JPWO2012132173A1 - Nonaqueous electrolyte secondary battery and vehicle - Google Patents

Abstract

An object of the present invention is to provide a non-aqueous electrolyte secondary battery using a sulfur-based positive electrode active material and further improved in cycle characteristics. The present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes a sulfur-based positive electrode active material containing carbon (C) and sulfur (S) as a positive electrode active material. The present invention relates to a drain electrolyte secondary battery containing metallic lithium or metallic sodium as a negative electrode active material.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery represented by a lithium secondary battery, a sodium secondary battery, and the like, and a vehicle equipped with the non-aqueous electrolyte secondary battery.

  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. 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.

  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 and sulfur (see Patent Document 1). By using a sulfur-based positive electrode active material made of a carbon material such as polyacrylonitrile 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. However, since the cycle characteristics are very important characteristics for the secondary battery, it is desired to develop a non-aqueous electrolyte secondary battery having further improved cycle characteristics.

International Publication No. 2010/044437

  The present invention has been made in view of the above circumstances, and provides a non-aqueous electrolyte secondary battery that uses a sulfur-based positive electrode active material and is further excellent in cycle characteristics, and a vehicle equipped with the non-aqueous electrolyte secondary battery. For the purpose.

As a kind of non-aqueous electrolyte secondary battery, one using a positive electrode active material other than a sulfur-based positive electrode active material and using metallic lithium or metallic sodium as a negative electrode active material is known. This type of non-aqueous electrolyte secondary battery is inferior in cycle characteristics. For example, in a non-aqueous electrolyte secondary battery using LiCoO ₂ as a positive electrode active material and metallic lithium as a negative electrode active material, if the charge / discharge is repeated, the surface of the metal lithium of the negative electrode deteriorates, and therefore the cycle characteristics are inferior. Conceivable. For this reason, metallic lithium has usually only been used as a negative electrode of an evaluation battery (also called a half battery) for evaluating the performance of a lithium ion secondary battery.

  As a result of extensive research, the inventors of the present invention have used a sulfur-based positive electrode active material as a positive electrode active material and metal lithium or metal sodium as a negative electrode active material. It has been found that the cycle characteristics can be greatly improved.

That is, the nonaqueous electrolyte secondary battery of the present invention that solves the above problems is
A positive electrode, a negative electrode, and a non-aqueous electrolyte;
The positive electrode includes a sulfur-based positive electrode active material containing carbon (C) and sulfur (S) as a positive electrode active material,
The negative electrode includes metallic lithium or metallic sodium as a negative electrode active material.
The vehicle of the present invention that solves the above problems is characterized by mounting the non-aqueous electrolyte secondary battery of the present invention.

  The nonaqueous electrolyte secondary battery of the present invention is excellent in cycle characteristics. Although the reason is not certain, it is thought that the film | membrane derived from a sulfur type positive electrode active material is formed in the surface of a negative electrode, and deterioration of metal lithium and metal sodium is suppressed by this film | membrane. Moreover, it is thought that the negative electrode contamination by sulfur is suppressed by the sulfur-based positive electrode active material stably capturing sulfur, and the above film is stabilized. Further, when the negative electrode active material is composed of metal lithium or metal sodium, a sufficient amount of lithium or sodium that can counter the irreversible capacity after the initial charge / discharge can be contained in the nonaqueous electrolyte secondary battery. By these cooperation, it is considered that the nonaqueous electrolyte secondary battery of the present invention exhibits extremely excellent cycle characteristics. In addition, the vehicle of the present invention equipped with the nonaqueous electrolyte secondary battery of the present invention having excellent cycle characteristics 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. 3 is a graph showing cycle characteristics of the nonaqueous electrolyte secondary battery of Example 1. FIG. 6 is a graph showing cycle characteristics of the nonaqueous electrolyte secondary battery of Example 2. 6 is a graph showing cycle characteristics of the nonaqueous electrolyte secondary battery of Example 3. It is a graph showing the cycle characteristic of the nonaqueous electrolyte secondary battery of a comparative example.

(Sulfur-based positive electrode active material)
The nonaqueous electrolyte secondary battery of the present invention contains a sulfur-based positive electrode active material in the positive electrode. Examples of the sulfur-based positive electrode active material include polysulfide carbon, elemental sulfur, heat-treated sulfur and plant materials such as coffee beans and seaweed, and composites thereof. As the carbon source compound, [1 ] Using at least one selected from polyacrylonitrile, [2] pitches, [3] polyisoprene, and [4] polycyclic aromatic hydrocarbons formed by condensation of six or more 6-membered rings, Particularly preferably used. These sulfur-based positive electrode active materials are composed of a carbon skeleton derived from any of the carbon source compounds described above and sulfur (S) bonded to the carbon skeleton. Hereinafter, a sulfur-based positive electrode active material using polyacrylonitrile as a carbon source compound is referred to as sulfur-modified PAN. A sulfur-based positive electrode active material using pitches as a carbon source compound is called sulfur-modified pitch. A sulfur positive electrode active material using polyisoprene as a carbon source compound is called sulfur-modified polyisoprene. A sulfur-based positive electrode active material using a polycyclic aromatic hydrocarbon formed by condensation of three or more six-membered rings as a carbon source compound is referred to as sulfur-modified PAH.

[1] Sulfur modified PAN
The sulfur-modified PAN is the same as that disclosed in Patent Document 1 above. Polyacrylonitrile (PAN) as a material for sulfur-modified PAN is preferably in the form of powder and preferably has a mass average molecular weight of about 10 ^{4 to} 3 × 10 ⁵ . Further, the particle size of PAN 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 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 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 manufacturing method of the sulfur type positive electrode active material currently disclosed by patent document 1, sulfur is heat-processed 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. In other words, sulfur-modified PAN has a carbon skeleton derived from PAN. By combining PAN and sulfur, elution of sulfur into the electrolyte can be suppressed, and cycle characteristics can be improved.

  Sulfur used for sulfur-modified PAN is preferably in the form of a powder, like PAN. Although it does not specifically limit about the particle size of sulfur, when classified using a sieve, those that do not pass through a sieve with a sieve opening of 40 μm and are within a size range that passes through a 150 μm sieve are 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 PAN powder and sulfur powder 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. More preferably, it is 1: 2 to 1: 5.

  Sulfur-modified PAN can be produced by the following method.

