JP2011060539A - Separator for secondary battery and lithium secondary battery using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a short-circuit problem and a low cycle characteristic problem due to the formation of a dendrite in a lithium secondary battery that uses a lithium metal for a negative electrode. <P>SOLUTION: For a separator of the lithium secondary battery that uses the lithium metal for the negative electrode, a porous resin film is used; the porous resin film has a porosity of 60% or above, wherein each pore has a three-dimensional ordered-arrayed structure and is communicated with each other by a communication hole; the porous resin film is fabricated by the processes: monodispersed spherical inorganic fine particles are dispersed in a solvent, and then a dispersion liquid is filtered by a filter to integrate the monodispersed spherical inorganic fine particles on the filter and form a close-packed deposit of the monodispersed spherical inorganic fine particles; the deposit is baked, to form a sintered material of the inorganic fine particles; and after filling in the spaces between the inorganic fine particles of the sintered material with resin, the sintered material is dipped in a solution that dissolves the inorganic fine particles but does not dissolve the resin so as to remove the inorganic fine particles by dissolution. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a separator for a secondary battery and a lithium secondary battery using the same.

In recent years, portable cordless products such as a CD player, a mobile phone, a notebook personal computer, and a video camera are becoming smaller and more portable. In addition, from the viewpoint of environmental problems such as air pollution and an increase in carbon dioxide, development of hybrid vehicles and electric vehicles has been promoted and it is in the stage of practical use. These electronic devices and electric vehicles are required to have excellent secondary batteries having characteristics such as high efficiency, high output, high energy density, and light weight. Various secondary batteries have been developed and researched as secondary batteries having such characteristics, and various researches have been conducted on lithium secondary batteries to provide those having such characteristics. At present, graphite is mainly used as a negative electrode of a lithium secondary battery. Graphite shows a theoretical capacity of 372 mAh / g by forming an intercalation compound LiC ₆ with lithium. However, since lithium secondary batteries currently commercialized use about 95% of this theoretical capacity, the battery In order to further increase the capacity of these, development of a new negative electrode material is required.

As a negative electrode material that replaces graphite, research using a metal material that is alloyed with lithium has been conducted intensively. To date, tin, silicon, and materials containing these have formed an alloy with lithium, and have a capacity larger than 372 mAh / g. It has been reported that it can be obtained. Among these, tin can be charged and discharged by repeating alloying / dealloying with lithium, and shows a high theoretical capacity of 993 mAh / g. Therefore, tin has recently attracted attention as a negative electrode material. However, since tin becomes Li _4.4 Sn at the time of alloying, its volume expands about 4 times. This causes a problem that tin is pulverized and the current collecting property is lowered, so that good cycle characteristics are not exhibited. Until now, it has been reported that volume expansion that occurs during charging and discharging is suppressed and cycle characteristics are improved by previously alloying a metal that does not react with lithium with tin (see Non-Patent Document 1). . However, when the amount of the active material per unit area is increased, there is a problem that capacity and cycle characteristics are deteriorated.

  On the other hand, lithium is the lightest of all materials, and metallic lithium is ideal because it has a very high theoretical energy density (weight capacity density 3861 mAh / g) and a low charge / discharge potential (−3.045 V vs. SHE). It is considered as a negative electrode material. For this reason, lithium metal has attracted attention as a negative electrode material for next-generation lithium secondary batteries, and its application has been studied. Since the electrode potential of lithium metal is very low, most electrolytes react with lithium metal to form an interface SEI (Solid Electrolyte Interface), and the electrode reaction proceeds through this lithium ion conductive SEI. Therefore, it is known that the properties of SEI greatly affect battery performance. For this reason, researches such as detailed analysis of SEI and development of new electrolytes that form good SEI have been extensively conducted.

  In a chargeable / dischargeable lithium battery, the positive electrode (cathode) and the negative electrode (anode) are usually separated by a porous polymer film containing an organic electrolyte, thereby preventing direct electrical contact between the anode and the cathode. It is said that. For example, lithium transition metal oxide is used as an anode, and metal lithium is used as a cathode. As the electrolyte, for example, a lithium salt dissolved in a non-aqueous organic solvent is used, which has good ionic conductivity and negligible electrical conductivity. During charging, lithium ions move from the positive electrode to the negative electrode (lithium). During discharge, lithium ions move in the opposite direction and are returned to the positive electrode.

  A battery using a lithium metal piece or sheet as a cathode is also called a lithium metal battery. Such a battery has a dendritic lithium metal as shown in FIG. Precipitation is known (see, for example, Patent Document 1). When charging and discharging are repeated, dendritic lithium grows, and the cycle characteristics deteriorate due to peeling from the lithium metal. In the worst case, it grows to the extent that it breaks through the separator, causing a short circuit of the battery and causing the battery to ignite. Therefore, in order to use lithium metal as a negative electrode, it is necessary to solve the problem of lithium dendrite.

