Negative electrode for secondary battery and secondary battery

Abstract

This secondary battery is provided with: a positive electrode; a negative electrode which comprises a plurality of fiber parts and a plurality of cover parts, while having a plurality of voids; a separator which is arranged between the positive electrode and the negative electrode; and an electrolyte solution. The plurality of fiber parts are connected to each other to form a three-dimensional network structure that comprises the plurality of voids; and each of the plurality of fiber parts contains carbon as a constituent element. The plurality of cover parts respectively cover the surfaces of the plurality of fiber parts; and each of the plurality of cover parts contains silicon as a constituent element. If the negative electrode is divided into equal halves in a direction in which the positive electrode and the negative electrode face each other with the separator being interposed therebetween, specifically into a first portion that is positioned on the side close to the separator and a second portion that is on the side far from the separator, at least one of the average fiber diameter of the plurality of fiber parts, the ratio of the weight of the plurality of cover parts to the sum of the weight of the plurality of fiber parts and the weight of the plurality of cover parts, and the void fraction is different between the first portion and the second portion.

Description

[Technical Field]

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

[0002] Due to the widespread use of various electronic devices such as mobile phones, secondary batteries have been developed as small, lightweight power sources with high energy density. These secondary batteries contain a positive electrode, a negative electrode, and an electrolyte, and various studies have been conducted on the configuration of these secondary batteries.

[0003] Specifically, a carbonaceous porous conductive substrate, a conductive agent (such as carbon nanotubes), and an active material (such as silicon) are used as materials for forming the negative electrode of a lithium ion secondary battery, and the porosity (void ratio) of the negative electrode is specified (see, for example, Patent Document 1).

[0004] A conductive substrate such as carbon fiber coated with silicon or the like is used as a material for forming the negative electrode of a lithium ion secondary battery, and the silicon content (weight ratio) in the negative electrode is specified (see, for example, Patent Document 2).

[0005] Materials used to form the negative electrode for a lithium ion secondary battery include a copper current collector and porous silicon having a three-dimensional network structure coated with a conductive substance such as a carbon material, and the average porosity of the porous silicon is specified (see, for example, Patent Document 3).

[0006] Inside a negative electrode for a lithium ion secondary battery, the silicon content, the carbon material content, and the porosity each have a gradient distribution (see, for example, Patent Document 4). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-335283 [Patent Document 2] Special Publication No. 2015-531977 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-084521 [Patent Document 4] Special Publication No. 2013-504168 Summary of the Invention

[0008] Although various studies have been conducted on the configuration of secondary batteries, the initial capacity characteristics, load characteristics, and cycle characteristics of the secondary batteries are still insufficient, and there is room for improvement.

[0009] Therefore, there is a demand for a negative electrode for a secondary battery that can provide excellent initial capacity characteristics, excellent load characteristics, and excellent cycle characteristics.

[0010] According to one embodiment of the present technology, a secondary battery negative electrode includes a plurality of fiber portions and a plurality of coating portions, and has a plurality of voids. The plurality of fiber portions are connected to each other to form a three-dimensional network structure having a plurality of voids, and each of the plurality of fiber portions contains carbon as a constituent element. Each of the plurality of coating portions coats the surface of each of the plurality of fiber portions and contains silicon as a constituent element. When the electrode is divided into a first portion and a second portion in the thickness direction, at least one of the average fiber diameter of the plurality of fiber portions, the ratio of the weight of the plurality of coating portions to the sum of the weight of the plurality of fiber portions and the weight of the plurality of coating portions, and the porosity is different between the first portion and the second portion.

[0011] According to one embodiment of the present technology, a secondary battery includes a positive electrode, a negative electrode including a plurality of fiber portions and a plurality of coating portions and having a plurality of voids, a separator disposed between the positive electrode and the negative electrode, and an electrolyte. The plurality of fiber portions are connected to each other to form a three-dimensional network structure having a plurality of voids, and each of the plurality of fiber portions contains carbon as a constituent element. Each of the plurality of coating portions coats the surface of each of the plurality of fiber portions and contains silicon as a constituent element. When the negative electrode is divided into two equal parts, a first part located closer to the separator and a second part located farther from the separator in a direction in which the positive electrode and the negative electrode face each other via the separator, at least one of the average fiber diameter of the plurality of fiber portions, the ratio of the weight of the plurality of coating portions to the sum of the weight of the plurality of fiber portions and the weight of the plurality of coating portions, and the porosity is different between the first part and the second part.

[0012] Details (such as definitions and calculation procedures) of the three types of physical properties mentioned above, namely, "average fiber diameter of the plurality of fiber parts," "ratio of the weight of the plurality of coating parts to the sum of the weight of the plurality of fiber parts and the weight of the plurality of coating parts," and "porosity," will be described later.

[0013] Furthermore, details (such as definitions) of the provision that "at least one of the average fiber diameter of the plurality of fiber parts, the ratio of the weight of the plurality of coating parts to the sum of the weight of the plurality of fiber parts and the weight of the plurality of coating parts, and the porosity differ between the first part and the second part" will be described later.

[0014] According to the negative electrode for a secondary battery or the secondary battery of one embodiment of the present technology, the negative electrode for a secondary battery includes the above-mentioned plurality of fiber portions and plurality of coating portions, and has a plurality of voids, and at least one of the above-mentioned average fiber diameter, ratio, and porosity is different between the first portion and the second portion, so that excellent initial capacity characteristics, excellent load characteristics, and excellent cycle characteristics can be obtained.

[0015] 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. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram illustrating a configuration of a negative electrode for a secondary battery according to an embodiment of the present technology. [Figure 2] 2 is an enlarged cross-sectional view showing the configuration of each of the carbon fiber portion and the covering portion shown in FIG. 1. FIG. [Figure 3] FIG. 2 is another schematic diagram illustrating the configuration of a negative electrode for a secondary battery. [Figure 4] 1 is a perspective view illustrating a configuration of a secondary battery according to an embodiment of the present technology. [Figure 5] FIG. 5 is an enlarged cross-sectional view showing the configuration of the battery element shown in FIG. [Figure 6] FIG. 10 is a cross-sectional view illustrating the configuration of a negative electrode for a secondary battery according to Modification 2. [Figure 7] FIG. 10 is a schematic diagram illustrating the configuration of a negative electrode for a secondary battery according to Modification 5. [Figure 8] FIG. 1 is a block diagram illustrating a configuration of an application example of a secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.Negative electrode for secondary batteries 1-1.Configuration 1-2.Configuration conditions 1-3. Manufacturing method 1-4. Action and effects 2. Secondary battery 2-1.Configuration 2-2.Operation 2-3. Manufacturing method 2-4. Action and effects 3. Variations 4. Uses of secondary batteries

[0018] <1. Negative electrode for secondary batteries> First, a negative electrode for a secondary battery (hereinafter simply referred to as "negative electrode") according to one embodiment of the present technology will be described.

[0019] This negative electrode is used in a secondary battery, which is an electrochemical device. However, the negative electrode may also be used in electrochemical devices other than secondary batteries. The type of other electrochemical device is not particularly limited, but specific examples include capacitors.

[0020] In addition, in electrochemical devices such as the secondary battery, the negative electrode absorbs and releases an electrode reactant during an electrode reaction. The type of electrode reactant is not particularly limited, but specifically, it is a light metal such as an alkali metal or alkaline earth metal. The alkali metal includes lithium, sodium, potassium, etc., and the alkaline earth metal includes beryllium, magnesium, calcium, etc.

[0021] <1-1.Configuration> Fig. 1 schematically illustrates the configuration of a negative electrode 10, which is an example of a negative electrode. Fig. 2 shows an enlarged cross-sectional configuration of each of the carbon fiber portion 1 and the coating portion 2 shown in Fig. 1. However, Fig. 1 illustrates only a portion of the negative electrode 10, and Fig. 2 illustrates each cross section of the carbon fiber portion 1 and the coating portion 2 intersecting the longitudinal direction of the carbon fiber portion 1.

[0022] 1, this negative electrode 10 includes a plurality of carbon fiber portions 1 and a plurality of coating portions 2, and also has a plurality of voids 10G. That is, the negative electrode 10 does not include a current collector such as a metal foil (hereinafter referred to as a "metal current collector"), and is therefore a so-called metal current collector-less electrode.

[0023] [Multiple carbon fiber sections] The plurality of carbon fiber portions 1 are a plurality of fiber portions having an average fiber diameter AD as shown in Fig. 1, and each of the plurality of carbon fiber portions 1 has a fiber diameter D as shown in Fig. 2. The plurality of carbon fiber portions 1 are connected to each other to form the three-dimensional mesh structure having the above-described plurality of voids 10G.

[0024] 1 shows a case where each of the plurality of carbon fiber portions 1 is linear for the sake of simplicity. However, the state (shape) of each of the plurality of carbon fiber portions 1 is not particularly limited, and therefore may be curved or branched, or may be a mixture of two or more of these.

[0025] Here, as described above, the plurality of carbon fiber parts 1 are connected to one another to form a three-dimensional network structure, more specifically, are randomly entangled with one another. The plurality of carbon fiber parts 1 may be bonded to one another via a carbide (not shown) such as a polymer compound. As a result, the plurality of carbon fiber parts 1 have a plurality of connection points, and the carbon fiber parts 1 are electrically connected to one another at the connection points.

[0026] Each of the plurality of carbon fiber portions 1 contains carbon as a constituent element, and therefore contains a so-called carbon-containing material, which is a general term for materials that contain carbon as a constituent element.

[0027] Specifically, the plurality of carbon fiber parts 1 includes carbon paper because the plurality of carbon fiber parts 1 are sufficiently connected to one another and the average fiber diameter AD is sufficiently large, so that a sufficient conductive network (three-dimensional mesh structure) is formed.

[0028] However, the plurality of carbon fiber portions 1 may be a material in which a plurality of fibrous carbon materials having the above-mentioned average fiber diameter AD are processed to form a three-dimensional network structure. The type of this fibrous carbon material is not particularly limited, but specific examples include vapor-grown carbon fiber (VGCF), carbon fiber (CF), and carbon nanofiber (CNF). Alternatively, the type of fibrous carbon material may be carbon nanotubes (CNT). The carbon nanotubes may be single-walled carbon nanotubes (single-walled carbon nanotubes (SWCNT)) or multi-walled carbon nanotubes (multi-walled carbon nanotubes (MWCNT)) such as double-walled carbon nanotubes (double-walled carbon nanotubes (DWCNT)).

[0029] Here, a predetermined condition is satisfied with respect to the average fiber diameter AD (nm) of the plurality of carbon fiber portions 1. Details of this predetermined condition will be described later.

[0030] [Multiple coated parts] As shown in FIG. 1, each of the plurality of covering portions 2 covers the surface of each of the plurality of carbon fiber portions 1, and as shown in FIG. 2, each of the covering portions 2 has a thickness T1.

[0031] The covering portion 2 may cover the entire surface of the carbon fiber portion 1, or may cover only a part of the surface of the carbon fiber portion 1. In the latter case, a plurality of covering portions 2 may cover the surface of the carbon fiber portion 1 at a plurality of locations that are spaced apart from one another. In Fig. 1, for the sake of simplicity, the case where the covering portion 2 covers the entire surface of the carbon fiber portion 1 is shown.

[0032] Furthermore, each of the multiple coating portions 2 contains silicon as a constituent element, and therefore contains a so-called silicon-containing material, because silicon has an excellent ability to absorb and release electrode reactants, and therefore a high energy density can be obtained.

[0033] This silicon-containing material is a general term for materials containing silicon as a constituent element. Therefore, the silicon-containing material may be elemental silicon, a silicon alloy, a silicon compound, a mixture of two or more of them, or a material containing one or more phases of them. However, elemental silicon may contain trace amounts of impurities. That is, the purity of elemental silicon does not have to be 100%. These impurities are impurities unintentionally contained in the manufacturing process of elemental silicon and oxides unintentionally formed due to oxygen in the atmosphere. The content of impurities in elemental silicon is preferably as small as possible, and more preferably 5% by weight or less.

[0034] The silicon alloy contains, as constituent elements other than silicon, any one or more of metal elements such as tin, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium. The silicon compound contains, as constituent elements other than silicon, any one or more of non-metal elements such as carbon and oxygen. However, the silicon compound may further contain any one or more of the series of metal elements described for the silicon alloy as constituent elements other than silicon.

[0035] Specific examples of the silicon alloy are Mg2Si, Ni2Si, TiSi2, MoSi2, CoSi2, NiSi2, CaSi2, CrSi2, Cu5Si, FeSi2, MnSi2, NbSi2, TaSi2, VSi2, WSi2, ZnSi2, and SiC, etc. However, the composition of the silicon alloy (mixing ratio of silicon and metal elements) can be arbitrarily changed.

[0036] Specific examples of the silicon compound are SiB4, SiB6, Si3N4, Si2N2O, SiO v (0 < v ≤ 2) and LiSiO, etc. However, the range of v may be, for example, 0.2 < v < 1.4.

