Positive electrode material and lithium secondary battery using the same

Abstract

The purpose of the present invention is to provide a lithium secondary battery comprising a sulfur-containing positive electrode active material, wherein a means for improving the capacity of the lithium secondary batteries is provided. One embodiment of the present invention is a positive electrode material: comprising: a positive electrode active material containing sulfur; and porous carbon. At least a portion of the positive electrode active material is packed into the pores of the porous carbon, and at least a portion of the outer pore surface of the porous carbon is exposed.

Description

Positive electrode material and lithium secondary battery using the same

[0001] The present invention relates to a positive electrode material and a lithium secondary battery using the same.

[0002] In recent years, research and development into all-solid-state lithium secondary batteries that use oxide-based or sulfide-based solid electrolytes has been actively conducted. Solid electrolytes are materials that are mainly composed of ion conductors that can conduct ions in a solid state. Therefore, in principle, all-solid-state lithium secondary batteries do not encounter the various problems that arise from flammable organic electrolytes, as in conventional liquid-based lithium secondary batteries. In addition, generally, the use of high-potential, large-capacity positive electrode materials and large-capacity negative electrode materials can significantly improve the output density and energy density of the battery. For example, elemental sulfur (S ８ ) has an extremely large theoretical capacity of about 1670 mAh / g, and has the advantages of being low cost and abundant in resources.

[0003] On the other hand, due to the low electronic conductivity of sulfur, the high capacity characteristic of sulfur-containing positive electrode active materials has not yet been fully utilized.

[0004] Japanese Patent Application Laid-Open Publication No. 2016-213006 discloses a method for producing a lithium sulfide composite, which comprises mixing a mixture containing lithium sulfide and sulfur with a conductive additive in an organic solvent, distilling off the organic solvent to obtain a polysulfide composite, and then subjecting the polysulfide composite to vacuum heat treatment at a temperature of 150 to 500° C. for 2 to 10 hours. According to the above document, by using this method, it is possible to mass-produce a lithium sulfide composite consisting of lithium sulfide and a conductive additive while ensuring safety during production.

[0005] However, according to the investigations of the present inventors, it has been found that when the lithium sulfide composite described in the above document is applied to a lithium secondary battery, sufficient capacity may not be obtained.

[0006] Therefore, an object of the present invention is to provide a means for improving the capacity of a lithium secondary battery that uses a positive electrode active material containing sulfur.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that the above-mentioned problems can be solved by providing a cathode material containing a sulfur-containing cathode active material and porous carbon, in which the cathode active material is filled into the pores of the porous carbon while the outer surfaces of the pores of the porous carbon are exposed, thereby completing the present invention.

[0008] That is, one embodiment of the present invention is a positive electrode material comprising a positive electrode active material containing sulfur and porous carbon, wherein at least a portion of the positive electrode active material is filled in the pores of the porous carbon, and at least a portion of the outer surfaces of the pores of the porous carbon are exposed.

[0009] Fig. 1 is a cross-sectional view schematically illustrating an enlargement of the surface of the positive electrode material obtained in Comparative Example 1. Fig. 2 is a cross-sectional view schematically illustrating an enlargement of the surface of the positive electrode material obtained in Example 1. Fig. 3 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (stacked-type secondary battery) according to one embodiment of the present invention.

[0010] A positive electrode material according to one embodiment of the present invention is a positive electrode material comprising a sulfur-containing positive electrode active material and porous carbon, wherein at least a portion of the positive electrode active material is filled into pores of the porous carbon and at least a portion of the outer surfaces of the pores of the porous carbon are exposed. The positive electrode material according to this embodiment can improve the capacity of a lithium secondary battery using a sulfur-containing positive electrode active material.

[0011] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, the technical scope of the present invention should be defined based on the description of the claims and is not limited to the following embodiments. In the description of the drawings, the same elements are given the same reference numerals and redundant explanations will be omitted. Furthermore, the dimensional proportions in the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0012] <Positive Electrode Material> Fig. 1 is a cross-sectional schematic diagram showing an enlarged view of the surface of the positive electrode material obtained in Comparative Example 1. In the positive electrode material 30a shown in Fig. 1, a sulfide solid electrolyte 35 covers the inside of the pores of the porous carbon 31 and a part of the surface outside the pores. In addition, elemental sulfur 33 fills the inside of the pores of the porous carbon 31 and covers the entire surface outside the pores.

