JP7313046B2 - lithium sulfur secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a lithium-sulfur secondary battery having a positive electrode containing a sulfur-based positive electrode active material.

With the popularization of mobile phone terminals and the research and development of electric vehicles that deal with environmental problems, high-capacity secondary batteries are in demand. As a secondary battery, a lithium-ion secondary battery is in widespread use.

As a secondary battery with a higher capacity than the lithium ion secondary battery, a lithium-sulfur battery containing sulfur as a positive electrode active material is attracting attention. Sulfur has a theoretical capacity of about 1670 mAh/g, which is about 10 times higher than LiCoO ₂ (about 140 mAh/g), which is a typical positive electrode active material for lithium ion batteries. Sulfur is also low cost and abundant in resources.

As shown in the following (reaction formulas 1 to 5), in a lithium-sulfur battery, at the time of discharge, for example, elemental sulfur (S ₈ ) is sequentially reduced to S ₈ ^2- (1), S ₆ ^2- (2), S ₄ ^2- (3), S ₂ ^2- (3) to become polysulfide anions, and finally Li ₂ S is generated (5). On the other hand, at the negative electrode, lithium in the negative electrode is released as lithium ions, reaches the positive electrode via the electrolyte, and becomes a Li source for Li ₂ S generation.

(Reaction formulas 1 to 5)

Lithium polysulfides composed of polysulfides such as S ₈ ^2- , S ₆ ^2- , S ₄ ^2- , and S ₂ ^2- , which are reduction products of sulfur, and lithium are easily soluble in organic solvents and are also eluted into the battery electrolyte.

During charging, the polysulfide anions eluted into the electrolyte are reduced when they reach the surface of the negative electrode, and are oxidized when they reach the surface of the positive electrode, causing a short circuit due to mass transfer in the electrolyte. Then, the coulomb efficiency (discharge capacity/charge capacity) decreases due to the so-called shuttle effect, in which charging is not performed even if the charging current is continuously applied.

Japanese Patent Application Laid-Open No. 2012-109223 discloses a lithium-sulfur secondary battery using, as an electrolyte, a glyme-based ionic liquid composed of a complex of glyme and a lithium salt. Since the glyme-based ionic liquid has a low solubility of lithium polysulfide, a decrease in coulombic efficiency is prevented. However, a battery with further improved coulombic efficiency has been desired.

Japanese Patent Application Laid-Open No. 2005-79096 discloses a lithium-sulfur secondary battery in which the surface of the positive electrode is coated with a polymer film by coating a monomer containing a non-aqueous electrolyte and then polymerizing it.

JP 2012-109223 A JP 2005-79096 A

An object of the present invention is to provide a lithium-sulfur secondary battery with high energy density and good coulomb efficiency.

A lithium-sulfur secondary battery according to an embodiment of the present invention comprises a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a separator disposed between the positive electrode and the negative electrode , wherein the separator has an ultraviolet curable resin that is a solid electrolyte containing an electrolytic solution, and the electrolytic solution is a glyme-based solvated ionic liquid.
A lithium-sulfur secondary battery according to an embodiment of the present invention includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a separator disposed between the positive electrode and the negative electrode, the separator having an ultraviolet curing resin that is a solid electrolyte containing an electrolytic solution, and the weight ratio of the electrolytic solution in the resin is 75 wt% or more and 95 wt% or less.

According to the embodiments of the present invention, it is possible to provide a lithium-sulfur secondary battery with high energy density and good coulomb efficiency.

1 is a configuration diagram of a lithium-sulfur secondary battery according to an embodiment; FIG. 4 is a photograph of a porous body having a separator function of the lithium-sulfur secondary battery of the first embodiment; FIG. 2 is a schematic cross-sectional view of a porous body having a separator function of the lithium-sulfur secondary battery of the first embodiment;

As shown in FIG. 1, a lithium-sulfur secondary battery 10 (hereinafter also referred to as a "battery") of the embodiment includes a positive electrode 20 containing a sulfur-based positive electrode active material, a separator 30, and a negative electrode 40 containing a negative electrode active material that absorbs and desorbs lithium ions as main constituent elements.

