JP2017027657A - Electrolytic solution for secondary battery and secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolytic solution for a secondary battery, having sufficient lithium ion conductivity, high in electrochemical stability, and providing excellent battery performance, and a lithium sulfur secondary battery using the electrolytic solution.SOLUTION: An organic solution of a sulfide-based solid electrolyte that exhibits lithium ion conductivity is employed as an electrolytic solution 5, the sulfide-based solid electrolyte being soluble into an organic solvent at a high concentration.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an electrolytic solution in a lithium sulfur secondary battery and a lithium sulfur secondary battery using the electrolytic solution.

  A lithium-sulfur secondary battery has been proposed as one of the next generation high capacity secondary batteries. This is because the theoretical capacity of the positive electrode active material of a general lithium ion secondary battery is about 180 mAh / g, whereas the theoretical capacity of the sulfur active material is as extremely large as 1675 mAh / g. For this reason, positive electrode materials for lithium-sulfur secondary batteries have been actively developed, and various reports have been made (for example, see Patent Documents 1 and 2).

International Publication No. 2012/070184 Pamphlet US Patent Publication No. 2011 / 0052998A1

  By the way, in such a lithium-sulfur secondary battery, further improvement is demanded for an electrolytic solution capable of obtaining sufficient battery characteristics.

  That is, an object of the present invention is to provide an electrolytic solution for a secondary battery that can provide sufficient lithium ion conductivity, has high electrochemical stability, and provides excellent battery performance. It is another object of the present invention to provide a lithium-sulfur secondary battery using such an electrolytic solution.

  In order to achieve the above object, the present invention finds a sulfide-based solid electrolyte exhibiting lithium ion conductivity that can be dissolved in an organic solvent at a high concentration, and sulfide exhibiting such lithium ion conductivity. An organic solution of a system solid electrolyte was applied as an electrolyte for a secondary battery.

That is, the electrolytic solution for a secondary battery disclosed herein, the organic solvent, shows the lithium ion conductivity, and the secondary of the general formula Li _a P _b at least a portion of the solid electrolyte represented by S _c was dissolved The battery electrolyte is characterized in that a is 3 <a <5, b is 1 <b <3, and c is 6 <c <8.

  In general, an inorganic solid electrolyte exhibiting lithium ion conductivity is used as an electrolyte layer or the like of an all-solid lithium secondary battery. This is because such an inorganic solid electrolyte has the property of showing high conductivity of lithium ions in the solid state and has high heat resistance and high electrochemical stability. It is known that such a solid electrolyte exhibiting lithium ion conductivity is generally hardly dissolved in an organic solvent.

  Here, as a result of intensive studies, the present inventors have found that some of the solid electrolytes exhibiting lithium ion conductivity exhibit sufficient solubility in organic solvents. An organic solution obtained by dissolving a solid electrolyte exhibiting lithium ion conductivity in an organic solvent has high lithium ion conductivity comparable to a general lithium salt used as an electrolyte for a lithium ion secondary battery. I found more to show.

  Therefore, according to the present invention, it is possible to provide an electrolytic solution for a secondary battery that has sufficient lithium ion conductivity, high electrochemical stability, and excellent battery performance.

Such a solid electrolyte exhibiting lithium ion conductivity is represented by the general formula Li _a P _b S _c, where a is 3 <a <5 and b is 1 <b <3. And c is 6 <c <8. In a preferred embodiment, at least a part of the solid electrolyte exhibiting lithium ion conductivity has a composition of Li ₄ P ₂ S ₇ . Thereby, sufficient solubility in an organic solvent is obtained, and battery performance is improved.

In a preferred embodiment, the lithium ion conductivity of the secondary battery electrolyte according to the present invention is 1 × 10 ⁻⁶ S / cm or more. Thereby, battery performance improves.

  Further, such a secondary battery electrolyte may be in the form of a gel containing a polymer or the like. Thereby, handling property improves.

  In a preferred embodiment, the secondary battery electrolyte includes a negative electrode including a material that absorbs and releases lithium ions, a positive electrode that uses sulfur as a positive electrode active material, and a separator that is disposed between the negative electrode and the positive electrode. It can be suitably used as the electrolytic solution of a lithium-sulfur secondary battery comprising an electrolytic solution filled between the negative electrode and the positive electrode. Thereby, the lithium sulfur secondary battery which has the outstanding battery performance can be brought about.

