Non-aqueous secondary batteries

Abstract

To provide a nonaqueous secondary battery which is low in volatility of nonaqueous liquid electrolyte and high in ion conductivity even in the case of using an acetonitrile-containing nonaqueous liquid electrolyte and a polypropylene-based separator having a particular pore structure.SOLUTION: A nonaqueous secondary battery comprises a nonaqueous liquid electrolyte which contains a nonaqueous solvent containing acetonitrile of 5-90 vol.%, a lithium salt and a cellulose-based compound (excluding carboxymethyl cellulose). The lithium salt includes a lithium-containing imide salt, and it is 1-2 mol / L to the nonaqueous liquid electrolyte. The nonaqueous secondary battery further comprises a separator including a polypropylene-based microporous film. The polypropylene-based microporous film has a polypropylene-containing polymer matrix extending in one direction perpendicular to a film thickness, and it has polypropylene-containing fibrils among the polymer matrix, and lots of pores among the fibrils.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Lithium-ion batteries and other non-aqueous secondary 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] In realizing such small, high-capacity batteries, the selection of separators placed between electrodes is crucial. Conventionally, paper, woven fabric, nonwoven fabric, and glass mats have been used as separators, but these have structural safety issues. In recent years, separators with small pore sizes, such as polyolefin microporous membranes, have been used, improving safety through their shutdown function and function of suppressing short circuits between electrodes.

[0004] Furthermore, conventional non-aqueous electrolytes for non-aqueous secondary batteries have used cyclic carbonate esters or cyclic ester solvents, which have resulted in poor impregnation of the electrolyte with the separator.

[0005] However, in recent years, the impregnation properties of separators have been improved by diluting the linear carbonate ester in a non-aqueous electrolyte solution using a cyclic carbonate ester or a cyclic ester solvent. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2019-164965 [Overview of the project] [Problems that the invention aims to solve]

[0007] As described above, compared to non-aqueous electrolytes using esters as solvents, non-aqueous electrolytes containing acetonitrile have high ionic conductivity, and therefore, when applied to non-aqueous secondary batteries, high output performance can be obtained. On the other hand, it has been newly discovered that when using polypropylene separators with small pore sizes that are manufactured by the dry process, there is a problem with the volatility of non-aqueous electrolytes containing acetonitrile. Furthermore, when manufacturing batteries containing polypropylene separators manufactured by the dry process, it is necessary to extend the vacuum process time to impregnate the separator with the electrolyte. As a result, it has been newly discovered that the low-boiling point solvent in the electrolyte volatilizes, causing initial defects.

[0008] Patent Document 1 reports that by adding polymer particles that swell when impregnated with the electrolyte to the electrolyte, the electrolyte can be replenished when the volume of the negative electrode shrinks during charging and discharging, thereby suppressing the depletion of the electrolyte. However, Patent Document 1 does not address the issue of electrolyte volatilization during battery manufacturing. Furthermore, the carboxymethylcellulose (CMC) used in this document does not dissolve in non-aqueous electrolytes containing acetonitrile and precipitates, so the effect of suppressing volatilization cannot be obtained.

[0009] The present invention has been made in view of the above problems, and aims to provide a non-aqueous secondary battery in which the non-aqueous electrolyte containing acetonitrile and a polypropylene separator having a specific pore structure have low volatility and high ionic conductivity. [Means for solving the problem]

[0010] The inventors of the present invention have diligently conducted research to solve the above-mentioned problems, and as a result have found that the above problems can be solved by using a non-aqueous electrolyte having the following configuration in a specific non-aqueous secondary battery, and have completed the present invention. That is, the present invention is as follows. (1) A non-aqueous secondary battery comprising a positive electrode containing one or more positive electrode active materials capable of intercalating and releasing lithium ions, a negative electrode containing a material capable of intercalating and releasing lithium ions, a non-aqueous electrolyte, and a separator, The non-aqueous electrolyte comprises a non-aqueous solvent, a lithium salt, and a cellulosic compound. The non-aqueous solvent contains 5% to 90% by volume of acetonitrile. The lithium salt contains a lithium-containing imide salt, and its content is between 1 mol / L and 2 mol / L relative to the non-aqueous electrolyte. The aforementioned cellulosic compounds, excluding carboxymethylcellulose, The separator has a polypropylene microporous membrane, The polypropylene-based microporous film has a polymer matrix containing polypropylene that extends in one direction perpendicular to the film thickness, and has polypropylene-containing fibrils between the polymer matrix and micropores between the multiple fibrils. A non-aqueous secondary battery characterized by the following features. (2) The non-aqueous secondary battery described in item (1), wherein the cellulosic compound is ethylcellulose. (3) The non-aqueous secondary battery according to item (1) or (2), wherein the cellulose compound is contained in an amount of 0.1% by mass or more and 10% by mass or less relative to the total amount of the non-aqueous electrolyte. (4) The non-aqueous electrolyte has a viscosity of 1.5 mPa·s or more and 40 mPa·s or less at 25°C, and an ionic conductivity of 10 mS / cm or more and 40 mS / cm or less, as described in any one of items (1) to (3). (5) A non-aqueous secondary battery according to any one of items (1) to (4), wherein the pore size ratio a / b of the polypropylene microporous film, which is the average elongated pore diameter a to the pore diameter b perpendicular to the average elongated pore diameter a, is 0.5 or more and 30 or less. (6) The separator has a porosity of 30% to 80% and an air permeability resistance of 100 seconds / 100 cm 3 More than 500 seconds / 100cm 3A non-aqueous secondary battery described in any one of the following items (1) to (5). (7) The non-aqueous secondary battery according to any one of items (1) to (6), wherein the lithium salt further comprises LiPF6, and the lithium-containing imide salt and LiPF6 are present in a molar concentration such that LiPF6 < the lithium-containing imide salt. [Effects of the Invention]

[0011] According to the present invention, even when using a non-aqueous electrolyte containing acetonitrile and a polypropylene separator having a specific pore structure, it is possible to provide a non-aqueous secondary battery that suppresses the volatility of the non-aqueous electrolyte and has high ionic conductivity. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic plan view showing an example of a non-aqueous secondary battery according to this embodiment. [Figure 2] Figure 1 is a cross-sectional view of a non-aqueous secondary battery along line AA. [Figure 3] This is a scanning electron microscope (SEM) image of a polypropylene microporous membrane. [Modes for carrying out the invention]

[0013] The following describes in detail embodiments for carrying out the present invention (hereinafter simply referred to as "this embodiment"). The present invention is not limited to the following embodiments, and various modifications are possible without departing from the spirit thereof. In this specification, numerical ranges described using "~" include the numerical values ​​described before and after them. In this embodiment, the upper or lower limit value described in a numerical range described in steps can be replaced with the upper or lower limit value of a numerical range described in another step. In this embodiment, the upper or lower limit value described in a numerical range can also be replaced with the value described in the example. In this embodiment, the scale, shape, length, and other configurations of each part shown in the drawings may be exaggerated in order to further enhance clarity.

[0014] [Non-aqueous secondary battery] The non-aqueous electrolyte according to this embodiment can be used to construct a non-aqueous secondary battery.

[0015] The non-aqueous secondary battery according to this embodiment is constructed by housing a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte in a suitable battery casing.

[0016] The non-aqueous secondary battery according to this embodiment may specifically be the non-aqueous secondary battery 100 shown in Figures 1 and 2. Here, Figure 1 is a schematic plan view of the non-aqueous secondary battery, and Figure 2 is a cross-sectional view taken along line AA of Figure 1.

[0017] The non-aqueous secondary battery 100 shown in Figures 1 and 2 is composed of pouch-type cells. The non-aqueous secondary battery 100 houses a laminated electrode body, which is constructed by stacking a positive electrode 150 and a negative electrode 160 with a separator 170 in between, and a non-aqueous electrolyte (not shown) within a space 120 of the battery casing 110. The battery casing 110 is made of, for example, aluminum laminate film, and is sealed by heat-sealing the upper and lower films at the outer periphery of the space formed by two aluminum laminate films. The laminate, in which the positive electrode 150, separator 170, and negative electrode 160 are stacked in order, is impregnated with the non-aqueous electrolyte. However, in Figure 2, in order to avoid complexity in the drawing, the individual layers constituting the battery casing 110, as well as the individual layers of the positive electrode 150 and negative electrode 160, are not shown separately.