A unit cell of the battery 10 is configured by arranging a positive electrode 20 and a negative electrode 40 with a separator 30 interposed therebetween. That is, coin cell case 51/gasket 52/negative electrode 40/separator 30/positive electrode 20/spacer 53/spring washer 54/top lid 55 are arranged in this order.

<Separator>
As shown in FIGS. 2 and 3, separator 30 includes porous body 31 and resin 32 filling the internal space of porous body 31 . Celmet (registered trademark) used for the porous body 31 having a separator mechanism is aluminum foam having a porosity of 98%, a specific surface area of 8500 m ² /m ³ , a pore diameter of 0.45 mm, and a thickness of 1.4 mm (see FIG. 3). In order to improve the strength and accommodate the coin cell case 51 of a predetermined size, Celmet was cut and compressed to a thickness of about 0.3 mm to be used as the porous body 31 . The compressed porous body 31 had a porosity of 85%.

The porous body 31 preferably has a porosity of 50% or more, has continuous pores, and contains almost no closed pores. Note that the porous body 31 having a porosity of more than 98% has weak mechanical strength and is not easy to handle.

For the porous body 31, foamed metal, metal nonwoven fabric, metal fiber aggregate, metal particle aggregate, or the like is used. For example, the porous body 31 is produced by coating the surface of foamed resin with a metal layer and then decomposing the foamed resin. As the foamed resin, urethane foam is preferable because of its high porosity, high uniformity of cell diameter, and excellent thermal decomposability. When the porous body 31 is made of nickel, the surface of the foamed resin is coated with a nickel-plated film by applying carbon powder or the like to make it conductive or by using a direct plating method. A metal layer, such as an aluminum layer, which is difficult to form by electroplating using an aqueous solution, is formed by electroplating using a molten salt bath.

The porous body 31 may be a glass fiber separator or a dendritic sheet such as a resin nonwoven fabric. Examples of the resin constituting the porous sheet include polyolefins such as polyethylene (PE) and polypropylene (PP); laminates having a three-layer structure of PP/PE/PP, polyimide, and aramid. In particular, polyolefin-based porous separators and glass fiber separators are preferable because they have the property of being chemically stable against organic solvents and can keep their reactivity with the electrolytic solution low.

The thickness of the porous body 31 is not limited, but in the application of a secondary battery for driving a vehicle motor, it is preferable that the total thickness of the porous body 31 is 4 μm to 60 μm with a single layer or multiple layers. Further, it is preferable that the porous body 31 has a maximum pore size of 10 μm or less (generally about 10 μm to 100 nm) and a porosity of 50% to 98%.

The electrolytic solution 35 is a glyme-based solvated ionic liquid in which a lithium salt and glyme as a solvent form a complex. The lithium salt is Li-TFSI (lithium (trifluoromethanesulfonyl)imide). The solvent is 0.5 mol of G1 (monoglyme), mol of G3 (triglyme), and 4 mol of hydrofluoroether (HFE), which is a fluorine solvent, and HF ₂ CF ₂ CH ₂ CO—CF ₂ CF ₂ H, per 1 mol of the lithium salt. HEF is a diluent.

The resin 32 is an ultraviolet curable resin that is a solid electrolyte containing an electrolytic solution 35 in which lithium salt is dissolved. Resin 32 is ethoxylated trimethylolpropane triacrylate (ETPTA).

The weight ratio of the electrolyte 35/ETPTA/polymerization initiator of the resin 32 is 85/15/0.15. Polymerization initiators are, for example, 2-hydroxy 2-methyl-1-phenyl-propan-1-one (HMPP), which forms radicals upon exposure to UV light.

The weight ratio of the electrolytic solution 35 in the resin 32 is preferably 75 wt % or more and 95 wt % or less in order to reduce electric resistance.

Since the resin 32 is liquid because it is not hardened, when it is dropped onto the main surface of the porous body 31, the internal space (void) is filled by interfacial tension.

By using an ultraviolet lamp to irradiate the porous body 31 filled with the resin 32 with ultraviolet rays of 365 nm and 280 mW/cm ² for 20 seconds, ETPTA is polymerized to form the resin 32 as a solid electrolyte.

