JP2022118649A - Separator used for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

To provide a technique capable of suppressing the occurrence of an internal short circuit while suppressing deterioration of battery performance.SOLUTION: In a technology disclosed herein, a separator for use in a lithium ion secondary battery is provided. This separator includes a base material and a heat-resistant layer containing a heat-resistant filler, formed on the base material. The heat-resistant layer contains a polycyclic aromatic hydrocarbon.SELECTED DRAWING: Figure 3

Description

TECHNICAL FIELD The present invention relates to a separator used in a lithium ion secondary battery and a lithium ion secondary battery. More specifically, it relates to the structure of a heat-resistant layer of a separator comprising a base material and a heat-resistant layer formed on the base material.

In recent years, secondary batteries such as lithium-ion secondary batteries have been used in portable power sources such as personal computers and mobile terminals, and power sources for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). is preferably used for

A secondary battery (for example, a lithium-ion secondary battery) for use as described above typically includes an electrode body having a positive electrode, a negative electrode, and a separator interposed therebetween to separate the two electrodes. . In order to use secondary batteries safely, various methods have been conventionally studied to suppress short-circuiting of positive and negative electrodes during charging and discharging of secondary batteries, for example. Patent Literature 1 proposes covering at least the surface of the positive electrode facing the negative electrode with an insulator. Further, Patent Literature 2 describes that a separator is provided with a heat-resistant layer.

JP-A-6-168739 WO 2018/037869

By the way, when a secondary battery is conventionally devised to prevent a short circuit, the detachment/insertion of lithium ions in the active material may be inhibited, which could be a factor in degrading the battery performance. Therefore, there is a demand for a technique for preventing short circuits without deteriorating battery performance.

The present invention was created in view of such circumstances, and an object of the present invention is to provide a technique capable of suppressing the occurrence of an internal short circuit while suppressing deterioration of battery performance.

The present inventor focused on deposition of metallic lithium, which is one of the causes of short circuits in lithium ion secondary batteries. When a lithium-ion secondary battery is charged and discharged, metallic lithium is deposited on one electrode (e.g., the negative electrode), and when this grows in a dendrite shape, it penetrates the separator and reaches the other electrode (e.g., the positive electrode). can reach and cause a short circuit. The present inventor has made extensive studies on suppressing the deposition of metallic lithium or reionizing the deposited metallic lithium. As a result, it was found that by including a polycyclic aromatic hydrocarbon in the heat-resistant layer of the separator used in a lithium ion secondary battery, it is possible to suppress the deterioration of the battery performance and suppress the occurrence of internal short circuit. I came to complete it.

That is, according to the technology disclosed herein, a separator for use in lithium ion secondary batteries is provided. This separator includes a base material and a heat-resistant layer containing a heat-resistant filler formed on the base material. Here, the heat-resistant layer contains a polycyclic aromatic hydrocarbon. The polycyclic aromatic hydrocarbon can suppress deposition of metallic lithium or reionize the deposited metallic lithium. Therefore, by using the separator having the above configuration, it is possible to prevent inhibition of desorption/insertion of lithium ions in the active material. It is possible to suppress the occurrence of an internal short circuit while suppressing deterioration of the battery performance of the lithium ion secondary battery.

In a preferred aspect, the heat-resistant layer is formed on one surface of the base material. According to such a configuration, it is possible to appropriately achieve the effects of the technology disclosed herein.

In another preferred embodiment, coronene or quaterrylene is included as the polycyclic aromatic hydrocarbon. By including these compounds in the heat-resistant layer, the effects of the technology disclosed herein can be appropriately realized.

Further, the technology disclosed herein includes an electrode body having a positive electrode having a positive electrode active material layer, a negative electrode having a negative electrode active material layer, and a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. and a lithium ion secondary battery. This lithium ion secondary battery includes the separator disclosed here. As described above, the separator suppresses the deterioration of the battery performance of the secondary battery (for example, a lithium ion secondary battery) (for example, the deterioration of the capacity retention rate), and suppresses the occurrence of an internal short circuit in the lithium ion secondary battery. can do. Therefore, in the lithium-ion secondary battery disclosed herein, both deterioration of battery performance and occurrence of internal short circuit are suppressed.