  Secondary battery separators include film thickness (thinness), mechanical strength, ionic conductivity (when containing electrolyte), electrical insulation, electrolyte resistance, shutdown effect, liquid retention against electrolyte, wetting Various characteristics are required. As a separator for a secondary battery having such properties, a polyolefin microporous membrane system (polymer film having micropores of 1 μm or less) such as polyethylene or polypropylene is generally used. The microporous membrane used as such a separator has random pores and a porosity of about 35 to 40%, and is widely used as a separator for lithium secondary batteries using carbon as an anode. Yes. These microporous membranes are classified into three types, dry one-component system, wet two-component system, and wet three-component system, depending on the production method. A dry one-component microporous membrane is produced by forming micropores starting from spherulites by stretching in the cooling process in forming a polymer melt extruded film. The wet two-component microporous membrane is produced by forming micropores by polymer microphase separation and plasticizer extraction in melt extrusion film formation of a polymer in which a plasticizer is previously kneaded. Furthermore, the wet three-component microporous membrane has fine pores formed by polymer microphase separation and extraction of the plasticizer and inorganic filler during the cooling process in the melt extrusion film formation of a polymer in which a plasticizer and inorganic filler fine particles are previously kneaded. It is manufactured by forming. As a representative separator, Asahi Kasei's Hypore (registered trademark) is known. There are various grades of hypopores having a film thickness of about 25 to several hundred μm and micropores of 0.05 to 0.5 μm. However, at present, it has not been possible to solve the problem of deterioration of cycle characteristics due to the formation of dendritic lithium metal and the problem of short circuit by using these conventionally known separators for lithium secondary batteries.

  In addition, as a method of suppressing the generation of dendritic lithium, a method of modifying the lithium electrodeposition with an additive that effectively acts on the surface of the lithium metal, or by pressing a separator against the lithium metal negative electrode, Research on the pressure effect that suppresses peeling has been made conventionally (see Non-Patent Document 2), but it has not led to effective performance improvement. Further, although a solid electrolyte has been studied as a method for forming a stable interface that does not cause a reaction with an electrolytic solution, a practical solid electrolyte has not yet been developed. Furthermore, it has been proposed to use a porous silica membrane that is considered to have a three-dimensionally ordered porous structure (3DOM structure) with a high porosity as a fuel cell separator (see Patent Document 2). No mention is made of the use for the next battery.

  As described above, lithium metal having a weight capacity density of 3861 mAh / g has attracted attention as a negative electrode material for the next generation lithium secondary battery, and its application has been studied. A lithium secondary battery using lithium metal as a negative electrode has been studied. However, there is a problem in safety and cycle characteristics, such as lithium metal depositing in a dendrite form, which breaks through the separator and short-circuits, and causes isolated lithium to deteriorate cycle characteristics. It has not yet been realized.

JP 2003-514355 A JP 2007-35300 A

H. Mukaibo, T .; Momma, M .; Mohamedi, T .; Osaka, Journal of the Electrochemical Society, 152 (2005) pp. A560-A565 T.A. Hirai, I .; Yoshishima, J. et al. Yamaki, Journal of the Electronic Chemical Society, 141 (1994) pp. 611-614

  As described above, lithium is a substance having the most basic electromotive force among the elements. By using it for the negative electrode of the battery, a high energy density battery can be expected, but it can be deposited in a dendrite form on the negative electrode surface during charging. In addition, it has been difficult to construct a secondary battery having a long charge / discharge cycle life. Dendritic lithium is not only extremely dangerous when it comes into contact with the positive electrode, causing internal short circuits, but it will fall off the negative electrode surface when repeated charging and discharging, generating particulate lithium that can not be used for charging and discharging, and charging and discharging capacity. Decrease.

  Therefore, an object of the present invention is to provide a lithium secondary battery having good battery safety and cycle characteristics, which has been a problem in the past in lithium secondary batteries using lithium metal as a negative electrode.

  Moreover, an object of this invention is to provide the separator used in order to comprise the lithium secondary battery with the said favorable characteristic, and its manufacturing method.

  As a result of intensive studies and studies to achieve the above object, the present inventors can control the electrodeposition reaction of lithium by using a separator having a three-dimensional ordered array porous structure (3DOM structure). Thus, it was found that a lithium secondary battery with improved cycle characteristics can be obtained, and that a porous film having such a 3DOM structure can be easily produced by the colloidal crystal template method, and based on these findings The present invention has been made.

  That is, the present invention relates to the following secondary battery separator and method for producing the same, and a lithium secondary battery using the secondary battery separator.

(1) A separator for a secondary battery, comprising a porous resin film having a porosity of 60% or more, having pores having a three-dimensional regular array structure, and pores communicating with each other through communication holes. .

(2) The separator for a secondary battery as described in (1) above, wherein the pore size is 50 to 2500 nm and the communication hole size is 1/2 to 1/100 of the pore size.

(3) The above (1) or (2), wherein the shape of the hole is substantially spherical, the hole is in a close-packed state, and the shape of the hole and the communication hole is a bottleneck shape ) Secondary battery separator.

(4) The secondary battery separator according to any one of (1) to (3), wherein the resin is a polyimide resin.

(5) Disperse monodispersed spherical inorganic fine particles in a solvent, and filter the dispersion with a filter to accumulate the monodispersed spherical inorganic fine particles on the filter to form a close-packed deposit of monodispersed spherical inorganic fine particles. Then, the deposited body is fired to form a sintered body of inorganic fine particles. After the resin is filled in the inorganic fine particle gaps of the sintered body, the sintered body is immersed in a solution in which the inorganic fine particles are dissolved but the resin is not dissolved. A method for producing a separator for a secondary battery, comprising dissolving and removing inorganic fine particles.

(6) The size of the communication hole formed in the separator after the inorganic fine particles are dissolved by controlling the sintering temperature and the sintering time of the monodispersed spherical inorganic fine particles (5) The manufacturing method of the separator for secondary batteries as described in).

(7) The monodispersed spherical inorganic fine particles are monodispersed spherical silica particles, the monodispersed spherical silica particles are sintered at 1000 to 1100 ° C. for 30 minutes to 12 hours, and a solution for dissolving silica is fluorinated. The method for producing a separator for a secondary battery as described in (5) or (6) above, wherein the method is a hydrogen acid aqueous solution.

(8) The filling of the resin is filling with a polyimide resin formed by firing after filling the gap between the inorganic fine particles with polyamic acid, and any of the above (5) to (7) Of manufacturing a secondary battery separator.