That is, the battery 10 is manufactured by a manufacturing method including a step of filling an internal space of a flat plate porous body having a first main surface and a second main surface with an uncured ultraviolet curable resin containing an electrolytic solution, and a step of curing and solidifying the resin.

Since the resin 32 is hardened, it is stable and does not swell with the electrolytic solution 35 . In addition, ultraviolet curing is a low-temperature and short-time process compared to thermal curing. Therefore, the electrolytic solution 35 does not evaporate or denature.

In addition, it was almost impractical to prove that the resin 32 is an ultraviolet curable resin because there was no suitable measurement and analysis means. In addition, no words specifying the structure or characteristics relating to the difference from non-ultraviolet curable resins could be found.

As the electrolytic solution 35, various glyme-based solvated ionic liquids in which a lithium salt and glyme form a complex can be used. Also, a plurality of types of glyme may be mixed and used. Solvents used in electrolytes of general lithium batteries may be used as the electrolyte 35 . However, it is preferable to use a glyme-based solvated ionic liquid in which lithium polysulfide has a low solubility as the electrolytic solution 35 in order to improve the coulombic efficiency.

As the resin, acrylic, trimethylolpropane triacrylate, vinyl, non-vinyl, and UV curable resin such as ETPT are used. The polymerization initiator used is one or more selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxylalkylketone, phenyl glyoxylate, benzyl dimethyl ketal, acyl phosphine and α-aminoketone.

<Positive electrode>
The positive electrode 20 uses elemental sulfur (S ₈ ) as a sulfur-based electrode active material, and includes an S/KB (sulfur/ketjenblack) composite in which 50% by weight of elemental sulfur (S) and 50% by weight of Ketjenblack (KB), which is a conductive agent, are mixed at a ratio of 50% by weight. A positive electrode 20 containing S/KB is produced by applying a slurry obtained by adding 10% by weight of polyvinylidene fluoride (PVdF) as a binder to the S/KB composite to a nickel foil (current collector) having a thickness of 20 μm and pressing the slurry. The thickness of the positive electrode 20 is 15 μm to 20 μm, and the weight ratio is S/KB/PVdF=4.5/4.5/1.0.

The positive electrode 20 may have a sulfur-based active material containing at least one selected from the group consisting of elemental sulfur, metal sulfides, metal polysulfides, and organic sulfur compounds. Metal sulfides include lithium polysulfides; _Li2Sn (1≤n≤8), and metal polysulfides include _TSn (T=Ni, Co, Cu, Fe, Mo, _Ti , 1≤n≤4).

The positive electrode 20 may contain a binder and a conductive agent in addition to the sulfur-based active material. Then, a slurry (paste) of these electrode materials is applied to a conductive carrier (current collector) and dried, whereby the carrier carries the electrode material to produce a positive electrode. Current collectors include foils, meshes, expanded grids (expanded metals), punched metals, porous bodies, and the like, made from conductive metals such as aluminum, nickel, copper, and stainless steel.

Celmet (registered trademark), which is foamed aluminum, may be used as the current collector. The positive electrode active material 11B contains elemental sulfur (S), a binder, and a conductive agent. The thickness of the current collector is 5 μm to 30 μm, but is not limited to this range.

The content of the positive electrode active material in the S/KB composite is preferably 50-98% by mass, more preferably 80-98% by mass. If the content of the active material is within the above range, the energy density can be increased, which is preferable. The thickness of the electrode material (the thickness of one side of the coating layer) is preferably 10 μm to 500 μm, more preferably 20 μm to 300 μm, still more preferably 10 μm to 50 μm.

Binders include polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), polyimide (PI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), lithium polyacrylate (PAALi), ethylene oxide, polyalkylene oxides such as ring-opening polymers of monosubstituted epoxides, or mixtures thereof.

<Negative Electrode>
The negative electrode 40 is a metallic lithium plate with a thickness of 200 μm.

