JP6238582B2 - Acetonitrile electrolyte containing high-concentration metal salt and secondary battery containing the electrolyte - Google Patents

Description

  The present invention relates to an acetonitrile electrolyte containing a high-concentration metal salt and a secondary battery containing the electrolyte.

  Lithium ion batteries have a larger theoretical energy density than conventional secondary batteries, and are therefore widely used as batteries for portable devices, notebook computers, and electric vehicles. In recent years, there has been a demand for smaller and lighter secondary batteries, and in particular for automobile applications, charging and discharging with large currents are necessary. There is a need for development of lithium ion batteries.

  In order to increase the rate of lithium ion batteries, it is necessary to improve the electrolyte material, but at present, in order to ensure the reversibility of the graphite negative electrode, non-aqueous electrolyte materials include cyclic esters and chain esters, etc. It is limited to the carbonate type solvent. In general, a carbon material such as graphite is used as a negative electrode active material in order to solve the problem of dendrite precipitation in the negative electrode of a lithium ion battery, but reversible insertion / extraction of lithium ions into / from the negative electrode carbon material. This is because it was considered that it can be achieved only in the presence of a carbonate-based solvent. However, when such a carbonate solvent is used, it is difficult to significantly improve the rate characteristics. Previous studies have shown that the rate of charge / discharge reaction is strongly influenced by the electrolyte solvent. That is, not only the lithium ion conductivity in the electrolyte bulk, but also the activation barrier of the lithium ion insertion reaction into the electrode strongly depends on the electrolyte solvent composition. Particularly, carbonate solvents such as ethylene carbonate and propylene carbonate, which are currently mainly used, are known to increase the activation barrier of electrode reaction (Non-Patent Documents 1 to 3 and the like). In order to improve, it is necessary to fundamentally review the electrolyte solvent composition.

  On the other hand, acetonitrile, which has the property of being difficult to decompose and having a low melting point, is a candidate for an electrolyte solvent, but has not yet been put into practical use due to problems such as being weak against reduction and unable to withstand the potential of lithium ion insertion. .

T.A. Abe et al. , J. et al. Electrochem. Soc. 151, A1120-A1123 (2004). T.A. Abe et al. , J. et al. Electrochem. Soc. , 152, A2151-A2154 (2005). Y. Yamada et al. , Langmuir, 25, 12766-12770 (2009). J. et al. T.A. Dudley et al. , J. et al. Power Sources, 35, 59-82 (1991). K. Xu et al. , J. et al. Phys. Chem. C, 111, 7411-7421 (2007). K. Xu, J .; Electrochem. Soc. , 154, A162-A167 (2007). M.M. S. Ding et al. , J. et al. Electrochem. Soc. , 150, A620-A628 (2003).

  This invention makes it a subject to provide the electrolyte solution which exhibits the rate characteristic which surpasses the present carbonate type electrolyte solution, maintaining the reversibility of a graphite negative electrode.

  As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention enable reversible insertion / extraction reaction of lithium ions to / from the negative electrode carbon material in acetonitrile electrolyte by containing a high concentration of lithium salt. And the inventors have found that excellent rate characteristics can be obtained by the electrolytic solution, and have completed the present invention.

That is, the present invention
(1) An electrolyte solution for a lithium secondary battery containing acetonitrile and LiN (SO ₂ F) ₂ , wherein the molar concentration of LiN (SO ₂ F) ₂ in the electrolyte solution is 3.0 mol / L or more. An electrolyte, characterized by:
(2) The electrolytic solution according to (1), wherein the volume molar concentration of the LiN (SO ₂ F) ₂ is in the range of 3.0 to 5.5 mol / L;
(3) molarity of the LiN _(SO 2 _{F) 2} is in the range of 3.5~5.0mol / L, about electrolytic solution according to the above (1).