(9) The method for producing a separator for a secondary battery as described in any one of (5) to (8) above, wherein the monodispersed spherical inorganic fine particles have a particle size of 50 to 2500 nm.

(10) A lithium secondary battery comprising the separator for secondary batteries described in any one of (1) to (4) above and a negative electrode made of lithium metal.

(11) In the lithium secondary battery according to (10), the electrolytic solution contains at least one compound selected from LiPF ₆ and LiClO ₄ , 1,2-dimethoxyethane, ethylene carbonate, propylene carbonate, A lithium secondary battery, which is at least one non-aqueous solvent solution selected from dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

  In the present invention, by using a porous film having a 3DOM structure as a separator, the ion current density is made uniform, and the electrodeposition reaction of lithium is uniformly controlled even under charge / discharge conditions with a high current density. For this reason, in a secondary battery using a lithium metal negative electrode, formation of lithium dendrite can be prevented, and as a result, a lithium secondary battery having high cycle characteristics and no short-circuit between positive and negative electrodes due to dendrite is obtained.

  In addition, since the 3DOM structure in the separator of the present invention is a porous body in which spherical holes regularly form adjacent holes (communication holes), these uniformized spaces A lithium secondary battery in which diffusion of lithium ions is controlled, high cycle characteristics, and no short circuit between positive and negative electrodes due to dendrite is obtained. The effect of making the current distribution of lithium ions uniform is considered to be due to the 3DOM structure in which the spherical holes have a three-dimensional, regularly continuous structure.

  The 3DOM structure in the separator of the present invention is a porous body having regularly arranged pores of a hexagonal close-packed structure, has a very high theoretical porosity, and a porous film having a large porosity is a separator. Therefore, since a large amount of electrolyte can be filled in the 3DOM structure, high ionic conductivity is obtained as compared with a conventional separator.

  The porous membrane having a 3DOM structure of the present invention can be easily produced by a method using monodispersed spherical inorganic fine particles as a template. In addition, by selecting the particle size of the monodispersed spherical inorganic fine particles serving as a template at the time of production, the pore size of the porous film can be easily controlled from the micro order to the nano order. In addition, by controlling the firing temperature and firing time of the monodispersed spherical inorganic fine particle aggregates, the size of the communication holes can be easily controlled, and a porous membrane for a separator having desired characteristics can be easily produced. be able to.

  Further, according to the method for producing a porous film of the present invention, any insulating material can be applied as a 3DOM structure by a colloidal crystal template method and applied as a 3DOM separator. Can also respond.

  Moreover, according to the manufacturing method of the porous membrane of this invention, the film thickness of the porous membrane separator which has 3DOM structure is controlled in the range of 30-500 micrometers, for example by changing the film thickness of the particle | grain assembly used as a casting_mold | template. It is possible to make it.

It is a drawing substitute photograph and is an SEM photograph of a lithium dendrite formed in a secondary battery using a conventional lithium metal as a negative electrode. It is a schematic diagram explaining one manufacturing method of the 3DOM structure porous membrane of this invention. It is a drawing substitute photograph, and is an SEM image of a 3DOM structure porous polyimide film produced by changing the particle size of monodispersed spherical silica and heat treatment conditions. FIG. 3 is a drawing-substituting photograph, which is a cross-sectional SEM image of the 3 DOM structure porous film obtained in Example 1. FIG. It is a cross-sectional schematic diagram which shows the electrode structure used for the cell for charging / discharging measurement. It is a cross-sectional schematic diagram of the cell for charging / discharging measurement which has the electrode structure of FIG. It is a figure which shows the cycling characteristics (Coulomb efficiency) in the charge / discharge depth of 10% of the cell using the separator of Example 1. FIG. It is a drawing-substituting photograph, and shows the deposited lithium surface (a) on the copper foil side of the cell after the cycle characteristics evaluation of Example 3 and the deposited lithium surface (b) SEM image of the lithium metal surface. It is a figure which shows the cycling characteristics (Coulomb efficiency) in the charge / discharge depth of 25% of the cell using the separator of Example 1. FIG. It is a schematic diagram of the coin cell used for cycle characteristics. It is a figure which shows the cycle characteristic (Coulomb efficiency) using the coin cell of FIG. It is a figure which shows the cycling characteristics (Coulomb efficiency) of the coin cell using 3DOM structure polyimide porous membrane. It is a drawing substitute photograph and is a surface SEM image of electrodeposited lithium when Celgard # 2400, 3DOM structure silica film, and 3DOM structure polyimide porous film are used as a separator.

  A lithium secondary battery using lithium metal as a negative electrode is generally used in which a lithium metal (for example, a foil or a plate) as a negative electrode and a positive electrode are opposed to each other with a separator interposed therebetween, and the separator is impregnated with an electrolytic solution. In the present invention, by using a specific separator as the separator, the purpose of the present invention, that is, the formation of dendritic lithium can be controlled, thereby preventing a short circuit between the negative electrode and the positive electrode due to dendrite, and cycle characteristics. In the following, the separator used in the lithium secondary battery of the present invention will be described.

  The basic functions of the lithium secondary battery separator are to prevent the short circuit by separating the positive and negative electrodes, to retain the electrolyte necessary for the battery reaction, to ensure high ionic conductivity, and to pass the battery reaction inhibitor For example, it has a current interruption characteristic for preventing and ensuring safety. Conventionally known separators are generally composed of a microporous polymer film having random pores with a porosity of about 40%. Thus, the present invention is not limited. However, when lithium metal is used for the negative electrode, a surface film called SEI (Solid Electrolyte Interface) is generated non-uniformly on the surface of the lithium metal, resulting in non-uniform current distribution. Occurs. The local lithium dendrite growth is considered to be caused by the reaction of the non-uniform lithium ion current distribution by the separator having random vacancies with the non-uniform lithium reaction layer due to the current distribution. That is, since the vacancies are random, when the lithium electrodeposition reaction becomes an ion diffusion-controlled reaction, the current density of lithium ions is locally concentrated, and as a result, the lithium dendrite breaks through the separator. Since it grows and causes a short circuit of the electrode, it is considered that it does not function as a separator.

