JP7735669B2 - Lithium-sulfur solid-state battery - Google Patents

Description

The present invention relates to a lithium-sulfur solid-state battery.

Amid a desire to develop secondary batteries with higher energy densities than conventional lithium-ion secondary batteries as power sources for electronic devices, electric vehicles, and other devices, lithium-sulfur solid-state batteries, which use sulfur, which has an extremely high theoretical capacity density, as a constituent material for the positive electrode, have attracted attention. For example, Patent Document 1 discloses a lithium-sulfur solid-state battery equipped with a solid electrolyte, and Patent Document 2 proposes a charging method for such a battery.

Patent No. 6578496 Japanese Patent Application Laid-Open No. 2019-140029

Lithium-sulfur solid-state batteries have the problem of internal short circuits occurring during repeated charge and discharge cycles. These short circuits occur when lithium metal dendrites precipitate and grow, eventually connecting the positive and negative electrodes. There is a need to improve the number of charge and discharge cycles (cycle characteristics) required before a short circuit occurs.

The present invention provides a lithium-sulfur solid-state battery with improved cycle characteristics.

[1] A lithium-sulfur solid state battery comprising: a sulfur positive electrode; a Li-Al alloy negative electrode; and a solid electrolyte, wherein the solid electrolyte is disposed between the sulfur positive electrode and the Li-Al alloy negative electrode; and wherein a molar ratio of lithium to aluminum in the Li-Al alloy negative electrode is 95:5 to 55:45.
[2] The lithium-sulfur solid state battery according to [1], wherein an electrolytic solution or an electrolytic gel is disposed between the Li-Al alloy negative electrode and the solid electrolyte.
[3] The lithium-sulfur solid state battery according to [2], wherein the electrolytic solution or the electrolytic gel contains an ionic liquid.
[4] The lithium-sulfur solid state battery according to [2] or [3], wherein a lithium ion conductive layer is further disposed between the Li-Al alloy negative electrode and the solid electrolyte, and the lithium ion conductive layer contains the electrolytic solution or the electrolytic gel.

The lithium-sulfur solid state battery of the present invention has improved cycle characteristics.

FIG. 1 is a cross-sectional view schematically illustrating an example of a lithium-sulfur solid state battery of the present invention. FIG. 2 is a cross-sectional view schematically showing another example of a lithium-sulfur solid state battery of the present invention. 1 is a bar graph showing the relationship between the number of cycles until charge/discharge becomes impossible and the Li/Al molar ratio of the negative electrode for the half cells produced in Examples 1 and 2 and Comparative Example 1. 10 shows SEM images of the surface of a negative electrode with a Li/Al molar ratio of 90/10 after charge and discharge. 10 shows SEM images of the surface of a negative electrode with a Li/Al molar ratio of 80/20 after charge and discharge. 10 shows SEM images of the surface of a negative electrode with a Li/Al molar ratio of 100/0 after charge and discharge.

<Lithium-sulfur solid-state battery>
A first aspect of the present invention is a battery comprising a sulfur positive electrode, a Li-Al alloy negative electrode, and a solid electrolyte,
the solid electrolyte is disposed between the sulfur positive electrode and the Li-Al alloy negative electrode;
The lithium-sulfur solid state battery has a lithium to aluminum molar ratio of 95:5 to 55:45 in the Li-Al alloy negative electrode.

From the viewpoint of reducing interfacial resistance, the lithium-sulfur solid state battery of this embodiment preferably has an electrolyte solution or electrolyte gel between the Li-Al alloy negative electrode and the solid electrolyte.

Hereinafter, an embodiment of a lithium-sulfur solid state battery will be described with reference to the drawings. For the sake of convenience, the drawings may be partially enlarged or reduced in size.
In the following description, unless otherwise specified, the electrolytic solution may be an electrolyte gel (a gel-like electrolyte containing a solvent and an electrolyte).

