JP2006156234A - Nonaqueous electrolyte secondary battery and its charging method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery enhancing a yield and reliability, and capable of charging at high voltage of 4.4-4.6 V vs. lithium, and to provide its charging method. <P>SOLUTION: In the nonaqueous electrolyte secondary battery equipped with a positive electrode 11 having a positive active material, a negative electrode 12 having a negative active material, and a nonaqueous electrolyte having a nonaqueous solvent and an electrolyte salt, the positive active material is comprised of a lithium cobalt composite oxide to which at least zirconium and magnesium are added and a lithium nickel manganese composite oxide having layer structure. The potential of the positive active material is 4.4-4.6 V vs. lithium. The content of each iron, chromium, copper and zinc as impurity elements contained in the positive electrode mix is respectively 10ppm or less. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

In the present invention, the positive electrode active material has a potential of 4.4 to 4.6 V based on lithium, preferably a combination of this positive electrode active material and a carbon material such as graphite as the negative electrode active material, so that the battery voltage is 4.3 V or higher. In particular, the present invention relates to a non-aqueous electrolyte secondary battery that is charged at a high voltage of 5 V or less and a charging method thereof, and in particular, has a layered structure and a lithium cobalt composite oxide containing at least both zirconium and magnesium in LiCoO ₂ as a positive electrode active material. The potential of the positive electrode active material using a mixture of a lithium manganese nickel composite oxide containing at least both manganese and nickel and having improved yield and reliability is 4.4 to 4.6 V based on lithium, Preferably, the positive electrode active material is combined with a carbon material such as graphite as the negative electrode active material, so that the battery voltage is 4.3V to 4.5V. The present invention relates to a non-aqueous electrolyte secondary battery that is charged with pressure and a charging method thereof.

  With the rapid spread of portable electronic devices, the required specifications for the batteries used for them are becoming stricter year by year, and in particular, small and thin, high capacity, excellent cycle characteristics, and stable performance are required. Yes. In the field of secondary batteries, lithium non-aqueous electrolyte secondary batteries, which have a higher energy density than other batteries, are attracting attention, and the proportion of lithium non-aqueous electrolyte secondary batteries shows a significant increase in the secondary battery market. ing.

  By the way, in a device in which this type of non-aqueous electrolyte secondary battery is used, the space for accommodating the battery is often a square (flat box shape), so the power generation element is accommodated in a rectangular outer can. The formed rectangular non-aqueous electrolyte secondary battery is often used. Such a rectangular nonaqueous electrolyte secondary battery is generally manufactured as follows.

  That is, a negative electrode plate in which a negative electrode mixture containing a negative electrode active material is applied on both sides of a negative electrode core (current collector) made of a long sheet-like copper foil, and a positive electrode core made of a long, thin sheet-like aluminum foil A separator made of a microporous polyethylene film or the like is disposed between a positive electrode plate coated with a positive electrode mixture containing a positive electrode active material on both sides, and a cylindrical shape with the negative electrode plate and the positive electrode plate insulated from each other by the separator. A cylindrical spiral electrode body is manufactured by winding it in a spiral shape. The cylindrical electrode body is crushed with a press machine and formed into a shape that can be inserted into a rectangular battery outer can. Then, the cylindrical electrode body is accommodated in the rectangular outer can, and an electrolyte is injected to inject the rectangular nonaqueous electrolyte. Next battery.

  The configuration of such a conventional rectangular nonaqueous electrolyte secondary battery will be described with reference to the drawings. FIG. 1 is a perspective view showing a rectangular nonaqueous electrolyte secondary battery disclosed in Patent Document 1 below, cut in the vertical direction. This non-aqueous electrolyte secondary battery 10 accommodates a flat spiral electrode body 14 in which a positive electrode plate 11 and a negative electrode plate 12 are wound via a separator 13 in a rectangular battery outer can 15. The battery outer can 15 is sealed with a sealing plate 16.

