JP2013037960A - Manufacturing method and apparatus for sulfur-based positive electrode active material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method and an apparatus for achieving mass production of a sulfur-based positive electrode active material capable of increasing a capacity of a nonaqueous electrolyte secondary battery.SOLUTION: In manufacturing sulfur-based positive electrode active material, while a polyacrylonitrile sheet is continuously being conveyed, sulfur (S) is deposited before heating. At this time, at least a part of sulfur (S) is deposited on the polyacrylonitrile sheet in a liquid manner. Using a sheet-like material as polyacrylonitrile, while the material is continuously being conveyed, sulfur is deposited before heating, thereby facilitating mass production of the sulfur-based positive electrode active material. Deposition of sulfur (S) on the polyacrylonitrile in a liquid manner enables production of the sulfur-based positive electrode active material excellent in battery characteristics.

Description

  The present invention relates to a method for producing a sulfur-based cathode active material that can be used as a cathode active material for a non-aqueous electrolyte secondary battery, and an apparatus for producing a sulfur-based cathode active material by this production method.

  A lithium secondary battery and a lithium ion secondary battery, which are types of non-aqueous electrolyte secondary batteries, are batteries having a large charge / discharge capacity, and are mainly used as batteries for portable electronic devices. Lithium ion secondary batteries are also expected as batteries for electric vehicles. A sodium secondary battery and a sodium ion secondary battery, which are a kind of non-aqueous electrolyte secondary battery, have an advantage that they can be provided at low cost because they use easily available sodium instead of lithium. Sodium has a standard oxidation-reduction potential of 0.33 V lower than lithium and a density of about 80%, but it is considered that the entire cell can develop 70 to 80% of the performance of a lithium ion secondary battery.

  As the positive electrode active material of these non-aqueous electrolyte secondary batteries, those containing rare metals such as cobalt and nickel are generally used. However, since these metals have a small circulation amount and are expensive, in recent years, a positive electrode active material using a substance replacing these rare metals has been demanded.

  As a positive electrode active material of a lithium ion secondary battery, a technique using sulfur is known. By using sulfur as the positive electrode active material, the charge / discharge capacity of the lithium ion secondary battery can be increased. However, in a lithium ion secondary battery using elemental sulfur as the positive electrode active material, a compound of sulfur and lithium is generated during discharge. This compound of sulfur and lithium is soluble in a non-aqueous electrolyte solution (for example, ethylene carbonate, dimethyl carbonate, etc.) of a lithium ion secondary battery. For this reason, a lithium ion secondary battery using sulfur as a positive electrode active material has a problem that, when charging and discharging are repeated, it gradually deteriorates due to elution of sulfur into the electrolytic solution, and the battery capacity decreases. That is, a lithium ion secondary battery using sulfur as a positive electrode active material is inferior in cycle characteristics.

  As a result of intensive studies, the inventors of the present invention have invented a positive electrode active material obtained by heat-treating a mixture of polyacrylonitrile (hereinafter abbreviated as PAN) and sulfur (see Patent Document 1). By using a sulfur-based positive electrode active material (hereinafter abbreviated as sulfur-modified PAN) made of PAN and sulfur as the positive electrode active material, the charge / discharge capacity of the lithium ion secondary battery is increased and the cycle characteristics are also improved. improves.

International Publication No. 2010/044437

  By the way, the above-mentioned sulfur-modified PAN is bulky and inferior in handleability. For this reason, the manufacturing method which can mass-produce sulfur modified PAN easily is desired.

  In recent years, a non-aqueous electrolyte secondary battery having a higher capacity than conventional ones has been desired. And the positive electrode active material which can further improve the capacity | capacitance of a nonaqueous electrolyte secondary battery is also desired.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for easily mass-producing sulfur-modified PAN capable of increasing the capacity of a nonaqueous electrolyte secondary battery, and an apparatus therefor. .

  As a result of intensive studies, the inventors of the present invention have found that sulfur-modified PAN having excellent battery characteristics can be produced by attaching sulfur to PAN in a liquid state when producing sulfur-modified PAN. Further, it has been found that sulfur-modified PAN can be easily mass-produced by continuously adhering sulfur to the PAN sheet and heating the sheet-like PAN, that is, the PAN sheet.

That is, the method for producing a sulfur-based positive electrode active material of the present invention that solves the above-mentioned problems uses polyacrylonitrile and sulfur (S) as raw materials, and a carbon skeleton derived from polyacrylonitrile and sulfur (S) bonded to the carbon skeleton. A sulfur-based positive electrode active material comprising:
Including a sulfur modification step of heating the polyacrylonitrile sheet with sulfur (S) attached thereto while continuously conveying the polyacrylonitrile sheet,
In the sulfur modification step, at least a part of the sulfur (S) is attached in liquid form to the polyacrylonitrile sheet.

The apparatus for producing a sulfur-based positive electrode active material of the present invention that solves the above-mentioned problems uses a polyacrylonitrile and sulfur (S) as raw materials, a carbon skeleton derived from polyacrylonitrile, and sulfur (S) bonded to the carbon skeleton. An apparatus for producing a sulfur-based positive electrode active material comprising:
Conveying means for continuously conveying the polyacrylonitrile sheet;
Sulfur supply means for attaching sulfur (S) to the polyacrylonitrile sheet being conveyed;
Heating means for heating the polyacrylonitrile sheet attached to sulfur,
The sulfur supply means is characterized in that at least a part of the sulfur (S) is adhered in liquid form to the polyacrylonitrile sheet.

  Hereinafter, unless otherwise specified, the production method of the sulfur-based positive electrode active material of the present invention is simply abbreviated as the production method of the present invention, and the production device of the sulfur-based positive electrode active material of the present invention is simply referred to as the production device of the present invention. Abbreviated.