  In the present invention, the separator vacancies have a three-dimensional ordered array porous structure (3DOM structure) having regularly arranged vacancies in a hexagonal close-packed structure, so that the current distribution of lithium ions is made uniform, By carrying out the precipitation reaction in a uniform and gentle manner, it was possible to deposit granular lithium metal without generating dendrites. Lithium ion diffusion is made uniform, so that even in the case of diffusion-controlled reaction, the ion current density is made uniform, so that the lithium electrodeposition reaction is controlled uniformly, and the 3DOM structure makes the ion current density uniform. As a result, even under charge / discharge conditions with a high current density, the electrodeposition reaction of lithium was uniformly controlled, and the cycle characteristics of a secondary battery using a lithium metal negative electrode could be improved. A separator has a very different structure. In the present invention, since the porosity of the porous membrane is extremely large compared to the porosity of the conventional separator, a large amount of electrolyte can be filled in the 3DOM structure. It has also become possible to have high ionic conductivity.

  The separator of the present invention has a porosity of 50% or more, preferably 60% or more, more preferably 65% or more, and preferably spherical pores have a 3DOM structure, and the pores are preferably spherical. The communication holes communicate with each other. The upper limit of the porosity may be any as long as the strength of the membrane functions as a separator, but is usually about 90%. Since the size of the pores is about 1 to 3 μm, the size of the lithium dendrite needs to be smaller than this. The pore size is usually preferably about 50 to 2500 nm, more preferably 100 to 2000 nm, and still more preferably 150 to 1500 nm. The porosity is determined by the size of the spherical particles used as a template. The communication hole is preferably smaller than the hole size (maximum hole diameter) and has a bottleneck shape. The communication hole size varies depending on the hole size and is not particularly limited, but is preferably about 1/2 to 1/100 of the normal hole size, more preferably 1/3 to 1/10, As a specific numerical value, it is preferable that it is about 20 nm-1000 nm, for example, More preferably, it is about 30-500 nm. If the communication hole size is too large, a problem of generation of dendrites may occur, and if it is too small, a problem of decrease in ionic conductivity may occur. Since the separator holds the electrolytic solution, the separator is preferably formed of a material excellent in liquid retention. The film thickness of the porous film is not particularly limited, but is about 30 to 500 μm.

  The porous membrane having a 3DOM structure of the present invention can be manufactured by the following method, for example. That is, first, monodispersed spherical inorganic fine particles are dispersed in a solvent, and the dispersion is filtered through a filter, whereby the monodispersed spherical inorganic fine particles are accumulated on the filter, and the finely dispersed rule of the monodispersed spherical inorganic fine particles serving as a template is obtained. An array is produced. Next, if necessary, the deposit is peeled off from the filter, fired and sintered. After the inorganic fine particle gaps of the sintered body thus obtained are filled with a resin constituting the separator, the inorganic fine particles are dissolved and removed by dipping in a solution in which the inorganic fine particles are dissolved but the resin is not dissolved. Then, the film | membrane which comprises the separator of this invention is formed by wash | cleaning and drying a film | membrane as needed.

  At this time, any solvent may be used as long as it does not dissolve the inorganic fine particles and the filter, and distilled water is generally used. Moreover, the thickness of the deposited particles, that is, the thickness of the porous film can be controlled by changing the filtration amount per unit area of the inorganic fine particles to be filtered, and the 3DOM structure can be obtained by changing the size of the inorganic fine particles. It is possible to control the pore diameter of the separator having the above. In the present invention, the pore size of the separator is preferably about 150 to 1500 nm as described above. However, the pore size after elution of the inorganic fine particles as a template is caused by the shrinkage of the resin and the like. Generally it is somewhat smaller than the particle size. Therefore, the diameter of the inorganic fine particles to be used may be selected in consideration of the finally required membrane porosity, resin shrinkage, and required pore diameter. The firing treatment is performed to increase the strength of the inorganic fine particle aggregate. That is, the firing process sinters the inorganic fine particles, melts the inorganic fine particles, and increases the strength of the inorganic fine particle aggregate. This also ensures the formation of the communication hole. Furthermore, by changing the heat treatment temperature and time at this time, the degree of sintering of the inorganic fine particles can be controlled, thereby controlling the size of the pores of the microporous resin film having a 3DOM structure. The heat treatment temperature may be a temperature equal to or higher than the temperature at which the inorganic fine particles to be used can be sintered, and the firing time may be an appropriate time according to the required communication hole size. Further, impregnation of the resin deposits may be performed by any conventionally known method, but is preferably impregnated by a vacuum impregnation filling method. At this time, the amount of the resin to be impregnated is preferably set so that the thickness of the resin film is equal to or less than the thickness of the sintered body. By controlling the amount of the resin to be impregnated, the film thickness of the porous resin film can be controlled.

  With reference to FIG. 2 (plan view) schematically shown, a method for producing a 3 DOM structure microporous membrane when monodispersed spherical silica is used as the monodispersed spherical inorganic fine particles and the resin is a polyimide resin will be described.

