JP2007141591A - Separator for lithium ion secondary battery, and lithium ion secondary battery using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce resistance of a separator and to improve mechanical strength thereof in order to provide high capacity and an excellent cycle characteristic even for a lithium ion secondary battery for a high-power application. <P>SOLUTION: This separator is used for a lithium ion secondary battery provided with a positive electrode formed of a lithium-containing composite oxide, a negative electrode formed of a material capable of retaining lithium, the separator, and a nonaqueous electrolyte. By dispersing a lithium ion conducting substance of 0.5-10 wt.% with respect to the weight of the whole separator in the separator, lithium ion conductivity of the separator is improved, so that resistance of the separator can be reduced and mechanical strength thereof can be improved at the same time. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an improvement in lithium ion conductivity and mechanical strength of a lithium ion secondary battery, particularly its separator.

In recent years, there has been a growing demand for secondary batteries that are small, light, and have high energy density as power sources for consumer electronic devices such as mobile phones, personal computers, and video cameras. Among them, a lithium-containing composite oxide is used as a positive electrode active material, a carbon material capable of occluding and releasing lithium ions, a silicon compound, a tin compound, or the like is used as a negative electrode material. A separator interposed between the positive electrode and the negative electrode is made of polyethylene or A lithium ion secondary battery using an aprotic organic solvent in which a lithium salt such as LiBF ₄ or LiPF ₆ is dissolved using a microporous membrane made of polypropylene or the like can obtain a high energy density at a high voltage. Widely used.

  In particular, in recent years, there has been an increasing demand for large-sized high-power lithium-ion secondary batteries for use in automobiles and small and lightweight high-power lithium-ion secondary batteries capable of rapid charging and large-current discharge for power tools. Development is actively underway. In order to maintain high capacity and cycle characteristics under severe conditions such as rapid charging and large current discharge, it is necessary that the ion conductivity of lithium ions in the battery is high. Since the original role of the separator in the battery is to separate the positive electrode and the negative electrode, the insulating polyolefin microporous film is used as described above, and it is said that the lithium ion conductivity is good. I want. In order to reduce the resistance of the separator as much as possible, the separator has been made thinner and higher in porosity. However, there is a limit to this approach from the viewpoint of the original role and safety of separating the positive electrode and the negative electrode.

Therefore, in order to provide high lithium ion conductivity and to reduce the resistance of the separator, the separator contains an inorganic compound having dielectric properties (Patent Document 1) or a lithium ion conductive glass ceramic powder. The use of a glass-ceramic composite electrolyte obtained by impregnating an electrolytic solution in a contained medium as a separator has been proposed (Patent Document 2).
JP 2001-283811 A Japanese Patent Laid-Open No. 2001-15160

  In the method of incorporating a dielectric inorganic compound described in Patent Document 1 into a separator, lithium ion conductivity can be improved by promoting dissociation of lithium salts existing in the pores of the separator or in the vicinity of the separator. It has been proposed. However, since the lithium salt concentration of the electrolytic solution itself is also limited, the separator cannot have sufficient lithium ion conductivity sufficient to follow particularly high rate charge / discharge. However, there is a problem that the battery cannot be obtained.

  In addition, a glass ceramic composite electrolyte obtained by impregnating an electrolyte in a medium containing lithium ion conductive glass ceramic powder described in Patent Document 2 is a lithium ion conductive composite in which the medium serves as a binder. Even if this solid electrolyte is used as a separator, the tensile strength and the tensile elongation are weaker than those of conventional polyolefin-based separators. Therefore, in a laminated battery in which a positive electrode, a negative electrode, and a separator are stacked and adhered, insulation between the positive electrode and the negative electrode can be secured. However, in a wound battery in which the positive electrode, the negative electrode, and the separator are wound, the positive electrode, The insulation between the negative electrodes cannot be sufficiently secured, and the insulation failure rate becomes high. In addition, as described above, the tensile strength and tensile elongation are weak compared to conventional polyolefin-based separators, and in particular, the expansion and contraction of the electrode active material due to repeated charge and discharge cannot be followed, and the battery cycle characteristics deteriorate. is there.

