TWI600195B - Nonaqueous electrolyte secondary battery and battery module using the same - Google Patents

Description

Nonaqueous electrolyte secondary battery and battery pack using same

The present invention relates to a nonaqueous electrolyte secondary battery and a battery pack using the same.

The present application is based on Japanese Patent Application No. 2011-268309 and claims priority.

In recent years, research and development of non-aqueous electrolyte secondary batteries for mobile machines, hybrid vehicles, electric vehicles, and household storage applications have gradually emerged. When the nonaqueous electrolyte secondary battery used in these fields is used, the charge and discharge cycle can be repeated for a long period of time.

When the charge/discharge cycle of the non-aqueous electrolyte secondary battery is repeated, the volume of the negative electrode material accompanying the intercalation or deintercalation of lithium deteriorates, and the negative electrode including the carbon-based material is deteriorated, and the battery capacity is lowered. In order to solve this problem, a non-aqueous "lithium titanate having a spinel structure" which is a solid material whose volume is hardly changed by embedding or deintercalation of lithium as a negative electrode material has been proposed as in Non-Patent Document 1. Electrolyte secondary battery.

At present, it is desirable to further improve the stability of the cycle depending on the materials and structures used.

In the non-aqueous electrolyte secondary batteries in which the negative electrode includes lithium titanate, various materials such as a lithium cobalt compound, a lithium nickel compound, and a lithium manganese compound are used as the positive electrode, and are used as the electrically isolated positive electrode. Examples of the negative electrode include various materials such as cellulose, polyethylene terephthalate, and polypropylene of a separator.

Prior technical literature Patent literature

Patent Document 1: International Publication No. 2003/081698

Patent Document 2: Japanese Laid-Open Patent Publication No. 2009-158396

Patent Document 3: Japanese Laid-Open Patent Publication No. 2009-205864

Patent Document 4: Japanese Laid-Open Patent Publication No. 2010-009898

Patent Document 5: Japanese Laid-Open Patent Publication No. 2010-153258

Patent Document 6: Japanese Laid-Open Patent Publication No. 06-060867

Patent Document 7: Japanese Laid-Open Patent Publication No. 2009-038017

Non-patent literature

Non-Patent Document 1: T. Ohzuku and K. Ariyoshi: "Lead-free accumulators for renewable and clean energy technologies", Chemistry Letters, Vol. 35, 848-849 (2006).

However, it is expected that the specificity of the compound to be used as the positive electrode and the specific material and structure to be used as the separator are optimized for cycle stability, and there is no evidence in Patent Documents 1 to 7.

An object of the present invention is to provide a nonaqueous electrolyte secondary battery which can exhibit excellent cycle stability and a battery pack using the same.

In view of the above circumstances, the inventors have found that by using a lithium manganese compound for the positive electrode, lithium titanate for the negative electrode, and a cellulose non-woven fabric containing polyethylene terephthalate fibers having a porosity of less than 50% by volume for the separator, Available The nonaqueous electrolyte secondary battery having the most excellent cycle stability is obtained, thereby completing the present invention.

That is, the present invention provides a nonaqueous electrolyte secondary battery which is a lithium ion battery including a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte; the positive electrode includes a lithium manganese compound, the negative electrode includes lithium titanate, and the separator is porous. A cellulose non-woven fabric containing polyethylene terephthalate having a rate of less than 50% by volume.

Preferably, in the nonaqueous electrolyte secondary battery of the present invention, the lithium manganese compound is Li _1+x M _y Mn _2-xy O ₄ (0≦x≦0.2, 0<y≦0.6; M is selected from the group consisting of A compound represented by a compound represented by at least one of Al and Ni; and particularly preferably, the lithium manganese compound is a compound represented by Li _1.1 Al _0.1 Mn _1.8 O ₄ or LiNi _0.5 Mn _1.5 O ₄ . In the nonaqueous electrolyte secondary battery of the present invention, the lithium titanate has a spinel structure.

In the nonaqueous electrolyte secondary battery of the present invention, the content of the polyethylene terephthalate fibers of the cellulose non-woven fabric containing the polyethylene terephthalate fibers is preferably 20% by weight or more and 80%. The weight% or less is more preferably 30% by weight or more and 70% by weight or less. Preferably, the cellulose non-woven fabric containing the polyethylene terephthalate fiber has a thickness of 15 μm or more and 35 μm or less, more preferably 20 μm or more and 30 μm or less.

The nonaqueous electrolyte secondary battery of the present invention has excellent cycle stability.

The above and other advantages, features, and advantages of the present invention will be apparent from the description of the appended claims. In addition, the scope of the invention It is disclosed by the scope of the patent application, and is intended to include all modifications within the scope and scope of the application.

<1. Negative electrode>

The nonaqueous electrolyte secondary battery of the present invention uses a negative electrode comprising "a lithium titanate having a spinel structure" as a negative electrode active material.

Preferably, it is represented by the molecular formula Li ₄ Ti ₅ O ₁₂ as lithium titanate. In the case of a spinel structure, the expansion and contraction of the active material due to the reaction of intercalation and deintercalation of lithium ions is small, which is particularly preferable. The lithium titanate may contain, for example, a trace amount of an element other than lithium or titanium such as Nb.

Preferably, the half width of the (400) plane of the powder X-ray diffraction of the lithium titanate using CuKα is 0.5 or less. When it is more than 0.5°, the crystallinity of lithium titanate is low, and the stability of the electrode is lowered.

It is preferable that the lithium content of the 8a lattice in the Rietveld analytical method using lithium X-ray diffraction of lithium titanate is 90% or more. If it is less than 90%, the defects in the crystal of lithium titanate are excessive, and the stability of the electrode is lowered.

