JP2005158719A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery excellent in input/output characteristics. <P>SOLUTION: The lithium ion secondary battery comprises a positive electrode having as a constituting component a positive electrode active substance that can absorb and desorb lithium ions, a negative electrode having as the constituting component a negative electrode active substance that can absorb and desorb lithium ions, and a nonaqueous electrolyte wherein the positive electrode contains the positive electrode active substance and porous carbon whose operating voltage is baser than 4 V vs. metal lithium potential, and the negative electrode contains the negative electrode active substance and porous carbon whose operating voltage is nobler than 1 V vs. the metal lithium potential. The positive electrode contains LiFePO<SB>4</SB>together with the porous carbon, and the negative electrode contains Li<SB>4</SB>Ti<SB>5</SB>O<SB>12</SB>together with the porous carbon. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a hybrid type lithium ion secondary battery having excellent input / output characteristics.

  In recent years, non-aqueous secondary batteries having high energy density, low self-discharge and good cycle characteristics have attracted attention as power sources for portable devices such as mobile phones and notebook computers and electric vehicles.

  Among such non-aqueous secondary batteries, lithium secondary batteries are currently most widely on the market.

The mainstream of lithium secondary batteries is for small-sized consumer use mainly for mobile phones of 2 Ah or less. Currently, there are many positive electrode active materials for lithium secondary batteries, but the most commonly known positive electrode active materials include lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide whose operating voltage is around 4V. It is a lithium-containing transition metal oxide having a basic structure of an oxide (LiNiO ₂ ) or a lithium manganese oxide (LiMn ₂ O ₄ ) having a spinel structure. Among them, lithium cobalt oxide is widely adopted as a positive electrode active material in a small-capacity lithium secondary battery up to a battery capacity of 2 Ah because of its excellent charge / discharge characteristics and energy density.

However, when considering future expansion to medium-sized and large-sized vehicles, especially mounting on HEVs where large demand is expected, the current small-sized specifications cannot satisfy the input / output characteristics required for HEV applications. Patent Document 1 describes an electrode having Li _x Ti _y O ₄ and carbon having a specific surface area of 500 m ² or more, and the counter electrode is LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _4, and Co, Ni, Mn thereof. It is described that what substituted one part etc. with other metals can be used. However, there is a problem that output characteristics and input characteristics are not sufficient.
JP 2002-158139 A JP 2000-294238 A JP 2000-302547 A F. Croce et. al. Electrochem and Solid-State Letters, 5 (3) A47-A50, 2002 S. Franger et. al. Electrochem and Solid-State Letters, 5 (10) A231-A233, 2002

The present invention has been made in view of the above problems, and an object thereof is to provide a lithium ion secondary battery having excellent input / output characteristics.

The configuration of the present invention and its operational effects are as follows. However, the action mechanism includes estimation, and the success or failure of the action mechanism does not limit the present invention.

  According to a first aspect of the present invention, there is provided a positive electrode having a positive electrode active material capable of occluding and releasing lithium ions as a constituent, and a negative electrode active material having a negative electrode active material capable of occluding and releasing lithium ions as a constituent. In the lithium ion secondary battery including a negative electrode as a constituent component and a non-aqueous electrolyte, the positive electrode contains a positive electrode active material and porous carbon whose operating potential is lower than 4 V with respect to the metallic lithium potential, The negative electrode is a lithium ion secondary battery including a negative electrode active material having an operating potential nobler than 1 V with respect to a metallic lithium potential and porous carbon.

  By adopting such an electrode configuration, it becomes possible to improve the input / output characteristics of the lithium ion secondary battery.

  There is a non-aqueous capacitor that combines activated carbon. This operates at 2.5 V to 0.5 V. At this time, the electrode corresponding to the positive electrode in the battery is about 4.0 V to 3.0 V / Li, and the electrode corresponding to the negative electrode is about Operating from 1.5V to 2.5V / Li.

  Therefore, the positive and negative electrode active materials used when the activated carbon is contained in the electrode are the active materials that can be charged and discharged within the potential window, respectively, so that the effects of the present invention can be exhibited most effectively. On the contrary, when a positive electrode active material that operates at a voltage exceeding 4.0 V / Li and a negative electrode active material that operates at a range below 1.5 V / Li are used, the same as a lithium battery using lithium cobaltate as a positive electrode. An oxidation reaction of the electrolytic solution occurs at the positive electrode, and a reduction reaction occurs at the negative electrode to form a film. In particular, activated carbon has a large specific surface area, so the amount of these side reactions is not the ratio of ordinary lithium batteries, and it is impossible to charge or discharge due to the increase in resistance accompanying the formation of a thick film, or the gas generated by side reactions. As a result, the battery may swell, or the liquid may be consumed by a side reaction, resulting in a shortage of liquid. Therefore, regulating the operating potential of the positive electrode and the negative electrode active material is an important requirement for allowing the electrode to contain porous carbon.

  Moreover, this invention is a lithium ion secondary battery in which the said positive electrode and a negative electrode contain 5 to 90% of said porous carbon by weight ratio, as described in Claim 2.

