JP2009146611A - Separator for nonaqueous lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator for a nonaqueous lithium secondary battery in which a short circuit hardly occurs and a transition metal contained in an electrode is stabilized and deterioration of the electrode and deterioration of the separator are prevented. <P>SOLUTION: In the separator for a nonaqueous lithium secondary battery in which a composite body containing an organic compound and a metallic compound is carried on a separator main body, the organic compound has a higher melting point than a substance composing the separator main body and the metallic compound includes at least one out of elements belonging to either group of 1A, 2A, 3A, 4A, 3B or 5B of the periodic table and is at least one selected from a group of hydroxide, carbonate, phosphate, sulfate, nitrate and alkoxide which are dissociated inside the nonaqueous lithium secondary battery. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a separator for a non-aqueous lithium secondary battery that is not easily deteriorated due to a transition metal contained in an electrode and is excellent in heat resistance.

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

Conventionally, transition metal compounds are used as active materials for positive electrodes and negative electrodes of non-aqueous lithium secondary batteries. For example, when a Li—Co-based oxide such as LiCoO ₂ is used as a positive electrode active material. In accordance with the insertion reaction of lithium ions accompanying charge / discharge, trivalent cobalt ions (Co ³⁺ ) are changed to unstable tetravalent cobalt ions (Co ⁴⁺ ). It reacts to oxidize the electrolyte and reduces itself to divalent cobalt ions (Co ²⁺ ). The divalent cobalt ions are dissolved in the electrolytic solution, transferred to the negative electrode side, reduced at the negative electrode, and deposited as metallic cobalt. When such metallic cobalt is deposited and accumulated on the negative electrode, the separator is damaged or the negative electrode is deteriorated, and the charge / discharge characteristics of the non-aqueous lithium secondary battery are deteriorated.

  Further, when the tetravalent cobalt ions are reduced to divalent cobalt ions, the separator is also oxidized and deteriorated. For example, when the separator is made of polyethylene, hydrogen atoms are removed from the polyethylene molecules by an oxidation reaction, leaving only the carbon skeleton. Such carbonized separators are weak in mechanical strength and easily broken.

  As described above, in the conventional non-aqueous lithium secondary battery, a positive electrode (or negative electrode) containing a transition metal and the electrolytic solution react with each other, so that an electrolytic solution decomposition product is deposited on the negative electrode (or positive electrode). There is a problem that the secondary battery is deteriorated by being oxidized and deteriorated.

  In addition, the conventional non-aqueous lithium secondary battery may be short-circuited inside and outside the battery due to high capacity and high energy density, for example, and 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 to solve these problems, for example, Patent Document 1 discloses a secondary battery including a separator including a heat-resistant nitrogen-containing aromatic polymer and ceramic powder. A secondary battery having a porous film comprising an inorganic oxide filler and a film binder on the surface of a positive electrode or a negative electrode is disclosed. Patent Document 3 discloses a filler having a melting point of 250 ° C. or higher and a melting point of 80 to A secondary battery including a separator made of a mixture with a 120 ° C. filler is disclosed.
JP2003-30686A JP-A-2005-327680 JP2007-95344

  However, in the secondary battery described in the above-mentioned patent document, deterioration of the secondary battery due to the transition metal contained in the electrode is not considered at all.

  Accordingly, an object of the present invention is to provide a separator for a non-aqueous lithium secondary battery that is less likely to cause a short circuit and that can stabilize the transition metal contained in the electrode and prevent the deterioration of the electrode and the separator. To do.

  That is, the separator for a non-aqueous lithium secondary battery according to the present invention is a separator for a non-aqueous lithium secondary battery in which a composite containing an organic compound and a metal compound is supported on a separator body, and the organic compound is The melting point is higher than that of the substance constituting the separator body, and the metal compound is at least one of elements belonging to any one of groups 1A, 2A, 3A, 4A, 3B, and 5B of the periodic table. And at least one selected from the group consisting of hydroxides, carbonates, phosphates, sulfates, nitrates, and alkoxides that dissociate inside the non-aqueous lithium secondary battery. .

  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 separator body as a base material, and a composite containing the metal compound is carried thereon. Even if the transition metal ions contained in the positive and negative electrodes become unstable due to the lithium ion insertion reaction associated with the charge and discharge of the metal, the metal ions dissociated and the metal ions eluted in the electrolyte transition Since metal ions can be captured and modified and stabilized, the positive electrode or negative electrode reacts with the electrolytic solution, and electrolyte decomposition products are deposited on the electrode (for example, Li electrodeposition). It can be suppressed that the positive electrode or the negative electrode reacts with the separator and the separator deteriorates.

