JP2009105017A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery using a positive electrode plate capable of improving continuous charging characteristics and high-temperature preservation characteristics, even with charging at a charging voltage of 4.40 V or more on a lithium criterion. <P>SOLUTION: In the nonaqueous electrolyte secondary battery having the positive electrode plate 11 wherein a positive electrode mixture agent is coated on a positive electrode collector, a negative electrode plate wherein a negative electrode mixture agent is coated on a negative electrode collector, and a nonaqueous electrolyte wherein electrolyte salt is dissolved in a separator and a nonaqueous solvent, a first-layered positive electrode mixture agent layer 11B formed on both sides or one side of the positive electrode collector 11A and a second-layered positive electrode mixture agent 11C with a composition different from the first layer formed on the surface of the first-layered positive electrode mixture agent layer 11B are formed on the positive electrode plate 11. A positive electrode active material in the second-layered positive electrode mixture agent layer 11C is a material wherein a dissolved amount of metal elements except lithium is less than a positive electrode active material of the first-layered positive electrode mixture agent layer 11B at the time of immersing into acid aqueous solution. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a positive electrode plate in which a positive electrode mixture containing a positive electrode active material containing lithium is applied to a positive electrode current collector, and a negative electrode electrode in which a negative electrode mixture containing a negative electrode active material is applied to a negative electrode current collector The present invention relates to a nonaqueous electrolyte secondary battery including a plate, a separator, and a nonaqueous electrolytic solution in which an electrolyte salt is dissolved in a nonaqueous solvent. More specifically, the present invention relates to a non-aqueous electrolyte secondary battery that can be charged at a high voltage of 4.40 V or higher on the basis of lithium, which is excellent in continuous charge capacity return rate and high temperature storage capacity return rate.

  Non-aqueous electrolyte secondary typified by lithium-ion secondary battery with high energy density and high capacity as a driving power source for portable electronic devices such as mobile phones, portable personal computers, portable music players, etc. Batteries are widely used. Among these, nonaqueous electrolyte secondary batteries using graphite particles as the negative electrode active material are widely used because of their high safety and high capacity.

  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 nonaqueous electrolyte secondary battery is often used. Such a rectangular non-aqueous 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 to both sides of a negative electrode current collector made of a long and thin sheet-like copper foil, and a positive electrode current collector made of a long and thin sheet-like aluminum foil and the like A separator made of a microporous polyethylene film or the like is placed between a positive electrode plate coated with a positive electrode mixture containing a positive electrode active material, and the negative electrode plate and the positive electrode plate are insulated from each other by the separator. A cylindrical wound electrode body is produced by spirally winding it around a columnar winding core. This cylindrical electrode body is crushed by a press machine and molded into a shape that can be inserted into a rectangular battery outer can. Then, the cylindrical electrode body is accommodated in a rectangular outer can, and an electrolyte is injected to form a rectangular nonaqueous electrolyte. Secondary battery is used.

  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 conventional rectangular nonaqueous electrolyte secondary battery cut in the longitudinal direction. In this nonaqueous electrolyte secondary battery 10, a flat wound electrode body 14 in which a positive electrode plate 11 and a negative electrode plate 12 are wound via a separator 13 is accommodated in a rectangular battery outer can 15. The battery outer can 15 is sealed with a sealing plate 16.

  The wound 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 formed on the inner surface of the battery outer can 15 that also serves as a positive electrode terminal. Direct contact and electrical connection. The negative electrode plate 12 is formed at the center of the sealing plate 16 and is electrically connected to a negative electrode terminal 18 attached via an insulator 17 via a negative electrode tab 19.

  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 wound electrode body 14 and the sealing plate 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 10, after the wound 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 injection hole The non-aqueous electrolyte is injected from 21 and the electrolyte injection hole 21 is sealed. Such a rectangular non-aqueous electrolyte secondary battery 10 has an excellent effect that there is little wasted space during use, and the battery performance and battery reliability are high.

  As the negative electrode active material used in this non-aqueous electrolyte secondary battery, carbonaceous materials such as graphite and amorphous carbon have a discharge potential comparable to that of lithium metal or lithium alloy, but dendrite grows. Therefore, it is widely used because it has excellent properties such as high safety, excellent initial efficiency, good potential flatness, and high density.

  In addition, as the nonaqueous solvent for the nonaqueous electrolyte, carbonates, lactones, ethers, esters and the like are used alone or in admixture of two or more, but among these, the dielectric constant is particularly large. Many carbonates with high non-aqueous electrolyte ionic conductivity are used

As a positive electrode active material in this nonaqueous electrolyte secondary battery, Li _x MO ₂ capable of reversibly inserting and extracting lithium ions (where M is at least one of Co, Ni, and Mn) A lithium transition metal composite oxide represented by: LiCoO ₂ , LiNiO ₂ , LiNi _y Co _1-y O ₂ (y = 0.01 to 0.99), LiMnO ₂ , LiMn ₂ O ₄ , LiCo _x Mn _y Ni _z O ₂ (x + y + z = 1), LiFePO ₄ or the like is used singly or in combination.

