Electrolyte solution for secondary battery, and battery using said electrolyte solution

Abstract

[Problem] The present invention addresses the problem of providing an electrolyte solution for a secondary battery, which has safety characteristics (flame retardance) capable of addressing ignition risk and which enables the achievement of excellent battery characteristics. [Solution] It has been newly found that an electrolyte solution for a secondary battery, which is capable of forming a passivation film on the surface of an electrode and is capable of achieving both low combustibility and excellent battery characteristics can be achieved by using a solvent system that contains a cyclic phosphoric acid ester having a six-membered ring structure and low combustibility, the formation of a passivation film having been impossible by a conventional phosphoric acid ester solvent system.

Description

Electrolyte for secondary battery and battery using said electrolyte

[0001] The present invention relates to an electrolyte for a secondary battery that is low in flammability and can provide excellent battery characteristics, and to a secondary battery containing the electrolyte.

[0002] In the lithium-ion batteries currently in practical use, LiPF is used in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC). ６ Organic solvent-based electrolyte solutions containing dissolved salts such as ammonium nitrate and ammonium hydroxide have been used (see, for example, Patent Document 1). These electrolyte solutions have the property of forming a passive film (also called a solid electrolyte interface (SEI) film) on the surface of the negative electrode, enabling reversible charging and discharging of lithium-ion batteries. Therefore, the basic composition of lithium-ion batteries has remained unchanged since they were commercialized in the 1990s.

[0003] On the other hand, conventional organic solvent-based electrolytes have two problems: flammability and oxidation stability. Because they use organic solvents, they are highly flammable. If the temperature of a lithium-ion battery rises suddenly due to overcharging, the electrolyte can ignite, resulting in a serious fire. Furthermore, the electrolyte's insufficient oxidation stability prevents lithium-ion batteries from exceeding the current voltage of 3.8 V.

[0004] Given this background, extensive research and development efforts have been made on next-generation electrolytes. One approach that has been considered is to add trace amounts of flame retardants to reduce the flammability of electrolytes. For example, adding phosphate ester compounds, known as flame retardants, to electrolytes can reduce flammability. However, these phosphate ester compounds cannot form a passive film on the negative electrode, which is essential for lithium-ion batteries. Therefore, adding a certain amount of phosphate ester flame retardants to reduce the flammability of electrolytes results in a trade-off: the charge-discharge cycle characteristics and lifespan of lithium-ion batteries are degraded. Therefore, organic phosphate esters have only been used as trace additives and have been difficult to use as the main solvent.

[0005] In addition, the use of nitrile-based, sulfone-based, and fluorine-based solvents has been investigated to improve oxidation stability and increase battery voltage. These solvents have higher oxidation stability than existing EC and DMC, and their use as electrolytes has been reported to enable reversible charging and discharging of next-generation cathode materials that operate at higher voltages than existing lithium-ion batteries. However, like the phosphate ester-based compounds, these solvents are unable to form a passive film on the anode, and their electrical properties have not yet reached a practical level.

[0006] JP 9-22722

[0007] Therefore, an object of the present invention is to provide a novel electrolyte solution that has safety (low flammability) that can address the risk of fire, has high oxidation stability, and is capable of forming a passive film on the surface of an electrode.

[0008] As a result of extensive research aimed at solving the above-mentioned problems, the present inventors have newly discovered that by using a low-flammability cyclic phosphate ester having a six-membered ring structure as a sole solvent or a mixed solvent system containing this as a main solvent, it is possible to form a passivation film on the electrode surface, which was previously impossible with phosphate ester solvent systems, thereby obtaining an electrolyte solution for a secondary battery that can achieve both low flammability and excellent battery performance. Furthermore, they have also discovered that such a mixed solvent-based electrolyte solution has excellent oxidation stability, and can achieve good charge-discharge cycle performance of the negative electrode even under high-temperature conditions such as 60°C or higher, as well as enable high-voltage charge-discharge reactions. Based on these findings, the present invention has been completed.

[0009] That is, in one aspect, the present invention relates to an electrolyte solution for a secondary battery that has low flammability, high oxidation stability, and passivation film forming ability, and specifically, the electrolyte solution includes: <1> an electrolyte solution for a secondary battery containing a solvent and an alkali metal salt, wherein the solvent contains at least a cyclic phosphate ester having a 6-membered ring structure containing a phosphorus atom and an oxygen atom; <2> the electrolyte solution for a secondary battery according to the above <1>, wherein the cyclic phosphate ester is represented by the following formula (1): (wherein X is a hydrogen atom, a halogen atom, C １ ~C １０Alkyl group or halogenated C １ ~C １０ each R independently represents a hydrogen atom, a halogen atom, or C １ ~C ５ Alkyl group or halogenated C １ ~C １０ represents an alkyl group. <3> The electrolyte solution for a secondary battery according to the above <2>, wherein X is a fluorine atom; <4> The electrolyte solution for a secondary battery according to the above <1>, wherein the cyclic phosphate ester has a melting point of 60°C or lower; <5> The electrolyte solution for a secondary battery according to the above <1>, wherein the cyclic phosphate ester is present in an amount of 10 mol% or more relative to the total solvent; <6> The electrolyte solution for a secondary battery according to the above <1>, wherein the solvent is a sole solvent for the cyclic phosphate ester; <7> The electrolyte solution for a secondary battery according to the above <1>, wherein the solvent is a mixed solvent system containing at least a first organic solvent and a second organic solvent, and the first organic solvent is a cyclic phosphate ester having a 6-membered ring structure containing a phosphorus atom and an oxygen atom; <8> The electrolyte solution for a secondary battery according to the above <7>, wherein the second organic solvent is selected from a chain ether, a chain carbonate, a chain ester, and a combination thereof, each of which has a halogen atom in the molecule; <9> The electrolyte solution for a secondary battery according to the above <1>, wherein the alkali metal salt is a lithium salt or a sodium salt; <10> The electrolyte solution for a secondary battery according to the above item <1>, wherein the anion constituting the alkali metal salt is an anion containing one or more groups selected from the group consisting of a fluorosulfonyl group, a trifluoromethanesulfonyl group, a perfluoroethanesulfonyl group, and a hexafluorophosphate group; <11> The electrolyte solution for a secondary battery according to the above item <1>, wherein the amount of the solvent is 4 mol or more per mol of the alkali metal salt.

