Non-aqueous secondary batteries

Abstract

To provide a nonaqueous electrolytic solution which, even when containing acetonitrile, is good in impregnation into a polypropylene-based separator having a specific porous structure and which also has high output performance, and a nonaqueous secondary battery containing the same.SOLUTION: Provided is a nonaqueous secondary battery including a nonaqueous electrolytic solution and a separator. The nonaqueous electrolytic solution contains: a nonaqueous solvent containing 5-90 vol.% of acetonitrile; a lithium salt; and a surfactant. The lithium salt contains a lithium-containing imide salt, and is contained by 1-2 mol / L relative to the nonaqueous electrolytic solution. The surfactant contains a nonionic surfactant and does not contain a hydroxy group as a substituent. The nonionic surfactant is a fatty acid ester compound and the like, and has a substituent with 3 or more carbon atoms. The separator includes a polypropylene-based microporous film. The polypropylene-based microporous film includes polypropylene-containing polymer matrices extending in one direction perpendicular to a film thickness, and has polypropylene-containing fibrils between the polymer matrices and porous holes between the fibrils.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous secondary battery. [Background technology]

[0002] BACKGROUND ART Non-aqueous secondary batteries such as lithium ion batteries are characterized by their small size, high capacity, and high output, and are widely used as power sources for various portable electronic devices.

[0003] To realize such a small, high-capacity battery, the selection of the separator placed between the electrodes is important. Traditionally, separators have been made of paper, woven fabric, nonwoven fabric, glass mat, etc., but these have safety issues due to their structure. In recent years, separators with small pores, such as polyolefin microporous membranes, have been used, which have improved safety by providing a shutdown function and preventing short circuits between electrodes.

[0004] Furthermore, conventionally, a solvent of cyclic carbonate or cyclic ester has been used in a non-aqueous electrolyte for a non-aqueous secondary battery, which has had the problem of poor impregnation of the separator with the electrolyte.

[0005] However, in recent years, the impregnation of the separator has been improved by diluting a chain carbonate ester in a non-aqueous electrolyte solution using a cyclic carbonate ester or a cyclic ester solvent and further adding a surfactant. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2-244565 [Patent Document 2] Japanese Patent Application Publication No. 7-263027 [Patent Document 3] Japanese Patent Application Publication No. 8-162155 [Patent Document 4] Japanese Patent Application Laid-Open No. 2008-71559 Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, compared to nonaqueous electrolytes using esters as solvents, nonaqueous electrolytes containing acetonitrile have higher ionic conductivity, and therefore, when applied to nonaqueous secondary batteries, high output performance can be achieved. However, it has been found that nonaqueous electrolytes containing acetonitrile pose a problem in battery output performance when combined with polypropylene separators having specific pore structures. After extensive investigation into this issue, it has been discovered that the impregnation or output performance of nonaqueous electrolytes containing acetonitrile decreases when polypropylene separators with small pore sizes are used, particularly when separators manufactured by a dry process are used.

[0008] Patent Documents 1 to 3 report that the impregnation of the separator can be improved by adding a phosphorus-based compound or a surfactant to a non-aqueous electrolyte solution, but they do not mention the difference in impregnation depending on the separator structure when an acetonitrile-containing electrolyte solution is used.

[0009] Patent Document 4 reports an electrolyte solution with high cycle life or thermal stability without reducing separator impregnation by adding a specific cyclic compound in addition to a surfactant to the nonaqueous electrolyte solution. However, in the case of a nonaqueous secondary battery containing the nonaqueous electrolyte solution described in Patent Document 4, if the solid electrolyte interface (SEI) coating is strengthened too much, increasing the coating resistance too much, output performance will be lost even if a nonaqueous electrolyte solution with high ionic conductivity is used. Furthermore, Patent Document 4 does not mention the difference in impregnation depending on the separator structure when an acetonitrile electrolyte solution is used.

[0010] The present invention has been made in view of the above-mentioned problems, and aims to provide a nonaqueous electrolyte solution that has good impregnation into a polypropylene-based separator having a specific pore structure and has high output performance, even if the nonaqueous electrolyte solution contains acetonitrile, and a nonaqueous secondary battery containing the same. [Means for solving the problem]

[0011] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result have found that the above problems can be solved by using a nonaqueous electrolyte solution having the following composition in a specific nonaqueous secondary battery, thereby completing the present invention. <1> A non-aqueous secondary battery comprising a positive electrode containing one or more positive electrode active materials capable of absorbing and releasing lithium ions, a negative electrode containing a material capable of absorbing and releasing lithium ions, a non-aqueous electrolyte solution, and a separator, the non-aqueous electrolyte solution contains a non-aqueous solvent, a lithium salt, and a surfactant; the non-aqueous solvent contains 5% by volume or more and 90% by volume or less of acetonitrile, the lithium salt includes a lithium-containing imide salt, and its content is 1 mol / L or more and 2 mol / L or less relative to the nonaqueous electrolyte solution; the surfactant is a compound that includes a nonionic surfactant and does not include a hydroxy group as a substituent, the nonionic surfactant is at least one compound selected from a fatty acid ester compound, a phosphate ester compound, an ether compound, an ester ether compound, and an alkanolamide compound; The nonionic surfactant has at least one substituent having 3 or more carbon atoms, the separator has a polypropylene-based microporous film, The polypropylene-based microporous membrane has a polymer matrix containing polypropylene extending in one direction perpendicular to the membrane thickness, and has fibrils containing the polypropylene between the polymer matrices and micropores between a plurality of the fibrils. A non-aqueous secondary battery characterized by: <2> 2. The non-aqueous secondary battery according to item 1, wherein the nonionic surfactant is the fatty acid ester compound or the phosphoric acid ester compound. <3> the fatty acid ester compound is ethyl laurate, or 3. The nonaqueous secondary battery according to item 2, wherein the phosphate ester compound is triamyl phosphate, tris(2-butoxyethyl) phosphate, or tris(2-ethylhexyl) phosphate. <4> 4. The non-aqueous secondary battery according to any one of items 1 to 3, wherein the content of the non-ionic surfactant is 0.1% by mass or more and 3% by mass or less with respect to the total amount of the non-aqueous electrolyte solution. <5> 5. The nonaqueous secondary battery according to any one of items 1 to 4, wherein the polypropylene-based microporous membrane has a pore size ratio a / b of an average pore size a to a pore size b perpendicular to the average pore size a, of 0.5 or more and 30 or less. <6> The separator has a porosity of 30% or more and 80% or less, and an air permeability of 100 seconds / 100 cm 3 More than 500 seconds / 100cm 3 6. The nonaqueous secondary battery according to any one of items 1 to 5, wherein: <7> 7. The nonaqueous secondary battery according to any one of items 1 to 6, wherein the lithium salt further contains LiPF6, and the lithium-containing imide salt and LiPF6 are contained at molar concentrations such that LiPF6<the lithium-containing imide salt. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a nonaqueous secondary battery that has good impregnation of a nonaqueous electrolyte solution in the thickness direction of a separator and has high output performance. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a plan view schematically illustrating an example of a nonaqueous secondary battery according to an embodiment of the present invention. [Figure 2] 2 is a cross-sectional view of the nonaqueous secondary battery of FIG. 1 taken along line AA. [Figure 3] 1 is a scanning electron microscope (SEM) image of a microporous polypropylene membrane. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail. The present invention is not limited to the following embodiment, and various modifications are possible without departing from the gist of the present invention. In this specification, a numerical range indicated using "to" includes the numerical values ​​before and after it.

[0015] [Non-aqueous secondary battery] The nonaqueous electrolyte solution according to this embodiment can be used to form a nonaqueous secondary battery.

[0016] The non-aqueous secondary battery according to this embodiment is configured by housing a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte solution in an appropriate battery exterior.

[0017] Specifically, the nonaqueous secondary battery according to this embodiment may be a nonaqueous secondary battery 100 shown in Figures 1 and 2. Here, Figure 1 is a plan view schematically showing the nonaqueous secondary battery, and Figure 2 is a cross-sectional view taken along line AA in Figure 1.

[0018] The nonaqueous secondary battery 100 shown in FIGS. 1 and 2 is a pouch-type cell. The nonaqueous secondary battery 100 accommodates a laminated electrode assembly formed by stacking a positive electrode 150 and a negative electrode 160 with a separator 170 interposed therebetween, and a nonaqueous electrolyte (not shown) within a space 120 of a battery exterior 110. The battery exterior 110 is made of, for example, an aluminum laminate film, and the outer periphery of the space formed by the two aluminum laminate films is sealed by heat-sealing the upper and lower films. The laminate, in which the positive electrode 150, the separator 170, and the negative electrode 160 are stacked in this order, is impregnated with a nonaqueous electrolyte. However, in FIG. 2, to avoid cluttering the drawing, the layers constituting the battery exterior 110 and the layers of the positive electrode 150 and the negative electrode 160 are not distinguished from one another.

[0019] The aluminum laminate film that constitutes the battery exterior 110 is preferably an aluminum foil coated on both sides with a polyolefin resin.

[0020] The positive electrode 150 is connected to the positive electrode lead body 130 within the nonaqueous secondary battery 100. Although not shown, the negative electrode 160 is also connected to the negative electrode lead body 140 within the nonaqueous secondary battery 100. One end of each of the positive electrode lead body 130 and the negative electrode lead body 140 is drawn out to the outside of the battery exterior 110 so that they can be connected to external devices, and the ionomer portions of these lead bodies are heat-sealed to one side of the battery exterior 110.

[0021] 1 and 2 has a laminated electrode body with one positive electrode 150 and one negative electrode 160, but the number of laminated positive electrodes 150 and negative electrodes 160 can be increased as needed depending on the capacity design. In the case of a laminated electrode body with multiple positive electrodes 150 and multiple negative electrodes 160, tabs of the same electrode may be joined together by welding or the like and then joined to a single lead body by welding or the like, and then taken out of the battery. The tabs of the same electrode may be formed from an exposed portion of a current collector, or may be formed by welding a metal piece to the exposed portion of a current collector, or the like.

[0022] The positive electrode 150 is composed of a positive electrode current collector and a positive electrode active material layer, and the negative electrode 160 is composed of a negative electrode current collector and a negative electrode active material layer.

[0023] The positive electrode active material layer contains a positive electrode active material, and the negative electrode active material layer contains a negative electrode active material.

[0024] The positive electrode 150 and the negative electrode 160 are arranged with the separator 170 interposed between them so that the positive electrode active material layer and the negative electrode active material layer face each other.

[0025] Hereinafter, each element constituting the nonaqueous secondary battery according to this embodiment will be described in order.

[0026] [Positive electrode] In the nonaqueous secondary battery according to this embodiment, the positive electrode contains one or more positive electrode active materials capable of absorbing and releasing lithium ions, and may have a positive electrode active material layer on one or both sides of the positive electrode current collector, as desired.

[0027] [Positive electrode current collector] The positive electrode current collector is made of a metal foil such as aluminum foil, nickel foil, or stainless steel foil. The surface of the positive electrode current collector may be coated with carbon or processed into a mesh. The thickness of the positive electrode current collector is preferably 5 to 40 μm, more preferably 7 to 35 μm, and even more preferably 9 to 30 μm.

[0028] [Cathode active material layer] The positive electrode active material layer contains a positive electrode active material, and may further contain a conductive additive and / or a binder as necessary.

[0029] (Cathode active material) The positive electrode active material layer preferably contains a material capable of absorbing and releasing lithium ions as the positive electrode active material, since use of such a material tends to enable a high voltage and a high energy density to be obtained.

