JPH11283612A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery in which charging/discharging cycle characteristics are enhanced and a high output is achieved by improving electronic conductivity of positive electrode active material so as to reduce inside resistance. SOLUTION: In a lithium battery, a battery case houses therein an inside electrode member 1 in which a positive electrode plate 2 and a negative electrode plate 3 are wound or layered via a porous polymer separator in such a manner as not to be brought into direct contact with each other, wherein an organic electrolyte is used. A metal coating film is formed at the surface of at least a part of positive electrode active material powder, and/or metal particles are fixed to the surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery in which charge-discharge cycle characteristics are improved and high output is achieved by improving the electron conductivity of a positive electrode active material and reducing the internal resistance. .

[0002]

2. Description of the Related Art In recent years, lithium secondary batteries have come into practical use as high energy density secondary batteries that supply power to small electronic devices such as portable communication devices and notebook personal computers. On the other hand, lithium secondary batteries
With the growing interest in resource and energy savings in the context of the international movement for global environmental protection, electric vehicles (EVs) and hybrid electric vehicles (HEVs) are being considered for active market introduction in the automotive industry. It is also expected to be used as a battery for driving a motor, etc., and attention has been focused on early commercialization of a large-capacity lithium secondary battery suitable for such a use.

A lithium secondary battery uses a lithium transition metal composite oxide or the like as a positive electrode active material, while using a carbonaceous material such as hard carbon or graphite as a negative electrode active material, and uses lithium in the positive electrode active material during charging. Ions move to the negative electrode active material via an electrolytic solution in which a lithium electrolyte is dissolved in a non-aqueous organic solvent and are trapped, and a reverse battery reaction occurs during discharge.

Here, specific examples of the lithium transition metal composite oxide used as the positive electrode active material include lithium cobaltate (LiCoO ₂ ) and lithium manganate (LiMn ₂ O ₄ ). , 1-20 μm
A powder having a particle size of the order is used. In the production of lithium secondary batteries for EVs and HEVs, generally, these positive electrode active material powders are made into a paste and applied to a metal foil as a current collector to produce a positive electrode plate.

[0005]

Here, the lithium transition metal composite oxide as the positive electrode active material is a mixed conductor having both lithium ion conductivity and electron conductivity.
Electronic conductivity is not always great. For this reason,
In preparing the paste, the internal resistance of the battery is reduced by adding a few percent of acetylene black as a conductive aid to the positive electrode active material powder.

Here, when acetylene black is not added, the contact between the positive electrode active material powders deteriorates, the internal resistance of the battery increases, and the utilization rate of each of the positive and negative electrode active materials decreases. The cycle characteristics also deteriorate, and the battery characteristics generally deteriorate. From this, it is clear that the addition of acetylene black contributes to a reduction in internal resistance of the battery and an improvement in cycle characteristics.

[0007] However, the addition of acetylene black reduces the filling amount of the positive electrode active material, and this is where the battery capacity is reduced. Acetylene black is a type of carbon and is a semiconductor, and its electronic conductivity is about three orders of magnitude lower than that of metals. Therefore, it is thought that there is a limit in the improvement of electron conductivity by acetylene black, and the amount of addition is set to an appropriate amount by comparing the positive effect of reducing the internal resistance and the negative effect of reducing the battery capacity. Will be done.

[0008] On the other hand, the particle size of acetylene black used is as very small as 10 to 100 nm, and it is doubtful whether all the particles of acetylene black added contribute to the improvement of electron conductivity, that is, the reduction of internal resistance. is there.

[0009]

Means for Solving the Problems The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object the three-dimensional bonding between positive electrode active material powders;
Another object of the present invention is to provide a lithium secondary battery in which the internal resistance of the positive electrode plate is reduced by reducing the resistance of the bond between the positive electrode active material powder and the metal foil serving as the current collector. That is, according to the present invention, a positive electrode plate and a negative electrode plate are wound or laminated so that the positive electrode plate and the negative electrode plate do not come into direct contact with each other via a separator made of a porous polymer. A lithium secondary battery containing an organic electrolyte and housed in a battery case, wherein a metal film is formed on at least a part of the surface of the positive electrode active material powder, and / or metal particles are fixed. A lithium secondary battery is provided.