[0010] In another aspect, the present invention also relates to a secondary battery containing the above-mentioned electrolyte solution, and specifically provides: <12> a secondary battery comprising a positive electrode, a negative electrode, and the electrolyte solution for a secondary battery according to any one of the above <1> to <11>; <13> a lithium ion secondary battery according to the above <12>, wherein the positive electrode contains an active material selected from a metal oxide having lithium, a polyanionic compound, or a sulfur-based compound; and the negative electrode contains an active material selected from a carbon material, metallic lithium, or a material capable of forming an alloy with lithium; <14> a sodium ion secondary battery according to the above <12>, wherein the positive electrode contains a transition metal oxide; and the negative electrode contains hard carbon; <15> the secondary battery according to the above <12>, wherein the charge-discharge cycle capacity retention rate after 50 cycles at 60°C or higher is in the range of 90 to 100%; <16> the secondary battery according to the above <12>, wherein the average coulombic efficiency up to 50 cycles at 60°C or higher is in the range of 99.0 to 100%.

[0011] According to the present invention, it is possible to provide an electrolyte solution for a secondary battery that has low flammability, high oxidation stability, and passivation film forming ability. These properties have not been achieved with conventional phosphate ester solvent systems and have been newly discovered in the present application. Such an electrolyte solution is highly useful because it simultaneously enables improved safety and high performance (high voltage).

[0012] Conventionally, the use of a carbonate ester-based solvent has been essential for stable operation of the negative electrode, but batteries using the secondary battery electrolyte of the present invention have excellent cycle characteristics and sufficiently high voltage tolerance. In particular, since good charge / discharge cycle characteristics can be obtained even at high temperatures such as 60°C or higher, they have the advantage of being able to withstand high-temperature operation in summer in automotive applications, etc. Therefore, by using the secondary battery electrolyte of the present invention, the risk of fire can be avoided even when the battery is overcharged, and a secondary battery with high safety and excellent battery characteristics such as a long life can be constructed.

[0013] FIG. 1 shows the PO prepared in the examples. ３2 is a graph showing the charge-discharge curves of a graphite electrode when an F / FEMC mixed solvent (LiFSI salt) is used. FIG. 3 is a graph showing the charge-discharge curves of a graphite electrode when an FEMC single solvent (LiFSI salt) is used as a comparative example. ３ 4 is a graph showing the charge-discharge curves of a graphite electrode when a F / PC mixed solvent (LiFSI salt) is used. ３ 5 is a graph showing charge-discharge curves for a graphite electrode when a F / DMC mixed solvent (LiFSI salt) is used. ３ 6 is a graph showing charge-discharge curves for a graphite electrode when a F / HFE mixed solvent (LiFSI salt) is used. ３ F / FEMC mixed solvent (LiFSI salt) and PO ３ F / FEMC mixed solvent (LiPF ６ 7 is a graph of the cycle number vs. capacity plot for a graphite electrode using PO in a high temperature operating environment at 60° C. ３ 8 is a graph of the cycle number vs. capacity plot for a graphite electrode using a F / FEMC mixed solvent (LiFSI salt) and a comparative EC / DMC mixed solvent (LiFSI salt). ６ / PO ３ F:FEMC (1:3:5) and LiPF ６ / EC: 2 wt% PO in DMC ３ 9 is a graph of the cycle number vs. capacity plot for graphite electrodes with added F. ３ F / FEMC mixed solvent (LiFSI salt) and comparative EC / DMC mixed solvent (LiPF ６ 10 is a graph showing charge-discharge curves of an LCO positive electrode when PO ３ 11 is a graph showing charge-discharge curves of an LCO positive electrode at a high voltage of 4.2 to 4.6 V when an F / FEMC mixed solvent (LiFSI salt) is used. FIG. 12 is a graph showing charge-discharge curves of an LCO positive electrode at a high voltage of 4.2 to 4.6 V when an EC / DMC mixed solvent (LiPF ６ 12 is a graph showing the charge-discharge curves of the LCO positive electrode at a high voltage of 4.2 to 4.6 V when using the PO salt of the present invention. ３ F sole solvent (LiPF ６When using LiNi1 / 3Co1 / 3Mn1 / 3O ２ (NCM111) A graph showing the charge-discharge curve at 70 ° C. of the positive electrode (right) and the EC / DMC mixed solvent (LiPF ６ 14 is a graph (left) showing the charge-discharge curves of the NCM111 positive electrode at 70° C. when the PO ３ F sole solvent (LiPF ６ salt) and a comparative DC / DMC mixed solvent (LiPF ６ 1 is a graph of cycle number vs. capacity plot at 70° C. for NCM111 cathode with ethylenediaminetetraacetic acid (E2) and ethylenediaminetetraacetic acid (E1) salt.