[0018] The aluminum laminate film constituting the battery casing 110 is preferably made by coating both sides of an aluminum foil with a polyolefin-based resin.

[0019] The positive electrode 150 is connected to the positive electrode lead 130 within the non-aqueous secondary battery 100. Although not shown in the diagram, the negative electrode 160 is also connected to the negative electrode lead 140 within the non-aqueous secondary battery 100. The positive electrode lead 130 and the negative electrode lead 140 each have one end extended outside the battery casing 110 so that they can be connected to external devices, and their ionomer portions are heat-sealed to one side of the battery casing 110.

[0020] The non-aqueous secondary battery 100 shown in Figures 1 and 2 has one stacked electrode body each for the positive electrode 150 and the negative electrode 160, but the number of stacked positive electrodes 150 and negative electrodes 160 can be appropriately increased depending on the capacity design. In the case of a stacked electrode body having multiple positive electrodes 150 and negative electrodes 160, the 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 taken out of the battery. The tabs of the same electrode can be made from the exposed part of the current collector, or from a metal piece welded to the exposed part of the current collector, and so on.

[0021] The positive electrode 150 consists of a positive electrode current collector and a positive electrode active material layer. The negative electrode 160 consists of a negative electrode current collector and a negative electrode active material layer.

[0022] The positive electrode active material layer contains the positive electrode active material, and the negative electrode active material layer contains the negative electrode active material.

[0023] The positive electrode 150 and the negative electrode 160 are arranged so that the positive electrode active material layer and the negative electrode active material layer face each other via a separator 170.

[0024] The following describes each element constituting the non-aqueous secondary battery according to this embodiment.

[0025] [Positive electrode] In the non-aqueous secondary battery according to this embodiment, the positive electrode contains one or more positive electrode active materials capable of intercalating and releasing lithium ions, and optionally, the positive electrode active material layer may be present on one or both sides of the positive electrode current collector.

[0026] [Positive current collector] The positive current collector is composed of, for example, a metal foil such as aluminum foil, nickel foil, or stainless steel foil. The positive current collector may have a carbon coating on its surface and may be processed into a mesh shape. The thickness of the positive current collector is preferably 5 to 40 μm, more preferably 7 to 35 μm, and even more preferably 9 to 30 μm.

[0027] [Positive electrode active material layer] The positive electrode active material layer contains a positive electrode active material and may further contain a conductive assistant and / or a binder as required.

[0028] (Positive electrode active material) The positive electrode active material layer preferably contains, as a positive electrode active material, a material capable of occluding and releasing lithium ions. When such a material is used, it is preferable because high voltage and high energy density tend to be obtained.

[0029] 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; are mentioned, and 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.

[0030] Also, examples of the positive electrode active material include Lithium cobalt oxide represented by LiCoO2; Lithium manganese oxide 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 0.05 O2, 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 MO2 (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) lithium-containing composite metal oxide; MnO2, FeO2, FeS2, V2O5, V6O 13 , TiO2, TiS2, MoS2, and NbSe2, metal oxides or metal chalcogenides having tunnel structures and layered structures; Sulfur; and Conductive polymers represented by polyaniline, polythiophene, polyacetylene, and polypyrrole; etc. are exemplified.

[0031] In particular, when the Ni content ratio q of the Li-containing metal oxide represented by the general formula (a) satisfies 0.5 < q < 1.2, it is preferable because both reduction of the amount of rare metal Co used and increase in the energy density are achieved.

[0032] Here, as the Ni content ratio of the Li-containing metal oxide increases, deterioration tends to progress at a lower voltage. In the positive electrode active material of the lithium-containing metal oxide represented by the general formula (a), there are active sites that oxidatively deteriorate the non-aqueous electrolyte, and these active sites may inadvertently consume the compound added to protect the negative electrode on the positive electrode side. Among them, acid anhydrides tend to be particularly susceptible to this effect. 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 a problem.

[0033] In addition, these additive decomposition products incorporated and deposited on the positive electrode side not only increase the internal resistance of the non-aqueous secondary battery, but also accelerate the deterioration of the lithium salt, and furthermore, the protection of the negative electrode surface becomes insufficient. To deactivate the active sites that essentially oxidatively deteriorate the non-aqueous electrolyte, it is preferable to control the Jahn-Teller distortion or coexist with components that play a role as a neutralizing agent. Therefore, the positive electrode active material preferably contains at least one metal selected from the group consisting of Al, Sn, In, Fe, V, Cu, Mg, Ti, Zn, Mo, Zr, Sr, and Ba.

[0034] For the same reason 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. Moreover, it is 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 because it does not inhibit the permeation of lithium ions.

[0035] The positive electrode active material may be a lithium-containing compound other than a lithium-containing metal oxide represented by general formula (a). Examples of such lithium-containing compounds include a composite oxide containing lithium and a transition metal element, a metal chalcogenide containing lithium, a metal phosphate compound containing lithium and a transition metal element, and a metal silicate compound containing lithium and a transition metal element. From the viewpoint of obtaining a higher voltage, 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 is particularly preferred as the lithium-containing compound.

[0036] More specifically, lithium-containing compounds are given by the following formula (XXa): Li v M I D2(XXa) {In the formula, D represents the chalcogen element, M I This represents one or more transition metal elements, including at least one other transition metal element, and the value of v is determined by the battery's charge / discharge state, representing a number between 0.05 and 1.10. The following equation (XXb): Li w M II PO4(XXb) {In formula, M II}, 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 by the battery's charge / discharge state and is a number between 0.05 and 1.10, and u is a number between 0 and 2. Examples of compounds represented by each of these are listed below.

[0037] The lithium-containing compound represented by formula (XXa) above has a layered structure, and the compounds represented by formulas (XXb) and (XXc) above have an olivine structure. These lithium-containing compounds may be modified in such ways as to stabilize the structure, by substituting some of the transition metal elements with Al, Mg, or other transition metal elements, incorporating these metal elements into the grain boundaries, substituting some of the oxygen atoms with fluorine atoms, or coating at least a portion of the surface of the positive electrode active material with another positive electrode active material.

[0038] The positive electrode active material can be used alone or in combination of two or more. It is preferable that the positive electrode active material layer contains at least one transition metal element selected from Ni, Mn, and Co, as this enables reversible and stable intercalation and release of lithium ions and achieves a high energy density.

[0039] When lithium-containing compounds and other positive electrode active materials are used in combination as positive electrode active materials, the ratio of the lithium-containing compounds to the total positive electrode active material is preferably 80% by mass or more, and more preferably 85% by mass or more.

[0040] (Conductive additive) Examples of conductive additives include graphite; carbon black, such as acetylene black and Ketjenblack; and carbon fibers. The content of the conductive additive is preferably 1 to 20 parts by mass or less, and more preferably 2 to 15 parts by mass, per 100 parts by mass of positive electrode active material.

[0041] If the conductive additive content is too high, the volumetric energy density decreases, but if it is too low, the formation of electron conduction paths becomes insufficient. In particular, since the positive electrode active material layer has lower electron conductivity than the negative electrode active material layer, if the amount of conductive additive is insufficient, it becomes impossible to obtain the predetermined battery capacity when discharging or charging a non-aqueous secondary battery at a high current value. Therefore, in applications requiring high power and / or rapid charging, it is preferable to increase the content of the conductive additive to a range in which the reaction based on electron transfer does not become the rate-limiting factor.

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

[0043] [Formation of positive electrode active material layer] The positive electrode active material layer is formed by dispersing a slurry containing a positive electrode mixture, which is a mixture of the positive electrode active material and, if necessary, a conductive additive and a binder, in a solvent, onto a positive electrode current collector, drying (solvent removal), and pressing as necessary. Known solvents can be used for this purpose. Examples include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, and water.

[0044] [Negative electrode] The negative electrode in the non-aqueous secondary battery according to this embodiment contains a material capable of intercalating and releasing lithium ions, and optionally has a negative electrode active material layer on one or both sides of the negative electrode current collector.

[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 negative electrode current collector may also have a carbon coating on its surface or be processed into a mesh shape. 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 needed.

