Positive electrode for secondary batteries, and secondary battery

Abstract

This secondary battery is provided with: a positive electrode that contains a positive electrode active material and a plurality of fibrous substances; a negative electrode; and an electrolyte solution. The positive electrode active material contains a sulfur-containing material; and the plurality of fibrous substances are intertwined with each other, thereby forming a three-dimensional network structure. The fibrous substances each comprise a carbon fiber part and a plurality of carbon covering parts that cover the surface of the carbon fiber part and hold the positive electrode active material. The plurality of carbon covering parts are separated from each other in the extending direction of the carbon fiber part; and the carbon fiber part has a plurality of exposed parts that are not covered by the plurality of carbon covering parts. The carbon covering parts each have a first end and a second end which is positioned on the opposite side of the first end in the extending direction of the carbon fiber part. The degree of flexibility of the fibrous substances expressed by formula (1) is 8.6 to 19.2.　(1): F = (L2 / L1) × T (In the formula, F denotes the degree of flexibility; L1 represents the length (µm) of the carbon fiber part in the extending direction of the carbon fiber part; L2 represents the sum (µm) of the respective lengths of the plurality of exposed parts in the extending direction of the carbon fiber part; and T represents the sum (number) of the respective numbers of the first ends and the second ends.)

Description

Positive electrode for secondary battery and secondary battery

[0001] The present technology relates to a positive electrode for a secondary battery and a secondary battery.

[0002] Due to the widespread use of various electronic devices such as mobile phones, secondary batteries have been developed as small, lightweight, and high-energy-density power sources. These secondary batteries contain a positive electrode (secondary battery electrode), a negative electrode, and an electrolyte.

[0003] In particular, secondary batteries using sulfur-containing materials as positive electrode active materials are known, and various studies have been conducted on the configuration of such secondary batteries. Specifically, in the positive electrode, multiple carbon nanotubes are entangled with each other, and a carbon layer coats the surfaces of the carbon nanotubes, and sulfur and nitrogen are co-doped into the carbon layer (see Patent Document 1). In addition, in the positive electrode, a highly graphitic porous carbon material coats a conductive core material, and sulfur is encapsulated inside the multiple pores (see Patent Document 2).

[0004] JP 2020-031045 A Patent No. 6726279

[0005] Although various studies have been conducted on the configuration of secondary batteries, the battery characteristics of the secondary batteries are still insufficient and there is room for improvement.

[0006] There is a demand for a positive electrode for a secondary battery and a secondary battery that can provide excellent battery characteristics.

[0007] According to one embodiment of the present technology, a positive electrode for a secondary battery includes a positive electrode active material and a plurality of fibrous materials. The positive electrode active material includes a sulfur-containing material, and the plurality of fibrous materials are entangled with each other to form a three-dimensional network structure. The fibrous material includes a carbon fiber portion and a plurality of carbon coating portions that cover the surface of the carbon fiber portion and hold the positive electrode active material. The plurality of carbon coating portions are spaced apart from each other in the extension direction of the carbon fiber portion, and the carbon fiber portion has a plurality of exposed portions that are not covered by the plurality of carbon coating portions. The carbon coating portion has a first end in the extension direction of the carbon fiber portion and a second end located opposite the first end. The flexibility of the fibrous material represented by formula (1) is 8.6 or more and 19.2 or less.

[0008] F = (L2 / L1) × T (1) (F is flexibility. L1 is the length (μm) of the carbon fiber portion in the extending direction of the carbon fiber portion. L2 is the sum (μm) of the lengths of the plurality of exposed portions in the extending direction of the carbon fiber portion. T is the sum (number) of the numbers of first ends and second ends.)

[0009] A secondary battery according to an embodiment of the present technology includes a positive electrode, a negative electrode, and an electrolyte solution, and the positive electrode has a configuration similar to the configuration of the positive electrode for a secondary battery according to the embodiment of the present technology described above.

[0010] The definition and calculation method of the flexibility shown in formula (1) will be described in detail later.

[0011] According to one embodiment of the present technology, the positive electrode for a secondary battery or the secondary battery includes a positive electrode active material and a plurality of fibrous materials, the positive electrode active material includes a sulfur-containing material, and the plurality of fibrous materials are entangled with each other to form a three-dimensional mesh structure, the fibrous materials include a carbon fiber portion and a plurality of carbon coating portions, and the plurality of carbon coating portions coat the surfaces of the carbon fiber portions and hold the positive electrode active material, the plurality of carbon coating portions are spaced apart from each other in the extending direction of the carbon fiber portions, the carbon fiber portions have a plurality of exposed portions that are not covered by the plurality of carbon coating portions, and the carbon coating portions have a first end and a second end in the extending direction of the carbon fiber portions, and the flexibility of the fibrous material shown in formula (1) is 8.6 or more and 19.2 or less, thereby enabling excellent battery characteristics to be obtained.

[0012] Note that the effects of the present technology are not necessarily limited to the effects described here, but may be any of a series of effects related to the present technology described below.

[0013] 1 is a cross-sectional view showing the configuration of a positive electrode for a secondary battery in an embodiment of the present technology; FIG. 2 is a plan view showing the configuration of a positive electrode active material layer shown in FIG. 1; FIG. 3 is a plan view showing the configuration of a fibrous material shown in FIG. 1; FIG. 4 is a plan view for explaining the definition of flexibility; FIG. 5 is a perspective view showing the configuration of a secondary battery in an embodiment of the present technology; FIG. 6 is a cross-sectional view showing the configuration of a battery element shown in FIG. 5; FIG. 7 is a block diagram showing the configuration of an application example of a secondary battery; and FIG. 8 is a cross-sectional view showing the configuration of a test secondary battery.

[0014] Hereinafter, an embodiment of the present technology will be described in detail with reference to the drawings. The description will be made in the following order: 1. Positive electrode for secondary battery 1-1. Configuration 1-2. Operation 1-3. Manufacturing method 1-4. Action and effect 2. Secondary battery 2-1. Configuration 2-2. Operation 2-3. Manufacturing method 2-4. Action and effect 3. Modification 4. Use of secondary battery

[0015] 1. Positive Electrode for Secondary Battery First, a positive electrode for a secondary battery (hereinafter simply referred to as "positive electrode") according to an embodiment of the present technology will be described.

[0016] The positive electrode described here is used in a secondary battery, which is an electrochemical device. However, the positive electrode may also be used in electrochemical devices other than secondary batteries. Specific examples of other electrochemical devices include primary batteries and capacitors.

[0017] The positive electrode absorbs and releases an electrode reactant during operation of the electrochemical device (during electrode reaction). The type of electrode reactant is not particularly limited, but specifically includes light metals such as alkali metals and alkaline earth metals. Specific examples of alkali metals include lithium, sodium, and potassium, and specific examples of alkaline earth metals include beryllium, magnesium, and calcium.

[0018] In the following, an example will be given in which the electrode reactant is lithium, whereby lithium is absorbed and released in an ionic state at the positive electrode during the electrode reaction.

[0019] <1-1. Structure> Fig. 1 shows a cross-sectional structure of a positive electrode 100, which is a specific example of a positive electrode. Fig. 2 shows a planar structure of the positive electrode active material layer 100B shown in Fig. 1. Fig. 3 shows a planar structure of the fibrous material 120 shown in Fig. 1.

[0020] As shown in FIG. 1, the positive electrode 100 includes a positive electrode current collector 100A and a positive electrode active material layer 100B.

[0021] 1, the positive electrode current collector 100A is a conductive support that supports the positive electrode active material layer 100B, and has a pair of surfaces on which the positive electrode active material layer 100B is provided. The positive electrode current collector 100A contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum.

[0022] 1, the positive electrode active material layer 100B is a layer that occludes and releases an electrode reactant, and is provided on one surface (upper surface) of the positive electrode current collector 100A. However, the positive electrode active material layer 100B may be provided on both surfaces (upper and lower surfaces) of the positive electrode current collector 100A.

[0023] 2 and 3, the positive electrode active material layer 100B includes a positive electrode active material 110 and a plurality of fibrous materials 120. The positive electrode active material layer 100B has a substantially flat plate-like structure and therefore has a thickness T (μm). This thickness T is not particularly limited and can be set arbitrarily.

[0024] (Positive Electrode Active Material) The positive electrode active material 110 contains a material that absorbs and releases lithium, and more specifically, contains one or more sulfur-containing materials. The positive electrode active material 110 is not shown in Fig. 2, and is shaded in Fig. 3.

[0025] As will be described later, the positive electrode active material 110 is held by a fibrous material 120 (carbon coating portion 122). Details of the configuration in which the positive electrode active material layer 100B is held by the fibrous material 120 will be described later.

[0026] The sulfur-containing material is a general term for materials containing sulfur as a constituent element. This sulfur-containing material may be a simple substance, an alloy, a compound, a mixture of two or more of these, or a material containing one or more of these phases. The term "simple substance" described here refers to a general simple substance, and therefore the simple substance may contain a trace amount of impurities. In other words, the purity of the simple substance is not necessarily limited to 100%.

[0027] 2 and 3, the plurality of fibrous materials 120 are entangled with one another while holding the positive electrode active material 110, thereby forming a three-dimensional mesh structure. That is, the positive electrode active material layer 100B has a three-dimensional mesh structure formed by the plurality of fibrous materials 120 being entangled with one another.

[0028] 3, in order to make it easier to understand the configuration of the fibrous material 120, the fibrous material 120 is shown extending linearly. In this example, the fibrous material 120 extends in an extension direction Z along the longitudinal direction.

[0029] As shown in FIG. 3, the fibrous material 120 includes a carbon fiber portion 121 and a plurality of carbon coating portions 122 .

[0030] (Carbon Fiber Portion) As shown in FIG. 3 , the carbon fiber portion 121 is a fibrous conductive member that supports the plurality of carbon coating portions 122 and extends in the extension direction Z.