  A mixed raw material in which PAN powder and sulfur powder are mixed is heated (heat treatment step). What is necessary is just to mix a mixing raw material with common mixing apparatuses, such as a mortar and a ball mill. As the mixed raw material, a material obtained by simply mixing sulfur and PAN may be used. For example, the mixed raw material may be formed into a pellet shape. The mixed raw material may be composed only of PAN and sulfur, or may be blended with a general material (such as a conductive aid) that can be blended with the positive electrode active material. The same applies to other sulfur-based positive electrode active materials described later.

  By heating the mixed raw material in the heat treatment step, sulfur and PAN contained in the mixed raw 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 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.

  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 a temperature which the coupling | bonding of sulfur and PAN advances, and a conductive material does not change in quality. Specifically, the heating temperature is preferably 250 ° C. or more and 500 ° C. or less, more preferably 250 ° C. or more and 450 ° C. or less, and further preferably 250 ° 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 raw material 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 mixed raw material is 1: 2 to 1:10 by mass ratio, the target object after the heat treatment step is heated at 200 ° C. to 250 ° C. while reducing the pressure. (Single element sulfur removal step) By taking a sufficient amount of sulfur into the PAN, it is possible to suppress adverse effects due to the remaining single sulfur. 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.

  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.

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 the Raman spectrum of the positive electrode of the present invention using sulfur-modified PAN as the positive electrode active material is measured, the same peak as the above peak, or a peak slightly different from the above peak in number and peak top position is confirmed. The

[2] Sulfur Modified Pitch A sulfur-based positive electrode active material using pitches as a carbon source compound is composed of a carbon skeleton derived from pitches and sulfur (S) bonded to the carbon skeleton. Pitches include coal pitch, petroleum pitch, mesophase pitch, asphalt, coal tar, coal tar pitch, organic synthetic pitch obtained by polycondensation of condensed polycyclic aromatic hydrocarbon compounds, and heteroatom-containing condensed polycyclic aromatic carbonization. At least one selected from the group consisting of organic synthetic pitches obtained by polycondensation of hydrogen compounds can be used.

  Coal tar, a kind of pitch, is a black viscous oily liquid obtained by high-temperature carbonization (coal carbonization) of coal. Coal pitch can be obtained by refining and heat treating (polymerizing) coal tar.

  Asphalt is a black-brown to black solid or semi-solid plastic material. Asphalt is broadly classified into what is obtained as a kettle residue when petroleum (crude oil) is distilled under reduced pressure and that which exists in nature. Asphalt is soluble in toluene, carbon disulfide and the like. Petroleum pitch can be obtained by refining and heat treating (polymerizing) asphalt.

  The pitch is usually amorphous and optically isotropic (isotropic pitch). An optically anisotropic pitch (anisotropic pitch, mesophase pitch) can be obtained by heat-treating the isotropic pitch in an inert atmosphere. Pitch is partially soluble in organic solvents such as benzene, toluene, carbon disulfide.

  Pitches are a mixture of various compounds and contain fused polycyclic aromatics as described above. The condensed polycyclic aromatic contained in the pitch may be a single species or a plurality of species. For example, the main component of coal pitch, which is a kind of pitches, is a condensed polycyclic aromatic. The condensed polycyclic aromatic can contain nitrogen and sulfur in addition to carbon and hydrogen in the ring. For this reason, the main component of coal pitch is considered to be a mixture of a condensed polycyclic aromatic hydrocarbon composed only of carbon and hydrogen and a heteroaromatic compound containing nitrogen, sulfur, etc. in the condensed ring.

  A sulfur-based positive electrode active material (sulfur-modified pitch) comprising a carbon skeleton derived from pitches and sulfur (S) bonded to the carbon skeleton can be produced by the same method as the above-described sulfur-modified PAN. When producing a sulfur-modified pitch, in the heat treatment step, at least a part of pitches and at least a part of sulfur are configured to be liquid. In other words, in the heat treatment step, at least a part of the pitches and at least a part of the sulfur are in liquid contact. For this reason, the contact area between pitches and sulfur in the heat treatment step can be made sufficiently large, and a sulfur-modified pitch that contains sulfur sufficiently and suppresses the elimination of sulfur can be obtained. In addition, when sulfur is refluxed in the heat treatment step, the contact frequency between pitches and sulfur can be increased, and a sulfur-modified pitch that further contains sulfur and further suppresses the elimination of sulfur can be obtained. .

  In the obtained sulfur-modified pitch, it is not clear how sulfur and pitches are bonded, but sulfur is taken in between the graphene layers of the pitches, or condensed polycyclic aromatics. It is presumed that hydrogen contained in the ring is substituted with sulfur to form a C—S bond.

  The temperature in the heat treatment step may be a temperature at which at least a part of pitches and at least a part of sulfur become liquid. In addition, regarding pitches, it is preferable that the temperature is such that the whole becomes a liquid. Further, regarding sulfur, the temperature is preferably such that the whole becomes a liquid, and more preferably a temperature at which a part becomes a gas and the rest becomes a liquid (that is, a temperature at which reflux is possible). The temperature in the heat treatment step is preferably 200 ° C. or higher, more preferably 300 ° C. or higher, and further preferably 350 ° C. or higher. For reference, the softening point of the coal pitch is about 200 to 350 ° C. For this reason, when using coal pitch as pitches, it is preferable to perform a heat treatment process at 350 degreeC or more. Moreover, if it is 350 degreeC or more, also when using pitches other than coal pitch, at least one part of pitches will soften (liquefy).

  By the way, when the temperature in the heat treatment process is excessively high, the pitches may be modified (graphitized). In this case, sulfur cannot be sufficiently taken into pitches. For this reason, it is preferable that the temperature in the heat treatment step is lower than the modification temperature of the pitches. If the temperature in the heat treatment step is 600 ° C. or lower, the modification of pitches can be suppressed. The temperature in the heat treatment step is more preferably 600 ° C. or less, and further preferably 500 ° C. or less. Furthermore, considering the above-mentioned softening of pitches, the temperature in the heat treatment step is preferably 200 ° C. or more and 600 ° C. or less, more preferably 300 ° C. or more and 500 ° C. or less, and 350 ° C. or more and 500 ° C. or less. Is more preferable.

  When sulfur is refluxed in the heat treatment step, the mixed raw material may be heated so that part of the mixed raw material becomes gas and part becomes 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.

  The heat treatment step is preferably performed in a non-oxidizing atmosphere as in the case of producing sulfur-modified PAN.

