JP2012204278A - Electrode for lithium secondary battery and secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode for a lithium ion secondary battery having high energy density and mounting energy density, superior cycle properties, and high potential flatness.SOLUTION: The electrode for the lithium secondary battery contains a transition metal compound having an olivine structure as a main electrode active material. A binding agent contained in an electrode mixture of the electrode for the lithium secondary battery is a high polymer material which shows high electron conductivity by occluding an electrolyte ion, and the density of the electrode mixture is 1.8 g/cm. As a result, high current collecting performance is given by improving the adhesion and density to a current collector of an electrode mixture layer containing the transition metal compound having the olivine structure as the main active material, and the energy density is improved.

Description

  The present invention relates to an electrode for a lithium secondary battery using a transition metal compound having an olivine structure as a main electrode active material and a lithium ion secondary battery using the same, and particularly to an electrode mixture for forming an electrode.

  Since a lithium ion battery having a high voltage and a high energy density was put into practical use in 1991, electronic devices have been rapidly reduced in size, weight, and cordlessness. On the other hand, against the backdrop of the increase in environmentally hazardous substances accompanying the consumption of fossil fuels and the rise in crude oil prices, the introduction of clean energy such as fuel cells and solar cells, and the spread of hybrid vehicles equipped with nickel-hydrogen batteries are rapidly spreading. Progressing. In particular, a lithium ion secondary battery has a high voltage and a high energy density, and therefore has been actively developed as a large-sized secondary battery for electric power storage and electric vehicles.

  Generally, a lithium secondary battery includes an electrolyte between a positive electrode, a negative electrode, and positive and negative electrodes. In the positive electrode and / or the negative electrode, an electrode mixture layer is formed on at least one surface of the current collector, and the positive / negative electrode mixture layer mainly includes a positive electrode active material or a negative electrode active material, a binder, and a conductive agent. It is composed of For example, as a positive electrode active material of a 18650 size standard small lithium ion secondary battery, a composite oxide made of a metal element such as lithium cobaltate, lithium, cobalt, nickel, and manganese is used. Higher energy density and lower cost of secondary batteries are being promoted. On the other hand, the negative electrode mixture uses a mixture of carbon materials such as graphite, hard carbon, and polyacene that can occlude lithium. However, in order to achieve higher energy density, silicon and tin oxide are used. Mixture electrodes and metal lithium negative electrodes using these as active materials have been studied.

In a small lithium ion secondary battery, the positive electrode active material is lithium cobaltate, and carbon such as graphite is most frequently used as the negative electrode active material. However, since the cobalt oxide of the positive electrode is unstable against heat, such as being thermally decomposed and releasing oxygen at high temperatures, spinel lithium manganate, which is highly safe in an overcharged state, is used instead. ing. However, in terms of effective energy density, cobalt oxide is 150 Ah / kg, whereas spinel type lithium manganese oxide is significantly inferior to 110 Ah / kg. Therefore, large lithium secondary battery electrodes are required to be safer than spinel lithium manganese oxide and have an effective energy density comparable to that of lithium cobalt oxide. A transition metal compound having an olivine type crystal structure represented by, for example, LiMPO ₄ (M: transition metal element) as a positive electrode active material for a lithium secondary battery excellent in safety and durability against such a spinel type manganese oxide. It is being considered.

JP 2005-514304 A JP 2006-032241 A JP 2009-263222 A JP 2005-302300 A Japanese Patent Application No. 6-132028

Zhaohui Chen and J. R. Dahn, "Reducing Carbon in LiFePO4 / C Composite Electrodes to Maximize Specific Energy.Volumetric Energy, and Tap Density", Journal of The Electrochemical Society, Vol. 149 (2002), pp. A1184- A1189 Y-H.Huang, K-S.Park, J.B.Goodenough, "Improving Lithium Batteries by Tethering Carbon-Coated LiFePO4 to Polypyrrole", Journal of The Electrochemical Society, Vol.153 No.12 (2006), pp. A2282-A2286