[0014] The following describes embodiments of the present invention. The scope of the present invention is not limited to these descriptions, and other than the following examples, the present invention can be implemented with appropriate modifications within the scope that does not depart from the spirit of the present invention.

[0015] The secondary battery electrolyte of the present invention is characterized by using a cyclic phosphate ester having a six-membered ring structure as a sole solvent or as a mixed solvent system containing the cyclic phosphate ester as a main solvent, and containing an alkali metal salt, thereby providing low flammability, high oxidation stability, and the ability to form a passivation film.

[0016] (1) Solvent: The essential organic solvent in the secondary battery electrolyte of the present invention is a cyclic phosphate ester having a six-membered ring structure containing a phosphorus atom and an oxygen atom. As described above, such a cyclic phosphate ester can form a passivation film (also called a solid electrolyte interface (SEI) film) on the surface of an electrode, particularly an anode (negative electrode). This molecular structure can be said to combine a cyclic structure similar to that of carbonate esters such as ethylene carbonate, which have traditionally been used as solvents capable of forming passivation films, with the structure of phosphate esters, which are known to be low-flammable substances (but cannot form passivation films). Furthermore, due to its six-membered ring structure, the cyclic phosphate ester of the present invention is characterized by its high thermal stability compared to other solvents, such as those with five-membered ring structures. As described below, such a cyclic phosphate ester is the solvent with the highest content in the electrolyte of the present invention, i.e., the main solvent. This also distinguishes it from conventional electrolytes to which trace amounts of phosphate esters are added as flame retardants.

[0017] The use of such a cyclic phosphate ester having a six-membered ring structure can provide not only excellent battery characteristics such as reversible charge and discharge due to its ability to form a passive film, but also safety due to its low flammability, a function that has not been achieved before.

[0018] More specifically, the cyclic phosphate ester is preferably a compound having a structure represented by the following formula (1).

[0019] In formula (1), X is a hydrogen atom, a halogen atom, or C １ ~C １０ Alkyl group or halogenated C １ ~C １０ Preferably, X is a halogen atom or a halogenated C １ ~C １０ alkyl group, more preferably a fluorine atom or a fluorinated C １ ~C ５The alkyl group may be a branched or straight chain alkyl group, and may have any substituent. In this specification, the term "halogen atom" refers to a fluorine atom, a chlorine atom, a bromine atom, or an iodine atom.

[0020] In formula (1), each R is independently a hydrogen atom, a halogen atom, or C １ ~C ５ Alkyl group or halogenated C １ ~C ５ Preferably, each of R is a hydrogen atom or a C １ ~C ５ An alkyl group is an alkyl group, which may be branched or linear, and may have an optional substituent.

[0021] In this specification, the "alkyl group" may be any of a linear, branched, or cyclic aliphatic hydrocarbon groups, or a combination thereof. The number of carbon atoms in the alkyl group is not particularly limited, but for example, the alkyl group may have 1 to 20 carbon atoms (C １～２０ ), 3 to 15 carbon atoms (C ３～１５ ), 5 to 10 carbon atoms (C ５～１０ When the number of carbon atoms is specified, it means "alkyl" having the number of carbon atoms in that range. For example, C １～８ Examples of alkyl include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, neo-pentyl, n-hexyl, isohexyl, n-heptyl, and n-octyl. In this specification, alkyl groups may have one or more optional substituents. Examples of the substituents include, but are not limited to, an alkoxy group, a halogen atom, an amino group, a mono- or di-substituted amino group, a substituted silyl group, and an acyl group. When an alkyl group has two or more substituents, the substituents may be the same or different. The same applies to the alkyl moiety of other substituents containing an alkyl moiety (e.g., an alkoxy group, an arylalkyl group, etc.).

[0022] In this specification, the term "alkoxy group" refers to a structure in which the alkyl group is bonded to an oxygen atom, and examples thereof include saturated alkoxy groups that are linear, branched, cyclic, or a combination thereof. Suitable examples include methoxy, ethoxy, n-propoxy, isopropoxy, cyclopropoxy, n-butoxy, isobutoxy, s-butoxy, t-butoxy, cyclobutoxy, cyclopropylmethoxy, n-pentyloxy, cyclopentyloxy, cyclopropylethyloxy, cyclobutylmethyloxy, n-hexyloxy, cyclohexyloxy, cyclopropylpropyloxy, cyclobutylethyloxy, and cyclopentylmethyloxy groups.

[0023] In this specification, when a functional group is defined as "optionally having a substituent," the type, substitution position, and number of the substituent are not particularly limited, and when two or more substituents are present, they may be the same or different. Examples of the substituent include, but are not limited to, alkyl groups, alkoxy groups, hydroxyl groups, carboxyl groups, halogen atoms, sulfo groups, amino groups, alkoxycarbonyl groups, and oxo groups. These substituents may further have a substituent. Examples of such substituents include, but are not limited to, halogenated alkyl groups and dialkylamino groups.

[0024] Preferable specific examples of the cyclic phosphate ester represented by formula (1) include the following compounds (PO ３ F; 2-fluoro-1,3,2-dioxaphosphorinane-2-oxide). However, the present invention is not limited to this. This compound is an example in which X is a fluorine atom and both R are hydrogen atoms.

[0025] The cyclic phosphate ester having a six-membered ring structure in the present invention preferably has a melting point of 60° C. or lower, and more preferably has a melting point of 40° C. or lower. In terms of such a melting point, it can be said that the cyclic phosphate ester is different from the phosphate ester compounds conventionally used as flame retardants.

