TWI625887B - Positive electrode for lithium-ion secondary battery and lithium-ion secondary battery - Google Patents

Abstract

An object of the present invention is to provide a novel positive electrode and a lithium ion secondary battery composed of the positive electrode, the positive electrode being produced using rubber as an inexpensive material and capable of enhancing the charging and discharging capacity of the lithium ion secondary battery. And recycling. In a lithium ion secondary battery, the positive electrode includes a current collector and an electrode layer formed on a surface of the current collector. The electrode layer includes an active material, a conductive additive, and a thermosetting resin binder that undergoes thermal curing. A sulfur-based positive electrode active material prepared by heat-treating a starting material containing rubber and sulfur in a non-oxidizing atmosphere.

Description

Positive electrode for lithium ion secondary battery and lithium ion secondary battery

The invention relates to a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery made using the positive electrode.

Since a lithium ion secondary battery (a type of non-aqueous electrolyte secondary battery) is lightweight and has a large charging and discharging capacity, the lithium ion secondary battery has been mainly used as a battery for portable electronic devices. Further, practical use of a lithium ion secondary battery as a battery for an automobile such as an electric automobile is expected. Generally, a material containing a rare metal such as cobalt or nickel is used as a positive electrode active material of a lithium ion secondary battery. However, due to the fact that the amount of rare metal distribution is small, it is not always easily available, and it is also expensive, there has been a need to use a positive electrode active material instead of a rare metal material.

A technique using elemental sulfur as a positive electrode active material is known. That is, sulfur is easier to buy and less expensive than rare metals, and has a further advantage: it can make the lithium ion secondary battery have a larger charge and discharge capacity than the current state. For example, it is known that a lithium ion secondary battery using sulfur as a positive electrode active material can achieve a charge and discharge capacity of about 6 times that of a lithium ion secondary battery using lithium cobalt oxide as a general positive electrode material.

However, a lithium ion secondary battery using elemental sulfur as a positive electrode active material has a problem that the battery capacity is deteriorated by repeated charging and discharging. That is, elemental sulfur may generate lithium-containing compounds during discharge, and since the generated compounds are soluble in non-aqueous electrolytes of lithium ion secondary batteries (eg, ethylene carbonate and dimethyl carbonate, etc.), Therefore, the charge and discharge capacity gradually decreases due to the sulfur flowing out of the electrolyte when it is repeatedly charged and discharged.

There is a technology using polyacrylonitrile as a starting material for a sulfur-based positive electrode active material. However, polyacrylonitrile is a relatively expensive material. In addition, in a lithium ion secondary battery using such a positive electrode active material for a positive electrode, battery performance such as charge and discharge capacity, and recyclability depend significantly on the quality of the starting polyacrylonitrile powder (especially particle size) ). Polyacrylonitrile with good quality is more expensive. Therefore, it is difficult to provide an inexpensive lithium-ion secondary battery having a large charge and discharge capacity and excellent cycleability.

It is known to improve the recycling efficiency by preventing sulfur from flowing into the electrolyte. A sulfur-based positive electrode active material prepared by heat-treating sulfur and a diene rubber together is mixed with a fluorine-containing resin as a binder, and The mixture is used as a positive electrode material (WO 2015/050086). However, sufficient recycling has not been achieved.

In a lithium ion secondary battery using a positive electrode material prepared by mixing a sulfur-based positive electrode active material with a fluorine-containing resin as a binder, it is recognized that when the charge and discharge are repeatedly performed, the recyclability decreases. It is caused by the active material being peeled or desorbed from the current collector due to the expansion of the active material, or the conductive path formed by the conductive additive is cut off. An object of the present invention is to provide a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery using the same, which does not have the above-mentioned disadvantages and exhibits excellent Charge and discharge capacity and recycling.

The present inventors have conducted intensive studies to solve the above-mentioned problems, and have therefore found that the sulfur-based positive electrode active material obtained by the following method provides a positive electrode for a lithium ion secondary battery exhibiting excellent characteristics. Electrode: heat treatment of rubber and sulfur, wherein the heat treatment is performed using a thermosetting resin as a binder and curing the resin. The present inventors have conducted further research and have completed the present invention.

That is, the invention relates to:

[1] A positive electrode for a lithium ion secondary battery, the lithium ion secondary battery including a current collector and an electrode layer formed on a surface of the current collector, wherein the electrode layer includes an active material and a conductive additive And a thermosetting resin binder undergoing thermal curing, and the active material includes a sulfur-based positive electrode active material, and the sulfur-based positive electrode active material is heat-treated by starting materials including rubber and sulfur in a non-oxidizing atmosphere To prepare.

[2] The positive electrode for a lithium ion secondary battery according to the above [1], wherein the thermosetting resin binder includes at least one of a polyimide resin and a polyimide-fluorimide resin.

[3] The positive electrode for a lithium ion secondary battery according to the above [1] or [2], wherein the total amount of the active material, the conductive additive, and the thermosetting resin binder is calculated The mixing ratio of the active material, the conductive additive, and the thermosetting resin binder in the electrode layer is 30% to 95% by mass, 2% to 40% by mass, and by mass, respectively. 3% to 30%.

[4] The positive electrode for a lithium ion secondary battery according to any one of [1] to [3] above, wherein a heat treatment temperature when preparing the sulfur-based positive electrode active material is 250 ° C. to 550 ℃.

[5] The positive electrode for a lithium ion secondary battery according to any one of the above [1] to [4], wherein the starting material of the sulfur-based positive electrode active material further contains a vulcanization accelerator .

[6] The positive electrode for a lithium ion secondary battery according to any one of [1] to [5] above, wherein the starting material of the sulfur-based positive electrode active material further includes a conductive carbon material .