In such a lithium secondary battery of the present invention, at least a part of the electric coupling between the positive electrode active material powders and at least a part of the positive electrode active material powder and the current collector of the positive electrode plate It is preferable from the viewpoint of reducing the internal resistance that the electrical connection of the above is made via the metal film or the metal particles. Further, as one means for obtaining such a structure, as a positive electrode active material powder, a powder having a metal coating formed on at least a part of its surface, and / or a metal particle is fixed on at least a part of its surface. The method using the powder obtained is preferably employed.

As such a metal coating and metal particles, at least one metal among aluminum, titanium, gold and platinum is preferably used. As the positive electrode active material, lithium cobaltate, lithium nickelate, or lithium manganate is preferably used. The configuration of such a lithium secondary battery of the present invention is as follows.
It is preferably used for batteries having a battery capacity of 2 Ah or more.
It can be suitably used for electric vehicles or hybrid electric vehicles.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The lithium secondary battery of the present invention
Higher output and improved cycle characteristics are achieved by reducing the internal resistance, and the device has a long service life. Hereinafter, embodiments of the present invention will be described, but it goes without saying that the present invention is not limited to the following embodiments.

The internal electrode body of the lithium secondary battery according to the present invention is formed by winding or laminating the positive electrode plate and the negative electrode plate via a separator made of a porous polymer film so that the positive electrode plate and the negative electrode plate do not come into direct contact with each other. It is configured.
Specifically, as shown in FIG.
Is manufactured by winding a positive electrode plate 2 and a negative electrode plate 3 with a separator 4 interposed therebetween, and usually a plurality of lead tabs 5 are provided on each of the electrode plates 2 and 3 according to the winding length. Such a wound internal electrode body 1 is formed by connecting a plurality of element batteries each composed of a part of a small area part of each of the electrode plates 2 and 3 connected to each lead tab 5 in parallel to form a large area positive electrode plate 2. It can be said that this is a structure in which one battery including the negative electrode plate 3 is formed.

On the other hand, as shown in FIG. 2, the laminated internal electrode body 7 has a positive electrode plate 8 and a negative electrode plate 9 alternately laminated with a separator 10 interposed therebetween, and a lead tab is provided on each of the electrode plates 8. 6 are connected. Such an internal electrode body 7 also has a structure in which a plurality of element batteries each consisting of a basically opposed positive electrode plate 8 and negative electrode plate 9 are connected in parallel.

In any of the above internal electrode bodies 1 and 7, the positive electrode plates 2 and 8 are formed by forming a layer made of a positive electrode active material (a positive electrode active material layer, hereinafter referred to as a “positive electrode layer”) on an aluminum foil as a current collector.
That. ) Is formed. The formation of the positive electrode layer on such a current collector is generally performed by adding a binder, a solvent, and the like to the electrode active material powder to form a paste, and applying the paste to a surface of the current collector using a device such as a reverse coater. This is done by:

Here, in the lithium secondary battery of the present invention, lithium cobalt oxide (Li) is used as a positive electrode active material.
CoO ₂ ) or lithium nickelate (LiNi
O ₂ ) or lithium manganate (LiMn ₂ O ₄ )
A powder of a lithium transition metal composite oxide such as the above is preferably used. Note that LiCoO ₂ or the like does not necessarily need to have a stoichiometric composition represented by such a chemical formula.

The positive electrode layer is formed by forming a metal film on at least a part of the surface of the positive electrode active material powder and / or fixing metal particles. Thus, the electrical coupling between at least a part of the positive electrode active material powder and the electrical coupling between at least a part of the positive electrode active material powder and the current collector are formed through the metal coating or the metal particles. You.

The metal coating or metal particles are not particularly limited as long as they are metals that do not corrode due to an electrode reaction at the positive electrode. In the present invention, aluminum (Al) and titanium (Ti) having particularly good corrosion resistance are used. ),
At least one kind of metal among gold (Au) and platinum (Pt) is used. Therefore, among these metals, only one kind may be used, or two or more kinds of metals may be used at the same time. It is preferable to use inexpensive aluminum among these metals from the viewpoint of cost.