[0013] FIG. 2 is a schematic cross-sectional view of an enlarged surface of the cathode material obtained in Example 1. The cathode material 30b shown in FIG. 2 differs from the cathode material 30a shown in FIG. 1 in that a portion of the surface outside the pores of the porous carbon 31 is exposed (i.e., there is a portion that is not covered by the sulfide solid electrolyte 35 and the elemental sulfur 33). This configuration facilitates the formation of an electron conduction path due to contact between adjacent porous carbons and an electron conduction path due to contact between the porous carbon and the conductive additive, thereby improving the utilization efficiency of the elemental sulfur 33. As a result, by applying this cathode material 30b to a lithium secondary battery, it is possible to improve the capacity of the lithium secondary battery.

[0014] The configuration of the positive electrode material according to this embodiment will be described in detail below.

[0015] [Porous Carbon] In this specification, porous carbon refers to a carbon material that has pores and is composed primarily of carbon. Here, "composed primarily of carbon" refers to containing carbon atoms as the main component, and is a concept that encompasses both "consisting solely of carbon atoms" and "consisting essentially of carbon atoms." "Consisting essentially of carbon atoms" means that the inclusion of impurities in an amount of about 2 to 3 mass % or less is acceptable.

[0016] The shape of the porous carbon is not particularly limited, but particulate is preferred. Examples of porous carbon particles include activated carbon, carbon black (e.g., Ketjen Black®), (oil) furnace black, channel black, acetylene black, thermal black, and lamp black; carbon particles (carbon supports) made of coke, natural graphite, artificial graphite, and the like. Alternatively, a ceramic or other mold may be mixed with a carbon raw material (e.g., resin), fired under an inert atmosphere, and then the mold may be dissolved with acid to synthesize a carbon material having a porous structure in which the shape of the mold has been transferred. This carbon material may then be used as the porous carbon particles. In this case, the pore size and pore volume of the resulting carbon material can be changed by appropriately adjusting the particle size of the mold and the compounding ratio of the carbon raw material.

[0017] The BET specific surface area of ​​the porous carbon is 200 m ２ / g or more, and ２ / g or more is more preferable, and 800m ２ / g or more, and more preferably 1200m ２ / g or more is particularly preferred, and 1500m ２ / g or more. The pore volume of the porous carbon having a pore diameter of 1 to 60 nm is preferably 0.3 cm ３ / g, and preferably greater than 0.74 cm ３ / g or more, and more preferably 1.0 cm ３ / g or more, and more preferably 2.0 cm ３ / g or more is particularly preferred, and 3.0 cm ３ The upper limit of the pore volume is not particularly limited, but is, for example, 10 cm ３ / g or less. If the BET specific surface area and pore volume of the porous carbon are within these ranges, a sufficient number of pores can be retained, and therefore a sufficient amount of positive electrode active material can be retained. In this specification, the BET specific surface area of ​​the porous carbon is a value measured by nitrogen adsorption / desorption measurement. The pore volume of porous carbon with pore diameters of 1 to 60 nm can be determined by the BJH method from the value measured by nitrogen adsorption / desorption measurement. This nitrogen adsorption / desorption measurement is performed using a BELSORP mini manufactured by Microtrac-Bell Corporation, at a temperature of -196°C, using a multipoint method. 0.01<P / P ０ The BET specific surface area is determined from the adsorption isotherm in the range of relative pressure <0.05. The pore volume is determined from the adsorption N at a relative pressure of 0.96. ２ Calculate from the volume.

[0018] The average pore diameter of the porous carbon is not particularly limited, but is preferably 1 to 50 nm, and more preferably 1 to 30 nm. If the average pore diameter of the porous carbon is within this range, electrons can be sufficiently supplied to the active material located far from the pore wall among the sulfur-containing positive electrode active material disposed inside the pores. The average pore diameter of the porous carbon can be calculated by nitrogen adsorption / desorption measurement, similar to the case of determining the BET specific surface area and pore volume.