  In order to solve the above-described problems, the lithium ion secondary battery separator and the lithium ion secondary battery of the present invention are characterized in that a lithium ion conductive material is dispersed in the separator. Thus, as a lithium ion permeation path in the separator in the lithium ion secondary battery, (1) the electrolyte solution in the pores of the separator, (2) in the lithium ion conductive material, (3) the lithium ion conductive material and electrolysis The three paths of the interface with the liquid can be considered, and it is considered that the lithium ion conductivity is improved and the resistance of the separator is reduced as compared with the conventional separator including only the path (1) without including the lithium ion conductive material. As a result, the lithium ion conductivity in the separator can follow the severe usage conditions of high rate charge and discharge for high output applications, and thus a lithium ion secondary battery with high capacity and good cycle characteristics can be obtained. It is done. In addition, lithium ion conductive material is uniformly dispersed in 0.5% to 10% by weight of the total weight of the separator, so that lithium ion can be maintained while maintaining the tensile strength and tensile elongation of the separator main material such as polyolefin. Conductivity and mechanical strength can be improved. For this reason, in the process of winding the positive electrode, the negative electrode, and the separator, the separator is not broken, and it can be easily manufactured, and the productivity is improved. Furthermore, even if high rate charge / discharge is repeated, a lithium ion secondary battery having good cycle characteristics with little deterioration of the separator can be obtained.

  According to the present invention, the lithium ion conductivity in the separator and the separator machine are dispersed by dispersing 0.5% to 10% by weight of lithium ion conductive material particles in the separator with respect to the total weight of the separator. Therefore, a lithium ion secondary battery having a high capacity and good cycle characteristics can be obtained even during high rate charge / discharge.

  The separator used in the present invention will be described.

  The main material of the separator is not particularly limited as long as it is a substance that can be used in a conventional lithium ion secondary battery. However, it is common to use a single or composite polyolefin resin such as polyethylene or polypropylene. However, it is preferable from the viewpoint of workability. The film thickness is preferably 9 μm to 25 μm. When the film thickness exceeds 25 μm, the volume of the separator in the battery increases, so that the volume of the active material decreases, which is disadvantageous in terms of battery capacity. On the other hand, if the film thickness is smaller than 9 μm, the mechanical strength of the separator is lowered, which is disadvantageous in terms of battery assembly and safety.

  Examples of the lithium ion conductive material include lithium ion conductive glass and lithium ion conductive crystal, which may be used, but lithium ion conductive glass is slightly superior in lithium ion conductivity.

Examples of the lithium ion conductive glass include Li ₃ PO ₄ —Li ₂ S—SiS ₂ , Li ₂ S—P ₂ S ₅ , LiPON, and the like. Can do.

Examples of the lithium ion conductive crystal include La _0.55 Li _0.33 TiO ₃ and Li _{4 + x} Si _1-x Al _x O ₄ (0 ≦ x ≦ 1), but are not limited thereto and are well known. Can be used.

  These lithium ion conductive materials can be obtained by mixing raw materials at a predetermined ratio, and then pulverizing and classifying them. The average particle size of the lithium ion conductive material dispersed in the separator is preferably 20 nm to 1000 nm. . By making the particles of the lithium ion conductive material fine particles having an average particle size up to 1000 nm, the contact area between the lithium ion conductive material and the electrolytic solution is increased, and the lithium ion conductivity in the separator is further improved. It is thought that you can.

  The separator in which particles of the lithium ion conductive material of the present invention are dispersed can be produced in the same manner as the conventional battery separator except that the lithium ion conductive material is first added to the conventional separator material which is the main material. For example, (1) an extraction method in which an additive that can be easily extracted in a subsequent process is added to a polymer material and a lithium ion conductive material, and the additive is removed with an appropriate solvent. (2) the polymer material and lithium ion Examples thereof include a stretching method in which a conductive material is mixed to form a crystalline polymer material, and then amorphous portions are selectively stretched to form micropores.

  The content of the lithium ion conductive material is 0.5% by weight to 10% by weight with respect to the total weight of the separator, thereby improving the lithium ion conductivity in the separator and at the same time, Since the tensile strength and tensile elongation of the battery are not impaired, workability when assembling the battery can be maintained. When the content of the lithium ion conductive material is less than 0.5% by weight, the lithium ion conductivity is not improved by the addition of the lithium ion conductive material. I can't get it. On the other hand, if the content of the lithium ion conductive material exceeds 10% by weight, the separator may be easily broken during battery assembly, and it may be difficult to form the separator itself.

  As the positive electrode, the negative electrode, and the electrolytic solution, those conventionally used in lithium ion secondary batteries can be used without any particular limitation.