Lithium titanate can be produced by heat-treating a lithium compound and a titanium compound at 500 ° C or higher and 1500 ° C or lower. If it is less than 500 ° C or more than 1500 ° C, there is a tendency that it is difficult to obtain lithium titanate having a desired structure. In order to improve the crystallinity of lithium titanate, it may be reheated at 500 ° C or more and 1500 ° C or less after the heat treatment. The temperature of the reheating treatment may be the same as or different from the temperature originally produced. The heat treatment environment can be carried out in air or in an inert gas such as nitrogen or argon. The heat treatment furnace is not particularly limited, and for example, a box furnace, a tubular furnace, a tunnel furnace, a rotary kiln, or the like can be used.

As the lithium compound, lithium hydroxide, lithium carbonate, lithium nitrate, lithium acetate, lithium oxalate, lithium halide or the like can be used. These lithium compounds may be used alone or in combination of two or more.

The titanium compound is not particularly limited, and for example, titanium oxide such as titanium oxide or titanium oxide can be used.

The ratio of the lithium compound to the titanium compound is such that the atomic ratio of lithium to titanium is about Ti/Li = 1.25, and the amount of adjustment can be adjusted depending on the properties of the raw material or the heating conditions.

The particle diameter of the produced lithium titanate is preferably 0.5 μm or more and 50 μm or less, and particularly preferably 1 μm or more and 30 μm or less from the viewpoint of handling. The above particle diameter is a value obtained by measuring the average particle diameter of each particle from the SEM and TEM images.

The lithium titanate has a specific surface area of preferably 0.1 m ² /g or more and 50 m ² /g or less from the viewpoint of easily obtaining a desired output density. The above specific surface area can be calculated by a mercury porosimeter and a BET method.

The bulk density of lithium titanate is preferably 0.2 g/cm ³ or more and 1.5 g/cm ³ or less. If it is less than 0.2 g/cm ³ , a large amount of solvent is required in the preparation of the slurry described later, which is disadvantageous in terms of cost; if it is more than 1.5 g/cm ³ , it is difficult to carry out mixing with the conductive auxiliary agent and the binder described later. problem.

<2. Positive electrode>

The positive electrode of the nonaqueous electrolyte secondary battery of the present invention contains a lithium manganese compound as a positive electrode active material.

Examples of the lithium manganese compound include Li ₂ MnO ₃ and Li _a M _b Mn _1-b N _c O ₄ (0<a≦2, 0≦b≦0.5, 1≦c≦2; M system belongs to 2~ Group 13 and elements of the 3rd to 4th cycles; N is a group of 14 to 16 and the 3rd cycle), Li _1+x M _y Mn _2-xy O ₄ (0≦x≦0.2, 0<y≦0.6 The M system is selected from the group consisting of lithium manganese compounds represented by at least one of the group of elements belonging to Groups 2 to 13 and having the 3rd to 4th cycles. Here, although M is at least one selected from the group consisting of Groups 2 to 13 and having the third to fourth periods, it is preferably Al, Mg, Zn, Ni, Co from the viewpoint of improving the stability. Further, Fe and Cr are more preferably Al, Mg, Zn, Ni or Cr, and particularly preferably Al, Mg, Zn and Ni. Further, from the viewpoint that the effect of improving the stability is large, N here is preferably Si, P and S.

Among them, from the point of high stability of the positive electrode active material, it is particularly preferable to use Li _1+x M _y Mn _2-xy O ₄ (0≦x≦0.2, 0<y≦0.6; M is selected from Al, Lithium manganese compound represented by at least one of Ni). If x < 0, there is a tendency that the capacity of the positive electrode active material decreases. Further, when x>0.2, there is a tendency to contain a large amount of impurities such as lithium carbonate. If y = 0, the stability of the positive electrode active material tends to decrease. Further, if y>0.6, there is a tendency to contain a large amount of impurities such as an oxide of M.

From the viewpoint of stability of the positive electrode active material, M is Al or Ni, and a material represented by Li _1.1 Al _0.1 Mn _1.8 O ₄ or LiNi _0.5 Mn _1.5 O ₄ is an optimum material.

The titanium manganese compound is preferably a spinel structure. The reason is that in the case of the spinel structure, the expansion and contraction of the active material caused by the reaction of the intercalation and deintercalation of titanium ions is small.

Preferably, the half-height width of the (400) plane of the powder X-ray diffraction of the CuKα of the lithium manganese compound is 0.5 or less. When it is more than 0.5°, the stability of the electrode may be lowered due to the low crystallinity of the positive electrode active material.

Preferably, the Litbert's analytical method using a X-ray diffraction of a lithium manganese compound accounts for 90% or more of the lithium content of the 8a crystal lattice. When it is less than 90%, the defects in the crystal of the positive electrode active material are excessive, and the stability of the electrode is lowered.

The particle diameter of the lithium manganese compound is preferably 0.5 μm or more and 50 μm or less, and particularly preferably 1 μm or more and 30 μm or less from the viewpoint of handling. The particle diameter here is a value obtained by measuring the size of each particle from the SEM and TEM images to calculate the average particle diameter.

The specific surface area of the lithium manganese compound is preferably from 0.1 m ² /g to 50 m ² /g or less from the viewpoint of easily obtaining the desired output density. The specific surface area can be calculated by the measurement of the BET method.

The bulk density of the lithium manganese compound is preferably 0.2 g/cm ³ or more and 2.0 g/cm ³ or less. When it is less than 0.2 g/cm ³ , a large amount of solvent is required for the preparation of the slurry to be described later, which is disadvantageous in cost, and if it is more than 2.0 g/cm ³ , it is difficult to mix with the conductive auxiliary agent and the binder described later. .