[0031] As a result, the carbon fiber portion 121 has a front end portion 121M (first end portion) located in front of the extension direction Z and a rear end portion 121N (second end portion) located behind the front end portion 121M in the extension direction Z. In other words, the rear end portion 121N is located on the opposite side of the front end portion 121M in the extension direction Z. Here, as is clear from FIG. 3 , since the extension direction Z is toward the right, the front end portion 121M is the right end portion and the rear end portion 121N is the left end portion.

[0032] The carbon fiber portion 121 has a front end 121E1 located in front of the extension direction Z and a rear end 121E2 located behind the front end 121E1 in the extension direction Z. That is, the rear end 121E2 is located on the opposite side of the front end 121E1 in the extension direction Z. As is clear from FIG. 3 , the extension direction Z is toward the right, so the front end 121E1 is the right end and the rear end 121E2 is the left end.

[0033] As will be described later, the plurality of carbon coating portions 122 coat the surface of the carbon fiber portion 121, but are spaced apart from one another in the extension direction Z. As a result, the carbon fiber portion 121 has a plurality of non-exposed portions 121X that are coated with the plurality of carbon coating portions 122 and a plurality of exposed portions 121Y that are not coated with the plurality of carbon coating portions 122.

[0034] As described above, in the positive electrode active material layer 100B, the plurality of fibrous materials 120 are entangled with one another to form a three-dimensional mesh structure. As a result, the plurality of carbon fiber portions 121 are entangled with one another and are connected to one another. As a result, the plurality of carbon fiber portions 121 are electrically connected to one another inside the positive electrode active material layer 100B, and a three-dimensional conductive network is formed using the plurality of carbon fiber portions 121.

[0035] As described above, the fibrous material 120 includes a plurality of carbon coating portions 122, and the carbon fiber portion 121 has a plurality of non-exposed portions 121X and a plurality of exposed portions 121Y.

[0036] In this case, the front end 121M may or may not be coated with the carbon coating portion 122. In particular, it is preferable that the front end 121M is exposed and not coated with the carbon coating portion 122, as shown in Fig. 3. This is because the front end 121M becomes more easily bent, and therefore the entire fibrous material 120 becomes more easily bent. Note that when the front end 121M is exposed, the front end 121M becomes one of the multiple exposed portions 121Y.

[0037] The same applies to the rear end 121N as described above with respect to the front end 121M. That is, the rear end 121N may be coated with the carbon coating portion 122, or may be exposed without being coated with the carbon coating portion 122. However, it is preferable that the rear end 121N be exposed as shown in Fig. 3. This is because the rear end 121N is more likely to bend, and therefore the entire fibrous material 120 is more likely to bend. Note that when the rear end 121N is exposed, the rear end 121N becomes one of the multiple exposed portions 121Y.

[0038] The carbon fiber portion 121 includes one or more types of fibrous carbon materials. The type of fibrous carbon material is not particularly limited, but specific examples include carbon nanotubes and carbon nanofibers. This is because a three-dimensional conductive network using multiple carbon fiber portions 121 can be stably formed. The carbon nanotubes may be single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), or both.

[0039] The average fiber diameter and average length of the carbon fiber portion 121 are not particularly limited and can be set arbitrarily.

[0040] 3 , the carbon coating portion 122 is a massive conductive member that coats the surface (surroundings) of the carbon fiber portion 121 while holding the positive electrode active material 110. However, as described above, the multiple carbon coating portions 122 are spaced apart from one another in the extension direction Z. That is, the multiple carbon coating portions 122 are not continuous in the extension direction Z, but are separated from one another in the extension direction Z.

[0041] As a result, the carbon coating portion 122 has a front end 122E1 (first end) located in front of the extension direction Z and a rear end 122E2 (second end) located behind the front end 122E1 in the extension direction Z. In other words, the rear end 122E2 is located on the opposite side of the extension direction Z from the front end 122E1. Here, as is clear from FIG. 3 , the extension direction Z is toward the right, so the front end 122E1 is the right end and the rear end 122E2 is the left end. Therefore, each of the multiple carbon coating portions 122 has a front end 122E1 and a rear end 122E2.

[0042] The carbon coating portion 122 coats the surface of the carbon fiber portion 121 and is therefore in contact with the surface of the carbon fiber portion 121. As a result, the carbon fiber portion 121 and the carbon coating portion 122 are electrically connected to each other.

[0043] As described above, the carbon coating portion 122 holds the positive electrode active material 110. The configuration in which the carbon coating portion 122 holds the positive electrode active material 110 is not particularly limited.

[0044] In particular, it is preferable that the carbon coating portion 122 has a plurality of pores 122K and the positive electrode active material 110 is inserted into the plurality of pores 122K. This is because the positive electrode active material 110 is more likely to be stably held by the carbon coating portion 122 and the positive electrode active material 110 and the carbon coating portion 122 are more likely to be in communication with each other, which facilitates stable and smooth absorption and desorption of lithium in the positive electrode active material layer 100B. In this case, the proportion of the positive electrode active material 110 that is deactivated due to a reaction with the electrolyte is particularly reduced, thereby improving the diffusibility of lithium and increasing the energy density.

[0045] The carbon coating portion 122 includes one or more types of non-fibrous carbon materials. The type of non-fibrous carbon material is not particularly limited, but specifically includes activated carbon. This is because the carbon coating portion 122 facilitates stable formation of the plurality of pores 122K, which facilitates stable insertion of the positive electrode active material 110 into the plurality of pores 122K.

[0046] [Flexibility] In this positive electrode 100, predetermined conditions are satisfied regarding the flexibility of the fibrous materials 120 in order to increase the packing property of the plurality of fibrous materials 120, more specifically, the density of the three-dimensional mesh structure formed by the plurality of fibrous materials 120. The flexibility F, which is an index representing the flexibility of the fibrous materials 120, will be described below.

[0047] Fig. 4 shows the planar configuration of fibrous material 120 corresponding to Fig. 3 in order to explain the definition of flexibility F. However, in Fig. 4, positive electrode active material 110 and pores 122K are omitted from illustration in order to simplify the content of the illustration. In the following description, Fig. 3, which has already been described, will be referenced as needed along with Fig. 4.

[0048] (Definition) The flexibility F is a coefficient expressed by the formula (1), and represents the flexibility of the fibrous material 120 as described above.

[0049] F = (L2 / L1) × T (1) (F is flexibility. L1 is the length (μm) of the carbon fiber portion 121 in the extension direction Z. L2 is the sum (μm) of the lengths L3 of the multiple exposed portions 121Y in the extension direction Z. T is the sum (number) of the numbers of the front ends 122E1 and the rear ends 122E2.)

[0050] As described above, the length L1 (μm) is the dimension (length) of the carbon fiber portion 121 in the extension direction Z. Thus, the length L1 is the distance from the front end 121E1 to the rear end 121E2 as shown in FIGS.

[0051] As described above, the sum L2 (μm) is the sum of the dimensions (lengths L3) of the multiple exposed portions 121Y in the extension direction Z. In other words, when the dimension (distance) of the exposed portion 121Y in the extension direction Z is length L3, the sum L2 is the sum of the multiple lengths L3.

[0052] 4, as described above, for the sake of simplification, only four lengths L3 are shown. In this case, since the front end 121M is the exposed portion 121Y as described above, the length L3 of the front end 121M (exposed portion 121Y) is also taken into consideration when calculating the sum L2. Similarly, since the rear end 121N is the exposed portion 121Y as described above, the length L3 of the rear end 121N (exposed portion 121Y) is also taken into consideration when calculating the sum L2.

[0053] As described above, the sum T is the sum of the number of the front ends 122E1 and the number of the rear ends 122E2. As a result, the value of the sum T is a multiple of 2, more specifically, the value is the number of the carbon coating portions 122 multiplied by 2.

[0054] As an example, if the number of carbon coating portions 122 is two, each of the two carbon coating portions 122 has a front end 122E1 and a rear end 122E2, so the sum T = (number of front ends 122E1 (= 1) × 2) + (number of rear ends 122E2 (= 1) × 2) = 4.

[0055] As another example, if the number of carbon coating portions 122 is three, each of the three carbon coating portions 122 has a front end 122E1 and a rear end 122E2, so the number T = (number of front ends 122E1 (= 1) × 3) + (number of rear ends 122E2 (= 1) × 3) = 6.

[0056] As a result, the greater the flexibility F, the easier it is for the fibrous material 120 to bend using the carbon fiber portion 121 (the multiple exposed portions 121Y), and the easier it is for the fibrous material 120 to become entangled with one another so that the multiple carbon coating portions 122 fill the voids. Therefore, the density of the three-dimensional mesh structure formed by the multiple fibrous materials 120 increases, and the packing property of the multiple fibrous materials 120 improves.

[0057] On the other hand, the smaller the flexibility F, the less likely the fibrous material 120 is to bend using the carbon fiber portion 121 (the plurality of exposed portions 121Y), and therefore the fibrous material 120 is less likely to become entangled with one another so that the plurality of carbon coating portions 122 fill the voids. Therefore, the density of the three-dimensional mesh structure formed by the plurality of fibrous materials 120 decreases, and the packing property of the plurality of fibrous materials 120 decreases.

[0058] (Range) The flexibility F is 8.6 to 19.2. This is because the flexibility of the plurality of fibrous materials 120 is optimized, thereby appropriately improving the packing property of the plurality of fibrous materials 120. As a result, the three-dimensional conductive network formed using the carbon fiber portion 121 is maintained, and the density of the positive electrode active material layer 100B is increased while ensuring the lithium absorption / desorption property of each of the plurality of carbon coating portions 122 (positive electrode active material 110), thereby increasing the electrode density of the positive electrode 100. Note that the value of the flexibility F is rounded to one decimal place.

