WO2013084840A1 - Nonaqueous electrolyte secondary battery and assembled battery using same - Google Patents

Abstract

A lithium-ion battery having a positive electrode, a negative electrode, a separator, and an organic electrolyte; wherein the positive electrode contains a lithium-manganese compound having a spinel structure, the negative electrode contains a complex oxide containing lithium titanate having a spinel structure, and the separator is a "polyethylene-terephthalate-fiber-containing cellulose nonwoven cloth" having a porosity of less than 50% by volume. Using a combination of such description makes it possible to obtain a nonaqueous electrolyte secondary battery having superior cycle stability.

Description

Nonaqueous electrolyte secondary battery and assembled battery using the same

The present invention relates to a nonaqueous electrolyte secondary battery and an assembled battery using the same.
This application claims priority based on Japanese Patent Application No. 2011-268309.

In recent years, non-aqueous electrolyte secondary batteries have been actively researched and developed for portable devices, hybrid vehicles, electric vehicles, and household power storage applications. In non-aqueous electrolyte secondary batteries used in these fields, charge and discharge cycles are repeated over a long period of time during actual use.
In a conventional nonaqueous electrolyte secondary battery, there is a problem in that when the charge / discharge cycle is repeated, the negative electrode made of carbon-based material deteriorates due to the volume change of the negative electrode material accompanying the insertion / extraction of lithium, and the capacity of the battery decreases. . In order to solve this problem, a non-patent document 1, such as a non-patent document 1 using a “lithium titanate having a spinel structure”, which is a robust material that hardly changes in volume due to insertion / extraction of lithium, is used. A water electrolyte secondary battery has been proposed.

Currently, further improvement in cycle stability is desired depending on the combination of materials and structures used.
Patent Documents 1 to 7 exemplify various materials such as a lithium cobalt compound, a lithium nickel compound, and a lithium manganese compound for use as a positive electrode in a non-aqueous electrolyte secondary battery in which the negative electrode includes lithium titanate. Various materials such as cellulose, polyethylene terephthalate, and polypropylene are exemplified for use as an electrically separating separator.

International Publication No. 2003/081698 Pamphlet JP 2009-158396 A JP 2009-205864 JP JP 2010-009898 A JP 2010-153258 JP Japanese Patent Laid-Open No. 06-060867 JP 2009-038017

However, in Patent Documents 1 to 7, what kind of specific compound is used as the positive electrode, and what kind of specific material and structure is used as the separator can maximize cycle stability. There is no document verified.
The subject of this invention is providing the non-aqueous electrolyte secondary battery which expresses the outstanding cycle stability, and an assembled battery using the same.

In view of the above circumstances, the present inventor achieved the most cycle stability by using a lithium manganese compound for the positive electrode, lithium titanate for the negative electrode, and a polyethylene terephthalate fiber-containing cellulose nonwoven fabric having a porosity of less than 50% by volume for the separator. The present inventors have found that an excellent nonaqueous electrolyte secondary battery can be obtained and have completed the present invention.
That is, the present invention is a lithium ion battery having a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode includes a lithium manganese compound, the negative electrode includes lithium titanate, and the separator has a porosity of 50% by volume. The present invention provides a nonaqueous electrolyte secondary battery which is a cellulose nonwoven fabric containing less than polyethylene terephthalate fibers.

In the non-aqueous electrolyte secondary battery of the present invention, the lithium manganese compound _{Li 1 + x M y Mn 2} - x - y O 4 (0 ≦ x ≦ 0.2, 0 <y ≦ 0.6, M is It is preferably a compound represented by the compound represented by the formula (1) selected from Al and Ni, and the lithium manganese compound is Li ₁ . A compound represented by 1Al _0.1 Mn _1.8 O ₄ or LiNi _0.5 Mn _1.5 O ₄ is particularly preferable.
In the nonaqueous electrolyte secondary battery of the present invention, the lithium titanate preferably has a spinel structure.
In the non-aqueous electrolyte secondary battery of the present invention, the polyethylene terephthalate fiber-containing cellulose nonwoven fabric preferably has a polyethylene terephthalate fiber content of 20 wt% to 80 wt%, preferably 30 wt% to 70 wt%. More preferably, it is as follows. The thickness of the polyethylene terephthalate fiber-containing cellulose nonwoven fabric is preferably 15 μm or more and 35 μm or less, and more preferably 20 μm or more and 30 μm or less.

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

It is sectional drawing which shows the state which enclosed the nonaqueous electrolyte secondary battery which concerns on the Example of this invention in the bag-shaped sheet | seat.

The above-described or further advantages, features, and effects of the present invention will be made clear by the following description of embodiments with reference to the accompanying drawings. The scope of the present invention is defined by the scope of the claims, and is intended to include any modifications within the scope and meaning equivalent to the scope of the claims.
<1. Negative electrode>
The nonaqueous electrolyte secondary battery of the present invention uses a negative electrode containing “lithium titanate having a spinel structure” as a negative electrode active material.