[0019] The average particle diameter (primary particle diameter) of the porous carbon particles is not particularly limited, but is preferably 0.05 to 50 μm, more preferably 0.1 to 20 μm, and even more preferably 0.5 to 10 μm. In this specification, the "particle diameter of the porous carbon particles" refers to the longest distance L between any two points on the contour line of the porous carbon particles. The value of the "average particle diameter of the porous carbon particles" is calculated as the arithmetic mean value of the particle diameters of particles observed in several to several tens of fields of view using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0020] [Sulfur-containing cathode active material] The type of sulfur-containing cathode active material is not particularly limited, but may be elemental sulfur (S ８ ) and lithium sulfide (Li ２ In addition to sulfur compounds (S), particles or thin films of organic sulfur compounds or inorganic sulfur compounds can be used, and any substance can be used as long as it utilizes the oxidation-reduction reaction of sulfur to release lithium ions during charging and absorb lithium ions during discharging. Examples of organic sulfur compounds include disulfide compounds, sulfur-modified polyacrylonitriles, sulfur-modified polyisoprenes, rubeanic acid (dithiooxamide), polycarbon sulfides, and the like, which are typified by the compounds described in International Publication No. 2010 / 044437. On the other hand, inorganic sulfur compounds are preferred because of their excellent stability, and specifically, elemental sulfur (S ８ ), lithium sulfide (Li ２ S), S-carbon composite, TiS ２ , TiS ３ , TiS4, NiS, NiS ２ , CuS, FeS ２ , MoS ２ , MoS ３ , MnS, MnS ２ , CoS, CoS ２ Among them, elemental sulfur (S ８ ), lithium sulfide (Li ２ S), S-carbon composite, TiS ２ , TiS ３ , TiS4, FeS ２ and MoS ２ is preferred, and elemental sulfur (S ８ ), lithium sulfide (Li ２ S), TiS ２ and FeS ２ is more preferable, and from the viewpoint of high capacity, elemental sulfur (S ８ ) and lithium sulfide (Li ２ S) is more preferred, and elemental sulfur (S ８ ) is particularly preferred. ８ ) as S ８ Alpha, beta, or gamma sulfur having the structure may be used.

[0021] As described above, the positive electrode material according to this embodiment is characterized in that at least a portion of the sulfur-containing positive electrode active material is filled into the pores of the porous carbon. The proportion of the sulfur-containing positive electrode active material filled into the pores is preferably 60% by mass or more, more preferably 60% by mass to 85% by mass, and even more preferably 65% ​​by mass to 80% by mass. This configuration improves the utilization efficiency of the sulfur-containing positive electrode active material, thereby improving the capacity of the lithium secondary battery. Whether or not at least a portion of the sulfur-containing positive electrode active material is filled into the pores of the porous carbon and the proportion of sulfur in the pores can be confirmed by thermogravimetry (TG) analysis, as described in the Examples.

[0022] The positive electrode material according to the present embodiment is also characterized in that at least a portion of the outer pore surface of the porous carbon is exposed. This configuration facilitates the formation of an electron conduction path due to contact between adjacent porous carbons and between the porous carbon and the conductive additive, thereby improving the utilization efficiency of the sulfur-containing cathode active material and ultimately improving the capacity of the lithium secondary battery. Whether or not at least a portion of the outer pore surface of the porous carbon is exposed can be confirmed by measuring the BET specific surface area using the method described in the Examples and confirming whether or not a peak in the pore volume value exists in the range where the pore diameter is greater than 3 nm.

[0023] [Solid Electrolyte] The positive electrode material according to the present embodiment preferably further contains a solid electrolyte, at least a portion of which is filled in the pores of the porous carbon. This configuration improves the lithium ion conductivity in the pores of the porous carbon, thereby further improving the capacity of the lithium secondary battery.

[0024] The type of solid electrolyte is not particularly limited, but is preferably a sulfide solid electrolyte containing an S element, as it exhibits excellent lithium ion conductivity, more preferably a sulfide solid electrolyte containing a Li element, an M element, and an S element, wherein the M element contains at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl, and I, and even more preferably a sulfide solid electrolyte containing an S element, a Li element, and a P element. One example is LPS (Li ２ S-P ２ S ５ ), Li ６ P.S. ５ X (wherein X is Cl, Br or I), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S and Li ３ P.S. ４ These sulfide solid electrolytes are preferably used because they have excellent lithium ion conductivity.

