JP5618112B2 - Sulfur-modified polyacrylonitrile, evaluation method thereof, positive electrode using sulfur-modified polyacrylonitrile, nonaqueous electrolyte secondary battery, and vehicle - Google Patents

Description

  The present invention relates to a sulfur-modified polyacrylonitrile suitably used as a positive electrode active material for a nonaqueous electrolyte secondary battery, and an evaluation method thereof.

  A lithium ion secondary battery, which is a type of nonaqueous electrolyte secondary battery, is a battery with a large charge / discharge capacity, and is mainly used as a battery for portable electronic devices. Lithium ion secondary batteries are also expected as batteries for electric vehicles.

  As a positive electrode active material of a lithium ion secondary battery, a material containing a rare metal such as cobalt or nickel is 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. For example, the charge / discharge capacity of a lithium ion secondary battery using sulfur as a positive electrode active material is approximately six times the charge / discharge capacity of a lithium ion secondary battery using a lithium cobaltate positive electrode material, which is a common positive electrode material. is there.

  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. Hereinafter, the characteristic of the lithium ion secondary battery in which the charge / discharge capacity decreases with repeated charge / discharge is referred to as “cycle characteristic”. The lithium ion secondary battery having a small decrease in charge / discharge capacity is a lithium ion secondary battery having excellent cycle characteristics, and the lithium ion secondary battery having a large decrease in charge / discharge capacity is a lithium ion secondary battery having inferior cycle characteristics.

  In order to improve cycle characteristics, a technique has been proposed in which a carbon material is blended with a positive electrode active material containing sulfur (hereinafter referred to as a sulfur-based positive electrode active material) (see, for example, Patent Document 1).

  Patent Document 1 introduces a technique using polysulfide carbon having carbon and sulfur as main components as a positive electrode active material. This polysulfide carbon is obtained by adding sulfur to a linear unsaturated polymer. According to this technique, elution of sulfur into the electrolytic solution can be suppressed by the carbon material. For this reason, it is thought that the cycle characteristic of the lithium ion secondary battery which uses this polysulfide carbon as a positive electrode active material improves. However, even with a lithium ion secondary battery using carbon polysulfide as the positive electrode active material, no significant improvement in cycle characteristics was observed. When carbon polysulfide is used as the positive electrode active material (when a linear unsaturated polymer is used as the carbon material for the positive electrode active material), it is considered that sulfur and lithium are combined during discharge. For this reason, it is considered that the —CS—CS— bond or —S—S— bond contained in the polysulfide carbon is cut and the polymer is cut. Therefore, a sulfur-based positive electrode active material that can further improve the cycle characteristics of the lithium ion secondary battery has been demanded.

  The inventors of the present invention invented a positive electrode active material obtained by heat-treating a mixture of polyacrylonitrile and sulfur (see Patent Document 2). The charge / discharge capacity of a lithium ion secondary battery using this positive electrode active material for the positive electrode is large, and the lithium ion secondary battery using this positive electrode active material for the positive electrode is excellent in cycle characteristics.

  However, on the other hand, lithium ion secondary batteries using sulfur-modified polyacrylonitrile as a positive electrode active material sometimes have different charge / discharge capacities depending on the polyacrylonitrile manufacturer and product lot. Therefore, in order to reduce the uneven quality of the lithium ion secondary battery, it is necessary to select and use sulfur-modified polyacrylonitrile suitable as a positive electrode active material for the lithium ion secondary battery.

JP 2002-154815 A International Publication No. 2010/044437

  The present invention has been made in view of the above circumstances, sulfur-modified polyacrylonitrile capable of improving the charge / discharge capacity of a lithium ion secondary battery, and an evaluation method for selecting such sulfur-modified polyacrylonitrile, and An object is to provide a positive electrode using the sulfur-modified polyacrylonitrile, a non-aqueous electrolyte secondary battery, and a vehicle equipped with the non-aqueous electrolyte secondary battery.

The sulfur-modified polyacrylonitrile of the present invention that solves the above problems is
A sulfur-modified polyacrylonitrile made from sulfur and polyacrylonitrile,
The area occupied by particles having a particle diameter of 1 μm or less and / or aggregates of the particles in the entire particles is 80% or more in terms of the imaging area ratio ,
The polyacrylonitrile is characterized in that an absorbance ratio D ₁₂₃₀ / D ₁₂₅₀ of absorbance D _{1230 at 1230} cm ⁻¹ and absorbance D _{1250 at 1250} cm ⁻¹ by Fourier transform infrared spectroscopy of the polyacrylonitrile is 0.75 or less.

Another sulfur-modified polyacrylonitrile of the present invention that solves the above problems is a sulfur-modified polyacrylonitrile using sulfur and polyacrylonitrile as a material,
The polyacrylonitrile is produced by a method other than emulsion polymerization.

  The positive electrode for a non-aqueous electrolyte secondary battery according to the present invention that solves the above-described problems includes the sulfur-modified polyacrylonitrile of the present invention as a positive electrode active material.

The non-aqueous electrolyte secondary battery of the present invention that solves the above problems includes the sulfur-modified polyacrylonitrile of the present invention in the positive electrode as a positive electrode active material.
A vehicle of the present invention that solves the above-described problems is characterized by mounting the nonaqueous electrolyte secondary battery of the present invention.

The method for evaluating the sulfur-modified polyacrylonitrile of the present invention that solves the above problems is as follows.
An evaluation method for sulfur-modified polyacrylonitrile used as a positive electrode active material for a non-aqueous electrolyte secondary battery,
Of all the particles, particles having a particle diameter of 1 μm or less and / or an area occupied by the aggregate of the particles are determined as conforming products when the imaging area ratio is 80% or more, and the other regions are regarded as non-conforming products. It is characterized by evaluating.

  The method for producing a nonaqueous electrolyte secondary battery of the present invention that solves the above problems includes the method for evaluating sulfur-modified polyacrylonitrile of the present invention.

  The nonaqueous electrolyte secondary battery using the sulfur-modified polyacrylonitrile of the present invention has a large charge / discharge capacity. Further, according to the method for evaluating sulfur-modified polyacrylonitrile of the present invention, sulfur-modified polyacrylonitrile that can improve the charge / discharge capacity of the non-aqueous electrolyte secondary battery can be selected. Therefore, if the method for evaluating sulfur-modified polyacrylonitrile of the present invention is used, sulfur-modified polyacrylonitrile that can be used for a non-aqueous electrolyte secondary battery having a large charge / discharge capacity can be produced. Moreover, if the evaluation method for sulfur-modified polyacrylonitrile of the present invention is used, a non-aqueous electrolyte secondary battery having a large charge / discharge capacity can be produced. Furthermore, since the vehicle of the present invention is equipped with the above-described nonaqueous electrolyte secondary battery of the present invention, it is excellent in various characteristics that require electric power.

