JP2007317675A - Separator for lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator for a lithium ion secondary battery high in safety when overcharged as compared with a conventional nonaqueous secondary battery to the extent allowing omission of a protective circuit and simplification for a thermistor system such as a thermo-sensitive switch and a thermo-sensitive sensor after keeping a practical battery characteristic, and capable of providing a low-cost nonaqueous secondary battery. <P>SOLUTION: This separator for a lithium ion secondary battery is formed of a sheet A having an average thickness of 10-35 μm, a basis weight of 6-20 g/m<SP>2</SP>, a Macmillan number not greater than 10, and (Macmillan number)×(average thickness) not greater than 200 μm, or a porous film including the sheet A, and including a porous organic polymer film B holding it by swelling it in an electrolytic solution, and having an average thickness of 10-35 μm, and a basis weight of 10-25 g/m<SP>2</SP>. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery with high safety and low cost during overcharge, a separator used therefor, a battery pack and an electric / electronic device including such a lithium ion secondary battery, and such a lithium ion. The present invention relates to a method for charging a secondary battery.

  With the recent spread of portable electronic devices and higher performance, secondary batteries having high energy density are desired. From such a background, a lithium ion secondary battery using a carbon material capable of electrochemically doping and dedoping lithium as a negative electrode active material and using a lithium-containing transition metal oxide as a positive electrode active material has been rapidly developed. It is becoming popular.

  This lithium ion secondary battery is charged and discharged by moving lithium ions between the positive and negative electrodes, and occludes and releases electrical energy. This lithium ion secondary battery has a high energy density because an average voltage of about 3.7 V, which is about three times that of a conventional secondary battery, can be obtained. However, an aqueous electrolyte like a conventional secondary battery can be obtained. Therefore, a non-aqueous electrolyte solution having sufficient oxidation-reduction resistance is used. For these reasons, lithium ion secondary batteries are also called non-aqueous secondary batteries.

  Since non-aqueous secondary batteries use flammable non-aqueous electrolytes as electrolytes, there is a risk of ignition and the like, and careful attention is paid to safety in their use. There are several possible cases of exposure to fire and other hazards, but overcharging is particularly dangerous.

  In order to prevent overcharging, current non-aqueous secondary batteries are charged at a constant voltage and a constant current, and the battery is equipped with a precise IC (protection circuit). The cost required for this protection circuit is large, and it is a factor that increases the cost of non-aqueous secondary batteries.

  When overcharging is prevented by the protection circuit, it is naturally assumed that the protection circuit does not operate well, and it is difficult to say that it is intrinsically safe. The current non-aqueous secondary battery has a protection circuit that breaks when overcharged, and is equipped with a safety valve / PTC element and a separator with a thermal fuse function to safely destroy the battery when overcharged. Has been made. However, even if equipped with the above-mentioned means, depending on the overcharge conditions, the safety during overcharge is not guaranteed, and in fact, non-aqueous secondary battery ignition accidents Is still happening.

  Thus, safety measures against overcharge of non-aqueous secondary batteries are not yet sufficient in terms of safety and cost, and are issues that need to be improved, with the aim of improving such issues. Various methods have been proposed.

  As one of such improvement methods, there is a method aiming at destroying the battery more safely when the protection circuit does not operate. For example, as proposed in Japanese Patent No. 2928779, Japanese Patent No. 3061759, Japanese Patent No. 3113652, Japanese Patent Application Laid-Open No. 2000-306610, etc., a method for quickly operating a safety valve by adding a compound that easily generates gas during overcharge As proposed in Japanese Patent No. 3061756 and the like, a method of adding a compound that polymerizes during overcharge and cutting off the current, as proposed in Japanese Patent Application Laid-Open No. 11-45740, etc. There is a method of adding a compound having an endothermic effect and the like, and a method using such an additive is partly put into practical use, and the safety of the non-aqueous secondary battery is improved.

  There is also a method aimed at ensuring safety while reducing costs by removing the protection circuit or simplifying it like a thermistor system. For example, as proposed in JP-A-6-338347, JP-A-2000-251932, JP-A-2000-277147, JP-A-2000-228215, etc., there is a method using a redox shuttle additive. . The redox shuttle additive causes an oxidation-reduction reaction between the positive and negative electrodes during overcharge, and prevents overcharge by a mechanism that consumes overcharge current. Such additives have been put into practical use and have contributed to improving the safety of non-aqueous secondary batteries, but have not yet reached the point where the protective circuit is removed or simplified.

  Japanese Patent Application Laid-Open No. 2000-67917 proposes a technique related to overcharge prevention using a gel polymer electrolyte membrane, and suggests the possibility of eliminating or simplifying the protection circuit. However, in this technique, the gel polymer electrolyte membrane needs to have a minimum film thickness of 30 μm or more, and in order to obtain a sufficient effect, it needs to be 40 μm or more. . However, considering that the separator thickness of the current general non-aqueous secondary battery is 25 μm and this separator thickness tends to be thinned with the aim of further increasing the energy density of the battery, this technology is versatile. Not something.

  Japanese Laid-Open Patent Publication No. 2000-123824 also proposes an overcharge prevention technique that uses a gel polymer electrolyte and suggests the possibility of eliminating or simplifying the protection circuit. In this technique, overcharge is prevented by using an oligomer of polyether, but the discharge characteristics of the battery are extremely poor compared to conventional non-aqueous secondary batteries, so it is difficult to say that it is general-purpose.

Japanese Patent No. 2928779 Japanese Patent 3061759 Japanese Patent No. 3113652 JP 2000-306610 A Japanese Patent No. 3061756 Japanese Patent Laid-Open No. 11-45740 JP-A-6-338347 JP 2000-251932 A JP 2000-277147 A JP 2000-228215 A JP 2000-67917 A JP 2000-123824 A

  The present invention solves the problems of the prior art as described above, and while maintaining practical battery characteristics, it is possible to eliminate the protection circuit or simplify thermistor system such as a thermal switch and a thermal sensor. Another object of the present invention is to provide a low-cost non-aqueous secondary battery that has higher safety during overcharge than conventional non-aqueous secondary batteries.

In order to solve the above problems, the present invention is a lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte,
1) The separator consists essentially of a porous sheet,
2) The positive electrode active material and the negative electrode active material can be reversibly doped and dedoped with lithium. The total amount of lithium contained in the positive electrode is Qp (mAh), and the amount of lithium that the negative electrode can be doped is Qn (mAh). ) And Qp> Qn,
3) When charged with a charging current Ic (mA) of 0.2Qn <Ic <2Qn, the charged electricity amount Qc (mAh) is within the range of 1 <Qc / Qn <Qp / Qn, and is charged on the negative electrode by charging the battery. Doping of the positive electrode of lithium is initiated through the resulting lithium species and continues until Qc> Qp.
Provide batteries.

  The present invention also provides a lithium ion secondary battery pack comprising the above-described lithium ion secondary battery and a thermal sensor or a thermal switch.

  The present invention further relates to a method for charging the above-described lithium ion secondary battery, wherein the lithium ion secondary battery is charged by a constant current charging method, and the end of the charging is determined as battery temperature rise, battery voltage drop and battery voltage. A method is provided that includes determining using at least one of the vibrations.

  The present invention also provides an electric / electronic device including the above-described lithium ion secondary battery or lithium ion secondary battery pack.

The present invention also provides a sheet having an average film thickness of 10 to 35 μm, a basis weight of 6 to 20 g / m ² , an air permeability (JIS P8117) of 100 seconds or less, a Macmillan number of 10 or less, and a Macmillan number × average film thickness of 200 μm or less. A separator for a lithium ion secondary battery.

The present invention further includes a porous organic polymer film (B) that encloses the above-described sheet (A) and swells and holds the electrolyte solution, and has an average film thickness of 10 to 35 μm and a basis weight of 10 to 25 g / A separator for a lithium ion secondary battery comprising an m ² porous film is provided.