  The spiral electrode body 14 is wound so that the positive electrode plate 11 is exposed at the outermost periphery, and the exposed outermost positive electrode plate 11 is in direct contact with the inner surface of the battery outer can 15 that also serves as a positive electrode terminal. And are electrically connected. Further, the negative electrode plate 12 is electrically connected via a current collector 19 to a negative electrode terminal 18 that is formed at the center of the sealing plate 16 and attached via an insulator 17 that also serves as a negative electrode terminal. .

  Since the battery outer can 15 is electrically connected to the positive electrode plate 11, in order to prevent a short circuit between the negative electrode plate 12 and the battery outer can 15, the upper end of the spiral electrode body 14 and the sealing plate 16 The insulating spacer 20 is inserted between the negative electrode plate 12 and the battery outer can 15 so as to be electrically insulated.

  In this rectangular nonaqueous electrolyte secondary battery, the spiral electrode body 14 is inserted into the battery outer can 15, the sealing plate 16 is laser welded to the opening of the battery outer can 15, and then the electrolyte solution is injected. The non-aqueous electrolyte solution is injected from the hole 21 and the electrolyte solution injection hole 21 is sealed. Such a rectangular non-aqueous electrolyte secondary battery has an excellent effect that there is little wasted space during use, and the battery performance and battery reliability are high.

  As a negative electrode active material used in such non-aqueous electrolyte secondary batteries, dendrites grow while carbonaceous materials such as graphite and amorphous carbon have discharge potentials comparable to lithium metals and lithium alloys. Therefore, it is widely used because it has excellent properties such as high safety, excellent initial efficiency, good potential flatness, and high density.

On the other hand, as a positive electrode active material, a high energy density 4V class non-aqueous electrolyte secondary battery is obtained by combining a lithium composite oxide such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ and LiFeO _{2 with} a negative electrode made of a carbon material. Is known to be obtained. Among them, LiCoO ₂ is often used because various battery characteristics are superior to others. However, since cobalt is expensive and abundant as a resource, this LiCoO ₂ is not used. In order to continue to be used as a positive electrode material for water electrolyte secondary batteries, further improvement in performance and life of non-aqueous electrolyte secondary batteries is desired, and by using other transition elements instead of cobalt. Many developments have been made so far to achieve various battery characteristics equivalent to or better than those obtained when cobalt is used.

For example, as disclosed in Patent Documents 2 and 3 below, as a method for improving the characteristics of a non-aqueous electrolyte secondary battery using LiCoO ₂ as a positive electrode active material, different elements such as Zr and Mg are added to LiCoO ₂ . The method is known. The following Patent Document 2, the addition of zirconium to LiCoO ₂ as a cathode active material, and generates a high voltage, and the non-aqueous electrolyte secondary battery exhibiting excellent charge and discharge characteristics and storage characteristics are disclosed . This zirconium-added LiCoO ₂ is stabilized by covering the surface of LiCoO ₂ particles with zirconium oxide ZrO ₂ or a composite oxide of lithium and zirconium, Li ₂ ZrO _3. As a result, even at a high potential, the electrolyte solution This is because a positive electrode active material exhibiting excellent cycle characteristics and storage characteristics is obtained without causing a decomposition reaction or crystal destruction of this, and this effect is simply obtained by adding zirconium or a zirconium compound to LiCoO ₂ after firing. It cannot be obtained only by mixing, but can be obtained by adding zirconium to a mixture of lithium salt and cobalt compound and baking.

In Patent Document 3 below, at least one element selected from not only zirconium (Zr) but also titanium (Ti) and fluorine (F) is added as a heterogeneous element to be added to the positive electrode active material LiCoO _2. It has been shown that the load characteristics and cycle characteristics of the lithium non-aqueous electrolyte secondary battery can be improved by the addition.

