Lithium-sulfur battery

Abstract

Polysulfides (Li2S) leach into the electrolyte from the surface of porous carbon materials in small amounts. x The present invention provides a battery with excellent battery characteristics by having a structure that suppresses the occurrence of ( [Solution] A lithium sulfur battery in which a positive electrode and a negative electrode, which use a substance containing sulfide as an active material, are immersed in a non-aqueous electrolyte for charging and discharging, characterized in that the surface of the positive electrode is covered with a capture layer, and the capture layer is an activated carbon layer.

Description

Technical Field

[0001] The present invention relates to a battery structure, and more particularly to a structure of a lithium-sulfur battery.

Background Art

[0002] Conventionally, in lithium-sulfur batteries, when a sulfur-based positive electrode active material is discharged in an electrolytic solution, polysulfide (Li2S x ) eluted from the positive electrode active material is reduced at the negative electrode and oxidized at the positive electrode, which causes a decrease in battery capacity (see, for example, Non-Patent Document 1). In order to prevent such a decrease in battery capacity, a technique for embedding a sulfur-based active material in the pores of a porous carbon material has been developed. However, in the conventional technique of embedding this sulfur-based active material in the pores of the porous carbon material, a phenomenon occurs in which a small amount of polysulfide (Li2S x ) elutes into the electrolytic solution, which causes a problem of deterioration of battery characteristics.

[0003] Further, as a conventional technique close to the present invention, a positive electrode material for lithium-sulfur secondary batteries containing composite activated carbon formed by mixing (a) sulfur-supporting activated carbon and (b) non-sulfur-supporting activated carbon is widely known (see, for example, Patent Document 1), but its configuration and effects are completely different from those of the present invention.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Non-Patent Documents

[0005]

Non-Patent Document 1

[0006] This invention was made in view of the above-mentioned conventional problems, and addresses the slight leaching of polysulfides (Li2S) from the surface of porous carbon materials into the electrolyte. x This invention provides a battery with superior battery characteristics by having a structure that suppresses the occurrence of ( [Means for solving the problem]

[0007] In other words, according to one aspect of the present invention for solving the above problems, the first aspect of the present invention is a lithium sulfur battery in which a positive electrode and a negative electrode, which use a substance containing sulfide as an active material, are immersed in a non-aqueous electrolyte for charging and discharging, characterized in that the surface of the positive electrode is covered with a trapping layer.

[0008] A second aspect of the present invention is a lithium-sulfur battery characterized in that the capture layer in the first aspect is an activated carbon layer. [Effects of the Invention]

[0009] This invention offers significant industrial benefits, including the ability to isolate polysulfides from the negative electrode by capturing them with activated carbon, and the suppression of capacity reduction because the polysulfides captured by the activated carbon are oxidized and reduced in response to changes in potential. [Brief explanation of the drawing]

[0010] [Figure 1]This is a schematic diagram of a battery according to an aspect of the present invention, in which the positive electrode has an activated carbon coating layer (trapping layer) on its surface and the active material is made of a sulfur-containing substance, and the positive electrode active material is sulfur (S8) and the negative electrode is metallic Li, making it a lithium-sulfur battery. [Figure 2] This is a schematic diagram of a conventional lithium-sulfur battery. [Modes for carrying out the invention]

[0011] The present invention will be described below using a battery according to the present invention, specifically a lithium-sulfur battery. A schematic diagram of lithium-sulfur battery 1 is shown in Figure 1, and a schematic diagram of a conventional lithium-sulfur battery 11 is shown in Figure 2. The lithium-sulfur battery 1 according to the embodiment of the present invention shown in Figure 1 has a coating layer 4 made of activated carbon that covers the positive electrode on the negative electrode 3 side of the positive electrode 2, and a separator 14 is provided between the coating layer 4 and the negative electrode 3 to electrically separate them. With this configuration, polysulfides generated at the positive electrode are trapped in the activated carbon, preventing the polysulfides from diffusing into the electrolyte and being reduced on the negative electrode surface.

[0012] In Figures 1 and 2, 1 is a lithium-sulfur battery according to an embodiment of the present invention, and 11 is a conventional lithium-sulfur battery. 2 and 12 are positive electrodes, 2a and 12a are positive electrode active materials, 3 and 13 are negative electrodes, 3a and 13a are negative electrode active materials, 4 is an activated carbon layer (capture layer) of the coating layer which is a feature of the present invention, 5 and 15 are electrolytes, 6 and 16 are lithium polysulfides, 16a is a deposit (lithium polysulfide and its reduced product), and 14 is a conventional separator that electrically separates the positive and negative electrodes.

[0013] The coating layer 4 composed of the above activated carbon does not inhibit the movement of lithium ions, and is constituted by a trapping layer that has the property of trapping polysulfides on its surface and inside and blocking the movement thereof. That is, by disposing the activated carbon trapping layer, which has the property of not inhibiting the movement of lithium ions, between the positive electrode and the negative electrode, particularly on the surface of the positive electrode as a coating layer, it is possible to prevent deterioration of battery characteristics caused by the polysulfides generated in the positive electrode moving to the vicinity of the negative electrode, being reduced on the surface of the negative electrode, and depositing reduced products on the surface of the negative electrode, without inhibiting the diffusion of lithium ions contained in the electrolyte around the positive electrode. The mechanism is that the polysulfides come into contact with the coating layer composed of activated carbon from the positive electrode, are trapped on the surface of the activated carbon through diffusion into the interior, so that the movement of the polysulfides is blocked, and the elution of the polysulfides into the electrolyte is suppressed.

