomposed, cobalt elution and gas generation occur due to high temperature storage and continuous charging, and the battery life is extremely deteriorated. There was a problem.

  The nonaqueous electrolyte secondary battery in the present embodiment is a nonaqueous electrolyte secondary battery comprising a positive electrode having lithium cobaltate as a positive electrode active material, a negative electrode having a negative electrode active material, and a nonaqueous electrolyte, In the positive electrode active material, a part of the surface of lithium cobalt oxide is coated with a metal oxide, and a part of the surface is exposed without being coated with the metal oxide, and the surface of the positive electrode active material is compared with lithium cobalt oxide particles. Thus, a sufficiently small lithium phosphate particle is dispersed over the entire surface. In particular, a part of lithium cobaltate is exposed without being covered with a metal oxide, and lithium phosphate is arranged in the exposed part.

Lithium cobaltate is described by the formula _{Li 1 + x Co 1-y} M y O 2, the M Mg, Al, Ti, W , Mo, Ni, Mn, Zr, V, at least one of Fe Included, and 0 ≦ x ≦ 0.3 and 0 ≦ y ≦ 0.02. In particular, it is preferable to add Ni because the effect of improving the life can be obtained while suppressing the initial capacity reduction. The lithium cobalt oxide has a substantially spherical particle shape, and the average particle diameter is obtained by measuring the particle diameter with an SEM and calculating the average of particles present in a predetermined region. The lithium cobalt oxide preferably has an average particle size of 5 μm or more and 15 μm or less.

  The metal oxide includes at least one of Zr, Al, Li, Ti, Ni, Mn, La, Zn, P, and B. In particular, Zr oxide and Al oxide are preferable because they are stable in an acidic solvent. The coverage of the surface of lithium cobaltate with the metal oxide is 10% or more and 80% or less, and preferably 40% or more and 60% or less. The coverage is the ratio of the entire surface of the lithium cobalt oxide that is coated with the metal oxide, and is defined by the amount of coating (mass%) decreased in the step after the coating treatment. And the coating amount of a metal oxide is 0.1 mass% or more and 0.5 mass% or less.

Lithium phosphate (Li ₃ PO ₄ ) is composed of nanoparticles having an average particle diameter of 10 nm to 100 nm. The weight ratio contained in the positive electrode of lithium phosphate is 0.1 or more and 2.5% by mass or less with respect to lithium cobaltate. The weight ratio of 0.1% by mass is the lowest amount that can be achieved by the addition of lithium phosphate, and the weight ratio of 2.5% by mass is the upper limit within a range that satisfies the battery capacity that can withstand actual specifications. .

  The positive electrode manufacturing method described above includes the first step of coating the surface of lithium cobalt oxide with a metal oxide, and mixing the lithium cobalt oxide with lithium phosphate to exfoliate part of the metal oxide, A second step of disposing lithium phosphate on the exposed portion of the substrate. In the second step, the lithium cobaltate and lithium phosphate are pressed and mixed. In addition, you may mix lithium cobaltate and lithium phosphate which were metal-coated by the spray-drying method.

  According to the nonaqueous electrolyte secondary battery in the present embodiment, since the coating made of a specific metal oxide is formed on a part of the surface of the positive electrode, the nonaqueous electrolyte solution directly touches lithium cobalt oxide. This prevents the electrolyte from decomposing when charged until the positive electrode potential is 4.3 V or higher with respect to lithium. In addition, the concentration of hydrofluoric acid in the battery is reduced by including lithium phosphate nanoparticles.

  The combination of the partial coating on the surface of the positive electrode active material and the lithium phosphate nanoparticles makes it difficult for lithium cobaltate to react with the electrolytic solution, and the decomposition of the electrolytic solution and the elution of cobalt on the surface of the positive electrode are greatly suppressed. Furthermore, by partially forming the metal oxide film, the capacity at a practical charge / discharge current value is ensured without inhibiting the diffusion of lithium ions. If the entire surface of the lithium cobalt oxide is covered with an oxide, the resistance increases, which is not preferable.