In a preferred embodiment, the heat-resistant layer is formed on the surface of the substrate on the positive electrode active material layer side. In the lithium-ion secondary battery having such a configuration, the effects of the technology disclosed herein are appropriately achieved.

1 is a cross-sectional view schematically showing the internal structure of a lithium ion secondary battery using a separator according to one embodiment; FIG. 1 is a schematic exploded view showing the configuration of a wound electrode body of a lithium-ion secondary battery using a separator according to one embodiment; FIG. FIG. 3 is a schematic diagram showing a part of the laminated structure of the wound electrode body of the lithium ion secondary battery using the separator according to one embodiment.

Hereinafter, embodiments according to the technology disclosed herein will be described with reference to the drawings. Matters not mentioned in this specification but necessary for implementing the technology disclosed here can be understood as design matters by those skilled in the art based on the prior art in the relevant field. The technology disclosed here can be implemented based on the content disclosed in this specification and common general technical knowledge in the field.

In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged, and is a term that includes so-called storage batteries and electricity storage elements such as electric double layer capacitors. In this specification, the term "lithium ion secondary battery" refers to a secondary battery that utilizes lithium ions as a charge carrier and that charges and discharges by the transfer of charge associated with the lithium ions between the positive and negative electrodes. In the drawings below, members and portions having the same function are denoted by the same reference numerals. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships. Hereinafter, one embodiment of the technology disclosed herein will be described in detail by taking a flat prismatic lithium ion secondary battery as an example, but the intention is to limit the technology disclosed here to such an embodiment. is not.

As shown in FIG. 1, a lithium ion secondary battery 100 is a flat prismatic battery in which a flat wound electrode body 20 (hereinafter also simply referred to as "electrode body 20") and a non-aqueous electrolyte 80 are formed. It is a sealed battery constructed by being housed in a case (that is, an outer container) 30 . The battery case 30 has a positive terminal 42 and a negative terminal 44 for external connection, and a thin safety valve 36 that is set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. is provided. Further, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte 80 . The positive terminal 42 is electrically connected to the positive collector plate 42a. The negative terminal 44 is electrically connected to the negative collector plate 44a. As the material of the battery case 30, for example, a metal material such as aluminum that is lightweight and has good thermal conductivity is used.

As shown in FIGS. 1 to 3, the electrode body 20 is a positive electrode sheet 50 in which a positive electrode active material layer 54 is formed on one side or both sides (here, both sides) of a long positive electrode current collector 52 along the longitudinal direction. and a negative electrode sheet 60 in which a negative electrode active material layer 64 is formed along one side or both sides (here, both sides) of a long negative electrode current collector 62 along the longitudinal direction, and two long separators 70 It has a configuration that is superimposed via and longitudinally turned. The positive electrode active material layer non-forming portions 52a (i.e., positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed) and the negative electrode active material layer non-formation portion 62a (that is, the portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64 being formed). , a positive collector plate 42a and a negative collector plate 44a are respectively joined.

For the positive electrode sheet 50 and the negative electrode sheet 60, materials similar to those used in conventional lithium ion secondary batteries can be used without particular limitation. A typical aspect is shown below.

Examples of the positive electrode current collector 52 forming the positive electrode sheet 50 include aluminum foil. The positive electrode active material layer 54 contains at least a positive electrode active material. Examples of positive electrode active materials include lithium transition metal oxides (eg, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNiO ₂ , LiCoO ₂ , LiFeO ₂ , LiMn ₂ O ₄ , LiNi _0.5 Mn _{1 .5O4} , etc.), lithium transition metal phosphate compounds ₍ eg, _LiFePO4 , etc.), and the like. The positive electrode active material layer 54 may contain components other than the active material, such as a conductive material and a binder. Carbon black such as acetylene black (AB) and other carbon materials (eg, graphite) can be suitably used as the conductive material. As the binder, for example, polyvinylidene fluoride (PVDF) or the like can be used.