  However, with the improvement in performance and functionality of portable electronic devices as described above, there is an increasing demand for higher capacity for nonaqueous electrolyte secondary batteries. As methods for meeting such demands, increasing the density of electrode materials, reducing the thickness of current collectors, separators, and the like, and increasing the battery voltage are generally known. Among these, when the electrode material is densified and the current collector and the separator are thinned, the productivity of the nonaqueous electrolyte secondary battery is lowered. On the other hand, increasing the charging voltage of the battery voltage is essential for the development of high-capacity batteries in the future, because it can minimize the impact on the productivity of non-aqueous electrolyte secondary batteries and increase the capacity. Technology.

Currently, 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 such as graphite as a negative electrode active material, the charging voltage is generally 4. 1 to 4.2 V (the positive electrode potential is 4.2 to 4.3 V with respect to 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.
JP 2005-317499 A JP 2006-134770 A JP 2006-156234 A JP 2006-228651 A

  However, the conventional non-aqueous electrolyte secondary battery has a problem that the positive electrode active material deteriorates quickly during continuous charging for a long time or storage for a long time at a high temperature, particularly when the positive electrode potential during charging is increased. That is, a conventional positive electrode active material of a non-aqueous electrolyte secondary battery that can be charged at a high charge voltage is used by mixing two or more types of positive electrode active material. For example, in the non-aqueous electrolyte secondary battery disclosed in Patent Document 1, a high voltage (up to 4.5 V) is obtained by adding at least Zr and Mg different elements to a lithium cobalt composite oxide as a positive electrode active material. In order to improve the structural stability, a layered lithium manganese nickelate having a high voltage and high thermal stability is mixed to ensure safety. The positive electrode plate used by mixing a plurality of such positive electrode active materials has continuous charge characteristics and high-temperature storage characteristics due to the preferential elution of metal components from materials with poor voltage resistance. It will decline.

On the other hand, Patent Document 2 discloses a positive electrode mixture layer applied to a positive electrode current collector for the purpose of improving long-time continuous charging and long-time storage characteristics of a nonaqueous electrolyte secondary battery. Apply a positive electrode mixture containing a positive electrode active material having a layer structure and a smaller weight loss rate at 400 ° C. by thermogravimetry to the second layer on the first layer near the current collector A non-aqueous electrolyte secondary battery using the positive electrode plate is disclosed. In the nonaqueous electrolyte secondary battery disclosed in Patent Document 2, specifically, as the positive electrode active material, LiNiO ₂ is used for the first layer, LiFePO ₄ is used for the second layer, and the negative electrode active material is used. Artificial graphite, CoSn alloy or CoSnC-containing material is used as the substance. And in patent document 2, the nonaqueous electrolyte secondary battery described here is a long time continuous charge or higher temperature than a nonaqueous electrolyte secondary battery in which only a layer containing LiNiO ₂ is formed as a positive electrode active material. It has been shown that the characteristics when stored for a long time can be improved.

  However, the continuous charge characteristics and the high-temperature storage characteristics of the nonaqueous electrolyte secondary battery disclosed in Patent Document 2 are those obtained with a charge upper limit voltage of 4.20 V (paragraphs [0067] to [0068]). However, the effect was not sufficient for high-temperature storage characteristics, continuous charge characteristics, or charge / discharge cycle life at higher charge voltages of 4.30 V or higher.

  As a result of investigating various combinations of the positive electrode active materials of the first layer and the second layer as described above, the inventor conducted a detailed investigation on the potential distribution in the thickness direction inside the positive electrode plate during charging. The potential distribution in the thickness direction of the electrode plate was not uniform, suggesting that the potential may be higher on the side facing the separator than on the current collector side. For this reason, by disposing a material with excellent oxidation resistance on the portion where the potential becomes higher during charging, that is, on the surface layer portion (second layer) of the positive electrode mixture coating surface of the positive electrode plate, the discharge capacity and discharge Since it was found that a non-aqueous electrolyte secondary battery excellent in continuous charge characteristics and high-temperature storage characteristics can be obtained without impairing performance, the oxidation resistance of the entire positive electrode plate can be improved, and the present invention has been completed. is there. The oxidation resistance referred to here is the degree of deterioration of the material when exposed to a high potential.

  That is, the object of the present invention is to use a positive electrode plate that can improve continuous charge characteristics and high-temperature storage characteristics even when charged at a battery voltage of 4.30 V or higher and a charging potential of 4.40 V or higher with respect to lithium. The object is to provide a non-aqueous electrolyte secondary battery.

  In order to achieve the above object, the nonaqueous electrolyte secondary battery of the present invention includes a positive electrode plate in which a positive electrode mixture containing a positive electrode active material containing lithium is applied to a positive electrode current collector, and a negative electrode active material. In a nonaqueous electrolyte secondary battery comprising a negative electrode plate in which a negative electrode mixture is applied to a negative electrode current collector, and a nonaqueous electrolyte solution in which an electrolyte salt is dissolved in a separator and a nonaqueous solvent, the positive electrode plate includes: The first positive electrode mixture layer formed on both sides or one side of the positive electrode current collector and the first layer formed on the surface of the first positive electrode mixture layer have a composition different from that of the first layer. A second positive electrode mixture layer is formed, and the positive electrode active material in the second positive electrode mixture layer is more acidic aqueous solution than the positive electrode active material in the first positive electrode mixture layer The amount of elution of metal elements excluding lithium when immersed in is small.

