CN101111956A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

Disclosed is a nonaqueous electrolyte secondary battery wherein the separator contains a heat-resistant resin having a chlorine atom as a terminal group and the positive electrode active material contains a lithium-containing complex oxide having an aluminum atom in the composition. In this nonaqueous electrolyte secondary battery, even if the chlorine atom is liberated into the nonaqueous electrolyte solution, aluminum contained in the positive electrode active material is selectively dissolved into the nonaqueous electrolyte solution, thereby suppressing dissolution of other constitutional elements. Consequently, there can be obtained a nonaqueous electrolyte secondary battery which is excellent in safety and high-temperature storage characteristics.

Description

Non-aqueous electrolyte secondary battery

technical field

The present invention relates to a nonaqueous electrolyte secondary battery, and more particularly, to a nonaqueous electrolyte secondary battery with improved safety.

Background technique

In recent years, the development of portable and cordless consumer electronic devices has grown rapidly. For energy sources to drive these electronic devices, smaller and lighter batteries with high energy density have been increasingly required. Among them, since lithium ion secondary batteries have high voltage and high energy density, development of lithium ion secondary batteries as energy sources for portable electronic devices such as notebook computers, mobile phones, and AV equipment is greatly expected.

For positive electrode active materials of lithium ion secondary batteries, lithium-containing composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ have been used. In the cathode active material, in association with expansion and contraction due to charging and discharging, destruction of crystal structure and cracking of particles occur. Therefore, repeated charge and discharge cycles lead to a decrease in capacity and an increase in internal resistance.

In order to improve the cycle characteristics and safety of batteries, for example, a proposal is to replace part of Co or Ni contained in lithium-containing composite oxides with elements such as Mg to stabilize the crystal structure of lithium-containing composite oxides (see Patent Document 1 ).

Among the cathode active materials mentioned above, _LiNiO has a large theoretical capacity; however, the reversibility of the crystal structure change associated with charge and discharge is significantly reduced. In order to solve the problem, another proposal is to replace part of Ni with an element such as Co to moderate the change of the crystal structure (see Patent Document 2, for example).

Moreover, there is also a proposal to replace nickel and/or cobalt in lithium-containing nickel-cobalt oxides with inexpensive Mn to obtain Li(NiMnCo)O ₂ and utilize this oxide as a positive electrode active material (see, for example, the patent document 3). As a result, inexpensive and high-performance batteries can be obtained.

In many cases, for a separator used in a lithium ion secondary battery, a porous film made of a thermoplastic resin such as polyolefin is used in view of safety. This is due to the so-called shut-down function of the diaphragm. Here, the blocking function refers to a function that when, for example, an external short circuit occurs and the battery temperature suddenly rises due to the short circuit, the separator is softened and its micropores are closed, resulting in a decrease in ion conductivity to stop current flow.

However, even when this shutoff function is activated, if the battery temperature rises further, so-called melt-down occurs in which the separator is melted and shrunk by heat, causing a large-scale short circuit between the positive electrode and the negative electrode. On the other hand, if the heat-fusibility of the separator is increased in order to improve the shutoff function, there arises another problem that the fusing temperature of the separator is lowered.

In view of the above problems, there have been many proposals for a composite separator including a porous layer made of polyolefin and a layer made of heat-resistant resin in order to simultaneously improve the shutoff performance and the fusing resistance. For example, one proposal is a separator obtained by laminating a layer composed of a heat-resistant nitrogen-containing aromatic polymer such as aramid or polyamideimide, and a ceramic powder with a layer of a porous membrane (see, for example, Patent file 4).

Patent Document 1: Japanese Laid-open Patent Application No.2002-198051

Patent Document 2: Japanese Patent Application No. 3,232,943

Patent Document 3: Japanese Laid-Open Patent Application No. 2004-31091

Patent Document 4: Japanese Patent Application No. 3,175,730.

Contents of the invention

Problems solved by the present invention

When the heat-resistant resin as described above is used, the safety of the battery can be improved. However, a great decrease in capacity occurs during high temperature storage. In particular, since an aromatic polyamide is obtained by polymerizing an organic substance containing an amine group such as p-phenylenediamine and an organic substance containing a chlorine group such as terephthaloyl chloride, the chlorine group exists as a terminal group in the produced aromatic polyamide. in polyamide. Also, since polyamideimide is obtained by the reaction between trimellitic anhydride monochloride and diamine, chlorine groups exist as terminal groups in the obtained polyamideimide. In a high temperature environment, the chlorine radicals liberate into the electrolyte. On the other hand, in the positive electrode active material, main components (transition metals such as Co) in the positive electrode active material are prone to leach in an environment of high temperature and high potential. When released chlorine exists in the vicinity of the positive electrode active material, complexation reactions continuously occur between the transition metal leaked from the positive electrode active material and chlorine. As a result, a large amount of component elements leaks from the cathode active material into the electrolyte, thereby reducing sites serving as the cathode active material. As a result, battery capacity drops significantly.

The present invention has been accomplished in view of the above-mentioned problems, and aims to provide a nonaqueous electrolyte secondary battery which is excellent in safety and capable of suppressing a decrease in capacity during high-temperature storage.

way of solving the problem

The present invention relates to a nonaqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, a nonaqueous electrolyte and a separator, wherein the separator includes a heat-resistant resin having chlorine atoms as terminal groups, and The cathode active material includes a lithium-containing composite oxide containing aluminum atoms in its composition.

Preferably, the above-mentioned heat-resistant resin includes at least one selected from the group consisting of aramid and polyamide-imide.

The aforementioned separator may include a film containing the heat-resistant resin and a film containing polyolefin laminated on the film containing the heat-resistant resin. Also, the above separator may include a polyolefin-containing film and a layer containing the heat-resistant resin and filler formed on the polyolefin-containing film.

Preferably, the above lithium-containing composite oxide is represented by the following formula:

Li _x M _1-y Al _y O ₂ (1)

Wherein, 1≤x≤1.05, 0.001≤y≤0.2 and M is at least one selected from Co, Ni, Mn and Mg.

Among the composite oxides represented by formula (1), the above-mentioned lithium-containing composite oxide may be a composite oxide represented by the following formula:

Li _a Co _1-bc Mg _b Al _c O ₂ (2)

wherein, 1≤a≤1.05, 0.005≤b≤0.1 and 0.001≤c≤0.2, or may be a composite oxide represented by the following formula:

Li _a Ni _1-bc Co _b Al _c O ₂ (3)

wherein, 1≤a≤1.05, 0.1≤b≤0.35 and 0.001≤c≤0.2, or may be a composite oxide represented by the following formula:

Li _a Ni _1-(b+c+d) Mn _b Co _c Al _d O ₂ (4)

Among them, 1≤a≤1.05, 0.1≤b≤0.5, 0.1≤c≤0.5, 0.001≤d≤0.2 and 0.2≤b+c+d≤0.75.

Invention effect

According to the present invention, during high-temperature storage, even when chlorine atoms are released from the heat-resistant resin contained in the separator into the non-aqueous electrolyte, the chlorine atoms preferentially react with aluminum contained in the positive electrode active material, so that the positive electrode active material Other components will not leak from the positive active material. As a result, a nonaqueous electrolyte secondary battery having improved safety and favorable high-temperature storage characteristics can be provided.

Description of drawings

FIG. 1 is a schematic vertical cross-sectional view showing a cylindrical lithium secondary battery prepared in Examples.

Detailed ways

The nonaqueous electrolyte secondary battery of the present invention includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, a nonaqueous electrolyte and a separator. The cathode active material includes a lithium-containing composite oxide containing aluminum atoms in its composition. The separator includes a heat-resistant resin having chlorine atoms as terminal groups.

In the present invention, the lithium-containing composite oxide as the cathode active material includes a predetermined amount of aluminum atoms. The stability constant of a complex composed of aluminum atoms and chlorine atoms is higher than that of a complex composed of main component elements (for example, transition metals such as Co, Ni, and Mn) and chlorine atoms in lithium-containing composite oxides. constant high. Based on this, the aluminum atom preferably easily forms a complex with the chlorine atom. Therefore, during high-temperature storage, even when chlorine atoms as terminal groups are released from the heat-resistant resin contained in the separator into the non-aqueous electrolyte, the chlorine atoms preferentially react with aluminum atoms contained in the positive electrode active material to form its complexes. This makes it possible to suppress leakage of component elements other than aluminum contained in the cathode active material into the non-aqueous electrolyte, thus preventing a decrease in battery capacity during high-temperature storage.

It is preferable that the heat-resistant resin having chlorine atoms as terminal groups include at least one selected from aramids and polyamideimides. This is because aromatic polyamides and polyamideimides are easy to form films since they are soluble in polar organic solvents, and porous films formed from them are excellent in nonaqueous electrolyte retention ability and heat resistance.

