JP3986744B2 - Lithium ion secondary battery separator and lithium ion secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium ion secondary battery that obtains an electromotive force when lithium ions enter and exit, and a separator used therefor. In particular, the present invention relates to an overcharge prevention technique for the purpose of improving safety.
[0002]
[Prior art]
With the recent spread of small portable devices typified by mobile phones and notebook computers, the demand for secondary batteries having high energy density is very high. A secondary battery having a high energy density can reduce the size and weight of these portable devices, and can be used for a long time with a single charge.
[0003]
From the viewpoint of increasing the capacity of the secondary battery, metallic lithium or a metal mainly composed of this has attracted attention as a negative electrode material. Theoretically, metallic lithium has the highest energy density as a negative electrode material. However, when charging / discharging is repeated, the negative electrode surface becomes rough, so that current concentration occurs and dendrites are gradually generated. There is a problem that sufficient cycle characteristics cannot be obtained due to an internal short circuit caused by this dendrite, and it has not been put into practical use. In addition, metallic lithium is a very active metal, and there are concerns about safety issues.
[0004]
These problems are solved by using a carbon material that is doped / undoped with lithium for the negative electrode. This is a so-called lithium ion secondary battery. Although these carbon materials are inferior in capacity to metallic lithium, the potential for doping and undoping lithium is very close to the oxidation-reduction potential of lithium, and a high electromotive force can be obtained. Higher energy density than secondary batteries. Further, since lithium is doped in the carbon material, dendrite is not generated with charge and discharge as in the case of using metallic lithium for the negative electrode, so that sufficient cycle characteristics can be obtained.
[0005]
A 4V class lithium ion secondary battery using a lithium-containing transition metal typified by lithium cobaltate as a positive electrode has been widely put into practical use because it has a very high energy density, but an organic solvent is used as an electrolyte. Therefore, careful attention is paid to ensuring safety because misuse causes ignition and the like.
[0006]
In particular, lithium ion secondary batteries are dangerous when overcharged. When a lithium ion secondary battery is overcharged, the electrolyte solution decomposes at the positive electrode interface, and the battery temperature rises due to this reaction heat. When the temperature rises, the electrolyte solution decomposes at the negative electrode interface, and the battery temperature further rises. Thus, when the battery temperature rises, the self-decomposition reaction of the electrolytic solution occurs explosively. This is a so-called runaway reaction. When a runaway reaction occurs, the battery temperature rises rapidly, and gas is generated due to decomposition of the electrolytic solution, so that the battery internal pressure also rises rapidly. At this time, the battery ignites, ruptures and explodes.
[0007]
Thus, prevention of overcharge is very important in securing the safety of the lithium ion secondary battery. In the lithium ion battery currently on the market, an electronic circuit (protection circuit) is mounted on the battery pack to prevent overcharging. In addition, assuming that this protection circuit is broken, safety devices such as a separator thermal fuse function, a safety valve, and a PTC element are also installed. However, these secondary safety devices suppress the ignition of the battery when overcharged and do not prevent overcharge itself.
[0008]
The overcharge prevention technology by the protection circuit has a problem of high cost. In the case of a single cell, about one third of the battery pack is the cost of the protection circuit. In addition, the structure becomes complicated as the unit cell is changed to the assembled cell, and this is one factor that a large-sized lithium ion secondary battery such as an electric vehicle has not been put into practical use. Moreover, since it is a control by an electronic circuit, there is a problem that it is broken, and it cannot necessarily be said to be safe. The secondary safety device as described above is installed assuming that it is broken. However, since these safety devices do not prevent overcharging itself, they cannot be said to be safe. For example, when the separator melts down, its function is lost. During this time, the battery temperature only needs to drop, but if not, it is assumed that it will explode.
[0009]
As an overcharge prevention technique different from the technique using an electronic circuit, there is a technique using a chemical reaction using an additive. These additives are added to various places in the battery such as an electrolyte and an electrode. Additives such as those that operate before the safety valve is overcharged by generating gas when the additive reacts, or technologies that increase the internal impedance of the battery and prevent overcharge by polymerizing the additive to form a film There are various effects. In addition, there are various driving forces that induce the reaction caused by the additive, such as heat and electricity (electrolysis).