Examples of the lithium titanate include those represented by the molecular formula Li ₄ Ti ₅ O ₁₂ is preferred. The spinel structure is particularly preferable because the expansion and contraction of the active material in the lithium ion insertion / extraction reaction is small. Lithium titanate may contain a small amount of elements other than lithium such as Nb and titanium, for example.
Lithium titanate preferably has a half width of (400) plane of powder X-ray diffraction by CuKα of 0.5 ° or less. If it is larger than 0.5 °, the crystallinity of lithium titanate is low, and the stability of the electrode may be lowered.

The lithium titanate preferably has a lithium content of 90% or more in the 8a site according to the Rietveld analysis by X-ray diffraction. If it is less than 90%, since there are many defects in the crystal of lithium titanate, the stability of the electrode may be lowered.
Lithium titanate can be prepared by heat-treating a lithium compound and a titanium compound at 500 ° C. or higher and 1500 ° C. or lower. When the temperature is lower than 500 ° C. or higher than 1500 ° C., lithium titanate having a desired structure tends to be difficult to obtain. In order to improve the crystallinity of lithium titanate, after the heat treatment, the heat treatment may be performed again at a temperature of 500 ° C. or higher and 1500 ° C. or lower. The temperature of the reheating treatment may be the same as or different from the initial temperature. The heat treatment atmosphere may be in the presence of 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 and the like can be used. These lithium compounds may be used alone or in combination of two or more.
Although it does not specifically limit as a titanium compound, For example, titanium oxides, such as titanium dioxide and a titanium monoxide, can be used.

The compounding ratio of the lithium compound and the titanium compound is preferably an atomic ratio of lithium and titanium, around Ti / Li = 1.25, and may have some width depending on the properties of the raw materials and heating conditions.
The particle size of the prepared lithium titanate is preferably 0.5 μm or more and 50 μm or less, and more preferably 1 μm or more and 30 μm or less from the viewpoint of handling. The particle diameter is a value obtained by measuring the size of each particle from SEM and TEM images and calculating the average particle diameter.

The specific surface area of lithium titanate is preferably 0.1 m ² / g or more and 50 m ² / g or less because a desired output density is easily obtained. The specific surface area may be calculated by measurement using a mercury porosimeter or 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. 0.2 g / cm in the case of less than ³ tend to be economically disadvantageous because it requires a large amount of solvent in the step of preparing the slurry described below, 1.5 g / cm ³ greater than the later of conductive agent, and a binder Tend to be difficult to mix.

<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.
The lithium manganese compound, for _{example, Li 2 MnO 3, Li a} M b Mn 1-b N c O 4 (0 <a ≦ 2,0 ≦ b ≦ 0.5,1 ≦ c ≦ 2, M 2 to 13 and a group elements belonging to third to fourth period, N is the element belonging to a and the third period 14-16 _{group), Li 1 + x M y} Mn 2-xy O 4 (0 ≦ x ≦ 0.2, 0 <y ≦ 0.6, and M is at least one selected from the group consisting of elements belonging to groups 2 to 13 and belonging to the third to fourth periods. Here, M is at least one selected from elements belonging to the groups 2 to 13 and belonging to the 3rd to 4th periods, but Al, Mg, Zn, Ni, Co, Fe and Cr are preferred, Al, Mg, Zn, Ni and Cr are more preferred, and Al, Mg, Zn and Ni are even more preferred. Further, N here is preferably Si, P, or S because the effect of improving the stability is large.

Among these, because of high stability of the cathode active _{material, Li 1 + x M y Mn} 2 - x - y O 4 (0 ≦ x ≦ 0.2, 0 <y ≦ 0.6, M is Al, and Ni It is particularly preferable that the lithium manganese compound be at least one selected. When x <0, the capacity of the positive electrode active material tends to decrease. Further, when x> 0.2, there is a tendency that many impurities such as lithium carbonate are included. When y = 0, the stability of the positive electrode active material tends to be low. Further, when y> 0.6, a large amount of impurities such as M oxide tends to be contained.

A material in which M is Al or Ni and Li _1.1 Al _0.1 Mn _1.8 O ₄ or LiNi _0.5 Mn _1.5 O ₄ is the most preferable material from the viewpoint of the stability of the positive electrode active material.
The lithium manganese compound preferably has a spinel structure. This is because in the case of the spinel structure, the expansion and contraction of the active material in the reaction of insertion / extraction of lithium ions is small.
The lithium manganese compound preferably has a half width of 0.5 ° or less on the (400) plane of powder X-ray diffraction by CuKα. When it is larger than 0.5 °, the crystallinity of the positive electrode active material is low, and thus the stability of the electrode may be lowered.

