Composite material, positive electrode, and battery

Abstract

A composite material according to the present disclosure comprises a carbon-sulfur composite and a protective layer formed on a surface of the carbon-sulfur composite. The carbon-sulfur composite contains a porous carbon material and a sulfur material supported by the porous carbon material. The protective layer contains at least one of non-graphitic carbon and a carbon precursor as a main component, and has a film thickness of 0.5 nm or more and a crystallite size Lc of 4.1 nm to 8.0 nm.

Description

Composite materials, cathodes, and batteries

[0001] This disclosure relates to composite materials, cathodes, and batteries.

[0002] With the widespread use of small electronic devices such as laptops and smartphones, and the electrification of automobiles, lithium-ion batteries are becoming increasingly popular as rechargeable batteries with large charge and discharge capacities. Generally, lithium-containing metal oxides are used as the positive electrode active material for lithium-ion batteries.

[0003] In recent years, lithium-sulfur batteries, a type of lithium-ion battery, have attracted attention in response to the need for even higher capacity. In lithium-sulfur batteries, sulfur and / or sulfur compounds (hereinafter also referred to as "sulfur materials") are used as the positive electrode active material. For example, sulfur has an extremely high theoretical capacity of approximately 1600 mAh / g. Therefore, lithium-sulfur batteries, which use sulfur materials such as sulfur as the positive electrode active material, are expected to be batteries with excellent charge and discharge capacity. Furthermore, they are also advantageous in terms of cost compared to conventional lithium-ion batteries that use lithium-containing metal oxides as the positive electrode active material.

[0004] On the other hand, sulfur materials have lower conductivity compared to metal oxides that have been conventionally used as positive electrode active materials. Therefore, when sulfur materials alone are used as positive electrode active materials, the electrochemical reaction between the sulfur material and lithium ions does not proceed, and almost no battery capacity is obtained. In addition, when sulfur materials are used as positive electrode active materials, lithium sulfide (Li) is produced as an intermediate product in the battery's charge-discharge reaction. ２ Lithium sulfide (S, etc.) may be generated. During the charge-discharge reaction process, which involves the generation of lithium sulfide, the positive electrode active material repeatedly expands and contracts. As a result, contact between the generated lithium sulfide and the electrolyte is promoted. When lithium sulfide comes into contact with the electrolyte, it readily dissolves into the electrolyte, causing the loss of sulfur material from the positive electrode. In this way, the battery's cycle characteristics are significantly reduced.

[0005] To solve the above-mentioned problems of lithium-sulfur batteries, a technology has been proposed, for example in International Publication No. 2014 / 141769 (Patent Document 1), in which a material in which sulfur material is supported on a porous carbon material is used as the positive electrode active material. By supporting the sulfur material on a porous carbon material, the sulfur material can be placed in the vicinity of the highly conductive carbon material. This promotes electron transfer and allows the electrochemical reaction between the sulfur material and lithium to proceed sufficiently. Furthermore, when the sulfur material supported in the pores of the porous carbon material reacts with lithium ions, lithium sulfide is also generated in the pores of the porous carbon material. Therefore, the elution of lithium sulfide into the electrolyte is suppressed. As a result, the deterioration of the battery's cycle characteristics can be suppressed.

[0006] International Publication No. 2014 / 141769

[0007] However, even when a material in which a sulfur material is supported on a porous carbon material, as proposed in Patent Document 1, is used as the positive electrode active material for a lithium-sulfur battery, excellent cycle characteristics may not be obtained.

[0008] The purpose of this disclosure is to provide a composite material that provides excellent discharge capacity and cycle characteristics when used as a positive electrode active material for a lithium-ion battery, a positive electrode using the composite material as the positive electrode active material, and a battery using the electrode.

[0009] The composite material according to this disclosure comprises a carbon-sulfur composite and a protective layer. The protective layer is formed on the surface of the carbon-sulfur composite. The carbon-sulfur composite includes a porous carbon material and a sulfur material. The sulfur material is supported on the porous carbon material. The protective layer mainly consists of at least one of non-graphitic carbon and a carbon precursor, has a film thickness of 0.5 nm or more, and a crystallite size Lc of 4.1 to 8.0 nm.

[0010] The cathode according to this disclosure comprises the composite material according to this disclosure.

[0011] The battery according to this disclosure comprises the positive electrode according to this disclosure.

[0012] The composite material according to this disclosure provides excellent discharge capacity and cycle characteristics when used as a positive electrode active material in a lithium-ion battery. The positive electrode according to this disclosure comprises the composite material according to this disclosure. The battery according to this disclosure comprises the electrode according to this disclosure.

[0013] The inventors conducted various studies on composite materials that can be used as positive electrode active materials for lithium-ion batteries to obtain excellent discharge capacity and cycle characteristics. As a result, the following findings were obtained.

[0014] As mentioned above, sulfur materials have high theoretical capacity. Furthermore, materials in which sulfur materials are supported on porous carbon materials (hereinafter also referred to as "carbon-sulfur composites") have sufficient conductivity. Therefore, when carbon-sulfur composites are used as the positive electrode active material of lithium-ion batteries, the electrochemical reaction between the sulfur material and lithium ions can proceed sufficiently. As a result, excellent discharge capacity can be obtained.

[0015] Furthermore, when a carbon-sulfur composite is used as the positive electrode active material for a lithium-ion battery, the elution of lithium sulfide into the electrolyte can be suppressed. The inventors considered that if the elution of lithium sulfide into the electrolyte could be further suppressed when a carbon-sulfur composite is used as the positive electrode active material for a lithium-ion battery, excellent cycle characteristics could be obtained. As a means to achieve this, the inventors focused on forming a protective layer on the surface of the carbon-sulfur composite.

[0016] Forming a protective layer on the surface of the carbon-sulfur composite can suppress contact between the composite and the electrolyte. Therefore, the elution of lithium sulfide into the electrolyte can be further suppressed. However, forming a protective layer on the surface of the carbon-sulfur composite inhibits the proximity of the sulfur material to lithium ions. As a result, the reaction between the sulfur material and lithium ions does not proceed efficiently, leading to a decrease in discharge capacity.

