JP2019046759A - Nonaqueous electrolyte and nonaqueous electrolyte power storage element - Google Patents

Abstract

To provide: a nonaqueous electrolyte which can raise a sulfur utilization rate and thus, increase a capacity density of a nonaqueous electrolyte power storage element when used in combination with a positive electrode including a predetermined amount of sulfur; and a nonaqueous electrolyte power storage element having a positive electrode including sulfur and a high capacity density.SOLUTION: A nonaqueous electrolyte according to an embodiment of the present invention is a nonaqueous electrolyte for a power storage element, which comprises a lithium salt, a polyether compound having four or more ether bonds, and fluorinated ether. The mole ratio of the lithium salt to the polyether compound is one or more. The mole ratio of the fluorinated ether to the lithium salt is five or more.SELECTED DRAWING: None

Description

  The present invention relates to a non-aqueous electrolyte and a non-aqueous electrolyte storage element.

  BACKGROUND OF THE INVENTION Non-aqueous electrolyte secondary batteries represented by lithium ion secondary batteries are widely used in personal computers, electronic devices such as communication terminals, automobiles, etc., due to their high energy density. The non-aqueous electrolyte secondary battery generally has a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte interposed between the electrodes, and delivers ions between the two electrodes. Configured to charge and discharge. Moreover, capacitors, such as a lithium ion capacitor and an electric double layer capacitor, are also prevailing widely as nonaqueous electrolyte electrical storage elements other than a nonaqueous electrolyte secondary battery.

  As a non-aqueous electrolyte storage element, a non-aqueous electrolyte storage element using sulfur as a positive electrode active material such as a Li-S battery is known. Sulfur has the advantage of having a large theoretical capacity. However, in a non-aqueous electrolyte storage device using sulfur, it is known that a phenomenon called a so-called polysulfide shuttle occurs. The polysulfide shuttle is a phenomenon in which polysulfide ions are eluted with charge and discharge, and the polysulfide ions cause a redox reaction between positive and negative electrodes. This polysulfide shuttle is one of the factors that degrade the charge and discharge cycle performance.

  In Non-Patent Document 1, lithium bis (trifluoromethanesulfonyl) imide (LiTFSI), tetraethylene glycol dimethyl ether (G4), and 1,1, as a technique for suppressing such polysulfide shuttle and improving charge-discharge cycle performance Described a Li-S battery using a non-aqueous electrolyte mixed with 1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE) in a molar ratio of 1: 1: 4 It is done. In Patent Document 1, LiTFSI, G4, and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoropropyl ether (TFEE) were mixed at a molar ratio of 1: 1: 5. A Li-S battery using a non-aqueous electrolyte is described.

JP 2014-41811 A

Kaoru Dokko et al., "Solvate Ionic Liquid Electrolyte for Li-S Batteries", Journal of The Electrochemical Society, 2013, 160, No. 8, A1304-A1310

  However, in the Li-S batteries using the non-aqueous electrolytes of Non-Patent Document 1 and Patent Document 1, it can not be said that the capacity density is sufficiently high. Development of a high capacity density non-aqueous electrolyte storage element using sulfur with a high theoretical capacity is expected.

  The present invention has been made based on the circumstances as described above, and its object is to increase the utilization of sulfur by using it in combination with a positive electrode containing a predetermined amount of sulfur, and as a result, a non-aqueous electrolyte It is an object of the present invention to provide a non-aqueous electrolyte storage element having a high capacity density and having a non-aqueous electrolyte capable of increasing the capacity density of the storage element and a positive electrode containing sulfur.

  A non-aqueous electrolyte according to one aspect of the present invention made to solve the above problems contains a lithium salt, a polyether compound having 4 or more ether bonds, and a fluorinated ether, and the above lithium with respect to the above polyether compound. It is a nonaqueous electrolyte for a storage element, in which the molar ratio of the salt is 1 or more and the molar ratio of the fluorinated ether to the lithium salt is larger than 5.

A non-aqueous electrolyte storage device according to another aspect of the present invention includes a positive electrode containing sulfur and the non-aqueous electrolyte, and the content of sulfur per unit area in the positive electrode is 2 mg / cm ² or more and 5 mg / cm. ^{It is 2} or less, and the viscosity at 25 degrees C of the said fluorinated ether is less than 1 mPa * s, It is a nonaqueous electrolyte electrical storage element.

  According to the present invention, by using in combination with a positive electrode containing a predetermined amount of sulfur, the utilization rate of the sulfur can be enhanced, and as a result, the nonaqueous electrolyte capable of enhancing the capacity density of the nonaqueous electrolyte storage element, and sulfur Thus, it is possible to provide a non-aqueous electrolyte storage element having a high positive electrode density and a high capacity density.

FIG. 1 is an external perspective view showing a non-aqueous electrolyte secondary battery according to an embodiment of the non-aqueous electrolyte storage device of the present invention. FIG. 2 is a schematic view showing a power storage device configured by assembling a plurality of non-aqueous electrolyte secondary batteries according to an embodiment of the non-aqueous electrolyte power storage element of the present invention.

  The non-aqueous electrolyte according to one embodiment of the present invention contains a lithium salt, a polyether compound having 4 or more ether bonds, and a fluorinated ether, and the molar ratio of the lithium salt to the polyether compound is 1 or more. It is a nonaqueous electrolyte for a storage element, in which the molar ratio of the fluorinated ether to the lithium salt is greater than 5.

  When the non-aqueous electrolyte is used in combination with a positive electrode containing a predetermined amount of sulfur, the utilization rate of the sulfur can be increased, and as a result, the capacity density of the non-aqueous electrolyte storage element can be increased. Although it is not certain as a reason which such an effect arises, the following is guessed. By using the fluorinated ether at a molar ratio of more than 5 with respect to the lithium salt, the viscosity of the non-aqueous electrolyte is sufficiently lowered. Therefore, it is presumed that the conductivity of lithium ions in the thickness direction in the positive electrode containing sulfur is enhanced, and the utilization of sulfur is enhanced. That is, according to the said non-aqueous electrolyte, it is estimated that lithium ion is easy to reach even the sulfur which exists in the inside of a positive electrode by viscosity reduction.