  Conventionally known lithium-containing composite oxides can be used as the positive electrode active material. For example, it is represented by the general formula LiMxOy (where 1 <x ≦ 2, 2 <y ≦ 4, M = Co, at least one selected from the group consisting of Ni, Mn, Fe, Al, V and Ti). Lithium-containing composite oxides and materials obtained by subjecting them to surface treatment are used.

  The positive electrode can be manufactured by a known method. For example, it can be produced by applying a positive electrode mixture paste prepared by mixing a binder, a conductive agent, and a solvent to this positive electrode active material on both sides of the current collector and rolling it after drying. As the conductive agent, natural graphite, artificial graphite, acetylene black, or the like can be used. As the binder, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose, or the like can be used. As the solvent, N-methylpyrrolidone, tetrahydrofuran, dimethylformamide and the like can be used. A metal such as aluminum or stainless steel is used for the current collector, but aluminum is preferred.

  Examples of the negative electrode active material include lithium metals, lithium alloys, and other lithium ions such as intermetallic compounds, carbon materials, organic compounds, inorganic compounds, metal complexes, and organic polymer compounds that can occlude and release lithium ions. The material which can hold | maintain is used. These may be used alone or in combination. Among these, a carbon material is particularly preferable.

The average particle size of the carbon material is preferably 0.1 μm to 60 μm, particularly preferably 0.5 μm to 30 μm. The specific surface area of the carbon material is preferably 1m ² / g~10m ² / g. Among them, graphite having a carbon hexagonal plane interval (d ₀₀₂ ) of 3.35 to 3.40 and a crystallite size (L _c ) in the c-axis direction of 100 mm or more is preferable.

  The negative electrode can be produced by a known method. For example, the negative electrode active material and a binder containing a binder can be applied to both surfaces of the negative electrode current collector, dried, and then rolled. As the binder, known materials such as polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, and styrene butadiene rubber are used. A known material is used for the current collector, but copper is preferred.

As the electrolytic solution, a non-aqueous solvent and a lithium salt dissolved therein are preferably used. Non-aqueous solvents are preferably cyclic carbonates such as ethylene carbonate and propylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, and cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone. Used. As the lithium salt, LiPF ₆ , LiBF _{4 and the} like are preferable, and these can be used alone or in combination.

  The form of the battery is not particularly limited, and may be any of a cylindrical shape, a flat shape, and a square shape. It is preferable that the battery is provided with, for example, an internal pressure relief type safety valve device or a current cutoff type safety valve device so that safety can be ensured even in the case of malfunction.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
(1) Fabrication of positive electrode The positive electrode is a mixture of Li ₂ CO ₃ , Co ₃ O ₄ , NiO, and MnO _2, and calcined at 900 ° C. for 10 hours. Li _0.94 Ni _0.35 Mn _0.35 Co _0.35 O ₂ powder 100 parts by weight Acetylene black (2.5 parts by weight) and a fluororesin-based binder (4 parts by weight) were mixed and suspended in a carboxymethylcellulose aqueous solution to obtain a paste. This paste is applied to both sides of an aluminum foil having a thickness of 0.03 mm, dried and rolled so that the porosity of the mixture portion is 25%, and the theoretical capacity per unit area is 52 mA with a width of 3.7 mAh. / cm ² and comprising electrode plate to obtain a positive electrode plate having a thickness of 99 .mu.m.

(2) Production of Negative Electrode The negative electrode is made of 100 parts by weight of mesophase graphite obtained by graphitizing mesophase spherules at 2800 ° C., a 1% by weight styrene / butadiene rubber emulsion as a solid content, and a 1% by weight carboxymethylcellulose aqueous solution as a solid content. The mixture was stirred to make a paste. This paste is applied to both sides of a 0.02 mm thick copper foil, dried and rolled so that the porosity of the mixture part becomes 35%, a negative electrode plate having a width of 57 mm and a load capacity of 250 mAh / g It was. Here, the load capacity (mAh / g) was a value obtained by dividing the capacity per unit area (mAh / cm ² ) of the positive electrode by the amount of active material (g / cm ² ) per unit area of the negative electrode.

(3) Production of Separator A separator composed of polyethylene as a main material and Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of a lithium ion conductive material of 100 nm was used. Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 100 nm was mechanically mixed with Li ₃ PO ₄ 2 wt%, Li ₂ S 49 wt%, and SiS ₂ 49 wt% using a planetary ball mill, and then pulverized. Obtained by classification.

