JP5087383B2 - Separator for non-aqueous lithium secondary battery - Google Patents

Description

  The present invention relates to a separator for a non-aqueous lithium secondary battery that can absorb a change in the volume of an electrode accompanying charge / discharge and also has excellent heat resistance.

  A conventional non-aqueous lithium secondary battery has a structure in which a separator made of an electrically insulating porous film is interposed between a positive electrode and a negative electrode, and a lithium salt is dissolved in the gap of the film. The impregnated electrolyte is impregnated. Such a non-aqueous lithium secondary battery has excellent characteristics such as high capacity and high energy density.

  However, in the conventional non-aqueous lithium secondary battery, the positive electrode and the negative electrode are repeatedly contracted and expanded in accordance with the charge / discharge cycle, so that the separator reacts with the positive electrode and the negative electrode to deteriorate them, and a short circuit occurs inside and outside the battery. The battery temperature may rise rapidly.

  Therefore, as a conventional separator, a porous film made of polyethylene having a melting point of 120 to 140 ° C. and excellent in shutdown characteristics, handleability, and cost has been widely used. Here, the shutdown means that the battery temperature rises due to overcharge, external or internal short circuit, etc., a part of the separator melts, the gap is closed, and the current is cut off.

  However, when the battery temperature further rises, the separator melts and rapidly contracts or breaks, so that a short circuit occurs again.

  For this reason, in order to improve the safety of non-aqueous lithium secondary batteries, attempts have been made to improve the heat resistance of electrode materials such as separators. Even in such a case, it is required to ensure safety.

As an attempt for this, for example, Patent Document 1 discloses a secondary battery including a separator containing a heat-resistant nitrogen-containing aromatic polymer and ceramic powder, and Patent Document 2 discloses a positive electrode or a negative electrode. A secondary battery having a porous film comprising an inorganic oxide filler and a membrane binder on its surface is disclosed. Patent Document 3 discloses a filler having a melting point of 250 ° C. or higher and a filler having a melting point of 80 to 120 ° C. A secondary battery including a separator made of a mixture of the above is disclosed.
JP2003-30686A JP-A-2005-327680 JP2007-95344

  However, in the secondary battery described in the above-mentioned patent document, it is said that problems such as separator oxidation / degradation, battery internal resistance increase, electrode degradation, leakage current due to electrolyte decomposition, short circuit, etc. can be improved. It ’s hard. That is, in the secondary battery described in Patent Document 1, the degree of adhesion between the heat-resistant aromatic polymer and the base material of the separator made of polyethylene terephthalate is low, and physical interface resistance exists between them. Battery characteristics and safety are insufficient. In the secondary battery described in Patent Document 2, an inorganic oxide filler is applied to the electrode using a binder, but there are problems in terms of productivity, handleability, and battery resistance. Moreover, the secondary battery described in Patent Document 3 is insufficient in terms of safety. For this reason, the secondary battery which was excellent in safety | security and the battery characteristic was improved is desired.

  Therefore, the present invention is for a non-aqueous lithium secondary battery that suppresses the reaction and deterioration of the separator and the electrode, and avoids / suppresses the rapid contraction and breakage of the separator while maintaining the shutdown characteristics. It is an object to provide a separator.

That is, the separator for a non-aqueous lithium secondary battery according to the present invention contains a separator body, and an organic compound and an inorganic compound provided on the surface of the separator body facing the positive electrode and / or the surface facing the negative electrode. A buffer layer made of a composite, wherein the organic compound has a higher melting point than the material constituting the separator body, and the inorganic compound includes LiAlO ₂ , LiAl ₅ O ₈ , _{Li 5 AlO 4, MgO, MgAl} 2 O 4, BaTiO 3, CoAl 2 O 4, Li 2 SiO 4, Li 2 B 4 O 7, Li 2 MoO 3, Li 3 PO 4, Al (OH) 3, AlPO 4 And at least one selected from the group consisting of Mg (OH) ₂ .

