WO2016140311A1 - Separator, manufacturing method for said separator, and lithium ion secondary battery using said separator - Google Patents

Abstract

Disclosed is a separator for a lithium ion secondary battery, said separator having a layer of a decomposed product of a supporting salt on at least one surface of a substrate. This separator can inhibit short-circuiting caused by dendrite deposition of a lithium metal during lithium deposition.

Description

Separator, manufacturing method thereof, and lithium ion secondary battery using the same

The present invention relates to a separator, and more particularly, to a separator capable of suppressing a short circuit due to lithium metal dendrite precipitation. The present invention further relates to a method for manufacturing the separator and a lithium ion secondary battery using the separator.

Non-aqueous secondary batteries such as lithium ion secondary batteries have already been put into practical use as batteries for notebook computers and mobile phones due to advantages such as high energy density, low self-discharge, and excellent long-term reliability. . However, in recent years, electronic devices have been enhanced in functionality and used in electric vehicles, and there has been a demand for the development of lithium ion secondary batteries having higher energy density and superior battery characteristics.

The separator is one of the basic components of a lithium ion secondary battery. In addition to the function of preventing a short circuit between the positive electrode and the negative electrode, with regard to battery characteristics such as a safety imparting function by meltdown, cycle characteristics, rate characteristics, etc. Various studies have been conducted.

For example, as a method for improving the rate characteristics of a lithium ion secondary battery, it is advantageous to employ a separator having a high porosity that allows easy movement of lithium ions. On the other hand, in a lithium ion secondary battery using a separator having a high porosity, when lithium is precipitated during charging, there is a problem in that metallic lithium penetrates the inside of the separator and easily causes a short circuit. For such a problem, as disclosed in Patent Document 1, a separator in which a resin film having fine holes and a non-woven fabric are overlapped, or as disclosed in Patent Document 2, between a negative electrode and a separator. In addition, a method of providing at least one of a metal conductor layer, a semiconductor layer, and an insulator layer has been studied. Further, in Patent Document 3, as a method of avoiding a micro-short due to dendrites, the initial charge voltage is (open circuit voltage at full charge−2000 mV) or more at the first charge (open circuit voltage at full charge−25 mV). A method for producing a lithium ion secondary battery is disclosed.

Japanese Patent No. 2732371 Japanese Patent No. 2943127 JP 2014-17209 A

However, the batteries described in Patent Documents 1 and 2 described above have problems such as an increase in resistance accompanying an increase in the thickness of the separator, and a man-hour and cost for manufacturing the separator. Further, the method described in Patent Document 3 is still insufficient in terms of preventing a short circuit due to lithium deposition while maintaining good battery characteristics.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a lithium ion secondary battery separator capable of suppressing a short circuit due to dendrite deposition of lithium metal during lithium deposition.

Means to solve the problem

One embodiment of the present invention relates to a separator for a lithium ion secondary battery, characterized by having a decomposition layer of a supporting salt on at least one surface of a substrate.

According to the present invention, it is possible to provide a lithium ion secondary battery separator capable of suppressing a short circuit due to lithium metal dendrite precipitation during lithium deposition.

It is typical sectional drawing which shows the structure of the battery element which the secondary battery which concerns on one Embodiment of this invention has. It is a SEM image of the separator which concerns on this invention in an Example. It is a graph which shows the result of the IR analysis of the layer formed in the separator surface which concerns on this invention in an Example (spectrum (lower)). In addition, the spectrum of the separator which does not have the decomposition layer of a support salt is shown together (spectrum (upper)). It is a schematic diagram which shows an example of the electric vehicle provided with the secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram which shows an example of the electrical storage equipment provided with the secondary battery which concerns on one Embodiment of this invention. It is a disassembled perspective view which shows the basic structure of a film-clad battery. It is sectional drawing which shows the cross section of the battery of FIG. 6 typically.

<Separator>
The separator according to the present invention is a separator for a lithium ion secondary battery having a decomposition product layer of a supporting salt on at least one surface of a substrate.

According to the present invention, it is possible to suppress short-circuiting due to lithium metal dendrite precipitation during lithium deposition and improve the short-circuit resistance of the lithium ion secondary battery. The reason for this is not clear, but is estimated as follows. That is, in the lithium ion secondary battery, when gas is generated from the positive electrode or the negative electrode during charging and discharging, the gas stays in the separator and obstructs the flow of ions, and as a result, lithium is liable to precipitate. When the metallic lithium deposited on the negative electrode further grows and reaches the positive electrode, a short circuit occurs. When the separator according to the present invention is used, the decomposition product layer of the supporting salt formed on the base material of the separator prevents air bubbles from entering the separator. For this reason, precipitation of lithium becomes difficult to occur. Furthermore, even when lithium deposition occurs, the deposited metal lithium can be prevented from reaching the positive electrode from the negative electrode, and a short circuit due to lithium deposition can be prevented. Furthermore, since the decomposition product layer is a decomposition product layer of the supporting salt, the Li ion conductivity is high, and the resistance is higher than that of a separator having a two-layer structure in which a polyolefin microporous film or the like is laminated on a substrate. Can be small.

Hereinafter, the structure of the separator according to the present invention will be described in more detail.

The separator according to the present invention uses a porous film or a nonwoven fabric as a base material, and a support salt (Li salt) decomposition product is supported on at least one side of the base material.