[0037] Among these, the silicon-containing material is preferably elemental silicon, as this allows for a higher energy density to be obtained. In this case, the silicon content in each of the multiple coating portions 2, i.e., the silicon content (purity) in the silicon-containing material, is not particularly limited, but is preferably 80% by weight or more, and more preferably 80% by weight to 100% by weight, as this allows for a significantly higher energy density to be obtained.

[0038] Although not specifically shown, a part or all of the surface of the covering portion 2 may be further covered with a coating layer. This coating layer contains one or more types of conductive materials such as carbon-containing materials and metal materials. This is because the conductivity of the negative electrode 10 is further improved. Details regarding the carbon-containing material are as described above. The type of metal material is not particularly limited.

[0039] When forming this coating layer, a silane coupling agent, a polymer-based material, or the like is used to ensure that the surface of the coating portion 2 can be sufficiently covered with the coating layer. By sufficiently covering the surface of the coating portion 2 with the coating layer, the decomposition reaction of the electrolyte on the surface of the coating portion 2 containing the silicon-containing material is suppressed.

[0040] Here, a weight ratio MA (wt %), which is the ratio of the weight M2 of the plurality of covering portions 2 to the sum of the weight M1 of the plurality of carbon fiber portions 1 and the weight M2 of the plurality of covering portions 2, satisfies a predetermined condition, and is calculated based on the formula MA=[M2 / (M1+M2)]×100. Details of this predetermined condition will be described later.

[0041] [Porosity] As described above, the negative electrode 10 has a three-dimensional network structure formed by a plurality of carbon fiber portions 1, and therefore has a plurality of voids 10G.

[0042] Here, the porosity R (volume %) determined based on the plurality of voids 10G satisfies a predetermined condition, which will be described in detail later.

[0043] [Other materials] The negative electrode 10 may further contain one or more of the other materials.

[0044] The type of other material is not particularly limited, but specifically, it is a binder, etc. This is because the plurality of carbon fiber portions 1 and the plurality of coating portions 2 are firmly connected to each other via the binder, thereby forming a strong conductive network.

[0045] The binder contains one or more polymer compounds, and specific examples of the polymer compounds include polyimide, polyvinylidene fluoride, polyacrylic acid, styrene butadiene rubber, and carboxymethyl cellulose.

[0046] <1-2. Configuration conditions> Regarding the configuration of the negative electrode 10, certain conditions are met, as will be explained below.

[0047] 3 is a schematic diagram showing another configuration of the negative electrode 10. However, unlike FIG. 1, FIG. 3 shows the entire negative electrode 10.

[0048] 3, the negative electrode 10 has a substantially plate-like or sheet-like structure and therefore has a thickness. This thickness is the dimension in the vertical direction (thickness direction H) in FIG.

[0049] Here, when focusing on three types of physical property values (average fiber diameter AD, weight fraction MA, and porosity R) that determine the configuration of negative electrode 10, predetermined conditions are satisfied for these three types of physical property values. Specifically, when negative electrode 10 is divided into lower portion 10X (first portion) and upper portion 10Y (second portion) in the thickness direction H, one or more of the average fiber diameter AD, weight fraction MA, and porosity R differ between lower portion 10X and upper portion 10Y. In FIG. 3, a dashed line is indicated at the boundary between lower portion 10X and upper portion 10Y to make them easier to distinguish from each other.

[0050] That is, the average fiber diameter AD may be different between the lower portion 10X and the upper portion 10Y. Alternatively, the weight percentage MA may be different between the lower portion 10X and the upper portion 10Y. Alternatively, the porosity R may be different between the lower portion 10X and the upper portion 10Y. Of course, any two or more of the average fiber diameter AD, weight percentage MA, and porosity R may be different between the lower portion 10X and the upper portion 10Y, or all of the average fiber diameter AD, weight percentage MA, and porosity R may be different between the lower portion 10X and the upper portion 10Y.

[0051] When the average fiber diameter AD differs between the lower portion 10X and the upper portion 10Y, the trend of change in the average fiber diameter AD is not particularly limited. Therefore, the average fiber diameter AD may change intermittently in the thickness direction H, or may change continuously in the thickness direction H.

[0052] What has been explained here regarding the change trend of the average fiber diameter AD also applies to the change trend of the weight proportion MA and the change trend of the porosity R.

[0053] That is, when the weight ratio MA is different between the lower portion 10X and the upper portion 10Y, the weight ratio MA may change intermittently in the thickness direction H, or may change continuously in the thickness direction H.

[0054] Furthermore, when the porosity R differs between the lower portion 10X and the upper portion 10Y, the porosity R may vary intermittently in the thickness direction H, or may vary continuously in the thickness direction H.

[0055] The lower portion 10X and the upper portion 10Y may be separate from each other or may be integrated with each other. When the lower portion 10X and the upper portion 10Y are separate from each other, the negative electrode 10 has a two-layer structure, and therefore a physical (actual) interface exists at the boundary between the lower portion 10X and the upper portion 10Y. In contrast, when the lower portion 10X and the upper portion 10Y are integrated with each other, the negative electrode 10 has a single-layer structure, and therefore no physical interface exists at the boundary between the lower portion 10X and the upper portion 10Y.

[0056] [Average fiber diameter AD] The average fiber diameter AD will be described in detail below.

[0057] (Definition of average fiber diameters ADX and ADY) As described above, the plurality of carbon fiber portions 1 have an average fiber diameter AD, and the negative electrode 10 includes a lower portion 10X and an upper portion 10Y as shown in Fig. 3. As a result, the plurality of carbon fiber portions 1 in the lower portion 10X have an average fiber diameter ADX, and the plurality of carbon fiber portions 1 in the upper portion 10Y have an average fiber diameter ADY, and therefore the average fiber diameters ADX and ADY are different from each other.

[0058] The average fiber diameters ADX and ADY are different from each other because the electrode reactant can easily move through the plurality of gaps 10G during the electrode reaction and the electrode reaction can easily proceed smoothly even when the electrode reaction is repeated. In this case, the electrode reactant can easily move smoothly even when the current value during the electrode reaction increases.

[0059] (Calculation procedure for average fiber diameters ADX and ADY) The procedure for calculating the average fiber diameter ADX is as follows. First, the negative electrode 10 is collected and then washed using a washing solvent such as dimethyl carbonate. When a secondary battery including the negative electrode 10 is obtained, the secondary battery is disassembled to collect the negative electrode 10. Next, the negative electrode 10 is cut using an ion milling device or the like to expose a cross section of the negative electrode 10.

[0060] Next, a cross section of the lower portion 10X is observed using a scanning electron microscope (SEM) or a transmission electron microscope (TEM) to obtain an observation result (observation image) of the cross section. This makes it possible to identify multiple carbon fiber portions 1 in the observation image. Observation conditions such as acceleration voltage and magnification can be set arbitrarily.

[0061] Next, 50 carbon fiber portions 1 are arbitrarily selected, and the fiber diameter D of each of the 50 carbon fiber portions 1 is measured. Finally, the average value of the 50 fiber diameters D is calculated to obtain the average fiber diameter ADX.

[0062] The procedure for calculating the average fiber diameter ADY is the same as the procedure for calculating the average fiber diameter ADX described above, except that the cross section of the upper portion 10Y is observed instead of the cross section of the lower portion 10X.

[0063] (Definition of the relationship between average fiber diameters ADX and ADY) The average fiber diameter ADX may be larger than the average fiber diameter ADY, or may be smaller than the average fiber diameter ADY.

[0064] Here, the definition of the case where the average fiber diameter ADX is larger than the average fiber diameter ADY is as explained below.

[0065] The average fiber diameter ADX being larger than the average fiber diameter ADY means that when 10 average fiber diameters ADX and 10 average fiber diameters ADY are calculated, all of the 10 average fiber diameters ADX are larger than the 10 average fiber diameters ADY. This means that the smallest value among the 10 average fiber diameters ADX is larger than the largest value among the 10 average fiber diameters ADY. Conversely, if any one average fiber diameter ADX among the 10 average fiber diameters ADX is smaller than any one average fiber diameter ADY among the 10 average fiber diameters ADY, this does not mean that the average fiber diameter ADX is larger than the average fiber diameter ADY.

[0066] When any of the ten average fiber diameters ADX is larger than any of the ten average fiber diameters ADY, the average fiber diameter ADX is considered to be larger than the average fiber diameter ADY in order to actively eliminate a configuration in which the average fiber diameter ADX accidentally becomes larger than the average fiber diameter ADY due to factors such as those involved in the manufacture of the negative electrode 10.

[0067] In other words, even if the average fiber diameter ADX calculated at any location in the lower portion 10X is larger than the average fiber diameter ADY calculated at any location in the upper portion 10Y, if the average fiber diameter ADX calculated at another location in the lower portion 10X is smaller than the average fiber diameter ADY calculated at another location in the upper portion 10Y, then the average fiber diameter ADX is not considered to be larger than the average fiber diameter ADY.

[0068] In contrast, no matter where in the lower portion 10X the average fiber diameter ADX is calculated, if that average fiber diameter ADX is larger than the average fiber diameter ADY calculated at any location in the upper portion 10Y, then that average fiber diameter ADX is larger than the average fiber diameter ADY.

[0069] The definition when the average fiber diameter ADX is smaller than the average fiber diameter ADY is the same as the definition when the average fiber diameter ADX is larger than the average fiber diameter ADY, except that the magnitude relationship is reversed.

[0070] That is, the average fiber diameter ADX being smaller than the average fiber diameter ADY means that when 10 average fiber diameters ADX and 10 average fiber diameters ADY are calculated, all of the 10 average fiber diameters ADX are smaller than the respective 10 average fiber diameters ADY. As a result, the maximum value of the 10 average fiber diameters ADX is smaller than the minimum value of the 10 average fiber diameters ADY.

[0071] When any of the ten average fiber diameters ADX is smaller than any of the ten average fiber diameters ADY, the average fiber diameter ADX is considered to be smaller than the average fiber diameter ADY in order to actively eliminate a configuration in which the average fiber diameter ADX accidentally becomes smaller than the average fiber diameter ADY due to factors such as those involved in the manufacture of the negative electrode 10.

[0072] (Optimal relationship between average fiber diameters ADX and ADY) As will be described later, when the negative electrode 10 is used in a secondary battery together with a positive electrode and a separator, the separator is disposed between the negative electrode 10 and the positive electrode, so that the negative electrode 10 and the positive electrode face each other with the separator interposed therebetween.

[0073] In this case, dividing the negative electrode 10 into the lower portion 10X and the upper portion 10Y in the thickness direction H means dividing the negative electrode 10 into two equal portions in the direction in which the positive electrode and the negative electrode 10 face each other with the separator interposed therebetween. As a result, in the negative electrode 10, the lower portion 10X is located closer to the separator, and the upper portion 10Y is located farther from the separator.

[0074] In particular, since the average fiber diameter AD is smaller in the lower portion 10X than in the upper portion 10Y, it is preferable that the average fiber diameter ADX is smaller than the average fiber diameter ADY, because this allows the electrode reactant to move more easily and the electrode reaction to proceed more smoothly even when repeated.

[0075] As long as the average fiber diameter ADX is smaller than the average fiber diameter ADY, the ratio of the average fiber diameter ADX to the average fiber diameter ADY (=ADX / ADY) is not particularly limited, but it is particularly preferred that the average fiber diameter ADX be 0.0003 to 0.5 times the average fiber diameter ADY. This is because the difference between the average fiber diameters ADX and ADY becomes sufficiently large, which facilitates the mobility of electrode reactants and allows the electrode reaction to proceed sufficiently even when repeated.

[0076] (Optimal range of average fiber diameters AD, ADX, ADY) The average fiber diameter AD of the entire negative electrode 10 is not particularly limited, but is preferably 10 nm to 12,000 nm. This is because the fiber diameter D becomes sufficiently large in the plurality of carbon fiber portions 1 that are the main portions of the negative electrode 10. This allows a sufficient conductive network (three-dimensional mesh structure) to be formed inside the negative electrode 10, thereby improving the conductivity of the negative electrode 10.

[0077] As long as the average fiber diameters ADX and ADY are different from each other, there are no particular limitations on the average fiber diameters ADX and ADY. In particular, when the average fiber diameter ADX is smaller than the average fiber diameter ADY, the average fiber diameter ADX is preferably 5 nm to 8,000 nm, and the average fiber diameter ADY is preferably 100 nm to 16,000 nm. Since the difference between the average fiber diameters ADX and ADY is sufficiently large, the electrode reactant can be easily transported, and the electrode reaction can proceed sufficiently even when repeated.

[0078] [Weight percentage MA] The weight ratio MA will be described in detail below.

[0079] [Definition of weight percentage MAX, MAY] As described above, the negative electrode 10 has the weight proportion MA, and also has a lower portion 10X and an upper portion 10Y as shown in Fig. 3. As a result, the lower portion 10X has the weight proportion MAX, and the upper portion 10Y has the weight proportion MAY, and therefore the weight proportions MAX and MAY are different from each other.

