Separator and lithium ion secondary battery

Abstract

To provide a separator and a lithium ion secondary battery that can prevent an electrolyte from running out (not being filled) on the electrode surface and improve the lithium ion migration speed.SOLUTION: A separator 20 for a lithium ion secondary battery includes an elastic resin foam 25 having a three-dimensional network structure, the 40% compression hardness of the elastic resin foam 25 is 10 kPa or more, and the thickness of the elastic resin foam 25 is 1.5 mm or more and 2.5 mm or less. The porosity of the elastic resin foam 25 is 92% or more. The elastic resin foam 25 is a melamine resin foam.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a separator for a lithium ion secondary battery and a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries have a high energy density and are widely used in electronic and electrical devices such as smartphones and laptops. In recent years, there has been a growing demand for higher energy density and higher output as storage batteries for drones and regenerative braking. This has led to a demand for higher discharge rate characteristics for lithium-ion secondary batteries.

[0003] In addition, the practical application of lithium-ion secondary batteries using lithium metal as the negative electrode has also been considered. Although lithium metal has the highest energy density of all metals, when lithium metal is used as the negative electrode, lithium precipitates in the form of dendrites during charging, causing partial short circuits, depletion of electrolyte, and the detachment of precipitated lithium. As a result, it is known that the cycle deterioration is significant, and there are safety issues in practical application. For this reason, lithium metal secondary batteries using lithium metal as the negative electrode are not currently in general use. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2011-68883 A [Patent Document 2] JP 2012-134145 A [Patent Document 3] Japanese Patent Application Publication No. 5-205717 Summary of the Invention [Problem to be solved by the invention]

[0005] Lithium ions move between the positive and negative electrodes via the electrolyte. The same is true between the electrodes and the separator. If the electrolyte is unevenly distributed and gaps are formed, lithium ions are less likely to move in the gaps. Therefore, compared to when there are no gaps, there is a risk of significant cycle deterioration due to a decrease in the mobility of lithium ions and the generation of dendrites.

[0006] For these reasons, research and development into secondary batteries has been actively conducted (for example, Patent Documents 1 to 3), but there is still room for improvement.

[0007] The present invention has been made in consideration of the above circumstances, and aims to provide a separator and a lithium ion secondary battery that can suppress the running out (lack of filling) of the electrolyte on the electrode surface and improve the migration speed of lithium ions. [Means for solving the problem]

[0008] (1) In order to achieve the above object, the separator of the present invention is a separator for a lithium ion secondary battery, characterized in that it has an elastic resin foam having a three-dimensional mesh structure, the elastic resin foam has a 40% compression hardness of 10 kPa or more, and the elastic resin foam has a thickness of 1.5 mm or more and 2.5 mm or less.

[0009] Since the separator has an elastic resin foam with a three-dimensional network structure, it can maintain a sufficient porosity even when compressed in a lithium ion secondary battery. In addition, since the thickness of the elastic resin foam is 1.5 mm or more and 2.5 mm or less, when the separator is sandwiched between electrodes in a lithium ion secondary battery and compressed at a pressure of 50 kPa or more and 250 kPa or less, the separator adheres closely to the electrodes without any gaps due to its restoring force. This makes it possible to suppress the electrolyte from running out (not filling) on ​​the electrode surface and to improve the migration speed of lithium ions in the separator. As a result, the discharge rate characteristics of the lithium ion secondary battery are improved, and the occurrence of dendrites in the lithium metal secondary battery is suppressed, thereby improving safety.

[0010] (2) In the separator described in (1) above, the elastic resin foam has a porosity of 92% or more, which allows the separator to maintain a good porosity even when compressed in the secondary battery.

[0011] (3) In the separator according to (1) or (2) above, the elastic resin foam is a melamine resin foam. This allows the elastic resin foam to maintain a good porosity when compressed in a secondary battery, and to adhere closely to the electrode without gaps due to compressive stress. This also prevents the electrolyte from running out (not filling) on ​​the electrode surface.