[7] The positive electrode for a lithium ion secondary battery according to the above [6], wherein the conductive carbon material is a carbon material having a graphite structure.

[8] The positive electrode for a lithium ion secondary battery according to the above [6] or [7], wherein the sulfur-based positive electrode active material is based on 100 parts by mass of the rubber The starting material includes 250 to 1500 parts by mass of sulfur, 3 to 250 parts by mass of a vulcanization accelerator, and 5 to 50 parts by mass of a conductive carbon material.

[9] The positive electrode for a lithium ion secondary battery according to any one of [1] to [8] above, wherein a total content of sulfur in the sulfur-based positive electrode active material is not less by mass At 50%, and

[10] A lithium ion secondary battery including the positive electrode for a lithium ion secondary battery according to any one of the above [1] to [9].

According to the present invention, a novel positive electrode and a lithium ion secondary battery made using the same can be provided. The positive electrode is produced using rubber as an inexpensive material and can enhance the charging and discharging of the lithium ion secondary battery. Capacity and recycling.

Hereinafter, a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery according to embodiments of the present invention are explained in detail.

＜ Positive electrode for lithium ion secondary battery ＞

A positive electrode for a lithium ion secondary battery includes a current collector and an electrode layer formed on a surface of the current collector.

[Current Collector] A current collector generally used for a positive electrode of a lithium ion secondary battery can be used. Examples of such current collectors include aluminum foil, aluminum mesh, perforated aluminum sheet, aluminum expanded sheet, stainless steel foil, stainless steel mesh, perforated stainless steel sheet, stainless steel expansion sheet, foamed nickel, non-woven nickel fabric, copper foil, Copper mesh, perforated copper plate, copper expansion plate, titanium foil, titanium mesh, non-woven carbon fabric, etc.

In particular, a current collector made of a non-woven carbon fabric and / or a woven carbon fabric formed of carbon having a high degree of graphitization can be suitably used as a current collector for a sulfur-based positive electrode active material because hydrogen-free and sulfur-containing Low reactivity. Examples of the starting material for carbon fibers having a high degree of graphitization include various pitches (ie, by-products of petroleum, coal, coal tar, etc.) as materials for carbon fibers, polyacrylonitrile fibers (PAN), and the like.

[Electrode layer] An electrode layer is an electrode layer formed on the surface of a current collector in such a manner that, in a non-oxidizing atmosphere, at a temperature higher than the curing temperature of the thermosetting resin binder, the active material for the electrode layer is contained, The mixture of the conductive additive and the thermosetting resin binder is heat-treated.

(Active Material) The active material includes a sulfur-based positive electrode active material, which is prepared by heat-treating a starting material containing rubber and sulfur in a non-oxidizing atmosphere. The active material is preferably an active material containing a sulfur-based positive electrode active material.

It is assumed that the sulfur-based positive electrode active material has a dithienoacene structure in the form of a long-chain polymer that is formed by condensation and connection of a thiophene ring and is represented by the following formula (i).

That is, under the conditions of an excitation wavelength λ = 532 nm, a grating of 600 gr / mm, and a resolution of 2 cm ^-1 , a laser Raman microscope (laser) commercially available from Nanophoton Corporation was used. (Raman microscope) The results of Raman-11 analysis of sulfur-based positive electrode active materials by Raman-11 showed peaks around Raman shifts of 500 cm ^-1 , 1250 cm ^-1 , and 1450 cm ^-1 . Although these spectra are different from those in the 6-membered graphite structure called the D-band around 1350 cm ^-1 and called the G-band around 1590 cm ^-1 , when comparing these spectra with those in the literature ( Chemical Physical Chemistry, 2009, 10, 3069-3076) When comparing the dithienothiophene spectra described above, the peak around 500 cm ^-1 is considered to be derived from the in-plane deformation of thiophene, and the peak around 1250 cm ^-1 It is the in-plane deformation of the CC derived from the thiophene ring, and the peak around 1450 cm ^-1 is the CC stretching derived from the thiophene.

It is considered that the sulfur-based positive electrode active material preferably meets the following characteristics. That is, in the FT-IR spectrum (under the following conditions, a Fourier transform infrared spectrophotometer, IR Affinity-1, commercially available from Shimadzu Corporation, was measured by a diffuse reflection method: Resolution: 4 cm ^-1 , cumulative number: 100 times, and measurement range: 400 cm ^-1 to 4000 cm ^-1 ), preferably about 917 cm ^-1 , about 1042 cm ^-1 , 1149 cm ^-1 There are peaks at around 1214 cm ^-1, around 1388 cm ^-1, around 1415 cm ^-1 , and around 1439 cm ^-1 .

<<< Rubber >> Examples of the rubber include diene rubber such as natural rubber, isoprene rubber, and butadiene rubber. The rubber may be used alone or in a combination of two or more thereof. Among them, natural rubber and high-cis polybutadiene rubber are particularly preferred. The two rubbers are apt to have an irregular structure including a curved molecular chain, and the intermolecular force between adjacent molecular chains may be relatively small, and thus hardly cause crystallization. Therefore, not only the structural body of the formula (i), but also the flexibility and processability of the sulfur-based positive electrode active material can be enhanced.

It is considered that the use of a butadiene rubber such as a high-cis polybutadiene rubber is particularly preferable for making the structure of the dithienothiophene body more homogeneous. Here, the high-cis polybutadiene rubber is a polybutadiene rubber having a cis-1,4 bond content of not less than 95% by mass. The sulfur-based positive electrode active material formed by this kind of high-cis polybutadiene rubber is characterized in that a Raman shift also has a peak with a Raman shift of about 1940 cm ^-1 , and is similar to sulfur derived from natural rubber. The positive electrode active materials are different. The peaks around 1400 cm ^-1 and the peaks around 1550 cm ^-1 are very small.

