Positive electrode for lithium secondary battery, and lithium secondary battery using same

Abstract

The purpose of the present invention is to provide, in a lithium secondary battery using a sulfur-containing positive electrode active material, a means whereby the resistance of said lithium secondary battery can be lowered. The present invention provides a positive electrode for a lithium secondary battery, said positive electrode containing a positive electrode active material layer including: a positive electrode material including a sulfur-containing positive electrode active material and porous carbon, with at least a portion of the positive electrode active material filling pores in the porous carbon; a conductivity aid; and a solid electrolyte. The conductivity aid includes a particulate conductivity aid having an average primary particle diameter that is smaller than the average pore diameter of the porous carbon or a fibrous conductivity aid having an average fiber diameter that is smaller than the average pore diameter of the porous carbon, and the total content of the porous carbon and the conductivity aid to the total mass of the positive electrode active material layer is greater than 7.7% by mass and less than 12% by mass.

Description

Positive electrode for lithium secondary battery and lithium secondary battery using the same

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

[0002] In recent years, research and development into lithium secondary batteries using 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. In general, 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] As a means for improving the electronic conductivity of the positive electrode active material layer of a lithium secondary battery, Japanese Patent Laid-Open Publication No. 2016-9679 discloses that fibrous carbon and spherical carbon are contained in the positive electrode active material layer as a conductive additive, which is said to suppress aggregation of the fibrous carbon, form a conductive network between the positive electrode active materials, and reduce the resistance of the positive electrode active material layer.

[0005] However, according to the investigations of the present inventors, when the technology described in the above document is applied to a lithium secondary battery using a positive electrode active material containing sulfur, it has been found that the resistance of the lithium secondary battery may not be sufficiently reduced in some cases.

[0006] Therefore, an object of the present invention is to provide a means for reducing the resistance 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 incorporating into a positive electrode active material layer a positive electrode material in which the pores of porous carbon are filled with a positive electrode active material containing sulfur, and a conductive additive having an average primary particle diameter or average fiber diameter smaller than the average pore diameter of the porous carbon, and by controlling the total content of the porous carbon and the conductive additive to a predetermined value, thereby completing the present invention.

[0008] That is, one embodiment of the present invention is a positive electrode for a lithium secondary battery, which contains a positive electrode active material layer including: a positive electrode material including a sulfur-containing positive electrode active material and porous carbon, wherein at least a portion of the positive electrode active material is filled into the pores of the porous carbon; a conductive additive; and a solid electrolyte, wherein the conductive additive includes a particulate conductive additive having an average primary particle diameter smaller than the average pore diameter of the porous carbon or a fibrous conductive additive having an average fiber diameter smaller than the average pore diameter of the porous carbon, and the total content of the porous carbon and the conductive additive with respect to the total mass of the positive electrode active material layer is more than 7.7 mass% and less than 12 mass%.

[0009] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) lithium secondary battery (stacked-type secondary battery) according to one embodiment of the present invention.

[0010] According to one embodiment of the present invention, there is provided a positive electrode for a lithium secondary battery, the positive electrode comprising a cathode active material layer including a cathode material comprising a sulfur-containing cathode active material and porous carbon, wherein at least a portion of the cathode active material is filled into the pores of the porous carbon, a conductive additive, and a solid electrolyte, wherein the conductive additive comprises a particulate conductive additive having an average primary particle diameter smaller than the average pore diameter of the porous carbon or a fibrous conductive additive having an average fiber diameter smaller than the average pore diameter of the porous carbon, and the total content of the porous carbon and the conductive additive relative to the total mass of the cathode active material layer is more than 7.7 mass% and less than 12 mass%. The positive electrode for a lithium secondary battery according to this embodiment can reduce the resistance of a lithium secondary battery using a sulfur-containing cathode 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] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) lithium secondary battery (hereinafter also simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. FIG. 1 shows a cross section of the stacked-type secondary battery during charging. The stacked-type secondary battery 10a shown in FIG. 1 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 are disposed on the surface of the negative electrode current collector 11'. 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. 1 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.

[0013] The main components of the positive electrode according to this embodiment are described below. The positive electrode according to this embodiment has a positive electrode active material layer containing a predetermined positive electrode material, a predetermined conductive additive, and a solid electrolyte. The positive electrode active material layer is formed on the surface of an optional current collector.

[0014] [Positive electrode current collector] The positive electrode current collector is an optional component and has the function of mediating the transfer of electrons from the positive electrode active material layer. There are no particular limitations on the material constituting the positive electrode current collector. Examples of materials that can be used for the positive electrode current collector include metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins. There are also no particular limitations on the thickness of the positive electrode current collector, but an example is 10 to 100 μm.