[0014] Incidentally, the thickness of the coating layer composed of the activated carbon layer depends on the shape and particle diameter of the activated carbon used, but requires a thickness of several times or more, more preferably 10 times or more the particle size of the activated carbon particles. The reason is that if the thickness of the coating layer is made smaller than that, the uniformity of the coating layer will decrease or the strength will decrease.

[0015] Note that in the lithium-sulfur battery according to the present invention, sulfur (S) is used for the positive electrode and lithium (Li) is used for the negative electrode as battery active material materials, and a conventional ether-based organic electrolyte is used as the electrolyte used for the description, but there is no particular limitation thereto. [Examples]

[0016] Hereinafter, the "lithium-sulfur battery" according to one embodiment of the present invention will be described in detail by way of Examples, but the present invention is not limited to these Examples. [Examples]

[0017] (Fabrication of positive electrode) Porous carbon (CMK-3 manufactured by ACS Material) and sulfur powder (Sigma-Aldrich) were weighed to a weight ratio of 1:1, mixed in a mortar for 10 minutes, then heated in an electric furnace at 150°C in an air atmosphere for 5 hours to support sulfur on the porous carbon. Thereafter, a conductive auxiliary agent (carbon black, Denka Black manufactured by Denka) and a binder (a solution of PVDF (polyvinylidene fluoride) in NMP (N-methyl-2-pyrrolidone), L#7208 manufactured by Kureha) were added such that the conductive auxiliary agent and the binder accounted for 10 wt% and 5 wt% respectively in the electrode, followed by kneading for 10 minutes. Subsequently, the mixture was dried for 10 minutes in an electric furnace under an air atmosphere at 100°C, then placed in a cylindrical resin mold with a diameter of 10 mm, and pressed and molded using a hand press while the sample was sandwiched between disc-shaped punches to obtain a disc-shaped positive electrode.

[0018] (Arrangement of Coating Layer) Activated carbon powder (MSC-30 manufactured by MC E-Tech) was pulverized for 1 hour using a ball mill, and classified with a classifier so that the maximum particle size was 2 μm. An aqueous suspension of PTFE (polytetrafluoroethylene) (Daikin Polyflon PTFE) was added to the classified activated carbon powder such that PTFE accounted for 5 wt% after drying, the mixture was kneaded for 10 minutes, then placed in a cylindrical resin mold with a diameter of 10 mm, and pressed with a hand press while the sample was sandwiched between disc-shaped punches, thereby producing a disc-shaped activated carbon membrane with a thickness of 10 μm. The activated carbon membrane and the positive electrode were stacked and integrated by pressing using a hand press, and a coating layer made of activated carbon with a thickness of 10 μm was provided on the positive electrode.

[0019] (Battery Fabrication) A battery was fabricated using the positive electrode provided with the coating layer that had been dried for 1 hour in a vacuum electric furnace at 100°C, a negative electrode made of lithium foil with a diameter of 10 mm and a thickness of 0.2 mm, and a separator made of a polyethylene microporous membrane. As the electrolytic solution, 1 M LiPF6 / (EC (ethylene carbonate)-EMC (ethyl methyl carbonate)) (EC:EMC = 3:7 (volume ratio)) was used.

[0020] (Conventional Example) A battery according to the conventional example was fabricated in the same manner as the battery of Example 1, except that the step of arranging the coating layer after fabricating the positive electrode was omitted.

[0021] (Battery Evaluation) The initial capacity of batteries prepared using the methods of Example 1 and the Conventional Example was measured by charging them to 2V at a charge rate of 1C and then discharging them to 0.5V at 1C. After measuring the initial capacity, the battery was charged and discharged 100 times under the same conditions. The discharge capacity after each cycle was measured, and the capacity retention rate was calculated by dividing the discharge capacity after each cycle by the initial capacity.

[0022] The initial capacity was (conventional battery) > (battery of Example 1). It is presumed that the initial capacity is influenced by the difference in the internal resistance of the battery, which is caused by the difference in the distance of the electrolyte between the positive and negative electrodes. Next, the capacity retention rate after cycle charging and discharging was (battery of Example 1) > (conventional battery). It is presumed that the capacity retention rate after this cycle of charging and discharging is influenced by differences in the ease with which lithium polysulfide moves to the negative electrode surface. [Explanation of Symbols]

[0023] 1. The present invention of a thermoplastic sulfur battery 2 Positive electrode 2a Cathode active material 3 negative electrode 3a Anode active material 4 Activated carbon layer / covering layer / trapping layer 5 Electrolyte 6. Lithium polysulfides 11 Conventional lithium-sulfur batteries 12 Positive electrode 12a Cathode active material 13 Negative electrode 13a Anode active material 14 Separator 15 Electrolyte 16 Lithium polysulfide 16a Sediments (Lithium polysulfides and their reduced forms)

Claims

1. In a lithium-sulfur battery in which a positive electrode and a negative electrode, which use a sulfide-containing substance as the active material, are immersed in a non-aqueous electrolyte for charging and discharging, A lithium sulfur battery characterized in that the surface of the positive electrode is covered with a trapping layer.

2. The lithium sulfur battery according to claim 1, characterized in that the capture layer is an activated carbon layer.

Images