As the substrate, a porous film or a nonwoven fabric having a thickness of 5 μm or more and 50 μm or less, more preferably 10 μm or more and 30 μm or less can be used. The base material preferably has a plurality of pores or voids continuously in the thickness direction of the base material, and does not have a straight path passing from one surface of the base material to the opposite surface.

The manifestation of the effect of the present invention does not depend on the physical properties of the base material itself. For example, polyamide, especially wholly aromatic polyamide (aramid), polyimide, polyethylene terephthalate (PET), polyphenylene sulfide (PPS) Resin materials such as cellulose, polyethylene, and polypropylene can be used.

In one embodiment, from the viewpoint of heat resistance of the lithium ion secondary battery, a high heat resistant resin material having a heat melting temperature or a thermal decomposition temperature of 160 ° C. or higher, preferably 180 ° C. or higher can be preferably used. Examples of the high heat-resistant resin material include polyethylene terephthalate, cellulose, aramid, polyimide, polyamide, polyphenylene sulfite resin, and the like, among which polyimide, polyphenylene sulfite, and aramid resin are preferable. In this specification, “thermal melting temperature” represents a temperature measured by differential scanning calorimetry (DSC) according to JIS K 7121, and “thermal decomposition temperature” refers to air using a thermogravimetric measuring device. Represents the temperature at which the 10% weight is reduced when the temperature is increased from 25 ° C. to 10 ° C./min in an air stream (10% weight loss temperature). The lower one of the thermal melting temperature and the thermal decomposition temperature represents 160 ° C. or higher.

Here, the aramid resin is an aromatic polyamide in which one or more aromatic groups are directly connected by an amide bond. Examples of the aromatic group include a phenylene group, and two aromatic rings may be bonded with oxygen, sulfur, or an alkylene group (for example, a methylene group, an ethylene group, a propylene group, etc.). . These divalent aromatic groups may have a substituent, and examples of the substituent include an alkyl group (for example, a methyl group, an ethyl group, a propyl group, etc.), an alkoxy group (for example, a methoxy group, Ethoxy group, propoxy group and the like), halogen (chloro group and the like) and the like. The aramid bond may be either a para type or a meta type. Examples of aramids include, but are not limited to, polymetaphenylene isophthalamide, polyparaphenylene terephthalamide, copolyparaphenylene 3,4'-oxydiphenylene terephthalamide.

The base material may be composed of two or more different components, and two or more porous films and / or nonwoven fabrics having different components and / or physical properties may be used in combination.

The porosity of the substrate is preferably more than 55% and 85% or less, and more preferably 60% or more and 80% or less. A separator with a large porosity is advantageous in that it has a large amount of electrolyte solution held inside and is advantageous for long-term use, a reduced amount of resin used for the base material, high ionic conductivity, and excellent rate characteristics. There is a point. On the other hand, if the porosity is too large, the mechanical strength may decrease.

When a nonwoven fabric or a porous film having a high porosity is used as the separator, the problem that the gas generated during charging / discharging penetrates into the separator and lithium is liable to occur becomes more prominent. However, according to the present invention, the decomposition layer of the supporting salt formed on the separator base material can suppress gas intrusion into the separator and suppress short-circuiting due to lithium deposition. Even when a base material is employed, a short circuit can be effectively prevented.

In addition, the porosity of a base material measured a bulk density according to JISP8118,
Porosity (%) = [1− (bulk density ρ (g / cm ³ ) / theoretical density of material ρ ₀ (g / cm ³ ))] × 100
Can be calculated as It can also be measured by a direct observation method using an electron microscope, a press-fitting method using a mercury porosimeter, or the like.

The average void diameter of the voids of the substrate is not particularly limited, but is preferably 0.05 μm or more and 10 μm or less, and more preferably 0.1 μm or more and 5 μm or less. The void diameter can be measured by the bubble point method or the mean flow method described in SIM-F-316. The average void diameter can be an average value of measured values at any five locations on the substrate.

The average thickness of the decomposed layer of the supporting salt is desirably 1 to 20 μm, and preferably 1 to 10 μm. When the thickness of the decomposition product layer is within the above range, a higher short-circuit prevention effect can be obtained, and a decrease in battery life characteristics can be suppressed. In addition, the thickness of the decomposition product layer of the supporting salt can be measured using a cross-sectional SEM (scanning electron microscope) image of the separator according to the present invention, and the average thickness is an arbitrary five-point thickness. It can be calculated as an average value.

The coverage of the decomposed layer of the supporting salt is preferably 5% or more, more preferably 10% or more, and more preferably 15% or more from the viewpoint of the effect of suppressing gas intrusion into the separator. Further preferred. Moreover, the upper limit of a coverage is not specifically limited, 100% may be sufficient. The coverage of the decomposed layer of the supporting salt can be measured by EDX analysis of the surface SEM (scanning electron microscope) image of the separator according to the present invention, and P or F is detected by EDX analysis. It can be obtained by the formula of area of area / area of visual field × 100 (%). The average coverage can be calculated as an average value of the coverage calculated from arbitrary five points of view.

In the present invention, the “supported salt decomposition product layer” means a layer containing a compound produced by decomposition of the supporting salt in the electrolyte solution of the lithium ion secondary battery by the first charge.

The layer containing the decomposition product of the supporting salt is formed on the substrate because the separator surface is subjected to infrared absorption spectrum measurement (IR measurement), nuclear magnetic resonance (NMR), time-of-flight secondary ion mass spectrometry (TOF- SIMS), energy dispersive X-ray analysis (EDX analysis), and the like, and can be confirmed by detecting elements or partial structures constituting the supporting salt.

