JPWO1998054778A1 - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

(57) [Abstract] Provided is a nonaqueous electrolyte secondary battery with high capacity and improved charge/discharge characteristics. The nonaqueous electrolyte secondary battery is a combination of a positive electrode (4) and a negative electrode (2) so as to satisfy the relationship 0.9≦Kp/Kn≦1.1, where Kp is the initial efficiency of the positive electrode (4) and Kn is the initial efficiency of the negative electrode (2). The active material of the positive _electrode ( ₄ ) is preferably a lithium _composite oxide having a composition of _LixNiyMzO2 (0.8<x<1.5, 0.8<y+z<1.2, 0≦z<0.35; M is at least one element selected from Co, Mg, Ca, Sr, Al, Mn, and Fe).

Description

[Detailed Description of the Invention] Non-aqueous Electrolyte Secondary Battery Technical Field The present invention relates to a non-aqueous electrolyte secondary battery, and more specifically, to a non-aqueous electrolyte secondary battery with high capacity and improved charge-discharge characteristics.

BACKGROUND ART The electronics field has seen remarkable development in recent years, resulting in the development of a wide variety of devices, including video cameras, LCD cameras, mobile phones, laptop computers, and word processors. Correspondingly, there is a growing demand for smaller, lighter, and more energy-dense batteries to power these electronic devices.

Traditionally, lead-acid batteries or nickel-cadmium batteries have been used in these electronic devices, but these batteries are unable to adequately meet the demands for miniaturization, weight reduction, and high energy density.

Therefore, non-aqueous electrolyte batteries, which use a non-aqueous electrolyte consisting of a lithium salt dissolved in a non-aqueous solvent, have been proposed. These non-aqueous electrolyte batteries, which use lithium, lithium alloys, or carbon materials capable of being doped and dedoped with lithium ions as the anode material and lithium-cobalt composite oxide as the cathode material, have already been put to practical use.

This type of non-aqueous electrolyte battery has the advantages of a high operating voltage of 3 to 4 V, allowing for high energy density, low self-discharge, and excellent cycle characteristics.

Furthermore, research and development into active materials for nonaqueous electrolyte batteries is being actively pursued to achieve further miniaturization, weight reduction, and higher energy density. Ni-containing lithium composite oxides, such as lithium nickel composite oxide and lithium nickel cobalt composite oxide, have also been proposed as positive electrode active materials.

For example, Japanese Patent Laid-Open Publication No. 5-290847 discloses that Li1 _{+ xCoO2} is used as a positive electrode active material, and lithium equivalent to this potential capacity is used for precharging the negative electrode, thereby increasing the capacity. However, the increase in capacity is insufficient.

DISCLOSURE OF THE INVENTION OBJECT OF THE INVENTION The present inventors have investigated the capacity and charge/discharge characteristics of non-aqueous electrolyte secondary batteries and found that simply increasing the capacity of each of the positive and negative electrodes does not improve the capacity or charge/discharge characteristics of the battery as designed.

Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art and to provide a non-aqueous electrolyte secondary battery having a high capacity and improved charge-discharge characteristics. SUMMARY OF THE INVENTION As a result of extensive research, the inventors have focused on the initial efficiencies of the positive electrode and the negative electrode, and have found that by combining the positive electrode and the negative electrode so that the initial efficiencies of both electrodes satisfy a specific relationship, it is possible to achieve a high battery capacity and improved charge-discharge characteristics, and have completed the present invention.

That is, the present invention relates to a nonaqueous electrolyte secondary battery having a positive electrode and a negative electrode capable of being doped and dedoped with lithium ions, in which the positive electrode and the negative electrode are combined so as to satisfy the relationship 0.9≦Kp/Kn≦1.1, where Kp is the initial efficiency of the positive electrode and Kn is the initial efficiency of the negative electrode.

In the present invention, the initial efficiency (Kp) of a positive electrode is the ratio of the discharge capacity to the charge capacity when the positive electrode is initially charged to 4.2 V and discharged to 3.0 V using lithium as the counter electrode, i.e., Kp = (initial discharge capacity) / (initial charge capacity).

