JP2018137100A - Electrolyte solution for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolyte solution for a lithium ion secondary battery, which contains an inorganic phosphate compound, which enables the achievement of high cycle characteristics of a lithium ion secondary battery and the achievement of a low battery resistance even if it contains no water.SOLUTION: An electrolyte solution for a lithium ion secondary battery, herein disclosed comprises: a non-fluorinated solvent; a fluorinated solvent; and an inorganic phosphate compound. The fluorinated solvent contains, in its molecule, a hydrogen atom and a fluorine atom. As to the fluorinated solvent, the ratio of the number of fluorine atoms to the number of hydrogen atoms in one molecule is 0.6-1.5. In the electrolyte solution, the content of the fluorinated solvent is 1 mass% or more and 10 mass% or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electrolytic solution for a lithium ion secondary battery.

  Lithium ion secondary batteries are lighter and have a higher energy density than existing batteries, and have recently been used as so-called portable power sources for vehicles and personal computers and power sources for driving vehicles. Lithium-ion secondary batteries are expected to become increasingly popular as high-output power sources for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). Yes.

  Lithium ion secondary batteries are required to have higher performance with the spread of the lithium ion secondary batteries, and various developments have been made for higher performance. For example, Patent Document 1 discloses that in a non-aqueous electrolyte secondary battery, the non-aqueous electrolyte contains 100 ppm or more and 1000 ppm or less of water with respect to the total weight of the positive electrode, the negative electrode, and the non-aqueous electrolyte. It is disclosed to coexist with an inorganic phosphate compound in a molar ratio of 0.5 to 10 times that of the water. Patent Document 1 describes that such a nonaqueous electrolyte secondary battery is excellent in cycle characteristics.

Japanese Patent Laying-Open No. 2015-090860

  However, in the technique described in Patent Document 1, an inorganic phosphate compound is used to improve cycle characteristics. However, in order to dissolve this inorganic phosphate compound in an electrolytic solution, a nonaqueous electrolyte secondary battery is used. The electrolytic solution needs to contain water. According to the study of the present inventor, as described in Patent Document 1, it has been found that there is a problem that the battery resistance increases when water is contained in the electrolytic solution.

  Accordingly, the present invention provides an electrolytic solution for a lithium ion secondary battery containing an inorganic phosphate compound, which can improve the cycle characteristics of the lithium ion secondary battery even if it does not contain water. It aims at providing the electrolyte solution for lithium ion secondary batteries which can make resistance low.

The electrolytic solution for a lithium ion secondary battery disclosed herein contains a non-fluorinated organic solvent, a fluorinated solvent, and an inorganic phosphate compound. The fluorinated solvent contains hydrogen atoms and fluorine atoms in the molecule. The ratio of the number of fluorine atoms to the number of hydrogen atoms in one molecule of the fluorinated solvent is 0.6 to 1.5. The content of the fluorinated solvent in the electrolytic solution is 1% by mass or more and 10% by mass or less.
According to such a configuration, by using a fluorinated solvent having a high fluorination rate in which the ratio of the number of fluorine atoms to the number of hydrogen atoms in one molecule is 0.6 to 1.5, The solubility in the electrolytic solution can be particularly enhanced. For this reason, even if electrolyte solution does not contain water, an inorganic phosphate compound can melt | dissolve. In addition, when such a fluorinated solvent having a high fluorination rate is used, the amount of the fluorinated solvent contained in the electrolyte solution can be reduced to contain the inorganic phosphate compound. When content in electrolyte solution is 1 mass% or more and 10 mass% or less, the improvement effect of the cycle characteristic by an inorganic phosphate compound can be acquired, suppressing the increase in battery resistance. That is, according to such a configuration, the electrolyte solution for a lithium ion secondary battery containing an inorganic phosphate compound can improve the cycle characteristics of the lithium ion secondary battery even if it does not contain water. Thus, it is possible to provide an electrolyte solution for a lithium ion secondary battery that can reduce battery resistance.

It is sectional drawing which shows typically an example of the lithium ion secondary battery comprised using the electrolyte solution for lithium ion secondary batteries which concerns on one Embodiment of this invention. It is a schematic diagram which shows the structure of the electrode body used for the lithium ion secondary battery of FIG.

