JP2002042817A - Lithium secondary battery and its positive electrode producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery having a high capacity, superior charge/discharge cycles and less increasing internal resistance by using a water-soluble polymer superior in organic solvent resistance, as a binder. SOLUTION: The lithium secondary battery comprises a positive electrode containing a positive electrode active material having LiNiO2 compound represented by a formula LixNi1-yMyO2, where M is at least one element selected from the group consisting of Co, Mn, All, B, Ti, Mg and Fe, 0<x<=1.2, and 0<y<=0.25, and the binder for binding the positive electrode active material, and an electrolyte, the BET specific surface area of the LiNiO2 compound being 0.65 m2/g or smaller, the binder having the water-soluble polymer having electrolyte non-swelling property and the positive electrode active material having the surface coated with the water-soluble polymer. The lithium secondary battery is superior in charge/discharge cycling property, with less lowering capacity, less increasing internal resistance and less lowering discharge capacity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery having a positive electrode active material and a binder.

[0002]

2. Description of the Related Art In recent years, a lithium battery is becoming mainstream because of its high weight energy density as a power source for electric equipment such as a cellular phone and a portable video camera.
This lithium battery has a positive electrode active material containing lithium and has a positive electrode capable of releasing lithium as lithium ions at the time of charging and occluding lithium ions at the time of discharging, and a negative electrode active material occluding lithium ions at the time of charging. It is composed of a negative electrode capable of releasing lithium ions at the time of discharge, and a non-aqueous electrolyte in which an electrolyte made of a supporting salt containing lithium in an organic solvent is dissolved.

Further, in such a lithium battery, in order to improve the weight energy density, the positive electrode and the negative electrode are formed in a sheet shape, and the sheet-shaped positive electrode and the negative electrode are formed via a separator formed in the same sheet shape. It is housed in a case in a wound or laminated state. The sheet-like positive electrode and negative electrode have a structure in which a mixture layer containing an active material is formed on the surface of a metal foil serving as a current collector.

Here, as a positive electrode active material used for a lithium battery, Li _x CoO ₂ , Li _x Ni
O ₂ , Li _x Mn ₂ O ₄ , Li _x FeO ₂ , V ₂ O ₅ , Cr
Transition metal oxides such as ₂ O ₅ , MnO ₂ , TiS ₂ and MoS ₂ and chalcogen oxides have been proposed.

As a positive electrode active material for this lithium battery,
Is especially Li_xCoO_TwoAnd Li_xNiO_Two, Li_xMn_TwoO_Four
As a positive electrode active material for 4V class non-aqueous electrolyte lithium batteries
It is known to be promising. Of these Li compounds
Among them, Li_xNiO_TwoHas the largest theoretical capacity, is inexpensive
Promising as a positive electrode active material for batteries that can be supplied stably
ing. Furthermore, this Li_xNiO_TwoIn addition, the additional element M
Li added_xNi_1-yM _yO_TwoUsing a compound represented by
Is also good.

As a method of forming an electrode having such a positive electrode active material into a sheet electrode, as disclosed in Japanese Patent Application Laid-Open No. 2-158055, a powdery active material, a conductive agent, a carboxymethylcellulose aqueous solution, and a polytetrafluoroethylene solution are used. There is known a method in which a paste is prepared by uniformly mixing an aqueous dispersion with a water-based dispersion, and the paste is applied, dried, and rolled on a film-like conductive foil such as a rolled aluminum foil.

At this time, polytetrafluoroethylene,
A resin having excellent organic solvent resistance such as carboxymethyl cellulose is used as a binder. These resins do not swell and dissolve in organic solvents such as ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC), which are widely used as solvents for electrolytes of lithium batteries. In addition, even when the active material expands and contracts by using the lithium battery, the state where the active material is firmly bound can be maintained. Here, when the binder swells or dissolves in the organic solvent, the binding force decreases, and the deterioration of the cycle characteristics increases. As a result, by using these binders having excellent organic solvent resistance, good cycle characteristics as a battery can be realized. Further, when the operating temperature of the battery becomes high, this difference becomes more noticeable.

Further, from the viewpoint of electrode production, the use of such a water-dispersible or water-soluble resin for the binder eliminates the use of an organic solvent during the production of the electrode.
An increase in cost due to the use of an organic solvent can be suppressed. In other words, the elimination of the use of an organic solvent reduces the cost of the solvent required for the organic solvent itself in the electrode manufacturing process, eliminates the need for solvent recovery processing equipment in the electrode manufacturing equipment, and reduces the industrial waste of organic solvent waste liquid. It has the effect of reducing the amount.

[0009]

However, Li _x
The NiO ₂ -based positive electrode active material can be used to increase the battery capacity of a lithium battery, but is manufactured using a water-dispersible or water-soluble resin having excellent organic solvent resistance as a binder. Lithium batteries have had problems such as a decrease in battery capacity and a decrease in large current discharge characteristics due to an increase in charge transfer resistance. This means that the Li _x NiO ₂ -based positive electrode active material has high reactivity with water, so that protons and lithium ions are present on the surface of the positive electrode active material during the paste preparation step of preparing an aqueous solution-based active material paste during electrode preparation. Exchange reaction occurs,
Alternatively, a phenomenon such as formation of a film formed by a reaction with water on the surface of the active material occurs, so that the battery performance is reduced.

[0010] In order to eliminate the influence of moisture, an attempt has been made to prepare a paste in which polytetrafluoroethylene is dispersed in a non-aqueous organic solvent to prepare an electrode. Has not been reached. In other words, polytetrafluoroethylene has poor dispersibility in a non-aqueous organic solvent, entanglement of polytetrafluoroethylene occurs during paste production, gelation occurs, aggregates are produced after electrode production, and short-circuiting occurs during battery assembly. This is because there are problems such as frequent occurrence.

For this reason, a method of adding a compound soluble in a non-aqueous organic solvent has been proposed for the purpose of improving the dispersibility of polytetrafluoroethylene in a non-aqueous organic solvent. For example, Japanese Patent Application Laid-Open No. 8-108687 describes a polyvinylidene fluoride (PV) that is soluble in a non-aqueous organic solvent.
DF), polyvinylidene chloride is added, and JP-A-10-12243 discloses a method of adding polyvinyl butyral.

However, since these added compounds have inferior electrolytic solution resistance to polytetrafluoroethylene, they swell and dissolve in the electrolytic solution.
There has been a problem that the binding force is reduced due to expansion and contraction of the electrode active material due to charge and discharge, and cycle deterioration is increased.

When a resin that swells and dissolves easily in the electrolyte is used as the binder, the coverage of the cathode active material with the binder is reduced, and the contact between the electrolyte and the cathode active material during charge and discharge is reduced. The area increases, and the reactivity between the electrolytic solution and the positive electrode active material increases. When the reaction between the electrolytic solution and the positive electrode active material occurs, an insulating film such as lithium fluoride is formed on the surface of the positive electrode active material by a decomposition reaction of the electrolytic solution or the like. The formation of the insulating film causes a problem that the internal resistance of the battery increases.

The present invention has been made in view of the above situation, and uses a water-soluble polymer having excellent organic solvent resistance as a binder, and has a high capacity and excellent charge / discharge cycles and suppresses an increase in internal resistance. It is an object to provide a rechargeable lithium secondary battery.

