JP5142415B2 - Positive electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

Disclosed is a positive electrode containing a sulfur-based positive electrode active substance and capable of increasing discharge rate characteristics and the cycle characteristics of lithium ion secondary batteries. The positive electrode for lithium ion secondary batteries contains a sulfur-based positive electrode active substance which contains carbon (C) and sulfur (S), and a conductive material which contains sulfur (S), wherein at least one part of said conductive material is a sulfide of at least one metal selected from the group consisting of the fourth-period metals, fifth-period metals, sixth-period metals, and rare earth elements.

Description

The present invention is a positive electrode for a lithium ion secondary battery containing the sulfur-based positive electrode active material, and relates to a lithium ion secondary batteries using the positive electrode.

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

  As a positive electrode active material of a lithium ion secondary battery, a material containing a rare metal such as cobalt or nickel is generally used. However, since these metals have a small circulation amount and are expensive, in recent years, a positive electrode active material using a substance replacing these rare metals has been demanded.

  As a positive electrode active material of a lithium ion secondary battery, a technique using sulfur is known. By using sulfur as the positive electrode active material, the charge / discharge capacity of the lithium ion secondary battery can be increased. For example, the charge / discharge capacity of a lithium ion secondary battery using sulfur as a positive electrode active material is approximately six times the charge / discharge capacity of a lithium ion secondary battery using a lithium cobaltate positive electrode material, which is a common positive electrode material. is there.

  However, in a lithium ion secondary battery using elemental sulfur as the positive electrode active material, a compound of sulfur and lithium is generated during discharge. This compound of sulfur and lithium is soluble in a non-aqueous electrolyte solution (for example, ethylene carbonate, dimethyl carbonate, etc.) of a lithium ion secondary battery. For this reason, a lithium ion secondary battery using sulfur as a positive electrode active material has a problem that, when charging and discharging are repeated, it gradually deteriorates due to elution of sulfur into the electrolytic solution, and the battery capacity decreases. Hereinafter, the characteristic of the lithium ion secondary battery in which the charge / discharge capacity decreases with repeated charge / discharge is referred to as “cycle characteristic”. The lithium ion secondary battery having a small decrease in charge / discharge capacity is a lithium ion secondary battery having excellent cycle characteristics, and the lithium ion secondary battery having a large decrease in charge / discharge capacity is a lithium ion secondary battery having inferior cycle characteristics.

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

  Patent Document 1 introduces a technique using polysulfide carbon having carbon and sulfur as main components as a positive electrode active material. This polysulfide carbon is obtained by adding sulfur to a linear unsaturated polymer. It is thought that the elution of sulfur to the electrolyte can be suppressed by the carbon material, and the cycle characteristics are improved.

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

  However, on the other hand, the positive electrode active material using the above-mentioned polyacrylonitrile or linear unsaturated polymer as a carbon material has a problem that the electrical discharge rate characteristic (so-called C rate) is not sufficient because of its high electrical resistance. .

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

  The present invention has been made in view of the above circumstances, and provides a positive electrode and a lithium ion secondary battery that contain a sulfur-based positive electrode active material and can improve cycle characteristics and discharge rate characteristics of a lithium ion secondary battery. Objective.

The positive electrode for a lithium ion secondary battery of the present invention that solves the above-mentioned problems contains a sulfur-based positive electrode active material containing carbon (C) and sulfur (S), and a conductive material containing sulfur (S). , at least a portion of said transmission conductive _{material, La 2 S 3, Sm 2} S 3, Ce 2 S 3, characterized in that Ru least one metal sulfide Tona selected from the group consisting of MoS _2.

The lithium ion secondary battery of the present invention that solves the above-described problems uses the positive electrode for a lithium ion secondary battery of the present invention .

As a first method of manufacturing a positive electrode for a lithium ion secondary battery of the present invention, a method of manufacturing a carbon (C) and sulfur (S) a positive electrode for a lithium ion secondary battery containing the sulfur-based positive electrode active material containing A heat treatment step of heating a mixed material containing a carbon material, sulfur, and a conductive material containing sulfur, the conductive material comprising a fourth period metal, a fifth period metal, a sixth period The method which is a sulfide of at least 1 type of metal chosen from the group which consists of a metal and rare earth elements can be mentioned .

As a second method for producing a positive electrode for a lithium ion secondary battery of the present invention, a method of manufacturing a carbon (C) and sulfur (S) a positive electrode for a lithium ion secondary battery containing the sulfur-based positive electrode active material containing A heat treatment step of heating a mixed material containing a carbon material, sulfur, and a conductive material not containing sulfur, the conductive material comprising a fourth period metal, a fifth period metal, and a sixth period The method which is at least 1 type of metal chosen from the group which consists of a metal and rare earth elements can be mentioned .

  The positive electrode of the present invention contains a sulfur-based positive electrode active material containing carbon (C) and sulfur (S), and a conductive material containing sulfur (S). As described above, the lithium ion secondary battery using elemental sulfur as the positive electrode active material is inferior in cycle characteristics. However, by mixing the carbon material with the positive electrode active material containing sulfur, elution of sulfur into the electrolytic solution can be suppressed, and cycle characteristics can be improved. The positive electrode of the present invention is blended with 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 may exhibit high electrical conductivity (conductivity) or may improve the lithium ion 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 with a sulfur type positive electrode active material, it may be sulfided by the sulfur contained in a sulfur type positive electrode active material at the time of manufacture of a positive electrode and / or charging / discharging of a battery. For this reason, when a material having low electrical conductivity in the state of sulfide or a material that cannot improve lithium ion conductivity is used as the conductive material, there is a problem that it is difficult to improve the discharge rate characteristics. However, in Rukoto used as capable of improving the condition in a high electric lithium ion conductive or positive indicating the conductivity of the sulfide as a conductive material, it may sufficiently increase the discharge rate characteristics.

  The positive electrode of the present invention can improve the cycle characteristics and discharge rate characteristics of the lithium ion secondary battery by these cooperation. Similarly, the lithium ion secondary battery of the present invention is excellent in cycle characteristics and discharge rate characteristics.