[0015] [Positive Electrode Active Material Layer] The positive electrode active material layer includes a positive electrode material including a sulfur-containing positive electrode active material and porous carbon, wherein at least a portion of the positive electrode active material is filled into the pores of the porous carbon, a conductive additive, and a solid electrolyte, wherein the conductive additive includes a particulate conductive additive having an average primary particle diameter smaller than the average pore diameter of the porous carbon or a fibrous conductive additive having an average fiber diameter smaller than the average pore diameter of the porous carbon, and the total content of the porous carbon and the conductive additive with respect to the total mass of the positive electrode active material layer is more than 7.7 mass% and less than 12 mass%.

[0016] In lithium secondary batteries using a sulfur-containing cathode active material, a composite material in which the cathode active material is filled into the pores of porous carbon can be used as the cathode material. This improves the utilization efficiency of the sulfur-containing cathode active material, thereby improving the capacity of the lithium secondary battery. However, the inventors' studies have found that a cathode active material layer containing such a cathode material, a solid electrolyte, and a conductive additive can sometimes result in high battery resistance. While the cause is not entirely clear, it is thought that the conductive additive blocks the pores of the porous carbon, resulting in insufficient contact between the cathode active material disposed in the pores and the solid electrolyte. This is thought to result in insufficient conduction of lithium ions to the cathode active material disposed in the pores, making it difficult for the reaction to proceed.

[0017] In contrast, in the positive electrode of this embodiment, the relationship between the average pore size of the porous carbon and the average primary particle size or average fiber size of the conductive additive is controlled so that the average pore size of the porous carbon is larger than the average primary particle size or average fiber size of the conductive additive. This prevents the pores of the porous carbon from being blocked by the conductive additive. The solid electrolyte then contacts the positive electrode active material present in the pores of the porous carbon, enabling lithium ions to be transported to the positive electrode active material. Furthermore, the conductive additive forms an electron conduction path between the porous carbon and the conductive additive. Furthermore, by controlling the total content of the porous carbon and the conductive additive in the positive electrode active material layer to a predetermined value, excellent electronic conductivity and ionic conductivity can be achieved in a well-balanced manner. These effects are believed to promote more efficient battery reactions and reduce battery resistance.

[0018] <Positive Electrode Material> The positive electrode material is formed by filling the pores of porous carbon with at least a part of a positive electrode active material containing sulfur.

[0019] [Porous Carbon] In this specification, porous carbon refers to a carbon material that has pores and is primarily composed of carbon. Here, "primarily composed of carbon" refers to containing carbon atoms as the primary 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 of approximately 2 to 3 mass % or less can be tolerated.

[0020] The shape of the porous carbon is not particularly limited, but particulate form 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, as well as carbon particles (carbon supports) made of coke, natural graphite, and artificial graphite. Alternatively, a carbon material having a porous structure in which the shape of the template is transferred may be synthesized by mixing a ceramic or other mold with a carbon raw material and firing the mixture in an inert atmosphere. The mold may then be dissolved with acid, and this may be used as the porous carbon particles. In this case, the pore size and pore volume of the resulting carbon material can be varied by appropriately adjusting the particle size of the mold and the compounding ratio of the carbon raw material.

[0021] 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 is more preferable, and 1000m ２ / g or more, and more preferably 1100m ２ / g or more is particularly preferred, and 1500m ２ The upper limit of the BET specific surface area is not particularly limited, but is, for example, 3000 m ２ / g or less. The total pore volume of the porous carbon is 1.0 cm ３ / g or more, and ３ / g or more, and 1.5 cm ３ / g or more, and more preferably 5 cm ３ / g or more, and more preferably 10 cm ３ The upper limit of the total pore volume is not particularly limited, but is, for example, 30 cm ３ / g or less. If the BET specific surface area and total pore volume of the porous carbon are within these ranges, a sufficient amount of pores can be retained, and therefore a sufficient amount of positive electrode active material can be retained. The BET specific surface area and total pore volume of the porous carbon can be determined by the BJH method from values ​​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.

[0022] The average pore diameter of the porous carbon is not particularly limited as long as it is larger than the average particle diameter or average fiber diameter of the conductive additive described below. The average pore diameter of the porous carbon is preferably 15 to 100 nm, more preferably 20 to 80 nm, even more preferably 25 to 70 nm, even more preferably 30 to 70 nm, even more preferably 35 to 70 nm, even more preferably 40 to 65 nm, and particularly preferably 45 to 60 nm. When the average pore diameter of the porous carbon is within these ranges, the effects of the present invention can be more significantly achieved. 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 total pore volume. In this case, the distribution of pore volume relative to pore diameter is obtained, and the pore diameter that accounts for 50% of the total pore volume in the cumulative pore volume distribution is defined as the average pore diameter.