  When the positive electrode is exposed to a high potential at a high temperature in a high-temperature storage test of a non-aqueous electrolyte secondary battery after charging, the lower the oxidation resistance of the positive electrode active material, the more the metal component is oxidized and the surface of the positive electrode active material particles As a result, the amount of the elution of the metal component from the catalyst increases, and the charge / discharge reaction activity on the surface of the positive electrode active material particles decreases. On the other hand, in the positive electrode active material having high oxidation resistance, the elution of metal components from the surface of the positive electrode active material particles hardly occurs even when exposed to high temperature or high voltage. The decrease in reaction activity is small. In the present invention, as a method for easily evaluating the oxidation resistance of the positive electrode active material, for example, when a constant mass of the positive electrode active material is immersed in a 0.1 mol / L hydrochloric acid aqueous solution at 25 ° C. and stored for about 1 hour. The method of measuring the elution amount (mol / L: number of moles per liter) of the metal component excluding Li in the hydrochloric acid aqueous solution per positive electrode active material can be used. That is, it can be determined that the smaller the elution amount of metal components (excluding Li) from the positive electrode active material into the aqueous hydrochloric acid solution, the higher the oxidation resistance. If the concentration and temperature of the acidic aqueous solution are too high, or if the immersion time is too long, most of the positive electrode active material will be eluted, making it difficult to see the difference. It is desirable to select the concentration, immersion time, and the like. For example, when hydrochloric acid is used, it is desirable that the concentration is 0.05 to 1.0 mol / L, the temperature is 20 to 60 ° C., and the time is about 10 minutes to 6 hours.

  In the positive electrode plate in the nonaqueous electrolyte secondary battery of the present invention, a positive electrode mixture layer having a two-layer structure is formed on the surface of the positive electrode current collector, and the positive electrode mixture layer in the second layer (surface layer) As the positive electrode active material, a material in which the elution amount of metal elements excluding lithium when immersed in an acidic aqueous solution is smaller than that of the positive electrode active material in the positive electrode mixture layer of the first layer (lower layer) is used. Therefore, even if the first positive electrode active material is more easily dissolved in the solvent than the second positive electrode active material, the first positive electrode mixture layer is the second positive electrode mixture. Since the nonaqueous electrolyte secondary battery of the present invention obtained is covered with a layer, the amount of elution of metals other than Li is reduced even when continuously charged or stored in a high temperature place. Therefore, according to the nonaqueous electrolyte secondary battery of the present invention, it is possible to suppress the capacity deterioration and the charge / discharge cycle life deterioration during continuous charging and high-temperature storage.

  The “weight reduction rate at 400 ° C. by thermogravimetry” disclosed in the above cited reference 2 is considered to indicate the resistance to the change in crystal structure as the temperature rises. This depends on the thermal stability of the crystal. On the other hand, the “elution amount of metal elements excluding lithium when immersed in an acidic aqueous solution” in the present invention has a part of which the detailed mechanism is unknown, for example, the metal contained in each positive electrode active material It is recognized that it changes under the influence of ionization tendency and the structure near the surface of the positive electrode active material. Therefore, between the “weight reduction rate at 400 ° C. by thermogravimetry” disclosed in the above cited reference 2 and the “elution amount of metal elements excluding lithium when immersed in an acidic aqueous solution” of the present invention. There is no correlation.

In the nonaqueous electrolyte secondary battery of the present invention, the positive electrode active material in the first and second positive electrode mixture layers is the second layer positive electrode active material is the first layer positive electrode active material. For example, Li _x MO ₂ (wherein M is at least one of Co, Ni, and Mn) is sufficient to achieve a predetermined effect if the amount of elution of metal elements excluding lithium when immersed in an acidic aqueous solution is small. ), That is, LiCoO ₂ , LiNiO ₂ , LiNi _y Co _1-y O ₂ (y = 0.01 to 0.99), LiMnO ₂ , LiMn ₂ O ₄ , LiCo _x Mn _y Ni _z O ₂ (x + y + z = 1) or LiFePO ₄ can be used in appropriate combination. Among these positive electrode compounds, in terms of capacity, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ (spinel type lithium manganate), LiNi _1-x Co _x O ₂ (0 <x <1), LiNi _{1-x— y} Co _x Mn _y O ₂ ) (0 <x, 0 <y, x + y <1) is desirable. These composite oxides may contain a trace amount of other elements such as aluminum, magnesium, zirconium, titanium, and the like. Further, LiFePO ₄ or a positive electrode active material containing other elements may be used.

  In addition, as the negative electrode plate used in the nonaqueous electrolyte secondary battery of the present invention, a material made of a carbonaceous material such as artificial graphite, natural graphite, or amorphous carbon known as a negative electrode active material can be used. .