[0017] Here, the inventors focused on non-graphitic carbon and carbon precursors as the main components of the protective layer formed on the surface of the carbon-sulfur composite. In carbon materials such as non-graphitic carbon and carbon precursors, a highly crystalline region (graphite region) with a graphite structure and a region with low crystallinity (amorphous region) where carbon atoms are arranged disorderly with defects such as voids coexist. The graphite region has a higher ability to suppress the elution of lithium sulfide into the electrolyte compared to the amorphous region. On the other hand, the amorphous region has a higher ability to permeate lithium ions compared to the graphite region. In non-graphitic carbon and carbon precursors, the proportion of the amorphous region is higher compared to graphitic carbon such as graphite (graphite) and graphene. Therefore, in non-graphitic carbon and carbon precursors, the amorphous region occupies a sufficient proportion and coexists with the graphite region. Therefore, if a protective layer with a sufficient film thickness mainly composed of at least one of non-graphitic carbon and carbon precursors is formed on the surface of the carbon-sulfur composite, it is possible to suppress the elution of lithium sulfide without inhibiting the approach of the sulfur material and lithium ions.

[0018] In the protective layer mainly composed of at least one of non-graphitic carbon and carbon precursors, the lithium ion permeation ability varies depending on the crystallite size of the protective layer. Specifically, the smaller the crystallite size of the protective layer, the higher the lithium ion permeation ability. Therefore, in order to increase the discharge capacity of the battery, it is preferable that the crystallite size is smaller.

[0019] On the other hand, when the carbon-sulfur composite is used as the positive electrode active material of a lithium ion battery, in the process of charge-discharge reaction accompanied by the generation of lithium sulfide, the carbon-sulfur composite as the positive electrode active material repeats expansion and contraction. When a protective layer is formed on the surface of the carbon-sulfur composite, defects may occur in the protective layer due to the expansion and contraction of the carbon-sulfur composite. In this case, lithium sulfide elutes into the electrolyte from the defects. As a result, the cycle characteristics of the battery deteriorate. That is, the protective layer formed on the surface of the carbon-sulfur composite requires sufficient strength.

[0020] In a protective layer mainly composed of at least one of non-graphitic carbon and a carbon precursor, the larger the crystallite size of the protective layer, the higher the strength. Therefore, when a carbon-sulfur composite in which a protective layer mainly composed of at least one of non-graphitic carbon and a carbon precursor is formed on the surface is used as a positive electrode active material of a lithium-ion battery, in order to obtain excellent cycle characteristics, the crystallite size of the protective layer needs to be somewhat large.

[0021] Based on the above findings, the inventors of the present invention further investigated the relationship between the crystallite size of the protective layer and the discharge capacity and cycle characteristics when the composite material including a carbon-sulfur composite and a protective layer formed on the surface of the carbon-sulfur composite, mainly composed of at least one of non-graphitic carbon and a carbon precursor and having a film thickness of 0.5 nm or more, is used as a positive electrode active material of a lithium-ion battery. As a result, it was found that when the crystallite size Lc of the protective layer is 4.1 to 8.0 nm, excellent cycle characteristics can be obtained while maintaining an excellent discharge capacity.

[0022] The composite material of the present embodiment is completed based on the above technical idea and has the following configuration.

[0023] The composite material of the first configuration includes a carbon-sulfur composite and a protective layer. The protective layer is formed on the surface of the carbon-sulfur composite. The carbon-sulfur composite includes a porous carbon material and a sulfur material. The sulfur material is supported on the porous carbon material. The protective layer is mainly composed of at least one of non-graphitic carbon and a carbon precursor, has a film thickness of 0.5 nm or more, and a crystallite size Lc of 4.1 to 8.0 nm.

[0024] The positive electrode of the first configuration includes the composite material of the first configuration.

[0025] The battery of the first configuration includes the positive electrode of the first configuration.

[0026] Hereinafter, the composite material, positive electrode, and battery of the present embodiment will be described in detail.

[0027] [Composite Material] The composite material of the present embodiment includes a carbon-sulfur composite and a protective layer.

[0028] [Carbon-Sulfur Composites] Carbon-sulfur composites consist of a porous carbon material and sulfur or a sulfur compound.

[0029] [Porous Carbon Materials] Porous carbon materials are well-known carbon materials that have multiple pores. In other words, porous carbon materials are not particularly limited as long as they have a porous structure that can support sulfur materials. For example, porous carbon materials may be one or more selected from the group consisting of graphite, carbon black, carbon nanotubes, carbon fibers, template carbon, and activated carbon.

[0030] The porous carbon material may consist of primary particles having any shape, or secondary particles formed by the aggregation of multiple primary particles. The size of the porous carbon material is also not particularly limited. For example, the size of the porous carbon material is 0.01 to 100 μm in terms of equivalent circle diameter. Here, the equivalent circle diameter refers to the diameter obtained when the area of ​​the porous carbon material observed by a transmission electron microscope (TEM) or the like is converted to a circle.

[0031] [Sulfur Material] The sulfur material is supported on a porous carbon material. In other words, the sulfur material is adsorbed on the surface of the porous carbon material, including both inside and outside the pores. To suppress the elution of lithium sulfide into the electrolyte, it is preferable that the sulfur material is adsorbed on the surface inside the pores of the porous carbon material.

[0032] The sulfur material is a well-known sulfur and / or sulfur compound that can be used as a positive electrode active material. The sulfur material may be one or more selected from the group consisting of, for example, sulfur, metal sulfides, and organic sulfur compounds. Sulfur is the elemental sulfur, for example S ８ The metal sulfide may be, for example, a compound of sulfur with one or more elements selected from the group consisting of Ti, Fe, Mo, Li, and Al. A compound of Ti and S is, for example, TiS ２ It may also be TiS ３ This may also be the case. For example, a compound of Fe and S is Fe ２ It may also be S. A compound of Mo and S is, for example, MoS ３ It may be Mo ３ S ４ This is also acceptable. For example, a compound of Li and S is Li２ S, Li ２ S ８、 Li ２ S ４ and Li ２ S ２ It may be one or more selected from the group consisting of. The compound of Al and S is, for example, Al ２ S ３ It may be. The organic sulfur compound may be, for example, sulfur-modified polyacrylonitrile. In order to increase the energy density calculated from the product of the discharge capacity and the discharge potential, the sulfur material is preferably sulfur.

[0033] The content of the sulfur material in the composite material of the present embodiment is not particularly limited. The higher the content of the sulfur material in the composite material, the higher the discharge capacity when the composite material is used as a positive electrode active material of a lithium ion battery. Therefore, the lower limit of the content of the sulfur material in the composite material is, for example, 65.0% by mass, 67.5% by mass, or 70.0% by mass. On the other hand, the lower the content of the sulfur material in the composite material, the more the decrease in cycle characteristics due to the elution of lithium sulfide into the electrolyte is suppressed. Therefore, the upper limit of the content of the sulfur material in the composite material is, for example, 80.0% by mass, 77.5% by mass, or 75.0% by mass.