[0080] The weight proportions MAX and MAY are different from each other because, during the electrode reaction, the carbon component (plurality of carbon fiber parts 1) suppresses the expansion and contraction of the negative electrode 10, while the silicon component (plurality of coating parts 2) makes it easier for the electrode reaction substance to be absorbed and released.

[0081] (Calculation procedure for weight percentage MAX, MAY) The procedure for calculating the weight ratio MAX is as follows. First, the negative electrode 10 is recovered and then washed using a washing solvent such as dimethyl carbonate. Next, a sample for analysis is obtained by sampling the lower portion 10X from the negative electrode 10. Next, the sample is analyzed using thermogravimetric differential thermal analysis (TG-DTA) to determine the weights M1 and M2. Note that any TG-DTA device can be used to analyze the sample.

[0082] In the analysis of this lower portion 10X, the weight loss when the heating temperature is increased to about 450°C is the weight of the electrolyte, binder, etc., and the weight loss when the heating temperature is increased to about 450°C to about 1350°C is the weight (weight M1) of the carbon component (plurality of carbon fiber portions 1). As a result, the weight of the remaining component is the weight (weight M2) of the silicon component (plurality of coating portions 2).

[0083] The temperature (approximately 450°C) at which the weight loss due to the electrolyte and other factors is detected may vary depending on the type of binder. Specifically, when the binder is polyvinylidene fluoride, the minimum value of the DTA differential curve is taken as the disappearance temperature, which is approximately 460°C.

[0084] Finally, the weight ratio MAX is calculated using the weights M1 and M2 based on the above formula.

[0085] The procedure for calculating the weight ratio MAY is the same as the procedure for calculating the weight ratio MAX described above, except that the upper portion 10Y is analyzed instead of the lower portion 10X.

[0086] (Definition of the relationship between weight percentage MAX and MAY) The weight proportion MAX may be greater than or less than the weight proportion MAY. The definition of the magnitude relationship between the weight proportions MAX and MAY is the same as the definition of the magnitude relationship between the average fiber diameters ADX and ADY described above.

[0087] Specifically, the weight proportion MAX being greater than the weight proportion MAY means that when the 10 weight proportions MAX and the 10 weight proportions MAY are calculated, all of the 10 weight proportions MAX are greater than each of the 10 weight proportions MAY. As a result, the smallest value of the 10 weight proportions MAX is greater than the largest value of the 10 weight proportions MAY.

[0088] When any of the 10 weight proportions MAX is greater than each of the 10 weight proportions MAY, the weight proportion MAX is considered to be greater than the weight proportion MAY in order to actively eliminate a configuration in which the weight proportion MAX accidentally becomes greater than the weight proportion MAY due to factors such as those involved in the manufacturing of the negative electrode 10.

[0089] The definition when the weight proportion MAX is smaller than the weight proportion MAY is the same as the definition when the weight proportion MAX is larger than the weight proportion MAY, except that the magnitude relationship is reversed.

[0090] In other words, when the weight proportion MAX is smaller than the weight proportion MAY, it means that when the 10 weight proportions MAX and the 10 weight proportions MAY are calculated, all of the 10 weight proportions MAX are smaller than the 10 weight proportions MAY. As a result, the maximum value of the 10 weight proportions MAX is smaller than the minimum value of the 10 weight proportions MAY.

[0091] When any of the 10 weight proportions MAX is smaller than each of the 10 weight proportions MAY, the weight proportion MAX is considered to be smaller than the weight proportion MAY in order to actively eliminate a configuration in which the weight proportion MAX accidentally becomes smaller than the weight proportion MAY due to factors such as those involved in the manufacturing of the negative electrode 10.

[0092] (Optimal relationship between weight ratio MAX and MAY) As described above, when the negative electrode 10 and the positive electrode face each other via a separator in a secondary battery, it is preferable that the weight ratio MAX is greater than the weight ratio MAY because the weight ratio MA is greater in the lower portion 10X than in the upper portion 10Y. This is because the electrode reactant is more likely to be absorbed and released while the expansion and contraction of the negative electrode 10 is more suppressed.

[0093] As long as the weight percentage MAX is greater than the weight percentage MAY, the ratio of the weight percentage MAX to the weight percentage MAY (=MAX / MAY) is not particularly limited, but it is preferable that the weight percentage MAX is 1.04 to 4.65 times the weight percentage MAY. This is because the difference between the weight percentages MAX and MAY becomes sufficiently large, which allows the electrode reactant to be sufficiently absorbed and released while sufficiently suppressing the expansion and contraction of the negative electrode 10.

[0094] (Optimal range of weight percentages MA, MAX, MA) The total weight proportion MA of the negative electrode 10 is not particularly limited, but is preferably 40% by weight to 80% by weight, because this allows the electrode reactant to be sufficiently absorbed and released while the expansion and contraction of the negative electrode 10 is sufficiently suppressed.

[0095] Note that, as long as the weight percentages MAX and MAY are different from each other, there are no particular limitations on each of them. In particular, when the weight percentage MAX is greater than the weight percentage MAY, the weight percentage MAX is preferably 42% by weight to 88% by weight, and the weight percentage MAY is preferably 12% by weight to 78% by weight. This is because the difference between the weight percentages MAX and MY is sufficiently large, which sufficiently suppresses the expansion and contraction of the negative electrode 10 and facilitates sufficient occlusion and release of the electrode reactant.

[0096] [Porosity R] Details regarding the porosity R are explained below.

[0097] [Porosity R] As described above, the negative electrode 10 has a porosity R, and also has a lower portion 10X and an upper portion 10Y as shown in Fig. 3. As a result, the lower portion 10X has a porosity RX, and the upper portion 10Y has a porosity RY, and therefore the porosities RX and RY are different from each other.

[0098] The reason why the porosities RX and RY are different from each other is that the electrode reactant can easily move during the electrode reaction by utilizing the distribution of the plurality of voids 10G, and the electrode reaction can easily proceed smoothly even when the electrode reaction is repeated. In this case, the electrode reactant can easily move smoothly, particularly even when the current value during the electrode reaction increases.

[0099] (Calculation procedure for porosity RX, RY) The procedure for calculating the porosity RX is as follows. After recovering and cleaning the negative electrode 10 using the same procedure as for calculating the average fiber diameter ADX described above, a three-dimensional image of the lower portion 10X is acquired using a focused ion beam scanning electron microscope (FIB-SEM), and the porosity RX is calculated based on the three-dimensional image using image analysis processing. For this image analysis processing, GeoDict, a comprehensive package software for innovative material development by Math2Market GmbH, or the like, can be used.

[0100] The procedure for calculating the porosity RY is the same as the procedure for calculating the porosity RY described above, except that a three-dimensional image of the upper portion 10Y is acquired instead of the lower portion 10X.

[0101] (Definition of the relationship between porosity RX and RY) The porosity RX may be greater than or less than the porosity RY. The definition of the magnitude relationship between the porosities RX and RY is the same as the definition of the magnitude relationship between the average fiber diameters ADX and ADY described above.

[0102] Specifically, the porosity RX being larger than the porosity RY means that when 10 porosities RX and 10 porosities RY are calculated, all of the 10 porosities RX are larger than the 10 porosities RY, respectively. As a result, the minimum value of the 10 porosities RX is larger than the maximum value of the 10 porosities RY.

[0103] When any of the ten porosities RX is greater than each of the ten porosities RY, the porosity R is considered to be greater than the porosity RY in order to actively eliminate a configuration in which the porosity RX accidentally becomes greater than the porosity RY due to factors such as those involved in the manufacture of the negative electrode 10.

[0104] The definition when the porosity RX is smaller than the porosity RY is the same as the definition when the porosity RX is larger than the porosity RY, except that the magnitude relationship is reversed.

[0105] That is, the porosity RX being smaller than the porosity RY means that when 10 porosities RX and 10 porosities RY are calculated, all of the 10 porosities RX are smaller than the 10 porosities RY. As a result, the maximum value of the 10 porosities RX is smaller than the minimum value of the 10 porosities RY.

[0106] When any of the ten porosities RX is smaller than each of the ten porosities RY, the porosity RX is considered to be smaller than the porosity RY in order to actively eliminate a configuration in which the porosity RX accidentally becomes smaller than the porosity RY due to factors such as those involved in the manufacture of the negative electrode 10.

[0107] (Optimal relationship between porosity RX and RY) As described above, when the negative electrode 10 and the positive electrode face each other via a separator in a secondary battery, it is preferable that the porosity RY is greater than the porosity RX because the porosity R is greater in the upper portion 10Y than in the lower portion 10X. This is because the electrode reactant is more likely to move and the electrode reaction is more likely to proceed smoothly even when repeated.

[0108] As long as the porosity RY is larger than the porosity RX, the ratio of the porosity RY to the porosity RX (=RY / RX) is not particularly limited, but it is preferable that the porosity RY is 1.1 to 4.5 times the porosity RX. This is because the difference between the porosities RX and RY becomes sufficiently large, which allows the electrode reactant to move sufficiently easily and also makes it easier for the electrode reaction to proceed sufficiently even when the electrode reaction is repeated.

[0109] (Optimal range of porosity R, RX, RY) The overall porosity R of the negative electrode 10 is not particularly limited, but is preferably 40% by volume to 70% by volume, because this allows the electrode reactant to move sufficiently easily and allows the electrode reaction to proceed sufficiently even when repeated.

[0110] Note that, as long as the porosities RX and RY are different from each other, there are no particular limitations on each of the porosities RX and RY. In particular, when the porosity RY is larger than the porosity RX, the porosity RX is preferably 20 volume % to 67 volume %, and the porosity RY is preferably 42 volume % to 90 volume %. This is because the difference between the porosities RX and RY is sufficiently large, which allows the electrode reactant to move sufficiently easily and allows the electrode reaction to proceed sufficiently even when repeated.

[0111] [Other physical properties] As described above, the lower portion 10X and the upper portion 10Y differ from each other in one or more of the average fiber diameter AD, the weight fraction MA, and the porosity R. In addition, although not described in detail here, the lower portion 10X and the upper portion 10Y may differ from each other in one or both of the average fiber length and the average curvature.

[0112] The average fiber length is the average value of the fiber lengths of the plurality of carbon fiber portions 1, and the average curvature is the average value of the curvatures of the plurality of carbon fiber portions 1.

[0113] [Average thickness AT1] The average thickness AT1 of the plurality of covering portions 2 is not particularly limited, but is preferably 1 nm to 3000 nm, because the amount of the covering portions 2 covering the surfaces of the carbon fiber portions 1 becomes sufficiently large, and the electrical conductivity of the negative electrode 10 is ensured, while a sufficient energy density can be obtained in the negative electrode 10.

[0114] The procedure for calculating the average thickness AT1 is as follows. First, an observation result (observation image) of the cross section of the negative electrode 10 is obtained using the same procedure as when calculating the average fiber diameter ADX described above. Next, 20 covering portions 2 are arbitrarily selected, and the thickness T1 of each of the 20 covering portions 2 is measured. Note that if the thickness T1 varies depending on the location within one covering portion 2, the maximum value of the thickness T1 is selected. Finally, the average value of the 20 thicknesses T1 is calculated to obtain the average thickness AT1.

[0115] <1-3. Manufacturing method> The negative electrode 10 is manufactured by the procedure described below.

[0116] [Manufacturing method for intermittent changes] The manufacturing procedure for intermittently varying the average fiber diameter AD, weight percentage MA, and porosity R in the thickness direction H is as follows. Here, the case where the average fiber diameter AD, weight percentage MA, and porosity R are made different between the lower portion 10X and the upper portion 10Y will be described.

[0117] (Preparation process of two types of multiple fibrous carbon materials) First, a plurality of fibrous carbon materials (with an average fiber diameter ADX) are prepared as the material for forming the lower portion 10X. Details regarding the plurality of fibrous carbon materials are as described above.

[0118] Next, a silicon-containing material is deposited on each surface of the plurality of fibrous carbon materials using a vapor phase method. The type of vapor phase method is not particularly limited, but specifically, it is one or more of vacuum deposition, chemical vapor deposition (CVD), sputtering, etc. As a result, a coating portion 2 is formed on each surface of the plurality of fibrous carbon materials, and each surface of the plurality of fibrous carbon materials is coated with the coating portion 2 (weight ratio MAX).

[0119] Next, a plurality of fibrous carbon materials (with an average fiber diameter ADY) that are materials for forming the upper portion 10Y are prepared.

[0120] Subsequently, by using the same procedure, a silicon-containing material is deposited on the surface of each of the plurality of fibrous carbon materials, thereby forming a coating portion 2 on the surface of each of the plurality of fibrous carbon materials (weight ratio MAY).

[0121] As a result, two types of multiple fibrous carbon materials are obtained that are used to form the lower portion 10X and the upper portion 10Y.

[0122] (Anode assembly process) Next, using a multi-layer weaving device, multiple fibrous carbon materials (average fiber diameter ADX, weight proportion MAX) on which the coating portion 2 is formed and multiple fibrous carbon materials (average fiber diameter ADY, weight proportion MAY) on which the coating portion 2 is formed are weaved together.