  In addition, when it dilutes more than 5 times with fluorinated ether in this way, the viscosity is lowered and the lithium ion concentration is also greatly reduced. Therefore, when diluted in this manner, the ion conductivity itself by the general measurement method tends not to improve so much or to decrease on the contrary. However, in the non-aqueous electrolyte, by focusing more on lowering the viscosity than the measured ion conductivity, the conductivity or diffusion of ions in the thickness direction in the positive electrode containing sulfur is enhanced, and the utilization rate of sulfur is improved. It is raising. Thus, the reason why the effect can be obtained even if the ion conductivity itself is not improved is that the conductivity or diffusivity of lithium ion in the positive electrode containing sulfur is different from the environment under which the measurement of ion conductivity is performed. It is presumed that the lower viscosity than the concentration largely affects the conductivity or diffusivity.

  Moreover, in the said non-aqueous electrolyte, since it dilutes using fluorinated ether, the shuttle phenomenon by elution of lithium polysulfide is suppressed, and favorable charge / discharge performance is exhibited. Furthermore, in the non-aqueous electrolyte, the molar ratio of the lithium salt to the polyether compound is 1 or more, and the solvated ionic liquid is formed by the coordination of the polyether compound to the lithium ion. This also suppresses the shuttle phenomenon due to the elution of lithium polysulfide.

  The viscosity at 25 ° C. of the fluorinated ether is preferably less than 1 mPa · s. Generally, when the sulfur content in the positive electrode is increased to achieve high capacity density, the layer containing sulfur becomes thicker. In this case, in the case of the conventional non-aqueous electrolyte, since the conductivity and diffusion of lithium ions are low, it becomes difficult to utilize sulfur present at a deep position, and the utilization of sulfur decreases. On the other hand, by using a fluorinated ether having a viscosity as low as less than 1 mPa · s at 25 ° C., the non-aqueous electrolyte in particular reduces its viscosity sufficiently sufficiently. Therefore, by using such a low viscosity fluorinated ether, the utilization rate of sulfur can be increased when the content of sulfur in the positive electrode is increased, and as a result, the capacity density can be remarkably increased.

A non-aqueous electrolyte storage device according to an embodiment of the present invention includes a positive electrode containing sulfur and the non-aqueous electrolyte, and the content of sulfur per unit area in the positive electrode is 2 mg / cm ² or more and 5 mg / cm ² It is the non-aqueous electrolyte electrical storage element which is the following and whose viscosity in 25 degreeC of the said fluorinated ether is less than 1 mPa * s.

  The non-aqueous electrolyte storage element has a high content of sulfur in the positive electrode. Furthermore, in the non-aqueous electrolyte storage element, since the non-aqueous electrolyte having high lithium ion diffusibility, which has been sufficiently reduced in viscosity as described above, is used, the utilization rate of sulfur is high. Therefore, the non-aqueous electrolyte storage element has a high capacity density due to the synergetic effect of the high sulfur content and the high utilization of sulfur.

The “content of sulfur per unit area in the positive electrode” is a value obtained by dividing the content (mg) of sulfur in the positive electrode by the area (cm ² ) of the region where sulfur is present on the surface of the positive electrode. For example, when sulfur is coated on the entire surface of one side of a sheet-like positive electrode substrate, the value obtained by dividing the total coating amount of sulfur by the area of one surface of the positive electrode substrate is “content of sulfur per unit area in positive electrode ". On the other hand, when sulfur is coated on both sides of the sheet-like positive electrode substrate, the value obtained by dividing the total coating amount of sulfur by the area of both surfaces of the positive electrode substrate is “the sulfur content per unit area in positive electrode ". The “content of sulfur per unit area in the positive electrode” can be translated into the coated amount of sulfur per unit area, the amount of lamination, etc., and the larger the value, the thicker the sulfur is laminated. Represents

<Non-aqueous electrolyte>
The non-aqueous electrolyte according to one embodiment of the present invention contains a lithium salt, a polyether compound having 4 or more ether bonds, and a fluorinated ether.

(Lithium salt)
Examples of the lithium salt include inorganic lithium salts such as LiPF ₆ , LiPO ₂ F ₂ , LiBF ₄ and LiClO ₄ , and organic lithium salts such as lithium imide salt. The lithium salt can be used alone or in combination of two or more.

  As a lithium salt, an organic lithium salt is preferable, and a lithium imide salt is more preferable. By using a lithium imide salt, a better solvated ionic liquid can be formed. The lithium imide salt includes not only a lithium imide salt having a structure in which two carbonyl groups are bonded to a nitrogen atom, but also a lithium imide salt having a structure in which two sulfonyl groups are bonded to a nitrogen atom, It is a meaning also including what has a structure which two phosphonyl groups were couple | bonded.

As said lithium imide salt,
LiN _(SO 2 _{F) 2} (lithium bis (fluorosulfonyl) _{imide: LiFSI), LiN (SO 2} CF 3) 2 ( lithium bis (trifluoromethanesulfonyl) _{imide: LiTFSI), LiN (SO 2} C 2 F 5) 2 (lithium bis (pentafluoroethane sulfonyl) _{imide: LiBETI), LiN (SO 2} C 4 F 9) 2 ( lithium bis (nonafluorobutanesulfonyl) _{imide), CF 3 -SO 2 -N-} SO 2 -N-SO ₂ CF ₃ Li ₂ , FSO ₂ -N-SO ₂ -C ₄ F ₉ Li, CF ₃ -SO ₂ -N-SO ₂ -CF ₂ -SO ₂ -N ₂ -SO ₂ -CF ₃ Li ₂ , CF _{3- SO 2 -N-SO 2 -CF 2} -SO 3 Li 2, CF 3 -SO 2 -N-SO 2 -CF 2 -SO 2 -C (- Lithium imide salts such as _{O 2 CF 3) 2 Li 2} ;
LiN _{(POF 2) 2} (lithium bis (difluoromethyl phosphonate) imide: LiDFPI) and lithium phosphonium imide salts such as.