The present invention will be described in detail below.
The lithium ion secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte,
1) The separator consists essentially of a porous sheet,
2) The positive electrode active material and the negative electrode active material can be reversibly doped and dedoped with lithium. The total amount of lithium contained in the positive electrode is Qp (mAh), and the amount of lithium that the negative electrode can be doped is Qn (mAh). ) And Qp> Qn,
3) When charged with a charging current Ic (mA) of 0.2Qn <Ic <2Qn, the charged electricity amount Qc (mAh) is within the range of 1 <Qc / Qn <Qp / Qn, and is charged on the negative electrode by charging the battery. Doping of the positive electrode of lithium is initiated through the resulting lithium species and continues until Qc> Qp.
A battery having the following characteristics.

[Overcharge prevention function]
In the lithium ion secondary battery of the present invention, when charged with a practical charging current Ic such as 0.2Qn <Ic <2Qn, the lithium species deposited without being doped on the negative electrode during overcharging is located near the positive electrode surface. And then this is doped into the positive electrode and this phenomenon is continued until Qc> Qp, thereby preventing the battery from being overcharged. The inventors of the present invention have proposed a shallow overcharge in which the amount of charged electricity Qc is 1 <Qc / Qn <Qp / Qn, where Qn is the amount of lithium that can be doped into the negative electrode and Qp is the amount of lithium contained in the positive electrode. By allowing lithium species deposited on the negative electrode surface to reach the vicinity of the positive electrode interface by the depth, not only can the safety during overcharging of the lithium ion secondary battery be ensured, but in this case, the lithium species that has reached the positive electrode interface. Has been found to be capable of discharging even after overcharging without being completely short-circuited between the positive and negative electrodes because it is doped in the positive electrode.

  The above overcharge prevention function prevents overcharge by doping the positive electrode of lithium species generated on the negative electrode during overcharge, and this phenomenon continues until Qc> Qp, thereby ensuring the safety of the battery during overcharge. However, in order to ensure a higher level of battery safety, it is preferable to continue the above phenomenon up to Qc / 3Qn.

  A lithium ion secondary battery in which this phenomenon occurs when Qc / Qn <1 is a battery that cannot be fully charged and is not preferable. Further, when this phenomenon does not occur when Qc / Qn <Qp / Qn, this phenomenon cannot occur when Qc / Qn> Qp / Qn because of the amount of lithium.

  Naturally, in order to obtain a lithium ion secondary battery having such an overcharge prevention function, the total lithium amount Qp contained in the positive electrode and the lithium amount Qn that can be doped by the negative electrode are Qp> Qn. There must be. The overcharge prevention function of the battery of the present invention is obtained by utilizing the lithium species deposited on the negative electrode surface during overcharge. This lithium species was originally contained in the positive electrode and used at this time. Since the amount of lithium that can be used is Qp-Qn, when Qp <Qn, it is impossible in principle to develop the overcharge prevention function of the present invention.

  Considering the higher level of safety of the lithium ion secondary battery of the present invention, this overcharge prevention is performed using half of the amount of lithium remaining in the positive electrode (Qp-Qn) when the negative electrode is completely doped with lithium. It is more preferable that the function is expressed. That is, it is more preferable that this overcharge prevention function is manifested at a shallower charging depth such that the amount of charge Qc is 1 <Qc / Qn <0.5 (Qp / Qn + 1).

  Here, the total amount of lithium Qp contained in the positive electrode and the amount of lithium Qn that can be doped by the negative electrode can be calculated from the weight of the positive electrode active material and the negative electrode active material contained in the unit area. It can also be obtained by measuring charge / discharge by assembling a three-electrode cell using a positive electrode or a negative electrode for the working electrode and lithium metal for the reference electrode and the counter electrode.

  In order to confirm such an overcharge prevention function in the battery of the present invention, there are the following methods. That is, the positive electrode and the negative electrode are taken out from the lithium ion secondary battery of the present invention, and Qp and Qn are measured by the above method to confirm that the relationship of Qp> Qn is satisfied. In addition, the positive electrode, the negative electrode, and the separator are taken out from the lithium ion secondary battery of the present invention, the positive electrode and the negative electrode are joined through the separator, and an electrolyte (electrolytic solution) is injected to produce the evaluation cell 1. When this evaluation cell 1 is charged with a charging current Ic that satisfies the condition of 0.2Qn <Ic <2Qn, the cell voltage drop, cell, and the like in the range of 1 <Qc / Qn <Qp / Qn, Observe that a voltage oscillation or a rough cessation of the cell voltage rise is observed. Let Q1 (mAh) be the amount of charge when this cell voltage drop, cell voltage oscillation, or a rough stop of the cell voltage rise starts. Further, it is confirmed that the cell voltage oscillation or the battery voltage increase is roughly stopped, and in some cases, the battery voltage decrease continues until Qc> Qp. Next, the separator and the positive electrode are taken out from the lithium ion secondary battery of the present invention, this positive electrode is joined to the negative electrode current collector (ie, Qn = 0) through this separator, and an electrolyte (electrolytic solution) is injected for evaluation. Cell 2 is fabricated. In such an evaluation cell 2, when the negative electrode current collector is regarded as a negative electrode and charged with a charging current Ic that satisfies 0.2Qn <Ic <2Qn, cell voltage drop, cell voltage oscillation, or cell voltage rise Let Q2 (mAh) be the amount of electricity when an approximate stop is started. Here, if there is a relationship of Q1> Q2 (ideally Q2 = Q1-Qn), the observed cell voltage drop, cell voltage oscillation, or rough stop of cell voltage rise is originally included in the positive electrode Therefore, the intended overcharge protection function can be confirmed.

  In the above test, if it is difficult to take out the negative electrode current collector, a copper foil or a lithium foil can be used instead of the negative electrode current collector. Moreover, as an electrolytic solution used in the above test, an electrolytic solution generally used in a lithium ion secondary battery can be used.

  In the lithium ion secondary battery of the present invention, when the overcharge prevention function is manifested, the electric energy injected by the charging operation is released out of the system as Joule heat. This Joule heat is expressed by charging current × battery voltage. For this reason, when a large charging current is used, the effect of Joule heat becomes dominant, and it becomes difficult to accurately evaluate the overcharge protection function. Considering that a practical range of 0.2Qn <Ic <2Qn is adopted as the charging current Ic, it is preferable that the capacity of the evaluation cell is small. The size of the evaluation cell is preferably about the size of a button type cell, but is not limited to this.

  The above-described overcharge prevention function can be explained with the most general lithium ion secondary battery system using a carbon material that can be doped and dedoped with lithium cobaltate on the positive electrode and lithium on the negative electrode. . This lithium ion secondary battery system is generally designed so that the amount of lithium (Qn) that can be doped into the negative electrode is about half of the amount of lithium (Qp) contained in the positive electrode. That is, 2Qn = Qp. In the conventional lithium ion secondary battery of this type, when it is charged with 1C (charging current Ic = Qn), the charging rate obtained by extracting all lithium from the positive electrode is 200% (charging). Rupture and ignition occur when the amount of electricity exceeds Qc = Qp).

  Based on the above, it is possible to prevent all lithium from being extracted from the positive electrode, that is, safety during overcharging can be secured by stopping the progress of charging, but overcharge prevention in the lithium ion secondary battery of the present invention The function is characterized by performing this via lithium species that deposit on the negative electrode during overcharge. That is, it is necessary to start the doping of the lithium species into the positive electrode when the charging rate is 200% or less (Qc <Qp), more preferably 150% (Qc <0.5 (Qp + Qn)) or less.

  Moreover, unless the doping of the lithium species deposited on the negative electrode to the positive electrode is continued, overcharging will not be prevented. That is, it is necessary to continue the charging rate of 200% or more (Qc> Qp), more preferably 300% or more (Qc> 3Qn).

  The lithium ion secondary battery of the present invention having an overcharge prevention function based on the mechanism as described above exhibits the following characteristics.