[0030] Examples of the positive electrode active material include: A positive electrode active material containing at least one transition metal element selected from the group consisting of Ni, Mn, and Co; include the following general formula (a): Li p Ni q Co r Mn s M t O u ·····(a) {In the formula, M is at least one metal selected from the group consisting of Al, Sn, In, Fe, V, Cu, Mg, Ti, Zn, Mo, Zr, Sr, and Ba, and 0 < p < 1.3, 0 < q < 1.2, 0 < r < 1.2, 0 ≤ s < 0.5, 0 ≤ t < 0.3, 0.7 ≤ q + r + s + t ≤ 1.2, 1.8 < u < 2.2, and p is a value determined by the charge and discharge state of the battery.} At least one selected from lithium-containing metal oxides represented by is preferable.

[0031] Further, as the positive electrode active material, for example, Lithium cobalt oxide represented by LiCoO2; Lithium manganese oxides represented by LiMnO2, LiMn2O4, and Li2Mn2O4; Lithium nickel oxide represented by LiNiO2; LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.8 Co 0.2 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.75 Co 0.15 Mn 0.15 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.85 Co 0.075 Mn 0.075 O2, LiNi 0.8 Co 0.15 Al 0.05O2, LiNi 0.81 Co 0.1 Al 0.09 O2, and LiNi 0.85 Co 0.1 Al 0.05 Li represented by O2 z A lithium-containing composite metal oxide represented by LiMO2 (where M contains at least one transition metal element selected from the group consisting of Ni, Mn, and Co, and represents two or more metal elements selected from the group consisting of Ni, Mn, Co, Al, and Mg, and z represents a number greater than 0.9 and less than 1.2); MnO2, FeO2, FeS2, V2O5, V6O 13 Metal oxides or metal chalcogenides having a tunnel structure and a layered structure, represented by, TiO2, TiS2, MoS2, and NbSe2; Sulfur; and Conductive polymers represented by polyaniline, polythiophene, polyacetylene, and polypyrrole; etc. can be mentioned.

[0032] In particular, when the Ni content ratio q of the Li-containing metal oxide represented by the general formula (a) is 0.5 < q < 1.2, it is preferable because both the reduction of the usage amount of Co, which is a rare metal, and the increase in the energy density are achieved.

[0033] Here, as the Ni content ratio of the Li-containing metal oxide increases, the deterioration tends to progress at a low voltage. In the positive electrode active material of the lithium-containing metal oxide represented by the general formula (a), there are active points that oxidatively deteriorate the non-aqueous electrolyte, and these active points may consume, unintentionally on the positive electrode side, the compound added to protect the negative electrode. Among them, acid anhydrides tend to be particularly affected. In particular, when the electrolyte contains acetonitrile as a non-aqueous solvent, the addition effect of the acid anhydride is extremely large, so the consumption of the acid anhydride on the positive electrode side is an issue.

[0034] Furthermore, the decomposition products of these additives that are incorporated into and deposited on the positive electrode side not only increase the internal resistance of non-aqueous secondary batteries, but also accelerate the degradation of the lithium salt and result in insufficient protection of the negative electrode surface. To deactivate the active sites that essentially cause oxidative degradation of non-aqueous electrolytes, it is preferable to incorporate a component that controls Jahn-Teller distortion or acts as a neutralizer. Therefore, it is preferable that the positive electrode active material contain at least one metal selected from the group consisting of Al, Sn, In, Fe, V, Cu, Mg, Ti, Zn, Mo, Zr, Sr, and Ba.

[0035] For the same reasons as above, it is preferable that the surface of the positive electrode active material is coated with a compound containing at least one metal element selected from the group consisting of Zr, Ti, Al, and Nb. It is also more preferable that the surface of the positive electrode active material is coated with an oxide containing at least one metal element selected from the group consisting of Zr, Ti, Al, and Nb. It is even more preferable that the surface of the positive electrode active material is coated with at least one oxide selected from the group consisting of ZrO2, TiO2, Al2O3, NbO3, and LiNbO2, as this does not inhibit the permeation of lithium ions.

[0036] The positive electrode active material may be a lithium-containing compound other than the lithium-containing metal oxide represented by general formula (a). Examples of such lithium-containing compounds include composite oxides containing lithium and a transition metal element, metal chalcogenides containing lithium, metal phosphate compounds containing lithium and a transition metal element, and metal silicate compounds containing lithium and a transition metal element. From the viewpoint of obtaining a higher voltage, the lithium-containing compound is preferably a metal phosphate compound containing lithium and at least one transition metal element selected from the group consisting of Co, Ni, Mn, Fe, Cu, Zn, Cr, V, and Ti.

[0037] More specifically, the lithium-containing compound is represented by the following formula (XXa): Li v M I D2(XXa) {wherein D represents a chalcogen element, and MI represents one or more transition metal elements including at least one transition metal element, and the value of v is determined by the charge / discharge state of the battery and represents a number from 0.05 to 1.10. The following formula (XXb): Li w M II PO4(XXb) {In formula, M II represents one or more transition metal elements, and the value of w is determined depending on the charge / discharge state of the battery and represents a number between 0.05 and 1.10.}, and The following formula (XXc): Li t M III u SiO4(XXc) {In formula, M III represents one or more transition metal elements, the value of t is determined depending on the charge / discharge state of the battery and represents a number from 0.05 to 1.10, and u represents a number from 0 to 2.} Examples of compounds include compounds represented by the following formulae:

[0038] The lithium-containing compound represented by the formula (XXa) has a layered structure, and the compounds represented by the formulas (XXb) and (XXc) have an olivine structure. These lithium-containing compounds may be those in which, for the purpose of stabilizing the structure, a part of the transition metal element is substituted with Al, Mg, or another transition metal element, those in which these metal elements are contained in the crystal grain boundaries, those in which a part of the oxygen atoms is substituted with fluorine atoms, or those in which at least a part of the surface of the positive electrode active material is coated with another positive electrode active material, or the like.

[0039] The positive electrode active material may be used alone or in combination of two or more. The positive electrode active material layer preferably contains at least one transition metal element selected from Ni, Mn, and Co, because this allows for reversible and stable absorption and desorption of lithium ions and a high energy density.

[0040] When a lithium-containing compound and another positive electrode active material are used in combination as the positive electrode active material, the ratio of the lithium-containing compound to the total positive electrode active material is preferably 80 mass% or more, and more preferably 85 mass% or more.

[0041] (Conductive additive) Examples of the conductive additive include graphite, carbon black such as acetylene black and ketjen black, and carbon fiber. The content of the conductive additive is preferably 10 parts by mass or less, more preferably 1 to 5 parts by mass, per 100 parts by mass of the positive electrode active material.

[0042] (binder) Examples of binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid, styrene-butadiene rubber, and fluororubber. The content of the binder is preferably 6 parts by mass or less, more preferably 0.5 to 4 parts by mass, per 100 parts by mass of the positive electrode active material.

[0043] [Formation of positive electrode active material layer] The positive electrode active material layer is formed by dispersing a positive electrode mixture, which is a mixture of a positive electrode active material and, if necessary, a conductive additive and a binder, in a solvent to form a positive electrode mixture-containing slurry, which is applied to a positive electrode current collector, dried (solvent removal), and, if necessary, pressed. Known solvents can be used. Examples of such solvents include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, and water.

[0044] [Negative electrode] The negative electrode in the nonaqueous secondary battery according to this embodiment contains a material capable of absorbing and desorbing lithium ions, and may have a negative electrode active material layer on one or both sides of the negative electrode current collector, as desired.

[0045] [Negative electrode current collector] The negative electrode current collector is made of a metal foil such as copper foil, nickel foil, or stainless steel foil. The surface of the negative electrode current collector may be coated with carbon or may be processed into a mesh. The thickness of the negative electrode current collector is preferably 5 to 40 μm, more preferably 6 to 35 μm, and even more preferably 7 to 30 μm.

[0046] [Negative electrode active material layer] The negative electrode active material layer contains a negative electrode active material, and may further contain a conductive additive and / or a binder as necessary.

[0047] (Negative electrode active material) Examples of negative electrode active materials include amorphous carbon (hard carbon); graphite such as artificial graphite and natural graphite; carbon materials such as pyrolytic carbon, coke, glassy carbon, fired bodies of organic polymer compounds, mesocarbon microbeads, carbon fiber, activated carbon, carbon colloid, and carbon black; as well as metallic lithium, metal oxides, metal nitrides, lithium alloys, tin alloys, Si materials, intermetallic compounds, organic compounds, inorganic compounds, metal complexes, and organic polymer compounds. The negative electrode active materials may be used alone or in combination of two or more. Examples of the Si materials include silicon, Si alloys, and Si oxides.

[0048] The negative electrode active material layer uses lithium ions as the negative electrode active material to increase the battery voltage. + It is preferable that the material contains a material capable of occluding at a potential lower than the potential of the metal.

[0049] The nonaqueous electrolyte solution according to this embodiment has the advantage of being able to suppress various deterioration phenomena that accompany volume changes in the negative electrode during repeated charge-discharge cycles, even when a Si material is used as the negative electrode active material. Therefore, in the nonaqueous secondary battery according to this embodiment, using a Si material, such as a silicon alloy, as the negative electrode active material is also a preferred embodiment, since it provides the battery with high capacity derived from the Si material and excellent charge-discharge cycle characteristics.

[0050] In this embodiment, a Si material, particularly SiO x (wherein 0.5≦x≦1.5). The Si material may be in any form of crystalline, low-crystalline, or amorphous. When a Si material is used as the negative electrode active material, it is preferable to coat the surface of the active material with a conductive material, since this improves the conductivity between the active material particles.

[0051] Silicon has an operating potential of approximately 0.5V (vsLi / Li + ), which is about 0.05 V (vs Li / Li) of the operating potential of graphite. + ) is slightly higher than that of the non-aqueous electrolyte solution containing acetonitrile. Therefore, the risk of lithium electrodeposition is reduced when a Si material is used. Acetonitrile, which is used as the non-aqueous solvent in this embodiment, may undergo a reduction reaction with lithium metal, potentially causing gas generation. Therefore, a negative electrode active material that is difficult to electrodeposit lithium is preferably used in combination with a non-aqueous electrolyte solution containing acetonitrile.

[0052] On the other hand, if the operating potential of the negative electrode active material is too high, the energy density of the battery will decrease. Therefore, from the viewpoint of improving the energy density, the negative electrode active material should be 0.4V vs. Li / Li. + It is preferable to operate at a potential less noble than that of the ion exchange reaction.

[0053] The content of the Si material, expressed as the amount relative to the total amount of the negative electrode active material layer, is preferably in the range of 0.1% by mass to 100% by mass, more preferably in the range of 1% by mass to 80% by mass, and even more preferably in the range of 3% by mass to 60% by mass. By adjusting the content of the Si material within the above range, a balance can be secured between the high capacity and the charge / discharge cycle performance of the nonaqueous secondary battery.

[0054] (Conductive additive) Examples of conductive additives include graphite, carbon black such as acetylene black and ketjen black, and carbon fiber. The content of the conductive additive is preferably 20 parts by mass or less, more preferably 0.1 to 10 parts by mass, per 100 parts by mass of the negative electrode active material.

[0055] (binder) Examples of binders include carboxymethyl cellulose, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid, and fluororubber. Also included are diene rubbers such as styrene-butadiene rubber. The binder content is preferably 10 parts by mass or less, more preferably 0.5 to 6 parts by mass, per 100 parts by mass of the negative electrode active material.

[0056] [Formation of negative electrode active material layer] The negative electrode active material layer is formed by dispersing a negative electrode mixture, which is a mixture of a negative electrode active material and, optionally, a conductive additive and / or a binder, in a solvent to form a negative electrode mixture-containing slurry, which is applied to a negative electrode current collector, dried (solvent removal), and, if necessary, pressed. Known solvents can be used. Examples of such solvents include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, and water.