[0015]

[0007] In order to solve the above-mentioned problems, as a result of repeated studies on the positive electrode active material and the binder, the specific surface area of the positive electrode active material is limited to thereby reduce the area involved in the reaction with water. It is possible to solve the above problem by suppressing the increase in internal resistance due to charge / discharge cycles by uniformly forming a water-soluble polymer film having excellent electrolyte resistance on the surface of the positive electrode active material. I found it.

That is, the lithium secondary battery of the present invention comprises:
Formula _{Li x Ni 1-y M y} O 2 (M is Co, Mn, Al, B, T
a positive electrode active material having at least one element selected from the group consisting of i, Mg, and Fe, and a LiNiO ₂ -based compound represented by 0 <x ≦ 1.2, 0 <y ≦ 0.25); In a lithium secondary battery having a positive electrode including a binder that binds to the electrolyte and an electrolyte, the BET specific surface area of the LiNiO ₂ -based compound is 0.65 m ² / g or less, and the binder is It has a water-soluble polymer that is non-swellable with respect to an electrolytic solution, and the surface of the positive electrode active material is coated with the water-soluble polymer.

In the lithium secondary battery of the present invention, the area involved in the reaction is reduced by reducing the specific surface area of the surface of the LiNiO ₂ -based compound serving as the positive electrode active material, and the battery capacity due to the reaction with water during battery manufacture. Of the decline. In addition, since the binder contains a water-soluble polymer having excellent organic solvent resistance, and the water-soluble polymer uniformly covers the surface of the positive electrode active material, the internal resistance increases and the discharge capacity decreases. The battery is suppressed and has excellent charge / discharge cycle characteristics.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION (Lithium Secondary Battery) The lithium secondary battery of the present invention is a lithium secondary battery having a positive electrode containing a positive electrode active material, a binder, and an electrolyte.

The positive electrode active material has the formula Li _x Ni _{1 -y My} O ₂ (M
Is at least one element selected from the group consisting of Co, Mn, Al, B, Ti, Mg, and Fe; 0 <x ≦ 1.2;
0 having LiNiO ₂ based compound represented by <y ≦ 0.25). Here, when a plurality of elements are used for M, y indicates the sum of the plurality of elements.

The LiNiO ₂ compound has a BET specific surface area of 0.65 m ² / g or less. By setting the BET specific surface area of the LiNiO ₂ -based compound to 0.65 m ² / g or less, the ion exchange reaction between protons and lithium generated by the water in the dispersion medium on the active material surface is suppressed. here,
When the BET specific surface area exceeds 0.65 m ² / g, the discharge capacity of the lithium secondary battery decreases.

Here, a battery having a positive electrode using a positive electrode active material made of a LiNiO ₂ -based compound having a different specific surface area and a binder containing polytetrafluoroethylene was prepared.
The battery initial capacity was measured. The specific surface area of the positive electrode active material was adjusted by changing the mixing ratio of the raw materials, the firing temperature, and the atmosphere (oxygen concentration, dew point, CO ₂ content, etc.) during the synthesis of the active material. A positive electrode was prepared using an aqueous dispersion of polytetrafluoroethylene and a water-based binder made of a water-soluble cellulose resin for an active material sample having a changed specific surface area, and a battery having this positive electrode was prepared. The initial discharge capacity of this battery was measured. As a comparison, a battery using an organic solvent-based binder composed of PVDF and N-methyl-2-pyrrolidone (NMP) as a binder was also prepared, and the discharge capacity was measured. The method for measuring the initial discharge capacity was the same as the method used in the evaluation of the examples. The measurement results are shown in Table 1.

[0022]

[Table 1]

As shown in Table 1, in the battery using the water-based binder,
It can be seen that the larger the specific surface area of the positive electrode active material, the lower the initial discharge capacity of the battery. This is considered to be because when the specific surface area of the positive electrode active material increases, the positive electrode active material reacts with moisture contained in the aqueous binder. In addition, the battery having a positive electrode active material having a specific surface area of 0.65 m ² / g has a battery initial capacity that is substantially the same as that obtained when an organic solvent-based binder is used, even when an aqueous binder is used as the binder. Has been realized.

[0024] The binder binds the positive electrode active material. The binder has a water-soluble polymer that is non-swellable with respect to the electrolytic solution, and uniformly covers the surface of the positive electrode active material. Since such a binder has excellent organic solvent resistance, it does not swell or dissolve in an electrolyte of a lithium battery, and firmly binds the active material when the positive electrode active material expands and contracts during charge and discharge of the battery. Since the battery can maintain a state of being bonded to the battery, good cycle characteristics as a battery can be realized. The binder may function as a thickener due to its own viscosity.

In addition, since the binder has a water-soluble polymer that is non-swellable with respect to the electrolytic solution, the water-soluble polymer can uniformly cover the surface of the positive electrode active material, and can be used in a lithium secondary battery. The reaction between the positive electrode active material and the electrolytic solution can be suppressed.

That is, when preparing the electrode paste during the manufacturing process of the lithium secondary battery, since the water-soluble polymer of the binder is uniformly dissolved in water, an electrode paste in which the water-soluble polymer is uniformly dissolved is prepared. In addition, the surface of the positive electrode active material is uniformly coated with the water-soluble polymer.

Since the water-soluble polymer film formed on the surface of the positive electrode active material is always in contact with the electrolyte, a non-swellable water-soluble polymer that does not swell in the electrolyte is used. Further, the water-soluble polymer does not dissolve in the electrolytic solution. That is, the film formed on the surface of the positive electrode active material by the water-soluble polymer is retained by not dissolving in the electrolytic solution.

Examples of such water-soluble polymers include methylcellulose (MC), carboxymethylcellulose (C
MC), ethylcellulose (EC), hydroxyethylcellulose (HEC), hydroxypropylmethylcellulose (HPMC), hydroxyethylmethylcellulose (HEMC) and other celluloses, and polyacrylates.

The water-soluble polymer is more preferably water-soluble cellulose.

The binder preferably has a resin that imparts flexibility to the positive electrode. That is, when the binder has a resin that imparts flexibility to the positive electrode, the positive electrode has flexibility, and handling of the positive electrode becomes easy during manufacturing and the like. Specifically, it is possible to bind the positive electrode active material, the conductive agent and the current collector using only a water-soluble polymer as a binder, but the prepared positive electrode has poor flexibility, so This is because the positive electrode is likely to be damaged such as cracks in the active material layer or falling off of the active material layer when the positive electrode in the form of a sheet is wound.

The resin for imparting flexibility to the positive electrode is as follows:
It is preferable to have excellent resistance to electrolyte. That is, since the resin that imparts flexibility to the positive electrode is used as a binder, when it swells or dissolves in the electrolytic solution, a film formed of the water-soluble polymer on the surface of the positive electrode active material causes damage. This is because

As a resin for imparting flexibility to the positive electrode, for example, polytetrafluoroethylene (PTF)
E), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PF
A), tetrafluoroethylene-ethylene copolymer (E
Examples include fluorine-based resins such as TFE), tetrafluoroethylene-hexafluoropropylene-perfluoroalkylvinylether copolymer (EPE), acrylic resins, olefins such as polyethylene and polyolefin, and resins such as polyimide resins.