[0025] In a positive electrode material containing a solid electrolyte (preferably a sulfide solid electrolyte), whether or not at least a portion of the solid electrolyte is filled in the pores of the porous carbon can be confirmed using various conventionally known methods. For example, an image of a cross section of the positive electrode material observed with a transmission electron microscope (TEM) is subjected to elemental mapping of each material using energy dispersive X-ray spectroscopy (EDX), and the arrangement of each material can be confirmed using the obtained elemental map and the count number of each element relative to the count number of all elements as an index.

[0026] <Method for producing a positive electrode material> The positive electrode material can be produced by a production method including, but not limited to, a step of impregnating porous carbon with a liquid containing a sulfur-containing positive electrode active material to obtain a composite material (hereinafter also referred to as step (1)), and a step of vacuum heat-treating the composite material to expose at least a portion of the outer surfaces of the pores of the porous carbon (hereinafter also referred to as step (2)). According to this production method, the positive electrode active material covering the outer surfaces of the pores can be removed while leaving the positive electrode active material filling the pores of the porous carbon. Each step of the production method will be described below.

[0027] In step (1), a composite is obtained by impregnating porous carbon with a liquid containing a sulfur-containing cathode active material. Examples of the liquid containing the sulfur-containing cathode active material include a molten liquid obtained by heating and melting the cathode active material itself, and a cathode active material solution obtained by dissolving the cathode active material in a solvent. When the sulfur-containing cathode active material is elemental sulfur, the liquid can be a molten liquid obtained by heating and melting elemental sulfur.

[0028] In step (1), when a composite material is obtained by impregnating porous carbon with a molten liquid obtained by heating and melting a positive electrode active material (e.g., elemental sulfur), it is preferable to premix the sulfur-containing positive electrode active material and porous carbon by a mixing process using a mixing means such as a mortar or a milling process using a pulverizing means such as a planetary ball mill, and then melt the sulfur-containing positive electrode active material (e.g., elemental sulfur) by a heat treatment. This heat treatment results in a composite material in which the sulfur-containing positive electrode active material is filled even inside the pores of the porous carbon.

[0029] In step (2), the composite material obtained in step (1) is subjected to a vacuum heat treatment to expose at least a portion of the outer pore surfaces of the porous carbon. The degree of vacuum and temperature during the vacuum heat treatment can be appropriately set depending on the type of positive electrode active material. When the sulfur-containing positive electrode active material is elemental sulfur, the treatment can be performed at a vacuum degree of 10 Pa or less, at a temperature of 180 to 240°C (preferably 190°C or higher but lower than 210°C), for 1 to 3 hours (preferably 1.5 hours to 2.5 hours), thereby selectively removing the positive electrode active material covering the outside of the pores of the porous carbon. As a result, it is possible to expose the outer pore surfaces while maintaining the positive electrode active material inside the pores.

[0030] In the case where the positive electrode material further contains a solid electrolyte, and at least a part of the solid electrolyte is filled in the pores of the porous carbon, the positive electrode material can be produced by adding a step of impregnating the composite material with a solution containing the solid electrolyte before the step (1) or after the step (1) and before the step (2).

[0031] As the solvent, from the viewpoints of solubility, operability, safety, etc., lower alcohols are suitable, and alcohols having 1 to 4 carbon atoms are preferred. Examples of alcohols having 1 to 4 carbon atoms include methanol, ethanol, 1-propanol, 2-propanol, n-butanol, 2-butanol, and tert-butanol. Among these, from the viewpoints of solubility, operability, safety, etc., methanol, ethanol, 1-propanol, and 2-propanol are preferred, methanol and ethanol are more preferred, and ethanol is particularly preferred.

[0032] The solvent preferably has a low water content, specifically, preferably less than 0.2% by mass. More preferably, the water content in the solvent is 0.1% by mass or less, even more preferably 0.05% by mass or less, even more preferably 0.02% by mass or less, even more preferably 0.01% by mass or less, even more preferably 0.005% by mass or less, and particularly preferably 0.002% by mass or less. The water content in the solvent can be measured, for example, by Karl Fischer coulometric titration.

[0033] The composite is impregnated with a solution containing a solid electrolyte, and then the solvent is removed while stirring the resulting dispersion. The solvent removal is preferably carried out under reduced pressure at a temperature of 70° C. or less. This prevents decomposition of the solid electrolyte and an increase in internal resistance.

