Positive electrode for secondary battery, and secondary battery

Abstract

This secondary battery comprises a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode current collector, and a positive electrode active material layer that is supported by the positive electrode current collector. The positive electrode active material layer includes a plurality of retention particles that include anatase-type titanium oxide, and a plurality of positive electrode active material particles that include a sulfur-containing material. The plurality of retention particles form a porous structure by being directly bonded to each other, the porous structure being directly connected to the positive electrode current collector. The plurality of positive electrode active material particles are retained by each of the plurality of retention particles, the average grain size of the plurality of retention particles being 100 nm or less.

Description

Positive electrode for secondary battery and secondary battery

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

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

[0003] Specifically, porous bodies formed of titanium nitride nanoparticles, porous nonwoven fabrics, conductive titanium oxide nanoparticles, or oxygen-reduced titanium oxide (TiO 2-x A sulfur composite (porous body) containing ) is used as a positive electrode (see, for example, Patent Documents 1 to 4).

[0004] JP 2020-529102 A JP 2011-198550 A JP 2020-534239 A JP 2018-526793 A

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

[0006] It is desired to provide a positive electrode for a secondary battery and a secondary battery having excellent battery characteristics.

[0007] According to one embodiment of the present disclosure, a positive electrode for a secondary battery includes a positive electrode current collector and a positive electrode active material layer supported by the positive electrode current collector. The positive electrode active material layer includes a plurality of support particles containing anatase titanium oxide and a plurality of positive electrode active material particles containing a sulfur-containing material. The support particles are directly bonded to each other to form a porous structure, and the porous structure is directly connected to the positive electrode current collector. The plurality of positive electrode active material particles are supported by the support particles, and the average particle size of the support particles is 100 nm or less.

[0008] Moreover, a secondary battery according to an embodiment of the present technology includes a positive electrode, a negative electrode, and an electrolyte solution, and the positive electrode has a configuration similar to that of the positive electrode for a secondary battery according to the embodiment of the present technology described above.

[0009] Here, the "average particle size of the plurality of retained particles" is calculated based on the observation results (electron microscope photographs) obtained by observing the cross section of the positive electrode active material layer using an electron microscope. The definition of this "average particle size," i.e., the details of the calculation procedure for the average particle size based on the electron microscope photographs, will be described later.

[0010] According to the positive electrode for a secondary battery or the secondary battery of one embodiment of the present technology, the positive electrode for a secondary battery includes a positive electrode current collector and a positive electrode active material layer, the positive electrode active material layer includes a plurality of retaining particles and a plurality of positive electrode active material particles, the plurality of retaining particles include anatase type titanium oxide, the plurality of positive electrode active material particles include a sulfur-containing material, the plurality of retaining particles are directly bonded to each other to form a porous structure, the porous structure is directly connected to the positive electrode current collector, the plurality of positive electrode active material particles are retained by each of the plurality of retaining particles, and the average particle size of the plurality of retaining particles is 100 nm or less, so that excellent battery characteristics can be obtained.

[0011] 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.

[0012] Fig. 2 is a cross-sectional view showing the configuration of a positive electrode for a secondary battery in an embodiment of the present technology. Fig. 3 is a cross-sectional view showing an enlarged configuration of a portion of the positive electrode for a secondary battery shown in Fig. 1. Fig. 4 is a schematic view showing an electron microscope photograph of a cross section of the positive electrode for a secondary battery shown in Fig. 1. Fig. 5 is a perspective view showing the configuration of a secondary battery in an embodiment of the present technology. Fig. 6 is a cross-sectional view showing the configuration of the battery element shown in Fig. 4. Fig. 7 is a cross-sectional view showing the configuration of a test secondary battery.

[0013] Hereinafter, an embodiment of the present technology will be described in detail with reference to the drawings. The description will be made in the following order: 1. Positive electrode for secondary battery 1-1. Configuration 1-2. Operation 1-3. Manufacturing method 1-4. Action and effect 2. Secondary battery 2-1. Configuration 2-2. Operation 2-3. Manufacturing method 2-4. Action and effect 3. Modification 4. Use of secondary battery

[0014] 1. Positive Electrode for Secondary Battery First, a positive electrode for a secondary battery (hereinafter simply referred to as "positive electrode") according to an embodiment of the present technology will be described.

[0015] The positive electrode described here is used in a secondary battery, which is an electrochemical device. However, the positive electrode may also be used in electrochemical devices other than secondary batteries. Specific examples of other electrochemical devices include primary batteries and capacitors.

[0016] This positive electrode operates by utilizing the electrochemical reaction of a sulfur-containing material in an electrochemical device. In a secondary battery using the positive electrode, the sulfur-containing material is electrochemically oxidized during discharge and electrochemically reduced during charge. Details of the sulfur-containing material will be described later.

[0017] <1-1. Configuration> Fig. 1 shows a cross-sectional configuration of a positive electrode 100, which is an example of a positive electrode. Fig. 2 shows an enlarged cross-sectional configuration of a portion of the positive electrode 100 shown in Fig. 1. Fig. 3 shows a schematic electron microscope photograph 200 of a cross section of the positive electrode 100 shown in Fig. 1.

[0018] As shown in FIG. 1, the positive electrode 100 includes a positive electrode current collector 110 and a positive electrode active material layer 120 .

[0019] [Positive Electrode Current Collector] As shown in FIG. 1, the positive electrode current collector 110 is a conductive support member that supports the positive electrode active material layer 120, and has a pair of surfaces (upper and lower surfaces) on which the positive electrode active material layer 120 is provided.

[0020] The positive electrode current collector 110 contains one or more types of conductive materials such as metal materials, and specific examples of the conductive materials include titanium, aluminum, titanium alloys, and aluminum alloys.

[0021] [Positive Electrode Active Material Layer] As shown in Fig. 1, the positive electrode active material layer 120 is a layer where the sulfur-containing material undergoes an electrochemical reaction, and is supported by the positive electrode current collector 110. The positive electrode active material layer 120 includes a positive electrode active material containing a sulfur-containing material and a support that holds the positive electrode active material. However, the positive electrode active material layer 120 may further include one or more of other materials, such as a positive electrode binder and a positive electrode conductive agent.

[0022] Here, the positive electrode active material layer 120 is provided on both surfaces (upper and lower surfaces) of the positive electrode current collector 110. However, the positive electrode active material layer 120 may be provided on only one surface (upper or lower surface) of the positive electrode current collector 110.

[0023] In particular, the positive electrode active material layer 120 contains two types of particles, as shown in Figures 1 and 2. The first type of particles is a plurality of retaining particles 121 that form the above-mentioned retainer (porous structure 124, which will be described later), and the second type of particles is a plurality of positive electrode active material particles 122 that are a plurality of particulate positive electrode active materials.

[0024] (Multiple Holding Particles) The holding particles 121 are made of anatase type titanium oxide (TiO 2 ) In other words, the titanium oxide contained in the holding particles 121 has an anatase-type crystal structure. The holding particles 121 described here are not secondary particles, which are aggregates of multiple primary particles, but are primary particles.

[0025] For example, anatase type titanium oxide contains one or more of the compounds represented by formula (1).

[0026] TiO w ... (1) (W satisfies 1.85≦W≦2.15.)

[0027] The anatase titanium oxide may contain one or more dopants. The dopant is an element that is doped into the anatase titanium oxide, and the type of dopant is not particularly limited as long as it is an element that can be doped into the anatase titanium oxide. The type of dopant can be appropriately selected to improve the conductivity of the porous structure 124 that serves as the support and to promote the formation of the porous structure 124. Specific examples of dopants include Nb, Ta, Fe, Zr, La, As, P, and B. However, from the standpoint of cost, anatase titanium oxide that does not contain a dopant is preferable to anatase titanium oxide that contains a dopant.

[0028] The reason why the retaining particles 121 contain anatase-type titanium oxide is that the electrochemical reaction of the multiple positive electrode active material particles 122 (sulfur-containing material) in the positive electrode active material layer 120 is more likely to proceed stably than when the retaining particles 121 contain rutile-type or brookite-type titanium oxide.

[0029] Specifically, anatase titanium oxide has the property of more easily promoting the electrochemical reaction of the sulfur-containing material than rutile or brookite titanium oxide, and therefore, the holding particles 121 containing anatase titanium oxide are more likely to stably promote the electrochemical reaction of the sulfur-containing material than the holding particles 121 containing rutile or brookite titanium oxide.

[0030] The plurality of retention particles 121 are directly bonded to one another to form a porous structure 124. As a result, the positive electrode active material layer 120 includes a porous structure 124 in which the plurality of retention particles 121, which are a plurality of primary particles, are directly bonded to one another, and the porous structure 124 has a plurality of voids (pores 123).