The resin for imparting flexibility to the positive electrode is preferably a fluorine resin. That is, since the fluorinated resin does not swell or dissolve in the electrolytic solution, the battery characteristics of the battery used as the binder can be improved. Here, in the lithium secondary battery of the present invention, it is preferable to use a fluorine-based resin as the binder, but it does not exclude the use of a resin or a polymer material other than the fluorine-based resin as the binder.

As the fluororesin, polytetrafluoroethylene is more preferable.

The LiNiO ₂ -based compound has the (006) plane, the (102) plane and the (1) plane in the X-ray powder analysis measurement results.
01) plane peak intensity ratio (I ₀₀₆ + I ₁₀₂ ) / I ₁₀₁
It is preferably 0.421 or less. That is, Li
By increasing the crystallinity of the NiO ₂ -based compound and strengthening the crystal structure, it is possible to suppress a decrease in the cycle characteristics of the lithium secondary battery. This is for the following reason.

That is, the ideal LiNiO_TwoStructure and
Then, Li between the oxygen layers⁺Layer, Ni ³⁺Layer where layer exists
Rock salt structure (α-NaFeO_TwoStructure). But,
Ni³⁺Is Ni²⁺Easily reduced to Ni²⁺Io
The radius (0.83Å) is Li⁺Ion radius (0.90
Å) is almost equal to LiNiO_TwoL generated during synthesis
i⁺Ni at the missing site²⁺Is easy to mix,
[Li⁺ _1-xNi²⁺ _x]_3b[Ni³⁺]_3a[O^2- _Two]_6CThat
It is easy to be structured. This Li⁺ _3bN on site
i²⁺Is locally electrochemically inactive when
And Li⁺Will prevent the spread of
For this reason, LiNiO with low crystallinity_TwoAs the electrode active material
If used, the battery capacity and durability will decrease.
Problems had arisen.

Therefore, as a method of quantitatively expressing the crystallinity of LiNiO ₂ , diffraction intensity I ₀₀₆ and (10) caused by the (006) plane by crystal structure analysis using X-ray diffraction are described.
2) The value obtained by dividing the sum of the diffraction intensity I ₁₀₂ caused by the plane and the diffraction intensity I ₁₀₁ caused by the (101) plane (I ₀₀₆ + I ₁₀₂ ).
/ I ₁₀₁ was used. The smaller this value is, the smaller the crystal defects are, and the stronger the bonding strength is.

Here, the relationship between the peak intensity ratio (I ₀₀₆ + I ₁₀₂ ) / I ₁₀₁ of the LiNiO ₂ -based compound and the battery characteristics was measured, and the measurement results are shown in Table 2. Specifically, a battery having a positive electrode using a positive electrode active material having a constant specific surface area and a different XRD peak intensity ratio and a binder containing polytetrafluoroethylene was prepared, and the discharge capacity after a charge / discharge test was performed. Was measured. First, the peak intensity ratio (I ₀₀₆ + I ₁₀₂ ) / I is maintained while maintaining the BET specific surface area by changing the mixing ratio of raw materials at the time of synthesis, the firing temperature, and the atmosphere (oxygen concentration, dew point, CO ₂ content, etc.). ₁₀₁ different L
A positive electrode active material sample composed of a LiNiO ₂ compound of iNi _0.81 Co _0.15 Al _0.04 O ₂ was prepared. A battery having a positive electrode using an aqueous binder composed of an aqueous dispersion of polytetrafluoroethylene and a water-soluble cellulose resin as the positive electrode active material was prepared, and the initial discharge capacity and the discharge capacity after the charge / discharge test were measured. . At this time, the BET specific surface area of the positive electrode active material was 0.65 m ² / g.
In addition, a lithium battery using an organic solvent-based binder composed of PVDF and NMP was also manufactured using each of the positive electrode active materials. The measurement of the initial discharge capacity and the cycle characteristics was performed by the measurement method described in the examples described later.

[0039]

[Table 2]

As shown in Table 2, the batteries using the aqueous binder were
Batteries that use organic solvent-based binders regardless of the peak strength ratio
158 (mAh / g) initial discharge capacity equivalent to
Have been. The discharge capacity ratio after the charge / discharge test is
Intensity ratio (I₀₀₆+ I₁₀₂) / I ₁₀₁Is less than 0.421
For batteries using substances, batteries using organic solvent-based binders
It turns out that it is superior.

The positive electrode active material, wherein _{Li a Mn b Me c O 4} (M
e is Co, Cu, Fe, Ni, Zn, Al, Cr, T
at least one element selected from the group consisting of i, Mg and V, 1 ≦ a ≦ 1.15, 0 ≦ c ≦ 0.35, a +
It is preferable to have a LiMnO ₄ -based compound represented by b + c = 3). By blending this LiMnO ₄ -based compound into the positive electrode active material, the charge / discharge curve of the lithium secondary battery can be adjusted. As a result, lithium secondary batteries having different charge / discharge characteristics can be obtained, and the use of the lithium secondary batteries can be expanded. Specifically, the deviation in the charge-discharge curve of the positive electrode active material is caused by blending a LiMn ₂ O ₄ compounds, the amount of displacement by changing the amount of LiMn ₂ O ₄ compounds is changed. If the charge / discharge curves are different, the charge / discharge capacity from the same potential changes. As a result, the charge / discharge characteristics of the lithium secondary battery change. Here, the LiMn ₂ O ₄ -based compound can be added to the positive electrode active material at an arbitrary ratio.

The electrolyte is used for a normal lithium secondary battery.
Electrolyte solution can be used. As this electrolyte
Is, for example, 1,2-dimethoxyethane, 1,2-die
Toxiethane, propylene carbonate, ethylene car
Bonate, γ-butyrolactone, tetrahydrafuran,
1,3-dioxolane, diethylene carbonate,
Simple materials such as chill carbonate and ethyl methyl carbonate
In a single or a mixture of two or more solvents, for example, LiCF_ThreeS
O_Three, LiC_FourF₉SO_Three, LiClO_Four, LiPF₆, Li
BF_Four, LiN (CF_ThreeSO_Two) (CF_ThreeSO_Two), Li
N (C_FourF₉SO _Two) (CF_ThreeSO_Two), LiN (C_TwoF_FiveS
O_Two) (C_TwoF_FiveSO_Two) Etc. alone or two or more
Use an organic solvent-based electrolyte solution prepared by dissolving the
Can be.

The lithium secondary battery of the present invention can be in a form used for ordinary lithium secondary batteries, except for the positive electrode active material, the binder and the electrolyte. In addition, the structure of the lithium secondary battery of the present invention is not particularly limited, and a stacked type having a stacked electrode body in which a positive electrode and a negative electrode are formed in a sheet shape and alternately stacked via a separator. The electrode battery may also be a wound electrode battery having a wound electrode body in which a sheet-shaped positive electrode and a sheet-shaped negative electrode are wound via a separator.

The sheet-like positive electrode and negative electrode are preferably formed by applying a mixture obtained by mixing an active material, a conductive material, a binder and the like to a current collector.

The negative electrode active material, current collector and separator that can be used in the lithium secondary battery of the present invention include:
For example, the following can be used.

Examples of the negative electrode active material include carbon materials capable of occluding and releasing lithium, such as flaky graphite, spherical graphite, and amorphous carbon, and lithium metal.

The current collector for the positive electrode is, for example, aluminum or stainless steel, and the current collector for the negative electrode is, for example, a net, punched metal, foam metal, foil processed into a plate shape, or the like made of copper, nickel, or the like. Can be used.

