JP3279091B2 - Organic electrolyte lithium secondary battery and method for producing separator thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electrolyte lithium secondary battery using lithium or a lithium alloy as a negative electrode active material, and more particularly to a separator for such a battery.

[0002]

2. Description of the Related Art A lithium secondary battery using lithium or a lithium alloy as a negative electrode active material and using an organic electrolyte has attracted attention because of its high energy density and excellent low-temperature characteristics as compared with an aqueous secondary battery. Are gathering.

However, active lithium deposited on the negative electrode upon charging reacts with an organic solvent as an electrolyte, or the deposited lithium grows in dendrite form,
An insulating layer is formed due to the reaction with the solvent to form lithium without electron conductivity (R. Selim and Bro, J. Electroc
hem. Soc, 121, 1457 (1974)), there is a problem that the charge and discharge efficiency of the lithium anode is poor. Further, there is a problem that lithium grown in a dendrite shape penetrates through the separator to cause an internal short circuit of the battery, and a practically sufficient lithium secondary battery has not been obtained.

[0004] The generation of dendritic lithium has a correlation with the charging current density, and when the charging current density is large, dendritic lithium is easily generated. A battery configuration is performed by enlarging a plate area and spirally winding a thin electrode plate and a separator. However, the localization of the reaction may occur due to the uneven surface of the electrode plate and the like, and dendritic lithium may be generated. The dendrite-like lithium generated in such a lithium negative electrode penetrates the fine holes of the thin and porous separator to cause an internal short circuit. Therefore, not only the performance of the battery is impaired, but also in the worst case, heat generation and ignition may occur.

To solve this problem, Japanese Patent Application Laid-Open No. HEI 3-129667 discloses a means for preventing a short circuit from an edge of an electrode plate.
No. 8 proposes making the width of the positive electrode wider than the width of the lithium negative electrode, and covering the edge of the positive electrode facing the edge of the negative electrode with an insulating member having the same thickness as that of the negative electrode. . Further, Japanese Patent Application Laid-Open No. 4-51473 discloses that the periphery of a positive electrode is sealed with a substance that is insoluble in an electrolytic solution and is electrically insulating.

[0006] However, the use of the above method complicates the manufacturing process, reduces the filling capacity of the active material, and inadequately improves the adhesion strength at the periphery of the positive electrode plate. There are problems such as the inability to prevent an internal short circuit due to the drop of the positive electrode mixture during the cycle, and none of them has been sufficiently improved.

Further, JP-A-1-319250 discloses that an ion-permeable polymer such as polyacrylamide is applied to a separator in order to prevent fine particles of a positive electrode mixture from passing through the separator. . However, polyacrylamide and the like are easily dissolved in the organic electrolyte, and if the polymer layer is formed to be honey, the resistance becomes high, which adversely affects battery characteristics. Japanese Patent Application Laid-Open No. Hei 2-162652 discloses that a solid polymer electrolyte film is integrally formed on an electrode plate in a battery having a configuration in which an electrode group is wound, for the purpose of reducing defects in the configuration. It has been disclosed. However, there is a problem in terms of reactivity with lithium or a lithium alloy as an active material of the solid polymer electrolyte film, mechanical strength, and the like. U.S. Pat. No. 5,281,491 discloses that a multi-layered microporous film having different physical properties is used as a separator, but it becomes complicated in battery configuration and process.

On the other hand, in the case of a polymer electrolyte, for example, as described in Japanese Patent Application Laid-Open No.
By using a gel electrolyte containing an organic electrolyte as a plasticizer, ionic conductivity can be dramatically improved.

It is also known to combine a polymer electrolyte and a separator in order to reinforce the mechanical strength of the polymer electrolyte. However, when a polymer electrolyte is used, the interface between the positive / negative electrode and the polymer electrolyte is not sufficiently bonded, particularly due to charge / discharge. When the positive and negative electrode plates expand and contract, a "gap" is formed at the interface, which causes a problem that a smooth charge / discharge reaction does not proceed.

[0010]

As described above, the above-mentioned method also has various problems. Although it is possible to prevent an internal short-circuit state due to dendrite, it is necessary to use the ion-conductive gel electrolyte layer alone. Has not been put into practical use due to its low mechanical strength, insufficient reliability and low-temperature ionic conductivity.