[Laminated structure]
The lithium-sulfur solid state battery 1 shown in Fig. 1 includes a sulfur positive electrode 11, a Li-Al alloy negative electrode 12, a solid electrolyte 13, and a lithium ion conductive layer 14, which are stacked in the thickness direction in the order shown in the figure. The lithium ion conductive layer 14 is impregnated with a part or all of an electrolytic solution 15.

For convenience, Figure 1 depicts layers of electrolyte solution 15 on both sides of the lithium ion conductive layer 14; however, the electrolyte solution 15 does not necessarily need to form a layer; it may simply be in a state of seeping out onto the surface of the lithium ion conductive layer 14. Therefore, the second surface 13b of the solid electrolyte 13 and the first surface 14a of the lithium ion conductive layer 14 may not be in contact with each other through a layer of electrolyte solution 15, or they may be in contact with each other without a layer of electrolyte solution 15 interposed. Similarly, the second surface 14b of the lithium ion conductive layer 14 and the first surface 12a of the Li-Al alloy negative electrode 12 may or may not be in contact with each other.

The laminated structure of the lithium-sulfur solid state battery 1 is preferably housed in a container containing an electrolyte solution 15. The sulfur positive electrode 11 and the Li-Al alloy negative electrode 12 are provided with connection terminals to an external circuit (not shown).

[Sulfur positive electrode 11]
The sulfur positive electrode 11 is not particularly limited as long as it contains sulfur and functions as a positive electrode, and sulfur positive electrodes of known lithium-sulfur solid state batteries can be used. A preferred sulfur positive electrode includes, for example, a conductive sheet having a large number of voids, the voids being open to the outside of the conductive sheet, and the conductive sheet containing sulfur and an electrolyte in the voids.

Examples of the conductive sheet include carbon felt and carbon cloth.
The thickness of the conductive sheet can be, for example, 50 μm to 30,000 μm, and the thickness of the sulfur positive electrode can be the same.

The voids may contain a carbon material such as graphite, carbon black, carbon nanotubes, graphene, or fullerene as a conductive additive.
The voids may contain a binder such as polyvinylidene fluoride, polyacrylic acid, or styrene-butadiene rubber.

[Li-Al alloy negative electrode 12]
The Li-Al alloy negative electrode 12 is not particularly limited as long as it contains an alloy of lithium and aluminum and functions as a negative electrode, and a known Li-Al alloy can be used. As the Li-Al alloy negative electrode, a negative electrode made of a Li-Al alloy is preferred, a negative electrode made of a Li-Al alloy having a flat or curved surface is more preferred, and a Li-Al alloy foil is even more preferred.

The thickness of the Li-Al alloy negative electrode can be, for example, 10 μm to 2000 μm, and preferably 100 μm to 1000 μm.

In the Li-Al alloy constituting the negative electrode, the molar ratio of Li atoms to Al atoms is Li:Al=95:5 to 55:45, preferably 95:5 to 70:30, more preferably 90:10 to 80:20, and even more preferably 90:10 to 85:15.
When the molar ratio is equal to or greater than the lower limit of the above range, the cycle characteristics of the lithium-sulfur solid state battery of this embodiment are improved. When the molar ratio is equal to or less than the upper limit of the above range, the output of the lithium-sulfur solid state battery of this embodiment can be increased. Here, output refers to the current density or voltage of the battery. When the Al content of the Li-Al alloy constituting the negative electrode is 50 or more in the above molar ratio, the Li content is too low, making it difficult to increase the battery output. Furthermore, since the Li-Al alloy has a small rate of change in volume expansion and contraction during charge and discharge, it also has the property (small volume change) required for a solid state battery in this respect.