This will be described in more detail with specific examples. For example, when the active material of the positive electrode is LiCoO ₂ and the metal compound is Al (OC ₂ H ₅ ) ₃ , the insertion and de-insertion reactions of lithium ions are performed. Accordingly, Co ions that are transition metals in the positive electrode material generate unstable valence Co ⁴⁺ , while part of Al contained in the metal compound is also dissociated, and ionized Al ³⁺ is contained in the electrolyte. To elute. Then, Al ³⁺ is combined with Co ²⁺ generated by reduction of Co ⁴⁺ in the electrolytic solution, and a Co—Al compound is generated on the positive electrode surface to be stabilized.

  In addition, since the melting point of the organic compound contained in the composite is higher than the melting point of the substance constituting the separator body, the temperature inside and outside the non-aqueous lithium secondary battery becomes equal to or higher than the melting point of the substance constituting the separator body. Even when the metal melts and contracts, the composite remains without being dissolved and can maintain the skeleton of the separator. 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.

The metal compound includes MgCO ₃ , BaCO ₃ , Li ₂ CO ₃ , Li ₃ PO ₄ , Al (OH) · nH ₂ O, Al ₂ (SO ₄ ) ₃ , MgSO ₄ , Al (OC ₂ H ₅ ) ₃ , It is preferably at least one selected from the group consisting of YPO ₄ , (ZrO) ₂ P ₂ O ₇ , ZrP ₂ O ₇ , and Al (NO ₃ ) ₃ .

  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.

  A non-aqueous lithium secondary battery according to the present invention includes a separator body, a positive electrode, a negative electrode, and a composite containing at least the positive electrode and the negative electrode, an organic compound and a metal compound, The organic compound has a melting point higher than that of the material constituting the separator body, and the metal compound includes 1A, 2A, 3A, 4A of the periodic table, A group consisting of hydroxide, carbonate, phosphate, sulfate, nitrate, and alkoxide, containing at least one of elements belonging to any of groups 3B and 5B and dissociating inside the battery It is at least one selected from the above.

  As described above, according to the present invention, it is possible to provide a non-aqueous lithium secondary battery in which rapid contraction and tearing of the separator hardly occur and the electrode and the separator are not easily deteriorated even after repeated charge and discharge.

  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 .

  In the separator according to the present embodiment, a composite body containing an organic compound and a metal compound is supported on 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 contained in the composite may be any compound having a higher melting point than the material constituting the separator body, preferably having a melting point of 180 ° C. or higher, more preferably having a melting point of 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 metal compound contained in the composite contains at least one of elements belonging to any group of 1A, 2A, 3A, 4A, 3B, and 5B of the periodic table, and is a non-aqueous lithium secondary battery Hydroxides, carbonates, phosphates, sulfates, nitrates, and alkoxides that dissociate internally.

Examples of such a metal compound include MgCO ₃ , BaCO ₃ , Li ₂ CO ₃ , Li ₃ PO ₄ , Al (OH) · nH ₂ O, Al ₂ (SO ₄ ) ₃ , MgSO ₄ , Al (OC _2). H ₅ ) ₃ , YPO ₄ , (ZrO) ₂ P ₂ O ₇ , ZrP ₂ O ₇ , Al (NO ₃ ) _{3 and the} like. These metal compounds may be used independently and 2 or more types may be used together.

Other metal compounds may be further mixed with the metal compound, and examples of such other metal compounds include AlPO ₄ , Al ₂ O ₃ , Al (OH) ₃ , Mg (OH) _{2, and the} like. Is mentioned.

  Such a composite containing an organic compound and a metal compound constitutes a porous body having a large number of voids.

  The content ratio of the organic compound and the metal compound in the pre-complex is preferably 95 to 50 parts by weight, more preferably 10 to 30 parts by weight of the organic compound with respect to 5 to 50 parts by weight of the organic compound. On the other hand, it is 90-70 weight part of metal compounds, More preferably, it is 85-75 weight parts of metal compounds with respect to 15-25 weight parts of organic compounds. When the amount of the metal compound is less than 50 parts by weight, the amount of the metal compound compounded is small and the effect of stabilizing the transition metal contained in the electrode is insufficient. Inhibited by the compound, the strength decreases and becomes brittle. Moreover, when the content of the metal 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.

  The aspect in which the composite is supported on the separator body is not particularly limited. For example, the composite covers the separator body, and faces the positive electrode of the separator main body and / or the negative electrode. Although the aspect provided with the buffer layer which consists of the said composite_body | complex on the surface may be sufficient, the aspect with which the said composite_body | complex is filled in the space | gap of the said separator main body may be sufficient, and also the positive electrode of the said separator main body A buffer layer made of the composite is provided on the surface facing the surface and / or the surface facing the negative electrode, and the composite body is filled in the voids of the separator body. In particular, when a buffer layer made of the composite is provided on the surface of the separator body that faces the positive electrode and / or the surface that faces the negative electrode, the volume change associated with expansion / contraction during charge / discharge of the electrode is absorbed. Thus, it is possible to suppress the electrode and the separator body from reacting and deteriorating.