Transition metal compounds having an olivine structure such as lithium iron phosphate (LiFePO ₄ ) are excellent in safety and thermal stability, have a high potential, and have good cycle characteristics as a secondary battery electrode.
FePO _{4 from} which lithium ions have been released is also attracting attention as a positive electrode active material for large batteries that replace spinel lithiated manganates because of its excellent thermal stability. However, since lithium iron phosphate has low electron conductivity, it is difficult to perform electrode reaction in charge and discharge as it is. In fact, the conductivity of lithium iron phosphate is about 10 ⁻¹ S / cm to 10 ⁻⁶ S / cm, while the conductivity at 25 ° C. of spinel type lithium manganate, lithium cobaltate and the like is about 10 ^{− 8} S / cm or less. In addition, transition metal compounds having an olivine-type crystal structure such as lithium iron phosphate are in powder form, and multiple primary particles form secondary particles without agglomerating closely with each other, so there are many spaces. is doing. For this reason, there is a problem that it is difficult to improve the adhesion of the electrode mixture layer to the current collector and the electrode density.
Therefore, it is possible to improve the adhesion of the mixture containing the transition metal compound having an olivine structure as the main particulate active material to the current collector, the density of the electrode layer, the improvement of the energy density, and the high current collection. Therefore, it is desired to improve output characteristics in high rate discharge.

In response to the problem of the transition metal compound having an olivine structure as an electrode active material, lithium iron phosphate has been developed to a practical level. For example, Patent Document 1 discloses a technique for reducing electrical resistance by adding a different metal element to LiFePO ₄ having an olivine structure. Non-Patent Document 1 and Patent Document 2 disclose a technique for improving the tap density during the production of olivine-type LiFePO ₄ and forming a composite with carbon having a particle size of several tens of nm to several μm or less. Patent Document 3 discloses an active material in which the surface of a lithium composite oxide having an olivine type crystal structure is coated with a conductive carbon material.

In general, an electrode of a lithium ion battery is a slurry prepared by preparing a mixture comprising an active material, a binder (binder), a conductive additive, and a solvent, and applied to a current collector sheet and dried to form a sheet electrode. It is produced by pressing for the purpose of density adjustment. In a mixture slurry using a transition metal compound having an olivine type crystal structure, the binder of the positive electrode enters into the space of the secondary particles, and as a result, the binding capacity of the binder decreases or the electrode There is a problem that it is difficult to improve the density. In order to solve this problem, Patent Document 4 describes using polyvinylidene fluoride having a molecular weight of 370,000 to 1,000,000 instead of the conventional polyvinylidene fluoride binder (PVdF: poly (vinylidene fluoride)) to increase the binding force between the particles. A method for improving the adhesion of the material to the current collector is disclosed.
On the other hand, a conductive polymer such as polyacetylene has been studied as a polymer electrode as an electrode active material, but has a low volumetric energy density and has not yet been put into practical use as a positive electrode of a secondary battery. In order to solve the problem of such a conductive polymer electrode, Patent Document 5 discloses a composite electrode of a transition metal compound and a conductive polymer. That is, although a composite positive electrode using vanadium pentoxide as a transition metal compound and polyaniline as a conductive polymer is disclosed, the energy density does not exceed spinel type lithium manganate. Non-Patent Document 2 discloses a case where the surface of olivine-type lithium iron phosphate is coated with polypyrrole or made conductive after coating, but as a binder for an active material having an olivine structure. It was difficult to easily predict the function of.

  Under such circumstances, an object of the present invention is to provide a high energy density electrode having high energy density and mounting energy density, excellent cycle characteristics, and high potential flatness, and a secondary battery using the electrode.

  As a result of earnestly examining these problems, the inventors of the present invention, in an electrode mixture for lithium secondary batteries using a transition metal compound having an olivine structure as a main electrode active material, in the binder of the electrode mixture, Focusing on the use of a polymer material that exhibits high electronic conductivity by occluding electrolyte ions, we succeeded in dramatically improving the mounting energy density of the electrode for a lithium secondary battery.

