JP6633855B2 - Electrolyte for secondary batteries and secondary batteries - Google Patents

Description

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

  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 a positive electrode active material of a general lithium ion secondary battery is about 180 mAh / g, while the theoretical capacity of a sulfur active material is extremely large, 1675 mAh / g. For this reason, the development of positive electrode materials for lithium-sulfur secondary batteries and the like has been actively conducted, and various reports have been made (for example, see Patent Documents 1 and 2).

WO 2012/070184 pamphlet US Patent Publication No. 2011/0052998 A1

  By the way, in such a lithium-sulfur secondary battery, further improvement is also required for an electrolyte solution which can obtain sufficient battery characteristics.

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

  In order to achieve the above object, in the present invention, among sulfide-based solid electrolytes exhibiting lithium ion conductivity, those which can be dissolved in an organic solvent at a high concentration have been found, and sulfides having such lithium ion conductivity have been found. The organic solution of the solid electrolyte was used as an electrolyte for a secondary battery.

That is, the electrolytic solution for a secondary battery disclosed herein, the tetrahydrofuran as the organic solvent, shows the lithium ion conductivity, and at least partially dissolve the solid electrolyte represented by the general formula Li _a P _b S _c (Where a is 3 <a <5, b is 1 <b <3, and c is 6 <c <8). .

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

  Here, as a result of earnest 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 such a solid electrolyte exhibiting lithium ion conductivity in an organic solvent has a high lithium ion conductivity comparable to a general lithium salt used as an electrolyte for a lithium ion secondary battery. We have further found that:

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

Solid electrolytes having such a lithium ion conductivity, are those represented by the general formula _Li a _P b _{S c,} wherein a is 3 <a <5, wherein 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 electrolyte solution for a secondary battery according to the present invention is 1 × 10 ⁻⁶ S / cm or more. Thereby, battery performance is improved.

  Further, such an electrolyte solution for a secondary battery may be a gel containing a polymer or the like. Thereby, handling properties are improved.

  In a preferred aspect, the electrolyte for a secondary battery includes a negative electrode including a material that occludes and releases lithium ions, a positive electrode including sulfur as a positive electrode active material, and a separator disposed between the negative electrode and the positive electrode. It can be suitably used as the electrolyte of a lithium-sulfur secondary battery including an electrolyte filled between the negative electrode and the positive electrode. As a result, a lithium-sulfur secondary battery having excellent battery performance can be provided.

Incidentally, before Symbol organic solvent is tetrahydrofuran. Thereby, sufficient solubility of the solid electrolyte exhibiting lithium ion conductivity can be obtained.

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

  As described above, according to the present invention, it is possible to provide an electrolyte for a secondary battery having sufficient lithium ion conductivity, high electrochemical stability, and excellent battery performance. Further, by using such an electrolytic solution, a lithium-sulfur secondary battery having excellent battery performance can be provided.

FIG. 1 is a cross-sectional view illustrating a schematic configuration of a lithium sulfur secondary battery according to an embodiment of the present invention. FIG. 2 is a diagram showing a Raman spectrum of Li ₄ P ₂ S ₇ of Example 1. FIG. 3 is a diagram illustrating a charge / discharge profile of the lithium-sulfur secondary battery according to the first embodiment. FIG. 4 is a diagram illustrating an initial discharge profile of the lithium-sulfur secondary batteries according to Example 1 and Comparative Examples 1 to 3. FIG. 5 is a diagram 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 description of the preferred embodiments below is merely exemplary in nature and is in no way intended to limit the invention, its applications, or its uses.

<Configuration of lithium sulfur secondary battery>
As shown in FIG. 1, a lithium-sulfur secondary battery 1 according to one embodiment includes a positive electrode 2 using sulfur as a positive electrode active material, a negative electrode 3 containing a material that occludes and releases lithium ions, 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, for example, by mixing a positive electrode active material, a conductive material, and a binder, adding an appropriate solvent, and forming a paste-like positive electrode material on the surface of the current collector. It may be formed by compression to increase the 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 metal polysulfide. When using elemental sulfur as the positive electrode active material, at least a part of the sulfur is, from the viewpoint of improving the dispersibility of sulfur in the above-described positive electrode material, a surfactant, a polymer pigment, and a surface treatment agent such as a silicone resin. 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 for 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, Li, an alloy of Li and Al or In, etc., or can be used Si doped with lithium ions, SiO, Sn, and SnO ₂ or a carbon material or the like.