  In the nonaqueous electrolyte secondary battery of the present invention, carbonates, lactones, ethers, esters, etc. can be used as the nonaqueous solvent (organic solvent) constituting the nonaqueous solvent electrolyte. Two or more of these solvents can be mixed and used. 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 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. The amount of solute dissolved in the non-aqueous solvent is preferably 0.5 to 2.0 mol / L.

In the nonaqueous electrolyte secondary battery of the present invention, the positive electrode active material in the first positive electrode mixture layer is composed of a composite oxide represented by a composition formula LiCoO ₂ containing at least cobalt and lithium. The positive electrode active material in the positive electrode mixture layer of the second layer is composed of a composite oxide represented by the composition formula LiNi _1-x Co _x O ₂ (0 <x <1) containing at least cobalt and nickel. It can be.

LiCoO ₂ is widely used as a positive electrode active material for non-aqueous electrolyte secondary batteries because it has various battery characteristics superior to others. However, when it is immersed in an acidic aqueous solution, a large amount of cobalt ions are eluted. On the other hand, the composite oxide represented by the composition formula LiNi _1-x Co _x O ₂ (0 <x <1) containing at least cobalt and nickel is more nickel ion than the case of LiCoO ₂ even when immersed in an acidic aqueous solution. And the elution amount of cobalt ions is small. Therefore, according to the nonaqueous electrolyte secondary battery of this aspect, it becomes possible to suppress deterioration in capacity and charge / discharge cycle life particularly during continuous charging and storage at high temperatures. In the non-aqueous electrolyte secondary battery of the embodiment according, during LiCoO ₂ is a positive electrode active material of the first layer, as is also shown in the Patent Documents 3 and 4, with respect to LiCoO ₂ In order not to cause decomposition reaction or crystal destruction of the electrolytic solution even at a high potential, a trace amount of different metal elements such as zirconium, magnesium, and aluminum may be contained.

In the nonaqueous electrolyte secondary battery of the present invention, the positive electrode active material in the first positive electrode mixture layer is composed of a composite oxide represented by a composition formula LiCoO ₂ containing at least cobalt and lithium. The positive electrode active material in the positive electrode mixture layer of the second layer has a composition formula LiNi _1-xy Co _x Mn _y O ₂ (0 <x <1, 0 <y <) containing at least cobalt, nickel, and manganese. The composite oxide represented by 1, 0 <x + y <1) may be used.

The composite oxide represented by the composition formula LiNi _1-xy Co _x Mn _y O ₂ (0 <x <1, 0 <y <1, 0 <x + y <1) containing cobalt, nickel, and manganese is Although it changes depending on the composition, the elution amount of nickel ions and cobalt ions when immersed in an acidic aqueous solution is significantly smaller than that of the composite oxide represented by the composition formula LiCoO ₂ . Therefore, even with the nonaqueous electrolyte secondary battery according to this aspect, it is possible to suppress the capacity deterioration during continuous charging or high-temperature storage and the deterioration of the charge / discharge cycle life.

  In the nonaqueous electrolyte secondary battery of the present invention, the end-of-charge voltage can be 4.40 V or more on the basis of lithium.

  In the nonaqueous electrolyte secondary battery of the present invention, since the charging voltage of the positive electrode can be 4.40 V or higher on the basis of lithium higher than that of the conventional one, it has a high capacity, but at the time of continuous charging or storage at high temperature. Capacitance deterioration and charge / discharge cycle life deterioration can be suppressed. Note that when a carbonaceous material is used as the negative electrode active material, the potential of the lithium-based carbonaceous material is 0.10 V, and thus the end-of-charge voltage is 4.30 V or more. In addition, although the upper limit of a charge end voltage changes also with a composition of a positive electrode active material, in order to ensure thermal stability and safety, 5.00 V or less is desirable on the basis of lithium.

  Hereinafter, the best mode for carrying out the present invention will be described in detail using examples and comparative examples. However, the following examples illustrate non-aqueous electrolyte secondary batteries for embodying the technical idea of the present invention, and are not intended to specify the present invention to these examples. The present invention can be equally applied to various modifications without departing from the technical idea shown in the claims.

[Investigation of solubility of each positive electrode active material]
First, in order to evaluate the oxidation resistance of each positive electrode material, LiCoO ₂ , LiNi _0.8 Co _0.2 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiNi _0.33 Co _{0. 33} 5 types of positive electrode active material samples of Mn _0.33 O ₂ and LiFePO ₄ were prepared, 1 g of each positive electrode active material was sampled, and a positive electrode having a constant mass in 10 mL of a 0.1 mol / L hydrochloric acid aqueous solution at 25 ° C. The active material was immersed and left for 1 hour. Next, the concentration (excluding Li) (mg / g) of the metal component eluted by ICP (inductively coupled plasma) emission spectrometry was measured for the water in each container, and the results were converted to mol / L units, and LiCoO ₂ The amount of metal (%) eluted from each positive electrode active material was determined by standardizing Co concentration eluted from 100%. The results are shown in Table 1.