The molar ratio of the Li-Al alloy constituting the negative electrode is preferably uniform throughout the entire volume of the Li-Al alloy, but it is important that it is within this range at least on the surface exposed to the positive electrode. This is because, when Li is repeatedly dissolved and absorbed on the surface during charge-discharge cycles, if the molar ratio on the surface is within the specified range, it is possible to prevent Li from depositing in an amorphous or dendritic form on the negative electrode surface. While the details of this mechanism are unclear, one possible reason is that when the molar ratio is within the specified range, an Al network (framework) is formed in the Li matrix, and after Li is dissolved, the Al network is maintained and easily absorbed back into the Al network, returning it to its original state.

The molar ratio of Li to Al on the surface of the negative electrode is determined as the ratio of the number of Li atoms to Al atoms (content ratio) per unit area (e.g., a circle with a diameter of 100 μm) using X-ray photoelectron spectroscopy (XPS) or energy dispersive X-ray analysis (EDX, EDS). Note that a film may form on the surface of the negative electrode during charge/discharge or exposure to the atmosphere. To accurately determine the molar ratio of Li to Al, it is desirable to remove this film before measurement or to take into account the effect of the film.

[Solid electrolyte 13]
The solid electrolyte 13 may be a solid electrolyte used in a known lithium-sulfur solid state battery.
The solid electrolyte 13 may be a crystalline material, an amorphous material, or a glass material. Examples of the material for the solid electrolyte 13 include an oxide-based material that does not contain sulfide and a sulfide-based material that contains at least sulfide.

_{Examples of} oxide- _based materials include _Li7La3Zr2O12 (abbreviation: _LLZ ) _{, Li2.9PO3.3N0.46 (} abbreviation: LIPON), _{La0.51Li0.34TiO2.94 , Li1.3Al0.3Ti1.7 ( PO4 ) 3 , 50Li4SiO4.50Li3BO3 , Li3.6Si0.6P0.4O4 , Li1.07Al0.69Ti1.46 ( PO4 ) 3 , and Li1.5Al0.5Ge1.5 ( PO4 ) 3 .} The LLZ may be doped with one or more elements such as aluminum, tantalum, niobium, and bismuth.

_{Examples of} sulfide-based materials include _{Li10GeP2S12 ( abbreviation : LGPS} ), _{Li3.25Ge0.25P0.75S4 , 30Li2S.26B2S3.44LiI , 63Li2S.36SiS2.1Li3PO4 , 57Li2S.38SiS2.5Li4SiO4 , 70Li2S.30P2S5 ( abbreviation : LISPS ) , 50Li2S.50GeS2 , Li7P3S11 , and Li3.25P0.95S4 .}

The constituent material of the solid electrolyte 13 is preferably an oxide-based material, since it has high stability in the atmosphere and can produce a highly dense solid electrolyte.
The solid electrolyte 13 may be made of one or more types of constituent materials.

The thickness of the solid electrolyte 13 can be, for example, 10 μm to 1200 μm.

[Lithium ion conductive layer 14]
The main body of the lithium ion conductive layer 14 has many voids (not shown) that extend from the first surface 14 a to the second surface 14 b, and the voids are impregnated with a part or all of the electrolyte solution 15. Therefore, the electrolyte solution containing lithium ions can move between the Li-Al alloy negative electrode 12 and the solid electrolyte 13 via the lithium ion conductive layer 14.

The main body of the lithium ion conductive layer 14 may be, for example, a porous body or a fiber aggregate formed by assembling fibrous materials to form a layer.
The main body may be made of a material that is used in conventional lithium-sulfur solid-state batteries, such as synthetic resin, glass, or paper. Polyimide is preferred as the synthetic resin. The main body may be made of one or more materials.

The thickness of the lithium ion conductive layer 14 can be, for example, 10 μm or more and 100 μm or less.
The main body constituting lithium ion conductive layer 14 may be a single layer or may have a laminated structure of two or more layers.
The thickness of the lithium ion conductive layer 14 made up of multiple layers means the total thickness of the multiple layers.

The amount of electrolyte solution 15 contained in the lithium ion conductive layer 14 is not particularly limited. For example, a guideline is an amount of approximately 80 to 120% of the volume of the voids in the main body of the lithium ion conductive layer 14.