[0047] (Negative electrode active material) Examples of negative electrode active materials include amorphous carbon (hard carbon, soft carbon); graphite such as artificial graphite and natural graphite; carbon materials such as pyrolytic carbon, coke, glassy carbon, calcined organic polymer compounds, mesocarbon microbeads, carbon fibers, activated carbon, carbon colloids, 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 material can be used alone or in combination of two or more. Examples of the Si materials mentioned above include silicon, Si alloys, and Si oxides.

[0048] From the perspective of increasing the battery voltage, lithium ions are used as the negative electrode active material in the negative electrode active material, resulting in a 0.4V vs. Li / Li + It is preferable to include a material that can absorb at a lower potential.

[0049] The non-aqueous electrolyte according to this embodiment has the advantage of being able to suppress various degradation phenomena associated with volume changes of the negative electrode when repeated charge-discharge cycles occur, even when a Si material is applied as the negative electrode active material. Therefore, in the non-aqueous 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 because it has high capacity derived from the Si material while also having excellent charge-discharge cycle characteristics.

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

[0051] Silicon has an operating potential of approximately 0.5V (vsLi / Li + ) becomes approximately 0.05V (vsLi / Li) of the operating potential of graphite. +) is slightly higher. Therefore, using Si material reduces the risk of lithium electrodeposition. The acetonitrile used as the non-aqueous solvent in this embodiment may react with lithium metal in a reduction reaction, causing gas generation. Therefore, a negative electrode active material that is difficult to electrodeposit with lithium is preferable when used in combination with a non-aqueous electrolyte containing acetonitrile.

[0052] On the other hand, negative electrode active materials with excessively high operating potentials result in a decrease in the energy density of the battery. Therefore, from the perspective of improving energy density, the negative electrode active material should be 0.4V vs. Li / Li. + It is preferable for it to operate at a lower potential.

[0053] The Si material content is preferably in the range of 0.1% to 100% by mass, more preferably in the range of 1% to 80% by mass, and even more preferably in the range of 3% to 60% by mass, as the amount per unit of the total amount of the negative electrode active material layer. By adjusting the Si material content within the above range, it is possible to ensure a balance between high capacity of the non-aqueous secondary battery and charge / discharge cycle performance.

[0054] (Conductive additive) Examples of conductive additives include graphite; carbon black, such as acetylene black and Ketjenblack; and carbon fibers. The content of the conductive additive is preferably 20 parts by mass or less, and 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 carboxymethylcellulose, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid, and fluororubber. Diene rubbers, such as styrene-butadiene rubber, are also acceptable. The binder content is preferably 10 parts by mass or less, and more preferably 0.5 to 6 parts by mass, per 100 parts by mass of the negative electrode active material.

[0056] [Formation of the negative electrode active material layer] The negative electrode active material layer is formed by dispersing a negative electrode mixture, which is a mixture of the negative electrode active material and, if necessary, a conductive additive and / or binder, in a solvent, onto a negative electrode current collector, drying (solvent removal), and pressing as necessary. Known solvents can be used for this purpose. Examples include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, and water.

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

[0058] The electrolyte 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 hinder the resolution of the problem of the present invention. The amount of water is 300 ppm by mass or less, and more preferably 200 ppm by mass or less, relative to the total amount of the non-aqueous electrolyte. As long as the non-aqueous electrolyte has the configuration necessary to achieve the resolution of the problem of the present invention, other components can be appropriately selected and applied from known non-aqueous electrolyte materials used in lithium-ion batteries.

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

[0060] <Non-aqueous solvents> In this embodiment, "non-aqueous solvent" refers to the elements of a non-aqueous electrolyte excluding lithium salts and various additives. If the non-aqueous electrolyte contains electrode protection additives, "non-aqueous solvent" refers to the elements of the non-aqueous electrolyte excluding lithium salts and additives other than electrode protection additives. 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 solvents other than aprotic solvents, as long as it does not hinder the resolution of the problem of the present invention.

[0061] For example, a non-aqueous solvent in a non-aqueous electrolyte can contain acetonitrile as an aprotic solvent. The inclusion of acetonitrile in the non-aqueous solvent improves the ionic conductivity of the non-aqueous electrolyte, thereby increasing the diffusibility of lithium ions within the battery. Therefore, when the non-aqueous electrolyte contains acetonitrile, lithium ions can diffuse well even to areas near the current collector, which are difficult for lithium ions to reach during high-load discharge, particularly in positive electrodes with a thicker positive electrode active material layer and increased positive electrode active material content. This allows for sufficient capacity to be extracted even during high-load discharge, resulting in a non-aqueous secondary battery with excellent load characteristics.

[0062] Furthermore, the inclusion of acetonitrile in the non-aqueous solvent can improve the rapid charging characteristics of non-aqueous secondary batteries. In constant current (CC)-constant voltage (CV) charging of non-aqueous secondary batteries, the capacity per unit time during the CC charging period is greater than the capacity per unit time during the CV charging period. When acetonitrile is used as the non-aqueous solvent in the non-aqueous electrolyte, the area in which CC charging can be performed can be enlarged (the CC charging time can be extended), and the charging current can also be increased, thus significantly shortening the time it takes to bring a non-aqueous secondary battery from the start of charging to a fully charged state.

[0063] Furthermore, acetonitrile is readily reductively decomposed electrochemically. Therefore, when using acetonitrile, it is preferable to use another solvent (for example, 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% to 90% by volume per total amount of the non-aqueous solvent. More preferably, the acetonitrile content is 10% or more by volume, 15% or more by volume, 20% or more by volume, 30% or more by volume, and even more preferably 40% or more by volume per total amount of the non-aqueous solvent. This value is more preferably 85% or less by volume, and even more preferably 66% or less by volume. When the acetonitrile content is 5% or more by volume per total amount of the non-aqueous solvent, the ionic conductivity tends to increase, leading to the development of high-power characteristics, and furthermore, the dissolution of lithium salt can be promoted. Because the additives described later suppress the increase in the internal resistance of the battery, when the acetonitrile content in the non-aqueous solvent is within the above range, it tends to be possible to further improve high-temperature cycle characteristics and other battery characteristics while maintaining the excellent performance of acetonitrile.

[0065] Examples of aprotic solvents other than acetonitrile include cyclic carbonates, fluoroethylene carbonates, lactones, organic compounds other than those of general formula (1) that have a sulfur atom, chain-like fluorinated carbonates, cyclic ethers, mononitriles other than acetonitrile, alkoxy-substituted nitriles, dinitriles, cyclic nitriles, short-chain fatty acid esters, chain-like ethers, fluorinated ethers, ketones, and compounds in which some or all of the H atoms of the aprotic solvent are 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] Examples of fluoroethylene carbonates include 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] Examples of lactones include γ-butyrolactone, α-methyl-γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, δ-caprolactone, and ε-caprolactone;

[0069] Examples of organic compounds containing a sulfur atom include ethylene sulfite, propylene sulfite, butylene sulfite, pentene sulfite, sulfolane, 3-sulfolene, 3-methylsulfolane, 1,3-propanesultone, 1,4-butanesultone, 1-propene-1,3-sultone, dimethyl sulfoxide, tetramethylene sulfoxide, and ethylene glycol sulfite;

[0070] Examples of linear 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] Examples of cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, and 1,3-dioxane;

[0072] Other mononitriles besides acetonitrile include, for example, propionitrile, butyronitrile, valeronitrile, benzonitrile, and acrylonitrile;

[0073] Examples of alkoxy-substituted nitriles include methoxyacetonitrile and 3-methoxypropionitrile;

[0074] Examples of dinitriles include malononitrile, succinonitrile, glutaronitrile, adiponitrile, 1,4-dicyanoheptane, 1,5-dicyanopentane, 1,6-dicyanohexane, 1,7-dicyanoheptane, 2,6-dicyanoheptane, 1,8-dicyanooctane, 2,7-dicyanooctane, 1,9-dicyanononane, 2,8-dicyanononane, 1,10-dicyanodecane, 1,6-dicyanodecane, and 2,4-dimethylglutaronitrile;

[0075] Examples of cyclic nitriles include 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 hydroangelicaate, methyl caproate, ethyl acetate, ethyl propionate, ethyl isobutyrate, ethyl butyrate, ethyl isovalerate, ethyl valerate, ethyl pivalate, ethyl hydroangelicaate, ethyl caproate, propyl acetate, propyl propionate, propyl isobutyrate, propyl butyrate, propyl isovalerate, propyl valerate, propyl pivalate, propyl hydroangelicaate, propyl caproate, isopropyl acetate, isopropyl propionate, isopropyl isobutyrate, isopropyl butyrate, isopropyl isovalerate, isopropyl valerate, isopropyl pivalate Propyl, isopropyl hydroangelicaate, isopropyl caproate, butyl acetate, butyl propionate, butyl isobutyrate, butyl butyrate, butyl isovalerate, butyl valerate, butyl pivalate, butyl hydroangelicaate, butyl caproate, isobutyl acetate, isobutyl propionate, isobutyrate, isobutyrate, isobutyl isovalerate, isobutyl valerate, isobutyl pivalate, isobutyl hydroangelicaate, 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 hydroangelicaate, and tert-butyl caproate;

[0077] Examples of linear ethers include 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 This represents an organic group that may contain a fluorine atom.