The lithium manganese compound preferably has a lithium content of 90% or more in the 8a site according to the Rietveld analysis by X-ray diffraction. If it is less than 90%, there are many defects in the crystal of the positive electrode active material, and the stability of the electrode may be lowered.
The particle size of the lithium manganese compound is preferably 0.5 μm or more and 50 μm or less, and more 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 and calculating the average particle diameter.

The specific surface area of the lithium manganese compound is preferably 0.1 m ² / g or more and 50 m ² / g or less because a desired output density is easily obtained. The specific surface area can be calculated by measurement by 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. 0.2 g / cm economically be disadvantageous because it requires a large amount of solvent in the step of preparing the slurry below in the case of less than ^3, 2.0 g / cm ³ conductive agent described later is greater than, it is mixed with a binder It tends to be difficult.

Lithium manganese compounds _{such, Li 1 + x M y Mn} 2 - x - y O 4 (0 ≦ x ≦ 0.2, 0 <y ≦ 0.6, preferably at least one M is Al, are selected from Ni ) Can be produced by heat-treating a lithium compound, a manganese compound, and a compound of M at 500 ° C. or higher and 1500 ° C. or lower. When the temperature is lower than 500 ° C. or higher than 1500 ° C., a positive electrode active material having a desired structure may not be obtained. The heat treatment may be performed by mixing a lithium compound, a manganese compound, and a compound of M, or may be heat-treated with a lithium compound after the manganese compound and the M compound are heat-treated. In order to improve the crystallinity of the positive electrode active material, after the heat treatment, the heat treatment may be performed again at 500 ° C. or more and 1500 ° C. or less. The temperature of the reheating treatment may be the same as or different from the initial temperature. The heat treatment atmosphere may be in the presence of air or in the presence of an inert gas such as nitrogen or argon. Although it does not specifically limit in a heat treatment furnace, For example, a box furnace, a tubular furnace, a tunnel furnace, a rotary kiln etc. can be used.

As the lithium compound, for example, lithium hydroxide, lithium carbonate, lithium nitrate, lithium acetate, lithium oxalate, lithium halide and 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, manganese oxide such as manganese dioxide, manganese carbonate, manganese nitrate, manganese hydroxide and 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, carbonate, oxide, nitrate, hydroxide, sulfate and the like can be used. M included in _{Li 1 + x M y Mn 2} -xy O 4 is, Al, be selected from at least one selected from Ni particularly preferred.
The compounding ratio of the lithium compound, the manganese compound, and the compound of M is 1 + x (lithium), 2-xy (manganese), and y (M), respectively, where 0 ≦ x ≦ It is selected in a range satisfying 0.2, 0 <y ≦ 0.6. For example, when a positive electrode active material having an atomic ratio of 1.5 of Mn / Li is produced, a slight width may be provided around the blending ratio of 1.5 depending on the properties of the raw materials and heating conditions.

<3. Conductive aid, binder>
The surface of the lithium titanate or lithium manganese compound used in the present invention may be covered with a carbon material, metal oxide, polymer, or the like in order to improve conductivity or stability.
A conductive additive may be used for the negative electrode and / or the positive electrode (hereinafter sometimes simply referred to as “electrode”) of the present invention. Although it does not specifically limit as a conductive support material, A carbon material is preferable. Examples thereof include natural graphite, artificial graphite, vapor grown carbon fiber, carbon nanotube, acetylene black, ketjen black, and furnace black. These carbon materials may be used alone or in combination of two or more.

In the present invention, the amount of the conductive additive 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 with respect to 100 parts by weight of the active material. If it is the said range, the electroconductivity of an electrode will be ensured. Moreover, adhesiveness with the below-mentioned binder is maintained, and adhesiveness with a collector can fully be obtained.
The binder that can be used in the electrode of the present invention is not particularly limited. For example, at least selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber, polyimide, and derivatives thereof. One type can be used. The binder is preferably dissolved or dispersed in a non-aqueous solvent or water from the viewpoint of ease of production of the electrode. The non-aqueous solvent is not particularly limited, and examples thereof include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, ethyl acetate, and tetrahydrofuran. You may add a dispersing agent and a thickener 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 with respect to 100 parts by weight of the active material. If it is the said range, the adhesiveness of an active material and a conductive support material will be maintained, and adhesiveness with a collector can fully be acquired.
<4. Electrode>
The electrode of the present invention is produced by applying a mixture of an active material, a conductive additive and a binder onto a current collector. From the ease of the production method, a slurry is produced by using the mixture and a solvent. A method is preferred in which the electrode is prepared by removing the solvent after coating the resulting slurry on the current collector.

The current collector that can be used for the electrode of the present invention is a metal material that is stable at 0.3 V (vs. Li ⁺ / Li) or higher and 2.0 V (vs. Li ⁺ / Li) or lower, such as copper, SUS, Nickel, titanium, aluminum, and alloys thereof are preferable, and aluminum is particularly preferable because of high stability. Aluminum is not particularly limited because it is stable in the electrode reaction atmosphere of the positive electrode and the negative electrode, but is preferably high-purity aluminum represented by JIS standards 1030, 1050, 1085, 1N90, 1N99 and the like.