[0059] (Calculation Method) The procedure for calculating the flexibility F is as follows. When a secondary battery is used to calculate the flexibility F, the positive electrode 100 is recovered from the secondary battery, and then the flexibility F is calculated based on the following procedure.

[0060] First, the positive electrode 100 is cut in the thickness direction to expose the cross section of the positive electrode 100. This thickness direction is the direction along the thickness T (the vertical direction in FIGS. 1 and 2). In this case, a precision electrode punch or the like is used as a cutting tool. However, if the cross section of the positive electrode 100 is crushed during cutting, making it difficult to observe the cross section of the positive electrode 100 in a subsequent process, one or more of processing methods such as a focused ion beam (FIB) method and a cross-section polisher (CP) method may be used.

[0061] Next, a cross section of the positive electrode 100 is observed using an electron microscope to obtain an observation result (electron microscope photograph) of the cross section. The observation magnification can be set arbitrarily as long as it allows the plurality of fibrous materials 120 to be visually recognized and identified. The type of electron microscope is not particularly limited, but specifically, it may be one or more of a scanning electron microscope, a transmission electron microscope, or the like.

[0062] Next, based on the electron microscope photograph, one fibrous material 120 is selected as the object of calculation of the flexibility F. In this case, any one fibrous material 120 is selected from among a plurality of fibrous materials 120 whose front end 121M and rear end 121N are both visible in the electron microscope.

[0063] That is, a plurality of fibrous materials 120 can be seen in the electron microscope, and the plurality of fibrous materials 120 are separated into four types, which will be described below.

[0064] The first type of fibrous material 120 is a fibrous material 120 in which both the front end 121M and the rear end 121N are present in the electron microscope, and therefore both the front end 121M and the rear end 121N are visible. The second type of fibrous material 120 is a fibrous material 120 in which both the front end 121M and the rear end 121N are not present in the electron microscope, and therefore both the front end 121M and the rear end 121N are not visible. The third type of fibrous material 120 is a fibrous material 120 in which the front end 121M is present but the rear end 121N is not present in the electron microscope, and therefore only the front end 121M is visible. The fourth type of fibrous material 120 is a fibrous material 120 in which the rear end 121N is present but the front end 121M is not present in the electron microscope, and therefore only the rear end 121N is visible.

[0065] That is, the first type of fibrous material 120 is a fibrous material 120 whose entirety can be seen in an electron microscope. In contrast, the second to fourth types of fibrous materials 120 are fibrous materials 120 whose entirety cannot be seen in an electron microscope, but whose only parts can be seen in an electron microscope.

[0066] When selecting one fibrous material 120 to calculate the softness F, only the first type of fibrous material 120 is selected, without selecting the second to fourth types of fibrous material 120.

[0067] The reason for selecting only the first type of fibrous material 120 whose front end 121M and rear end 121N are visible is that if only that first type of fibrous material 120 is not selected, the reproducibility of counting the number of each of the front end 121M and rear end 121N will decrease, resulting in variations in the value of the sum T shown in equation (1).

[0068] Therefore, in order to ensure the accuracy of the sum T and thereby the calculation accuracy of the flexibility F, it is necessary to select only the first type of fibrous material 120, without selecting the second to fourth types of fibrous material 120.

[0069] After selecting one fibrous material 120 for which the flexibility F is to be calculated, a series of parameters described below are examined based on an electron microscope photograph.

[0070] First, by focusing on the carbon fiber portion 121, the dimension of the carbon fiber portion 121 in the extension direction Z, i.e., the distance (length L1) from the front end 121E1 to the rear end 121E2, is measured. The value of length L1 is rounded to one decimal place.

[0071] Second, by focusing on the multiple exposed portions 121Y, the dimension (length L3) of each of the multiple exposed portions 121Y in the extension direction Z is measured, and then the sum L2 of the multiple lengths L3 is calculated. Note that the values ​​of the sum L2 and the length L3 are rounded to one decimal place.

[0072] Third, by focusing on the plurality of carbon coating portions 122, the number of the front ends 122E1 and the number of the rear ends 122E2 are counted to calculate the sum T of the number of the front ends 122E1 and the number of the rear ends 122E2. As described above, this sum T is the number of carbon coating portions 122 multiplied by 2.

[0073] When measuring the lengths L1 and L3, the lengths L1 and L3 may be measured manually, or the lengths L1 and L3 may be measured mechanically using image processing software or the like.

[0074] Furthermore, when counting the number of each of the front ends 122E1 and rear ends 122E2, the number of each of the front ends 122E1 and rear ends 122E2 may be counted manually, or the number of each of the front ends 122E1 and rear ends 122E2 may be counted mechanically using image processing software or the like.

[0075] The type of image processing software is not particularly limited, but specifically, it is image editing software such as GIMP (v2.10.10). Of course, the image editing software described here may be of a different version.

[0076] Next, the flexibility F is calculated based on the length L1, the sum L2, and the number T using formula (1).

[0077] Next, the procedure for calculating the softness F described above is repeated 10 times. In this case, each time the softness F is calculated, the fibrous material 120 for which the softness F is to be calculated is changed. As a result, the softness F is calculated for each of the 10 fibrous materials 120, and therefore 10 softnesses F are calculated.

[0078] Finally, the average of the 10 flexibility degrees F is calculated to obtain the final flexibility degree F. The reason for using the average as the final flexibility degree F is to ensure the accuracy (reproducibility) of the flexibility degree F by suppressing the variation in the flexibility degree F. In this way, the flexibility degree F is calculated.

[0079] [Diameter] As described above, the carbon coating portion 122 coats the surface of the carbon fiber portion 121, and therefore has a diameter D (nm) as shown in Fig. 4. As is clear from Fig. 4, this diameter D is the dimension of the carbon coating portion 122 in a direction intersecting the extension direction Z (the vertical direction in Fig. 4).

[0080] The diameter D is not particularly limited, but is preferably 75 nm to 250 nm, because the diameter D is optimized, which makes it easier for the fibrous material 120 to bend in the carbon coating portion 122 as well, and makes it easier for the carbon coating portion 122 to hold a sufficient amount of the positive electrode active material 110.

[0081] The procedure for calculating the diameter D is as follows. As in the case of calculating the flexibility F, when a secondary battery is used to calculate the diameter D, the positive electrode 100 is recovered from the secondary battery, and then the diameter D is calculated based on the following procedure.

[0082] First, an observation result (electron microscope photograph) of the cross section of the positive electrode 100 is obtained using the same procedure as when calculating the flexibility F. Next, one carbon coating portion 122 for which the diameter D is to be calculated is selected, and the diameter D of that carbon coating portion 122 is measured. If the diameter D varies depending on the measurement position due to the shape of the carbon coating portion 122, the maximum value of the multiple diameters D measured at multiple measurement positions is selected. In this case, the diameter D may be measured manually, or the diameter D may be measured mechanically using image processing software or the like. Details regarding the image processing software are as described above. The value of the diameter D is rounded to one decimal place.

[0083] Finally, the above-described procedure for calculating the diameter D is repeated 10 times, and the average value of the 10 diameters D is calculated to obtain the final diameter D. In this case, each time the diameter D is calculated, the carbon coating portion 122 for which the diameter D is to be calculated is changed. The reason for using the average value as the final diameter D is to ensure the accuracy (reproducibility) of the diameter D by suppressing the variation in the diameter D. In this way, the diameter D is calculated.

[0084] [Other Constituent Elements] The positive electrode active material layer 100B may further contain one or more of other materials such as a positive electrode conductive agent and a positive electrode binder.

[0085] The positive electrode conductive agent contains one or more conductive materials such as a carbon material, a metal material, and a conductive polymer compound. Specific examples of the carbon material include activated carbon, graphite, carbon black, acetylene black, and ketjen black.

[0086] The positive electrode binder contains one or more of materials such as synthetic rubber and polymer compounds. Specific examples of synthetic rubber include styrene-butadiene rubber, fluorine-containing rubber, and ethylene-propylene-diene. Specific examples of polymer compounds include polyvinylidene fluoride, polyimide, and carboxymethyl cellulose.

[0087] <1-2. Operation> In this positive electrode 100, during the electrode reaction, lithium is absorbed in an ionic state into the positive electrode active material 110, and lithium is released in an ionic state from the positive electrode active material 110.

[0088] <1-3. Manufacturing method> The positive electrode 100 is manufactured by the following example procedure. In the following, a case will be described in which the carbon coating portion 122 has a plurality of pores 122K, and the positive electrode active material 110 is inserted into the plurality of pores 122K, as shown in FIG.

[0089] [Manufacturing Procedure] When manufacturing the positive electrode 100, first, a solution containing the forming materials (plurality of fibrous carbon materials) of the carbon fiber portion 121 and a solution containing the forming materials (powdered carbon source) of the plurality of carbon coating portions 122 are mixed together to obtain a mixed solution. In this case, a dispersant may be added to the mixed solution to improve the dispersibility of each of the plurality of fibrous carbon materials and the powdered carbon source. The type of dispersant is not particularly limited, but specifically, it is a polymer compound such as carboxymethyl cellulose.

[0090] Next, the mixed solution is poured into a pressure vessel, and hydrothermal treatment is performed using the mixed solution. The type of pressure vessel is not particularly limited, but specifically, it is an autoclave or the like. Note that conditions such as the heating temperature and heating time in the hydrothermal treatment can be set as desired. Next, the mixed solution after the hydrothermal treatment is filtered to recover a powdery reaction product, which is then dried.

[0091] Subsequently, the reactants and zinc chloride (ZnCl 2 ) are mixed with each other to obtain a mixture, and then the mixture is heated in an inert atmosphere. The type of inert atmosphere is not particularly limited, but a specific example is a nitrogen atmosphere. Note that the heating conditions, such as the heating temperature and heating time, can be set as desired.