  The lithium imide salt preferably has a fluorine atom, and specifically preferably has, for example, a fluorosulfonyl group, a difluorophosphonyl group, a fluoroalkyl group and the like. Among the lithium imide salts, lithium sulfonylimide salts are preferable, LiTFSI and LiFSI are more preferable, and LiTFSI is more preferable.

(Polyether compound)
The said polyether compound is a compound which has 4 or more ether bond (-O-). The lower limit of the number of ether bonds is preferably 5. On the other hand, the upper limit of the number of ether bonds is, for example, 20, preferably 12 and more preferably 6. The polyether compound is a non-aqueous solvent that dissolves a lithium salt, and coordinates to a lithium ion. The lithium ion and the polyether compound form a solvated ionic liquid. The polyether compound can be used alone or in combination of two or more.

The polyether compound is preferably a compound represented by the following formula (1).
R ^1- (OCH ₂ CH ₂ ) _x -OR ² (1)
In Formula (1), R ¹ and R ² are each independently an alkyl group having 1 to 3 carbon atoms. x is an integer of 3 to 5;

Examples of the alkyl group having 1 to 3 carbon atoms in ^{R 1} and ^{R 2} in the formula (1) include a methyl group, an ethyl group, n- propyl, and i- propyl. As R ¹ and R ² , a methyl group is preferable respectively. As x in Formula (1), 4 is preferable.

The polyether compound is also preferably a crown ether. The crown ether, e.g. _{(-CH 2 -CH 2 -O-) n} (n is an integer of 4-6.) Can be mentioned compounds represented by.

  As the polyether compound, tetraethylene glycol dimethyl ether, triethylene glycol dimethyl ether, crown ether or a combination thereof is preferable, and tetraethylene glycol dimethyl ether is more preferable. By using such a polyether compound, a complex with the formed lithium ion (solvated ionic liquid) is in a particularly stable state.

  The molar ratio of the lithium salt to the polyether compound is 1 or more. Note that in this molar ratio "1", significant figures are 1 digit, meaning that 0.5 or 0.9 which rounds off the 1st decimal place to 1 is included (the same applies to the other The numerical value represented by the integer of, rounded to the first decimal place. The lower limit of the molar ratio is preferably 0.8, more preferably 0.9, and still more preferably 1.0. On the other hand, the upper limit of the molar ratio is, for example, 3, preferably 2, 1.2, more preferably 1.1. By the molar ratio being at least the lower limit or the upper limit, a more favorable solvated ionic liquid is formed.

(Fluorinated ether)
The fluorinated ether is a compound in which part or all of the hydrogen atoms of the ether are substituted by fluorine atoms. The fluorinated ether does not include compounds having 4 or more ether bonds. The fluorinated ether preferably has one or two ether bonds, and only one ether bond. The said fluorinated ether is also contained as a non-aqueous solvent. The fluorinated ether can be used alone or in combination of two or more.

As said fluorinated ether, the compound represented, for example by following formula (2) can be mentioned.
R ³ -O-R ⁴ (2)
In formula (2), ^{R 3} is a fluorinated hydrocarbon group having 1 to 8 carbon atoms. R ⁴ is a hydrocarbon group having 1 to 8 carbon atoms or a fluorinated hydrocarbon group having 1 to 8 carbon atoms.

As the R ^3, a fluorinated alkyl group is preferable. The upper limit of the carbon number of R ³ is preferably 5, more preferably 3, and still more preferably 2. The lower limit of the carbon number of R ³ is preferably 2.

As the R ^4, preferably a fluorinated hydrocarbon group, and more preferably a fluorinated alkyl group. The upper limit of the carbon number of R ⁴ is preferably 5, more preferably 3, and still more preferably 2. The lower limit of the carbon number of R ⁴ is preferably 2.

Moreover, as an upper limit of the total carbon number of said R < ³ > and R < ⁴ >, 6 is preferable, 5 is more preferable, and 4 is more preferable. On the other hand, as a lower limit of this total carbon number, 3 is preferable and 4 is more preferable. By using a fluorinated ether having such a carbon number, the viscosity and the ion conductivity become suitable, and the effect of the non-aqueous electrolyte can be further enhanced.