A lithium manganese compound, for example, Li _1+x M _y Mn _2-xy O ₄ (0≦x≦0.2, 0<y≦0.6; M system is at least one selected from the group consisting of Al and Ni) can be used at 500 ° C or higher and 1500 It is produced by heat-treating a lithium compound, a manganese compound, and a compound of M at ° C or lower. If it is less than 500 ° C or more than 1500 ° C, there is a case where a positive electrode active material having a desired structure cannot be obtained. The heat treatment may be carried out by mixing a lithium compound, a manganese compound, and a compound of M, and heat-treating the manganese compound and the compound of M, followed by heat treatment with the lithium compound. In order to improve the crystallinity of the positive electrode active material, it may be reheated at 500 ° C or higher and 1500 ° C or lower after the heat treatment. The temperature of the reheating treatment may be the same as or different from the temperature originally produced. The heat treatment environment may be in the air or in the presence of an inert gas such as nitrogen or argon. The heat treatment furnace is not particularly limited, and for example, a box furnace, a tubular furnace, a tunnel furnace, a rotary kiln, or the like can be used.

As the lithium compound, for example, lithium hydroxide, lithium carbonate, lithium nitrate, lithium acetate, lithium oxalate, lithium halide or the like can be used. These lithium compounds may be used alone or in combination of two or more.

As the manganese compound, for example, a manganese oxide such as manganese dioxide, manganese carbonate, manganese nitrate, manganese hydroxide or the like can be used. These manganese compounds may be used alone or in combination of two or more.

As the compound of M, for example, a carbonate, an oxide, a nitrate, a hydroxide, a sulfate or the like can be used. More preferably, the M system included in Li _1+x M _y Mn _2-xy O ₄ is at least one selected from the group consisting of Al and Ni.

The ratio of the lithium compound, the manganese compound and the compound of M is such that the atomic ratios of lithium, manganese and M are 1+x (lithium), 2-xy (manganese), and y(M), respectively, wherein 0≦x≦ The range of 0.2, 0 < y ≦ 0.6 is selected. For example, when a positive electrode active material having an atomic ratio of Mn/Li of 1.5 is produced, the above ratio can be adjusted to a certain extent at about 1.5 depending on the properties of the raw material or the heating conditions.

<3. Conductive auxiliaries, adhesives>

In order to improve conductivity or stability, the surface of the lithium titanate or lithium manganese compound used in the present invention may be covered with a carbon material, a metal oxide or a polymer.

A conductive auxiliary agent can also be used in the negative electrode and/or the positive electrode of the present invention (hereinafter, simply referred to as "electrode"). Although not particularly limited, it is preferably carbon The material is used as a conductive additive. For example, natural black lead, artificial black lead, vapor-grown carbon fiber, carbon nanotube, acetylene black, conductive carbon black, and furnace black are exemplified. These carbon materials may be used alone or in combination of two or more.

In the present invention, the amount of the conductive auxiliary agent contained in each electrode is preferably 1 part by weight or more and 30 parts by weight or less, more preferably 2 parts by weight or more and 15 parts by weight or less, based on 100 parts by weight of the active material. If it is in the above range, the conductivity of the electrode can be ensured. Further, the adhesion to the binder described later can be maintained, and sufficient adhesion to the current collector can be obtained.

The binder which can be used for the electrode of the present invention is not particularly limited, but may be selected from, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber, and polyfluorene. At least one of an imine and a group of such inducers. The binder is preferably dissolved or dispersed in a non-aqueous solvent or water in terms of ease of electrode fabrication. The nonaqueous solvent is not particularly limited, and examples thereof include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, and ethyl acetate. Ester, and tetrahydrofuran. A dispersant or a thickener may also be added to these.

In the present invention, the amount of the binder contained in each electrode is preferably 1 part by weight or more and 30 parts by weight or less, more preferably 2 parts by weight or more and 15 parts by weight or less, based on the weight of the active material 100. When it is in the above range, the adhesion between the active material and the conductive auxiliary agent can be maintained, and sufficient adhesion to the current collector can be obtained.

<4. Electrode>

The electrode of the present invention is produced by coating a current collector with a mixture of an active material, a conductive auxiliary agent and a binder, but the degree of difficulty in the production method is In a preferred embodiment, a slurry is prepared by using the mixture and the solvent, and the obtained slurry is applied onto a current collector to remove the solvent to prepare an electrode.

The current collector used in the electrode of the present invention can preferably be a stable metal material of 0.3 V (vs. Li ⁺ /Li) or more and 2.0 V (vs. Li ⁺ /Li) or less, such as copper, SUS, nickel. , titanium, aluminum and these alloys; especially from the point of high stability, aluminum is preferred. Since aluminum is relatively stable in the electrode reaction environment of the positive electrode and the negative electrode, it is preferably a high-purity aluminum represented by JIS standards 1030, 1050, 1085, 1N90, 1N99, etc., although it is not particularly limited.

As the current collector, a surface in which aluminum is coated on a metal material other than aluminum (copper, SUS, nickel, titanium, and the like) may be used.

Preferably, the current collector has a surface roughness Ra of 0.05 μm or more and 0.5 μm or less. If it is less than 0.05 μm, the adhesion to the electrode may be lowered. If it is more than 0.5 μm, it may be difficult to form the electrode uniformly. Further, the surface roughness Ra can be measured by a light wave interference type surface roughness measuring device.

The current collector has a resistance of preferably 5 μΩ·cm or less. If it is higher than 5 μΩ ‧ cm, there is a problem that the battery performance is lowered. The resistance can be measured by the four-terminal method.

The thickness of the current collector is not particularly limited, but is preferably 10 μm or more and 100 μm or less. When it is less than 10 μm, it is difficult to handle from the viewpoint of production, and when it is larger than 100 μm, it is disadvantageous from the economic point of view.

