JP2013161653A - Sulfur-based cathode active material and nonaqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sulfur-based cathode active material capable of improving cycle characteristics and discharge rate characteristics of a nonaqueous secondary battery, and to provide a nonaqueous secondary battery using the sulfur-based cathode active material for a cathode.SOLUTION: The sulfur-based cathode active material comprises: a carbon skeleton derived from a carbon source compound selected from polyacrylonitrile, pitches, polyisoprene, plant raw materials, and polycyclic aromatic hydrocarbon obtained by condensing three or more six-membered rings; and sulfur (S) and iodine (I) which are bonded to the carbon skeleton.

Description

  The present invention relates to a sulfur-based positive electrode active material useful as a positive electrode of a non-aqueous secondary battery such as a lithium ion secondary battery, and a non-aqueous secondary battery including the sulfur-based positive electrode active material.

  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 material 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.

  In order to suppress the elution of sulfur into the electrolyte, a technique has been proposed in which a positive electrode active material containing sulfur (hereinafter referred to as a sulfur-based positive electrode active material) is blended with a material other than sulfur, such as a carbon material. (For example, see Patent Document 1).

  Patent Document 1 introduces a technique that uses polysulfide carbon containing carbon and sulfur as main components as a sulfur-based positive electrode active material. In this polysulfide carbon, sulfur is added to a linear unsaturated polymer. According to Patent Document 1, it is said that this sulfur-based positive electrode active material can suppress a decrease in charge / discharge capacity of a lithium ion secondary battery due to repeated charge / discharge. 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.

  However, even with the sulfur-based positive electrode active material introduced in Patent Document 1, the cycle characteristics of the lithium ion secondary battery could not be sufficiently improved. This is thought to be due to the fact that the polymer is cleaved by the —CS—CS— bond or —S—S— bond contained in the polysulfide carbon being broken by the binding of sulfur and lithium during discharge.

  Therefore, the inventors of the present invention invented a sulfur-based 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 sulfur-based 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.

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

  However, the lithium ion secondary battery using the sulfur-based positive electrode active material described in Patent Documents 1 and 2 has a problem that the discharge rate characteristic (so-called C rate) is low. The present invention has been made in view of such circumstances, a sulfur-based positive electrode active material capable of improving cycle characteristics and discharge rate characteristics of an aqueous secondary battery, and a non-aqueous system using the sulfur-based positive electrode active material as a positive electrode. An object is to provide a secondary battery.

  The feature of the sulfur-based positive electrode active material of the present invention that solves the above problems is that it comprises a carbon skeleton and sulfur (S) and iodine (I) bonded to the carbon skeleton. The carbon skeleton is desirably derived from a carbon source compound selected from polyacrylonitrile, pitches, polyisoprene, plant raw materials, and polycyclic aromatic hydrocarbons formed by condensation of three or more six-membered rings.

  The non-aqueous secondary battery of the present invention is characterized in that it includes a positive electrode including the sulfur-based positive electrode active material of the present invention and a negative electrode.

  The sulfur-based positive electrode active material of the present invention contains iodine (I) together with sulfur (S). Therefore, the conductivity is improved, and the discharge rate characteristic is improved according to the nonaqueous secondary battery of the present invention.

It is explanatory drawing which represents typically the reaction apparatus used with the manufacturing method of the sulfur type positive electrode active material of an Example. 2 shows a Raman spectrum of a carbon-sulfur reactant comprising a PAN-derived carbon skeleton and sulfur (S) bonded to the carbon skeleton. It is a graph which shows the temperature rising conditions at the time of the heating at the time of making a carbon-sulfur reactant and iodine react in an Example. It is a graph showing a discharge rate characteristic. It is a graph showing a 1C discharge curve.

  The sulfur-based positive electrode active material of the present invention comprises a carbon skeleton and sulfur (S) and iodine (I) bonded to the carbon skeleton. It is possible to use a part of sulfur of polysulfide carbon substituted with iodine, but (1) polyacrylonitrile (hereinafter referred to as PAN), (2) pitches, (3) plant material, (4) polyisoprene. And (5) it is preferable to use a carbon skeleton derived from a carbon source compound selected from polycyclic aromatic hydrocarbons formed by condensation of three or more six-membered rings.

  In order to produce the sulfur-based positive electrode active material of the present invention from these carbon source compounds, the carbon source compound, sulfur, and iodine are mixed, and in a non-oxidizing atmosphere in a sealed system in which sulfur and iodine do not flow out. There is a method of heating. Alternatively, as described in Patent Document 2, the carbon source compound and sulfur are first reacted, and the obtained carbon-sulfur reactant and iodine are mixed and heated in a sealed system to introduce iodine. The sulfur type positive electrode active material of this invention can be manufactured. In the following description of the manufacturing method, the case where the latter method is used will be mainly described.

