JPH11260405A - Solid electrolyte-type lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent internal short-circuit caused by dendrite-shaped deposited lithium after long-period use, by forming a diaphragm arranged between a positive electrode and a negative electrode with a micro-porous film having a specific average vacancy fraction and a specific pore size, and by filling the vacancies with a solid electrolyte. SOLUTION: A diaphragm has an average vacancy fraction of 40%-80% and an average pore size of 1 μm or less, and the vacancies are filled with a solid electrolyte, therefore diffusion of lithium caused by charge/discharge is excellent and internal short-circuit caused by dendrite-shaped lithium is prevented. Preferably, a circumferential edge of the diaphragm is positioned outside circumferential edges of a positive electrode and a negative electrode and a vacancy fraction of the outside part is made 30% or less by thermal deposition or the like, to thereby securely prevent the internal short-circuit on the periphery of the positive electrode and the negative electrode where electric current is liable to concentrate. Charge/discharge cycle characteristics are improved by prevention of the internal short-circuit. A micro-porous film is produced from polyethylene, polypropylene, polyvinyl alcohol, polyester or the like by a drawing method or a wet method or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid electrolyte type lithium secondary battery, and more particularly, to a solid electrolyte capable of preventing internal short circuit, improving charge / discharge cycle characteristics, and thereby improving reliability. The present invention relates to a lithium secondary battery.

[0002]

2. Description of the Related Art A solid electrolyte type lithium secondary battery has a negative electrode in which a negative electrode active material is disposed on a flat negative electrode current collector, and a positive electrode in which a positive electrode active material is disposed on a flat positive electrode current collector. Are laminated via an isolating film, and a polymer solid electrolyte which is lightweight, has excellent spreadability, and has good workability is used for the isolating film.

[0003] The above-mentioned solid electrolyte membranes include a polymer matrix such as polyethylene oxide, polypropylene oxide, polyvinylidene fluoride, polyacrylonitrile, etc., in which lithium perchlorate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate are used. A solid electrolyte in which a lithium salt such as an imide salt is dissolved, or a gel electrolyte in which a plasticizer such as γ-butyl lactone, ethylene carbonate, propylene carbonate, or acetonitrile is mixed in the polymer matrix, and the lithium salt is dissolved therein. Is used.

[0004] Such a solid electrolyte membrane is formed by coating the above-mentioned solid electrolyte or gel electrolyte on a positive electrode or a negative electrode and then solidifying the solid electrolyte to a thickness of 10 to 100 µm. In general, one is manufactured and interposed between the positive electrode and the negative electrode. In this case, the solid electrolyte membrane also serves as a separator.

Further, Japanese Patent Application Laid-Open No. 63-40270 proposes to use a separator in which voids of a fibrous nonconductive resin or voids of a microporous film are filled with a solid electrolyte. ing.

[0006]

In the case where only the solid electrolyte membrane is used as the separator as described above, when charge and discharge are repeated, lithium is deposited in a dendrite shape at the periphery of the positive electrode or the negative electrode where current tends to concentrate. Internal short-circuit, or if used continuously at high temperatures, the solid electrolyte or gel electrolyte is softened and the mechanical strength is reduced, which may cause a decrease in the reliability of the solid electrolyte type lithium secondary battery. Had become.

Further, in the case of a separator in which voids of a fibrous non-conductive resin or pores of a microporous film are filled with a solid electrolyte, the diameter of the void is several tens for a fibrous non-conductive resin. To several hundred microns, there is a problem that it is not possible to reliably prevent internal short circuit due to lithium deposited in a dendritic state for long-term use, and if the film is a microporous film, the average porosity is low. Since it is 20% to 40%, there is a problem that the membrane resistance is increased and the battery performance is reduced.

[0008]

According to a first aspect of the present invention, there is provided a solid electrolyte type lithium secondary battery in which a positive electrode and a negative electrode are disposed via an isolation film. Is composed of a microporous film having an average porosity of 40% or more and 80% or less, an average pore size of 1 μm or less, and wherein the pores are filled with a solid electrolyte; Thereby, the microporous film has an average porosity of 40% or more and 80% or less, so that diffusion of lithium during charge and discharge can be improved, and since the average pore size is 1 μm or less, it can be used for a long time. In contrast, an internal short circuit due to lithium precipitated in a dendrite shape can be prevented.

