JP2009032410A - Lithium secondary cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary cell with high loading characteristics applicable to an auxiliary power source such as a hybrid car or a fuel cell vehicle, with a high output, high energy density and longevity. <P>SOLUTION: In the lithium secondary cell structured by: a cathode containing a cathode active material consisting of lithium transition metal composite oxide; an anode containing a material occluding/releasing lithium; and nonaqueous electrolyte containing lithium salt, a cathode active material volume M (g/cm<SP>2</SP>) per unit area of one side face of a collector foil with both faces coated with the cathode active material, is 0.014 g/cm<SP>2</SP>or less, and a ratio E/M of the cathode active material volume M (g/cm<SP>2</SP>) to a lithium ion volume E (mol/cm<SP>3</SP>) contained in the lithium salt per unit volume of the nonaqueous electrolyte, is 0.07 or more and 0.2 or less. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery.

  The development of power supply devices such as lithium secondary batteries or capacitors for the purpose of application to fuel cell vehicles, hybrid vehicles, and the like has been active. In-vehicle applications such as fuel cell vehicles and hybrid vehicles require performance such as high load characteristics, high output characteristics, and long life characteristics. In recent years, from the viewpoint of environmental problems such as carbon dioxide reduction, there is an increasing expectation for the practical application of these power supply devices for auxiliary power supplies of fuel cell vehicles and hybrid vehicles. For practical application to such an automobile field, it is important to further improve the high load characteristics, increase the output, and extend the life of these power supply devices. Furthermore, excellent input characteristics are also required for effective use of regenerative energy.

  In order to apply the lithium secondary battery as an auxiliary power source of the fuel cell vehicle, it is desirable that the lithium secondary battery can be driven only by the electricity of the lithium secondary battery until the fuel cell is started. In recent years, there has been a demand for a so-called dual mode capable of electric driving in urban areas. In order to meet such various demands and put into practical use with a battery mounted in a limited space of a vehicle, further improvement in battery performance such as load characteristics and output characteristics is desired.

  Against this background, various techniques relating to battery performance improvement by improving battery materials such as positive and negative electrode materials and electrolytes have been disclosed.

For example, in Patent Document 1, in order to provide a lithium secondary battery in which input / output characteristics and high-temperature cycle characteristics are balanced, the non-aqueous electrolyte is optimized. This lithium secondary battery uses a non-aqueous electrolyte solution containing lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1.2 M to 3 M, but was applied to one surface of a positive electrode current collector foil. The amount of the positive electrode active material is about 0.0035 g / cm ² , and the ratio of the lithium ion amount of LiPF _{6 to} the amount of the positive electrode active material is 0.358 to 0.458.

Patent Documents 2 and 3 also provide a positive electrode coated with a positive electrode active material mixture containing lithium manganate (LiMn ₂ O ₄ ) in order to provide a cylindrical lithium ion battery with high capacity, high output, and high safety. A lithium secondary battery having an electrolyte containing LiPF ₆ as a solute at a concentration of 1.5 mol / liter or less or 1.3 mol / liter or less is disclosed, but applied to one side of a positive electrode current collector foil The amount of the positive electrode active material is 0.0242 g / cm ² , and the thickness of the application part (both sides) is 210 μm.

In Patent Document 4, in order to provide a high-power and long-life non-aqueous electrolyte secondary battery, optimization of the non-aqueous electrolyte is attempted. In a lithium secondary battery having a positive electrode using lithium manganate as the positive electrode active material and a negative electrode using a carbon material as the negative electrode active material, the concentration of LiPF ₆ with respect to the organic solvent of the non-aqueous electrolyte is 1.2 mol / Liter to 1.6 mol / liter.

Japanese Patent Laid-Open No. 2002-25606 JP 2000-311706 A JP 2000-311707 A JP 2003-243029 A

  An object of the present invention is to provide a lithium secondary battery having high load characteristics, high output, high energy density, and long life that can be applied to an auxiliary power source such as a hybrid vehicle and a fuel cell vehicle.