  Such a separator for a non-aqueous lithium secondary battery according to the present invention (hereinafter also simply referred to as a separator) has a buffer layer made of a composite containing a specific organic compound and a specific inorganic compound on the surface of the separator body. When the separator according to the present invention is used for a non-aqueous lithium secondary battery, the buffer layer is interposed between the positive electrode and the separator main body and between the negative electrode and the separator main body. Inhibiting contact between the separator body and the electrode. For this reason, according to this invention, it is suppressed that a separator main body and an electrode react and deteriorate.

  In addition, since the composite is porous having a large number of voids, the buffer layer made of the composite is a cushioning material even if the volume of the electrode expands and contracts due to the lithium ion insertion reaction. And can absorb the volume change. Therefore, according to the present invention, this also suppresses the deterioration of the separator main body and the electrode due to the reaction.

  In addition, since the melting point of the organic compound used in the buffer layer is higher than the melting point of the material constituting the separator body, the temperature inside and outside the non-aqueous lithium secondary battery becomes higher than the melting point of the substance constituting the separator body, and the separator body melts. Even in the case of contraction, the buffer layer remains undissolved and can maintain the separator skeleton. For this reason, according to the present invention, it is possible to avoid / suppress the rapid contraction and breakage of the separator while maintaining the shutdown characteristics.

  Furthermore, since the buffer layer is composed of a composite containing the organic compound and the inorganic compound, the skeleton of the separator is further increased compared to the case where the buffer layer is composed of the organic compound alone. Can be easily retained, and the thermal stability of the secondary battery can be improved.

  The organic compound preferably has a melting point of 180 ° C. or higher.

  The thickness of the buffer layer is preferably 2 μm or more.

The nonaqueous lithium secondary battery according to the present invention includes a separator body, a positive electrode, a negative electrode, and between the separator body and the positive electrode and / or between the separator body and the negative electrode. And a buffer layer made of a composite containing an inorganic compound, wherein the organic compound has a higher melting point than the material constituting the separator body, and the inorganic compound is LiAlO. ₂ , LiAl ₅ O ₈ , Li ₅ AlO ₄ , MgO, MgAl ₂ O ₄ , BaTiO ₃ , CoAl ₂ O ₄ , Li ₂ SiO ₄ , Li ₂ B ₄ O ₇ , Li ₂ MoO ₃ , Li ₃ PO ₄ , Al It is at least one selected from the group consisting of (OH) ₃ , AlPO ₄ , and Mg (OH) ₂ .

  As described above, according to the present invention, the separator and the electrode are not easily deteriorated even after repeated charge and discharge, the rapid contraction and breakage of the separator are avoided / suppressed, and the short circuit does not easily occur. A secondary battery can be provided.

  A nonaqueous lithium secondary battery according to an embodiment of the present invention will be described below.

  Examples of the form of the non-aqueous lithium secondary battery according to this embodiment include a coin, a button, a sheet, a cylinder, a flat shape, and a square shape. These secondary batteries are composed of a positive electrode, a negative electrode, an electrolyte, a separator, and the like.

  Examples of the positive electrode include Li-containing Ti, Mo, W, Nb, V, Mn, Fe, Cr, Ni, Co and other transition metal composite oxides, composite sulfides, vanadium oxides, and conjugated polymers. Examples thereof include organic conductive materials such as those having a chevrel phase compound as an active material.

  Examples of the negative electrode include carbon-based active materials such as graphite and coke, metal lithium, lithium vanadium oxide, lithium transition metal nitride, silicon and the like as active materials.

  In the positive electrode and the negative electrode, additives such as a conductive agent, a binder, a filler, a dispersant, an ionic conductive agent, and a pressure enhancer are appropriately selected and blended in the powder made of the active material. Also good.

  Examples of the conductive agent include graphite, carbon black, acetylene black, ketjen black, carbon fiber, and metal powder. Examples of the binder include polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. Is mentioned.

  In order to produce the positive electrode or the negative electrode, for example, a mixture of the active material and various additives is added to a solvent such as water or an organic solvent to form a slurry or paste, and the obtained slurry or paste is used as a doctor blade method. Etc. are applied to the electrode support substrate, dried, and consolidated with a rolling roll or the like to obtain a positive electrode or a negative electrode.