  1) When charging is performed with a charging current Ic that satisfies 0.2Qn <Ic <2Qn, the charge electricity quantity Qc at which the battery voltage drops, the battery voltage oscillates, or the battery voltage rise almost stops is 1 <Qc / Qn <Qp / Qn.

  Here, the drop in battery voltage occurs when the lithium species produced on the negative electrode are doped into the positive electrode. Moreover, the vibration of the battery voltage indicates that the doping of the lithium species occurs intermittently. The drop and vibration of the battery voltage depend on the internal resistance of the battery, and are difficult to observe when the internal resistance of the battery is small. In such a case, observation may be made by minimizing the voltage sampling time. In addition, even when the cycle rate of deposition of lithium species on the negative electrode and doping of the positive electrode is extremely high, the battery voltage drop and the battery voltage vibration are hardly observed. When this cycle is fast and the internal resistance of the battery is small, a phenomenon that the increase in the battery voltage almost stops is apparently observed.

  2) When charging is performed with a charging current Ic that satisfies 0.2Qn <Ic <2Qn, the battery voltage starts to oscillate at 10 mV or more in the range where the charge quantity Qc is 1 <Qc / Qn <Qp / Qn, and Qc > Continues until Qp.

  As described above, the oscillation of the battery voltage proves that the cycle of deposition of lithium species on the negative electrode and doping of the positive electrode is repeated, but it is sufficient that this continues until Qc> Qp. The overcharge prevention function is manifested.

  3) Charging is performed with a charging current Ic satisfying 0.2Qn <Ic <2Qn such that the charge electric quantity Qc becomes 2 <Qc / Qn <3, and discharging is performed with a discharging current Id of 0.1Qn <Id <0.5Qn. When carried out, the discharge electricity quantity Qd is in the range of 1 <Qd / Qn <Qp / Qn.

  Since the lithium ion secondary battery of the present invention does not cause a complete internal short circuit, it can be discharged thereafter. If this overcharge prevention mechanism is working, the amount of discharged electricity Qd is in the range of 1 <Qd / Qn <Qp / Qn in consideration of the lithium remaining on the negative electrode.

  4) When charged with a charging current Ic satisfying 0.2Qn <Ic <2Qn, the battery voltage is 5.5 V or less in the entire charge quantity Qc range of 1 <Qc / Qn <1.5Qp / Qn. .

  Since the lithium ion secondary battery of the present invention prevents the progress of charging by the mechanism as described above, the battery voltage does not rise to a voltage at which the electrolyte (electrolytic solution) is decomposed. From this, when this overcharge prevention function is working, as long as a positive electrode active material generally used at present is used, the voltage never exceeds 5.5V.

5) Battery internal resistance R _0.5 at 1 kHz when the amount of charged electricity Qc is Qc / Qn = 0.5, and battery internal resistance R ₂ at 1 kHz when Qc = Qp is 1.5R _0.5 > is _{R 2.}
When this overcharge prevention function is working, the electrolytic solution does not decompose as in normal overcharge, and the internal resistance of the battery does not increase significantly.

  6) The longest diameter of the lithium species generated on the negative electrode by charging the battery is 100 μm or less in the range where the charge electric quantity Qc is in the range of Qp <Qc <1.5Qp.

  This overcharge prevention function is manifested because the lithium species deposited on the negative electrode is made fine and rapidly reaches the vicinity of the positive electrode interface.

  The lithium ion secondary battery of the present invention has all the features 1) to 6) in principle, but if the charging current increases with the increase in battery capacity, it does not satisfy all the features due to the Joule heat problem. A case also comes out. In general, by satisfying one of the above features 1) to 6), more preferably two, it can be confirmed that the battery has an overcharge prevention function necessary for the lithium ion secondary battery of the present invention.

  Further, in the lithium ion secondary battery of the present invention, it is natural that the overcharge prevention function is considered, but abnormal decomposition of the electrolyte does not occur at the time of overcharge, and the crystal structure of the positive electrode may be destroyed. Absent. It can be confirmed by a method such as GC-MS that abnormal electrolyte decomposition does not occur. Moreover, it can be confirmed from the peak pattern of X-ray diffraction that the crystal structure of the positive electrode does not break.

[electrode]
For the positive electrode and negative electrode used in the lithium ion secondary battery of the present invention, an active material capable of reversibly doping and dedoping lithium is used, and the total lithium amount Qp contained in the positive electrode described above and the negative electrode are used. As long as Qp> Qn, which is the relationship of the amount of lithium Qn that can be doped, is satisfied, there is no particular limitation, and materials generally used for lithium ion secondary batteries can be used.

  The positive electrode and the negative electrode are generally composed of an active material, a binder polymer that binds the active material and holds an electrolytic solution, and a current collector. It is also possible to add a conductive aid for the purpose of improving the electrical conductivity of the electrode.

In the lithium ion secondary battery of the present invention, lithium-containing transition metal oxides such as LiCoO ₂ , LiMn ₂ O ₄ , and LiNiO ₂ are preferably used as the positive electrode active material. The negative electrode active material includes polyacrylonitrile, phenol resin, phenol novolac resin, sintered organic polymer compounds such as cellulose, coke, sintered pitch, carbon represented by artificial graphite and natural graphite. Materials are preferably used.

  As binder polymers, PVdF copolymer resins such as polyvinylidene fluoride (PVdF), PVdF and hexafluoropropylene (HFP), a copolymer of perfluoromethyl vinyl ether (PFMV) and tetrafluoroethylene, polytetrafluoroethylene, Fluorine resins such as fluoro rubber, hydrocarbon polymers such as styrene-butadiene copolymer and styrene-acrylonitrile copolymer, carboxymethyl cellulose, polyimide resin and the like can be used, but are not limited thereto. These may be used alone or in combination of two or more.

  As for the current collector, a material excellent in oxidation resistance is used for the positive electrode, and a material excellent in reduction resistance is used for the negative electrode. Specifically, examples of the positive electrode current collector include aluminum and stainless steel, and examples of the negative electrode current collector include copper, nickel, and stainless steel. Moreover, about a shape, a foil shape and a mesh shape can be used. In particular, an aluminum foil is suitably used as the positive electrode current collector, and a copper foil is suitably used as the negative electrode current collector.

  Carbon black (acetylene black) is preferably used as the conductive assistant, but is not limited thereto.

  The mixing ratio of the active material, the binder polymer, and the conductive assistant is preferably in the range of 3 to 30 parts by weight of the binder polymer with respect to 100 parts by weight of the active material, and preferably in the range of 0 to 10 parts by weight of the conductive assistant.

  The method for producing the electrode as described above is not particularly limited, and a known method can be adopted.

[Separator]
There are two types of separators preferably used in the lithium ion secondary battery of the present invention.

The first form has an average film thickness of 10 to 35 μm, basis weight of 6 to 20 g / m ² , and air permeability (JIS P8117: time required for 100 cc of air to pass through a 1 in ² area at a pressure of 2.3 cmHg) The sheet (A) has a Macmillan number of 10 or less and a Macmillan number × average film thickness of 200 μm or less for 100 seconds or less. Such a sheet (A) not only has a large gap, but also the ratio (curvature) between the shortest distance between the positive and negative electrodes measured through the through hole of the separator when the battery is assembled and the film thickness of the separator. Since there are many through holes close to 1, it is easy for lithium species generated on the negative electrode during overcharge to approach the vicinity of the positive electrode interface. For this reason, it is suitable for expressing the overcharge prevention function as described above.

Sheets with an average film thickness of less than 10 μm and a basis weight of less than 6 g / m ² tend to exhibit an overcharge prevention function, but they are insufficient in strength or frequently cause short-circuits. It is not preferable.