As an example, when the supporting salt contains LiPF ₆ , the decomposition layer of the supporting salt formed on the separator substrate has an absorption exhibiting a PF bond at 825 to 865 nm (around 845 nm) in IR analysis, preferably maximum Shows absorption. Therefore, the decomposition product layer includes at least Li, P, F, O, and C, and includes a compound having a PF bond as at least a part thereof, and is a complex inorganic layer (including organic matter) insoluble in the electrolytic solution. It may be out). The compound having a PF bond is, for example, a phosphate ester compound in which at least one oxygen atom is substituted with fluorine.

Further, when the decomposition product layer was analyzed in detail by EDX analysis of a cross-sectional SEM image, the surface was mainly composed of a layer having a PF bond, but the inside was formed of a layer mainly composed of oxygen. . Since the strength of P, F, etc. is weak from the inner layer, it can be estimated that the layer is mainly composed of Li ₂ O or Li ₂ CO ₃ . Therefore, the decomposition product layer preferably has a different composition between the surface side of the layer and the inside of the layer, and a large proportion of the compound having a PF bond on the surface side, and a proportion of the compound containing P and / or F inside. less, for example, the proportion of such Li, compounds containing O and C as Li ₂ O or _Li 2 CO ₃ is higher inorganic layer.

The present invention further relates to a method for producing a separator for a lithium ion secondary battery having a decomposition layer of a supporting salt on at least one side of a substrate.

The separator according to the present invention can be produced, for example, by the following method. A laminated body in which only the base material portion of the separator is sandwiched between the positive electrode and the negative electrode is prepared, and an electrolytic solution containing a supporting salt (Li salt) is enclosed to prepare at least two cells. The first cell is charged with a predetermined current value, the voltage at the point where the voltage starts to drop temporarily is measured, and this voltage is defined as V ₀ . The voltage V ₀ is estimated to be a voltage that causes a short circuit due to lithium deposition. In the second and subsequent cells, the Li salt decomposition layer is formed by performing initial charging until the voltage reaches within V ₀ ± 0.1 V by constant current charging under the same conditions as in the first cell. Can be formed. If the voltage at the end of the initial charge is too low, an effective Li salt decomposition product layer cannot be formed, and the short-circuit prevention effect decreases. In addition, when the voltage at the end of the initial charge is too high, the supporting salt is excessively consumed, resulting in deterioration of the subsequent cycle characteristics, and an excessively decomposed layer is formed, resulting in an increase in resistance and a decrease in life. There is a risk of connection.

The charging rate employed in the above manufacturing method is not particularly limited, but from the viewpoint of production efficiency, the current value for charging the initial charge capacity of the cell in 1 hour is 1 C, and is 0.05 C to 0.5 C. A constant current is preferred. Further, from the viewpoint of preventing short circuit, it is preferable to discharge quickly after the voltage reaches V ₀ ± 0.1 V in the first charge of the second and subsequent cells. Note that one or more charging / discharging cycles may be performed before the voltage reaches V ₀ ± 0.1 V, but from the viewpoint of production efficiency, up to the voltage V ₀ ± 0.1 V in the first charging. It is preferable to perform charging.

Hereinafter, the configuration of the lithium secondary battery using the separator according to the present invention will be described.

There are various types of secondary batteries such as a cylindrical type, a flat wound square type, a laminated square type, a coin type, a flat wound laminate type, and a laminated laminate type, depending on the structure and shape of the electrode. The present invention is applicable to any of these types. Among these, the shape of the secondary battery to which the present invention is applied is preferably a laminated laminate type from the viewpoint of excellent heat dissipation when the battery element generates heat. Hereinafter, a laminated laminate type secondary battery will be described.

A laminated laminate type secondary battery has a battery element and an outer package in which the battery element is sealed. A schematic cross-sectional view of the battery element is shown in FIG. As shown in FIG. 1, the battery element has a configuration in which a plurality of negative electrodes a and a plurality of positive electrodes c are alternately stacked with separators b each having a decomposed layer of a supporting salt according to the present invention. Can do. According to the separator according to the present invention, the decomposition product layer of the supporting salt on the base material can prevent the gas from entering the separator, and can prevent lithium precipitation. Furthermore, even when lithium is deposited, the deposited metal lithium can be prevented from reaching the positive electrode from the negative electrode. Therefore, it is more preferable that the decomposition product layer is configured to exist on at least the negative electrode side surface of the separator. The electrolytic solution is sealed in the exterior body together with the negative electrode a, the positive electrode c, and the separator b. The negative electrode a has an extension (also called a tab) protruding from the separator b. The extension is an end of the negative electrode current collector d of the negative electrode a that is not covered with the negative electrode active material. Similarly, in the positive electrode c, an extension portion (tab) that is an end portion of the positive electrode current collector e of the positive electrode c that is not covered with the positive electrode active material protrudes from the separator b. The extension part of the positive electrode c and the extension part of the negative electrode a are formed at positions that do not interfere with each other when the positive electrode c and the negative electrode a are laminated. The extensions of all negative electrodes a are collected together and connected to the negative terminal g by welding. Similarly, all the positive electrode c extensions are gathered together and connected to the positive electrode terminal f by welding.

As another aspect, a secondary battery having a structure as shown in FIGS. 6 and 7 may be used. The secondary battery includes a battery element 20, a film outer package 10 that houses the battery element 20 together with an electrolyte, and a positive electrode tab 51 and a negative electrode tab 52 (hereinafter also simply referred to as “electrode tabs”). .