In the first method for producing a positive electrode for a lithium ion secondary battery, a metal sulfide is used as a conductive material. For this reason, as described above, the conductive material and the positive electrode active material can be dispersed substantially uniformly, and the discharge rate characteristics can be improved. In the second method for producing a positive electrode for a lithium ion secondary battery, an unsulfurized metal is used. Also in this case, the discharge rate characteristics are improved because the conductive material is sulfided during the production and exhibits high electrical conductivity or the lithium ion conductivity of the positive electrode is improved.

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 a graph showing the result of having carried out X-ray diffraction of the sulfur modification pitch. It is a graph showing the result of Raman spectrum analysis of sulfur-modified pitch. It is explanatory drawing which represents typically the reaction apparatus used with the manufacturing method of the positive electrode of a test example . 4 is a graph showing the result of X-ray diffraction of a sulfur-based positive electrode active material-conductive material composite used for the positive electrode of Test Example 1. 4 is a graph showing the result of X-ray diffraction of a sulfur-based positive electrode active material-conductive material composite used for the positive electrode of Test Example 2. 6 is a graph showing the result of X-ray diffraction of a sulfur-based positive electrode active material-conductive material composite used for the positive electrode of Test Example 4. 14 is a graph showing the result of X-ray diffraction of a sulfur-based positive electrode active material-conductive material composite used for the positive electrode of Test Example 7. 10 is a graph showing the result of X-ray diffraction of a sulfur-based positive electrode active material-conductive material composite used for the positive electrode of Test Example 9. 10 is a graph showing the result of X-ray diffraction of the sulfur-based positive electrode active material used for the positive electrode of Test Example 10 . It is a graph showing the result of having carried out X-ray diffraction of the sulfur type positive electrode active material-conductive material (Fe) composite. 4 is a graph showing a discharge rate characteristic (charge / discharge curve) of a lithium ion secondary battery using the positive electrode of Test Example 1. FIG. 4 is a graph showing discharge rate characteristics (cycle characteristics) of a lithium ion secondary battery using the positive electrode of Test Example 1. 5 is a graph showing discharge rate characteristics (cycle characteristics) of a lithium ion secondary battery using the positive electrode of Test Example 2. FIG. 10 is a graph showing discharge rate characteristics (cycle characteristics) of a lithium ion secondary battery using the positive electrode of Test Example 3. 10 is a graph showing discharge rate characteristics (cycle characteristics) of a lithium ion secondary battery using the positive electrode of Test Example 4. 10 is a graph showing a discharge rate characteristic (charge / discharge curve) of a lithium ion secondary battery using the positive electrode of Test Example 5. 10 is a graph showing discharge rate characteristics (cycle characteristics) of a lithium ion secondary battery using the positive electrode of Test Example 5. 10 is a graph showing a discharge rate characteristic (charge / discharge curve) of a lithium ion secondary battery using the positive electrode of Test Example 6. 14 is a graph showing discharge rate characteristics (cycle characteristics) of a lithium ion secondary battery using the positive electrode of Test Example 6. 10 is a graph showing a discharge rate characteristic (charge / discharge curve) of a lithium ion secondary battery using the positive electrode of Test Example 7. FIG. 10 is a graph showing discharge rate characteristics (cycle characteristics) of a lithium ion secondary battery using the positive electrode of Test Example 7. 10 is a graph showing a discharge rate characteristic (charge / discharge curve) of a lithium ion secondary battery using the positive electrode of Test Example 8. 10 is a graph showing discharge rate characteristics (cycle characteristics) of a lithium ion secondary battery using the positive electrode of Test Example 8. 10 is a graph showing a discharge rate characteristic (charge / discharge curve) of a lithium ion secondary battery using the positive electrode of Test Example 9. 10 is a graph showing discharge rate characteristics (cycle characteristics) of a lithium ion secondary battery using the positive electrode of Test Example 9. 10 is a graph showing discharge rate characteristics (cycle characteristics) of a lithium ion secondary battery using the positive electrode of Test Example 10 .

The positive electrode for lithium ion secondary batteries of the present invention (hereinafter referred to as the positive electrode of the present invention) contains a positive electrode active material and a conductive material. The lithium ion secondary battery of the present invention is a battery using the positive electrode of the present invention. The method for producing a positive electrode for a lithium ion secondary battery of the present invention (hereinafter referred to as the production method of the present invention) includes a heat treatment step of heating a mixed raw material containing a carbon material, sulfur, and a conductive material, A positive electrode containing a positive electrode active material and a conductive material is manufactured. According to the production method of the present invention, the positive electrode of the present invention can be produced.

[Conductive material]
The conductive material, that is, the material of the conductive material used in manufacturing the positive electrode of the present invention, is 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 sulfides thereof. In addition, the 4th periodic metal, the 5th periodic metal, and the 6th periodic metal as used in this specification are based on a periodic table. For example, the fourth periodic metal refers to a metal included in the fourth periodic element in the periodic table. The conductive material is preferably one that exhibits high electrical conductivity in a sulfide state or that can greatly improve the lithium ion conductivity of the positive electrode. For example, Ti, Fe, La, Ce, Pr, It is at least one selected from the group consisting of Nd, Sm, V, Mn, Fe, Ni, Cu, Zn, Mo, Ag, Cd, In, Sn, Sb, Ta, W, and Pb, or a sulfide thereof. preferable. In the positive electrode, the conductive material consists of both the metal and its sulfide, or consists only of the metal sulfide. These conductive material materials preferably contain a large amount of sulfide, and more preferably consist only of sulfide. This is because the conductive material and the sulfur-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, the conductive material having high electric conductivity and / or high lithium ion conductivity is TiS ₂ , FeS ₂ , Me ₂ S ₃ (wherein Me is selected from Ti, La, Ce, Pr, Nd, and Sm). 1), MeS (wherein, Me is one selected from Ti, La, Ce, Pr, Nd, Sm), Me ₃ S ₄ (wherein, Me is Ti, La, Ce, Pr, Nd) , S _x ), Me _x S _y (wherein Me is Ti, Fe, V, Mn, Fe, Ni, Cu, Zn, Mo, Ag, Cd, In, Sn, Sb, Ta, And a kind selected from 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 materials, the electrical conductivity and / or lithium ion conductivity of the entire positive electrode can be improved, and the discharge rate characteristics of the lithium ion 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 compounding ratio of a carbon material such as polyacrylonitrile, which will be described later, and the conductive material is preferably 10: 0.5 to 10: 5, and more preferably 10: 1 to 10: 3. More preferred. 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 a 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. .