  By setting the content ratio of the porous carbon to 5% by weight or more, the effect of improving input / output characteristics can be sufficiently exhibited. Further, by setting the content ratio of the porous carbon to 90% by weight or less, even when using porous carbon having a small electrochemical capacity, sufficient high output pulse characteristics can be obtained without greatly reducing the energy density of the battery. It can be set as the provided lithium ion secondary battery. The content ratio of the porous carbon is more preferably 10% to 80%, and further preferably 20% to 70%.

According to a third aspect of the present invention, in the lithium ion secondary battery according to the third aspect, the positive electrode active material is olivine type lithium iron phosphate represented by a composition formula LiFePO ₄ .

As described above, regulating the working potential of the positive electrode is an important requirement for incorporating porous carbon into the electrode. Since the olivine-type lithium iron phosphate represented by the above composition has a flat Li insertion / extraction potential in the vicinity of 3.3 to 3.5 V / Li, the positive electrode active material in the case of containing porous carbon in the electrode Suitable as Moreover, since all the oxygen in lithium iron phosphate is covalently bonded to phosphorus to form a polyanion, oxygen in the positive electrode is not released as the temperature rises, and the electrolyte does not burn. For this reason, safety in a high-temperature charged state is dramatically improved as compared with LiCoO ₂ or the like. In addition, it has extremely excellent chemical and mechanical stability and meets the requirements of medium and large applications such as excellent long-term storage performance. The operating potential is 4 V / Li or less. It is classified as the highest potential among the positive electrode active materials possessed, and the true density is as high as 3.6 g / cc, which is extremely preferable.

The present invention is the lithium ion secondary battery according to claim 4, wherein the negative electrode active material is a spinel type lithium titanate represented by a composition formula Li ₄ Ti ₅ O _12. .

  As with the positive electrode, regulating the working potential of the negative electrode is an important requirement for allowing the electrode to contain porous carbon.

  A negative electrode active material used in a general lithium ion secondary battery is a carbon material, and insertion / extraction of Li ions is performed at a potential close to a lithium metal potential of 1.0 V / Li or less. Formation of the film accompanying the reductive decomposition of proceeds. Since this coating is not conductive, the load characteristics of the negative electrode are reduced. In particular, since activated carbon has a large specific surface area, the reduction reaction amount of the electrolytic solution is not the ratio of a normal lithium battery, and charge / discharge rate characteristics may be significantly impaired due to an increase in resistance accompanying the formation of a thick film. .

For these reasons, a titanium-based negative electrode material in which insertion / extraction of Li ions is performed in the vicinity of 1.4 to 1.7 V / Li is suitable. Among these, Li ₄ Ti ₅ O ₁₂ is preferable from the viewpoint that the crystal distortion due to charging / discharging is small and the life of the battery can be expected to be extended.

  Furthermore, the present invention is the lithium ion secondary battery according to claim 5, wherein the non-aqueous electrolyte contains a room temperature molten salt.

  As a general electrolyte, an electrolytic solution in which a lithium salt is dissolved in an organic solvent that is liquid at room temperature is used. However, these organic solvents are volatile and highly flammable, and are therefore classified as flammable substances. Therefore, safety and high temperature environment during overuse such as overcharge, overdischarge, and short circuit There was a problem with the stability below.

  On the other hand, room temperature molten salt is an electrolyte that is in a liquid state at room temperature, as its name suggests, and it has almost no volatility and is flame retardant, so it is safer by mixing room temperature molten salt with a general electrolyte. The effect of improving the stability at high temperatures can be expected.

  However, the room temperature molten salt has the characteristic that the viscosity of the liquid is high in addition to the above characteristics. Therefore, when lithium salt is included in this room temperature molten salt, the transport number of Li ions is smaller than that of a general lithium secondary battery electrolyte. Charging / discharging can hardly be performed.

  However, since the ionic conductivity is superior to that of a general electrolytic solution, the cation originally contained in the room temperature molten salt can move quickly in the solution.

  In the present invention, since the electrode contains porous carbon in addition to the active material for the lithium battery, it is possible to develop capacitor-like characteristics by forming an electric double layer on the porous carbon surface. In this case, when the room temperature molten salt having high ionic conductivity is contained in the liquid, this characteristic works effectively, and the input / output characteristics of the lithium ion secondary battery of the present invention can be further improved. Further, a higher concentration of the room temperature molten salt is preferable because an increase in the electric double layer capacity can be expected by increasing the amount of ions adsorbed on the porous carbon contained in the electrode, and the salt concentration is preferably 2 mol / liter or more. . For this reason, it is better that the ratio of the ambient temperature molten salt contained in the non-aqueous electrolyte is large, and even if the non-aqueous electrolyte is composed only of a lithium salt and an ambient temperature molten salt, the effect of the present invention is improved. There is no adverse effect and there is no problem.

  According to a sixth aspect of the present invention, the positive electrode active material comprises particles having a 50% particle size of 6 μm or less and a 90% particle size of 20 μm or less. It is.

  According to such a configuration, since the overvoltage of the positive electrode during discharge can be further reduced, the flatness of the voltage during battery use can be further improved. In particular, the flatness of the voltage at the end of discharge of the battery can be improved.