Examples of the negative electrode current collector 62 forming the negative electrode sheet 60 include copper foil. The negative electrode active material layer 64 contains at least a negative electrode active material. As the negative electrode active material, for example, carbon materials such as graphite, hard carbon, and soft carbon can be used, and graphite is preferred. The negative electrode active material layer 64 may contain components other than the active material, such as binders and thickeners. As the binder, for example, styrene-butadiene rubber (SBR) or the like can be used. As a thickening agent, for example, carboxymethyl cellulose (CMC) or the like can be used.

As the separator 70, the separator disclosed here is used. As shown in FIG. 3, the separator 70 includes a base material 74 and a heat-resistant layer 72. As shown in FIG. The heat-resistant layer 72 is formed on at least one surface of the substrate 74 in the stacking direction of the positive electrode sheet 50 , the negative electrode sheet 60 and the separator 70 . By providing the heat-resistant layer 72, for example, thermal contraction of the base material 74 during charging and discharging of the lithium ion secondary battery 100 can be suppressed, and the occurrence of an internal short circuit can be prevented. In FIG. 3, the heat-resistant layer 72 is formed on the surface of the base material 74 on the positive electrode active material layer 54 side. Such a configuration is preferable because the effect of suppressing thermal shrinkage of the base material 74 by the heat-resistant layer 72 can be easily obtained.

The base material 74 is typically a porous resin sheet. Examples of the resin material forming the porous resin sheet include polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide and the like. The base material 74 may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer).

The heat-resistant layer 72 contains a heat-resistant filler, a binder, and a polycyclic aromatic hydrocarbon. As the heat-resistant filler and binder, materials typically used in lithium-ion secondary batteries of this type can be used without particular limitation. Examples of heat-resistant fillers include metal oxides such as alumina (Al ₂ O ₃ ), magnesia (MgO), silica (SiO ₂ ) and titania (TiO ₂ ); metal nitrides such as aluminum nitride and silicon nitride; Metal hydroxides such as calcium oxide, magnesium hydroxide and aluminum hydroxide can be mentioned. Examples of binders include acrylic resins.

The separator disclosed herein is characterized by the heat-resistant layer containing a polycyclic aromatic hydrocarbon. A polycyclic aromatic hydrocarbon is a hydrocarbon having a structure in which a plurality of aromatic rings (e.g., six-membered rings) are condensed. In the present embodiment, the polycyclic aromatic hydrocarbon has at least four six-membered rings. A compound having The six-membered ring may be fused with another ring. In counting the number of six-membered rings, for example, the number of six-membered rings possessed by tetracene in which four six-membered rings are two-dimensionally condensed is counted as four. In addition, the number of six-membered rings possessed by coronene, in which six six-membered rings are cyclically condensed to form a further six-membered ring in the central portion, is counted as seven.

Polycyclic aromatic hydrocarbons include compounds in which a substituent is introduced into a structure in which multiple aromatic rings (eg, six-membered rings) are condensed. The substituent may be, for example, one aromatic ring (eg, six-membered ring) or a group in which multiple aromatic rings are condensed (eg, naphthyl group). For example, the number of six-membered rings in dinaphthoanthracene (1-dinaphthoanthracene, 2-dinaphthoanthracene) in which two naphthyl groups are bonded to anthracene in which three six-membered rings are condensed is counted as seven. .

Examples of the polycyclic aromatic hydrocarbons include pyrene (4 six-membered rings, 16 carbon atoms), chrysene (4 six-membered rings, 18 carbon atoms), tetracene (4 six-membered rings, 4 carbon atoms), 18), triphenylene (4 six-membered rings, 18 carbon atoms), perylene (5 six-membered rings, 20 carbon atoms), pentacene (5 six-membered rings, 22 carbon atoms), benzopyrene (6-membered 5 rings, 20 carbon atoms), benzoperylene (6 6-membered rings, 22 carbon atoms), corannulene (5 6-membered rings, 20 carbon atoms), coronene (7 6-membered rings, 20 carbon atoms) 24), 1-dinaphthoanthracene (six-membered ring number 7, carbon number 34), 2-dinaphthoanthracene (six-membered ring number 7, carbon number 34), rubrene (six-membered ring number 8, carbon number 42), terylene (8 six-membered rings, 30 carbon atoms), ovalene (10 six-membered rings, 32 carbon atoms), hexabenzotetracene (10 six-membered rings, 40 carbon atoms), quaterrylene (6 11 membered rings and 40 carbon atoms). These can be used alone or in combination of two or more. The number of six-membered rings in the polycyclic aromatic hydrocarbon can be 5 or more (for example, 6 or more). There is preferably one, and it can be eight or more, nine or more, or ten or more. Although not particularly limited, the number of six-membered rings in the polycyclic aromatic hydrocarbon can be 15 or less.