It is a graph showing the result of having carried out X-ray diffraction of sulfur modification polyacrylonitrile. It is a graph showing the result of having performed a Raman spectrum analysis of sulfur-modified polyacrylonitrile. It is explanatory drawing which represents typically the reaction apparatus used with the manufacturing method of the sulfur modified polyacrylonitrile of an Example. 2 is an SEM image of the surface of sulfur-modified polyacrylonitrile of Sample 1. 2 is a SEM image of the surface of sulfur-modified polyacrylonitrile of Sample 2. 3 is an SEM image of the surface of sulfur-modified polyacrylonitrile of Sample 3. 3 is an SEM image of the surface of sulfur-modified polyacrylonitrile of sample 4. 6 is a SEM image of the surface of sulfur-modified polyacrylonitrile of sample 5. 2 is an SEM image of the surface of sulfur-modified polyacrylonitrile of Sample 6. 3 is a SEM image of the surface of sulfur-modified polyacrylonitrile of Sample 7. 2 is a SEM image of the surface of a sulfur-modified polyacrylonitrile of sample 8. 2 is an SEM image of the surface of sulfur-modified polyacrylonitrile of sample 9. 3 is a graph showing a charge / discharge curve of a lithium ion secondary battery of Sample 1. FIG. 4 is a graph showing cycle characteristics of a lithium ion secondary battery of Sample 1. 5 is a graph showing a charge / discharge curve of a lithium ion secondary battery of Sample 8. FIG. It is a graph showing the result of having analyzed the sulfur modification polyacrylonitrile of sample 1 by FT-IR. It is a graph showing the result of having analyzed sulfur modification polyacrylonitrile of sample 2 by FT-IR. It is a graph showing the result of having analyzed sulfur modification polyacrylonitrile of sample 3 by FT-IR. It is a graph showing the result of having analyzed sulfur modification polyacrylonitrile of sample 4 by FT-IR. It is a graph showing the result of having analyzed sulfur modification polyacrylonitrile of sample 5 by FT-IR. It is a graph showing the result of having analyzed the sulfur modification polyacrylonitrile of the sample 6 by FT-IR. It is a graph showing the result of having analyzed sulfur modification polyacrylonitrile of sample 7 by FT-IR. It is a graph showing the result of having analyzed sulfur modification polyacrylonitrile of sample 8 by FT-IR. It is a graph showing the result of having analyzed sulfur modification polyacrylonitrile of sample 9 by FT-IR. It is a graph showing the relationship between the light absorbency ratio ( _D1230 / _D1250 ) by FT-IR of the sulfur modification polyacrylonitrile of samples 1-9, and the discharge capacity of the 2nd time. 4 is a graph showing the result of thermal mass-differential thermal analysis of sulfur-modified polyacrylonitrile of Sample 1. 4 is a graph showing the result of thermal mass-differential thermal analysis of sulfur-modified polyacrylonitrile of Sample 2. 4 is a graph showing the result of thermal mass-differential thermal analysis of sulfur-modified polyacrylonitrile of Sample 3. 4 is a graph showing the result of thermal mass-differential thermal analysis of sulfur-modified polyacrylonitrile of Sample 4. 6 is a graph showing the result of thermal mass-differential thermal analysis of sulfur-modified polyacrylonitrile of Sample 5. 6 is a graph showing the result of thermal mass-differential thermal analysis of the sulfur-modified polyacrylonitrile of Sample 6. It is a graph showing the result of having conducted the thermal mass-differential thermal analysis of the sulfur modified polyacrylonitrile of the sample 7. FIG. It is a graph showing the result of having carried out the thermal mass-differential thermal analysis of the sulfur modification polyacrylonitrile of the sample 8. FIG. It is a graph showing the result of having carried out the thermal mass-differential thermal analysis of the sulfur modification polyacrylonitrile of the sample 9. FIG.

  The positive electrode for a nonaqueous electrolyte secondary battery of the present invention (hereinafter referred to as the positive electrode of the present invention) contains the sulfur-modified polyacrylonitrile of the present invention as a positive electrode active material in the positive electrode. The nonaqueous electrolyte secondary battery of the present invention is a battery using the positive electrode of the present invention, and contains the sulfur-modified polyacrylonitrile of the present invention as a positive electrode active material in the positive electrode.

<Sulfur modified polyacrylonitrile>
The sulfur-modified polyacrylonitrile of the present invention is the same as that disclosed in Patent Document 2 above. Specifically, the sulfur-modified polyacrylonitrile of the present invention is made of sulfur and polyacrylonitrile, and contains carbon element (C) and sulfur element (S). Hereinafter, polyacrylonitrile is abbreviated as PAN as necessary.

The polyacrylonitrile as a material for sulfur-modified polyacrylonitrile is preferably in the form of powder and preferably has a mass average molecular weight of about 10 ^{4 to} 3 × 10 ⁵ . The particle size of polyacrylonitrile is preferably about 0.5 to 50 μm, more preferably about 1 to 10 μm, when observed with an electron microscope. If the molecular weight and particle size of polyacrylonitrile are within these ranges, the contact area between polyacrylonitrile and sulfur can be increased, and polyacrylonitrile 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 polyacrylonitrile as the positive electrode active material of a lithium ion secondary battery, the high capacity inherent in sulfur can be maintained, and elution of sulfur into the electrolyte is suppressed, so the cycle characteristics are greatly improved. . This is presumably because sulfur does not exist as a simple substance in sulfur-modified polyacrylonitrile, but exists in a stable state combined with polyacrylonitrile. In the manufacturing method of the sulfur type positive electrode active material currently disclosed by patent document 2, sulfur is heat-processed with polyacrylonitrile. When polyacrylonitrile is heated, it is considered that polyacrylonitrile is three-dimensionally crosslinked to form a condensed ring (mainly a 6-membered ring) and close the ring. For this reason, it is considered that sulfur is present in the sulfur-modified polyacrylonitrile in a state of being bonded to the polyacrylonitrile that has progressed in the ring closure. For this reason, sulfur-modified polyacrylonitrile has a carbon skeleton derived from polyacrylonitrile. By combining polyacrylonitrile and sulfur, elution of sulfur into the electrolytic solution can be suppressed, and cycle characteristics can be improved.