[0023] The average particle diameter (primary particle diameter) of the porous carbon is not particularly limited, but is preferably 0.5 to 500 μm, and more preferably 1 to 200 μm. In this specification, the "particle diameter of the porous carbon" refers to the longest distance L between any two points on the contour line of the porous carbon. The value of the "average particle diameter of the porous carbon" is calculated as the arithmetic mean value of 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).

[0024] The content of porous carbon relative to the total mass of the positive electrode active material layer is not particularly limited, but is preferably 5 to 11 mass%, more preferably 6 to 11 mass%, and even more preferably 7 to 10 mass%. Within this range, the sulfur-containing positive electrode active material can be efficiently retained in the pores, which can further improve the battery capacity. Furthermore, the electron conductivity and ion conductivity in the positive electrode active material layer can be easily ensured, and the effects of the present invention can be obtained more significantly.

[0025] [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 , TiS ４ , NiS, NiS ２ , CuS, FeS ２ , MoS２ , 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 , TiS ４ , 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.

[0026] The positive electrode material is formed by filling at least a portion of the sulfur-containing positive electrode active material into the pores of the porous carbon. This configuration improves the utilization efficiency of the sulfur-containing positive electrode active material, thereby enabling an increase in the capacity of the lithium secondary battery. The proportion of the sulfur-containing positive electrode active material filling 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. Thermogravimetry (TG) analysis can be used to confirm whether at least a portion of the sulfur-containing positive electrode active material has filled the pores of the porous carbon and to measure the proportion of sulfur in the pores.

[0027] Specifically, a TG thermal analyzer (TG-DTA8122 Smart Loader, manufactured by Rigaku) ​​was placed in an argon atmosphere glove box maintained at a dew point of -60 ° C. or less, and 10 mg of the positive electrode material as a sample was placed in an aluminum pan for the TG thermal analyzer to perform TG thermal analysis. The program applied 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 weight change rate (dTG) exceeds a threshold value, and temperature control is performed when it falls below the threshold value again. Since the temperature rise stops when dTG is higher than the threshold value, in a graph with time on the horizontal axis and weight loss on the vertical axis, the weight loss is linear at temperatures where dTG exceeds the threshold value. The measurement conditions were a temperature rise of 10 ° C. / min in the temperature range from 30 ° C. to 300 ° C. The threshold for dTG is set at 0.2% wt / min.

[0028] When the positive electrode active material is sulfur alone, under the above conditions, the linear weight loss of sulfur alone occurs near 200 ° C. Therefore, focusing on the initial linear weight loss at temperatures above 200 ° C., the end point of that linear weight loss (i.e., the temperature at which the dTG threshold is exceeded and the temperature rise resumes) is taken 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 is taken as sulfur outside the pores, and the weight of sulfur inside the pores is determined by subtracting the weight of sulfur outside the pores from the charged amount. Then, the ratio (percentage) of sulfur inside the pores to the charged amount is calculated to obtain the ratio of sulfur present in the pores (intrapore sulfur ratio).

[0029] <Method for producing a cathode material> The cathode material can be produced by a production method including a step of impregnating porous carbon with a liquid containing a cathode active material containing sulfur to obtain a composite cathode material. According to the production method, the cathode active material can be filled into the pores of the porous carbon. That is, in one embodiment, the cathode material is obtained by impregnating porous carbon with a liquid containing a cathode active material containing sulfur.

[0030] Specifically, a composite material 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.

[0031] 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 mixing using a mixing means such as a mortar or milling using a grinding means such as a planetary ball mill, and then melt the sulfur-containing positive electrode active material (e.g., elemental sulfur) by heat treatment. The temperature during 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, heat treatment can be performed at a temperature of 175 to 195°C for 5 to 10 hours. This heat treatment results in a composite material in which the sulfur-containing positive electrode active material is filled into the pores of the porous carbon.

[0032] The content of the positive electrode material relative to the total mass of the positive electrode active material layer is preferably 40% by mass or more, more preferably 40% by mass or more and 95% by mass or less, even more preferably 50% by mass or more and 80% by mass or less, and particularly preferably 55% by mass or more and 70% by 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.

[0033] [Conductive additive] The positive electrode active material layer contains a conductive additive. Any known material can be used as the conductive additive as long as it does not retain the positive electrode active material or solid electrolyte inside the pores. In the positive electrode of this embodiment, the conductive additive includes a particulate conductive additive having an average primary particle diameter smaller than the average pore diameter of the porous carbon or a fibrous conductive additive having an average fiber diameter smaller than the average pore diameter of the porous carbon.