[0034] [Method for measuring the content of sulfur material] The content of the sulfur material in the composite material can be measured by the following method using combustion ion chromatography. Put 3 mg of the composite material into a quartz boat and insert it into a combustion furnace. Then, heat and burn the composite material while sharing oxygen gas in the combustion furnace. The heating temperature is, for example, 650 to 1200 °C. By burning the composite material, sulfur in the sulfur material contained in the composite material is converted into sulfur oxides (SO ２ and SO ３ ). The combustion gas containing the generated sulfur oxides is collected in an absorption liquid. In the absorption liquid, sulfur contained in the sulfur oxides is converted into sulfate ions (SO ４ ２－It exists as ). The sulfate ion concentration in the absorption solution is measured by ion chromatography, and the sulfur content (mass%) in the composite material is calculated. From the obtained sulfur content in the composite material, the sulfur content (mass%) in the composite material is calculated based on the type and composition of the sulfur material.

[0035] [Protective layer] The protective layer is formed on the surface of the carbon-sulfur composite. The protective layer may be formed on a part of the surface of the carbon-sulfur composite, or on the entire surface of the carbon-sulfur composite.

[0036] The protective layer mainly consists of at least one of non-graphitic carbon and a carbon precursor. In other words, the protective layer may mainly consist of non-graphitic carbon only, or only a carbon precursor, or both non-graphitic carbon and a carbon precursor.

[0037] Non-graphitic carbon is obtained by calcining thermoplastic organic materials such as thermoplastic resins. Non-graphitic carbon consists of one or more types selected from the group consisting of amorphous carbon and randomly layered carbon. Amorphous carbon refers to carbon that has short-range order (on the order of a few atoms to a dozen or so atoms) and does not have long-range order (on the order of hundreds to thousands of atoms). Randomly layered carbon refers to carbon consisting of carbon atoms that have a random layered structure parallel to the hexagonal network plane direction of graphene and do not show crystallographic regularity in the three-dimensional direction. Randomly layered carbon is preferably confirmed by a transmission electron microscope (TEM) or the like.

[0038] Carbon precursors are substances that exist before thermoplastic organic materials are converted to non-graphitic carbon when heated. Carbon precursors have lower conductivity compared to non-graphitic carbon. Therefore, in order to improve the cycle characteristics when composite materials are used as positive electrode active materials in lithium-ion batteries, it is preferable that the protective layer be mainly composed of non-graphitic carbon.

[0039] The protective layer may contain other components such as graphite, conductive carbonaceous fine particles, or tin particles, to the extent that they do not adversely affect the composite material of this embodiment.

[0040] As described above, non-graphitic carbon and carbon precursors have a higher proportion of amorphous regions compared to graphitic carbon such as graphite and graphene. In other words, in a protective layer mainly composed of at least one of non-graphitic carbon and carbon precursors, amorphous regions occupy a sufficient proportion while coexisting with graphite regions. Compared to amorphous regions, graphite regions have a higher ability to suppress the elution of lithium sulfide into the electrolyte. On the other hand, amorphous regions have a higher ability to permeate lithium ions compared to graphite regions. Therefore, in the composite material of this embodiment, in which a protective layer mainly composed of at least one of non-graphitic carbon and carbon precursors is formed on the surface of the carbon-sulfur composite, assuming that the film thickness and crystallite size of the protective layer satisfy the conditions described later, the elution of lithium sulfide is suppressed without hindering the approach of sulfur material and lithium ions. As a result, excellent discharge capacity and cycle characteristics can be obtained when used as a positive electrode active material for lithium-ion batteries.

[0041] Here, if the G / D ratio of the protective layer obtained by Raman spectroscopy is 1.0 or less, it is determined that the protective layer is mainly composed of at least one of non-graphitic carbon and a carbon precursor.

[0042] The G / D ratio is the intensity ratio of the G-band peak to the D-band peak in the Raman spectrum obtained by Raman spectroscopy. The G-band peak originates from the graphite region in the protective layer, and in this embodiment, it is 1580 cm⁻¹. －１ It is obtained in the vicinity. The peak in the D band is a peak originating from the amorphous region in the protective layer, and in this embodiment, it is 1360 cm. －１It can be obtained in the vicinity. The smaller the G / D ratio, the smaller the G-band peak intensity relative to the D-band peak intensity. In this case, the protective layer has a high content of non-graphitic carbon and carbon precursors, which contain more amorphous regions compared to graphitic carbon. If the G / D ratio of the protective layer is 1.0 or less, the content of non-graphitic carbon and carbon precursors is sufficiently high. In this case, it is judged that the protective layer is mainly composed of at least one of non-graphitic carbon and carbon precursors. There is no particular lower limit to the G / D ratio of the protective layer, but considering normal industrial production, it is, for example, 0.5.

[0043] [Method for Measuring G / D Ratio] The G / D ratio of the protective layer can be measured by the following method. The composite material is filled into a designated sample holder and Raman spectroscopy is performed. The Raman spectrometer used for Raman spectroscopy is, for example, the NRS-7100 manufactured by JEOL Ltd. For Raman spectroscopy, the measurement setup is 180° backscatter configuration, the excitation wavelength is 532 nm, the diffraction grating lines are 600 lines / nm, the ND filter is 25%, the power is 2.3 mW, and the objective lens magnification is 50x. From the Raman spectrum obtained by Raman spectroscopy, the G / D ratio is 1585–1595 cm⁻¹. －１ Determine the intensity of the peak present in the region and use it as the peak intensity of the G band. Also, 1355–1365 cm －１ The intensity of the peak present in the region is determined and defined as the peak intensity of the D band. The ratio of the peak intensity of the G band to the peak intensity of the D band is calculated. The obtained peak intensity ratio is defined as the G / D ratio of the protective layer. Note that the G / D ratio of the protective layer is the value obtained by rounding the second decimal place of the obtained value to one decimal place.

[0044] On the other hand, when the composite material of this embodiment is provided as a positive electrode, the composite material is contained in the positive electrode mixture layer as a positive electrode active material. In addition to the composite material, the positive electrode mixture layer may contain conductive materials such as acetylene black, carbon black, carbon nanotubes, or graphene, as well as thickeners such as carboxymethylcellulose, binders such as styrene-butadiene rubber, or PVDF (polyvinylidene fluoride). Furthermore, decomposition products of the electrolyte of the battery in which the positive electrode is provided may adhere to the surface of the composite material. If the battery is a lithium-ion battery, such decomposition products may include, for example, Li. When measuring the G / D ratio of the protective layer for such a composite material provided as a positive electrode, for example, the following method can be used.