  As a result of intensive studies to solve the above problems, the present inventors have found that the above problems can be achieved by setting the ratio (ratio) of the amount of the positive electrode active material and the amount of lithium ions in the non-aqueous electrolyte to a suitable range. As a result, the present invention has been completed.

The outline of the present invention is as follows.
(1) In a lithium secondary battery composed of a positive electrode including a positive electrode active material composed of a lithium transition metal composite oxide, a negative electrode including a material that absorbs and releases lithium, and a non-aqueous electrolyte containing a lithium salt, The positive electrode active material amount M (g / cm ² ) per unit area of one surface of the current collector foil coated with the positive electrode active material on both sides is 0.014 g / cm ² or less, and the positive electrode active material amount M ( g / cm ² ) and a ratio E / M of lithium ion amount E (mol / cm ³ ) contained in the lithium salt per unit volume of the non-aqueous electrolyte is 0.07 or more and 0.2 or less. The lithium secondary battery is characterized by the above.
(2) The lithium secondary battery according to (1), wherein the lithium transition metal composite oxide is represented by a chemical formula LiMO ₂ (M is at least one transition metal).
(3) The lithium secondary battery according to (1) or (2), wherein the lithium transition metal composite oxide is represented by a chemical formula LiNi _a Mn _b Co _c O ₂ (a + b + c = 1).

  According to the present invention, a lithium secondary battery having good load characteristics and high output is provided, and a lithium secondary battery suitable for a hybrid vehicle, a fuel cell vehicle, an electric vehicle, and the like can be provided. Furthermore, it is possible to provide a lithium secondary battery that can be widely applied to fields such as electric tools that require high load characteristics and high output.

A lithium secondary battery according to the present invention includes a positive electrode including a positive electrode active material made of a lithium transition metal composite oxide, a negative electrode including a material that absorbs and releases lithium, and a lithium composed of a non-aqueous electrolyte containing a lithium salt. In the secondary battery, the positive electrode active material amount M (g / cm ² ) per unit area of one surface of the current collector foil coated with the positive electrode active material on both surfaces is 0.014 g / cm ² or less, and The ratio E / M of the positive electrode active material amount M (g / cm ² ) to the lithium ion amount E (mol / cm ³ ) contained in the lithium salt per unit volume of the non-aqueous electrolyte is 0.07 or more. It is 0.2 or less.

Generally, since the battery capacity of a lithium secondary battery is based on the amount of lithium ions contained in the positive electrode active material present in the battery container, the current that can be taken out from the battery is also the amount of lithium in the positive electrode active material. It depends on. Therefore, the current value represented by the time rate can be considered as a scale indicating how long the amount of lithium in the positive electrode active material that can move as a battery reaction is moved between the electrodes. In the field of automobiles, a time rate of 1/10 to 1/20 (10C to 20C) is required, which corresponds to a large current capable of moving lithium ions in the positive electrode in 3 to 6 minutes. . In order to improve the battery characteristics under such a high load, it is important to consider the amount of lithium ions used for the battery reaction under a large current. In other words, if the amount of lithium ions present in the vicinity of the reaction interface in the electrolyte is sufficiently larger than the amount of lithium ions used in the battery reaction at the reaction interface, the battery reaction proceeds smoothly even at a large current at high load. In particular, it is considered that the battery characteristics under a high load can be secured. That is, by making the amount of lithium ions (mol / cm ³ ) in the electrolyte solution suitable for the amount of positive electrode active material (g / cm ² ), and by allowing a sufficient amount of lithium ions to exist in the vicinity of the reaction interface, The battery reaction can proceed smoothly, and the load characteristics can be improved.