As the current collector, a metal material other than aluminum (copper, SUS, nickel, titanium, and alloys thereof) coated with aluminum can also be used.
The surface roughness Ra of the current collector is preferably 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 reduced, and if it is more than 0.5 μm, it may be difficult to form the electrode uniformly. The surface roughness Ra can be measured using a light wave interference type surface roughness measuring instrument.

The electrical resistance of the current collector is preferably 5 μΩ · cm or less. If it is higher than 5 μΩ · cm, the battery performance may be reduced. Electrical resistance can be measured by the four probe method.
The thickness of the current collector is not particularly limited, but is preferably 10 μm or more and 100 μm or less. If it is less than 10 μm, it may be difficult to handle from the viewpoint of production, and if it is thicker than 100 μm, it may be disadvantageous from an economic viewpoint.

The method for preparing the slurry is not particularly limited, but it is preferable to use a ball mill, a planetary mixer, a jet mill, or a thin-film swirl mixer because the active material, the conductive additive, the binder, and the solvent can be mixed uniformly. The slurry may be prepared by mixing the active material, the conductive additive, and the binder and then adding a solvent, or may be prepared by mixing the active material, the conductive additive, the binder, and the solvent together.

The solid content concentration of the slurry is preferably 30 wt% or more and 80 wt% or less. If it is less than 30 wt%, the viscosity of the slurry tends to be too low, whereas if it is higher than 80 wt%, the viscosity of the slurry tends to be too high, and it may be difficult to form an electrode described later.
The solvent used for the slurry is preferably a non-aqueous solvent or water. The non-aqueous solvent is not particularly limited, and examples thereof include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, ethyl acetate, and tetrahydrofuran. Moreover, you may add a dispersing agent and a thickener to these.

The method for forming the electrode on the current collector is not particularly limited. For example, the slurry is applied to the current collector with a doctor blade, die coater, comma coater, etc., and then the solvent is removed, or the solvent is applied after spraying. The method of removing is preferable. The method for removing the solvent is preferable because it is easy to dry using an oven or a vacuum oven. Examples of the atmosphere for removing the solvent include room temperature or high temperature air, an inert gas, and a vacuum state. Although the temperature which removes a solvent is not specifically limited, It is preferable that they are 60 degreeC or more and 250 degrees C or less. If it is less than 60 ° C., it may take time to remove the solvent, and if it is higher than 250 ° C., the binder may be deteriorated. After producing the electrode, the electrode may be compressed using 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. If it is less than 10 μm, it may be difficult to obtain a desired capacity, and if it is thicker than 200 μm, it may be 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. If it is less than 1.0 g / cm < ³ >, the contact with an active material and a conductive support material may become inadequate, and electronic conductivity may fall. When it is larger than 4.0 g / cm ³ , an electrolyte solution described later is less likely to penetrate into the electrode, and lithium conductivity may decrease. The electrode may be compressed to a desired thickness and density. Although compression is not specifically limited, For example, it can carry out using a roll press, a hydraulic press, etc.

In the present invention, the electric capacity 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 a desired capacity may be increased. On the other hand, if it is more than 3.6 mAh, it may be difficult to obtain a desired output density.
In the present invention, it is preferable that the electric capacity per 1 cm 2 of the positive electrode is 0.5 mAh or more and 3.0 mAh or less. When it is less than 0.5 mAh, the size of a battery having a desired capacity tends to increase, whereas when it exceeds 3.0 mAh, it tends to be difficult to obtain a desired output density.

The electric capacity calculation per 1 cm ^{2 of} each of the negative electrode and the positive electrode can be calculated by measuring charge / discharge characteristics after preparing each electrode and then preparing a half-cell using lithium metal as a counter electrode, as in the examples described later.
The electric capacity per 1 cm ^{2 of the} electrode is not particularly limited, but can be controlled by a method of controlling by the weight of the electrode formed per unit area of the current collector, for example, the coating thickness at the time of electrode coating described above.

<5. Non-aqueous electrolyte>
The non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery of the present invention is not particularly limited, but a polymer is impregnated with an electrolytic solution in which a solute is dissolved in a non-aqueous solvent, or an electrolytic solution in which a solute is dissolved in a non-aqueous solvent. A gel electrolyte can be used.
The non-aqueous solvent preferably includes a cyclic aprotic solvent and / or a chain aprotic solvent. Examples of the cyclic aprotic solvent include cyclic carbonates, cyclic esters, cyclic sulfones and cyclic ethers. Examples of the chain aprotic solvent include chain carbonates, chain carboxylic acid esters and chain ethers. In addition to the above, a solvent generally used as a solvent for nonaqueous electrolytes such as acetonitrile may be used. More specifically, dimethyl carbonate, methyl ethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyl lactone, 1,2-dimethoxyethane, sulfolane, dioxolane, propion For example, methyl acid can be used. These solvents may be used alone or as a mixture of two or more. However, in view of the ease of dissolving the solute described below and the high conductivity of lithium ions, a mixture of two or more of these solvents. Is preferably used. For example, dimethyl carbonate and ethylene carbonate are exemplified as a preferred combination. A gel electrolyte in which an electrolyte is impregnated in a polymer can also be used.