[0057] [Non-aqueous electrolyte] In this specification, the term "nonaqueous electrolyte solution" (hereinafter simply referred to as "electrolyte solution") refers to an electrolyte solution in which the water content is 1 mass % or less relative to the total amount of the electrolyte solution.

[0058] The electrolyte solution according to this embodiment preferably contains as little water as possible, but may contain a very small amount of water as long as it does not impede the solution of the problems of the present invention. The water content is 300 mass ppm or less, more preferably 200 mass ppm or less, based on the total amount of the nonaqueous electrolyte solution. As long as the nonaqueous electrolyte solution has a configuration that achieves the solution of the problems of the present invention, the other components can be appropriately selected and applied from the constituent materials of known nonaqueous electrolyte solutions used in lithium-ion batteries.

[0059] The electrolyte solution according to this embodiment can contain acetonitrile, a non-aqueous solvent, a lithium salt, an electrode protection additive, and a surfactant.

[0060] <Non-aqueous solvent> In this embodiment, the term "non-aqueous solvent" refers to the elements remaining in the non-aqueous electrolyte solution excluding the lithium salt and various additives. When the non-aqueous electrolyte solution contains an electrode protection additive, the term "non-aqueous solvent" refers to the elements remaining in the non-aqueous electrolyte solution excluding the lithium salt and additives other than the electrode protection additive. Examples of non-aqueous solvents include alcohols such as methanol and ethanol; and aprotic solvents. Among these, aprotic solvents are preferred as non-aqueous solvents. The non-aqueous solvent may contain a solvent other than an aprotic solvent, as long as it does not impede the solution of the problems of the present invention.

[0061] For example, the nonaqueous solvent for the nonaqueous electrolyte solution can contain acetonitrile as an aprotic solvent. The inclusion of acetonitrile in the nonaqueous solvent improves the ionic conductivity of the nonaqueous electrolyte solution, thereby increasing the diffusibility of lithium ions within the battery. Therefore, when the nonaqueous electrolyte solution contains acetonitrile, lithium ions can be effectively diffused even to the region near the current collector, which is difficult for lithium ions to reach during high-load discharge, especially in a positive electrode in which the positive electrode active material layer is thickened to increase the loading amount of the positive electrode active material. Therefore, it is possible to extract sufficient capacity even during high-load discharge, and a nonaqueous secondary battery with excellent load characteristics can be obtained.

[0062] Furthermore, the inclusion of acetonitrile in the nonaqueous solvent can improve the rapid charging characteristics of nonaqueous secondary batteries. In constant current (CC)-constant voltage (CV) charging of nonaqueous secondary batteries, the charge capacity per unit time during the CC charging period is greater than the charge capacity per unit time during the CV charging period. When acetonitrile is used as the nonaqueous solvent in a nonaqueous electrolyte, the range in which CC charging is possible can be expanded (CC charging time can be extended), and the charging current can also be increased, significantly shortening the time from the start of charging to fully charging a nonaqueous secondary battery.

[0063] Acetonitrile is easily electrochemically reductively decomposed, and therefore, when acetonitrile is used, it is preferable to use another solvent (e.g., an aprotic solvent other than acetonitrile) in combination with acetonitrile as a non-aqueous solvent, and / or to add an electrode protection additive for forming a protective film on the electrode.

[0064] The acetonitrile content is preferably 5% by volume or more and 90% by volume or less, based on the total amount of the non-aqueous solvent. The acetonitrile content is more preferably 10% by volume or more, more preferably 15% by volume or more, more preferably 20% by volume or more, more preferably 30% by volume or more, and even more preferably 40% by volume or more, based on the total amount of the non-aqueous solvent. This value is more preferably 85% by volume or less, and even more preferably 66% by volume or less. When the acetonitrile content is 5% by volume or more, based on the total amount of the non-aqueous solvent, the ionic conductivity tends to increase, resulting in high-power performance, and the dissolution of the lithium salt can be promoted. Since the additives described below suppress an increase in the internal resistance of the battery, when the acetonitrile content in the non-aqueous solvent is within the above range, the high-temperature cycle characteristics and other battery characteristics tend to be further improved while maintaining the excellent performance of acetonitrile.

[0065] Examples of aprotic solvents other than acetonitrile include cyclic carbonates, fluoroethylene carbonate, lactones, organic compounds having sulfur atoms other than those represented by general formula (1), chain fluorinated carbonates, cyclic ethers, mononitriles other than acetonitrile, alkoxy group-substituted nitriles, dinitriles, cyclic nitriles, short-chain fatty acid esters, chain ethers, fluorinated ethers, ketones, and compounds in which some or all of the H atoms of the aprotic solvents have been substituted with halogen atoms.

[0066] Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, trans-2,3-butylene carbonate, cis-2,3-butylene carbonate, 1,2-pentylene carbonate, trans-2,3-pentylene carbonate, cis-2,3-pentylene carbonate, vinylene carbonate, 4,5-dimethylvinylene carbonate, and vinylethylene carbonate;

[0067] Fluoroethylene carbonates include, for example, 4-fluoro-1,3-dioxolan-2-one, 4,4-difluoro-1,3-dioxolan-2-one, cis-4,5-difluoro-1,3-dioxolan-2-one, trans-4,5-difluoro-1,3-dioxolan-2-one, 4,4,5-trifluoro-1,3-dioxolan-2-one, 4,4,5,5-tetrafluoro-1,3-dioxolan-2-one, and 4,4,5-trifluoro-5-methyl-1,3-dioxolan-2-one;

[0068] Lactones include γ-butyrolactone, α-methyl-γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, δ-caprolactone, and ε-caprolactone;

[0069] Examples of organic compounds having a sulfur atom include ethylene sulfite, propylene sulfite, butylene sulfite, pentene sulfite, sulfolane, 3-sulfolene, 3-methyl sulfolane, 1,3-propane sultone, 1,4-butane sultone, 1-propene 1,3-sultone, dimethyl sulfoxide, tetramethylene sulfoxide, and ethylene glycol sulfite;

[0070] Examples of chain carbonates include ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, dipropyl carbonate, methyl butyl carbonate, dibutyl carbonate, and ethyl propyl carbonate;

[0071] Cyclic ethers include, for example, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, and 1,3-dioxane;

[0072] Examples of mononitriles other than acetonitrile include propionitrile, butyronitrile, valeronitrile, benzonitrile, and acrylonitrile;

[0073] Alkoxy-substituted nitriles include, for example, methoxyacetonitrile and 3-methoxypropionitrile;

[0074] Dinitriles include, for example, malononitrile, succinonitrile, glutaronitrile, adiponitrile, 1,4-dicyanoheptane, 1,5-dicyanopentane, 1,6-dicyanohexane, 1,7-dicyanoheptane, 2,6-dicyanoheptane, 1,8-dicyanoctane, 2,7-dicyanoctane, 1,9-dicyanononane, 2,8-dicyanononane, 1,10-dicyanodecane, 1,6-dicyanodecane, and 2,4-dimethylglutaronitrile;

[0075] Cyclic nitriles include, for example, benzonitrile;

[0076] Examples of short-chain fatty acid esters include methyl acetate, methyl propionate, methyl isobutyrate, methyl butyrate, methyl isovalerate, methyl valerate, methyl pivalate, methyl hydroangelate, methyl caproate, ethyl acetate, ethyl propionate, ethyl isobutyrate, ethyl butyrate, ethyl isovalerate, ethyl valerate, ethyl pivalate, ethyl hydroangelate, ethyl caproate, propyl acetate, propyl propionate, propyl isobutyrate, propyl butyrate, propyl isovalerate, propyl valerate, propyl pivalate, propyl hydroangelate, propyl caproate, isopropyl acetate, isopropyl propionate, isopropyl isobutyrate, isopropyl butyrate, isopropyl isovalerate, isopropyl valerate, and iso pivalate. propyl, isopropyl hydroangelate, isopropyl caproate, butyl acetate, butyl propionate, butyl isobutyrate, butyl butyrate, butyl isovalerate, butyl valerate, butyl pivalate, butyl hydroangelate, butyl caproate, isobutyl acetate, isobutyl propionate, isobutyl isobutyrate, isobutyl butyrate, isobutyl isovalerate, isobutyl valerate, isobutyl pivalate, isobutyl hydroangelate, isobutyl caproate, tert-butyl acetate, tert-butyl propionate, tert-butyl isobutyrate, tert-butyl butyrate, tert-butyl isovalerate, tert-butyl valerate, tert-butyl pivalate, tert-butyl hydroangelate, and tert-butyl caproate;

[0077] Chain ethers include, for example, dimethoxyethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme, and tetraglyme;

[0078] Examples of fluorinated ethers include Rf aa -OR bb (In the formula, Rf aa represents an alkyl group containing a fluorine atom, and R bb represents an organic group which may contain a fluorine atom);

[0079] Ketones, for example, acetone, methyl ethyl ketone, and methyl isobutyl ketone;

[0080] Examples of the compound in which some or all of the H atoms of the aprotic solvent have been substituted with halogen atoms include compounds in which the halogen atoms are fluorine; Examples include:

[0081] Examples of fluorinated chain carbonates include methyl trifluoroethyl carbonate, trifluorodimethyl carbonate, trifluorodiethyl carbonate, trifluoroethylmethyl carbonate, methyl 2,2-difluoroethyl carbonate, methyl 2,2,2-trifluoroethyl carbonate, and methyl 2,2,3,3-tetrafluoropropyl carbonate. The fluorinated chain carbonates are represented by the following general formula: R cc -OC(O)OR dd (In the formula, R cc and R dd are CH3, CH2CH3, CH2CH2CH3, CH(CH3)2, and CH2Rf ee and Rf ee is an alkyl group having 1 to 3 carbon atoms in which a hydrogen atom is substituted with at least one fluorine atom, and R cc and / or R dd contains at least one fluorine atom).

[0082] In addition, examples of fluorinated short-chain fatty acid esters include fluorinated short-chain fatty acid esters such as 2,2-difluoroethyl acetate, 2,2,2-trifluoroethyl acetate, and 2,2,3,3-tetrafluoropropyl acetate. Fluorinated short-chain fatty acid esters are represented by the following general formula: R ff -C(O)OR gg (In the formula, R ffis CH3, CH2CH3, CH2CH2CH3, CH(CH3)2, CF3CF2H, CFH2, CF2Rf hh , CFHRf hh , and CH2Rf ii and R is at least one selected from the group consisting of gg are CH3, CH2CH3, CH2CH2CH3, CH(CH3)2, and CH2Rf ii and Rf hh is an alkyl group having 1 to 3 carbon atoms in which a hydrogen atom may be substituted with at least one fluorine atom, and Rf ii is an alkyl group having 1 to 3 carbon atoms in which a hydrogen atom is substituted with at least one fluorine atom, and R ff and / or R gg contains at least one fluorine atom, and R ff If is CF2H, R gg can be expressed as (not CH3).

[0083] In the present embodiment, the aprotic solvent other than acetonitrile may be used alone or in combination of two or more.

[0084] In the present embodiment, the non-aqueous solvent is preferably one or more of cyclic carbonates and chain carbonates in combination with acetonitrile from the viewpoint of improving the stability of the non-aqueous electrolyte solution. From this viewpoint, the non-aqueous solvent in the present embodiment is more preferably one of cyclic carbonates in combination with acetonitrile, and further preferably one of cyclic carbonates and chain carbonates in combination with acetonitrile.

[0085] When a cyclic carbonate is used with acetonitrile, it is particularly preferred that the cyclic carbonate comprises ethylene carbonate, vinylene carbonate and / or fluoroethylene carbonate.