When considering short-circuit resistance, puncture strength can be an index. The puncture strength is preferably 3 g / μm or more and the effective puncture strength (puncture strength × film thickness) is preferably 80 g or more. Here, the puncture strength is set on a fixed frame of 11.3 mmφ, a needle having a tip radius of 0.5 mm is vertically projected at the center of the sheet, and the needle is pushed in at a constant speed of 50 mm / min. This is a value obtained by standardizing the force applied to the needle when a hole is formed in the sheet with the average film thickness.
In a sheet having an average film thickness of less than 10 μm and a basis weight of less than 6 g / m ² , it is difficult to obtain a sheet that satisfies such conditions as piercing strength.

A sheet having an average film thickness larger than 35 μm, a basis weight larger than 20 g / m ² , and an air permeability (JIS P8117) exceeding 100 seconds is not only disadvantageous with respect to the overcharge prevention function, but also has an internal resistance. This is also not preferable from the viewpoint of a decrease in battery characteristics and an energy density due to an increase in the density. Considering the battery characteristics, it is preferable that the sheet (A) has a Macmillan number of 10 or less and the Macmillan number × average film thickness is 200 μm or less. More preferably, the Macmillan number × average film thickness is 150 μm or less. Here, the Macmillan number is an index indicating the ionic conductivity of the battery separator, and is a ratio between the impedance when the sheet (A) is impregnated with the electrolytic solution and the impedance of the electrolytic solution alone. In this specification, the value measured at 25 ° C. is adopted as the Macmillan number. For sheets with an average film thickness of more than 35 μm and a basis weight of more than 20 g / m ² and an air permeability (JIS P8117) exceeding 100 seconds, the above conditions of Macmillan number and Macmillan number × average film thickness must be satisfied. It becomes difficult.

  Specific examples of the sheet (A) include a sheet having a structure having a structure like a polyolefin microporous film used for a separator of a general lithium ion secondary battery, a hole having a hard protrusion or a laser, and a nonwoven fabric. Although the sheet | seat formed from a simple fiber is mentioned, the sheet | seat (A) should just satisfy the above conditions, and is not limited to these.

  When the sheet (A) is composed of fibers, it is preferable that the average fiber diameter of the fibers constituting the sheet is 1/2 to 1/10 of the average film thickness of the sheet (A). When the average fiber diameter is less than 1/10 of the average film thickness of the sheet (A), the curvature increases, and not only sufficient battery characteristics cannot be obtained, but also an overcharge prevention function is disadvantageous. Moreover, when an average fiber diameter exceeds 1/2 of the average film thickness of a sheet | seat (A), the entanglement point of fibers will decrease and the sheet | seat which has sufficient intensity | strength cannot be obtained. In addition, there is a problem that a short circuit occurs simultaneously when the opening of the sheet (A) is too large to produce a battery. Furthermore, when such a sheet | seat (A) is used in the 2nd form in the separator of this invention mentioned later, there also exists a problem that pinholes generate frequently and sufficient separator cannot be obtained.

  When the sheet (A) is composed of fibers, a non-woven fabric is preferable. Examples of the method for producing the nonwoven fabric include commonly used dry methods, spunbond methods, water needle methods, spunlace methods, wet papermaking methods, and melt blow methods. Of these production methods, a wet papermaking method that facilitates obtaining a uniform, thin-leaf nonwoven fabric is particularly preferred.

  The overcharge prevention function in the lithium ion secondary battery of the present invention is largely related to the structure of the separator, and is considered not to be particularly related to the material constituting the sheet (A). That is, any material having sufficient oxidation resistance and reduction resistance can be suitably used for the sheet (A). Examples of such a material include polyester, aromatic polyamide, polyphenylene sulfide, and polyolefin. Here, these may be used alone or in combination of two or more thereof. Moreover, the material which comprises a sheet | seat (A) should just have molecular weight sufficient to obtain a molded object, and if it is about 5000 or more molecular weight (weight average molecular weight: Mw), it will use suitably. Is possible.

The second form includes the porous organic polymer film (B) that encloses the sheet (A) and swells and holds the electrolyte solution, and has an average film thickness of 10 to 35 μm and a basis weight of 10 to 25 g / a porous membrane m ^2. Compared to the case where the separator is constituted only by the sheet (A) as described above, this second embodiment is close to the surface of the positive electrode of the lithium species deposited on the negative electrode during overcharge by the porous organic polymer film (B). May be slightly inferior from the viewpoint of the overcharge prevention function, but as long as the above conditions are satisfied, the lithium-ion secondary battery is fully safe when overcharged. Can be secured. On the other hand, the advantage of the second embodiment over the first embodiment is that the electrolyte retention and short-circuit resistance are improved, and it is severe for film-clad batteries and short circuits that require severe conditions against liquid leakage. This is effective for a battery having a structure in which a separator such as a square battery that requires conditions is wound in a flat shape.

In the second embodiment, if the average film thickness is less than 10 μm and the basis weight is less than 10 g / m ² , sufficient strength as a separator may not be obtained, and problems such as a short circuit may occur at the same time. As in the case of the first embodiment, with such a porous film, it may be difficult to obtain characteristic values of a puncture strength of 3 g / μm or more and an effective puncture strength of 80 g or more.

Further, if the average film thickness is larger than 35 μm and the basis weight is larger than 25 g / m ² , not only the above-described overcharge prevention function is disadvantageous, but also the battery characteristics may be deteriorated. In particular, low temperature characteristics may not be preferable. As in the case of the first embodiment, with such a porous film, it may be difficult to obtain a porous film having a Macmillan number of 10 or less and a film thickness × Macmillan number of 200 μm or less.

  As described above, since the overcharge prevention function in the lithium ion secondary battery of the present invention hardly depends on the material constituting the separator, the porous organic polymer film (B) swells in the electrolyte solution. It is a material that holds this, and it is sufficient if it has sufficient redox resistance for use in a battery. From such a viewpoint, a suitable material for the porous organic polymer film (B) includes a PVdF copolymer mainly composed of polyvinylidene fluoride (PVdF). Here, the molecular weight of the PVdF copolymer is preferably in the range of 10,000 to 1,000,000 in terms of weight average molecular weight (Mw).

  As a suitable vinylidene fluoride (VdF) copolymerization ratio of a PVdF copolymer, the range of 92-98 mol% is mentioned as a molar fraction of VdF. If the molar fraction of VdF exceeds 98%, the crystallinity of the polymer is too high, and it is not only difficult to form a separator, but also the degree of swelling with respect to the electrolytic solution may decrease, which is not preferable. On the other hand, when the molar fraction of VdF is less than 92%, the polymer crystallinity is excessively lowered, and the mechanical properties and heat resistance of the porous membrane supporting the electrolytic solution may be deteriorated.

  A particularly preferable PVdF copolymer includes a terpolymer composed of VdF, HFP, and CTFE. The copolymer composition of such a copolymer is VdF / HFP (a) / CTFE (b) [where (a) = 2-8 wt%, (b) = 1-6 wt%]. Is particularly preferred.

  If the copolymerization fraction (a) of HFP is less than 2% by weight, the swelling degree of the non-aqueous electrolyte tends to decrease, which is not preferable. On the other hand, if it exceeds 8% by weight, the elastic modulus of the film decreases, and a large amount of electrolytic solution cannot be retained sufficiently, and the heat resistance in the state of retaining the electrolytic solution tends to decrease, which is not preferable.

  Moreover, as a copolymerization fraction (b) of CTFE, 1 to 6 weight% is suitable. If the CTFE fraction is less than 1% by weight, the effect of adding CTFE is not sufficient, and it tends to be difficult to maintain heat resistance and improve electrolyte retention. On the other hand, when the amount added is more than 6% by weight, the degree of swelling of the electrolytic solution tends to decrease, which is not preferable.

  These PVdF copolymers may be used alone or as a mixture of two or more kinds of copolymers. If necessary, an electrolyte-swellable non-fluorine polymer such as polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), or polyethylene oxide (PEO) may be blended.

  In addition to the sheet (A) and the porous organic polymer film (B), the porous film may contain a porous inorganic filler as necessary. By including a porous inorganic filler, it is possible to improve the short-circuit resistance of the film without sacrificing ionic conduction characteristics. Suitable inorganic fillers include porous particles having a particle diameter of 0.1 to 10 μm such as silica and alumina.