[0079] Examples of ketones include acetone, methyl ethyl ketone, and methyl isobutyl ketone;

[0080] Examples of compounds in which some or all of the H atoms in the aprotic solvent are substituted with halogen atoms include compounds in which the halogen atom is fluorine; These are some examples.

[0081] Examples of fluorinated chain carbonates include methyltrifluoroethyl 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 above fluorinated chain carbonates have the following general formula: R cc -OC(O)OR dd (In the formula, R cc and R dd These are CH3, CH2CH3, CH2CH2CH3, CH(CH3)2, and CH2Rf ee At least one selected from the group consisting of Rf ee is an alkyl group having 1 to 3 carbon atoms in which at least one fluorine atom replaces a hydrogen atom, and R cc and / or R dd It can be represented as (containing at least one fluorine atom).

[0082] Furthermore, 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 At least one selected from the group consisting of R gg These are CH3, CH2CH3, CH2CH2CH3, CH(CH3)2, and CH2Rf ii At least one selected from the group consisting of Rf hh Rf is a C1-C3 alkyl group in which at least one fluorine atom may substitute for a hydrogen atom. ii is an alkyl group having 1 to 3 carbon atoms in which at least one fluorine atom replaces a hydrogen atom, and R ff and / or R gg It contains at least one fluorine atom, R ff If CF2H, then R gg It can be represented as (not CH3).

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

[0084] In this embodiment, the non-aqueous solvent preferably contains one or more cyclic carbonates and linear carbonates together with acetonitrile, from the viewpoint of improving the stability of the non-aqueous electrolyte. From this viewpoint, it is more preferable to use a cyclic carbonate together with acetonitrile in this embodiment, and even more preferable to use both cyclic carbonates and linear carbonates together with acetonitrile.

[0085] When using a cyclic carbonate with acetonitrile, it is particularly preferable that such cyclic carbonate includes ethylene carbonate, vinylene carbonate, and / or fluoroethylene carbonate.

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

[0087] In this embodiment, the lithium salt includes a lithium-containing imide salt as an imide salt, and the lithium-containing imide salt is LiN(SO2C m F 2m+1 It is preferable that the imide salt is represented by the formula 2{wherein m is an integer from 0 to 8}.

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

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

[0090] When the non-aqueous solvent contains acetonitrile, the lithium salt preferably contains LiPF6. Since the saturation concentration of the imide salt relative to acetonitrile is higher than the saturation concentration of LiPF6, it is preferable to include LiPF6 and the imide salt at a molar concentration such that LiPF6 ≤ imide salt, as this suppresses the association and precipitation of the lithium salt and acetonitrile at low temperatures. Furthermore, it is preferable that the imide salt content be between 0.5 moles and 3.0 moles per liter of the non-aqueous solvent, from the viewpoint of ensuring sufficient ion supply to the non-aqueous electrolyte according to this embodiment.

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

[0092] Furthermore, as described above, by limiting the content of imide salts and LiPF6, it becomes possible to more effectively suppress the increase in resistance during high-temperature heating.

[0093] (Fluorine-containing inorganic lithium salt) The lithium salt in this embodiment may include a fluorine-containing inorganic lithium salt. Here, "fluorine-containing inorganic lithium salt" refers to a lithium salt that does not contain carbon atoms as anions, contains fluorine atoms as anions, and is soluble in acetonitrile. Fluorine-containing inorganic lithium salts are excellent in that they form a passive film on the surface of the positive electrode current collector, thereby suppressing 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 Examples include {where b represents an integer between 0 and 3}, and one or more of these can be selected and 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 these, the use of a fluorine-containing inorganic lithium salt having a phosphorus atom is more preferable because it makes it easier to release free fluorine atoms. A typical fluorine-containing inorganic lithium salt is LiPF6, which dissolves and releases the PF6 anion. When a fluorine-containing inorganic lithium salt having a boron atom is used as the fluorine-containing inorganic lithium salt, it is preferable because it makes it easier to capture excess free acid components that may lead to battery degradation, and from this viewpoint, LiBF4 is preferred.

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

[0097] The amount of fluorine-containing inorganic lithium salt in the non-aqueous electrolyte according to this embodiment may be, for example, 0.05 moles or more and 1.0 moles or less per liter of non-aqueous solvent.

[0098] (Organolithium salts) The lithium salt in this embodiment may include organolithium salts. An "organolithium salt" is a lithium salt other than an imide salt that contains a carbon atom as an anion and is soluble in acetonitrile.

[0099] Examples of organolithium salts include organolithium salts having an oxalic acid group. Specific examples of organolithium salts having an oxalic acid group include, for example, organolithium salts represented as LiB(C2O4)2, LiBF2(C2O4), LiPF4(C2O4), and LiPF2(C2O4)2, respectively. Among these, at least one lithium salt selected from the lithium salts represented as LiB(C2O4)2 and LiBF2(C2O4) is preferred. Furthermore, it is more preferable to use one or more of these together with a fluorine-containing inorganic lithium salt. These organolithium salts having an oxalic acid group may be added to a non-aqueous electrolyte or incorporated into the negative electrode (negative electrode active material layer).

[0100] In this embodiment, the amount of organolithium salt added to the non-aqueous electrolyte is preferably 0.005 moles or more, more preferably 0.01 moles or more, even more preferably 0.02 moles or more, and particularly preferably 0.05 moles or more, per liter of non-aqueous solvent, from the viewpoint of ensuring better effects from its use. However, if the amount of organolithium salt having an oxalic acid group in the non-aqueous electrolyte is too large, precipitation may occur. Therefore, the amount of organolithium salt having an oxalic acid group added to the non-aqueous electrolyte is preferably less than 1.0 mole, more preferably less than 0.5 moles, and even more preferably less than 0.2 moles, per liter of non-aqueous solvent.

[0101] Organolithium salts having an oxalic acid group are known to be poorly soluble in low-polarity organic solvents, particularly in linear carbonates. The content of the organolithium salt in the non-aqueous electrolyte according to this embodiment may be, for example, 0.01 moles or more and 0.5 moles or less per liter of non-aqueous solvent.

[0102] Furthermore, organolithium salts containing oxalic acid groups may contain trace amounts of lithium oxalate, and when mixed as a non-aqueous electrolyte, they may react with trace amounts of water contained in other raw materials, potentially generating a white precipitate of lithium oxalate. Therefore, it is preferable to limit the lithium oxalate content in the non-aqueous electrolyte according to this embodiment 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, for example, LiClO4, LiAlO4, LiAlCl4, LiB 10 Cl 10 , inorganic lithium salts that do not contain a fluorine atom as an anion, such as chloroborane Li; LiCF3SO3, LiCF3CO2, Li2C2F4(SO3)2, LiC(CF3SO2)3, LiC n F (2n+1) SO3 (where n ≥ 2), lithium salts of lower aliphatic carboxylic acids, lithium tetraphenylborate, lithium salts of organic lithium such as LiB(C3O4H2)2; LiPF such as LiPF5(CF3) n (C p F 2p+1 ) 6-n 〔where n is an integer from 1 to 5 and p is an integer from 1 to 8〕, lithium salts of organic lithium represented by; LiBF such as LiBF3(CF3) q (C s F 2s+1 ) 4-q 〔where q is an integer from 1 to 3 and s is an integer from 1 to 8〕, lithium salts of organic lithium represented by; 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 from each other 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 from each other 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 from each other and represent a perfluoroalkyl group having 1 to 8 carbon atoms.} Examples include organolithium salts represented by each of the above, and one or more of these can be used together with a fluorine-containing inorganic lithium salt.