[0086] <Lithium salt> The non-aqueous electrolyte solution according to this embodiment contains a lithium salt, and the content of the lithium salt may be 1 mol / L or more and 2 mol / L or less relative to the non-aqueous electrolyte solution.

[0087] The lithium salt in this embodiment includes a lithium-containing imide salt as an imide salt, and the lithium-containing imide salt is LiN(SO2C m F 2m+1 )2 {wherein m is an integer of 0 to 8}.

[0088] The lithium salt in this embodiment may further include one or more selected from fluorine-containing inorganic lithium salts, organic lithium salts, and other lithium salts, in addition to the imide salt.

[0089] (imide salt) Specifically, the imide salt preferably contains at least one of LiN(SO2F)2 and LiN(SO2CF3)2.

[0090] When acetonitrile is contained in the non-aqueous solvent, the lithium salt preferably contains LiPF6. Since the saturation concentration of the imide salt relative to acetonitrile is higher than that of LiPF6, it is preferable to contain LiPF6 and the imide salt at a molar concentration such that LiPF6≦imide salt, since this can suppress association and precipitation of the lithium salt and acetonitrile at low temperatures. Furthermore, it is preferable that the content of the imide salt is 0.5 mol or more and 3.0 mol or less per 1 L of non-aqueous solvent from the viewpoint of ensuring the supply of ions to the non-aqueous electrolyte solution according to this embodiment.

[0091] A non-aqueous electrolyte solution containing acetonitrile containing at least one of LiN(SO2F)2 and LiN(SO2CF3)2 can effectively suppress the decrease in ionic conductivity at low temperatures such as -10°C or -30°C, thereby achieving excellent low-temperature properties.

[0092] Furthermore, by limiting the contents of the imide salt and LiPF6 as described above, it is possible to more effectively suppress the increase in resistance when heated to high temperatures.

[0093] (Fluorine-containing inorganic lithium salt) The lithium salt in this embodiment may include a fluorine-containing inorganic lithium salt. Here, the term "fluorine-containing inorganic lithium salt" refers to a lithium salt that does not contain a carbon atom in the anion but contains a fluorine atom in the anion and is soluble in acetonitrile. The fluorine-containing inorganic lithium salt is excellent in that it forms a passive film on the surface of the positive electrode current collector and inhibits corrosion of the positive electrode current collector.

[0094] Examples of fluorine-containing inorganic lithium salts include LiPF6, LiBF4, LiAsF6, Li2SiF6, LiSbF6, and Li2B 12 F b H 12-b {wherein b represents an integer of 0 to 3}, and one or more selected from these can be used.

[0095] As the fluorine-containing inorganic lithium salt, a compound that is a double salt of LiF and a Lewis acid is desirable, and among them, a fluorine-containing inorganic lithium salt having a phosphorus atom is more preferred because it easily releases free fluorine atoms. A typical fluorine-containing inorganic lithium salt is LiPF6, which dissolves and releases PF6 anions. When a fluorine-containing inorganic lithium salt having a boron atom is used as the fluorine-containing inorganic lithium salt, it is preferred because it easily captures excess free acid components that may cause battery degradation, and from this viewpoint, LiBF4 is preferred.

[0096] The content of the fluorine-containing inorganic lithium salt in the non-aqueous electrolyte solution according to this embodiment is preferably 0.01 mol or more, more preferably 0.1 mol or more, and even more preferably 0.25 mol or more, per 1 L of non-aqueous solvent. When the content of the fluorine-containing inorganic lithium salt is within the above range, the ionic conductivity tends to increase and high-power characteristics tend to be realized. Furthermore, the amount per 1 L of non-aqueous solvent is preferably 2.8 mol or less, more preferably 1.5 mol or less, and even more preferably 1.0 mol or less. When the content of the fluorine-containing inorganic lithium salt is within the above range, the ionic conductivity tends to increase and high-power characteristics can be realized, and the decrease in ionic conductivity due to an increase in viscosity at low temperatures tends to be suppressed. Therefore, the high-temperature cycle characteristics and other battery characteristics tend to be further improved while maintaining the excellent performance of the non-aqueous electrolyte solution.

[0097] The content of the fluorine-containing inorganic lithium salt in the nonaqueous electrolyte solution according to this embodiment may be, for example, 0.05 mol or more and 1.0 mol or less per 1 L of the nonaqueous solvent.

[0098] (organic lithium salts) The lithium salt in this embodiment may include an organic lithium salt. The term "organic lithium salt" refers to a lithium salt other than an imide salt that contains a carbon atom in the anion and is soluble in acetonitrile.

[0099] Examples of organic lithium salts include organic lithium salts having an oxalic acid group. Specific examples of organic lithium salts having an oxalic acid group include organic lithium salts represented by LiB(C2O4)2, LiBF2(C2O4), LiPF4(C2O4), and LiPF2(C2O4)2. Among these, at least one lithium salt selected from the group consisting of LiB(C2O4)2 and LiBF2(C2O4) is preferred. It is more preferred to use one or more of these together with a fluorine-containing inorganic lithium salt. This organic lithium salt having an oxalic acid group may be added to a nonaqueous electrolyte solution or incorporated into the negative electrode (negative electrode active material layer).

[0100] In this embodiment, the amount of the organic lithium salt added to the non-aqueous electrolyte solution is preferably 0.005 mol or more, more preferably 0.01 mol or more, even more preferably 0.02 mol or more, and particularly preferably 0.05 mol or more, per 1 L of non-aqueous solvent, in order to better ensure the effects of its use. However, if the amount of the organic lithium salt having an oxalic acid group in the non-aqueous electrolyte solution is too large, precipitation may occur. Therefore, the amount of the organic lithium salt having an oxalic acid group added to the non-aqueous electrolyte solution is preferably less than 1.0 mol, more preferably less than 0.5 mol, and even more preferably less than 0.2 mol, per 1 L of non-aqueous solvent.

[0101] It is known that organic lithium salts having an oxalic acid group are poorly soluble in organic solvents with low polarity, particularly in chain carbonates. The content of the organic lithium salt in the nonaqueous electrolyte solution according to this embodiment may be, for example, 0.01 mol or more and 0.5 mol or less per 1 L of the nonaqueous solvent.

[0102] In addition, the organic lithium salt having an oxalic acid group may contain a small amount of lithium oxalate, and when mixed to form a non-aqueous electrolyte solution, it may react with a small amount of water contained in other raw materials to generate a new white precipitate of lithium oxalate. Therefore, it is preferable that the content of lithium oxalate in the non-aqueous electrolyte solution according to this embodiment be limited to a range of 500 ppm or less.

[0103] (Other lithium salts) The lithium salt in this embodiment may include other lithium salts in addition to those mentioned above.

[0104] Other specific examples of lithium salts include: LiClO4, LiAlO4, LiAlCl4, LiB 10 Cl 10 inorganic lithium salts that do not contain fluorine atoms in the anion, such as chloroborane Li; LiCF3SO3, LiCF3CO2, Li2C2F4(SO3)2, LiC(CF3SO2)3, LiC n F (2n+1) SO3 (wherein n≧2), organic lithium salts such as lower aliphatic carboxylic acids Li, tetraphenylborate Li, and LiB(C3O4H2)2; LiPF such as LiPF5(CF3) n (C p F 2p+1 ) 6-n an organic lithium salt represented by the formula (wherein n is an integer of 1 to 5, and p is an integer of 1 to 8); LiBF such as LiBF3(CF3) q (C s F 2s+1 ) 4-q an organic lithium salt represented by the formula (wherein q is an integer of 1 to 3, and s is an integer of 1 to 8); Lithium salts combined with polyvalent anions; The following formula (YYa): LiC(SO2R jj )(SO2R kk )(SO2R ll ) (YYa) {where, R jj , R kk , and R ll may be the same or different and represent a perfluoroalkyl group having 1 to 8 carbon atoms. The following formula (YYb): LiN(SO2OR mm )(SO2OR nn ) (YYb) {where, R mm , and R nn may be the same or different and represent a perfluoroalkyl group having 1 to 8 carbon atoms.}, and The following formula (YYc): LiN(SO2R oo )(SO2OR pp ) (YYc) {where, R oo , and R pp may be the same or different and represent a perfluoroalkyl group having 1 to 8 carbon atoms.} One or more of these can be used together with the fluorine-containing inorganic lithium salt.

[0105] The amount of the other lithium salt added to the non-aqueous electrolyte solution may be appropriately set within the range of, for example, 0.01 mol or more and 0.5 mol or less per liter of non-aqueous solvent.

[0106] <Electrode protection additive> The non-aqueous electrolyte solution according to this embodiment may contain an additive for protecting the electrodes (electrode protection additive). The electrode protection additive may substantially overlap with the substance that serves as a solvent for dissolving the lithium salt (i.e., the non-aqueous solvent described above). The electrode protection additive is preferably a substance that contributes to improving the performance of the non-aqueous electrolyte solution and the non-aqueous secondary battery, but also includes substances that are not directly involved in the electrochemical reaction.

[0107] Specific examples of the electrode protection additive include: fluoroethylene carbonates represented by 4-fluoro-1,3-dioxolan-2-one, 4,4-difluoro-1,3-dioxolan-2-one, cis-4,5-difluoro-1,3-dioxolan-2-one, trans-4,5-difluoro-1,3-dioxolan-2-one, 4,4,5-trifluoro-1,3-dioxolan-2-one, 4,4,5,5-tetrafluoro-1,3-dioxolan-2-one, and 4,4,5-trifluoro-5-methyl-1,3-dioxolan-2-one; unsaturated bond-containing cyclic carbonates, such as vinylene carbonate, 4,5-dimethylvinylene carbonate, and vinylethylene carbonate; Lactones represented by γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, δ-caprolactone, and ε-caprolactone; cyclic ethers, such as 1,4-dioxane; cyclic sulfur compounds represented by ethylene sulfite, propylene sulfite, butylene sulfite, pentene sulfite, sulfolane, 3-sulfolene, 3-methyl sulfolane, 1,3-propane sultone, 1,4-butane sultone, 1-propene 1,3-sultone, and tetramethylene sulfoxide; These may be used alone or in combination of two or more.

[0108] The content of the electrode protection additive in the non-aqueous electrolyte solution is preferably 0.1 to 30% by volume, more preferably 0.3 to 15% by volume, even more preferably 0.4 to 8% by volume, and particularly preferably 0.5 to 4% by volume, relative to the total amount of the non-aqueous solvent.

[0109] In this embodiment, the higher the content of the electrode protection additive, the more the deterioration of the nonaqueous electrolyte solution is suppressed. However, the lower the content of the electrode protection additive, the more the high-power performance of the nonaqueous secondary battery in a low-temperature environment is improved. Therefore, by adjusting the content of the electrode protection additive within the above range, it is possible to exhibit excellent performance based on the high ionic conductivity of the electrolyte solution without impairing the basic functions of the nonaqueous secondary battery. Furthermore, by preparing a nonaqueous electrolyte solution with such a composition, it is possible to further improve the cycle performance of the nonaqueous secondary battery, the high-power performance in a low-temperature environment, and other battery characteristics.

[0110] Acetonitrile is easily electrochemically reductively decomposed, and therefore, the nonaqueous solvent containing acetonitrile preferably contains one or more cyclic aprotic polar solvents, more preferably one or more unsaturated bond-containing cyclic carbonates, as an electrode protection additive for forming a protective coating on the negative electrode.

[0111] The unsaturated bond-containing cyclic carbonate is preferably vinylene carbonate, and the content of vinylene carbonate in the nonaqueous electrolyte is preferably 0.1% by volume or more and 4% by volume or less, more preferably 0.2% by volume or more and less than 3% by volume, and even more preferably 0.5% by volume or more and less than 2.5% by volume. This makes it possible to more effectively improve low-temperature durability and provide a secondary battery with excellent low-temperature performance.