[0031] In detail, the positive electrode active material layer 120 includes a sintered body of a plurality of retention particles 121 formed using a firing method, and the sintered body is the above-mentioned retention body (porous structure 124). As a result, as described above, the plurality of retention particles 121 are directly bonded to one another inside the positive electrode active material layer 120. Details of the method for forming the positive electrode active material layer 120 using this firing method will be described later.

[0032] As described above, "directly bonded to each other" means that the porous structure 124 is a sintered body of a plurality of retention particles 121. That is, the plurality of retention particles 121 are not indirectly bonded to each other via a binder, but are directly bonded to each other without the binder in order to form the porous structure 124. Furthermore, the plurality of retention particles 121 are not electrically connected to each other via the conductive agent due to being indirectly bonded to each other via the conductive agent in order to form the porous structure 124, but are electrically connected to each other without the conductive agent due to being directly bonded to each other without the conductive agent in between.

[0033] The reason why the positive electrode active material layer 120 includes the porous structure 124, which is a sintered body of the plurality of retention particles 121, is that the plurality of retention particles 121 are physically and electrically connected to one another. This increases the energy density per volume of the positive electrode active material layer 120 and improves the electronic conductivity between the plurality of retention particles 121. Therefore, the energy density is ensured in the positive electrode 100 while the electrical resistance is reduced, and a high discharge capacity can be obtained in a secondary battery using the positive electrode 100.

[0034] This porous structure 124 is directly connected to the positive electrode current collector 110. As a result, not only are the multiple retention particles 121 directly bonded to one another to form the porous structure 124, but some of the multiple retention particles 121 forming the porous structure 124 are directly connected to the positive electrode current collector 110.

[0035] This "directly connected to each other" means that some of the multiple retaining particles 121 are not indirectly connected to the positive electrode current collector 110 via a binder or a conductive agent, but rather some of the multiple retaining particles 121 are directly connected to the positive electrode current collector 110 without via a binder or a conductive agent.

[0036] For these reasons, as shown in Fig. 1, the plurality of retention particles 121 are directly bonded to one another to form a porous structure 124 that serves as the skeleton of the positive electrode active material layer 120. In this case, the retention particles 121 retain a plurality of positive electrode active material particles 122, as shown in Fig. 2. That is, each of the plurality of retention particles 121 retains a plurality of positive electrode active material particles 122. Note that, in Fig. 2, only one retention particle 121 is shown to simplify the illustration.

[0037] Here, the average particle size AS of the multiple retention particles 121 forming the porous structure 124 is sufficiently small, specifically, 100 nm or less. That is, since the average particle size AS is a value on the so-called nano-order, the retention particles 121 are so-called nanoparticles. This is because the energy density per weight of the positive electrode active material layer 120 is improved and migration paths (multiple pores 123) for the sulfur-containing material are easily formed inside the positive electrode active material layer 120. This promotes the electrochemical reaction of the multiple positive electrode active material particles 122 (sulfur-containing material) on the surfaces of the retention particles 121, making the electrochemical reaction more stable. Therefore, the battery capacity of a secondary battery using the positive electrode 100 is increased.

[0038] In particular, the average particle size AS is preferably 30 nm or less, because this further promotes the electrochemical reaction of the plurality of positive electrode active material particles 122 (sulfur-containing material) on the surface of the retention particle 121. This is also because this further improves the energy density per weight of the positive electrode active material layer 120, and makes it easier for the plurality of pores 123 to be formed inside the positive electrode active material layer 120.

[0039] The lower limit of the average particle size AS is not particularly limited. Specifically, the average particle size AS is preferably 7 nm or more, because this makes it easier for the plurality of holding particles 121 to be formed stably.

[0040] The procedure for calculating the average particle size AS is as follows: To calculate the average particle size AS, an electron microscope photograph 200 is used.

[0041] Specifically, first, the positive electrode 100 is cut in the thickness direction (the vertical direction in FIG. 1 ) to expose a cross section of the positive electrode 100. In this case, the positive electrode 100 is cut using a cutting device such as an ion milling device to expose a cross section of the positive electrode active material layer 120. Note that, as the ion milling device, an ion milling device such as the ArBlade (registered trademark) 5000 manufactured by Hitachi High-Tech Corporation can be used.

[0042] Next, an electron microscope is used to observe the cross section of the positive electrode active material layer 120, thereby obtaining an electron microscope photograph 200. The type of electron microscope is not particularly limited, but specifically, it is one or more of a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The observation conditions are not particularly limited, but specifically, an acceleration voltage of 5.0 kV and a magnification of 150,000 times.

[0043] In the electron microscope photograph 200, as shown in Fig. 3, a plurality of retention particles 121 are directly bonded to one another, and therefore a porous structure 124 having a plurality of pores 123 is observed. In Fig. 3, in order to simplify the content of the illustration, the planar shape of each of the plurality of retention particles 121 is rectangular, and the plurality of positive electrode active material particles 122 is not shown.

[0044] Next, 50 retained particles 121 are arbitrarily selected from the plurality of retained particles 121 visible in the electron microscope photograph 200, and then the particle size S (maximum outer diameter) of each of the 50 retained particles 121 is measured. This provides the particle sizes S of the 50 particles.

[0045] When selecting 50 retained particles 121, the retained particle 121 located closest to the user is selected from among the multiple overlapping retained particles 121. In other words, a retained particle 121 (121Y) whose entire outer edge is not visible because it overlaps with one or more other retained particles 121 is not selected. In contrast, a retained particle 121 (121X) whose entire outer edge is visible because it does not overlap with one or more other retained particles 121 is selected. In Figure 3, some of the retained particles 121X to be selected are shaded.

[0046] Finally, the average value of the particle diameters S of the 50 particles is calculated, and the average value is set as the average particle diameter AS.

[0047] The set of configuration conditions for the positive electrode active material layer 120 can be set arbitrarily. The set of configuration conditions includes the volume density (g / cm 3 ), specific surface area (m 2 / g) and porosity (%).

[0048] (Multiple Positive Electrode Active Material Particles) As shown in Fig. 1 and Fig. 2 , the multiple positive electrode active material particles 122 are held by the multiple holding particles 121, respectively. Here, the multiple positive electrode active material particles 122 are adsorbed to the surfaces of the multiple holding particles 121, respectively. Note that the multiple positive electrode active material particles 122 are not shown in Fig. 1 .

[0049] The reason why the plurality of positive electrode active material particles 122 are held by the plurality of holding particles 121 is that the electrochemical reaction of the plurality of positive electrode active material particles 122 tends to proceed sufficiently and stably on the surfaces of the holding particles 121. As a result, a secondary battery using the positive electrode 100 can stably obtain a sufficient battery capacity.

[0050] As described above, the positive electrode active material particles 122 contain one or more sulfur-containing materials. The sulfur-containing material is a general term for materials containing sulfur as a constituent element. That is, the sulfur-containing material may be elemental sulfur (hereinafter referred to as "elemental sulfur"), a sulfur alloy, a sulfur compound, a mixture of two or more of these, or a material containing one or more of these phases.

[0051] However, the "element" described here means a general element, and the element may contain a trace amount of impurities. In other words, the purity of the element is not necessarily limited to 100%.

[0052] The sulfur compound may contain a salt. The salt contains an anion containing sulfur as a constituent element (hereinafter referred to as a "sulfur-containing anion") and an arbitrary cation. The type of cation is not particularly limited, but a cation that promotes an electrochemical reaction in a secondary battery using the positive electrode 100 is preferable, because this improves the energy density.

[0053] Specifically, the cations are cations of light metals such as alkali metals and alkaline earth metals, with specific examples of alkali metals being lithium, sodium, and potassium, and specific examples of alkaline earth metals being beryllium, magnesium, and calcium.

[0054] Specific examples of sulfur-containing materials include elemental sulfur, alkali metal sulfides, alkaline earth metal sulfides, alkali metal polysulfides, and alkaline earth metal polysulfides. Among these, the sulfur-containing material preferably contains an alkali metal polysulfide, and more preferably contains lithium polysulfide. This is because the electrochemical reaction of the multiple positive electrode active material particles 122 can proceed more easily. Specific examples of lithium polysulfides include lithium sulfide (LiS).

[0055] The sulfur-containing anion is a polysulfide ion (S n x- In the case where the compound contains a cyclic alkyl group, the ranges of n and x are not particularly limited. Specifically, n preferably satisfies 2≦n≦20, and more preferably satisfies 2<n≦12. x preferably satisfies 0<x≦2, and more preferably is 2.