It is preferable that the metal film formed on the surface of such a positive electrode active material powder be formed thin enough not to hinder the movement of lithium ions due to the charge / discharge reaction of the positive electrode active material. That is, the thickness is preferably about 1 to 100 nm so that lithium ions can penetrate the metal coating. In addition, the thickness of the metal film may vary, and the metal film may not be partially formed.

Similarly to the above-mentioned metal coating, the thickness of the metal particles fixed on the surface of the positive electrode active material powder, when the metal particles are dense and cover the entire surface of the positive electrode active material powder, have a thickness of The thickness is preferably about 1 to 100 nm, but the thickness may vary, and the metal particles do not have to adhere to a part of the surface of the positive electrode active material powder.

The microstructure of the positive electrode active material powder in the present invention is such that the electron conduction path from the positive electrode active material powder to the current collector is limited to these metal films or metal particles. It does not mean. It goes without saying that it is preferable that all of the positive electrode active material powder be conducted three-dimensionally by these metal coatings and the like, but such a situation is necessarily required to reduce the internal resistance. It is not something.

The formation of such a metal film or the fixation of metal particles on the surface of the positive electrode active material powder can be performed either before or after applying the positive electrode active material powder to the current collector. However, in the case of large-capacity lithium secondary batteries such as EV and HEV, the positive electrode plates 2 and 8 require a correspondingly large area, so that the positive electrode active material powder is applied to the current collector. Forming a metal film or the like after the above may cause problems in equipment.

Further, after the positive electrode active material powder is applied to the current collector, it may be difficult to form a metal film or the like at the boundary between the positive electrode active material powder and the current collector, and Is not practical because of the need to perform in a temperature range in which binder combustion does not occur, and various restrictions are added to maintain the properties of the formed positive electrode layer. Therefore, in the present invention, a method of modifying the surface of the positive electrode active material powder with a metal before applying the positive electrode active material powder to the current collector is preferably used.

That is, a method in which a powder having a metal film formed on at least a part of its surface and / or a powder having metal particles fixed on at least a part of its surface is preferable as the positive electrode active material powder. Adopted to. As a method of forming a metal film on the surface of the positive electrode active material powder (metal coating method), an evaporation method, a sputtering method, a plasma coating, a plating method, or the like can be used.
Metal particles can be fixed to the positive electrode active material powder by a mechanochemical surface modification technique such as thermal plasma coating or mechanical grinding.

Thus, a metal film is formed on the surface,
Alternatively, the electrical connection between the positive electrode active material powder and between the positive electrode active material powder and the current collector can be easily secured by applying the positive electrode active material powder to which the metal particles are fixed to the current collector. Will be able to

Since the positive electrode active material powder has such a microstructure, for example, when lithium ions are released from the positive electrode active material to the electrolyte during charging, electrons generated in the positive electrode active material are reduced. The positive electrode active material having low electron conductivity as in the related art because it can easily move to the current collector through the metal film or metal particles having high electron conductivity or through these metals. As compared with the case where the current has been transferred to the current collector through the powder or acetylene black, the resistance is extremely reduced, that is, the internal resistance of the positive electrode plate is extremely reduced.

Next, the production of the negative electrodes 3 and 9 will be described. The negative electrode plates 3 and 9 are formed in a thin plate shape by forming a negative electrode active material layer (hereinafter, referred to as a “negative electrode layer”) on a copper foil, a nickel foil, or an alloy foil thereof as a negative electrode current collector. The method of forming the negative electrode layer is the same as that of the positive electrode plate 2 described above.
The method is the same as that in the method of No. 8, but as the negative electrode active material, an amorphous carbonaceous material such as soft carbon or hard carbon, or a carbonaceous powder such as natural graphite is preferably used.

As the separators 4 and 10, those having a three-layer structure in which a lithium-ion permeable polyethylene film having micropores is sandwiched between porous lithium-ion permeable polypropylene films are preferably used. When the temperature of the internal electrode body rises, the polyethylene film softens at about 130 ° C. and the micropores are crushed, which also serves as a safety mechanism for suppressing the movement of lithium ions, that is, the battery reaction. And
By sandwiching the polyethylene film with a polypropylene film having a higher softening temperature, contact and welding between the separators 4 and 10 and the positive and negative electrode plates (2.3) and (8.9) can be prevented.