In another aspect, the invention provides:
(4) A positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material capable of occluding and releasing lithium ions, and the electrolyte for a lithium secondary battery according to any one of (1) to (3) above A secondary battery having:
(5) The secondary battery according to (4), wherein the negative electrode active material is a carbon material, metallic lithium, a lithium alloy, or a metal oxide;
(6) The secondary battery according to (4), wherein the positive electrode active material is a metal oxide having a lithium element, a polyanion compound, or a sulfur compound;
About.

  According to the present invention, by including a high concentration of lithium salt, the reduction resistance of acetonitrile is improved, and reversible insertion / extraction reaction of lithium ions to / from a negative electrode carbon material, which has been difficult in the conventional acetonitrile electrolyte. Is possible. In addition, despite the fact that no carbonate-based solvent or additive is used, excellent charge / discharge cycle characteristics can be obtained, and the rate characteristics significantly surpassing conventional commercial electrolytes can be obtained. .

  In addition, since relatively inexpensive acetonitrile can be used as an electrolyte solvent, it is not only significant in terms of cost but also has a low melting point, and is currently used mainly in lithium ion batteries. It is superior to ethylene carbonate.

FIG. 1 is a graph showing a charge / discharge curve in a graphite electrode when a 4.5 M LiFSA / acetonitrile electrolyte is used. FIG. 2 is a graph showing a comparison of charge and discharge curves when 3.0M and 4.5M LiFSA / acetonitrile electrolytes are used. FIG. 3 is a graph showing a comparison of charge / discharge curves when LiFSA / acetonitrile electrolyte and carbonate electrolyte are used. FIG. 4 is a graph showing a comparison of rate characteristics when LiFSA / acetonitrile electrolyte and carbonate electrolyte are used.

  Hereinafter, embodiments of the present invention will be described. The scope of the present invention is not limited to these descriptions, and other than the following examples, the scope of the present invention can be appropriately changed and implemented without departing from the spirit of the present invention.

1. Electrolytic Solution (1) Solvent The solvent used in the electrolytic solution of the present invention is most preferably acetonitrile (AN). However, other non-aqueous solvents can also be used, for example, ethers such as ethyl methyl ether and dipropyl ether; nitriles of methoxypropionitrile; esters such as methyl acetate; amines such as triethylamine; methanol and the like Alcohols; ketones such as acetone; and fluorine-containing alkanes. Among these, one type may be used alone, or two or more types may be used in combination. However, it is not limited to these.

  The non-aqueous solvent is preferably an aprotic organic solvent, and examples thereof include 1,2-dimethoxyethane, tetrahydrofuran, dimethyl sulfoxide, γ-butyrolactone, and sulfolane in addition to the acetonitrile. . In addition to these, it is also possible to use a mixed solvent including other nonaqueous solvents.

(2) Alkali Metal Salt The alkali metal salt used as the supporting electrolyte in the electrolytic solution of the present invention is LiN (SO ₂ F) ₂ (lithium bis (fluorosulfonyl). Hereinafter, it may be referred to as “LiFSA”). Most preferred. However, other lithium salts can be used as long as they can be dissociated in the electrolytic solution to supply lithium ions. Such a lithium salt is not particularly limited. For example, LiN (CF ₃ SO ₂ ) ₂ (hereinafter sometimes referred to as “LiTFSA” or “LiTFSI”), LiN (C ₂ F ₅ SO ₂ ) ₂ (hereinafter sometimes referred to as “LiBETI”), LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) (C ₂ F _{5 SO 2), LiN (CF 3} SO 2) (C 3 F 7 SO 2), LiN (CF 3 SO 2) (C 4 F 9 SO 2), LiPF 6, LiBF 4, LiClO 4, and any of these What is selected from the combination is mentioned. In addition, when the secondary battery to which the electrolytic solution of the present invention is applied is a sodium ion battery or a potassium ion battery, a sodium salt or potassium salt made of the same anion as the above lithium salt can also be used. Moreover, when the secondary battery to which the electrolytic solution of the present invention is applied is a magnesium ion battery or a calcium ion battery, a magnesium salt or a calcium salt made of an anion similar to the above lithium salt can also be used.