The initial efficiency (Kn) of the negative electrode is the ratio of the charge capacity to the discharge capacity when the battery is initially discharged to +0.0 V and then charged to 2.0 V using lithium as the counter electrode. In other words, Kn = (initial charge capacity) / (initial discharge capacity).

Nickel-containing lithium composite oxides as active materials have high capacity, but their initial efficiency tends to be lower than that of lithium cobalt oxide. Non-graphitizable carbons and polymeric carbons also have high capacity but lower initial efficiency compared to graphite-based active materials. Traditionally, when there is a difference in initial efficiency between the positive and negative electrodes, batteries are built by packing fewer electrodes with poor initial efficiency or by placing a load on electrodes with good initial efficiency. While increasing capacity tends to result in poor initial charge/discharge efficiency, efforts have traditionally been made to improve initial efficiency. Furthermore, in conventional lead-acid batteries, for example, the concentration of electrolyte in the electrolyte solution changes during charging and discharging. In batteries that generate electricity by reacting electrolyte anions and cations with the positive and negative electrodes, capacity varies significantly depending on initial efficiency, so increasing initial efficiency is effective in improving capacity.

However, in lithium-ion secondary batteries, electricity is extracted externally by lithium ions moving between the positive and negative electrodes, so the electrolyte concentration in the electrolyte solution remains essentially unchanged. In other words, since the electrolyte in the electrolyte solution is not consumed during charging and discharging, there is no need to forcefully increase the initial efficiency. Rather, it has been found that the ratio of the initial efficiencies of the positive and negative electrodes is important. In the present invention, by ensuring that the ratio of the initial efficiencies of the positive and negative electrodes falls within the above-mentioned specific range, a battery with high capacity and improved charge and discharge characteristics is provided.

In the present invention, the active material of the positive _{electrode is LixNiyMzO2} (where x is 0.8<x<1.5, y+z is 0.8<y+z<1.2, and z is 0≦z<0.35 ₎ .

M represents at least one element selected from Co, Mg, Ca, Sr, Al, Mn, and Fe.Detailed Description of the InventionIn the present invention, a lithium composite oxide is used as the positive electrode active material. Examples of lithium composite oxides include Li_xCoO₂(0<x≦1.0), Li_xNiO₂( 0<x≦1.0), Li_1+xMn_2-xO₄(0≦x≦1/3), Li (M, Mn)₂ O₄(M=Cr, Co, Al, B), etc.

In the present invention, the _lithium composite oxide is _{particularly preferably LixNiyMzO2 (} where x is 0.8<x<1.5, y+z is 0.8<y+z<1.2, z is 0≦z<0.35, and M represents at least one element selected from Co, Mg, Ca, Sr, Al, Mn, and Fe) from the viewpoints of high capacity and low cost. In this case, the metal M is more preferably Co, and may be two or more metals.

Examples of methods for producing such lithium composite oxides include the LiMetal³⁺O₂ (Here, Metal refers to Ni as the main component and at least one element selected from Co, Mg, Ca, Sr, Al, Mn, and Fe.) One example of a method for producing a slurry is to react an alkaline water-soluble lithium compound containing an anion that volatilizes during firing with a basic metal salt in an aqueous medium, and then dry and fire the resulting slurry.

The basic metal salt is represented by Metal ²⁺ (OH) _2-nk (A ^n- ) _k ·mH ₂ O. Here, Metal ²⁺ mainly contains Ni and may contain at least one element selected from Co, Mg, Ca, Sr, Al, Mn, and Fe. ^{A n-} represents an n-valent anion (n = 1 to 3) such as nitrate ion, chloride ion, bromide ion, acetate ion, or carbonate ion. Furthermore, k is 0.03≦k≦0.3, and m is 0≦m<2.