  In particular, the positive electrode active material is more preferably composed of particles having a 50% particle diameter of 3 μm or less and a 90% particle diameter of 10 μm or less.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery excellent in the input / output characteristic can be provided.

  Embodiments of the present invention will be exemplified below, but the present invention is not limited to the following embodiments.

  The lithium secondary battery according to the present invention includes a positive electrode having a positive electrode active material as a main constituent, a negative electrode having a negative electrode active material as a main constituent, and a nonaqueous electrolyte containing an electrolyte salt in a nonaqueous solvent. In general, a separator and an outer package for packaging them are provided between the positive electrode and the negative electrode.

  For the positive electrode of the lithium secondary battery of the present invention, an electrode composed of a positive electrode active material whose operating potential is lower than 4 V with respect to the metallic lithium potential is preferably used.

Examples of the positive electrode active material include group I metal compounds such as CuO, Cu ₂ O, Ag ₂ O, CuS, and CuSO ₄ , group IV metal compounds such as TiS ₂ , SiO ₂ , and SnO, V ₂ O ₅ , and V ₆ O. Group V metal compounds such as ₁₂ , VO _x , Nb ₂ O ₅ , Bi ₂ O ₃ , Sb ₂ O ₃ , and Group VI metals such as CrO ₃ , Cr ₂ O ₃ , MoO ₃ , MoS ₂ , WO ₃ and SeO ₂ Compounds, Group VII metal compounds such as MnO ₂ and Mn ₂ O ₃ , Group VIII metal compounds such as Fe ₂ O ₃ , FeO, Fe ₃ O ₄ , Ni ₂ O ₃ , NiO, CoO ₃ and CoO, or a general formula Li _x MX ₂ , Li _x MN _y X ₂ (M, N is a group I to VIII metal, X is a chalcogen compound such as oxygen, sulfur, etc.) And lithium-containing transition metal oxides such as lithium-manganese complex oxides , Li _x FePO ₄ and Li _x FeSO ₄ having an olivine structure, further, disulfide, polypyrrole, polyaniline, polyparaphenylene, polyacetylene, conductive polymer compounds such as polyacene-based material, but pseudo-graphite structure carbon material and the like However, it is not limited to these. Among these, lithium-containing transition metal oxide, Li _x FePO ₄ having an olivine structure is a positive electrode active material having a high operating voltage and capable of releasing lithium, so that a negative electrode capable of occluding lithium at the time of assembly. Easy to combine with active material. Furthermore, Li _x FePO ₄ having an olivine structure is preferable because it has the ability to release lithium even when the end-of-charge voltage is 3.6 V or less.

  As the negative electrode used in the present invention, an electrode composed of a negative electrode active material having an operating potential nobler than 1 V with respect to the metallic lithium potential is preferably used.

Examples of the negative electrode active material include group I metal compounds such as CuO, Cu ₂ O, Ag ₂ O, CuS and CuSO ₄ , group IV metal compounds such as TiS ₂ , SiO ₂ and SnO, V ₂ O ₅ and V ₆ O. Group V metal compounds such as ₁₂ , VO _x , Nb ₂ O ₅ , Bi ₂ O ₃ , Sb ₂ O ₃ , and Group VI metals such as CrO ₃ , Cr ₂ O ₃ , MoO ₃ , MoS ₂ , WO ₃ and SeO ₂ Compounds, Group VII metal compounds such as MnO ₂ and Mn ₂ O ₃ , Group VIII metal compounds such as Fe ₂ O ₃ , FeO, Fe ₃ O ₄ , Ni ₂ O ₃ , NiO, CoO ₃ and CoO, or a general formula Li _x MX ₂ , Li _x MN _y X ₂ (M, N represents a group I to VIII metal, X represents a chalcogen compound such as oxygen or sulfur), etc., for example, Li _y TiO ₂ , Li _{4 +} y Ti ₅ O _12, lithium titanate, such as _{Li 4 + y Ti 11 O 20} , further di- Rufido, polypyrrole, polyaniline, polyparaphenylene, polyacetylene, conductive polymer compounds such as polyacene-based material, but pseudo-graphite structure carbon material and the like, but is not limited thereto. Among these, Li _{4 + y} Ti ₅ O ₁₂ has a flat potential of 1.4 to 1.6 V with respect to the lithium potential. Furthermore, most lithium can be occluded at 1.2 V or more with respect to the lithium potential, and the negative electrode active material is suitable for detecting the end-of-charge potential at 1.2 V or more. Li _{4 + y} Ti ₅ O ₁₂ is preferable because, unlike graphite, the crystal distortion associated with insertion and extraction of lithium is small, so that the life of the lithium ion secondary battery can be improved.

  An electrode composed of lithium iron phosphate is suitably used as the positive electrode active material of the lithium ion secondary battery of the present invention, and an electrode composed of lithium titanate is preferably used as the negative electrode.

In synthesizing Li _x FePO ₄ used in the present invention, the production method is not particularly limited as long as the composition formula is satisfied and a single phase of LiFePO ₄ is generated. Actually, a solid phase method (for example, see Patent Document 2), a sol-gel method (for example, see Non-Patent Document 1), and a hydrothermal method (for example, see Non-Patent Document 2) are generally known.