[0010]
Examples of adding the above-described additives include those in which redox shuttles proposed in JP-A-6-338347 and JP-A-7-302614 are added. This is because a chemical species having a redox potential of 4.0 to 4.5 V is added to the electrolytic solution, and overcharge current is consumed by an oxidation-reduction reaction between the positive and negative electrodes of this chemical species to prevent overcharging. It is a technology. However, considering the electrode reaction rate of the redox species and the diffusion in the electrolyte, it is necessary to add a large amount in order to consume the overcharge current in the rapid charge of 1C or more. Such a large amount of addition causes a decrease in battery characteristics, which is not preferable. In addition, there is a problem in that it cannot be added as much as 1 C of current is consumed due to solubility problems. Under such circumstances, the addition of an oxidation-reduction reagent called a redox shuttle has not been adopted.
[0011]
In addition, in the case of high-rate charging in terms of kinetics, it is difficult to ensure safety, and a runaway reaction may occur before overcharge is sufficiently protected. In addition, overcharge prevention means using additives are difficult to control and are not employed at present, such as adding a large amount of additives in an attempt to sufficiently protect overcharge tends to lead to deterioration of battery characteristics.
[0012]
[Problems to be solved by the invention]
Overcharging is a very dangerous state in a lithium ion secondary battery, and the only way to protect the battery from overcharging is to control the electronic circuit. However, the method of controlling by an electronic circuit is expensive and the structure is complicated in the assembled cell. In addition, the electronic circuit may be broken, and overcharge cannot be reliably protected.
[0013]
In addition to electronic circuits, secondary safety devices such as safety valves are also mounted on lithium-ion secondary batteries, but they are designed to prevent over-ignition of the battery when this safety device is overcharged. It does not prevent charging itself. Rather, for example, a state where the safety valve is open is dangerous, and it is difficult to say that the battery is intrinsically safe.
[0014]
That is, an object of the present invention is to provide a lithium ion secondary battery separator that is intrinsically safe against overcharging, and a lithium ion secondary battery using the same.
Furthermore, the objective of this invention is providing the separator for lithium ion secondary batteries which protects an overcharge reliably at lower cost, and a lithium ion secondary battery using the same.
[0015]
[Means for Solving the Problems]
The present inventors have found that such a problem can be solved when a film having specific physical properties is used as a separator for a lithium ion secondary battery, and have reached the present invention.
That is, the present invention has a through-hole having a Macmillan number of 2 to 10, a film thickness of 5 to 40 μm, a porosity of 30 to 50%, an opening area of 1 to 100 μm ² and a curvature of 1. A separator for a lithium ion secondary battery in which the total opening area of the through-holes is 0.1 to 10% of the surface area, and a carbon-based material capable of doping and dedoping lithium in the negative electrode, and containing lithium in the positive electrode In a lithium secondary battery using a transition metal oxide, the separator is used.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the separator for lithium ion secondary batteries and the lithium ion secondary battery of the present invention will be described in detail.
The Macmillan number of the separator for lithium ion secondary batteries of the present invention is a measure of membrane resistance, and is obtained by dividing the resistivity when the separator is impregnated with the electrolyte by the resistivity when only the electrolyte is used. It is done. The resistivity when the separator is impregnated with the electrolytic solution can be obtained by a normal AC impedance method. In addition, the resistivity of the electrolytic solution alone can be obtained as the reciprocal of the conductivity obtained by a conductivity meter. Although the electrolyte solution used at this time is not specifically limited, What is used for a lithium secondary ion battery is preferable.
[0017]
The film thickness of the lithium ion secondary battery separator of the present invention is in the range of 5 to 40 μm. If the film thickness is too thick, not only the internal impedance of the battery increases but also the volume occupied by the separator increases, which is not advantageous in terms of battery capacity. On the other hand, if the film thickness is too thin, the mechanical strength is insufficient, causing problems such as internal short circuit of the battery.
The porosity of the lithium ion secondary battery separator of the present invention is in the range of 30 to 50%. If the porosity is low, the ion permeability is poor and sufficient battery characteristics cannot be obtained. Moreover, when the porosity is too high, the mechanical strength is insufficient.