[0026] In a preferred embodiment, the secondary battery electrolyte of the present invention can be a system in which the cyclic phosphate ester having the six-membered ring structure is the sole solvent. In this embodiment, a high-voltage cell using a lithium metal anode (e.g., NCM111 cathode / lithium metal anode) can provide extremely high coulombic efficiency and capacity retention even at high temperatures. This is believed to be due to the formation of a stable passive film on the positive electrode surface by the cyclic phosphate ester. Furthermore, even when a carbon material such as graphite is used as the anode, the electrolyte can be used in a similar manner by using an appropriate mixed solvent system or an appropriate binder, as described below.

[0027] In another preferred embodiment, the secondary battery electrolyte of the present invention can be a mixed solvent system containing a second organic solvent in addition to the cyclic phosphate ester (first organic solvent). Any organic solvent known in the art can be used as the second organic solvent. Examples of such organic solvents include ethers such as ethyl methyl ether and dipropyl ether; carbonates such as ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC); nitriles such as methoxypropionitrile; esters such as methyl acetate; amines such as triethylamine; alcohols such as methanol; ketones such as acetone; and fluorine-containing alkanes. For example, aprotic organic solvents such as 1,2-dimethoxyethane, acetonitrile, tetrahydrofuran, dimethyl sulfoxide, γ-butyrolactone, and sulfolane can also be used. Without being bound by theory, the second organic solvent solvates lithium ions together with the first organic solvent, thereby suppressing lithium ion-solvent co-intercalation between graphite layers, which occurs when only the first organic solvent is used, thereby enabling reversible and high-capacity charge-discharge reactions.

[0028] Non-limiting examples of such second organic solvents include chain ethers, chain carbonates, and chain esters, each of which has a halogen atom in the molecule. The second organic solvent is typically a chain carbonate having a halogen atom in the molecule, and preferably a chain carbonate having a halogenated alkyl group.

[0029] A preferred example of the second organic solvent is methyl-2,2,2-trifluoroethyl methyl carbonate (FEMC), but is not limited to this.

[0030] In some cases, the secondary battery electrolyte solution of the present invention may further contain other solvents known in the art in addition to the first and second organic solvents. As described above, such other solvents may include, for example, ethers such as ethyl methyl ether and dipropyl ether; carbonates such as ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC); nitriles such as methoxypropionitrile; esters such as methyl acetate; amines such as triethylamine; alcohols such as methanol; ketones such as acetone; and fluorine-containing alkanes. For example, aprotic organic solvents such as 1,2-dimethoxyethane, acetonitrile, tetrahydrofuran, dimethyl sulfoxide, γ-butyrolactone, and sulfolane may also be used.

[0031] When the secondary battery electrolyte solution of the present invention is a mixed solvent system, the first organic solvent is present preferably in a ratio of 10 mol % or more, more preferably 15 mol % or more, and even more preferably 20 mol % or more, based on the total solvent.

[0032] In the secondary battery electrolyte solution of the present invention, the molar ratio of the first organic solvent to the second organic solvent may preferably be in the range of 10:90 to 90:10.As described above, the secondary battery electrolyte solution of the present invention may contain 100% of the cyclic phosphate ester having a six-membered ring structure, i.e., may be used as the sole solvent.

[0033] (2) Alkali Metal Salt The secondary battery electrolyte of the present invention contains a predetermined amount of alkali metal salt together with the solvent. However, the present invention is also characterized in that it does not require a high concentration of alkali metal salt, which was required in the case of conventional flame-retardant solvents, and can provide excellent battery characteristics with a lower concentration of alkali metal salt.

[0034] Specifically, the mixing ratio of the alkali metal salt to the solvent in the electrolyte solution for a secondary battery of the present invention is 4 mol or more, preferably 6 mol or more, and more preferably 7 mol or more, per 1 mol of the alkali metal salt. The upper limit of the amount of the solvent is not particularly limited as long as the electrochemical reaction at the positive electrode and the negative electrode proceeds, but may be, for example, 12 mol or less, preferably 10 mol or less, per 1 mol of the alkali metal salt.

[0035] The alkali metal salt used in the secondary battery electrolyte of the present invention is preferably a lithium salt or a sodium salt. Depending on the type of secondary battery using the electrolyte of the present invention, for example, if the secondary battery is a lithium ion battery, a lithium salt is preferred, and if the secondary battery is a sodium ion battery, a sodium salt is preferred. Also, a mixture of two or more alkali metal salts can be used.

[0036] The anion constituting the alkali metal salt is preferably an anion containing one or more groups selected from the group consisting of a fluorosulfonyl group, a trifluoromethanesulfonyl group, a perfluoroethanesulfonyl group, and a hexafluorophosphate group. For example, bis(fluorosulfonyl)imide ([N(FSO ２ ) ２ ] － ), (fluorosulfonyl)(trifluorosulfonyl)imide ([N(CF ３ SO ２ ) (FSO ２ )] － ), bis(trifluoromethanesulfonyl)imide ([N(CF ３ SO ２ ) ２ ] －), bis(perfluoroethanesulfonyl)imide ([N(C ２ F ５ SO ２ ) ２ ] － ), (perfluoroethanesulfonyl)(trifluoroethanemethanesulfonyl)imide ([N(C ２ F ５ SO ２ ) (CF ３ SO ２ )] － ), or hexafluorophosphate (PF ６ － ) is preferred.