  By making the PAN into a sheet shape such as a nonwoven fabric or a woven fabric, the bulky PAN can be easily handled. By attaching sulfur to such a PAN sheet and heating it, sulfur and PAN are combined to obtain sulfur-modified PAN. At this time, sulfur-modified PAN can be easily mass-produced by attaching sulfur while continuously conveying the PAN sheet. At this time, sulfur-modified PAN having a high sulfur content can be obtained by using sulfur in a liquid state. This is because liquid sulfur has a large contact area with PAN, and is thus easily bonded to PAN. A non-aqueous electrolyte secondary battery using the sulfur-modified PAN produced by this method as a positive electrode active material has a high capacity. Therefore, according to the production method and apparatus of the present invention, it is possible to easily mass-produce sulfur-modified PAN that can increase the capacity of the nonaqueous electrolyte secondary battery.

It is a graph showing the result of having carried out X-ray diffraction of sulfur modified PAN sulfur modified PAN. It is a graph showing the result of the Raman spectrum analysis of sulfur-modified PAN. It is a graph. FIG. 3 is an explanatory diagram schematically illustrating the manufacturing apparatus of Example 1. 3 is an enlarged view of a main part schematically showing a sulfur modification means in the manufacturing apparatus of Example 1. FIG. 6 is an enlarged view of a main part schematically showing a manufacturing apparatus of Example 2. FIG. 10 is an enlarged view of a main part schematically showing a manufacturing apparatus of Example 3. FIG. 2 is a charge / discharge curve of a lithium secondary battery using the sulfur-modified PAN of Example 1 as a positive electrode. 2 is a cycle characteristic of a lithium secondary battery using the sulfur-modified PAN of Example 1 as a positive electrode. It is a charging / discharging curve of the lithium secondary battery which made the sulfur modified PAN of the comparative example the positive electrode. It is a cycle characteristic of the lithium secondary battery which made the sulfur modified PAN of the comparative example the positive electrode.

(Sulfur-based positive electrode active material)
The sulfur-based positive electrode active material produced by the production method and production apparatus of the present invention is the above-described sulfur-modified PAN, and comprises a carbon skeleton derived from PAN and sulfur (S) bonded to the carbon skeleton.

The sulfur-modified PAN is the same as that disclosed in Patent Document 1 above. PAN as a material for sulfur-modified PAN can be made into various shapes such as a fiber and a powder. In any case, it is preferable to use those having a mass average molecular weight of about 10 ^{4 to} 3 × 10 ⁵ . Further, the particle diameter of PAN (fiber diameter in the case of fibers) is preferably about 0.1 to 50 μm, more preferably about 0.1 to 10 μm, when observed with an electron microscope. If the molecular weight and particle diameter (or fiber diameter) of PAN are within these ranges, the contact area between PAN and sulfur can be increased, and PAN and sulfur can be reacted with high reliability. For this reason, the elution of sulfur to the electrolytic solution can be more reliably suppressed.

  By using sulfur-modified PAN as the positive electrode active material of the non-aqueous electrolyte secondary battery, the high capacity inherent in sulfur can be maintained and the elution of sulfur into the electrolyte is suppressed, so that the cycle characteristics are greatly improved. . This is probably because sulfur does not exist as a simple substance in the sulfur-modified PAN, but exists in a stable state combined with PAN. In the production method disclosed in Patent Document 1, sulfur is heat-treated together with PAN. When PAN is heated, it is considered that PAN is three-dimensionally cross-linked and closed while forming a condensed ring (mainly a six-membered ring). For this reason, it is considered that sulfur is present in the sulfur-modified PAN in a state of being bonded to the PAN that has progressed in the ring closure. By combining PAN and sulfur, elution of sulfur into the electrolyte can be suppressed, and cycle characteristics can be improved.

  When powdery sulfur is used for sulfur-modified PAN, the particle size of sulfur is not particularly limited, but when classified using a sieve, it does not pass through a sieve having a sieve opening of 40 μm and 150 μm. Those having a size within a range of passing through a sieve of 10 μm are preferable, and those having a size of not passing through a sieve having a sieve opening of 40 μm and passing through a sieve of 100 μm are more preferable.

  The compounding ratio of PAN and sulfur used for the sulfur-modified PAN is not particularly limited, but is preferably 1: 0.5 to 1:10 by mass ratio and 1: 0.5 to 1: 7. Is more preferably 1: 2 to 1: 5.

  As a result of elemental analysis, sulfur-modified PAN contains carbon, nitrogen, and sulfur, and may contain small amounts of oxygen and hydrogen. Further, as shown in FIG. 1, as a result of X-ray diffraction of sulfur-modified PAN with CuKα rays, only a broad peak having a peak position in the vicinity of 25 ° was confirmed in the diffraction angle (2θ) range of 20-30 °. It was. For reference, X-ray diffraction was measured by X-ray diffraction using CuKα rays with a powder X-ray diffractometer (manufactured by MAC Science, model number: M06XCE). The measurement conditions were voltage: 40 kV, current: 100 mA, scan speed: 4 ° / min, sampling: 0.02 °, number of integrations: 1, measurement range: diffraction angle (2θ) 10 ° -60 °.

  Furthermore, the mass loss by thermogravimetric analysis when sulfur-modified PAN is heated from room temperature to 900 ° C. at a rate of temperature increase of 20 ° C./min is 10% or less at 400 ° C. On the other hand, when a mixture of sulfur powder and PAN powder is heated under the same conditions, a mass decrease is observed from around 120 ° C., and when the temperature is 200 ° C. or higher, a large mass loss due to the disappearance of sulfur is recognized.

  That is, in sulfur-modified PAN, it is considered that sulfur does not exist as a simple substance, but exists in a state of being combined with PAN that has advanced ring closure.