  By the way, the method for producing polyacrylonitrile includes a step of polymerizing an acrylonitrile monomer (hereinafter simply referred to as a monomer). Examples of general polymerization methods include bulk polymerization, suspension polymerization, solution polymerization, and emulsion polymerization.

  Bulk polymerization is a method in which a solvent is used and polymerization is carried out by adding only a monomer or a small amount of a polymerization initiator to the monomer. The product is mainly composed of a polymer and an unreacted monomer, and contains impurities derived from a polymerization initiator, but is pure as compared with other polymerization methods. On the other hand, when the monomer is in a liquid state, the viscosity of the reaction system increases with the progress of polymerization, and stirring and flow (removal from the reactor) and removal of heat of reaction become difficult.

  Suspension polymerization is a polymerization method in which a monomer and a solvent (water) are mechanically stirred and suspended for polymerization. In this method, it is common to use a radical generator soluble in the monomer as the polymerization initiator. In the reaction system, the polymerization disclosure agent is present in the monomer droplets dispersed in the solvent. For this reason, the polymerization reaction proceeds in a state close to a bulk polymerization occurring in each monomer droplet. Since the reaction occurs in the monomer droplets, there is an advantage that a polymer having a small molecular weight (that is, a small particle size) and few impurities can be obtained.

  Solution polymerization is a method of performing a polymerization reaction in a solvent. As the solvent for the solution polymerization, a solvent that does not easily react with either the monomer or the catalyst (polymerization initiator) is used. According to this method, since the solvent absorbs heat, the reaction heat of polymerization is easy to adjust, but the reaction rate is slow. Since it is difficult to manage the solvent, it is not an industrially used method. It is technically difficult to produce a solution polymerized product having a uniform particle size.

Emulsion polymerization is a type of radical polymerization. A polymerization initiator (usually a radical generator) that can be dissolved in a medium such as water, a monomer that is hardly soluble in the medium, and an emulsifier (surfactant). ) Is added for polymerization. Emulsion polymerization is suitable for making the particle shape uniformly at the submicron level, and is also optimal in terms of production efficiency. On the other hand, high molecular weight (that is, increase in particle size) is unavoidable in emulsion polymerization. Further, since some emulsifier (surfactant) remains in the emulsion polymerization product, the polyacrylonitrile obtained by the emulsion polymerization method contains an emulsifier as an impurity. There is also a possibility that acrylamide generated by hydrolysis of acrylonitrile (CH ₂ ═CHCN) may be contained as an impurity.

  Of these polymerization methods, emulsion polymerization is considered industrially advantageous. However, as a result of intensive studies, the inventors of the present invention have found that polyacrylonitrile obtained by emulsion polymerization is not preferable as a material for sulfur-modified polyacrylonitrile for a positive electrode active material. That is, when sulfur-modified polyacrylonitrile using polyacrylonitrile obtained by emulsion polymerization as a raw material is used as a positive electrode active material, sulfur-modified polyacrylonitrile obtained using polyacrylonitrile obtained by other methods as a positive electrode active material Compared with the case where it uses as, the capacity | capacitance of a nonaqueous electrolyte secondary battery becomes small. The reason is not clear, but it is expected that the above-mentioned impurities such as emulsifiers and acrylamide are partly responsible. Therefore, it is preferable to use polyacrylonitrile produced by a method other than emulsion polymerization (bulk polymerization, suspension polymerization, solution polymerization, etc.).

  Polyacrylonitrile produced by emulsion polymerization (hereinafter referred to as non-conforming PAN) and polyacrylonitrile produced by other methods (hereinafter referred to as conforming PAN) are: particle size, presence of impurities, stereoregularity Etc.

  For example, the particle size of conforming PAN is very small compared to the particle size of non-conforming PAN. This is considered to be because the polymerization reaction proceeds in the micelle formed in the presence of the emulsifier in the emulsion polymerization. The particle size of the incompatible PAN produced by emulsion polymerization is a size corresponding to the size of micelles formed in the emulsion polymerization reaction system. Specifically, the particle size of the conforming PAN is as small as approximately 1 μm or less, whereas the particle size of the nonconforming PAN is a large diameter exceeding 5 μm. The particle size of the sulfur-modified polyacrylonitrile can be measured by the measurement method described later in the column of Examples.

  The presence or absence of impurities can be confirmed by the presence or absence of the C = O peak. That is, the incompatible PAN includes acrylamide having a carboxylic acid-based surfactant remaining or synthesized during polymerization. Therefore, when the sulfur-modified polyacrylonitrile is subjected to IR analysis (for example, Fourier transform infrared spectroscopy, FT-IR), a peak derived from C═O of the carboxylic acid surfactant and / or acrylamide is confirmed. The presence or absence of this peak makes it possible to discriminate between compatible PAN and non-compatible PAN.

It is known that the stereoregularity of substances varies depending on the synthesis method. Also, the higher the tacticity of the polyacrylonitrile, the absorbance of 1230 cm ^-1 as measured by FT-IR _{(D 1230)} and the absorbance of 1250cm ^-1 _{(D 1250)} and absorbance ratio of _(D 1230 _/ D ₁₂₅₀₎ is It is supposed to grow. Furthermore, it is known that polyacrylonitrile (compatible PAN) synthesized by a solution polymerization method becomes atactic (a structure in which the absolute configuration of asymmetric carbon atoms is completely random).

As a result of intensive studies, the inventors of the present invention have found that the absorbance ratio (D ₁₂₃₀ / D ₁₂₅₀ ) by FT-IR is small in the conforming PAN and large in the non-conforming PAN. Specifically, it is 0.75 or less in conforming PAN, and exceeds 0.75 in non-conforming PAN. For this reason, the conforming PAN and the non-conforming PAN can be distinguished also by the absorbance ratio (D ₁₂₃₀ / D ₁₂₅₀ ) by FT-IR. Examples of stereoregularity include regularity of configuration and regularity of conformation. Of these, if the regularity of the configuration is low, it becomes atactic. If the regularity of the three-dimensional structure is low, the structure is not a straight chain but an intricate structure. In any case, the lower regularity results in a more complicated structure.