As a separator, for example, a thickness of 10 to
A microporous polypropylene film or a microporous polyethylene film having a pore size of 50 (μm) and a porosity of 30 to 70% can be used.

In the lithium secondary battery of the present invention, the area involved in the reaction is reduced by reducing the specific surface area of the surface of the positive electrode active material, and the reduction in battery capacity due to the reaction with water is suppressed. In addition, a water-soluble polymer with excellent organic solvent resistance is used as the binder, and the surface of the positive electrode active material is uniformly coated, thereby suppressing an increase in internal resistance and swelling and dissolving the electrolyte. This does not occur, and the state in which the active material is firmly bound can be maintained even if the electrode active material expands and contracts during charging and discharging of the battery. Therefore, the battery has an excellent charge / discharge cycle.

(Method for Producing a Positive Electrode for a Lithium Secondary Battery) The method for producing a positive electrode for a lithium secondary battery according to the present invention is represented by the formula Li _{x
Ni 1-y M y O 2} (M is Co, Mn, Al, B, Ti, M
g, at least one element selected from the group consisting of Fe, 0 <x ≦ 1.2, 0 <y ≦ 0.25), and B
LiNiO ₂ having an ET specific surface area of 0.65 m ² / g or less
A positive electrode active material having a base compound, a binder having a water-soluble polymer that is non-swelling with respect to an electrolyte used in a lithium secondary battery, and water are mixed to form a paste-like active material mixture. A paste adjusting step of adjusting the paste; and a coating step of applying the paste-like active material mixture to the current collector.

The method for producing a positive electrode for a lithium secondary battery of the present invention includes a paste adjusting step of adjusting a paste-like active material mixture by mixing a positive electrode active material, a binder, and water. In this paste adjustment step, water can be used as a medium because a water-soluble polymer that is non-swellable with respect to an electrolyte used for a lithium secondary battery is used as a binder. As a result, since no organic solvent is used, the cost required for manufacturing the battery can be reduced. Further, in the paste preparation step, the coating of the binder is formed on the surface of the positive electrode active material by adjusting the paste-like active material mixture by mixing the positive electrode active material, the binder, and water. be able to.

The paste adjusting step is a step of adjusting a paste-like active material mixture in which the positive electrode active material, the binder, and water are mixed. The adjustment may be made in such a manner.

The positive electrode active material has the formula Li _x Ni _{1 -y My} O ₂ (M
Is at least one element selected from the group consisting of Co, Mn, Al, B, Ti, Mg, and Fe; 0 <x ≦ 1.2;
0 <y ≦ 0.25).

The LiNiO ₂ compound has a BET specific surface area of 0.65 m ² / g or less. It is considered that by setting the BET specific surface area of the LiNiO ₂ -based compound to 0.65 m ² / g or less, the occurrence of ion exchange reaction between protons and lithium due to moisture on the surface of the LiNiO ₂ -based compound is suppressed. Further, the BET specific surface area is 0.65 m ^2.
/ G, the reaction with water occurs, and the initial discharge capacity decreases.

The binder has a water-soluble polymer which is non-swellable with respect to the electrolyte used for the lithium secondary battery. Since the binder has a water-soluble polymer that is non-swellable with respect to the electrolytic solution, the water-soluble polymer can uniformly cover the surface of the positive electrode active material in the paste preparation step. The reaction between the positive electrode active material and the electrolyte can be suppressed in the lithium secondary battery.

That is, since the binder has a water-soluble polymer, it is uniformly dissolved in water in the paste preparation step, and the positive electrode active material is dispersed in this solution, so that the water-soluble polymer is uniformly coated. The obtained positive electrode active material can be obtained.

The water-soluble polymer film formed on the surface of the positive electrode active material is always in contact with the electrolyte when the battery is formed, so that the water-soluble polymer does not swell in the electrolyte. A swellable water-soluble polymer is used. Further, the water-soluble polymer does not dissolve in the electrolytic solution. That is, the film formed on the surface of the positive electrode active material by the water-soluble polymer is retained by not dissolving in the electrolytic solution.

Since such a binder is excellent in organic solvent resistance, it does not swell or dissolve in an electrolyte of a lithium battery, and is active even if the active material expands and contracts by charging and discharging. Because it is possible to maintain a state where substances are firmly bound,
Causes lithium batteries to have good cycling characteristics. Also, the binder may act as a thickener due to its own viscosity.

Examples of such a water-soluble polymer include methyl cellulose (MC) and carboxymethyl cellulose (C
MC), ethylcellulose (EC), hydroxyethylcellulose (HEC), hydroxypropylmethylcellulose (HPMC), hydroxyethylmethylcellulose (HEMC) and other celluloses, and polyacrylates.

The water-soluble polymer is more preferably water-soluble cellulose.

The binder preferably has a resin that imparts flexibility to the positive electrode. That is, when the binder has a resin that imparts flexibility to the positive electrode, the positive electrode has flexibility, and handling of the positive electrode becomes easy during manufacturing and the like. Specifically, it is possible to bind the positive electrode active material, the conductive agent and the current collector using only a water-soluble polymer as a binder, but the prepared positive electrode has poor flexibility, so This is because the positive electrode is likely to be damaged such as cracks in the active material layer or falling off of the active material layer when the positive electrode in the form of a sheet is wound.

The resin that imparts flexibility to the positive electrode is:
It is preferable to have excellent resistance to electrolyte. That is, since the resin that imparts flexibility to the positive electrode is used as a binder, when it swells or dissolves in the electrolytic solution, a film formed of the water-soluble polymer on the surface of the positive electrode active material causes damage. This is because

As a resin for imparting flexibility to the positive electrode, for example, polytetrafluoroethylene (PTF)
E), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PF
A), tetrafluoroethylene-ethylene copolymer (E
Examples include fluorine-based resins such as TFE), tetrafluoroethylene-hexafluoropropylene-perfluoroalkylvinylether copolymer (EPE), acrylic resins, olefins such as polyethylene and polyolefin, and resins such as polyimide resins.

The resin that imparts flexibility to the positive electrode is preferably a fluororesin. That is, since the fluorinated resin does not swell or dissolve in the electrolytic solution, the battery characteristics of the battery used as the binder can be improved. Here, in the present invention, it is preferable to use a fluorine-based resin as the binder, but it is not excluded that a non-fluorine-based resin or a polymer material is used as the binder.

As the fluororesin, polytetrafluoroethylene is more preferable.

As the electrolyte used for the lithium secondary battery in which the binder exhibits non-swelling properties, an electrolyte used for a normal lithium secondary battery can be used. Examples of the electrolyte include 1,2-dimethoxyethane, 1,2-diethoxyethane, propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrafuran,
A single solvent such as 1,3-dioxolane, diethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, or a mixture of two or more solvents, for example, LiCF ₃ S
O ₃ , LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiPF ₆ , Li
BF ₄ , LiN (CF ₃ SO ₂ ) (CF ₃ SO ₂ ), Li
N (C ₄ F ₉ SO ₂ ) (CF ₃ SO ₂ ), LiN (C ₂ F ₅ S
An organic solvent-based electrolytic solution prepared by dissolving an electrolyte such as O ₂ ) (C ₂ F ₅ SO ₂ ) alone or by dissolving two or more thereof can be used.

