WO2021166771A1 - Electrolytic solution for secondary cell containing cyclic phosphoric acid ester - Google Patents

Abstract

[Problem] To provide an electrolytic solution for a secondary cell that has safety characteristics capable of addressing ignition risk (flame retardance) and that makes it possible to provide exceptional cell properties. [Solution] An electrolytic solution for a secondary cell containing an organic solvent and an alkali metal salt, the organic solvent containing a cyclic phosphoric acid ester that has a 5-membered ring structure containing a phosphorus atom and an oxygen atom, and the makeup of the electrolytic solution being such that there are at least five moles of solvent per mole of alkali metal salt.

Description

Electrolyte for secondary batteries containing cyclic phosphoric acid ester

The present invention relates to a highly safe electrolytic solution for a secondary battery and a secondary battery containing the electrolytic solution.

Lithium-ion batteries with high energy density are expected to be widely used as large storage batteries for electric vehicles and power storage applications, in addition to small portable devices such as mobile phones and laptop computers. In recent years, there has been a demand for secondary batteries that can be manufactured at low cost. Therefore, with the aim of increasing the energy density of secondary batteries, active research is being conducted on various secondary batteries such as derivative types of lithium-ion batteries and sodium-ion batteries that use sodium, which is cheaper and has more resources than lithium. It is done.

However, conventionally, in secondary batteries such as lithium ion batteries and sodium ion batteries, a flammable organic electrolyte must be used in order to obtain battery characteristics that can withstand practical use. Many ignition and explosion accidents of lithium ion batteries caused by the use of such flammable organic electrolytes have been reported, which is a major obstacle to the expansion of the market and applications of secondary batteries. Further, in a sodium ion battery, sodium metal may be generated by overcharging or the like, but such sodium metal is extremely reactive and there is a concern about a risk of ignition or the like. Most of these safety issues are mainly due to the use of highly volatile and flammable organic carbonates as solvents and the chemically unstable lithium hexafluorophosphate (LiPF _{6) as an electrolyte salt.} Due to being used.

On the other hand, although organic phosphate esters such as trimethyl phosphate (TMP), triethyl phosphate (TEP) and diethyl ethyl phosphate (DEEP) have nonflammable properties, these organic phosphate esters are of graphite anodes. It is known that a stable solid electrolyte interface (SEI) film cannot be formed on the surface, causing exfoliation of graphite and continuous electrolyte decomposition during the initial charging process. These organic phosphoric acid esters were used only as a trace amount of additives, and it was difficult to use them as a main solvent.

On the other hand, the inventors of the present application have reported that they can function as an electrolytic solution by adding a high-concentration alkali metal salt to a flame-retardant solvent such as trimethyl phosphate (non-patent). Document 1). However, the electrolytic solution containing such a high-concentration salt also has problems such as high cost, high viscosity, and low wettability. Therefore, the current situation is that a nonflammable electrolyte solution capable of passivating a carbonaceous anode or the like and forming an SEI film even under normal low-concentration electrolyte (for example, 1 M or less) conditions has not been realized so far. Is. Therefore, there is a need to develop a nonflammable electrolytic solution having high thermal stability and electrochemical stability.

Therefore, the present invention provides an electrolytic solution for a secondary battery that has safety (flame retardancy) that can cope with the risk of ignition and can provide excellent battery characteristics without using a high-concentration electrolyte. That is the subject.

As a result of diligent studies to solve the above problems, the present inventors used a cyclic phosphoric acid ester having a specific structure as a solvent to suppress the addition amount of the alkali metal salt and to passivate the electrode surface. We have newly found that an electrolytic solution for a secondary battery capable of achieving both a film (SEI film) forming ability and flame retardancy can be obtained, and have completed the present invention.

That is, the present invention is, in one aspect, an electrolytic solution for a secondary battery containing <1> an organic solvent and an alkali metal salt, wherein the organic solvent has a cyclic structure having a 5-membered ring structure containing a phosphorus atom and an oxygen atom. The electrolytic solution containing a phosphoric acid ester and having a composition of the electrolytic solution having a solvent amount of 5 mol or more with respect to 1 mol of the alkali metal salt;
<2> The electrolytic solution for a secondary battery according to <1>, wherein the cyclic phosphoric acid ester is represented by the formula (1) shown below.

(In the formula, X represents an oxygen or nitrogen atom; Y represents one or two alkyl groups that may have substituents.);
<3> The electrolytic solution for a secondary battery according to <2> above, wherein Y is an alkyl halide group having 1 to 10 carbon atoms;
<4> The electrolytic solution for a secondary battery according to <1> above, wherein the cyclic phosphoric acid ester is selected from a compound having the following structure;

<5> The electrolyte for a secondary battery according to any one of <1> to <4> above, wherein the alkali metal salt is a lithium salt or a sodium salt;
<6> 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. The electrolyte for a secondary battery according to any one of <1> to <5>above;
<7> The anion is a 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 ] ^- ), (Perfluoro ethanesulfonyl) (trifluoroethane) imide _{([N (C 2 F 5} SO 2) (CF 3 SO 2)] -), or hexafluorophosphate (PF ₆ ^-) is, according to the above <6> Electrolyte for secondary batteries;
<8> The electrolytic solution for a secondary battery according to any one of <1> to <7>, wherein the ratio of the cyclic phosphoric acid ester in the total solvent is 10 to 100 mol%;
<9> The electrolytic solution for a secondary battery according to any one of <1> to <8> above, which further contains an alkyl carbonate or an alkyl carbonate containing a halogen atom as a co-solvent;
<10> The electrolyte for a secondary battery according to <9> above, wherein the co-solvent is 2,2,2-trifluoroethylmethyl carbonate (FEMC); and <11> The secondary battery is lithium. The electrolyte solution for a secondary battery according to any one of <1> to <10> above, which is an ion secondary battery or a sodium ion secondary battery, is provided.