[0034] <Lithium secondary battery> By applying the above-described positive electrode material to a lithium secondary battery, it is possible to improve the capacity of the lithium secondary battery. Hereinafter, a lithium secondary battery to which the positive electrode material of this embodiment is applied will be briefly described.

[0035] FIG. 3 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. FIG. 3 shows a cross section of the stacked-type secondary battery during charging. The stacked-type secondary battery 10a shown in FIG. 3 has a structure in which a substantially rectangular power-generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior. Here, the power-generating element 21 has a structure in which a negative electrode, a solid electrolyte layer 17, and a positive electrode are stacked. The negative electrode has a structure in which a negative electrode current collector 11' and a negative electrode active material layer 13 made of lithium metal deposited on the surface of the negative electrode current collector 11' are stacked. The positive electrode has a structure in which a positive electrode active material layer 15 is disposed on the surface of a positive electrode current collector 11". As a result, the negative electrode current collector 11', the negative electrode active material layer 13, the solid electrolyte layer 17, the positive electrode active material layer 15, and the positive electrode current collector 11" constitute one unit cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 3 can be said to have a structure in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel. A negative electrode current collector 25 and a positive electrode current collector 27 that are electrically connected to the respective electrodes (negative and positive electrodes) are attached to the negative electrode current collector 11' and the positive electrode current collector 11", respectively, and are structured so as to be sandwiched between the ends of the laminate film 29 and extended to the outside of the laminate film 29. A restraining pressure is applied to the stacked secondary battery 10a in the stacking direction of the power generating element 21 by a pressure member (not shown). Therefore, the volume of the power generating element 21 is kept constant.

[0036] The main components of the lithium secondary battery according to this embodiment will be described below.

[0037] [Current Collector] The current collector (negative electrode current collector, positive electrode current collector) has the function of mediating the movement of electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There are no particular restrictions on the material that constitutes the current collector. Examples of materials that can be used for the current collector include metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins. There are also no particular restrictions on the thickness of the current collector, but an example is 10 to 100 μm.

[0038] [Negative Electrode Active Material Layer] The negative electrode active material layer contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but includes carbon materials, metal oxides, and metal active materials. Furthermore, a lithium-containing metal may be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is a lithium-containing active material, and examples thereof include lithium metal and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li with at least one of In, Al, Si, and Sn. In some cases, two or more negative electrode active materials may be used in combination. Of course, negative electrode active materials other than those described above may also be used. The negative electrode active material preferably includes lithium metal or a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and more preferably includes lithium metal or a lithium-containing alloy. When lithium metal or a lithium-containing alloy is used as the negative electrode active material, the lithium secondary battery as the electrical device may be a so-called lithium deposition type in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during charging. Therefore, in this configuration, the thickness of the negative electrode active material layer increases as the charging process progresses and decreases as the discharging process progresses. The negative electrode active material layer does not need to be present during full discharge, but in some cases, a negative electrode active material layer made of a certain amount of lithium metal may be present during full discharge.

[0039] The negative electrode active material layer may further contain at least one of a solid electrolyte, a conductive additive, and a binder in addition to the above-mentioned negative electrode active material.

[0040] The thickness of the negative electrode active material layer varies depending on the intended configuration of the lithium secondary battery, but is preferably within the range of 0.1 to 1000 μm, for example.

[0041] [Solid Electrolyte Layer] The solid electrolyte layer is interposed between the negative electrode and the positive electrode and contains a solid electrolyte (usually as a main component). The solid electrolyte contained in the solid electrolyte layer is not particularly limited, and any solid electrolyte known in the art can be appropriately adopted. Examples of solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes. This solid electrolyte is preferably a sulfide solid electrolyte because it exhibits excellent lithium ion conductivity. As the sulfide solid electrolyte in the solid electrolyte layer, the same as that described above for the positive electrode material can be applied, and therefore, description here is omitted. The solid electrolyte layer may further contain a binder in addition to the solid electrolyte.

[0042] The thickness of the solid electrolyte layer varies depending on the configuration of the intended lithium secondary battery, but from the viewpoint of improving the volumetric energy density of the battery, it is preferably 600 μm or less, more preferably 500 μm or less, and even more preferably 400 μm or less. On the other hand, there is no particular restriction on the lower limit of the thickness of the solid electrolyte layer, but it is preferably 1 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more.