[0037] Therefore, specific examples of the alkali metal salt include lithium bis(fluorosulfonyl)imide (LiFSI), lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(perfluoroethanesulfonyl)imide (LiBETI), lithium (perfluoroethanesulfonyl)(trifluoromethanesulfonyl)imide, and lithium hexafluorophosphate (LiPF ６ ); or sodium bis(fluorosulfonyl)imide (NaFSI), sodium (fluorosulfonyl)(trifluoromethanesulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium bis(perfluoroethanesulfonyl)imide (NaBETI), sodium (perfluoroethanesulfonyl)(trifluoromethanesulfonyl)imide, or sodium hexafluorophosphate (NaPF ６ ) are listed.

[0038] Particularly preferred alkali metal salts are lithium bis(fluorosulfonyl)imide (LiFSI), sodium bis(fluorosulfonyl)imide (NaFSI), lithium hexafluorophosphate (LiPF ６ ), sodium hexafluorophosphate (NaPF ６) This is because these salts have weak cation-anion interactions and have high ionic conductivity even at high concentrations.

[0039] In addition to these alkali metal salts, supporting electrolytes known in the art can be included, such as LiPF when the secondary battery is a lithium ion battery. ６ , LiBF ４ , LiClO ４ , LiNO ３ , LiCl, Li ２ SO ４ and Li ２ S, etc., and any combination thereof.

[0040] (3) Other Components The secondary battery electrolyte solution of the present invention may also contain other components as necessary for the purpose of improving its functionality, etc. However, it is preferable that the secondary battery electrolyte solution of the present invention does not contain a room-temperature molten salt having a melting point of 50°C or less as a simple salt. Specific examples of such molten salts include imidazolium salts and tetrafluoroborates. This is because the secondary battery electrolyte solution of the present invention already has sufficient ionic conductivity without the addition of such a molten salt. More preferably, the secondary battery electrolyte solution of the present invention consists only of a mixed solvent of the first and second organic solvents and an alkali metal salt.

[0041] Examples of other components include conventionally known overcharge inhibitors, dehydrating agents, deoxidizing agents, and property improving aids for improving capacity retention and cycle characteristics after high-temperature storage.

[0042] Examples of overcharge inhibitors include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partially fluorinated compounds of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; and fluorine-containing anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, and 2,6-difluoroaniol. One type of overcharge inhibitor may be used alone, or two or more types may be used in combination.

[0043] When the electrolyte solution contains an overcharge inhibitor, the content of the overcharge inhibitor in the electrolyte solution is preferably 0.01 to 5% by mass. By adding 0.1% by mass or more of the overcharge inhibitor to the electrolyte solution, it becomes easier to prevent the secondary battery from exploding or catching fire due to overcharge, and the secondary battery can be used more stably.

[0044] Examples of dehydrating agents include molecular sieves, sodium sulfate, magnesium sulfate, calcium hydride, sodium hydride, potassium hydride, and lithium aluminum hydride. The solvent used in the electrolytic solution of the present invention may be one that has been dehydrated with the dehydrating agent and then rectified. Alternatively, a solvent that has been dehydrated with the dehydrating agent alone without rectification may be used.

[0045] Examples of the characteristic improving aid for improving the capacity retention characteristics and cycle characteristics after high-temperature storage include carboxylic acid anhydrides such as succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, and phenylsuccinic anhydride; sulfur-containing compounds such as ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, diphenyl sulfone, methyl phenyl sulfone, dibutyl disulfide, dicyclohexyl disulfide, tetramethylthiuram monosulfide, N,N-dimethylmethanesulfonimide, and N,N-diethylmethanesulfonimide; hydrocarbon compounds such as heptane, octane, and cycloheptane; and fluorine-containing aromatic compounds such as fluoroethylene carbonate (FEC), fluorobenzene, difluorobenzene, hexafluorobenzene, and benzotrifluoride. These property improvement assistants may be used alone or in combination of two or more. When the electrolytic solution contains a property improvement assistant, the content of the property improvement assistant in the electrolytic solution is preferably 0.01 to 5 mass %.

[0046] 2. Secondary Battery The secondary battery of the present invention comprises a positive electrode, a negative electrode, and the above-described electrolyte solution for secondary batteries. The secondary battery can be a lithium-ion secondary battery or a sodium-ion secondary battery. In the case of a lithium-ion secondary battery, the secondary battery of the present invention preferably has an operating voltage of 2.3 V or higher. In the case of a sodium-ion secondary battery, the secondary battery of the present invention preferably has an operating voltage of 2 V or higher. The electrolyte solution of the present invention can be used not only in these secondary batteries but also in electricity storage devices such as capacitors.

[0047] As described above, by using the electrolyte solution of the present invention, a secondary battery can be provided that not only has low flammability but also has high oxidation resistance and high-temperature operating capability. For example, the secondary battery of the present invention can have a charge / discharge cycle capacity retention rate of 90 to 100% after 50 cycles at 60°C or higher. Furthermore, the secondary battery of the present invention can have an average coulombic efficiency of 99.0 to 100% up to 50 cycles at 60°C. Preferably, the secondary battery has excellent charge / discharge cycle capacity retention rate and average coulombic efficiency even at 70°C.

[0048] (1) Negative Electrode: The negative electrode in the secondary battery of the present invention may have an electrode configuration known in the art. For example, when the secondary battery is a lithium-ion battery, an electrode containing a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions may be used. Examples of such a negative electrode active material include known negative electrode active materials for lithium-ion secondary batteries, such as natural graphite, highly oriented pyrolytic graphite (HOPG), and amorphous carbon. Further examples include metal compounds such as lithium metal and metal nitrides, and materials capable of forming alloys with lithium. Examples of alloys containing lithium include lithium-aluminum alloys, lithium-tin alloys, lithium-lead alloys, and lithium-silicon alloys. Examples of metal nitrides containing lithium include lithium cobalt nitride, lithium iron nitride, and lithium manganese nitride. These negative electrode active materials may be used alone or in combination. Preferably, carbonaceous materials such as natural graphite (graphite), highly oriented graphite (HOPG), amorphous carbon, etc. From the viewpoint of increasing the voltage and energy density of the secondary battery and reducing the number of series connections required to drive the device, it is desirable to use a negative electrode whose operating potential is lower than the potential of metallic lithium by 0.5 V or less.