  Monodispersed spherical silica particles (Seahoster; manufactured by Nippon Shokubai Co., Ltd.) are dispersed in, for example, distilled water (the upper left diagram in FIG. 2), and this is filtered under reduced pressure using a filter to accumulate the silica particles on the filter. At this time, the silica particles are in a close packed state. The deposit is peeled off from the filter, and heat treatment is performed at 1000 to 1100 ° C. for 30 minutes to 12 hours, for example, to sinter the silica fine particle deposit (upper right diagram in FIG. 2). Next, the interstices of the sintered body are impregnated with a polyamic acid, which is a polyimide precursor, by a vacuum impregnation filling method (lower right diagram in FIG. 2). Thereafter, the polyamic acid is thermally imidized by firing, and the silica used as a template is dissolved and removed, for example, by impregnating it in a low-concentration hydrofluoric acid aqueous solution for 12 hours, for example, and used as a separator. Is produced (lower left diagram in FIG. 2).

  By changing the particle diameter of monodispersed spherical silica and heat treatment conditions used as described in Table 1 by such a method, the pore size, communication pore size, pore size of the produced 3DOM structure porous polyimide membrane was changed. Porosity can be controlled. In FIG. 3, the SEM image of the communicating hole of 3DOM polyimide film produced on the conditions of the following table | surface is shown. 3, (a) is a pore size (pore diameter) 1300 nm, (b) is a pore size 860 nm, (c) is a pore size 400 nm, (d) is a pore size 200 nm, and (e) is a pore. The size is 150 nm.

  FIG. 3 shows that the 3DOM polyimide film is a porous body in which pores reflecting the particle diameter of the silica particles used as a template are regularly arranged in three dimensions. Also, it can be seen that the size of each communication hole (small black dot) is about one-fourth to one-fifth of the hole.

  In the above example, monodispersed spherical silica particles are used as the monodispersed spherical inorganic fine particles, and the resin is a polyimide resin. However, the monodispersed spherical inorganic fine particles and the resin are not limited to these. Monodispersed spherical inorganic fine particles are used as a template when forming pores, and therefore have monodispersed spherical shapes that do not react with the resin that forms the porous resin film, and that can be dissolved and eluted. As long as it is not limited to inorganic fine particles, any type may be used. For example, examples of the monodispersed spherical inorganic fine particles include polystyrene, polymethyl methacrylate and the like in addition to the monodispersed spherical silica.

  Furthermore, as the resin for forming the 3DOM structure porous film, any of the resins that have been known to be excellent in liquid retention and have been conventionally used as separators for secondary batteries can be used. Resins that can be used include, for example, polyamide resins such as nylon, polyester resins such as polyethylene terephthalate and polyethylene naphthalate, polyolefin resins such as acrylic resin, polyethylene and polypropylene, and polyvinyl chloride. Examples thereof include resins, polyvinylidene chloride resins, polyurethane resins, polyoxymethylene resins, fluorine-based resins such as polytetrafluoroethylene, and polyparaphenylenebenzbisthiazole resins. When the resin has a low melting point or a low softening point, when the temperature of the secondary battery rises, the separator is easily melted and shrinks easily. Since the problem that a short circuit occurs between the electrodes when the thermal shrinkage of the separator occurs, a resin having a high melting point or softening point, for example, a resin having a melting point or softening point of 140 ° C. or higher is preferable.

  In addition, although the preferable manufacturing method of the separator was mentioned above, the manufacturing method of the separator of this invention is not limited to said thing.

  Next, the lithium secondary battery of the present invention will be described. A lithium secondary battery includes a negative electrode plate made of lithium, a separator according to the present invention, a nonaqueous electrolyte solution made of an aprotic organic solvent and a lithium salt, a positive electrode plate, and a gasket, a current collector as other battery components , Sealing plate, cell case and the like. In the secondary battery of the present invention, lithium metal is used as the negative electrode and the 3 DOM structure porous membrane of the invention is used as the separator. Any other known or well-known battery components can be used. is there. Further, the shape of the battery may be any shape including a conventionally known shape such as a cylindrical shape, a square shape, and a coin shape, and is not particularly limited. When the lithium secondary battery is, for example, a coin-type battery, usually, a negative electrode plate is placed on the cell floor plate, an electrolyte and a separator are placed thereon, and a positive electrode is placed so as to face the negative electrode. The secondary battery is caulked together with the plate, but the structure or manufacturing method of the lithium ion battery of the present invention is not limited to this.

  As described above, in the lithium secondary battery of the present invention, the positive electrode constituting the battery, the non-aqueous electrolyte composed of an aprotic organic solvent and a lithium salt, the conventional positive electrode of the lithium secondary battery, the non-aqueous electrolyte, etc. Any known or well-known material can be used.

Although it does not specifically limit as a material used as a positive electrode in the lithium secondary battery of this invention, The metal chalcogen compound etc. which can occlude and discharge | release lithium ion at the time of charging / discharging are preferable. Examples of such metal chalcogen compounds include vanadium oxide, vanadium sulfide, molybdenum oxide, molybdenum sulfide, manganese oxide, chromium oxide, titanium oxide, titanium sulfide, and the like. And composite oxides and sulfides. Examples of such compounds include Cr ₃ O ₈ , V ₂ O ₅ , V ₅ O ₁₈ , VO ₂ , Cr ₂ O ₅ , MnO ₂ , TiO ₂ , MoV ₂ O ₈ , TiS ₂ V ₂ S ₅ MoS _2. MoS ₃ VS ₂ , Cr _0.25 V _0.75 S ₂ , Cr _0.5 V _0.5 S _{2 and the} like. Further, LiMY ₂ (M is a transition metal such as Co and Ni, Y is a chalcogen compound such as O and S), LiM ₂ Y ₄ (M is Mn and Y is O), an oxide such as WO ₃ , CuS, It is also possible to use sulfides such as Fe _0.25 V _0.75 S ₂ and Na _0.1 CrS ₂ , phosphorus such as NiPS ₈ and FePS ₈ , sulfur compounds, selenium compounds such as VSe ₂ and NbSe ₃ , and iron compounds such as iron oxide. it can. Further, manganese oxide and lithium / manganese composite oxide having a spinel structure are also preferable.