  The shape of the pitches and sulfur, the particle size, etc. are not particularly limited. This is because the pitches and sulfur are brought into contact with each other in a liquid state in the heat treatment step, so that the pitches and sulfur are in sufficient contact even when the pitches have a non-uniform or large particle size. The pitches and sulfur in the mixed raw material are preferably uniformly dispersed, but may be non-uniform. The mixed raw material may be composed only of pitches and sulfur, or may be blended with a general material (such as a conductive aid) that can be blended with the positive electrode active material.

  The heating time in the heat treatment step may be appropriately set according to the heating temperature, and is not particularly limited. When heating at the preferable temperature mentioned above, it is preferable to heat about 10 minutes-10 hours, and it is more preferable to heat for 30 minutes-6 hours.

  There is also a preferred range for the mixing ratio of pitches and sulfur in the mixed raw material. If the amount of sulfur added to the pitches is too small, a sufficient amount of sulfur cannot be taken into the pitches. If the amount of sulfur added to the pitches is too large, free sulfur (single sulfur) is contained in the sulfur-modified pitch. This is because a large amount remains to contaminate the electrolytic solution in the nonaqueous electrolyte secondary battery. The mixing ratio of pitches and sulfur in the mixed raw material is preferably 1: 0.5 to 1:10 by mass ratio, more preferably 1: 1 to 1: 7, and 1: 2 A ratio of 1: 5 is particularly preferred.

  Even when the amount of sulfur added to pitches is excessive, a sufficient amount of sulfur can be taken into the pitches in the heat treatment step. For this reason, when adding sulfur excessively with respect to pitches, the single-piece | unit sulfur removal process should just be given to the to-be-processed body after a heat processing process similarly to the manufacturing method of the sulfur modified PAN mentioned above.

When Raman spectroscopy of sulfur-modified pitch obtained by the above production method, there is a major peak in the vicinity of 1557cm ^-1 of Raman shift, ^{and, 1371cm} -1 in the range of 200cm ^-1 ~1800cm -1, 1049cm -1, ^{994cm -1, 842cm -1, 612cm -1} , 412cm -1, 354cm -1, the peak respectively is present in the vicinity of 314 cm ^-1. Note that the Raman spectrum of sulfur-modified pitch is different from the Raman spectrum of sulfur-modified PAN described above.

  It is desirable that the sulfur-modified pitch further includes sulfur-modified PAN as a second sulfur-based positive electrode active material. By further including this second sulfur-based positive electrode active material, the cycle characteristics are further improved when used in a non-aqueous electrolyte secondary battery. The reason is not clear, but it is considered that sulfur is immobilized because of the large binding force between PAN and sulfur.

  In order to produce a sulfur-modified pitch further containing the second sulfur-based positive electrode active material, the sulfur-modified pitch and the second sulfur-based positive electrode active material (that is, sulfur-modified PAN) can be physically mixed. However, since stability may be a concern, in order to increase the stability, a mixing step of mixing raw materials containing pitches, PAN powder, and sulfur powder to form a mixed raw material, It is desirable to perform a heat treatment step of heating. As the PAN powder, those having a mass average molecular weight in the range of about 10,000 to 300,000 are preferable. Moreover, about the particle size of PAN, when it observes with an electron microscope, what is in the range of about 0.5-50 micrometers is preferable, and what is in the range of about 1-10 micrometers is more preferable.

  As a result of elemental analysis of the sulfur-modified pitch, carbon, nitrogen, and sulfur were detected. In some cases, small amounts of oxygen and hydrogen were detected. Accordingly, the sulfur-modified pitch contains at least one of nitrogen, oxygen, sulfur compounds and the like as impurities in addition to C and S.

[3] Sulfur-modified polyisoprene As the polyisoprene, both natural rubber and synthetic polyisoprene can be used. However, cis-type polyisoprene tends to take an irregular shape with a structure in which molecular chains are bent, A cis type is preferable to a trans type because many gaps are generated between the molecular chains and the intermolecular force is relatively small, so that the molecules do not crystallize and have a soft property.

  The sulfur-modified polyisoprene can be produced by performing a mixing step of mixing a raw material containing polyisoprene and sulfur powder to make a mixed raw material, and a heat treatment step of heating the mixed raw material. In the mixing step, the dried product of polyisoprene may be pulverized and mixed with sulfur powder, a solution obtained by dissolving polyisoprene in a solvent and sulfur powder may be mixed, and latex such as natural rubber or raw rubber may be mixed. It is also possible to mix with sulfur powder. A mixer, various mills, etc. can be used for a mixing means.

  In the heat treatment step, polyisoprene and sulfur are reacted. This reaction is generally referred to as vulcanization, but it is desirable to react with an excessive amount of sulfur relative to the amount of polyisoprene to obtain a positive electrode active material containing sulfur at a high concentration. The temperature of this heat treatment step is desirably performed under the condition that at least a part of polyisoprene and at least a part of sulfur are liquid. By doing so, it is possible to obtain a sulfur-modified polyisoprene in which the contact area between polyisoprene and sulfur can be sufficiently increased, sulfur is sufficiently contained and sulfur desorption is suppressed.

  In the heat treatment process, if the temperature is too high, sulfur vaporizes, so the sulfur concentration in the reaction system may be lowered. In such a case, it is desirable to react while refluxing sulfur. By doing in this way, it becomes easy to obtain sulfur-modified polyisoprene that contains sulfur sufficiently. When sulfur is refluxed in the heat treatment step, since the melting point of polyisoprene is as low as about 30 ° C., the temperature may be higher than the temperature at which sulfur vaporizes.

  Note that vulcanization of a general rubber material is performed in a temperature range of 100 ° C to 190 ° C. Vulcanization at around 120 ° C. is called low-temperature vulcanization, and from around 180 ° C. is called high-temperature over-vulcanization. The temperature of the heat treatment performed in the present invention is higher than the above temperature range, and the heating temperature is preferably 250 ° C to 500 ° C, more preferably 300 ° C to 450 ° C. The heat treatment atmosphere can be performed in the same manner as in the case of the pitches described above.

  There are no particular limitations on the shape, particle size, etc. of polyisoprene and sulfur in the mixed raw material. Since it is preferable that polyisoprene and sulfur come into contact with each other in the heat treatment step, polyisoprene and sulfur come into contact with each other even when the particle size of polyisoprene or sulfur is uneven or large. This is because the polyisoprene and sulfur are sufficiently in contact with each other. The polyisoprene and sulfur in the mixed raw material are preferably uniformly dispersed, but may be non-uniform.