  Embodiments according to the present invention will be described below with reference to the drawings. It should be noted that matters other than the matters specifically mentioned in the present specification and necessary for the implementation of the present invention (for example, a general configuration of an electrolytic solution for a lithium ion secondary battery that does not characterize the present invention and Manufacturing process) can be understood as a design matter of those skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field. Moreover, in the following drawings, the same code | symbol is attached | subjected and demonstrated to the member and site | part which show | plays the same effect | action. In addition, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships.

In the present specification, the “secondary battery” refers to a general power storage device that can be repeatedly charged and discharged, and is a term including a so-called storage battery such as a lithium ion secondary battery and a power storage element such as an electric double layer capacitor.
Further, in the present specification, the “lithium ion secondary battery” refers to a secondary battery that uses lithium ions as a charge carrier and is charged / discharged by movement of charges accompanying the lithium ions between the positive and negative electrodes.

  The electrolytic solution for a lithium ion secondary battery according to this embodiment contains a non-fluorinated organic solvent, a fluorinated solvent, and an inorganic phosphate compound.

As a non-fluorinated organic solvent (that is, an organic solvent that does not contain a fluorine atom) used in the electrolyte solution for a lithium ion secondary battery according to the present embodiment, a conventional electrolyte solution for a lithium ion secondary battery is used. The same non-fluorinated organic solvent used as the non-aqueous solvent (organic solvent) can be used without particular limitation.
For example, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones that do not contain fluorine atoms can be used without limitation. Specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and the like.

As the fluorinated solvent used in the electrolyte solution for the lithium ion secondary battery according to the present embodiment, a solvent containing hydrogen atoms and fluorine atoms in the molecule is used. And what has ratio (F / H ratio) of the number of fluorine atoms with respect to the number of hydrogen atoms in 1 molecule is 0.6-1.5 is used.
Examples of such fluorinated solvents include fluorinated cyclic carbonates such as difluoroethylene carbonate (F / H ratio: 1), 4- (trifluoromethyl) -ethylene carbonate (F / H ratio: 1); trifluoro Methyl methyl carbonate (F / H ratio: 1), fluoromethyl difluoromethyl carbonate (F / H ratio: 1), methyl (2,2,2-trifluoroethyl) carbonate (F / H ratio: 0.6), (2,2-difluoroethyl) fluoromethyl carbonate (F / H ratio: 0.6), (2-fluoroethyl) difluoromethyl carbonate (F / H ratio: 0.6), ethyl trifluoromethyl carbonate (F / H H ratio: 0.6), 2,2,2-trifluoroethyl-2′-fluoroethyl carbonate (F / H ratio: 0.67), 2,2, 2-trifluoroethyl-2 ′, 2′-difluoroethyl carbonate (F / H ratio: 1), bis (2,2,2-trifluoroethyl) carbonate (F / H ratio: 1.5), pentafluoro Examples include fluorinated carbonates such as fluorinated chain carbonates such as ethyl ethyl carbonate (F / H ratio: 1). Of the fluorinated carbonates, fluorinated chain carbonates are preferred.
The fluorinated solvent preferably has 2 to 4 carbon atoms.

  The content of the fluorinated solvent in the electrolytic solution (that is, the content relative to the total mass of the electrolytic solution) is 1% by mass or more and 10% by mass or less.

Examples of the inorganic phosphate compound used in the electrolyte for the lithium ion secondary battery according to this embodiment include Li ₃ PO ₄ , Na ₃ PO ₄ , K ₃ PO ₄ , Ca ₃ (PO ₄ ) ₂ , and AlPO. _{4 and the} like, and Li ₃ PO ₄ is particularly preferable.
There is no restriction | limiting in particular about content of the inorganic phosphate compound in electrolyte solution, and the upper limit of content is decided by the kind and content (namely, solubility of the inorganic phosphate compound of electrolyte solution) of the said fluorinated solvent. Therefore, the content of the inorganic phosphate compound in the electrolytic solution exceeds 0 and is not more than this upper limit value.

The electrolytic solution for a lithium ion secondary battery according to the present embodiment usually contains a supporting salt. As the supporting salt, a lithium salt (preferably LiPF ₆ ) such as LiPF ₆ or LiBF ₄ can be suitably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less (for example, 1.0 mol / L).