[0092] As a result, a carbon coating portion 122 (activated carbon) is formed on the surface of the carbon fiber portion 121 using the carbon source, and the surface of the carbon fiber portion 121 is coated with the carbon coating portion 122. In this case, a plurality of pores 122K are formed in the carbon coating portion 122. Thus, a precursor for forming a plurality of fibrous materials 120 is obtained.

[0093] This precursor has the same configuration as the plurality of fibrous materials 120, except for the points described below. That is, the carbon coating portion 122 covers the entire surface of the carbon fiber portion 121, so the carbon fiber portion 121 does not have a plurality of exposed portions 121Y. In addition, the carbon coating portion 122 does not hold the positive electrode active material 110.

[0094] Next, the precursor is placed in an aqueous acid solution, and then ultrasonic waves are applied to the aqueous acid solution. The type of aqueous acid solution is not particularly limited, but a specific example is an aqueous hydrochloric acid solution. The conditions for ultrasonic irradiation, such as the irradiation intensity and irradiation time, can be set as desired. This removes zinc-based residues, such as zinc oxide, from the precursor.

[0095] In this ultrasonic irradiation process, portions of the carbon coating 122 that coats the surface of the carbon fiber portion 121 fall off in multiple locations, exposing the carbon fiber portion 121 at the locations where the carbon coating 122 has fallen off. As a result, multiple carbon coating portions 122 are formed that are spaced apart from each other in the extension direction Z, and the carbon fiber portion 121 has multiple non-exposed portions 121X and multiple exposed portions 121Y. Thus, multiple fibrous materials 120 are formed that include the carbon fiber portion 121 and the multiple carbon coating portions 122.

[0096] Next, the hydrochloric acid aqueous solution is filtered to recover the plurality of fibrous materials 120, and the plurality of fibrous materials 120 are then washed using a cleaning solvent. The type of cleaning solvent is not particularly limited, but specific examples include aqueous solvents such as pure water and alcohol. A specific example of alcohol is ethanol. In this case, the plurality of fibrous materials 120 may be ultrasonically cleaned, or the cleaning process for the plurality of fibrous materials 120 may be repeated multiple times. When performing multiple cleaning processes, the type of cleaning solvent and whether or not ultrasonic cleaning is performed may be changed for each cleaning process. Note that cleaning conditions, such as the cleaning time, can be set as desired. After the plurality of fibrous materials 120 have been washed, the plurality of fibrous materials 120 are dried.

[0097] Next, the plurality of fibrous materials 120 and the positive electrode active material 110 (powdered sulfur-containing material) are mixed to obtain a mixture, and the mixture is then heated. The heating conditions, such as the heating temperature and heating time, can be set as desired. As a result, the positive electrode active material 110 is inserted into the plurality of pores 122K, and the positive electrode active material 110 is held by the carbon coating portion 122. Thus, a plurality of fibrous materials 120 (carbon fiber portion 121 and a plurality of carbon coating portions 122) that hold the positive electrode active material 110 are formed.

[0098] Next, the plurality of fibrous materials 120 holding the positive electrode active materials 110, the positive electrode binder, and the positive electrode conductive agent are mixed together to form a positive electrode mixture. Next, the positive electrode mixture is poured into a solvent to prepare a paste-like positive electrode mixture slurry. This solvent may be an aqueous solvent or an organic solvent. Next, the positive electrode mixture slurry is applied to both surfaces of the positive electrode current collector 100A to form the positive electrode active material layer 100B. Finally, the positive electrode active material layer 100B may be compression-molded using a roll press or the like. In this case, the positive electrode active material layer 100B may be heated, or the compression molding may be repeated multiple times.

[0099] As a result, the positive electrode active material layer 100B is formed on the positive electrode current collector 100A, and thus the positive electrode 100 is completed.

[0100] [Adjustment Procedure] When manufacturing the positive electrode 100, the flexibility D and the diameter D can each be adjusted by the procedure described below.

[0101] To adjust the flexibility D, conditions such as the ultrasonic irradiation intensity and irradiation time are changed in the process of irradiating the precursor-containing acid aqueous solution with ultrasonic waves. Specifically, while the length L1 is constant, the above-mentioned conditions such as the ultrasonic irradiation intensity and irradiation time are changed. This changes the number of exposed portions 121Y, the length L3, the number of front ends 122E1, and the number of rear ends 122E2, and therefore the flexibility F. In this case, increasing the irradiation intensity and lengthening the irradiation time increases the flexibility F, while decreasing the irradiation intensity and shortening the irradiation time decreases the flexibility F.

[0102] To adjust the diameter D, in the process of obtaining the mixed solution, the mixing ratio of a solution containing the forming materials (plurality of fibrous carbon materials) of the carbon fiber portion 121 and a solution containing the forming materials (powdered carbon source) of the carbon coating portion 122 is changed. Specifically, the mixing amount of the latter solution is kept constant, and the mixing amount of the former solution is changed. This changes the amount of carbon coating portion 122 formed on the surface of the carbon fiber portion 121, and therefore the diameter D. In this case, increasing the mixing amount of the former solution increases the diameter D, and decreasing the concentration of the former solution decreases the diameter D.

[0103] <1-4. Actions and Effects> According to this positive electrode 100, the positive electrode 100 includes a positive electrode active material 110 and a plurality of fibrous materials 120. The positive electrode active material 110 includes a sulfur-containing material, and the plurality of fibrous materials 120 are entangled with each other to form a three-dimensional network structure. The fibrous material 120 includes a carbon fiber portion 121 and a plurality of carbon coating portions 122. The plurality of carbon coating portions 122 coat the surface of the carbon fiber portion 121 and hold the positive electrode active material 110. The plurality of carbon coating portions 122 are spaced apart from each other in the extension direction Z. The carbon fiber portion 121 has a plurality of exposed portions 121Y that are not coated by the plurality of carbon coating portions 122. The carbon coating portions 122 have a front end 122E1 and a rear end 122E2. Furthermore, the flexibility F of the fibrous material 120 is 8.6 to 19.2.

[0104] In this case, as described above, the flexibility of the plurality of fibrous materials 120 is optimized. This maintains the three-dimensional conductive network formed using the carbon fiber portion 121, and ensures the lithium absorption / desorption properties of each of the plurality of carbon coating portions 122 (the positive electrode active material 110). This increases the density of the positive electrode active material layer 100B, thereby increasing the electrode density of the positive electrode 100. Therefore, excellent battery characteristics can be obtained in a secondary battery using the positive electrode 100, and a secondary battery having such excellent battery characteristics can be realized.

[0105] In particular, if the carbon fiber portion 121 has a front end portion 121M and a rear end portion 121N, and one or both of the front end portion 121M and the rear end portion 121N are exposed portions 121Y, the entire fibrous material 120 is more likely to bend. Therefore, the electrode density of the positive electrode 100 is further increased, and a greater effect can be obtained.

[0106] Furthermore, if the diameter D of the carbon coating portion 122 is 75 nm to 250 nm, the fibrous material 120 can be easily curved in the carbon coating portion 122, and the carbon coating portion 122 can easily hold a sufficient amount of the positive electrode active material 110. Therefore, the electrode density of the positive electrode 100 is sufficiently increased while the energy density is ensured, and thus a greater effect can be obtained.

[0107] Furthermore, if the carbon coating portion 122 has a plurality of pores 122K and the positive electrode active material 110 (sulfur-containing material) is inserted into the plurality of pores 122K, the positive electrode active material 110 is more easily held by the carbon coating portion 122, and the positive electrode active material 110 and the carbon coating portion 122 are more easily connected to each other. Therefore, lithium is more easily absorbed and released stably and smoothly in the positive electrode active material layer 100B, and a greater effect can be obtained.

[0108] Furthermore, if the carbon fiber portion 121 contains one or both of carbon nanotubes and carbon nanofibers, a three-dimensional conductive network is likely to be stably formed using the plurality of carbon fiber portions 121. Furthermore, if the carbon coating portion 122 contains activated carbon, the plurality of pores 122K is likely to be stably formed in the carbon coating portion 122, and the positive electrode active material 110 is likely to be stably inserted into the plurality of pores 122K. Therefore, a greater effect can be obtained.

[0109] 2. Secondary Battery Next, a secondary battery according to an embodiment of the present technology to which the positive electrode 100 is applied will be described.

[0110] This secondary battery is a secondary battery that obtains battery capacity by utilizing the absorption and desorption of an electrode reactant, and includes a positive electrode, a negative electrode, and an electrolyte. In the following, as described above, the case where the electrode reactant is lithium will be taken as an example.

[0111] <2-1. Configuration> Fig. 5 shows a perspective configuration of a secondary battery, and Fig. 6 shows a cross-sectional configuration of the battery element 20 shown in Fig. 5. However, Fig. 5 shows a state in which the exterior film 10 and the battery element 20 are separated from each other, and the cross section of the battery element 20 along the XZ plane is shown by a dashed line.

[0112] As shown in FIGS. 5 and 6, this secondary battery includes an exterior film 10, a battery element 20, a positive electrode lead 31, a negative electrode lead 32, and sealing films 41 and .

[0113] The secondary battery described here is a so-called laminate film type secondary battery, since it uses the flexible or pliable exterior film 10 as an exterior member for housing the battery element 20, as described above.

[0114] 5, the exterior film 10 has a bag-like structure that is sealed when the battery element 20 is housed therein. As a result, the exterior film 10 houses a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte solution, which will be described later.

[0115] Here, the exterior film 10 is a single film-like member that is folded in a folding direction F. The exterior film 10 is provided with a recessed portion 10U (a so-called deep drawn portion) for accommodating the battery element 20.

[0116] Specifically, the exterior film 10 is a three-layer laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order from the inside out, and when the exterior film 10 is folded, the outer peripheral edges of the opposing fusion layers are fused to each other. The fusion layer contains a polymer compound such as polypropylene. The metal layer contains a metal material such as aluminum. The surface protection layer contains a polymer compound such as nylon.

[0117] However, the configuration (number of layers) of the exterior film 10 is not particularly limited, and may be one layer, two layers, or four or more layers.