[0049] When a cyclic phosphate ester having a six-membered ring structure is used as the sole solvent for the electrolyte, the negative electrode is preferably lithium metal, an alloy containing lithium element, or a metal nitride containing lithium element.

[0050] When the secondary battery is a sodium ion battery, an electrode containing a negative electrode active material capable of electrochemically absorbing and releasing sodium ions can be used. Examples of such negative electrode active materials include known negative electrode active materials for sodium ion secondary batteries, such as hard carbon, soft carbon, carbon black, ketjen black, acetylene black, activated carbon, carbon nanotubes, carbon fiber, and amorphous carbon. Sodium ion metals, or alloys, metal oxides, and metal nitrides containing sodium ion elements can also be used. Among these, carbonaceous materials such as hard carbon with a disordered structure are preferred as negative electrode active materials.

[0051] The negative electrode may contain only the negative electrode active material, or may contain at least one of a conductive material and a binder in addition to the negative electrode active material, and may be in the form of a negative electrode mixture attached to a negative electrode current collector. For example, when the negative electrode active material is foil-shaped, the negative electrode may contain only the negative electrode active material. On the other hand, when the negative electrode active material is powder-shaped, the negative electrode may contain the negative electrode active material and a binder. Methods for forming a negative electrode using a powder-shaped negative electrode active material include a doctor blade method and a molding method using a pressure press.

[0052] Examples of conductive materials that can be used include carbon materials, conductive fibers such as metal fibers, metal powders such as copper, silver, nickel, and aluminum, and organic conductive materials such as polyphenylene derivatives. Examples of carbon materials that can be used include graphite, soft carbon, hard carbon, carbon black, ketjen black, acetylene black, graphite, activated carbon, carbon nanotubes, and carbon fibers. Furthermore, mesoporous carbon obtained by burning synthetic resins containing aromatic rings, petroleum pitch, and the like can also be used.

[0053] As the binder, for example, fluorine-based resins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), etc., or polyethylene, polypropylene, etc. As the negative electrode current collector, a rod-shaped body, plate-shaped body, foil-shaped body, mesh-shaped body, etc. made mainly of copper, nickel, aluminum, stainless steel, etc. can be used.

[0054] (2) Positive Electrode The positive electrode of the secondary battery of the present invention may have an electrode structure known in the art. For example, when the secondary battery is a lithium ion battery, the positive electrode active material may be lithium cobalt oxide (LiCoO ２ ), lithium manganese oxide (LiMn ２ O ４ ), lithium nickel oxide (LiNiO ２ lithium-containing transition metal oxides containing one or more transition metals, such as transition metal sulfides, metal oxides, lithium iron phosphate (LiFePO ４ ) and lithium iron pyrophosphate (Li ２ FeP ２ O ７ ), a lithium-containing polyanion-based compound containing one or more transition metals such as sulfur-based compounds (Li ２ Specific examples include LiNi, which is used as a high-capacity electrode or a high-voltage electrode. ｘ Mn ｙ Co ｚ O ２ (x+y+z=1) (NMC), LiNi ｘ Co ｙ Al ｚ O ２ (x+y+z=1) (NCA) or LiNi ０．５ Mn １．５ O ４ (LNMO). The positive electrode may contain a conductive material or a binder. When the secondary battery is a sodium ion battery, known positive electrode active materials can be used similarly.

[0055] The conductive material and binder may be the same as those used for the negative electrode.

[0056] Examples of the metal for the positive electrode current collector include copper, nickel, aluminum, and stainless steel.

[0057] (3) Separator The separator used in the secondary battery of the present invention is not particularly limited as long as it has the function of electrically separating the positive electrode layer and the negative electrode layer. Examples of the separator include a porous sheet made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyimide, and a porous insulating material such as a nonwoven fabric or a glass fiber nonwoven fabric.

[0058] (4) Shape, etc. The shape of the secondary battery of the present invention is not particularly limited as long as it can accommodate the positive electrode, the negative electrode, and the electrolyte solution. Examples of the shape include a cylindrical shape, a coin shape, a flat shape, and a laminate shape.

[0059] Although the electrolyte solution and secondary battery of the present invention are suitable for use as a secondary battery, this does not exclude their use as a primary battery.

[0060] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0061] 1. Cyclic phosphate ester (PO ３Ｆ）の合成 As a six-membered cyclic phosphate ester that serves as a solvent for the electrolyte, 2-fluoro-1,3,2-dioxaphosphorinane-2-oxide (PO ３ F) was synthesized.