More specific materials include LiCoO ₂ , LiCo _1-x Al _x O ₂ , LiCo _1-x Mg _x O ₂ , LiCo _1-x Zr _x O ₂ , LiMn ₂ O ₄ , Li _1-x Mn _{2− x O 4, LiCr x Mn 2} -x O 4, LiFe x Mn 2-x O 4, LiCo x Mn 2-x O 4, LiCu x Mn 2-x O 4, LiAl x Mn 2-x O 4, LiNiO _{2, LiNi x Mn 2-x} O 4, Li 6 FeO 4, NaNi 1-x Fe x O 2, NaNi 1-x Ti x O 2, FeMoO 4 Cl, LiFe 5 O 8, FePS 3, FeOCl, FeS 2 _{, Fe 2 O 3, Fe 3} O 4, β-FeOOH, α-FeOOH, γ-FeOOH, α-LiFeO 2, α-NaFeO 2, LiFe 2 (MoO 4) 3, LiFe 2 (WO 4) 3, LiFe _{2 (SO 4) 3, Li} 3 Fe 2 (PO 4) 3, Li 3 Fe 2 (AsO 4) 3 Like _{Li 3 V 2 (AsO 4)} 3, Li 3 FeV (AsO 4) 3, Li 1.5 Al 0.5 Ge 1.5 (PO 4) 3, LiFePO 4, Li 2 FeSiO 4, FeBO 3, FeF 3 .

  Nonaqueous solvents that can be used in the electrolyte of the lithium secondary battery of the present invention include acetonitrile (AN), γ-butyrolactone (BL), γ-valerolactone (VL), γ-octanoic lactone (OL), diethyl Ether (DEE), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), dimethyl sulfoxide (DMSO), 1,3-dioxolane (DOL), ethylene carbonate (EC), propylene carbonate ( PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate (MF), tetrahydrofuran (THF), 2-methyltetrahydrofuran (MTHF), 3-methyl-1,3-oxaziridine -2-one (MOX), sulfolane (S And the like, which may be used alone or as a mixture of two or more.

As the lithium salt used in the electrolyte of a lithium secondary _{battery, LiPF 6, LiAsF 6, LiClO} 4, LiBF 4, LiCF 3 SO 3, Li (CF 3 SO 2) 2 N, LiC 4 F 9 SO ₃ or the like, and one or more of these may be dissolved in the non-aqueous solvent at a concentration of about 0.5 to 2.0 M to form a non-aqueous electrolyte.

In the present invention, there are differences in cycle characteristics depending on the type of separator and electrolyte used. For example, when a 3 DOM structure porous imide membrane is used as the separator, 1 moldm ⁻³ LiClO ₄ / EC (ethyl carbonate): DEC (Diethyl carbonate) = 1: 1 showed particularly high performance.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not limited at all by this.

In the following examples, the charge / discharge test was performed using a copper foil as the positive electrode, lithium metal as the negative electrode, 3DOM polyimide as the separator, 1 moldm ⁻³ LiPF ₆ / EC: DEC = 1: 2 and 1 moldm ⁻³ LiClO ₄ / The test was conducted in a beaker cell and a coin cell using EC: DEC = 1: 1.

[Example 1]
<Production of polyimide porous membrane>
0.3 g silica monodispersed spherical particles with a diameter of 550 nm manufactured by Nippon Shokubai were dispersed in 10 cc of distilled water to prepare a suspension. This suspension was filtrated under reduced pressure using a 100 μm membrane filter, and was closely packed and deposited in a thickness of 200 μm on the filter. Next, the silica was dried, removed from the filter, and calcined at 1100 ° C. for 3 hours to increase the strength.

  On the other hand, as a polyimide precursor to be filled, 0.5 mol of pyromellitic acid and 0.5 mol of 4,4'-diaminodiphenyl ether were reacted in dimethylacetamide to synthesize a 10% by weight polyamic acid. The synthesized polyamic acid was filled in the baked silica film as a template and imide cured by heating imidization. The polyimide-silica composite film was immersed in a 10% hydrogen fluoride aqueous solution for 12 hours to dissolve and elute the silica to obtain a 3DOM structure polyimide film having continuous pores. The film thickness of the obtained 3 DOM structure polyimide porous membrane was 150 μm, the pore size was 400 nm, the porosity was 80%, and the communication pore size was 100 nm. A cross-sectional SEM (scanning electron microscope) image of the obtained 3 DOM structure polyimide film is shown in FIG. 4 (right). In addition, the left figure of FIG. 4 is a surface SEM image.