[0018]
When the separator for a lithium ion secondary battery of the present invention is provided with a separator between a positive electrode and a negative electrode to form a lithium ion secondary battery, the separator film, for example, from the positive electrode side to the negative electrode side is passed through the separator. It is characterized by having a plurality of through holes having a curvature rate of 1 through the surface. Here, the through-hole having a curvature of 1 refers to a through-hole whose shortest distance from one side of the separator to the other side through the through-hole is equal to the film thickness. Such a through-hole can be observed as a portion through which light is transmitted by a transmission optical microscope.
[0019]
Polyolefin microporous membranes have been put to practical use as ordinary lithium ion secondary battery separators, but there are through holes with a high curvature in this polyolefin microporous membrane. There is no hole, and this is a significant difference between the separator for a conventional lithium ion secondary battery and the present invention.
The opening area of each through hole is in the range of 1 to 100 μm ² . If the opening area of the through-hole exceeds 100 μm ² , the probability of causing an internal short circuit when the battery is manufactured is not preferable. Further, if the opening area of the through hole is less than 1 μm ² , the function of protecting overcharge cannot be obtained sufficiently. This overcharge protection function is achieved by depositing fine metal lithium and reaching the positive electrode interface. This is because metal lithium is selectively applied to the portion where current easily flows and physical resistance to metal lithium deposition is low. Is made possible by precipitation. A through-hole having an opening area of less than 1 μm ² is too small as a site for selectively depositing metallic lithium.
[0020]
The total area of the through holes is 0.1 to 10% of the surface area of the separator. If the total area of the through-holes exceeds 10%, there is too much space in the separator, causing an internal short circuit of the battery. On the other hand, if it is less than 0.1%, there are too few sites where metal lithium is selectively deposited, so that overcharge cannot be sufficiently protected.
The opening area of each of the through holes and the ratio of the total opening area of the through holes can be calculated from this image by observing the separator with an optical microscope, an electron microscope, or the like.
The separator of this invention should just have the above characteristics, and the form can be considered variously. Although an example is given below, it is not limited to this.
[0021]
For example, a form in which the through hole of the present invention is formed in a polyolefin microporous film (for example, Celgard TM2400: manufactured by Celgard) used in a normal lithium ion secondary battery is conceivable. However, it is difficult to obtain a sufficient Macmillan number in the form in which the through-hole is opened in the dense film as defined above, but even if a sufficient Macmillan number can be obtained, a short circuit is not possible. Problems and bias of charge and discharge become remarkable, which is not preferable from the viewpoint of battery life and load characteristics. For example, a separator as described in JP-A-11-250890 does not provide sufficient battery characteristics.
[0022]
There are various methods for making a through-hole in a microporous membrane. For example, there are a method of physically making a hole with something like a needle and a method of using an electron beam.
Further, a form in which fibers are entangled two-dimensionally is also conceivable. In such a form, it is also possible to process a two-dimensional sheet in which fibers are appropriately entangled by a method such as pressing so that the features of the present invention can be obtained.
[0023]
Since the overcharge protection function as described above is important in the features defined in the present invention and is not related to the material constituting the separator, the material constituting the lithium secondary battery separator of the present invention Is not particularly limited. For example, polyolefin materials typified by polyethylene and polypropylene, polyamide, polyester, polytetrafluoroethylene, polyphenylene sulfide, polyvinyl chloride, polyimide, polysulfone and the like can be mentioned. However, when considering oxidation resistance, reduction resistance, and moldability, polyolefin-based materials are considered suitable.
[0024]
The lithium ion secondary battery of the present invention is a lithium ion secondary battery separator of the present invention described above, using a carbon-based material capable of doping and undoping lithium as a negative electrode and a lithium-containing transition metal oxide as a positive electrode. It is characterized by using.
The electrode used in the lithium ion secondary battery of the present invention includes an active material that is doped / undoped with lithium ions, a binder polymer that binds the active material and swells in the electrolyte, a conductive aid for improving electronic conductivity, Consists of a current collector. The electrode may be gelled and have a structure capable of holding the electrolytic solution.