From the viewpoint of efficiently realizing the effects of the technology disclosed herein, preferred examples of the polycyclic aromatic hydrocarbon include coronene (formula (1) below) and 1-dinaphthoanthracene (formula (2) below). , 2-dinaphthanthracene (formula (3) below), rubrene (formula (4) below), terylene (formula (5) below), and quaterylene (formula (6) below). Among them, it is preferable to use coronene or quaterrylene.

Formula (1):

Formula (2):

Formula (3):

Formula (4):

Formula (5):

Formula (6):

Although not particularly limited, the inclusion of polycyclic aromatic hydrocarbons can reduce deposited metallic lithium (Li) by the following mechanism, for example. The polycyclic aromatic hydrocarbon can effectively coordinate lithium ions to an aromatic ring (for example, a six-membered ring). Therefore, when the polycyclic aromatic hydrocarbon is contained in the heat-resistant layer of the separator, the following formula (A):
Li+PAH→LiPAH (A)
, the polycyclic aromatic hydrocarbon (PAH) reacts with the deposited metal lithium (Li) and extracts Li as ions to form LiPAH in which Li and PAH are coordinated.
In addition, LiPAH produced as described above can be represented by the following formula (B):
LiPAH → Li ⁺ +PAH ^- (B)
, can be dissociated into lithium ions (Li ⁺ ) and PAH ions (PAH ⁻ ) in the non-aqueous electrolyte. In this way, since the deposited metallic lithium (Li) is reionized as described above, it is possible to suppress internal short circuits while suppressing deterioration in battery performance.

As the number of aromatic rings (eg, six-membered rings) contained in the compound of the polycyclic aromatic hydrocarbon increases, the extraction of lithium ions from metallic lithium increases. That is, it can be said that the more the number of aromatic rings, the higher the effect of suppressing deterioration of battery performance and the effect of suppressing short circuit. On the other hand, large polycyclic aromatic hydrocarbons have low solubility in non-aqueous electrolytes. Therefore, by including such a compound in the heat-resistant layer of the separator, the above effect can be realized more favorably.

From the viewpoint of suppressing deterioration in battery performance, it is preferable that the polycyclic aromatic hydrocarbon be contained only in the heat-resistant layer 72 . As a result, the polycyclic aromatic hydrocarbon can be prevented from clogging the pores of the separator, and lithium ions can smoothly move between the electrodes. By including the polycyclic aromatic hydrocarbon in the heat-resistant layer, the deposited metallic lithium is reionized without suppressing the desorption/insertion of lithium ions into the active material, and the deterioration of the battery performance is suppressed. , the occurrence of a short circuit can be suppressed. However, the polycyclic aromatic hydrocarbon may be contained in the base material 74 in addition to the heat-resistant layer 72 as long as the effects of the technology disclosed herein can be realized. When the polycyclic aromatic hydrocarbon is included in the positive electrode or the negative electrode, the polycyclic aromatic hydrocarbon is arranged on the surface of the positive electrode active material or the negative electrode active material, and lithium ions are desorbed/inserted into the active material. can be inhibited. In addition, a potential difference is generated between the active material and the polycyclic aromatic hydrocarbon arranged on the surface of the active material (the battery is locally formed), and the battery reaction between the positive electrode and the negative electrode may not be performed smoothly.

A method for incorporating the polycyclic aromatic hydrocarbon into the heat-resistant layer of the separator is not particularly limited, but a spin coating method can be employed as described in Examples below. Alternatively, it may be contained in the heat-resistant layer by using a heat-resistant layer-forming slurry containing a predetermined amount of polycyclic aromatic hydrocarbon. The content of the polycyclic aromatic hydrocarbon in the heat-resistant layer 72 is not particularly limited, and can be appropriately changed as necessary.