[0056] (Positive Electrode Binder) The positive electrode binder contains one or more of synthetic rubbers, polymer compounds, etc. Specific examples of synthetic rubbers include styrene-butadiene rubbers, and specific examples of polymer compounds include polyethylene glycol, polyvinylidene fluoride, and polyimide.

[0057] (Positive Electrode Conductive Agent) The positive electrode conductive agent contains one or more conductive materials such as a carbon material, a metal material, and a conductive polymer compound. Specific examples of the carbon material include graphite, carbon black, acetylene black, and ketjen black.

[0058] <1-2. Operation> The positive electrode 100 operates as follows.

[0059] During an electrode reaction (during discharge of a secondary battery using an electrolyte together with the cathode 100 described below), a plurality of cathode active material particles 122 (sulfur-containing material) are electrochemically reduced on the surfaces of a plurality of retention particles 121 in the cathode active material layer 120. In this case, if the sulfur-containing material contains a sulfur-containing anion having high solubility, a salt containing the sulfur-containing anion may be eluted from the plurality of retention particles 121 into the electrolyte.

[0060] Furthermore, during an electrode reaction (when a secondary battery using an electrolyte together with the cathode 100 described below is charged), a plurality of cathode active material particles 122 (sulfur-containing material) are electrochemically oxidized on the surfaces of a plurality of retention particles 121 in the cathode active material layer 120. In this case, if the sulfur-containing material contains a sulfur-containing anion having high solubility, a salt containing the sulfur-containing anion contained in the electrolyte is electrochemically oxidized, and the salt may precipitate on the surfaces of the plurality of retention particles 121, resulting in the formation of some or all of the plurality of cathode active material particles 122.

[0061] As an example, the sulfur-containing material may be elemental sulfur (S 8 ), the electrochemical reaction represented by formula (2) proceeds.

[0062] n / 8S 8 +x e- = S n x- ... (2) (where n satisfies 2≦n≦20, and x satisfies 0<x≦2.)

[0063] As is clear from formula (2), elemental sulfur is reduced to polysulfide ions during discharge and oxidized to elemental sulfur during charge. During discharge, the positive electrode active material particles 122 are reduced, so that the elemental sulfur becomes a salt containing polysulfide ions, and some or all of the salt may be dissolved in the electrolyte. During charge, the salt containing polysulfide ions, which is the positive electrode active material particles 122, may be oxidized, or the salt containing polysulfide ions dissolved in the electrolyte may be oxidized, so that some or all of the positive electrode active material particles 122 may be formed.

[0064] <1-3. Manufacturing Method> The positive electrode 100 is manufactured by the following example procedure.

[0065] First, a paste is obtained by mixing a plurality of retaining particles 121 (anatase-type titanium oxide), a positive electrode binder, and a solvent. In this case, a plurality of retaining particles 121 having an average particle size AS of 100 nm or less are used. The type of solvent is not particularly limited, but specifically, it is an aqueous solvent such as water, and a paste using an aqueous solvent as the solvent is a so-called aqueous paste. The composition (mixing ratio) of the paste can be set as desired.

[0066] Next, the paste is applied to both surfaces of the positive electrode current collector 110 to form a coating film, and then the positive electrode current collector 110 on which the coating film has been formed is pressed. The pressing conditions can be set arbitrarily. As a result, the coating film is pressure-bonded to both surfaces of the positive electrode current collector 110.

[0067] Next, the positive electrode current collector 110 on which the coating film has been formed is fired in the atmosphere. The firing conditions, such as the firing temperature and firing time, can be set as desired. In this case, the firing conditions are adjusted so that the plurality of holding particles 121 containing anatase-type titanium oxide are directly bonded to one another while maintaining their primary particle state. As an example, the maximum temperature during firing is 500°C to 1200°C. Note that the environmental conditions during firing are not particularly limited, and therefore the firing process may be performed in an oxygen atmosphere.

[0068] In this firing treatment, the positive electrode binder is degreased during firing. As a result, the plurality of retention particles 121 are directly bonded to one another, forming a porous structure 124 having a plurality of pores 123. In this case, part of the porous structure 124, i.e., part of the plurality of retention particles 121, is directly connected to the positive electrode current collector 110.

[0069] Subsequently, the sulfur-containing material is added to the solvent, and the solvent is stirred to prepare a sulfur-containing solution. The type of the solvent is not particularly limited, but specifically, it is an organic solvent or the like.

[0070] Finally, the positive electrode current collector 110 with the porous structure 124 formed thereon is immersed in a sulfur-containing solution, and then the positive electrode current collector 110 with the porous structure 124 formed thereon is removed from the sulfur-containing solution and dried. Immersion conditions such as immersion time can be set as desired. As a result, the plurality of positive electrode active material particles 122 (sulfur-containing material) are held by the plurality of holding particles 121, respectively.

[0071] Thus, the positive electrode active material layer 120 including the plurality of holding particles 121 (porous structure 124) and the plurality of positive electrode active material particles 122 is formed, and the positive electrode 100 is completed.

[0072] It should be noted that a method other than the above-described method of firing a coating film containing a positive electrode binder may be used to fabricate the positive electrode 100. As long as the porous structure 124 can be formed by using a firing process, the fabrication procedure for the positive electrode 100 can be changed as appropriate.

[0073] Specifically, a plurality of retention particles 121 may be press-molded without using a positive electrode binder, and then the plurality of retention particles 121 may be fired. Alternatively, a dispersion liquid in which a plurality of retention particles 121 is dispersed may be applied to both surfaces of the positive electrode current collector 110, and the dispersion liquid may be dried, and then the positive electrode current collector 110 to which the dispersion liquid has been applied may be fired.

[0074] <1-4. Actions and Effects> According to this positive electrode 100, the positive electrode 100 includes a positive electrode current collector 110 and a positive electrode active material layer 120. The positive electrode active material layer 120 includes a plurality of retention particles 121 (anatase-type titanium oxide) and a plurality of positive electrode active material particles 122 (sulfur-containing material). The positive electrode active material layer 120 includes a porous structure 124, which is formed by the plurality of retention particles 121 being directly bonded to one another and is directly connected to the positive electrode current collector 110. The plurality of positive electrode active material particles 122 are retained by the plurality of retention particles 121, respectively, and the average particle size AS of the plurality of retention particles 121 is 100 nm or less.

[0075] In this case, as described above, a series of actions described below are obtained.

[0076] First, because the positive electrode active material layer 120 includes a porous structure 124 that is a sintered body of a plurality of retention particles 121, the plurality of retention particles 121 are physically and electrically connected to one another. In this case, the energy density of the positive electrode active material layer 120 increases, and electronic conductivity between the plurality of retention particles 121 improves. This reduces electrical resistance while ensuring energy density.

[0077] Secondly, since the holding particles 121 contain anatase-type titanium oxide, the electrochemical reaction of the plurality of positive electrode active material particles 122 (sulfur-containing material) in the positive electrode active material layer 120 tends to proceed stably.

[0078] Third, since the average particle size AS of the plurality of retaining particles 121 is 100 nm or less, the energy density per weight of the positive electrode active material layer 120 is improved, and migration paths (plurality of pores 123) for the sulfur-containing material are easily formed inside the positive electrode active material layer 120.

[0079] For these reasons, the electrochemical reaction of the sulfur-containing material is likely to proceed in the positive electrode 100. Moreover, the positive electrode 100 including the porous structure 124 can be easily manufactured using an existing firing method. Therefore, a sufficient discharge capacity can be easily and stably obtained during charge and discharge, and a secondary battery having excellent battery characteristics can be realized using the positive electrode 100.

[0080] In particular, when the average particle size AS is 30 nm or less, the sulfur-containing material can move more easily inside the positive electrode active material layer 120, the energy density per weight of the positive electrode active material layer 120 is improved, and migration paths (multiple pores 123) for the sulfur-containing material can be more easily formed inside the positive electrode active material layer 120, thereby achieving a greater effect.

[0081] Furthermore, if the sulfur-containing material contains an alkali metal polysulfide, the electrochemical reaction of the sulfur-containing material can be facilitated sufficiently, thereby achieving a higher effect. In this case, if the alkali metal polysulfide contains lithium polysulfide, the electrochemical reaction of the plurality of positive electrode active material particles 122 can be facilitated more easily, thereby achieving an even higher effect.

[0082] 2. Secondary Battery Next, a secondary battery according to an embodiment of the present technology to which the positive electrode 100 is applied will be described.

[0083] The secondary battery described here is a secondary battery that obtains battery capacity by utilizing an electrochemical reaction of an electrode reactant, and includes a positive electrode, a negative electrode, and an electrolyte.