  In addition, since lithium iron phosphate has poor electronic conductivity, a carbon coat is applied to the particle surface to compensate for this, or a part of Fe is replaced with another element typified by Nb. Even if such treatment is performed, the effect of the present invention is not impaired, and it contributes sufficiently to the improvement of the input / output characteristics. A lithium ion secondary battery using carbon-coated lithium iron phosphate and other element-substituted lithium iron phosphate subjected to such treatment as a positive electrode active material is also within the scope of the present invention.

In synthesizing Li ₄ Ti ₅ O ₁₂ used in the present invention, the production method is not particularly limited as long as the above composition formula is satisfied. As an example of a manufacturing method, the method of patent document 3 is mentioned, for example.

Examples of the porous carbon contained in the positive and negative electrodes include activated carbon and carbon nanotubes, but are not particularly limited as long as they are generally used in capacitors. However, since the electric capacity of the porous carbon depends on its specific surface area, it is better that the specific surface area is large. In the case of activated carbon, it is preferably at least 1000 m ² / g or more, preferably 1500 m ² / g or more. In the case of carbon nanotubes, it is preferably 100 m ² / g or more. Further, in the case of carbon nanotubes, it is preferable to have a multi-layer structure rather than a single-layer structure in view of electric double layer capacity. These porous carbons may contain a different element such as nitrogen.

  In addition to the method of physically mixing the positive and negative active materials and the porous carbon at the time of producing the electrode, the method of adding the porous carbon to the electrode can be applied directly to the positive and negative electrode active materials or directly to the porous carbon. -The electrode which can fully exhibit the effect of this invention can be produced also by coating a negative electrode active material. Examples of the coating method include a sol-gel method and a dip coating method. However, the coating method is not particularly limited thereto, and it is sufficient that the coating substance is adhered to the surface of the particles to be coated. It is also effective to mix porous carbon during the synthesis of the positive and negative electrode active materials.

An anion constituting a room temperature molten salt that can be used for a non-aqueous electrolyte has at least one of an organic anion having a perfluoroalkyl group or an anion consisting only of a nonmetallic element having no perfluoroalkyl group. It is preferable to do. Examples of the organic anion having a perfluoroalkyl group include CF ₃ SO ₃ ⁻ , CF ₃ COO ⁻ , N (CF ₃ SO ₂ ) ₂ ⁻ , N (C ₂ F ₅ SO ₂ ) ₂ ⁻ , and N (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ) ⁻ , C (CF ₃ SO ₂ ) ₃ ^— and C (C ₂ F ₅ SO ₂ ) ₃ ^— are preferably selected, and particularly N (CF ₃ SO ₂ ) ₂ ^— is preferable, but not limited thereto. As an anion consisting of only a nonmetallic element having no perfluoroalkyl group, one or more anions from the group consisting of BF ₄ ⁻ , PF ₆ ⁻ , AsF ₆ ⁻ , ClO ₄ ⁻ , CN ⁻ and CH ₃ COO ⁻ are used. It is preferable to select, and in particular, BF ₄ ⁻ is preferable, but not limited thereto. These may be used alone or in combination of two or more.

  Further, the room temperature molten salt preferably has a quaternary ammonium organic cation. Examples of the quaternary ammonium organic cation include imidazolium cation, tetraalkylammonium cation, pyridinium cation, pyrrolium cation, pyrazolium cation, pyrrolinium cation, pyrrolidinium cation, and piperidinium cation. Among these, an imidazolium cation having a skeleton represented by (Chemical Formula 1) is particularly preferable.

  Examples of the imidazolium cation include 1,3-dimethylimidazolium ion, 1-ethyl-3-methylimidazolium ion, 1,3-diethylimidazolium ion, 1-butyl-3-yl as dialkylimidazolium cation. Methyl imidazolium ion, etc., and trialkyl imidazolium cation, 1,2,3-trimethylimidazolium ion, 1,2-dimethyl-3-ethylimidazolium ion, 1,2-dimethyl-3-propylimidazolium ion Ions, 1-butyl-2,3-dimethylimidazolium ions, and the like, but are not limited thereto.

  Examples of the tetraalkylammonium cation include, but are not limited to, trimethylethylammonium ion, trimethylpropylammonium ion, trimethylhexylammonium ion, and tetrapentylammonium ion.

  Examples of the pyridinium cation include N-methylpyridinium ion, N-ethylpyridinium ion, N-propylpyridinium ion, N-butylpyridinium ion 1-ethyl-2-methylpyridinium, 1-butyl-4-methylpyridinium, 1-butyl Examples include -2,4-dimethylpyridinium, but are not limited thereto.

  Examples of the pyrrolium cation include 1,1-dimethylpyrrolium ion, 1-ethyl-1-methylpyrrolium ion, 1-methyl-1-propylpyrrolium ion, 1-butyl-1-methylpyrrolium ion, It is not limited to these.

  Examples of the pyrazolium cation include 1,2-dimethylpyrazolium ion, 1-ethyl-2-methylpyrazolium ion, 1-propyl-2-methylpyrazolium ion, 1-butyl-2-methylpyrazolium ion, and the like. However, it is not limited to these.