Batteries for mobile devices, especially mobile phones, have a large capacity per unit volume and are required to maintain a high voltage state in any state of charge. A lithium ion secondary that satisfies such specifications is required. Lithium cobaltate (LiCoO ₂ ) is used as the positive electrode active material of the battery. Deterioration due to elution of metal ions from the positive electrode, which is an object of the present invention, is a phenomenon represented by lithium cobaltate as long as the above specifications (high capacity and high voltage) are satisfied.

  Ensuring that the surface of lithium cobaltate is 100% coated with metal oxide leads to higher costs. According to the nonaqueous electrolyte secondary battery of the present embodiment, even when the coating with the metal oxide is not complete, the exposed portion of the surface of the lithium cobaltate is covered with the lithium phosphate nanoparticles, so that in the battery It is possible to suppress damage caused by the attack of the generated (hydrogen fluoride) HF. Therefore, it is not necessary to guarantee 100% coating with metal oxide, leading to cost reduction.

[Example 1]
[Synthesis of positive electrode active material]
After heat treatment at 850 ° C. for 40 hours in an air atmosphere using Li ₂ CO ₃ and Co ₃ O ₄ , the mixture was pulverized in a mortar to obtain lithium cobaltate having a particle diameter of 5 to 10 μm, Li _1.03 CoO ₂ .

Next, Li _1.03 CoO ₂ as a positive electrode active material and Zr (OC ₃ H ₇ ) ₄ were stirred in a 2-propanol solvent, dried at 80 ° C., and heated at 400 ° C. for 10 hours to obtain Zr. A positive electrode active material coated with about 0.1% by mass of oxide (ZrO _x ) was obtained. The coating amount (adhesion amount) of ZrO _x was measured by an ICP (Inductivity Coupled Plasma) method. Lithium phosphate (Li ₃ PO ₄ ) having an average particle size of 50 nm was added to and mixed with this ZrO _x -coated LiCoO ₂ at a mass ratio of 99.5: 0.5.

In addition, the mixture of Li ₃ PO ₄ may be added at the time of preparing the positive electrode slurry, which has been previously ultrasonically dispersed in N-methyl-2-pyrrolidone (NMP).

A similar structure can also be obtained by mixing a metal-coated positive electrode active material and lithium phosphate by spray drying. After stirring and mixing ammonium phosphate and lithium hydroxide hydrate in ultrapure water for 1 hour so as to have a weight of 1: 1, lithium phosphate nanoparticles are prepared, and then a metal-coated positive electrode active material is added thereto. The mixture was added and further stirred. At this time, it adjusted with lithium hydroxide or ammonia water so that pH in water might be kept at 10-11. After the metal-coated positive electrode active material was added, the mixture was further stirred and mixed for 1 hour, and then dried by spray drying to obtain a structure in which lithium phosphate adhered to the surface of the metal-coated positive electrode active material. Finally, by drying in vacuum at 120 ° C. for 2 hours, a ZrO _x coated-Li ₃ PO ₄ mixed positive electrode active material is obtained.

  Note that heat treatment at a temperature of 500 ° C. or higher is not preferable because the coating material and the lithium cobalt oxide particles are partly dissolved and charge / discharge characteristics are deteriorated. This is because, when a coating layer with high crystallinity partially dissolved on the surface of lithium cobaltate is formed, it is difficult to remove a part of the coating layer by a mixing process as described later. If the coating layer made of an oxide has no defect, the resistance increases and the charge / discharge characteristics decrease.

[Production of positive electrode]
ZrO _x coating-Li ₃ PO ₄ mixed positive electrode active material, lithium phosphate, acetylene black as a conductive additive, and polyvinylidene fluoride as a binder are mixed at a weight ratio of 95: 2.5: 2.5 Then, a slurry was prepared using NMP as a solvent. It is possible to increase the density by mixing using a mortar while gradually reducing the solid content ratio.

As shown in FIG. 2, part of the ZrO _x coating (metal oxide 22) is peeled off on the surface of the positive electrode active material during the mixing process, and lithium phosphate (Li ₃ PO ₄ ) is formed around the peeled surface together with a conductive auxiliary agent. The structure is protected by 23 nanoparticles. Since the lithium phosphate fine particles are not covered with the oxide or disposed at the location where the coating is peeled off when the electrode is produced, an excellent deterioration suppressing effect is obtained. In addition, ZrOx is formed on the surface of LiCoO ₂ as shown in FIG. 2, part of the ZrOx is not formed, LiCoO ₂ is exposed, and lithium phosphate is dispersed in both the covering part and the exposed part. Is observed by spectroscopy using an electron microscope. The coverage of the surface of the positive electrode active material by ZrO _x coating is 50% because the amount of coating was reduced to about 0.05% by mass when the positive electrode active material was extracted after IC preparation and analyzed by ICP. The coating amount can be measured not only by the above-described comparison by ICP analysis but also by observation using ToF-SIMS or an electron microscope.