[0045] First, the positive electrode composite layer is physically removed from the positive electrode current collector to obtain a mixture containing the composite material. The mixture is washed with water or a solvent such as N-methylpyrrolidone to remove the binder. The mixture, from which the binder has been removed, is further immersed in water to dissolve and remove the electrolyte decomposition products adhering to the surface of the composite material. In this way, a mixture consisting of the composite material and a conductive additive is obtained.

[0046] A sample is prepared by spreading the obtained mixture smoothly onto two glass plates or the like. The measurement surface is defined as the area on the sample where the mixture is smoothly spread and exposed. The particle sizes of the composite material and conductive additive in the mixture are often smaller than the spatial resolution of micro-Raman spectroscopy, making detection difficult even by this analysis. Therefore, first, Raman spectroscopy mapping is performed in an arbitrary measurement area on the measurement surface to obtain the G / D ratio distribution. Raman spectroscopy mapping is performed by measuring a 50 μm square measurement area in 5 μm steps. For example, conditions such as an exposure time of 20 seconds, two integrations, an excitation wavelength of 532.38 nm, a slit width of 100 μmφ, and an aperture of 4000 μmφ can be set during measurement. Next, elemental mapping is performed in the measurement area using SEM (scanning electron microscope)-EDS (energy-dispersive X-ray spectroscopy). After mapping by Raman spectroscopy and before elemental mapping by SEM-EDS, a coating of Au or Pt may be applied to the sample to enhance conductivity. In elemental mapping by SEM-EDS, the sulfur concentration within the measurement region is mapped at the same magnification as in Raman spectroscopy. The acceleration voltage for SEM-EDS analysis is set to 10 kV. A well-known method can be used to quantify the sulfur concentration; for example, the ZAF correction method can be used. The obtained Raman mapping results and elemental mapping results are superimposed. Any position where a sulfur concentration of 20% by mass or more is detected by elemental mapping is considered to be the position where the composite material exists, and the G / D ratio at that position is determined. The obtained G / D ratio can be considered as the G / D ratio of the protective layer provided on the composite material.

[0047] [Film Thickness] The protective layer must have a film thickness of 0.5 nm or more. If the film thickness of the protective layer is less than 0.5 nm, even if the protective layer mainly consists of at least one of non-graphite carbon and a carbon precursor, and the crystallite size of the protective layer is appropriate, the elution of lithium sulfide into the electrolyte cannot be sufficiently suppressed. As a result, when the composite material is used as the positive electrode active material of a lithium-ion battery, excellent cycle characteristics cannot be obtained.

[0048] On the other hand, if the thickness of the protective layer is 0.5 nm or more, assuming that the crystallite size Lc of the protective layer satisfies the conditions described later, the elution of lithium sulfide can be sufficiently suppressed. As a result, excellent cycle characteristics can be obtained when the composite material is used as the positive electrode active material of a lithium-ion battery. The preferred lower limit for the thickness of the protective layer is 1.0 nm.

[0049] There is no particular upper limit to the thickness of the protective layer. Considering normal industrial production, the upper limit of the protective layer thickness is 50.0 nm. Note that the thinner the protective layer, the higher the lithium ion permeability. As a result, the reactivity between the sulfur material and lithium ions increases. Therefore, the preferred upper limit of the protective layer thickness is 30.0 nm, more preferably 20.0 nm, even more preferably 15.0 nm, even more preferably 10.0 nm, and even more preferably 5.0 nm.

[0050] [Method for Measuring Film Thickness] The film thickness of the protective layer can be measured using a transmission electron microscope (TEM) in the following manner. First, the composite material is sealed in a sample holder, and an electron microscope image is observed using the TEM. For TEM observation, the observation magnification is set to 2,000,000 times, and the acceleration voltage is set to 200 kV. From the obtained observation image, the outer periphery of the composite material is determined based on the contrast. Then, starting from an arbitrary position on the outer periphery of the composite material, electron diffraction measurements are performed at a pitch of 0.1 nm toward the interior of the composite material in a direction perpendicular to the tangent to the outer periphery at that position. In electron diffraction measurements, no clear diffraction rings originating from the crystal structure are observed in the halo pattern caused by the protective layer. On the other hand, clear diffraction rings originating from the crystal structure are observed in the halo pattern caused by the carbon-sulfur composite. Therefore, the halo pattern caused by the protective layer and the halo pattern caused by the carbon-sulfur composite can be easily distinguished by those skilled in the art. The length t (nm) from the point where a halo pattern caused by the protective layer is observed in electron diffraction measurements to the point where a halo pattern caused by the carbon-sulfur composite is observed is measured. Similar electron diffraction measurements are performed starting from a total of five points on the outer circumference determined by TEM observation. The arithmetic mean of the lengths t (nm) obtained from the measurements starting from these five points is considered to be the thickness (nm) of the protective layer.

[0051] On the other hand, when measuring the thickness of the protective layer on the composite material provided in the positive electrode, the following method can be used, for example. First, the positive electrode composite layer is physically peeled off from the current collector of the positive electrode to obtain a mixture containing the composite material. The mixture is washed with water or a solvent such as N-methylpyrrolidone to remove the binder. The mixture from which the binder has been removed is further immersed in water to dissolve and remove the electrolyte decomposition products adhering to the surface of the composite material. In this way, a mixture consisting of the composite material and a conductive additive is obtained.

[0052] TEM observation is performed on the obtained mixture using the method described above. Within the observation field, the composite material and the conductive additive are distinguished using the EDS attached to the TEM. Specifically, particles in which the sulfur content of 20% by mass or more is confirmed by elemental analysis using the EDS are identified as the composite material. A well-known method can be used to quantify the sulfur concentration, for example, the ZAF correction method can be used. The thickness of the protective layer is measured on the identified composite material using the method described above.

[0053] [Crystallite Size Lc] The protective layer further has a crystallite size Lc of 4.1 to 8.0 nm. The crystallite size Lc of the protective film is the size in the hexagonal network stacking direction of crystallites that form a graphite structure in the protective layer, which is mainly composed of at least one of non-graphite carbon and a carbon precursor. The smaller the crystallite size Lc of the protective layer, the higher the lithium ion permeability, but the lower the strength of the protective layer.