A method for detecting the polycyclic aromatic hydrocarbon contained in the separator 70 (typically, the heat-resistant layer 72) is not particularly limited. As an example, a separator test piece is prepared and immersed in tetrachloroethane to extract polycyclic aromatic hydrocarbons. Next, the polycyclic aromatic hydrocarbon extract is concentrated, and the concentrated residue is subjected to 1H-NMR measurement using tetrachloroethane-d4 as a heavy solvent, MALDI-TOF-MS analysis using a THF suspension, etc. Polycyclic aromatic hydrocarbons can be detected by In addition, as described in formula (B) above, ions of polycyclic aromatic hydrocarbons may be present in the non-aqueous electrolyte. Such polycyclic aromatic hydrocarbon ions can be detected by concentrating the non-aqueous electrolyte to be tested and performing the above-mentioned 1H-NMR measurement, MALDI-TOF-MS analysis, etc. on the concentrated residue. can be done.

The separator disclosed herein can be used in lithium ion secondary batteries according to known methods. By using the separator disclosed herein in a lithium ion secondary battery, deposited metallic lithium can be reionized in the lithium ion secondary battery. Therefore, it is possible to suppress an internal short circuit due to dendrite growth of deposited metallic lithium. In addition, deterioration of battery performance due to precipitation of metallic lithium in the lithium ion secondary battery can be suppressed.

Non-aqueous electrolyte 80 typically contains a non-aqueous solvent and a supporting salt. As the non-aqueous solvent and the supporting salt, various solvents used in the electrolytic solution of this type of lithium ion secondary battery can be used without particular limitation. Non-aqueous solvents include, for example, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoro Carbonates such as methyldifluoromethyl carbonate (F-DMC) and trifluorodimethyl carbonate (TFDMC) can be mentioned. Such non-aqueous solvents can be used singly or in combination of two or more. Lithium salts such as LiPF ₆ , LiBF ₄ and LiClO ₄ (preferably LiPF ₆ ) can be used as the supporting salt. The concentration of the supporting salt is preferably 0.7 mol/L or more and 1.3 mol/L or less. Moreover, the non-aqueous electrolytic solution 80 may contain conventionally known additives such as various film-forming agents, thickeners, and dispersing agents, if necessary. Note that FIG. 1 does not strictly show the amount of non-aqueous electrolyte 80 injected into battery case 30 .

The lithium ion secondary battery 100 can be used for various purposes. Suitable applications include drive power supplies mounted in vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). Moreover, the lithium ion secondary battery 100 can be used as a storage battery such as a small power storage device. The lithium ion secondary battery 100 can also be used typically in the form of an assembled battery in which a plurality of batteries are connected in series and/or in parallel.

As an example, the prismatic lithium ion secondary battery 100 including the flattened wound electrode assembly 20 has been described. However, the lithium ion secondary battery can also be configured as a lithium ion secondary battery including a laminated electrode body (that is, an electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated). Also, the lithium ion secondary battery can be configured as a cylindrical lithium ion secondary battery, a laminated lithium ion secondary battery, or the like.

EXAMPLES Examples relating to the present invention will be described below, but the present invention is not intended to be limited to the examples shown below.

<Production of lithium-ion secondary battery for evaluation>
Evaluation lithium ion secondary batteries according to Examples 1 to 10 were produced as follows.
-Example 1-
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (LNCM) as a positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder, LNCM: AB : PVdF = 87:10:3, and mixed with N-methylpyrrolidone (NMP) to prepare a slurry for forming a positive electrode active material layer. This slurry was applied to an aluminum foil and dried to prepare a positive electrode sheet having a positive electrode active material layer. The dimensions of the positive electrode active material layer were 47 mm×45 mm.

Graphite (C) as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed at a mass ratio of C:SBR:CMC=98:1:1. A negative electrode active material layer forming slurry was prepared by mixing with ion-exchanged water. A negative electrode sheet having a negative electrode active material layer was produced by applying this slurry to a copper foil and drying it. The dimensions of the negative electrode active material layer were 49 mm×47 mm.