The sulfur-based positive electrode active material derived from natural rubber may be considered to include a graphite structure in a part of a dithienothiophene structure, and thus a part of the structure may be considered to be heterogeneous. At the same time, it can be considered that the sulfur-based positive electrode active material derived from butadiene rubber does not contain such a graphite structure, and its structure is considered to be homogeneous, and therefore the effects mentioned above are particularly excellent.

In an example, the rubber is provided in a non-vulcanized state as a starting material for a sulfur-based positive electrode active material.

<< Sulfur> Various forms of sulfur can be used, such as powdered sulfur, precipitated sulfur, insoluble sulfur, colloidal sulfur, and the like. From the perspective of uniform diffusion of sulfur into rubber, it is suitable to use colloidal sulfur with fine particles.

The compounding ratio of sulfur is preferably not less than 250 parts by mass, and more preferably not less than 300 parts by mass based on 100 parts by mass of the rubber. On the other hand, although there is no upper limit of the compounding ratio of sulfur, the compounding ratio is usually not more than 1500 parts by mass, and preferably not more than 1,000 parts by mass. Since the compounding ratio is within the above-mentioned range, there is a tendency that the charging and discharging capacity and the recyclability can be completely increased. Even if the ratio exceeds 1500 parts by mass, it is difficult to sufficiently improve the charging and discharging capacity or the recyclability, and there is a tendency that a ratio of not more than 1500 parts by mass is advantageous from a cost perspective.

＜＜ Vulcanization accelerator ＞＞

Preferably, the starting material for the sulfur-based positive electrode active material further includes a vulcanization accelerator. This is because a vulcanization accelerator can be used to enhance the recycleability of a lithium ion secondary battery.

Examples of the vulcanization accelerator include a thiourea-based vulcanization accelerator, a guanidine-based vulcanization accelerator, a thiazole-based vulcanization accelerator, a sulfenimide-based vulcanization accelerator, a thiuram-based vulcanization accelerator, and a dithiocarbamate-based (Dithiocarbamate-based) one or more of a vulcanization accelerator and a xanthate-based vulcanization accelerator. Examples of the thiuram compound include tetramethylthiuram disulfide (TT), tetraethylthiuram disulfide (TET), and tetrabutylthiuram disulfide Tetrabutylthiuram disulfide (TBT), tetrakis (2-ethylhexyl) thiuram disulfide (TOT-N), tetramethylthiuram monosulfide (TS) Or dipentamethylenethiuram tetrasulfide (TRA), etc.

Particularly preferred as the thiuram compound are tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide in which any of the terminal substituents is a linear alkyl group. , Tetramethylthiuram monosulfide, etc. A preferred dithiocarbamate compound is its zinc salt. Examples of the zinc salt of dithiocarbamic acid include zinc diethyldithiocarbamate (EZ), zinc dibutyldithiocarbamate (BZ), dimethyldithioamino One or more of zinc dimethyldithiocarbamate (PZ), zinc N-ethyl-N-phenyldithiocarbamate (PX), and the like.

When these compounds are selected and used as a vulcanization accelerator, a sulfur-based positive electrode active material that exhibits an effect that not only enhances the charge and discharge capacity but also enhances the recycleability can be formed.

When the vulcanization accelerator is mixed, the compounding ratio of the vulcanization accelerator is preferably not less than 3 parts by mass, and more preferably not less than 10 parts by mass based on 100 parts by mass of the rubber. On the other hand, the compounding ratio is preferably not more than 250 parts by mass, and more preferably not more than 50 parts by mass. When the compounding ratio of the vulcanization accelerator is within the above-mentioned range, the recycling efficiency of the lithium ion secondary battery tends to be enhanced.

<< Conductive carbon material> The starting material for the sulfur-based positive electrode active material may further include a conductive carbon material. This is because the electrical conductivity can be enhanced by mixing a conductive carbon material. Any of various conductive carbon materials for enhancing the recycling of lithium ion secondary batteries can be used as the conductive carbon material.

Examples of the conductive carbon material include one or more carbon materials having a thick and aromatic ring structure, such as carbon black, graphite, carbon nanotube (CNT), carbon fiber (CF), graphene, fullerene Wait. In addition, a conductive carbon material having a nitrogen-containing heterocyclic ring structure can also be used as the conductive carbon material. Particularly preferred as the conductive carbon material are carbon materials having a graphite structure, and examples thereof include the above-mentioned carbon materials having a thick and aromatic ring structure.

Among them, carbon black is preferable because carbon black is inexpensive and excellent in diffusibility. Moreover, a small amount of carbon nanotubes or graphene can be combined with carbon black. According to such a combination, the recyclability of the lithium ion secondary battery can be further improved without significantly increasing the cost. The combined amount of carbon nanotubes or graphene is preferably not less than 8% by mass and not more than 12% by mass based on the total amount of the conductive carbon material.

When the conductive carbon material is compounded, the compounding ratio of the conductive carbon material is preferably not less than 5 parts by mass, and more preferably not less than 10 parts by mass based on 100 parts by mass of the rubber. On the other hand, the compounding ratio of the conductive carbon material is preferably not more than 50 parts by mass, and more preferably not more than 30 parts by mass. When the compounding ratio is within the mentioned range, it tends to achieve enhanced recycling efficiency by compounding conductive carbon materials, and at the same time sufficiently enhance the charging and discharging capacity of the lithium ion secondary battery.