[0084] In this negative electrode, capacity is obtained by utilizing the electrochemical reaction of cations contained in the electrolyte. In this case, it is preferable to utilize the electrochemical reaction of cations capable of forming salts with sulfur-containing anions, because this improves the energy density of the secondary battery. In this case, the negative electrode active material is a material capable of electrochemically reacting with cations capable of forming salts with sulfur-containing anions.

[0085] Specifically, the cations are ions of light metals such as alkali metals and alkaline earth metals, with specific examples of alkali metals being lithium, sodium, and potassium, and specific examples of alkaline earth metals being beryllium, magnesium, and calcium.

[0086] In this case, the negative electrode active material contains one or both of an alkali metal material and an alkaline earth metal material. The alkali metal material is a material containing an alkali metal element as a constituent element, and the alkaline earth metal material is a material containing an alkaline earth metal element as a constituent element. However, the alkali metal element and the alkaline earth metal material may each be of one type or two or more types. Furthermore, each of the alkali metal material and the alkaline earth metal material may be a simple substance, an alloy, a compound, or two or more types thereof. The meaning of the simple substance is as described above.

[0087] In the following, an example will be given in which the cation is a lithium ion. A secondary battery that obtains capacity by utilizing the absorption and release of lithium in the negative electrode is known as a lithium-sulfur secondary battery.

[0088] <2-1. Configuration> Fig. 4 shows a perspective configuration of a secondary battery, and Fig. 5 shows a cross-sectional configuration of the battery element 20 shown in Fig. 4. However, Fig. 4 shows a state in which the exterior film 10 and the battery element 20 are separated from each other, and the cross section of the battery element 20 along the XZ plane is shown by a dashed line.

[0089] As shown in FIGS. 4 and 5, this secondary battery includes an exterior film 10, a battery element 20, a positive electrode lead 31, a negative electrode lead 32, and sealing films 41 and .

[0090] The secondary battery described here is a so-called laminate film type secondary battery, since it uses a flexible or pliable exterior film 10 as an exterior member for housing the battery element 20, as described above.

[0091] 4, the exterior film 10 has a bag-like structure that is sealed when the battery element 20 is housed therein. As a result, the exterior film 10 houses a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte solution, which will be described later.

[0092] Here, the exterior film 10 is a single film-like member that is folded in a folding direction F. The exterior film 10 is provided with a recessed portion 10U (a so-called deep drawn portion) for accommodating the battery element 20.

[0093] Specifically, the exterior film 10 is a three-layer laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order from the inside out, and when the exterior film 10 is folded, the outer peripheral edges of the opposing fusion layers are fused to each other. The fusion layer contains a polymer compound such as polypropylene. The metal layer contains a metal material such as aluminum. The surface protection layer contains a polymer compound such as nylon.

[0094] However, the configuration (number of layers) of the exterior film 10 is not particularly limited, and may be one layer, two layers, or four or more layers.

[0095] [Battery Element] The battery element 20 is housed in an exterior film 10. The battery element 20 is a so-called power generation element, and as shown in Figures 4 and 5, includes a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte (not shown).

[0096] Here, battery element 20 is a so-called wound electrode body. That is, positive electrode 21 and negative electrode 22 are wound around winding axis P while facing each other with separator 23 interposed therebetween. As is clear from FIG. 4 , winding axis P is a virtual axis extending in the Y-axis direction.

[0097] There are no particular limitations on the three-dimensional shape of battery element 20. Here, battery element 20 has a flat three-dimensional shape, and therefore the shape of a cross section (cross section along the XZ plane) of battery element 20 intersecting winding axis P is a flat shape defined by major axis J1 and minor axis J2.

[0098] The major axis J1 is an imaginary axis extending in the X-axis direction and has a length greater than that of the minor axis J2. The minor axis J2 is an imaginary axis extending in the Z-axis direction intersecting the X-axis direction and has a length less than that of the major axis J1. Here, the three-dimensional shape of the battery element 20 is a flattened cylinder, and therefore the cross-sectional shape of the battery element 20 is a flattened, approximately elliptical shape.

[0099] (Positive Electrode) The positive electrode 21 has a configuration similar to that of the positive electrode 100. That is, as shown in Fig. 5 , the positive electrode 21 includes a positive electrode current collector 21A and a positive electrode active material layer 21B. The configurations of the positive electrode current collector 21A and the positive electrode active material layer 21B are similar to the configurations of the positive electrode current collector 110 and the positive electrode active material layer 120, respectively.

[0100] (Negative Electrode) As shown in FIG. 5, the negative electrode 22 includes a negative electrode current collector 22A and a negative electrode active material layer 22B.

[0101] The negative electrode current collector 22A has a pair of surfaces on which the negative electrode active material layer 22B is provided. The negative electrode current collector 22A contains a conductive material such as a metal material, and a specific example of the conductive material is copper.

[0102] The anode active material layer 22B includes one or more anode active materials capable of absorbing and releasing lithium. However, the anode active material layer 22B may further include one or more other materials, such as an anode binder and an anode conductor. The method for forming the anode active material layer 22B is not particularly limited, and specifically includes one or more of a coating method, a vapor phase method, a liquid phase method, a thermal spraying method, and a firing method (sintering method).

[0103] Here, the anode active material layer 22B is provided on both sides of the anode current collector 22A. However, the anode active material layer 22B may be provided on only one side of the anode current collector 22A on the side where the anode 22 faces the cathode 21.

[0104] The type of negative electrode active material is not particularly limited, but specifically includes carbon materials, metal materials, and metallic lithium (so-called lithium elemental substance), etc. This is because a high energy density can be obtained. The meaning of elemental substance is as described above.

[0105] The type of carbon material is not particularly limited, but specific examples include non-graphitizable carbon, graphitizable carbon, graphite (natural graphite and artificial graphite), pyrolytic carbons, cokes, glassy carbons, organic polymer compound calcined bodies, carbon fiber, and activated carbon. Examples of cokes include pitch coke, needle coke, and petroleum coke. Organic polymer compound calcined bodies are materials in which polymer compounds such as phenolic resins and furan resins are carbonized by calcining them at an appropriate temperature. However, some organic polymer compound calcined bodies may be classified as non-graphitizable carbon or graphitizable carbon.

[0106] Carbon materials are preferred because they undergo very little change in their crystal structure during charge and discharge, resulting in high charge and discharge capacities and good cycle characteristics. Graphite is particularly preferred because it has a large electrochemical equivalent and a high energy density. Non-graphitizable carbon is also preferred because it provides excellent cycle characteristics. Furthermore, carbon materials with low charge and discharge potentials, more specifically, carbon materials with charge and discharge potentials close to that of lithium metal, are preferred because they facilitate the realization of high energy densities in secondary batteries.

[0107] The metal-based material is a material that contains, as a constituent element, one or more of metal elements and semi-metal elements that can form an alloy with lithium. Specific examples of the metal elements and semi-metal elements include magnesium, boron, aluminum, titanium, gallium, indium, silicon, germanium, tin, lead, bismuth, cadmium, silver, zinc, hafnium, zirconium, yttrium, palladium, and platinum.

[0108] This metallic material may be crystalline or amorphous. More specifically, the metallic material may be a simple substance, an alloy, a compound, a mixture of two or more of these, or a material containing two or more of these phases. The meaning of "simple substance" is as described above. The alloy may be not only a material containing two or more metallic elements as constituent elements, but also a material containing one or more metallic elements and one or more metalloid elements as constituent elements. The alloy may also contain one or more non-metallic elements as constituent elements. The structure of the alloy is not particularly limited, but may be, for example, a solid solution, a eutectic (eutectic mixture), an intermetallic compound, or a mixture of two or more of these.

[0109] Among these, the metallic material preferably contains a metal element or a metalloid element of Group 4B in the short periodic table as a constituent element, and more preferably contains one or both of silicon and tin as constituent elements, because a sufficiently high energy density can be obtained.

[0110] Silicon alloys contain, as constituent elements other than silicon, one or more of tin, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, niobium, molybdenum, aluminum, phosphorus, gallium, chromium, etc. Tin alloys contain, as constituent elements other than tin, one or more of silicon, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, niobium, molybdenum, aluminum, phosphorus, gallium, and chromium.

[0111] Each of the silicon compound and the tin compound contains one or more of oxygen, carbon, etc. as a constituent element. Note that each of the silicon compound and the tin compound may contain one or more of the elements described for the silicon alloy as a constituent element.

[0112] In particular, when the metallic material contains tin as a constituent element, it is preferable that the metallic material contains cobalt, tin, and carbon as constituent elements and has low crystallinity or amorphousness, because a sufficiently high energy density can be obtained.