  The electrolyte solution for the lithium ion secondary battery according to the present embodiment contains the specific amount of the fluorinated solvent having the specific fluorination rate (F / H ratio), so that the cycle characteristics of the lithium ion secondary battery are obtained. Can be increased, and the battery resistance can be decreased. The reason is considered as follows.

  One of the causes of the deterioration of the cycle characteristics of the lithium ion secondary battery is oxidative decomposition of the electrolytic solution. As a method for suppressing this oxidative decomposition, a method of forming an insulating protective film on the surface of the positive electrode of the lithium ion secondary battery and suppressing the reaction between the positive electrode and the electrolytic solution can be mentioned. When the electrolytic solution contains an inorganic phosphate compound, a protective coating derived from this inorganic phosphate compound can be formed on the surface of the positive electrode, thereby suppressing the reaction between the positive electrode and the electrolytic solution and It is possible to improve the cycle characteristics of the ion secondary battery (increase the capacity retention rate when charging and discharging are repeated).

However, in order to obtain an effect of improving the cycle characteristics of the lithium ion secondary battery by the inorganic phosphate compound, the inorganic phosphate compound needs to be dissolved in the electrolytic solution. Here, a nonaqueous solvent that hardly dissolves the inorganic phosphate compound is usually used for the electrolytic solution. Therefore, in patent document 1 which is a prior art, the electrolyte solution contains water. As a result, the inorganic phosphate compound can be dissolved directly in water, and hydrofluoric acid produced by involving water (for example, fluorination produced by reaction of water with a supporting salt such as LiPF ₆ ). It is also possible to dissolve in (hydrogen acid).
However, when water is contained in the electrolytic solution, a side reaction between water and a supporting salt such as LiPF ₆ occurs, which causes a decrease in the supporting salt concentration, accumulation of by-products on the electrode, and the like. As a result, the battery resistance is increased.

  On the other hand, according to the study of the present inventor, it was found that the inorganic phosphate compound can be dissolved in the electrolytic solution even when a fluorinated solvent is used in addition to using water. This is presumably because hydrofluoric acid is generated when the fluorinated solvent undergoes oxidative decomposition, and the inorganic phosphoric acid compound dissolves therein. Therefore, by using a fluorinated solvent in the electrolytic solution, the effect of improving the cycle characteristics of the lithium ion secondary battery by the inorganic phosphate compound can be obtained.

  However, since the molecular size of the fluorinated solvent is larger than that of the non-fluorinated solvent, when the electrolytic solution contains a fluorinated solvent, the viscosity of the electrolytic solution increases. An increase in the viscosity of the electrolyte causes an increase in battery resistance. Therefore, only by replacing the non-fluorinated solvent used in the electrolytic solution with a fluorinated solvent, the two battery performances of high cycle characteristics and low battery resistance cannot be obtained simultaneously.

  Therefore, in this embodiment, a fluorinated solvent having an F / H ratio of 0.6 to 1.5 and a high fluorination rate is used. When such a fluorinated solvent having a high fluorination rate is used, the solubility of the inorganic phosphate compound can be particularly enhanced. As a result, the amount of the fluorinated solvent contained in the electrolyte solution can be reduced in order to contain the inorganic phosphate compound. And, when the content of the fluorinated solvent in the electrolytic solution is 1% by mass or more and 10% by mass or less, the cycle characteristics are improved by forming a protective coating derived from the inorganic phosphate compound while suppressing an increase in battery resistance. An effect can be obtained.

  Thus, the electrolytic solution for a lithium secondary battery according to the present embodiment does not need to contain water, and therefore it is preferable that the electrolyte does not substantially contain water. In the present specification, “substantially free of water” means “not containing water or having an amount of water mixed as an inevitable impurity even if it contains water”, for example, The water content is less than 100 mass ppm or 50 mass ppm or less in the electrolytic solution.

  The electrolyte solution for a lithium secondary battery according to this embodiment is a component other than the above-described components, for example, a gas generating agent such as biphenyl (BP) or cyclohexylbenzene (CHB), as long as the effects of the present invention are not significantly impaired. Various additives such as a dispersant, a thickener, and the like may be included.