[0054] If the crystallite size Lc of the protective layer is less than 4.1 nm, the protective layer will not have sufficient strength. Therefore, when the composite material is used as the positive electrode active material of a lithium-ion battery, defects may occur in the protective layer due to the expansion and contraction of the carbon-sulfur composite during the charge-discharge reaction. In this case, even if the protective layer is mainly composed of at least one of non-graphitic carbon and carbon precursor and has an appropriate film thickness, the elution of lithium sulfide into the electrolyte cannot be sufficiently suppressed. As a result, when the composite material is used as the positive electrode active material of a lithium-ion battery, excellent cycle characteristics cannot be obtained. If the crystallite size Lc of the protective layer exceeds 8.0 nm, even if the protective layer is mainly composed of at least one of non-graphitic carbon and carbon precursor, lithium ions cannot sufficiently permeate the protective layer, and the approach between the sulfur material and lithium ions is inhibited. As a result, when the composite material is used as the positive electrode active material of a lithium-ion battery, excellent discharge capacity cannot be obtained.

[0055] If the crystallite size Lc of the protective layer is 4.1 to 8.0 nm, then, assuming the thickness of the protective layer satisfies the above conditions, the elution of lithium sulfide can be sufficiently suppressed without hindering the approach of the sulfur material to lithium ions. As a result, when the composite material is used as the positive electrode active material of a lithium-ion battery, excellent discharge capacity and cycle characteristics can be obtained. The preferred lower limit of the crystallite size Lc is 4.5 nm. The preferred upper limit of the crystallite size Lc is 7.0 nm, and more preferably 6.0 nm.

[0056] [Method for measuring crystallite size Lc] The crystallite size Lc of the protective layer can be measured by the following method. First, the composite material is dispersed in an organic solvent. The organic solvent is, for example, quinoline. The dispersion is filtered and the filtrate is collected. The organic solvent is thoroughly removed from the filtrate using a well-known method. X-ray diffraction measurement is performed on the solid remaining after removal of the organic solvent. In the X-ray diffraction measurement, the radiation source is CuKα, the acceleration voltage is 40kV, the acceleration current is 40mA, the scan speed is 1° / min, and the sampling width is 0.02°. From the diffraction profile obtained by the X-ray diffraction measurement, the peak of the (002) plane of the graphite structure located at diffraction angles of 23 to 27° is identified. From the identified (002) plane peak, the crystallite size Lc (nm) is determined based on Scherrer's equation as follows: Lc = Kλ / βcosθ In the above equation, K is the shape constant and 0.9 is substituted. λ is the wavelength (nm) of the X-rays used in the measurement, and 1.54 is substituted for it. β is substituted for the full width at half maximum (°) of the peak of the (002) plane, and θ is substituted for the diffraction angle (°) of the peak of the (002) plane. The crystallite size Lc obtained by the above equation is defined as the crystallite size Lc of the protective layer. Note that the crystallite size Lc is the value obtained by rounding the second decimal place of the obtained value to one decimal place.

[0057] On the other hand, when measuring the crystallite size Lc of the protective layer of the composite material provided in the positive electrode, the following method can be used, for example. First, the positive electrode composite layer is physically peeled off from the current collector of the positive electrode to obtain a mixture containing the composite material. The mixture is washed with water or a solvent such as N-methylpyrrolidone to remove the binder. The mixture from which the binder has been removed is further immersed in water to dissolve and remove the electrolyte decomposition products adhering to the surface of the composite material. In this way, a mixture consisting of the composite material and a conductive additive is obtained.

[0058] The protective layer components are extracted from the obtained mixture. Specifically, the mixture is immersed in quinoline. After allowing it to stand for a sufficient amount of time, the elution of the protective layer can be observed in the supernatant. This supernatant is collected by filtration. The collected supernatant is dried to remove the quinoline. The crystallite size Lc of the remaining solid sample is measured by X-ray diffraction in the same manner as described above.

[0059] [Applications of the Composite Material] The composite material of this embodiment can suppress the elution of lithium sulfide without hindering the approach of sulfur material and lithium ions. Therefore, it is suitable as a positive electrode active material for lithium-ion batteries. The composite material of this embodiment can also be applied to applications other than positive electrode active materials for lithium-ion batteries.

[0060] [Method for Manufacturing Composite Materials] An example of a method for manufacturing the composite material of this embodiment will be described. The method for manufacturing the composite material described below is just one example for manufacturing the composite material of this embodiment. Therefore, the composite material having the above-described configuration may be manufactured by other manufacturing methods other than the method described below. However, the method described below is a preferred example of a method for manufacturing the composite material of this embodiment.

[0061] An example of a method for manufacturing the composite material of this embodiment includes the following steps: (Step 1) Composite formation step (Step 2) Protective layer formation step Each step will be described below.

[0062] [(Step 1) Compounding Step] In the compounding step, a sulfur material is supported on a porous carbon material to obtain a carbon-sulfur composite. Specifically, first, the porous carbon material and the sulfur material are mixed. The mixing method is not particularly limited, but dry mixing is preferred. The mixing ratio of the porous carbon material and the sulfur material is also not particularly limited. In the compounding step, for example, the mixing ratio of the sulfur material to the porous carbon material is 50 to 90% by mass.

[0063] The porous carbon material and sulfur material, which have been thoroughly mixed, are heated. The heating temperature is, for example, 150 to 400°C, and the holding time at the heating temperature is, for example, 5 to 30 hours. After the holding time has elapsed, the resulting carbon-sulfur composite is allowed to cool to room temperature. In order to suppress oxidation of the porous carbon material and sulfur material, the process from heating to cooling is carried out under an inert gas atmosphere. The inert gas is, for example, nitrogen.

[0064] [(Step 2) Protective Layer Formation Step] In the protective layer formation step, a protective layer mainly composed of non-graphitic carbon and a carbon precursor is formed on the surface of the carbon-sulfur composite obtained in the composite step. Specifically, a thermoplastic resin is added to and mixed with the carbon-sulfur composite obtained in the composite step. The mixture is then heated under an inert gas atmosphere to obtain composite particles. Examples of thermoplastic resins used in the protective layer formation step include petroleum-based pitch, coal-based pitch, synthetic thermoplastic resins, natural thermoplastic resins, and mixtures thereof. A preferred thermoplastic resin is pitch powder. Pitch powder melts and carbonizes during the heating process. Therefore, the protective layer obtained by heating and mixing pitch powder is easily formed uniformly on the surface of the carbon-sulfur composite.