  The separator 4 serves to insulate the positive electrode 2 and the negative electrode 3 in the electrolytic solution 5, and a known separator for a lithium ion secondary battery or a lithium sulfur secondary battery can be used. For example, the separator 4 is formed of a porous film made of a synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous film made of a ceramic, and has a structure in which two or more kinds of these 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 due to the shutdown effect (the effect of closing the pores when an excessive current flows and closing the current). This is preferable because

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

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

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, and particularly preferably 1 × 10 ⁻⁷ S / cm, from the viewpoint of improving battery performance. ⁻⁶ S / cm or more.

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

Further, the electrolyte solution 5 may include an additional additive from the viewpoint of improving the charge / discharge characteristics and safety of the secondary battery. Examples of the additive include an additive such as LiNO ₃ for forming a coating film on the surface of the metal Li negative electrode to prevent a shuttle phenomenon, and for the purpose of forming a coating film, improving safety, and improving durability, for example, fluoride ion. Alkali metal salts, alkaline earth metal salts containing at least one halide ion among (F ⁻ ), chloride ion (Cl ⁻ ), bromide ion (Br ⁻ ), and iodide ion (I ⁻ ); And inorganic additives such as ammonium salts. In addition, organic additives for the same purpose can also be mentioned. These additives may be used alone or in combination of two or more. The concentration of the additive is preferably 0.01 wt% or more.

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

<Operation mechanism of lithium sulfur secondary battery>
When the negative electrode 3 is made of metallic lithium, the lithium-sulfur secondary battery 1 having the above configuration operates by the following mechanism. That is, at the time of discharging, the 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 via the separator 4 and reacts with a sulfur active material such as S ₈ of the positive electrode 2 by a reduction reaction represented by the following formula (2) to generate a discharge product Li ₂ Produces S. Then, a current can be taken out of the lithium-sulfur secondary battery 1.
_{^{16Li + + S 8 + 16e -}} → 8Li 2 S ··· (2)
On the other hand, during charging, Li ₂ S and the like, which are discharge products, are oxidized in the positive electrode 2 by the reverse reaction of the above formula (2), and Li ⁺ is released into the electrolyte 5. Li ⁺ moves to the negative electrode 3 side via the separator 4, and at the negative electrode interface, Li ⁺ is reduced by the reverse reaction of the above formula (1).

  Next, a specific embodiment 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 (99.9% manufactured by Alfa) as a raw material and 1.061 g of P ₂ S ₅ (99.9% manufactured by Aldrich) are weighed to be 66.6 mol% and 33.4 mol%. I made it. Li ₂ S and P ₂ S ₅ were put in a 45 ml zirconia container under an Ar atmosphere, 7 zirconia beads having a diameter of 10 mm, and 10 zirconia beads having a diameter of 10 mm were added and sealed, and ball milling treatment was performed 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 spectroscopy measurement. Since a peak derived from the structure of P ₂ S ₇ ^{4- at} a wave number of 403 cm ⁻¹ , Li ₄ P ₂ S ₇ was obtained. It was confirmed.

<Ionic 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 under a load of 4 t to form 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. Then, it was set in a Teflon (registered trademark) cell (electrodes can be taken out from both sides via a metal plate) to which a certain pressure can be applied by a spring. Thereafter, the outside of the Teflon (registered trademark) cell was wrapped with a laminate film to form a vacuum pack, and the electrochemical impedance was measured. The ionic conductivity of the obtained Li ₄ P ₂ S ₇ was 6.47 × 10 ⁻⁵ S / cm at 25 ° C.

<About 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 . Table 1 shows the results.

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, Li ₄ P ₂ S ₇ was found to exhibit solubility in THF.