As shown in FIG. 7, the battery element 20 is formed by alternately stacking a plurality of positive electrodes 30 and a plurality of negative electrodes 40 with separators 25 therebetween. In the positive electrode 30, the electrode material 32 is applied to both surfaces of the metal foil 31. Similarly, in the negative electrode 40, the electrode material 42 is applied to both surfaces of the metal foil 41.

The secondary battery in FIG. 1 has electrode tabs drawn out on both sides of the outer package. However, in the secondary battery to which the present invention can be applied, the electrode tab is drawn out on one side of the outer package as shown in FIG. It may be a configuration. Although detailed illustration is omitted, each of the positive and negative metal foils has an extension on a part of the outer periphery. The extensions of the negative electrode metal foil are collected together and connected to the negative electrode tab 52, and the extensions of the positive electrode metal foil are collected together and connected to the positive electrode tab 51 (see FIG. 7). The portions gathered together in the stacking direction between the extension portions in this way are also called “current collecting portions”.

The film outer package 10 is composed of two films 10-1 and 10-2 in this example. The films 10-1 and 10-2 are heat sealed to each other at the periphery of the battery element 20 and sealed. In FIG. 7, the positive electrode tab 51 and the negative electrode tab 52 are drawn out in the same direction from one short side of the film outer package 10 sealed in this way.

Of course, electrode tabs may be drawn from two different sides. Further, regarding the film configuration, FIGS. 6 and 7 show an example in which the cup portion is formed on one film 10-1 and the cup portion is not formed on the other film 10-2. In addition, a configuration in which a cup portion is formed on both films (not shown) or a configuration in which neither cup portion is formed (not shown) may be employed.

Hereinafter, each element will be described in detail.

<Negative electrode>
The negative electrode has a negative electrode current collector formed of a metal foil, and a negative electrode active material coated on both surfaces of the negative electrode current collector. The negative electrode active material is bound so as to cover the negative electrode current collector with a negative electrode binder. The negative electrode current collector is formed to have an extension connected to the negative electrode terminal, and the negative electrode active material is not applied to the extension.

The negative electrode active material in the present embodiment is not particularly limited. For example, a carbon material that can occlude and release lithium ions, a metal that can be alloyed with lithium, a metal oxide that can occlude and release lithium ions, and the like. Is mentioned.

Examples of the carbon material include graphite (natural graphite, artificial graphite, etc.), amorphous carbon, diamond-like carbon, carbon nanotube, or a composite thereof. Here, carbon with high crystallinity has high electrical conductivity, and is excellent in adhesiveness and voltage flatness with a negative electrode current collector made of a metal such as copper. On the other hand, since amorphous carbon having low crystallinity has a relatively small volume expansion, it has a high effect of relaxing the volume expansion of the entire negative electrode, and deterioration due to non-uniformity such as crystal grain boundaries and defects hardly occurs.

Examples of the metal include Al, Si, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, Zn, La, and alloys of two or more thereof. Moreover, you may use these metals or alloys in mixture of 2 or more types. These metals or alloys may contain one or more non-metallic elements.

Examples of the metal oxide include silicon oxide, aluminum oxide, tin oxide, indium oxide, zinc oxide, lithium oxide, and composites thereof. The negative electrode active material preferably contains tin oxide or silicon oxide, and more preferably contains silicon oxide. This is because silicon oxide is relatively stable and hardly causes a reaction with other compounds. Moreover, it is preferable that the whole or one part has an amorphous structure. An amorphous structure is considered to have relatively few elements due to non-uniformity such as grain boundaries and defects. It can be confirmed by X-ray diffraction measurement (general XRD measurement) that all or part of the metal oxide has an amorphous structure. Specifically, when the metal oxide does not have an amorphous structure, a peak specific to the metal oxide is observed. However, the metal oxide may have a case where all or part of the metal oxide has an amorphous structure. Inherent peaks are broad and observed.

In addition, a carbon material, a metal, and a metal oxide can be mixed and used independently. For example, the same kind of materials such as graphite and amorphous carbon may be mixed, or different kinds of materials such as graphite and silicon may be mixed.

Examples of the binder for the negative electrode include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer rubber, polytetrafluoroethylene, polypropylene, polyethylene, Polyimide, polyamideimide, or the like can be used. The amount of the binder for the negative electrode used is 0.5 to 25 parts by mass with respect to 100 parts by mass of the negative electrode active material from the viewpoints of “sufficient binding force” and “high energy” which are in a trade-off relationship. Is preferred.

As the negative electrode current collector, aluminum, nickel, stainless steel, chromium, copper, silver, and alloys thereof are preferable in view of electrochemical stability. Examples of the shape include foil, flat plate, and mesh.

<Positive electrode>
The positive electrode has a positive electrode current collector formed of a metal foil, and a positive electrode active material coated on both surfaces of the positive electrode current collector. The positive electrode active material is bound so as to cover the positive electrode current collector with a positive electrode binder. The positive electrode current collector is formed to have an extension connected to the positive electrode terminal, and the positive electrode active material is not applied to the extension.