The basic metal salt represented by this formula can be produced by adding about 0.7 to 0.95 equivalents, preferably about 0.8 to 0.95 equivalents, of an alkali relative to Metal ²⁺ to an aqueous solution of Metal ²⁺ at reaction conditions below about 80°C, followed by aging at 40 to 70°C for 0.1 to 10 hours and removing by-products by washing with water. The alkali used here includes alkali metal hydroxides such as sodium hydroxide, alkaline earth metal hydroxides such as calcium hydroxide, and amines.

A basic metal salt selected from the compounds represented by this formula is reacted with one or more lithium compounds selected from lithium hydroxide, lithium carbonate, or hydrates thereof in water at a reaction solution concentration of 5 to 25% by weight and at a reaction temperature of room temperature to 100°C to obtain a slurry. The resulting composition is then spray-dried to improve the uniformity of its shape.

The dried product is calcined by heat treatment in an oxidizing gas atmosphere containing air, oxygen, ozone, or the like at a temperature range of about 700 to 1000°C for about 0.1 to 20 hours, thereby obtaining a lithium composite oxide.

Another example of a method for producing the lithium composite oxide used in the present invention is a method using a basic metal carbonate obtained from a water-soluble metal compound and a water-soluble lithium compound.

The water-soluble metal compound used here is a nitrate, sulfate, metal chloride, or the like. This water-soluble metal compound may be a mixture of a nickel compound as the main component and a predetermined amount of another water-soluble metal compound so that at least one element selected from Co, Mg, Ca, Sr, Al, Mn, and Fe is incorporated.

Basic metal carbonates can be obtained by filtering and drying the precipitate obtained by reacting the mixture of water-soluble metal compounds described above with a compound selected from the group consisting of alkali carbonates, alkali bicarbonates, ammonium carbonate, and ammonium bicarbonate in water, or by further reacting the same reaction system in the presence of sodium hydroxide. In this case, to produce a good precipitate, it is best to use a slight excess of carbonate radicals, and it is also important to control the stirring conditions in order to control the specific surface area of the precipitate.

The basic metal carbonate thus obtained is mixed with a powder of a water-soluble lithium compound, such as lithium carbonate or lithium hydroxide, in the desired ratio of metal to Li. This mixture is first heated in the powder form to 300-500°C in the presence of an inert gas or an oxygen-containing gas. This heating only promotes decomposition of the basic metal carbonate, and carbon dioxide gas within the crystalline structure is released. This heating is continued until the evolution of carbon dioxide gas substantially ceases, converting all of the basic metal carbonate into a metal oxide having numerous fine pores.

After the evolution of carbon dioxide gas has substantially ceased, the temperature is further increased, and the molten water-soluble lithium compound penetrates into the micropores of the metal oxide, bringing them into very intimate contact. When the mixture is then fired at a temperature of 700 to 900°C in the presence of oxygen gas or oxygen-enriched air, the Ni becomes divalent and trivalent, producing a Li composite oxide.

The basic metal carbonate used here is preferably one with a larger specific surface area (for example, 100 m ² /g or more) since this increases the efficiency of gas release and the generation of micropores after pre-baking.

This positive electrode active material, a conductive agent such as acetylene black or graphite, and a binder such as polytetrafluoroethylene or vinylidene polyfluoride are mixed with an organic solvent such as N-methyl-2-pyrrolidone to prepare a positive electrode mixture paint. This paint can then be applied to a current collector such as aluminum foil and dried to obtain a positive electrode.

The conductive agent, binder, organic solvent, and current collector are not particularly limited and may be selected from a variety of materials.

Next, a negative electrode active material that is used in combination with such a positive electrode active material will be described.

The negative electrode active material can be any material capable of being doped or dedoped with lithium, lithium alloys, or lithium ions, without any particular limitations. Examples of such materials include carbon materials and tin oxides. Specific examples include graphite, glassy carbons, and polymer carbons (hard carbons obtained by carbonizing synthetic resins such as cellulose, phenolic resin, furfural resin, polyparaphenylene, and polyacrylonitrile), which are carbon materials obtained by heat-treating high polymers having a cross-linked structure in an inert atmosphere. Polymer carbons are particularly suitable due to their high capacity.