A mixture of 47.5% by weight of high-density polyethylene having a weight average molecular weight of 800,000, 2.5% by weight of Li ₃ PO ₄ -Li ₂ S-SiS ₂ and 50% by weight of mineral oil is kneaded, heated and melted to produce a biaxial The film was formed into a film with an extruder. Next, this film was stretched in the width direction and the length direction by a tenter stretching machine heated to 145 ° C. and heat-treated in an atmosphere of 148 ° C. for 10 seconds. Thereafter, the membrane was immersed in a trichlorethylene solvent to extract and remove the mineral oil in the membrane, and the attached solvent was removed by drying. Further, the film was stretched in the width direction by a tenter stretching machine heated to 110 ° C. to prepare a separator having a film thickness of 20 μm, high-density polyethylene 95% by weight, and Li ₃ PO ₄ —Li ₂ S—SiS ₂ 5% by weight.

(4) Assembly of battery A cylindrical lithium ion secondary battery (diameter 26 mm, height 65 mm) was assembled using a predetermined positive electrode and the negative electrode. FIG. 1 shows a longitudinal sectional view of a cylindrical lithium ion battery produced in this example. The battery was assembled as follows.

  First, after attaching the aluminum positive electrode lead 10 to the positive electrode current collector 7 of the predetermined positive electrode 3 and the nickel negative electrode lead 9 to the negative electrode current collector 6 of the predetermined negative electrode 2, winding it through the predetermined separator 4, A wound electrode group was constructed. A polypropylene insulating plate 8 is arranged at the bottom of the electrode group, and the negative electrode lead 9 is welded to the bottom of a nickel-plated iron battery can 5 and the positive electrode lead 10 is connected to the battery lid 11 via an internal pressure-operated safety valve device 13. Welded to. Thereafter, a non-aqueous electrolyte was injected into the battery can 5 by a reduced pressure method. Finally, the cylindrical lithium ion secondary battery 1 of the present invention having a capacity of 2.6 Ah was completed by caulking the open end of the battery can 5 with an insulating sealing plate gasket 12.

As the non-aqueous electrolyte, a solution in which LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 15:15:70 to a concentration of 1.40 mol / l was used.

(Example 2)
For the separator, Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 100 nm as in Example 1 was used, 49.75% by weight of high-density polyethylene having a weight average molecular weight of 800,000, and Li ₃ A separator was prepared in the same manner as in Example 1 except that a mixture of 0.25% by weight of PO ₄ -Li ₂ S—SiS ₂ and 50% by weight of mineral oil was used, and a film thickness of 20 μm and high density polyethylene of 99.5% were used. %, Li ₃ PO ₄ —Li ₂ S—SiS ₂ 0.5 wt% separator was obtained. A lithium ion secondary battery was produced in the same manner as in Example 1 except that this separator was used.

(Example 3)
For the separator, Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 100 nm as in Example 1 was used, and 45% by weight of high-density polyethylene having a weight average molecular weight of 800,000, and Li ₃ PO ₄ A separator was prepared in the same manner as in Example 1 except that a mixture of 5% by weight of -Li ₂ S-SiS ₂ and 50% by weight of mineral oil was used, and a film thickness of 20 μm, high-density polyethylene 90% by weight, Li ₃ PO ₄ A separator composed of 10% by weight of —Li ₂ S—SiS ₂ was obtained. A lithium ion secondary battery was produced in the same manner as in Example 1 except that this separator was used.

Example 4
For the separator, using Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 10 nm instead of Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 100 nm, a film thickness of 20 μm, A lithium ion secondary battery was produced in the same manner as in Example 1 except that a separator composed of 95% by weight of high density polyethylene and 5% by weight of Li ₃ PO ₄ —Li ₂ S—SiS ₂ was produced.

(Example 5)
For the separator, Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 20 nm was used instead of Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 100 nm, and the film thickness was 20 μm, A lithium ion secondary battery was produced in the same manner as in Example 1 except that a separator composed of 95% by weight of high density polyethylene and 5% by weight of Li ₃ PO ₄ —Li ₂ S—SiS ₂ was produced.

(Example 6)
For the separator, using Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 500 nm instead of Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 100 nm, a film thickness of 20 μm, A lithium ion secondary battery was produced in the same manner as in Example 1 except that a separator composed of 95% by weight of high density polyethylene and 5% by weight of Li ₃ PO ₄ —Li ₂ S—SiS ₂ was produced.