The present invention solves such a problem, and prevents dendrite-like lithium generated in a lithium anode from penetrating through a separator to cause an internal short-circuit state of a battery, thereby ensuring safe and reliable operation. An object is to provide a high secondary battery.

[0012]

To solve these problems, an organic electrolyte lithium secondary battery according to the present invention comprises a negative electrode using lithium or a lithium alloy as an active material and a positive electrode using a metal oxide as an active material. And an organic electrolyte solution and a separator in a battery container, wherein the separator uses an ion-conductive substance having a porous thin film as a skeleton. It is one in which a gel electrolyte is integrated. The separator has a two-layer structure of a porous resin layer and an ion-conductive gel electrolyte layer,
The surface in contact with the negative electrode of the separator may be an ion-conductive gel electrolyte layer.

The separator may be treated with a surfactant, and the microporous pores of the separator substrate having hydrophilicity may be integrated with an ion-conductive gel electrolyte.

The method for producing a separator according to the present invention is characterized in that a mixture of an ultraviolet-polymerizable resin and an organic electrolyte containing a lithium salt is impregnated or coated on a separator substrate treated with a surfactant. Filling the pores of the base material with the mixed liquid, and irradiating ultraviolet rays through a glass plate coated on the contact surface with the mixed liquid with a fluororesin to cure the resin, whereby the separator base material and the ions are ionized. A step of integrating the conductive gel solid electrolyte with the conductive gel solid electrolyte.

As the resin material for gelling, those having a structure having an acrylate group at the end of a polyethylene oxide skeleton and a polyolefin skeleton which are hardly expanded or dissolved in an organic electrolyte solution are desirable. A solution in which a monomer and oligomer polymerizable with ultraviolet light, a photopolymerization initiator and an organic electrolyte are mixed is uniformly impregnated or applied to a hydrophilic separator substrate treated with a surfactant, and the resin is polymerized by irradiation with ultraviolet light. By curing, a thin separator in which the separator substrate and the ion-conductive gel electrolyte layer are integrated can be obtained.

In order to form a thin layer having a thickness of 100 μm or less using an ultraviolet curable resin, the influence of oxygen acting as a polymerization inhibitor is removed, and the pores of a thermoplastic resin such as polyethylene are irradiated with ultraviolet rays. It is important to remove the thermal effects so as not to be clogged by deformation due to the heat of the time. For this purpose, it is preferable that the resin is cured by irradiating ultraviolet rays through a glass plate whose contact surface with the liquid is coated with a fluororesin.

When a hydrophobic separator base material not treated with a surfactant is used, the separator base material repels an ion-conductive gel electrolyte solution, so that the pores cannot be sufficiently impregnated. When a hydrophilic separator substrate having good properties is used, the ion-conductive gel electrolyte solution can enter the pores of the separator substrate, completely fill the pores, and cover both surfaces of the separator substrate.

[0018]

The internal short circuit caused by the lithium dendrite penetrating through the separator hole and reaching the positive electrode is prevented by using an integrated thin separator in which the fine holes of the separator base material are filled with the ion-conductive gel electrolyte. Therefore, a lithium secondary battery having excellent safety and reliability can be provided.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings.

The separator base material has a pore size of 0.27 μm.
m, a porosity of 60%, and a microporous polyethylene film (Celgard, K878) having a thickness of 20 to 22 μm.

On the other hand, the ion-conductive gel electrolyte layer used is a mixture of an organic electrolytic solution and an ultraviolet curable resin at a weight ratio of 80:20. The organic electrolyte is obtained by dissolving 1 mol / dm ^{3 of} lithium perchlorate LiClO ₄ as an electrolyte in a mixed solvent obtained by mixing propylene carbonate and ethylene carbonate at a volume ratio of 50:50. UV-curable resin is an aliphatic polyether oligomer,
A monomer having a polyethylene oxide skeleton and having acrylate groups at both ends is mixed at a weight ratio of 50:50, and 1.0 wt% of a photopolymerization initiator is further added. After the above polyethylene film is treated with a nonionic surfactant polyethylene glycol alkyl ether to impart sufficient hydrophilicity, the film is coated and impregnated with the above mixture at a concentration of 2.5 μl / cm ^2. I let it. Next, an ion conductive gel electrolyte layer is formed by irradiating ultraviolet rays. When a thin ultraviolet curable resin film is formed, oxygen acts as a polymerization inhibitor.
In addition, there is a problem that the film is deformed by heat due to the irradiation of ultraviolet rays and the holes are crushed. Therefore, in order to improve the releasability, the impregnated film was adhered to a glass plate whose contact surface with a liquid was coated with a fluororesin to prevent the effects of oxygen and heat, and then irradiated with ultraviolet rays. In this way, the ion conductive gel electrolyte 2 was integrated on both sides of the polyethylene film 1 as shown in FIG.