[0012] (4) The separator according to any one of the above (1) to (3) is characterized in that it has a coating film covering the outer periphery of the elastic resin foam, thereby improving the retention of the electrolyte and suppressing cycle deterioration.

[0013] (5) The lithium ion secondary battery of the present invention is a lithium ion secondary battery including a negative electrode, a positive electrode, and a separator, the separator having an elastic resin foam with a three-dimensional mesh structure, and the elastic resin foam has a porosity of 40% or more and a compressive stress of 50 kPa or more.

[0014] The separator has a porosity of 40% or more and a compressive stress of 50 kPa or more, so it adheres tightly to the electrodes without gaps due to the compressive stress against external pressure while ensuring sufficient porosity, and it is possible to prevent the electrolyte from running out (lack of electrolyte) on the electrode surface. Effect of the Invention

[0015] According to the present invention, it is possible to suppress the electrolyte from running out (lack of filling) on ​​the electrode surface, and to improve the migration speed of lithium ions. [Brief description of the drawings]

[0016] [Figure 1]1A and 1B are conceptual diagrams showing cross sections of the negative electrode and the separator in a lithium ion secondary battery according to the present invention, in which (a) shows the state during charging, and (b) shows the state during discharging. [Diagram 2] 1 is an explanatory diagram showing characteristics of an elastic resin foam according to the present invention. [Diagram 3] FIG. 2 is a diagram showing the configuration of an example of a laminate enclosed in the lithium ion secondary battery according to the embodiment. [Figure 4] FIG. 2 is a diagram showing the configuration of the negative electrode and the separator in the lithium ion secondary battery according to the present invention. [Diagram 5] 1 is a table summarizing the test conditions and test results of each example and each comparative example. [Figure 6] 4 is a graph showing discharge curves for each discharge current in Example 1. [Figure 7] 11 is a graph showing discharge curves for each discharge current in Example 2. [Figure 8] 11 is a graph showing discharge curves at various discharge currents in Example 3. [Figure 9] 13 is a graph showing discharge curves at various discharge currents in Comparative Example 2. [Figure 10] 1 is a graph showing the capacity retention rate for each cycle in Examples 2 and 4. [Figure 11] 1 is a graph showing charge / discharge curves in Example 2. [Figure 12] 13 is a graph showing charge / discharge curves in Comparative Example 2. [Figure 13] FIG. 1 is a conceptual diagram showing a cross section of the negative electrode and separator of a conventional lithium-ion secondary battery, where (a) shows the state during charging, and (b) shows the state during discharging. [Figure 14] FIG. 1 is a conceptual diagram showing a cross section of a negative electrode and a separator in a conventional lithium metal secondary battery. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] [principle] 1, 13 and 14, a cause of the formation of gaps between the electrodes and the separator and the principle of the present invention will be described.

[0018] Figure 13 is a conceptual diagram showing a cross section of the negative electrode and separator of a conventional lithium-ion secondary battery, where (a) shows the state during charging and (b) shows the state during discharging. As shown in Figures 13(a) and (b), the negative electrode 30 absorbs lithium ions during charging, causing a reduction reaction and locally expanding in volume. On the other hand, during discharging, lithium ions are released by an oxidation reaction, and the expanded area contracts.

[0019] At this time, the expansion and contraction of the electrodes causes electrolyte 40 to concentrate locally, resulting in uneven distribution of electrolyte 40. As a result, electrolyte 40 does not spread evenly between the electrodes and separator 120, resulting in gaps 50 in areas where electrolyte 40 is not present.

[0020] In the portion where the gap 50 occurs between the electrode and the separator 120, it becomes difficult for the lithium ions to move between the separator 120 and the electrode. As a result, the mobility of the lithium ions decreases. In addition, in a lithium metal secondary battery using lithium metal as the negative electrode 30, the lithium ions are absorbed on the surface of the electrode, so that the electrode expands and contracts significantly. As shown in FIG. 14, in a lithium metal secondary battery, the lithium ions are concentrated in the area where the electrolyte 40 exists, and the lithium ions may precipitate in a dendrite shape.