The various internal electrode members 1 and 7 are
The battery case is fitted in a battery case corresponding to each shape. Here, as an electrolytic solution impregnated in the internal electrode bodies 1 and 7 and filled in the battery case, ethylene carbonate (E) is used.
C), diethyl carbonate (DEC), dimethyl carbonate (DMC) in addition to carbonate-based ones, propylene carbonate (PC), γ-butyrolactone, tetrahydrofuran, acetonitrile, etc., in a single solvent or a mixed solvent of an organic solvent, as an electrolyte. Li
Lithium complex fluorine compounds such as PF ₆ and LiBF ₄ or lithium halides such as LiClO ₄
A non-aqueous organic electrolytic solution in which one or more kinds are dissolved is preferably used. Further, a solid polymer electrolyte or the like obtained by gelling the electrolyte solution thus prepared can also be used.

FIG. 3 is a sectional view showing another embodiment of the internal electrode body. The internal electrode body 19 having a laminated structure has a positive electrode layer 14 formed on one surface of a plate-shaped or foil-shaped positive electrode current collector 11, while a negative electrode layer 15 is formed on one surface of a negative electrode current collector 12. Formed, each of the current collectors 11 and 12
The surfaces on which the electrode layers 14 and 15 are not formed are electrically connected to each other, and the surface of the positive electrode layer 14 and the surface of the negative electrode layer 15 face each other via the separator 17 or the solid electrolyte 18. It is configured by being laminated in a plurality of stages. The internal electrode body 19 in this case is different from the internal electrode bodies 1 and 7 described above in that the element batteries are connected in series.

In such an internal electrode body 19, it is preferable that the positive electrode layer 14 is formed by applying the above-described positive electrode active material powder on which a metal film is formed or metal particles are fixed. Needless to say. The same applies to the negative electrode layer 15.

As described above, the lithium secondary battery of the present invention, in which the internal resistance of the positive electrode plate is reduced, has a large discharge output, particularly when it is suitably used for a battery having a battery capacity of 2 Ah or more. In addition, the effect of improving the charge / discharge cycle characteristics and prolonging the battery life is remarkable. Therefore, it can be suitably used for EV or HEV. Hereinafter, the present invention will be described with reference to Examples, but it goes without saying that the present invention is not limited to the following Examples.

[0033]

EXAMPLES (Measurement of Resistance of Positive Electrode Layer) Average particle size is 20 μm
In order to coat the surface of the LiMn ₂ O ₄ cathode active material powder with a metal, 100 g of the powder was placed in a chamber of a sputtering apparatus, and a sputtering process was performed once for about 1 minute using Au as a target for 10 to 10 minutes. Performed 50 times. At this time, every time one sputtering process was completed, the powder was taken out of the chamber and an operation of stirring the entire powder was performed so that the entire powder was uniformly coated. .

[0034] For each positive electrode active material powder having a different number of times of sputtering, a polyvinylidene fluoride (PVDF) binder dissolved in normal methylpyrrolidone (NMP) as a solvent is externally provided with respect to a powder weight of 100. After adding 8 wt% to obtain a slurry having an appropriate viscosity, a 25 μm-thick aluminum foil is coated with a thick film by a reverse coater, dried, and further roll-pressed to form a positive electrode layer having a final thickness of 80 to 100 μm. Was prepared. The positive electrode layer thus obtained is referred to as a positive electrode layer according to an example.

Next, in order to measure the resistance of the positive electrode layer according to these examples, one copper rod having a diameter of 6 mmφ and a length of 100 mm was prepared, and the end face of this rod was set upright to the positive electrode layer. By pressing one side of the tester probe to a copper rod and the other to an aluminum foil, the resistance of the positive electrode layer corresponding to an area having a diameter of 6 mmφ was measured. FIG. 4 shows the measurement results.