In another aspect, the invention
<12> A secondary battery including a positive electrode, a negative electrode, and an electrolytic solution for a secondary battery according to any one of <1> to <11>above;
<13> The secondary battery according to <12> above, which is a lithium ion secondary battery;
<14> The secondary battery according to <13>, wherein the positive electrode contains an active material selected from a metal oxide having a lithium element, a polyanionic compound, or a sulfur compound;
<15> The secondary battery according to <13> above, wherein the negative electrode contains an active material selected from a carbon material, metallic lithium, or a substance capable of forming an alloy with lithium;
<16> The secondary battery according to <12> above, which is a sodium ion secondary battery;
<17> The secondary battery according to <16>, wherein the positive electrode is a transition metal oxide; and <18>, the secondary battery according to <16>, wherein the negative electrode is hard carbon. It is something to do.

According to the electrolytic solution for a secondary battery of the present invention, it is possible to achieve both the ability to form a passivation film (SEI film) on the electrode surface and flame retardancy while suppressing the amount of alkali metal salt added.

Conventionally, it has been essential to use a carbonic acid ester solvent for stable operation of the negative electrode, but the battery using the electrolytic solution for the secondary battery of the present invention has excellent cycle characteristics and voltage resistance. Is also high enough. Therefore, by using the electrolytic solution for the secondary battery of the present invention, it is possible to avoid the risk of ignition even when the battery is overcharged, and the safety is high and the life is extended. It is possible to construct a secondary battery having the same battery characteristics.

FIG. 1a is a graph showing the LSV measurement results for the electrolytic solution prepared in the examples. FIG. 1b is an image showing a flammability test of the electrolytic solution prepared in the examples. FIG. 2a is a graph showing a charge / discharge curve of a coin battery (graphite | Li) using the electrolytic solution prepared in the examples. FIG. 2b is a graph showing the result of the cycle test of the coin battery. FIG. 3 is a graph showing the results of chronoamperometry (CA) measurement for a coin battery (Al | Li) using the electrolytic solution prepared in the examples. FIG. 4 is a chart showing the results of XPS measurement of the aluminum electrode after CA measurement. FIG. 5 is a graph showing a charge / discharge curve of a coin battery (NMC | Li) using the electrolytic solution prepared in the examples. FIG. 6 is a graph showing a charge / discharge curve of a coin battery (LNMO | Li) using the electrolytic solution prepared in the examples. FIG. 7 is a graph showing a charge / discharge curve of a coin battery (graphite | Li) using an electrolytic solution _{having LiPF 6 as a supporting salt.} FIG. 8 is a graph showing a charge / discharge curve of a coin battery (graphite | Li) using an electrolytic solution using a TFEP single solvent (100 mol%) at 25 ° C. FIG. 9 is a graph showing a charge / discharge curve of a coin battery (graphite | Li) using an electrolytic solution using a TFEP single solvent (100 mol%) at 45 ° C. FIG. 10 is a graph showing a charge / discharge curve of a coin battery (graphite | Li) using an electrolytic solution of a mixed solvent of TFEP and EMC (TFEP: EMC = 2: 8). FIG. 11 is a graph showing a charge / discharge curve of a coin battery (graphite | Li) using an electrolytic solution of a mixed solvent of TFEP and DMC (TFEP: DMC = 2: 8). FIG. 12 is a graph showing a charge / discharge curve of a coin battery (graphite | Li) using an electrolytic solution of a mixed solvent of TFEP and DMC (TFEP: DMC = 1: 1). FIG. 13 is a chart showing a comparison of the overall performance of the LiFSI / TFEP / FEMC electrolytic solution of the present invention and the commercial LiPF _{6 / EC / DMC electrolytic solution.} FIG. 14 is a graph showing a charge / discharge curve of a graphite | Li half cell using a DMAP / EMC electrolytic solution (supporting salt: LiFSI). FIG. 16 is a graph showing a charge / discharge curve of a graphite | Li half cell using a DMAP / EMC electrolytic solution (supporting salt: LiPF _6).

Hereinafter, embodiments of the present invention will be described. The scope of the present invention is not limited to these explanations, and other than the following examples, the scope of the present invention can be appropriately modified and implemented as long as the gist of the present invention is not impaired.

1. 1. Electrolyte The electrolyte for a secondary battery of the present invention is characterized by containing a cyclic phosphoric acid ester having a specific structure as an organic solvent and containing a low concentration of an alkali metal salt. This makes it possible to achieve both excellent battery characteristics and flame retardancy.

(1) Solvent The organic solvent contained in the electrolytic solution for a secondary battery of the present invention contains a cyclic phosphoric acid ester having a 5-membered ring structure containing a phosphorus atom and an oxygen atom. Such a cyclic phosphoric acid ester has a cyclic structure similar to a carbonate ester such as ethylene carbonate, which has been conventionally used as a solvent capable of forming an inactive film (SEI film) on the anode (negative electrode), and a nonflammable substance (but inactive). It is characterized by having a molecular structure fused with the structure of a phosphoric acid ester known as (cannot form a film). By having such a molecular structure, as described above, it has a function that has not been achieved in the past, that is, it has excellent battery characteristics due to the passivation film (SEI film) forming ability and safety due to its flame retardancy. Can be provided.

More specifically, the cyclic phosphoric acid ester is preferably a compound having a structure represented by the following formula (1).

Here, in the formula, X represents an oxygen atom or a nitrogen atom, and Y represents one or two alkyl groups which may have a substituent. For example, since an oxygen atom is an element having a valence of 2, when X is an oxygen atom, Y is an alkyl group. In this case, "XY" forms an alkoxy group (-OY). Further, since the nitrogen atom is an element having a valence of 3, when Y is a nitrogen atom, Y may be one alkyl group or two alkyl groups. That is, when Y is one alkyl group, "XY" forms an alkylamino group (-NH-Y), and when Y is two alkyl groups, "XY" is a dialkylamino group (-". N (Y) ₂ ) is formed.