One embodiment of the electrode for a lithium secondary battery according to the present invention uses a transition metal compound having an olivine structure as a main electrode active material. The binder contained in the electrode mixture of the electrode for the lithium secondary battery is a polymer material exhibiting high electron conductivity by occluding electrolyte ions, and the density of the electrode mixture is 1.8 g / cm ^3. That's it. By using a polymer material exhibiting high electronic conductivity as a binder, it penetrates between active material particles as a conductive agent and is combined. In addition, it improves adhesion not only between the particles of the active material but also in binding with the current collector surface. Further, the polymer material itself can store energy as an active material by reversible doping and dedoping with electrolyte ions. This improves the conductivity of the electrode mixture, increases the output characteristics, current collection performance, and energy density as an electrode, and enables the electrode composition layer to be thicker. As a result, the lithium secondary battery electrode Mounting energy density can be improved.

Since transition metal compound particles having an olivine type crystal structure form secondary particles by primary particles and there are many spaces, if this hole can be filled with other active materials, Density and energy density can be improved.
It is preferable that the polymer material exhibiting high electron conductivity by occlusion of the electrolyte ions described above is dissolved and / or uniformly dispersed in water or an organic solvent in an oxidized or reduced state. The polymer material is desirably used as a binder by being dissolved and / or uniformly dispersed, and uniformly penetrates between the particles of the active material, thereby improving the adhesion and density of the electrode mixture. Can do.
Further, when the electrode mixture described above is used as a positive electrode of a lithium secondary battery electrode, the lithium transition metal compound is a lithium composite oxide containing iron and phosphorus, and the electrochemical potential at the time of charging the positive electrode is It is preferable to be base on the most noble side redox potential of the polymer material.

  In the electrode for a lithium secondary battery according to the present invention, the electrode mixture preferably contains carbon or a metal oxide having conductivity as a conductive assistant within a range of 5% by weight or less of the electrode mixture layer. By containing a suitable amount of conductive aid, the conductivity of the electrode mixture layer can be improved. Further, according to one aspect of the lithium ion secondary battery according to the present invention, an electrode mixture containing the above-described lithium transition metal compound having an olivine structure as a main active material is used as an electrode mixture layer having a thickness of 100 μm or more as a current collector. It is formed on a sheet.

  According to the present invention, it is possible to provide a high energy density electrode having high energy density and mounting energy density, excellent cycle characteristics, high potential flatness, and a secondary battery using the electrode.

  Hereinafter, embodiments of the present invention will be described. One aspect of the electrode mixture for a lithium secondary battery of the present invention uses a transition metal compound (1) having an olivine structure as a main electrode active material. A polymer material (2) that exhibits high electron conductivity by occluding electrolyte ions in the binder of the electrode mixture is used.

First, the electrode mixture (active material, binder, conductive additive, etc.) of the electrode for a lithium secondary battery according to the present invention will be described.
[Active material: transition metal compound having olivine structure]
The transition metal compound (1) having an olivine structure used in the present invention is LiFePO ₄ , LiCoPO ₄ , LiVPO ₄ , LiVPO ₄ F, LiMnPO ₄ , LiNiVO ₄ , LiFeP ₂ O ₇ , LiTePO ₄ , Li ₂ MnSiO _4. , Li ₂ MnSiO ₄ , Li ₃ V ₂ (PO ₄ ) ₃ , Li ₂ FeSiO ₄ , LiMnBO ₃ , LiFeBO ₃ , and general formulas Li _x Fe _1-y B _y C (0 ≦ x, y ≦ 1) ) Is preferable, and in particular, x is preferably 0.9 or more, and y is preferably 0.9 ≦ y ≦ 1.0. In the general formula, “B” is selected from Co, Mn, Ni, Te, Mo, Cu, Cr and the like. “C” is represented by PO ₄ , SiO ₄ , VO ₄ , and BO ₃ . Furthermore, LiFePO ₄ and LiFe _y B _1-y PO ₄ are particularly preferable from the viewpoint of matching performance with the binder in the present invention. The transition metal compound (1) may be one obtained by forming a complex with a conductive agent in the particle production process, or one whose surface is conductively coated with a metal oxide, carbon or the like.