[0034] In this specification, the term "fibrous conductive additive" refers to a conductive additive having a fibrous shape. The term "fibrous" encompasses, for example, elongated shapes such as columnar shapes, and is not particularly limited to linear or curved shapes. Furthermore, the term "fibrous conductive additive" may refer to a hollow tubular shape as long as it has a fibrous shape. More specifically, the "fibrous conductive additive" refers to a conductive additive having an aspect ratio (fiber length / fiber diameter) of 10 to 1,000 in an image obtained by observing the cross section of a positive electrode active material layer using a scanning electron microscope (SEM). The average fiber length of the fibrous conductive additive is preferably 10 μm or more. The average fiber length of the fibrous conductive additive is preferably 10 to 100 μm, more preferably 10 to 50 μm, even more preferably 10 to 40 μm, and particularly preferably 15 to 40 μm. By setting the average fiber length of the fibrous conductive additive within this range, the conductivity between the positive electrode active materials can be improved, and cycle durability can be further improved. The fiber length of the fibrous conductive additive can be the average value of several to several tens of fiber lengths measured using a transmission electron microscope (TEM) or a scanning electron microscope (SEM). The average fiber diameter of the fibrous conductive additive is not particularly limited as long as it is smaller than the average pore diameter of the porous carbon, but is, for example, 5 to 80 nm, preferably 10 to 80 nm, more preferably 10 to 70 nm, even more preferably 10 to 55 nm, and particularly preferably 10 to 30 nm. Within the above range, the effects of the present invention can be more pronounced. The average fiber diameter of the fibrous conductive additive can be the average value of several to several tens of fiber diameters measured using a transmission electron microscope (TEM) or a scanning electron microscope (SEM). Examples of fibrous conductive additives that can be used include carbon nanotubes (single-walled carbon nanotubes, multi-walled carbon nanotubes, etc.), carbon nanohorns, carbon nanofibers, carbon nanofilaments, carbon fibrils, and vapor-grown carbon fibers. The fibrous conductive assistant may be used alone or in combination of two or more kinds.

[0035] On the other hand, the term "particulate conductive additive" refers to a conductive additive having a particle-like shape. The term "particulate" encompasses shapes such as spherical, hemispherical, ellipsoidal, short chain, scale, cylindrical, and polygonal prism, and may be linear or curved. Furthermore, the term "particulate conductive additive" refers to a conductive additive having a particle-like shape, and may be hollow inside. More specifically, the term "particulate conductive additive" refers to a conductive additive other than the above-mentioned "fibrous conductive additive," which has an aspect ratio (major axis / minor axis) of less than 10 in an image obtained by observing the cross section of a positive electrode active material layer using a scanning electron microscope (SEM).

[0036] The average primary particle diameter of the particulate conductive additive contained in the positive electrode active material layer is not particularly limited as long as it is smaller than the average pore diameter of the porous carbon, but is preferably 10 to 80 nm, more preferably 10 to 70 nm, even more preferably 10 to 55 nm, and particularly preferably 30 to 45 nm. Within the above range, the effects of the present invention can be more pronounced. The average primary particle diameter of the particulate conductive additive can be the average particle diameter of several to several tens of primary particles measured using a transmission electron microscope (TEM) or a scanning electron microscope (SEM). The average aspect ratio of the particulate conductive additive contained in the positive electrode active material layer is preferably closer to 1, and is preferably 1 to 5, and more preferably 1 to 2. The average aspect ratio of the particulate conductive additive can be the average aspect ratio of several to several tens of particulate conductive additives measured using a scanning electron microscope (SEM). The particulate conductive additive is not particularly limited, and examples thereof include carbon powders such as carbon blacks such as acetylene black, ketjen black (furnace black), channel black, and thermal black. Among these, acetylene black, ketjen black (furnace black), channel black, and thermal black are preferably used from the viewpoint of being able to more closely follow the volume change of the positive electrode active material accompanying charge and discharge. Only one type of particulate conductive additive may be used alone, or two or more types may be used in combination.

[0037] In a preferred embodiment, the porous carbon contained in the positive electrode active material layer has an average pore diameter of 25 to 70 nm, and the particulate conductive additive has an average primary particle diameter or the fibrous conductive additive has an average fiber diameter of 10 to 55 nm. With this configuration, the effects of the present invention can be more significantly obtained.

[0038] The ratio of the average primary particle diameter of the particulate conductive additive or the average fiber diameter of the fibrous conductive additive to the average pore diameter of the porous carbon is, for example, 0.1 to 0.95, and preferably 0.2 to 0.9. When a particulate conductive additive is used, the ratio of the average primary particle diameter of the particulate conductive additive to the average pore diameter of the porous carbon is more preferably 0.5 to 0.9. Within the above range, the effects of the present invention can be obtained even more significantly.

[0039] The proportion of the total content of the particulate conductive assistant having an average primary particle size smaller than the average pore size of the porous carbon and the fibrous conductive assistant having an average fiber size smaller than the average pore size of the porous carbon relative to the total mass of the conductive assistant contained in the positive electrode active material layer is, for example, 80% by mass or more, preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 97% by mass or more, and most preferably 100% by mass. That is, when a particulate conductive assistant and a fibrous conductive assistant are used in combination as the conductive assistant, it is preferable to use a particulate conductive assistant having an average primary particle size smaller than the average pore size of the porous carbon and a fibrous conductive assistant having an average fiber size smaller than the average pore size of the porous carbon.