  The concentration range of the alkali metal salt in the electrolyte solution should be a high concentration that allows a reversible insertion / desorption reaction of the metal ion to / from the negative electrode carbon material unless precipitation of the alkali metal salt occurs. it can. For example, when LiFSA is used in a lithium secondary battery, a salt concentration at which the volume molar concentration of LiFSA in the electrolytic solution is 3.0 mol / L or more is preferable. More preferably, the salt concentration is in the range of 3.0 to 5.5 mol / L, and still more preferably in the range of 3.5 to 5.0 mol / L.

(3) Other components Moreover, the electrolytic solution of the present invention may contain other components as necessary for the purpose of improving the function thereof. Examples of the other components include conventionally known overcharge inhibitors, dehydrating agents, deoxidizing agents, capacity maintenance characteristics after high-temperature storage, and property improvement aids for improving cycle characteristics.

  Examples of the overcharge inhibitor include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; 2-fluoro Partially fluorinated products of the above aromatic compounds such as biphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene; fluorinated anisole such as 2,4-difluoroanisole, 2,5-difluoroanisole and 2,6-difluoroaniol Compounds. An overcharge inhibitor may be used individually by 1 type, and may use 2 or more types together.

  When electrolyte solution contains an overcharge inhibitor, it is preferable that content of the overcharge inhibitor in electrolyte solution is 0.01-5 mass%. By containing 0.1% by mass or more of the overcharge inhibitor in the electrolytic solution, it becomes easier to suppress the rupture / 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 performing rectification after dehydrating with the above dehydrating agent may be used. Moreover, you may use the solvent which performed only the dehydration by the said dehydrating agent, without performing rectification.

Examples of the characteristic improvement aid for improving capacity maintenance characteristics and cycle characteristics after high-temperature storage include succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, dihydrate Carboxylic anhydride such as glycolic acid, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, phenylsuccinic anhydride; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methanesulfonic acid Methyl, busulfan, sulfolane, sulfolene, dimethylsulfone, diphenylsulfone, methylphenylsulfone, dibutyldisulfide, dicyclohexyldisulfide, tetramethylthiuram monosulfide, N, N-dimethylmethanesulfonamide, N, N-diethylmethanes Sulfur-containing compounds such as honamide; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, N-methylsuccinimide, etc. Nitrogen compounds; hydrocarbon compounds such as heptane, octane and cycloheptane; and fluorine-containing aromatic compounds such as fluorobenzene, difluorobenzene, hexafluorobenzene and benzotrifluoride. These characteristic improvement aids may be used alone or in combination of two or more. When the electrolytic solution contains a property improving aid, the content of the property improving aid in the electrolytic solution is preferably 0.01 to 5% by mass.

2. Secondary battery The secondary battery of this invention is equipped with a positive electrode and a negative electrode, and the electrolyte solution of this invention.

(1) Negative electrode As a negative electrode in the secondary battery of this invention, the electrode containing the negative electrode active material which can occlude / release lithium ion electrochemically is mentioned. As such a negative electrode active material, known negative electrode active materials for lithium ion secondary batteries can be used. For example, natural graphite (graphite), highly oriented graphite (HOPG), amorphous Examples thereof include carbonaceous materials such as carbon. Still other examples include metal compounds such as lithium metal or alloys containing lithium elements, metal oxides, metal sulfides, and metal nitrides. For example, examples of the alloy having a lithium element include a lithium aluminum alloy, a lithium tin alloy, a lithium lead alloy, and a lithium silicon alloy. The metal oxide having a lithium element can be, for example, lithium titanate _(Li 4 Ti ₅ O _12, etc.) and the like. 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 individually by 1 type, and may use 2 or more types together.

  Especially, as a negative electrode active material, carbonaceous materials, such as a graphite, are preferable. Further, as the carbonaceous material, graphite and a carbonaceous material in which the surface of graphite is coated with amorphous carbon as compared with the graphite are particularly preferable.