[0025]
Examples of the positive electrode active material include various lithium-containing transition metal oxides, but are not particularly limited thereto, and any active material may be used as long as it is used for a so-called 4V class lithium secondary battery. Examples of lithium-containing transition metal oxides include lithium-containing cobalt oxides such as LiCoO ₂ , lithium-containing nickel oxides such as LiNiO _2, and lithium-containing manganese oxides such as LiMn ₂ O ₄ .
A carbon material that absorbs and releases lithium ions is used as the negative electrode active material. Examples of the carbon material include polyacrylonitrile, phenol resin, phenol novolac resin, a sintered organic polymer compound such as cellulose, artificial graphite, and natural graphite.
[0026]
PVdF such as polyvinylidene fluoride (PVdF), PVdF and hexafluoropropylene (HFP), perfluoromethyl vinyl ether (PFMV), and a copolymer of tetrafluoroethylene as a binder polymer that binds the active material and swells in the electrolyte. Fluorocarbon resins such as copolymer resins, polytetrafluoroethylene, fluororubbers, hydrocarbon polymers such as styrene-butadiene copolymers and styrene-acrylonitrile copolymers, carboxymethylcellulose, polyimide resins, etc. can be used. However, the present invention is not limited to this. These may be used alone or in combination of two or more.
[0027]
As the current collector, a foil or mesh made of a material having excellent oxidation stability is suitably used for the positive electrode, and a foil or mesh made of a material having excellent reduction stability is suitably used for the negative electrode. Specifically, examples of the positive electrode include aluminum, stainless steel, nickel, and carbon, and examples of the negative electrode include metallic copper, stainless steel, nickel, and carbon. In particular, an aluminum foil or mesh is suitably used for the positive electrode, and a copper foil or mesh is suitably used for the negative electrode.
[0028]
As the conductive assistant, artificial graphite, carbon black (acetylene black), nickel powder and the like are preferably used. The negative electrode may not contain this conductive additive.
In the lithium ion secondary battery of the present invention, an electrolytic solution in which a lithium salt is dissolved as an electrolyte in a polar organic solvent is preferably used.
[0029]
The organic solvent to be used is not particularly limited as long as it is a polar organic solvent having 10 or less carbon atoms that is generally used in lithium ion secondary batteries. For example, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), 1,2-dimethoxyethane (DME), 1 , 2-diethoxyethane (DEE), γ-butyrolactone (γ-BL), sulfolane, acetonitrile and the like. These polar organic solvents may be used alone or in combination of two or more. In particular, at least one organic solvent selected from PC, EC, γ-BL, DMC, DEC, MEC, and DME is preferably used.
[0030]
Examples of the lithium salt dissolved in the organic solvent include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium borotetrafluoride (LiBF ₄ ), and lithium arsenic hexafluoride (LiAsF). ₆ ), lithium trifluorosulfonate (LiCF ₃ SO ₃ ), lithium perfluoromethylsulfonylimide [LiN (CF3SO ₂ ) ₂ ], lithium perfluoroethylsulfonylimide [LiN (C ₂ F ₅ SO ₂ ) ₂ ] and the like Can be mentioned. These may be used in combination. The concentration of the dissolved lithium salt is preferably in the range of 0.2 to 2M.
[0031]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
[0032]
[Example 1]
<Preparation of separator>
Through-holes with a curvature of 1 were drilled in a cell guard TM2400 (manufactured by Celgard) having a thickness of 25 μm with a needle having a diameter of 2 μm so that the total opening area of the through-holes was 1% of the surface area of the separator. This separator had a porosity of 38.6% and a Macmillan number of 6.7.
[0033]
This separator was punched into a circle having a diameter of 19 mm, sandwiched between a copper electrode and a lithium electrode having a diameter of 19 mm, and impregnated with a PC / EC (1/1 weight ratio) electrolyte solution in which 1M LiBF ₄ was dissolved to produce a cell. As a result of depositing lithium metal on the copper electrode at a current density of ² mA / cm ² and measuring the impedance of the cell by the alternating current method, the impedance of the cell at a frequency of 1 Hz was 50 Ω · m or less within 1 hour.