According to a second aspect of the present invention, in the solid electrolyte type lithium secondary battery of the first aspect, the outer peripheral edge of the separator is located outside the outer peripheral edges of the positive electrode and the negative electrode, and The partial porosity of the portion located on the outer side is characterized by being 30% or less, whereby an internal short circuit caused by lithium deposited in a dendrite shape at the periphery of the positive electrode or the negative electrode where current tends to concentrate. Can be reliably prevented.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described based on its embodiments.

A feature of the solid electrolyte type lithium secondary battery according to the embodiment of the present invention is that the separator interposed between the positive electrode and the negative electrode has an average porosity of 40% or more and 80% or less, It consists of a microporous film having an average pore diameter of 1 μm or less, and the pores are filled with a solid electrolyte.

[0012] The microporous film as the separator is
A resin material such as polyethylene, polypropylene, polyvinyl alcohol, polyester, or polyamide is produced by a stretching method, a wet method, or the like, and has an average porosity of 40.
% Or more and 80% or less, more preferably 45% or more and 7% or less.
It is preferably 0% or less, and the average pore diameter is 1 μm or less, more preferably 0.5 μm or less.

Further, the microporous film is cut to be larger than the outer dimensions of the positive electrode and the negative electrode so that the outer peripheral edge is located outside the outer peripheral edges of the positive electrode and the negative electrode. Before or after interposition between the partial porosity of the portion located outside is 30% or less,
More preferably, a treatment such as heat welding is performed to reduce the number of vacancies to 20% or less.

[0014]

FIG. 1 is a sectional view of a solid electrolyte type lithium secondary battery according to an example of the present invention and a comparative example.

The solid electrolyte type lithium secondary battery shown in FIG. 1 has a negative electrode 5 in which metallic lithium as a negative electrode active material 3 is supported on a negative electrode current collector 1 made of copper foil, and acetylene black as a conductive agent. And lithium cobalt oxide as a positive electrode active material 4 containing polyvinylidene fluoride as a binder is supported on a positive electrode current collector 2 made of aluminum foil,
On the surface of the positive electrode active material 4 not in contact with the positive electrode current collector 2, there is provided a solid electrolyte layer 7 formed by impregnating and polymerizing and curing a 1 mol solution of ethylene carbonate of diacrylate polyethylene oxide and lithium hexafluorophosphate. A positive electrode 6 and a microporous film 8 made of polyethylene are impregnated with a 1 mol solution of ethylene carbonate of diacrylate polyethylene oxide and lithium hexafluorophosphate described above, and an isolation membrane 9 polymerized and cured. A laminate in which the membrane 9 is interposed between the negative electrode 5 and the positive electrode 6 and the outer peripheral edge of the separator 9 is made to have a partial porosity of 20% or less by a heat sealer is formed of aluminum 10 and polyethylene terephthalate 11. This was inserted into a laminate made of and sealed by heat welding.

(Evaluation Test 1) A battery A ₁ using the polyethylene microporous film 8 having an average pore diameter of 1 μm or less and an average porosity of 30%, and an average porosity of 45% was used. Battery B ₁ , with an average porosity of 52%
Cell C ₁ using ones, the average porosity of cell D ₁ that was used for 64%, the average porosity of the battery E ₁ which was used in 70%, an average porosity of 80% A battery G ₁ using a battery F ₁ using an average porosity of 85%
Comparative Example in which 10 pieces of each were prepared and only a 1 mol solution of ethylene carbonate of diacrylate polyethylene oxide and lithium hexafluorophosphate was polymerized and cured without using the microporous film 8 made of polyethylene. Were manufactured and subjected to a charge / discharge cycle test, and the discharge capacity after one cycle and the presence or absence of an internal short circuit after 100 cycles were examined. The results are shown in Table 1. The conditions of the charge / discharge cycle test were as follows: in an atmosphere at an ambient temperature of 20 ° C., charging was performed at a constant current of 4 mA, at a constant voltage of 4.1 V for 6 hours, and discharging was performed at a constant current of 4 mA at a constant current of 2.7 V. Up to the end voltage.

