JP5677885B2 - Positive electrode for lithium secondary battery and lithium secondary battery - Google Patents

Description

The present invention relates to an improvement of a lithium secondary battery, particularly to a lithium secondary battery having improved safety and a positive electrode thereof.

Lithium secondary batteries have high capacity, high electromotive force, high energy density, and are suitable for miniaturization and thinning. However, in terms of safety, lithium with high chemical activity or highly flammable electrolyte As a battery material, it is necessary to ensure sufficient safety even when an abnormality occurs. In particular, when an overdischarge of a battery occurs, the copper normally used as a current collector for the negative electrode may dissolve or the electrolytic solution may undergo reductive decomposition, which may cause explosion or ignition. It is necessary to take sufficient measures against discharge. As a countermeasure, a protection circuit for monitoring cell voltage changes is provided in the battery pack. Thereby, when an abnormal overdischarge occurs and the voltage decreases, the battery circuit can be shut off.

On the other hand, lithium iron phosphate (LiFePO ₄ ) having an olivine type structure and related compounds are used as the positive electrode active material among various inorganic compounds in practical use as the positive electrode active material of the lithium secondary battery. Positive electrode materials have been developed. (The following prior art documents)

The olivine-type lithium iron phosphate compound can be easily obtained by a so-called solid phase synthesis method in which a mixture comprising a lithium salt, an iron salt, a phosphate compound, and the like is heated. Since this olivine-type lithium iron phosphate has iron as a main component and has advantages in terms of material cost and resources, it has attracted attention as a next-generation positive electrode active material.

This lithium iron phosphate compound has a potential (as opposed to Li / Li ⁺ ) operating as a positive electrode active material of about ² to 3.5 V, and has been put to practical use as a positive electrode active material: lithium cobaltate, lithium nickelate, spinel type It is low compared with lithium manganate and the like.

US Pat. No. 5,910,382 JP-A-9-134724 Japanese Patent Laid-Open No. 9-171827 JP 2001-110414 A JP 2003-338992 A Japanese Patent Application Laid-Open No. 2004-514639 JP 2007-35358 A

Journal of Electrochemical Society, 144, 1188, 1997 (J. Electrochem. Soc., 144, 1188, 1997) Journal of Electrochemical Society, 144, 1609, 1997 (J. Electrochem. Soc., 144, 1609, 1997)

The olivine-type lithium iron phosphate compounds for potential (vs. Li / Li ⁺⁾ occurs a change in the crystal phase becomes 2V below the potential (vs. Li / Li ⁺⁾ is extremely slow voltage drop rate in less than 2V Therefore, it is not easy to detect a voltage drop by the protection circuit. Alternatively, there has been no effective countermeasure for local overdischarge caused by non-uniform current distribution in the battery for some reason.
In addition, it has been difficult to perform recharging after the voltage has dropped to a voltage that is regarded as an overdischarge region.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a lithium secondary battery using a positive electrode using olivine type lithium iron phosphate particles as an active material in an overdischarge region (2 to 0.5 V). ), A lithium ion secondary battery positive electrode capable of quickly dropping the voltage and a lithium secondary battery comprising the positive electrode are provided.

As a result of intensive studies to achieve the above object, the inventors of the present application have found that a positive electrode of a lithium secondary battery using an olivine-type lithium iron phosphate having a chemical composition represented by LiFePO ₄ as an active material under the following conditions: It has been found that the above problems can be solved by a positive electrode having a measured discharge potential drop average speed of a specific speed or more, and the present invention has been completed.
<Measurement conditions of average discharge potential drop>
Cell configuration
-Bipolar pouch cell-Counter electrode: metallic lithium-Electrolyte: 1M LiPF ₆ in EC + EMC + DMC = 1: 1: 1 (vol.%)
Measurement conditions and temperature: 30 ° C
・ Discharge range: 2 V to 0.5 V (vs. Li / Li ⁺ )
・ Discharge current: 20 mA / g-positive electrode

That is, in the present invention, a positive electrode active material layer and a conductive polymer layer whose electric resistance rapidly increases at a potential of 2 V or less (vs. Li / Li ⁺ ) are laminated, and the positive electrode active material is lithium comprising olivine type lithium iron phosphate. The present invention relates to a positive electrode for a lithium secondary battery, characterized in that the discharge potential drop average rate measured under the above conditions is 0.3 V / Hr or more.

According to the electrode of the present invention, a lithium ion secondary battery using olivine-type lithium iron phosphate particles as an active material can obtain a sufficient voltage drop rate when overdischarge is occurring. The voltage drop can be detected quickly by the protection circuit, and safety against overdischarge can be ensured. In addition, local overdischarge, which is difficult to detect by voltage measurement, can be prevented, and recharging after dropping to a voltage that is normally regarded as an overdischarge region is possible.

