JP2010021033A - Separator for nonaqueous secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator for a nonaqueous secondary battery improved in adhesion and binding performance of a substrate and a heat resistant layer, and superior in thermal resistance. <P>SOLUTION: This is a separator of which the surface of the separator substrate is coated with a heat resistant layer, and the surface of the separator substrate has a wettability of 40 mN/m or more. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a separator for a non-aqueous secondary battery that has improved adhesion and binding properties between a base material and a heat-resistant layer and is excellent in heat resistance.

  A conventional non-aqueous 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 placed in the gap of the porous film. The dissolved electrolyte is impregnated.

  Such a non-aqueous secondary battery has excellent characteristics such as high capacity and high energy density. However, the separator and the electrolytic solution are positive and negative by repeating contraction and expansion of the positive and negative electrodes in accordance with the charge / discharge cycle. And react with the negative electrode to deteriorate them, causing a short circuit inside and outside the battery, and the battery temperature may rise rapidly.

  For this reason, as a conventional separator, what consists of a porous film made from polyethylene excellent in shutdown characteristics, handling property, and price is 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.

Therefore, in order to suppress the reaction between the electrode and the separator or the electrolytic solution, and to prevent the separator from being broken due to a rapid temperature rise, it has been attempted to provide a heat-resistant layer on the separator. Discloses a secondary battery including a separator having a heat-resistant layer containing a heat-resistant nitrogen-containing aromatic polymer and ceramic powder, and Patent Document 2 includes a separator having a porous heat-resistant layer. A secondary battery is also disclosed.
JP2000-30686 JP 2000-327680 A

  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 batteries described in Patent Document 1 and Patent Document 2, it is difficult to uniformly provide a heat-resistant layer on the surface of the base material, and adhesion and binding properties between the heat-resistant layer and the base material are low. Since physical interface resistance exists between them, battery characteristics and safety are insufficient.

  Then, this invention makes it a subject to provide the separator for non-aqueous secondary batteries which the adhesiveness and binding property of a base material and a heat resistant layer improve, and were excellent in heat resistance.

  That is, the separator for a non-aqueous secondary battery according to the present invention is a separator in which the separator base material surface is covered with a heat-resistant layer, and the separator base material surface has a wettability of 40 mN / m or more. And

  Such a separator for a non-aqueous secondary battery according to the present invention (hereinafter also simply referred to as a separator) is provided with sufficient hydrophilicity on the separator substrate surface. It can apply | coat uniformly, As a result, a heat-resistant layer can be uniformly formed in the whole separator base-material surface. In addition, by providing sufficient hydrophilicity to the separator substrate surface, the adhesion and binding properties between the separator substrate surface and the heat-resistant layer are improved, and the heat-resistant layer does not peel even after repeated charge and discharge. , Heat resistance can be stably maintained.

  The melting point of the heat resistant layer is preferably higher than the melting point of the separator substrate.

  Examples of the heat-resistant layer include those containing at least one inorganic compound selected from the group consisting of oxides, carbonate compounds, hydroxides, and phosphate compounds, and aromatic polyamides. Can be mentioned.

  Such a non-aqueous secondary battery including the separator according to the present invention is also one aspect of the present invention.

  As a method for producing such a separator according to the present invention, for example, the step of modifying the surface of the separator substrate with a hydrophilic group to make the separator substrate surface have a wettability of 40 mN / m or more; And a step of applying a coating solution for a heat-resistant layer to the surface of the separator base material modified by the above. The manufacturing method of the separator is also one aspect of the present invention.

  Examples of the method of modifying the surface of the separator substrate with a hydrophilic group include those using corona discharge, plasma treatment, ion implantation, ion beam mixing, and the like.

  As described above, according to the present invention, the contact between the separator substrate and the electrode is hindered by the heat-resistant layer, and the separator and the electrode react and deteriorate even when charging / discharging is repeated or a high voltage is applied. Therefore, a non-aqueous secondary battery that has excellent battery characteristics such as safety, output characteristics, and cycle characteristics is provided. be able to.

  Hereinafter, a non-aqueous secondary battery according to an embodiment of the present invention will be described.

  The nonaqueous secondary battery according to the present embodiment takes, for example, a coin, a button, a sheet, a cylinder, a flat shape, a square shape, and the like, and includes 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 this embodiment consists of a separator base material and a heat-resistant layer formed on the separator base material surface. The heat-resistant layer needs to be formed at least on the surface of the separator base on the positive electrode side, but may be formed on both the negative electrode side and the positive electrode side surface.