[0123] In this case, the former plurality of fibrous carbon materials form a three-dimensional mesh structure having a plurality of voids 10G, thereby forming a lower portion 10X (porosity RX) including a plurality of carbon fiber portions 1 and a plurality of covering portions 2. The latter plurality of fibrous carbon materials form a three-dimensional mesh structure having a plurality of voids 10G, thereby forming an upper portion 10Y (porosity RY) including a plurality of carbon fiber portions 1 and a plurality of covering portions 2. As a result, the lower portion 10X and the upper portion 10Y are stacked on top of each other and are connected to each other.

[0124] This completes the assembly of the negative electrode 10. The negative electrode 10 has a two-layer structure, since it includes a lower portion 10X and an upper portion 10Y that are physically separate from each other.

[0125] (baking of negative electrodes, etc.) Finally, if necessary, the negative electrode 10 is pressed using a press or the like, and then the negative electrode 10 is fired. In this case, the porosities RX and RY can be adjusted by changing the pressing pressure. The firing temperature can be set arbitrarily.

[0126] This completes the negative electrode 10, which includes a plurality of carbon fiber portions 1 and a plurality of coating portions 2 and has a plurality of voids 10G. In this case, the average fiber diameter AD, weight percentage MA, and porosity R can be adjusted according to the average fiber diameters ADX and ADY, weight percentages MAX and MAY, and porosities RX and RY, respectively.

[0127] [Manufacturing method for continuous change] The manufacturing procedure for continuously varying the average fiber diameter AD, weight percentage MA, and porosity R in the thickness direction H is as follows. Here, the case where the weight percentage MA and porosity R are made different between the lower portion 10X and the upper portion 10Y will be described.

[0128] (Preparation process of multiple carbon fiber parts) First, as described above, carbon paper, which is a plurality of carbon fiber portions 1, is prepared.

[0129] (Process for forming multiple covering portions) Next, the powder of the silicon-containing material is added to the solvent. This disperses the powder of the silicon-containing material in the solvent, thereby preparing a dispersion liquid. This solvent may be an aqueous solvent or a non-aqueous solvent (organic solvent). In this case, a binder may be added to the solvent. Details regarding this binder are as described above.

[0130] Next, the dispersion liquid is applied to the plurality of carbon fiber parts 1, and then the dispersion liquid is dried. As a result, the dispersion liquid containing the powder of the silicon-containing material is impregnated into the interior of the plurality of carbon fiber parts 1, and the powder of the silicon-containing material is fixed to the surface of each of the plurality of carbon fiber parts 1. Therefore, the surface of each of the plurality of carbon fiber parts 1 is coated with the powder of the silicon-containing material, and a plurality of coated parts 2 is formed. However, instead of applying the dispersion liquid to the plurality of carbon fiber parts 1, the plurality of carbon fiber parts 1 may be immersed in the dispersion liquid.

[0131] In this case, when the dispersion liquid is impregnated into the interior of the plurality of carbon fiber parts 1, the amount of the dispersion liquid impregnated decreases as the distance (depth) required for the impregnation increases, and therefore the amount of powder of the silicon-containing material adhered to the surface of each of the plurality of carbon fiber parts 1 decreases.

[0132] As a result, the average fiber diameter AD, weight percentage MA, and porosity R each change continuously in the thickness direction H, resulting in the assembly of a negative electrode 10 including a lower portion 10X and an upper portion 10Y. This negative electrode 10 has a single-layer structure because it includes the lower portion 10X and the upper portion 10Y that are physically integrated with each other. However, the average fiber diameter ADX, weight percentage MAX, and porosity RX are different from the average fiber diameter ADY, weight percentage MAY, and porosity RY, respectively.

[0133] In this case, the weight ratios MAX and MAY can be adjusted by changing the dispersion concentration, impregnation speed, drying conditions, etc. The porosities RX and RY can be adjusted by changing the dispersion concentration, impregnation speed, drying conditions, etc. along with the initial porosity R.

[0134] When the dispersion liquid is impregnated into the interiors of the plurality of carbon fiber parts 1, a suction device or the like may be used to suck the dispersion liquid from the side opposite to the side where the dispersion liquid is impregnated into the interiors of the plurality of carbon fiber parts 1. This facilitates impregnation of the dispersion liquid into the interiors of the plurality of carbon fiber parts 1, thereby facilitating the formation of a plurality of coated parts 2. In this case, the weight proportions MAX and MY can each be adjusted by changing the suction conditions or the like.

[0135] (Baking of the negative electrode 10, etc.) Finally, if necessary, the negative electrode 10 is pressed using a press or the like, and then the negative electrode 10 is fired. In this case, the porosities RX and RY can be adjusted by changing the pressing pressure. The firing temperature can be set arbitrarily.

[0136] This completes the negative electrode 10, which includes a plurality of carbon fiber portions 1 and a plurality of coating portions 2 and has a plurality of voids 10G. In this case, the average fiber diameter AD, weight percentage MA, and porosity R can be adjusted according to the average fiber diameters ADX and ADY, weight percentages MAX and MAY, and porosities RX and RY, respectively.

[0137] <1-4. Actions and Effects> This negative electrode 10 includes the above-mentioned multiple carbon fiber portions 1 and multiple coating portions 2, and has multiple voids 10G, and one or more of the average fiber diameter AD, weight proportion MA, and porosity R are different between the lower portion 10X and the upper portion 10Y.

[0138] In this case, as described above, the difference between the physical properties of the lower portion 10X and the physical properties of the upper portion 10Y is utilized to obtain a series of actions described below.

[0139] First, inside the negative electrode 10, a conductive network (three-dimensional mesh structure) is formed by the plurality of carbon fiber parts 1 containing a conductive carbon-containing material, thereby improving the conductivity.

[0140] Secondly, since each of the plurality of covering portions 2 contains a silicon-containing material that is excellent in occluding and releasing the electrode reactant, a high energy density can be obtained.

[0141] Third, since a plurality of voids 10G having different inner diameters are formed inside the negative electrode 10, the electrode reactant is more likely to move through the plurality of voids 10G during the electrode reaction, and the electrode reaction is more likely to proceed smoothly even when the electrode reaction is repeated. In this case, the movement rate of the electrode reactant is likely to be rate-determining, particularly in the upper portion 10Y located farther from the separator in the secondary battery, but the electrode reactant is more likely to move smoothly even when the current value during the electrode reaction increases.

[0142] Fourth, since multiple voids 10G having inner diameters of discontinuous sizes are distributed inside the negative electrode 10, the electrode reactant material can move more easily during the electrode reaction, and the electrode reaction can proceed more smoothly even when the electrode reaction is repeated.

[0143] For these reasons, a high energy density can be obtained, the electrode reactant can be remarkably easily moved during the electrode reaction, and the electrode reaction can proceed remarkably smoothly even when repeated. Therefore, a secondary battery using the negative electrode 10 can have excellent initial capacity characteristics, excellent load characteristics, and excellent cycle characteristics.

[0144] In addition, since the above-described negative electrode 10 does not require a metal current collector, it is possible to reduce the weight and increase the weight energy density (Wh / kg) compared to when a metal current collector is used.

[0145] In particular, if the average fiber diameter ADX is smaller than the average fiber diameter ADY, multiple carbon fiber portions 1 having a relatively small fiber diameter AD are more likely to be arranged near the coating portion 2 (silicon-containing material) in the lower portion 10X located closer to the separator in the secondary battery, which makes it easier to eliminate poor electronic contact within the negative electrode 10 during the electrode reaction. This makes it easier for the electrode reactant to migrate and allows the electrode reaction to proceed more smoothly even when repeated, resulting in even greater effects. In this case, if the average fiber diameter ADY is 0.0003 to 0.5 times the average fiber diameter ADX, the electrode reactant can migrate sufficiently and the electrode reaction can proceed sufficiently even when repeated, resulting in even greater effects.

[0146] Furthermore, if the weight percentage MAX is greater than the weight percentage MAY, the electrode reactant material is more likely to be occluded and released while the expansion and contraction of the negative electrode 10 is more effectively suppressed, thereby achieving a higher effect. In this case, if the weight percentage MAX is 1.04 to 4.65 times the weight percentage MAY, the electrode reactant material is more likely to be occluded and released while the expansion and contraction of the negative electrode 10 is sufficiently suppressed, thereby achieving an even higher effect.

[0147] Furthermore, if the porosity RY is greater than the porosity RX, the electrode reactant material moves more easily and the electrode reaction proceeds more smoothly even when repeated, thereby achieving a higher effect. In this case, if the porosity RY is 1.1 to 4.5 times the porosity RX, the electrode reactant material moves more easily and the electrode reaction proceeds more smoothly even when repeated, thereby achieving an even higher effect.

[0148] Furthermore, if the average fiber diameter AD of the entire negative electrode 10 is 10 nm to 12,000 nm, the weight percentage MA of the entire negative electrode 10 is 40 wt % to 80 wt %, and the porosity R of the entire negative electrode 10 is 40 vol % to 70 vol %, the expansion and contraction of the negative electrode 10 is sufficiently suppressed, the electrode reactant material is sufficiently easy to move, and the electrode reaction proceeds sufficiently even when repeated, thereby achieving a greater effect.

[0149] Furthermore, if the silicon content in each of the multiple coating portions 2 (silicon-containing material) is 80% by weight or more, a significantly high energy density can be obtained while ensuring electrical conductivity, thereby achieving a greater effect.

[0150] <2. Secondary battery> Next, a secondary battery according to an embodiment of the present technology, more specifically, an example of a secondary battery using the above-described negative electrode 10 will be described.

[0151] As described above, the secondary battery described here is a secondary battery that obtains battery capacity by utilizing the occlusion and release of an electrode reactant, and includes a positive electrode, a negative electrode, a separator, and an electrolytic solution that is a liquid electrolyte. As described above, the type of electrode reactant is not particularly limited.

[0152] In the following, we will take the case where the electrode reactant is lithium as an example. A secondary battery that obtains battery capacity by utilizing the absorption and desorption of lithium is called a lithium ion secondary battery. In this lithium ion secondary battery, lithium is absorbed and desorbed in the ionic state.

[0153] In this case, the charge capacity of the negative electrode is larger than the discharge capacity of the positive electrode. That is, the electrochemical capacity per unit area of the negative electrode is set to be larger than the electrochemical capacity per unit area of the positive electrode. This is to prevent deposition of electrode reactants on the surface of the negative electrode during charging.

[0154] <2-1.Configuration> Fig. 4 shows a perspective view of the secondary battery. Fig. 5 shows an enlarged cross-sectional view of the battery element 30 shown in Fig. 4. However, Fig. 4 shows a state in which the exterior film 20 and the battery element 30 are separated from each other, and Fig. 5 shows only a portion of the battery element 30. Below, reference will be made occasionally to Figs. 1 to 3, which have already been described, and the components of the negative electrode 10, which have already been described, will be cited.

[0155] 4 and 5, this secondary battery includes an exterior film 20, a battery element 30, a positive electrode lead 41, a negative electrode lead 42, and sealing films 51 and 52. The secondary battery described here is a laminate film type secondary battery that uses a flexible exterior film 20.

[0156] [Exterior film] 4, the exterior film 20 is a flexible exterior member that houses the battery element 30, and has a sealed bag-like structure with the battery element 30 housed inside. Therefore, the exterior film 20 houses an electrolyte solution together with a positive electrode 31 and a negative electrode 32, which will be described later.

[0157] Here, the exterior film 20 is a single film-like member that is folded in a folding direction F. The exterior film 20 is provided with a recessed portion 20U (so-called deep drawn portion) for accommodating the battery element 30.

[0158] Specifically, the exterior film 20 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, and when the exterior film 20 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.

[0159] However, the configuration (number of layers) of the exterior film 20 is not particularly limited, and may be one layer, two layers, or four or more layers.

[0160] [Battery element] As shown in FIGS. 4 and 5, the battery element 30 is a power generating element that includes a positive electrode 31, a negative electrode 32, a separator 33, and an electrolyte (not shown), and is housed inside the exterior film 20.

[0161] This battery element 30 is a so-called laminated electrode body, and therefore the positive electrodes 31 and the negative electrodes 32 are laminated one upon the other with separators 33 interposed therebetween. The number of positive electrodes 31, negative electrodes 32, and separators 33 laminated is not particularly limited. Here, a plurality of positive electrodes 31 and a plurality of negative electrodes 32 are alternately laminated with separators 33 interposed therebetween.

[0162] (positive electrode) As shown in FIG. 5, the positive electrode 31 includes a positive electrode current collector 31A and a positive electrode active material layer 31B.

[0163] The positive electrode current collector 31A has a pair of surfaces on which the positive electrode active material layer 31B is provided. The positive electrode current collector 31A contains a conductive material such as a metal material, and a specific example of the metal material is aluminum.

[0164] 4, the positive electrode current collector 31A includes a protrusion 31AT on which the positive electrode active material layer 31B is not provided, and the plurality of protrusions 31AT are joined to each other to form a single lead. Here, the protrusion 31AT is integrated with the portion other than the protrusion 31AT. However, since the protrusion 31AT is separate from the portion other than the protrusion 31AT, it may be joined to the portion other than the protrusion 31AT.