In addition, it is considered that a vulcanization accelerator is used to enhance the recycleability of a lithium ion secondary battery by mixing a large amount of sulfur into the rubber and preventing the polymer from being cut off during discharge. On the other hand, it can be considered that the conductive carbon material is used to enhance the recyclability of the lithium ion secondary battery by enhancing the conductivity of the sulfur-based positive electrode active material, thereby improving the reactivity with lithium ions.

〈〈 Preparation of sulfur-based positive electrode active material 〉〉 Rubber is used in various applications, is easily available and inexpensive, has high reactivity with sulfur, and has a high content of double bonds for mixing sulfur therein. Therefore, it can be considered that a sulfur-based positive electrode active material in which a large amount of sulfur is mixed into its molecule can be prepared by heat-treating rubber and sulfur at a temperature higher than a temperature at which vulcanization is conventionally performed. In addition, it is assumed that when the sulfur-based positive electrode active material has a structure of the above formula (i), the structure forms a three-dimensional network, thereby sealing and fixing elemental sulfur.

Sulfur-based positive electrode active materials can be prepared by mixing rubber, sulfur, and, if necessary, a vulcanization accelerator with a conductive carbon material at a given mixing ratio to produce a starting material, and at a temperature higher than a conventional vulcanization temperature. The starting material is heat-treated at a temperature.

<< Heat Treatment> It is preferable to perform heat treatment by performing heat treatment in a non-oxidizing atmosphere so as to prevent oxidative degradation or excessive thermal decomposition of the components. Therefore, a sulfur-based positive electrode active material having excellent effects of enhancing the charge and discharge capacity and the recyclability of the lithium ion secondary battery can be formed. A non-oxidizing atmosphere means an atmosphere that is substantially free of oxygen. Examples of heating under a non-oxidizing atmosphere include performing a heat treatment in an inert atmosphere in a silicon dioxide tube filled with an inert gas such as nitrogen or argon. The temperature of the heat treatment is preferably not less than 250 ° C, and more preferably not less than 300 ° C. On the other hand, the heat treatment temperature is preferably not more than 550 ° C, and more preferably not more than 450 ° C. When the heat treatment temperature is within the mentioned range, there is a tendency to prevent decomposition of the starting compound while performing a sufficient sulfurization reaction, which is advantageous in achieving sufficient charge and discharge capacity of the lithium ion secondary battery.

The time period for the heat treatment is preferably 2 hours to 6 hours. When the heat treatment time is within the mentioned range, there is a tendency that the heat treatment can be sufficiently advanced and the component can be prevented from being excessively thermally decomposed.

Mixing and heat treatment of the starting materials can also be performed by performing heat treatment while kneading rubber, sulfur, and the like in a continuous device such as a twin-screw extruder.

<< Removal of Unreacted Sulfur >> The so-called unreacted sulfur caused by the deposition caused by cooling the sublimed sulfur during heat treatment may remain in the prepared sulfur-based positive electrode active material. It is desirable to remove such unreacted sulfur because the unreacted sulfur can cause deterioration in recycling. Examples of a method for removing unreacted sulfur include removal by performing heat treatment under reduced pressure, removal by warm air, removal by washing with a solvent, and the like.

(Crushing and Classification) The sulfur-based positive electrode active material is pulverized into a predetermined particle size, and the sulfur-based positive electrode active material is classified into particles suitable for producing a positive electrode. A preferred median size of the particles is about 5 μm to 25 μm. Note that in the heat treatment method using the twin-screw extruder explained above, since the shearing is performed at the time of kneading, the produced sulfur-based positive electrode active material can also be pulverized at the same time.

<<< Sulfur-based positive electrode active material> In the sulfur-based positive electrode active material prepared by the heat treatment mentioned above, when the total content of sulfur is increased, the recycling efficiency of the lithium ion secondary battery tends to be improved. Therefore, as large a total content of sulfur as possible is preferable. The total content of sulfur obtained by elemental analysis is preferably not less than 50% by mass, more preferably not less than 51% by mass, further preferably not less than 53% by mass, and still more preferably not less than mass At 55%. Furthermore, preferably, the content of hydrogen is not more than 1.6% by mass, especially not more than 1.0% by mass.

In a system in which a carbon material having a graphite structure is compounded as a conductive carbon material, there are cases where the sulfur content is lower than the mentioned range due to the influence of the carbon constituting the carbon material. However, the effect of enhancing the recyclability of the lithium ion secondary battery can still be exhibited. In this case, the sulfur content is preferably not less than 45% by mass in order to maintain the effect of enhancing the recyclability of the lithium ion secondary battery.

<< Conductive Additives> Examples of the conductive additives include vapor grown carbon fiber (VGCF), carbon powder, carbon black (CB), acetylene black (AB), and Ketjen Black ( KETJENBLACK, KB), graphite, fine powders of metals (such as aluminum and titanium) that are stable at the positive electrode potential. One or more of the above may be used as a conductive additive.

(Binder) The binder is used as a binding agent for fixing an active material and a conductive additive to a current collector, and in the embodiment of the present invention, a thermosetting resin is used. By using a thermosetting resin as a binder, not only the active material and the conductive additive are firmly fixed to the current collector, but also the active material can be prevented from being peeled from the current collector and separated from the current collector due to the expansion of the active material, and the conductive additive can be prevented from forming. The conductive path is cut off.

The binder is a binder containing at least one of polyimide resin and polyamido-imide resin. Polyimide resins and polyamido-imide resins have high heat resistance and high adhesive properties. Therefore, when a polyfluorene resin and a polyfluorene-fluorene resin are contained in the binder, a positive electrode having high heat resistance and a long service life can be produced. A polyimide resin has higher heat resistance and adhesion characteristics than a polyimide-amibide resin, and therefore, it is preferable that the binder includes a polyimide resin.