From the results shown in Table 1, LiNi _0.8 Co _0.2 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ and LiNi _0.33 Co _0.33 Mn _0.33 O ₂ It can be seen that the elution amount of the metal component is very small, but the elution amount of the metal component from LiCoO ₂ and LiFePO ₄ is large. Therefore, LiCoO ₂ and LiFePO ₄ are preferably added to the positive electrode mixture layer of the first layer (lower layer side) formed on the surface of the positive electrode current collector, LiNi _0.8 Co _0.2 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ and LiNi _0.33 Co _0.33 Mn _0.33 O ₂ are desirably added to the positive electrode mixture layer of the second layer (surface side). I understand that.

[Creation of positive electrode plate]
The positive electrode plate used in each example and comparative example was prepared as follows. That is, 85 parts by mass of each positive electrode active material, 5 parts by mass of graphite powder as a conductive agent, and 5 parts by mass of carbon black were sufficiently mixed. Thereafter, a vinylidene fluoride polymer as a binder dissolved in N-methyl-2-pyrrolidone (NMP) was mixed to a solid content of 5 parts by mass to prepare a positive electrode mixture slurry. In the batteries of Examples 1 to 4 using the positive electrode mixture slurry thus prepared, as shown in FIG. 2, different types of positive electrode active materials on both surfaces of the positive electrode current collector 11A made of aluminum foil or the like. The first positive electrode mixture layer 11 </ b> B and the second positive electrode mixture layer 11 </ b> C including substantially the same thickness were formed. FIG. 2 is a partial cross-sectional view of the positive electrode plates of Examples 1 to 4. The active material in 11B in the first positive electrode mixture layer and the active material in the second positive electrode mixture layer 11C of each battery of Examples 1 to 4 are as shown in Table 2 below. Further, based on the results in Table 1, the amount of elution of the positive electrode active material in the second positive electrode mixture layer 11C with respect to the acidic aqueous solution is larger than the positive electrode active material in the first positive electrode mixture layer 11B. Are combined so that there are few.

  That is, of the positive electrode mixture slurry produced as described above, the positive electrode mixture slurry applied as the first positive electrode mixture layer 11B is formed on both surfaces of the aluminum positive electrode current collector 11A having a thickness of 20 μm by the doctor blade method. And then sufficiently drying, a positive electrode mixture slurry containing a positive electrode active material of a type different from that of the first positive electrode mixture layer 11B as the second positive electrode mixture layer 11C is applied to the first layer. Similarly, it was applied to substantially the same thickness so as to cover the positive electrode mixture layer 11B of the eye. Thereafter, the positive electrode mixture layer 11C of the second layer is also sufficiently dried and then rolled with a roller press until a predetermined thickness is obtained, and then cut into a strip shape having a width of 40 mm to produce the positive electrode plate 11. did. In addition, the active material in the positive mix layer of each nonaqueous electrolyte secondary battery of Examples 1-4 was put together in the following Table 2, and was shown.

As shown in Table 2, the positive electrode plate of each battery of Comparative Examples 1 to 6 was a positive electrode plate (Comparative Example 1) coated with one layer of a positive electrode mixture containing LiCoO ₂ , LiNi _{O 2.} A positive electrode plate (Comparative Example 2) coated with one layer of a positive electrode mixture containing ₃₃ Co _0.33 Mn _0.33 O ₂ , LiCoO ₂ and LiNi _{O 2 .} A positive electrode plate (Comparative Example 3) coated with one layer of a positive electrode mixture containing a positive electrode active material in which ₈ Co _0.2 O ₂ is mixed at a mass ratio of 1: 1, LiCoO ₂ and LiNi _0.5 Co _0.2 Mn _O. A positive electrode plate (Comparative Example 4) coated with one layer of a positive electrode mixture containing a positive electrode active material in which ₃ O ₂ is mixed at a mass ratio of 1: 1, LiCoO ₂ and LiNi _0.33 Co _0.33 Mn _0.33 O ₂ mass ratio of 1: positive electrode plate was 1 layer coating positive electrode mixture containing a mixed positive electrode active material in 1 (ratio Norei _{5), LiNiO 0.33 Co 0.33 Mn} O. to the first-layer A positive electrode plate (a reverse pattern of Comparative Example 6 and Example 3) in which a positive electrode mixture layer containing ₃₃ O ₂ and a positive electrode mixture layer containing LiCoO ₂ were formed in the second layer was prepared. . In the batteries of Comparative Examples 1 to 5, a positive electrode plate having only one positive electrode mixture layer was formed. In this case, the positive electrode mixture was made to have the same thickness as the positive electrode plates of Examples 1 to 4. An agent slurry was applied and produced in the same manner as in Examples 1 to 4. In addition, the active material in the positive mix layer of each nonaqueous electrolyte secondary battery of Comparative Examples 1 to 6 is shown in Table 2 below.