[0062] 1,3-Propanediol (15.00 g, 0.20 mol) and dichloromethane (400 mL) were placed in a 1 L four-neck round-bottom flask and stirred in a water bath (20°C). Phosphoryl chloride (30.22 g, 0.20 mol) was added dropwise over approximately 10 minutes, and the mixture was stirred in a water bath (20°C) for 13 hours. The solvent was then distilled off under reduced pressure. The resulting pale yellow liquid was distilled to give 2-chloro-2-oxo-1,3,2-dioxaphosphorinane as a colorless liquid (25.33 g, 0.16 mol, yield: 81%). Sodium fluoride (48.30 g, 1.15 mol) and acetonitrile (250 mL) were added to a 500 mL round-bottom flask, and 2-chloro-2-oxo-1,3,2-dioxaphosphorinane (60.00 g, 0.383 mol) was added while stirring at 20°C. The mixture was then heated to 75°C and stirred at 75°C for 14 hours. The solvent (acetonitrile) was distilled off under reduced pressure, and then chloroform (100 mL) was added. The solid matter was removed by filtration through Kiriyama filter paper. The filtrate was concentrated under reduced pressure to obtain a light brown liquid. This was distilled to obtain 2-fluoro-2-oxo-1,3,2-dioxaphosphorinane (PO ３ A colorless liquid of F) was obtained (34.43 g, 0.124 mol, yield: 64%).

[0063] 2. Preparation of electrolyte: LiFSI salt (Nippon Shokubai Co., Ltd.), LiPF ６ The salt (Kishida Chemical Co., Ltd.) and the cosolvent 2,2,2-trifluoroethyl methyl carbonate (FEMC) (Kishida Chemical Co., Ltd.) were commercially available lithium battery grade products and used as they were. ６ PO ３ The salt was dissolved in a F / FEMC mixed solvent (molar ratio 3:5) to prepare an electrolyte solution. The molar ratio of salt to solvent was 1:8. Similarly, electrolyte solutions were prepared using propylene carbonate (PC) (Kishida Chemical Co., Ltd.), dimethyl carbonate (DMC) (Kishida Chemical Co., Ltd.), and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE) (Kishida Chemical Co., Ltd.) as co-solvents.

[0064] As a comparative example, an electrolyte solution was prepared by dissolving LiFSI only in FEMC (molar ratio 1:8). ６ / Ethylene carbonate (EC):DMC (volume ratio 3:7) (Kishida Chemical Co., Ltd.) was used.

[0065] 3. Electrode Preparation: The electrode material, natural graphite, was purchased from SEC Carbon Co., Ltd. The negative electrode was fabricated by mixing natural graphite with polyvinylidene fluoride (PVdF, Kureha) in N-methylpyrrolidone (NMP) (weight ratio: natural graphite:PVdF = 90:10). The slurry was coated onto a Cu foil current collector using a Baker-type applicator. The resulting electrode was then dried in an oven at 80°C for 3 hours, and then further dried at 120°C under vacuum for 6 hours before use. LiCoO ２ (LCO) was purchased as a sheet electrode coated on Al foil from Hosen Co., Ltd. The mass of active material per unit area was 10.5 to 11.5 mg / cm for the positive electrode. ２ , 2 to 3.5 mg / cm for the negative electrode ２ It was decided.

[0066] 4. Electrochemical Measurement Conditions: The negative electrode half-cell (natural graphite|Li) and the positive electrode half-cell (LCO|Li) were both 2032-type coin cells with glass separators (GC50, ADVANTEC). Constant current charge / discharge tests were performed using a TOSCAT-3100 charge / discharge unit (Toyo Systems Co., Ltd.).

[0067] 5. Evaluation of charge-discharge characteristics The charge-discharge behavior of the graphite negative electrode was evaluated using the electrolyte prepared in 2. above. The temperature was 25°C, the voltage range was 2.5 V to 0.01 V, and the current value was 37.2 mA / g based on the weight of the graphite electrode. ３ The charge-discharge curves for the graphite negative electrode obtained using the F:FEMC electrolyte are shown in Figure 1. As a comparative example, the results for the case where FEMC was used alone as the solvent are shown in Figure 2.

[0068] As a result, LiFSI / PO ３ It was found that the F:FEMC electrolyte allowed for reversible charging and discharging, whereas the LiFSI / FEMC electrolyte did not allow charging.

[0069] 6. Consideration of co-solvent combinations Next, PO ３ Charge-discharge cycle tests were conducted on graphite anodes using PC, DMC, and HFE as co-solvents in addition to FEMC. The temperature was 25°C, the voltage range was 2.5 V to 0.01 V, and the current value was 37.2 mA / g based on the graphite electrode weight. The charge-discharge curves are shown in Figures 3 to 5, respectively.

[0070] As a result, it was found that reversible charging and discharging was possible even when a cosolvent other than FEMC was used.

[0071] 7. Study of alkali metal salts PO ３ F: In the FEMC mixed solvent, LiPF was used instead of LiFSI. ６ A similar charge-discharge cycle test was performed on a graphite anode using salt. A comparison with LiFSI is shown in Figure 6. The results showed that LiPF ６ It was found that even when using the battery, charging and discharging was possible for 50 cycles.

[0072] 8. Evaluation of high-temperature characteristics The temperature condition was set to 60°C, and the charge-discharge cycle characteristics of the graphite negative electrode in a high-temperature operating environment were evaluated. ３ F: FEMC electrolyte solution and LiPF as a comparative example ６ The results of the comparison when using the LiFSI / PO / EC:DMC electrolyte are shown in Figure 7. ３ The F:FEMC electrolyte maintained its capacity even after 100 cycles, whereas the LiPF ６ In the case of the LiFSI / PO / EC:DMC electrolyte, the capacity tended to decrease as the number of charge-discharge cycles increased. ３ The F:FEMC electrolyte showed a higher coulombic efficiency. This result indicates that the use of the electrolyte of the present invention enables a long cycle life even in a high temperature operating environment.