In the present specification, the "alkyl group" may be any of a linear, branched, cyclic, or a combination thereof, an aliphatic hydrocarbon group. The number of carbon atoms of the alkyl group is not particularly limited, but for example, the number of carbon atoms is 1 to 20 (C _{1 to 20} ), the number of carbon atoms is 3 to 15 (C _{3 to 15} ), and the number of carbon atoms is 5 to 10 (C _{5 to 10).} ). When the number of carbon atoms is specified, it means "alkyl" having the number of carbon atoms in the range of the number of carbon atoms. For _example, the _{C 1 ~ 8} alkyl, methyl, ethyl, n- propyl, isopropyl, n- butyl, isobutyl, sec- butyl, tert- butyl, n- pentyl, isopentyl, neo-pentyl, n- hexyl, isohexyl, Includes n-heptyl, n-octyl and the like. As used herein, the alkyl group may have one or more arbitrary substituents. Examples of the substituent 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. If the alkyl groups have more than one substituent, they may be the same or different. The same applies to the alkyl moiety of other substituents containing the alkyl moiety (eg, alkane group, arylalkyl group, etc.).

In the present specification, the "alkoxy group" has a structure in which the alkyl group is bonded to an oxygen atom, and examples thereof include a linear, branched, cyclic, or a combination thereof, a saturated alkoxy group. For example, methoxy group, ethoxy group, n-propoxy group, isopropoxy group, cyclopropoxy group, n-butoxy group, isobutoxy group, s-butoxy group, t-butoxy group, cyclobutoxy group, cyclopropylmethoxy group, n- Pentyloxy group, cyclopentyloxy group, cyclopropylethyloxy group, cyclobutylmethyloxy group, n-hexyloxy group, cyclohexyloxy group, cyclopropylpropyloxy group, cyclobutylethyloxy group, cyclopentylmethyloxy group and the like are preferable. Take as an example.

In the present specification, when it is defined that a functional group "may have a substituent", the type of substituent, the position of substitution, and the number of substituents are not particularly limited, and two If they have the above substituents, they may be the same or different. Examples of the substituent include, but are not limited to, an alkyl group, an alkoxy group, a hydroxyl group, a carboxyl group, a halogen atom, a sulfo group, an amino group, an alkoxycarbonyl group, an oxo group and the like. .. Further substituents may be present in these substituents. Examples of such include, but are not limited to, alkyl halide groups, dialkylamino groups, and the like.

Y in the formula (1) can be preferably an alkyl group having 1 to 10 carbon atoms, and more preferably an alkyl group having 1 to 5 carbon atoms. Typically, Y is preferably a linear alkyl group. When X is a nitrogen atom, two Ys can be present, and the two Ys in this case may be the same or different, and each independently has 1 to 10 carbon atoms, more preferably carbon atoms. It can be an alkyl group of 1-5.

In a preferred embodiment, the alkyl group of Y can also be substituted with one or more halogen atoms at any position. In this case, Y is an alkyl halide group (haloalkyl group). Due to the electrostatic attraction of the alkyl halide group, better battery characteristics can be obtained. As used herein, the term "halogen atom" means a fluorine atom, a chlorine atom, a bromine atom, or an iodine atom. As the position of substitution, it is preferable that the terminal of the alkyl group of Y is substituted with one or more halogen atoms. The halogen atom is preferably fluorine, in which case the alkyl halide group is a fluoroalkyl group. Preferably, Y is an alkyl halide group having 1 to 10 carbon atoms, and more preferably a fluoroalkyl group having 1 to 10 carbon atoms. In particular, an alkyl group having a perfluoromethyl group at the terminal is preferable.

Preferred specific examples of the cyclic phosphoric acid ester represented by the formula (1) include the following compound (TFEP). However, the present invention is not limited to this. The compound is an example in which X is an oxygen atom and Y is a trifluoroethyl group.

Further, as another preferable specific example of the cyclic phosphoric acid ester represented by the formula (1), the following compound (DMAP) can be mentioned. However, the present invention is not limited to this. The compound is an example in which X is a nitrogen atom and two Ys are methyl groups, respectively.

In a preferred embodiment, the electrolytic solution for a secondary battery of the present invention can further contain an alkyl carbonate or an alkyl carbonate containing a halogen atom as a co-solvent. Examples of such alkyl carbonates include ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and 2,2,2-trifluoroethyl methyl carbonate (FEMC).

In some cases, the electrolytic solution for a secondary battery of the present invention may be a mixed solvent containing a solvent other than the above. Examples of such other solvents include ethers such as ethyl methyl ether and dipropyl ether; nitriles of methoxypropionitrile; esters such as methyl acetate; amines such as triethylamine; alcohols such as methanol; acetone and the like. Ketones; fluorine-containing alcohols and the like can be used. For example, aprotic organic solvents such as 1,2-dimethoxyethane, acetonitrile, tetrahydrofuran, dimethyl sulfoxide, γ-butyrolactone, and sulfolane can also be used.

In the electrolytic solution for a secondary battery of the present invention, the cyclic phosphoric acid ester is preferably present in a proportion of 10 to 100 mol%, more preferably 30 to 100 mol%. Particularly preferably, the cyclic phosphate ester is used as the single solvent (ie, 100 mol%).

(2) Alkali metal salt The electrolytic solution for a secondary battery of the present invention contains a predetermined amount of alkali metal salt. However, in the present invention, the amount of the alkali metal salt added at a lower concentration is excellent without requiring the high concentration of the alkali metal salt required in the case of the conventional flame-retardant solvent as in Non-Patent Document 1. It is also characterized in that it can provide the characteristics of a battery.

Specifically, the mixing ratio of the alkali metal salt and the solvent in the electrolytic solution for the secondary battery of the present invention is such that the amount of the solvent is 5 mol or more, preferably 6 mol or more, with respect to 1 mol of the alkali metal salt. It is preferably 7 mol or more. 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 is, for example, 12 mol or less of the solvent with respect to 1 mol of the alkali metal salt, and preferably the solvent with respect to 1 mol of the alkali metal salt. It can be 10 mol or less.