  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. When the mixed raw material is heated at the above-mentioned preferable temperature, it is preferably heated for about 1 minute to 10 hours, more preferably 5 minutes to 60 minutes. The vulcanization time for a general rubber material is several minutes to several tens of minutes depending on the heating temperature. Vulcanization time exceeding 1 hour is called over-vulcanization, and it is said that the performance as a rubber is lowered. The sulfur-modified polyisoprene used in the present invention does not need the flexibility required for rubber materials, and there is no problem even if the heat treatment time is longer than the time called overvulcanization.

  In the above production method, there is a preferable range for the blending ratio of polyisoprene and sulfur in the mixed raw material. If the amount of sulfur in the polyisoprene is too small, sufficient amount of sulfur cannot be taken into the polyisoprene, and if the amount of sulfur in the polyisoprene is excessive, free sulfur (single sulfur) is contained in the sulfur-modified polyisoprene. This is because a large amount remains to contaminate the electrolytic solution in the nonaqueous electrolyte secondary battery. The blending ratio of polyisoprene and sulfur in the mixed raw material is preferably 1: 0.5 to 1:10, and more preferably 1: 1 to 1: 7, in terms of mass ratio. 1 to 2: 5 is particularly preferred.

In addition, the general rubber vulcanization treatment using natural rubber as a main raw material changes the ratio of adding sulfur to the rubber to change the expansion and contraction of the rubber. An elastic rubber (for example, a rubber band) is produced by adding about 3 to 6% of sulfur to a chain-structured raw rubber and heat-treating it. When sulfur is about 30 to 40%, hard rubber (ebonite, use example: Light bulb socket, fountain pen). Usually, vulcanization of rubber is performed at a temperature of about 140 ° C., but in the above production method, it is performed at a high temperature of 250 to 500 ° C. Hydrogen sulfide gas is generated by extracting hydrogen from CH _{2 and the} like, and a reaction in which S is added instead of the extracted hydrogen occurs, and a substance having a high S content (sulfur content) is obtained.

  If the blending amount of sulfur with respect to polyisoprene is excessive, a sufficient amount of sulfur can be easily taken into the polyisoprene in the heat treatment step. And even if the amount of sulfur added to polyisoprene is more than necessary, the above-mentioned adverse effects due to the elemental sulfur are suppressed by performing the elemental sulfur removal step for removing excess elemental sulfur from the object to be treated after the heat treatment step. it can. Specifically, when the blending ratio of polyisoprene and sulfur in the mixed raw material is 1: 2 to 1:10 by mass ratio, the workpiece after the heat treatment step is heated at 200 ° C. to 250 ° C. while reducing the pressure. By performing (single sulfur removal step), it is possible to suppress an adverse effect caused by the remaining simple sulfur while incorporating a sufficient amount of sulfur into the polyisoprene. When the single sulfur removal step is not performed on the target object after the heat treatment step, the target object may be used as it is as the sulfur-modified polyisoprene. Moreover, what is necessary is just to use the to-be-processed body after a simple sulfur removal process as sulfur modified polyisoprene, when performing the simple substance sulfur removal process to the to-be-processed body after a heat treatment process.

  According to the above manufacturing method, instead of blending rare metals such as cobalt as a material for the positive electrode active material, a material obtained by reacting polyisoprene and sulfur is compared, so that the positive electrode active material of the nonaqueous electrolyte secondary battery is compared. Since it can be procured easily, it can be manufactured at low cost.

  Natural rubber is a material that has not been completely refined and is very inexpensive. For this reason, according to the said manufacturing method, it can manufacture cheaply compared with the case where carbon materials, such as PAN, are used, for example. Generally, natural rubber contains about 6-7% of non-rubber components such as proteins, fatty acids, carbohydrates, and ash. Even when such materials are used, sulfur functions as a sulfur-based positive electrode active material. Modified polyisoprene can be obtained.

  Also, polyisoprene can be easily converted into a liquid state by heating. For this reason, polyisoprene and sulfur are sufficiently brought into contact in the heat treatment step, and there is no need to particularly consider the particle size of polyisoprene or sulfur.

The sulfur-modified polyisoprene is considered to have a structure similar to ebonite, for example, as represented by the chemical formula 1, but the structure is not clear. However, having a carbon skeleton derived from polyisoprene, in FT-IR spectrum, a near 1452cm ^-1, and around 1336cm ^-1, and around 1147cm ^-1, and around 1067cm ^-1, and around 1039cm ^-1, 938cm ^- and near ^1, and around 895cm ^-1, and around 840 cm ^-1, and around 810 cm ^-1, and around 584cm ^-1, respectively main peak is present.

On the other hand, polyisoprene, in FT-IR spectrum, a near 3279cm ^-1, and around 3034cm ^-1, and around 2996cm ^-1, and around 2931cm ^-1, and around 2864cm ^-1, and around 2728cm ^-1, 1653cm ^- and near ^1, and around 1463cm ^-1, and around 1378 cm ^-1, and around 834cm ^-1, and around 579cm ^-1, respectively main peak is present.

Moreover, the substance obtained by heat-treating polyisoprene at 400 ° C. has a FT-IR spectrum of about 2962 cm ⁻¹ , about 2872 cm ⁻¹ , about 2723 cm ⁻¹ , about 1701 cm ⁻¹ , and about 1458 cm ^−1. When the vicinity of 1377 cm ^-1, and around 968cm ^-1, and around 885cm ^-1, and around 816cm ^-1, respectively main peak is present.

Further, general ebonite having a sulfur content of about 30% has an FT-IR spectrum of 2928 cm ⁻¹ , 2858 cm ⁻¹ , 1735 cm ⁻¹ , 1643 cm ⁻¹ , 1599 cm ⁻¹ , 1518cm and around ^-1, and around 1499Cm ^-1, and around 1462Cm ^-1, and around 1454Cm ^-1, and around 1447Cm ^-1, and around 1375 cm ^-1, and around 1310Cm ^-1, and around 1277cm ^-1, 1254cm ^{- 1} neighborhood, 1194 cm ^-1 neighborhood, 1115 cm ^-1 neighborhood, 1088 cm ^-1 neighborhood, 1031 cm ^-1 neighborhood, 953 cm ^-1 neighborhood, 835 cm ^-1 neighborhood, 739 cm ^-1 neighborhood, 696 cm ^-1 neighborhood and, each main and around 654cm ^-1, and around 592cm ^-1, to There is a peak.