<Preparation of coin battery sample>
5.0 g of sulfur (Sulfax PS manufactured by Tsurumi Chemical Co., Ltd.) modified with 1% organic component, 0.56 g of polyethylene oxide (PEO) having a molecular weight of 3,000,000, and 30 g of zirconia beads having a diameter of 2 mm were weighed, placed in a poly container, and stirred. And 20 g of acetonitrile, and further stirred, and then ball-milled at 90 rpm for 12 hours to produce a yellow viscous slurry. The zirconia beads were removed with a mesh, the slurry was formed on a PET film coated with a release agent, and dried to produce a self-supporting sheet of sulfur. The produced sheet was molded into the same shape as a carbon electrode having a diameter of 14 mm and formed into a circle, and a sulfur film was pressed against the electrode surface and heated to produce a sulfur / carbon positive electrode. The PEO film after sulfur introduction was peeled off from the electrode surface, and subjected to a battery experiment using a positive electrode into which sulfur was introduced at 8 to 10 mg / cm ² . A Li-Al alloy foil having a diameter of 15 mm and a thickness of 400 µm (Al concentration: 20 vol%) was used as a negative electrode, and Celgard # 2400 (manufactured by Celgard) was used as a separator. As the electrolyte, 150 μL of Li ₄ P ₂ S ₇ / THF electrolyte adjusted to be 0.08 M 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 prepared in the same manner as in Example 1, except that Li ₄ P ₂ S ₇ was changed to LiBF ₄ .

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

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

<Evaluation of charge / discharge characteristics of lithium sulfur secondary battery>
The characteristics of the lithium-sulfur secondary battery were evaluated for the coin battery samples of Example 1 and Comparative Examples 1 to 3. The results are shown in FIGS.

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

  As shown in FIG. 3, the coin battery sample of Example 1 was capable of charging and discharging at a high charge / discharge capacity exceeding 700 mAh / g.

In addition, as shown in FIG. 4 and Table 2, Comparative Examples 1 to 3 have clearly lower capacities than the initial discharge capacity of Example 1, and Li ₄ P ₂ S ₇ electrolysis in the 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 ionic conductivity of the Li electrolyte salt / THF electrolyte used in Comparative Examples 1 to 3 were measured and compared. Using a glass cell having SUS electrodes provided on both electrodes inside, about 5 ml of each electrolyte solution adjusted to 0.08 M was added thereto, and AC impedance was measured to measure ionic conductivity. The ionic conductivity was calculated by calculating the cell constant in a similar experiment using a 0.1 N aqueous KCl solution. The results are shown in FIG.

As shown in FIG. 5 and Table 2, the Li ₄ P ₂ S ₇ / THF electrolytic solution used in Example 1 was higher than the electrolytic solutions containing LiBF ₄ and LiPF ₆ of Comparative Examples 1 and 2. In addition to the ionic conductivity, it was found that the ionic conductivity was lower than that of the electrolyte containing LiTFSI of Comparative Example 3.

<Summary>
From the above results, the Li ₄ P ₂ S ₇ / THF electrolytic solution 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. Battery performance. The electrolyte containing LiTFSI of Comparative Example 3 is superior in Li ₄ P ₂ S ₇ / THF electrolyte in Example 1 in terms of ionic conductivity, but is different in terms of 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 was useful as an electrolyte for a secondary battery.

  The present invention provides an electrolyte for a secondary battery having sufficient lithium ion conductivity, high electrochemical stability, and excellent battery performance, and a lithium sulfur secondary using such an electrolyte. It is very useful because a battery can be provided.

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

Claims (6)

In tetrahydrofuran as the organic solvent, 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
(Where a is 3 <a <5, b is 1 <b <3, and c is 6 <c <8.) The solid at least a portion of the electrolyte is characterized by having a composition of Li ₄ P ₂ S _7, the electrolyte solution for a secondary battery according to claim 1. The electrolyte solution for a secondary battery according to claim 1, wherein the lithium ion conductivity is 1 × 10 ⁻⁶ S / cm or more. 4.   The electrolyte for a secondary battery according to any one of claims 1 to 3, wherein the electrolyte is in a gel form. A negative electrode including a material that absorbs and releases lithium ions, a positive electrode including sulfur as a positive electrode active material, a separator disposed between the negative electrode and the positive electrode, and filled between the negative electrode and the positive electrode Lithium sulfur secondary battery comprising an electrolyte solution,
The lithium electrolyte secondary battery according to claim 1, wherein the electrolyte is the electrolyte for a secondary battery according to claim 1. The lithium sulfur secondary battery according to claim 5, wherein the electrolyte for a secondary battery includes an additive.

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