Also, it is preferable that the glass transition temperature, melting point, and thermal degradation initiation temperature involving chemical changes of the above-mentioned heat-resistant resin be sufficiently high, and more particularly, that its mechanical strength at high temperature be high.

For example, according to ASTM-D648 of the American Society for Testing Materials, the heat deflection temperature of the above-mentioned heat-resistant resin, that is, the heat deflection temperature determined by a load deflection temperature (load deflection temperature) test at 1.82 MPa is preferably 260° C. or higher . The higher the heat distortion temperature, the more surely the shape of the separator can be maintained even when the separator is subjected to heat shrinkage or the like. In the case where the heat distortion temperature is 260°C or higher, the separator can exhibit sufficiently high thermal stability even when the temperature of the battery is further increased (usually about 180°C) by heat accumulated during battery overheating .

Preferably, the content of chlorine contained in the separator is 300-3000 μg per 1 g of the separator. The content of chlorine contained in a predetermined weight of the heat-resistant resin is affected by the degree of polymerization of the heat-resistant resin. In the case where the content of chlorine is too small, the degree of polymerization of the heat-resistant resin is too high, so its elasticity decreases. This results in reduced processability of the heat-resistant resin. In the case where the content of chlorine is too large, the degree of polymerization of the heat-resistant resin is too small, and thus the heat deflection temperature of the heat-resistant resin is lowered. Based on this, it is expected that when the content of chlorine is within the above-mentioned range, the effect of the heat-resistant resin can be fully exhibited.

In the present invention, a porous film comprising the above heat-resistant resin can be used as a separator. Also, for example, the separator may be a laminated film including a porous film containing polyolefin such as polyethylene and polypropylene, and a porous film containing the above-mentioned heat-resistant resin. Also, the separator may be a laminated matter including a porous film containing polyolefin and a porous layer containing the above-mentioned heat-resistant resin and filler formed thereon.

For example, a porous membrane containing the above-mentioned heat-resistant resin can be produced as follows:

First, a heat-resistant resin is dissolved in a polar solvent such as N-methylpyrrolidone. The resulting solution is applied to substrates such as glass and stainless steel sheets, and then dried. The resulting porous membrane was peeled off from the substrate. In this way, a porous membrane containing the above-mentioned heat-resistant resin can be obtained.

A laminated film comprising a porous film containing polyolefin and a porous film containing the above-mentioned heat-resistant resin can be formed by dissolving the above-mentioned heat-resistant resin in a polar solvent, applying the resulting solution to the porous film containing polyolefin, and then drying preparation.

A laminate comprising a porous film comprising polyolefin and a porous layer formed thereon comprising the above-mentioned heat-resistant resin and filler can be produced as follows:

The above-mentioned heat-resistant resin is dissolved in a polar solvent, and a filler is added to the resulting solution. The mixture thus obtained was applied to a polyolefin-containing porous film, followed by drying. In this way, a laminate including a porous film containing polyolefin and a porous layer containing the above-described heat-resistant resin and filler formed thereon can be obtained.

Preferably, the filler used is chemically stable and of high purity so as not to adversely affect battery performance even when impregnated with a non-aqueous electrolyte or exposed to the redox potential of the active material. The filler is, for example, an inorganic oxide filler. Examples of the inorganic oxide filler include inorganic porous materials such as alumina, zeolite, silicon nitride, silicon carbide, titania, zirconia, magnesia, zinc oxide and silica.

Among the above, preferably used for the separator is a laminate including a porous film containing polyolefin and a porous layer containing the above-mentioned heat-resistant resin and filler formed thereon because it is more excellent in heat resistance.

In the case where the separator is a laminate including a porous film containing polyolefin and a porous layer containing a heat-resistant resin and a filler, the thickness of the porous layer containing a heat-resistant resin and a filler is not particularly limited, however, in view of The thickness is preferably 1-20 μm, more preferably 2-10 μm, in terms of preventing the occurrence of internal short circuit and ensuring a balance between safety and battery capacity. When the thickness is less than 1 μm, the effect of the porous layer containing the heat-resistant resin and the filler to suppress shrinkage of the polyolefin-containing porous film in a high-temperature environment decreases. When the thickness exceeds 20 μm, the porosity of the porous layer containing the heat-resistant resin and the filler becomes lower and thus lowers the ion conductivity of the porous layer. This increases the resistance somewhat and degrades the charge/discharge characteristics of the battery.

In view of securing ionic conductivity, the porosity of the porous layer containing the heat-resistant resin and filler is preferably 20-70%. The porosity can be controlled by adjusting the application rate or drying conditions (temperature or air flow rate) of the mixture comprising the heat-resistant resin and the filler, and the particle size or shape of the filler.

In the case where the separator has a porous film containing polyolefin and a porous layer formed thereon containing a heat-resistant resin and a filler, the total thickness of the separator is not particularly limited, however, if overall safety is considered, various batteries Characteristics and battery set capacity, the thickness is preferably 5-35 μm.

The pore diameter of the micropores of the polyolefin-containing porous membrane is preferably 0.01 to 10 μm in view of obtaining both ionic conductivity and mechanical strength.

In the case where the separator contains the above heat-resistant resin, the thickness of the separator is preferably 5-20 μm, more preferably 10-20 μm, in view of securing a balance between safety and battery capacity by preventing occurrence of internal short circuit. The porosity of the separator comprising a heat-resistant resin is preferably 20-70%. The porosity of the separator can be controlled by adjusting the application rate or drying conditions of the solution containing the heat-resistant resin.

Next, a lithium-containing composite oxide containing aluminum atoms in its composition will be described.

As described above, in the present invention, a lithium-containing composite oxide containing a predetermined amount of aluminum is used so that when a chlorine atom existing as a terminal group in a heat-resistant resin is released under a high-temperature and high-potential environment, the chlorine atom preferentially interacts with Aluminum atoms form complexes.

Among the lithium-containing composite oxides described above, a lithium-containing composite oxide represented by the following formula is used:

Li _x M _1-y Al _y O ₂ (1)

Wherein, 1≤x≤1.05, 0.001≤y≤0.2, and M is at least one selected from Co, Ni, Mn and Mg. The lithium-containing composite oxide represented by formula (1) has a high capacity and can absorb and desorb lithium ions even at high voltage.

The molar ratio x of lithium is preferably 1≤x≤1.05. When the molar ratio x of lithium is less than 1, the lithium salt content in the raw material mixture used in the preparation of the lithium-containing composite oxide is low. This allows electrochemically inert impurities such as cobalt oxides to be present in the resulting product, leading to a possible reduction in battery capacity. When the molar ratio x of lithium exceeds 1.05, excess lithium salt exists in the raw material mixture. This allows lithium salts to exist as impurities in the product, leading to a possible reduction in battery capacity.

Here, the molar ratio x of lithium is a value obtained immediately after the lithium-containing composite oxide represented by formula (1) is prepared.

The molar ratio y of aluminum is preferably 0.001≤y≤0.2, more preferably 0.005≤y≤0.2. When the molar ratio y of aluminum is less than 0.001, the above-mentioned effects cannot be sufficiently obtained, resulting in that the improved effect may not be sufficiently realized. When the molar ratio y exceeds 0.2, the content of metal atoms M contributing to the charge/discharge reaction decreases, resulting in a possible decrease in battery capacity.

There is no particular limitation on the preparation method of the lithium-containing composite oxide represented by formula (1), however, for example, it can be prepared in the following manner:

At least one salt selected from cobalt salts, nickel salts, manganese salts, and magnesium salts is mixed with lithium salts and magnesium salts in a predetermined ratio. The resulting raw material mixture is baked at a high temperature in an oxidizing atmosphere, whereby a lithium-containing composite oxide represented by formula (1) can be obtained.

Among the lithium-containing composite oxides represented by formula (1), lithium-containing composite oxides represented by the following formula can be used:

Li _a Co _1-bc Mg _b Al _c O ₂ (2)

Among them, 1≤a≤1.05, 0.005≤b≤0.1 and 0.001≤c≤0.2. The lithium-containing composite oxide represented by formula (2) contains magnesium. Even when the positive electrode active material repeats expansion and contraction due to charge and discharge, the addition of magnesium makes it possible to prevent lattice deformation, structural breakdown thereof, or cracking of active material particles. Thereby, the decrease in discharge capacity is alleviated, and the cycle characteristics are improved.

The molar ratio b of magnesium is preferably 0.005≦b≦0.1. When the molar ratio b is lower than 0.005, the above effects cannot be achieved. When the molar ratio b exceeds 0.1, the battery capacity will decrease to some extent.

The molar ratio c of aluminum is preferably 0.001≦c≦0.2. When the molar ratio c is lower than 0.001, the effect of Al cannot be sufficiently exhibited. When the molar ratio c exceeds 0.2, the amount of metal atoms contributing to the charge/discharge reaction becomes insufficient to some extent.