[0021] Although the above description is based on the negative electrode, the same phenomenon occurs in the positive electrode. However, the positive electrode undergoes a paired reaction with the negative electrode during charging and discharging. That is, the positive electrode contracts during charging and expands during discharging.

[0022] In contrast, in the present invention, by compressing an elastic separator that is thicker and has a higher porosity than conventional separators in a battery, the generation of gaps can be suppressed by the compressive stress of the separator, thereby improving the discharge rate characteristics of the secondary battery and suppressing the generation of dendrites in lithium metal secondary batteries, thereby improving safety.

[0023] FIG. 1 is a conceptual diagram showing a cross section of the negative electrode and separator in a secondary battery using lithium ions according to the present invention, where (a) shows the state during charging, and (b) shows the state during discharging. As shown in FIG. 1, during charging, the expansion of the negative electrode 30 and the compressive stress of the separator 20 cause the negative electrode 30 to adhere closely to the electrode without any gaps. This prevents the electrolyte from running out (not being filled) on the electrode surface. During discharging, the compressive stress of the separator 20 can prevent the electrolyte 40 from becoming unevenly distributed due to the compression and shrinkage of the electrode.

[0024] Next, an embodiment of the present invention will be described with reference to the drawings. In order to facilitate understanding of the description, the same reference numerals are used for the same components in each drawing, and duplicated descriptions will be omitted. In the configuration diagrams, the size of each component is conceptually shown, and does not necessarily represent the actual dimensional ratio.

[0025] [Separator configuration] The separator of the present invention is a separator for a lithium ion secondary battery or a lithium metal secondary battery, and has an elastic resin foam 25. FIG. 2 is a schematic diagram showing an elastic resin foam holding an electrolyte. The elastic resin foam 25 has a three-dimensional network structure in which a skeleton is formed by crosslinking a plurality of linear polymer chains with a crosslinking agent or a functional group possessed by the polymer chain itself. The three-dimensional network structure refers to a structure in which fibrous or rod-like parts are three-dimensionally connected to form a network skeleton, and makes it possible to realize a high porosity. The elastic resin foam 25 exhibits elasticity within a deformation range to the extent that it is inserted between electrodes as a separator, and when a force is applied, it is distorted, and when the force is removed, it returns to its original dimensions.

[0026] The elastic resin foam 25 has a thickness of 1.5 mm or more and 2.5 mm or less, preferably 2 mm. The elastic resin foam 25 is manufactured in a size larger than the dimension between the electrodes into which the separator 20 is to be inserted, and is adjusted for use as a separator by cutting it out with a cutter or the like. Since the thickness is 1.5 mm or more, the elastic resin foam can be easily processed. Since the thickness is 2.5 mm or less, the movement of lithium ions can be ensured when compressed in a battery, and good discharge rate characteristics can be obtained when applied to a lithium ion secondary battery. The 40% compression hardness of the elastic resin foam 25 is 10 kPa or more. This provides a sufficient repulsive force for the elastic resin foam 25 to adhere closely to the electrodes when compressed in a battery.

[0027] The elastic resin foam 25 preferably has a porosity of 92% or more, and more preferably 98.5% or more. Since the porosity is 92%, a good porosity can be maintained even when compressed inside the lithium ion secondary battery.

[0028] The elastic resin foam 25 may be any material that satisfies the above-mentioned conditions, but it is assumed that the elastic resin foam 25 is formed of a material that is hardly dissolved or swelled in the electrolyte and is highly stable in the electrolyte. The elastic resin foam 25 is preferably a melamine resin foam (MLM). The melamine resin foam has a three-dimensional network structure as shown in FIG. 2, since the bubbles are formed to be continuous. Furthermore, the melamine resin foam can exert a good compressive stress when placed in a lithium ion secondary battery, and can suppress the formation of gaps between the electrodes and the separator. Furthermore, since the melamine resin foam does not dissolve even at the electrode potential in a lithium metal secondary battery, it is also suitable as a separator for a lithium metal secondary battery. The separator may have a coating that covers the outer periphery of the elastic resin foam. Details of the coating will be described later.