  (1) A carbon-sulfur reactant comprising a PAN-derived carbon skeleton and sulfur (S) bonded to the carbon skeleton can be produced by the production method described in Patent Document 2. That is, it can be produced by mixing a raw material powder containing sulfur powder and PAN powder into a mixed raw material and heating in a non-oxidizing atmosphere while preventing the outflow of sulfur vapor. Thus, at the same time as the PAN ring-closing reaction, sulfur in the vapor state reacts with PAN, and PAN modified with sulfur is obtained.

  (1) The PAN powder preferably has a weight average molecular weight in the range of about 10,000 to 300,000. Further, the particle size of PAN is preferably within the range of about 0.5 to 50 μm, more preferably within the range of 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, sulfur and iodine can be increased, and PAN, sulfur and iodine can be reacted with high reliability.

  The particle size of the sulfur powder is not particularly limited, but when it is classified using a sieve, it is preferably in the range of about 150 μm to 40 μm, more preferably in the range of about 100 μm to 40 μm. preferable. Moreover, since iodine reacts in a sublimated state, the particle size is not particularly limited.

  When PAN and sulfur are first reacted, the mixing ratio of sulfur powder and PAN powder in the mixed raw material is not particularly limited, but about 50 to 1000 parts by mass of sulfur powder with respect to 100 parts by mass of PAN powder. Preferably, the amount is about 50 to 500 parts by mass, and more preferably about 150 to 350 parts by mass.

  As an example of a method of heating while preventing the outflow of sulfur, a method of heating in a sealed atmosphere can be employed. In this case, the sealed atmosphere may be maintained in a sealed state to the extent that sulfur vapor generated by heating is not dissipated. Further, the non-oxidizing atmosphere may be a reduced pressure state with a low oxygen concentration such that the oxidation reaction does not proceed; an inert gas atmosphere such as nitrogen or argon; a sulfur gas atmosphere or 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. What is necessary is just to heat as a state or inert gas atmosphere. In addition, a mixed raw material of sulfur powder and PAN powder may be heated in a vacuum packaged state with a material that does not react with sulfur vapor such as 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 that the pressure is applied. 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.

  The sulfur powder and the PAN powder may be simply mixed, but for example, the mixed raw material may be formed into a pellet. The mixed raw material may be composed of only 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 heating temperature is preferably about 250 to 500 ° C, more preferably about 250 to 450 ° C, and further preferably about 250 to 400 ° C. The heating time is not particularly limited and varies depending on the actual heating temperature. Usually, the heating time may be maintained for about 10 minutes to 10 hours, and preferably about 30 minutes to 6 hours. According to the method described above, it is possible to form sulfur-modified PAN in such a short time.

  In addition, as another example of the method of heating while preventing the outflow of sulfur, mixing containing sulfur powder and PAN powder while refluxing sulfur vapor in a reaction vessel having an opening for discharging hydrogen sulfide produced by the reaction A method of heating the raw material can be employed. In this case, the opening for discharging the hydrogen sulfide may be provided at a position where the generated sulfur vapor is liquefied and recirculated almost completely and the outflow of sulfur vapor from the opening can be prevented. For example, by providing an opening in a part where the temperature in the reaction vessel is about 100 ° C. or less, hydrogen sulfide generated by the reaction is discharged from the opening, but sulfur vapor is condensed in the opening part. Thus, it can be returned to the reaction vessel without being discharged to the outside.

  Among the production methods described above, according to the method of heating in a sealed atmosphere, the obtained sulfur-modified PAN has a carbon content of 40 to 60% by mass as the content in the sulfur-modified PAN from the results of elemental analysis. , Sulfur is 15 to 30% by mass, nitrogen is 10 to 25% by mass, and hydrogen is about 1 to 5% by mass.

  In addition, among the above-described production methods, in the method of heating while discharging hydrogen sulfide gas, the obtained sulfur-modified PAN has a large sulfur content, and the peak area ratio calculation results from elemental analysis and XPS measurement As the content in the sulfur-modified PAN, carbon is 25 to 50% by mass, sulfur is 25 to 55% by mass, nitrogen is 10 to 20% by mass, oxygen is 0 to 5% by mass, and hydrogen is about 0 to 5% by mass. It becomes the range.

  Further, the obtained sulfur-modified PAN has a weight loss by thermogravimetric analysis of 10% or less at 400 ° C. when heated from room temperature to 900 ° C. at a heating rate of 20 ° C./min. On the other hand, when the mixed raw material of sulfur powder and PAN powder is heated under the same conditions, a weight decrease is observed from around 120 ° C., and a large weight loss due to the disappearance of sulfur is rapidly observed at 200 ° C. or higher. Furthermore, in the sulfur-modified PAN, as a result of X-ray diffraction by CuKα ray, the sulfur-based peak disappears, and only a broad peak having a diffraction angle (2θ) of around 20 to 30 ° C. is confirmed.

  From these points, in the sulfur-modified PAN obtained by the above-described method, it is considered that sulfur does not exist as a simple substance but is present in a state of being bonded to the PAN that has progressed in the ring closure.