[0017]

[Table 1]

From Table 1, the discharge capacity after one cycle was 26 to 30 mAh for all of the batteries B ₁ , C ₁ , D ₁ , E ₁ , F ₁ , G ₁ , and H, Battery A ₁
Was 12 mAh. The presence or absence of an internal short circuit after 100 cycles has elapsed is determined by batteries A ₁ , B ₁ , C ₁ , D ₁ ,
Neither E ₁ nor F ₁ was found, but it was found that battery G ₁ was found in 5 out of 10 batteries and battery H was found in all.

From this, the microporous film 8 as the separator 9 has an average pore diameter of 1 μm or less and an average porosity of 4 μm.
From 0% to 80%, it can be seen that the charge / discharge cycle characteristics can be improved without reducing the discharge capacity after one cycle.

(Evaluation Test 2) The above battery E ₁ using the polyethylene microporous film 8 having an average porosity of 70% and an average pore diameter of 1 μm, and having an average pore diameter of 0.1 μm A battery E ₂ , which has an average pore size of 0.
Battery E ₃ using 3 μm, average pore size 0.5 μm
Battery E ₄ which was used for m, the average pore size of the battery E ₅ which was used in the 1.8 .mu.m, the average pore size to produce 10 pieces each battery E ₆ that was used in the 3.2 .mu.m, the evaluation test 1 and A charge / discharge cycle test was conducted under the same conditions, and the discharge capacity after one cycle and the presence or absence of an internal short circuit after 100 cycles were examined. The results are shown in Table 2.

[0021]

[Table 2]

From Table 2, the discharge capacity after one cycle was 27 to 30 mAh for all of the batteries E _{1 to} E ₆ . In addition, the presence or absence of an internal short circuit after 100 cycles has elapsed was not confirmed for any of the batteries E ₁ , E ₂ , E ₃ , and E ₄ , but for one of the ten batteries E ₅ , E ₆ it was found that was observed in six of the ten.

Accordingly, the microporous film 8 as the isolation membrane 9 has an average pore diameter of 1 μm or less,
It can be seen that the charge / discharge cycle characteristics can be improved without lowering the discharge capacity after one cycle.

(Evaluation Test 3) In contrast to the battery E ₁ using the microporous film 8 having an average porosity of 70% and an average pore diameter of 1 μm used in the evaluation test 2 as the separator 9, A battery E ₁₀ whose outer peripheral edge is located outside the outer peripheral edges of the positive electrode 6 and the negative electrode 5, and a battery E ₁₀ in which a portion located outside the isolation film 9 has a partial porosity of 30%.
_11, each ten batteries E ₁₂ which portion porosity part was set to 20% located outside of the separator 9, the average porosity of 70% was used in the evaluation test 2, the average pore diameter 0.5μ
E using a microporous film 8 of m as a separator 9
_{On the} other hand, the battery E ₄₀ in which the outer peripheral edge of the separator 9 is located outside the outer peripheral edge of the positive electrode 6 and the negative electrode 5, the partial porosity of the portion located outside the separator 9 is 30. battery E ₄₁ was set to%, the battery E ₄₂ which partially porosity of a portion located outside was made to be 20% of the isolating layer 9
And the average porosity used in the evaluation test 1 was 45
%, A microporous film 8 having an average pore diameter of 1 μm
In the battery B ₁ used as the battery B ₁ , a portion of the battery B ₁₀ in which the outer peripheral edge of the separator 9 is located outside the outer peripheral edges of the positive electrode 6 and the negative electrode 5, and a portion located outside the separator 9 porosity battery B ₁₁ was made to be 30%, the partial porosity of a portion located outside of the separator 9 is a battery B ₁₂ produced 10 pieces each which was set to 20%, for each A charge / discharge cycle test was performed, and the presence or absence of an internal short circuit after a lapse of 100 cycles was examined. The results are shown in Table 3. The conditions for the charge / discharge cycle test were as follows: in an atmosphere at an ambient temperature of 20 ° C., charging was performed at a constant current of 10 mA, at a constant voltage of 4.1 V for 3.5 hours, and discharging was performed at a constant current of 4 mA at a constant current of 2. Up to a final voltage of 7V.