A microporous polypropylene sheet having a thickness of 12 μm was prepared as a separator base material. Alumina powder as an inorganic filler, PVDF as a binder, and carboxymethyl cellulose (CMC) as a thickener were dispersed in water to prepare a slurry for forming a heat-resistant layer. This slurry was applied to one side of the microporous polypropylene sheet to form a heat-resistant layer having a thickness of 4 μm. Next, ethyl methyl carbonate containing 0.1 M concentration of coronene as a polycyclic aromatic hydrocarbon was prepared and spin-coated on the heat-resistant layer. The separator base material with the heat-resistant layer after the drying treatment was processed into a bag shape so that the heat-resistant layer was on the inside. Thus, a separator containing a positive electrode sheet was produced. The dimensions of the separator were 51×49 mm.

A microporous polypropylene sheet having a thickness of 12 μm was prepared as a separator base material. The microporous polypropylene sheet was processed into a bag. Thus, a separator containing a negative electrode sheet was produced. The dimensions of the separator were 51×49 mm.

After each of the positive electrode sheet and the negative electrode sheet was accommodated in a bag-like separator, the positive electrode active material layer and the negative electrode active material layer were laminated so as to face each other to prepare an electrode body. A collector terminal was attached to this electrode body, and this was housed in a laminate case. A lithium ion secondary battery for evaluation according to Example 1 was obtained by injecting a non-aqueous electrolyte into the laminate case and heat-sealing the laminate case for sealing. As the non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of EC:EMC:DMC=3:3:4 is supported. LiPF ₆ as a salt dissolved at a concentration of 1.16M was used. Moreover, in this example, the polycyclic aromatic hydrocarbon is contained only in the heat-resistant layer. In Table 1, the polycyclic aromatic hydrocarbon-containing site (heat-resistant layer in Example 1) is marked with "○", and "-" indicates that it does not contain polycyclic aromatic hydrocarbons. there is

-Examples 2 to 5-
The heat-resistant layer was spin-coated with ethyl methyl carbonate containing coronene at a concentration shown in the corresponding column of Table 1. Other than that, using the same materials and procedures as in Example 1, lithium ion secondary batteries for evaluation according to Examples 2 to 5 were produced.
-Example 6-
A lithium ion secondary battery for evaluation according to Example 6 was produced using the same materials and procedures as in Example 1, except that quaterrylene was used as the polycyclic aromatic hydrocarbon.

-Example 7-
The positive electrode sheet having the positive electrode active material layer prepared as described above was immersed in ethyl methyl carbonate containing 1.0 M concentration of coronene as the polycyclic aromatic hydrocarbon for 10 minutes. A lithium-ion secondary battery for evaluation according to Example 7 was produced using the same materials and procedures as in Example 1, except that this was dried before use.

-Example 8-
The negative electrode sheet having the negative electrode active material layer prepared as described above was immersed in ethyl methyl carbonate containing 1.0 M concentration of coronene as the polycyclic aromatic hydrocarbon for 10 minutes. A lithium-ion secondary battery for evaluation according to Example 8 was produced using the same materials and procedure as in Example 1, except that this was dried before use.

-Example 9-
A lithium-ion secondary battery for evaluation according to Example 9 was prepared using the same materials and procedures as in Example 1, except that the heat-resistant layer of the separator containing the positive electrode sheet was not spin-coated with the polycyclic aromatic hydrocarbon. was made.

<Short-circuit suppression evaluation>
The short-circuit suppressing effect was evaluated for each lithium-ion secondary battery for evaluation produced as described above. One cycle of constant current charging at a current value of 1 C from 3.0 V to 4.3 V and constant current discharging at 0.5 C from 4.3 V to 3.0 V was performed for each lithium ion secondary battery for evaluation. The battery was charged and discharged. The voltage of each lithium ion secondary battery for evaluation was measured when charging and discharging were performed for a predetermined number of cycles, and when a voltage drop of 200 mV was confirmed, it was determined that a short circuit had occurred. The number of charge/discharge cycles was 100 cycles at maximum. The results are shown in the corresponding columns of Table 1.

The evaluation criteria for the short-circuit suppression effect are as follows.
⊚ (excellent short-circuit suppressing effect): No short-circuit after 100 charge/discharge cycles.
○ (good short-circuit suppressing effect): no short-circuit after 90 cycles of charging and discharging.
x (no short-circuit suppressing effect): Short-circuit occurred in less than 90 charge-discharge cycles.