[Binder: A polymer material that exhibits high electronic conductivity by occlusion of electrolyte ions]
The polymer material (2), which is a binder in the present invention and exhibits high electron conductivity by occluding electrolyte ions, is a polymer material that exhibits electronic conductivity, such as polypyrrole, polythiophene, polyaniline, polyacene, Examples include Redox active conductive polymer materials such as polydiphenylbenzidine, polyvinylcarbazole, and polytriphenylamine.

(Conductivity of binder)
These electronically conductive polymer materials exhibit high electrical conductivity by forming a complex with a donor or acceptor, but as a binder in the present invention, it is desirable to have an electrical conductivity of 1 S / cm or more. . In addition, high ion diffusibility in the polymer is also required. These polymers preferably have high electrical conductivity and current collecting ability, and are excellent in binding performance to a current collector as a polymer, and further have a function as an active material. Among these, π-electron conjugated polymers having high electron affinity such as polypyrrole, polyaniline, or derivatives thereof are preferable for the positive electrode binder in that it can be charged and discharged relatively stably in a general-purpose non-aqueous electrolyte. . In addition, a π-electron conjugated polymer having a high ionization potential such as polyphenylene, polythiophene, polyphenylene vinylene, or a derivative thereof is suitable as a negative electrode binder. For example, in a battery system in which the active material of the positive electrode mixture is lithium iron phosphate with an olivine structure and polyaniline as a binder, the supporting electrolyte anion in the electrolytic solution is doped into the polyaniline by charging, and at the same time the lithium iron lithium phosphate Ions move to the negative electrode as deintercalation, and energy is simultaneously accumulated in the binder and the active material. Since this reaction proceeds reversibly, during discharge, lithium ions intercalate with iron phosphate, and at the same time, anions are dedoped from polyaniline, and the conductivity of the binder decreases.

Among these polymers, especially those with high electron affinity used for the positive electrode binder, active doping of the electrolyte anion occurs from the undoped state even if there is no electron transfer through the external circuit, and it is stably conductive. It becomes a state.
Among these polymers, polymers having excellent dispersibility are particularly suitable as binders, particularly soluble or meltable polymers. Examples of the positive electrode binder include reduced polyaniline, polypyrrole alkylated at the 2 or 5 position, polypyrrole doped with styrene alkyl sulfonic acid as a dopant, polyphenylene vinylene, poly (3,4-ethylenedioxythiophene) (PEDOT), and the like. It is done.

  The combination of a binder and a transition metal compound having an olivine structure is extremely important, and it is difficult to operate with a composite of lithium cobaltate or spinel manganese dioxide and polyaniline, which is the redox potential of polyaniline as a binder and these inorganic active materials. It is thought that this is due to the difference, but the details are unknown.

(Binder adhesion, density)
Although the powder having an olivine structure has a large number of spaces of secondary particles and it is difficult to improve the density, according to the present invention, the binder uniformly penetrates as a conductive agent, can be combined, and the active material particles are used as the binder. Extremely high adhesion can be obtained not only in the interval but also in binding to the current collector surface. Conventionally used PVdF and SBR (styrene butadiene rubber) emulsion used as a water-based binder cannot be said to have sufficient adhesion to an aluminum current collector, but in the present invention, extremely high adhesion is obtained.
As a result, even if the thick film after pressing the mixture layer is 60 microns or more, it does not peel in the 90-degree bending test. Furthermore, the density after charge / discharge is 2.0 g / cm ³ or more with respect to the density of the conventional electrode film, and the conventional volume energy density can be improved by 10% or more. Moreover, since the polymer material (2) which is a binder also contributes to charging / discharging, it leads also to the improvement of the energy density of the electrode itself.
If the mixture layer density, for example of the lithium iron phosphate, the density of the lithium iron phosphate itself is 2.7 g / cm ^3, but it does not exceed it, the energy of 1.8 g / cm ³ in the following prior art The density cannot be exceeded, and in particular for large batteries, a density of 2.0 g / cm ³ or more is preferred in order to improve the mounting density.