The preferable range of the molar ratio a of lithium and the reason why the range is preferable are the same as in the case of the lithium-containing composite oxide represented by formula (1).

There is no particular limitation on the preparation method of the lithium-containing composite oxide represented by formula (2), however, for example, it can be prepared in the following manner:

Lithium salt, magnesium salt, cobalt salt and aluminum salt are mixed in a predetermined ratio. The obtained raw material mixture is baked at a high temperature in an oxidizing atmosphere, whereby a lithium-containing composite oxide represented by formula (2) can be obtained.

A compound salt containing two or more elements selected from cobalt, magnesium, and aluminum may be used instead of the salts of the respective elements contained in the compound salt. For example, instead of cobalt salts, magnesium salts, and aluminum salts, eutectic hydroxides containing cobalt, magnesium, and aluminum or eutectic oxides thereof may be used.

Also, a lithium-containing composite oxide represented by the following formula can be used:

Li _a Ni _1-bc Co _b Al _c O ₂ (3)

Among them, 1≤a≤1.05, 0.1≤b≤0.35 and 0.001≤c≤0.2. It is known that LiNiO ₂ -based materials have a high capacity density, in which the change of crystal structure associated with charge and discharge is large and the reversibility of the structure change is low. On the other hand, the lithium-containing composite oxide represented by formula (3) further contains cobalt and aluminum in its composition. The presence of cobalt atoms or aluminum atoms in its crystal structure, particularly in the lithium diffusion layer, makes it possible to suppress contraction of the crystal lattice when lithium is released from the composite oxide.

Also, the lithium-containing composite oxide represented by formula (3) is inexpensive compared to LiCoO ₂ -based materials, and is particularly used as a positive electrode material in large batteries.

The molar ratio b of cobalt is preferably 0.1≦b≦0.35. When the molar ratio b is lower than 0.1, the above effects are difficult to achieve. When the molar ratio b exceeds 0.35, the battery capacity will decrease to some extent.

The preferred ranges of the molar ratio a of lithium and the molar ratio c of aluminum and the reasons why these ranges are preferred are the same as in the case of the lithium-containing composite oxide represented by formula (1).

The lithium-containing composite oxide represented by formula (3) can be produced, for example, in the following manner:

A nickel salt, a cobalt salt and an aluminum salt are dissolved in water at a predetermined mixing ratio. The resulting aqueous solution was subjected to a neutralization process to be precipitated as a nickel-cobalt-aluminum ternary system composite hydroxide by coprecipitation. The obtained composite hydroxide and lithium salt are mixed in a predetermined mixing ratio, and then the mixture is baked to obtain a lithium-containing composite oxide represented by formula (3).

A compound salt containing two or more elements selected from nickel, cobalt, and aluminum may be used instead of the salts of the respective elements contained in the compound salt.

Also, a lithium-containing composite oxide represented by the following formula can be used:

Li _a Ni _1-(b+c+d) Mn _b Co _c Al _d O ₂ (4)

Among them, 1≤a≤1.05, 0.1≤b≤0.5, 0.1≤c≤0.5, 0.001≤d≤0.2 and 0.2≤b+c+d≤0.75. The lithium-containing composite oxide represented by formula (4) is inexpensive and can maintain stable battery performance.

The molar ratio b of manganese is preferably 0.1≦b≦0.5. When the molar ratio b is less than 0.1, the content of manganese in the composite oxide is small, making it difficult to obtain cost reduction. When the molar ratio b exceeds 0.5, the battery capacity will decrease to some extent.

The molar ratio c of cobalt is preferably 0.1≦c≦0.5. When the molar ratio c is lower than 0.1, the crystal of the composite oxide becomes slightly unstable, resulting in a possible reduction in cycle characteristics or safety of the battery may be reduced to some extent. When the molar ratio c exceeds 0.5, the battery capacity will decrease to some extent.

In order to achieve and simultaneously balance various battery performances, it is preferred that 0.2≤b+c+d≤0.75.

The preferred ranges of the molar ratio a of lithium and the molar ratio d of aluminum are the same as in the case of the lithium-containing composite oxide represented by formula (1).

The lithium-containing composite oxide represented by formula (4) can be prepared, for example, by mixing lithium salt, nickel salt, cobalt salt, manganese salt, aluminum salt, etc. in a predetermined mixing ratio, followed by baking at a high temperature in an oxidizing atmosphere mixture.

Similar to the above, a compound salt containing two or more elements selected from nickel, cobalt, manganese, and aluminum may be used instead of the salts of the respective elements contained in the compound salt. For example, instead of nickel salts, cobalt salts, manganese salts, and aluminum salts, eutectic hydroxides containing cobalt, magnesium, manganese, and aluminum and eutectic oxides thereof may be used.

As the lithium salt used for synthesizing the above lithium-containing composite oxide, for example, lithium carbonate, lithium hydroxide, lithium nitrate, lithium sulfate, and lithium oxide can be used.

As the magnesium salt, for example, magnesium oxide, basic magnesium carbonate, magnesium chloride, magnesium fluoride, magnesium nitrate, magnesium sulfate, magnesium acetate, magnesium oxalate, magnesium sulfide and magnesium hydroxide can be used.

As cobalt salts, for example, cobalt oxide and cobalt hydroxide can be used.

As the aluminum salt, for example, aluminum hydroxide, aluminum nitrate, aluminum oxide, aluminum fluoride and aluminum sulfate can be used.

As the nickel salt, for example, nickel oxide and nickel hydroxide can be used.

As the manganese salt, for example, manganese oxide, manganese hydroxide, manganese carbonate, manganese nitrate, manganese sulfate, manganese fluoride, manganese chloride and manganese oxyhydroxide can be used.

In the lithium-containing composite oxide represented by formula (1), the effects of the present invention can be achieved when the composite oxide contained therein is used alone or when two or more kinds are used in combination. For example, a mixture containing two or more lithium-containing composite oxides represented by formulas (1) to (4) may be used for the cathode active material.

In the case of a mixture of lithium-containing composite oxides whose composition does not contain an aluminum atom, for example, a mixture of Li _a Co _1-b O ₂ and Li _a Ni _1-(b+c) Mn _b Co _c O ₂ is used For positive electrode active materials, the potential of each composite oxide in a charged state is determined according to the valence of the transition metal contained therein. In these two composite oxides, since the kinds of transition metals contained therein are different, the potentials of the composite oxides are also different from each other. Therefore, in the mixture, the potential distribution is easily changed. Therefore, when chlorine atoms contained in the heat-resistant resin as terminal groups are released, main component elements (transition elements such as Co) in the cathode active material may easily leak into the non-aqueous electrolyte. Also, in the case where the charging voltage is high, transition metals contained in the cathode active material are easily oxidized, and in particular, main component elements (transition metals such as Co) are easily leaked in a high voltage environment.

On the contrary, since the lithium-containing composite oxide used in the present invention contains Al, even if the chlorine atoms contained in the heat-resistant resin are released into the non-aqueous electrolyte, Al selectively leaks from the composite oxide of the positive electrode, thereby Inhibits the leakage of other main component elements. As a result, a battery excellent in safety in which reduction in capacity during high-temperature storage is suppressed can be obtained.

Next, the positive electrode, negative electrode, and nonaqueous electrolyte will be described.

For example, the positive electrode may include a positive electrode current collector and a positive electrode material mixture layer supported thereon.

The positive electrode material mixture layer includes a positive electrode active material, a conductive agent, a binder, and the like. As described above, positive electrode active materials include lithium-containing composite oxides containing aluminum atoms in their composition.

For the binder for the positive electrode, for example, polytetrafluoroethylene; modified acrylonitrile rubber particles (for example, BM-500B obtained from Zeon Corporation, Japan); Polyvinylidene fluoride and its modified substances. They can be used alone or in combination of two or more.

The above-mentioned polytetrafluoroethylene and modified acrylonitrile rubber particles can be combined with carboxymethyl cellulose, polyethylene oxide or soluble modified acrylonitrile rubber (for example, available from Zeon Corporation, Japan) having a thickening effect. BM-720H) used in combination.

As the conductive agent, acetylene black, Ketjen Black or various graphites can be used. They can be used alone or in combination of two or more.

In the case of a negative electrode, the negative electrode may include a negative electrode current collector and a negative electrode material mixture layer supported thereon. The negative electrode material mixture layer contains a negative electrode active material. The negative electrode material mixture layer may contain a binder, a conductive agent, and the like as needed.

For the negative electrode active material, lithium metal; materials capable of forming alloys with lithium; various natural graphites and artificial graphites; silicon-based composite materials such as silicides; containing at least one element selected from the group consisting of tin, aluminum, zinc, and magnesium can be used Lithium alloys; and various alloy materials. They can be used alone or in combination of two or more.