As specific fluorinated ether, for example, CF ₃ OCH ₃ , CF ₃ OC ₂ H ₅ , F (CF ₂ ) ₂ OCH ₃ , F (CF ₂ ) ₂ OC ₂ H ₅ , CF ₃ (CF ₂ ) CH ₂ O (CF ₂ ) CF ₃ , F (CF ₂ ) ₃ OCH ₃ , F (CF ₂ ) ₃ OC ₂ H ₅ , F (CF ₂ ) ₄ OCH ₃ , F (CF ₂ ) ₄ OC ₂ H ₅ , F ( CF ₂ ) ₅ OCH ₃ , F (CF ₂ ) ₅ OC ₂ H ₅ , F (CF ₂ ) ₈ OCH ₃ , F (CF ₂ ) ₈ OC ₂ H ₅ , F (CF ₂ ) ₉ OCH ₃ , CF ₃ CH ₂ OCH ₃ , CF ₃ CH ₂ OCHF ₂ , CF ₃ CF ₂ CH ₂ OCH ₃ , CF ₃ CF ₂ CH ₂ OCHF ₂ , CF ₃ CF ₂ CH ₂ O (CF ₂ ) ₂ H, CF ₃ CF ₂ CH ₂ O _{(CF 2)} 2 F, HC _{2 CH 2 OCH 3, (CF} 3) (CF 2) CH 2 O (CF 2) 2 H, H (CF 2) 2 OCH 2 CH 3, H (CF 2) 2 OCH 2 CF 3 ( abbreviation: TFEE) H (CF ₂ ) ₂ CH ₂ OCHF ₂ , H (CF ₂ ) ₂ CH ₂ O (CF ₂ ) ₂ H (abbreviation: HFE), H (CF ₂ ) ₂ CH ₂ O (CF ₂ ) ₃ H, H (CF ₂ ) ₃ CH ₂ O (CF ₂ ) ₂ H, H (CHF) ₂ CH ₂ O (CF ₂ ) ₂ H, (CF ₃ ) ₂ CHOCH ₃ , (CF ₃ ) ₂ CHCF ₂ OCH ₃ , CF ₃ CHFCF ₂ OCH ₃ , CF ₃ CHFCF ₂ OCH ₂ CH ₃ , CF ₃ CHFCF ₂ CH ₂ OCHF ₂ , CF ₃ CHFCF ₂ OCH ₂ (CF ₂ ) ₂ F, CF ₃ CHFCF ₂ OCH ₂ CF ₂ CF ₂ H, H (CF ₂ ) ₄ CH ₂ O (CF ₂ ) ₂ H, CH ₃ CH ₂ O (CF ₂ ) ₄ F, F (CF ₂ ) ₄ CH ₂ O (CF ₂ ) ₂ H, H (CF ₂₎ ) ₂ CH ₂ OCF ₂ CHFCF ₃ , F (CF ₂ ) ₂ CH ₂ OCF ₂ CHFCF ₃ , H (CF ₂ ) ₄ CH ₂ O (CF ₂ ) H, CF ₃ OCH ₂ (CF ₂ ) ₂ F, CF ₃ CHFCF ₂ OCH ₂ (CF ₂ ) ₃ F, CH ₃ CF ₂ OCH ₂ (CF ₂ ) ₂ F, CH ₃ CF ₂ OCH ₂ (CF ₂ ) ₃ F, CH ₃ O (CF ₂ ) ₅ F, F (CF ₂ ) ₃ CH ₂ OCH ₂ (CF ₂ ) ₃ F, F (CF ₂ ) ₂ CH ₂ OCH ₂ (CF ₂ ) ₂ F, H (CF ₂ ) ₂ CH ₂ OCH ₂ (CF ₂ ) ₂ H, CH ₃ CF ₂ OCH ₂ and _{(CF 2)} 2 H etc. It can gel.

The viscosity at 25 ° C. of the fluorinated ether may be, for example, less than 2 mPa · s, but is preferably less than 1 mPa · s. The upper limit of the viscosity is more preferably 0.8 mPa · s, further preferably 0.7 mPa · s. When the content of sulfur in the positive electrode is increased by using a fluorinated ether having a viscosity of not more than the above upper limit (for example, the content of sulfur per unit area in the positive electrode is 2 mg / cm ² or more and 5 mg / cm ² or less) When the sulfur utilization rate is increased, the capacity density can be more significantly increased. The lower limit of the viscosity is, for example, 0.2 mPa · s, and may be 0.4 mPa · s or 0.6 mPa · s.

As such low viscosity fluorinated ether,
1,1,2,2-tetrafluoroethyl-2,2,2-trifluoropropyl ether (TFEE: viscosity 0.65 mPa · s at 25 ° C.),
Ethyl-1,1,2,2-tetrafluoroethyl ether (viscosity 0.486 mPa · s at 23 ° C.),
Hexafluoroisopropyl methyl ether (viscosity 0.498 mPa · s at 23 ° C.) can be mentioned. Among these, TFEE is preferred. TFEE has low viscosity and good ionic conductivity.

In addition, as said fluorinated ether, that whose viscosity in 25 degreeC is 1 mPa * s or more can also be used. In this case, the utilization rate of sulfur can be increased when the content of sulfur in the positive electrode is small (for example, when the content of sulfur per unit area in the positive electrode is less than 2 mg / cm ² ).

  The content of the fluorinated ether is more than 5 in molar ratio to the lithium salt. The lower limit of the molar ratio of fluorinated ether to lithium salt is preferably 5.5, more preferably 8, further preferably 12, and even more preferably 15. By setting the molar ratio to the above lower limit or more, more sufficient dilution occurs, and the viscosity of the non-aqueous electrolyte decreases. Therefore, the utilization rate of sulfur can be increased, and the capacity density of the non-aqueous electrolyte storage element can be increased. On the other hand, the upper limit of the molar ratio of the fluorinated ether to the lithium salt is, for example, 40, preferably 30 and more preferably 25. By making this molar ratio below the said upper limit, lithium salt of sufficient density | concentration can be made to exist, and charge / discharge performance can be made more favorable.

(Other ingredients)
The non-aqueous electrolyte may contain components other than the lithium salt, the polyether compound and the fluorinated ether as long as the effects of the present invention are not impaired. Examples of the other components include various electrolyte salts other than lithium salts, polyether compounds, nonaqueous solvents other than fluorinated ethers, and various additives contained in other nonaqueous electrolytes for general storage devices. It can be mentioned.

  As other electrolyte salt, sodium salt, potassium salt, magnesium salt, onium salt etc. can be mentioned. However, the upper limit of the content of the electrolyte salt other than the lithium salt in the total electrolyte salt is preferably 10 mol%, more preferably 1 mol%, and still more preferably 0.1 mol%.

  Other non-aqueous solvents include cyclic carbonates, linear carbonates, esters, amides, sulfones, lactones, nitriles and the like. However, the upper limit of the content of the non-aqueous solvent other than the polyether compound and the fluorinated ether in the total non-aqueous solvent is preferably 10 mol%, more preferably 1 mol%, and 0.1 mol%. More preferable. The fact that the non-aqueous solvent consists essentially of only the polyether compound and the fluorinated ether suppresses the elution of polysulfide ions, and the viscosity becomes more suitable, etc. It can be enhanced more.