Furthermore, in Patent Document 4 below, a Li transition metal composite oxide having a layer structure in which the composition ratio of Ni and Mn is substantially equal has a voltage in the vicinity of 4 V equivalent to LiCoO ₂ and is excellent in high capacity. Further, it is disclosed that the following patent document 5 contains at least Ni and Mn as transition metals and further contains fluorine in a lithium transition metal composite oxide having a layered structure. When a positive electrode active material is used, a non-aqueous electrolyte secondary battery that can be charged at a charge voltage of 4.4 V or more and combined with a negative electrode containing a carbon material as a negative electrode active material and has excellent thermal stability In addition, the following Non-Patent Document 1 discloses that among lithium transition metal composite oxides containing Ni, Co, and Mn, the chemical formula LiM having the same composition ratio of Mn and Ni n _x Ni _x Co _(1-2x) material represented by _{O 2} have been shown to exhibit specific high thermal stability even charged state (high oxidation state).

JP-A-2001-273931 (Claims, paragraphs [0003] to [0004], FIG. 1) JP-A-4-319260 (Claims, paragraphs [0006], [0008] to [0011]) JP 2004-299975 A (claims, paragraphs [0006] to [0008]) JP 2002-042813 A (claims, paragraphs [0011] to [0016]) JP 2004-296098 A (Claims) Electrochemical and Solid-State Letters, 4 (12) A200-A203 (2001)

At present, in a non-aqueous electrolyte secondary battery using a lithium-containing transition metal oxide such as lithium cobaltate LiCoO ₂ as a positive electrode active material and a carbon material as a negative electrode active material, When combined, the charging voltage is generally 4.1 to 4.2 V (the potential of the positive electrode active material is 4.2 to 4.3 V based on lithium). Under such charging conditions, the positive electrode is used only 50 to 60% of the theoretical capacity. Therefore, if the charging voltage can be further increased, the capacity of the positive electrode can be utilized at 70% or more of the theoretical capacity, and the capacity and energy density of the battery can be increased.

  Based on the development status of the positive electrode active material for a non-aqueous electrolyte secondary battery as described above, the present applicant has conducted various studies to obtain a positive electrode active material that can achieve a more stable and high charge voltage. A novel non-aqueous electrolyte secondary battery using a mixture of lithium cobaltate to which different elements are added as active materials and layered lithium manganese oxide is developed. Japanese Patent Application Nos. 2004-094475 and 2004-320394 have been developed. (Hereinafter collectively referred to as “prior application”).

  The positive electrode active material of the non-aqueous electrolyte secondary battery according to the invention of the prior application improves the structural stability at high voltage (up to 4.5V) by adding at least Zr and Mg different elements to lithium cobaltate, Furthermore, safety is ensured by mixing layered lithium manganese oxide with high voltage and high thermal stability. When a positive electrode using this positive electrode active material is combined with a negative electrode using a carbon material as a negative electrode active material, a non-aqueous electrolyte secondary battery that can be charged at a high voltage of 4.3 V to 4.5 V is obtained.

  However, such non-aqueous electrolyte secondary batteries according to the invention of the prior application are subject to problems in terms of production yield and reliability due to the occurrence of defective test products in which the battery voltage decreases even if the frequency of occurrence is low. The inventors of the present application, as a result of various investigations of the cause of the occurrence of this defective product, are based on such a voltage drop in a nonaqueous electrolyte secondary battery in which a positive electrode active material is produced using a highly purified raw material. Since the generation of defective products was not observed, it was found that the metal impurities in the positive electrode plate were the cause. This phenomenon is considered to be caused by the fact that the metal impurities in the positive electrode mixture were eluted during the charge / discharge cycle to cause a micro short circuit.

  Industrially, it is unacceptable in terms of cost to manufacture a non-aqueous electrolyte secondary battery using a raw material purified to a high purity. Therefore, as a result of further experiments to determine the impurities that cause such a decrease in battery voltage, the inventors have controlled at least four impurity concentrations of Fe, Cr, Cu, and Zn within a predetermined range. As a result, the inventors have found that the frequency of occurrence of a decrease in battery voltage as described above can be drastically reduced, and the present invention has been completed.