  In a particularly preferred embodiment, the organic solvent is tetrahydrofuran. Thereby, sufficient solubility of the solid electrolyte which shows lithium ion conductivity is obtained.

  In a particularly preferred embodiment, the secondary battery electrolyte contains an additive. Thereby, battery performance can be improved effectively.

  As described above, according to the present invention, it is possible to provide an electrolytic solution for a secondary battery that has sufficient lithium ion conductivity, high electrochemical stability, and excellent battery performance. Moreover, the use of such an electrolytic solution can provide a lithium-sulfur secondary battery having excellent battery performance.

FIG. 1 is a cross-sectional view showing a schematic configuration of a lithium-sulfur secondary battery according to an embodiment of the present invention. FIG. 2 is a diagram showing the Raman spectrum of Li ₄ P ₂ S _{7 in} Example 1. FIG. 3 is a diagram showing a charge / discharge profile of the lithium-sulfur secondary battery according to Example 1. FIG. 4 is a diagram illustrating initial discharge profiles of lithium-sulfur secondary batteries according to Example 1 and Comparative Examples 1 to 3. FIG. 5 is a graph showing the ionic conductivity of the electrolytes of Example 1 and Comparative Examples 1 to 3.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or its application.

<Configuration of lithium-sulfur secondary battery>
As shown in FIG. 1, a lithium-sulfur secondary battery 1 according to an embodiment includes a positive electrode 2 using sulfur as a positive electrode active material, a negative electrode 3 containing a material that absorbs and releases lithium ions, and a positive electrode 2 and a negative electrode 3. And an electrolyte 5 filled between the positive electrode 2 and the negative electrode 3 and having lithium ion conductivity.

  The positive electrode 2 is prepared by, for example, mixing a positive electrode active material, a conductive material, and a binder, adding a suitable solvent to form a paste-like positive electrode material, applying and drying on the surface of the current collector, and if necessary You may compress and form in order to raise an electrode density. The positive electrode active material contains sulfur. Sulfur may be contained in any form, but is preferably at least one of elemental sulfur and metal sulfide. The metal sulfide includes a metal polysulfide. When elemental sulfur is used as the positive electrode active material, at least a part of the sulfur is a surface treatment agent such as a surfactant, a polymer pigment, or a silicone resin from the viewpoint of improving the dispersibility of sulfur in the positive electrode material. It may be modified with an organic component containing In this case, the concentration of the organic component in the sulfur is preferably 0.01% by mass or more and 10% by mass or less.

As the negative electrode 3, for example, a general negative electrode of a lithium ion secondary battery or a lithium sulfur secondary battery can be used. Specifically, as the material of the negative electrode 3, for example, an alloy of Li, Li and Al or In, or Si, SiO, Sn, SnO ₂ or a carbon material doped with lithium ions can be used.

  The separator 4 insulates between the positive electrode 2 and the negative electrode 3 in the electrolyte solution 5, and a well-known thing can be used as a separator of a lithium ion secondary battery or a lithium sulfur secondary battery. For example, the separator 4 is composed of a porous film made of synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous film made of ceramic, and has a structure in which these two or more kinds of porous films are laminated. You may have. Among these, the porous film made of polyolefin not only has an excellent short-circuit prevention effect, but also improves the safety of the battery by the shutdown effect (the effect of closing the vacancies and closing the current when an excessive current flows). This is preferable.

  As shown in FIG. 1, the electrolyte 5 is filled between the positive electrode 2 and the separator 4, inside the separator 4, and between the separator 4 and the negative electrode 3, and is a solid that exhibits lithium ion conductivity in an organic solvent. At least a part of the electrolyte is dissolved.

As the solid electrolytes having lithium ion conductivity, in which from the viewpoint of solubility and battery performance improvement in an organic solvent, represented by the general formula Li _a P _b S _c. Here, a is 3 <a <5, b is 1 <b <3, and c is 6 <c <8. Moreover, it is particularly preferable that at least a part of the solid electrolyte exhibiting lithium ion conductivity has a composition of Li ₄ P ₂ S ₇ .