  The porous film can be obtained by impregnating and applying a polymer solution constituting the porous organic polymer film (B) to the sheet (A) and then removing the solvent. Specific examples of the method for producing a porous membrane include the following methods.

  1. The polymer constituting the porous organic polymer membrane (B), a solvent that dissolves the polymer and is compatible with water, and a phase separation agent (gelling agent or pore opening agent) are mixed and dissolved, and the solution state The dope was impregnated and applied to the sheet (A), and the resulting film was then immersed in an aqueous coagulation bath to coagulate the polymer constituting the porous organic polymer film (B), followed by washing and drying. A method for forming a porous film.

  2. A polymer constituting the porous organic polymer film (B), a volatile solvent for dissolving the polymer, and a plasticizer are mixed and dissolved, and the dope in the solution state is impregnated and applied to the sheet (A) and then dried. After removing the volatile solvent, the plasticizer is dissolved, the plasticizer is extracted with a volatile solvent that does not dissolve the polymer constituting the porous organic polymer film (B), and then dried to form a porous film. .

  3. The polymer constituting the porous organic polymer film (B) and the plasticizer are mixed and then heated to plasticize and melt the polymer constituting the porous organic polymer film (B). After the impregnated dope is impregnated on the sheet (A), the film is cooled to solidify the coated film, the plasticizer is dissolved, and plasticized with a volatile solvent that does not dissolve the polymer constituting the porous organic polymer (B). A method of extracting the agent and drying it to form a porous membrane.

  The second embodiment of the separator of the present invention is advantageous in terms of the first embodiment, the electrolyte solution impregnation property, the retention property, and the like. The impregnation property of the electrolytic solution can be evaluated by the amount of the electrolytic solution impregnated. In this specification, the amount of electrolyte impregnation is shown as a weight fraction of the impregnated electrolyte with respect to the dry weight of the separator. Specifically, the electrolytic solution retainability is obtained by centrifuging a separator impregnated with a non-aqueous electrolyte solution with a centrifugal force of 1400 × g (gravity acceleration) for 20 minutes to remove the non-aqueous electrolyte solution having a weak retention force. Thus, the retention of the electrolytic solution in the separator can be evaluated. In this specification, the electrolyte retention is indicated by the weight fraction of the separator after centrifugation relative to the separator before centrifugation. The electrolytic solution retainability is preferably 70% by weight or more, and more preferably 80% by weight or more.

The separator as described above used in the lithium ion secondary battery of the present invention is 1.5 × 10 ² N / m or more, particularly 3.0 × 10 ² N / m or more in consideration of handling properties in manufacturing the battery. It is preferable to have a proof stress strength of. Here, the yield strength is the strength of the elastic limit and gives an index of how much tension the film can be handled. The higher this value, the easier the handling and the higher the productivity. means.

  The yield strength is determined by a normal tensile test. In the present invention, the strength of the yield strength was calculated from a stress-elongation curve obtained by cutting the separator into 1 cm × 3 cm strips and performing a tensile test with Tensilon at a tensile speed of 20 mm / min.

  The heat distortion temperature of the separator of the present invention is preferably 150 ° C. or higher, and more preferably 170 ° C. or higher. Here, the thermal deformation temperature can be evaluated by thermomechanical analysis (TMA). The evaluation of the heat distortion temperature by TMA is performed by cutting a separator into a strip of 4 mm width, applying a load of 0.01 N, and raising the temperature at a rate of 10 ° C./min. The temperature at which this occurs can be used as the heat distortion temperature.

[Non-aqueous electrolyte]
As the non-aqueous electrolyte of the lithium ion secondary battery of the present invention, a solution obtained by dissolving a lithium salt in a non-aqueous solvent used in a general lithium ion secondary battery is used.

  Specific examples of the non-aqueous solvent include propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate. (MEC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), γ-butyrolactone (γ-BL), sulfolane, acetonitrile and the like. These non-aqueous solvents may be used alone or in combination of two or more. In particular, at least one solvent selected from PC, EC, γ-BL, DMC, DEC, MEC and DME is preferably used.

Examples of the lithium salt dissolved in the non-aqueous solvent include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium borotetrafluoride (LiBF ₄ ), lithium arsenic hexafluoride (LiAsF _6). ), Lithium trifluorosulfonate (CF ₃ SO ₃ Li), lithium perfluoromethylsulfonylimide [LiN (CF ₃ SO ₂ ) ₂ ] and lithium perfluoroethylsulfonylimide [LiN (C ₂ F ₅ SO ₂ ) ₂ ]. However, it is not limited to these. Moreover, these may be used in mixture of 2 or more types. The concentration of the lithium salt to be dissolved is preferably in the range of 0.2 to 2M (mol / L).

[Manufacture of lithium ion secondary batteries]
There is no limitation in particular as a manufacturing method of the lithium ion secondary battery of this invention, The manufacturing method of a well-known lithium ion secondary battery is employable.

  Specifically, a method is generally used in which a joined body in which a positive electrode and a negative electrode are joined via a separator is put in an exterior, an electrolyte is injected, and then sealed. Here, a vacuum injection method is preferably used for the injection of the electrolyte, but is not particularly limited thereto. In addition, this joined body may be impregnated with an electrolytic solution before being put in the exterior.

  In a so-called film-clad battery in which the exterior is a pack made of an aluminum laminate film, it is preferable that the electrode and the separator are bonded and integrated. In this case, the porous film of the above-described second form is suitable as the separator. Adhesion between the separator and the electrode is mainly performed by a thermocompression bonding method, and this may be performed in a dry state not including an electrolyte, or may be performed in a wet state including an electrolyte. Moreover, when the adhesiveness between the separator and the electrode is good, it is possible to manufacture a battery without going through the thermocompression bonding process.

[Lithium ion secondary battery]
The shape of the lithium ion secondary battery of the present invention is not particularly limited, and may be any shape such as a cylindrical shape, a flat shape such as a square shape, and a button shape.

  Examples of the exterior include a pack made of a steel can, an aluminum can, and an aluminum laminate film, but are not particularly limited thereto.

  In the lithium ion secondary battery of the present invention, when the overcharge prevention function as described above is exhibited, the electric energy injected by the operation of charging is released out of the system as Joule heat. This Joule heat is expressed by charging current × battery voltage. Since the battery voltage when the overcharge prevention function is exhibited is substantially constant, Ic is dominant in this Joule heat. Although it depends on the Ic to be used, it is considered that the battery may be exposed to dangers such as ignition due to the temperature inside the battery rising due to Joule heat. Therefore, depending on the Ic employed, it is preferable to select a shape with good heat dissipation efficiency for the lithium ion secondary battery of the present invention. From the viewpoint of improving the heat dissipation efficiency, a film-clad battery using an aluminum laminate pack for the exterior is suitable. Moreover, the method of attaching a heat sink to a battery can also be employ | adopted.

  In the lithium ion secondary battery of the present invention, gas generation due to oxidative decomposition of the electrolyte is suppressed by the above-described overcharge prevention function. In a film-clad battery, severe conditions are required with respect to the swelling of the battery. From this viewpoint, the lithium ion secondary battery of the present invention is preferably used in the form of a film-clad battery. In addition, a safety valve is generally attached to a conventional lithium ion secondary battery. However, in the lithium ion secondary battery of the present invention, gas generation during overcharging is suppressed, so even when a safety valve is not provided Can be sufficiently secured. However, it is a matter of course that safety is further improved if there is a safety valve.

  In the lithium ion secondary battery of the present invention, safety against overcharging may not be ensured depending on the charging current Ic employed due to the Joule heat problem. The overcharge prevention function in the lithium ion secondary battery of the present invention is not preferable in principle for large current charging. However, the use of known additives that have been proposed for improving the safety during overcharge is insufficient to ensure the safety during overcharge in large current charging. In the lithium ion secondary battery of the present invention, the overcharge prevention function is based on a mechanism fundamentally different from that in the case of using a known additive, and therefore it is also possible to use such an additive in combination. is there. Thus, by using a known additive system in combination, it is possible to sufficiently ensure safety during overcharging in large current charging.