  The positive electrode mixture slurry thus obtained was applied to an aluminum current collector foil having a gap of 200 μm and a thickness of 15 μm using a Baker type applicator, and dried at 80 ° C. for 1 hour. Next, the positive electrode formed into a disk shape of 15 mmφ was pressed at a pressure of about 30 MPa, and then dried at 100 ° C. for 20 hours in a vacuum dryer.

[Production of negative electrode]
A metal lithium rolled sheet having a predetermined thickness was punched into a disk shape having a diameter of 16 mm to produce a negative electrode.

[Non-aqueous electrolyte]
A nonaqueous electrolytic solution was prepared by dissolving 1 mL of lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 2.

[Battery assembly]
A flat battery was assembled using the positive and negative electrodes and the non-aqueous electrolyte. In addition, as a separator, the microporous film made from a polypropylene was used, and this was impregnated with the previous non-aqueous electrolyte.

  FIG. 1 is a cross-sectional view schematically showing a produced nonaqueous electrolyte secondary battery of the present invention.

  In the flat type nonaqueous electrolyte secondary battery 1 of the present invention, the insulating ring 8 is inserted into the side surface of the SUS container 13 and laminated in the order of the negative electrode 4, the separator 3, and the positive electrode 2, and then the nonaqueous electrolyte is applied to the separator 3. After impregnating and placing the aluminum pressing plate 5 and the leaf spring 6 in this order on the positive electrode 2, the SUS lid 7 on which the insulating sleeve 10 is installed is tightened with the bolt 12 and the nut 11 via the insulating packing 9. Assembled by.

  The positive electrode 2 is connected to the SUS lid 7 via the aluminum holding plate 5 and the leaf spring 6, and the negative electrode 4 is connected to the bolt 12 via the SUS container 13, and the chemical energy generated in the battery is SUS covered. 7 and the bolt 12 can be taken out from both terminals as electric energy.

[Example 2]
A positive electrode active material was produced in the same manner as in Example 1 except that Li ₃ PO ₄ having an average particle diameter of 50 nm was mixed at a mass ratio of 99: 1. A nonaqueous electrolyte secondary battery according to Example 2 was produced in the same manner as in Example 1 except that the positive electrode active material was changed.

[Example 3]
A positive electrode active material was produced in the same manner as in Example 1 except that Li ₃ PO ₄ having an average particle diameter of 50 nm was mixed at a mass ratio of 97.5: 2.5. A nonaqueous electrolyte secondary battery according to Example 3 was produced in the same manner as in Example 1 except that the positive electrode active material was changed.

[Example 4]
A positive electrode active material was produced in the same manner as in Example 1 except that the coating amount of ZrO _x was about 0.5% by mass. A nonaqueous electrolyte secondary battery according to Example 4 was produced in the same manner as in Example 1 except that the positive electrode active material was changed.

[Example 5]
A positive electrode active material was produced in the same manner as in Example 1 except that Al oxide (Al ₂ O ₃ ) was coated at about 0.1% by mass instead of ZrO _x . The Al ₂ O ₃ coating was prepared by stirring LiCoO ₂ and Al (OOC ₈ H ₁₅ ) (OC ₃ H ₇ ) ₂ in a 2-propanol solvent, followed by drying at 80 ° C. and 10 ° C. at 400 ° C. LiCoO ₂ coated with about 0.1% by mass of Al ₂ O ₃ was obtained by heating for a period of time. In addition, the adhesion amount of Al ₂ O ₃ was measured by an ICP (Inductivity Coupled Plasma) method. A nonaqueous electrolyte secondary battery according to Example 5 was produced in the same manner as in Example 1 except that the positive electrode active material was changed.