[0165] The positive electrode active material layer 31B contains one or more types of positive electrode active materials capable of absorbing and releasing lithium, but may also contain one or more types of other materials such as a positive electrode binder and a positive electrode conductive agent.

[0166] Here, the positive electrode active material layer 31B is provided on both sides of the positive electrode current collector 31A. However, the positive electrode active material layer 31B may be provided on only one side of the positive electrode current collector 31A, on the side where the positive electrode 31 faces the negative electrode 32. The method for forming the positive electrode active material layer 31B is not particularly limited, and specifically, it may be one or more of coating methods or the like.

[0167] The type of positive electrode active material is not particularly limited, but specifically includes a lithium-containing compound. This lithium-containing compound is a compound containing lithium and one or more transition metal elements as constituent elements, and may further contain one or more other elements as constituent elements. The type of other element is not particularly limited as long as it is an element other than lithium and transition metal elements, but specifically includes elements belonging to Groups 2 to 15 of the long period periodic table. The type of lithium-containing compound is not particularly limited, but specifically includes oxides, phosphate compounds, silicate compounds, borate compounds, etc.

[0168] Specific examples of oxides are LiNiO2, LiCoO2, LiCo 0.98 Al 0.01 Mg 0.01 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O2, Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 )O2 and LiMn2O4. Specific examples of phosphate compounds include LiFePO4, LiMnPO4, LiFe 0.5 Mn 0.5 PO4 and LiFe 0.3 Mn 0.7 PO4, etc.

[0169] The positive electrode binder contains one or more of synthetic rubbers and polymeric compounds. Specific examples of synthetic rubbers include styrene-butadiene rubber, fluorine-containing rubber, and ethylene-propylene-diene. Specific examples of polymeric compounds include polyvinylidene fluoride, polyimide, and carboxymethyl cellulose.

[0170] The positive electrode conductive agent contains one or more conductive materials such as carbon materials, and specific examples of the carbon materials include graphite, carbon black, acetylene black, ketjen black, and carbon nanotubes. However, the conductive material may also be a metal material or a polymer compound.

[0171] (Negative electrode) 5, the negative electrode 32 faces the positive electrode 31 via the separator 33 and is capable of absorbing and desorbing lithium. The negative electrode 32 has a configuration similar to that of the negative electrode 10 (the lower portion 10X and the upper portion 10Y) described above, and therefore includes a plurality of carbon fiber portions 1 and a plurality of coating portions 2, and has a plurality of voids 10G. As described above, the lower portion 10X is located closer to the separator 33 than the upper portion 10Y, and the upper portion 10Y is located farther from the separator 33 than the lower portion 10X.

[0172] In this negative electrode 32, lithium is absorbed and released mainly in each of the plurality of coating portions 2. However, lithium may be absorbed and released not only in each of the plurality of coating portions 2 but also in the plurality of carbon fiber portions 1.

[0173] As shown in FIG. 4, the negative electrode 32 includes a protrusion 31AT consisting of a portion of the carbon fiber portion 1 that is not provided with a plurality of coating portions 2, and the plurality of protrusions 31AT are joined together to form a single lead.

[0174] (separator) 5, the separator 33 is an insulating porous film interposed between the positive electrode 31 and the negative electrode 32, and allows lithium ions to pass through while preventing contact (short circuit) between the positive electrode 31 and the negative electrode 32. The separator 33 contains a polymer compound such as polyethylene.

[0175] (electrolyte) The electrolyte solution is impregnated into each of the positive electrode 31, the negative electrode 32, and the separator 33, and contains a solvent and an electrolyte salt.

[0176] The solvent contains one or more of non-aqueous solvents (organic solvents) such as carbonate ester compounds, carboxylic acid ester compounds, and lactone compounds, and the electrolyte solution containing such a non-aqueous solvent is a so-called non-aqueous electrolyte solution.

[0177] Carbonate compounds include cyclic carbonates and chain carbonates. Specific examples of cyclic carbonates include ethylene carbonate and propylene carbonate. Specific examples of chain carbonates include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0178] The carboxylic acid ester compound is a chain carboxylic acid ester, etc. Specific examples of the chain carboxylic acid ester include methyl acetate, ethyl acetate, trimethylmethyl acetate, methyl propionate, ethyl propionate, and propyl propionate.

[0179] The lactone compound is a lactone, etc. Specific examples of lactones include γ-butyrolactone and γ-valerolactone.

[0180] The electrolyte salt contains one or more types of light metal salts such as lithium salts.

[0181] Specific examples of lithium salts include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium bis(fluorosulfonyl)imide (LiN(FSO)), lithium bis(trifluoromethanesulfonyl)imide (LiN(CFSO)), lithium bis(oxalato)borate (LiB(C0)), lithium difluoro(oxalato)borate (LiB(C0)F), lithium monofluorophosphate (LiPFO), and lithium difluorophosphate (LiPF0).

[0182] 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.

[0183] The electrode solution may further contain one or more additives, including, but not limited to, unsaturated cyclic carbonates, halogenated carbonates, phosphates, acid anhydrides, nitrile compounds, and isocyanate compounds.

[0184] Specific examples of unsaturated cyclic carbonates include vinylene carbonate, vinylethylene carbonate, and methyleneethylene carbonate. Specific examples of halogenated carbonates include halogenated cyclic carbonates and halogenated chain carbonates. Specific examples of halogenated cyclic carbonates include monofluoroethylene carbonate and difluoroethylene carbonate. Specific examples of halogenated chain carbonates include fluoromethylmethyl carbonate. Specific examples of phosphates include trimethyl phosphate and triethyl phosphate.

[0185] Acid anhydrides include dicarboxylic acid anhydrides, disulfonic acid anhydrides, and carboxylic acid sulfonic acid anhydrides. A specific example of a dicarboxylic acid anhydride is succinic acid anhydride. A specific example of a disulfonic acid anhydride is ethanedisulfonic acid anhydride. A specific example of a carboxylic acid sulfonic acid anhydride is sulfobenzoic acid anhydride.

[0186] Nitrile compounds include mononitrile compounds, dinitrile compounds, and trinitrile compounds. A specific example of a mononitrile compound is acetonitrile. A specific example of a dinitrile compound is succinonitrile. A specific example of a trinitrile compound is 1,2,3-propanetricarbonitrile. A specific example of an isocyanate compound is hexamethylene diisocyanate.

[0187] [Positive lead] As shown in Fig. 4, the positive electrode lead 41 is a positive electrode terminal connected to the assembly of the multiple protrusions 31AT of the positive electrode 31, and is led from the inside to the outside of the exterior film 20. The positive electrode lead 41 contains a conductive material such as a metal material, and a specific example of the metal material is aluminum. The shape of the positive electrode lead 41 is not particularly limited, but specifically may be either a thin plate shape or a mesh shape.

[0188] [Negative lead] As shown in FIG. 4 , the negative electrode lead 42 is a negative electrode terminal connected to the assembly of the multiple protrusions 32AT of the negative electrode 32, and is led from the inside to the outside of the exterior film 20. In particular, the negative electrode lead 42 is preferably connected to the carbon fiber portion 1 of the negative electrode 32. This is because electrical conductivity between the negative electrode 32 and the negative electrode lead 42 is improved. The negative electrode lead 42 contains a conductive material such as a metal material, and a specific example of the metal material is copper. Here, the lead-out direction of the negative electrode lead 42 is the same as the lead-out direction of the positive electrode lead 41. The details regarding the shape of the negative electrode lead 42 are the same as the details regarding the shape of the positive electrode lead 41.

[0189] [Sealing film] The sealing film 51 is inserted between the exterior film 20 and the positive electrode lead 41, and the sealing film 52 is inserted between the exterior film 20 and the negative electrode lead 42. However, one or both of the sealing films 51 and 52 may be omitted.

[0190] The sealing film 51 is a sealing member that prevents outside air and the like from entering the inside of the exterior film 20. The sealing film 51 also contains a polymer compound such as polyolefin that has adhesiveness to the positive electrode lead 41, and the polyolefin is polypropylene or the like.

[0191] The configuration of the sealing film 52 is the same as the configuration of the sealing film 51, except that the sealing film 52 is a sealing member that has adhesiveness to the negative electrode lead 42. That is, the sealing film 52 contains a polymer compound such as polyolefin that has adhesiveness to the negative electrode lead 42.

[0192] <2-2. Operation> When the secondary battery is charged, lithium is released from the positive electrode 31 in the battery element 30 and is absorbed into the negative electrode 32 via the electrolyte. On the other hand, when the secondary battery is discharged, lithium is released from the negative electrode 32 in the battery element 30 and is absorbed into the positive electrode 31 via the electrolyte. During these charging and discharging times, lithium is absorbed and released in an ionic state.

[0193] <2-3. Manufacturing method> When manufacturing a secondary battery, the positive electrode 31 and the negative electrode 32 are each produced and the electrolyte solution is prepared according to the procedure described below as an example, and then the secondary battery is assembled and subjected to a stabilization process after assembly.

[0194] [Preparation of positive electrode] First, a paste-like cathode mixture slurry is prepared by adding a mixture (cathode mixture) of a cathode active material, a cathode binder, and a cathode conductive agent to a solvent. This solvent may be an aqueous solvent or an organic solvent. Next, the cathode mixture slurry is applied to both surfaces (excluding the protrusions 31AT) of the cathode current collector 31A including the protrusions 31AT to form the cathode active material layer 31B. Finally, the cathode active material layer 31B is compression-molded using a roll press or the like. In this case, the cathode active material layer 31B may be heated, or the compression molding may be repeated multiple times. As a result, the cathode active material layer 31B is formed on both surfaces of the cathode current collector 31A, thereby producing the cathode 31.

[0195] [Preparation of negative electrode] The negative electrode 32 including the protrusion 32AT is produced by a procedure similar to that for producing the negative electrode 10 described above.

[0196] [Preparation of electrolyte] An electrolyte salt is added to a solvent, whereby the electrolyte salt is dispersed or dissolved in the solvent, thereby preparing an electrolytic solution.

[0197] [Secondary battery assembly] First, a laminate (not shown) is prepared by alternately stacking positive electrodes 31 and negative electrodes 32 with separators 33 interposed therebetween. This laminate has the same configuration as that of the battery element 30, except that the positive electrodes 31, negative electrodes 32, and separators 33 are not impregnated with an electrolyte solution.

[0198] Next, the plurality of protrusions 31AT are joined to one another, and the plurality of protrusions 32AT are joined to one another. Next, the positive electrode lead 41 is joined to the joined body of the plurality of protrusions 31AT, and the negative electrode lead 42 is connected to the joined body of the plurality of protrusions 32AT.

[0199] Next, after the laminate is housed inside the recessed portion 20U, the exterior films 20 (adhesive layer / metal layer / surface protection layer) are folded to face each other. Next, the outer peripheral edges of two sides of the facing exterior films 20 (adhesive layers) are joined together using a heat fusion method or the like, thereby housing the laminate inside the bag-shaped exterior film 20.

[0200] Finally, after injecting an electrolyte solution into the bag-shaped exterior film 20, the outer peripheral edges of the remaining side of the exterior film 20 (bonding layer) are joined together using a heat fusion method or the like. In this case, a sealing film 51 is inserted between the exterior film 20 and the positive electrode lead 41, and a sealing film 52 is inserted between the exterior film 20 and the negative electrode lead 42.

[0201] This allows the laminate to be impregnated with the electrolyte, producing a laminated electrode body, the battery element 30. The battery element 30 is then sealed inside the bag-shaped exterior film 20, and a secondary battery is assembled.

[0202] [Secondary battery stabilization] 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 forms a coating on the surface of each of the positive electrode 31 and the negative electrode 32, electrochemically stabilizing the state of the secondary battery. This completes the secondary battery.

[0203] <2-4. Actions and Effects> In this secondary battery, the negative electrode 32 has a configuration similar to that of the above-described negative electrode 10. Therefore, for the same reasons as those described for the negative electrode 10, excellent initial capacity characteristics, excellent load characteristics, and excellent cycle characteristics can be obtained.

[0204] Furthermore, if the secondary battery is a lithium ion secondary battery, a sufficient battery capacity can be stably obtained by utilizing the absorption and release of lithium, and therefore a greater effect can be obtained.

[0205] Other functions and effects of the secondary battery are the same as those of the negative electrode 10 described above.

[0206] <3. Modifications> Next, a modified example will be described.

[0207] The configurations of the negative electrode 10 and the secondary battery described above can be modified as appropriate, as described below, although any two or more of the series of modifications described below may be combined with each other.

[0208] [Variation 1] In the above-described manufacturing method of the negative electrode 10 (manufacturing method relating to intermittent changes), the negative electrode 10 is manufactured to have a two-layer structure using a lower portion 10X and an upper portion 10Y that are physically separated from each other in order to intermittently change the average fiber diameter AD, the weight fraction MA, and the porosity R in the thickness direction H. However, the layer structure of the negative electrode 10 is not limited to two layers and may be three or more layers.

[0209] In this case, the same effect can be obtained as long as one or more of the average fiber diameter AD, weight proportion MA, and porosity R are different between the lower portion 10X and the upper portion 10Y.