[0113] Alternatively, the negative electrode active material may be a metal oxide or polymer compound capable of absorbing and releasing lithium. The type of metal oxide is not particularly limited, but specific examples include lithium titanium oxide, iron oxide, ruthenium oxide, and molybdenum oxide. Specific examples of lithium titanium oxide include lithium titanate (Li 4 Ti 5 O 12 ) and specific examples of the polymer compound include polyacetylene, polyaniline, and polypyrrole.

[0114] The details regarding the negative electrode binder are the same as those regarding the positive electrode binder, and the details regarding the negative electrode conductive agent are the same as those regarding the positive electrode conductive agent.

[0115] 5, the separator 23 is an insulating porous film interposed between the positive electrode 21 and the negative electrode 22, and allows lithium to pass through in an ionic state while preventing the occurrence of a short circuit due to contact between the positive electrode 21 and the negative electrode 22. The separator 23 contains a polymer compound such as polyethylene.

[0116] (Electrolyte) The electrolyte is a liquid electrolyte, and is impregnated into each of the positive electrode 21, the negative electrode 22, and the separator 23. The electrolyte contains a solvent and an electrolyte salt.

[0117] The solvent contains one or more types of non-aqueous solvents (organic solvents), and the electrolyte containing the non-aqueous solvent is a so-called non-aqueous electrolyte.

[0118] The non-aqueous solvent is an ester, an ether, or the like, and more specifically, is one or more of a carbonate ester compound, a carboxylic acid ester compound, and a lactone compound, because the dissociation of the electrolyte salt and the mobility of ions are improved.

[0119] The carbonate ester compounds include cyclic carbonate esters and chain carbonate esters. Specific examples of the cyclic carbonate esters include ethylene carbonate and propylene carbonate, and specific examples of the chain carbonate esters include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0120] The carboxylic acid ester compound is a chain carboxylic acid ester, etc. Specific examples of the chain carboxylic acid ester include ethyl acetate, ethyl propionate, propyl propionate, and ethyl trimethylacetate.

[0121] The lactone compound is lactone, etc. Specific examples of lactone include γ-butyrolactone and γ-valerolactone.

[0122] The ethers may be tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, etc. Alternatively, the ethers may be a compound represented by formula (3). This compound is a linear ether having an ethyleneoxy structural unit as a repeating unit, and specific examples of this compound include 1,2-dimethoxyethane, diethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0123] (R1 and R2 are each an alkyl group having 1 to 10 carbon atoms, and n is an integer of 1 to 10.)

[0124] The nonaqueous solvent is one or more of unsaturated cyclic carbonates, fluorinated cyclic carbonates, sulfonates, phosphates, acid anhydrides, nitrile compounds, isocyanate compounds, etc. This is because the dissociation of the electrolyte salt and the mobility of ions are similarly improved.

[0125] Specific examples of unsaturated cyclic carbonates include vinylene carbonate, vinylethylene carbonate, and methyleneethylene carbonate. Specific examples of fluorinated cyclic carbonates include monofluoroethylene carbonate and difluoroethylene carbonate. Specific examples of sulfonic acid esters include propane sultone and propene sultone. Specific examples of phosphate esters include trimethyl phosphate and triethyl phosphate. Specific examples of acid anhydrides include succinic anhydride, 1,2-ethanedisulfonic anhydride, and 2-sulfobenzoic anhydride. Specific examples of nitrile compounds include succinonitrile. Specific examples of isocyanate compounds include hexamethylene diisocyanate.

[0126] The electrolyte salt contains one or more types of light metal salts such as lithium salts.

[0127] A specific example of the lithium salt is lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium trifluoromethanesulfonate (LiCF 3 SO 3 ), lithium bis(fluorosulfonyl)imide (LiN(FSO 2 ) 2 ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF 3 SO 2 ) 2 ), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF 3 SO 2 ) 3 ), lithium bis(oxalato)borate (LiB(C 2 O 4 ) 2 ), lithium monofluorophosphate (Li 2 PFO 3 ), lithium difluorophosphate (LiPF 2 O 2 ) and lithium nitrate (LiNO 3 ) etc. This is because a high battery capacity can be obtained.

[0128] 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.

[0129] 4 and 5, the positive electrode lead 31 is a positive electrode wiring connected to the positive electrode current collector 21A of the positive electrode 21, and is led out of the exterior film 10. The positive electrode lead 31 contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum. The positive electrode lead 31 has a shape such as a thin plate or a mesh.

[0130] [Negative Electrode Lead] As shown in Figures 4 and 5 , the negative electrode lead 32 is a negative electrode wiring connected to the negative electrode 22, and is led out of the exterior film 10. Here, the lead-out direction of the negative electrode lead 32 is the same as the lead-out direction of the positive electrode lead 31. This negative electrode lead 32 contains a conductive material such as a metal material, and a specific example of the conductive material is copper. Note that the details regarding the shape of the negative electrode lead 32 are the same as the details regarding the shape of the positive electrode lead 31.

[0131] [Sealing Film] As shown in Fig. 4, the sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31. As shown in Fig. 4, the sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32. However, one or both of the sealing films 41, 42 may be omitted.

[0132] The sealing film 41 is a sealing member that prevents outside air and the like from entering the inside of the exterior film 10. This sealing film 41 contains a polymer compound such as polyolefin that has adhesiveness to the positive electrode lead 31, and a specific example of the polymer compound is polypropylene.

[0133] The configuration of the sealing film 42 is the same as the configuration of the sealing film 41, except that the sealing film 42 is a sealing member that has adhesiveness to the negative electrode lead 32. That is, the sealing film 42 contains a polymer compound such as polyolefin that has adhesiveness to the negative electrode lead 32.

[0134] <2-2. Operation> This secondary battery operates as follows during charging and discharging.

[0135] During charging, lithium is released from the positive electrode 21 of the battery element 20 and is absorbed into the negative electrode 22 via the electrolyte. During discharging, lithium is released from the negative electrode 22 of the battery element 20 and is absorbed into the positive electrode 21 via the electrolyte. During both discharging and charging, lithium is absorbed and released in an ionic state.

[0136] <2-3. Manufacturing Method> When manufacturing a secondary battery, the positive electrode 21 and the negative electrode 22 are each prepared and an electrolytic solution is prepared according to the procedure described below as an example. Thereafter, the positive electrode 21, the negative electrode 22, and the electrolytic solution are used to assemble a secondary battery, and a stabilization process is performed on the assembled secondary battery.

[0137] [Fabrication of Positive Electrode] The positive electrode 21 is fabricated by the same procedure as that for fabricating the positive electrode 100. In this case, the positive electrode active material layers 21B are formed on both surfaces of the positive electrode current collector 21A.

[0138] [Fabrication of Negative Electrode] First, a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent are mixed together to form a negative electrode mixture. Next, the negative electrode mixture is poured into a solvent to prepare a paste-like negative electrode mixture slurry. This solvent may be an aqueous solvent or an organic solvent. Finally, the negative electrode mixture slurry is applied to both surfaces of the negative electrode current collector 22A to form the negative electrode active material layer 22B. Then, the negative electrode active material layer 22B may be compression-molded using a roll press or the like. In this case, the negative electrode active material layer 22B may be heated, or the compression molding may be repeated multiple times. As a result, the negative electrode active material layer 22B is formed on both surfaces of the negative electrode current collector 22A, thereby fabricating the negative electrode 22.

[0139] [Preparation of Electrolyte Solution] An electrolyte salt is added to a solvent, whereby the electrolyte salt is dispersed or dissolved in the solvent, thereby preparing an electrolyte solution.

[0140] [Assembly of Secondary Battery] First, the positive electrode lead 31 is connected to the positive electrode current collector 21A of the positive electrode 21 using a joining method such as welding, and the negative electrode lead 32 is connected to the negative electrode current collector 22A of the negative electrode 22 using a joining method such as welding.

[0141] Next, the positive electrode 21 and the negative electrode 22 are stacked one on top of the other with the separator 23 interposed therebetween, and then the positive electrode 21, the negative electrode 22, and the separator 23 are wound together to produce a wound body (not shown). Next, the wound body is pressed using a press or the like to form a flat shape. The wound body after this formation has a configuration similar to that of the battery element 20, except that the positive electrode 21, the negative electrode 22, and the separator 23 are not impregnated with an electrolyte solution.

[0142] Next, after the roll is housed inside the recess 10U, the exterior film 10 (fusion layer / metal layer / surface protection layer) is folded to face each other. Next, the outer peripheral edges of two sides of the facing fusion layers are joined together using an adhesive method such as heat fusion, thereby housing the roll inside the bag-shaped exterior film 10.