[Positive electrode active material]
The positive electrode active material used for the positive electrode of the present invention is a sulfur-based positive electrode active material containing carbon (C) and sulfur (S). Examples of the sulfur-based positive electrode active material include those disclosed in Patent Document 1 (one using a linear unsaturated polymer as a carbon material) and those disclosed in Patent Document 2 described above (carbon material). And those using various pitch-based carbon materials as the carbon material are preferably used. Hereinafter, a sulfur-based positive electrode active material using polyacrylonitrile as a carbon material is referred to as sulfur-modified polyacrylonitrile. A sulfur-based positive electrode active material using a pitch-based carbon material as a carbon material is referred to as sulfur-modified pitch. Furthermore, polyacrylonitrile is abbreviated as PAN as necessary.

<Sulfur modified polyacrylonitrile>
When polyacrylonitrile is used as the carbon material, the high capacity inherent in sulfur can be maintained, and elution of sulfur into the electrolyte is suppressed, so that the cycle characteristics are greatly improved. This is presumably because sulfur does not exist as a simple substance in the sulfur-based positive electrode active material but exists in a stable state combined with polyacrylonitrile. In the manufacturing method of the sulfur type positive electrode active material currently disclosed by patent document 2, sulfur is heat-processed with polyacrylonitrile. When polyacrylonitrile is heated, it is considered that polyacrylonitrile is three-dimensionally cross-linked to form a condensed ring (mainly a 6-membered ring) and close the ring. For this reason, it is considered that sulfur is present in the sulfur-based positive electrode active material in a state of being bonded to the polyacrylonitrile that has progressed in the ring closure. By combining polyacrylonitrile and sulfur, elution of sulfur into the electrolytic solution can be suppressed, and cycle characteristics can be improved.

The polyacrylonitrile used as the carbon material is preferably in the form of powder, and preferably has a mass average molecular weight of about 10 ^{4 to} 3 × 10 ⁵ . The particle size of polyacrylonitrile is preferably about 0.5 to 50 μm and more preferably about 1 to 10 μm when observed with an electron microscope. If the molecular weight and particle size of polyacrylonitrile are within these ranges, the contact area between polyacrylonitrile and sulfur can be increased, and polyacrylonitrile and sulfur can be reacted with high reliability. For this reason, the elution of sulfur to the electrolytic solution can be more reliably suppressed.

  The sulfur used for the sulfur-based positive electrode active material is also preferably in the form of powder. 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 polyacrylonitrile powder and sulfur powder used for the sulfur-based positive electrode active material is not particularly limited, but is preferably 1: 0.5 to 1:10 by mass ratio, and 1: 0.5 to 1 : 7 is more preferable, and 1: 2 to 1: 5 is still more preferable.

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

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

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

An example of the Raman spectrum of sulfur-modified polyacrylonitrile is shown in FIG. In the Raman spectrum shown in FIG. 2, there are major peak near 1331cm ^-1 of Raman shift, ^{and, 1548cm} -1 in the range of ^{200cm -1 ~1800cm -1, 939cm -1,} 479cm -1, 381cm -1, There is a peak near 317 cm ⁻¹ . The Raman shift peak described above is observed at the same position even when the ratio of elemental sulfur to polyacrylonitrile is changed. Thus, these peaks characterize sulfur-modified polyacrylonitrile. Each of the above-described peaks exists in a range of approximately ± 8 cm ⁻¹ with the above-described peak position as the center. In the present specification, the “main peak” refers to a peak having the maximum peak height among all peaks appearing in the Raman spectrum.

For reference, the Raman shift described above was measured with RMP-320 (excitation wavelength λ = 532 nm, grating: 1800 gr / mm, resolution: 3 cm ⁻¹ ) manufactured by JASCO Corporation. Note that the number of Raman spectrum peaks may change or the position of the peak top may be shifted depending on the wavelength of incident light or the difference in resolution. Therefore, when measuring the Raman spectrum of the positive electrode of the present invention using sulfur-modified polyacrylonitrile as the positive electrode active material, the same peak as the above peak or a peak slightly different from the above peak in number and peak top position is confirmed. Is done.

<Sulfur modified pitch>
In this specification, the pitch-based carbon material is obtained by polycondensation of coal pitch, petroleum pitch, mesophase pitch (anisotropic pitch), asphalt, coal tar, coal tar pitch, and condensed polycyclic aromatic hydrocarbon compounds. It refers to at least one selected from the group consisting of an organic synthetic pitch or an organic synthetic pitch obtained by polycondensation of a heteroatom-containing condensed polycyclic aromatic hydrocarbon compound. These are known as carbon materials containing condensed polycyclic aromatics.

  Coal tar, a kind of pitch-based carbon material, 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.

  The pitch-based carbon material is a mixture of various compounds, and includes a condensed polycyclic aromatic as described above. The condensed polycyclic aromatic contained in the pitch-based carbon material may be a single species or a plurality of species. For example, the main component of coal pitch, which is a kind of pitch-based carbon material, 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.

  When pitch-based carbon materials are used as the carbon material, the high capacity inherent in sulfur can be maintained and the elution of sulfur into the electrolyte is suppressed, as in the case of using polyacrylonitrile as the carbon material. Is greatly improved. This is because sulfur does not exist as a simple substance in the sulfur-based positive electrode active material, but sulfur is taken in between the graphene layers of the pitch-based carbon material, or hydrogen contained in the condensed polycyclic aromatic ring is converted into sulfur. It is presumed that this is because it is replaced with a CS bond.

  The particle size of the pitch-based carbon material is not particularly limited. Moreover, when using a pitch-type carbon material as a carbon material, the particle size of sulfur is also not specifically limited. The mixing ratio of the pitch-based carbon material and sulfur is not particularly limited, but the mixing ratio of the pitch-based carbon material and sulfur in the mixed raw material is 1: 0.5 to 1:10 by mass ratio. Preferably, it is 1: 1 to 1: 7, more preferably 1: 2 to 1: 5.