That is, the present invention provides a positive electrode active material in which the charging voltage when combined with a negative electrode using a carbon material with improved yield and reliability as a negative electrode active material can be a high voltage of 4.3 V or more and 4.5 V or less. Non-coating using a mixture of a lithium cobalt composite oxide containing at least both zirconium and magnesium in LiCoO ₂ and a lithium manganese nickel composite oxide having a layered structure and containing at least both manganese and nickel An object is to provide a water electrolyte secondary battery and a charging method thereof.

  The above object of the present invention can be achieved by the following configurations. That is, the invention of a non-aqueous electrolyte secondary battery according to claim 1 includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and a non-aqueous electrolyte having a non-aqueous solvent and an electrolyte salt. In the water electrolyte secondary battery, the positive electrode active material comprises a lithium cobalt composite oxide to which at least zirconium and magnesium are added, and a lithium nickel manganese composite oxide having a layered structure, and the potential of the positive electrode active material Is 4.4 to 4.6 V based on lithium, and the content of each of iron, chromium, copper, and zinc as impurity elements contained in the positive electrode mixture is 10 ppm or less. .

In the present invention, it is essential that the content of each of the impurity elements iron, chromium, copper, and zinc contained in the positive electrode mixture is 10 ppm or less, and at least one of these impurity elements is 10 ppm. If it is contained in excess, the number of batteries judged as defective by battery voltage inspection increases. Such a decrease in impurity concentration can be achieved by refining the positive electrode active material and suppressing contamination during electrode plate production. In addition, even if the content of each of the impurity elements iron, chromium, copper, and zinc exceeds 10 ppm, the same as in the case of the conventional non-aqueous electrolyte secondary battery using the positive electrode active material made of LiCoO ₂ In addition, when the potential of the positive electrode active material is less than 4.3 V on the basis of lithium, the number of batteries judged to be defective by the battery voltage inspection does not increase, but such a non-aqueous electrolyte secondary battery It is outside the scope of the present invention.

In the present invention, Li _a Co _1-xy- Zr _x Mg _y M _z O ₂ (0 ≦ a ≦ 1.1, x ≧ 0.0001, y ≧ 0.0001) is used as the lithium cobalt composite oxide. Z ≧ 0, 0.0002 ≦ x + y + z ≦ 0.03, M = Al, Ti, Sn). If the added amount of Zr, Mg, Al, Ti, Sn is small, the effect of improving the cycle characteristics is small, and if the added amount is large, the capacity is reduced. As the layered lithium-nickel-manganese composite _{oxide, Li b Mn s Ni t Co} u M 'v O 2 (0 ≦ b ≦ 1.2,0.1 ≦ s ≦ 0.5,0.1 ≦ t ≦ 0.5, u ≧ 0, 0.0001 ≦ v ≦ 0.03, s + t + u + v = 1, M ′ = Mg, Zr, Al, Ti, Sn, Ni and Mn are substantially equal in molar ratio (0.95 ≦ s / t ≦ 1.05)) is preferred,
An active material having high thermal stability can be obtained with the above composition. Although the cycle and storage characteristics are further improved by adding M ′, the effect is small when the added amount is small, and the initial capacity is lowered when the added amount is large.

  In the present invention, carbonates, lactones, ethers, esters and the like can be used as the nonaqueous solvent (organic solvent) constituting the nonaqueous solvent electrolyte, and two or more of these solvents can be used. Can also be used in combination. Among these, carbonates, lactones, ethers, ketones, esters and the like are preferable, and carbonates are more preferably used.