[0067] The positive electrode active material, wherein _{Li a Mn b Me c O 4} (M
e is Co, Cu, Fe, Ni, Zn, Al, Cr, T
at least one element selected from the group consisting of i, Mg and V, 1 ≦ a ≦ 1.15, 0 ≦ c ≦ 0.35, a +
It is preferable to have a LiMn ₂ O ₄ compound represented by b + c = 3). By having a LiMn ₂ O ₄ compound as the positive electrode active material, the battery characteristics of the lithium secondary battery can be changed.

The application step is a step of applying the paste-like active material mixture to the current collector. The application of the paste-like active material mixture to the current collector can be performed by a method used for the usual application of the paste-like active material mixture. Examples of the application method include an application method using a die coater, a comma coater, a reverse roller, a doctor blade, or the like.

The coating step preferably includes a drying step of drying the paste-like active material mixture applied to the current collector. By drying the paste-like active material mixture applied to the current collector, the paste-like active material mixture flows on the surface of the current collector during subsequent handling, so that the applied thickness does not change.

The method for producing a positive electrode for a lithium secondary battery according to the present invention preferably includes a pressure bonding step of pressing the current collector coated with the paste-like active material mixture. That is, by pressing the paste-like active material mixture applied to the surface of the current collector, the paste is pressed against the current collector and the weight energy density of the positive electrode is improved. In the pressing step, it is preferable to press the paste-like active material mixture that has been subjected to the drying step.

The positive electrode manufactured by the method for manufacturing a positive electrode for a lithium secondary battery according to the present invention can be used for an ordinary lithium secondary battery. That is, in the lithium secondary battery using the positive electrode manufactured by the manufacturing method of the present invention, a negative electrode other than the positive electrode, a separator, and the like may be the same as those used in the conventional lithium secondary battery. it can.
At this time, as a method for manufacturing the lithium secondary battery, a method similar to the conventional method for manufacturing a lithium secondary battery can be used.

In the method for producing a positive electrode for a lithium secondary battery of the present invention, the area involved in the reaction is reduced by using the positive electrode active material having a reduced specific surface area, and the reduction in battery capacity due to the reaction with water is prevented. I am holding it down. In addition, a water-soluble polymer with excellent organic solvent resistance is used as the binder, and the surface of the positive electrode active material is uniformly coated, thereby suppressing an increase in internal resistance and swelling and dissolving the electrolyte. This does not occur, and the state in which the active material is firmly bound can be maintained even if the electrode active material expands and contracts during charging and discharging of the battery. For this reason, the lithium secondary battery using the positive electrode manufactured by the manufacturing method of the present invention is a battery excellent in charge and discharge cycles.

Further, since a water-soluble polymer having excellent organic solvent resistance is used for the binder of the positive electrode, the positive electrode can be manufactured without using an organic solvent. For this reason,
The cost of safety management required for the use of organic solvents,
The material cost of the organic solvent itself can be reduced, and the positive electrode can be manufactured safely and inexpensively.

[0074]

The present invention will be described below with reference to examples.

As an example of the present invention, a lithium secondary battery was manufactured.

The BET specific surface area and XRD of the positive electrode active material used in the examples were measured by the following measuring methods.

(BET specific surface area) manufactured by Shimadzu Corporation
Model: BET specific surface area by nitrogen adsorption method (hereinafter referred to as nitrogen BET specific surface area) was measured using Flow Soap II2300.

(XRD) Rigaku Co., Ltd., Model: RINT
2000, X-ray source: CuKα ₁ , tube voltage: 50
(KV), tube current: 100 (mA), divergence slit: 1
/ 2 (deg), scattering slit: 1/2 (deg), light receiving slit: 0.15 (mm), scanning mode: continuous, scanning range: 15 ° to 75 °, and the diffraction intensity was measured.

After the data was subjected to background removal at a curvature of 5.0 using a RISM qualitative analysis program, the intensity ratio of (Kα ₁ / Kα ₂ ) was set to 0.5 and Kα
_The effect of ₂ was removed.

The X-ray diffraction intensity corresponding to each index plane was read from the obtained data, and the 006, 102, and 101 planes, which are indicative of the crystallinity of the LiNiO ₂ cathode active material, also had peak intensity ratios: (I ₀₀₆ + I ₁₀₂ ) / I
I asked for ₁₀₁ .

Example 1 Example 1 is a cylindrical lithium secondary battery 100 shown in FIG.

This cylindrical lithium secondary battery 100 has a positive electrode active material containing lithium, and releases lithium as lithium ions during charging and can occlude lithium ions during discharging. A negative electrode 2 having a negative electrode active material that can occlude lithium ions during charging and release lithium ions during discharging, and a non-aqueous electrolyte 3 in which a supporting salt containing lithium is dissolved in an organic solvent. Lithium secondary battery.

The positive electrode 1 is joined to a positive electrode current collector 11, a positive electrode mixture layer 12 formed on the surface of the positive electrode current collector 11 and having a positive electrode active material and a binder, and a positive electrode current collector 11. And the positive electrode current collecting lead 13 thus formed.

The positive electrode active material was LiNi _0.81 Co _0.15 Al
_0.04 O ₂ , and the nitrogen BET specific surface area is 0.65 m ² /
g, peak intensity ratio by XRD: (I ₀₀₆ + I ₁₀₂ ) / I
₁₀₁ is 0.421 lithium nickel oxide, and the binder is a mixture of carboxymethylcellulose sodium salt aqueous solution and PTFE.

The negative electrode 2 includes a negative electrode current collector 21, a negative electrode mixture layer 22 having a negative electrode active material formed on the surface of the negative electrode current collector 21, and a negative electrode current collector bonded to the negative electrode current collector 21. And a lead 23.

The positive electrode 1 and the negative electrode 2 are
And is held in the case 7 in a state wound. Also, the current collecting leads 13 and 23 of the positive electrode 1 and the negative electrode 2
Are the positive terminal part 5 and the negative terminal part 6 of the case, respectively.
Is connected to

The separator is a polyethylene porous membrane having a thickness of 25 μm. The electrolyte is ethylene carbonate (EC) and diethylene carbonate (DEC).
1 mo as an electrolyte in a solvent mixed at a volume ratio of 0:70
A solution in which 1 / liter of LiPF ₆ was dissolved was used.

The lithium secondary battery of Example 1 was manufactured according to the following procedure.

(Production of Positive Electrode) First, 87 w
t%, acetylene black as conductive agent (Model: HS-
100) An aqueous solution of carboxymethylcellulose sodium salt of 10% by weight and 2% by weight of carboxymethylcellulose sodium salt as a binder which also functions as a thickener is mixed so that the solid content of sodium carboxymethylcellulose is 2% by weight, and the mixture is mixed with a twin screw stirrer.
Stir for hours. Then, as a binder, a solid content ratio of about 50
% PTFE aqueous dispersion is added so that the solid content of PTFE becomes 1 wt%, and the mixture is stirred for 30 minutes using a vacuum emulsification stirrer. The paste obtained in this manner is coated on an aluminum foil with a comma coater, and the basis weight per one side is 8.
Apply on both sides at 56 (mg / cm ² ).

Next, this electrode was passed through a roll press machine,
A load of a linear pressure of 0.74 (ton / cm) is applied to bring the electrode density to 2.41 (g / cm ² ). Thereafter, this electrode was cut into a width of 5.4 (cm) and a length of 86 (mm), and a length of 2.5 (c) was used as a lead tab weld for current extraction.
m) of the electrode mixture was scraped off. The effective reaction area of this electrode is 5.4 (cm) × 83.5 (cm) × 2 = 901.
8 (cm ² ).