As the positive electrode active _{material, LiMnO 2, Li x Mn 2} O 4 (0 <x <2), Li 2 MnO 3, Li x Mn 1.5 Ni 0.5 O 4 (0 <x <2) layer, such as Lithium manganate having a structure or lithium manganate having a spinel structure, LiCoO ₂ , LiNiO ₂ or a part of these transition metals replaced with another metal, LiNi _1/3 Co _1/3 Mn _1/3 O Lithium transition metal oxides in which a specific transition metal such as ₂ does not exceed half, Li in excess of the stoichiometric composition in these lithium transition metal oxides, LiFePO ₄ and other olivine structures, etc. Is mentioned. Further, these metal oxides were partially substituted with Al, Fe, P, Ti, Si, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, Zn, La, etc. Materials can also be used. In particular, Li _α Ni _β Co _γ Al _δ O ₂ (1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.7, γ ≦ 0.2) or Li _α Ni _β Co _γ Mn _δ O ₂ (1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.6, γ ≦ 0.2) are preferable. A positive electrode active material can be used individually by 1 type or in combination of 2 or more types.

The positive electrode active material can be selected from several viewpoints. From the viewpoint of increasing the energy density, it is preferable to include a high-capacity compound. Examples of the high-capacity compound include nickel-lithium oxide (LiNiO ₂ ) or lithium-nickel composite oxide obtained by substituting a part of nickel in nickel-lithium oxide with another metal element. The layered structure represented by the following formula (A) Lithium nickel composite oxide is preferred.

Li _y Ni _(1-x) M _x O ₂ (A)
(However, 0 ≦ x <1, 0 <y ≦ 1.2, and M is at least one element selected from the group consisting of Co, Al, Mn, Fe, Ti, and B.)

From the viewpoint of high capacity, the Ni content is high, that is, in the formula (A), x is preferably less than 0.5, and more preferably 0.4 or less. Examples of such a compound include Li _α Ni _β Co _γ Mn _δ O ₂ (0 <α ≦ 1.2, preferably 1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.7, γ ≦ 0. .2), Li _α Ni _β Co _γ Al _δ O ₂ (0 <α ≦ 1.2, preferably 1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.6, preferably β ≧ 0.7, γ ≦ 0.2), etc., especially LiNi _β Co _γ Mn _δ O ₂ (0.75 ≦ β ≦ 0.85, 0.05 ≦ γ ≦ 0.15, 0.10 ≦ δ ≦ 0.20). ). More specifically, for example, LiNi _0.8 Co _0.05 Mn _0.15 O ₂ , LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , LiNi _0.8 Co _0.15 Al _0.05 O _{2, LiNi} 0.8 _Co 0.1 _Al can be preferably used 0.1 _{O 2} or the like.

From the viewpoint of thermal stability, it is also preferable that the Ni content does not exceed 0.5, that is, in the formula (A), x is 0.5 or more. It is also preferred that the number of specific transition metals does not exceed half. Such compounds include Li _α Ni _β Co _γ Mn _δ O ₂ (0 <α ≦ 1.2, preferably 1 ≦ α ≦ 1.2, β + γ + δ = 1, 0.2 ≦ β ≦ 0.5, 0 0.1 ≦ γ ≦ 0.4, 0.1 ≦ δ ≦ 0.4). More specifically, LiNi _0.4 Co _0.3 Mn _0.3 O ₂ (abbreviated as NCM433), LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (abbreviated as NCM523), LiNi _0.5 Co _0.3 Mn _0.2 O ₂ (abbreviated as NCM532), etc. (however, the content of each transition metal in these compounds varies by about 10%) Can also be included).

In addition, two or more compounds represented by the formula (A) may be used as a mixture. For example, NCM532 or NCM523 and NCM433 range from 9: 1 to 1: 9 (typically 2 It is also preferable to use a mixture in 1). Furthermore, in the formula (A), a material having a high Ni content (x is 0.4 or less) and a material having a Ni content not exceeding 0.5 (x is 0.5 or more, for example, NCM433) are mixed. As a result, a battery having a high capacity and high thermal stability can be formed.

Also, radical materials or the like can be used as the positive electrode active material.

As the positive electrode binder, the same as the negative electrode binder can be used. The amount of the positive electrode binder to be used is preferably 2 to 15 parts by mass with respect to 100 parts by mass of the positive electrode active material from the viewpoints of “sufficient binding force” and “high energy” which are in a trade-off relationship. .

As the positive electrode current collector, for example, aluminum, nickel, silver, or an alloy thereof can be used. Examples of the shape of the positive electrode current collector include a foil, a flat plate, and a mesh. As the positive electrode current collector, an aluminum foil can be suitably used.

A conductive auxiliary material may be added to the positive electrode active material coating layer for the purpose of reducing impedance. Examples of the conductive auxiliary material include carbonaceous fine particles such as graphite, carbon black, and acetylene black. The amount of the conductive auxiliary material is preferably 0.1 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material.

<Electrolyte>
As the electrolytic solution used in the present embodiment, a nonaqueous electrolytic solution containing a supporting salt (lithium salt) and a nonaqueous solvent that dissolves the supporting salt can be used.

As the non-aqueous solvent, an aprotic organic solvent such as carbonate ester (chain or cyclic carbonate), carboxylic acid ester (chain or cyclic carboxylic acid ester), and phosphate ester can be used.

Examples of carbonate solvents include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and vinylene carbonate (VC); dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate. (EMC), chain carbonates such as dipropyl carbonate (DPC); and propylene carbonate derivatives.

Examples of the carboxylic acid ester solvent include aliphatic carboxylic acid esters such as methyl formate, methyl acetate, and ethyl propionate; and lactones such as γ-butyrolactone.