As a method for measuring stereoregularity, ^13C NMR or the like may be used in addition to FT-IR.

  The sulfur used in the sulfur-modified polyacrylonitrile is preferably in the form of a powder, like polyacrylonitrile. Although it does not specifically limit about the particle size of sulfur, When classifying using a sieve, what is in the range of the magnitude | size which does not pass a sieve with a sieve opening of 40 micrometers and passes a 150 micrometers sieve is preferable. It is more preferable that the mesh size does not pass through a 40 μm sieve and is within a size range that passes through a 100 μm sieve.

  The compounding ratio of the polyacrylonitrile powder and sulfur powder used for the sulfur-modified polyacrylonitrile 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 preferable, and 1: 2 to 1: 5 is even more preferable.

  As a result of elemental analysis, sulfur-modified polyacrylonitrile 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 polyacrylonitrile with CuKα rays, only a broad peak having a peak position near 25 ° was confirmed in the diffraction angle (2θ) range of 20-30 °. It was done. 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 °.

  Further, the mass loss by thermogravimetric analysis when sulfur-modified polyacrylonitrile 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 polyacrylonitrile powder is heated under the same conditions, a mass decrease is recognized from around 120 ° C., and a large mass decrease due to the disappearance of sulfur is recognized suddenly at 200 ° C. or higher.

  That is, in sulfur-modified polyacrylonitrile, sulfur does not exist as a simple substance, but is considered to exist in a state of being bonded to polyacrylonitrile that has progressed in ring closure.

An example of the Raman spectrum of sulfur-modified polyacrylonitrile 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 Raman shift peak described above is observed at the same position even when the ratio of elemental sulfur to polyacrylonitrile is changed. Thus, these peaks characterize sulfur-modified polyacrylonitrile. 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.

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. Note that the number of Raman spectrum peaks may change or the position of the peak top may be shifted depending on the wavelength of incident light or the difference in resolution. Therefore, when measuring the Raman spectrum of the positive electrode of the present invention using sulfur-modified polyacrylonitrile as the positive electrode active material, the same peak as the above peak or a peak slightly different from the above peak in number and peak top position is confirmed. Is done.

(Positive electrode for non-aqueous electrolyte secondary battery)
The positive electrode of the present invention contains the sulfur-modified polyacrylonitrile of the present invention as a positive electrode active material.

  The positive electrode can have the same structure as that of a general positive electrode for a nonaqueous electrolyte secondary battery (for example, a positive electrode for a lithium ion secondary battery) except for the positive electrode active material. For example, the positive electrode of the present invention can be manufactured by applying a positive electrode material, which is a mixture of sulfur-modified polyacrylonitrile, a conductive additive, a binder, and a solvent, to a current collector. Alternatively, a mixed raw material in which sulfur powder and polyacrylonitrile powder are mixed can be heated (a heat treatment step is performed) after filling the positive electrode current collector. According to this method, polyacrylonitrile and sulfur are reacted to obtain sulfur-modified polyacrylonitrile, and at the same time, the sulfur-modified polyacrylonitrile and the current collector can be integrated without using a binder. If no binder is used, the amount of the positive electrode active material per positive electrode mass can be increased, and the capacity per positive electrode mass can be improved.

  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 difluoride (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 polyacrylonitrile. As another method, a mixture of the sulfur-modified polyacrylonitrile of the present invention, the above-described conductive additive and binder is kneaded with a mortar or a press machine to form a film, and the film mixture is collected with a press machine or the like. The positive electrode for a nonaqueous electrolyte secondary battery of the present invention can also be produced by pressure bonding to the body.

  What is necessary is just to use what is generally used for the positive electrode for nonaqueous electrolyte secondary batteries as a collector. For example, current collectors include aluminum foil, aluminum mesh, punched aluminum sheet, aluminum expanded sheet, stainless steel foil, stainless steel mesh, punched stainless steel sheet, stainless steel expanded sheet, foamed nickel, nickel non-woven fabric, copper foil, copper mesh Examples thereof include a punching copper sheet, a copper expanded sheet, a titanium foil, a titanium mesh, a carbon nonwoven fabric, and a carbon woven fabric. Among these, the carbon non-woven fabric / woven fabric current collector made of carbon having a high graphitization degree is suitable as a current collector for sulfur-modified polyacrylonitrile 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.) and polyacrylonitrile fiber, which are carbon fiber materials, can be used.

  The non-aqueous electrolyte secondary battery of the present invention preferably contains a conductive material in the positive electrode. The conductive material refers to a material that exhibits high electrical conductivity or that can greatly improve the lithium ion conductivity of the positive electrode. By including a conductive material in the positive electrode, the electrical conductivity of the entire positive electrode and / or the conductivity of charge carriers such as lithium ions 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 as a raw material of sulfur-modified polyacrylonitrile.

  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 a sulfide state or that can greatly improve the lithium ion conductivity of the positive electrode. For example, Ti, Fe, La, Ce, Pr, It is at least one selected from the group consisting of Nd, Sm, V, Mn, Fe, Ni, Cu, Zn, Mo, Ag, Cd, In, Sn, Sb, Ta, W, and Pb, or a sulfide thereof. 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 polyacrylonitrile are easily combined by blending the metal with 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.

(Non-aqueous electrolyte secondary battery)
Hereinafter, the structure of the nonaqueous electrolyte secondary battery using the positive electrode of the present invention will be described. Hereinafter, the non-aqueous electrolyte secondary battery using the positive electrode of the present invention is simply abbreviated as a non-aqueous electrolyte secondary battery. Further, a lithium ion secondary battery using the positive electrode of the present invention is simply abbreviated as a lithium ion secondary battery. The positive electrode is as described above.