  The sulfur-modified PAN obtained by the above method can be removed by heating in a non-oxidizing atmosphere when unreacted sulfur is present. Thereby, higher purity sulfur-modified PAN can be obtained.

  The non-oxidizing atmosphere may be, for example, a reduced pressure state with a low oxygen concentration that does not cause an oxidation reaction; an inert gas atmosphere such as nitrogen or argon. The heating temperature is preferably about 150 to 400 ° C, more preferably about 150 to 300 ° C, and still more preferably about 200 to 300 ° C. Note that if the heating time is too high, the sulfur-modified PAN may decompose. The heat treatment time is not particularly limited, but is usually preferably about 1 to 6 hours.

  (2) As pitches, coal pitch, petroleum pitch, mesophase pitch, asphalt, coal tar, coal tar pitch, organic synthetic pitch obtained by polycondensation of condensed polycyclic aromatic hydrocarbon compounds, heteroatom-containing condensed polycycle At least one selected from the group consisting of organic synthetic pitches obtained by polycondensation of aromatic hydrocarbon 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.

  (2) A carbon-sulfur reactant comprising a carbon skeleton derived from pitches and sulfur (S) bonded to the carbon skeleton can be produced by the following production method. That is, it includes a heat treatment step of heating a mixed raw material containing pitches and sulfur, and in the heat treatment step, at least a part of the pitches and at least a part of the sulfur are made 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 sufficiently increased, and a carbon-sulfur reactant containing sulfur sufficiently and suppressing sulfur desorption can be obtained. When sulfur is refluxed in the heat treatment step, the frequency of contact between the pitches and sulfur can be increased, and a carbon-sulfur reactant that further contains sulfur and further suppresses sulfur desorption is obtained. Can do.

  In the obtained carbon-sulfur reactant, it is not clear how sulfur and pitches are bonded, but sulfur is taken in between the graphene layers of pitches or condensed polycycles. It is presumed that hydrogen contained in the aromatic ring is replaced with sulfur to form a CS 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 coal pitch is about 60-350 ° 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 lower, and further preferably 500 ° C. or lower. Furthermore, in consideration of 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. For reference, the melting point of α sulfur (orthogonal sulfur, which is the most stable structure near room temperature) is 112.8 ° C, melting point of β sulfur (monoclinic sulfur) is 119.6 ° C, and melting point of γ sulfur (monoclinic sulfur) is 106.8 ° C. The boiling point of sulfur is 444.7 ° C. By the way, since the vapor pressure of sulfur is high, the generation of sulfur vapor can be visually confirmed when the temperature of the mixed raw material is 150 ° C. or higher. 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 recirculation | reflux apparatus of a known structure when recirculating | refluxing sulfur in a heat treatment process.

  Here, 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 pitches and sulfur (for example, an atmosphere that does not contain hydrogen or a non-oxidizing atmosphere). For example, if 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 addition, the non-oxidizing atmosphere here includes a reduced pressure state in which the oxygen concentration is low enough that the oxidation reaction does not proceed, an inert gas atmosphere such as nitrogen or argon, a sulfur gas atmosphere, and the like.

  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. Further, 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 for about 10 minutes to 10 hours, and it is more preferable to heat for 30 minutes to 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 in the pitches is too small, a sufficient amount of sulfur cannot be taken into the pitches, and if the amount of sulfur in the pitches is excessive, free sulfur (single sulfur in the sulfur-based positive electrode active material) This is because a large amount of) remains and contaminates the electrolyte in the non-aqueous 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 to 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 bad influence by the above-mentioned simple substance sulfur can be controlled by removing simple substance sulfur from the processed object after a heat treatment process. 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 performing (single sulfur removal step), it is possible to suppress an adverse effect due to the remaining single sulfur while incorporating a sufficient amount of sulfur into the pitches. 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 a carbon-sulfur reactant as it is. Moreover, what is necessary is just to use the to-be-processed body after a simple sulfur removal process as a carbon-sulfur reactant, when performing the simple substance sulfur removal process to the to-be-processed body after a heat treatment process.

  As a result of elemental analysis of the sulfur-based positive electrode active material, carbon, nitrogen, and sulfur were detected. In some cases, small amounts of oxygen and hydrogen were detected. Therefore, this carbon-sulfur reactant contains at least one of nitrogen, oxygen, sulfur compounds and the like as impurities in addition to C and S.

  (3) As plant materials, at least one selected from coffee beans, tea leaves, seaweeds, sugar cane, corns, fruits, cereals, straws and chaffs can be used.

  (3) A carbon-sulfur reactant comprising a carbon skeleton derived from a plant raw material and sulfur bonded to the carbon skeleton includes a mixing step of mixing the plant raw material and sulfur to form a mixture, and the mixture into a sealed container. It can manufacture by performing the heating process heated at 250 to 600 degreeC. The mixing ratio of the plant raw material and sulfur is preferably 5: 1 to 1: 5 in terms of mass ratio.