  As the organic solvent, for example, at least one selected from ether organic solvents such as tetrahydrofuran, glyme, diglyme, triglyme and tetraglyme, and ester solvents such as diethyl carbonate and propylene carbonate, or from these A mixture of dioxolane for viscosity adjustment with at least one selected (for example, glyme, diglyme or tetraglyme) can be used. Particularly preferably, the organic solvent is tetrahydrofuran.

  The concentration of the solid electrolyte in the electrolytic solution 5 is preferably 0.005M or more, more preferably 0.01M or more, and particularly preferably 0.03M or more from the viewpoint of improving battery performance.

Further, the lithium ion conductivity of the electrolytic solution 5 is preferably 1 × 10 ⁻⁷ S / cm or more, more preferably 5 × 10 ⁻⁷ S / cm or more, particularly preferably 1 × 10 from the viewpoint of improving battery performance. ^-6 S / cm or more.

  Further, such an electrolytic solution 5 may be in a liquid form or a gel containing a polymer or the like. Examples of the polymer to be contained when using the gel electrolyte include polyethylene oxide (PEO), polyacrylonitrile (PAN), poly (vinylidene fluoride) (PVDF), and polymethyl methacrylate (PMMA).

Moreover, the electrolyte solution 5 may contain an additional additive from a viewpoint of the charging / discharging characteristic and safety | security improvement of a secondary battery. Examples of the additive include an additive such as LiNO ₃ that forms a coating film on the surface of the metal Li negative electrode to prevent the shuttle phenomenon, and for the purpose of forming a coating film, improving safety, and improving durability, for example, fluoride ion An alkali metal salt, an alkaline earth metal salt containing at least one or more halide ions among (F ⁻ ), chloride ions (Cl ⁻ ), bromide ions (Br ⁻ ), and iodide ions (I ⁻ ), Examples thereof include inorganic additives such as ammonium salts. Moreover, the organic additive of the same objective is also mentioned. These additives may be used alone or in combination. The concentration of the additive is preferably 0.01 wt% or more.

  Such an electrolytic solution 5 has sufficient lithium ion conductivity and high electrochemical stability. Therefore, a lithium-sulfur secondary battery having excellent battery performance can be provided by using such an electrolytic solution 5.

<Operating mechanism of lithium-sulfur secondary battery>
The lithium-sulfur secondary battery 1 having the above configuration operates by the following mechanism when the negative electrode 3 is made of metallic lithium. That is, at the time of discharging, metallic lithium of the negative electrode 3 is oxidized by the following formula (1), and Li ⁺ is released into the electrolytic solution 5.
Li → Li ⁺ + e ⁻ (1)
The released Li ⁺ moves to the positive electrode 2 side through the separator 4 and reacts with a sulfur active material such as S ₈ of the positive electrode 2 by the reduction reaction shown in the following formula (2), thereby causing the discharge product Li _2. S is produced. And an electric current can be taken out of the lithium sulfur secondary battery 1 outside.
16Li ⁺⁺ S ₈ + 16e ⁻ → 8Li ₂ S (2)
On the other hand, at the time of charging, Li ₂ S or the like that is a discharge product in the positive electrode 2 is oxidized by the reverse reaction of the above formula (2), and Li ⁺ is released into the electrolytic solution 5. Li ⁺ moves to the negative electrode 3 side via the separator 4, and Li ⁺ is reduced by the reverse reaction of the above formula (1) at the negative electrode interface.

  Next, specific examples will be described.

[Example 1]
<Preparation of Li ₄ P ₂ S ₇ >
In Example 1, Li ₄ P ₂ S ₇ was used as a solid electrolyte exhibiting lithium ion conductivity. Li ₄ P ₂ S ₇ was synthesized by the following method. 0.439 g of Li ₂ S (Alfa 99.9%) and 1.061 g of P ₂ S ₅ (99.9% Aldrich), which are raw materials, are weighed to be 66.6 mol% and 33.4 mol%. I made it. Li ₂ S and P ₂ S ₅ were placed in a 45 ml zirconia container under an Ar atmosphere, and 7 zirconia beads having a diameter of 10 mm and 10 zirconia beads having a diameter of 3 mm were added and sealed, followed by ball milling at 380 rpm for 40 hours ( Frich P-7) was performed to obtain 1.5 g of Li ₄ P ₂ S ₇ . As shown in FIG. 2, the obtained sample was subjected to Raman spectroscopic measurement, and there was a peak derived from the structure of P ₂ S ₇ ^{4- at} a wave number of 403 cm ^−1, and thus Li ₄ P ₂ S ₇ was obtained. It was confirmed.