[Production of negative electrode]
Vinylidene fluoride polymer as a binder dissolved in N-methyl-2-pyrrolidone (NMP) in 95 parts by mass of natural graphite (Lc value is 150 Å or more and d value is 3.38 Å or less) powder Were mixed so as to be 5 parts by mass as a solid content to prepare a negative electrode mixture slurry. Thereafter, the obtained negative electrode mixture slurry was applied to both surfaces of a negative electrode current collector (copper foil) having a thickness of 18 μm by a doctor blade method to form a negative electrode mixture layer on both surfaces of the negative electrode current collector. Next, after drying, the product was rolled with a roller press until a predetermined thickness was obtained, cut into a strip shape having a width of 42 mm, and a negative electrode lead was welded to the end portion to prepare a negative electrode plate 12 (see FIG. 1).

[Production of battery]
Each of the positive electrode plates of Examples 1 to 4 and Comparative Examples 1 to 6 prepared as described above and the negative electrode plate are opposed to each other through a polyethylene microporous membrane separator 13 having a width of 44 mm and a thickness of 25 μm. After being arranged in such a manner, it was wound around a cylindrical winding core to produce a cylindrical wound electrode body. Furthermore, the outermost periphery was fixed with a polypropylene tape so that the cylindrical wound electrode body could not be unwound. Next, this cylindrical wound electrode body was pressed to obtain a wound electrode body having an oval cross-sectional shape. The press pressure was controlled so that the thickness of the oval wound electrode body after pressing was 4.2 mm. Thereafter, a tape was affixed to the surface of the oval wound electrode body that contacted the bottom of the can to insulate it from the can bottom. These oval wound electrode bodies were inserted into a rectangular battery can as shown in FIG. 1, and the sealing body provided with the liquid inlet was sealed by laser welding. Then, by injecting 2.5 g of electrolyte into a hole for electrolyte injection provided in the sealing body portion, an aluminum plate is installed in the injection port, and sealed by laser welding to a height of 45 mm. Non-aqueous electrolyte secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 6 having a width of 54 mm and a thickness of 5.0 mm were produced. As the electrolytic solution, a solution obtained by dissolving 1 mol / L LiPF ₆ in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a mass ratio of 3: 7 was used.

[Continuous charging test at normal charging voltage]
First, various measurement data were obtained when 4.20 V (the positive electrode potential is 4.30 V with respect to lithium), which is normally used as the upper limit voltage for charging, was employed. First, the batteries of Examples 1 to 4 and Comparative Examples 1 to 6 were charged in a 25 ° C. atmosphere at a constant current of 0.60 A until the battery voltage reached 4.20 V, and then 4.20 V. The battery was charged at a constant voltage until the charging current reached 0.03 A. After 10 minutes of rest, the battery was discharged at a constant current of 0.12 A until the battery voltage reached 2.75 V, and the initial discharge capacity was measured. Thereafter, the battery was charged at a constant current of 0.60 A until the battery voltage reached 4.20 V in an atmosphere of 25 ° C., and then continuously charged for 60 days at a constant voltage of 4.20 V. After the continuous charging, the battery is discharged at a constant current of 0.12 A until the battery voltage reaches 2.75 V in an atmosphere of 25 ° C., and further charged until the battery voltage reaches 4.20 V at a constant current of 0.60 A. Thereafter, the battery was charged at a constant voltage of 4.20 V until the charging current reached 0.03 A. Then, after resting for 10 minutes, the battery was discharged at a constant current of 0.12 A until the battery voltage reached 2.75 V, and the return discharge capacity after continuous charging was measured. From these results, the capacity recovery rate (%) after continuous charging was determined according to the following equation (1). The results are summarized in Table 2.
Capacity recovery rate after continuous charging (%)
= (Reset discharge capacity after continuous charge / initial discharge capacity) × 100 (1)

[High temperature storage test at normal charging voltage]
The batteries of Examples 1 to 4 and Comparative Examples 1 to 6 were charged at a constant current of 0.60 A until the battery voltage reached 4.20 V in an atmosphere at 25 ° C., and then at a constant voltage of 4.20 V. The battery was charged until the charge current reached 0.03A. After 10 minutes of rest, the battery was discharged at a constant current of 0.12 A until the battery voltage reached 2.75 V, and the initial discharge capacity was measured. Next, in an atmosphere of 25 ° C., the battery was charged with a constant current of 0.60 A until the battery voltage reached 4.20 V, and then charged with a constant voltage of 4.20 V until the charging current became 0.03 A, It preserve | saved for 60 days in 60 degreeC atmosphere. After the storage test is completed, the battery is discharged at a constant current of 0.12 A until the battery voltage reaches 2.75 V in an atmosphere at 25 ° C., and further charged until the battery voltage reaches 4.20 V at a constant current of 0.60 A. Thereafter, the battery was charged at a constant voltage of 4.20 V until the charging current reached 0.03 A. Then, after resting for 10 minutes, the battery was discharged at a constant current of 0.12 A until the battery voltage reached 2.75 V, and the return discharge capacity after high temperature storage was measured. From these results, the capacity recovery rate (%) after high-temperature storage was determined according to the following formula (2). The results are summarized in Table 2.
Capacity recovery rate after storage at high temperature (%)
= (Reset discharge capacity after high temperature storage / Initial discharge capacity) × 100 (2)