[0143] Finally, after injecting the electrolyte solution into the bag-shaped exterior film 10, the outer peripheral edges of the remaining sides of the opposing fusion layers are joined together using an adhesive method such as heat fusion. In this case, a sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and a sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32.

[0144] As a result, the wound body is impregnated with the electrolyte, forming the wound electrode body, which is the battery element 20. The battery element 20 is then sealed inside the bag-shaped exterior film 10, completing the secondary battery.

[0145] <2-4. Actions and Effects> In this secondary battery, the positive electrode 21 has a configuration similar to that of the positive electrode 100. Therefore, for the reasons described above, a sufficient discharge capacity can be stably obtained during charging and discharging, and excellent battery characteristics can be obtained.

[0146] In particular, if the secondary battery is a lithium-sulfur secondary battery, a sufficient battery capacity can be stably obtained by utilizing the absorption and desorption of lithium, and therefore a greater effect can be obtained.

[0147] 3. Modifications The configuration of the secondary battery can be modified as appropriate, as described below. However, the series of modifications described below may be combined with each other.

[0148] [Variation 1] In the above-described positive electrode 100, the sulfur-containing material contains an alkali metal polysulfide, and the alkali metal polysulfide contains lithium polysulfide. However, the alkali metal polysulfide may contain sodium polysulfide.

[0149] A secondary battery that includes a negative electrode 22 together with a positive electrode 21 to which the positive electrode 100 is applied, and obtains capacity by utilizing the absorption and release of sodium in the negative electrode 22, is a so-called sodium-sulfur secondary battery.

[0150] The configuration of this sodium-sulfur secondary battery is the same as that of the lithium-sulfur secondary battery described above, except as described below. The type of negative electrode active material is not particularly limited, but specific examples include carbon materials and metallic sodium (so-called elemental sodium). The meaning of elemental sodium is as described above. The electrolyte salt of the electrolyte solution contains one or more types of sodium salts. Specific examples of sodium salts include sodium salts corresponding to the specific examples of lithium salts described above.

[0151] In this case as well, a sufficient discharge capacity can be easily and stably obtained when the secondary battery is charged and discharged, and the same effect can be obtained.

[0152] [Variation 2] In the above-described positive electrode 100, the sulfur-containing material includes an alkali metal polysulfide. However, the sulfur-containing material may include an alkaline earth metal polysulfide, more specifically, may include magnesium polysulfide.

[0153] A secondary battery that includes a negative electrode 22 together with a positive electrode 21 to which the positive electrode 100 is applied, and obtains capacity by utilizing the absorption and release of magnesium in the negative electrode 22, is a so-called magnesium-sulfur secondary battery.

[0154] The configuration of this magnesium-sulfur secondary battery is similar to that of the lithium-sulfur secondary battery described above, except as explained below.

[0155] The type of negative electrode active material is not particularly limited, but specifically includes magnesium-based materials. This magnesium-based material is a material containing magnesium as a constituent element, and may be magnesium alone, a magnesium alloy, a magnesium compound, a mixture of two or more of these, or a material containing two or more of these phases. The meaning of "single element" is as described above. The type of metal element (excluding magnesium) contained as a constituent element in the magnesium alloy is not particularly limited and can be selected arbitrarily. The magnesium compound contains one or more non-metallic elements such as carbon, oxygen, sulfur, and halogens as constituent elements. Specific examples of halogens include fluorine, chlorine, bromine, and iodine.

[0156] The electrolyte salt of the electrolytic solution contains one or more types of magnesium salts. The type of magnesium salt is not particularly limited. When the electrolyte salt contains a magnesium salt, the solvent of the electrolytic solution preferably contains an ether (linear ether) shown in formula (3). This is because the electrolyte salt is more likely to be stably dissolved in the solvent.

[0157] Specifically, the magnesium salt is a magnesium salt represented by formula (4).

[0158] MgX n ...(4) (X is a monovalent or divalent anion. n is 1 or 2.)

[0159] When X in formula (4) is a halogen ion, the magnesium salt is a so-called halogen metal salt. Specific examples of halogen include F, Cl, Br, and I.

[0160] Of course, X may be an ion other than a halogen ion. A specific example of a magnesium salt in which X is an ion other than a halogen ion is magnesium perchlorate (Mg(ClO) 4 ) 2 ), magnesium nitrate (Mg(NO 3 ) 2 ), magnesium sulfate (MgSO 4 ), magnesium acetate (Mg(CH 3 COO) 2 ), magnesium trifluoroacetate (Mg(CF 3 COO) 2 ), magnesium tetrafluoroborate (Mg(BF 4 ) 2 ), magnesium tetraphenylborate (Mg[B(C 6 H 5 ) 4 ] 2 ), magnesium hexafluorophosphate (Mg(PF 6 ) 2 ), magnesium hexafluoroarsenate (Mg(AsF 6 ) 2 ), magnesium salt of perfluoroalkylsulfonic acid (Mg(RfSO 3 ) 2 ), magnesium salt of perfluoroalkylsulfonylimide (Mg(Rf 2 SO 2 ) 2 N) 2 ) and magnesium salts of hexaalkyldisilaZides (Mg[N(SiR 3 ) 2 ), Mg[B(OCH(CRff 3 ) 2 ) 4 ] 2 etc. Rf is a perfluoroalkyl group, and R is an alkyl group. Each of the 48 Rff is either H or F, and at least one of the 48 Rff is F.

[0161] Among these, it is preferable that the magnesium salt contains one or both of a halogen-based magnesium salt and an imide-based magnesium salt, because this further improves the energy density. In this case, the magnesium salt may contain only one of a halogen-based magnesium salt and an imide-based magnesium salt, or may contain both a halogen-based magnesium salt and an imide-based magnesium salt.

[0162] The halogen-based magnesium salt is a magnesium salt in which X is a halogen ion in the formula (4). Specific examples of the halogen-based magnesium salt include magnesium fluoride (MgF 2 ), magnesium chloride (MgCl 2 ), magnesium bromide (MgBr 2 ) and magnesium iodide (MgI 2 Among them, the halogen-based magnesium salt is preferably magnesium chloride, because it can provide a sufficiently high energy density.

[0163] The imide-based magnesium salt is a magnesium salt having an imide-type molecular structure, preferably a magnesium salt having a sulfonylimide-type molecular structure, because it can provide a higher energy density. In particular, when the imide-based magnesium salt is used in combination with a halogen-based magnesium salt, an even higher energy density can be obtained.

[0164] Specific examples of imide-based magnesium salts include the above-mentioned magnesium salts of perfluoroalkylsulfonylimides. Among them, magnesium salts of perfluoroalkylsulfonylimides have the structural formula (Mg(Rf 2 SO 2 ) 2 In the perfluoroalkyl group (Rf), the number of carbon atoms in the perfluoroalkyl group (Rf) may be 1 to 10, 1 to 8, 1 to 6, 1 to 4, 1 to 3, or 1 or 2. A specific example of the magnesium salt of a perfluoroalkylsulfonylimide is magnesium bis(trifluoromethanesulfonyl)imide (Mg(CF 3 SO 2 ) 2 In particular, when magnesium bis(trifluoromethanesulfonyl)imide and magnesium chloride are used in combination, an even higher energy density can be obtained.

[0165] In this case as well, a sufficient discharge capacity can be easily and stably obtained when the secondary battery is charged and discharged, and the same effect can be obtained.

[0166] 5, a porous film separator 23 is used. However, although not specifically shown here, a laminated separator including a polymer compound layer may be used instead of the porous film separator 23.

[0167] Specifically, the laminated separator includes a porous membrane having a pair of surfaces and a polymer compound layer provided on one or both surfaces of the porous membrane. This is because the separator's adhesion to the positive electrode 21 and the negative electrode 22 is improved, thereby suppressing misalignment of the battery element 20, i.e., miswinding of the positive electrode 21, the negative electrode 22, and the separator. This suppresses swelling of the secondary battery even if a decomposition reaction of the electrolyte occurs. The polymer compound layer includes a polymer compound such as polyvinylidene fluoride. This is because polymer compounds such as polyvinylidene fluoride have excellent physical strength and are electrochemically stable.

[0168] One or both of the porous film and the polymer compound layer may contain a plurality of insulating particles. This is because the plurality of insulating particles dissipate heat when the secondary battery generates heat, thereby improving the safety (heat resistance) of the secondary battery. The plurality of insulating particles contain one or more types of insulating materials, such as inorganic particles and resin particles. The inorganic particles include aluminum oxide, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide, and zirconium oxide. The resin particles include acrylic resin and styrene resin.

[0169] When a laminated separator is produced, a precursor solution containing a polymer compound and an organic solvent is prepared, and then the precursor solution is applied to one or both sides of a porous film. In this case, the precursor solution may contain a plurality of insulating particles.