[0040] The content of the conductive additive relative to the total mass of the positive electrode active material layer is not particularly limited, but is preferably 0.1 to 5 mass%, more preferably 0.3 to 3 mass%, and even more preferably 0.5 to 2 mass%. Within this range, the electron conductivity and ion conductivity in the positive electrode active material layer can be easily ensured, and the effects of the present invention can be obtained more significantly.

[0041] In the positive electrode for a lithium secondary battery of this embodiment, the total content of the porous carbon and the conductive additive relative to the total mass of the positive electrode active material layer is more than 7.7 mass% and less than 12 mass%. If the total content of the porous carbon and the conductive additive relative to the total mass of the positive electrode active material layer is 7.7 mass% or less, the electronic conductivity is insufficient, and the reactivity in the region close to the solid electrolyte layer is reduced. This can increase the resistance of the battery. If the total content of the porous carbon and the conductive additive relative to the total mass of the positive electrode active material layer is 12 mass% or more, the ionic conductivity is insufficient, and the reactivity in the region close to the positive electrode current collector is reduced. This can increase the resistance of the battery. The content is preferably 8 to 11.5 mass%, more preferably 8 to 11 mass%.

[0042] [Solid Electrolyte] In the lithium secondary battery according to this embodiment, the positive electrode active material layer further contains a solid electrolyte.

[0043] 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.

[0044] The solid electrolyte may or may not be at least partially filled inside the pores of the porous carbon, but it is preferable that the solid electrolyte is not filled inside the pores of the porous carbon, which makes it possible to obtain the effects of the present invention more significantly.

[0045] 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.

[0046] The content of the solid electrolyte in the positive electrode active material layer is preferably 1 mass % or more and 70 mass % or less, more preferably 5 mass % or more and 50 mass % or less, and even more preferably 10 mass % or more and 45 mass % or less, relative to the total mass of the positive electrode active material layer. When the content of the solid electrolyte in the positive electrode active material layer is within the above range, both the ionic conductivity and the energy density of the positive electrode active material layer can be achieved.

[0047] The positive electrode active material layer may further include a binder. The type of binder is not particularly limited, and binders known in the art may be appropriately used. Examples include fluorine-based resins such as polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements) and polytetrafluoroethylene (PTFE), as well as styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC).

[0048] The total content of the positive electrode material, the conductive additive, and the solid electrolyte relative to the total mass of the positive electrode active material layer is preferably 90 mass% or more, more preferably 95 mass% or more, even more preferably 98 mass% or more, particularly preferably 99 mass% or more, and most preferably 100 mass%.

[0049] 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. The 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 or less. ２ It is preferable that this is equal to or greater than this.

[0050] By applying the above-described positive electrode for a lithium secondary battery to a lithium secondary battery, it is possible to reduce the resistance of the lithium secondary battery. That is, one embodiment of the present invention is a lithium secondary battery including a positive electrode for a lithium secondary battery according to one embodiment of the present invention. Hereinafter, the main components of a lithium secondary battery to which the positive electrode for a lithium secondary battery according to this embodiment is applied will be described.

[0051] [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 the solid electrolyte 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 one described above for the positive electrode active material layer can be similarly applied, and therefore a description thereof will be omitted here.

[0052] The content of the solid electrolyte in the solid electrolyte layer is preferably 50% by mass or more and 100% by mass or less, more preferably 90% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 99% by mass or less.

[0053] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte. The binder that can be used in the solid electrolyte layer is the same as that described above for the positive electrode active material layer.

[0054] 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 800 μm or less, more preferably 700 μm or less, and even more preferably 600 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, even more preferably 10 μm or more, and even more preferably 100 μm or more.

[0055] [Negative electrode current collector] There are no particular limitations on the material constituting the negative electrode current collector, and conventionally known knowledge can be used. For example, metals and conductive resins can be used as the material constituting the negative electrode current collector. There are no particular limitations on the thickness of the negative electrode current collector, but an example is 10 to 100 μm.

[0056] [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.

[0057] 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-described negative electrode active material. The solid electrolyte, conductive additive, and binder that can be used in the solid electrolyte layer are the same as those described above for the positive electrode active material layer.

[0058] 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.

[0059] [Positive Electrode Current Collector Plate and Negative Electrode Current Collector Plate] The material constituting the current collector plates (25, 27) is not particularly limited, and any known highly conductive material conventionally used as a current collector plate for lithium secondary batteries can be used.

[0060] [Positive Electrode Lead and Negative Electrode Lead] Although not shown, the current collector and the current collector plate may be electrically connected via a positive electrode lead or a negative electrode lead. As the constituent materials of the positive electrode and the negative electrode lead, materials used in known lithium secondary batteries can be similarly adopted.