[0034]
<Production of battery>
N-methylpyrrolidone (NMP) of 5.1% by weight PVdF so that the dry weight of the lithium cobaltate powder 89.5 parts by weight, carbon black 4.5 parts by weight and polyvinylidene fluoride (PVdF) 6 parts by weight. Using the solution, a positive electrode material paste was prepared. The obtained paste was applied onto an aluminum foil having a thickness of 20 μm, dried and pressed to produce a positive electrode having a thickness of 120 μm.
As a carbonaceous negative electrode material, an NMP solution of 5.8 wt% PVdF was added so that 87 parts by weight of mesophase carbon microbead (MCMB6-28) powder, 3 parts by weight of carbon black, and 10 parts by weight of PVdF were dried. A negative electrode material paste was prepared. The obtained paste was applied onto a copper foil having a thickness of 18 μm, dried and pressed to prepare a negative electrode having a thickness of 125 μm.
[0035]
The positive electrode and the negative electrode were punched to a diameter of 15 mm, and the produced separator was punched to a diameter of 18 mm. Using these, a button type battery (CR2032) was produced. As the electrolytic solution, 1M LiBF ₄ EC / DEC (1/1 weight ratio) (manufactured by Kishida Chemical) was used.
When this battery was subjected to a current density of 0.56 mA / cm ² , 4.2 V constant current, constant voltage charging, 0.56 mA / cm ² , 2.75 V cut-off constant current discharge, a capacity of 5.0 mAh was obtained. .
<Overcharge evaluation>
When this battery was charged with a constant current at a current density of 2.8 mA / cm ² , the battery voltage continued to vibrate in the vicinity of 4.4 V from the point where the amount of charged electricity exceeded 5.3 mAh, and this state was maintained at 15 mAh or more. .
The open circuit voltage thereafter was 4.31 V, and a discharge of 5.4 mAh was possible.
[0036]
[Comparative Example 1]
<Preparation of separator>
Through-holes with a curvature of 1 were drilled in a Celgard TM2400 with a film thickness of 25 μm (manufactured by Celgard) with a needle having a diameter of 30 μm so that the total opening area of the through-holes was 10% of the surface area of the separator. This separator had a porosity of 44.2% and a Macmillan number of 6.8.
[0037]
This separator was punched into a circle having a diameter of 19 mm, sandwiched between a copper electrode and a lithium electrode having a diameter of 19 mm, and impregnated with a PC / EC (1/1 weight ratio) electrolyte solution in which 1M LiBF ₄ was dissolved to produce a cell. As a result of depositing lithium metal on the copper electrode at a current density of ² mA / cm ² and measuring the impedance of the cell by the alternating current method, the impedance of the cell at a frequency of 1 Hz was 50 Ω · m or less within 1 hour.
<Production of battery>
A button battery was produced using the separator produced in the same manner as in Example 1, but the battery was short-circuited internally and could not be charged / discharged.
[0038]
[Comparative Example 2]
<Preparation of separator>
Through-holes with a curvature of 1 were drilled in a Celgard TM2400 (made by Celgard) having a thickness of 25 μm with a needle having a diameter of 0.4 μm so that the total area of the through-holes was 5% of the surface area of the separator. This separator had a porosity of 41.1% and a Macmillan number of 6.5.
This separator was punched into a circle having a diameter of 19 mm, sandwiched between a copper electrode and a lithium electrode having a diameter of 19 mm, and impregnated with a PC / EC (1/1 weight ratio) electrolyte solution in which 1M LiBF ₄ was dissolved to produce a cell. As a result of depositing lithium metal on the copper electrode at a current density of ² mA / cm ² and measuring the impedance of the cell by the alternating current method, the impedance of the cell at a frequency of 1 Hz did not become 50 Ω · m or less within 1 hour.
[0039]
<Production of battery>
A button battery was produced using the separator produced in the same manner as in Example 1. When charged and discharged in the same manner as in Example 1, a capacity of 5.1 mAh was obtained.
<Overcharge evaluation>
When the produced battery was charged with a constant current at a current density of 2.8 mA / cm ² , the battery voltage continued to rise and reached 5 V or higher. Thereafter, the battery was not dischargeable.