  The plant raw material is carbonized by heating and becomes a low-temperature fired body. This low-temperature fired body is not completely carbonized and contains an uncarbonized product in which some heteroatoms such as oxygen and nitrogen remain.

Sulfur has a melting point of 113 ° C or 119 ° C, becomes a fluid liquid around 300 ° C, and boils at 444.6 ° C. Sulfur reacts with hydrogen, carbon, chlorine, etc. at high temperatures to produce H ₂ S, CS ₂ , S ₂ Cl _2, etc. Sulfur combines with almost all metals except Au to form sulfides. Therefore, it is considered that the plant raw material and sulfur undergo various reactions during heating, and the low-temperature fired body of the plant raw material and sulfur form some complex. Sulfur generates radicals when heated to 159 ° C or higher. This radical promotes carbonization of plant raw materials.

  Gases are generated by heating plant materials and sulfur alone. The generated gas does not leak from the container because the container is sealed. Therefore, pressure is applied in the sealed container by the gases. The pressure promotes the various reactions and carbonization described above.

  It is preferable to further include a drying step for drying the plant raw material before the mixing step. By previously drying the plant material, moisture can be removed, and the heating process time can be shortened. Furthermore, it is preferable to have the impurity removal process which removes the impurity which generate | occur | produced in the heating process by heating the mixture after a heating in vacuum at 150 to 400 degreeC after a heating process. By this impurity removal step, impurities such as excess sulfur reactant generated in the heating step can be easily removed.

  (4) As the polyisoprene, both natural rubber and synthetic polyisoprene can be used. However, the cis-type polyisoprene has a structure in which the molecular chain is bent and easily forms an irregular shape. Since a large number of gaps are formed between the chains and the intermolecular force is relatively small, crystallization between molecules does not occur and the polymer has a soft property. Therefore, the cis type is preferable to the trans type.

  (4) a carbon-sulfur reactant comprising a carbon skeleton derived from polyisoprene and sulfur bonded to the carbon skeleton is a mixing step in which a raw material containing polyisoprene and sulfur powder is mixed to form a mixed raw material; It can manufacture by performing the heat processing process which heats 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, or 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 in this way, the contact area of polyisoprene and sulfur can be made sufficiently large, and a carbon-sulfur reactant containing sulphur and suppressing sulfur desorption can be obtained.

  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 a carbon-sulfur reactant containing sulfur sufficiently. When sulfur is refluxed in the heat treatment step, the melting point of polyisoprene is as low as about 30 ° C., so that 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 it is called high-temperature overvulcanization. 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. Further, the polyisoprene and sulfur in the mixed raw material are preferably dispersed uniformly, 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 preferred temperature described above, 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 the performance as a rubber is said to decrease. The carbon-sulfur reactant 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, a 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) in the sulfur-based positive electrode active material This is because a large amount of remains and contaminates the electrolyte in the non-aqueous secondary battery. The blending ratio of polyisoprene and sulfur 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. : 2 to 1: 5 is particularly preferable.

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. Elastic rubber (for example, rubber band) is produced by adding about 3 to 6% sulfur to chain-structured raw rubber and heat treatment. 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. However, in the above production method, since it is performed at a high temperature of 250 to 500 ° C., addition reaction of S to —C═C—double bond, and — 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 object to be treated 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, this target object may be used as a carbon-sulfur reactant as it is. Moreover, what is necessary is just to use the to-be-processed body after a simple sulfur removal process as a carbon-sulfur reactant, when performing the simple substance sulfur removal process to the to-be-processed body after a heat treatment process.

  The mixed raw material may be composed only of polyisoprene 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.

  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, ash, etc., but even when such materials are used, it functions as a sulfur-based positive electrode active material. 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.

  (4) When elemental analysis of a sulfur-based positive electrode active material comprising a carbon skeleton derived from polyisoprene and sulfur (S) bonded to the carbon skeleton, sulfur (S) and carbon (C) dominate. 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 deteriorate when used for a positive electrode for a non-aqueous secondary battery.

  (5) Polycyclic aromatic hydrocarbon (PAH) formed by condensation of three or more six-membered rings is a generic 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. Among these, in the present invention, a structure in which three or more six-membered rings, which are benzene ring structures, are connected in a straight chain. It is preferable to use sulfur and at least one of acenes having a structure in which three or more six-membered rings are not linear but bent.

  As acenes which are polycyclic aromatic hydrocarbons in which a plurality of aromatic rings share a side and are connected in a straight chain, 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, and at least one selected from these groups can be used. Among them, those having 3 to 6 rings having high stability are desirable.

  In addition, polycyclic aromatic hydrocarbons having a structure in which three or more six-membered rings are not linear 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.