(Example 7)
With respect to the separator, Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 1000 nm was used instead of Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 100 nm, and the film thickness was 20 μm. A lithium ion secondary battery was produced in the same manner as in Example 1 except that a separator composed of 95% by weight of high density polyethylene and 5% by weight of Li ₃ PO ₄ —Li ₂ S—SiS ₂ was produced.

(Example 8)
For the separator, using Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 2000 nm instead of Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 100 nm, a film thickness of 20 μm, A lithium ion secondary battery was produced in the same manner as in Example 1 except that a separator composed of 95% by weight of high density polyethylene and 5% by weight of Li ₃ PO ₄ —Li ₂ S—SiS ₂ was produced.

Example 9
As a separator, it was used having an average particle diameter of the polyethylene and the lithium ion conductive material of the main material is composed of Li ₂ S-P ₂ S ₅ is 100 nm.

Li ₂ S—P ₂ S ₅ having an average particle size of 100 nm is obtained by mechanically mixing Li ₂ S and P ₂ S ₅ at a mass ratio of 6: 4 by a planetary ball mill, then pulverizing and classifying. Obtained.

A separator was prepared in the same manner as in Example 1 except that a mixture of 46% by weight of high-density polyethylene having a weight average molecular weight of 800,000, 4% by weight of Li ₂ S—P ₂ S ₅ and 50% by weight of mineral oil was used. A separator made of 20 μm thick, 92% by weight of high-density polyethylene, and 8% by weight of Li ₂ S—P ₂ S ₅ was obtained. Further, a lithium ion secondary battery was produced in the same manner as in Example 1 except that this separator was used.

(Example 10)
As the separator, a separator made of La _0.55 Li _0.33 TiO ₃ having an average particle size of 100 nm of polyethylene and a lithium ion conductive material was used.

La _0.55 Li _0.33 TiO ₃ having an average particle diameter of 100 nm is prepared by mixing La ₂ O ₃ , Li ₂ CO ₃ , and TiO ₂ at a mass ratio of 11: 40: 7, respectively, at 800 ° C. and 1150 ° C., respectively. It was obtained by pulverizing and classifying after time baking.

A separator was prepared in the same manner as in Example 1 except that 46% by weight of high-density polyethylene having a weight average molecular weight of 800,000, 4% by weight of La _0.55 Li _0.33 TiO ₃ and 50% by weight of mineral oil were used. A separator having a thickness of 20 μm, high-density polyethylene 92% by weight, and La _0.55 Li _0.33 TiO ₃ 8% by weight was obtained. Further, a lithium ion secondary battery was produced in the same manner as in Example 1 except that this separator was used.

(Comparative Example 1)
For the separator, Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 100 nm as in Example 1 was used, 49.9% by weight of high density polyethylene having a weight average molecular weight of 800,000, and Li ₃ A separator was prepared in the same manner as in Example 1 except that a mixture of 0.1% by weight of PO ₄ -Li ₂ S—SiS ₂ and 50% by weight of mineral oil was used, and a film thickness of 20 μm and high density polyethylene of 99.8% were used. %, Li ₃ PO ₄ —Li ₂ S—SiS ₂ 0.2 wt% separator was obtained. A lithium ion secondary battery was produced in the same manner as in Example 1 except that this separator was used.

(Comparative Example 2)
For the separator, Li ₃ PO ₄ —Li ₂ S—SiS ₂ having an average particle diameter of 100 nm as in Example 1 was used, and 40% by weight of high-density polyethylene having a weight average molecular weight of 800,000, and Li ₃ PO ₄ A separator was prepared in the same manner as in Example 1 except that a mixture of 10% by weight of Li ₂ S—SiS ₂ and 50% by weight of mineral oil was used, and a film thickness of 20 μm, high-density polyethylene 80% by weight, Li ₃ PO ₄ A separator composed of 20% by weight of —Li ₂ S—SiS ₂ was obtained. A lithium ion secondary battery was produced in the same manner as in Example 1 except that this separator was used.

(Comparative Example 3)
A polyethylene microporous membrane separator was prepared in the same manner as in Example 1 except that no lithium ion conductive material was added to the separator, and a lithium ion secondary battery was prepared using this separator.

  About the separator for lithium ion secondary batteries and the lithium ion secondary battery of Examples 1 to 10 and Comparative Examples 1 to 3, evaluation of separator electrical resistance, initial battery capacity, and cycle characteristics was performed by the following methods. Went.