  Moreover, as an upper limit of content of said lithium salt in the said non-aqueous electrolyte, a polyether compound, and components other than fluorinated ether, 5 mass% is preferable, 1 mass% is more preferable, and 0.1 mass% is It may be even more preferable. By suppressing the content of other components, the capacity density can be further increased.

  The upper limit of the viscosity of the non-aqueous electrolyte at 25 ° C. is preferably 4 mPa · s, more preferably 3 mPa · s, and still more preferably 2 mPa · s. By setting the viscosity of the non-aqueous electrolyte to the upper limit or less, ion diffusion in the thickness direction of the positive electrode containing sulfur can be enhanced, and the utilization rate of sulfur can be further enhanced. On the other hand, the lower limit of the viscosity may be, for example, 0.5 mPa · s, or 0.8 mPa · s.

The lower limit of the ionic conductivity at 25 ° C. of the non-aqueous electrolyte is preferably 0.1 mS / cm ^2, more preferably 0.5 mS / cm ^2, more preferably 0.8 mS / cm ^2. By setting the ion conductivity of the non-aqueous electrolyte to the above lower limit or more, sufficient ion conductivity can be secured, and charge / discharge performance can be further enhanced. On the other hand, the upper limit of the ion conductivity, for example, preferably 10 mS / cm ^2, more preferably 5 mS / cm ^2, more preferably 2 mS / cm ^2, even more preferably from 1.5 mS / cm ^2. In the non-aqueous electrolyte, even with such ion conductivity, sufficient viscosity reduction can be achieved, and therefore, the capacity density can be increased.

The ionic conductivity is a value measured by the following method. A bipolar symmetric cell is produced using a polyethylene separator (thickness 25 μm, porosity 45%, air permeability 310 seconds / 100 ml) containing a non-aqueous electrolyte and stainless steel plates as both electrodes. On the other hand, the AC impedance is measured under the conditions of an applied voltage amplitude of 5 mV, a frequency range of 1 MHz to 100 mHz, and a temperature of 25 ° C. This measured value is referred to as the ion conductivity. The distance is the thickness of the separator, and the area is the electrode area (cm ² ) divided by the porosity (%) of the separator.

  The non-aqueous electrolyte can be prepared by mixing a lithium salt, a polyether compound, a fluorinated ether and, if necessary, other components in a predetermined ratio.

<Non-aqueous electrolyte storage element>
A non-aqueous electrolyte storage device according to an embodiment of the present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. Hereinafter, a non-aqueous electrolyte secondary battery (hereinafter, also simply referred to as "secondary battery") will be described as an example of the non-aqueous electrolyte storage element. The positive electrode and the negative electrode usually form an electrode body alternately stacked or wound via a separator. The electrode body is housed in a battery case, and the non-aqueous electrolyte is filled in the battery case. The non-aqueous electrolyte is interposed between the positive electrode and the negative electrode. As said battery container, the well-known metal battery container normally used as a battery container of a non-aqueous electrolyte secondary battery, a resin battery container etc. can be used.

(Positive electrode)
The positive electrode has a positive electrode substrate, and a positive electrode active material layer disposed directly or via an intermediate layer on the positive electrode substrate. A part or all of the positive electrode active material layer may be impregnated in the positive electrode base material or the intermediate layer.

  The said positive electrode base material has electroconductivity. A sheet-like thing can be suitably adopted as the above-mentioned positive electrode substrate. As a material of a positive electrode base material, metals, such as aluminum, titanium, a tantalum, stainless steel, or alloys thereof, carbon, etc. are used. Among these, aluminum, an aluminum alloy, and carbon are preferable in terms of the balance of potential resistance, conductivity height and cost. Moreover, a foil, a vapor deposition film, etc. are mentioned as a formation form of a positive electrode base material, From the surface of cost, a foil is preferable. That is, an aluminum foil is preferable as the positive electrode substrate. In addition, as aluminum or aluminum alloy, A1085P, A3003P etc. which are prescribed | regulated to JIS-H-4000 (2014) can be illustrated. Moreover, porous materials, such as carbon paper, are suitably used for the said positive electrode base material.

The intermediate layer is a coating layer on the surface of the positive electrode base material, and includes conductive particles such as carbon particles to reduce the contact resistance between the positive electrode base material and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and can be formed of, for example, a composition containing a resin binder and conductive particles. In addition, having "conductivity" means that the volume resistivity measured based on JIS-H-0505 (1975) is 10 < ⁷ > ohm * cm or less, and "non-conductivity" Means that the above-mentioned volume resistivity is more than 10 ⁷ Ω · cm.

  The positive electrode active material layer is formed of a so-called positive electrode mixture containing a positive electrode active material. Moreover, the positive electrode mixture which forms a positive electrode active material layer contains arbitrary components, such as a conductive agent, a binder (binder), a thickener, and a filler, as needed.

  The positive electrode contains sulfur as a positive electrode active material. That is, the positive electrode active material layer contains sulfur. The sulfur as the positive electrode active material may be a simple substance of sulfur or a sulfur compound. As said sulfur compound, metal sulfides, such as lithium sulfide, organic disulfide compounds, organic sulfur compounds, such as a carbon sulfide compound, etc. can be mentioned. Sulfur has advantages such as high theoretical capacity and low cost.

  It is also preferable to use a composite of sulfur and a conductive agent (a material having a higher conductivity than sulfur). Examples of this composite include one in which sulfur is supported on a conductive agent as a carrier, and more specifically, a composite of sulfur and porous carbon (sulfur-porous carbon composite: SPC Etc. can be mentioned. 50 mass% is preferable and, as for the minimum of content of sulfur in this SPC, 60 mass% is more preferable. On the other hand, 90 mass% is preferable and 80 mass% of the upper limit of this content is more preferable. By making the content of sulfur in the SPC into the above range, it is possible to achieve both a large electric capacity and good conductivity, and the like.