  The negative electrode may contain only a negative electrode active material. In addition to the negative electrode active material, the negative electrode contains at least one of a conductive material and a binder, and 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 has a foil shape, a negative electrode containing only the negative electrode active material can be obtained. On the other hand, when the negative electrode active material is in a powder form, a negative electrode having a negative electrode active material and a binder (binder) can be obtained. As a method for forming a negative electrode using a powdered negative electrode active material, a doctor blade method, a molding method using a pressure press, or the like can be used.

  Examples of the conductive material 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. As the carbon material, graphite, soft carbon, hard carbon, carbon black, ketjen black, acetylene black, graphite, activated carbon, carbon nanotube, 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, fluororesin such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylenetetrafluoroethylene (ETFE), polyethylene, polypropylene, or the like can be preferably used. As the negative electrode current collector, a rod-like body, a plate-like body, a foil-like body, a net-like body or the like mainly composed of copper, nickel, aluminum, stainless steel, or the like can be used.

(2) Positive electrode In the positive electrode of the secondary battery of the present invention, a known positive electrode active material for a lithium ion secondary battery can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ) Lithium-containing transition metal oxides, transition metal sulfides, metal oxides, lithium iron phosphate (LiFePO ₄ ), and lithium iron pyrophosphate (Li ₂ ), including one or more transition metals such as lithium nickelate (LiNiO ₂ ) Examples thereof include lithium-containing polyanion compounds containing one or more transition metals such as FeP ₂ O ₇ ), sulfur compounds (Li ₂ S), and the like. The positive electrode may contain a conductive material or a binder, or may contain a catalyst that promotes an oxidation-reduction reaction of oxygen.

  As the conductive material and the binder (binder), the same materials as the negative electrode can be used.

As the catalyst, MnO ₂ , Fe ₂ O ₃ , NiO, CuO, Pt, Co, or the like can be used. Further, as the binder (binder), the same binder as that of the negative electrode 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. For example, polyethylene (PE) Examples thereof include a porous sheet made of a resin such as polypropylene (PP), polyester, cellulose, and polyamide, and a porous insulating material such as a nonwoven fabric such as a nonwoven fabric and a glass fiber nonwoven fabric.

(4) Shape, etc. The shape of the secondary battery of the present invention is not particularly limited as long as it can accommodate a positive electrode, a negative electrode, and an electrolytic solution. For example, a cylindrical type, a coin type, a flat plate type, a laminate type Etc.

  In addition, although the electrolyte solution and secondary battery of this invention are suitable for the use as a secondary battery, using as a primary battery is not excluded.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited by these.

1. Evaluation of reversible charge-discharge reaction at graphite electrode (charge-discharge test)
Using an electrolytic solution containing 4.5 M LiFSA, the monopolar potential at the graphite electrode was measured, and the charge / discharge behavior was compared at the C / 10 rate. The measurement was carried out using a natural graphite binder electrode (average particle size 10 μm) containing 10% by weight of polyvinylidene fluoride (PVdF) as a working electrode, and a half cell equipped with metallic lithium as a counter electrode and a reference electrode. VMP-3) manufactured by the company. The temperature is 25 ° C. The obtained results are shown in FIG.

From the results shown in FIG. 1, in an acetonitrile solution containing 4.5 M LiFSA, a reversible lithium insertion and desorption reaction to graphite can be obtained even when an acetonitrile solvent having low reduction resistance is used by setting a high salt concentration. Has been demonstrated. Further, it was shown that even when the number of cycles increased, the discharge capacity was hardly decreased, and cycle deterioration did not occur. It was found that good cycle characteristics can be obtained. Reversible charge / discharge is obtained with a discharge capacity close to the theoretical capacity (372 mAhg ⁻¹ ) of a graphite electrode, and this is an electrolyte solution that does not contain vinylene carbonate as an ethylene carbonate solvent or additive commonly used in lithium ion batteries. It can be said that the performance is unprecedented.

