JP3996226B2 - Nonaqueous battery separator and nonaqueous battery using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a separator for a non-aqueous solvent battery such as a lithium battery, and more particularly to a separator for a non-aqueous battery having excellent permeability and mechanical strength and excellent charge / discharge characteristics.
[0002]
[Prior art]
Polyolefin microporous membranes are used in various separation membranes, battery separators, electrolytic capacitor separators, and the like. Particularly in lithium batteries, since a lithium metal and lithium ions are used, an aprotic polar organic solvent is used as an electrolyte solvent, and a lithium salt is used as an electrolyte. Therefore, for the separator installed between the positive electrode and the negative electrode, a polyolefin microporous film such as polyethylene or polypropylene that is insoluble in an organic solvent and stable to an electrolyte or an electrode active material is used as the separator.
[0003]
A lithium secondary battery is expected to be one of the secondary batteries with the highest energy density because the electromotive force is as high as 2.5 to 4 V and the molecular weight of lithium, which is the main component of the active material, is small. Coin-type and AA-size small-capacity lithium secondary batteries are used in memory backup batteries and mobile phones. However, the capacity of several tens of kWh is required for electric vehicles and small-scale load adjustment, and increasing capacity and increasing output are issues. Lithium secondary batteries cannot use aqueous electrolytes due to high electromotive force, etc., and use organic solvent electrolytes or polymer solid electrolytes, so their electrical conductivity is low, so the current density is low. . For this reason, it is necessary to increase the electrode area in a large-capacity high-power battery.
[0004]
The separator used in these batteries needs to have a rough separator surface and a large pore size so as not to reduce the effective sectional area of the electrode by the contact between the electrode and the separator. In consideration of safety, a moderately thick film thickness is necessary in order to appropriately take the distance between the two electrodes. Furthermore, in order to improve the discharge characteristics and the cycle characteristics, which are battery characteristics, a film having a large electrolyte solution holding amount is required.
[0005]
With a polyolefin microporous film alone, it is difficult to increase the film thickness and the cycle characteristics are not good. The nonwoven fabric alone has a problem that the pore diameter is too large to meet the demand.
[0006]
[Problems to be solved by the invention]
Therefore, the subject of this invention is providing the separator for non-aqueous batteries which was excellent in the permeation | transmission performance and mechanical strength suitable for a large sized battery, and excellent in charging / discharging characteristics.
[0007]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the present invention has been conducted as a result of intensive investigations on a membrane as a separator having a rough structure on the surface and a dense structure in the center layer so as not to reduce the effective cross-sectional area of the surface in contact with the electrode. It has been found that by laminating a polyethylene microporous membrane having physical properties and a polypropylene non-woven fabric having specific properties, a battery separator having excellent permeability and mechanical strength and excellent charge / discharge characteristics can be obtained. I came up with the invention.
[0008]
That is, the present invention provides a polyethylene microporous membrane having a thickness of 25 to 30 μm, a porosity of 30 to 50%, an average through-hole diameter of 0.001 to 0.1 μm, and a tensile breaking strength of 500 kg / cm ² or more. The present invention relates to a separator for non-aqueous batteries having a basis weight of 20 to 50 g / m ² , a film thickness of 100 to 200 μm laminated with a polypropylene nonwoven fabric having an air permeability of 1 second / 100 cc or less and an air permeability of 500 to 1500 seconds / 100 cc. Is . Furthermore, the present invention provides a non-aqueous battery using the same.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The separator for non-aqueous batteries of the present invention can be obtained by laminating a polyethylene microporous film having specific physical properties and a polypropylene nonwoven fabric having specific physical properties. The thickness of the separator for a non-aqueous battery of the present invention is 100 to 200 μm, preferably 130 to 170 μm. If the thickness is less than 100 μm, the cycle characteristics deteriorate, and if it exceeds 200 μm, the volume of the film occupies the battery volume increases. Is high, and the battery performance decreases. The air permeability is 500 to 1500 seconds / 100 cc, preferably 800 to 1200 seconds / 100 cc. When the air permeability exceeds 1500 seconds / 100 cc, the film resistance is large, and when current is passed, the voltage drop or the battery generates heat. There is a risk.