<Ion conductivity of Li ₄ P ₂ S ₇ >
The ionic conductivity of Li ₄ P ₂ S _{7 in} the solid state was measured by electrochemical impedance measurement using a Teflon (registered trademark) cell system (in a glove box). That is, 200 mg of a solid electrolyte (Li ₄ P ₂ S ₇ ) was pressed for 1 minute at a load of 4 t and molded into a pellet having a diameter of 13 mm. Next, an indium (In) foil having a thickness of 0.1 mm was punched out with a diameter of 13 mm and attached to both surfaces of the electrolyte pellet. And it set to the cell made from Teflon (trademark) which can provide a fixed pressure with a spring (The electrode can be taken out via a metal plate from both surfaces). Thereafter, the outside of the Teflon (registered trademark) cell was wrapped with a laminate film to form a vacuum pack, and electrochemical impedance measurement was performed. The ionic conductivity of the obtained Li ₄ P ₂ S ₇ was 6.47 × 10 ⁻⁵ S / cm at 25 ° C.

<Solubility of Li ₄ P ₂ S ₇ >
The solubility of Li ₄ P ₂ S ₇ in tetrahydrofuran (THF) as an organic solvent was compared with the solubility of other lithium ion conductive solid electrolytes, namely Li ₃ PS ₄ and Li ₄ P ₂ S ₆ . The results are shown in Table 1.

As shown in Table 1, it was found that Li ₃ PS ₄ and Li ₄ P ₂ S ₆ showed almost no solubility in THF. On the other hand, it was found that Li ₄ P ₂ S ₇ exhibits solubility in THF.

<Production of coin battery sample>
5.0 g of sulfur (Sulfax PS Tsurumi Chemical Co., Ltd.) modified with 1% organic components, 0.56 g of polyethylene oxide (PEO) with a molecular weight of 3 million, and 30 g of zirconia beads with a diameter of 2 mm are weighed in a plastic container and stirred. Then, 20 g of acetonitrile was added and further stirred, and then ball milled at 90 rpm for 12 hours to prepare a yellow viscous slurry. Zirconia beads were removed with a mesh, a slurry was formed on a PET film coated with a release agent, and dried to prepare a self-supporting sheet of sulfur. The produced sheet was formed into the same shape as a circular carbon electrode having a diameter of 14 mm, a sulfur film was pressure-bonded to the electrode surface, and heated to produce a sulfur / carbon positive electrode. The PEO film after the introduction of sulfur was peeled off from the electrode surface and subjected to a battery experiment using a positive electrode into which 8 to 10 mg / cm ^{2 of} sulfur was introduced. A Li—Al alloy foil (Al concentration of 20 vol%) having a diameter of 15 mm and a thickness of 400 μm was used as the negative electrode, and Celgard # 2400 (manufactured by Celgard) was used as the separator. As the electrolytic solution, 150 μL of Li ₄ P ₂ S ₇ / THF electrolytic solution adjusted to be 0.08M was used. A CR2032-type coin battery sample was prepared using these materials by a known method.

[Comparative Example 1]
A coin battery sample was produced in the same manner except that Li ₄ P ₂ S ₇ in Example 1 was changed to LiBF ₄ .

[Comparative Example 2]
A coin battery sample was produced in the same manner except that Li ₄ P ₂ S ₇ of Example 1 was changed to LiPF ₆ .

[Comparative Example 3]
A coin battery sample was prepared in the same manner except that Li ₄ P ₂ S ₇ of Example 1 was changed to lithium bis (trifluoromethanesulfonyl) imide (LiTFSI).

<Evaluation of charge / discharge characteristics of lithium-sulfur secondary battery>
About the coin battery sample of Example 1 and Comparative Examples 1-3, the characteristic of the lithium sulfur secondary battery was evaluated. The results are shown in FIGS. 3 and 4 and Table 2.