The positive electrode of the present invention is a positive electrode using the olivine type lithium iron phosphate as an active material, wherein the discharge potential drop average rate measured by the following method is 0.3 V / Hr or more. It is.

<Measurement conditions of average discharge potential drop>
[Cell structure]
・ Bipolar pouch cell
・ Counter electrode: Lithium metal
Electrolyte: 1M LiPF ₆ in EC + EMC + DMC = 1: 1: 1 (vol.%)
Here, EC is an abbreviation for ethylene carbonate, EMC is an abbreviation for ethyl methyl carbonate, and DMC is an abbreviation for dimethyl carbonate.

[Measurement condition]
・ Measurement temperature: 30 ℃
・ Discharge range: 2 V to 0.5 V (vs. Li / Li ⁺ )
・ Discharge current: 20 mA / g-positive electrode

The average discharge potential drop rate measured by the above measurement method is more preferably 0.4 V / Hr or more, and further preferably 0.5 V / Hr or more. When the discharge potential drop average speed is less than 0.3 V / Hr, detection of the voltage drop by the protection circuit is delayed, it is difficult to ensure safety against overdischarge, and further, recharge after overdischarge becomes difficult. .

The positive electrode of the present invention can be obtained, for example, by forming the positive electrode structure by laminating a positive electrode active material layer and a conductive polymer layer whose electrical resistance rapidly increases at a potential of 2 V or less (vs. Li / Li ⁺ ). .

That is, a conductive polymer layer on the surface of a positive electrode current collector made of a metal foil such as an aluminum foil, and a positive electrode active material layer in which a binder and a conductive material are blended with an olivine-type lithium iron phosphate that is a positive electrode active material are sequentially formed. It can be obtained by laminating.

The olivine-type lithium iron phosphate has a general formula of Li _1-x Fe _{1- y My} PO ₄ (where −0.2 ≦ x ≦ 0.2, 0 ≦ y ≦ 0.5, M represents Li, Ni , Co, Mn, Mg, Al, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y. In the above general formula, the value of y can be set to y = 0. In this case, the general formula is represented by Li _1-x FePO ₄ .

As a method for producing the olivine-type lithium iron phosphate particles, for example, a lithium phosphate compound, an iron (II) phosphate compound, and a compound containing a metal element M added as necessary are mixed with water. It is obtained by dispersing in a polar solvent such as alcohol to prepare a slurry, and heat-treating the slurry in a closed container at a temperature of 120 to 280 ° C. in an inert gas atmosphere. The average particle size of the olivine type lithium iron phosphate is preferably 10 μm or less, and more preferably 3 μm or less.

Further, at least a part of the olivine-type lithium iron phosphate surface can be coated with a conductive substance such as carbon or noble metal, if necessary. Thereby, the electronic conductivity of the said olivine type lithium iron phosphate can be improved more. The conductive material is preferably carbon. In this case, the conductivity of the positive electrode active material can be improved without significantly increasing the manufacturing cost and weight of the positive electrode active material. Commercially available products can also be used as the olivine type lithium iron phosphate particles.

As the conductive material, for example, carbon black, natural graphite, artificial graphite, or a mixture of one or more of carbonaceous powders such as cokes can be used, and natural graphite powder or artificial graphite powder and A mixture with carbon black is preferred. As a compounding quantity of a electrically conductive material, 5-40 mass% is preferable with respect to olivine type lithium iron phosphate, and 10-30 mass% is more preferable.

Moreover, as said binder, a polyvinylidene fluoride, a polytetrafluoroethylene, a polyethylene / polymethacrylate copolymer etc. can be used, for example. As a compounding quantity of a binder, 1-20 mass% is preferable with respect to olivine type lithium iron phosphate, and 5-15 mass% is more preferable.

The positive electrode active material layer of the present invention is a paste obtained by blending the lithium iron phosphate particles having the olivine type structure with the binder, the conductive material and a solvent, and applying this to a current collector such as an aluminum foil, followed by drying. Can be obtained.

The thickness of the positive electrode active material layer is usually 300 μm or less, preferably 20 to 200 μm, but is not limited thereto.

The positive electrode of the present invention can be obtained by forming a laminate in which a conductive polymer layer is provided on the surface of the positive electrode active material layer.

Any conductive polymer can be used as long as its electric resistance rapidly increases at a potential of 2 V or less (vs. Li / Li ⁺ ). Specific examples of such a conductive polymer include polyaniline, polypyrrole, polythiophene, polyacetylene, polyphenylene, polyphenylene vinylene, and derivatives thereof. In the present invention, polyaniline is preferably used. As for polyaniline, polyaniline having the following characteristics is particularly preferably used.