  Examples of the pyrrolium cation include 1,2-dimethylpyrrolium ion, 1-ethyl-2-methylpyrrolium ion, 1-propyl-2-methylpyrrolium ion, 1-butyl-2-methylpyrrolium ion, and the like. Although it is mentioned, it is not limited to these.

  Examples of the pyrrolidinium cation include 1,1-dimethylpyrrolidinium ion, 1-ethyl-1-methylpyrrolidinium ion, 1-methyl-1-propylpyrrolidinium ion, 1-butyl-1-methylpyrrolidinium ion, and the like. However, it is not limited to these.

Examples of the piperidinium cation include 1,1-dimethylpiperidinium ion, 1-ethyl-1-methylpiperidinium ion, 1-methyl-1-propylpiperidinium ion, 1-butyl-1-methylpiperidinium ion, and the like. However, it is not limited to these.
In addition, the normal temperature molten salt which has these quaternary ammonium organic substance cations may be used independently, and may be used in mixture of 2 or more types.

  Hereinafter, the present invention will be described with reference to examples and comparative examples, but the present invention is not limited to the following description.

(Example 1)
(Production of LiFePO ₄ )
A molar ratio of iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) in a 2: 2: 1 ratio Weighed out and mixed. Thereafter, the mixture was pulverized and mixed for 10 hours with a ball mill using 2-propanol as a solvent. Next, this mixture was vacuum-dried to remove 2-propanol as a solvent to obtain a precursor.

The obtained precursor was put into an incense pot made of alumina, and pre-baked in an annular baking furnace at 300 ° C. for 5 hours under an argon flow (0.5 liter / min). Further, LiFePO ₄ powder was synthesized by baking at 650 ° C. for 20 hours under an argon flow (0.5 liter / min).

(Preparation of positive electrode)
The LiFePO ₄ (positive electrode active material), acetylene black (conductive agent), polyvinylidene fluoride (PVdF, binder) and activated carbon (porous carbon, specific surface area 2000 m ² / g) were used in a weight ratio of 40:10:10: 4
The mixture was mixed at a ratio of 0, N-methyl-2-pyrrolidone (NMP) was added and sufficiently kneaded to obtain a positive electrode paste. The positive electrode paste was applied to both surfaces of an aluminum foil current collector with a thickness of 20 μm, dried, and then pressed to obtain a positive electrode. A positive electrode terminal was welded to the positive electrode by ultrasonic welding.

(Production of Li ₄ Ti ₅ O ₁₂ )
Titanium oxide (TiO ₂ rutile ratio 90%) and Li ₂ CO ₃ were weighed so that the Li / Ti ratio was 0.80 and mixed for 30 minutes in an automatic mortar. This mixture was put in an alumina casserole, and calcined in an electric furnace at 700 ° C. for 4 hours under oxygen flow (0.1 l / min). Further, this temporarily fired body was fired in an electric furnace at 800 ° C. for 5 hours under oxygen flow (0.1 l / min). The synthesized powder was pulverized in an automatic mortar to obtain Li ₄ Ti ₅ O ₁₂ powder.

(Preparation of negative electrode)
Weight ratio of Li ₄ Ti ₅ O ₁₂ (negative electrode active material), acetylene black (conductive agent), polyvinylidene fluoride (PVdF, binder) and activated carbon (porous carbon, specific surface area 2000 m ² / g) obtained as above. 40:10
Was mixed at a ratio of 10:40, and N-methyl-2-pyrrolidone (NMP) was added and sufficiently kneaded to obtain a negative electrode paste. Next, the negative electrode paste was applied on both surfaces of an aluminum foil current collector with a thickness of 20 μm, dried, and then pressed to form a negative electrode. A negative electrode terminal was welded to the negative electrode by ultrasonic welding.

(Preparation of electrolyte)
A nonaqueous electrolyte A was prepared by dissolving LiBF ₄ as a fluorine-containing electrolyte salt at a concentration of 2 mol / l in a mixed solvent in which ethylene carbonate and γ-butyllactone were mixed at a volume ratio of 1: 1.

(Production of battery)
A laminate battery was produced using the above-described members in a dry atmosphere having a dew point of −40 ° C. or lower. A cross-sectional view of the battery is shown in FIG. The positive electrode and the negative electrode were wound in an oval shape through a polypropylene separator having a thickness of 20 μm. A metal resin composite film made of polyethylene terephthalate (15 μm) / aluminum foil (50 μm) / metal adhesive polypropylene film (50 μm) is used as the outer package, and the open ends of the positive electrode terminal and the negative electrode terminal are exposed to the outside. Thus, it was hermetically sealed except for the portion to be the injection hole.

  After injecting a certain amount of non-aqueous electrolyte A from the injection hole, the injection hole part is heat sealed in a vacuum state to produce a flat lithium secondary battery. This was designated as the battery of Example 1. The battery was placed in a SUS battery case, and positive and negative electrodes were connected to terminals, and then a lid was attached and laser welding was performed. Subsequently, after introducing a predetermined amount of the above-described electrolytic solution, the liquid inlet was sealed. The lithium ion secondary battery thus produced is referred to as the battery 1 of the present invention.