[Example 6]
A positive electrode active material was produced in the same manner as in Example 5 except that Li ₃ PO ₄ having an average particle diameter of 50 nm was mixed at a mass ratio of 99: 1. A nonaqueous electrolyte secondary battery according to Example 6 was produced in the same manner as in Example 1 except that the positive electrode active material was changed.

[Example 7]
A positive electrode active material was produced in the same manner as in Example 5 except that Al ₂ O ₃ was coated at about 0.5% by mass. A nonaqueous electrolyte secondary battery according to Example 7 was produced in the same manner as in Example 1 except that the positive electrode active material was changed.

[Example 8]
Instead of Li _1.03 CoO _2, except that the application of the Li _1.03 Co _0.99 Mg _0.01 O ₂ was prepared the positive electrode active material in the same manner as in Example 2. Li _1.03 Co _0.99 Mg _0.01 O ₂ was prepared by heat treatment at 850 ° C. for 40 hours in an air atmosphere using Li ₂ CO ₃ , Co ₃ O ₄ , and MgO, and then pulverizing in a mortar to obtain a particle size of 5 10 μm of Li _1.03 Co _0.99 Mg _0.01 O ₂ was obtained. The amount of Mg added was measured by an ICP (Inductivity Coupled Plasma) method. A nonaqueous electrolyte secondary battery according to Example 8 was produced in the same manner as in Example 1 except that the positive electrode active material was changed.

[Example 9]
Instead of Li _1.03 CoO _2, except that the application of the _{Li 1.03 Co 0.99 Al 0.005 Mg 0.005} O 2 was prepared the positive electrode active material in the same manner as in Example 2. Li _1.03 Co _0.99 Mg _0.01 O ₂ was prepared by heat treatment at 850 ° C. for 40 hours in an air atmosphere using Li ₂ CO ₃ , Co ₃ O ₄ , MgO, Al ₂ O ₃ and then pulverizing in a mortar. Thus, Li _1.03 Co _0.99 Mg _0.01 O ₂ having a particle diameter of 5 to 10 μm was obtained. The addition amount of Mg and Al was measured by an ICP (Inductivity Coupled Plasma) method. A nonaqueous electrolyte secondary battery according to Example 9 was produced in the same manner as in Example 1 except that the positive electrode active material was changed.

[Example 10]
Instead of Li _1.03 CoO _2, except that the application of the _{Li 1.03 Co 0.98 Al 0.01 Mg 0.01} O 2 was prepared the positive electrode active material in the same manner as in Example 2. Li _1.03 Co _0.99 Mg _0.01 O ₂ was prepared by heat treatment at 850 ° C. for 40 hours in an air atmosphere using Li ₂ CO ₃ , Co ₃ O ₄ , MgO, Al ₂ O ₃ and then pulverizing in a mortar. Thus, Li _1.03 Co _0.99 Mg _0.01 O ₂ having a particle diameter of 5 to 10 μm was obtained. The addition amount of Mg and Al was measured by an ICP (Inductivity Coupled Plasma) method. A nonaqueous electrolyte secondary battery according to Example 9 was produced in the same manner as in Example 1 except that the positive electrode active material was changed.

[Comparative Example 1]
Comparative Example 1 was carried out in the same manner as in Example 1 except that the positive electrode active material Li _1.03 CoO ₂ which was not coated with a metal oxide and did not contain lithium phosphate and was obtained as described above was used. A non-aqueous electrolyte secondary battery was produced.

[Comparative Example 2]
Except for using a positive electrode active material Li _1.03 CoO ₂ that was not coated with a metal oxide and contained lithium phosphate at a mass ratio of 97.5: 2.5 with respect to the positive electrode active material, the same as in Example 1 above. A nonaqueous electrolyte secondary battery according to Comparative Example 2 was produced.

[Comparative Example 3]
Example 10 is the same as Example 10 except that the positive electrode active material Li _1.03 Co _0.98 Al _0.01 Mg _0.01 O ₂ which was not coated with a metal oxide and did not contain lithium phosphate and was obtained as described above was used. Similarly, a nonaqueous electrolyte secondary battery according to Comparative Example 3 was produced.

[Battery evaluation]
About each battery of Examples 1-10 and Comparative Examples 1-3 produced as mentioned above, the capacity | capacitance confirmation test and the continuous charge test were done on condition of the following. The results are shown in Table 1 below.