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

[0106] <Additive for electrode protection> The non-aqueous electrolyte 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 acts as a solvent for dissolving the lithium salt (i.e., the non-aqueous solvent mentioned above). The electrode protection additive is preferably a substance that contributes to improving the performance of the non-aqueous electrolyte and the non-aqueous secondary battery, but it also includes substances that do not directly participate in the electrochemical reaction.

[0107] Specific examples of electrode protection additives include, for example, 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, such as ethylene sulfite, propylene sulfite, butylene sulfite, pentene sulfite, sulfolane, 3-sulfolene, 3-methylsulfolane, 1,3-propanesultone, 1,4-butanesultone, 1-propene-1,3-sultone, and tetramethylene sulfoxide; These include, and they can be used individually or in combination of two or more types.

[0108] The content of the electrode protection additive in the non-aqueous electrolyte 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, based on the total amount of the non-aqueous solvent.

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

[0110] Acetonitrile is readily reductively decomposed electrochemically. Therefore, non-aqueous solvents containing acetonitrile preferably contain one or more cyclic aprotic polar solvents as electrode protective additives for forming a protective film on the negative electrode, and more preferably contain one or more unsaturated bond-containing cyclic carbonates.

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

[0112] As an electrode protection additive, vinylene carbonate 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, the interfacial (film) resistance can be kept low, and cycle degradation at low temperatures can be suppressed.

[0113] <Acid anhydride> In the non-aqueous secondary battery according to this embodiment, a portion of the non-aqueous electrolyte decomposes during the initial charge, stabilizing the negative electrode surface by forming a SEI (Semi-Electrolyte Integrity) layer. To more effectively enhance this SEI, an acid anhydride can be added. When acetonitrile is included as the non-aqueous solvent, the strength of the SEI tends to decrease with increasing temperature, but the addition of an acid anhydride promotes the enhancement of the SEI. Therefore, by using such an acid anhydride, the increase in internal resistance over time due to thermal history can be effectively suppressed.

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

[0115] In this embodiment, since it is preferable to enhance the SEI before the reductive decomposition of the non-aqueous solvent, the non-aqueous secondary battery preferably contains at least one cyclic acid anhydride that acts early during the initial charge. These cyclic acid anhydrides may contain only one type or multiple types. Alternatively, other cyclic acid anhydrides may be included. Furthermore, it is preferable that the cyclic acid anhydride contains at least one of succinic anhydride, maleic anhydride, and phthalic anhydride.

[0116] A non-aqueous electrolyte containing at least one of succinic anhydride, maleic anhydride, and phthalic anhydride allows for the formation of a robust SEI on the negative electrode, more effectively suppressing the increase in resistance during high-temperature heating. The inclusion of succinic anhydride is particularly preferable. This allows for the more effective formation of a robust SEI on the negative electrode while suppressing side reactions.

[0117] If the non-aqueous electrolyte according to this embodiment contains an acid anhydride, the amount of acid anhydride is preferably in the range of 0.01 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the non-aqueous electrolyte, 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.

[0118] It is preferable that the non-aqueous electrolyte contains the acid anhydride. On the other hand, as long as the acid anhydride can act 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. As for how to incorporate the acid anhydride into the battery component, for example, it may be incorporated into the battery component during manufacturing, or it may be impregnated into the battery component by post-treatment such as coating, immersion, or spray drying.

[0119] <Optional additives> In this embodiment, optional additives (additives other than acid anhydrides and electrode protection additives) may be appropriately included in the non-aqueous electrolyte for purposes such as improving the charge-discharge cycle characteristics of the non-aqueous secondary battery, improving high-temperature storage capabilities, and enhancing safety (e.g., preventing overcharging).

[0120] Optional additives include, for example, sulfonic acid esters, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, tert-butylbenzene, and nitrogen-containing cyclic compounds without steric hindrance around lone pairs of electrons [such as pyridine, 1-methyl-1H-benzotriazole, and 1-methylpyrazole]. In particular, phosphate esters are effective as optional additives because they suppress side reactions during storage.

[0121] If the non-aqueous electrolyte according to this embodiment contains other optional additives, the amount of such additives is preferably in the range of 0.01% to 10% by mass, more preferably 0.02% to 5% by mass, and even more preferably 0.05 to 3% by mass, based on the total amount of the non-aqueous electrolyte. By adjusting the amount of other optional additives within the above range, it is possible to add even better battery characteristics without impairing the basic functions of the non-aqueous secondary battery.

[0122] <Surfactants> The non-aqueous electrolyte according to this embodiment may contain a surfactant. In this embodiment, the surfactant is a compound having one or more hydrophilic groups and one or more lipophilic groups in a single molecule. Examples of surfactants include anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants. The ionizing surfactant is limited to those that do not inhibit the insertion and removal of lithium ions from the negative electrode. Furthermore, from the viewpoint of the solubility of the non-aqueous electrolyte, a nonionic surfactant is preferred as the surfactant.

[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 diethyl phosphonoacetate {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=CHCH2O)3P=O}, triamyl phosphate, tris(2-butoxyethyl) phosphate, and triphosphate. Examples 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 glycol and polyoxyethylene sorbitan fatty acid esters; and alkanolamide compounds such as fatty acid alkanolamides. From the viewpoint of improving the stability of non-aqueous electrolytes, fatty acid ester compounds or phosphate ester compounds are preferred. Ethyl laurate is preferred as a 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 in at least one substituent constituting the hydrophobic portion of a 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 non-aqueous electrolyte. Furthermore, from the viewpoint that a higher number of carbon atoms in the substituent increases the boiling point of the compound and makes purification difficult, the number of carbon atoms in the substituent is preferably 25 or less, and even more preferably 20 or less. If the hydrocarbon chain of the substituent is branched, the total number of carbon atoms in the substituent is used. Specific examples of substituents with 3 or more carbon atoms include propyl group, isopropyl group, butyl group, isobutyl group, s-butyl group, t-butyl group, amyl group, isoamyl group, s-amyl group, t-amyl group, 2-butoxyethyl group, 2-ethylhexyl group, and phenyl group.

[0125] In this embodiment, the surfactant content is 0.1% by mass or more and 3% by mass or less relative to the total amount of the non-aqueous electrolyte. From the viewpoint of improving impregnation, the surfactant content is more preferably 0.3% by mass or more, and from the viewpoint of maintaining high ionic conductivity, it is more preferably 2% by mass or less, relative to the total amount of the non-aqueous electrolyte. Similarly, the nonionic surfactant content is preferably 0.1% by mass or more and 3% by mass or less, and more preferably 0.3% by mass or more and 2% by mass or less, relative to the total amount of the non-aqueous electrolyte. If the surfactant or nonionic surfactant content is 0.1% by mass or more, the effect of suppressing output reduction is more effectively obtained, and if the content is 3% by mass or less, the decrease in ionic conductivity is less and the impact on battery characteristics such as input / output characteristics and battery life is less.

[0126] Here, the surfactant content is the same as that of a non-aqueous electrolyte at room temperature. 1 This can be determined by 1H-NMR measurement (normalized with a standard substance (C6F4H2), and the surfactant content is calculated from the integrated signal value of each detected component).

[0127] Through diligent research by the inventors, it was found that while the presence of a hydroxyl group in the surfactant improves impregnation into the separator, it also makes the surfactant more susceptible to reductive decomposition at the negative electrode during charging, leading to a deterioration in battery performance. Furthermore, it was newly discovered that when a hydroxyl group is present in the surfactant, bubbles are more likely to form during the reduced pressure step after injecting the non-aqueous electrolyte. If the initial charge is performed while bubbles are still present, the SEI (Single-End Integrity) will not be uniformly formed on the negative electrode surface, leading to initial charge / discharge failure. Non-aqueous electrolytes containing acetonitrile enable subsequent charge / discharge operations by uniformly forming the SEI during the initial charge. Therefore, if there are areas where SEI formation is insufficient during the initial charge due to bubble formation, non-aqueous electrolytes containing acetonitrile will directly lead to initial charge failure or a reduction in battery life compared to non-aqueous electrolytes that do not contain acetonitrile. Accordingly, it is important that the surfactant in this embodiment does not contain a hydroxyl group as a substituent, or more specifically, as a terminal substituent.