The solute is not particularly limited. For example, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , LiBOB (Lithium Bis (Oxalato) Borate), LiN (SO ₂ CF ₃ ) _2, etc. are dissolved in the solvent. It is preferable because it is easy to. The concentration of the solute contained in the electrolytic solution is preferably 0.5 mol / L or more and 2.0 mol / L or less. If it is less than 0.5 mol / L, the desired lithium ion conductivity may not be exhibited. On the other hand, if it is higher than 2.0 mol / L, the solute may not be dissolved any more. The non-aqueous electrolyte may contain a trace amount of additives such as a flame retardant and a stabilizer.

<6. Separator>
The separator is a substance that is placed between the positive electrode and the negative electrode, has no electronic conductivity, and has lithium ion conductivity. “Polyethylene terephthalate fiber-containing cellulose nonwoven fabric” is used in the nonaqueous electrolyte secondary battery of the present invention.
In the present invention, it is necessary to use a separator having a porosity of less than 50% by volume from the viewpoint of improving cycle characteristics. Here, “porosity” is defined as “porosity = (1−total volume of fibers constituting separator / volume of separator)”.

Conventionally, when a separator having a porosity of less than 50% by volume is used, the internal resistance between the positive and negative electrodes in the battery is further increased and the load is increased as compared with the case of using a separator having a porosity of 50% by volume or more. When it became heavy, good current load characteristics could not be expected. However, even if the porosity is less than 50% by volume, the present invention makes use of a cellulose nonwoven fabric containing polyethylene terephthalate fibers in a predetermined blending ratio, thereby dramatically improving cycle characteristics without sacrificing current load characteristics. Can be improved.

Examples of the type of cellulose that is an essential material for the present invention include organic fibers such as natural cellulose fibers, regenerated cellulose fibers, and solvent-spun cellulose fibers from the viewpoint of affinity with the electrolyte. Cellulose fibers are advantageous in that the selectivity of the electrolytic solution is widened compared to microporous separators limited to PP and PE, and are superior to conventional microporous separators from the viewpoint of heat resistance.

In addition, the cellulose fibers contain polyethylene terephthalate fibers that can secure a certain level of strength and have heat resistance, thereby improving the strength deficiencies compared to using separators made only of cellulose fibers and improving the strength of the separators. be able to.
Polyethylene terephthalate fiber can be added in any amount, but in order to ensure a certain level of strength, polyethylene terephthalate fiber needs to be 20% by weight or more based on the total weight of the separator, so that the electrolyte retainability In consideration, the proportion of the polyethylene terephthalate fiber needs to be 80% by weight or less. 30% by weight or more and 70% by weight or less are more preferable from the viewpoints of strength and electrolyte solution retention, and 50% by weight is the most balanced blend, and shows good battery characteristics.

The method of adding the polyethylene terephthalate fiber is not limited, but for example, it can be produced by mixing polyethylene terephthalate fiber and cellulose fiber and using paper making such as wet paper making or dry paper making. Wet papermaking can be performed by a conventional method. For example, the paper may be made using a wet paper machine equipped with a hand-made paper machine or a perforated plate. Dry papermaking can also be made using conventional methods such as airlaid and card manufacturing.

The separator may contain various plasticizers, antioxidants, flame retardants, and may be coated with a metal oxide or the like.
The thickness of the separator is preferably 10 μm or more and 100 μm or less. When the thickness is less than 10 μm, the positive electrode and the negative electrode may contact each other, and when the thickness is more than 100 μm, the internal resistance of the battery may increase. The thickness of the separator is particularly preferably 15 μm or more from the viewpoint of strength, and particularly preferably 35 μm or less from the viewpoint of battery characteristics. Most preferably, it is 20 μm or more and 30 μm or less.

According to the present invention, by improving the strength of the separator, it is possible to reduce the thickness of the separator as compared with a cellulose nonwoven fabric having the same thickness, which leads to improvement of current characteristics. In addition, the pore volume is reduced, the amount of electrolyte used can be reduced, and the energy density of the battery is improved.
<7. Non-aqueous 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 in which the same electrode is formed on both sides of the current collector, and the positive electrode is formed on one side of the current collector and the negative electrode is formed on one side. Alternatively, it may 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 and An insulating material is disposed around the negative electrode.