[Capacity confirmation test]
-Charging: Constant current charging was performed at a current of 0.1 CA until the battery voltage reached 4.45 V, and then charging was performed at a constant voltage of 4.45 V until a current of 0.01 CA.
Discharge: Constant current discharge was performed at a current of 0.1 CA until the battery voltage reached 3.00V.
-Pause: The pause interval between charging and discharging was 1 hour.

[60 ° C continuous charge test]
After measuring the discharge capacity before the continuous charge test at room temperature (25 ° C.) by the above-mentioned method, the battery is left in a constant temperature bath at 60 ° C. for 30 minutes and then kept at 60 ° C. with a constant current of 0.1 CA. The battery was charged until the battery voltage reached 4.5 V, and further charged continuously at a constant voltage of 4.5 V until the current value rose to 0.1 CA again after decaying, and the time was measured.

From the results shown in Table 1, the following can be understood. That is, the batteries of Examples 1 to 10 according to the present invention show equivalent discharge capacities as compared with the batteries of Comparative Examples 1 to 3, and Comparative Example 1 in which the surface is not coated with a metal oxide or metal oxide Compared to Comparative Example 2 in which nanoparticles of lithium phosphate (Li ₃ PO ₄ ) were mixed without coating the material, and Comparative Example 3 in which a part of Co was replaced with Mg and Al, 60 ° C., 4 ° C. The leakage current generation time (endurance time) at the time of continuous charge of 5 V is extended.

  In Comparative Example 1, since the surface of the positive electrode active material does not include a metal oxide coating, decomposition of the non-aqueous electrolyte on the active material surface under high temperature and high voltage is promoted, and Co elution cannot be suppressed. The continuous charge endurance time is shortened.

Further, in Comparative Example 2, the surface of the positive electrode active material does not include a metal oxide coating, but the concentration of hydrofluoric acid in the battery is reduced because the positive electrode active material mixed with 2.5% by mass of Li ₃ PO ₄ nanoparticles is used. doing. Thereby, decomposition of the electrolyte and elution of Co are suppressed, and the durability time of continuous charging is longer than that of Comparative Example 1, but the nanoparticles of lithium phosphate (Li ₃ PO ₄ ) do not contribute to charging and discharging. The amount of lithium phosphate (Li ₃ PO ₄ ) nanoparticles mixed is reduced to 0.5% by mass, and the discharge capacity is small compared to the case where the positive electrode surface is coated with ZrO ₂ , and the continuous charge durability time is short. It has become.

  In addition, when a part of Co shown in Comparative Example 3 was replaced with Mg and Al, the effect of improvement was slight. Although the crystal structure is stabilized by substitution of Mg and Al, it can be said that under a high voltage of 4.5 V, it was insufficient to suppress decomposition of the nonaqueous electrolyte and Co elution at the interface. Therefore, it was confirmed that the effects were exhibited for the first time in the combinations as shown in Examples 1 to 10.

  Although the details that the results as shown in Examples 1 to 10 are obtained are not clear, the concentration of hydrogen fluoride that triggers the dissolution of Co when phosphorus is continuously charged at a high voltage of 4.3 V or higher is set to phosphorus. The endurance time of continuous charging is improved by the synergistic effect of the effect of reducing by lithium oxide nanoparticles and the effect of reducing the area where lithium cobaltate directly touches the non-aqueous electrolyte by coating the surface of the positive electrode active material with metal oxide. It is thought. Moreover, it is thought that the cause of the large capacity reduction is due to the effect of partially covering a metal oxide having no lithium ion conductivity.

  As described above, according to the non-aqueous electrolyte secondary battery of the present invention, the effect of suppressing the decomposition of the electrolytic solution when charged until the positive electrode potential is 4.3 V or higher with respect to lithium is excellent. In addition, by mixing lithium phosphate nanoparticles, the concentration of hydrofluoric acid in the battery is reduced, making it difficult for lithium cobaltate to react with the electrolyte solution. Elution is suppressed. It was found that by combining the partial coating of the metal oxide film and the lithium phosphate nanoparticles, the capacity at a practical charge / discharge current value was ensured without significantly inhibiting the diffusion of lithium ions.