[0210] [Variation 2] As shown in FIG. 6, which corresponds to FIG. 2, the negative electrode 10 may further include a plurality of surface portions 3.

[0211] Each of the surface portions 3 is provided on the surface of each of the coating portions 2 and has a thickness T2. Each of the surface portions 3 contains one or more types of ion-conductive materials, which improves the ion conductivity of the negative electrode 10. The type of ion-conductive material is not particularly limited.

[0212] Specifically, the ion-conducting material is a solid electrolyte such as lithium phosphate nitride and lithium phosphate (Li3PO4). The composition of the lithium phosphate nitride is not particularly limited, but specifically, Li 3.30 PO 3.90 N 0.17 And so on.

[0213] The ion-conductive material is a gel electrolyte in which an electrolyte solution is held by a matrix polymer compound. The composition of the electrolyte solution is as described above. Specific examples of the matrix polymer compound include polyethylene oxide and polyvinylidene fluoride.

[0214] In particular, the ion-conductive material preferably contains a solid electrolyte, that is, preferably contains one or both of lithium phosphate nitride and lithium phosphate, because this sufficiently improves the ion conductivity of the negative electrode 10.

[0215] The surface portion 3 may be provided on the entire surface of the covering portion 2, or on only a part of the surface of the covering portion 2. In the latter case, a plurality of surface portions 3 spaced apart from one another may be provided on the surface of the covering portion 2.

[0216] The average thickness AT2 of the plurality of surface portions 3 is not particularly limited and can be set arbitrarily. The procedure for calculating the average thickness AT2 is the same as the procedure for calculating the average thickness AT1 described above, except that the thickness T2 of the surface portion 3 is measured instead of the thickness T1 of the covering portion 2.

[0217] The procedure for forming the multiple surface portions 3 is as follows. When a solid electrolyte is used as the ion-conductive material, the surface portions 3 are formed directly on the surface of the coating portion 2 using a gas phase method such as sputtering. When a gel electrolyte is used as the ion-conductive material, a solution containing an electrolyte solution, a matrix polymer compound, and a dilution solvent is applied to the surface of the coating portion 2, and then the solution is dried. Details regarding the type of solvent are as described above. Alternatively, the coating portion 2 may be immersed in the solution.

[0218] In this case, the ion conductivity of lithium ions is improved by utilizing the plurality of surface portions 3 inside the negative electrode 10, and therefore a greater effect can be obtained.

[0219] In particular, by utilizing a plurality of surface portions 3 containing an ion-conductive material, the negative electrode 10 can be applied to an all-solid-state battery. This is because the expansion and contraction of the negative electrode 10 is suppressed, thereby suppressing an increase in the interfacial resistance between the negative electrode 10 and the solid electrolyte. This allows the all-solid-state battery to achieve both ensuring safety and improving energy density.

[0220] [Variation 3] When the negative electrode 10 includes a plurality of surface portions 3 (Variation 2), the average thickness AT2 may be the same between the lower portion 10X and the upper portion 10Y, or may be different between the lower portion 10X and the upper portion 10Y. When the average thickness AT2 is different between the lower portion 10X and the upper portion 10Y, the average thickness AT2 in the lower portion 10X may be larger than the average thickness AT2 in the upper portion 10Y, or the average thickness AT2 in the lower portion 10X may be smaller than the average thickness AT2 in the upper portion 10Y. This is because the ionic conductivity of lithium ions inside the negative electrode 10 is further improved. The definition of the magnitude relationship regarding the average thickness AT2 is the same as the definition of the magnitude relationship regarding the average fiber diameter AD(ADX, ADY) described above.

[0221] In particular, it is preferable that the average thickness AT2 in the upper portion 10Y is larger than the average thickness AT2 in the lower portion 10X. This is because, although the migration rate of the electrode reactant tends to be rate-determining in the upper portion 10Y located farther from the separator in the secondary battery, the ion conductivity of lithium ions is improved in the upper portion 10Y, and therefore, lithium ions tend to migrate more smoothly even when the current value during charging and discharging increases.

[0222] [Variation 4] Furthermore, when the negative electrode 10 includes a plurality of surface portions 3 (Variation 2), the weight ratio MB (wt %), which is the ratio of the weight M3 of the plurality of surface portions 3 to the sum of the weight M1 of the plurality of carbon fiber portions 1, the weight M2 of the plurality of coating portions 2, and the weight M3 of the plurality of surface portions 3, may be the same between the lower portion 10X and the upper portion 10Y, or may be different between the lower portion 10X and the upper portion 10Y. This weight ratio MB is calculated based on the formula MB=[M3 / (M1+M2+M3)]×100.

[0223] Specifically, the negative electrode 10 has the weight proportion MB as described above, and also has a lower portion 10X and an upper portion 10Y as shown in Fig. 3. As a result, the lower portion 10X has the weight proportion MBX, and the upper portion 10Y has the weight proportion MBY, and therefore the weight proportions MBX and MBY are different from each other.

[0224] When the weight proportions MBX and MBY are different from each other, the electrode reactant is more likely to be occluded and released during the electrode reaction, unlike when the weight proportions MBX and MBY are the same.

[0225] The weight percentage MBX may be greater than or less than the weight percentage MBY. The definition of the magnitude relationship between the weight percentages MBX and MBY is the same as the definition of the magnitude relationship between the weight percentages MAX and MAY described above.

[0226] As described above, when the negative electrode 10 and the positive electrode face each other via a separator in a secondary battery, it is preferable that the weight percentage MB is larger in the upper portion 10Y than in the lower portion 10X, and therefore the weight percentage MBY is larger than the weight percentage MBX, because this makes it easier for the electrode reactant to be occluded and released during the electrode reaction.

[0227] [Variation 5] As shown in FIG. 7, which corresponds to FIG. 1, the negative electrode 10 may further include a plurality of additional carbon fiber portions 4.

[0228] 7, the plurality of additional carbon fiber portions 4 are a plurality of additional fiber portions having an average fiber diameter smaller than the average fiber diameter AD of the plurality of carbon fiber portions 1. Here, each of the plurality of additional carbon fiber portions 4 is fixed to the surface of each of the plurality of covering portions 2, and therefore is connected to the surface of each of the plurality of covering portions 2.

[0229] 7 shows a case where each of the plurality of additional carbon fiber portions 4 is linear for the sake of simplicity. However, the state (shape) of each of the plurality of additional carbon fiber portions 4 is not particularly limited, as in the case of the state of the plurality of carbon fiber portions 1 described above.

[0230] When the negative electrode 10 includes a plurality of additional carbon fiber parts 4 together with a plurality of carbon fiber parts 1, not only does the plurality of carbon fiber parts 1 form a conductive network, but the plurality of additional carbon fiber parts 4 also form a dense conductive network, thereby significantly improving the conductivity of the negative electrode 10.

[0231] In particular, it is preferable that some or all of the multiple additional carbon fiber portions 4 (multiple additional carbon fiber portions 4R) are each connected to two or more covering portions 2, respectively. This is because the two or more covering portions 2 are electrically connected to each other via one or more additional carbon fiber portions 4R. This forms a denser conductive network, further improving the conductivity of the negative electrode 10.

[0232] The average fiber diameter of the plurality of additional carbon fiber parts 4 is smaller than the average fiber diameter AD of the plurality of carbon fiber parts 1, specifically, 1 / 10,000 to 1 / 2 times, and preferably 1 / 300 to 1 / 5 times, the average fiber diameter AD. More specifically, the average fiber diameter of the plurality of additional carbon fiber parts 4 is 1 nm to 300 nm. This is because the plurality of additional carbon fiber parts 4 are easily dispersed inside the negative electrode 10, and the plurality of additional carbon fiber parts 4 are easily able to form a dense conductive network.

[0233] The procedure for calculating the average fiber diameter of the plurality of additional carbon fiber portions 4 is the same as the procedure for calculating the average fiber diameter AD described above, except that the fiber diameter of each of any 20 additional carbon fiber portions 4 is measured and the average value of the 20 fiber diameters is used as the average fiber diameter. However, when the fiber diameter is small, it is preferable to use a TEM rather than an SEM to observe the cross section of the negative electrode 10.

[0234] Each of the plurality of additional carbon fiber portions 4 contains carbon as a constituent element, and therefore contains a carbon-containing material, similar to each of the plurality of carbon fiber portions 1. Details regarding this carbon-containing material are as described above.

[0235] In particular, each of the plurality of additional carbon fiber parts 4 is preferably one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes, and vapor-grown carbon fibers, because the average fiber diameter becomes sufficiently small, making it easier for the plurality of additional carbon fiber parts 4 to be sufficiently dispersed inside the negative electrode 10 and for a denser conductive network to be formed.

[0236] In this case, as described above, the conductivity of the negative electrode 10 is significantly improved, and therefore a greater effect can be obtained.

[0237] [Variation 6] When the negative electrode 10 includes a plurality of additional carbon fiber portions 4 (Variation 5), the average fiber diameters of the plurality of additional carbon fiber portions 4 may be the same between the lower portion 10X and the upper portion 10Y, or may be different between the lower portion 10X and the upper portion 10Y. When the average fiber diameters are different between the lower portion 10X and the upper portion 10Y, the average fiber diameter in the lower portion 10X may be larger than the average fiber diameter in the upper portion 10Y, or the average fiber diameter in the lower portion 10X may be smaller than the average fiber diameter in the upper portion 10Y. This is because a dense conductive network is more likely to be formed inside the negative electrode 10, thereby further improving the conductivity of the negative electrode 10. The definition of the magnitude relationship regarding the average fiber diameter is the same as the definition of the magnitude relationship regarding the average fiber diameter AD (ADX, ADY) described above.

[0238] In particular, it is preferable that the average fiber diameter in the lower portion 10X is smaller than the average fiber diameter in the upper portion 10Y, because a dense conductive network is more likely to be formed in the lower portion 10X located closer to the separator in the secondary battery, thereby further improving the conductivity of the negative electrode 10.

[0239] [Variation 7] A porous film separator 33 was used. However, although not specifically shown here, a laminated separator including a polymer compound layer may be used instead of the separator 33.

[0240] 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 separators improve their adhesion to the positive electrode 31 and the negative electrode 32, respectively, thereby preventing the battery element 30 from slipping out of its winding. This reduces the likelihood of swelling of the secondary battery even if a decomposition reaction of the electrolyte occurs. The porous membrane has the same structure as the porous membrane described for the separator 33. The polymer compound layer includes a polymer compound such as polyvinylidene fluoride. This is because polyvinylidene fluoride and other polymers have excellent physical strength and are electrochemically stable.

[0241] 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 promote heat dissipation when the secondary battery generates heat, thereby improving the safety (heat resistance) of the secondary battery. The insulating particles may be one or both of inorganic particles and resin particles. Specific examples of inorganic particles include particles of aluminum oxide, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide, and zirconium oxide. Specific examples of resin particles include particles of acrylic resin and styrene resin.

[0242] When fabricating a laminated separator, a precursor solution containing a polymer compound and a solvent is prepared, and then the precursor solution is applied to one or both sides of a porous membrane. In this case, instead of applying the precursor solution to the porous membrane, the porous membrane may be immersed in the precursor solution. In addition, the precursor solution may contain a plurality of insulating particles.

[0243] Even when this laminated separator is used, the same effect can be obtained because lithium ions are able to move between the positive electrode 31 and the negative electrode 32. In this case, the safety of the secondary battery is particularly improved as described above, and therefore, a greater effect can be obtained.

[0244] [Variation 8] An electrolyte solution, which is a liquid electrolyte, was used. However, although not specifically shown here, an electrolyte layer, which is a gel electrolyte, may be used instead of the electrolyte solution.

[0245] In a battery element 30 using an electrolyte layer, positive electrodes 31 and negative electrodes 32 are alternately stacked with separators 33 and electrolyte layers interposed therebetween. In this case, an electrolyte layer is interposed between the positive electrode 31 and separator 33, and an electrolyte layer is interposed between the negative electrode 32 and separator 33. However, the electrolyte layer may be interposed only between the positive electrode 31 and separator 33, or only between the negative electrode 32 and separator 33.

[0246] 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 prevented. 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 the electrolytic solution, the polymer compound, and a dilution solvent is prepared, and then the precursor solution is applied to one or both surfaces of each of the positive electrode 31 and the negative electrode 32. Details regarding the type of solvent are as described above.

[0247] Even when this electrolyte layer is used, the same effect can be obtained because lithium ions can move between the positive electrode 31 and the negative electrode 32 via the electrolyte layer. In this case, leakage of the electrolyte solution is prevented as described above, and therefore a greater effect can be obtained.

[0248] <4. Uses of secondary batteries> Finally, the uses (application examples) of the secondary battery will be described.

[0249] The use of the secondary battery is not particularly limited. The secondary battery used as a power source may be a main power source for electronic devices, electric vehicles, etc., or an auxiliary power source. A main power source is a power source that is used preferentially regardless of the presence or absence of other power sources. An auxiliary power source is a power source that is used in place of the main power source or a power source that can be switched from the main power source.