[0065] The protective layer formation process must satisfy the following conditions: (Condition 1) The mixing ratio of the thermoplastic resin to the carbon-sulfur composite is 5% or more by mass. (Condition 2) The proportion of easily graphitizable components in the thermoplastic resin is 80% or more by mass. (Condition 3) The heating temperature T is 200 to 600°C. Conditions 1 to 3 will be explained below.

[0066] [(Condition 1) Mixing ratio of thermoplastic resin] In the protective layer formation process, the smaller the mixing ratio of thermoplastic resin to carbon-sulfur composite, the thinner the protective layer will be. If the mixing ratio of thermoplastic resin to carbon-sulfur composite is less than 5% by mass, the protective layer thickness in the manufactured composite material may be less than 0.5 nm. Therefore, the mixing ratio of thermoplastic resin to carbon-sulfur composite should be 5% or more by mass. There is no particular upper limit to the mixing ratio of thermoplastic resin to carbon-sulfur composite, but for example, it is 45% by mass.

[0067] [(Condition 2) Proportion of easily graphitizable components] In the protective layer formation process, heating converts the thermoplastic resin into a carbon precursor, and then into non-graphitizable carbon. During this conversion process, the hexagonal network structure of graphene units in the molecular structure of the thermoplastic resin is rearranged three-dimensionally, growing into a highly crystalline graphite structure. Easily graphitizable components in thermoplastic resins are those with large graphene units in their molecular structure that readily grow into a graphite structure when heated at high temperatures of 2000°C or higher. Components other than easily graphitizable components in thermoplastic resins are called difficult-to-graphitize components.

[0068] The easily graphitizable components in thermoplastic resins can be measured, for example, by the following method: A sample is prepared by embedding the thermoplastic resin in epoxy resin or the like. The measurement surface of the sample is mirror-polished. The measurement surface after mirror polishing is observed using a polarizing microscope. The observation magnification is adjusted arbitrarily according to the size of the sample. For example, the observation magnification is 1000x. In this case, the size of the observation field is, for example, 13 μm × 13 μm. The non-graphitizable regions consisting of the non-graphitizable components in the thermoplastic resin exhibit optical isotropy. On the other hand, the easily graphitizable regions consisting of the easily graphitizable components in the thermoplastic resin exhibit optical anisotropy. Therefore, the non-graphitizable regions and easily graphitizable regions in the thermoplastic resin can be easily distinguished in the observation field of a polarizing microscope. Specifically, in the observation field of a polarizing microscope, the brightness of the isotropic non-graphitizable regions hardly changes depending on the direction from which light is shone. The brightness of the anisotropic easily graphitizable regions increases significantly when light is shone from a specific direction. The observation field is photographed under conditions where the non-graphitizing and easily graphitizing regions can be distinguished. The captured images are analyzed using a known method, and the area of ​​the easily graphitizing region is measured. The area of ​​the easily graphitizing region is divided by the total area of ​​the observation field to calculate the area percentage (%) of the easily graphitizing region. The area percentage of the easily graphitizing region is considered to be the percentage of easily graphitizable components (%) in the thermoplastic resin.

[0069] If the proportion of easily graphitizable components in the thermoplastic resin added during the protective layer formation process is less than 80% by mass, the growth of crystallites with a graphite structure in the hexagonal network stacking direction will not proceed sufficiently in the formed protective layer. As a result, the crystallite size Lc of the protective layer in the manufactured composite material may be less than 4.1 nm. Therefore, the proportion of easily graphitizable components in the thermoplastic resin should be 80% or more by mass. There is no particular upper limit to the proportion of easily graphitizable components in the thermoplastic resin, but for example, it can be 100% by mass.

[0070] [(Condition 3) Heating Temperature T] The heating time in the protective layer formation process is defined as T (°C). As described above, in the protective layer formation process, heating converts the thermoplastic resin into a carbon precursor, and further into non-graphite carbon. At this time, the higher the heating temperature T, the easier it is for the graphite structure to grow. If the heating temperature T is less than 200°C, the growth of crystallites taking the graphite structure in the hexagonal network stacking direction does not proceed sufficiently in the formed protective layer. As a result, the crystallite size Lc of the protective layer in the manufactured composite material may be less than 4.1 nm. Furthermore, if the heating temperature T exceeds 600°C, the growth of crystallites taking the graphite structure in the hexagonal network stacking direction is excessively promoted in the formed protective layer. As a result, the proportion of the graphite region becomes excessive. As a result, the crystallite size Lc may exceed 8.0 nm, and furthermore, the G / D ratio of the protective layer may exceed 1.0. In this case, the main component of the protective layer is not non-graphite carbon or carbon precursor, but highly crystalline graphite carbon. Therefore, the heating temperature T should be 200 to 600°C. While the holding time at heating temperature T (°C) is not particularly limited, considering typical industrial production, it is, for example, 0.5 to 2.0 hours.

[0071] The composite material of this embodiment is manufactured by the manufacturing method described above.

[0072] [Positive Electrode] The positive electrode of this embodiment comprises the composite material of this embodiment. In the positive electrode of this embodiment, the composite material of this embodiment is used as the positive electrode active material. Therefore, the positive electrode of this embodiment is suitable for lithium-ion batteries. As long as the positive electrode of this embodiment is provided with the composite material of this embodiment as the positive electrode active material, the other components may be well-known configurations and are not particularly limited. For example, the positive electrode of this embodiment comprises a current collector and a positive electrode mixture layer.

[0073] [Current Collector] In the positive electrode of this embodiment, the current collector is not particularly limited, and a well-known current collector can be used. The current collector may be one or more selected from the group consisting of, for example, aluminum foil and stainless steel foil.

[0074] [Positive Electrode Compound Layer] The positive electrode compound layer is formed on the current collector. The composite material of this embodiment, which is provided in the positive electrode of this embodiment, is included in the positive electrode compound layer as the positive electrode active material. In the positive electrode of this embodiment, the positive electrode compound layer may contain substances other than the composite material of this embodiment. The positive electrode compound layer may contain, for example, a binder and a conductive additive.

[0075] [Method for Manufacturing the Positive Electrode] The method for manufacturing the positive electrode of this embodiment is not particularly limited. The positive electrode of this embodiment is manufactured by a well-known method using the composite material of this embodiment as the positive electrode active material. The method for manufacturing the positive electrode of this embodiment includes, for example, a positive electrode mixture slurry preparation step and a positive electrode mixture layer formation step.