An example of the Raman spectrum of sulfur-modified PAN is shown in FIG. In the Raman spectrum shown in FIG. 2, there are major peak near 1331cm ^-1 of Raman shift, ^{and, 1548cm} -1 in the range of ^{200cm -1 ~1800cm -1, 939cm -1,} 479cm -1, 381cm -1, There is a peak near 317 cm ⁻¹ . The above-described Raman shift peak is observed at the same position even when the ratio of elemental sulfur to PAN is changed. Thus, these peaks characterize sulfur-modified PAN. Each of the above-described peaks exists in a range of approximately ± 8 cm ⁻¹ with the above-described peak position as the center. In the present specification, the “main peak” refers to a peak having the maximum peak height among all peaks appearing in the Raman spectrum. In the Raman spectrum, the number of peaks may change or the position of the peak top may be shifted due to the difference in the wavelength or resolution of incident light. For example, the peak of 1548cm ^-1, which was described above, in some cases deviate substantially within a range of ± ^{8 cm -1} centered at 1531cm ^-1.

For reference, the Raman shift described above was measured with RMP-320 (excitation wavelength λ = 532 nm, grating: 1800 gr / mm, resolution: 3 cm ⁻¹ ) manufactured by JASCO Corporation.

(Method for producing sulfur-modified PAN)
The production method of the present invention is a method for producing the above-described sulfur-modified PAN. The production apparatus of the present invention is an apparatus for producing the above-described sulfur-modified PAN by the production method of the present invention.

In the production method of the present invention, a sheet-like material (PAN sheet) is used as the PAN. The PAN sheet may be a sheet shape. The sheet form referred to here does not limit the thickness, hardness, width, length, or the like. For example, the thickness of the PAN sheet is not particularly limited, but is usually about 20 μm to 200 μm. In particular, as the battery capacity per unit area upon the electrode becomes 0.5mAh ^/ cm ² ~10mAh ^/ cm 2 or so, and weighs a basis weight per unit area of ^6g / m 2 ^~110g / m PAN sheet is preferably a ² in the range of about, as battery capacity is 3mAh ^/ cm ² ~7mAh ^/ cm 2 or so, more is PAN sheet basis weight is ^30g / ^{m 2 ~70g} / m 2 range of about preferable.

  The PAN sheet is preferably a porous sheet in order to contact sulfur uniformly. Porous PAN sheets include non-woven sheets and woven sheets. Especially, the porosity, pore diameter, etc. can be easily controlled, and the reaction rate of polyacrylonitrile and sulfur with the amount of sulfur filling corresponding to the porosity. The nonwoven fabric sheet is preferable in that it can control the electric capacity. In view of handling at the time of manufacture, the PAN sheet is preferably flexible. Also in this respect, the PAN sheet is preferably a non-woven sheet or a woven sheet. The porosity of the porous PAN sheet is preferably about 40 to 90% by volume, and more preferably about 50 to 90% by volume. In addition, the porosity said here is a ratio of the volume of pores (void portions partitioned by PAN fibers) when the volume of the porous PAN sheet is 100% by volume. The pore diameter is preferably about 1 μm to 500 μm, more preferably about 1 μm to 300 μm.

  Furthermore, in consideration of handleability and bulkiness, it is preferable to use a PAN sheet wound in a roll shape, but other shapes may be used. For example, the accordion pleats may be folded back and stacked.

  In addition, the PAN sheet may contain a conductive additive and a conductive material described later. Further, the PAN sheet may be integrated with a sheet-like current collector, that is, a current collector sheet, before the sulfur modification step described later. For example, a current collector sheet and a PAN sheet may be laminated to form a composite sheet, or PAN may be dissolved or dispersed in a solvent to form a liquid, slurry, or solid PAN. It may be attached to the current collector sheet by dipping, dropping, spraying, applying, depositing, etc., and integrating.

  The material of the current collector sheet is not particularly limited, and a material generally used for a positive electrode for a nonaqueous electrolyte secondary battery may be used. Specifically, aluminum foil, aluminum mesh, punched aluminum sheet, aluminum expanded sheet, stainless steel foil, stainless steel mesh, punched stainless steel sheet, stainless steel expanded sheet, nickel foam, nickel nonwoven fabric, copper foil, copper mesh, punching Examples include a copper sheet, a copper expanded sheet, a titanium foil, a titanium mesh, and carbon paper (nonwoven fabric / woven fabric). Among these, a carbon paper current collector made of carbon having a high graphitization degree is suitable as a current collector for a positive electrode in a non-aqueous electrolyte secondary battery because it does not contain hydrogen and has low reactivity with sulfur. As a raw material for carbon fiber having a high degree of graphitization, various pitches (that is, by-products such as petroleum, coal, coal tar, etc.), PAN fibers, etc., which are carbon fiber materials can be used.

  The production method of the present invention includes a sulfur modification step. The sulfur modification step is a step in which sulfur is attached to the sulfur PAN sheet while the PAN sheet is continuously conveyed and heated. The sulfur modification step includes at least two steps: a step of attaching sulfur to the PAN sheet (referred to as a sulfur supply step) and a step of heating the PAN sheet attached with the sulfur element (referred to as a heat treatment step). The heat treatment step may be performed after the sulfur supply step or may be performed simultaneously with the sulfur supply step.

  In the sulfur supply step, at least a part of sulfur may be attached in liquid form to the PAN sheet. For example, the PAN sheet may be immersed in liquid sulfur, or liquid sulfur is dropped and sprayed on the PAN sheet. It may be equal. Further, a sulfur slurry in which solid sulfur (solid phase) and liquid sulfur (liquid phase) are mixed may be applied to the PAN sheet. Furthermore, the PAN sheet may be exposed to a mixed atmosphere in which liquid sulfur (liquid phase) and gaseous sulfur (gas phase) are mixed. The liquid phase in the mixed atmosphere may contain a liquid inert to sulfur and PAN. The same applies to the gas phase.