  As the separator substrate, a porous film made of a polyolefin such as polypropylene or polyethylene, an olefin nonwoven fabric, an aramid nonwoven fabric, an olefin film, an aramid film, or the like is used. A porous film made of a polyolefin such as polypropylene or polyethylene is excellent in terms of handling property and price in addition to shutdown characteristics, and is preferably used.

  The porosity of the separator substrate 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 nonaqueous 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 substrate 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, so that the energy density of the nonaqueous secondary battery decreases.

  The separator substrate has a surface modified with a hydrophilic group such as a COOH group or an OH group, and has a wettability of 40 mN / m or more. When the wettability is less than 40 mN / m, it is difficult to uniformly apply a highly polar coating solution for the heat-resistant layer to the surface of the separator substrate, and a uniform heat-resistant layer is formed on the entire surface of the separator substrate. In addition, since the adhesion and binding properties between the separator base material surface and the heat-resistant layer are insufficient, the heat-resistant layer peels off during repeated charge and discharge. For this reason, the heat resistance of the separator becomes insufficient, and when a high current is passed, the resistance increases and the IR drop increases, resulting in a decrease in output characteristics.

  The heat-resistant layer preferably has a melting point higher than that of the separator substrate. For example, the heat-resistant layer preferably contains an inorganic compound and an aromatic polyamide.

Examples of the inorganic compound include 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. Oxides such as MoO ₃ , Li ₃ PO ₄ , AlPO ₄ , Li ₃ PO ₄ , YPO ₄ , (ZrO) ₂ P ₂ O ₇ , ZrP ₂ O _{7 and} other phosphoric acid compounds, Al (OH) ₃ , Al ( OH) .nH ₂ O, hydroxides such as Mg (OH) ₂ , and carbonate compounds such as MgCO ₃ , BaCO ₃ , and Li ₂ CO ₃ . These inorganic compounds may be used independently and 2 or more types may be used together.

  Examples of the aromatic polyamide include poly (phenylene terephthalamide), poly (benzamide), poly (4,4′-benzanilide terephthalamide), 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. The optical properties of these aromatic polyamides may be meta or para.

  These aromatic polyamides have a melting point of 180 ° C. or higher, the battery temperature rises, and a separator base material made of polyethylene having a melting point of 120 to 130 ° C. or polypropylene having a melting point of 140 to 150 ° C. melts. The separator skeleton can be maintained to prevent short circuit. Among these, an aromatic polyamide having a melting point of 250 ° C. or higher is preferable.

  Such a heat-resistant layer made of an inorganic compound and an aromatic polyamide is porous having a large number of voids.

  The porosity of the heat-resistant 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 heat-resistant layer is 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 90% by volume, the mechanical strength is insufficient and it is easily broken.

  The heat-resistant layer preferably has a thickness (one side) of 2 μm or more, more preferably 2 to 20 μm, and still more preferably 2 to 10 μm. When the thickness of the heat-resistant layer is less than 2 μm, the volume change due to the expansion / contraction of the electrode accompanying charge / discharge cannot be sufficiently absorbed, the reinforcing effect on the separator substrate is not sufficient, and the battery temperature is too high. It may be difficult to prevent the short circuit by maintaining the separator skeleton after the separator substrate is melted. On the other hand, if the thickness of the heat-resistant layer is excessive, the resistance increases and the battery characteristics deteriorate. However, when the safety of the secondary battery is given top priority, the thicker the heat-resistant layer may be, the better.

  The content ratio of the aromatic polyamide to the inorganic compound in the heat-resistant 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 aromatic polyamide. It is 90-70 weight part of inorganic compounds with respect to weight part, More preferably, it is 85-75 weight parts of inorganic compounds with respect to 15-25 weight parts of aromatic polyamide. When the inorganic compound is less than 50 parts by weight, the blending amount is small, the heat resistance enhancing effect on the porous formed by the aromatic polyamide is insufficient, and the effect of stabilizing the transition metal contained in the electrode is insufficient. Or On the other hand, if it exceeds 95 parts by weight, the formation of the porous material by the aromatic polyamide is inhibited by the inorganic compound, the strength is lowered, and the material becomes brittle. Moreover, when the content of the inorganic compound is out of 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.

  In order to manufacture such a separator, the following method can be used. First, the surface of a separator substrate made of polyethylene, polypropylene, or the like is modified with a hydrophilic group so that the wettability of the separator substrate surface is 40 mN / m or more. The method of modifying the surface of the separator substrate with a hydrophilic group is not particularly limited, and examples thereof include methods using corona discharge, plasma treatment, ion implantation, ion beam mixing, and the like. The material can be selected as appropriate according to the material. By modifying with hydrophilic groups using these methods, even for base materials made of materials that are difficult to uniformly apply a coating solution for heat-resistant layers with low inherent polarity and high polarity such as polypropylene. A heat-resistant layer can be uniformly formed on the entire separator substrate surface.