The alkali metal salt used in the electrolytic solution for a secondary battery of the present invention is preferably a lithium salt or a sodium salt. Depending on the type of secondary battery using the electrolytic solution of the present invention, for example, a lithium salt is preferable when the secondary battery is a lithium ion battery, and a sodium salt is preferable when the secondary battery is a sodium ion battery. It is also possible to use a mixture in which two or more kinds of alkali metal salts are combined.

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 (trifluo). b) imide _{([N (CF 3 SO 2} ) 2] -), bis (perfluoroethanesulfonyl) imide _{([N (C 2 F 5} SO 2) 2] -), ( perfluoro ethanesulfonyl) (tri tetrafluoroethane) imide _{([N (C 2 F 5} SO 2) (CF 3 SO 2)] -), or hexafluorophosphate (PF ₆ ^-) is preferred.

Therefore, specific examples of the alkali metal salt include lithium bis (fluorosulfonyl) imide (LiFSI), lithium (fluorosulfonyl) (trifluorosulfonyl) imide, lithium bis (trifluoromethanesulfonyl) imide (LiTFSI), and lithium bis (fluorosulfonyl) imide (LiTFSI). Perfluoroethanesulfonyl) imide) (LiBETI), lithium (perfluoroethanesulfonyl) (trifluoroethanemethanesulfonyl) imide, or lithium hexafluorophosphate (LiPF ₆ ); or sodium bis (fluorosulfonyl) imide (NaFSI), Sodium (fluorosulfonyl) (trifluorosulfonyl) imide, sodium bis (trifluoromethanesulfonyl) imide (NaTFSI), sodium bis (perfluoroethanesulfonyl) imide) (NaBETI), sodium (perfluoroethanesulfonyl) (trifluoroethanemethane) Sulfonyl) imide or sodium hexafluorophosphate (NaPF ₆ ) can be mentioned.

Particularly preferred alkali metal salts are lithium bis (fluorosulfonyl) imide (LiFSI) or sodium bis (fluorosulfonyl) imide (NaFSI). This is because these salts have a weak cation-anion interaction and have high ionic conductivity even at high concentrations.

In addition to these alkali metal salts, supporting electrolytes known in the art can be included. Such supporting electrolytes include, for example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiNO ₃ , LiCl, Li ₂ SO ₄ and Li ₂ S and any of these, when the secondary battery is a lithium ion battery. The one selected from the combination can be mentioned.

(3) Other Components The electrolytic solution for a secondary battery of the present invention may contain other components as necessary for the purpose of improving its function. However, the electrolytic solution for a secondary battery of the present invention preferably does not contain a room temperature molten salt having a melting point of 50 ° C. or lower as a simple substance. Specific examples of such molten salts include imidazolium salts and tetrafluoroborates. This is because the electrolytic solution for a secondary battery of the present invention already has sufficient ionic conductivity without adding such a molten salt. More preferably, in the electrolytic solution for a secondary battery of the present invention, the electrolytic solution consists only of an organic solvent and an alkali metal salt.

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

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

When the electrolytic solution contains an overcharge inhibitor, the content of the overcharge inhibitor in the electrolytic solution is preferably 0.01 to 5% by mass. By containing 0.1% by mass or more of the overcharge inhibitor in the electrolytic solution, it becomes easier to suppress the explosion and ignition of the secondary battery due to overcharge, and the secondary battery can be used more stably.

Examples of the dehydrating agent include molecular sieves, mirabilite, magnesium sulfate, calcium hydride, sodium hydride, potassium hydride, lithium aluminum hydride and the like. As the solvent used in the electrolytic solution of the present invention, a solvent obtained by dehydration with the dehydrating agent and then rectification can also be used. Further, a solvent obtained by only dehydrating with the dehydrating agent without rectification may be used.

Examples of the property improving aid for improving the capacity retention property and the cycle property after high temperature storage include succinic anhydride, glutacon anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, and dianhydride. Carboic acid anhydrides such as glycolic acid, cyclohexanedicarboxylic acid anhydride, cyclopentanetetracarboxylic acid dianhydride, phenylsuccinic anhydride; ethylene sulphite, 1,3-propanesulton, 1,4-butanesulton, methanesulfonic acid Includes methyl, busulfane, sulfolane, sulfolene, dimethylsulfone, diphenylsulfone, methylphenylsulfone, dibutyldisulfide, dicyclohexyldisulfide, tetramethylthium monosulfide, N, N-dimethylmethanesulfonimide, N, N-diethylmethanesulfonimide, etc. Sulfur compounds; hydrocarbon compounds such as heptane, octane and cycloheptane; fluorocarbon aromatic compounds such as fluoroethylene (FEC), fluorobenzene, difluorobenzene, hexafluorobenzene and benzotrifluoride. These property improving aids may be used alone or in combination of two or more. When the electrolytic solution contains a characteristic improving auxiliary agent, the content of the characteristic improving auxiliary agent in the electrolytic solution is preferably 0.01 to 5% by mass.

2. Secondary Battery The secondary battery of the present invention includes a positive electrode and a negative electrode, and the above-mentioned electrolyte for a secondary battery. 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 more. 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 more.

(1) Negative electrode As the negative electrode in the secondary battery of the present invention, an electrode configuration known in the art can be used. For example, when the secondary battery is a lithium ion battery, an electrode containing a negative electrode active material capable of electrochemically occluding and releasing lithium ions can be mentioned. As such a negative electrode active material, a known negative electrode active material for a lithium ion secondary battery can be used, for example, natural graphite (graphite), highly oriented pyrolytic graphite (HOPG), and amorphous. Examples include carbonic materials such as carbon. Still other examples include lithium metals, metal compounds such as metal nitrides, and substances capable of forming alloys with lithium. For example, as an alloy having a lithium element, for example, a lithium aluminum alloy, a lithium tin alloy, a lithium lead alloy, a lithium silicon alloy and the like can be mentioned. Examples of the metal nitride containing a lithium element include lithium cobalt nitride, lithium iron nitride, and lithium manganese nitride. These negative electrode active materials may be used alone or in combination of two or more. Preferably, a carbonaceous material such as natural graphite (graphite), highly oriented graphite (HOPG), and amorphous carbon can be used. From the viewpoint of increasing the voltage and energy density of the secondary battery and reducing the number of series required to drive the device, it is desirable to use a negative electrode whose operating potential of the negative electrode is lower than 0.5 V with respect to the metallic lithium potential.