[0170] Even when this laminated separator is used, the same effect can be obtained because lithium can move in an ionic state between the positive electrode 21 and the negative electrode 22. In this case, as described above, in particular, the displacement of the battery element 20 is suppressed, thereby further suppressing swelling of the secondary battery, thereby achieving a greater effect.

[0171] [Modification 4] An electrolytic solution, which is a liquid electrolyte, is used in Fig. 5. However, although not specifically shown here, an electrolyte layer, which is a gel electrolyte, may be used instead of the electrolytic solution.

[0172] In the battery element 20 using the electrolyte layer, the positive electrode 21 and the negative electrode 22 are stacked with the separator 23 and the electrolyte layer interposed therebetween, and the positive electrode 21, the negative electrode 22, the separator 23, and the electrolyte layer are wound together. The electrolyte layer is interposed between the positive electrode 21 and the separator 23, and also between the negative electrode 22 and the separator 23.

[0173] Specifically, the electrolyte layer contains a polymer compound together with an electrolytic solution, and the electrolytic solution is held by the polymer compound. This is because leakage of the electrolytic solution is suppressed. The composition of the electrolytic solution is as described above. The polymer compound contains polyvinylidene fluoride, etc. When forming the electrolyte layer, a precursor solution containing an electrolytic solution, a polymer compound, an organic solvent, etc. is prepared, and then the precursor solution is applied to one or both surfaces of each of the positive electrode 21 and the negative electrode 22.

[0174] Even when this electrolyte layer is used, the same effect can be obtained because lithium can move in an ionic state through the electrolyte layer between the positive electrode 21 and the negative electrode 22. In this case, leakage of the electrolyte solution is particularly suppressed as described above, and therefore a greater effect can be obtained.

[0175] 4. Uses of Secondary Batteries There are no particular limitations on the uses (application examples) of secondary batteries. A secondary battery used as a power source may be a main power source for electronic devices, electric vehicles, and the like, or may be 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 a main power source, or a power source that can be switched from a main power source.

[0176] 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.

[0177] 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, it is possible to use household electrical appliances by using the power stored in the secondary battery, which is a power storage source.

[0178] Of course, the secondary battery may be used for purposes other than the series of purposes exemplified here.

[0179] An embodiment of the present technology will be described.

[0180] Examples 1 to 4 and Comparative Examples 1 to 3 As will be explained below, secondary batteries were fabricated, and then the battery characteristics of the secondary batteries were evaluated.

[0181] [Manufacture of Secondary Battery] A secondary battery was manufactured according to the procedure described below.

[0182] Here, a test secondary battery was fabricated to perform a simple evaluation of battery characteristics. Fig. 6 shows the cross-sectional structure of the test secondary battery (a coin-type lithium-sulfur secondary battery).

[0183] 6, this secondary battery includes a test electrode 61, a counter electrode 62, a separator 63, an exterior cup 64, an exterior can 65, a gasket 66, and an electrolyte (not shown). Here, the test electrode 61 corresponds to the positive electrode, and the counter electrode 62 corresponds to the negative electrode.

[0184] The test electrode 61 is housed in an exterior cup 64, and the counter electrode 62 is housed in an exterior can 65. The test electrode 61 and the counter electrode 62 are stacked together with a separator 63 interposed therebetween, and the test electrode 61, the counter electrode 62, and the separator 63 are each impregnated with an electrolyte. The exterior cup 64 and the exterior can 65 are crimped together with a gasket 66, so that the test electrode 61, the counter electrode 62, and the separator 63 are sealed by the exterior cup 64 and the exterior can 65.

[0185] (Fabrication of Test Electrode) The test electrode 61 was fabricated by a firing method. Specifically, first, a plurality of holding particles 121 (anatase type titanium oxide (anatase type TiO 2 ) or anatase-type titanium oxide doped with a dopant (boron) at 4 mol % relative to titanium (anatase-type TiO 2 The aqueous paste was obtained by mixing the boron-doped cathode binder (polyethylene glycol) and the aqueous solvent (water) at a mixing ratio (weight ratio) of the plurality of retaining particles 121 to the cathode binder of 90:10.

[0186] Next, the aqueous paste was applied to one surface of a positive electrode current collector 110 (titanium foil having a thickness of 20 μm) to form a coating film. Next, the positive electrode current collector 110 on which the coating film had been formed was pressed using a roll press, thereby pressure-bonding the coating film to the positive electrode current collector 110.

[0187] Subsequently, the positive electrode current collector 110 on which the coating film was formed was fired in the atmosphere (firing temperature = 750°C, firing time = 1 hour). As a result, the positive electrode binder was degreased, and the plurality of retention particles 121 were sintered to each other, thereby forming a porous structure 124.

[0188] Subsequently, the positive electrode current collector 110 having the porous structure 124 formed thereon was punched out into a disk shape (diameter=15 mm).

[0189] Next, a sulfur-containing material (lithium polysulfide (Li)) was dissolved in a solvent (1,2-dimethoxyethane). 2 S 8 )) was added to the solvent, and the solvent was stirred to prepare a sulfur-containing solution. In this case, the concentration of the sulfur-containing solution was 200 mmol / L (= 200 mmol / dm 3 )

[0190] Finally, the positive electrode current collector 110 with the porous structure 124 formed thereon was immersed in a sulfur-containing solution (immersion time = 1 hour), and then the positive electrode current collector 110 with the porous structure 124 formed thereon was removed from the sulfur-containing solution and dried. As a result, the plurality of positive electrode active material particles 122 (sulfur-containing material) were held by the plurality of holding particles 121, respectively, and thus the test electrode 61 was produced.

[0191] For comparison, the material for forming the plurality of holding particles 121 was rutile titanium oxide (rutile TiO 2 A test electrode 61 was fabricated in the same manner except that the test electrode 61 was fabricated using the same material.

[0192] For comparison, a test electrode 61 was fabricated using substantially the same procedure, except that a carbon material (Ketjen black) was used instead of anatase titanium oxide as the material for forming the plurality of retention particles 121. In this case, a coating method was used as the fabrication method for the test electrode 61. Specifically, a paste-like mixture slurry was prepared by adding a positive electrode active material (carbon material), a positive electrode binder (styrene butadiene rubber), and a dispersant (carboxymethyl cellulose) to a solvent (a mixture of water, which is an aqueous solvent, and ethanol, which is an organic solvent). The mixture slurry was then applied to one side of the positive electrode current collector 110 and dried.

[0193] The "porous structure" column in Table 1 indicates whether or not the porous structure 124 is formed. "Formed" means that a sintered body of the plurality of retention particles 121 was formed, so that the porous structure 124 was formed. "Non-formed" means that a sintered body of the plurality of retention particles 121 was not formed, so that the porous structure 124 was not formed.

[0194] (Fabrication of Counter Electrode) An alkali metal (lithium metal plate) serving as a negative electrode active material was punched out into a disk shape (diameter = 16 mm), thereby obtaining a counter electrode 62.

[0195] (Preparation of Electrolyte Solution) An electrolyte salt (lithium bis(trifluoromethanesulfonyl)imide (LiN(CF))) was dissolved in a solvent (1,2-dimethoxyethane, an ether). 3 SO 2 ) 2 ) and lithium nitrate (LiNO 3 In this case, the content of the electrolyte salt (lithium bis(trifluoromethanesulfonyl)imide) was adjusted to 1 mol / L (=1 mol / dm 3 ), and the content of electrolyte salt (lithium nitrate) was 1 mol / l (= 1 mol / dm 3 ) Thus, the electrolyte solution was prepared.

[0196] (Assembly of Secondary Battery) First, the test electrode 61 was placed in the exterior cup 64, and the counter electrode 62 was placed in the exterior can 65. Next, the test electrode 61 placed in the exterior cup 64 and the counter electrode 62 placed in the exterior can 65 were stacked together with an electrolyte-impregnated separator 63 (a glass fiber separator with a thickness of 200 μm) interposed therebetween. In this case, the test electrode 61 was positioned so that the positive electrode active material layer 120 faced the counter electrode 62 with the separator 63 interposed therebetween. Finally, with the test electrode 61 and the counter electrode 62 stacked together with the separator 63 interposed therebetween, the exterior cup 64 and the exterior can 65 were crimped together with the gasket 66. As a result, the test electrode 61 and the counter electrode 62 were sealed in the exterior cup 64 and the exterior can 65, completing the secondary battery.

[0197] After the secondary battery was completed, the secondary battery was disassembled to recover the test electrode 61. Using this test electrode 61, the average particle size AS (nm) of the multiple retained particles 121 was calculated according to the procedure described above. The calculation results of the average particle size AS are shown in Table 1.