From the above viewpoint, the positive electrode active material amount M (g / cm ² ) per unit area of one surface of the positive electrode current collector foil, the lithium ion amount E (mol / cm ³ ) in the electrolytic solution per unit volume, As a result of various investigations on the relationship with the battery characteristics, it is possible to obtain good load characteristics by making E / M fall within the range of 0.07 to 0.2, more preferably 0.10 to 0.15. It became clear that lithium secondary batteries can be provided. If E / M exceeds 0.2, there are more ions in the vicinity of the reaction interface than the amount of lithium ions required for the battery reaction, and there is a large amount of lithium ions. Therefore, the viscosity of the electrolytic solution is increased, the ionic conductivity is lowered, and it is difficult to obtain good battery characteristics. On the other hand, if the E / M is less than 0.07, the absolute amount of lithium ions in the vicinity of the reaction interface is insufficient with respect to the amount of lithium ions required for the reaction, so that a smooth battery reaction does not proceed and a good battery is obtained. Performance cannot be obtained.

In the present invention, as described above, the load characteristics are improved by setting the amount of lithium ions in the electrolytic solution within a preferable range with respect to the amount of the positive electrode active material. Increase or decrease according to the amount. As the amount of the positive electrode active material increases, the amount of lithium ions in the electrolytic solution is required accordingly, and for that purpose, it is necessary to dissolve more lithium salt. However, when the amount of the positive electrode active material per unit area of one surface of the current collector foil coated with the positive electrode active material exceeds 0.014 g / cm ² , the amount of lithium salt dissolved becomes extremely large, and thus the electrolysis As the viscosity of the liquid increases, it becomes difficult to ensure good battery characteristics. Therefore, a high-performance lithium secondary battery can be provided by setting the positive electrode active material amount to 0.014 g / cm ² or less. If the amount of the positive electrode active material is reduced, good characteristics can be secured, and therefore the lower limit of the amount of the positive electrode active material is not particularly limited. However, as the amount of the positive electrode active material is decreased, the thickness of the coating film of the positive electrode active material is reduced accordingly, and it may be difficult in production technology to obtain a uniform film. When such production technical also considered, the positive electrode active material weight is preferably 0.007 g / cm ² or more 0.014 g / cm ² or less, more preferably 0.007 g / cm ² or more 0.013 g / cm ² or less , further preferably 0.007 g / cm ² or more 0.012 g / cm ² or less.

  The positive electrode used in the lithium secondary battery of the present invention may have a known configuration. For example, a positive electrode mixture in which a positive electrode active material, a conductive agent, and a binder are mixed is applied to both surfaces of a metal foil and dried.・ It is formed by pressurization.

A known lithium transition metal composite oxide may be applied to the positive electrode active material, and LiCoO ₂ , LiNiO ₂ , LiNi _1-x Co _x O ₂ , LiMn ₂ O ₄ , LiNi _0.5 Mn _0.5 O _2. , LiFePO ₄ , Li ₂ MO ₃ (M is at least one transition metal) and the like are not particularly limited. Examples of the transition metal include Ni, Co, and Mn. In the present invention, a material represented by the chemical formula LiMO ₂ (M is at least one transition metal, more preferably Ni, Co) can be used. Further, a part of Ni, Co, etc. such as LiNiO ₂ and LiCoO ₂ can be substituted with one or more transition metals, for example, LiNi _0.5 Mn _0.3 Co _0.2 O ₂ , Examples include LiNi _0.5 Mn _0.5 O ₂ , LiNi _0.7 Co _0.3 O ₂ , and LiNi _1/3 Mn _1/3 Co _1/3 O ₂ . In the present invention, for example, LiNi _0.5 Mn _0.3 Co _0.2 O ₂ and LiNi _0.7 Co _0.3 O ₂ are preferably used.

  The conductive agent may be a known conductive agent, for example, a carbon-based conductive agent such as graphite, acetylene black, carbon black, carbon fiber, and is not particularly limited. In the present invention, for example, graphite is preferably used.