  Next, a heat-resistant layer coating solution is applied to the surface of the separator base material modified with a hydrophilic group to form the heat-resistant layer. Specifically, first, the aromatic polyamide is dissolved in a polar organic solvent such as N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone or tetramethylurea. Next, the inorganic compound is dispersed in the obtained aromatic polyamide solution to prepare a slurry solution. And the obtained slurry solution is apply | coated to the said separator base-material surface as said heat-resistant layer coating liquid.

  Subsequently, the separator formed by applying the slurry solution to the substrate surface is placed in an atmosphere controlled at a constant humidity at 20 ° C. or higher. Thereby, the aromatic aramid in which the inorganic compound is dispersed is deposited on the separator substrate surface. 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 evaporation or the like.

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

  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.

<Example 1>
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, and LiNi _0.80 Co _0.15 Al _0.05 was added to this solution. 85 parts by mass of O ₂ (hereinafter abbreviated as NCA) and 10 parts by mass of conductive carbon were added to form a slurry. 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 NCA: conductive carbon: polyvinylidene fluoride in the positive electrode was 85: 10: 5. The density of the positive electrode mixture was adjusted to 2.4 g / cm ³ by rolling. The final thickness was 60 μm. The electrode plate was cut to have a length of 210 mm and a width of 54 mm, the positive electrode mixture was peeled off at the 10 mm portion in the length direction, and an aluminum lead was welded thereto. Further, a positive electrode plate was produced by vacuum drying at 100 ° C. for 10 hours.

Next, graphite powder is used as a negative electrode active material, 96 parts by weight of this graphite powder and 4 parts by weight of polyvinylidene fluoride as a binder are mixed and dispersed in N-methyl-2-pyrrolidone to form a negative electrode slurry. . And this negative electrode slurry was uniformly apply | coated on the 10-micrometer-thick copper foil, it dried, and it was set as the negative electrode. The negative electrode mixture density was adjusted to 1.2 g / cm ³ by rolling. The final thickness was 70 μm. The electrode plate was cut to have a length of 250 mm and a width of 58 mm, the negative electrode mixture was peeled off at the 10 mm portion in the length direction, and a nickel lead was welded thereto. Furthermore, the negative electrode plate was produced by vacuum-drying at 100 degreeC for 10 hours.

  Subsequently, a separator base material made of a polypropylene (PP) microporous film was subjected to corona discharge at the voltage and processing speed shown in Table 1 below using ASA-4 manufactured by Shinko Electric Instrument Co., Ltd. The substrate surface was modified with hydrophilic groups.

  The wettability of the separator substrate surface after corona discharge was evaluated as follows. The results are shown in Table 1 below.

<Evaluation of wettability>
The wettability is evaluated in accordance with the wet tension test method (JIS K 6768), and a commercially available wet tension test mixture 22.6 to 73 mN / m is immersed in a cotton swab and the sample is corona. It apply | coated to the surface of the separator base material after discharge. At this time, the minimum value of the test liquid mixture number (mN / m) that was wet without water repellency was evaluated as the wetting index. The wettability of Example 1 was 47 mN / m.

With respect to the separator substrate after corona discharge, a coating solution for a heat-resistant layer containing aromatic polyamide (poly (phenylene terephthalamide)) and Al (OH) ₃ at a weight ratio of 15:85 has a thickness of 4 μm on one side. Was applied to prepare a separator having a heat-resistant layer formed thereon.

  Furthermore, the following tests were performed on the separator on which the heat-resistant layer was formed in order to evaluate the adhesion and binding properties of the heat-resistant layer. The results are shown in Table 1 below.

<Evaluation of heat-resistant layer remaining state after peel test>
The evaluation of the heat-resistant layer was performed by attaching a vinyl tape (JIS C 2336) to a separator with a 1 cm square grid, checking the state of the grid after peeling, and measuring the residual rate of the heat-resistant layer. . The residual rate of Example 1 was 80%.

  The positive electrode plate, the negative electrode plate, and the separator manufactured as described above are arranged in the order of the negative electrode plate, the separator, and the positive electrode plate, the positive electrode aluminum lead is on the inner peripheral side, and the negative electrode nickel lead is on the outermost outer periphery. Laminated and wound from one end to form an electrode group. At this time, the negative electrode plate serving as the outermost periphery was cut with the separator after applying tension until the negative electrode mixture coating portion was completely wound and the negative electrode lead was wound around the separator. Then, after winding up so that there was no looseness in the outermost periphery part, the negative electrode lead of the outermost periphery was fixed with tape, and the electrode element was produced.