[Conductive aid]
A conductive additive can be added as a positive electrode mixture component. Conductive aids include conductive carbon powders such as acetylene black, aniline black, activated carbon, ketjen black and graphite powder, carbon bodies and carbon fibers starting from resin raw materials such as polyacrylonitrile, pitch, cellulose and phenol. , Ti, Sn, In and other metal oxide powders, stainless steel, nickel and other metal powders and fibers. In addition to high electrical conductivity, an effect with a small addition amount is required as a characteristic required for these conductive aids. In general, about 5 to 8% by weight (wt%) of a conductive additive is added to the electrode mixture. However, it is necessary to add a large amount of a transition metal compound having an olivine structure. This decreases the energy density because it does not directly reduce the adhesion strength of the material and does not directly contribute to energy storage. On the other hand, in the present invention, the conductive battery is not required for basic battery operation, and the binder polymer imparts conductivity. Therefore, when the conductive assistant is added, the addition amount is kept to a minimum. be able to. That is, an addition amount of 1 to 5% by weight or less is preferable for improving the output characteristics of the electrode while suppressing a decrease in energy density as much as possible. More preferably, it is 2 to 4% by weight or less.

[Others]
The amount of the polymer material (2) in the positive electrode mixture of the present invention is 1% by weight or more and less than 20% by weight. More preferably, it is 3% by weight or more and less than 10% by weight. If it is less than 1% by weight, it is difficult to expect both the binding force and the conductivity, and the present invention is not achieved. Moreover, the adhesiveness as an electrode can be stored by blending other binders, but there is no other effect. On the other hand, when the amount of the polymer material (2) is 20% by weight or more, the electrode density is less than 1.8 g / cm ^3, and it is difficult to make use of the high volume energy density of the olivine structure.
The thickness of the positive electrode mixture layer is 60 μm or more and less than 300 μm, more preferably 100 μm or more and less than 200 μm. If it is less than 100 μm, it is disadvantageous for improving the mounting energy density of a large battery, and if it is 500 μm or more, it is difficult to maintain the adhesion, which is also disadvantageous in terms of current collection efficiency.

  As described above, according to the lithium secondary battery electrode of the present invention, the improvement in the adhesion and density improvement of the mixture electrode layer comprising the transition metal compound having an olivine structure as the main particulate active material to the current collector is improved. In addition, a high current collecting property can be imparted. Thereby, the energy density of the electrode for lithium secondary batteries can be improved even in high-rate discharge.

Next, a lithium ion secondary battery using the above-described electrode for a lithium secondary battery will be described.
A secondary battery can be assembled by using the electrode thus obtained and combining elements such as an electrolyte and a separator as other elements of the secondary battery.
As the separator, a porous film such as paper, polypropylene, polyethylene, glass, or ceramic, or a woven or non-woven fabric of fibers can be used. In addition, a solid electrolyte can be used as a constituent element instead of the separator, but it is used as needed for the purpose of isolating the positive and negative electrodes, and is not limited thereto.

The electrolytic solution is obtained by dissolving a supporting electrolyte in a solvent.
As these non-aqueous organic solvents, commonly used electrolytic solutions can be used, but the invention is not limited thereto. A solvent may be used individually by 1 type and may be used in mixture of 2 or more types. Among them, those having a large dielectric constant, electrochemical stability, a wide potential window, a wide use temperature range, and excellent safety are preferable.

  Specific examples of the organic solvent include lactones such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, 3-methyl-1,3-oxazolin-2-one; N-methylformamide, N, N-dimethyl Amides such as formamide, N-methylacetamide, N-methylpyrrolidinone; carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, ethylene carbonate, styrene carbonate, vinylene carbonate; 1,3-dimethyl-2- Examples thereof include imidazolidinones such as imidazolidinone or fluorine-based solvents in which hydrogen atoms or alkyl groups of these various organic solvents are substituted with fluoroalkyl groups.