In the FT-IR spectrum, a region of 1300 to 650 cm ⁻¹ is called a fingerprint region, and many fine peaks are observed in this region, and the pattern is unique to the substance. Therefore, it is possible to identify what the substance is by comparing the absorption in this region with known samples and spectral databases. The FT-IR spectrum of sulfur-modified polyisoprene, the material obtained by heat-treating polyisoprene and polyisoprene at 400 ° C., and ebonite are completely different from each other. Can be identified. In particular, peaks near 1067 cm ⁻¹ and 895 cm ⁻¹ are only found in sulfur-modified polyisoprene and can be identified by FT-IR spectrum.

  When elemental analysis of sulfur-modified polyisoprene is performed, sulfur (S) and carbon (C) are mostly occupied, and small amounts of oxygen and hydrogen are detected. It is desirable that the composition ratio of sulfur (S) and carbon (C) is included in the range of 1/5 or more in terms of atomic ratio (S / C). If the amount of sulfur is less than this range, the charge / discharge characteristics may be degraded when used in a positive electrode for a non-aqueous electrolyte secondary battery.

  It is desirable that the sulfur-modified polyisoprene further contains a second sulfur-based positive electrode active material (sulfur-modified PAN) as in the case where the pitches described above are used. The mixing amount, production method, and the like are the same as in the case of sulfur-modified pitch.

  The compounding ratio of the total amount of polyisoprene and PAN in the mixed raw material and sulfur can be 1: 0.5 to 1:10 by mass ratio. When the blending amount of sulfur with respect to the total amount of polyisoprene and PAN is too small, a sufficient amount of sulfur cannot be taken into polyisoprene and PAN, and when the blending amount of sulfur with respect to the total amount of polyisoprene and PAN is excessive, This is because a large amount of free sulfur (elemental sulfur) remains in the sulfur-modified polyisoprene and contaminates the electrolyte in the non-aqueous electrolyte secondary battery. The mixing ratio of sulfur with respect to the total amount of polyisoprene and PAN in the mixed raw material is preferably 1: 0.5 to 1:10, more preferably 1: 1 to 1: 7 in terms of mass ratio. 1 to 2: 5 is particularly preferred.

  The heat treatment step in the case of further containing PAN powder in the mixed raw material can be performed in the same manner as the above-described method for producing sulfur-modified PAN.

  The mixing amount of the second sulfur-based positive electrode active material is not particularly limited, but from the viewpoint of cost, it is preferably about 0 to 80% by mass, and about 5 to 60% by mass with respect to the entire positive electrode active material. Is more preferable, and it is still more preferable to set it as about 10-40 mass%.

[4] Sulfur modified PAH
Sulfur-modified PAH has a carbon skeleton derived from at least one polycyclic aromatic hydrocarbon (PAH) formed by condensation of three or more six-membered rings. PAH is a general term for hydrocarbons condensed with aromatic rings that do not contain heteroatoms or substituents. There are four-membered rings, five-membered rings, six-membered rings, and seven-membered rings. In the invention, among the acenes having a structure in which three or more six-membered rings that are benzene rings are connected in a straight chain, and compounds having a structure in which three or more six-membered rings are not straight but bent, etc. It is preferable to use at least one kind and sulfur.

  Examples of acenes that are polycyclic aromatic hydrocarbons in which a plurality of aromatic rings share a side and are connected in a straight chain include bicyclic naphthalene, tricyclic anthracene, tetracyclic tetracene, pentacyclic pentacene, 6 There are ring hexacene, 7 ring heptacene, 8 ring octacene, 9 ring nonacene, and 10 or more aromatic rings linked together, and at least one selected from these groups can be used. Among them, those having 3 to 6 rings having high stability are desirable.

  Polycyclic aromatic hydrocarbons having a structure in which three or more six-membered rings are not straight but bent include phenanthrene, benzopyrene, chrysene, pyrene, picene, perylene, triphenylene, coronene, and more rings. There are those in which the above aromatic rings are linked, and at least one selected from these groups can be used.

  Sulfur-modified PAH can be produced by the same method as sulfur-modified pitch or sulfur-modified polyisoprene.

  In the heat treatment step, the polycyclic aromatic hydrocarbon and sulfur are reacted. In this reaction, it is desirable to react with the amount of sulfur being excessive with respect to the amount of polycyclic aromatic hydrocarbons to obtain a positive electrode active material containing sulfur at a high concentration. It is desirable that the temperature of the heat treatment step be such that at least a part of the polycyclic aromatic hydrocarbon and at least a part of sulfur are liquid. By doing so, it is possible to obtain a sulfur-modified PAH in which the contact area between the polycyclic aromatic hydrocarbon and sulfur can be sufficiently increased, sulfur is sufficiently contained, and sulfur desorption is suppressed.

  There is a preferable range for the blending ratio of the polycyclic aromatic hydrocarbon and sulfur in the mixed raw material. If the amount of sulfur in the polycyclic aromatic hydrocarbon is too small, a sufficient amount of sulfur cannot be taken into the polycyclic aromatic hydrocarbon, and if the amount of sulfur in the polycyclic aromatic hydrocarbon is excessive, sulfur modification This is because a large amount of free sulfur (single sulfur) remains in the PAH and contaminates the electrolyte solution in the non-aqueous electrolyte secondary battery. The blending ratio of the polycyclic aromatic hydrocarbon and sulfur in the mixed raw material is preferably 1: 0.5 to 1:10 of polycyclic aromatic hydrocarbon: sulfur in a mass ratio, and 1: 1 to 1 : 7 is more preferable, and 1: 2 to 1: 5 is particularly preferable.

  If the amount of sulfur added to the polycyclic aromatic hydrocarbon is excessive, a sufficient amount of sulfur can be easily taken into the polycyclic aromatic hydrocarbon in the heat treatment step. And even if it mix | blends sulfur more than necessary with respect to polycyclic aromatic hydrocarbon, by performing the simple substance sulfur removal process which removes excess simple sulfur from the to-be-processed object after a heat treatment process, the simple substance sulfur mentioned above The adverse effect by can be suppressed. Specifically, when the mixing ratio of the polycyclic aromatic hydrocarbon and sulfur in the mixed raw material is 1: 2 to 1:10 by mass ratio, the object to be treated after the heat treatment step is 200 ° C. to By heating at 250 ° C. (single sulfur removal step), a sufficient amount of sulfur can be taken into the polycyclic aromatic hydrocarbon, and adverse effects due to the remaining simple sulfur can be suppressed. 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 PAH. Moreover, what is necessary is just to use the to-be-processed body after a simple sulfur removal process as sulfur modified PAH, when performing the simple substance sulfur removal process to the to-be-processed body after a heat treatment process.