  The binder may be a known binder such as polyvinylidene fluoride, polytetrafluoroethylene, fluororubber, polypropylene, polyethylene, and the like, and is not particularly limited. In the present invention, for example, polyvinylidene fluoride is preferably used.

  The solvent is appropriately used, and an organic solvent such as N-methyl-2-pyrrolidone is preferably used.

  The mixing ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode mixture is not particularly limited. For example, when the positive electrode active material is 1 by weight, the ratio is 1: 0.05 to 0.20: 0.02. ~ 0.10.

  An aluminum metal foil is used for the current collector foil of the positive electrode, and the thickness is preferably about 10 μm to about 30 μm, for example.

  The negative electrode used in the lithium secondary battery of the present invention may have a known configuration, for example, a negative electrode mixture in which a material that absorbs and releases lithium and a binder are mixed are applied to both surfaces of a metal foil, It is formed by drying and pressing.

  A known material may be used as a material that absorbs and releases lithium, that is, a negative electrode active material, and examples thereof include amorphous carbon materials, carbon materials such as coke, and graphite, and are not particularly limited. In the present invention, for example, an amorphous carbon material is preferably used.

  As the binder, for example, the same one as the positive electrode is used, and is not particularly limited. In the present invention, for example, polyvinylidene fluoride is preferably used.

  The solvent is appropriately used. For example, an organic solvent such as N-methyl-2-pyrrolidone is preferably used.

  The mixing ratio of the material that occludes / releases lithium in the negative electrode mixture and the binder is not particularly limited, and is, for example, 95 to 85: 5 to 15 by weight.

  A copper foil is preferably used for the current collector foil of the negative electrode. The thickness of the metal foil is preferably about 5 μm to about 20 μm, for example.

The non-aqueous electrolyte used for the lithium secondary battery of the present invention is not particularly limited as long as it has a known configuration. For example, a non-aqueous solvent selected from at least one selected from propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, tetrahydrofuran, 1,2-diethoxyethane, and the like, for example, LiPF ₆ , An organic electrolytic solution in which at least one lithium salt selected from LiBF ₄ , LiClO _{4 and the} like is dissolved can be used. In the present invention, for example, LiPF ₆ is preferably used for ethylene carbonate or methyl ethyl carbonate.

As described above, the concentration of lithium ions in the non-aqueous electrolyte is the ratio E / M of the lithium ion amount E (mol / cm ³ ) to the positive electrode active material M (g / cm ² ) on one side of the current collector foil. Is in a range of 0.07 or more and 0.2 or less. Therefore, for example, when the positive electrode active material amount M is preferably 0.007 g / cm ² or more and 0.014 g / cm ² or less, the lithium ion amount E is preferably 0.00049 mol / cm ³ or more and 0.0028 mol / cm ^3. It is.

  Further, a microporous separator, for example, a polyolefin-based microporous polymer film, may be used according to the structural requirements of the battery, and the effects of the present invention are not impaired at all.

  Examples of the shape of the lithium secondary battery include a cylindrical shape, a stacked shape, a coin shape, and a card shape, and are not particularly limited.

  If the lithium secondary battery of this invention is a cylindrical type, it can be manufactured as follows, for example.

A positive electrode active material is added with a conductive agent such as graphite and a binder such as polyvinylidene fluoride dissolved in a solvent such as N-methyl-2-pyrrolidone in the above weight ratio and kneaded to obtain a positive electrode slurry. Next, this slurry is apply | coated to both surfaces of aluminum metal foil. At this time, the positive electrode active material per unit area of one surface is coated to be less than 0.014 g / cm ^2. Then, it presses and dries and produces a positive electrode.

  Polyvinylidene fluoride or the like dissolved in N-methyl-2-pyrrolidone or the like is added as a binder in the above weight ratio to an amorphous carbon material that is a negative electrode active material, and kneaded to obtain a negative electrode slurry. Next, after apply | coating this slurry to both surfaces of copper foil, it presses and dries and produces a negative electrode.