[0073] 9. PO ３ Consideration of the amount of F added Next, PO ３ The same charge-discharge cycle test as in 8. above was carried out by changing the amount of F added. The temperature condition was 70°C. ６ / PO３ F:FEMC (1:3:5) electrolyte and LiPF ６ / EC: 2 wt% PO in DMC ３ A comparison with the electrolyte solution containing F is shown in Figure 8. As a result, the LiPF ６ / PO ３ The F:FEMC (1:3:5) electrolyte maintained its capacity even after 150 cycles, whereas the PO ３ When a small amount of F (2 wt%) was added as an additive, a decrease in charge / discharge capacity was observed with the passage of cycles. ３ It was found that using F as the main solvent rather than as a small amount of additive resulted in a long cycle life in a high temperature operating environment.

[0074] 10. Evaluation of the resistance (polarization) of the positive electrode LiFSI / PO ３ F: FEMC electrolyte and comparative LiPF ６ The positive electrode resistance was evaluated for the LiFSI / PO / EC:DMC electrolyte (LCO|Li cell (4.2 V), 0.1 C charge / discharge). As shown in FIG. ３ It was found that the F:FEMC electrolyte had a small difference between the charge voltage and the discharge voltage (polarization) and a low resistance.

[0075] 11. Evaluation of high voltage resistance LiFSI / PO ３ F: FEMC electrolyte and comparative LiPF ６ The high voltage resistance of the LiPF / EC:DMC electrolyte solution was evaluated using an LCO|Li cell. As a result, as shown in FIGS. ６ In the case of the LiFSI / PO / EC electrolyte, the capacity degradation was observed when the upper limit voltage was increased to 4.4 V or more. ３ With the F:FEMC electrolyte, no capacity deterioration was observed even when the upper limit voltage was increased to 4.4 V or more, and it was found that the battery had sufficient cycle characteristics.

[0076] 12. PO ３ As a single solvent electrolyte, instead of the above mixed solvent, 2-fluoro-1,3,2-dioxaphosphorinane-2-oxide (PO ３Similar experiments were conducted using NCM111 as the sole solvent. Specifically, a high-voltage half-cell (NCM111 positive electrode | lithium metal negative electrode) with metallic lithium as the negative electrode was used, and the charge-discharge cycle characteristics were evaluated in the same manner as in 5 and 6 above. The temperature condition was 70°C. The results are shown in Figures 12 and 13, along with the results for a commercial electrolyte, EC / DMC solvent system, as a comparative example.

[0077] As a result, cyclic phosphate ester (PO ３ Even when using EC / DMC as the sole solvent, the capacity degradation was minimal and the cycle characteristics were excellent, even under high-temperature operating conditions. On the other hand, the capacity degradation was significant when using the EC / DMC solvent system, which is a commercial electrolyte.

Claims

1. An electrolyte solution for a secondary battery comprising a solvent and an alkali metal salt, wherein the solvent contains at least a cyclic phosphate ester having a six-membered ring structure containing a phosphorus atom and an oxygen atom.

2. The electrolyte solution for a secondary battery according to claim 1, wherein the cyclic phosphate ester is represented by the following formula (1): (wherein X is a hydrogen atom, a halogen atom, C １ ~C １０ Alkyl group or halogenated C １ ~C １０ each R independently represents a hydrogen atom, a halogen atom, or C １ ~C ５ Alkyl group or halogenated C １ ~C ５ represents an alkyl group.

3. The electrolyte for a secondary battery according to claim 2, wherein X is a fluorine atom.

4. The electrolyte for a secondary battery according to claim 1, wherein the cyclic phosphate ester has a melting point of 60°C or less.

5. The electrolyte solution for a secondary battery according to claim 1, wherein the cyclic phosphate ester is present in an amount of 10 mol % or more relative to the total solvent.

6. The electrolyte solution for a secondary battery according to claim 1, wherein the solvent is a sole solvent for the cyclic phosphate ester.

7. The secondary battery electrolyte solution according to claim 1, wherein the solvent is a mixed solvent system containing at least a first organic solvent and a second organic solvent, and the first organic solvent is a cyclic phosphate ester having a six-membered ring structure containing a phosphorus atom and an oxygen atom.

8. The secondary battery electrolyte solution according to claim 7, wherein the second organic solvent is selected from a chain ether, a chain carbonate, a chain ester, and a combination thereof, each of which has a halogen atom in the molecule.

9. The electrolyte for a secondary battery according to claim 1, wherein the alkali metal salt is a lithium salt or a sodium salt.

10. The secondary battery electrolyte solution according to claim 1, wherein the anion constituting the alkali metal salt is an anion containing one or more groups selected from the group consisting of a fluorosulfonyl group, a trifluoromethanesulfonyl group, a perfluoroethanesulfonyl group, and a hexafluorophosphate group.

11. The electrolyte solution for a secondary battery according to claim 1, wherein the amount of the solvent is 4 mol or more per mol of the alkali metal salt.

12. A secondary battery comprising a positive electrode, a negative electrode, and the electrolyte solution for secondary batteries according to any one of claims 1 to 11.

13. A lithium ion secondary battery according to claim 12, wherein the positive electrode contains an active material selected from a metal oxide having lithium, a polyanion-based compound, or a sulfur-based compound; and the negative electrode contains an active material selected from a carbon material, metallic lithium, or a material capable of forming an alloy with lithium.

14. The secondary battery according to claim 12, which is a sodium ion secondary battery, wherein the positive electrode comprises a transition metal oxide; and the negative electrode comprises hard carbon.

15. The secondary battery according to claim 12, wherein the charge-discharge cycle capacity retention rate after 50 cycles at 60° C. or higher is in the range of 90 to 100%.

16. The secondary battery according to claim 12, wherein the average coulomb efficiency up to 50 cycles at 60°C or higher is in the range of 99.0 to 100%.

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