[0076] [Preparation Step for Cathode Compound Slurry] In the preparation step for the cathode compound slurry, a composition for forming the cathode compound layer (cathode compound slurry) is prepared. The cathode compound slurry should be prepared according to the cathode compound layer to be obtained. For example, it may be prepared by kneading the composite material of this embodiment with a binder and a solvent. For example, it may also be prepared by kneading the composite material of this embodiment with a conductive additive, a binder and a solvent. The kneading method should be adjusted as appropriate depending on the conductive additive, binder and solvent. In other words, the preparation step for the cathode compound slurry can be carried out by a well-known method.

[0077] [Positive Electrode Mixture Layer Formation Process] In the positive electrode mixture layer formation process, a positive electrode mixture layer is formed on the current collector. Specifically, the kneaded positive electrode mixture slurry is coated onto the current collector. The coating method is not particularly limited and any well-known method may be used. For example, coating may be performed using an applicator with a gap. For example, coating may also be performed by spraying using a sprayer.

[0078] The positive electrode of this embodiment can be manufactured through the above process.

[0079] [Battery] The battery of this embodiment is equipped with the positive electrode of this embodiment. Therefore, the battery of this embodiment is preferably a lithium-ion battery. As long as the battery of this embodiment is equipped with the positive electrode of this embodiment, the other components may be well-known configurations and are not particularly limited. The battery of this embodiment is equipped with, for example, a negative electrode and an electrolyte. The electrode of this embodiment may further include, for example, a separator. The shape of the battery of this embodiment is not particularly limited and may be cylindrical, rectangular, coin-shaped, or sheet-shaped.

[0080] [Negative Electrode] In the battery of this embodiment, the negative electrode is not particularly limited and may have a well-known configuration. The negative electrode comprises, for example, a current collector and a negative electrode mixture layer. The current collector of the negative electrode may be one or more selected from the group consisting of, for example, copper foil and stainless steel foil. If the battery of this embodiment is a lithium-ion battery, the negative electrode active material contained in the negative electrode mixture layer may be, for example, a carbon-based active material represented by graphite, an alloy active material represented by CuSn alloy, Li ４ Ti ５ O １２ The active material may be one or more selected from the group consisting of oxide-based active materials represented by and Si-based active materials represented by Si and SiO. In the battery of this embodiment, the negative electrode may be, for example, made of lithium metal. When the negative electrode is made of lithium metal, the negative electrode may further include copper foil or stainless steel foil as a current collector. On the other hand, when the negative electrode is made of lithium metal, the negative electrode may not have a current collector and may consist only of lithium metal.

[0081] [Electrolyte] The electrolyte conducts ions between the positive and negative electrodes. In the battery of this embodiment, the electrolyte is not particularly limited, and well-known electrolytes can be used. If the battery of this embodiment is a lithium-ion battery, the electrolyte is, for example, a non-aqueous electrolyte obtained by dissolving a lithium salt in an organic solvent. The lithium salt is, for example, LiClO ４ LiBF ４ LiPF ６ LiAsF ６ LiB(C) ６ H ５ ), LiCF ３ SO ３ LiCH３ SO ３ , Li(CF ３ SO ２ ) ２ N, LiC ４ F ９ SO ３ , Li(CF ２ SO ２ ) ２ , LiCl, LiBr, LiI, Li(FSO ２ ) ２ It is one or more selected from the group consisting of N.

[0082] [Battery Manufacturing Method] The manufacturing method of the battery of this embodiment is not particularly limited. The battery of this embodiment may be manufactured, for example, by a well-known method, by placing a laminate formed by stacking the positive electrode, negative electrode, and electrolyte of this embodiment into a battery case.

[0083] The effects of the composite material, positive electrode, and battery of this embodiment will be further explained by the following examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effectiveness of the composite material, positive electrode, and battery of this embodiment. Therefore, the composite material, positive electrode, and battery of this embodiment are not limited to this one example of conditions.

[0084] Batteries corresponding to each test number shown in Table 1 were manufactured, and their characteristics were evaluated.

[0085]

[0086] [Manufacturing of Composite Material] A porous carbon material and a sulfur material were dry-mixed using a mortar. As the porous carbon material, Ketjenblack manufactured by Lion Specialty Chemicals Co., Ltd., with a primary particle size of 40 nm in equivalent circular diameter, was used. As the sulfur material, sulfur powder manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. was used. Sulfur powder is a sulfur powder with a particle size of approximately 70 μm. The mixing ratio of the sulfur material to the porous carbon material was 80.7% by mass.

[0087] The mixture was heated and held at 155°C under a nitrogen atmosphere for 10 hours. The heated product was then allowed to cool to room temperature. In this manner, a carbon-sulfur complex was produced.

[0088] To a carbon-sulfur composite that had been cooled to room temperature, coal-based pitch powder was added as a thermoplastic resin and dry-mixed using a mortar. The coal-based pitch powder had a softening point of 86°C and an average particle size of 20 μm. The mass percentage of the coal-based pitch powder to the carbon-sulfur composite was as shown in the "Thermoplastic Resin Mixing Ratio (%)" column in Table 1. The mass percentage of the easily graphitizable component in the coal-based pitch powder was as shown in the "Easily Graphitizable Component (%)" column in Table 1.

[0089] The mixture was heated to T (°C) under a nitrogen atmosphere. The heating temperature T (°C) was as shown in the "Heating Temperature T (°C)" column in Table 1. The mixture was then held at heating temperature T (°C) for 1 hour. After the holding time, the heated product was allowed to cool to room temperature. The composite particles for each test number were produced using the above process.

[0090] [Measurement of Sulfur Content] Based on the method described in [Method for Measuring Sulfur Content] above, the sulfur content in the composite material for each test number was measured. The obtained sulfur content in mass percent is shown in the "Content (%)" column of Table 1.

[0091] [Measurement of G / D Ratio] Based on the method described in [Method for Measuring G / D Ratio] above, the G / D ratio of the protective layer in the composite material for each test number was measured. The obtained G / D ratios are shown in the "G / D Ratio" column of Table 1. If the obtained G / D ratio is 1.0 or less, it was determined that the protective layer mainly consists of at least one of non-graphitic carbon and carbon precursors (indicated as "non-graphitic" in the "main component" column of Table 1). If the obtained G / D ratio exceeds 1.0, it was determined that the protective layer mainly consists of graphitic carbon, rather than at least one of non-graphitic carbon and carbon precursors (indicated as "graphitic" in the "main component" column of Table 1).

[0092] [Measurement of Film Thickness] Based on the method described in [Method for Measuring Film Thickness] above, the film thickness of the protective layer in the composite material for each test number was measured. The obtained film thickness (nm) is shown in the "Film Thickness (nm)" column in Table 1.