  (5) Polycyclic aromatic hydrocarbon (PAH) formed by condensation of three or more six-membered rings is a generic 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. Among these, in the present invention, a structure in which three or more six-membered rings, which are benzene ring structures, are connected in a straight chain. It is preferable to use sulfur and at least one of acenes having a structure in which three or more six-membered rings are not linear but bent.

  As acenes which are polycyclic aromatic hydrocarbons in which a plurality of aromatic rings share a side and are connected in a straight chain, 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, and at least one selected from these groups can be used. Among them, those having 3 to 6 rings having high stability are desirable.

  In addition, polycyclic aromatic hydrocarbons having a structure in which three or more six-membered rings are not linear 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.

  (5) To produce a carbon-sulfur reactant comprising a carbon skeleton derived from a compound selected from polycyclic aromatic hydrocarbons formed by condensation of three or more six-membered rings, and sulfur bonded to the carbon skeleton. Can be performed as in the case of pitches or 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 carbon-sulfur reactant 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 in this way, the contact area of a polycyclic aromatic hydrocarbon and sulfur can be made large enough, and the carbon-sulfur reactant which contains sulphur enough and the elimination of sulfur can be obtained.

  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 added to 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 added to the polycyclic aromatic hydrocarbon is excessive, A large amount of free sulfur (single sulfur) remains in the sulfur reactant. The mixing 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. Even if sulfur is added to the polycyclic aromatic hydrocarbon in an amount more than necessary, the single sulfur removal step of removing excess single sulfur from the object to be treated after the heat treatment step will have an adverse effect due to single sulfur. Can be suppressed. Specifically, when the mixing ratio of the polycyclic aromatic hydrocarbon and sulfur in the mixed raw material is set to 1: 2 to 1:10 by mass ratio, the object to be treated after the heat treatment step is reduced from 200 ° C. to 200 ° C. 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 a carbon-sulfur reactant as it is. Moreover, what is necessary is just to use the to-be-processed body after a simple sulfur removal process as a carbon-sulfur reactant, when performing the simple substance sulfur removal process to the to-be-processed body after a heat treatment process.

  The mixed raw material may be composed only of polycyclic aromatic hydrocarbons 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.

(5) A carbon-sulfur reactant comprising a carbon skeleton derived from a compound selected from polycyclic aromatic hydrocarbons formed by condensation of three or more six-membered rings, and sulfur bonded to the carbon skeleton is, for example, When pentacene is selected as the starting polycyclic aromatic hydrocarbon, it is considered that the structure is similar to hexathiapentacene, but the structure is not clear. Further, polycyclic aromatic carbon using anthracene as the hydrocarbon - sulfur reactant, the FT-IR spectrum, a near 1056cm ^-1, there are respectively a peak and around 840 cm ^-1, the, FT-IR anthracene Since it is completely different from the spectrum, it can be identified by the FT-IR spectrum.

  (5) Elemental analysis of a carbon-sulfur reactant comprising a carbon skeleton derived from a compound selected from polycyclic aromatic hydrocarbons formed by condensation of three or more six-membered rings, and sulfur bonded to the carbon skeleton Sulfur (S) and carbon (C) dominate, 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 deteriorate when used for a positive electrode for a non-aqueous secondary battery.

  The carbon-sulfur reactant obtained as described above is mixed with iodine and heated in a sealed system, whereby iodine is introduced into the carbon skeleton and the sulfur-based positive electrode active material of the present invention is obtained. It is unclear at this time whether iodine binds to carbon not bound to sulfur or whether sulfur bound to carbon is replaced by iodine. The heating temperature is preferably in the range of 50 ° C to 700 ° C, and preferably in the range of 100 ° C to 500 ° C.

  The obtained sulfur-based positive electrode active material contains sulfur (S) in an amount of 20 to 70% by mass and iodine (I) in an amount of 0.01 to 20% by mass. Is preferred. If the content of sulfur (S) is less than 20% by mass, the charge / discharge characteristics when a non-aqueous secondary battery is used may be deteriorated, and if it exceeds 70% by mass, the conductivity may be decreased. Further, when the content of iodine (I) is less than 0.01% by mass, the discharge rate characteristics when a non-aqueous secondary battery is formed are deteriorated, and when it exceeds 20% by mass, the discharge capacity is decreased.

(Positive electrode for non-aqueous secondary batteries)
The positive electrode used for the non-aqueous secondary battery of the present invention contains the above-described sulfur-based positive electrode active material. The positive electrode for a non-aqueous secondary battery can have the same structure as a general positive electrode for a non-aqueous secondary battery except for the positive electrode active material. For example, it can be manufactured by applying a positive electrode material in which the above-described sulfur-based positive electrode active material, conductive additive, binder, and solvent are mixed to a current collector.

  Conductive aids include vapor grown carbon fiber (VGCF), carbon powder, carbon black (CB), acetylene black (AB), ketjen black (KB), graphite, aluminum, titanium and other positive electrodes Examples thereof include fine metal powders stable in potential.