[0061] [Battery Exterior Material] As the battery exterior material, a known metal can case can be used. Alternatively, a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power generating element as shown in FIG. 1 can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of achieving high output and excellent cooling performance, making it suitable for use in batteries for large equipment such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable for the exterior body because it allows for easy adjustment of the collective pressure applied to the power generating element from the outside.

[0062] The lithium secondary battery according to the present embodiment has a configuration in which a plurality of unit cell layers are connected in parallel, and therefore has high capacity and excellent cycle durability, and is therefore suitable for use as a driving power source for EVs and HEVs.

[0063] Although one embodiment of a lithium secondary battery has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0064] For example, the type of battery to which the lithium secondary battery according to the present embodiment is applied includes a bipolar battery including a bipolar electrode having a positive electrode active material layer electrically coupled to one surface of a current collector and a negative electrode active material layer electrically coupled to the opposite surface of the current collector.

[0065] Furthermore, the lithium secondary battery according to the present embodiment does not have to be an all-solid-state type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolytic solution). There is no particular limitation on the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferably an amount that allows the shape of the solid electrolyte layer formed by the solid electrolyte to be maintained and prevents leakage of the liquid electrolyte (electrolytic solution).

[0066] The following embodiments are also included within the scope of the present invention: a positive electrode for a lithium secondary battery according to claim 1 having the features of claim 2; a positive electrode for a lithium secondary battery according to claim 1 or 2 having the features of claim 3; and a lithium secondary battery using the positive electrode for a lithium secondary battery according to any one of claims 1 to 3.

[0067] 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 -68°C or less.

[0068] <Examples of Manufacturing Evaluation Cells> [Example 1] (Manufacturing of Positive Electrode Material) In a glove box in an argon atmosphere with a dew point of -68°C or less, 49 parts by mass of sulfur (manufactured by Aldrich Chemicals) as a positive electrode active material and 7 parts by mass of porous carbon (manufactured by Toyo Tanso Co., Ltd., Knobel (registered trademark)) having an average pore diameter of 45 nm were thoroughly mixed in an agate mortar, and the mixed powder was then placed in a sealed pressure-resistant autoclave and heated at 185°C for 8 hours to melt the sulfur, thereby preparing a positive electrode material. The porous carbon had an average particle diameter of 4 μm and a BET specific surface area of ​​1600 m ２ / g, and the total pore volume is 17.9 cm ３ / g.

[0069] (Production of Positive Electrode Mixture) In a glove box with an argon atmosphere having a dew point of −68° C. or less, 56 parts by mass of the positive electrode material prepared above and a sulfide solid electrolyte (manufactured by Apcera, Li ６ P.S. ５43 parts by mass of sulfur, solid electrolyte, and carbon material were mixed in a zirconia container containing 5 mm diameter zirconia balls and milled at 190 rpm for 8 hours in a planetary ball mill (Fritsch, Premium Line P-7) to obtain a powder of a positive electrode mixture. The composition (mass ratio) of the positive electrode mixture was 49:43:8 (sulfur:solid electrolyte:carbon material). The mass ratio of the carbon material was the sum of the porous carbon and the conductive additive.

[0070] (Production of Evaluation Cell) The battery was produced in a glove box in an argon atmosphere with a dew point of −68° C. or less. A SUS cylindrical convex punch (10 mm diameter) was inserted into one side of a cylindrical tube jig (manufactured by Macor) (tube inner diameter 10 mm, outer diameter 23 mm, height 20 mm), and a sulfide solid electrolyte (manufactured by Apcera, Li ６ P.S. ５ 80 mg of SUS304 (C1) was placed inside the cylindrical tube jig. Then, another SUS cylindrical convex punch was inserted to sandwich the solid electrolyte, and the tube was pressed for 3 minutes at a pressure of 75 MPa using a hydraulic press to form a solid electrolyte layer with a diameter of 10 mm and a thickness of approximately 0.6 mm in the cylindrical tube jig. Next, the cylindrical convex punch inserted from above was temporarily removed, and 5.4 mg of the positive electrode mixture prepared above was placed on one side of the solid electrolyte layer in the cylindrical tube. A cylindrical convex punch (also serving as a positive electrode current collector) was inserted again from above, and the tube was pressed for 3 minutes at a pressure of 300 MPa to form a positive electrode active material layer with a diameter of 10 mm and a thickness of approximately 0.06 mm on one side of the solid electrolyte layer. Next, the lower cylindrical convex punch (which also served as the negative electrode current collector) was removed, and a lithium foil (manufactured by Honjo Metals, thickness 0.20 mm) punched to a diameter of 8 mm and an indium foil (manufactured by Nilaco Corporation, thickness 0.30 mm) punched to a diameter of 9 mm were stacked as the negative electrode, and the cylindrical tube jig was inserted from the bottom so that the indium foil was positioned on the solid electrolyte layer side.The cylindrical convex punch was then inserted again, and pressed at a pressure of 75 MPa for 3 minutes to form a lithium-indium negative electrode.In this way, an evaluation cell (all-solid-state lithium secondary battery) was produced in which the negative electrode current collector (punch), lithium-indium negative electrode, solid electrolyte layer, positive electrode active material layer, and positive electrode current collector (punch) were stacked in this order.