  The lower limit of the content of sulfur (sulfur element) in the positive electrode active material layer is preferably 40% by mass, and more preferably 50% by mass. On the other hand, 90 mass% is preferable as an upper limit of this content, and 70 mass% is more preferable. When using SPC, as a minimum of content of SPC in the above-mentioned positive electrode active material layer, 60 mass% is preferred, and 80 mass% is more preferred. On the other hand, as an upper limit of this content, 95 mass% is preferred, and 90 mass% is more preferred. By making content of sulfur or SPC into the said range, coexistence with large electric capacity and favorable electroconductivity can be aimed at, and a capacity | capacitance density can be raised more.

The lower limit of the content of sulfur (elemental sulfur) per unit area in the positive electrode is 2 mg / cm ² , preferably 3 mg / cm ² , and more preferably 4 mg / cm ² . By making the content of sulfur per unit area in the positive electrode equal to or more than the above lower limit and using the non-aqueous electrolyte, the utilization rate of sulfur can be increased and the capacity density can be increased. On the other hand, the upper limit of the content of sulfur per unit area in the positive electrode is 5 mg / cm ² , preferably 4.5 mg / cm ² .

  As said positive electrode active material, other positive electrode active materials other than sulfur may be contained. However, as a lower limit of the content rate of sulfur in all the positive electrode active materials, 50 mass% is preferable, 90 mass% is more preferable, and 99 mass% is more preferable.

  The conductive agent is not particularly limited as long as the conductive material does not adversely affect the battery performance. Examples of such a conductive agent include natural or artificial graphite; carbon blacks such as furnace black, acetylene black and ketjen black; metals; conductive ceramics and the like. The shape of the conductive agent may, for example, be powdery or fibrous.

  As said binder (binder), Thermoplastic resins, such as fluorine resin (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) etc.), polyethylene, a polypropylene, a polyimide, a polyacrylic acid etc .; ethylene-propylene-diene rubber Elastomers such as (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, and polysaccharide polymers.

  Examples of the thickener include polysaccharide polymers such as carboxymethylcellulose (CMC) and methylcellulose. When the thickener has a functional group that reacts with lithium, it is preferable to deactivate the functional group in advance by methylation or the like.

  The filler is not particularly limited as long as it does not adversely affect the battery performance. Examples of the main component of the filler include polyolefins such as polypropylene and polyethylene, silica, alumina, zeolite, glass and the like.

  The positive electrode can be obtained, for example, by coating a slurry of the positive electrode mixture on a positive electrode substrate or a positive electrode substrate provided with an intermediate layer, and drying the slurry.

(Negative electrode)
The negative electrode has a negative electrode base material, and a negative electrode active material layer disposed directly or via an intermediate layer on the negative electrode base material. The intermediate layer may have the same configuration as the intermediate layer of the positive electrode.

  The said negative electrode base material has electroconductivity. The negative electrode substrate can be made to have the same configuration as the positive electrode substrate, but as the material, a metal such as copper, nickel, stainless steel, nickel plated steel or an alloy thereof is used, and copper or a copper alloy is preferable. . That is, copper foil is preferable as the negative electrode substrate. As a copper foil, a rolled copper foil, an electrolytic copper foil, etc. are illustrated.

  The negative electrode active material layer is formed of a so-called negative electrode mixture containing a negative electrode active material. Moreover, the negative electrode composite material which forms a negative electrode active material layer contains arbitrary components, such as a conductive agent, a binder (binder), a thickener, and a filler, as needed. As the optional components such as the conductive agent, the binder (binder), the thickener, and the filler, the same components as those of the positive electrode active material layer can be used.

  Examples of the negative electrode active material include alkali metals such as lithium and sodium; compounds containing alkali metals such as lithium alloy, sodium alloy and lithium composite oxide; carbon materials such as graphite and non-graphitic carbon; Sn, Ge, Si And metals other than alkali metals, such as Sn oxides and Ge oxides, oxides or metalloids of metals other than alkali metals, and polyphosphate compounds. Among these, an alkali metal or a compound containing an alkali metal is preferable, and a compound containing lithium or lithium is preferable. The negative electrode active material can be used alone or in combination of two or more. In addition, when using the negative electrode active material which does not contain an alkali metal, it is preferable that a positive electrode active material contains the compound which contains alkali metal in addition to sulfur.

  Furthermore, the negative electrode active material layer is typically a nonmetal element such as B, N, P, F, Cl, Br, or I; a typical metal such as Li, Na, Mg, Al, K, Ca, Zn, Ga, or Ge The element may contain a transition metal element such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, W and the like.

(Separator)
As a material of the said separator, a woven fabric, a nonwoven fabric, a porous resin film etc. are used, for example. Among these, a porous resin film is preferable from the viewpoint of strength, and a nonwoven fabric is preferable from the viewpoint of liquid retention of the non-aqueous electrolyte. As a main component of the separator, polyolefins such as polyethylene and polypropylene are preferable from the viewpoint of strength, and polyimide and aramid are preferable from the viewpoint of resistance to oxidative degradation. In addition, these resins may be composited. Furthermore, it may be a multilayer separator in which an inorganic porous layer is laminated on a porous resin film.

(Non-aqueous electrolyte)
The non-aqueous electrolyte used in the secondary battery is the non-aqueous electrolyte according to one embodiment of the present invention described above, and is a non-aqueous electrolyte having a viscosity at 25 ° C. of less than 1 mPa · s of the fluorinated ether. . The details of this non-aqueous electrolyte are as described above.