[0010]
Hereinafter, the structure of the separator for nonaqueous batteries and the manufacturing method thereof will be described.
[0011]
(1) Polyethylene microporous membrane The polyethylene microporous membrane of the present invention has a thickness of 25 to 30 µm. If the thickness is less than 25 μm, the mechanical strength of the film is small, and the safety at the time of short circuit is lacking. On the other hand, if it exceeds 30 μm, the air permeability value becomes high and the air resistance becomes large, which is not preferable. The basis weight is required to be smaller than the basis weight of the laminated polypropylene nonwoven fabric.
[0012]
The porosity of the microporous membrane is 30 to 50%, preferably 35 to 45%. If the porosity is less than 30%, the amount of electrolyte filled in the pores decreases, which is not preferable. On the other hand, if it exceeds 50%, the film strength is undesirably lowered.
[0013]
The average through-hole diameter of the microporous membrane is 0.001 to 0.1 μm, preferably 0.01 to 0.05 μm. When the average through-hole diameter is less than 0.001 μm, it becomes physically difficult to fill the pores of the electrolytic solution, and the passage of ions is hindered. On the other hand, when it exceeds 0.1 μm, it becomes difficult to prevent the diffusion of the active material and the reaction product.
[0014]
The tensile breaking strength is 500 kg / cm ² or more. Thereby, it can be set as the separator which is high intensity | strength and is hard to tear. As a preferred method for obtaining the polyethylene microporous membrane of the present invention, a polyethylene solvent is supplied to the polyethylene composition to prepare a solution of the polyethylene composition, and this solution is extruded into a sheet form from a die of an extruder. There is a method in which a gel-like molded product is formed by cooling, the gel-like molded product is heated and stretched, and then the remaining solvent is removed.
[0015]
(2) The polypropylene nonwoven fabric used for the polypropylene nonwoven fabric laminate has a basis weight of 20 to 50 g / m ² , preferably 25 to 45 g / m ² and an air permeability of 1 second / 100 cc or less. Those having a fiber diameter of 0.1 to 5 μm and a thickness of 50 to 150 μm are preferred. The basis weight of the nonwoven fabric needs to be larger than the value corresponding to the basis weight of the polyethylene microporous membrane.
[0016]
(3) Laminate The separator for nonaqueous batteries of the present invention can be obtained by laminating the polyolefin microporous membrane and the polypropylene nonwoven fabric into a composite membrane. The lamination process is performed by a normal calendar process. The calendering heat compression roll temperature is 50 to 140 ° C., preferably 50 to 120 ° C., and the roll pressure is 5 to 50 kg / cm ² , preferably 5 to 30 kg / cm ² . However, care must be taken so that the permeability of the microporous membrane is not impaired.
[0017]
Lamination may be performed by laminating the nonwoven fabric on one side or both sides of the microporous membrane, such as two layers of microporous membrane / nonwoven fabric and three layers of nonwoven fabric / microporous membrane / nonwoven fabric.
[0018]
It is preferable to control the heat compression roll so that the thickness of the laminate is 100 to 200 μm, which is a range that can be used as a battery separator.
[0019]
The battery separator of the present invention is used for non-aqueous batteries, particularly non-aqueous secondary batteries. As the positive electrode active material of the nonaqueous secondary battery, V ₂ O ₅ , Cr ₂ O ₅ , MnO ₂ , TiS ₂ , LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ or the like is used. On the other hand, the negative electrode material is not particularly limited as long as it can be doped and dedoped with metallic lithium or lithium ions, and a carbon material or the like can also be used. Further, as the non-aqueous electrolyte solution, lithium salts such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ are used alone or in combination, and as the solvent of the electrolyte solution, propylene carbonate, ethylene carbonate, 1,2-dimethoxy is used. One or more of methane, 1,2-dimethoxyethane, γ-butyrolactone, tetrahydrofuran, acetonitrile, vinyl formate and the like can be used. The shape of the battery can is not particularly limited, and cans such as an oval shape, a cylindrical shape, and a square shape are common. The material of the outer can is not particularly limited, but a rope, stainless steel or the like is used.