The negative electrode active material, conductive agent, and binder are mixed or kneaded together with an organic solvent to prepare a negative electrode mixture coating. This coating is applied to a current collector such as copper foil and dried to obtain a negative electrode. The conductive agent, binder, organic solvent, and current collector are not particularly limited and can be selected from a variety of materials.

In the present invention, when combining such positive and negative electrodes, the relationship 0.9≦Kp/Kn≦1.1 must be satisfied, where Kp is the initial efficiency of the positive electrode and Kn is the initial efficiency of the negative electrode. If the ratio Kp/Kn of the initial efficiency of the positive electrode to the initial efficiency of the negative electrode is either less than 0.9 or greater than 1.1, the efficiency and capacity of the battery will be reduced when used in such a battery. In other words, even if the initial efficiency of only one of the positive and negative electrodes is good, the battery capacity will not be improved. In the present invention, it is more preferable that Kp and Kn satisfy the relationship 0.95≦Kp/Kn≦1.05.

The initial efficiency of each positive electrode and negative electrode can be adjusted by the characteristics of the active material itself. For example, in the positive electrode, increasing the value of x in Li _x Ni _y M _z O ₂ decreases the initial efficiency. Furthermore, reducing defects in the crystal can improve the initial efficiency. In the negative electrode, the initial efficiency can be adjusted by attaching a substance that reacts with lithium to form a compound to the negative electrode surface. The initial efficiency also varies depending on the specific surface area, shape, and firing conditions of the active material. In terms of the internal structure, the initial efficiency can be changed by introducing elements other than carbon into the interior. Alternatively, the initial efficiency can be adjusted by mixing materials with different initial efficiencies and changing the mixing ratio.

The initial efficiency of each positive and negative electrode can be adjusted by the type and amount of conductive agent, the amount of binder, the pressing pressure, the degree of dispersion of the active material mixture coating, and other factors. For example, the initial efficiency of the electrode can be improved by increasing the amount of conductive agent within a range that does not reduce the electrode capacity, decreasing the amount of binder within a range that maintains the strength and adhesiveness of the active material layer, or increasing the pressing pressure. To decrease the initial efficiency of the electrode, simply make the opposite adjustments.

In the nonaqueous electrolyte secondary battery of the present invention, an electrolyte solution in which a lithium salt is used as a supporting electrolyte and is dissolved in an organic solvent is used.

The organic solvent is not particularly limited, but examples thereof include propylene carbonate, ethylene carbonate, dimethoxyethane, γ-butyrolactone, tetrahydrofuran, diethyl carbonate, methyl ethyl carbonate, and dipropyl carbonate, which may be used alone or in combination.

The supporting electrolyte is not particularly limited, but LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , etc. may be used alone or in combination of two or more.

The nonaqueous electrolyte secondary battery of the present invention may be in a variety of forms. For example, in addition to coin-type batteries, a jelly roll formed by using a positive electrode, a negative electrode, and a separator and then housed in a round or rectangular can may be used.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of the cell used in the examples for measuring positive and negative electrode characteristics.

FIG. 2 is a cross-sectional view of a coin-type battery as an example of a nonaqueous electrolyte secondary battery of the present invention.

MODES FOR CARRYING OUT THE INVENTION The present invention will be further described in detail with reference to the following examples, but the present invention is not limited to these examples.

[Example 1] (Positive Electrode Fabrication) Nickel nitrate and cobalt nitrate were mixed in an aqueous solution at a Ni/Co molar ratio of 0.8/0.2 to form a precipitate, which was then dried in a spray dryer to obtain roughly spherical particles. These particles were mixed with lithium hydroxide at a Li/Ni/Co molar ratio of 1/0.8/0.2 and calcined. The resulting particles had a particle size of 15 μm.

Using this lithium composite oxide LiNi _0.8 Co _0.2 O ₂ as the active material, a positive electrode mixture paint having the following composition was prepared.