[Electrolyte 15]
The electrolyte solution 15 contains a solvent and lithium ions (Li ⁺ ). The lithium ions may be contained as a salt together with counter ions.

The lithium ions contained in the electrolyte solution 15 are the main ion carriers for the electrochemical reaction, and their content may be the same as that in conventional lithium-sulfur solid-state batteries, or may be at a higher concentration. In conventional lithium-sulfur solid-state batteries, for example, a solvated ionic liquid consisting of a glyme-lithium salt complex serves as a carrier for the lithium ions.

The solvent contained in the electrolytic solution 15 is not particularly limited as long as it can dissolve lithium ions, and preferably contains an ionic liquid.
The inclusion of an ionic liquid in electrolytic solution 15 further improves the battery characteristics of lithium-sulfur solid state battery 1. Electrolytic solution 15 may contain only one type of ionic liquid, or two or more types of ionic liquid.

The ionic liquid used in conventional lithium-sulfur solid-state batteries can be used. Among the various ionic liquids, a solvated ionic liquid consisting of a glyme-lithium salt complex is preferred because it can improve battery characteristics.

Examples of the lithium salt in the glyme-lithium salt complex include lithium bis(fluorosulfonyl)imide (LiN(SO ₂ F) ₂ , abbreviated as LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiN(SO ₂ CF ₃ ) ₂ , abbreviated as LiTFSI).

Examples of glymes in glyme-lithium salt complexes include triethylene glycol dimethyl ether (abbreviated as triglyme) and tetraethylene glycol dimethyl ether (abbreviated as tetraglyme).

Preferred glyme-lithium salt complexes include, for example, triglyme-LiFSI complex, tetraglyme-LiFSI complex, triglyme-LiTFSI complex, and tetraglyme-LiTFSI complex.

The electrolyte solution 15 may contain other additives as long as they do not impair the spirit of the present invention. Examples of other additives include lithium borohydride (LiBH ₄ ), lithium bis(oxalato)borate (LiBOB), etc. These additives can suppress the precipitation of metallic lithium.

<Other embodiments>
The lithium ion conductive layer 14 of the lithium-sulfur solid state battery 1 in Fig. 1 has an optional configuration and can be omitted. The lithium-sulfur solid state battery of this embodiment may have, for example, a layered structure shown in Fig. 2. The same components in Fig. 2 as those in Fig. 1 are denoted by the same reference numerals as those in Fig. 1.
The electrolyte 15 of the lithium-sulfur solid state battery 2 is held between the solid electrolyte 13 and the Li—Al alloy negative electrode 12 by a container (not shown).

<Method of manufacturing a lithium-sulfur solid-state battery>
The lithium-sulfur solid state battery of this embodiment can be manufactured in the same manner as known lithium-sulfur solid state batteries, except that a Li—Al alloy is used as the negative electrode.

The following describes examples, but the present invention is not limited to these examples.

<Manufacturing half cells>
[Example 1]
The electrolyte solution was prepared as follows: 1 g of LiTFSI and 0.774 g of tetraglyme were mixed to obtain 1.774 g of solvated ionic liquid. The molar ratio of LiTFSI to tetraglyme in the electrolyte solution was 1:1.
A main body made of a polyimide sheet (diameter 17 mm, thickness 30 μm) was thoroughly impregnated with 80 μL of the electrolyte solution obtained above to obtain a lithium ion conductive layer.

A coin-shaped half cell was produced by concentrically stacking a negative electrode (diameter 15 mm, thickness 600 μm) made of a Li-Al alloy (Li:Al molar ratio = 90:10), a lithium ion conductive layer containing the above electrolyte, and a positive electrode (diameter 15 mm, thickness 600 μm) made of a Li-Al alloy (Li:Al molar ratio = 90:10) in this order in the thickness direction.