When the secondary battery is a sodium ion battery, an electrode containing a negative electrode active material capable of electrochemically occluding and releasing sodium ions can be used. As such a negative electrode active material, a known negative electrode active material for a sodium ion secondary battery can be used, and for example, hard carbon, soft carbon, carbon black, Ketjen black, acetylene black, activated carbon, carbon nanotubes, and carbon. Examples include carbonic materials such as fiber and amorphous carbon. Further, a sodium ion metal, an alloy containing a sodium ion element, a metal oxide, a metal nitride, or the like can also be used. Among them, as the negative electrode active material, a carbonaceous material such as hard carbon having a disordered structure is preferable.

The negative electrode may contain only the negative electrode active material, and contains at least one of a conductive material and a binder (binder) in addition to the negative electrode active material, and is a negative electrode current collector as a negative electrode mixture. It may be in the form of being attached to. For example, when the negative electrode active material is in the form of a foil, it can be a negative electrode containing only the negative electrode active material. On the other hand, when the negative electrode active material is in the form of powder, it can be a negative electrode having a negative electrode active material and a binder. As a method for forming the negative electrode using the powdered negative electrode active material, a doctor blade method, a molding method by a crimp press, or the like can be used.

As the conductive material, for example, conductive fibers such as carbon materials and metal fibers, metal powders such as copper, silver, nickel and aluminum, and organic conductive materials such as polyphenylene derivatives can be used. As the carbon material, graphite, soft carbon, hard carbon, carbon black, Ketjen black, acetylene black, graphite, activated carbon, carbon nanotubes, carbon fiber and the like can be used. In addition, mesoporous carbon obtained by firing a synthetic resin containing an aromatic ring, petroleum pitch, or the like can also be used.

As the binder, for example, a fluororesin such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), or ethylene tetrafluoroethylene (ETFE), polyethylene, polypropylene, or the like can be preferably used. As the negative electrode current collector, a rod-shaped body, a plate-shaped body, a foil-shaped body, a net-like body or the like mainly made of copper, nickel, aluminum, stainless steel or the like can be used.

(2) Positive Electrode As the positive electrode of the secondary battery of the present invention, an electrode configuration known in the art can be used. For example, when the secondary battery is a lithium ion battery, the positive electrode active material is _{one or more of lithium cobalt oxide (LiCoO 2} ), lithium manganate (LiMn ₂ O ₄ ), lithium nickel oxide (LiNiO ₂ ), and the like. Lithium-containing transition metal oxides containing transition metals, transition metal sulfides, metal oxides, one or more transition metals such as _{lithium iron oxide (LiFePO 4} ) and lithium iron pyrophosphate (Li ₂ FeP ₂ O _7). lithium-containing polyanionic compounds containing, sulfur-based compounds (Li 2 _S) and the like. Specific _{examples, LiNi x Mn y Co z O} 2 used as a high-capacity electrode or the high voltage electrode _{(x + y + z = 1} ) (NMC), LiNi x Co y Al z O 2 (x + y + z = 1) (NCA) and LiNi _0.5 Mn _1.5 O ₄ (LNMO) can be mentioned. The positive electrode may contain a conductive material or a binder. Similarly, when the secondary battery is a sodium ion battery, a known positive electrode active material can be used.

As the conductive material and the binder, the same material as the above negative electrode can be used.

As the positive electrode current collector metal, for example, copper, nickel, aluminum, stainless steel, or the like can be used.

(3) Separator The separator used in the secondary battery of the present invention is not particularly limited as long as it has a function of electrically separating the positive electrode layer and the negative electrode layer, but for example, polyethylene (PE). , A porous sheet made of a resin such as polypropylene (PP), polyester, cellulose, or polyimide, and a porous insulating material such as a non-woven fabric such as a non-woven fabric and a glass fiber non-woven fabric.

(4) Shape, etc. The shape of the secondary battery of the present invention is not particularly limited as long as it can store the positive electrode, the negative electrode, and the electrolytic solution, but for example, it is a cylindrical type, a coin type, a flat plate type, or a laminated type. And so on.

Although the electrolytic solution and the secondary battery of the present invention are suitable for use as a secondary battery, the use as a primary battery is not excluded.

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

1. 1. Synthesis of cyclic phosphate [1-1. Synthesis of TFEP]
2- (2,2,2-trifluoroethoxy) -1,3,2-dioxaphosphoran 2-oxide (TFEP) having the following structure was synthesized as a cyclic phosphoric acid ester used as a solvent for the electrolytic solution. ..

2,2,2-Trifluoroethanol (21.07 g, 210.6 mmol) and triethylamine (18.65 g, 184.3 mmol) were added to anhydrous tetrahydrofuran (THF, 250 mL) in an ice-water bath. Next, 2-chloro-1,3,2-dioxaphosphoran 2-oxide (25 g, 175.5 mmol) was added dropwise to the mixture over 30 minutes and the reaction mixture was stirred overnight. Then, triethylamine hydrochloride was removed by filtration, and THF was removed by a rotary evaporator. After vacuum distillation (130 ° C., 0.4 mbar), a colorless oily product (22.4 g, yield 62.1%) was obtained. The resulting TFEP was stored on a 4Å molecular sieve for at least 2 days prior to use.
¹ H NMR (CDCl ₃ , ppm): λ 4.56-4.42 (m, 6H); ¹³ C NMR (CDCl ₃ , ppm): λ 124.6-120.2 (CF ₃ ), 66.5 (OCH ₂ CH ₂ O), 64.6 ( OCH ₂ )

[1-2. DMAP synthesis]
2- (Dimethylamino) -1,3,2-dioxaphosphoran-2-oxide (DMAP) having the following structure was synthesized as a cyclic phosphoric acid ester used as a solvent for the electrolytic solution.