  The sulfur-modified pitch contains a plurality of types of polycyclic aromatic hydrocarbons. The polycyclic aromatic hydrocarbon (PAH) referred to in this specification is from the group consisting of the above-mentioned various pitch-based carbon materials themselves and the various polycyclic aromatic hydrocarbons contained in the above-mentioned various pitch-based carbon materials. It refers to at least one carbon material selected.

  Further, sulfur-modified pitch (coal pitch: sulfur = 1: 1, 1: 5, 1:10), simple coal pitch and simple sulfur were X-ray diffracted by CuKα rays. The diffraction conditions are the same as those of the above sulfur-modified polyacrylonitrile.

  As shown in FIG. 3, in the range of diffraction angle (2θ) of 10 to 60 °, the main peak of elemental sulfur was present at around 22 °, and the main peak of elemental coal pitch was present at around 26 °. The peak of the sulfur-modified pitch with a coal pitch / sulfur mixing ratio of 1: 1 was a single peak and was present at around 26 °. The main peak of sulfur-modified pitch having a blending ratio of coal pitch and sulfur of 1: 5 and sulfur-modified pitch having a blending ratio of coal pitch and sulfur of 1:10 was present at around 22 °.

  Sulfur-modified pitch is excellent in thermal stability. When the sulfur-modified pitch is heated from room temperature to 550 ° C. at a heating rate of 10 ° C./min, the mass reduction by thermogravimetric analysis is about 25% at 550 ° C. For reference, the mass loss of the coal pitch is about 30% at 550 ° C. In the case of elemental sulfur, the mass gradually decreases from around 170 ° C., and rapidly decreases when the temperature exceeds 200 ° C. Coal pitch is also difficult to decrease in mass, and in the vicinity of 250 ° C to 450 ° C, coal pitch tends to be less likely to decrease in mass than sulfur-modified pitch. Above 450 ° C., the mass of sulfur-modified pitch is less likely to be reduced than that of coal pitch.

  An example of the Raman spectrum of sulfur-modified pitch is shown in FIG. For reference, this Raman spectrum is measured under the same conditions as the Raman spectrum of the above-described sulfur-modified polyacrylonitrile.

In the Raman spectrum shown in FIG. 4, there are major peak near 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. These peaks are observed at similar positions even when the ratio of elemental sulfur to the pitch-based carbon material is changed, and are peaks that characterize the sulfur-modified pitch. When the Raman spectrum of the positive electrode of the present invention using sulfur-modified pitch as the positive electrode active material is measured, peaks that are the same as these peaks or slightly different in number and peak top position are confirmed. The Raman spectrum of sulfur-modified pitch is different from the Raman spectrum of sulfur-modified polyacrylonitrile.

  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.

<Other sulfur-based positive electrode active materials>
Other sulfur-based positive electrode active materials used in the positive electrode of the present invention include the above-mentioned polysulfide carbon, elemental sulfur, sulfur-modified polycyclic aromatic hydrocarbons, sulfur-modified rubber, coffee raw materials such as coffee beans and seaweed, and sulfur. The heat-treated thing or these composites etc. can be mentioned.

  In addition, when using sulfur-modified polyacrylonitrile or sulfur-modified pitch as the positive electrode active material, when using polysulfide carbon as the positive electrode active material (when using a linear unsaturated polymer as the carbon material for the positive electrode active material). In comparison, the cycle characteristics can be further improved. This is because when carbon polysulfide is used as the positive electrode active material, sulfur and lithium are bonded at the time of discharge, so that -CS-CS- bond and -S-S- bond contained in the polysulfide carbon are broken. This is probably because the polymer is cut. Therefore, in the method for producing a positive electrode of the present invention, it is preferable to use polyacrylonitrile or a pitch-based carbon material as the carbon material. Polyacrylonitrile is more preferably used in terms of cycle characteristics and capacity, and pitch-based carbon material is more preferably used in terms of cost. Further, as the carbon material, sulfur-modified polyacrylonitrile and sulfur-modified pitch may be used in combination.

(Method for producing positive electrode for lithium ion secondary battery)
In the production method of the present invention, a mixed material obtained by mixing the above-described sulfur-based positive electrode active material (ie, carbon material and sulfur) and a conductive material is heated. What is necessary is just to mix a mixing material with general mixing apparatuses, such as a mortar and a ball mill. As the mixed raw material, a material obtained by simply mixing sulfur, a carbon material, and a conductive material may be used. For example, the mixed raw material may be formed into a pellet shape.

  By heating the mixed raw material in the heat treatment step, sulfur contained in the mixed raw material and the carbon material are combined. When an unsulfurized metal is used as the conductive material, metal sulfidation also occurs. The heat treatment step may be performed in a closed system or an open system, but in order to suppress the dissipation of sulfur vapor, it is preferably performed in a closed system. In addition, the atmosphere in which the heat treatment step is performed is not particularly limited, but it is preferably performed in an atmosphere that does not hinder the bonding between the carbon material and sulfur (for example, an atmosphere not containing hydrogen or a non-oxidizing atmosphere). For example, when hydrogen is present in the atmosphere, sulfur in the reaction system reacts with hydrogen to form hydrogen sulfide, so that sulfur in the reaction system may be lost. In particular, when polyacrylonitrile is used as a carbon material, the vaporized sulfur reacts with the polyacrylonitrile at the same time as the polyacrylonitrile ring-closing reaction by heat treatment in a non-oxidizing atmosphere. It is believed that polyacrylonitrile is obtained. The non-oxidizing atmosphere referred to here includes a reduced pressure state in which the oxygen concentration is low enough not to cause an oxidation reaction, an inert gas atmosphere such as nitrogen or argon, a sulfur gas atmosphere, and the like.

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

  What is necessary is just to set suitably the heating time of the mixed raw material in a heat processing process according to heating temperature, and it does not specifically limit it. The preferable heating temperature mentioned above should just be a temperature which the coupling | bonding of sulfur and a carbon material advances, and a conductive material does not change in quality.