  By heating sulfur attached to the PAN sheet and the PAN sheet in the heat treatment step (hereinafter referred to as a sulfur-attached PAN sheet), sulfur and PAN are combined to obtain sulfur-modified PAN. The heat treatment step may be performed in an open system, but in order to suppress the dissipation of sulfur vapor, it is preferably performed in a semi-enclosed system in which only the entrance / exit of the PAN sheet is open. In addition, the atmosphere in which the heat treatment step is performed is not particularly limited, but it is preferably performed in an atmosphere that does not hinder the binding of PAN and sulfur (for example, an atmosphere containing no hydrogen or a non-oxidizing atmosphere). For example, when hydrogen is present in the atmosphere, sulfur in the reaction system reacts with hydrogen to form hydrogen sulfide, so that sulfur in the reaction system may be lost. Further, it is considered that by performing heat treatment in a non-oxidizing atmosphere, vapor-state sulfur reacts with PAN simultaneously with the PAN ring-closing reaction, and PAN modified with sulfur is obtained. The non-oxidizing atmosphere referred to here includes a reduced pressure state in which the oxygen concentration is low enough not to cause an oxidation reaction, an inert gas atmosphere such as nitrogen or argon, a sulfur gas atmosphere, and the like.

  What is necessary is just to set suitably the heating time of the sulfur adhesion PAN sheet | seat in a heat processing process according to heating temperature, and it does not specifically limit it. The preferable heating temperature described above may be a temperature at which the coupling between sulfur and PAN proceeds and the auxiliary material such as the conductive material and the current collector does not deteriorate. Specifically, the heating temperature is preferably 250 to 500 ° C, more preferably 250 to 450 ° C, and further preferably 250 to 400 ° C.

  In the heat treatment step, sulfur is preferably refluxed. In this case, what is necessary is just to heat a sulfur adhesion PAN sheet so that a part of sulfur turns into gas and a part turns into liquid. In other words, the temperature of the sulfur-attached PAN sheet may be a temperature equal to or higher than the temperature at which sulfur vaporizes. Vaporization here refers to the phase change of sulfur from a liquid or solid to a gas, and may be any of boiling, evaporation, and sublimation. For reference, the melting point of α sulfur (orthogonal sulfur, which is the most stable structure near room temperature) is 112.8 ° C, the melting point of β sulfur (monoclinic sulfur) is 119.6 ° C, and γ sulfur (monoclinic sulfur). ) Has a melting point of 106.8 ° C. The boiling point of sulfur is 444.7 ° C. By the way, since the vapor pressure of sulfur is high, generation | occurrence | production of sulfur vapor | steam can also be confirmed visually when the temperature of a sulfur adhesion PAN sheet will be 150 degreeC or more. Therefore, if the temperature of the sulfur-attached PAN sheet is 150 ° C. or higher, sulfur can be refluxed. In addition, what is necessary is just to recirculate | reflux sulfur using the recirculation | reflux apparatus of a known structure when recirculating | refluxing sulfur in a heat treatment process.

  A PAN sheet in contact with sulfur is a flexible sheet (hereinafter, referred to as Teflon (registered trademark) sheet, aluminum sheet, polyimide sheet, stainless steel sheet, platinum sheet, etc.) A method of heating at the same time as pressing by a hot press method in a state of being sandwiched from both sides with a “sealing sheet”) may be employed. In this case, liquid sulfur is attached to the PAN sheet, and then the laminated sheet of the PAN sheet and the sealing sheet is heated by sandwiching the PAN sheet to which sulfur has been attached between the sealing sheets.

  The heating time in the hot press method is preferably about 5 minutes to 2 hours, more preferably about 10 minutes to 1 hour. In this method, by pressing a PAN sheet in contact with sulfur between the sealing sheets and pressurizing, the air in the space sandwiched between the sealing sheets is removed, and a substantially sealed non-oxidizing atmosphere and Become. In addition, the PAN sheet and sulfur are sufficiently adhered by being pressed. By heating in this state, the target sheet-like sulfur-modified PAN can be obtained. The hot press method is a simple method and is suitable for mass production because a sheet-like sulfur-modified PAN can be produced in a short time.

There is no limitation in pressing pressure in the hot press method, by sandwiching the both sides sheet, as the PAN sheet being in contact with sulfur can be sufficiently ^{sealed, 100kgf / cm 2 ~1500kgf / cm} 2 about the press preferably to ^pressure, and more preferably to 200kgf / cm ² ~1000kgf / cm 2 about the press pressure. According to the hot press method, since the target sheet-like sulfur-modified PAN can be obtained in a very short time of 5 minutes to 2 hours, the target product can be obtained very efficiently.

  In addition, depending on the apparatus used for sulfur-modifying the PAN sheet, the above-described sealing sheet may be unnecessary. For example, if the heat treatment step in the sulfur modification step can be performed in a non-oxidizing atmosphere, the sheet-like sulfur similar to that obtained by the above hot press method can be obtained simply by heating while pressing the PAN sheet to which sulfur has adhered. Modified PAN can be obtained.

  Even when the amount of sulfur in the sulfur-attached PAN sheet is excessive, a sufficient amount of sulfur can be taken into the PAN in the heat treatment step. For this reason, when adding sulfur excessively with respect to PAN, the bad influence by the elemental sulfur mentioned above can be suppressed by removing elemental sulfur from the to-be-processed body after a heat treatment process. Specifically, when the mixing ratio of PAN and sulfur in the sulfur-attached PAN sheet is 1: 2 to 1:10 by mass ratio, the object to be treated after the heat treatment step is at 200 ° C. to 250 ° C. while reducing the pressure. By heating (single sulfur removal step), it is possible to suppress an adverse effect due to the remaining single sulfur while taking a sufficient amount of sulfur into the PAN. When the single sulfur removal step is not performed on the target object after the heat treatment step, this target object may be used as it is as the sulfur-modified PAN. Moreover, what is necessary is just to use the to-be-processed body after a single sulfur removal process as sulfur modified PAN, when performing the single-piece | unit sulfur removal process to the to-be-processed body after a heat treatment process. The time for the elemental sulfur removal step is not particularly limited, but is preferably about 1 to 6 hours.