<Evaluation of capacity retention rate after cycle test>
The capacity (initial capacity) of each lithium-ion secondary battery for evaluation produced above was measured. One cycle of constant current charging at a current value of 1 C from 3.0 V to 4.3 V and constant current discharging at 0.5 C from 4.3 V to 3.0 V was performed for each lithium ion secondary battery for evaluation. Three cycles of charging and discharging were performed. After that, the capacity of each evaluation lithium ion secondary battery was measured, and the value obtained using the following formula (1) was defined as the capacity retention rate (%) of each example. The results are shown in the corresponding columns of Table 1.
Formula (1): Capacity retention rate (%) = (capacity after cycle test/initial capacity) x 100

As shown in Table 1, in the lithium ion secondary batteries for evaluation according to Examples 1 to 6 having a separator containing a polycyclic aromatic hydrocarbon in the heat-resistant layer, the lithium ion secondary battery for evaluation according to Example 9 containing no polycyclic aromatic hydrocarbon It was found that the short-circuit suppressing effect was improved when compared with the lithium-ion secondary battery for evaluation. In addition, it was found that both the effect of suppressing a short circuit and the effect of suppressing a decrease in capacity retention rate can be achieved. Further, it was found that Examples 1 to 5 containing coronene as the polycyclic aromatic hydrocarbon realized the above effect depending on the concentration of the polycyclic aromatic hydrocarbon. Further, when comparing Example 6 containing quaterrylene as the polycyclic aromatic hydrocarbon and Example 2 containing coronene, the above effects are more effectively realized when the number of six-membered rings contained in the polycyclic aromatic hydrocarbon increases. found to get

In addition, in the lithium ion secondary batteries for evaluation according to each of Example 7 containing the polycyclic aromatic hydrocarbon only in the positive electrode and Example 8 containing the polycyclic aromatic hydrocarbon only in the negative electrode, the short circuit suppressing effect and the capacity retention rate were improved. No decrease suppressing effect was confirmed. In these examples, the polycyclic aromatic hydrocarbon was arranged on the surface of the positive electrode active material and the negative electrode active material, and it is considered that desorption/insertion of lithium ions in the active material was inhibited.

As described above, according to the technology disclosed herein, there is provided a separator for use in a lithium ion secondary battery that includes a base material and a heat-resistant layer formed on the base material. The heat-resistant layer of this separator contains a polycyclic aromatic hydrocarbon. This makes it possible to suppress the occurrence of an internal short circuit while suppressing deterioration of battery performance in a lithium ion secondary battery including the separator.

Although specific examples of the technology disclosed herein have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. For example, the above embodiments have been described with the separator disclosed herein as a separator for a lithium ion secondary battery, but the separator is not limited to this. For example, the separator may be used as a separator for a sodium ion secondary battery.

20 Wound electrode assembly 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector 50 Positive electrode sheet (positive electrode)
52 positive electrode current collector 52a positive electrode active material layer non-formed portion 54 positive electrode active material layer 60 negative electrode sheet (negative electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-forming portion 64 Negative electrode active material layer 70 Separator 72 Heat resistant layer 74 Base material 80 Non-aqueous electrolyte 100 Lithium ion secondary battery

Claims (5)

A separator used in a lithium ion secondary battery,
a substrate;
a heat-resistant layer containing a heat-resistant filler formed on the substrate;
and
Here, the separator, wherein the heat-resistant layer contains a polycyclic aromatic hydrocarbon. 2. The separator according to claim 1, wherein said heat-resistant layer is formed on one surface of said base material. 3. The separator according to claim 1, wherein the polycyclic aromatic hydrocarbon contains coronene or quaterrylene. an electrode assembly having a positive electrode comprising a positive electrode active material layer, a negative electrode comprising a negative electrode active material layer, and a separator interposed between the positive electrode and the negative electrode;
a non-aqueous electrolyte;
A lithium ion secondary battery comprising
A lithium ion secondary battery comprising the separator according to any one of claims 1 to 3 as the separator. 5. The lithium ion secondary battery according to claim 4, wherein said heat-resistant layer is formed on a surface of said base material facing said positive electrode active material layer.

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