(Production of Negative Electrode) 92.5 wt% of flaky graphite as a negative electrode active material and PVDF7.
5% by weight of PV in N-methyl-2-pyrrolidone
A paste in which graphite was dispersed in a solution in which DF was dissolved was prepared, and the paste was applied to both sides of the copper foil using a comma coater in the same manner as the positive electrode so that the basis weight per side was 5.1 (mg / cm ² ). Was applied. Thereafter, a load of a linear pressure of 0.25 (ton / cm) was applied through a roll press to produce an electrode whose electrode density was increased to 1.25 (g / cm ² ). Next, this electrode was made to have a width of 5.6
m) and cut to a length of 90.5 (cm), and the electrode mixture for a length of 0.5 (cm) was scraped off as a reed tab welding part for extracting current. The effective reaction area of this electrode is 5.
6 (cm) × 90 (cm) × 2 = 1,008 (cm ² ).

(Assembly of Battery) The positive electrode and the negative electrode obtained as described above were wound with a separator interposed therebetween to form a wound electrode, and this electrode was inserted into the case. At this time, the current collecting lead was joined to the positive terminal and the negative terminal of the case.

Thereafter, the electrolytic solution was injected into the case, and the case was closed.

By the above procedure, the cylindrical lithium secondary battery of Example 1 was manufactured.

Comparative Example 1 Comparative Example 1 is a cylindrical lithium secondary battery similar to Example 1 except that polyvinylidene fluoride (PVDF) was used as a binder for the positive electrode. Here, the positive electrode when PVDF was used as the binder was prepared by dissolving PVDF in N-methyl-2-pyrrolidone, forming a paste in which the positive electrode active material and the conductive agent were dispersed in this solution, and forming this paste into aluminum. Manufactured by applying to foil.

Example 2 Example 2 is a cylindrical lithium secondary battery similar to Example 1 except that the nitrogen BET specific surface area of the positive electrode active material was 0.55 (m ² / g). .

Comparative Example 2 Comparative Example 2 is a cylindrical lithium secondary battery similar to Example 1 except that the nitrogen BET specific surface area of the positive electrode active material is 0.83 (m ² / g). .

Comparative Example 3 Comparative Example 3 is a cylindrical lithium secondary battery similar to Example 1 except that the nitrogen BET specific surface area of the positive electrode active material is 1.04 (m ² / g). .

Comparative Example 4 Comparative Example 4 is a cylindrical lithium secondary battery similar to Comparative Example 3 except that PVDF was used as a binder for the positive electrode.

Example 3 In Example 3, the positive electrode active material X
Peak intensity ratio by RD: (I₀₀₆+ I₁₀₂) / I ₁₀₁But
Except for 0.412, the same cylindrical shape as in Example 1 was used.
Secondary battery.

Comparative Example 5 Comparative Example 5 shows that the positive electrode active material X
Peak intensity ratio by RD: (I₀₀₆+ I₁₀₂) / I ₁₀₁But
Same as Example 1 except that 0.449 was used
Cylindrical lithium secondary battery.

Comparative Example 6 Comparative Example 6 is a cylindrical lithium secondary battery similar to Comparative Example 5, except that PVDF was used as a binder for the positive electrode.

Example 4 In Example 4, the positive electrode active material had a nitrogen BET specific surface area of 0.55 (m ² / g) and a peak intensity ratio by XRD: (I ₀₀₆ + I ₁₀₂ ) / I _101.
Is a cylindrical lithium secondary battery similar to that of Example 1 except that the value is 0.412.

Example 5 In Example 5, hydroxyethyl cellulose (HE) was used instead of CMC as a binder for the positive electrode.
A cylindrical lithium secondary battery similar to that of Example 1 except that C) was used. In Example 5, the production of the positive electrode was performed in the same manner as in Example 1 except that HEC was used instead of CMC.
Was done in a similar manner.

Example 6 Example 6 is a cylindrical lithium secondary battery similar to Example 1 except that hydroxypropylmethylcellulose (HPMC) was used instead of CMC as the binder for the positive electrode. In addition, in Example 6,
The production of the positive electrode was performed in the same manner as in Example 1 except that HPMC was used instead of CMC.

Example 7 Example 7 is a cylindrical lithium secondary battery similar to Example 1 except that hydroxyethyl methylcellulose (HEMC) was used instead of CMC as the binder for the positive electrode. In Example 7, the production of the positive electrode was performed in the same manner as in Example 1 except that HEMC was used instead of CMC.

Example 8 Example 8 was the same as Example 1 except that tetrafluoroethylene-hexafluoropropylene copolymer (FEP) was used instead of PTFE as the binder for the positive electrode. It is a lithium secondary battery.
In Example 8, the production of the positive electrode was performed in the same manner as in Example 1, except that an aqueous FEP dispersion was used instead of the aqueous PTFE dispersion.

(Example 9) Example 9 is the same as Example 1 except that tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA) was used instead of PTFE as the binder for the positive electrode. Type lithium secondary battery. In Example 9, the production of the positive electrode was performed using PT
The procedure was performed in the same manner as in Example 1, except that the aqueous PFA dispersion was used instead of the aqueous FE dispersion.

Example 10 Example 10 is a cylindrical lithium battery similar to Example 1 except that an acrylic emulsion resin was used instead of PTFE as a binder for the positive electrode. In Example 10, the production of the positive electrode was P
The procedure was performed in the same manner as in Example 1 except that an acrylic emulsion resin was used instead of TFE.

(Example 11) In Example 11, a positive electrode active material was used.
Is LiNi_0.80Co_0.15Al_0.05O_TwoAnd nitrogen
The BET specific surface area is 0.65 (m^Two/ G), by XRD
Peak intensity ratio: (I_{0 06}+ I₁₀₂) / I₁₀₁Is 0.421
, Except that the following was used.
This is a rechargeable lithium battery.

Example 12 In Example 12, a positive electrode active material was used.
Is LiNi_0.90Co_0.06Al_0.04O_TwoAnd nitrogen
The BET specific surface area is 0.65 (m^Two/ G), by XRD
Peak intensity ratio: (I_{0 06}+ I₁₀₂) / I₁₀₁Is 0.421
, Except that the following was used.
This is a rechargeable lithium battery.

(Example 13) In Example 13, a positive electrode active material was used.
Is LiNi_0.75Co_0.21Al_0.04O_TwoAnd nitrogen
The BET specific surface area is 0.65 (m^Two/ G), by XRD
Peak intensity ratio: (I_{0 06}+ I₁₀₂) / I₁₀₁Is 0.421
, Except that the following was used.
This is a rechargeable lithium battery.

Example 14 In Example 14, the positive electrode active materials were LiNi _0.81 Co _0.15 Al _0.04 O ₂ and Li _1.12 Mn.
_This is a cylindrical lithium secondary battery similar to that of Example 1 except that a mixture of _1.88 O ₄ was used.