(Sulfur modified PAN production equipment)
A manufacturing apparatus for manufacturing a sheet-like sulfur-modified PAN by the manufacturing method of the present invention described above includes at least a conveying means for conveying a PAN sheet, a sulfur supplying means for performing a sulfur supplying process, and a heat treatment process. Heating means for performing the above.

  The conveying means for conveying the PAN sheet is not particularly limited as long as the PAN sheet can be continuously conveyed. For example, a plurality of pairs of rollers that rotate while sandwiching the PAN sheet may be used. When the PAN sheet is in the form of a roll, the PAN sheet may be conveyed while being pushed out by feeding the PAN sheet with a rotating shaft or a bearing for the rotating shaft that rotates while holding the core of the rolled PAN sheet. Alternatively, the PAN sheet (sheet-like sulfur-modified PAN) after the heat treatment step is wound around a rotating shaft in a roll shape, and the sheet-like sulfur-modified PAN is wound around the rotating shaft or bearing for the roll, thereby You may convey while pulling. However, the present invention is not limited to this, and various known conveying means may be used.

  The sulfur supply means only needs to allow at least a part of the liquid sulfur to adhere to the PAN sheet. For example, various types of spray such as a spray for spraying liquid sulfur and a liquid tank for storing liquid sulfur. Can be used. When performing a sulfur supply process and a heat treatment process simultaneously, for example, a liquid tank containing liquid sulfur may be heated and the PAN sheet may be heated together with liquid sulfur in the liquid tank. In the sulfur modification step, at least a part of sulfur attached to the PAN sheet may be liquid. For this reason, the sulfur supply means may attach solid sulfur and / or gaseous sulfur to the PAN sheet together with liquid sulfur.

  As the heating means, a known means capable of heating the PAN sheet to about 400 ° C. can be used.

(Non-aqueous electrolyte secondary battery)
Hereinafter, the structure of the nonaqueous electrolyte secondary battery using the sulfur-modified PAN manufactured by the above-described manufacturing method of the present invention as a positive electrode active material will be described.

[Positive electrode]
The positive electrode in the nonaqueous electrolyte secondary battery may contain a binder, a conductive auxiliary agent, and the like in addition to the sulfur-modified PAN as the positive electrode active material. For example, as described above, when the PAN sheet is integrated with the current collector sheet and subjected to the sulfur modification step, a composite of the sulfur modified PAN and the current collector in the form of a sheet is obtained. This composite can be used as a positive electrode as it is. In this case, since the binder etc. which are mentioned later become unnecessary, the energy density of a nonaqueous electrolyte secondary battery improves.

  In addition, you may produce a positive electrode by apply | coating the positive electrode mixture which mixed the conductive support agent, the binder, and the solvent to what crushed sheet-like sulfur-modified PAN to a collector. In addition, as another method, the positive electrode mixture described above may be kneaded with a mortar or a press and formed into a film shape, and the positive electrode may be manufactured by pressing the film-like mixture onto a current collector with a press machine or the like. it can. The current collector has been described above.

  Examples of the conductive assistant include vapor grown carbon fiber (VGCF), carbon powder, carbon black (CB), acetylene black (AB), ketjen black (KB), graphite, positive electrodes such as aluminum and titanium. Examples thereof include fine metal powders stable in potential.

  As the binder, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamideimide (PAI), carboxymethylcellulose (CMC), polychlorinated Examples include vinyl (PVC), methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), and polypropylene (PP).

  Examples of the solvent include N-methyl-2-pyrrolidone, N, N-dimethylformaldehyde, alcohol, water and the like. These conductive assistants, binders and solvents may be used as a mixture of plural kinds. The blending amount of these materials is not particularly limited. For example, it is preferable to blend about 20 to 100 parts by mass of the conductive additive and about 10 to 20 parts by mass of the binder with respect to 100 parts by mass of the sulfur-modified PAN.

  In addition, you may mix | blend conductive materials other than a conductive support agent with a positive mix. The conductive material refers to a material that exhibits high electrical conductivity or that can greatly improve the ionic conductivity of the positive electrode. By including a conductive material in the positive electrode, the electrical conductivity and / or ionic conductivity of the entire positive electrode can be improved, and the discharge rate characteristics of the nonaqueous electrolyte secondary battery can be improved. As the material for the conductive material (conductive material), it is preferable to use a material that exhibits the above function in the state of sulfide. This is because the function of the conductive material is not impaired even if it is sulfurized by sulfur, which is a raw material of sulfur-modified PAN. In this case, the conductive material may be mixed into the PAN sheet together with PAN.

  As the conductive material, at least one metal selected from the group consisting of a fourth periodic metal, a fifth periodic metal, a sixth periodic metal, and a rare earth element, or a sulfide thereof can be used. In addition, the 4th periodic metal, the 5th periodic metal, and the 6th periodic metal as used in this specification are based on a periodic table. For example, the fourth periodic metal refers to a metal included in the fourth periodic element in the periodic table. The conductive material is preferably one that exhibits high electrical conductivity in the state of sulfide or that can greatly improve the ionic conductivity of the positive electrode. For example, Ti, Fe, La, Ce, Pr, Nd Sm, V, Mn, Fe, Ni, Cu, Zn, Mo, Ag, Cd, In, Sn, Sb, Ta, W, Pb, or a sulfide thereof is preferable. . In the positive electrode, the conductive material consists of both the metal and its sulfide, or consists only of the metal sulfide. These conductive material materials preferably contain a large amount of sulfide, and more preferably consist only of sulfide. This is because the conductive material and the sulfur-modified PAN are easily blended by blending the metal into the positive electrode in a sulfide state, and the conductive material and the positive electrode active material are dispersed substantially uniformly. Further, by using sulfide as the conductive material, there is an advantage that the ratio of the metal and sulfur in the conductive material can be easily controlled within a desired range.