  Examples of the electrode support substrate include a foil, a sheet or net made of copper, nickel, stainless steel, aluminum, or the like: a sheet or net made of carbon fiber. Note that compaction molding may be performed in a pellet form without using the electrode support substrate.

  Examples of the electrolyte include a nonaqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent, a polymer electrolyte, an inorganic solid electrolyte, a composite material of a polymer electrolyte and an inorganic solid electrolyte, and the like.

  Examples of the solvent for the non-aqueous electrolyte include chain esters such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate: γ-lactones such as γ-butyllactone: 1,2-dimethoxy Chain ethers such as ethane, 1,2-diethoxyethane, and ethoxymethoxyethane: Cyclic ethers of tetrahydrofuran: Nitriles such as acetonitrile.

Examples of the lithium salt that is a solute of the non-aqueous electrolyte include LiAsF ₆ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiClO ₄ , LiCF ₃ SO ₃ , LiSbF ₆ , LiSCN, LiCl, LiC ₆ H ₅ SO ₃ , Examples thereof include LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC ₄ P ₉ SO ₃ .

  The separator in the present embodiment includes a separator main body and a buffer layer made of a composite containing an organic compound and an inorganic compound formed on the surface of the separator main body. The buffer layer is formed on both the negative electrode side and the positive electrode surface of the separator body.

  As the separator body, for example, a porous film made of a polyolefin such as polypropylene or polyethylene, or a porous material such as a glass filter or a nonwoven fabric can be used. Among them, a porous film made of polyethylene having a melting point of 120 to 140 ° C. The film is preferably used because it has excellent shutdown characteristics and is excellent in handling and price.

  The porosity of the separator body is preferably 40 to 90% by volume, more preferably 50 to 80% by volume. When the porosity is less than 40% by volume, the ionic conductivity of the separator is lowered, and the high rate discharge characteristics of the non-aqueous lithium secondary battery may be deteriorated. On the other hand, when the porosity exceeds 90% by volume, the mechanical strength of the separator is insufficient and the separator is easily broken.

  The thickness of the separator body is preferably 60 μm or less, more preferably 10 to 30 μm. When the thickness exceeds 60 μm, the volume of the electrode plate group increases, and the energy density of the non-aqueous lithium secondary battery decreases.

  The organic compound used in the buffer layer may be any compound having a higher melting point than the material constituting the separator body, but preferably has a melting point of 180 ° C. or higher, more preferably 250 ° C. or higher. If the melting point is less than 180 ° C., the temperature difference from the melting point of the material constituting the separator body is small, and after the battery temperature rises and the separator body melts, the separator skeleton cannot be maintained to prevent a short circuit. .

  Examples of such organic compounds include polypropylene, poly (phenylene terephthalamide), poly (benzamide), poly (4,4′-benzanilide terephthalamide), and poly (phenylene-4,4′-biphenylenedicarboxylic acid). Amide), poly (phenylene-2,6-naphthalenedicarboxylic acid amide), poly (2-chloro-phenylene terephthalamide), phenylene terephthalamide / 2,6-dichlorophenylene terephthalamide copolymer, etc. Aramid)). The optical properties of these aramids may be meta or para.

The inorganic compound used for the buffer layer is stable inside the non-aqueous lithium secondary battery and does not cause side reactions. Specifically, LiAlO ₂ , LiAl ₅ O ₈ , Li ₅ AlO ₄ , MgO, MgAl ₂ O ₄ , BaTiO ₃ , CoAl ₂ O ₄ , Li ₂ SiO ₄ , Li ₂ B ₄ O ₇ , Li ₂ MoO ₃ , Li ₃ PO ₄ , Al (OH) ₃ , AlPO ₄ , and Mg (OH) ₂ is there. These inorganic compounds may be used independently and 2 or more types may be used together.