(Example 2)
(Preparation of electrolyte)
LiBF ₄ is dissolved at a concentration of 2 mol / l in 1 liter of a room temperature molten salt (EMIBF ₄ ) composed of 1-ethyl-3-methylimidazolium ion (EMI ⁺ ) and tetrafluoroborate ion (BF4-), Nonaqueous electrolyte B was produced.
Nonaqueous electrolyte C and nonaqueous electrolyte B were mixed at a weight ratio of 1: 1 to prepare nonaqueous electrolyte C. The amount of water in the electrolyte was less than 30 ppm.

(Production of battery)
A lithium ion secondary battery produced in the same manner as in Example 1 except that the nonaqueous electrolyte C described above was injected was designated as the battery 2 of the present invention.

(Comparative Example 1)
(Preparation of positive electrode)
LiFePO ₄ (positive electrode active material), acetylene black (conductive agent) and polyvinylidene fluoride (PVdF, binder) are mixed at a weight ratio of 75:15:10, and N-methyl-2-pyrrolidone (NMP) is mixed. Was added and kneaded sufficiently to obtain a positive electrode paste. The positive electrode paste was applied to both surfaces of an aluminum foil current collector with a thickness of 20 μm, dried, and then pressed to obtain a positive electrode. A positive electrode terminal was welded to the positive electrode by ultrasonic welding.

(Production of battery)
Using the positive electrode described above and the negative electrode produced in Example 1, a square battery having a design capacity of 200 mAh was produced in a dry atmosphere with a dew point of −40 ° C. or lower. A cross-sectional view of the battery is shown in FIG. The positive electrode and the negative electrode were wound in an oval shape through a polypropylene separator having a thickness of 20 μm. A metal resin composite film made of polyethylene terephthalate (15 μm) / aluminum foil (50 μm) / metal adhesive polypropylene film (50 μm) is used as the outer package, and the open ends of the positive electrode terminal and the negative electrode terminal are exposed to the outside. Thus, it was hermetically sealed except for the portion to be the injection hole.

  After pouring only a certain amount of non-aqueous electrolyte A from the liquid injection hole, the liquid injection hole part is thermally sealed in a vacuum state to produce a flat lithium secondary battery having a design capacity of 200 mAh. This was designated as the battery of Example 1. The battery was placed in a SUS battery case, and positive and negative electrodes were connected to terminals, and then a lid was attached and laser welding was performed. Subsequently, after introducing a predetermined amount of the above-described electrolytic solution, the liquid inlet was sealed. The lithium ion secondary battery produced in this way is referred to as comparative battery 1.

(Comparative Example 2)
(Preparation of positive electrode)
LiCoO ₂ (positive electrode active material), acetylene black (conductive agent), polyvinylidene fluoride (PVdF, binder) and activated carbon (porous carbon) are mixed at a weight ratio of 40: 10: 10: 40, and N -Methyl-2-pyrrolidone (NMP) was added and sufficiently kneaded to obtain a positive electrode paste. The positive electrode paste was applied to both surfaces of an aluminum foil current collector with a thickness of 20 μm, dried, and then pressed to obtain a positive electrode. A positive electrode terminal was welded to the positive electrode by ultrasonic welding.

(Production of battery)
A lithium ion secondary battery produced in the same manner as in Example 1 except that the positive electrode described above was used is referred to as Comparative Battery 2.

(Initial charge / discharge)
For the purpose of initial activation of the batteries, the batteries 1 and 2 of the present invention and the comparative batteries 1 and 2 were each charged and discharged at a constant current of 5 cycles. The current was set to 0.1 ItmA (10 hour rate) for both charging and discharging. Here, the batteries 1 and 2 of the present invention and the comparative battery 1 have an end-of-charge voltage of 2.5 V and an end-of-discharge voltage of 0.5 V, and the comparative battery 2 has an end-of-charge voltage of 2.7 V and an end-of-discharge voltage of 1.5 V. did. Here, the discharge capacity at the fifth cycle is defined as the nominal capacity.

(Output test)
Following the initial charge / discharge, batteries that were charged at 0.1 ItA were prepared, and each current of 1, 5, 10, 20, 40, 60, 80, and 100 mA / cm ² in a constant temperature bath at 20 ° C. A constant current discharge was performed. After discharging at each current value, the battery was charged at 0.1 ItA, and then discharged at the next current value.

  At each current value, the output W was calculated from the voltage 10 seconds after the start of discharge and the current value at that time, and the value was divided by the battery weight to determine the mass output density W / kg.

(Input test)
Following the initial charge / discharge, batteries that were discharged at 0.1 ItA were prepared, and each current of 1, 5, 10, 20, 40, 60, 80, and 100 mA / cm ² in a constant temperature bath at 20 ° C. The constant current charging was performed. After completion of charging at each current value, the battery was discharged at 0.1 ItA, and then charged at the next current value.

  At each current value, the output W was calculated from the voltage 10 seconds after the start of charging and the current value at that time, and the value was divided by the battery weight to determine the mass input density W / kg.