Although the method for producing the slurry is not particularly limited, it is preferable to use a ball mill, a planetary mixer, a jet mill, or a film rotary mixer since the active material, the conductive auxiliary agent, the binder, and the solvent are uniformly mixed. Slurry It can be produced by mixing an active material, a conductive auxiliary agent, and a binder, and then adding a solvent, and can also be produced by mixing an active material, a conductive auxiliary agent, a binder, and a solvent together.

The solid content concentration of the slurry is preferably 30% by weight or more and 80% by weight or less. If it is less than 30% by weight, the viscosity of the slurry tends to be too low. On the other hand, if it is more than 80% by weight, the viscosity of the slurry tends to be too high, so that it is difficult to form an electrode to be described later.

The solvent used in the slurry is preferably a non-aqueous solvent or water. The nonaqueous solvent is not particularly limited, and examples thereof include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, and acetic acid. Ethyl ester, tetrahydrofuran, and the like. Further, a dispersing agent or a thickener may be added to the materials.

The method of forming the electrode on the current collector is not particularly limited, but a preferred method is to remove the solvent by applying the slurry to the current collector by a doctor blade, a die coater or a knife coater, or by spraying. Remove the solvent. The method of removing the solvent is preferably simple to dry by using an oven or a vacuum oven. Examples of the environment in which the solvent is removed include air at room temperature or high temperature, an inert gas, a vacuum state, and the like. The temperature at which the solvent is removed is not particularly limited, but is preferably 60° C. or higher and 250° C. or lower. When the temperature is less than 60 ° C, it takes a long time to remove the solvent, and if it is more than 250 ° C, the binder may be deteriorated. After the electrode is fabricated, the electrode may be compressed by a roll press or the like.

In the present invention, the thickness of the electrode is preferably 10 μm or more and 200 μm or less. When it is less than 10 μm, it is difficult to obtain a desired capacity, and when it is more than 200 μm, it is difficult to obtain a desired output density.

In the present invention, the density of the electrode is preferably 1.0 g/cm ³ or more and 4.0 g/cm ³ or less. When it is less than 1.0 g/cm ³ , the contact with the active material or the conductive auxiliary agent is insufficient, and the electron conductivity is lowered. When it is more than 4.0 g/cm ³ , it is difficult for the electrolyte solution to be described later to permeate into the electrode to lower the lithium conductivity. The electrodes can also be compressed in the desired thickness and density. The compression is not particularly limited, but can be carried out, for example, by drum compression, hydraulic compression, or the like.

In the present invention, the capacitance per 1 cm ² of the negative electrode is preferably 0.5 mAh or more and 3.6 mAh or less. If it is less than 0.5 mAh, the size of the battery having the required capacitance becomes large; on the other hand, if it is larger than 3.6 mAh, it is difficult to obtain the desired output density.

In the present invention, the capacitance per 1 cm ² of the positive electrode is preferably 0.5 mAh or more and 3.0 mAh or less. If it is less than 0.5 mAh, the size of the battery having the required capacitance becomes large; on the other hand, if it is more than 3.0 mAh, it is difficult to obtain a desired output density.

The calculation of the capacitance per 1 cm ² of each of the negative electrode and the positive electrode can be calculated by measuring the charge/discharge characteristics of the half-cell using lithium metal as the counter electrode after the respective electrodes are produced, as will be described later.

Although the electrode capacitance per 1 cm ² is not particularly limited, a method of controlling the weight of the electrode formed per unit area of the current collector can be used, for example, the coating thickness at the time of applying the electrode can be controlled.

<5. Nonaqueous electrolyte>

The nonaqueous electrolyte used in the nonaqueous electrolyte secondary battery of the present invention is not particularly limited, but an electrolyte solution in which a solute is dissolved in a nonaqueous solvent can be used. The electrolyte in which the solute is dissolved in the non-aqueous solvent is impregnated into the gel electrolyte of the polymer.

The nonaqueous solvent preferably contains a cyclic aprotic solvent and/or a chain aprotic solvent. Examples of the cyclic aprotic solvent include a cyclic carbonate, a cyclic ester, a cyclic oxime, and a cyclic ether. Examples of the chain-shaped aprotic solvent include a chain carbonate, a chain carboxylate, and a chain ether. Further, in addition to the above, a solvent such as acetonitrile or the like which is generally used as a solvent for a nonaqueous electrolyte may be used. More specifically, dimethyl carbonate, diethyl methyl carbonate, dipropyl carbonate, methyl propyl ester, methyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, Γ-butyrolactone, 1,2-dimethoxyethane, cyclobutyl hydrazine, dioxolane, 3-pentolone acid methyl group and the like. These solvents may be used singly or in combination of two or more kinds. However, it is preferred to use two or more kinds of solvents in combination with the ease of dissolving the solute and the lithium ion conductivity. For example, dimethyl carbonate and ethylene carbonate are exemplified as preferred combinations. Further, a gel electrolyte in which an electrolyte is mixed with a polymer can also be used.

The solute is not particularly limited, but is preferably, for example, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , LiBOB (Lithium Bis (OxaIato) Borate), LiN (equivalent to the ease of dissolution in the solvent). SO ₂ CF ₃ ) ₂ and the like. Preferably, the concentration of the solute contained in the electrolytic solution is 0.5 mol/L or more and 2.0 mol/L or less. If it is less than 0.5 mol/L, it may be difficult to exhibit the desired lithium ion conductivity; on the other hand, if it is more than 2.0 mol/L, the solute will not continue to dissolve. The non-aqueous electrolyte may also contain a trace amount of an additive such as a flame retardant or a stabilizer.