[0118] [Battery Element] The battery element 20 is housed in an exterior film 10. This battery element 20 is a so-called power generation element, and as shown in Figures 5 and 6, includes a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte (not shown).

[0119] Here, battery element 20 is a so-called wound electrode body. That is, positive electrode 21 and negative electrode 22 are wound around winding axis P while facing each other with separator 23 interposed therebetween. As is clear from FIG. 5 , winding axis P is a virtual axis extending in the Y-axis direction.

[0120] There are no particular limitations on the three-dimensional shape of battery element 20. Here, battery element 20 has a flat three-dimensional shape, and therefore the shape of a cross section (cross section along the XZ plane) of battery element 20 intersecting winding axis P is a flat shape defined by major axis J1 and minor axis J2.

[0121] The major axis J1 is an imaginary axis extending in the X-axis direction and has a length greater than that of the minor axis J2. The minor axis J2 is an imaginary axis extending in the Z-axis direction intersecting the X-axis direction and has a length less than that of the major axis J1. Here, the three-dimensional shape of the battery element 20 is a flattened cylinder, and therefore the cross-sectional shape of the battery element 20 is a flattened, approximately elliptical shape.

[0122] (Positive Electrode) The positive electrode 21 has a configuration similar to that of the positive electrode 100. That is, as shown in Fig. 6, the positive electrode 21 includes a positive electrode current collector 21A and a positive electrode active material layer 21B. The configurations of the positive electrode current collector 21A and the positive electrode active material layer 21B are similar to the configurations of the positive electrode current collector 100A and the positive electrode active material layer 100B, respectively.

[0123] Here, the positive electrode active material layer 21B is provided on both sides of the positive electrode current collector 21A, but the positive electrode active material layer 21B may be provided on only one side of the positive electrode current collector 21A.

[0124] (Negative Electrode) As shown in FIG. 6, the negative electrode 22 contains a negative electrode active material. This negative electrode active material contains an alkali metal material, which is a material containing one or more types of alkali metal elements as constituent elements. However, the type of alkali metal element may be only one type or two or more types. Furthermore, the alkali metal material may be a simple substance, an alloy, a compound, or two or more types thereof.

[0125] Here, the alkali metal material contains an alkali metal (a so-called elemental alkali metal). This is because sufficient battery capacity can be obtained. The meaning of the elemental alkali metal described here is as described above. That is, the elemental alkali metal may contain any amount of impurities, and therefore the purity of the elemental alkali metal is not necessarily limited to 100%.

[0126] The type of alkali metal is not particularly limited, but specific examples include lithium, sodium, and potassium.

[0127] Among these, as described above, lithium is preferable as the alkali metal, because this further improves the Coulomb efficiency. A secondary battery in which the positive electrode 21 contains a positive electrode active material (sulfur-containing material) and the negative electrode 22 contains a negative electrode active material (lithium) is a so-called lithium-sulfur secondary battery. In this case, since the negative electrode 22 is a lithium metal plate, the negative electrode 22 may contain lithium metal as the negative electrode active material (alkali metal).

[0128] 6, the separator 23 is an insulating porous film interposed between the positive electrode 21 and the negative electrode 22, and allows lithium to pass through in an ionic state while preventing the occurrence of a short circuit due to contact between the positive electrode 21 and the negative electrode 22. The separator 23 contains a polymer compound such as polyethylene.

[0129] (Electrolyte) The electrolyte is a liquid electrolyte, and is impregnated into each of the positive electrode 21 and the separator 23. The electrolyte contains a solvent and an electrolyte salt.

[0130] The solvent contains one or more types of non-aqueous solvents (organic solvents), and the electrolyte containing the non-aqueous solvent is a so-called non-aqueous electrolyte.

[0131] The non-aqueous solvent is an ester, an ether, or the like, and more specifically, is one or more of a carbonate ester compound, a carboxylic acid ester compound, and a lactone compound, because the dissociation of the electrolyte salt and the mobility of ions are improved.

[0132] The carbonate ester compounds include cyclic carbonate esters and chain carbonate esters. Specific examples of the cyclic carbonate esters include ethylene carbonate and propylene carbonate, and specific examples of the chain carbonate esters include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0133] The carboxylic acid ester compound is a chain carboxylic acid ester, etc. Specific examples of the chain carboxylic acid ester include ethyl acetate, ethyl propionate, propyl propionate, and ethyl trimethylacetate.

[0134] The lactone compound is lactone, etc. Specific examples of lactone include γ-butyrolactone and γ-valerolactone.

[0135] The ethers may be 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, or the like.

[0136] The nonaqueous solvent is one or more of unsaturated cyclic carbonates, fluorinated cyclic carbonates, sulfonates, phosphates, acid anhydrides, nitrile compounds, isocyanate compounds, etc. This is because the dissociation of the electrolyte salt and the mobility of ions are similarly improved.

[0137] Specific examples of unsaturated cyclic carbonates include vinylene carbonate, vinylethylene carbonate, and methyleneethylene carbonate. Specific examples of fluorinated cyclic carbonates include monofluoroethylene carbonate and difluoroethylene carbonate. Specific examples of sulfonic acid esters include propane sultone and propene sultone. Specific examples of phosphate esters include trimethyl phosphate and triethyl phosphate. Specific examples of acid anhydrides include succinic anhydride, 1,2-ethanedisulfonic anhydride, and 2-sulfobenzoic anhydride. Specific examples of nitrile compounds include succinonitrile. Specific examples of isocyanate compounds include hexamethylene diisocyanate.

[0138] Of course, the composition of the non-aqueous solvent is not particularly limited as long as it contains one or more of the above-mentioned series of non-aqueous solvent candidates, and can be set arbitrarily.

[0139] The electrolyte salt contains one or more types of light metal salts such as lithium salts.

[0140] A specific example of the lithium salt is lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium trifluoromethanesulfonate (LiCF 3 SO 3 ), lithium bis(fluorosulfonyl)imide (LiN(FSO 2 ) 2 ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF 3 SO 2 ) 2 ), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF 3 SO 2 ) 3 ), lithium bis(oxalato)borate (LiB(C 2 O 4 ) 2 ), lithium monofluorophosphate (Li 2 PFO 3 ) and lithium difluorophosphate (LiPF 2 O 2 ) etc. This is because a high battery capacity can be obtained.

[0141] The content of the electrolyte salt is not particularly limited, but specifically, it is 0.3 mol / kg to 3.0 mol / kg relative to the solvent, because high ionic conductivity can be obtained.

[0142] 5 and 6, the positive electrode lead 31 is a positive electrode wiring connected to the positive electrode current collector 21A of the positive electrode 21, and is led out of the exterior film 10. The positive electrode lead 31 contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum. The positive electrode lead 31 has a shape such as a thin plate or a mesh.

[0143] [Negative Electrode Lead] As shown in Figures 5 and 6, the negative electrode lead 32 is a negative electrode wiring connected to the negative electrode 22 and is led out of the exterior film 10. Here, the lead-out direction of the negative electrode lead 32 is the same as the lead-out direction of the positive electrode lead 31. The negative electrode lead 32 contains a conductive material such as a metal material, and a specific example of the conductive material is copper. Note that the details regarding the shape of the negative electrode lead 32 are the same as the details regarding the shape of the positive electrode lead 31.

[0144] [Sealing Film] The sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and the sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32. However, one or both of the sealing films 41 and 42 may be omitted.

[0145] The sealing film 41 is a sealing member that prevents outside air and the like from entering the inside of the exterior film 10. This sealing film 41 contains a polymer compound such as polyolefin that has adhesiveness to the positive electrode lead 31, and a specific example of the polymer compound is polypropylene.

[0146] The configuration of the sealing film 42 is the same as the configuration of the sealing film 41, except that the sealing film 42 is a sealing member that has adhesiveness to the negative electrode lead 32. That is, the sealing film 42 contains a polymer compound such as polyolefin that has adhesiveness to the negative electrode lead 32.

[0147] <2-2. Operation> The secondary battery operates in the battery element 20 as follows.

[0148] During discharge, lithium is released from the negative electrode 22 and is absorbed into the positive electrode 21 via the electrolyte. On the other hand, during charge, lithium is released from the positive electrode 21 and is absorbed into the negative electrode 22 via the electrolyte. During both discharge and charge, lithium is absorbed and released in an ionic state.

[0149] <2-3. Manufacturing Method> When manufacturing a secondary battery, the positive electrode 21 is fabricated and an electrolytic solution is prepared according to the procedure described below as an example. Thereafter, the positive electrode 21, the negative electrode 22, and the electrolytic solution are used to assemble a secondary battery, and a stabilization process is performed on the assembled secondary battery.

[0150] [Fabrication of Positive Electrode] First, the positive electrode 21 is fabricated by the same procedure as that for fabricating the positive electrode 100. In this case, the positive electrode active material layer 21B is formed on the positive electrode current collector 21A.

[0151] [Preparation of Electrolyte Solution] An electrolyte salt is added to a solvent, whereby the electrolyte salt is dispersed or dissolved in the solvent, thereby preparing an electrolyte solution.

[0152] [Assembly of Secondary Battery] First, an alkali metal-containing anode 22 is prepared. In this case, a lithium metal plate or the like is used as the anode 22. Next, a cathode lead 31 is connected to the cathode current collector 21A of the cathode 21 using a joining method such as welding, and an anode lead 32 is connected to the anode 22 using a joining method such as welding.

[0153] Next, the positive electrode 21 and the negative electrode 22 are stacked one on top of the other with the separator 23 interposed therebetween, and then the positive electrode 21, the negative electrode 22, and the separator 23 are wound together to produce a wound body (not shown). Next, the wound body is pressed using a press or the like to form a flat shape. The wound body after this formation has a configuration similar to that of the battery element 20, except that the positive electrode 21, the negative electrode 22, and the separator 23 are not impregnated with an electrolyte solution.