Dimethylamine (72.0 ml, 124 mmol) was added to anhydrous tetrahydrofuran (THF, 250 ml) in an ice water bath. Next, a mixture of 2-chloro-1,3,2-dioxaphosphoran-2-oxide (10 g, 70 mmol) and THF (50 ml) was added dropwise to the mixture over 30 minutes and mixed overnight. Then, dimethylamine hydrochloride was removed by filtration, and THF was removed by a rotary evaporator. DMAP was extracted from the oily product using an ether solvent. The ether solvent was removed by a rotary evaporator to obtain DMAP.
¹ H NMR (CDCl ₃ , ppm): δ 4.1-4.2 (OCH ₂ CH ₂ O), 2.8 (6H); ¹³ C NMR (CDCl ₃ , ppm): δ 66.5 (OCH ₂ CH ₂ O), 36.5 (CH) ₃ )

δ
2. Preparation of Electrolyte As the LiFSI salt (Nippon Shokubai Co., Ltd.) and the co-solvent 2,2,2-trifluoroethylmethyl carbonate (FEMC) (Halocarbon Products Co., Ltd.), lithium battery grade commercial products were used as they were. An electrolytic solution was prepared by dissolving LiFSI in a TFEP / FEMC mixed solvent (1: 3 by volume) in a glove box filled with argon gas. The molar ratio of salt to solvent was 1: 8, and the molar concentration was 0.95 M.

As a comparative example, an electrolytic solution (molar ratio 1: 8, concentration 0.98 M) in which LiFSI was dissolved only in FEMC was prepared. Similarly, as a comparative example, _{a commercially available EC / DMC electrolytic solution containing 1.0 M LiPF 6} was purchased (Kishida Chemical Co., Ltd.).

3. 3. Electrode preparation The electrode materials natural graphite, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ (NMC), LiNi _0.5 Mn _1.5 O ₄ (LNMO) are SEC Carbon, Toyoshima Seisakusho, and Toyoshima Seisakusho, respectively. Purchased from Hosen Co., Ltd. NMC or LNMO was mixed with polyvinylidene fluoride (PVdF, Kureha) and acetylene black (AB, Li-400, Denka) in N-methylpyrrolidone (NMP, Wako) to prepare cathodes. The weight ratio was 80:10:10 for active material: AB: PVdF. In addition, natural graphite was mixed with PVdF in NMP to prepare an anode (weight ratio is graphite: PVdF = 90:10). A doctor blade was used to coat the slurry on the Al foil current collector for the cathode and the Cu foil current collector for the anode. The resulting electrodes were then dried in an oven at 60 ° C. for 2 hours and then further dried under vacuum at 120 ° C. before use. Mass loading of the active material, the cathode at 3 ~ 4mg / cm ^2, and a 1.4 ~ 2mg / cm ² at the anode.

4. Electrochemical measurement conditions LSV measurement (Linear Swep Voltammetry) was performed using a three-electrode cell using a Pt or Al disk as a working electrode and a lithium metal as a reference electrode and a counter electrode. Cyclic voltammetry measurement (CV) was performed at a scan rate of ^{0.1 mVs -1} using a two-electrode coin cell using a graphite anode, an NMC or LNMO cathode as the working electrode, and a lithium metal as the reference / counter electrode. A VMP3 potentiostat (BioLogic) was used in both the LSV measurement and the CV measurement. The graphite | Li, NMC | Li, and LNMO | Li half cells were all 2032 type coin cells using a glass fiber separator. The constant current charge / discharge test and the rate capacity test were performed using a TOSCAT-3100 charge / discharge unit (Toyo System Co.). The charge / discharge measurement was performed at the same C rate without using the constant voltage mode. Electrochemical impedance spectroscopy (EIS) was performed after 3 and 100 cycles at an open circuit potential with an amplitude of 10 mV in the frequency range of 10 MHz to 1 MHz.

5. Evaluation of Oxidative Stability of Electrolyte Solution 2. The oxidative stability of the electrolytic solution prepared in the above was evaluated by LSV measurement using a Pt electrode. As shown in FIG. 1a, the EC / DMC electrolyte containing _{1.0 M LiPF 6 of} ^{Comparative Example showed an oxidation potential of 4.4 V at 0.02 mAcm-2} , but the 0.95 M LiFSI of the present invention was obtained. It was found that the TFEP / FEMC electrolyte contained was stable up to 4.9 V and was also useful for high potential positive electrodes. Further, as a result of measuring the temperature dependence of the ionic conductivity and the viscosity, the values of the TFEP / FEMC electrolytic solution (2.19 mScm ^-1 and 6.20 mPa, respectively) have the same values as those of the EC / DMC electrolytic solution. , It was found that the characteristics required for battery applications were satisfied.

6. Evaluation of flame retardancy of electrolyte solution 2. A flammability test was conducted on the electrolytic solution prepared in 1. As shown in FIG. 1b, the conventional EC / DMC electrolyte was very volatile and flammable, ignited immediately and continued to burn even after the torch was removed. The self-extinguishing time (SET) was 68 ± 3 s / g. The electrolyte of FEMC alone was also flammable, as shown in FIG. 1c. On the other hand, the TFEP / FEMC electrolytic solution of the present invention did not ignite at all, and the SET was zero (s / g). This result shows the excellent nonflammability of the electrolytic solution of the present invention containing the cyclic phosphate TFEP.