Other inorganic compounds may be further mixed with the inorganic compound, and examples of such other inorganic compounds include Al ₂ O ₃ , ZrO, and SiO ₂ .

  A composite containing an organic compound and an inorganic compound constituting such a buffer layer is porous having a large number of voids.

  The porosity of the buffer layer is preferably 50 to 90% by volume, more preferably 60 to 90% by volume. When the porosity is less than 50% by volume, the cushioning property of the buffer layer may be lowered, and the volume change accompanying expansion / contraction during charge / discharge of the electrode may not be sufficiently absorbed. On the other hand, when the porosity exceeds 90% by volume, the mechanical strength is insufficient and it is easily broken.

  The thickness (one side) of the buffer layer is preferably 2 μm or more, more preferably 2 to 20 μm, and further preferably 2 to 10 μm. If the thickness of the buffer layer is less than 2 μm, the volume change due to the expansion / contraction of the electrode due to charge / discharge cannot be sufficiently absorbed, the reinforcing effect on the separator body is not sufficient, and the battery temperature rises. Then, after the separator main body is melted, it may be difficult to prevent the short circuit by maintaining the skeleton of the separator. On the other hand, if the thickness of the buffer layer is excessive, the resistance increases and the battery characteristics deteriorate. However, when the safety of the secondary battery is given the highest priority, the thicker the buffer layer, the better.

  The content ratio of the organic compound and the inorganic compound in the buffer layer is preferably 95 to 50 parts by weight, more preferably 10 to 30 parts by weight with respect to 5 to 50 parts by weight of the organic compound. On the other hand, it is 90-70 weight part of inorganic compounds, More preferably, it is 85-75 weight parts of inorganic compounds with respect to 15-25 weight parts of organic compounds. When the amount of the inorganic compound is less than 50 parts by weight, the amount of the inorganic compound is small, and the heat resistance enhancement effect on the porous formed by the organic compound is insufficient. Formation is hindered by inorganic compounds and the strength decreases and becomes brittle. Moreover, when the content of the inorganic compound is outside the range of 95 to 50 parts by weight, when such a separator is incorporated in a secondary battery, the battery resistance may be increased and deteriorate easily.

  Although it does not specifically limit as a manufacturing method of the said buffer layer, For example, the following method is mentioned. When using aramid as the organic compound, first, the aramid is dissolved in a polar organic solvent such as N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone, tetramethylurea and the like.

  Next, an inorganic compound is dispersed in the obtained polar organic solution to prepare a slurry solution. And the obtained slurry solution is apply | coated to the said separator main body surface.

  Subsequently, the separator formed by applying the slurry solution to the surface of the main body is placed in an atmosphere controlled at a constant humidity at 20 ° C. or higher. Thereby, aramid in which the inorganic compound is dispersed is deposited on the surface of the separator body. Thereafter, the separator is immersed in a coagulating liquid made of an aqueous solution or an alcohol solution.

  Next, the polar organic solvent is removed from the separator surface by evaporating it.

  And if the separator from which the polar organic solvent was removed is dried below the melting point of the separator body, a separator having a buffer layer formed on the surface of the body is obtained.

  The present invention is not limited to the above embodiment.

  In the above embodiment, a separator having a buffer layer formed on the surface of the separator body is used. However, the buffer layer only needs to be interposed between the separator body and the electrode, and the buffer layer is formed on the electrode surface. The buffer layer may be provided on a base material independent of the separator main body and the electrode.

  In addition, it is needless to say that the present invention may appropriately combine some or all of the above-described embodiments and modified embodiments, and various modifications can be made without departing from the spirit of the present invention.

  The present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to these examples.

<Manufacture of secondary batteries>
An N-methyl-2-pyrrolidone solution in which polyvinylidene fluoride as a binder (# 1100 manufactured by Kureha Chemical Industry Co., Ltd.) was dissolved was prepared. In this solution, 95 parts by mass of LiCoO ₂ and 2 parts by mass of conductive carbon were prepared. And was slurried. The prepared positive electrode slurry was uniformly applied onto an Al foil having a thickness of 15 μm and dried to obtain a positive electrode. The weight ratio of LiCoO ₂ : conductive carbon: polyvinylidene fluoride in the positive electrode was 95: 2: 3.