Among these, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), dipropyl carbonate Carbonic acid esters (cyclic or chain carbonates) such as (DPC) are preferred.

Examples of the phosphate ester include trimethyl phosphate, triethyl phosphate, tripropyl phosphate, trioctyl phosphate, triphenyl phosphate, and the like.

Other solvents that can be contained in the non-aqueous electrolyte include, for example, ethylene sulfite (ES), propane sultone (PS), butane sultone (BS), dioxathilane-2,2-dioxide (DD), and sulfolene. 3-methylsulfolene, sulfolane (SL), succinic anhydride (SUCAH), propionic anhydride, acetic anhydride, maleic anhydride, diallyl carbonate (DAC), dimethyl 2,5-dioxahexanoate, 2,5 Dimethyl hexane hexanoate, furan, 2,5-dimethylfuran, diphenyl disulfide (DPS), dimethoxyethane (DME), dimethoxymethane (DMM), diethoxyethane (DEE), ethoxymethoxyethane, chloroethylene carbonate , Dimethyl ether, methyl Tyl ether, methyl propyl ether, ethyl propyl ether, dipropyl ether, methyl butyl ether, diethyl ether, phenyl methyl ether, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), tetrahydropyran (THP), 1,4-dioxane (DIOX), 1,3-dioxolane (DOL), methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, methyl difluoroacetate, methyl propionate, ethyl propionate, propyl propionate, methyl formate , Ethyl formate, ethyl butyrate, isopropyl butyrate, methyl isobutyrate, methyl cyanoacetate, vinyl acetate, diphe Rudisulfide, dimethylsulfide, diethylsulfide, adiponitrile, valeronitrile, glutaronitrile, malononitrile, succinonitrile, pimonitrile, suberonitrile, isobutyronitrile, biphenyl, thiophene, methyl ethyl ketone, fluorobenzene, hexafluorobenzene, carbonate electrolyte, Glyme, ether, acetonitrile, propiononitrile, γ-butyrolactone, γ-valerolactone, dimethyl sulfoxide (DMSO) ionic liquid, aliphatic carboxylic acid esters such as phosphazene, methyl formate, methyl acetate, ethyl propionate, or the like A compound in which a part of hydrogen atoms of a compound is substituted with a fluorine atom.

Non-aqueous solvents can be used alone or in combination of two or more.

As the supporting salt in this embodiment, LiPF ₆ , LiAsF ₆ , LiAlCl ₄ , LiClO ₄ , LiBF ₄ , LiSbF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN ( A lithium salt that can be used for a normal lithium ion secondary battery such as CF ₃ SO ₂ ) ₂ can be used. The supporting salt can be used alone or in combination of two or more. The concentration of the supporting salt in the electrolytic solution is preferably 0.5 M or more and 2 M or less, and preferably 0.7 M or more and 1.5 M or less.

In one embodiment of the present invention, the electrolytic solution may further contain an additive. Although it does not specifically limit as an additive, An overcharge inhibitor, surfactant, film formation additive, etc. are mentioned.

Examples of additives include fluorinated cyclic carbonates, unsaturated cyclic carbonates, cyclic disulfonic acid esters, and the like. These compounds can form a film on the surface of the electrode active material during charging and discharging of the secondary battery, and can improve battery characteristics such as cycle characteristics.

On the other hand, these additives may cause gas generation during charging and discharging, and as a result, may increase lithium deposition. However, in the present invention, the reason is not clear, but there is a tendency that a decomposed layer of the supporting salt is more effectively formed by gas entering the separator, and when these compounds are used, A more excellent short-circuit prevention effect may be obtained.

Examples of the fluorinated cyclic carbonate include a compound represented by the following formula (1).

In the formula (1), A, B, C and D are each independently a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms or a halogenated alkyl group, and at least one of A, B, C and D One is a fluorine atom or a fluorinated alkyl group. The number of carbon atoms of the alkyl group and the halogenated alkyl group is more preferably 1 to 4, and further preferably 1 to 3.

Examples of the fluorinated cyclic carbonate include compounds in which some or all of the hydrogen atoms are substituted with fluorine atoms, such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC). -Fluoro-1,3-dioxolan-2-one (fluoroethylene carbonate: FEC) is preferred.

The content of the fluorinated cyclic carbonate is not particularly limited, but is preferably 0.01% by mass or more and 10% by mass or less, and 0.05% by mass or more and 5% by mass or less in the electrolytic solution. Is more preferable, and it is preferably 0.05% by mass or more and 3% by mass or less.

The unsaturated cyclic carbonate is a cyclic carbonate having at least one carbon-carbon unsaturated bond in the molecule. For example, vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, 4,5-dimethyl vinylene carbonate, 4,5- Vinylene carbonate compounds such as diethyl vinylene carbonate; 4-vinylethylene carbonate, 4-methyl-4-vinylethylene carbonate, 4-ethyl-4-vinylethylene carbonate, 4-n-propyl-4-vinylene ethylene carbonate, 5-methyl -4-vinylethylene carbonate, 4,4-divinylethylene carbonate, 4,5-divinylethylene carbonate, 4,4-dimethyl-5-methyleneethylene carbonate, 4,4-diethyl-5-methyle Vinyl ethylene carbonate compounds such as ethylene carbonate. Among these, vinylene carbonate or 4-vinylethylene carbonate is preferable, and vinylene carbonate is particularly preferable.