From the results shown in Table 2, the following can be understood. That is, the nonaqueous electrolyte secondary battery of Examples 1 to 3 using a positive electrode active material with a small amount of metal element elution when immersed in an acidic aqueous solution in the second layer as a positive electrode plate, and these materials were mixed When comparing the results of the non-aqueous electrolyte secondary batteries of Comparative Examples 3 to 5 using the coated electrode plate, the initial capacity is almost the same, but the capacity recovery rate after continuous charging and the capacity recovery after high temperature storage Both the non-aqueous electrolyte secondary batteries of Examples 1 to 3 gave better results. In addition, the nonaqueous electrolyte secondary battery of Comparative Example 1 in which the positive electrode plate is a single layer containing LiCoO ₂ as the positive electrode active material has a good initial discharge capacity, but the capacity is restored after continuous charging. The capacity recovery rate after storage at high temperature is poor. Furthermore, the nonaqueous electrolyte secondary battery of Comparative Example 2 in which the positive electrode plate is a single layer containing LiNi _0.33 Co _0.33 Mn _0.33 O ₂ as a positive electrode active material has a capacity recovery rate after continuous charging, Although a good result has been obtained for the capacity recovery rate after high-temperature storage, the initial discharge capacity is inferior.

Furthermore, when comparing the results of the nonaqueous electrolyte secondary battery of Example 3 and the nonaqueous electrolyte secondary battery of Comparative Example 6 in which the arrangement relationship of the first layer and the second layer is opposite to each other, the initial discharge capacity is Although good results were obtained for both, the non-aqueous electrolyte secondary battery of Comparative Example 6 was inferior in terms of the capacity recovery rate after continuous charging and the capacity recovery rate after high-temperature storage. In addition, the nonaqueous electrolyte secondary battery of Example 4 in which the first layer is a layer containing LiFePO ₄ as a positive electrode active material and the second layer is a layer containing LiCoO ₂ as a positive electrode active material has an initial discharge capacity. However, the capacity recovery rate after continuous charging and the capacity recovery rate after high-temperature storage are good. The measurement result of Example 4 shows that even when LiFePO ₄ , which is the positive electrode active material having the largest amount of elution of metal elements excluding lithium when the first layer is immersed in an acidic aqueous solution, When the second layer containing a positive electrode active material with a small amount of metal element elution when immersed in an acidic aqueous solution than LiFePO ₄ is formed on the surface, the capacity recovery rate after continuous charging and the capacity recovery rate after high-temperature storage are It shows that it becomes good.

[Continuous charge test at high charge voltage]
Next, various measurement data were obtained when 4.30 V (the positive electrode potential was 4.40 V on the basis of lithium) higher than the 4.20 V that is normally employed as the charge upper limit voltage. First, each of the batteries of Examples 1 to 4 and Comparative Examples 1 to 6 was charged in a 25 ° C. atmosphere at a constant current of 0.60 A until the battery voltage reached 4.30 V, and then 4.30 V. The battery was charged at a constant voltage until the charging current reached 0.03 A. After 10 minutes of rest, the battery was discharged at a constant current of 0.12 A until the battery voltage reached 2.75 V, and the initial discharge capacity was measured. Thereafter, the battery was charged at a constant current of 0.60 A until the battery voltage reached 4.30 V in an atmosphere of 25 ° C., and then continuously charged for 60 days at a constant voltage of 4.30 V. After the end of continuous charging, the battery is discharged at a constant current of 0.12 A until the battery voltage reaches 2.75 V in an atmosphere of 25 ° C., and further charged until the battery voltage reaches 4.30 V at a constant current of 0.60 A. Thereafter, the battery was charged at a constant voltage of 4.30 V until the charging current reached 0.03 A. Then, after resting for 10 minutes, the battery was discharged at a constant current of 0.12 A until the battery voltage reached 2.75 V, and the return discharge capacity after continuous charging was measured. From these results, the capacity recovery rate (%) after continuous charging was determined according to the above equation (1). The results are summarized in Table 3.

[High temperature storage test at high charging voltage]
For the batteries of Examples 1 to 4 and Comparative Examples 1 to 6, the battery was charged at a constant current of 0.60 A at a constant current of 0.60 A until the battery voltage reached 4.30 V, and then at a constant voltage of 4.30 V. The battery was charged until the charge current reached 0.03A. After 10 minutes of rest, the battery was discharged at a constant current of 0.12 A until the battery voltage reached 2.75 V, and the initial discharge capacity was measured. Next, in an atmosphere of 25 ° C., the battery was charged with a constant current of 0.60 A until the battery voltage reached 4.30 V, and then charged with a constant voltage of 4.30 V until the charging current became 0.03 A. It preserve | saved for 60 days in 60 degreeC atmosphere. After the storage test is completed, the battery is discharged at a constant current of 0.12 A until the battery voltage reaches 2.75 V in an atmosphere of 25 ° C., and further charged until the battery voltage reaches 4.30 V at a constant current of 0.60 A. Thereafter, the battery was charged at a constant voltage of 4.30 V until the charging current reached 0.03 A. Then, after resting for 10 minutes, the battery was discharged at a constant current of 0.12 A until the battery voltage reached 2.75 V, and the return discharge capacity after high temperature storage was measured. From these results, the capacity recovery rate (%) after high-temperature storage was determined according to the above equation (2). The results are summarized in Table 3.