  Next, a mixture of lithium vanadium oxide (LVO) powder and carbon material powder is used as a negative electrode active material, 90 parts by weight of the mixed powder of LVO powder and carbon material powder, and 10 parts by weight of polyvinylidene fluoride serving as a binder, Were mixed and dispersed in N-methyl-2-pyrrolidone to obtain a negative electrode slurry. And this negative electrode slurry was uniformly apply | coated on the 20-micrometer-thick copper foil, it dried, and it was set as the negative electrode.

Made of polyethylene including a buffer layer in which a composite containing meta-aramid polymer, which is an organic compound, and various inorganic compounds shown in Table 1 at a weight ratio of 15:85 is applied between the positive electrode and the negative electrode in a thickness of 4 μm on one side. A nonaqueous electrolytic solution in which LiPF ₆ is dissolved at a concentration of 1.50 mol / L in a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed in a ratio of 3: 7 as a nonaqueous electrolyte with a separator interposed. This was poured into a 18650 type cylindrical lithium secondary battery.

<Evaluation of capacity maintenance characteristics>
Each non-aqueous lithium secondary battery of each example and control was charged at a constant current (0.5 C) -constant voltage (4.3 V), and then 0.5 C discharge was performed for 200 cycles to a discharge starting voltage of 2.75 V. The capacity retention rate after the end of 200 cycles was measured. The results are shown in Table 1 below.

  From the results shown in Table 1, when a buffer layer made of a composite containing a predetermined organic compound and a predetermined inorganic compound is formed on both sides of the separator body (Examples 1 to 6), only the separator (separator body) As compared with the case where the battery is interposed between the negative electrode and the positive electrode (control), the capacity maintenance performance upon repeated charge / discharge was significantly improved. Therefore, according to the present invention, it has been clarified that a non-aqueous lithium secondary battery in which the separator and the battery are not deteriorated and the battery characteristics are not deteriorated even when charging and discharging are repeated is obtained.

<Evaluation of separator cushion>
A weight of 1.0 kg / cm ² was applied to the separator of Example 1 and the control separator, and the thickness and compression rate at that time were evaluated. The results are shown in Table 2 below.

  From the results shown in Table 2, the separator of Example 1 having buffer layers on both sides is superior in shrinkability compared to a control separator made of only polyethylene, and therefore, an excellent cushioning material when arranged between electrodes. Was found to function as.

  According to the present invention, it is possible to provide a non-aqueous lithium secondary battery that maintains high safety even when charging and discharging are repeated.

Claims (4)

A separator body;
A buffer layer made of a composite containing an organic compound and an inorganic compound, provided on a surface facing the positive electrode and / or a surface facing the negative electrode of the separator body, for a non-aqueous lithium secondary battery A separator,
The organic compound has a higher melting point than the material constituting the separator body,
The inorganic _{compound, AlPO 4, Li 3 PO 4} , MgAl 2 O 4, Li 2 ZrO 3, _and, at least is one non-aqueous lithium secondary battery separator selected from the group consisting of Li 2 SiO _4.   The separator for a non-aqueous lithium secondary battery according to claim 1, wherein the organic compound has a melting point of 180 ° C. or higher.   The separator for a non-aqueous lithium secondary battery according to claim 1 or 2, wherein the buffer layer has a thickness of 2 µm or more. A separator body;
A positive electrode;
A negative electrode,
A non-aqueous lithium comprising a buffer layer made of a composite containing an organic compound and an inorganic compound, provided between the separator main body and the positive electrode and / or between the separator main body and the negative electrode. A secondary battery,
The organic compound has a higher melting point than the material constituting the separator body,
The non-aqueous lithium secondary battery, wherein the inorganic compound is at least one selected from the group consisting of AlPO ₄ , Li ₃ PO ₄ , MgAl ₂ O ₄ , Li ₂ ZrO ₃ , and Li ₂ SiO ₄ .