<6. Isolation film>

The separator is provided between the positive electrode and the negative electrode and has lithium ion conductivity without electron conductivity. In the nonaqueous electrolyte secondary battery of the present invention, "cellulose non-woven fabric containing polyethylene terephthalate fibers" is used.

In the present invention, from the viewpoint of improving cycle characteristics, it is necessary to use a separator having a porosity of less than 50% by volume. Here, "porosity" is defined as "porosity = (1 - total volume of fibers constituting the separator / volume of separator)".

In the past, when a separator having a porosity of less than 50% by volume and a separator having a porosity of 50% by volume or more were used, when the internal resistance between the positive and negative electrodes in the battery increases and the load is aggravated, Expect good current load characteristics. However, in the present invention, even if the porosity is less than 50% by volume, by using a cellulose non-woven fabric containing a specific ratio of polyethylene terephthalate fibers, the cycle characteristics can be greatly improved without sacrificing current load characteristics.

The type of cellulose which is an essential material of the present invention is an organic fiber such as a natural cellulose fiber, a regenerated cellulose fiber or a solvent-spun cellulose fiber from the viewpoint of affinity with an electrolytic solution. Cellulose fibers are advantageous in comparison with microporous separators defined by PP or PE in that they are more selective than electrolytes, and are superior to the prior microporous separators even from the viewpoint of heat resistance.

Further, since the cellulose fiber contains a polyethylene terephthalate fiber which can ensure a certain degree of strength and has heat resistance, the insulation film which is only used for the cellulose fiber can be improved in strength, thereby achieving isolation. membrane The increase in strength.

Although the polyethylene terephthalate fiber can be added in any amount, in order to ensure a certain degree of strength, the weight of the polyethylene terephthalate fiber relative to the entire separator should be 20% by weight or more; The liquid retention property of the liquid is such that the ratio of the polyethylene terephthalate fibers is 80% by weight or less. From the viewpoint of the strength and the liquid retention property of the electrolyte, it is more preferably 30% by weight or more and 70% by weight or less, and 50% by weight is a formulation which achieves an optimum balance, and exhibits good battery characteristics.

Although the method of adding the polyethylene terephthalate fiber is not limited, for example, polyethylene terephthalate fiber and cellulose fiber may be mixed and produced by papermaking such as wet papermaking or dry papermaking. Wet papermaking can be achieved in a conventional manner. For example, papermaking can be carried out using a wet paper machine having a handsheet, a perforated plate, or the like. Dry papermaking can also be carried out by a conventional method such as an air-laid method, a carding method, or the like.

Further, the separator may also contain various plasticizers, antioxidants, flame retardants, and may be covered with a metal oxide or the like.

Preferably, the thickness of the separator is 10 μm or more and 100 μm or less. If it is less than 10 μm, the positive electrode and the negative electrode may be in contact with each other. If it is larger than 100 μm, the internal resistance of the battery may become high. The thickness of the separator is particularly preferably 15 μm or more from the viewpoint of strength, and is preferably 35 μm or less from the viewpoint of battery characteristics. The optimum is 20 μm or more and 30 μm or less.

According to the present invention, by increasing the strength of the separator, it is thinner than the cellulose non-woven fabric of the same thickness, so that the current characteristics can be improved. And can Reducing the pore volume reduces the amount of electrolyte used, thereby increasing the energy density of the battery.

<7. Nonaqueous electrolyte secondary battery>

The positive electrode and the negative electrode of the nonaqueous electrolyte secondary battery of the present invention may be in the form of the same electrode formed on both surfaces of the current collector, or may be in the form of forming a positive electrode on one surface of the current collector and forming a negative electrode on the other surface; It can also be a bipolar electrode. For example, in the case of a bipolar electrode, a separator is disposed between the positive electrode side and the negative electrode side of the adjacent bipolar electrode, and the positive electrode side and the negative electrode side face each other to prevent liquid from being connected to the positive electrode and the negative electrode. The peripheral portion is provided with an insulating material.

The nonaqueous electrolyte secondary battery of the present invention may be one in which a separator is disposed between the positive electrode side and the negative electrode side. The positive electrode, the negative electrode, and the separator include a nonaqueous electrolyte that is responsible for lithium ion conduction.

Preferably, the ratio of the capacitance of the positive electrode of the nonaqueous electrolyte secondary battery of the present invention to the capacitance of the negative electrode satisfies the following formula (a).

1 ≦ B / A ≦ 1.2 ( a) wherein the above equation (1), A represents the capacitance per 1cm ² of the positive electrode, B represents a capacitance per 1cm ² of the negative electrode.

If B/A is less than 1, there is a case where the potential of the negative electrode becomes a lithium deposition potential when the battery is overcharged. On the other hand, if B/A is more than 1.2, the negative electrode active material which is not related to the battery reaction is excessively induced. The situation of side reactions.

The area ratio of the positive electrode to the negative electrode of the nonaqueous electrolyte secondary battery of the present invention is not particularly limited, but preferably satisfies the following formula (b).

1≦D/C≦1.2 (b) (where C represents the positive electrode area and D represents the negative electrode area).

When D/C is less than 1, for example, in the case of B/A=1 described above, since the capacitance of the negative electrode is smaller than that of the positive electrode, the potential of the negative electrode becomes the deposition potential of lithium when the charge is excessively charged. On the other hand, if D/C is larger than 1.2, since the negative electrode of the portion which is not in contact with the positive electrode is large, there is a case where the negative electrode active material which is not involved in the reaction of the battery induces a side reaction. Although the area control of the positive electrode and the negative electrode is not particularly limited, it can be carried out, for example, by controlling the coating width when slurry coating is performed.

The area ratio of the separator and the negative electrode used in the nonaqueous electrolyte secondary battery of the present invention is not particularly limited, but preferably satisfies the following formula (c).