(Composition) Positive electrode active material: 93 _parts by weight of the above _{LiNi0.8Co0.2O2} Conductive agent: 4 parts by weight of Graphite KS44 manufactured by LONZA Binder: 3 parts by weight of KYNAR741 from Elf Atochem _Japan Polyvinylidene fluoride (PVDF) Solvent: 67 parts by weight of N-methyl-2-pyrrolidone (NMP) 3 parts by weight of PVDF was dissolved in 27 parts by weight of NMP to prepare 30 parts by weight of binder solution. 93 parts by weight of active material and 4 parts by weight of conductive material were dry mixed in a hypermixer, and the mixture was placed in a pressure kneader. 13 parts by weight of the above binder solution was added to the mixture, and the mixture was kneaded for 30 minutes while water-cooling the jacket of the pressure kneader. The kneaded product was removed, and 17 parts by weight of binder solution and 40 parts by weight of NMP were added and dissolved in a hypermixer to obtain an active material mixture coating.

The prepared mixture coating was applied to one side of a current collector made of 20 μm thick aluminum foil using a blade coater, dried, and then compression-molded using a roller press and cut to a specified size to obtain an electrode (P1) with an active material amount per unit volume of 3 g/cm ³ and a mixture layer thickness of 67 μm.

(Evaluation of Positive Electrode Characteristics) The electrode (P1) was cut into a length of 25 mm and a width of 20 mm, and the electrode layer was removed by a 5 mm width from the top end, leaving a 20 mm square electrode layer. A stainless steel wire was spot-welded to the top end from which the electrode layer was removed as a lead. Furthermore, a PVDF lacquer was applied to the back surface of the electrode (the side of the current collector without the active material layer) and allowed to dry to form a PVDF coating, creating an electrode for evaluation (working electrode).

A cell for measuring charge/discharge capacity was prepared as shown in FIG. 1, and charge/discharge was carried out as follows.

That is, referring to Figure 1, the electrode (13) prepared above was placed in a beaker (11). Facing the active material layer (13a) of the electrode (13), a counter electrode (14) made of a lithium plate connected to a stainless steel wire and a Luggin capillary (16) with a similar reference electrode (15) were placed in the beaker (11). The electrolyte (17) was prepared by dissolving 1 mol/L lithium perchlorate as the electrolyte salt in a 1:1 (volume ratio) mixture of ethylene carbonate and diethyl carbonate. The beaker (11) and the Luggin capillary (16) were sealed with silicone stoppers (12) and (18), respectively, to prepare a measurement cell.

The cell was charged at a constant current of 1 mA to a cut-off voltage of 4.2 V (potential vs. Li/Li ⁺ ) and discharged at a constant current of 1 mA to a cut-off voltage of 3.0 V (potential vs. Li/Li ⁺ ), and the charge/discharge capacity was determined. The discharge capacity of the positive electrode was 188 mA/g, and the initial efficiency K _P1 = (initial discharge capacity)/(initial charge capacity) = 0.88.

(Preparation of negative electrode) A negative electrode mixture paint was prepared according to the following composition:

(Composition) Anode active material: Non-graphitizable carbon (average particle size 11 μm) 82 parts by weight Conductive agent: LONZA Graphite KS25 9 parts by weight Binder: Elf Atochem Japan KYNAR741 9 parts by weight Polyvinylidene fluoride (PVDF) Solvent: N-methyl-2-pyrrolidone (NMP) 150 parts by weight 9 parts by weight of PVDF were dissolved in 81 parts by weight of NMP to prepare 90 parts by weight of binder solution. 82 parts by weight of active material and 9 parts by weight of conductive material were dry-mixed in a hypermixer, and the mixture was placed in a pressure kneader. 50 parts by weight of the binder solution was added to the mixture, and the mixture was kneaded for 60 minutes while the pressure kneader jacket was water-cooled. This kneaded mixture was taken out, and 40 parts by weight of the binder solution and 69 parts by weight of NMP were added, followed by dissolution in a hypermixer to obtain an active material mixture coating.

The prepared mixture coating was applied to one side of a current collector made of rolled copper foil 18 μm thick using a blade coater, dried, and then compression-molded using a roller press and cut to a specified size to obtain an electrode (N1) with an active material amount per unit volume of 1 g/cm ³ and a mixture layer thickness of 83 μm.