Moreover, the result of having performed the same measurement about the acetonitrile solution containing 3.0M LiFSA is shown in FIG. Here, only the lithium insertion curve of the first cycle is displayed for comparison, and the region up to 100 mAhg ⁻¹ is enlarged. As described above, an electrolyte solution with a 4.5 M LiFSA concentration could insert Li in an amount close to the theoretical capacity, while at 3.0 M, it was 10 mAhg ^-1 or less, and reversible charge / discharge was impossible. there were. From this, it can be said that the discharge capacity obtained at a LiFSA concentration of 4.5 M is due to the presence of a high concentration of lithium salt. Conversely, it is suggested that a LiFSA concentration of 3.0 M or higher is necessary to ensure the reaction reversibility of the graphite electrode.

  These results demonstrate the effectiveness of the negative electrode reaction with the electrolyte of the present invention containing a high concentration of lithium salt.

2. Evaluation of Rate Characteristics Using the cell of Example 1, the rate characteristics in the discharge capacity were evaluated. FIG. 3 shows a voltage curve when lithium is inserted into graphite at a 2C rate using an acetonitrile solution containing 4.5 M LiFSA. The lower limit voltage was set to 0 V before lithium metal deposition occurred. As a comparative example, the results obtained in an ethylene carbonate / dimethyl carbonate (EC / DMC) electrolyte solution containing 1.0 M LiPF ₆ are also shown in FIG. In the carbonate electrolyte, only about 100 mAhg ⁻¹ could be inserted, but in the LiFSA / acetonitrile electrolyte of the present invention, it became clear that 300 mAhg ⁻¹ or more lithium was inserted.

  Similarly, the discharge capacity at various rates in the range of C / 20 to 5C was measured for an acetonitrile electrolyte containing 4.5 M LiFSA and an EC / DMC electrolyte. FIG. 4 shows the results of plotting the capacity in the desorption direction obtained by measuring the same rate (current value) in both the lithium insertion direction and the desorption direction in graphite under the same conditions for each of three cycles. In order to confirm reversibility, the 22nd and 23rd cycles are performed again at C / 20).

From the results in FIG. 4, the LiFSA / acetonitrile electrolyte of the present invention showed a higher reversible capacity at any rate. In addition, as the rate increased, the capacity of the EC / DMC electrolyte of the comparative example decreased significantly, whereas the electrolyte of the present invention showed a capacity that remained almost unchanged up to 2C, and was almost the same even at 5C. It was found that a capacity of 300 mAhg ⁻¹ can be maintained.

In lithium ion batteries, the rate of lithium insertion into the graphite negative electrode determines the charging rate, and it is clear from the results that the lithium ion battery can be charged at high speed by using the electrolytic solution of the present invention. It became.

  In the above examples, the acetonitrile electrolytic solution containing the high concentration LiFSA of the present invention solves the problems in the lithium metal negative electrode of the conventional acetonitrile-based electrolytic solution, and is superior to the carbonate-based electrolytic solution currently on the market. It demonstrates that it has rate characteristics and can function as a suitable electrolyte.

  Although specific embodiments of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. In addition, the invention described in the claims may include various modifications of the specific embodiments described above.

Claims (6)

An electrolytic solution for a lithium secondary battery containing acetonitrile and LiN (SO ₂ F) ₂ , wherein a volume molar concentration of the LiN (SO ₂ F) ₂ in the electrolytic solution is 3.0 mol / L or more. Characteristic electrolyte. The electrolytic solution according to claim 1, wherein the volume molar concentration of LiN (SO ₂ F) ₂ is in the range of 3.0 to 5.5 mol / L. The electrolytic solution according to claim 1, wherein the volume molar concentration of LiN (SO ₂ F) ₂ is in the range of 3.5 to 5.0 mol / L. The secondary battery which has the positive electrode containing a positive electrode active material, the negative electrode containing the negative electrode active material which can occlude and discharge | release lithium ion, and the electrolyte solution for lithium secondary batteries of any one of Claims 1-3. The secondary battery according to claim 4, wherein the negative electrode active material is a carbon material, metallic lithium, a lithium alloy, or a metal oxide. The secondary battery according to claim 4, wherein the positive electrode active material is a metal oxide containing lithium element, a polyanion compound, or a sulfur compound.

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