[0112] Vinylene carbonate, an additive for electrode protection, suppresses the reductive decomposition reaction of acetonitrile on the negative electrode surface. On the other hand, excessive film formation leads to a decrease in low-temperature performance. Therefore, by adjusting the amount of vinylene carbonate added within the above range, it is possible to keep the interfacial (film) resistance low and suppress cycle degradation at low temperatures.

[0113] <Acid anhydride> The nonaqueous secondary battery according to this embodiment is stabilized by the formation of an SEI on the negative electrode surface due to partial decomposition of the nonaqueous electrolyte during initial charging. To more effectively strengthen this SEI, an acid anhydride can be added. When acetonitrile is included as the nonaqueous solvent, the strength of the SEI tends to decrease with increasing temperature, but the addition of an acid anhydride promotes strengthening of the SEI. Therefore, by using such an acid anhydride, it is possible to effectively suppress the increase in internal resistance over time due to thermal history.

[0114] Specific examples of acid anhydrides include chain acid anhydrides such as acetic anhydride, propionic anhydride, and benzoic anhydride; cyclic acid anhydrides such as malonic anhydride, succinic anhydride, glutaric anhydride, maleic anhydride, phthalic anhydride, 1,2-cyclohexanedicarboxylic anhydride, 2,3-naphthalenedicarboxylic anhydride, and naphthalene-1,4,5,8-tetracarboxylic dianhydride; and mixed acid anhydrides having a structure formed by dehydration condensation of different types of acids, such as two different types of carboxylic acids or a carboxylic acid and a sulfonic acid. These may be used alone or in combination of two or more types.

[0115] Since the nonaqueous secondary battery according to this embodiment preferably strengthens the SEI before the reductive decomposition of the nonaqueous solvent, it is preferable that the acid anhydride contains at least one cyclic acid anhydride that acts early during the first charge. These cyclic acid anhydrides may be contained alone or in combination. Alternatively, a cyclic acid anhydride other than these cyclic acid anhydrides may be contained. Furthermore, the cyclic acid anhydride preferably contains at least one of succinic anhydride, maleic anhydride, and phthalic anhydride.

[0116] A nonaqueous electrolyte solution containing at least one of succinic anhydride, maleic anhydride, and phthalic anhydride can form a strong SEI on the negative electrode and more effectively suppress an increase in resistance during high-temperature heating. It is particularly preferable to include succinic anhydride. This allows for more effective formation of a strong SEI on the negative electrode while suppressing side reactions.

[0117] When the non-aqueous electrolytic solution according to the present embodiment contains an acid anhydride, the content thereof is preferably in the range of 0.01 parts by mass or more and 10 parts by mass or less, more preferably 0.05 parts by mass or more and 1 part by mass or less, and even more preferably 0.1 parts by mass or more and 0.5 parts by mass or less, per 100 parts by mass of the non-aqueous electrolytic solution.

[0118] The acid anhydride is preferably contained in the non-aqueous electrolyte solution. On the other hand, as long as the acid anhydride can function in a non-aqueous secondary battery, at least one battery component selected from the group consisting of a positive electrode, a negative electrode, and a separator may contain the acid anhydride. The acid anhydride may be contained in the battery component during production of the battery component, or may be impregnated into the battery component by post-treatment such as coating, immersion, or spray drying.

[0119] Optional Additives In this embodiment, for the purpose of improving the charge / discharge cycle characteristics, high-temperature storage properties, and safety (for example, preventing overcharging) of the nonaqueous secondary battery, optional additives (additives other than the acid anhydride and the electrode protection additive) may be appropriately contained in the nonaqueous electrolyte solution.

[0120] Examples of optional additives include sulfonic acid esters, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, tert-butylbenzene, nitrogen-containing cyclic compounds with no steric hindrance around the unshared electron pair (pyridine, 1-methyl-1H-benzotriazole, 1-methylpyrazole, etc.), etc. In particular, phosphate esters have the effect of suppressing side reactions during storage and are effective as optional additives.

[0121] When the non-aqueous electrolyte solution according to this embodiment contains other optional additives, the content thereof is preferably in the range of 0.01% by mass to 10% by mass, more preferably 0.02% by mass to 5% by mass, and even more preferably 0.05 to 3% by mass, relative to the total amount of the non-aqueous electrolyte solution. By adjusting the content of the other optional additives within the above ranges, it tends to be possible to impart even better battery characteristics without impairing the basic functions of a non-aqueous secondary battery.

[0122] <Surfactant> In this embodiment, the surfactant is a compound having one or more hydrophilic groups and one or more lipophilic groups in one molecule. Examples of surfactants include anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants. Ionizable surfactants are limited to those that do not inhibit the insertion and desorption of lithium ions into and from the negative electrode. Furthermore, from the viewpoint of solubility in nonaqueous electrolytes, nonionic surfactants are preferred.

[0123] Nonionic surfactants include fatty acid ester compounds such as glycerin fatty acid esters, sorbitan fatty acid esters, sucrose fatty acid esters, and saturated fatty acid esters; ethyl diethylphosphonoacetate {EDPA; (C2H5O)2(P=O)-CH2(C=O)OC2H5}, tris(trifluoroethyl) phosphate {TFEP; (CF3CH2O)3P=O}, triphenyl phosphate {TPP; (C6H5O)3P=O}, triallyl phosphate {CH2=CHCHO)3P=O}, triamyl phosphate, tris(2-butoxyethyl) phosphate, and tris(2-butoxyethyl) phosphate. Examples of suitable non-aqueous electrolytes include phosphate ester compounds such as tris(2-ethylhexyl); ether compounds such as polyoxyethylene alkyl ethers, polyoxyalkylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene polycyclic phenyl ethers, polyoxyethylene distyrenated phenyl ethers, and polyoxyethylene polyoxypropylene glycol; ester ether compounds such as fatty acid polyethylene glycols and polyoxyethylene sorbitan fatty acid esters; and alkanolamide compounds such as fatty acid alkanolamides. Fatty acid ester compounds or phosphate ester compounds are preferred from the viewpoint of improving the stability of non-aqueous electrolytes. Ethyl laurate is preferred as the fatty acid ester compound. Phosphate ester compounds are particularly preferred, and among them, at least one selected from the group consisting of triamyl phosphate, tris(2-butoxyethyl) phosphate, and tris(2-ethylhexyl) phosphate is more preferred, with tris(2-ethylhexyl) phosphate being particularly preferred.

[0124] The number of carbon atoms contained in at least one substituent constituting the hydrophobic portion of the nonionic surfactant is preferably 3 or more, more preferably 5 or more, and even more preferably 8 or more, from the viewpoint of reducing the surface tension of the nonaqueous electrolyte. Furthermore, since a large number of carbon atoms in the substituent increases the boiling point of the compound and makes purification difficult, the number of carbon atoms contained in the substituent is preferably 25 or less, and even more preferably 20 or less. When the hydrocarbon chain of the substituent is branched, the total number of carbon atoms contained in the substituent is used. Specific examples of substituents having 3 or more carbon atoms include a propyl group, an isopropyl group, a butyl group, an isobutyl group, a s-butyl group, a t-butyl group, an amyl group, an isoamyl group, a s-amyl group, a t-amyl group, a 2-butoxyethyl group, a 2-ethylhexyl group, and a phenyl group.

[0125] In this embodiment, the content of the surfactant is 0.1% by mass or more and 3% by mass or less, based on the total amount of the non-aqueous electrolyte solution. The content of the surfactant is more preferably 0.3% by mass or more, based on the total amount of the non-aqueous electrolyte solution, from the viewpoint of improving impregnation, and more preferably 2% by mass or less, based on the total amount of the non-aqueous electrolyte solution, from the viewpoint of maintaining high ionic conductivity. Similarly, the content of the nonionic surfactant is preferably 0.1% by mass or more and 3% by mass or less, based on the total amount of the non-aqueous electrolyte solution, and more preferably 0.3% by mass or more and 2% by mass or less. When the content of the surfactant or nonionic surfactant is 0.1% by mass or more, the effect of suppressing output degradation can be more effectively obtained, and when the content is 3% by mass or less, the degradation of ionic conductivity is small, resulting in less impact on battery characteristics such as input / output characteristics and battery life.

[0126] Here, the surfactant content is the amount of the non-aqueous electrolyte at room temperature. 1 It can be determined by H-NMR measurement (normalization is performed using a standard substance (C6F4H2) and the surfactant content is calculated from the integrated value of the signal of each detected component).

[0127] As a result of extensive research, the inventors found that while the presence of hydroxyl groups in surfactants improves separator impregnation, they also facilitate reductive decomposition at the negative electrode during charging, resulting in deterioration of battery performance. Furthermore, it was newly discovered that when hydroxyl groups are present in surfactants, bubbles are more likely to form during the decompression process after the nonaqueous electrolyte is injected. If the first charge is performed without removing all the bubbles, the SEI is not uniformly formed on the negative electrode surface, leading to initial charge / discharge failures. Nonaqueous electrolytes containing acetonitrile form a uniform SEI during the first charge, enabling subsequent charge / discharge operations. Therefore, if there are areas where SEI formation is insufficient during the first charge due to bubble formation, nonaqueous electrolytes containing acetonitrile directly lead to initial charge failures or reduced battery life compared to nonaqueous electrolytes without acetonitrile. Therefore, it is important that the surfactant of this embodiment does not contain hydroxyl groups as a substituent, more specifically, as a terminal substituent.

[0128] <Separator> The separator of this embodiment has a polypropylene-based microporous film.

[0129] The polypropylene-based microporous membrane according to this embodiment has a polypropylene-containing polymer matrix extending in one direction perpendicular to the membrane thickness, with polypropylene-containing fibrils between the polymer matrix and micropores between the fibrils. The "one direction perpendicular to the membrane thickness" refers to the machine direction (hereinafter sometimes abbreviated as MD) of continuous microporous membrane molding. The structure of the polypropylene-based microporous membrane can be obtained by production using a dry method, more specifically, by a dry method including a step of dry-porizing a polypropylene-containing resin molded body. Figure 3 shows an example of an SEM image of a polypropylene microporous membrane. For example, the polypropylene-containing polymer matrix structure extending in the "one direction perpendicular to the membrane thickness" can be observed in Figure 3.

[0130] The microporous membrane is a monolayer membrane consisting of a single polyolefin-based microporous layer, a microporous monolayer membrane consisting of a resin layer other than a polyolefin-based microporous layer, or a composite microporous monolayer membrane having a polyolefin-based resin and another resin, and is sometimes called a microporous layer.

[0131] The polyolefin-based microporous film is formed from a polyolefin resin composition containing a polyolefin resin as a main component.

[0132] The polypropylene-based microporous film is formed from a polypropylene resin composition containing polypropylene as a main component.

[0133] In this specification, when a film contains a particular material as a main component, it means that the film contains 50% or more of the material based on the mass of the film.

[0134] In this specification, a microporous membrane containing a polyolefin resin as a main component means that the proportion of the polyolefin resin in the microporous membrane is 50% by mass or more relative to the mass of the microporous membrane, and therefore the polyolefin resin can form a polymer network in the microporous membrane.

[0135] The polypropylene resin composition containing polypropylene as a main component means that the proportion of polypropylene in the polypropylene resin composition is 50% by mass or more relative to the mass of the polypropylene resin composition.

[0136] The microporous laminate film means a multilayer film in which multiple polyolefin-based microporous layers are laminated, or a composite microporous film in which a polyolefin-based microporous film and a microporous film containing another resin are laminated, and may be simply referred to as a laminate hereinafter. When the separator of this embodiment is a laminate, any of the layers contains the polypropylene-based microporous film containing polypropylene as a main component.