  The manufacturing method of the said secondary battery is not specifically limited, It can carry out combining the well-known method. The manufacturing method of the secondary battery includes, for example, a step of manufacturing a positive electrode and a negative electrode, a step of preparing a non-aqueous electrolyte, and an electrode body alternately stacked or wound by laminating the positive electrode and the negative electrode through a separator. Forming the positive electrode and the negative electrode (electrode assembly) in a battery case, and injecting the non-aqueous electrolyte into the battery case. The above injection can be performed by a known method. After injection, the secondary battery can be obtained by sealing the inlet.

<Other Embodiments>
The present invention is not limited to the above embodiment, and can be implemented in various modifications and improvements in addition to the above embodiment. For example, in the above embodiment, the non-aqueous electrolyte storage device has been described focusing on the form of a lithium ion secondary battery that is a non-aqueous electrolyte secondary battery, but other non-aqueous electrolyte storage devices may be used. Examples of other non-aqueous electrolyte storage elements include capacitors (electric double layer capacitors, lithium ion capacitors) and the like.

  Further, in the positive electrode, a clear positive electrode active material layer containing sulfur may not be formed. For example, sulfur may be supported in a dispersed manner on the surface of the positive electrode substrate or inside the porous positive electrode substrate.

  FIG. 1 is a schematic view of a rectangular non-aqueous electrolyte secondary battery 1 which is an embodiment of the non-aqueous electrolyte storage device according to the present invention. In addition, the same figure is a view seen through the inside of the battery container. In the non-aqueous electrolyte secondary battery 1 shown in FIG. 1, the electrode body 2 is housed in a battery case 3. The electrode body 2 is formed by winding a positive electrode including a positive electrode active material and a negative electrode including a negative electrode active material via a separator. The positive electrode is electrically connected to the positive electrode terminal 4 via the positive electrode lead 4 ′, and the negative electrode is electrically connected to the negative electrode terminal 5 via the negative electrode lead 5 ′.

  The configuration of the non-aqueous electrolyte storage element according to the present invention is not particularly limited, and a cylindrical storage element, a square storage element (rectangular storage element), a flat storage element, etc. may be mentioned as an example. The present invention can also be realized as a power storage device provided with a plurality of the above non-aqueous electrolyte power storage elements. One embodiment of a power storage device is shown in FIG. In FIG. 2, power storage device 30 includes a plurality of power storage units 20. Each storage unit 20 includes a plurality of non-aqueous electrolyte secondary batteries 1. The power storage device 30 can be mounted as a power source for vehicles such as an electric vehicle (EV), a hybrid vehicle (HEV), and a plug-in hybrid vehicle (PHEV).

  Hereinafter, the present invention will be more specifically described by way of examples, but the present invention is not limited to the following examples.

<Preparation of Nonaqueous Electrolyte>
Comparative Example 1
Lithium salts lithium bis (trifluoromethanesulfonyl) imide (LiTFSI), tetraethylene glycol dimethyl ether (G4) and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoropropyl ether (TFEE: 25 The non-aqueous electrolyte of Comparative Example 1 was obtained by mixing a viscosity of 0.65 mPa · s at ° C.) at a molar ratio of 1.0: 1.0: 4.0.

[Comparative Example 2, Examples 1 to 4]
Each non-aqueous electrolyte of Comparative Example 2 and Examples 1 to 4 was obtained in the same manner as Comparative Example 1 except that the type and molar ratio of each component used were as shown in Table 1. In the table, HFE represents 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (viscosity 1.60 mPa · s at 25 ° C.).

[Measurement of ion conductivity]
Each obtained non-aqueous electrolyte was impregnated into a polyethylene separator, and a bipolar symmetric cell was formed using this and a stainless steel plate as a positive electrode and a negative electrode. AC impedance measurement was performed on this bipolar symmetric cell to measure the ion conductivity. The measurement conditions were: applied voltage amplitude: 5 mV, frequency domain: 1 MHz to 100 mHz, temperature 25 ° C. The measurement results are shown in Table 1.

[Measurement of viscosity]
The viscosity at 25 ° C. of each of the obtained non-aqueous electrolytes was measured using a falling ball viscometer Lovis 2000 ME manufactured by Anton Paar. The measurement results are shown in Table 1.

  As shown in Table 1, the non-aqueous electrolytes of Examples 1 to 4 have lower viscosity than the non-aqueous electrolytes of Comparative Examples 1 and 2. In particular, from the comparison between Comparative Example 1 and Examples 1 and 2 using the same fluorinated ether, and the comparison between Comparative Example 2 and Examples 3 and 4, the viscosity is more than 5 times the dilution ratio It turns out that it is falling sharply. On the other hand, regarding the ion conductivity, it can be seen that the non-aqueous electrolytes of Examples 1 to 4 are comparable to or slightly lower than the non-aqueous electrolytes of Comparative Examples 1 and 2. In addition, when TFEE and HFE are compared, it can be seen that the viscosity is lower and the ion conductivity is higher when TFEE is used.

<Fabrication of secondary battery>
Comparative Example 3
Magnesium citrate was carbonized by heating at 900 ° C. for 1 hour in an air atmosphere. Magnesium oxide was eluted by immersing the obtained carbide in a 1 MH ₂ SO ₄ aqueous solution. Thereafter, the carbide was washed and dried to obtain porous carbon. The porous carbon and sulfur were mixed at a mass ratio of 30:70. This mixture was sealed in a closed vessel under an argon atmosphere, allowed to cool at 150 ° C. for 5 hours, and heat treated at 300 ° C. for 2 hours to obtain a sulfur-porous carbon composite (SPC). The temperature rising rate at each heat treatment was 5 ° C./min.