[0154] Next, after the roll is housed inside the recess 10U, the exterior film 10 (fusion layer / metal layer / surface protection layer) is folded to face each other. Next, the outer peripheral edges of two sides of the facing fusion layers are joined together using an adhesive method such as heat fusion, thereby housing the roll inside the bag-shaped exterior film 10.

[0155] Finally, after injecting the electrolyte solution into the bag-shaped exterior film 10, the outer peripheral edges of the remaining sides of the opposing fusion layers are joined together using an adhesive method such as heat fusion. In this case, a sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and a sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32.

[0156] As a result, the wound body is impregnated with the electrolyte, forming the wound electrode body, which is the battery element 20. The battery element 20 is then sealed inside the bag-shaped exterior film 10, and the secondary battery is assembled.

[0157] [Stabilization of Secondary Battery] The assembled secondary battery is charged and discharged. Various conditions, such as the ambient temperature, the number of charge / discharge cycles (number of cycles), and the charge / discharge conditions, can be set as desired. This electrochemically stabilizes the states of the positive electrode 21 and the negative electrode 22, thereby completing the secondary battery.

[0158] <2-4. Actions and Effects> In this secondary battery, the positive electrode active material layer 21B of the positive electrode 21 has a configuration similar to that of the positive electrode 100. Therefore, for the reasons described above, the three-dimensional conductive network is maintained in the positive electrode 21, and lithium absorption / desorption properties are ensured, while the density of the positive electrode active material layer 21B increases, thereby increasing the electrode density of the positive electrode 21. Therefore, a secondary battery with excellent battery characteristics can be realized.

[0159] In particular, if the negative electrode 22 contains an alkali metal, the absorption and release of lithium can be utilized to stably obtain sufficient battery capacity, thereby achieving a higher effect. In this case, if the alkali metal contains lithium, the coulomb efficiency is further improved, thereby achieving an even higher effect.

[0160] Furthermore, if the secondary battery is a lithium-sulfur secondary battery, a sufficient battery capacity can be stably obtained by utilizing the absorption and desorption of lithium, and therefore a greater effect can be obtained.

[0161] 3. Modifications The configuration of the secondary battery can be modified as appropriate, as described below. However, the series of modifications described below may be combined with each other.

[0162] 1 , the positive electrode 100 includes a positive electrode current collector 100A and a positive electrode active material layer 100B. However, the positive electrode 100 may include only the positive electrode active material layer 100B without including the positive electrode current collector 100A. Even in this case, as described above, the three-dimensional conductive network is maintained and lithium absorption / desorption properties are ensured, while the density of the positive electrode active material layer 100B increases, thereby increasing the electrode density of the positive electrode 100. Therefore, a similar effect can be obtained.

[0163] 6, a porous film separator 23 is used. However, although not specifically shown here, a laminated separator including a polymer compound layer may be used instead of the porous film separator 23.

[0164] Specifically, the laminated separator includes a porous membrane having a pair of surfaces and a polymer compound layer provided on one or both surfaces of the porous membrane. This is because the separator's adhesion to the positive electrode 21 and the negative electrode 22 is improved, thereby suppressing misalignment of the battery element 20, i.e., miswinding of the positive electrode 21, the negative electrode 22, and the separator. This suppresses swelling of the secondary battery even if a decomposition reaction of the electrolyte occurs. The polymer compound layer includes a polymer compound such as polyvinylidene fluoride. This is because polymer compounds such as polyvinylidene fluoride have excellent physical strength and are electrochemically stable.

[0165] One or both of the porous film and the polymer compound layer may contain one or more types of insulating particles. This is because the insulating particles dissipate heat when the secondary battery generates heat, improving the safety (heat resistance) of the secondary battery. The insulating particles include inorganic particles and resin particles. The inorganic particles include one or more types of inorganic materials such as aluminum oxide, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide, and zirconium oxide. The resin particles include one or more types of resin materials such as acrylic resin and styrene resin.

[0166] When a laminated separator is produced, a precursor solution containing a polymer compound and an organic solvent is prepared, and then the precursor solution is applied to one or both sides of a porous film. In this case, the precursor solution may contain a plurality of insulating particles.

[0167] Even when this laminated separator is used, the same effect can be obtained because lithium can move in an ionic state between the positive electrode 21 and the negative electrode 22. In this case, as described above, in particular, the displacement of the battery element 20 is suppressed, thereby further suppressing swelling of the secondary battery, thereby achieving a greater effect.

[0168] [Modification 3] An electrolytic solution, which is a liquid electrolyte, is used in Fig. 6. However, although not specifically shown here, an electrolyte layer, which is a gel electrolyte, may be used instead of the electrolytic solution.

[0169] In the battery element 20 using the electrolyte layer, the positive electrode 21 and the negative electrode 22 are stacked with the separator 23 and the electrolyte layer interposed therebetween, and the positive electrode 21, the negative electrode 22, the separator 23, and the electrolyte layer are wound together. The electrolyte layer is interposed between the positive electrode 21 and the separator 23, and also between the negative electrode 22 and the separator 23.

[0170] Specifically, the electrolyte layer contains a polymer compound together with an electrolytic solution, and the electrolytic solution is held by the polymer compound. This is because leakage of the electrolytic solution is suppressed. The composition of the electrolytic solution is as described above. The polymer compound contains polyvinylidene fluoride, etc. When forming the electrolyte layer, a precursor solution containing an electrolytic solution, a polymer compound, an organic solvent, etc. is prepared, and then the precursor solution is applied to one or both surfaces of each of the positive electrode 21 and the negative electrode 22.

[0171] Even when this electrolyte layer is used, the same effect can be obtained because lithium can move in an ionic state through the electrolyte layer between the positive electrode 21 and the negative electrode 22. In this case, leakage of the electrolyte solution is particularly suppressed as described above, and therefore a greater effect can be obtained.

[0172] 4. Uses of Secondary Batteries Finally, uses (application examples) of secondary batteries will be described.

[0173] The use of the secondary battery is not particularly limited. The secondary battery used as a power source may be a main power source or an auxiliary power source in electronic devices, electric vehicles, etc. The main power source is a power source that is used preferentially regardless of the presence or absence of other power sources. The auxiliary power source may be a power source used in place of the main power source or a power source that can be switched from the main power source.

[0174] Specific examples of uses for secondary batteries are as follows: Electronic devices such as video cameras, digital still cameras, mobile phones, laptop computers, headphone stereos, portable radios, and portable information terminals; Storage devices such as backup power supplies and memory cards; Power tools such as power drills and power saws; Battery packs installed in electronic devices; Medical electronic devices such as pacemakers and hearing aids; Electric vehicles such as electric cars (including hybrid cars); Power storage systems such as home or industrial battery systems that store power in preparation for emergencies, etc. In these uses, one secondary battery may be used, or multiple secondary batteries may be used.

[0175] The battery pack may include a single cell or a battery pack. The electric vehicle is a vehicle that runs on a secondary battery as a driving power source, and may be a hybrid vehicle that also includes a driving source other than the secondary battery. In a home power storage system, power stored in a secondary battery, which is a power storage source, can be used to power home electrical appliances, etc.

[0176] Here, an example of the use of the secondary battery will be specifically described. The configuration described below is merely an example and can be modified as appropriate.

[0177] 7 shows the block diagram of a battery pack, which is an example of an application of a secondary battery. The battery pack described here is a battery pack (a so-called soft pack) that uses one secondary battery, and is installed in electronic devices such as smartphones.

[0178] 7, the battery pack includes a power supply 51 and a circuit board 52. The circuit board 52 is connected to the power supply 51 and includes a positive terminal 53, a negative terminal 54, and a temperature detection terminal 55.

[0179] The power source 51 includes one secondary battery. The positive electrode lead of this secondary battery is connected to a positive electrode terminal 53, and the negative electrode lead is connected to a negative electrode terminal 54. The power source 51 is connected to the outside via the positive electrode terminal 53 and the negative electrode terminal 54, and is therefore capable of charging and discharging. The circuit board 52 includes a control unit 56, a switch 57, a thermosensitive resistor element (a so-called PTC element) 58, and a temperature detection unit 59. However, the PTC element 58 may be omitted.

[0180] The control unit 56 includes a central processing unit (CPU) and memory, and controls the operation of the entire battery pack. The control unit 56 detects and controls the usage state of the power source 51.

[0181] When the voltage of power supply 51 (secondary battery) reaches the overcharge detection voltage or the overdischarge detection voltage, control unit 56 turns off switch 57 to prevent charging current from flowing through the current path of power supply 51. The overcharge detection voltage is not particularly limited, but specifically, it is 4.20 V±0.05 V, and the overdischarge detection voltage is not particularly limited, but specifically, it is 2.40 V±0.10 V.

[0182] Switch 57 includes a charge control switch, a discharge control switch, a charging diode, a discharging diode, etc., and switches between the connection and disconnection of power supply 51 and an external device in response to instructions from control unit 56. Switch 57 includes a metal oxide semiconductor field effect transistor (MOSFET), etc., and the charge current and the discharge current are detected based on the ON resistance of switch 57.

[0183] Temperature detection unit 59 includes a temperature detection element such as a thermistor. Temperature detection unit 59 measures the temperature of power supply 51 using temperature detection terminal 55 and outputs the temperature measurement result to control unit 56. The temperature measurement result measured by temperature detection unit 59 is used when control unit 56 performs charge / discharge control in the event of abnormal heat generation and when control unit 56 performs correction processing when calculating the remaining capacity.

[0184] An embodiment of the present technology will be described.

[0185] Examples 1 to 10 and Comparative Examples 1 to 4 As will be described below, secondary batteries were fabricated, and then the battery characteristics of the secondary batteries were evaluated.

[0186] [Manufacture of Secondary Battery] A secondary battery was manufactured according to the procedure described below.