LiPF ₆ or the like is dissolved in a non-aqueous solvent such as ethylene carbonate so as to have the above-mentioned lithium ion amount to prepare a non-aqueous electrolyte solution.

  Next, a porous insulating material separator is sandwiched between the positive electrode and the negative electrode, wound, and then inserted into a battery can molded of stainless steel or aluminum. After connecting the electrode lead piece and the battery can, a non-aqueous electrolyte solution is injected, and the battery can is sealed to obtain a lithium ion secondary battery.

  An example of a cylindrical lithium secondary battery of the present invention is shown in FIG.

  A positive electrode 1 formed by applying the positive electrode mixture on both sides of an aluminum foil; a negative electrode 2 formed by applying the negative electrode mixture on both sides of a copper foil; and a separator 3 disposed between the positive electrode 1 and the negative electrode 2; The positive electrode current collecting lead piece 5 connecting the positive electrode 1 and the positive electrode current collecting lead portion 7, the negative electrode current collecting lead piece 6 connecting the negative electrode 2 and the negative electrode current collecting lead portion 8, and the negative electrode current collecting lead portion 8 The battery can 4 connected to the bottom surface, the battery lid 9 caulked to the opening end of the battery can 4 with the gasket 12, the positive electrode terminal portion 11 in contact with the back surface of the battery lid 9, and the positive electrode terminal portion 11 The rupture valve 10 is formed. The positive electrode 1 and the negative electrode 2 are wound through a separator 3 and disposed inside the battery can 4 as an electrode group. A space formed by the battery can 4 and the battery lid 9 is filled with an electrolytic solution (not shown).

  The lithium secondary battery according to the present invention can be applied to, for example, a hybrid vehicle, a fuel cell vehicle, an electric vehicle, etc., and can also be applied as a power source for electric tools that require high load characteristics and high output. It is.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples, but these examples do not limit the scope of the present invention.

Example 1
LiNi _0.5 Mn _0.3 Co _0.2 O ₂ was used as the positive electrode active material, and the positive electrode active material, the conductive agent graphite, and the binder polyvinylidene fluoride were mixed at a weight ratio of 85: 10: 5. Was mixed for 30 minutes to obtain a positive electrode mixture. The positive electrode mixture was applied to both surfaces of a 20 μm thick aluminum foil as a current collector. Positive electrode mixture per unit area of one side of the collector, the positive electrode active material weight, respectively 0.0092g ^/ cm 2, was 0.0078g / cm ^2. On the other hand, an amorphous carbon material was used as the negative electrode active material and polyvinylidene fluoride was used as the binder, and the mixture was kneaded at a weight ratio of negative electrode active material: binder = 90: 10. The obtained negative electrode mixture was applied to both surfaces of a copper foil having a thickness of 10 μm. Each of the produced positive and negative electrodes was roll-formed with a press machine and then vacuum-dried at 150 ° C. for 5 hours.

After drying, the positive electrode 1 and the negative electrode 2 were wound through the separator 3 and inserted into the battery can 4. The negative electrode current collecting lead piece 6 was collected on the nickel negative electrode current collecting lead portion 8 and ultrasonically welded, and the current collecting lead portion was welded to the bottom of the can (FIG. 1). On the other hand, the positive electrode current collecting lead piece 5 was ultrasonically welded to the aluminum positive electrode current collecting lead portion 7 and then the aluminum positive electrode current collecting lead portion 7 was resistance welded to the battery lid 9. After injecting the electrolytic solution (LiPF ₆ / EC (ethylene carbonate): MEC (methyl ethyl carbonate) = 1: 2), the battery lid 9 was sealed with the caulking of the battery can 4 to obtain a battery. A gasket 12 was inserted between the upper end of the battery can 4 and the lid.