  For example, when polyacrylonitrile is used as the carbon material, the heating temperature is preferably 250 to 500 ° C., more preferably 250 to 400 ° C., and even more preferably 250 to 300 ° C. When a pitch-based carbon material is used as the carbon material, the heating temperature is preferably 200 ° C. or higher and 600 ° C. or lower, more preferably 300 ° C. or higher and 500 ° C. or lower, and 350 ° C. or higher and 500 ° C. or lower. Is more preferable. When a pitch-based carbon material is used as the carbon material, at least a part of the pitch-based carbon material and at least a part of sulfur become liquid in the heat treatment step. In other words, in the heat treatment step, at least a part of the pitch-based carbon material and at least a part of sulfur are in liquid contact. Therefore, the contact area between the pitch-based carbon material and sulfur in the heat treatment step is large, the pitch-based carbon material and sulfur are sufficiently bonded, and the desorption of sulfur from the sulfur-based positive electrode active material is suppressed.

    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 recirculation | reflux apparatus of a known structure when recirculating | refluxing sulfur in a heat treatment process.

  Even when the amount of sulfur in the mixed material is excessive, a sufficient amount of sulfur can be taken into the carbon material in the heat treatment step. For this reason, when sulfur is excessively added to the carbon material, the elemental sulfur described above is removed by removing the elemental sulfur from the object to be treated (sulfur-based positive electrode active material-carbon material composite) after the heat treatment step. The adverse effect by can be suppressed. 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 carbon material. 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-based positive electrode active material. Moreover, what is necessary is just to use the to-be-processed body after a single sulfur removal process as a sulfur type positive electrode active material, when performing a single-piece | unit sulfur removal process to the to-be-processed body after a heat treatment process.

(Positive electrode for lithium ion secondary battery)
The positive electrode of the present invention is produced by the production method of the present invention described above, and contains a sulfur-based positive electrode active material and a conductive material. When the positive electrode of the present invention contains sulfur-modified polyacrylonitrile and / or sulfur-modified pitch as a sulfur-based positive electrode active material, the Raman spectrum of the positive electrode shows peaks derived from the sulfur-modified polyacrylonitrile shown in FIG. The peak derived from the sulfur-modified pitch shown in FIG.

  The positive electrode can have the same structure as a general positive electrode for a lithium ion secondary battery except for the positive electrode active material and the conductive material. For example, the positive electrode of the present invention comprises a positive electrode material obtained by mixing a mixture of a sulfur-based positive electrode active material and a conductive material (that is, an object to be processed obtained by a heat treatment process), a conductive additive, a binder, and a solvent. It can be manufactured by applying to. Alternatively, the mixed raw material in which the sulfur powder, the carbon material powder, and the conductive material powder are mixed can be heated (a heat treatment step is performed) after filling the positive electrode current collector. According to this method, a mixture of a sulfur-based positive electrode active material and a conductive material can be produced, and at the same time, the mixture 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 positive electrode includes a conductive material. The content ratio between the sulfur-based positive electrode active material and the conductive material in the positive electrode is preferably 10: 0.1 to 10: 5, more preferably 10: 0.3 to 10: 2, in terms of mass ratio. .

  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. Depending on the configuration of the conductive material, it may not be necessary to add a conductive additive.

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

  Examples of the solvent include N-methyl-2-pyrrolidone, N, N-dimethylformaldehyde, alcohol, water and the like. These conductive assistants, binders and solvents may be used as a mixture of plural kinds. The blending amount of these materials is not particularly limited. For example, it is preferable to blend about 20 to 100 parts by mass of the conductive additive and about 10 to 20 parts by mass of the binder with respect to 100 parts by mass of the sulfur-based positive electrode active material. As another method, a mixture of the sulfur-based positive electrode active material of the present invention, the above-described conductive additive and binder is kneaded with a mortar or a press machine to form a film, and the film-like mixture is collected with a press machine or the like. The positive electrode for a lithium ion secondary battery of the present invention can also be produced by pressure bonding to an electric body.

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

(Lithium ion secondary battery)
Hereinafter, the structure of the lithium ion secondary battery using the positive electrode of the present invention will be described. Hereinafter, a lithium ion secondary battery using the positive electrode of the present invention is simply abbreviated as a lithium ion secondary battery. The positive electrode is as described above.

[Negative electrode]
As the negative electrode material, a known carbon-based material such as lithium metal or graphite, a silicon-based material such as a silicon thin film, or an alloy-based material such as copper-tin or cobalt-tin can be used. When a negative electrode material that does not contain lithium, for example, a carbon-based material, a silicon-based material, an alloy-based material, etc., among the negative electrode materials described above, short-circuiting between the positive and negative electrodes due to the generation of dendrites is unlikely to occur. This is advantageous. 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 lithium. For this reason, a lithium pre-doping process in which lithium is inserted in advance into either one or both of the negative electrode and the positive electrode is necessary. The lithium pre-doping method may be a known method. For example, when lithium is doped into the negative electrode, a half-cell is assembled using metallic lithium as the counter electrode, and lithium is inserted by an electrolytic doping method in which lithium is electrochemically doped, or a metallic lithium foil is attached to the electrode After that, there is a method in which lithium is inserted by a pasting pre-doping method in which it is left in an electrolytic solution and doped using diffusion of lithium to the electrode. Also, when the positive electrode is predoped with lithium, the above-described electrolytic doping method can be used.

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

〔Electrolytes〕
As the electrolyte used for the lithium ion secondary battery, an electrolyte in which an alkali metal salt as an electrolyte is dissolved 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, gamma-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 lithium ion secondary battery is a lithium polymer secondary battery, the electrolyte is in a solid state (for example, a polymer gel).

[Others]
The lithium ion 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 lithium ion secondary battery is a sealed type, the separator is also required to have a function of holding an electrolytic solution. As the separator, it is preferable to use a thin, microporous or non-woven membrane made of polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide, cellulose, glass or the like. The shape of the lithium ion secondary battery is not particularly limited, and can be various shapes such as a cylindrical shape, a stacked shape, and a coin shape.

Test example

  Hereinafter, the positive electrode, the conductive material, and the method for producing the positive electrode of the present invention will be specifically described.