[Negative electrode]
As the negative electrode active material, a known carbon-based material such as lithium metal or graphite, an element that can occlude / release lithium ions and can be alloyed with lithium, and / or a compound containing the element can be used. In this case, the charge carrier is lithium, and the nonaqueous electrolyte secondary battery of the present invention is a lithium secondary battery, a lithium ion secondary battery, or a lithium polymer secondary battery. As another negative electrode active material, metallic sodium, an element that can occlude / release sodium ions and can be alloyed with sodium, and / or a compound containing the element can also be used. In this case, the charge carrier is sodium, and the nonaqueous electrolyte secondary battery of the present invention is a sodium secondary battery, a sodium ion secondary battery, or a sodium polymer secondary battery.
The elements that can be alloyed with lithium are Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Si, It is preferably at least one selected from the group consisting of Ge, Sn, Pb, Sb and Bi. Of these, silicon (Si) or tin (Sn) is particularly preferable. The elemental compound having an element capable of alloying with lithium is preferably a silicon compound or a tin compound. The silicon compound is preferably SiO _x (0.5 ≦ x ≦ 1.5). Silicon has a large theoretical capacity, but has a large volume change during charge and discharge. Therefore, it can be used in a compound state (ie, SiO _x ) to reduce the volume change.
As the tin compound, for example, a tin alloy (Cu—Sn alloy, Co—Sn alloy, etc.), a tin alloy (Cu—Sn alloy, Co—Sn alloy, etc.) and the like are preferably used. In addition, silicon-based materials such as silicon thin films and alloy-based materials such as copper-tin and cobalt-tin can be preferably used. When the charge carrier is sodium, it is preferable to use hard carbon, soft carbon, or a tin compound as the negative electrode active material.
When a negative electrode active material, such as a carbon-based material, a silicon-based material, an alloy-based material, or the like that does not contain lithium, for example, among the negative electrode active materials described above, a short circuit occurs between the positive and negative electrodes due to the generation of dendrites. It is advantageous in that it is difficult. However, when the negative electrode using these negative electrode active materials is used in combination with the positive electrode of the present invention, a substance such as Li or Na that is involved in charge / discharge by ionizing and moving between the positive electrode and the negative electrode (So-called charge carriers) are not included in either the positive electrode or the negative electrode. For this reason, a pre-doping process in which charge carriers are inserted in advance into either one or both of the negative electrode and the positive electrode is necessary. The charge carrier pre-doping method may be a known method. For example, in the case where lithium, which is a kind of charge carrier, is doped into the negative electrode, an electrolytic doping method in which a half cell is assembled using metallic lithium as the counter electrode and electrochemically doped with lithium in the negative electrode can be used. Alternatively, an adhesive pre-doping method in which a metal lithium foil attached to an electrode is allowed to stand in an electrolytic solution and lithium is doped into the negative electrode by utilizing diffusion of lithium. Also, when the positive electrode is predoped with lithium, the above-described electrolytic doping method can be used. The same applies to sodium.

  As the negative electrode material that does not contain lithium, a silicon-based material that is a high-capacity negative electrode material is particularly preferable, and among these, thin-film silicon that is advantageous in terms of capacity per volume due to thin electrode thickness is more preferable.

〔Electrolytes〕
As the electrolyte used for the non-aqueous electrolyte secondary battery, an electrolyte obtained by dissolving an alkali metal salt as a supporting electrolyte (supporting salt) 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, γ-butyrolactone, and acetonitrile. When the charge carrier is lithium, for example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiI, LiClO ₄ or the like can be used as the supporting electrolyte. The concentration of the electrolyte may be about 0.5 mol / l to 1.7 mol / l. The electrolyte is not limited to liquid. For example, when the non-aqueous electrolyte secondary battery is a lithium polymer secondary battery, the electrolyte is in a solid state (for example, a polymer gel). When the charge carrier is Na, sodium salts such as NaPF ₆ , NaBF ₄ , NaAsF ₆ , NaCF ₃ SO ₃ , NaI, NaClO ₄ can be used for the electrolyte.

[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, polyacrylonitrile, 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.

(Method for producing positive electrode for non-aqueous electrolyte secondary battery)
The production method of the present invention includes a step (heat treatment step) of heating a mixed material obtained by mixing the above-described sulfur-modified polyacrylonitrile material (that is, polyacrylonitrile powder and sulfur powder). What is necessary is just to mix a mixing material with general mixing apparatuses, such as a mortar and a ball mill. As the mixed raw material, one obtained by simply mixing sulfur and a carbon material may be used. For example, the mixed raw material may be formed into a pellet shape and used. When manufacturing a positive electrode containing a conductive material, a mixture of polyacrylonitrile powder, sulfur powder and conductive material may be used as a mixed material.

  By heating the mixed raw material in the heat treatment step, sulfur contained in the mixed raw material and the carbon material are combined. The heat treatment step may be performed in a closed system or an open system, but in order to suppress the dissipation of sulfur vapor, it is preferably performed in a closed system. 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 bonding between the carbon material and sulfur (for example, an atmosphere not containing 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. In particular, when polyacrylonitrile is used as a carbon material, the vaporized sulfur reacts with the polyacrylonitrile at the same time as the polyacrylonitrile ring-closing reaction by heat treatment in a non-oxidizing atmosphere. It is believed that polyacrylonitrile 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.

  There is no particular limitation on the specific method for creating a non-oxidizing atmosphere in a sealed state. For example, the mixed raw material is placed in a container that is kept tight enough not to dissipate sulfur vapor, and the inside of the container is decompressed. Alternatively, heating may be performed in an inert gas atmosphere. In addition, the mixed raw material may be heated in a vacuum packaged state with a material that does not easily react with sulfur vapor (for example, an aluminum laminate film). In this case, in order to prevent the packaging material from being damaged by the generated sulfur vapor, for example, the packaged raw material is put in a pressure vessel such as an autoclave containing water and heated, and the generated steam is added from the outside of the packaging material. It is preferable to press. According to this method, since pressure is applied by water vapor from the outside of the packaging material, the packaging material is prevented from being swollen and damaged by sulfur vapor.

  What is necessary is just to set suitably the heating time of the mixed raw material in a heat processing process according to heating temperature, and it does not specifically limit it. The preferable heating temperature mentioned above should just be temperature which the coupling | bonding of sulfur and a carbon material advances. Specifically, the heating temperature is preferably 250 ° C. or more and 500 ° C. or less, more preferably 250 ° C. or more and 400 ° C. or less, and further preferably 300 ° C. or more and 400 ° C. or less.

  In the heat treatment step, sulfur is preferably refluxed. In this case, the mixed raw material may be heated so that a part of the mixed raw material becomes a gas and a part becomes a liquid. In other words, the temperature of the mixed raw material may be a temperature equal to or higher than the temperature at which sulfur is vaporized. 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 mixed raw material will be 150 degreeC or more. Therefore, if the temperature of the mixed raw material is 150 ° C. or higher, sulfur can be refluxed. In addition, what is necessary is just to recirculate | reflux sulfur using the reflux apparatus of a known structure, when recirculating | refluxing sulfur in a heat treatment process.