The supporting electrolyte salt includes a lithium salt, and is not particularly limited as long as it is a stable lithium salt in the operating voltage range of a lithium battery or a lithium ion battery.
Specific examples of the lithium salt used in the present invention include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), and lithium bis (trifluoromethane). Sulfonyl) amide (LiN (SO ₂ CF ₃ ) ₂ ), lithium bis (pentafluoroethanesulfonyl) amide (LiN (SO ₂ C ₂ F ₅ ) ₂ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), and the like. .
These lithium salts can be used singly or in combination of two or more.

In the current lithium battery system in which a non-aqueous electrolyte is used, since the electrolyte is a flammable liquid, a flame retardant and an ionic liquid are also used. In addition, as for polymer electrolytes, in addition to ion-conducting glasses such as NASICON and LISICON in the inorganic system, a lithium ion conductor of Li ₂ S—P ₂ S-based glass ceramics has recently recorded a conductivity of 10 ³ Siemens.

  By forming an electrode mixture mainly composed of the above-described lithium transition metal compound (1) having an olivine structure on the current collector sheet as an electrode mixture layer having a thickness of 60 μm or more, energy density and mounting energy A large-sized lithium ion secondary battery having a high energy density electrode with high density, excellent cycle characteristics, and high potential flatness can be provided. Here, the large-sized lithium ion secondary battery includes, for example, a lithium ion secondary battery used for power storage, automobiles (electric cars, hybrid cars, etc.), notebook computers, and the like.

The electrode of the present invention is particularly advantageous as a positive electrode material, but is not limited thereto. Examples of the negative electrode material include negative electrodes such as carbon negative materials and metal oxides in addition to transition metals having an olivine structure. For example, examples of the carbon material include natural carbides such as pitch, coke, and coal, or a fired body, synthetic polymer, carbon body that is a fired body of natural polymer, graphite, and the like. Oxides such as silicon, tin, and titanium (Li _{4 + z} Ti ₅ O ₁₂ ) are also excellent as negative electrode materials. However, in the present invention, the negative electrode is preferably a carbon or silicon negative electrode from the viewpoint of cost.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.
-Production of positive electrode-
Table 1 shows the configurations of the positive electrode synthesis agent layer electrodes of Examples and Comparative Examples.

[Example 1]
(Binder synthesis example)
5 mL (55 mmol) of aniline purified by distillation into a 300 mL eggplant flask and 100 mL of 1 MH ₂ SO ₄ were added, and the mixture was cooled to 0 ° C. or lower in an ice bath containing saline. 50 mL of 0.25 M (NH ₄ ) ₂ S ₂ O ₈ was slowly added dropwise over 1 hour, followed by stirring overnight. The obtained green precipitate was filtered and washed well with 1MH ₂ SO ₄ , water and methanol to obtain 3.24 g of polyaniline doped with sulfate anions. All reagents used were manufactured by Wako Pure Chemical Industries, Ltd.
Polyaniline doped with sulfate anions was added to a 5 wt% hydrazine-methanol solution and stirred overnight under a nitrogen atmosphere. The obtained purple powder was filtered, washed well with methanol, and then heated and dried in vacuum at 80 ° C. to obtain 2.11 g of white reduced polyaniline. The reduced polyaniline was stored in a glove box under an inert gas atmosphere immediately after drying.

(Preparation of Example Positive Electrode)
Lithium iron phosphate LiFePO ₄ (manufactured by Sumitomo Osaka Cement Co., Ltd.) as a positive electrode active material in a binder solution prepared by dissolving 20 parts by weight of polyaniline synthesized in the binder synthesis example in 80 parts by weight of N-methylpyrrolidone (hereinafter also referred to as NMP) Disperse 81 parts by weight to make a paste, adjust the viscosity by adding an appropriate amount of NMP, and then use a blade coater on a 10 cm wide, 30 cm long, 20 μm thick aluminum foil (manufactured by Nippon Denki Kogyo Kogyo Co., Ltd.). And then dried in an oven at 100 ° C. for 10 minutes, further dried at 110 ° C. for 10 minutes, allowed to stand at 85 ° C. for 12 hours under reduced pressure, uniaxially pressed at a pressure of 100 kgf · cm ² , and inert It preserve | saved under gas atmosphere and it punched on the 14 mm diameter disk with the Thomson blade. In Example 1, the ratio of the active material (olivine-type lithium iron phosphate): binder was 81:19, and no conductive auxiliary agent (carbon auxiliary agent) was added.