For example, when pentacene is selected as the polycyclic aromatic hydrocarbon that is the starting material, the sulfur-modified PAH is considered to have a structure similar to hexathiapentacene as shown in Chemical Formula 2, The structure is not clear. In addition, the sulfur positive electrode active material using anthracene as the polycyclic aromatic hydrocarbon has peaks in the vicinity of 1056 cm ⁻¹ and 840 cm ^{−1 in} the FT-IR spectrum, and the FT-IR spectrum of anthracene. Since it is completely different from FT, it can be identified by FT-IR spectrum.

  When elemental analysis of sulfur-modified PAH is performed, sulfur (S) and carbon (C) occupy most, and small amounts of oxygen and hydrogen are detected. It is desirable that the composition ratio of sulfur (S) and carbon (C) is included in the range of 1/5 or more in terms of atomic ratio (S / C). If the amount of sulfur is less than this range, the charge / discharge characteristics may be degraded when used in a positive electrode for a non-aqueous electrolyte secondary battery.

  It is desirable that the sulfur-modified PAH further contains a second sulfur-based positive electrode active material (sulfur-modified PAN) as in the case of using the polyisoprene described above. The mixing amount, production method, and the like are the same as in the case of sulfur-modified polyisoprene.

(Positive electrode for non-aqueous electrolyte secondary battery)
The positive electrode in the nonaqueous electrolyte secondary battery of the present invention contains the sulfur-based positive electrode active material described above as the 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 except for the positive electrode active material. For example, a positive electrode can be manufactured by applying a positive electrode material mixed with sulfur-modified PAN, a conductive additive, a binder, and a solvent to a current collector. Alternatively, the mixed raw material in which the sulfur powder and the PAN powder are mixed can be heated (a heat treatment step is performed) after filling the positive electrode current collector. According to this method, PAN and sulfur are reacted to obtain sulfur-modified PAN, and at the same time, sulfur-modified PAN 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. The same applies to sulfur-modified pitch, sulfur-modified polyisoprene, sulfur-modified PAH and the like.

  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 PAN. As another method, a mixture of the above-described sulfur-based positive electrode active material, the above-described conductive additive and binder is kneaded with a mortar or a press to form a film, and the film-like mixture is collected with a press or the like. A positive electrode for a nonaqueous electrolyte secondary battery 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 nonwoven fabric, copper foil, copper mesh Punching copper sheet, copper expanded sheet, titanium foil, titanium mesh, carbon paper (nonwoven fabric / woven fabric) and the like are exemplified. Among these, a carbon paper current collector made of carbon having a high degree of graphitization does not contain hydrogen and has low reactivity with sulfur, and thus is suitable as a current collector for a sulfur-based positive electrode active material. 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 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 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 sulfided by sulfur as a raw material of the sulfur-based positive electrode active material.

  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-based positive electrode active material are easily blended by blending the metal into the positive electrode in the form of sulfide, 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.

Specifically, as a conductive material having high electrical conductivity and / or ionic conductivity (more specifically, lithium ion conductivity, sodium ion conductivity), TiS ₂ , FeS ₂ , Me ₂ S ₃ (wherein, Me is a kind selected from Ti, La, Ce, Pr, Nd, and Sm), MeS (wherein Me is a kind selected from Ti, La, Ce, Pr, Nd, and Sm), Me ₃ S ₄ (wherein Me is a kind selected from Ti, La, Ce, Pr, Nd, Sm), Me _x S _y (wherein Me is Ti, Fe, V, Mn, Fe, Ni, Cu, Zn, Mo, Ag, Cd, In, Sn, Sb, Ta, W, and Pb, and x and y are arbitrary integers. In this case, as the conductive material, Ti, Fe, La, Ce, Pr, Nd, Sm, V, Mn, Fe, Ni, Cu, Zn, Mo, Ag, Cd, In, Sn, Sb, Ta, W, What is necessary is just to use at least 1 type chosen from Pb as it is or in the state of sulfides like said conductive material. By using these conductive material materials, 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. In view of the raw material cost, ease of procurement, and the amount of resources, TiS _z (wherein z is 0.1 to 2) is more preferably used, and TiS ₂ is particularly preferably used.

  The mixing ratio of the above-mentioned sulfur-based positive electrode active material and conductive material is preferably 10: 0.5 to 10: 5, more preferably 10: 1 to 10: 3, in terms of mass ratio. This is because if the blending amount of the conductive material is excessive, the amount of the positive electrode active material relative to the entire positive electrode is excessively small. In order to disperse the conductive material substantially uniformly in the sulfur-based positive electrode active material, the conductive material is preferably in the form of powder. The conductive material preferably has a particle size measured with an electron microscope or the like of 0.1 to 100 μm, more preferably 0.1 to 50 μm, and even more preferably 0.1 to 20 μm.

  In addition, in order to identify mixing of a sulfur type positive electrode active material and a conductive material, it can carry out by X-ray diffraction analysis as follows.