  Specific examples include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), cyclopentanone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl. -1,3-oxazolidine-2-one, dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), methyl propyl carbonate, methyl butyl carbonate, ethyl propyl carbonate, ethyl butyl carbonate, dipropyl carbonate, γ -Butyrolactone, γ-valerolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, 1,4-dio Xanthan can be mentioned. In the present invention, a mixed solvent containing EC is preferably used from the viewpoint of increasing the charge / discharge efficiency. However, since cyclic carbonate is generally easily oxidized and decomposed at a high potential, the EC content in the non-aqueous electrolyte is 5% by volume or more and 25% by volume. % Or less is preferable.

In addition, as a solute of the nonaqueous electrolyte in the present invention, a lithium salt generally used as a solute in a nonaqueous electrolyte secondary battery can be used. Such lithium salts include LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , LiAsF ₆ , LiClO ₄ , Li ₂ B ₁₀ Cl ₁₀ , Li ₂ B ₁₂ Cl ₁₂ , and mixtures thereof Illustrated. Among these, LiPF ₆ (lithium hexafluorophosphate) is preferably used. When charged with a high charging voltage, although aluminum is a current collector of the positive electrode is easily dissolved in the presence of LiPF _6, by LiPF ₆ decomposes, coating is formed on the aluminum surface, the aluminum by the coating Can be dissolved. Therefore, it is preferable to use LiPF ₆ as the lithium salt. The amount of solute dissolved in the non-aqueous solvent is preferably 0.5 to 2.0 mol / L.

  The invention according to claim 2 is the non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material is made of a carbonaceous material.

The invention according to claim 3 is the nonaqueous electrolyte binary battery according to claim 1 or 2, wherein the nonaqueous electrolyte further contains 0.5 to 5% by mass of VC.

  The invention according to claim 4 is the nonaqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the lithium manganese nickel composite oxide of the positive electrode active material further contains cobalt. And

  Furthermore, the invention of the method for charging a non-aqueous electrolyte secondary battery according to claim 5 includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, a non-aqueous electrolyte having a non-aqueous solvent and an electrolyte salt, The positive electrode active material comprises a lithium cobalt composite oxide to which at least zirconium and magnesium are added, and a lithium nickel manganese composite oxide having a layered structure, and is included in the positive electrode mixture In the method of charging a non-aqueous electrolyte secondary battery in which the content of each of the impurity elements iron, chromium, copper, and zinc is 10 ppm or less, the potential of the positive electrode active material is 4.4 to 4.6 V based on lithium. It is characterized by charging.

  By providing the above configuration, the present invention has the following excellent effects. That is, according to the invention of claim 1, since the concentration of at least four impurities of Fe, Cr, Cu, and Zn, which are impurities that cause a decrease in battery voltage, is maintained at 10 ppm or less, the battery has a high capacity and a high energy density. The non-aqueous electrolyte secondary battery having a good manufacturing yield and excellent reliability can be obtained while reducing the generation of defective products with reduced battery voltage.

  According to the second aspect of the present invention, since the negative electrode active material is made of a carbonaceous material, if a low-potential carbonaceous material (about 0.1 V based on lithium) is used as the negative electrode active material, the battery voltage is high and the positive electrode active material is high. A battery with a high utilization rate of the substance can be obtained.

  According to the invention of claim 3, VC is conventionally used conventionally as an additive for suppressing reductive decomposition of an organic solvent. By adding this VC, the negative electrode by the first charge is used. Before insertion of lithium into the negative electrode active material layer, a negative electrode surface coating (SEI: Solid Electrolyte Interface) is formed on the negative electrode active material layer, and this SEI prevents the insertion of solvent molecules around the lithium ions. Since it functions as a barrier, the negative electrode active material does not directly react with the organic solvent, so that an effect of improving cycle characteristics is seen and a long-life nonaqueous electrolyte secondary battery is obtained. The addition amount of VC is 0.5-5 mass% with respect to the whole electrolyte solution, Preferably it is 1-3 mass%. If the amount of VC added is less than 0.5% by mass, the effect of improving the cycle characteristics is small, and if it exceeds 3% by mass, the initial capacity decreases and the battery swells at high temperatures, which is not preferable.