(Evaluation of Negative Electrode Characteristics) The electrode (N1) was cut into a length of 25 mm and a width of 20 mm, and the electrode layer was removed by a 5 mm width from the top end, leaving a 20 mm square electrode layer. A stainless steel wire was spot-welded to the top end from which the electrode layer was removed as a lead. Furthermore, PVDF lacquer was applied to the back surface of the electrode (the side of the current collector without the active material layer) and dried to form a PVDF coating, creating an electrode for evaluation (working electrode).

A charge/discharge capacity measurement cell similar to that shown in Figure 1 was prepared. That is, a similar cell was prepared except that the evaluation electrode was placed in place of electrode (13) in Figure 1. In this cell, a counter electrode (14) was placed facing the surface of the evaluation electrode on which the active material layer was formed.

The cell was discharged at a constant current of 1 mA to a cut-off voltage of 0.0 V (potential vs. Li/Li ⁺ ), and then charged at a constant current of 1 mA to a cut-off voltage of 2.0 V (potential vs. Li/ ^{Li +} ), to determine the charge/discharge capacity. The charge capacity of the negative electrode was 398 mA/g, and the initial efficiency K _N1 = (initial charge capacity)/(initial discharge capacity) = 0.81.

(Battery Fabrication and Evaluation of Battery Characteristics) Using the above-described positive electrode (P1) and negative electrode (N1), a coin-type battery was fabricated as shown in Figure 2, as an example of a nonaqueous electrolyte secondary battery.

The positive electrode (P1) was punched out to a diameter of 15 mm, and the negative electrode (N1) was punched out to a diameter of 15.5 mm. A nonaqueous electrolyte was prepared by dissolving _LiPF6 at a concentration of 1 mol/L in a 1:1 (volume ratio) mixture of ethylene carbonate and diethyl carbonate.

Using this nonaqueous electrolyte, positive and negative electrodes, a thin polypropylene separator, a negative electrode cup, a positive electrode can, and a gasket, a coin-type battery with a diameter of 20 mm and a thickness of 2.5 mm was fabricated, as shown in Figure 2. In Figure 2, this coin-type battery consists of a positive electrode (P1) (4) housed in a positive electrode can (6) and a negative electrode (N1) (2) housed in a negative electrode cup (1), stacked with a separator (3) in between, and the positive electrode can (6) and the negative electrode cup (1) are crimped together with a gasket (5) to seal them.

The current collector is not shown in the illustration.

The battery thus fabricated was charged at a charging current of 1 mA until the battery voltage reached 4.2 V, and then charged for 20 hours until the battery voltage reached 4.2 V. The battery was then discharged at a discharge current of 1 mA and a cut-off voltage of 2.5 V to determine the discharge capacity, which was 5.9 mAh.

[Example 2] (Preparation of Positive Electrode) The LiNi used in Example 1_0.8Co_0.2O₂Instead of Li_1.17Ni_0.8Co_0.2 O₂The same procedure as in Example 1 was repeated except that the amount of active material per unit volume was 3 g/cm.³An electrode (P2) with a mixture layer thickness of 64 μm was obtained. The discharge capacity of this positive electrode was 187 mA/g, and the initial efficiency was K_P2=0.72.

(Fabrication of Negative Electrode) An electrode (N2) having an active material amount per unit volume of 1 g/cm3 and a mixture layer thickness of 86 μm was obtained in the same manner as in Example 1, except that non-graphitizable carbon (average particle size 4.2 μm) was used as the active material instead of the non-graphitizable carbon (average particle size 11 μm) used in Example ^1. The charge capacity of this negative electrode was 397 mA/g, and the initial efficiency K _N2 = 0.72.

(Battery Fabrication and Evaluation of Battery Characteristics) A coin-type battery was fabricated using the positive electrode (P2) and negative electrode (N2) in the same manner as in Example 1. The discharge capacity was determined to be 6.0 mAh.