More specifically, LiNi _0.81 Co _0.15 Al _0.04
O ₂ has a nitrogen BET specific surface area of 0.65 (m ² / g), X
Peak intensity ratio by RD: (I ₀₀₆ + I ₁₀₂ ) / I ₁₀₁ is 0.421 and Li _1.12 Mn _1.88 O ₄ is nitrogen BET
The specific surface area was 0.5 (m ² / g). The mixing ratio of these components is LiNi _0.81 Co _0.15 Al _{0.04 by} weight.
O ₂ : Li _1.12 Mn _1.88 O ₄ was 60:27.

(Embodiment 15) In Embodiment 15, LiNi is used.
Example 14 except that the mixing ratio of _0.81 Co _0.15 Al _0.04 O ₂ and Li _1.12 Mn _1.88 O ₄ was 27:60 by weight.
This is a cylindrical lithium secondary battery similar to.

Comparative Example 7 Comparative Example 7 is a cylindrical lithium secondary battery similar to Example 14 except that PVDF was used as a binder for the positive electrode.

Comparative Example 8 Comparative Example 8 is a cylindrical lithium secondary battery similar to Example 15 except that PVDF was used as a binder for the positive electrode.

(Evaluation) As evaluations of the lithium secondary batteries of Examples and Comparative Examples, the initial discharge capacity, high-temperature cycle characteristics and internal resistance increase rate were measured. Table 3 shows the measurement results. In addition, these measuring methods were as follows. In the batteries of Comparative Examples 3 to 6, only the initial discharge capacity was measured.

(Initial Discharge Capacity) First, CC-CV charging is performed up to 4.1 (V) at a charging current of 250 (mA),
CC discharge was performed with a discharge current of 333 (mA) to 3.0 (V).

Next, when the charging current is 1000 (mA), the charging current is 4.1.
CC-CV charge and discharge current up to (V) 1000 (mA)
After performing CC discharge four times to 3.0 (V), CC-CV charge is performed to 4.1 (V) at a charging current of 1000 (mA).
CC discharge was performed to 3.0 (V) with a discharge current of 333 (mA).

The discharge capacity at this time was defined as the initial capacity of the battery, and a comparison was made using a value standardized by the weight of the positive electrode active material filled in the battery. The measurement was performed in a 25 ° C. atmosphere.

(High Temperature Cycle Characteristics) The lithium secondary battery after the initial capacity measurement was put in a constant temperature bath at an ambient temperature of 60 ° C., and was charged at a charging current of 1984 (mA) up to 4.1 (V).
A cycle of charging and CC charging with a discharge current of 1984 (mA) to 3.0 (V) was repeated 500 times,
The ratio of the discharge capacity at the 00th cycle was compared.

(Internal Resistance) First, a charging current of 1000 (m
In A), CC-CV charging was performed for 1.5 hours to 3.750 (V). After that, discharge at 300 (mA) for 10 seconds,
Charge at 300 (mA) for 10 seconds, 900 (mA) for 10 seconds
Discharge for 2 seconds, charge at 900 (mA) for 10 seconds, 2700
(MA) for 10 seconds, 2700 (mA) for 10 seconds, 5400 (mA) for 10 seconds, 5400 (m)
A): Charged for 10 seconds, discharged at 8100 (mA) for 10 seconds, charged at 8100 (mA) for 10 seconds, and after each discharge, the current after charging and discharging was plotted on the vertical axis and the charging and discharging current was plotted on the horizontal axis.
The internal resistance was determined from the slope of the approximate linear line of the voltage plot value.

(Resistivity increase rate) The internal resistance increase rate is determined by measuring the internal resistance after 500 cycles of the charge / discharge performed in the high-temperature cycle characteristics and measuring the increase in the internal resistance before and after the charge / discharge. Obtained by dividing by the internal resistance of

More specifically, {resistance increase rate (%)} = {(internal resistance after high-temperature cycle test) − (initial internal resistance)}
/ (Initial internal resistance).

[0126]

[Table 3]

From Table 3, it is found that Example 1 has an initial discharge capacity of 1
The battery was as high as 58 mAh / g, and the discharge capacity ratio was as high as 82.0%, indicating that the battery had little capacity deterioration. The resistance increase rate was 1.7%, and almost no increase in internal resistance was observed.

In Comparative Example 1, the initial discharge capacity was sufficiently high, but the discharge capacity ratio was reduced to 75.0%. That is, since PVDF is used as the binder, the binding force of the positive electrode active material decreases due to swelling and dissolution with the electrolytic solution,
The discharge capacity ratio has decreased. Also, the surface of the positive electrode active material is not uniformly covered with the binder, or a gap is generated in the binder due to swelling due to the electrolyte during charge / discharge, which promotes a decomposition reaction of the electrolyte and insulates the electrolyte such as lithium fluoride. The resistive film grew and the resistance increase rate after the high-temperature cycle test was as large as 24%.

Example 2 is a battery in which the specific surface area of the positive electrode active material was reduced, and the initial discharge capacity was 160 mAh / g,
The discharge capacity ratio also shows a high value of 82.3%. Also,
The rate of increase in resistance is almost unchanged at -1.1%.

In Comparative Example 3, since the nitrogen BET specific surface area was large, the initial discharge capacity was reduced to 138 mAh / g. The rate of increase in resistance was -4.7%, and almost no change in internal resistance was observed.

In Comparative Example 4, since the binder was PVDF, no decrease in the initial capacity was observed even when the nitrogen BET specific surface area was increased. However, the binder is PVDF
Therefore, the discharge capacity ratio is greatly reduced to 74.7%. That is, the binding agent PVDF swells and dissolves with the electrolytic solution, so that the binding force of the positive electrode active material is reduced and the discharge capacity ratio is reduced. Further, since the binder was only PVDF, the resistance increase rate was 75.
That was up 3%.

In Example 3, the initial discharge capacity was 158 mAh.
/ G, and the discharge capacity ratio is as high as 85.0%. That is, in Example 3, since the crystallinity of the positive electrode active material was increased, a decrease in cycle characteristics was suppressed. The rate of increase in resistance is almost unchanged at 0.6%.

In Comparative Example 5, the initial discharge capacity was 158 mA.
Although it shows a high value of h / g, the discharge capacity ratio after the cycle test is reduced to 65.2%. That is, since the crystallinity of the positive electrode active material is low, the cycle characteristics are deteriorated. The rate of increase in resistance is almost unchanged at -0.5%.

In Comparative Example 6, since PVDF was used as the binder for the positive electrode, the initial discharge capacity was as high as 160 mAh / g, but the discharge capacity ratio after the cycle test was 7%.
It has dropped to 2.0%. The binder is PVDF
And the resistance increase rate is 7 as in Comparative Example 1.
It rose considerably to 9.2%.

In Example 4, since the specific surface area of the positive electrode active material was reduced and the crystallinity was enhanced, the initial discharge capacity was 160 mAh / g and the discharge capacity ratio was 85.1%, both of which were high. I have. In addition, the rate of increase in resistance hardly changed to 1.8%.

In Example 5, the initial discharge capacity was 158 mAh.
/ G and a discharge capacity ratio of 81.1%, both show high values. Further, even when HEC is used as the binder for the positive electrode, the rate of increase in resistance is sufficiently suppressed to 8.2%.

In Example 6, the initial discharge capacity was 158 mAh.
/ G and a discharge capacity ratio of 80.7%, both show high values. Further, even when HPMC is used as the binder for the positive electrode, the rate of increase in resistance is sufficiently suppressed to 7.9%.