If the binder contains at least one of a polyamidoimide resin and a polyamido-amimine resin even in a small amount, the above-mentioned effects are exhibited depending on the content thereof. The content of at least one of the polyfluorene resin and the polyfluorene-fluorene resin based on the entire binder is preferably not less than 50% by mass, more preferably not less than 70% by mass, and even more preferably Not less than 80% by mass, more preferably not less than 90% by mass. From the standpoint of adhesive properties, preferably, the adhesive is an adhesive composed of at least one of a polyimide resin and a polyimide-fluorimide resin.

(Production of Positive Electrode) A method of producing a positive electrode includes a coating step and a curing step.

The coating step is a step of mixing an active material, a conductive additive, and a binder and adding a solvent or the like thereto as necessary to produce a mixture for an electrode layer in the form of a slurry, and applying the mixture on a surface of a current collector. Any one of the coating methods generally used for producing an electrode for a lithium ion secondary battery can be suitably used as the coating method. Examples of such a coating method include a roll coating method, a dip coating method, a doctor blade coating method, a spray coating method, a curtain coating method, and the like.

Examples of the solvent include one or more of N-methyl-2-pyrrolidone, N, N-dimethylformaldehyde, alcohol, water, and the like.

Regarding the mixing ratio of the active material, the conductive additive, and the binder, it is preferable to mix 30% to 95% of the activity by mass relative to the total content of the three components of the active material, the conductive additive, and the binder. Materials, 2% to 40% by mass of conductive additives, and 3% to 30% by mass of binders. The mixing ratio of the active material is more preferably 50% to 95% by mass, especially 70% to 95% by mass, and the mixing ratio of the conductive additive is more preferably 2% to 30% by mass, especially 2% by mass. 15%, and the compounding ratio of the binder is more preferably 3% to 20% by mass, especially 3% to 15% by mass.

The coating thickness of the electrode layer is preferably 10 μm to 150 μm.

The curing step is a step of heat-treating a mixture for an electrode layer at a temperature higher than a curing temperature of a thermosetting resin, and the mixture is applied on a surface of a current collector. The curing of the thermosetting resin adhesive can be achieved by performing a heat treatment at a temperature higher than the curing temperature of the thermosetting resin to be used. A typical range of such a temperature is, for example, 120 ° C to 350 ° C. Further, it is preferable to perform the heat treatment in a non-oxidizing atmosphere. The non-oxidizing atmosphere is an atmosphere that does not substantially contain oxygen, and is, for example, an atmosphere in which the partial pressure of oxygen is reduced to such an extent that it does not cause an oxidation reaction to progress, or an inert gas atmosphere such as nitrogen and argon.

An example of another method for producing a positive electrode is the following method: kneading a sulfur-based positive electrode active material with a conductive additive, a thermosetting resin binder, and a small amount of a solvent in a mortar or the like; molding the kneaded product into a film form; The film is pressure-bonded to a current collector with a press or the like; and then, a heat treatment is performed at a temperature higher than the curing temperature of the thermosetting resin.

The positive electrode thus obtained includes a current collector and an electrode layer formed on a surface of the current collector, and the electrode layer is an electrode layer composed of an active material, a conductive additive, and a thermosetting thermosetting resin binder. In this positive electrode, the active material is in the form of a powder and is fixed on the surface of the current collector by a binder. The median diameter of the particle diameter of the active material powder is not more than 25 μm.

<Lithium-ion secondary battery> In the embodiment of the present invention, the lithium-ion secondary battery is a lithium-ion secondary battery prepared using the above-mentioned positive electrode, and can be used in addition to the positive electrode by a common method. It is produced using components commonly used in this field (ie, using a negative electrode, an electrolyte, and optionally a separator). The shape of the lithium ion secondary battery is not particularly limited, and the lithium ion secondary battery may have various shapes, such as a cylindrical shape, a laminated type, a coin shape, and a button shape. Lithium-ion secondary batteries have large charge and discharge capacities and are excellent in recyclability.

(Negative Electrode) Examples of the negative electrode material include known metal lithium, carbon-based materials such as graphite, silicon-based materials such as silicon thin films, alloy-based materials such as copper-tin or cobalt-tin, and the like. Among the above-mentioned negative electrode materials, in the case where a carbon-based material, a silicon-based material, an alloy-based material, etc. which do not contain lithium is used, it is advantageous from the following points: it is less likely that a The resulting short circuit between the positive and negative electrodes.

However, in the case where a negative electrode material not containing lithium is used in combination with the positive electrode of the embodiment of the present invention, neither the positive electrode nor the negative electrode contains lithium, and therefore lithium is added to both the negative electrode or the positive electrode Either one or a pre-doping process inserted into both the negative electrode and the positive electrode becomes necessary. For a method of pre-doping lithium, a known method can be used. For example, in the case where the negative electrode is doped with lithium, the following method of inserting lithium can be given: a method of doping electrolytically, in which metallic lithium is used as a counter electrode and then electrochemically doped To assemble a half-cell with hybrid lithium; and to apply a pre-doping method in which lithium is diffused to the electrode by applying a metal lithium foil to the electrode and then leaving the electrode with the applied metal lithium foil in an electrolytic solution Doping. Further, also in the other case where the positive electrode is doped with lithium, the aforementioned method of electrolytically doping may be used.

A silicon-based material as a high-capacity negative electrode material is preferable as a negative electrode material not containing lithium. Among them, a silicon thin film that can make the thickness of the electrode thinner and is advantageous in terms of capacity per volume is particularly preferable.