The electrolyte for a lithium ion secondary battery according to this embodiment is prepared by preparing an electrolyte containing a non-fluorinated organic solvent, a fluorinated solvent, and an inorganic phosphate compound in advance. It may be used for secondary batteries. In this case, the electrolyte solution prepared in advance may be injected into a lithium ion secondary battery assembly (that is, an assembly that does not contain the electrolyte solution) in which an electrode is accommodated in the battery case.
Alternatively, a mixed liquid containing a non-fluorinated organic solvent and a fluorinated solvent is prepared, while at least one of the positive electrode, the negative electrode, and the separator contains an inorganic phosphate compound, the electrode is placed in the battery case. A accommodated lithium ion secondary battery assembly is prepared. Then, this mixed solution is injected into the lithium ion secondary battery assembly to produce a lithium ion secondary battery, and the inorganic phosphate compound is dissolved in the mixed solution in the lithium ion secondary battery. The electrolyte solution for lithium ion secondary batteries which concerns on a form may be produced | generated.
Alternatively, an electrolyte for a lithium ion secondary battery according to this embodiment is prepared in advance, and at least one of the positive electrode, the negative electrode, and the separator contains an inorganic phosphate compound, and the electrode is accommodated in the battery case The lithium ion secondary battery assembly may be injected.

The lithium ion secondary battery using the electrolytic solution for a lithium ion secondary battery according to the present embodiment has two battery performances of high cycle characteristics and low battery resistance.
Hereinafter, the configuration example of the lithium ion secondary battery using the electrolyte for the lithium ion secondary battery according to the present embodiment will be described with reference to the drawings. The electrolyte for the lithium ion secondary battery according to the present embodiment will be described below. The lithium ion secondary battery constituted by using is not limited to the lithium ion secondary battery described below. The electrolyte solution for a lithium ion secondary battery according to this embodiment can be used for various lithium ion secondary batteries according to a known method.

  FIG. 1 is a cross-sectional view schematically showing a lithium ion secondary battery 10 according to a configuration example. FIG. 2 is a schematic diagram showing the configuration of the electrode body 40 used in the lithium ion secondary battery 10. As shown in FIG. 1, the lithium ion secondary battery 10 includes a battery case 20 and an electrode body 40 (in FIG. 1, a wound electrode body).

  The battery case 20 includes a case body 21 and a sealing plate 22. The case main body 21 has a box shape having a rectangular opening at one end. Here, the case main body 21 has a bottomed rectangular parallelepiped shape in which one surface corresponding to the upper surface in the normal use state of the lithium ion secondary battery 10 is opened. The sealing plate 22 is a member that closes the opening of the case body 21. The sealing plate 22 is a rectangular plate. The sealing plate 22 is welded to the peripheral edge of the opening of the case body 21 to form a substantially hexahedral battery case 20. The battery case 20 is mainly composed of, for example, a metal material that is lightweight and has good thermal conductivity. Examples of such a metal material include aluminum, stainless steel, nickel-plated steel, and the like. Although a rectangular case is illustrated here, the battery case 20 is not limited to such a shape. For example, the battery case 20 may be a cylindrical case. Further, the battery case 20 may have a bag shape that wraps the electrode body, or may be a so-called laminate type battery case 20.

  In the example shown in FIG. 1, a positive electrode terminal 23 (external terminal) and a negative electrode terminal 24 (external terminal) for external connection are attached to the sealing plate 22. A safety valve 30 and a liquid injection port 32 are formed on the sealing plate 22. The safety valve 30 is configured to release the internal pressure when the internal pressure of the battery case 20 rises to a predetermined level (for example, a set valve opening pressure of about 0.3 MPa to 1.0 MPa) or more. Further, FIG. 1 shows a state in which the injection port 32 is sealed with a sealing material 33 after the electrolyte solution 80 is injected.

  As shown in FIG. 2, the electrode body 40 includes a strip-shaped positive electrode (positive electrode sheet 50), a strip-shaped negative electrode (negative electrode sheet 60), and strip-shaped separators (separators 72 and 74).