The negative electrode may contain one or more negative electrode active materials selected from the group consisting of lithium, lithium alloys, carbon or metal capable of intercalating and deintercalating lithium, lithium/inert sulfur composites, and sodium alloys. The negative electrode active material contained in the negative electrode acts to absorb and desorb lithium ions. As the negative electrode active material, conventionally known negative electrode materials such as metal materials such as lithium titanate, lithium gold, sodium metal, lithium aluminum alloy, lithium tin alloy, lithium silicon alloy, sodium silicon alloy, lithium antimony alloy, natural graphite, artificial graphite, carbon black, acetylene black, graphite, activated carbon, carbon fiber, coke, soft carbon, hard carbon and other crystalline carbon materials and amorphous carbon materials can be used. Among them, it is preferable to use a carbon material, lithium, or a lithium-transition metal composite oxide because a battery having excellent capacity and input/output characteristics can be constructed.

<Evaluation>
The cycle characteristics of the battery 10 of the embodiment produced by the above method were evaluated. For comparison, a battery 110 of a comparative example having a structure similar to that of the battery 10 was also produced and evaluated and analyzed in the same manner. The battery 110 has a separator in which the internal space of the porous body 31 is filled with a liquid electrolytic solution 35 .

The charge/discharge evaluation was performed with a cutoff potential of 1.5 V-3.0 V (vs. Li/Li ⁺ ), a charge/discharge rate of 3.0 C, and a current density of 25 μA/cm ² . Cyclic voltammetry measurements (CV) were performed with a cutoff potential of 1.5 V-3.0 V (vs. Li/Li ⁺ ) and a scanning rate of 0.1 mV/s.

The capacity of battery 10 at the second cycle was 700 mAh/g-S. The second cycle capacity of battery 110 was similarly 700 mAh/g-S.

Battery 10 had a coulombic efficiency of 100% after 10 cycles, while battery 110 had a coulombic efficiency of 80% or less after 10 cycles.

The battery 10 has good coulomb efficiency because the resin 32 that is the solid electrolyte of the separator 30 prevents sulfur from flowing out to the negative electrode 40 .

<Modified Example of First Embodiment>
Since the lithium-sulfur secondary battery 10A of the modified example of the first embodiment is similar to the lithium-sulfur secondary battery 10 and has the same effects, components having the same functions are denoted by the same reference numerals and descriptions thereof are omitted.

The separator 30 of the battery 10A is a sheet-shaped resin 32 having a first principal surface and a second principal surface which is the reverse side of the first principal surface. It is preferable that the thickness of the resin 32, which is a solid electrolyte containing the electrolytic solution 35 and cured by ultraviolet rays, is 10 μm or more and 500 μm or less.

A porous sheet is disposed on at least one of the main surfaces of the sheet-shaped resin 32, and the internal space of the porous sheet may be filled with the electrolyte 35 or the UV-curable resin 32, which is a solid electrolyte containing the electrolyte 35.

In the above description, the battery 10 having a simple structure has been described. However, an assembled battery having a structure in which a plurality of unit cells are laminated like the battery 10, or a battery having a structure in which cells having the same laminated structure are wound and housed in a case may be used. Also, the electrolytic solution 35 may be a gel electrolyte or a solid electrolyte.

The present invention is not limited to the above-described embodiments and the like, and of course various modifications, combinations, and applications are possible without departing from the scope of the invention.

10, 10A Lithium-sulfur secondary battery 11B Positive electrode active material 20 Positive electrode 30 Separator 31 Porous body 32 Resin 35 Electrolyte 40 Negative electrode

Claims (5)

A positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a separator disposed between the positive electrode and the negative electrode,
The separator has an ultraviolet curable resin that is a solid electrolyte containing an electrolytic solution,
A lithium-sulfur secondary battery , wherein the electrolytic solution is a glyme-based solvated ionic liquid . A positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a separator disposed between the positive electrode and the negative electrode,
The separator has an ultraviolet curable resin that is a solid electrolyte containing an electrolytic solution,
A lithium-sulfur secondary battery , wherein the weight ratio of the electrolyte in the resin is 75 wt % or more and 95 wt % or less . The separator includes a porous body and the resin disposed in the internal space of the porous body,
3. The lithium-sulfur secondary battery according to claim 1, wherein said porous body has a porosity of 50% or more and 98% or less. 3. The lithium-sulfur secondary battery according to claim 1, wherein the separator is a sheet made of the resin . 5. The lithium-sulfur secondary battery according to claim 4, wherein a porous body having an internal space in which the electrolyte or the resin is disposed is disposed on at least one of the main surfaces of the sheet.

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