(Electrolyte) As the electrolyte to be used on the lithium ion secondary battery, those electrolytes in which an alkali metal salt used as the electrolyte is dissolved in an organic solvent can be used. Examples of preferred organic solvents include at least one selected from non-aqueous solvents, such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl ether, γ-butane Lactone and acetonitrile. Examples of usable electrolytes include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiI, LiClO ₄ and the like. The concentration of the electrolyte may be about 0.5 mol / liter to 1.7 mol / liter. Note that the electrolyte is not limited to a liquid form. For example, in the case where the lithium ion secondary battery is a lithium polymer secondary battery, the electrolyte is in a solid form (for example, in the form of a polymer gel)

(Separator) In addition to the negative electrode, the positive electrode, and the electrolyte described above, the lithium ion secondary battery may also be equipped with other members, such as a separator. The separator is interposed between the positive electrode and the negative electrode, thereby not only enabling ions to move between the positive electrode and the negative electrode, but also for preventing the positive electrode and the negative electrode from shorting each other internally. When the lithium ion secondary battery is hermetically sealed, the separator needs to have a function of storing an electrolytic solution. As for the separator, it is preferable to use a thin and microporous or non-woven film made of a material such as polyethylene, polypropylene, polyacrylonitrile, aromatic polyimide, polyimide, cellulose, glass Wait.

<Explanation of Terms> Recyclability is a characteristic in which the charge and discharge capacity of a lithium ion secondary battery decreases with repeated charge and discharge. A lithium ion secondary battery with a small degree of reduction in charge and discharge capacity is excellent in recycling, and a lithium ion secondary battery with a large degree of reduction in charge and discharge capacity has poor cycleability.

DSC stands for differential scanning calorimetry.

The cis-1,4 bond content of the butadiene rubber is the amount (mass%) of the cis-1,4 bond unit in the entire butadiene rubber, and can be obtained by 13C-NMR spectrum.

Examples

＜ Materials for testing ＞

Rubber: High cis-butadiene rubber (UBEPOL (registered trademark) BR150L, available from Ube Industries, Ltd.), cis-1,4 bond content: by mass 98%)

Sulfur: colloidal sulfur, available from Tsurumi Chemical Industry Co., Ltd.

Conductive carbon material: acetylene black (Denka Black, available from Denki Kagaku Kogyo Kabushiki Kaisha)

Vulcanization accelerator: NOCCELOR TS (tetramethylthiuram monosulfide, available from OUCHI SHINKO CHEMICAL INDUSTRIAL CO., LTD.)

Conductive additive: Acetylene black (Denka Black, available from Denka Chemical Co., Ltd.)

Binder 1 (thermosetting resin): polyimide resin (curing temperature: 200 ° C to 250 ° C (DSC))

Binder 2 (thermoplastic resin): polyvinylidene fluoride (available from Wako Pure Chemical Industries, Ltd.)

Example 1

<Preparation of sulfur-based positive electrode active material>

To 100 parts by mass of butadiene rubber, 1,000 parts by mass of sulfur, 25 parts by mass of a vulcanization accelerator, and 20 parts by mass of a conductive carbon material were used, and a kneading tester [MIX-LABO, by Moriyama Company, Ltd. .) Manufacturing] The kneaded mixture is kneaded to prepare a kneaded product. The kneaded product thus obtained was manually cut into small pieces of not more than 3 mm to prepare a starting compound to be subjected to a heat treatment.

(Reaction equipment)

The starting compound is heat-treated using a reaction apparatus 1 as illustrated in FIG. 1. The reaction device 1 includes a reaction container 3 having an outer diameter of 60 mm, an inner diameter of 50 mm, and a height of 300 mm, and is made of quartz glass, which is formed into a flat-bottomed cylindrical shape to face The starting compound 2 is contained and heat-treated; a silicone plug 4 is used to close the upper opening of the reaction vessel 3; an alumina protective tube 5 ("alumina SSA-S", available from Nikkota Corporation, outside Diameter 4 mm, inner diameter 2 mm, and length 250 mm) and gas introduction pipe 6 and exhaust pipe 7 (both of which are "alumina SSA-S", available from NIKKATO), Two tubes with an outer diameter of 6 mm, an inner diameter of 4 mm, and a length of 150 mm), these three tubes penetrate the plug 4; and the electric furnace 8 (crucible furnace, opening width: 80 mm in diameter, heating height: 100 mm) for heating the reaction vessel 3 from the bottom surface.

The alumina protection tube 5 is formed in such a length that its lower portion touches the starting compound 2 contained in the bottom of the reaction container 3 from the plug 4, and a thermocouple 9 is inserted through the inside of the alumina protection tube 5. The alumina protection tube 5 is used as a protection tube for the thermocouple 9. The front end of the thermocouple 9 is inserted into the starting compound 2 while being protected by the closed front end of the alumina protection tube 5, and is used to measure the temperature of the starting compound 2. The output of the thermocouple 9 is input to the temperature controller 10 of the electric furnace 8 as indicated by the solid line arrows in the figure, and the temperature controller 10 is used to control the heating temperature of the electric furnace 8 based on the input from the thermocouple 9.

The gas introduction pipe 6 and the exhaust pipe 7 are formed so that the bottom ends thereof protrude downward from the plug 4 by 3 mm. In addition, the upper part of the reaction vessel 3 protrudes from the electric furnace 8 to be exposed to the atmosphere. Therefore, the steam of sulfur generated from the starting compound due to the heating of the reaction vessel 3 rises to the upper part of the reaction vessel 3 as shown by the long and short dashed arrows in the figure, and is transformed into droplets while being cooled as shown in the figure. Dropping and reflow are indicated by broken arrows in the formula. Therefore, the sulfur in the reaction system does not leak to the outside through the exhaust pipe 7.