[0128] <Cellulose-based compounds> In this embodiment, a cellulosic compound is included to prevent the volatilization of the non-aqueous electrolyte. The cellulosic compound is limited to those that dissolve in the non-aqueous electrolyte and do not inhibit the insertion / desorption of lithium ions to the negative electrode.

[0129] The cellulosic compound according to this embodiment excludes carboxymethylcellulose from the viewpoint of solubility in non-aqueous electrolytes.

[0130] Cellulosic compounds, from the viewpoint of solubility in non-aqueous electrolytes and not inhibiting the insertion / desorption of lithium ions to the negative electrode, are defined by the following general formula: [ka] {In the formula, R is independently a hydrogen (H) atom, a linear or branched alkyl group, a linear or branched hydroxyalkyl group, a carboxyalkyl group, or a cyanoalkyl group, provided that the carboxyalkyl group is selected when R ≠ -CH2CO2H} It is preferable to have a constituent unit represented by .

[0131] Examples of cellulosic compounds include ethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, and cyanoethylcellulose. These can be used individually or in combination of two or more.

[0132] The content of the cellulosic compound is 0.1% by mass or more and 10% by mass or less, relative to the total amount of the non-aqueous electrolyte. From the viewpoint of improving volatility, 0.3% by mass or more is preferred, and 0.5% by mass or more is more preferred. Furthermore, from the viewpoint of suppressing a decrease in ionic conductivity, 6% by mass or less is preferred, and 3% by mass or less is more preferred.

[0133] <Separator> The separator in this embodiment has a polypropylene-based microporous membrane.

[0134] The polypropylene microporous film according to this embodiment has a polymer matrix containing polypropylene extending in one direction perpendicular to the film thickness, and has polypropylene-containing fibrils between the polymer matrix and micropores between the multiple fibrils. The "one direction perpendicular to the film thickness" refers to the mechanical direction of continuous molding of the microporous film (hereinafter sometimes abbreviated as MD). The structure of the polypropylene microporous film can be obtained by a dry process, and more specifically, by a dry process that includes a step of dry-porousing a polypropylene-containing resin molded body. Figure 3 shows an example of an SEM image of a polypropylene microporous film. For example, from Figure 3, the polypropylene-containing polymer matrix structure extending in the "one direction perpendicular to the film thickness" can be observed.

[0135] A microporous membrane is a single-layer membrane consisting of a single polyolefin-based microporous layer, a microporous single-layer membrane consisting of a resin layer other than a polyolefin-based microporous layer, or a composite microporous single-layer membrane having a polyolefin-based resin and another resin, and is sometimes also called a microporous layer.

[0136] Polyolefin-based microporous membranes are formed from polyolefin resin compositions that primarily contain polyolefin resin.

[0137] Polypropylene microporous membranes are formed from a polypropylene resin composition containing polypropylene as the main component.

[0138] In this specification, the presence of a specific material as the main component of a film means that the film contains 50% or more of that material, based on its mass.

[0139] In this specification, the fact that a microporous membrane contains polyolefin resin as its main component means that the proportion of polyolefin resin in the microporous membrane is 50% by mass or more relative to the mass of the microporous membrane. Therefore, the polyolefin resin can form a polymer network in the microporous membrane.

[0140] For a polypropylene resin composition to contain polypropylene as its 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.

[0141] A microporous multilayer film refers to 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 is sometimes simply referred to as a laminate below. In the case of the separator in this embodiment being a laminate, one of the layers contains the polypropylene-based microporous film mainly composed of polypropylene.

[0142] A separator is a component placed between multiple electrodes in an energy storage device and having ion permeability and, if necessary, shutdown properties. The separator includes a microporous film and / or a microporous multilayer film, and may further include any functional layer as desired.

[0143] The inventors of this invention have found that when a polypropylene microporous membrane is combined with a non-aqueous electrolyte containing acetonitrile, there is a problem with the volatility of the electrolyte. After diligent investigation into this problem, it was found that when a polypropylene separator with a small pore size and a pore structure parallel to the thickness direction, manufactured by a dry process, is used, the impregnation of the non-aqueous electrolyte containing acetonitrile becomes extremely low. As a result, a new problem was discovered: the non-aqueous electrolyte volatilizes rather than impregnates the separator. Furthermore, during battery manufacturing, it is necessary to extend the time of the reduced pressure process to impregnate the separator with the electrolyte. As a result, it was newly discovered that the low-boiling point solvent in the electrolyte volatilizes, causing initial defects.

[0144] From the viewpoint of increasing strength, the pore size ratio a / b of the polypropylene microporous film, which is the ratio of the average elongated pore diameter a to the pore diameter b perpendicular to the average elongated pore diameter a, is preferably 0.5 or more and 30 or less. 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.

[0145] 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 The following is preferable:

[0146] If the porosity is lower than 30%, it becomes difficult to ensure sufficient ion permeability when the microporous membrane is used in battery applications. Furthermore, if the porosity is higher than 80%, the microporous membrane may not be able to maintain sufficient strength. From this viewpoint, 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. Also, the air permeability resistance of the separator is 100 seconds / 100 cm. 3 Adjusting the parameters as described above is preferable from the viewpoint of obtaining a separator containing a homogeneous microporous membrane without defects. Furthermore, the air permeability resistance of the separator should be set to 500 seconds / 100 cm. 3The following adjustments are preferable because they can contribute to ensuring sufficient ion permeability. From this perspective, the air permeability resistance of the separator should be 120 seconds / 100 cm. 3 Over, 450 seconds / 100cm 3 The following is more preferable: 150 seconds / 100cm 3 More than 400 seconds / 100cm 3 The following is even more preferable:

[0147] In non-aqueous secondary batteries, when a separator having the properties described above is combined with a non-aqueous electrolyte, the lithium ion transfer rate may be limited not by the separator structure, but by the ionic conductivity of the non-aqueous electrolyte. It is also known that if the viscosity of the non-aqueous electrolyte is high, the impregnation of the separator decreases. Therefore, from the viewpoint of improving battery characteristics while maintaining the excellent performance of the non-aqueous electrolyte, it is preferable that the non-aqueous electrolyte has a viscosity of 1.0 mPa·s or more and 40 mPa·s or less at 25°C, and / or an ionic conductivity of 10 mS / cm or more and 40 mS / cm or less at 25°C, and more preferably a viscosity of 1.5 mPa·s or more and 30 mPa·s or less at 25°C, and an ionic conductivity of 15 mS / cm or more and 35 mS / cm or less at 25°C.

[0148] <Battery casing> The battery casing of the non-aqueous secondary battery in this embodiment can be a known configuration. For example, a battery can or a laminate film casing may be used as the battery casing.

[0149] For the battery casing, a metal casing made of steel, stainless steel, aluminum, or clad material can be used, for example.

[0150] The laminate film casing can be used by stacking two sheets with the heat-melt resin side facing inward, or by folding the film so that the heat-melt resin side faces inward, and sealing the ends with heat seal. When using the laminate film casing, the positive electrode current collector may be connected to the positive electrode lead (or positive electrode terminal and lead tab connected to the positive electrode terminal), and the negative electrode current collector may be connected to the negative electrode lead (or negative electrode terminal and lead tab connected to the negative electrode terminal). In this case, the laminate film casing may be sealed with the ends of the positive electrode lead and negative electrode lead (or lead tab connected to the positive electrode terminal and negative electrode terminal, respectively) extended to the outside of the casing.

[0151] As the laminate film outer casing, for example, a laminate film consisting of a three-layer structure of heat-melt resin / metal film / resin can be used.

[0152] The aluminum laminate film that makes up the battery casing is preferably made by coating both sides of aluminum foil with a polyolefin-based resin.

[0153] <Shape of a non-aqueous secondary battery> The shape of the non-aqueous secondary battery according to this embodiment can be applied to, for example, a rectangular, rectangular tube, cylindrical, elliptical, button-shaped, coin-shaped, flat, laminated, etc.