The obtained electrode element was inserted into a battery can, and LiPF ₆ was dissolved in a mixed solution of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate in a volume ratio of 30:40:30 as a non-aqueous electrolyte solution so as to be 1 mol / L. And a battery lid that also serves as a positive electrode terminal was caulked through a gasket to obtain a 18650 size cylindrical battery.

  In order to evaluate the battery characteristics of the obtained secondary battery, the following tests were performed. The results are shown in Table 1 below.

<Evaluation of capacity maintenance characteristics>
After charging the secondary battery at a constant current (0.5 C) -constant voltage (4.3 V), a 0.5 C discharge is performed for 300 cycles to a discharge starting voltage of 2.75 V, and the capacity is maintained after the end of the 300 cycles The rate was measured. The capacity retention rate of Example 1 was 92%.

<Evaluation of output characteristics>
The secondary battery was adjusted to SOC (charged state) 60%, discharged at 3C for 10 seconds, paused for 10 seconds, charged at 3C for 10 seconds, and paused for 10 seconds. Similarly, the measurement was continuously performed at 5C and 10C, and the OCV (open potential) before the test and the voltage after 10 seconds discharge of 3C, 5C and 10C were plotted on the Y axis, and the current value was plotted on the X axis. , The current value at 2 V was obtained, and the output characteristic was obtained by setting output (W) = current value (A) × 2 (V). The output characteristic of Example 1 was 11 W.

<Examples 2, 3, and 4>
Examples 2, 3, and 4 are the same as Example 1 except that the speed of the corona discharge treatment is changed.

<Example 5>
Example 5 is the same as Example 1 except that polyethylene (PE) is used for the separator substrate and the conditions of the corona discharge treatment are changed.

<Comparative Example 1>
Comparative Example 1 is the same as Example 1 except that the heat-resistant layer was applied without performing the corona discharge treatment.

<Comparative Examples 2 and 3>
Comparative Examples 2 and 3 are the same as Example 1 except that the conditions of the corona discharge treatment are changed.

<Comparative example 4>
Comparative Example 4 is the same as Example 1 except that polyethylene is used for the separator base material and the corona discharge treatment conditions are changed.

<Comparative Example 5>
Comparative Example 5 is the same as Example 1 except that polyethylene was used for the separator substrate and the corona discharge treatment was not performed when the heat-resistant layer was applied.

  From the results shown in Table 1, in Examples 1 to 5 using the separator base material whose surface wettability was improved to 40 mN / m or more by corona discharge treatment, the heat-resistant layer formed on the base material surface was subjected to a peel test. However, the battery using these separators was excellent in cycle characteristics and output characteristics. On the other hand, in Comparative Examples 1 to 5 where the wettability of the separator base material surface was less than 40 mN / m, it was difficult to uniformly apply the heat-resistant layer coating liquid to the separator base material surface. The heat-resistant layer was almost peeled off by the peel test, and batteries using these separators were inferior in cycle characteristics and output characteristics.

  From such a result, according to the present invention, since the adhesion and binding properties between the separator substrate and the heat-resistant layer are improved, even if charging and discharging are repeated, the heat-resistant layer is maintained on the separator substrate surface, For this reason, it became clear that reaction with a separator and a battery and degradation by it were suppressed, and the nonaqueous secondary battery excellent in the battery characteristic was obtained.

Claims (7)

A separator substrate surface is coated with a heat-resistant layer,
The separator substrate surface is a non-aqueous secondary battery separator having a wettability of 40 mN / m or more.   The non-aqueous secondary battery separator according to claim 1, wherein a melting point of the heat-resistant layer is higher than a melting point of the separator base material.   The non-aqueous secondary battery according to claim 1, wherein the heat-resistant layer contains at least one inorganic compound selected from the group consisting of oxides, carbonate compounds, hydroxides, and phosphate compounds. Separator.   The non-aqueous secondary battery separator according to claim 1, wherein the heat-resistant layer contains an aromatic polyamide.   A nonaqueous secondary battery comprising the separator according to claim 1, 2, 3 or 4. A method for producing a separator for a non-aqueous secondary battery in which a separator substrate surface is coated with a heat-resistant layer,
Modifying the surface of the separator substrate with a hydrophilic group, and making the wettability of the separator substrate surface 40 mN / m or more; and
The manufacturing method of the separator for non-aqueous secondary batteries which has the process of apply | coating the coating liquid for heat-resistant layers to the surface of the said separator base material modified by the hydrophilic group.   The method for producing a separator for a non-aqueous secondary battery according to claim 6, wherein the method of modifying the surface of the separator substrate with a hydrophilic group uses corona discharge, plasma treatment, ion implantation, or ion beam mixing.