[0250] 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. In these uses, one secondary battery may be used, or multiple secondary batteries may be used.

[0251] The battery pack may use a single cell or a battery pack. The electric vehicle is a vehicle that operates (travels) using a secondary battery as a driving power source, and may be a hybrid vehicle that also has a driving source other than the secondary battery. In a home power storage system, the power stored in the secondary battery, which is a power storage source, can be used to power home electrical appliances, etc.

[0252] Here, an example of an application of a secondary battery will be specifically described. The configuration described below is merely an example and can be modified as appropriate.

[0253] Figure 8 shows the block diagram of a battery pack. 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.

[0254] 8, the battery pack includes a power supply 61 and a circuit board 62. The circuit board 62 is connected to the power supply 61 and includes a positive terminal 63, a negative terminal 64, and a temperature detection terminal 65.

[0255] The power source 61 includes one secondary battery. In this secondary battery, the positive electrode lead is connected to a positive electrode terminal 63, and the negative electrode lead is connected to a negative electrode terminal 64. The power source 61 is connected to the outside via the positive electrode terminal 63 and the negative electrode terminal 64, and is therefore capable of charging and discharging. The circuit board 62 includes a control unit 66, a switch 67, a thermosensitive resistor (PTC) element 68, and a temperature detection unit 69. However, the PTC element 68 may be omitted.

[0256] The control unit 66 includes a central processing unit (CPU) and memory, and controls the operation of the entire battery pack. The control unit 66 detects and controls the usage state of the power source 61 as necessary.

[0257] When the voltage of the power supply 61 (secondary battery) reaches the overcharge detection voltage or the overdischarge detection voltage, the control unit 66 turns off the switch 67 to prevent the charging current from flowing through the current path of the power supply 61. The overcharge detection voltage is not particularly limited, but specifically, it is 4.2±0.05V, and the overdischarge detection voltage is not particularly limited, but specifically, it is 2.4±0.1V.

[0258] Switch 67 includes a charge control switch, a discharge control switch, a charge diode, a discharge diode, etc., and switches between the presence and absence of a connection between power supply 61 and an external device in response to an instruction from control unit 66. Switch 67 includes a field effect transistor (MOSFET) using a metal oxide semiconductor, etc., and the charge / discharge current is detected based on the ON resistance of switch 67.

[0259] The temperature detection unit 69 includes a temperature detection element such as a thermistor, measures the temperature of the power supply 61 using the temperature detection terminal 65, and outputs the temperature measurement result to the control unit 66. The temperature measurement result measured by the temperature detection unit 69 is used when the control unit 66 controls charging and discharging in the event of abnormal heat generation, and when the control unit 66 performs correction processing when calculating the remaining capacity. [Example]

[0260] An embodiment of the present technology will be described.

[0261] <Examples 1 to 20 and Comparative Examples 1 to 3> After the secondary batteries were fabricated, the characteristics of the secondary batteries were evaluated. Here, two types of secondary batteries (first secondary battery and second secondary battery) were fabricated to evaluate the characteristics of the secondary batteries.

[0262] [Preparation of the first secondary battery] First secondary batteries (Examples 1 to 20 and Comparative Example 3) were fabricated according to the procedure described below. The first secondary batteries were laminate film-type lithium ion secondary batteries (battery capacity = 7 mAh to 12 mAh) shown in Figs. 4 and 5.

[0263] In the following description, the components of the negative electrode 10 shown in FIGS. 1 to 3 will be referenced where appropriate to explain the process of producing the negative electrode 32.

[0264] (Preparation of positive electrode) First, the positive electrode active material (LiNi 0.8 Co 0.15 Al 0.05A positive electrode mixture was prepared by mixing 97 parts by mass of ethylenediamine dinitrate (O2), 2.2 parts by mass of a positive electrode binder (polyvinylidene fluoride), and 0.8 parts by mass of a positive electrode conductive agent (Ketjen black). The positive electrode mixture was then added to a solvent (N-methyl-2-pyrrolidone, an organic solvent), and the solvent was stirred using a planetary mixer to prepare a paste-like positive electrode mixture slurry. The positive electrode mixture slurry was then applied to one side (excluding the protrusions 31AT) of a positive electrode current collector 31A (aluminum foil, thickness = 15 μm) including the protrusions 31AT using a coating device, and the positive electrode mixture slurry was then dried (drying temperature = 120°C) to form a positive electrode active material layer 31B. Finally, the positive electrode active material layer 31B was compression-molded using a hand press (volume density of the positive electrode active material layer 31B = 3.5 g / cm). 3 ) In this way, the positive electrode 31 including the protrusion 31AT was produced.

[0265] (Preparation of negative electrode) First, a plurality of fibrous carbon materials (average fiber diameter ADX) were prepared to form the lower portion 10X. Vapor-grown carbon fiber (VGCF), carbon nanotubes (CNT), and carbon fiber (CF) were used as the fibrous carbon materials according to the average fiber diameter ADX. The average fiber diameters ADX (nm) are as shown in Tables 1 and 2.

[0266] Next, a vacuum deposition method was used to deposit a silicon-containing material (elemental silicon (Si)) on each surface of the plurality of fibrous carbon materials to form a plurality of coating portions 2 (weight proportion MAX). In this case, silicon (purity = 99.9%) was used as the deposition source, and two deposition sources were arranged to sandwich the plurality of fibrous carbon materials. In addition, by not depositing the silicon-containing material on some of the plurality of fibrous carbon materials, the portion of the plurality of fibrous carbon materials on which the plurality of coating portions 2 were not formed became the protrusion 32AT. The weight proportion MAX (weight %) is as shown in Tables 1 and 2.

[0267] Subsequently, in order to form the upper portion 10Y, a plurality of covering portions 2 (weight proportion MAY) were formed using a plurality of fibrous carbon materials (average fiber diameter ADY) by the same procedure.

[0268] Subsequently, using a multi-layer laminating device, the two types of fibrous carbon materials on which the above-mentioned multiple coating portions 2 were formed were interwoven, thereby forming a lower portion 10X including multiple carbon fiber portions 1 and multiple coating portions 2, and an upper portion 10Y including multiple carbon fiber portions 1 and multiple coating portions 2, and the lower portion 10X and the upper portion 10Y were laminated together. Thus, the negative electrode 32 was assembled.

[0269] Finally, the negative electrode 32 was pressed in a room temperature environment (temperature=23° C.), and then heated in a nitrogen (N 2 ) atmosphere (heating temperature=350° C., heating time=3 hours).

[0270] This completed the negative electrode 32 having a two-layer structure including a lower portion 10X (porosity RX) and an upper portion 10Y (porosity RY) and having a plurality of voids 10G. The porosity RX (volume %) is as shown in Tables 1 and 2.

[0271] When producing this negative electrode 32, the weight ratios MAX and MAY were changed by adjusting the deposition amount of the silicon-containing material, and the porosities RX and RY were changed by adjusting both the deposition amount of the silicon-containing material and the pressing pressure of the negative electrode 32.

[0272] Here, one or more of three physical property values (average fiber diameter AD, weight percentage MA, and porosity R) were made different between the lower portion 10X and the upper portion 10Y, as shown in Tables 1 and 2. The "magnification ratios" shown in Tables 1 and 2 represent the magnification ratios that determine the magnitude relationship of each physical property value (average fiber diameters ADX, ADY, weight percentages MAX, MAY, and porosities RX, RY).

[0273] Specifically, the "magnification factor" for the average fiber diameter AD represents the magnification factor (ADX / ADY) of the average fiber diameter ADX to the average fiber diameter ADY. Therefore, a magnification factor smaller than 1 means that the average fiber diameter ADX is smaller than the average fiber diameter ADY.

[0274] The "magnification factor" for the weight proportion MA represents the magnification of the weight proportion MAX to the weight proportion MAY (=MAX / MAY). Therefore, a magnification factor greater than 1 indicates that the weight proportion MAX is greater than the weight proportion MAY.

[0275] The "magnification factor" regarding the porosity R represents the magnification of the porosity RY relative to the porosity RX. Therefore, a magnification factor greater than 1 indicates that the porosity RY is greater than the porosity RX.

[0276] (Preparation of Electrolyte) After adding an electrolyte salt (lithium hexafluorophosphate) to the solvent, the solvent was stirred. The solvent used was a mixture of ethylene carbonate, a cyclic carbonate, dimethyl carbonate, a chain carbonate, and monofluoroethylene carbonate, an additive (halogenated cyclic carbonate). The mixing ratio (weight ratio) of the solvents was ethylene carbonate:dimethyl carbonate:monofluoroethylene carbonate = 30:60:10. The content of the electrolyte salt relative to the solvent was 1 mol / kg. In this way, an electrolytic solution was prepared.

[0277] (First secondary battery assembly) First, a positive electrode 31 including a protrusion 31AT and a negative electrode 32 including a protrusion 32AT were alternately stacked with a separator 33 (microporous polyethylene film, thickness = 20 μm) interposed therebetween to prepare a laminate (positive electrode 31 / separator 33 / negative electrode 32).

[0278] Subsequently, a positive electrode lead 41 (aluminum foil) was joined to the protruding portion 31AT, and a negative electrode lead 42 (copper foil) was joined to the protruding portion 32AT.

[0279] Next, the exterior film 20 (adhesive layer / metal layer / surface protection layer) was folded so as to sandwich the laminate housed inside the recessed portion 20U, and then the outer peripheral edges of two sides of the exterior film 20 (adhesive layer) were heat-sealed to each other, thereby housing the laminate inside the bag-shaped exterior film 20. The exterior film 20 used was an aluminum laminate film in which a adhesive layer (polypropylene film, thickness = 30 μm), a metal layer (aluminum foil, thickness = 40 μm), and a surface protection layer (nylon film, thickness = 25 μm) were laminated in this order from the inside.

[0280] Finally, after pouring the electrolyte solution into the bag-shaped exterior film 20, the outer peripheral edges of the remaining side of the exterior film 20 (bonding layer) were heat-sealed together in a reduced pressure environment. In this case, a sealing film 51 (polypropylene film, thickness = 5 μm) was inserted between the exterior film 20 and the positive electrode lead 41, and a sealing film 52 (polypropylene film, thickness = 5 μm) was inserted between the exterior film 20 and the negative electrode lead 42.

[0281] As a result, the laminate was impregnated with the electrolyte solution, thereby producing the battery element 30. Therefore, the battery element 30 was sealed inside the exterior film 20, and a secondary battery was assembled.

[0282] When assembling the first secondary battery, the thickness of the positive electrode active material layer 31B was adjusted so that the capacity ratio, i.e., the ratio of the positive electrode charge capacity to the negative electrode charge capacity (= positive electrode charge capacity / negative electrode charge capacity), was 0.7.

[0283] (Stabilization of the first secondary battery) The first secondary battery was charged and discharged for one cycle in a room temperature environment (temperature = 23°C). During charging, the battery was charged at a constant current of 0.1 C until the voltage reached 4.2 V, and then at a constant voltage of 4.2 V, the battery was charged at a constant current of 0.1 C until the voltage reached 2.0 V. 0.1 C is the current value at which the battery capacity (theoretical capacity) is fully discharged in 10 hours, and 0.025 C is the current value at which the battery capacity is fully discharged in 40 hours.

[0284] As a result, a coating was formed on the surface of each of the positive electrode 31 and the negative electrode 32, and the state of the first secondary battery was electrochemically stabilized, thereby completing the first secondary battery.

[0285] [Preparation of the second secondary battery] A second secondary battery (battery capacity = 10 mAh to 15 mAh) was fabricated using the same procedure as the first secondary battery described above, except that a lithium metal plate (thickness = 100 μm) was used instead of the positive electrode 31.

[0286] Here, the first secondary battery using the positive electrode 31 as the counter electrode for the negative electrode 32 is a so-called full cell, whereas the second secondary battery using a lithium metal plate as the counter electrode for the negative electrode 32 is a so-called half cell.

[0287] [Preparation of a secondary battery for comparison] For comparison, two types of secondary batteries for comparison (Comparative Examples 1 and 2) were fabricated in the same manner except that a metal current collector was used to fabricate a comparative negative electrode.

[0288] To fabricate this negative electrode, 82 parts by mass of a negative electrode active material (silicon (Si), purity = 95%, median diameter D50 = 50 nm), 10 parts by mass (solid content equivalent) of a negative electrode binder (polyimide), 3 parts by mass of a negative electrode conductive agent (carbon black), and 5 parts by mass of another negative electrode conductive agent (carbon nanotube dispersion) were mixed together to form a negative electrode mixture. This carbon nanotube dispersion contained 0.8 parts by mass of carbon nanotubes and 4.2 parts by mass of a dispersion medium (polyvinylidene fluoride).

[0289] Next, the negative electrode mixture was added to a solvent (organic solvent N-methyl-2-pyrrolidone), and the organic solvent was stirred using a planetary mixer to prepare a paste-like negative electrode mixture slurry. The negative electrode mixture slurry was then applied to both sides of a metal current collector (copper foil (Cu), thickness = 10 μm or 6 μm) using a coating device, and the negative electrode mixture slurry was then dried to form a negative electrode active material layer. This completed the assembly of the negative electrode.