[Battery pack]
The battery pack of the present invention comprises at least the lithium ion secondary battery of the present invention and a thermal sensor or a thermal switch.

  In the lithium ion secondary battery of the present invention, as described above, Joule heat generation occurs when the overcharge prevention function is exhibited. In the lithium ion secondary battery of the present invention, it is possible to detect overcharge by this thermal energy, and it is effective in terms of safety to provide the battery pack with a thermal sensor or a thermal switch.

  In the lithium ion secondary battery of the present invention, safety at the time of overcharging can be sufficiently secured only by the thermal sensor or the thermal switch, but in addition to that, a protection circuit may be provided. When equipped with a protection circuit, the safety of the battery is further improved.

[How to charge]
The charging method of the present invention charges the lithium ion secondary battery of the present invention by a constant current charging method, and uses at least one of the rise of the battery temperature, the drop of the battery voltage, and the vibration of the battery voltage to complete the charge. Judgment.

  The lithium ion secondary battery of the present invention has a feature that even if overcharge is performed, not only can the safety be ensured by the overcharge prevention function as described above, but the battery can be discharged thereafter. Moreover, the onset of the overcharge prevention function can be detected by the battery temperature rise due to Joule heat, the battery voltage drop or the battery voltage vibration start. Furthermore, the expression of the overcharge prevention function means that the battery is fully charged. For this reason, constant current charging is possible in which the end of charging is determined by an increase in battery temperature, a decrease in battery voltage, or vibration of the battery voltage.

  However, the charging method of the lithium ion secondary battery of the present invention is not limited to the above-described charging method, and a constant current / constant voltage charging method generally employed is also preferably used.

[Electrical and electronic equipment]
According to the present invention, an electric / electronic device including the lithium ion secondary battery or the lithium ion secondary battery pack of the present invention is also provided.

  The lithium ion secondary battery and lithium ion secondary battery pack of the present invention are suitably used for portable electronic devices such as mobile phones and notebook computers. In particular, when the charging method as described above is employed, the charging time can be greatly shortened as compared with the case of using the normal constant current / constant voltage charging method.

Hereinafter, the present invention will be further described by examples.
[Separator]

Example 1
Polyethylene terephthalate (PET) short fibers having a fineness of 0.33 dtex (average fiber diameter of about 5.5 μm) and 6/4 PET short fibers for binder having a fineness of 0.22 dtex (average fiber diameter of about 4.5 μm) are used. Were blended at a weight ratio of 10 to form a film with a basis weight of 12 g / m ² by a wet papermaking method, and calendered at 200 ° C. to obtain a nonwoven sheet. The characteristics of the obtained sheet were as follows.

  Average film thickness 18 μm, air permeability 0.07 seconds, puncture strength 5.0 g / μm (90 g), Macmillan number 5.0 (Macmillan number × film thickness = 90 μm).

Example 2
A large number of holes were evenly opened in a polypropylene (PP) microporous membrane (Celgard TM2400, manufactured by Celgard) with a needle having a diameter of 2 μm. The characteristics of the obtained sheet were as follows.

Average film thickness 25 μm, basis weight 13.5 g / m ² , air permeability 80 seconds, puncture strength 12 g / μm (300 g), Macmillan number 5.8 (Macmillan number × film thickness = 145 μm).

Example 3
A wet papermaking method by blending PET short fibers for binder with a fineness of 1.22 dtex (average fiber diameter of about 11 μm) at a weight ratio of 5/5 to oriented crystallized PET short fibers with a fineness of 0.55 dtex (average fiber diameter of about 7 μm). Was formed into a film with a basis weight of 12 g / m ² and calendered at 160 ° C. to obtain a nonwoven sheet. The characteristics of the obtained sheet were as follows.

  Average film thickness 18 μm, air permeability 0.04 seconds, puncture strength 6.5 g / μm (117 g), Macmillan number 9.0 (Macmillan number × film thickness = 162 μm).

  A PVdF copolymer having VdF: HFP: CTFE = 95.5: 2.3: 2.2 (molar ratio) was converted into N, N-dimethylacetamide (DMAc) and polypropylene glycol (PPG-400) having an average molecular weight of 400. Was dissolved in a 6/4 (weight ratio) mixed solvent at 60 ° C. to prepare a dope for film formation having a copolymer concentration of 14% by weight. After the obtained dope was impregnated and applied to the nonwoven sheet, the obtained film was immersed in an aqueous solution having a solvent concentration of 40% by weight to be solidified, then washed with water and dried to obtain a porous film. The characteristics of the obtained porous membrane were as follows.

Average film thickness 26 μm, basis weight 21.1 g / m ² , piercing strength 5.5 g / μm (144 g), Macmillan number 5.9 (Macmillan number × film thickness = 153 μm), proof stress 3.5 × 10 ² N / M.

Example 4
Using a crystallized m-aramid short fiber having a fineness of 0.9 dtex (fiber diameter of about 10 μm), a dry paper-making method is used to form a film with a basis weight of 15 g / m ² and calender roll at 320 ° C. Obtained. The characteristics of the obtained sheet were as follows.

  Average film thickness 30 μm, air permeability 0.04 seconds, puncture strength 5.6 g / μm (95 g), Macmillan number 5.8 (Macmillan number × film thickness = 98.6 μm).

  A PVdF copolymer having VdF: HFP: CTFE = 95.5: 2.3: 2.2 (molar ratio) was converted into N, N-dimethylacetamide (DMAc) and polypropylene glycol (PPG-400) having an average molecular weight of 400. Was dissolved in a 6/4 (weight ratio) mixed solvent at 60 ° C. to prepare a dope for film formation having a copolymer concentration of 10% by weight. After the obtained dope was impregnated and applied to the nonwoven sheet, the obtained film was immersed in an aqueous solution having a solvent concentration of 40% by weight to be solidified, then washed with water and dried to obtain a porous film. The characteristics of the obtained porous membrane were as follows.

Average film thickness 34 μm, basis weight 20.9 g / m ² , puncture strength 9.7 g / μm (330 g), Macmillan number 4.6 (Macmillan number × film thickness = 156 μm), proof stress strength 6.4 × 10 ² N / M.

Example 5
Blended PET short fibers for binder with a fineness of 0.22 dtex (average fiber diameter of about 4.5 μm) at a weight ratio of 6/4 to crystallized m-aramid short fibers with a fineness of 0.55 dtex (average fiber diameter of about 7 μm) Then, it was formed into a film with a basis weight of 11 g / m ² by a wet papermaking method, and calendered at 200 ° C. to obtain a nonwoven sheet. The characteristics of the obtained sheet were as follows.

  Average film thickness 17 μm, air permeability 0.06 seconds, puncture strength 5.6 g / μm (95 g), Macmillan number 5.8 (Macmillance × film thickness = 99 μm).

  After impregnating and applying the same dope prepared in Example 3 to the nonwoven sheet, the obtained film was immersed in an aqueous solution having a solvent concentration of 40% by weight to be solidified, then washed with water and dried to form a porous film. Obtained. The characteristics of the obtained porous membrane were as follows.

Average film thickness 24 μm, basis weight 16.7 g / m ² , puncture strength 5.0 g / μm (120 g), Macmillan number 5.4 (Macmillan number × film thickness = 130 μm), proof stress strength 3.5 × 10 ² N / M.

Example 6
A PVdF copolymer having VdF: HFP: CTFE = 95.5: 2.3: 2.2 (molar ratio) was converted into N, N-dimethylacetamide (DMAc) and polypropylene glycol (PPG-400) having an average molecular weight of 400. Was dissolved in a mixed solvent of 6.5 / 3.5 (weight ratio) at 60 ° C. to prepare a dope for film formation having a copolymer concentration of 12% by weight. After impregnating and applying the obtained dope to the non-woven sheet formed in Example 1, the obtained film was immersed in an aqueous solution having a solvent concentration of 40% by weight to be solidified, and then washed with water and dried to obtain a porous film. It was. The characteristics of the obtained porous membrane were as follows.