7. Evaluation of lithium insertion behavior into graphite negative electrode 2. The charge / discharge curves of the graphite | Li coin battery prepared using the three types of electrolytes prepared in FIG. 2a are shown in FIG. 2a. Cells based on FEMC-only electrolyte showed a long plateau at 0.9 V due to continuous electrolyte decomposition and / or solvent co-insertion, indicating that the SEI layer could not be formed on the graphite surface (SEI). Upper part of FIG. 2a). On the other hand, in the TFEP / FEMC electrolytic solution of the present invention, some voltage plateaus were observed in the region of 0.05 to 0.25 V (lower part of FIG. 2a), which was almost the same as that of the EC / DMC electrolytic solution (middle part of FIG. 2a). The same charge / discharge curve was obtained. This is characteristic of continuous step formation by the Li-graphite intercalation compound and can be said to be approximately equal to the theoretical volume (372 mAhg ^-1 _{) of fully lithium-plated graphite (LiC 6).}

Furthermore, the initial Coulomb efficiency of the TFEP / FEMC electrolyte of the present invention was 84.6%, which was almost the same as that of the EC / DMC electrolyte (85.4%). The small plateau observed at about 1 V during the initial charge is believed to be due to the irreversible decomposition of TFEP forming a stable SEI film. In fact, as shown in FIG. 2b, the cycle characteristics (91.4%) of the TFEP / FEMC electrolyte are significantly higher than those of the EC / DMC electrolyte (80.8%) over 400 cycles at a C / 2 rate. Improved to. Further, even at the rate of 2C, the average Coulomb efficiency of the TFEP / FEMC electrolytic solution was 99.85%, showing a capacity retention rate of 60%.

Further, as a result of examining the surface of the graphite electrode charged and discharged in the TFEP / FEMC electrolytic solution of the present invention using X-ray photoelectron spectroscopy (XPS), a compound containing phosphorus produced by decomposition of TFEP was detected. .. Therefore, it was demonstrated that TFEP contributes to SEI film formation.

8. Suppression of corrosion of positive electrode aluminum current collector 2. The results of chronoamperometry (CA) measurement of the aluminum electrode in the LiFSI / FEMC electrolyte and the LiFSI / TFEP / FEMC electrolyte prepared in FIG. 3 are shown in FIG. The holding potential was 4.9 V based on lithium. When the LiFSI / FEMC electrolyte was used, the current value increased with time. From the scanning electron microscope (SEM) image of the aluminum electrode after the measurement, traces of oxidative corrosion were confirmed. This is a commonly observed phenomenon when LiFSI salts are used and is a side reaction that causes deterioration of lithium-ion batteries. On the other hand, when the LiFSI / TFEP / FEMC electrolytic solution was used, the current value temporarily increased but then decreased. No trace of corrosion was confirmed from the SEM image of the aluminum electrode after the measurement. From the above, it was clarified that the use of the TFEP solvent can suppress the oxidative corrosion of aluminum caused by LiFSI.

The result of XPS measurement of the aluminum electrode after the CA measurement is shown in FIG. _{It was confirmed that aluminum fluoride (AlF 3} ) and lithium fluoride (LiF) were produced in the commercial LiPF ₆ / EC / DMC electrolyte. These are derived from the LiPF ₆ salt, it is said to be contributing passive film components generally corrosion inhibition. _{On the other hand, AlF 3} was not produced in the FEMC electrolytic solution containing the LiFSI salt. Therefore, as observed in the CA measurement, it is considered that aluminum corrosion occurred. On the other hand, in the LiFSI / TFEP / FEMC electrolytic solution, _{the formation of LiF and AlF 3} was confirmed as in the case of using the _{LiPF 6} salt, demonstrating that TFEP also enables passivation of aluminum.

8. Application to positive electrode
_{LiNi 1/3} Mn _1/3 Co _1/3 O ₂ (NMC) | Charging / discharging of Li coin battery produced by using the LiFSI / TFEP / FEMC electrolytic solution of the present invention and the commercial LiPF ₆ / EC / DMC electrolytic solution. The curve is shown in FIG. The charge cutoff voltage was 4.5 V, the first 3 cycles were set to C / 10 rate, and the subsequent 3 cycles were set to C / 2 rate. In the commercial LiPF ₆ / EC / DMC electrolyte, volume deterioration was observed with the cycle, and the volume retention rate after 200 cycles was 28.3%. On the other hand, when the LiFSI / TFEP / FEMC electrolytic solution of the present invention was used, the capacity retention rate after 200 cycles was 86.5%, and a long-term stable charge / discharge cycle was possible. This is because the electrolytic solution has both high oxidative stability and aluminum passivation function.

FIG. 6 shows a charge / discharge curve when the LiFSI / TFEP / FEMC electrolytic solution of the present invention is applied to a LiNi _0.5 Mn _1.5 O _{4 (LNMO) | Li coin battery showing a higher voltage.} The charge cutoff voltage was set to 4.9 V, the first 3 cycles were set to C / 10 rate, and the subsequent 3 cycles were set to C / 2 rate. The capacity retention rate after 200 cycles was 70%, and a stable charge / discharge cycle was possible.

9. Charge / discharge measurement of TFEP / FEMC electrolyte using LiPF ₆
Above 2. With respect to the TFEP / FEMC electrolytic solution prepared in the above, an electrolytic solution in which the supporting salt _{was changed from LiFSI to LiPF 6} (salt: solvent molar ratio was 1: 8) was prepared, and charge / discharge measurement of a graphite | Li coin battery was performed. The results are shown in FIG. Measured at C / 20 rate. When using LiPF ₆ salt are also observed similar charge-discharge curve, it can be seen that SEI film formed on the graphite electrode. Therefore, it was demonstrated that the SEI film forming effect of the TFEP solvent of the present invention appears regardless of the type of supporting salt.

10. Charge / discharge measurement of TFEP electrolyte without using co-solvent
Above 2. For the TFEP / FEMC electrolytic solution prepared in the above, prepare an electrolytic solution (salt: solvent molar ratio of 1: 8) using TFEP alone solvent (100 mol%) without using FEMC, and charge a graphite | Li coin battery. The results of the discharge measurement are shown in FIGS. 8 and 9. The C / 20 rate was set and the temperature was 25 ° C. or 45 ° C. It was found that the viscosity was higher than that when FEMC was used as the co-solvent, so that the polarization was large and the obtained capacity was small, but a reversible charge / discharge cycle was possible. Therefore, it was demonstrated that the TFEP solvent of the present invention can be used with or without the presence of a co-solvent.