[0093] [Measurement of Crystallite Size Lc] Based on the method described in [Method for Measuring Crystallite Size Lc] above, the crystallite size Lc of the protective layer in the composite material for each test number was measured. The obtained crystallite size Lc (nm) is shown in the "Crystallite Size Lc (nm)" column in Table 1.

[0094] [Manufacturing of the positive electrode] A mixture was prepared by mixing 75 parts by mass of the manufactured composite material with 5 parts by mass of styrene-butadiene rubber (SBR) (binder), 5 parts by mass of carboxymethylcellulose (CMC) (binder), and 15 parts by mass of acetylene black powder (conductive additive). Distilled water was then added to the mixture and kneaded to prepare a positive electrode slurry.

[0095] The prepared cathode mixture slurry was coated onto 18 μm thick aluminum foil using a doctor blade with a 150 μm gap. The coating amount of cathode mixture slurry on the aluminum foil was 2–4 mg / cm². ２ The coated cathode mixture slurry was dried to form a cathode mixture layer. The aluminum foil with the cathode mixture layer formed on it was punched out using a circular punch with a diameter of 13 mm to produce cathodes for each test number.

[0096] [Battery Manufacturing] Using the manufactured positive electrode, batteries of each test number were manufactured. Specifically, a 17 mm diameter separator was placed on the positive electrode of each manufactured test number. Furthermore, a 19 mm diameter, 1.0 mm thick metallic lithium foil was placed on the separator as the counter electrode (negative electrode). The laminate thus formed was placed inside a case. The outer circumference of the case containing the laminate was press-formed with a dedicated crimping machine to produce a coin-type battery (2016 type). The supporting electrolyte was LiTFSI (bis(trifluoromethanesulfonyl)imide lithium:Li(CF ３ SO ２ ) ２ As N), a mixed solution of LiTFSI:tetraglyme (G4):hydrofluoroether = 10:8:40 (molar ratio) was used as the electrolyte.

[0097] [Evaluation of Battery Discharge Capacity and Cycle Characteristics] The sulfur content in the positive electrode composite layer of each test number battery was calculated from the sulfur content in the composite material of each test number battery. Furthermore, the capacity of the positive electrode of each test number battery was calculated based on the theoretical capacity of sulfur (1672 mAh / g). Using a current value of 1 / 10 of the calculated positive electrode capacity, constant current doping was performed with respect to the counter electrode until a potential difference of 1.0 V was achieved. Then, doping with a constant voltage of 1.0 V was continued with respect to the counter electrode until the above current value was reduced to 1 / 10, and the doping capacity was measured. Next, dedoping was performed with a constant current value of 1 / 10 of the calculated positive electrode capacity with respect to the counter electrode until a potential difference of 3.0 V was achieved, and the dedoping capacity was measured. The doping capacity and dedoping capacity at this time correspond to the discharge capacity and charge capacity when the positive electrode of each test number is used as the positive electrode of a lithium-ion battery. Therefore, the measured doping capacity was taken as the discharge capacity, and the measured dedoping capacity was taken as the charge capacity. The doping and dedoping cycles were repeated 20 times under the same conditions.

[0098] Based on the discharge capacity of the first cycle, the discharge capacity per unit mass (mAh / g) of the composite material alone was calculated, taking into account the volumes of the binder and conductive additive. If the obtained value was 400 mAh / g or higher, it was judged that excellent discharge capacity had been obtained (indicated as "E" in the "Discharge Capacity" column of Table 1). On the other hand, if the obtained value was less than 400 mAh / g, it was judged that excellent discharge capacity had not been obtained (indicated as "B" in the "Discharge Capacity" column of Table 1).

[0099] The ratio of the discharge capacity at cycle 2 to the discharge capacity at cycle 2 was calculated and defined as the capacity retention rate (%). If the obtained capacity retention rate was 65.0% or higher, it was judged that excellent cycle characteristics were obtained (indicated as "E" in the "Cycle Characteristics" column in Table 1). If the obtained capacity retention rate was less than 65.0%, it was judged that excellent cycle characteristics were not obtained (indicated as "B" in the "Cycle Characteristics" column in Table 1).

[0100] [Evaluation Results] Referring to Table 1, in tests 1 to 5, the protective layer provided on the composite material consisted mainly of at least one of non-graphite carbon and a carbon precursor, had a film thickness of 0.5 nm or more, and a crystallite size Lc of 4.1 to 8.0 nm. Therefore, lithium-ion batteries using the composite material as the positive electrode active material exhibited excellent discharge capacity and cycle characteristics.

[0101] On the other hand, in test number 6, the mixing ratio of thermoplastic resin to carbon-sulfur composite in the protective layer formation process during the manufacturing of the composite material was too low. As a result, the thickness of the protective layer on the composite material was too thin. Consequently, lithium-ion batteries using the composite material as the positive electrode active material did not exhibit excellent cycle characteristics.

[0102] In test number 7, the proportion of easily graphitizable components in the thermoplastic resin was too low during the protective layer formation process when manufacturing the composite material. As a result, the crystallite size Lc of the protective layer on the composite material was too small. Consequently, lithium-ion batteries using the composite material as the positive electrode active material did not exhibit excellent cycle characteristics.

[0103] In test number 8, the heating temperature T was too low during the protective layer formation process when manufacturing the composite material. As a result, the crystallite size Lc of the protective layer on the composite material was too small. Consequently, lithium-ion batteries using the composite material as the positive electrode active material did not exhibit excellent cycle characteristics.

[0104] In test number 9, the heating temperature T was too high during the protective layer formation process when manufacturing the composite material. As a result, the G / D ratio of the protective layer on the composite material was too high. In other words, the protective layer on the composite material was not primarily composed of at least one of non-graphite carbon and carbon precursors. Furthermore, the crystallite size Lc of the protective layer was too large. Consequently, lithium-ion batteries using the composite material as the positive electrode active material could not obtain a good discharge capacity.

[0105] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. A composite material comprising a carbon-sulfur composite and a protective layer formed on the surface of the carbon-sulfur composite, wherein the carbon-sulfur composite comprises a porous carbon material and a sulfur material supported on the porous carbon material, and the protective layer mainly consists of at least one of non-graphitic carbon and a carbon precursor, has a film thickness of 0.5 nm or more, and has a crystallite size Lc of 4.1 to 8.0 nm.

2. A positive electrode comprising the composite material described in claim 1.

3. A battery comprising the positive electrode described in claim 2.

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