Then, a thickness of 40 μm comprising a polyethylene film and an ion-conductive electrolyte layer closing the pores.
Was created. This separator is referred to as a.

The separator base material is made of polypropylene and has a thickness of 25 μm, a major axis of 0.125 μm, and a minor axis of 0.1 μm.
A microporous membrane separator (Celgard, 2400) having a porosity of 38% and having elliptical holes of 05 μm is used. This was treated for 5 minutes in an air atmosphere at a degree of vacuum of 200 Pa or less using a plasma irradiation device (manufactured by Nippon Vacuum, EP4759) to oxidize the separator and improve lyophilicity. Then, as in the case of the separator a, the separator base material and the ion-conductive gel electrolyte were integrated, and the thickness was 45 μm.
Was created. This separator is referred to as b.

Further, as a separator substrate, a polyethylene made of 25 μm thick, a maximum pore diameter of 0.03 μm, and a porosity of 3
Using an 8% microporous membrane separator (manufactured by Tonen Kagaku), the surface of the separator was oxidized by plasma irradiation treatment in the same manner as for the separator b to improve the lyophilicity. Then, as shown in FIG. 2, a gel electrolyte layer was provided only on one side of the separator, and an ultraviolet curing treatment was performed to form a two-layer structure of the separator and the ion-conductive gel electrolyte. The thickness of the separator on which the gel electrolyte layer was formed was about 30 μm. This separator is denoted by c.

Next, as a comparative example, a separator using only microporous polyethylene is referred to as d. Next, using the separators a, b, and c according to the present invention and the separator d of the comparative example, coin-type lithium secondary batteries A, B, C, and D as shown in FIG.

In FIG. 3, reference numeral 3 denotes a positive electrode,
The manganese dioxide as the positive electrode active material, the carbon as the conductive material, and the polytetrafluoroethylene resin as the binder were mixed at a weight ratio of 90: 5: 5 and molded into a disc having a diameter of 14.2 mm. Reference numeral 4 denotes a stainless steel case, and reference numeral 5 denotes a separator. Reference numeral 6 denotes metallic lithium as a negative electrode active material, 7 denotes a stainless steel sealing plate, and 8 denotes a polypropylene gasket. The electrolytic solution is a mixed solvent of propylene carbonate and 1,2-dimethoxyethane in a volume ratio of 50:50 in which lithium perchlorate is 1 mol / d.
m ³ dissolved.

For the above-mentioned batteries A, B, C and D, a charging / discharging test with a constant current / constant electric quantity of 1.8 mA and an electric quantity of 5.4 mAh (upper limit voltage 3.8 V, lower limit voltage 2.0 V) And the results are shown in FIG.

It can be seen that the batteries A, B, and C of the present invention all have better charge / discharge cycle characteristics than the battery D of the comparative example. In particular, by using a lyophilic separator substrate having good wettability treated with a surfactant, the pores of the separator substrate could be completely and uniformly filled with the ion-conductive gel electrolyte. Can prevent internal micro short circuit by penetrating through the hole of the separator,
It is considered that the cycle characteristics of Battery A became the best.

Also, in the batteries B and C, since the cycle characteristics are greatly improved as compared with the battery D, the pores of the separator substrate are filled with the ion-conducting gel electrolyte by plasma irradiation, thereby causing an internal short circuit. Battery deterioration was prevented.