[0187] Here, a test secondary battery was fabricated to perform a simple evaluation of the battery characteristics. Fig. 8 shows the cross-sectional structure of the test secondary battery (a coin-type lithium ion secondary battery).

[0188] 8, this secondary battery includes a test electrode 61, a counter electrode 62, a separator 63, an exterior cup 64, an exterior can 65, a gasket 66, and an electrolyte (not shown). Here, the test electrode 61 corresponds to the positive electrode, and the counter electrode 62 corresponds to the negative electrode.

[0189] The test electrode 61 is housed in an exterior cup 64, and the counter electrode 62 is housed in an exterior can 65. The test electrode 61 and the counter electrode 62 are stacked together with a separator 63 interposed therebetween, and the test electrode 61, the counter electrode 62, and the separator 63 are each impregnated with an electrolyte. The exterior cup 64 and the exterior can 65 are crimped together with a gasket 66, so that the test electrode 61, the counter electrode 62, and the separator 63 are sealed by the exterior cup 64 and the exterior can 65.

[0190] (Fabrication of Test Electrode (Positive Electrode)) When fabricating the test electrode 61, first, an aqueous solution (concentration of the fibrous carbon material = 0.2 wt %) containing the material for forming the carbon fiber portion 121 (single-walled carbon nanotubes (SWCNTs) which are a plurality of fibrous carbon materials) and a dispersant (carboxymethyl cellulose) and an aqueous solution (concentration = 1 mol / L (= 1 mol / dm )) containing the material for forming the plurality of carbon coating portions 122 (xylose which is a powdered carbon source) were dissolved in water. 3 In this case, the mixing ratio (weight ratio) of the two types of aqueous solutions was changed in order to change the diameter D (nm) of the carbon coating portion 122 formed in a later step.

[0191] The mixed solution was then placed in a pressure vessel (autoclave) and subjected to hydrothermal treatment (heating temperature: 220°C, heating time: 5 hours). The mixed solution after the hydrothermal treatment was then filtered to recover a powdery reaction product, which was then dried.

[0192] Subsequently, the reactants and zinc chloride (ZnCl 2 ) were mixed with each other to obtain a mixture, and then the mixture was heated in an inert atmosphere (nitrogen atmosphere) (heating temperature = 800°C and heating time = 1 hour). As a result, a carbon coating portion 122 (activated carbon) was formed on the surface of the carbon fiber portion 121 (SWCNT) using the carbon source, and a plurality of pores 122K were formed in the carbon coating portion 122, thereby obtaining a precursor.

[0193] Next, the precursor was placed in an acid aqueous solution (hydrochloric acid aqueous solution), and then ultrasonic waves were applied to the acid aqueous solution. In this case, the ultrasonic irradiation conditions (irradiation intensity and irradiation time) were changed to change the flexibility F of the fibrous material 120 formed in a subsequent process. As a result, a plurality of fibrous materials 120 including a carbon fiber portion 121 and a plurality of carbon coating portions 122 were formed.

[0194] Next, the hydrochloric acid aqueous solution was filtered to recover the plurality of fibrous materials 120. Next, the plurality of fibrous materials 120 were ultrasonically cleaned (cleaning time = 1 hour) using a cleaning solvent (ethanol), and the cleaning solvent was then filtered. Next, the plurality of fibrous materials 120 were ultrasonically cleaned (cleaning time = 1 hour) using a cleaning solvent (pure water), and the cleaning solvent was then filtered. Next, the plurality of fibrous materials 120 after cleaning were dried.

[0195] Next, the plurality of fibrous materials 120 and the positive electrode active material 110 (sulfur element, which is a powdered sulfur-containing material) were mixed to obtain a mixture, and the mixture was then heated (heating temperature = 150°C and heating time = 1 hour). In this case, the mixing ratio (weight ratio) of the plurality of fibrous materials 120 to the positive electrode active material 110 was set to 40:60. As a result, the positive electrode active material 110 was inserted into the plurality of pores 122K, and the positive electrode active material 110 was held by each of the plurality of carbon coating portions 122.

[0196] Thus, a plurality of fibrous materials 120 (carbon fiber portions 121 and carbon coating portions 122) holding the positive electrode active material 110 were formed.

[0197] Next, the plurality of fibrous materials 120 holding the positive electrode active material 110, the positive electrode binder (styrene butadiene rubber), the positive electrode conductive agent (acetylene black), and the thickener (carboxymethyl cellulose) were mixed together to prepare a positive electrode mixture in a mixing ratio (weight ratio) of 68:17:9:6.

[0198] Next, the positive electrode mixture was added to a solvent (pure water, an aqueous solvent), and the solvent was stirred to prepare a paste-like positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to one side of a positive electrode current collector 21A (aluminum foil with a thickness of 21 μm) using a coating device, and the positive electrode mixture slurry was then dried to form a positive electrode active material layer 21B. Finally, the positive electrode current collector 21A on which the positive electrode active material layer 21B was formed was punched out into a disk shape (diameter = 15 mm). This produced a test electrode 61 including the positive electrode current collector 21A and the positive electrode active material layer 21B.

[0199] After the test electrode 61 was completed, the flexibility F and diameter D (nm) were examined, and the results are shown in Table 1. The procedures for adjusting the flexibility F and diameter D, the procedures for calculating the flexibility F, and the procedures for measuring the diameter D are as described above.

[0200] For comparison, a test electrode 61 was fabricated using the same procedure except that the acid aqueous solution containing the precursor was not irradiated with ultrasonic waves. In this case, the flexibility F was 0 because a part of the carbon coating portion 122 was not detached from the carbon fiber portion 121 and therefore the multiple exposed portions 121Y were not formed.

[0201] (Fabrication of Counter Electrode (Negative Electrode)) An alkali metal (lithium metal plate) serving as a negative electrode active material was punched out into a disk shape (diameter = 16 mm), thereby obtaining a counter electrode 62.

[0202] (Preparation of Electrolyte Solution) An electrolyte salt (lithium bis(trifluoromethanesulfonyl)imide) was added to a solvent (vinylene carbonate, an unsaturated cyclic carbonate, and monofluoroethylene carbonate, a fluorinated cyclic carbonate), and the solvent was then stirred. In this case, the mixing ratio (volume ratio) of the solvents was 50:50, and the content of the electrolyte salt relative to the solvent was 1 mol / L (=1 mol / dm 3 ) Thus, the electrolyte solution was prepared.

[0203] (Assembly of Secondary Battery) First, the test electrode 61 was placed in the exterior cup 64, and the counter electrode 62 was placed in the exterior can 65. Next, the test electrode 61 placed in the exterior cup 64 and the counter electrode 62 placed in the exterior can 65 were stacked together with an electrolyte-impregnated separator 63 (Celgard 3501, a dry separator manufactured by Celgard Co., Ltd.) interposed therebetween. Finally, with the test electrode 61 and the counter electrode 62 stacked together with the separator 63 interposed therebetween, the exterior cup 64 and the exterior can 65 were crimped together with a gasket 66. As a result, the test electrode 61 and the counter electrode 62 were sealed in the exterior cup 64 and the exterior can 65, and a secondary battery was assembled.

[0204] (Stabilization of Secondary Battery) The secondary battery was subjected to three cycles of charge and discharge in a room temperature environment (temperature = 25° C.). In this case, during discharge, the battery was discharged at a constant current of 0.05 C until the voltage reached 1.0 V, and during charge, the battery was charged at a constant current of 0.05 C until the voltage reached 3.0 V. Note that 0.05 C is a current value at which the battery capacity (theoretical capacity) is fully discharged in 20 hours.

[0205] As a result, the test electrode 61 and the counter electrode 62 were electrochemically stabilized, and the secondary battery was completed.

[0206] [Evaluation of Battery Characteristics] When the electrode characteristics and discharge characteristics were evaluated as the battery characteristics, the results shown in Table 1 were obtained.

[0207] (Electrode Characteristics) First, the secondary battery was disassembled to recover the test electrode 61, and then the weight (g) and thickness (cm) of the test electrode 61 were measured. Next, the positive electrode current collector 21A was peeled off from the positive electrode active material layer 21B using a spatula, and then the weight (g) and thickness (cm) of the positive electrode current collector 21A were measured. Next, the weight (g) and volume (cc (=cm)) of the positive electrode active material layer 21B were measured. 3 ) was calculated. When measuring the weight of the positive electrode active material layer 21B, the weight of the positive electrode current collector 21A was subtracted from the weight of the test electrode 61. When measuring the volume of the positive electrode active material layer 21B, the thickness (cm) of the positive electrode active material layer 21B was calculated by subtracting the thickness of the positive electrode current collector 21A from the thickness of the test electrode 61, and then the volume was calculated based on the diameter (cm) and thickness of the positive electrode active material layer 21B. This gave the electrode density (g / cm 3 The electrode density, which is an index for evaluating the electrode characteristics, was calculated based on the following calculation formula: % (weight of positive electrode active material layer 21B) / volume of positive electrode active material layer 21B.

[0208] (Discharge Characteristics) First, the secondary battery was charged and discharged three times at room temperature (temperature = 25°C) to measure the discharge capacity (mAh) at the third cycle. The charge and discharge conditions were the same as those used for stabilizing the secondary battery.

[0209] Subsequently, the positive electrode active material 110 was recovered from the secondary battery by the procedure described below, and then the weight (g) of the positive electrode active material 110 was measured.

[0210] In this case, first, the secondary battery was disassembled inside a glove box (in an inert atmosphere) to recover the test electrode 61. Next, the test electrode 61 was immersed in a solvent (dimethyl carbonate, an organic solvent) to clean the test electrode 61. Next, the test electrode 61 was thoroughly dried inside the glove box, and then the weight (g) of the test electrode 61 was measured.