[0043] [Positive Electrode Active Material Layer] The positive electrode active material layer contains the above-mentioned positive electrode material. The content of the positive electrode material relative to the total mass of the positive electrode active material layer is preferably 40 mass% or more, more preferably 40 mass% to 99 mass% or less, even more preferably 50 mass% to 90 mass% or less, and particularly preferably 60 mass% to 80 mass% or less. If the content of the positive electrode material is within the above range, the capacity of the lithium secondary battery can be further improved.

[0044] The positive electrode active material layer may further contain a solid electrolyte, a conductive additive (one that does not hold the positive electrode active material or solid electrolyte inside the pores), and / or a binder.

[0045] The thickness of the positive electrode active material layer varies depending on the configuration of the intended lithium secondary battery, but is preferably within the range of 0.1 to 1000 μm, for example. The basis weight of the positive electrode active material layer is preferably 20 mg / cm from the viewpoint of suppressing an increase in internal resistance and improving the utilization efficiency of the positive electrode active material. ２ Preferably, it is 15 mg / cm or less. ２ More preferably, it is 10 mg / cm or less. ２ The lower limit of the basis weight is not particularly limited, but it is preferably 3 mg / cm ２ It is preferable that this is equal to or greater than this.

[0046] The above describes an embodiment of the present invention, but the present invention is not limited to the configurations described in the above embodiment, and can be modified as appropriate based on the claims.

[0047] The following embodiments are also included within the scope of the present invention: a cathode material according to claim 1 having the characteristics of claim 2; a cathode material according to claim 1 or 2 having the characteristics of claim 3; a cathode material according to any one of claims 1 to 3 having the characteristics of claim 4; a lithium secondary battery using a cathode material according to any one of claims 1 to 4 having the characteristics of claim 5; a method for producing a cathode material according to any one of claims 1 to 4 having the characteristics of claim 6; a method for producing a cathode material according to claim 6 having the characteristics of claim 7; and a cathode material obtained by the method for producing a cathode material according to claim 6 or 7 having the characteristics of claim 8.

[0048] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. In the following, the operations were carried out in an argon atmosphere with a dew point controlled to -60°C or less.

[0049] <Examples of Manufacturing Evaluation Cells> [Example 1] (Manufacturing of Positive Electrode Material) An argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ 600 mg of SiO2 (Cl) was dissolved in 15 mL of ultra-dehydrated ethanol. The resulting ethanol-solid electrolyte solution contained a pore size distribution peak of 14 nm and a BET specific surface area of ​​1600 m. ２ / g, and the pore volume of pores with diameters of 1 to 60 nm measured by the BJH method is 3.23 cm ３ 400 mg of porous carbon with a ethanol-solid electrolyte solution of 1000 mg / g was placed in the flask. After confirming that the surface of the porous carbon was completely covered with the ethanol-solid electrolyte solution, the flask was stirred at 300 rpm using a magnetic stirrer and heated to 50°C on a hot plate, while the flask was evacuated to a vacuum of 2000 Pa using a diaphragm vacuum pump for 8 hours to remove the ethanol. After confirming that no ethanol remained, the solid electrolyte-porous carbon composite was recovered. 167 mg of the solid electrolyte-porous carbon composite and sulfur (S ８ ) powder and 333 mg of the sulfur-containing powder were mixed in an agate mortar for 5 minutes and then placed in a sealable autoclave. The autoclave containing the powder was placed in an electric furnace at 185°C for 9 hours, and a heat treatment was performed to impregnate the pores of the solid electrolyte-porous carbon with sulfur. The sulfur-solid electrolyte-porous carbon composite was recovered, and 2 g of the composite was weighed and placed in a sealed stainless steel (SUS) container. A mantle heater was placed around the SUS container, and a vacuum line using an oil rotary vacuum pump was connected. A vacuum heat treatment was performed at 200°C for 2 hours at a vacuum of 10 Pa or less, yielding a cathode material. The temperature of the vacuum heat treatment was observed by inserting a thermocouple between the mantle heater and the SUS container.