  The positive electrode sheet 50 includes a strip-shaped positive electrode current collector foil 51 and a positive electrode active material layer 53. For the positive electrode current collector foil 51, for example, an aluminum foil or the like can be used. On one side of the positive electrode current collector foil 51 in the width direction, an exposed portion 52 is provided along the edge. In the illustrated example, the positive electrode active material layer 53 is formed on both surfaces of the positive electrode current collector foil 51 except for the exposed portion 52 provided on the positive electrode current collector foil 51. However, the positive electrode active material layer 53 may be formed only on one side of the positive electrode current collector foil 51.

The positive electrode active material layer 53 typically includes a positive electrode active material, a conductive material, a binder (binder), and the like. Examples of the positive electrode active material include lithium composite metal oxides such as a layered structure and a spinel structure (for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNiO ₂ , LiCoO ₂ , LiFeO ₂ , LiMn ₂ O ₄ , LiNi _0.5 Mn _1.5 O ₄ , LiFePO _4, etc.). The higher the operating potential of the positive electrode active material, the higher the effect of using the electrolytic solution for the lithium ion secondary battery according to this embodiment. Therefore, it is preferable to use a positive electrode active material having an operating potential of 4.3 V or higher with respect to metallic lithium. As the conductive material, a carbon material such as carbon black (for example, acetylene black or ketjen black) can be adopted. As the binder, various polymer materials such as polyvinylidene fluoride (PVdF) and polyethylene oxide (PEO) can be employed. The positive electrode active material layer 53 may contain the above inorganic phosphate compound. When the positive electrode sheet 50 (particularly, the positive electrode active material layer 53) contains the above inorganic phosphate compound, it is advantageous in that a protective film derived from the inorganic phosphate compound is formed on the positive electrode sheet 50.

  The negative electrode sheet 60 includes a strip-shaped negative electrode current collector foil 61 and a negative electrode active material layer 63. For the negative electrode current collector foil 61, for example, a copper foil or the like can be used. On one side in the width direction of the negative electrode current collector foil 61, an exposed portion 62 is provided along the edge portion. In the illustrated example, the negative electrode active material layer 63 is formed on both surfaces of the negative electrode current collector foil 61 except for the exposed portion 62 provided on the negative electrode current collector foil 61. However, the negative electrode active material layer 63 may be formed only on one side of the negative electrode current collector foil 61.

  The negative electrode active material layer 63 typically includes a negative electrode active material, a binder, a thickener, and the like. As the negative electrode active material, for example, carbon materials such as graphite (natural graphite, artificial graphite) and low crystalline carbon (hard carbon, soft carbon) can be used, and among them, graphite can be preferably used. As the binder, various polymer materials such as styrene butadiene rubber (SBR) can be adopted. As the thickener, various polymer materials such as carboxymethyl cellulose (CMC) can be employed. The negative electrode active material layer 63 may contain the above inorganic phosphate compound.

  As the separators 72 and 74, for example, a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide can be used. Preferably, a polyolefin resin (eg, PE , PP) is used. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). The separators 72 and 74 may be provided with a heat resistant layer (HRL). Separator 72,74 may contain the above-mentioned inorganic phosphate compound, for example in a heat-resistant layer.

In the electrode body 40, as shown in FIG. 2, the positive electrode sheet 50, the negative electrode sheet 60, and the separators 72 and 74 are wound in order, and the wound electrode body (wound electrode body). 40 has a shape that is flatly pushed and bent along one plane including the winding axis WL. The width b1 of the negative electrode active material layer 63 is slightly wider than the width a1 of the positive electrode active material layer 53. Furthermore, the widths c1 and c2 of the separators 72 and 74 are slightly wider than the width b1 of the negative electrode active material layer 63 (c1, c2>b1> a1). In the example shown in FIG. 2, the exposed portion 52 of the positive electrode current collector foil 51 and the exposed portion 62 of the negative electrode current collector foil 61 are spirally exposed on both sides of the separators 72 and 74, respectively. In the present embodiment, as shown in FIG. 1, the electrode body 40 is formed by gathering the intermediate portions of the positive and negative exposed portions 52 and 62 protruding from the separators 72 and 74, and arranging the positive and negative electrodes disposed inside the battery case 20. The inner terminals 23 and 24 are welded to the tip portions 23a and 24a.
In the present embodiment, the electrode body 40 is configured as a wound electrode body, but may be configured as a stacked electrode body.