[0029] [Lithium-ion secondary battery] The lithium ion secondary battery according to this embodiment is a lithium ion secondary battery including the separator according to this embodiment. FIG. 3 is a schematic diagram for explaining an example of a laminate constituting the lithium ion secondary battery according to this embodiment. The laminate shown in FIG. 3 is enclosed in a case (not shown) to produce a lithium ion battery. Examples of the case include a cylindrical or rectangular metal can, a laminate film, and a heat shrink tube. In the lithium ion secondary battery, a certain pressure is applied inside the case to reduce the volume fluctuation caused by charging and discharging. Examples of the method of applying pressure include pressing the battery with the case and wrapping the battery with a shrink tube. The pressure inside the case is determined by the strength and size of the case, the degree of vacuum when filling the electrolyte, and the like. It is preferable that the pressure inside the case is adjusted to be the same pressure as that when a spring with a spring constant of 3 kgf / mm or more and 5 kgf / mm or less is used in the test cell.

[0030] The laminate 1 includes a positive electrode 10, a separator 20, and a negative electrode 30. The separator 20 is provided to separate the positive electrode 10 and the negative electrode 30, and holds an electrolyte solution 40 as shown in Fig. 2. The separator 20 may be any of the separators in the above-described embodiments, and includes an elastic resin foam having a three-dimensional network structure.

[0031] A positive electrode used in a conventionally known lithium ion secondary battery can be used as the positive electrode 10. For example, lithium cobalt oxide or the like can be used as the positive electrode 10.

[0032] The separator 20 includes an elastic resin foam 25 and a coating 27. The elastic resin foam 25 has a porosity of 40% or more and a compressive stress of 50 kPa or more inside the case. Since the porosity is 40% or more, short circuits due to clogging can be suppressed and the discharge rate characteristics can be improved. Since the compressive stress is 50 kPa or more, the occurrence of gaps between the electrodes and the separator can be suppressed. Since the volume ratio before and after arrangement is 3.0 or more and 10 or less, the occurrence of gaps between the electrodes and the separator 20 can be suppressed by the restoring force when the separator is compressed.

[0033] Coating 27 is preferably made of polyvinylvinylidene fluoride for the purpose of reducing the amount of electrolyte solution retained inside the elastic resin foam after it has flowed out due to evaporation or the like. Coating 27 may reduce the porosity if it penetrates into elastic resin foam 25. Therefore, coating 27 is preferably formed so as to cover the outer periphery of elastic resin foam 25. The outer periphery of elastic resin foam includes at least the outer surface of the elastic resin foam, and refers to a position where evaporation of electrolyte solution is prevented. Coating 27 also has a property of allowing lithium ions to pass therethrough, and the effect on the mobility of lithium ions is suppressed.

[0034] Examples of a method for forming the coating 27 on the outer periphery of the elastic resin foam 25 include dipping, coating with a brush, spray, coating gun, etc. Note that it is preferable for the separator 20 to have the coating 27, but it is not essential that the separator 20 has the coating 27.

[0035] As the negative electrode 30, a negative electrode used in a conventionally known lithium ion secondary battery can be used. As the negative electrode 30, for example, carbon-based materials such as graphite and graphite can be used. In addition, in the lithium ion secondary battery according to this embodiment, lithium metal can also be used as the negative electrode 30. Here, as described above, in order to achieve high energy density, the use of lithium metal, which is a material having the highest energy density, has been studied in the past. However, when lithium metal is used for the negative electrode, it is known that lithium precipitates in a dendrite shape during charging, resulting in significant cycle deterioration due to partial short circuit, depletion of the electrolyte medium, and falling off of the precipitated lithium. In particular, in high-rate and large-sized batteries, there are safety problems such as short circuiting, so that lithium metal secondary batteries have not been generally used so far.