1) NMP-soluble polyaniline that can be dissolved in NMP (N-methyl-2-pyrrolidone) at 3 to 20 mass% to form a solution in the undoped state.
2) A highly polymerized polyaniline having an intrinsic viscosity [η] measured in sulfuric acid at 30 ° C. of 0.4 to 1.5 dl / g in a dedope state.

The polyaniline having a high polymerization degree can be produced from aniline or an aniline derivative by an oxidation polymerization method. It is known that polyaniline takes the structure of leucoemeraldine, emeraldine and pernigranyline depending on its oxidation state. Among these, when polyaniline having an emeraldine structure is doped, it is useful because it has the highest electron conductivity. On the other hand, polyaniline having a leuco emeraldine structure does not provide high electron conductivity even when doped, but has an advantage of excellent solubility. Therefore, in the present invention, emeraldine type high molecular weight polyaniline is converted to a hydrazine compound such as phenylhydrazine, hydrazine, hydrazine hydrate, hydrazine sulfate, hydrazine hydrochloride, and a reductive metal hydride compound such as lithium aluminum hydride and lithium borohydride. By reducing the perniguraniline structure using a compound such as the above, the solubility in a solvent is increased, and then a doping treatment is performed as necessary to dissolve 3-20% by mass of polyaniline in the solvent. Use polyaniline solution. In this case, NMP (N-methyl-2-pyrrolidone) is preferable as the solvent.

The high polymerization degree polyaniline preferably has [η] in the range of 0.4 to 1.5 dl / g. When [η] is less than 0.4 dl / g, the strength of the polyaniline as a coating film is insufficient, which is not preferable. Moreover, when it exceeds 1.5 dl / g, the solubility with respect to NMP as a polyaniline falls and it is unpreferable.

The polymerization of emeraldine type high degree of polymerization polyaniline used in the present invention is performed by oxidizing an aniline with an oxidizing agent while maintaining a temperature of 5 ° C. or less, preferably 0 ° C. or less in a solvent in the presence of a protonic acid. Polymerization is performed to produce doped polyaniline, which is then dedoped by a basic material. Thereby, the high degree-of-polymerization polyaniline which can be used for the positive electrode of this invention whose intrinsic viscosity [(eta)] measured at 30 degreeC in the sulfuric acid is 0.4-1.5 dl / g is obtained.

Examples of the protonic acid include hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, borofluoric acid, phosphorous hydrofluoric acid, hydrofluoric acid, hydroiodic acid, and other inorganic acids, benzenesulfonic acid, p-toluenesulfonic acid, and the like. Aromatic sulfonic acids, alkanesulfonic acids such as methanesulfonic acid, ethanesulfonic acid, phenols such as picric acid, aromatic carboxylic acids such as m-nitrobenzoic acid, aliphatic carboxylic acids such as dichloroacetic acid, malonic acid, etc. Is preferably used. Further, as the oxidizing agent, manganese dioxide, ammonium peroxodisulfate, hydrogen peroxide, ferric salt, iodate and the like are preferably used. These oxidizing agents are usually used in amounts ranging from 0.5 to 1.5 moles per mole of aniline. Moreover, water is preferably used as the solvent.

In the present invention, in order to increase conductivity by doping polyaniline, an acidic compound such as protonic acid can be used as a dopant in the NMP solution of polyaniline. As such a protonic acid, a protonic acid having an acid dissociation constant pKa of 3 or less is preferable. For example, a sulfonic acid compound can be used.