  The separator according to the present embodiment preferably has a porosity of 40 to 60% by volume. If the porosity is less than 40% by volume, the cushioning property may be lowered, and the volume change accompanying expansion / contraction during charging / discharging of the electrode may not be sufficiently absorbed. On the other hand, when the porosity exceeds 60% 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.

  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, a slurry solution is prepared by dispersing the metal compound in the obtained polar organic 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 metal 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 embodiment, a separator in which the composite is supported on the separator body is used. However, in the non-aqueous lithium secondary battery according to the present invention, a buffer layer made of the composite is formed on the electrode surface. Alternatively, the composite may be supported on a substrate independent of the separator main body and the electrode, and may exist between the separator main body and the negative electrode or the positive 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 between a positive electrode and a negative electrode, a composite containing a meta-aramid polymer, which is an organic compound, and various metal compounds listed in Table 1 in a weight ratio of 15:85 is applied at 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 injected to prepare 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 separator in which a buffer layer made of a composite containing a predetermined organic compound and a predetermined metal compound is formed on both sides of the separator body (Examples 1 to 4), polyethylene is used. As compared with the case where only the separator (separator main body) is interposed between the negative electrode and the positive electrode (control), the capacity maintenance performance upon repeated charge / discharge was remarkably 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 by FT-IR>
Before and after the capacity maintenance characteristic evaluation experiment, the separator of Example 2 and the control separator were analyzed by FT-IR. In addition, about the separator of Example 2, only the buffer layer was analyzed by FT-IR before the capacity maintenance characteristic evaluation experiment, and only the separator main body was analyzed by FT-IR after the capacity maintenance characteristic evaluation experiment. The results are shown in FIG.

  From the results shown in FIG. 1, in the separator made only of polyethylene used as a control, hydrogen was released after the capacity maintenance characteristic evaluation experiment, and a peak derived from unsaturated carbide and oxide was observed at 1200 to 1800 nm. In the separator of Example 2 having the buffer layers on both sides, the polyethylene as the separator body was not oxidized even after the capacity maintenance characteristic evaluation experiment. Therefore, it has been found that the buffer layer suppresses the oxidation and deterioration of the separator body.

  Further, when the control separator and the separator main body of Example 2 were visually observed after the capacity maintenance characteristic evaluation experiment, the control separator was carbonized and turned black, but the separator main body of Example 2 There was little change.

  According to the present invention, it is possible to provide a non-aqueous lithium secondary battery having excellent charge / discharge characteristics and improved safety.

The chart which shows the result of having analyzed the separator of Example 2 and the separator of control by FT-IR.

Claims (5)

A separator for a non-aqueous lithium secondary battery in which a composite containing an organic compound and a metal compound is supported on a separator body,
The organic compound has a higher melting point than the material constituting the separator body,
The metal compound contains at least one of elements belonging to any one of groups 1A, 2A, 3A, 4A, 3B, and 5B of the periodic table, and dissociates inside the non-aqueous lithium secondary battery. A separator for a non-aqueous lithium secondary battery, which is at least one selected from the group consisting of oxides, carbonates, phosphates, sulfates, nitrates, and alkoxides. The metal compound includes MgCO ₃ , BaCO ₃ , Li ₂ CO ₃ , Li ₃ PO ₄ , Al (OH) · nH ₂ O, Al ₂ (SO ₄ ) ₃ , MgSO ₄ , Al (OC ₂ H ₅ ) ₃ , 2. The separator for a non-aqueous lithium secondary battery according to claim 1, wherein the separator is at least one selected from the group consisting of YPO ₄ , (ZrO) ₂ P ₂ O ₇ , ZrP ₂ O ₇ , and Al (NO ₃ ) ₃ .   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. A buffer layer made of a composite containing an organic compound and a metal compound is provided on the surface facing the positive electrode and / or the surface facing the negative electrode of the separator body,
The separator for a nonaqueous lithium secondary battery according to claim 1, 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 secondary battery comprising at least a composite containing an organic compound and a metal compound interposed between the positive electrode and the negative electrode,
The organic compound has a higher melting point than the material constituting the separator body,
The metal compound contains at least one of elements belonging to any one of groups 1A, 2A, 3A, 4A, 3B, and 5B of the periodic table, and dissociates inside the battery, hydroxide, carbonate A non-aqueous lithium secondary battery that is at least one selected from the group consisting of phosphate, sulfate, nitrate, and alkoxide.