In view of improving capacity, preferably used among the above-mentioned materials for the negative electrode active material is at least one material selected from the above-mentioned materials capable of forming alloys with lithium, lithium metal, and lithium alloys.

Examples of materials capable of forming an alloy with lithium are simple substances of silicon, oxides of silicon (for example, _SiOx (0<x<2)), simple substances of tin, oxides of tin (for example, SnO), and Ti.

The negative electrode material mixture layer may be formed by directly vapor-depositing the negative electrode active material onto the current collector. Alternatively, the negative electrode material mixture layer may be formed by applying a material mixture containing the negative electrode active material and a small amount of arbitrary components onto the current collector and then drying.

For the binder used for the negative electrode, similar to the case of the positive electrode, various resin materials including polyvinylidene fluoride and its modified substances can be used.

Among them, in view of improving safety against overcharge, it is more preferable to use, for example, a mixture of a water-soluble binder containing a styrene-butadiene copolymer or a modified substance thereof and a cellulose-based resin such as carboxymethylcellulose .

The nonaqueous electrolyte contains a nonaqueous solvent and a solute dissolved therein. As the non-aqueous solvent, solvents commonly used in this field can be used. Examples of the solvent include ethylene carbonate, dimethyl carbonate, diethyl carbonate, and methylethyl carbonate. They can be used alone or in combination of two or more.

As the solute, lithium salts commonly used in the art can be used. Examples of lithium salts include LiPF ₆ and LiBF ₄ . These lithium salts may be used alone or in combination of two or more.

In order to form a favorable coating film on the positive and negative electrodes, the non-aqueous electrolyte may further contain, for example, vinylene carbonate, cyclohexylbenzene, and/or modified substances thereof.

Next, the present invention will be described by way of examples, but the present invention is not limited to these examples.

Example

Example 1

(Example 1-1)

(a) Preparation of positive electrode

An aqueous solution containing cobalt sulfate at a concentration of 0.999 mol/L and aluminum sulfate at a concentration of 0.001 mol/L was continuously supplied to the reaction bath. Sodium hydroxide was dropped into the reaction bath until the pH of the aqueous solution became 10 to 13, thereby synthesizing Co _0.999 Al _0.001 (OH) ₂ . Subsequently, the obtained hydroxide was sufficiently washed with water, and then dried to obtain a precursor of a positive electrode active material.

The thus obtained precursor and lithium carbonate were mixed so that the molar ratio of lithium, cobalt and aluminum became 1.02:0.999:0.001. The resulting mixture was prebaked at 600° C. for 10 hours and then crushed. Subsequently, the ground baked material was baked again at 900° C. for 10 hours, and then ground and classified to obtain a lithium-containing composite oxide represented by the formula Li _1.02 Co _0.999 Al _0.001 O ₂ . The obtained lithium-containing composite oxide is referred to as positive electrode active material 1-1.

3 kg of the positive electrode active material thus obtained, 1 kg of polyvinylidene fluoride (#1320 (trade name) containing 12% by weight as a positive electrode binder, manufactured by Kureha Chemical Industry Co., Ltd., were stirred with a double-arm kneader. Obtained) a solution of N-methyl-pyrrolidone (hereinafter referred to as NMP), 90 g of acetylene black as a conductive agent and an appropriate amount of NMP to prepare the positive electrode material mixture coating.

This coating agent was applied to both surfaces of a 15 μm thick aluminum foil serving as a positive electrode current collector. At this time, the above-mentioned paint was not applied to the positive electrode lead connection portion.

Next, the paint thus applied was dried, and then rolled to form a cathode material mixture layer having an active material density (weight of active material/volume of material mixture layer) of 3.3 g/cm ³ . The total thickness of the positive electrode current collector and the positive electrode material mixture layer was 160 μm.

After that, the precursor for electrode sheet thus obtained was cut to a width capable of being inserted into a battery case for a cylindrical battery (diameter: 18 mm and length: 65 mm), thereby obtaining a positive electrode sheet.

(b) Preparation of negative electrode

Stir 3kg of artificial graphite as the negative electrode active material, 75g of modified material (by Zeon Corporation, Japan obtained "BM -400B (trade name)) aqueous dispersion, 30 g of carboxymethyl cellulose as a thickener and an appropriate amount of water to prepare the negative electrode material mixture coating. The coating thus obtained is applied to a 10 μm negative electrode current collector Both sides of the thick copper foil. At this time, the paint was not applied to the negative electrode lead connection portion.

Next, the paint thus applied was dried, and then rolled with a roll to form a negative electrode material mixture layer having an active material density of 1.4 g/cm ³ . The total thickness of the copper foil and negative electrode material mixture layer was 180 μm.

After that, the precursor for an electrode sheet thus obtained was cut to a width so as to be inserted into a battery case for a cylindrical battery as described above, thereby obtaining a negative electrode sheet.

(c) Preparation of diaphragm

A laminated film including a 16 μm thick porous film made of polyethylene (PE) and a film made of a heat-resistant resin aramid resin was prepared. This laminated film was used as a separator.

The preparation method of above-mentioned laminated film is as follows:

To 100 parts by weight of NMP, 6.5 parts by weight of dry anhydrous calcium chloride was added. The resulting mixture was heated to 80° C. in a reaction bath to completely dissolve anhydrous calcium chloride in NMP.

After the temperature of the solution thus obtained returned to normal temperature, 3.2 parts by weight of p-phenylenediamine was added to the solution and completely dissolved therein. Subsequently, the reaction bath containing the solution containing p-phenylenediamine was placed in a normal temperature bath at 20°C. While keeping the temperature at 20°C, 5.8 parts by weight of terephthaloyl dichloride was dropped into the solution for reaction within 1 hour, thereby obtaining poly-p-phenylene-p-benzamide (hereafter referred to as for PPTA).

Subsequently, the solution containing PPTA was left to stand in a normal temperature bath at 20° C. for 1 hour. After the reaction was complete, the PPTA-containing solution was placed in a vacuum bath and then degassed for 30 minutes while stirring under reduced pressure.

Furthermore, the polymerization solution thus obtained was diluted with a calcium chloride-added NMP solution, thereby preparing a PPTA-NMP solution having a PPTA concentration of 1.4% by weight.

The NMP solution of PPTA thus obtained was thinly applied to a porous film made of polyethylene with a spatula, and then dried with hot air at 80° C. (air flow rate: 0.5 m/sec). The PTAA film thus obtained was sufficiently washed with pure water to remove calcium chloride so that the film became porous, and then dried again. In this way, a laminated film comprising a porous film made of polyethylene and a porous film of PTAA formed thereon was prepared.

Measurement of the residual chlorine content in the laminated film by chemical analysis revealed that the residual chlorine content was 650 μg per 1 g of the laminated film.

(d) Preparation of non-aqueous electrolyte

LiPF ₆ was dissolved at a concentration of 1 mol/L in a solvent mixture containing ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate in a volume ratio of 2:3:3. To the resulting solution was added vinylene carbonate to prepare a nonaqueous electrolyte. The content of the vinylene carbonate was 3 parts by weight per 100 parts by weight of the non-aqueous electrolyte.

(e) Preparation of battery

A cylindrical battery as shown in Figure 1 was prepared.

First, the positive electrode sheet and the negative electrode sheet obtained in the above manner were cut into predetermined lengths to provide the positive electrode 11 and the negative electrode 12 . One end of the positive electrode lead 14 is connected to the positive electrode lead connection portion of the positive electrode 11 . One end of the negative electrode lead is connected to the negative electrode lead connecting portion of the negative electrode 12 .

The separator 13 was inserted between the positive electrode 11 and the negative electrode 12, and the whole was rolled to prepare a cylindrical electrode assembly. At this time, the separator 13 was placed between the positive electrode 11 and the negative electrode 12 so that the PTAA layer was on the positive electrode side. The outermost part of the coiled electrode is covered with a separator 13 .

The electrode assembly thus obtained was sandwiched between an upper insulating ring 16 and a lower insulating ring 17 , and the whole was placed in a battery case 18 . Subsequently, 5 g of the above-mentioned non-aqueous electrolyte (not shown) was injected into the battery case 18 . Subsequently, the internal pressure of the battery case 18 was lowered to 133 Pa, and left to stand until no residue of the nonaqueous electrolyte was observed on the surface of the electrode assembly, thereby impregnating the nonaqueous electrolyte into the electrode assembly.

Subsequently, the other end of the positive electrode lead 14 is welded to the back of the battery cover with insulating packing 20 placed around it, and the other end of the negative electrode lead 15 is welded to the bottom surface of the interior of the battery case 18. Finally, the open end of the battery case 18 was crimped onto the insulating package 20 of the battery cover 19 to seal the opening of the battery case 18, thereby completing the cylindrical lithium ion secondary battery. The battery thus produced is referred to as the battery of Example 1-1.