[0137] A separator is a component disposed between multiple electrodes in an electricity storage device, and has ion permeability and, if necessary, shutdown properties. The separator includes a microporous membrane and / or a microporous laminate membrane, and may further include any functional layer, if desired.

[0138] The present inventors have found that a combination of a polypropylene-based microporous membrane and a nonaqueous electrolyte containing acetonitrile results in a problem with output performance. After extensive research into this problem, they discovered that, among polypropylene-based separators with small pore sizes, separators manufactured by a dry process and having pores parallel to the thickness direction are used, resulting in extremely low impregnation with a nonaqueous electrolyte containing acetonitrile. Furthermore, they have newly discovered that low impregnation in the thickness direction of the separator increases the resistance of the electrode, which leads to a decrease in output performance.

[0139] In the polypropylene-based microporous membrane, the pore size ratio a / b, where a is the average long pore size a of the microporous membrane and b is the pore size perpendicular to the average long pore size a, is preferably 0.5 or more and 30 or less, from the viewpoint of increasing strength. From the viewpoint of increasing strength, the pore size ratio a / b is more preferably 1 or more and 20 or less, and even more preferably 1.5 or more and 15 or less.

[0140] The separator preferably has a porosity of 30% or more and 80% or less, and / or an air permeability (hereinafter also referred to as "air permeability resistance") of 100 seconds / 100 cm 3 Over, 500 seconds / 100cm 3 It is preferable that:

[0141] If the porosity is lower than 30%, it becomes difficult to ensure sufficient ion permeability when the microporous membrane is used for a battery. Furthermore, if the porosity is higher than 80%, the microporous membrane may not be able to maintain sufficient strength. From this perspective, the porosity of the separator is more preferably 33% or more and 77% or less, and even more preferably 37% or more and 73% or less. Furthermore, the air permeation resistance of the separator is preferably 100 seconds / 100 cm. 3Adjusting the temperature to the above level is preferable from the viewpoint of obtaining a separator containing a defect-free, homogeneous microporous membrane. 3 It is preferable to adjust the air permeability of the separator to 120 sec / 100 cm or less, as this can contribute to ensuring sufficient ion permeability. 3 Over, 450 seconds / 100cm 3 More preferably, it is 150 seconds / 100 cm or less. 3 More than 400 seconds / 100cm 3 It is more preferable that:

[0142] <Battery exterior> The battery exterior of the nonaqueous secondary battery of this embodiment may have a known structure, for example, a battery can or a laminate film exterior.

[0143] The battery can may be a metal can made of, for example, steel, stainless steel, aluminum, or a clad material.

[0144] The laminate film exterior can be used as an exterior by stacking two sheets with the heat-melt resin side facing inward, or by folding the laminate film exterior so that the heat-melt resin side faces inward and sealing the edges with heat sealing. When using a laminate film exterior, a positive electrode lead (or a positive electrode terminal and a lead tab connected to the positive electrode terminal) may be connected to the positive electrode current collector, and a negative electrode lead (or a negative electrode terminal and a lead tab connected to the negative electrode terminal) may be connected to the negative electrode current collector. In this case, the laminate film exterior may be sealed with the ends of the positive electrode lead and the negative electrode lead (or the lead tabs connected to the positive electrode terminal and the negative electrode terminal, respectively) extended outside the exterior.

[0145] As the laminate film exterior body, for example, a laminate film having a three-layer structure of heat-melting resin / metal film / resin can be used.

[0146] The aluminum laminate film constituting the battery exterior is preferably an aluminum foil coated on both sides with a polyolefin resin.

[0147] <Shape of non-aqueous secondary battery> The shape of the nonaqueous secondary battery according to this embodiment can be, for example, a square, rectangular tube, cylindrical, elliptical, button, coin, flat, laminate, or the like.

[0148] The nonaqueous secondary battery according to this embodiment can be particularly preferably applied to a prismatic, rectangular tubular, and laminated type battery.

[0149] <Method of manufacturing non-aqueous secondary battery> The nonaqueous secondary battery of this embodiment can be manufactured using the above-mentioned nonaqueous electrolyte, a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, a separator, and, if necessary, a battery exterior.

[0150] First, a laminate consisting of a positive electrode, a negative electrode, and a separator can be formed. For example, possible methods include a method of preparing a laminate of long positive electrodes and negative electrodes with a long separator interposed between them, and then winding the laminate to form a wound laminate; a method of cutting the positive electrodes and negative electrodes into multiple sheets having a certain area and shape, and alternately stacking the resulting positive electrode sheets and negative electrode sheets with a separator sheet interposed therebetween to form a laminate with a layered structure; or a method of folding a long separator zigzag, and alternately inserting positive electrode sheets and negative electrode sheets between the zigzag folded separators to form a laminate with a layered structure.

[0151] Next, the above-mentioned laminate is housed in a battery exterior (battery case), an electrolyte solution is poured into the battery case, and the laminate is immersed in the electrolyte solution and sealed, thereby producing a nonaqueous secondary battery of this embodiment. Alternatively, a gel-state electrolyte membrane may be produced in advance by impregnating a substrate made of a polymer material with the electrolyte solution, and a laminate having a laminate structure may be formed using sheet-like positive electrodes, negative electrodes, and electrolyte membranes, as well as a separator, and then housed in the battery exterior to produce a nonaqueous secondary battery.

[0152] If the electrodes are arranged so that there is a portion where the outer peripheral edge of the negative electrode active material layer overlaps with that of the positive electrode active material layer, or there is a portion where the width is too small in the non-facing portion of the negative electrode active material layer, misalignment of the electrodes may occur during battery assembly, which may result in a deterioration in the charge-discharge cycle characteristics of the non-aqueous secondary battery. Therefore, it is preferable to fix the positions of the electrodes of the electrode body used in the non-aqueous secondary battery in advance using tapes such as polyimide tape, polyphenylene sulfide tape, and polypropylene (PP) tape, adhesives, etc.

[0153] In this embodiment, when a nonaqueous electrolyte containing acetonitrile is used, its high ionic conductivity may cause lithium ions released from the positive electrode during the initial charge of the nonaqueous secondary battery to diffuse throughout the negative electrode. In nonaqueous secondary batteries, the area of ​​the negative electrode active material layer is generally larger than that of the positive electrode active material layer. However, if lithium ions diffuse and are absorbed in a portion of the negative electrode active material layer that does not face the positive electrode active material layer, these lithium ions will not be released during the initial discharge and will remain in the negative electrode. Therefore, the contribution of these unreleased lithium ions will be the irreversible capacity. For these reasons, nonaqueous secondary batteries using a nonaqueous electrolyte containing acetonitrile may have low initial charge / discharge efficiency.

[0154] On the other hand, if the area of ​​the positive electrode active material layer is larger than that of the negative electrode active material layer or if the two are the same, current tends to concentrate at the edge portions of the negative electrode active material layer during charging, making it easier for lithium dendrites to form.

[0155] Although there are no particular limitations on the ratio of the area of ​​the entire negative electrode active material layer to the area of ​​the portion where the positive electrode active material layer and the negative electrode active material layer face each other, for the reasons described above, it is preferably greater than 1.0 and less than 1.1, more preferably greater than 1.002 and less than 1.09, even more preferably greater than 1.005 and less than 1.08, and particularly preferably greater than 1.01 and less than 1.08. In a nonaqueous secondary battery using a nonaqueous electrolyte solution containing acetonitrile, the initial charge-discharge efficiency can be improved by reducing the ratio of the area of ​​the entire negative electrode active material layer to the area of ​​the portion where the positive electrode active material layer and the negative electrode active material layer face each other.

[0156] Reducing the ratio of the area of ​​the entire negative electrode active material layer to the area of ​​the portion where the positive electrode active material layer and the negative electrode active material layer face each other means limiting the area of ​​the portion of the negative electrode active material layer that does not face the positive electrode active material layer. This makes it possible to minimize the amount of lithium ions released from the positive electrode during the initial charge that are absorbed in the portion of the negative electrode active material layer that does not face the positive electrode active material layer (i.e., the amount of lithium ions that are not released from the negative electrode during the initial discharge and result in irreversible capacity). Therefore, by designing the ratio of the area of ​​the entire negative electrode active material layer to the area of ​​the portion where the positive electrode active material layer and the negative electrode active material layer face each other within the above range, it is possible to improve the load characteristics of the battery by using acetonitrile, increase the initial charge / discharge efficiency of the battery, and further suppress the formation of lithium dendrites.

[0157] The nonaqueous secondary battery of this embodiment can function as a battery after initial charging, but is stabilized by partial decomposition of the electrolyte during initial charging. While there are no particular limitations on the initial charging method, the initial charging is preferably performed at 0.001 to 0.3 C, more preferably at 0.002 to 0.25 C, and even more preferably at 0.003 to 0.2 C. Performing the initial charging via a constant voltage charge during the initial charging also produces favorable results. By setting the voltage range in which the lithium salt participates in the electrochemical reaction to a long range, a stable and strong SEI is uniformly formed on the electrode surface, suppressing an increase in internal resistance. Furthermore, the reaction product is not firmly fixed only to the negative electrode, and in some way, it also has a favorable effect on components other than the negative electrode, such as the positive electrode and separator. Therefore, it is very effective to perform the initial charging while taking into account the electrochemical reaction of the lithium salt dissolved in the nonaqueous electrolyte.

[0158] The nonaqueous secondary battery of this embodiment can also be used as a battery pack in which a plurality of nonaqueous secondary batteries are connected in series or in parallel. From the viewpoint of managing the charge / discharge state of the battery pack, the operating voltage range per battery is preferably 2 to 5 V, more preferably 2.5 to 5 V, and particularly preferably 2.75 V to 5 V.

[0159] As the size of the nonaqueous secondary battery increases, it becomes more difficult for the nonaqueous electrolyte to permeate the entire separator, so it is preferable to leave the battery to stand for 12 hours or more after the nonaqueous electrolyte has been poured.

[0160] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit and scope of the present invention. [Example]

[0161] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, experiments and measurements in the examples were carried out at room temperature.

[0162] (1) Preparation of non-aqueous electrolyte Various non-aqueous solvents were mixed in an inert atmosphere to give the concentrations shown in Tables 1 and 2. A surfactant was added to the resulting non-aqueous solvent in the proportions shown in Tables 1 and 2. Furthermore, various lithium salts were added to give the respective concentrations shown in Tables 1 and 2 to prepare non-aqueous electrolyte solutions (S01) to (S12). The compositions of these non-aqueous electrolyte solutions are shown in Tables 1 and 2.

[0163] The abbreviations for non-aqueous solvents, lithium salts, and additives in Tables 1 and 2 have the following meanings: Furthermore, the mass % of the surfactant indicates the mass parts relative to 100 mass parts of the non-aqueous electrolyte solution.

[0164] (lithium salts) LiPF6: Lithium hexafluorophosphate LiFSI: Lithium bis(fluorosulfonyl)imide (LiN(SO2F)2) (non-aqueous solvent) AcN: acetonitrile EMC: Ethyl methyl carbonate EC: Ethylene carbonate VC: vinylene carbonate ES: Ethylene sulfite (surfactant) L0013: Ethyl laurate P0265: Triamyl phosphate P0270: Triethyl phosphate P0683: Tris(2-butoxyethyl) phosphate P1022: Tris(2-ethylhexyl) phosphate

[0165] [Table 1]

[0166] [Table 2]

[0167] (1-1) Impregnation test 10 μl of the prepared electrolyte was dropped onto a separator punched into a 19 mm diameter disk, and the impregnation in the thickness direction was evaluated. The impregnation was evaluated as ◯ if it penetrated to the back side of the separator within 15 minutes, and × if it did not penetrate at all.