[0036] In contrast, in the lithium ion secondary battery including the separator according to the present embodiment, the occurrence of gaps between the electrodes and the separator can be suppressed by compressive stress, and the occurrence of dendrites can be suppressed. Therefore, lithium metal can be used as the negative electrode 30. For the same reason, a lithium metal alloy can also be used as the negative electrode.

[0037] The lithium ion secondary battery may further include a positive electrode current collector and a negative electrode current collector. The positive electrode current collector and the negative electrode current collector may be made of, for example, stainless steel, gold, platinum, copper, zinc, nickel, tin, aluminum, or an alloy thereof, and may have a plate-like, foil-like, mesh-like, or other shape.

[0038] The electrolyte solution is composed of an electrolyte solvent and an electrolyte salt. As the electrolyte solvent and the electrolyte salt, electrolyte solvents and electrolyte salts used in conventionally known lithium ion secondary batteries can be used. The electrolyte solvent preferably contains a chain carbonate and a cyclic carbonate.

[0039] As the chain carbonate, for example, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) can be used.

[0040] As the cyclic carbonate, for example, ethylene carbonate (EC) and propylene carbonate (PC) can be used.

[0041] The electrolyte salt preferably contains a lithium salt, and any lithium salt that can be used in conventionally known lithium secondary batteries can be used. Specifically, lithium salts such as lithium hexafluorophosphate: LiPF6, lithium tetrafluoroborate: LiBF4, and lithium bis(fluorosulfonyl)imide: LiFSI can be used.

[0042] In addition, it is preferable that the electrolyte salt further contains a polyvalent cation salt. For example, magnesium trifluoromethanesulfonate (Mg(TFS)2):(Mg(SO3CF3)2, magnesium oxide:(MgO), magnesium fluoride (MgF2), magnesium bis(trifluoromethanesulfonyl)imide (MgTFSI2):Mg[N(SO2CF3)2]2 can be used as the polyvalent cation salt. As a result, as shown in FIG. 4, an organic coating or an inorganic coating is formed at the interface between the negative electrode 30 and the electrolyte medium. The formation of an organic coating or an inorganic coating at the interface between the negative electrode 30 and the electrolyte medium suppresses decomposition of the electrolyte.

[0043] The electrolyte may further contain a binder, such as carboxymethyl cellulose (CMC), polyethylene oxide (PEO), or polyvinylidene fluoride (PVdF).

[0044] The lithium ion secondary battery of this embodiment can be manufactured by preparing a separator using an elastic resin foam having a three-dimensional network structure, and sandwiching it between electrodes at a pressure of 50 kPa or more.

[0045] [Example] Next, a test cell of the lithium ion secondary battery was produced and a discharge characteristic test was carried out. [Preparation of test cells for lithium-ion secondary batteries] The conditions for producing each test cell of the lithium ion secondary battery are as follows.

[0046] Example 1 The negative electrode was a graphite electrode (3.6 mAh, 2 cm 2 A lithium cobalt oxide electrode (3 mAh, 2 cm2) coated on an aluminum foil was used as the positive electrode. 2The battery was made of a 1.0M LiPF6-containing electrolyte, manufactured by Mic-Lab. The electrolyte was a mixture of ethylene carbonate and dimethyl carbonate (volume ratio 1:2) with 1.0M LiPF6 added. The separator was made of a white-type consumer melamine resin foam (manufactured by BASF, 40% compression hardness 11 kPa) sold under the name "Gekiochikun (registered trademark)" that was cut to a thickness of 2 mm and an outer diameter of 16 mmφ.

[0047] The positive electrode, negative electrode, separator, and electrolyte were placed in a jig cell (SB2A, a two-pole cell for battery evaluation, manufactured by EC Frontier) to prepare a test cell for Example 1. A spring with a spring constant of 1 kgf / mm was used. The compressive stress in the jig cell was 50 kPa.