(Preparation of negative electrode)
Graphite (manufactured by Nippon Graphite Co., Ltd.) as the negative electrode active material, PVdF (manufactured by Kureha Co., Ltd.) having an average molecular weight of 600,000 to 700,000 as the binder, N-methylpyrrolidone as the coating solvent, negative electrode active material: binder: NMP is mixed at a predetermined ratio (mass ratio, but the binder is converted into a solid content) to form a paste, applied to a copper foil (made by Fukuda Metal Foil Co., Ltd.) having a width of 10 cm and a length of 30 cm, with a predetermined thickness. After drying in an oven for 10 minutes, drying at 100 ° C. for another 10 minutes, allowing to stand at 85 ° C. under reduced pressure for 12 hours, uniaxial pressing, storing in an inert gas atmosphere, and on a 16 mmφ disc with a Thomson blade Punched out.

(Preparation of non-aqueous electrolyte)
A solution obtained by dissolving LiPF _{6 at} a ratio of 1 mol per liter of a mixed solution of diethyl carbonate: ethylene carbonate = 1: 1 was used as a non-aqueous electrolyte.

(Production of lithium ion secondary battery)
Negative electrode metal lithium (Honjo Metal Co., Ltd.), polyolefin separator (Celgard Co., Ltd.), composite punched into a 16mmφ size in a bipolar simple evaluation cell (Toyo System Co., Ltd.) under inert atmosphere They were superposed in the order of positive electrode and used as an electrolyte.

[Example 2]
As an active material, a binder, and a conductive assistant, except that the paint was adjusted so that the weight ratio of (olivine lithium iron phosphate) :( polyaniline synthesized by chemical polymerization) :( acetylene black) was 85: 10: 5 Produced a lithium ion secondary battery in the same manner as in Example 1.

[Example 3]
As an active material, a binder and a conductive additive, except that the paint was adjusted so that the weight ratio of (olivine type lithium iron phosphate) :( polyaniline synthesized by chemical polymerization) :( acetylene black) was 90: 6: 4 Produced a lithium ion secondary battery in the same manner as in Example 1.

[Example 4]
Other than adjusting the paint so that the weight ratio of (olivine type lithium iron phosphate) :( polyaniline synthesized by chemical polymerization) :( acetylene black) is 92: 5: 3 as the active material, binder and conductive assistant Produced a lithium ion secondary battery in the same manner as in Example 1.

[Example 5]
As an active material, a binder, and a conductive auxiliary agent, a binder such that the weight ratio of (olivine type lithium iron phosphate) :( polypyrrole-polystyrenesulfonic acid synthesized by chemical polymerization) :( acetylene black) is 85: 10: 5. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the N-methylpyrrolidone solution was dispersed.

[Comparative Example 1]
(Preparation of the positive electrode of Comparative Example 1)
10 parts by weight of PVdF (manufactured by Kureha Co., Ltd.) having an average molecular weight of 600,000 to 700,000 is dissolved in 90 parts by weight of N-methylpyrrolidone as a coating solvent, and lithium iron phosphate LiFePO ₄ (Sumitomo Osaka Cement Co., Ltd.) as a positive electrode active material Company), acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) and PVdF in a ratio of 88: 6: 6 (mass ratio, but the binder is solid content conversion) to make a paste, and an appropriate amount of NMP In addition, after adjusting the viscosity, it was applied to an aluminum foil having a width of 10 cm, a length of 30 cm, and a thickness of 20 μm (manufactured by Nippon Denki Kogyo Co., Ltd.) with a predetermined thickness, dried in an oven at 100 ° C. for 10 minutes, and further at 110 ° C. for 10 minutes. dried and left for 12 hours under reduced pressure 85 ° C., after uniaxial pressing at a pressure of 100 kgf · cm ^2, and stored in an inert gas atmosphere, Thomson blade He was punched on the disk of 14mmφ. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the positive electrode described above was used.