[0050] (Production of Evaluation Cell) The above positive electrode material and a sulfide solid electrolyte (Li ６ P.S. ５ The solid electrolyte (LiCl) and Ketjen black were weighed to a mass ratio of 74:25:1 and mixed in an agate mortar for 5 minutes to obtain a positive electrode mixture. An insulating container with a φ10 mm hole and two SUS pins that could be inserted into the φ10 mm hole as current collectors were prepared. ６ P.S. ５ Cl) at 101.9 mg / cm ２ The positive electrode mixture was weighed to have a basis weight of 6.8 mg / cm, placed between the two SUS pins, and pressed at 75 MPa for 1 minute to form a solid electrolyte layer. One of the SUS pins was temporarily removed, and the positive electrode mixture was placed in a weight ratio of 6.8 mg / cm. ２The laminate was weighed to have a basis weight of 10 ...

[0051] [Example 2] In the above (production of a positive electrode material), a porous carbon having a pore volume of 0.74 cm3 and a pore diameter of 1 to 60 nm measured by the BJH method was used. ３ The evaluation cell of this example was obtained in the same manner as in Example 1, except that porous carbon of 0.1g / g was used.

[0052] Example 3 An evaluation cell for this example was obtained in the same manner as in Example 1, except that in the above (production of a positive electrode material), the temperature during the vacuum heat treatment of the sulfur-solid electrolyte-porous carbon composite was set to 210°C.

[0053] [Example 4] In the above (production of evaluation cell), the basis weight of the positive electrode mixture was 8.5 mg / cm ２ The evaluation cell of this example was obtained in the same manner as in Example 1, except that:

[0054] [Example 5] An evaluation cell for this example was obtained in the same manner as in Example 1, except that in the above (production of a positive electrode material), the porous carbon was not impregnated with a solid electrolyte, and the pores of the porous carbon were subjected to an impregnation heat treatment with sulfur.

[0055] Comparative Example 1 An evaluation cell for this comparative example was obtained in the same manner as in Example 1, except that the vacuum heat treatment of the sulfur-solid electrolyte-porous carbon composite was not performed in the above (production of the positive electrode material).

[0056] Comparative Example 2 In the above (production of a positive electrode material), a porous carbon having a pore volume of 0.3 cm3 and a pore diameter of 1 to 60 nm measured by the BJH method was used. ３ The evaluation cell of this comparative example was obtained in the same manner as in Comparative Example 1, except that porous carbon of 0.1g / g was used.

[0057] Comparative Example 3 An evaluation cell for this comparative example was obtained in the same manner as in Example 1, except that in the above (production of a positive electrode material), the temperature during the vacuum heat treatment of the sulfur-solid electrolyte-porous carbon composite was set to 150°C.

[0058] <Confirmation of whether the outer pore surfaces of porous carbon are exposed> The BET specific surface area of ​​the positive electrode material produced above was measured to confirm whether the outer pore surfaces of the porous carbon were exposed. 10 g of the positive electrode material as a sample was placed in a pressure-resistant glass container. Drying before measuring the BET specific surface area was not performed because there was a risk of sulfur drying. While cooling with liquid nitrogen, the BET specific surface area of ​​the sample was measured using a BET specific surface area measuring device (Belsorp-mini II, manufactured by Japan BEL Corporation). N was used as the adsorption gas. ２ was used.

[0059] From the obtained data, a graph was created with the pore diameter on the horizontal axis and the pore volume on the vertical axis. When a peak in the pore volume value exists in the range of pore diameters larger than 3 nm, the surface of the porous carbon is considered to be exposed. When the surface of the porous carbon is not exposed, sulfur covers the surface, resulting in almost no pore volume, and therefore no peak in the pore volume value appears.

[0060] <Proportion of sulfur in pores> Thermogravimetric (TG) analysis was performed on the cathode material produced above to confirm the amount of sulfur present in the pores of the porous carbon. A TG thermal analyzer (TG-DTA8122 Smart Loader, manufactured by Rigaku) ​​was placed in a glove box under an argon atmosphere with a dew point maintained at -60 ° C or below. 10 mg of the cathode material as a sample was placed in an aluminum TG thermal analyzer pan and subjected to TG thermal analysis. The program used was the stepwise isothermal TG method (SIA method) of the TG thermal analyzer. The SIA method is a measurement method in which isothermal control is performed when the rate of weight change (dTG) is greater than a threshold value, and temperature-rising control is performed when the rate of weight change (dTG) is lower than the threshold value. Since the temperature rise stops when the dTG concentration is higher than the threshold, a graph with time on the horizontal axis and weight loss on the vertical axis shows a linear weight loss at temperatures above the threshold. The measurement conditions were a temperature rise of 10°C / min in the range of 30°C to 300°C. The dTG threshold was set at 0.2% wt / min.