[0290] Finally, the negative electrode was pressed in a room temperature environment (temperature = 23°C), and then heated in a nitrogen atmosphere (heating temperature = 350°C, heating time = 3 hours).

[0291] In addition, the column "Metal current collector (thickness)" in Tables 1 and 2 indicates whether or not a metal current collector is used, and if so, the material and thickness (μm) of the metal current collector.

[0292] [Characteristic evaluation of secondary batteries] The characteristics of the secondary battery (initial capacity characteristics, load characteristics, and cycle characteristics) were evaluated, and the results shown in Tables 1 and 2 were obtained.

[0293] In this case, the initial capacity characteristics were evaluated using the second secondary battery (half cell), and the load characteristics and cycle characteristics were evaluated using the first secondary battery (full cell) according to the procedures described below.

[0294] (Initial capacity characteristics) The discharge capacity was measured by charging and discharging the secondary battery for one cycle while applying pressure to the secondary battery in a room temperature environment (temperature = 23 ° C.) From this, the initial capacity, which is an index for evaluating the initial capacity characteristics, was calculated based on the formula: initial capacity (mAh / g) = discharge capacity (mAh) / total weight (g) of the negative electrode 32.

[0295] In this case, pressure was applied to the secondary battery in the direction in which the positive electrode 31 and the negative electrode 32 were stacked together with the separator 33 interposed therebetween, and the secondary battery was charged and discharged while the positive electrode 31 and the negative electrode 32 were in close contact with each other via the separator 33. Note that the total weight of the negative electrode 32 described above includes the weight of a metal current collector when one is used, but does not include the weight of the metal current collector when one is not used.

[0296] During charging, the battery was charged at a constant current of 0.1 C until the voltage reached 0.005 V, and then at a constant voltage of 0.005 V, the battery was charged at a constant current of 0.1 C until the voltage reached 0.01 C. During discharging, the battery was discharged at a constant current of 0.1 C until the voltage reached 1.5 V. 0.01 C is the current value that fully discharges the battery capacity in 100 hours.

[0297] (Load characteristics) First, the secondary battery was charged and discharged for one cycle in a room temperature environment (temperature = 23°C) to measure the discharge capacity (discharge capacity at the first cycle).

[0298] During charging, the battery was charged at a constant current of 0.2 C until the voltage reached 4.2 V, and then at a constant voltage of 0.025 C at that voltage. During discharging, the battery was discharged at a constant current of 0.2 C until the voltage reached 2.5 V. 0.2 C is the current value that fully discharges the battery capacity in 5 hours.

[0299] Next, the secondary battery was charged and discharged for one cycle in the same environment to measure the discharge capacity (discharge capacity at the second cycle). The charge and discharge conditions were the same as those for the first cycle, except that the charge and discharge currents were both changed to 5C. 5C is the current value at which the battery capacity is fully discharged in 0.2 hours.

[0300] Finally, the load retention rate, which is an index for evaluating load characteristics, was calculated based on the formula: load retention rate (%)=(discharge capacity at second cycle / discharge capacity at first cycle)×100.

[0301] (Cycle characteristics) First, the secondary battery was charged and discharged for one cycle in a room temperature environment (temperature = 23°C) to measure the discharge capacity (discharge capacity at the first cycle). Next, the secondary battery was charged and discharged for 199 cycles in the same environment to measure the discharge capacity (discharge capacity at the 200th cycle). The charge and discharge conditions were the same as those for the first cycle when the load characteristics were evaluated.

[0302] Finally, the capacity retention rate, which is an index for evaluating cycle characteristics, was calculated based on the formula: capacity retention rate (%)=(discharge capacity at 200th cycle / discharge capacity at 1st cycle)×100.

[0303] (Normalization of characteristic values) The initial capacity values shown in Tables 1 and 2 are normalized values with the initial capacity value for the secondary battery of Comparative Example 1 using a metal current collector (copper foil with a thickness of 10 μm) set to 100. The same applies to the values of the load retention rate and capacity retention rate, which are normalized based on the secondary battery of Comparative Example 1.

[0304] [Table 1]

[0305] [Table 2]

[0306] [Consideration] As shown in Tables 1 and 2, the initial capacity, load retention rate, and capacity retention rate each varied significantly depending on the configuration of the negative electrode. In the following, the initial capacity, load retention rate, and capacity retention rate values in Comparative Example 1 are used as the comparison standard.

[0307] Specifically, when a metal current collector was used, reducing the thickness of the metal current collector (Comparative Example 2) increased the initial capacity, but both the load retention rate and capacity retention rate decreased.

[0308] In contrast, when multiple carbon fiber parts 1 and multiple coating parts 2 were used without using a metal current collector (Examples 1 to 20 and Comparative Example 3), the initial capacity, load retention rate, and capacity retention rate each varied depending on their configuration.

[0309] That is, when the average fiber diameter AD, weight ratio MA, and porosity R were the same between the lower portion 10X and the upper portion 10Y (Comparative Example 3), the load retention rate and capacity retention rate increased, but the initial capacity decreased significantly.

[0310] However, when one or more of the average fiber diameter AD, weight ratio MA, and porosity R were different between the lower portion 10X and the upper portion 10Y (Examples 1 to 20), the initial capacity, load retention rate, and capacity retention rate all increased.

[0311] In this case, when the average fiber diameter ADX was smaller than the average fiber diameter ADY, the initial capacity, load retention rate, and capacity retention rate all increased. Furthermore, when the weight percentage MAX was greater than the weight percentage MAY, the initial capacity, load retention rate, and capacity retention rate all increased. Furthermore, when the porosity RY was greater than the porosity RX, the initial capacity, load retention rate, and capacity retention rate all increased.

[0312] Furthermore, when the multiplier for the average fiber diameter AD was 0.0003 to 0.5, the initial capacity, load retention rate, and capacity retention rate all increased sufficiently. Furthermore, when the multiplier for the weight proportion MA was 1.04 to 4.65, the initial capacity, load retention rate, and capacity retention rate all increased sufficiently. Furthermore, when the multiplier for the porosity R was 1.1 to 4.5, the initial capacity, load retention rate, and capacity retention rate all increased sufficiently.

[0313] <Examples 21 to 23> As shown in Table 3, a secondary battery was produced using the same procedure as in Example 1, except that multiple surface portions 3 containing an ion-conductive material were formed in the process of producing the negative electrode 32, and then the characteristics of the secondary battery (initial capacity characteristics, load characteristics, and cycle characteristics) were evaluated.

[0314] As an ion-conducting material, lithium phosphate nitrate (Li 3.30 PO 3.90 N 0.17 ) and lithium phosphate (Li3PO4). The average thickness AT2 (nm) of the plurality of surface portions 3 in the lower portion 10X is as shown in Table 3.

[0315] The "magnification" shown in Table 3 represents the magnification of the average thickness AT2 in the upper portion 10Y to the average thickness AT2 in the lower portion 10X (=average thickness AT2 in the upper portion 10Y / average thickness AT2 in the lower portion 10X). Therefore, a magnification greater than 1 indicates that the average thickness AT2 in the upper portion 10Y is greater than the average thickness AT2 in the lower portion 10X.

[0316] When forming a plurality of surface portions 3, a sputtering method was used to deposit an ion conductive material on the surface of each of the plurality of covering portions 2. However, when forming a plurality of surface portions 3 containing lithium phosphate, lithium phosphate was used as the target, and when forming a plurality of surface portions 3 containing lithium oxynitride phosphate, lithium phosphate was used as the target in a nitrogen atmosphere.

[0317] [Table 3]

[0318] As shown in Table 3, when a plurality of surface portions 3 were formed (Examples 21 to 23), the initial capacity, load retention rate, and capacity retention rate all increased more than when a plurality of surface portions 3 were not formed (Example 1). In particular, when a plurality of surface portions 3 were formed, the initial capacity, load retention rate, and capacity retention rate all increased further as the magnification ratio for the average thickness AT2 increased.

[0319] [summary] The results shown in Tables 1 to 3 indicate that when the negative electrode 32 (negative electrode 10) includes the above-mentioned carbon fiber portions 1 and coating portions 2 and has multiple voids 10G, and when one or more of the average fiber diameter AD, weight fraction MA, and porosity R are different between the lower portion 10X and the upper portion 10Y, the initial capacity, load retention rate, and capacity retention rate all increase. Therefore, the secondary battery can achieve excellent initial capacity characteristics, excellent load characteristics, and excellent cycle characteristics.

[0320] 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.

[0321] Specifically, the secondary battery has been described as having a laminate film structure, but the structure of the secondary battery is not particularly limited, and other battery structures such as a cylindrical type, a prismatic type, a coin type, and a button type may also be used.

[0322] The battery element has been described as having a stacked structure. However, the structure of the battery element is not particularly limited, and other structures such as a wound structure and a zigzag structure may be used. In the wound structure, the positive electrode and the negative electrode are wound with a separator interposed therebetween, and in the zigzag structure, the positive electrode and the negative electrode are folded in a zigzag pattern while facing each other with the separator interposed therebetween.

[0323] 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.

[0324] The effects described in this specification are merely examples, and 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.

Claims

1. A positive electrode and a negative electrode including a plurality of fiber portions and a plurality of coating portions and having a plurality of voids; a separator disposed between the positive electrode and the negative electrode; Electrolyte and Equipped with the plurality of fiber portions are connected to each other to form a three-dimensional network structure having the plurality of voids, and each of the plurality of fiber portions contains carbon as a constituent element; each of the plurality of coating portions covers a surface of each of the plurality of fiber portions and contains silicon as a constituent element; when the negative electrode is divided into two equal parts, a first part located closer to the separator and a second part located farther from the separator in a direction in which the positive electrode and the negative electrode face each other via the separator, an average fiber diameter of the plurality of fiber parts in the first part is smaller than an average fiber diameter of the plurality of fiber parts in the second part; Secondary battery.

2. At least one of the ratio of the weight of the plurality of coating portions to the sum of the weight of the plurality of fiber portions and the weight of the plurality of coating portions, and the porosity is different between the first portion and the second portion. The secondary battery according to claim 1 .

3. an average fiber diameter of the plurality of fiber portions in the first portion is 0.0003 to 0.5 times the average fiber diameter of the plurality of fiber portions in the second portion; The secondary battery according to claim 1 or 2.

4. the proportion in the first portion is greater than the proportion in the second portion; The secondary battery according to claim 2 .

5. the ratio in the first portion is 1.04 times or more and 4.65 times or less than the ratio in the second portion; The secondary battery according to claim 4 .

6. the porosity of the second portion is greater than the porosity of the first portion; The secondary battery according to claim 2 .

7. The porosity of the second portion is 1.1 times or more and 4.5 times or less than the porosity of the first portion. The secondary battery according to claim 6.

8. the average fiber diameter throughout the negative electrode is 10 nm or more and 12,000 nm or less, the proportion of the negative electrode in the entire negative electrode is 40% by weight or more and 80% by weight or less, the porosity of the entire negative electrode is 40% by volume or more and 70% by volume or less; The secondary battery according to claim 2 .

9. The silicon content in each of the plurality of coating portions is 80% by weight or more. The secondary battery according to claim 1 .

10. the negative electrode further includes a plurality of surface portions provided on the surfaces of the plurality of covering portions, Each of the plurality of surface portions includes an ion-conductive material. The secondary battery according to claim 1 .

11. the ion-conductive material includes at least one of lithium phosphate oxynitride and lithium phosphate; The secondary battery according to claim 10.

12. The average thickness of the plurality of surface portions is different between the first portion and the second portion. The secondary battery according to claim 10.

13. the average thickness in the second portion is greater than the average thickness in the first portion; The secondary battery according to claim 12.

14. the negative electrode further includes a plurality of additional fiber parts having an average fiber diameter smaller than the average fiber diameter of the plurality of fiber parts, At least some of the plurality of additional fiber portions are connected to surfaces of the plurality of coating portions, and each of the plurality of additional fiber portions contains carbon as a constituent element. The secondary battery according to claim 1 .

15. the average fiber diameters of the plurality of additional fiber portions are different between the first portion and the second portion; The secondary battery according to claim 14.

16. the average fiber diameter of the plurality of additional fiber portions in the first portion is smaller than the average fiber diameter of the plurality of additional fiber portions in the second portion; The secondary battery according to claim 15.

17. It is a lithium-ion secondary battery. The secondary battery according to claim 1 .

18. The fiber optic cable includes a plurality of fiber portions and a plurality of coating portions, and has a plurality of voids; the plurality of fiber portions are connected to each other to form a three-dimensional network structure having the plurality of voids, and each of the plurality of fiber portions contains carbon as a constituent element; each of the plurality of coating portions covers a surface of each of the plurality of fiber portions and contains silicon as a constituent element; when the fiber is divided into a first portion and a second portion in the thickness direction, an average fiber diameter of the plurality of fiber portions in the first portion is smaller than an average fiber diameter of the plurality of fiber portions in the second portion; Negative electrode for secondary batteries.

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