1≦F/E≦1.5 (c) (where E represents the area of the negative electrode and F represents the area of the separator).

If F/E is less than 1, the positive electrode and the negative electrode will be in contact, and if it is greater than 1.5, the volume required for the package is increased, so that the output density of the battery is lowered.

The amount of the nonaqueous electrolyte used in the nonaqueous electrolyte secondary battery of the present invention is not particularly limited, but is preferably 0.1 mL or more per 1 Ah. If it is less than 0.1 mL, the conduction of lithium ions accompanying the electrode reaction may not proceed, and the battery performance required may not be exhibited.

The nonaqueous electrolyte may be contained in advance in the positive electrode, the negative electrode, and the separator, or may be added after being wound or laminated between the positive electrode side and the negative electrode side.

The nonaqueous electrolyte secondary battery of the present invention may be packaged by a composite film after winding or laminating a plurality of the above laminated bodies, or may be an angular, elliptical, cylindrical, coin-shaped, button-shaped or flaky metal can. Package. It is also possible to have a mechanism for discharging the gas generated at the time of packaging. Product of laminated body The number of layers can be layered enough to perform the required voltage values and capacitance.

The nonaqueous electrolyte secondary battery of the present invention can be a battery pack by connecting a plurality of them. The assembled battery of the present invention can be produced by appropriately connecting in series and in parallel with a desired size, capacitance, and voltage. Furthermore, in order to confirm the state of charge of each battery and improve safety, it is preferable to add a control circuit to the above battery.

Example

In the following, the present invention will be specifically described by way of examples, and the present invention is not limited thereto, and may be appropriately modified without departing from the spirit and scope of the invention.

<Example 1>

(Manufacture of positive electrode)

Li _1.1 Al _0.1 Mn _1.8 O ₄ of the positive active material is described in the literature ("Lithium Aluminum Manganese Oxide Having Spinel-Framework Structure for Long-Life Lithium-Ion Batteries" Electrochemical and Solid-State Letters Volume 9, Issue 12, Pages A557 (2006) The method disclosed is produced.

That is, an aqueous dispersion of manganese dioxide, lithium carbonate, aluminum hydroxide, and boric acid was prepared, and a mixed powder was produced by a spray drying method. At this time, each amount of manganese dioxide, lithium carbonate, and aluminum hydroxide was adjusted so that the molar ratio of lithium, aluminum, and manganese was 1.1:0.1:1.8. Next, the mixed powder was heated at 900 ° C for 12 hours in an air atmosphere, and then heated at 650 ° C for 24 hours. Finally, the powder was washed with water at 95 ° C, and then dried to prepare a powdery positive electrode active material.

A slurry was prepared by mixing 100 parts by weight of a positive electrode active material, 6.8 parts by weight of a conductive auxiliary agent (acetylene black), and 6.8 parts by weight of PVdF (solid content concentration: 12 wt%, NMP solution) as a binder. . The slurry was applied on an aluminum foil (20 μm), and dried under vacuum at 150 ° C to prepare a positive electrode (50 cm ² ).

The positive electrode capacitance was measured by a subsequent charge and discharge test apparatus.

The positive electrode coated on one surface of the aluminum foil in the same manner as described above was punched into a disk shape of 16 mmφ as an operating electrode, and Li metal was punched into a disk shape of 16 mmφ as a counter electrode. Using these electrodes, the test electrode (HS battery, manufactured by Baoquan Co., Ltd.) was laminated in the order of the operating electrode (single-side coating)/separator (Celgard #2500)/Li metal, and 0.15 mL of non-water was added. An electrolyte (ethylene carbonate / dimethyl carbonate = 3/7 vol%, LiPF ₆ 1 mol / L) was prepared into a half-cell. The half-cell was allowed to stand at 25 ° C for one day, and then connected to a charge and discharge tester (HJ1005SD8, manufactured by Hokuto Denko Corporation). The half-cell was subjected to constant current charging (termination voltage: 4.5 V) and constant current discharge (termination voltage: 3.5 V) five times at 25 ° C and 0.4 mA, and the result of the fifth time was taken as a positive electrode capacitance. As a result, the positive electrode capacitance was 1.0 mAh/cm ² .

(Manufacture of negative electrode)

The method disclosed in the literature ("Zero-Strain Insertion Material of Li[Li1/3Ti5/3]04 for Rechargeable Lithium Cellq" J. Electrochem. Soc., Volume 142, Issue 5, pp. 1431-1435 (1995)) Li ₄ Ti ₅ O _{12 of the} negative electrode active material was produced.

That is, first, manganese dioxide and lithium hydroxide are mixed at a molar ratio of titanium to lithium of 5:4; then, the mixture is heated at 800 ° C under a nitrogen atmosphere. After 12 hours, it was pulverized to prepare a powdery negative electrode active material.

100 parts by weight of the prepared negative electrode active material, 6.8 parts by weight of the conductive auxiliary agent (acetylene black), and 6.8 parts by weight of the solid component of PVdF (solid content concentration: 12 wt%, NMP solution) as a binder Make a slurry. The slurry was applied on an aluminum foil (20 μm), and dried under vacuum at 150 ° C to prepare a negative electrode (50 cm ² ).

The negative electrode capacitance was measured by a subsequent charge and discharge test apparatus.