[Negative electrode]
Examples of the negative electrode active material for a non-aqueous electrolyte secondary battery using sulfur-modified PAN as the positive electrode active material include known carbon-based materials such as metallic lithium, metallic sodium, and graphite, silicon-based materials such as silicon thin films, and copper-tin. And alloy materials such as cobalt-tin can be used. Note that the non-aqueous electrolyte secondary battery in the case of using metallic lithium as the negative electrode active material is a lithium secondary battery. The non-aqueous electrolyte secondary battery in the case of using metallic sodium as the negative electrode active material is a sodium secondary battery (so-called sodium sulfur battery). When a negative electrode material that does not contain lithium, for example, a carbon-based material, a silicon-based material, an alloy-based material, etc., among the negative electrode materials described above, a short circuit between the positive and negative electrodes due to the generation of lithium dendrites is required It is advantageous in that it does not easily occur. However, when these negative electrode materials not containing lithium are used in combination with sulfur-modified PAN, if the lithium ion secondary battery is used, neither the positive electrode nor the negative electrode contains lithium. For this reason, a lithium pre-doping process in which lithium is inserted in advance into either one or both of the negative electrode and the positive electrode is necessary. The lithium pre-doping method may be a known method. For example, when lithium is doped into the negative electrode, a half-cell is assembled using metallic lithium as the counter electrode, and lithium is inserted by an electrolytic doping method in which lithium is electrochemically doped, or a metallic lithium foil is attached to the electrode After that, there is a method in which lithium is inserted by a pasting pre-doping method in which it is left in an electrolytic solution and doped using diffusion of lithium to the electrode. Also, when the positive electrode is predoped with lithium, the above-described electrolytic doping method can be used. The same applies to sodium.

  As a negative electrode material that does not contain lithium, a silicon-based material that is a negative electrode material having a high capacity is particularly preferable. Among them, thin-film silicon that is advantageous in terms of capacity per unit volume due to thin electrode thickness, and high capacity and cycle characteristics. Excellent silicon monoxide (SiO) is more preferred.

  As the current collector for the negative electrode, aluminum foil, aluminum mesh, punched aluminum sheet, aluminum expanded sheet, stainless steel foil, stainless steel mesh, punched stainless steel sheet, stainless steel expanded sheet, nickel foam, nickel non-woven fabric, copper foil, Examples include copper mesh, punched copper sheet, copper expanded sheet, titanium foil, titanium mesh, carbon paper (nonwoven fabric / woven fabric) and the like.

〔Electrolytes〕
As the electrolyte used for the nonaqueous electrolyte secondary battery, an electrolyte obtained by dissolving an alkali metal salt as an electrolyte in an organic solvent can be used. As the organic solvent, it is preferable to use at least one selected from non-aqueous solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl ether, isopropyl methyl carbonate, vinylene carbonate, γ-butyrolactone, and acetonitrile. . As the electrolyte, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiI, LiClO ₄ or the like can be used when lithium is used as the negative electrode active material. When sodium is used as the negative electrode active material, it is selected from NaPF ₆ , NaBF ₄ , NaClO ₄ , NaAsF ₆ , NaSbF ₆ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) ₂ , lower fatty acid sodium salt, NaAlCl _{4 and the} like. One kind or a plurality of kinds can be used. Among them, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , NaPF ₆ , NaBF ₄ , NaAsF ₆ , NaSbF ₆ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) _{2 and the} like contain fluorine (F). Therefore, it is preferably used. The concentration of the electrolyte may be about 0.5 mol / L to 1.7 mol / L. Note that the electrolyte is not limited to liquid, and may be solid (for example, a polymer gel).

[Others]
The nonaqueous electrolyte secondary battery may include a member such as a separator in addition to the above-described negative electrode, positive electrode, and electrolyte. The separator is interposed between the positive electrode and the negative electrode, allows ions to move between the positive electrode and the negative electrode, and prevents an internal short circuit between the positive electrode and the negative electrode. If the non-aqueous electrolyte secondary battery is a sealed type, the separator is also required to have a function of holding an electrolytic solution. As the separator, it is preferable to use a thin, microporous or non-woven membrane made of polyethylene, polypropylene, PAN, aramid, polyimide, cellulose, glass or the like. The shape of the nonaqueous electrolyte secondary battery is not particularly limited, and can be various shapes such as a cylindrical shape, a stacked shape, and a coin shape.

  Hereafter, the manufacturing method and manufacturing apparatus of this invention are demonstrated concretely.

Example 1
<Production of sulfur-modified PAN>
[1] PAN Sheet A PAN sheet having a basis weight of 9.2 g / m ² and a thickness of 22 μm was prepared. This PAN sheet is a non-woven sheet made of PAN fibers, has a thickness of 22 μm, a width of 300 mm, and a length of 100 m, and is wound around a roll core.

[2] Apparatus FIG. 3 is an explanatory diagram schematically showing the manufacturing apparatus of the first embodiment. FIG. 4 is an enlarged view of a main part schematically showing the sulfur modification means of FIG.

  As shown in FIG. 3, the manufacturing apparatus of Example 1 includes a first transport unit 11, a second transport unit 12, a third transport unit 13, a press unit 2, a sulfur modification unit 3, and a plurality of auxiliary rollers 4. The first transport means 11 is a rotating shaft that rotates counterclockwise in FIG. The second conveying means 12 is a rotating shaft that rotates clockwise in FIG. The third transport means 13 is a rotating shaft that rotates counterclockwise in FIG. A PAN sheet 5 wound in a roll shape is fixed to the first transport unit 11. Due to the rotation of the first conveying means 11, the PAN sheet 5 is sent out in the right direction in FIG. A current collector sheet 6 wound in a roll shape is fixed to the second conveying means 12. The current collector sheet 6 is carbon paper (model number: TGP-H-030, thickness 0.11 mm, width 300 mm, length 800 mm) manufactured by Toray Industries, Inc. The current collector sheet 6 is sent out in the right direction in FIG. 2 by the rotation of the second conveying means 12. The 1st conveyance means 11, the 2nd conveyance means 12, and the 3rd conveyance means 13 rotate in synchronization. The 1st conveyance means 11, the 2nd conveyance means 12, and the 3rd conveyance means 13 are equivalent to the conveyance means in this invention. In addition, the manufacturing method and manufacturing apparatus of this invention should just provide at least 1 conveyance means. When the manufacturing method and the manufacturing apparatus of the present invention are provided with only one conveying means, for example, only the third conveying means 13 may be provided.