The measurement was performed by charging and discharging at a constant current of 0.77 mA (0.50 mA / cm ² ), and the cut voltage was set to discharge 1.5V and charge 2.37V.

  As shown in FIG. 3, the coin battery sample of Example 1 was able to be charged / discharged at a high capacity with a charge / discharge capacity exceeding 700 mAh / g.

Further, as shown in FIG. 4 and Table 2, Comparative Examples 1 to 3 have clearly lower capacities than the initial discharge capacity in Example 1, and Li ₄ P ₂ S ₇ electrolysis in a lithium sulfur secondary battery. The superiority of the liquid was shown.

<Measurement of ionic conductivity of electrolyte>
The ionic conductivity of the Li ₄ P ₂ S ₇ / THF electrolyte used in Example 1 and the Li electrolyte salt / THF electrolyte used in Comparative Examples 1 to 3 were measured and compared. Using a glass cell having SUS electrodes on both electrodes inside, about 5 ml of each electrolyte adjusted to 0.08 M was added thereto, and the ionic conductivity was measured by measuring the AC impedance. The ionic conductivity was calculated by obtaining the cell constant in the same experiment with a 0.1 N KCl aqueous solution. The results are shown in FIG.

As shown in FIG. 5 and Table 2, the Li ₄ P ₂ S ₇ / THF electrolyte used in Example 1 is higher than the electrolyte containing LiBF ₄ and LiPF ₆ in Comparative Examples 1 and 2. It was found that the ionic conductivity was exhibited and a low ionic conductivity was exhibited as compared with the electrolytic solution containing LiTFSI of Comparative Example 3.

<Summary>
From the above results, the Li ₄ P ₂ S ₇ / THF electrolyte in Example 1 exhibits high ionic conductivity comparable to a general lithium salt used as an electrolyte for a lithium ion secondary battery, and is excellent. It has been found to bring about battery performance. In addition, although the electrolyte solution containing LiTFSI of Comparative Example 3 is superior in ionic conductivity to the Li ₄ P ₂ S ₇ / THF electrolyte solution in Example 1, it is the initial discharge capacity of the lithium-sulfur secondary battery. It was shown that the Li ₄ P ₂ S ₇ / THF electrolyte of Example 1 is useful as an electrolyte for a secondary battery.

  The present invention provides an electrolyte solution for a secondary battery having sufficient lithium ion conductivity, high electrochemical stability, and excellent battery performance, and a lithium-sulfur secondary battery using such an electrolyte solution. Since a battery can be provided, it is extremely useful.

DESCRIPTION OF SYMBOLS 1 Lithium sulfur secondary battery 2 Positive electrode 3 Negative electrode 4 Separator 5 Electrolytic solution (electrolytic solution for secondary batteries)

Claims (7)

In an organic solvent, it shows the lithium ion conductivity, and the general formula Li _a P _b S solid electrolyte at least partially dissolved liquid electrolyte for a secondary battery represented _c. In
(Wherein a is 3 <a <5, b is 1 <b <3, and c is 6 <c <8). _{2. The} electrolyte for a secondary battery according to claim 1, wherein at least a part of the solid electrolyte has a composition of Li ₄ P ₂ S ₇ . The electrolyte solution for a secondary battery according to claim 1 or 2, wherein the lithium ion conductivity is 1 x ^10-6 S / cm or more.   The electrolyte solution for a secondary battery according to any one of claims 1 to 3, wherein the electrolyte solution is in a gel form. Filled between the negative electrode containing a material that absorbs and releases lithium ions, a positive electrode using sulfur as a positive electrode active material, a separator disposed between the negative electrode and the positive electrode, and the negative electrode and the positive electrode A lithium-sulfur secondary battery comprising an electrolyte solution,
The said electrolyte solution is the electrolyte solution for secondary batteries described in any one of Claims 1-4, The lithium sulfur secondary battery characterized by the above-mentioned. The lithium-sulfur secondary battery according to claim 5, wherein the organic solvent is tetrahydrofuran. The lithium-sulfur secondary battery according to claim 5, wherein the electrolyte for a secondary battery includes an additive.

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