According to the invention of claim 4, a lithium transition metal composite oxide containing Ni, Co, and Mn, particularly represented by the chemical formula LiMn _x Ni _x Co _(1-2x) O _{2 in} which the composition ratio of Mn and Ni is equal. Since the material to be used exhibits a specific high thermal stability even in a charged state (high oxidation state), a safe non-aqueous electrolyte secondary battery can be obtained even when charged at a high charging voltage.

  Furthermore, according to the invention of claim 5, since the potential of the positive electrode active material whose charging voltage is higher than that of the conventional one can be 4.4 to 4.6 V on the basis of lithium, high capacity and high energy density are achieved. A non-aqueous electrolyte secondary battery is obtained.

  Hereinafter, the best mode for carrying out the present invention will be described in detail using examples and comparative examples. However, the examples described below are merely examples of non-aqueous electrolyte secondary batteries and their charging methods for embodying the technical idea of the present invention, and the present invention is specified in this example. The present invention can be equally applied to various modifications without departing from the technical idea shown in the claims.

<Preparation of positive electrode>
The heterogeneous element-added lithium cobalt oxide was produced as follows. As a starting material, lithium carbonate is used as lithium source (Li ₂ CO ₃ ), and cobalt source is zirconium (Zr) as a heterogeneous element at the time of cobalt carbonate synthesis, 0.2 mol% with respect to cobalt and magnesium (Mg). Coprecipitation was performed from an aqueous solution added with 0.5 mol%, and then zirconium and magnesium-added tricobalt tetroxide (Co ₃ O ₄ ) obtained by a thermal decomposition reaction were used. A predetermined amount of these were weighed and mixed, and then fired at 850 ° C. for 20 hours in an air atmosphere to obtain zirconium and magnesium-added lithium cobalt oxide. This was pulverized with a mortar to an average particle size of 14 μm to obtain a positive electrode active material A.

The layered lithium nickel manganate was prepared as follows. As starting materials, lithium carbonate (Li ₂ CO ₃ ) is used for the lithium source, and co-precipitated hydroxide represented by Ni _0.33 Mn _0.33 Co _0.34 (OH) ₂ is used for the transition metal source. It was. A predetermined amount was weighed and mixed, and then fired at 1000 ° C. for 20 hours in an air atmosphere. This was pulverized to a mean particle size of 5 μm in a mortar to obtain a positive electrode active material B.

  The positive electrode active material A and the positive electrode active material B obtained as described above were mixed so that the mass ratio was 7: 3. Next, the mixed positive electrode active material was 94 parts by mass, and carbon powder as a conductive agent. Was mixed with 3 parts by mass of polyvinylidene fluoride powder as a binder, and this was mixed with an N-methylpyrrolidone (NMP) solution to prepare a slurry. This slurry was applied to both sides of an aluminum current collector having a thickness of 15 μm by a doctor blade method and dried to form active material layers on both sides of the positive electrode current collector. Then, it compressed using the compression roller and produced the positive electrode whose length of a short side is 43.5 mm. Contamination during the production of the positive electrode active material and the electrode plate was suppressed, and the amount of impurity metal elements (Fe, Cr, Cu, Zn) in the positive electrode mixture was adjusted. As a result of measurement by an inductively coupled plasma (ICP) emission spectrometer, it was found that the positive electrode mixture contained Fe: 5 ppm, Cr: 3 ppm, Cu: 2 ppm, Zn: 2 ppm.