[0168] (2) Fabrication of small non-aqueous secondary batteries

[0169] (2-1) Preparation of the positive electrode

[0170] (2-1-1) Preparation of the positive electrode (P1) (A) Lithium iron phosphate (LiFePO4) having an olivine structure as a positive electrode active material, (B) carbon black powder as a conductive additive, and (C) polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 89:3:8 to obtain a positive electrode mixture.

[0171] N-methyl-2-pyrrolidone was added as a solvent to the resulting positive electrode mixture to a solids content of 68% by mass, and the mixture was further mixed to prepare a positive electrode mixture-containing slurry. The positive electrode mixture-containing slurry was applied to one side of a 15 μm-thick, 280 mm-wide aluminum foil serving as a positive electrode current collector, adjusting the basis weight of the slurry, using a three-roll transfer coater to create a coating pattern with a coating width of 240-250 mm, a coating length of 125 mm, and an uncoated length of 20 mm. The solvent was then dried and removed in a hot air drying oven. The resulting electrode roll was trimmed on both sides and dried under reduced pressure at 130°C for 8 hours. The density of the positive electrode active material layer was then adjusted to 1.8 g / cm using a roll press. 3 The positive electrode (P1) was obtained by rolling the positive electrode active material layer and the positive electrode current collector so that the weight per unit area excluding the positive electrode current collector was 11.0 mg / cm. 2 It was.

[0172] (2-1-2) Preparation of the positive electrode (P2) (A) A composite oxide of lithium, nickel, manganese, and cobalt (LiNi 0.8 Mn 0.1 Co 0.1O2), (B) carbon black powder as a conductive additive, and (C) polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 94:3:3 to obtain a positive electrode mixture.

[0173] N-methyl-2-pyrrolidone was added as a solvent to the resulting positive electrode mixture to a solids content of 68% by mass, and the mixture was further mixed to prepare a positive electrode mixture-containing slurry. The positive electrode mixture-containing slurry was applied to one side of a 15 μm-thick, 280 mm-wide aluminum foil serving as a positive electrode current collector, adjusting the basis weight of the slurry, using a three-roll transfer coater to create a coating pattern with a coating width of 240-250 mm, a coating length of 125 mm, and an uncoated length of 20 mm. The solvent was then dried and removed in a hot air drying oven. The resulting electrode roll was trimmed on both sides and dried under reduced pressure at 130°C for 8 hours. The density of the positive electrode active material layer was then adjusted to 2.9 g / cm using a roll press. 3 The positive electrode (P2) was obtained by rolling the positive electrode active material layer and the positive electrode current collector so that the weight per unit area excluding the positive electrode current collector was 16.6 mg / cm. 2 It was.

[0174] (2-2) Preparation of negative electrode

[0175] (2-2-1) Preparation of negative electrode (N1) (a) Graphite powder as the negative electrode active material, (b) carbon black powder as a conductive additive, and (c) polyvinylidene fluoride (PVDF) as a binder were mixed in a solid mass ratio of 90:3:7 to obtain a negative electrode mixture. N-methyl-2-pyrrolidone was added as a solvent to the resulting negative electrode mixture to a solids content of 45% by mass and further mixed to prepare a negative electrode mixture-containing slurry. The negative electrode mixture-containing slurry was applied to one side of a copper foil with a thickness of 8 μm and a width of 280 mm, which served as a negative electrode current collector, while adjusting the basis weight, using a three-roll transfer coater to obtain a coating pattern with a coating width of 240 to 250 mm, a coating length of 125 mm, and an uncoated length of 20 mm. The solvent was then dried and removed in a hot air drying oven. The resulting electrode roll was trimmed on both sides and dried under reduced pressure at 80°C for 12 hours. The density of the negative electrode active material layer was then adjusted to 1.3 g / cm using a roll press.3 The negative electrode (N1) was obtained by rolling the negative electrode active material layer and the negative electrode current collector so that the coating weight excluding the negative electrode current collector was 5.4 mg / cm. 2 It was.

[0176] (2-2-2) Preparation of negative electrode (N2) (a) Graphite powder as the negative electrode active material, (b) carbon black powder as the conductive additive, and (c) polyvinylidene fluoride (PVDF) as the binder were mixed in a mass ratio of 90:3:7 to obtain a negative electrode mixture. N-methyl-2-pyrrolidone was added as a solvent to the resulting negative electrode mixture to a solids content of 45% by mass and further mixed to prepare a negative electrode mixture-containing slurry. The negative electrode mixture-containing slurry was applied to one side of a copper foil with a thickness of 8 μm and a width of 280 mm, which served as a negative electrode current collector, while adjusting the basis weight, using a three-roll transfer coater to obtain a coating pattern with a coating width of 240 to 250 mm, a coating length of 125 mm, and an uncoated length of 20 mm. The solvent was then dried and removed in a hot air drying oven. The resulting electrode roll was trimmed on both sides and dried under reduced pressure at 80°C for 12 hours. The density of the negative electrode active material layer was then adjusted to 1.4 g / cm using a roll press. 3 The negative electrode (N2) was obtained by rolling the negative electrode active material layer and the negative electrode current collector so that the weight per unit area excluding the negative electrode current collector was 11.9 mg / cm. 2 It was.

[0177] (2-3) Preparation of separator

[0178] (2-3-1) Preparation of separator (F1)

[0179] <Preparation of Polypropylene Resin Composition> Ultra-high molecular weight polypropylene resin (PP, MFR = 0.25) and ethylene / 1-butene copolymer (C2 / C4: density = 0.893 g / cm 2Pellets of PP (C2 / C4) with a melting point of 80°C and a molecular weight of 6.7 were dry-blended in a 50:50 (mass%) ratio of PP to C2 / C4, and then melt-mixed using a ZSK40 (Coperion, L / D = 46). To minimize resin decomposition and denaturation, the resin inlet hopper and raw material tank were completely sealed, and nitrogen was continuously flowed from the bottom of the hopper to maintain the oxygen concentration near the raw material inlet at 50 ppm or less. All vents were also completely sealed to prevent air leakage into the cylinder. This oxygen concentration reduction significantly suppressed polymer decomposition and denaturation even under high-temperature conditions, further enabling the fine dispersion of the ethylene / 1-butene copolymer. After melt-mixing, the strand was pulled from an 8-hole die and cooled in a water-cooled bath. The molten mixture was then cut into pellets using a pelletizer.

[0180] <Preparation of microporous membrane (single layer)> The pellets were dry-blended with ultra-high molecular weight polypropylene resin (PP, MFR = 0.25) at a mass ratio of PP:pellets = 90:10 (mass%), then melted in a 2.5-inch extruder and fed into an annular die using a gear pump. This resulted in a resin ratio of PP:C2 / C4 = 95:5 (mass%). The die temperature was set to 230°C, and the molten polymer was cooled by blown air and then wound up on a roll. The extruded precursor (raw film) had a thickness of 15 μm, and the raw film was then annealed at 130°C for 15 minutes. The annealed film was then cold-stretched to 21% at room temperature, then hot-stretched to 156% at 120°C, and relaxed to 126% at 125°C to form a separator with a single layer of microporous membrane. After the above stretching and perforation, the physical properties of the microporous membrane were measured.

[0181] (2-3-2) Preparation of separator (F2)

[0182] <Preparation of Polypropylene Resin Composition> A polypropylene resin composition was prepared in the same manner as for the separator (F1).

[0183] <Preparation of microporous membrane (separator with three-layer microporous membrane)> The pellets were dry-blended with ultra-high molecular weight polypropylene resin (PP, MFR = 0.25) at a mass ratio of PP:pellets = 90:10 (mass%), then melted in a 2.5-inch extruder and fed into an annular die using a gear pump. This resulted in a resin ratio of PP:C2 / C4 = 95:5 (mass%).

[0184] The die temperature was set at 240°C, and the molten polymer was cooled by blown air and then wound up on a roll. The extruded PP precursor (raw film) had a thickness of 6 µm.

[0185] High molecular weight polyethylene resin (PE, MFR=0.38) was melted in a 2.5-inch extruder and fed to an annular die using a gear pump.

[0186] The die temperature was set at 210°C, and the molten polymer was cooled by blown air and then wound up on a roll. The extruded PE precursor (raw film) had a thickness of 6 µm.

[0187] The resulting PP precursor and the resulting PE precursor were stacked together in a triple-layer structure (PP precursor / PE precursor / PP precursor) and laminated at 120°C to obtain a triple-layered raw film. This raw film was annealed at 125°C for 20 minutes. The annealed film was then cold-stretched to 15% at room temperature, then hot-stretched to 150% at 115°C, and relaxed to 103% at 125°C to form a separator with a triple-layered microporous membrane structure. After the stretching and perforation, the separator's physical properties were measured.

[0188] [Average long pore diameter (nm), pore diameter ratio] The average pore diameter of the microporous membrane was obtained by observing the surface of the microporous membrane with an SEM and measuring the pore diameter in the SEM image. Specifically, the surface of the microporous membrane was observed with an SEM at 30,000 magnification, and the pore diameters of the micropores within an image area of ​​4.0 μm × 2.8 μm were measured and averaged to obtain the average pore diameter a. The pore diameters in the direction perpendicular to the pore diameter direction of each pore were also measured and averaged to obtain the average pore diameter b. The pore diameter ratio (a / b) was calculated by dividing the average pore diameter by the average pore diameter. Figure 3 shows an example of an SEM image of a polypropylene microporous membrane used as the separator (F1). For example, the pore diameter and pore diameter ratio can be calculated from the observation in Figure 3.

[0189] [Thickness measurement (μm)] The thickness of the separator including the microporous membrane was measured at room temperature of 23±2°C using a Digimatic Indicator IDC112 manufactured by Mitutoyo Corporation.

[0190] [Porosity (%)] A 5 cm x 5 cm square sample was cut out from the separator including the microporous membrane, and the porosity was calculated from the volume and mass of the sample using the following formula. Porosity (%) = (Volume (cm 3 ) - mass (g) / density of resin composition (g / cm 3 )) / volume(cm 3 ) x 100

[0191] [Air permeability resistance (sec / 100cm) 3 )] The air resistance of the separator including the microporous membrane was measured using a Gurley air permeability meter in accordance with JIS P-8117.

[0192] (2-4) Assembly of small non-aqueous secondary batteries The positive electrode (P1) obtained as described above was punched into a disk shape with a diameter of 15.958 mm, and the negative electrode (N1) obtained as described above was punched into a disk shape with a diameter of 16.156 mm. These were then used to form the separator (F1) obtained as described above (pore size ratio 8.1, film thickness 25 μm, air permeation resistance 217 sec / 100 cm). 3A laminate was obtained by stacking two sheets of aluminum foil on both sides of a sheet with a porosity of 55%. The laminate was inserted into a disk-shaped battery case made of SUS. Next, 200 μL of non-aqueous electrolyte (S01, S06 to S10) was poured into the battery case, and the laminate was immersed in the non-aqueous electrolyte. After that, the battery case was sealed and the non-aqueous electrolyte was allowed to fully penetrate the laminate, thereby obtaining a small non-aqueous secondary battery (P1 / F1 / N1).

[0193] Using the same procedure, P1 was used for the positive electrode, N1 for the negative electrode, and separator F2 (pore size ratio 2.5, film thickness 12 μm, air resistance 214 s / 100 cm 3 A small nonaqueous secondary battery (P1 / F2 / N1) was obtained, using S09 to S10 as the nonaqueous electrolyte, with a porosity of 45%. A small nonaqueous secondary battery (P2 / F1 / N2) was also obtained, using P2 as the positive electrode, N2 as the negative electrode, separator F1, and S11 to S12 as the nonaqueous electrolyte.