[Example 2]
<Preparation of beaker cell for charge / discharge measurement>
Cut two pieces of Permacell Kapton (registered trademark) tape (width: 1.9 cm, thickness: 100 μm) to a length of 5 cm, paste one on the cutting mat, and use a punch with a diameter of 8 mm to make a hole in the center of the Kapton tape. It was. Cut the copper foil into strips (length 7cm, width 1.2cm), apply Kapton tape to cover more than the upper half of one side of the copper foil, and paste Kapton tape with holes on the opposite side. By attaching, the copper foil side electrode as shown in FIG. 5 was produced. All subsequent operations were performed in a glove box under an Ar atmosphere. Place a 4 cm ² square PP plate, place a copper foil side electrode on it, immerse a 3 DOM polyimide film with a size of 1 cm ² or more in the electrolyte, and install it so that the hole of the Kapton tape is completely hidden on the copper foil side electrode. did. Punching Honjo Metal lithium foil (thickness 600 mm) in the punch of 9 mm, it was placed together center hole on previous polyimide film, placing a strip-shaped copper foil as a lead, further PP plate ² square 4cm The cell shown in FIG. 6 was produced by placing both sides of the PP plate alternately with clips. Two PP spacers and electrolyte were poured into a snap cup with a lid (50 mL), a cell was placed between the spacers, a copper foil was put out from the snap cup to make a lead, and a measurement terminal was attached. The cell was placed in a tight box (1.3 L), the lead copper foil was connected to the terminals of the positive electrode and the negative electrode, and the lid of the tight box was closed to obtain a charge / discharge measurement cell.

[Example 3]
<Charge / discharge measurement: 10% depth of charge / discharge>
Charging / discharging measurement was performed using a battery charging / discharging device (HJ1001SM8A) manufactured by Hokuto Denko. As the electrolytic solution, 1 moldm ⁻³ LiPF ₆ / EC: DEC = 1: 2 was used, and lithium metal was electrodeposited on the copper foil side electrode by performing electrodeposition at a current density of 1 mAhcm ⁻² for 50 hours. A charge / discharge test was conducted for 15 minutes at a current density of ² mAcm ⁻² to evaluate cycle characteristics. FIG. 7 shows cycle characteristics (Coulomb efficiency) at a charge / discharge depth of 10%. As shown in FIG. 7, the beaker cell using the 3DOM structure polyimide porous membrane (pore size 550 nm) of Example 1 was subjected to Coulomb efficiency in a charge / discharge test of 10% depth of discharge (DOD; depth of discharge). At 100%, a cycle stability of 800 cycles or more was exhibited.

  FIG. 8 shows a deposited lithium surface SEM image (a) on the copper foil side of the cell after the cycle characteristics evaluation and a precipitated lithium surface SEM image (b) of the negative electrode lithium metal surface. From FIG. 8, it can be confirmed that lithium is electrodeposited without forming dendrite. It can also be seen that lithium is electrodeposited in the pores of the 3DOM polyimide porous film.

[Example 4]
<Charge / discharge measurement: charge / discharge depth 25%>
A cell similar to that of Example 3 was prepared, and charge / discharge measurement was performed using a battery charge / discharge device (HJ1001SM8A) manufactured by Hokuto Denko. The electrolyte used was 1 moldm ⁻³ LiClO ₄ / EC: DEC = 1: 1, and was subjected to 50 hours of electrodeposition at a current density of 1 mAcm ⁻² to deposit 9C equivalent lithium (1.29 mg film thickness 50 mm) on the copper foil side. Electrodeposited on the electrode. A charge / discharge test for 38 minutes was conducted at a current density of ² mAcm ⁻² (charge / discharge depth of 25%), and the cycle characteristics were evaluated. FIG. 9 shows the Coulomb efficiency at a charge / discharge depth of 25%. As shown in FIG. 9, the beaker cell using the 3DOM structure polyimide porous membrane (pore size 550 nm) of Example 1 is 400 at a Coulomb efficiency of 100% in a charge / discharge test with a DOD (charge / discharge depth) of 25%. It showed cycle stability over the cycle.

[Comparative Example 1]
<Production of coin cell for charge / discharge measurement>
Honjo Metal Lithium Foil (thickness 300 μm) is punched with 1.4 mm punches, PP (polypropylene) porous membrane separator (thickness 25 μm) is punched with 1.7 mm punch, 1 moldm ^{− for} electrolyte ^{Using 3} LiClO ₄ / EC: DEC = 1: 1 and incorporating these ingredients into a 2032 coin cell made by Hosen in a glove box, the coin cell of FIG. 10 was produced.

In the charge / discharge test, charge / discharge measurement was performed using a battery charge / discharge device (HJ1001SM8A) manufactured by Hokuto Denko. As the electrolytic solution, 1 moldm ⁻³ LiClO ₄ / EC: DEC = 1: 1 was used, and a charge / discharge test was performed for 15 minutes at a current density of 25 mAhm ⁻² to evaluate cycle characteristics. As a result, the cell was short-circuited at the 123rd cycle. FIG. 11 shows the Coulomb efficiency of the charge / discharge test.

[Example 5]
Two castle metal lithium foils (thickness 300 μm) were punched out with 1.4 mm punches, and the 3DOM polyimide porous membrane (pore diameter 550 nm, thickness 100 μm) prepared in Example 1 was 1.7 mm punched. By punching out, using Celgard # 2400 as the separator and 1 moldm ⁻³ LiClO ₄ / EC: DEC = 1: 1 as the electrolyte solution, these materials are incorporated into a 2032 coin cell made by Hosen in a glove box. Ten coin cells were produced.

In the charge / discharge test, charge / discharge measurement was performed using a battery charge / discharge device (HJ1001SM8A) manufactured by Hokuto Denko. A charge / discharge test was performed at a current density of 16.2 mAcm ⁻² for 15 minutes to evaluate cycle characteristics. FIG. 12 shows cycle characteristics (Coulomb efficiency) in the charge / discharge test. The coin cell using the 3DOM structure polyimide porous film (550 nm) showed cycle stability of 200 cycles or more with a Coulomb efficiency of 100%.