Average film thickness 24 μm, basis weight 19.7 g / m ² , piercing strength 6.3 g / μm (151 g), Macmillan number 6.5 (Macmillan number × film thickness = 156 μm), proof stress strength 3.8 × 10 ² N / M.

Example 7
A PET short fiber for binder with a fineness of 1.22 dtex (average fiber diameter of about 11 μm) is blended in a weight ratio of 6/4 to a PET short fiber having a fineness of 0.11 dtex (average fiber diameter of about 3.5 μm). Then, a film was formed with a basis weight of 12 g / m ² by a wet papermaking method, and calendered at 130 ° C. to obtain a nonwoven sheet. The characteristics of the obtained sheet were as follows.

  Average film thickness 14 μm, air permeability 0.60 seconds, puncture strength 8.9 g / μm (124 g), Macmillan number 5.0 (Macmillance × film thickness = 70 μm).

  After impregnating and applying the same dope prepared in Example 3 to the nonwoven sheet, the obtained film was immersed in an aqueous solution having a solvent concentration of 40% by weight to be solidified, then washed with water and dried to form a porous film. Obtained. The characteristics of the obtained porous membrane were as follows.

Average film thickness 24 μm, basis weight 18.8 g / m ² , puncture strength 6.8 g / μm (164 g), Macmillan number 4.9 (Macmillan number × film thickness = 118 μm), proof stress strength 3.3 × 10 ² N / M.

  About the separator of Examples 1-7, when those heat deformation temperature etc. were measured, it was as Table 1 below.

Comparative Example 1
A polypropylene (PP) microporous membrane (Celgard TM2400, manufactured by Celgard) was used as a separator. The characteristics of this film were as follows.

Average film thickness 25 μm, basis weight 14.8 g / m ² , air permeability 350 seconds, puncture strength 15.2 g / μm (380 g), Macmillan number 6.5 (Macmillan number × film thickness = 163 μm).

Comparative Example 2
A wet paper-making method in which crystallized m-aramid short fibers having a thickness of 0.9 dtex (average fiber diameter of about 10 μm) and m-aramid fibrites (synthetic pulp-like particles) are blended at a ratio of 8/2 (weight ratio). Was formed into a film with a basis weight of 30 g / m ² and calendered at 320 ° C. to obtain a paper-like sheet. The characteristics of the obtained sheet were as follows.

  Average film thickness 35 μm, air permeability 38 seconds, puncture strength 16 g / μm (550 g), Macmillan number 18.0 (Macmillan number × film thickness = 630 μm).

Comparative Example 3
A melt blown nonwoven fabric having an average fiber diameter of 1.5 μm and a basis weight of 35 g / m ² was formed using PET as a raw material. The nonwoven fabric was calendered at 130 ° C. to make the film thickness 50 μm. The characteristics of this nonwoven fabric were as follows.

  Air permeability 40 seconds, puncture strength 5.5 g / μm (275 g), Macmillan number 3.8 (Macmillan number × film thickness = 190 μm).

  After impregnating and applying the same dope prepared in Example 3 to the above nonwoven sheet, the resulting film was immersed in an aqueous solution having a solvent concentration of 40% by weight to be solidified, then washed with water and dried to obtain a porous film Got. The characteristics of the obtained porous membrane were as follows.

Average film thickness 60 μm, basis weight 43.5 g / m ² , puncture strength 60 g / μm (360 g), Macmillan number 3.3 (Macmillan number × film thickness = 198 μm).

[Evaluation using button batteries]
Example 8
"Positive electrode"
Lithium cobaltate (LiCoO ₂ , Nippon Chemical Industry Co., Ltd.) powder 89.5 parts by weight, acetylene black 4.5 parts by weight, and PVdF dry weight of 6 parts by weight 6% by weight of PVdF A positive electrode paste was prepared using an N-methyl-pyrrolidone (NMP) solution. The obtained paste was applied onto an aluminum foil having a thickness of 20 μm, dried and pressed to obtain a positive electrode having a thickness of 97 μm.

The total lithium content Qp of this positive electrode calculated from the weight of the positive electrode was 5.4 mAh / cm ² .

"Negative electrode"
As a negative electrode active material, mesophase carbon microbeads (MCMB, manufactured by Osaka Gas Chemical Co., Ltd.) powder 87 parts by weight, 3 parts by weight of acetylene black, and 6% by weight of PVdF so that the dry weight of PVdF is 10 parts by weight. An NMP solution was used to produce a negative electrode agent paste. The obtained paste was applied onto a copper foil having a thickness of 18 μm, dried and pressed to obtain a negative electrode having a thickness of 90 μm.

The amount of lithium Qn that can be doped into this negative electrode measured by a three-electrode cell was 2.6 mAh / cm ² .

"Production of button batteries"
The above positive electrode and negative electrode were punched into a circle with a diameter of 14 mm, and the separators produced in Examples 1 to 7 were punched into a diameter of 16 mm and used. The positive electrode and the negative electrode were joined via a separator, impregnated with an electrolyte (electrolytic solution), and sealed in a battery can. The electrolyte used here is 1M LiPF ₆ EC / DEC (1/1 weight ratio). The size of the battery can is CR2032.

"Overcharge evaluation"
Overcharge evaluation using the manufactured button battery was conducted at a charging current of 0.52 mA / cm ² , a constant current and a constant voltage of 8 hours at 4.2 V, a discharge current of 0.52 mA / cm ² , and a constant current discharge of 2.75 V cutoff. 1 cycle charging / discharging measurement was performed under the condition of, and overcharging was performed under the condition of constant current charging for 10 hours at a charging current of 2.6 mA / cm ² . Moreover, after resting for 2 hours after overcharge, discharge was performed under the conditions of a discharge current of 0.52 mA / cm ² and a 2.75 V cutoff. Here, the sampling time of the battery voltage was every 30 seconds.

  In all the batteries, vibration of the battery voltage was observed during charging, and the battery voltage did not exceed 5.5 V during charging. In all the batteries, the open circuit voltage after overcharging was in the range of 4.2 to 4.5 V, and discharging after overcharging was possible.

  Table 1 shows the amount of electricity (Q1) when the battery voltage starts to oscillate and the discharge capacity Qd obtained after overcharging. As an example, FIG. 1 shows a voltage change during overcharge in a battery using the separator of Example 7, and FIG. 2 shows a discharge behavior after overcharge.

Comparative Example 4
In the same manner as in Example 8, a button battery was produced using the separators of Comparative Examples 1 to 3. This button battery was tested in the same manner as in Example 8.

  In the battery using the separator of Comparative Example 2, sufficient characteristics were not obtained in the first charge / discharge cycle. This is because the resistance of the separator is large. Therefore, the battery using the separator of Comparative Example 2 was not subjected to an overcharge test.

  In the battery using the separators of Comparative Examples 1 and 3, the initial charge / discharge was good, so an overcharge test was performed. However, no vibration of the battery voltage was observed, the battery voltage increased to 5.5 V or higher, and subsequent discharge was impossible. As an example, FIG. 1 shows a voltage change during overcharging in a battery using the separator of Comparative Example 1.

  From the above results, it can be seen that the batteries using the separators of Comparative Examples 1 and 3 clearly do not have the overcharge prevention function as in the lithium ion secondary battery of the present invention.

Example 9
Using the separators of Examples 1 to 7, a button battery similar to that of Example 8 was produced using a copper foil instead of the negative electrode. In this button battery, constant current charging was performed at a charging current of 0.56 mA / cm ² with the copper foil as a negative electrode. At that time, the battery voltage drop, the battery voltage oscillation, or the rough stop of the battery voltage rise was observed in all the batteries.
Table 2 shows the amount of electricity Q2 at which the above phenomenon has started.