The nonaqueous electrolyte secondary battery of the present invention may be one obtained by winding or laminating a separator disposed between the positive electrode side and the negative electrode side. The positive electrode, the negative electrode, and the separator contain a nonaqueous electrolyte that is responsible for lithium ion conduction.
The ratio of the electric capacity of the positive electrode and the electric capacity of the negative electrode in the nonaqueous electrolyte secondary battery of the present invention preferably satisfies the following formula (a).

1 ≦ B / A ≦ 1.2 (a)
However, in said Formula (1), A shows the electrical capacity per 1 cm < ² > of positive electrodes, B shows the electrical capacity per 1 cm < ² > of negative electrodes.
When B / A is less than 1, the potential of the negative electrode may become the deposition potential of lithium during overcharge. On the other hand, when B / A is greater than 1.2, a negative electrode active material that does not participate in the battery reaction is present. Side reactions may occur due to the large amount.

The area ratio between the positive electrode and the negative electrode in 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)
(However, C represents the area of the positive electrode, and D represents the area of the negative electrode.)
When D / C is less than 1, for example, when B / A = 1 as described above, the capacity of the negative electrode is smaller than that of the positive electrode, so that the potential of the negative electrode may become a lithium deposition potential during overcharge. On the other hand, if D / C is greater than 1.2, the negative electrode active material not involved in the battery reaction may cause a side reaction because the portion of the negative electrode that is not in contact with the positive electrode is large. Although control of the area of a positive electrode and a negative electrode is not specifically limited, For example, in the case of slurry coating, it can carry out by controlling the coating width.

The area ratio between 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)
(However, E represents the area of the negative electrode, and F represents the area of the separator.)
When F / E is less than 1, the positive electrode and the negative electrode are in contact with each other, and when F / E is greater than 1.5, the volume required for the exterior increases, and the output density of the battery may decrease.

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 of battery capacity. If it is less than 0.1 mL, the conduction of lithium ions accompanying the electrode reaction may not catch up, and the desired battery performance may not be exhibited.
The nonaqueous electrolyte may be added to the positive electrode, the negative electrode, and the separator in advance, or may be added after winding or laminating a separator disposed between the positive electrode side and the negative electrode side.

The non-aqueous electrolyte secondary battery of the present invention may be wound or laminated with a laminate film after the laminate is wound, or may be rectangular, elliptical, cylindrical, coin-shaped, button-shaped, or sheet-shaped. It may be packaged with a metal can. The exterior may be provided with a mechanism for releasing the generated gas. The number of stacked layers can be stacked until a desired voltage value and battery capacity are exhibited.

The non-aqueous electrolyte secondary battery of the present invention can be an assembled battery by connecting a plurality of the non-aqueous electrolyte secondary batteries. The assembled battery of the present invention can be produced by appropriately connecting in series or in parallel according to a desired size, capacity, and voltage. Moreover, it is preferable that a control circuit is attached to the assembled battery in order to confirm the state of charge of each battery and improve safety.

EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited at all by these Examples, In the range which does not change the summary, it can change suitably.
<Example 1>
(Manufacture of positive electrode)
The positive electrode active material Li _1.1 Al _0.1 Mn _1.8 O ₄ has been published 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). ).

That is, an aqueous dispersion of manganese dioxide, lithium carbonate, aluminum hydroxide, and boric acid was prepared, and a mixed powder was prepared by a spray drying method. At this time, each amount of manganese dioxide, lithium carbonate, and aluminum hydroxide was prepared such 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 again heated at 650 ° C. for 24 hours. Finally, the powder was washed with water at 95 ° C. and dried to prepare a powdery positive electrode active material.

100 parts by weight of the prepared positive electrode active material, 6.8 parts by weight of conductive additive (acetylene black), and 6.8 parts by weight of PVdF (solid content concentration 12 wt%, NMP solution) as a binder were mixed to prepare a slurry. Produced. The slurry was applied to an aluminum foil (20 μm) and then vacuum dried at 150 ° C. to produce a positive electrode (50 cm ² ).
The capacity of the positive electrode was measured with the following charge / discharge test apparatus.

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

(Manufacture of negative electrode)
The negative electrode active material Li ₄ Ti ₅ O ₁₂ can be found in the literature ("Zero-Strain Insertion Material of Li [Li1 / 3Ti5 / 3] O4 for Rechargeable Lithium Cells" J. Electrochem. Soc., Volume 142, Issue 5, pp. 1431-1435 (1995)).
That is, first, titanium dioxide and lithium hydroxide are mixed so that the molar ratio of titanium and lithium is 5: 4, and then this mixture is heated and pulverized at 800 ° C. for 12 hours in a nitrogen atmosphere. A negative electrode active material was prepared.

100 parts by weight of the prepared negative electrode active material, 6.8 parts by weight of conductive additive (acetylene black), and 6.8 parts by weight of PVdF (solid content concentration 12 wt%, NMP solution) as a binder were mixed. A slurry was prepared. The slurry was applied to an aluminum foil (20 μm) and then vacuum dried at 150 ° C. to prepare a negative electrode (50 cm ² ).
The capacity of the negative electrode was measured by the following charge / discharge test.