(Evaluation of separator electrical resistance)
The separator of this example and the comparative example was immersed in an electrolyte solution of 1.25 mol / l LiPF ₆ , ethylene carbonate: ethyl methyl carbonate = 1: 3 (volume ratio) and sandwiched between SUS electrodes, voltage amplitude 10 mV, frequency 1 kHz. The AC impedance was measured under a condition of ˜100 kHz, and the value obtained from the Cole-Cole plot was evaluated as the electrical resistance of the separator.

(Evaluation of initial battery capacity)
The manufactured lithium ion secondary battery reached a charging voltage of 4.2 V by constant current charging at 10 A at an environmental temperature of 20 ° C., and then charged at a constant voltage of 4.26 A at a constant voltage of 4.2 V. After a rest of minutes, a constant current discharge at a discharge end voltage of 2.0 V was performed at a discharge current of 20 A to obtain an initial battery capacity. At that time, the ratio of the battery capacity in each example and the comparative example to the battery capacity in Comparative Example 3 was obtained at 100 minutes.

(Evaluation of capacity maintenance rate after 500 cycles)
The produced lithium ion secondary battery reached 4.2V by constant current charging at 10 A at an environmental temperature of 20 ° C., and then charged at constant voltage of 4.26 A under constant constant 4.2 V, and charged for 20 minutes. After the rest, a constant current discharge with a discharge end voltage of 2.0 V was performed with a discharge current of 20 A to obtain an initial battery capacity. Then, after a 20-minute pause, the same charge / discharge cycle was repeated, and the battery capacity at the 500th cycle was defined as the battery capacity after 500 cycles of this battery. The ratio of the obtained battery capacity after 500 cycles with respect to the initial capacity was determined in terms of 100 minutes.

  Table 1 shows the evaluation results of the electrical resistance of the separator, the initial battery capacity, and the capacity retention rate after 500 cycles for each example and comparative example.

  According to Table 1, when 0.5% to 10% by weight of lithium ion conductive material particles are dispersed in the separator, the electrical resistance of the separator is lowered, and high rate charge / discharge is achieved in a lithium ion secondary battery using the separator. It was found that good results were obtained with the initial battery capacity and cycle characteristics even under such severe conditions. As described above, this is considered to be due to the improvement of the lithium ion conductivity and mechanical strength of the separator.

  Further, by comparing Example 1, Example 4 and Example 5, the separator and battery characteristics did not change even when the average particle size of the lithium ion conductive material was reduced to 10 nm. From the viewpoint of making it easier, it can be said that it is sufficient to grind the average particle size to 20 nm, further to 100 nm.

  In Comparative Example 1, since the content of the lithium ion conductive material is small, the lithium ion conductivity is not improved, and a battery using a conventional separator not containing the lithium ion conductive material (Comparative Example 3) There was no change in battery characteristics.

  Further, in Comparative Example 2, since the content of the lithium ion conductive material is large, the lithium ion conductivity is improved and the initial battery capacity is improved, but the cycle characteristics are not improved, and as described above. It is considered that the separator is greatly deteriorated when high rate charge / discharge is repeated.

  Similar effects of the present invention were also obtained in a separator in which particles of a lithium ion conductive material other than those shown in Examples were dispersed and a lithium ion secondary battery using the separator.

  According to the present invention, a separator that improves lithium ion conductivity and mechanical strength, and a lithium ion secondary battery that has a high capacity and good cycle characteristics even in high output applications can be obtained.

Longitudinal section of cylindrical lithium ion secondary battery

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 2 Negative electrode 3 Positive electrode 4 Separator 5 Battery can 6 Negative electrode collector 7 Positive electrode collector 8 Insulating plate 9 Negative electrode lead 10 Positive electrode lead 11 Battery cover 12 Insulation sealing plate gasket 13 Safety valve device

Claims (3)

A separator for a lithium ion secondary battery comprising a positive electrode made of a lithium-containing composite oxide, a negative electrode made of a material capable of holding lithium, a separator, and a non-aqueous electrolyte. On the other hand, a lithium ion secondary battery separator in which 0.5 to 10% by weight of a lithium ion conductive material is dispersed. 2. The separator for a lithium ion secondary battery according to claim 1, wherein an average particle diameter of the lithium ion conductive material is 20 nm to 1000 nm. A lithium ion secondary battery comprising a positive electrode made of a lithium-containing composite oxide, a negative electrode made of a material capable of holding lithium, a separator, and a nonaqueous electrolyte, wherein the separator is claimed in claim 1 or claim 2. A lithium ion secondary battery using the separator described in 1.

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