  The binders include Polyvinylidene Fluoride (PVDF), Polytetrafluoroethylene (PTFE), Styrene-Butadiene Rubber (SBR), Polyimide (PI), Polyamideimide (PAI), Carboxymethylcellulose (CMC), Polychlorinated Examples include vinyl (PVC), methacrylic resin (PMA), PAN (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 that about 100 to 100 parts by mass of the conductive auxiliary agent and about 10 to 20 parts by mass of the binder are blended with respect to 100 parts by mass of the sulfur-based positive electrode active material. As another method, a mixed raw material of a sulfur-based positive electrode active material, the above-described conductive additive and binder is kneaded with a mortar or a press machine to form a film, and the mixed raw material in a film form is collected with a press machine or the like. The positive electrode for non-aqueous secondary batteries can also be manufactured by crimping | bonding to a body.

  What is necessary is just to use what is generally used for the positive electrode for non-aqueous 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, nickel foam, nickel nonwoven fabric, copper foil, copper Examples thereof include mesh, punched copper sheet, copper expanded sheet, titanium foil, titanium mesh, carbon nonwoven fabric, carbon woven fabric, and carbon paper. Among these, the carbon non-woven fabric / woven fabric current collector made of carbon having a high degree of graphitization is suitable as a current collector for a sulfur-based positive electrode active material because it does not contain hydrogen and has low reactivity with sulfur. As a raw material for carbon fiber having a high degree of graphitization, various pitches (that is, by-products such as petroleum, coal, coal tar, etc.), PAN fibers, etc., which are carbon fiber materials can be used.

  The positive electrode for a non-aqueous secondary battery of the present invention includes the above-described sulfur-based positive electrode active material as a positive electrode active material. Therefore, a non-aqueous secondary battery using the positive electrode has a large charge / discharge capacity, excellent cycle characteristics, and can be manufactured at low cost.

  The positive electrode including the above-described sulfur-based positive electrode active material may include a sulfide of 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. These metal sulfides can exhibit high electrical conductivity (conductivity) or can improve the ionic conductivity of the positive electrode. For this reason, these metal sulfides function as a conductive material. And discharge rate characteristic can be improved by mix | blending these metal sulfides with a positive electrode.

  In addition, since a conductive material is mix | blended with a positive electrode active material with a sulfur type positive electrode active material, it may sulfidize by the sulfur contained in a sulfur type positive electrode active material at the time of manufacture of a positive electrode and / or charge / discharge of a battery. For this reason, when a material having low electrical conductivity in the state of sulfide or a material that cannot improve ion conductivity is used as the conductive material, there is a problem that it is difficult to improve the discharge rate characteristics. However, if a conductive material that exhibits high electrical conductivity in the state of sulfide or that can improve the ionic conductivity of the positive electrode is used, the discharge rate characteristics can be sufficiently improved.

  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 contained 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 is preferably at least one selected from the group consisting of, or a sulfide thereof. . 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, TiS ₂ , FeS ₂ , Me ₂ S ₃ (wherein Me is a kind selected from Ti, La, Ce, Pr, Nd, Sm) ), MeS (wherein Me is a kind selected from Ti, La, Ce, Pr, Nd, Sm), Me ₃ S ₄ (wherein Me is Ti, La, Ce, Pr, Nd, Me _x S _y (wherein Me is Ti, Fe, V, Mn, Fe, Ni, Cu, Zn, Mo, Ag, Cd, In, Sn, Sb, Ta, W) , 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 ion conductivity of the entire positive electrode can be improved, and the discharge rate characteristics of the non-aqueous secondary battery can be improved. In view of raw material costs, ease of procurement, and resource amount, TiS _z (wherein z is 0.1 to 2) is more preferably used, and TiS ₂ is particularly preferably used.

  The mixing ratio between the sulfur-based positive electrode active material of the present invention and the 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 sulfur-based 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 using 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 TiS ₂ 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 detected. Since it can be detected, the addition of TiS ₂ can be estimated even when no peak is confirmed by X-ray diffraction. When MoS ₂ is used as the conductive material, MoS ₂ peaks appear around 14.4, 32.7, 33.5, 35.9, 39.6, 44.2, 49.8, 56.0, and 58.4 °. 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, FeS2 peaks appear in the vicinity of 28.5, 33.0, 37.1, 40.8, 47.4, 56.3, and 59.0 °. This peak confirms that Fe was used as the conductive material (that is, the positive electrode contains at least one of FeS, FeS ₂ , and Fe ₂ S ₃ as the conductive material).