(Example 1-2 to 1-4)

A battery was prepared in the same manner as in Example 1-1, except that the concentration ratio of cobalt sulfate to aluminum sulfate was changed to 0.95:0.05, 0.80:0.20 or 0.75:0.25 when synthesizing the precursor of the cathode active material. The batteries thus obtained are referred to as batteries of Examples 1-2 to 1-4.

(Example 1-5)

A battery was prepared in the same manner as in Example 1-1, except that when synthesizing the precursor of the positive electrode active material, iron sulfate was additionally added, and the concentration ratio of cobalt sulfate, iron sulfate, and aluminum sulfate was changed to 0.9:0.05:0.05. The battery thus obtained is referred to as the battery of Example 1-5.

(Example 1-6 to 1-9)

A battery was prepared in the same manner as in Example 1-2, except that when synthesizing the positive electrode active material, the precursor of the positive electrode active material and lithium carbonate were mixed so that the molar ratio of lithium, cobalt, and aluminum was 0.98:0.95:0.05, 1: 0.95:0.05, 1.05:0.95:0.05 or 1.08:0.95:0.05. The batteries thus obtained are referred to as batteries of Examples 1-6 to 1-9.

(Example 1-10)

A battery was prepared in the same manner as in Example 1-2, except that a laminated film in which a film made of polyamideimide resin was formed on a porous film made of polyethylene was used instead of the PTAA film as a separator. The batteries thus produced are referred to as batteries of Examples 1-10.

A method of producing a laminated film comprising a porous film made of polyethylene and a film made of polyamideimide resin formed thereon will be described below:

At room temperature, trimellitic anhydride monochloride and diamine were added to NMP and mixed to obtain an NMP solution of polyamic acid. The resulting NMP solution was thinly applied to a porous film made of polyethylene with a spatula. The coated film was dried with hot air at 80° C. (air flow rate: 0.5 m/sec), thereby dehydrating and cyclizing the polyamic acid to convert into polyamideimide. A laminated film comprising a porous film made of polyethylene and a film made of polyamideimide formed thereon was thus produced. The total thickness of the laminated film was 20 μm.

Measurement of the residual chlorine content in the laminated film by chemical analysis revealed that the residual chlorine content was 830 µg per 1 g of the laminated film.

(Example 1-11)

A battery was prepared in the same manner as in Example 1-2, except that a porous film made of aramid was used as a separator. The battery thus obtained is referred to as the battery of Example 1-11.

The preparation method of the porous membrane made of aramid will be described below:

A predetermined amount of aramid resin was dissolved in NMP in the manner described above. Subsequently, the NMP solution was applied to a smooth stainless steel sheet with a spatula. The thus-obtained coated film was dried with hot air at 80° C. (air flow rate: 0.5 m/sec) to provide a film made only of aramid. The thickness of the porous membrane was 20 μm.

Measurement of the residual chlorine content in the porous film by chemical analysis revealed that the residual chlorine content was 1800 µg per 1 g of the laminated film.

(Example 1-12)

A battery was produced in the same manner as in Example 1-2 except that a laminate comprising a porous film made of polyethylene and a layer containing a filler and an aramid resin formed thereon was used as a separator. The batteries thus produced were referred to as batteries of Examples 1-12.

The preparation method of the above-mentioned laminate will be described below:

To the NMP solution of the aramid resin prepared in Example 1-1 above, alumina fine particles were added and stirred. The amount of alumina fine particles added was 200 parts by weight per 100 parts by weight of the aramid resin contained in the NMP solution.

The dispersion thus obtained was thinly applied onto a porous film made of polyethylene with a doctor blade, and then the coated film was dried with hot air at 80° C. (air flow rate: 0.5 m/sec). A laminate comprising a porous film made of polyethylene and a layer containing filler and aramid formed thereon was thus produced.

The determination of the residual chlorine content in the laminate by chemical analysis showed that the residual chlorine content was 600 μg per 1 g of the separator.

(comparative example 1)

Lithium-containing composite oxide was synthesized in the same manner as in Example 1-1, except that cobalt hydroxide was synthesized from cobalt sulfate only, and then lithium carbonate and cobalt hydroxide were mixed so that the molar ratio of lithium to cobalt became 1.02:1. A battery was produced in the same manner as in Example 1-1, except that the lithium-containing composite oxide thus obtained was used as the cathode active material. The battery thus produced is referred to as a battery of Comparative Example 1.

(comparative example 2)

A battery was prepared in the same manner as in Example 1-2 except that a 20 μm thick porous film made of polyethylene was used as a separator. The battery thus produced is referred to as a battery of Comparative Example 2.

Each battery thus obtained was subjected to precharge and discharge twice, in which each battery was discharged at a constant current of 400 mA until the battery voltage dropped to 3V, and then charged at a constant current of 1400 mA until the battery voltage reached 4.2V. Subsequently, the charged battery was stored at 45° C. for 7 days. The batteries after storage were evaluated as follows.

[evaluate]

(i) Measuring discharge capacity

The stored battery was subjected to a first charge and discharge cycle once. In the first charge and discharge cycle, the battery was charged at a constant voltage of 4.2V at 20°C until the current value decreased to 100mA, and then the charged battery was discharged at a constant current of 2000mA until the battery voltage decreased to 3V. The discharge capacity at this time is called an initial discharge capacity. The results are shown in Table 1.

(ii) Security testing

Charge the stored battery at a constant voltage of 4.2V at 20°C until the current value drops to 100mA. Subsequently, the charged battery was placed in a constant temperature bath at 130° C. to measure the maximum temperature of the battery surface. The results are shown in Table 1.

(iii) High temperature storage characteristics

First, the initial discharge capacity was measured in the manner described above. Subsequently, the battery was charged at a constant voltage of 4.2V at 20°C until the current value decreased to 100mA. Then, the charged battery was placed in a constant temperature bath at 90 °C and stored for 24 hours. The stored battery was discharged at a constant current of 2000 mA to obtain the stored discharge capacity. The ratio of the stored discharge capacity to the initial discharge capacity was calculated as a percentage. This ratio is called the capacity recovery ratio. The results are shown in Table 1.

It should be noted that Table 1 also shows the composition of the positive electrode active material and the type of separator used in Examples and Comparative Examples.

Table 1

Li _a Co _1-bc Fe _b Al _c O ₂ diaphragm Initial discharge capacity (mAh) Maximum battery surface temperature (°C) Capacity recovery rate (%) a 1-b-c b c Example 1-1 1.02 0.999 0 0.001 Aramid+PE 2050 142 66 Example 1-2 1.02 0.95 0 0.05 Aramid+PE 2020 139 70 Example 1-3 1.02 0.8 0 0.2 Aramid+PE 2000 140 71 Example 1-4 1.02 0.75 0 0.25 Aramid+PE 1890 139 73 Example 1-5 1.02 0.9 0.05 0.05 Aramid+PE 2015 141 72 Example 1-6 0.98 0.95 0 0.05 Aramid+PE 1900 144 71 Example 1-7 1 0.95 0 0.05 Aramid+PE 1950 143 70 Example 1-8 1.05 0.95 0 0.05 Aramid+PE 1970 141 72 Example 1-9 1.08 0.95 0 0.05 Aramid+PE 1880 142 70 Example 1-10 1.02 0.95 0 0.05  Polyamideimide+PE 2020 144 69 Example 1-11 1.02 0.95 0 0.05 Aramid 2020 137 72 Example 1-12 1.02 0.95 0 0.05 (aramid+filler)+PE 2020 142 71 Comparative example 1 1.02 1 0 0 Aramid+PE 2050 141 50 Comparative example 2 1.02 0.95 0 0.05 PE 2020 156 72

As shown in Table 1, in the battery of Comparative Example 2, which did not contain a heat-resistant resin in the separator, the maximum temperature of the battery surface rose to 156°C. This indicates that when the separator does not contain the heat-resistant resin, the safety of the battery is reduced.

As apparent from the results of Comparative Example 1, even when the separator contained a heat-resistant resin, in the case where the positive electrode active material did not contain aluminum atoms, the capacity recovery rate was significantly lowered. It is conceivable that this is due to the release of chlorine groups as terminal groups contained in the heat-resistant resin into the non-aqueous electrolyte under a high-temperature environment, and the acceleration of the main component elements of the positive electrode active material (in the case of Comparative Example 1, Cobalt) leakage.