[Example 3] (Preparation of Positive Electrode) An electrode (P3) was obtained with the same composition as in Example 2, but with an active material amount per unit volume of 3 g/ ^cm3 and a mixture layer thickness of 60 μm. The discharge capacity of this positive electrode was 187 mA/g and the initial efficiency K _P3 = 0.72.

(Preparation of Negative Electrode) An electrode (N3) having an active material amount per unit volume of 1 g/cm3 and a mixture layer thickness of 90 μm was obtained in the same manner as in Example 1, except that a mixture of 64 parts by weight of the non-graphitizable carbon (average particle size 11 μm) used in Example 1 and ¹⁶ parts by weight of the non-graphitizable carbon (average particle size 4.2 μm) used in Example 2 was used as the active material. The charge capacity of this negative electrode was 398 mA/g, and the initial efficiency K _N3 was 0.79.

(Battery Fabrication and Evaluation of Battery Characteristics) A coin-type battery was fabricated using the positive electrode (P3) and negative electrode (N3) in the same manner as in Example 1. The discharge capacity was determined to be 5.8 mAh.

[Example 4] (Preparation of Positive Electrode) An electrode (P4) having an active material amount per unit volume _{of 3} g _/ cm3 and a mixture layer thickness _{of 65} μm was obtained in _the same manner as in Example 1, except that the active material used was a mixture of ₇₄ parts by weight of _{LiNi0.8Co0.2O2} used in Example ¹ and 19 parts by weight of Li1.17Ni0.8Co0.2O2 used in Example 2. The discharge capacity of this positive electrode was 188 mA/g, and the initial efficiency _KP4 was 0.85.

(Preparation of Negative Electrode) A negative electrode mixture coating having the same composition as that used in Example 1 was applied to one side of a current collector made of rolled copper foil 18 μm thick using a blade coater, dried, and then compression-molded using a roller press and cut to a predetermined size to obtain an electrode (N4) with an active material amount per unit volume of 1 g/ ^cm3 and a mixture layer thickness of 85 μm. The charge capacity of this negative electrode was 398 mA/g and the initial efficiency K _N4 = 0.81.

(Battery Fabrication and Evaluation of Battery Characteristics) A coin-type battery was fabricated using the positive electrode (P4) and negative electrode (N4) in the same manner as in Example 1. The discharge capacity was determined to be 6.0 mAh.

[Comparative Example 1] (Preparation of Positive Electrode) A positive electrode mixture coating having the same composition as that used in Example 1 was applied to one side of a 20 μm thick aluminum foil current collector using a blade coater, dried, and then compression-molded using a roller press and cut to a predetermined size to obtain an electrode (P5) with an active material amount per unit volume of 3 g/cm ³ and a mixture layer thickness of 71 μm. The discharge capacity of this positive electrode was 188 mA/g and the initial efficiency K _P5 = 0.88.

(Preparation of Negative Electrode) A negative electrode mixture coating having the same composition as that used in Example 2 was applied to one side of a current collector made of rolled copper foil 18 μm thick using a blade coater, dried, and then compression-molded using a roller press and cut to a predetermined size to obtain an electrode (N5) with an active material amount per unit volume of 1 g/ ^cm3 and a mixture layer thickness of 79 μm. The charge capacity of this negative electrode was 397 mA/g and the initial efficiency K _N5 = 0.72.

(Battery Fabrication and Evaluation of Battery Characteristics) A coin-type battery was fabricated using the positive electrode (P5) and negative electrode (N5) in the same manner as in Example 1. The discharge capacity was determined to be 5.6 mAh.

[Comparative Example 2] (Preparation of Positive Electrode) A positive electrode mixture coating having the same composition as that used in Example 2 was applied to one side of a 20 μm thick aluminum foil current collector using a blade coater, dried, and then compression-molded using a roller press and cut to a predetermined size to obtain an electrode (P6) with an active material amount per unit volume of 3 g/cm ³ and a mixture layer thickness of 60 μm. The discharge capacity of this positive electrode was 187 mA/g and the initial efficiency K _P6 = 0.72.