Next, the weight 1 of the LiMn ₂ O ₄ positive electrode active material powder having an average particle diameter of 20 μm and not subjected to the sputtering treatment was used.
Acetylene black (hereinafter, referred to as “AB”) is added in an amount of 0 to 6 wt%, and PVDF dissolved in NMP as a solvent is added.
8 wt%, 5 wt% up to 1-4 wt% AB
%, 9 wt%, and 6 wt%, about 10 wt%, respectively, to obtain a slurry having an appropriate viscosity. These slurries were thick-coated on an aluminum foil having a thickness of 25 μm, dried, and then roll-pressed to produce a positive electrode layer having a final thickness of 80 to 100 μm. The positive electrode layer thus obtained is used as a positive electrode layer according to a comparative example.

The resistance of the obtained positive electrode layer according to the comparative example is
It measured similarly to the positive electrode layer which concerns on the said Example. FIG. 5 shows the measurement results. From FIGS. 4 and 5, 10 times of the sputtering process is performed to reduce the resistance of the positive electrode layer by 1 wt% of AB.
It was found to have a remarkable effect corresponding to the addition. Thereby, the amount of added AB can be reduced, and the packing density of the positive electrode active material powder in the positive electrode layer can be improved.

(Production and Evaluation of Battery) LiM having a specific surface area of 1.4 m ² / g and an average particle diameter of 20 μm by a BET method.
The n ₂ O ₄ positive electrode active material powder was prepared in the same manner as in the procedure for the resistance measurement test of the positive electrode layer according to the above-described example.
0 g was placed in a chamber of a sputtering apparatus, and a sputtering process for about 1 minute was performed 30 times once using an Au target, thereby performing metal coating on the surface of the powder. At this time, every time one sputtering process was completed, the powder was taken out of the chamber and an operation of stirring the entire powder was performed so that the entire powder was uniformly coated. .

Further, AB was added to the obtained positive electrode active material powder.
Was added by about 2 wt%, NMP was used as a solvent, and PVDF was mixed as a binder to prepare a paste. This paste is applied to a thickness of 25 using a reverse coater.
Apply to both sides of aluminum foil of μm, length 3 in the winding direction
A positive electrode plate having an electrode surface shape of 400 mm × 200 mm in width was produced.

On the other hand, a highly graphitized carbon fiber having a diameter of about 5 μm and a length of about 10 μm is used as a negative electrode active material, and 2 wt% of artificial graphite for increasing conductivity is added to the negative electrode active material. A paste was prepared by mixing PVDF as a binder, and was applied to both surfaces of a copper foil having a thickness of 20 μm using a reverse coater to prepare a negative electrode plate having an electrode surface shape of 3400 mm in length in the winding direction × 200 mm in width.

The positive electrode plate and the negative electrode plate thus produced were
The internal electrode body as shown in FIG. 1 was produced by winding while insulating using a 210-mm-wide polypropylene separator. During the production of the internal electrode body, as a current collection lead tab, an aluminum foil lead tab on the positive electrode plate, a copper foil lead tab on the negative electrode, at an appropriate interval, and one end on one end of the internal electrode body. Ultrasonic welding was performed on each of the inner electrode body end faces so as to form electrodes.

Subsequently, the negative electrode lead tab is fitted to the battery terminal, a negative electrode terminal cap is attached to the battery case, and the negative electrode terminal side of the battery case is sealed. Then, an electrolytic solution in which the electrolyte LiPF ₆ was dissolved to a concentration of 1 mol% was injected into a mixed solvent of EC and DEC, and the inside of the glove box was maintained at a vacuum for 2 hours to allow the electrolytic solution to penetrate into the battery. .
Thereafter, the positive electrode lead tab was fitted to the positive terminal of the battery, and the positive terminal cap was attached to the battery case and sealed. The five batteries thus manufactured are referred to as Examples 1 to 5.

On the other hand, for the batteries of Examples 1 to 5,
Except that the metal coating by sputtering of the positive electrode active material powder was not performed, another five batteries were manufactured through the same steps as in Examples 1 to 5 described above. These batteries are referred to as Comparative Examples 1 to 5.