[0048] Before being placed in the jig cell, the separator had a porosity of 98.5% and a density of 1.5 gcm 3 The separator placed in the jig cell had a thickness of 0.2 mm, a porosity of 83%, and a volume ratio of 10% before and after placement. The porosity was calculated by calculating the weight from the volume, porosity, and density of the separator before being placed in the jig cell, and by calculating the volume from the thickness of the separator placed in the jig cell.

[0049] Example 2 A test cell was produced under the same process and conditions as in Example 1, except that a spring (3 kgf / mm, SUS316) attached to the jig cell was used as the spring. The compressive stress in the jig cell was 150 kPa. The separator placed in the jig cell had a thickness of 0.09 mm, a porosity of 63%, and a volume ratio before and after placement of 4.5%.

[0050] Example 3 A test cell was produced under the same process and conditions as in Example 1, except that a spring with a spring constant of 5 kgf / mm was used as the spring. The compressive stress in the jig cell was 250 kPa. The separator placed in the jig cell had a thickness of 0.06 mm, a porosity of 45%, and a volume ratio before and after placement of 3.5%.

[0051] Comparative Example 1 A test cell was produced under the same process and conditions as in Example 1, except that a spring with a spring constant of 7 kgf / mm was used as the spring. The compressive stress in the jig cell was 350 kPa. The separator placed in the jig cell had a thickness of 0.04 mm, a porosity of 17%, and a volume ratio before and after placement of 2.0%.

[0052] Comparative Example 2 Except for the fact that a sheet-shaped separator (polypropylene) was used as the separator, a test cell was produced using the same steps and conditions as in Example 1. The separator had a thickness of 0.02 mm and a porosity of 40%.

[0053] (Discharge characteristic test) A discharge characteristic test was carried out using each of the above-mentioned Examples and Comparative Examples. The discharge characteristic test was carried out in a thermostatic chamber at 25°C with a constant current applied and a cut-off voltage of 4.2 V to 3.0 V. In addition, the discharge characteristic test was carried out at a current of 0.75 mAcm -2 (0.5C) and discharge current 1.5mAcm -2 (1C), 3mAcm -2 (2C), 6mAcm -2 (4C), 12mAcm -2 (8C) and measured in each case.

[0054] Battery with a discharge capacity of less than 2.5mAh at 8C or less than 1.5mAh at 1C was rated as pass (C), battery with a discharge capacity of 2.5mAh or more at 8C or 1.5mAh or more at 1C was rated as good (B), and battery with a discharge capacity of 2.5mAh or more at 8C or 2.0mAh or more at 1C was rated as excellent (A).

[0055] (result) Fig. 5 is a table summarizing the test conditions and test results of each Example and each Comparative Example. Fig. 6 is a graph showing the discharge curves at each discharge current in Example 1, and Fig. 7 is a graph showing the discharge curves at each discharge current in Example 2. Fig. 8 is a graph showing the discharge curves at each discharge current in Example 3, and Fig. 9 is a graph showing the discharge curves at each discharge current in Comparative Example 2. As shown in Fig. 5, Comparative Example 1 was inoperable, so a discharge curve could not be obtained.

[0056] It was confirmed that Examples 1 to 3 and Comparative Example 2 operated normally. Moreover, Examples 2 and 3 showed better discharge capacity at 8C than Example 1 and Comparative Example 2. Since Example 3 with a porosity of 45% showed better discharge capacity than Comparative Example 2 with a porosity of 40%, it was confirmed that even with the same porosity, the rate performance was higher when there was no gap between the separator and the electrode due to the repulsive force. Moreover, Example 2 had a smaller difference in discharge capacity at 1C and 8C than Example 3. From this, it was confirmed that Example 2 had better rate characteristics than Example 3.

[0057] Next, a charge-discharge cycle test was carried out to verify the effect of the polyvinylvinylidene fluoride coating on the cycle characteristics.