<Preparation of negative electrode>
A slurry was prepared by dispersing 95 parts by mass of graphite powder, 3 parts by mass of carboxymethyl cellulose (CMC) as a thickener, and 2 parts by mass of styrene-butadiene rubber (SBR) as a binder in water. This slurry was applied to both surfaces of a copper current collector having a thickness of 8 μm by a doctor blade method and dried to form active material layers on both surfaces of the negative electrode current collector. Then, it compressed using the compression roller and produced the negative electrode whose length of a short side is 44.5 mm. The potential of graphite is 0.1 V with respect to lithium. For example, when the positive electrode active material potential is 4.4 V based on lithium, the battery voltage using graphite as the negative electrode is 4.3 V. The coating amount of the positive electrode and the negative electrode was measured with a tripolar cell (counter electrode: lithium metal, reference electrode: lithium metal) as the charge capacity per 1 g of the positive electrode active material at the charging voltage as the design standard. Based on the theoretical charge capacity, the charge capacity ratio (negative electrode charge capacity / positive electrode charge capacity) was adjusted to 1.1. Table 1 shows the positive electrode active material and the charge capacity.

<Relationship between charging positive electrode potential and positive electrode capacity>
Zirconium and magnesium-added lithium cobalt oxide / layered lithium nickel manganate (mixing ratio 7: 3)

<Preparation of electrolyte>
A mixed solvent was prepared so as to be 20% by volume of ethylene carbonate, 50% by volume of ethyl methyl carbonate, and 30% by volume of diethyl carbonate, and 1 mol / L of LiPF ₆ was dissolved therein to obtain an electrolyte.

<Production of battery>
Using the above positive electrode, negative electrode and electrolyte, and using a polyethylene microporous membrane as a separator, 500 rectangular nonaqueous electrolyte secondary batteries (6 mm × 34 mm × 50 mm) according to Example 1 were produced.

<Measurement of inspection defects>
About each battery produced as mentioned above, it charges with the constant current of 1 It (1C) at 25 degreeC, and after the voltage of a battery becomes 4.3V, a charging current value is 1 with a constant current of 4.3V. The battery was initially charged until / 50 It (1/50 C). This initially charged battery was allowed to stand at 60 ° C. for 24 hours, and the battery voltage drop exceeding 0.1 V was regarded as defective, and the number of batteries determined to be defective was examined. The results are shown in Table 2.

<Measurement of cycle characteristics>
About the battery judged to be a non-defective product by the measurement of the inspection failure, discharging was performed at 25 ° C. at a constant current of 1 It until the battery voltage reached 3 V, and the discharge capacity at this time was obtained as the battery capacity. The cycle characteristics were measured for each battery whose battery capacity was measured at 25 ° C. until the battery voltage reached 4.3 V at a constant current of 1 It, and then the current value was reduced to 1/50 It at a constant voltage of 4.3 V. Then, charging was performed until the battery voltage reached 3 V at a constant current of 1 It as one cycle, and the discharge capacity after 300 cycles was obtained repeatedly until reaching 300 cycles. And the capacity | capacitance residual ratio (%) after 300 cycles in 25 degreeC was calculated | required about each battery based on the following formulas. The results are shown in Table 2 together with the battery capacity and the capacity remaining rate.
Capacity remaining rate (%) = (discharge capacity after 300 cycles / battery capacity) × 100

<Reference Examples 1 and 2>
A battery having the same configuration as in Example 1 was prepared, and the measurement of defective inspection was performed in the same manner as in Example 1 except that the charging voltage was 4.2 V (Reference Example 1) and 4.6 V (Reference Example 2). The battery capacity and cycle characteristics were measured. The results are shown in Table 2 together with the results of Example 1.

<Examples 2-5, Comparative Examples 1-7>
The batteries of Examples 2-5 and Comparative Examples 1-7 were changed in the same manner as in Example 1 except that the impurity content was changed during the production of the positive electrode and the negative electrode, and the charging voltage as the design standard was changed. Each of these batteries was subjected to measurement of defective inspection, measurement of battery capacity, and measurement of cycle characteristics in the same manner as in Example 1. The results are shown in Table 2 together with the results of Example 1 together with the impurity content of each battery.