( Test Example 1)
[1] Mixed raw material A sulfur powder having a particle size of 50 μm or less when classified using a sieve was prepared. A polyacrylonitrile powder having a particle size in the range of 0.2 μm to 2 μm when prepared with an electron microscope was prepared. As the conductive material, La ₂ S ₃ having a particle size of 50 μm or less when classified using a sieve was prepared.

  5 g of sulfur powder, 1 g of polyacrylonitrile powder and 0.1 g of conductive material material powder were mixed and pulverized in a mortar to obtain a mixed raw material.

[2] Apparatus As shown in FIG. 5, 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 and 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 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 330 ° C. After stopping the heating, the temperature of the mixed raw material 9 increased to 350 ° C. and then decreased. Therefore, in this heat treatment step, the mixed raw material 9 was heated to 350 ° 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 350 ° 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 250 ° C. for 3 hours while being vacuumed. The temperature elevation temperature at this time was 10 ° C./min. By this step, the elemental sulfur remaining in the object to be treated after the heat treatment step is evaporated and removed, and a sulfur-based positive electrode active material-conductive material composite that does not contain (or substantially does not contain) elemental sulfur is obtained.

<Production of lithium ion secondary battery>
[1] Positive electrode A mixture of 3 mg of a sulfur-based positive electrode active material-conductive material complex, 2.7 mg of acetylene black, and 0.3 mg of polytetrafluoroethylene (PTFE) is added into a film in an agate mortar while adding an appropriate amount of hexane. The film was kneaded until a film-like positive electrode material was obtained. The total amount of the positive electrode material was press-bonded to an aluminum mesh (mesh roughness # 100) punched into a circle having a diameter of 14 mm with a press machine and dried at 80 ° C. overnight. In this step, the positive electrode for the lithium ion secondary battery of Test Example 1 was obtained. Note that conductive material in the positive electrode is La ₂ S _3, the content ratio of sulfur-based positive electrode active material and conductive material (mass ratio) was 10: 0.6.

[2] Negative Electrode As the negative electrode, a metal lithium foil (a disk shape having a diameter of 14 mm and a thickness of 500 μm, manufactured by Honjo Metal) was used.

[3] Electrolytic Solution As the electrolytic solution, a nonaqueous electrolyte in which LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate was used. Ethylene carbonate and diethyl carbonate were mixed at a 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 and the negative electrode obtained in [1] and [2]. Specifically, in a dry room, a separator (Celgard 2400 made by Celgard, a polypropylene microporous membrane having a thickness of 25 μm) and a glass nonwoven fabric filter (thickness 440 μm, made by ADVANTEC, GA100) are placed between the positive electrode and the negative electrode. To be an electrode body battery. This electrode body battery was accommodated in a battery case (CR2032-type coin battery member, manufactured by Hosen Co., Ltd.) made of a stainless steel container. The electrolyte solution obtained in [3] was injected into the battery case. The battery case was sealed with a caulking machine to obtain a lithium ion secondary battery of Test Example 1.

( Test Example 2)
The manufacturing method of the positive electrode of Test Example 2 is the same as the manufacturing method of the positive electrode of Test Example 1 except that a mixture of 5 g of sulfur powder, 1 g of polyacrylonitrile powder, and 0.3 g of conductive material powder is used as a mixed raw material. It is. In the production method of Test Example 2, the mass ratio of polyacrylonitrile to the conductive material in the mixed raw material was 1: 0.3. The conductive material in the positive electrode of Test Example 2 was La ₂ S ₃ as in Test Example 1, and the content ratio (mass ratio) between the sulfur-based positive electrode active material and the conductive material was 10: 1.8. The lithium ion secondary battery of Experimental Example 2, except for using the positive electrode of Test Example 2 is the same as the lithium ion secondary batteries of Experimental Example 1.

( Test Example 3)
The manufacturing method of the positive electrode of Test Example 3 is the same as the manufacturing method of the positive electrode of Test Example 1 except that a mixture of 5 g of sulfur powder, 1 g of polyacrylonitrile powder, and 0.5 g of conductive material powder is used as the mixed raw material. It is. In the production method of Test Example 3, the mass ratio of polyacrylonitrile to the conductive material in the mixed raw material was 1: 0.5. Moreover, the conductive material in the positive electrode of Test Example 3 was La ₂ S ₃ as in Test Examples 1 and 2, and the content ratio (mass ratio) between the sulfur-based positive electrode active material and the conductive material was 10: 2.9. . The lithium ion secondary battery of Experimental Example 3, except for using the positive electrode of Test Example 3 is the same as the lithium ion secondary batteries of Experimental Example 1.

( Test Example 4)
The manufacturing method of the positive electrode of Test Example 4 is a test example except that TiS ₂ is used as a conductive material, and a mixture of 5 g of sulfur powder, 1 g of polyacrylonitrile powder and 0.1 g of conductive material material powder is used as a mixed raw material. It is the same as the manufacturing method of 1 positive electrode. In the manufacturing method of Test Example 4, the mass ratio of polyacrylonitrile to the conductive material in the mixed raw material was 1: 0.1. Further, conductive material in the positive electrode of Test Example 4 is TiS _2, the content ratio of sulfur-based positive electrode active material and conductive material (mass ratio) was 10: 0.6. The lithium ion secondary battery of Test Example 4, except for using the positive electrode of Test Example 4 is the same as the lithium ion secondary batteries of Experimental Example 1.

( Test Example 5)
The manufacturing method of the positive electrode of Test Example 5 uses Sm ₂ S ₃ as a conductive material, and uses a mixture of 5 g of sulfur powder, 1 g of polyacrylonitrile powder and 0.1 g of conductive material material as a mixed raw material. This is the same as the manufacturing method of the positive electrode of Test Example 1. In the production method of Test Example 5, the mass ratio of polyacrylonitrile to the conductive material in the mixed raw material was 1: 0.1. Moreover, the conductive material in the positive electrode of Test Example 5 was Sm ₂ S ₃ , and the content ratio (mass ratio) between the sulfur-based positive electrode active material and the conductive material was 10: 0.6. The lithium ion secondary battery of Test Example 5, except for using the positive electrode of Test Example 5 is the same as the lithium ion secondary batteries of Experimental Example 1.