[0198] [Evaluation of Battery Characteristics] When the initial charge / discharge characteristics and the battery capacity characteristics were evaluated as the battery characteristics, the results shown in Table 1 were obtained.

[0199] (Initial Charge-Discharge Characteristics) First, the charge capacity (mAh) and discharge capacity (mAh) of the secondary battery were measured using cyclic voltammetry in a room temperature environment (temperature = 25° C.). In this case, the current (mA) was measured while sweeping the potential (V) in the range of 1.9 V to 2.8 V at a sweep rate of 0.01 mV / sec, and the correlation between the potential and the current (cyclic voltammogram) was obtained.

[0200] Next, the cyclic voltammogram was divided into a discharge region (negative current region) and a charge region (positive current region). The discharge capacity was calculated by integrating the current in the discharge region with respect to time, and the charge capacity was calculated by integrating the current in the charge region with respect to time.

[0201] Finally, the initial efficiency, which is an index for evaluating the initial charge-discharge characteristics, was calculated based on the formula: initial efficiency (%)=charge capacity (mAh) / discharge capacity (mAh).

[0202] (Battery Capacity Characteristics) After measuring the discharge capacity (mAh) described above, the secondary battery was first disassembled to recover the test electrode 61 and peel the positive electrode current collector 110 from the positive electrode active material layer 120. Next, the positive electrode active material layer 120 was placed in an organic solvent (1,2-dimethoxyethane), and the organic solvent was stirred and then filtered. As a result, the plurality of positive electrode active material particles 122 were dissolved in the organic solvent, and a filtrate consisting of undissolved components was recovered. This filtrate contained the porous structure 124 (the plurality of retention particles 121).

[0203] Next, the surface area (m 2 ) was measured. In this case, a fully automatic specific surface area measuring device, Macsorb (registered trademark), manufactured by Mountec Co., Ltd. was used as the surface area measuring device. After degassing the porous structure 124 (heating temperature = 200°C and heating time = 30 minutes), the surface area was measured using the BET method (nitrogen gas).

[0204] Finally, the basic capacity (mAh / m 2 ) = discharge capacity (mAh) / surface area (m 2 ) was used to calculate the basic capacity, which is an index for evaluating the battery capacity characteristics.

[0205]

[0206] [Discussion] As shown in Table 1, the initial efficiency and basic capacity each varied greatly depending on the configuration of the test electrode 61.

[0207] Specifically, when the material of the retaining particles 121 was rutile-type titanium oxide and a porous structure 124 was formed (Comparative Example 2), the initial efficiency decreased significantly and the basic capacity also decreased significantly.

[0208] Furthermore, when the material of the retaining particles 121 was a carbon material (Ketjen black) and the porous structure 124 was not formed (Comparative Example 3), the initial efficiency increased significantly, but the basic capacity decreased significantly.

[0209] In contrast, when the material of the retaining particles 121 was anatase-type titanium oxide and a porous structure 124 was formed (Examples 1 to 4 and Comparative Example 1), the initial efficiency and basic capacity each varied depending on the average particle size AS.

[0210] When the average particle size AS was larger than 100 nm (Comparative Example 1), the charge / discharge reaction did not proceed, and therefore the initial efficiency and the basic capacity could not be calculated.

[0211] However, when the average particle size AS was 100 nm or less (Examples 1 to 3), the charge-discharge reaction proceeded, and the initial efficiency and basic capacity could be calculated. In this case, the initial efficiency and basic capacity increased significantly.

[0212] In particular, when the average particle size AS was 100 nm or less (Examples 1 to 3), when the average particle size AS was 30 nm or less, more specifically, 7 nm to 30 μm, the initial efficiency and the basic capacity were further increased.

[0213] Furthermore, when boron-doped anatase-type titanium oxide was used as the material for the retaining particles 121 (Example 4), the basic capacity was increased while maintaining a significantly high initial efficiency compared to when anatase-type titanium oxide that was not doped with boron was used (Example 2).

[0214] [Summary] From the results shown in Table 1, it can be seen that when the positive electrode 100 includes a positive electrode current collector 110 and a positive electrode active material layer 120, the positive electrode active material layer 120 includes a plurality of retention particles 121 (anatase-type titanium oxide) and a plurality of positive electrode active material particles 122 (sulfur-containing material), the positive electrode active material layer 120 includes a porous structure 124, the porous structure 124 is formed by the plurality of retention particles 121 being directly bonded to one another and directly connected to the positive electrode current collector 110, the plurality of positive electrode active material particles 122 are retained by each of the plurality of retention particles 121, and the plurality of retention particles 121 have an average particle size AS of 100 nm or less, high initial efficiency and high basic capacity were obtained. Therefore, both the initial charge / discharge characteristics and the battery capacity characteristics were improved, and excellent battery characteristics could be obtained in the secondary battery.

[0215] The configuration of the secondary battery according to the present technology has been described above with reference to an embodiment and examples. However, the configuration of the secondary battery according to the present technology is not limited to the configuration described in the embodiment and examples, and various modifications are possible.

[0216] Specifically, the battery structure of the secondary battery has been described as being of a laminate film type and a coin type. However, the battery structure of the secondary battery is not particularly limited, and may be of a cylindrical type, a square type, a button type, or the like.

[0217] The battery element has been described as having a wound structure. However, the structure of the battery element is not particularly limited, and may be a stacked structure or a zigzag structure. In the stacked structure, the positive and negative electrodes are stacked on top of each other, and in the zigzag structure, the positive and negative electrodes are folded in a zigzag pattern.

[0218] The effects described in this specification are merely examples, and therefore the effects of the present technology are not limited to the effects described in this specification. Therefore, other effects may be obtained with respect to the present technology.

[0219] The present technology may also be configured as follows. <1> A secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes: a positive electrode current collector; and a positive electrode active material layer supported by the positive electrode current collector, wherein the positive electrode active material layer includes: a plurality of retention particles including anatase-type titanium oxide; and a plurality of positive electrode active material particles including a sulfur-containing material, wherein the plurality of retention particles are directly bonded to each other to form a porous structure, the porous structure is directly connected to the positive electrode current collector, the plurality of positive electrode active material particles are retained by each of the plurality of retention particles, and the average particle size of the plurality of retention particles is 100 nm or less. <2> The secondary battery according to <1>, wherein the average particle size is 30 nm or less. <3> The secondary battery according to <1> or <2>, wherein the sulfur-containing material includes an alkali metal polysulfide. <4> The secondary battery according to <3>, wherein the alkali metal polysulfide includes lithium polysulfide. <5> The secondary battery according to any one of <1> to <4>, which is a lithium-sulfur secondary battery. <6> A positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode active material layer supported by the positive electrode current collector, wherein the positive electrode active material layer comprises a plurality of retention particles containing anatase-type titanium oxide and a plurality of positive electrode active material particles containing a sulfur-containing material, the plurality of retention particles are directly bonded to each other to form a porous structure, the porous structure is directly connected to the positive electrode current collector, the plurality of positive electrode active material particles are retained by each of the plurality of retention particles, and the plurality of retention particles have an average particle size of 100 nm or less.

Claims

1. a positive electrode, a negative electrode, and an electrolyte; The positive electrode is a positive electrode current collector; a positive electrode active material layer supported by the positive electrode current collector; Including, The positive electrode active material layer is a plurality of support particles including anatase titanium oxide; a plurality of positive electrode active material particles including a sulfur-containing material; Including, the plurality of holding particles are directly bonded to each other to form a porous structure; the porous structure is directly connected to the positive electrode current collector; the plurality of positive electrode active material particles are held by the plurality of holding particles, The average particle size of the plurality of retained particles is 100 nm or less. Secondary battery.

2. The average particle size is 30 nm or less. The secondary battery according to claim 1 .

3. The sulfur-containing material includes an alkali metal polysulfide. The secondary battery according to claim 1 or 2.

4. The alkali metal polysulfide includes lithium polysulfide. The secondary battery according to claim 3 .

5. It is a lithium-sulfur secondary battery. The secondary battery according to claim 1 or 2.

6. a positive electrode current collector; a positive electrode active material layer supported by the positive electrode current collector; Including, The positive electrode active material layer is a plurality of support particles including anatase titanium oxide; a plurality of positive electrode active material particles including a sulfur-containing material; Including, the plurality of holding particles are directly bonded to each other to form a porous structure; the porous structure is directly connected to the positive electrode current collector; the plurality of positive electrode active material particles are held by the plurality of holding particles, The average particle size of the plurality of retained particles is 100 nm or less. Positive electrode for secondary batteries.