On the other hand, as in the case of the batteries of Examples 1-1 to 1-12, in the case of using a separator containing a heat-resistant resin and a positive electrode active material containing an aluminum atom in its composition, the safety at high temperature Both safety and storage properties are favorable. It is conceivable that this is due to the fact that the aluminum atoms in the positive electrode active material form a stable complex with the chlorine atoms released from the aramid (or polyamideimide), so the aluminum atoms are selectively removed from the positive electrode active material. Leakage, so that the leakage of other component elements of the positive electrode active material can be suppressed. It is to be noted that the effect can also be obtained as in the case of the batteries of Examples 1 to 5 in which a positive electrode active material containing a metal such as iron in addition to cobalt is used in its composition.

The results of Examples 1-1 to 1-4 show that the greater the aluminum content in the positive electrode active material, the lower the maximum temperature of the battery, and the more the capacity recovery rate increases. However, as shown in Examples 1-4, the excessive increase of the aluminum content decreased the ratio of the main component elements in the cathode active material, thus reducing the initial discharge capacity.

Also, the results of Examples 1-2 and 1-6 to 1-8 show that when the lithium content in the cathode active material is too large or too small, the initial discharge capacity decreases. It is conceivable that when the lithium content in the positive electrode active material is too small, the content of impurities such as cobalt oxide that does not contribute to the battery capacity increases, and the battery capacity is reduced; when the lithium content is too large, excess lithium exists in the positive electrode active material as impurities. materials, reduce the initial discharge capacity.

For this reason, in _the lithium-containing composite oxide represented by _{LixCo1 -yAlyO2 ,} 1≤x≤1.05 and 0.001≤y≤0.2 are preferable.

Furthermore, the results of Examples 1-10 to 1-12 show that the effects as described above can also be obtained in the case where a laminated film comprising a porous film and a film containing a heat-resistant resin is used as a separator; A porous film made of a thermal resin is used as the separator; and a laminate having a porous film and a layer containing a filler and an aramid resin is used therein as the separator.

(Example 2-1 to 2-12)

Precursors 2-1 to 2-12 were synthesized in the same manner as in Example 1-1, except that when synthesizing the precursor of the positive electrode active material, magnesium sulfate was additionally added, and cobalt sulfate and magnesium sulfate were changed as shown in Table 2. and the concentration ratio of aluminum sulfate. Then, positive electrode active materials 2-1 to 2-12 were synthesized in the same manner as in Example 1-1 except that the mixing ratio of the precursors 2-1 to 2-12 thus obtained and lithium carbonate was changed as shown in Table 2. Batteries were prepared using these cathode active materials in the same manner as in Example 1-1. The batteries thus obtained are referred to as batteries of Examples 2-1 to 2-12.

The respective batteries thus obtained were subjected to the same precharging and discharging as in Example 1 twice. The charged batteries were stored at 45°C for 7 days. For the stored battery, the initial discharge capacity, the maximum temperature of the battery surface, and the capacity recovery rate were measured in the same manner as in Example 1. The results are shown in Table 2.

(iv) Capacity retention rate

In this embodiment, the capacity retention rate of the battery stored at 45°C for 7 days was further measured. The capacity retention rate was determined as follows. For the batteries after storage, the above charge and discharge cycles were repeated 200 times. The ratio of the discharge capacity at the 200th cycle to the discharge capacity at the first cycle was calculated as a percentage. This ratio is called the capacity retention ratio. The results are shown in Table 2.

Table 2

Li _a Co _1-bc Mg _b Al _c O ₂ Initial discharge capacity (mAh) Maximum battery surface temperature (°C) Capacity recovery rate (%) Capacity retention rate (%) a 1-b-c b c Example 2-1 1.02 0.979 0.02 0.001 2050 143 67 88 Example 2-2 1.02 0.93 0.02 0.05 2020 140 71 91 Example 2-3 1.02 0.78 0.02 0.2 2000 142 73 92 Example 2-4 1.02 0.77 0.02 0.21 1950 140 74 93 Example 2-5 1.02 0.949 0.001 0.05 2015 142 73 80 Example 2-6 1.02 0.945 0.005 0.05 1970 144 73 93 Example 2-7 1.02 0.85 0.1 0.05 1950 144 71 92 Example 2-8 1.02 0.8 0.15 0.05 1900 142 72 87 Example 2-9 0.98 0.93 0.02 0.05 1946 141 72 91 Example 2-10 1 0.93 0.02 0.05 1999 141 73 93 Example 2-11 1.05 0.93 0.02 0.05 1999 142 73 92 Example 2-12 1.08 0.93 0.02 0.05 1946 142 73 91

In Table 2, the capacity retention ratios of the respective batteries were 80% or higher. This indicates that the addition of magnesium to the positive electrode active material moderates the expansion and contraction of the positive electrode active material associated with charge and discharge, and thus suppresses the decrease in discharge capacity.

The results of Examples 2-2 and 2-5 to 2-8 showed that the higher the molar ratio b of magnesium in the positive electrode active material, the more the capacity retention was improved. However, in Example 2-5 in which the molar ratio b was 0.001, the capacity retention rate was 80%, indicating that sufficient cycle characteristics could not be obtained.

On the other hand, when the molar ratio b of magnesium increases, the ratio of the main component elements in the cathode active material decreases, and thus the initial discharge capacity tends to decrease. In other words, in Example 2-8 in which the molar ratio b was 0.15, sufficient initial discharge capacity could not be obtained.

Also, as shown in Examples 2-1 to 2-4 and Examples 2-9 to 2-12, with respect to the aluminum content and the lithium content, the same trends as in Example 1 were observed.

For this reason, among lithium-containing composite oxides represented by Li _a Co _1-bc Mg _b Al _c O ₂ , 1≤a≤1.05, 0.005≤b≤0.1 and 0.001≤c≤0.2 are preferable.

(Example 3-1 to 3-12)

Precursors 3-1 to 3-12 were synthesized in the same manner as in Example 1-1, except that nickel sulfate, cobalt sulfate, and aluminum sulfate were used when synthesizing the precursor of the positive electrode active material, and changed as shown in Table 3 their concentration ratio. Then, positive electrode active materials 3-1 to 3-12 were synthesized in the same manner as in Example 1-1, except that the mixing ratio of the thus obtained precursors 3-1 to 3-12 and lithium carbonate was changed as shown in Table 3 . Batteries were prepared using these cathode active materials in the same manner as in Example 1-1. The batteries thus obtained are referred to as batteries of Examples 3-1 to 3-12. The respective batteries thus obtained were subjected to the same precharging and discharging as in Example 1 twice. The charged batteries were stored at 45°C for 7 days. For the batteries after storage, the initial discharge capacity, the maximum temperature of the battery surface, the capacity recovery rate and the capacity retention rate were measured in the same manner as in Example 2. The results are shown in Table 3.

table 3

Li _a Ni _1-bc Co _b Al _c O ₂ Initial discharge capacity (mAh) Maximum battery surface temperature (°C) Capacity recovery rate (%) Capacity retention rate (%) a 1-b-c b c Example 3-1 1.01 0.849 0.15 0.001 2250 144 48 87 Example 3-2 1.01 0.8 0.15 0.05 2100 141 77 88 Example 3-3 1.01 0.65 0.15 0.2 2069 142 83 89 Example 3-4 1.01 0.64 0.15 0.21 2030 141 85 91 Example 3-5 1.01 0.945 0.005 0.05 2350 143 73 81 Example 3-6 1.01 0.85 0.1 0.05 2150 145 74 87 Example 3-7 1.01 0.6 0.35 0.05 2100 145 73 91 Example 3-8 1.01 0.5 0.45 0.05 1950 143 74 93 Example 3-9 0.98 0.8 0.15 0.05 2009 141 82 87 Example 3-10 1 0.8 0.15 0.05 2082 142 83 88 Example 3-11 1.05 0.8 0.15 0.05 2054 142 84 89 Example 3-12 1.08 0.8 0.15 0.05 1917 141 81 88

The results in Table 3 show that when the positive electrode active material contains nickel and cobalt, and the content of nickel is large, the initial discharge capacity and capacity retention rate are improved.

Also, the results of Examples 3-5 to 3-8 show that the greater the content of nickel contained in the positive electrode active material, that is, the smaller the content of cobalt, the more the initial discharge capacity is improved. However, in the case of Example 3-8 in which the molar ratio b of cobalt was 0.45, sufficient initial discharge capacity could not be obtained.

Also, in the case of Example 3-5 in which the molar ratio b of cobalt was 0.005, the capacity retention rate was lowered to some extent. It is conceivable that this is because the expansion and contraction of the positive electrode active material associated with charge and discharge cannot be sufficiently moderated.

Also, as shown in Examples 3-1 to 3-4 and Examples 3-9 to 3-12, with respect to the aluminum content and the lithium content, the same trends as in Example 1 were observed.

For this reason, among lithium-containing composite oxides represented by Li _a Ni _1-bc Co _b Al _c O ₂ , 1≤a≤1.05, 0.1≤b≤0.35 and 0.001≤c≤0.2 are preferable.