  Even when the amount of sulfur in the mixed material is excessive, a sufficient amount of sulfur can be taken into the carbon material in the heat treatment step. For this reason, in the case where sulfur is excessively added to the carbon material, the above-described adverse effects due to the elemental sulfur can be suppressed by removing the elemental sulfur from the object to be treated (sulfur-modified polyacrylonitrile) after the heat treatment step. Specifically, when the mixing ratio of the carbon material and sulfur in the mixed raw material is 1: 2 to 1:10 by mass ratio, the object to be processed after the heat treatment step is heated at 200 ° C. to 250 ° C. while reducing the pressure. By doing so, a sufficient amount of sulfur can be taken into the carbon material, and adverse effects due to the remaining single sulfur can be suppressed. When omitting the step of removing simple sulfur, the object to be treated may be used as it is as sulfur-modified polyacrylonitrile. Moreover, what is necessary is just to use the to-be-processed body after single-piece | unit sulfur removal as sulfur modified polyacrylonitrile, when removing single-piece | unit sulfur from the to-be-processed body after a heat treatment process.

  Hereinafter, the sulfur-modified polyacrylonitrile, the positive electrode for a non-aqueous electrolyte secondary battery, the non-aqueous electrolyte secondary battery, and the method for evaluating the sulfur-modified polyacrylonitrile of the present invention will be specifically described.

<Sulfur modified polyacrylonitrile>
[1] Mixed raw material A sulfur powder having a particle size of 50 μm or less when classified using a sieve was prepared. A polyacrylonitrile powder having a particle size in the range of 0.2 μm to 2 μm when prepared with an electron microscope was prepared. 5 g of sulfur powder and 1 g of polyacrylonitrile powder were mixed and pulverized in a mortar to obtain a mixed raw material.
[2] Apparatus As shown in FIG. 3, the reaction apparatus 1 includes a reaction vessel 2, a lid 3, a thermocouple 4, an alumina protective tube 40, two alumina tubes (gas introduction tube 5, gas discharge tube 6), argon gas It has a pipe 50, a gas tank 51 containing argon gas, a trap pipe 60, a trap tank 62 containing a sodium hydroxide aqueous solution 61, an electric furnace 7, and a temperature controller 70 connected to the electric furnace.

  As the reaction vessel 2, a bottomed cylindrical glass tube (made of quartz glass) was used. In the heat treatment step described later, the mixed raw material 9 was accommodated in the reaction vessel 2. The opening of the reaction vessel 2 was closed with a glass lid 3 having three through holes. An alumina protective tube 40 (alumina SSA-S, manufactured by Nikkato Co., Ltd.) accommodating the thermocouple 4 was attached to one of the through holes. A gas introduction pipe 5 (alumina SSA-S, manufactured by Nikkato Corporation) was attached to the other one of the through holes. A gas exhaust pipe 6 (alumina SSA-S, manufactured by Nikkato Corporation) was attached to the remaining one of the through holes. The reaction vessel 2 had an outer diameter of 60 mm, an inner diameter of 50 mm, and a length of 300 mm. The alumina protective tube 40 had an outer diameter of 4 mm, an inner diameter of 2 mm, and a length of 250 mm. The gas introduction pipe 5 and the gas discharge pipe 6 had an outer diameter of 6 mm, an inner diameter of 4 mm, and a length of 150 mm. The tips of the gas introduction pipe 5 and the gas discharge pipe 6 were exposed to the outside of the lid 3 (inside the reaction vessel 2). The length of this exposed portion was 3 mm. The tips of the gas introduction pipe 5 and the gas discharge pipe 6 become approximately 100 ° C. or less in a heat treatment process described later. For this reason, the sulfur vapor generated in the heat treatment step does not flow out of the gas introduction pipe 5 and the gas discharge pipe 6 but is returned (refluxed) to the reaction vessel 2.

  The tip of the thermocouple 4 placed in the alumina protective tube 40 indirectly measured the temperature of the mixed raw material 9 in the reaction vessel 2. The temperature measured by the thermocouple 4 was fed back to the temperature controller 70 of the electric furnace 7.

  An argon gas pipe 50 was connected to the gas introduction pipe 5. The argon gas pipe 50 was connected to a gas tank 51 containing argon gas. One end of a trap pipe 60 was connected to the gas discharge pipe 6. The other end of the trap pipe 60 was inserted into the sodium hydroxide aqueous solution 61 in the trap tank 62. The trap pipe 60 and the trap tank 62 are traps for hydrogen sulfide gas generated in a heat treatment process to be described later.

[3] Heat treatment step The reaction vessel 2 containing the mixed raw material 9 was placed in an electric furnace 7 (crucible furnace, opening width φ80 mm, heating height 100 mm). At this time, argon was introduced into the reaction vessel 2 through the gas introduction tube 5. The flow rate of argon gas at this time was 100 ml / min. Ten minutes after the start of the introduction of the argon gas, heating of the mixed raw material 9 in the reaction vessel 2 was started while continuing the introduction of the argon gas. The temperature rising rate at this time was 5 ° C./min. When the mixed raw material 9 reached 100 ° C., the introduction of argon gas was stopped while continuing to heat the mixed raw material 9. Gas was generated when the mixed raw material 9 reached about 200 ° C. The heating was stopped when the mixed raw material 9 reached 360 ° C. After stopping the heating, the temperature of the mixed raw material 9 increased to 400 ° C. and then decreased. Therefore, in this heat treatment step, the mixed raw material 9 was heated to 400 ° C. Thereafter, the mixed raw material 9 was naturally cooled, and when the mixed raw material 9 was cooled to room temperature (about 25 ° C.), the product (that is, the object to be treated after the heat treatment step) was taken out from the reaction vessel 2. The heating time at this time was about 10 minutes at 400 ° C., and sulfur was refluxed.

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

  The object to be treated after the heat treatment step was pulverized in a mortar. 2 g of the pulverized product was 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 step, the elemental sulfur remaining in the object to be treated after the heat treatment step was evaporated and removed, and the sulfur-modified polyacrylonitrile of Example 1 not including (or substantially not including) elemental sulfur was obtained.