[0154] The non-aqueous secondary battery according to this embodiment can be preferably applied to prismatic, rectangular tubular, and laminated types.

[0155] <Manufacturing method for non-aqueous secondary batteries> The non-aqueous secondary battery in this embodiment can be manufactured using the above-mentioned non-aqueous 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 casing.

[0156] First, a laminate consisting of a positive electrode, a negative electrode, and a separator can be formed. For example, a method of forming a laminate with a wound structure is made by creating a laminate with a long positive electrode and a negative electrode with a long separator interposed between the positive and negative electrodes, and then winding it up; a method of forming a laminate with a laminate structure is made by cutting the positive electrode and negative electrode 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 in between; a method of forming a laminate with a laminate structure is made by folding a long separator in a zigzag pattern and alternately inserting positive electrode sheets and negative electrode sheets between the zigzag-folded separators; and so on.

[0157] Next, the laminate described above is housed in a battery casing (battery case), the electrolyte is poured into the battery case, and the laminate is immersed in the electrolyte and sealed to produce the non-aqueous secondary battery according to this embodiment. Alternatively, a gel-like electrolyte membrane can be prepared in advance by impregnating a substrate made of polymer material with the electrolyte, and a laminated structure can be formed using a sheet-like positive electrode, negative electrode, electrolyte membrane, and separator, and then housed in a battery casing to produce the non-aqueous secondary battery.

[0158] Furthermore, if the electrode arrangement is designed such that there is an overlap between the outer edge of the negative electrode active material layer and the outer edge of the positive electrode active material layer, or if there is a section of the non-opposing part of the negative electrode active material layer that is too narrow, then misalignment of the electrodes may occur during battery assembly, potentially degrading the charge-discharge cycle characteristics of the non-aqueous secondary battery. Therefore, it is preferable to fix the position of the electrodes in the electrode body used in the non-aqueous secondary battery in advance using tapes such as polyimide tape, polyphenylene sulfide tape, or polypropylene (PP) tape, or adhesives.

[0159] In this embodiment, when a non-aqueous electrolyte containing acetonitrile is used, due to its high ionic conductivity, lithium ions released from the positive electrode during the initial charge of a non-aqueous secondary battery may diffuse throughout the negative electrode. In non-aqueous secondary batteries, it is common to have a larger negative electrode active material layer than the positive electrode active material layer. However, if lithium ions diffuse and are intercalated in areas of the negative electrode active material layer that do not face the positive electrode active material layer, these lithium ions will remain in the negative electrode without being released during the initial discharge. As a result, the contribution of these unreleased lithium ions becomes irreversible capacity. For these reasons, non-aqueous secondary batteries using a non-aqueous electrolyte containing acetonitrile may have low initial charge-discharge efficiency.

[0160] 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 areas are the same, current concentration is more likely to occur at the edges of the negative electrode active material layer during charging, making it easier for lithium dendrites to form.

[0161] There are no particular restrictions on the ratio of the total area of ​​the 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. However, for the reasons mentioned above, it is preferable that the ratio be 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 non-aqueous secondary battery using a non-aqueous electrolyte containing acetonitrile, the initial charge-discharge efficiency can be improved by reducing the ratio of the total area of ​​the 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.

[0162] Reducing the ratio of the total area of ​​the 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 proportion of the negative electrode active material layer that does not face the positive electrode active material layer. This makes it possible to reduce as much as possible the amount of lithium ions absorbed by the portion of the negative electrode active material layer that does not face the positive electrode active material layer from the lithium ions released from the positive electrode during the initial charge (i.e., the amount of lithium ions that are not released from the negative electrode during the initial discharge and become irreversible capacity). Therefore, by designing the ratio of the total area of ​​the 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.

[0163] The non-aqueous secondary battery in this embodiment can function as a battery after the initial charge, but it is stabilized by the decomposition of a portion of the electrolyte during the initial charge. There are no particular restrictions on the method of initial charging, but it is preferable to perform the initial charge at 0.001 to 0.3C, more preferably at 0.002 to 0.25C, and even more preferably at 0.003 to 0.2C. Performing the initial charge via constant voltage charging in between also yields favorable results. By setting a long voltage range in which the lithium salt is involved in the electrochemical reaction, a stable and robust SEI is uniformly formed on the electrode surface, which has the effect of suppressing the increase in internal resistance. In addition, the reaction products are not firmly fixed only on the negative electrode, but also have a good effect on components other than the negative electrode, such as the positive electrode and separator, in some way. For this reason, it is very effective to perform the initial charge considering the electrochemical reaction of the lithium salt dissolved in the non-aqueous electrolyte.

[0164] In this embodiment, the non-aqueous secondary battery can also be used as a battery pack in which multiple non-aqueous secondary batteries are connected in series or parallel. From the viewpoint of managing the charge and discharge state of the battery pack, the operating voltage range per battery is preferably 2 to 5V, more preferably 2.5 to 5V, and particularly preferably 2.75V to 5V.

[0165] As the size of non-aqueous secondary batteries increases, it becomes more difficult for the non-aqueous electrolyte to penetrate the entire separator. Therefore, it is preferable to allow the battery to stand for 12 hours or more after injecting the non-aqueous electrolyte.

[0166] Although embodiments for carrying out the present invention have been described above, the present invention is not limited to the embodiments described above. The present invention can be modified in various ways without departing from its spirit. [Examples]

[0167] 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, the experiments and measurements in the examples were performed under room temperature conditions.

[0168] (1) Preparation of non-aqueous electrolyte Under an inert atmosphere, various non-aqueous solvents were mixed to the predetermined concentrations shown in Table 1. Cellulose compounds were added to the resulting non-aqueous solvents in the proportions shown in Table 1. Furthermore, various lithium salts were added to the predetermined concentrations to prepare non-aqueous electrolytes (S01) to (S04). The compositions of these non-aqueous electrolytes are shown in Table 1.

[0169] The abbreviations for non-aqueous solvents, lithium salts, and cellulosic compounds in Table 1 have the following meanings. Furthermore, the mass percentage of the cellulosic compound represents parts by mass relative to 100 parts by mass of the non-aqueous electrolyte. Note that mass percentage can be converted to volume percentage using the following formula 1. The value obtained by dividing the mass of the cellulosic compound by its specific gravity / (100 + the value obtained by dividing the mass of the cellulosic compound by its specific gravity) × 100 (Equation 1)

[0170] (Lithium salt) LiPF6: Lithium hexafluorophosphate LiFSI: Lithium bis(fluorosulfonyl)imide (LiN(SO2F)2) (Non-aqueous solvents) Acn: Acetate EMC: Ethyl methyl carbonate EC: Ethylene carbonate VC: Vinylen carbonate ES: Ethylene sulfite (Cellulose-based compounds) C0603: Carboxymethylcellulose

[0171] [Table 1]

[0172] The ionic conductivity and viscosity of the non-aqueous electrolytes prepared above were measured by the following method. Note that in non-aqueous electrolyte S04, carboxymethylcellulose did not dissolve and precipitate was observed, so further testing was discontinued.

[0173] (2) Measurement of ionic conductivity A non-aqueous electrolyte was prepared in a polypropylene container, and an ion conductivity measuring cell "CT-58101B" (product name), manufactured by Toa DKK Co., Ltd., which was connected to an ion conductivity meter "CM-41X" (product name), was inserted into the container containing the non-aqueous electrolyte, and the ion conductivity of the non-aqueous electrolyte was measured at 25°C.

[0174] (3) Viscosity measurement A non-aqueous electrolyte was prepared in a polypropylene container, and the detection terminal of a vibrating viscometer manufactured by Sekonic Corporation was inserted into the container containing the non-aqueous electrolyte to measure the viscosity of the non-aqueous electrolyte at 25°C.

[0175] Table 2 below shows the results of measuring the ionic conductivity and viscosity of non-aqueous electrolytes S01 to S04 at 25°C.

[0176] [Table 2]

[0177] As shown in Table 2 above, the non-aqueous electrolyte to which the cellulose compound according to this embodiment is added maintains high ionic conductivity of 20 mS / cm or more, and its viscosity is kept below 10 mPa·s.