  The pressing means 2 includes a pair of rollers 20 and 21. The pair of rollers 20 and 21 are urged in a direction approaching each other. The PAN sheet 5 sent out from the first transport unit 11 and the current collector sheet 6 sent out from the second transport unit 12 are stacked and pass through the press unit 2. And when passing the press means 2, it is pressed and integrated.

  The laminate (composite sheet 50) of the PAN sheet 5 and the current collector sheet 6 that has passed through the pressing means 2 is conveyed to the sulfur modification means 3 located on the right side of the pressing means 2 in FIG.

  As shown in FIG. 3, the sulfur modification unit 3 includes a heating furnace 30 and a sulfur supply unit 31 attached to the heating furnace 30. A heating means 32 is embedded in the side wall of the heating furnace 30. The heating means 32 heats the inside of the heating furnace 30 (particularly the liquid tank 33) to 400 ° C. The heating furnace 30 has a substantially box shape in which a carry-in port 34 and a carry-out port 35 are opened. The bottom of the heating furnace 30 constitutes a liquid tank 33. The sulfur supply means 31 is attached to the upper part of the heating furnace 30 and injects sulfur gas into the heating furnace 30. Since the sulfur vapor pressure inside the heating furnace 30 gradually increases, liquid sulfur accumulates in the liquid tank 33 at the bottom of the heating furnace 30. Part of the sulfur is vaporized again by being heated by the heating means 32, and the vaporized sulfur moves to the upper part of the heating furnace 30. For this reason, the composite sheet 50 described later is refluxed in the heating furnace 30. Further, an inert gas (argon gas) is supplied to the heating furnace 30 by an inert gas supply means (not shown). For this reason, a sulfur liquid phase 90 is formed at the bottom of the heating furnace 30 (that is, the liquid tank 33), and a mixed gas phase 91 of sulfur and argon gas is formed at the top of the heating furnace 30.

  The composite sheet 50 passed through the pressing means 2 and conveyed to the heating furnace 30 through the carry-in port 34 is conveyed to the liquid tank 33 of the heating furnace 30 and immersed in liquid sulfur accumulated in the liquid tank 33. . The time for which the composite sheet 50 stays in the heating furnace 30 is about 10 to 20 minutes.

  The PAN sheet 5 conveyed to the outside of the heating furnace 30 through the carry-out port 35 is fixed to the third conveying means 13 and wound up. A predetermined tension is applied to the PAN sheet 5 and the current collector sheet 6 at each stage by the auxiliary roller 4.

[3] Sulfur modification step The composite sheet 50 integrated by the pressing means 2 is conveyed to the heating furnace 30 and is subjected to a sulfur supply step and a heat treatment step in the heating furnace 30. In Example 1, the sulfur supply step and the heat treatment step were performed simultaneously. Immediately after conveyance, sulfur gas present in the mixed gas phase 91 adheres to the composite sheet 50 located above the liquid tank 33. Liquid sulfur adheres to the composite sheet 50 immersed in the sulfur liquid phase 90 existing in the liquid tank 33. Since the composite sheet 50 is heated in a state where sulfur is adhered, PAN and sulfur contained in the composite sheet 50 are combined to obtain a sheet-like sulfur-modified PAN.

[4] Elemental sulfur removal step In order to remove elemental sulfur (free sulfur) remaining in the sulfur-modified PAN after the heat treatment step, the following steps were performed.

  The sheet-like sulfur-modified PAN after the heat treatment step was punched out, placed in a glass tube oven, and heated at 200 ° C. for 3 hours while being vacuumed. The temperature elevation temperature at this time was 10 ° C./min. By this process, the elemental sulfur remaining in the object to be treated after the heat treatment process was evaporated and removed, and the sheet-like sulfur-modified PAN of Example 1 not including (or substantially not including) elemental sulfur was obtained.

(Example 2)
The manufacturing method of Example 2 is substantially the same as the manufacturing method of Example 1 except that the PAN sheet 5 is conveyed while being formed on the current collector sheet 6. The manufacturing apparatus according to the second embodiment is the same as the manufacturing apparatus according to the first embodiment except that the manufacturing apparatus includes a fibrous PAN supply unit 70 and does not include the first conveying unit 11. The principal part enlarged view which represents typically the manufacturing apparatus of Example 2 is shown in FIG.

  As shown in FIG. 5, the fibrous PAN supply means 70 in the manufacturing apparatus of Example 2 blows out the electrospun 51 obtained by the electrospinning means (not shown) onto the current collector sheet 6. For this reason, the PAN sheet 5 is formed directly on the current collector sheet 6. The composite sheet 50 in which the PAN sheet 5 is formed on the current collector sheet 6 is transported to the press means in the same manner as in Example 1, and then transported to the sulfur modification means.

(Example 3)
The manufacturing method of Example 3 is substantially the same as the manufacturing method of Example 1 except that the PAN sheet 5 is conveyed while being formed on the current collector sheet 6. The manufacturing apparatus of the third embodiment is the same as the manufacturing apparatus of the first embodiment except that the liquid PAN supply unit 71 is provided and the first transport unit 11 is not provided. The principal part enlarged view which represents typically the manufacturing apparatus of Example 3 is shown in FIG. The liquid PAN described above was prepared by dissolving PAN powder in an N, N-dimethylformamide (DMF) solvent.

  As shown in FIG. 6, the liquid PAN supply means 71 in the manufacturing apparatus of Example 3 sprays the PAN solution 52 directly on the current collector sheet 6. For this reason, the PAN sheet 5 is formed on the current collector sheet 6 (or integrally with the current collector sheet 6). The obtained composite sheet 50 is conveyed to the press means in the same manner as in Example 1, and then conveyed to the sulfur modification means.