SPC, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder are weighed at a mass ratio of 85: 5: 10, and N-methyl-2-pyrrolidone (NMP) is used as a dispersion medium It mixed, and obtained the slurry of positive mix. The positive electrode mixture slurry was applied to carbon paper as a positive electrode substrate and dried. Thus, a sheet-like positive electrode was obtained in which a positive electrode active material layer having a sulfur content per unit area of 0.78 mg / cm ² was formed. In addition, the area (positive electrode area) which coated the said positive electrode compound material slurry in the positive electrode was 1.13 cm < ² >.

  On the other hand, sheet-like metal lithium was prepared as a negative electrode. In addition, a polyethylene microporous film was prepared as a separator. Using the obtained positive electrode, the negative electrode, the separator, and the nonaqueous electrolyte of Comparative Example 1, a secondary battery as a nonaqueous electrolyte storage element of Comparative Example 3 was obtained.

[Comparative Examples 4 to 6, Examples 5 to 7, Reference Examples 1 to 9]
Comparative Examples 4 to 6, Examples 5 to 7 and Comparative Examples 3 and 4 except that the content of sulfur per unit area in the non-aqueous electrolyte used and the positive electrode was as shown in Table 2. The respective secondary batteries of Reference Examples 1 to 9 were obtained. The adjustment of the sulfur content was performed by adjusting the coating amount of the positive electrode mixture slurry containing the SPC. Table 2 also shows the sulfur content in each secondary battery and the theoretical capacity density based on the theoretical capacity of sulfur. The theoretical capacity density was calculated based on the theoretical capacity of sulfur being 1675 mAh / g.

[Evaluation]
About each obtained secondary battery, the charging / discharging test was done in the thermostat set to 25 degreeC. The charge was constant current (CC) charge, and the discharge was constant current (CC) discharge. The constant current values for charge and discharge were 0.1 C. The charge upper limit voltage and the discharge end voltage were 3.0 V and 1.0 V, respectively. The measured value of the discharge capacity in each secondary battery is shown in Table 2 as "actual capacity". In addition, the discharge capacity per unit area of the positive electrode determined from the actual capacity and the area of the positive electrode was determined as "measured capacity density", and the ratio of "measured capacity density" to "theoretical capacity density" was determined as "sulfur utilization rate". The "measured capacity density" and the "sulfur utilization rate" are shown in Table 2.

From the results of Table 2, by using the non-aqueous electrolyte of Examples 1 to 4 in combination with the positive electrode containing a predetermined amount of sulfur, the utilization rate of the sulfur is enhanced, and the capacity density of the secondary battery (measured capacity density) It can be seen that it can enhance the Moreover, it turns out that the secondary battery of Examples 5-7 has a high capacity density over 3 mAh / cm < ² >. Specifically, the following can be understood from each Comparative Example, Example, and Reference Example.

First, as shown in Comparative Examples 3 to 6, when the diluted amount of fluorinated ether is 4 times, the measured capacity density of the secondary battery is as low as about 0.9 mAh / cm ² or less. Furthermore, as can be seen from the comparison between Comparative Example 3 and Comparative Example 4 and the comparison between Comparative Example 5 and Comparative Example 6, the sulfur content in the positive electrode is increased, that is, the sulfur utilization rate is large when sulfur is thickly coated As a result, the measured capacity density and the actual capacity are conversely reduced.

On the other hand, as can be seen from the comparison of Comparative Example 4 and Examples 5 and 6, when the dilution amount with low viscosity TFEE is increased to more than 5 times, the content of sulfur in the positive electrode is high. Sulfur utilization is increasing. Also, from Example 7 etc., when a non-aqueous electrolyte diluted more than 5 times with TFEE is used, high sulfur utilization is maintained even if the sulfur content is thickly applied to more than 4 mg / cm ² It is shown. For this reason, in each of the secondary batteries of Examples 5 to 7, the measured capacity density and the actual capacity are significantly increased due to the synergistic effect of the high sulfur content and the high utilization rate. As can be seen from the comparison of Comparative Example 3 and Reference Examples 1 and 2, when the content of sulfur in the positive electrode is small, the utilization of sulfur reaches a plateau even if the dilution amount with TFEE is increased to more than 5 times. When a non-aqueous electrolyte diluted by more than 5 times with low viscosity TFEE is used, high capacity density is exhibited by setting the content of sulfur per unit area in the positive electrode to 2 mg / cm ² or more and 5 mg / cm ² or less It can be seen that (Examples 5 to 7).

  On the other hand, as can be seen from the comparison of Comparative Example 5 and Reference Examples 4 and 5, when the dilution amount is increased to more than 5 times with HFE having a relatively high viscosity, when using a positive electrode with a low sulfur content, It can be seen that the utilization rate of sulfur is increased. As can be seen from the comparison of Comparative Example 6 and Reference Examples 7 and 8, when the content of sulfur in the positive electrode is high, the utilization of sulfur is not improved even if the amount of dilution with HFE is increased more than 5 times. I understand that.

  The present invention can be applied to non-aqueous electrolyte storage devices used as power supplies for personal computers, electronic devices such as communication terminals, etc., and automobiles.

DESCRIPTION OF SYMBOLS 1 non-aqueous electrolyte secondary battery 2 electrode body 3 battery container 4 positive electrode terminal 4 ′ positive electrode lead 5 negative electrode terminal 5 ′ negative electrode lead 20 storage unit 30 storage device

Claims (3)

Lithium salt,
It contains a polyether compound having 4 or more ether bonds, and a fluorinated ether,
The molar ratio of the lithium salt to the polyether compound is 1 or more,
Nonaqueous electrolyte for a storage element, wherein the molar ratio of the fluorinated ether to the lithium salt is greater than 5.   The non-aqueous electrolyte according to claim 1, wherein the viscosity at 25 ° C of the fluorinated ether is less than 1 mPa · s. A positive electrode containing sulfur, and the non-aqueous electrolyte according to claim 2.
The content of sulfur per unit area in the positive electrode, 2 mg / cm ² or more 5 mg / cm ² or less is the non-aqueous electrolyte energy storage device.

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