[Example 2]
A coin-shaped half cell was produced in the same manner as in Example 1, except that the Li:Al molar ratio of the Li-Al alloy used in the positive electrode and negative electrode was changed to 80:20.

[Comparative Example 1]
A coin-shaped half cell was produced in the same manner as in Example 1, except that a Li negative electrode made of lithium foil (Li:Al molar ratio = 100:0) was used instead of the negative electrode made of Li-Al alloy.

<Evaluation of cycle characteristics>
Using the half cells produced in Examples 1 and 2 and Comparative Example 1, charge/discharge cycles were repeated at a temperature of 80°C, with one cycle consisting of a discharge rate of 2C and a charge rate of 2C, and the number of cycles until charging or discharging became impossible was measured. The results are shown in a graph in Figure 3.
The measurement results clearly show that the cycle characteristics of the half cells using negative electrodes made of Li-Al alloys in Examples 1 and 2 are superior to those of the half cell of Comparative Example 1 using a negative electrode made of Li foil.
Furthermore, when comparing Example 1 and Example 2, it was confirmed that the negative electrode made of the Li-Al alloy in Example 1 has a high Li content, and therefore not only can a high output be obtained, but the cycle characteristics are also further improved.

<Observation of the surface condition of the negative electrode>
As in Examples 1 and 2 and Comparative Example 1, newly produced half cells were used, and charge/discharge cycles were repeatedly performed at a temperature of 80°C, with one cycle consisting of a discharge rate of 2C and a charge rate of 2C. The charge/discharge cycle was terminated after 90 cycles (i.e., immediately before the half cell of Comparative Example 1 was short-circuited), and the negative electrode was removed from each half cell.
When the surface of the removed negative electrode located on the lithium ion conductive layer side was observed with an SEM, numerous large irregularities presumably formed by the deposition of metallic lithium were confirmed on the surface of the Li negative electrode of Comparative Example 1, as shown in FIG. 6. On the other hand, as shown in FIGS. 4 and 5, the surfaces of the negative electrodes made of the Li-Al alloys of Examples 1 and 2 were smoother than those of Comparative Example 1, with only small irregularities being confirmed. The surface of the negative electrode made of the Li-Al alloy of Example 1 was partially uneven, but was mostly smoother than the surfaces of Example 2 and Comparative Example 1. It is believed that this state of the negative electrode surface reflects the cycle characteristics.

From the above results, it is clear that the lithium-sulfur solid state battery of the present invention has improved cycle characteristics by using a Li-Al alloy with a specific molar ratio as the negative electrode.
It should be noted that Example 2 is a comparative example.

It is common to use half cells in testing secondary batteries, and the test results using the half cells manufactured above are similar to those obtained in tests using the lithium-sulfur solid state battery according to the present invention.

1, 2... Lithium-sulfur solid state battery, 11... sulfur positive electrode, 12... Li-Al alloy negative electrode
13... solid electrolyte, 14... lithium ion conductive layer, 15... electrolyte solution or electrolyte gel

Claims (4)

The battery comprises a sulfur positive electrode, a Li-Al alloy negative electrode, and a solid electrolyte;
the solid electrolyte is disposed between the sulfur positive electrode and the Li-Al alloy negative electrode;
The lithium-sulfur solid state battery, wherein the molar ratio of lithium to aluminum in the Li-Al alloy negative electrode is 95:5 to 85:15 . The lithium-sulfur solid state battery according to claim 1, further comprising an electrolytic solution or electrolytic gel between the Li-Al alloy negative electrode and the solid electrolyte. The lithium-sulfur solid state battery according to claim 2, wherein the electrolyte solution or the electrolyte gel contains an ionic liquid. The lithium-sulfur solid state battery according to claim 2 or 3, further comprising a lithium ion conductive layer disposed between the Li-Al alloy negative electrode and the solid electrolyte, the lithium ion conductive layer containing the electrolyte solution or the electrolyte gel.