Next, the mixing ratio of the organic electrolyte solution of the gel electrolyte used in the present embodiment and the ultraviolet curable resin was changed to
FIG. 5 shows the results of measuring the ionic conductivity at ° C. However, when the ratio of the organic electrolyte was 85% by weight or more, the gel electrolyte could not be sufficiently cured. As can be seen from the figure, the ionic conductivity of the gel electrolyte is governed by the electrolyte ratio,
Since the conductivity sharply decreases as the electrolyte solution ratio decreases, the electrolyte solution ratio is desirably 50% by weight or more, and the gel electrolyte having mechanical strength is desirably 80% by weight or less. In addition, although the viscosity depends on the mixing ratio of the monomer and the oligomer of the resin, the viscosity may be changed within a range that does not hinder the film formation. Furthermore, by adjusting the compounding ratio and thickness of the gel electrolyte, it is possible to change the internal resistance of the battery, and to change the short-circuit current of the battery without changing the electrode surface and structure. It can be.

From the above results, by using a separator having a structure in which the fine pores of the olefin resin separator of the porous thin film of the present invention are filled with an ion-conductive gel electrolyte, the dendritic lithium penetrates through the separator and is It was confirmed that the occurrence of a short circuit could be prevented.

In this embodiment, a coin-type battery was used. However, the separator of the present invention in which the gel electrolyte was integrated had sufficient flexibility and thinness, and had a cylindrical shape in which the electrode group was wound. It can also be used for batteries. Further, it is possible to use other than those used in this embodiment for the ultraviolet curing resin, the electrolytic solution, and the positive electrode active material.

[0033]

As described above, the fine pores of the separator were filled with the ion-conductive gel electrolyte, and the dendritic lithium deposited on the lithium negative electrode during charging was prevented from penetrating the separator and causing an internal short circuit. Further, by controlling the mixing ratio, the thickness, and the like of the gel electrolyte, the internal resistance of the battery can be controlled, and the short-circuit current can be reduced.

As a result, it is possible to obtain a highly reliable and safe lithium secondary battery in which no internal short circuit occurs during a charge / discharge cycle.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a separator of the present invention.

FIG. 2 is a schematic sectional view showing another example of the separator of the present invention.

FIG. 3 is a cross-sectional view of a coin-type lithium secondary battery.

FIG. 4 is a charge / discharge cycle life characteristic diagram of a battery.

FIG. 5 is a diagram showing the relationship between the ratio of the organic electrolyte solution of the gel electrolyte and the ionic conductivity.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Polyethylene film 2 Gel electrolyte 3 Positive electrode 4 Case 5 Separator 6 Negative electrode 7 Sealing plate 8 Gasket

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-82457 (JP, A) JP-A-5-190208 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 10/40 H01M 2/16 H01M 2/18

Claims (3)

(57) [Claims] 1. A lithium battery comprising a negative electrode containing lithium or a lithium alloy as an active material, a positive electrode containing metal oxide as an active material, an organic electrolyte, and a separator made of an olefin-based porous resin sealed in a battery container. a secondary battery, the separator has a two-layer structure of the porous resin layer and the ion-conductive gel electrolyte layer, and Ri surface in contact with the negative electrode of the separator Do from an ion conducting gel electrolyte layer and the ion transmission
The neutral gel electrolyte contains 50 to 80% by weight of an organic electrolyte and 20% by weight.
An organic electrolyte lithium secondary battery comprising about 50% by weight of an ultraviolet curable resin . 2. A microporous separator substrate treated with a surfactant is impregnated or coated with a mixture of an ultraviolet-polymerizable resin and an organic electrolyte solution containing a lithium salt to form pores in the separator substrate. The step of filling with the mixed solution, and curing the resin by irradiating ultraviolet rays through a glass plate coated with a fluororesin on the contact surface of the mixed solution, the separator substrate and the ion-conductive gel electrolyte A method for producing a separator for an organic electrolyte lithium secondary battery, comprising a step of integrating the same. 3. An olefin-based porous resin separator is subjected to plasma irradiation in an atmosphere containing a trace amount of oxygen to oxidize the surface of the separator, and then coated with a solution comprising an organic electrolytic solution and an ultraviolet curable resin. A separator for an organic electrolyte lithium secondary battery, in which a thin film layer is formed and then the ultraviolet curable resin is cured by irradiating ultraviolet rays to cure the organic electrolyte gel layer with the separator or to have a two-layer structure with the separator. Manufacturing method.