In Example 7, the initial discharge capacity was 158 mAh.
/ G and a discharge capacity ratio of 80.6%, both of which show high values. Further, even if HEMC is used as the binder of the positive electrode, the resistance increase rate is sufficiently suppressed to 2.6%.

In Example 8, although the FEP binder was used, the initial discharge capacity was 158 mAh / g, and the discharge capacity ratio was 8
It shows a high value of 1.5%. The rate of increase in resistance is almost unchanged at + 0.8%.

In Example 9, the PFA binder was used, but the initial discharge capacity was 158 mAh / g and the discharge capacity ratio was 8
It shows a high value of 1.7%. Further, the resistance increase rate is hardly changed to + 2.1%.

In Example 10, the initial discharge capacity was 158 mA.
h / g and the discharge capacity ratio were 80.2%, both showing high values. In addition, even if an acrylic resin is used as the binder for the positive electrode, the rate of increase in resistance hardly changes to 1.6%.

Examples 11 to 13 are batteries in which the composition of the positive electrode active material was changed. The initial discharge capacity and the discharge capacity ratio were sufficiently high, and the resistance increase rate was suppressed to + 2.0% or less. I have.

Examples 14 and 15 are batteries using a mixture of lithium nickel oxide and lithium manganese oxide as the positive electrode active material, and using PTFE / CMC as the binder for the positive electrode. In Example 14 in which this manganese oxide was mixed at a ratio of 27/87, the initial capacity and the discharge capacity ratio showed sufficiently higher values as compared with Comparative Example 7 in which the positive electrode binder was PVDF. . In Example 15 in which the manganese oxide was mixed at a ratio of 60/87, the initial capacity and the discharge capacity ratio showed sufficiently higher values as compared with Comparative Example 8 in which the positive electrode binder was PVDF. I have.

From the examples and comparative examples, the initial discharge was achieved by using a water-soluble polymer as a binder for the positive electrode and a positive electrode active material made of lithium nickel oxide having a small specific surface area and high crystallinity. A lithium secondary battery having excellent capacity, cycle characteristics, and resistance increase rate can be obtained.

[0145]

According to the lithium secondary battery of the present invention, by reducing the specific surface area of the positive electrode active material, the area involved in the reaction is reduced, and the reduction in battery capacity due to the reaction with moisture is suppressed. In addition, since the binder contains a water-soluble polymer with excellent organic solvent resistance, and the water-soluble polymer uniformly covers the surface of the positive electrode active material, the internal resistance increases and the discharge capacity decreases. Is suppressed, and the battery has an excellent charge / discharge cycle.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a battery manufactured in an example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 cylindrical lithium secondary battery 1 positive electrode 11 positive electrode current collector 12 positive electrode mixture layer 13 positive electrode current collector lead 2 negative electrode 21 negative electrode current collector 22 negative electrode mixture layer 23 negative electrode current collector lead 3 Non-aqueous electrolyte 4 Separator 5 Positive electrode terminal 6 Negative electrode terminal 7 Case

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kenichiro Kami 1-1-1, Showa-cho, Kariya-shi, Aichi Prefecture Inside Denso Corporation (72) Inventor Manabu Yamada 1-1-1, Showa-cho, Kariya City, Aichi Prefecture Denso Corporation F term (reference) 5H029 AJ03 AJ05 AJ06 AK03 AL06 AL12 AM03 AM04 AM05 AM07 BJ02 BJ14 CJ08 CJ22 DJ08 DJ12 DJ17 EJ12 5H050 AA07 AA08 AA12 BA17 CA07 CB07 CB12 DA02 DA11 EA23 EA24 FA12 FA19 GA10 GA12

Claims (14)

[Claims] The formula Li _x Ni _{1- y My} O ₂ (M is Co, M
at least one element selected from the group consisting of n, Al, B, Ti, Mg and Fe, 0 <x ≦ 1.2, 0 <y ≦
0.25) in a lithium secondary battery including a positive electrode including a positive electrode active material having a LiNiO ₂ -based compound represented by formula ( ₂ ), a binder for binding the positive electrode active material, and an electrolyte solution. The BET specific surface area of the LiNiO ₂ compound is 0.65 m ²
/ G or less, and the binder has a water-soluble polymer that is non-swellable with respect to the electrolyte, and the surface of the positive electrode active material is coated with the water-soluble polymer. Rechargeable lithium battery. 2. The LiNiO ₂ -based compound has a peak intensity ratio (I ₀₀₆ + I ₁₀₂ ) / I of a (006) plane, a (102) plane and a (101) plane in an X-ray powder analysis measurement result.
The lithium secondary battery according to claim 1, wherein ₁₀₁ is 0.421 or less. 3. The lithium secondary battery according to claim 1, wherein the water-soluble polymer is water-soluble cellulose. 4. The lithium secondary battery according to claim 1, wherein the binder has a resin that imparts flexibility to the positive electrode. 5. The lithium secondary battery according to claim 4, wherein the resin that imparts flexibility to the positive electrode is a fluorinated resin. 6. The lithium secondary battery according to claim 5, wherein said fluorine-based resin is polytetrafluoroethylene. Wherein said positive electrode active material has the formula Li _a Mn _b Me _c
O ₄ (Me is Co, Cu, Fe, Ni, Zn, Al, C
at least one element selected from the group consisting of r, Ti, Mg and V, 1 ≦ a ≦ 1.15, 0 ≦ c ≦ 0.3
5, a + b + c = 3) The lithium secondary battery according to claim 6, further comprising a LiMn ₂ O ₄ compound represented by. 8. The formula Li _x Ni _{1- y My} O ₂ (M is Co, M
at least one element selected from the group consisting of n, Al, B, Ti, Mg and Fe, 0 <x ≦ 1.2, 0 <y ≦
0.25), and the BET specific surface area is 0.65 m ² /
g of a positive electrode active material having a LiNiO ₂ -based compound or less, a binder having a water-soluble polymer that is non-swellable with respect to an electrolytic solution used for a lithium secondary battery, and water, and mixed to form a paste. A method for producing a positive electrode for a lithium secondary battery, comprising: a paste adjusting step of adjusting a paste-like active material mixture; and an application step of applying the paste-like active material mixture to a current collector. 9. The LiNiO ₂ -based compound has a peak intensity ratio (I ₀₀₆ + I ₁₀₂ ) / I of a (006) plane, a (102) plane and a (101) plane in X-ray powder analysis measurement results.
_The method for producing a lithium secondary battery according to claim 8, wherein ₁₀₁ is 0.421 or less. 10. The method for producing a positive electrode of a lithium secondary battery according to claim 8, wherein the water-soluble polymer is water-soluble cellulose. 11. The method for producing a positive electrode for a lithium secondary battery according to claim 8, wherein the binder has a resin that imparts flexibility to the positive electrode. 12. The method according to claim 11, wherein the resin that imparts flexibility to the positive electrode is a fluorine-based resin. 13. The method according to claim 12, wherein the fluororesin is polytetrafluoroethylene. 14. The positive electrode active material has a formula of Li _a Mn _b Me.
_c O ₄ (Me is Co, Cu, Fe, Ni, Zn, Al, C
at least one element selected from the group consisting of r, Ti, Mg and V, 1 ≦ a ≦ 1.15, 0 ≦ c ≦ 0.3
5, a + b + c = 3) in the manufacturing method of the positive electrode of a lithium secondary battery according to claim 8 to 14, further comprising a LiMn ₂ O ₄ compound represented.