[0020]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not particularly limited to the examples.
[0021]
Example 1
Using a polyethylene microporous membrane having a thickness of 30 μm, a pore diameter of 0.03 μm, a porosity of 40%, a longitudinal tensile breaking strength of 1088 kg / cm ² , and a transverse tensile breaking strength of 806 kg / cm ² , a melt blown polypropylene nonwoven fabric (mesh weight) : 40 g / m ² , air permeability: 0 sec / 100 cc, fiber diameter: 4 μm, thickness: 440 μm) treated at 110 ° C., roll pressure 30 kg / cm ² , 3 m / min preheated compression roll to 120 μm Thereafter, the polyethylene microporous membrane was calender laminated at 70 ° C. and a roll pressure of 30 kg / cm ² , 3 m / min to obtain a composite membrane for a separator having a film thickness of 153 μm and an air permeability of 960 seconds / 100 cc.
[0022]
Example 2
An active material mainly composed of lithium metal as a negative electrode active material and manganese as a positive electrode, and LiPF ₆ dissolved as 1 mol in one liter of ethylene carbonate as an electrolyte is used as an electrolyte. The separator of Example 1 was placed as a separator, and this was spirally wound and housed in a metal case to prepare a battery.
[0023]
The battery was charged and discharged at a constant current in a voltage range between 4V and 2V, thereby measuring the charge / discharge efficiency for each cycle. The result is shown in graph 1 of FIG. In FIG. 1, the horizontal axis represents the number of charge / discharge cycles, and the vertical axis represents the charge / discharge efficiency (%), which is the ratio of the capacity at the time of charging / discharging when the capacity at the time of charging of each cycle is 100%. It is a plot.
[0024]
Comparative Example 1
For comparison, a battery using only the polyethylene microporous membrane used in Example 1 as a separator was prepared in the same manner as in Example 2, and a charge / discharge cycle test was performed. The result is shown in the graph 2 of FIG.
[0025]
As is apparent from FIG. 1, the efficiency of the battery using the separator of the present invention does not decrease even when charging and discharging are repeated. It can be seen that the efficiency is significantly reduced after a short cycle.
[0026]
【The invention's effect】
When the laminated composite film of the polyethylene microporous membrane and the polypropylene nonwoven fabric of the present invention is used as a separator for a non-aqueous battery, the charge / discharge cycle property is good and it can sufficiently cope with a high capacity.
[Brief description of the drawings]
FIG. 1 shows the number of charge / discharge cycles and the charge / discharge efficiency of a non-aqueous battery using a separator of the present invention (Example 2) and a non-aqueous battery using only a polyethylene microporous membrane as a separator (Comparative Example 1). It is a graph which shows the relationship with (%).
[Explanation of symbols]
1. Example 2
2. Comparative Example 1

Claims (2)

A polyethylene microporous membrane having a thickness of 25 to 30 μm, a porosity of 30 to 50%, an average through hole diameter of 0.001 to 0.1 μm, and a tensile breaking strength of 500 kg / cm ² or more, and a basis weight of 20 to 50 g / M ² , a separator for a non-aqueous battery formed by laminating a polypropylene nonwoven fabric having an air permeability of 1 second / 100 cc or less, wherein the separator has a film thickness of 100 to 200 μm and an air permeability of 500 to 1500 seconds. Charging / discharging which measures charging / discharging efficiency for each cycle by charging / discharging at a constant current between 4V and 2V in a non-aqueous battery in which the separator is arranged. In a cycle test, a non-aqueous battery separator characterized by having a charge / discharge efficiency that does not substantially decrease even when the number of charge / discharge cycles is repeated at least 23 times is used. Non-aqueous battery. The non-aqueous battery using a separator for a non-aqueous battery according to claim 1, wherein the polypropylene non-woven fabric is a polypropylene non-woven fabric that has been subjected to a preheating compression roll treatment.

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