The negative electrode was applied to one surface of the aluminum foil under the same conditions as above, and punched into a disk shape of 16 mmφ to prepare an operating electrode. Li metal was die-cut into a 16 mmφ disk shape as a counter electrode. Using these electrodes, the test electrode (HS battery, manufactured by Baoquan Co., Ltd.) was laminated in the order of the operating electrode (single-side coating)/separator (Celgard #2500)/Li metal, and 0.15 mL of non-aqueous solution was added. An electrolyte (ethylene carbonate / dimethyl carbonate = 3/7 vol%, LiPF ₆ 1 mol / L) was prepared into a half-cell. After the half-cell was allowed to stand at 25 ° C for one day, it was connected to a charge and discharge tester (HJ1005SD8, manufactured by Hokuto Denko Co., Ltd.). The half-cell was subjected to constant current discharge (termination voltage: 1.0 V) and constant current charge (termination voltage: 2.0 V) five times at 25 ° C and 0.4 mA, and the result of the fifth time was taken as a negative electrode capacitance. As a result, the negative electrode capacitance was 1.2 mAh/cm ² .

(isolation film)

A cellulose non-woven fabric containing polyethylene terephthalate fibers (containing the same amount of polyethylene terephthalate fibers and cellulose fibers having a thickness of 25 μm and a porosity of 47% by volume, used in the separator) 55 cm ² separator).

(Manufacture of nonaqueous electrolyte secondary battery)

A nonaqueous electrolyte secondary battery was fabricated in the following manner.

First, a positive electrode (one side coating) / separator / negative electrode The positive electrode (one side coating; 50 cm ²⁾ (surface coated) of the layer of the finished product of the order of the negative electrode (one side coating; 50 cm ² ), and the separator.

Next, the aluminum sheets 1a and 3a were vibrated and fused to the positive electrode 1 and the negative electrode 3 at both ends, and then placed in a bag-shaped aluminum laminate 4. The nonaqueous electrolyte 5 was prepared by dissolving a mixed nonaqueous solvent of ethylene carbonate/dimethyl carbonate = 3:7 (vol ratio) at a concentration of 1 mol/L and LiPF ₆ . After adding 2 mL of the obtained nonaqueous electrolyte 5, it was sealed under reduced pressure to prepare a nonaqueous electrolyte secondary battery. Fig. 1 shows a completed nonaqueous electrolyte secondary battery (cross-sectional view).

<Example 2>

(Tsutomu Ohzuku, Sachio Takeda, Masato Iwanaga "Solid-state redox potentials for Li[Me1/2Mn3/2]04(me:3d-transition metal)having spinel-framework structures:a series of 5 volt materials for advanced lithium -ion batteries" Journal of Powersources, Vol. 81-82, pp. 90-94 (1999)), LiNi _0.5 Mn _1.5 O _{4 was} produced as a positive electrode active material. That is, first, lithium hydroxide, manganese hydroxide oxide, and nickel hydroxide were mixed at a molar ratio of lithium, manganese, and nickel of 1:1.5:0.5. Next, the mixture was heated at 550 ° C in an air atmosphere, and then heated at 750 ° C to prepare LiNi _0.5 Mn _1.5 O ₄ .

A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the produced LiNi _0.5 Mn _1.5 O _{4 was} used as the positive electrode active material.

<Comparative Example 1>

In the literature (ARArmstrong, et al., "The layered intercalation compounds Li (Mn _1-y Co _y ) O ₂ : Positive electrode materials for lithium-ion batteries." J. Electrochem. Soc., 1994. Vol. 141 (11 ): pp. 2972-2977), LiCoO _{2 was} produced as a positive electrode active material. That is, lithium carbonate and cobalt oxide were mixed at a molar ratio of lithium to cobalt of 1:1. Next, the mixture was heated at 650 ° C in an air atmosphere, and then heated at 850 ° C to produce LiCoO ₂ . A nonaqueous electrolyte secondary battery was produced in the same manner as in the Example except that the produced LiCoO _{2 was} used as the positive electrode active material.

<Comparative Example 2>

A nonaqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 1, except that polyethylene terephthalate fibers (25 μm, porosity: 48% by volume, and 55 cm ² ) were used for the separator.

<Comparative Example 3>

A nonaqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 1, except that a cellulose nonwoven fabric (25 μm, porosity: 70% by volume, and 55 cm ² ) was used for the separator.

<Comparative Example 4>

In the literature (T. Ohzuku, A. Ueda, and M. Nagayama, "Electrochemistry and structural chemistry of LiNiO ₂ (R3m) for 4 volt secondary lithium cells," Journal of Electrochemical Society, vol. 140, no. 7, pp. In the method disclosed in 1862-1870, (1993)), LiNiO _{2 was} produced as a positive electrode active material. That is, lithium hydroxide and nickel carbonate are mixed at a molar ratio of lithium to cobalt of 1:1. Next, the mixture was heated at 600 ° C in an oxygen atmosphere, and then heated at 750 ° C to prepare LiNiO ₂ . A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the produced LiNiO _{2 was} used for the positive electrode active material.

<Comparative Example 5>

A nonaqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 4, except that polyethylene terephthalate fibers (25 μm, porosity: 48% by volume, and 55 cm ² ) were used for the separator.

<Comparative Example 6>

A nonaqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 4 except that a cellulose nonwoven fabric (25 μm, porosity: 70% by volume, and 55 cm ² ) was used for the separator.

<Comparative Example 7>

A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that a microporous film made of polypropylene (Celgard #2500) was used as the separator.

<Comparative Example 8>

A nonaqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 7, except that polyethylene terephthalate fibers (25 μm, porosity: 48% by volume, and 55 cm ² ) were used for the separator.

<Comparative Example 9>

A nonaqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 7, except that a cellulose nonwoven fabric (25 μm, porosity: 70% by volume, and 55 cm ² ) was used for the separator.

<Comparative Example 10>

A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 2 except that a microporous film made of polypropylene (Celgard #2500) was used as the separator.

<Comparative Example 11>

A nonaqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 10 except that polyethylene terephthalate fibers (25 μm, porosity: 48% by volume, and 55 cm ² ) were used for the separator.