Although the battery capacity of each of the produced batteries was 22 Ah, the charge / discharge characteristics were evaluated by a cycle test. Here, charging is performed by constant current charging at 22 A of 1 C and 4.1 V constant voltage, and discharging is performed up to a discharge end voltage of 2.5 V by constant current discharging at a discharging rate of 22 A of 1 C. I decided to repeat. Then, the discharge capacity and internal resistance after 500 cycles were standardized with the initial value being 100%. The internal resistance was calculated from the drop in battery terminal voltage when switching to discharging after charging was completed.

Table 1 shows the measurement results. Although the initial discharge capacity is not different between Examples 1 to 5 and Comparative Examples 1 to 5, the internal resistance of Examples 1 to 5 is about 2 compared to Comparative Examples 1 to 5.
It can be seen that it is smaller by 0%. Therefore,
It can be seen that the metal coating on the metal active material powder has an effect on reducing the internal resistance without reducing the battery capacity.

[0046]

[Table 1]

On the other hand, after 500 cycles, in Comparative Examples 1 to 5, the discharge capacity is significantly reduced and the internal resistance is significantly increased. One possible cause of the increase in internal resistance due to the cycle test is that the volume change of the positive electrode active material powder during charge and discharge breaks the bond between the powders and increases the internal resistance. In Examples 1 to 5, since the metal coating was applied to the positive electrode active material powder itself, the disconnection of the electrical connection between the powders was suppressed, and the increase in the internal resistance was suppressed. It is considered something.

[0048]

As described above, according to the lithium secondary battery of the present invention, since the internal resistance of the positive electrode plate is reduced,
Output loss is reduced, and higher output is achieved. Thus, an excellent effect that a large current can be stably discharged can be obtained. In addition, there is an excellent effect that the charge / discharge cycle characteristics are improved and the battery life is prolonged.

[Brief description of the drawings]

FIG. 1 is a perspective view showing the structure of a wound internal electrode body.

FIG. 2 is a perspective view showing an embodiment of a structure of a laminated internal electrode body.

FIG. 3 is a cross-sectional view showing another embodiment of the structure of the laminated internal electrode body.

FIG. 4 is a graph showing the effect of metal coating on the positive electrode active material powder in the positive electrode plate according to the present invention.

FIG. 5 is a graph showing the effect of adding acetylene black to a positive electrode active material in a conventional positive electrode plate.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Internal electrode body, 2 ... Positive electrode plate, 3 ... Negative electrode plate, 4 ... Separator, 5 ... Lead tab, 6 ... Lead tab, 7 ... Internal electrode body, 8 ... Positive electrode plate, 9 ... Negative electrode plate, 10 ... Separator, 11
... positive electrode current collector, 12 ... negative electrode current collector, 14 ... positive electrode layer, 15
... Negative electrode layer, 17 ... Separator, 18 ... Solid electrolyte, 19
... internal electrode body.

Claims (7)

[Claims] An internal electrode body wound or laminated on a battery case so that the positive electrode plate and the negative electrode plate do not come into direct contact with each other via a separator made of a porous polymer. A lithium secondary battery containing and using an organic electrolyte, wherein a metal film is formed on at least a part of the surface of the positive electrode active material powder, and / or metal particles are fixed. Lithium secondary battery. 2. The electric connection between at least a part of the positive electrode active material powder and an electric connection between at least a part of the positive electrode active material powder and a current collector of the positive electrode plate are made of the metal. 2. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is formed through a coating or the metal particles. 3. A powder having a metal film formed on at least a part of its surface and / or a powder having metal particles fixed on at least a part of its surface as said positive electrode active material powder. The lithium secondary battery according to claim 1, wherein: 4. The lithium according to claim 1, wherein the metal coating and the metal particles are at least one metal selected from aluminum, titanium, gold, and platinum. Rechargeable battery. 5. The lithium secondary battery according to claim 1, wherein the positive electrode active material is lithium cobaltate, lithium nickelate, or lithium manganate. 6. The lithium secondary battery according to claim 1, wherein the battery capacity is 2 Ah or more. 7. An electric vehicle or a hybrid electric vehicle.
7. The lithium secondary battery according to claim 6.