Example 10
Using the separators of Examples 1 to 7, a button battery similar to that of Example 8 was produced. The button battery was subjected to an overcharge test under the same conditions as in Example 8. During the overcharge test, impedance R _0.5 at 1 kHz was measured when the amount of charged electricity reached 1.3 mAh / cm ² . Further, impedance R2 at 1 kHz was measured when the amount of charged electricity was 5.4 mAh.
Table 1 shows the values of _R 2 _{/ R 0.5.}

Example 11
Using the separators of Examples 1 to 7, a button battery similar to that of Example 8 was produced. The button battery was subjected to an overcharge test under the same conditions as in Example 8. When the amount of charged electricity in the overcharge test reached 7 mA / cm ² , overcharge was stopped, the battery was broken, and the negative electrode surface was observed with a scanning electron microscope (SEM). FIG. 3 shows, as an example, an SEM photograph when the separator of Example 5 is used. In any battery, it was observed that lithium species having the longest diameter of 100 μm or less were scattered.

  Further, in the separators of Examples 3 to 7, holes corresponding to this lithium species were observed in those in contact with the negative electrode, and no holes were observed in those in contact with the positive electrode.

  From the results of Examples 8 to 11 described above, the lithium ion secondary battery of the present invention using the lithium ion secondary battery separator of the present invention has an overcharge prevention function through lithium species as described above. I understand that.

[Evaluation in film-clad batteries]
Example 12
Using the positive electrode and the negative electrode produced in Example 8 in the same area, a film-clad battery was produced using the separator of Example 5. This film-clad battery was produced by joining a positive electrode and a negative electrode through a separator, placing them in an aluminum laminate pack, and injecting an electrolytic solution. In this film-clad battery, an electrolytic solution in which 1 M of LiPF ₆ was dissolved in a mixed solvent having a composition of EC: DEC: MEC = 1: 1: 1 (weight ratio) was used. Here, the size of the film-clad battery was 55 mm × 35 mm × 3.7 mm.

  When the first charge / discharge measurement was performed in the same manner as in Example 8, the capacity of this film-clad battery was 650 mAh.

This film-clad battery was overcharged by constant current charging at an ambient temperature of 25 ° C. until the charge electricity amount reached 1950 mAh (charging rate: 350%) at a charging current of 2.6 mA / cm ² . At this time, the increase in the battery voltage almost stopped at a charging rate of 120% and became steady at about 4.5 to 4.6V. Further, the battery surface temperature began to increase at the same time as the increase in battery voltage stopped, became steady at about 50 ° C., and the maximum battery surface temperature was 51 ° C. (FIG. 4). In this overcharge, the film battery did not swell. Here, the charging rate is a percentage of the duration of the charging operation with respect to the time required for full charging.

After this overcharge, constant current discharge was performed to 2.75 V at a discharge current of 0.52 mA / cm ² . At this time, a discharge capacity of 656 mAh was obtained.

  From the above results, it can be seen that the overcharge prevention function in the lithium ion secondary battery of the present invention can be effectively obtained even in a practical battery size.

Example 13
A film-clad battery having the same configuration as the film-clad battery produced in Example 12 was overcharged at a charging current of 2.6 mA / cm ² to a charge rate of 186%, and an oven heating test was performed on the battery. The temperature rising rate of the oven was 5 ° C./min, and when the temperature reached 150 ° C., the oven was left for 1 hour. As a result, the battery swelled but did not rupture or ignite.

Comparative Example 5
A film-sheathed battery was prepared in the same manner as in Example 12 except that the separator of Comparative Example 1 was used, and overcharged to a charge rate of 186% at a charging current of 2.6 mA / cm ^2. An oven heating test was performed under the same conditions. When the oven temperature reached 125 ° C., the battery burst and ignited.

Example 14
A film-clad battery having the same configuration as that of the film-clad battery produced in Example 12 was produced. This film battery was charged with a charging current of 2.6 mA / cm ² to a charge electricity amount of 1300 mAh and subjected to a constant current discharge of 0.52 mA / cm ² to 2.75 V for 5 cycles. During this test, the battery did not swell, burst or ignite, and a discharge capacity of 448 mAh was obtained even in the fifth cycle.

  From the results of Examples 13 and 14 and Comparative Example 5, it can be seen that the lithium ion secondary battery of the present invention is extremely safe against overcharging compared to the conventional lithium ion secondary battery.

Example 15
A film-clad battery having the same configuration as that of the film-clad battery produced in Example 12 was produced, and a chalk marker for detecting heat generation was attached to the surface of the film-clad battery. This battery was overcharged with a charge current of 2.6 mA / cm ² and an electric charge of 900 mAh. At this time, the attached chalk marker was discolored.

Comparative Example 6
A film-clad battery having the same configuration as the film-clad battery produced in Comparative Example 5 was produced, and a chalk marker for detecting heat generation was attached to the surface of the film-clad battery. This battery was overcharged with a charge current of 2.6 mA / cm ² and an electric charge of 900 mAh. At this time, the attached chalk marker did not change color.

  In the battery of Example 15, when the overcharge prevention function was manifested, the electric energy injected into the battery by the operation of charging was not stored in the battery, but was released outside the system as Joule heat, so the choke marker was discolored. On the other hand, in the battery of Comparative Example 6, since all the electric energy injected into the battery by charging is stored in the battery, there is no heat generation and the choke marker does not change color.

  From the results of Example 15 and Comparative Example 6, the lithium ion secondary battery of the present invention has the advantage that the thermistor type overcharge protection circuit that could not be achieved by the conventional lithium ion secondary battery can be used effectively. I understand.

  According to the present invention, in a lithium ion secondary battery, it becomes possible to prevent overcharge by doping lithium into the positive electrode via lithium species generated on the negative electrode during overcharge, and safe lithium ions during overcharge A secondary battery can be provided.

It is a figure which shows the voltage change during the overcharge in Example 8 and Comparative Example 6. FIG. It is a figure which shows the discharge behavior after the overcharge of Example 8. FIG. It is an electron micrograph which shows the shape of the lithium seed | species produced on a negative electrode at the time of overcharge. It is a figure which shows the voltage change in the overcharge of Example 12, and the change of battery surface temperature.

Claims (9)

It consists of a sheet (A) having an average film thickness of 10 to 35 μm, a basis weight of 6 to 20 g / m ² , an air permeability (JIS P8117) of 100 seconds or less, a Macmillan number of 10 or less at 25 ° C., and a Macmillan number × average film thickness of 200 μm or less. Separator for lithium ion secondary battery.   The separator according to claim 1, wherein the sheet (A) is composed of fibers, and an average fiber diameter of fibers constituting the sheet is 1/2 to 1/10 of an average film thickness of the sheet (A).   The separator according to claim 2, wherein the sheet (A) is a nonwoven fabric.   The separator according to any one of claims 1 to 3, wherein the sheet (A) is made of polyester, aromatic polyamide, polyphenylene sulfide, polyolefin, or a mixture of two or more thereof. A porous organic polymer film (B) that encloses the sheet (A) according to any one of claims 1 to 3 and swells and retains the electrolyte solution, and has an average film thickness of 10 to 35 μm and a basis weight of 10 A separator for a lithium ion secondary battery comprising a porous film of ˜25 g / m ² .   The separator according to claim 5, wherein the porous organic polymer film (B) is mainly composed of polyvinylidene fluoride (PVdF).   The separator according to claim 6, wherein the porous organic polymer film (B) is made of a polyvinylidene fluoride copolymer containing 92 to 98 mol% of vinylidene fluoride (VdF).   The separator according to claim 7, wherein the porous organic polymer film (B) comprises a terpolymer of vinylidene fluoride, hexafluoropropylene (HFP), and chlorotrifluoroethylene (CTFE).   The copolymerization composition of the terpolymer is VdF / HFP (a) / CTFE (b), where (a) = 2-8 wt% and (b) = 1-6 wt%. 8. The separator according to 8.