The gas introduction pipe 6 is continuously supplied with argon gas from a gas supply system not shown. The exhaust pipe 7 is connected to a trapping tank 12 containing an aqueous solution 11 of sodium hydroxide. The exhaust gas that has moved from the reaction vessel 3 to the outside through the exhaust pipe 7 passes through the aqueous solution 11 of sodium hydroxide in the trap solution tank 12 and is released to the outside. Therefore, even if the exhaust gas contains hydrogen sulfide gas generated by the sulfurization reaction, the hydrogen sulfide gas can be removed from the exhaust gas by neutralization with an aqueous solution of sodium hydroxide.

[Heat treatment step] After continuously supplying argon (Ar) gas from the gas supply system at a flow rate of 80 ml / min to the reaction vessel 3 containing the starting compound 2 in the bottom thereof for 30 minutes, heating was started in the electric furnace 8 . The temperature rise rate was 5 ° C / min. Since gas starts to be generated when the temperature of the starting compound becomes 200 ° C, heating is continuously performed while adjusting the flow rate of argon so that the flow rate of the exhaust gas becomes as constant as possible. When the temperature of the starting compound 2 reached 450 ° C, heat treatment was performed for two hours while maintaining the temperature at 450 ° C. Then, the reaction product was naturally cooled to 25 ° C. under an argon atmosphere while adjusting the flow rate of the argon gas, and the reaction product was taken out from the reaction container 3.

[Removal of Unreacted Sulfur] In order to remove unreacted sulfur (free element sulfur) remaining in the product after the heat treatment step, the following steps were performed. That is, the reaction product is pulverized in a mortar, and 2 g of the pulverized product is put into a glass tube oven and heated at 250 ° C. for three hours while performing vacuum suction to produce unreacted sulfur in which is removed (or A sulfur-based positive electrode active material containing only trace amounts of unreacted sulfur). The temperature rise rate was 10 ° C / min.

<Positive electrode> The sulfur-based positive electrode active material, conductive additive (acetylene black), and binder (polyimide resin) produced above are measured so that the mixing ratio becomes a sulfur-based positive electrode active material: Conductive additive: binder = 87: 3: 10 (% by mass), and put the sulfur-based positive electrode active material, conductive additive, and binder into the container. When using N-methyl-2-pyrrolidone (commercially available from Kishida Chemical Co., Ltd., battery grade) as a diffusing agent to adjust the viscosity of the mixture, the mixture is stirred and rotated / revolved. A mixer (ARE-250, commercially available from Thinky) was mixed to prepare a uniform slurry. The prepared slurry was applied to a 20 μm-thick aluminum foil (current collector) with an applicator (at a thickness of 60 μm), and then heated at 250 ° C. for 3 hours with a dryer to thermally cure the adhesive. Thus, a positive electrode of Example 1 was obtained.

<Negative electrode> A metal lithium foil (manufactured by Honjo Metal Co., Ltd.) having a thickness of 500 μm was punched into a circle having a diameter of 14 mm to prepare a negative electrode.

<Electrolyte> As the electrolyte, a nonaqueous electrolyte in which LiPF ₆ has been dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate is used. The volume ratio of ethylene carbonate to diethyl carbonate was 1: 1. The concentration of LiPF ₆ was 1.0 mol / liter.

<Lithium-ion secondary battery> A separator composed of a polypropylene microporous membrane having a thickness of 25 μm and a glass nonwoven filter having a thickness of 500 μm [Celgard (registered trademark) 2400, by Carl Made by Gede Company], the above positive electrode and negative electrode are laminated in a battery case made of a stainless steel container (for example, for a CR2032 coin cell (a product of HOSEN Co., Ltd.) Component), and an electrolyte solution was added thereto, followed by hermetically sealing with a folding machine to obtain a CR2032 coin-type lithium ion secondary battery. All operations are performed in a dry space.

Example 2 A positive electrode of Example 2 and a lithium ion secondary battery using the same were prepared in the same manner as in Example 1 except that a conductive carbon material and a vulcanization accelerator were not used.

Comparative Example 1 A positive electrode of Comparative Example 1 and a lithium ion secondary battery using the same were prepared in the same manner as in Example 1, except that polyvinylidene fluoride was used as a binder. It should be noted that since polyvinylidene fluoride is a thermoplastic resin, when heating is performed in the final stage of preparing the positive electrode, "heating at 250 ° C for 3 hours" is changed to "heating at 150 ° C for 10 hours" .

Comparative Example 2 A positive electrode of Comparative Example 2 and a positive electrode of Comparative Example 2 were prepared in the same manner as in Example 1 except that the final temperature of the starting material for the sulfur-based positive electrode active material was changed to 600 ° C. A lithium ion secondary battery using the positive electrode.

Comparative Example 3 A positive electrode of Comparative Example 3 and a positive electrode of Comparative Example 3 were prepared in the same manner as in Example 1, except that the final temperature of the starting material for the sulfur-based positive electrode active material was changed to 200 ° C. A lithium ion secondary battery using the positive electrode.

<Measurement of charge and discharge capacity and capacity retention rate> Regarding each lithium ion secondary battery prepared in Examples 1 and 2 and Comparative Example 1, the charge and discharge capacity thereof were measured. Specifically, under the condition of a test temperature of 30 ° C, charging and discharging were repeatedly performed at a current value equal to 250 mA (corresponding to 0.5 C) per 1 g of the positive electrode active material. The discharge terminal voltage is 1.0 V and the charging terminal voltage is 3.0 V. Measure each discharge capacity (mAh / g). The results are shown in FIG. 2.