The main diffraction peak positions of La ₂ S ₃ by ASTM card are 24.7, 25.1, 26.9, 33.5, 37.2, 42.8 °, and the like. The main diffraction peak positions of TiS ₂ are 15.5, 34.2, 44.1, 53.9 °, and the like. The main diffraction peak positions of Ti are 35.1, 38.4, 40.2, 53.0 °, and the like. The main diffraction peak positions of MoS ₂ are 14.4, 32.7, 33.5, 35.9, 39.6, 44.2, 49.8, 56.0, 58.4 °, and the like. The main diffraction peak positions of Fe are 44.7, 65.0, 82.3 °, and the like. In the sulfur-based positive electrode active material using PAN, a broad single peak is observed in the vicinity of 25 ° when the diffraction angle (2θ) is in the range of 20 to 30 °. In contrast, in a sulfur-based positive electrode active material-conductive material composite containing a conductive material, a peak derived from the conductive material appears. For example, when La ₂ S ₃ is used as the conductive material, La ₂ S ₃ peaks appear in the vicinity of 24.7, 25.1, 33.5, and 37.2 °. This peak can be confirmed using the _La 2 _{S 3} as conductive material (i.e. the positive electrode contains _La 2 _{S 3} as conductive material). Further, when TiS ₂ was used as the conductive material, almost no peak was confirmed. When Ti is used as the conductive material, Ti peaks appear in the vicinity of 35.1, 38.4, 40.2, and 53.0 °. From this peak, it can be confirmed that Ti was used as the conductive material. As described above, when TiS ₂ is used as the conductive material, its presence cannot be confirmed by X-ray diffraction. However, if other analysis methods such as ICP elemental analysis and fluorescent X-ray analysis are used, Ti is not present. Since it can be detected, addition of TiS ₂ can be estimated even when no peak is confirmed by X-ray diffraction. Further, when MoS ₂ is used as the conductive material, it is around 14.4, 32.7, 33.5, 35.9, 39.6, 44.2, 49.8, 56.0, 58.4 °. The peak of MoS ₂ appears. This peak can be confirmed with MoS ₂ as a conductive material (i.e. the positive electrode contains MoS ₂ as conductive material). When Fe is used as the conductive material, FeS ₂ peaks appear in the vicinity of 28.5, 33.0, 37.1, 40.8, 47.4, 56.3, and 59.0 °. From this peak, it can be confirmed that Fe is used as the conductive material (that is, the positive electrode includes at least one of FeS, FeS ₂ , and Fe ₂ S ₃ as the conductive material).

(Non-aqueous electrolyte secondary battery)
Hereinafter, the configuration of a non-aqueous electrolyte secondary battery using the above-described sulfur-based positive electrode active material for the positive electrode will be described. The positive electrode is as described above.

[Negative electrode]
As the negative electrode active material for the non-aqueous electrolyte secondary battery of the present invention, metallic lithium or metallic sodium can be used. When metallic lithium is used for the negative electrode active material, the nonaqueous electrolyte secondary battery of the present invention is a lithium secondary battery. When metallic sodium is used for the negative electrode active material, the nonaqueous electrolyte secondary battery of the present invention is a sodium secondary battery (so-called sodium sulfur battery).

  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.

  Hereinafter, the nonaqueous electrolyte secondary battery of the present invention will be specifically described.

Example 1
The nonaqueous electrolyte secondary battery of Example 1 is a lithium secondary battery. The nonaqueous electrolyte secondary battery of Example 1 was produced as follows.

[1] Mixed raw material A sulfur powder having a particle size of 50 μm or less when classified using a sieve was prepared. A PAN powder having a particle size in the range of 0.2 μm to 2 μm when prepared with an electron microscope was prepared. 5 parts by mass of sulfur powder and 1 part by mass of PAN 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 process, the elemental sulfur remaining in the object to be treated after the heat treatment process was evaporated and removed, and the sulfur-modified PAN of Example 1 not including (or substantially not including) elemental sulfur was obtained.

<Production of lithium secondary battery>
[1] Positive electrode 60 parts by mass of the sulfur-based positive electrode active material obtained in the elemental sulfur removal step, 20 parts by mass of ketjen black (KB), 20 parts by mass of polyimide (PI), N-methyl-2-pyrrolidone ( NMP) to prepare a slurry.

  On the other hand, a current collector prepared by punching carbon paper (“TGP-H-030” manufactured by Toray Industries, Inc.) to a diameter of 11 mm is prepared, and after filling the slurry, dried at 200 ° C. under reduced pressure for 2 hours to produce a positive electrode. did.

[2] Negative electrode As the negative electrode, a disc-shaped lithium foil formed of metallic lithium with a thickness of about 0.5 mm and a diameter of 13 mm was used.

[3] Electrolytic Solution As the electrolytic solution, a nonaqueous electrolyte in which LiPF ₆ was dissolved in propylene carbonate was used. 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, the negative electrode and the electrolytic solution obtained in [1], [2] and [3] above. Specifically, in a dry room, a separator made of a polypropylene microporous membrane having a thickness of 25 μm (“Celgard 2400” manufactured by Celgard) and a glass nonwoven fabric filter having a thickness of 500 μm are sandwiched between a positive electrode and a negative electrode. Thus, an electrode body battery was obtained. 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 the nonaqueous electrolyte secondary battery of Example 1.

(Example 2)
The nonaqueous electrolyte secondary battery of Example 2 is a sodium secondary battery. The nonaqueous electrolyte secondary battery of Example 2 is the same as Example 1 except for the composition of the negative electrode and the electrolytic solution. As the negative electrode, a disc-shaped sodium foil formed by slicing metallic sodium and forming a thickness of about 0.5 mm and a diameter of 13 mm was used. As the electrolytic solution, a non-aqueous electrolyte in which NaClO ₄ was dissolved in propylene carbonate was used. The concentration of NaClO ₄ in the electrolytic solution was 1.0 mol / L.

(Example 3)
The non-aqueous electrolyte secondary battery of Example 3 uses SUS (stainless steel) foil instead of carbon paper as a positive electrode current collector. Specifically, 70 parts by mass of the same sulfur-based positive electrode active material as in Example 1, 15 parts by mass of KB, 15 parts by mass of PI, and N-methyl-2-pyrrolidone (NMP) were mixed to prepare a slurry. A slurry prepared by applying this slurry to a SUS foil having a diameter of 11 mm and a thickness of 20 mm was dried at 80 ° C. overnight to produce a positive electrode. Other than that, the nonaqueous electrolyte secondary battery of Example 3 is the same as the nonaqueous electrolyte secondary battery of Example 1.

(Comparative example)
The nonaqueous electrolyte secondary battery of the comparative example is the same as the nonaqueous electrolyte secondary battery of Example 1 except that silicon oxide (SiO) is used as the negative electrode active material and carbon paper is used as the current collector for the negative electrode. It is. The negative electrode in the nonaqueous electrolyte secondary battery of the comparative example was manufactured as follows.

[1] Negative electrode First, SiO powder (manufactured by Osaka Titanium Technologies, average particle size 6 μm), KB as a conductive additive and PI as a binder were mixed to prepare a slurry. The composition ratio of each component in the slurry was SiO powder: KB: PI: = 80: 5: 15 (mass ratio) as a solid content. This slurry was applied to the surface of a 20 μm thick copper foil (current collector), temporarily dried at 80 ° C. for 20 minutes, heat treated at 300 ° C. for 1 hour in a reduced pressure, and then punched out to a diameter of 11 mm in diameter. A negative electrode was obtained.