[0071] [Example 2] In the above (production of the positive electrode material), the amount of porous carbon used was changed to 8 parts by mass. Furthermore, in the (production of the positive electrode mixture), the conductive additive was changed to 1 part by mass of acetylene black having an average primary particle diameter of 30 nm, and the amount of sulfide solid electrolyte used was changed to 42 parts by mass. The composition (mass ratio) of the positive electrode mixture was sulfur:solid electrolyte:carbon material = 49:42:9. Except for the above, an evaluation cell for this example was obtained in the same manner as in Example 1.

[0072] [Example 3] In the above (production of the positive electrode material), the amount of porous carbon used was changed to 9 parts by mass. Furthermore, in the (production of the positive electrode mixture), the conductive additive was changed to 1 part by mass of acetylene black having an average primary particle diameter of 40 nm, and the amount of sulfide solid electrolyte used was changed to 41 parts by mass. The composition (mass ratio) of the positive electrode mixture was sulfur:solid electrolyte:carbon material = 49:41:10. Except for the above, an evaluation cell for this example was obtained in the same manner as in Example 1.

[0073] [Example 4] In the above (production of the positive electrode material), the amount of porous carbon used was changed to 10 parts by mass. In addition, in the (production of the positive electrode mixture), the amount of sulfide solid electrolyte used was changed to 40 parts by mass. The composition (mass ratio) of the positive electrode mixture was sulfur:solid electrolyte:carbon material = 49:40:11. Except for the above, an evaluation cell for this example was obtained in the same manner as in Example 1.

[0074] [Example 5] In the above (production of a positive electrode material), the porous carbon was changed to 9 parts by mass of porous carbon (Triporous (trademark), manufactured by Sony Corporation) having an average pore diameter of 60 nm. This porous carbon had an average particle diameter of 200 μm and a BET specific surface area of ​​1100 m. ２ / g, and the total pore volume is 14.3 cm ３ / g. In addition, in (production of the positive electrode mixture), the amount of sulfide solid electrolyte used was changed to 41 parts by mass. The composition (mass ratio) of the positive electrode mixture was sulfur:solid electrolyte:carbon material = 49:41:10. Except for the above, the evaluation cell of this example was obtained in the same manner as in Example 1.

[0075] [Example 6] An evaluation cell for this example was obtained in the same manner as in Example 5, except that in the above (production of positive electrode mixture), the conductive additive was changed to 1 part by mass of acetylene black having an average primary particle diameter of 30 nm.

[0076] [Example 7] An evaluation cell for this example was obtained in the same manner as in Example 5, except that in the above (production of positive electrode mixture), the conductive additive was changed to 1 part by mass of acetylene black having an average primary particle diameter of 45 nm.

[0077] [Example 8] An evaluation cell for this example was obtained in the same manner as in Example 3, except that in the above (production of positive electrode mixture), the conductive additive was changed to 1 part by mass of carbon nanotubes having an average fiber diameter of 10 nm (multi-walled carbon nanotubes, manufactured by Sanyo Pigment Co., Ltd.).

[0078] Comparative Example 1 In the above (production of the positive electrode material), the amount of porous carbon used was changed to 10 parts by mass. Furthermore, in the (production of the positive electrode mixture), no conductive additive was used, and the amount of sulfide solid electrolyte used was changed to 41 parts by mass. The composition (mass ratio) of the positive electrode mixture was sulfur:solid electrolyte:carbon material = 49:41:10. Aside from the above, an evaluation cell for this comparative example was obtained in the same manner as in Example 1.

[0079] Comparative Example 2 In the above (production of the positive electrode material), the amount of porous carbon used was changed to 9 parts by mass. Furthermore, in the (production of the positive electrode mixture), the conductive additive was changed to 1 part by mass of acetylene black having an average primary particle diameter of 45 nm, and the amount of sulfide solid electrolyte used was changed to 41 parts by mass. The composition (mass ratio) of the positive electrode mixture was sulfur:solid electrolyte:carbon material = 49:41:10. Except for the above, an evaluation cell for this comparative example was obtained in the same manner as in Example 1.