[0211] Next, the positive electrode current collector 21A was peeled off from the positive electrode active material layer 21B using a spatula, and the weight (g) of the positive electrode current collector 21A was measured. When measuring the weight of the positive electrode current collector 21A, the positive electrode current collector 21A was immersed in a beaker containing a large amount of water and then ultrasonically cleaned. This removed residue adhering to the surface of the positive electrode current collector 21A, and the weight of the positive electrode current collector 21A was then measured.

[0212] Next, the positive electrode active material layer 21B was crushed using a mortar to obtain a powdery crushed product, and the crushed product was analyzed using energy dispersive X-ray analysis (EDX) to measure the sulfur content (mixing ratio) in the crushed product.

[0213] As a result, the weight of the positive electrode active material layer 21B was calculated by subtracting the weight of the positive electrode current collector 21A from the weight of the test electrode 61, and then the weight of the positive electrode active material layer 21B was multiplied by the mixing ratio of sulfur to calculate the weight of the positive electrode active material 110.

[0214] After measuring the weight of the positive electrode active material 110, the gravimetric discharge capacity, which is an index for evaluating discharge characteristics, was finally calculated based on the formula: gravimetric discharge capacity (mAh / g) = discharge capacity at the third cycle / weight of the positive electrode active material 110. This gravimetric discharge capacity is the discharge capacity per unit weight of the positive electrode active material 110.

[0215]

[0216] [Discussion] As shown in Table 1, the electrode density and the gravimetric discharge capacity each varied greatly depending on the configuration of the test electrode 61.

[0217] Specifically, when the flexibility F is within the appropriate range (flexibility F = 8.6 to 19.2) (Examples 1 to 4), the flexibility F is 0.40 g / cm 3 , unlike when the flexibility F is outside the appropriate range (Comparative Examples 1 to 4). 3 As a result, a high electrode density of 1475 mAh / g or more was obtained, and a high gravimetric discharge capacity of 1475 mAh / g or more was also obtained.

[0218] In particular, when the flexibility F was within an appropriate range (Examples 3, 5 to 10), and the diameter D was within an appropriate range (diameter D = 75 nm to 250 nm) (Examples 3, 6 to 9), a higher electrode density and a higher weight discharge capacity were obtained.

[0219] [Summary] From the results shown in Table 1, when the flexibility F was 8.6 to 19.2 in the secondary battery using the positive electrode 100, the electrode density increased and the weight discharge capacity also increased. Therefore, both the electrode characteristics and discharge characteristics were improved, and therefore, excellent battery characteristics could be obtained in the secondary battery.

[0220] The present technology has been described above with reference to an embodiment and examples. However, the configuration of the present technology is not limited to the configuration described in the embodiment and examples, and can be modified in various ways.

[0221] Specifically, the battery structure of the secondary battery has been described as being a laminate film type and a coin type. However, the battery structure of the secondary battery is not particularly limited, and may be a cylindrical type, a square type, a button type, or the like.

[0222] The battery element has been described as having a wound structure. However, the structure of the battery element is not particularly limited, and may be a stacked structure or a zigzag structure. In the stacked structure, the positive and negative electrodes are stacked on top of each other, and in the zigzag structure, the positive and negative electrodes are folded in a zigzag pattern.

[0223] Furthermore, although the electrode reactant is lithium in the above description, the electrode reactant is not particularly limited. Specifically, as described above, the electrode reactant may be other alkali metals such as sodium and potassium, or alkaline earth metals such as beryllium, magnesium, and calcium. In addition, the electrode reactant may be other light metals such as aluminum.

[0224] The effects described in this specification are merely examples, and therefore the effects of the present technology are not limited to the effects described in this specification. Therefore, other effects may be obtained with respect to the present technology.

[0225] The present technology may also be configured as follows: <1> A secondary battery comprising: a positive electrode including a positive electrode active material and a plurality of fibrous materials; a negative electrode; and an electrolyte solution, wherein the positive electrode active material includes a sulfur-containing material, and the plurality of fibrous materials are entangled with each other to form a three-dimensional mesh structure, the fibrous material includes: a carbon fiber portion; and a plurality of carbon coating portions that cover a surface of the carbon fiber portion and hold the positive electrode active material, the plurality of carbon coating portions are spaced apart from each other in an extending direction of the carbon fiber portion, the carbon fiber portion has a plurality of exposed portions that are not covered by the plurality of carbon coating portions, and the carbon coating portion has a first end in the extending direction of the carbon fiber portion and a second end located opposite the first end, and the flexibility of the fibrous material represented by formula (1) is 8.6 or more and 19.2 or less. F = (L2 / L1) × T (1) (F is flexibility. L1 is the length (μm) of the carbon fiber portion in the extension direction of the carbon fiber portion. L2 is the sum of the lengths (μm) of each of the multiple exposed portions in the extension direction of the carbon fiber portion. T is the sum of the numbers of first ends and second ends.) <2> The secondary battery according to <1>, wherein the carbon fiber portion has a first end in the extension direction of the carbon fiber portion and a second end located opposite to the first end, and at least one of the first end and the second end is the exposed portion. <3> The secondary battery according to <1> or <2>, wherein a diameter of the carbon coating portion in a direction intersecting the extension direction of the carbon fiber portion is 75 nm or more and 250 nm or less. <4> The secondary battery according to any one of <1> to <3>, wherein the carbon coating portion has a multiple pores, and the positive electrode active material is inserted into the multiple pores. <5> The secondary battery according to any one of <1> to <4>, wherein the carbon fiber portion includes at least one of a carbon nanotube and a carbon nanofiber, and the carbon coating portion includes activated carbon. <6> The secondary battery according to any one of <1> to <5>, wherein the negative electrode includes an alkali metal. <7> The secondary battery according to <6>, wherein the alkali metal includes lithium.<8> The secondary battery according to any one of <1> to <7>, which is a lithium-sulfur secondary battery. <9> A positive electrode for a secondary battery, comprising: a positive electrode active material and a plurality of fibrous materials, wherein the positive electrode active material comprises a sulfur-containing material, and the plurality of fibrous materials are entangled with each other to form a three-dimensional mesh structure, the fibrous materials include: a carbon fiber portion; and a plurality of carbon coating portions that cover a surface of the carbon fiber portion and hold the positive electrode active material, the plurality of carbon coating portions are spaced from each other in an extension direction of the carbon fiber portion, the carbon fiber portion has a plurality of exposed portions that are not covered by the plurality of carbon coating portions, and the carbon coating portion has: a first end in the extension direction of the carbon fiber portion and a second end located opposite the first end, and the flexibility of the fibrous material represented by formula (1) is 8.6 or more and 19.2 or less. F = (L2 / L1) x T (1) (F is flexibility. L1 is the length (μm) of the carbon fiber portion in the extending direction of the carbon fiber portion. L2 is the sum (μm) of the lengths of the multiple exposed portions in the extending direction of the carbon fiber portion. T is the sum (number) of the numbers of first ends and second ends.)

Claims

1. a positive electrode including a positive electrode active material and a plurality of fibrous materials; A negative electrode; Electrolyte and Equipped with The positive electrode active material includes a sulfur-containing material, the plurality of fibrous substances are entangled with each other to form a three-dimensional mesh structure, The fibrous material is A carbon fiber portion; a plurality of carbon coating portions that coat the surfaces of the carbon fiber portions and hold the positive electrode active material; Including, The carbon coating portions are spaced apart from one another in an extending direction of the carbon fiber portion, the carbon fiber portion has a plurality of exposed portions that are not covered by the plurality of carbon coating portions, The carbon coating portion is a first end in an extension direction of the carbon fiber portion; a second end located opposite the first end; having The softness of the fibrous material represented by formula (1) is 8.6 or more and 19.2 or less. Secondary battery. F=(L2 / L1)×T...(1) (F is flexibility. L1 is the length (μm) of the carbon fiber part in the extension direction of the carbon fiber part. L2 is the sum (μm) of the lengths of the multiple exposed parts in the extension direction of the carbon fiber part. T is the sum (pieces) of the numbers of the first ends and the second ends.)

2. The carbon fiber portion is a first end portion in an extension direction of the carbon fiber portion; a second end located opposite the first end; having At least one of the first end and the second end is the exposed portion. The secondary battery according to claim 1 .

3. The diameter of the carbon coating portion in a direction intersecting the extension direction of the carbon fiber portion is 75 nm or more and 250 nm or less. The secondary battery according to claim 1 .

4. The carbon coating portion has a plurality of pores, The positive electrode active material is inserted into the plurality of pores. The secondary battery according to claim 1 .

5. the carbon fiber portion includes at least one of a carbon nanotube and a carbon nanofiber, The carbon coating portion includes activated carbon. The secondary battery according to claim 1 .

6. The negative electrode contains an alkali metal. The secondary battery according to claim 1 .

7. The alkali metal includes lithium. The secondary battery according to claim 6.

8. It is a lithium-sulfur secondary battery. The secondary battery according to claim 1 .

9. A positive electrode active material and a plurality of fibrous materials, The positive electrode active material includes a sulfur-containing material, the plurality of fibrous substances are entangled with each other to form a three-dimensional mesh structure, The fibrous material is A carbon fiber portion; a plurality of carbon coating portions that coat the surfaces of the carbon fiber portions and hold the positive electrode active material; Including, The carbon coating portions are spaced apart from one another in an extending direction of the carbon fiber portion, the carbon fiber portion has a plurality of exposed portions that are not covered by the plurality of carbon coating portions, The carbon coating portion is a first end in an extension direction of the carbon fiber portion; a second end located opposite the first end; having The softness of the fibrous material represented by formula (1) is 8.6 or more and 19.2 or less. Positive electrode for secondary batteries. F=(L2 / L1)×T...(1) (F is flexibility. L1 is the length (μm) of the carbon fiber part in the extension direction of the carbon fiber part. L2 is the sum (μm) of the lengths of the multiple exposed parts in the extension direction of the carbon fiber part. T is the sum (pieces) of the numbers of the first ends and the second ends.)