(Comparative example)
The manufacturing method of the comparative example is an example except that the composite sheet 50 was sulfur-modified in sulfur gas (that is, in the mixed gas phase 91 in the heating furnace 30) and was not immersed in liquid sulfur in the liquid tank 33. This is the same method as the first manufacturing method.

<Test lithium secondary battery>
A lithium secondary battery for evaluation using the sulfur-modified PAN of Example 1 as a positive electrode and a lithium secondary battery for evaluation using the sulfur-modified PAN of Comparative Example as a positive electrode were prepared, and the cycle characteristics were evaluated. The positive electrode used was a sheet-shaped sulfur-modified PAN punched out to a diameter of 11 mm. The negative electrode used was a metal lithium foil having a thickness of 500 μm punched to a diameter of 14 mm. As the electrolytic solution, a solution in which LiPF ₆ was dissolved at 1.0 mol / L in a mixed solvent of ethylene carbonate and diethyl carbonate (mass ratio 1: 1) was used. Separator (Celgard 2400, manufactured by Celgard) made of a polypropylene microporous film with a thickness of 25 μm and a glass nonwoven fabric filter with a thickness of 440 μm using a battery case made of a stainless steel container (CR 2032 type coin battery member, manufactured by Hosen Co., Ltd.) The positive electrode and the negative electrode were laminated in a dry room through (GA-100, manufactured by ADVANTEC). Thereafter, an electrolytic solution was injected and sealed with a caulking machine to produce a lithium secondary battery.

<Composition analysis of sulfur-modified PAN>
About the sulfur-modified PAN of Example 1 and the sulfur-modified PAN of the comparative example, using a carbon / sulfur gas analyzer (EMIA-720, manufactured by HORIBA, Ltd.) and an oxygen / nitrogen gas analyzer (EMGA-550FA, manufactured by HORIBA, Ltd.) The composition was analyzed. The results of the composition analysis are shown in Table 1.

  As shown in Table 1, the sulfur-modified PAN of Example 1 had a much higher sulfur content than the sulfur-modified PAN of the comparative example. This is because by attaching liquid sulfur to PAN, the contact area between PAN and sulfur element becomes larger than when gaseous sulfur is attached to PAN. This is thought to be due to the combination. From this result, the sulfur-modified PAN produced by the method of the present invention contains a large amount of sulfur, and when the total amount of hydrogen, carbon, sulfur, oxygen and nitrogen is 100 mol%, the amount of sulfur is 10 mol%. It can be seen that it can be determined that the sulfur-modified PAN produced by the production method of the present invention exceeds.

<Cycle characteristics>
This battery was charged and discharged at a current value corresponding to 50 mA per 1 g of the positive electrode active material. At that time, the final discharge voltage was 1.0V, and the final charge voltage was 3.0V. FIG. 7 shows a charge / discharge curve of the lithium secondary battery using the sulfur-modified PAN of Example 1 as a positive electrode, and FIG. 8 shows the cycle characteristics. Further, FIG. 9 shows a charge / discharge curve of a lithium secondary battery using the sulfur-modified PAN of the comparative example as a positive electrode, and FIG. 10 shows the cycle characteristics.

  As shown in FIGS. 7 to 10, the lithium secondary battery using the sulfur-modified PAN of the example is far superior in charge / discharge capacity and cycle compared to the lithium secondary battery using the sulfur-modified PAN of the comparative example. The characteristics are shown. From this result, it can be seen that according to the production method and production apparatus of the present invention, sulfur-modified PAN capable of increasing the capacity of the nonaqueous electrolyte secondary battery can be easily mass-produced.

2: Pressing means 3: Sulfur modifying means 4: Auxiliary roller 5: PAN sheet 6: Current collector sheet 11: First conveying means 12: Second conveying means 13: Third conveying means 30: Heating furnace 31: Sulfur supply means 32: Heating means 33: Liquid tank 34: Carry-in port 35: Carry-out port 50: Composite sheet 70: Fibrous PAN supply means 71: Liquid PAN supply means

Claims (5)

A method for producing a sulfur-based positive electrode active material comprising polyacrylonitrile and sulfur (S) as raw materials and comprising a carbon skeleton derived from polyacrylonitrile and sulfur (S) bonded to the carbon skeleton,
Including a sulfur modification step of heating the polyacrylonitrile sheet with sulfur (S) attached thereto while continuously conveying the polyacrylonitrile sheet,
In the sulfur modification step, at least a part of the sulfur (S) is attached in liquid form to the polyacrylonitrile sheet.   The method for producing a sulfur-based positive electrode active material according to claim 1, wherein in the sulfur modification step, the polyacrylonitrile sheet is passed through a liquid tank containing liquid sulfur (S).   The manufacturing method of the sulfur type positive electrode active material of Claim 1 or 2 which performs the said sulfur modification | denaturation process to the composite sheet which integrated the said polyacrylonitrile sheet | seat and the collector sheet. An apparatus for producing a sulfur-based positive electrode active material comprising polyacrylonitrile and sulfur (S) as raw materials and comprising a carbon skeleton derived from polyacrylonitrile and sulfur (S) bonded to the carbon skeleton,
Conveying means for continuously conveying the polyacrylonitrile sheet;
Sulfur supply means for attaching sulfur (S) to the polyacrylonitrile sheet being conveyed;
Heating means for heating the polyacrylonitrile sheet adhered to sulfur, and
The sulfur supply means is an apparatus for producing a sulfur-based positive electrode active material, characterized in that at least a part of the sulfur (S) is attached in liquid form to the polyacrylonitrile sheet. The sulfur supply means includes a liquid tank containing liquid sulfur (S),
The apparatus for producing a sulfur-based positive electrode active material according to claim 4, wherein the transport unit allows the polyacrylonitrile sheet to pass through the liquid tank.

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