  In the form shown in FIG. 1, a flat wound electrode body 40 is accommodated in the battery case 20 along one plane including the winding axis WL. The battery case 20 further contains an electrolytic solution 80. The electrolytic solution 80 enters the electrode body 40 from both axial sides of the winding shaft WL (see FIG. 2). Note that FIG. 1 does not strictly indicate the amount of the electrolytic solution 80 injected into the battery case 20.

As the electrolytic solution 80, the electrolytic solution for the lithium ion secondary battery according to the present embodiment described above is used. As a modification, when at least one of the positive electrode active material layer 53, the negative electrode active material layer 63, and the separators 72 and 74 contains the above inorganic phosphate compound, the electrolyte solution 80 contains an inorganic phosphate compound. When the lithium ion secondary battery 10 is completed or used, the inorganic phosphoric acid compound is dissolved in the electrolytic solution 80, and the electrolyte for the lithium ion secondary battery according to this embodiment is used. It is good also as composition to generate.
In any case, since the inorganic phosphate compound dissolved in the electrolytic solution 80 is consumed by forming a protective cost film on the positive electrode sheet 50, the inorganic electrolytic compound for the lithium ion secondary battery according to the present embodiment is inorganic. It will be understood that the lower limit of the phosphate compound content is not critical.

  The lithium ion secondary battery 10 configured as described above can be used for various applications, and preferred applications include an electric vehicle (EV), a hybrid vehicle (HV), a plug-in hybrid vehicle (PHV), and the like. Driving power source mounted on the vehicle. The lithium ion secondary battery 10 can also be used in the form of a battery pack typically formed by connecting a plurality of lithium ion secondary batteries 10 in series and / or in parallel.

  EXAMPLES Examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.

<No. 1. Preparation of lithium ion secondary battery for evaluation>
A metal sulfate other than Li was dissolved in a predetermined amount of water, and crystallization was performed while neutralizing with NaOH to obtain a precursor hydroxide. This was mixed with a predetermined amount of lithium carbonate, fired at 900 ° C. for 15 hours, and then pulverized to obtain a lithium nickel manganese composite oxide having an average particle diameter of 10 μm as a positive electrode active material.
This positive electrode active material, acetylene black (AB) as a conductive material, polyvinylidene fluoride (PVdF) as a binder, and Li ₃ PO ₄ as an inorganic phosphate compound are mixed into the positive electrode active material: AB: PVdF: Li ₃ PO ₄ = 87: 8: 2: 3 was mixed with NMP at a mass ratio to prepare a positive electrode active material layer forming slurry. This slurry was applied on an aluminum foil and dried. The obtained sheet was cut so that the design capacity of the battery was about 14 mAh, and a positive electrode sheet was produced.
Further, natural graphite (C) having an average particle diameter of 20 μm as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener, C: SBR: CMC = The slurry for negative electrode active material layer formation was prepared by mixing with ion exchange water at a mass ratio of 98: 1: 1. This slurry was applied onto a copper foil and dried. The obtained sheet was cut so that the design capacity of the battery would be about 14 mAh to produce a negative electrode sheet.
A porous polyolefin sheet was prepared as a separator.
The produced positive electrode sheet and the negative electrode sheet were opposed to each other through a separator to produce an electrode body.
A current collector was attached to the produced electrode body, and the lithium ion secondary battery for evaluation was obtained by housing in a laminate case together with the electrolyte and sealing. In addition, what mixed ethylene carbonate (EC) and ethyl methyl carbonate (EMC) by the volume ratio 30/70 was used for the solvent of electrolyte solution. Further, LiPF ₆ was added to the electrolyte so as to have a concentration of 1.0M.

<No. Production of Lithium Ion Secondary Battery for Evaluation 2>
When preparing the slurry for forming the positive electrode active material layer, Li ₃ PO ₄ which is an inorganic phosphate compound was not added, and ethylene carbonate (EC) and bis (2,2,2) were used as the solvent of the electrolytic solution. -No. 1 except that a mixture of (trifluoroethyl) carbonate (BTFEC) at a volume ratio of 30/70 was used. No. 1 in the same manner as the evaluation lithium ion secondary battery. 2 lithium ion secondary batteries for evaluation were obtained.