The discharge capacity (mAh / g) at the second discharge is regarded as the initial capacity. The larger the initial capacity, the larger the charge and discharge capacity of the lithium ion secondary battery, and this was evaluated as preferable. Further, according to the discharge capacity at the tenth discharge DC ₁₀ (mAh / g) and a discharge capacity at the twentieth discharge DC ₂₀ (mAh / g), by the formula (a) to calculate the capacity retention rate (%) . Capacity retention rate (%) = (DC ₂₀ (mAh / g) / DC ₁₀ (mAh / g)) ´ 100 (a)

It can be said that the higher the capacity retention rate, the more excellent the lithium ion secondary battery's recyclability. A capacity retention rate of not less than 90% is considered satisfactory. In addition, the capacity retention ratio is preferably not less than 95%.

<Elemental analysis> Elemental analysis was performed on the sulfur-based positive electrode active materials produced in the examples and comparative examples. Regarding carbon, hydrogen, nitrogen, and sulfur, it is calculated based on the mass quantities measured with the vario MICRO cube, a fully automatic elemental analysis device that can be constructed from Elementar Japan KK. Mass ratio (%) of the total amount of the sulfur-based positive electrode active material.

The results are shown in Table 1. [Table 1]

As can be seen from Table 1, in the lithium ion secondary battery of Example 1 using a polyimide resin as a thermosetting resin as a binder, the discharge capacity at the 20th time (number of cycles: 20) was maintained at 466 mAh / g. At the same time, in the lithium ion secondary battery of Comparative Example 1 using polyvinylidene fluoride as a thermoplastic resin as a binder, the discharge capacity deteriorated from the initial stage, and it was almost at the 20th time (number of cycles: 20). Nothing left.

Therefore, the lithium-ion secondary battery of Example 1 has significantly improved cycleability compared with the lithium-ion secondary battery of Comparative Example 1. In addition, both the discharge capacity and the recyclability of Example 1 were more satisfactory than the discharge capacity and the recyclability of Example 2. It can be considered that this is because more sulfur is mixed into the active material due to the addition of a vulcanization accelerator when the positive electrode active material is prepared, and also because the conductivity of the active material is improved due to the addition of a conductive carbon material.

The mass ratio of sulfur in Example 1 was 55.8%, while the mass ratio of sulfur in Example 2 was 53.7%. In Comparative Example 2, the discharge capacity was hardly exhibited at the 20th time (number of cycles: 20), while the discharge capacity of Comparative Example 3 was 155 mAh / g at the 20th time (number of cycles: 20). Therefore, the discharge capacity of Comparative Example 2 and Comparative Example 3 was largely deteriorated compared to Example 1. It is considered that this is because in Comparative Example 2, the reaction between rubber and sulfur did not proceed sufficiently, and the amount of sulfur incorporated was small, while in Comparative Example 3, the positive electrode active material was decomposed.

1‧‧‧ reaction equipment
2‧‧‧ starting compounds
3‧‧‧ reaction container
4‧‧‧ stop
5‧‧‧ alumina protection tube
6‧‧‧Gas introduction pipe
7‧‧‧ exhaust pipe
8‧‧‧ Electric stove
9‧‧‧ Thermocouple
10‧‧‧Temperature Controller
11‧‧‧Sodium hydroxide in water
12‧‧‧Capturing tank

FIG. 1 is a cross-sectional view schematically illustrating a reaction apparatus used for the production of a sulfur-based positive electrode active material in an example. FIG. 2 is a graph showing the results of cycle charging and discharging in Examples 1 and 2 and Comparative Example 1. FIG.

Claims (8)

A positive electrode for a lithium ion secondary battery includes a current collector and an electrode layer formed on a surface of the current collector, wherein the electrode layer includes 30% to 95% by mass of an active material, and 2% by mass To 40% by mass of a conductive additive and 3% to 30% by mass of a thermosetting resin binder that undergoes thermal curing, and the active material includes a sulfur-based positive electrode active material, and the sulfur-based positive electrode active material is The non-oxidizing atmosphere is prepared by heat-treating a starting material containing rubber and sulfur, wherein the sulfur-based positive electrode active material contains not less than 250 parts by mass of sulfur relative to 100 parts by mass of the rubber, Not more than 1500 parts by mass, the heat treatment temperature is 250 ° C to 550 ° C, and the heat treatment time is 2 hours to 6 hours. The positive electrode for a lithium ion secondary battery according to item 1 of the scope of the patent application, wherein the thermosetting resin binder comprises at least one of a polyimide resin and a polyimide-fluorimide resin. The positive electrode for a lithium ion secondary battery according to item 1 of the scope of patent application, wherein the starting material of the sulfur-based positive electrode active material further includes a vulcanization accelerator. The positive electrode for a lithium ion secondary battery according to item 1 of the scope of patent application, wherein the starting material of the sulfur-based positive electrode active material further includes a conductive carbon material. The positive electrode for a lithium ion secondary battery according to item 4 of the scope of the patent application, wherein the conductive carbon material is a carbon material having a graphite structure. The positive electrode for a lithium ion secondary battery according to item 4 or 5 of the scope of application for a patent, wherein the starting material of the sulfur-based positive electrode active material includes 250 based on 100 parts by mass of the rubber. Part by mass to 1500 parts by mass of the sulfur, 3 parts by mass to 250 parts by mass of a vulcanization accelerator, and 5 parts by mass to 50 parts by mass of the conductive carbon material. The positive electrode for a lithium ion secondary battery according to item 1 of the scope of patent application, wherein the total content of the sulfur in the sulfur-based positive electrode active material is not less than 50% by mass. A lithium ion secondary battery includes the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 7 of the scope of patent application.