(Preparation of Negative Electrode) A negative electrode mixture coating having the same composition as that used in Example 1 was applied to one side of a current collector made of rolled copper foil 18 μm thick using a blade coater, dried, and then compression-molded using a roller press and cut to a predetermined size to obtain an electrode (N6) with an active material amount per unit volume of 1 g/ ^cm3 and a mixture layer thickness of 90 μm. The charge capacity of this negative electrode was 398 mA/g and the initial efficiency K _N6 = 0.81.

(Battery Fabrication and Evaluation of Battery Characteristics) A coin-type battery was fabricated using the positive electrode (P6) and negative electrode (N6) in the same manner as in Example 1. The charge/discharge capacity was determined to be 5.6 mAh.

The above results are summarized in Table 1.

As can be seen from Table 1, the batteries of Examples 1 to 4 all have high capacities. In particular, in Example 2, the initial efficiency of the positive and negative electrodes is 0.72, which is not very good, but the battery capacity is high. The batteries of Comparative Examples 1 and 2 have poor combinations of the positive and negative electrodes, so their capacities are smaller than those of the batteries of the Examples.

In the examples, coin-type batteries were fabricated as an example of nonaqueous electrolyte secondary batteries. However, batteries of various shapes, such as cylindrical, pin, and paper types, can also be fabricated using the present invention. Therefore, the above examples are merely illustrative in all respects and should not be interpreted as limiting. Furthermore, all modifications within the scope of the claims are within the scope of the present invention.

INDUSTRIAL APPLICABILITY As described above, the nonaqueous electrolyte secondary battery of the present invention has a positive electrode and a negative electrode combined to satisfy the relationship 0.9≦Kp/Kn≦1.1, where Kp is the initial efficiency of the positive electrode and Kn is the initial efficiency of the negative electrode. This results in high capacity and excellent charge/discharge characteristics.

The present invention contributes to improving the charge-discharge characteristics of non-aqueous electrolyte secondary batteries by increasing their capacity.

───────────────────────────────────────────────────── Continued from the front page (81) Designated Countries EP(AT,BE,CH,CY, DE, DK, ES, FI, FR, GB, GR, IE, I T, LU, MC, NL, PT, SE), OA (BF, BJ , CF, CG, CI, CM, GA, GN, ML, MR, NE, SN, TD, TG), AP(GH, GM, KE, L S, MW, SD, SZ, UG, ZW), EA (AM, AZ , BY, KG, KZ, MD, RU, TJ, TM), AL , AM, AT, AU, AZ, BA, BB, BG, BR, BY, CA, CH, CN, CU, CZ, D E, DK, E E, ES, FI, GB, GE, GH, GM, GW, HU , ID, IL, IS, JP, KE, KG, KR, KZ, LC, LK, LR, LS, LT, LU, LV, MD, M G, MK, MN, MW, MX, NO, NZ, PL, PT , RO, RU, SD, SE, SG, SI, SK, SL, TJ, TM, TR, TT, UA, UG, US, UZ, V N, YU, ZW (Note) This publication is based on the international publication by the International Bureau of International Trade and Industry (WIPO). Please note that the effect of the international publication of the Japanese-language patent application (Japanese-language utility model registration application) related to this publication arises pursuant to Article 184-10, Paragraph 1 of the Patent Act (Article 48-13, Paragraph 2 of the Utility Model Act), and is unrelated to this publication.

Claims (2)

[Claims] 1. A nonaqueous electrolyte secondary battery comprising a positive electrode and a negative electrode capable of doping and dedoping lithium ions, the positive electrode and the negative electrode being combined so as to satisfy the relationship 0.9≦Kp/Kn≦1.1, where Kp is the initial efficiency of the positive electrode and Kn is the initial efficiency of the negative electrode. 2. The nonaqueous electrolyte secondary battery according to claim ₁ , wherein the positive electrode active material comprises a _{lithium composite oxide} having a composition of LixNiyMzO2 (wherein x is 0.8<x<1.5, y+z is 0.8<y+z<1.2, z is 0≦z<0.35, and M represents at least one element selected from Co, Mg, Ca, Sr, Al, Mn, and Fe).