Table 1 summarizes the amount of lithium ions E (mol / cm ³ ) and the E / M ratio in the electrolyte of the prototype batteries (battery numbers 1-1 to 1-6).

  The battery capacity was determined by charging / discharging at a charge end voltage of 4.2 V, a discharge end voltage of 2.7 V, and a charge / discharge rate of 1 C (1 hour rate). Next, the high load characteristics were examined by setting the charging conditions to a charge end voltage of 4.2 V and a charging current of 1 C. The discharge rate at the time of measuring the load characteristics was set to 20 C (three fractions). FIG. 2 shows the relationship between the E / M ratio and the load characteristics. In addition, the maintenance factor (%) of the discharge capacity of 20 C with respect to the discharge capacity of 1 C was used as an index representing the load characteristics. In all of the batteries 1-1 to 1-6, the discharge capacity maintenance rate was about 80%, but in the battery 1-6, the maintenance rate was as low as about 60%.

(Example 2)
A battery was fabricated in the same manner as in Example 1 using LiNi _0.7 Co _0.3 O ₂ as the positive electrode active material and amorphous carbon as the negative electrode active material. Table 2 summarizes the positive electrode active material amount M (g / cm ² ), the lithium ion amount E (mol / cm ³ ), and the E / M ratio of the prototype battery.

The load characteristics were evaluated at a discharge rate of 20 C in the same manner as in Example 1. The results are shown in FIG. Batteries 2-1 to 2-4 having a positive electrode active material amount of 0.0073 g / cm ^{2 to} 0.0126 g / cm ² had a capacity retention ratio of 70 to 80% and showed good characteristics. -5 had a capacity retention rate of about 30%.

Next, in the state of SOC (state of charge) 50%, currents of 1C, 5C, 10C, and 20C were applied for 10 seconds, and the voltage at the 10th second at each current value was measured to examine the output performance. Using battery discharge end voltage (V _D) and the current value when the extrapolated straight line of the current-voltage characteristic to a final discharge voltage (I _D), was determined output from the equation _{P O = I D × V D} . The weight output density is 3260 W / kg for battery 2-1, 3040 W / kg for battery 2-2, 2830 W / kg for battery 2-3, 2510 W / kg for battery 2-4, and 2040 W / kg for battery 2-5. The battery 2-5 with a large amount of the positive electrode active material had a lower output density than the other batteries.

1 is a side sectional view showing a cylindrical lithium secondary battery according to the present invention. It is a figure which shows the relationship between E / M ratio of a lithium secondary battery, and a capacity | capacitance maintenance factor. It is a figure which shows the relationship between the amount of positive electrode active materials of a lithium secondary battery, and a capacity | capacitance maintenance factor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Battery can 5 Positive electrode current collection lead piece 6 Negative electrode current collection lead piece 7 Positive electrode current collection lead part 8 Negative electrode current collection lead part 9 Battery cover 10 Rupture valve 11 Positive electrode terminal part 12 Gasket

Claims (3)

A positive electrode including a positive electrode active material made of a lithium transition metal composite oxide, a negative electrode including a material that absorbs and releases lithium, and a lithium secondary battery including a non-aqueous electrolyte containing a lithium salt, the positive electrode active material Is the positive electrode active material amount M (g / cm ² ) per unit area of one surface of the current collector foil coated on both sides is 0.014 g / cm ² or less, and the positive electrode active material amount M (g / cm ² ) ² ) and the ratio E / M of the lithium ion content E (mol / cm ³ ) contained in the lithium salt per unit volume of the non-aqueous electrolyte is 0.07 or more and 0.2 or less. The lithium secondary battery. The lithium secondary battery according to claim 1, wherein the lithium transition metal composite oxide is represented by a chemical formula LiMO ₂ (M is at least one transition metal). The lithium secondary battery according to claim 1, wherein the lithium transition metal composite oxide is represented by a chemical formula LiNi _a Mn _b Co _c O ₂ (a + b + c = 1).

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