[0058] Example 4 A test cell was produced using the same process and conditions as in Example 2, except that a polyvinylvinylidene fluoride coating was provided on the outer periphery of the separator by immersing the separator in polyvinylvinylidene fluoride (5 wt % NMP solution).

[0059] (Charge-discharge cycle test) A charge-discharge cycle test was carried out using Examples 2 and 4. The charge-discharge cycle characteristic test was carried out by applying a constant current in a thermostatic chamber at 25°C with a cut-off voltage of 4.2 V to 3.0 V. The charge was performed at 0.75 mA cm -2 (0.5C) discharge is 6mAcm -2The cycle was repeated 50 times under the condition of (4C). For the test cells of Examples 1 and 4, the percentage of the discharge capacity in each cycle was calculated based on the discharge capacity at the first charge / discharge, and the calculated value was taken as the capacity retention rate.

[0060] The changes in capacity retention rate over time for the test cells of Examples 2 and 4 are shown in Fig. 10. In Fig. 10, open diamonds indicate the results of the charge capacity for Example 2, and open circles indicate the results of the discharge capacity for Example 2. In addition, striped diamonds indicate the results of the charge capacity for Example 4, and black circles indicate the results of the discharge capacity for Example 4.

[0061] 10, in both charging and discharging, the capacity retention rate was higher in Example 4 than in Example 2. Therefore, it was confirmed that the polyvinyl vinylidene fluoride coating film improves the cycle characteristics.

[0062] Next, in Example 2 and Comparative Example 2, the cycle characteristics of the lithium metal secondary batteries were examined. [Preparation of test cells for lithium metal secondary batteries] The conditions for producing each test cell of the lithium metal secondary battery are as follows. The test cell for the lithium metal secondary battery had a lithium metal electrode (thickness 20 μm, area 1.5 cm) as the negative electrode. 2 Except for the fact that a 100% ZnO battery (manufactured by Honjo Chemical Co., Ltd.) was used, the test cells were fabricated using the same process and under the same conditions as those for the lithium-ion secondary battery test cells.

[0063] (Charge-discharge cycle test) The charge / discharge characteristic test was performed in a thermostatic chamber at 25°C with a constant current applied and a cutoff voltage of 4.2 V to 3.0 V. The charge was performed at 0.75 mA cm -2 (0.5C), 6mAcm -2 The discharge cycle under the condition (4C) was repeated 30 times.

[0064] Fig. 11 is a diagram showing charge / discharge curves in the test cell of Example 2. Fig. 12 is a diagram showing charge / discharge curves in the test cell of Comparative Example 2. Example 2 and Comparative Example 2 show similar capacities at 5 cycles, but Example 2 maintains a larger capacity value than Comparative Example 2 at 30 cycles. Therefore, it was confirmed that the separator of the present invention shows superior cycle characteristics in lithium metal secondary batteries compared to conventional sheet-like separators. [Explanation of symbols]

[0065] 1. Laminate 10 positive electrode 20 Separator 25 Elastic resin foam 27 Coating 30 negative electrode 40 Electrolyte 50 Gap (between electrode and separator) 120 Conventional separator

Claims

1. A separator for lithium-ion secondary batteries, It has an elastic resin foam having a three-dimensional network structure, A separator characterized in that the elastic resin foam has a 40% compression hardness of 10 kPa or more.

2. The separator according to claim 1, characterized in that the porosity of the elastic resin foam is 92% or more.

3. The separator according to claim 1 or 2, characterized in that the elastic resin foam is a melamine resin foam.

4. The separator according to claim 1 or 2, characterized by having a coating that covers the outer circumference of the elastic resin foam.

5. A lithium-ion secondary battery comprising a negative electrode, a positive electrode, and a separator, The separator has an elastic resin foam having a three-dimensional network structure, The lithium-ion secondary battery is characterized in that the elastic resin foam has a porosity of 40% or more and a compressive stress of 50 kPa or more.

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