[Example 6, Comparative Example 2, Comparative Example 3]
In order to confirm the precipitation form of lithium metal when a three-dimensional ordered porous body was used, Celgard (registered trademark) # 2400 (Comparative Example 2) and the 3DOM structure polyimide porous film of Example 1 (Example 6) were used as separators. ) And a 3DOM structure silica film (Comparative Example 3) prepared according to the procedure described in Example 1 of Japanese Patent Application Laid-Open No. 2007-35300, and a beaker cell for measurement test is produced in the same manner as Example 2. Then, an electrodeposition test for lithium was conducted. For the electrodeposition, a battery charging / discharging device (HJ1001SM8A) manufactured by Hokuto Denko was used. As the electrolytic solution, 1 moldm ⁻³ LiClO ₄ / EC: DEC = 1: 1 was used, and lithium metal was electrodeposited on the copper foil side electrode by performing electrodeposition at a current density of 20 mAcm ⁻² for 15 minutes.

  When each separator after electrodeposition was visually observed, Celguard # 2400 having random pores was transparent, and thus it was considered that no electrodeposited lithium was contained inside the separator. Moreover, in 3DOM silica, since the lithium ion permeation site is gray, it was confirmed that electrodeposited lithium was inside the porous body. In the 3DOM structure polyimide porous membrane of the present invention, the lithium ion permeation site is green, so it is confirmed that electrodeposited lithium enters the porous body and some reaction occurs between lithium and polyimide. It was done.

  On the other hand, when the electrodeposited lithium on the copper foil was visually confirmed, in Cellguard # 2400, the electrodeposited lithium was white. Moreover, in the separator of the three-dimensional regular arrangement structure, all electrodeposited lithiums were gray. FIG. 13A shows a surface SEM image of electrodeposited lithium when using Celgard # 2400, and FIG. 13B shows a surface SEM image of electrodeposited lithium when using a 3DOM structure silica film. The surface SEM image of electrodeposited lithium at the time of using a 3DOM structure polyimide porous membrane for c) is shown. As shown in FIG. 13A, in the cell guard # 2400, the electrodeposited lithium was in a dendrite shape. Further, from FIGS. 13B and 13C, in the separator having the three-dimensional regular arrangement structure, any electrodeposited lithium was in a short granular form, and dendrite was not formed. The difference in color in electrodeposited lithium is considered to have arisen from the difference in the deposited form of electrodeposited lithium. Moreover, when the SEM image of the electrodeposition surface of the 3DOM structure silica film | membrane and 3DOM structure polyimide porous film | membrane after electrodeposition was confirmed, the mode that lithium had entered in the porous body was able to be confirmed. In both the 3DOM structure silica film and the 3DOM structure polyimide porous film, electrodeposited lithium is contained inside the porous body, whereas in the 3DOM structure silica film, the color of the electrodeposited lithium is the same, whereas in the 3DOM structure polyimide porous film The film appeared green and changed in appearance. From this, it is considered that some kind of reaction occurs between the polyimide and the lithium metal. However, since the 3DOM structure silica film is rigid and difficult to deform, the separator cannot be freely deformed according to the shape of the lithium secondary battery. There is also a problem that reacts.

  As described above in detail, by using the 3DOM structure porous membrane of the present invention, it is possible to prevent the precipitation of lithium dendrite, which has been a problem in the past in lithium secondary batteries using lithium metal for the negative electrode, and safety. A lithium secondary battery having excellent properties and cycle characteristics can be provided.

Claims (11)

  A separator for a secondary battery, comprising a porous resin film having a porosity of 60% or more, the pores having a three-dimensional regular array structure, and the pores communicating with each other through the communication holes.   The separator for a secondary battery according to claim 1, wherein the pore size is 50 to 2500 nm, and the communication hole size is 1/2 to 1/100 of the pore size.   3. The secondary according to claim 1, wherein the shape of the hole is substantially spherical, the hole is in a close-packed state, and the shape of the hole and the communication hole is a bottleneck shape. Battery separator.   The separator for a secondary battery according to any one of claims 1 to 3, wherein the resin is a polyimide resin.   By dispersing the monodispersed spherical inorganic fine particles in a solvent and filtering the dispersion with a filter, the monodispersed spherical inorganic fine particles are accumulated on the filter to form a close-packed deposit of monodispersed spherical inorganic fine particles, The deposited body is fired to form a sintered body of inorganic fine particles, and after filling the gap between the inorganic fine particles of the sintered body with a resin, the inorganic fine particles are dissolved but the resin is not dissolved. A method for producing a separator for a secondary battery, comprising dissolving and removing the battery.   The size of the communication holes formed in the separator after the inorganic fine particles are dissolved is controlled by controlling the sintering temperature and the sintering time of the monodispersed spherical inorganic fine particles. A method for producing a separator for a secondary battery.   The monodispersed spherical inorganic fine particles are monodispersed spherical silica particles, and the monodispersed spherical silica particles are sintered at 1000 to 1100 ° C. for 30 minutes to 12 hours, and a solution for dissolving silica is a hydrofluoric acid aqueous solution. The method for producing a separator for a secondary battery according to claim 5 or 6, wherein:   The separator for a secondary battery according to any one of claims 5 to 7, wherein the filling of the resin is filling with a polyimide resin formed by firing after filling the gap between the inorganic fine particles with polyamic acid. Manufacturing method.   The method for producing a separator for a secondary battery according to any one of claims 5 to 8, wherein the monodispersed spherical inorganic fine particles have a particle size of 50 to 2500 nm.   A lithium secondary battery comprising the secondary battery separator according to claim 1 and a negative electrode made of lithium metal. The lithium secondary battery according to claim 10, wherein the electrolytic solution contains at least one compound selected from LiPF ₆ and LiClO ₄ , 1,2-dimethoxyethane, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl A lithium secondary battery comprising at least one non-aqueous solvent solution selected from carbonate and ethyl methyl carbonate.