  From the results shown in Table 2 above, the following can be understood. That is, when a mixture of lithium cobalt oxide and lithium nickel manganate added with different elements is used as the positive electrode active material, if the charging voltage is in the range of 4.3 V to 4.5 V, Fe, Cr, Cu as impurities , Zn is made to be 10 ppm or less for all (Examples 1 to 5), the defective test value is 1 or less out of 500, the battery capacity is 1150 mAh or more, and the cycle characteristic is 87% or more. Obtained. On the other hand, if the charge voltage is as low as 4.2 V even if it is 10 ppm or less for all of Fe, Cr, Cu, and Zn as impurities (reference example 1), the results of the defective inspection value and the cycle characteristics are good. However, the result of the battery capacity is only 1080 mAh. On the other hand, even if the impurities are 10 ppm or less for all of Fe, Cr, Cu, and Zn, when the charging voltage is as high as 4.6 V (Reference Example 2), the results of the defective inspection value and the battery capacity are good. However, the cycle characteristics are greatly deteriorated to 60%.

  Further, if any one of Fe, Cr, Cu, and Zn as impurities exceeds 10 ppm (Comparative Examples 1 to 7), good results are obtained in the battery capacity and cycle characteristics, but the inspection defect value is 500. It can be seen that the number is 5 or more, and the manufacturing yield is poor and the reliability is high.

  As described above, according to the present invention, in order to obtain a high-capacity non-aqueous electrolyte secondary battery that has a high capacity by high-voltage charging, a high yield in manufacturing, and a high reliability, the metal impurities ( In particular, control of the amount of Fe, Cr, Cu, Zn) is important, and it is clear that good results can be obtained by suppressing the content of each impurity element to 10 ppm or less by refining the positive electrode active material and preventing contamination. It is.

  In Examples and Comparative Examples, examples in which a mixed solvent composed of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC) was used as an electrolyte solvent were shown. It will be apparent that the following solvents can be used. However, it is better to add the cyclic carbonate from the viewpoint of increasing the charge / discharge efficiency, but since it is easily oxidatively decomposed at a high potential, the content in the nonaqueous electrolyte solvent is 5% by volume particularly when EC is used. It is desirable to set it as 25 volume% or less.

It is a perspective view which cut | disconnects and shows the conventional square nonaqueous electrolyte secondary battery to the vertical direction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Nonaqueous electrolyte secondary battery 11 Positive electrode plate 12 Negative electrode plate 13 Separator 14 Flat spiral electrode body 15 Rectangular battery outer can 16 Sealing plate 17 Insulator 18 Negative electrode terminal 19 Current collector 20 Insulating spacer 21 Electrolyte injection Liquid hole

Claims (5)

In a nonaqueous electrolyte secondary battery comprising a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and a nonaqueous electrolyte having a nonaqueous solvent and an electrolyte salt,
The positive electrode active material comprises a lithium cobalt composite oxide to which at least zirconium and magnesium are added, and a lithium nickel manganese composite oxide having a layered structure,
The positive electrode active material has a potential of 4.4 to 4.6 V based on lithium,
And each content of iron, chromium, copper, and zinc which are impurity elements contained in the said positive electrode mixture is 10 ppm or less, The nonaqueous electrolyte secondary battery characterized by the above-mentioned.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material is made of a carbonaceous material.   The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the non-aqueous electrolyte further contains 0.5 to 5% by mass of vinylene carbonate.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the lithium manganese nickel composite oxide of the positive electrode active material further contains cobalt. A positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and a non-aqueous electrolyte having a non-aqueous solvent and an electrolyte salt,
The positive electrode active material comprises a lithium cobalt composite oxide to which at least zirconium and magnesium are added, and a lithium nickel manganese composite oxide having a layered structure,
And in the method for charging a non-aqueous electrolyte secondary battery in which each content of iron, chromium, copper, and zinc as impurity elements contained in the positive electrode mixture is 10 ppm or less,
The non-aqueous electrolyte secondary battery charging method, wherein the positive electrode active material is charged at a potential of 4.4 to 4.6 V based on lithium.

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