Example 4

(Example 4-1 to 4-19)

Precursors 4-1 to 4-19 were synthesized in the same manner as in Example 1-1, except that nickel sulfate, manganese sulfate, cobalt sulfate and aluminum sulfate were used when synthesizing the precursor of the positive electrode active material, and as in Table 4 Change their concentration ratios as shown. Then, positive electrode active materials 4-1 to 4-19 were synthesized in the same manner as in Example 1-1, except that the mixing ratio of the thus obtained precursors 4-1 to 4-19 and lithium carbonate was changed as shown in Table 4 . Batteries were prepared using these cathode active materials in the same manner as in Example 1-1. The batteries thus obtained are referred to as batteries of Examples 4-1 to 4-19.

The respective batteries thus obtained were subjected to the same precharging and discharging as in Example 1 twice. The charged batteries were stored at 45°C for 7 days. For the stored battery, the initial discharge capacity, the maximum temperature of the battery surface, and the capacity recovery rate were measured in the same manner as in Example 1. The results are shown in Table 4.

It should be noted that Table 4 also shows the values of b+c+d.

Table 4

Li _a Ni _1-(b+c+d) Mn _b Co _c Al _d O ₂ Initial discharge capacity (mAh) Maximum battery surface temperature (°C) Capacity recovery rate (%) a 1-(b+c+d) b c d b+c+d Example 4-1 1.01 0.339 0.33 0.33 0.001 0.661 1890 144 69 Example 4-2 1.01 0.31 0.32 0.32 0.05 0.69 1862 138 73 Example 4-3 1.01 0.26 0.27 0.27 0.2 0.74 1710 139 74 Example 4-4 1.01 0.27 0.26 0.26 0.21 0.73 1690 138 76 Example 4-5 1.01 0.44 0.19 0.32 0.05 0.56 1950 141 72 Example 4-6 1.01 0.19 0.57 0.19 0.05 0.81 1650 143 73 Example 4-7 0.98 0.31 0.32 0.32 0.05 0.69 1781 137 74 Example 4-8 1 0.31 0.32 0.32 0.05 0.69 1846 137 73 Example 4-9 1.05 0.31 0.32 0.32 0.05 0.69 1822 137 72 Example 4-10 1.08 0.31 0.32 0.32 0.05 0.69 1700 138 73 Example 4-11 1.01 0.85 0.05 0.05 0.05 0.15 2100 148 76 Example 4-12 1.01 0.75 0.10 0.10 0.05 0.25 1972 143 75 Example 4-13 1.01 0.5 0.20 0.25 0.05 0.5 1930 142 73

Example 4-14 1.01 0.25 0.50 0.20 0.05 0.75 1700 139 72 Example 4-15 1.01 0.25 0.20 0.50 0.05 0.75 1750 140 71 Example 4-16 1.01 0.25 0.60 0.10 0.05 0.75 1630 138 71 Example 4-17 1.01 0.25 0.10 0.60 0.05 0.75 1695 141 73 Example 4-18 1.01 0.15 0.60 0.20 0.05 0.85 1620 137 72 Example 4-19 1.01 0.15 0.20 0.60 0.05 0.85 1640 142 72

When the positive electrode active material contains manganese in addition to nickel and cobalt, an inexpensive positive electrode active material can be obtained while maintaining stable battery performance. In order to achieve cost reduction, a certain amount or more of manganese is required. In the case of Example 4-1 in which the molar ratio b of manganese was 0.05, the maximum temperature of the battery surface increased, and the safety of the battery decreased to some extent. In the case of Examples 4-16 and 4-18 in which the molar ratio b of manganese was 0.6, the initial discharge capacity decreased.

Also, as evident from Examples 4-11, when the cobalt molar ratio c was 0.05, the maximum temperature of the battery surface increased. In the case of Examples 4-17 and 4-19 in which the molar ratio c of cobalt was 0.6, the initial discharge capacity decreased.

Also, as shown in Examples 4-1 to 4-4 and Examples 4-7 to 4-10, with respect to the aluminum content and the lithium content, the same trends as in Example 1 were observed.

For this reason, in lithium-containing composite oxides represented by Li _a Ni _1-(b+c+d) Mn _b Co _c Al _d O ₂ , preferably 1≤a≤1.05, 0.1≤b≤0.5, 0.1≤c ≤0.5 and 0.001≤d≤0.2.

In addition, in Examples 4-18 and 4-19 in which b+c+d was 0.85, it was observed that the initial discharge capacity tended to decrease. In Examples 4-11 in which b+c+d is 0.15, it was observed that the maximum temperature of the battery surface tended to increase, and the safety of the battery tended to decrease to a certain extent. Therefore, this indicates that when 0.2≤b+c+d≤0.75, a battery in which the three characteristics are well-balanced can be obtained.

In the following examples, battery performance was evaluated in cases where a mixture of multiple lithium-containing composite oxides was used as the positive electrode active material; where the positive electrode active material was exposed to a high-pressure environment; and where the kind of the negative electrode active material was changed.

Example 5

(Example 5-1)

50 parts by weight of the positive electrode active material (Li _1.02 Co _0.95 Al _0.05 O ₂ ) used in Example 1-2 and 50 parts by weight of the positive electrode active material used in Example 4-2 (Li _1.01 Ni _0.32 Mn _0.32 Co _0.32 Al _0.05 O ₂ ) were mixed to obtain a powder used as the positive electrode active material 5-1. A battery was prepared in the same manner as in Example 1-1, except that the cathode active material was used. The battery thus obtained is referred to as the battery of Example 5-1.

(Example 5-2)

A battery was prepared in the same manner as in Example 1-2, except that the density of the positive electrode active material in the positive electrode material mixture layer was changed to 3.3 g/cm ³ , and the thickness of the positive electrode sheet was changed to 144 μm. The battery thus obtained is referred to as the battery of Example 5-2.

(Example 5-3)

A battery was prepared in the same manner as in Example 4-2, except that the density of the positive electrode active material in the positive electrode material mixture layer was changed to 3.3 g/cm ³ and the thickness of the positive electrode sheet was changed to 144 μm. The battery thus obtained is referred to as the battery of Example 5-3.

(Example 5-4)

In a double-arm kneader, 3 kg of elemental silicon (Si) powder (median particle size: 10 μm) as a negative electrode active material, 750 g of modified styrene-butadiene rubber particles containing 40% by weight as a binder were stirred (BM-400B (trade name) obtained by Zeon Corporation, Japan), 600 g of acetylene black as a conductive agent, 300 g of carboxymethylcellulose as a thickener, and an appropriate amount of water as a dispersion medium to prepare Anode material mixture paste. This anode material mixture paste was applied to both sides of a 10 μm thick strip-shaped anode current collector made of copper foil. The negative electrode material mixture paste thus applied was dried and rolled with a roll to obtain a negative electrode sheet. A battery was prepared in the same manner as in Example 3-2, except that the negative electrode sheet was used. The battery thus obtained is referred to as the battery of Example 5-4.

(Example 5-5)

A battery was prepared in the same manner as in Example 5-4, except that SiO powder (median particle diameter: 8 μm) was used instead of silicon powder, and the sizes of the positive and negative electrodes were appropriately changed. The battery thus obtained is referred to as the battery of Example 5-5.

(Example 5-6)

Negative electrodes as described below were prepared using a vacuum vapor deposition apparatus comprising a vacuum chamber in which a water-cooled roll was disposed.

An electrolytic Cu foil (manufactured by FURUKAWA CIRCUIT FOILCO., Ltd., thickness: 20 μm) serving as a current collector was adhered and fixed to a water-cooled roll in a vacuum vapor deposition apparatus. Under the roller, a crucible made of graphite in which silicon (obtained from Furuuchi Chemical Corporation, ingot purity of 99.999%) was placed was placed. Install a nozzle in the vacuum chamber so that oxygen can be introduced between the crucible and the Cu foil. The flow rate of oxygen (obtained from NIPPON SANSO CORPORATION with a purity of 99.7%) from the nozzle was set at 20 scm (20 cm ³ flow per minute). In order to prevent excessive adhesion of silicon, a shield plate made of stainless steel with openings was placed between the crucible and the water-cooled roll. The opening has a width of 10 mm in the direction of rotation of the roll. On the opening of the shielding plate, a shutter is provided to prevent evaporation and adhesion until the evaporation temperature is reached.

An electron gun is used to vapor-deposit silicon on the current collector. The accelerating voltage of the electron beam was set at -8 kV, and the emission of the electron beam was set at 150 mA.

The vacuum degree in the vacuum chamber is 1.5×10 ^-1 Pa, and the rotation speed of the water-cooled roller is 10 cm/min. The surface temperature of the water-cooled roll was set at 20°C.