[0194] (3) Evaluation of small non-aqueous secondary batteries The small nonaqueous secondary batteries obtained as described above were first subjected to an initial charge treatment and initial charge / discharge capacity measurement according to the procedure in (3-1) below. Next, each small nonaqueous secondary battery was evaluated according to the procedure in (3-2). Charge / discharge was performed using a charge / discharge device ACD-M01A (product name) manufactured by Asuka Electronics Co., Ltd. and a programmable thermostatic bath IN804 (product name) manufactured by Yamato Scientific Co., Ltd.

[0195] In this specification, 1 C means the current value at which a fully charged battery is expected to be discharged to completion in 1 hour when discharged at a constant current.

[0196] Specifically, for a small nonaqueous secondary battery (P1 / F1 or F2 / N1), 1C means the current value expected to be discharged from a fully charged state of 3.5 V to 2.0 V at a constant current, with the discharge expected to be completed in one hour. For a small nonaqueous secondary battery (P2 / F1 / N2), 1C means the current value expected to be discharged from a fully charged state of 4.2 V to 3.0 V at a constant current, with the discharge expected to be completed in one hour.

[0197] The small nonaqueous secondary battery (P1 / F1 or F2 / N1) assembled according to the above procedures (2-4) and (3) is a 3 mAh class cell, with a battery voltage of 3.5 V in a fully charged state and a current value of 3 mA equivalent to 1 C. Similarly, the small nonaqueous secondary battery (P2 / F1 / N2) is a 6 mAh class cell, with a battery voltage of 4.2 V in a fully charged state and a current value of 6 mA equivalent to 1 C. Hereinafter, unless otherwise specified, the notation of current value and voltage will be omitted for convenience.

[0198] (3-1) Initial charge / discharge treatment of small non-aqueous secondary batteries The ambient temperature of the small nonaqueous secondary battery (P1 / F1 or F2 / N1) was set to 25°C, and the battery was charged at a constant current equivalent to 0.1 C until it reached a fully charged state. Then, the battery was charged at a constant voltage for 1.5 hours. The battery was then discharged to a predetermined voltage at a constant current equivalent to 0.1 C. The initial efficiency was calculated by dividing the discharge capacity by the charge capacity. For batteries with an initial efficiency of less than 80%, the battery capacity was not met, making it difficult to obtain accurate evaluation results. Therefore, subsequent evaluation tests could not be performed. The initial charge / discharge process of the small nonaqueous secondary battery (P2 / F1 / N2) was performed using the same procedure.

[0199] (3-2) Output test of small non-aqueous secondary batteries (P1 / F1 or F2 / N1) The small nonaqueous secondary batteries (P1 / F1 or F2 / N1) that had undergone the initial charge-discharge treatment by the method described in (3-1) above were charged at a constant current of 1 C at an ambient temperature of 25°C until they reached 3.5 V, and then charged at a constant voltage of 3.5 V until the current value decayed to 0.025 C. They were then discharged at 0.1 C to 2.0 V.

[0200] Next, the battery was charged at a constant current of 1 C until it reached 3.5 V, and then charged at a constant voltage of 3.5 V until the current value decayed to 0.025 C. It was then discharged at 0.5 C to 2.0 V. Next, the battery was charged at a constant current of 1 C until it reached 3.5 V, and then charged at a constant voltage of 3.5 V until the current value decayed to 0.025 C. It was then discharged at 1 C until it reached 2.0 V. Next, the battery was charged at a constant current of 1 C until it reached 3.5 V, and then charged at a constant voltage of 3.5 V until the current value decayed to 0.025 C. It was then discharged at 5 C to 2.0 V. Next, the battery was charged at a constant current of 1 C until it reached 3.5 V, and then charged at a constant voltage of 3.5 V until the current value decayed to 0.025 C. It was then discharged at 10 C to 2.0 V. Next, the battery was charged at a constant current of 1 C until it reached 3.5 V, and then charged at a constant voltage of 3.5 V until the current value decayed to 0.025 C. It was then discharged at 20 C to 2.0 V. The 20C discharge capacity was calculated as the 20C capacity retention rate when the 0.1C discharge capacity in the first cycle was taken as 100%.

[0201] The 20C capacity retention rate is an index of output performance at 25°C, and is preferably 70% or more, more preferably 73.5% or more, and even more preferably 75% or more.

[0202] [Examples 1 to 8, Comparative Examples 1 to 4] The impregnation of the separator was evaluated according to the method described in (1-1) above using the non-aqueous electrolyte solutions shown in Table 1. The test results are shown in Table 3.

[0203] [Table 3] ○: Impregnated in the thickness direction of the separator ×: Not impregnated in the thickness direction of the separator

[0204] As shown in Table 3, Comparative Examples 1 to 4, in which the surfactant according to this embodiment was not added to the electrolyte solution, exhibited poor impregnation of the separator in the thickness direction, resulting in a decrease in initial efficiency or output performance even after the batteries were fabricated. On the other hand, Examples 1 to 8, in which the surfactant according to this embodiment was added to the electrolyte solution, exhibited good impregnation of the separator in the thickness direction, and are expected to suppress a decrease in initial efficiency or output performance.

[0205] [Examples 9 to 13, Comparative Examples 5 to 7] Here, we will explain the interpretation of the test results for the small nonaqueous secondary batteries (P1 / F1 or F2 / N1). Small nonaqueous secondary batteries (P1 / F1 or F2 / N1) were fabricated according to the method described in (2) above. Next, each small nonaqueous secondary battery (P1 / F1 or F2 / N1) was evaluated according to the procedures described in (3-1) and (3-2) above. The test results are shown in Table 4.

[0206] [Table 4]

[0207] As shown in Table 4, Examples 9 to 13 exhibited 20C capacity retention rates of 72% or more. Among them, Examples 12 and 13 exhibited 20C capacity retention rates of 75% or more, exhibiting superior output performance to Examples 9 to 11. On the other hand, Comparative Example 5 exhibited a 20C capacity retention rate of less than 70%. For Comparative Examples 6 and 7, the initial efficiency was less than 79%, so output tests after the initial charge-discharge treatment were not performed.

[0208] (3-3) AC impedance measurement of small non-aqueous secondary batteries (P2 / F1 / N2) The small nonaqueous secondary battery (P2 / F1 / N2) that had been subjected to the initial charge-discharge treatment by the method described in (3-1) above was charged at a constant current of 1 C at an ambient temperature of 25°C until it reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value decayed to 0.025 C. Then, AC impedance measurements were performed.

[0209] The AC impedance measurements were performed using a Solartron Frequency Response Analyzer 1400 (trade name) and a Solartron Potentio-Galvanostat 1470E (trade name). An AC signal was applied while varying the frequency from 1000 kHz to 0.01 Hz, and the impedance was measured from the voltage-current response signal. The value where the signal intersected the horizontal axis of a complex impedance plane plot (Cole-Cole plot) was calculated as the bulk resistance. The resistance value was the real part (horizontal axis). The amplitude of the applied AC voltage was ±5 mV. The ambient temperature of the battery during AC impedance measurements was 25°C.

[0210] The bulk resistance is an index of the separator's ability to be impregnated with electrolyte, and a battery with a higher bulk resistance indicates poorer separator impregnation with electrolyte, while a battery with a lower bulk resistance indicates better separator impregnation with electrolyte. From this perspective, the bulk resistance is preferably 1.5 Ω or less, more preferably 1.3 Ω or less, and even more preferably 1.0 Ω or less.

[0211] [Example 14, Comparative Example 8] Here, we will explain the interpretation of the test results for the small nonaqueous secondary battery (P2 / F1 / N2). The small nonaqueous secondary battery (P2 / F1 / N2) was fabricated according to the method described in (2) above. Next, each small nonaqueous secondary battery (P2 / F1 / N2) was evaluated according to the procedures described in (3-1) and (3-3) above. The test results are shown in Table 5.

[0212] [Table 5]

[0213] As shown in Table 5, Example 14 exhibited a good bulk resistance of 1.0Ω, while Comparative Example 8 exhibited a high bulk resistance of 1.7Ω.

[0214] As shown by these results, it has become clear that by using a nonaqueous electrolyte solution within the range of the constituent requirements of the present invention, impregnation into the separator is good, and therefore a nonaqueous secondary battery with high output performance can be provided. [Industrial Applicability]

[0215] The nonaqueous secondary battery using the nonaqueous solvent of the present invention is expected to be used, for example, as a rechargeable battery for portable devices such as mobile phones, portable audio devices, personal computers, and IC (Integrated Circuit) tags; a rechargeable battery for automobiles such as hybrid vehicles, plug-in hybrid vehicles, and electric vehicles; a low-voltage power supply such as a 12V-class power supply, a 24V-class power supply, or a 48V-class power supply; a residential power storage system; an IoT device; etc. Furthermore, the nonaqueous secondary battery using the nonaqueous solvent of the present invention can also be used in cold regions and outdoors in summer. [Explanation of symbols]

[0216] 100 Nonaqueous secondary battery 110 Battery casing 120 Battery enclosure space 130 Positive electrode lead body 140 Negative electrode lead body 150 positive electrode 160 negative electrode 170 Separator

Claims

1. A non-aqueous secondary battery comprising a positive electrode containing one or more positive electrode active materials capable of absorbing and releasing lithium ions, a negative electrode containing a material capable of absorbing and releasing lithium ions, a non-aqueous electrolyte solution, and a separator, the non-aqueous electrolyte solution contains a non-aqueous solvent, a lithium salt, and a surfactant; the non-aqueous solvent contains 5% by volume or more and 90% by volume or less of acetonitrile, the lithium salt includes a lithium-containing imide salt, and the content of the lithium salt relative to the nonaqueous electrolyte solution is 1 mol / L or more and 2 mol / L or less; the surfactant is a compound that includes a nonionic surfactant and does not include a hydroxy group as a substituent, the nonionic surfactant is at least one compound selected from a fatty acid ester compound, a phosphate ester compound, an ether compound, an ester ether compound, and an alkanolamide compound; The nonionic surfactant has at least one substituent having 3 or more carbon atoms, the separator has a polypropylene-based microporous film, The polypropylene-based microporous membrane has a polymer matrix containing polypropylene extending in one direction perpendicular to the membrane thickness, and has fibrils containing the polypropylene between the polymer matrices and micropores between a plurality of the fibrils. A non-aqueous secondary battery characterized by:

2. 2. The non-aqueous secondary battery according to claim 1, wherein the nonionic surfactant is the fatty acid ester compound or the phosphoric acid ester compound.

3. the fatty acid ester compound is ethyl laurate, or 3. The nonaqueous secondary battery according to claim 2, wherein the phosphate ester compound is triamyl phosphate, tris(2-butoxyethyl) phosphate, or tris(2-ethylhexyl) phosphate.

4. 4. The non-aqueous secondary battery according to claim 1, wherein the nonionic surfactant is contained in an amount of 0.1 mass % or more and 3 mass % or less with respect to the total amount of the non-aqueous electrolyte solution.

5. The nonaqueous secondary battery according to any one of claims 1 to 4, wherein the polypropylene-based microporous membrane has an average pore diameter a and a pore diameter b perpendicular to the average pore diameter a, and the pore diameter ratio a / b is 0.5 or more and 30 or less.

6. The separator has a porosity of 30% or more and 80% or less and an air permeability of 100 seconds / 100 cm ３ More than 500 seconds / 100cm ３ The nonaqueous secondary battery according to any one of claims 1 to 5, wherein:

7. The lithium salt is LiPF ６ and further comprising LiPF ６ <The lithium-containing imide salt and LiPF at a molar concentration that forms the lithium-containing imide salt. ６ The non-aqueous secondary battery according to any one of claims 1 to 6, comprising:

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