<Production of lithium ion secondary battery>
[1] Positive electrode 3 mg of the sulfur-modified polyacrylonitrile of Example 1, 2.1 mg of acetylene black (AB), and 0.9 mg of TAB were mixed. TAB is a mixture of AB and polytetrafluoroethylene (PTFE) mixed at AB: PTFE = 2: 1 (mass ratio). This mixture was kneaded with an agate mortar while adding an appropriate amount of hexane until it became a film, to obtain a film-like positive electrode material. The total amount of the positive electrode material was press-bonded to an aluminum mesh punched into a circle having a diameter of 14 mm with a press machine and dried at 100 ° C. for 3 hours. In this step, the positive electrode for the lithium ion secondary battery of Example 1 was obtained.

[2] Negative electrode As the negative electrode, a metal lithium foil having a thickness of 500 μm punched to a diameter of 14 mm was used.

[3] Electrolytic Solution As the electrolytic solution, a nonaqueous electrolyte in which LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate was used. Ethylene carbonate and diethyl carbonate were mixed at a mass ratio of 1: 1. The concentration of LiPF ₆ in the electrolytic solution was 1.0 mol / l.

[4] Battery A coin battery was manufactured using the positive electrode and the negative electrode obtained in [1] and [2]. Specifically, in a dry room, a separator (Celgard 2400) made of a polypropylene microporous membrane with a thickness of 25 μm and a glass nonwoven fabric filter with a thickness of 500 μm are sandwiched between a positive electrode and a negative electrode, and an electrode body battery It was. This electrode body battery was accommodated in a battery case (CR2032-type coin battery member, manufactured by Hosen Co., Ltd.) made of a stainless steel container. The electrolyte solution obtained in [3] was injected into the battery case. The battery case was sealed with a caulking machine to obtain a lithium ion secondary battery of Example 1.

<Measurement of particle size of sulfur-modified polyacrylonitrile>
Seven types of polyacrylonitrile having different manufacturers and / or different production lots were prepared. The breakdown consists of three types produced by suspension polymerization (samples 1 to 3), one produced by bulk polymerization (sample 4), one produced by solution polymerization (sample 5), and Two types (samples 7 and 8) were produced by emulsion polymerization. Also, the same polyacrylonitrile as sample 8 was subjected to dry milling (milling using a ball mill) (sample 6), and wet milling (polyacrylonitrile dispersed in ethanol and milling using a ball mill) (sample 9). ) Was prepared.

  Each sulfur-modified polyacrylonitrile of Samples 1 to 9 was imaged at a magnification of 5000 with a scanning electron microscope (SEM). The acceleration voltage at this time was 5 to 10 kV, and platinum was used for coating. SEM images of the surface of each sulfur-modified polyacrylonitrile are shown in FIGS.

  As shown in FIGS. 4 to 12, the appearance of the sulfur-modified polyacrylonitrile of Samples 1 to 5 and the appearance of the sulfur-modified polyacrylonitrile of Samples 6 to 9 are greatly different.

  As shown in FIGS. 4 to 8, the sulfur-modified polyacrylonitrile of Samples 1 to 5 was observed as an aggregate of fine particles having a particle size of 1 μm or less in the SEM image. Because it is an aggregate of a large number of particles, the aggregate as a whole appeared to be in the form of secondary particles of irregular shape. Assuming that the entire area where the sulfur-modified polyacrylonitrile was imaged was 100 area%, the area occupied by particles having a particle diameter of 1 μm or less and / or aggregates of these particles was 80 area% or more. Preferably it is 90 area% or more. On the SEM image, no granular material having a particle size exceeding 5 μm was confirmed.

  As shown in FIGS. 9 to 12, the sulfur-modified polyacrylonitriles of Samples 6 to 9 were observed as granular bodies having a particle size exceeding 5 μm in the SEM images. Although cracks were observed on the surface, the entire granule was not indeterminately shaped and was almost granular. Little or no aggregates of particles having a particle diameter of 1 μm or less and / or aggregates of these particles were confirmed.

  Based on the SEM images, it was evaluated that the sulfur-modified polyacrylonitrile of Samples 1 to 5 was compatible PAN, and the sulfur-modified polyacrylonitrile of Samples 6 to 9 was non-compatible PAN.

  In this evaluation test, the entire region where polyacrylonitrile was imaged in the SEM image obtained by imaging each sulfur-modified polyacrylonitrile at a magnification of 5000 was used as a parameter, that is, “entire particle”. And the area | region which the area | region where the particle diameter of 1 micrometer or less and / or its aggregate imaged in this was occupied was computed. However, the parameter in the sulfur-modified polyacrylonitrile and the evaluation method of the present invention is not limited to this. For example, “the entire region where polyacrylonitrile is imaged in an SEM image captured at 3000 to 7000 times” may be used as a parameter.

<Evaluation of charge / discharge characteristics>
Using each of the sulfur-modified polyacrylonitriles of Samples 1 to 9, lithium ion secondary batteries of Samples 1 to 9 were manufactured by the above-described method for manufacturing a lithium ion secondary battery.

  About each lithium ion secondary battery of samples 1-9, charging / discharging was performed repeatedly at 30 degreeC. Specifically, CC discharge (low current discharge) was first performed at 0.1 C to 1.0 V. In the subsequent cycles, charge and discharge in which CC discharge was performed at 0.1 C to 1.0 V after CC charge to 0.1 V at 0.1 C was repeated. And the discharge capacity of the 2nd time of each lithium ion secondary battery was compared. The second discharge capacity of each lithium ion secondary battery is 764.7 mAh / g for sample 1, 747.5 mAh / g for sample 2, 732.2 mAh / g for sample 3, 594.3 mAh / g for sample 4, The sample 5 was 384.3 mAh / g, the sample 6 was 339.0 mAh / g, the sample 7 was 316.8 mAh / g, the sample 8 was 286.1 mAh / g, and the sample 9 was 129.6 mAh / g. For reference, the results of the cycle test of the lithium ion secondary battery of Sample 1 are shown in FIGS. The result of the cycle test of the lithium ion secondary battery of Sample 8 is shown in FIG. Table 1 shows the second discharge capacity of each lithium ion secondary battery.

  The second discharge capacity of the lithium ion secondary batteries of Samples 1 to 5 was larger than the second discharge capacity of the lithium ion secondary batteries of Samples 6 to 9. This result confirmed that the sulfur-modified polyacrylonitrile of Samples 1 to 5 was a compatible PAN, and the sulfur-modified polyacrylonitrile of Samples 6 to 9 was a non-compatible PAN.