The content of the unsaturated cyclic carbonate is not particularly limited, but is preferably 0.01% by mass or more and 10% by mass or less, and 0.05% by mass or more and 5% by mass or less in the electrolytic solution. Is more preferable, and it is preferably 0.05% by mass or more and 3% by mass or less.

Examples of the cyclic disulfonic acid ester include a compound represented by the following formula (2).

In the formula (2), R ₁ and R ₂ are each independently a substituent selected from the group consisting of a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, a halogen group, and an amino group. R ₃ is an alkylene group having 1 to 5 carbon atoms, a carbonyl group, a sulfonyl group, a fluoroalkylene group having 1 to 6 carbon atoms, or an alkylene group or a fluoroalkylene unit having 2 to 6 carbon atoms bonded via an ether group. A divalent group is shown. )

In the formula (2), R ₁ and R ₂ are preferably each independently a hydrogen atom, an alkyl group having 1 to 3 carbon atoms or a halogen group, and R ₃ is an alkylene group having 1 or 2 carbon atoms. Or it is more preferable that it is a fluoroalkylene group.

Examples of preferred compounds of the cyclic disulfonic acid ester represented by the formula (2) include the following compounds, but are not limited thereto.

The content of the cyclic disulfonic acid ester is preferably 0.005% by mass or more and 10% by mass or less in the electrolytic solution, and more preferably 0.01% by mass or more and 5% by mass or less. By containing 0.005% by mass or more, a sufficient film effect can be obtained. Moreover, the raise of the viscosity of electrolyte solution and the accompanying increase in resistance can be suppressed as content is 10 mass% or less.

One additive can be used alone, or two or more additives can be used in combination. When two or more additives are used in combination, the total content of the additives is 0. It is preferable to add so that it may become 5 to 5 mass%.

<Exterior body>
The exterior body can be appropriately selected as long as it is stable to the electrolytic solution and has a sufficient water vapor barrier property. For example, in the case of a laminated laminate type secondary battery, it is preferable to use a laminate film of aluminum and resin as the outer package. An exterior body may be comprised with a single member and may be comprised combining several members.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1
A battery as shown below was prepared using an aramid nonwoven fabric film having a thickness of 25 μm and a porosity of 70% as the base material.

(Positive electrode)
LiNi _0.8 Co _0.15 Al _0.05 , a carbon conductive agent (acetylene black), and polyvinylidene fluoride as a binder are dispersed in N-methyl-2-pyrrolidone at a weight ratio of 92: 4: 4. A slurry was prepared, applied to a current collector foil made of aluminum, and dried to form a positive electrode active material layer. Similarly, after forming an active material layer on the back surface of the current collector foil made of aluminum, it was rolled to obtain a positive electrode plate.

(Negative electrode)
Natural graphite, sodium carboxymethyl methylcellulose as a thickener, and styrene butadiene rubber as a binder are mixed in an aqueous solution at a weight ratio of 98: 1: 1 to prepare a slurry, which is applied to a copper current collector foil. And dried to form a negative electrode active material layer. Similarly, after forming an active material layer on the back surface of the current collector foil made of copper, a negative electrode plate was obtained by rolling.

(Electrolyte)
As the non-aqueous solvent for the electrolytic solution, a non-aqueous solvent in which EC and DEC were mixed at a volume ratio of 30:70 was used. LiPF ₆ was dissolved as a supporting salt to a concentration of 1M. Further, Compound (2-1), vinylene carbonate, and fluoroethylene carbonate were added by 1% by weight.

(Production of battery)
A positive electrode plate and a negative electrode plate were laminated via a separator base material to produce an electrode body. A current extraction terminal was connected to each of the laminated positive electrode plate and negative electrode plate, and accommodated in a laminate film outer package of aluminum and resin. After injecting the electrolyte into the outer package, the outer package was sealed under reduced pressure to obtain a battery. The size of the electrode body was adjusted so that the initial charge capacity of the cell was 100 mAh.

(First charging process)
When the first battery was charged at 30 mA, the voltage temporarily dropped at 3.8V. The second battery was charged until it reached 3.8 V with a similar current value. Thereafter, discharge was performed at 100 mA until 2.5 V was reached, and a Li salt decomposition product layer was formed on the substrate. The following measurements were performed using this second cell.

(Battery characteristics evaluation)
The produced battery was charged in a constant current constant voltage mode at a current value of 30 mA up to a battery voltage of 4.2 V. By charging, the voltage of the battery increased to 4.2 V, and no decrease in battery voltage was observed during or after charging.

When the above cell was disassembled and the separator was observed, it was found that a microporous layer was formed on the substrate (FIG. 2). Further, in the IR analysis of the surface of the formed layer, the maximum absorption was observed at around 845 nm (FIG. 3), which is considered to be a component containing a PF bond derived from a LiPF ₆ decomposition product. For IR analysis, Spectrum Spotlight 200 (manufactured by PerkinElmer) and an MCT detector as a detector were used. The frequency region of 4000 to 7000 cm −1 was measured by the ATR method. The number of integrations was 43. The average thickness of the decomposed layer of the supporting salt was 5 μm, and the coverage was 15%.

Reference example 1
A battery was produced in the same procedure as in Example 1.

(First charging process)
When the first battery was charged at 30 mA, the voltage temporarily dropped at 3.8V. The second battery was charged with similar current values until it reached 3.5V. Thereafter, discharging was performed at 100 mA until 2.5V was reached.

When the above cell was disassembled and the separator was observed, the average thickness of the support salt decomposition product layer was 0.5 μm, and the coverage was 2%.