(High temperature storage test)
Subsequent to the initial charge / discharge, batteries that were charged at 0.1 ItA were prepared, and left in a constant temperature bath at 80 ° C. for 24 hours. Thereafter, the battery was sufficiently cooled by being left in a constant temperature bath at 20 ° C. for 5 hours, and the swelling of the battery was visually confirmed.
Table 1 shows the results of the output test, the high output pulse test, and the high temperature storage test. The result of the high temperature storage test is indicated by ○ when the battery is not swollen and × when the battery is swollen.

As can be seen from the results in Table 2, only the comparative battery 2 using LiCoO ₂ as the positive electrode active material was swollen, and no other battery swelling occurred. This is because the final charge potential of LiCoO ₂ used as the positive electrode active material is 4.3 V / Li, which is higher than the anodic potential of a normal non-aqueous capacitor, so that the specific surface area of porous carbon contained in the positive electrode is large. This is probably because a large amount of gas was generated by the activated carbon due to the oxidation reaction of the electrolyte solution at the positive electrode field. The other materials using LiFePO ₄ as the positive electrode active material are considered to have no potential for gas generation and swelling due to the potential at which no oxidation reaction between the positive electrode and the electrolyte occurs even at the end of charging. Comparing the battery 1 of the present invention and the comparative battery 1 in Table 1, it can be seen that the battery 1 of the present invention containing activated carbon in both electrodes has improved input / output characteristics than the comparative battery 1. This is considered to be due to the fact that activated carbon coexists in the electrode, so that the polarization resistance at the time of discharge is relaxed and a large current discharge in a short time becomes possible.

  Furthermore, the battery 2 of the present invention has improved input / output characteristics compared to the battery 1 of the present invention, and the difference is obvious when compared with the comparative battery 1. It is thought that the addition of room temperature molten salt to the non-aqueous electrolyte effectively improves the input / output characteristics by utilizing the electric double layer capacity due to the porous carbon surface in the electrode. .

  Following the initial charge / discharge, the battery 1 of the present invention which was brought to the end of charge at 0.1 ItA was separately prepared, and a quantity of electricity of 100% of the nominal capacity was discharged at a constant current of 0.1 ItA. FIG. 1 shows a discharge curve at this time. The curve shown in FIG. 1 is a change in the closed circuit voltage. Here, the potential change is within 0.1 V over 50% or more of the nominal capacity.

(Example 3)
(Preparation of positive electrode active material M)
The molar ratio of iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) is 2: 2: 1. Weighed out and mixed. Next, 2-propanol was added as a solvent to this mixture, and pulverized and mixed for 2 hours using a ball mill. A precursor was thus obtained.

The precursor obtained above was put in an alumina sagger and calcined at 600 ° C. for 10 hours under a nitrogen flow (0.5 dm ³ / min) in an annular firing furnace to synthesize LiFePO ₄ powder. Next, as a pulverization treatment, the obtained LiFePO ₄ powder was put in a pot of a ball mill, and the inside of the pot was purged with argon, followed by grinding for 24 hours.

The pulverized LiFePO ₄ powder and polyvinyl alcohol powder are mixed at a weight ratio of 9: 7, and this mixture is put in an alumina sagger, and 700 in an annular baking furnace under nitrogen flow (1.5 dm ³ / min). The surface of the particles of LiFePO ₄ powder was coated with carbon by heat treatment at 2 ° C. for 2 hours. By this treatment, about 5 wt% carbon is coated on the LiFePO ₄ particles. In order to release the aggregation state of the carbon-coated LiFePO ₄ , a pulverization treatment for 20 minutes was further performed using a ball mill. This is designated as a positive electrode active material M. As a result of measuring the particle size distribution of the positive electrode active material M using a sedimentation-type laser diffraction type particle size distribution analyzer (manufactured by Beckman Co., Ltd., counter LS13320 type) using 2-propanol as a dispersion medium, the 90% particle diameter (D90 ) Was 1.83 μm, 50% particle size (D90) was 1.15 μm, and 10% particle size (D10) was 0.65 μm.

(Preparation of positive electrode active material N)
The molar ratio of iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) and lithium carbonate (Li ₂ CO ₃ ) is 2: 2: 1. Weighed out and mixed. Next, 2-propanol was added as a solvent to this mixture, and pulverized and mixed for 2 hours using a ball mill. A precursor was thus obtained.

The precursor obtained above was put in an alumina sagger and calcined at 600 ° C. for 10 hours under a nitrogen flow (0.5 dm ³ / min) in an annular firing furnace to synthesize LiFePO ₄ powder.

The obtained LiFePO ₄ powder and polyvinyl alcohol powder were mixed at a weight ratio of 9: 7, and this mixture was placed in an alumina sagger, and 700 ° C. under a nitrogen flow (1.5 dm ³ / min) in an annular baking furnace. The surface of the particles of LiFePO ₄ powder was coated with carbon by heat treatment at 2 ° C. for 2 hours. By this treatment, about 5 wt% carbon is coated on the LiFePO ₄ particles. In order to release the aggregation state of LiFePO ₄ , a pulverization treatment for 20 minutes was further performed using a ball mill. This is designated as a positive electrode active material N. As a result of measuring the particle size distribution of the positive electrode active material N in the same manner as described above, the 90% particle size (D90) of the positive electrode active material N is 33.5 μm, the 50% particle size (D90) is 7.25 μm, and the 10% particle size. (D10) was 1.69 μm.