( Test Example 6)
The manufacturing method of the positive electrode of Test Example 6 uses Ce ₂ S ₃ as a conductive material, and uses a mixture of 5 g of sulfur powder, 1 g of polyacrylonitrile powder and 0.1 g of conductive material material as a mixed raw material. This is the same as the manufacturing method of the positive electrode of Test Example 1. In the manufacturing method of Test Example 6, the mass ratio of polyacrylonitrile to the conductive material in the mixed raw material was 1: 0.1. Moreover, the conductive material in the positive electrode of Test Example 6 was Ce ₂ S ₃ , and the content ratio (mass ratio) between the sulfur-based positive electrode active material and the conductive material was 10: 0.6. The lithium ion secondary battery of Test Example 6, except for using the positive electrode of Test Example 6 is the same as the lithium ion secondary batteries of Experimental Example 1.

( Test Example 7)
The manufacturing method of the positive electrode of Test Example 7 uses unsulfurized Ti as a conductive material, and uses a mixture of 5 g of sulfur powder, 1 g of polyacrylonitrile powder and 0.1 g of conductive material material as a mixed raw material. This is the same as the manufacturing method of the positive electrode of Test Example 1. In the manufacturing method of Test Example 7, the mass ratio of polyacrylonitrile to the conductive material in the mixed raw material was 1: 0.1. Further, conductive material in the positive electrode of Test Example 7 is TiS _2, the content ratio of sulfur-based positive electrode active material and conductive material (mass ratio) was 10: 0.6. The lithium ion secondary battery of Experimental Example 7, except for using the positive electrode of Test Example 7 is the same as the lithium ion secondary batteries of Experimental Example 1.

( Test Example 8)
The manufacturing method of the positive electrode of Test Example 8 uses TiS ₂ as a conductive material, 5 g of sulfur powder as a mixed raw material, 1 g of coal pitch powder (isotropic pitch, CAS number 65996-93-2) and conductive material powder 0 The same method as in the positive electrode manufacturing method of Test Example 1, except that a mixture with 0.1 g was used and the electrode was dried under vacuum at 200 ° C. for 3 hours. In the manufacturing method of Test Example 8, the mass ratio of the pitch-based carbon material and the conductive material in the mixed raw material was 1: 0.1. Further, conductive material in the positive electrode of Test Example 8 is TiS _2, the content ratio of sulfur-based positive electrode active material and conductive material (mass ratio) was 10: 0.5. The lithium ion secondary battery of Test Example 8, except for using the positive electrode of Test Example 8 is the same as the lithium ion secondary batteries of Experimental Example 1.

( Test Example 9)
The manufacturing method of the positive electrode of Test Example 9 is a test example except that MoS ₂ is used as a conductive material, and a mixture of 5 g of sulfur powder, 1 g of polyacrylonitrile powder, and 0.1 g of conductive material material powder is used as a mixed raw material. It is the same as the manufacturing method of 1 positive electrode. In the manufacturing method of Test Example 9, the mass ratio of polyacrylonitrile to the conductive material in the mixed raw material was 1: 0.1. Further, conductive material in the positive electrode of Test Example 9 is MoS _2, the content ratio of sulfur-based positive electrode active material and conductive material (mass ratio) was 10: 0.6. The lithium ion secondary battery of Test Example 9, except for using the positive electrode of Test Example 9 is the same as the lithium ion secondary batteries of Experimental Example 1.

( Test Example 10 )
The manufacturing method of the positive electrode of Test Example 10 is the same as the manufacturing method of the positive electrode of Test Example 1 except that no conductive material was used. The positive electrode of Test Example 10 is the same as the positive electrode of Test Example 1 except that it does not contain a conductive material. The lithium ion secondary battery of the Experimental Example 10, except for using the positive electrode of Test Example 10 is the same as the lithium ion secondary batteries of Experimental Example 1.

[Analysis of sulfur-based positive electrode active materials by X-ray diffraction]
X-ray diffraction analysis was performed on the sulfur-based positive electrode active material-conductive material composites of Test Examples 1-4, 7, and 9 and the sulfur-based positive electrode active material of Test Example 10 . A powder X-ray diffractometer (manufactured by MAC Science, M06XCE) was used as the apparatus. The measurement conditions were CuKα line, voltage: 40 kV, current: 100 mA, scan speed: 4 ° / min, sampling: 0.02 °, number of integrations: 1, diffraction angle (2θ): 10 ° to 60 °. . Diffraction patterns obtained by X-ray diffraction are shown in FIGS.

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. As shown in FIG. 11, in the sulfur positive electrode active material ( Test Example 10 ) using polyacrylonitrile as a carbon material without blending a conductive material, the diffraction angle (2θ) is in the range of 20 to 30 ° and around 25 °. A broad single peak is observed. 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, as shown in FIGS. 6 and 7, 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, as shown in FIG. 8, when TiS ₂ was used as the conductive material, almost no peak could be confirmed. As shown in FIG. 9, 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 TiO 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, the addition of TiO ₂ can be estimated even when no peak is confirmed by X-ray diffraction. As shown in FIG. 10, when MoS ₂ is used as the conductive material, 14.4, 32.7, 33.5, 35.9, 39.6, 44.2, 49.8, 56. A MoS ₂ peak appears around 0, 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). For reference, a diffraction pattern of a sulfur-based positive electrode active material-conductive material composite (polyacrylonitrile: Fe = 1: 0.1) using Fe as a conductive material is shown in FIG. As shown in FIG. 12, when Fe is used as the conductive material, FeS _{2 is in} the vicinity of 28.5, 33.0, 37.1, 40.8, 47.4, 56.3, 59.0 °. A peak appears. 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).

[Discharge rate characteristics test]
The discharge rate characteristics of the lithium ion secondary batteries of Test Examples 1 to 8 and Test Example 10 were measured. Specifically, for each lithium ion secondary battery, the current value per gram of the positive electrode active material is changed to 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C,. Went. The cut-off voltage at this time was 3.0V to 1.0V. The temperature was 25-30 ° C. The results of the discharge rate characteristic test are shown in FIGS. 13 and 14 Test Example 1, 15 Test Example 2, FIG. 16 Test Example 3, Figure 17 Test Example 4, 18 and 19 Test Example 5, Fig. 20 and 21 Test Example 6, FIG. 22 , 23 relates to the test example 7, FIGS. 24 and 25 relate to the test example 8, FIGS. 26 and 27 relate to the test example 9, and FIG. 28 relates to the lithium ion secondary battery of the test example 10 . Of these, the graphs of FIGS. 13, 18, 20, 22, and 24 are charge / discharge curves, and the graphs of FIGS. 14 to 17, 19, 21, 23, 25, 27, and 28 represent cycle characteristics.