  As is clear when the results of Table 2 and Table 3 are compared at the same time, when charging is performed at the charging upper limit voltage 4.30V, compared with the case where charging is performed at the charging upper limit voltage 4.20V, Examples 1-4 and comparison In all the non-aqueous electrolyte secondary batteries of Examples 1 to 6, the initial discharge capacity is increased. And the relationship between the result of the nonaqueous electrolyte secondary battery of Examples 1-4 at the time of charging with charge upper limit voltage 4.30V and the result of the nonaqueous electrolyte secondary battery of Comparative Examples 1-6 is The result of the tendency similar to the result at the time of charging with charge upper limit voltage 4.20V is obtained. However, in the nonaqueous electrolyte secondary batteries of Examples 1 to 4, the capacity recovery rate after continuous charging in the case of charging at a charging upper limit voltage of 4.30 V and in the case of charging at a charging upper limit voltage of 4.20 V In addition, substantially the same results were obtained for both the capacity recovery rate after high-temperature storage. On the other hand, in the nonaqueous electrolyte secondary batteries of Comparative Examples 1 to 6, when charging was performed at the charging upper limit voltage of 4.30 V, the capacity recovery rate after continuous charging when charging was performed at the charging upper limit voltage of 4.20 V In addition, compared with the capacity recovery rate after storage at high temperature, it is greatly degraded. Therefore, when a positive electrode active material with a small amount of metal element eluted when immersed in an acidic aqueous solution in the second layer 11C is used as the positive electrode plate, the charge upper limit voltage is 4.20V or 4.30V, It can be seen that good results are obtained for both the capacity recovery rate after continuous charging and the capacity recovery rate after high-temperature storage.

  In Examples 1 to 4, an example in which only one type of positive electrode active material is used as the first layer active material is shown, but a mixture of two or more types of positive electrode active materials may be used. Moreover, the same material as the positive electrode active material of the second layer may be mixed in the positive electrode active material of the first layer. Furthermore, the positive electrode active material of the second layer may also be one kind, or two or more kinds of active materials may be mixed. In Examples 1 to 4, the first layer and the second layer were formed so that the coating thicknesses were the same, but the thicknesses of the first layer and the second layer may not be the same. Furthermore, in Examples 1-4, although the example which used aluminum as a material of a positive electrode electrical power collector was shown, an aluminum alloy and another metal can be used. Further, as the structure of the positive electrode current collector, punching metal can be used in addition to the foil.

It is a perspective view which cut | disconnects and shows the conventional square nonaqueous electrolyte secondary battery to the vertical direction. It is a fragmentary sectional view of the positive electrode plate of an Example.

Explanation of symbols

10: Nonaqueous electrolyte secondary battery 11: Positive electrode plate 11A: Positive electrode current collector 11B: First positive electrode mixture layer 11C: Second positive electrode mixture layer 12: Negative electrode plate 13: Separator 14 : Flat wound electrode body 15: Rectangular battery outer can 16: Sealing plate 17: Insulator 18: Negative electrode terminal 19: Negative electrode tab 20: Insulating spacer 21: Electrolyte injection hole

Claims (4)

A positive electrode plate in which a positive electrode mixture containing a positive electrode active material containing lithium is applied to a positive electrode current collector, a negative electrode plate in which a negative electrode mixture containing a negative electrode active material is applied to a negative electrode current collector, and a separator And a non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte solution in which an electrolyte salt is dissolved in a non-aqueous solvent,
The positive electrode plate includes a first positive electrode mixture layer formed on both sides or one side of a positive electrode current collector, and a first layer formed on a surface of the first positive electrode mixture layer. A second positive electrode mixture layer having a composition different from that of
The amount of elution of metal elements excluding lithium when the positive electrode active material in the second positive electrode mixture layer is immersed in an acidic aqueous solution rather than the positive electrode active material in the first positive electrode mixture layer A non-aqueous electrolyte secondary battery characterized by having a low content. The positive electrode active material in the first positive electrode mixture layer is composed of a composite oxide represented by a composition formula LiCoO ₂ containing at least cobalt and lithium, and the positive electrode in the second positive electrode mixture layer _2. The non-aqueous electrolyte 2 according to claim 1, wherein the active material is composed of a composite oxide represented by a composition formula LiNi _1-x Co _x O ₂ (0 <x <1) containing at least cobalt and nickel. Next battery. The positive electrode active material in the first positive electrode mixture layer is composed of a composite oxide represented by a composition formula LiCoO ₂ containing at least cobalt and lithium, and the positive electrode in the second positive electrode mixture layer The active material is a composite oxidation represented by the composition formula LiNi _1-xy Co _x Mn _y O ₂ (0 <x <1, 0 <y <1, 0 <x + y <1) including at least cobalt, nickel, and manganese. The non-aqueous electrolyte secondary battery according to claim 1, comprising a product.   The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein an end-of-charge voltage of the non-aqueous electrolyte secondary battery is 4.40 V or more based on lithium.

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