[0040]
[Comparative Example 3]
<Preparation of separator>
Through-holes with a curvature of 1 were drilled in a Celgard TM2400 with a film thickness of 25 μm (Celgard) with a needle having a diameter of 10 μm so that the total opening area of the through-holes was 50% of the surface area of the separator. This separator had a porosity of 69.0% and a Macmillan number of 4.0.
[0041]
This separator was punched into a circle having a diameter of 19 mm, sandwiched between a copper electrode and a lithium electrode having a diameter of 19 mm, and impregnated with a PC / EC (1/1 weight ratio) electrolyte solution in which 1M LiBF ₄ was dissolved to produce a cell. As a result of depositing lithium metal on the copper electrode at a current density of ² mA / cm ² and measuring the impedance of the cell by the alternating current method, the impedance of the cell at a frequency of 1 Hz was 50 Ω · m or less within 1 hour.
<Production of battery>
A button battery was produced using the separator produced in the same manner as in Example 1, but it was short-circuited internally and could not be charged / discharged.
[0042]
[Comparative Example 4]
<Preparation of separator>
Open a through hole with a curvature of 1 in a 25 μm cell guard TM2400 (manufactured by Celgard) with a 1.5 μm diameter needle so that the total area of the through holes is 0.01% of the surface area of the separator. It was. This separator had a porosity of 38% and a Macmillan number of 6.8.
This separator was punched into a circle having a diameter of 19 mm, sandwiched between a copper electrode and a lithium electrode having a diameter of 19 mm, and impregnated with a PC / EC (1/1 weight ratio) electrolyte solution in which 1M LiBF ₄ was dissolved to produce a cell. As a result of depositing lithium metal on the copper electrode at a current density of ² mA / cm ² and measuring the impedance of the cell by the alternating current method, the impedance of the cell at a frequency of 1 Hz did not become 50 Ω · m or less within 1 hour.
[0043]
<Production of battery>
A button battery was produced using the separator produced in the same manner as in Example 1. As a result of charging and discharging this battery in the same manner as in Example 1, a capacity of 5.1 mAh was obtained.
<Overcharge evaluation>
When this battery was charged with a constant current at a current density of 2.8 mAh / cm ² , the battery voltage continued to rise and became 5 V or higher. Thereafter, the battery was not dischargeable.
[0044]
[Comparative Example 5]
<Preparation of separator>
A polypropylene film having a film thickness of 25 μm was prepared by the melt extrusion method. A through hole having a curvature of 1 was formed in the film with a needle having a diameter of 10 μm so that the total area of the through holes was 9% of the surface area of the separator. This separator had a porosity of 9% and a Macmillan number of 12.
[0045]
This separator was punched into a circle having a diameter of 19 mm, sandwiched between a copper electrode and a lithium electrode having a diameter of 19 mm, and impregnated with a PC / EC (1/1 weight ratio) electrolyte solution in which 1M LiBF ₄ was dissolved to produce a cell. As a result of depositing lithium metal on a copper electrode at a current density of ² mA / cm ² and measuring the impedance of the cell by an alternating current method, the impedance of the cell at a frequency of 1 Hz was 50 Ω · m or less within 1 hour.
<Production of battery>
A button battery was produced using the separator produced in the same manner as in Example 1. As a result of charging and discharging the battery in the same manner as in Example 1, only 70% of the capacity of the battery in Example 1 was obtained.
[0046]
【The invention's effect】
As described above in detail, by using the separator of the present invention, a lithium secondary battery having excellent overcharge characteristics and high safety can be provided without impairing battery characteristics.

Claims (2)

McMillan number 2-10, thickness 5 to 40 m, a porosity of 30-50%, and tortuosity opening area at 1 to 100 [mu] m ² has a plurality of through holes of 1, of the through hole A separator for a lithium ion secondary battery, wherein the total area of the apertures is 0.1 to 10% of the surface area. A lithium secondary battery using the separator according to claim 1 in a lithium secondary battery using a carbon-based material capable of doping and dedoping lithium for a negative electrode and a lithium-containing transition metal oxide for a positive electrode. battery.