A negative electrode was applied to one side of an aluminum foil under the same conditions as described above, and a punching working electrode was produced in a 16 mmφ disk shape. Li metal was punched into a disk shape of 16 mmΦ and used as a counter electrode. Using these electrodes, the working electrode (single-sided coating) / separator (# 2500 manufactured by Celgard) / Li metal were laminated in this order in the test cell (HS cell, manufactured by Hosen Co., Ltd.), and a non-aqueous electrolyte (ethylene carbonate) / Dimethyl carbonate = 3/7 vol%, LiPF ₆ 1 mol / L) was added in an amount of 0.15 mL to prepare a half-cell. The half-cell was allowed to stand at 25 ° C. for one day, and then connected to a charge / discharge test apparatus (HJ1005SD8, manufactured by Hokuto Denko). This half-cell was subjected to constant current discharge (end voltage: 1.0 V) and constant current charge (end voltage: 2.0 V) at 25 ° C. and 0.4 mA five times, and the fifth result was defined as the negative electrode capacity. As a result, the capacity of the negative electrode was 1.2 mAh / cm ² .

(Separator)
As the separator, a polyethylene terephthalate fiber-containing cellulose nonwoven fabric (separator having a thickness of 25 μm, a porosity of 47 vol%, and 55 cm ² containing the same amount of polyethylene terephthalate fiber and cellulose fiber) was used.
(Manufacture of non-aqueous electrolyte secondary batteries)
A non-aqueous electrolyte secondary battery was produced as follows.

First, the produced positive electrode (single-sided coating; 50 cm ² ), negative electrode (single-sided coating; 50 cm ² ), and separator were laminated in the order of positive electrode (single-sided coating) / separator / negative electrode (single-sided coating).
Next, aluminum tabs 1 a and 3 a were vibration welded to the positive electrode 1 and the negative electrode 3 at both ends, and then put into a bag-like aluminum laminate sheet 4. The nonaqueous electrolyte 5 was prepared by dissolving a mixed nonaqueous solvent of ethylene carbonate / dimethyl carbonate = 3: 7 (vol ratio) and LiPF ₆ at a concentration of 1 mol / L. After 2 mL of the obtained nonaqueous electrolyte 5 was added, the nonaqueous electrolyte secondary battery was produced by sealing while reducing the pressure. FIG. 1 shows the produced nonaqueous electrolyte secondary battery (cross-sectional view).

<Example 2>
LiNi _0.5 Mn _1.5 O ₄ as a positive electrode active material (Tsutomu Ohzuku, Sachio Takeda, Masato Iwanaga "Solid-state redox potentials for Li [Me1 / 2Mn3 / 2] O4 (Me: 3d-transition metal) having spinel-framework It was made by the method described in "structure: a series of 5 volt materials for advanced lithium-ion batteries" Journal of Powersources, Vol. 81-82, pp. 90-94 (1999)). That is, lithium hydroxide, manganese oxide hydroxide, and nickel hydroxide were first mixed so that the molar ratio of lithium, manganese, and nickel was 1: 1.5: 0.5. Next, this mixture was heated at 550 ° C. in an air atmosphere, and then heated again 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>
As a positive electrode active material, LiCoO ₂ has been described in literature (AR Armstrong, 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). That is, lithium carbonate and cobalt oxide were mixed so that the molar ratio of lithium to cobalt was 1: 1. Next, this mixture was heated at 650 ° C. in an air atmosphere, and then heated again at 850 ° C. to prepare LiCoO ₂ . A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 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 fiber (25 μm, porosity 48 volume%, 55 cm ² ) was 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 volume%, 55 cm ² ) was used as the separator.

<Comparative Example 4>
LiNiO ₂ is used as a positive electrode active material in the literature (T. Ohzuku, A. Ueda, and M. Nagayama, “Electrochemistry and structural chemistry of LiNiO ₂ (R 3 m) for 4 volt secondary lithium cells,” Journal of the Electrochemical Society, vol. 140, no. 7, pp. 1862-1870, (1993)). That is, lithium hydroxide and nickel carbonate were mixed so that the molar ratio of lithium to nickel was 1: 1. Next, this mixture was heated at 600 ° C. in an oxygen atmosphere, and then heated again 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 as 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 fiber (25 μm, porosity 48 volume%, 55 cm ² ) was 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 volume%, 55 cm ² ) was used as the separator.

<Comparative Example 7>
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that a polypropylene microporous membrane (Celgard # 2500) was used as the separator.
<Comparative Example 8>
A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 7, except that polyethylene terephthalate fiber (25 μm, porosity 48 volume%, 55 cm ² ) was used as 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 non-woven fabric (25 μm, porosity 70 volume%, 55 cm ² ) was used as the separator.
<Comparative Example 10>
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 2 except that a polypropylene microporous membrane (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 fiber (25 μm, porosity 48 volume%, 55 cm ² ) was used as 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 volume%, 55 cm ² ) was used as the separator.