<No. 3. Preparation of lithium ion secondary battery for evaluation>
Except for using a mixture of ethylene carbonate (EC) and methyl (2,2,2-trifluoroethyl) carbonate (MTFEC) at a volume ratio of 30/70 as the solvent of the electrolytic solution, the above No. 1 was used. No. 1 in the same manner as the evaluation lithium ion secondary battery. 3 lithium ion secondary batteries for evaluation were obtained.

<No. Of 4 lithium ion secondary battery for evaluation>
Except for using a mixture of ethylene carbonate (EC) and methyl (2,2,2-trifluoroethyl) carbonate (MTFEC) at a volume ratio of 50/50 as the solvent of the electrolytic solution, the above No. 1 was used. No. 1 in the same manner as the evaluation lithium ion secondary battery. 4 lithium ion secondary batteries for evaluation were obtained.

<No. Of lithium ion secondary battery for evaluation 5>
Except for using a mixture of ethylene carbonate (EC) and bis (2,2,2-trifluoroethyl) carbonate (BTFEC) at a volume ratio of 30/70 as the solvent of the electrolytic solution, the above No. 1 was used. No. 1 in the same manner as the evaluation lithium ion secondary battery. 5 lithium ion secondary batteries for evaluation were obtained.

<No. Preparation of Lithium Ion Secondary Battery for Evaluation 6>
Except for using a mixture of fluoroethylene carbonate (FEC) and bis (2,2,2-trifluoroethyl) carbonate (BTFEC) at a volume ratio of 30/70 as the solvent of the electrolytic solution, the above No. 1 was used. No. 1 in the same manner as the evaluation lithium ion secondary battery. 6 lithium ion secondary batteries for evaluation were obtained.

<No. Of lithium ion secondary battery for evaluation of No. 7>
In the electrolyte solution, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) mixed at a volume ratio of 30/70, so that fluoroethylene carbonate (FEC) is 10% by mass in the electrolyte solution. Except for using the added one, the above No. 1 was used. No. 1 in the same manner as the evaluation lithium ion secondary battery. 7 lithium ion secondary batteries for evaluation were obtained.

<No. 8 Production of Lithium Ion Secondary Battery for Evaluation>
Bis (2,2,2-trifluoroethyl) carbonate (BTFEC) is electrolyzed in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 30/70 in the solvent of the electrolytic solution. Except for using what was added so that it might become 15 mass% in a liquid, said No .. No. 1 in the same manner as the evaluation lithium ion secondary battery. Eight lithium ion secondary batteries for evaluation were obtained.

<No. 9 Production of Lithium Ion Secondary Battery for Evaluation>
Methyl (2,2,2-trifluoroethyl) carbonate (MTFEC) is electrolyzed in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 30/70 in the solvent of the electrolytic solution. Except for using what was added so that it might become 1 mass% in a liquid, said No .. No. 1 in the same manner as the evaluation lithium ion secondary battery. Nine lithium ion secondary batteries for evaluation were obtained.

<No. Production of 10 lithium ion secondary batteries for evaluation>
Methyl (2,2,2-trifluoroethyl) carbonate (MTFEC) is electrolyzed in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 30/70 in the solvent of the electrolytic solution. Except having used what was added so that it might become 10 mass% in a liquid, said No .. No. 1 in the same manner as the evaluation lithium ion secondary battery. Ten lithium ion secondary batteries for evaluation were obtained.

<No. 11 Evaluation Lithium Ion Secondary Battery Production>
Bis (2,2,2-trifluoroethyl) carbonate (BTFEC) is electrolyzed in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 30/70 in the solvent of the electrolytic solution. Except for using what was added so that it might become 1 mass% in a liquid, said No .. No. 1 in the same manner as the evaluation lithium ion secondary battery. 11 lithium ion secondary batteries for evaluation were obtained.

<No. Production of 12 lithium ion secondary batteries for evaluation>
Bis (2,2,2-trifluoroethyl) carbonate (BTFEC) is electrolyzed in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 30/70 in the solvent of the electrolytic solution. Except having used what was added so that it might become 10 mass% in a liquid, said No .. No. 1 in the same manner as the evaluation lithium ion secondary battery. Twelve lithium ion secondary batteries for evaluation were obtained.

<Activation>
Each of the obtained lithium ion secondary batteries for evaluation was subjected to activation by performing constant current charging at a current value of 1 C up to 4.9 V and maintaining the constant voltage charging as it is.