In the lithium ion secondary battery of Test Example 10 (FIG. 28) in which no conductive material was blended, the discharge capacity at 2C was 350 to 400 mAh / g, and the discharge capacity at 5C was 120 to 150 mAh / g. It was about. On the other hand, in the lithium ion secondary battery of Test Example 1 (FIG. 14) in which La ₂ S ₃ as the conductive material is blended with the positive electrode material (specifically, the mixed raw material), the discharge capacity at 2C is 500 mAh / The discharge capacity at 5C exceeding 5 g was as high as about 250 mAh / g. This result shows that the discharge rate characteristic of the lithium ion secondary battery using a sulfur type positive electrode active material can be improved by mix | blending a conductive material material with a positive electrode material. Furthermore, as shown in FIG. 14, the capacity | capacitance fall was hardly seen after 28 cycles (0.1C) by using a sulfur type positive electrode active material. From these results, it can be said that the positive electrode for a lithium ion secondary battery of the present invention using a sulfur-based positive electrode active material and a conductive material in combination is excellent in discharge rate characteristics and cycle characteristics.

Further, the lithium ion secondary battery (Fig. 14) of Test Example 1 in which 0.1 part by mass of the conductive material is blended with 1 part by mass of polyacrylonitrile and the lithium of Test Example 2 in which 0.3 part by mass of the conductive material is blended When comparing the ion secondary battery (FIG. 15) and the lithium ion secondary battery (FIG. 16) of Test Example 3 containing 0.5 parts by mass of the conductive material, the discharge rate characteristic is Test Example 1> Test Example 2. > Test Example 3. This is because the capacity of the conductive material is lower than that of the sulfur-based positive electrode active material, or the conductive material is inactive as the active material of the lithium ion secondary battery. This is thought to be because the capacity of the whole positive electrode is reduced due to the fact that the compounding amount of the substance is reduced. And from this result, when the balance between capacity and output is taken into consideration, the preferable blending ratio of the conductive material is within the range of 0.1 to 0.3 parts by mass of the conductive material with respect to 1 part by mass of the sulfur-based positive electrode active material. I understand that.

Further, the lithium ion secondary battery of Test Example 4 using TiS ₂ as the conductive material (FIG. 17), the lithium ion secondary battery of Test Example 5 using Sm ₂ S ₃ as the conductive material (FIG. 19), All of the lithium ion secondary batteries of Test Example 6 (FIG. 21) using Ce ₂ S ₃ as the conductive material had a discharge capacity exceeding 500 mAh / g at 2C, and were excellent in discharge rate characteristics.

Further, regarding the lithium ion secondary battery of Test Example 7 using Ti as the conductive material (FIG. 23), the discharge capacity at 2C was about 500 mAh / g. From this result, it is understood that the discharge rate characteristics of the lithium ion secondary battery can be improved even when non-sulfides are used as the conductive material. This is thought to be because unsulfurized Ti reacts with sulfur in the heat treatment step and is sulfurized. Note that the lithium ion secondary battery of Test Example 4 (FIG. 17) using TiS ₂ as the conductive material is more discharged than the lithium ion secondary battery of Test Example 7 using Ti as the conductive material (FIG. 23). Excellent rate characteristics. From this result, it can be seen that metal sulfide is more preferably used as the conductive material.

Furthermore, the lithium ion secondary battery of Test Example 8 using sulfur-modified pitch as the sulfur-based positive electrode active material (FIG. 25) is the lithium ion secondary battery of Test Example 4 using sulfur-modified polyacrylonitrile as the sulfur-based positive electrode active material. Although the capacity itself is lower than that of the battery (FIG. 17), the discharge capacity decrease from 0.1 C to about 2.0 C is small by adding the conductive material. Therefore, it can be seen that even when a sulfur-modified pitch is used as the sulfur-based positive electrode active material, the discharge rate characteristics can be enhanced by using the sulfur-based positive electrode active material and the conductive material together. A lithium ion secondary battery using a sulfur-modified pitch as a positive electrode active material has a smaller capacity and inferior cycle characteristics than a lithium ion secondary battery using a sulfur-modified polyacrylonitrile as a positive electrode active material. Compared to lithium ion secondary batteries using simple sulfur as a substance, the cycle characteristics are much better.
Further, in the lithium ion secondary battery of Test Example 9 (FIG. 27) in which MoS ₂ as a conductive material is blended with a positive electrode material (specifically, a mixed raw material), the discharge capacity at 2C exceeds 500 mAh / g, and 5C In this case, the discharge capacity was as high as about 200 mAh / g. From this result, it is understood that when MoS ₂ is used as the conductive material, the discharge rate characteristics of the lithium ion secondary battery can be improved to the same extent as when La ₂ S ₃ is used as the conductive material.

  From the above results, it can be seen that the positive electrode of the present invention can improve the cycle characteristics and discharge rate characteristics of the lithium ion secondary battery. It can also be seen that the lithium ion secondary battery of the present invention exhibits excellent cycle characteristics and discharge rate characteristics. Furthermore, according to the positive electrode manufacturing method of the present invention, it can be seen that a positive electrode capable of improving the cycle characteristics and discharge rate characteristics of a lithium ion secondary battery can be manufactured.

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

Claims (2)

A sulfur-based positive electrode active material containing carbon (C) and sulfur (S), and a conductive material containing sulfur (S),
At least a portion of said transmission conductive _{material, La 2 S 3, Sm 2} S 3, Ce 2 S 3, lithium ion secondary, wherein Ru least one metal sulfide Tona selected from the group consisting of MoS ₂ primary Battery positive electrode. A lithium ion secondary battery using the positive electrode for a lithium ion secondary battery according to claim 1 .