(Preparation of positive electrode)
Mixing positive electrode active material, acetylene black (conductive agent), polyvinylidene fluoride (PVdF, binder) and activated carbon (porous carbon, specific surface area 2000 m ² / g) in a weight ratio of 40: 10: 10: 40 Then, N-methyl-2-pyrrolidone (NMP) was added and sufficiently kneaded to obtain a positive electrode paste. The positive electrode paste was applied on both sides of an aluminum foil current collector with a thickness of 20 μm, dried, and then pressed, and cut into a size of 30 mm × 30 mm to obtain a positive electrode. A positive electrode terminal was welded to the positive electrode by ultrasonic welding. Here, the positive electrode produced using the positive electrode active material M was designated as positive electrode M, and the positive electrode produced using the positive electrode active material N was designated as positive electrode N.

(Preparation of electrolyte)
LiPF ₆ as a fluorine-containing electrolyte salt was dissolved at a concentration of 1 mol / l in a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 to obtain a nonaqueous electrolyte in Example 3.

(Evaluation of positive electrode)
A lithium ion secondary battery was produced in the same manner as in Example 1. However, a reference electrode was inserted to observe unipolar behavior. The electrochemical performance of the positive electrode M and the positive electrode N was evaluated using this battery. In the following tests, charging was performed at 0.1 ItmA (10 hour rate) and constant current / constant voltage charging for 15 hours, and the end of discharging was performed when the positive electrode potential reached 2.0 V (vs. Li ^/ Li ⁺ ). In addition, a pause of 30 minutes was provided when switching from charging to discharging and when switching from discharging to charging. First, after charging and discharging with a discharge current of 0.1 ItmA (10 hour rate) for 4 cycles, the discharge current was set to 2 ItmA for the fifth cycle. The above test was performed on lithium ion secondary batteries (battery M and battery N, respectively) produced using the positive electrode M and the positive electrode N, respectively, and the results were compared.

  FIG. 2 shows changes in the positive electrode potential during the discharge of the fourth cycle and the fifth cycle of the battery M using the positive electrode active material M subjected to the pulverization treatment, and FIG. 3 shows the positive electrode active material not subjected to the pulverization treatment. The change of the positive electrode potential at the time of the discharge of the 4th cycle of the battery N using N and the 5th cycle is shown, respectively.

  As can be seen from FIG. 2 and FIG. 3, when the transition of the constant current continuous discharge potential of 0.1 ItmA is compared, in the battery N using the positive electrode active material not subjected to the pulverization treatment, the discharge potential is 3 V at about 135 mAh / g. On the other hand, in the battery M using the positive electrode active material subjected to the pulverization treatment, it was possible to discharge nearly 150 mAh / g before the discharge potential reached 3V. Similarly, when the constant current continuous discharge potential transition of 2 ItmA is compared, in the battery N using the positive electrode active material not subjected to the pulverization treatment, the discharge potential reaches 3 V at about 70 mAh / g, whereas the pulverization is performed. In the battery M using the treated positive electrode active material, it was possible to discharge nearly 90 mAh / g before the discharge potential reached 3V. Thus, the positive electrode active material powder is composed of particles having a 50% particle size of 6 μm or less and a 90% particle size of 20 μm or less, more preferably, a 50% particle size of 3 μm or less and 90% particles. It can be seen that, by using particles having a diameter of 10 μm or less, the flatness of the voltage during use of the battery can be further improved, and in particular, the flatness of the voltage at the end of discharge of the battery can be improved.

It is a figure which shows the discharge voltage change of this invention battery. It is a figure which shows the positive electrode electric potential change at the time of discharge of this invention battery. It is a figure which shows the positive electrode electric potential change at the time of discharge of this invention battery.

Claims (6)

A positive electrode having a positive electrode active material capable of occluding and releasing lithium ions as a constituent component, a negative electrode having a negative electrode active material having a negative electrode active material capable of occluding and releasing lithium ions as a constituent component, and a non-aqueous electrolyte In the lithium ion secondary battery provided, the positive electrode contains a positive electrode active material and porous carbon whose operating potential is lower than 4 V with respect to the metallic lithium potential, and the negative electrode has an operating potential with respect to the metallic lithium potential. A lithium ion secondary battery comprising a negative electrode active material nobler than 1 V and porous carbon. The lithium ion secondary battery according to claim 1, wherein the positive electrode and the negative electrode contain 5% to 90% of the porous carbon by weight. 3. The lithium ion secondary battery according to claim 1, wherein the positive electrode active material is an olivine-type lithium iron phosphate represented by a composition formula LiFePO _{4. 4} . The lithium ion secondary battery according to any one of claims 1 to 3, wherein the negative electrode active material is spinel type lithium titanate represented by a composition formula Li ₄ Ti ₅ O ₁₂ . The lithium ion secondary battery according to any one of claims 1 to 4, wherein the non-aqueous electrolyte contains a room temperature molten salt. 6. The lithium ion secondary battery according to claim 3, wherein the positive electrode active material is composed of particles having a 50% particle diameter of 6 μm or less and a 90% particle diameter of 20 μm or less.

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