<Analysis of sulfur-modified polyacrylonitrile by FT-IR>
With respect to the sulfur-modified polyacrylonitrile of Samples 1 to 9, the presence or absence of impurities and stereoregularity were measured by FT-IR. As an apparatus, Affinity-1 (manufactured by Shimadzu Corporation) was used, and measurement by a diffuse reflection method was performed. The resolution was 8 cm ⁻¹ and the number of integrations was 50 times. Charts obtained by FT-IR are shown in FIGS. FIG. 25 shows a graph showing the relationship between the absorbance ratio (D ₁₂₃₀ / D ₁₂₅₀ ) by FT-IR and the second discharge capacity.

As shown in FIGS. 16 to 24, the C = O peak was not observed in the sulfur-modified polyacrylonitrile of Samples 1 to 5, whereas the C = O peak was observed in the sulfur-modified polyacrylonitrile of Samples 6 to 9. It was. Further, the sulfur-modified polyacrylonitrile of Samples 1 to 5 almost coincided with the fingerprint region. Furthermore, the absorbance ratio (D ₁₂₃₀ / D ₁₂₅₀ ) by FT-IR is 0.73 for sample 1, 0.75 for sample 2, 0.71 for sample 3, 0.73 for sample 4, and 0.75 for sample 5. The sample 6 was 1.04, the sample 8 was 1.09, and the sample 9 was 1.04. In Sample 7, D ₁₂₃₀ / D ₁₂₅₀ could not be calculated because the peak of D _{1230 and} the peak of D ₁₂₅₀ were not separated.

  Since the C = O peak was not recognized in the samples 1 to 5 having the conforming PAN, and the C = O peak was observed in the samples 6 to 9 being the non-conforming PAN, It can be seen that a non-conforming PAN can be identified.

In addition, D ₁₂₃₀ / D ₁₂₅₀ of samples 1 to 5 that are compliant PANs is a very small value of 0.75 or less, whereas D ₁₂₃₀ / D ₁₂₅₀ of samples 6, 8, and 9 that are non-compliant PANs is Since it was a large value exceeding 1.00, it can be understood that the conforming PAN and the non-conforming PAN can also be identified by D ₁₂₃₀ / D ₁₂₅₀ .

As shown in FIG. 25, there is a strong correlation between the absorbance ratio (D ₁₂₃₀ / D ₁₂₅₀ ) by FT-IR and the second discharge capacity. Specifically, the correlation coefficient between the two is −0.83, and both have a negative correlation.

<Analysis of sulfur-based positive electrode active materials by thermal mass spectrometry>
The thermal mass change (TG) of the sulfur-modified polyacrylonitrile of Samples 1 to 9 was measured. A Rigaku thermal analyzer (Thermo Plus TG8120) was used as the measuring device. Specifically, by supplying high purity nitrogen gas at a flow rate of 100 ml / min, each sample was heated from room temperature to 550 ° C. at a rate of temperature increase of 10 ° C./min, and the relationship between temperature and mass change was measured. Thermal mass-differential thermal analysis was performed. The analysis results are shown in FIGS.

  As shown in FIGS. 26 to 34, mass changes and temperature changes were observed in samples 6 to 9 in a low temperature range of 100 ° C. or lower, but such mass changes and temperature changes were observed in samples 1 to 5. I couldn't. Since the mass change and temperature change in this low temperature region indicate the presence of water, it can be seen that Samples 6 to 9 contain water. Samples 4 and 7 showed temperature changes in the vicinity of 240 and in a high temperature range of 320 ° C. to 370 ° C. or higher. Since this temperature change indicates the presence of impurities, it can be seen that the samples 4 and 7 contain impurities. From these results, it can be seen that compatible PAN and non-compatible PAN can also be distinguished by water and / or impurities.

1: Reactor 2: Reaction vessel 3: Lid 4: Thermocouple

Claims (10)

A sulfur-modified polyacrylonitrile made from sulfur and polyacrylonitrile,
The area occupied by particles having a particle diameter of 1 μm or less and / or aggregates of the particles in the entire particles is 80% or more in terms of the imaging area ratio ,
Sulfur, characterized in that the absorbance ratio D ₁₂₃₀ / D ₁₂₅₀ of absorbance D _{1230 at 1230} cm ⁻¹ and absorbance D _{1250 at 1250} cm ⁻¹ by Fourier transform infrared spectroscopy of the polyacrylonitrile is 0.75 or less Modified polyacrylonitrile.   A sulfur-modified polyacrylonitrile made from sulfur and polyacrylonitrile,
  The area occupied by particles having a particle diameter of 1 μm or less and / or aggregates of the particles in the entire particles is 80% or more in terms of the imaging area ratio,
  The polyacrylonitrile is a sulfur-modified polyacrylonitrile characterized in that a peak derived from C═O by Fourier transform infrared spectroscopy is not confirmed. In the Raman spectrum, there is a main peak near 1331cm ^-1 of Raman shift, ^{and, 1548cm} -1 in the range of ^{200cm -1 ~1800cm -1, 939cm -1,} 479cm -1, 381cm -1, 317cm -1 The sulfur-modified polyacrylonitrile according to claim 1 or 2 , wherein a peak exists in the vicinity thereof . The sulfur-modified polyacrylonitrile according to claim 1 or 3 , wherein the D ₁₂₃₀ / D ₁₂₅₀ is 0.73 or less. A positive electrode for a nonaqueous electrolyte secondary battery comprising the sulfur-modified polyacrylonitrile according to any one of claims 1 to 4 as a positive electrode active material. A non-aqueous electrolyte secondary battery comprising the sulfur-modified polyacrylonitrile according to any one of claims 1 to 4 as a positive electrode active material in a positive electrode. A vehicle equipped with the nonaqueous electrolyte secondary battery according to claim 6 . An evaluation method for sulfur-modified polyacrylonitrile used as a positive electrode active material of a nonaqueous electrolyte secondary battery,
Of all the particles, particles having a particle diameter of 1 μm or less and / or an area occupied by the aggregate of the particles are determined as conforming products when the imaging area ratio is 80% or more, and the other regions are regarded as non-conforming products. A method for evaluating sulfur-modified polyacrylonitrile, which is characterized by being evaluated. A method for producing a nonaqueous electrolyte secondary battery, comprising the method for evaluating a sulfur-modified polyacrylonitrile according to claim 8 . A sulfur-modified polyacrylonitrile made from sulfur and polyacrylonitrile,
A sulfur-modified polyacrylonitrile, wherein the polyacrylonitrile is produced by bulk polymerization, suspension polymerization or solution polymerization.

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