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

(Negative electrode)
As the negative electrode material, known carbon-based materials such as metallic lithium, metallic sodium, non-graphitizable carbon (hard carbon) and alloy materials capable of occluding and releasing lithium ions can be used. When the negative electrode material is a material that does not contain lithium, for example, among the negative electrode materials described above, carbon-based materials, tin-based materials, other alloy-based materials, etc., a short circuit between the positive and negative electrodes due to the generation of dendrites It is advantageous in that it does not easily occur. However, when these negative electrode materials not containing lithium are used in combination with the positive electrode of the present invention, neither the positive electrode nor the negative electrode contains a metal that becomes an ion such as lithium. For this reason, a pre-doping process in which lithium or the like is inserted in advance into either one or both of the negative electrode and the positive electrode is necessary.

  For example, when lithium is doped into the negative electrode, a half battery is assembled using metallic lithium as the counter electrode, and lithium is inserted by an electrolytic doping method in which lithium is electrochemically doped, or a metallic lithium foil is attached to the electrode. There is a method of inserting lithium by a pasting pre-doping method in which it is left in an electrolytic solution after being attached and doped by utilizing diffusion of lithium to the electrode. The above-described electrolytic doping method can also be used when the positive electrode is pre-doped with lithium or the like.

  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 nonwoven fabric, carbon woven fabric, and carbon paper.

(Electrolytes)
As an electrolyte used for the non-aqueous 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, 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. 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 secondary battery is a lithium polymer secondary battery, the electrolyte is in a solid state (for example, a polymer gel).

(Other)
The non-aqueous 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 secondary battery is a sealed type, the separator is also required to have a function of holding the 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 non-aqueous 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 sulfur-based positive electrode active material of the present invention, a method for producing the same, and the non-aqueous secondary battery of the present invention will be specifically described with reference to Examples and Comparative Examples.

[1] Mixed raw material A sulfur powder having a particle diameter of 50 μm or less when classified using a sieve was prepared. As the PAN powder, a 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. 1, 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), and argon gas. It has a pipe 50, a gas tank 51 containing argon gas, a trap pipe 60, a trap tank 62 containing a lithium 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. In one of the through holes, an alumina protective tube 40 (alumina SSA-S, manufactured by Nikkato Co., Ltd.) containing the thermocouple 4 was attached. A gas introduction pipe 5 (alumina SSA-S, manufactured by Nikkato Co., Ltd.) was attached to the other through hole. A gas exhaust pipe 6 (alumina SSA-S, manufactured by Nikkato Co., Ltd.) 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 part 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 lithium 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. 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 5 minutes at 400 ° C., and sulfur was refluxed.

[4] Elemental sulfur removal process In order to remove elemental sulfur (free sulfur) remaining in the object to be treated after the heat treatment process, the following processes 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 is evaporated and removed, and a sulfur-modified PAN as a carbon-sulfur reactant containing no elemental sulfur (or including a minute amount of elemental sulfur) was obtained. .

The sulfur-modified PAN was subjected to Raman analysis using RMP-320 (excitation wavelength λ = 532 nm, grating: 1800 gr / mm, resolution: 3 cm ⁻¹ ) manufactured by JASCO Corporation. The obtained Raman spectrum is shown in FIG. In FIG. 2, the horizontal axis is the Raman shift (cm ⁻¹ ), and the vertical axis is the relative intensity. As can be seen from Figure 2, according to the Raman analysis results of the product, there are a main peak in the vicinity of 1331cm ^-1, and, 1548cm ^-1 in the range of ^{200cm -1 ~1800cm -1, 939cm -1,} 479cm - ^{1, 381cm -1,} the peak was present around 317cm ^-1.

[5] Iodine introduction step 0.3040 g of the above sulfur-modified PAN and 0.0027 g of iodine (I ₂ ) were mixed well in a mortar and heated to 400 ° C. at a temperature increase rate shown in FIG. After holding at 0 ° C. for 1 hour, it was allowed to stand until it reached room temperature. Table 1 shows the composition analysis results of the obtained sulfur-based positive electrode active material of this example.

  The composition of carbon (C), hydrogen (H) and nitrogen (N) is “vario MICRO cube” (manufactured by Elementar), combustion furnace temperature: 1150 ° C, reduction furnace temperature: 850 ° C, helium flow rate: 200ml / The sample was precisely weighed to 0.0001 mg under the conditions of min, oxygen flow rate: 25-30 ml / min, and simultaneous analysis of carbon (C), hydrogen (H), and nitrogen (N) was performed.

For the composition of sulfur (S), a sample was precisely weighed to 0.001 mg and analyzed by flask combustion to ion chromatography. Using “DX320” (DIONEX), column: IonPacAS12A, mobile phase: 2.7 mmol / L Na ₂ CO ₃ +0.3 mmol / L NaHCO ₃ , flow rate: 1.5 ml / min, detector: conductivity detector, Injection volume: Measured under conditions of 25 μL.

The composition of iodine (I) is weighed according to JIS Z7302 and burned in the combustion tube, the generated gas is absorbed into the solution, and a part of the absorption liquid is analyzed by ion chromatography using an ultraviolet absorption detector. did. Using “ICA2000” (manufactured by Toa DKK), mobile phase: 1.8 mmol / L Na ₂ CO ₃ +1.7 mmol / L NaHCO ₃ , flow rate: 1.2 ml / min, detector: conductivity detector, injection volume: 100 μL It measured on condition of this.