(Measurement of cycle characteristics)
The nonaqueous electrolyte secondary batteries produced in the examples and comparative examples were connected to a charge / discharge device (HJ1005SD8, manufactured by Hokuto Denko), and 55 ° C., 50 mA constant current charge, and 50 mA constant current discharge were repeated 100 times. The charge end voltage and discharge end voltage at this time were 3 V and 2 V, respectively. Table 1 shows the 100th discharge capacity when the first discharge capacity is 100.

As is apparent from Table 1, the nonaqueous electrolyte secondary batteries of Examples 1 and 2 of the present invention have improved 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 employed as the positive electrode active material, the battery using the polyethylene terephthalate fiber-containing cellulose nonwoven fabric (Example 1) as the separator is a polypropylene microporous membrane (Comparative Example 7), polyethylene terephthalate fiber ( The cycle stability is superior to the battery using Comparative Example 8) and cellulose (Comparative Example 9).

When LiNi _0.5 Mn _1.5 O ₄ is used as the positive electrode active material, the battery using the polyethylene terephthalate fiber-containing cellulose nonwoven fabric (Example 2) as the separator is a polypropylene microporous membrane (Comparative Example 10), polyethylene terephthalate fiber (Comparative). The cycle stability is superior to the battery using Example 11) and cellulose (Comparative Example 12).
However, when the positive electrode active material is LiCoO ₂ , a battery using polyethylene terephthalate fiber-containing cellulose nonwoven fabric (Comparative Example 1) as a separator is a battery using polyethylene terephthalate fiber (Comparative Example 2) and cellulose (Comparative Example 3). Compared to, the cycle stability is not greatly superior.

Even when the positive electrode active material is LiNiO ₂ , the battery using the polyethylene terephthalate fiber-containing cellulose nonwoven fabric (Comparative Example 4) as the separator is the same as the battery using the polyethylene terephthalate fiber (Comparative Example 5) and cellulose (Comparative Example 6). In comparison, the cycle characteristics are not greatly superior.
Therefore, the combination of using Li _1.1 Al _0.1 Mn _1.8 O ₄ or LiNi _0.5 Mn _1.5 O ₄ as the positive electrode active material and using the cellulose non-woven fabric containing polyethylene terephthalate fiber as the separator greatly improves the cycle stability of the battery. I understand that

The cause of the synergistic effect between the specific positive electrode active material and the specific structure / specific material separator is presumed. The inventor assumes that free radicals generated at the interface between the electrolyte and the positive electrode pass through the separator and cause a precipitation reaction on the negative electrode surface as a mechanism for causing cycle deterioration of the battery. Therefore, the combination of the specific positive electrode active material and the specific structure / specific material separator suppresses free radical generation, promotes trapping of the generated free radical by the separator, and suppresses the precipitation reaction on the negative electrode surface. And so on.

Claims (10)

1. A lithium ion battery having a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte,
   The positive electrode includes a lithium manganese compound;
   The negative electrode comprises lithium titanate;
   A nonaqueous electrolyte secondary battery, wherein the separator is a polyethylene terephthalate fiber-containing cellulose nonwoven fabric, and the porosity thereof is less than 50% by volume.
2. The lithium manganese _{compound, Li 1 + x M y Mn} 2 - x - y O 4 (0 ≦ x ≦ 0.2, 0 <y ≦ 0.6, preferably at least one M is Al, are selected from Ni The nonaqueous electrolyte secondary battery of Claim 1 which is a compound represented by this.
3. 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 (0 ≦ x ≦ 0.2, 0 <y ≦ 0.6).
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 (0 ≦ x ≦ 0.2, 0 <y ≦ 0.6).
5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the lithium titanate has a spinel structure.
6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the polyethylene terephthalate fiber content of the polyethylene terephthalate fiber-containing cellulose nonwoven fabric is 20 wt% or more and 80 wt% or less.
7. The non-aqueous electrolyte secondary battery according to claim 1, wherein the polyethylene terephthalate fiber-containing cellulose nonwoven fabric has a polyethylene terephthalate fiber content of 30 wt% or more and 70 wt% or less.
8. The nonaqueous electrolyte secondary battery according to claim 1, wherein the polyethylene terephthalate fiber-containing cellulose nonwoven fabric has a thickness of 15 μm or more and 35 μm or less.
9. The nonaqueous electrolyte secondary battery according to claim 1, wherein the polyethylene terephthalate fiber-containing cellulose nonwoven fabric has a thickness of 20 μm or more and 30 μm or less.
10. An assembled battery formed by connecting a plurality of the nonaqueous electrolyte secondary batteries according to claim 1.

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