[0061] In preliminary experiments, it was found that under the above conditions, the linear weight loss of sulfur alone occurs near 200 ° C. Therefore, we focused on the initial linear weight loss at temperatures higher than 200 ° C., and the end point of that linear weight loss (i.e., the temperature at which the dTG threshold is exceeded and the temperature rise resumes) was used as the dividing point between sulfur inside the pores and sulfur outside the pores. Since sulfur outside the pores evaporates before sulfur inside the pores, the weight loss from room temperature (25 ° C.) to the dividing point was determined as sulfur outside the pores, and the weight of sulfur outside the pores was determined by subtracting the weight of sulfur outside the pores from the charged amount. The ratio (percentage) of sulfur inside the pores to the charged amount was then calculated and used as the ratio of sulfur present in the pores (intrapore sulfur ratio). The obtained values ​​are shown in Table 1.

[0062] <Charge-Discharge Test> The evaluation cell manufactured above (applied with a confining pressure of 100 MPa) was subjected to a charge-discharge test at a temperature of 25°C at 1.1 to 3.1 V vs. Li / Li. ＋A charge-discharge test was conducted at 100°C. CC discharge was first performed at 0.03 C, followed by CC-CV charge (0.01 C cut) and CC discharge at 0.03 C. Thereafter, a cycle of CC-CV charge (0.01 C cut) and CC discharge at 0.05 C was repeated four times, and the final discharge capacity (discharge capacity at the fourth cycle) was measured. The obtained discharge capacity value was divided by the mass of sulfur contained in the positive electrode active material layer to determine the discharge capacity per unit mass of sulfur. The results are shown in Table 1 below. The discharge capacity values ​​shown in Table 1 are relative values ​​when the discharge capacity per unit mass of sulfur in Example 1 is set to 100.

[0063]

[0064] As shown in Table 1, it can be seen that the present invention can improve the capacity of lithium secondary batteries using a cathode active material containing sulfur. On the other hand, in Comparative Examples 1 and 2 in which the vacuum heat treatment was not performed, and Comparative Example 3 in which the temperature of the vacuum heat treatment was low, the outer surfaces of the pores of the porous carbon were not exposed, and a decrease in capacity was observed.

[0065] REFERENCE SIGNS LIST 10a laminated secondary battery, 11' negative electrode current collector, 11" positive electrode current collector, 13 negative electrode active material layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 19 single cell layer, 21 power generating element, 25 negative electrode current collector, 27 positive electrode current collector, 29 laminate film, 30a, 30b positive electrode material, 31 porous carbon, 33 elemental sulfur, 35 sulfide solid electrolyte.

Claims

1. A positive electrode material comprising a positive electrode active material containing sulfur and porous carbon, wherein at least a part of the positive electrode active material is filled in pores of the porous carbon, and at least a part of an outer surface of the pores of the porous carbon is exposed.

2. The positive electrode material according to claim 1, wherein a pore volume of pores having a pore diameter of 1 to 60 nm in the porous carbon is larger than 0.3 cm ３ / g.

3. The positive electrode material according to claim 1, wherein 60% by mass or more of the positive electrode active material is filled in the pores.

4. The positive electrode material according to claim 1, further comprising a solid electrolyte, wherein at least a part of the solid electrolyte is filled in pores of the porous carbon.

5. A lithium secondary battery comprising a positive electrode having a positive electrode active material layer containing the positive electrode material according to claim 1, a negative electrode, and a solid electrolyte layer containing a solid electrolyte interposed between the positive electrode and the negative electrode, wherein a content of the positive electrode material with respect to a total mass of the positive electrode active material layer is 40% by mass or more.

6. A method for manufacturing a positive electrode material, comprising: impregnating a porous carbon with a liquid containing a positive electrode active material containing sulfur to obtain a composite material; and vacuum heat-treating the composite material to expose at least a part of an outer surface of the pores of the porous carbon.

7. The method for manufacturing a positive electrode material according to claim 6, wherein the liquid is a molten liquid obtained by heating and melting elemental sulfur.

8. A positive electrode material obtained by the manufacturing method according to claim 6.

Images