An active material containing silicon and oxygen is coated on one face of the current collector by vapor deposition, and then, in the same manner, the active material is coated on the other face. Thus, an anode in which a thin film composed of an active material is supported on both sides of a current collector was produced.

The composition of the negative active material was quantified by elemental analysis. The results showed that the composition of the negative electrode active material was SiO _0.6 .

A battery was prepared in the same manner as in Example 5-4, except that the negative electrode thus obtained was used and the sizes of the positive and negative electrodes were appropriately changed. The battery thus produced was referred to as the battery of Example 5-6.

Each of the batteries of Examples 5-1 and 5-4 to 5-6 was subjected to the same precharging and discharging as in Example 1 twice. The charged batteries were stored at 45°C for 7 days. For the stored battery, the initial discharge capacity, the maximum temperature of the battery surface, and the capacity recovery rate were measured in the same manner as in Example 1. The results are shown in Table 5.

Each of the batteries of Examples 5-2 and 5-3 was subjected to the same precharge and discharge twice as in Example 1 except that the end-of-charge voltage was changed to 4.4V. The charged batteries were stored at 45°C for 7 days. Subsequently, the batteries after storage were evaluated as follows.

(v) Measuring discharge capacity

The stored battery was charged at a constant voltage of 4.4V at 20°C until the current value dropped to 100mA, and then the charged battery was discharged at a constant current of 2000mA until the battery voltage dropped to 3V to obtain an initial discharge capacity. The results are shown in Table 5.

(ii) Security testing

The stored battery was charged at a constant voltage of 4.4V at 20°C until the current value decreased to 100mA. Subsequently, the charged battery was placed in a constant temperature bath at 130° C. to measure the maximum temperature of the battery surface. The results are shown in Table 5.

(iii) High temperature storage characteristics

First, the initial discharge capacity was measured in the manner described above. Subsequently, the battery was charged at a constant voltage of 4.4V at 20°C until the current value decreased to 100mA. Then, the charged battery was placed in a constant temperature bath at 90 °C and stored for 24 hours. The stored battery was discharged at a constant current of 2000 mA to obtain the stored discharge capacity. The ratio of the stored discharge capacity to the initial discharge capacity was calculated as a percentage. This ratio is called the capacity recovery ratio. The results are shown in Table 5.

table 5

positive active material Negative active material Charging voltage (V) Initial discharge capacity (mAh) Maximum battery surface temperature (°C) Capacity recovery rate (%) Example 5-1 Li _1.02 Co _0.95 Al _0.05 O ₂ +Li _1.01 Ni _0.32 Mn _0.32 Co _0.32 Al _0.05 O ₂ Artificial graphite 4.2 1941 138 72 Example 5-2 Li _1.02 Co _0.95 Al _0.05 O ₂ Artificial graphite 4.2 2222 141 66 Example 5-3 Li _1.01 Ni _0.32 Mn _0.32 Co _0.32 Al _0.05 O ₂ Artificial graphite 4.4 2048 139 67 Example 5-4 Li _1.01 Ni _0.8 Co _0.15 Al _0.05 O ₂ silicon 4.2 2310 140 70 Example 5-5 Li _1.01 Ni _0.8 Co _0.15 Al _0.05 O ₂ SiO 4.2 2373 140 72 Example 5-6 Li _1.01 Ni _0.8 Co _0.15 Al _0.05 O ₂ SiO _0.6 4.2 2394 138 74

As shown in Table 5, each battery of Examples was excellent in initial discharge capacity and capacity recovery rate, and the maximum temperature of the battery surface was not high. This indicates that a battery excellent in safety and high-temperature storage characteristics can be obtained in the case where a mixture containing two types of lithium-containing composite oxides is used as the positive electrode active material (Example 5-1); where the positive electrode active The material was exposed to a high-pressure environment (Examples 5-2 to 5-3); and wherein a negative electrode with a high capacity was used (Examples 5-4 to 5-6).

Industrial Applicability

According to the present invention, since the positive electrode active material contains an appropriate amount of aluminum, if chlorine atoms as terminal groups in the heat-resistant resin contained in the separator are released into the nonaqueous electrolyte, leakage of the main component elements of the positive electrode active material into the nonaqueous electrolyte can be suppressed. in non-aqueous electrolytes. As a result, a nonaqueous electrolyte secondary battery having excellent safety and improved high-temperature storage characteristics can be obtained. The battery can be used as an energy source for devices requiring excellent battery performance even in a high-temperature environment.

claims

(Amended in accordance with Article 19 of the Treaty)

1. A nonaqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, a nonaqueous electrolyte and a separator, wherein:

the separator comprises a heat-resistant resin having chlorine atoms as terminal groups, and

The positive electrode active material contains a lithium-containing composite oxide containing an aluminum atom in its composition, and

The lithium-containing composite oxide is represented by the following formula:

Li _x M _1-y Al _y O ₂ (1)

Wherein, 1≤x≤1.05, 0.001≤y≤0.2 and M is at least one selected from Co, Ni, Mn and Mg.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the heat-resistant resin includes at least one selected from the group consisting of aramid and polyamideimide.

3. The nonaqueous electrolyte secondary battery as claimed in claim 1, wherein the separator includes a film containing the heat-resistant resin and a film containing polyolefin laminated on the film containing the heat-resistant resin .

4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the separator includes a polyolefin-containing film and a layer containing the heat-resistant resin and filler formed on the polyolefin-containing film.

5. The non-aqueous electrolyte secondary battery as claimed in claim 1, wherein the lithium-containing composite oxide is represented by the following formula:

Li _a Co _1-bc Mg _b Al _c O ₂ (2)

Among them, 1≤a≤1.05, 0.005≤b≤0.1 and 0.001≤c≤0.2.

6. The nonaqueous electrolyte secondary battery as claimed in claim 1, wherein the lithium-containing composite oxide is represented by the following formula:

Li _a Ni _1-bc Co _b Al _c O ₂ (3)

Among them, 1≤a≤1.05, 0.1≤b≤0.35 and 0.001≤c≤0.2.

7. The nonaqueous electrolyte secondary battery as claimed in claim 1, wherein the lithium-containing composite oxide is represented by the following formula:

Li _a Ni _1-(b+c+d) Mn _b Co _c Al _d O ₂ (4)

Among them, 1≤a≤1.05, 0.1≤b≤0.5, 0.1≤c≤0.5, 0.001≤d≤0.2 and 0.2≤b+c+d≤0.75.

Claims (8)

1. rechargeable nonaqueous electrolytic battery, it comprises the positive pole that contains positive electrode active materials, contains the negative pole of negative active core-shell material, nonaqueous electrolyte and barrier film, wherein:

Described barrier film comprise have the chlorine atom as the heat stable resin of end group and

Described positive electrode active materials comprises the lithium-contained composite oxide that contains the aluminium atom in its composition.

2. rechargeable nonaqueous electrolytic battery as claimed in claim 1, wherein, described heat stable resin comprises and is selected from least a in aromatic polyamides and the polyamidoimide.

3. rechargeable nonaqueous electrolytic battery as claimed in claim 1, wherein, described barrier film comprises the film that contains described heat stable resin and laminatedly contains the polyolefinic film that contains on the film of described heat stable resin in this.

4. rechargeable nonaqueous electrolytic battery as claimed in claim 1, wherein, described barrier film comprises and contains polyolefinic film and be formed at the described layer that contains described heat stable resin and filler that contains on the polyolefinic film.

5. rechargeable nonaqueous electrolytic battery as claimed in claim 1, wherein, described lithium-contained composite oxide is represented by following formula:

Li _xM _1-yAl _yO ₂ (1)

Wherein, 1≤x≤1.05,0.001≤y≤0.2 and M are selected from least a among Co, Ni, Mn and the Mg.

6. rechargeable nonaqueous electrolytic battery as claimed in claim 5, wherein, described lithium-contained composite oxide is represented by following formula:

Li _aCo _1-b-cMg _bAl _cO ₂ (2)

Wherein, 1≤a≤1.05,0.005≤b≤0.1 and 0.001≤c≤0.2.

7. rechargeable nonaqueous electrolytic battery as claimed in claim 5, wherein, described lithium-contained composite oxide is represented by following formula:

Li _aNi _1-b-cCo _bAl _cO ₂ (3)

Wherein, 1≤a≤1.05,0.1≤b≤0.35 and 0.001≤c≤0.2.

8. rechargeable nonaqueous electrolytic battery as claimed in claim 5, wherein, described lithium-contained composite oxide is represented by following formula:

Li _aNi _1-(b+c+d)Mn _bCo _cAl _dO ₂ (4)

Wherein, 1≤a≤1.05,0.1≤b≤0.5,0.1≤c≤0.5,0.001≤d≤0.2 and 0.2≤b+c+d≤0.75.

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