11. Charge / discharge measurement of TFEP electrolyte with different co-solvent
Above 2. For the TFEP / FEMC electrolyte prepared in the above, an electrolyte (salt: solvent molar ratio of 1: 8) was prepared using a co-solvent other than FEMC, and charge / discharge measurement of a graphite | Li coin battery was performed. Is shown in FIGS. 10 to 12. Ethyl methyl carbonate (EMC) or dimethyl carbonate (DMC) was used as a co-solvent. FIG. 10 is a mixed solvent of TFEP: EMC = 2: 8, FIG. 11 is a mixed solvent of TFEP: DMC = 2: 8, and FIG. 12 is a mixed solvent of TFEP: DMC = 1: 1. The C / 20 rate was set and the temperature was 25 ° C. It was found that the initial irreversible capacitance is larger than that when FEMC is used, but reversible charging / discharging is possible after the second cycle. Therefore, it has been demonstrated that the TFEP solvent of the present invention can be used with various co-solvents.

12. Comparison of overall performance of electrolytes 2. FIG. 13 shows a comparison of the overall performance of the LiFSI / TFEP / FEMC electrolyte prepared in the above and the commercial LiPF _{6 / EC / DMC electrolyte.} The LiFSI / TFEP / FEMC electrolyte of the present invention far exceeds the commercial electrolyte in terms of potential window, safety and charge / discharge cycle characteristics. In addition, the ionic conductivity, viscosity, and thermal stability are the same as those of the commercial electrolyte. From the above, the LiFSI / TFEP / FEMC electrolytic solution of the present invention has an overall advantage over the commercial organic electrolytic solution.

13. Charge / discharge measurement of DMAP electrolytic solution Further, TFEP is described in the above 1. Charge / discharge measurement was performed on the electrolytic solution replaced with DMAP synthesized in 1. Above 2. The DMAP / EMC electrolyte (DMAP: EMC = 1: 3) was prepared in the same procedure as in the above procedure, and the charge / discharge reversibility of the graphite negative electrode was measured for the graphite | Li half cell, and the results are shown in FIGS. 14 to 15. In FIG. 14, LiFSI was used as the supporting salt, and in FIG. 15, LiPF ₆ was used as the supporting salt. As a result, it was demonstrated that reversible charging / discharging is possible even when DMAP, which is a cyclic phosphoric acid ester compound having an alkylamino group, is used, as in the case of TFEP. It was also shown that not only LiFSI but also LiPF ₆ can function similarly when used as a supporting salt. These results show the versatility of the cyclic phosphoric acid ester electrolytic solution of the present invention.

Claims (18)

1. An electrolytic solution for a secondary battery containing an organic solvent and an alkali metal salt.
   The organic solvent contains a cyclic phosphate ester having a 5-membered ring structure containing a phosphorus atom and an oxygen atom.
   The composition of the electrolytic solution is such that the amount of solvent is 5 mol or more with respect to 1 mol of the alkali metal salt.
   The electrolytic solution.
2. The electrolytic solution for a secondary battery according to claim 1, wherein the cyclic phosphoric acid ester is represented by the formula (1) shown below.
   

   

   
   
   (In the formula, X represents an oxygen or nitrogen atom; Y represents one or two alkyl groups that may have substituents.)
3. The electrolytic solution for a secondary battery according to claim 2, wherein Y is an alkyl halide group having 1 to 10 carbon atoms.
4. The electrolytic solution for a secondary battery according to claim 1, wherein the cyclic phosphoric acid ester is selected from a compound having the following structure.
   

   

   
   
5. The electrolytic solution for a secondary battery according to any one of claims 1 to 4, wherein the alkali metal salt is a lithium salt or a sodium salt.
6. 1. 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. The electrolyte for a secondary battery according to any one of 5 to 5.
7. The anions are bis (fluorosulfonyl) imide ([N (FSO ₂ ) ₂ ] ^- ), (fluorosulfonyl) (trifluorosulfonyl) imide ([N (CF ₃ SO ₂ ) (FSO ₂ )] ^- ), and bis. (Trifluoromethanesulfonyl) imide ([N (CF ₃ SO ₂ ) ₂ ] ^- ), bis (perfluoroethanesulfonyl) imide ([N (C ₂ F ₅ SO ₂ ) ₂ ] ^- ), (perfluoroethanesulfonyl) (trifluoroethane) imide _{([N (C 2 F 5} SO 2) (CF 3 SO 2)] -), or hexafluorophosphate (PF ₆ ^-) is a rechargeable battery as claimed in claim 6 Electrolyte.
8. The electrolytic solution for a secondary battery according to any one of claims 1 to 7, wherein the ratio of the cyclic phosphoric acid ester in the total solvent is 10 to 100 mol%.
9. The electrolytic solution for a secondary battery according to any one of claims 1 to 8, further comprising an alkyl carbonate or an alkyl carbonate containing a halogen atom as a co-solvent.
10. The electrolytic solution for a secondary battery according to claim 9, wherein the co-solvent is 2,2,2-trifluoroethylmethyl carbonate (FEMC).
11. The electrolytic solution for a secondary battery according to any one of claims 1 to 10, wherein the secondary battery is a lithium ion secondary battery or a sodium ion secondary battery.
12. A secondary battery including a positive electrode, a negative electrode, and an electrolytic solution for a secondary battery according to any one of claims 1 to 11.
13. The secondary battery according to claim 12, which is a lithium ion secondary battery.
14. The secondary battery according to claim 13, wherein the positive electrode contains an active material selected from a metal oxide having a lithium element, a polyanionic compound, or a sulfur compound.
15. The secondary battery according to claim 13, wherein the negative electrode contains an active material selected from a carbon material, metallic lithium, or a substance capable of forming an alloy with lithium.
16. The secondary battery according to claim 12, which is a sodium ion secondary battery.
17. The secondary battery according to claim 16, wherein the positive electrode is a transition metal oxide.
18. The secondary battery according to claim 16, wherein the negative electrode is hard carbon.

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