  The composition analysis of each element is performed twice, and the average value is shown as the analysis value.

It can be seen that this sulfur positive electrode active material contains 1.9% by weight of iodine (I) and about 1% as iodine (I ₂ ). Even if free iodine is contained, it is considered that the iodine contained in the analyzed composition is bonded to the carbon skeleton because it is sublimated and separated before combustion decomposition.

<Production of lithium ion secondary battery>
[1] Positive electrode 3 parts by mass of the above-described sulfur-based positive electrode active material, 2.1 parts by mass of acetylene black (AB), and 0.9 parts by mass of TAB (a mixture of AB: polytetrafluoroethylene = 2: 1 by mass ratio) While adding an appropriate amount of hexane, the mixture was kneaded in an agate mortar until it became a film to obtain a film-like positive electrode material. The entire amount of the positive electrode material was pressed with a press machine onto an aluminum mesh (mesh roughness # 100) punched into a circle with a diameter of 11 mm, and dried at 80 ° C. overnight to form the positive electrode for the lithium ion secondary battery of Example 1. Obtained.

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

[3] Electrolytic Solution As the electrolytic solution, a solution obtained by dissolving LiPF ₆ in a mixed solvent obtained by mixing ethylene carbonate and diethyl carbonate was used. Ethylene carbonate and diethyl carbonate were mixed at a volume 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, negative electrode, and electrolytic solution obtained in [1], [2], and [3]. Specifically, in a dry room, a glass nonwoven fabric filter having a thickness of 500 μm and a separator were sandwiched between a positive electrode and a negative electrode to obtain an electrode body battery. This electrode body battery was housed 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.

In the same manner as in Example 1 except that 25 parts by mass of iodine (I ₂ ) was added to 100 parts by mass of the sulfur-modified PAN produced in the same manner as in Example 1, about 50% by weight of iodine (I). That is, a sulfur-based positive electrode active material containing about 25% as iodine (I ₂ ) was prepared. A lithium ion secondary battery was manufactured in the same manner as in Example 1 except that this sulfur-based positive electrode active material was used when manufacturing the positive electrode.

[Comparative Example 1]
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the sulfur-modified PAN produced in the same manner as in Example 1 was used as the sulfur-based positive electrode active material when the positive electrode was produced.

<Discharge rate characteristics test>
The discharge rate characteristics of the lithium ion secondary batteries of Examples 1 and 2 and Comparative Example 1 were measured. Each lithium ion secondary battery was repeatedly charged and discharged by changing the current value at a C rate of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C,. In addition, 11 cycles of charge and discharge were performed at 1.8 mA in advance, and the C rate was determined based on the capacity of the 11th cycle. The cut-off voltage at this time was 1.0V. The temperature was 25 ° C. The results of the discharge rate characteristic test are shown in FIG.

<Discharge characteristics test>
FIG. 5 and Table 2 show the discharge characteristics when the lithium ion secondary batteries of Examples 1 and 2 and Comparative Example 1 were discharged by 1C.

  The lithium ion secondary battery of Example 1 has a large discharge capacity and a high average potential compared to Comparative Example 1, and the energy density at the 1C rate is improved by 30% compared to Comparative Example 1. It is clear that this is an effect obtained by including iodine in the sulfur-based positive electrode active material. In the lithium ion secondary battery of Example 2, the discharge rate characteristics and average potential are higher than those of Comparative Example 1, but the discharge capacity is low. This is presumably because the proportion of iodine that seems not to be involved in Li desorption (capacity) in the sulfur-based positive electrode active material increased, and the proportion of sulfur that participated in Li desorption (capacity) decreased.

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

Claims (6)

  A sulfur-based positive electrode active material comprising a carbon skeleton and sulfur (S) and iodine (I) bonded to the carbon skeleton.   The carbon skeleton is derived from a carbon source compound selected from polyacrylonitrile, pitches, plant raw materials, polyisoprene, and polycyclic aromatic hydrocarbons formed by condensation of three or more six-membered rings. The sulfur-based positive electrode active material described.   3. The sulfur-based positive electrode according to claim 2, wherein the sulfur (S) is contained in an amount of 20% by mass or more and less than 70% by mass, and the iodine (I) is contained in an amount of 0.01% by mass or more and less than 20% by mass. Active material.   It has a carbon skeleton derived from polyacrylonitrile, the sulfur (S) is contained in an amount of 30 to 70% by mass, and the iodine (I) is contained in an amount of 0.01 to 20% by mass. 3. The sulfur-based positive electrode active material according to claim 2.   A non-aqueous secondary battery comprising a positive electrode including the sulfur-based positive electrode active material according to claim 1 and a negative electrode.   8. A vehicle equipped with the non-aqueous secondary battery according to claim 7.

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