<Comparative Example 12>

A nonaqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 10 except that a cellulose nonwoven fabric (25 μm, porosity: 70% by volume, and 55 cm ² ) was used for the separator.

(Measurement of cycle characteristics)

The non-aqueous electrolyte secondary battery prepared in the examples and the comparative examples was connected to a charge and discharge device (HJ1005SD8, manufactured by Hokuto Electric Co., Ltd.), and 100 times of 50 mA constant current charging and 50 mA constant current discharging were repeated at 55 °C. . It is assumed that the charge termination voltage and the discharge termination voltage are 3 V and 2 V, respectively. Table 1 shows the 100th discharge capacitance when the first discharge capacitance is 100.

As described in Table 1, the nonaqueous electrolyte secondary batteries of Examples 1 and 2 of the present invention can improve the cycle stability as compared with the nonaqueous electrolyte secondary batteries of Comparative Examples 1 to 12.

When Li _1.1 Al _0.1 Mn _1.8 O _{4 is} used as the positive electrode active material, a cell using a cellulose non-woven fabric containing polyethylene terephthalate fibers (Example 1) as a separator is used as compared with a polypropylene microporous film ( The battery of Comparative Example 7), polyethylene terephthalate fiber (Comparative Example 8) and cellulose (Comparative Example 9) was more excellent in cycle stability.

Further, when LiNi _0.5 Mn _1.5 O _{4 is} used as the positive electrode active material, a battery comprising a cellulose non-woven fabric of polyethylene terephthalate fibers (Example 2) as a separator is used as compared with a polypropylene microporous film. (Comparative Example 10), a polyethylene terephthalate fiber (Comparative Example 11), and a cellulose (Comparative Example 12) battery were more excellent in cycle stability.

However, when LiCoO _{2 is} used as the positive electrode active material, a battery of a cellulose non-woven fabric containing polyethylene terephthalate fibers (Example 1) is used as a separator, and a polyethylene terephthalate fiber is used. In Example 2), the cycle stability of the cellulose (Comparative Example 3) was not significantly superior.

Even when LiNiO _{2 was used} as the positive electrode active material, a battery containing a cellulose non-woven fabric of polyethylene terephthalate fibers (Comparative Example 4) and a polyethylene terephthalate fiber were used as a separator (Comparative Example) 5) Compared with the battery of the cellulose (Comparative Example 6), the cycle stability was not greatly superior.

From these conditions, it is known that Li _1.1 Al _0.1 Mn _1.8 O ₄ or LiNi _0.5 Mn _1.5 O _{4 is} used as the positive electrode active material, and a combination of cellulose non-woven fabrics containing polyethylene terephthalate fibers can be used to greatly increase the battery. Cycle stability.

It is presumed that such a specific positive electrode active material has a synergistic effect with a separator of a specific structure or a specific substance. The inventors conceived that the mechanism for causing deterioration of the battery cycle is that a radical generated at the interface between the electrolyte and the positive electrode penetrates the separator to induce a precipitation reaction on the surface of the negative electrode. On the other hand, it is presumed that the combination of the specific positive electrode active material and the specific structure and the separator of the specific substance causes generation of a suppressing radical, promotes the radical generated by the trapping of the separator, and suppresses the precipitation reaction on the surface of the negative electrode. .

1‧‧‧ positive

1a‧‧‧Aluminum sheet

2‧‧‧Separator

3‧‧‧negative

3a‧‧‧Aluminum sheet

4‧‧‧Package film

5‧‧‧Non-aqueous electrolyte

Fig. 1 is a cross-sectional view showing a state in which a nonaqueous electrolyte secondary battery according to an embodiment of the present invention is sealed in a bag-shaped film.

1‧‧‧ positive

1a‧‧‧Aluminum sheet

2‧‧‧Separator

3‧‧‧negative

3a‧‧‧Aluminum sheet

4‧‧‧Package film

5‧‧‧Non-aqueous electrolyte

Claims (10)

A nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte lithium ion battery, wherein the positive electrode comprises a lithium manganese compound; the negative electrode comprises lithium titanate; and the separator comprises polyparaphenylene A cellulose non-woven fabric of ethylene formate fibers having a porosity of less than 50% by volume. The nonaqueous electrolyte secondary battery according to claim 1, wherein the lithium manganese compound is Li _1+x M _y Mn _2-xy O ₄ (0≦x≦0.2, 0<y≦0.6, M is selected from Al, A compound represented by at least one of Ni). The nonaqueous electrolyte secondary battery according to claim 1, wherein the lithium manganese compound is represented by Li _1.1 Al _0.1 Mn _1.8 O ₄ . The nonaqueous electrolyte secondary battery according to claim 1, wherein the lithium manganese compound is represented by LiNi _0.5 Mn _1.5 O ₄ . The nonaqueous electrolyte secondary battery of claim 1, wherein the lithium titanate has a spinel structure. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the polyethylene terephthalate fiber of the cellulose non-woven fabric containing the polyethylene terephthalate fiber is 20% by weight or more and 80% by weight. the following. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the polyethylene terephthalate fiber of the cellulose non-woven fabric containing the polyethylene terephthalate fiber is 30% by weight or more and 70% by weight. the following. The nonaqueous electrolyte secondary battery according to claim 1, wherein the cellulose non-woven fabric containing the polyethylene terephthalate fiber has a thickness of 15 μm. Up and 35 μm or less. The nonaqueous electrolyte secondary battery according to claim 1, wherein the cellulose non-woven fabric containing the polyethylene terephthalate fiber has a thickness of 20 μm or more and 30 μm or less. A battery pack in which a plurality of nonaqueous electrolyte secondary batteries as claimed in claim 1 are connected.