[0080] Comparative Example 3 In the above (production of the positive electrode material), the amount of porous carbon used was changed to 9 parts by mass. Furthermore, in the (production of the positive electrode mixture), the conductive additive was changed to 1 part by mass of acetylene black having an average primary particle diameter of 60 nm, and the amount of sulfide solid electrolyte used was changed to 41 parts by mass. The composition (mass ratio) of the positive electrode mixture was sulfur:solid electrolyte:carbon material = 49:41:10. Except for the above, an evaluation cell for this comparative example was obtained in the same manner as in Example 1.

[0081] Comparative Example 4 In the above (production of the positive electrode material), the amount of porous carbon used was changed to 9 parts by mass. Furthermore, in the (production of the positive electrode mixture), the conductive additive was changed to 1 part by mass of vapor-grown carbon fiber (VGCF-H manufactured by Resonac Co., Ltd., average fiber length 4 μm) with an average fiber diameter of 150 nm, and the amount of sulfide solid electrolyte used was changed to 41 parts by mass. The composition (mass ratio) of the positive electrode mixture was sulfur:solid electrolyte:carbon material = 49:41:10. Except for the above, an evaluation cell for this comparative example was obtained using the same method as in Example 1.

[0082] Comparative Example 5 In the above (production of the positive electrode material), the amount of porous carbon used was changed to 9 parts by mass. Furthermore, in the (production of the positive electrode mixture), the conductive additive was changed to 1 part by mass of graphite having an average primary particle diameter of 175 nm, and the amount of sulfide solid electrolyte used was changed to 41 parts by mass. The composition (mass ratio) of the positive electrode mixture was sulfur:solid electrolyte:carbon material = 49:41:10. Aside from the above, an evaluation cell for this comparative example was obtained in the same manner as in Example 1.

[0083] Comparative Example 6 In the above (production of the positive electrode material), the amount of porous carbon used was changed to 6 parts by mass. Furthermore, in the (production of the positive electrode mixture), the amount of sulfide solid electrolyte used was changed to 44 parts by mass. The composition (mass ratio) of the positive electrode mixture was sulfur:solid electrolyte:carbon material = 49:44:7. Aside from the above, an evaluation cell for this comparative example was obtained in the same manner as in Example 1.

[0084] Comparative Example 7 In the above (production of the positive electrode material), the amount of porous carbon used was changed to 11 parts by mass. Furthermore, in the (production of the positive electrode mixture), the amount of sulfide solid electrolyte used was changed to 39 parts by mass. The composition (mass ratio) of the positive electrode mixture was sulfur:solid electrolyte:carbon material = 49:39:12. Aside from the above, an evaluation cell for this comparative example was obtained in the same manner as in Example 1.

[0085] <Charge / Discharge Test> A charge / discharge test was performed on the evaluation cell manufactured above using a charge / discharge tester (HJ-SD8, manufactured by Hokuto Denko Corporation) in a constant temperature bath set at 25°C. The 1C capacity was determined based on the cell capacity calculated from the theoretical capacity, and the cells were discharged at 0.03C, 0.05C, and 0.1C in a charged state of 50% SOC. The DCR (cell resistance) was calculated from the voltage change after 10 seconds. The results are shown in Table 1 below. The DCR values ​​shown in Table 1 are relative values ​​when the DCR in Comparative Example 1 is set to 1.0. In Table 1, AB represents acetylene black, CNT represents carbon nanotubes, VGCF represents vapor-grown carbon fiber, and Gr represents graphite.

[0086]

[0087] As shown in Examples 1 to 8 in Table 1, according to the present invention, the resistance of a lithium secondary battery using a positive electrode active material containing sulfur is reduced. On the other hand, it was found that the effect of reducing resistance was not obtained in Comparative Examples 2 to 5, in which the average primary particle diameter or average fiber diameter of the conductive additive was equal to or larger than the average pore diameter of the porous carbon. Furthermore, it was found that the effect of reducing resistance was not obtained when the content of the carbon material was outside the specified range, as in Comparative Examples 6 and 7.

[0088] 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.

Claims

a positive electrode material including 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; A conductive additive; a solid electrolyte; A positive electrode for a lithium secondary battery containing a positive electrode active material layer comprising: the conductive assistant includes a particulate conductive assistant having an average primary particle diameter smaller than the average pore diameter of the porous carbon, or a fibrous conductive assistant having an average fiber diameter smaller than the average pore diameter of the porous carbon, a total content of the porous carbon and the conductive additive relative to the total mass of the positive electrode active material layer being more than 7.7 mass % and less than 12 mass %; 　 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the porous carbon has an average pore diameter of 25 to 70 nm, and the particulate conductive additive has an average primary particle diameter or the fibrous conductive additive has an average fiber diameter of 10 to 55 nm.

2. The positive electrode for a lithium secondary battery according to claim 1, wherein the solid electrolyte is a sulfide solid electrolyte. 　 A lithium secondary battery comprising the positive electrode for lithium secondary batteries according to claim 1.

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