Specific examples of the sulfonic acid compound include phenol sulfonic acid, benzene sulfonic acid, aminonaphthol sulfonic acid, methanyl acid, sulfanilic acid, allyl sulfonic acid, lauryl sulfuric acid, xylene sulfonic acid, chlorobenzene sulfonic acid, methane sulfonic acid, and ethane. Sulfonic acid, 1-propanesulfonic acid, 1-butanesulfonic acid, 1-hexanesulfonic acid, 1-heptanesulfonic acid, 1-octanesulfonic acid, 1-nonanesulfonic acid, 1-decanesulfonic acid, 1-dodecanesulfonic acid , Styrenesulfonic acid, p-toluenesulfonic acid, naphthalenesulfonic acid, ethylbenzenesulfonic acid, propylbenzenesulfonic acid, butylbenzenesulfonic acid, pentylbenzenesulfonic acid, hexylbenzenesulfonic acid, heptylbenzenesulfonic acid, Cutylbenzenesulfonic acid, nonylbenzenesulfonic acid, decylbenzenesulfonic acid, undecylbenzenesulfonic acid, dodecylbenzenesulfonic acid, pentadecylbenzenesulfonic acid, octadecylbenzenesulfonic acid, diethylbenzenesulfonic acid, dipropylbenzenesulfonic acid, dibutylbenzenesulfone Acid, methylnaphthalenesulfonic acid, ethylnaphthalenesulfonic acid, propylnaphthalenesulfonic acid, butylnaphthalenesulfonic acid, pentylnaphthalenesulfonic acid, hexylnaphthalenesulfonic acid, heptylnaphthalenesulfonic acid, octylnaphthalenesulfonic acid, nonylnaphthalenesulfonic acid, decylnaphthalenesulfone Acid, undecylnaphthalenesulfonic acid, dodecylnaphthalenesulfonic acid, pentadecylnaphthalene Acid, octadecylnaphthalenesulfonic acid, dimethylnaphthalenesulfonic acid, diethylnaphthalenesulfonic acid, dipropylnaphthalenesulfonic acid, dibutylnaphthalenesulfonic acid, dipentylnaphthalenesulfonic acid, dihexylnaphthalenesulfonic acid, diheptylnaphthalenesulfonic acid, dioctylnaphthalenesulfonic acid, Dinonylnaphthalenesulfonic acid, trimethylnaphthalenesulfonic acid, triethylnaphthalenesulfonic acid, tripropylnaphthalenesulfonic acid, tributylnaphthalenesulfonic acid, camphorsulfonic acid, acrylamide-t-butylsulfonic acid, polyvinylsulfonic acid, polyvinylsulfuric acid, polystyrenesulfonic acid , Sulfonated styrene-butadiene copolymer, polyallylsulfonic acid, polymethallylsulfonic acid, poly-2 -Acrylamide-2-methylpropane sulfonic acid, polyisoprene sulfonic acid, etc. can be mentioned. Of these dopants, phenolsulfonic acid and benzenesulfonic acid are particularly preferable.

These dopants are added in a range of 0.05 to 0.6 equivalents, more preferably 0.1 to 0.3 equivalents, relative to the structural unit units of polyaniline.

By applying the conductive polymer solution such as polyaniline to the surface of the positive electrode active material layer described above and drying, the conductive polymer layer such as polyaniline whose electrical resistance increases rapidly at a potential of 2 V or less (vs. Li / Li ⁺ ). Can be obtained. The lithium ion secondary battery positive electrode of the present invention in which is stacked on the positive electrode active material layer can be obtained.

The thickness of the conductive polymer layer is usually in the range of 0.01 to 10 μm, and preferably in the range of 0.1 to 2 μm. When the thickness of the conductive polymer layer is less than 0.01 μm, even if the resistance of the conductive polymer increases rapidly, the insulation between the positive electrode layer and the current collector is not sufficient, and on the other hand, 10 μm When it is thicker, the resistance of the conductive polymer may cause an increase in internal resistance during normal charge / discharge.

As described above in detail, according to the present invention, a lithium secondary battery using a positive electrode using olivine-type lithium iron phosphate particles as an active material has become an overdischarged region (2-0.5 V). In this case, it is possible to provide a lithium ion secondary battery positive electrode and a lithium secondary battery including the positive electrode that can quickly drop the voltage.

Claims (3)

A positive electrode for a lithium secondary battery, in which a positive electrode active material layer and a conductive polymer layer whose electric resistance rapidly increases at a potential of 2 V or less (vs. Li / Li ⁺ ) are laminated, and the positive electrode active material is olivine type lithium iron phosphate A positive electrode for a lithium secondary battery, characterized in that the average discharge potential drop rate measured under the following conditions is 0.3 V / Hr or more.
<Measurement conditions of average discharge potential drop>
Structure of cell / bipolar pouch cell / counter electrode: lithium metal / electrolyte: 1M LiPF ₆ in EC + EMC + DMC = 1: 1: 1 (vol.%)
Measurement conditions and temperature: 30 ° C
・ Discharge range: 2 V to 0.5 V (vs. Li / Li ⁺ )
・ Discharge current: 20 mA / g-positive electrode A positive electrode for a lithium secondary battery according to claim 1, wherein the conductive polymer layer is characterized by comprising the polyaniline. The positive electrode for a lithium secondary battery according to claim 2, wherein the polyaniline has the following characteristics.
1) NMP-soluble polyaniline that can be dissolved in NMP (N-methyl-2-pyrrolidone) at 3 to 20 mass% to form a solution in the undoped state.
2) A highly polymerized polyaniline having an intrinsic viscosity [η] measured in sulfuric acid at 30 ° C. of 0.4 to 1.5 dl / g in a dedope state.