  In Examples 1-10 above, examples of Zr and Al oxide coatings have been shown, but it has been found that Li, Ti, Ni, Mn, La, Zn, P, B, and the like can achieve the same effect. In addition, it is possible to use not only a single case but also a suitable mixture.

  In addition, as an example of Co substitution material, Mg and Al are shown as examples, but it is known that Ti, W, Mo, Ni, Mn, Zr, V, Fe, etc. can achieve the same effect. Further, these substitution elements can also be used in appropriate mixture.

DESCRIPTION OF SYMBOLS 1 ... Flat type nonaqueous electrolyte secondary battery 2 ... Positive electrode 3 ... Separator 4 ... Negative electrode 5 ... Aluminum pressing plate 6 ... Leaf spring 7 ... SUS lid 8 ... Insulating ring 9 ... Insulating packing 10 ... Insulating sleeve 11 ... Nut 12 ... Bolt 13 ... SUS container 21 ... Lithium cobalt oxide 22 ... Metal oxide 23 ... Lithium phosphate

Claims (10)

A nonaqueous electrolyte secondary battery comprising a positive electrode having lithium cobaltate as a positive electrode active material, a negative electrode having a negative electrode active material, and a nonaqueous electrolyte,
The positive electrode has a part of the surface of the lithium cobalt oxide coated with a metal oxide, a part of the surface is exposed without being coated with the metal oxide, and contains lithium phosphate. Are dispersed throughout the surface of the lithium cobaltate,
The lithium cobalt oxide has an average particle size of 5 μm or more and 15 μm or less,
The lithium phosphate has an average particle size of 10 nm or more and 100 nm or less, and is a nonaqueous electrolyte secondary battery. The metal oxide, Li, Zr, Ti, Al , Ni, Mn, La, Zn, according to claim 1, characterized in that includes at least one element selected from the group consisting of P and B Non-aqueous electrolyte secondary battery. The coverage of the metal oxide by the surface of the lithium cobalt oxide, a non-aqueous electrolyte secondary battery according to claim 1 or claim 2, characterized in that 80% or less than 10%. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein a coating amount of the metal oxide is 0.1% by mass or more and 0.5% by mass or less. The lithium cobalt oxide is described by the formula _{Li 1 + x Co 1-y} M y O2, the M Mg, Al, Ti, W , Mo, Ni, Mn, Zr, V, at least one of Fe 5. The nonaqueous electrolyte secondary battery according to claim 1 , wherein 0 ≦ x ≦ 0.3 and 0 ≦ y ≦ 0.02. The weight ratio contained in the positive electrode of the said lithium phosphate is 0.1-2.5 mass% with respect to lithium cobaltate, As described in any one of Claims 1-5 characterized by the above-mentioned. Non-aqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery according to claim 1,
The non-aqueous electrolyte secondary battery, wherein the lithium phosphate has a structure in which lithium phosphate is disposed on an exposed portion of the surface of the lithium cobalt oxide. A positive electrode for a non-aqueous electrolyte secondary battery having lithium cobaltate as a positive electrode active material,
A part of the surface of the lithium cobaltate is coated with a metal oxide, a part of the surface is exposed without being coated with the metal oxide, and contains lithium phosphate. Dispersed throughout the surface of the lithium acid ,
The lithium cobalt oxide has an average particle size of 5 μm or more and 15 μm or less,
The positive electrode for a non-aqueous electrolyte secondary battery , wherein the lithium phosphate has an average particle diameter of 10 nm to 100 nm . A method for producing a positive electrode for a non-aqueous electrolyte secondary battery having lithium cobaltate as a positive electrode active material,
A first step of coating the entire surface of the lithium cobaltate with a metal oxide;
The lithium cobalt oxide is mixed with lithium phosphate was peeled partially the metal oxide, seen containing a second step, the disposing the lithium phosphate on the exposed portion of the surface of the lithium cobalt oxide,
The lithium cobalt oxide has an average particle size of 5 μm or more and 15 μm or less,
The lithium phosphate has an average particle size of 10 nm or more and 100 nm or less, and the method for producing a positive electrode for a non-aqueous electrolyte secondary battery. The method for producing a positive electrode for a non-aqueous electrolyte secondary battery according to claim 9 , wherein in the second step, the lithium cobalt oxide and the lithium phosphate are pressed and mixed.

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