[Comparative Example 2]
10 parts by weight of PVdF (manufactured by Kureha Co., Ltd.) having an average molecular weight of 600,000 to 700,000 is dissolved in 90 parts by weight of N-methylpyrrolidone as a coating solvent, and lithium iron phosphate LiFePO ₄ (Sumitomo Osaka Cement Co., Ltd.) as a positive electrode active material Lithium ion secondary battery was produced in the same manner as in Comparative Example 1 except that the ratio was 84: 8: 8.

-Evaluation of positive electrode-
The charge / discharge test measurement method uses HJ-1005SH8A charge / discharge measurement device manufactured by Hokuto Denko Corporation. First, the battery is charged with a current of 0.2 in the charge direction until the battery voltage becomes 3.9 V, and after 10 minutes of rest. The battery was discharged at a current of 0.2 until the battery voltage became 2.5 V, and rested for 10 minutes. Thereafter, charging and discharging were repeated at a predetermined current value, and battery characteristics were evaluated. At this time, the initial characteristic data of each is the data of the fifth cycle, and the discharge capacity is calculated from the weight of the active material and the inactive material (binder and conductive aid) of the entire electrode as the combined electrode total capacity. did. The energy capacity, volumetric energy density, weight energy density, output characteristics, shutdown characteristics at the time of short-circuiting, and cycle characteristics (capacity after 100 cycles) were examined.

The evaluation results of the lithium ion secondary batteries shown in Table 1 are shown in Table 2.

Table 2 shows the following items.
-Adhesion: 90 degree bend test ◎: No crack / peeling, ○: Crack at the crease,
Δ: Partial peeling at the crease, ×: Peeling to the periphery of the crease
・ Energy density:
Capacity when discharged at 0.2C, then discharged at each rate of 0.2C, 1C, 3C, 5C ・ Energy capacity after 100 cycles:
Capacitance ratio to initial capacity after 100 cycles of charge / discharge: Short test: Change when short-circuited by external circuit ◎: No change, reusable,
○: No change in appearance and temperature, capacity degradation within 50%,
Δ: No change in appearance, unusable,
×: Appearance change, Temperature rise, Unusable • Olivine iron weight energy density (mAh / g):
Expression energy density of olivine structural compound (active material ratio conversion)

  As described above, the phosphorus-containing oxide having an olivine structure according to the present invention can be manufactured at a low cost with a low environmental load, and the secondary battery using this as a positive electrode active material has a high capacity even in the current load characteristics. Is obtained. Further, by using a polymer material exhibiting high electronic conductivity by occluding electrolyte ions as a binder, the conductivity is improved and it can be used as a positive electrode material for high-speed charge / discharge.

  In addition, this invention is not limited to embodiment shown above. Within the scope of the present invention, it is possible to change, add, or convert each element of the above-described embodiment to a content that can be easily considered by those skilled in the art.

Claims (5)

An electrode for a lithium secondary battery having a transition metal compound having an olivine structure as a main electrode active material,
The binder contained in the electrode mixture is a polymer material that exhibits high electron conductivity by occluding electrolyte ions,
The electrode for a lithium secondary battery, wherein the density of the electrode mixture is 1.8 g / cm ³ or more.   2. The electrode for a lithium secondary battery according to claim 1, wherein the electrode mixture contains carbon or metal oxide having conductivity as a conductive assistant within a range of 5% by weight or less of the electrode mixture layer. .   3. The electrode for a lithium secondary battery according to claim 1, wherein the polymer material is dissolved and / or uniformly dispersed in water or an organic solvent in an oxidized or reduced state. The lithium secondary battery electrode is a positive electrode,
The lithium transition metal compound is a lithium composite oxide containing iron and phosphorus, and the electrochemical potential during charging of the positive electrode is lower than the most noble-side redox potential of the polymer material. The electrode for a lithium secondary battery according to any one of claims 1 to 3.   An electrode mixture comprising the lithium transition metal compound having an olivine structure according to any one of claims 1 to 4 as a main active material is formed on a current collector sheet as an electrode mixture layer having a thickness of 60 µm or more. Lithium ion secondary battery.