(Battery characteristics evaluation)
The produced battery was charged in a constant current constant voltage mode at a current value of 30 mA up to a battery voltage of 4.2 V. During charging, it was observed that the voltage temporarily dropped around 3.9 V and the positive and negative electrodes were short-circuited.

In the battery of Example 1, the layer containing the decomposition product of the supporting salt is effectively formed on the separator by charging up to a voltage immediately before the short-circuit due to lithium deposition at the time of initial charging and discharging immediately after that. It is presumed that the decrease in battery voltage due to a short circuit could be suppressed in the second and subsequent charging / discharging. On the other hand, in the battery of Reference Example 1, the effective support salt decomposition layer was not formed at the time of the initial charge, so that invasion of gas into the separator could not be suppressed during the second charge and discharge, and the positive and negative electrodes were short-circuited. Estimated.

Comparative Example 1
A cell was prepared in the same procedure as in Example 1 except that a polypropylene film having a thickness of 25 μm and a porosity of 55% was used as the substrate, and an initial charging process was performed.

When the above cell was disassembled and the base material was observed, a decomposition layer of Li salt could not be confirmed.

Comparative Example 1 suggests that when a separator that does not contain gas is used, a decomposition layer of the supporting salt is not formed.

The battery according to the invention can be used, for example, in all industrial fields that require a power source, as well as in industrial fields related to the transport, storage and supply of electrical energy. Specifically, power supplies for mobile devices such as mobile phones and notebook computers; power supplies for transportation and transportation media such as trains, satellites, and submarines, including electric vehicles such as electric cars, hybrid cars, electric bikes, and electric assist bicycles A backup power source such as a UPS; a power storage facility for storing power generated by solar power generation, wind power generation, etc .;

FIG. 4 and FIG. 5 show an electric vehicle 200 and a power storage facility 300, respectively, as examples of the various devices and power storage facilities described above. Electric vehicle 200 and power storage facility 300 have assembled batteries 210 and 310, respectively. The assembled batteries 210 and 310 are configured by connecting a plurality of batteries having the above-described separator according to the present invention in series and in parallel to satisfy required capacity and voltage.

a negative electrode b separator c positive electrode d negative electrode current collector e positive electrode current collector f positive electrode terminal g negative electrode terminal 10 film outer package 20 battery element 25 separator 30 positive electrode 40 negative electrode 200 electric vehicle 210, 310 assembled battery 300 power storage equipment

Claims (15)

1. A separator for a lithium ion secondary battery, comprising a base salt decomposition product layer on at least one side of a substrate.
2. The lithium ion secondary battery separator according to claim 1, wherein the porosity of the substrate is 60% or more.
3. The separator for a lithium ion secondary battery according to claim 1 or 2, wherein the substrate includes a nonwoven fabric.
4. The lithium ion secondary battery separator according to any one of claims 1 to 3, wherein the base material contains a resin having a heat melting temperature or a heat decomposition temperature of 160 ° C or higher.
5. The lithium ion secondary battery separator according to any one of claims 1 to 4, wherein the base material contains an aramid resin.
6. The lithium ion secondary battery separator according to any one of claims 1 to 5, wherein the decomposition product layer of the supporting salt has an absorption detectable at 825 to 865 nm in IR measurement.
7. The lithium ion secondary battery separator according to claim 6, wherein the decomposition layer of the supporting salt has a maximum absorption at 825 to 865 nm in IR measurement.
8. The separator for a lithium ion secondary battery according to any one of claims 1 to 7, wherein an average thickness of a decomposition product layer of the supporting salt is 1 µm or more and 20 µm or less.
9. The separator for a lithium ion secondary battery according to any one of claims 1 to 8, wherein an average coverage of the substrate by the decomposed layer of the supporting salt is 10% or more and 100% or less.
10. A lithium ion secondary battery comprising the lithium ion secondary battery separator according to any one of claims 1 to 9.
11. The lithium ion secondary battery according to claim 10, wherein the decomposition product layer of the supporting salt is present at least on the negative electrode side of the separator.
12. An electric vehicle comprising the lithium ion secondary battery according to claim 10 or 11.
13. A power storage facility comprising the lithium ion secondary battery according to claim 10 or 11.
14. A method for producing a separator for a lithium ion secondary battery having a decomposition product layer of a supporting salt on at least one side of a substrate,
    Producing two or more lithium ion secondary batteries having a positive electrode, a negative electrode, a separator substrate, and an electrolyte containing a supporting salt;
    Charging a first lithium ion secondary battery at a predetermined current value (I) and measuring a voltage (V ₀ ) at which the voltage temporarily decreases;
    Charging the remaining lithium ion secondary battery until the voltage reaches V ₀ ± 0.1 V at the current value (I), and then discharging,
    A separator manufacturing method, comprising:
15. A method for producing a lithium ion secondary battery comprising a separator having a decomposition product layer of a supporting salt on at least one side of a substrate,
    Producing two or more lithium ion secondary batteries having a positive electrode, a negative electrode, a separator substrate, and an electrolyte containing a supporting salt;
    Charging a first lithium ion secondary battery at a predetermined current value (I) and measuring a voltage (V ₀ ) at which the voltage temporarily decreases;
    Charging the remaining lithium ion secondary battery at the current value (I) until the voltage reaches V ₀ ± 0.1 V, and then discharging,
    A method for producing a lithium ion secondary battery, comprising:
    

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