[0025]

[Table 3]

[0026] From Table 3, the presence or absence of internal short circuit after 100 cycles elapsed, the four of the battery E ₁₀ is 10, the battery E ₄₀
In but two of the 10, to one of the battery B ₁₀ is 10 pieces, the two of the battery E ₁₁ is 10, to one of the battery E ₄₁ is 10, the battery B ₁₁ 10 Battery E
₁₂ was found in one of the ten batteries, while battery E ₄₂
And it was not observed in the battery B _12.

From this, if the microporous film 8 having an average porosity of 70% is used as the isolation film 9, the one having an average pore diameter of 1 μm is more likely to cause an internal short circuit.
It can be seen that an internal short circuit is more likely to occur when the partial porosity of the portion located outside of the sample is 30%. Also, the isolation film 9
When the microporous film 8 having an average porosity of 45% is used, it is understood that an internal short circuit hardly occurs even if the average porosity is 1 μm.

That is, the separator 9 has an average porosity of 40%.
As long as it is at least 80% and the microporous film 8 has an average pore size of 1 μm or less, it is more preferable that the average porosity is 45% or more and 70% or less. Made of a microporous film 8 having a particle size of 0.5 μm or less, good characteristics can be obtained even for high-rate charging. The above-mentioned isolation film 9 has its outer peripheral edge located outside the outer peripheral edges of the positive electrode 6 and the negative electrode 5, and has a partial porosity of 30% or less, more preferably a portion located outside the outer peripheral edge. If it is 20% or less,
This can be said to be even better because it is possible to prevent an internal short circuit at the peripheral portion of the positive electrode 6 or the negative electrode 5 where current tends to concentrate.

In the above embodiment of the present invention, lithium metal is used as the negative electrode active material 3, lithium cobalt oxide is used as the positive electrode active material 4, and the solid electrolyte layer 7 and the separator 9 are used.
Has been described as an example impregnated with a 1 molar solution of ethylene carbonate of diacrylate polyethylene oxide and lithium hexafluorophosphate. However, the present invention is not limited thereto, and other materials may be used.

[0030]

As described above, according to the first aspect of the present invention, the separator comprises a microporous film having an average porosity of 40% or more and 80% or less and an average pore size of 1 μm or less,
In addition, since the solid electrolyte is filled in the vacancies, lithium diffusion accompanying charge / discharge can be improved, and an internal short circuit due to lithium deposited in a dendrite shape for a long-term use can be prevented. According to the second aspect of the present invention, the outer peripheral edge of the separator is located outside the outer peripheral edges of the positive electrode and the negative electrode, and the partial porosity of the portion located outside the outer edge is 30% or less. Since an internal short circuit at the periphery of the positive electrode or the negative electrode where current tends to concentrate can be reliably prevented, it is possible to contribute to the improvement of the charge / discharge cycle characteristics and the reliability thereof.

[Brief description of the drawings]

FIG. 1 is a sectional view of a solid electrolyte type lithium secondary battery according to an example of the present invention and a comparative example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 negative electrode current collector 2 positive electrode current collector 3 negative electrode active material 4 positive electrode active material 5 negative electrode 6 positive electrode 7 solid electrolyte layer 8 microporous film 9 separator

Claims (2)

[Claims] 1. A solid electrolyte type lithium secondary battery in which a positive electrode and a negative electrode are disposed via a separator, wherein the separator has an average porosity of 40% or more and 80% or less, and an average pore size of 40% or less. A solid electrolyte type lithium secondary battery comprising a microporous film having a thickness of 1 μm or less and having pores filled with a solid electrolyte. 2. The solid electrolyte type lithium secondary battery according to claim 1, wherein the outer peripheral portion of the separator is located outside the outer peripheral edges of the positive electrode and the negative electrode, and a portion of the outer peripheral portion is located outside the outer peripheral portions. A solid electrolyte type lithium secondary battery having a porosity of 30% or less.