[0178] (4) Volatility Test A glass dish was placed on a weighing scale. A separator punched into a 19mm diameter disc was placed on the glass dish, and 5 μl of the prepared electrolyte was dropped onto it. The weight A immediately after dropping was measured. The weight B was measured 1 minute after dropping, and the volatility of the electrolyte was evaluated from the weight change using the following formula. (Formula) (Weight B - Weight A) / Weight A x 100

[0179] (5) Fabrication of separators

[0180] <Preparation of polypropylene resin composition> Ultra-high molecular weight polypropylene resin (PP, MFR=0.25) and ethylene / 1-butene copolymer (C2 / C4: density=0.893g / cm³) 2 Pellet I (melting point = 80°C, MFR = 6.7) was dry-blended in a mass ratio of PP:C2 / C4 = 50:50 (mass%), and then melt-kneaded using ZSK40 (manufactured by Coperion, L / D = 46). To minimize resin decomposition and modification, the resin input hopper opening was completely sealed from the raw material tank, and nitrogen was continuously flowed from the bottom of the hopper to control the oxygen concentration near the raw material input opening to 50 ppm or less. In addition, all vents were completely sealed to eliminate any air leakage into the cylinder. This reduction in oxygen concentration significantly suppressed polymer decomposition and modification even under high-temperature conditions, and further enabled fine dispersion of the ethylene / 1-butene copolymer. After melt-kneading, the strand was pulled from a die (8 holes), the molten mixture was cooled in a water-cooled bath, and then cut using a pelletizer to obtain pellet II.

[0181] <Fabrication of microporous membrane (separator with three layers of microporous membrane)> Ultra-high molecular weight polypropylene resin (PP, MFR=0.25) and the above-mentioned pellet II were dry-blended in a mass ratio of PP:pellet II = 90:10 (mass%), then melted in a 2.5-inch extruder and supplied to an annular die using a gear pump. As a result, the input resin ratio was PP:C2 / C4 = 95:5 (mass%).

[0182] The die temperature was set to 240°C, and the molten polymer was cooled by blown air before being wound onto a roll. The extruded PP precursor (raw film) had a thickness of 6 μm.

[0183] High molecular weight polyethylene resin (PE, MFR=0.38) was melted in a 2.5-inch extruder and supplied to an annular die using a gear pump.

[0184] The die temperature was set to 210°C, and the molten polymer was cooled by blown air before being wound onto a roll. The extruded PE precursor (raw film) had a thickness of 6 μm.

[0185] The obtained PP precursor and PE precursor were layered in a PP precursor / PE precursor / PP precursor configuration and laminated at 120°C to obtain a three-layer laminated raw material film. This raw material 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 three-layer microporous membrane structure. After the stretching and porosification process, the physical properties of the separator were measured. The physical properties of the separator fabricated as described above were measured by the following method.

[0186] [Average long pore diameter (nm), pore diameter ratio] The average pore diameter of a microporous membrane was obtained by observing the surface of the microporous membrane with a scanning electron microscope (SEM) and measuring the pore diameter in the SEM image. Specifically, the surface of the microporous membrane was observed with an SEM at 30,000x magnification, and the pore diameters of micropores within a 4.0 μm × 2.8 μm image area were measured and the average was calculated to obtain the average pore diameter a. In addition, the pore diameter in the direction perpendicular to the pore diameter direction of each pore was measured and the average was calculated 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 a separator. For example, pore diameter and pore diameter ratio can be calculated from the observation in Figure 3.

[0187] [Measurement of thickness (μm)] The thickness of a separator containing a microporous membrane was measured at room temperature (23±2°C) using a Mitutoyo IDC112 digital indicator.

[0188] [Porosity (%)] A 5cm x 5cm square sample was cut from a separator containing a microporous membrane, and the porosity was calculated from the volume and mass of the sample using the following formula. Porosity (%) = (volume (cm³) 3 )- Mass (g) / Density of resin composition (g / cm³) 3 )) / Volume (cm³ 3 ) × 100

[0189] [Air resistance (sec / 100cm) 3 )] The air permeability resistance of a separator containing a microporous membrane was measured using a Gurley type air permeability meter compliant with JIS P-8117. The physical properties of the separator measured according to the above method were: pore size ratio 8.1, film thickness 25 μm, and air permeability resistance 217 seconds / 100 cm. 3 The porosity was 55%.

[0190] [Examples 1-2, Comparative Examples 1-2] The volatility of the non-aqueous electrolyte was evaluated using the non-aqueous electrolyte shown in Table 1 and the separator prepared according to the method described in (5) above, according to the method described in (4) above. The test results are shown in Table 3.

[0191] [Table 3]

[0192] As shown in Table 3, Comparative Example 1, in which the cellulose-based compound according to this embodiment was not added to the electrolyte, showed a large weight change of 12.3% from the time the electrolyte was added to one minute, indicating high volatility. Therefore, even if a battery is manufactured, acetonitrile volatilizes during the vacuum process, resulting in a decrease in initial efficiency or output performance. On the other hand, Examples 1 and 2, in which the cellulose-based compound according to this embodiment was added to the electrolyte, showed a small weight change of less than 10.5% from the time the electrolyte was added to one minute, indicating suppressed volatility. Therefore, it can be expected that initial efficiency or performance variations after battery manufacturing will be suppressed.

[0193] As these results demonstrate, it has become clear that the volatility of the electrolyte can be suppressed by using a non-aqueous electrolyte within the scope of the constituent elements of the present invention. [Industrial applicability]

[0194] Since non-aqueous electrolytes within the scope of the constituent elements of the present invention can be used in batteries where suppression of volatility is required, non-aqueous secondary batteries using the non-aqueous solvent of the present invention are particularly expected to be used in applications such as: rechargeable batteries for portable devices such as mobile phones, portable audio devices, personal computers, and IC (Integrated Circuit) tags; rechargeable batteries for automobiles such as hybrid vehicles, plug-in hybrid vehicles, and electric vehicles; low-voltage power supplies such as 12V, 24V, and 48V power supplies; residential energy storage systems, IoT devices, etc. Furthermore, non-aqueous secondary batteries using the non-aqueous solvent of the present invention can also be applied to applications in cold regions and outdoor applications in summer. [Explanation of Symbols]

[0195] 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 intercepting and releasing lithium ions, a negative electrode containing a material capable of intercepting and releasing lithium ions, a non-aqueous electrolyte, and a separator, The non-aqueous electrolyte comprises a non-aqueous solvent, a lithium salt, and a cellulosic compound. The non-aqueous solvent contains 5% to 90% by volume of acetonitrile. The lithium salt contains a lithium-containing imide salt, and its content is between 1 mol / L and 2 mol / L relative to the non-aqueous electrolyte. The aforementioned cellulosic compound is excluding carboxymethylcellulose, and the aforementioned cellulosic compound is ethylcellulose. The separator has a polypropylene microporous membrane, The polypropylene-based microporous film has a polymer matrix containing polypropylene that extends in one direction perpendicular to the film thickness, and has polypropylene-containing fibrils between the polymer matrix and micropores between the multiple fibrils. A non-aqueous secondary battery characterized by the following features.

2. The non-aqueous secondary battery according to claim 1, wherein the cellulose compound is contained in an amount of 0.1% by mass or more and 10% by mass or less based on the total amount of the non-aqueous electrolyte.

3. The non-aqueous secondary battery according to claim 1 or 2, wherein the non-aqueous electrolyte has a viscosity of 1.0 mPa·s or more and 40 mPa·s or less at 25°C, and an ionic conductivity of 10 mS / cm or more and 40 mS / cm or less.

4. The non-aqueous secondary battery according to any one of claims 1 to 3, wherein the pore size ratio a / b of the average elongated pore size a of the polypropylene microporous film to the pore size b perpendicular to the average elongated pore size a is 0.5 or more and 30 or less.

5. The separator has a porosity of 30% to 80% and an air permeability resistance of 100 seconds / 100 cm. ３ More than 500 seconds / 100cm ３ The non-aqueous secondary battery according to any one of claims 1 to 4, which is as follows:

6. The lithium salt is LiPF ６ It further includes LiPF ６ <The lithium-containing imide salt and LiPF at the molar concentration that constitutes the lithium-containing imide salt. ６ A non-aqueous secondary battery according to any one of claims 1 to 5, including the above.

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