<Cycle test>
The cycle characteristics of the nonaqueous electrolyte secondary batteries of Examples 1 to 3 and the comparative example were evaluated. Specifically, for Example 1, the current value per 1 g of the positive electrode active material was charged and discharged repeatedly for 550 cycles at 0.1 C (500 mAh / g conversion). For Example 2, charge and discharge were repeated at 0.1 C for up to 11 cycles, 0.05 C for 12 cycles and 15 cycles, and again at 0.1 C for 16 cycles and thereafter. About Example 3, it charged / discharged repeatedly at 265 cycles at 1.0C. About the comparative example, it charged / discharged repeatedly for 630 cycles at 0.1C. The cut-off voltages at this time are 3.0 to 1.0 V in Example 1, 2.67 to 0.67 V in Example 2, 3.0 to 1.0 V in Example 3, and 3.0 to 1.0 in Comparative Example. It was 0.6V. The temperature was 25 ° C. The cycle characteristics of the nonaqueous electrolyte secondary battery of Example 1 are shown in FIG. The cycle characteristics of the nonaqueous electrolyte secondary battery of Example 2 are shown in FIG. The cycle characteristics of the nonaqueous electrolyte secondary battery of Example 3 are shown in FIG. FIG. 7 shows the cycle characteristics of the nonaqueous electrolyte secondary battery of the comparative example.

  As shown in FIGS. 3 to 6, each non-aqueous electrolyte secondary battery was reversibly charged and discharged. However, as shown in FIG. 7, the capacity of the nonaqueous electrolyte secondary battery of the comparative example gradually decreased after 450 cycles. In contrast, the non-aqueous electrolyte secondary battery of each example hardly experienced a decrease in capacity. In particular, as shown in FIG. 3, the nonaqueous electrolyte secondary battery of Example 1 maintained a very high capacity (544 mAh / g) even after 550 cycles. The difference between the nonaqueous electrolyte secondary battery of Example 1 and the nonaqueous electrolyte secondary battery of the comparative example is the negative electrode active material. For this reason, the nonaqueous electrolyte secondary battery of Example 1 is considered to have improved cycle characteristics by using metallic lithium as the negative electrode active material.

  Further, as shown in FIG. 4, the nonaqueous electrolyte secondary battery of Example 2 maintained a very high capacity of 550 mAh / g or more even at 15 cycles or more. The nonaqueous electrolyte secondary battery of Example 2 is obtained by replacing the negative electrode active material of the nonaqueous electrolyte secondary battery of Example 1 with metal sodium from metal lithium. Thus, the nonaqueous electrolyte secondary battery of Example 2 exhibited excellent cycle characteristics by using metallic sodium as the negative electrode active material. In order to give particularly excellent cycle characteristics to the nonaqueous electrolyte secondary battery, it is necessary to select metallic lithium or metallic sodium as the negative electrode active material, or to use carbon paper as the current collector for the positive electrode. It seems to be particularly effective. In addition, by using carbon paper as the current collector for the positive electrode and using metal lithium or metal sodium as the negative electrode active material, the active material and the current collector are compared with the case where the metal foil current collector is used. And the electrode composition is uniform in the thickness direction. For this reason, an electrode with little cycle deterioration is obtained. Also, since carbon paper has voids, the electrode using the carbon paper current collector has better retention of the electrolyte in the electrode than the electrode manufactured by applying the electrode material to the foil current collector. Deterioration due to electrolyte depletion is unlikely to occur.

  As shown in FIG. 6, the nonaqueous electrolyte secondary battery of Example 3 maintained a very high capacity of 600 mAh / g or more even at 260 cycles or more. The nonaqueous electrolyte secondary battery of Example 3 uses an electrode obtained by replacing the current collector of the nonaqueous electrolyte secondary battery of Example 1 with SUS foil and applying a slurry of a sulfur-based positive electrode active material. It is. Thus, even when the positive electrode by a general coating method was used, excellent cycle characteristics were obtained by using metallic lithium or metallic sodium as the negative electrode active material.

  Further, the nonaqueous electrolyte secondary battery of the present invention has an advantage that it can be easily manufactured. That is, the non-aqueous electrolyte secondary battery in which the positive electrode active material is a sulfur-based positive electrode active material and does not contain lithium or sodium as the negative electrode active material needs to be doped with lithium or sodium during the manufacturing process. On the other hand, the nonaqueous electrolyte secondary battery of the present invention does not need to be doped and can be easily manufactured. Furthermore, by using metallic lithium or metallic sodium as the negative electrode active material, an amount of lithium or sodium capable of canceling the irreversible capacity that occurs during the first charge / discharge can be contained in the negative electrode, thereby reducing the capacity of the nonaqueous electrolyte secondary battery. Can be suppressed.

1: Reactor 2: Reaction vessel 3: Lid 4: Thermocouple 5: Gas introduction pipe 6: Gas discharge pipe 7: Electric furnace

Claims (6)

A positive electrode, a negative electrode, and a non-aqueous electrolyte;
The positive electrode includes a sulfur-based positive electrode active material containing carbon (C) and sulfur (S) as a positive electrode active material,
The non-aqueous electrolyte secondary battery, wherein the negative electrode contains metallic lithium or metallic sodium as a negative electrode active material.   The sulfur-based positive electrode active material includes a carbon skeleton derived from a carbon source compound selected from polyacrylonitrile, pitches, polyisoprene, and a polycyclic aromatic hydrocarbon formed by condensation of three or more six-membered rings, and the carbon The nonaqueous electrolyte secondary battery according to claim 1, comprising sulfur (S) bonded to a skeleton. The sulfur-based positive electrode active material has a carbon skeleton derived from polyacrylonitrile, 1548Cm in the Raman spectrum, the main peak is present near 1331cm ^-1 of Raman shift, ^and within the range of 200cm ^-1 ~1800cm -1 ^-1 , 939 cm ⁻¹ , 479 cm ⁻¹ , 381 cm ⁻¹ , and 317 cm ⁻¹ , respectively, have peaks in the vicinity of the nonaqueous electrolyte secondary battery according to claim 1 or 2.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material is metallic lithium.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode includes carbon paper as a current collector.   A vehicle comprising the nonaqueous electrolyte secondary battery according to any one of claims 1 to 5.

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