<IV resistance measurement>
Each activated lithium ion secondary battery for evaluation was adjusted to a SOC (State of Charge) 60% charge state, and then placed in an environmental atmosphere of 25 ° C. The discharge was performed for 10 seconds at a current value of 20 C, the voltage value 10 seconds after the start of discharge was measured, and the IV resistance (battery resistance) was calculated. The results are shown in Table 1.

<Initial capacity measurement>
Each activated lithium ion secondary battery for evaluation was placed in an environment of 25 ° C. After carrying out constant current charge to 4.9V with the electric current value of 1 / 5C, constant voltage charge was performed until the electric current value became 1 / 50C, and it was set as the full charge state. Thereafter, a constant current was discharged to 3.5 V at a current value of 1/5 C, and the discharge capacity at this time was measured to obtain an initial capacity.

<Charge / discharge cycle test>
Each evaluation lithium ion secondary battery was placed in an environment of 60 ° C., and charged and discharged 300 cycles at a constant current of 2 C to 4.9 V and then a constant current of 2 C to 3.5 V. Then, the capacity | capacitance measurement was performed by the same method as initial capacity | capacitance measurement, and this was made into the capacity | capacitance after 300 cycles of charging / discharging. The capacity retention rate was calculated according to the following formula.
Capacity maintenance rate (%) = (capacity after 300 cycles of charge / discharge / initial capacity) × 100
The results are shown in Table 1.

From Table 1, no. No. 1 lithium ion secondary battery for evaluation; Compared with the evaluation lithium ion secondary batteries 3 to 6, when the solvent as the basic composition is replaced with a fluorinated solvent (that is, when the amount of the fluorinated solvent is large), the capacity retention rate is improved, but the resistance It can be seen that increases significantly.
No. No. 2 lithium ion secondary battery for evaluation and No. 2 Compared with the lithium ion secondary battery for evaluation of No. 6, even when the solvent of the electrolytic solution contains a fluorinated solvent, the capacity retention rate is low when Li ₃ PO ₄ which is an inorganic phosphate compound is not used. I understand that.
On the other hand, no. From the evaluation results of the evaluation lithium ion secondary batteries 9 to 12, it can be seen that the addition of a small amount of the fluorinated solvent greatly increases the capacity retention rate while suppressing an increase in resistance.
However, no. From the evaluation result of the lithium ion secondary battery for evaluation No. 7, it is found that the capacity retention rate cannot be sufficiently improved when the fluorination rate (F / H ratio) of the fluorinated solvent is small.
No. From the evaluation results of the lithium ion secondary battery for evaluation of No. 8, it can be seen that when the addition amount of the fluorinated solvent exceeds 10% by mass, the battery resistance is increased.

  From the above, the fluorinated solvent having an F / H ratio of 0.6 to 1.5 is added to an electrolyte for a lithium ion secondary battery containing an inorganic phosphate compound, the content of which is 1% by mass or more. It can be seen that the addition of 10% by mass or less makes it possible to achieve both battery performance, ie, high cycle characteristics and low battery resistance of the lithium ion secondary battery.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

DESCRIPTION OF SYMBOLS 10 Lithium ion secondary battery 20 Battery case 21 Case main body 22 Sealing plate 23 Positive electrode terminal 24 Negative electrode terminal 30 Safety valve 32 Injection port 33 Sealing material 40 Winding electrode body 50 Positive electrode sheet 51 Positive electrode current collecting foil 52 Exposed part 53 Positive electrode activity Material layer 60 Negative electrode sheet 61 Negative electrode current collector foil 62 Exposed portion 63 Negative electrode active material layer 72, 74 Separator 80 Electrolytic solution WL Winding shaft

Claims (1)

A non-fluorinated organic solvent, a fluorinated solvent, an inorganic phosphate compound,
An electrolyte for a lithium ion secondary battery containing
The fluorinated solvent contains hydrogen atoms and fluorine atoms in the molecule,
The ratio of the number of fluorine atoms to the number of hydrogen atoms in one molecule of the fluorinated solvent is 0.6 to 1.5,
The content of the fluorinated solvent in the electrolyte solution is 1% by mass or more and 10% by mass or less,
Electrolytic solution for lithium ion secondary battery.

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